Fluidized Bed Opposed Jet Mill Research Articles

Research on the Grinding Process of Superhard Particles in the Fluidized Bed Opposed Jet Mill Based on the CFD-DEM Methodology

The process of superhard particle breakage in the grinding zone of the fluidized bed opposed jet mill is investigated using the CFD-DEM (computational fluid dynamics-discrete element method) coupling method with the Tavares UFRJ Breakage Model in the present study. The effects of structural and operational parameters, such as target plate structure, nozzle position, air inlet velocity, and feed rate, on the equipment stress distribution, airflow velocity, pressure field, particle velocity, and cumulative particle size distribution are thoroughly studied to determine the optimal structural and operational parameters. Experimental validation is conducted, including scanning electron microscope (SEM) observation of particle morphology and analysis of particle size distribution of ground product particles. The simulation results indicate that the wear rate of the structure without a target plate is lower than that of the structure with a target plate in the grinding central zone. Therefore, the structure without a target plate is chosen for further investigation. The cumulative particle size distribution after grinding is influenced by nozzle position, air inlet velocity, and feed rate. The particle D50 is positively correlated with nozzle spacing and feed rate, while it is negatively correlated with air inlet velocity. The optimal grinding effect is achieved when the distance between the nozzle and the center of the grinding zone ranges from 52.5 mm to 72.55 mm, the air inlet velocity is 950 m/s, and the feed rate is 10.5 g/s. Through experimental investigation, it has been observed that when the feed rate is 10 g/s, the particle size distribution becomes more uniform. Furthermore, consistent trends in the cumulative particle size distribution in the experiment and simulation results can be found, which validates the present numerical model. It was observed that particles at low feed rates retain certain angular edges, while particle roundness increases at high feed rates.

Quantitative evaluation of energy efficiency for steel slag comminution in a fluidized bed opposed jet mill

Exploring a scientific quantitative evaluation for energy efficiency and its improvement in jet milling process is of great significance for energy saving and emission reduction. In this study, three classical comminution breakage models, E-S breakage probability model, A-b breakage model and E-M breakage model, were modified by conducting comminution experiments with an improved single particle impact comminution equipment. Based on modified breakage models, new formulas, describing relationship between the comminution energy efficiency and breakage characteristic, were proposed and verified with the variation of system holdup, inlet pressure and feeding initial particle size on energy efficiency in a lab scale fluidized bed opposed jet mill. Results show that modified breakage models all well fitted the comminution results, and modified E-M breakage model was with the biggest accuracy. The energy efficiency calculated from the three new models is basically consistent with the change of comminution conditions, varying from 0.15% to 1.35%, which represents great improvement potentiality. The proposal of energy efficiency model would provide foundation for the subsequent quantitatively evaluation on energy utilization and efficiency improvement of steel slag ultra-fine grinding by fluidized bed opposed jet mill with waste heat steam.

Simulation of Particle Motion Behavior in Fluidized Bed Opposed Jet Mill

In order to study the influence of the internal flow field of the fluidized bed opposed jet mill on the motion behavior of particles, Computational Fluid Dynamics (CFD) and Discrete Dlement Method (DEM) are used for coupling calculations. By adjusting the nozzle spacing and inlet pressure, Numerical simulation is carried out on the process of particles collisions with each other after accelerating under the high-speed jet produced by the nozzle. The trajectory of the particles in the flow field of the collision area and the change of the collision state of the particles are analyzed. Finally, the best parameters are selected based on the total collision energy. The results show that the particles will gradually shift and spread during the acceleration process. The reduction of the nozzle spacing is beneficial to increase the probability of particle collisions. However, if the spacing is too small, the particles cannot be fully accelerated; the increase in inlet pressure will increase the kinetic energy of the particles, and number of collisions is almost unaffected. By comparing the total collision energy, the best-simulated preparation conditions are selected as 110mm and 1.1MPa.

Eulerian Multiphase Simulation of the Particle Dynamics in a Fluidized Bed Opposed Gas Jet Mill

The compressible and turbulent gas–solid multiphase flow inside a fluidized bed opposed jet mill was systematically investigated through numerical simulations using the Euler–Euler approach along with the kinetic theory of granular flow and frictional models. The solid holdup and nozzle inlet air velocity effects on the gas–solid dynamics were assessed through a detailed analysis of the time-averaged volume fraction, the time-averaged velocity, the time-averaged streamlines, and the time-averaged vector field distributions of both phases. The simulated results were compared with the experimental observations available in the literature. The numerical simulations contributed to a better understanding of the particle–flow dynamics in a fluidized bed opposed gas jet mill which are of fundamental importance for the milling process performance.

