cyanide remediation: current and past technologies

Proceedings of the 10th Annual Conference on Hazardous Waste Research104

CYANIDE REMEDIATION: CURRENT ANDPAST TECHNOLOGIES

C.A. Young§ and T.S. Jordan, Department of Metallurgical Engineering, Montana Tech,Butte, MT 59701

ABSTRACTCyanide (CN-) is a toxic species that is found predominantly in industrial effluentsgenerated by metallurgical operations. Cyanide's strong affinity for metals makes itfavorable as an agent for metal finishing and treatment and as a lixivant for metalleaching, particularly gold. These technologies are environmentally sound but requiresafeguards to prevent accidental spills from contaminating soils as well as surface andground waters. Various methods of cyanide remediation by separation and oxidation aretherefore reviewed. Reaction mechanisms are given throughout. The methods arecompared in regard to their effectiveness in treating various cyanide species: freecyanide, thiocyanate, weak-acid dissociables and strong-acid dissociables.

KEY WORDScyanide, metal-cyanide complex, thiocyanate, oxidation, separation

§ Corresponding author.

INTRODUCTIONWaste waters from industrial operationstransport many chemicals that have ad-verse effects on the environment. Variouschemicals leach heavy metals which wouldotherwise remain immobile. The chemicalsand heavy metals may be toxic and thuscause aquatic and land biota to sicken ordie. Most waste-water processing tech-nologies that are currently available or arebeing developed emphasize the removal ofthe chemicals or heavy metals as cations.However, the anions associated with heavymetal cations can be equally as toxic butare largely ignored. In this regard, theremediation of cyanide has been consid-ered paramount [1].

Cyanide is a singly-charged anion contain-ing unimolar amounts of carbon and nitro-gen atoms triply-bonded together: C≡N- orCN-. It is a strong ligand, capable of com-plexing at low concentrations with virtuallyany heavy metal. Because the health andsurvival of plants and animals are depend-

ent on the transport of these heavy metalsthrough their tissues, cyanide is very toxic.The mean lethal dose to the human adult isbetween 50 and 200 mg [2]. U.S. EPAstandards for drinking and aquatic-biotawaters regarding total cyanide are 200 and50 ppb, respectively, where total cyaniderefers to free and metal-complexed cya-nides [3]. According to RCRA, all cyanidespecies are considered to be acute haz-ardous materials and have therefore beendesignated as P-Class hazardous wasteswhen being disposed of. P-Class hazard-ous wastes are the most regulated waste inregard to amounts accumulated in a givenyear as compared to the other class desig-nations: F, K and U.

Metal-complexed cyanides are classifiedaccording to the strength of the metal-cyanide bond. Free cyanide refers to themost toxic forms of cyanide: cyanide anionand hydrogen cyanide. Weak-acid disso-ciables (WADs) refer to cyanide complexeswith metals such as cadmium, copper,nickel and zinc. Although thiocyanate

Proceedings of the 10th Annual Conference on Hazardous Waste Research 105

(SCN-) is a WAD, it is often considered inits own category. Strong-acid dissociables(SADs) refer to cyanide-complexes withmetals such as cobalt, gold, iron and silver.This nonspecific, complexing nature makescyanide attractive for (1) extracting metals,particularly gold, from ores via variousleaching operations; (2) finishing/treatingmetal products and their surfaces, and (3)preventing certain particles from becominghydrophobic in the flotation separation ofminerals. Cyanide concentrations forleaching and finishing/treating are severalorders of magnitude higher than those en-countered in flotation whereas cyanide so-lutions are more voluminous in leaching ascompared in finishing/treating. Over a bil-lion tons of gold ore are leached each yearwith cyanide. Consequently, in order toprevent surface and ground water contami-nation, procedures for the safe and propertreatment, storage and handling of effluentsare of primary concern for cyanide leachingoperations [4, 5].

In this study, current and past remediationmethods for waste and process waterscontaining cyanide are reviewed. Emphasisis placed on the oxidation methods whichactually destroy the cyanide.

CYANIDE REMEDIATIONVarious procedures exist for treating cya-nide and are comprised of several physical,adsorption, complexation and/or oxidationmethods. These methods predominantlyinvolve separation or destruction processesand may occur naturally. Separation proc-esses include the physical, adsorption andcomplexation methods that are used toconcentrate and thereby recover cyanidefor recycling. On the other hand, destruc-tion processes are used to sever the car-bon-nitrogen triple bond thereby destroyingthe cyanide and producing non-toxic orless-toxic species. Because the carbonand/or nitrogen atoms in the cyanide mole-cule undergo changes in oxidation state,destruction processes are commonly re-

ferred to as oxidation methods. Thesephysical, adsorption, complexation and oxi-dation methods are described in the ensu-ing discussions.

Physical methodsPhysical methods for cyanide treatmentcan be accomplished using dilution, mem-branes, electrowinning and hydroly-sis/distillation.

Dilution

Dilution is the only treatment method whichdoes not separate or destroy cyanide. Thismethod involves combining a toxic cyanidewaste with an effluent that is low in or freeof cyanide to yield a waste water belowdischarge limits. Consequently, dilution issimple and cheap and is often used as astand-alone or back-up method to insurethat discharge limits are satisfied [3]. Dilu-tion is usually considered to be unaccept-able since the total amount of cyanide dis-charge is not altered and since naturally-occurring processes such as adsorptionand precipitation can attenuate and therebyconcentrate the cyanide in ground andsurface waters.

Membranes

Cyanide can be separated from water us-ing membranes with either electrodialysisor reverse osmosis. In electrodialysis, apotential is applied across two electrodesseparated by a membrane permeable tocyanide. The cyanide solution requiringpurification is placed in the half-cell contain-ing the cathode or negative electrode.Cyanide, because it is negatively charged,will diffuse through the membrane andconcentrate in the half-cell containing theanode or positive electrode. In reverse os-mosis, pressure is applied to a cyanide so-lution needing treatment but, in this case,water is forced through a membrane im-permeable to cyanide. Both of these meth-ods have been shown to be applicable to


solutions containing free and metal-complexed cyanide [6-11].

