AA TRBR:2000 Edition

For decades, the aluminum industry has been investigating options for treating, disposing, and using bauxite residue (or red mud), a byproduct of the Bayer process to extract aluminum oxide from bauxite ore. This report examines the issues surrounding bauxite residue, discusses different approaches for treatment/disposal/utilization, and identifies research and development needs for some of the most promising options.
Alumina Production and Bauxite Residue
The starting material for electrolytic smelting of aluminum is pure, anhydrous aluminum oxide (Al2O3), also known as alumina. The principal ore from which aluminum is extracted is called bauxite. The principal ingredients contained in the bauxite are hydrated aluminum oxide, iron oxide (which gives it its reddish-brown color), silicates (e.g., clay, quartz), and titanium dioxide.
The Bayer process is used to produce alumina from bauxite. Because of variability in ores, each alumina plant is almost tailored to suit a particular bauxite. In the Bayer process, the bauxite is crushed and ground, then mixed with a solution of caustic soda and pumped into large autoclaves. There, under pressure and at a temperature of 110°C to 270°C, the alumina contained in the ore is dissolved to form sodium aluminate. The silica in the bauxite reacts and precipitates from solution as sodium-aluminum-silicate.
The remaining residues consist partly of minerals that do not dissolve during the caustic treatment of the bauxite (e.g., iron and titanium oxide), calcium carbonate and calcium aluminates from lime addition, and also of what is known as the dedication product (DSP), which contains not only silica but considerable quantities of unrecoverable alumina and soda. Some bauxite residues contain as much as 24% TiO2, 43% Al2O3, and 54% Fe2O3, depending on the mineralogical make-up of the bauxite. The residue also contains trace amounts of the metals niobium, gallium, zirconium, thorium, scandium, and vanadium. The residue settles out of solution and is separated from the sodium aluminate solution, washed to recover the caustic soda, and pumped to disposal areas.
The worldwide alumina industry produces over 70 million dry metric tons of bauxite residue annually. Australia is the largest alumina refiner in the world, processing around 30% of the total alumina (see text box next page).
The production of one ton of aluminum metal yields 1.5 to 4 tons of bauxite residue (on a dry basis) to be disposed. The worldwide average of residue in disposal sites is 50% solids in the residue.
Overview of Bauxite Residue Treatment
Bauxite residue is removed during the Bayer process by gravity settling or filtering the solids from the sodium aluminate liquor. Storage methods, which involve the dewatering of the residue, are tailored to suit local situations and legislation. Methods that do not preclude the future exploitation of the residue can be considered under the headings stacking and nonstacking.
Stacking calls for residue that is dense enough to require (if it is to be conveyed) vacuum, pressure, or high-pressure filtration to ensure the desired degree of dewatering. Alternatively, a common stacking method consists of spreading a thin layer of residue over a broad area and then extending the next layer only when the first is completely dry.
Nonstacking methods provide for the discharge of bauxite residue into lagoons. Such lagoons may not be naturally impervious but can be made so by means of plastic sheeting or clay to prevent seepage of the caustic liquid into the subsoil. The mud that settles on the bottom can be drained and stored. A layer of water is kept over the surface of the mud, or the lake is sprayed periodically with water to keep the dust hazard to a minimum. Eventually, lagoons can be covered with a layer of topsoil when they are closed.