WATER POLLUTION CONTROL RESEARCH SERIES • 12010 EIE 03/71
An Investigation of Techniques
for Removal of Chromium
from Electroplating Wastes
ENVIRONMENTAL PROTECTION AGENCY • WATER QUALITY OFFICE
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WATER POLLUTION CONTROL RESEARCH SERIES
The Water Pollution Control Research Series describes the
results and progress in the control and abatement of pollu-
tion of our Nation's waters. They provide a central source
of information on the research, development, and demon-
stration activities of the Water Quality Office, Environ-
mental Protection Agency, through inhouse research and grants
and contracts with Federal, State, and local agencies, re-
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Inquiries pertaining to the Water Pollution Control Research
Reports should be directed to the Head, Project Reports
System, Office of Research and Development, Water Quality
Office, Environmental Protection Agency, Washington, B.C. 20242.
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AN INVESTIGATION OF TECHNIQUES FOR REMOVAL
OF CHROMIUM FROM ELECTROPLATING WASTES
Sponsored by
INDUSTRIAL POLLUTION CONTROL BRANCH
ENVIRONMENTAL PROTECTION AGENCY
and
METAL FINISHERS' FOUNDATION
Prepared by
BATTELLE MEMORIAL INSTITUTE
Columbus Laboratories
Columbus, Ohio 43201
Program #12010 EIE
Grant #WPRD 201-10-68
March 1971
For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 - Price $1.00
Stock Number 5501-0087
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WQO Review Notice
This report has been reviewed by the Water Quality
Office and approved for publication. Approval does
not signify that the contents necessarily reflect the
views and policies of the Water Quality Office, nor
does mention of trade names or commercial products
constitute endorsement or recommendation for use.
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ABSTRACT
This report describes work which was conducted on the removal of hexa-
valent chromium from plating rinse waters employing various treatment
processes. The study consisted of an initial phase in which information
was sought by questionnaire and by wastewater analyses on the type of
waste produced by smaller electroplating plants. Laboratory studies were
conducted on several nonconventional methods for treatment of these
wastewaters including ion flotation, adsorption on activated carbon, and
solvent extraction. A demonstration pilot-plant study also was conducted
on the activated carbon process employing actual rinse waters from a hard
chrome plating operation.
The results of the various phases of the study indicated that activated
carbon adsorption for chromium removal may have practical application in
many small plating plants. Further development of the process was recom-
mended in actual plating plant installations.
This report was submitted by Battelle Memorial Institute, Columbus
Laboratories, in partial fulfillment of Grant Project WPRD 201-01-68
by the Industrial Pollution Control Branch, Environmental Protection
Agency to the Metal Finishers' Foundation.
Key Words: Electroplating wastes
Chromium
Waste treatment
Activated carbon
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CONTENTS
Section Page
I CONCLUSIONS 1
II RECOMMENDATIONS 3
III INTRODUCTION 5
IV BACKGROUND INFORMATION 7
State of the Art of Metal-Finishing Waste Treatment • • 7
Current Practices in Small Electroplating Plants • • • 7
Production Characteristics 7
Waste-Effluent Volumes and Compositions 9
V EXPERIMENTAL WORK 13
Phase 1: Preliminary Experimental Study 13
General Scope of Investigation 13
Ion Flotation Studies 13
General Description of Method 13
Equipment and Procedure 14
Results 17
Activated Carbon Adsorption Studies 25
General Description of Method 25
Equipment and Procedure 26
Results 29
Liquid-Liquid Extraction Studies 40
General Description of Method 40
Chemistry of Liquid-Liquid Extraction • • • • 42
Requirements for Feasibility 43
Equipment and Procedure 44
ii
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CONTENTS (Cont.)
Section Page
Results 46
Regeneration 48
Treatability of the Strip Solution 49
Other Techniques Evaluated 51
Reduction of Chromium with Activated Carbon. . 51
Reverse Osmosis for Chromium Removal 51
Adsorption of Chromium on Activated Alumina . . 52
Preliminary Evaluation of Treatment Costs 52
Ion Flotation 53
Adsorption on Activated Carbon 54
Liquid-Liquid Extraction 56
Comparison of Experimental and Conventional . . 59
Processes
Phase 2: Demonstration Plant Study 60
Pilot-Scale Investigation 62
Equipment and Procedure 62
Results of Sulfuric Acid Regeneration Method . 64
Results of Caustic Regeneration Method .... 67
Discussion of Pilot-Plant Runs 67
VI ECONOMIC EVALUATION OF PROCESS 77
VII ACKNOWLEDGMENTS 81
VIII BIBLIOGRAPHY £3
85
IX APPENDICES
Conventional Methods for Treatment of Chromium Wastewater. 87
Methods used for Control Analyses 89
iii
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FIGURES
1 CALCULATED LEVELS OF CHROMIUM AND CYANIDE DISCHARGED BY NAMF
MEMBER PLANTS 10
2 WATER USAGE BY NAMF MEMBER PLANTS 11
3 STANDARD LABORATORY FLOTATION CELL FOR THE ION FLOTATION
EXPERIMENTS 15
4 SKETCH OF FLOTATION CELL 16
5 EFFICIENCY OF VARIOUS COLLECTORS FOR CHROMIUM REMOVAL FROM
100 PPM SOLUTION 21
6 EFFICIENCY OF VARIOUS COLLECTORS FOR CHROMIUM REMOVAL FROM
10 PPM SOLUTION 22
7 BENCH-SCALE CONTINUOUS COLUMNS FOR ADSORPTION EXPERIMENTS 28
8 AMOUNT OF POWDERED ACTIVATED CARBON REQUIRED FOR HEXAVALENT
CHROMIUM REMOVAL 31
9 EFFECT OF CONTACT TIME ON CHROMIUM REMOVAL 32
10 EFFECT OF pH ON CHROMIUM REMOVAL 33
11 TYPICAL ADSORPTION ISOTHERMS FOR HEXAVALENT CHROMIUM 35
12 EXAMPLE OF TWO-STAGE EXTRACTION, SINGLE-STAGE STRIPPING
LIQUID-LIQUID EXTRACTION PROCESS 41
13 BENCH-SCALE MORRIS-TYPE CONTACTOR FOR CONTINUOUS LIQUID-
LIQUID EXTRACTION EXPERIMENTS 45
14 FLOWSHEET ASSUMED FOR COST ESTIMATES 57
15 ALTERNATIVE PROCESSES FOR CHROMIUM REMOVAL BY ACTIVATED CARBON 61
16 PILOT-PLANT CARBON ADSORPTION SYSTEM 63
17 PILOT-PLANT DATA ON ADSORPTION AND ACID REGENERATION 68
18 PILOT-PLANT DATA ON ADSORPTION AND CAUSTIC REGENERATION 71
19 ADSORPTION CAPACITIES FOR HEXAVALENT CHROMIUM AFTER MULTIPLE
CYCLES WITH INTERMEDIATE CAUSTIC REGENERATION 73
20 ADSORPTION CAPACITIES FOR HEXAVALENT CHROMIUM AFTER MULTIPLE
CYCLES WITH INTERMEDIATE ACID REGENERATION 74
21 EFFECT OF OPERATING CYCLES ON CHEMICAL COSTS FOR TWO
REGENERATION METHODS 79
22 STANDARD CHROMIUM CURVE 91
iv
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TABLES.
No.,
1 SUMMARY OF NAMF MEMBER PLANTS IN WHICH CHROMIUM AND/OR
CYANIDES ARE USED 8
2 SUMMARY OF RINSING PRACTICES IN NAMF MEMBER PLANTS 8
3 SUMMARY OF DISPOSAL METHODS USED BY NAMF MEMBER PLANTS 8
4 CONCENTRATIONS OF CHROMIUM AND CYANIDE IN VARIOUS WASTE
STREAMS FROM NAMF MEMBER PLANTS 12
5 TYPICAL CONCENTRATION OF HEAVY METALS IN COMBINED EFFLUENTS
FROM SEVERAL NAMF MEMBER PLANTS 12
6 TECHNIQUES EVALUATED EXPERIMENTALLY 13
7 EXPERIMENTAL DATA ON VARIOUS COLLECTORS FOR FLOTATION OF
HEXAVALENT CHROMIUM 18
8 FLOTATION DATA SHOWING THE EFFECT OF pH ON REMOVAL OF
HEXAVALENT CHROMIUM 19
9 FLOTATION DATA ON CHROMIUM-CONTAINING SOLUTIONS AT VARIOUS
INITIAL CONCENTRATIONS 20
10 EXPERIMENTAL DATA ON THE REGENERATION AND REUSE OF FLOTATION
COLLECTOR FOR HEXAVALENT CHROMIUM 24
11 FLOTATION DATA ON SOLUTIONS CONTAINING TRIVALENT CHROMIUM 25
12 TYPES OF ACTIVATED CARBON EVALUATED 27
13 REMOVAL OF HEXAVALENT CHROMIUM WITH VARIOUS TYPES OF
ACTIVATED CARBON 30
14 COMPARISON OF GRANULAR AND PULVERIZED ACTIVATED CARBON FOR
CHROMIUM REMOVAL AT VARIOUS CONCENTRATIONS 36
15 CONTINUOUS EXPERIMENTS ON ADSORPTION OF CHROMIUM WITH
ACTIVATED CARBON 37
16 REGENERATION OF CARBON FROM CHROMIUM ADSORPTION EXPERIMENTS 39
17 PERFORMANCE OF SEVERAL AMINES IN THE BATCH EXTRACTION OF
HEXAVALENT CHROMIUM 47
18 BATCH EXTRACTION OF HEXAVALENT CHROMIUM FROM MORE CONCENTRATED
SOLUTIONS USING ALAMINE 336 46
19 EXTRACTION OF HEXAVALENT CHROMIUM IN CONTINUOUS OPERATIONS 48
20 EXPERIMENTAL DATA ON CHROMIUM REMOVAL BY REVERSE OSMOSIS 51
21 EFFICIENCY AND COSTS OF ION FLOTATION COLLECTORS FOR
CHROMIUM REMOVAL 53
22 ESTIMATED REAGENT COSTS FOR ACTIVATED CARBON REGENERATION
AFTER ADSORPTION OF CHROMIUM 55
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TABLED (Cont.)
No..
23 ECONOMIC FACTORS IN OPERATING COSTS FOR LIQUID-LIQUID
EXTRACTION 58
24 PRELIMINARY COMPARISON OF COSTS FOR SELECTED EXPERIMENTAL
AND CONVENTIONAL TREATMENT METHODS 59
25 PILOT-PLANT ADSORPTION DATA ON COLUMN NO. 1, ACID
REGENERATION 65
26 PILOT-PLANT REGENERATION DATA ON COLUMN NO. 1 66
27 PILOT-PLANT ADSORPTION DATA ON COLUMN NO. 2, CAUSTIC
REGENERATION 69
28 PILOT-PLANT REGENERATION DATA ON COLUMN NO. 2 70
29 COMPARISON OF CHEMICAL COSTS FOR ACTIVATED CARBON ADSORPTION
AND CONVENTIONAL TREATMENT 78
30 ESSENTIAL PLANT AND OPERATING COSTS FOR CARBON ADSORPTION
AND CONVENTIONAL TREATMENT PROCESSES 80
vi
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SECTION I
CONCLUSIONS
(1) The use of waste treatment systems by small electroplating plants
is not yet required in many locations. Adequate technology, however, is
available for treating these wastes to any required degree of detoxifica-
tion by conventional processes prior to discharge into sewers or other
receiving waters.
(2) On the basis of laboratory studies, several nonconventional pro-
cesses are technically feasible for the treatment of chromium rinse
waters. These are: activated-carbon adsorption, ion flotation, and
solvent extraction.
(3) From a practical standpoint, an activated-carbon process has promise
of excelling the conventional methods for treatment of chromium rinse
waters from small plating shops. The carbon process may have significant
advantages in low capital investment, simple and continuous operation,
and minimum floor space requirements, all of which are particularly
important to the small plater.
(4) The carbon process has been shown feasible on a pilot-plant scale
with actual chromium rinse waters from a small plating company. The
system can be operated with removal efficiencies greater than 99 percent
for hexavalent chromium. Removal of total chromium is about 95 percent.
The process also provides for partial recovery of the removed chromium by
caustic regeneration techniques or, alternately, the loaded carbon can
be disposed of conveniently as a solid waste material. The carbon-
adsorption process, as presently developed, is not optimum and very
probably can be improved with additional study. The regeneration step,
while workable, was not as efficient as originally anticipated. Improved
regeneration, as might be realizable with continued operation, would
enhance significantly the practical and economic feasibility of this
process.
(5) Operating costs for a carbon process employing caustic regeneration
are about $5.00 per 8-hour day on the basis of a 15-gpm waste stream
containing 100 ppm of hexavalent chromium. This compares to an opera-
ting cost of about $7.00 for the conventional treatment method employing
sodium bisulfite.
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SECTION II
RECOMMENDATIONS
This program has involved an evaluation of both conventional and noncon-
ventional processes for treating chromium rinse waters considering the
technical and economic criteria believed applicable to small electro-
plating plants. The scope of the program was to develop processes for
demonstration phase utilization. Based on the results obtained in the
study, it was recommended that field testing of an activated-carbon
process on chromium rinse waters be carried out.
For accurate definition of the full potential of the carbon adsorption
process, the data obtained in this study should be extended. The pilot-
plant runs, while demonstrating that carbon adsorption has the promise
of becoming a very simple and economic process especially suitable for
the smaller plater, have not provided a complete answer to several
important questions:
(1) How can stripping be made more effective and complete?
(2) With better stripping, how much greater adsorptive capacity can be
obtained per cycle and what effect will this have on the longevity of
the carbon?
(3) What will be the precise effect of operation with waste solutions
more dilute and more concentrated than the waste streams used in this
study?
(4) To what extent will improved stripping and higher adsorption capac-
ities enhance the practical and economic feasibility of the process?
(5) What is the impact of other metals, such as copper and nickel which
were present to some extent in pilot-scale runs on the adsorption and
stripping of chromium?
It is recommended that these factors be established-firmly, not by any
extension of a formal research program, but by installing units similar
to the one used in this study in several plants and to have them
operated over a period of at least 1 year.
Such continuing effort would be aimed at establishing design criteria in
terms of the economic optimization of the carbon process in a variety of
possible installations. These installations are recommended where (1)
the waste flow is small and contains low concentrations of hexavalent
chromium, (2) low-to-moderate quantities of waste chromium are involved,
and a simple and convenient method of disposal is required, and (3)
moderate quantities of chromium wastes are produced and the possibility
exists for recovery of sodium chromate.
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Very probably, some guidance and back-up laboratory work would be
required. It is believed that much if not all of this could be done at
the plant site by the selected companies themselves.
Battelle suggests that a plan for these extended runs be discussed in
detail with officials of the Metal Finishers' Foundation.
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SECTION III
INTRODUCTION
Many industries in the heavily industrialized areas of the United States
are substantial contributors to the growing problem of water pollution.
In many cases the wastes generated by industrial sources are carbonaceous
and can be treated by biological methods or by incineration techniques.
In other cases, however, the wastes are largely inorganic materials which
cannot be disposed of by conventional techniques. Some of the most
troublesome wastes are those generated in the production of inorganic
chemicals and in various facets of the metallurgical industry. In the
latter category, considerable attention has been directed toward abate-
ment of pollution from waste pickle liquor from the steel industry and
from metal-finishing wastes.
Electroplating and metal-finishing waste streams can contribute to stream
pollution, either directly, owing to their content of toxic and corrosive
materials, such as cyanide, acids, and metals; or indirectly, owing to
the deleterious effect these components exert on biological sewage treatment sys-
tems. Federal, state, and municipal regulations fixing the allowable
concentrations of the harmful components of these wastes already have been
established. The restrictions are fairly rigorous at present. There is
indication that they will, in many places, be made more rigorous in the
future. Enforcement of regulations may be expected to become increasingly
strict.
There is ample technology available for treating chromium and cyanide
wastewaters to any required degree of detoxification. The conventional
treatment methods, however, have been developed primarily for large
electroplating plants where the relatively high cost of treatment can be
absorbed. Economical methods have not been developed for the treatment
of wastes from the relatively small electroplating shops. Currently many
of these plants discharge their wastes into the city sewers and depend
upon the municipal sewage plant for the removal of toxic materials.
More restrictive sewer ordinance and receiving water standards are major
factors that make the development of economical techniques for treating
the effluents from small electroplating facilities highly desirable. Another
factor is the potential recovery of valuable metals from these waste streams.
The Pollution Abatement Committee of the Metal Finishers' Foundation,
being acutely aware of this problem, authorized Battelle to undertake a
study of existing hydrometallurgical techniques to determine their appli-
cability to the treatment of wastes from smaller electroplating plants.
