CA1209523A - Detoxication of thiocyanate solutions by electrochemical oxidation - Google Patents

Abstract

ABSTRACT

A process for the recovery of cyanide from thiocyanate. An aqueous solution containing thiocyanate is introduced into an electrochemical reactor. The reactor is activated for an appropriate time period while the pH of the solution is maintained in the range of 1 to 4. The cyanide formed can be recovered, in various ways including recovery as hydrocyanic acid.
Where desired, the thiocyanate can be oxidized in the reactor for a longer period while the pH of the solution is maintained in the range of 10 to 12, to produce relatively harmless products, for example cyanate, ammonia, carbon dioxide and nitrogen.
The process is particularly useful to process thiocyanate containing wastes produced in the processing of gold and silver ores and concentrates, unit operations related to base metal processing, coking operations and petroleum refining.

Description

5~3 This invention rela~es ~o a novel electrochemical process for oxidizing thiocyanate ~SCN ). In particularp this invention relates to a process fnr recovering cyanide (CN ) from aqueous solutions containing thio-cyanate by controlled partial electrooxidation ofthiocyanate.
Aqueous solutions containing thiocyanate arise from many industrial processes, the principal sources being hydrometallurgical pxocessing of gold and silver 10 ores and concentrates and certain unit operations related to base metal processing. Very large volumes of effluent containing somewhat lower levels of thiocyanate emanate from coking operations either from the quenching waters or gas cleaning installations. The refining of petroleum 15 produces dilute thiocyanate solutions and thiocyanate is a common component of many inorganic waste streams generated by the chemical industry. Waste effluents rontaining thiocyanate are environmentally objectionable because in the natural environment thiocyanate is o~idized 20 by various pathways yielding highly toXic cyanide com-pounds.
It is helpful to consider an example of a typical thiocyanate containing waste li~uor that could be treated by the present process. In gold recovery 25 by cyanidation of sulfidic concentrates obtained by Eroth flotation of copper ore tailings, the waste liquor effluent may contain 1000-1200 milligrams of CN per litre and 1200-1400 milligramsper litre of SCN . The presence of thiocyanate in the effluent represents a 30 significant loss of reagent cyanide.
The formation of thiocyanate is a result of the release of sulfide (S2 ~ present in compounds of copper, ironr nickel and other metals during the cyanidation leaching of the tailings. Sulfide undergoes chemical oxidation in the oxygen rich leach liguor to form a series of oxysulfur species including thiosulfates and thionates. It is believed that thiocyanate is formed by reaction of cyanide with thionates. A reaction sugyested for the formation of thiocyanate by the action of tri-thionate ion (S302 6) with cyanide is shown in equation 11) .

(1) S3O 6 + CN SCN ~ S2O6 In addition to the irreversible consumption of reagent cyanide~ there is evidence to suggest that the presence of thiocyanate in gold cyanidation solution inhibits the oxidation of gold and therefore retards its solubilization. This effect could possibly be due to the formation of unstable gold sulfides on the metallic gold surface thereby reducing the rate of mass transport of the reactants, cyanide and dissolved oxygen resulting in a reduction of the gold leaching rate. A common practice in gold mills which serves to maintain the thiocyanate at an appropriate and acceptably low level is to discharge up to 20% of the thiocyanate fouled leach liqu~r from the cyanidation circuit pex day. The remaining liquor is then regenerated by additio'n of reagent cyanide. Let us assume for the purposes of this example that the volume of fouled leach solution discharged per day is 250 metric tons. This represents approximately 350 kilogxams of free and complexed cyanide per day.
Another source of waste effluent occurs in the processing of a concentrate fraction obtained from complex zinc-copper-lead sulfide ores. In this example, it is necessary to use a cyanide concentration of twenty times ~2~5~3 the conventional level in order to effect dissolution of contained silver values. Under these conditions, it is found that a significant fraction of the cyanide is converted to thiocyanate. The barren discharge solution can be acidified allowing the expurgation of cyanide as hydrocyanic acid ~HCN). The cyanide depleted residual acidic solution may contain up to 1000 milli-grams/litre of thiocyanate. The silver recovery process may produce up to 1800 kilograms of thiocyanate pex day.
The two examples given above demonstrate the large quantity of thiocyanate bearing waste liquor produced by cyanidation of sulfide ores and concentrates.
The conventional method of processing this type of effluent (aside from natural oxidation in holding ponds which is reported to be relatively slow when compared to the natural oxidation of cyanide) is by chemical oxidation using aqueous hypochlorite or using chlorine gas and aqueous caustic - the latter is usually termed alkaline chlorination. The stoichiometry for the alkaline chlorination of thiocyanate to cyanate (CNO ) and sulfate (SO4 ) is often represented by equation ~2).

