Leaching
Selective leaching of precious metals
 
Kazakova Yu.V., Novikov O.N.
 
Article is written for the next aspects: non-cyanic leaching, gold, goldextracting plant (GEP), hydrometallurgy of gold


Technology of gold extracting by cyanic method is designed for use of harmful for human and dangerous for environment reagent - cyanide - which is cumulative toxin of the 2-nd class of danger. Thus, even minimal pollutions affect environment essentially. For neutralization of cyanide calcium hypochlorite has been used. This measure is resolution of main cyanide pollution, but isn't it for toxic vapours and chemical discharge into atmosphere. Use of sodium hypochlorite and chlorine as a method of gold extracting is widely known. These works have been executed as experiments and enlarged laboratory tests. This problem is sufficiently learned, monographies are available [1,2]. Universal complex «Alpha” will help to use this technology in industry. Equipment of this complex may be used for every task in technology of gold extracting by chlorine. One of perspective methods of gold extracting may be a heap leaching with use of “Alpha”complex. Electrolysis has been executed with inert cathode (for example, graphite) in the next conditions: current density 50 - 100 A/m², specific surface area of cathode 10 - 20 m²/m³, electrolysis duaration up to 4 -12 hours, size value of pulp solid phase 0,15 mm. Rate of gold regeneration by electrolysis is 98 - 99,3 %, approximate current output 0,5 - 1,4 %, specific electricity output discharge 40 - 170 kW/h for 1 kg of regenerated gold. Electrolyte gold extracting from hypochlorination pulps of goldcontaining concentrates of various compounds. Gold and silver extracting by “Alpha” complex has been made by non-toxic pollution ecologically pure technology with minimal danger for air of work zone. Mission of this article is developing of thermodynamical, kinetic process model for transmitting free gold into liquid form in chloride solutions to apply it in projecting of GEPs. In this article theoretical background of transmitting from technological process of goldextracting from goldcontaining raw minerals to the chlorine technology. Solution of sodium hypochlorite has been formed during electrolysis of highly concentrated solution of natrium chloride. Solution of natrium salts dissolves gold, which is contained in raw material by equation of chemical reaction 1. Reaction 1: 2Au + 3NaClO + 5NaCl + 3H2O → 2Na[AuCl4] + 6NaOH. Equation of reaction shows that molar ratio of active gold amount and that of transmitted into complex form in solution equals to stoichiometric amount. Because salt concentration of original reagents has an overage amount balance of reaction will be directed to creating of chemical products. By this way, all gold in raw mineral shall to be transferred into solution. After that parameters of goldcontaining raw material chlorination including concentration of dissolved gold have to be calculated. Mathematical calculations are in electronic form by Mathcad software program. Results of calculations are shown in table of indications. On the first stage of calculations coefficient of conductivity (κᵢ) of concentrated sodium chloride solution with concentration value Cᵢ(NaCl) by equation (1). κi = (zkk + zaa)*α*Ci(NaCl) (1), where λk and λa – mobility value of cations and anions accordingly, zk and za – electrical potential of cations and anions accordingly,
 
Λk = 50,14*10-4 m2*Sm/molar unit С1(NaCl) = 10 g/l,
Λa= 76,3*10-4 m2*Sm/molar unit С2(NaCl) = 50 g/l,
zk= +1 С3(NaCl) = 100 g/l,
za= -1 С4(NaCl) = 150 g/l,
α= 0,52 С5(NaCl) = 200 g/l,
  С6(NaCl) = 250 g/l,
molar unit = gram molecule weight
 
