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Cyanide Remediation: Current and Past Technologies C.A

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Cyanide Remediation: Current and Past Technologies C.A CYANIDE REMEDIATION: CURRENT AND PAST TECHNOLOGIES C.A. Young§ and T.S. Jordan, Department of Metallurgical Engineering, Montana Tech, Butte, MT 59701 ABSTRACT Cyanide (CN-) is a toxic species that is found predominantly in industrial effluents generated by metallurgical operations. Cyanide's strong affinity for metals makes it favorable as an agent for metal finishing and treatment and as a lixivant for metal leaching, particularly gold. These technologies are environmentally sound but require safeguards to prevent accidental spills from contaminating soils as well as surface and ground waters. Various methods of cyanide remediation by separation and oxidation are therefore reviewed. Reaction mechanisms are given throughout. The methods are compared in regard to their effectiveness in treating various cyanide species: free cyanide, thiocyanate, weak-acid dissociables and strong-acid dissociables. KEY WORDS cyanide, metal-cyanide complex, thiocyanate, oxidation, separation INTRODUCTION ent on the transport of these heavy metals through their tissues, cyanide is very toxic. Waste waters from industrial operations The mean lethal dose to the human adult is transport many chemicals that have ad- between 50 and 200 mg [2]. U.S. EPA verse effects on the environment. Various standards for drinking and aquatic-biota chemicals leach heavy metals which would waters regarding total cyanide are 200 and otherwise remain immobile. The chemicals 50 ppb, respectively, where total cyanide and heavy metals may be toxic and thus refers to free and metal-complexed cya- cause aquatic and land biota to sicken or nides [3]. According to RCRA, all cyanide species are considered to be acute haz- die. Most waste-water processing tech- ardous materials and have therefore been nologies that are currently available or are designated as P-Class hazardous wastes being developed emphasize the removal of when being disposed of. P-Class hazard- the chemicals or heavy metals as cations. ous wastes are the most regulated waste in However, the anions associated with heavy regard to amounts accumulated in a given metal cations can be equally as toxic but year as compared to the other class desig- are largely ignored. In this regard, the nations: F, K and U. remediation of cyanide has been consid- ered paramount [1]. Metal-complexed cyanides are classified according to the strength of the metal- Cyanide is a singly-charged anion contain- cyanide bond. Free cyanide refers to the ing unimolar amounts of carbon and nitro- most toxic forms of cyanide: cyanide anion gen atoms triply-bonded together: C≡N- or - and hydrogen cyanide. Weak-acid disso- CN . It is a strong ligand, capable of com- ciables (WADs) refer to cyanide complexes plexing at low concentrations with virtually with metals such as cadmium, copper, any heavy metal. Because the health and nickel and zinc. Although thiocyanate survival of plants and animals are depend- § Corresponding author. 104 Proceedings of the 10th Annual Conference on Hazardous Waste Research (SCN-) is a WAD, it is often considered in ferred to as oxidation methods. These its own category. Strong-acid dissociables physical, adsorption, complexation and oxi- (SADs) refer to cyanide-complexes with dation methods are described in the ensu- metals such as cobalt, gold, iron and silver. ing discussions. This nonspecific, complexing nature makes cyanide attractive for (1) extracting metals, Physical methods particularly gold, from ores via various Physical methods for cyanide treatment leaching operations; (2) finishing/treating can be accomplished using dilution, mem- metal products and their surfaces, and (3) branes, electrowinning and hydroly- preventing certain particles from becoming sis/distillation. hydrophobic in the flotation separation of minerals. Cyanide concentrations for Dilution leaching and finishing/treating are several orders of magnitude higher than those en- Dilution is the only treatment method which countered in flotation whereas cyanide so- does not separate or destroy cyanide. This lutions are more voluminous in leaching as method involves combining a toxic cyanide compared in finishing/treating. Over a bil- waste with an effluent that is low in or free lion tons of gold ore are leached each year of cyanide to yield a waste water below with cyanide. Consequently, in order to discharge limits. Consequently, dilution is prevent surface and ground water contami- simple and cheap and is often used as a nation, procedures for the safe and proper stand-alone or back-up method to insure treatment, storage and handling of effluents that discharge limits are satisfied [3]. Dilu- are of primary concern for cyanide leaching tion is