Air Jet Mill Research Articles

Application of a simplifying model to the breakage of different materials in an air jet mill

This paper describes the application of a unifying ‘master curve’ model for single impact size reduction in an air jet mill. Based on a dimensional analysis and a modified fracture mechanical theory, it enables to relate the weight percentage of broken particles under sieve cut size to the mean sieve cut size of the studied material, the net kinetic energy provided to the material, the number of impacts and a global parameter fMat.. Altogether 11 sieve cuts of different materials are impacted in an experimental air jet mill. Minimum breakage energies are derived and are shown to vary with a power function of the sieve cut size. The simplifying model is then successfully applied and enables to derive the fMat. parameters. In our situation, the experimental data do not show a direct relationship between fMat. and the minimum breakage energy and sieve cut size for all the materials.

Ultrafine wool powders and their bulk properties

Wool powder may be utilized in advanced applications, notably composite materials, biomedical and cosmetics depending on the development of a suitable powder fabrication process and understanding of powder properties. This paper discusses a novel approach to mill viscoelastic wool fibres using a combined wet attritor and air jet milling (AJM) process. Results show that 5h attritor milling followed by spray drying can produce wool powder with a BET surface area of 14.89m2/g and volume based d(0.5) of 4μm. Despite absence of any milling pre-treatments, the particles are much smaller than previously prepared wool particles using other methods. Subsequent AJM further reduces d(.5) to 1.5μm, but creates loose aggregates which results in their low bulk density. The aggregates can be disintegrated using a strong consolidating force and such AJM powder can be compressed to 45% of the original volume. Rheological measurements suggest strong cohesion and poor flowability of wool powders. Consolidated AJM powder is even more cohesive with a flow function of 1.64 compared to spray dried powder without AJM which has a flow function of 3.74. The results are important for processing and applications of these novel organic fibre powders, where ultrafine particles are needed and understanding of powder packing and flow behaviour is important.

Influence of Solvent on the Morphology and Subsequent Comminution of Ibuprofen Crystals by Air Jet Milling

Crystal morphology plays an important role in drug processing and delivery, which may be controlled during crystallisation. In this study, ibuprofen particles with different size and morphology were produced by controlled crystallisation in order to evaluate their impact on particle size reduction. Results suggest that the micronisation behaviour of ibuprofen was markedly influenced by the morphology and size of starting materials. It was possible to reduce the size of ibuprofen particles to sizes less than 5 µm during dry milling, which is markedly below the reported brittle-ductile transition size. Results also indicate that the particle size reduction mechanism is influenced by the size and morphology of the starting ibuprofen crystals. Dissolution behaviour of ibuprofen was shown to be influenced by the solid surface chemistry of micronised drug particles. The molecular modelling study provided deeper understanding of the experimental findings observed in this study.

Milling effect on the control efficacy of spray‐dried Bacillus thuringiensis technical powder against diamondback moths

The relationship between the particle size of spray-dried Bacillus thuringiensis NT0423 technical powder and its insecticidal activity against diamondbackmoth was investigated in laboratory and glasshouse conditions. A variety of sizes of Bt particles were made by homogenisation, followed by pilot-scale milling to optimise further the current Bt formulation process. In laboratory conditions, homogenised Bt suspension with smaller particles had higher control efficacy against diamondback moth. However, in glasshouse conditions, the smallest particle treatment did not follow the size-dependent insecticidal activity, rather showing decreased control efficacy. Secondly, air-jet-milled Bt powder (16.9 μm) had higher control efficacy than hammer-milled Bt powder (35.1 μm) in glasshouse conditions. However, double air-jet milling (5.3 μm) had lower control efficacy compared with one-time milling. These results suggest that NT0423 technical powder with smaller particles is better in controlling diamondback moths, but excessively small particles (<10 μm) possibly reduce the insecticidal activity.

