Effect of bauxite properties on the settling of red mud

Abstract

Abstract Most bauxite deposits are fossil soils in which gibbsite or boehmite are the ore minerals. In the alumina production process with sodium hydroxide (Bayer process), the ore minerals go into solution and the iron-oxide impurities remain insoluble (red mud tailings). The settling behaviour of the tailings is an important economic factor as it controls purity, and an important environmental factor as it controls final liquid content of the red mud tailings for disposal. In the Bayer process, the operating factors determining different settling rates are not at all clear. It is known that two bauxites from a few metres apart may produce muds with different settling rates. Thus, the mineralogical and chemical compositions of the bauxite would seem to be important factors in determining mud settlement patterns. In this work, a total of 7 bauxites and 7 red muds from Southern Brazil are examined for their mineralogical, chemical and physical properties by employing X-ray diffraction (XRD), X-ray fluorescence (XRF), dissolution technique (dithionite-citrate), inductively coupled plasma (ICP) and, Brunauer, Emmett and Teller (B.E.T.) specific surface area, for comparison with the measured settling behaviour of the muds. The results of this work reveal the differences in settling rates, and mineralogical, physical and chemical properties which exist among the studied red muds. Since there was no technological variation in the Bayer process used for the bauxites, these noticeable differences in settling behaviours of their derived red muds are due mainly to differences in their source bauxites. The mineralogical composition of the bauxite is one of the most important factors that significantly influences the red mud properties. Muds containing hematite, settle faster than those which contain goethite as the main iron mineral. Specific surface area and specific gravity determinations of the red mud indicate that the content of cryptocrystalline iron oxides and nature of their mineralogical composition may be used as a guide to settling rate. Amorphous and cryptocrystalline iron mineral is another significant factor in affecting the settling of the studied Brazilian red muds. This amorphous and cryptocrystalline iron (Fed) possibly acts as a bonding agent for the red mud particles and the synthetic flocculant.