(2) SCN ~ 4C12 + 10 OH -~ CNO ~ 8Cl ~ So2 + 5H20 The cyanate species (CNO ) may undergo further oxidation with additional chlorine and base but will also dissociate via a hydrolysis reaction producing in receiving waters, ammonia and carbonate. Using the stoichiometry of equation (2), an estimate of the chemical requirements can be made for treating by conventional means the thiocyanate contained in the effluent of example 1. If a typical 10% reagent excess is assumed, approximately 0.85 - 1.0 metric tons per day ~2~5~3 of chlorine is required together wi~h 2.3 - 2.7 metric tons of sodium hydroxide per day (a portion of the base requirement may already be available in the effluent).
The treated waste would contain approximately 2.4 - 2.8 metric tons per day of sodium chloride which often is unacceptable in receiving waters. For the purposeSof comparison, the chemical re~uirements for oxidation of 300 kilograms per day of cyanide would be 0.9 metric tons per day of chlorine and 1.0 metric ton per day of sodium hydroxide. The s~oichiometry of the alkaline chlorination of cyanide is given by equation (3).

(3) CN + C12 ~ 20H -~ CN0 + 2C~ ~ H20 These es~imates of reagent requirements indicate that the oxidation of thiocyanate by chemical means is an inherently expensive and hazardous proposition and is generally regarded a~ being much more expensive than alkaline chlorination of the cyanide which often accompanies the thiocyanate oxidation.
It is an object of the present invention to provide a process whereby thiocyanate can be electro-chemically oxidized more economically than by conven-tional means and to recover, for credit and reuse, cyanide which forms as an intermediate product of the electrooxidation. The process of the present invention can be carried out on a batch or continuous basis with a variety of effluent compositions. With many thio-cyanate effluents no chemical pretreatment such as p~
adjustments or adjustment of the buffer index or capacity of the effluent before electrochemical treatment is required. Also, when thiocyanate or cyanide is treated in the conventional manner by chemical oxidation, the 5 ~
waste contains a large amount of sodium chloride and may very well contain undesirable levels of free chlorine or sodium hydro~ide ~rom chemical overdosage. In additiony when treated in the conventional manner, the ~olume of the effluent may be considerably increased by the large volume of reagents added.
A process for the racovery of cyanide from thio-cyanate, said process comprising introducing an a~ueous solution containing thiocyanate into a suitable electro-chemical reactor, applying a direct current electricalpotential to said reactor, aarrying out a reaction under controlled conditions around room temperature for an appropriate time period so that during the period shortly after the electrochemical reaction begins and for the remainder of said process, the p~ of the a~ueous solution is maintained in an acid range to facilitate conversion of a major proportion of the thiocyanate to cyanide and recovering the cyanide so formed~
Pre~erably, the process of the present invention includes the steps of introducing the aqueous solution into th~ electrochemical reactor at a temperature around room temperature and carrying out the reaction without significant heat input.
Still more preferably, the pH of the aqueous solution shortly after the electrochemical reaction begins, is maintained in the range of 1 to 4.
There is further provided a process for electxo-chemically oxidizing thioqyanate. An a~ueous solution containing thiocyanate ions is introduced into a suit-able reactor. A direct current electrical potential isapplied to said reactor to convert the thiocyanate ions to relatively harless reaction products, while main-taining the pH of the solution in a range ~rom 10 to 12.

