Datos estructurales

Selenium concentration in the earth’s crust is similar to antimony and cadmium which are approximately 0.08 mg each per kilogram of ore. Selenium is found in most sulphide minerals largely due to similarities with sulphur such as ionic radii (Harold et al., 1965). Table 3.1 illustrates the similarities between selenium and sulphur. Sulphide (S2-_{) ion has a radius of 184 pm while a selenide has a radius of 198 pm} (Harold et al., 1965). Though in small quantities, selenium occurs in different minerals i.e., galena (PbS), chalcopyrite (CuFeS2) arsenopyrite (FeAsS), sphalerite (Zn,Fe)S, pyrrhotite Fe(1-x)S [where x = 0 to 0.2] and pyrite FeS2.

Other minerals with naturally occurring selenium are eucairite (CuAgSe), clausthalite (PbSe), crookesite (Cu7(Tl,Ag)Se4), and berzelianite (Cu2Se). However, these minerals contain low quantities of selenium and are too rare to be used for selenium production. Thus, the study of selenium chemistry of these minerals has received little attention.

Table 3.1: Similarities between selenium and sulphur

Sulphur Selenium Tetrahedral covalent radius 1.04Å 1.14Å Ionic radius (divalent)2 _{1.81Å 1.98Å} Electronegativity 2.5 2.4 (Harold et al., 1965)

Kilic et al. (2013) conducted investigations of copper and selenium recovery from copper anode slimes. The process consisted of two hydrometallurgical steps; decopperisation in sulphuric acid media and dissolution of selenium in a caustic solution. Temperature, caustic concentration, oxygen partial pressure and reaction time were all found to increase selenium dissolution. The caustic concentration was varied between 0.36 and 5 mol/L. Maximum selenium extraction was obtained with 4 mol/L caustic concentration. The maximum temperature of 90°C used in the

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investigation yielded the maximum selenium dissolution. The following reactions were proposed for the dissolution of selenium producing sodium selenite and sodium selenate respectively (Kilic et al., 2013):

 s O2 g NaOH aq Na2SeO3 aq H2O l

Se    

 s 3O2 g 4NaOH aq 2Na2SeO4 aq 2H2O l Se

2    

Figure 3.2 and Figure 3.3 illustrate the speciation of selenate and selenite as a function of pH (Scott, 1991). The fully deprotonated selenate is formed at a pH of about 2-14. On the other hand, selenite is only formed at a pH of about 8. The redox potential determines which species will predominate over the other (Baur and Johnson 2003). Cornelis et al. (2008) obtained 79% selenate and 21% selenite during geochemical modelling of selenium leaching in alkaline media at a pH value of 11. Baur and Johnson (2003) also showed that the thermodynamic stability of the selenate increases with pH, as can also be seen from the Eh-pH diagram (Figure 2.2). Other researchers have also shown that an increase in pH results in increased selenium dissolution (Zheng and Chen, 2013; Otero-Rey et al., 2005). Zheng and Chen (2013) and Otero-Rey et al. (2005) obtained results that showed a relative influence of temperature on selenium and arsenic dissolution. The selenium leaching extent also improved with an increase in reaction time.

Figure 3.2: Equilibrium diagram of selenate as a function of pH at 25°C (Scott, 1991) 0 0.2 0.4 0.6 0.8 1 0 2 4 6 8 10 12 14 M o lar f ract ion pH SeO4 2- HSeO4 -

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Figure 3.3: Equilibrium diagram of selenite as a function of pH at 25°C (Scott, 1991)

3.4.1 Kinetics of selenium oxidation

Kinetic studies on the oxidation of selenium and selenium bearing minerals in acidic, neutral and alkaline conditions have been conducted by a number of researchers. Kilic et al. (2013) conducted kinetic studies on the dissolution of selenium from copper anode slimes. The experimental data fitted the ash diffusion model. However, 62 kJ/mol activation energy was obtained which is typical of a chemical controlled reaction. No further discussion was made on this effect. Zheng and Chen (2013) conducted a kinetic study on the leaching of selenium-tellurium rich material in 1 to 2.5 mol/L sodium sulphite solution and 25-95°C. It was reported that the dissolution reaction was controlled by diffusion through a boundary layer with an activation energy of about 20 kJ/mol. Leaching kinetics of selenium from nickel-molybdenum smelter dust using sodium chlorate, in a mixture of hydrochloric and sulphuric acid was studied by Hou et al. (2010). 98% leaching efficiency of selenium was obtained at 95°C, initial H+ _{concentration of 8 mol/L and 150 rev/min in 150 minutes. The apparent} activation energy for the dissolution of selenium was 44.4 kJ/mol and is consistent with surface chemical reaction control.

