**1. Introduction**

The mineral industry consumes significant energy from the national energy demand. The energy consumed in mining as a percentage of national energy consumption in countries such as Australia, USA, Canada, China, Saudi Arabia, and South Africa are 11 [1], 12 [2], 8 [3], 10 [4], 8 [5], and 17.4 [6], respectively. The sources of the energy (for the USA) and the cost trend are presented in Figure 1. In the mineral industry, comminution accounts for the largest share of energy consumption [7,8]. Comminution involves two-unit operations—crushing and grinding. Grinding is always a great concern because it accounts for about 50–70% of the total energy consumption [7,8]. Researchers have elaborately clarified why energy in grinding is so high. The main reason is that as material becomes smaller, more energy is required to create new surfaces. Unfortunately, most of the applied energy is usually lost as noise, heat, and electromechanical during the grinding operation; only about 1% is used for the materials' grinding [9–12]. Hence, there is a need to look for means of rationalizing energy consumption in the grinding process (Figure 2a) [12,13]. Comminution energy depends on minerals to be comminuted as per their type, amount, and properties. Moreover, it relies on the used machine and its operation parameters and skills of operating manpower [14]. The trend of comminution energy (Canada mining industry) between 1990 and 2015 is as presented in Figure 2b. The mean and standard deviation of the energy consumed within these years are 70.71 petajoules and 7.14, respectively. The comminution energy increased by 6.97 petajoules within the period, with a possible increase in future due to declined ore grades and the increase in demand for minerals [14].

**Figure 1.** (**a**) Mining operations energy sources (data sources: [13,15]). (**b**) Energy costs trend [16].

**Figure 2.** (**a**) Energy-saving opportunity for energy intensive process in mining [13]. (**b**) Comminution energy trend (Canada) between 1990 and 2015 (calculated based on 50% of total mining energy) (data source: [17]).

To ameliorate the situation, early studies have been tailored towards the optimization of grinding circuits. Some of the approaches are the optimization of the mill load, filling ratio [18], ore to media ratio, media size distribution [19], mill length to diameter ratio, and adding mill riffles [20]. In the last few decades, the pretreatment of ore prior to comminution has been proposed [21]. Pretreatment denotes an independent operation performed on ores prior to grinding. The main objective of pretreating ore is to create intergranular cracks so that the grinding operation can become easier, which can lead to a reduction in the grinding energy [22–26]. Among the pretreatment methods are thermal (via furnace, microwave, or radio frequency), chemical additives, electric, magnetic, ultrasonic, radio frequency, and bio-milling. These methods can be categorized as presented in Figure 3. Some of the pretreatment methods have showed positive results towards the reduction in ore's grinding energy requirement but are yet to be commercialized. Previously, some review works had been performed that focused on a single method—chemical additive [27], thermal via furnace [28], thermal via microwave [29], and electric pretreatments [30]. Somani et al. (2017) provided a combined review of thermal, ultrasonic, and electrical pretreatments [21]. In this work, a holistic review of well-known pretreatment methods is attempted in order to give a concise overview of the development in comminution energy reduction and provide for future research needs.

**Figure 3.** Classification of pretreatment methods.

#### **2. Thermal Pretreatment**

Thermal pretreatment is a method that applies heat energy through conduction, convection, or radiation to create granular or inter-granular cracks in rocks [21]. When the temperature of the material changes, the physical properties of the material are altered, which leads to the displacement of some grains leading to fractures. Basically, there are two means of applying heat energy to rocks—through conventional (furnace) and electromagnetic (radiofrequency and microwave) or dielectric heating.

#### *2.1. Thermal Pretreatment via Furnace*

The use of a furnace to heat rock is usually referred to as conventional heating. When rock is subjected to heating, stresses are developed within the rock matrix that leads to cracking as a result of thermal expansion and contraction. Thermal pretreatment was introduced in mineral comminution with the prime objective to increase liberation and reduce the energy used in the process [31]. Since rock is an aggregate of minerals, the heating of rocks causes crystals to expand in different orientations depending on the mineral constituents. The expansion of crystals may cause the internal cracking of rocks, which can reduce the competence of rocks before comminution. Inter-crystalline may open up around 200 ◦C upward, which can increase the existing discontinuity and create new ones in granite [32], gypsum, and celestite [33]; however, this may not be the case for all rock types because ore texture, crystallinity, and size has a significant effect on ore response to thermal pretreatment [34,35]. In addition, a crystal shape may contribute to the crack pattern when grains are displaced due to the expansion of the minerals. The extent of the cracking of rock in different directions depends on the thermal confined stress at any direction. This approach has been used in different studies to create cracks in rocks since early work in the 20th century [36,37]. Omran et al. (2015) demonstrated that, as the temperature of the furnace increases (400 ◦C and above, at 1 h residence time), the cracks developing in the iron ore matrix increase (approximately 10% increase for every 100 ◦C) and consequently, the particles' liberation is improved (Figure 4) [34]. However, the maximum temperature after which increasing the temperature has no effect on the particles' liberation from the iron ore still needs to be established. The thermal pretreatment of rock is of interest in rock drilling and excavation, underground storage, nuclear waste, deep petroleum boring, tunneling, dam, reservoir [38], geothermal [39], archeological study [40], building construction [41], and ore comminution and beneficiation [42].

