Analysis of grinding kinetics in a laboratory ball mill using population-balance-model and discrete-element-method

Grinding processing consumes a lot of energy in mineral processing, but it is a low-efficiency process in which only approximately 1% of the total energy is used to reduce the actual particle size. Therefore, an efficient operation in the grinding process increases the competitiveness of the production and is an essential for enhancing the energy efficiency of the entire mineral processing procedure.Therefore, the study will focus on to finding a different method to predicting the particle size distribution of the ball mill, by using the PBM which reflects the actual size distributions of ground product and the DEM which can understand the internal particle behavior in the mill chamber. First, the grinding parameters were calculated by applying size distributions of ground product under various conditions to PBM and the behaviors of the particles inside the ball mill obtained through DEM were analyzed to predict the distribution of the impact energy used for grinding. Next, the relational expression between the grinding rate parameter and the normal force applied to the grinding materials was derived. Using the relational expression derived from this study, it was confirmed that the size distributions in other conditions can be predicted.     

Impact of grinding aids on dry grinding performance,bulk properties and surface energy

In dry fine grinding processes the relevance of particle-particle interactions rises with increasing product fineness. These particle-particle interactions reduce the grinding efficiency and complicate the process control. The adsorption of grinding aid molecules on the product particle surface is a common measure to handle these effects. To ensure an efficient grinding aid application, the impacts of additives on particle and bulk properties, which influence the micro-processes inside the mill, need to be understood. Within this study the effects of several grinding aids on dry fine grinding of limestone in a laboratory vibration mill were investigated. Unlike in many other scientific works, the impacts of grinding aids were analyzed on different levels simultaneously: Grinding success and agglomerate size distributions were evaluated by wet and dry particle size measurements, respectively. Additionally, material coating on the grinding media, powder flowabilities and particle specific surface energies were measured. It was shown that all of the investigated grinding aids influence the grinding efficiency. However, the formation of agglomerates is not necessarily linked to the product fineness. Furthermore, a strong impact of certain grinding aids on the flowability of the product powder was determined. Thereby, the bulk flow behavior also determines the grinding result as it affects the stress mechanism inside the mill. Moreover, a direct relation between surface energy and powder flowability as well as agglomeration behavior could be demonstrated.     

Influence of processing parameters on grinding mechanism in planetary mill by employing discrete element method

Evaluation of process parameters and performance of planetary mills is considered an important issue in optimization of their predicted efficiencies. The current research deals with a computer simulation based on discrete element method (DEM) employed to evaluate the grinding mechanism of a planetary mill. The effect of rotational speed, the volume percentage of the balls and powders on impact energy, the frequency of the impacts, the abrasion of the balls and the dissipated energy have been investigated. It was realized that by increasing the rotational speed, the impact energy and the number of impacts of ball–ball, ball–container, ball–powder, powder–powder and dissipated energy were increased. By increasing the balls filling ratio, the impact energy value of ball–ball remained almost constant after a critical point while those of ball–powder impact energy and the dissipated energy were slightly increased. With an increase in powders filing ratio, a maximum value was observed for the ball–powder impact energy; however, the ball–ball impact energy remained almost constant and the dissipated energy indicates a reduction in rate.     

Effect of Various Operating Factors on Wet Stirred Mill Performance

In this study, the effects of various operating factors, such as initial particle size, feed quantity, filling volume of the ball, and ball size distribution on fine grinding of calcite powders (CaCO₃) were studied using a laboratory stirred mill under wet conditions. A series of laboratory experiments using 2⁴ full factorial designs was conducted to determine the optimum grinding parameters. The main and interaction effects on the specific surface area (m²/g) of the ground product were evaluated using Yates' analysis. The test results showed that the main factors that influence the mill grinding performance are the feed quantity and the ball size distribution.     

