Mechanical properties of moist agglomerates in relation to granulation mechanisms part II. Effects of particle size distribution

Stress—strain relationships of moist samples of dicalcium phosphate qualities with different particle size distributions are determined and compared with the granule growth pattern in a high-speed mixer. It is shown that granule growth by coalescence proceeds when a limiting agglomerate strength is achieved due to densification of the moist agglomerates. In the region of growth by coalescence, the growth rate is improved by an improved deformability of the moist agglomerates, while increasing strength delays the growth rate. The effects of agglomerate strength and plasticity are described mathematically by a model which outlines the main effects of the stress—strain relationship upon granule growth by coalescence.     

Some surface energy effects in the strength of moist polymer agglomerates

The crushing strengths of agglomerates made by tumbling cellulose acetate and cellulose acetate butyrate powders wetted by water-methanol solutions of various concentrations were measured. After normalization for the effects of variable pore saturation and porosity, agglomerate strength factors exhibited minima as methanol concentration was increased. These minima also corresponded approximately to maximum pore saturation by the wetting liquid. It is proposed that these experimental maxima and minima correspond to a condition under which surface tensions of the liquid and solid phases are just matched, so that maximum capillary suction and zero van der Waals attraction exist in the wetted particle systems. It is concluded that reliable solids surface tension data can thus be generated from compressive strength measurements on agglomerates of low energy particulate solids.     

Correlation of moist strength with contact angles for polymer powder agglomerates

The results of an experimental study of the effect of liquid-solid contact angle on the crushing strength of wet agglomerates are presented. Agglomerates formed from model systems of polymer particles were used so that contact angles measured on film-cast and/or heat-pressed surfaces derived from the powders could be relatively easily obtained. It is shown that, for the polymer systems studied, the effect of contact angles on the crushing strength of moist agglomerates is well correlated by the available capillary theories. The relationship between crushing strength and tensile strength is a part of this theoretical prediction which is also considered here.     

Hardness of moist agglomerates in relation to interparticle friction, capillary and viscous forces

Wet agglomerates deform plastically until they break through crack propagation. On the particulate level, liquid bridges are responsible for the strength of the wet agglomerate as they hold the particles together. Recent micro-scale studies have identified the role of liquid surface tension, bridge Laplace pressure and liquid viscosity, which, in combination, explain the axial strength of pendular liquid bridges. Different situations exist depending on the degree the liquid wets the particles and on the saturation of the agglomerate mass.On the wet agglomerate level, the hardness is related to three factors: the liquid binder surface tension and viscosity and the interparticle friction. A simple model is developed in this paper, based on the powder and liquid binder properties, which shows that the forces due to interparticle friction are generally predominant in wet agglomerates made from non-spherical particles. Although mechanical interlocking is not accounted for, the model yields accurate prediction of wet agglomerate hardness independently measured on wet masses of varying composition. This theoretical hardness could prove an interesting tool for wet granulation research and technology.     

The strength of agglomerates bound by salt bridges

The correlation between the drying behavior and the tensile strength of agglomerates bound by salt bridges in the dry state was investigated. The pellets examined had been prepared by compacting limestone powders, which were premoistened with a salt solution, in a manually operated die press. The percentage liquid saturation of the moist agglomerate, the concentration of the salt solution and the drying temperature were altered. The influence of the surface incrustation on the drying process could be examined by measuring the temperatures within the cylindrical pellet at different radial positions during drying and by determining the drying rate curves. The tensile strength of the dried pellets, which was usually measured after removing the salt crust, depends on the quantity and the local distribution of the crystallized salt, and the strength of the salt bridges. The latter depends on the crystal structure which is influenced by the drying rate.     