#### Effect of Thermal Pretreatment (via Furnace) on Ore Grindability

A review of early work on conventional heating as related to rock grindability was discussed by Fitzgibbon and Veasey (1990) [28]. It was reported that the grinding resistance (grindability) of rock can be lowered using a thermal pretreatment approach. Thermal alteration in rocks has different effects, such as structural damage, phase-change, decomposition, and desorption [43]. The anisotropic nature of minerals causes thermal stress concentrations at points and grain boundaries, which may lead to the fracturing of individual mineral grains when heat is applied to rocks [43].

**Figure 4.** Effect of thermal treatment via furnace on iron ore grinding (100 g, 2.45 GHz, and 1 h) (data source: [34]).

The effect of thermal pretreatment on the grindability of celestite and gypsum was studied [33]. The rock sample of size fractions −1.168 + 0.6 mm (500 g) was ground using a ceramic ball (20 and 25 mm diameters, 588 g). The grinding time ranged between 5 and 30 min at 5 min intervals. The work indexes of untreated celestite and gypsum calculated using the Hardgrove method were 6.76 kWh/ton and 5.18 kWh/ton, respectively [33]. It was reported that there was no significant effect of heat pretreatment on celestite's grindability within 200 ◦C, while that of gypsum decreased by 22.8% within the same temperature range. This result was expected because gypsum, being one of the hydrate minerals, can decompose easily due to the removal of water molecules when exposed to temperatures within 10–250 ◦C. When dehydration occurs, gypsum (CaSO4·2H2O) may transformed to plaster of Paris (hemihydrate mineral: CaSO4·O·5H2O), which usually leads to structural failure [33,44]. This shows that the existence of water molecules in the rock structure may help ore response to thermal treatment. Conversely, celestite required higher temperatures up to 1140 ◦C before effective changes could be observed [33,45].

Recently, the impact of the thermal pretreatment of manganese ore (selected from Qom mining site, Iran) was investigated [44]. Different size fractions (−1.7 + 1.18 mm, −1.18 + 0.6 mm, −0.6 + 0.3 mm, and −0.3 + 0.15 mm) were separately investigated to determine the appropriate size range in which thermal pretreatment can cause a significant effect on the breakage characteristic of manganese ore. A sample was thermally treated in a furnace for 60 min at 750 ◦C. The treated sample was ground in a ball mill (diameter; 20 cm, height; 25 cm) using ball charges of different sizes of 8.869 kg. An untreated sample of the same mass was ground under the same grinding conditions and the results were compared. The specific rate of breakage approach was adopted in the study, using the first order kinetic model. The slope of the semi-logarithm curve for mass retained against grinding time was used to estimate the breakage characteristic of the ore. The results of the breakage characteristics of treated (thermal treatment) and non-treated manganese ore were compared. An improvement in grinding rate of 37% was obtained for the size fraction −0.30 + 0.15 mm. This result may be attributed to the fact that heat better penetrated to the lower size range than the higher one when treated under the same conditions. The results from different studies related to the pretreatment of ore via furnace are presented in Table 1.


**Table 1.** Summary of thermal pretreatment via furnace on the grindability of minerals/ores.

Downstream Benefits, Economic Assessment and Industrial Applications of Thermal Pretreatment (via Furnace)

Despite improvements in grindability that may reach up to 45% for some ores (Table 1), thermal pretreatment via furnace has neither been piloted nor adopted in the mining industry. The following challenges have been associated with the method: 1) non-uniformity in rock heating; 2) surface heating; 3) not environmentally friendly—it releases gases to the environment; 4) safety issues related to high temperature; and 5) high energy consumption. Despite all these challenges, thermal pretreatment through furnace is still being pursued, not only to improve the grindability of ores but also to improve downstream operations. Dash et al. (2019) demonstrated that thermal treatment can improve the magnetic separation of low-grade hematite ore [47]. The representative samples (200 g, 10 mm) were treated in a laboratory furnace and the samples were water quenched after the treatment times were reached. It was then ground to −75 μm using a ball mill. The analysis of samples after magnetic separation (wet high intensity magnetic separation (WHIMS); solid % = 25) showed that the iron yield can be improved within a range of 15–20% when hematite is treated between 500 and 800 ◦C [47]. Early studies show that the thermal treatment of tin ore can save 93 W/t of the ore processed, however 117 kWh/t will be consumed by the furnace at 100% efficiency [48]. This shows that the method may not be economically viable. The issue of the high energy consumption of the furnace is the major challenge; however, further investigation shows that an improvement in liberation usually leads to a high recovery, which may offset the energy consumed during the pretreatment. In fact, a 1% increase in recovery has been argued to be enough to cover the expended energy on the pretreatment via furnace [28]. Even when the improvement in liberation is insignificant, thermal pretreatment can still lead to a better recovery, especially for iron ores due to increases in their magnetic properties [47]. Nevertheless, further research is still needed to investigate the downstream benefits of the thermal pretreatment (via furnace) which will lead to thorough economic assessments and possible applications in the mining industry.
