The effect of interstitial gas on milling Part 3: Correlation between ball and powder behavior and the milling characteristics

In two earlier papers [1, 2] the effect of interstitial gas on milling was reported. It showed that these effects were well correlated with changes in powder porosity and consequently with the changes in powder mobility due to variation of gas viscosity and pressure.In this paper the cause of this correlation is further investigated. Starting from a close visual inspection of the processes in a ball mill, a model is developed in which the milling characteristics are correlated to the ball flow and the powder behavior during ball collisions. From experiments, it follows that both are influenced by powder mobility.     

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.     

Characteristics and pozzolanic reactivity of glass powders

This paper deals with the morphology, fineness and pozzolanic activity of four glass powders: one (GP-fine) from the screening of crushed waste glasses, one (GP-dust) from a dust collector for the glass crushing process and two (GP-4000 and GP-6000) from further grinding of the powder from the dust collector in a ball mill. GP-fine and GP-dust consist mainly of large flaky particles, while GP-4000 and GP-6000 consist mainly of small angular particles. The finenesses of these glass powders are measured by particle size distribution and Blaine fineness method. For a similar particle size distribution, ground glass powder has a higher Blaine specific surface area than Portland cement due to the angular morphology of glass particles. Finely ground glass powders exhibited very high pozzolanic activity. The finer the glass powder is, the higher its pozzolanic reactivity is. An increase in curing temperature accelerates the activation of pozzolanic reactivity of both glass powder and coal fly ash in terms of strength development rate. Mortar cube strength results (ASTM C109) indicated that curing temperature has a greater influence on the glass powder than on fly ash. The rapid mortar bar expansion test (ASTM C1260) results indicate that the replacement of Portland cement with ground glass powder also reduces the expansion due to alkali-aggregate reactions, although it is not as effective as coal fly ash.     

The effect of interstitial gas on milling

In a research program on the influence of interstitial gas on the handling of fine powders, particle diameter less than 100 μm, the effect on milling is also investigated.The influence of the interstitial gas is exhibited through the drag force, due to velocity differences, which the gas exerts on the solid particles of the powder. These forces strongly influences the behavior of the powder.Our investigations of milling showed that the milling parameters, i.e. the specific rates of breakage and the breakage parameter, were dependent on the powder flow behavior. Two extremes were the regime of free-flowing powder, where the rate of breakage was high and the grinding of the individual particles was rather ineffective, and the regime in which the powder did not flow at all, where the rate of breakage was low, but where the grinding of the single particles was rather fine.     

Dense phase viscosities of fluidised beds at elevated pressures

The dense phase viscosities of gas-fluidised beds have been measured at pressures up to 20 bar by a falling-sphere technique. Glass powders of mean sizes 64, 101 and 475 μm were fluidised by carbon dioxide or nitrogen. An increase in gas pressure leads to a substantial decrease in the viscosity of the fine powder when fluidised but the viscosity of fluidised beds of powders larger than about 100 μm is almost independent of pressure. The implications of the experimental observations on the stability of bubbles in high-pressure fluidised beds are discussed.     

Increased chemisorption onto activated carbon after ball-milling

Activated carbon has been milled for up to 1000 h in a laboratory-scale tumbling ball mill under a vacuum. Thermogravimetric (TG) analysis of the powder in argon showed an increasing mass loss with milling time indicating the presence of chemisorbed gas. TG Fourier transform infrared spectrometry showed the gas was a mixture of water, CO₂ and an unidentified gas (probably oxygen). BET surface area measurements showed a decreasing surface area with milling time, however, this was shown to be massively in error for the longer milling times due to the presence of the chemisorbed gas. The area occupied by the chemisorbed gas increased from 40 to 80% of the true surface area which was almost constant at 1258±27 m² g⁻¹ for all three powders. These results show that extremely large errors may be made when using BET analysis to determine the surface area of powders, especially those where the surface activity is substantially increased during processing.     

Effect of Processing Parameters on Phase and Microstructure Evolution in RBAO Ceramics

Important parameters controlling the reaction-bonding of Al₂O₃ (RBAO) process are Al content, particle size, and green density. Successful fabrication of high-strength bodies requires fine and homogeneous powders. Low milling intensity does not lead to the required particle fineness, whereas overmilling causes extensive oxidation and hydrolyzation of Al. Oxidation of Al during milling is enhanced by ZrO₂ additions. As much of the Al should be oxidized by solid/gas reaction as possible. Milled RBAO precursor powder contains physically-adsorbed and chemically-bonded (aluminum hydroxide) water. Decomposition of chemically-bonded water promotes cracking in the temperature range 450–550°C. Bloating of RBAO occurs when the surface region becomes dense before complete decomposition of aluminum hydroxide in the interior of the body.     

