Al(III) speciation distribution and transformation in high concentration PACl solutions

Effects of Al(III) concentration and pH on the speciation of Al(III) in polyaluminum chloride (PACl) solutions especially on the Al13 fraction were investigated. A series of PACl samples were prepared over the range of Al(III) concentration from 0.01 to 2.0 mol/L with the B (OH/AI ratio) value from 1.0 to 2.5 by forced hydrolysis of AlCl3. The samples were characterized by ferron assay, pH and 27Al NMR. It was shown that the Al(III) concentration had a dramatic effect on the hydrolysis processes and the species distribution of PACl was in relate to the decrease of pH. The fraction of Al species, Alb (or Al13) decreased and Alc increased with increase of total Al(III) concentration. Under the condition of Al(III) 2.0 mol/L, B = 2.5, the pH value was 2.73 and no Al13 could be detected. During diluting and aging, the species distribution evolved. The Al13 could then be detected again and the amounts increased with time. If the diluted samples were concentrated by freeze dry at −35deg;C or heating at 80°C, the pH value and Al13 content would decrease with the increased concentration. It demonstrated that the key factor for formation of Al13 in concentrated PACl was pH value.