The effect of interstitial gas on milling. Part 2

In a previous paper results were presented on the effect of interstitial gas on the milling characteristics of one specific fine powder in a ball mill. This second paper gives more data on two other powders, cracking catalyst and hematite, together with those on the powder used in the earlier experiments, quartz sand. The effects found are similar for each of the three powders: increasing gas pressure or viscosity of the gas or both inside the mill increases the rate of breakage and decreases the fineness of the daughter particles of a milling event. The overall milling speed or production rate as well as the ultimate fineness of the product are both improved by increasing pressure or viscosity.On the basis of these results a comparison is made with wet milling. It appears that pressurized milling, pressure around 10 bar, is a good alternative for the milling of fine powders.     

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.     

Influence of charge on the parameters of the batch grinding equation and its implications in simulation

This paper examines the influence of charge on the weight—size distribution and distribution modulus in the context of the batch grinding model. Variation of the distribution modulus with grinding time is obtained for most of the charges used in this study. It is shown that the scheme for the determination of parameters of the model based on the zero order production of fine particles which minimizes experimental measurements is adequate for a restricted range of charge of the ball mill. Successful simulation of grinding behavior with the parameter values obtained from such a scheme is then limited to the charges which give a maximum absolute rate of breakage. The physical explanation of this behavior is briefly discussed.     

Experimental and numerical analysis of a lab-scale fluid energy mill

This study investigates the grinding performance of Fluid Energy Mill (FEM) through experimental studying and numerical simulation. The experimental parametric study shows that the mean product particle size decreases with grinding pressure (GP) and increases with the solid feed rate (SFR). In comparison, the influence of the feed pressure (FP) on the product size is much less significant. Visualization study indicates the existence of a particle-concentrated layer near the peripheral wall region, named the grinding region in this article since most of the collision-induced size reduction occurred in this region. The grinding air streams re-orient the particles, facilitating particle-particle and particle-wall collisions downstream in the grinding region. To understand the influence of the particle-wall collision, the peripheral wall of FEM was coated with a foam film in some experiments. The particle-wall collision was found to play a significant role in size reduction, especially under low air pressure.The gas flow inside the grinding chamber was simulated as the initial step to the ongoing 2-phase flow simulation of the milling process in the FEM. The simulation results show that eddies are formed at the feed air entrance, which explains the tendency of fine particle deposition in this region. The simulation results also suggest a strong relationship between the GP and the mean gas velocity in the grinding region.     

The Effects of Operating Conditions on the Milling of Microcrystalline Cellulose

There is little detailed work relating the physical process that occurs during milling to the mechanical properties and mechanism of particle breakage. Very often, the selection of an appropriate mill and subsequently the determination of its optimum operating conditions are by trial and error. This paper look into optimizing the operating conditions of a ball mill through statistical analysis and the effect of temperature on the milling behavior of a common pharmaceutical excipient, microcrystalline cellulose (MCC). In addition, the bulk milling behavior of MCC is compared to its single particle breakage behavior. In this work, milling is conducted in a Retsch single ball mill where a bed of powder is subjected to impact by a steel ball in a horizontal cylindrical container. The container is vibrated horizontally at a set frequency, causing the ball to impact on the bed of particles. It is found that the finest MCC product can be achieved by milling a 2 g batch of material using a 12 mm ball size and at a frequency of 18 Hz. Temperature is found to have insignificant effect on the extent of breakage of MCC in both bulk milling and single particle impact testing. Milling and single particle impact experiments have both shown that MCC is more susceptible to breakage with increasing strain rate. In conclusion, the single impact tests could be used successfully for predicting the bulk milling behavior of the material, as shown in the case of MCC.     

