Neutralization and cation dissolution characteristics of bauxite refining residue

Abstract

The neutralization reaction of red mud with various concentrations of HCI acid and the dissolution of cations were studied in order to evaluate the mechanism of Na release from red mud. The changes in the mineralogy of the desilication products (DSP) was simultaneously followed by XRD and SEM. From the neutralization curve, four buffer regions were identified: pH 10.0-8.3 resulting from the neutralization of carbonate and free hydroxyl; pH 7.8-6.2 due to the dissolution of carbonates of Ca and Mg and accompanied by the release of Na from the desilication products; pH 35-3.2 resulting from the dissolution of the aluminum hydroxide precipitated from the decomposition of DSP taking place between pH about 7.0 and 5.4; and pH 0.8-0.4 resulting from a further dissolution of aluminum and iron oxide. The mineralogical study of the DSP particles of red mud revealed it to be a zeolitic mineral akin to natrodavyne consisting of crystalline rods and blobs forming a spherical ball-like network. The rods curved and wrapped on top of each other in a meridian orientation producing what appears to be poles from which rods radiated. The X-ray diffraction analysis only showed the major peaks of natrodavyne at pH 6.2. Scanning electron micrographs on the DSP particles at pH 6.2 showed no observable change in the external morphology of DSP with reduction in pH. In addition, the maximum release of Na was achieved at pH 5.4, thus Al and Si must have been released at a pH higher than 5.4 but immediately re-precipitated and therefore not observed in solution. It can be concluded that Na in the DSP particles were released at pH 7.0- 5.4 by acid hydrolysis. Hence, neutralization of red mud to a pH above 5.4 would produce a maximum release of Na without the possibility of excess dissolution of Al, although in practice we would not necessarily reduce the pH to as low as 5.4