Cyanide Removal From Aqueous Streams

The invention is directed to a process for the removal of cyanide from aqueous solutions, such as waste waters, and more particularly, to a process which can be applied to industrial effluents containing cyanide to remove cyanide and associated ions effectively at a high reaction rate.

Cyanides are useful materials industrially and have been employed for years in fields such as electroplating of metals, gold recovery from ores, treatment of sulfide ore slurries in flotation, etc. The toxicity of cyanides is, of course, well known and control of liquid effluents from plants employing cyanides industrially have been actively explored with some success. It is now recognized that effluent waters from industrial plants should not contain cyanides at levels in excess of about 0.1 milligram per liter (ppm) upon discharge. It is known that in many installations, cyanide ion will form complexes with heavy metals [Me(CN).sub.x.sup.V- ] and that related ionic species such as thiocyanates (CNS.sup.-), cyanates (CNO.sup.-) and associated species, e.g., thiosulfates (S.sub.2 O.sub.3.sup.--) may also occur in the waste waters. Processes involving the use of cyanide generate vaste quantities of water bearing cyanide and associated ions in quantities exceeding those regarded as being safe for discharge into streams. Gold mining, alone, is responsible for the generation of large amounts of cyanide laden water.

General awareness of the problem has resulted in a number of techniques for cyanide disposal in waste waters including oxidation by chemical or electrochemical means, ion exchange, and AVR (acidification-volatilization-reneutralization). These processes are operative but are accompanied by serious practical disadvantages including high costs. Thus, chemical oxidation of cyanide can be achieved with strong oxidants such as chlorine gas or hypochlorite, ozone, peroxide, peroxysulfates or oxygen in the presence of a catalyst. Alkaline chlorination is the most commonly used approach. In addition to high reagent costs, chloride ion contamination of the water being treated and potential formation of cyanogen chloride can result. Ozonation treatment of effluents is clean and efficient, but involves high initial capital costs as well as high energy cost. Peroxides and peroxysulfates are expensive reagents for cyanide oxidation in dilute water solution. Pressure oxidation using oxygen and a catalyst, such as nickel metal or activated carbon, can be employed to destroy the cyanide content of waste waters, but the reaction rates are low and the use of expensive pressure vessels is required.

Electrolytical oxidation of aqueous cyanide solutions, while being feasible, involves the use of sophisticated electrolytic cells and high capital investment.

Ion exchange offers the potential for recovering metal values and cyanide but is accompanied by the possibility of precipitation of metals within the resin bed and fouling, a high requirement for sulfuric acid, high capital cost and a hazard associated with HCN recovery. It involves sophisticated technology and careful control of a kind not customarily available at remote mine sites or in waste water treatment facilities.

The AVR approach also offers the potential for recovering cyanide through the generation of HCN vapor. This is inherently a hazardous operation in which energy requirements are high, reaction kinetics are slow and it is difficult to achieve acceptably low residual cyanide levels in the effluent.

While progress has been made in dealing with the problem, it is clear that better, cheaper, more effective methods are needed to cope with a problem having the scope of the cyanide contamination of waste waters. Thus, a process which effectively removes cyanides and associated ions from waste waters but which uses low cost reagents, has high reaction kinetics, and can be carried out in simple equipment with low capital investment and low energy requirement is needed.

In accordance with the invention, free and complex cyanide species present in waste water streams are decomposed by treatment of the waste water with a mixture of sulfur dioxide and air or oxygen in the presence of a water-soluble copper catalyst. The treatment is effective at any pH in the range of about 5 to about 12, but is preferably conducted in a pH range between about 5 and 10, e.g., around 9. Removal of the free cyanide and complex metal cyanide species from waste water streams is very slow with SO.sub.2 and air alone. The presence of copper catalyzes the removal of free cyanide, complex heavy metal cyanide cyanate and thiosulfate species from the stream. Once the cyanide species are removed, related species can be removed by continued treatment with sulfur dioxide and oxygen or air in the presence of a metal ion (simple or complex) such as nickel, cobalt, or manganese which then acts catalytically in the stream. The thiocyanate species is 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 the pH range about 5 to about 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 about 10. The metal species employed as catalyst can thus be recovered and recycled, if desired.

Alkali or alkaline-earth metal sulfites (including bisulfites and metabisulfites) can be employed in place of the sulfur dioxide-air or oxygen mixture.

