Structural influence of sphalerite on their leaching from complex concentrate by sodium nitrate in sulphuric acid

Nonferrous metals and iron usually appear in a form of sulphide complex ores. The large deposits of complex ores may contain chalcopyrite, sphalerite, galena and pyrite in disseminated form with complex mineralogical composition and fine-grained structures. Complex ores are often characterized by particulary fine intergrowth of the mineral values. For instance, chalcopyrite and sphalerite are frequently intergrown, micro-size grains of 10-20 m are being dispersed within the pyrite. Sometimes it is not possible to prepare flotation concentrates of the individual minerals, but only bulk concentrates. Although treatments by pyrometallurgical processes are not attractive because a large amount of SO2 is produced, approximately 80 to 85% of the world’s total copper is produced pyrometallurgically; in terms of the world’s total zing production, 80% is produced by roasting-leaching-electrowinning process and 20% is produced by the Imperial smelting process (Antonijevic et al., 2004; Aydogan et al., 2006).

Hydrometallurgical process offers a great potential for treating complex sulphides and it results in improving metal recoveries and eliminates air pollution hazards.

In recent years, there has been a heightened interest in the possible application of various reagents in the hydrometallurgical processing of sulphide concentrates. Ferric and cupric ions, oxygen, other oxidants and bacteria have been used as oxidative leaching agents of sulphides in sulphate and chloride media, under atmospheric or pressure conditions (Prasad and Pandey, 1998; Dreisinger, 2006; Hackl et al., 1995; McDonald and Muir, 2007; Aydogan et al., 2006; Dutrizac, 2006; Aydogan et al., 2005, Santos et al., 2010)

Oxidative dissolution of a sulphide concentrate using nitrate as leaching agent in an acidic medium takes place with formation of elemental sulphur and it can be represented by one of the following chemical reactions (Bredenhann and Van Vuuren, 1999; Peng et al., 2005, Sokić et al., 2009; Sokić et al., 2012):

3MeS + 2NO₃^- + 8H⁺ = 3Me²⁺ + 3S⁰ + 2NO + 4H₂O (1)

or

MeS + 2NO₃^- + 4H⁺ = Me²⁺ + S⁰ + 2NO₂ + 2H₂O (2)

The aim of the research reported in this paper was to investigate the structural characteristics of sphalerite grains on their leaching by sulphuric acid solution in the presence of sodium nitrate from complex concentrate.

In this paper, chemical, XRD and qualitative and quantitative microscopic analysis were used to determine the characteristics of the samples of complex concentrate and leach residue.

The content of copper, zinc, lead and iron in complex concentrate and leach residues were determined by the atomic-absorption spectrophotometer (AAS) Perkin Elmer model ANALYST 300. The samples were prepared by dissolving in mineral acids to determine the content of copper, zinc, lead, iron and sulfur in them to AAS.

The phase composition of complex concentrate and solid residues was determined by X-ray analysis using diffractometer PHILIPS PW-1710.

Quantitative microscopic investigation was performed in reflected light on Carl Zeiss-Jena, JENAPOL-U microscope by parallel profile method (interval 1 mm). Objective magnification was from 10 to 50x. The surface of observed grain is determined using OZARIA software and system for microphotography (Tomanec et al., 1997). Tailing minerals were not determined individually but generally.

Structural characteristics of aggregates are divided in five motifs:

1. 
   Liberated grains (self-determining mineral grains with some 100% visible surfaces).
   
2. 
   Mineral grains with inclusions (the observed mineral which contains some other mineral species with maximum 10-30% of surface, mostly inclusions).
   
3. 
   Impregnations (the observed mineral which contains some other mineral species with maximum 70-90% of surface).
   
4. 
   Simple intergrowths (the observed mineral is grown to some different mineral where its surface is some 30-70%, mostly exsolution).
   
5. 
   Complex intergrowths (the observed mineral is grown to several minerals where its surface is some 10-50%, mostly multiple intergrowths, and exsolution).
   

The complex concentrate enriched during the flotation of a CuFeS₂-PbS-ZnS polymetallic ore in the Rudnik flotation plant (Rudnik – Serbia) was used. Four particle size fractions were obtained by wet sieving. The chemical analysis of each size fraction is presented in Table 1.

X-ray diffraction (XRD) analysis was used for the phase fraction determination in the complex concentrate; the result is shown in Figure 1. The presence of chalcopyrite, sphalerite, galena, pyrrhotite and quartz was registered.

Table 2

Structural characteristics of sphalerite aggregates are shown in Figure 3. Sphalerite occurrence of the liberated sulphide grains is 59.3%. The rest mineral grains of sphalerite are mostly in a form of inclusions and simple and complex intergrowths.

Sphalerite leaching tests included the investigation of the influence of temperature and leaching time, the solid-liquid ratio, stirring speed, acid and nitrate concentration on the leaching rate and determination of their optimal values. The leaching of zinc achieved 93,3 % at temperature 80 ^oC and time 240 min. It was observed that during leaching after a certain time there was a significant decrease of leaching rate and sphalerite remained in the solid residues. In order to explain the decrease in leaching rate in the final stage, characterization of leach residue was performed and presented.