Analysis of grinding aid performance effects on dry fine milling of calcite

The control mechanism in the production of fine particles is important in dry grinding processes, which are popular in the grinding of some minerals. The behavior of the fine particles with each other and with the mill environment is complex and needs to be investigated in detail (on a micro-scale) in every application. One of the parameters controlling the fine particles in a mill is the use of the correct amount and type of grinding aid. Mechanisms of action must be understood well to develop grinding aids and fully utilize their positive effects. As a general explanation, with the use of grinding aids, the production tonnage can be increased at the desired product fineness, as well as achieving a finer product at the same production tonnage. However, it is necessary to know other mechanisms and effects to increase efficiency and correct use. The influence of grinding aids on the dry fine grinding of calcite using several analysis methods was revealed in the present study. Within the context, the results were assessed considering grinding performance (particle size distribution and size reduction ratio), product quality (color properties), flowability, adsorption properties (FTIR), and agglomeration behavior (SEM). Moreover, the tested effect of each grinding aid was discussed for each analysis, and the collected results were combined around a summary core diagram.     

Analysis of ball mill grinding operation using mill power specific kinetic parameters

With a view to developing a sound basis for the design and scale-up of ball mills, a large amount of data available in the literature were analyzed for variation of the two key mill performance parameters: power specific values of the ‘absolute breakage rate of the coarsest size fraction’, S^*, and ‘absolute rate of production of fines’, F^*, with some of the important operating and design variables such as the mill speed, ball load, particle load, ball diameter and mill diameter. In general, values of both the mill performance parameters were found to vary significantly with the mill operating conditions. The nature and relative magnitude of variation for the two parameters also differed significantly. Moreover, the effect of any particular variable on the S^* and F^* values was found to be significantly different for different sets of operating conditions. It has been emphasized that, as the purpose of grinding is to produce fine particles, the mill design and scale-up work should be based mainly on the F^* parameters. Moreover, it is not correct to regard the S^* values to be independent of the mill design and operating variables as a general rule, especially for a fine analysis of the performance of the grinding systems.     

Wet Grindability of Calcite to Ultra-Fine Sizes in Conventional Ball Mill

An experimental practice on the ultra-fine wet grinding of calcite ore in a conventional batch ball mill is reported. In this study, the effect of wet grinding conditions on the production of fine particles was researched. The influence of operating parameters such as operation speed (% of critical speed), ball filling ratio, calcite filling ratio, pulp density, ball size distribution, and grinding time on the grindability of calcite ore was systematically examined. Experimental results were evaluated on the basis of d₈₀ product size. As a result of this study, optimum experimental conditions were found to be 80% of critical speed for operation speed, 35% for ball filling ratio, 15% for calcite filling ratio, 75% for pulp density, 50% (1 cm) and 50% (2 cm) for ball size distribution, and 60 min for grinding time. It was found that the best product has d₁₀ = 1.51, d₅₀ = 12.53, and d₈₀ = 30.02 µm particle size and its steepness factor is 3.75. The outcomes indicate that the wet grinding technique in conventional ball mill for calcite ore is effective to obtain ultra-fine size products.     

Size reduction performance evaluation of HPGR/ball mill and HPGR/stirred mill for PGE bearing chromite ore

In the present study, size reduction experiments were performed on High-Pressure Grinding Rolls (HPGR), ball mill and stirred mill of PGE bearing chromite ore. The performance of HPGR was evaluated in two stages of size reduction to reduce energy consumption. In the first stage of HPGR, the effect of operating variables such as the gap between the rolls, roll speed, and specific pressing force on product size (P₈₀) and energy consumption (E_CS) was investigated. The process to get the smallest product size was optimized within the experimental range of investigation. The crushed product of HPGR was subjected to grinding in the second stage in a ball mill and stirred mill. The effect of mill speed, grinding time, and ball size on the performance of the ball mill was investigated and the product was further investigated in the second stage. A comparative analysis of the ball mill and stirred mill performance and energy consumption at different grinding time intervals was also performed. It was found that the ball mill consumed 54.67 kWh/t energy to reduce the F₈₀ feed size of 722.2 µm to P₈₀ product size of 275.4 µm while stirred mill consumed 32.45 kWh/t of energy to produce the product size of 235.6 µm. It also showed that stirred mill produced finer product than the ball mill at around 40% lesser consumption of energy. It can be concluded that the HPGR-Stirred mill combination was a more energy-efficient grinding circuit than the HPGR-Ball mill combination for PGE bearing chromite ore.     