Influence of process parameters on breakage kinetics and grinding limit at the nanoscale

In long‐term milling experiments, in a stirred media mill, a grinding limit where no further particle breakage occurs was identified. During mechanical stressing of the particles, defects are generated in the crystalline lattice, which allows real fracture of nanoparticles. Below a critical size, defects cannot be stored or generated in the crystallites and the overall limit of grinding is reached. This limit is strongly influenced by material properties and hardly affected by most of the process conditions. However, the breakage kinetics strongly depend on the process parameters and suspension conditions as long as the grinding limit is not reached. Based on these findings, two mechanisms of nanoparticle breakage are proposed. Proper choice of process parameters saves not only up to 90% of the energy input to reach the grinding limit but also leads to a higher product quality in terms of crystallinity and less milling bead wear. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Bulk coarse particle arching phenomena in a moving bed with fine particle presence

Bridging or arching of flowing solids particles is a serious hazard in the operation of moving bed systems. The mechanics of the arching has been extensively analyzed in the context of particle discharge from a hopper with conical geometry by considering the particulate layer stress distribution. However, bridging can also occur in a moving bed system with cylindrical geometry during the continuous mass flow of solids particles. Experimental work conducted in this study reveals that the appearance of solids bridging is normally accompanied by the presence of fine particles in the coarse moving particles as well as by the countercurrent interstitial gas flow. In this study, a stress analysis of the layered particles distributed in a cylindrical, vertical moving bed that flows downward opposing to upward flow of the interstitial gas is developed to quantify the bridging phenomenon. The analysis takes into account of the effects of presence of fine powder in the coarse particle flows and properties, such as particle‐size distribution, bed voidage, and interstitial gas flow rate. The experimental validation of the present stress analysis for moving bed systems with varied fine and coarse particle concentration distributions, and interstitial gas velocities is also conducted. The stress distributions of the particles under flowing and arching conditions are obtained. An arching criterion is formulated, which indicates that the critical radius of the standpipe to avoid arching phenomenon is only related to the property of the bulk solids in the present geometric configuration of the flow system. © 2014 American Institute of Chemical Engineers AIChE J, 60: 881–892, 2014     

Powder‐shape analysis and sintering behavior of high‐density polyethylene powders for rotational molding

The quality of rotational molded products is strongly affected by the sintering behavior of the powders used in the process. In turn, for a given material, the sintering behavior of polymer powders is dependent on the size and the shape of particles obtained in the milling apparatus. The quality of powders for rotational molding is usually determined by means of size distribution, dry flow, and bulk density tests. However, these tests do not provide insight into the relationship between the shape of powders, the milling conditions, and the sintering behavior during the rotational molding cycle. Nevertheless, the application of mathematical tools to powder analysis can significantly improve the efficiency of the grinding process, looking not only at the size but also at the shape of the powder. This can in turn result in a higher reliability of rotational molding and in better performances of the products obtained in processes dominated by the sintering behavior of polymer powders. In this work the grinding process of recycled high‐density polyethylene was analyzed using a quantitative approach to the shape and size of the powders. In particular, shape factors, capable of characterizing powders obtained in different milling conditions, were studied. Finally, the influence of the powders' shape and size on sintering behavior was studied by thermomechanical analysis. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 449–460, 2004     

Ball-mill grinding kinetics of master alloys for steel powder metallurgy applications

The grinding kinetics of three newly developed master alloys for steel powder metallurgy applications were investigated using a laboratory ball-mill. Non-first order grinding kinetic is observed for the three master alloys as the breakage rate increases with grinding time due to the work hardening of the ductile phase in the microstructure. Agglomeration of fine particles is observed after a critical time at which d₉₀ reaches its lowest value (~ 30 μm). Critical times are related to the hardness and the microstructure of the different master alloys. Agglomeration of fine particles can be overcome with the use of a process control agent. In this study, the addition of stearic acid to master alloy powders prior to grinding successfully eliminates agglomeration for long grinding times (d₉₀ ~ 16 μm after grinding for 270 min).     

