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.     

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     

Formulation of a physically motivated specific breakage rate parameter for ball milling via the discrete element method

A physically based specific breakage rate parameter of the population balance model for batch dry‐milling is formulated, which explicitly accounts for the impact energy distribution calculated by the discrete element method (DEM). Preliminary DEM simulations of particle impact tests were first performed, which concluded that dissipation energy should be used in contrast to collision energy to accurately define the impact energy distribution. Subsequently, DEM simulations of the motion of spheres representing silica glass beads in a ball mill were performed to determine the specific breakage rate parameter, which was in good agreement with those found experimentally. An analysis of the impact energy distribution, which was only possible within context of the physically motivated specific breakage rate parameter, emphasized the importance of accounting for a threshold impact energy. Without proper assessment of the impact energy distribution, DEM simulations may lead to an erroneous evaluation of milling experiments. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2404–2415, 2014     

Analysis of the milling rate of pharmaceutical powders using the Distinct Element Method (DEM)

The milling behaviour of microcrystalline cellulose (MCC) and α-lactose monohydrate (αLM) in an oscillatory single ball mill has been analysed by using the Distinct Element Method (DEM). The experimental results suggest that the milling behaviour of αLM is more strongly influenced by the milling frequency as compared to MCC. A similar conclusion is also drawn from the DEM results. The milling behaviour of MCC and αLM is described by a first order rate process, and its rate constant, K_p, is found to correlate very well with the milling power, P_n, determined by the DEM simulation, except for the milling behaviour of αLM at 18 Hz. For the latter, there appears to be an incubation time after which the milling rate increases substantially. The results presented here provide a basis for predicting the milling behaviour of a material systematically based on the fundamental material properties and the machine dynamics without the need for extensive experiment and use of large quantities of materials.     

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.     

The influence of grinding technique on the liberation of clinker minerals and cement properties

This paper describes a study of the relationship between the physical, chemical and mineralogical parameters of cement products obtained by different grinding mechanisms namely high pressure grinding rolls (HPGR) and ball milling, and their effects upon the properties of cements prepared from the ground clinker. Samples were prepared as narrow size fractions and also as distribution samples. Characterization parameters were ascertained by using XRF, laser sizing, Blaine and BET surface area and image analysis methods. HPGR grinding resulted in higher degrees of liberation of clinker phases arising from the intergranular breakage along the grain boundaries compared to ball mill grinding. As for service properties, water demand of HPGR products was higher than ball mill products resulting from high micro fissured structure. Despite high liberation of particularly alite mineral in HPGR grinding, the compressive strength of ball mill products was slightly higher than HPGR products for narrow size samples. Finally, particle size distribution effect on strength was more obvious for distribution samples; generally ball milling gave higher strength values.     

An analysis of fine dry grinding in ball mills

The kinetics of dry grinding of several cement clinkers and two coals were investigated in a laboratory tumbling ball mill. The kinetic process is first-order at first, but the rates of breakage decrease as fines accumulate in the bed. It was demonstrated that the slowing of the breakage rates applies to all sizes in the mill, indicating that the cushioning action of fines affects the whole breakage process, even though mill power remains constant. Tests on cleaning or non-cleaning the balls showed that the major factor was not the build-up of a coating on the balls. Radio-tracing tests showed that the effect was not due to pelletizing of fines into larger particles. The quantitative magnitude of the cushioning action was different for different materials. It is, therefore, postulated that cushioning is affected not only by air trapped in the bed of fine particles but also by the cohesive attraction of fine particles, which is a function of the material.     

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。     

Modelling of comminution processes in Spex Mixer/Mill

It is well known that mathematical models which simulate comminution processes represent a useful tool in several fields of academic and industrial research, with particular emphasis on nano-material and pharmaceutical production. In the present work a mathematical model which is able to quantitatively describe comminution processes in a ball milling system (i.e., Spex-Mixer/Mill) has been developed. The proposed approach takes into account three different contributions: dynamics of the vial, dynamics of spheres motion and simulation of the comminution process. The vial dynamics has been modelled by taking advantage of an appropriate roto-translation matrix. Model results have been successfully compared with literature experimental data. The spheres motion within the Spex Mixer/Mill has been simulated using a 3D dynamic model based on classical mechanics as well as the so-called discrete element method, which is widely adopted to quantitatively describe multi-body collision behaviour. In particular, existing models of impact with dissipation as well as the classical Hertz impact theory have been taken into account. This part of the global model allows one to obtain, for different operating conditions, the impact specific energy and impact velocity as a function of time. The latter ones represent input parameters for the simulation of comminution processes that is performed through suitable population balances, where different breakage functions as well as appropriate breakage probabilities have been considered. Model results are reported in terms of granulometric distribution of powders within the mixer-mill as a function of time, minimal grain size obtainable and time needed to complete the comminution process for various operating conditions (i.e., mill frequency and charge ratio).     

