Leaching of Polymetallic Cu-Zn-Pb Concentrate with Sodium Nitrate in Sulphuric Acid

MIROSLAV D. SOKIĆ, Institute for Technology of Nuclear Original scientific paper and Other Mineral Raw Materials, Belgrade UDC: 622.234.4 BRANISLAV R. MARKOVIĆ, Institute for Technology of Nuclear DOI: 10.5937/tehnika2104426S and Other Mineral Raw Materials, Belgrade DUŠAN V. MILOJKOV, Institute for Technology of Nuclear and Other Mineral Raw Materials, Belgrade ALEKSANDRA S. PATARIĆ, Institute for Technology of Nuclear and Other Mineral Raw Materials, Belgrade MLADEN D. BUGARČIĆ, Institute for Technology of Nuclear and Other Mineral Raw Materials, Belgrade GVOZDEN B. JOVANOVIĆ, Institute for Technology of Nuclear and Other Mineral Raw Materials, Belgrade KATARINA R. PANTOVIĆ-SPAJIĆ, Institute for Technology of Nuclear and Other Mineral Raw Materials, Belgrade


INTRODUCTION
Non-ferrous metals and iron usually appears in a form of sulphide complex ores. Large deposits of complex ores often contain chalcopyrite, sphalerite, galena and pyrite, which are fine-grained structures complexly fused with tailings minerals in complex mineral forms [1]. Sometimes, when it is difficult to prepare flotation concentrates of the individual minerals, then it is easier to prepare bulk concentrates [2]. Although treatments by pyrometallurgical process are not attractive, because a large amounts of SO2 is produced, approximatelly 80-85% of worlds total copper is produced pyrometallurgically, and 80% of worlds total zinc is produced by roasting-leaching-electrowinning process, and 20% is produced by Imperial Smelting Process [3][4].
Van Weert et al. [46] developed the NITROX process, which utilizes nitric acid to recover gold from refractory pyrite and arsenopyrite ores. They found that sulphide sulphur formed mainly elemental sulphur, while smaller part formed sulphate. The NO gas that is produced is directly oxidized by the air into the NO2, which dissolves in the solution and reacts with water to form HNO3, thereby completing the NITROX cicle. The most significant problem was high sulphate production. Droppert and Shang [47] showed that sulphate formation could be minimized by addi-tion of a small excess of HNO3, followed by slow addition of HNO3 keeping the acid concentration at a constant low level.
Some industrial operations use the nitric or nitrous acid added in a small concentration to the sulphuric acid, e.g. in oxigen extended pressure leach processes. At Sunshine Precious Metals, silver and copper are recovered from a complex sulphide concentrate at temperatures between 145 and 155C and at total pressure of 709kPa [24,39,48]. In that case, nitrous acid were found to enhance the solubilization of minerals at lower temperatures and pressures, and the nitrous/sulphuric acid leach process is used with success.
Following successful operation in the Sunshine Pressure Leach plant, the catalysis under the extended pressure oxidation, using nitrogen species, is promoted as a nitrogen species catalised (NSC) technology [23,48]. It was demonstrated that the addition of nitrite ion in a small amounts catalyses the oxidation of sulfides in the presence of oxigen; recent data show that it is a fast reaction, typicaly less than 30 min, for slurry containing 100 g/L of solids [23]. The leaching process of metal sulphides by nitric acid as oxidant is more efficient in presence of NO + ions. The addition of NO 2ions instead of NO 3ions accelerates the formation of NO + ions, which further oxidises sulphide minerals at lower temperatures to the elemental sulfur [40,48].
Oxidative dissolution of a sulphide concentrate using nitrate as the leaching agent in an acid medium take place with formation of elemental sulphur, and it can be reperesented by one of the following chemical reactions [20,21,49]: or Initially the rate of reaction is controled by a surface chemical reaction and later on changes into a diffusion controled.
General flow sheet for the treatment for the complex Cu-Zn-Pb concentrates is presented in Fig 1. The objactive of this work is performance of the leaching process using the selected complex Cu-Zn-Pb sulphide concentrates, from Rudnik flotation plant, by sulphuric acid solution in the presence of sodium nitrate.