This study, as proposed to the Metal Finishers' Foundation, was comprised
of two phases:
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Phase 1. Preliminary Experimental Study.
Phase 2. Demonstration Plant Study.
In each of these phases, chromium wastes and cyanide wastes were given
separate study. This report presents the results, conclusions, and
recommendations of the study on chromium wastes. A similar report will
be prepared at a later date when the work on cyanide waste has been
completed,
The first phase of the work on chromium was, in effect, a screening
study, the overall objective of which was to pinpoint the processes best
adapted technically and economically to the treatment of chromium wastes
typical of those generated by the smaller plater. In this phase of the
study, a thorough review of the present state of the art in the treat-
ment of chromium waste was conducted; wastes from a selected sample of
smaller plating establishments were characterized for volume and compo-
sition; and the various nonconventional approaches available to the
smaller plater for treating his wastes were evaluated experimentally.
As a result of this preliminary phase study, it was concluded that an
activated carbon adsorption technique, a somewhat novel approach, gave
promise of excelling the conventional process as a practical and economic
method for treating chromium wastes from the typical smaller shop.
In the second phase cf the work on chromium wastes, this carbon adsorp-
tion process was investigated on a pilot-plant scale. The pilot plant
was set up in an actual plating plant (a member of the National Association
of Metal Finishers) and operated over a 5-month period on this plants'
normal reject rinse waters.
This study on chromium and cyanide emphasized only these two contaminants
as contributing to the overall waste treatment problem. This, of course,
overlooks other important contaminants, particularly heavy metals, which
would be carried over in the combined wastewater stream. The development
of an overall waste treatment approach which would remove these contaminants
was beyond the scope of the current program.
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SECTION IV
BACKGROUND INFORMATION
State of the Art of Metal-Finishing Waste Treatment
One of the initial efforts in this program was devoted to a survey of
the open literature pertaining to waste treatment in the metal-finishing
industry. This state-of-the-art survey emphasized such aspects as the
nature of electroplating and metal-finishing wastes, current restrictions
on their disposal, and conventional methods available for treatment of
these wastes. The survey is presented in detail as a separate progress
report on this program, designated 12010 EIE, dated November 15, 1968.
It is not included within this report. However, for the purpose of
comparing waste-treatment processes, a description of the conventional
methods determined by this survey is included in the appendix. The review
of these methods is intended to provide facts for the guidance of the
smaller plater in the selection of a waste-treatment process. It should
be pointed out, however, that these methods were developed more or less
for use in larger electroplating plants having large volumes of waste-
water. The use of these methods for treating low volumes of wastewater
would certainly be feasible technically, but .could be impractical or un-
economical for the smaller plater.
Current Practices in^ Smaj^l Electroplating Plants
Production Characteristics
One of the major efforts during the initial phase of the program was the
accumulation of considerable data on the operating characteristics of
small electroplating plants. These data were obtained by surveying mem-
ber plants in NAMF via questionnaires. The survey emphasized primarily
the extent of chromium- or cyanide-plating operations and certain aspects
of these operations such as rinsing methods, disposal methods, chemical
and water usage, etc. Questionnaires were sent to 655 U.S. members
(foreign members were not included). In addition, 30 firms were asked
to submit samples of combined and segregated rinse waters from their
plant. Although only about 200 questionnaires were returned, a large
number of firms were not involved in chromium or cyanide plating.
The basic production characteristics of NAMF member plants based on this
survey are summarized in Tables 1, 2, and 3. Table 1 shows the number
of plants which have chromium and/or cyanide plating operations. These
data indicate that a large majority of the plants (81 percent) do both
chromium and cyanide plating.
Table 2 shows the number of plants which currently have separate or com-
bined rinsing circuits and whether or not it would be practical to separate
these wastes. Table 3 indicates the current distribution of plants that
employ sanitary sewers, lagoons, natural water bodies, etc., for
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disposal of wastewaters. These data indicate that most of the plants use
sanitary or storm sewers for disposal.
TABLE 1. SUMMARY OF NAMF MEMBER PLANTS IN WHICH
CHROMIUM AND/OR CYANIDES ARE USED
Plants with chromium only
Plants with cyanide only
Plants with both chromium and cyanides
Total
Number
of Plants
24
11
111
186
Percent
of Total
13
6
_JLL
100
TABLE 2. SUMMARY OF RINSING PRACTICES IN NAMF MEMBER PLANTS
Separate chromium and cyanide rinse circuits
Combined chromium and cyanide rinse circuits
Practical to separate wastes
Impractical to separate wastes
Number
of Plants
84
94
71
100
Percent
of Total
47
53
42
58
TABLE 3. SUMMARY OF DISPOSAL METHODS USED BY
NAMF MEMBER PLANTS
Municipal sanitary sewer
Storm sewer
Land disposal
Lagooning
Natural stream, lake, etc.
Others
Number
of Plants
143
24
5
3
10
6
Percent
of Total
75
12
3
2
5
3
Total
191
100
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It is reasonable to presume that the response to the questionnaires con-
stitutes a valid sample and that the information obtained is applicable
to the association as a whole and possibly to the entire smaller shop
plating industry.
Waste-Effluent Volumes and Compositicms
The level of contaminants in wastewater from electroplating operations
and the total volume of these wastes are also important factors in the
evaluation of waste-treatment processes for small plating plants. . Since
this information is almost unavailable in the literature, the waste sur-
vey was used to provide data on these factors. NAMF member companies
were asked to report their annual consumption of plating chemicals con-
taining chromium and cyanide and the total annual water usage within the
plant. For the purpose of estimating the amount of these plating chemi-
cals which eventually appear in the wastewater, a loss factor of 80 per-
cent was selected for both the chromium and cyanides.
Based on this assumption, calculations were made of the total chromium,
cyanide, and water discharged by each particular plant. A summarization
of these data for all plants surveyed is shown in Figures 1 and 2. These
graphs are important in establishing the approximate position of a par-
ticular plant within the industry and can be used to provide the follow-
ing generalizations with regard to waste treatment methods. They are of
importance in the assessment of:
(1) The general levels of chromium and cyanides likely to be discharged
in the plant's wastewater
(2) The approximate annual cost for destruction of these contaminants
within various segments of the industry
(3) Areas where recovery of plating chemicals and water should be
considered.
In addition to the data shown in Figures 1 and 2, considerable data also
were obtained on the actual composition of rinse waters generated by
NAMF member plants. Selected plants submitted samples of their waste
streams and these samples were analyzed to determine concentrations of
cyanide, chromium, and other metals. Analytical data on chromium and
cyanide levels in various waste streams are summarized in Table 4. Typi-
cal analyses for heavy metals in effluents from various plants are shown
in Table 5. The data on chromium and cyanide indicate that cyanide con-
centrations are two to three times higher than chromium concentrations
for combined effluents, which supports those data shown previously in
Figure 1. Generally, it can be stated that concentrations will range
from 10 to 100 ppm for either chromium or cyanide. Heavy metal concen-
trations, by contrast, generally fall below 10 ppm with the possible
*
U.S. Bureau of Mines Information Circular 8058.
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Cti
01
"d
C
cti
cn
o
C
0)
14-1
w
C
•H
0)
-G
•H
C
n)
>>
u
(-1
o
•H
e
o
u
o
H
L 2 5 10 20 30 40 50 60 70 80 90 95 98 99
Plants with Less Than the Indicated Amount of Contaminant, %
FIGURE 1. CALCULATED LEVELS OF CHROMIUM AND CYANIDE DISCHARGED BY
NAMF MEMBER PLANTS
10
-------
CO
t>0
C
O
•H
S
-------
exceptions of copper and nickel. These elements were the major heavy
metals found besides chromium in the particular samples which were
analyzed.
TABLE 4. CONCENTRATIONS OF CHROMIUM AND CYANIDE IN VARIOUS
WASTE STREAMS FROM NAMF MEMBER PLANTS(a>
Total Chromium,
_ Type of Sample
Separate chromium stream
Separate cyanide stream
Combined effluent
Number of
Samples
9
7
28
ppi
Range
15-70
—
0-49
nW
Average
41
—
11
Total Cyanide,
ppi
Range
•••»
9-115
1-103
n\c)
Average
..... ^
72
28
(a) Based on actual analyses by atomic adsorption and standard chemical
techniques.
(b) Includes chromium contained in both the liquid and solid fractions
of the sample.
(c) Cyanides in the liquid fraction, only.
TABLE 5. TYPICAL CONCENTRATION OF HEAVY METALS IN COMBINED
EFFLUENTS FROM SEVERAL NAMF MEMBER PLANTS
Concentration of Indicated Component, ppm
(a)
Cu
Plant
Plant
Plant
Plant
Plant
No.
No.
No.
No.
No.
4
7
12
15
21
31
2
10
36
7
.0
.0
.0
.0
.0
Zn
5
10
3
<0
0
.0
.0
.0
.5
.2
Cd
<0
1
4
1
<0
.5
.0
.0
.0
.5
Fe
2
2
<2
4
2
.0
.0
.0
.0
.0
Ni
9.0
5.0
58.0
24.0
23.0
(a) Based on analyses of samples by atomic adsorption
techniques.
12
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SECTION V
EXPERIMENTAL WORK
j*hase 1: PreliminaryiJSxperimental Study
General Scope of Investigation
The initial phase of experimental work was limited to the investigation
of "nonconventional" methods that might be applicable to the treatment
of chromium rinse waters—the conventional methods of treatment having
been thoroughly studied by numerous investigators for many years. At
the outset of the program, the two most promising of the "nonconventional"
approaches appeared to be:
(1) Ion flotation
(2) Liquid-liquid extraction.
These two approaches and a third—activated carbon adsorption--received
major attention in the experimental program. During the course of the
study, however, other approaches that would meet the qualifications of
"nonconventional" and "suitable for rinse waters" suggested themselves
and these were examined briefly. The "nonconventional" processes dealt
with experimentally are listed in Table 6 and are described in the
following sections of the report.
TABLE 6. TECHNIQUES EVALUATED EXPERIMENTALLY
Applicability;
Hexavalent Trivalent
Chromium Chromium
Ion Flotation x
Activated Carbon Adsorption x
Liquid-Liquid Extraction x
Reduction by Activated Carbon x
Reverse Osmosis x
Adsorption on Activated Alumina x
Ion Flotation Studies
General Description of Method. Of the various techniques studied during
this program, ion flotation probably is the most recently developed pro-
cess. The technique of separating ions from aqueous solutions by
13
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flotation has been recognized only for about 20 years. During this time,
however, it has received significant development effort and is now becom-
ing known as one of the basic chemical engineering unit processes.
Ion flotation is basically a combination of conventional mineral-flota-
tion and ion-exchange processes. In mineral flotation, for example,
finely divided solid particles are separated from a bulk solution by
attachment to small air bubbles introduced into the liquid. The bubbles
rise to the liquid surface, collapse, and form a froth which contains
the solid material in concentrated form. An identical procedure is used
in ion flotation to separate and concentrate ions from solution rather
than solid particles.
The mechanism of bubble attachment in both flotation techniques is accom-
plished through the addition of a suitable collector. These collectors
usually are some type of organic chemical having surface-active proper-
ties and are selective for only certain compounds. In addition to sur-
face activity, the collectors for ion flotation have an inorganic group
which ionizes in aqueous solutions and tends to make the collector
partially soluble. The exchange of ions between the collector and solu-
tion is the basis for separation of certain ions from the solution.
Because ion flotation is especially effective for removing ions from
extremely dilute solutions, it was given major attention during the
experimental program. Its simplicity and low equipment costs enhanced
its attractiveness for meeting the needs of the smaller plater.
Equipment and Procedure. The experimental apparatus for the study of
the ion-flotation technique comprised both a specially designed glass
flotation cell and a standard Denver laboratory cell which is designed
for conventional flotation work. A photograph of the Denver cell is
shown in Figure 3. Figure 4 is a sketch of the special glass cell.
The glass cell shown in Figure 4 was fabricated from a standard 500-cc
graduated cylinder by installing ports for injecting flotation collector
via a syringe and for withdrawing samples of the bulk solution. The
cell contained an air-dispersion tube which was made of fritted glass
with a porosity of 25 to 50 microns. During flotation, the air was
generally admitted at a constant rate and was measured by a rotameter.
The cell also was provided with an overflow tube which could be used to
collect any foam produced during an experiment.
Generally, experiments in the small cell were conducted with a 300-cc
volume of solution to be floated. The procedure used in conducting the
majority of experiments was as follows:
(1) The cell was filled with 300 cc of the particular solution under
study.
(2) The required flow of air was started (in most cases, a rate of 450
cc/minute was used).
14
-------
FIGURE 3. STANDARD LABORATORY FLOTATION CELL FOR
THE ION FLOTATION EXPERIMENTS
15
-------
Air
Out
_o .
_?_-!•-
O
<
o
Syringe
Injection
Port
d
•V
— Foam Level
— Solution Level
A ,<
;+ !••
I !•'
L,....J*
Sample Port
FIGURE 4. SKETCH OF FLOTATION CELL
16
-------
(3) A preselected volume of collector solution was injected via the
syringe port.
(4) After sufficient time had elapsed (generally 10 to 20 minutes),
samples of the treated solution were obtained and saved for chemical
analyses.
(5) Steps (3) and (4) were repeated until no further change occurred or
until a specified total quantity of collector had been added.
(6) Samples were analyzed and the percentage extraction calculated.
The same general procedure was used for those experiments in the Denver
laboratory cell except that the solution volume was usually 1 liter.
Also, in several experiments involving regeneration or recovery, attempts
were made to collect the entire flotation product which concentrated in
the foam layer, whereas in the small glass cell this material was simply
discarded after the run.
Results. Chromium Removal (Hexavalent). The initial experiments on ion
flotation were conducted with the objective of screening several possible
collectors which might be effective for removal of hexavalent chromium.
The dichromate ion, being negatively charged, requires the addition of
an anionic collector in order to effect flotation. Previous investiga-
tors have shown that certain anionic collectors, such as the long-chain
quaternary-ammonium compounds, were effective for flotation of hexavalent
chromium from aqueous solutions.
For the purpose of selecting a suitable collector, a total of seven
anionic collectors were evaluated during this preliminary experimenta-
tion. The compounds selected for study consisted of primary, tertiary,
or quaternary-ammonium compounds containing a long chain organic group
of between 10 and 16 carbon atoms. Experiments were conducted with 10-
ppm hexavalent chromium solution in the small glass flotation cell shown
in Figure 4.
The particular collectors evaluated and the results of preliminary
chromium-removal experiments are shown in Table 7. As shown by these
data, significant extractions of chromium were achieved with several
different collectors. The basic difference observed was that the pri-
mary amines [Collectors CD, (2), and(3) in Table 7] tended to form precipi-
tates with dichromate ions and the operation could be classified essenti-
ally as precipitate flotation. Removal of chromium with the primary
amines varied from 89 to 94 percent. The remaining collectors evaluated
either did not significantly remove chromium or caused excessive foam
formation and subsequent loss of solution from the cell. Collector (6),
for example, which has been used in previous studies, removed about 70
percent of the chromium; however, the foam fraction amounted to more than
half of the initial solution volume. This foaming tendency no doubt
would produce a large volume of collapsed-foam solution in a continuously
operated system and prevent the attainment of a high concentration of
17
-------
TABLE 7. EXPERIMENTAL DATA ON VARIOUS COLLECTORS FOR
FLOTATION OF HEXAVALENT CHROMIUM
00
(1)
(2)
(3)
(4)
(5)
(6)
(7)
Collector Used
Dodecylamine HC1
Tetradecylamine HC1
Hexadecy lamina HC1
N,N-Dimethyldodecylamine HC1
Decyltrimethylammonium bromide
Ethylhexadecyldimethylammonium bromide
Hexadecylpyridinium chloride
Amount
Used,
cc'a'
4.0
3.0
3.0
4.0
4.0
0.5
0.5
Chromium
Concentration, ppm
pH
6.3
6.3
6.3
6.3
6.3
6.3
6.3
Initial
10.0
10.0
10.0
10.0
10.0
10.0
10.0
Final
0.8
0.6
1.1
6.5
8.5
(b)
3.2W
6.6(b)
Indicated
Percent
Removal
92
94
89
35
15
68
34
(a) Solutions were made by dissolving chemical in isopropanol and adjusting to 20 gpl. The
amine collectors also were neutralized with HC1 to a pH of 7.
(b) Excessive foaming occurred during these runs causing loss of some solution.
-------
chromium for final disposal. Collectors (4) through (7), although exhi-
biting true ion"flotation properties, were therefore eliminated from
further consideration.
Subsequent experiments on hexavalent chromium solutions were conducted
to investigate the effects of flotation variables on chromium extraction
with a primary amine as collector. Among the variables believed to have
significant effects on the process were solution pH, chromium concentra-
tion, and quantity of collector added.
