Milling dynamics: Part I. Attritor dynamics: Results of a cinematographic study

The motions of grinding media and powder in an attritor canister were studied by means of filming the agitated charge and frame-by-frame scrutiny of the footage. In conjunction with auxiliary experiments, this permitted semiquantitative analysis of the milling action. In particular, the mill can be divided into several regions characterized by different balances between direct impacts and rolling/sliding of the grinding media. Simple calculations suggest that impacts are more capable of effecting mechanical alloying (MA) than are rolling or sliding events in an attritor. Powder circulation within an operating mill was also investigated. Based on the results and the accompanying analysis, concepts for improved attritor design are presented. D. Maurice is formerly a Graduate Student, Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903     

The effects of ball size distribution on attritor efficiency

A study was undertaken to determine how media dynamics are altered when differently sized grinding balls are used in an attritor. Cinematographic techniques identify the extent of segregation/mixing of the differently sized balls within the attritor as a function of impeller rotational velocity and small ball number fraction. This permits determination of rotational velocities needed to most efficiently use the tactic of milling with differently sized media. Cinematographic observations show that the close-packed media array, assumed when balls of the same size are used for milling, is disrupted when differently sized balls are used. Monitoring powder particle numbers as a function of milling time for the situations when the same and differently sized balls are used can be used to assess relative milling efficiencies. Results indicate powder deformation, fracture, and welding are enhanced through employment of differently sized balls. This conclusion is reinforced by observations of microstructural characteristics of powder processed with the different type of media.     

Effect of the method of introduction of Y2O3 into NiAl-based powder alloys on their structure: II. Mechanical activation

Effect of mechanical activation of NiAl powders produced by calcium hydride reduction in an attritor and a ball mill on the specific surface, the oxygen concentration, the strain hardening, and the coherent domain size (CDS) of the powders is studied. It is found that the powder specific surface milled in the attritor for 10–15 h is larger by a factor of 1.7–1.8 and the oxygen concentration in a powder is lower by a factor of 1.35 as compared to the its milling in the ball mill for 150 h. The powders milled in the attritor for 15 h have the level of microstresses higher by a factor of ～2.4 and the CDS smaller by a factor of 2 as compared to the powder treated in the ball mill for 150 h. When milling a powder in the attritor, the milling time decreases by a factor of 10 and the degree of powder refinement increases, which improves the technological characteristics of the powders. As a result of the combination (in one operation) of mechanical activation of an NiAl intermetallic matrix powder in the attritor and the introduction of dispersed particles of a refractory oxide Y₂O₃ powder, the produced composite alloy has a density close to the theoretical one and has no aggregates of dispersed oxide particles at grain boundary junctions. Submicro- and nanosized oxide particles are homogenously distributed in the intermetallic matrix volume, which is characterized by a homogeneous distribution of nickel and aluminum.     

Compaction and characterization of mechanically alloyed nanocrystalline titanium aluminides

Blended elemental (BE) Ti-24 at. pct Al-11 at. pct Nb (Ti-24-11) and Ti-55 at. pct Al (Ti-55) powders and prealloyed (PA) Ti-24-11 powders were mechanically alloyed in a SPEX mill or an attritor. After SPEX milling for 10 hours, the BE Ti-24-11 powder contained the B2/bcc phase, while the BE Ti-55 powder showed the presence of an amorphous phase. The PA Ti-24-11 powder containing the B2 phase showed a decrease of crystal size on milling. These powders were consolidated by hot isostatic pressing (“hipping”), Ceracon process, and dynamic methods. On compaction, the B2/bcc phase in the Ti-24-11 sample transformed to a mixture of the B2 and orthorhombic (“O”) phases, while the amorphous phase in the Ti-55 powder crystallized to a mixture of the γ-TiAl and α ₂-Ti₃Al phases. The finest grain size in compacted material was obtained in the dynamically consolidated powder, and the grain size in the hot isostatic pressed (“hipped”) powder became larger with the increasing hipping temperature.     

