Solvothermal synthesis of high value copper powder from Copper bleed solution

Abstract

More restrictive pollution abatement regulations, feedstock shortages, rising energy costs, inflationary pressures and the need of high value products at lower costs are forcing industries to recycle wastes and processes secondaries. Copper bleed solution (CBS) is one such rich source of copper and nickel, which is generated during the electrorefining of copper anode wherein various impurities present in anode, dissolve and/or settle as sludge in the cell. Generally, the composition of CBS is Cu: 35-45g/L, Ni: 20g/L, Fe: 2g/L, Bi : 100-150ppm, As :150ppm – 10g/L, Sb : 300-400 ppm, H2SO4 : 170-200g/L. A typical copper bleed solution obtained from ICC, HCL, Ghatsila, was treated to recover copper and nickel as value added products such as metal powders. Here we report a bench scale investigation on the preparation of high purity copper powder from copper bleed solution by solvent extraction separation of copper followed by H2 precipitation stripping of loaded organic. From a purified (iron and acid free) copper bleed solution containing 38.76 g/L Cu and 0.54 g/L Ni, copper was separated by solvent extraction using Versatic 10 acid as an extractant in kerosene. At O:A phase ratio of 1.5 and pH 4.25 copper was completely extracted from the copper bleed solution in five counter current extraction stages. From the loaded organic containing 30.5 g/L Cu, preparation of high purity copper powder by direct H2 precipitation stripping in an autoclave was studied. Effect of various parameters viz. H¬2 pressure, rpm and temperature was investigated to optimize the process. The recovery of copper powder increased with the increase in H2 pressure at a constant temperature and stirring speed. Kinetics of copper powder recovery also imoroved with increase in H2 pressure. At 20 bar pressure 80% Cu powder was recovered in 15 min, whereas at 15 bar only 29% copper powder was recovered. Kinetics and recovery were improved with the increase in reaction temperature and rpm. At 20 bar H2 pressure, 403 K and 400 rpm, a maximum of ~95 % Cu powder was recovered from loaded Versatic 10 containing 30.5 g/L Cu. Kinetic study of the process shows that synthesis of copper powder follow nucleation-growth model as well as chemical control kinetic model. The activation energy values for solvothermal stripping of copper for the nucleation and growth model (50.2 kJ/mol) and also for the chemical reaction control (30.4 kJ/mol) were found to be within the range reported for such models, although the difference in these values are nearly 20 kJ/mol. This clearly indicates that the stripping of loaded copper from the solvent by hydrogen may alternatively be represented by chemical controlled shrinking core kinetic model very well and not only by the nucleation and growth model as reported previously. The purity of annealed copper powder was found to be 99.9%. Properties such as apparent density, flow rate, green density, hydrogen loss and acid insoluble matter etc. were also evaluated and were found to be suitable for P/M applications.