Production of finely ground natural graphite particles with high electrical conductivity by controlling the grinding atmosphere

Natural graphite particles with high crystallinity sieved to obtain a particle size range of under 63 μm were ground with a ball mill, under various well-controlled grinding atmospheres such as N₂, O₂, He, H₂, and vacuum. The ratio, X_dif50/X_st50, i.e. between the 50 wt.% Stokes diameter and the 50 wt.% laser diffraction diameter, of the ground particles, was used as an index of the flakiness of the particles. The specific resistance of films composed of the ground graphite particles was systematically measured. The rate of reduction in the size of the particles by grinding was slow under an O₂-rich atmosphere such as 100% O₂ and dry air. On the other hand, it was relatively fast in vacuum, or under an N₂ or He atmosphere, and a gas mixture of 99% N₂ and 1% O₂. The rate of size reduction by grinding under a H₂ atmosphere was intermediate. In our experimental conditions, the flakiness of the ground particles increased with the decrease in the particles’ sizes. The electrical conductivity of the ground particles, however, tended to decrease with the decrease in their sizes. Under the condition that the Stokes diameter of the ground particles remains constant, the electrical conductivity of films made from the ground particles increases with the increase in the flakiness of the particles. It was finally determined from our systematic grinding experiments that small flaky particles, which had a size, X_st of ∼1 μm, with a high electrical conductivity can be produced by grinding in a gas mixture of 99% N₂ and 1% O₂. In this case, the flaky shape of the ground particles was visually confirmed by scanning electron microscopy.     

Colloid Mills: Theory and Experiment

In a colloid mill utilizing a rotating and a stationary plate, a high shear field exists. Particles rotate, generating a lift force moving them to the rotating plate. When particles collide, a substantial energy transfer occurs because their surface velocities are opposite. A mathematical model has been developed relating the particle rotational speed to the parameters of the colloid mill such as gap size, speed, slip viscosity, and particle size distribution. A slurry of the material being ground is forced into the gap. Grinding is autogenous as a result of collisions between rotating particles. All of the material in the process stream is ground finer than the gap setting and grinding can be optimized by adjusting mill operating parameters. However, the mill is not able to grind the incoming stream to submicrometer sizes and there is molecular contamination from the surface of theB₄C cones.     

Analysis of nonlinear batch grinding in stirred media mills using self-similarity solution

Generally, particle breakage rate is considered to be independent of the grinding environment, and hence, the system is referred to as a linear time-invariant grinding system with first-order grinding kinetics. However, time-dependent breakage rate exists and perhaps, is more critical for fine grinding of particles. The time-dependent breakage rate also introduces nonlinearity in the grinding phenomena. In the present work, a self-similarity based approach is described to model the evolution of fine particle size distributions in a batch stirred media milling with an emphasis on the nonlinear breakage rate function by considering the breakage rate to be a function of the grind time. The present approach yields analytical expressions for cumulative weight percent finer distributions for the continuous-size continuous-time population balance equation. The breakage parameters in the analytical solution can be estimated for a given system from any three measured size distributions that show self-similarity and these parameters can be used to predict distributions evolving at higher grind times. Several sets of published data of stirred media milling are employed to validate the model.     

研磨介质在细磨煤系煅烧高岭土中的行为与影响

利用戒严型湿式搅拌磨,研究了磨矿介质各行为－－介质种类、介质尺雨配比和介南物料比等对煤系煅烧高土超细磨矿的影响。在优化研磨介质等条件的基础上,进行了模拟生产状态的连续磨矿试验,实现了磨矿产品－２μｍ含量大于９０％的预期结果。     

水泥熟料的粉磨动力学研究

采用行星磨对水泥熟料进行不同时间段的研磨,通过测定粉磨产物的粒度分布、比表面积和45 μm筛余值,以Rosin-Rammler-Bennet(RRB)方程作为粒度分布模型,运用线性回归分析和粉磨动力学的特征粒径分析的方法,对熟料进行了粉磨动力学研究和粉磨性能评价.实验结果表明:经过研磨后,水泥熟料粉体的比表面积、特征粒径和均匀性系数与粉磨时间的对数或双对数呈线性关系,熟料的粉磨速度方程为dS/dt=170.66/t,且熟料粉磨细度不宜高于550 m~2·kg~(-1).     

