United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Cincinnati OH 4'5268
 Research and Development
EPA-600/S2-81-081  Mar. 1982
Project Summary
Characterization  of
Boliden's  Sulfide-Lime
Precipitation  Plant
 D. Bhattacharyya, C. Sund-Hagelberg, K. Schwitzgebel, G. M. Blythe, J. C.
 Terry, and F. B. Craig
  Sulfide precipitation is an effective
process for the treatment of industrial
wastes containing highly toxic heavy
metals. The attractive features of the
sulfide precipitation process are: attain-
ment of high degree of metal removals
over a broad pH range, effective pre-
cipitation of certain metals (such as:
As, Cu, Cd, Hg) even at very low pH,
low detention time requirement in the
reaction tank because'of the high reac-
tivity of sulfides, and the feasibility of
selective metal recovery. With sulfide
precipitation, the high reactivity of
sulfides  (S2-, HS-, H2S) with heavy
metal ions and the very low solubili-
ties of the heavy metal sulfides over a
broad pH range are features not found
with the hydroxide precipitation pro-
cesses.
  Sulfide precipitation processes to
remove heavy metals have gained con-
siderable importance. I1~61 Bhattachary-
ya, et al.I34) have done extensive bench-
scale sulfide precipitation work at the
University of Kentucky. High degree
of separation of heavy metal cations
and arsenic from actual smelter efflu-
ents was obtained with a combination
lime-sulfide precipitation process. Dur-
ing the second phase of the project a
joint work (by University of Kentucky,
Radian Corporation, and Boliden Metall
Corporation, Sweden) was undertaken
to obtain full-scale sulfide precipita-
tion data from  a  unique  system (to
remove arsenic, heavy metals and flu-
oride) recently developed by a Swed-
ish nonferrous metal production (cop-
per and lead smelter) company. The
full-scale plant (200 mVhour capac-
ity) was,put into operation in 1978
and was designed to precipitate (at pH
3-5) As, Zn, Cu, Pb, Cd, and Hg as
sulfide for possible recycle to "roast-
er," and remove fluoride separately by
lime (at pH > 10) as CaF2 (uncontami-
nated with heavy metals) for landfill.
The of an investigation involving both
full-scale results and laboratory-scale
precipitation of metals (from nonfer-
rous metal production operations) are
presented.
  This Project Summary was devel-
oped by EPA's 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 order-
ing information at back).

Experimental
  The  full-scale  data was obtained at
Boliden Metall Corporation, Skellefte-
hamn, Sweden  The system was de-
signed to treat process water, and rain
and wash waters as a single waste
stream to maximize containment of
heavy metals at a reasonable cost. The
process water was typically 30 to 40%
of the total flow.
  Plant Description  A schematic dia-
gram of the process is shown in Figure
1. The wastewaters contained heavy
metals and were normally acidic. The
major constituents were arsenic (100-
500 mg/l) and zinc (25-200 mg/l). Cop-

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                                              NaOH
Rain and Wash
    Water
                                                                           Polymer
                                                                                                      Lime
Source:
• Floor Washings
• Arsenic Plant
• Electrostatic
  Precipitator Washing
                                                    Reaction Tanks
                                                  (AszSs. HgS, CdS,
                                                 ZnS, etc. Formation]

                                                  * Samp/ing Points
Process Water

Source:
• Central Gas Scrubber
  Water (Cu & Pb
  Smelter Gases Con-
  tainment SO?)
• ZnO Plant Gas
  Washer
• Scrubber from Cu
  Matte Tapping
figure 1.   Schematic diagram of the full-scale sulfide-lime treatment process.
                                                                                To Drumfilter
                                                                                (intermittent
                                                                                  operation)
                                                                              Metal Sulfide Cake
                                                                                                       Fluoride
                                                                                                     Precipitation
                                                                                        Discharge
 per, lead, mercury, and selenium were
 present in smaller amounts (a total of
 40 mg/l). The incoming water was first
 partly neutralized (with NaOH) to pH
 2.5-3.0, then sodium sulfide was added.
 The amount of reagent added was con-
 trolled by monitoring the  pH, i.e., the
 addition of sodium sulfide was stopped
 when a  predetermined  pH value was
 reached.
   Sodium sulfide was added as a 1 5
 percent solution  to  the first reaction
 tank, which had a 55 m3 capacity. This
 resulted  in a  wastewater residence
 time of between 20 and 30 minutes at
                                        flowratesof 110to170mVh Thethird
                                        reaction tank was also equipped to allow
                                        the addition of more sulfide. This was
                                        sometimes necessary in order to com-
                                        pensate for secondary  reactions The
                                        wastewater-precipitate   slurry   was
                                        pumped from the third reaction tank to a
                                        thickener where flocculant was added
                                        to enhance sedimentation properties.
                                        The sludge from the thickener under-
                                        flow was further concentrated by using
                                        a  belt drum filter. The  filtrate  was
                                        returned to the reaction tanks. The over-
                                        flow from the thickener was polished in
                                        one of two parallel multilayer filters.
The effluent  from the polishing step
was fed to the fluoride treatment plant.
A 10 percent lime slurry was added to
adjust the pH to 10 or greater. The CaF2
slurry was pumped to a settling pond.
The overflow from this settling  pond
was fed to an inter mediate holding pond
for discharge to the smelter effluent
receiver.
  The 15 percent Na2S solution and the
10 percent lime slurry were prepared
within the plant. The 40 percent sodium
hydroxide solution was purchased com-
mercially. The mixing tanks were instru-
mented with level controls. Occasiona