Effect of process parameters on the crush strength of granular fertiliser

The granulation characteristics of nitro-phosphate-based NPK fertiliser have been investigated in a bench scale drum granulation unit. It was found that the granulation of the fertiliser took place in the growth regime. Crush strength analysis of the final granulate was undertaken and was correlated with particle size and fractional saturation. The experimental data and analysis in this study indicate that that the rounded, regular-shaped granules produced by granulating with higher liquid phase ratio and fractional saturation result in the granules having stronger bonds between sub-granules and lower porosity.     

The effect of wetting liquid droplet size on the growth of agglomerates during wet drum granulation

Results of investigation of the effect of wetting liquid droplet size on the growth of agglomerates during wet drum granulation of dolomite flour of selected grain-size composition which guaranteed geometrical similarity were discussed.

The process of granulation was carried out batch-wise in a drum granulator 0.5 m in diameter and 0.4 m long at rotational speed 0.33 s⁻¹ and constant volumetric drum filling degree =0.1. On the bed tumbling in the drum, the wetting liquid (distilled water) was supplied at a constant flow rate Q_w=12×10⁻³ m³/h. The size of wetting droplets was changed using various air flow rates through pneumatic spray nozzles in the range Q_a=1.0 to 3.0 m³/h and applying a sprinkler which supplied (drop-wise) the liquid uniformly along the entire drum length. In the whole experimental cycle, constant mean saturation degree of the feed equal to S=0.293 was used. Relationships determining the effect of wetting droplet size and particle size distribution of the raw material on the rate of agglomerate growth during drum granulation were developed.     

Granulation in high-speed mixers Part V. Power consumption and temperature changes during granulation

Granulation in a Fielder^® PMAT 25 VG laboratory-scale mixer is studied by recording power consumption and temperature increase during addition of the binder liquid. It is shown that the consumed energy is converted completely into heat in the moist mass. It is suggested that the heat development which is caused by friction between particle surfaces during particle rearrangements and compaction controls the recorded increase of the power consumption. It is further shown that the time—power consumption curve is closely related to the granule growth process. This is interpreted as an effect of an improved ability of the moist agglomerates to be strained as the liquid saturation increases. Heat development and the corresponding power consumption are caused by straining the moist agglomerates during agitation. Since granule growth by coalescence is improved by an improved ability to be strained, there must exist a close relationship between heat development and thus power consumption and granule growth.     

流化床喷雾造粒技术进展

本文介绍了流化床喷雾造粒技术,包括造粒机理、操作方式、影响因素等。流化床喷雾造粒是将溶液、悬浮液或熔融液喷雾到已经干燥或不完全干燥颗粒的流化床床层内,在同一设备内一步完成蒸发、结晶、干燥或化学反应的造粒过程。流化床造粒操作对颗粒的尺寸、形状要求不高,所得到的产品无结块、具有良好的流动性能。与其他传统的造粒方法相比,流化床造粒具有工艺流程简单、设备装置紧凑、投资省、生产强度大等优点,越来越引起人们的关注。     

Granulation in high-speed mixers Part VI. Effects of process conditions on power consumption and granule growth

Effects of impeller rotation speed, liquid flow rate and liquid amount upon power consumption and granule growth are investigated by granulation in a Fielder^® PMAT 25 VG high-speed mixer. The records of power consumption demonstrate effects of the process conditions which are coincident with the effects on the granule growth. This is attributed to the effect of the liquid saturation on the mechanical properties of the moist agglomerates. Granulation end-point control by power consumption meters requires the correlation between granule growth and power consumption being established for the particular process conditions. It is suggested that the differentiated power consumption signal be recorded in order to detect the characteristic phases of the granulation process. With regard to the control of the amount of binder solution required for an uncritical process, the differentiated signal can be useful.     

