鞍山科技大学本科生毕业设计（论文） 第 1 页THE SELF-GRINDING MECHANISM AND AFFECTING FACTORS OF BULK MATERIAL IN FLUID MOTIONAbstract: The fluidity and classfication of bulk material (loose body) were introduced, the self-grinding mechanism and the affecting factors bulk materials in various forms of phase, state and motion were investigated. A rotational-flow-state centrifugal autogenous grinder was developed on the basis of applying self-grinding mechanism of bulk material, the result tested by the autogenous grinder was compared with extremely high specific area were obtained. The feasibility lf the developed new-type artogenous grinder in the view of fluid motion of bulk material was proved.Key words: motion of bulk material; self-grinding mechanism; new developed mill1 IntroductionComminution except coarse grinding in most commintors or crushing machines is performed in the course of motion of bulk materials. Only in a few comminting equipments such as rollermill and extruding milll, the vomminuted materials are stationary or fluid motion was provided through grasping the characteristic of fluid motion in the course of comminuting.2 Fluid Motion of Bulk Materials2.1Fluidity of bulk materialsIn unconsolidated media mechanics, bulk material is also named loose body, it is the aggregate of interrelated solid particles, where a single particle presents the characteristic of solid and is the skeleton of the loose body. However, in macroscopic view, it also presents fluidity and some characteristics of liquid: (a) being the same with liquid, bulk material can not keep a certain shape; (b) both bulk material and liquid can not bear a tension force but bear a pressure force. The difference between bulk material and liquid is that there exists an inner friction (inner friction angle) in the bulk material. This is to say, if an external condition is exerted on bulk material to alleviate or eliminate the inner friction angle, the bulk material will be fluidized. For example, adding some media such as water and colloid materials to bulk material or exerting special external forces (resonant force etc.) on bulk material, the bulk 鞍山科技大学本科生毕业设计（论文） 第 1 页material can be fluidized.2.2 Affecting facters of bulk material motionAs stated above, the most predominant factor affecting the motion of bulk material is the existence of inner friction angle. The smaller the inner friction angle is, the easier the motion of bulk material becomes. In the conctrte, the factors can be: (a) the lumpiness of single particle in bulk material, the lumpier the particle is, the more difficult the motion of bulk material will be;(b) the unit weight of bulk material, the beavier the rnit weight is, the more difficult the motion will be;(c) the looseness (porosity) of bulk material, the looser the bulk material is, the easier the motion will be;(d) the humidity of bulk material, bulk material starts to flow when the humidity exceeds a critical, whereas, for some bulk materials, the increment in humidity conversely brings about the increment of inner friction angle and leads it difficult to