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overflows from these mix tanks were
directed to the exit stream of the CaF2
precipitation tank.
  Although,  the full-scale plant  had
been in operationforapproximatelyone
year, the plant was still considered to be
in a startup mode, due to the opera-
tional problems related to a higher than
anticipated arsenic level in the waters
treated.
  Process Evaluation. The full-scale
process was evaluated in 1979. A test
plan was devised to determine the heavy
metal and arsenic removal of the overall
process in three short tests (ST-01, ST-
02, ST-03), and to  determine  the per-
formance of the individual process ves-
sels in two more detailed or "long" tests
(LT-01 and LT-02). The important sam-
pling  points are shown in Figure 1. For
the short tests only the incoming streams
(Points 1 and 2) and the effluent stream
(Points 7 and 8) were sampled
  Laboratory Studies.  Bench-scale studies
were also conducted at the University of
Kentucky to identify  the effects of pH
and sulfide dosage on arsenic and zinc
separations. In the full-scale process
considerable variations of reagent dos-
age and pH values were observed. The
effects  of dissolved  S02  (present in
smelter effluents) on  metal sulfide pre-
cipitation were also established.


Process Chemistry
  The extent of metal sulfide precipita-
tion is expected to be a function of pH,
type of metal, sulfide dosage, and other
interfering ions (such as dissolved S02.
SO2") that might be present. The follow-
ing types of reactions take place in the
sulfide reaction tanks:

       M2+ + S2" = M S (Solid)

        M = Hg, Cu, Cd, Pb, Zn

With  arsenic (III) the  primary reactions
are-

      As2O3 + 3 S2" + 3H20 ~
        As2S3 (Solid) + 6 OH"

If the pH is allowed to rise above 6,
AsaS3 solubilizes as:

      2 As2S3 (Solid)+ 2H20  =
        3H+ + As3Si" + HAs02

HgS, As2S3, CuS, CdS, and PbS can be
completely precipitated even at pH 2,
whereas ZnS  precipitation would  be
incomplete at pH < 5 Figure 2 shows
\


I
.c
 0

-2

-4

-6
S-a
  -14

  -16
        CuzS
     02    4   6    8   10  12  14
                   pH

  Figure  2.    Calculated solubilities of
              metal sul fides as a function
              of pH.
 the theoretical solubilities  (calculated
 with a, computer program which included
 all possible reactions) of various metal
 sulfides. Except arsenic, the solubilities
 of other metals decrease with an in-
 crease  in pH values.  Arsenic precipi-
 tates only in acidic pH. In the presence
 of dissolved sulfur dioxide, side  reac-
 tions  between  sulfite  (SO2, S03) and
 sulfide  will also consume some  Na2S
 reagent to form elemental  sulfur and
 thiosulfate (H2S03 + 2H2S - 3S + 3H2);
 4HSO3  + 2HS~ - 3S2O3"  + 3H20).
 These reactions are particularly impor-
 tant for metals with  higher solubility
 (such as, ZnS) in the acidic pH values