usually considered to be unaccept- operations [4, 5]. able since the total amount of cyanide dis- charge is not altered and since naturally- In this study, current and past remediation occurring processes such as adsorption methods for waste and process waters and precipitation can attenuate and thereby containing cyanide are reviewed. Emphasis concentrate the cyanide in ground and is placed on the oxidation methods which surface waters. actually destroy the cyanide. Membranes CYANIDE REMEDIATION Cyanide can be separated from water us- Various procedures exist for treating cya- ing membranes with either electrodialysis nide and are comprised of several physical, or reverse osmosis. In electrodialysis, a adsorption, complexation and/or oxidation potential is applied across two electrodes methods. These methods predominantly separated by a membrane permeable to involve separation or destruction processes cyanide. The cyanide solution requiring and may occur naturally. Separation proc- purification is placed in the half-cell contain- esses include the physical, adsorption and ing the cathode or negative electrode. complexation methods that are used to Cyanide, because it is negatively charged, concentrate and thereby recover cyanide will diffuse through the membrane and for recycling. On the other hand, destruc- concentrate in the half-cell containing the tion processes are used to sever the car- anode or positive electrode. In reverse os- bon-nitrogen triple bond thereby destroying mosis, pressure is applied to a cyanide so- the cyanide and producing non-toxic or lution needing treatment but, in this case, less-toxic species. Because the carbon water is forced through a membrane im- and/or nitrogen atoms in the cyanide mole- permeable to cyanide. Both of these meth- cule undergo changes in oxidation state, ods have been shown to be applicable to destruction processes are commonly re- Proceedings of the 10th Annual Conference on Hazardous Waste Research 105 solutions containing free and metal- Progress is continuing to make electrowin- complexed cyanide [6-11]. ning technology economically viable to di- lute solutions. Direct applications to cya- Electrowinning nide remediation may then be possible. SADs and WADs can be reduced to corre- Hydrolysis/distillation sponding metals by applying a potential across two electrodes immersed in the Free cyanide naturally hydrolyzes in water same solution: to produce aqueous hydrogen cyanide [HCN(aq)]: y-x - o - M(CN)x + ye → M + xCN (1) CN- + H+ → HCN(aq) (4) Thiocyanate does not respond. Free cya- nide is liberated which makes solutions The aqueous hydrogen cyanide can then more amenable to other recovery and volatilize as hydrocyanic gas [HCN(g)]: remediation processes. Because this elec- trowinning reaction involves the reduction HCN(aq) → HCN(g) (5) of an anion at the cathode, a negatively- charged electrode, metal recoveries and Because hydrocyanic gas has a vapor current efficiencies are low. This is com- pressure of 100 kPa at 26ºC, which is pensated for by using steel wools as high- above that of water (34 kPa at 26ºC), and a surface area cathodes, increasing agitation boiling point of 79ºC, which is below that of to further increase mass transport, increas- water (100ºC), cyanide separation can be ing solution temperatures, using appropri- enhanced at elevated temperatures and/or ate solution pHs and conductivities, in- reduced pressures [14-16]. Distillation rates creasing metal concentrations if possible, can also be increased by increasing the and redesigning the electrowinning cell of agitation rate, the air/solution ratio, and the which four designs have been developed surface area at the air/solution interface. for gold processing: Zadra, AARL, NIM Hydrocyanic gas can be captured and con- graphite-chip and MINTEK parallel plate centrated for recycling in conventional ab- cells [3, 12]. Furthermore, Reaction 1 must sorption-scrubbing towers. It can also be be accompanied by a corresponding oxida- vented to the open atmosphere and has tion reaction, usually oxygen evolution, at been noted to occur naturally in tailings the anode with an equivalent number of ponds, especially in warm and arid envi- electrons, e-, being donated: ronments [17]. In such cases, it is para- mount that environmental regulations be - - y/4.{4OH → 2H2O + O2 + 4e } (2a) satisfied. Thiocyanate, WADs and SADs are not affected. + - y/4.{2H2O → O2 + 4H + 4e } (2b) Complexation methods Electrowinning is predominantly used for gold processing; however, it has been used Cyanide treatment can also be done with for cyanide regeneration, in which case it is several complexation methods such as often referred to as the Celec or HSA proc- acidification/volatilization, metal addition, ess [3, 13]. Electrowinning performs well in flotation and solvent extraction. concentrated solutions;
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