flow;(e) the morphology and surface roughness of single particle, the inner friction angle is colsely related to the morphology and roughness of particle of bulk material;(f) it is more difficult for momideal bulk material to flow than for ideal bulk material to.2.3 Classification of fluid motion of bulk materialThe fluid motion of bulk material can be classified according to whether there is energy-carrier medium or not:(Ⅰ) Single-phase flow. When there is not energy-carrier medium in the bulk material, or there are media, for example, air and water, but the media do not play the role of energy-carrying, the flow is all regarded as single-phase flow.(Ⅱ) Biphase flow, when there are quantities of energy-carrier media in the bulk material, the particles of bulk material are suspending or near to suspending, the flow is biphase flow.The flow velocity of bulk material is an impoetant parameter for self-grinding. According to the velocity, the fluid motion can be classified:(ⅰ) ultimate low velocity (ν<9m/s); (ⅱ) low velocity (9<ν<100m/s);(ⅲ) medium velocity (20<ν<100m/s);(ⅳ)high velocity (100<ν<200m/s); (ⅴ) ultrahigh velocity (250<ν<1000m/s). The efficiency of self-鞍山科技大学本科生毕业设计（论文） 第 1 页grinding is very low when the flow velocity is in the ultimate low velocity range, the velocity is hardly chosen as a parameter in autogenous grinder. The low velocity is often chosen as a parameter in horizontal cylindrical autogenous grinders. The medium velocity is usually chosen as a parameter in vertial shaft centrifugal comminutors, and high velocity and ultrahigh velocity are adopted in ultrafine comminution.3 Analysis of Fluid Motion Mechanisn3.1 Classification of self-grinding modesThe self-grinding modes of fluid motion can be classified into:(a) impact self-grinding. In this mode, particles collide each other and reduction takes place;(b) delaminating self-grinding. Particles impact and shear each other, the particles are delaminated and reduction take place;(c) fatigue rupture self-grinding. Materials are fatigued to rupture under the condition of high-frequency altenating pulse stresses. The fatigue rupture self-grinding can make tough mateials to be comminuted in the way in which brittle materials are comminuted.3.2 Analysis of self-grinding mechanismThe flowing forms of every kinds of bulk materials are composed of two basic flowing forms:linear flow and rotational flow. In practice, an independent flowing form is usually present in grinding machine, in a very few case, two flowing forms are compositely present in centrfugal autogenous grinder. Hence, respectively studying the self-grinding mechanism of the two basic flowing forms is the basis for investigating the self-grinding mechanism of bulk material. Moreover, difference in phase and state of flowing bulk material must also be considered.3.2.1 linear flow(a) Single-phase linear flow. The forms of self-grinding are impacting and delaminating of particles. For example, he principle of vertical shaft impact comminutor is that a strong centrifugal force field caused by a high-speed rotating centrifugal disk brings about a high-speed linear jet of bulk material, the jet collides and impacts the particles remained the wall of cylinder. Meantime, the difference in velocity of jet particles caused by different sizes and morphologies also brings about impacting and delaminating of particles, but the degree of wearing and delaminaing is limited.