|dc.description.abstract||The industrial importance of the chromium industry to South Africa is
emphasised by the fact that it is considered the largest chromite (chromium
ore) and ferrochrome (chrome-iron alloy) producing country in the world.
Although South Africa holds three quarters of the world's chromite ore
reserves, the chrome-to-iron (Cr-to-Fe) ratio of the local chromite ores is
relatively low (1.47 to 1.55), compared to other deposits in the world (2.6 to
3.5). Additionally, iron is more readily reduced than chromium. The
combination of these two factors implies that ferrochrome produced from
South African chromite ore contains 47-53% chromium. Current pricing
practises in the world ferrochrome industry dictate that ferrochrome producers
are only paid for the chromium content in the ferrochrome, which implies that
South African ferrochrome producers export a large percentage of their
product without any financial benefit. Research to improve the Cr-to-Fe ratio
is therefore essential to support sustainability of the local ferrochrome
Conventional beneficiation methods such as gravity concentration, magnetic
separation and floatation are unlikely to increase the Cr-to-Fe ratio, since both
iron and chromium are part of the same mineral phase, i.e. the spinel, which
requires structural dissociation. It has been proven on laboratory scale that
high temperature carbochlorination (CO and Clz atmosphere) can be used to
selectively remove iron from chromite. However, such methods are unlikely to
be implemented on an industrial scale due to health, environmental and cost
considerations. In light of this, an alternative approach to chromite
chlorination, avoiding the use of chlorine and other toxic gasses, was
investigated during this study. Since it was found that NaCI addition
significantly improved the effectiveness of carbochlorination of chromite, the
effect of adding only NaCI during high temperature treatment of chromite was
investigated. The material utilised during this investigation consisted of local chromite,
anthracite (source of carbon) and attapulgite clay (serving as a binder).
These materials were mixed in a ratio and subsequently milled to 0 90 = 75fJm
to represent materials and specifications similar to those used during
pelletisation of the chromite in the pre-reduction ferrochrome production
process. This mixture could also be used to generate a partially reducing
atmosphere (CO rich) during high temperature treatment, which was similar to
the reaction conditions utilised during carbochlorination. The abovementioned
milled mixture was pelletised into cylindrical pellets with a die set
and a hydraulic press.
This experimental investigation was based on a mono-variance procedure, in
that the four different variables investigated, i.e. maximum pellet treatment
temperature, exposure time, wt% NaCI addition to the pellets and the
atmosphere the pellets were exposed to, were varied one at a time during
experimentation. After each alteration of the afore-mentioned variables, the
Cr-to-Fe ratios, together with other parameters, were measured. Analyses
undertaken included Scanning Electron Microscopy, with Energy-Dispersive
X-ray Spectroscopy (SEM-EDS), Inductively Coupled Plasma Optical
Emission Spectrometry (ICP-OES) and cured breaking strength.
Although this investigation did not focus on the cured breaking strength of the
pellets, it is a very important industrial parameter and was therefore
measured. Results indicated that the addition of NaCI had a definite effect. In
both oxidising and partially reducing atmospheres the cured breaking strength
of the cured pellets increased up to 800°C exposure, whereafter it decreased.
This was attributed to melting of NaCI at 801 °C. In the oxidising atmosphere,
the cured breaking strength increased again at temperatures higher than
1 000°C, due to the formation of a thin oxldised layer on the outside of the
pellets, which could be confirmed by SEM analysis.
Fine, metallic-like crystals were noticed inside and on the lids of crucibles in
which pellets containing NaCI as an additive were cured at temperatures of 900°C or higher. SEM-EDS analysis and weight-ratio calculations revealed
that these crystals were pure FeO. This indicated that some iron might have
been liberated from the chromite spinel matrix. However, ICP-OES analyses
revealed that Cr-to-Fe ratios did not change significantly under any of the
experimental conditions (NaCI addition 5wt% to 15wt%, curing between 500'C
and 1200'C, and oxidative/partially reducing atmospheres).
The observed FeO crystals did not make any meaningful difference to the Crto-
Fe ratio of the chromite, but was of great academic interest as iron was
extracted from the chromite spinel. This indicated that it is not only the
formation of low melting point species, such as those proposed in previous
mechanistic studies of carbochlorination of chromite, but that molten NaCI
alone could also initiate the extraction of iron out of chromite. According to
the knowledge of the author, this is the first report of its nature in open
SEM and SEM-EDS analyses also proved that the addition of NaCI to the
chromite/carbon/clay mixtures enhanced the rate of chromite pre-reduction.
This finding was in agreement with earlier literature reports.
In conclusion, it can be stated that the addition of NaCI alone cannot alter the
Cr-to-Fe ratio of chromite during high temperature treatment. NaCI addition
did, however, have an effect on other important parameters i.e. initiation of
iron removal, cured breaking strength and the rate of chromite pre-reduction.
From the results and experience gained in this study, certain
recommendations with regard to possible future studies could also be made.
This included investigating i) other single component additives to possibly
alter the Cr-to-Fe ratio during high temperature treatment, ii) the effect of
industrially relevant additives such as CaO/CaC03, Mg03 and SiOz on the rate
of chromite pre-reduction and iii) the effect of different clays (e.g. attapulgite,
bentonite, etc.) on the rate of chromite pre-reduction.||