Ultra-fine grinding mechanism of inorganic powders in a stirred ball mill

An experimental investigation on grinding mechanism for calcite used in a stirred ball mill was carried out. The slurry concentration and the amount of grinding aids were chosen as main experimental factors of the grinding process. The effect of grinding aids on particle size distribution and grinding efficiency, defined as the increases of specific surface area per the specific grinding energy, was investigated. It was demonstrated that the grinding rate for calcite could be improved by addition of grinding aids. The grinding energy efficiency by adding a specific grinding aids was improved approximately 45.2% in comparison with and without grinding aids (n=700rpm, J=0.7, d_B= 1.0 mm, C_s=60wt%). This paper was presented at the ‘First Asian Particle Technology Symposium’ held at Bangkok between December 13 and 15, 2000.     

Empirical and scale-up modeling in stirred ball mills

Stirred ball mills are frequently used for ultrafine- and nanogrinding in food, pharmaceutical and chemical industry, but only few investigations have been published on empirical or scale-up modeling of stirred ball mills. Experiments have been carried out with a laboratory scale stirred ball mill. During the experiments the main technical parameters such as stirrer speed, grinding media, filling ratio, grinding time and the solid mass concentration have been systematically adjusted. The particle size distribution of mill products can be well estimated by empirical functions, so an empirical model has been prepared for the laboratory mill. The relation between the grinding fineness, grinding time and specific grinding work was represented for several materials such as pumice, andesite, limestone and tailings of ore mining industry. The power consumption of the stirred ball mill for scale-up was determined by a method based on the dimensional analysis. A new scale-up model has been presented as well by with industrial size stirred ball mills can be designed on the basis of the laboratory measurements.     

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.     

Ultra-fine grinding mechanism of inorganic powders in a stirred ball mill

Recently, in various industrial processes, the need for fine particles, especially submicron-sized particles, has increased in the field of preparing raw powders such as fine ceramics and high value added products. Therefore, the research in fine grinding has gained more importance, especially, in submicron grinding. In the previous paper, a series of wet grinding experiments using inorganic powders by a stirred ball mill were performed. The grinding consumption power was measured, and the grinding rate constant, K, in the grinding kinetics equation was examined, based on a grinding kinetics analysis of experimental specific surface area with particle size distribution of ground products obtained under various grinding conditions. Also the effect of grinding aids on grinding rate constant K was investigated. It was confirmed that the grinding rate constant K, when using grinding aids improved by 1.95% and 25.6% for a 60 wt% and 70wt% slurry concentrate, respectively, when compared with the case of the absence of grinding aids. It was found that grinding aids have an important effect on increasing the grinding rate, especially for a slurry of high concentration.     

Application of the population balance model to the grinding of mixtures of minerals

Batch experiments were carried out on the dry ball mill grinding of calcite, hematite and quartz, individually and as binary mixtures. In terms of the population balance grinding kinetics model, the breakage rate functions of the individual minerals were found to be time-independent but environmental-dependent. The breakage distribution functions of the individual minerals were the same, irrespective of whether the mineral was ground alone or as a component of a mixture. A modified form of the Charles energy—size reduction equation was used to resolve the net energy expended by the mill into that consumed by each of the components of the mixture. The breakage rate function, when normalized in terms of specific energy (energy consumed per unit mass of mineral), was found to be both time- and environment-independent for each of the minerals studied. These observations have potential for application in mill design, scale-up and control in the processing of complex ores.     

Surface modification of calcite by wet-stirred ball milling and its properties

This paper investigates the surface modification of calcite from the Ni?de region of Turkey with sodium oleate (SDO) as a modification agent, which is incorporated into the calcite with wet ultra-fine grinding in a laboratory stirred ball mill. The effect of surface modification is evaluated by a floating test, which measures the active ratio (AR), fourier transform infrared spectrometry (FT/IR) and thermogravity analyses (TG-DTA). The results indicate that the hydrophilic surface of calcite becomes hydrophobic after the incorporation of SDO through wet-stirred ball milling.     

