DEVELOPMENT OF FLUXES FOR REFINING OF COPPER

Abstract

Due to depletion of high grade copper ores, metallurgists are forced to use lower grade copper ores. This results in higher proportions of impurities being encountered during copper processing. Impurities such as As, Sb, Se, Te, Bi, Ni etc, besides getting carried over to copper refining stage and thereby affecting the process efficiency,are also occluded in copper produced thereby affecting its electrical and mechanical properties. The literature survey on copper refining shows that elements As, Sb, Di, Ni, Se, Te, etc can be effectively removed from metal using soda bearing slags in a slag-metal reaction process. The use of sodium carbonate as a flux for removing the impurities has been significant recently. But use of sodium carbonate in pure form has posed many problems on industrial scale. A critical study of the published work has indicated that no systematic studies have been conducted on thermodynamic and mass transport aspects of soda-base slags and copper metal systems. The present work has, therefore,' been undertaken to establish the practical operating parameters for refining of copper using soda-base fluxes. The investigation has been confined to the removal of only Se and Te impurities from iv copper using sodium- borate slags. The entire work has been reported in five chapters. Chapter I has been devoted to the brief introduction on the problem and critical review of literature comprising thermodynamic and kinetic studies including slag viscosity measurements. Finally formulation of the problem has been taken up. Chapter II deals with the thermodynamics of copper refining by flux and includes the theoretical analysis experimental set-up and procedure used for the determination of selenium and tellurium equilibrium distribution • coefficients for various Cu-Se and Cu-Te alloys, using sodium-borate slags of different compositions at 1473K . Effect of Ca0 and CaF2 additions to sodium-borate slags on Se and Te distribution coefficients has also been studied. This chapter also includes the results obtained and their interpretations. It has been found that, i) with increase in . slag basicity (i.e. wt'/. Na20/wt% 8203 ratio), equilibrium distribution coefficient for both Se and Te increases, ii) addition of 50 wt'/. Ca0 to sodium -borate slag (wt'/ Na20/wt% B203 = 0.86) increases the distribution coefficient values by about 2 times for both Se and Te elements, iii) with addition of 20 wt% CaF2 to the slag (30.9 wt% Na20, 35.8 wt% B203,33.3 wt'/ Ca0), the distribution coefficient further increases by around 1.8 times for Se and 2.2 times for Te. Chapter III has been devoted to the study of physico-chemical property i.e viscosity of various slag systems developed. It includes the theoretical aspects, experimental set- up and procedure used for measuring the viscosity of slag as a function of temperature and slag composition. It has been found from the results and their interpretations that, i) with increase in temperature, the viscosity decreases for all slag compositions, ii) with increase in Na20 content (34.5wt'/.to 46.4wt%) in sodium-borate slag compositions developed for the present work, the viscosity decreases from 0.31 Pa-s to 0.29 Pa-s at 1473K, iii) with addition of Ca0 (10 wt% to 50 wt'/.) to sodium-borate slag (wt'/. Na20 /wt'/ B203 = 0.86), its viscosity increases from 0.76 Pa-s to 1.12 Pa-s at 1473K and iv) with addition of CaF2 (5wt/. to 20wt%) to slag composition (30.9 wt% Na20, 35.8 wt./43203,33.3 wt'/.CaO), the viscosity decreases from 0.44 Pa-s to 0.38 Pa - s at 1473K. Chapter IV deals with the kinetics of copper refining by soda-base flux and includes the theoretical aspects, experimental set-up and procedure used to study the Se and Te transfer as a function of gas flow rate in N2 gas bubble agitated slag metal system at 1473K for CU-Se and Cu-Te alloys using slag composition (25.7wt% Na20,29.8wt% B203, 27.8wt%Ca0,16.7wt%CaP2). Results obtained and their interpretations are also included in this chapter. It has vi been observed that with increase in gas flow rate (5.0 to 17.0 cm3/sec), the overall mass transfer coefficient increases from 2.04x103 to 3.06x10-3 cm/sec for Se while for Te it increases from 3.41x10-3 cm/sec to 5.02x10-3 cm/sec. Chapter V includes conclusions while scope for future work is indicated separately. vii