The effect of pressure on the flow of gas in fluidized beds of fine particles

The division of gas flow between the bubble and dense phases of fluidized beds of six different types of Group A powders has been studied at pressures of up to 20 bar using surface collapse and X-ray absorption measurements. It was found that with these fine powders as pressure increases at constant volumetric gas flowrate so the size and hold-up of bubbles decrease while their frequency increases. Contrary to previous measurements the average bubb velocity appears to decrease with increasing pressure. The dominant mode of bubble break-up in all the powders was found to be division from the rear, contrast to that observed with Group B powders at atmospheric pressure. Interstitial phase voidage was found to increase with increasing pressure.The results are interpreted in terms of a model which assumes a difference between the voidages, and hence the gas flow, of powder in the wakes behind     

Modeling of continuous self‐classifying spiral jet mills part 1: Model structure and validation using mill experiments

The objective of this work is to develop a milling model for a continuous self‐classifying spiral air jet mill. Its foundation is a population balance model with selection and breakage distribution functions that have been related to a minimal number of mill‐dependent and powder‐dependent parameters. Initially, experimentation is required to determine the mill‐dependent parameters for a specific mill, by milling a “base” powder at multiple operating conditions. Powder‐dependent parameters can be determined from either mill experiments or from material characterization measurements that require small amounts of powder (presented in Part 2). Ultimately, the milling model presented successfully predicts the product particle size using as inputs the feed particle‐size distribution and mill operating conditions. Three crystalline powders, sodium bicarbonate, lactose monohydrate, and sucrose, have been used to test the proposed milling model. © 2014 American Institute of Chemical Engineers AIChE J 60: 4086–4095, 2014     

Influence of the deagglomeration procedure on aqueous dispersion,slip casting and sintering of Si3N4-based ceramics

The influence of the deagglomeration procedure on the rheological behaviour of Si₃N₄-based aqueous suspensions, the slip casting performance and the final properties after sintering were investigated. Ball milling and planetary milling performed deagglomeration of powders. The experimental results showed that the time required to obtain the same degree of deagglomeration was considerably shorter in the case of planetary milling. The decrease in viscosity during the milling procedure enabled well dispersed and relatively high-concentrated (55-vol.%), suspensions to be obtained by adding successive 5-vol.% increments of solids to an initial 45-vol.% suspension. The time required to achieve complete deagglomeration of the starting suspension or after adding each 5-vol.% solids' increments was of 4 h for planetary mill and varied from 24 to 48 h for ball mill, with increasing solids loading, reaching total deagglomeration times of 12 and 96 h, respectively. The results have shown that, for a given solids volume fraction, both the degree of deagglomeration and the time required to achieve it, i.e. the duration of the contact between the powders and the dispersing aqueous solution, are key factors for achieving dense and homogeneous green microstructures, and for improving the densification behaviour and final properties of sintered bodies.     

Effects of processing parameters on material properties of mechanically processed polyamide

A semicrystalline polyamide polymer was processed using a new technique known as mechanical alloying. The material processed by this technique was first introduced into a high-energy ball mill and ground over long periods of time, resulting in the production of extremely fine powders. These powders were subsequently consolidated well below the materials' melting-point temperature. The effect of processing parameters including mechanical milling time, consolidation temperature, and the length of consolidation time were studied fairly extensively. The investigation shows that polyamide powders are continually refined with increasing mechanical milling time and the resulting materials have improved mechanical properties. The influence of both consolidation temperature and the length of consolidation time on material properties indicate that materials with higher density, crystallinity, hardness, strength, and ductility are produced when consolidated using higher temperatures and longer times. © 1995 John Wiley & Sons, Inc.     

气－固－固循环流化床填料层中压降及细颗粒含率的轴向分布

在气－固－固循环流化床中，尺寸较大的固相（通常作为催化剂）被固定在床中形成一段填料层，较小的固相一般为细颗粒（通常用作吸附剂或热载体）被气流携带穿过填料层。本文从讨论此类流化床中气－固两相并流流过填料层时压降的数学模型入手，应用实验测得的填料段的压降，细颗粒的平均动含率及细颗粒的循环流率等实验结果，回归了数学模型中的有关参数。在此基础上，应用这一数学模型对细粉的截面平均动含率在填料层轴向的分布行为进行了研究     

Influence of particle size distribution on rheology and particle packing of silica-based suspensions

The effect of particle size, particle size distribution and milling time on the rheological behaviour and particle packing of silica suspensions was investigated using slurries containing total solids loading of 46 vol.%. Three silica powders with different average particle sizes (2.2, 6.5 and 19 μm), derived from dry milling of sand, and a colloidal fumed silica powder with 0.07 μm were used. Different proportions of colloidal fumed silica powder were added to each of the coarser silica powders and the mixtures were ball-milled for different time periods. The influence of these factors and of the particle size ratio on the rheological behaviour of the suspensions and densities of green slip cast bodies was studied.The results show that the flow properties of slips are strongly influenced by the particle size distribution. The viscosity of suspensions increases with the addition of fine particles, imposing some practical limitations in terms of volume fraction of fines that can be added. On the other hand, increasing the size ratio enhanced the shear thinning character of the suspensions, while decreasing the size ratio led to an accentuation of the shear thickening behaviour. For all mixed suspensions, green densities increased with increasing milling time, due to size reduction of silica powders and a more efficient deagglomeration of fumed silica. Increasing amounts of fumed silica led to a first increase of particle packing up to a maximum, followed by a decreasing trend for further additions. Good relationships could be observed between rheological results and packing densities.     

