Microwave pretreatment of a low grade copper ore to enhance milling performance and liberation
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.