The process can be carries out batchwise or continuously 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% SO.sub.2, 1-5% CO.sub.2 with lime or limestone as base to produce a suspension or slurry containing calcium sulfite or bisulfite. Alternatively, a 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 pH 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 water solution results in the production of sulfuric acid which must be neutralized resulting in gypsum formation when lime or limestone is used as base to control pH. A low operating pH of 5 to 7 is preferred when using sparingly soluble sulfites such as calcium sulfite 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 required air or oxygen addition can be added separately. In similar fashion, (and bearing in mind the need for pH control) the required amount of sulfur dioxide can be added initially with the air or oxygen requirement added separately. At least about 2.5 grams of sulfur dioxide are required per gram of CN.sup.- to be removed; whereas at least about 4.5 grams of sulfur dioxide are required per gram of CNS.sup.- to be removed. The rate of oxygen supply can be used to control reaction kinetics.

Where recovery of high grade metal hydroxides is desired, soluble alkaline sulfites such as sodium sulfite or SO.sub.2 and sodium hydroxide or carbonate can be used. Metals which can be recovered, if present in the waste water treated in accordance with the invention include copper, nickel, cobalt, zinc, cadmium, manganese, iron, arsenic, silver, gold, platinum and palladium. The presence of a small amount of xanthate in the waste water assists in gold recovery. To obtain a precipitate of metal hydroxide at the conclusion of the cyanide removal reactions the pH of the solution desirably should be elevated to the range of 9-10. When this is done precipitation of the metal hydroxide results, with high metal recoveries.

It is to be appreciated that the waste waters treated in accordance with the invention are indeed very dilute with respect to the species to be removed. Thus, with respect to the cyanide species the waters treated rarely contain more than about 1000 ppm and more ordinarily will contain no more than about 200 ppm. Cyanide species can be reduced to as low as 0.1 ppm (0.1 milligram per liter) in a very short treatment time in accordance with the invention, when a copper catalyst is co-present with the reactants sulfur dioxide and oxygen. Cyanide removal to levels well below 0.1 ppm has been achieved.

In removal of free cyanide and cyano-metal complexes 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.degree. to 100.degree. C. and the operating pH about 5 to about 9 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 SO.sub.2 --air mixtures or for adding air alone to water solutions or pulps.

The catalyst for free cyanide and complex cyanide removal 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. Thiosulfate ion can also be removed from the solution under these preferred conditions once cyanide is removed.

In the case of thiocyanate ion removal, the same reagents are preferred using a temperature in the range of about 25.degree. C. to 100.degree. C. and with a pH being less than about 9. The SO.sub.2 addition rate should be at least about 2 grams per liter per hour as a 0.1 to 1% volume proportion of sulfur dioxide in air. Again a metal catalyst, preferably nickel, should be present to provide high reaction kinetics.

For purposes of this specification and claims, the expression "total cyanide" used herein includes free cyanide and metal cyanide complexes. The term "metal catalyst" refers to metal in solution. Essentially water-insoluble compounds such as copper hydroxide, per se, are not effective catalysts.

Some examples will now be given.

This example illustrates the effect of pH in the range 6.5 to 10 on cyanide removal in cyanide-containing solution using SO.sub.2 gas, air and lime. Feed solution analyzing in mg/l, CN.sup.- 164, CNS.sup.- 127, S.sub.2 O.sub.3.sup.-- 437, Ni.sup.++ 71 and Cu.sup.+ 34 was placed in a two liter capacity resin reaction kettle and agitated at 1,000 rpm by means of a titanium impeller. Sulfur dioxide gas, pre-mixed with air (1.2% by volume SO.sub.2) was added at a constant rate of 2 g per liter of solution per hour through a fritted glass filter 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 Radiometer Titrator pH controller.

Results are shown in Table I. The optimum pH for cyanide removal was around 9. At this pH, CN.sup.-, S.sub.2 O.sub.3.sup.--, Ni.sup.++ and Cu.sup.+ in solution were all removed to relatively low values. At pH's below 9, some of the Ni remains in solution. This Ni, however, can be 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.sup.- decomposition was incomplete in the time allowed for the tests.

This example illustrates the effect of SO.sub.2 addition rate on the removal of cyanide from cyanide containing water using SO.sub.2, air and lime. Feed solution analyzing in mg/l, CN.sup.- 164, CNS.sup.- 127, S.sub.2 O.sub.3.sup.-- 437, Ni.sup.++ 71 and Cu.sup.+ 34 was placed in a two liter capacity resin reaction kettle and agitated at 1,000 rpm by means of a titanium impeller.

