Chemistry for Sustainable Development 12 (2004) 431–436 431

Extraction of Cyanides from Waste Solutions of Cyanidation of lotation Concentrates from Kholbinskoye Gold Deposit

A. A. KOCHANOV1, A. A. RYAZANTSEV1, A. A. BATOEVA2, D. B. ZHALSANOVA2, A. M. BADALYAN3 and O. V. POLYAKOV3 1Siberian Transport University, Ul. D. Kovalchuk 191, Novosibirsk 630049 (Russia) E-mail: [email protected] 2Baikal Institute of Nature Management, Siberian Branch of the Russian Academy of Sciences, Ul. Sakhyanovoy 6, Ulan Ude 670047 (Russia) 3A. V. Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences, Pr. Akademika Lavrentyeva 3, Novosibirsk 630090 (Russia)

(Received April 21, 2003; in revised form January 6, 2004)

Abstract

Processes that occur during extraction of cyanides from cyanidation solutions using centrifugal bubbling apparatuses (CBA) as reactors have been studied. In the eddy chamber of CBA (ðÍ < 3), one can observe virtually complete removal of HCN from solution and precipitation of heavy metals in the form of insoluble x - 1 compounds. The electronic absorption spectra of solutions treated in CBA suggest that destruction of [Cu(CN)x] , + 2+ 2– oxidation of Cu to Cu by air oxygen, and oxidation of thiocyanates in the presence of SO23, forming 2– – HCN and SO4 , are also accompanied by the emergence of stable intermediate products (SCN)2 and (SCN)x of oxidation in solution.

INTRODUCTION formed after acidification of the waste process- ing solutions of cyanidation to ðÍ 6–2.5 fol- Cyanides are widely used in extraction of lowed by absorption of HCN by alkaline solu- precious metals, mainly fine gold and silver, tions. The obtained sodium cyanide is recycled. from sulphide-bearing ores. A significant part Acidification is accompanied not only by bind- of cyanides in the form of simple and com- ing of free cyanides into HCN, but also by plex compounds remain in recycled solutions. destruction of complex cyanides of heavy me- Excess recycled water is subjected to purifica- tals to form additional hydrogen cyanide. tion including destructive oxidation of free and The stability of complex cyanides of heavy complex cyanides, and then it is released into metals depends strongly on ðÍ of water solu- open reservoirs or onto the land. At the same tions (3ig. 1) [5]. time, the waste solutions from processing of Au3+, Co2+, and 3e2+ cyanides are stable in flotation concentrates contain cyanides in such acidic media, while Zn2+ and Ag+ complexes quantities that recovery and recycling of these decompose practically completely even at ðÍ compounds in the form of NaCN become prof- 6. This behaviour of complex cyanides of itable. heavy metals is in good agreement with the The acidification – volatilization – reneu- corresponding Eh – pH diagrams [5]. Obvious- tralization method (AVR process) is commonly ly, the most complete removal of cyanides in used for cyanide recovery [1–3]. The method is the form of HCN by distilling would be ex- based on distilling or blowing out volatile HCN pected for solutions acidified to ðÍ < 3. 432 A. A. KOCHANOV et al.

treatment using CaOCl2 as an oxidation agent is provided in case of emergency.

