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.     

A model for continuous grinding in a laboratory hammer mill

The equations of fully mixed, steady continuous grinding with rapid ideal classification through a discharge screen have been modified to allow for non-rapid removal of material less than the screen size. Values of specific rates of breakage, daughter fragment distributions and the variation of specific rates with mill hold-up determined in prior batch tests on a hammer mill were used in the equations, to predict hold-up versus feed rate and product size distribution versus feed rate. The predictions were in reasonable agreement with the experimental values from continuous grinding. There is a maximum production rate which can obtained, and energy utilization is inefficient at low production rates.     

Investigations of autogenous and semi-autogenous grinding in tumbling mills

A model based on the specific rates of breakage of each size in a mill as affected by balls and large lumps has been developed. Three mechanisms of breakage are recognized: ‘normal’ fracture of smaller sizes by balls and larger pebbles, ‘chipping’ of raw feed to give rounded pebbles and ‘abrasion’ from the surfaces of pebbles. A tracing technique was developed to distinguish these contributions to autogenous breakage in batch grinding tests. It was found that the accumulation of finer material strongly cushions the impact between pebbles and pebbles and pebbles and balls. Empirical relations were developed to describe this cushioning effect. It was also found that chipping breakage is first order for short times of grinding. The mathematical model lumps fracture and chipping in a single contribution and considers the material as if it were a mixture of fast- and slow-breaking components. Breakage rates and primary breakage distributions measured on an ore in batch tests were used to predict the performance of a 1.8-m diameter pilot-scale mill, and predictions compared to the pilot-scale data on the same ore.     

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.     

An analysis of ball-and-race milling part II. The babcock E 1.7 mill

A mathematical model was developed for a ball-and-race mill based on specific rates of breakage and primary fragment distributions. The model includes internal classification of particles falling back into the race and external classification due to the built-on classifier. It was demonstrated that the normalized primary fragment distribution produced in a pilot-scale Babcock E-type mill of 17 in. race diameter was the same as in the Hardgrove laboratory test mill and that the specific rates of breakage varied with particle size in the same manner. Steady-state continuous tests on the pilot-scale mill showed that breakage rates depended on the rate of feed, since the mill pulled less power at low feed rates. This effect plus the residence time effect gave coarser product size distributions at low and high feed rates than at a medium feed rate. Model simulations based on parameters measured in the Hardgrove mill correctly predicted the product size distribution from the E-type mill.     

利用立磨粉磨高湿原料的理论与实践

通常认为立磨对水分适中的原料有较强的适应性。安徽省皖维高新材料股份有限公司水泥分厂1000t/d预热预分解窑生产线,因采用该公司化工生产废渣——电石渣配料生产,其原料综合水分高达13%￣15%。在该线的高湿原料粉磨系统设计中,采用了MLS2619立磨方案。实践证明,采用立磨方案粉磨高湿原料是完全可行的。     

A comparison of the Bond method for sizing wet tumbling ball mills with a size—mass balance simulation model

A comparison is made of the results from a ball mill model simulation with those of the conventional Bond ball mill design method, for a material whose breakage characteristics and Work Index have been determined. In order to perform the simulation, ‘normal’ values were chosen for make-up feed size distribution, mill residence time distribution, ball mix, classifier behaviour, etc. At high flow rates through a mill (low reduction ratio), it is necessary to allow for the reduction in breakage rates caused by over-filling of the mill, using an empirical mass transfer relation: filling

. By suitable choice of the constant in this relation, it was found possible to duplicate the variation of mill capacity with feed size and product size (in wet closed-circuit grinding) predicted by the Bond method. The simulation model is then used to predict the variation of performance with design variables not included in the Bond method.     

Evaluation of a laboratory rod mill when grinding bituminous coal

Laboratory testing was conducted to evaluate the breakage of bituminous coal in a rod mill. A Pittsburgh seam coal, which had been obtained from the product stream of an operating coal cleaning facility, was used as the feed material. This material had an ash value of 6.7% and had been crushed to a nominal ?6.35 mm. Grinding tests were carried out on a mono-size (2.38 × 1.68 mm) fraction. The tests were conducted using a 193.5 mm diameter and 245.0 mm long laboratory rod mill, which contained steel rods at a volume loading of 23%. The mill was operated at 70% of critical speed for various grind times. A locked-cycle test was also performed to simulate continuous grinding conditions.The grinding was characterized using the population (or size–mass) balance model. The breakage distribution and rates of breakage were determined from the grinding data. The results indicated first-order breakage. The locked-cycle results indicated that steady state was achieved in several iterations. The simulated results matched the experimental results very well.     

