Rhenium was the last naturally occurring element to be discovered and the last element discovered having a stable isotope. Rhenium is typically recovered as a byproduct of molybdenum refinement. Since recovery of rhenium from molybdenite is difficult and the concentrations of rhenium in molybdenite are very low, typically from about 0.002% to about 0.02%, rhenium is one of the most expensive metals available in commodity markets. Rhenium has several characteristics that make it unique, such as, for example, the second highest melting point amongst metals, amongst the densest metals, a super conductor, and the greatest number of oxidation states of any element. Industrial applications include the use of rhenium in catalysts, electronics, thermocouples, high temperature turbine blades, and photoflash devices.
Rhenium may be extracted from ores that contain copper and molybdenum. Common practice for leaching copper from low-grade copper ore is to place the ore in a heap leach pad and leach the ore with dilute sulfuric acid solution. The resulting copper-bearing solution is typically concentrated via solvent extraction and/or electrowon to produce pure copper cathode. Typically, the copper-bearing solution has less than one part per million of dissolved rhenium and may contain significant amounts of other metals in the copper-bearing solution. Recovery of rhenium from the copper-bearing solution is not economically feasible and hence rhenium is, along with other metal values, typically not recovered from the copper-bearing solution before the electrowinning stage.
Generally, rhenium is recovered as a result of the molybdenite roasting to produce molybdenum. The acid blow-down from the molybdenite roasting off-gas contains concentrations of rhenium which are much higher than the concentrations of rhenium in the copper-bearing solution. In addition, the acid blow-down stream does not contain the metal values such as copper or molybdenum since they have already been recovered upstream, and this allows rhenium to be recovered from the acid blow-down stream by ion exchange, solvent extraction and/or crystallization.
Since the demand for rhenium continues to increase on a year-by-year basis, new methods for rhenium recovery from sources other than molybdenum roasting processes are needed.