Date：2014-12-08 07:45:48 ?? Source：Clirik
Introduction
Vertical roller mills (VRM) 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.
Mill 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.
Product quality
One of the perceived difficulties with the VRM when they were first proposed as an alternative for cement milling was the particle size distribution (PSD) of the product. Due to the high circulation factor in the mill and the high separation efficiency, VRMs have the tendency to produce with a steep PSD curve resulting in cement that is dissimilar to that produced in a ball mill (a steep PSD cement results in high water demand cement). Again, the VRM manufacturers have adapted the conventional design of the raw mill VRM to address this issue.
Firstly, the table profile curves upwards on the outside edges to retain a certain amount of material on the table. In addition, the mills have an adjustable dam ring on the outside of the table, which has the same effect of retaining material on the table. In doing this, the material has a longer residence time on the table and a wider range of particles are produced. Other variables such as the grinding pressure, the mill airflow and the separator speed can also be tuned to change the PSD curve. Usually, a compromise is found between the PSD curve and energy consumption, as the above actions reduce the grinding efficiency and therefore increase the mill’s power consumption.
Investment cost
A VRM 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 mills are built at the supplier’s factory and transported to the cement producer’s site. In contrast, the vertical mill 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.
Deciding factors
Bearing in mind the development work in adapting the VRM to cement grinding and solving any perceived quality issues with the mills, as well as its lower energy consumption, it is strange that the VRM has not been fully embraced by the cement industry as the system of choice.
The increased capital cost may be a factor for many companies in current times when capital is short; certain countries still have very low power costs and therefore the power saving may not provide sufficient payback for the additional capital investment in a VRM. The lack of full adaption of VRMs may be down to an attitude of perceived risk, with cement manufacturers wanting to ensure that there is not a change to the quality of cement produced with the new system and not realising the efforts that have been made to adapt the VRM to cement grinding. Some cement producers may perceive that VRMs are difficult to maintain and operate and may not want to retrain their employees, who already have experience with ball mill systems.
One potential development that may see the increase in adaptation in VRMs could be scale. The capacity of VRMs for raw material grinding has been increasing over the years since they became the machine of choice. Such a development in cement grinding – a high capacity VRM that would replace two or three cement mills, with a low specific power consumption – may be enough to convince cement manufacturers to install a VRM for this purpose.
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