Sulfur dioxide gas was added at rates in the range 0.36 to 17 g per liter per hour, pre-mixed with air which was added at a constant rate of 60 liters per liter per hour. Therefore the SO.sub.2 -air mixture varied in composition from 0.2 to 10% by volume SO.sub.2. 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 SO.sub.2 efficiency for CN.sup.- removal decreased with increasing SO.sub.2 addition rate. This may be attributed to both inadequate dispersion of air and a lack of available oxygen at the higher SO.sub.2 addition rates. Both CN.sup.- and S.sub.2 O.sub.3.sup.-- can be decomposed in the range of SO.sub.2 addition rates studied. Decomposition of SCN.sup.-, however, was only achieved with SO.sub.2 addition rates below about 2 g per liter per hour.

This example shows that complete cyanide decomposition can also be attained in 40 minutes of reaction using calcium or sodium sulfite instead of sulfur dioxide gas in combination with air. In each test, 1.1 g of SO.sub.2 was added slowly either as CaSO.sub.3 slurry containing 18.3 g/l CaO and 22.2 g/l SO.sub.2 or as Na.sub.2 SO.sub.3 crystals respectively, to feed cyanide water analyzing in mg/l, CN.sup.- 110, SCN.sup.- 120, S.sub.2 O.sub.3.sup.-- 274, Cu.sup.+ 38.4 and Ni.sup.++ 47. The reaction mixture was then aerated at 1 liter per liter per minute and H.sub.2 SO.sub.4 was added slowly to lower the pH 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.

An example of one stage continuous decomposition of cyanide in cyanide-bearing waste water using SO.sub.2, 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 SO.sub.2 and air through a fritted glass inlet tube and lime addition on pH demand at pH 9. About 1.5 g/l of SO.sub.2 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 concentrations of CN.sup.-, Ni.sup.++ and Cu.sup.+ in solution were all lowered to below 0.5 mg/l. About 80% of the thiosulfate was decomposed. Only a minor amount of the thiocyanate was removed.

Decomposition of CNS.sup.- species would require longer retention time and higher pH to be completed.

An example of a two stage continuous treatment of cyanide containing effluent using synthetic stack gas (0.6% SO.sub.2, 1% CO.sub.2 and air) and lime at 24.degree. C. is shown in Table VI. The bulk of the cyanide is decomposed in the first stage with a solution retention time of 4.6 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 destruction, recovery of all dissolved Ni and Cu in a high grade precipitate at approximately 20% Ni+Cu by weight and recovery of part of the Pt and Pd and 80% of the Ag in the precipitate. Gold was not recovered in this particular test. Most of the thiosulfate but none of the thiocyanate is removed under the above reaction conditions.

Examples given in Table VII illustrate the effect of temperature on the decomposition of cyanide species and thiosulfate. The tests were carried out batchwise using synthetic stack gas containing 0.6% SO.sub.2, 1% CO.sub.2 in air and lime for pH control. The examples of tests 2, 3 and 4 indicate that temperature does not affect the removal efficiency of toxic 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 1.degree. C. Examples of tests 1 and 2 show that at 1.degree. C., cyanide species and thiosulfate are removed more efficiently at pH 9 than at pH 5.

Examples illustrating the effect of copper on decomposition of cyanide species and thiosulfate are given in Table VIII. With no copper present, cyanide was removed at a very slow rate (Test C-1, C-3, C-4 and C-6).

The examples of tests C-2 and C-5 show that copper acts as a catalyst not only for CN.sup.- removal, but also for thiosulfate decomposition. The tests show that copper is a catalyst for CN.sup.- and S.sub.2 O.sub.3.sup.-- decomposition (C-5), while nickel acts as a catalyst for the removal of thiocyanate once CN.sup.- has been removed (C-8).

Examples illustrating removal of cyanide from gold mill effluents are shown in Table IX. As can be seen all species except SCN.sup.- were decomposed and all metal values were hydrolyzed out of solution, including Zn, Fe and As. In all cases, Cu was present initially.

This example illustrates removal of cyanide from zinc effluent. As can be seen, CN.sup.- removal was very slow and inefficient in the absence of added copper (Test P-1). In the presence of some copper (Cu/CN.sup.- tot=0.075) removal of CN.sup.- was complete in one hour. In addition, iron was removed to a very low level of 0.01 mg/l.

