Shape, mineral particles

Consider a cube-shaped mineral particle (sides of length L= 100 pm) placed on the surface of a container of water of surface tension y = TL mN/m. 

The characteristics which determine the properties filler that will impart to a composite are particle shape, particle size, surface area, and particle-matrix compatibility (Fig. 1). Particle-matrix compatibility relates to the ability of the polymer to coat and adhere to the filler. The shape of most mineral filler particles can be a sphere, cube, block, plate, needle, or fiber whereas some filler also contain a mixture of shapes. Mineral particles resembling plates, needles, and fibers are further characterized by their aspect ratio (http //www.rtvanderbilt.com/ fillersintroweb.pdf). In rubber/polymer composites, applied stress is transferred from the rubber/polymer matrix to the strong and stiff mineral. It seems reasonable that this stress transfer will be better affected if the mineral particles are smaller, because greater surface is thereby exposed for a given mineral concentration. Moreover, if these particles have a high aspect ratio (are needle-like, fibrous or platy in shape), they will better intercept the stress propagation through the matrix (Fig. 2) (http //www.rtvanderbilt.com/fillersintroweb.pdf). 

Figure. 5.100. A glass fiber reinforced polyamide with elastomer was cryoultrathin sectioned with an older diamond knife, due to the presence of the glass fibers, and images of the sections unstained (A) and stained with RUO4 vapor (B) showed the dark dispersed phase of the elastomer in the latter and the irregularly shaped mineral particles in the former. The broken glass fibers are the larger structures in the image. At higher magnification (C), the dark dispersed phase is more clearly resolved as fairly round particles about 0.2/tm in diameter. (From Wood [186] used with permission of the American Chemical Society Rubber Division.)...

Some of the common terms used for defining or describing particle shapes in a qualitative way are presented in Table 2.1. It is by now quite clear that particle shape cannot be very precisely defined. However, it is fortunate that mineral particles occur in a variety of generally simple shapes some are acicular, several are plate-like, most are convex, others are mildly concave, and in this manner a phraseological descriptive list is built for the different shapes that are formed or produced or generally encountered of powders of fragments of... 

Root exudation and microbial action produce organic compounds with a range of composition and molecular weights. These compounds interact with the mineral particles, which also vary in size, shape, ciystallinity, and electric charge (Emerson et al. 1986). Interactions between soil mineral particles, organic matter and microbes can occur at many different size scales, because these materials have a large size range in soils (Fig. 7). 

The above processes involve separation based either on bulk properties (for example, size, density, shape, etc.) directly or by subtle control of the chemistry of the narrow interfacial region between the mineral particle and the aqueous solution in which it is suspended. In the processing of certain ores, such as those of uranium, gold or oxidized copper, chemical alteration of the minerals may be required to recover the valuable metals. These techniques are not discussed here, except to include those aspects which are directly related to surfaces and interfaces. 

Colloid behavior in natural soil-water systems is controlled by dispersion-flocculation processes, which are multifaceted phenomena. They include surface electrical potential (El-Swaify, 1976 Stumm and Morgan, 1981), solution composition (Quirk and Schofield, 1955 Arora and Coleman, 1979 Oster et al., 1980), shape of particles, initial particle concentration in suspension (Oster et al., 1980), and type and relative proportion of clay minerals (Arora and Coleman, 1979). When suspended in water, soil colloids are classified according to their settling characteristics into settleable and nonsettleable solids. 

Chang, F.-R.C, and Sposito, G., The electrical double layer of a disk-shaped clay mineral particle Effect of particle size, J. Colloid Interface Sci., 163, 19, 1994. 

Production of organic acids by EMF has also been suggested to result in penetration of mineral particles by EM hyphae (Jongmans et al., 1997), although recent estimates have found that tunnel-shaped structures within mineral grains contribute very little (less than 1 %) of the total weathering in the soil (Smits et al., 2005 see Smits, Chapter 13 this volume). 

As it can be seen from the Table 3 that for the saturation region, S=0%, the shear strength is independent of the shape factor, namely independent of the type of the mineral. In contrast to that, as the saturation degree increases, the shear strength is absolutely dependent on the shape factor of the mineral particles. The reason is probably the change in the adhesion forces and liquid bridges between the particles with different shape factors in the more saturated regions. 

Fig. 4. SE images of organic polymer binders (a, b) in the primaiy form as dispersion and (c, d) coalesced in silicone resin coatings, a, b Typical particle shape of the primaiy latex particles after rapid diying in a vacuum, c, d Plaster/paint samples etched with 10% HNO3. c Plan view of a coalesced polymer binder of a silicone resin plaster and d cross-section of of a polymer film partly covering a clay mineral particle within a silicone resin emulsion paint.

AI2O3 or coarse a-Al203 + H2O) as a function of surface area for boehmite (circles), yAl203 (triangles), and corundum (squares). Surface enthalpy is equal to the slope of each line, shown for easier comparison in the inset. Data for anhydrous oxides from McHale et al. (1997a). (b) Surface enthalpy as a function of particle size for corundum and boehmite, assuming spherical shape of particles. [Used by permission of the editor of Clays and Clay Minerals, from Majzlan et al. (2000), Fig. 1, p. 702.]... 

The best combination of properties, especially Impact strength, was obtained with very small mineral particles in the range of one to two microns. The acicular shaped Wollastonlte particles contributed greater stiffness to a product, sometimes with a sacrifice of impact strength. 

In most cases, the shape of mineral particles could be considered more or less spherical, especially for small particle size, while the waste particles are of variant shape (spherical, wiry, platy etc). This shape variance in some cases hinders their effective separation, but in others it has a beneficial effect. 

A characteristic feature of flame heated ash is that the particles are spherical in shape as shown in Figure 2. The transformation of the angular silicate mineral particles in pulverized coal to spherical particle ash is a result of the surface tension force acting on the vitrified species. The stress (f) on a non-spherical surface section of the particle is ... 

Table II shows that the small irregularly shaped particles transform to spheres in coal flame when the viscosity of the material is several orders higher than that required for bulk flow under gravity, which is about 25 N s m". A laboratory technique was used to determine the minimum temperature at which coal mineral species are transformed to spherical shapes (17). Particles of 10 to 200 pm in diameter were introduced into a gas stream and then passed through a vertical furnace. The temperature of the furnace was varied from 1175 to 2025 K and was measured by a radiation pyrometer and by thermocouples placed in the furnace. The residence time of particles in the furnace was between 0.2 and 0.5 sec. depending on the particle size.

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