Grinding Limit Research Articles

Particle aggregation and the grinding limit in high energy ball mill

Dust aggregates — clusters of individual particles — are formed during mechanochemical processing of brown coal in a planetary ball mill. The aggregate structure can be identified by comparing the average sizes of aggregates and individual particles. We showed that dust aggregates are generally arranged in a regular way: the number of individual particles located along the aggregate coincides with the number of particles located across the aggregate. Power of power-law distribution of the monomers per aggregate revealed that individual particles stick together in extremely dense clusters. The average number of monomers per claster is ~104. In addition, it turned out that the dependence of the monomer's “stickiness” on applied energy dose has a kink simultaneously with other characteristics of the mechanochemical process (the monomer size and the chemical reaction depth). Speaking using mechanochemistry terms, it corresponds to transition from the brittle grinding mode to the plastic deformation mode after reaching the grinding limit.

A Novel PBM for Nanomilling of Drugs in a Recirculating Wet Stirred Media Mill: Impacts of Batch Size, Flow Rate, and Back-Mixing.

We examined the evolution of fenofibrate (FNB, drug) particle size distribution (PSD) during the production of nanosuspensions via wet stirred media milling (WSMM) with a cell-based population balance model (PBM). Our objective was to elucidate the potential impacts of batch size, suspension volumetric flow rate, and imperfect mixing in a recirculating WSMM. Various specific breakage rate functions were fitted to experimental PSD data at baseline conditions assuming perfect mixing. Then, the best function was used to simulate the PSD evolution at various batch sizes and flow rates to validate the model. A novel function, which is a product of power-law and logistic functions, fitted the evolution the best, signifying the existence of a transition particle size commensurate with a grinding limit. Although larger batches yielded coarser and wider PSDs, the suspensions had identical PSDs when milled for the same effective milling time. The flow rate had an insignificant influence on the PSD. Furthermore, the imperfect mixing in the mill chamber was simulated by considering more than one cell and different back-mixing flow ratios. The effects were weak and restricted to the first few turnovers. These insights contribute to our understanding of recirculating WSMM, providing valuable guidance for process development.

Grinding limit and profile optimization in the high-incidence section of rail corrugation in mountainous city metro

As the most common and direct control method for rail corrugation, the rail grinding can effectively improve the service life of rails, but requires scientific theoretical guidance to achieve the efficient grinding. Therefore, it is necessary to formulate corresponding grinding limits and profile optimizations for different characteristics of rail corrugations. According to the on-site investigation of rail corrugation in the overlapping section of horizontal-vertical curves of the mountainous city metro, the corresponding dynamic model of vehicle-track system is constructed. Through the safety evaluation of the dynamic characteristics of the vehicle-track system, the safety limit of the rail corrugation grinding is preliminarily determined. Then, based on the wheel-rail friction-coupled vibration theory, the finite element model of the wheel-rail system is built. By exploring the wheel-rail friction-coupled vibration characteristics in the corrugated section with typical characteristics, the optimal limit of rail corrugation grinding is further confirmed. Finally, the treatment effects before and after rail grinding are compared using the profile optimization. Results show that when the corrugation wavelengths in the overlapping section are 30 mm, 40 mm, 50 mm, 60 mm, and 70 mm, the safety limits for rail corrugation grinding are 0.03 mm, 0.04 mm, 0.05 mm, 0.08 mm, and 0.15 mm. For the typical rail corrugation with the wavelength of 50 mm, the optimal limit of the rail corrugation grinding is 0.02 mm. Moreover, the earlier the rail corrugation is polished, the better the relative optimization effect is, but it is necessary to consider the grinding cost for targeted grinding.

