PURPOSE: To prevent Fe dissolved in an aqueous solution of a caustic alkali from precipitating, by adding an alkaline earth metal hydroxide or an alkaline earth metal compound to become a hydroxide in the aqueous solution of a caustic alkali to the aqueous solution of a caustic alkali containing an alkali aluminate.
CONSTITUTION: A member of Al or Al alloy is etched with an aqueous solution of a strong alkali such as NaOH, etc. as a pretreatment before anodic oxidation is carried out. Al as sodium aluminate is dissolved in the treated solution, and Fe, etc. as impurities in the member of Al is dissolved in it. When the waste liquor of the treated solution is used as a raw material for preparation of alumina by Bayer's process, Fe(OH)2 and Fe(OH)3 as well as Al(OH)3 are precipitated to damage the quality, so that 1W10g/l calculated as hydroxide of a hydroxide of an alkaline earth metal such as Ca, Mg, etc. or an alkaline earth metal compound to be reacted with NaOH to form a hydroxide is added to the solution, to prevent Fe from precipitating as a hydroxide.
MINAMIDA TOMEO
TSUJI KOUICHIROU
| JPS5731527A | 1982-02-20 |