Grinding mills: fine grinding in ball and rod mills
The fine comminution of materials is carried out in mills of different designs operating by abrasion of material or simultaneous effect of impact and abrasive forces. The number of types and designs of mills for fine comminution is rather significant; the most common are ball and rod mills. The Samara Strommashina plant manufactures and sells both industrial mill types.
The mills used most often for fine comminution are the so-called ball mills (grinding mill, grinding machine) in which the product is processed with balls contained with it inside a hollow rotating drum. In ball mills the material is comminuted under the action of falling steel or silicon balls impacts an by abrasion of it between the balls and the internal surface of the mill housing. If a ball mill filled with balls is driven, then balls are lifted in the rotation direction due to friction between the mill wall and balls until the ascent angle exceeds their natural slope angle, after which they roll down. One can determine the mill drum rotation frequency at which the balls would fall from the maximum height and would have the maximum falling speed.
The ball weight shall be sufficient to grind largest lumps of the charged material. It is necessary to observe the proper ratio between the ball size and the size of the material being loaded into the mill for efficient operation of the ball mills.
If the comminuted material contains many large pieces which will not be ground in the mill, they will be accumulated gradually between the balls until they stop the mill operation. In such cases it is necessary to decrease the source material lumps size or increase the balls size, but in the latter case the mill operating surface area decreases which reduces the mill capacity.
The degree of filling of the drum with balls also influences the mill capacity and grinding efficiency, because if the filling degree is too large, the lifting balls collide with falling balls. Usually the maximum degree of filling of the drum with balls shall not exceed 30 to 35% of its volume.
Ball mill capacity and power consumption
The ball mill power consumption depends on the drum diameter and drum diameter/length ratio. When the drum length is small, the grinding is coarse, and to achieve the required grinding fineness, one has to return much amount of the material from the separator to the mill which results in the mill overloading.
If the drum length is too great, grinding is only effected in the front part of the drum; the presence of balls in the other part of the drum just increases the power consumption. The optimum ratio between the drum length L and diameter D is not determined precisely, but usually it is assumed as L : D = 1.56 to 1.64.
Also the mill capacity depends on many other factors: filling of the drum with balls, their size, armour shape, drum rpm, grinding fineness, moisture content and size of the material to be comminuted, timeliness of removal of the finished product.
Power consumption for ball mills is a function of many factors: physical properties of the comminuted material and, first of all, its specific gravity and hardness, degree of filling of the mill drum with grinding balls, drum rpm, etc.
The ball mills consume much power; when the mill filled with balls is in idle mode, the power consumption is approximately equal to power consumption in full-load (i.e. material grinding) mill operation mode. Therefore, it is highly disadvantageous to operate the mill in the underloaded state.
The ball mill efficiency is low – no more than 15%. Power is mainly consumed for ball and mill housing wear and friction, material heating, etc.
The advantages of ball mills are:
- versatility, i.e. possibility to comminute a wide range of materials, and high capacity;
- consistency of the set grinding fineness at a definite mill capacity for a long time period (with periodical adding of balls to compensate their wear);
- reliable and safe operation ease of maintenance.
Ball mills can operate without a magnetic separator, as ingress of foreign objects into the mill is not dangerous; besides, grinding can be combined with hot air drying in them.
Drawbacks of ball mills:
- bulkiness and heavy weight;
- large specific power consumption for grinding;
- noise during operation.
Rod mills are also used for fine grinding. Material is ground using 40 to 80 mm rods in them. The grinding rods contact the material being ground along the line, but not in separate points as is the case in ball grinding process. Besides, coarse particle of the material being ground in the rod mills protect fine particles against smashing which results in a more homogeneous product. Thus, equipment is selected based on numerous factors. The plant's specialists are available for providing services on selection and design of ball and rod mills.