Narrow Size Fractions Research Articles

96/06654 National strategy statement on natural refrigerants (NRs)

Char made from narrow size fractions of lignite in a fluidized bed was briquetted at 140 MPa with various binder tars produced from the lignite. The raw briquettes were cured in air at 200 °C and then carbonized, usually at 925 °C. The tensile strength of the resulting formed coke could be as much as three times that of the best metallurgical coke, or as low as 2–3 MPa, depending on the type of binder used, or even a factor of ten lower than this if the curing stage was omitted. Curing is thus a crucial stage. Distinct optima of binder percentage and char particle size were found, giving maximum briquette strengths (≈ 13 MPa). A corresponding ‘strength surface’ suggested four factors associated with weakness of a briquette. Binders produced by air-blowing rather than nitrogen-blowing gave stronger briquettes. Fluidized bed tar was greatly ‘improved’ in this way, but Gray-King tar only slightly so, probably because the former contained more generally reactive species. Cured-only briquettes were usually surprisingly strong and carbonization increased this strength ≈ 1.5 times. Curing is considered to ‘fix’ the binder; carbonization changes the chemical nature of the ‘cement’ between the char surfaces but does not appreciably alter its spatial distribution. The carbonized briquette thus has substantially the same structural flaws as were present in the cured state and so has a strength closely correlated with that achieved merely by curing.

Modelling of interparticle breakage

A model was developed for interparticle breakage considering especially the influence of the feed particle size distribution. Based on experimental results with particle beds consisting of narrow size fractions, a consistent description of the breakage behaviour of a material can be achieved in terms of the fraction of broken particles and the breakage function, both depending on the particle size and the energy absorption. The interactions between particles of different sizes can be quantified by introducing an energy split function expressing the distribution of the energy to the size fractions in the bed. This model enables us to develop a method for predicting the results of interparticle breakage with different feed particle size distribution. The investigations were conducted with a piston press using quartz in the size range from 100 to 4000 μm by variation of the energy absorption between 1 and 20 J/g.

The Distribution of Magnetic Susceptibility in Crushed Ores

Particles of the crushed products form a dispersive system in which the continuous phase (waste rock) constitutes a matrix for inclusions of dispersed phases (metal–bearing minerals). In the process of crushing multicomponent ores, the inclusions of dispersed phases are bound in random to the consecutive particles of the crushed product. Value of of the magnetic susceptibility of particles depends on the number of inclusions of magnetic minerals. Starting from the assumption that the number of inclusions in the particles of narrow size fraction has the Poisson distribution, the author has derived an equation of the distribution function of the magnetic susceptibility of particles of narrow size fraction. The distribution function is expressed by the Pearson function. This paper presents a detailed form of this function for weakly and strongly magnetic ores. Parameters of the distribution function are connected to the values characterising a given sample.

Fractal assessment of finely ground limestone for flue gas desulphurisation

Industrial minerals, those utilised for their physical properties and chemical content, e.g., limestone, often employ comminution during their preparation. A study was performed to investigate if the type of pulveriser used to comminute limestone, a grade suitable for flue gas desulphurisation, influenced its morphology and subsequent behaviour. Samples of the limestone were pulverised by three different devices a hammer mill, rolls crusher and ball mill. A narrow size fraction was removed from each of the products by elutriation. These fractions were then subjected to sulphuric acid digestion tests, to simulate the desulphurisation process. The tests revealed that the amount of acid consumed, i.e., limestone utilisation, varied with the pulverisers under study, with the hammer mill having the highest limestone utilisation. From SEM micrographs of the limestone fragments their overall geometric shape and fractal dimension were determined, using image analysis procedures. The fractal dimension is a useful descriptive parameter for characterising boundaries that take on a randomly rugged appearance. Using the classical geometric shape factors, aspect ratio and circularity, one could not distinguish between the three comminution products. However, fractal analysis was sensitive enough to differentiate between the fragments produced by the different pulverisers. Hammer milling produced the most rugged particles followed by rolls crushing and ball milling respectively.

Development and evaluation of sustained-release ibuprofen-wax microspheres. II. In vitro dissolution studies.