Highly Active Calcium Oxide Derived from Scallop Shell for Used Cooking Oil Transesterification Reaction

The liquid additive grinding method was used to improve the specific surface area (SSA) of calcium oxide (CaO) derived from scallop shells as a sustainable resource. Grinding was carried out using a planetary ball mill on a laboratory scale by stepwise addition of some polar and non-polar solvents to determine CaO grinding limit. The crystalline phase of CaO was determined by using X-ray Diffraction (XRD) and SSA was observed based on Brunauer, Emmet, and Teller (BET) method. The mechanism of grinding was obtained from the Scanning Electron Microscope (SEM). The SSA of CaO was increased in proportion to the amount of the liquid additive and grinding time up to 102 m2 g-1 while the CaO phase was retained even after the addition of liquid according to the spectra of Fourier Transform-Infrared (FT-IR). The high conversion of triglyceride from used cooking oil to fatty acid methyl esters (FAMEs) up to 99% was obtained within an hour at 65℃.

Reduction of submicron particle agglomeration via melt foaming in solid crystalline suspension

Particle size reduction of poorly water-soluble drug crystals down to the submicron region (0.1–1 µm) is a focus strategy to increase bioavailability. In this context, solid crystalline suspensions (SCS) manufactured in an adapted wet milling process have been introduced as a promising formulation technique. However, for such particle size, agglomeration due to the prevailing attractive interactions poses a major limitation to product quality. As a countermeasure, steric and electrostatic stabilizers have been extensively used in conventional wet grinding applications to introduce kinetic stability to primary particles. In this study the formulation of SCS, containing Xylitol and Griseofulvin, is optimized using the surfactant sodium dodecyl sulfate (SDS). The stabilizer did not impact suspension stability during grinding, as a constant apparent grinding limit (d50=0.35-0.40 µm) was observed over a wide range of concentrations. Nevertheless, redispersion of primary particles from the solidified SCS in water was enhanced at concentrations of 1 wt.% and above. This behavior of the optimized formulation positively impacted the dissolution kinetics of the Griseofulvin particles compared to the binary system. Different imaging techniques revealed the morphology of the solidified Xylitol matrix and the particle location inside it. At 1 wt.% SDS a porous matrix was obtained, which is caused by a foaming process during grinding. Further investigations indicated the importance of this foam regarding the improved particle redispersion, rather than an electrostatic stabilization. A mechanism, which is inspired by particle stabilized foams and emulsions, is proposed.

Wet nanomilling of naproxen using a novel stabilization mechanism via zirconium complexation

Using a novel electrostatic stabilization mechanism that utilizes complexation of Zr(IV) species by carboxylic acid groups at the particle surface, the true grinding limit of monocrystalline naproxen nanoparticles was identified at 30 nm under an optimized naproxen/Zr(IV)-salt formulation. In a stirred media mill, we studied the influence of stress energy and number of stress events by varying the milling bead diameter. Small milling beads provide a compromise between sufficiently high stress energy and stress number and are beneficial for nanomilling of naproxen in terms of grinding kinetics. It is shown that the product particle size can be controlled by adjusting the colloidal stability via the Zr(IV) concentration. Regardless of formulation properties, particle fracture propagates along the grain boundaries as revealed by crystallite size analysis. Investigations of the solvent phase and the particle geometry reveal that mechanical stress leads to enhanced solubility of the drug, which in return promotes recrystallization and thus particle growth. The solubility can be reduced by lowering the process temperature and stabilizer concentration. In contrast to inorganic nanoparticles, which are almost free of lattice imperfections at the true grinding limit, we find a clear indication of lattice strain in the stressed naproxen nanoparticles. Since the nanoparticle yield is still limited to 40% due to particle ripening, fast removal of the nanoparticles from the mill must be a further next step to be solved in nanoparticle processing.

On the grinding effects of high-silicon iron tailings.