Synthesis of dispersed CaCO3 nanoparticles by the ultrafine grinding

A one-step grinding process to obtain CaCO₃ nanoparticles from a micrometer-sized CaCO₃ was studied. A high-speed beads mill was employed to grind the particles, and poly(acrylic acid, sodium salt) was used to disperse the ground particles. The main parameters, which were investigated, were the slurry concentration, the rotor speed, the bead size, and the surfactant concentration. The larger bead size, higher slurry concentration, and faster rotor speed showed higher grinding efficiencies. However, there was severe agglomeration of the ground particles resulting in larger secondary particles as the grinding time increased after the certain point. The dispersion and enhanced grinding of particles were achieved by the surfactant. The particle size distribution of the ground particles had a narrow peak around 190 nm that was measured by the diffraction method. The primary particle size of the ground particles was around 40 nm.     

The effect of mechanical grinding on the mesoporosity of steam‐activated palm kernel shell activated carbons

BACKGROUND: Palm kernel shell activated carbon (OPSA) produced by steam gasification at high temperatures generally results in high surface areas of 1146 to 1600 m² g^?1, attributed to the high volume of micropores (0.43 to 0.56 cm³ g^?1). The mesoporosity of naturally occurring activated carbons is observed to increase with decreasing particle size. Mechanical grinding was therefore performed to investigate its effect on the mesoporosity and microporosity of OPSA. RESULTS: Mechanical grinding had a strong effect on mesopore volume and average pore diameter, with an increase in mesopore volume from 47 to 66% as particle size decreases. Interestingly, no significant effect on the micropore fraction was observed in ground OPSA particles. CONCLUSIONS: The mechanically ground OPSA particles possessed dual adsorption capabilities due to the high microporosity and moderate mesoporosity contained in the structures. This results in interesting porosity behaviour of palm kernel shell activated carbons and the potential to provide materials of distinct sorption capacities with minimal treatment. Copyright © 2009 Society of Chemical Industry     

分别粉磨与共同粉磨对石灰石硅酸盐水泥颗粒分布及水化性能的影响

试验采用分别粉磨工艺和共同粉磨工艺配制石灰石硅酸盐水泥,分析了石灰石和熟料的粉磨特性差异,并研究了在单位能耗相同情况下,两种粉磨制度下石灰石硅酸盐水泥各组份颗粒分布及水化情况。结果表明,分别粉磨制度能合理控制石灰石硅酸盐水泥颗粒的分布,使熟料处于更细的颗粒范围内,提高了粉磨效率;细的石灰石粉能促进水泥的早期水化反应,而中后期的水化速率主要取决于熟料颗粒的细度;采用合理的分别磨粉工艺配制的石灰石硅酸盐水泥,在相同龄期下,其水化产物含量较多,水化产物之间的连结更紧密,水泥石整体结构更密实。     

行星振动磨机的作用原理与特性分析

行星振动磨机是集行星磨与振动磨于一体的新型超细粉磨设备。本文通过分别在行星振动磨和振动磨中粉磨氧化锆、氯化铝和石英等物质，对比其粒度分布，比表面积和平均粒度的结果，说明行星振动磨机的粉磨特性。通过在不同转速，不同振幅下的粉磨，进一步分析行星振动磨机的工作原理和工作参数。运用Z射线衍射分析，说明在超微粉碎过程中发生机械力化学效应，从而增强了超细粉体的化学活性。     

Optimization of some parameters of stirred mill for ultra-fine grinding of refractory Au/Ag ores