Preparation and surface properties of cashmere guard hair powders

Cashmere guard hair, a by-product from the cashmere dehairing industry is used for low value applications because the guard hairs are not suitable for spinning as they are coarse, contain large medullation and lack crimp or curvature. To find new uses in high value-added applications, cashmere guard hairs were milled into fine particles using the processing sequence Chopping→Attritor milling→Spray Drying→Air Jet milling. The guard hairs were partially hydrolysed with hydrochloric acid which increased the pulverisation rate due to the deterioration in mechanical properties. The volume median particle size d(0.5) was reduced from 2.328μm, for untreated cashmere guard hair powder to 0.461μm for powder from the acid hydrolysed guard hairs. FTIR and XPS studies revealed the breakdown of the cashmere guard hair cuticle cells and the exposure of the cortex on the powder surface along with the oxidation of the cashmere guard hair during milling.

The integration of carborundum powder comminution and surface modification in an air jet mill

Surface modification of carborundum (SiC) particles with a modifying agent, with simultaneous ultrafine comminution in an air jet mill, was studied in this work. The production rate of SiC and the viscosity of the SiC-paraffin oil suspension were used to evaluate the performance of comminution and modification. The experimental results indicated that the modifying agent bonded onto the surface of the SiC to form an organic layer in the integrated process of modification and comminution. The types and content of modifying agents and the temperature of the compressed air have a great effect on the performance of ultrafine modified SiC, and there are some contradictory aspects between modification and comminution in the integrated process. According to the targets of both surface modification and ultrafine comminution, the optimum temperature of compressed air and feed rate of the modifying agent solution was 60 °C and 2 ml/min respectively, when the modifying agent A-151 was adopted to obtain the modified ultrafine SiC in one step.

Effect of crystallisation conditions and feedstock morphology on the aerosolization performance of micronised salbutamol sulphate

Salbutamol sulphate (SS) used in dry powder inhalers requires drug particles in the respirable size range of 1–5 μm to achieve a suitable therapeutic effect. The aim of this study was therefore to determine strategies for controlling drug substance characteristics pre and post-crystallisation to facilitate the production of micronised SS with desirable particle attributes for optimal delivery as an inhaled aerosol. SS batches were crystallised using an antisolvent method to produce a range of crystal morphologies. Air jet milling was then used to reduce the size of crystallised SS particles. Starting materials and micronised batches of SS were characterised in the solid state using a range of techniques with subsequent assessment of aerosol properties. Assessment of the aerodynamic characteristics of micronised SS delivered by DPI (without any carrier) indicated that fine particle fraction and emitted dose as a percentage of the total recovered dose were dependent on the quality attributes of the micronised SS, which were directly linked to the degree of imperfections and the morphology of the crystalline feedstock used in micronisation. Aerosolization performance of micronised SS can be optimised by manipulation of feedstock characteristics through crystal engineering and through definition of optimal processing conditions for micronisation.

Application of a simplifying model to the breakage of different materials in an air jet mill

This paper describes the application of a unifying ‘master curve’ model for single impact size reduction in an air jet mill. Based on a dimensional analysis and a modified fracture mechanical theory, it enables to relate the weight percentage of broken particles under sieve cut size to the mean sieve cut size of the studied material, the net kinetic energy provided to the material, the number of impacts and a global parameter f Mat. . Altogether 11 sieve cuts of different materials are impacted in an experimental air jet mill. Minimum breakage energies are derived and are shown to vary with a power function of the sieve cut size. The simplifying model is then successfully applied and enables to derive the fMat. parameters. In our situation, the experimental data do not show a direct relationship between fMat. and the minimum breakage energy and sieve cut size for all the materials.

Dephosphorization Treatment of High Phosphorus Iron Ore by Pre-reduction, Air Jet Milling and Screening Methods

This paper proposes a novel process for the pre-treatment of high phosphorus iron ore by pre-reduction, air jet milling and screening methods. The dephosphorization behavior was evaluated as a function of such parameters as the milling gas kinetic energy, Ek and specific energy consumption, Esp. At a fixed milling gas kinetic energy, the dephosphorization treatment behavior was highly dependent on the solids feed rate and the degree of sintering of the pre-reduced phases. The proposed pre-treatment process involves a pre-reduction process by carbonaceous materials at 1200°C, and air jet milling using Ek of 325.24 kJ and Esp of 3.24 kJkg–1h followed by fine screen separation using a 25 μm screen size.