OPTIMIZATION OF OPERATING VARIABLES FOR PRODUCTION OF NANOPARTICLES USING RESPONSE SURFACE MODELING

The objective of this study is to investigate the effect of operating parameters such as ball loading (J), solid mass fraction (C_m), pH of the suspension, and grinding time (t) on particle size distribution for the production of nano mineral particles in a stirred ball mill using response surface modeling. The particle size is arrived at by the method of Rosin-Rammler-Bennett distribution rather than by the average diameter method. Box-Behnken design is employed to arrive at the number of trials required. The results obtained show that particle size decreased with the increase in ball loading, pH, and grinding time, but not solid mass fraction. Moreover, optimum (i.e., minimum particle size) value of pH and solid mass fraction is found to be 12 and 0.3 respectively for the studied material, silica.     

The effect of ball and mill diameters on grinding rate parameters in dry grinding operation

For dry ball mill grinding operation, the effect of ball and mill diameters on grinding rate parameters of the size-discretized population balance model has been investigated for quartz, limestone, a soft cement clinker and a hard cement clinker. Experiments were performed in three mills of 29.2, 40.6 and 61.0 cm diameter. The diameter of the balls used ranged from 1.27 to 3.81 cm. The particle size range covered was $\text{8}\text{10}$ to $\text{100}\text{150}$ mesh. The rate parameter values were determined very accurately using a special technique. It has been shown that the particle size exponent α in equation S_i = Ax_i^α is independent of ball and mill diameters. Based on this fact, a new correlation has been developed to describe the effect of ball and mill diameters on the rate parameters. The various constants in this correlation are strongly material dependent.     

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.     

A study on the specific rate of breakage of cement materials in a laboratory ball mill

The specific rate of breakage (S_i) in the widely accepted first-order expression of grinding rate is one of the important factors required to evaluate a grinding process, particularly for the initial grinding stage in various mill types.In this study, the effects of ball diameter and feed size on the specific rate of breakage were investigated on limestone, trass and clinker samples at batch grinding conditions based on a kinetic model. Eight different monosize fractions were prepared between 1.7 and 0.106 mm, using a √2 sieve series. The specific rates of breakage (S_i) were determined from the size distributions at different grinding times, and the specific rates of breakage were compared for three different ball diameters (41, 25.4 and 9.5 mm).The results indicated that the variation of the specific rate of breakage with feed size of cement materials could be expressed. For the specific rate of breakage of each material, empirical equations were developed to express it as a function of feed size and ball diameter.     

Analysis of ball movement for research of grinding mechanism of a stirred ball mill with 3D discrete element method

A simulation of the three-dimensional motion of grinding media in the stirred media mill for the research of grinding mechanism has been carried out by 3-dimensional discrete element method (DEM). The movement of ball assemblies was graphically displayed with some snapshots from start of the milling to 0.20 s. From these simulation results, the grinding zone in the mill was confirmed to be distributed into two regions, which is near the stirrer and the side wall of mill around the stirrer. The power changing the rotation speed of stirrer was examined based on the micro interactive forces at all the contact points between ball-to-ball and between ball-to-stirrer. DEM is a very powerful tool for the microanalysis of movement of balls, which could not have been solved by a conventional experimental method.     

Kinetics of solid-state polymorphic transition of glycine in mechano-chemical processing

The kinetics of solid-state polymorphic transition of the stable polymorph of glycine (Form γ) to an unstable Form α were studied using a planetary ball mill to demonstrate the effect of mechano-chemical processing. The kinetics were analyzed in terms of operating variables, i.e. revolution speeds, the number of balls, mass of sample, materials of jar and balls, and grinding time. The rate of the transition was well correlated with the two-dimensional Avrami equation, and the apparent rate constants empirically determined were correlated with the above-mentioned variables. For the transition in a stainless steel jar and balls, the changes of polymorphic compositions were successfully correlated over the ranges of experimental conditions examined.     

