A low-cost titanium production process, the CSIR-Ti powder process, which aims to produce
titanium powder directly by metallothermic reduction of titanium tetrachloride with lithium, has been
under development at the Council for Industrial and Scientific Research (CSIR). Crude titanium
powder produced using the CSIR-Ti process is inevitably contaminated with by-products such as
lithium chloride, lithium and titanium dichloride. These by-products tend to become sources of
impurities in titanium powder, specifically oxygen and chloride impurities. The presence of oxygen
and chloride impurities has marked effects on the mechanical properties of titanium finished
products. Consequently, for the crude titanium powder to be rendered useful downstream, it must
be purified and the by-products reduced to concentrations specified in the commercial standards.
The present study was undertaken to examine whether acid leaching could be used to selectively
dissolve and prevent hydrolysis of the by-products—specifically excess lithium and unreacted
titanium dichloride in the crude titanium powder produced by the CSIR-Ti process. A further
objective was to determine whether a purified product that meets both oxygen and total residual
chloride content as specified by the standards can be achieved. The effects of key leaching
variables and their interaction were also investigated to gain fundamental understanding of these
effects on the by-products leaching behaviour.
A literature study to select a suitable lixiviant and to establish the aqueous chemistry of the byproducts
and their effect on the leaching conditions was undertaken. It showed that of the various
acids suggested in the literature, hydrochloric acid was the cheapest and that it was more suited
for the CSIR-Ti leaching process than nitric acid, due to the common ion chloride. This simplifies
the leachate purification process downstream. The literature study established that Ti(II) has no
aqueous chemistry but instead is oxidised to Ti(III) in solution. It was found that Ti(III) is easily
oxidised to TiO2+ by dissolved oxygen and water. However, the oxidation rate was slow in
hydrochloric acid solutions with the advantage that hydrolysis of the ions could be minimised and
the precipitation of the oxides or oxychlorides prevented. It was further revealed that the lithium
neutralisation reaction is highly exothermic, with the possibility of raising the leachate temperature
to 60°C, resulting in the contamination of the titanium powder particles by the oxide layer and
precipitated hydrolysis products.
Batch leaching tests were carried out using factorial design of experiments to investigate the effect
of initial hydrochloric acid concentration, which was estimated by varying the concentration between 0.032 M and 1 M; particle size, which was varied between −10 mm and +10 mm; and the
initial temperature, varied between 14°C and 30°C. The resulting data were modelled and
analysed using the analysis of variance statistical method. The solid residues were analysed for
oxygen and total residual chloride content. The solid residue was also characterised by scanning
electron microscopy (SEM) to examine the morphology of the leached particles. Leaching kinetics
model fitting was also conducted.
The statistical analysis showed that of the three factors investigated, temperature was the factor
with the most statistical significance on both the oxygen and chloride concentration in the purified
product, followed by particle size. The effect of acid concentration proved to be minimal, a
phenomenon attributed to low concentrations of acid-consuming impurities, specifically excess
lithium in the crude product. Thus, the two concentrations of hydrochloric acid investigated were
found to be efficient to prevent hydrolysis product formation.
Scanning electron micrographs revealed that crushing the crude product with a jaw crusher
occluded crude titanium pores, thus locking in some by-products in addition to the pores locked
by sintering during the metallothermic reduction. The observation showed that residual chloride
impurities in the purified product are not just a consequence of hydrolysis products but also byproducts
locked deeper in the pores of the product.
Based on the parameter ranges evaluated in the study, a product that satisfied both oxygen and
chloride standard specifications was achieved when the crude product was leached in both 1 M
and 0.032 M initial HCl concentrations, temperature of 30°C and particle size of +10 mm. The
combination of (−10 mm and 14°C) at all concentrations also yielded acceptable oxygen and
chloride content levels. Overall, it was concluded from the present work that purification of crude
CSIR-Ti product by leaching in dilute HCl is technically feasible. / Dissertation (MSc Applied Sciences (Metallurgy))--University of Pretoria, 2014. / Materials Science and Metallurgical Engineering / MSc (Applied Sciences (Metallurgy)) / Unrestricted
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:up/oai:repository.up.ac.za:2263/84127 |
| Date | January 2021 |
| Creators | Serwale, Matsie Rinny |
| Contributors | Sole, Kathryn, Coetsee, Theresa, Fazluddin, S., matsie.serwale@gmail.com |
| Publisher | University of Pretoria |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Rights | © 2021 University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria. |
Page generated in 0.0023 seconds