 Results and Discussions
   For  the  five full-scale test runs the
 combined (process water + wash water)
inlet concentrations (in mg/l) of metals
were As = 130-450,  Cu = 3-5,  Pb =
20-40, Cd = 3-16, Zn = 30-60,  Hg =
2-4,  Fe - 5-20, Sulfite = 600-1000,
Sulfate = 1500-2000, and Fluoride =
90-130. The five tests represented typi-
cal operation on five separate days, with
no deliberate attempt to vary operating
conditions Table 1 summarizes the oper-
ating conditions for the five tests The
sulfide precipitation pH was in the range
of 3 7 to 4 8 and the CaF2 precipitation
pH was in the range of 11.2 to 11 5 The
approximate sodium sulfide addition rate
(in mg/l of wastewater) ranged between
400  to 900 mg/l. This corresponds to
Na2S dosage fluctuations of 0.8 to 3
times  the  theoretical stoichiometric
dosages
  Figures 3 and 4  show the  results of
the two long-test runs at various sam-
pling points. Point 7 corresponds to the
sulfide precipitation effluent, whereas
Point 8 corresponds to the effI uent from
the entire treatment process The multi-
layer filter effluent contained less than
2  mg/l suspended solids, hence  the
soluble and total concentrations at Point
7 were approximately the same  The
thickener solid  removal efficiency was
about 92%. Figures 3 and 4 show that
during sulfide precipitation  no signifi-
cant improvement in soluble metal con-
centrations is observed  after reaction
tank II. Hg, Cu, Cd, and Pb removal by
sulfide precipitation was always excel-
lent, whereas As an Zn separation was
complete only after lime precipitation.
The  range  of separations and treated
effluent concentrations (for all five tests)
from the overall sulfide-lime treatment
process is shown in Table 2.
  The  removal of  all heavy  metals
(except As and Zn) was excellent (>
99%) even only with Na2S precipitation
atlowpH Table 3 shows the As and Zn
  Table  1.    Operating Data Summary of Heavy Metal Removal Tests

Total Inlet Rate (m3/hr)
Process Water Flow Rate (m3/hr)
Runoff Water Flow Rate (m3/hrl
Runoff Water pH
Reaction Tank 1 pH
Reaction Tank II pH
Reaction Tank III pH
Thickener Overflow pH
Lime Treatment pH
Test 1
ST-01
114
40
74
2.4
3.7
3.4
3.7
4.0
11.3
Test 2
ST-02
128
60
68
2.7
3.7
4.2
4.1
4.0
11.3
Test 3
LT-01
108
31
77
2.3
3.9
4.3
4.0
4.0
11.2
Test 4
ST-03
143
43
100
2.3
4.9
4.8
3.9
3.8
11.5
Test 5
LT-02
131
39
92
2.9
4.2
4.3
4.1
3.8
11.4

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    woo
 I
 .o
 2
 0)
 -Q

 "
                            \         \         \

                         Test Run: LJ-01 IBoliden Metal I)
                            Sulfide Precipitation
                              pH = 3 9 to 4.3
              Precipitation
               pH= 11.2
                         Solid Symbols: Indicate Less Than The
                                      Concentration Shown
                  345678