Variations in solution pH were made within the limits of 2.0 and 10.0
using 10-ppm chromium solution with a C]^ amine (tetradecylamine) as
collector. The results of these experiments are summarized in Table 8.
These data show that a wide range of pH could be used and still achieve
effective removal of hexavalent chromium. Extractions greater than 90
percent were obtained within a pH range of 4.0 and 10.0. Below a pH of
4.0, however, the efficiency of removal decreased significantly. There
was also an increased tendency for foaming at the lower pH values.
TABLE 8. FLOTATION DATA SHOWING THE EFFECT OF pH
ON REMOVAL OF HEXAVALENT CHROMIUM
Initial
pH
2.0
3.0
4.0
4.0
6.0
8.0
10.0
Amount of
Collector
Added ^ ' , cc
2
2
2
3
3
3
3
Chromium
Concentration, ppm
Initial Final
10 2.90
10 1.75
10 0 . 85
10 0.67
10 0.60
10 0.27
10 0.23
Indicated
Percent
Removal
71.0
82.5
91.5
93.3
94.0
97.3
97.7
(a) The collector was tetradecylamine HC1, 20 "gpl in
isopropanol.
A series of experiments then was conducted to investigate the effect of
chromium concentration and reagent consumption at various concentrations.
Data obtained from three experiments in the small glass cell with start-
ing solutions containing 1, 10, and 100 ppm chromium are shown in Table
9. These data show—as expected—that more efficient removal of chromium
was obtained in more concentrated solutions. (Note greater amounts of
chromium per mg of collector added were removed at the higher concentra-
tions.) Also of importance was that residual chromium concentrations
could be reduced to a very low value (0.07 ppm chromium) if the starting
solution was sufficiently dilute.
19
-------
TABLE 9. FLOTATION DATA ON CHROMIUM-CONTAINING SOLUTIONS
AT VARIOUS INITIAL CONCENTRATIONS
Initial Chromium
Concentration 'a' ,
ppm
1
10
100
Amount of
Collector
Added (b>,
cc
1.0
2.0
1.0
2.0
3.0
2.0
4.0
6.0
8.0
Final Chromium
Concentration ,
ppm
0.13
0.07
3.72
1.27
0.87
77.0
54.5
32.0
20.0
Indicated
Percent
Removal
87
93
63
87
91
23
45
68
80
Removal
Efficiency,
mg Cr/mg
Collector
0.013
0.007
0.094
0.065
0.046
0.173
0.171
0.170
0.150
(a) Initial solution volume was 300 cc; pH at 6.0.
(b) Collector was tetradecylamine HC1, 20 gpl in isopropanol.
Because the removal efficiency is a major factor in determining the
economics of the process, additional data were obtained on this variable
for each of the three primary amine flotation collectors. These addi-
tional experiments were conducted in the Denver laboratory flotation
cell shown previously in Figure 3. Flotation runs were made with 1-
liter volumes of solution containing either 10-ppm or 100-ppm hexavalent
chromium. The results obtained during these runs are summarized graphi-
cally in Figures 5 and 6 in which the weight of chromium removed is
plotted against the weight of collector added. The removal efficiency
at any point during the extraction can be determined by calculating the
slope of the particular curve under consideration. In this manner, the
maximum removal efficiency was found to be about 0.27 mg of chromium
removed per mg of C]^ amine added. This value was close to that pre-
dicted based on a reaction between the amine and dichrornate ions; thus,
the attainment of a higher removal efficiency even in large-scale equip-
ment was,therefore, believed unlikely. It also should be noted that these
data show a somewhat lower efficiency if other amine collectors are used
or if the extraction is conducted to a point where very dilute chromium
solutions are encountered.
Regeneration. The need for study of possible regeneration procedures
arose from the fact that high extractions of chromium had been demonstra-
ted during the experimentation, but that the efficiency of extraction was
such that the process would be uneconomical for certain applications.
The relatively high cost of the primary amine collectors, which were the
only collectors found suitable for removal of hexavalent chromium,
20
-------
100
80
I? 60
c
o
•H
4J
a
a
•H
O
40
20
, ,
Amine
Amine
Amine
100
200 300 400
Collector Used, mg
500
600
FIGURE 5. EFFICIENCY OF VARIOUS COLLECTORS FOR CHROMIUM REMOVAL FROM
100-ppm SOLUTION
21
-------
10.0
8.0
ao
&
C
o
o
en
e
3
•H
u
6.0
4.0
2.0
C12 Amine
GH+ Amine
C16 Amine
I
20
40 60
Collector Used, rag
80
100
FIGURE 6. EFFICIENCY OF VARIOUS COLLECTORS FOR CHROMIUM REMOVAL
FROM 10-ppm SOLUTION
22
-------
indicated that the recovery and reuse of these collectors was mandatory.
In order to make a valid evaluation of the process economics, a brief
study was made of possible regeneration procedures which could be included
in the process. The regeneration procedures which were considered
included the use of strong acid or dilute caustic for treatment of the
recovered spent flotation collector containing hexavalent chromium.
The initial experiments with acid regeneration, however, indicated that
this method was not feasible. Although hexavalent chromium was liberated
from the amine collector by acid treatment, the recovered amine was evi-
dently degraded and did not remove chromium during a subsequent flotation
cycle.
In subsequent experiments, effective regeneration was obtained by treat-
ment of the flotation collector with dilute caustic solution. The caustic
solution apparently liberated chromate from the precipitated complex of
primary amine and chromic acid, and freed the amine which could be fil-
tered and reused for subsequent flotation.
Experimental data shown in Table 10 illustrate the general results
obtained with dilute caustic regeneration when applied after flotation
of chromium from a 100-ppm solution with a Cj^ amine as collector. Dur-
ing this experiment,the entire spent flotation collector was recovered
as completely as possible, regenerated with caustic, and reused for a
total of three flotation/regeneration cycles.
The results shown indicate a loss of flotation efficiency after two
regenerations (apparent percent removal decreased from 90 to 61), but
almost complete recovery of hexavalent chromium. This decreased effi-
ciency probably was due to decreased amounts of collector available
during subsequent flotation cycles since about 60 percent of the initial
collector was lost after three regenerations. The causes of reagent
losses in the small-scale experimentation were not determined. It is
possible that the amine was sufficiently soluble in water to permit some
losses to the aqueous phase during the flotation cycles. The loss of
materials in handling the small quantities of reagents involved in bench-
scale experiments and the incomplete separation of phases also are possi-
ble routes for the decreased amounts of collector available in succeeding
cycles. An accurate measure of the efficiency of reagent utilization can
be obtained only by larger scale, continuous experimentation.
Chromium Removal (Trivalent). A possible alternative method for treating
chromium-bearing effluents by ion flotation also was studied during the
program. Basically, this technique involved the reduction of hexavalent
chromium to the trivalent form and the subsequent removal of the triva-
lent chromium as hydroxide. The removal of the hydroxide by flotation
might be more economical than removal of hexavalent chromium since the
flotation could be effected with less costly collectors eliminating the
need for possible regeneration procedures. The method is essentially the
same as that used in large plating plants except that the chromium hydro-
xide is removed by sedimentation and clarification. For treatment of
dilute rinse waters, especially, the flotation technique may have some
advantage over sedimentation.
23
-------
TABLE 10. EXPERIMENTAL DATA ON THE REGENERATION AND REUSE
OF FLOTATION COLLECTOR FOR HEXAVALENT CHROMIUM
NJ
4S
Solution Data
Cycle 1
Flotation
Regeneration
Cycle 2
Flotation
Regeneration
Cycle 3
Flotation
Regeneration
Collector
Weight,
mg
500
(a)
(a)
(a)
(a)
214.7
Volume,
cc
1000,, ,
170 (b)
1000,
170 (b)
1000,, ,
170 (b)
Initial
Chromium,
ppm
100
0
100
0
100
0
Final
Chromium,
ppm
10
500
17
485
39
330
Extracted
Chromium,
mg
90
85.0
83
82.5
61
56.1
Apparent
Percent
Chromium
Removal
90
83
61
Apparent
Percent
Chromium
Recovery
94
99
92
(a) Collector present in froth was recovered entirely after each flotation run and reused in
subsequent cycles.
(b) The regeneration solution was 1 percent NaOH.
-------
A brief series of experiments on the flotation of trivalent chromium
showed that this form of chromium could be removed effectively by intro-
ducing a neutralized-fatty-acid collector such as potassium laurate.
Typical data illustrating this result are shown in Table 11. As shown,
the procedure was effective in removing up to about 95 percent of the
chromium at pH 9.0. In one instance the chromium content of the solution
was decreased to 0.6 ppm.
TABLE 11. FLOTATION DATA ON SOLUTIONS CONTAINING
TRIVALENT CHROMIUM(a>
Expt.
No.
22A
22B
22C
22D
25A
25B
25C
Initial
pH
5.0
7.0
9.0
11.0
7.0
9.0
10.9
^JPpm
Initial
100
100
100
100
10
10
10
Cr+3
Final
N.A.
12.9
5.3
N.A.
2.1
0.6
1.4
Indicated
Percent Removal
0
87.1
94.7
0
79
94
86
(a) Experiments were conducted by adding 0.2 cc of a solu-
tion containing 10 gpl of potassium laurate collector
to 300 cc of initial chrome solution.
N.A. = Not analyzed (These final solutions showed no appar-
ent extraction and were therefore not analyzed.)
Activated-Carbon-Adsorption Studies
General Description of Method. During the search of the literature for
the state-of-the-art review, references were found in which the use of
activated carbon for the adsorption of chromium frotn solution was cited.
Activated carbon has been studied by various investigators for the
tertiary treatment of domestic wastewaters. It also has been used for
the adsorption of various materials from solution, including metal ions.
For these reasons it was concluded that adsorption on activated carbon
should be one of the techniques to be evaluated during this phase of the
program.
Modern theories hold that the adsorption of materials from solution by
activated carbon is accomplished by "van der Waal" or "dispersion" forces,
These forces exist among all molecules and atoms, whether or not they are
chemically combined, and may be compared to the gravitational force the
earth exerts upon objects near it.
25
-------
The ability of activated carbon to adsorb a given material depends largely
on its surface area. Each particle of activated carbon has a vast inter-
connecting network of many-sized pores, providing a very large surface
area for adsorption. Consequently, the pore structure of activated car-
bons is extremely important in determining their adsorptive properties.
Although the reasons are not completely understood, it is known that
other substances have an influence on the adsorptive properties of acti-
vated carbons. Oxygen combined with the carbon can be particularly
important and can increase its affinity for polar compounds and decrease
its affinity for nonpolar compounds. In some cases, the inorganic-ash
portion of the carbon can also influence the adsorption process. These
latter factors may play a major role in the adsorption of inorganic
materials.
Activated carbons are usually classified according to their physical form
(e.g., powdered or granular) and according to their use (e.g., water
grade, decolorizing, liquid phase, or gas phase). Granular carbons are
those materials which are over 150 mesh in particle size and powdered
carbons are those which are smaller in particle size.
Activated carbons are produced from various carbonaceous raw materials
(e.g., bituminous coal, nut shells, lignite, pulp-mill residue, and wood).
During this experimental program, many types and sizes of activated car-
bon were studied for the removal of chromium from waste solutions.
Equipment and Procedure^ Preliminary Batch Experiments. In preliminary
batch experiments on the adsorption of chromium, various amounts of car-
bon were mixed with a dichromate solution and the mixture was stirred
for a predetermined time at room temperature. The mixture then was
filtered on glass filter paper and the filtrate analyzed for hexavalent
chromium.
During the preliminary batch experiments, several sizes of activated car-
bon, as well as those produced from various materials, were evaluated.
The carbons studied are listed in Table 12.
Continuous Carbon Columns. In subsequent continuous-adsorption experi-
ments, a series of four glass columns arranged as shown in Figure 7 was
employed. The columns were constructed of 2-inch-diameter Pyrex glass
tubing and were approximately 30 inches in length. By pumping the solu-
tion through the four columns in series, a total carbon-bed depth of 10
feet was attainable. The granular carbon was supported on a 65-mesh
stainless steel grid. Oscillating pumps were used to pump the solution
through the columns. The solution flow was down through the carbon. The
columns were constructed with a sampling port at the bottom of each one
so that effluent samples could be obtained for analysis. The type carbon
selected for the continuous experiments was Pittsburgh Activated Carbon
Company's Type OL which is a granular carbon with a particle size of 20
x 50 mesh. This selection was based on the overall results of prelimi-
nary batch experiments.
26
-------
TABLE 12. TYPES OF ACTIVATED CARBON EVALUATED
to
Trade Name
Nuchar
it
Pittsburgh
ii
ir
it
Absorbite
ii
Company
West Va. Pulp & Paper
ditto
Pittsburgh Act. Carbon Co.
ditto
ii
ii
Barneby-Cheney
ditto
Source Material
Pulp mill residue
ditto
Bituminous coal
ditto
ii
n
Nut shells
ditto
Type and /or Size
Granular (8 x 30)
Pulverized
Pulverized (RC)
8 x 30 (SGL)
12 x 40 (CAL)
20 x 50 (OL)
(WVL)
Pulverized, nonactive (XB)
Granular nonactive (BB)
it
tr
it
ii
Cliffchar
it
$
Royal Oak Charcoal Co.
ditto
II
II
II
If
II
II
Wood charcoal
ditto
Pulverized, low-active (YD)
Granular, low-active, 12x30 (PA)
Pulverized, high-active (XZ)
Granular, high-active, 10x50 (PC)
Pulverized, acid wash (JF)
Granular, acid wash, 10x50 (PK)
Pulverized
Granular, 10 x 20
-------
FIGURE 7. BENCH-SCALE CONTINUOUS COLUMNS FOR ADSORPTION EXPERIMENTS
28
-------
The procedure used in the continuous experiments was to pump a synthetic
waste solution containing varying amounts of chromium through the column
at a given rate with periodic analysis of effluent samples. In some
experiments only one column was used while in others two, three, or all
four were used. In this manner the time of "breakthrough", when the
effluent contained the element being adsorbed, could be observed as well
as the time when the carbon was completely loaded. From the data collec-
ted during these experiments, the capacity of the carbon for adsorbing
chromium could be determined. The major variables studied during these
experiments were:
(1) pH of the feed solution
(2) Concentration of the feed solution
(3) Rate of flow or residence time
(4) Ionic form of the element in solution (e.g., dichromate, chromate,
or chromic acid).
S.esuits. Batch Experiments. A series of experiments to compare the
effectiveness of various types of carbon was conducted in the initial
stages of the study. The results are shown in Table 13. Carbons pro-
duced from nut shells and wood charcoal (e.g., Absorbite and Cliffchar)
effected very little chromium removal. Those produced from pulp mill
residue (Nuchar) adsorbed or removed approximately 50 percent of the
chromium present in the solution. Carbon produced from bituminous coal
(Pittsburgh) removed from 52 to 89 percent of the chromium.
To study some of the variables that could affect chromium removal (car-
bon, concentration time, pH), several additional series of experiments
were conducted with Pittsburgh RC pulverized carbon. The results obtained
on the effect of carbon concentration are shown in Figure 8. When the
concentration was increased to 35 grams per liter, the percentage removed
reached 94 percent and appeared to level out since a further increase to
40 grams per liter did not increase the amount of chromium removal.
The effect of contact time on chromium adsorption was determined by con-
tacting a solution containing 10 ppm chromium with 10«gpl of carbon for
varying periods up to 2 hours. Results, shown graphically in Figure
9 indicate that maximum adsorption occurred within the first 10
minutes and that increasing contact time beyond this point had very
little effect.
The effect of pH value on chromium adsorption is shown in Figure 10. The
normal pH value of the synthetic waste solution containing 10-ppm chromium
as dichromate using distilled water is 6,0. During this series of experi-
ments, the pH value was varied from 2 to 12 by the addition of either
sodium hydroxide or sulfuric acid. The results indicated that low pH
values were required for effective chromium removal.
Laboratory experiments also were conducted to determine the maximum
adsorptive capacity of various carbon samples at chromium concentrations
between 10 and 2000 ppm. Typical data obtained in these experiments are
29
-------
TABLE 13. REMOVAL OF HEXAVALENT CHROMIUM WITH VARIOUS TYPES OF ACTIVATED CARBON
CO
o
Type of Activated Carbon
Physical Characteristics
Apparent Percent
Chromium Removal(a'
Pittsburgh - RC
Pittsburgh - OL
Pittsburgh - CAL
Pittsburgh - SGL
Nuchar - Aqua A
Nuchar - WVL
Absorbite - XB
Absorbite - BB
Absorbite - YD
Absorbite - PA
Absorbite - XZ
Absorbite - PC
Absorbite - JF
Absorbite - PK
Cliffchar
Cliffchar
Pulverized
Granular (20 x 50 mesh)
Granular (12 x 40 mesh)
Granular (8 x 30 mesh)
Pulverized
Granular (8 x 30 mesh)
Pulverized (nonactive)
Granular (nonactive)
Pulverized (low-active)
Granular (low-active, 12 x 30)
Pulverized (high-active)
Granular (high-active, 10 x 50)
Pulverized (acid washed)
Granular (acid washed, 10 x 50)
Pulverized
Granular (10 x 20 mesh)
89
80
67
52
50
47
12
26
14
17
19
19
17
17
6
10
(a) In all cases, the experimental conditions were 10-gram - per-liter carbon concen-
tration and 5"minute contact time.