A modified USP paddle method using minibaskets was used to study the effects of various formulations on in vitro dissolution of ibuprofen microspheres. Formulations containing waxes such as paraffin or ceresine wax without modifiers exhibited very slow dissolution profiles and incomplete release, which did not improve with increased drug loading or the preparation of smaller microspheres. The addition of modifiers such as stearyl alcohol and glyceryl monostearate greatly increased the dissolution rate, with 20% (w/w) near the optimum for predictable dissolution. Higher drug loading and decreased microsphere size increased the dissolution rate from microspheres containing modifier. Optimum formulations contained ceresine wax or microcrystalline wax and stearyl alcohol as a modifier, with a drug content of 17%. An increase in the encapsulation dispersant concentration had little effect on the dissolution profiles. The dissolution data from narrow size fractions of microspheres indicated spherical matrix drug release kinetics; the 50% dissolution time decreased with the square of the microsphere diameter. With appropriate modifiers, wax microsphere formulations of drugs with solubility characteristics similar to those of ibuprofen can offer a starting basis for predictable sustained release dosage forms.

SEPARATION AND BEHAVIOR OF NONSPHERICAL PARTICLES IN SEDIMENTATION/STERIC FIELD-FLOW FRACTIONATION

ABSTRACT Sedimentation/steric field-flow fractionation (Sd/StFFF) is an elution separation technique capable of measuring the size distribution of 0.3–100 µm spherical and near-spherical particles with advantages including high resolution, fast analysis, and the ready collection of narrow size fractions. In this study we investigate the applicability of Sd/StFFF to various nonspherical particles including the doublets of spheres, rod-shaped glass fibers, compressed latex discs, and quartz particles (BCR 67 and 70) with complex mixed shapes. Some fractionation, retention, and selectivity features of these particles are defined and measured in relationship to those of spheres (latex beads), which are better understood. While the relative behavior of these two particle types depends on many factors, especially the distance of the particle from the Sd/StFFF channel wall, in most cases the nonspherical particles are eluted before spheres of equal volume and they often display higher selectivity than spherical ...

Rapid Separation and Measurement of Particle Size Distribution of Starch Granules by Sedimentation/Steric Field‐Flow Fractionation

ABSTRACTSedimentation/steric field‐flow fractionation (Sd/StFFF) is introduced as a new technique for the size analysis of starch granules. Sd/StFFF provides both high speed and high resolution, and narrow size fractions of starch granules can be collected from the eluting stream for further characterization by microscopy or other techniques. The sedimentation FFF instrument was calibrated and applications were made to 12 starch samples derived from wheat, durum wheat, corn, oat, tapioca, and potato. The instrument response curve (the fractogram) and the size distribution curves for several of these samples were recorded. The mode or bimodal diameters and the percentages of A‐and B‐granules were compiled.

Upper explosive limit of dusts: Experimental evidence for its existence under certain circumstances

An experimental study of the explosibility of cornstarch using the 20-L vessel shows that, for a narrow size fraction near its limiting particle size for explosibility, an upper explosive limit exists. Upper explosive limits were also observed with both cornstarch and Pittsburgh Standard coal dust at low oxygen concentrations. These upper limits occur at accessible dust concentrations. A simple mechanism, “the oxygen depletion” phenomenological concept, is described. The optimum dust concentration, calculated using this mechanism, gives reasonable agreement with the experimental values, as well as explaining the variation of optimum concentration with particle size and oxygen concentration.

Settling and rheology of suspensions of narrow‐sized coal particles

AbstractSettling rates, yield stresses, and shear‐stress/shear‐rate dependences were determined as functions of solids concentration for suspensions in water of coal particles of narrow size fractions. Particles and suspensions are characterized by coal analyses, determination of heating value, solid heat capacity and thermal conductivity, particle size analyses, and determination of densities and maximum packing concentrations for each particle size range. Analysis of the settling rate data establishes that the usually strong retardation of (hindered) settling due to concentration is compounded by particle shape effects. In addition, observed particle shapes are subsumed within particle shape classes inferred from the settling data. Shear‐stress/shear‐rate data for concentrated suspensions of the three larger and relatively narrower sized particles over the broad range of shear rates from about 1 to 104S−1 suggest the behavior to be essentially Newtonian, albeit with a marked particle concentration augmentation. Suspensions of the smallest coal particles of broader size distribution were strongly non‐Newtonian, possessing a yield stress at low shear, essentially power‐low shear‐thinning behavior over the intermediate range and approaching high‐shear Newtonian limiting behavior above about 103S−1. Relations developed include viscosity‐and yield stress‐concentration correlations incorporating the maximum packing volume fraction of solids. These are useful generally for suspensions and particularly as the basis for guiding the formulation of coal‐water fuels.

