*E*ff*ect of Grinding Aids on Ore Grindability*

Considerably more research has been carried out on the use of grinding aids to improve the grindability of material in dry grinding operations than in wet grinding. This has occurred as a result of the amount of cement clinker needed to be ground daily in tumbling mills. The grinding of cement clinker to the desired fine size is one the greatest concerns in the cement industry and accounts for huge operation costs in term of energy consumption (Figure 10a). The use of grinding aids in the cement industry to improve the grindability and product throughput of the cement clinker dates to 1930 [81]. Most grinding aids are organic liquids, such as tri-ethanol amine, glycerol, alcohols, propylene glycol, organosilicones, diethylene glycol, and resins, etc. [27,81]. Grinding aids are added to the clinker during the final grinding stage (Figure 10b) to reduce the comminution energy. The summary of the findings from the literature on the use of some grinding additives for the improvement of cement clinker's grindability is as presented in Table 6.

**Figure 10.** (**a**) Energy distribution in cement production equipment (data source: [82]). (**b**) Schematic of dry cement production (modified after [82]).


**Table 6.** Improvement in grindability of cement clinker using grinding aids.

Apart from cement clinker fine grinding where chemical additives have shown a beneficial effect on the grindability of material mostly in dry grinding operations, they have also been used for ore comminution in the mining industry. The use of CaO (200 g/t) as an additive for the grinding of magnetite ore (40% Fe, quartz as gangue) has been demonstrated, and has caused the differential grinding of quartz from Fe content [86].

As per the reduction in comminution energy in the wet grinding operation of minerals, sodium silicate, aero801, and sodium oleate have been demonstrated for celestite [87]. At concentrations of 100 g/t and 1000 g/t, sodium silicate and sodium oleate improved the grinding of celestite. At a 10 g/t concentration, both sodium silicate and sodium oleate caused adverse effects (cumulative 80%, passing size is larger than referenced celestite ground at the same operating conditions) on the grinding of celestite at all grinding periods. Aero801 improved the grindability of celestite at 10 g/t, 100 g/t, and 1000 g/t; however, 100 g/t was reported as the appropriate concentration for the grinding of celestite [87].

Downstream Benefits, Economic Assessment, and Industrial Applications of Chemical Additive

Toprack et al. (2014) conducted an industrial scale study to investigate the appropriate chemicals that showed significant improvements in cement production [80]. In the study, six chemicals were investigated for the improvement in production and 28-day strength enhancement of cement (Table 7). The raw materials considered for the cement production were gypsum, limestone, clinker, and fly ash. An existing closed grinding circuit that consisted of a two-compartment ball mill and dynamic air classifier was used for the study. The dynamic air classifier allowed fine particles to move to the silo, while coarse particles returned to the mill for further grinding. The energy consumptions of the reference and the chemically treated samples were compared, as presented on Table 7. In terms of energy saving, all the six chemical additives used in the study reduced the comminution energy significantly. However, the organic and inorganic modified amines, hydroxylamine, and the mixture of polycarboxylate and amines had adverse effects on the 28-day cement strength [80]. An economic analysis of the whole system showed that cement production using any of the chemical additives used in the study was more profitable than cement production without the chemical additive (Table 7).

**Table 7.** Effect of chemical additives on the comminution energy and cement strength enhancement [80].


#### **4. Electrical Pretreatment**

Electrical pretreatment is among the most targeted technologies that has been studied and reported in literature, after microwave pretreatment [21]. Its principle is based on the passage of a high-voltage electrical pulse (HVEP) into the rock matrix to cause fragmentation. The variation in the electrical conductivity of rock causes the expansion and explosion of rock grains when a high voltage is passed into the rock matrix. The non-conductive part of the rock resists the current flow, which leads to a structural change due to HVEP. The expansivity of the mineral grain varies, and therefore micro cracks can be generated in different degrees. A high pressure is also built up within the rock matrix, such that the tensile strength of the rock is exceeded (electrical disintegration (ED) method). This pressure occurs as a result of the change of state (from solid to gas) of some particles within the rock when the electric current passes through the rock lumps [88]. These amount to the deformation and weakening of the rock due to the high temperature (about 104 K) generated by the charge displacement current [89,90]. Different technical terms are found in the literature to represent electrical pretreatment; however, there are slight variations in the procedures or parameters used in creating micro cracks. Some of the technical names are as listed in Table 8.


