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Structural Differences of Rod Mill and Ball Mill

The structure of the rod mill is quite similar to ball grinder, but there are three points of difference:

1. Rod mill uses the steel rod whose diameter is 50-100mm as grinding medium but the latter mill uses steel ball as grinding medium. The steel rod is 25-50mm shorter than the barrel and the rod is made by high-carbon steel whose steel content is 0.8%-1%. The charge weight of rod is about 35%-45% of the effective volume of rod milling machine. When observed by eyes, the horizontal plane of rod is about 100-200 m lower to the center line of barrel.

2. In the rod mill, the ratio between barrel length and diameter is about 1.5-2.0. The inside surface of lining board on the shell cover must be vertical plane so that in the barrel, the steel rod can not do chaos movement, be bent or broken and the steel rod can move regularly. In the ball grinding machine, the ratio between barrel length and diameter is less, which is often a little larger than 1.

3. Rod mill uses the overflow type or opening type ore discharging instead of the waffle slab ore discharging. The diameter of hollow shaft neck in ore discharging terminal is larger than the ball mill with same specification. The barrel rotating speed of the rod mill should be less than the latter mill with same specification so that the medium inside can work in the falling type.

The lining board of the rod mill often adopts waveform, stepped-form or wedge. According to the different ore discharging ways, the rod mill can be classified into two types: center ore discharging type and surrounding ore discharging type. The former is widely used and the latter did not manufactured any more. The diameter of hollow shaft neck in ore discharging terminal is larger than the ball mill with same specification so that the ore slurry level can be reduced and the speed that the ore slurry come through the rod mill can be improved. The discharging opening of the large rod mill can reach 1200mm.

Rod mill can grind the ore stones by the pressure and grinding force of rod. When the rod hit the ore stones, the coarse grains will be hit and ground at first and then the smaller ore grains will be ground. Line contact is between the rods and the point contact is between the balls. So when the rod is rotating and rising along the barrel, there are coarse grains between the rod and barrel, which is more like the bar screen to make the fine grains come across the intervals between the rods hammer crusher. What's more, it is also useful to crush the coarse grains and make the coarse grains get together in the place where is hit by the grinding medium. So the rod mill can do selected grinding. The grain forms of the products are uniform and there is less over-grinding.

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