Salt fluxes for alkali and alkaline earth element removal from molten aluminum

Tightening regulations for emissions of HCl, Cl-2, and particulate matter have been a driving force for aluminum ingot plants to reduce their use of chlorine for refining metal. Solid chloride salts containing MgCl2 can be used to remove alkali and alkaline earth elements to ppm or lower levels, and in many instances can replace chlorine for this application. MgCl2 is very hygroscopic and even in the driest climate needs to be handled properly to be used safely in an ingot plant. Tests showed that fusing MgCl2 with KCI and other chloride components does not eliminate moisture pick-up. Rates of Na and Ca removal by salts containing MgCl2 were measured during melting and holding processes in a 4.5 metric ton reverberatory furnace. When present in the furnace during melting, salts enhance trace element removal. Removal of these elements during holding with salt was a slow process. Agitation of the metal through truck stirring increased the removal rates by a factor of 30 or more, and reaction rates were somewhat higher than those achieved by fluxing with an Ar-Cl-2 mixed gas. Injection of the salt with a lance was not as effective as mechanical stirring. Using fused salts gave no benefit in trace element removal as compared to blended salt mixtures.