Structure Characterization of Sulfate and Oxysulfate Phases Formed During Sulfuric Acid Digestion of Ilmenites

Abstract

Laboratory digestion of natural and upgraded ilmenites with concentrated sulfuric acid has been carried out under conditions simulating the sulfate-route pigment process. X-Ray diffraction studies on the solid digestion cakes led to the characterization of a number of different iron titanium sulfate and oxysulfate compounds, whose formation was dependent on the ilmenite composition and digestion conditions. Two different sulfate phases were identified, with structures related to those for Fe₂(SO₄)₃ and H₂O[ Zr (HPO₄)₂]. Four different iron titanium oxysulfates , ( Fe,Ti )(O,OH)SO₄, were identified as major digestion products, three having structures related to those for β-NbOPO₄, GeOHPO₄ and lazulite , Mg[AlOHPO₄], and the fourth being a new struture type. The structures of the oxysulfate phases were refined by the Rietveld method. Both the sulfates and the oxysulfates have three-dimensional framework structures formed by corner linking of ( Ti,Fe )O₆ octahedra and SO₄ tetrahedra, and containing channels or interlayer regions that can be occupied by cations such as H₃O⁺ and Fe²⁺. The different structures have many features in common. Their structural relationships are discussed and mechanisms are proposed for the phase transformations encountered in the digestion studies.