Analysis of Mechanical Milling in Simoloyer: An Energy Modeling Approach

A model was developed to estimate the energy transfer from milling media to the powder during milling carried out in Simoloyer CM 01 (Zoz GmbH, Wenden, Germany), a horizontal attritor high-energy ball mill. The model was then used to estimate the energy transfer in milling of iron at 1000 rpm. Furthermore, the time required to achieve a particular grain size was formulated as a function of milling speed, using the model developed for the energy transfer. The results were verified at 500 rpm and 1500 rpm for iron and aluminum.     

The physics of mechanical alloying: A first report

In this paper, we present a first attempt to define the basic geometry, mechanics, and physics of the process of mechanical alloying. The geometry of the collision events which lead to particle fragmentation and coalescence is modeled on the basis of Hertzian contacts between the grinding media which entrap a certain amount of material volume between the impacting surfaces. This geometry essentially defines the volume of material affected per collision, and from this information and characteristics of the specific mill and the material being processed, impact times, powder strain rates and strains, powder temperature increase, powder cooling times, and milling times can be approximated.     

艾砂磨机在遂昌金矿的试验应用

遂昌金矿二段球磨闭路的磨矿产品细度已经达到极限,致使尾矿中金、银品位难以进一步降低。对艾砂磨机与二段球磨机的磨矿效果及产品的氰化浸出效果进行了对比,并介绍了艾砂磨机在选冶车间二段磨矿中所进行的工业试验。结果表明:采用艾砂磨机开路磨矿替代原二段球磨闭路磨矿,其磨矿产品-74μm从75%提高到95%,氰化尾渣金品位从0.35 g/t降低到0.12 g/t,银品位从13.17 g/t降低到7.4 g/t,指标较好,经济效益显著。     

关于振动磨整机功率消耗的研究

分析了振动磨筒体内研磨介质的运动状态及粉磨机理，在此基础上，从能量观点出发，对振动磨整要的功率消耗进行了研究，并提出了计算振动磨功率的理论公式。     

Modeling and Analysis of High-Energy Ball Milling Through Attritors

The effects of major processing parameters of attritor mills on ball milling efficiency (i.e., minimum energy consumption with maximum milling progress) are investigated using discrete element modeling (DEM). The major processing parameters investigated include the size of balls, ball volume fraction inside the canister, ball milling velocity, and design of the impeller shaft of the attritor mill. Their effects are studied through examination of the output parameters including the average speed of balls, maximum speed of balls, and torque applied on the impeller shaft. The torque on the impeller shaft represents the energy consumption during ball milling, while the difference between the maximum and average speeds of balls scales with the compressive pressure during “mini-forging” of powder particles trapped between the colliding balls and thus scales with milling progress (particle deformation and size reduction). The simulation reveals that the ball milling velocity, ball volume fraction inside the canister, ball size, and impeller shaft design are all important parameters for energy-efficient ball milling. In particular, high ball milling velocities can lead to larger particle deformation and faster size reduction with minimum energy consumption. Further, ball sizes smaller than the gap that will not be hit by impellers directly are good for high-energy-efficient ball milling. Otherwise, energy consumption increases substantially.

     

高能球磨对新型TiC钢结硬质合金组织和性能影响的研究

采用行星式球磨机对Fe-3.0Cr-3.0Mo-0.5Cu-0.5C-33TiC新型钢结硬质合金混合粉末进行高能球磨,对不同球磨时间粉末的形貌和粒度进行观察,测定了烧结后合金的密度、硬度和抗弯强度,并对其组织结构进行了分析.结果表明：球磨初期,粉末粒度迅速减小,粉末出现片状形貌,随着球磨时间增加,粉末粒度减小速度变缓,最后趋于稳定,片状形貌逐渐消失,不规则球形形貌增多.球磨过程中,Fe与其它添加元素（C、Mo、Cu）发生合金化反应.在一定时间内,随着球磨时间的增加,混合粉末成分均匀性增加,合金的密度、硬度和抗弯强度也明显提高.     

