Thermodynamics of removing selenium and tellurium from liquid copper by sodium carbonate slags

Abstract

Even small amounts of selenium or tellurium in copper products impair their properties. Removal of these elements in conventional copper fire-refining by oxidation is difficult. In the softening process of crude lead, selenium and tellurium can be removed in reducing conditions by treating the metal bath with a mixture of NaOH and metallic sodium. An analogous process has been suggested for copper, where molten copper is treated with metallic sodium and/or with sodium-bearing slags in the presence of a reductant. 2'3'4 The thermodynamic background of these processes is still inadequately known, and thus the practical conditions for the optimum refining effect can not be established. Due to its good thermal stability, sodium carbonate has been found to be a suitable flux for removing antimony and arsenic from crude copper. 5"6 Therefore, research was directed to the thermodynamics of selenium and tellurium at low concentrations in pure sodium carbonate slags. The thermodynamic properties of selenium and tellurium species dissolved in liquid sodium carbonate were approached by their distribution equilibria measurements. The distribution equilibria were measured by a sampling technique in a vertical laboratory tube furnace at 1473 K, as a function of the oxygen partial pressure of the system in pure CO2 (g; 1 bar). 99.99+ pct pure HCOF grade copper (Outokumpu Oy) containing selenium and tellurium was equilibrated with pure sodium carbonate slag (E. Merck; anhydrous, extra pure) in an alumina crucible. ~ Oxygen activity of the system was controlled by Cu20 additions or by immersing a graphite rod into the crucible. It was measured in most runs by an oxygen concentration cell described in detail earlier. 6 In some cases, the oxygen activity was calculated from the analyzed oxygen concentrations of the copper alloy, 7 by using the thermodynamic data for the solution of oxygen in dilute copper-selenium and coppertellurium alloys. 8'9't~ The initial concentrations of selenium and tellurium in the alloy were 0.5 pct in three runs and 0.25 pct Se in one run. The slag-metal ratio was initially 60 g Na2CO3/150 g Cu. After equilibration, which took 20 to 30 minutes as a result of an intensive stirring of the system, samples were sucked with an alumina tube (i.d. 4 mm) separately from the slag and metal phases and quenched in flowing nitrogen gas. The selenium and tellurium concentrations of the phases were analyzed by using standard AAS methods. Previous calculations on the stability of the slag in copper fire-refining conditions s show that sodium carbonate is essentially stable under pure CO2(g) down to Po_, = 10 -~5 bar, where its sodium pressure reaches the value p.s, ~ 0.02 bar. The observed distribution coefficients, defined in general as