Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2006. / As easy to mine high grade ore bodies are being depleted, many mining industries are
experiencing an increasing need to process lower grade ores, and thus the high costs
involved in the mineral recovery from these ores (of which comminution energy costs are
a large component) are of major concern. It has been estimated that up to 70% of the total
energy consumption in mineral processing is used up by comminution processes, which
characteristically may have efficiencies of less than 0.1% in terms of the transfer of
electrical energy into particle breakage. In many cases, very fine grinding is required to
liberate the valuable inclusions in such low grade ores, which also leads to slimes losses
of valuable minerals due to the inefficiencies of recovery methods in the ultra-fine size
ranges.
For many years the use of thermal pretreatment has been suggested as a way to decrease
the costs of size reduction, and improve the liberation of valuable minerals in ores to aid
later beneficiation technologies, but it was not until exploration into the use of
microwaves to selectively heat only some of the minerals in ores, that this form of
treatment became economically viable.
A low grade copper ore from Palabora was subjected to microwave treatment and then
tested for ore strength in a laboratory rod mill, using the developing cumulative size
distributions of the rod mill products with time to quantitatively determine the effects of
microwave treatment on ore strength. It was seen that after microwave treatment the ore
responded more readily to milling, producing a finer grind than for untreated ore at every
measured time interval of milling. From this data, comminution models were created to
describe the grinding of this ore in various flowsheet simulations.
An investigation was also performed to determine the effect of the application of
microwave treatment on the liberation of minerals, due to the preferential breakage induced along grain boundaries during the selective thermal expansion of certain mineral
inclusions in ores during microwave treatment. To ensure consistency between results for
microwave treated and untreated material, it was decided to use the same grinding time
for both when preparing ore for the next stage of testing. A grinding time was chosen
which would produce an 80% passing size of 800 μm for the microwave treated ore. This
time was determined from the previous grinding tests and was found to be approximately
16 minutes. After particle size classification of the mill products through sieving, a size
range suitable for gravity separation processes was chosen for sink-float testing, with the
aim of investigating whether microwave treatment had liberated enough gangue material
at large particle sizes to offer the possibility of removing this hard gangue material early
on in the process, before costly fine grinding is required. XRF analysis of the products
showed little difference in recoveries of gangue material to the floats between treated and
untreated material, and that while most of the copper reported to the sinks, that some of
the copper was always entrained in the floats. These losses of valuable minerals to the
gravity tailings will lead to overall losses in copper mineral recovery from the plant.
QEMSCAN® analysis showed that there was a significant increase in mineral liberation
in the size ranges associated with flotation as a result of the microwave treatment. An
increase in liberation of the copper minerals which are easily recovered by flotation (i.e.
chalcopyrite, cubanite, bornite, chalcocite and digenite) of 8.4% over that of the untreated
ore was seen. This indicates that significant increases in copper recovery are possible
after microwave treatment, and also that less fine grinding is then required to extract the
valuable minerals from the ore, which leads to a reduction in loss of these valuable
minerals to slimes.
Palabora Mining Company supplied enough data on their plant operations from 1989 to
enable models to be built to describe the operation of the mills and classifiers used in
their comminution circuit. This data, together with the work performed to compare the
performance of microwave treated and untreated Palabora ore in both milling and
liberation (which allowed for basic recovery models to be built), allowed flowsheet
simulations of the plant operations. Simulations of the plant after the addition of microwave pretreatment of the ore showed that the total energy used in comminuting the
ore (including that of the microwave treatment) to the correct size distribution for mineral
recovery by flotation were reduced by 19% from that required for untreated ore, and was
mainly due to reductions in the circulating loads over the mills. By exploiting the greater
milling capacity allowed for by these lower circulating loads, it was shown that it was
theoretically possible to obtain increases of up to 46% in maximum throughput after
microwave treatment, while retaining the same final grind size in the feed sent to
flotation as is required for untreated ore.
The addition of gravity separation processes to remove liberated gangue material from
the comminution circuit early on, led to further savings in energy and also grinding
media, and also decreased the requirements for flotation reagents and smelter fuel later on
in the flowsheet. Unfortunately, the losses of entrained copper to the gravity separation
tailings were such that overall economic losses were incurred by the operation. It was
concluded that when dealing with low grade ores, only the implementation of very
efficient and mineral specific separation technologies could make the removal of gangue
material at large particle sizes (i.e. > 1 mm) viable.
Economic analyses based on the simulations of the plant under various operating
conditions showed potential increases in plant profitability after the addition of
microwave pretreatment of the ore before milling, and were reported using net present
value (NPV) calculations for the plant over a 10 year period with monetary values
discounted at 20%. When operating under the same conditions and throughput as in the
1989 data provided by Palabora Mining Company, an increase in the NPV of the plant of
23% over that for the reported operation was seen after the addition of microwave
pretreatment, and an increase of 72% in NPV given a 10% increase in throughput which
is made possible by microwave pretreatment of the ore. In real money terms, after 10
years of operation the increase in NPV of the plant with the addition of microwave
pretreatment of the ore was seen to be around R259 million (under the conditions
reported for the plant operation in 1989), and around R795 million if the 10% increase in
throughput which is only made possible by microwave pretreatment is realized. Current conditions at Palabora are very different from those supplied by the plant for the
operation in 1989, however, as the mining operation has since been moved underground
resulting in the throughput of the plant being greatly reduced, with the consequence that
the plant is currently operating at a loss. Palabora mining company posted a net loss of
R158 million over the 6 months leading up to June 2004, while an economic analysis of
the proposed addition of microwave pretreatment of the ore at an increased throughput of
10% made possible by this treatment, indicated that a loss of only R138 million would
have been incurred over the same 6 month period had this been implemented. Thus, while
benefits from the introduction of microwave pretreatment of the ore before milling can
still be seen under the operating conditions of the plant during the time period
investigated, these alone would not have be able to bring the plant to profitable operation.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/1832 |
| Date | 03 1900 |
| Creators | Scott, Grant |
| Contributors | Bradshaw, S. M., Eksteen, J. J., University of Stellenbosch. Faculty of Engineering. Dept. of Process Engineering. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
| Format | 1114362 bytes, application/pdf |
| Rights | University of Stellenbosch |
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