Method for improving the brightness of aluminum hydroxide

摘要

There is disclosed a method for improving the brightness level of aluminum hydroxide removed from a caustic solution. The method comprises:(a) pre-filtering the solution to remove contaminants therefrom; and (b) contacting the filtered solution with an adsorbent consisting essentially of a calcined compound having the formula: AwBx(OH)yCz.nH2O wherein A represents a divalent metal cation selected from the group consisting of: Mg2+, Ni2+, Fe2+, Ca2+ and Zn2+; B represents a trivalent metal cation selected from the group consisting of: Al3+, Fe3+ and Cu3+; C represents a mono- to tetravalent anion selected from the group consisting of: OH-, Cl-, Br-, NO3- CH3COO-, C2O42-, CO32-, SO42-, PO43-, Fe(CN)63- and Fe(CN)64-; and w, x, y, z and n satisfy the following: 0/=n>/=+E,fra 1/2+EE (w-x); and (c) adsorbing additional contaminants onto the calcined compound. On a preferred basis, this method also includes: (d) filtering or sepiating contacted compound and additional contaminants from the solution. Still further preferred method steps include: adding to the filtered solution as seed material an aluminum hydroxide of high brightness, preferably with a whiteness level of about 85% or higher based on a 100% TiO2 reference standard. Preferably, the method further includes: (i) separating calcined compound from the solution; (ii) recalcining the separated compound; and (iii) contacting recalcined compound with additional solution. A preferred adsorbent for step (b) consists essentially of calcined hydrotalcite. There is also provided an improved aluminum hydroxide product having a whiteness level of 85% or higher based on a 100% TiO2 reference standard, said compound having been removed from a caustic solution and decolorized by the foregoing method.