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Hammer mills

Hammer mills are used as disagglomerators or as size reduction machines. Combined with a downstream flash dryer, they represent an alternative process to combined grinding and drying for soft to medium-hard materials, such as limestone, chalk, coal, clay, magnesite, phosphate ores, sewage sludges and filtercake.

As a size reduction machine that breaks the feed material by impact and percussion, the hammer mill can be used for feed sizes of up to 50 mm and product finenesses of up to approx. 1 mm, depending on the feed material involved. To achieve the desired product fineness, the separator installed downstream of the flash dryer can be either static or dynamic. While the finished material is collected in a filter, the tailings are returned to the hammer mill.

 

Depending on the type of material feeding (counterflow or uniflow), drying gas of approx. 350 °C or approx. 800 °C are employed. Systems already in operation are dealing with feed moisture contents of up to 40 % and attaining residual moisture contents of 1 - 2 % while achieving a maximum throughput rate of 400 tph.

 

The operating hammer mills have rotors with diameters of up to 4.5 m and a width of 3.5 m. Hammer mills are generally driven with a fixed speed by squirrel cage or slip ring motors, but in some cases frequency-controlled squirrel cage motors are used, e.g. in coal power stations.