Mechanical strength and damage tolerance of highly porous alumina ceramics produced from sintered particle stabilized foams

Highly porous alumina particle stabilized foams were prepared by combining the concepts of particle stabilized foams and gelcasting, using sulfonate surfactants and poly vinyl alcohol (PVA) as the gelcasting polymer. The ceramic samples sintered at 1500 °C for 2 h had porosities from 65% to 93%, with pore sizes in two categories: “big pore” around 300 μm and “small pore”, around 100–150 μm, depending on the type and amount of surfactant added. The mechanical behaviour of the foams (axial and diametral compression) depended on the overall porosity and pore size. On average, tensile and compressive strengths around 5 and 16 MPa respectively were measured for samples with bigger pore sizes and larger porosities. Samples with smaller pore sizes and lower porosities produced average values of 12 and 57 MPa for tensile and compressive strengths, respectively. The elastic modulus reached a maximum around 3GPa for “small pore” size samples. The effect of increasing amount of PVA in the samples had a strong effect on the green mechanical strength, but it did not significantly affect the mechanical response of the sintered alumina foams. Large and complex shape sintered components produced using this route showed a remarkable damage tolerance, due to crack tip blunting.     

Correlation of aqueous-physical, structural-mechanical, and thermophysical properties of moist disperse materials

It is shown that the cohesiveness and plastic strength of moist materials and their thermophysical and drying properties are determined by the ratio of capillary-mobile and capillary-immobile moisture forms and natural properties of materials which can be best estimated based on the least capillary moisture or the maximum molecular moisture capacity which are the most significant aqueous-physical parameters of materials. Formulas are proposed for the calculation of the dependence of cohesiveness and plastic strength of materials on their moisture content and drying sensitivity, as well as formulas for the calculation of thermophysical properties of moist materials. Translated from Steklo i Keramika, No. 5, pp. 17–21, May, 2000.     

Wet granule breakage in a breakage only high-hear mixer: Effect of formulation properties on breakage behaviour

Wet granule breakage can occur in the granulation process, particularly in granulators with high agitation forces, such as high-shear mixers. In this paper, the granule breakage is studied in a breakage only high-shear mixer. Granule pellets made from different formulations with precisely controlled porosity and binder saturation were placed in a high-shear mixer in which the bulk medium is a non-granulating cohesive sand mixture. After subjecting the pellets to different mixing time in the granulator, the numbers of whole pellets without breakage are counted and taken as a measure of granule breakage. The experimental results showed that binder saturation, binder viscosity and surface tension as well as the primary powder size have significant influence on granule breakage behaviour. It is postulated that granule breakage is closely related to the granule yield strength, which can be calculated from a simple equation which includes both the capillary and viscous force of the liquid bridges in the granule. The Stokes deformation number calculated from the impact velocity and the granule dynamic strength gives a good prediction of whether the granule of certain formulation will break or not. The model is completely based on the physical properties of the formulations such as binder viscosity, surface tension, binder saturation, granule porosity and particle size as well as particle shape.     

Tensile strength of agglomerates

Firstly, this paper gives a review of tensile strength of agglomerates; i.e. a discussion of the most important theoretical considerations, the various methods of measurement, a comparison of measuring techniques by experimental results and a brief look at deformation behaviour under unidirectional loading. In the second part new developments are dealt with.For the tensile strength of agglomerates in which forces are transmitted at points of contact, a new relationship is shown in which the displacements due to individual forces are taken into consideration. Experiments in which not only the tensile strength but also the force distribution are measured demonstrate the necessity of the new approach when short-range forces are present. However, the force—displacement characteristics of the attractive forces need not to be taken into account when long-range forces — e.g forces due to suitability large liquid bridges — act. The attractive forces may then be linearly superposed.The measurement of the tensile strength of agglomerates is discussed in detail. In particular the problematic nature of measurements using the split plate apparatus is pointed out, and a new method of interpreting results is suggested. The applicability of this method is tested by comparable experiments.     

