Coarse particle

The method is used for determination of the amount of coarse particles in a particulate material or their dispersion. The particles are considered coarse if they do not pass through a 45 im sieve. The process of sieving is conducted with wet material and it is aided by water flushing and brushing. The material retained on the sieve is determined gravimetrically after drying. 

The Part 1 of ISO 787 gives a color comparison method for pigments and extenders. The specimen and the standard pigment are dispersed in a specific binder under controlled conditions. The resultant pastes of pigments are spread on a substrate and visually compared. 

The Part 25 of ISO 787 specifies a colorimetric method of comparison. The similar method of dispersion is used but a more precise definition of binder is given. In addition, fumed silica is used as an ingredient in the dispersion. The results of testing give relative hue and lightness differences for a broad range of materials from white to black. 

The ASTM standard specifies details of method which is in principle a method of using color computer to determine CIE tristimulus values and other parameters of color which can be calculated. 

Each method discussed in this section has different precision of determination and results are not comparable. In evaluation of this data it is essential to take note of the method used. 

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The StabUity or persistence of a poUutant in the atmosphere depends on the poUutant s atmospheric residence time. Mean residence times and principal atmospheric sinks for a variety of species are given in Table 2. Species like SO2, (NO and NO2), and coarse particles have lifetimes less than... 

For smaller particles, the theory indicates that efficiency decreases according to the dotted line of Figure 7. Experimental data (134) (sofld line of Eig. 7) for a cyclone of Eig. 9 dimensions show that equation 15 tends to overstate collection efficiency for moderately coarse particles and understate efficiency for the finer fraction. The concept of particle cut-size, defined as the size of particle collected with 50% mass efficiency, determined by equation 16 has been proposed (134). 

Fig. 4. Geldart group particle classification diagram for air at ambient conditions (6). Group A consists of fine particles B, coarse particles C, cohesive,...

Transport Disengaging Height. When the drag and buoyancy forces exerted by the gas on a particle exceed the gravitational and interparticle forces at the surface of the bed, particles ate thrown into the freeboard. The ejected particles can be coarser and more numerous than the saturation carrying capacity of the gas, and some coarse particles and clusters of fines particles fall back into the bed. Some particles also coUect near the wall and fall back into the fluidized bed. 

For fine pulverization, both dry and wet processes are utilized, but increasingly the dry process is more popular because wet grinding ultimately requires drying and is much more energy intensive. A sensitive fan swirls the dust sizes into the air separator and permits coarse particles to recycle to the grinding mill or be rejected as tailings the fines are drawn into cyclones where the dust is collected. 

Early fluorescent pigments were promoted and adopted for use in screen inks for poster boards and paints for safety applications. These thermoset pigments were not well-suited because of their poor fightfastness. Also, because of their relatively coarse particle size, their use in thinner film applications, such as gravure or flexo, was limited. 

The characteristics of a powder that determine its apparent density are rather complex, but some general statements with respect to powder variables and their effect on the density of the loose powder can be made. (/) The smaller the particles, the greater the specific surface area of the powder. This increases the friction between the particles and lowers the apparent density but enhances the rate of sintering. (2) Powders having very irregular-shaped particles are usually characterized by a lower apparent density than more regular or spherical ones. This is shown in Table 4 for three different types of copper powders having identical particle size distribution but different particle shape. These data illustrate the decisive influence of particle shape on apparent density. (J) In any mixture of coarse and fine powder particles, an optimum mixture results in maximum apparent density. This optimum mixture is reached when the fine particles fill the voids between the coarse particles. 

Mechanical comminution may be used to form metal powders. Relatively coarse particles are produced by machining, whereas ball mills, impact mills, gyratory cmshers, and eddy mills give fine powders of britde materials. 

Fig. 14. Reinforcement and crack tip kiteractions ki a particulate composite (a) coarse particles ki a strong particle—matrix kiterface, and (b) fine particles ki...