ɑ – level of dissociation of concentrated sodium chloride solution. As original initiative data the in scientific literature published information [3] has been practically used, and original concentration value of natrium chloride are taken into account. After that output amount of natrium hypochlorite Cᵢ(NaCl) depending on electrical field strength Uᵢ by equations (1, 2) has been calculated. Cᵢ(NaCl) = Ui*S*ά*η/(W(H2O)*ro*Mr(NaClO)) (2), where S – electrode square significance, S = 2 m²ά – empirical coefficient, ά = 1,386 g/(A*h)η – current output, η = 0,7W(H2O) – water-recirculation, W(H2O) = 1 m³/hro - relative resistance, ro = 0,044 kg*m³/(c³*A²)Mr(NaClO) – sodium hypochlorite molecular mass, Mr(NaClO) = 74,5 g/gram-molecular weight. Time of location of sodium chloride in electro reactor ti is calculated by equation (3) [1]. ti=1/Ui*u0i (3), where u0i – absolute mobility speed of chloride-ions for various concentrations of natrium chloride. Equations (2, 3) describes calculations of electrolyzer parameters. By such a way, at condition W(H2O) = 1 m³/h, a tank unit shall have a volume capability equal to 4 m³. Rapidity of hypochlorite generation is calculated by equation (4), Gi = ά*η*Ii (4). Equilibrium constant of gold dissolve reaction by sodium hypochlorite has been calculated by formula (5)
Ke= k*en*F*(ENaClO+EAu)/R*T (5), where k – coefficient of proportionality of equilibrium constant, k = 1 (gram molecule weight/m³)-5n (n – quantity of electrons in reaction, n = 6F (F – Faraday constant, F=96500 Cl (Coulomb) /gram molecule weight)) [3]. ENaClO - electrode potential EclO-/Сl- = 1,494 В [3]. EAu – electrode potential Eau0/Au+3 = -1,498 В [3]. R – universal gas constant, R = 8,314 J (Jole) / (gram molecule weight*K) [3]. T - absolute temperature, T=293K. Equilibrium constant value is directly proportional to significance of temperature. Variation range of equilibrium constant within interval of temperature variation is relatively not sufficient, therefore, there is no necessity to warm up working (functional) solution. Formula for calculation of dissolved gold concentration (6) is a simple algebraical consequence from equation of equilibrium constant. C(Na[AuCl4]) = {Ke[C(NaClO)]3[C(H2O)]3[C(NaCl)]5[C(Au0)]2/[C(NaOH)]6}0.5 (6) C(Na[AuCl4]) = 1 gram molecule weight/m³. This concentration value equals to estimated concentration of metal gold in ore. Result confirms ability of 100% gold extraction. Energy consumption Wi has been resulted by formula Wi=Ii*Ui (7). Table 1.
 
i Сi(NaCl)g/m-unit κi, с32/ (kg*m3) Ci(NaClO)g/m-unit Ui, V Ii, А Gi, kg/s Wi, Watt ti,s
1 10 1,124 3,7*10-3 0,001 0,281 7,5*10-8 2,8*10-4 8,1*105
2 50 5,62 0,183 0,01 14,049 3,8*10-6 0,14 8,1*104
3 100 11,239 3,659 0,1 280,978 7,6*10-5 28,1 8,1*103
4 150 16,859 54,89 1 4,2*103 1,1*10-3 4,2*103 809,3
5 200 22,478 731,8 10 5,6*104 0,015 5,6*105 80,9
6 250 28,098 1,8*105 20 1,4*105 0,038 2,8*106 40,5
 
Energy consumption calculation has shown accordingly to the table 1 that a time of sojourn in electro reactor is a reversely proportional to energy consumption. Optimal energy consumption in range of 3-5 kW insures certainly sufficient quantity of hypochlorite for gold extraction from raw mineral. Taking into account a specific rate of gold dissolving (see above), time of dissolving, that in accordance to kinetic limitations and experimental data is not more than 2 hours, it may be expected that the model reflects the leaching process adequately and may be used practically. The model shows that equilibrium value of concentration of dissolved gold is reflected in the range of maximal significances in all interval of acidity scale. With this condition a process may be practised in leach substance. Before the expert methods have shown that a process is suitable strongly in acid substance [1]. The processing model has shown that reaction is executable in other conditions too. A leach substance is prefferable, because inside metals stay in raw material as non-dissolutionable hydrooxides. Experimental data, resulted at the test exercises of “Alpha-7ВС” module (electrolyzer) of the first modification, have shown that regarding leach substance a chlorine technology – extracting of gold, contained in silicium as a cover layer, - may be used successfully and their effectiveness have resulted in significance 95,1%ア4,1%. Developed model can be a basis of further scientific researches on modeling of boring processes to enable metal extracting by method of heap leaching, and for utilization of various wastes, which contain metals in combined principle. Adoption of chlorine technology with use of “Alpha” complex standard equipment contributes to significant reduce of material, working and energy consumptions, as well as a time period for projecting and installation of gold extracting factories. Naturally, collection of industrial effluents and recycling of discharge waste waters are hereby regarded.
Conclusions:

 
  1. Thermodynamical model of a free gold transfer into liquid form in chloride solutions in leach substance is designed.
  2. Calculations of various factors influence on to process of dissolving gold by electrochemical processing of activated natrium chloride solution are executed.
  3. Percentage of sodium hypochloride and percentage of sodium chloride and gold mixed substance, dissolved by hypochlorite, are equal, subsequently, gold of solid substance accordingly to thermodynamical calculations has been extracted for 100%.
  4. Regarding data, published in methodical literature, thermodynamical calculations don't affect experimental data for describing of gold dissolving.




Scientific studies:
 
  1. Chloride metallurgy of gold. Zyryanov M.N., Leonov S.B. - M.: “INTERMET ENGINEERING Publishing”, 1997. – 288 pages (p).
  2. Electromechanical recycling of discharge waste waters. Krasnoborod'ko I.G., Svetashkova Ye.S. - Leningrad, 1978. - 90 p.
  3. Quick reference guide of physical-chemical values. Edition 8, renewed. /Editors: A.A.Ravdelya and A.M.Ponomaryova. - L.: Chemistry, 1983. - 232 p.

 
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