Micronization of a Soft Material: Air-Jet and Micro-Ball Milling

The air-jet and ball-mill are frequently used in fine micronization of active pharmaceutical ingredients to the order of 1-5 microm, which is important for increasing dissolution rates, and also for pulmonary delivery. In this study, we investigated the ability of air-jet and ball-mill to achieve adequate micronization on the lab scale using a model soft material, Pluronic F-68. Material mechanical properties were characterized using the nanometer 600. Pluronic F-68 was ball-milled in a micro-mill at different material weights and durations in liquid nitrogen vapor. In comparison, a lab scale air-jet mill was used at various milling parameters according to a full factorial design, where the response factors were particle yield and particle size distribution, which was analyzed using laser diffraction and scanning electron microscopy. The yield achieved with the micro-ball mill was 100% but was ~80% for the air-jet mill, which reduced the size of Pluronic F-68 from 70 microm to sizes ranging between 23-39 microm median diameters. Ball milling produced particles less than 10 microm after 15 min. Although air-jet milling proved capable of particle size reduction of the relatively soft material Pluronic F-68, limitations to the lower size range achievable were observed. The feed rate of the material into the air jet mill was a significant factor and slower feed rates lead to smaller sizes by allowing more time for particle collisions and subsequent particle breakage to occur. Micro-ball milling under cold condition was more successful at achieving a lower range particle size reduction of soft materials.

Environmental Benefits and Energy Efficiency of Superheated Steam as Media in Processing Ultra-Fine Fly Ash Technology

With superheated steam from thermal power plants for medium, the fluidized bed jet mill which had improved vertical turbine classifier was used to study Superheated steam as media in processing ultra-fine fly ash technology. Experiment tested the energy consumption of processing fly ash with superheated steam, which was converted into standard coal compared with the air jet mill technology to prove the environment and energy saving advantages in superheated steam-gas processing ultra-fine fly ash. Experimental results showed that this technology has low energy consumption, high grinding force, and its grinding, grading, collection process is completely in dry method, so this technology is an ideal method for processing ultra-fine fly ash by low-cost and large-scale. At the same time, this paper assessed the environmental benefits of some environmental indicators. Finally, the applications of this technology are summarized.

CFD simulation and experimental analysis of flow dynamics and grinding performance of opposed fluidized bed air jet mill

This paper presents a three dimensional Computational Fluid Dynamics (CFD) model to investigate the flow dynamics of solid–gas phases during fine grinding in an air jet mill. Alpine 100AFG fluidized bed air jet mill is considered for the study and the jet milling model is simulated using FLUENT 6.3.2 using a standard k-ε model. The model is developed in GAMBIT 2.3.16 and meshed by tet/hybrid (T-Grid) and Triangular (Pave) meshes. The effects of operating parameters such as solid feed rate, grinding air pressure and internal classifier speed on the performance of the jet mill are analyzed. The CFD simulation results are presented in the forms of dual phase vector plot, volume fraction of phases and particle trajectories during fine grinding process. The mass of ground feed entering and leaving the cyclone (underflow) is also computed by simulation. The proposed model gives realistic predictions of the flow dynamics within the jet mill. Experiments are conducted on the Alpine 100AFG jet mill to study the particle size, morphology and mass of the ground product. The numerical results are found in good agreement with the experimental results.

Molecular weight and secondary structure change in eri silk during alkali degumming and powdering

AbstractChanges in molecular weight and secondary structure of eri silk during alkali degumming and silk powdering were studied. An increase in silk degumming intensity, through increased alkali concentration, treatment temperature, and time, reduced the fibroin molecular weight and, therefore, the fiber tenacity, but at the same time, increased the β‐sheet fraction. These changes reduced the time required to mill the degummed silk fibers into fine powders. Mechanical forces used in wet attritor and air jet milling disturbed intermolecular packing along the direction of side chains, but the conformation remained essentially β‐sheet even in the sub‐micron silk particles. Dry milling drastically reduced molecular weight and changed the conformation of the fibroin chains. The rate of the spontaneous conformation transition in regenerated fibroin solution prepared from fibers and powders increased with a decrease in fibroin molecular weight, affecting the time fibroin solutions could be stored before gelling. Overall, the study showed that molecular weight and secondary structure of silk powders could be manipulated by suitably changing the degumming and milling conditions. It also suggests that wet media milling and air jet milling are better than dry media milling to prepare less degraded and more crystalline ultrafine silk particles. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Preparation and evaluation of thiomer nanoparticles via high pressure homogenization