-------
100
90
4J
C
QJ
O
M
CO
X
80
70
60
50
40
30
20
10
0
10 15 20 25 30 25 40
Powdered Activated Carbon Concentration, gram/liter
FIGURE 8. AMOUNT OF POWDERED ACTIVATED CARBON REQUIRED FOR
HEXAVALENT CHROMIUM REMOVAL
31
-------
100
OJ
u
5-i
0)
a.
o
B
<
•H
e
o
c
01
r-H
CO
ct)
XI
-------
100
80
c
0)
0
!-l
0)
(3.
Cti
O
01
60
§
•H
B
O
40
c
01
rH
n)
cfl
x
0)
as
20
6
pH of Solution
10
12
FIGURE 10. EFFECT OF pH ON CHROMIUM REMOVAL
33
-------
shown In Figure 11. These data show adsorption isotherms (equilibrium
adsorption capacities versus solution concentration) for the two types
of carbon which gave the best results. The isotherms shown in this
figure are important in selecting the best carbon for a specific appli-
cation. For example, at high concentrations of about 2000 ppm, both
carbons have identical capacities; thus, the selection would be based on
carbon cost. In this case, Nuchar carbon would be selected since it is
significantly cheaper than Pittsburgh OL carbon. On the other hand, the
capacities of each carbon at low concentration are quite different.
Pittsburgh OL, for example, has over twice the capacity of Nuchar at
concentrations less than 70 ppm. Thus, Pittsburgh OL would be preferred
in this case since the higher capacity more than offsets the higher unit
cost.
In addition to comparative data, the isotherms show the ultimate adsorp-
tion capacities attainable with each type of carbon in a column system
operated for single adsorption and disposal. Actual data points obtained
in this series of experiments indicate capacities of up to 21 percent of
the weight of carbon as chromium can be adsorbed at a feed concentration
of 2000 ppm of hexavalent chromium. Operation at these concentrations
is considered feasible if rinse water is recirculated in a closed loop
around the rinsing tank. Thus, adsorption capacities of 20 percent might
be practical in a commercial system.
In summary, the batch experimentation on the removal of hexavalent
chromium from synthetic waste solutions indicated that
(1) Pittsburgh pulverized carbon, type RC, effects the largest percent-
age of chromium removal with Pittsburgh granular, type OL, being second
best. The difference between the pulverized and granular carbon is
almost insignificant (Table 14).
(2) Thirty-five grams of carbon per liter of waste solution adsorbs
maximum amount of hexavalent chromium.
(3) No reduction of chromium occurs during carbon contact at pH values
in the range of about 3 or greater.
(4) Maximum adsorption is obtained within 10 minutes.
(5) Low pH values (about 2 to 3) are required for effective chromium
removal.
34
-------
i.o cr
i i i 1
i I I I
0.5
I
•H
6
o
0.1
O
ti
O
o
CO
§
•H
cr
Ed
0.5
— D
0.01
Legend
D - Pittsburgh OL carbon, 100 gn
• - Pittsburgh OL carbon, 5 g^
O - Nuchar carbon, 100 gn
• - Nuchar carbon, 5 gi
. , I . , I, I
I I
I I I I
10
50 100 500 1000
**-
Hexavalent chromium concentration, ppm
5000
FIGURE 11. TYPICAL ADSORPTION ISOTHERMS FOR HEXAVALENT CHROMIUM
35
-------
TABLE 14. COMPARISON OF GRANULAR AND PULVERIZED ACTIVATED CARBON
FOR CHROMIUM REMOVAL AT VARIOUS CONCENTRATIONS
Carbon Concentration, Hexavalent Chromium Removed, percent
grams per liter
5
10
15
20
25
30
35
40
Pittsburgh Granular
57
80
86
91
93
94
96
98
Pittsburgh Pulverized
74
89
96
96
96
96
96
97
Continuous Carbon Columns. A total of ten experiments was conducted in a
continuous fashion using one or more of the carbon columns shown in
Figure 7. The simulated waste solutions fed to the columns contained 10,
100, and 1000-ppm chromium prepared from either dichromate or chromic
acid. In the first seven experiments, distilled or deionized water was
used to make up the solution. The last three experiments were conducted
using tap water. The pH value of the feed solutions ranged from 1.8 to
6.3. Feed rates varied from 0.5 to 5.0 gallons per minute per square
foot of bed surface area. These rates correspond to retention times from
about 2 to 15 minutes.
The ^data in Table 15 show the conditions employed and the results obtained
during the continuous experiments on chromium removal.
In the first experiment (1), all four columns were used. Each column was
filled with 700 grams of type OL carbon. The feed solution was made up
with distilled water and potassium dichromate and contained 10-ppm
chromium at a pH value of 6.3. A total of 1324.8 liters of solution was
fed at a rate of 0.5 gallon per minute per square foot of carbon surface
area which gave a retention time of about 15 minutes. During this
experiment 1.2 grams of the 1.3 grams of chromium fed were adsorbed pri-
marily in the first column. This amounted to over 90 percent adsorption
of the incoming chromium and gave a carbon-loading capacity of about
0.002 (weight of chromium per weight of carbon). This ratio was con-
siderably better than that obtained in any of the batch experiments.
Although only 92 percent was adsorbed on the first column, the other 8
percent was adsorbed by the following column. This experiment was dis-
continued since it was felt that solutions of higher concentration fed
at faster rates would give the needed data more quickly.
In Experiment 2 all four columns were also employed. The solution con-
centration was increased to 100 ppm and the feed rate was increased to
give a retention time of about 4 minutes. About 75 percent of the 15.5
36
-------
TABLE 15. CONTINUOUS EXPERIMENTS ON ADSORPTION OF CHROMIUM WITH ACTIVATED CARBON
Expt
No.
1
2
3
4(f,
6
^ ' i i
IT
I o vfij
•i J x ^
(a)
(b)
(c)
(d)
(e)
(f)
(g)
Feed Solution
No. of Chromium Effluent
Total
Chromium
. Columns * . Rate,. Volume, Content Volume, Chromium Content Adsorbed
Used Type^ gpm/ft pH liters ppm g pH liters ppm<^
4 K2Cr207 0.5 6.3 1324.8 10 1.3 >8.0 1324.8 0-2
4 K2Cr207 2.0 5.4 155.4 100 15.5 3-9 155.4 0-50
1 K2Cr207 2.0 2.1 291.4 100 28.5 2-7 287.1 0-30
. 1 K.2Cr207 3.0 4.0 203.5 100 20.4 2-7 203.5 0-50
' 1 K2Cr20? 3.0 4.0 18.9 100 1.9 >8.0 18.9 0-20
|K2Cr207 3.0 4.0 18.9 100 1.9 6.8 18.9 0-10
1 1.2 92.2
=3.9 1.5 79.0
" 1
0.3 h 38.0 95.8
1.4, ,J
1.7
3.4fej 91.2 96.4
5.5^ 77.8 93.4
1.9)e^ 30.5 94.0
4.4W 32.8 88.1
Carbon
Loading
Capacity,
Grams Cr/
g Carbon
0.002
0.017
0.032
0.023
0.002
0.055
0.125
0.111
0.043
0.048
r,07 = potassium dichromate,
^ /
This experiment was conducted using a different column of carbon than had been used previously.
The feed solution for these experiments was made from tap water instead of
distilled or deionized water used
previously.
Conditions: Carbon content, 700 grams per column, 2800 grams in four columns.
-------
grams of chromium fed during this run was adsorbed to give a carbon
loading capacity of 0.017. The increased capacity was believed due to
operation at a lower pH (5.4) and a higher chromium concentration.
The following two experiments (3 and 4) were conducted using only one of
the columns. The pH value in these experiments was 2.1 and 4.0, respec-
tively, and the retention time was 4 and 3 minutes. In both cases about
80 percent of the chromium fed was adsorbed on the carbon. The carbon-
loading capacities were even better than in the two initial experiments
(i.e., 0.032 and 0.023).
Experiment 5 was conducted using the same conditions except that the
column was loaded with fresh carbon. This experiment was terminated due
to the high chromium content (8 to 10 ppm) of the effluent. It was
believed that, since the carbon had not been washed with acid previously,
any basic materials present in the carbon caused the pH value of the
solution to increase to a value greater than 8. It was theorized that
by prewashing the carbon with acid these basic materials would be leached
out.
In Experiment 6, three different feed solutions were used (100 ppm, pH 4;
1000 ppm, pH 4; and 1000 ppm, pH 2.5). A total of 38 grams of the 39.7
grams of chromium fed was adsorbed on the carbon. This amounted to about
96 percent adsorption and a chromium to carbon ratio of 0.055. This
experiment appeared to confirm the previous finding that the adsorptive
capacity of the carbon increases with increased concentration of hexava-
lent chromium in the feed and with lower pH values.
The next two experiments (7 and 11) were conducted to determine the
maximum capacity of the carbon while feeding a solution containing 1000
ppm Cr. From the data in Table 15 it can be seen that with low pH values
and a high chromium concentration in the feed solution, the capacity
reached much higher levels (chromium to carbon ratios of 0.125 and 0.111,
respectively) with excellent removal efficiency (95 percent).
In Experiments 13 and 14 the column was operated under conditions which
would be practical in a commercial-scale unit. After the effluent con-
tained 1 ppm chromium, the experiment was stopped, the carbon was regen-
erated, and a second adsorption cycle conducted (Run 14). The adsorption
efficiency for chromium in this second cycle by the regenerated carbon
was apparently unimpaired.
These final experiments indicated that, with waste solutions containing
as high as 1000 ppm chromium and at pH values of 4 or less, virtually
all of the chromium could be adsorbed. They also indicated that it is
possible to load the carbon with at least 10 percent of its weight of
chromium.
Regeneration. During the experimentation on the adsorption of chromium
on activated carbon, various techniques were studied to regenerate the
carbon for reuse. Several types of regenerants at various concentrations
were tried. These regenerants were
38
-------
(1) Sodium hydroxide (NaOH)
(2) Sulfuric acid (H2S04)
(3) Hydrochloric acid (HC1)
(4) Water (H20)
(5) Acetic acid (CH3COOH).
Table 16 shows the conditions and results of the experiments on regenera-
tion of carbon and recovery of chromium. The data indicated that
(1) Both sodium hydroxide and sulfuric acid were effective regenerants.
Used alone, sulfuric acid was capable of removing all of the adsorbed
chromium (Experiment 4). When sodium hydroxide was used alone, the rate
of chromium removal from the carbon became low after about two-thirds
of the chromium was extracted (Experiments 1, 6, 7, 11).
(2) Regeneration by means of sulfuric acid results in the reduction of
hexavalent chromium to trivalent chromium (Experiments 3, 4, 6, 7, 11).
If chromium recovery were desired, sulfuric acid therefore would not make
a suitable regenerant. However, for simple disposal, acid regeneration
might be feasible since trivalent chromium must be produced prior to
disposal.
(3) Regeneration by sodium hydroxide leaves the chromium in the hexava-
lent state as sodium chromate possibly suitable for recovery.
(4) Regeneration of the carbon with either acid or sodium hydroxide
appeared to be a technically feasible method of operation. This is
borne out by the fact that the partially regenerated carbon from Experi-
ment 13 was reused with no observable impairment in efficiency* It was
suspected, however, that numerous cycles of regeneration and reuse might
significantly affect the adsorption efficiency of the carbon, and should
be investigated in subsequent studies.
(5) The maximum concentration of chromium possible in the regenerating
solutions remained to be established. In the laboratory work, concentra-
tion ratios of 25:1 were obtained, with evidence that much higher ratios
may be possible.
TABLE 16. REGENERATION OF CARBON FROM CHROMIUM ADSORPTION EXPERIMENTS
Chromium Recovered
Expt.
No.
1
Chromium
Adsorbed on.
Carbon, g
1.2
Regeneration Volume
Solution, Used,
Type(a) liters
1% NaOH
— H20
12.9
6.0
totals 19.9
bi Regeneration
Grams
0.79
o]l2(b)
1.06
Percent
of Cr Fed
65.8
12.5
10.0
88.3
Grams of
Regenerant
Required/
Gram of Cr
129
83
39
-------
TABLE 16. (Cont.)
2 11.6 10% H2S04
3 22.9 10% H2S04
4 16.2 10% H2S04
5 1.5 10% NaOH
6 38.0 1% NaOH
5% H2S04
7 91.2 1% NaOH
5% H2S04
11 77.8 1% NaOH
5% H2S04
5% HC1
20% CH3COOH
13 30.5 1% NaOH
0.8% H2S04
14 32.8 1% NaOH
— H20
18.4
18.0
7.0
4.0
8.3
4.0
totals 12.3
11.5
8.5
totals 20.0
17.0
11.7
1.8
6.0
totals 36.5
5.9
7._1
totals 13.0
5.1
18.7
totals 23.8
9.41
19.1
16.3
-------
(12)
Make-up
Organic Extractant
To Compensate for
Losses
(1)
Waste Feed
(Aqueous)
M
Make-up
Stripping
Solution
Partially
Extracted
Aqueous
Phase to
Second St
Stage
Extraction
1
1
1
1
1
t
> r*
* i
i
(5) ! ^L 1 Light --
1 '^fe'X. 1 Or8anic
! ^S?^ Phase
0«0 \ (3)
Heavier
L . 1 Aqueous
Phase -»•
i
\_ Mixer Settler s
Second-Stage Extraction
1
f
\
-> (2) 1 ^
do'
Mixed
Phases
N Stripped Organic ((
1 Light
i^^v^^ Organic
f^v^-l Phase
^^ (4)
Heavier
Aqueous
Phase
t
>
3/ /
i
(8) j ,
Mixed
Phases
y Mixer Settler ^ v,
First-Stage Extraction
r
(9)
N _
1
r
U Light __^J
Organic
Phase
Heavier
Aqueous
S
Stripping or Regeneration
\
k
\
f
Extracted Aqueous
Phase
To Disposal (7)
Containing substantially
all, or most of, the
material extracted
from the waste feed
(11)
To Chemical Destruction
or Recovery
FIGURE 12. EXAMPLE OF TWO-STAGE EXTRACTION, SINGLE-STAGE STRIPPING
LIQUID-LIQUID EXTRACTION PROCESS
-------
As it issues from the settler (4) it is sent to a regeneration stage (8
and 9) where it is stripped of its load of contaminant and restored to
its original composition. The regenerated organic material is then recycled
back to the extraction stage (6). In the stripping or regeneration section,
the stripping agent is constantly recirculated through the mixer-settler
(8 and 10). Periodically, enough of the stripping solution now loaded with
the contaminant is removed from the circuit for subsequent treatment (11).
The process outlined in Figure 12 is merely an example. Numerous varia-
tions in flow patterns, number of stages, etc., are possible.
Equipment for liquid-liquid extraction too can be varied. The equipment
implied in Figure 12 is the so-called horizontal type, mechanically agi-
tated mixer-settler, in which in each stage the phases are intimately
dispersed in one vessel by agitation and then permitted to flow by grav-
ity to another settling vessel for phase separation. Each stage, there-
fore, requires two separate vessels. Other types of mechanical mixer-
settlers may be used. These include the vertical type in which the
stages are superimposed in a single vertical shaft with a central axle
for driving the agitators in each mixing section and baffled tanks simi-
lar in nature to the apparatus used in the laboratory. There are several
other types of liquid-liquid extraction equipment which probably also
would be applicable to the treatment of metal-finishing waste. These
include
(1) Various types of columns in which no agitation is employed, such as
perforated plate columns, bubble cap columns, packed columns, and spray
columns. This apparatus might possibly be applicable to the treatment
of metal-finishing wastes.
(2) Mechanically agitated columns, such as the so-called pulsed column.
These would be applicable to the treatment of metal-finishing wastes
also, but would be more costly than spray columns.
(3) Centrifuge types in the contacting or mixing. Settling is greatly
accelerated making possible high treatment rates and consequently reduced
equipment sizes. These types generally are expensive, but require less
floor space than the other types.
Chemistry of Liquid-Liquid Extraction. The chemistry involved in liquid-
liquid extraction can probably be most clearly explained by an example.
