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     

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.     

干法超细加工重钙粉的研究

讨论了加工重钙超细粉的方法，分析了以过热蒸汽为介质的气流磨的特性，理论与实验证明，过热蒸汽气流磨可以大规模，低成本，干法地超细加工重钙粉。     

Comminution and structural changes in a jet mill

The energy of collision in jet mills which grind by means of high-velocity collisions results not only in highly intensive comminution, but certain changes take place in the structure of the material. Even the crystalline particles of so-called rigid materials are known to become microplastic and to accumulate a certain energy when their size is below 1 μm. The energy of collision may remove ions or atoms from the surface of the crystal lattice, producing thereby a mechanochemically activated state. Due to mechanical damage of the crystal structure and to plastic deformation the material may become partly or totally amorphous, and this too may be accompanied by an increase in free energy.Some mineral materials, such as dolomite, limestone, crystalline sulphur and, of the plastics, PVC powder, were ground in a jet mill type Fryma JMRS-80. Our work is part of the research aimed at the utilization of the rich sources of natural high pressure CO₂ occurring in Hungary. Changes in the structure of the material due to comminution were determined by means of differential thermal analysis and X-ray diffractometry.The results confirmed that in addition to producing intensive comminution, high-speed collision affects also the crystal structure of the materials and their internal energy content. If the ground products are destined for further processing, their more active state must be taken into consideration, that is, mechanochemical activation must be taken account of as forming part of the preparatory process.Cooling due to the expansion of the gas entering the milling zone over-compensates for the thermal energy generated there by the friction of the colliding particles. The high milling gas to solid ratio in the jet mill ensures the conduction of any eventual locally arising thermal energy, in contrast to systems in which milling is performed without a gas phase.If a structural change, i.e. disintegration of the pargicles, occurs in course of jet milling, the suggested mechanism is as follows: the mechanical energy introduced into the particles by their collision gives rise in the crystal lattice to oscillations which act by causing a mechanical strain on certain sites (lattice points, bonds) in such a way that local energy accumulation may lead to the disintegration of the particle or to other structural changes [1].Accumulation of the oscillation energy is analogous to the accumulation process of thermal energy, which can be expressed as a temperature rise. The observation of sharp chipped surfaces on jet milled PVC particles, without any sign of local softening, disagrees with the possibility of a local temperature increase.     

Fluidised-bed jet milling of pharmaceutical powders

Jet milling is often employed to produce very fine product or to tackle materials that are difficult to mill. This paper analyses particle breakage in a single jet region in a fluidised bed with a view to identify the role of jet hydrodynamics and material properties. By making a number of simplifying assumptions, the particle breakage in the jet can be estimated by coupling a hydrodynamic model of the jet with the kinetics of single-particle impact breakage. This approach works satisfactorily for a number of particulate solids, e.g., α-lactose monohydrate, but poorly for some others, e.g., paracetamol.The underlying cause of this feature is unclear, although it is likely to be due to the hydrodynamic mechanism. In this work, a sensitivity analysis has been carried out which highlights the importance of some of the hydrodynamic parameters on particle flow and breakage in the jet region. A more general model of jet hydrodynamics is needed to predict reliably the particle behaviour in this region.     

Molecular weight and secondary structure change in eri silk during alkali degumming and powdering

Changes in molecular weight and secondary structure of eri silk during alkali degumming and silk powdering were studied. An increase in silk degumming intensity, through increased alkali concentration, treatment temperature, and time, reduced the fibroin molecular weight and, therefore, the fiber tenacity, but at the same time, increased the β‐sheet fraction. These changes reduced the time required to mill the degummed silk fibers into fine powders. Mechanical forces used in wet attritor and air jet milling disturbed intermolecular packing along the direction of side chains, but the conformation remained essentially β‐sheet even in the sub‐micron silk particles. Dry milling drastically reduced molecular weight and changed the conformation of the fibroin chains. The rate of the spontaneous conformation transition in regenerated fibroin solution prepared from fibers and powders increased with a decrease in fibroin molecular weight, affecting the time fibroin solutions could be stored before gelling. Overall, the study showed that molecular weight and secondary structure of silk powders could be manipulated by suitably changing the degumming and milling conditions. It also suggests that wet media milling and air jet milling are better than dry media milling to prepare less degraded and more crystalline ultrafine silk particles. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Fine milling of fused mullite and mullite-corundum

Conclusions Milling time during the fine grinding of fused mullite and mullite-corundum powders in a vibromill instead of a ballmill can be reduced by a factor of five. The addition of sulfite lye or ethyl silicate in amounts of 0.1% reduces by a factor of 12–18 the vibromilling time, but iron pickup is increased. Grinding in a jet mill causes the specific surface to be much less than milling in a vibromill; the capacity for sintering such powders is impaired.Translated from Ogneupory, No. 6, pp. 17–18, June, 1981.     