This example illustrates the catalytic effect of some metals in the decomposition of thiosulfate in thiosulfate-containing effluents using SO.sub.2, air and lime (Table XI). In the absence of metal, decomposition of S.sub.2 O.sub.3.sup.-- is slow (Test S-1). Of the metals tested, copper was the best (Test S-2), and nickel had no catalytic effect (Test S-3). Cobalt, although present as a precipitate at pH 9, also acts as a catalyst for S.sub.2 O.sub.3.sup.-- decomposition (Test S-4). Both iron and manganese have some catalytic effect on S.sub.2 O.sub.3.sup.-- 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-7).

Examples of cyanide removal from effluents also containing between 15 and 62% by weight solids (mainly pyrrhotite-iron sulfide) using SO.sub.2, air and lime are shown in Table XII. No CN.sup.- 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-1, P-2 and P-3 indicate that CN.sup.-, complexed metal cyanide and S.sub.2 O.sub.3.sup.-- are all decomposed when adding a CuSO.sub.4 solution continuously during the treatment with SO.sub.2, air and lime.

In the presence of 15.3% solids, about 0.25 grams of copper per gram of CN.sup.- is required and in the presence of 61.8% solids, about 1 gram of copper per gram of CN.sup.- is required.

An example of a two stage continuous treatment of gold mill effluent liquor using SO.sub.2, air and lime at pH 9 and at 23.degree. C. is shown in Table XIII. The bulk of the cyanide is decomposed in the first stage with a solution retention time of 9 minutes and an addition of 0.67 g SO.sub.2 per liter of feed. The remaining cyanide is oxidized in the second stage with a solution retention time of 9 minutes and an addition of 0.33 g SO.sub.2 per liter of feed. The treated effluent at pH 9 is neutralized with lime to pH 10 and filtered to recover the precipitated metal hydroxides. The precipitate analyzed 31% Ni+Cu+Fe+Zn+As by weight.

This example illustrates the beneficial effect of recycling precipitated metal hydroxide solids upon the removal of cyanide from cyanide waste waters. Results are given in Table XIV. Batch treatment of the feed cyanide liquor with SO.sub.2, air and lime at 23.degree. C. and pH 9 in the absence of metal hydroxide solids required 115 minutes to remove the cyanide from 334 mg/l down to 0.5 mg/l. The metal hydroxide precipitate resulting from this experiment was settled and was added as underflow slurry (5% of feed volume) to fresh feed. The feed cyanide liquor containing the metal hydroxides was treated with SO.sub.2, air and lime at 23.degree. C. and pH 9. In this instance, cyanide was removed from 335 mg/l to .about.0.5 mg/l in 20 minutes.

This example illustrates treatment of cyanide containing effluents with either Na.sub.2 SO.sub.3 or CaSO.sub.3 without artificial aeration, depending on oxygen absorption through the surface of a quiescent liquor. In these experiments 1 l portion of solution analyzing in mg/l: CN.sub.TOT.sup.- 68, Cu8, Ni7.5, Fe8 and Zn7.5 were treated with 0.5 g of SO.sub.2 added respectively as Na.sub.2 SO.sub.3 and a CaSO.sub.3 suspension, at 23.degree. C. The pH of the cyanide solution after addition of the SO.sub.2 was about 9.3 in each case. The solution was left standing, open to the air and the assays shown in Table XV were obtained.

After 4 days, all CN.sup.- was removed in both cases. The removal is faster in the presence of Na.sub.2 SO.sub.3 then in the presence of CaSO.sub.3.

This example illustrates the treatment of effluent with SO.sub.2, air and lime at 23.degree. C. and pH 9 to remove CN.sub.TOT.sup.-, followed by chlorination at pH 11 to remove SCN.sup.-. Results are shown in Table XVI. In another test the same effluent has been subjected only to alkaline chlorination and chlorine consumption was approximately double, while cyanide removal was incomplete, due to the stability of ferrocyanide.

This example illustrates treatment of cyanide containing slurry effluent with Na.sub.2 SO.sub.3 without artificial aeration, depending on oxygen absorption through the surface of a quiescent liquor. In the experiment, 1 l of slurry containing 35% by weight sulfidic solids and analyzing in mg/l: CN.sub.TOT 440, and Cu292 was treated with 5 g of SO.sub.2 added as Na.sub.2 SO.sub.3 at 23.degree. C. The slurry was left standing, open to the air and assays shown in Table XVII were obtained.