                            Sampling Point Number

Figure 3.    Residual metal and fluoride concentrations at various sampling points
             (full-scale process) for test run LT-01
separations by the sulfide process (prior
to lime precipitation step) were consid-
erably  lower. The poorer  separations
were due to the pH and sulfide dosage
variations, and consumption of  a por-
tion of the sulfide reagent by 863 pres-
ent in  the inlet wastewater. The zero
separation of zinc and poor separation
of As for the  ST-01 run was primarily
due to the insufficient addition of NazS
reagent.
  Several  observations indicated  the
partial  disappearance of  sulfide from
solution through a pathway otherthan heavy
metal precipitation. These observations
were based on the facts that: (a) no H2S
gas loss from the solutions occurred, in
spite of  apparent stoichiometric over-
doses at acidic  pH  values, (b)  non-
closure  of sulfide  material balance
without  the consideration of thiosu If ate
and elemental sulfur formation; and (c)
the reduction of sulfite concentration in
the sulfide  reaction tanks  The inlet
wastewater to the sulfide precipitation
plant contained  high  concentration
(600-1000 mg/l) of sulfite. Bhattachary-
ya and Sun'71 have conducted extensive
bench-scale stojdies with synthetic and
Boliden Plant wastewaters to establish
the effects of sulfite-sulfide side reac-
tion, and pH variation on arsenic, zmc,
and other heavy metals precipitation.
  Bench-scale studies conducted at the
University of  Kentucky showed that
proper control of pH and sulfide reagent
addition are necessary for effective pre-
cipitation of arsenic and  zinc. With
arsenic  precipitation the pH effect  is
more complicated because the reaction
of arsenic and sulfide  produces OI-T
ions (by the reaction 2HAs02 +  3S2~ =
As2S3(S) +60H~), thus  instantaneous
pH  rise will occur unless maintained
constant by acid addition. Studies with
1000 mg/l As (III) at 1.0 x Na2S dosage
showed that at pH 3 (if not maintained
constant)  if pH  was  allowed to rise
before adjusting to pH 3, the residual
arsenic  concentration  was  ]5  times
higher (30 mg/l instead of residual As
of 2 mg/l with constant pH). Figure 5
(conducted with actual Boliden  waste-
water) shows that arsenic precipitation
is best at low pH, whereas Zn precipita-
tion  is best above pH 5  Hence  a two-
stage precipitation (with As2S3 removal
in the first stage) would be most ideal
  The removal of heavy metals in the
presence  of high sulfite is required  in
various  industrial wastewaters  Exten-
sive bench-scale studies conducted with
As, Zn,  and other heavy metals in the
presence  of 0-1000 mg/l S03 showed
that at pH < 5, side reaction between
sulfite and sulfide consumed a signifi-
cant portion of the Na2S reagent partic-
ularly during precipitation of metals of
higher solubility (such  as  ZnS, NiS)
Figure 6 shows the reduction of SO3(by
sulfide reaction) concentration and the
resultant  formation  of thiosulfate dur-
ing ZnS precipitation at pH 3 Table 4
shows (experiments  with single salts)
that for metals with low solubility pro-
duct (such as, CuS) M2+ -S2~ reaction
predominates over S03 -S2~ reaction.
Higher  reaction pH  also reduced the
importance  of sulfite-sulfide reaction.
Results with  actual  Boliden  wastes
showed that if the reaction was carried
out at pH 4 with 1  OX  stoichiometric
dosage Zn residuals can be reduced to
5-7  mg/l (instead of 15-36 mg/l Zn
obtained with full-scale tests). The con-
sumption of sulfide by sulfite was also
verified in two special full-scale tests,
which showed the formation of thiosul-
fate and elemental sulfur in the reaction

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    1000


    400

     200
till
Test Hun: L T-02 (Boliden Metal I)
       Sulfide Precipitation
         pH = 3.8to4.2
,_[L  / imr>   I
Precipitation
 pH= 11.4
                                                               Fluoride
                                     F
                                  O As
                                  A In
                                  V Pb
                                  D Cd
                                  O Cu
                                     Hg
                             Solid Symbols: Indicate Less
                                           Than The
                                           Concentration
                                           Shown
     001
        1+23        4        5         6        7        8
                              Samp/ing Point Number

Figure 4.    Residual metal and fluoride concentrations at various sampling points
             (full-scale process) for test run L T-02.


Table 2.    Overall Removal of Metals by Sulfide-Lime Precipitation Process (Full-
            Scale Process)
Component
As
Cu
Pb
Cd
Zn
Hg
F
% Removal
96 to 99
>90
97 to 99
99
99
99
70- 78
Effluent Concentration, mg/l
1 8 to 4.1
<0.1
0. 1 to 02.
0.01
0.1 to 03
0.008-0.01
20 to 34
products. Laboratory results also showed
that with excess Na2S dosages (>1 4X)
the sulfide not consumed by metal pre-
cipitation  reacted with sulfite to form
primarily elemental sulfur

Summary and Conclusions
  The full-scale data provided the fol-
lowing results, (a) the combined suIfide-
hme precipitation process provided ex-
cellent removals  for all heavy metals
and arsenic, (b) with sulfide  precipita-
tion (no lime treatment) process at pH
3 5-5 0  although Cu,  Cd, Hg, and Pb
removals were excellent (98-99%), ar-
senic and zinc removals were not always
satisfactory due to improper  operating
conditions andsuIf ite-suIf ide reactions;
(c) excess sulfide dosage caused no H2S
odor problem because of the presence
of dissolved SOs in wastewater (excess
sulfide reacts with sulfite to form thio-
sulfate and sulfur), (d) anionic polymer
provided excellent flocculation of sul-
fide precipitates, and (e) sulfide sludge
was easy to dewater, vacuum filtration
provided 25-30% solids  Laboratory
studies (bench-scale) showed that proper
control of pH (particularlyfor As precipi-
tation) and  sulfide reagent  addition
improved arsenicandzmc precipitation.
The side reaction between sulfite-sulfide
consumed a significant portion of the
Na2S reagent  (to  form  thiosulfate and
sulfur) particularly during precipitation
of metals of higher solubility (such as
ZnS).


References
  1   "Control  and Treatment Technol-
     ogy for the Metal Finishing Indus-
     try: Sulfide Precipitation," EPA
     Technology  Transfer Report No.
     EPA-625/8-80-003 (April 1980).