Fine grinding of aggregated powders

In many cases, including natural ores as well as synthetic powders, fine grinding involves the breakage of bound aggregates rather than solid particles. The characteristics of breakage in such systems have been investigated by experimental studies of grinding kinetics, in a model system of partially sintered alumina particles, ground in a laboratory centrifugal ball mill. The effects of aggregate strength (extent of sintering) and energy input (mill speed) on the breakage rates and breakage distributions have been evaluated. Breakage appears to occur primarily through splitting of the aggregated mass into two or three smaller aggregates accompanied by release of the primary particles, leading to strongly bimodal breakage distributions.     

Characterization of intermetallic Fe-Mn-Si powders produced by casting and mechanical ball milling

Mechanical milling is a common method used to produce different powders. Milling time is one of the most important factors in the process, which affects characteristics such as particle size distribution and morphology. Four compositions of mechanically milled Fe-Mn-Si master alloy powders were investigated in the present paper. Milling times from 10 to 120 min were used. Particle size distribution and milling kinetics of Fe-Mn-Si powders were studied, and the parameters in breakage function have been determined. The results show that powder characteristics vary with the contents of silicon and manganese. During milling, the particle size initially decreases. At longer milling times, however, small particles agglomerate to larger particles (overmilling). The optimum milling time to get powders with very fine particle sizes is alloy-dependent. Apart from the agglomeration, the milling process of Fe-Mn-Si powders can be described by a classic batch-grinding equation based on the population balance model.     

Rapid Process for Manufacturing Aluminum Nitride Powder

A rapid, direct nitridation process for the manufacture of sinterable aluminum nitride (A1N) powder was developed at the pilot scale. Atomized aluminum metal and nitrogen gas were heated and reacted rapidly to synthesize A1N while they passed through the reaction zone of a transport flow reactor. The heated walls of the reactor simultaneously initiated the reaction and removed the generated heat to control the exotherm. Several variations of the process were required to achieve high conversion and reduce wall deposition of the product. The fine A1N powder produced did not require a postreaction grinding step to reduce particle size. However, a secondary heat treatment, following a mild milling step to expose fresh surface, was necessary to ensure complete conversion of the aluminum. In some instances, a final air classification step to remove large particles was necessary to promote densification by pressureless sintering. The A1N powder produced was pressureless sintered with 3 wt% yttria to fabricate fully dense parts which exhibited high thermal conductivity. The powder was shown to be less sinterable than commercially available car-bothermally produced powders.     

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.     

Flow regime determination in horizontal pneumatic transport of fine powders using non-intrusive acoustic probes

A variety of flow regimes may be observed in dilute phase pneumatic transport of fine powders. As the gas flow rate is reduced or the solids flow rate increased, particles may settle on the bottom of the horizontal sections, forming either a stagnant layer or slowly moving dunes. This change in flow regime leads to such problems as flow instabilities and very long residence times for some particles. Maintaining a consistent operation and product quality requires rapid detection of any change in flow regime. In many applications, particularly in pharmaceutical processes, the installation of intrusive sensors is undesirable. The objective of this study was to develop reliable flow regime detection through the on-line analysis of signals from non-invasive acoustic sensors.Non-intrusive microphones were used to record acoustic emissions generated by powder flow through a horizontal, 0.1 m diameter, stainless steel, pneumatic transport pipe, at various solids fluxes and superficial gas velocities. Measurements were recorded simultaneously on the top and the bottom of the pipe, to record the flow of solids as they hit and are reflected from the pipe walls. To confirm the flow regimes, high speed video imaging in a section of clear acrylic pipe allowed for detailed analysis of the flow structure. Two flow regimes were observed: dilute phase flow and conveying over settled solids. Cycle or frequency analysis of the acoustic measurements recorded from the top or the bottom of the pipe provides reliable, on-line detection of these flow regimes.     