The aim of this study was to establish and evaluate a high pressure homogenization method for the preparation of thiomer nanoparticles. Particles were formulated by incorporation of the model protein horseradish peroxidase in chitosan-glutathione (Ch-GSH) and poly(acrylic acid)-glutathione (PAA-GSH) via co-precipitation followed by air jet milling. The resulting microparticles were suspended in distilled water using an Ultraturax and subsequently micronized by high pressure homogenization. Finally, resulting particles were evaluated regarding size distribution, shape, zeta potential, drug load, protein activity and release behaviour. The mean particle size after 30 cycles with a pressure of 1500 bar was 538 ± 94 nm for particles consisting of Ch-GSH and 638 ± 94 nm for particles consisting of PAA-GSH. Nanoparticles of Ch-GSH had a positive zeta-potential of +1.03 mv, whereas nanoparticles from PAA-GSH had a negative zeta potential of −6.21 mv. The maximum protein load for nanoparticles based on Ch-GSH and based on PAA-GSH was 45 ± 2% and 37 ± %, respectively. The release profile of nanoparticles followed a first order release kinetic. Thiolated nanoparticles prepared by a high pressure homogenization technique were shown to be stable and provide controlled drug release characteristics. The preparation method described here might be a useful tool for a more upscaled production of nanoparticulate drug delivery systems.

Reactive Vaporization of Crude Glycerol in a Fluidized Bed Reactor

Crude glycerol is a coproduct of the trans-esterification of vegetable oil and animal fat to biodiesel that contains as much as 5% salt. Processing the glycerol to value added products often requires an expensive distillation step. In this work, we propose a new process in which crude liquid glycerol is fed directly to a fluidized bed, vaporizes, and then reacts to form acrolein over a tungsten doped zirconia catalyst. Salt in the crude glycerol crystallizes and accumulates in the fluid bed. In a commercial process, the salt crystals will be subject to significant mechanical stresses that will cause them to attrit. The attrited powder will be elutriated from the bed and collected by filters. The focus of this work has been to evaluate the effect of salt on catalyst activity as well as its attrition resistance. Thus far, there is little evidence of salt crystallization or migration to the interior of the catalyst particles—after more than 1 day of continuous operation, catalyst activity remained unchanged. Tests on a standard air jet mill suggest that the mechanical stresses typical of a fluidized bed will be sufficient to attrit the salt and thus minimize its accumulation in the reactor with time.

Gastroretentive particles formulated with thiomers: development and in vitro evaluation

The objective of this study is to develop and evaluate gastroretentive particulate delivery systems using Riboflavin-5′-monophosphate sodium salt dihydrate (RF5′PNa) as model drug. Poly(acrylic acid)-cysteine and chitosan-4-thiobuthylamidine were evaluated and compared as anionic and cationic polymers for gastroretentive particles.Permeation studies were performed with freshly excised stomach mucosa from rats. Polymers and combination with glutathione were evaluated for permeation enhancing properties. Furthermore, particles were prepared by air jet milling and characterized.Permeation studies showed that the apparent permeability coefficients for RF5′PNa with thiomers and glutathione are 1.511-fold and 2.354-fold higher than control, respectively. It can be seen from the results glutathione in combination with thiomers has a significant influence for increasing permeation. Poly(acrylic acid)-cysteine and chitosan-4-thiobuthylamidine particles demonstrated a mean diameter of 336.5 ± 16.5 and 396.3 ± 17.0 nm and zeta potential of −19.98 ± 1.015 and 27.15 ± 0.500 mV, respectively. The drug loading of Poly(acrylic acid) particles was significantly higher than chitosan particles. The release rate of RF5′PNa from the thiolated particles was slower compared with unmodified particles. Moreover, thiolated particles showed higher mucoadhesive properties compared to unmodified particles.Overall, thiolated particles of both anionic and cationic polymers had improved mucoadhesive and controlled release properties. Therefore, they could be promising for gastroretentive delivery systems.