EXPERIMENTAL

Reagents and methods for determining concentration of substances in solutions

3or experiments with model solutions, rea- gents of chemically pure or analytically pure grade were used. The composition of the real waste solutions of cyanidation, mg/l: the total content of cyanides and thiocyanates was 580– 3ig. 1. Effect of ðÍ on the stability of metal cyanide 3600, including thiocyanates 780–2360, iron 2– complexes in water solutions: 1 – Zn(CN)4 , 46.0–126.0, copper 300–800, zinc 2–50, nickel 2 – 2– , 3 – 2– , 4 – 4– , 5 – – , Ni(CN) 4 Cu(CN)3 3e(CN)6 Au(CN)2 9–32, silver 10–25, thiosulphates 1500–3300, 6 – Co(CN)2– . 4 sulphites 60–80, sulphates >1000, chlorides >300, ðÍ 9.5–11.0. Nevertheless, even in the case of ðÍ 2–3, The concentrations of thiocyanates in solu- it is generally possible to extract and recycle tions were determined by photometric meas- no more than 75 % of cyanides from the real urement of 3e(SCN) [6]; that of cyanides, by waste solutions of cyanidation containing 400– 3 the Bucksteg method with volatilization in the 900 mg/l of copper [3]. This is attributable to form of HCN; and that of thiosulphates, by the formation of sparingly soluble and rather the iodometric method [8]. Atomic absorption stable (in acid solutions) compounds CuCN and analysis for heavy metals was conducted on an copper, nickel, zinc, and iron ferrocyanides. AAS Perkin Elmer 3110 device. Absorption spec- In addition, the completeness and the charac- tra of solutions were registered on a UV-VIS ter of decomposition of complex cyanides de- Aglent-8453 instrument. pend not only on ðÍ and on the chemical com- position of these compounds, but also on the availability of other anions and solute oxygen Centrifugal bubbling apparatuses in solution, and also on the intensity of mass for AVR processes transfer in the reaction zone. These circum- stances should be taken into account when us- The design of CBA (3ig. 2) involves a cylind- ing the AVR method. rical case whose upper part has a separation This work presents the results of our stu- dies on processes that occur during extraction of cyanides from recycled processing solutions of the hydrometallurgy department of the gold- processing plant (Kholbinskiy mine) using cen- trifugal-bubbling apparatuses (CBA) as reactors for AVR processes. Winning of gold at the mine is realized by flotation concentration of ore fol- lowed by leaching of the metal in cyanide form from the concentrates. The plant uses closed- loop recycled water supply without fresh water make-up and without waste disposal. The dam 3ig. 2. Schematic diagram of a centrifugal bubbling ap- and the bottom of the tailings pond are perma- paratus: 1 – cylindrical case; 2– separation zone; 3 – fan; 4 – gas outlet; 5, 6 – fitting pipes for liquid feed nently in the frozen state to prevent under- and discharge; 7 – gas inlet; 8 – swirler; 9 – rotating ground filtration of solutions. Reagent sewage gas-liquid layer. EXTRACTION O CYANIDES ROM WASTE SOLUTIONS O CYANIDATION O LOTATION CONCENTRATES 433 zone, a fan, and a gas outlet and the bottom As noted above, acidification of waste so- part has fitting pipes for liquid feed and dis- lutions leads to dissociation of the complex charge, a gas inlet and a guiding device (swir- cyanides of heavy metals: ler). The swirler is a ring with tangential slots Mx+ (–)xy↔ Mx+ + yCN– (1) for gas twisting. (CN)y The operating principle of CBA is letting and to binding of cyanide ions into HCN: the gas pass through a rotating liquid layer + – held by the gas stream in the eddy chamber. H + CN = HCN (2) When the fan is switched on, the gas passes The liberated metal ions react with iron through the slots of the swirler and entrains 4– cyanides [3e(CN)6] (if any) in solution to the liquid in the swirl, forming a rotating gas- form ferrocyanides sparingly soluble in acid liquid layer inside the apparatus. Through the media: central fitting pipe the gas and liquid enter 4– õ+ the separator that uses the centrifugal and in- [3e(CN)6] + 4/õÌ = Ì4/õ [3e(CN)6] (3) ertial forces to let the gas and liquid out of the or M(CN)x type compounds: reactor. 3 x+ – A CBA with an air-flow rate of 140–160 m /h M + xCN = M(CN)x (4) (liquid flow rate 60-80 l/h) and an industrial (M = Ag+, Zn2+, Ni2+, Cu+) module of two devices mounted in series with an air-flow rate of up to 8000 m3/h were used The precipitation sequence Zn2+ > 3e2+ > Ni2+ for our experiments. Air stripping of solutions > Cu+ is controlled, on the one hand, by disso- to remove HCN was conducted in a device with ciation of cyanide complexes at different ðÍ two swirlers installed in succession; HCN ab- values (see 3ig. 1) [4] and on the other hand, by sorption by a NaOH solution was carried out the stability of ferrocyanides in acid media [9]. in a CBA with three swirlers. The industrial 3igure 3 shows the kinetic curves of re- module afforded 2.5–3 m3 of cyanide solutions moval of common cyanides, thiocyanates, and to be processed per hour by means of the AVR copper from processing media. The data ob- method. tained testify that virtually complete removal of HCN and heavy metals from the solution is observed within a minute in the eddy cham- RESULTS AND DISCUSSION ber of CBA under conditions of intense mass Air stripping of cyanidation solutions transfer at ðÍ < 3. This leads to 100 % absorp- to remove HCN in CBA tion of hydrogen cyanide in an alkali solution. Copper thiocyanates comprise the bulk of the Previously [8], we have demonstrated that deposit; they form by the reaction the rates of chemical reactions in CBA and the [Cu(CN) ]x – 1 + xH+ + SCN– completeness of cyanide removal by the AVR x method depend strongly on the intensity of = xHCN + CuSCN (5) mass transfer, which, in turn, is controlled substantially by the rotation rate of the gas- liquid layer. Residence time of the gas-liquid Oxidation of thiocyanates in CBA layer in CBA during air stripping is 15-60 s (one to four treatments in the swirler). The 3igure 4 presents the results of air strip- volumetric rates of reactions involving the ping and absorption of HCN during the AVR transformations of thiocyanates and cyanides process in CBA. Curve 1 is plotted from data in the rotating gas-liquid layer of CBA were for common cyanides in solutions before and calculated from the loss of the total concentra- after treatment in CBA; curve 2 is drawn from – tion of cyanides and thiocyanates and com- the results of CN determination in the ab- prised 9.5-13.7 mol/(l h), which is 45–65 times sorbing solution of NaOH. It is evident that higher than the rates of the same processes in the contents of cyanides in the absorbing so- a bubble type column. lution of NaOH are, on the average, 30 % 434 A. A. KOCHANOV et al.