鞍山科技大学本科生毕业设计（论文） 第 1 页(b) Biphase linear flow. Similar to flow of liquid, the biphase linear flow also includes laminar flow and turbulent cuttent. The flow is stable when it is in the district of laminar flow, the velocity of particles in a layer is same but that in various layers is different. The friction of particles between different layers takes place. However, as well-known, the flow velocity in laminar flow district is very low, so the degree of self-grinding caused by laminar flow is limited.The comminution mainly takes place in the district of turbulent vurrent due to higher or very high velocity of current and occurrence of violent turbulence in the district. Violent collision between particles exists in the trubulence, and impact comminution is formed. If several jets intersect each other, the intersected particles will violently collide and impact. The higher the velocity is, the more efficient the self-grinding will be. For example, air-current comminutor works in gas-solid biphase self-grinding.3.2.2 Rotational flowRotational flow is formed by an external force such as in an anular pipe and cylindrical container. The centrifugal force field caused by rotating flow radially acts on the particles, the particle jet pressures the walls of pipe or container and particles remained on the walls, thus lesds to friction and shear force between the particles. The peculiar shear force in rotational flow is a predominant factor contributing self-grinding.(a) Single-phase rotational flow. The state and velocity of each particle of bulk material are different owing to the difference in size and morphology of each particle. Consequently, the fricting shear force between particles caused centrifugal pressure is a kind of alternating and pulse stress, the higher the flowing velocity is, the higher the altermating frequency and intension wil be. The high-frequency alternating and pulse shear stress make particles fatigue ruptured, the fatigur rupture which is expressed as brittle fatigue comminution is also a predominant form of self-grinding in rotational flow.(b)Biphase rotational flow. The self-grinding foem stated in single-phase flow above obviously is present in biphase flow. However, the self-grinding form is relatively weak owing to the fact that the viscid effect of energy-carrier medium hinders the flow of particles. Similar to those of biphase linear flow, the occurrence of violent turbulence ta high rotational 鞍山科技大学本科生毕业设计（论文） 第 1 页velocity brings about the collision of particles and impact comminution is formed. The self-grinding form also occupies a place in biphase rotational flow.4 Factors Affecting Self-grindingOn the basis of analyses of comminution mechanism stated above, the