~Z~$5;~3 Whether the pro~ess in accordance with th~ pxesent invention i8 utilized to recover cyanide or to con~ert the thiocyanate solution into relatively harmless reaction products, as described abo~e, depends on the level of thiocyanate prese~t in the efflu~nt. A waste liquor with a high concentration of thiocyanate would normally be treated under conditions to allow maximum recovery of the intermediate cyanide formed during the electrooxidation process. ~owever, if the waste liquor contains only low levels of thiocyanate, ~wo options for processing would be possible. The dilute thiocyanate containing li~uor may be completely electro-$5Z3 oxidized producing an environmentally acceptable wasteor the dilute thiocyanate containing liquor may be concentrated by a convenient physical or chemical method.
The concentrated thiocyanate solution then may be treated by the method of the presen~ invention which allows for cyanide recovery.
In discussing the invention in greater detail, it is helpful to re~er to the possible electrochemical reactions that occur. In the elec~rochemical treatment of thiocyanate, electrooxidation of thiocyanate occurs at anodic surfaces, and at cathodic surfaces electro~
reduction of hydrogen ion occur to produce hydrogen gas.
If the thiocyanate solution contains other electrooxidizable species such as cyanide, thiosulfate, thionates, etc., the reactions at the anodic surfaces will consist of a number of parallel electrooxidation reactions. Further, the parallel electrooxidation of water (or hydroxyl ions) will also occur at anodic surfaces. Similarly, if the thiocyanate solution contains platable metals such as copper, zinc, nickel etcr, the reactions at the cathodic surfaces will consist of number of parallel electroreduction reactions comprising the simultaneous production of hydrogen and the cathodic deposition of metals. For the purpose of explaining the electro-oxidation of thiocyanate it is useul to consider thatthe solution is essentially a pure thiocyanate solution.
Since the cyanide moiety in thiocyanate can be anodically converted to a series of products such as cyanide ion, cyanate ion, ni~rogen gas and carbon dioxide or carbonate and bicarbonate ion, it is helpful to considex the electrooxidation reactions in sequence.
Although the stoichiometrics of the various thiocyanate reactions have not been uneguivocally established, ~L2!t~523 considerable analysis of anodic products of electro-oxidation of thiocyanate indicates that under a range of electrolysi~ conditions the fate of thiocyanate may be represented by the following equations:
s Electrooxidation of SCN to CN and SO4

(4) SCN + 4H20 ~ CN + SO4 + 8H ~ 6e Electrooxidation of SCN through to CNO and So2

(5) SCN + 5H~O-~ CNO ~ SO4 + lOH + 8e Electrooxidation of SCN throuqh to CO2r.N2 and SO~

(6) SCN + 6H20~ O-5N2 + C2 + 12H + SO4 + lle The above reactions represent stoichiometries and ~he form of the species in solution will, of course, depend on the pH. For example, cyani.de in acidic solution will be present almost entirely in the neutral HCN form while in highly basic solution it will he present almost entirely as CN ion. Similarly, the weak base sulfate ion will partially protonate in acidic solutions, and except in low pH solutions, carbon dioxide will be present as a mixture of bicarbonate and carbonate ions.
The stiochiometry of the anodic production of oxygen gas by the electrooxidation of water (or hydroxyl ion) is represented bY equation (7) or equation (8) ~7) 2H2~ 2 + 4H + 4e (8) 4 -~ + 2H2 + 4e (g) .523 In the absenee of electroreducible species sueh as plata~le metals, the predominant reaciion at the cathode is the production of hydrogen gas by the electroxeduetion of hydrogen (hydronium) ion or, equivalently, from the stoichiometric viewpoint, the electroreduction of water. The reaction may be written as follows (9) 2H20 2e--~H2(g) From the standpoint of recovering cyanide from thiocyanate, the relevant electrode reactions are (4) and ~8). From the standpoint of converting thiocyanate to relatively nontoxic cyanate and to nontoxic nitrogen gas and carbon dioxide, the relevant electrode reactions are respectively (5) and (9) and (6) and (9). The anodie formation of oxygen gas operates in parallel with all thiocyanate anodic reactions. At high thiocyanate concentrations,the current efficieney for oxygen pro-duction is relatively low. At low thiocyanate concen~trations (and cyanide),oxygen production becomes the predominant anodic reaction.
The overall electrochemi~al cell reaction leading to the production of cyanide from thiocyanate is obtained b~ combining e~uations (4) and (9) to yield reaction equation (10), (10~ SCN + 4H20-~ CN + H2S04 + 3H~