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.     

中国超细粉碎和精细分级技术现状及发展

综述了中国超细粉碎与精细分级技术与设备的现状、近5年的进展 .20世纪90年代中期以来,中国超细粉碎和分级技术取得了显著进步,现已具备了研制和生产气流粉碎机、高速机械冲击式超细粉碎机、搅拌球磨机、振动球磨机、塔式搅拌磨、行星球磨机、高压射流磨、旋风自磨机等各类超细粉碎及涡轮式气流分级机和离心式水力分级机等设备的能力,并在流态化床式气流粉碎机、飓风自磨机、搅拌球磨机和砂磨机、行星球磨机、高压水射流磨机以及精细分级原理和分级设备等方面取得了一些进展,具有自主知识产权的新技术、新设备显著增多.指出中国目前在超细粉碎和精细分级领域仍然存在大型设备不足、工艺控制技术落后、磨耗和单位产品能耗偏高、特殊粒形超细粉体的生产工艺和设备落后等问题.最后对21世纪初中国超细粉碎和分级技术的主要发展趋势进行了展望.     

Analysis of individual and interaction effects of processing parameters on wet grinding performance in ball milling of alumina ceramics using statistical methods

It is of great interest and necessary to closely investigate and understand how ball milling parameters can affect the overall quality of milled ceramic powders. In this paper, we analyzed the individual and interaction effects of the key processing parameters on the wet grinding performance in a ball mill for alumina powder using statistical methods. The grinding performance was evaluated based on three quality characteristics of milled alumina powder: the median particle size (d₅₀), width and skewness reflecting the shape of particle size distributions (PSDs). For this determination, the volume percentage of slurry, solid content, milling speed, milling time, and ball size are regarded as the key parameters affecting these characteristics. Milling experiments were designed by central composite design, and an experimental dataset was collected systematically. The analysis results show that it is worthwhile to consider not only the d₅₀ but also the shape of the PSDs when assessing the ball milling performance. These results are useful for the production of high-quality alumina powders in ceramic industries.     

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.     

A novel process for simultaneous milling and coating of particulates

Ascorbic acid particulates are simultaneously milled and coated with wax in this study using a novel process based on fluid energy milling. A premix of ascorbic acid particulates and wax powders are fed into a fluid energy mill (FEM), inside which ascorbic acid particulates frequently collide with each other, wax powder particles and the wall. Consequently, ascorbic acid particulates are broken down to smaller sizes and coated with wax simultaneously. This novel simultaneous milling and coating process has several advantages compared to traditional separate milling and coating processes such as elimination of solvent usage, reduction of agglomeration, and vastly improved production efficiency. In this study the influences of the grinding air pressure and wax content on the process are investigated. The results suggest that milling and coating should be optimized collectively for this process. In general, higher energy input leads to smaller particulate size and makes it more difficult to coat, whereas higher wax content leads to higher coating coverage and larger particulate size. The wax coating slows the dissolution rate of ascorbic acid in water, especially when the wax content passes a certain threshold value.     

Effect of Milling on the Intrinsic Coercivity of Barium Ferrite Powders

The effect of milling on the intrinsic coercive force of Ba ferrite powders has been ascribed to (1) thermal fluctuations caused by superparamagnetic fine particles and (2) lattice defects introduced by milling. To test these interpretations, the Moessbauer absorption spectra and angular variation of coercive force for ball-milled powders were determined. With increasing milling time, the Moessbauer absorption spectrum becomes ambiguous, and the absorption lines are broadened; the quadrupole-split center line corresponding to superparamagnetic BaFe₁₂O₁₉ cannot be detected. The angular variation of coercive force shows that milling considerably lowers either the nucleation field of the reverse domain or the effective anisotropy field or both. It is concluded that lattice defects, rather than superparamagnetic fine particles introduced by milling, greatly affect the intrinsic coercive force of Ba ferrite powders.     

Interfacial reaction and side effect of MgB2 superconducting material through low-rotation mechanical milling

Powder processing by ball milling is an effective approach for materials engineering. Although various methods for material processing are available, only high-energy shaker/vibratory or planetary mills have been intensively utilized to develop mechanical milling or alloying routes for structural control of MgB₂ superconducting materials. Herein, we have attempted structural modification by using a low-rotation shaker, which is categorized as a low-energy and economical mill in terms of industrial applications. The operation speed was kept constant at 40 rpm, which is much lower than typical conditions employed for planetary mills. Instead of adjusting the low rotational speed, the other processing parameters were controlled to enhance the energy transfer from the balls to powders. The applied milling conditions were ultimately found to cause severe plastic deformation of the raw powders. The shape and size changed drastically, depending on the processing time. The morphological variation of the processed powders as precursors for the MgB₂ materials influenced the void structure and the composition including amorphous phases. By considering these results, we also elucidated the mechanism underlying the structural changes upon ball milling and their effects on the transport critical current performance. The present approach for powder processing offers potential as an effective milling route for structural modification of superconducting materials.