生产水煤浆用新型超细搅拌球磨机的研究

介绍一种新型超细搅拌球磨机,用于磨制生产水煤浆用超细煤粉;分析了该机的结构特点、工作原理、性能特点以及磨矿介质粒径的选取等;论述了磨机各组成部分的技术特点。     

A Comparative Study on a Vertical Stirred Mill Agitator Design for Fine Grinding

Comminution is an energy intensive process. A small change in efficiency can lead to substantial benefits in an overall economy of the process plant. This study focused on the comparison of vertical stirred mill agitator designs. A double helical screw agitator was designed for this purpose. A series of stirred mill experiments were performed with two types of agitator designs a standard pin type and CSIRO’s designed double helical screw stirrers. The effects of operating parameters such as grinding time, stirrer speed, and pulp density on grinding performance was investigated using a magnetite concentrate. Grinding performance was analyzed by considering the product fineness and the energy consumption. The test results show that the grinding time and stirrer speed played a significant role; however, the pulp density had little impact on grinding performance in both cases of agitator designs. The 80% passing target product size of 38 μm was obtained with double helical screw agitator in 20 min of grinding with an expend of 10.53 kWh/t specific energy, whereas, the target product size of 38 μm was achieved with the pin type stirrer at the rate of 21.73 kWh/t. It is evident that grinding in a vertical stirred mill with a double helical screw is more efficient than that using a pin type stirrer in terms of the product size distribution and the specific energy consumption. It is concluded that the double helical screw design provides better energy efficiency compared to the pin type stirrer design. The models were developed for the responses P₈₀ and E_cs. Both models show high regression coefficients thus ensuring a satisfactory of models with experimental data. The model equations developed were then optimized using a quadratic programming to minimize the P₈₀ size at minimum specific energy.     

Analysis of energy-size reduction relationships in batch tumbling ball mills

A theoretical energy-size reduction relationship is derived for tumbling ball mills based on a solution of the integro-differential equation of comminution kinetics, in which the proportional relationship is applied between the grinding rate constant and the net mill power. The derived formula is similar to an empirical energy law, dW ∝ dx_r/x_rⁱ, where W is the specific energy input, x_r is the particle size of product and the exponent i is shown to be a variable depending upon the ground material, the type of mill and the method to measure energy. Derived results are confirmed with reported data in reasonable agreement. Also, the Bond's energy law is examined and a method for the correction of the Bond work index is discussed.     

Continuous grinding in a small wet ball mill. Part IV. A study of the influence of grinding media load and density

TXX influence of changes in grinding media load and density on the grinding behaviour of trace quantities of quartz within an environment of calcite in a small continuous wet ball mill have been studied using (a) ball loads ranging from 45% to 100% of the standard load of 1-in. balls, (b) a standard load of 1-in. pebbles and (c) standard loads of equicylinders with specific gravities ranging from 2.82 to 9.49. A mill overload condition (125% ball load) has also been studied using a feed comprising 5% by weight of full-size-range quartz and 95% standard calcite.The results show that variations in breakage behaviour were best followed by a comparison of the adjusted breakage rate constants (k′) for individual sizes corresponding to a standard hold-up-weight. The changes in k′ with ball load could be explained in terms of changes in number of impacts per unit time and in the environmental size distribution. A reduction in the density of the grinding media caused a disproportionate decrease in k′ for the coarse sizes compared with the fine sizes, and breakage rates tended to zero for all sizes as the density of the medium approached that of the pulp.Whilst under some of the test conditions the flow of tracer solids behaved anomalously, under most conditions there was an approximately linear relationship between average residence time and total weight of media charged to the mill.Media density per se had no effect on breakage function, but there was a distinct change due to change in media shape.     

Reduction of the porcelain firing temperature by preparation of the raw materials

An innovative way of reduction of firing temperature of porcelain tableware is reached by preparation of raw materials down to submicron- and nanoscaled powder for higher reactivity. In this study a common slurry was ground in an agitator ball mill from d₅₀ = 5.0 μm to 0.9 μm, green bodies were prepared, and glost firing was simulated in a dilatometer. The sintering temperature has been decreased by approximately 180 °C. A reflection between ball mill and agitator ball mill regarding the grinding cost shows no difference which means that the ball mill could be replaced. The energy consumption during the grinding process will be discussed regarding to energy savings resulting from reduced firing temperature. Furthermore a comparison between experimental and literature data will be done. The effect of grinding of raw material is finally evaluated concerning sintering behaviour and material properties.     

Effect of media size in stirred ball mill grinding of coal

As a prerequisite to producing super-clean coal with any physical coal-cleaning process, such as microbubble flotation, the feed coal must be micronized to liberate finely disseminated mineral matter. The stirred ball mill is regarded as one of the most efficient devices for micronizing coal. Using a 13.4 cm batch mill, the optimum operating conditions have been determined in terms of media size, feed size and media type. The rate of breakage determined with monosized feeds are compared on the basis of specific energy consumption. It has been found that a 20:1 ball size/particle size ratio gives optimum grinding conditions.     

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.