Patent ID: 7022302

Claim:
A process for the preparation of hydrotalcite which comprises, (i) diluting bittern with water to obtain a solution with about 2% (w/v) magnesium ion concentration and clarifying the diluted bittern by treating with equivalent amounts of alum solution and caustic soda solution to precipitate aluminium and magnesium hydroxide in the range of 75–100 ppm and 200–300 ppm respectively; (ii) separating the sedimented hydroxides and obtaining a clear solution of bittern with a magnesium ion concentration in the range of 0.53–0.90 molar; (iii) reacting clear bittern obtained in step (ii) above with alkali carbonate while keeping mole ratio of carbonate ions to magnesium ions in the range of 1.1–1.4 to obtain an aqueous slurry of a magnesium precursor; (iv) reacting a solution of alum or soluble aluminium salts having not more than 100 ppm of iron with alkali carbonate under continuous stirring while keeping mole ratio of carbonate ions to aluminium ions in the range of 1.70–1.95 to obtain an aqueous slurry of aluminium precursor; (v) separating the precursors obtained in steps (iii) and (iv) above in form of cakes, and washing the cakes till washed liquor of magnesium precursor and aluminium precursor show less than 0.1% (w/v) of chloride and sulphate respectively; (vi) heating well dispersed aqueous slurry of magnesium precursor and adding caustic soda equivalent to 0.90–0.95 of magnesium ion present under continuous stirring; (vii) maintaining the temperature of slurry obtained in step (vi) and then allowing it to cool; (viii) re-dispersing aluminium precursor in 0.05 molar sodium carbonate solution to obtain an aqueous aluminium slurry; (ix) adding aqueous aluminium slurry obtained in step (viii) to a magnesium slurry obtained in step (vii) keeping ratio of mole of aluminium ions to total moles of aluminium and magnesium ions in the range of 0.28–0.33 under continuing agitation to obtain an amorphous magnesium-aluminium complex; (x) crystallizing mixed slurry of 3–5% (w/v on product basis) under agitation and cooling; (xi) adding stearic acid of 3–6% (w/v on product basis) to a slurry obtained in step (x) with continuous stirring; (xii) agitating the slurry obtained in step (xi) while allowing to cool, filtering the slurry and washing the cake till filtrate indicated pH of 8.5 to 10.5; (xiii) drying the said wet cake and pulverizing to obtain hydrophobic hydrotalcite in powder form.