CHEMICAL REACTIONS AND THERMODYNAMICS
Based on the literature data and on the characterization of both produced solution and leaching solid residues, obtained by leaching of pollymetalic concentrate containing chalcopyrite, sphalerite, galena and pyrrhotite in oxidized acidic medium, following chemical reactions were chosen in this study and than analyzed in the H2SO4-NaNO3-H2O system.
Chalcopyrite (CuFeS2) leaching system (14) Thermodynamic analysis included calculation of standard Gibbs energy change and E-pH diagrams. In order to clarify the occurrence probability of quoted reactions (3)-(14), Gibbs energy change was calculated within a temperature range of 25 up-to 90 o C and the obtained values are given in Table 2. HSC Chemistry software and its data base of thermodinamic values of reaction participants were used in calculations.
The negative values of Gibbs energy change GT for reactions (3)- (14) show that they are all thermodinamically feasible at standard pressure and temperature range of 30-100 o C. Higher negative GT value of the reaction (4) than that of the reaction (3) suggests its occurrence with higher thermodinamic probability. NO and NO2 ratio in gaseous products and elemental sulphur and sulphate ratio after leaching depend on concentration of nitrate ion (Droppert at al. 1995). E-pH diagrams show the thermodynamic stability of water solution components and the correlations electrochemical potential -pH values. E-pH diagrams for the behavior of copper, zinc, lead and iron in Cu-Zn-Pb-Fe-S-H2O system are presented in Fig. 2.

Figure 2 -E-pH diagrams for Cu-Zn-Pb-Fe-S-H2O
system at 80 o C From Fig. 2 it can be concluded that copper, zinc, lead and iron leach from their minerals at low pH values and under the given oxidizing conditions. Under these oxidizing conditions at high electrode potential and low pH values, Cu 2+ , Zn 2+ , Fe 2+ and Fe 3+ ions exist in water solutions, while Pb 2+ exists in sulphate form in the resi-due. The increase of temperature lowers the possibility of Fe 3+ ion existance in the system.

EXPERIMENTAL
The concentrate enriched during the flotation of a CuFeS2-PbS-ZnS polymetallic ore in the Rudnik flotation plant (Rudnik -Serbia) was used. All leaching experiments of polymetallic concentrate at atmospheric pressure and temperatures up-to 90 o C were carried out using experimental set-up, which provides the stable hermetic conditions and allow the heating at constant temperature.
The liquid volume was kept co-nstant during the experiments. The calculated volumes of H2SO4 and TEHNIKA -RUDARSTVO, GEOLOGIJA I METALURGIJA 72 (2021) 4 NaNO3 solutions were put into the glass reactor and heated-up to the selected temperature. When the temperature was reached, the solid conce-ntrate was added and that moment is taken for the begi-nning of reaction.
After finite time intervals, during the leaching process, the solution samples were taken for chemical analysis which was carried out with AAS (Perkin Elmer).
The solid residues were carefully fil-tered out, washed with distilled water, dried and their phase content was determined by AAS, X-ray analysis using diffractometer (Siemens D500), light micro-scopy (Carl Zeiss-Jena JENAPOL-U) and thermal DTA / TG analysis on a NETZSH 409 Ep.

Characterization of Polymetallic Concentrate
Chemical composition of the polymetallic concentrate, which was used in the leaching process, is presented in Table 3. X-ray diffraction (XRD) analysis was used for the phase fraction determination in the polymetallic concentrate; the result is shown in Figure 3. 0 2, deg 90 Figure 3 -X-ray diffraction (XRD) analysis of the polymetallic concentrate from "Rudnik" flotation plant [20] The presence of chalcopyrite, sphalerite, galena, pyrrhotite and qu-artz is registered. Prepared sample microphotograph of polyme-ta-llic concentrate from "Rudnik" flotation plant is presented in Figure 4.  fides that oxidize differently. Further heating leads to a sudden loss of mass, which also occurs in several stages, which indicates that more different sulfates are present. These mass losses are accompanied by endothermic effects, of which they are clearly expressed at 776 o C (CuSO4 dissociation) and 855 o C (CuSO4·CuO dissociation).