Assessing stress conditions and impact velocities in fluidized bed opposed jet mills

Fluidized bed opposed jet mills are capable of meeting the continuously growing demand for contamination-free fine particles. In this type of jet mill, the solid material is entrained and accelerated by expanding gas jets that are focused onto a focal point inside a fluidized bed. The resulting particle collisions induce breakage. The process is affected by the relative particle velocities and the number of particle–particle collisions. Clearly, both quantities are distributed. However, to date, neither relative particle velocities nor collision frequencies in such units have been determined. The present work introduces an innovative method to assess the stressing conditions in jet mills experimentally. To this end, mixtures of glass and ductile metal microspheres were used, with the latter employed in small amounts. Inter-particle collisions between the aluminum and glass spheres lead to the formation of dents on the microparticles. The size and number of these dents are associated with the individual collision velocities and overall collision frequencies. The correlation between dent size and collision velocity was obtained from finite element calculations based on empirical data. The proposed approach was validated using particle image velocimetry during secondary gas injection into a fluidized bed reactor. In this case the effect of the distance between two opposed nozzles was examined. For a lab-scaled fluidized bed opposed jet mill the effects of gas pressure and hold-up were investigated. Relative particle velocities were found to be significantly lower than the gas velocities, while the number of contacts per particle was determined to be extremely high.

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.

超微粉分級機搭載 カウンタジェットミルAFG-CRS

超微粉分級機搭載 カウンタジェットミルAFG-CRS

Dynamics of fine grinding in a fluidized bed opposed jet mill

Today the modeling of the process dynamics and the complex two-phase flows in mills still remain a challenging task. In this study the dynamic comminution process in a fluidized bed opposed jet mill is investigated. For this purpose, well-defined model glass beads were ground in a lab-scale mill. The particle size distributions (PSDs) inside the mill and at the outlet, solid concentrations and the holdup of the mill were recorded. By comparing the morphology of the ground material with that of particles examined in single particle stressing experiments, we conclude that the predominant stressing mechanism in the mill is determined by a high number of low intensity stressing events. Furthermore, the grinding kinetics is strongly influenced by the solid holdup within the mill and the initial PSD of the feed material. A higher holdup leads to a higher product mass flow rate with slightly finer particles. The determined high solid concentration in the transport section of the mill indicates a high loading of the classifier which increases with the holdup. Our study shows that the internal recirculation is crucial for the overall performance of the mill:Due to imperfect classification, particles accumulate inside the mill which is further enhanced by a high holdup of fine particles. Our results clearly show that the classification and transport processes must be studied in detail for any in-depth understanding of dry fine grinding in fluidized bed opposed jet mills.

Fluidized Bed Opposed Jet Mill System for Processing Inorganic Materials

A jet mill system was built aiming to give values for processing inorganic materials, to be used for different industry. The milling housing of the system is composed of; milling chamber, compressed air nozzles which deliver compressed air in the milling chamber to accelerate sample particles. The classifier wheel is composed of two concentric pieces welded together under argon and coupled to a AC Motor, 0 - 9000 rpm, 2 kW, with AC frequencies convertor. The performances of this jet mill system were tried on five cheap locally available materials, viz. white sand, glass, iron oxide, black carbon and alum. It is possible to get particle sizes of less than 1 μm with narrow distribution of particle sizes.

Modeling the Multi-Stage Classification in a Fluidized Bed Jet Mill

The research and modeling the classification effects in a fluidized bed opposed jet mill are presented. Particle distribution is the result of the interaction between two stages of classification in this mill: gravity classification above the bed, and centrifugal classification in a mill classifier. The objective of the study is to develop a model of multi-stage classification and apply in to the process in question. The universal algorithm of constructing the matrix model of arbitrary complex classification circuit is proposed and the example of its application to modeling the classification in the mill is presented. The experimental identification of the model parameters was carried out.

Parameter effects on dry fine pulverization of alumina particles in a fluidized bed opposed jet mill

The paper presents the experimental results on dry fine pulverization of alumina particles in a fluidized bed opposed jet mill. The effect of operating parameters (i.e., feed load, inlet air pressure and the distance between nozzle outlet and jet meeting point) on the grinding results was investigated in detail. The breakage behaviors of alumina particles with respect to the effects of different parameters were discussed via the selection and breakage functions calculated by the first Kapur function method. In addition, the product size distributions could be simulated by a proposed model.

Dry Comminution of Silicon Carbide Particles in a Fluidized Bed Opposed Jet Mill: Kinetics of Batch Grinding

This study investigated the batch grinding kinetics of silicon carbide (SiC) particles in a fluidized bed opposed jet mill by population balance modeling. The selection and breakage functions were obtained by the first Kapur function method. The breakage behaviors for various SiC particles obtained under different experimental conditions (such as inlet air pressure, feed load, and distance between nozzle outlet and jet meeting point) in the jet mill were discussed. In addition, a polynomial model was proposed to predict the relation between the Kapur function and the particle size in the jet mill. The product size distributions obtained under various operating conditions from the jet mill could be simulated by modeling.