Effect of sample preparation method on quantification of polymorphs using PXRD

The purpose of this study was to improve the sensitivity and accuracy of quantitative analysis of polymorphic mixtures. Various techniques such as hand grinding and mixing (in mortar and pestle), air jet milling and ball milling for micronization of particle and mixing were used to prepare binary mixtures. Using these techniques, mixtures of form I and form II of clopidogrel bisulphate were prepared in various proportions from 0–5% w/w of form I in form II and subjected to x-ray powder diffraction analysis. In order to obtain good resolution in minimum time, step time and step size were varied to optimize scan rate. Among the six combinations, step size of 0.05° with step time of 5 s demonstrated identification of maximum characteristic peaks of form I in form II. Data obtained from samples prepared using both grinding and mixing in ball mill showed good analytical sensitivity and accuracy compared to other methods. Powder x-ray diffraction method was reproducible, precise with LOD of 0.29% and LOQ of 0.91%. Validation results showed excellent correlation between actual and predicted concentration with R2 > 0.9999.

The characterization and chemical reactivity of powdered wool

Wool powders with various particle sizes have been produced using different milling techniques. Scanning electron microscopy (SEM) showed gradual breakdown of the fibre as it was progressively converted into powder form. Chlorination enhanced the effectiveness of subsequent air-jet milling. X-ray photoelectron spectroscopy (XPS) revealed an increase in the surface concentrations of oxygen and nitrogen, and a decrease in carbon and sulphur on conversion of the fibres into powders, as the cortex became exposed on the powder surface. An increased surface concentration of cysteic acid was observed for the chlorinated powder. Rapid uptake of dye by wool powders was observed in situations where there was virtually no uptake by the original fibre. Hydrophobic dyes were more readily sorbed than were hydrophilic dyes. The chlorination treatment led to a decrease in the sorption of acid dyes. Confocal microscopy, used in conjunction with a fluorescent stain, showed that chemicals were able to penetrate wool particles, even at room temperature. The rate and extent of uptake of dye by the finer powders were comparable to that obtained with activated charcoal, even though the surface area of the charcoal was 100 times greater.

Characterization of poly(ethylene- co-vinyl acetate) (EVA) filled with low grade magnesium hydroxide

Low-grade magnesium hydroxide (LG-MH) is a solid by-product that undergoes an endothermic decomposition in the temperature range of 300–750 °C. Due to its thermal behaviour and its lower cost relative to pure Mg(OH) 2, it was studied as a non-halogenated flame retardant filler in a 28% vinyl acetate (VA) content poly(ethylene- co-vinyl acetate) matrix. The solid was characterized by XRF and the crystalline phases determined by XRD, composed predominantly of Mg(OH) 2 and calcium and magnesium carbonates. Particle size reduction was performed by both mechanical as well as air jet milling in order to optimize the particle size distribution. Composites with different filler concentrations were prepared to evaluate the mechanical properties and flame retardancy by means of limiting oxygen index tests. LOI was also determined in specimens filled with commercial flame-retardants to analyse the effectiveness of this solid.

Fabrication of ultrafine powder from eri silk through attritor and jet milling

Fibroin protein derived from silk fibres has been extensively studied with exciting outcomes for a number of potential advanced biomaterial applications. However, one of the major challenges in applications lies in engineering fibroin into a desired form using a convenient production technology. In this paper, fabrication of ultrafine powder from eri silk is reported. The silk cocoons were degummed and the extracted silk fibres were then chopped into snippets prior to attritor and air jet milling. Effects of process control agents, material load and material to water ratio during attritor milling were studied. Compared to dry and dry–wet attritor milling, wet process emerged as the preferred option as it caused less colour change and facilitated easy handling. Ultrafine silk powder with a volume based particle size d(0.5) of around 700 nm could be prepared following the sequence of chopping ➔ wet attritor milling ➔ spray drying ➔ air jet milling. Unlike most reported powder production methods, this method could fabricate silk particles in a short time without any pre-treatment on degummed fibre. Moreover, the size range obtained is much smaller than that previously produced using standard milling devices. Reduction in fibre tenacity either shortened the milling time even further or helped bypassing media milling to produce fine powder directly through jet milling. However, such reduction in fibre strength did not help in increasing the ultimate particle fineness. The study also revealed that particle density and particle morphology could be manipulated through appropriate changes in the degumming process.