Primary particle properties

Zheng, W. Influence of primary particle properties on fluidization quality, M.S. thesis, Institute of Chemical Metallurgy (1987). 

The characterization of primary particle properties in a particulate system and their correlation with the secondary bulk properties of the system is a problem common to all branches of particle technology. As pointed out above, it is still necessary in powder handling to measure the secondary properties directly and it can be argued, therefore, that particle characterization can be... 

The above listed characteristics depend on many primary particle properties and operation conditions, such as the nature of the material (brittle or soft), particle size, kind of loading in the mill, intensity and velocity of loading, the shape and hardness of the loading surfaces and the environmental conditions, namely temperature. 

Particle characterization, i.e. the description of the primary properties of particles in a particulate system, underlies all work in particle technology. Primary particle properties such as the particle size distribution, particle shape, density, surface properties and others, together with the primary properties of the liquid (viscosity and density) and also with the concentration and the state of dispersion, govern the other, secondary properties such as the settling velocities of the particles, the permeability of a bed or the specific resistance of a filter cake. Knowledge of these properties is vital in the design and operation of equipment for solid-liquid separation. 

Before any crystallization process is designed, the goals of the process and desired particle properties have to be defined. Table 10.1 lists some of these properties. One can distinguish goals that are directly influenced by the crystallization process such as yield, particle size, and/or particle size distribution (PSD) and properties that can be derived from these properties such as the bulk density. It is not always easy to predict the influence of primary particle properties on the derived ones flowability, for example, is a very complex property that can depend on many primary particle properties such as shape, particle size, and particle size distribution or roughness. Some of these parameters can be influenced by the crystallization process, for example, particle size and particle size distribution, some are not that easy to control, for example, amorphous content and roughness, and others are more or less intrinsic and cannot be influenced by the process, for example, hardness and plasticity of the crystals. 

In discussions of the surface properties of solids having a large specific surface, it is convenient to distinguish between the external and the internal surface. The walls of pores such as those denoted by heavy lines in Fig. 1.8 and 1.11 clearly comprise an internal surface and equally obviously the surface indicated by lightly drawn lines is external in nature. In many cases, however, the distinction is not so clear, for the surfaces of the primary particles themselves suffer from imperfections in the forms of cracks and fissures those that penetrate deeply into the interior will contribute to the internal surface, whereas the superficial cracks and indentations will make up part of the external surface. The line of demarcation between the two kinds of surface necessarily has to be drawn in an arbitrary way, but the external surface may perhaps be taken to include all the prominences and all of those cracks which are wider than they are deep.,The internal surface will... 

The carbon black in semiconductive shields is composed of complex aggregates (clusters) that are grape-like stmctures of very small primary particles in the 10 to 70 nanometer size range (see Carbon, carbon black). The optimum concentration of carbon black is a compromise between conductivity and processibiUty and can vary from about 30 to 60 parts per hundred of polymer (phr) depending on the black. If the black concentration is higher than 60 phr for most blacks, the compound is no longer easily extmded into a thin continuous layer on the cable and its physical properties are sacrificed. Ionic contaminants in carbon black may produce tree channels in the insulation close to the conductor shield. 

In sintering, the green compact is placed on a wide-mesh belt and slowly moves through a controlled atmosphere furnace (Fig. 3). The parts are heated to below the melting point of the base metal, held at the sintering temperature, and cooled. Basically a solid-state process, sintering transforms mechanical bonds, ie, contact points, between the powder particles in the compact into metallurgical bonds which provide the primary functional properties of the part. 

In the early days of the commercial development of PVC, emulsion polymers were preferred for general purpose applications. This was because these materials exist in the form of the fine primary particles of diameter of the order of 0.1-1.0 p,m, which in the case of some commercial grades aggregate into hollow secondary particles or cenospheres with diameters of 30-100 p,m. These emulsion polymer particles have a high surface/volume ratio and fluxing and gelation with plasticisers is rapid. The use of such polymers was, however, restricted because of the presence of large quantities of soaps and other additives necessary to emulsion polymerisation which adversely affect clarity and electrical insulation properties. 

Carbon blacks are synthetic materials which essentially contain carbon as the main element. The structure of carbon black is similar to graphite (hexagonal rings of carbon forming large sheets), but its structure is tridimensional and less ordered. The layers of carbon blacks are parallel to each other but not arranged in order, usually forming concentric inner layers (turbostratic structure). Some typical properties are density 1.7-1.9 g/cm pH of water suspension 2-8 primary particle size 14-250 nm oil absorption 50-300 g/100 g specific surface area 7-560 m /g. 

Primary particles characteristics Packing characteristics Bulk properties... 

The most challenging part of rubber mixing is the dispersion of the filler The filler agglomerates have to be broken into smaller particles, the aggregates, but not completely to the level of primary particles. An optimal particle size distribution has to be achieved in order to obtain the best properties of the final rubber product [14]. 

Most particles of the dispersion phase occur in a wide distribution of sizes consisting of aggregates of the primary particles. To ensure maximum stability, these aggregates must be reduced to an acceptable minimum size. When particles larger than the accepted minimum size are present in a dispersion, the physical properties of the dispersion are influenced by the size of the larger aggregates. 

Catalytic activity and electrochemical performance generally increase as the NiO and YSZ particle sizes are reduced. However, ultrafine powders are prone to agglomeration during the milling and mixing process the distributions of the phases (and hence the percolation threshold and many other important properties) are determined by the agglomeration size, not by the primary particle size. 

Grade efficiency data are usually derived from experimental trials which provide sufficient information to allow the material balance to be closed for particles of all sizes. Sufficient information for determination of G(d) is provided by a combination of any three of the following four system properties E, Ff(d), Fu(d), F0(d), the remaining property being determined by the material balance. Size distribution data for primary particles, rather than floes or aggregates, are required for the inventory. 

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