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Chemical and electrochemical leaching studies of synthetic and natural ilmenite in hydrochloric acid solutions

Jabit, Nurul Ain (2017) Chemical and electrochemical leaching studies of synthetic and natural ilmenite in hydrochloric acid solutions. PhD thesis, Murdoch University.

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The research interest for upgrading ilmenite to synthetic rutile has increased over the years due to the increasing demand for titanium dioxide white pigment and titanium metal. Sulfate and chloride leaching processes in the absence or presence of reducing agents are the most commonly tested leaching routes. However, chloride leaching is more economical and hence the preferred option. It allows relatively easy treatment of waste solutions and therefore the regeneration and recycling of hydrochloric acid. Despite the many attempts to develop chloride processes to upgrade ilmenite to synthetic rutile, systematic kinetic and mechanistic studies based on chemical and electrochemical dissolution are lacking in the literature. In addition, no comparison has been made between flat natural or synthetic ilmenite surfaces and particles of ilmenite concentrates of different origin and composition in chloride solutions. The main objective of this thesis is to bridge this gap and improve the understanding of the kinetics and reactions for the dissolution of synthetic ilmenite and natural ilmenite concentrates of different compositions, under non-reducing and reducing conditions.

The measured dissolution rates of flat surfaces of synthetic ilmenite were found to decrease with time and obey a parabolic rate law, due to the blockage of the surface by an insoluble product. The dissolution rates of both iron and titanium were higher in the presence of tin(II) chloride as a reducing agent and remained unaffected over time. The reductive role of tin(II) ions appears to be that of inhibiting the formation of surface blocking solids, which can be used to propose a reaction sequence for ilmenite leaching.

In electrochemical studies the measured rest potential of an ilmenite electrode in hydrochloric acid solutions was found to be lower compared to that measured in sulfuric acid solutions, indicating higher dissolution rates in the former case, due to reducing conditions. The measured potentials and surface characterisation studies indicated the reductive dissolution of titanium(IV) in the solid phase to titanium(III) in the aqueous phase, which facilitates the reductive leaching process by removing the insoluble product layers. Hematite mineral, which is found associated with weathered ilmenite, was also found to have a higher dissolution rate under the cathodic conditions which facilitated leaching.

The leaching of three ilmenite samples of different compositions collected from local producers over the years, labelled as North Capel, Iluka and Tiwest, gave different results with regards to the iron and titanium dissolution as a result of different degrees of alteration. The leaching efficiency appeared to be in the
descending order of North Capel > Iluka > Tiwest, which appeared to be inversely proportional to the degree of alteration. The highest extraction in hydrochloric acid alone was obtained from North Capel ilmenite (99% Ti and ~100% Fe) under conditions of 11 M HCl, pulp density of 4 g/L, 80 -110 °C and 2 h of leaching time. However, in the presence of a reducing agent, dissolution rates of iron and titanium increased in solutions of low acid concentrations and temperature.

The initial rate of ilmenite dissolution in hydrochloric acid under reducing conditions was found to be half order with respect to the concentration of reducing agent, which suggests the involvement of an electrochemical reaction. However, the chemical/electrochemical leaching studies of rotating flat surfaces of ilmenite, based on the parabolic rate law and kinetic study revealed the same diffusivity of protons through the product layer.

The batch leaching of ilmenite particles obeyed a shrinking core kinetic model, supporting the view that the proton diffusion through a product layer is the rate controlling step. The magnitude of proton diffusivity obtained using chemical leaching studies of rotating flat surfaces of synthetic or natural ilmenite or batch
leaching studies of different ilmenite concentrates agree reasonably well despite the difference in mineral composition.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Engineering and Information Technology
Supervisor(s): Senanayake, Gamini
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