Electrowinning

SADs and WADs can be reduced to corre-sponding metals by applying a potentialacross two electrodes immersed in thesame solution:

M(CN)xy-x + ye- → Mo + xCN- (1)

Thiocyanate does not respond. Free cya-nide is liberated which makes solutionsmore amenable to other recovery andremediation processes. Because this elec-trowinning reaction involves the reductionof an anion at the cathode, a negatively-charged electrode, metal recoveries andcurrent efficiencies are low. This is com-pensated for by using steel wools as high-surface area cathodes, increasing agitationto further increase mass transport, increas-ing solution temperatures, using appropri-ate solution pHs and conductivities, in-creasing metal concentrations if possible,and redesigning the electrowinning cell ofwhich four designs have been developedfor gold processing: Zadra, AARL, NIMgraphite-chip and MINTEK parallel platecells [3, 12]. Furthermore, Reaction 1 mustbe accompanied by a corresponding oxida-tion reaction, usually oxygen evolution, atthe anode with an equivalent number ofelectrons, e-, being donated:

y/4.{4OH- → 2H2O + O2 + 4e-} (2a)

y/4.{2H2O → O2 + 4H+ + 4e-} (2b)

Electrowinning is predominantly used forgold processing; however, it has been usedfor cyanide regeneration, in which case it isoften referred to as the Celec or HSA proc-ess [3, 13]. Electrowinning performs well inconcentrated solutions; at dilute concentra-tions, hydrogen evolution predominates,possibly masking Reaction 1 completely:

2H+ + 2e- → H2(g) (3)

Progress is continuing to make electrowin-ning technology economically viable to di-lute solutions. Direct applications to cya-nide remediation may then be possible.

Hydrolysis/distillation

Free cyanide naturally hydrolyzes in waterto produce aqueous hydrogen cyanide[HCN(aq)]:

CN- + H+ → HCN(aq) (4)

The aqueous hydrogen cyanide can thenvolatilize as hydrocyanic gas [HCN(g)]:

HCN(aq) → HCN(g) (5)

Because hydrocyanic gas has a vaporpressure of 100 kPa at 26ºC, which isabove that of water (34 kPa at 26ºC), and aboiling point of 79ºC, which is below that ofwater (100ºC), cyanide separation can beenhanced at elevated temperatures and/orreduced pressures [14-16]. Distillation ratescan also be increased by increasing theagitation rate, the air/solution ratio, and thesurface area at the air/solution interface.Hydrocyanic gas can be captured and con-centrated for recycling in conventional ab-sorption-scrubbing towers. It can also bevented to the open atmosphere and hasbeen noted to occur naturally in tailingsponds, especially in warm and arid envi-ronments [17]. In such cases, it is para-mount that environmental regulations besatisfied. Thiocyanate, WADs and SADsare not affected.

Complexation methodsCyanide treatment can also be done withseveral complexation methods such asacidification/volatilization, metal addition,flotation and solvent extraction.

Acidification/volatilization

Because Reaction 4 is in equilibrium at pH9.3, cyanide predominates above this pHvalue and aqueous hydrogen cyanide pre-dominates below. Consequently, cyanide


will remain in solution at pH values greaterthan approximately 11, which explains whyoperations maintain cyanide solutionsabove this pH. On the other hand, cyanidewill volatilize at pH values less than ap-proximately 8; the lower the pH, the higherthe rate of volatilization. If the pH is loweredbelow pH 2, hydrocyanic gas will also beevolved from WADs:

M(CN)xy-x + xH+ → xHCN(g) + My+ (6)

Thiocyanate and SADs react similarly butsolutions must typically be adjusted belowpH 0 and consequently are only moderatelyaffected [18]. Since this would increaseacid consumption beyond reason, typicalacidification/volatilization operations use apH between 1.5 and 2. Volatilization ratescan also be increased in the same manneras distillation. In addition, volatilization hasbeen observed naturally due to the dissolu-tion of carbon dioxide and subsequent for-mation of carbonic acid [15, 16, 19].

After acidification, the solution is predomi-nantly cyanide-free but must be reneutral-ized so it can be discharged or recycled forfurther use. Reneutralization causes themetals which were liberated via Reaction 6to be precipitated as hydroxides:

My+ + yOH- = M(OH)y (s) (7)

If lime [Ca(OH)2] is used for reneutralizationand sulfuric acid (H2SO4) is used for acidifi-cation, gypsum (CaSO4-2H2O) will also beprecipitated:

Ca2+ + SO42- = CaSO4-2H2O (s) (8)

Sludges of gypsum and metal hydroxidesare often difficult to separate from water[20], which is a problem that can be mini-mized by using other acids such as nitric,HNO3 [21], or other bases such as caustic,NaOH [22]. Nitric and caustic are more ex-pensive but savings can be realized insludge separation and treatment.

Acidification/volatilization (AV) and reneu-tralization (R) processes are collectivelyreferred to as the AVR or Mills-Croweprocess that was originally developed circa1930 at the Flin Flon operation in Canada[2, 3, 22]. Different versions were imple-mented later at the Real del Monte mine inMexico, the CANMET facility in Canada,and the Golconda CRP venture in Australia[23-25]. A similar technology, the Cyani-sorb™ process, has recently been tested atthe Golden Cross operation in New Zea-land and the De Lamar mine in Idaho andfound to be more efficient: (1) acidificationwas conducted at pH 6 using better de-signed packed towers, and (2) absorptionwas conducted using caustic to preventfouling from gypsum precipitation [26, 27].It is apparent that the AVR and Cyani-sorb™ processes consume tremendousamounts of acids and bases but are pre-ferred over the hydrolysis/distillation proc-ess due to reduced energy consumptionand increased volatilization rates.

Metal addition

Cyanide can be rendered unreactive by theaddition of various metals and metalcations to promote the formation of metal-cyanide complexes or precipitates. For ex-ample, in the Merrill-Crowe process devel-oped in 1890 for gold recovery, zinc isadded to gold cyanide solutions resulting inthe precipitation or cementation of the gold[28]:

2Au(CN)2- + Zno → 2Auo + Zn(CN)4

- (9)

In essence, a gold SAD-complex is ex-changed in solution for a zinc WAD-complex thus making the solution moreamenable to other remediation methods.Cementation of gold and other metals incyanide solutions has also been accom-plished with aluminum, copper and iron [29-33].

