&ER&
United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-81-157 Oct. 1981
Project Summary
Alternatives for Sodium
Cyanide for Flotation Control
E. J. Mezey, D. W. Neuendorf, G. Ray Smithson, Jr., and James F. Shea
Cyanide has long been looked upon
as the classical poison and has been
listed by EPA as a priority pollutant.
The mineral dressing industry has long
used cyanide in its concentration and
extractive metallurgy operations.
Cyanide plays a role of varying
importance in the metallurgy of gold,
silver, copper, nickel, cobalt, lead,
zinc, molybdenum, and cadmium. In
the copper, lead, and zinc industries
the primary uses for cyanides are as
depressants for the flotation of iron
and zinc minerals and for the reactiva-
tion of copper minerals in the flotation
of molybdenite.
This study was made to determine
the technical and, if possible, the
economic feasibility of substituting
alternate reagents for cyanides as
depressants for iron in the flotation of
copper, lead, and zinc ores. The study
was carried out in two phases. The
first consisted of a literature search,
an evaluation of the data obtained in
the search and, finally, a suite of
laboratory-scale flotation experiments
to test the three selected reagents.
These experiments were made on
copper ores, copper-lead-zinc ores,
and zinc ores supplied by industry. The
three reagents selected and tested
were sodium sulfite, sodium sulfide,
and sodium thiosulfate. Phase II
consisted of interviewing company
officials and operators in the copper-
lead-zinc industry. Visits were made
to 11 companies operating 17 mills in
these fields.
This Project Summary was devel-
oped by EPA't Industrial Environmen-
tal Research Laboratory, Cincinnati,
OH, to announce key findings of the
research project that is fully docu-
mented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
The economic recovery of copper,
lead, and zinc from the sulfide ores of
these metals in the United States
always requires an initial concentration
step, or steps. The flotation process Is
almost always a part of concentrating
operations, often the major part. Flota-
tion is a process for physically separating
and collecting valuable minerals from
other minerals that may be present in an
ore. There are many ways to modify this
process and one such modification is
the basis for the work undertaken on
this project. It is the use of cyanide in the
copper, lead, and zinc milling industries
as a depressant for pyrite in the flotation
of copper, lead, and zinc materials.
Many of the collectors used in the
flotation of these minerals also coat iron
sulfide particles, with the result that
they too float with the valuable minerals,
significantly lowering the grade of
concentrates obtained. The use of
cyanide has been found to prevent this
co-flotation of pyrite.
Procedure
The first phase of this program
consisted of a literature search and
limited experimental work to identify
possible substitutes for cyanide as a
pyrite depressant. The literature of the
period 1968-1978 dealing with pyrite
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depressants was surveyed using> the
computer search of the following 'data
bases: Chemical Abstracts, Smithson-
ian, Dissertation Abstracts, Engineering
Index, and NTIS (National Technical
Information Service). Key words which
were used in the broadly defined search
were the following: flotation, ore
treatment, chalcocite, chalcopyrite,
sphalerite, galena, pyrite, and depress.
A total of 38 articles relevant to the
topic of pyrite depression were identified
in the computer search. Fourteen
candidate depressants were identified
and grouped into one of four classes:
metal salts, reducing agents, oxidizing
agents or miscellaneous. In each case,
information was sought concerning the
depressant's performance on pyrite,
selectivity, environmental considera-
tions, economics, and state of develop-
ment. In addition, the theory of depres-
sant action, when known, was summa-
rized.
This information was then evaluated
to determine three alternative depres-
sants with the best combination of the
following: (1) effective pyrite depression,
(2) selective depressant action, (3) low
environmental pollution potential, (4)
cost per tonne of ore processed similar
to cyanide, (5) high state of development
as a practical depressant, and (6) low
toxicity.
The approach to the basic assessment
strategy was to score each candidate
based on possession of the attributes
noted above and to rank the depressants
according to the scores thus assigned. A
reagent was given a "+" rating on
performance (see Table 1) if a relatively
small reagent concentration produced a
high degree of pyrite rejection in
flotation tests. A "-" performance rating
was given if a relatively large reagent
concentration was needed to produce a
high or low degree of pyrite rejection. If
a "+" or"-" rating could not be assigned
on the basis of information found in the
literature, a "0" rating was given. The
other evaluation criteria were treated in
the same way, according to the criteria
descriptions set forth in Table 1.
