Oversized particles

Testing. Various test methods are provided by ASTM (16). These iaclude pigment tests of importance such as chemical analysis, presence of oversize particles, oil absorption, particle size distribution, degree of dispersion, presence of soluble components, etc. Numerous tests are also given by ASTM for the properties of filled and unfilled polymers. These iaclude, for example, such properties as impact resistance, stiffness, viscosity, tear resistance, hardness, color, and electrical resistivity. 

Inks. Refined kaolin is a common ingredient in a large variety of printing inks (qv). In addition to extending the more expensive polymers present, ka olin also contributes to improved color strength, limits the penetration of the ink into the paper, controls rheology, and improves adhesion. Kaolin for this appHcation must usually be as white as possible and free from oversize particles. Surface treated clays are used to improve compatibiHty with oil-based ink. Clays can also be an ingredient in the newer water-based or uv-cured inks. 

Pulp screening removes the remaining oversized particles such as bark fragments, oversized chips, and uncooked chips. In open screen rooms, wastewater from the screening process receives... 

French Also known as the Indirect process. A process for making zinc oxide, in the form of a white pigment, from metallic zinc. The zinc is melted and vaporized in a current of carbon monoxide. The vapor is oxidized with air in a second chamber, forming zinc oxide and carbon dioxide. The fume passes through a settling chamber, where oversized particles settle out, and from there go to a bag house, where the product is collected. Confusingly, the French process has been operated by North American Oxide Company, at Clarksville, TN. See also American. 

The separators in Figure 2.3 may be either a cyclone type, as typified by the Bradley microsizer or a mechanical air separator. Cyclone separators, the theory of operation and application of which are fully discussed in Chapter 1, may be used. Alternatively, a whizzer type of air separator such as the NEI air separator shown in Figures 1.29 and 1.30 is often included as an integral part of the mill, as shown in the examples of the NEI pendulum mill in Figure 2.21. Oversize particles drop down the inner case and are returned directly to the mill, whilst the fine material is removed as a separate product stream. 

MPa, corresponding to about 90% conversion, excess monomer is vented off to be recycled. Removal of residual monomer typically involves passing the reaction mixture through a countercurrent of steam. The reaction mixture is then cooled, and the polymer separated, dried in hot air at about 100°C, sieved to remove any oversized particles, and stored. Typical number-average molecular weights for commercial PVC are in the range 30,000-80,000. 

Oversize particles agglomerates Segregation of Undesirable grain... 

In the PhoSAI process of Scottish Agricultural Industries (Figure 12.1), ammonia reacts with phosphoric acid (minimum concentration 42 % P2O5) in a stirred-tank reactor to produce a slurry with an N/P ratio of 1.4 (the point of maximum solubility) under atmospheric pressure. The slurry flows to a pin mixer in which the N/P ratio is brought back to 1.0 by the addition of more concentrated phosphoric acid. During this step the solubility decreases and more water vaporizes due to the heats of reaction and crystallization. A solid product is formed that typically contains 6% to 8 % moisture. The product is screened, the oversize particles are ground and the product is sent to storage296. 

For the dry milling process, the atomized metal is separated in suitable fractions to which fatty acids are added to prevent the fusion of the particles (Fig. 14.5). The milling is done either in a continuing process, and the oversized particles are fed back into the mill, or by the batch method. The particles are separated by size after the grinding process is completed. 

The design of fluid energy mills provides an internal classification system according to their particle size in which the finer and lighter particles are discharged and the heavier, oversized particles, under the effect of centrifugal force, are retained until reduced to a significantly smaller size. 

Information on purchased catalysts should include the lot numbers and dates of manufacture, and the amount of water and other volatile matter still contained on the catalyst. Specifications should provide data for chemical analyses, and physical-mechanical and physical-chemical properties. The latter information should include data concerning the average shape and sizes of particles, including oversized particles, fines content, and other measures of physical integrity. An example of information which could be included in purchase specifications for a fixed bed catalyst is shown in the following Table II. 

Centrifugal force deposits the oversize particles against the bowl wall, from which they are conveyed by the helix. The overflow fractions flow around the helix to the liquid-discharge ports. Size of separation is controlled by feed rate and degree of centrifugal force. 

Particle characteristics Size, size distribution, shape, mechanical properties, surface chemistry, dispersion stability, concentration, agglomeration, and oversize particle count... 

In addition to the hardness and shape, particle size is another important property for an abrasive. Three relevant parameters that characterize a CMP slurry are mean particle size, particle-size distribution, and oversized particle count. The particle size modulates the material removal rate, WIWNU, and surface quality for both metal and nonmetal CMP. There are two CMP mechanisms that describe the relationship between particle size and removal rate. For a slurry containing extremely large particles, the indentation mechanism may dominate. More specifically, the material removal rate depends on the indentation volume. In other words, the volume of material removal per particle is directly proportional to the particle size. The net effect is that an increase in particle size leads to an increase in material removal rate. A slurry containing smaller particles will follow the contact-area mechanism... 

Mahajan and coworkers [85] studied the impact of abrasive size at different particle concentrations on the oxide removal rate. It was found that the removal rate was a direct function of the particle concentration for monosize abrasives of size 0.2 pm, thereby supporting the contact-area mechanism. The mechanism shifted to indentation for a monodispersed system at 1.5 pm, resulting in reduced removal rates. At 0.5 pm, the removal rate initially increased and then decreased with particle concentration, suggesting a shift in the removal rate mechanism. Particle-size distribution [86] has an equally important effect as the particle size. A larger number of oversized particles in the distribution also cause a shift in the mechanism of material removal. Mahajan and coworkers conducted studies to evaluate the impact of size distribution on oxide removal rates. Baseline commercial slurry was spiked with different concentrations of impurities in the range of 0.5-1.5 pm. The size at different concentrations resulted in removal rates lower than that obtained with the original slurry. Slurry spiked with 1.1 % of 1.5pm particles resulted in a removal rate equal to the baseline slurry, suggesting the predominance of indentation mechanism. Slurries spiked with other concentrations and sizes resulted in a decrease in the removal rate explained by the reduction in the contact area of the abrasives with the oxide substrate. 

Another practical difficulty in characterizing the correlation between slurry properties and defect level is that the absolute number of these oversized particles is very less in relation to the overall number of particles in the slurry. As shown in Fig. 13.29, the number of large particles that is capable of causing defects is in the range of 2.0 x 10 to 9.0 x 10. A typical slurry with mean size of 130 nm has an average of 1.1x10 oversize particles >0.5 pm and 2.3 X 10 total abrasive particles. 

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