Separation of gold from platinum metals on cation exchangers in concentrated hydrobromic acid solutions

Abstract

Separation of gold from platinum metals in the Dowex 50W × 8 [H⁺]-hydrobromic acid plus bromine system was investigated. Distribution coefficients of gold (in trace amounts) increase almost linearly with hydrobromic acid concentrations from 0·5 to 5 N, reach nearly constant value at 5 and 6 to 7 N and decrease slightly with further increase in hydrobromic acid concentration. Distribution coefficients decrease with loading and with temperature. Apparent enthalpy change for the distribution reaction of gold between resin and solution is about 3 kcal. mole^–1. The absorption capacity of Dowex 50W × 8 [H⁺] for gold (in 6 N hydrobromic acid), as determined from break-through graphs, increases considerably with the increase in gold concentration in the solution. Separation of both carrier-free and milligram amounts of platinum and gold can be achieved with 6 N hydrobromic acid plus 0·0035 M bromine solution at 15° C. Platinum is eluted in the solvent front and the gold, retained by the resin, is rapidly stripped off with acetyl-acetone. Statistical analysis of the results of the separations by use of tracers has proved that no systematic error is involved. Iridium, palladium and rhodium behave in a similar manner to platinum and can be separated from gold with this system. A possible mechanism of the absorption of gold by cation-exchange resin, i.e., the partition of undissociated bromoauric acid with the formation of a molecular (charge transfer) complex between the aromatic rings of resin network and complex acid, is suggested.