Use of the attainable region analysis to optimize particle breakage in a ball mill

Particle size reduction is one of the most widely encountered, yet least energy efficient, processes. Therefore, potentially significant energy and cost savings exist with even the slightest increase in milling efficiency. Often one would like to mill particles to a certain size, and no smaller, while minimizing energy use and milling time. We use the attainable region (AR) analysis to optimize the comminution of silica sand particles in a bench top laboratory ball mill. When the mill is loaded with a large number of grinding media (J=volume of media/mill volume=10.7%), the breakage profiles are indistinguishable over all rotation rates investigated. However, operation at lower grinding media fill level (J=1.5%) reveals separation between the grinding profiles for different rotation rates, suggesting more efficient breakage occurs at a lower grinding media fill level for a given rotation rate. Our results show that operation at multiple speeds, fast at first and then slower (φ_c=0.03), takes advantage of the initially overlapping grinding profiles and produces a similar particle size distribution with a decreased amount of processing time—less than half the time required for the single rotation rate milling. A natural extension of this idea is continuous milling, where the first mill can operate at a higher energy input for a shorter amount of time and the second mill can operate at a lower energy input for a longer amount of time.     

Production of Fine Polymer Powder under Cryogenic Conditions

In order to produce fine polymer powders, a special and unconventional cryogenic grinding system was established using liquid nitrogen, where a jet‐vortex mill was used as the grinding mill. The major feature of this grinding process is that heat generation during the grinding period was eliminated. The results suggest that this cryogenic grinding system may be suitable for studying the grinding properties of polymeric materials. It may also be helpful in understanding mechanochemistry, e.g., the t‐P‐T conditions for different mechanochemical processes under cryogenic conditions (where T is the temperature, and P the pressure of the gas mixture in the grinding chamber). In addition, an Elbow‐jet classifier was attached to the jet‐vortex mill so that fine, medium and coarse products of polymeric powders could be obtained simultaneously. Chitin, a type of renewable natural polymer, was ground in the system and XRD analysis of ground powders showed they displayed highly activated properties. Unlike a high‐energy mechanical milling process, such as a vibratory (bead) mill which requires more milling time t, the final properties of the ground polymer in the cryogenic grinding system were highly dependent on the temperature in the chamber of the jet‐vortex mill. The grinding results of chitin also showed that the minimum diameters of the ground polymer products are larger than several tens of micrometers (e.g., 75 μm). The developed method offers a new choice for the production of materials, polymer modification (e.g., degradation), and recycling of wasted rubber and plastic.     

Effect of solid circulation rate on coating efficiency and agglomeration in circulating fluidized bed type coater

Circulating fluidized bed was proposed to be used as a coater, and coating experiments of glass beads with silica powder were performed in a circulating fluidized bed. Glass beads and silica powder were chosen as model particles, because their shape was almost spherical. The respective effects of gas flow rates supplied from a distributor and from an air nozzle for solid circulation, feed rate of powder suspension and particle content in the bed on coating efficiency and agglomeration are mainly discussed. Coating efficiency in circulating fluidized bed coater was correlated well with solid circulation time rather than with gas flow rates or solid circulation rate, while the agglomeration among core particles was mainly governed by solid circulation rate.     

基于颗粒间相互作用的细颗粒粉体料仓下料过程分析

以玻璃微珠、流化床裂化催化剂颗粒、褐煤和聚氯乙烯颗粒为实验物料,开展粉体流动性表征与料仓下料实验。研究发现,不同粉体的流动性差异较大,相应的料仓重力下料结果也不同;实验所用粉体的下料流率远低于传统Brown and Richards模型的预测值。分析表明,颗粒间相互作用导致的粉体黏附团聚是阻碍细颗粒粉体下料流动的主要原因。基于上述分析,利用剪切测试结合摩尔应力圆理论获得床层拉伸应力,并借助Rumpf方程构建的颗粒间相互作用与粉体床层应力之间的模型来获得不同粉体的颗粒间作用力;继而采用Bond数对粉体床层空隙率进行修正,揭示了颗粒间相互作用对粉体床层结构的影响,并在此基础上建立了粉体下料流率预测模型。新建立的耦合颗粒间作用力的粉体流率模型,有效改善了传统模型对细颗粒粉体流率预测值偏高的弊端,显著降低了流率预测偏差。