Scanning Electron Microscopic Study of Microstructure of Gala Apples During Hot Air Drying

Microscopic changes that occur in plant food materials during drying significantly influence the macroscopic properties and quality factors of the dried food materials. It is critical to study the microstructure to understand the underlying cellular mechanisms to improve the performance of food drying techniques. However, there is limited research on such microstructural changes of plant food material during drying. In this work, Gala apple parenchyma tissue samples were studied using a scanning electron microscope for gradual microstructural changes as affected by temperature, time, and moisture content during hot air drying at two drying temperatures: 57 and 70°C. For fresh samples, the average cellular parameter values were as follows: cell area, 20,000 µm²; ferret diameter, 160 µm; perimeter, 600 µm; roundness, 0.76; elongation, 1.45; and compactness, 0.84. During drying, a higher degree of cell shrinkage was observed with cell wall warping and an increase in intercellular space. However, no significant cell wall breakage was observed. The overall reductions in cell area, ferret diameter, and perimeter were about 60, 40, and 30%. The cell roundness and elongation showed overall increments of about 5% and the compactness remained unchanged. Throughout the drying cycle, cellular deformations were mainly influenced by the moisture content. During the initial and intermediate stages of drying, cellular deformations were also positively influenced by the drying temperature and the effect was reversed at the final stages of drying, which provides clues regarding case hardening of the material.     

Modeling of a grinding circuit with a swing-hammer mill and a twin-cone classifier

A model was formulated for a continuous, air-swept milling system, assuming the mill to be fully mixed, with all particles leaving the mill in the air stream. The air-sweeping effect was treated as an internal classification which allows fine particles to leave and returns coarse particles for regrinding. The kinetic parameters for the continuous model were estimated using experimental data from a continuous pilot-plant air-swept swing-hammer mill. Functional forms that provide information regarding the power, the internal classification action of the mill, and the action of the external classifier, were deduced for different plant conditions. It was found that the specific rates of breakage could be assumed to vary with particle size according to S_i = a(x_i/x_o)^α, where the value of α was 1.3 for the coal used in the tests. The values of a varied with mill hold-up W, giving a maximum of aW as W became large. The primary breakage distribution values were assumed to be normalized, giving a value of characteristic slope of γ = 0.60.     

Using the discrete element method to analyze the breakage rate in a centrifugal/vibration mill

The grinding characteristics of a centrifugal mill with varying G/D (gyration/mill diameter) ratios were investigated using the population balance model and the discrete element method (DEM). A series of grinding tests were conducted on illite samples using a centrifugal mill under various conditions, and the breakage parameters were calculated. Three-dimensional DEM simulations were also conducted. It was found that the specific rates of breakage estimated for various grinding conditions correlated well with the impact energy calculated from DEM simulations. This information was used to develop scale-up functions for the centrifugal mill in terms of G/D ratio, rotational speed, mill diameter, grinding media diameter, and ball loading.     

Comminution of Wood – Influence of Process Parameters

An essential element for sustainable use of renewable resources is an efficient comminution process for which the relation between energy input and size reduction is of great importance. Fine comminution of beech, oak, and spruce wood chips in cutting and hammer mills at different moisture content levels is investigated. The influence of the different process parameters as well as the size reduction performed by the hammer and cutting mill on the specific comminution energy is reported and the particulate properties of the comminution products are reported. Considering the energy requirements, functional relations were derived from the experimental results, which describe the relation between comminution energy and size reduction.     

E-90-P COMPARATIVE TEST OF HAMMER MILL PULVERIZER AND DRY PAN FOR CRUSHING STREATOR SHALE1

In the machines as described below the following points are discussed: (1) the grinding action, (2) energy consumption per ton ground, (3) capacity to produce fines six-mesh and under, (4) to determine the operating characteristics of the hammer mill. The conclusions given are: (1) The dry pan will not grind shale chunks larger than 15 inches maximum diameter, nor chert nodules of any size. The hammer mill grinds easily in one operation all chunks small enough to go through the 20 × 42-inch hopper opening and all chert nodules. (2) The energy consumption of the dry pan as compared to the hammer mill in kw. hrs. per ton of fines through six-mesh is approximately as 1.40 to 1.25. (3) The maximum capacity of the Frost %foot dry pan with 1/4-inch screen plate openings was known to be approximately 25 tons through six-mesh per hour. The maximum capacity of the 30 × 42-inch Williams hammer mill as reached in these tests with hand feeding was found to be 56 tons total feed and 42 tons through six-mesh per hour. This figure could have been greatly exceeded if the material could have been fed more rapidly. The dry pan tailings coarser than six-mesh compared to the hammer mill tailings of the same coarseness are as 40 to 25%. (4) The hammer mill is more efficient than the dry pan in practically all respects if properly set up and operated, but it is easily possible to secure less efficient results from the hammer mill, particularly if the feeding is not properly regulated.     

