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A special case of the Grate Ball Millis known as the pebble mill. Media include porcelain balls. Non-metallics such as flint pebbles and sometimes the ore itself. (When the ore itself is used as media, the mill is termed autogenous.) The bulk density of a pebble charge is less than that found in steel grinding media. Consequently, the volume within a pebble mill must be larger than that of a Ball Mill for the same power draw.
Pebble mills; therefore tend to be longer than Ball Mills of the same diameter. Pebble mills can be used in applications where material is alternately ground and then subjected to a processing step such as flotation and then ground again until a satisfactory product is obtained. Such grinding is called “stage grinding”. An example of stage grinding would be a regrind circuit where middling or tails from flotation or other separation processes are reground to liberate additional material. Pebble mills are often used where iron contamination in the product is detrimental.
When metal balls are used in what would otherwise be a pebble mill, the mill is termed a tube mill. The length of a tube mills is normally at least twice its diameter. Tube mills are well suited to regrind applications where a very fine grind is required, or were a high surface area is required such as in the ceramic industry. rotary kiln,ball mill,ball mill http://www.rawballmill.com