Note: Descriptions are shown in the official language in which they were submitted.
PC-2130/CAN/l
THIOSULFATE ~EMOVAL FRO~ AQ~EO~S STRRAMS
The invention is directed to a process for the removal
of thiosulfate from aqueous solution~, such as waste waters, and
more particularly, to a process which can be applied to industrial
effluents containing thiosulfate to remove thiosulfate and associ-
ated ions effectively at a high reaction rate.
Thiosulfate ion occurs in various types of waste water
streams resulting from processes in which thiosulfates are used,
e.g., photographic processing, or from processes in which it may
be generated, e.g., froth flotation of metal sulfides. Awareness
of thiosulfate ion content of waste water streams is required
since excessive con~ent of this ion is biologically undesirable.
Thus, thiosulfate ion can impose an excessive oxygen demand in a
stream containing the same. Methods are known which are capable
of reducing the thiosulfate ion content of aqueous streams, for
example, alkaline chlorinationO The known methods are expensive
in terms of reagent cost. Improved and cheaper methods are needed.
BRIEF DESCRIPTION OF THE INVENTION
In accordance with the invention, thiosulfate species
present in waste water streams are decomposed by treatment of the
waste water with a mixture of sulfur dioxide and air or oxygen.
The treatment is effective at any pH in the range of 2 to 14 but
is preferably conduct2d in a pH range between about 5 and 10~
Removal of the thiosulfate species other than copper from waste
water streams i5 very slow with SO2 and air alone. The presence
of copper catalyzes the removal of thiosulfate species from the
stream. Once the cyanide species are removed if present, thio-
sulfate and related species can be removed by continued treatment
with sulfur dioxide and oxygen or air in the presence of copper
with or without an addi~ional metal such as nickel, cobalt, or
manganese which then act catalytically in the stream. The
PC-2130/CAN/l
-- 2 --
thiocyanate species i5 removed effectively using nickel as a
catalyst with or without copper.
Control of pH is effected by any alkali or alkaline-
earth metal hydroxide or carbonate. Limestone can be used in khe
pH range 2 to 6.5. Metals present in the effluents treated in
accordance with the invention can be recovered as oxides or
hydroxides by adjusting the pH of the treated waste water to the
range of about 9 to abou~ 10. The metal species employed as
catalyst can thus be recovered and recycled, if desired.
Alkali or alkaline-earth metal sulfites can be employed
in place of the sulfur dioxide - air or oxygen mixture.
The process can be carried out batchwise or continu-
ously using one or several stages, depending on the objectives
with respect to species to be decomposed and metals to be
recovered.
The necessary reagent can be prepared, for example by
scrubbing a stack gas containing typically 0.1 to 5~ S02, 1-5%
C2 with lime or limestone as base to produce a suspension or
slurry containing calcium sulfite or bisul~ite. Alternatively, a
~o stack gas, as before described, can be used as a primary reagent
along with lime or limestone as a base. When using calcium sulfite
or bisulfite, an operating p~ of about 5 to about 7 is desirable,
since at higher pH dissolution of calcium sulfite becomes too
slow~ It will be appreciated in this connection that the action
of sulfur dioxide and oxygen in wa~er solution results in the
production of sulfuric acid whlch must be neutralized resulting
in gypsum formation when lime or limestone is used as base to
control pH~ A low operating p~ of 5 to 7 is preferred when using
sparingly soluble sulfites such as calcium sulfite or sulfur
dioxide and lime so as to reduce the amount of unreacted sulfite
and gypsum in the metal, including gold, silver and platinum-
group metal precipitates. The required amount of sulfite can be
added at once and the r~quired air or oxygen addition can be added
separately. In similar fashion, (and bearing in mind the need
for pH con~rolj the required amount of sulfur dioxide can be added
initially with the air or oxygen requirement added separatelyO
~he rate of oxygen supply can be used ~o control reaction kinetics.