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.     

Breakage and Mass Transfer Models During Drying of Rough Rice

A breakage model was investigated for thin-layer drying of rough rice. The breakage model developed can predict the percentage of head rice (E/E_o) as an exponential function of the grain moisture. Experimental data of the rice moisture content during drying were fitted with a theoretical model of the drying process to obtain the parameters. Experimental data of percentage head rice and moisture content were fitted to obtain the parameters of breakage model. Both functions were used together to obtain a drying-breakage model. This model allows us to predict the drying time required to achieve a rice moisture content desired and to estimate the head rice yield percentage for this moisture content.     

Druckverhalten feuchter kugelförmiger Zeolith 4A‐Granulate

The material behavior of three particle sizes of elastic‐plastic zeolite 4A granules has been experimentally studied using compression tests. The recorded force‐displacement curves have been approximated by mechanical models from the literature. Moreover, the influence of particle size and moisture content on the material behavior has been investigated. Furthermore, the specific fracture energy distribution and the distribution of the equivalent impact velocity at fracture have been determined. At impact stressing the energetic equivalent breakage energy corresponds to the impact velocity of a particle on a rigid wall initiating breakage of the particle.     

Further studies of ball and powder filling effects in ball milling

The specific rates of breakage of quartz have been studied in three tumbling ball mills, two of 195 mm i.d. and one of 0.6 m i. d., as a function of fractional ball and powder loading. It was found that power results from a small mill with small lifters were anomalous, probably due to slippage, so that results reported previously from tests in this mill were not correct as a function of ball load. New results are presented for both wet and dry grinding.     

Physical and frictional properties of non-treated and steam exploded barley, canola, oat and wheat straw grinds

During storage and handling, accurate knowledge of the physical and frictional behaviors of biomass grinds is essential for the efficient design of equipment. Therefore, experiments were performed on non-treated and steam exploded barley, canola, oat and wheat straw grinds to determine their coefficient of internal friction and cohesion at three hammer mill screen sizes of 6.4, 3.2 and 1.6 mm, three normal stress values of 9.8, 19.6 and 39.2 kPa at 10% moisture content (wb). At any specific hammer mill screen size, the geometric mean particle size and bulk density of non-treated straw was significantly larger than steam exploded straw. The bulk density of ground straw significantly increased with a decrease in hammer mill screen sizes. The steam exploded straw grinds resulted in higher coefficient of internal friction compared to non-treated straw grinds primarily because of lower bulk densities. The coefficient of friction for non-treated barley, canola, oat and wheat straw were in the range of 0.505 to 0.584, 0.661 to 0.665, 0.498 to 0.590, and 0.532 to 0.591, respectively. Similarly, the coefficient of friction for steam exploded barley, canola, oat and wheat straw were in the range of 0.562 to 0.738, 0.708 to 0.841, 0.660 to 0.860, and 0.616 to 1.036, respectively, which were higher than non-treated straw of the kind. Power, logarithmic or exponential equations were developed to predict the coefficient of internal friction and cohesion with respect to average geometric mean particle sizes for non-treated and steam explode barley, canola, oat and wheat straw grinds.     

Development of empirical drying correlations for ground wood chip and ground wood pellet particles

In this research, the drying rates of ground chip and ground pellet particles of pine were investigated. Pulp wood chips and whole pellets were ground in a hammer mill. Grinder screens with perforations 3.2, 6.3, 12.7, and 25.4?mm were installed in the hammer mill to produce a range of ground particle sizes. Particle size of ground chips had a broad range of 0.25–6?mm, while pellet particles had a smaller range of 0.5–2?mm. Initial moisture contents of ground particles were conditioned to 0.1, 0.3, and 0.5 (dry mass basis) using sprayed water. The moistened particles were dried isothermally in a thin-layer dryer set at the temperatures of 50, 100, 150, and 200°C. Chip particles dried faster than pellet particles of the same particle sizes. In addition to water loss, the particles exposed to a temperature of 200°C lost 3–7% of their dry mass. Smaller particles lost more dry mass than larger particles. The modeling revealed that the drying constants of pellet particles are less than those of chip particles with the same particle size.     

Analysis of the kinetics of coal breakage by wet grinding in the szego mill

This paper deals with the mechanism of breakage of Pittsburgh coal in the Szego mill, for the preparation of coal-grade 2 oil and coal-water slurries. Following a brief introduction to the mill and its operation, the kinetics of coal breakage is discussed. The non-first-order breakage in the mill is due to material and environment effects.A two-component mechanistic model, considering the coal in a size interval to be composed of crushed feed particles and flaky particles, is used to explain the breakage. The selection functions for both components in various

size intervals are estimated from experimental data using an optimization technique. Finally, the variation of the selection functions with particle size is discussed.