Great Wall News

Cement Grinding Mill:Compared Vertical Roller Mill with Ball Mill

2015-09-30 14:37:18

  Vertical roller mill equipments have become the standard for grinding raw materials in the cement manufacturing process. However, despite the fact that more than 10 years has passed since the installation of the first VRM for clinker grinding was almost completed, many cement producers are still favouring ball mills for cement production when designing new integrated cement plants or grinding plants. The industry has failed to fully adapt to this technological development, which offers significant energy savings, as it did with other technical developments such as the precalciner kiln or high efficiency coolers.

  1.From the Investment cost

  Vertical grinding mill is a compact unit, making the footprint of the installation smaller and reducing the civil engineering costs when compared to a ball mill system. Savings are also made due to the method of construction of the two systems. Ball grinding mills are built at the supplier’s factory and transported to the cement producer’s site. In contrast, the vertical mill machine is built onsite at the cement plant, avoiding difficult logistical issues and associated costs. The equipment costs are higher for a VRM than a ball mill, reflecting the greater complexity of the system, which includes items such as the rollers, table and the hydraulic system. Overall, when equipment, erection and civil costs are taken into account, the cost of a VRM project is around 20 – 25% greater than a ball mill system of the same capacity.

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  2.From Operation and Maintenance

  Operation of a ball mill is relatively simple, with no moving mechanical parts within the mill itself. The system is tolerant to variations in both mill feed quality and quantity. By contrast, the VRM is a more complex piece of equipment with a sophisticated hydraulics system that operates the rollers. The material bed between the rollers and the table is thin and it is essential that the rollers and table do not come into contact. A variable bed of material will cause vibrations on the mill; this effect is somewhat reduced in VRMs grinding kiln raw materials as the feed is larger than the cement mill feed. The moisture content on the kiln raw materials is also higher than the cement mill feed, which is generally dry. In order to reduce these vibration issues and adequately prepare the material, the rollers in the cement VRM have been modified. In some cases, grooved rollers are used to prepare to de-aerate the material in a low pressure zone prior to it passing into a high pressure grinding zone where the particles are fractured. In other cases, smaller rollers prior to the main rollers are used to achieve the same result.

  In terms of ongoing operations, the grinding media in a ball mill has a much higher wear rate compared to that of the table and rollers in a VRM. With a ball mill, it is important to monitor the performance through axial testing, to ensure that the media grading is correct and that there is a sufficient quantity of top-size media in the mill to sufficiently grind the mill feed materials. If this is not the case, ‘nibs’ (large, unground material) will collect at the end of the first chamber and block the slots of the mill diaphragm. However, topping up the mill with additional media is a simple task and most cement manufacturers will re-grade their cement mill media on an annual basis.

  The other major repairs that take place on ball mills are to replace the liners and diaphragms and on a VRM to replace (or turn) the rollers and replace the table. The liners on a ball mill generally last up to 5 years in the first chamber and as long as 8 years in the second chamber and therefore are infrequently changed. Diaphragm plates will require changing more frequently, but certainly not on an annual basis. Whilst the wear parts on a VRM will not require changing on an annual basis, work will be required. This could be hard-facing the roller and table or reversing the roller segments. Eventually the wear parts will require replacement. Therefore the maintenance requirements of the VRM are higher than those of the ball mill. Overall, the maintenance costs are not dissimilar for both types of mills: while the wear rates for a ball mill are higher than for a VRM, the cost of the replacement parts is lower, and the opposite applies for the VRM – i.e. lower wear rate but higher replacement costs.