Grinding is a conventional process in the cement industry and still draws a lot of interest. It is mainly due to costs involved in the process. ICR tried to give a perspective of the existing scenario coupled with the two interviews from experts.
In the entire process of manufacturing of cement, grinding still draws a lot of attention from the plant management. For cement grinding, the technology development has not taken major strides. The ball mill continues to be the main hardware. What has changed is, it has now pre-grinders to improve efficiency and to conserve energy. The development of roller press was used to retrofit the existing systems. Today the process of grinding has gone further with vertical roller mills but the capital cost remains to be higher and operations are not so easy. Present grinding systems will give lower Blaine's value but better particle size distribution so that the plant is able to derive full potential of the clinker. The latest generation dynamic separators together with a roller press can produce finished cement with high energy efficiency. The pattern of typical energy consumption in grinding operation can be tabulated as seen in the table.
There are couple of important factors to monitor the operations of ball mill. These are ball charge and its level, mill lining, diaphragm condition, etc. need to be maintained in proper condition. The stoppages of the mill are to be measured and recorded as mean time between failures (MTBF). These have become important operational parameters of a smooth running of plant.
Instability, where ball mill feed is stopped and the mill does not produce, is also not frequently recorded or acted upon. When it comes to mill control, operators rarely concentrate on mill production but the kiln is given importance. Expert systems on mills should be universal and well-tuned.
The option of vertical roller mill has been slower to enter the cement grinding market. The success of vertical roller mill largely depends on a team of good process engineer, maintenance engineer and the coordination between the two. It is highly sophisticated piece of expensive equipment needs to run under correct process parameters to give you the best of the results. Today with confidence we can say that Indian engineering community has successfully faced the challenge. Many plants today have been using vertical roller mills very successfully to get best of the results. Grinding bed stability problems offered a challenge which many plants have struggled with, until a significant number of mills began to be installed in the late 1990s, and this has multiplied in the years after. However, in pure energy efficiency terms, the benefit of grinding power reduction is countered by the very high power required by mill fans. In addition, the absence of the heat generated in a ball mill and the high volume of air required by the vertical mill have required the provision of waste heat from cooler exhausts and/or auxiliary furnaces to dry raw materials and achieve a limited dehydration of gypsum.
Grinding aids have become handy to improve the performance of ball mills. No doubt it adds to the cost of production. It should not be a fashion to use grinding aid but there has to be a need and the cost benefit analysis should be able to prove the use of grinding aid. The benefit of grinding aids on cement flowability has to be considered, along with the added scope for reduction of cement clinker content with some modern additives.
Correct timing on the maintenance of a first chamber cement mill lining and the successful implementation of an expert system on a cement mill both offer benefits in terms of power consumption. Accurate process measurements are also key to energy saving opportunities. Air compressors are another area for attention.
We draw attention of the readers to the two interviews given by the experts from cement industry. One talk about the grinding as such and the other talks about the concept of split location of the plant. Both have covered the subject in such a way that will certainly interest the readers.