Unit-2-comminution省名师优质课赛课获奖课件市赛课百校联赛优质课一等奖课件.ppt

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2-lesson 1,Principles of comminution,1/32,1,Introduction 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 size of the ore is,progressively,逐步地,reduced until the clean particles of mineral can be separated by,such methods as are available,.,2/32,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 the case of,quarry,采石厂,products,to produce material of controlled particle size.,3/32,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.,4/32,Section 2,Comminution in the mineral processing plant,or“mill”,takes place 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.,5/32,Section 3,Crushing is accomplished by,compression,挤压,of the ore against,rigid,坚硬,surfaces,or by,impact against,撞击,surfaces in a rigidly,constrained,限制,motion path.This is,contrasted with,（与此不一样是，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,.,6/32,Section 4,Crushing is 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 other definitions are sometimes used.,7/32,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 applications.,8/32,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,reduction,can be achieved for roughly half the energy and costs required by tumbling mills.,Which one is cheaper?,9/32,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 rolls,10/32,Principles of comminution,Most minerals are crystalline materials 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.,11/32,Section 2-1,In the crystalline lattice of minerals,these,inter-atomic bonds,are effective only over small 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,12/32,Strain,应变,of a crystal lattice resulting from tensile or compressive stresses,13/32,Section 2-2Even when rocks are uniformly,loaded,the internal stresses are not,evenly distributed,as the rock consists of a variety of minerals dispersed as grains of various sizes.,14/32,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,缺点,in the matrix,which act as sites for,stress concentration,.,15/32,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,the stress direction.Therefore,there is a,critical value,for the crack length at any particular level of stress at which the increased stress level at the crack tip is sufficient to break the atomic bond at that point.,16/32,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,.,17/32,Section 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,.,18/32,Section 2-4,When fracture does occur,some of,the stored energy,is transformed 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.,19/32,Section 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.,20/32,Section 2-5,Brittle materials relieve the strain energy mainly by,crack propagation,whereas,tough 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.,21/32,Section 2-5,Crack propagation,can also be,inhibited,抑制,by,encounters 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?,22/32,Section 2-6,The energy required for comminution is reduced,in the presence(absence),of water,and can be further reduced by chemical additives which adsorb onto,the solid.,How to reduce the energy for comminution of ores?,23/32,Section 2-6,This may be due to the lowering of the,surface energy on adsorption,providing that,the surfactant can,penetrate,渗透,into a crack and reduce the bond strength at the crack tip before rupture.,24/32,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.,25/32,Section 2-7,Breakage,is achieved mainly by crushing,impact,and attrition,and all three modes of fracture can be,discerned,识别,depending on the,rock mechanics,and the type of loading.,26/32,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,or by shear at,projections,发射,-,凸起处,.,27/32,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?,28/32,Section 2-9,In impact breaking,due to the,rapid loading,a particle,experiences,a higher average stress(while undergoing strain),than is necessary,to achieve simple fracture,and tends to break apart rapidly,mainly by tensile failure.The products are often very similar in size and shape.,29/32,Section 2-10,Attrition(shear failure)produces much fine,material,and may be,undesirable,depending on the comminution stage and industry sector.,30/32,Section 2-10,Attrition occurs mainly in practice due to particle-particle interaction(inter-particle comminution),which may occur if a crusher is fed too fast,contacting particles thus increasing the degree of compressive stress and hence shear failure.,31/32,共勉,The End,不积跬步无以成千里,不积小流无以成江海,A journey of a thousand miles begins with a single step,Not a little flow without taking jianghai,32/32,