In this study, optimization of some parameters of stirred mill on ultra-fine grinding of refractory Au/Ag ores was performed. A three-level Box-Behnken design combining a response surface methodology (RSM) with quadratic programming (QP) was employed for modelling and optimization of some operating parameters in ultra-fine grinding. Grinding tests were carried out in a laboratory scale pin-type vertical stirred mill. The relationship between the response, i.e. d₈₀ size, and four grinding parameters, i.e. ball diameter, grinding time, ball charge ratio and stirrer revolution was presented as empirical model equations. Analysis of variance showed a high coefficient of determination value (R² = 0.9698), thus ensuring a satisfactory of the second-order regression model with the experimental data.The model equations were then optimized using the quadratic programming method to minimize for d₈₀ size within the experimental range studied. The optimum conditions were found to be 1.61 mm for ball diameter, 11.50 min for grinding time, 80% for ball charge ratio and 745 rpm for stirrer revolution for this grinding process.In order to verify the improvement of grinding performance using the optimal level of control factors three verification experiments were conducted, and the results for d₈₀ was 3.37 μm, which were smaller than those obtained in the initial tests.     

一水硬铝石的机械化学处理

通过X射线衍射、颗粒粒度测量、红外光谱以及N2吸附技术分析干磨对一水硬铝石结构的影响。球磨可增加一水硬铝石的BET(Brunauer—Emmet—Teller)比表面积。但随着球磨时间的延长,颗粒粒径的减小速率会逐渐降低。球磨2h后,一水硬铝石颗粒粒径的减少存在一个极限。研究表明,球磨过程会引发颗粒的团聚。随着球磨时间的增加,所引发的机械化学效应显著加剧一水硬铝石的X射线衍射谱中各晶面衍射峰强度的降低,但在1～2h内没有新的物相生成,一水硬铝石的α—AlOOH结构保持完整,这对于一水硬铝石深加工制备功能矿物材料十分有利。N2吸附研究表明,球磨12h后一水硬铝石颗粒的孔径比球磨6h和原始样品的孔径稍小,这充分说明干磨有利于颗粒微孔结构的形成,为制备催化和多孔材料奠定了较好的基础。红外分析结果显示,机械化学处理后一水硬铝石的特征峰也会发生一定的变化。     

A comparison of dry and wet grinding of a quartzite ground in a small batch ball mill

Classical grinding models involve the selection function (S), which gives the rates of breakage of particles of each screen size fraction, and the breakage function (B), which describes the instantaneous size distributions of fragments produced when the particles of each fraction are broken. In order to investigate the differences between dry and wet grinding as far as the selection and breakage functions are concerned, batch grinding experiments were performed on both dry and wet bases, on the same material, a quartzite, in a small ball mill under similar experimental conditions.On a dry basis, the rates of breakage were found to be time invariant and independent of the size environment in the mill. It is logical to postulate a similar behavior for the breakage function. On a wet basis (65% solids), an increase of the rates of breakage was observed as grinding proceeds. This behavior is essentially due to the variation of the size environment within the mill. This increase in breakage rates was, however, less and less important as the particle size decreased and was not observed for the smallest particles tested. These points were confirmed by considering the disappearance kinetics of samples of different screen size fractions of quartzite injected in the mill during the batch grinding of a limestone. Moreover, it is not impossible that the breakage function could also vary with grinding time, giving rise to finer instantaneous size distributions of fragments as the size environment in the mill becomes finer. As an overall result, wet grinding has appeared more selective than dry grinding for coarse material, while it did not produce more schlamms.     