Batch and continous biooxidation kinetics of a refractory gold-bearing pyrite concentrate

Batch and continuous biooxidation studies have been carried out on a refractory, gold-bearing pyrite concentrate. The batch results were obtained using approximately the same surface area concentration for three different narrow size fractions. These showed an almost constant mass biooxidation rate for pyrite of 3,22 kg m −3 3 d −1 or an area rate of 1,53 × 10 −3 kg m −2 2 d −1 . The logistic equation was found to fit the batch data well and gave values of km ranging from 1,259 to 1,277 d −1 with values of Xm from 0,597 to 0,942 for the largest and smallest size refractions respectively. For the single stage continuous bioreactor the logistic equation also gave a good fit of the data with mean values for km of 0,305 d −1 and a mean X m of 0,762. These are close to those obtained for the batch biooxidation data. Biooxidation of the pyrite proceeds with the development of pores which were observed both on polished sections and scanning electron micrographs. This biooxidation and pore formation appears to be selective in the gold-rich regions with the consequence that only 50% sulphide oxidation is required for 90% gold liberation. The concentrate contained traces of arsenic which appears to be closely associated with the gold.

Some considerations of coherency in topoenergetic terms

The spatial coherency of composite structures is discussed in topoenergetic terms by considering mixing experiments on water and aqueous solutions with HRMC. Two series of experiments are presented: on normal and shaken successively diluted aqueous solutions of Na3PO4· 12H2O with methanol as structure developer, and on the solubility behaviour in water of a narrow size fraction of KCl crystals irradiated at 546 nm for different time multiples of 5 seconds. The amplitude of relative standard deviation of the integral mixing energy as a function of mixing time shows the contribution of different elementary processes by revealing the specific spectrum of the composite structure in water and aqueous solutions.

Batch biooxidation of a gold-bearing pyrite-arsenopyrite concentrate

Batch biooxidation data were obtained for a gold-bearing arsenopyrite-pyrite concentrate at 12% solids, pH of 1.6 and temperature of 40°C. The specific rates of biooxidation of arsenopyrite and pyrite were found to be very similar at 0,15 d −1 or as mass rates, for arsenopyrite 2,1 kg m −3 d −1 and pyrite 6,5 kg m −3 d −1. However, it was found that the biooxidation of arsenopyrite preceded that of pyrite and was almost complete before the oxidation of pyrite commenced. Data were also obtained for four narrow size fractions prepared from the bulk concentrate. Sequential oxidation of arsenopyrite and pyrite was observed for all size fractions. The oxidation rate obtained from the linear portion of the curve was found to have a linear dependence on surface area concentration suggesting that area-based rates are more appropriate for characterizing biooxidation kinetics. The logistic equation was found to be a good fit to all the data obtained. It was found that gold liberation was linearly dependent upon the extent of arsenic oxidation. However, sulphide oxidation appeared to be preferential in the gold-rich regions, leading to a non-linear dependence of gold liberation on extent of sulphide oxidation.

Combustion Synthesis of the YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-x</sub> Superconductor

Abstract Synthesis of the YBa 2 Cu 3 O 7− x (Y123) superconductor was achieved using two different modes of combustion synthesis, viz. self-propagating high-temperature synthesis and thermal explosion. The effects of several processing variables, including the green density, the oxygen pressure, and the Cu and BaO 2 particle sizes, on the formation of Y123 were investigated. The use of Cu particles in relatively narrow size fractions, ranging from

Combustion synthesis of the YBa 2Cu 3O 7− x superconductor

Synthesis of the YBa 2 Cu 3 O 7− x (Y123) superconductor was achieved using two different modes of combustion synthesis, viz. self-propagating high-temperature synthesis and thermal explosion. The effects of several processing variables, including the green density, the oxygen pressure, and the Cu and BaO 2 particle sizes, on the formation of Y123 were investigated. The use of Cu particles in relatively narrow size fractions, ranging from <10 μm to 44 μm, along with quenching of the reaction during its progress, provided insight into the mechanism of the synthesis process.

Rheological and fluidization behaviour of powders of different particle size distribution

While the importance of fines in determining the rheological and fluidization behaviour of powders has been recognized by many research workers, there has been little previous work aimed at understanding and characterizing the influence of fines content. In this project two particulate materials, spent FCC and glass, were examined. For each material, the behaviour of three size distributions of essentially identical surface-volume mean diameter (53 and 73 μm) but different fines contents and size spectra was studied. In addition, two narrow-sized fractions with smaller and larger average size were investigated for comparison. Characterization to study the role of fines included shear, cohesion, fluidization and collapse tests. These tests were performed under carefully controlled conditions to minimize possible disturbances from surface effects due to moisture and attrition, and from changes in size distribution by elutriation and comminution. The flow and fluidization properties of powders were found to be influenced by the particle size distribution. Collapse test results are affected by the bed height chosen. The expansion capability of bubble-free fluidized beds correlate reasonably with the interparticle adhesion force found from shear tests. The common assumption that minimum bubbling occurs by separation of particle contacts following complete balancing of interparticle adhesion forces by the overall force of hydrodynamic drag is questionable.