Fractionation of fibrous fraction from steam-exploded rice straw

Fractionation of fibrous fraction from steam-exploded rice straw (SERS) of high moisture content have been studied with respect to the separation degree of fibrous tissue from non-fibrous tissue including epidermal, parenchymatous and vessel tissue using a fluidized bed opposed jet mill. Chemical composition and fiber characteristics, such as fiber size and composed cell proportion were studied for the separated fibrous fraction of SERS. 70.4% high cellulose fibrous fraction with 63.1% fiber cell content and 65.6% cellulose content was produced from the fractionation process. The separation degree of fibrous tissue of the fractionated fibrous fraction was 2.04. The mean fiber length was 0.97 mm, slightly longer than the untreated rice straw. The new method is suitable to produce high fiber tissue content fractions without damaging the raw fibers extensively.

Superfine grinding of steam-exploded rice straw and its enzymatic hydrolysis

The combination of low severity steam explosion and superfine grinding has been studied with respect to side products generation and enzymatic hydrolysis efficiency. Chemical compositions, fiber characteristics and composed cells contents in the superfine ground product and the ground residue particles produced by superfine grinding were also studied. At the determined parameters using FJM-200 fluidized bed opposed jet mill, 78% superfine ground steam-exploded rice straw (SERS) products with the mean fiber length of 60 μm were obtained, the particles yield was 179% higher than that from the native rice straw (RS). Enzymatic hydrolysis, chemical composition, fiber characteristics and composed cells proportion of the superfine ground SERS product and the ground residue all show great differences. The difference in enzymatic hydrolysis and structural properties indicates that superfine grinding is a good way to fractionate SERS into easily bio-converted part and difficultly hydrolysed part.

Ultrafine Grinding Using a Fluidized Bed Opposed Jet Mill: Effects of Feed Load and Rotational Speed of Classifier Wheel on Particle Shape

ABSTRACTCircularity, aspect ratio, modelx, and pellips were employed to study the effects of process parameters, namely varying feed loads and rotational speeds of the classifier wheel, of the fluidized bed opposed jet mill on the shape of the micronized particles produced. The Shapiro–Wilk statistical test showed that 80.0% of the shape distributions of the four descriptors were not normal. Therefore, the Kruskal–Wallis test, which is a nonparametric statistical test, was employed to analyze the data. Micronized particles were more spherical and less elongated, as indicated respectively by higher median circularity and lower median modelx values when compared to unmilled lactose. These descriptors were able to indicate that the particles had been micronized. When feed loads of 250 and 350 g were used, increasing the rotational speed of the classifier wheel was found to bring about a decrease in span values of all the shape descriptors, indicating that the micronized particles were more uniform in shape. Micronized particles produced had lower median aspect ratio values than the unmilled lactose, whereas a higher feed load of 450 g resulted in the production of micronized particles that were less uniform in shape and more elliptical in nature, as reflected by the lower median pellips values. A higher feed load of 450 g caused a high level of impingement of particles on to the rotating classifier wheel, causing decreased classifier wheel efficiency, and this resulted in a less well-controlled micronization process. Thus, aspect ratio and pellips were sensitive to the changes in performance of the classifier wheel. The four shape descriptors were proposed to be used collectively as indicators for the monitoring of the micronization process.

Modelling fine grinding in a fluidized bed opposed jet mill: Part I: Batch grinding kinetics

A methodology for characterising grinding kinetics is developed based on a new criterion, `the residual fraction' to represent the performance of a grinding process. This is applied to pseudo-batch grinding of hydrargillite in a fluidised bed opposed jet mill Alpine 100 AFG. It appears to give an adequate representation and a simple mathematical expression for the change in particle size distribution with time. Approximate values of breakage and selection matrices are then calculated on the basis of previous studies, showing that the breakage function is not normalisable over the whole range of particle size. These results are used to propose a fragmentation mechanism for hydrargillite which includes cleavage of the larger particles and destructive breakage of the fine particles.

Modelling fine grinding in a fluidized bed opposed jet mill: Part II: Continuous grinding

An overall model of a continuous grinding process is proposed which combines experimentally-determined breakage kinetics, a model of solids flow in the active zone and the grade efficiency curve of the classifier. This model is developed and tested for an Alpine 100 AFG fluidised bed opposed jet air mill with an integral classifier. The classifier appears to have a predominant role with respect to the product quality and the efficiency of the process but the existence of a draw off results in a dead flux of particles less than 4 μm. The solids flow in the mill is represented by a model consisting of two zones: a perfectly mixed zone where grinding takes place, followed by a plug flow transport zone where no grinding takes place. The single adjustable parameter model appears to give a good representation of the change in particle size distribution in the process, expressed in terms of the residual fraction (a concept introduced in part I). The a posteriori calculation of the space time in the grinding zone supports the value used for the time range used in the batch grinding kinetics.