气流粉碎机粉碎室速度场研究

本文建立了气流粉碎机粉碎室结构与流动参数间的函数关系,确立了速度场的变化规律,所得结论不仅与Rink等人采用静压测孔推算的速度分布十分吻合,而且还得到作者用激光多普勒测速技术对流场进行速度测试的进一步验证.     

Characterization of media mills based on mechanical energy applied to particles

This paper deals with the evaluation of characteristics of media mills having a different milling mechanism based on the mechanical energy applied to the particles to be processed during a milling treatment. Spherical copper powder was used as a stress-sensitive material and the milling treatment of the copper powder was carried out under various operating conditions using three types of media mills, a horizontal tumbling ball mill, vertical agitating ball mill and bead mill. The size distributions of copper powder before and after the milling treatment were measured and the deformation of copper particles was determined experimentally. The net energy applied to the copper powder was estimated from the plastic deformation of copper particles. It has been clarified that the applied energy depends strongly on the motion of media in the mill. By introducing two dimensionless parameters, which express the energy transfer efficiency from the kinetic energy of media to the particles and the motion of media in the mill, respectively, the media mills could be characterized on a uniform scale based on the applied energy regardless of milling mechanism.     

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 the method and degree of milling on the rate of milling and leaching of industrial alumina

Conclusions An investigation was made of the influence of the method and degree of milling on the time required to obtain a given mean particle size, and on the leaching of refired industrial alumina.It was found that much more time is required for milling of alumina in a ball mill than in a vibrational mill, the more so, the higher the temperature of the preliminary firing of the alumina. At the same time, the mean surface diameter of the powder increases, together with the content of grains of radius greater than 5, while the content of grains of radius less than 2, which is important to ensure good properties of the fired ceramic, decreases.During milling in a ball mill, the amount of ground iron is increased considerably compared with the amount during vibrational milling to the same particle size of the alumina.Leaching of alumina after milling in a ball mill is less satisfactory than after milling in a vibrational mill. As a result, the Al₂O₃ content in the washed material is decreased and the amount of Na₂O, Fe₂O₃, and other impurities is increased.Translated from Ogneupory, No. 1, pp. 43–50, January, 1969.     

Effective comminution of diatomaceous earth in a vertical type jet mill

Comminution characteristics of diatomaceous earth have been investigated in a vertical type jet mill of pilot plant scale; the diameter of the chamber of mill is 26.3 cm. Fluidization of the particles in the chamber has been proposed to enhance the comminution efficiency of the jet mill by promoting the effective contacting of powders with compressed air as well as the chamber wall. Effects of the pressures or densities of pulverizing-air and fluidizing-air and comminution time on the mean size of the particles and the rate of comminution have been examined. The statistical analysis of pressure fluctuations in the chamber of the mill has been utilized to predict the characteristics of the vertical-type jet mill taking advantage of the fluidization technique. The particle size and comminution rate have been correlated with the operating variables, respectively.     

闭路磨磷矿石制浆的设计及应用

某磷肥项目磷酸装置的原料制备采用棒磨机十分级机＋球磨机的湿法闭路磨矿工艺。介绍了该工艺的原料粒度分布、工艺流程和系统控制；论述了棒磨机、斜窄流分级机等主要设备的工作原理和结构特征；通过磨矿系统产品分析数据，总结了湿法闭路磨矿工艺的应用效果和相关注意事项。     

The effect of ball milling time and rotational speed on ultra high molecular weight polyethylene reinforced with multiwalled carbon nanotubes

Ultra high molecular weight polyethylene (UHMWPE) composites reinforced with multiwalled carbon nanotubes (MWCNT) were produced using planetary ball milling. The aim was to develop a more wear resistant composite with improved mechanical properties to be used in stress bearing joints. The effect of manufacturing parameters such as the effect of ball milling time and rotational speed on the final composite was analyzed by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), particle size distribution, and contact angle measurements. Ball milling as a mixing technique for UHMWPE based composites is not a new approach but yet, the effect of time, rotational speed, loading of milling jar, and type of ball mill has not been reported properly for UHMWPE. Composites with 0.5 and 1.0 wt% UHMWPE/MWCNTs were manufactured with different rotational speed and mixing times. The results indicate that rotational speed rather than mixing time is important for dispersing MWCNTs using planetary ball milling. Tensile test showed a slight decrease for the MWCNT concentration of 1 wt% suggesting that this amount is the threshold for a satisfactory distribution of the fillers in the matrix. POLYM. COMPOS. 37:1128–1136, 2016. © 2014 Society of Plastics Engineers     

A novel process for preparation of ultra-clean micronized coal by high pressure water jet comminution technique

A novel process for the preparation of ultra-clean micronized coal is presented in this paper. High pressure water jet mill replacing the ball mill is employed for coal comminution in the new preparation process, which is the essential difference from the traditional one. To compare the new preparation process with the traditional one, the comparison experiments were performed, with froth flotation tests of the fine particles ground by both mills using 0# diesel oil and n-dodecane as collector, 2-octanol as frother, and sink–float separation tests using mixtures of carbon tetrachloride–benzene and carbon tetrachloride–bromoform as dense liquid. Different parameters including combustible recovery, ash content of the clean coal, separation efficiency, and energy consumption were investigated based on the two different preparation processes. The results show that the new preparation process has high combustible recovery, low ash content of the product, high separation efficiency, and low energy consumption compared with the traditional one.The comminution mechanism of high pressure water jet mill is introduced in this paper. The high pressure water jet comminution technique has great potential in coal pulverization, having the advantages of low energy consumption, low iron content, and low equipment wear.     

