CA1188250A - Electrochemically treating aqueous thiocyanate solutions - Google Patents
Electrochemically treating aqueous thiocyanate solutionsInfo
- Publication number
- CA1188250A CA1188250A CA000404388A CA404388A CA1188250A CA 1188250 A CA1188250 A CA 1188250A CA 000404388 A CA000404388 A CA 000404388A CA 404388 A CA404388 A CA 404388A CA 1188250 A CA1188250 A CA 1188250A
- Authority
- CA
- Canada
- Prior art keywords
- cyanide
- thiocyanate
- aqueous solution
- reactor
- solution
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
- C25B1/00—Electrolytic production of inorganic compounds or non-metals
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
- C22B19/00—Obtaining zinc or zinc oxide
- C22B19/20—Obtaining zinc otherwise than by distilling
- C22B19/22—Obtaining zinc otherwise than by distilling with leaching with acids
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/20—Recycling
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 time period while the pH of the solution is maintained in the range of 10 to 12, to produce relative-ly 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.
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 time period while the pH of the solution is maintained in the range of 10 to 12, to produce relative-ly 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
This invention rela~tes to a novel electrochemical process for oxidizing -thiocyanate (SCN ). In particular, this invention relates to a process for reco~ering cyanide (CN ) from aqueous solutions containing thio-cyanate by controlled partial eiectrooxidation ofthiocyanate.
~ queous solutions containing thiocyanate arise from many industrial processes, the principal so~lrces being hydrometallurgical processing 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 containing 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 liquor that could be treated by the present process. In gold recovery 25 by cyanidation of sulfidic concentrates obtained by froth flotation of copper ore tailings, the waste liquor effluent may contain 1000-1200 milligrams of CN per litre and 1200-1400 milligra~sper 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 (S ) presen-t in compounds of copper, iron, nickel and other metals during the cyanidation ~`
leaching of the tailings. Sulfide undergoes chemical oxidation in the oxygen rich leach li~uor to form a series of oxysulfur species incll-ding thiGsulfates and thionates. It is believed that thiocyanate is formed by reaction of cyanide with thionates. A reaction suggested for the formation of thiocyanate by the action of tri-thionate ion (S302 6) with cyanide is shown in equation (1) (1) 3 6 SCN ~ S2O6 In addition to the irreversible consumption of reagent cyanide, there is evidence to suggest that the presence ofthiocyanate 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 liquor from the cyanidation circuit per 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 ~50 metric tons. This represents approximately 350 kilograms 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 5~
the conventional level in order to effect dissolution of contained silver values. Under these cGnditions, 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 per day.
The two examples given above demonstrate the large quantity of thiocyanate bearing waste liquor produced by cyanidation of sulfide ores and concentrates.
The ~onventional 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).
~ queous solutions containing thiocyanate arise from many industrial processes, the principal so~lrces being hydrometallurgical processing 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 containing 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 liquor that could be treated by the present process. In gold recovery 25 by cyanidation of sulfidic concentrates obtained by froth flotation of copper ore tailings, the waste liquor effluent may contain 1000-1200 milligrams of CN per litre and 1200-1400 milligra~sper 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 (S ) presen-t in compounds of copper, iron, nickel and other metals during the cyanidation ~`
leaching of the tailings. Sulfide undergoes chemical oxidation in the oxygen rich leach li~uor to form a series of oxysulfur species incll-ding thiGsulfates and thionates. It is believed that thiocyanate is formed by reaction of cyanide with thionates. A reaction suggested for the formation of thiocyanate by the action of tri-thionate ion (S302 6) with cyanide is shown in equation (1) (1) 3 6 SCN ~ S2O6 In addition to the irreversible consumption of reagent cyanide, there is evidence to suggest that the presence ofthiocyanate 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 liquor from the cyanidation circuit per 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 ~50 metric tons. This represents approximately 350 kilograms 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 5~
the conventional level in order to effect dissolution of contained silver values. Under these cGnditions, 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 per day.
The two examples given above demonstrate the large quantity of thiocyanate bearing waste liquor produced by cyanidation of sulfide ores and concentrates.
The ~onventional 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 ~ SO~ ~ 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 (~), an estimate of the chemical reguirements 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 of chlorine is required together with 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 receivin~ 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 l.0 metric ton per day of sodium hydroxide. The stoichiometry of the alkaline chlorination of cyanide is given by equation ~3).
The treated waste would contain approximately 2.4 - 2.8 metric tons per day of sodium chloride which often is unacceptable in receivin~ 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 l.0 metric ton per day of sodium hydroxide. The stoichiometry of the alkaline chlorination of cyanide is given by equation ~3).
(3) CN + C12 + 20H -~ CN0 + 2Cl + H~0 These estimates of reagent requirements indicate that the oxidation of thiocyanate by chemical means is an inherently expensive and hazardous proposition and is generally regarded as being much more expensive than alkaline chlorination of the cyanide which often ~0 accompanies the thiocyanate oxidation.
Through the use of the process of the present invention it is possible to electrochemically oxidize thiocyanate more economically than by conventional means and,in addition, it is possible to recover for credit and reuse, cyanide which forms as an intermediate product of the electrooxidation. In addition, the present process 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 pH
3~ adjustments or adjustment of the buffer index or capacity o~ the e~fluent before electrochemical treatment is re~uired. Also, when thiocyanate or cyanide is treated in the conventional manner by chemical oxidation, the ? 5 (3~
waste contains a lar~e amount of sodium chloride and may very well contain undesirable levels of free chlorine or sodium hydroxide from chemical overdosage. In addition, when treated in the conventional manner, the volume of the effluent may be considerably increased by the large volume of reagents added.
