This study evaluates the effect of using high pressure grinding rolls (HPGR) in new comminution circuit designs in comparison to conventional comminution devices such as SAG and ball mills, in terms of energy efficiency. The total energy to achieve similar product sizes was measured directly in the process by using a new digital energy meter. The difference between the measured energy as a percentage of the conventional ball mill energy constituted the energy efficiency or energy saving of the circuit configuration.
The analysis is based on laboratory data that compares the performance of hybrid HPGR/ball mill circuits against that of conventional grinding mill circuits. The circuits are evaluated in terms of the total comminution energy, resultant mineral liberation and the eco-efficient effect of significantly reducing grinding media consumption. Three different ore types are studied, viz. a lead/zinc ore, a bauxite/aluminum ore and a platinum/chrome ore. Total circuit energy is compared using both Bond’s third theory and work index values, and a newly developed methodology of direct energy measurement.
In order to validate the results of the digital energy meter, measured energy results made in the Bond ball mill were compared to several other energy evaluation techniques that included mill power models, DEM and Bond energy “back-calculation” methods. Comparable results have confirmed that the digital energy meter can measure specific comminution energy directly.
Bond’s empirical method is also critically reviewed. The Bond method which uses an empirical equation and the well known Bond work index to predict comminution energy requirements appears to be based on 60 J/rev (Bond, 1952a 1952 b). This so-called mill energy defines an equivalent net energy in the Bond ball mill test to realise the same for a 2.4 meter wet grinding mill. Bond’s empirical equation results can thus be reproduced using 60J/rev and the mill test data.
Bond’s original paper published in 1949 stated that the net energy input to the laboratory scale ball mill is 93 J/rev (Bond, 1949). This is comparable to the digital energy meter measurement of 91.4 J/rev. Bond’s empirical relationship thus suggests that there is a built in scale factor of 0.645 which accounts for the differences between dry laboratory milling tests and a full scale (2.4m) wet grinding mill and is the reason for the stated 60 J/rev.
The digital energy meter was subsequently used to measure the energy consumption in the ball mill and compares this with the energy required to produce the same product size distribution in several hybrid HPGR/ball mill circuits. It was observed that on average a measured energy saving of 25-40% was achieved through the hybrid HPGR/ ball milling circuit when compared with the conventional ball mill circuit.
Results obtained from measuring energy directly are compared with results obtained by using Bond’s work index and third theory equation. When Bond’s method is used, a method frequently used in research, negative energy savings sometimes result. The Bond third theory and work index should not be used under these circumstances.
The characteristics of the liberated mineral products were measured using the JKMRC mineral liberation analyser (MLA). For each of the experimental circuit products, liberation characteristics concluded that the enhancement of liberated mineral through the use of HPGR technology was not founded. However, extensive particle micro-cracking was visible in the HPGR produces that were visible in the MLA backscatter images. This was not observed in the conventionally crushed products. The HPGR micro cracks are believed to be responsible for particle weakening and the resultant reduced milling energy requirements of ball mill processes that follow with the total circuit energy requirements significantly reduced.
Greater eco-efficiency can also be realised by reducing the consumption of mill liners and grinding media. Though the “dollar cost” of comminution is normally accounted for as a direct electricity expense in the process and is rarely considered for its overall energy cost or “embodied energy” of manufacturing the steel which amounts to up to 4-6 kWh/t. Eco-efficient and sustainable development initiatives are linked to “energy cost” and not always “dollar cost” savings. Rather the direct and indirect energy cost savings and the impact on the environment should be targeted.
Direct energy use in comminution processes is reviewed. It is shown that 0.56% (87 TWh) of the global net electrical energy consumption of 15,500 TWh per annum is used to crush and grind non-ferrous ores. Of this, 33% and 53% of the energy is required to process gold and copper ores respectively. This suggests that the HPGR should be targeted at gold and copper mining operations in the future to be effective in reducing carbon emissions. As such new eco-efficient flowsheets that use multiple HPGR’s in series could be considered. HPGR would be employed as primary comminution devices followed by small ball mills.
All of the data presented in this thesis is provided in the accompanying CD.