0 0.2 0.4 0.6 0.8 1 0 2 4 6 8 10 12 14 16 M o lar f ract ion pH H2SeO3- HSeO3 - _SeO32-

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3.5 Other studies on alkaline oxidative leaching of sulphur, selenium and

arsenic bearing materials

Hoffmann (1984) studied the caustic leaching of selenium from copper anode slimes. He stipulated a leaching temperature of approximately 200°C and caustic concentration in the range 2.5 to 12.5 mol/L. The concentration of the caustic solution was based on the concentration of the selenium in the slimes and the solids content of the slimes in the leach slurry. Due to the importance of oxygen in the dissolution rate of selenium, the oxygen partial pressures were varied from 200 to 2000 kPa. The reaction time was varied between 4 to 7 hours. Under these operating conditions, the conversion of selenium to the hexavalent form is represented by Reaction 3.14:

 s ₂3O2 g 2NaOH aq Na2SeO4 aq H2O l

Se     3.14

All the metal sulphides present in the slimes were converted to sodium sulphate and their respective oxides, hydroxides or sodium salts. Although no attempt was made to determine the various factors affecting the leaching rate or the various factors that might affect the downstream operations e.g. the precious metal recoveries, the alkaline leaching process was rated as appropriate.

A caustic leaching process is described in a European patent (Thomas et al., 1986) for the leaching of selenium and platinum selenide (PtSe) from a product of sulphur dioxide reduction process of selenium and PGMs. Selenium was dissolved and tellurium remained in the residue. In this process, the filter cake from the sulphur dioxide reduction step was slurried in a caustic solution between 100 and 250 g/L solids. The amount of caustic used for the leach was 1 mol/L in excess of the stoichiometric amount with respect to selenium. A caustic pressure leach was conducted at 180 to 220°C and a total gauge pressure of 1725 to 2410 kPa (gauge). The oxygen partial pressure was maintained between 340 to 690 kPa. Sufficient oxygen was provided to oxidise selenium and tellurium to the hexavalent state. The maximum selenium extraction was yielded at the optimal operating conditions of 200°C, oxygen partial pressure of 690 kPa and an excess of 1 mol/L.

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Steenekamp and Dunn (1999) showed that tellurium and selenium can be selectively leached from a copper selenide/telluride precipitate, leaving the PGMs as part of the residue. The treatment consisted of two leaching stages; first to leach tellurium, followed by the leaching of selenium. The approximate leach conditions for the first stage were as follows:

Temperature 80°C Oxygen partial pressure 65 kPa

Time 15-20 minutes NaOH 0.5 mol/L

Tellurium was recovered in solution as sodium tellurate according to the stoichiometry of the following reaction:

 s 2O2 g 2NaOH aq Na2TeO4 aq 2CuO s H2O l

CuTe      3.15

It was also noted that increased caustic concentration and residence time resulted in an insoluble sodium tellurate (Na2TeO4). The residue was then subjected to another stage of caustic leaching, although the leaching conditions were not stated. The extent of the selenium, sulphur and arsenic leaching was 90%, 90% and 75%, respectively. Rappas and Waterman (1990) utilised alkali solutions to leach smelter flue dust. This resulted in the separation of metals in a water-insoluble or solid group, containing bismuth and several oxides, and a water-soluble or liquid group containing soluble oxyanions, including arsenic. Additionally, a large fraction of the sulphur was removed as sodium sulphate. This selective caustic leaching of flue dust yielded a solid residue containing most of the valuable metals, i.e., copper, gold and silver, which was suitable for recycling back to the smelter, and a caustic solution containing soluble sodium salts of the oxy-anion forming elements. Concentrated caustic solution of approximately 2 mol/L, leaching temperature of 50° to 100°C and a residence time of one hour were utilised. It was found that in order to achieve good leaching of sulphur and arsenic in the flue dust, without dissolving the valuable elements, the caustic mass ratio had to be kept between 0.3 and 1.4 g NaOH per gram flue dust. This

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conclusion is supported by Subramanian et al. (1980) who found the optimum dosage of caustic to be 0.5 to1.07 g NaOH per g flue dust.

According to a study conducted by Saydut et al (2011) on the chemical leaching of sulphur and mineral matter from asphaltite (a mixture of hydrocarbons), a total of about 83% dissolution of sulphur was achieved after 16 hours in 1 mol/L caustic solution at 180°C. Bhakta et al. (1989) investigated the kinetics of alkaline oxidative leaching of gold arsenopyrite ores. The test conditions of the study were 100°C, 2 mol/L NaOH, oxygen pressure of 276 kPa, 10% solids loading and 4 hours residence time. Under these conditions 90% arsenic extraction was achieved.

The following reactions have been proposed (Subramanian et al., 1979) for elemental selenium, tellurium, sulphur and arsenic oxidation, respectively.

 s 3O2 g 4NaOH aq 2Na2SeO4 aq 2H2O l Se 2     3.16  s 3O2 g 4NaOH aq 2Na2TeO4 aq 2H2O l Te 2     3.17  s 3O2 g 4NaOH aq 2Na2SO4 aq 2H2O l S 2     3.18  s 5O2 g 12NaOH aq 4Na3AsO4 aq 6H2O l As 4     3.19

3.6 Platinum group metals chemistry