When considering the overall reaction (10) and assumin~
a current efficiency of 100% ~that is no other anodic and cathodic reactions of significance are occurring), there is a net acid production of 0.33 moles o H+ per $S~3 Faraday o charge through the cell. Therefore, as the electrochemical processing of thiocyanate solution proceeds the solution tends ~o become more and more acidic. Reaction (10) stoichiometry has been verified by analysis for thiocyanate, cyanide and acid during the course of electrolysis.
~ he production of acid is beneficial from the standpoint o~ the specific cyanide yield since (except where the thiocyanate solution has a high buffering capacity) it has the efect of preserving the cyanide produced from undergoing further rapid electrooxidation to cyanate or through to nitrogen gas and carbon dioxide~
I~itially, the conversion of thiocyanate at the anode can be represented by the reaction ~4). When the thio-cyanate solution does not have a high huffering capacityin the acidic direction, the large amount of acid produced (8 moles of H~ per mol of cyanide produced) will tend to cause a substantial decxease in the pH of the anolyte ~olution adjacent the anode surace. Similarly, ~0 the hydroxyl ion produced by the cathodic reaction will increase the p~ în the catholyte adjacent the cathode surfaces although this effect will be resisted if the thiocyanate solution has substantial buffering capacity in the basic direction. This suggests that an acidic anode boundary layer and a basic cathode boundary layer may exist.
It is the establishment of an acidic anode bollndary layer which is believed to he the main reason why the cyanide product is protected from rapid electro-oxidation at the anode. It has been established thatthe free anionic CN is much more easily electrooxidi~ed than the neutral protonated HCN form of cyanide. As thiocyanate is ~lectrooxidized at the anode to produce ~2~SZ3 cyanide ion, the cyanide ion is immediately protonated by the anodically produced acid. Conse~uentl~, the acidic anode boundary layer functions to preserve cyanide from rapid electrooxidation a~ the anode by converting the cyanide ion into the much more dificult to electrooxidi~e neutral protonated ~orm. This explanation is considered in a quantitative way in the discussion below on data Tables 1 - 4.
The protonated form of thiocyanate is similarly made less easily electrooxidized than the free anionic SCN form of thioc~anate. However, in this case the acidic anode boundary layer appears to have little efect on the current efficiency of thiocyanate conversion to cyanide. An explanation is found in the fact that HSCN
lS is an extremely strong acid CQmpare~ to HCN. The pKa of HSCN is less than 1.0 (pKa of HCN is 9.32) which means that even if the pH of the acidic anode boundary layer dropped as low as pH 1.0~ more than 50% of th~ thio-cyanate in the acidic boundary layer would still exist in the SCN , which is much less difficult to electro-oxidize.
Th~ explanation given above relating to the boundary layers appears to have some validity as demon-strated by the data in the following tables, each re-presenting a separate run. There may be different, butequally plausible, theories to explain why the process of the present invention occurs. The explanation given above is not intended to be conclusive.

5~3 Run ~1; Bulk pH_- 11.1; 0 . 5~1 in carbonate buf fer t ~min)SCN ~mg/l ) CN (mg/l) o 2920 1~ 1910 199 200 B ~0 Rlm ~t 2; Bulk pH = 9 . 5; o . 5M carbonate buf f er _ _ _ t ~min) SCN (mg/l) CN (mg/l) 5~1180 482 2û011 12 Rurl #3; Bulk pH = 9~6; 0.05M carbonate buffer t (min ) SCN (mg/l ) CN ~mg/l ) 0 ~868 0 ~0 54~ û14 Run ~4; Bulk pH = 4.2; no buffer salts added t (min) SCN (m~/l) CN (mg/l) n 2930 0 1402 64û