For the extraction of hexavalent chromium, a number of organic compounds
would be satisfactory. The one investigated most thoroughly in the
laboratory campaign was the so-called Alamine 336, manufactured by
General Mills. Another which was investigated to a lesser extent, but
which exhibited as much promise, was Amberlite LAI, manufactured by Rohm
and Haas. The former is a tertiary amine, the latter a secondary amine.
These amines have the capacity for reacting with hexavalent chromium in
acidic solution by the following mechanism.
-------
2R3N
Alamine 336
Cr2°7 =
Bichromate
ion as
would exist
in acidic
chromium
rinses
+ 2H
Hydrogen
ions as
obtained
in acidic
conditions
Compound of amine
with dichromate
Soluble in kerosene or
other organic solvents;
low or negligible solu-
bility in water;
remains dissolved in
the organic phase after
separating
Soluble in
kerosene or
other solvents;
very low or
negligibly solu-
ble in water;
generally used
in dilute solu-
tion in kerosene,
etc., at strengths
up to about 5 percent
Another possible mechanism is to first convert the amine to an amine sul-
fate and then react it with the chromium solution; but the overall effect,
the conditions required, etc., do not vary significantly from those exhi-
bited in the above reaction.
After the reaction of chromium with the amine is completed in the mixing
section of the extraction equipment and the phases are separated, the
chromium compound with the amine preferentially dissolved in the organic
phase is removed from the system to be stripped. In the stripping opera-
tion, the aim is to remove the chromium from combination with the amine
into a highly concentrated aqueous phase and to restore the amine to its
original composition so that it can be recirculated to extract chromium
from additional waste solution. Where the extraction of chromium from
the waste solution requires an acidic condition, the stripping reaction
requires an alkaline condition. Stripping takes place according to the
following reaction:
(R3NH)2Cr20?
Compound of
amine with
dichromate
dissolved
in organic
phase
4NaOH H
Sodium
hydroxide
in aqueous
stripping
solution
Sodium chromate
dissolved in
aqueous solution
after stripping;
may be recircu-
lated to build
Cr content to
10 or 20 g/1
Alamine 336
dissolved
in kerosene
after
stripping;
recycled to
additional
extraction
3H20
Water
Requirements for Feasibility. For liquid-liquid extraction to be feasi-
ble in the treatment of metal-finishing wastes, these conditions would
have to be met:
43
-------
(1) The extraction of chromium from rinse waters should be virtually
complete. Although there may at present be a certain laxity in legisla-
tion and enforcement, the day may come when every plater may be required
to satisfy limits as low as 1 or 2 ppm of chromium. An acceptable
liquid-liquid extraction process would have to be capable of achieving
such limits.
(2) Reagent recovery by stripping should be efficient.
(3) The stripping operation should produce a greatly concentrated solu-
tion of the contaminant either for recovery or chemical destruction.
(4) The treated effluent solution should be sufficiently free from
organic contaminants such as oil to satisfy local restrictions. Present
restrictions on oil are variable, ranging from zero to some nonspecific
quantity described in such terms as "none making the water unsuitable for
the use indicated". It is the very nature of liquid-liquid extraction in
which oily substances, such as kerosene, are intimately dispersed in the
waste, to introduce some oil into the water phase. The process should
operate, therefore, in such a manner that the "oily substance" content of
the discharged waste is within permissible limits. The control of the
oil content of the treated effluent might be important if the water were
reused, even in part, for rinsing, owing to the possible effects on work
quality.
(5) The operation should be relatively simple and routine requiring a
minimum of time and attention by the plater.
(6) Costs, both capital and operating, should be reasonable.
In the following sections of this report, the feasibility of liquid-
liquid extraction for chromium removal is discussed on the basis of the
data obtained in the experimental program.
Equipment and Procedure. Preliminary batch experiments were made in
separatory funnels, by the conventional single-stage procedure. In these
runs, measured quantities of the aqueous and organic phases adjusted to
the desired pH level were mixed. The mixture was permitted to stand for
20 minutes to effect phase separation, and the treated aqueous phase was
analyzed to determine its metal content.
Subsequent mixer-settler runs were made in the apparatus shown in Figure
13, which provided three stages of extraction, with the aqueous phase
being fed into the righthand mixing compartment. Agitation by laboratory
stirrers was provided in each stage. The treated aqueous solution was
overflowed from the lefthand settling compartment and the "loaded"
organic was aspirated by suction from the righthand settling compartment.
The runs were made at various flow rates, volumetric ratios of aqueous to
organic, and composition of aqueous and organic solutions.
44
-------
FIGURE 13. BENCH-SCALE MORRIS-TYPE CONTACTOR FOR
CONTINUOUS LIQUID-LIQUID EXTRACTION
EXPERIMENTS
45
-------
Results.. The program was designed to determine how well the liquid-
liquid extraction process might meet the requirements for feasibility.
Preliminary screening experiments were conducted to select suitable
reagents. The results of these experiments indicated that at least
three of the reagents tried would be effective for chromium removal
under proper conditions of pH, etc. These were
(1) Aliquat 336, a quaternary amine salt, manufactured by General Mills,
Its price was quoted at about $1 per pound.
(2) Alamine 336, a tertiary amine produced by the same company, said to
be available at 80 cents per pound.
(3) Amberlite LAI, a secondary amine manufactured by Rohm and Haas,
reported to cost 60 cents per pound.
Results obtained in preliminary experiments with these three reagents
are summarized in Table 17.
In most of the subsequent work, Alamine 336 was employed. Aliquat 336
was not used because of a reported high solubility in water (1 percent).
It is believed that Amberlite LAI may have performed as well as Alamine
336, provided the pi. was not allowed to exceed 2.0.
Undoubtedly there are a number of other reagents, probably all amines,
which would be as suitable as those investigated.
Degree of Chromium Removal Obtainable. The batch experiments in Table
17 were all made on solutions containing 10 ppm of chromium. Additional
batch experiments were made by the same procedures on more concentrated
chromium solutions, using a 2.5 percent solution of Alamine 336 in a
kerosene-isodecanol solvent. The results are shown in Table 18.
TABLE 18. BATCH EXTRACTION OF HEXAVALENT CHROMIUM FROM
MORE CONCENTRATED SOLUTIONS USING ALAMINE 336
Expt.
No.
100A
58A
100B
Cr6 in-
Feed,
ppm
1000
100
27
Volume
of Feed,
ml
400
50
100
Volume of
Extracting
Solution, ml
200
25
25
6
Cr in Treated
Solution, jpm
27
3
1.5
pH
1.5
2.0
1,8
Extraction,
percent
97
97
93+
Indications from these experiments were that liquid-liquid extraction by
Alamine should function efficiently over a wide range of chromium
46
-------
TABLE 17. PERFORMANCE OF SEVERAL AMINES IN THE BATCH
EXTRACTION OF HEXAVALENT CHROMIUM
Reagent
Aliquat 336
Aliquat 336
Aliquat 336
Alamine 336
Alamine 336
Alamine 336
Amberlite LAI
Amberlite LAI
Chromium
in Feed,
ppm
10
10
10
10
10
10
10
10
Concentration
of Reagent in
Extracting
Solution,
percent
2.0
0.2
0.2
2.0
2.0
0.2
2.0
0.2
Volume Ratio
Aqueous to
Extracting
Solution
2:1
2:1
10:1
2:1
2:1
2:1
2:1
2:1
Chromium
in Treated
Solution,
ppm
<0.2
0.2-1.0
0.5-1.2
0.2-0.8
2.4-3.6
0.4-1,1
<2.0
0.9-1.3
pH Range
0.3-8.0
1-3.8
2
0.3-1.0
2.0-4.0
1.0-1.5
0.3-2.0
0.3-1.5
Extraction,
percent
98+
90-98
88-95
92-98
64-72
89-96
98+
87-91
-------
concentration provided the pH of the solution was maintained at about 2
or lower and that there was sufficient reagent available for reaction
with the chromium. Other investigators have obtained similar high
extractions from solutions containing as much as 4000 ppm of hexavalent
chromium.*
A number of continuous extraction experiments also were conducted in the
mixer-settler unit shown in Figure 12. The results of these experiments
are summarized in Table 19.
TABLE 19. EXTRACTION OF HEXAVALENT CHROMIUM IN
CONTINUOUS OPERATIONS
50-1
51-1
Cr6 in
Cr6 in
Feed Treated Chromium
Expt. Solution, Solution, Extraction,
No.
100
100
Remarks
52-1
53-1
56-1
56-A
59-1
61-1
62-1
r ^
14
10
10
10
10
10
10
r *
0.05
0.10
0.09
0.70
0,28
0.04
1.3
99+
99
99+
93
97
99+
89
Lower extraction due to
operation at pH 4
Lower extraction due to
0.36
0.40
excessive feed rate through
mixer-settler
99+
99+
On the basis of these data, it was concluded that the requirement of
reducing chromium to the required level of about 1 ppm or lower could be
accomplished by liquid-liquid extraction.
Regeneration. The regenerability of Alamine 336 was investigated both
in batch and continuous runs. In a typical batch run (Experiment 100A)
200 ml of a 2.5 percent solution of Alamine 336 in kerosene-isodecanol
was partially loaded by extracting 400 ml of a chromic acid solution
containing 1000 ppm of hexavalent chromium. After loading, the organic
phase was stripped with 95 ml of 10 percent NaOH solution. The follow-
ing tabulation shows the results obtained:
"Chromium", General Mills Publication, Kankakee, Illinois (1961).
48
-------
Weight, g Percenjt
Chromium In Feed Solution, 0.400 100
400 ml @ 1000 ppm Cr6
Chromium Accounted for in 0.418 104
Final Strip Solution,
95 ml @ 4400 ppm Cr6
The stripped organic phase, in this experiment, was completely white and
clear and contained at most only a few ppm of hexavalent chromium. This
solution was reused once to extract an additional lot of chromic acid
solution containing 1000 ppm of hexavalent chromium. The treated solu-
tion contained 35 ppm of hexavalent chromium. This compares with 27 ppm
obtained in the first treatment.
The regenerability of "loaded" Alamine 336 solutions also was tested in
the continuous mixer-settler runs. In one experiment the organic phase
which had been collected separately was stripped with 4 percent sodium
hydroxide by agitation for 15 minutes in the mixer-settler. Stripping
was extremely rapid and, judging by the disappearance of the typical
orange color in the organic phase, was completed within 5 minutes. The
stripped solution then was employed in a subsequent run and yielded
almost complete extraction of chromium.
On the basis of these data and considering the work of other investiga-
tors, there appeared to be little doubt that the Alamine reagent could
be stripped and reused indefinitely. However, Alamine solutions loaded
or even partially loaded with chromium, when allowed to stand overnight,
lost the characteristic yellow-orange color and turned green. This was
caused by the reduction of chromium, either by the kerosene or by the
Alamine itself. Experiments with 2.5 percent solutions of Alamine 336
in solvents other than kerosene (mineral spirits and toluene) resulted
in similar reduction. It is believed that the reduction is effected by
the Alamine itself. If so, then an equivalent portion of the Alamine is
in turn oxidized and may or may not still possess its full effectiveness.
Treatability of the Strip_Solution. The regenerating solution, i.e.,
the sodium hydroxide solution that has been contacted with the "loaded"
organic solution and now contains sodium chromate, should be sufficiently
concentrated in chromium to minimize costs in further treatment. The
laboratory work did not establish the maximum degree of concentration
possible in a system in which dilute wastewaters (10-100 ppm of hexava-
lent chromium) are extracted and the organic phase is stripped with
sodium hydroxide. On the basis of laboratory data, concentrations as
great as 50:1 and possibly 100:1 should be readily obtainable. This is
based on the fact that virtually complete extraction of chromium into
the organic phase can be obtained at aqueous-organic ratios of 10:1, and
on information from chemical producers that the organic phase can be
stripped at aqueous-to-organic ratios of 1:5. In the case of stronger
waste solutions, such as might be encountered in "save" rinse tanks or in
countercurrent rinsing, similar concentration ratios might be achieved.
49
-------
Oil content of the treated solutions. In all cases in the laboratory
experiments, the treated solutions as produced either from the separa-
tory funnel batch tests or the mixer-settler, carried off some of the
organic phase. The amount of organic material in the treated solution
was measured by total carbon analyses. Analytical data showed that the
effluents, as produced, contained from about 150 to more than 400 ppm of
carbon which corresponds to about 200 to 500 ppm of kerosene. In some
instances there was a distinguishable layer of the organic fraction on
the solution and in practically all other cases a thin, discontinuous
iridescent film of oil was discernible.
Undoubtedly much of the contamination of the effluent by oil, or kero-
sene, was due to limitations of the laboratory mixer-settler which did
not provide much settling area. It also was believed that the agitation
employed in the mixer-settler operation was more violent than needed and
that this contributed to the formation of more or less stable emulsions.
In order to gain some idea of the amount of kerosene contributed by the
inadequate settling area in the mixer-settler, the effluent samples were
shaken and then permitted to settle for about 1 hour. A sample was then
taken from the bottom of the sample container and analyzed for carbon.
Typical results are shown in the following tabulation:
Carbon Analysis, ppm
Sample As_ Produced After Settling 1 Hour
50-1 400+ 284
50-3 400+ 320
50-6 168 144
51-1 148 94
51-2 112 55
51-4 400+ 220
51-6 356 142
52-1 296 168
52-3 100 74
62-1 400+ 128
These results, in general, indicated that a significant decrease in the
carbon (i.e., kerosene) content was possible by providing additional
settling time. The persistence of the carbon remaining after settling,
however, should be regarded as a cautionary factor, which merits addi-
tional consideration.
This loss of carbonaceous material to the effluent may also indicate a
serious loss of the extractant. For the purposes of the economic esti-
mates which are presented later, it was assumed that reagent losses to
the effluent will not exceed 10 ppm. An analysis for nitrogen (which is
a distinctive component of the amine reagent) on one effluent solution
indicates this loss may be several times as great as what has been allowed.
Additional work on reagent loss is therefore believed to be very
important.
50
-------
Other Techniques Evaluated
Reduction of Chromium With Activated Carbon. During the experimentation
on the adsorption of hexavalent chromium by activated carbon, it was
noted that activated carbon reduced hexavalent chromium to the trivalent
form under acidic conditions. In Experiment 14 (Table 15), for example,
effluents were obtained which contained, on the average, 10 ppm triva-
lent chromium, indicating a reduction of 10 percent of the incoming
chromium. The extent of reduction was affected primarily by pH and
reduction could be eliminated by increasing the pH above 3.0. Although
this effect was not studied in detail, one test was made to acquire some
measure of the capacity of carbon for reducing hexavalent chromium. The
results of this test indicated that 0.8 gram of chromium was reduced per
gram of activated carbon added; however, the reaction did not appear to
be completed. No additional study was made to determine the maximum
amount of chromium which could be reduced per gram of activated carbon.
.Reverse Osmosis for Chromium Removal. Considering the newness of the
field of reverse osmosis, a very brief study was made of this technique
to determine its possible application to the treatment of metal-finishing
wastes. Research on reverse osmosis has been primarily in the field of
desalinization of brackish water; however, some effort has been devoted
to the treatment of various industrial wastes.
The preliminary evaluation of reverse osmosis for chromium removal was
conducted with a ROGA Model III Laboratory Reverse Osmosis Unit using
chromic acid solutions containing about 100 ppm chromium. A total of
eight reverse osmosis experiments on chromic acid solutions were made.
Among the variables which were studied were the effects of pressure and
feed concentration. The experimental results are given in Table 20.
TABLE 20. EXPERIMENTAL DATA ON CHROMIUM REMOVAL
BY REVERSE OSMOSIS
Expt.
No.
1
2
3
4
Pressure,
Concentration,
_ ppm Cr(a) ^
Concentrate Permeate Concentrate Permeate
Flow, gph
500
350
200
200
100
17.6
19.0
13.6
15.8
1.8
1.3
0.8
0.5
104
104
104
83
94
97
70
44
45
46
35
36
37
39
Indicated
Percent
Removal
58
57
56
58
62
62
44
51
-------
TABLE 20. (Cont.)
Concentration,
Expt. Pressure, Flow, gph ppm Cr(a'
No. psig
6 500
7(b) 500
8(c) 500
Concentrate Permeate Concentrate
23.8 1.6 76
97
34
42
112
120
140
182
Permeate
54
46
9.5
12.5
40
43
49
64
Indicated
Percent
Removal
29
53
72
70
64
64
65
65
(a) Test solutions were prepared from chromic acid and deionized water.
(b) Mixed permeates from Experiments 4-6 were used as feed solution for
this experiment.
(c) In this experiment, the concentrate stream was continuously fed back
to the feed vessel during the run.
As shown in Table 20, percentage reductions of hexavalent chromium were
only "fair", ranging from a low of 39 percent to a high of 72 percent.
