Patent ID: 8287636

Claim:
A process of preparing a pearlescent pigment, comprising: (a) preparing an aqueous metal precursor solution by mixing a precursor aqueous solution comprising 0.1-5 weight parts of a zinc precursor with an aluminum precursor aqueous solution comprising an aqueous flux relative to 100 weight parts of the aluminum precursor; (b) preparing a mixed gel by titrating the aqueous metal precursor solution with an aqueous sodium salt solution to pH 6.0-7.5, followed by hydrolysis; (c) aging the mixed gel at 60-100Â° C. for 5-30 hours to form an aged mixed gel; (d) drying the aged mixed gel at 60-200Â° C. for 5-30 hours to form a dried mixed gel; (e) preparing a crystallized cake comprising flak alpha-alumina crystals and the aqueous flux by crystallizing the dried mixed gel at 850-1,300Â° C. for 1-10 hours; (f) preparing an alpha-alumina crystal by (i) cooling the crystallized cake to room temperature, (ii) removing the aqueous flux by dissolving in water and filtering the crystallized cake, (iii) dispersing the filtrate in 0.1-30% sulfuric acid solution at 20-90Â° C., and (iv) filtrating, washing and drying; and (g) forming a coating layer on the surface of the alpha-alumina crystal by adding an acid to aqueous suspension of the alpha-alumina crystal to adjust the pH to 1.0-3.0 and by adding 30-60 weight parts of a metal or a metal precursor and 30-65 weight parts of an alkaline aqueous solution relative to 100 weight parts of the alpha-alumina crystal, followed by calcination.