PC-2130/CAN/l
-- 3 --
With respect to the thiosulfate species the waters
treated rarely contain more than about 1000 ppm and more ordinarily
will contain no more than about 200 ppm. Thiosulfate species can
be reduced to as low as 0.1 ppm (0.1 milligram per liter) or lower
in a very short treatment time in accordance with the invention.
PREFERRED CONDITIONS
In removal of thiosulfate from waste waters in accordance
with the invention, the preferred ingredients are sulfur dioxide,
air and lime. The temperature may be in the range of 0 to 100C
and the operating pH about 5 to about ~ or 10. Sulfur dioxide
preferably is dispersed in the water to be treated as a mixture
of 0.1 to 6% by volume in air. For this purpose, reactors used
in flotation technology are entirely suitable either for adding
SO2-air mixtures or for adding air alone to water solutions or
pulps.
The preferred catalyst is copper which should be present
preferably in a weight ratio of copper to total cyanide of at
least about 0.25 gm/gm to obtain efficient utilization of sulfur
dioxide and air, together with high reaction kinetics.
Some examples will now be given.
~XAMPLE 1
This example illustrates the effect of pEI in the range
6.5 to 10 on cyanide removal in cyanide-containing solution using
502 gas, air and lime. Feed solution analyzing in mg/l, CN- 164,
CNS- 127, S2O3 437~ Ni~+ 71 and Cu+ 34 was placed in a two liter
capacity resin reaction kettle and agitated at l,000 rpm by means
of a titanium impeller. Sulur dioxide gas, pre-mixed with air
(1.2% by volume SO2) was added at a constant rate of 2g per liter
of solution per hour ~hrough a fritted glass filtPr inlet placed
adjacent to the impeller blades. The pH of the reaction mixture
was controlled by addition of a one molar lime-water slurry on
demand by means of a Radiome~er Titrator pH controller.
Results are shown in Table I. The opti~um pH for cyanide
removal was around g. At this pH, CN-, 5203, Ni++ and Cu+ in
solution were all removed to relatively low values. At pH I 5 below
9, some of the Ni remains in solution~ This Ni, however, can be
~3
PC-2130/CAN/l
.~ _
hydrolyzed at higher pH. At pH's above 9, longer reaction times
are required and the efficiency of the reagents decreases. In
all cases, CNS- decomposition was incomplete in the time allowed
for the tests.
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PC-2130/C~N/l
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PC-2130/CAN/l
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EX~MPLE 2
This example illustrates the efect of SO2 addition
rate. Feed solution analyzing in mg/l, CN- 164, CNS ~ 127, S2O3
437, Ni~ 71 and Cu~ 34 was placed in a two liter capacity resin
reaction kettle and agitated at ltO00 rpm by means of a titanium
impeller. Sulfur dio~ide gas was added at rates in the range
0.36 to 17g per liter per hour, pre-mixed with air which was added
at a constant rate of 60 liters per liter per hour. Therefore
the SO2-air mixture varied in composition from 0.2 to 10% by volume
SO2. The pH of the reaction mixture was maintained constant at 9
with a one molar lime-water slurry added on demand by means of a
Radiometer pH controller. Results are given in Table II.
The SO2 efficiency for CN- removal decreased wi~h
increasing SO2 addition rate~ This may be attributed to both
inadequate dispersion of air and a lack of available oxygen at
the higher SO2 addition rates~ ~th CN- and S2O-3 can be decomposed
in the range of SO2 addition rates studied. Decomposition of SCN-,
however, was only achieved with SO2 addition rates below about ~g
per liter per hourr
PC-2130/CAN/l
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PC-2130/C~/l
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EXAMPLE 3
This example shows that thiosulfate decomposition can
also be attained in 40 minutes of reaction using calcium or sodium
sulite instead of sulfur dioxide gas in combination with air.
In each test, 1.19 of SO2 wa5 added slowly either as CaS03 slurry
containing 18.3 9/1 CaO and 22.2 9/1 SO2 or as Na2SO3 crystals
respectively, to feed cyanide water analyzing in mg/l, CN- 110,
SCN- 1~0, S2o3 274, Cu+ 38.4 and Nit+ 47. The reaction mixture
was then aerated at 1 liter per liter per minute and H2S04 was
added slowly to lower the p~ from about 9 to 5.5. After 40 minutes
the final liquor was re-neutralized with lime to pH 10 and the
precipitated metal hydroxides were removed by filtration.