Usually the rate, Q, is far in excess of the required rate, especially if the bulk material consists primarily of coarse particles. Slowing down the discharge rate requires a feeder. Fine powders, on the other hand, have considerably lower maximum discharge rates when exiting from a mass flow bin, because of the interaction between air (or gas) and solid particles as reflected in the permeabiUty of the material. 

On a chute, higher drag results in lower particle velocity which can be accentuated by stratification on the chute surface because of the sifting mechanism. Concentrations of smaller particles close to the chute surface and larger particles at the top of the bed of material, combined with the typically higher frictional drag of finer particles, often result in a concentration of fine particles close to the end of the chute, and coarse particles farther away. This can be particulady detrimental if portions of the pile go to different processing points, as is often the case with multiple outiet bins or bins with vertical partitions. 

Air Entrainment. Fine particles generally have a lower permeabiUty than coarse particles, and therefore tend to retain air longer in void spaces. Heavier particles settie more quickly in a duidized mixture than lighter particles. Thus, when a mixture of particles is charged into a bin, it is not uncommon to find a vertical segregation pattern, where the coarser, heavier particles concentrate at the bottom of the bed and the finer, lighter particles concentrate near the top. 

Particle Size Distribution. Almost every feed slurry is a mixture of fine and coarse particles. Performance depends on the frequency of distribution of particle size ia the feed. Figure 5 shows that whereas all of the coarse particles having a diameter greater than some are separated, fewer of the very fine particles are, at any given feed rate. The size distribution frequency of particles ia feed and centrate for a fine and coarse feed are quite different. More coarse particles separate out than fine ones. Classification of soHds by size is often done by centrifugal sedimentation. 

Fig. 5. Particle distribution (upper line) before and (lower line) after action of the separator where the cross-hatched areas represent the particles separated out. By definition, all particles of d > are separated out. A number of particles having d < d are also separated, (a) Fiae and (b) coarse particle...

Owing to the feed pan distance usually maintained on wet dmm cobbers, the wear encountered with coarser particles, and the feed velocities requited to move coarse particles, the recommended upper size limits for cobber separators is 10 mm in diameter. Individual ore characteristics requited to obtain Hberation determine the feed size in rougher and finisher feeds. For finishers, where all the nonmagnetics must be overflowed, a sufficiently fine size to accomplish the overflow must be obtained. Typical feed sizes for cobbers are from —841 fim (—20 mesh) to 10 mm for rougher, —420 fim (—35 mesh) to —297 fim (—48 mesh) and for finishers, —63 fim (—270 mesh) to —44 fim (—325 mesh). The magnetic content of the iton ores to be concentrated varies over fairly wide limits. Ores as low as 10 wt % Fe have been successfully treated, as have ores having up to 50 wt % or more Hon. 

The reaction of C2S with CaO to form C S depends on dissolution of the lime Hi the clinker Hquid. When sufficient Hquid is present, the rate of solution is controUed by the size of the CaO particles, which depends Hi turn on the sizes of the particles of ground limestone. Coarse particles of siHca or calcite fail to react completely under commercial burning conditions. The reaction is governed by the rate of solution (10) ... 

Glassification. Classification (2,12,26,28) or elutriation processes separate particles by the differences in how they settle in a Hquid or moving gas stream. Classification can be used to eliminate fine or coarse particles, or to produce a narrow particle size distribution powder. Classification by sedimentation iavolves particle settling in a Hquid for a predetermined time to achieve the desired particle size and size distribution or cut. Below - 10 fim, where interparticle forces can be significant, gravitational-induced separation becomes inefficient, and cyclone and centrifugation techniques must be used. Classification also separates particles by density and shape. Raw material separation by differential sedimentation is commonly used in mineral processiag. 

Fig. 2. (a) Brightness improvement obtained by the use of kaolin as a filler in paper and (b), opacity improvement obtained as pulp fibers are replaced with various kaolins. In both cases, A represents fine-particle calcined clay B, high brightness No. 1 kaolin C, coarse-particle water-washed kaolin and D,... 

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