(8e) Characterization of Bauxite-Residue Classified by Wet Cyclone System

The Bayer process used for alumina refining produces huge amount of bauxite residue (BR) as a solid residue. Generally, the BR consists of hematite, anatase, goethite, boehmite, quartz and sodalite type desilication product (SOD). The BR also contains chlorine at a non-trivial level. The chlorine is an obstacle to potential applications such as auxiliary raw materials in cement industry. The problematic chlorine content in the BR of 0.1-0.2wt% was found to be present. The BR samples were classified using a wet cyclone system, the cyclosizer. And then, the reduction of chlorine by the hydraulic classification was investigated. The chlorine mainly existed in the SOD particles smaller than 15 mm in diameter and the SOD particles had recrystallized during the desilication stage of the Bayer process. This implies that the reduction of the chlorine content in the BR may be achievable by the separation of the fine SOD particles from the BR. By the hydraulic classification using the cyclosizer, the recovery of the BR with the chlorine content <0.02% was 16.1%. This product was rarely contained the SOD particles. The classified particles were individually characterized by image analysis using a Computer Controlled Scanning Electron Microscopy (CC-SEM). The analysis using a ternary diagram of Na-Al-Si system showed that the finest fraction contained a little of Na but the other fractions did not contain it. The Na contents agreed with the chlorine contents in the BR because Na was mainly contained in the SOD. This Na in the SOD was also responsible for chlorine in the BR. Thus, it is clarified that the chlorine contents in the BR can be estimated by the element analysis using CC-SEM.