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The Chemical Behaviour of Cyanide in the Extraction of Gold. 1. Kinetics of Cyanide Loss in the Presence and Absence of Activated Carbon by M.D

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The Chemical Behaviour of Cyanide in the Extraction of Gold. 1. Kinetics of Cyanide Loss in the Presence and Absence of Activated Carbon by M.D J. S. Afr. Inst. Min. Metal/., vol. 90, no. 2. Feb. 1990. pp. 37-44. The chemical behaviour of cyanide in the extraction of gold. 1. Kinetics of cyanide loss in the presence and absence of activated carbon by M.D. ADAMS * SYNOPSIS The kinetics of cyanide loss were found to be first order under any given set of conditions, allowing rate con- stants to be easily determined. At room temperature and in the absence of activated carbon, some cyanide is lost by hydrolysis to hydrogen cyanide, but this does not occur to any significant extent at pH values higher than about 10. A higher rate of hydrolysis occurs when air agitation is used. When activated carbon is present, an additional carbon-catalysed oxidation reaction is responsible for a fairly high loss of cyanide. The evidence indicates a reaction mechanism that consumes oxygen and produces cyanate ion. Some of the cyanate thus produced decomposes to form a mixture of ammonia, carbonate, and urea, depend- ing on the solution conditions. Additional cyanide is lost as a result of the adsorption of sodium cyanide by the activated carbon. At high temperatures, an additional hydrolysis reaction, which involves the formation of ammonium formate as an intermediate, occurs an/j is responsible for.a high loss of cyanide, leading ultimately to the formation of ammonia, hydrogen, and carbon dioxide. The presence of activated carbon has no effect on the rate of this reaction. SAMEV A TTING Daar is gevind dat die kinetika van sianiedverlies onder enige gegewe omstandighede van die eerste orde is wat dit moontlik maak om die tempokonstantes maklik te bepaal. By kamertemperatuur en in die aanwesigheid van geaktiveerde koolstof, gaan van die sianied deur hidrolise verlore aan waterstofsianied, maar dit vind nie in enige beduidende mate by pH-waardes bo ongeveer 10 plaas nie. 'n Hoer hidrolisetempo kom voor wanneer lugroering gebruik word. Wanneer daar geaktiveerde koolstof aanwesig is, is 'n bykomende koolstofgekataliseerde oksidasiereaksie verant- woordelik vir 'n betreklik hoe sianiedverlies. Daar is aanduidings van 'n reaksiemeganisme wat suurstof verbruik en 'n sianaatioon produseer. Van die sianaat aldus geproduseer, ontbind om 'n mengsel van ammoniak, karbonaat en ureum te vorm, afhangende van die oplostoestande. Verdere sianied gaan verlore as gevolg van die adsorpsie van natriumsianied deur die geaktiveerde koolstof. By hoe temperature vind daar 'n bykomende hidrolisereaksie plaas wat die vorming van ammoniumformiaat as 'n tussenstof behels, vir 'n hoe sianiedverlies verantwoordelik is, en uiteindelik tot die vorming van ammoniak, waterstof en koolstofdioksied lei. Die aanwesigheid van geaktiveerde koolstof het geen uitwerking op die tempo van hierdie reaksie nie. INTRODUCTION A knowledge of the chemical behaviour and stability temperatures: of the cyanide ion in aqueous solution is of particular importance to the gold-processing industry in terms of CN- + 2HP - NH3 + HCO;. (2) process control and the detoxification of tailings solu- It is reported! that reaction (2) occurs very slowly at tions. However, the mechanisms involved in the reaction ambient temperatures, resulting in the gradual decom- of the cyanide ion in aqueous solution can be complex, position of cyanide in alkaline solutions. Additional particularly in the presence of activated carbon. The hydrolytic reactions have been reported7,s to result in present work was undertaken in an effort to clarify the various ill-defined polymeric forms of hydrogen cyanide kinetics and mechanisms of cyanide loss under various and cyanogen. As these reactions are not important in conditions. gold-plant solutions, the species involved are not dealt Two general types of reaction have been proposed for with in any detail in this paper. the loss of cyanide from aqueous solution, viz hydrolytic The second general type of reaction, viz oxidative reac- and oxidative reactions. tion, is also responsible for the loss of cyanide from At low pH values, the well-known hydrolysis of cyanide aqueous solution. Cyanide is fairly easily oxidized to occurs as follows: cyanate or cyanogen. In alkaline solutions, these reac- CN- + HP ... HCN + OH-. (I) tions are9 The hydrogen cyanide thus formed will volatilize to some 2CN- ... (CN)ig) + 2e- (EO = -O,182V), (3) "" extene. At high pH values, another hydrolysis reaction and has been postulated2-6 to occur, particularly at high 20H- + CN- ... CNO- + HP + 2e- (EO -O,97V). (4) * Mintek, Private Bag X3015, Randburg, 2125 Transvaal. @ The South African Institute of Mining and Metallurgy, 1990. Paper received 9th March, 1989. SA ISSN 0038-223X/3.00 + 0.00. All Eo values refer to standard reduction potentials versus JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY FEBRUARY 1990 37 the standard hydrogen electrode (SHE), even though the grade. Oxygen of commercial purity and high-purity reactions to which they refer may, for convenience sake, nitrogen were used, and all the solutions were made up be written as oxidation reactions. with deionized water. The activated carbon used was Le A diagram of potential versus pH for CN-H2O at Carbone 0210, which was purified by continual washing 25 QC is shown in Fig. 1, together with the associated with cold deionized water. thermodynamic functions represented by the curves. It In the experiments conducted at ambient temperature, is clear that the oxidation of cyanide to cyanate is thermo- 500 ml of a 0,5 g/l solution of sodium cyanide was dynamically favourable at room temperature. However, employed, of which the natural pH value was 10,2. The the kinetics of this reaction are exceedingly slow in the pH values of the solution were adjusted with sodium absence of any catalyst, which is why cyanide can be used hydroxide or sulphuric acid. The solution was stirred with economically in large-scale hydrometallurgical processes. a magnetic or overhead stirrer, and the required amount of activated carbon was added. The pH value was 2 measured by use of a Knick pH-meter, and the solution 2 potential was measured with a Labion Model 15 poten- tiometer, which uses a platinum electrode in conjunction ? ...... with a saturated calomel reference electrode. The dis- solved oxygen was measured with a YSI dissolved oxygen br-- meter model 54A, which was calibrated with atmosphere- ...... ? equilibrated deionized water, and corrected for altitude """....... and ambient temperature. When different temperatures HOCN """'...... """...... were used, a thermostatically controlled water-bath was employed in which the variation in temperature did not ;;- - OCN- f.ti 0 exceed :t 0,01 QC. For the adsorption experiments, 5,0 g of activated carbon was contacted in a rolling bottle with 50 ml of solution for 24 hours. Various concentrations of NaCN and NaOH were employed, and the ionic strength was kept constant at 0,2 mol/kg (adjusted with NaCl). HCN -1 In the experiments conducted at higher temperatures, 300 ml of solution containing 0,2 M sodium cyanide and 0,2 M sodium hydroxide were employed. The solution was refluxed in a round-bottomed flask with a water- -2 0 2 4 6 8 10 12 14 16 cooled condenser. pH Cyanide was determined titrimetrically with a silver nitrate solution, as described by VogellO. Iodide ion (as Reactions KI) was used as the indicator, and aqueous ammonia was a 2H,O+2e- 20H- + H, £= -0,0591 pH -0,0295 log PH, introduced to solubilize the silver cyanide that formed. '" The end-point is indicated by a golden-yellow turbidity. b O,+4H++4e-"'2H,O £= -1,228 -0,0591 pH+O,OI47 log Po, Cyanate was determined by ion chromatography. The [CWJ cyanide content of some of the samples was also check- I HCN(aq) CN- +H+ log -9,39+pH '" [HCN] = ed in this manner, and a good correlation between the [OCW] two methods was noted, even at cyanate concentrations 2 HOCN(aq) log 3,89 pH "'OCN-+H+ [HOCN] = - + as high as 0,1 M. The carbons were analysed for nitrogen by use of a Heraeus elemental analyser and a Leco nitro- 3 HOCN(aq)+2H+ +2e- HCN(aq)+H,O '" gen analyser. [HOCN] £=0,021 -0,0591 pH +0,0295 log [HCN] RESULTS AND DISCUSSION 4 OCN- + 3H + + 2e- HCN(aq)+ H,O '" The experimental conditions selected were designed to [OCN-] £=0,136 -0,0885 pH +0,0295 log simulate the various stages of a typical carbon-in-pulp [HCN] (CIP) process. In this paper, the fundamental chemistry 5 OCN-+2H++2e- "'CN-+H,O involved in the loss of cyanide will be discussed. In the [OCN-] second paperIl in this series, these fundamental findings £=0,141 -0,0591 pH +0,0295 log [CW] will be related to the practical situation in gold-processing ? Reaction not identified by Bard. plants. Recent work by Hoecker and Muirl2 presents a superficial study of some of the aspects investigated in Fig. 1-Homogeneous equilibrium diagram of potential versus detail in the present study. A comprehensive review of pH for the CN-H2O system at 25 GC(after Bard~ methods for the destruction of cyanide can also be found in that paperl2. The results of a typical experiment, plotted in semi- EXPE~ENTALPROCEDURE logarithmic form, are shown in Fig. 2. This linear rela- The sodium cyanide, sodium hydroxide, and potassium tionship was found to be representative of the kinetics cyanate were of A.R. grade, and were supplied by SAAR- of cyanide loss under all the conditions studied. This Chem and Merck. All the other chemicals were of A.R. suggests that a first-order rate law adequately describes 38 FEBRUARY 1990 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY the kinetics of cyanide loss for all the reactions that occur. cyanate proceeds directly-reaction (4)-or via a cyano- Hence, gen intermediate-reaction (3). d[CN-] - = k1[CN-], (5) Effect of Carbon Concentration dt """"""""""""""'" The presence of activated carbon in the cyanide solu- where kl is the first-order rate constant.
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