Breif Introduction to Ball Mill

The following article describes types of ball mill, affecting factors, ball mill grinding media, and working principle.

The final stages of comminution are performed in tumbling mills using steel balls as the grinding medium and so designated “ball mills.” Since balls have a greater surface area per unit weight than rods, they are better suited for fine finishing. The term ball mill is restricted to those having a length to diameter ratio of 1.5 to 1 and less. Ball mills in which the length to diameter ratio is between 3 and 5 are designated tube mills. These are sometimes divided into several longitudinal compartments, each having a different charge composition; the charges can be steel balls or rods, or pebbles, and they are often used dry to grind cement clinker, gypsum, and phosphate. Tube mills having only one compartment and a charge of hard, screened ore particles as the grinding medium are known as pebble mills. They are widely used in the South African gold mines. Since the weight of pebbles per unit volume is 35-55% of that of steel balls, and as the power input is directly proportional to the volume weight of the grinding medium, the power input and capacity of pebble mills are correspondingly lower. Thus in a given grinding circuit, for a certain feed rate, a pebble mill would be much larger than a ball mill, with correspondingly higher operating cost. However, it is claimed that the increment in capital cost can be justified economically by a reduction in operating cost attributed to the lower cost of the grinding medium. This may, however, be partially offset by higher energy cost per tonne of finished product.
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What is ball mill?

Ball mills are a similar shape to that of the rod mills except that they are shorter with length to diameter ratios of 1 to 1.5. As the name implies, the grinding media in these mills are steel balls. The particles size of the feed usually does not exceed 2.5 cm. The grinding is carried out by balls being carried up the side of the mill such that they release and fall to the point where they impact the ore particles in trailing bottom region of the slurry. If the mill is rotated too fast, the balls can be thrown too far and just strike the far end of the mill and conversely, if the mill is rotated to slow, the efficiency of the grinding process significantly reduced. Ball mills are suited for finer grinding as larger particles do not impede the impact on to smaller particle as in rod mills.

What is Rod Mill?

Rod mills are long cylinders filled with steel rods that grind by compressive forces and abrasion. The length of the cylinder is typically 1.5 to 2.5 times longer than the diameter. As the mill turns, the rods cascade over each other in relatively parallel fashion. One of the primary advantages of a rod mill is that it prevents over-grinding of softer particles because coarser particles act as bridges and preferentially take the compressive forces. Rod mills can take particles as coarse as 5 cm. Many of the newer operations tend to install ball mills in combination with SAG mills and avoid rod mills due the cost of the media, the cost of replacing rods and general maintenance costs. Many older operations have rod mills in combination with ball mills.

Rod mill

Rod mills are charged initially with a selection of rods of assorted diameters, the proportion of each size being calculated to provide maximum grinding surface and to approximate to a seasoned or equilibrium charge. A seasoned charge will contain rods of varying diameters ranging from fresh replacements to those which have worn down to such a size as to warrant removal. Actual diameters in use range from 25 to 150mm. The smaller the rod the larger is the total surface area and hence the greater is the grinding efficiency. The largest diameter should be no greater than that required to break the largest particle in the feed. A coarse feed or product normally requires larger rods. Generally, rods should be removed when they are worn down to about 25 mm in diameter or less, depending on the application, as small ones tend to bend or break. High carbon steel rods are used as they are hard, and break rather than warp when worn, so do not entangle with other rods. Optimum grinding rates are obtained with new rods when the volume is 35% of that of the shell. Thus reduces to 20-30% with wear and is maintained at this figure by substitution of new rods for worn ones. This proportion means that with normal voidage, about 45% of the mill volume is occupied. Overcharging results in inefficient grinding and increased liner and rod consumption. Rod consumption varies widely with the characteristics of the mill feed, mill speed, rod length, and product size; it is normally in the range 0.1-1.0 kg of steel per tonne of ore for wet grinding, being less for dry grinding. Rod mills are normally run at between 50 and 65% of the critical speed, so that the rods cascade rather than cataract; many operating mills have been sped up to close t0 80% of critical speed without any reports of excessive wear. The feed pulp density is usually between 65 and 85u/o solids by weight, finer feeds requiring lower pulp densities. The grinding action results from line contact of the rods on the ore particles; the rods tumble in essentially a parallel alignment, and also spin, thus acting rather like a series of crushing rolls. The coarse feed tends to spread the rods at the feed end, so producing a wedge- or cone-shaped array. This increases the tendency for grinding to take place preferentially on the larger particles, thereby producing a minimum amount of extremely fine material. This selective grinding gives a product of relatively narrow size range, with little oversize or slimes. Rod mills are therefore suitable for preparation of feed to gravity concentrators, certain flotation processes with slime problems, magnetic cobbing, and ball mills. They are nearly always run in open circuit because of this controlled size reduction.