Unit 2 comminution

1、Chapter 2 -lesson 1 Principles of comminution1Introduction secton 1 Because most minerals are finely disseminated 分布and intimately associated with the gangue, they must be initially “unlocked” or “liberated” before separation can be undertaken. This is achieved by comminution in which the particle s

2、ize of the ore is progressively逐步地 reduced until the clean particles of mineral can be separated by such methods as are available. Comminution in its earliest stages is carried out in order to make the freshly excavated采掘 material easier to handle by scrapers电铲, conveyors, and ore carriers, and in t

3、he case of quarry 采石厂products to produce material of controlled particle size.Section 2 Explosives炸药 are used in mining to remove ores from their natural beds, and blasting can be regarded as the first stage in comminution.Section 2 Comminution in the mineral processing plant, or “mill”, takes place

4、 as a sequence of crushing and grinding processes. Crushing reduces the particle size of run-of-mine ore to such a level that grinding can be carried out until the mineral and gangue are substantially基本上 produced as separate particles.Section 3 Crushing is accomplished by compression挤压 of the ore ag

5、ainst rigid坚硬的 surfaces, or by impact against撞击 surfaces in a rigidly constrained限制 motion path. This is contrasted with （与此不同的是，对比而言）grinding which is accomplished by abrasion摩擦 and impact of the ore by the free motion of unconnected media such as rods, balls, or pebbles鹅卵石、砾石.Section 4 Crushing is

6、 usually a dry process, and is performed执行 进行 in several stages, reduction ratios 破碎比being small, ranging from three to six in each stage. The reduction ratio of a crushing stage can be defined as the ratio of maximum particle size entering to maximum particle size leaving the crusher, although othe

7、r definitions are sometimes used.Section 5 Tumbling 翻滚mills 滚筒磨with either steel rods or balls, or sized ore as the grinding media, are used in the last stages of comminution. Grinding is usually performed wet to provide a slurry feed to the concentration process, although dry grinding has limited a

8、pplications.Section 5 There is an overlapping size area where it is possible to crush or grind the ore. From a number of case studies, it appears that at the fine end of crushing operations equivalent相同的 reduction can be achieved for roughly half the energy and costs required by tumbling mills. Whic

9、h one is cheaper?Section 5 It has been concluded by steane that it is cheaper to grind, and that crushing can be carried no further than is necessary to provide suitable feed for the grinding mills. Stirred mills High pressure grinding rollsPrinciples of comminution Most minerals are crystalline mat

10、erials in which the atoms are regularly arranged in three-dimensional arrays. The configuration排布 of atoms is determined by the size and types of physical and chemical bonds holding them together.Section 2-1 In the crystalline lattice of minerals, these inter-atomic bonds are effective only over sma

11、ll distances, and can be broken if extended by a tensile stress扩张应力. Such stresses may be generated by tensile or compressive loading拉伸/挤压载荷Strain应变 of a crystal lattice resulting from tensile or compressive stressesSection 2-2 Even when rocks are uniformly loaded, the internal stresses are not even

12、ly distributed, as the rock consists of a variety of minerals dispersed as grains of various sizes.Section 2-2 The distribution of stress depends upon the mechanical properties of the individual minerals, but more importantly upon the presence of cracks裂纹 or flaws缺陷 in the matrix, which act as sites

13、 for stress concentration.Section 2-3 It has been shown that the increase in stress at such a site is proportional to 与 成正比the square root of the crack length perpendicular to垂直方向的the stress direction. Therefore, there is a critical value for the crack length at any particular level of stress at whi

14、ch the increased stress level at the crack tip is sufficient to break the atomic bond at that point.Section 2-3 Such rupture of the bond will increase the crack length, thus increasing the stress concentration and causing a rapid propagation of the crack through the matrix, thus causing fracture.Sec

15、tion 2-4 Although the theories of comminution assume that the material is brittle, crystals can, in fact, store energy without breaking, and release this energy when the stress is removed. Such behaviour is known as elastic.Section 2-4 When fracture does occur, some of the stored energy is transform

16、ed into free surface energy, which is the potential energy of atoms at the newly produced surfaces. Due to this increase in surface energy, newly formed surfaces are often more chemically active, and are more amenable to更易于 the action of flotation reagents, etc., as well as oxidising more readily.Se

17、ction 2-5 Griffith (1921) showed that materials fail by crack propagation裂纹扩展 when this is energetically feasible, i.e. when the energy released by relaxing the strain energy is greater than the energy of the new surface produced.Section 2-5 Brittle materials relieve the strain energy mainly by crac

18、k propagation, whereastough materials can relax strain energy without crack propagation by the mechanism of plastic flow, where the atoms or molecules slide over each other and energy is consumed in distorting the shape of the material.Section 2-5 Crack propagation can also be inhibited抑制 by encount

19、ers with other cracks or by meeting crystal boundaries. Fine-grained rocks, such as taconites, are therefore usually tougher than coarse-grained rocks. Why the fine rocks is tougher than the coarse rocks?Section 2-6 The energy required for comminution is reduced in the presence (absence)of water, an

20、d can be further reduced by chemical additives which adsorb onto the solid. How to reduce the energy for comminution of ores?Section 2-6 This may be due to the lowering of the surface energy on adsorption providing that the surfactant can penetrate渗入 into a crack and reduce the bond strength at the

21、crack tip before rupture.Section 2-7 Real particles are irregularly不规则地 shaped, and loading is not uniform but is achieved through points, or small areas, of contact.Section 2-7 Breakage is achieved mainly by crushing, impact, and attrition, and all three modes of fracture can be discerned识别 dependi

22、ng on the rock mechanics and the type of loading.Section 2-8 When an irregular particle is broken by compression, or crushing, the products fall into two distinct size ranges- coarse particles resulting from the induced tensile failure, and fines from compressive failure near the points of loading,

23、or by shear at projections发射-凸起处.Section 2-8 The amount of fines produced can be reduced by minimising the area of loading and this is often done in compressive crushing machines by using corrugated波纹 crushing surfaces. How to reduce the amount of fines particles in comminution?Section 2-9 In impact breaking, due to the rapid loading,