A process for the reco~ery of cyanide from thio-cyanate, said process comprising introducing an aqueous solution containing thiocyanate into a suitable electro-chemical reactor, applying a direct current electricalpotential to said reactor, carrying 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 pH of the aqueous 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~
Preferably, the process of the present invention includes the steps of introducing the aqueous solution into the 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 2S solution shortly after the electrochemical reaction begins, is maintained in the range of 1 to 4.
There is further provided a process for electro-chemically oxidizing thiocyanate. An aqueous solution containing thiocyanate ions is introduced into a suit-able reactor for an appropriate time period. The pHof the aqueous solution is maintained in a range from lO to 12 during the reaction and the aqueous solution is removed from the reactor once the thiocyanate has been converted to cyanate and sulfate.
- 5~ -Whether the process in accordance with the pxesent invention is utili~,ed to recover c~anide or to convert the thiocyanate solution into relatively harmless reaction products, as described above, depends on the level of thiocyanate present in the effluent. ~ waste liquor with a high concentration of thiocyanate would normally be treated under conditions to allow maxi.mum recovery of the intermediate cyanide formed during the electrooxidation process. However, if the waste liquor contains only low levels of thiocyanate, two options for processing would be possible. The dilute thiocyanate containing liquor may be completely electro-38i~5(~
oxidized producing an environmen-tally acceptable waste or the dilute thiocyanate containing liquor may be concentrated by a convenient physical or chemical method.
The consentrated thiocyanake solution then may be treated by the method of the present invention which allows for cyanide recovery.
In discussing the invention in greater ~etail, it is helpful to refer to the possible electrochemical reactions that occur. In the electrochemical treatment of thiocyanate, electrooxidation of thiocyanate occurs at anodic surfaces, and a~ cathodic surfaces electro-reduction of hydrogen ion occur to produce hydrogen gas.
I the thiocyanate solution contains other electrooxidizable species such as cyanide, thiosulfate, thionates, etc.
the reactions at the anodic surfaces will consist of a nu~ber of parallel eleetrooxidation 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 etc., the reactions at the cathodic surfaces will consist of number of parallel electroreduetion reactions comprising the simultaneous ; production of hydrogen and the cathodic deposition of metals. For the purpose of explaining the electro-~5 oxidation of thiocyanate it is useful to consider that the solution is essentially a pure thiocyanate solution.
Since the cyanide moiety in thiocyana~e can be anodically converted to a series of products such as cyaniae ion, cyanate ion, nitrogen gas and carbon dioxide or earbonate and bicarbonate ion, it is helpful to considex the electrooxidation reactions in sequence~
~lthough the stoichiometrics of the various thiocyanate reactions have not been unequivocally established, 5(3 considerable analy~is of anodic products of electro-oxidation of thiocyanate indicates that under a range bf electrolysis conditions the fate of thiocyanate may be represented by the following equations:
Electrooxidation of SCN to CN and S04
Through the use of the process of the present invention it is possible to electrochemically oxidize thiocyanate more economically than by conventional means and,in addition, it is possible to recover for credit and reuse, cyanide which forms as an intermediate product of the electrooxidation. In addition, the present process 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 pH
3~ adjustments or adjustment of the buffer index or capacity o~ the e~fluent before electrochemical treatment is re~uired. Also, when thiocyanate or cyanide is treated in the conventional manner by chemical oxidation, the ? 5 (3~
waste contains a lar~e amount of sodium chloride and may very well contain undesirable levels of free chlorine or sodium hydroxide from chemical overdosage. In addition, when treated in the conventional manner, the volume of the effluent may be considerably increased by the large volume of reagents added.
A process for the reco~ery of cyanide from thio-cyanate, said process comprising introducing an aqueous solution containing thiocyanate into a suitable electro-chemical reactor, applying a direct current electricalpotential to said reactor, carrying 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 pH of the aqueous 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~
Preferably, the process of the present invention includes the steps of introducing the aqueous solution into the 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 2S solution shortly after the electrochemical reaction begins, is maintained in the range of 1 to 4.
There is further provided a process for electro-chemically oxidizing thiocyanate. An aqueous solution containing thiocyanate ions is introduced into a suit-able reactor for an appropriate time period. The pHof the aqueous solution is maintained in a range from lO to 12 during the reaction and the aqueous solution is removed from the reactor once the thiocyanate has been converted to cyanate and sulfate.
- 5~ -Whether the process in accordance with the pxesent invention is utili~,ed to recover c~anide or to convert the thiocyanate solution into relatively harmless reaction products, as described above, depends on the level of thiocyanate present in the effluent. ~ waste liquor with a high concentration of thiocyanate would normally be treated under conditions to allow maxi.mum recovery of the intermediate cyanide formed during the electrooxidation process. However, if the waste liquor contains only low levels of thiocyanate, two options for processing would be possible. The dilute thiocyanate containing liquor may be completely electro-38i~5(~
oxidized producing an environmen-tally acceptable waste or the dilute thiocyanate containing liquor may be concentrated by a convenient physical or chemical method.
The consentrated thiocyanake solution then may be treated by the method of the present invention which allows for cyanide recovery.