The main chemical component of high-silicon iron tailings (HSITs) is SiO2; HSITs also include some oxides such as Al2O3 and CaO. Mechanical activation can reduce the particle size of HSITs and enhance their pozzolanic activity such that they can be used as a type of mineral admixture for cement-based materials (CBMs). This study aims to investigate the mechanical activation (ultrafine grinding) effects of HSITs, including physical and crystallization structure effects. The particle distribution, specific surface area, density, and solubility of HSITs were tested using laser particle size analysis and other routine physical testing methods. Their crystal structures were analyzed using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry-thermogravimetry (DSC-TG). Grinding reduced the particle size of HSIT particles and increased their specific surface area, wherein the minimum D50 was 5.75 μm, the maximum specific surface area was 7608 m2/kg, and the corresponding grinding time was 3.5 h. With an increase in grinding time, the solubility showed an increasing trend; however, the density showed a decreasing trend. The change was fast before 3.5 h or 4 h and then slowed down, but the final solubility was still higher than its initial level, while the final density was still lower than its initial level. Grinding reduced the degree of crystallization of the minerals in HSITs and increased the microscopic strain and disorder of its crystal structure. These changes were significant for a grinding time of 0-3.5 h, after which the changes tended to be slow. The symmetry and integrity of the SiO2 structure decreased with grinding. The activity index of the HSIT powder was higher than 0.6. Ultrafine grinding improves the particle size distribution of HSITs and reduces the crystallinity of their main minerals, which in turn increases their chemical reactivity. It can be said that ultra-finely ground HSIT powder is pozzolanic and can be used as a mineral admixture for CBMs, and its grinding limit can be inferred to be 3.5 h.

Effect of Grinding Media Size on Ferronickel Slag Ball Milling Efficiency and Energy Requirements Using Kinetics and Attainable Region Approaches

The aim of this study is to evaluate the effect that the size of grinding media exerts on ferronickel slag milling efficiency and energy savings. A series of tests were performed in a laboratory ball mill using (i) three loads of single size media, i.e., 40, 25.4, and 12.7 mm and (ii) a mixed load of balls with varying sizes. In order to simulate the industrial ball milling operation, the feed to the mill consisted of slag with natural size distribution less than 850 μm. Grinding kinetic modeling and the attainable region (AR) approach were used as tools to evaluate the data obtained during the ball milling of slag. Particular importance was given to the determination of the specific surface area of the grinding products, the identification of the grinding limit, and the maximum specific surface area which could be achieved when different grinding media sizes were used. The results showed that, in general, the breakage rates of particles obey non-first-order kinetics and coarse particles are ground more efficiently than fines. The AR approach proved that there is an optimal grinding time (or specific energy input) dependent on the ball size used for which the volume fraction of the desired size class is maximized. The use of either 25.4 mm balls or a mixed load of balls with varying sizes results in 31 and 24% decrease in energy requirements, compared to the use of balls with small size (12.7 mm).

OPTIMUM WORKHOLDING CONDITIONS FOR PRECISION CYLINDRICAL GRINDING OPERATIONS

Grinding process is a very important process in machining industry being one of the most popular processes when high quality parts must be manufacture. Likewise, workholding is a critical issue on cylindrical grinding. The use of the driving dog is common when the workpiece is held between centers. However, one of the handicaps of this workholding is that the cylindrical workpiece cannot be ground along the complete length. In order to tackle this issue, in the present work the workpiece is held between centers avoiding the use of the driving dog. To this end, a methodology to obtain the grinding limit parameters that ensure that the transmitted torque is higher that the resistance torque is presented, being the aim of these tests is to avoid the sliding between the point and the workpiece. Finally, non-destructive tests are designed, which, using a safety coefficient of about 0.77, the tests allow the correct design of each specific grinding process. Keywords: cylindrical grinding, workholding, driving dog, sliding

Investigation on rail corrugation grinding criterion based on coupled vehicle–track dynamics and rolling contact fatigue model

Based on the measured spectra of rail roughness and track structures longitudinal roughness, the rail grinding limit is studied with the help of an established coupled dynamic metro vehicle–track model and a rolling contact fatigue model. The results indicate that metro rail grinding control should be regulated according to corrugation wavelength range and operating speed. Based on the rolling contact fatigue model, longer wavelength of rail corrugation has less influence on the wheel rolling contact fatigue. For the metro lines with a maximum operating speed of 80 km/h, the average levels of rail corrugation in the wavelength ranges of 30–65 mm, 65–125 mm, and 125–250 mm should be less than 5.4, 24.8, and 33.8 dB re 1 μm, respectively; for the ones with the operating speed of 80–120 km/h, the corresponding average corrugation levels in the three wavelength ranges should be less than 4.4, 9.8, and 29.8 dB re 1 μm, respectively.