A shear pan mill for preparation of ultrafine polyamide 66 powder using sodium sulfate ionic crystals as grinding aid

Fine polyamide 66 (PA66) powder was prepared in a shear pan mill that was designed and developed in our laboratories. In contrast with discontinuous impacts in the ball mill, the size reduction operation in the pan mill is characterized by application of uninterrupted strong shear forces on particles. It has been found that as compared to the ball mill, much finer pulverization of the polymer material is achieved in the pan mill, in spite of the high strength and ductility of the polymer. Sodium sulfate ionic crystals with hard and pointed edge were co-milled with PA66 to further cut the fine polymer particles into ultrafine powder (2–5?µm). Besides acting as a grinding aid, the ionic crystals also prevented conglutination and aggregation of the polymer powder. Furthermore, PA66 powder could be conveniently separated and purified from the co-milled grinding aid-polymer mixture by simple water washing and filtration. Thus, an energy efficient and environment-friendly technology has been developed to prepare the ultrafine plastic powder at room temperature. This process holds a good promise for commercial applications.     

Analysis of abrasion mechanism of grinding media in a planetary mill with DEM simulation

In order to investigate the abrasion phenomena in a planetary ball mill, we conducted the grinding operation without a powder and sought a correlation between the ball abrasion and the ball impact energy estimated by a Discrete Element Method (DEM) simulation. Experimental results showed that the mass of abraded balls increased in proportion to the grinding time in the early stage of grinding up to 75 min. The abrasion rate increases quadratically with the mill rotation speed. It decreases with an increase in ball diameter up to 12.7 mm and then slightly increases when the diameter is 15.8 mm. It also increases with an increase in the ball-filling ratio of the mill up to 50%. Similar tendencies are found in the impact energy calculated from the balls motion simulated by DEM. Therefore, it is said that the abrasion rate has a strong correlation to the impact energy of balls under any milling conditions.     

Studying the effect of different operation parameters on the grinding energy efficiency in laboratory stirred mill

In this study, considering different operational parameters for stirred media mill, change in specific energy was compared to the change in R_x values, i.e. the cumulative weight of the material undersized to a specific sieve. R values, namely R₃₈, R₇₅, R₁₀₆, were measured before and after grinding in stirred mill. The change in R_x (ΔR_x, %) values were calculated and they were used to evaluate the certain unit of effectual energy (Ecb). This abovementioned calculation is performed by proportioning the Specific Energy (SE) to ΔR_x values. The effectual part of SE is considered to be the ratio of the energy needed only for size reduction in grinding and it should be related to the ΔR_x. The relative Ecb ratios of different grinding conditions give the relative specific energy efficiency ratio (SEe). The relative specific energy efficiency ratio is inversely proportional to specific grinding parameters and ground product particle sizes. The relative specific energy efficiency can be considered as the relative amount of energy for various grinding conditions. The variation between relative energy amount and the previously specified particle size provides a realistic comparison of different grinding parameters. The abovementioned variation could be employed to understand the resistance particle size which is a new concept to describe the particle size at which the maximum effectual SE is directly used. In the context of this study, it was aimed to figure out the interrelation between specific energy efficiency and PSD variation along with the resistance particle size.     