In discussing the invention in greater ~etail, it is helpful to refer to the possible electrochemical reactions that occur. In the electrochemical treatment of thiocyanate, electrooxidation of thiocyanate occurs at anodic surfaces, and a~ cathodic surfaces electro-reduction of hydrogen ion occur to produce hydrogen gas.
I the thiocyanate solution contains other electrooxidizable species such as cyanide, thiosulfate, thionates, etc.
the reactions at the anodic surfaces will consist of a nu~ber of parallel eleetrooxidation 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 etc., the reactions at the cathodic surfaces will consist of number of parallel electroreduetion reactions comprising the simultaneous ; production of hydrogen and the cathodic deposition of metals. For the purpose of explaining the electro-~5 oxidation of thiocyanate it is useful to consider that the solution is essentially a pure thiocyanate solution.
Since the cyanide moiety in thiocyana~e can be anodically converted to a series of products such as cyaniae ion, cyanate ion, nitrogen gas and carbon dioxide or earbonate and bicarbonate ion, it is helpful to considex the electrooxidation reactions in sequence~
~lthough the stoichiometrics of the various thiocyanate reactions have not been unequivocally established, 5(3 considerable analy~is of anodic products of electro-oxidation of thiocyanate indicates that under a range bf electrolysis conditions the fate of thiocyanate may be represented by the following equations:
Electrooxidation of SCN to CN and S04
(4) SCN + 4H20 ~ CN + S04 + 8H + 6e Electrooxidation of SCN through to CNO and S04
(5) SCN + 5H20-~ CNO + SO~ + lOH -~ 8e Electrooxidation of SCN through to C02,N2 and S04 ; 16) SCN '~ 6H2~ 0-5N2 + C02 + 12H -~ SO~ + lle The above reactions represent stoichiometries and the form of the ,species in solution will, of course, depend on the pH. For example, cyanide in acidic solution will be present almost entirely in the neutral HCN form while in highly basic solution it will be 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) 2H20~2 + 4H + ~e : (g) (8) 4 ~ + 2H20 + 4e (g) ;'5~
In the absence of electroreducible species such as platable ~etals the predominant reaction at the cathode is the pxoduction of hydrogen gas by the electroreduction of hydrogen (hydronium) ion or equivalently, from the stoichiometric viewpoint, the electroreduction of water. The reaction may be written as ~9) 2H20 ~ 2e -~H2 -~ 20H
; 10 (g) From the standpoint of recovering cyanide from thiocyanate, the relevant electrode reactions are (4) and (8)o From the standpoint of ~onverting thiocyanate to relatively nontoxic cyanate and to nontoxic nitrogen gas and carbon dioxide, the relevant electrode reactions are recpectively (5) and (9) and (6) and (9). The anodic formation of oxygen gas operates in parallel with all thiocyanate anodic reactions. At high thiocyanate concentrations the current efficiency for oxygen pro duction is relatively lowO At low thiocyanate concen-trations (and cyanide) oxygen production becomes the predominant anodic reac~ion.
The overall electrochemical cell reaction leading to the production of cyanide from thiocyanate is obtained by combing ~quations ~4) and (9) to yield reaction equation (1~3 (10) SCN + 4H2O-~ CN + H2SO~ + 3H2 When considering the overall reaction (10) and assuminy a current efficiency of 100% (that is no o~her anodic and cathodic reactions of significance are occurring), there is a net acid production of 0.33 moles of H~ per 5~3 Faraday of charge through the cell. Therefore, as the electrochemical processing of thiocyanate solution proceeds the solution tends to become more and more acidic. Reaction (10) stoichiometry has been verified by analysis for thiocyanater cyanide and acid during the course of e]ectro]ysisO
The production of acid is beneicial from the standpoint of the specific cyanide yield since (except where thethiocyanate solution has a high buffering capacity) it has the effect of preserving the cyanide produced from undergoing further rapid electrooxidation to cyanate or through to nltrogen gas and carbon dioxide.
Initially, the conversion of thiocyanate at the anode can be represented by the reaction (4). When the thio-cyanate solution does not have a high buffering capacityin the acidic direction, the large amount of acid produced (~ moles of H+ per mol of cyanide produced) will tend to cause a substantial decrease in the pH of the anolyte solution adjacent the anode surface. Similarly, the hydroxyl ion produced by the cathodic xeaction will increase the pH in the catholyte adjacent the cathode suxfaces although this effect will be resisted if the thiocyana~e solution has substantial buf-fering capacity in the basic direction. This suygests that an acidic ~5 anode boundary layer and a basic cathode boundary layer may exist.
It is the establishment of an acidic anode boundary layer which is believed to be the main reason why the cyanide product is protected from xapid electro-o~idation 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 electrooxidi2ed at the anode to produce o cyanide ion, the cyanide lon is immediately protonated by the anodically produced acid. Consequently, the acidic anode boundary layer functions to preserve cyanide from rapid electrooxidation at the anode by converting the cyanide ion into the much moxe diEEicult to electrooxidize neutxal protonated form. This explanation is considered in a quantitative way in the discussion below on data Tables 1 - ~.
The protonated form of thiocyanate is similarly made less easily electrooxidized than the free anionic SCN form of thiocyanate. However, in this case the acidic anode boundary layer appears to have little effect on the current efficiency of thiocyanate conversion to cyanide. An explanation is found in the fact that HSCN
is an extremely strong acid compared 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 the thio-cyanate in the acidic boundary la~er would still exist in the much less difficult to electrooxidize SCN form.
