Colloids and Surfaces A: Physicochemical and Engineering Aspects
The surface chemistry of Bayer process solids: a review
Introduction
The Australian aluminium industry was born in 1955 when a State/Commonwealth joint venture began producing alumina and aluminium at Bell Bay in Tasmania. The venture was the result of the desire of Governments of the day to end Australia’s dependence on overseas supplies of aluminium, while also acquiring the skills and technology to produce this valuable commodity. The initial capacity of the plant was a (relatively) meagre 12 kilotonness (kt) per year, with all the bauxite used imported.
Today, the Australian aluminium smetting and alumina refining industries contribute AUD$2.2 billion and AUD$2.5 billion, respectively to Australia’s export income. Current forecasts suggest this contribution is likely to increase to a total of AUD$5.7 billion by the end of 1998 [1]. Regrettably, the refining industry experiences significant restrictions due to limitations imposed on the process by surface chemical reactions. Currently, these reactions either directly or indirectly cost the Australian alumina refining industry at least AUD$150 million per year in lost production. Development of an understanding of the chemical nature of these reactions is vital if Australian refiners are to remain competitive on the world stage. Planned capacity expansions in China, India and Vietnam, combined with a continuing push for increased capacity utilisation at existing refineries, demand this of Australian refiners.
Australia is the worlds largest producer of bauxite and refiner of alumina. In 1988, it was estimated that Australia had bauxite stocks of over half a billion tonnes [2], with the major reserves located in the Darling Ranges (Western Australia), Gove (Northern Territory) and Weipa (Queensland) areas (Fig. 1). Currently, Australia mines 43 megatonnes (Mt) of bauxite per annum, producing 13.3 Mt of alumina, or just under 30% of annual world production [1]. Given this large production tonnage, Australia produces a relatively small amount of aluminium per annum (1.4 Mt, or just over 5% of annual world production; [1]). Most indicators suggest aluminium production will gradually increase with time, however the (relatively) high cost of electrical power in Australia will always be somewhat of a limiting factor in this respect.
Many bauxite ores are of poor quality (<40% available alumina; [3]) and have a relatively high organic content. Bauxites having 60% available alumina are relatively rare (pure gibbsite—A1(OH)3—has only 65.4% available alumina), and ores having 50% available alumina are considered high quality by most alumina refiners. Unfortunately, many Australian ores are at the lower end of the scale, with Darling Range bauxite being particularly low in available alumina content (30–35%) and having high silica and iron oxide content [4]. The high organic content of Australian ores (total organic carbon—TOC≈0.15–0.30 (w/w)%) is of great concern because of the surface-active nature of many of the organic compounds that find their way into the Bayer process. These surface-active compounds are major factors in the limiting surface chemical reactions mentioned previously and, as such, are the subject of ongoing investigation at a number of academic and/or industry supported research centres.
Section snippets
Aluminium and alumina—a global perspective
Aluminium is electrochemically produced from alumina using the Hall–Héroult process. Alumina is in turn produced from the ore bauxite using the Bayer process. Aluminium’ properties are well documented, and include its light weight, low melting point, corrosion resistance, good thermal and electrical conductivity, and high reflectivity. In addition, the physical properties of aluminium can be enhanced through alloying, mechanical working and heat treatment. Aluminium is thus extensively used
The Bayer process
In 1888, Karl Josef Bayer developed and patented a process, which has become the cornerstone of the aluminium production industry worldwide. The Bayer process, as it has become known, is used for refining bauxite (named after Les Baux, the district in France where the ore was first mined) to smelting grade alumina, the precursor to aluminium. Typically, depending upon the quality of the ore, between 1.9 and 3.6 tonnes of bauxite is required to produce 1 tonne of alumina.
The Bayer process
Bayer Process solids
Two main chemical reactions limit the productivity and efficiency of alumina refineries in Australia and the rest of the world. These reactions are the precipitation of gibbsite (2) and the crystallisation of sodium oxalate (4). Additionally, the flocculation of fine red mud is an expensive and sometimes limiting operation in the overall refining process.
The aforementioned chemical reactions and the flocculation of red mud are surface reactions.
Conclusions
The specific aim of the research described in this review has been the continued development of understanding of the surface chemistry of Bayer process solids. Continued research into this important area will significantly enhance our understanding of the nature of surfaces and adsorptive processes occurring under Bayer-like conditions, whilst also contributing to the conceptual advancement of our knowledge and understanding of the nature of surfaces (and adsorption processes occurring thereon)
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Formerly of Alcoa of Australia, Kwinana, WA; and Comalco Minerals and Alumina, Brisbane, QLD.