Classifier performance during dynamic fine grinding in fluidized bed opposed jet mills

Dry grinding of particles to sizes below 10 µm can be realized in fluidized bed opposed jet mills. In these mills the energy for comminution is supplied by pressurized gas, which is introduced through focused nozzles. The gas transports the material towards an internal classifier, which separates the fines from the coarse material. The fines are discharged whereas the coarse material is recycled. Within this study special attention is paid to separation at the classifying wheel. Limestone batch grinding experiments were performed in a fluidized bed opposed jet mill equipped with on-line and in-line probes: The particle size distributions (PSDs) of the product flow and the solid concentration below the classifier were determined on-line. The flow field around the classifier was recorded by a high-speed camera and off-line measurements of the mill inventory and its PSD were performed. Our measurements reveal that the solid transport from the milling zone to the classifier and the classifier performance strongly depend on solid concentration. Increasing the solid feed concentration or the classifier wheel speed leads to unwanted accumulation of product-sized particles inside the mill. In particular, we find high solid loadings of up to 1.05 g?g^?1 and strong cluster formation (local zones of high solid concentration) in the vicinity of the classifier blades. Estimated separation efficiency curves of the classifier show a strong “fish-hook effect” which increases with the solid concentration. Our findings are relevant for future process optimization by careful tuning of grinding performance, holdup and classifier speed.     

Evaluating the effect of feed particles size and their hardness on the particle size distribution of semi-autogenous (SAG) mill’s product

The main target of this research was to provide the optimal size of mills’ feed with acceptable accuracy, in order to find the size range of particles to achieve the maximum grinding product. Nowadays, experimental semi-autogenous (SAG) mill and drop-weight test are used for evaluation of grinding circuit regarding changes in feed particle size distribution, size of the ball, speed of mill, prediction of energy required, and product size distribution for complete grinding in AG and SAG mills. In this study, the effect of initial size ranges of feed (+13.2–315?mm) on the amount of grinding product and its size distribution has been evaluated, in order to find the size range of particles to achieve the maximum grinding product. The results showed that the feed size range of 19–22.4?mm had the highest amount of grinding product and breakage index number in different energy levels and the validity of results was evaluated with ball drop-weight test, as well.     

Influence of dry and wet grinding conditions on fineness and shape of particle size distribution of product in a ball mill

The influence of grinding conditions on the production of fine particles and the width of the particle size distribution produced during ball mill grinding was investigated. The grinding experiments were carried out varying the grinding ball diameter under dry and wet conditions. The relation between the weight passing size observed in an arbitrary cumulative undersize fraction and the grinding time was expressed by modifying Tanaka’s semi-theoretical equation for the grinding limit. The fineness of the product was evaluated by the median particle size in undersize distribution, and the shape of the particle size distributions by three different size ratios calculated using 10%, 20%, 50%, 80% and 90% cumulative weight passing sizes, that showed the width of the particle size distribution. The median particle sizes of product obtained for the grinding limit in wet and dry conditions were around 0.5–0.6 μm and 2 μm, respectively. The width of the particle size distribution in wet grinding decreased with decreasing median particle size of the ground product, and the size distribution in dry grinding became nearly constant. The particle size distribution width was lowered by using smaller grinding balls in wet condition and larger grinding balls in dry condition.     

超细硅酸锆磨矿工艺优化与节能

为扩展球磨机、搅拌磨在物料粉磨过程中的应用,通过对比球磨机与搅拌磨的特性,分析两者在硅酸锆生产中的研磨效率,发现在磨矿过程中采用球磨机与搅拌磨分段研磨、优化组合的方式能充分发挥两种设备的优点,提高研磨效率。结果表明:使用该技术能大规模地生产出中位粒径为1μm的超细硅酸锆产品,并能显著降低磨矿能耗,节约生产成本。     

A study on the effect of process parameters in stirred ball mill

An experimental study on the fine grinding of calcite powder (d₅₀ = 62.16 μm) using a 0.75 l laboratory stirred ball mill has been carried out. The effects of various operating factors, such as grinding time (min), stirrer speed (rpm), slurry density (wt.%) and ball filling ratio on fine grinding was studied under batch wet conditions using alumina balls, 95% purity with diameters 3.5–4.0 mm. A series of laboratory experiments using 2⁴ full factorial designs was conducted to determine the optimum grinding parameters. The test results showed that the stirrer speed and grinding time have strong effects on the grinding efficiency, based on the value of specific surface area (m²/g).     