The 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, but equally plausible, theories to explain why the process of the present invention occurs. The explanation given above is not intended to be conclusive~
:.., ~ 5 ~
Run #l; Bulk pH = 11.1; 0.5M in carbonate buffer t (min)SCN (~g/l) CN (mg/l) 2160 ~195 200 8 ~0 :
Run #2; Bulk pH ~ 9.5; 0.5M carbonate buffer t ~min)SCN tmg/l) CN (mg/l) o 2860 0 . 50 1180 ~82 100 303 . ~20 8~5~(~
Run #3; Bulk pH = 9. 6? - 05M carbonate buffer (rr.in ) SCN (mg/l ) CN (mg/l ) o 2868 0 1330 ~21 Run #4; Bulk pH = 4.2; no buffer salts added t ~min)SCN (mg/l ) CN (mg/.l ) 1a~02 640 - 100 ~82 952 250 - ~ 9 892 5~3 The ~boye data were obtained b~ processin~ 300 litre batches of thiocyanate solution in an industrial size electrochemical reactox described below and xe~erred to as Reactor 2. The solutions were made up using tap watPr 5 and technical grade salts. Electrochemical processing was caxried out on a batch recirculation basis. The temperature was maintained in the range 24 - 29C. The operatinq current and the recirculation flow rate ~ere the same in all runs.
The buffer capacities for the bulk solutions are 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 differencP in the 15 rates in all four runs.
Considering the extreme r~ms 1 and 4, the higher buffer index in run 1 would effectively prevent signifi-cant acidication of the anode houndary layer~ However, in run 4 the absence of buffPr would result in strong 20 acidification of the anode boundary layer - estimated drop in pH i5 about3 pEI units to pH 1.2 the fact that thiocyanate electrooxidation rate is essentially the same in anode bo~mdary layers at pH 11 and 1.2 suggests the explanation given above for the lack of variation of 25 thiocyanate electro~xidation rate with pH might be valid.
~ omparing the accumulation rates of cyanide in four runs, it is seen that very little cyanide accumulates in run 1 and close to the theoretical amount calculated from equation (10) accumulates in run 4 ~ at least in the 30 first part of *he run. A possible explanation is as follows. In run 1, the acidification of the boundary layer will be resisted by the strong bufferin~ capacicy of the solution and consequently the boundary layer will not drop much below pH 11. At p~ 11 the fraction of 5~
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 thiocyanate, it is electrooxidized in a parallel anodic reaction, hence the low rate of cyanide accumulation and the rapid disappearance o~ 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 oE
the solution will not greatly resist the acidification of 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. At 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 capacity is 10 times higher than in run 3 and very little anode boundary layer acidification would be expected. If this obtains, then the rate and level of cyanide accumulation according to our proposed theory should be greater in run 2 than in rur. 3 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 solu-tion is essen~ially 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 bulk solution and in the acidic boundary layer. Therefore, the rate and level of cyanide accumulation should be highest in this run 3;Z5~.~
which is confirmed by the data.
The process of 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 (S0~ ).
Chemical analyses on process solutions after both partial and complete oxidation has determined that virtually all sulphur is present as sulfate, which is an environmentally acceptable form. This is important where it is desired to use the process of the present invention to treat indus-trial effluents that initially co~tain intermediate oxy-sulfur species as well as thiocyanate.
~arious electrochemical reactors will be suit~
able for use with the process according to the present invention. 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 process in accordance with the present invention. Various other suitable reactors will be r~adily apparent to those skilled in the art. However, while it will be possible to use various electroch~mical reactors including a conventional electrochemical reactor, the efficiency of the process - will vary greatly with the type of reactor used.
~5 While the reactor described in Canadian Patent No . 1~ 016 r 495 is suitable to carry out the process according to the present invention, when the process is to be carried out on a large scale, this reactor ~s presently too expensive and too fragile to be economically feasible.
Since the process oE 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~
A second suitable reactor that can be used to carry out the process in accordance with the present invention is a discrete, fixed layer, particulater bi-polar reactor (henceforth referred to as reactor No. 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 particlesO Of course, the reactor vessel must contain means for supporting the various layers within it. Preferably, the base o the reactor v~ssel is strong enough to support the various layers.
In reactor No. 2, the reactor vessel can be constructed of virtually any suitable material and any reasona~le shapP but is preferably circular in cross section. For example, the reactor vessel can be made of steel with the inside wall being rubber-lined so that i~
is electrically insulated. Also, the reactor wall could be made of concrete. The reactor vessel could - also be constructed in modular form so that additional sections could be added as required. The two primary electroaes 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 various materials~ for exam~le~ graphite, metallur~.ical coke or anthracite. The particles can be specifically arranged in a fi~ed relationship to form a layer, or, where crushed particles are used, sufficient particles can simply be poured onto an insulating spacer to form one layer. One type o~ 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 of the cylinder ~ie. in an upright position) in a fixed rela-~tionship forming one layer of particles. Each layer is topped by a poly-vinyl chloride coated Fibreglas (a trade mark) mesh and therl the next layer of tumbled cylinders is placed immediately on top of that Fibreglas (a txade mark) mesh. Ultimately, a series of fixed 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 than .25cm.