20~ 37 950 250 ~9 8~2 ~2~ 3 The above dat~ were obtained by processin~ 300 litre batches o~ thio~yanabe solution in an industrial size electrochemical reactor described below and xeferred to as Reactor 2. The solutions were made up using tap water 5 and technical grade salts. Electrochemical processing was carried out on a batch recirculation basis. The temperature was maintained in the range 24 - 29C. The operating current and the recirculation flow rate were the same in all runs.
The buffer capacities for the bulk solutions were respectively 0.25, 0.14, 0.014 and 0.002 mol H
per litre per unit decrease in pH for runs 1 to 4. Com-paring the rate of thiocyanate electrooxidation, it is apparent that there is no significant difference in the 15 rates in all four runs.
Considering the extreme runs 1 and 4, the higher buffer index in run 1 would effectively prevent signifi-cant acidication of the anode ~oundary layer. However, in run 4 the absence ~f buffer would result in strong 20 acidification of the anode boundary layer - estimated drop in pH is about3 pH unit~ to pH 1.2 the fact that thiocyanate electrooxidation rate is esse~tially the same in anode boundary layers at pH 11 and 1.2 suggests the explanation given above for th~ lack of variation of - 25 thiocyanate electrooxidation rate with pH might be ~alid.
Comparing the accumulation rates of cyanide in the four runs, it is seen that very little cyanide accumu~ates in run 1 and close to the theoretical amount calculated from equation (10) accumulates in run 4 - at least in the 30 first paxt of the run. A possible explanation is as ol~ows. In run 1, the acidification of the boundary layer will be resisted by the strong buffering capacity of the solution and consequently the boundary layer will not drop much below p~ 11. At pH 11 the fractio~ of ~2~ Z3 cyanide product in the more easily electrooxidizable form, cyanide ion, will approach 100%. Therefore conditions are ideal in run 1 for electrooxidation of cyanide. Thus as cyanide is produced from thiocya~abe, it is electrooxidized in a parallel anodic reaction, hence the low rate of cyanide accumulation and the rapid disappearance of cyanide as the run proceeds.
In run 3, the fraction of cyanide in the cyanide ion form in the bulk solution at pH 9.6, is equivalent to about 60%. However, the moderate buffering capacity of the solution will no~ greatly resist the acidification o the anode boundary layer. It is estimated for this case that the pH of the boundary layer can drop about 1 pH units to about pH 8.6. A~ pH 8.6 approximately 13%
of the cyanide product will exist in the more easily electrooxidizable cyanide ion form. Thus the rate and level of cyanide accumulation in run 3 should be more than in run 1, which is apparent from the data. This explanation is validated by the data of run 2. In run 2 the buffer capaci~y is 10 times higher than in run 3 and very little anode boundary layer acidiication would be expected. If this obtains, then the rate and level of cyanide accumulation/according to our proposed theory, should be greater in run 3 than in run 2 which the data confirms. In run 4 as noted above, the anode boundary layer acidification down to an estimated pH 1.2 could occur because the solution is acidic initially and the buffering capacity of the solution is essentially neg-ligible. In this run it would be expected that essen-tially all the cyanide found will be in the less easilyelectrooxidiable HCN form in the bul~ solution and in the acidic boundary layer. Thereore, the rate and level of cyanide accumulation should be highest in this run, ~9~ ~ 3 which is confirmed by the data.
The process o the present invention has an additional advantage in that the sulphur present in thiocyanate appears in the stoichiometry of the half-cell reaction (4) and ~6) in the form of sulphate (50~ ).
Chemical analyses on process solutions after both partial and complete oxidation has determined that virtually all sulphur is present as sulate, which is an environmentally acceptable form. This is important where it is desired to use the process of the present invention to treat industrial effluents that initially co~tain intermediate oxy-sulfur species as well as thioc~anate.
Various electrochemical reactors will be suit-able for use with the process according to the presentinvention. For example, the electrochemical reactor or electric cell described in Canadian Patent No. 1,016,495 is a suitable reactor that can be used to carry out the pxocess in accordance with the present invention. Various other sui~able reactors will be readily apparent to those skilled in the art. However, while it will be possible to use various electrochemical reactors including a conventional electrochemical reactor, the efficiency of the process will vary yreatly with the type of reactor used.
While the reactor described in Canadian Patent No. 1,016,~95 is suitable to carry out the process according to the present invention, when the process is to be carried out on a laxge scale, this reactor IS presently too expensive and too fragile to be economically feasible.
Since the process of the present invention will often be utilized in a large scale operation~ the reactor is preferably one that has durable components and is cap-able of being fully erected at the site.

