Contribution to the quaternary system H 2 O–Al 3+ , Ca 2+ //O 2− ,SO42−: Solid–liquid equilibria in the ternary systems Al 2 (SO 4 ) 3 –CaSO 4 –H 2 O and Al 2 O 3 –SO 3 –H 2 O at 25 °C

Abstract

During the acid processing of aluminosilicate ores, the temporary precipitation of a white solid phase principally consisting in calcium sulfate and aluminium hydroxide and/or sulfates may be observed. This study aimed to determine the nature of the solid phases, their stability and their solubility. Understanding why precipitation occurs and being able to prevent it would help avoid the clogging that may occur during acid ore mining. Thus the solid-liquid equilibria of the two ternary systems Al2(SO4)3-CaSO4-H2O and Al2O3-SO3-H2O were defined. The first ternary highlights very low gypsum solubility, which explains why this mineral is observed so often during acid mining. However no double Al-Ca compound, in equilibrium with a liquid phase, was found. The determination of the second system was more complex as a large number of aluminium hydroxysulfates, which could appear during the ore processing, have been reported in the literature. Most of these solid phases are poorly crystallised and not well defined as they decompose rapidly and are often metastable. The use of different techniques of characterisation (X-ray diffraction, thermogravimetry, scanning electron microscopy...) enabled the chemical composition of the solid phases to be found and also their solubility curves to be delimited. One of the aluminium hydroxysulfates decomposes rapidly when the mother solution is removed. This underlines the poor aluminium hydroxysulfate stability, which explains the difficulties in characterising them.