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 similarly shaped cexamic 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 o~ the present invention, can be recovered ~or , re-use. Also, it is sometimes necessary to pre-treat the ef~luent or aqueous solution prior to carrying out the electrochemical reaction within the suitable reactor.
' :' Some of these procedures are discussed 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 above, 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.
A portion of the thiocyanate containing cyanidation leach solution is continuously fed to a suitable electro~
chemical raactor where partial electrooxidation takes place 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.
EX~MPLE 3 Cyanide can be recovered using a strong base ion exchanger on a batch or semi-continuous basis.
EXAMæLE 4 Cyanide can be recovered by utilizing the electrochemical reactor in conjunction with an air 5(3 -- 19 -- . ' stripper to recover the cyanide as hydrocyanic acid.
The elec-trochemical reaction products are fed into an air stripper where air, hydrocyanic acid, water and hydrogen are separated from the electrochemical reaction produc-ts. The cyanide can then be recovered from the hydrocyanic acid by neutrali~ation with lime water or sodium hydroxide in an adsorption tower.
~XP~LE 5 Cyanide can be recovered by utilizing the electrochemical reactor in conjunction with a steam stripper to recover the cyanide as hydrccyanic acid.
This is similar to the use of the air stripper except that steam and air are used with steam stripping. Once lS the hydrocyanic açid is recovered, it can be neutralized : with lime or sodium hydroxide to recover the cyanide.
, Cyanide can be recovered by directly 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)4 solutlon is used for make up for further cyanidation and the zinc collected in the reactor 5~3 -- ~o - , is leached out with sulphuric acid.
__ Where the effluent contains an acidic solution of zinc and thiocyanate, a cation 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 ~he sulphuric acid and hydrocyanic acid formed. The solid calcium sulphate is thickened by settling and the clear super-natant Ca(CN~4 solution is used for make up for further - cyanidation.
Any cathodic zinc that has been deposited is removed from the reactor by sulphuric acid. Zinc is eluted from the cation exchanger with sulphuric acid.
E ~PLE 8 -An acidic zinc-thiocyanate solution i6 treated electrochemically to convert most of the thiocyanate to cyanide and sulphate and simultaneously recover a good ~5 portion of the zinc cathodically.
The electrochemically 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 neutral~ze the sulphuric acid and hydrocyanic acid. The solid calcium sulphate is thickened by settling and the clear supernatant Ca~CN)4 solution is used for make up for further cyanidation.
5~
The zinc collected in the reactor is leached ou~ with sulphuric acid and zinc is eluted from the cation exchanger with sulphuric acid.
EXAMPI.E 9 Prior to carrying out the electrochemical reaction, where the cyanidation waste is basic, it is acidified to a pH ranging from 5 to 6.5 and any solids are filtered out.
The waste is then treated on a weak 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 essentially 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 set out in Examples 1 and 2.
This would be necessary only where the alkaline cyanidation waste has a high buffer index.
EXP~LE 10 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.
~: ' l:L~50 EXAMæLE 11 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 dioxid~ to produce a solution with a low buffer index.
The electrochemicàl reaction of the present invention can then be carried out on t~e xesulting solution to convert the.thiocyanate to cyanide and the cyanide so formed can be recovered.
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) 2H20~2 + 4H + ~e : (g) (8) 4 ~ + 2H20 + 4e (g) ;'5~
In the absence of electroreducible species such as platable ~etals the predominant reaction at the cathode is the pxoduction of hydrogen gas by the electroreduction of hydrogen (hydronium) ion or equivalently, from the stoichiometric viewpoint, the electroreduction of water. The reaction may be written as ~9) 2H20 ~ 2e -~H2 -~ 20H
; 10 (g) From the standpoint of recovering cyanide from thiocyanate, the relevant electrode reactions are (4) and (8)o From the standpoint of ~onverting thiocyanate to relatively nontoxic cyanate and to nontoxic nitrogen gas and carbon dioxide, the relevant electrode reactions are recpectively (5) and (9) and (6) and (9). The anodic formation of oxygen gas operates in parallel with all thiocyanate anodic reactions. At high thiocyanate concentrations the current efficiency for oxygen pro duction is relatively lowO At low thiocyanate concen-trations (and cyanide) oxygen production becomes the predominant anodic reac~ion.
The overall electrochemical cell reaction leading to the production of cyanide from thiocyanate is obtained by combing ~quations ~4) and (9) to yield reaction equation (1~3 (10) SCN + 4H2O-~ CN + H2SO~ + 3H2 When considering the overall reaction (10) and assuminy a current efficiency of 100% (that is no o~her anodic and cathodic reactions of significance are occurring), there is a net acid production of 0.33 moles of H~ per 5~3 Faraday of charge through the cell. Therefore, as the electrochemical processing of thiocyanate solution proceeds the solution tends to become more and more acidic. Reaction (10) stoichiometry has been verified by analysis for thiocyanater cyanide and acid during the course of e]ectro]ysisO
The production of acid is beneicial from the standpoint of the specific cyanide yield since (except where thethiocyanate solution has a high buffering capacity) it has the effect of preserving the cyanide produced from undergoing further rapid electrooxidation to cyanate or through to nltrogen gas and carbon dioxide.
