Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Group iiia metal or beryllium
Patent
1996-10-17
1998-07-07
Bos, Steven
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Group iiia metal or beryllium
423119, 423131, C01F 100, B01D 1100
Patent
active
057764263
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for treating solid material containing fluoride and sodium. The invention is especially suitable for the treatment of spent potlining from aluminium smelting.
Aluminium is conventionally produced by the reduction of alumina. The reduction is carried out in a reduction pot containing fused cryolite. The reduction pot is lined on its side and bottom with a carbonaceous lining. During operation of the pot, the lining gradually deteriorates with the carbonaceous material being penetrated by materials in the pot. The lining eventually deteriorates to such an extent that the pot must be shut-down, the contaminated lining removed and the pot re-lined. The potlining also consists of an outer refractory wall which is removed with the carbonaceous material.
Spent potlinings, as a result of their use, deterioration and penetration by bath materials, contain significant quantities of fluorides, as well as aluminium, alkali metals, nitrides and cyanide. Large quantities of spent potlining are generated each year, with approximately 35,000 tonnes per annum being generated in Australia alone. Disposal of spent potlining, which contains significant quantities of hazardous and/or water leachable material, presents numerous problems.
A number of methods for treating spent potlinings have been previously described. These methods include combustion of the spent potlining, roasting of spent potlining and various treatments with acidic solutions, dilute caustic solutions or with lime.
One such prior art treatment is disclosed in British Patent application No. 2,056,425 by Alcan Research & Development Limited, published 18th March 1981.
The process described in GB 2,056,425 involves treating raw spent pot lining material with lime and caustic soda in such a way as to separate the solid coarse residue from the fine, insoluble CaF.sub.2 resulting from lime digestion. According to the application, sodium values from the wastes are converted to sodium hydroxide, some of which react with caustic-soluble alumina in the waste to form sodium aluminate. The resultant caustic/sodium aluminate liquor is stated as being able to be employed in any manner known in the art.
The process of GB 2,056,425 is based on the observation that the particle size of the calcium fluoride deposited in the lime digestion stage is very fine and that if the spent potlining material is ground rather coarsely, the insoluble residues of the spent potlining material may be separated from the calcium fluoride by a screening procedure. This allows for the recovery of a calcium fluoride product and a solids residue. The calcium fluoride may then be treated to recover fluoride values as AlF.sub.3 or mixed with sand and subjected to hydrolysis at 1000.degree. C. to recover the fluorine values and leave an inert. residue suitable for landfill.
GB 2,056,425 requires a residence time in the digestion step of 3-4 hours in order to obtain satisfactory extraction of fluorides from the spent potlining. The weight of lime used is preferably 1.2 to 1.6 times the fluoride content of the spent potlining, which is equivalent to a Ca:F molar ratio of 0.4 to 0.6.
Although the process of GB 2,056,425 is described as a lime digestion process, at page 3, the specification states that caustic solution is recycled from a later stage of the process to the digestion step in order to maintain a slurry having a solids content of 15-30%. This caustic soda solution recirculated to the leaching step has a caustic concentration of about 50 g/l caustic soda, calculated as Na.sub.2 CO.sub.3. In some instances, the recycled caustic liquor may have a caustic concentration of up to 100 g/l, but lower quality products are obtained.
A part of the caustic liquor is also bled out of the system. This liquor contains NaOH, Al.sub.2 O.sub.3 and F in solution, as well as appreciable quantities of dissolved cyanide. Indeed, the cyanide concentration may be up to 200 ppm. This level of cyanide would cause the recovered caustic soda liquor to be classified as a hazardo
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Kidd Ian Lewis
Rodda Darren Paul
Wellwood Grant Ashley
Bos Steven
Comalco Aluminium Limited
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