factors affecting self-grinding can be summarized as following:(a) The instinctive structure and physical properties of bulk material such as fragility, hardness, brittleness, toughness, joint, cleavage and natural defects. All these factors are of importance to every comminution form. (b) The state of flowing. Linear flow and biphase rotational flow are applicable to brittle material, and single-phase rotational flow is applicable to tough material, the reason is that the high-frequency pulse shear effect renders tough material brittle failute. (c) The velocity of flowing. Whether in linear flow state or in rotation flow state, the velocity of flowing is one of the most important factors affecting the efficiency of self-giending. The higher the velocity is, the more efficient the self-grind will be, and the finer the obtained particles will be.(d) The concentration (for biphase flow) or the looseness(for single-phase flow). On the basis of probability theory, the increment in concentration raises the probability of collision between particles, thus improves the efficiency of comminution. Comsequently, increment in comcentration fo bulk material in biphase flow state and decrement in loosemess of bulk material in single-phase state are effective ways to improve the effectiveness of self-grinding.5 Practical Application鞍山科技大学本科生毕业设计（论文） 第 1 页A new-type rotational-flow-state centrifugal autogenous grinder has successfully developed by applying self-grinding machanism of bulk material in Wuhan University of Technology. The schematic diagram of principal machine is shown in Fig.1. The evident difference betweeen the developed autogenous grinder and other vertical shaft centrifugal comminutor is that the working pan of the developed autogenous grinder is in conical shape. When the conical working pan rotates at a high speed, the resolutes of centrifugal force field acting on material including horizontal force and vertical force, thus the material in the ore-grinding cylinder cyclically and spirally flows upwards, and the material can be fully comminuted in the rotational flow. The horizontal flow and radial flow are shown in Fig.2 and Fig.3 respectively. 鞍山科技大学本科生毕业设计（论文） 第 1 页The results of comminution efficency by the developed autogenous grinder were compared with those(listed in Table 1).The results in Table 1 show that such indexes as the granularity and fineness of product, throughput, enery-consumption and noise by the centrifugal autogenous grinder are all superior to those by 4R Raymond mill. This reveals that the new type autogenous grinder which is develop on the basis of the viewpoint of fluid motion is feasible.鞍山科技大学本科生毕业设计（论文） 第 1 页自动磨碎机以及散装流体材料对其的影响摘要：在不同阶段的形状、状态和运动的情况下，通过被采用的液体和散装材料（主体松软材料） ，对自动磨碎机以及散装流体材料的影响所做的调查。在散装材料磨碎机应用的基础上，一种新型的循环流体状态自生磨碎机已经发展起来了，自生磨碎机的实验结果与 4R 雷德蒙工厂的实验结果通过比较，及其高精确地获得了更小的微粒。在对散装材料磨碎方面，这种新发展起来的自动研磨机的可行性是被证明成功的。1、介绍粉碎除了在大多数粉碎机的磨碎过程或是压缩机在散装材料运动中被压碎的过程，只有在很少的一些设备例如轧辊机和挤压机粉碎时采用的材料才是固定的或可以说是比较固定的。因此，一种关于流体状态的散装材料自动磨碎机的新的构想，通过深刻领会流体材料在研磨过程中的特性而被提出来了。2、散装材料的流体状态2.1 流体散装材料在机械学的分支介质机械学中，散装材料同样被叫做主体松软材料，它是相互关联的固体微粒的集合体，在这里，每一个单独的微粒都代表着固体的特性，并且是主体松软材料的骨骼。