PROCESSING OF CAKE OF MOLYBDENUM MINING AND METALLURGICAL PRODUCTION BY ELECTROOXIDATION IN CHLORIDE-CONTAINING SOLUTIONS

One of the types of technogenic metal-containing raw materials of Almalyk MMC JSC (Uzbekistan) is waste processing of molybdenite concentrate of previous years, based on nitric acid opening of sulfide concentrate: cakes stored in the tailings of the plant, allowing to some extent to increase the output of the plant's main products. In their composition, wt%: 4.1 Mo; 2.0 Fe, 3.87 S, 78.98 SiO 2 , 5.26 CaO, others 4.79 (including Au 20-25 g/t). Purpose: development of technology for the extraction of Mo. Au, other components from molybdenum cakes by electrooxidation in hydrochloric acid medium. Interest in the method is caused by the modernization of hydrometallurgical technologies, due to the increase in environmental requirements for them, and also due to the emergence of new hydrometallurgical equipment. We used a two-chamber electrolysis plant for Mo-cake leaching, with a membrane, a direct current source, 20 A, generating active chlorine: hypochlorite ions at the anode. Mo(VI), Аu(III), Cu(II) ions were analyzed by photocolorimetric and spectrometric methods (ICP - Agilent 7500 IСP MS). The concentration of NaClO was determined according to the State Standard 18190-72, free chlorine was determined by titration with methyl orange. The behavior of gold in Mo-cake in contact with hypochlorite ions is considered. The oxidation of Mo cakes in a hydrochloric acid medium by hypochlorite ions generated anode at the anode has been studied, the parameters of their electrooxidation have been calculated, and the leaching products have been studied. The method of electrooxidation, in the presence of HCl, on the basis of 3-factor planning of the experiment, regression equations were obtained. Checking their adequacy, according to the Fisher criterion, by comparing the calculated and experimental data on the degree of extraction of these metals, showed the adequacy of the proposed process models. A technology for the electrooxidation of molybdenum cake has been developed, which is an alternative to the existing scheme of oxidative roasting of molybdenite concentrate, followed by soda leaching of molybdenum from the cinder, with cyanide processing of cake for gold. It has been established that the electrooxidation of Mo-cakes makes it possible to process them efficiently, with through extraction of gold up to 90%, molybdenum - up to 72.6%.


INTRODUCTION
Previously, the production of molybdenum at the Scientific Production Association "Production of rare metals and hard alloys" (SPA PRM&HA) JSC "Almalyk MMC" was based on nitric acid opening of molybdenite concentrate (MoC) [1][2][3], later replaced by oxidative roasting followed by ammonia [4], then soda [5] by leaching the cinder [6].The Mo cakes tailings [7] accumulated in the tailings are not yet processed, although the concentration of gold in them is higher than in gold-bearing ore [8] processed by oxidative leaching in solutions of cyanide, ferric chloride [9], and hypochlorite [10].The last reagent, due to chemical instability, is anodically generated directly in the pulp (electrooxidation method) [11][12], in a 10% NaCl, H2O, Na2CO3 medium [13].For Mo concentrate, in this case, it is known that the product yield depended on: NaCl concentration, temperature, stirring, L:S (liquid:solid) ratio, pH, anode current density (hypochlorite concentration), pH [14].When the solution was acidified, MoO3•(H2O)x precipitated (together with Re).The low emission of chlorine made it possible to consider the process environmentally friendly, and the leaching of Mo concentrate [15] is the result of its interaction with chlorine, and not with the anode current.However, Mocake, sharply depleted in Mo, is more attractive only as a gold-bearing raw material, for which it is expedient to replace Na2CO3, as a Mo(VI) leaching agent, with HCl, as an agent leaching Au 3+ ions.It should be noted that interest in the electrooxidative processing of Au-, Mo-containing raw materials [16] is caused by the modernization of hydrometallurgical technologies, due to increased environmental requirements for them, and also due to the emergence of new hydrometallurgical equipment.
Purpose: Evaluation of the efficiency of Mo-cake proc processing in hypochlorite hydrochloric acid electrolyte using the experimental design method.