Initially, the conversion of thiocyanate at the anode can be represented by the reaction (4). When the thio-cyanate solution does not have a high buffering capacityin the acidic direction, the large amount of acid produced (~ moles of H+ per mol of cyanide produced) will tend to cause a substantial decrease in the pH of the anolyte solution adjacent the anode surface. Similarly, the hydroxyl ion produced by the cathodic xeaction will increase the pH in the catholyte adjacent the cathode suxfaces although this effect will be resisted if the thiocyana~e solution has substantial buf-fering capacity in the basic direction. This suygests that an acidic ~5 anode boundary layer and a basic cathode boundary layer may exist.
It is the establishment of an acidic anode boundary layer which is believed to be the main reason why the cyanide product is protected from xapid electro-o~idation 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 electrooxidi2ed at the anode to produce o cyanide ion, the cyanide lon is immediately protonated by the anodically produced acid. Consequently, the acidic anode boundary layer functions to preserve cyanide from rapid electrooxidation at the anode by converting the cyanide ion into the much moxe diEEicult to electrooxidize neutxal protonated form. This explanation is considered in a quantitative way in the discussion below on data Tables 1 - ~.
The protonated form of thiocyanate is similarly made less easily electrooxidized than the free anionic SCN form of thiocyanate. However, in this case the acidic anode boundary layer appears to have little effect on the current efficiency of thiocyanate conversion to cyanide. An explanation is found in the fact that HSCN
is an extremely strong acid compared 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 the thio-cyanate in the acidic boundary la~er would still exist in the much less difficult to electrooxidize SCN form.
The 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, but equally plausible, theories to explain why the process of the present invention occurs. The explanation given above is not intended to be conclusive~
:.., ~ 5 ~
Run #l; Bulk pH = 11.1; 0.5M in carbonate buffer t (min)SCN (~g/l) CN (mg/l) 2160 ~195 200 8 ~0 :
Run #2; Bulk pH ~ 9.5; 0.5M carbonate buffer t ~min)SCN tmg/l) CN (mg/l) o 2860 0 . 50 1180 ~82 100 303 . ~20 8~5~(~
Run #3; Bulk pH = 9. 6? - 05M carbonate buffer (rr.in ) SCN (mg/l ) CN (mg/l ) o 2868 0 1330 ~21 Run #4; Bulk pH = 4.2; no buffer salts added t ~min)SCN (mg/l ) CN (mg/.l ) 1a~02 640 - 100 ~82 952 250 - ~ 9 892 5~3 The ~boye data were obtained b~ processin~ 300 litre batches of thiocyanate solution in an industrial size electrochemical reactox described below and xe~erred to as Reactor 2. The solutions were made up using tap watPr 5 and technical grade salts. Electrochemical processing was caxried out on a batch recirculation basis. The temperature was maintained in the range 24 - 29C. The operatinq current and the recirculation flow rate ~ere the same in all runs.
The buffer capacities for the bulk solutions are 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 differencP in the 15 rates in all four runs.
Considering the extreme r~ms 1 and 4, the higher buffer index in run 1 would effectively prevent signifi-cant acidication of the anode houndary layer~ However, in run 4 the absence of buffPr would result in strong 20 acidification of the anode boundary layer - estimated drop in pH i5 about3 pEI units to pH 1.2 the fact that thiocyanate electrooxidation rate is essentially the same in anode bo~mdary layers at pH 11 and 1.2 suggests the explanation given above for the lack of variation of 25 thiocyanate electro~xidation rate with pH might be valid.
~ omparing the accumulation rates of cyanide in four runs, it is seen that very little cyanide accumulates in run 1 and close to the theoretical amount calculated from equation (10) accumulates in run 4 ~ at least in the 30 first part of *he run. A possible explanation is as follows. In run 1, the acidification of the boundary layer will be resisted by the strong bufferin~ capacicy of the solution and consequently the boundary layer will not drop much below pH 11. At p~ 11 the fraction of 5~
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 thiocyanate, it is electrooxidized in a parallel anodic reaction, hence the low rate of cyanide accumulation and the rapid disappearance o~ 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 oE
the solution will not greatly resist the acidification of 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. At 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 capacity is 10 times higher than in run 3 and very little anode boundary layer acidification would be expected. If this obtains, then the rate and level of cyanide accumulation according to our proposed theory should be greater in run 2 than in rur. 3 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 solu-tion is essen~ially 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 bulk solution and in the acidic boundary layer. Therefore, the rate and level of cyanide accumulation should be highest in this run 3;Z5~.~
which is confirmed by the data.
The process of 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 (S0~ ).
Chemical analyses on process solutions after both partial and complete oxidation has determined that virtually all sulphur is present as sulfate, which is an environmentally acceptable form. This is important where it is desired to use the process of the present invention to treat indus-trial effluents that initially co~tain intermediate oxy-sulfur species as well as thiocyanate.
~arious electrochemical reactors will be suit~
able for use with the process according to the present invention. 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 process in accordance with the present invention. Various other suitable reactors will be r~adily apparent to those skilled in the art. However, while it will be possible to use various electroch~mical reactors including a conventional electrochemical reactor, the efficiency of the process - will vary greatly with the type of reactor used.
~5 While the reactor described in Canadian Patent No . 1~ 016 r 495 is suitable to carry out the process according to the present invention, when the process is to be carried out on a large scale, this reactor ~s presently too expensive and too fragile to be economically feasible.