3ig. 4. Air stripping to remove HCN (1) and HCN absorption (2) in CBA (solution feed rate in CBA is 1 l/min, ðÍ 2.47).

At inlet to CBA: ÑCN- = 633 mg/l, ÑSCN- = 1837 mg/l; at outlet from CBA: ÑCN- = 245.5 mg/l, ÑSCN-= 1008 mg/l.

+ + 2+ 2Cu + 1/2O2 + 2H = 2Cu + H2O (6) E0 = 1.076 V HCN formation:

2– 2Cu(SCN)2 + SO23+ H2O = 2CuSCN + 3S 2– + SO4 + 2HCN(7) and oxidation of sulphur:

3ig. 3. Kinetics of removal of common cyanides (a), thi- 2– + – ocyanates, and copper (b) during treatment of process- S + 4H2O = SO4 + 8H + 6å (8) 0 ing solutions in CBA (Ñ0 is the initial concentration, Å = –0.357 V and C is current concentration, mg/l): 1, 2 – CN–; 3, 4 – SCN–; 5, 6 – Cu; 7 – 2– ; pH: 3.7 (1, 3, 6, 7), 2.4 (2, 4, 5). SO2 3 This mechanism is supported by the character of variation of 2– and copper concentra- SO2 3 higher than one might expect even in the case tions after treatment of cyanide solutions in of complete dissociation of complex cyanides CBA (see 3ig. 3). of heavy metals. Hence, the additional quan- tity of cyanides formed in the absorbing solu- tion due to oxidation of SCN–. The diagram Åh – ðÍ (3ig. 5) plotted with thermodynamic data [10] confirms that thiocy- anates are unstable in the HSCN – H2O sys- tem. Oxidation of thiocyanates can form sul- phur [11] or thiocyanogen (SCN)2 [12] as inter- mediate products, which depends on condi- tions. The first scheme is realized in the pres- ence of Cu2+ and with thiosulphates or sul- phites present in solution. Under these condi- tions, thiocyanates are actively oxidized to HCN 2– and SO4 . Treatment of cyanidation solutions in CBA under conditions of intense mass transfer at x – 1 ðÍ < 3 results in decomposition of [Cu(CN)x] , + oxidation of Cu to Cu2+ by air oxygen ac- 3ig. 5. Diagram potential – ðÍ for the HSCN – H2O cording to the scheme system [11]. EXTRACTION O CYANIDES ROM WASTE SOLUTIONS O CYANIDATION O LOTATION CONCENTRATES 435