RESULTS AND ITS DISCUSSION
During the electrolysis of chloride solutions, chlorine is released at the anode (1), which, upon hydrolysis, forms hypochlorous acid (2): Cl2 + H2O → HCl + HClO (2) which is also a strong oxidizing agent (3): it is possible to oxidize molybdenite MoS2 directly with chlorine: The task of the stage of work was to optimize the Mo-cake electrooxidation process in terms of the following parameters: anode current density ia, concentration of sodium chloride, hydrochloric acid, L:S ratio, duration, solution-pulp temperature.It was performed using the Matlab program by planning a three-factor experiment with variation intervals: X1, X2, X3 and calculating their effect on the extraction of Mo (Y1, %) and Au (Y2, %) into solution (Table 1), where: Х1concentration of sodium chloride: 100-250 g/dm3 ; X2anode current density: 100-200 mA/cm 2 ; Х3concentration of hydrochloric acid: 10-100 g/dm 3 .moreover, in view of the fact that Mo cake is a middling product poor in Mo, and dilution of the pulp with electrolyte (an increase in L:S value) is favorable from the point of view of increasing the completeness of the extraction of valuable components from it, but it is not economically feasible due to the increase in filtration costs, pulp pumping, electricity, then for the 3-factor experiment a fixed ratio L:S = 10:1 was chosen.Analyzing the regression equations ( 5) and ( 6), we see that the extraction of molybdenum and gold in solutions was positively affected by all three factors, although to a different extent.Y1 depended to the greatest extent on the concentration of hydrochloric acid X3, somewhat less on the anodic density current ia (factor X2): an increase in ia led to an intensification of chlorine release, stimulating the mechanism (4) of MoS2 oxidation by atomic chlorine.
For the extraction of Au ( 6), the growth of X3 is more significant than X3 in (5).Given that in a hypochloritechloride electrolyte, the electrooxidation of gold proceeds according to the reaction equation ( 7), the role of chloride ions is to shift its equilibrium to the right:   The next task of the study was to assess the adequacy of models ( 5) and (6).It was performed by comparing the experimental data Y1 (ex), Y2 (ex) and data from the substitution of experimental conditions in ( 5) and ( 6) (Table 2 and Figure 3a, b, c).The dependences of Y1 (ex), Y2 (ex) on the concentration of NaCl (Figure 3a), HCl (Figure 3b), ia (Figure 3c) were obtained at fixed values of 2 of the 3 factors, with a variable 3 th value, which varied in a wider range of variation than with mathematical planning (Table 1).Figure 3a shows a reduced level of Y1 (ex) and Y2 (ex), in comparison with the calculation.When moving to the lower boundary of the X1 variation interval, the discrepancy between them increased.With an increase in the concentration of NaCl (Figure 3a) in the range: 25-138 g/dm 3 , an increase in Y2 (ex) from 45.8 to 61.5% was observed, caused by the intensification of chlorine release and pulp saturation with it.But a further increase in X1 led to a decrease in the concentration of Cl2 in the solution and a drop in the recovery of Au.The effect of ia on Y1(ex), Y2 (ex) (Figure 3b, Table 3) was considered: their monotonic increase with increasing ia was established and at the lower boundary of the X2 variation interval, a significant discrepancy between Y1 (ex), Y2 (ex) with the calculated data (with a lag of the first from the second), but when moving upwards within the interval, up to 200 mA/cm 2 , the indicated discrepancy was leveled, then increased again.An increase in the HCl concentration increased the recovery of Au (Figure 3c, Table 2), both in accordance with the calculation and in the experiment.Moreover, in the absence of HCl in the pulp, Y1 (ex), Y2 (ex) are much less than the calculated ones and are minimal: 3.5% Mo and 0.5% Au, respectively.
An increase in the L:S index increased Y1 (ex), Y2 (ex) up to 70-80% (Figure 3e) only in the initial section, up to L:S= 10:1, but did not further increase the values of Y1(ex), Y2 (ex): dilution of the pulp more than 10:1 did not cause a noticeable increase in the extraction of Mo and Au.
To check the adequacy of the obtained regression equations ( 5) and ( 6) in addition to the visual convergence of curves Y1 (ex) and Y2 (ex) with the calculated dependences (curves 1, 3), confirming their adequacy to each other, was of interest to compare the values of the residual and selected variances.The multiple correlation index reflects convergence: it takes into account the combined influence of factors on the result.Unlike the paired correlation coefficient, which can take negative values, the multiple index takes values from 0 to 1. Therefore, the correlation coefficient r*(Xi) (Table 4) cannot be used to interpret the nature of the relationship.The closer to each other the calculated Y1, Y2 and the actual values of Y1 (ex) and Y2 (ex) (2.4), relative to the regression line, the smaller the residual variance and, therefore, the greater the value of the multiple correlation coefficient r*(X1, X2, X3).Thus, with a value of r* close to 1, the regression equation better describes the actual data and the factors X1, X2 and X3 have a greater effect on the result.With an r* value close to 0, the regression equation does not describe the actual data well.The value of the correlation coefficient for ( 5) and ( 6) is close to unity, so the relationship between Y and X is close to linear: Y1 r*av = 0.92, Y2 r*av = 0.97.The adequacy variance was calculated for (5) and for (6).The adequacy of equations ( 5) and ( 6) was successfully checked by the Fisher criterion [17][18].Statistical analysis showed that the mathematical models for ( 5) and ( 6) are adequate.

CONCLUSION
The efficiency of leaching of molybdenum and gold from the Mo-cakes of "Almalyk MMC JSC" in NaCl solutions was studied by electrooxidation in the presence of HCl, based on 3-factor experiment planning, regression equations were obtained.Checking their adequacy according to the Fisher criterion, by comparing the calculated and experimental data on the degree of extraction of these metals, showed the adequacy of the proposed models.It has been established that the electrooxidation of Mo-cakes makes it possible to process them efficiently, with through extraction of gold up to 90%, molybdenum -up to 72.6%.Optimal process conditions are: L:S = 10:1; T = 25 °C; CHCl = 100 g/dm 3 ; СNaCl = 100-138 g/dm 3 ; ia = 150-200 mA/cm 2 ; τ = 6 h.It is advisable to continue the study by extending the technology of Mo-cake leaching in NaCl solutions to other supporting electrolytes.

Table 1
Matrix of a three-factor experiment for Mo and Au leaching from Mo-cake in HCl solution, when generating hypochlorite in it, τ = 6 h, samples initial weight is 100 g

Table 3
Dependence