A combined tracer and back-calculation method for determining particulate breakage functions in ball milling.: Part III. Simulation of an open-circuit continuous milling system

The combined tracer and backcalculation method for determining particulate breakage parameters described in Part I of this paper and applied to batch ball milling in Part II is extended here to the open-circuit continuous ball milling of martite iron ore. Breakage parameters are determined by this method in the continuous ball mill in the batch mode of operation. Residence time distributions are obtained in the continuous mode of operation with radioactive tracers. Segregated flow, models based on the dispersion model and the tanks-in-series model are given for use with the breakage parameter and residence time distribution data to predict product weight—size distributions. A particular model based on three perfectly mixed tanks in series is used to describe a mill with entrance and exit trunnions and is compared to both transient and steady-state experimental results. The comparison indicates that this method of simulation will be useful.     

Influence of mechanical properties on impact fracture: Prediction of the milling behaviour of pharmaceutical powders by nanoindentation

The impact grinding behaviour of materials can be characterized by the two breakage parameters f_Mat and xW_m,min [Vogel and Peukert, Powder Technol. 129 (2003) 101-110]. These parameters are usually determined by single particle milling tests. The parameters are useful for predicting the selection function and the breakage function and thus enable modelling of impact milling processes. So far, no detailed correlations have been established between the breakage parameters f_Mat and xW_m,min and intrinsic material properties. In this work, we study the correlation between the breakage parameters of pharmaceutical powders and their mechanical properties (hardness, Young's modulus and fracture toughness) that are determined from indentation experiments. It will be shown that f_Mat and xW_m,min can be expressed in terms of the brittleness index (defined as the ratio of hardness to fracture toughness H/K_c). This correlation allows the prediction of the breakage probability of a material by using only a small number of crystals.     

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.     

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.     

Continuous grinding in a small wet rod mill Part I. Comparison with a small ball mill

Tests have been done with a small continuous wet rod mill in order to characterise its dynamic behaviour in terms of the type of model previously developed for a small continuous wet ball mill. The three functions studied were the distribution of residence time, the breakage function and the breakage rate as a function of size.Distribution of residence time data showed that flow through the rod mill was similar to that through the ball mill and the contents of the mill were very well mixed. The breakage function results showed that any breakage event statistically produced less fines than in a ball mill. Breakage rate data showed that the coarsest sizes were broken more selectively, thus resulting in less fines in the product than would have been obtained if the breakage had been a result of ball action. Nevertheless it was shown that, overall, breakage in the rod mill could be described in similar terms to breakage in the ball mill, and the models were very similar.     

On Predicting Roller Milling Performance VI: Effect of Kernel Hardness and Shape on the Particle Size Distribution from First Break Milling of Wheat

Models based on the breakage equation for roller milling have been developed to predict the output particle size distribution delivered by First Break roller milling of wheat from distributions of single kernel characteristics. These models allow prediction of the breakage of mixtures of kernels of unknown origin or varieties and varying in size and hardness, based solely on Perten Single Kernel Characterisation System (SKCS) characteristics. Predictions have been developed for both Sharp-to-Sharp and Dull-to-Dull roll dispositions, and show good agreement with independent data. Milling under a Dull-to-Dull disposition is more sensitive to kernel hardness and gives a more pronounced U-shaped distribution of output particle sizes (i.e., large proportions of both small and large particles, with few in the mid-size range) than Sharp-to-Sharp milling. Similarly, softer wheats break to give a more U-shaped distribution than harder wheats. These findings also demonstrate that kernel hardness as reported by the SKCS is meaningful in relation to wheat breakage during roller milling. Previous work has shown that single kernel moisture measurements can be included in predictive equations; further work reported here demonstrates the potential to add the fourth SKCS parameter, kernel mass, to predictions in order to allow for the effect of kernel shape on breakage.     

Numerical Simulation of Open‐Circuit Continuous Mills Using a Non‐Linear Population Balance Framework: Incorporation of Non‐First‐Order Effects

Population balance modeling has been used as a tool for simulating, optimizing, and designing various particulate processes, including milling. A fundamental tenet of the traditional models for milling processes is the first‐order breakage kinetics. Ample data obtained from batch milling studies show that this assumption is not necessarily valid for certain milling systems. In the present theoretical investigation, an attempt has been made to incorporate these experimentally observed non‐first‐order effects into continuous mill models within the context of a novel non‐linear population balance framework. In view of two idealized flow regimes, i.e., perfect mixing and plug‐flow, continuous mills operating in the open‐circuit mode are numerically simulated. The simulations indicate that not only does the product size distribution depend on the degree of mixedness in a continuous mill, but also on the non‐first‐order effects arising from multi‐particle interactions.     

Comparison of Comminution by Impact of Particle Collectives and Other Grinding Processes

This article presents the innovative grinding apparatus Pulsar in which comminution is caused by impact of a particle plug on an impact plate. Advantages of the Pulsar principle in comparison to other types of mills are discussed. The aim of the work is to classify the Pulsar system in the field of grinding apparatus and machines in terms of energy consumption. Experiments were carried out in a Pulsar, a cross beater mill and a ball mill for comparison purposes. The results show that, for the material quartz sand, grinding in the Pulsar at a medium pressure of compressed air (p_car,i = 7.5 bars) and a medium magnetic valve opening time (t_o = 70 ms) is as efficient as in the ball mill. The grinding energy consumption of both mills, the Pulsar and the ball mill, is remarkably higher than that of the cross beater mill.     

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.     

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.