A mössbauer study of the iron mineralogy of acid-leached bauxite

Abstract

The removal of iron from bauxite via some form of leaching step is particularly desirable because of the greatly enhanced value of low-iron bauxite in the refractory, abrasives and chemical industries, Previous investigations have shown that the iron minerals in Weipa bauxite are selectively dissolved under certain conditions, but that the extent of iron removal is limited unless total extraction of other mineral components takes place. In an effort to understand this behaviour, Mössbauer effect measurements have been made on samples of Weipa bauxite both before and after leaching with hydrochloric acid. Because of the finely divided state of the goethite present it has been necessary to take measurements at both 300 K and 4.2 K in order to fully characterize the iron mineralogy of the bauxites. Prior to leaching, the majority of the iron is present as hematite, which may be aluminium substituted by up to about 10%. Smaller quantities of aluminous goethite (usually about 15% substituted) and iron present within the structure of kaolin associated with the bauxite have also been found. About 95% of the hematite is removed during leaching, together with some of the gibbsite and boehmite present. A significant precipitation of goethite, containing a variable amount of aluminium, takes place during the leaching. From the limited number of samples investigated it appears that at higher acid concentrations the degree of alumiuium substitution of the precipitated goethite may decrease. Other studies of the forms of iron precipitated in aciiic chloride media have shown β-FeOOH (akaganéite) to be the principal product. It is believed that the finely divided goethite initially present in the bauxite has nucleated the precipitation thus leading to the formation of goethite rather than akaganéite.