In another process first employed in 1909[34], ferrous (Fe2+) or ferric (Fe3+) cations


are added to cyanide solutions to form therespective SAD-complexes Fe(CN)6

4- andFe(CN)6

3-. These ferrohexacyanide andferrihexacyanide anions are considered tobe essentially nontoxic due to their highstability; however, concerns regarding theirphotodecomposition (see photolysis meth-ods), and therefore their long term stability,have been raised [2]. Nevertheless, proc-esses have been developed where theseanions are precipitated as double salts. Forexample, ferrohexacyanide anions precipi-tate as Prussian Blue upon addition of ferriccations [35-37]:

3Fe(CN)64- + 4Fe3+ → Fe4[Fe(CN)6]3(s) (10)

Prussian blue precipitation is commonlyobserved in process circuits. Ferrohexacy-anide anions can also be precipitated asCu2Fe(CN)6, Ni2Fe(CN)6 and Zn2Fe(CN)6

upon addition of cupric, nickel and zinccations, respectively [3, 37, 38]:

Fe(CN)64- + 2M2+ → M2Fe(CN)6(s) (11)

Similarly, ferrihexacyanide anions can beprecipitated as Prussian Brown with ferrouscations or as Prussian Green with ferriccations [39-41]:

2Fe(CN)63- + 3Fe2+ → Fe3[Fe(CN)6]2(s) (12a)

Fe(CN)63- + Fe3+ → Fe2(CN)6(s) (12b)

In other precipitation processes, cuprousand argentous cations have been added inexcess to form CuCN and AgCN singlesalts from free cyanide [42-44]. Silver pre-cipitation is used commonly as a titrationtechnique for determining free cyanideconcentrations [45].

Each of the cemented metals, SAD-complexes and cyanide precipitates formedby metal addition processes is stable, but itis paramount that the pH be maintainedduring and after their formation to preventtheir decomposition and dissolution. Thio-cyanate has limited response to metal ad-

dition but can be removed via substitutionfor cyanide in the products [46]. Clearly,metal addition is usually not a stand-alonemethod for cyanide treatment.

Flotation

Flotation was first implemented in 1880 forconcentrating minerals from ores [47, 48]and has since been adapted for remedia-tion. In regards to cyanide remediation, ionand conventional flotation practices havebeen used to separate SAD-complexesand precipitates formed naturally or bymetal addition. Flotation is usually con-ducted quickly to prevent these speciesfrom decomposing and redissolving [18,49-52]. In ion flotation, a heteropolar surfac-tant, usually a cationic amine such as trica-prylmethyl ammonium chloride (R4NCl), isadded to react with the anionic SAD-complex to precipitate as an organic (R)double salt:

(y-x)R4NCl + M(CN)xy-x →

(R4N)y-xM(CN)6(s) + (y-x)Cl- (13)

Usually, the double salt precipitate will nu-cleate to form colloids or larger-sized parti-cles. Ion flotation works well for separatingSADs but only partially for WADs. The fateof thiocyanate is unknown. By comparison,the same or different surfactant is added inconventional flotation to adsorb at the pre-cipitate surface. In both cases, hydrophobicspecies are created that can be separatedinto a froth phase upon injection of airbubbles into the system. Various chemicalssuch as frothers and modifiers can beadded to maximize the flotation response.

Solvent extraction

In solvent extraction (SX), an organic sol-vent or diluent is used to solubilize at leastone of many organic extractants of solvat-ing, chelating or ion-exchange capabilities.The diluent must be immiscible in and lessdense than water, the aqueous phase, andthe extractant must remain in the diluentand have selectivity for the aqueous spe-


cies being remediated. The diluent and ex-tractant are collectively referred to as theorganic phase. When the organic andaqueous phases are mixed, the aqueousspecies is stripped from the aqueous phaseby the extractant in reactions similar to Re-action 13 above. The species has loadedinto the organic phase. Various chemicalstermed modifiers can be added to the or-ganic and aqueous phases to maximize theextraction. Mixing is ceased at the appro-priate time, and the stripped water andloaded organic are allowed to disengage,thus effecting a separation of the species.The process is repeated to transfer thespecies from the loaded organic to a sec-ond aqueous phase of lower volume andappropriate chemistry. As a result, the con-centration of the species is increased andthe organic phase is stripped and recycled.Recycling is necessary to minimize ex-penses due to organic loss and is possiblebecause the extraction is reversible.

Solvent extraction technology was originallydeveloped for uranium processing in themid-1950s [53] and received world-wideattention when the technology was suc-cessfully applied to copper processing tenyears later [54, 55]. Applications have sincebeen developed for selectively extractinggold cyanide using amine and phosphorousester solvent extractants [56-58]. Otherapplications of solvent extraction to cyanideare the analytical detection proceduresused in colorimetry [59-61] and, conse-quently, of industrial interest for possibleadaptation to future processing and reme-diation. In this regard, very little is knownabout the use of solvent extraction for theremediation of nearly all cyanide species.

Adsorption methodsMinerals, activated carbons and resins ad-sorb cyanide from solution. Several typesof contact vessels can be used for this pur-pose and include elutriation columns, agi-tation cells, packed-bed columns andloops, etc. Once the cyanide is adsorbed,

the material is separated from the solutionby, for example, screening, gravity separa-tion or flotation. The material is then placedinto another vessel where the cyanide isdesorbed into a low volume solution andthus concentrated. Finally, the material isseparated again and, in most cases, reacti-vated and recycled for further use.

Minerals

Soils, wastes and ores containing mineralssuch as ilmenite (FeTiO3), hematite(Fe2O3), bauxite [AlO.OH/Al(OH)3] and py-rite (FeS2), as well as mineral-groups suchas feldspars, zeolites and clays have beenshown to effectively adsorb free and metal-complexed cyanides [62-66]. Depending onthe mineral, cyanide adsorption is usually acombination of two mechanisms: ion ex-change, precipitation or coulombic interac-tion. Cyanide attenuation by such mineralcommodities purifies ground and surfacewaters but increases cyanide consumptionin leaching operations.

Activated carbon

Active or activated carbons are typicallyprepared by partial thermochemical de-composition of carbonaceous materials—predominantly wood, peat, coal and coco-nut shells. Adsorption characteristicschange with preparation method and mate-rial. Because activated carbons have highporosity and high surface area, adsorptioncapacities and rates are high. Adsorption,however, is not very selective; cations, ani-ons, and neutral species can be adsorbedsimultaneously at various sites via ion ex-change, solvation, chelation and coulombicinteractions. Activated carbons are com-monly used in packed-bed systems fortreating waste waters and gases. Althoughthe use of activated carbons dates back toancient Egypt and India, the first commer-cial application was in gas masks circa1915 [67]. Applications to cyanide wastewaters have been reported with packed-bed systems and shown to be applicable atdilute cyanide concentrations [68, 69] with


increased adsorption of WADs and SADswith copper or silver pretreatment [18, 70,71].

