Development of methods to assess the efficiency of comminution devices and operations.

For the past 50 years, the Bond Operating Work Index method has been the preferred measure of energy efficiency of comminution processes. This method calculates the specific energy from a given feed 80% passing size to a given product 80% passing size. Because the Bond method relies on the condition specified by Bond that the slope of feed and product size distributions are parallel on a log-log scale, the method usually works well for ball mills and rod mills but does not work well for devices which do not produce a parallel product size distribution. An error of more than +/- 20% is expected when applied to AG/SAG mills. The variation between the slope of the feed and product size distributions has been found to be the reason why the use of a single point (80% passing) is not ideal for AG/SAG mills. The Bond method estimates the power drawn by the mill motor (direct energy). In an environment where sustainability and greenhouse gases are becoming an important factor, other energy input such as energy related to grinding media consumption and other wear parts and material transport within the grinding circuit (indirect energy) should also be included in the assessment of comminution energy efficiency. Three new methods to assess the energy efficiency of comminution processes have been developed and tested. These methods were devised to compare the utilization of energy during the comminution process. The first method - which is the Operating Efficiency method - estimates energy efficiency based on the new surface area generated. Traditionally, it has been assumed that the total surface area per unit mass increases rapidly with decreasing particle size which means that the total surface area per unit mass may never come to a limit. However, with modern laser sizing equipment and computer modelling, a surface area analysis on cement samples with particle size ranging from 37.5 mm to 1.4 microns has shown that the total surface area reaches a well-defined limit with decreasing size. The surface area occurs between 10 microns and 100 microns. Hence, the energy efficiency of comminution process can be assessed by estimating the energy required to generate material in the ultra-fine regions. A marker size can be used if it represents more than 80% of the total surface area in the final comminution product. For typical mineral grinding processes, 75 microns is a suitable marker. However, in coarse grinding or very fine grinding, a coarser or finer marker size may be appropriate. Similar to the Bond operating work index, the comminution efficiency can be estimated by comparing the energy calculated from plant data and the energy required to achieve the same amount of grinding from laboratory test. At present, there is no specific test to estimate the energy required to generate one tonne of new material in minus 75 micron fraction in the lab. However, the detailed information available from a Bond grindability test can be used. This new method has been found to work well for many different comminution devices including the IsaMill and the Tower mill. The second method is the efficiency method which is based on the comparison of energy consumed in comminution devices/mills with single particle comminution. The Whiten crusher model has been used for many years to estimate the efficiency of industrial crushers. The crusher model uses a classification function and a breakage function determined from laboratory test to calculate the amount of material selected for breakage in the crusher. This information together with an energy-size reduction characteristic determined from the JK drop weight test is used to calculate the comminution energy required to crush material selected for breakage in the mill. A new breakage function developed by Shi and Kojovic (2007) was used in this model as it was found to provide a more accurate prediction of crusher power draw. This size dependent equation also allows the crusher power prediction approach to be extended to other comminution mills without the limitation of parallel size distributions. Using the Perfect Mixing model (Andersen, 1988) and the new breakage model equation, the efficiency of different comminution devices was calculated as follows: crushers (70 to 80%), ball mills (25 to 35%) and SAG mills (around 40%). The third method extends assessment of energy efficiency to the energy related to grinding media consumption and their environmental effects as materials consume substantial quantities of energy during their manufacture. The eco-efficiency method links both direct energy (energy to run the mill) and indirect energy (energy required to manufacture grinding media) to the environmental impact. This method employs the concept of a Life Cycle Assessment. The methodology defines the system boundary and limits of the comminution circuit to be assessed. Using data inventory of the direct and indirect energy consumption of the circuit, the impact assessment of the energy to the environment is calculated. This method provides a more balanced assessment of the energy consumed by a comminution circuit. Typically the indirect energy consumption is 40-60% of the direct energy consumption