Biomorphic SiC: porous SiC and ultrafine SiC powder formation through biomimicking route

ABSTRACT Natural wood is a ready-to-use template for making porous silicon carbide (SiC) body. Biomorphic β-SiC was synthesised from wood of a mango (Mangifera Indica) tree. Carbon template was made by carbonising this wood at 1000°C in argon gas atmosphere. Thus formed template was repeatedly infiltrated with a mixture of tetraethyl-orthosilicate (TEOS) (in alcohol) and oxalic acid solution (aqueous). Heat treatment at 1600°C for 1 h in vacuum converted the infiltrated template to β-SiC with a complex hierarchical cellular structure- a mimic of the plant structure. To produce phase pure SiC, unreacted carbon (C) and silica (SiO2) (if any) were removed by burning in air followed by treating with hydrofluoric acid. Mono dispersed SiC powder having particle size of ∼225 nm has been obtained by crushing, grinding and milling of thus formed biomorphic SiC body. A grinding limit is reported for the first time in SiC system.

Numerical investigation of the fatigue performance of elastic rail clips considering rail corrugation and dynamic axle load

To analyze the reasons for rail clip fracture, the characteristics of rail corrugation were first measured using a rail corrugation meter. The vibration acceleration of the fastener clips at the sections with/without rail corrugation was measured, and the effects of rail corrugation on the clip vibration were analyzed. After this, a vehicle--track coupling dynamic model and a refined model for the fastener system were established in order to study the effects of rail corrugation on the vibration acceleration and stress on critical points. Finally, the rail grinding limits were determined based on the fatigue analysis method and the damage accumulation theory from the aspect of the fatigue life of the clip. The results of the study showed that the main wavelength of rail corrugation at the rail clip fracture section was approximately 40 mm. The vibration acceleration of the clip caused by rail corrugation was too large. Under normal installation conditions, the maximum clip stress was 1490 MPa at the small circular arc on the rear arch, which was identical to the on-site fracture location. The intrinsic frequency of the clip was approximately 810 Hz. Rail corrugation excited and triggered the forced vibration of the clip, and induced resonance at a speed of 120 km/h and a wavelength of 40 mm. The large cyclic stress amplitude of the clip with rail corrugation increased from 44 MPa to 68 MPa when compared with the clip without rail corrugation. Rail clip fracture was caused by the naturally occurring resonance fatigue arising from rail corrugation. For metro lines designed with a maximum speed of 120 km/h, it was suggested to control the rail corrugation amplitudes with a wavelength of 40 mm, 50 mm, 30 mm, 120 mm and 160 mm to below 0.04, 0.08, 0.16, 0.19 and 0.2 mm, respectively, taking into account the fatigue life of the clip.

Rapid development of API nano-formulations from screening to production combining dual centrifugation and wet agitator bead milling.

Various wet ball nanomilling-screening tools for poorly soluble APIs are available which differ in their milling principle, batch size and number of samples. Here, the transferability of results from screening (small to medium-scale) to pharmaceutical production (largescale) was investigated. Wet ball milling in a dual centrifuge (DC) (10-100 mg API, 40 samples in parallel) was used to identify stable nanoformulations. In addition different sized agitator bead mills were used for scale-up to industrial scales. DC-and small-scale agitator milling (AM) resulted in small and virtually identical API-particles. Additionally, similar API-particles were obtained using two different sized agitator bead mills (batch size 1.5 and 30 kg) and applying comparable specific grinding energies (SGE). The SGE used in the trials represents the grinding limit for this API-suspension. Using lower SGEs, AM results in larger API-particles. All used milling tools had no influence on the APIs crystal structure and wear of grinding media (Zr/Y) is low. The study confirmed the importance to choose the right formulation and process parameters, which positively affect grinding efficacy, particle size distribution and wear contamination. The excellent comparability of results obtained from DC-milling and AM significantly reduces the duration for successful and predictable formulation development.