Deformation behaviour of agglomerates under tensile stress

A new instrument has been developed in which the stress—strain behaviour of agglomerates can be measured in uniaxial tension. The instrument permits tension to be measured under predetermined stresses, and further enables the tensile stress—strain behaviour to be determined in the agglomerate in the unstable region.Agglomerates with different bonding mechanisms were studied. In the case where agglomerates are held together by liquid bridges, a model has been developed which permits the stress—strain behaviour to be predicted. This is confirmed by experiments. For agglomerates with a high degree of liquid saturation, the tensile stress—strain behaviour can be predicted from the capillary pressure relation. The tensile stress—strain curves of dry agglomerates and of agglomerates with solid bridges between particles are shown and compared. From the different stress—strain characteristics, the physical nature of the bonds can be elucidated.     

Effect of lubrication on the distribution of force between spherical agglomerates during compression

We employ the carbon paper technique to aid the understanding of in die force and spatial distributions, upon compression of approximately 1 mm sized spherical agglomerates (pellets) of microcrystalline cellulose (MCC). The aim in this study was to test for the effect of lubricant film on force and spatial distributions. Pellets of MCC were formed via granulation and extrusion/spheronisation. Investigation of pellet bed compression was performed on a materials tester. Prior to compression studies the pellets were characterised for bulk density, size and deformability. Two pellet types were investigated; MCC and MCC lubricated with magnesium stearate. The carbon paper technique relies upon carbon paper as the medium for transferring imprints from compressed pellets onto photo quality paper. The digitised images of these imprints form the basis of analysis through the use of image processing software. Using the carbon paper technique within the range of 10-30 MPa indicates that lubrication does not have a significant effect on the distribution of forces between spherical agglomerates during uniaxial compression. Spatial analysis of the imprints revealed that the lubricated pellets exhibited a higher packing order than the unlubricated ones at low applied pressures (10 and 20 MPa), a difference that could not be observed at 30 MPa. Hence interparticle friction and/or cohesion appear to influence the initial particle rearrangement, whereas confinement is suggested to dominate at higher pressures.     

喷涂聚脲弹性体与潮湿混凝土基材粘接性能研究

针对水利水电混凝土工程常处于潮湿状态的特点．介绍了应用于潮湿混凝土基材的喷涂聚脲弹性体涂层的粘接性能,试验结果表明,SK型表面处理剂与潮湿混凝土基材粘接强度能达到5．0MPa．施用表面处理剂后再喷涂聚脲弹性体涂层粘接强度能达到2．5MPa。     

A theoretical model of the effect of distributed loading on the tensile strength of agglomerates as measured in the diametral compression test

The tensile strength of particle agglomerates is analyzed to indicate the effect of distributed loading through contact flattening during the diametral compression test. It is assumed that only the contact regions of the agglomerate are flattened and that the free boundary maintains its original position during loading. The increased packing density so produced is related to the total loading as a reaction force through an empirical relationship used to describe die compaction of powders. Agglomerate failure occurs when the maximum tensile stress caused by the platen loading exceeds the cohesive strength of the particle assemblage. Theoretical predictions of the effects of parameters such as bulk powder properties and the extent of load distribution on agglomerate strength are presented from the analysis.     

The Athy—Heckel equation applied to granular agglomerates of basic tricalcium phosphate [3Ca3PO4)2·Ca(OH)2]

It is shown that direct compression grade tricalcium phosphate yields linear Heckel plots only if the particle density used is that obtained by liquid pycnometry or mercury porosimetry. If the true density is used (i.e. that obtained by gas displacement), then linearization is not obtained in a pressure range where bonding evidently occurs. It is shown that the pore volume in the range of diameters from 0.5 to 2 μm decreases as tableting pressure increases, and that bonding is associated with the loss of pores of this size.     

钢渣制品的最佳碳化制度

通过对钢渣制品进行碳化养护,确定其最佳的碳化制度.实验结果表明:当液态外加剂掺入量为9.00%,初始成型压力f为3.0 MPa时,钢渣制品在CO2分压为fco2=0.25 MPa、fco2=0.55 MPa和fco2=0.75 MPa的压强下各碳化养护1 h后,其碳化增重率迭11.02%,在最佳的碳化制度下,相对于未碳...