Results are shown in Tables III and IV.
PC- 2 1 3 O/CAN/l
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PC-2130/CA~/1
EXAMPLE ~
An example of one stage continuous decomposition of
cyanide in cyanide-bearing waste water using SO2, air and lime is
given in Table V. The reaction was carried out in a 450 ml
capacity stirred beaker with continuous addition of pre-mixed SO2
and air through a fritted glass inlet tube and lime addition on
pH demand at pH 9. About 1.5 9/1 of SO2 was added as a 1.75%
mixture in air with a solution retention time of 30 minutes. The
reacted suspension was collected in beakers where the pH was
adjusted to 10 with lime. Under the above conditions the concen-
trations of CN-, Ni~+ and Cu+ in solution were all lowered to
be~ow 0.5 mg/l. About 80~ of the thiosulfate was decomposed.
Only a minor amount of the thiocyanate was removed~ Decomposition
of CNS- species would require longer retention time and higher pH
to be completed.
PC-21 3u/CAN/l
-- 12 --
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PC-2130/CAN/l
-- 13 --
EXAMPLE 5
An example of a two stage continuous treatment of cyanide
containing effluent using synthetic stack gas (0.6% SO2, 1% CO2
and air) and lime at 24~C is shown in Table VIo The bulk of the
cyanide is decomposed in the first stage with a solution retention
time of 4n6 minutes and an addition of stack gas of 40.6 liter
per liter of solution, maintaining the pH at 6.5 to 7.0 with lime.
The remaining cyanide is decomposed in the second oxidation stage
with a solution retention time of 2.8 minutes and a stack gas
addition of 24.8 liter per liter of solution treated, maintaining
the pH at 6.5 with lime. The treated effluent is then neutralized
with lime to pH 10 and filtered to recover the precipitated metal
hydroxides. The above treatment leads to complete cyanide destruc-
tion, recovery of all dissolved Ni and Cu in a high grade precipi-
tate at approximately 20% Ni + Cu by weight and recovery of part
of the Pt and Pd and 80% of the Ay in the precipitate. Gold was
not recovered in this paxticular test. Most of the thiosulfate
but none of the thiocyanate is removed under the above reaction
conditions.
PC-2130/CAN/l
-- 14 --
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PC- 213 0/CA N/l
-- 15 --
EX~PLE 6
Examples given in Table VII illustrate the effect of
temperature on the decomposition of cyanide species and thio-
sulfate. The tests were carried out batchwise using synthetic
stack gas containing 0.6~ SO~ CO2 in air and lime for pH
control. The examples of tests 2, 3 and 4 indicate that tempera-
ture does not affect the removal efficiency of to~ic cyanide
species over the temperature range investigated, at pH 9, but
affects the efficiency of thiocyanate and thiosulfate removal.
Almost no thiocyanate is decomposed at 1C. Examples of tests 1
and 2 show that at 1C, cyanide species and thiosulfate are
removed more efficiently at pH 9 than at pH 5.
3 PC-2130/C~
- 16 -
TABLE VI I
C~ D~
Cc~ndition~: b~t~h, S02 ~daed as simulated 3tack gas
(- 6~ SC32 t 1% C02 t air3 at rate of
O . 86 g/l/h (tests 1, 2 and 4 ) and of
0.34 g/l/h~test 3~; lime as ba e.
