Grinding is a highly energy intensive process in the cement industry. Approximately 60-70 per cent of electrical energy used in a cement plant is utilised for grinding of raw materials, coal and clinker. According to R Vasudevan, a freelance cement industry consultant, the efficiency of energy utilisation in a cement plant is hardly 15 -20 per cent on an average. Therefore, any improvement in energy utilisation is always of high priority.
The various types of grinding systems currently being used for cement grinding in a cement plant are:
- Conventional tube mills (open and closed circuits)
- Tube mills with pre-crushing unit, i.e., vertical shaft impactor (VSI) or horizontal impact rusher (HIC)
- Roller presses in semi-finish and finish grinding modes
- Vertical roller mills (VRMs)
- Ring roller mills or Horo mills
In spite of low efficiency, ball mills still continue to be used because of reliability, easy maintenance and cost considerations. Jayanta Saha, a cement process consultant based in Navi Mumbai, says that ball mills are not being used as standalone units currently, but are integrated with other equipment like separators, along with roller presses.
Taking a look inside a ball mill is critical to understanding its performance. Dr Raghuraj K Rao, from Belgaum-based Aqua Alloys, has written on the internals of a ball mill.
Today´s single chamber ball mills (mostly converted from previously available twin chamber mills) generally provide larger grinding volume and mainly drive power for high throughput finish grinding, essential to meet high Blaine requirements of additive cement. As observed by Dr Rao, during the retrofitting exercise, not enough attention is given to the mill internals despite significant change in granulometry and grinding characteristics of feed entering the ball mill. He further says that there is a need to go for unconventional liner design. In his article, Dr Rao outlines the conceptualisation of a new liner design, lists the advantages over conventional liners, and also summarises the results of trials at different sites.
On the other hand, PR Raghavarao, a cement industry veteran, has talked about his own experience in carrying out diagnostic studies on mill optimisation. Experience has shown that the potential for optimisation is greatest in the cement grinding process in a cement plant The benefits that can be achieved due to the optimisation of the cement grinding system through a process diagnostic study are many. As Raghavarao says, in a given diagnostic study, the mill performance is evaluated and recommendations are formulated based on the past history of the mill operations, inspections, process measurements, feed/product characteristics and grinding performance, in order to improve mill output and reduction in power consumption in the ball mill system.
While discussing the diagnostic study, Raghavarao has also touched upon the methodology, PMA (Project Management Approach), to execute such small projects. It is an interesting tool which takes into account not only the client, but also all the stakeholders which are critical for the success of a project.
Experience over the years has shown that a ball mill grinding circuit has a fixed range of operating parameters in which optimum performance exists. Deviating from these parameters may alter the end product or lead to higher power consumption, which is often forgotten by the plant operators.
Vertical Roller Pre-grinding Mill
Saha says that there is no doubt that presently the most preferred pre-grinder is the roller press. However, Vertical Roller Pre-grinder Mills (VRPMs) are gradually gaining popularity, day by day. In the VRPM system, a centrifugal force produced by the rotation of the table distributes the material over the table surface. Clinker is caught between rotating table and roller and effectively ground by a combination of compressive force from roller and shear force generated by difference in circumferential speed of table and roller. Finally, ground material is extracted by gravity with the assistance of scrapers. The grinding principle is similar to that in efficient grinding in vertical roller mills, with the key difference in magnitude of applied pressure on rollers. In VRPM, the pressure is on the higher side. However, it has been observed that newer plants are going in for only VRM for finished grinding, and the performance has been more than satisfactory. However, Saha prefers VRPM over roller press because of reasons like a good grinding method, in principle, and commonly used equipment for clinker pre-grinding systems (until the debut of high pressure vertical roller mills). In case of roller press, the gap between rollers is not stable, resulting in unstable product fineness. Further, as maintenance is complicated, this cannot be called a ´perfect´ machine. R Vasudevan, a freelance cement consultant, is of the view that nothing great has happened in the last ten years in the field of cement grinding - only some incremental improvements have been taking place.
The choice of one specific type of equipment in a combination cannot be termed as a successful combination, mainly because it depends on clinker properties. Vasudevan further says that the choice of equipment will depend mainly on the type of clinker. He says that for slag cement, grinding slag and clinker separately makes more business sense. Vasudevan says that composite cement (BIS has recently introduced a new set of specifications which can have both flay ash and slag) is a product where economic benefits will be more than the sum of its parts, if properly designed and executed.
A process diagnostic study can give you:
- Reduction in specific power consumption
- Increase in production
- Stable and sustained operations
- Increase in availability of the equipment (less downtime)
- Improved and consistent product quality.