Coupled transport membranes : I. Copper separations

Abstract A new process for separating and concentrating metals is described. It is based on the principle of coupled transport. Metal ions such as copper can be “chemically pumped” across a membrane against large concentration gradients by allowing the counterfiow of a coupled ion such as hydrogen ion. The process is carried out within a microporous merebrane containing within its pores an organic, water-immiscible complexing agent. The complexing agent acts as a shuttle, picking up metal ions on one side of the membrane, carrying them across the membrane as a complex, and preserving electrical neutrality by carrying hydrogen ions in the opposite direction. Because the flows of metal and hydrogen ions are coupled, the metal ion can be “chemically pumped” from a dilute into a concentrated solution. The energy for the process derives from the flow of hydrogen ions in the opposite direction. The process could find application, for example, in the purification of dump leach liquors in the hydrometallurgical processing of low grade ores Coupled transport of cupric ion was demonstrated with a Celgard 2400/LIX 64N membrane. Under favorable conditions, copper was concentrated against a 4000-fold concentration difference, and copper was separated from iron with a separation factor greater than 1000. A pH difference of one to two units is sufficient to drive the copper through the membrane at a high rate and separate it from iron. For a given pH gradient across the membrane, the flow of metal ion initially varies linearly with the metal ion concentration gradient, but at high metal ion gradients the flow reaches a limiting value. This appears to be the result of the limited solubility of the metal complex in the organic phase. Typical transport rates are on the order of 3 μg/cm 2 -min or 3 lb/ft 2 -year. A short economic assessment based on this flux indicates that processing with coupled transport membranes should be competitive with the best current technology