Effect of Energy Input in a Ball Mill on Dimensional Properties of Grinding Products

This study investigates the evolution of dimensional properties of grinding products, namely, the mass, the surface area, the length, and the number of particle distributions with the energy input in a ball mill. The size analysis of the mill products enables the calculation of the mass distribution of each material at predetermined size classes and then the determination of the other dimensional properties by taking into consideration the geometry of the particles. Based on the distribution of dimensional properties, an effort was made to determine whether the grinding products are fractal in nature. The effect of material type and grinding conditions on the relationship between the dimensional properties and the specific energy input was also investigated. Special importance was assigned to the study of the specific surface area and the surface area production rate, both during the initial grinding stages and at higher energy levels. The determination of the surface area production rate allows the identification of the grinding limit and the maximum specific surface area which could theoretically be achieved. This approach has as ultimate objective the maximization of energy efficiency and subsequently the minimization of the high grinding cost.

Revisiting the thermal recrystallization of mechanically amorphized lignocellulose

The products of mechanical activation of lignocellulose are subject to the process of recrystallization of amorphized cellulose sections in some cases, this process reduces the efficiency of subsequent chemical and enzymatic treatment of plant raw materials. The process of recrystallization was studied on lignin-containing raw material - wheat straw. The crystal structure, particle and crystallite size of wheat straw ware studied by X-ray diffraction and granulometric analysis. It was found that the degree of crystallinity of cellulose in the composition of wheat straw decreases from 70 to 19 % in the process of mechanical activation in an AGO-2 planetary ball mill, the destruction of cellulose chains is observed predominantly along the [010] direction. The grinding limit was reached after 10 minutes of activation. Thermocycling at the boiling point of liquid nitrogen (-196 °C ) and at 180 °C of lignocellulose revealed that under these conditions the degree of recrystallization of amorphous regions of cellulose is negligible, which can be explained by the presence of lignin and a low water content in a material with a material moisture content (about 5.0 wt. %).

Effect of prolonged dry grinding on size distribution, crystal structure and thermal decomposition of ultrafine particles of dolostone

The effect of ultrafine dry grinding for up to 1920 min in a planetary ball mill at 300 rpm on size distribution, particle agglomeration and bulk structural changes in a dolomite-rich (CaMg(CO3)2) rock was studied. The size and shape of the ground particles were characterized by laser scattering and scanning electron microscopy (SEM) respectively. The uniformity index of the size distribution evaluated using the Rosin-Rammler equation showed that the apparent grinding limit was reached after 480 min. Differential- and thermo-gravimetric (DTA-TG) analyses showed that the decomposition temperature of magnesite (MgCO3) was affected to a far greater extent by the input energy of grinding than that of calcite (CaCO3). The activation energy of the total decomposition decreased for the aliquots ground above 480 min. This behavior was explained by the storage of extensive lattice distortions in dolomite crystal structure evaluated by X ray line profile analysis of major diffracting peaks. The infrared (IR) bands related to the bending (877 cm−1) and stretching (1418 cm−1) vibrations of the anionic CO32– groups increased with the particle size decreasing as well as being sensitive to particle coarsening caused by agglomeration. Coupled to morphological and Rosin-Rammler size distribution analyses, IR data confirmed extensive agglomeration above 480 min. The transformation between calcite and aragonite polymorphs was also noticed in those aliquots ground for periods ≥480 min. The bulk structural changes documented here were useful to show that the inversion from the breakage to the agglomeration regime observed in dolostone powder system is directly associated with the change in the energy relaxation mechanism inside the dolomite grains.

Impact of Process Parameters on the Grinding Limit in High-Shear Wet Milling

Knowledge of the impact of process parameters on the minimum achievable (critical) particle size below which breakage is no longer observed for high shear rotor stator wet milling (HSWM) operations is vital to design and optimize milling processes of active pharmaceutical ingredients. The grinding limit is a result of a balance between material properties and the energy imparted to particles during the milling processes. In turn, the energy imparted to particles depends on rotation rate, generator geometry, mill configuration, flow rate, etc. In this communication, a master curve was constructed by normalizing critical particle size curves obtained at different cumulative breakage energies, using a shift factor that can be determined with minimal experimentation.

