Kinetics of sulfation of chalcopyrite with steam and oxygen in the presence of ferric oxide
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INTRODUCTION
THE roasting of chalcopyrite with oxygen is generally employed to produce copper sulfate. I~ The kinetics and mechanism of oxidation of chalcopyrite with air/oxygen have been widely studied. Agarwal and Gupta ~zl studied the oxidation kinetics of pelletized agglomerates between 773 and 973 K and showed the applicability of the shrinking core model with an activation energy of 50 kJ/mol. The nucleation and growth kinetics was reported by Bumazhnov and Lenchev, [31 who showed three different activation energies in three temperature ranges. Between 673 and 689 K, the activation energy of 176 kJ/mol was acquired, between 789 to 973 K, it was 113 kJ/mol, and above 973 K, the value was observed to be 42 kJ/mol. Recently, Chaubal and Sohn t41 studied the intrinsic kinetics of oxidation of chalcopyrite particles for a temperature up to 1150 K. They observed that the reaction proceeded via the pore blocking model. The activation energy was found to be 71 kJ/tool in the temperature range 754 to 873 K and 215 kJ/mol below 754 K. For a commercial roaster, LeungE~ suggested oxidation of chalcopyrite by either direct oxidation of chalcopyrite to Fe304 or by oxidation of pyrrhotite obtained from the decomposition of chalcopyrite. Razouk et al. I61 reported that below 673 K, covellite was first formed which was then oxidized to oxide and sulfate. The formation of copper sulfate was primarily due to the interaction of cupric oxide (CuO) and ferric sulfate. The sulfate thus formed was reported to decompose above 873 K, and ferrites formed above 1173 K. Wadsworth et at. [71 noticed the formation of copper sulfate from the cuprous sulfide by weight gain with oxygen as the oxidant obeying the parabolic rate law and calculated an activation energy of 27.21 kJ/mol. Apart from
oxidation of chalcopyrite with oxygen, Roberts and WilliamsC~l studied the reaction kinetics with steam, resulting in the formation of cuprous oxide and elemental sulfur as the product. The oxidation reaction followed first-order kinetics with respect to steam concentration and with a corresponding activation energy of 42.27 kJ/mol. It is reported that sulfide ore weathered slowly above the ground to the metal sulfate.t9] Hence, the weathering phenomenon can be considered to take place using steam and oxygen at high temperature, thereby creating aggressive conditions to favor copper sulfate formation. In view of this, an attempt was recently made for the direct sulfation of chalcopyrite with a steam-oxygen mixture on the bench scale in our laboratoryY ~ An optimum recovery of 82.32 pct could be achieved in the presence of 10 wt pct Fe203 at 773 K. In continuation of this work, it was considered worthwhile to study the kinetics and mechanism of sulfation of chalcopyrite with steam and oxygen in the presence of ferric oxide. The results of these studies are reported and discussed in this work.
II.
THERMODYNAMIC CONSIDERATIONS
Figure 1 gives the predominance area diagram of the CuFe-S-O system which indicates the stability of different phases includin
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