Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Group iiia metal or beryllium
Patent
1983-08-31
1985-12-24
Valentine, Donald R.
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Group iiia metal or beryllium
423111, 423126, C01F 100, C01F 754
Patent
active
045605408
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
1. Technical Field
The present invention relates to a method for recovering useful products from waste products obtained when producing aluminium fluoride from aluminium hydroxide and fluosilicic acid; and more particularly to recovering sodium silicoaluminates and sodium fluoroaluminates from silica waste, mother liquor and wash-water deriving from the manufacture of aluminium fluoride.
An object of the present invention is to obtain a possibility of recovering useful and economically valuable products from the waste products obtained in the aluminium fluoride production.
2. Background of the Invention
When manufacturing aluminium fluoride, several troublesome waste products are obtained which must be deposited and, optionally, rendered harmless. Thus, there is obtained a silica waste which contains fluorine and aluminium impurities, and a mother liquor and a wash-water containing fluorine, aluminium and silicon. These waste products present a serious problem, since they occur in relatively large quantities and require complicated and expensive treatment before they can be deposited. It is true that silica residues per se can be deposited, but--as will be understood--the direct deposit of silica contaminated with large quantities of harmful impurities cannot be tolerated. The mother liquor and the wash water must be neutralized with calcium hydroxide to a pH-value of about 10, at which the fluorine and silicate contents thereof precipitate. This precipitate is then separated from the mother liquor and wash water, for example by centrifugation, and subsequently deposited.
Thus, the aforementioned waste products constitute a serious environmental problem, even though they may be treated in the manner indicated before being deposited. In addition, substantial amounts of silicon and fluorine are lost when the waste is treated in said manner.
The reactions involved in the production of aluminium fluoride are given below, the waste products being underlined. ##STR1## Prior art recognizes some different methods for recovering active silica and cryolite as well as other compounds from fluosilicic acid and waste products from the preparation of aluminium fluoride.
Austrain,B, 314 474 discloses the production of aluminium fluoride from fluosilicic acid recovered from the production of phosphate containing fertilizers. Hereby 8-25% fluosilicic acid is reacted with aluminium hydroxide in a relation which is 0.95-1.05:1 of the stoichiometric relationship according to the formula. H.sub.2 SiF.sub.6 +Al(OH).sub.3 .fwdarw.2AlF.sub.3 +SiO.sub.2 +H.sub.2 O at a temperature of 60.degree.-100.degree. C. The aluminium fluoride obtained can be used for the production of cryolite. Aluminium hydroxide is thereby reacted with fluosilicic acid in two steps, using one third of the amount of aluminium hydroxide requested in the first step, and two thirds thereof in the second step, whereby a mixture containing aluminium fluoride, which is precipitated and separated off, and aluminium hydrofluoric acid is obtained. The aluminium hydrofluoric acid is then treated with alkali at pH 3-6.5, preferably at pH 6-6.5 and at a temperature of 60.degree. C. to form cryolite. This process, however, requires very pure components as otherwise contaminations of i.a. phosphates will precipitate and contaminate the cryolite. Any silica present will also form complexes which contaminates the cryolite and deteriorates the quality thereof.
Using fluosilicic acid and ammonia as starting materials active silica and cryolite can be prepared (Austrain, B, 315128). Thereby ammonium fluoride and silica are primarily prepared, whereafter the ammonium fluoride is reacted with sodium aluminate to the formation of cryolite.
Swedish,B, 366723 relates to a process to recover fluorine and aluminium from dilute, aqueous solutions, particularly mother liquors obtained in the crystallization of aluminium fluoride, whereby the mother liquors are treated with ammonia, or ammonium dicarbonate to give a chiolite-analogous ammonium compound, (NH.sub.4).sub.5 Al
REFERENCES:
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patent: 2783128 (1957-02-01), Wendt
patent: 2916352 (1959-12-01), Fitch et al.
patent: 3175882 (1965-03-01), Derr
patent: 3493330 (1970-02-01), Vancil et al.
patent: 3755532 (1973-08-01), Harrison et al.
Boliden Aktiebolag
Valentine Donald R.
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