Effect of lime additions and bulk chromium content on chromium deportment in smelter matte-slag systems
Date
2010-03
Authors
Du Preez, Rudolph C.
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: More mining houses are turning their attention to the processing of ore mined
from the UG2 reef. This is mainly due to the depletion of the Merensky reef
or the low availability of ore from the Platreef. With the higher UG2 ore
concentration in the process feed, companies experience more problems with
the processing of the ore due to its high chrome content. Although there are
various possible solutions to the problems experienced in the processing of UG2
ore, very little of thise solutions are actually implemented in the industry.
When smelting chrome-bearing ore, the chrome can go to any of three
phases: matte phase, spinel phase or glass phase. If it reports to the furnace
matte phase it can lead to problems in the down stream processing. When
chrome forms part of the spinel phase it forms a solid, refractory-like material
which, to an extent, is an unwanted material inside the furnace. Lastly the
chrome can report to the glass phase (liquid phase) which is the more desireable
phase to which chrome should report to since it will have little or no impact
on downstream processing.
There were four main objectives for this research project namely to conduct
a literature review to understand the problem of chrome in the smelting
process, to do an experimental simulation of a matte and slag phase in one
crucible, to interpret the experimental results and compare the experimental
results to thermodynamic predictions obtained using FactSageTM.
From literature the following aspects were found to be important:
Silica structures
Research has shown that silica incorporates different metal ions into its
structures. When the metal ions are incorporated into the silica structures
they are seen as part of the glass phase. By incorporating cations such as
chromium into the silica structure the structure is able to maintain its neutral
electrical charge.
Basicity of slag
Oxides once melted will either donate or accept oxide ions. The group of
oxides that are in excess will determine whether that specific slag is acidic or
non acidic. This is important to our study since the basicity can determine
the stability of the spinel phase. Partial pressures
The partial pressure of the system is important since it forms part of the
equilibrium constant calculations - meaning that partial pressures determine
the stability of certain species. Currently different arguments exist as to how
the sulphur pressure inside a molten bath is maintained. What is important,
however, is that for the system in this research project a log oxygen partial
pressure of -8 and a log sulphur partial pressure of -3.5 was chosen. These
values were based on previous research done on PGM and copper smelting
processes.
Chrome deportment
Previous research on chrome deportment shows a relationship between temperature
and the amount of chromium dissolving into the glass phase as well as
an increased chromium(II)oxide solubility (when compared to chromium(III)oxide)
in a silicate melt. Regarding slag chemistry and chrome deportment very little
work has been published for the system found in the PGM industry but it is
mentioned that by adjusting certain slag additives (alumina, lime and silica)
the chrome deportment can be manipulated.
The scope of this project was to investigate the effect of chromium, lime
and silica on chrome deportment. More specifically, the effect of lime. The
reason being that lime was originally added as 10%wt of the feed stream in the
processing of Merensky ore to act as a fluxing agent. With the new furnace
design and higher power densities the slag are maintained at 200oC to 350oC
higher than when Merensky ore was smelted. With these higher slag temperatures
fluxing agents will play a smaller role meaning that lime additions
become less important. Literature studies also showed that additional lime in
a slag system can stabilize the spinel structure (which is an unwanted phase).
Removing lime would be advantageous out of a chrome presepective as well as
an economic point of view.
The research was conducted in three sections namely the determination of
the time required for this particular system to reach equilibrium, the investigation
of different additives on chrome deportment using a controlled atmosphere
and synthetic slags and, lastly, a comparison of the experimental results obtained
to thermodynamic predictions.
For the equilibration studies reaction time periods of 4,7,11 and 16 hours
were used. From the results it was found that the alumina crucible dissolved
into the glass phase continually. This indicated that equilibrium was not
reached. However, a reaction time, rather than an equilibrium time, was chosen
where the species and phases had enough time to react. This was based
on literature, on observations of species diffusing between the matte and slag
phase as well as on two-point analysis (diffusion gradients). A reaction time
of nine hours was chosen With the reaction time fixed, the effect of different slag additives on chrome
deportment was investigated. For the addition of chrome it was seen that an
1.5%wt increase in the starting material increased the chrome content of the
slag phase by 0.025%wt. For the same increase in chromium in the starting
materials the chrome content of the spinel phase increased by 2.1% indicating
that chromium has a tendency to report to the spinel phase. The increase
in chromium had a minor effect on the chrome content of the matte phase,
however, since the chrome content only increased from 0.025%wt to 0.028%wt.
Plotting the results showed that increasing the lime in the starting material
decreases the chromium content in the glass phase. For the 39%wt silica system
the chromium content in the glass phase decreased from 0.75%wt to 0.46%wt
for an increase in the lime content from 1.7%wt to 7.3%wt. The same trend was
seen for the 33%wt silica system. For the spinel phase an increase of 1.27%wt
was seen when the lime content of the starting materials was increased from
1.7%wt to 10.1%wt for a 33%wt silica system. The same increase in lime
increased the chrome content of the matte phase from 0.03%wt to 0.06%wt for
a 33%wt silica system.
Silica also proved to affect chrome deportment. Increasing the silica content
of the starting materials from 25%wt to 39%wt increased the amount of
chromium in the glass phase from 0%wt to 0.46%wt for a 10%wt lime system.
The same effect is seen for a lower lime content except that more chromium
were incorporated into the silica structure. A silica increase from 32.4%wt
to 39%wt resulted in a chromium decrease from 5.2%wt to 0% in the spinel
phase for a 10%wt lime system. The same trend was seen for the 1.5%wt lime
system. An increase in the silica levels lead to an increase in the chrome level
of the matte phase. When silica is increased from 32%wt to 39%wt the chrome
content of the matte phase increased from 0.06%wt to 0.07%wt.
The last part of the research project entailed the comparison of the trends
observed with the experimental results to trends obtained from thermodynamic
predictions. FactSageTM is a program that uses model equations to predict
the Gibbs free energies for different phases. The program is therefore also able
to predict the amount of different phases present at equilibrium. This is called
thermodynamic ”optimization”.
In section 6 trends that were observed from FactSageTM results are compared
to the trends found in the experimental results. It is important to note
that it is only trends that are evaluated and not actual values since FactSageTM
calculations are for a system that is at equilibrium and (as explained above)
this system is not at complete equilibrium. The comparison however was good.
Several trends found in the experimental results were confirmed by the results
from FactSageTM. These included the relationships of chromium fed versus
chromium spinel, lime fed versus chromium spinel, lime fed versus chromium
in glass, silica fed versus chromium in glass, silica fed versus chromium in
spinel and silica fed versus chromium in matte. To conclude, slag additions can be used to manipulate chrome deportment
to an extent. Secondly, FactSageTM can be used for thermodynamic predictions
but a proper understanding as well as some form of validation of the
specific system investigated is still needed.
Due to time constraints and the difficulty of experimentally simulating this
multi-phase system in the lab, only a few parameters were investigated. In
order to obtain a more complete understanding of the system the effect of
partial pressures and temperature should also be investigated.
Description
Thesis (MScEng) –Stellenbosch University, 2010
Keywords
Matte phase, Smelter systems, Lime, Chrome bearing ore, Dissertations -- Process engineering, Theses -- Process engineering, Ore-dressing, Smelting, Pyrometallurgy, Chromium