Reduction of copper with cellulose in an autoclave; An alternative to electrolysis?

Copper is widely used in electricity wire and electronic devices, which makes it one of the most important base metals. Hydrometallurgical processing routes are typical for ores with low copper content. The low-grade copper ores are stacked in large heaps and sprinkled with dilute sulfuric acid; this is called heap leaching. The collected leach liquor is concentrated and purified in a solvent extraction step, and subsequently processed by electrowinning. Copper plates out and the generated acid is recycled to the heap leach. This paper deals with an alternative for the electrolytic processing step by autoclaving a mixture of cellulose and copper sulfate solution. In the laboratory a 600 ml. Parr autoclave was used. First the exact ratio of cellulose and copper ions was determined. Cellulose was suspended in a copper sulfate solution and heated in an autoclave till 250 degrees C. After a retention time of 60 minutes at this temperature the pressure vessel was quenched with water. The liquid was analyzed by ICP and the solids by ICP and XRD. The copper ions were precipitated as elemental copper with a particle size around 80 mu m A solid byproduct was an easily separable, black organic phase, probably carbon. The generated dilute sulfuric acid contained dissolved organic carbon. The low pH of this acid restricts the choice of construction materials for the autoclave. Kinetic results indicate a retention time of 30 minutes, after reaching 250 degrees C. The process is selective for copper with respect to nickel and iron, but arsenic partly precipitates with the copper. This technology might have its potential as an alternative for electrowinning or can be used for small waste streams containing copper in relatively high concentrations. (C) 1999 Published by Elsevier Science Ltd All rights reserved.