Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Group iiia metal or beryllium
Reexamination Certificate
2000-08-07
2002-11-12
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Group iiia metal or beryllium
C423S130000
Reexamination Certificate
active
06479024
ABSTRACT:
This invention relates to a process for producing alumina. In particular, the invention relates to a process for extracting alumina from bauxite.
Aluminium is produced, almost exclusively, from the alumina ore bauxite by a combination of the Bayer and Hall-Heroult processes. In the Bayer process, bauxite is typically crushed, washed, dried and ground before being treated with aqueous sodium hydroxide at an elevated temperature. The sodium hydroxide dissolves the alumina to form a solution from which insoluble impurities (“red muds”) are removed. Alumina trihydrate (Al
2
O
3
.3H
2
O or Al(OH)
3
) as gibbsite is precipitated from the solution and is calcined to produce alumina (Al
2
O
3
) for electrolytic reduction to aluminium in the Hall-Heroult process. An outline of the Bayer process is given in
Chemistry and Industry,
Jul. 18, 1988, p.445-451.
The precipitation of the trihydrate from the solution (“the Bayer process liquor”) involves seeding the solution with crystals of the trihydrate (in the form of gibbsite) at about 80° C. and maintaining the temperature at this level for a significant period of time whilst crystallisation takes place. If the trihydrate is precipitated more rapidly, for example at lower temperatures, it is found to be less pure and the impurities which it contains, such as silicates, sulphate, oxalate, carbonate, sodium and organic compounds, make it unsuitable for electrolysis because of the adverse effects which these impurities have on the electrolytic cell and on the resulting aluminium.
An essential feature of the Bayer process is that the liquor stream is recycled; as a consequence, there can be a build up of contaminants, impurities and additives with time. In Bayer liquors containing relatively high levels of oxalate as impurity, this problem is remedied to some extent by crystallising sodium oxalate from the liquor.
The contaminants in the Bayer liquor may be conveniently considered under three separate headings: organic impurities, inorganic impurities and additives.
Organic materials are introduced into the Bayer process by almost all bauxites. Whilst initially much of this is in the form of humic materials, with time in the liquor, degradation occurs and a vast number of organic compounds is generated, with a wide range of molecular weights and chemical structures and functionalities. The largest concentration of any single organic compound is the oxalate anion (C
2
O
4
2−
), the penultimate degradation product of the humates present in the liquor prior to the formation of CO
2
/CO
3
2−
. Not all of the organic compounds have equal effects on the Bayer process. Many of the organic compounds strongly absorb visible light, turning an otherwise colourless liquor various shades from pale straw to virtually black. The most serious problem associated with many of the organic compounds is the undesirable effects they have on the precipitation of gibbsite and sodium oxalate.
Certain compounds are known to be very strong poisons of gibbsite precipitation. For example, organic molecules containing specific hydroxyl (—OH) and carboxyl (—COOH) groups can dramatically affect gibbsite precipitation. Negative effects of these compounds can include reduction in precipitation rate, precipitation yield, product purity (both colour, and also increased sodium content, which remains in the alumina produced by calcining, and causes problems in subsequent aluminium production), morphology and strength (resistance to attrition) and an increase in the relative amount of smaller particles (fines) in the product.
In all Bayer plants where there is an increasing concentration of oxalate in the liquor stream with time, the sodium oxalate must be removed, because above certain concentrations the liquor becomes unstable with respect to the solubility of sodium oxalate, and the latter can precipitate out spontaneously, causing various problems in plant operation, including flotation and solids overflow, generation of undesired fine particles of gibbsite and contamination of the gibbsite product by co-crystallization with sodium oxalate. The controlled removal of sodium oxalate is usually achieved by controlled precipitation, either in a separate side stream or by deliberate co-crystallization with gibbsite, in a controlled manner, in the main process stream. Alternative removal processes include “liquor burning” and adsorption onto an inert solid phase e.g., activated carbon. Some of the organic compounds in the Bayer liquor, apart from oxalate, can affect the precipitation of sodium oxalate by raising its apparent solubility (and hence lowering the driving force to make it crystallize), thereby reducing yield, as well as precipitation rate, by poisoning sodium oxalate crystal growth sites. The compounds which have a negative effect on the crystallization of sodium oxalate are referred to by the generic term oxalate seed poisons.
Various inorganic anions are introduced into the Bayer liquor during plant operation, some of which build up in concentration, through digestion of the bauxite, from naturally occurring impurities in the bauxite and/or reagents and water, and, in the case of carbonate (CO
3
2−
), also through reaction of atmospheric carbon dioxide with the caustic soda solution.
Inorganic anions derived from impurities in bauxite include oxoanions of metals (e.g., oxoanions of transition metals) such as vanadate (VO
4
3−
), ferrate (FeO
4
2−
) and silicate (SiO
4
4
). Other inorganic anions in the liquor include sulphate (SO
4
2−
) and phosphate (PO
4
3−
), for example.
In the case of some Bayer plants, using certain bauxites as feedstocks, the concentration of sulphate in the liquor can build up to unacceptable levels. The problems caused by the presence of sulphate in the liquor stream include raising the ionic strength of the liquor whilst reducing the effective level of free caustic, and potential problems of scale formation (e.g., by gypsum, CaSO
4
.2H
2
O) through reaction with calcium added during causticisation.
During the digestion stage of the Bayer process, soluble silica is taken up into solution in the Bayer liquor, and is subsequently precipitated as de-silication product (DSP), which partially or wholly consists of one or more sodium alumino-silicates. Whilst this precipitation step effectively removes the silicon from solution, it does so at the expense of removing sodium and aluminium from useful production.
The conversion of sodium hydroxide in the Bayer liquor to sodium carbonate greatly reduces the effectiveness and productivity of plant operation, and so is currently treated in many refineries by the process of causticisation, in which quicklime (calcium oxide, CaO) and/or slaked lime (calcium hydroxide, Ca(OH)
2
) is added to the liquor. In practice, not all of the lime is converted to calcium carbonate, and some reacts to form other compounds, including ones which use up aluminium. The large quantities of lime used, combined with the inefficiencies usually associated with its use, leads to causticisation being a large expense in the operation of a Bayer plant, as well as there being associated negative environmental impacts.
Various chemicals are added to the Bayer liquor at different parts of the process to achieve a range of improvements; for example, crystal growth modifiers and flocculants. It is sometimes advantageous to be able to remove some of these chemicals at certain stages of the process, and yet not remove others.
In view of the huge volume of bauxite which is processed around the world, even small improvements in the process, such as a reduction in the time taken for precipitation or a reduction in the temperature which needs to be maintained, can lead to vast cost savings.
Layered double hydroxides are a class of compounds which comprise two metal ions and have a layered structure. A brief review of layered double hydroxides is provided in
Chemistry in Britain,
September 1997, p. 59 to 62. The hydrotalcites, perhaps the most well-known of the layered double hydroxides, have been
Fogg Andrew Michael
O'Hare Dermot Michael
Parkinson Gordon Michael
Bos Steven
Curtin University of Technology
Volpe and Koenig P.C.
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