**Table 8.** Technical terms used for the electrical pretreatment [30].

Electrical pretreatment equipment consists of a high voltage (HV) power source, a sample chamber, and an HV pulse generator that has an arrangement of capacitors with a rectifier. The arrangement of the capacitors depends on the expected capacitance that gives the required voltage. The rock sample to be tested is usually put in water because it has a high dielectric strength and creates a plasma which prevents electrical discharge outside the rock. The rise in voltage is the same for all techniques except electrohydraulic disintegration (EHD), which may result in a lower energy efficiency [30]. For ED, an HV pulse is directly passed into a rock lump that has been immersed in water through the electrode, which makes this procedure quite different from other approaches that require the dipping of the electrode into water in order to generate a shock wave [91]. Electrical pulse disaggregation (EPD), electrodynamic disintegration (EDD), and EHD require water, but more energy is needed for the EHD method and the deformation is generally due to exceeding the compressive strength of the rock [92]. There are divergent opinions on the classification of electrical pretreatment. Some researchers are of the opinion that electrical fragmentation is divided into two categories—one that requires water for breakage and the other without water [93,94]. In this regard, EPD, EDD, and EHD belong to the same group, while ED is the second type. Another view is that it is quite difficult to distinguish between the methods because the electrode gap that is usually associated with EPD, EDD, and EHD may not occur due to rock shape variation [91]. In this case, the classification is based on the voltage rising time.

### *E*ff*ect of Electrical Pretreatment on Ore Comminution*

The investigation of EPD and ED to be used in mineral processing was started in the early 1970s and research continued until 2002, when an EPD-suitable device (CNT EPD Spark-2) was designed by the research team of CNT Mineral Consulting Inc. (Ottawa, ON, Canada) [95]. The machine has been used to liberate undamaged diamond crystal from the host rock and emerald from quartzite. The good thing about the machine is that the original shape of the crystal is retained, unlike in conventional crushers that can deform the crystal or break it into fine particles. The ED technique was used to disintegrate granite, copper, kimberlite, and nickel sulfide rocks [89]. The feature associated with ED is that the disintegration occurs at the grain boundary without causing unnecessary fine products and liberating valuable minerals [89]. This can reduce the amount of ore to be crushed in a conventional crusher. This method is even more appropriate to be referred to as secondary blasting or pre-crushing, since it is more suitable for larger rock sizes (boulders).

A comparison of ED with a roll-crusher was performed using coal feeds of different specific gravities (1.35–1.45) and size distributions (4.0–5.6 and 5.6–8.0) [94]. The cathode and anode electrodes of the ED device are stainless steel (with a 2 mm sieve size) and brass disks, respectively. The coal samples (Nantun, China) were crushed and sieved to obtain different size distributions, as earlier stated. Representative samples from the two size distributions were mixed (1:1) to get a 200 g feed sample. A total of 100 g of the sample was fed into the sample chamber, such that there existed five layers with 200 g each. The initial voltage supplied through the cathode was 16 kV, and the value was increased up to 56 kV before the sample disintegration occurred. The voltage and current waveforms were studied using the oscilloscope. The ED test was repeated 60 times to arrive at good conclusions. A representative sample prepared as that of the ED test was crushed using a roll-crusher, and a size distribution analysis of both test methods was performed. The results of optical images showed that rough and smooth surfaces were generated for the ED and the roll-crusher, respectively. In addition,

mineral matter was exposed in the case of the ED test products, which indicates that the disintegration occurred at the grain boundaries [94].

The EPD technique was used to liberate minerals from copper (New South Wales, Australia), gold–copper, and lead–zinc (Queensland, Australia) ores [96]. A sample size in the range of 12–45 mm (3600 kg) was collected from mine sites and each ore type was divided into two (one half for the EPD test while the other half was for the conventional crusher test). The products from the two tests were used to carry out a standard bond rod mill test. The closing screen aperture considered for the test was 1.18 mm. The results showed that the percentage changes in the work indices (improvement) between the EPD and conventional crusher for the copper, gold–copper, and lead–zinc ores were 18%, 24%, and 6%, respectively [96]. Similar research was carried out using a platinum group metal ore (South Africa) and samples from Australia, as earlier mentioned. Coarser products were generated in the EPD method, with less fine materials and valuable minerals liberated than that of the conventional crusher [97].