Grinding of Fe-B4C powder charge in an inertial cone mill

The use of an inertial cone mill to grind iron and boron carbide powder mixtures is examined. The dependence of the particle-size distribution in the mixture after grinding on the maximum particle size is analyzed. It is established that the content of coarse-grained fractions (larger than 100 µ m) substantially decreases as compared with the initial powder even after three grinding cycles. If the number of grinding cycles is increased to 20, the content of finer fractions (smaller than 63 µ m) is only 6 to 8% higher than in powders subjected to 3–12 grinding cycles in the inertial cone mill. Therefore, more than 3 or 6 cycles to produce Fe-B₄C powder with the optimal content of the ≤100≤63 µ m fraction are unreasonable.     

Investigation of a discus-milling process using a powder mixture of Al and TiO2

In this study, a powder mixture of Al and TiO₂ was employed to investigate the milling process in a discus mill. In this first report on this novel mechanical mill, several variables, including the milling time and powder charge and their effects on the microstructural evolution of powder particles, are monitored and studied. The study reveals that the dominant parameters of the milling process are the milling time and the starting powder charge, similar to the other high-energy ball-milling processes. The longer the milling time and the smaller the starting powder charge, the more homogeneous the mixing and the finer the microstructure of the powder particles. The reaction between Al and TiO₂ was not observed with a milling period as long as 6 hours, for the present materials. However, the reaction between Al and TiO₂, during the subsequent heat treatment, is influenced by the milling condition. The powders with the longer milling times and finer mixing microstructures also form a finer microstructure, after the reaction between Al and TiO₂ during heat treatment. The methods for achieving an optimal milling efficiency for the Al-TiO₂ system are discussed.     

Dissolution behavior and bioavailability of phenytoin from a ground mixture with microcrystalline cellulose

The ground mixture of phenytoin and microcrystalline cellulose was prepared by grinding in a vibrational ball mill. The X-ray diffraction patterns indicated the amorphous nature of the ground mixture. Comparative studies were made concerning the in vitro dissolution and in vivo absorption of fine phenytoin powder, phenytoin sodium powder, and the ground mixture. The ground mixture showed a greater dissolution rate than the fine powder and attained supersaturation in the pharmacopeial disintegration media at pH 1.2 and 7.4. In vivo absorption studies of each preparation were carried out in five subjects, using a crossover design, by measuring the urinary excretion rate of a main metabolite, 5-(p-hydroxyphenyl)-5-phenylhydantoin. The blood levels of phenytoin and the corresponding urinary excretion patterns of the metabolite were determined in two subjects. The ground mixtures significantly improved the bioavailability of phenytoin. The drug was completely and rapidly absorbed after oral administration of the ground mixture. The vibrational ball milling technique for a poorly water-soluble drug with microcrystalline cellulose provides a promising way of improving the in vivo drug absorption.     

基于智能磨矿介质及CNN和优化SVM模型的球磨机负荷识别方法

当前球磨机负荷检测方法难以准确评估磨机内部变化,给磨机综合运行状态的控制和优化带来较大难度。本文设计了一款内嵌加速度传感器且与钢球介质物理性质相一致的智能磨矿介质用于识别磨机负荷,开展了不同充填率等磨矿条件下的磨矿试验,设计磨矿效果系数划分磨机负荷状态;分别采用了卷积神经网络方法(CNN)和优化的支持向量机(SVM)模型对智能磨矿介质获取的加速度信号进行球磨机负荷识别。基于优化的SVM模型将获取的一维加速度信号进行互补集合经验模态分解算法（CEEMD）去噪、时域特征值和样本熵提取等预处理,将上述磨机负荷的特征向量分别输入GA?SVM、GS?SVM、PSO?SVM分类模型进行训练,研究表明,PSO?SVM模型的识别准确率可达98.33%,但存在训练过程繁琐,耗费时间长的问题。在图像识别领域具有优秀应用能力的CNN模型是把智能磨矿介质检测加速度信号数据转换为二维图片后直接输入基于VGG19网络的CNN模型进行分类识别,磨机负荷分类识别准确率高于优化的SVM模型,可达98.89%,在保证识别准确率的同时有效节约了计算时间。基于CNN的智能磨矿介质球磨机负荷识别方法可为实现球磨机负荷检测与在线评估提供重要解决方案与技术保障。      