The results of these limited experiments suggest that reverse osmosis
may have application as part of an overall wastewater treatment/recycle
system for plating wastes, but considerably more experimental work would
be required to confirm this.
Adsorption of Chromium on Activated Alumina. Because activated carbon
was effective in adsorbing chromium from the waste solutions and because
several other elements are reported to have been adsorbed on activated
alumina, it was believed that this technique should be screened. In
this assessment, powdered activated alumina with a particle size of 8 x
14 mesh was used. The procedure comprised mixing 10 grams of the alumina
with 1 liter of solution containing 10 ppm chromium as dichromate and
stirring the mixture for 30 minutes. The mixture then was filtered and
the filtrate analyzed for hexavalent chromium content. The experimental
results did not show any measurable chromium removal.
Preliminary Evaluation of Treatment Costs
In order to compare the various treatment processes studied during the
initial phase of the program, an attempt was made to estimate the costs
of the various techniques for removal of chromium. Although cost factors
were not precise at this point in the program, and could change perhaps
52
-------
significantly, depending on the outcome of future work, they were used,
by necessity, as a primary basis for comparing the processes. The
following sections discuss the procedures used in arriving at appropriate
cost factors for each particular process.
Ion Flotation. The chemicals consumed in operating an ion flotation
system were assumed to include the flotation collector solutions and any
regenerating chemicals required if the collector was recovered. Air for
flotation and associated supply costs were considered insignificant with
respect to these materials.
Sufficient data were obtained in laboratory studies to determine the
efficiency of certain collectors for removing chromium. Based on these
data, the costs of flotation collectors for hexavalent chromium or triva-
lent chromium were calculated for solutions containing 10 ppm and 100 ppm
chromium. The results of these calculations are summarized in Table 21.
Also included are the collector costs for 100 ppm hexavalent chromium
with the provision for regeneration and recovery of the collector which
was studied very briefly.
TABLE 21. EFFICIENCY AND COSTS OF ION FLOTATION
COLLECTORS FOR CHROMIUM REMOVAL
Removal Collector
Attained'3', Requirement^,
percent lb/lb_Cr
Costs(b)
$/lb Cr $/1000 gal
Hexavalent Chromium:
@ 100 ppm
@ 10 ppm
@ 100 ppm, with
regeneration
91
78
90
5.0
8.0
1.3
1.65
2.64
0.43
1.32
0.21
0.34
Trivalent Chromium:
@ 100 ppm
@ 10 ppm
95
94
0.07
0.70
.»0.01 0.01
0.14 0.01
(a) Based on experimental data shown in Figures 5 and 6 and in Tables
10 and 13.
(b) Based on mixed primary amine collectors @ $0.33/lb for hexavalent
chromium and fatty acid collectors @ $0.20/lb for trivalent
chromium.
53
-------
The data in Table 21 indicate a 90 percent removal of hexavalent chromium
at 100 ppm for $1.65 per pound of chromium. With regeneration and recovery
of the collector, this is reduced to about $0.55 per pound of chromium
including the added cost of regenerating chemicals, which would amount to
an additional 20 to 30 percent over the values shown in Table 21. These
results are based on limited small-scale experiments. Thus, if more
effective reagent recovery could be obtained in larger scale equipment, a
considerable further reduction in reagent cost could be realized.
For the removal of trivalent chromium, by contrast, the cost of the col-
lector ranges from $0.01 to $0.14 per pound of chromium depending on the
concentration. This cost would be relatively insignificant with respect
to the cost of other reagents required, such as reductants for hexavalent
chromium and caustic for precipitation of trivalent chromium. These
latter chemicals are the same as those required in conventional treatment,
thus the costs of the two processes would be similar.
None of these estimates take into account the cost of solvent if the
flotation collectors are added in alcoholic solution. No estimate could
be determined on this factor on the basis of the small-scale experiments.
Whether or not a solvent is needed and the required concentration must be
determined in larger scale equipment.
On the basis of a typical plant having a waste flow of 15 gallons per
minute containing 100 ppm of chromium, it was assumed that the flotation
process could be conducted with limited equipment comprising a 500-gallon
flotation cell, a pump, and a small reagent feeder. An agitated tank and
filter also would be needed for the regeneration and recovery of the spent
flotation collector. It was estimated that the installed cost of this
equipment would be about $4,500.
Adsorption on Activated Carbon. In evaluating the economic feasibility
of an activated carbon process for the removal of chromium from waste
plating rinse waters, several factors must be considered. These factors
include such items as material costs, equipment costs, labor costs, whether
materials can be regenerated for reuse, and whether any valuable contaminants,
such as chromium, could be recovered.
Pittsburgh Type OL granular activated carbon is quite expensive compared
with various other types of carbon available. The manufacturer's f.o.b. price
is 39 cents per pound in lots of over 30,000 pounds. Other brands of
activated carbons with approximately the same relative activity are avail-
able at about 50 percent less cost. It is possible that some of these less
expensive materials also could effect high chromium removal.
Estimated costs were calculated for chromium removal by activated carbon
and for the regeneration of the carbon, based on the bench-scale
54
-------
experimental work with Pittsburgh Type OL granular activated carbon. The
results are shown in Table 22. These data show the costs involved in two
separate methods of carbon regeneration. The first method involves the
use of sodium hydroxide to remove the chromium from the carbon as hexava-
lent chromium and a sulfuric acid wash to control the pH value of the
carbon. The sodium hydroxide solution containing high concentrations of
hexavalent chromium (as high as 30 grams per liter) then is either recov-
ered or destroyed by conventional methods. Destruction would involve the
treatment of relatively small volumes of the concentrated solution with
sodium bisulfite to produce a chromium hydroxide sludge that could be
hauled to dumps. As can be seen the material costs for this method would
amount to about 39 cents per pound of chromium contained in the feed
solution.
TABLE 22. ESTIMATED REAGENT COSTS FOR ACTIVATED CARBON
REGENERATION AFTER ADSORPTION OF CHROMIUM
Amount of Estimated
Reagent Reagent
Required, Cost,
Reagents Ib/lb Cr $/lb Cr
Method I (sodium hydroxide)
Regeneration:
NaOH (sodium hydroxide) @ 3c/lb 1.5
H2S04 (sulfuric acid) @ 2c/lb 2.0
Total cost of reagents for regeneration/lb Cr
Conventional Treatment:
N32S205 (sodium bisulfite) @ 7c/lb 2.8
H2S04 @ 2c/lb 1.4
NaOH @ 3c/lb 2.3
Total cost of conventional treatment/lb Cr
Total cost of Method I, per Ib Cr 0.39
Method II (sulfuric acid)
Regeneration:
H2S04 @ 2c/lb 3.0
Activated carbon @ 39
-------
The second method of carbon regeneration involves the use of sulfuric
acid to remove the chromium from the carbon as reduced chromium. With
this method, some of the carbon would be consumed since it is believed
that the reduction is carried out according to the following equation.
-I- SI^SO^ + 3C -*- 2Cr2 (S0^)3 + 3C02
In this reaction about 0.2 pound of carbon would be used per pound of
chromium reduced. The highly concentrated acid solution could then be
treated with sodium hydroxide to neutralize and precipitate a chromium
hydroxide sludge. The cost of regeneration by this method, including
carbon costs, would amount to about 21 cents per pound of chromium fed
to the process in the waste effluent.
On the basis of the "average" waste flow conditions cited previously
(15 gallons per minute at 100 ppm of chromium) and an 8-hour operating
day, it was estimated that a 21 cubic foot carbon column would be required
for the process. This size would permit regenerations at intervals of
about one week. If hexavalent chromium is destroyed and not recovered,
a 250-gallon agitated tank also would be needed for this purpose.
Equipment for this process was estimated at about $5,500.
Liquid-Liquid Extraction. Although there are a number of uncertainties con-
cerning the precise type and size of equipment, optimum operating conditions,
etc., required for the liquid-liquid extraction treatment of metal-
finishing wastes, enough information was obtained in the laboratory cam-
paign to project preliminary capital and operating costs. Figure 14 is
a flowsheet of the speculative process for which the estimates were made.
Capital costs were arrived at by estimating the installed cost of equipment
that would be necessary to treat 15 gallons per minute of waste rinse
water containing from 1 to 100 ppm of hexavalent chromium.
Installed costs of the equipment including the necessary pumps, piping,
etc., were estimated to be about $8400.
A major factor in operating cost is the quantity of chromium that must
be treated. The operating cost estimates were made therefore for levels
of chromium concentration; 10, 30, and 100 ppm at a flow rate of 15
gallons per minute. Details of these estimates are shown in Table 23.
56
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Rinse Water
14,400 gallons; 15 gpm
(10 to 100 ppm of Cr6)
I
Extractant_
1.5 gpm
Extraction Column
8-minute retention time
20" x 8f tall, SS
2 or more stages
T
Circulating Stripping
Solution, 1.5 gpm
Stripping Column
30-minute retention time
12" <|> x 8' tall
single stage
Stripped
Extractant
1.5 gpm
I
Jr
Settling Tank
20-minute retention time
300 gallon, mild steel
Settling
Tank
100 gallon, mild steel
Waste Effluent
15 gpm
1 ppm Cr
100 ppm kerosene
10 ppm reagent
Bleed-off Stripping Solution
for Chemical Destruction
(10,000 ppm
Sulfuric acid—-
Sodium bisulfite-
Treatment Tank
100-gallon, mild steel
Treated Strip Solution
to Waste
Average 0.15 gpm
FIGURE 14. FLOWSHEET ASSUMED FOR COST ESTIMATES
57
-------
Ul
00
TABLE 23. ECONOMIC FACTORS IN OPERATING COSTS
FOR LIQUID-LIQUID EXTRACTION
(a)
Ib Chromium/Day
Chromium, ppm:
Sodium hydroxide
Kerosene
Reagent
Sulfuric acid
Sodium bisulfite
Lab or (b>
Power
Total
in Effluent:
Unit Cost,
$
0.03/lb
0.16/gal.
0.60/lb
0.02/lb
0.07/lb
3.00/hr
0.03/kwhr
Ib
5
2
1.2
7
3.4
0.2
20
1.2
10
Cost/hr,
Cost, $ $
0.15
0.32
0.96
0.14
0.24
0.60
0.60
3.01 0.19
Ib
15
2
1.2
20
10
0.5
20
3.6
30
Cost/hr,
Cost, $ $
0.45
0.32
0.96
0.40
0.70
1.50
0.60
4.93 0.31
Ib
50
2
1.2
65
34
1.0
20
12.0
100
Cost/hr,
Cost, $ $
1.50
0.32
0.96
1.30
2.40
3.00
0.60
10.08 0.67
(a) All costs based on 16-hour day.
(b) Labor cost calculated by assuming that practically all labor is used in treating stripped solution
at 1 hour per batch treatment. At 10 ppm treatment estimated to be every 5 days; at 30 ppm every
other day; at 100 ppm, daily.
-------
Liquid-liquid extraction would afford a good opportunity for chromium
recovery, because it could produce a strip solution containing at least
10 and possibly up to 20 grams per liter of hexavalent chromium. Provided
enough chromium is involved, significant credit might be realized from
the recovery of chromium.
Provided the treated effluent did not contain undesirably high concentra-
tions of oil, at least some of it might be reusable in rinsing. The
attendant savings in water and sewerage costs would constitute a credit
for the operation.
Comparison of Experimental and Conventional Processes. In addition to the above
cost estimates for the experimental processes, two conventional techniques
also were evaluated for comparative purposes. The conventional methods
selected were ion exchange and conventional reduction with sodium bisulfite.
Estimates of the operating and equipment costs for each of these methods
was made using the same basis as mentioned previously (15 gallons per
minute at 100 ppm chromium). A comparison of these costs with those
determined for the experimental techniques is shown in Table 24.
TABLE 24. PRELIMINARY COMPARISON OF COSTS FOR SELECTED
EXPERIMENTAL AND CONVENTIONAL TREATMENT METHODS
Reagent Capital Operating
Cost, Cost'3', Cost'b',
$/lb Cr § $/dayri
Removal Only:
Conventional Method with Na^S^O,. 0.30 8,000 10.00
Ion Exchange 0.58 8,500 12.50
Carbon Adsorption 0.21 5,500 8.00
Liquid-Liquid Extraction 0.54 8,500 11,00
Ion Flotation 0.55 4,500 10.50
Removal Plus Recovery of Chromium: ,
Ion Exchange 0.29 10,000* 7.50)cf
Carbon Adsorption 0.16 7,000 5.50^°'
• IT-— __.-•• _ - _-.- L- _ _ -i ji. -i.u.i. . L ~ i -" .i u w~i .. i _L - -• L., _.. T r • •
(a) Based on waste flow rate of 15 gallons per minute and contaminant
level up to 100 ppm.
(b) Based on 8-hour day, 250 days/year; waste flow rate of 15 gallons
per minute, and contaminant level of 100 ppm. Includes cost of
reagents, labor, power, fuel, maintenance at 5 percent per year,
and depreciation at 13 percent per year.
(c) Includes in addition to the items listed in (b) a credit for
recovery based on chromium at $0.65/lb.
59
-------
This economic comparison indicated that the carbon adsorption process
for chromium removal may have economic advantage over the other methods
listed. Carbon adsorption also appeared competitive with ion exchange
when recovery of chromium is considered; however, the possible saving in
rinse water by these methods was not included.
Phase 2; Demonstration Plant Study
The activated carbon adsorption technique was selected for further devel-
opment on a pilot-plant scale for the following reasons:
(1) Apparent economic advantage over conventional waste treatment pro-
cesses within the range of rinsewater flow rates and chromium concentra-
tions normally encountered in small plating plants
(2) Operational simplicity
(3) Low floor-space requirements.
A preliminary evaluation of the process on the basis of Phase 1 results
indicated that three basic techniques or process modifications employing
activated carbon adsorption might be developed for use by the small
plater. These process modifications are illustrated in Figure 15.
The first method constitutes the use of carbon as a single adsorption
material followed by disposal of the loaded carbon as a solid waste.
During the adsorption cycle, the chromium rinse water is recirculated
from the rinse tank, through the carbon columns, and then reused for
rinsing. This permits the attainment of high carbon loading capacities,
since the chromium content of the water can be allowed to increase to
the point where it impairs rinsing efficiency. The ultimate adsorption
capacities for this method of operation were determined experimentally
in previous laboratory studies.
The second and third methods of operation also may be conducted as des-
cribed above, except that the loaded carbon is regenerated chemically
after each adsorption cycle and reused for subsequent cycles. Various
methods for regeneration were investigated in detail during both labora-
tory work and pilot-plant operations. In Method II, regeneration is
carried out by recirculating dilute sulfuric acid through the carbon bed.
Chromium is reduced during this treatment, and with a subsequent caustic
addition to the effluent, is precipitated as trivalent chromium hydroxide
which can be removed by sedimentation. Chromium ultimately ends up as a
sludge which would be disposed of as a solid waste material.
With caustic regeneration, as shown in Method III, chromium is removed
from the carbon as hexavalent chromium, possibly suitable for recovery
and reuse within the plant. The spent regenerating solution contains
primarily sodium chromate with small amounts of excess caustic. It
might be reusable in post-treatment solutions for zinc and cadmium plating
60
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Make-up
Carbon
RINSE
TANK
Rinse Water
\
Spent
Carbon
Disposal
CARBON COLUMNS
METHOD I: SINGLE ADSORPTION AND DISPOSAL
Acid
RINSE
TANK
M
RINSE
TANK
4 Rinse Water
C
ETHOD II: ACID RE(
^.Rinse Water
^ Acid
Solution^
ARBON COLUMNS REGENERATING
TANKS
GENERATION Al
ID DISPOSAL
Caust
1
_- Caustic
Solution ^
S
iic
Lii
Ca
mmtmm
1
me
us
M
or
tic
r
^. Sludge
Disposal
ETTLING
TANK
Sodium
Recovery
CARBON COLUMNS REGENERATING TANKS
METHOD III: CAUSTIC REGENERATION AND RECOVERY
FIGURE 15. ALTERNATIVE PROCESSES FOR CHROMIUM REMOVAL
BY ACTIVATED CARBON
61
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operations or converted to chromic acid in a single, small ion exchange
unit. Regeneration with caustic, however, reclaimed only about 2/3 of
the chromium in previous laboratory studies, and ultimately the remain-
ing chromium must be removed by acid regeneration or the spent carbon
disposed of as in Method I.
Pilot-Scale Investigation
Equipment and Procedure. The second phase of experimental work on activated
carbon was conducted in pilot-plant equipment employing carbon columns of
1-foot-ID by 6 feet in length. The pilot-plant system was installed at
a plating plant in Columbus, Ohio, and operated on actual chromium rinse
waters produced by this company. These rinse waters contained generally
between 100 and 820 ppm hexavalent chromium at pH values ranging from
2.0 to 3.0. The water was used directly as feed to the carbon columns with
no prior treatment or pH adjustment.
