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The Szego Grinding Mill
The Szego grinding mill is a planetary ring-roller mill shown schematically in FIG.1. It consists principally of a stationary, cylindrical grinding surface inside which a number of helically grooved rollers rotate. These radically mobile rollers are suspended from flanges connected to the central drive shaft; they are pushed outward by centrifugal force and roll on the grinding surface.
FIG.1 Grinding Mill
The material is fed by gravity, or pumped into a top feed cylinder and is discharged at the bottom of the mill. The particles, upon entering the grinding section, are repeatedly crushed between the rollers and the stationary grinding surface. The crushing force is created mainly by the radial acceleration of the rollers. Shearing action is induced by the high velocity gradients generated in the mill, and hence the primary forces acting on the particles are the crushing and shearing force caused by rotational motion of the rollers. Attrition is also important, particularly in dry grinding, and impaction also occurs at higher rotational speeds.
An important feature is the ability of the roller grooves to aid the transport of material through the mill, thus providing a means to control residence time and mill capacity. This transporting action is particularly important with materials which would not readily flow by gravity. Pastes and sticky materials fall into this category.
The mill has several design variables which may be utilized to meet specific product requirements. The important variables are the number of rollers, their mass, diameter and length, and the shape, size and number of starts of the helical grooves no the rollers. As the number of rollers is increased, the product becomes finer. Heavier rollers and higher rotational speeds generate greater crushing forces and give higher reduction ratios. The ridge: groove size ratio may also be changed to increase or decrease the effective pressure acting on the particles. the common groove shapes are rectangular and tapered and the latter will decrease the chance of particles getting stuck in the grooves.
The Szego grinding mill been used for dry as well as wet grinding of coal , in both water and oil, for the preparation of coal-slurry fuels. to grind coal in water to the standard boiler size(80 percent<74μm)takes about 7 MJ/Mg(20KWH/tonne).
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