Since the process oE 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~
A second suitable reactor that can be used to carry out the process in accordance with the present invention is a discrete, fixed layer, particulater bi-polar reactor (henceforth referred to as reactor No. 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 particlesO Of course, the reactor vessel must contain means for supporting the various layers within it. Preferably, the base o the reactor v~ssel is strong enough to support the various layers.
In reactor No. 2, the reactor vessel can be constructed of virtually any suitable material and any reasona~le shapP but is preferably circular in cross section. For example, the reactor vessel can be made of steel with the inside wall being rubber-lined so that i~
is electrically insulated. Also, the reactor wall could be made of concrete. The reactor vessel could - also be constructed in modular form so that additional sections could be added as required. The two primary electroaes 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 various materials~ for exam~le~ graphite, metallur~.ical coke or anthracite. The particles can be specifically arranged in a fi~ed relationship to form a layer, or, where crushed particles are used, sufficient particles can simply be poured onto an insulating spacer to form one layer. One type o~ 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 of the cylinder ~ie. in an upright position) in a fixed rela-~tionship forming one layer of particles. Each layer is topped by a poly-vinyl chloride coated Fibreglas (a trade mark) mesh and therl the next layer of tumbled cylinders is placed immediately on top of that Fibreglas (a txade mark) mesh. Ultimately, a series of fixed 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 than .25cm.
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 similarly shaped cexamic 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 o~ the present invention, can be recovered ~or , re-use. Also, it is sometimes necessary to pre-treat the ef~luent or aqueous solution prior to carrying out the electrochemical reaction within the suitable reactor.
' :' Some of these procedures are discussed 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 above, 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.
A portion of the thiocyanate containing cyanidation leach solution is continuously fed to a suitable electro~
chemical raactor where partial electrooxidation takes place 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.
EX~MPLE 3 Cyanide can be recovered using a strong base ion exchanger on a batch or semi-continuous basis.
EXAMæLE 4 Cyanide can be recovered by utilizing the electrochemical reactor in conjunction with an air 5(3 -- 19 -- . ' stripper to recover the cyanide as hydrocyanic acid.
The elec-trochemical reaction products are fed into an air stripper where air, hydrocyanic acid, water and hydrogen are separated from the electrochemical reaction produc-ts. The cyanide can then be recovered from the hydrocyanic acid by neutrali~ation with lime water or sodium hydroxide in an adsorption tower.
~XP~LE 5 Cyanide can be recovered by utilizing the electrochemical reactor in conjunction with a steam stripper to recover the cyanide as hydrccyanic acid.
This is similar to the use of the air stripper except that steam and air are used with steam stripping. Once lS the hydrocyanic açid is recovered, it can be neutralized : with lime or sodium hydroxide to recover the cyanide.
, Cyanide can be recovered by directly 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)4 solutlon is used for make up for further cyanidation and the zinc collected in the reactor 5~3 -- ~o - , is leached out with sulphuric acid.
__ Where the effluent contains an acidic solution of zinc and thiocyanate, a cation 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 ~he sulphuric acid and hydrocyanic acid formed. The solid calcium sulphate is thickened by settling and the clear super-natant Ca(CN~4 solution is used for make up for further - cyanidation.
Any cathodic zinc that has been deposited is removed from the reactor by sulphuric acid. Zinc is eluted from the cation exchanger with sulphuric acid.
E ~PLE 8 -An acidic zinc-thiocyanate solution i6 treated electrochemically to convert most of the thiocyanate to cyanide and sulphate and simultaneously recover a good ~5 portion of the zinc cathodically.
The electrochemically 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 neutral~ze the sulphuric acid and hydrocyanic acid. The solid calcium sulphate is thickened by settling and the clear supernatant Ca~CN)4 solution is used for make up for further cyanidation.
5~
The zinc collected in the reactor is leached ou~ with sulphuric acid and zinc is eluted from the cation exchanger with sulphuric acid.
EXAMPI.E 9 Prior to carrying out the electrochemical reaction, where the cyanidation waste is basic, it is acidified to a pH ranging from 5 to 6.5 and any solids are filtered out.
The waste is then treated on a weak 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 essentially 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 set out in Examples 1 and 2.
This would be necessary only where the alkaline cyanidation waste has a high buffer index.
EXP~LE 10 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.
~: ' l:L~50 EXAMæLE 11 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 dioxid~ to produce a solution with a low buffer index.
The electrochemicàl reaction of the present invention can then be carried out on t~e xesulting solution to convert the.thiocyanate to cyanide and the cyanide so formed can be recovered.
Claims (18)
1. A process for the recovery of cyanide from thio-cyanate, said process comprising introducing an aqueous solution containing thiocyanate into a suitable electro-chemical reactor, applying a direct current electrical potential to said reactor carrying 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 pH of the aqueous 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.
2. A process as claimed in Claim 1 including the steps of introducing the aqueous solution into the electro-chemical reactor at a temperature around room temperature and carrying out the reaction without significant input.
3. A process as claimed in Claim 1 wherein the reaction is carried in a range from 24°C to 29°C.
4. A process as claimed in any one of Claims 1, 2 or 3 wherein said process is carried out on a continuous basis.
5. A process as claimed in Claim 1 wherein the pH of the aqueous solution, shortly after the electrochemical reaction begins, is maintained in the range of 1 to 4.
6. A process as claimed in any one of Claims 2, 3 or 5 wherein the aqueous solution contains a carbonate species that causes the solution to have a high buffer capacity, and prior to applying a direct current electrical potential to the reactor, removing substantially all of the carbonate species contained in said aqueous solution to substantially reduce said buffer capacity.
