A Methodology for the Design of Energy Efficient Comminution Circuits

Zeljka Pokrajcic (2010). A Methodology for the Design of Energy Efficient Comminution Circuits PhD Thesis, Sustainable Minerals Institute, The University of Queensland.

       
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Author Zeljka Pokrajcic
Thesis Title A Methodology for the Design of Energy Efficient Comminution Circuits
School, Centre or Institute Sustainable Minerals Institute
Institution The University of Queensland
Publication date 2010-08
Thesis type PhD Thesis
Supervisor Dr. Rob Morrison
Dr. Bill Johnson
Total pages 320
Total colour pages 75
Total black and white pages 245
Subjects 09 Engineering
Abstract/Summary Comminution, or particle size reduction, involving crushing and grinding, is a highly energy intensive process. This is largely due to the prevailing use of tumbling mills for grinding, where the nature of the particle breakage process is unconstrained and random. This indiscriminate nature of breakage contributes to the inefficient use of energy and may not promote liberation of valuable minerals in the ore. It results in high energy consumption and the generation of critical size material and ultra fines, both of which are difficult to process. The total demand for minerals is increasing to meet the requirements of the developing world and beyond. As a result, large reserves of high grade ore deposits have already been depleted. This has seen the need to mine and process ever lower grade, more competent and finely disseminated ores at much higher throughputs and finer product sizes. The inefficient nature of the comminution process coupled with ore reserves of declining head grade and the imminent introduction of a carbon trading scheme, suggests that the long term viability of the mineral processing industry depends on developing new methods and strategies to improve the efficiency of all processes. This will facilitate higher throughput rates at lower energy consumption. This is particularly important for the comminution process, as it is the most energy intensive part of a mineral processing circuit. A review of the current comminution circuit design practices reveals a general view that the comminution circuit design process is complicated and inexact involving the consideration of many interacting factors. A trend has been the repeated design and installation of large, low capital cost and uncomplicated crushing and grinding circuits. These normally feature one or two stages of crushing, followed by one or two large tumbling mills and a single classification stage. For most new comminution circuits, the general configuration and layout are more similar than different, where the size of the tumbling mills are dictated by the target product size and the throughput rate. The methodology for energy efficient comminution circuit design presented in this thesis challenges the popular trend of large and generic comminution circuits. The methodology promotes a better understanding and use of the ore properties and options to reduce energy consumption, including: • The inclusion of pre-concentration techniques at the head of a comminution circuit. The purpose of pre-concentration is to remove coarse size liberated gangue from later stages of processing and energy intensive particle size reduction. The selection of pre-concentration techniques is based on inherent ore textures and ore properties at a large scale. • The use of more efficient comminution equipment and autogenous grinding techniques. Through the re-distribution of comminution energy in a comminution circuit, the generation of problematic size fractions such as critical size material and ultra fines can be minimised or even eliminated. This aids the overall efficiency of the comminution process and minimises power usage for the same throughput and product size. It also limits the consumption of grinding media and the associated embedded energy resulting from grinding media manufacture. • The selection of the coarsest possible target product size(s) for a comminution circuit. This is achieved by thorough analysis and understanding of mineral liberation data of a coarse stream in a comminution circuit and the options and methods available to separate composite valuable mineral bearing particles from mainly liberated gangue in the stream. When these energy efficient strategies are combined in the design and configuration of a comminution circuit, energy savings are likely to be significant. However, it is difficult to accurately quantify the impact on energy savings and circuit operation when all three energy efficient strategies feature in the design of a comminution circuit. Instead, studies in this thesis have shown that the use of just one these strategies in the design of a comminution circuit can: • Reduce the total energy consumption by up to 42% (when compared to a conventional crushing and grinding circuit) through the use of more efficient comminution equipment and autogenous grinding techniques. This equates to grinding energy and grinding consumables savings in excess of $11M per annum for a typical 5Mtpa concentrator. It also can reduce the operating costs by up to a half. • Reduce the energy required for size reduction by 46% when liberated non-sulphide gangue is removed from a coarse stream in the comminution circuit and from further size reduction. This opportunity is identified by the thorough examination of mineral liberation data of this stream. The rejection of gangue at this point in the comminution circuit can provide a saving of approximately $3.5m per year in total grinding costs. It is noted that these energy savings can be at the cost of greater capital cost investment and, some may say, increased operating complexity. Incorporating the energy efficient strategies in a design methodology for comminution circuits can have a few significant implications: • It can increase size and value of the resource being treated as the rejection of liberated gangue by pre-concentration can decrease the mine cut-off grade. • It can improve performance of downstream separation processes as less gangue is present in the feed and the feed size distribution is narrower, allowing for better separation efficiency. • It can reduce the consumption of grinding consumables and minimise the associated energy used it their manufacture. • It may require higher capital cost investment but this is likely to be off set by the reduced energy and grinding media consumption as well as increasing energy costs and the introduction of a carbon trading scheme. This thesis develops and demonstrates a methodology, or a conceptual framework, for comminution circuit design which links together energy efficient design strategies which can reduce all forms of energy consumption and their cost.
Keyword comminution circuit design
mineral liberation
gangue rejection
ore sorting,
target product size,
energy efficiency
eco-efficiency
crushing and grinding
Additional Notes colour pages: 46 74 82 83 85 90 91 92 94 96 97 99 100 102 103 104 105 106 110 111 113 114 115 119 125 127 129 135 136 140 141 144 149 161 166 167 171 173 174 175 177 178 179 180 181 183 184 185 186 188 189 191 192 193 194 195 198 199 200 201 204 208 210 216 217 222 227 232 233 234 235 239 254 260 261

 
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Created: Mon, 09 Aug 2010, 13:53:08 EST by Ms Zeljka Pokrajcic on behalf of Library - Information Access Service