An example of a two stage continuous treatment of gold mill effluent using SO.sub.2 air and lime at pH 9 and 23.degree. C. is shown in Table XVIII. The bulk of the cyanide is removed in the first stage from which the effluent is filtered to separate the solids. The filtrate is then treated in a second stage to achieve final cyanide removal (polishing stage). The above approach yields a high grade metal hydroxide precipitate and low reagent consumption.

This example illustrates the removal of cyanide from dilute cyanide waste waters with pre-addition of SO.sub.2 at pH 9, followed by aeration. The SO.sub.2 was introduced in the waste water as a 2.5 volume % mixture in nitrogen. After SO.sub.2 addition, the cyanide was analyzed at <0.5 mg/l, suggesting that very little air is required for the removal of low levels cyanide from effluents (air initially present in the water may be sufficient).

The foregoing Examples set forth in terms of experimental data the ranges of operation of the process of the present invention. With respect to the reaction of simple and complex cyanide with SO.sub.2 and oxygen in the presence of copper ion, a pH range of about 5 to about 12 supported by the data in Examples 1, 4 and 9; a practical temperature range of 0.degree. to 50.degree. C. is supported by Example 6 and; a practical pressure range for applied air or oxygen and SO.sub.2 ranging from atmospheric up to that minor pressure above atmospheric sufficient to overcome hydrostatic pressure and permit sparging is supported by all the examples related to reaction with CN ion and especially Examples 1 and 16. With respect to pressurized operation above the aforestated range of applied gas pressures, it is deemed that this is not necessary and involves considerable expense. However, the appended claims, in their broadest scope, are not to be deemed so limited as to exclude pressurized operation and optionally operating at temperatures above about 100.degree. C. at super atmospheric pressures if, for any reason, a person desiring to destroy cyanide would wish to do so. It has been noted in a particular commercial installation that when sulfur dioxide and air are fed into a cyanide-containing aqueous effluent also containing copper and suspended solids and that effluent is subjected to hydrostatic pressures of the order of 2000 KN/m.sup.2 rapid reaction occurs resulting in destruction of cyanide in a very few seconds. Destruction is essentially complete in that amount of time necessary for transfer of the slurry effluent uphill to a tailings pond. When gaseous SO.sub.2 and air are employed in the practice of present invention for destruction of cyanide ion, Example 2 shows that volume ratios of oxygen to SO.sub.2 of about 1.25 to 100 are operable. In addition, Example 1 and many of the other Examples show that lime (Ca(OH).sub.2) can be used for pH control over the range of about 5 to 11.

The following Examples describe the application of the process of the present invention to gold mine (or mill) effluent and tailings. Basically gold recovery from ore involves treating a finely comminuted ore with aqueous cyanide and recovering the soluble gold from cyanide solution. Barren liquor and tailings are treated with sulfur dioxide or sulfites and air prior to disposal to a waste site and after essentially all gold recovery operations have been carried out. It will be appreciated that secondary metal (e.g., copper, cobalt, nickel, etc.) recovery might be carried out on cyanide free material such as precipitation as described in Example 12. Detrimental species other than cyanide, e.g., arsenic, can also be removed as described in one or more of the following examples.

In carrying out the process of the present invention on a continuous manner, it is usually required that a volume of water be established which contains cyanide at most at the discharge level and also contains copper as a solid in cyanide-soluble form. Experience has shown that the process of the present invention at least in part involves a reaction wherein cyanide is oxidized to the less harmful cyanate. This reaction is catalyzed by the presence of the soluble copper and proceeds most expeditiously when all reactable cyanide is complexed as copper, zinc or nickel complex cyanides. Cyanide complexed with iron as ferrocyanide is not oxidized in the present process.

As a method of initiating a continuous process in accordance with the present invention, we prefer to initially operate the process as a batch in whatever reaction vessel is available. When the batch is fully reacted, the cyanide content is low e.g. below 1 ppm and copper hydroxide as a product is present in the reacted batch, continuous operation is started. If, for any reason, copper hydroxide or any cyanide-soluble solid copper compound (other than copper ferrocyanide) disappears from the reaction volume, the continuous process can be reestablished by introduction of such solid copper compound into the reaction volume and restarting as a batch, if necessary.

