A study has been carried out on the Becher and Summit
Processes with the aim of understanding the mechanism and critical
parameters required for the production of a range of pure iron oxide
pigments, as well as high quality synthetic rutile from reduced
ilmenite .
The Becher Process currently produces a large quantity of
worthless mixed phase iron oxides. However, this study has shown that
the range of iron oxides formed are all derived from the
transformation of lepidocrocite (y-FeOOH) through the solution phase
in iron(II) solutions. The results of a kinetic study of the
transformation of lepidocrocite found that the rate exhibited an
induction period at low pH, was dependent on temperature and was
linearly related to log [H+] and log [Fe2+]. The rate determining step
was found to be the formation of suitable product nuclei, following
dissolution of the initial oxide at the surface of the crystal
lattice.
An electrochemical study of these reactions showed that the
product formed from the transformation of lepidocrocite was a function
of the solution potential and an experimental Eh-pH diagram was
constructed to predict the iron oxide phase produced from hydrolysis
and transformation reactions. The results from this fundamental study
were then applied on both a laboratory and plant scale to produce pure
iron oxide phases.
A modified Summit Process, involving the removal of metallic
iron from the porous reduced ilmenite matrix using FeCl 3, regeneration
of iron(III) and the production of pure iron oxide pigments from the
waste iron(II) chloride solution, was also investigated in detail.
A kinetic study of pure iron dissolution in iron(III)
solutions, comparing three electrochemical techniques and a standard
solution sampling method, gave consistent rate constants provided
allowance was made for the reaction with the proton. The iron
dissolution mechanism was found to be iron(III) diffusion controlled,
while the dissolution in HC1 was under mixed control. A study using
both pure iron and pressed reduced ilmenite discs found that acid
consumption could be minimised by the addition of citrate or by the
addition of A1 3+ or Fe 2+ , which are believed to block the adsorption
of the proton. It was found that iron(III)-citrate complexes inhibited
iron(II1) hydrolysis in the reduced ilmenite pores and enhanced the
purity of the synthetic rutile product.
A study of the oxidation of iron(II) by atmospheric oxygen
using copper(II) and activated carbon catalysts found that these
catalysts were inefficient for complete iron(III) regeneration. The
heating of carbon in the presence of cu2+ was found to enhance the
initial rate of iron(II) oxidation, however it is believed that
surface oxide redox couples formed on the carbon control the iron(II)/
iron(III) ratio in solution, and prevent complete iron(I1) oxidation.
The production of iron oxide pigments under the controlled conditions
afforded by the Summit Process, resulted in superior quality pigments
than are presently attainable from the Becher Process. However,
controlled ageing and crystal growth using waste lepidocrocite from
the Becher Process would result in similar quality pigments being
produced.
| Identifer | oai:union.ndltd.org:ADTP/221884 |
| Date | January 1990 |
| Creators | Christopher Ward |
| Publisher | Murdoch University |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://www.murdoch.edu.au/goto/CopyrightNotice, Copyright Christopher Ward |
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