Air attrition mills

Series Designates air attrition milled products the lower the number the finer the grind. 

The general pieces of equipment used in grinding flake mica or mica concentrate into saleable mica products are hammer mills of various types, fluid energy mills, Chaser or Muller mills for wet grinding, and Raymond or WiUiams high side roUer mills. Another method is being developed, called a Duncan mill (f. M. Huber, Inc.), that is similar in many respects to an attrition mill. AH of these mills are used in conjunction with sieves, and all but some types of hammer mills incorporate air classifiers as a part of the circuit. 

The method used for grinding pressed cakes depends upon the nature of the cake, its purity, residual oil, and moisture content. If the whole cake is to be pulverized without removal of fibrous particles, it may be ground in a hammer mill with or without air classification. A 15-kW (20-hp) hammer mill with an air classifier, grinding pressed cake, had a capacity of 136 kg/h (300 Ib/h), 90 percent through No. 200 sieve a 15-kW (20-lm) screen-hammer mill grinding to 0.16-cm (Vi6-in) screen producea 453 kg/h (1000 Ib/h). In many cases the hammer mill is used merely as a preliminaiy disintegrator, followed by an attrition mill. Typical performance of the attrition mill is given in Table 20-25. A finer product may be obtained in a hammer mill in closed circuit with an external screen or classifier. 

The following materials may be ground at ordinary temperatures if only the regular commerci fineness is required amber, arabac, tragacanth, rosin, olibanum, gum benzoin, myrrh, guaiacum, and montau wax. If a finer product is required, hammer mills or attrition mills in closed circuit, with screens or air classifiers, are used. 

Fig. 12.9. Flow diagram for manufacture of cellulose acetate yarn (1) wood pulp (2) attrition mill (3) cyclone (4) 35% acetic acid (5) pretreater (6) magnesium acetate solution (7) precooled acetylation mix (8) sulfuric acid (9) acetylator (10) ripener (11) steam (12) blender (13) precipitator (14) dilute acetic acid (15) hammer mill (16) water (17) rotary screen washer (18) squeeze rolls (19) drying oven (20) blender (21) storage bins (22) silos (23) weight bins (24) acetone (25) wood pulp (26) pigment (27) mixers (28) hold tanks (29) filter press (30) pump (31) filter (32) air (33) jet (34) acetone recovery (35) oiling wheel (36) feed roll (37) bobbin (38) inspection.

If particle sizes in the micrometer range are required, then the attrition mill can be used. Here particles are bled into a chamber in which great turbulence has been created by two inlets of air at different pressure. The particles hit one another and are removed by centrifugal means and collected in a cyclone setup or in a bag above the cyclone. Such particles become highly electrically charged during the operation and also become very cohesive. 

The first preparation method consists in coating active phases on alumina foams. Commercial spinel powder is attrition-milled to break agglomerates. Attrition-milled MgAl204 spinel powder is dispersed in an aqueous solution of rhodium nitrates under stirring at room temperature, for 2h. The amount of Rh is calculated to produce 20wt% Rh loaded catalysts. Water is then evaporated from the slurry and residues are calcined in air (450°C, 4h). Powders obtained are used to prepare slurries which are later used to coat alumina foams. 

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