After a depressant received ratings on
all criteria, the individual "+", "-", and
"0" ratings were added to produce a
total evaluation score. When all alterna-
tive depressants have received a total
score, ranks from 1 through 14 were
assigned, based on the total evaluation
scores. All of the individual criterion
scores, total evaluation scores, and
ranks for all of the alternative depres-
sants are summarized in Table 2.
The second phase of this program
consisted of interviewing officials from
11 companies which operated 17 mills
treating copper, copper-lead-zinc, and
zinc sulfide ores in Arizona, Utah,
Missouri, and Tennessee. The visits
were carried out during the period of
October 29 to November 9,1979. Topics
discussed included the company's
experience, if any, with the pyrite
depressants under study, and the com-
pany's opinion on the technical and
economic feasibility of using alternative
depressants.
Results
The three reagents with the highest
ranking in the literature evaluation were
sodium sulfide, sodium thiosulfate, and
sodium sulfite, with respective evalua-
tion scores of +3, +5, and +5. Sodium
cyanide, evaluated according to the
same criteria, would have a total
evaluation score of +3 and a rank of 3.
Thus, these three reducing agents were
selected as the most promising alterna-
tives to cyanide as a pyrite depressant
and were compared to cyanide in
laboratory tests of performance and
selectivity on three different ore types.
Kr6D ranked fourth in the evaluation
(+1, rank = 4). Because the lack of
published information on this reagent is
primarily responsible for its poor
showing, new information as it becomes
available may well change its standing.
As a natural product, used in low
concentrations (5-100 ppm), it may
prove to be economical.
The results from the preliminary
laboratory screening experiments sug-
gest that sodium sulfide, Na2S, and
sodium sulfite, Na2S03, could approach
the effectiveness of sodium cyanide,
NaCN, as a pyrite depressant under the
constraints of this experimental program
and the ores studied.
Results of the interviews conducted
during the second phase of the program
included the following:
(1) None of the six mills visited or
contacted by phone in Arizona (all
copper producers) used cyanide or
sulfite to depress the flotation of
iron sulf ides, and none discharged
to waters of the state or operated
under NPDES permits.
(2) Most of the Arizona plants used
cyanide for the separation, by
flotation, of molybdenum sulfide
from the copper molybdenum
concentrates. This particular use
of cyanide was not addressed in
this study.
(3) One large copper producer in
Utah was found to use cyanide to
depress pyrite flotation, discharged
to waters of the state, and was
under the NPDES permit system.
Officials of this company stated
that there was no problem with
cyanide in the discharges from
the three mills the company
operated.
(4) All but one of the six lead-zinc
mills visited in Missouri and the
four in Tennessee use cyanide. All
but one mill (in Tennessee) used
cyanide in almost "starvation"
amounts. That one, with extremely
high iron sulfide in its feed, used
0.18 Ib (0.082 kg/metric ton) of
cyanide per ton or ore processed.
These Missouri and Tennessee
mills are discharged to waters of
the state and operated under the
NPDES permit system. Most of
the plants practice recycling of
mill waters. According to officials,
most of the NPDES permits did not
limit cyanide, and the measure-
ment for cyanide in effluents was
consistently below detectable
levels.
Conclusions and
Recommendations
The preliminary experimental work
conducted during Phase I of this
program suggested that sulfides and
sulfites are promising as alternative
depressants for pyrite in the flotation
treatment of copper, lead, and zinc ores.
Work by other investigators, cited in the
first phase, also indicated that sodium
sulfite would be an effective depressant
in the flotation of chalcopyrite ores.
