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Optimization of the Mechanical Comminution - The Crushing Stage

Ngonidzashe Chimwani

Department of Mining Engineering, University of South Africa (UNISA), Florida Campus, Johannesburg, South Africa

Abstract

The mineral processing industry is faced with various challenges that hinder the efficient extraction as well as the marketing of metals. Among those challenges are the depletion of high-grade ores, rise in energy costs, and reduction of metal prices. In view of that, the need to improve efficiency becomes eminent in the mining and mineral processing industry. Improving efficiency can be achieved through carefully reviewing all the mining and mineral processing stages to identify technical innovations that enable the establishment of high-efficiency and low-energy consumption processes. This chapter thus reviews different crushers and the parameters that influence their potential to process low-grade ores efficiently. The review unveiled that downstream processes-oriented research should be preferred to those that focus on the optimization of a particular unit to improve efficiency. Other initiatives, such as managing the demand side of power and optimizing the idling time of the crushers, have shown great potential to reduce the energy consumption of impact crushers. This is because the energy expenditure of a gyratory crusher during idling is 30% of the full load power consumption, and jaw crushers not only lose a significant portion of energy into noise and heat, but have no-load power ranging from 40% to 50%.

Keywords: Comminution, jaw crusher, gyratory crusher, cone crusher, impact crusher, HPGR crusher, low-grade ore, optimization

1.1 Introduction

Comminution involves processes that reduce rocks from the infinite size of the half-space to sizes that allow valuable minerals to be separated from the gangue [1] using different separation methods. Rock blasting is the first process of comminution followed by crushing and then grinding, and the efficiency of each process is dependent on the preceding process. The blasting being on the utmost upstream has to provide not only the desired particle sizes for the crushers and mills but particle grains with reduced internal resistance owing to the existence of macro and microfissures within the particle. Although the presence of flaws within the blasted particles enhances productivity and lowers the grinding energy during mechanical comminution generally, they cause a reduction in productivity in autogenous mills that use the blasted particles as the grinding media. Thus, the control of the blasting stage should be guided by the requirements of the downstream process.

Requirements of the downstream processes are further complicated by the challenges exacerbated by the processing of low-grade ores resulting from the depletion of high-grade ores. Low-grade ores contain small grainsized minerals dispersed through the rock, which require high energy as well as the knowledge of the ore texture to free the minerals from the host, and worse still, high fine losses are unavoidable in the process. The high energy requirements of low-grade ore processing add to the headache that the mineral processing industry has had since its inception. The comminution stage has, over the years, been widely known to be characterized by low efficiency [2] and high energy consumption, among other things. The continuous rise in energy costs and the increased concerns over global warming have sparked efforts to minimize CO2 emissions, albeit with costs. Thus, it is imperative to be proactive than reactive, that is, to address the problems at the design and process stages.

To that end, several efforts have been made in that direction, at the crushing and milling stages including the development of comminution tests. The tests conducted are the drop weight tests, which provide the abrasion properties of rocks; the bond crushability work index, which determines the energy requirements of a crusher; the bond abrasion tests, which estimate mill/crusher liners; and the point load tests, which measure crushability by correlating the mechanical strength of the rock and the crusher reduction ratio. All these tests play an important role in the simulation and design of crushers and mills. The information drawn from these tests is used by the designers and manufacturers of comminution equipment to determine the load and non-load power requirements of each crusher/mill. From the wealth of research also emerged that the random application of forces inside the crushing and grinding machines between particles themselves or particles and the machine parts is fundamental to the disruptive nature of the comminution process [3]. These random forces make it impossible to achieve 100% efficiency in the comminution machines. Variations of the interactions of those particles and the products obtained are influenced by the equipment and operational parameters, such as the shaft velocities, equipment geometry and material properties, which ultimately affect the process efficiency in general, and the equipment in particular. Some equipment effect single impact, while others apply multiple impacts until particle sizes are disintegrated and reduced to acceptable values. The challenge, however, is that those processes are not efficient as mentioned in the foregoing, worse still with the processing of low-grade ores. This is especially considering that the biggest fraction of the input energy is dissipated as noise and heat with the other fraction channeled towards disrupting the forces that bind particles constituting the ore [4]. Richardson and Harker [5] compiled a list of the breakdown of the energy expenditure in comminution processes, and in that list, energy was observed to be consumed through the production of elastic deformation of the particles before fracture occurrence, inelastic deformation that leads to size reduction, elastic equipment distortion, friction between particles and between particles and the machine, and in the vibration, heat, and noise in the plant.

The proper definition of energy expenditure has been the subject of research over the years, and the need to deepen the enquiry cannot be overemphasized especially with the current dominance of low-grade ore processing. It is important to mention that the widely accepted theory used to determine the actual amount of energy used in comminution is derived from the inverse proportionality relationship between the mean particle size of the particles and the newly produced surface area, as the milling progresses. This thus means that the determination of surface areas at different stages of milling provides a reliable inquiry into the energy expenditure of the comminution process. In that vein, the Bond work index has been widely used and found to be reliable [6]. From comparing the total ideal fracture surface energy needed to generate new surfaces during the comminution of covalent materials and the standard Bond work index, Tromans [7] observed that crushing and grinding operations have 1% energy efficiency or even less.

Modeling energy consumption of a comminution equipment requires the knowledge of the design and operational parameters of the equipment [6]. The parameters range from that of the feed material to those of the equipment comminuting the materials. Some researchers have suggested altering the parameters of the feed through pre-treatment of the material to be comminuted to reduce Bond index values [3]. Although the pre-treatment increases cracks in the particles, it has the possibility of consuming more energy than the comminution process itself [6]. The most followed procedure, however, is to optimize the comminution process through fine-tuning the equipment parameters in a manner that redirects energy to be spent through vibration, heat, and noise into strain energy, which alters the internal forces of particles, hence creating new surface areas. But optimizing the process undoubtedly requires an understanding of Tromans' [7] detailed account of the energy expenditure. The author observed energy as utilized through the elastic deformation of particles before fracturing occurs, the inelastic deformation that results in the breakage of particles, elastic distortion of the equipment, friction between particles and between particles and the machine, and in noise, heat, and vibration of the equipment. It is also important to note that energy efficiency during comminution, as noted by Tromans [7] depends on impact efficiency, which, in turn, is influenced by the particle loading force, size, and orientation of inherent fissures in the particle with respect to the loading axis.

Over the years, the design and optimization of comminution equipment were achieved through experimental work and important milestones were reached. However, the experimental approach could not show how the particles flow and interact among themselves and with the equipment parts. Researchers thus found computer modeling applicable to unpack the internal dynamics of the particles that cause breakage. Advanced tools that allow particle flows to be simulated and the breakage process to be predicted, such as the discrete element modeling (DEM), were utilized. The tool found use in soil mechanics [8], and then later applied to milling and mineral processing [9]. Its ability to address a wide range of particle-related problems is well-documented in the literature [10-12]. Recently, it was used to simulate solid flows and energy transfer in the vertical impact crusher [13], configure ball mill...