Method of manufacturing aluminum fluoride anhydride

A method of manufacturing aluminum fluoride anhydride having an average particle diameter of 100 .mu.m or more, which comprises the steps of:

• (a) preparing an initial super-saturated solution of aluminum fluoride;
• (b) preparing primary AlF.sub.3.3H.sub.2 O seed crystals containing not more than 5 weight % of fine particles of 40 .mu.m or less in particle diameter;
• (c) adding said AlF.sub.3.3H.sub.2 O seed crystals into the super-saturated solution of aluminum fluoride in an amount such that a total surface area of all the added seed crystals is in a range of 40-100 m.sup.2 per 1 kg of AlF.sub.3.3H.sub.2 O calculated as the amount of AlF.sub.3.3H.sub.2 O necessary to be precipitated from the initial super-saturated solution of AlF.sub.3 to reduce the concentration of AlF.sub.3 in the initial super-saturated solution to 1.6 weight %;
• thereafter,
• (d) if the concentration of the AlF.sub.3 dissolved in the resultant super-saturated solution after addition of said seed crystals is above 13 weight % or below 9 weight %, adjusting the concentration to 9 to 13 weight %;
• thereafter
• (e) heating the resultant aluminum fluoride super-saturated solution to a temperature ranging from 75.degree. to 106.degree. C.;
• thereafter
• (f) allowing crystals to precipitate from the resultant super-saturated solution of aluminum fluoride under agitation, using a batch crystallization method in a crystallizer;
• (g) repeating at least once the steps of (c) to (f) by using the crystals obtained in the step (f) and containing not more than 5 weight % of fine particle of 40 .mu.m or less in particle diameter as seed crystals;
• (h) separating AlF.sub.3.3H.sub.2 O having an average particle diameter of 100 .mu.m or more from a resultant slurry of AlF.sub.3.3H.sub.2 O after crystallization;
• (i) drying the AlF.sub.3.3H.sub.2 O thus separated;
• and
• (j) dehydrating the AlF.sub.3.3H.sub.2 O thus dried to remove combined water to produce aluminum fluoride anhydride;
• and
• wherein the aluminum fluoride anhydride has a fluidity factor "a" which is less than 0.135, where fluidity factor "a" is a constant defined by the following empirical equation:
• P/C=1/ab+P/a
• P . . . =pressure (Pa)
• C . . . =Degree of decrease in apparent volume (-) defined by
• C=(Vo-V)/Vo
• Vo=initial volume (m.sup.3)
• V=A volume under pressure (m.sup.3)
• a,b=Constants.
• 2.