7. A process as claimed in Claim 5 wherein the pH of the aqueous solution during the process is maintained in the range of 1 to 4 by the generation of acid during the electrochemical reaction.
8. A process as claimed in Claim 5 wherein the pH
of the aqueous solution during the process is maintained in the range of 1 to 4 by the addition of concentrated sulphuric acid.
of the aqueous solution during the process is maintained in the range of 1 to 4 by the addition of concentrated sulphuric acid.
9. A process as claimed in any one of Claims 2, 3 or 5 wherein the cyanide is recovered by utilizing the electrochemical reactor in conjunction with an air stripper to recover the cyanide as hydrocyanic acid and neutralizing the hydrocyanic acid to recover the cyanide.
10. A process as claimed in any one of Claims 2, 3 or 5 wherein the cyanide is recovered by utilizing the electrochemical reactor in conjunction with a steam stripper to recover the cyanide as hydrocyanic acid and neutralizing the hydrocyanic acid to recover the cyanide.
11. A process as claimed in any one of Claims 2, 3 or 5 wherein the aqueous solution is acidic and contains zinc which is deposited cathodically during the electrochemical reaction, the cyanide being recovered by treating the solution containing the electrochemical reaction products with lime to neutralize the sulphuric acid and hydrocyanic acid and leaching any zinc collected in the reactor with sulphuric acid.
12. A process as claimed in any one of Claims 2, 3 or 5 wherein the aqueous solution contains zinc, including the steps of removing the zinc by using a cation exchanger before the aqueous solution is introduced into the electro-chemical reactor, cathodically depositing any residual zinc during the electrochemical reaction, treating the solution resulting from the electrochemical reactor with lime to neutralize the sulphuric acid and hydrocyanic acid, leaching any zinc collected in the reactor with sulphuric acid and recovering zinc from the cation exchanger with sulphuric acid.
13. A process as claimed in any one of Claims 2, 3 or 5 including the steps, prior to the electrochemical reaction, of acidifying the aqueous solution to a pH
ranging from 5 to 6.5 any solids being filtered out, treating the solution on a weak base anion exchanger to first extract any anionic metal cyanide species and then collecting the thiocyanate on a second weak base anion exchanger and then recovering the thiocyanate by eluting the thiocyanate from the ion exchanger with aqueous sodium hydroxide or lime water.
ranging from 5 to 6.5 any solids being filtered out, treating the solution on a weak base anion exchanger to first extract any anionic metal cyanide species and then collecting the thiocyanate on a second weak base anion exchanger and then recovering the thiocyanate by eluting the thiocyanate from the ion exchanger with aqueous sodium hydroxide or lime water.
14. A process as claimed in any one of Claims 2, 3 or 5 wherein the aqueous solution has a high buffering index and prior to the electrochemical reaction, the high buffer index of the solution is reduced by adding calcium chloride to precipitate calcium carbonate.
15. A process as claimed in any one of Claims 2, 3 or 5 wherein the aqueous solution has a high buffer capacity which is reduced prior to the electrochemical reaction by acidification to substantially convert all of the leach CO3? and CO2? to CO2? and expurgating the CO2.
16. A process as claimed in any one of Claims 2, 3 or 5 wherein cyanide is recovered by the direction return of the cyanide enriched aqueous solution to a cyanide consuming process.
17. A process as claimed in any one of Claims 2, 3 or 5 wherein cyanide is recovered in concentrated form using a weak base anion exchanger on a batch or continuous basis.
18. A process as claimed in any one of Claims 2, 3 or 5 wherein the cyanide is recovered by expurgation of cyanide as hydrocyanic acid on a batch or continuous basis.
Priority Applications (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000404388A CA1188250A (en) | 1982-06-03 | 1982-06-03 | Electrochemically treating aqueous thiocyanate solutions |
| CA000474748A CA1209523A (en) | 1982-06-03 | 1985-02-20 | Detoxication of thiocyanate solutions by electrochemical oxidation |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000404388A CA1188250A (en) | 1982-06-03 | 1982-06-03 | Electrochemically treating aqueous thiocyanate solutions |
Related Child Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000474748A Division CA1209523A (en) | 1982-06-03 | 1985-02-20 | Detoxication of thiocyanate solutions by electrochemical oxidation |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1188250A true CA1188250A (en) | 1985-06-04 |
Family
ID=4122914
Family Applications (2)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000404388A Expired CA1188250A (en) | 1982-06-03 | 1982-06-03 | Electrochemically treating aqueous thiocyanate solutions |
| CA000474748A Expired CA1209523A (en) | 1982-06-03 | 1985-02-20 | Detoxication of thiocyanate solutions by electrochemical oxidation |
Family Applications After (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000474748A Expired CA1209523A (en) | 1982-06-03 | 1985-02-20 | Detoxication of thiocyanate solutions by electrochemical oxidation |
Country Status (1)
| Country | Link |
|---|---|
| CA (2) | CA1188250A (en) |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| MX2023002015A (en) | 2020-08-18 | 2023-04-11 | Enviro Metals Llc | Metal refinement. |
-
1982
- 1982-06-03 CA CA000404388A patent/CA1188250A/en not_active Expired
-
1985
- 1985-02-20 CA CA000474748A patent/CA1209523A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| CA1209523A (en) | 1986-08-12 |
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