Effluent Treatment

The present invention is concerned with a process for removing both cyanide and arsenic and/or antimony from effluent solutions and in particular for removing these contaminants simultaneously from effluent solutions.

In Canadian patent application Ser. No. 395,013 filed Jan. 27, 1982, now Canadian Pat. No. 1,165,474 there is described a process for removal of cyanide from aqueous effluent solutions which comprises reacting the cyanide with sulfur dioxide (or its equivalent as an alkali or alkaline earth sulfite, bisulfite or pyrosulfite) and oxygen in the presence of soluble copper at a pH in the range of about 4 to 12. On occasion cyanide-containing effluent solutions from gold mine operations also contain arsenic or antimony which should be removed in order to provide treated effluent solution which can be discharged with minimal damage to the local environment.

It is the object of the present invention to provide a process for simultaneous removal of arsenic and/or antimony and cyanide from aqueous solutions.

The process of the present invention comprises including along with the reagents SO.sub.2 and oxygen in the presence of soluble copper as taught in Canadian application Ser. No. 395,013 "effective ferric ion" in an amount at least about three times the weight of total arsenic and antimony in the effluent solution while maintaining the solution at a pH of about 5 to 9 and continuing the reaction until cyanide is reduced to the designed treated effluent level.

The term "effective ferric ion" employed in the previous paragraph means ferric ion in soluble form, e.g., in the form of an aqueous solution of ferric sulfate, ferric chloride, ferric alum or the like. The requirement for the ferric ion (or perhaps more accurately the hydrated ferric ion) to be in water-soluble form is set because arsenic and antimony are removed by precipitation which is more complete in the presence of freshly precipitated ferric hydroxide. If ferric ion is introduced as aged ferric hydroxide or ferric oxide relatively few sites will be available for occlusion and the equilibrium amount of ferric ion in solution will be too small at a pH in the range of interest to effectively precipitate arsenic and antimony.

The SO.sub.2 -oxygen reaction is carried out as disclosed in Canadian application Ser. No. 395,013. Thus, free and complex cyanide species, other than ferrocyanides, present in waste water streams are decomposed by treating 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 a pH in the range of about 5 to about 11. 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 and complex heavy metal cyanide species from the stream. Once the cyanide species are removed, e.g., thiosulfate, etc. related species can be removed by continued treatment with sulfur dioxide and oxygen and 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. In some instances, a low pH can be used to produce said oxides and hydroxides. The metal species employed as catalyst can thus be recovered and recycled, if desired.

Alkali or alkaline-earth metal sulfites (including bisulfites, meta-bisulfites and pyrosulfites) can be employed in place of the sulfur dioxide reagent.

The necessary reagent for cyanide waste water treatment 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 particularly advantageous. It will be appreciated in this connection that the action of sulfur dioxide and oxygen in water solution during the practice of the process of this invention results in the production of sulfuric acid which must be neutralized resulting in calcium sulfate (gypsum) formation when lime or limestone is used as base to control pH. 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- to be removed; whereas at least about 4.5 grams of sulfur dioxide are required per ram of CNS- to be removed. The rate of oxygen supply to the solution sets the maximum for the reaction kinetics.

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 total cyanide in solution and more ordinarily will contain no more than about 200 ppm total cyanide in solution. This total cyanide is, of course, reduced by precipitation of ferrocyanide.

In removal of free cyanide and cyanometal complexes from waste waters containing arsenic and/or antimony, the preferred ingredients are sulfur dioxide, air and lime. The temperature may be in the range of 0.degree. to 100.degree. C. Preferably the pressure is about atmos-pheric but super-atmospheric pressures can be used. 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 weight ratio of copper to total cyanide of at least about 0.25 gm/gm to obtain 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. 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. 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 re-established by introduction of such solid copper compound into the reaction volume and restarting as a batch, if necessary.

The effective ferric ion used in the process of the present invention is preferably added as ferric sulfate to incoming effluent and lime or Ca(OH).sub.2 is used as a base for pH maintenance. The result of this is that sulfate will be precipitated as calcium sulfate (gypsum) and iron will be insolubilized as ferric hydroxide containing occluded arsenic and antimony compounds. As a practical embodiment one may opt to carry out the treatment of effluent in two continuous stages. The first stage could be carried out as a pH of 9 with SO.sub.2 and air in the presence of soluble copper (as preferred in Canadian application Ser. No. 395,013) to provide a relatively low total soluble cyanide content, any ferrocyanide present being precipitated as a metal ferrocyanide in the first stage. In the second stage, maintained at a pH of about 6 ferric sulfate is added and SO.sub.2 -air treatment in the presence of soluble copper is continued to destroy remaining cyanide and simultaneously precipitate arsenic and antimony along with occluding ferric hydroxide.

As 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++, Fe+++, air and lime as the reagents is shown in Table I. In this Example, after establishing a reaction volume containing low cyanide and cyanide soluble copper in an aqueous medium, reaction was carried out at 23.degree. C. with a 60 minute retention time.

An example of two stage continuous cyanide and arsenic removal from gold mill effluent containing a high arsenic concentration is solution using Na.sub.2 S.sub.2 O.sub.5, Cu++, Fe+++, air and lime as the reagents is shown in Table II. After establishing two sequential volumes of aqueous liquid-containing cyanide soluble copper, the approach con- sisted of treating the effluent, at 23.degree. C. and pH 8.5, with Na.sub.2 S.sub.2 O.sub.5, Cu++, air and lime in a first stage to remove the cyanide, Cu, Fe, Zn and part of the As, followed by treatment with Fe+++ (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.

Continuous tests were conducted on a synthetic effluent stream containing cyanide, copper, antimony and arsenic. After having established a reacted volume of effluent by batch treatment, continuous operation was started for three different tests as set forth in Table III using SO.sub.2 /Air.

The data in Table III shows that along with destruction of cyanide by SO.sub.2 /air, arsenic can be lowered to less than 0.2 ppm. in effluent solution at a pH of 6 or 9 by adding ferric ion as aqueous ferric sulfate; that at a pH of 6, antimony in the effluent stream can be lowered to 2 ppm.; and than when the pH of the stream containing 2 ppm. antimony is raised to 9, the content of antimony is lowered still further.