The leach residue, obtained at 80^oC under the following conditions: stirring speed 300 rpm, 1.5 M H₂SO₄, 0.6 M NaNO₃, 20 g concentrate/1.2 dm³ and 240 min, was chosen for X-ray and qualitative and quantitative mineralogical analyses.

The phases identified by XRD were elemental sulphur, anglezite and unleached sulphides (Fig. 5), which confirms the expectation that the elemental sulphur is formed during leaching.

0 2, (^o) 90

The presence of sulphur and greater amounts of anglesite, which shines milky white, is evident in Figure 8; corroded chalcopyrite and pyrrhotite are situated next to the right figure’s margin.

The structural assembly of sphalerite grains in the leach residue is favorable and no reason to reduce the leaching rate in the final stage of reaction. Reason for this is elemental sulfur, which was precipitated at the particle surfaces during the leaching. Similar results were obtained during the leaching of chalcopyrite in the same solution (Sokić et al., 2014). The structural assembly of mineral grains of chalcopyrite is not a reason for the reduction of leaching rate, but elemental sulfur, which was precipitated at the particle surfaces.

During the sphalerite leaching by sodium nitrate and sulfuric acid solution from complex concentrate, leaching rate decreases with increasing the time and a part of sphalerite mineral grains remains in the leach residue. The chemical, XRD and qualitative and quantitative mineralogical analyses were used for the characterization of the complex concentrate and leach residue. In complex concentrate, 59.3 % of sphalerite mineral occurs as in liberated grains, and the rest is in association with gangue minerals. In the leach residue, 21.8 % sphalerite mineral grains occur as liberated with highly corroded surfaces. Therefore, the structural assembly of sphalerite grains in the leach residue no reason for reduce the leaching rate in the final stage of reaction. The XRD and qualitative and quantitative mineralogical analyses confirmed formation of elemental sulphur, precipitated at the particle surfaces. The leaching rate in the final stage of leaching is controlled of the lixiviant diffusion through the sulphur layer, which is formed as the reaction product during the leaching.

S. Aydogan, A. Aras, M. Canbazoglu, Dissolution kinetics of sphalerite in acidic ferric chloride leaching, Chemical Engineering Journal, 114 (2005) 67-72.

Aydogan, S., Ucar, G., Canbazoglu, M. (2006). Dissolution kinetics of chalcopyrite in acidic potassium dichromate solution, Hydrometallurgy, 81, 45-51.

Bredenhann, R., Van Vuuren, C. (1999). The leaching behaviour of a nickel concentrate in an oxidative sulphuric acid solution, Minerals Engineering, 12, 687-692.

Dreisinger, D. (2006). Copper leaching from primary sulfides: Options for biological and chemical extraction of copper, Hydrometallurgy, 83, 10-20.

Dutrizac, J.E., 2006. The dissolution of sphalerite in ferric sulfate media. Metallurgical and Materials Transactions B, 37B, 161-171.

Hackl, R., Dreisinger, D., Peters, E., King, J. (1995). Passivation of chalcopyrite during oxidative leaching in sulfate madia, Hydrometallurgy, 39, 25-48.

McDonald, R., Muir, D. (2007). Pressure oxidation leaching of chalcopyrite. Part I. Comparison of high and low temperature reaction kinetics and products, Hydrometallurgy, 86, 191-205.

Peng, P., Xie, H., Lu, L., 2005. Leaching of a sphalerite concentrate with H₂SO₄-HNO₃ solutions in the presence of C₂Cl₄. Hydrometallurgy, 80, 265-271.

Prasad, S., Pandey, B. (1998). Alternative processes for treatment of chalcopyrite – a review, Minerals Engineering, 11, 763-781.

Radosavljević, S., Stojanović, J., Kašić, V. (2006). Mineralogy of the Polymetallic Ore from Rudnik Mine. Proceedings of 20^th International Serbian Symposium on Mineral Processing, Sokobanja, Serbia, 8-12.

Santos, S.M.C., Machado, R.M., Correia, M.J.N., Reis, M.T.A., Ismael, M.R.C., Carvalho, J.M.R., 2010. Ferric sulphate/chloride leaching of zinc and minor elements from a sphalerite concentrate. Minerals Engineering, 23, 606-615

Sokić, M., Marković, B., Živković, D. (2009). Kinetics of chalcopyrite leaching by sodium nitrate in sulphuric acid, Hydrometallurgy, 95 (2009) 273-279.

Sokić, M., Marković, B., Matković, V., Živković, D., Štrbac, N., Stojanović, J. (2012). Kinetics and mechanism of sphalerite leaching by sodium nitrate in sulphuric acid solution, Journal of mining and metallurgy Section B: Metallurgy, 48 (2) B 185-195.

Sokić, M., S. Radosavljević, B. Marković, V. Matković, N. Štrbac, Ž. Kamberović, D. Živković, Influence of chalcopyrite structure on their leaching by sodium nitrate in sulphuric acid, Metallurgical and Materials Engineering, 20, 1 (2014) 53-60.

Tomanec, R., Vučinić, D., Radosavljević, S., Jovanić, P., Miljanović, F. (1997). The characterization of ores and minerals products with the help of imige analysis, Proceedings of 7^th Balkan Conference On Mineral Processing, Vatra Dornej, Romania, 382-387.