A. the classification of mills  

After the coarse, medium and tertiary crushing of the crushers, the granularity of material should be between 8 mm and 20mm.In order to reach the required granularity, the crushed materials need to be pulverized by the pulverizer. Grinding is an important process in the production of the modern industry. 

Grinding is needed in some industrial department such as ore dressing, building materials, cement, coal, chemical industry, electric power, metallurgy and so on, The major component of ball mill is a cylindrical shell which rotates slowly, in whom there are some grinding medium. Ball mill has the following advantages: simple structure, reliable operation, simple management and maintenance, long-time operation, strong adjustment to materials, big crushing ratio (it can reach 300 and large production capacity. As a result, it can meet the demands of modern large-scale industrial production.

 1, according to L/D (length to diameter) 

(1) Short-type mill 

The short-type mill, whose L/D is below 2, is also called ball mill. Most of them are with single chamber, and can be used in coarse grinding or primary grinding. We can also use 2 or 3 series ball mills. 

(2) Medium-type mill

              The mill, whose L/D is about 3, can be called medium mill. 

(3) Long-type mill 

The mill, whose L/D is above 4, can be called long-type mill or tube mill. There are 2 or 4 chambers in most Medium-type and long-type mills. They are widely used in cement plants. 

2, According to the grinding medium in the mill 

(1) Ball mill 

The grinding medium is steel balls or cylpebs.This kind of mill is the most widely used kind of mill. 

(2) Rod mill 

The grinding medium in the mill is rods. The L/D of the rod mill is usually from 1.5 to 2. 

(3) Gravel mill 

The grinding mediums in the mill are gravel, cobbles, and ceramic balls and so on. The liners of the gravel mill are made of granite and porcelain. It can be used in the process manufacturing of raw materials, in which some industries such as ceramic have certain requirements about the impurities, colour and luster of the metal. 

B. The working principle of mills  

Ball mill is mainly made up of cylindrical shell, end cover, bearing, and big ring gear and so on. We can load some steel balls or steel rods, whose diameter is from 25mm to 150mm,the intake quality is about the 25～50％ of the effective volume of the shell. There are end covers on both ends of the shell, to be connected by bolt and flange at shell end. There are holes at the center of end covers, called hallow shaft necks propped up to bearings, therefore, shell can turn. Main gear ring is fixed on shell. In driving unit, main gear ring and shell are driven to turn by motor, shaft coupling, reducer and pinion. When shell turning, grinding medium rise to a certain height along the shell wall and drop in form of parabola or in torrents. 

Material is fed into shell from hallow shaft neck of input end, and transfer gradually towards output end. During the movement of material, steel balls will be lift to a certain height by the rotating shell to crush the material by fall. Part of steel balls will grind the material by dropping in torrents in shell. Whole movement procedure of material is the procedure of crushing. 

In the working principle of ball mill, as the rotary of the shell and the movement of grinding media, the materials gradually spread to the discharging direction, finally overflow from the hollow shaft of discharging end, this kind of ball mill is called as overflow type ball mill. Another kind of ball mill has the grid plate in the discharging end, the grid plate consist of several pieces of fan-shaped grate plates, whose hole opening is 7-20mm, generally 7-8mm suitable. Materials enter into the space between the grid plate and end cover through the holes, and then to be lift by lifting plate, fall along lifting plate to hallow shaft neck through conical block, finally, discharged by discharge unit at hallow shaft neck.  This kind of ball mill is called as grate type ball mill.