Ion exchange between solid phases: material balance and mechanism of interaction of aluminium hydroxide gel in aqueous suspension with a strong acid cation exchange resin

Abstract

Ion exchange between aluminium hydroxide gel, prepared from aluminium sulphate and aqueous ammonia, and a strong acid cation exchange resin (Amberlite IR 120) was studied to provide fundamental data in order to underpin potential applications in water treatment or hydrometallurgical processing. The overall ion exchange was an acid-base reaction which approached completion relatively slowly (over several hours) because both of the main reactants were solid in form. Preliminary work showed that an increase in ionic strength from zero to 1.5 by stepwise addition of ammonium sulphate increased the rate of ion exchange while decreasing the proportion of aluminium loaded on the resin at equilibrium. Quantitative kinetic and equilibrium data describing the distribution of aluminium, hydrogen and ammonium ions over the gel, aqueous and resin phases were obtained on the basis of a new experimental and charge/material balance design requiring the minimum of analytical measurements on (i) the initial pH and concentration (and molecular formula) of the hydroxide precipitate suspension in the mother liquor from precipitation; (ii) the initial hydrogen ion content of the resin; and (iii) the variation with time of pH and suspended and dissolved aluminium concentration in the aqueous phase. Resulting composite plots of mole distribution against time up to 24 h gave information on the mechanism occurring (several overlapping stages in the initial reaction over the first 20 min, followed by slower changes, governed by small, roughly steady-state, concentrations of hydrogen and aluminium ions in aqueous solution, up to pseudo equilibrium at 300 min) and indicated that the overall reaction was 60–70% complete within 1–2 h and that the practical ion exchange capacity for aluminium at equilibrium was greater than 70% of the theoretical maximum. The results also indicated that aluminium ions hydrolyse similarly in both aqueous and resin phases.