T~st Stream T pH SO2 Analyses
(min3 t&) Adaed (mg/l.L
~gJl~CMToT SCN S2O3 Cu Ni
F~ed 0 1 9 . 5 0 173 410 ~S0 24 64
Effluent 240 1 5 3.4 1.7 376 170 14 2
Effluent 300 1 5 4.3 0.2 364 63 4 23
2 ~ed û 1 9 . 5 0 173 398 368 26 73
Effluent 60 1 9 0 . 86 0. 8 372 215 24 62
E:fluent 300 1 9 4 . 3 0 . 3 313 36 0 . 31. 8
3 Feed 0 24 9 . 5 0 162 231 224 29 79
E~fluerlt150 24 - 9 8 . 35 1. 4 182 1~ ~ 31. 9
~Sffluent300 24 9 1.7 3.5 2 9 0.2 2.1
4 Feed 0 24 9. 5 0 173 381 224 2B :104
~fluent 60 S0 9 0 . 86 0 . 8 371 18 0 . 60 . 5
luent 180 50 g 2.6 0.6 2 s5 0.3 0.4
PC-2130/CA~/l
- 17 -
EXAMPLE 7
Examples illustrating the effect of copper on decomposi-
tion of cyanide species and thiosulfate are given in Table VIII.
With no copper present, cyanide was removed at a very slow rate
(Test C-l, C-3, C-~ and C-6). The examples of tests C-2 and C-5
show that copper acts as a catalyst not only for CN- removal, but
also for thio~ulfate decomposition. The tests show that copper
is a catalyst for CN ~ and S20~ decomposition (C-5), while nickel
acts as a catalyst for the removal of thiocyanate once CN- has
been removed (C-8~.
PC-2130/CAN/l
TABLE VIII
EFFECT OF COPPER
CONDITION5: batch, 22~23C, pH 9, SO2 addition rate of 4.3 g/l/h,
air addition rate of 60 l/l/h.
S2 Analyses
Time Added (mg/l~
Test Stream (min) (g/lj CNtot SCN- S2O~ Cu Ni
C-l FEED 0 0 178 0 0 0 0
EFFLUENT40 2 . 9 100 0 0 0 0
C--2 FEED 0 0 218 0 0 87 0
EFFLUENT20 1. 4 0.4 0 0 2.8 0
C--3 FEED 0 0 239
EFFLUENT40 2 . 9 190 0 0 0 49
C-4 FEED 0 150 164 330 0 0
EFFLUENT50 3.6 99 200 212 0 0
C-5 F~ED 0 0 150 153 362 21
EFFLUENT35 6 0.5 195 3 0.2 0
C-6 FEED 0 0 160 156 352 0 74
EFFLUENT50 3 . 6120 129 242 0 67
C-7 FEED 0 0 150 154 341 19 79
EFFLUENT50 3.6 55 127 132 14 36
C-8 FEED 0 0 200 173 350 40 77
EFFLUENT33 2 . 40.5 112 77 0.2 2.5
EFFLllENT50 3 . 6 1 1 1 0 .1 2 . 0
PC-2130/CAN/l
-- 19 --
EXAMPLE 8
E~ample illustrating removal of cyanide and thiosulfate
from gold mill effluents is shown in Table IX. As can be seen
all species except SCN- were decomposed and all metal values were
hydrolyzed out of solution, including Zn, Fe and As. In all cases,
Cu was present initially.
PC-2130/CAN/ï
~ 20 -
TABLE I X
CYANIDE REMOVAL FROM SIMULATED GOLD MILL_EFFLUENT
CONDITION5: - batch 22C, pH 9, air addition rate of 60 l/h/l,
lime as base
~ S2 addition rate: Test G-6: 107 g/l/h;
S2 Analyse
Time added (mg/l)
Test (min) ~g/l~CNToT SCN-S2O3- Cu Ni Zn Fe As
G-6 0 0 225 390 232 48.3 1.6 5.5 95 N.D.