高能机械化学法制备超微氮化钼粉体

在室温下通过自行设计的高能机械化学球磨机,使钼粉在NH3气氛下经过高能球磨得到超微氮化钼粉体.分别利用X射线衍射仪、扫描电镜和透射电镜对制得的粉体结构和粒度进行分析.结果表明,在球料质量比为8∶1的情况下经过30 h球磨,得到了FCC结构的Mo2N粉体,粉体平均粒度在100 nm以内.机械化学反应过程中,一方面,NH3分子在清洁的钼金属表面的化学吸附起着重要的作用,为球磨过程中由于介质球碰撞所产生的储存于钼粉中的能量(界面能和缺陷能)提供了Mo-N化学吸附向氮化钼转变所需的激活能.另一方面,钼粉晶粒细化改变了Mo原子表面电子的不饱和性,从而促进了Mo与N的键合作用.在Mo与NH3的高能机械化学反应过程中,球磨转速的高低对整个反应的速度起决定性作用.     

Investigation of the process of grinding reduced sponge iron and development of the technology of producing iron powders of different bulk density

Summary The following grinding technology is recommended for the purpose of increasing the fine-fraction yield of powder: a) grinding of reduced briquets in a hammer mill and the sifting of powder into coarse and fine fractions; b) grinding of the coarse fraction in a ball mill over a period of 16 h and the sifting of powder by means of a vibrating screen; c) mixing of the fine fractions obtained in the first and the second screenings.In order to increase the bulk density of the powder, it is recommended to perform triple grinding of the powder in a hammer mill or to grind it in a ball mill.     

半自磨机顽石作为实验室立磨机介质的磨损与磨矿研究

随着半自磨流程在国内大型矿山的日益普及, 对如何处理半自磨机磨矿过程中产生的顽石, 各选厂采用的方式不尽相同.利用半自磨机顽石充当立磨机磨矿介质处理这类顽石并对此开展顽石的磨矿条件试验研究, 包括:磨矿时间、充填率、磨矿浓度和磨矿处理量等条件试验, 以及顽石、钢球磨矿介质对比试验.试验结果表明:半自磨机顽石可以作为磨矿介质在立磨机内使用, 其损耗量相比较于传统钢球的损耗量 (大约0.05 %) 是较大的, 一般在1 %~2 %之间.在磨矿时间3 min、填充率60 %、磨矿浓度65 %、磨矿量5 kg时可获得满足浮选要求的磨矿产品.在磨矿时间3 min、立磨机转速210 r/min、磨矿浓度65 %、磨矿量5 kg、磨矿介质直径10~20 mm、充填率60 %的条件下进行顽石和钢球磨矿介质的磨矿对比试验, 结果表明, 钢球磨矿产品的细度比顽石磨矿高16.72 %, 过粉碎粒级产率高2.35 %, 钢球的磨矿效率大于顽石, 但顽石磨矿产品中0.045~0.038 mm的产率高于钢球磨矿, 说明顽石充当磨矿介质时易使被磨矿物磨细到嵌布粒度的大小, 会使产品粒度分布更合理.      

球磨机安装问题探析

磨矿作业是金属矿选矿工艺的重要工序,球磨机是磨矿作业的主要设备,球磨机能否长期稳定运行,其安装工程质量至关重要。从基础工作,中心标板和基准点的埋设,底座的安装,主轴承的安装,筒体、端盖、大齿圈的安装,大小齿轮的装配等方面详细介绍了球磨机的安装步骤及安装过程中需注意的问题,值得借鉴。     

Friability and crushing strength of micrometer-size diamond abrasives used in microgrinding of optical glass

In abrasive grinding, the properties of the abrasives and their response to impact loading play a significant role in determining the results achievable. For micrometer-size diamond abrasives used for bound-abrasive microgrinding of optical glass, friability testing is used to estimate the related particle properties. Friability and crushing strength of diamond abrasives are estimated based on the data from comminution of sample powders on a commercial SPEX mixer/mill. Different diamond abrasives as well as a CBN abrasive are tested. Evolution of powder size and size distribution with comminution time is characterized with a HORIBA laser scattering analyzer. Correlation is established for the impact stress and the probability of fracture during comminution. This study demonstrates how to combine the ease of data acquisition found in a conventional friability test with the capability of predicting specific mechanical properties normally found only by crushing individual abrasive particles.