然而，肉眼可见的方面分析，它同样代表流体和流体的一些特性。（a） 与流体一样，散装材料不能保持固定的形状；（b） 散装材料和流体都不能承受拉力但可以承受压力。散装材料与流体的不同是散装材料可以承受较小的正切力，而流体不能，这是由于在散装材料里存在内部摩擦（内部摩擦角） 。也就是说，如果用外部条件施加在散装材料上用以减轻或削减内部摩擦角，散装材料将被流体化。举例来说，把一些如水和胶体材料的介质加入到散装材料中或是在外部施加特殊的力（谐振力等）在散装材料上，散装材料将会被流体化。2.2 散装材料状态的影响因素如上所述，影响散装材料状态最主要的因素是其内部存在摩擦角。摩擦角越小，散装材料的状态越容易得到。具体说来，影响因素包括：（a） 散装材料单独微粒体的块状程度。微粒的块状程度越大，散装材料的状态越难得到。（b） 散装材料单元体的重量。单元体的重量越大，散装材料的状态越难得到。鞍山科技大学本科生毕业设计（论文） 第 1 页（c） 散装材料的疏松（多孔性）程度。散装材料越疏松，散装材料的状态越容易得到。（d） 散装材料的潮湿程度。当湿度超过临界值时，散装材料开始流动，然而，对于某些散装材料，在湿度反面附加的东西反过来也带来摩擦角影响因素并且导致散装材料不易流动。（e） 微粒的形态和表面粗糙程度。内部的摩擦角与散装材料微粒的形态和粗糙程度有着密切的关系。（f） 对于理想状态的散装材料，实际状态的散装材料更难流动。2.3 散装材料流体状态的分类散装材料流体状态根据是否有承载能量的介质可分类为：（Ⅰ）单阶段流动。但散装材料中没有承载能量的介质，又或者是有介质，如水和空气，但不能起到承载能量作的介质，这样的流动形式都被认为是单阶段流动。（Ⅱ）双阶段流动。当散装材料中存在大量的能量承载介质，散装材料的微粒是悬浮的或是接近悬浮的，这样的流动形式被认为是双阶段流动。散装材料的流动速度对于自磨来说是主要参数。根据其速度，流体的状态可分为：（a） 最低速度(ν<9m/s) ；（b） 低速(9<ν<100m/s)；（c） 中速(20<ν<100m/s)；（d） 高速(100<ν<200m/s)；（e） 超高速(250<ν<1000m/s)。当流动速度在最低速度范围内时，自磨的效率是非常低的，作为自动磨碎机来说这种速度几乎不能作为参数。低速经常被拿来作为水平圆柱磨碎机的参数。介质的速度经常被选来作为纵向离心磨碎机的参数，并且极细粉碎中采用高速和超高速。3．流体磨碎机的分析3.1 自磨形式的分类流体的自磨形式可以分为：（a） 自磨的冲击。在这种形式下，微粒互相碰撞并且减小；（b） 自磨的分层。微粒互相撞击互相削减，发生微粒分层、减少的现象；（c） 自磨的疲劳破裂。在高频率交替脉冲的重压下，材料因疲劳而导致破裂；疲鞍山科技大学本科生毕业设计（论文） 第 1 页劳破裂的自磨过程，用脆性材料被研磨的方式，可以使较硬的材料被研磨。3.2 自动磨碎机的分析各种各样的散装材料流动形式都是由以下两种流动形式组成：直线流动和旋转流动。实际上，独立的流动形式常出现在自磨机上，在一些情况下，两种流动形式在离心自磨机中合成。因此，为了研究散装自磨机，把自磨机的这两种流动形式分开来研究。而且，流动时不同的阶段和状态会带来不同的自磨形式。因此，散装材料的阶段和状态也必须考虑进去。3.2.1 直线流动（a）单直线流动。自磨的形式是微粒的撞击和分层。纵向冲击自磨机的原理是：一个高速的旋转离心圆盘产生的巨大的离心力场，带动散装材料产生高速直线喷雾，喷雾互相撞击，并且微粒停留在圆筒壁上。同时，不同大小和形态导致喷雾微粒在速度上的不同，致使微粒撞击和分层，但是磨损和分层的程度是有限的。（b）双直线流动。与流体的流动形式相似，双直线流动同样包括层流和乱流。在层流区域流动是稳定的，微粒的速度在同一层上是相同的，而不同层上的速度不同。不同层上的微粒产生摩擦。然而，总所周知，层流区域的速度是非常慢的，因此层流自磨的程度是有限的。研磨主要发生在乱流区域，因为在这个区域发生高速流动和强烈骚动。乱流中产生强烈的微粒撞击，形成撞击粉碎。如果几条喷雾相交，交错的微粒会强烈的碰撞和冲击。速度越高，自磨得效率越高。如，气流自磨机以固体、气体双直线流动的自磨下工作。3.2.2 旋转流动旋转流动是在如密闭管子或圆筒形容器中的产生的外力的作用下形成的。由旋转流动产生的微粒的径向运动形成的离心力场，使微粒喷雾对管壁和容器壁产生压力，并且使微粒停留在壁上，因此导致微粒间摩擦力和剪切力。旋转流动中产生的特殊的剪切力是促成自磨的主要因素。（a） 单旋转流动。由于散装材料的每个微粒大小和形态的不同，导致每个微粒的状态和速度不同。因此，微粒间的摩擦剪切力导致的离心压力是一种交替和脉冲的压力，流动的速度越高，交替的频率和强度越高。高频率的的交替和脉冲剪切力使微粒疲劳破裂，这种被认为是脆性疲劳自磨的疲劳破鞍山科技大学本科生毕业设计（论文） 第 1 页裂是自磨在旋转流动的主要形式。（b） 双旋转流动。单旋转流动的自磨形式，明显地存在于双旋转流动中。然而，由于承载能量的介质的粘度妨碍介质的流动，自磨得形式相对较弱。与双直线流动相似，高旋转速度的乱流导致微粒的撞击和自磨得冲击。双旋转流动在自磨中同样占据一个重要位置。4．自磨的影响因素如上所述的自磨机的基本分析，自磨机的影响因素可以总结如下：（a） 散装材料的本质结构和物理性质有脆性、硬度、易变性、强度、连接、裂开和自然缺陷。这些因素对每个自磨形式都非常重要。（b） 流动状态。直线流动和双旋转流动适用于脆性材料，单循环流动适用于硬性材料，原因是高频率脉冲剪切的影响使硬性材料脆性断裂。（c） 流动速度。无论是在直线流动状态或是在旋转流动状态，流动的速度是影响自磨效率的一个主要因素。速度越高，自磨得效率越高，获得的微粒越细。（d） 集中性（双流动）松散度（但流动） 。在可行性理论的基础上，集中材料的增加会加剧微粒间的撞击，因此影响自磨得效率。因此，双循环中集中材料的增加和单循环状态松散材料的减少是提高自磨效率的有效途径。5．实际应用鞍山科技大学本科生毕业设计（论文） 第 1 页图 1 机器原理略图 1-纵轴 2-底盘 3-工作盘 4-锤板 5-安装网 6-刮削器 7-自磨筒 8 空气进入筒 9-分离器 10-材料入口一种旋转流动离心自磨机已经被武汉科技大学成果的研制出。机器的主要图表如图1。这种自磨机与其它纵向自磨机的明显区别是它的工作盘是圆锥形的而其它的事平的。当圆锥工作盘在高速下旋转，强烈的横向的和纵向的离心力作用在材料上，材料在圆筒中处于螺旋循环上升状态，材料可以在旋转流中被完全磨碎。图 2 和 3 分别是水平和轴向运动方向。图 2 水平流动 图 3 纵向流动这种自磨机工作效率的结果与 4R 雷蒙德工业公司的相比较：表 1 与 4R 雷德蒙工厂的实验结果的比较工厂 加入材料的大小产品的颗粒 产量t/h能量消耗 噪音4R 雷德蒙工厂150mm-4mm 96%过-200网眼3.2 18.2kw 99dBФ800 离心自磨机150mm 98.5%过-200网眼3.52 15.8kw 82dB从表 1 中可以看出这种产品的粒度和细度、产量、消耗量和噪音等指数都好于 4R鞍山科技大学本科生毕业设计（论文） 第 1 页雷蒙德工厂的。显示出在流体观点的基础上研制的新型自磨机是可行的。