Effect of drug loading and molecular weight of cellulose acetate propionate on the release characteristics of theophylline microspheres.

Microspheres with 40, 50, and 60% drug loading of anhydrous theophylline core material were prepared by the emulsion-solvent evaporation method. Three different molecular weights of cellulose acetate propionate were used as encapsulating polymers. The geometric mean diameter of the microspheres increased with drug loading for all polymers. Dissolution rate for a given particle size fraction also increased with drug loading for all polymers. Higuchi/Baker-Lonsdale spherical matrix dissolution kinetics were followed by narrow particle size fractions of the microspheres. A linear relationship between the T-50% (time required for 50% of the drug to be released) and the square of microsphere diameter was observed with all three molecular weights of the encapsulants. The slowest drug release was obtained with the high molecular weight polymer, which also produced the smoothest microspheres.

Mechanical properties of silicon nitride-SiC platelet composites

Abstract Silicon nitride and SiC platelet-silicon nitride composites have been prepared by hot-pressing and by pressureless sintering. As-received, large SiC platelets behave as critical flaws and were separated into two narrow size fractions for the production of composites. The mechanical behaviour of the composites has been assessed and results are discussed taking into account the raw materials' properties, the microstructure of the dense sintered ceramics and fracture mechanisms.

ON THE PREDICTION OF EXTRUDATE PORE STRUCTURE FROM PARTICLE SIZE

Measurements have been made of the amount of water required to saturate a range of powders made by blending, in different ratios, three narrow-sized fractions of α-alumina. From these results the porosities of the powders have been calculated. Some powder mixtures were formed into pastes with water and various aids to extrusion; the porosities and densities of dried extruded blends of the same three fractions have been measured by mercury injection. Similar measurements have been made on the same extrudates following firing. A simple liquid saturation test provided a guide to the determination of porosity in the whole system. Particle size distribution results on the original powder blends have been related to the pore size distribution of the dried and fired extrudates. In particular, a detailed comparison between the porosity predicted by the method of Ouchiyama and Tanaka from only the porosities of the three primary fractions and the particle size distributions of the blends, and the porosities calcul...

Factors affecting the strength of formed coke made from lignite char

Char made from narrow size fractions of lignite in a fluidized bed was briquetted at 140 MPa with various binder tars produced from the lignite. The raw briquettes were cured in air at 200 °C and then carbonized, usually at 925 °C. The tensile strength of the resulting formed coke could be as much as three times that of the best metallurgical coke, or as low as 2–3 MPa, depending on the type of binder used, or even a factor of ten lower than this if the curing stage was omitted. Curing is thus a crucial stage. Distinct optima of binder percentage and char particle size were found, giving maximum briquette strengths (≈ 13 MPa). A corresponding ‘strength surface’ suggested four factors associated with weakness of a briquette. Binders produced by air-blowing rather than nitrogen-blowing gave stronger briquettes. Fluidized bed tar was greatly ‘improved’ in this way, but Gray-King tar only slightly so, probably because the former contained more generally reactive species. Cured-only briquettes were usually surprisingly strong and carbonization increased this strength ≈ 1.5 times. Curing is considered to ‘fix’ the binder; carbonization changes the chemical nature of the ‘cement’ between the char surfaces but does not appreciably alter its spatial distribution. The carbonized briquette thus has substantially the same structural flaws as were present in the cured state and so has a strength closely correlated with that achieved merely by curing.

Dispersed solid dynamics in a slurry bubble column

New data and theory are presented for describing dispersed solids in slurry bubble columns. Axial solids concentration distributions were measured in a 0.108 m.i.d. slurry bubble column apparatus operated at steady-state conditions. Slurry and gas superficial velocities ranged from 0.0 to 0.02 m/s and 0.03 to 0.20 m/s, respectively. The liquid-phase was either water or ethanol, and the solid-phase consisted of narrow-sized fractions of glass spheres. With a one-dimensional sedimentation-dispersion model, the data have been used to develop a method for predicting average solids loading and axial distribution of solids in a bubble column. Correlations are given for the hindred settling velocity, the solids dispersion coefficient, and the solids concentration at the top and bottom of the column. A solids distribution function is derived using one parameter that contains the hindred settling velocity and the solids dispersion coefficient. The new method is applied to data from the literature for three slurry bubble columns with diameters in the range of 0.066–0.286 m. The new method predicts axial distribution of solids with improved accuracy over existing methods, especially for non-aqueous systems and for those with low slurry feed concentration or high settling velocity.