In gold leaching with cyanide, packed-bedsystems are usually avoided and havegiven rise to the use of continuous opera-tions exemplified by carbon-in-column(CIC), carbon-in-leach (CIL) and carbon-in-pulp (CIP) processes. Although these proc-esses have been designed for gold-cyanideadsorption, it does not preclude applyingthem for the remediation of free or othermetal-cyanide complexes as implied byMuir et al. [15]. High adsorption capacitiesand rates as well as strong ability for reacti-vation are important parameters for all acti-vated carbons but, when used in CIC, CILand CIP processes, they must also havehigh mechanical strength, high wear resis-tance, and a consistent particle size.

Resins

Resins are usually polymeric beads con-taining a variety of surface functionalgroups with either chelation or ion-exchange capabilities, somewhat similar tosolvent extraction processes discussedearlier. They can be selective and havehigh adsorption capacities depending onwhether the chelating or ion-exchangeproperties are strong or weak. Resins thatare deposited on substrates as thin filmsare predominantly used in packed-bedsystems, whereas resins that do not requirea substrate are mostly used in continuousprocesses like those used with activatedcarbons for gold (i.e., RIC and RIP). Gold-blatt [72] developed the first resin columnfor cyanide recovery. Metal-cyanide com-plexes have since been found to adsorbmore strongly but this adsorption is de-pendent on which resin is being used andhow the solution and/or resin are pretreated[73-77]. Thiocyanate appears to adsorbweakly. In a comparison of resins to acti-vated carbons, Fleming and Cromberge[78] showed that resins can be more costeffective since they resist organic fouling,

regenerate more efficiently, have longerlife, and desorb faster. At present, the onlyresin being used in industry for cyanide re-covery is the chelation resin, Vitrokele V912[79].

Oxidation methodsExcept for the dilution process, the physi-cal, complexation and adsorption methodsyield a high concentration product that stillmust be treated prior to discharge. Thetoxic cyanide species remain. Conse-quently, these methods are used predomi-nantly for separation, recovery and recy-cling. In order to destroy the cyanide, sub-sequent oxidation methods are needed.Cyanide oxidation can be conducted usingbiological, catalytic, electrolytic, chemicaland photolytic methods.

Bio-oxidation

Various species of bacteria, fungi, algae,yeasts and plants, along with their associ-ated enzymes and amino acids, are knownto oxidize cyanide naturally [80-91]. Thepredominant mechanism of bio-oxidation isthe metabolic conversion of cyanide to cy-anate, OCN-, a species less toxic thancyanide:

CN- + ½O2(aq) → OCN- (14)

Oxygen is usually considered to be aque-ous but may be present as a gas. Severalintermediate species are formed during thisreaction depending on which biomass isused, what cyanide species are present,and what the solution conditions are regard-ing cyanide concentration, pH, and tem-perature. Once the cyanate is producedand released, it will hydrolyze to ammo-nium and bicarbonate ions:

OCN- + 3H2O → NH4+ + HCO3

- + OH- (15)

Reaction 15 is favorable under conditionsless than approximately pH 7 at room tem-peratures [92, 93]. Ammonium is also con-sidered toxic and must also be treated prior


to discharge, usually by nitrification or deni-trification processes [94].

Of the various biomasses examined, bac-teria are perhaps the best known. Bacterialoxidation of cyanide has been developedfor effluent treatment at the Homestakegold processing plant in Lead, South Da-kota [89, 90]. A biofilm composed primarilyof Pseudomonas paucimobilis bacteria onrotating biological contactors is used tometabolize cyanide in water from tailingsand underground mines. Unlike most otherbiomasses, this strain of bacteria can treatmetal cyanide complexes including theSAD species:

M(CN)xy-x + 3xH2O + x/2O2(aq) →

My+ + xNH4+ + xHCO3

- + xOH- (16)

Bacterial accumulation of the metal cations(My+) occurs by absorption, adsorption orhydroxide precipitation (see Reaction 7).The metal-loaded bacteria continuallyslough off the contactors and are removedby clarification. In addition, thiocyanate isremediated:

SCN- + 3H2O + 2O2(aq) →SO4

2- + NH4+ + HCO3

- + H+ (17)

Since Reactions 15 and 16 produce hydrox-ides, thiocyanate bio-oxidation can helpbuffer the system. Ammonium produced byReactions 15-17 is then treated by bacterialnitrification with a biofilm of aerobic, autot-rophic bacteria to produce nitrite (NO2

-)which, in the presence of oxygen, quicklyoxidizes to nitrate (NO3

-). Lime is added toprecipitate the sulfate and bicarbonate asgypsum and calcite (CaCO3), respectively(see Reaction 8). The water is clarified anddischarged into a stream.

Other bacterial remediation processes arebeing developed. In one process, alginate-encapsulated Pseudomonas putida bacte-ria is used to treat decommissioned heapleach pads and other sources of groundwater contamination [95, 96]. Column tests

on ore from the Summitville Mine, a Super-fund SITE in Colorado, proved superior toperoxide rinse tests and, as a result, fieldtests are underway at the mine site. Fur-thermore, the technology is being adaptedfor use in fluidized bed reactors for treatingeffluents [97]. In other processes, anaero-bic bacteria have been used to oxidizecyanide in oxygen-starved conditions [98-100] and could therefore be applied toground waters and stagnant tailings ponds.In these cases, Reactions 14, 16 and 17would occur with water and not oxygen.

Catalysis

As noted previously, activated carbon is astrong adsorbent for cyanide. However,when adsorption takes place in the pres-ence of oxygen, the activated carbon willcatalytically oxidize cyanide to cyanate asshown in Reaction 14 [15, 18, 68, 71, 101-104]. When copper is present either in so-lution or preadsorbed on the activated car-bon, the rate of catalysis increases, a phe-nomenon attributed to the increased ad-sorption of cyanide. If the copper co-catalyst is in excess, the cyanate hydrolysisshown as Reaction 15 is enhanced. Goldprocessing plants therefore minimize oxy-gen input into their carbon adsorption cir-cuits. Although catalysis by activated car-bon is effective for treating free cyanideand WADs as well as some SADs, it hasnot been developed into a cyanide destruc-tion process. The fate of thiocyanate is un-known. A similar system using a PbO2

catalyst has been favorably tested [105].

