PRECIPITATION AND CALCINATION OF MONOHYDRATE ALUMINA FROM THE BAYER PROCESS LIQUORS I. Paspaliaris*, D. Panias, A. Amanatidis, Jacques Mordini, D. Werner, G. Panou, D. Ballas *Author for correspondence. Lab. of Metallurgy, National Technical University of Athens, GR-157 10 Zografos, P.O. Box 64056, Greece ABSTRACT The Bayer process is universally applied for the production of alumina from bauxites. The process has three main steps: Digestion, where bauxite is digested with caustic soda at ele- vated temperature and pressure to produce aluminate liquor; precipitation, where the alumi- nate liquor is hydrolysed at 55-60°C to produce crystalline alumina trihydrate, (gibbsite, Ah03.3H20) and calcination, when gibbsite is calcined at 900-1200°C to produce anhydrous alumina (Ah03). The present project aims at the development of a highly innovative variation of the Bayer process whereby, at the precipitation step, alumina monohydrate (boehmite, AI203.H20) rather than trihydrate will be precipitated at atmospheric conditions and subse- quently calcined to produce anhydrous alumina. Boehmite can be produced economically and in substantial quantities under conditions and with equipment similar to these used for gibbs- ite precipitation in the current practice of the Bayer process, resulting in significant energy savings during the calcination stage. The amount of energy saving is estimated to be 1.8 GJlt Ah03. This amount represents 60% reduction over the current calcination practice or 15% total energy saving in the Bayer process. 1. INTRODUCTION Bayer process is used almost exclusively for the extraction of alumina from bauxites. Ap- proximately 90% of anhydrous alumina produced by the Bayer process is used for the produc- tion of primary aluminium[l] (metallurgical grade alumina). The remainder (non-metallurgical grade alumina) is used for the production of refractories, abrasives, ceramics, cement, white- ware, aluminium chemicals, flame retardants, detergent zeolites, adsorbents and fillers. The essential steps of the Bayer process are: I. Digestion of bauxite by using sodium hydroxide at elevated temperatures (220-260°C) in autoclaves. 2. Separation of insoluble impurities (red mud) from the pregnant solution. 3. Precipitation of aluminum hydroxide (gibbsite, AI203.3H20) from the pregnant liquor. Precipitation is effected by cooling the solution to 58°C, thus causing supersaturation, and by introducing small gibbsite particles as seeds. The crystalline gibbsite precipitate is separated into a fine and a coarse fraction; the fine fraction, representing approximately 300% by weight of the coarse, is recycled to the precipitators as a seed to promote crystal-