Yield stress- and zeta potential-pH behaviour of washed α-Al2O3 suspensions with relatively high Ca(II) and Mg(II) concentrations: Hydrolysis product and bridging

• Hydrolysis products of Ca(II) and Mg(II) adsorbed at pH below the IEP of α-Al2O3
• Precipitate bridging by Ca(II) and Mg(II) hydroxides at the maximum yield stress
• Zeta potential-pH trend can be used to determine onset pH of adsorption.
• It can also be used to deduce the pH at the onset of hydrolysis product formation.
• Results have direct relevance to the effects of lime addition in mineral processing.

The abrupt change in the zeta potential-pH trend at pH 7 showed that positively charged hydrolysis products of both Mg(II) and Ca(II) ions are adsorbed on washed positively charged alumina particles. This pH is much lower than that predicted pH from equilibrium data for the formation of these hydrolysis products. Surface induced hydrolysis has been attributed by others as being responsible. The yield stress–pH curve was also shifted to a much higher pH in the presence of Mg(II) and Ca(II). The maximum yield stress of the washed alumina suspension at pH 9.5 was shifted to pH 12 and 13 by Mg(II) and Ca(II) additives respectively. The magnitude of the maximum yield stress in the presence of Mg(II) and Ca(II) appeared to be larger or as large as that without these additives. The species diagrams of Mg(II) and Ca(II) were used to explain change in the zeta potential and yield stress behaviour with pH. Insoluble Mg(OH)2 and Ca(OH)2 species are the predominant products at the pH of maximum yield stress. Precipitate bridging was invoked to explain the additional attractive force responsible for the larger maximum yield stress. Steric effect will also be present due to the adsorbed layer. The new phenomena observed here are due to the concentration of Mg(II) and Ca(II) employed, ranging from 0.002 to 0.41 M, being generally more than an order of magnitude larger than that used in previous studies.

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