Electrolysis

Principles of electrowinning are applicableto electrolysis and, as a result, operatingparameters are basically the same. How-ever, cyanide remediation occurs as anoxidation of free cyanide at the anoderather than a reduction of a metal-cyanidecomplex at the cathode. Furthermore, be-cause cyanide and the anode are oppo-sitely charged, current efficiencies aremuch better. Cyanate is usually produced


using one of two techniques: electro-oxidation and electro-chlorination. In elec-tro-oxidation, the cyanide reaction occursdirectly [13, 18, 106-108]:

CN- + 2OH- → OCN- + H2O + 2e- (18)

whereas, in electro-chlorination, the reac-tion is the same overall but occurs via areaction sequence [18, 109]:

2Cl- → Cl2(g) + 2e- (19a)

CN- + Cl2(g) → CNCl(aq) + Cl- (19b)

CNCl(aq) + 2OH- → OCN- + H2O + Cl- (19c)

Chlorocyanogen (CNCl), or tear gas, isformed as an intermediate and chlorideanions are regenerated and therefore serveas a catalyst. The reaction should be car-ried out between 40ºC and 50ºC to mini-mize the formation of chlorate (O3Cl-). Un-der these conditions, hypochlorite (OCl-) isproduced [108]:

Cl- + H2O → OCl- + 2H+ + 2e- (20)

which aids in chemical oxidation as dis-cussed in the next section (see alkalinechlorination). Hypochlorite is toxic, espe-cially to fish, and must be controlled prior todischarge. Since hydroxide anions are con-sumed in both reaction processes, pH con-trol above 11 is necessary to prevent hy-drocyanic and chlorocyanic gases fromvolatilizing. Electro-oxidation does not per-form as well as electro-chlorination. Bothare effective against thiocyanate andWADs but ineffective against SADs.

Chemical addition

The most popular method for cyanide de-struction is by addition of oxidants. Oxi-dants have high electron affinity andtherefore strip cyanide of electrons pre-dominantly yielding cyanate as a reactionproduct. Oxygen, ozone, hydrogen perox-ide, chlorine, hypochlorite and sulfur diox-ide are the most common oxidants.

Natural degradation of cyanide can occurvia reactions with gaseous and aqueousoxygen (see Reaction 14). The reactionoccurs quickly in agitated slurries, slowly intailings ponds, and is catalyzed in the pres-ence of activated carbon and certain aero-bic bacteria as previously noted. It issomewhat effective for thiocyanate andWADs but ineffective for SADs. These re-actions are often referred to as atmosphericoxidation reactions [2, 3].

Ozonation for cyanide destruction has beenexamined extensively because it is a supe-rior oxidant to oxygen [110-116]. Like oxy-gen, ozone reacts with cyanide to producecyanate; however two mechanisms havebeen proposed:

CN- + O3(aq) → OCN- + O2(aq) (21a)

3CN- + O3(aq) → 3OCN- (21b)

which are referred to as simple and cata-lytic ozonation, respectively. Simple ozona-tion yields oxygen which can further oxidizecyanide. On the other hand, catalyticozonation represents a high reaction effi-ciency and has been observed at high ad-dition rates, although rarely. Continuedaddition of ozone will convert cyanate tocarbonate and nitrogen gas:

2OCN- + 3O3(aq) + H2O →2HCO3

- + N2(g) + 3O2(aq) (22a)

2OCN- + O3(aq) + H2O → 2HCO3- + N2(g) (22b)

Because continued ozonation prevents cy-anate hydrolysis via Reaction 15 and doesnot oxidize the cyanate to nitrite or nitrate,neither nitrification nor denitrification areneeded. Furthermore, ozonation is highlydestructive towards thiocyanate and WADsbut is not destructive towards SADs. Inaddition, because hydroxide decomposesozone, it becomes less efficient at pH val-ues greater than approximately 11. This isa “Catch 22” situation since cyanide solu-tions should be maintained above pH 11 toprevent hydrocyanic gas volatilization.


Other disadvantages of ozonation includeits high expense for generating ozone.

In the Degussa process, hydrogen peroxideis used to chemically oxidize cyanide [117].Hydrogen peroxide is an oxidant strongerthan oxygen but weaker than ozone. It isoften considered better because it is rela-tively cheap, water soluble, and easy tohandle and store. The Degussa processhas been examined in several researchefforts, especially for comparing to otherprocesses [118-125]. Results of thesestudies show that hydrogen peroxide reactswith cyanide to produce cyanate and, whenadded in excess, nitrite and carbonate and,eventually, nitrate form:

CN- + H2O2 → OCN- + H2O (23a)

OCN- + 3H2O2 →NO2

- + CO32- + 2H2O + 2H+ (23b)

NO2- + H2O2 → NO3

- + H2O (23c)

Otherwise, the cyanate will hydrolyze ac-cording to Reaction 15. The Degussa proc-ess is successful at oxidizing most WADsbut has little or no effect on thiocyanateand SADs. Even so, it has been incorpo-rated into numerous mining operations inCanada and the U.S. as a primary cyanidedestruction process as well as a stand-byprocess for emergency situations.

Other chemicals can be added to increasereaction rates and efficiencies of hydrogenperoxide: cupric cation, formalde-hyde/Kastone reagent and sulfuric acid [1].Cupric cations act as catalysts for Reac-tions 23a-c [2, 3] but can be consumed byprecipitating ferrohexacyanide anions (seeReaction 11) [2]. Cyanide reacts with for-maldehyde (HOCH) in the presence ofDuPont's proprietary Kastone reagent toform glycolnitrile (HOCH2CN) which, in turn,hydrolyses to glycolic acid amide(HOCH2CONH2) in the presence of hydro-gen peroxide [126-128]:

CN- + 2HOCH(aq) + H2O →HOCH2CN(aq) + OH- (24a)

HOCH2CN(aq) + H2O →HOCH2CONH2(aq) (24b)

Cyanate and ammonium ions are also ob-tained as by-products. (3) When sulfuricacid and hydrogen peroxide are mixed, ahighly exothermic reaction occurs, formingpersulfuric or Caro's acid, H2SO5. Caro'sacid readily decomposes to oxygen andsulfuric acid and, therefore, must be madeon site at the time of use. It reacts withcyanide to produce cyanate [123, 129]:

CN- + H2SO5(aq) → OCN- + H2SO4(aq) (25)

Similar reactions occur for thiocyanate andWADs; however, SADs do not react. Testwork has also shown that hydrocyanic gasvolatilization is negligible in spite of the factthat an acid is being added and produced.This is attributed to fast reaction rates andthe inability of SADs to volatilize since equi-librium pH is established between 6.5 and8 (see Reaction 6). Reactions similar toCaro's acid were also noted with persulfate,S2O8

2-.