超细粉磨技术在氧化铁生产中的应用

针对普通开流氧化铁管磨机在氧化铁生产过程中存在电耗高、流速快、料球比低、严重糊磨等问题,通过在磨内设置高效筛分装置,采用料位调节、微型钢段等技术措施,极大地改善了磨内粉磨工况,提高了粉磨效率。该技术在工业生产中得到应用,它能提高台时产量30%以上,降低粉磨电耗25%以上,并大幅降低钢材的消耗。     

Enhancing the capacity of large-scale ball mill through process and equipment optimization: An industrial test verification

The production capacity of the large-scale ball mill in the concentrator is a crucial factor affecting the subsequent separation and the economic benefits of the operation. The main aim of this study is to improve the processing capacity of the large-scale ball mill. Taking a Φ5.49 × 8.83 m ball mill as the research object, the reason for the low processing capacity of the ball mill was explored via process mineralogy, physicochemical analysis, workshop process investigation, and the power consumption method. Based on this framework, a series of laboratory grinding optimization tests were conducted and verified via industrial tests. The results show that the ore primarily contained hematite and magnetite, the disseminated particle size of magnetite was primarily a coarse-grained inlay that was easy to separate from gangue, while the disseminated particle size of hematite was primarily an uneven and medium-sized inlay, which increased the grinding difficulty. Under optimum conditions of +6.0 mm material suitable for a 100 mm ball diameter, −6.0 + 2.0 mm material suitable for an 80 mm ball diameter, −2.0 mm material suitable for a 70 mm ball diameter; medium ratio of Φ90 mm 34.62%, Φ70 mm 26.92%, Φ60 mm 23.08%, Φ40 mm 15.38%; filling ratio of 32%; material ball ratio of 1.0; rotation speed rate of 80%; and grinding concentration of 78%, the −0.074 mm content in the grinding product increased from 55.10% to 58.86% and the processing capacity of the ball mill increased from 310 to 320 t/h to 350 t/h. Scanning electron microscopy/energy dispersive X-ray spectrometry (SEM-EDS) micrograph analysis shows that the fineness of the ore and dissociation degree of useful minerals were apparently improved by optimizing the process and equipment.     

A case study on energy and exergy analyses for an industrial-scale vertical roller mill assisted grinding in cement plant

In this study, the analyses of energy and exergy were implemented for an industrial-scale vertical roller mill (VRM) of Kerman Momtazan Cement Company (KMCC) of Iran. The energy and exergy analyses demonstrated the first law efficiency of the VRM is 62.1%, while the second law efficiency of the VRM is 34.6%. Comparing to the widely applied ball milling, the second law efficiency is 16.4% higher for the VRM than the ball mill. Results also showed when the classifier rotor speed increases from 53 to 65 rpm, the particle size of the product decreases from P_90µm = 18.2% to P_90µm = 10.8%, but the power consumption of the VRM unit increases from 19.7 to 22.3 kWh/t of raw materials. Finally, the power consumption of the VRM unit compared with 14 raw mill units around Iran and the international best available technology (IBAT). The results demonstrated that the VRM unit consumes around 81% (9.75 kWh/t of raw materials), and 36% (5.8 kWh/t of raw materials) more energy to grind raw material than the IBAT unit and domestic best raw mill (DBRM), respectively.     

Influence of nozzle angle and classifier height on the performance of a spiral air jet mill

We study the effect of varying air jet mill design parameters, the nozzle angle and the classifier tube height, on the performance of the mill, operated in the semi-batch mode, using quartz and limestone particles as feed. Particle size distributions of the product and the material in the mill are measured along with the mass in the mill. The product size distribution and the specific mass flux are found to be constant with time. The breakage rate is found to increase with nozzle angle. The efficiency of classification, characterized by the fraction of coarse particles larger than the cut size, increases with nozzle angle and classifier tube height. The energy for grinding per unit mass of the product decreases with nozzle angle and increases with classifier tube height and optimal values of the parameters, at which the specific energy is minimized, are obtained.