The roller press is often used where energy-efficient grinding of large product quantities is required. The two rollers, rotating in opposite directions, exert very high pressure on the material, effectively crushing and weakening the particles by causing micro cracks, so that the subsequent fine grinding is easier. When used as a pre-grinding system in ball mill circuits, production increases of up to 100 % and more can be achieved.
The diameter of a ball mill has a great influence on the power consumption of the mill and thus on the possible production output. This is because the ratio of the diameter to the grinding path length determines the residence time of the material in the mill and thus the theoretically achievable fineness of the material at the discharge.
Coarser products would be ground too finely in long mills. They require the short residence time in a short mill. Fine products, on the other hand, need a long residence time. By combining a mill with a separator, it is possible to control the product fineness and to return only those fractions that are still too coarse to the mill. For this reason, very long mills are no longer common today.
Common length-to-diameter ratios:
Short mills: L/D <2.5
Normal mills: L/D 2.5 - 3.5
Long mills: L/D 3.5 - 5.5
When ball mills are supported on individual bearing jewels via races or surfaces on the mill shell, this is referred to as a slide shoe bearing.
This modern bearing is effective and inexpensive because the head linings of the mill do not have to be designed to carry the load. Depending on the mill size, two, three or four sliding shoes are used per bearing side. The mill, with its heavy weight, rests statically on the slide shoe surfaces. To lift the mill a high-pressure pump pumps lubricating oil into the space between the mill shell and the bearing. This process is referred to as hydrostatic lubrication. When the mill rotates the movement of the mill running surface creates hydrodynamic lubrication. A low-pressure pump maintains the oil flow.
The slide shoes are made of a material with emergency running properties to avoid damages at the bearing and the mill shell, if the oil film should be interrupted.
The filling level of 30 % often turns out to be the most economical nominal value. However, filling degrees around 24 – 26 % are energetically the better option for most plants, although the production is lower. Hence, the ideal filling degree always depends on the operator's target. For products that are very easy to grind, filling levels around 20 % are often sufficient.
Making grinding processes as efficient as possible is a science in itself – one to which we dedicate our entire expertise and spirit of innovation. From the invention of the first MONOBLOC diaphragm to the technical fine-tuning of the separator QDK. How we became what we are today.