  2  Lantz, J. B.,  "Evaluation of  a
     Developmental Heavy Metal Waste
     Treatment  System," Naval Con-
     struction Battalion Center Report
     No. ER-314-40-6(1 979).

  3.  Bhattacharyya, D ,etal., "Separa-
     tion of Toxic Heavy Metals by Sul-
     fide  Precipitation," Sep. Sci. and
     Technology. 14, 441 (1979)

  4.  Bhattacharyya, D , era/., "Sulfide
     Precipitation of Heavy Metalsfrom
     Non-Ferrous  Metal Production
     Wastes," 51st Annual Conference
     - Water Pollution Control Federa-
     tion, California (1978).

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Table 3.    Arsenic and Zinc Removals by Sulfide Process (Full-Scale Process)
Test Number
LT-01
LT-02
ST-01
ST-02
ST-03
/Va25 Dosage Reaction pH
~2.7X* 3.9-4.3
~3.7X* 4.7-4.3
~0.8X* 3.4-3.7
~2.0X* 3.7-4.2
=O.9X* 3.9-4.9
% Removal
As Zn
78
82
67
87
78
51
75
0
69
55
*Even with excess Na^S no H2S gas was formed because of sulfite-sulfide reactions.
 t

.0
 «J
 c
 u
 o
o
N
0)
cc
   30
   20
    10
I  0
    65

    60

.O
    50
    40
O  30
 c
 CD
    20
    10
Figure 5.
                                                Raw Feed
                                      4
                                     pH
                                      i
            NazS dosage = 1 .OX
                                               Q  Boliden ST-03 Feed
                                               A  Boliden LT-01  Feed
                                               D  Boliden ST-01 Feed
                      3456

                                      pH

             Residual arsenic  and zinc concentrations after sulfide precipitation
             (bench-scale process).
                                                                                  5.   Larson, H. P.,  and Ross, L.  W.,
                                                                                      "Two-Stage  Process  Chemically
                                                                                      Treats Mine Drainage to Remove
                                                                                      Dissolved Metals," Operating Hand-
                                                                                      book of Mineral Processing, 349
                                                                                      (February 1976).

                                                                                  6.   Schlauch, R. M., and Epstein, A.
                                                                                      C., "Treatment of Metal Finishing
                                                                                      Wastes by Sulfide Precipitation,"
                                                                                      EPA-600/2-77-049 (1979).

                                                                                  7.   Bhattacharyya, D., and Sun, G.,
                                                                                      "Precipitation of Heavy Metals and
                                                                                      Arsenic with Sodium Sulfide: Sulfite-
                                                                                      Sulfide Interaction," In Prepara-
                                                                                      tion. (1980).
                                  6

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     16
oo

 5
 a}

1
 00
 o
 3
 CO
 0
 c
 Q)
 O

 O
 CJ
  5
 Q)
                  I
            System:
                           \
       Zn, = 500 mg/l (7.7 mM)
       (S03),= 1000 mg/1(12.5 mM)
       NazS Dosage =1.0X(7.7 mM)
                            10
                          15        20

                     Time, Minutes
                25
                                                                      30
Figure 6.
Consumption of su/fite by sulfide during ZnS precipitation in the pres-
ence of Na2SO3(bench-scale process).
Table 4.    Consumption of Sulfide by
             Sulfite  During  Metal-
            Sulfide Precipitation

Initial metal concentration =100 mg/l
Initial NaiSOyConcentration = 1000 mg/l
NazS dosage = 1 .OX
System
               Reaction
                 PH
  Na2S03
As(lll)-Na2S-
  Na2S03
  Na2S03
             % Sulfide
            Consumed
             by Sulfite
     3
     4
     3
                            33.6
                            12.8
                             2.1
D. Bhattacharyya is with the University of Kentucky, Lexington, KY; C. Sund-
  Hagelbergis with BolidenMetallCorporation, Sweden; K. Schwitzgebel, G. M.
  Blythe, and J. C.  Terry are with Radian Corporation. Austin, TX; F. B. Craig
  (also the EPA Project Officer, see below) is with the Industrial Environmental
  Research Laboratory, Cincinnati, OH 45268.
The complete report, entitled "Characterization of Boliden's Sulfide-Lime Pre-
  cipitation Plant," (Order No. PB 81 -209 2 72; Cost: $10.50 (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
                                                                                  S. GOVERNMENT PRINTING OFFICE: 1982/559 -092/338I

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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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Fees Paid
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Agency
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Penalty for Private Use $300

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