~2'~S~

A second suitable reactor that can be used to carry out the process in accordance with the present invention is a discrete, fixed layer, particulate, bi-polax reactor ~hencefoxth referred to as reactor NOr 2)~
Reactor No. 2 has at least two layers of electrically conductive particles, each layer being discrete in that it is separated from adjacent layers by an electrically insulating spacer or screen wedged between adjacent layers of particles. Electrically insulating spacers are also located immediately beneath the lower most layer and immediately above the upper most layer of particles~ The various layers are maintained in a fixed relationship by said spacers. Except for that taken by the spacers themselves, there is no gap, distance or space between adjacent layers of particles. Of course, the reactor vessel must contain means for suppor~ing the various layers within it. Preferably, the base of the reactor vessel is strong enough to support the variou~ layers.
In reactor No. 2, the reactor vessel can be constructed of virtually any suitable material and any reasonable shape but is preferably circular in cross section. For example, the reactor vessel can be made of steel with the inside wall being ruhber-lined so that it is electrically insulated. Also, the reactor wall could be mada of concrete. The reactor v~ssel could also be constructed in modular form so that additional sections could be added as required. The two primary electrodes can be fabricated from various materials for example, graphite plates, stainless steel, lead or even mild steel.
The conducting or semi-conducting material for use as layers of particles in reactor No. 2 can be -~12~

various material~r f~r ~xample~ graphite, metallurgical coke or anthracite. The particles can be specifically arranged in a ~ixed relationship to ~orm a layer, or, where crushed particles are used, su~ficient particles can simply be poured on~o an insulating spacer to form one layer. One type of particle that works well consists of 2.5 X 2.5 cm graphite cylinders that have been tumbled wet in a rotating drum. The rotating drum produced graphite nodules approaching spherical shape as the edges are rounded by the tumbling action. These nodules are placed on what remains of the flat portion sf the cylinder (ie. in an upright position~ in a fixed rela-tionship forming one layer of particles. ~ach layer i5 topped by a poly-vinyl chloride coated Fibreglas (a trade mark) mesh and then the next layer of tumbled cylinders is placed immediately on top of that Fibreglas ~a trade mark) mesh. Ultimately, a series of ixed conducting layers is created, each separated by a non-conducting membrane, all interposed between a primary anode and cathode. Particle sizes are screened so that no particles are smaller ~han . 25cmO
With reactor No. 2, in addition to Fibreglas (a trade mark) mesh, various other materials can be used as the insulating spacer. For example, crushed ston~, coarse granular plastic nodules, ceramic burl saddles or similaxly shaped ceramic or plastic shaped or glass fabric with poly-vinyl chloride coating.
There are various ways that the cyanide formed as an intermediate product in accordance with the process of the present invention, can be recovered ~or re-use. Also, it is sometimes necessary to pre-treat the effluent or aqueous solution prior to carrying out the electrochemical reaction within the suitable reactor.

~l2~ 3 Some of these procedures are di cussed in the following examples. Other processes for recovering the cyanide formedor pre-treating the aqueous solution will be readily apparent to those skilled in the art; but will still be within the scope of the claims.

Cyanide can be recovered by expurgation as hydro-cyanic acid. As stated abov0, the condition of low pH, while not influencing the rate of thiocyanate oxidation promotes the protonation of cyanide ion, which in turn inhibits its further oxidation. By allowing the pH of the processing solution to decrease as acid is generated, the hydrocyanic acid may be continuously recovered by expurgation.

EXAMæLE 2 _ .
A portion of the thiocyanate containing cyanidation leach solution is continuously fed to a suitable electro-chemical reactor where partial electrooxidation takesplace forming cyanide as an intermediate product. This leach solution with its enriched cyanide concentration is returned to the cyanidation circuit. With appropriate process control, a steady state thiocyanate/cyanide concentration is maintained in the leach circuit.

Cyanide can be recovered using a strong base ion axchanger on a batch or semi-continuous basis.

Cyanide can be recovered by utilizing the electrochemical reactor in conjunction with an air 5~3 stripper to recover the cyanide as hydrocyanic acid.
The electrochemical reaction products are fed into an air stripper where air, hydrocyanic acid, water and hydrogen are separated from the electrochemical reaction products. The cyanide can then be recovered from the hydrocyanic acid by neutralization with lime water or sodium hydroxide in an adsorption tower.

Cyanide ~an be recovered by utilizing the electrochemical reactor in conjunction with a steam stripper to recover the cyanide as hydrocyanic acid.
This is similar to the use of the air stripper except that steam and air are used with steam stripping. Once the hydrocyanic acid is recovered, it can be neutralized with lime or sodium hydroxide to recover the cyanide.

