The kinetics of selenium removal from molten copper by powder injection

Chemical thermodynamic calculations show that selenium removal from copper melts using sodium carbonate (soda ash) is only effective under reducing conditions. Reducing conditions can be generated by carbon, but even more effectively by calcium carbide which has not been used previously for such a purpose. To clarify the kinetics of these multiphase, multicomponent reactions, various mixtures were either placed on top of or injected into 70 kg heats of molten copper. The following reagents were found to be effective in removing selenium: soda ash-graphite mixtures, calcium carbide, and calcium carbide-soda ash mixtures, in increasing order of effectiveness. Experiments were also performed with synthetic blister copper containing oxygen, selenium, tellurium, bismuth, nickel, silver, and lead. As expected from the thermodynamic analysis, only the first three of these elements were removed. A mathematical model was developed to describe the diffusion-controlled reaction kinetics of selenium and oxygen removal at calcium carbide particle interfaces. Very good agreement between the model and experiments was achieved for the reaction paths of selenium and oxygen when 35 pct of the particles were in contact with the melt. The utilization of powder varied over a wide range (0 to 10 pct), depending on the selenium and oxygen contents. The industrial implications of this work are discussed in terms of multielement removal, refractory erosion, temperature loss, and reagent utilization.