| 摘要 |
The soda content of soda-contaminated alumina hydrate is reduced and a substantially 100% crystalline alpha-alumina product having a crystal size in the range 5-12 microns produced by calcining at an elevated temperature from 1000 DEG C. up to the melting point of alumina in the presence of chlorine-containing material and fluorine-containing material, said material being employed in the range 0.5 to 10% chlorine or chlorine content and 0.01 to 5% fluorine or fluorine content by weight based on the resulting treated alumina and being capable of yielding with the soda-contaminant at the calcining temperature a volatile sodium compound, the calcining operation being carried out for a time sufficiently long to reduce the soda-content by at least about 30%, and recovering the crystalline alumina product. Specified chlorine-containing materials include gaseous chlorine itself, ammonium chloride, hydrogen chloride, aluminium chloride, phosgene, carbon tetrachloride, chlorinated C1-12 hydrocarbons and organic chlorine containing compounds capable of decomposition to chlorine-containing forms which will react with the soda contaminant to form a volatile sodium compound. Specified fluorine-containing materials include hydrogen fluoride, aluminium fluoride, ammonium fluoride, elemental fluorine, carbon tetrafluoride and fluorinated C1-12 hydrocarbons and organic fluorine-containing compounds which will decompose at the calcining temperature to yield forms capable of reacting with the soda-contaminant. The alumina may be supplied dry or as a slurry to the kiln. If desired, reaction may take place in a fluidized bed of alumina particles. The fluorine-containing material and/or chlorine-containing material may be introduced at either end of the kiln, depending upon the physical state of the alumina, and on the volatility of and the physical state of the chlorine-containing and fluorine-containing materials. Reference has been directed by the Comptroller to Specification 864,009. |