Effects of concentration of dispersions on particle sizing during production of fine particles in wet grinding process

Stirred media milling is a prospective technology for producing colloidal dispersions by means of wet grinding process. In the past, many researchers have studied the effects of different operating parameters such as size, shape, nature and quantity of grinding medium, the speed of agitator in grinding chamber, the feed rate of dispersions, etc. in stirred media mills. However, it is still less known how particle sizing which generates valuable information of particle size of the product to interpret, control and optimize the grinding process, is influenced by the concentration of the dispersion during stirred media milling where particles change their size from micron to colloidal range rapidly. One of the reasons of this lack had been our incapability in the past to study the particle size distribution of dispersions without dilution. The recent advent of acoustic attenuation spectroscopy is known to be capable of studying dispersions without dilution, under real process conditions and on line. The study employs acoustic attenuation spectroscopy to investigate the effects of concentration of dispersions of CaCO₃ on its particle sizing during size reduction process in a stirred media mill (LabStar manufactured by NETZSCH). The dispersions of CaCO₃ at 5%, 10%, 20% and 30% (m/m) were studied about six hours under a selected set of operating conditions. Contrary to the existing knowledge obtained through other techniques of particle sizing that are based on the principle of dilution, acoustic attenuation spectroscopy shows that, under certain grinding time at given operating conditions, increase in concentration of dispersion results in better grinding results yielding smaller particles. The causes behind the differences in results of acoustic attenuation spectroscopy and dynamic light scattering have been thoroughly investigated. We find certain limitations of acoustic attenuation spectroscopy in particle sizing. A typical phenomenon which causes misleading trends in particle sizing is multiple scattering in acoustic measurements. Multiple scattering, particularly, influences acoustic results when particles approach to fine size range during size reduction process.     

Pre-Concentration of Ultrafine Crushed Hematite Ores

In this study, a lean hematite ore was crushed to produce ultrafine particles using a high pressure grinding roll and then pre-concentrated using a high gradient magnetic separator with a steel rod matrix to reduce the ore tonnage for milling and grinding energy consumption. Pilot test results showed that the primary concentrate grade was increased by 6.98 percentage points from the crude ore with 18.34% Fe at an optimum pre-concentration condition obtained using a matrix with a rod diameter and a rod gap that both measure 4 mm. The concentrate recovery reached a maximum of 86.13% with a tailings yield of 37.61%. Primary concentrates were separated further, and clean concentrates with a grade of 67.15% were obtained.

In a magnetic field around a matrix rod, the magnetic capturing force on coarse hematite particles was increased, but such force on fine hematite particles was decreased as the matrix rod diameter increased at the same background magnetic field intensity. Considering gravity and viscous resistance of locked particles, the matrix with large diameter rods (e.g., 4 mm) could effectively and completely recover these particles. By contrast, the matrix with small diameter rods (e.g., 1 mm) could efficiently recover fine particles but not coarse particles such as lean-locked particles.     

An investigation of carbon nanotube jet grinding

Since their observation in 1976 and 1991, carbon nanotubes (CNTs) have generated much interest due to their properties and potential applications. CNTs are tubular carbon molecules with remarkable mechanical, electrical, chemical and thermal properties, which make them useful in various applications. Industries producing CNTs via the fluidized bed chemical vapor deposition technique face challenges related to the size of CNT bundles. The two main challenges are agglomeration and agglomerate size distribution control. A solution to these challenges involves the use of jet mills to grind the CNT agglomerates. The goal of this study was to determine whether the nanotubes could be ground with air jets using a commercial jet mill and apply a two-parameter model to describe the grinding process. The present study has indicated that air-jet grinding of CNTs is feasible with a typical commercial jet mill. This paper presents the effect of operational parameters on the arithmetic mean diameter of the ground product. Sonic velocity through the grinding nozzles was required to obtain reasonable grinding rates and relatively narrow particle size distributions. This occurs at high air to solids feedrate ratios. Additionally, a simple attrition model can describe the grinding process in the spiral jet mill.     