The HVEP technique was used to investigate the liberation of magnetite ore using a −2 mm (200 g) representative feed sample. The sample treated with HVEP and the untreated one were ground under the same grinding conditions using a rod mill. An improvement of 13.19% in the liberation of iron minerals was achieved using HVEP when compared to the untreated sample [98].

Recently, a high-voltage electric pulse crusher (HVEPC) was designed and used for the crushing of phosphate ore [99]. The size fractions of the phosphate ore used in the study ranged between −75 and 50 mm. The bond crushability index of the phosphate ore reduced by 10.6% (compared with the conventional crusher) when the HV pulse-specific energy ranged between 3 and 5 kWh/t. The effects of capacitance, voltage, and PSD of the sample on crushing using the HVEPC were also investigated. It was found that an increase in the capacitance and voltage lead to an improvement in the crushing of the phosphate ore at size ranges of −19 + 12.5, −12.5 + 6.35, and −6.35 + 3.35 mm. The summary of some of the laboratory experiment successes of electrical pretreatment are presented in Table 9.


**Table 9.** Summary of the electrical pretreatment.

Downstream Benefits, Economic Assessment, and Industrial Applications of Electrical Pretreatment

Recently, a pilot scale HV pulses (HVP or EPD) testing machine (Figure 11) developed by SELFRAG AG (Kerzers, Switzerland) was used for the investigation of the particle weakening behavior of ores (gold–copper ore, New South Wales, Australia; iron oxide copper–gold (IOCG) ore, South Australia; and hematite ore) [100]. The machine had a setting system that allowed capacitance and voltage regulation. The pulse energy (ranged 50–200 kV) could be kept constant when varying the voltage or capacitance of the machine. The machine could process ore up to 10 th−<sup>1</sup> (3–10 th<sup>−</sup>1), depending on the pulse energy, PSD, and density of the ore. The PSDs of the tested ores were 22.4–26.5, 31.5–37.5, and 45–53 mm. It was found that the higher the specific energy of the HVP machine, the better the breakage characteristics measured using the fineness indicator (*t*10), which connotes a cumulative percentage passing size equivalent to one tenth of the original size before the pretreatment and pre-weakening assessments. It was reported that the HVP pretreatment caused a reduction in the competency of gold–copper and IOCG ores by 81.7% and 131.8% respectively, while that of hematite increased by 40.7% [100]. However, an economic evaluation of this method was not performed in the study, which calls for further research. Nevertheless, the HVP machine produced at Julius Kruttschnitt Mineral Research Centre (JKMRC, Indooroopilly, Queensland, Australia) in 2009 consumed considerable energy (1–3 kWh/t) during the ore pretreatment [96]. Safety due to high-voltage generation for rock weakening has usually being

associated with electrical pretreatment; however, this has been put into consideration in the JKMRC machine, and electromagnetic shielding against high electric voltages has been introduced [101]. The electrical method has been applauded for its reduction in the ore to be comminuted; this can assist with rejecting gangue after treatment (pre-concentration) [102,103], and for the reduction in fine particles in the final product after comminution [104]. The former has been suggested to be performed at the mining site so that the haulage cost can be reduced and the rejected can be used as back filling [105]. With that approach, the energy cost can be lowered not only for haulage but also for comminution and processing. The inclusion of the HVP machine in the mining cycle also has environmental benefits, as tailing can be reduced since some wastes would have been rejected from the mining site. A simulation of this approach suggested that 5 kWh/t can be saved for 2000 t/h copper–gold operations [105]. Parker et al. (2015) discussed that electrical pretreatment (electro-comminution) has the potential to improve mineral liberation, which may increase the recovery using the floatation method. In the study, the authors compared the surface chemistry of the untreated and electrical treated samples and found that the latter improved the surface chemistry of the ore as well as the liberation of chalcopyrite in the coarse size range [106].

**Figure 11.** High voltage pulse pilot scale testing machine at Kerzers, Switzerland (reproduced with permission) [100].
