Patent ID: 8658119
Filing Date: 2014-02-25
Classification: C01F,C01P,F23G

Abstract:
1. A method of producing calcium carbonate from a waste sludge containing calcium carbonate and at least one silicate mineral, the method comprising the steps of: (i) incinerating the sludge to an ash in which at least 60% of the calcium carbonate present in the sludge is converted to free calcium oxide which is unreacted with the silicate mineral(s), (ii) optionally slaking the ash to convert at least a portion of the free calcium oxide therein to calcium hydroxide, (iii) treating either the ash from step (i) or the slaked ash from step (ii) with an aqueous solution of a polyhydroxy compound to provide a solution of dissolved calcium hydroxide comprising 10% to 35% by weight of dissolved polyhydroxy compound and 1% to 5% by weight of dissolved calcium hydroxide (expressed as Ca(OH) (iv) separating insoluble material from the solution of calcium ions obtained in step (iii), (v) dispersing carbon dioxide through the solution obtained from step (iv) so as to form a mixture containing calcium carbonate, (vi) terminating the dispersion of carbon dioxide before the pH falls below 9.5 and adding an alkaline reagent to maintain the pH for the product mixture at a value of at least 9.5, and (vii) separating the calcium carbonate produced in step (v).