Coarse product

Calcium carbonate is one of the most common filler/extenders used in the paint and coatings industry. Consumer and contractor paint formulas can include products from submicrometer size to coarse mesh sizes. The main function of calcium carbonate in paint is as a low cost extender. It is also used to improve brightness, appHcation properties, stabiHty, and exposure resistance. Coarse products help to lower gloss and sheen or even provide textured finishes. The selection of product type and particle size is deterrnined by the desired performance and cost of the coating. 

The majority of spray dryers in commercial use employ cocurrent flow of gas and solids. Countercurrent-flow diyers are used primarily for diying soaps and detergents. Their classif ng ability is useful in these applications. Air flow is upward, cariying entrained fines from the top of the chamber. The coarse product settles and is removed... 

Very clean coarse product, but relatively low capacity for a given... 

Feed to be sized is put into hindered-settling condition by hydraulic water in quantity only sufficient to teeter the smallest particle wanted in the coarse product. The finer fractions report to the overflow or pass into the upper column for removal in a three-product unit. 

Three parameters define the performance of a classifier. These are cut size, shaipness of cut, and eapaeity. Cut size, X50, is the size at which 50 percent of the material goes into the coarse product and 50 percent into the fine. (This should not be confused with the cutoff size, a name sometimes given to the top size of the fine product .)... 

It has been suggested that the circulating load can be calculated by a material balance from size analyses of the feed, fine product, and coarse product of the classifier in a closed-circuit grinding system [Bond, Rock Prod., 41, 64 (January 1938)]. However, since size analyses are subject to error, it is better to use this information to check the size analyses (Vaillant, op. cit.). The appropriate equation is (Dahl, Classifier Test Manual, Portland Cem. Assoc. BuU. MRB-53, 1954)... 

Typical separation efficiency curves of an air classifier versus particle size are given in Fig. 20-14. The amount of top size in the fines may be very low, but there is typically 10 to 30 percent fines in the coarse product that is, the low end of the cui ve tends to flatten out at 10 to 30 percent. In addition, the separation at the cut size is typically a gradu cui ve. Data of this sort, which are needed to evaluate closed-circuit mill performance, are seldom available. See subsection on characteristics of size classifiers for a testing method. 

A continuous particle separator, operating at steady state, is now considered in which particles are introduced in suspension at a volumetric feedrate <2/ at a solids concentration of Cf on a volume basis. Fluid and particles are divided between a coarse product outlet (underflow) and a fine product outlet (overflow), which have volumetric flowrates of... 

Feed to Classifier Fine Product Coarse Product... 

The ratio of growth rate to nucleation rate is a measure of the crystal size obtained from a given process the larger this ratio, the coarser the product. It is also clear that production rate falls off with decreasing supersaturation. The operation of a crystallizer is a compromise between these two factors. If size is of little importance, high supersaturations will result in high production rates and small crystals. Conversely, coarse product is obtained at lower supersaturation, but at the expense of production rate. Crystallizer operation will be discussed in greater detail in Section VI. 

Control of water into the classifiers is important since separation into fine and coarse products is made largely by the buoyancy, viscosity, and degree of agitation in the pool. 

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