Analysis of ground rice straw with a hydro-textural approach

The lignocellulosic material contained in agricultural residues like straws, represents a resource with many points of biomolecular interest but their extraction is subject to a succession of treatments highly energy-consuming and generating effluents. Due to the strength of the supramolecular structure of the lignocellulosic matrix at various levels, it is difficult to improve the efficiency of separation processes for these materials. The utilization of biomass constituents often requires pretreatment, which is a crucial step in the separation into constituents. The rice straw must firstly be fractionated by grinding, which can also require pretreatment to improve efficiency. The question arises as to how best to identify and model the mechanisms involved in the grinding process with or without pretreatment. The aim of this study was to complete the characterization of the grinding process using a hydro-textural approach applied to biopowders to help with identification of the mechanisms involved. Experimental trials were conducted with rice straw through several milling steps, which led to decreasing particle sizes. The physical properties (density, cohesion, coefficient of friction, ability to flow) were characterized for some of the obtained powders, which were then described with a hydro-textural diagram. The results reveal that the breakdown process led to a “loss of porosity” regardless of the size of the powder particles. The fragmentation seemed to be located in the “zones of stronger porosities”. These structures containing residual water, decreased weakly with grinding. Assuming that the water was located within cells of the straw (intracellular water), given the power law, which correlates compactness to median diameter (d50), we introduced a characteristic size, which corresponds to the physical limit of optimal grinding. Its value, calculated from the model, is nearly equal to the cell wall thickness: limϕ→1d50=ecell.~2μm.

The extreme mobility of debris avalanches: A new model of transport mechanism

AbstractLarge rockslide‐debris avalanches, resulting from flank collapses that shape volcanoes and mountains on Earth and other object of the solar system, are rapid and dangerous gravity‐driven granular flows that travel abnormal distances. During the last 50 years, numerous physical models have been put forward to explain their extreme mobility. The principal models are based on fluidization, lubrication, or dynamic disintegration. However, these processes remain poorly constrained. To identify precisely the transport mechanisms during debris avalanches, we examined morphometric (fractal dimension and circularity), grain size, and exoscopic characteristics of the various types of particles (clasts and matrix) from volcanic debris avalanche deposits of La Réunion Island (Indian Ocean). From these data we demonstrate for the first time that syn‐transport dynamic disintegration continuously operates with the increasing runout distance from the source down to a grinding limit of 500 µm. Below this limit, the particle size reduction exclusively results from their attrition by frictional interactions. Consequently, the exceptional mobility of debris avalanches may be explained by the combined effect of elastic energy release during the dynamic disintegration of the larger clasts and frictional reduction within the matrix due to interactions between the finer particles.

The onset of particle agglomeration during the dry ultrafine grinding of limestone in a planetary ball mill

This study investigates structural and morphological changes in limestone particles ground in a planetary ball mill. The grinding tests were carried out as a function of the revolution speed (100–300rpm) and time (7–120min). The feed size, the grinding media and the filling rate were kept constant. The size and shape of the particles were characterized by laser scattering and scanning electron microscopy (SEM), respectively. The uniformity index given by the Rosin–Rammler particle size distribution combined with SEM micrographs was useful to show the decrease of the grinding rate caused by the agglomeration of fines lying on partially broken particles. The effect of the mechanical action of grinding on the crystalline structure of calcite grains was investigated by X-ray diffraction (XRD) and electron paramagnetic resonance (EPR) spectroscopy. EPR spectroscopy was performed before and after the samples being irradiated with a dose of 5kGy of gamma rays. XRD analysis only showed a net reduction in the intensity of the (101¯4) peak and a slight increase in the line broadening of this peak. On the other hand, the energy input provided by the grinding action modified the population of paramagnetic defects existing in calcite lattice. In addition to the creation of electron traps responsible for a signal with a g-factor equal to 1.9999, the EPR measurements showed that the intensity of the hyperfine lines of Mn2+ substituting Ca2+ was affected by the grinding action. The morphological and particle size analysis together with the relationship found between the intensity of paramagnetic defects and specific surface area of limestone particles provided a picture of the onset of the agglomeration process. These results were discussed in order to explain the apparent grinding limit observed in ultrafine milling of limestone.