Pilot-plant equipment was installed as shown in Figure 16. Basically,
the system consisted of two 1000-gallon rubber-lined tanks, an adsorption
module containing two carbon columns, and associated pumps, piping, and
instruments to operate the system. The adsorption module was a standard
ion exchange package unit purchased from Illinois Water Treatment Company
but charged with activated carbon beds instead of ion exchange resins.
Each column was filled with 100 pounds of Pittsburgh Type OL activated
carbon. The unit also was modified by installing additional piping and
valves to permit recirculation of the regenerating solutions. Piping
was arranged so that one column could be regenerated while the other
column was undergoing adsorption.
In operating the system, rinse water was pumped at about 5 gpm into one
or both of the holding tanks. The water was then recirculated in these
tanks to equalize concentrations and pumped to the adsorption unit.
Effluent water from the carbon columns was sampled periodically and dis-
carded to the sewer. Samples of the feed also were obtained periodically.
These samples were analyzed to determine pH and chromium concentration.
Adsorption was continued until a "breakthrough" was achieved, e.g.,
effluent samples reached about 10 ppm in hexavalent chromium concentration.
Following adsorption, the loaded carbon columns were regenerated by one
of two techniques, e.g., either dilute sulfuric acid or caustic solution
was recirculated through the appropriate column. Normally, regeneration
was carried out at flow rates of 1 to 2 gpm and for various periods up to
16 continuous hours. The regenerating solution was then pumped from the
column and analyzed to determine the amount of chromium stripped. Extens-
ive washing of the column with water also was employed to complete a
material balance on chromium. Analyses for chromium were conducted
according to the procedures outlined in Appendix B.
62
-------
OJ
FIGURE 16. PILOT-PLANT CARBON ADSORPTION SYSTEM
-------
The initial experiment in the pilot-plant system consisted of a prelimi-
nary operation to check out the system and to develop operating proce-
dures for subsequent detailed runs. Column No. 1 was used for this
preliminary run and was charged with 50 pounds of Pittsburgh Type OL
activated carbon. Prior to feeding, the carbon was acid washed by
recirculating a 5 percent sulfuric acid solution through it for several
hours. This acid wash procedure was found necessary from laboratory
work to remove basic materials from the carbon and to lower the column
pH to a level conducive to high adsorption of hexavalent chromium.
Following the acid wash treatment, the column was operated by feeding
rinse water containing 352 ppm of hexavalent chromium at a pH of 2.3 and
at flow rates of 1 to 2 gpm, A total of 471 gallons of rinse water from
the plating plant was treated during the initial experiment. Analyses
of the effluent samples indicated that essentially 100 percent of the
chromium was removed except for the final 40 gallons when the chromium
content of the effluent rose to about 1.0 ppm.
Following the preliminary operation, a series of continuous adsorption
and regeneration runs were made employing both carbon columns of the
system. Each column was completely filled with carbon (100 pounds) and
acid washed with sulfuric acid prior to beginning each adsorption cycle.
Results of Sulfuric Acid Regeneration Method. Initial experiments were
concerned primarily with the acid regeneration technique in which the
adsorbed chromium was stripped from the carbon as trivalent chromium
sulfate with dilute sulfuric acid. The operation included a total of 6
adsorption cycles and 5 regeneration cycles under various operating
conditions.
The pertinent data from these experiments are shown in Tables 25 and 26.
During adsorption runs (Table 25), the removal of hexavalent chromium
from the rinse water was high (greater than 99 percent) through four
cycles of operation. In Cycle 3, however, somewhat poorer operation was
obtained because of the presence of reduced chromium in effluent samples.
This lowered adsorption efficiencies in Cycle 3 to about 90 percent for
total chromium. The poorer results evidently were caused by insufficient
time being allowed for acid recirculation and generally ineffective
washing of the reduced chromium from the column.
,In Cycle 5, the very low adsorption 'capacity could not be compared with
other results because a change was made in operating procedure to evalu-
ate adsorption under less acid conditions (pH = 6.0). As a result of
this change, very little chromium was adsorbed in Cycle 5. This run
demonstrated that lower pH values were necessary for efficient adsorp-
tion. In Cycle 5a (starting pH = 3.8), adsorption efficiencies again
increased to about 95 percent.
Significantly, the results of these experiments showed that acid regener-
ation was not completely effective in stripping chromium once it had been
64
-------
TABLE 25. PILOT-PLANT ADSORPTION DATA ON COLUMN NO. 1, ACID REGENERATION
Effluent Data
Cycle 1
Cycle 2
Cycle 3
Cycle 4
Cycle 5
Cycle 5a
Duration
Operating
days
5
4
2
3
1
3
of Run
Feeding
hours
28
15
10
15
4
28
Feed Data
Total
gal.
2766
1950
950
1380
374
2978
PH
2.3-2.6
1.9-2.0
2.0
2.1-2.4
2.4
2.2-3.1
C^
250-460
410-820
725-750
275-375
250
100-260
Start
PH
1.6
1.1
2.1
2.0
6.0
3.8
Final
pH
8.6
5.4
4.7
4.7
6.0
6.5
Typical
B*
<0.05
0.1
3
<0.05
130
<0.05
Final
ppm
Cr+6
9.7
1.0
5.4
6.1
195
41
Typical
ppm
Cr+3
™
5.5
75
30
—
1.3
Calculated
Chromium Adsorbed
Percent
Cr+6
>99
>99
>99
>99
50
95
Percent
Total Cr
>99
>99
90
90
50
95
Ib
9.45
10.11
5.34
3.80
0.40
3.57
Apparent
Carbon
Loading
Capacity
Ib Cr/lb
Carbon
0.095
0.147
0.126
0.094
0.077
0.105
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TABLE 26. PILOT-PLANT REGENERATION DATA ON COLUMN NO. 1
Regenerating Solutions
Cycle 1
Cycle 2
Cycle 3
Cycle 4
Cycle 5
Ib
42
24
44
24
6
gal
30
45
90
45
15
Percent
3-21
6
3-6
6
5
Wash
Water
gal
80
100
120
105
35
Chromium Concentrations
„ +6 n +3
ppm Cr ppm Cr
0 1,000-18,300
1200-4100 500-20,000
0 14-12,000
72 2,600-6,200
1800
Chromium Stripped From Carbon
Ib Cr+6
0
0.51
0
0.10
0.75
Ib Cr+3 Total Cr
4.87 4.87
6.88 7.39
7.00 7.00
2.05 2.15
0.75
(a) In this run, 10 pounds of caustic was used prior to treatment with acid.
-------
adsorbed by the carbon. This is illustrated in Figure 17 which compares
adsorption and regeneration data for each run. Overall, only 76 percent
of the adsorbed chromium was removed through 5 cycles of acid regeneration.
Results_of_Caustic Regeneration Method. Considerably more data were obtained
on the second method of operation involving regeneration with caustic to
remove hexavalent chromium followed by an acid wash treatment to remove
residual caustic and condition the column for subsequent adsorption cycles.
Operating data and results through 10 cycles of adsorption and caustic re-
generation are shown in Tables 27 and 28.
The adsorption data in Table 27 indicate generally that chromium adsorption
capacities were somewhat better with caustic regeneration than with acid
regeneration. The weight of chromium adsorbed per cycle remained fairly
constant at about 5.0 pounds through 7 cycles of operation neglecting the
initial run which was low because of insufficient acid washing of the
carbon prior to undertaking this series of experiments. Adsorption
efficiency ranged from about 90 to over 99 percent through all runs.
The lower efficiencies were traced to the presence of trivalent chromium
in effluent samples. This probably was caused by insufficient washing of
the column leaving residual trivalent chromium or to the formation of
trivalent chromium during adsorption by employing high chromium concen-
trations in the feed.
Adsorption capacities in two of the final runs (Cycles 8 and 9) decreased
to about 3.0 pounds for unexplained reasons. It was suspected, however,
that the poorer adsorption was the result of a gradual buildup of chromium
on the carbon which was not removed by regeneration. This is illustrated
in Figure 18. As was found in previous runs, caustic regeneration also
was not completely effective in stripping chromium from the carbon. Over-
all only 76 percent of the adsorbed chromium was removed through 10 cycles
of operation. In only two runs was the amount of chromium stripped equal
to or greater than the amount adsorbed in the same cycle.
Discussion of Pilot-Plant Runs. The results of the pilot-plant study
demonstrated that actual chromium rinse waters can be effectively treated
with activated carbon but also that important areas exist for additional
study or refinement of the process. One area which needs additional
study is in the apparent loss of adsorptive capacity after multiple cycles
of operation with either caustic or acid regeneration. In the caustic
regeneration runs, for example, there was indication that after 8 and 9
cycles, serious loss of adsorptive capacity occurred. However, in Cycle
10, there was a partial recovery in the system with adsorptive capacities
of 4 pounds of chromium per 100 pounds of carbon again being achieved. The
reasons for the improved results were not fully determined but were believed
due to more effective regeneration procedures employed in the preceding
cycle. Not enough cycles were conducted during the campaign to fully evaluate at
67
-------
12
10
Pi
O
•H
W
CO
H
QJ
PI
0)
CO
OJ
t>0
.3
n
O
T3
0)
P.
8
J-l
O
[0
§
•H
3
O
Vl
Legend
O Adsorption Data
• Regeneration Data
0
Operating Cycles
FIGURE 17. PILOT-PLANT DATA ON .ADSORPTION AND ACID REGENERATION
68
-------
TABLE 27. PILOT-PLANT ADSORPTION DATA ON COLUMN NO. 2, CAUSTIC REGENERATION
Effluent Data
Duration
Operating
days
Cycle 1
Cycle 2
Cycle 3
Cycle 4
Cycle 5
Cycle 6
Cycle 7
Cycle 8
Cycle 9
Cycle 10
2
4
4
2
1
3
3
2
1
2
of Run
Feed Data
Feeding Total
hours gal.
10
32
23
11
11
21
23
14
8
16
1123
3860
2055
1204
1260
2457
2535
1557
927
1933
PH
2.2
2.6-3.0
2.0-2.2
2.0-2.2
2.2
2.5
2.3-2.6
2.4
2.2
2.2-2.6
ppm
Cr+6
310
110-240
320-725
500-560
400-600
190-343
176-346
142-441
340
171-351
Start
PH
1.2
2.0
2.5
1.5
2.3
2.7
2.8
2.5
3.3
2.1
Final
PH
9.3
7.3
6.9
5.0
5.1
6.3
5.3
3.7
4.6
4.8
Typical
ppm
Cr+6
<0.05
<0.05
<0.05
1
1
0.1
1
0.3
1
<0.05
Final Typical
ppm ppm
Cr+6 cr+3
10
1.6
3.5
2.5
6.6
9.4
63
102
38
25
—
—
0.8
28
60
3
4
25
16
10
Calculated
Chromium Adsorbed
Percent
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
Percent
Total
Cr
>99
>99
>99
95
90
98
97
90
95
95
C
Ib
2.91
5.07
7.51
4.96
4.72
5.40
5.22
3.31
2.49
4.01
Apparent
Carbon
Loading
Ib Cr/lb
Carbon
0.029
0.067
0.110
0.116
0.112
0.128
0.155
0.157
0.155
0.146
(a) Includes chromium not removed in preceding runs.
-------
TABLE 28. PILOT-PLANT REGENERATION DATA ON COLUMN NO. 2
Regenerating
Solution
Cycle 1
Cycle 2
Cycle 3
Cycle 4
Cycle 5
Cycle 6
Cycle 7
Cycle 8
Cycle 9
Cycle 10
Ib
NaOH
5
14
15
15
13
12
14
20
25
25
gal.
15
30
15
15
15
15
15
15
15
15
Acid
Ib
9
16
16
16
12
12
20
16
44
20
Wash
gal.
H20
15
15
15
15
15
15
30
15
30
15
Wash
Water
gal.
55
70
35
70
120
70
200
75
150
75
Chromium Concentrations
ppm Cr
3,800
480-10,200
18,000
8,500-19,000
8,100-13,700
4,800-11,700
5,600-16,800
1,700-16,400
80-1,800
320-9,500
ppm Cr
240-3000
3,800
1,600
3600-5800
21-2400
970
1,800
200-1000
160-11,700
1000-5 , 400
Chromium Stripped
Ib Cr+6
0.58
1.63
3.53
3.55
3.38
2.02
2.69
2.47
0.35
2.35
Ib Cr+3
0.70
1.63
0.81
1.50
0.50
0.43
0.46
0.25
4.48
1.35
From Carbon
Ib Total Cr
1.28
3.26
4.34
5.05
3.88
2.45
3.15
2.72
4.83
3.70
-------
12
tn
a
o
•H
4J
to
t-l
-------
what point the original carbon would become ineffective and need replace-
ment. This also applies to the acid regeneration runs in which only 5
cycles were conducted and a much more serious loss of adsorptive capacity
was noted.
In an effort to evaluate further the loss of adsorption efficiency
experienced in pilot-plant runs, several confirmatory experiments also
were conducted in the laboratory. In these experiments, series of adsorp-
tion and regeneration treatments were made on a 5-gram sample of activated
carbon similar to that used in the pilot-plant runs. Both adsorption and
regeneration cycles were conducted by magnetically stirring the samples in
a beaker for periods up to 1 hour. The solution used in the adsorption
cycles contained 1000 ppm hexavalent chromium. Solution pH was maintained
at 3.0. These starting conditions were selected to achieve final equilib-
rium conditions closely approximating those obtained in the pilot-plant
operation, e.g., 300-500 ppm Cr and 3.0 pH.
The results of these laboratory experiments are compared with the pilot-
plant runs in Figures 19 and 20. Significantly, the laboratory data
points could be approximated closely by the linear relationships shown
in these figures. The pilot-plant data generally reflect a much greater
variation between individual runs, but indicate the same trend after
several cycles. Variations were explained as being due to changes in
operating conditions such as feed concentration, pH, etc.
On the basis of the extrapolated linear relationship shown in these fig-
ures, it appears that adsorption capacities gradually decrease under
either method of regeneration. Degradation of the carbon with acid is
much greater than with caustic regeneration. For example, after 100
operating cycles, the predicted adsorption capacities are more than 0.03
pound per pound of carbon with caustic compared to less than 0.01 pound
with acid regeneration. These data indicate a much shorter life of the
carbon bed with acid treatment; however, replacement of the carbon prob-
ably would be required in either case. The optimum point at which carbon
would be replaced cannot be established solely on the basis of the data
presently available. Extrapolated data from these figures indicate that
more than 100 cycles might be achieved with caustic treatment before
replacing the bed. Actual experimental data, however, were obtained
through only 10 cycles of operation. An extensive operating campaign
would be needed to adequately extend the experimental data to a point
where an accurate prediction of carbon life could be made.
Another aspect of the process which is not fully understood involves
stripping of the adsorbed chromium from the carbon during the regenera-
tion cycle. Incomplete stripping generally occurred during all runs
with up to 10 percent chromium remaining on the carbon in several caustic
regeneration runs. Even though this residual chromium was present, an
additional 4 to 5 pounds of chromium could be adsorbed in the subsequent
cycles. Whether or not adsorption capacities could be increased signifi-
cantly if complete regeneration were achieved, remains to be established.
The reasons for incomplete stripping were not investigated during this
study; however, it could possibly be caused by operation with feeds
72
-------
§
,0
J-l
ra
cu
H
o
V-l
OJ
ex
S-l
O
CO
T3
E
•H
I
V-i
6
i.o
0.8
0.6
0.4
0.2
0.1
0.08
0.06
0.04
0.02
0.01
Legend
Pilot-Plant Runs
Laboratory Stirring Tests
I
I
I
6 8 10
Operating Cycles
20
40
60 80 100
FIGURE 19. ADSORPTION CAPACITIES FOR HEXAVALENT CHROMIUM AFTER
MULTIPLE CYCLES WITH INTERMEDIATE CAUSTIC REGENERATION
73
-------
c
o
•8
«
o
,0
-------
containing high chromium concentrations and at low pH values. Reduction
of chromium during adsorption was found to occur at pH values below
about 3.0. During reduction, chromium oxide ((^203) probably was pro-
duced. This material, which is only partially soluble in dilute sulfuric
acid, would be stripped very slowly during the regeneration cycle. Added
evidence that this might occur was observed when complete stripping of
the loaded carbon was obtained in boiling dilute sulfuric acid in labora-
tory experiments.
Feed rinse waters obtained during the pilot-plant study contained
considerably higher concentrations of chromium than were originally
anticipated. This also could have influenced reduction of chromium
during adsorption and inability to completely strip the carbon. Use
of other feed streams containing less than 100 ppm hexavalent chromium
might have improved results.