The second scheme of thiocyanate oxida- tion in aqueous solutions through thiocyanogen formation,

– – 0 2SCN = (SCN)2 + 2å , E = –0.77 V (9) 2– also gives rise to HCN and SO4 as end pro- ducts. Hydrolysis of thiocyanogen in aqueous so- lutions according to the scheme

3(SCN)2 + 5Í2Î – 2– + 3ig. 6. Absorption spectra of thiocyanogen: 1, 3 – solu- = 5SCN + HCN + SO4 + 7H (10) tions are obtained by oxidation of Pb(SCN) 2 with bro- mine in CCl and ÑÍ ÑÎÎÍ; 2, 4 – solutions 1, 3 after described in the literature may be considered 4 3 an excess of KSCN has been added. to be an integrated process as there is much evidence in favour of intermediate compounds, 4), our data (3ig. 6) reproduce the results of often more stable than (SCN)2 [13–18]. [13]. The difference lies in the fact that the It was shown that, e. g., thiocyanogen can peaks corresponding to the absorption maxima form (SCN) polymers not only in non-aque- x lie at λmax = 254 and 273 nm. The absorption ous solvents such as glacial acetic acid [13] and peak at 254 nm, whose intensity increases con- acetonitrile [16], but also in aqueous media when siderably after addition of excess KSCN to the thiocyanates are oxidized with hydrogen per- – solution, is attributed to (SCN)3 or to some oxide [17] or 3e(III) compounds [18]. other, more condensed thiocyanate. The emer- Thiocyanogen is generally identified by its gence of a compound of this kind in solution characteristic absorption at λmax = 295 nm in causes the second peak to shift in the long- the UV-VIS spectra [19]. The wide absorption wavelength range of the spectrum (λmax = band with λmax = 300 nm in the region 280– 315 nm). 320 nm, whose intensity increases appreciably When the AVR process is conducted in CBA – with the content of SCN in aqueous solution (ðÍ < 3), reactions (6) and (7) proceed quickly during oxidation of HSCN, was explained by and the concentration of thiocyanates decreases the presence of thiocyanogen together with sharply in solutions even after 15 s of treat- (SCN)– the stable intermediate 3 [17]: ment in the eddy chamber of CBA (see 3ig. 3). After further treatment, the concentration of – (SCN)– (SCN)2 + SCN = 3 (11) thiocyanates decreases slowly as practically all Moreover, the intense peak appearing at copper precipitates from the solution. The peaks that are responsible for the emergence of the λmax = 320 nm in the absorption spectrum was – oxidation products of thiocyanates (thiocyano- attributed to the formation of (SCN)3 as an intermediate during electrooxidation of iron gen or its polymeric modifications (SCN)x) are (III) thiocyanate in an aqueous 0.1 M solution absent from the absorption spectra of the so- lutions (3ig. 7, curve 1). However, after the of HClO4 [18]. The two peaks (at 320 and 283 nm) ob- precipitate has been separated and allowed to served in the absorption spectra of the oxida- settle for a few days, the liquid contains a tion products of SCN– in acetonitrile were at- small amount of amorphous suspended solid, tributed to the trithiocyanate ion and thiocya- percolating through the paper filter and dis- nogen, respectively [20]. Two peaks (at 250 and solving in NaOH. Addition of NaOH to the 288 nm) were observed in the absorption spec- solution gives rise to two maxima (at 267 and tra of fresh solutions of thiocyanogen in ace- 285 nm), whose intensity increases with ðÍ (see tic acid [13]. 3or thiocyanogen obtained by ox- 3ig. 7, curves 2, 3). One maximum (λmax = 285 nm) was attributed to thiocyanogen (SCN) , and the idation of Pb(SCN)2 with bromine in CCl4 2 – (curves 1, 2) and in glacial acetic acid (curves 3, other to (SCN)x , where x ≥ 3. This was in- 436 A. A. KOCHANOV et al.

also to production of more hydrogen cyanide due to regenerative oxidation. A mechanism is suggested whereby the con- version of thiocyanates in CBA occurs both by oxidation by air oxygen in the presence of copper ions and thiosulphates and through for- mation of thiocyanogen.

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