Grinding equipment: types of mills
The main process in production of cement, lime, ceramic materials, etc., is grinding of different materials to particles sized less than tenths of millimetre. Drum (ball and rod), medium-speed impact, vibratory and jet-type mills are used for material grinding in modern production.
In terms of their operating principle mills are divided into:
1) drum type where material is ground in a rotary of vibrating drum using grinding media charged into the drum or without grinding media by impact or abrasion action of material particles against each other and the drum lining;
In their turn, the drum mills are classified based on operation mode – intermittent and continuous operation mode, based on grinding type – dry and wet grinding; in terms of material charging and discharging – with charging and discharging through the hatch, with charging and discharging through hollow spigots, with charging through the spigot and discharging through the drum walls. The mill drum is driven via a girth gear or a spigot.
The drum mills can operate in unclosed or closed cycles. In the latter case the material removed from the mill is screened (separated), and coarse particles (over-size) are returned into the mill for remilling. When such operation pattern is used the material ground to the required particle size is continuously removed from the mill which improves its efficiency.
2) medium-speed type in which material is ground by smashing and partial abrasion between some base and operating surface of a ball, roll or roller.
In ball ring mills used for grinding clays, chalk, talc and lime the material is ground as a result of smashing and abrasion during relative movement of balls and rings. The feeder feeds the material to be ground to the groove of the lower spring driven by the motor via a gearbox. The balls are forced against the lower rings with springs through the upper ring. The ground material is spilled over the external edge of the lower rings and is carried away by the air flow delivered through the nozzle at the speed of 20 to 30 m/s to the separator where is is sized.
3) impact type in which the material is ground by impact of hinged (shaft kilns) or rigidly fixed (paddle-type mills) hammers. The product which achieved a definite grinding fineness is carried from the hammer action zone by the air flow;
In impact action mills the material is ground under the action of impacts of rotary operating members – breaker bars against material particles, re-collisions of the particles with breaker plates and with each other. Mills are divided into those with breaker bars rigidly fixed on the rotating rotor and those with hinged hammers - breaker bars. The main parameter of the impact mills is diameter of the circumference describes by breaker bars.
4) jet-type where the material is ground as a result of friction and inter-collision of material particles against each other and against chamber walls as the particles are carried by a high-speed air flow.
In terms of energy carrier the jet-type mills are divided into air jet and steam jet mills. The schematic diagrams of jet mills are shown below. A jet mill with vertical annular chamber is designed for ultrafine (less than 1µm) grinding.
The above listed grinding methods, despite their large number are related to mechanical grinding method in terms of operating principle. In this case the operating member acts on the material to be ground directly or material particles inter-collide. Some methods based on other physical phenomena are being developed, in particular: using electrohydraulic effect (high-voltage discharge in liquid), ultrasonic oscillations, quickly-changing high and low temperatures, laser beams, water jet energy, etc.
Despite variety of types and kinds of material grinding machines, there are some common requirements to be met by such machines: ease of design, convenience and safety of its servicing, minimum amount of wearing parts design of which and attachment design would ensure their easy replacement; protective devices which would be destroyed (toggle plates, bolts, etc.) or deformed (springs) if the permissible loads are exceeded protecting more complex and expensive assemblies against damage; ensure compliance with sanitary and hygienic standards concerning noise, vibration and air dustiness.
The Samara Strommashina plant has been manufacturing grinding equipment for over 70 years already. We design a variety of industrial mill types and calculate the grinding equipment output rate according to the client's process requirements.