Mechanism for performance of energetically modified cement versus corresponding blended cement

The microstructure of cement paste of 50/50 mixes of cement/quartz and cement/fly ash, both ground in a special mill [energetically modified cement (EMC) process] and simply blended, have been studied under sealed curing conditions. The grinding process reduced the size of both cement grains and quartz/fly ash markedly and created flaky agglomerates of high inner surface for the finer particles. EMCs had much higher degree of hydration at 1 day, but similar as blends at 28 days. The pores were much finer for EMC paste due to smaller particles as also reflected in the strength. The morphology of calcium hydroxide in EMC paste appeared more mass like. Pozzolanic reaction was insignificant for quartz in EMC, but increased for fly ash. Thus, improved performance of EMC versus OPC can be explained by increased early hydration and extensive pore size refinement of the hardened binder resulting in reduced permeability and diffusivity for concrete.     

湿法超细研磨中无机材料浆体的流变性研究

概述了无机材料超细粉体机械研磨制备中浆体的流变性能、流变性能的表征方法和流变学行为的经验方程模型.对影响超细粉浆体流变性能的参数(如固相含量、颗粒粒径和分布、形状、温度、pH值及分散剂)进行了讨论.对浆料流变学性能进行最优化以提高超细粉体的产量,降低能耗,提高产品细度.提出了今后的研究重点应放在湿法超细研磨过程中分散剂种类及用量对浆体流变学性能改变机理的研究,并建立可描述浆体流变性能与粉碎参数、分散剂用量、能耗和颗粒大小的数学关系.     

粉磨工艺对水泥厂煤粉中矿物分布规律的影响

采集了山东泰安中联水泥厂立式辊磨和管式球磨两种不同粉磨方式的出磨煤粉,通过浮沉、工业分析、粒度分析,研究了不同粉磨工艺对于煤粉粒度-灰分规律的影响。研究发现,两种不同粉磨工艺的出磨煤粉都呈现出灰分随密度级别的增大而递增,随着粒度的增加而递减的规律,各密度级别浮沉产率和粒度-灰分曲线相似,说明粉磨方式对煤粉中矿物规律没有影响。     

超精密表面研抛磨粒的研究进展

半导体材料的超精密加工是一种获得高表面质量和表面完整性的加工技术,研抛磨粒是实现半导体材料超精密加工的关键耗材之一。从研抛磨粒的组成方式和结构特点,概述了研抛磨粒的研究现状和发展趋势。首先,构建了研抛界面内半导体材料工件-研抛磨粒-研抛垫的接触模型,讨论了研抛磨粒的材质、形状、浓度、粒径等因素对半导体材料研抛质量和研抛效率的影响;其次,从材质和粒径等方面介绍了混合磨粒的研抛性能,以及相应的研抛机理;然后,从材料结构和化学作用等角度总结了复合磨粒在超精密加工技术中的应用;最后,展望了研抛磨粒未来的研究方向。     

Application of grinding kinetics analysis of inorganic powders by a stirred ball mill

The need for ultra fine particles has been increasing in the preparation field of raw powders such as fine ceramics and high functional products. A series of wet grinding experiments were carried out on inorganic powders such as calcite, pyrophyllite and talc by a stirred ball mill. The grinding rate constant K’ in the equation of grinding kinetics was examined based on the grinding kinetics analysis as the same type of function of a previous paper on a vertical type planetary ball mill. The experimental particle size distribution of the ground products was obtained in various grinding conditions. The grinding rate constants K and K’ were expressed by empirical equation involving experimental conditions by a stirred ball mill. The empirical equation on the grinding rate constant was expressed in terms of a function involving the ball diameter of grinding balls, the median diameter of feed material, and Bond’s work index of material, in the experimental conditions. The values of empirical constants C1 and C₂ were 21.13 and 0.0109 on K, while C₁ and C₂ were 120.99 and 0.0192 on K′, respectively. And the particle size distribution of ground products of each test material for a given grinding time was found to be expressing the selection function (the specific rate of breakage) which was obtained from the grinding kinetics analysis. In this study, the grinding rate change on calcite and pyrophyllite was similar at the same experimental operation condition. However, in the case of talc, it was observed that the grinding rate was not increased compared with other samples.