Alkaline chlorination has been practicedever since cyanide leaching of gold wascommercially developed in 1889 and, con-sequently, has been the most commonlyapplied technique for cyanide destruction[18, 28, 109, 130-132]. As will be ex-plained, alkaline chlorination removes toxicWAD and free cyanide species completely,quickly and economically. However, it suf-fers from high reagent costs regarding al-kaline pH control and chlorinegas/hypochlorite consumption, high dis-charge concentrations of chloride and hy-pochlorite anions of which both are toxic,and inability to remediate SADs.

Alkaline chlorination can be accomplishedusing chlorine gas as discussed earlier forelectro-chlorination (see Reactions 19b and19c):


CN- + Cl2(g) + 2OH- → OCN- + H2O + 2Cl- (26)

Chlorine gas also reacts with thiocyanateand metal-complexed cyanides to producecyanate:

SCN- + 4Cl2(g) + 10OH- →OCN- + SO4

2- + 8Cl- + 5H2O (27a)

M(CN)xy-x + xCl2(g) + (2x+y)OH- →

xOCN- + 2xCl- + M(OH)y + xH2O (27b)

Due to the high pH, sulfate and metal hy-droxide precipitates are formed; however,only WADs react according to Reaction27b. SADs are inert. When in excess, chlo-rine gas can also react with cyanate to pro-duce nitrogen and carbon dioxide:

2OCN- + 3Cl2(g) + 4OH- →N2(g) + 2CO2(g) + 6Cl- + 2H2O (28)

In such a case, chlorine and hydroxideconsumptions become excessive, only in-creasing the difficulty in maintaining alkalin-ity above pH 10 to avoid the volatilization ofhydrocyanic and chlorocyanic gases.

To compensate for this high base con-sumption, the hypochlorite alkaline chlori-nation process was adapted due, in princi-ple, to observations that the hypochloritethat was produced by the hydrolysis ofchlorine gas also remediated cyanide:

Cl2(g) + 2OH- → Cl- + OCl- + H2O (29)

In this regard, hypochlorite reactions aresimilar to those depicted for chlorine gas(see electro-chlorination). For example, hy-pochlorite and cyanide react, producingchlorocyanogen which, in turn, hydrolysesto cyanate (see Reaction 19c):

CN- + OCl- + H2O → CNCl(aq) + 2OH- (30a)

CNCl(aq) + 2OH- → OCN- + Cl- + H2O (30b)

Cyanate is also formed via hypochlorite re-actions involving thiocyanate and metal-complexed cyanides:

SCN- + 4OCl- + 2OH- →OCN- + SO4

2- + 4Cl- + H2O (31a)

M(CN)xy-x + xOCl- + yOH- →

xOCN- + xCl- + M(OH)y (31b)

As with chlorine gas reactions, only WADsreact in this manner; SADs are inert. Fi-nally, when in excess, hypochlorite canalso react with cyanate to produce nitrogenand carbon dioxide:

2OCN- + 3OCl- + H2O →N2(g) + 2CO2(g) + 3Cl- + 2OH- (32)

Clearly, hydroxide consumption is minimaland only becomes high when concentra-tions of thiocyanate and WADs are high, asindicated by Reactions 31a and 31b. To-day, hypochlorite is predominantly addedas sodium or calcium salts since thesemetal cations have natural buffering ten-dencies.

The INCO process for cyanide destructioninvolves mixing sulfur dioxide and air withfree and metal-complexed cyanide as wellas thiocyanate to yield cyanate [133-137]:

CN- + SO2(g) + H2O + O2(g) →OCN- + H2SO4(aq) (33a)

M(CN)xy-x + xSO2(g) + xH2O + xO2(g) →

xOCN- + xH2SO4(aq) + My+ (33b)

SCN- + 4 SO2(g) + 5H2O + 4O2(g) →OCN- + 5H2SO4(aq) (33c)

Reactions 33a-c can be catalyzed with cu-pric or nickel cations; however, SADs donot react. Since the reactions generatesulfuric acid and are most efficient near pH9, lime is added for pH control. This gen-erates sludge due to metal hydroxide andgypsum precipitation which, as noted ear-lier, can be difficult to clarify (see Reactions7 and 8). The INCO process has been in-corporated into numerous mining opera-tions in Canada and the U.S. since thetechnology was commissioned in 1983.


Other remediation processes have beenconsidered for cyanide oxidation. Theseinclude reacting cyanicides, a term used forspecies which consume cyanide in leachcircuits, with cyanide to produce less-toxicspecies. For example, solid polysulfideminerals such as elemental sulfur (S8) andpyrrhotite (Fe1-xS) and aqueous sulfur-oxyanions such as polythionate (S2O6

2-) andthiosulfate (S2O3

2-) are known to react natu-rally with cyanide to produce thiocyanate[3, 18, 138, 139]. However thiocyanate isdifficult to remediate, as already discussed,and is also a good lixivant for gold [43,140]. Consequently, cyanide remediationvia thiocyanate formation is usually notconsidered viable.

Photolysis

Photolysis can enhance reduction/oxidation(redox) reactions by providing energy fromelectromagnetic radiation to catalyze elec-tron transfer processes. Electromagneticradiation is absorbed, causing an electronin the absorbing compound to pass fromthe ground state to an excited state, possi-bly separating paired electrons from oneanother. The electrons become more sus-ceptible to the chemical environment andare therefore more apt to participate in re-dox reactions. Photoreduction occurs whenthe absorbing compound donates the ex-cited electron to another species. Photo-oxidation occurs when the absorbing com-pound accepts an electron from anotherspecies in order to fill its electron vacancy.The excited electron will eventually relax tothe ground state of the reaction products.