凹凸棒石粘土助磨超细粉碎的研究

分别选择无机系列硅酸钠等、有机系列硬脂酸等试剂作为盱眙产优质凹凸棒石粘土（下简称凹土）超细粉碎的助磨剂,选用行星式球磨粉碎机作为凹土超细粉碎的研究设备,分别考察助磨剂对凹土机械法超细粉碎的影响。用激光粒度仪检测粉碎样品的粒度变化,用扫描电镜、傅立叶红外光谱考察了晶体结构和形貌的变化。     

Effects of multiwall carbon nanotubes on the thermal and mechanical properties of medium density polyethylene matrix nanocomposites produced by a mechanical milling method

Medium‐density polyethylene/multiwall carbon nanotube (MDPE/MWCNT) nanocomposites were produced by a mechanical milling method using a high‐energy ball mill. The MDPE and MWCNTs were added to the ball mill at a constant 20:1 weight ratio of ball/powders and milled for 10 h to obtain polyethylene matrix nanocomposites reinforced with 0.5, 1, 2.5, and 5 weight percent of MWCNTs. To clarify the role of both MWCNT content and milling time on the morphology of MDPE, some nanocomposite samples were investigated by using a scanning electron microscope. To evaluate the role of milling on the microstructure of the nanocomposites, very thin films of MDPE/MWCNTs were prepared and studied by transmission electron microscopy. Thermal behavior of these nanocomposites was investigated by using differential scanning calorimetry (DSC). Standard tensile samples were produced by compression molding. The dependence of the tensile properties of MDPE on both milling time and MWCNT content was studied by using a tensile test. The results of the microscopic evaluations showed that the milling process could be a suitable method for producing MDPE/MWCNT nanocomposites. The addition of carbon nanotubes to MDPE caused a change in its morphology at constant milling parameters. The results of the DSC tests showed that the crystallization temperature of MDPE increased as MWCNTs were added, although no dependency was observed as milling time increased. Crystallization index changed from 50 to 55% as MWCNT content increased from 0 to 5%. The results of the tensile tests showed that both the Young's modulus and the yield strength of MDPE increased as MWCNTs were added. J. VINYL ADDIT. TECHNOL., 2010. © 2010 Society of Plastics Engineers     

Degradation of natural rubber during mill mastication

Changes in the molecular weight distribution of natural rubber, after mill mastication, have been studied by using a Water's gel permeation chromatography. This study indicates that as the polymer is milled the distribution is narrowed primarily through the breaking down of large molecular weight molecules. The peak of the distribution curve shifts to lower and lower molecular weight with increased milling time but it appears to be approaching a limiting molecular weight after 76 min. of milling time. One unexpected result is the development of a secondary hump in the distribution curve at the high molecular weight end. This hump gets larger and moves to lower molecular weight with increasing milling time until 76 min. of milling, at which time it disappears entirely.     

Effect of mill diameter on the rate of initial grinding in vibration ball mills

Vibration ball milling was carried out using three kinds of mills with the same internal volume but different diameters. The variations of specific surface area produced with time were investigated at a constant vibration intensity using feldspar with an initial size range of 74 – 149 μm. As a result, it was found that the specific surface area of product was markedly influenced by the difference in mill diameter even at the same vibrating conditions. According to an empirical equation proposed in a previous paper, the rate of initial grinding was also found to vary by a factor of 1.4 to 2.0, depending upon the mill used. The dependence of the rate of grinding on mill diameter can be explained by considering an effective area of mill shell capable of colliding with balls and a motion of balls in each mill. An available suggestion for designing a mill pot was obtained from this work.     

Characterization and comparison of gray cast iron powder produced by target jet milling and high energy ball milling of machining scraps

Target jet milling and conventional ball milling were used to produce powders from gray cast iron scraps. Powders of similar size distribution produced by the two methods were pressed at different compacting pressures. Green compacts were made at the compacting pressures of 500, 600, 700 and 800 MPa. Jet milled powder showed good compaction behavior while ball milled powder showed very poor compressibility. Also, balanced compacts composed of 50% Hoganas SC100.26 iron powder and each of the cast iron powders produced in this work were made at 500 and 800 MPa and their green properties were determined. Results showed that, green properties of the jet milled powder were acceptable and superior compared to ball milled powder. The jet milling process proved to be a much more efficient process compared to ball milling in terms of time and production capacity.