Examples of two stage continuous treatment of gold mill effluent containing nickel in solution using SO.sub.2, air and lime, with and without solids separation between stages are given in Table XX. The results show a substantial improvement in the effectiveness of the second stage when solids have been removed, as the total cyanides (CN.sub.T) in the treated effluent was decreased from 6.4 mg/L to 0.7 mg/L.

An example of two stage continuous removal of cyanide from gold mill tailings slurry (36.8 wt. % solids) using H.sub.2 SO.sub.3, air and lime is given in Table XXI. The H.sub.2 SO.sub.3 solution was prepared by sparging SO.sub.2 gas (mixture of 15% SO.sub.2 and 85% N.sub.2) in water at 23.degree. C. The preferred pH is 8 for slurry treatment. Neither H.sub.2 SO.sub.3 nor lime was added in the second stage. Although copper addition (as CuSO.sub.4) was not required, it was added at the equivalent of 50 mg/L to the first stage and this resulted in improved reaction rate. With no copper addition, a retention time of 60 minutes per stage was required.

An example of continuous cyanide removal from gold mill barren solution and tailings slurry using Na.sub.2 SO.sub.3 and/or Na.sub.2 S.sub.2 O.sub.5 and air is given in Table XXII. The treatment comprises two stages. In the first stage barren solution is treated at 23.degree. C. and pH 9.05 with Na.sub.2 SO.sub.3, air and 100 mg/L Cu.sup.++. The treated barren solution is then reacted together with tailings slurry in the second stage using a mixture of Na.sub.2 SO.sub.3 and Na.sub.2 S.sub.2 O.sub.5 and air at 23.degree. C. and pH 8. No lime addition is required. In this approach advantage is taken of the copper which is present in the treated barren solution, which upon redissolution in the tailings slurry can act as additional copper catalyst in stage 2. The approach also shows that adequate choice of SO.sub.2 reagents (Na.sub.2 SO.sub.3, Na.sub.2 S.sub.2 O.sub.5 or mixtures thereof) can be used advantageously for pH control.

An example of two stage continuous cyanide and arsenic removal from gold mill effluent containing a high arsenic concentration in solution using Na.sub.2 S.sub.2 O.sub.5, Cu.sup.++, Fe.sup.+++, air and lime as the reagents is shown in Table XXIII.

The approach consists of treating the effluent, at 23.degree. C. and pH 8.5, with Na.sub.2 S.sub.2 O.sub.5, Cu.sup.++, air and lime in a first stage to remove the cyanide, Cu, Fe, Zn and part of the As, followed by treatment with Fe.sup.+++ (added as a solution of Fe.sub.2 (SO.sub.4).sub.3) in a second stage at 23.degree. C. and pH 7, to remove the remaining arsenic.

An example of single stage continuous cyanide and arsenic removal from gold mill effluent containing a high arsenic concentration in solution, using Na.sub.2 S.sub.2 O.sub.5, Cu.sup.++, Fe.sup.+++, air and lime as the reagents is shown in Table XXIV.

In this approach, all reagents are added simultaneously to perform cyanide and metal removal at pH 6.

This example demonstrates by data in Table XXV the continuous removal of cyanide from gold mill barren solution and tailings slurry at 23.degree. C. using Na.sub.2 S.sub.2 O.sub.5, air and lime as the reagents. The example relates to a plant scale experiment treating 120 liter/minute of barren solution and 2,100 liter/min of tailings slurry at .about.30 wt. % solids (.about.1,800 liter/min contained liquor). In this approach, the barren solution is treated in a stirred reactor at 22.degree. C. and pH 9.7 with Na.sub.2 S.sub.2 O.sub.5, air and lime. The treated barren effluent overflows into a tailings pump box also receiving the tailings slurry. The total tailings are pumped at an estimated pressure of 450 psi (about 3100 KN/m.sup.2) to a tailings pond 300 feet uphill with a retention time of .about.8 minutes in pipe.

The example shows that with no addition of Na.sub.2 S.sub.2 O.sub.5 to the tailings, the CN.sub.T at the end of the pipe is 19 mg/L. With addition of Na.sub.2 S.sub.2 O.sub.5 to the tailings, estimated at 0.1 g per L of total tailings, the CN.sub.T was removed to a level of <0.3 mg/L at the tailings pond discharge. The example shows the ability of Na.sub.2 S.sub.2 O.sub.5 to react with CN in a closed pipe using the dissolved oxygen and dispersed air present in the tailings.