Energy needed to promote electrons fromthe ground state to the excited state isusually equivalent to that possessed by ul-traviolet (UV) radiation. Many redox reac-tions can be catalyzed upon exposure toartificial sources such as arcs, lamps andlasers or to natural sunlight, which is freebut not necessarily available 24 hours aday. Higher energies such as gamma ra-diation and lower energies such as visible

light can be used but applications are lim-ited [141]. Photolytic reactions can be in-duced directly if the absorbing compound isthe species being remediated or indirectly ifthe absorbing compound is available fortransferring the photon-energy to the spe-cies being remediated. Photolysis can,therefore, be conducted in the absence ofphotosensitizers (direct photolysis) or intheir presence as aqueous species(homogeneous photolysis and photocata-lysis) or solid semi-conductors(heterogeneous photocatalysis). Thesephotolytic methods are relatively new tech-nologies and are thus referred to as ad-vanced oxidation processes. Recently, theyhave been reviewed but with primary em-phasis on oxidation of organic compounds[142, 143].

Direct photolysis is not applicable to freecyanides but does occur with some WADsand SADs, particularly the ferric and ferroushexacyanide complexes [124, 125, 144-146]. Although subject to considerable de-bate, the following reaction mechanismwas proposed for ferrihexacyanide:

Fe(CN)63- + H2O hυ →

[Fe(CN)5H2O]2- + CN- (34a)

[Fe(CN)5H2O]2- + 2H2O hυ →Fe(OH)3(s) + 5CN- + 3H+ (34b)

These reactions are reversible; however,the ferrihydroxide [Fe(OH)3] can react fur-ther with free cyanide and ferrihexacyanideto form Prussian Blue precipitates, similarto Reaction 10. Reactions 34a and b arereferred to as photo-aquation reactions andhave also been observed for cobalthexacy-anide, Co(CN)6

3-, the strongest SAD [146,147]. Furthermore, ferrihexacyanide can beproduced via the direct photolysis of ferro-hexacyanide:

Fe(CN)64- hυ → Fe(CN)6

3- + e- (35)

or the homogeneous photolysis of ferro-hexacyanide with oxygen [148]:


4Fe(CN)64- + 2H2O + O2 hυ →

4Fe(CN)63- + 4OH- (36)

Combinations of Reactions 34a-36 liberatecyanide at UV-wavelengths less than ap-proximately 420 nm, explaining why fishare commonly killed during the day and notthe night as well as why adding ferric andferrous cations to cyanide for detoxificationis highly suspect (see metal addition meth-ods) in spite of the fact that UV-radiationdoes not significantly penetrate water [2].Clearly, direct photolysis can remediateWADs and SADs but dangerously liberatesfree cyanide during the reaction.

Homogeneous photolysis has also beenused in combination with several of thechemical oxidation methods discussed ear-lier in order to increase reaction efficien-cies, thereby minimizing reagent consump-tion and making SADs more amenable todestruction. For example, homogeneousphotolysis with ozone has been shown toremediate free cyanide, WADs and SADsto concentrations below 0.1 ppm from con-centrations ranging between 1 and 100,000ppm [112, 149-151]. Increases in UV in-tensities, ozone concentrations (i.e.,flowrates), and solution temperatures im-prove reaction kinetics. Reactions 21a, band 22 occur but to a much lesser extentdue to the production of hydroxyl radicals(OH.) and their subsequent reaction withcyanide:

H2O + O3(aq) hυ → 2OH. + O2(aq) (37a)

CN- + 2OH. → OCN- + H2O (37b)

Because hydroxyl radicals are devoid ofcharge and have a high affinity for elec-trons, they can quickly strip virtually anychemical of electrons including thiocyanate,WADs and SADs, thus causing their oxida-tion. Resulting cyanate will hydrolyze ac-cording to Reaction 15 or will react withcontinued photolytic ozonation to producebicarbonate and either nitrogen gas (seeReaction 22a), nitrite or nitrate:

OCN- + 3OH. → HCO3- + ½N2(g) + H2O (38a)

OCN- + 6OH. →HCO3

- + NO2- + H+ + 2H2O (38b)

OCN- + 8OH. →HCO3

- + NO3- + H+ + 3H2O (38c)

Cyanide destruction by photolytic ozonationrequires 1 mole of ozone for every mole ofcyanide (i.e., 1:1) to form cyanate but thisratio increases to 5:1 when nitrate is pro-duced. Such high reagent consumptions,when equated with high temperatures andhigh UV intensities, can make photolyticozonation costly.

Free cyanide, thiocyanate, WADs andSADs can also be oxidized by the homo-geneous photolysis of hydrogen peroxide[122-125, 152, 153]. The photolytic peroxi-dation reaction also yields hydroxyl radi-cals:

H2O2 hυ → 2OH. (39)

and, as a result, Reactions 15, 37b, and38a, b and c can be observed dependingon the amount of excess hydrogen perox-ide that is present. Remediation of freecyanide, thiocyanate, WADs and SADs isnear 100%, independent of concentration.Because Reaction 39 occurs quickly, Re-actions 23a, b and c do not occur. Likephotolytic ozonation, increasing UV intensi-ties, hydrogen peroxide concentrations,and solution temperatures enhance reac-tion rates. Furthermore, reaction stoi-chiometries are 1:1 when cyanate formsand 5:1 when nitrate forms. Nevertheless,hydrogen peroxide is more attractive due toits relative inexpense and ease of storageand handling.

Homogeneous photolysis with ozone, hy-drogen peroxide, and other aqueous pho-tosensitizers is disadvantageous becausethe photosensitizers are consumed andoften added in excess to ensure completeoxidation occurs. As already noted, photo-sensitizer consumption and costs are


therefore high. In addition, the resulting ef-fluent must be processed to treat the ex-cess photosensitizer. In regards to ozoneand hydrogen peroxide, this can be ac-complished by giving the hydroxyl radicalstime to decompose to water and oxygen ina holding pond; however, such a simplesolution is not always available for otherphotosensitizers. These problems can bealleviated using catalytic photosensitizersthat can be recycled. Homogeneous photo-catalysis with aqueous photosensitizersand heterogeneous photocatalysis withsolid photosensitizers result. Homogeneousphotocatalysis, however, is rarely practiceddue to the difficulty in separating aqueousspecies for recycling. On the other hand,heterogeneous photocatalysis is more vi-able because standard solid-liquid separa-tion processes or fixed-bed systems can beused.