The results suggested that the use of
these chemicals as alternative depres-
sants should be evaluated as an
alternative to the treatment of large
volumes of mining wastewater contain-
ing low concentrations of cyanide. The
breadth of applicability of such alterna-
tive depressants remains unknown and
must be determined. The benefits of
reduced environmental effects must be
weighed against possible effects on
processing the continually leaner
sulfide ore bodies adaptable to benefici-
ation by froth flotation. Losses of metal
value to railings or the misplacement of
metal values into the beneficiated ore
due to less selective flotation separations
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Table 1. Evaluation and flanking Criteria '" for Pyrite Depressants
Performance rating
Attribute +
Performance
Selectivity
Environmental
considerations
Cost considerations™
Developmental status
High pyrite rejection levels, reasonably
low depressant concentration required
No interference with subsequent flotation
steps, high recovery of nonpyrite minerals
Generally considered nonhazardpus
Cost per ton of ore processed similar to cyanide
Used commercially in pilot plant or in
extensive laboratory-scale optimization
Low to moderate pyrite rejection levels,
high depressant concentration required
Interference with subsequent flotation
steps, low recovery of nonpyrite minerals
Potentially hazardous
High cost per ton of ore processed
Subjected to screening tests only
(m>A value of "0" will be assigned to a particular depressant if insufficient information is available for assignment
of a "+" or "-" value.
™A favorable impact on cost would taken into consideration not only the cost of the depressant but also the need and
complexity of wastewater treatment if it were used.
Table 2. Pyrite Depressant Evaluation and Ranking
Depressant Performance™ Selectivity™
Heavy Metals
(as nitrate salts)
Ag + +
Cr(lll) +
Hgdlll +
CufllJ
Al
Feflll)
Reducing Agents
NaaS +
NaaSaOa + +
NaHtPOz + 0
NatClO* + 0
NaaSOa + +
Oxidizing Agents
KMnQ* + 0
NaaCraO? + 0
Miscellaneous
Kr6D 0 +
Environmental Total
Considerations Cost Developmental Evaluation
and Toxicity™ Considerations™ Status™ Score
.1
.3
-3
.5
+ + -/
+ + -/
+ + + +3
+ + + +5
0 - - -/
+ - 0
+ + + +5
.2
.2
o o o +r
Rank
6
12
12
14
6
6
£(•>
7"'
6
5
/'*'
10
10
4
fa) Selected for screening study.
(b)Each "+" or "-" has a value of 1.
also may produce undesirable environ-
mental and economic effects. Therefore,
any displacement of optimized beneficia-
tion processes based on the use of
sodium cyanide as a depressant by one
using an alternative pyrite depressant
can have a substantial impact on an
important sector of the mining industry.
The conclusions of the plant survey,
however, have shown that the problem
is not a simple one. Most of the copper-
producing plants visited in the western
United States did not use cyanide to
depress pyrite flotation. Virtually all of
the lead-zinc producing plants in
Missouri and Tennessee use cyanide.
but in almost "starvation" amounts.
Most of these plants are operated in
closed-cycle or nearly-closed-cycle.
None admitted to having a cyanide
problem related to their effluents.
None of the officials visited in plants
using cyanide and discharging to waters
of the state, knew of, or had done any
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work to determine, the mode of occur-
rence of cyanide in their effluents (i.e.,
free cyanide, complex cyanides, cyanate,
thiocyanate).
The general impression received from
these visits was that cyanide in plant
discharges did not constitute a signif-
icant problem or even a discernable
problem in most cases.
It wquld be unwise, however, to
generalize from this impression. Only 4
states were represented out of about 20
in which copper, lead, and zinc ores are
treated for flotation. Only 17 of the
estimated 90 to 100 copper-lead-zinc
concentrators in the United States were
covered. Although the survey failed to
disclose any significant problem with
cyanide in mill discharges, the possibility
that problems do exist in other mills
cannot be ruled out.
E. J. Mezey, D. W. Neuendorf, G. Ray Smithson. Jr.. and James F. Shea are with
Battelle Columbus Laboratories, Columbus, OH 43201.
Roger C. Wilmoth is the EPA Project Officer (see below).
The complete report, entitled "Alternatives for Sodium Cyanide for Flotation
Control." (Order No. PB 81-247 439; Cost: $9.50, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U. S. Environmental Protection Agency
Cincinnati, OH 45268
US GOVERNMENT PRINTING OFFICE, 1981 — 559-017/7375
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Environmental Protection
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