Cyanide can be recovered by dixectly recycling it in solution to a cyanide leaching process. Since the conversion of thiocyanate to cyanide results in virtually all sulphur species being converted to sulfate, the acceptability of sulfate must be considered. In the leaching of zinc sulfide containing residues, the acidic `
zinc-thiocyanate solution is treated electrochemically to convert most of the thiocyanate to cyanide and sulfate and simultaneously to recover a large portion of the zinc cathodically. The electrochemically converted acidic solution is then treated with lime to neutralize the sulphuric acid and the hydrocyanic acid. The solid calcium sulphate is thickened by settling and the clear supernatant Ca(CN)2 solution is used for make up for further cyanidation and ~he zinc collected in the reactor ~r'2 ~ 5;~3 is leached out with sulphuric acid.

Where the effluent contains an acidic solution of æinc and thiocyanate, a c~tion exchanger could be used operating on the acid cycle to remove the zinc from the solution. The essentially zinc free solution is then treated electrochemically to convert most of the thio-cyanate to cyanide and sulphate and to cathodically deposit any residual zinc.
The electrochemically converted acidic solution is then treated with lime to neutralize the sulphuric acid and hydrocyanic acid formed. l~he solid calcium sulphate is thickened by settling and the clear super-lS natant Ca(CN~4 solution is used for make up for furthercyanidation.
Any cathodic zinc that ha~ been deposited is removed from the reactor by sulphuric acid. Zinc is eluted from the cation exchanger with sulphuric acid.
~0 _ An acidic zinc-thiocyanate solution is treated electrochemically to convert most of the thiocyanate to cyanide and sulphate and simultaneously recover a good portion of the zinc cathodically.
The electxochemically treated solution is then rendered essentially zinc free by using a cation exchanger operating on an acid cycle.
The solution is then treated with lime to neutralize the sulphuric acid and hydrocyanic acid. The solid calcium sulphate i5 thickened by settling and the clear supernatant CatCN)2 solution is used for make up for further cyanidation.

5i2:~

The zinc collected in the reactor is leached out with sulphuric acid and zinc is eluted from the cation exchanger with sulphuric acidn Pxior to carrying out the electrochemical reaction, where the cyanidation waste is basic, it is acidified to a pH ranging from ~ to Ç.5 and any solids are filtered out.
The waste is then treated on a weaX base anion exchanger to extract the anionic metal cyanide species (eg. copper, nickel, iron and/or cobalt) and the thiocyanate is collected on a second-weak base anion exchanger. The weak base anion exchanger con~
taining ~ssentially thiocyanate is then eluted with base such as sodium hydroxide or lime water to produce an effluent with a low buffer index.
The electrochemical reaction can then be carried out together with air stripping or steam stripping as ~0 set out in Examples 1 and 2.
This would be necessary only where the alkaline cyanidation waste has a high buffer index.

Where the effluent or aqueous solution contains a high buffer index based on the bi-carbonate/carbonate concentration~ the buffer capacity can be substantially reduced by adding calcium chloride to precipitate the carbonate as calcium carbonate.
The electrochemical reaction to convert the thiocyanate to cyanide can then be carried out on the resulting solution.

" ll2~S~3 EXAMPLE _ Where the effluent or aqueous solution has a high buffering index because of the bi-carbonate/car-bonate concentration, the buffering index can be substantially reduced by adding acid to substantially convert all of the bi-carbonate and carbonate to carbon dioxide and then expurgating the carbon dioxide to produce a solution with a low buffer index.
The electrochemical reaction of the present invention can then be carried out on the resulting solution to convert the thiocyanate to cyanide. and the cyanide so for~ed can be recovered.

Claims (4)

1. The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
   l. A process for electrochemically oxidizing thio-cyanate, said process comprising introducing an aqueous solution containing thiocyanate ions into a suitable electro-chemical reactor, applying a direct current electrical potential to said reactor to convert the thiocyanate ions to relatively harmless reaction products, while maintaining the pH of the solution in a range from 10 to 12.
2. 2. A process as claimed in Claim 1 wherein the reaction products are cyanate, ammonia, carbon dioxide and nitrogen.
3. 3. A process as claimed in Claim 1 wherein the pH
   is maintained by utilizing the buffer capacity of the aqueous solution.
4. 4. A process as claimed in Claim 1 wherein the buffer capacity of the solution is enhanced by adding carbonate.