The solid photosensitizers used in hetero-geneous photocatalysis are usually cheap,nontoxic and inert semiconductors. UV-absorbance promotes electrons in the va-lence band across the bandgap and intothe conductance band of the semiconduc-tor which creates a hole, h+, in the valenceband and an excited electron, e-, in theconductance band:

semiconductor hυ → (e-h+) (40)

However, the impingent UV radiation musthave energy equal to or greater than thebandgap of the semiconductor. The elec-tron-hole (e-h+) pair can then induce redoxreactions provided that the reacting speciesare adsorbed at the semiconductor surfaceand that the reaction products are inert tothe reverse reaction and/or desorb quickly.

Heterogeneous photocatalysis has beenexamined for cyanide oxidation and threemechanisms have been proposed [122-125, 152-161]. The first mechanism pro-duces hydroxyl radicals via the reduction ofeither adsorbed water or adsorbed hydrox-ide by holes in the valence band:

H2O + h+ hυ → OH. + H+ (41a)

OH- + h+ → OH. (41b)

On the other hand, hydroxyl radicals areproduced in the second mechanism by re-acting dissolved oxygen with excited elec-trons in the conduction band through a su-peroxide (O2

-) intermediate:

O2(aq) + e- → O2- (42a)

2O2- + 2H+ → 2OH. + O2(aq) (42b)

Both reaction mechanisms are concludedwith cyanide oxidizing to cyanate, nitrogengas, nitrite and/or nitrate by the hydroxylradicals (see Reactions 37b and 38a, b, c)and are also effective for remediating thio-cyanate, WADs and SADs. Early work hadindicated that bicarbonate and ammoniawere produced by cyanate hydrolysis ac-cording to Reaction 15; however, ammoniais readily oxidized in these systems. Thethird mechanism involves reducing cyanideby holes in the valence band to producecyanyl radicals, CN.:

CN- + h+ → CN. (43)

Hydroxyl radicals produced according toReactions 41a, b and/or 42a, b then reactwith the cyanyl radicals:

CN. + OH. → OCN- + H+ (44)

Resulting cyanate further reacts with hy-droxyl radicals as depicted earlier in Reac-tions 38a, b and c.

Spectroscopic evidence supports thesethree reaction mechanisms and is believedto be independent of the semiconductorphotocatalysts that were tested—TiO2,ZnO, FeTiO3, SiO2, Fe2O3, ZnS and CdS, toname a few. The metal oxides worked bestbecause of their chemical stability; how-ever, anatase, a polymorph of TiO2, waspreferred due to its high quantum efficiencyfor photoconversion, its stable formation of


electron-hole pairs, and its large bandgapof 3.2 eV. This band gap equates to amaximum wavelength of approximately387.5 nm which implies that virtually allportions of the UV spectrum can be used.In addition, electron-hole pairs were alsomade more stable by depositing metal is-lands of, for example, platinum and rho-dium onto the surfaces of the photocata-lysts.

Presently, photolysis is used in at least sixremediation systems, aside from thoseemployed for disinfection. These includethe perox-pure™ process of Vulcan Per-oxidation Systems, Inc. [162], the Rayox®

Process of Solarchem Environmental Sys-tems, Inc. [163], the Ultrox™ Process ofUltrox, Inc. [164], the PhotoReDox™ Sys-tem of ClearFlow, Inc. [165, 166], and thetwo systems developed at Sandia NationalLaboratories in Albuquerque, NM [167],and National Renewable Energy Labora-tory in Golden, CO [168]. The first threeprocesses use homogeneous photolysiswith hydrogen peroxide and have alsobeen adapted for use with ozone and otheraqueous photosensitizers. Differences be-tween the systems are in cell design(horizontal versus vertical) and artificial UV-intensity (3 kW versus 60 W). On the other

hand, the other processes use heteroge-neous photocatalysis with TiO2. In thesesystems, the UV-radiation is artificial or so-lar and the TiO2 is either suspended orfixed on one of several supports (glassbeads or meshes); however, the supportscan decrease photoconversion efficienciesdue to reflection and refraction of the inci-dent UV-radiation [169]. Although all six ofthese systems were principally designed fororganic destruction, several have beendemonstrated for cyanide oxidation.

CONCLUSIONSCyanide remediation can be accomplishedusing numerous separation and oxidationprocesses. Separation processes includephysical methods with membranes, elec-trowinning and hydrolysis/distillation; com-plexation methods with acidifica-tion/volatilization, metal addition, flotationand solvent extraction; and adsorptionmethods with various minerals, activatedcarbons and resins. These methods areused to purify effluents for discharge byconcentrating and recovering the cyanidefor recycling. On the other hand, oxidationprocesses are used to destroy the cyanideand include various biological, catalytic,electrolytic, chemical and photolytic meth-

Table 1. Effectiveness of separation processes for cyanide remediation as discussed in this paper as modified andupdated from Ingles and Scott [18].


ods. Several investigations compared theeffectiveness of separation and oxidationprocesses and found them to be dependenton the cyanide species that were present;free cyanide (CN- and HCN), thiocyanate(SCN-), weak-acid dissociables (WADs),and strong-acid dissociables (SADs) maybe remediated by one process and not an-other. The advanced oxidation processesassociated with photolytic methods workwell for all cyanide species and conse-quently are the preferred choice for reme-diation; however, if certain cyanide speciesare not present, remediation can be ac-complished by more appropriate alterna-tives. This is clearly illustrated in Tables 1and 2. Consequently, the proper choice forcyanide remediation must be made on acase-by-case basis; then and only then cana real decision be made regarding the needfor further cyanide treatment.

ACKNOWLEDGMENTThis work was conducted as part of theMine Waste Technology Program (MWTP)under an interagency agreement, IAG IDNo. DW89935117-01-0, between the U.S.Environmental Protection Agency (EPA)and the U.S. Department of Energy (DOE),Contract No. DE-AC22-88ID12735. This

paper was not subjected to MWTP peerand administrative review and, therefore,may not necessarily reflect the views of theprogram. No official endorsement shouldbe inferred.

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