Ball attrition

The white cast irons and their low alloys have good abrasion resistance properties [2,3]. White cast irons are used for grinding balls, segments for mill liners and slurry pumps. In the ceramic industry they are used for muller tyres and augers in the pulp and paper industry for attrition mill plates and chip feeders and in the paint industry for balls for grinding pigments. 

Feed material in ball mills is nsnally smaller than about 50 p.m, and the solids contents of slurries range from 30% to 70%. The size of the spherical grinding media is in the range of 0.5-5 mm. Very rapid attrition is produced in ball mills by the intense combination of compression and shearing forces and the frequency of collisions, which is very high. 

The second class of grinding equipment is used to prepare dispersions. Typical of this class are ball and pebble mills, ultrasonic mills, and attrition mills. Solids, eg, sulfur, antioxidants, accelerators, and zinc oxide, are generally ground on this equipment (see Size reduction). Ball mill action is assisted in some mills by a combination of dispersion circulation by an external pump and mechanical oscillation of an otherwise fixed nonrotary mill chamber. Where ball mill chambers are rotated it is necessary to experimentally establish an optimum speed of rotation, the size and weight of the ball charge, and ensure the mills do not overheat during the grinding period. 

Various methods are used for evaluating the quality, ie, physical strength and enzyme dust formation, of the granulate. In the elutriation process, a sample of product is fluidized in a glass tube with a perforated bottom plate for 40 minutes. Dust from the sample is collected on a filter and the enzyme activity measured. An acceptable dust level is when less than 5—10 ppm of the activity of the sample has been collected. In the so-called Heubach method, 20 g of granulate is elutriated. During the elutriation, four steel balls are rotated in the bed in order to evaluate the impact of attrition on the dust release of the enzyme. The dust is collected on a filter and measured. The acceptable dust level is very low. 

Mixing is usually carried out in a ceramic ball mill with steel balls. The inevitable attrition of the mill balls leads to an addition of 0.5-1 wt% to the iron content for which allowance must be made. Care must be taken to maintain the quantity and size distribution of the mill balls and to remove those that have become so small that they cannot be separated from the slip or powder on sieving. 

Salt Attrition. The crude is ground in a ball mill or a double-arm, sigma blade mixer in the presence of a large excess of sodium chloride and an organic solvent such as diethylene glycol or xylene to give the green-shade, beta copper phthalocyanine blue. 

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