Sodium tungstate preparation process

Details Sodium tungstate preparation process Sodium tungstate preparation process

1998-05-04

1999-11-30

Bos, Steven

Chemistry of inorganic compounds

Treating mixture to obtain metal containing compound

423 61, 423593, 423DIG12, C01B 1930, C01C 1930, C01D 1930

Patent

active

059937564

DESCRIPTION:

BRIEF SUMMARY
FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to a method for the preparation of sodium tungstate by the oxidation of hard metal scrap and/or heavy metal scrap in a molten salt.
Methods for the recovery of useful materials from ("working up of") hard metal scrap (WC--Co or WC--Co--TaC--TiC) and heavy metal scrap (W--Cu--Ni--Fe) are described in numerous patent publications. Known methods include the oxidation of hard metal scrap at elevated temperatures (U.S. Pat. No. 3,887,680), treatment with liquid zinc (U.S. Pat. No. 3,595,484) or the anodic dissolution of contained binder materials (AT-A 380 495). These methods are either expensive as regards apparatus or are suitable only for certain types of scrap.
Moreover the reaction of hard metal scrap in molten salts is important industrially.
Thus U.S. Pat. No. 4,603,043 describes the working up of W-containing materials in a molten salt consisting of NaNO.sub.3 and NaOH at temperatures of 500 to 700.degree. C. Here, in order better to control the reaction, NaOH and the W-containing component are first of all heated to 560 to 600.degree. C. The NaNO3 required as oxidising agent is subsequently added over a period of 2 to 3 hours.
According to DE 3144295, the working up of hard metal scrap is likewise carried out in an alkali hydroxide/alkali nitrate melt, in a molten salt consisting of 40 to 80% alkali nitrate and 20 to 60% alkali hydroxide. 5% of NaCl is added in order to lower the melting point. The melting temperature is 550.degree. C. The melt is subsequently poured into water.
DD-A 207 932 discloses the working up of hard metal scrap in pure sodium nitrate melts or pure sodium nitrite melts. Here the stoichiometric excess of the alkali component is to be 10 to 15% and the melting temperature 900.degree. C. The reacted melt is cooled to room temperature prior to being dissolved in water.
According to IN-A 157 146, a molten salt consisting of alkali hydroxide, preferably KOH or NaOH, and an alkali nitrate as oxidising agent, preferably KNO.sub.3 or NaNO.sub.3, is employed for the working up of hard metal scrap. The melting temperature varies between 350 and 460.degree. C. In this connection it was established that at 400 to 420.degree. C. the yields of tungsten are 90 to 94%. At 440 to 460.degree. C. the yield of tungsten can be increased to 99%. In this case the reaction can be controlled only with difficulty. In addition emission of nitrogen oxides occurs.
The disadvantage of all reactions in nitrate and/or nitrite media is that the highly exothermic reaction is difficult to control. Consequently the overall reaction is considered to be problematic as regards industrial safety. In addition there is a frequently uncontrollable emission of nitrous gases. Nitrite and nitrate contained in the sodium tungstate solution and in the filtration residues render working up extremely difficult when a solvent extraction has to be subsequently carried out.
The object of the present invention, therefore, is to provide a method for the working up of lumps of hard metal scrap and heavy metal scrap, which does not have the disadvantages described.


SUMMARY OF THE INVENTION

This object was fulfilled according to the invention by the working up by oxidation of tungsten-containing scrap of various compositions in a molten salt consisting of NaOH and Na.sub.2 SO.sub.4.
The present invention accordingly provides a method for the preparation of sodium tungstate by the oxidation of hard metal scrap and/or heavy metal scrap in a molten salt mixture consisting of from 60 to 90 wt. % of NaOH and from 10 to 40 wt. % of Na.sub.2 SO.sub.4.
The reaction is preferably carried out in a moving melt, particularly preferably in a batchwise operated directly fired rotary kiln. As protection against stresses the rotary kiln should be lined with a refractory material. The oxidation is advantageously carried out by blowing air into the melt. It is technically efficient to carry out the reaction at temperatures of between 800 and 1100.degree. C. Particularly good digestio

REFERENCES:
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patent: 1800758 (1931-04-01), Schwarzkope
patent: 3595484 (1971-07-01), Barnard et al.
patent: 3887680 (1975-06-01), MacInnis et al.
patent: 4298581 (1981-11-01), Douglas et al.
patent: 4406866 (1983-09-01), Reilly
patent: 4603043 (1986-07-01), Douglas et al.

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