Influence of the Operating Parameters
The experimental results on determination of leaching parameters of polymetallic concentrate from "Rudnik" flotation plant with sulphuric acid in the presence of sodium nitrate were performed in the te-mperature range of 20-90 o C and during the time in-tervals 60-240min. Further, the optimum values were: the phase ratio solid/liquid (S:L) = 1:5, the starting sulphuric acid concentration 225g/dm 3 , the sodium nitrate content exceeds 30% of the stoichiometric calculated value.
The influences of temperature and time on the leaching degrees of the zinc, copper and iron are presented in Figure 6. It is well known that the oxidative dis-solution of galena produces insoluble PbSO4 in the sulphuric acid leaching medium [51].Low leaching degree is obvious for all three metals, but for copper it is extremly low (Fig. 6.a). The similar situation occures for the influence of time on the leaching degrees at the temperature of 40 o C (Fig. 6.b).
Onset of leaching degrees is noti-cable at the temperature of 60 o C (Fig. 6.c), where the zinc leaching degree is the highest. Further tempe-rature increasing contributes that all three metals leac-hing degres increase (Fig. 6.d), and finally reaching the tem-perature of 90 o C at time of 240min (Fig. 6.e). It is important to notice that in contrast to the lower temperatures, when the process goes on above 50 o C the zinc leaching degree is higher than that for iron.

Characterization of the Solid Residuals
Characterization of the solid residuals after the leaching process included chemical and mineralogical analyses. The samples were chosen to be representative regarding a wide range of possible leaching products. Chemical analysis of the solid residual after the leaching is presented in Table 4, as well as the overall mass of the solid residual after the leaching process.  Microphotographs were taken on a prepared sample of the solid residuals after the leaching process for detection of the present phases. A microphotograph of a selected sample is shown in Fig. 8. For the reason of better phases recognition, the cedar oil was used.
The presence of sulphur and greater amount of anglesite, which shines milky white, is evident in Fig.  8, and corroded chalcopyrite and pyrrhotite are situated next to the right figure's margin. Thermogram of the solid residual after leaching obtained at a heating rate of 10 o C/min in air and shown in Figure 9. ntrate When the sample is heated, an endothermic peak characteristic of elemental sulfur is observed at a temperature of 118 o C. After that, there is a large loss of mass in the range of 250-350 o C, which is a consequence of the oxidation of a large amount of sulfur to SO2. The increase in mass followed by exo-thermic effects takes place in the range of 350-600 o C and is much smaller than the increase in mass in polymetallic concentrate, which indicates the presence of a small amount of unleaded sulfides. The mass loss above 700 o C is the result of the dissociation of the present sulphates (mainly anglesites) and is much sma-ller in relation to the analogous mass loss in concen-trates before leaching.

CONCLUSION
Based on the choosen leaching chemism of the polymetallic sulphide Zn-Pb-Cu concentrate and its thermodynamical analysis, the assumed chemical reactions mechanism for zinc, copper and iron leaching is confirmed.
The phases detection in both, the starting concentrate and the products after the leaching process with H2SO4 and NaNO3, were performed for better understanding of the chemical reactions that took place in the system.
The presence of the anglesite, elemental sulphur, gangue and unleached sulphide minerals, was registered using X-ray diffraction (XRD) analysis of the solid residual. This fact points out that leaching product of any sulphide mineral is elemental sulphur, which does not oxidize to sulphate in the temperature range (20-90 o C) and the time interval (60-240 min).
After the leaching process, copper and zinc are in the form of copper(II) sulphate and zinc(II) sulphate. Iron is being oxidized to form iron(III) sulphate, which then acts as a leaching agent for the present sulphide minerals, and at the same time become reduced to iron(II) sulphate. Lead from the galena reacts to form the lead sulphate (anglesite), which is insoluble and remains in the precipitate.
Detailed mineralogical investigations indicate a po-lymetallic concentrate complexity and explain weak leaching effect of suplhide minerals in the final leaching stage. The main reasons for that are:  elemental sulphur and anglesite, formed during the process and precipitate at the grain boundaries, fine grained mineral structure and complex mutual inter-growth of chalcopyrite, sphalerite, galena and pyrrho-tite (inclusion, impregnation, simple and complex in-tergrowth),  complex adhered beneficial suplhide minerals with gangue minerals (predominantly quartz). The accomplished leaching degrees under the given conditions (temperature of 90 o C, time of 4 hours, phase ratio S:L=1:5, the starting H2SO4 concentration of 225g/dm 3 , with sodium nitrate addition in the content of 30% above the stoichiometric needed) are as follow: Zn -89.25%, Cu -73.08% and Fe -70.80%.