Although some questions were not resolved during the campaign, sufficient
data were obtained to demonstrate the technical feasibility of the pro-
cess. Adsorption and removal of hexavalent chromium was greater than 99
percent in all but two runs. In these two runs, lower adsorption could
be accounted for by changes in operating procedure. Removal of total
chromium was somewhat poorer (90 to 99 percent) due to the presence of
trivalent chromium observed during several runs. This probably was the
result of insufficient washing of the carbon after regeneration or to the
reduction of incoming chromium during adsorption. Operating performance
generally was excellent during the entire pilot-plant study in which more
than 29,000 gallons of rinse water and about 80 pounds of chromium were
treated in the system. In many cases, the system was operated unattended
with no problems being experienced. Operation in a continuous manner is
therefore believed practical for commercial plating plant installations.
75
-------
SECTION VI
ECONOMIC EVALUATION OF PROCESS
On the basis of pilot-scale and laboratory studies conducted in Phase 2,
operating costs were reevaluated for the carbon adsorption process and
compared with the conventional treatment method using sodium bisulfite.
The results of Phase 2 indicated the necessity for carbon replacement at
regular intervals which was not considered in the previous evaluation.
Calculations of chemical costs for the acid regeneration and the caustic
regeneration processes were made assuming the linear relationships shown
in Figures 19 and 20. The results for 20-cycle and 50-cycle replacement
intervals are shown in Table 29 and compared with conventional treatment.
Costs for other operating cycles are illustrated in Figure 21. It should
be noted that unit costs of raw material (caustic and acid) were selected
to represent closely the actual costs incurred by small plating shops in
purchasing limited quantities of materials. These costs are significantly
higher than used in the previous economic evaluation.
The data shown in Table 29 indicate that chemical costs for the acid
regeneration process are about $0.91 and $0.73 per pound of chromium if
25 and 50 cycles of adsorption, respectively, can be achieved before
carbon replacement. Above 50 operating cycles, the carbon replacement
cost has very little effect on the overall economics (see Figure 21).
On the same basis, the chemical costs for caustic regeneration are about
$0.69 and $0.50 per pound of chromium removed for 20 and 50 cycles,
respectively. This cost, however, does not include any credits for
recovery of sodium chromate.
In addition to these data, costs also were estimated for the single
adsorption process in which chromium would be disposed of in combination
with spent carbon. For this estimate, the use of Nuchar "carbon ($0.25/
Ib) was selected because it is significantly cheaper and showed loading
capacities almost equivalent to Pittsburgh OL carbon (see Figure 11). On
the basis of the 20 percent loading capacity that was demonstrated in the
laboratory work, the raw material cost for this treatment would be $1.25
per pound of chromium, solely for make-up carbon. If partial recovery of
carbon or chromium could be realized from the spent material, this cost
would be reduced considerably.
In general, the cost data indicated that chemical costs for the carbon
adsorption process range from $0.50 to $1.25 per pound of chromium
removed, depending on whether or not the carbon is regenerated or dis-
posed of after a single adsorption. The lower cost can be achieved with
caustic regeneration, recovery of sodium chromate, and reuse of the
sodium chromate within the plating plant for other operations such as
conversion coatings. The possibility also exists for regeneration
77
-------
TABLE 29. COMPARISON OF CHEMICAL COSTS FOR ACTIVATED CARBON
ADSORPTION AND CONVENTIONAL TREATMENT
Amount of
Reagent Used, Cost,
_ Ib/lb Cr $/lb Cr:
Carbon Adsorption and Acid Regeneration:
20 Cycle Operation
3.1 0.09
NaOH @ 6.5c/lb 2.8 0.18
Carbon @ 40<:/lb 1.6 0.64
0.91
jO Cycle Ope ration
HS0 @ 3£/lb 4.2 0.13
NaOH @ 6.5c/lb 3.4 0.22
Carbon @ 40
-------
10.00
8.0C
6.0G
4.0C
•co-
to
Q
U
2.0C
§ 1.00
•H
£ 0.80L
0.6C
0.4C
0.2C
0.1C
Unit Cost Data
Sulfuric Acid: 3c/lb
Caustic: 6.5£/lb
Carbon: 40
-------
of chromic acid by ion exchange methods. Whether or not recovery is
feasible must be determined in each specific plating plant. Because of
the higher cost of the acid regeneration method, which does not include
costs for disposal of sludge, this method is considered the least practical
of those studied during the program.
An economic comparison of the carbon process with conventional treatment
also was made including other operating costs besides chemicals such as
labor, amortization, etc. This comparison is shown in Table 30. The
data indicate that operating costs for the carbon process are $5.37 per
day based on the particular waste stream under consideration. Costs for
conventional treatment are $7.34 per day.
TABLE 30. ESSENTIAL PLANT AND OPERATING COSTS FOR CARBON
ADSORPTION AND CONVENTIONAL TREATMENT PROCESSES
Carbon Conventional
Process Treatment
Essential Plant Cost; dollars'3^ 5,500 8,000
Operating Costs; dollars/day
(1) ChemicalsU) 3.00 3.30
(2) Labor @ $2.00/man-hr 1.00 2.00
(3) Supplies and Maintenance @ 0.003
percent of Plant Cost 0.17 0.24
(4) Amortization @ 0.0224 percent of
Plant Cost
Totals
(a) Based on 8-hour day, waste flow rate of 15 gallons per minute, and
contaminant level of 100 ppm, assuming unit cost data shown in Table 29,
80
-------
SECTION VII
ACKNOWLEDGMENTS
This research program was conducted during the period of April, 1968,
through June, 1970. Battelle personnel participating in the program
were A. K. Reed, T. L. Tewksbury, J. F. Shea, R. G. Brown, J. G. Price,
M. F. Nichols, and G. R. Smithson, Jr.
The cooperation and assistance of the following is gratefully acknowledged:
Metal Finishers' Foundation Pollution Abatement Committee:
Walter V. Turner (Eric S. Turner & Co., New Rochelle,
N.Y.) Chairman
A. T. Leonard (Superior Plating, Inc., Minneapolis,
Minn.) Foundation President
Charles Levy (Swift Laboratories, Inc., Waltham,
Mass.)
Raymond E. Morris (Remco Finishing Corp., Fernwood,
Pa.)
A. T. Marinaro (Masters' Electro-Plating Assn., Inc.,
Long Island City, N.Y.)
C. W. Shriver (Superior Plating Co., Columbus, Oh.)
P. Peter Kovatis (National Assn. of Metal Finishers)
Messers William J. Lacy, Chief and Edward L. Dulaney, P.E., Staff
Engineer, Industrial Pollution Control Branch, Environmental Protection
Agency.
81
-------
BIBLIOGRAPHY
(1) Lewis, C. J., "Liquid-Liquid Extraction", Industr. Wastes, Vol. 2,
pp 137-141 (1957).
(2) Blake, Charles A., Baes, Charles, F., Jr., and Brown, Keith B.,
"Solvent Extraction with Alkyl Phosphoric Compounds", Ind. & Eng.
Chem., Vol. 50, pp 1763-7 (1958).
(3) Katz, Sidney Allen, "The Extraction of Hexavalent Chromium from
Aqueous Media by Methyl Isobutyl Ketone", Univ. Microfilms Order
No. 62-4312, p 145 (1962).
(4) Sebba, F., Ion Flotation, Elsevier Publishing Company, New York,
Chapter 9 (1962).
(5) Baarson, R. E., and Ray, C. L., Unit Processes in Hydrometallurgy
Edited by M. E. Wadsworth and F. T. Davis, First Edition, Gordon
and Breach Science Publishers, New York (1964); Metallurgical Soc.
Conf., Vol. 24, "Precipitate Flotation—A New Metal Extraction and
Concentration Technique", pp 659-679.
(6) Grieves, R. B., and Wood, R. K., "Continuous Foam Fractionation:
The Effect of Operating Variables on Separation", A.I.Ch.E. Journal
Vol. 10, No. 4, p 456 (1964).
(7) Grieves, R. B., "Foam Separation Processes for Industrial Waste
Treatment: Phenol, Phosphate, and Hexavalent Chromium", Proc.
21st Industr. Waste Conf., Purdue Univ. Engng. Ser. No. 121,
p 192 (May, 1966).
(8) Grieves, R. B., et al., "Ion Flotation for Industrial Wastes,
Separation of Hexavalent Chromium", Proc. 21st Ann. Waste Disp.
Conf., Purdue Univ. Engng. Extn. No. 121, p 110 (May 2, 1966).
(9) Grieves, R. B., and Schwartz, S. M., "Continuous Ion Flotation
of Bichromate", A.I.Ch.E. Journal, Vol. 12, No. 4, pp 746-750
(1966).
-------
APPENDIX
Conventional Methods for Treatment
of Chromium Wastes
Methods used for Control Analyses
-------
SECTION IX
APPENDIX
Conventional Methods for Treatment
of Chromium Wastes
The treatment of rinse waters from chromium plating operations usually
consists of one or a combination of the following procedures:
(1) Reduction of hexavalent chromium to the trivalent form followed by
the precipitation of the reduced chromium as the hydroxide
(2) Reclamation of chromic acid from the more concentrated rinse stream
by evaporative or ion exchange techniques
(3) Removal of hexavalent chromium by the addition of compounds which
form insoluble salts, e.g., barium chromate.
Reduction of Hexavalent Chromium _and Precipitation
of Chromic Hydroxide^
Methods for reduction of hexavalent chromium vary with each particular
plant. Common reducing agents are gaseous sulfur dioxide; sodium bisul-
fite, metabisulfite, or hydrosulfite; and ferrous sulfate.
Reduction with S02 is the method most commonly employed by many large
plating plants. Basic equipment for this method consists of sulfonators
for combining sulfur dioxide with water and agitated tanks for conduct-
ing the reduction. During reduction sulfuric acid is normally added to
maintain an acid solution with a pH range of 2.0-3.0. Under these con-
ditions, the reactions which occur are:
Sulfur Water Sulfurous
dioxide acid
2Cr03 + 3H2S03 -> Cr2(S04)3 + 3H20
Chromic Sulfurous Chromic
acid acid sulfate
The approximate chemical usage is 1 pound of S02 per pound of chromic
acid (Cr03) in the waste solution.
Other reducing agents, such as bisulfites or ferrous sulfate, also are
used by plating plants for treatment of chromium wastewaters. Bisulfite
may be added as a solid or as a solution. As with sulfur dioxide, the
addition of sulfuric acid is required to maintain a pH of about 2 to 3
87
-------
to obtain rapid and complete reaction. The anhydrous form of sodium
bisulfite (^28205) or sodium metabisulfite also may be used. With
ferrous sulfate larger quantities of sludge are produced than with
sulfur dioxide or bisulfites. Some economic advantage for reduction
with ferrous sulfate may be realized, however, if quantities of waste
pickle liquor are readily available at the plating plant.
After reduction it may be possible to dispose directly of the effluent.
The effluent may, however, require neutralization and precipitation of
the now trivalent chromium (and other metals) before disposal to reduce
its corrosiveness and whatever toxicity it may possess from such metals
as nickel, copper, etc. If the precipitated solids after neutralization
are too high or too potentially toxic to meet local regulations, the
neutralized effluent may have to be given some sort of solids-liquid
separation, such as sedimentation or filtration, prior to disposal.
Reclamation of Chromic Acid
In addition to the chemical operations discussed in the foregoing
section, there are two commonly used physical methods that have been
used in the treatment of rinse waters. These are ion exchange and
evaporation.
Ion exchange has been widely used in the treatment of chromium and mixed
rinse waters, not only for detoxification, but also for the recovery of
chromium, water, and, in the case of hot rinses, of heat. Ion exchange
processes may be particularly attractive as a means for detoxifying
chromium rinse waters. Very little attention is required, except when
regeneration of the exchange resins is necessary.
Regeneration, which amounts to removing the load of metals that the
resins have absorbed, must be done periodically. This is accomplished
by passing sulfuric acid through the resins to redissolve the metals.
The regenerated solutions are greatly concentrated, but they are still
toxic. If they are to be discharged to waste, then they require the
chemical treatments previously discussed, but because they are concen-
trated and relatively low in volume compared to the original rinse
waters, treatment can be carried out batchwise in small tanks. Recovery
of chromium and other metals from the regenerating solutions also is a
possibility; however, its economic feasibility for the smaller plater
would require study. Water recovery for reuse in rinsing is a built-in
feature of the ion exchange process.
Evaporative processes also have found some use for recovery of chromium
from rinse waters. Generally, evaporative processes are economical only
on concentrated rinses, such as those produced in still tanks or multi-
stage countercurrent rinsing. If the smaller plater employs such rinsing
techniques, evaporation processes may be worth consideration not only
for the recovery of plating chemicals but also of rinse water.
-------
Precipitation of Hexavalent Chromium
Removal of toxic chromium from wastewaters also can be effected by pre
cipitation as hexavalent chromium. This method of treatment usually
involves the use of barium salts to effect the formation of insoluble
barium chromate. With barium chloride, for example, the following
reaction takes place:
BaCl2 + Na2Cr04 •> BaCr04 + 2NaCl
Barium Sodium Insoluble Sodium
chloride chromate barium chromate chloride
The major disadvantage of this method is that the additions of barium
chloride must be strictly controlled, as this chemical is highly toxic.
The sludges produced also are toxic and may result in an additional
disposal problem.
The process involving the precipitation of highly insoluble barium chro-
mate generally will require a solids-liquid separation step before the
effluent is disposed. Relatively few plants employ this process.
Methods used for Control Analyses
Simplified Test for Hexavalent Chromium
(1) Transfer a 50-ml sample, containing no more than 1.25 ppm of hexava-
lent chromium (predilute if necessary) to a 100-ml Erlenmeyer flask.
(2) Add 0.1 gram of solid reagent mixture made by grinding together
0.25 gram of 1,5 diphenylcarbohydrazide and 9.75 grams of tartaric acid.
(3) Shake vigorously until all the reagent is dissolvedt
(4) Let stand 5 minutes to develop full color and measure the absorbance
in a spectrophotometer at 540 IBM wavelength.
(5) Determine the hexavalent chromium concentration of the solution by
comparison with a standard calibration curve (see Figure A-l).
(6) The color also may be compared against a set of permanent standards
prepared in steps of 0.5 ppm by mixing together various proportions of
crystal violet and safrinin.
89
-------
Titrametric Method for Total Chromium
A control procedure for total chromium also was developed. This method
involved the oxidation of chromium by ammonium persulfate in the
presence of silver nitrate and titration of the chromium by the conven-
tional ferrous sulfate-permanganate titration.
90
-------
G
-------
1
Accession Number
w
5
2
Subject Field & Group
SELECTED WATER RESOURCES ABSTRACTS
INPUT TRANSACTION FORM
Organization
Metal Finisher's Foundation, Upper Montclair, New Jersey
Title
AN INVESTIGATION OF TECHNIQUES FOR REM3VAL OF CHROMIUM FROM ELECTROPLATING WASTES
] Q Authors)
Smithson, G.R. , Jr.
16
21
Project Designation
Project No.
12010 EIE
Note
TIV-.-? o v.ovx^v.4- T om ri TTTT? rva /T\ •; ^ -i-v
TO fn yi oT
report on Phase II Chromate System Work. Phase I
report 12010 EIE 11/68, "A State-of-the-Art Review
of Metal Finishing Waste Treatment is available from GPO
22
Citation
Battelle Memorial Institute, Columbus, Ohio
Descriptors (Starred First)
•^Activated Carbon, -^Chromium, -«Waste Water Treatment
25
Identifiers (Starred First)
^Electroplating wastes
27
Abstract
This report describes work which was conducted on the removal of hexa-
valent chromium from plating rinse waters employing various treatment
processes. The study consisted of an initial phas e in which information
was sought by questionnaire and by waste water analyses on the type of
waste produced by smaller electroplating plants. Laboratory studies were
conducted on several nonconventional methods for treatment of these
wastewaters including ion flotation, adsorption on activated carbon, and
solvent extraction. A demonstration pilot-plant study also was conducted
on the activated carbon process employing actual rinse waters from a hard
chrome plating operation.
The results of the various phases of the study indicated that activated
carbon adsorption for chromium removal may have practical application in
may small plating plants. Further development of the process was recom-
mended in actual plating plant installations.
This report was submitted by Battelle Memorial Institute, Columbus
Laboratories, in partial fulfillment of Grant Project 12010 EIE by the
Industrial Pollution Control Branch, Environmental Protection Agency to
Abstractor tH6 MlrtaJ-
Edward L. Dulaney
Water Quality Research, EPA
WR:102 (REV JULY 1969)
WRSIC
SEND. WITH COPY OF DOCUMENT, TO: WATER RESOURCES SCIENTIFIC INFORMATION CENTER
U.S. DEPARTMENT OF THE INTERIOR
WASHINGTON, D. C. 20240
* CPO: 1970-369-030
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|