0.56 204 350 -- 48.3 1.40 5.6 92 --
1.12 31 -- -- 14.0 0.80 4.~ 60 --
1.41 0.~ - 1.0 0.40 c0.~1 2.4 --
1.700.06 336 <0.5 1.0 <0.40 <0.4 0.9 --
N.D. 0.01 mg/l
J~
PC- 2 1 3 O/CAN/l
-- 21 --
EXAMPLE 9
This example illustrates the catalytic effect of some
metals in the decomposi~ion of thiosulfate in thiosulfate-
containing effluents using S02, air and lime (Table X). In the
absence of metal, decomposition of S20~ is slow (Test S-l)~ Of
the metals tested, copper was the best (Test S-2), and nickel had
no cataly~ic effect (Test S-3). Cobalt, although present as a
precipitate at pH 9, also acts as a catalyst for S2Q~ decom-
position (Test S-4). Both iron and manganese have some catalytic
effect on S20~ decomposition at pH 4 and 6 respectively, but no
catalytic effect of manganese was observed at pH 9 (Tests S-5, S-
6 and S-73.
PC-2130/CAN/l
- 22 ~
TABLE X
CA~ALYTIC DECOMPOSITION OF THIOSULFATE
U5ING S02, AIR AND LIME
CONDITIONS: batch, 22-23C, pH controlled with lime,
addition of 4.2 g SO2 per l/per h, air addition at 80 1/h/1
EFFLUENT ANALYSES
Catalyst
S2 Amount In
Time Added 520~ Solution
Test (min) ~ ~H (mg71) ~y~ (mg~13
S-l 0 0 9.0301 None 0
9.0240 ~-- 0
S-2 0 0 9.0320 Cu 33
1.40 9.034 0.6
S-3 0 0 9.0378 Ni 45
~0 2.80 9.0350 0.
S-4 0 0 9.0373 Co 0.5
2.80 9.01 0.2
S-5 0 0 5.6456 Fe --
1.13 4.0157 --
S-6 0 0 6.0431 Mn --
6D 1.13 6.0161
5-7 0 0 9.0373 Mn N.~.
2.80 9.0310 N.D.
* 50 mg/l of metal catalyst was added as metal sulfate
~ 3~
P~-2130/CA~J/l
- 23 -
EXAMPLE 10
Examples of cyanide removal from effluents also
containing between 15 and 62% by weight solids (mainly pyrrhotite-
iron sulfide~ using S02, air and lime are shown in Table XI. No
CN- is decomposed in the absence of copper addition (Test P-4).
In this case, a large amount of thiosulfate is produced through
oxidation of the pyrrhotite. The examples of Tests P-l, P-2 and
P-3 indicate that CN-, complexed metal cyanide and S203 are all
decomposed when adding a CuS04 solution continuously during the
treatment with SO~, air and lime. In the presence of 15.3% solids,
about 0.25 grams of copper per gram of CN- is required and in the
presence of 61.8% solids, about 1 gram of copper per gram of CN~
i5 required.
PC-2130/CAN/l
- 24 -
TABL$ XI
CYANIDE REMOVAL FROM EFFLUENT IN THE
PRES~N OE OF SULFIDES (P~RR~OTIT~:FeS~
COND~TIO~S: batch, 22-23C, R~ 8 controlled with lime, SO2 addition rate; 2.1
o~ 4.2 g/l/h; alr addition rate: 60 l/h/l; copper addition on a continuous
basis as CuSO4 solution.
cu
added EFFLUENT ANALYSES
SO2 as (mg/l)
~ s~lids Time added CuSO4
Test (weight) (min) ~g/l? (mg/l) ~ SCN- S203= Cu Ni Fe
P-l 15.3 ~ 0 0 160 ~1 80 ~.9 56.1 1.1
1~4 50 41 54 23 lB.7 36.3 0.2
1.7 64 1 50 4 2.5 15.4 1.0
~.2156 1 48 2 0.7 2.7 0.2
P 2 15.3 0 0 0 145 60 76 5.6 56.1 1.3
2.8 26 92 60 25 15.4 45~1 0.~
3.5 32 1 53 6 2~6 17.6 0.2
4.2 3g 1 50 8 0.3 6.2 0.2
P-3 61.8 0 0 0 152 452457 11.9 29.7 10.6
1.~112 8 400 62 14.3 10.1 0.7
2.1164 0.8 372 33 0.8 2.3 0.5
P~4 6108 ~ 0 0 113 35U480 17.1 33~0 13.1
2.1 0 113 4361,1820.9 ~7~3 0.4
