Chlorination

 

 The free energies of formation and melting and boiling points of the chlorides involved in the reaction are shown below,

 

Given equal concentrations or activities, the chloride with the highest negative free energy of formation would be expected to be removed first. In this way, assuming that the reactions are irreversible, the elements would be expected to react in the order zinc, lead, iron, silver, copper, at a temperature of 1,150 ^oC. Nevertheless, the elements are not present in equal concentrations and their vapor pressures appear to influence the order of removal. In this way, in practice iron is the first to be removed, followed closely by lead and zinc. Their chlorides are all gaseous at refining temperature, causing high turbulence and bubbling, and the chlorine injection rate is of necessity slow during this time. Iron almost certainly form ferric chloride, which boils at 315 ^oC, and not ferrous chloride, and for this reason displaced first.

After these elements are almost completely removed, copper and silver start to form chlorides. Being liquids at refining temperature, no turbulence occurs, and the chlorine injection rate can be increased. As the molten chlorides fill the crucible they are bailed off into a previously heated crucible for subsequent treatment for the recovery of silver and traces of gold. After bailing, a further quantity of flux is added and the chlorination proceeds. Probably, four or five bailing operations, depending on the quality of the bullion being refined are needed. The chlorine injection rate is again reduced towards the end of refining period, when red gold chloride fume can be detected above the melt, and considerable turbulence occurs. A dip sample must be taken from the melt, through the slag, 30 mm below the gold surface by means of a special crucible.  

Chlorination of silver and base metals

(initial concentrations 10.1%Ag, 1.35% Cu, 0.41%Pb, 0.13% Fe, 0.09% Zn)

 

Sampling Crucible