CA1186423A - Thiosulfate removal from aqueous streams - Google Patents

Abstract

ABSTRACT OF THE INVENTION

The thiosulfate content of waste waters containing the same is removed by treatment with sulfur dioxide or an alkali or alkaline earth metal sulfite or bisulfite in the presence of excess oxygen and a metal catalyst which is preferably copper.

Description

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 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 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/CAN/l 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 U~ ~ N
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tn h PC-2130/C~/l 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.

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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.

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PC-2130/CAN/l 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.

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t~ h PC- 213 0/CA N/l 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~

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 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 EFFLUENT20 1. 4 0.4 0 0 2.8 0 EFFLUENT40 2 . 9 190 0 0 0 49 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 EFFLUENT50 3 . 6120 129 242 0 67 EFFLUENT50 3.6 55 127 132 14 36 EFFLUENT33 2 . 40.5 112 77 0.2 2.5 EFFLllENT50 3 . 6 1 1 1 0 .1 2 . 0 PC-2130/CAN/l 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 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 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 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

Claims (15)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The method for reducing the thiosulfate content of an aqueous solution containing the same which comprises treating said solution with a reagent from the group consisting of sulfur dioxide, an alkali or alkaline earth metal sulfite or bisulfite, and an oxygen-containing gas at a pH of about 2 to about 14 in the presence of a metal catalyst.

2. The method in accordance with claim 1 wherein the catalyst is copper.

3. The method in accordance with either of claims 1 or 2 wherein said solution is treated in continuous fashion in one or more stages.

4. The method in accordance with claim 1 wherein the pH is about 5 to about 10.

5. The method in accordance with any of claims 1, 2 or 4 wherein pH control is effected by an alkali or alkaline earth metal base.

6. The method in accordance with either of claims 1 or 2 wherein the pH is below 6.5 and the base is limestone.

7. The method in accordance with any of claims 1, 2 or 4 wherein treated solution containing catalyst metal in precipitated form is contacted with fresh solution containing thiosulfate species whereby said catalyst effects further impurity removal.

8. The method in accordance with claim 1 wherein the mixture of sulfur dioxide and air employed contains about 0.1 to about 6%
sulfur dioxide, by volume.

9. The method in accordance with claim 1 wherein said solution contains up to 1000 ppm of thiosulfate.

10. The method in accordance with claim 1 wherein sulfur dioxide and air are employed in admixture.

11. The method in accordance with claim 8 wherein said sulfur dioxide and air are introduced with agitation.

12. The method in accordance with claim 1 wherein an agent from the group consisting of sulfur dioxide and alkali and alkaline earth metal sulfites and bisulfites is first introduced into said solution and oxygen is thereafter introduced.

13. The method in accordance with claim 12 wherein said solution is quiescent during said oxygen introduction.

14. The method in accordance with claim 1 wherein an agent from the group consisting of calcium sulfite, calcium bisulfite, sulfur dioxide and lime are employed and the solution pH is maintained in the range of 5 to 7.

15. The method in accordance with claim 1 wherein the process is carried out at a pH in the range of 2 to about 8 and the pH is raised to about pH 10 after thiosulfate removal to hydrolyze metals contained in the solution, thereby permitting metal recovery by solid-liquid separation.