Cement Energy and Environment

All this is possible because a closed circuit mill operation permits manufacture or quality clinker at a coarser grind. The top size is controlled to 0.02 per cent 295 microns (48 mesh) and most of the superfines are eliminated which makes the slurry less viscous. Hence less water is needed to grind and to transport the slurry. In open circuit, most of the mill power is absorbed in making these superfines. Thus, eliminating them saves power and increases mill capacity. Thus many of the small grindi ng media is replaced by larger size balls as a large percentage of oversize is returned to the mill. Most of the mill maintenance is in the smaller grinding media section. An automatic system with magnetic flow meters and slurry density gauges can control the water content in the kil n feed slurry to within 0.5 per cent. Typical savings in a raw mill department can be as much as US $ I m a year for an an nual clinker production or 500,000 t. The level of savings depends on the water reduction possible, the cost or fuel and power. A typical investment cost tor one 4-m diameter raw mill system is US $ 1.8m and the capital investment can usually be recovered in two years. GRI:O.'DJ:-;G :1-lJLl. ' o\UIOnllhC 1 C\d ( OtHtt•l 6 Elrctnc [ar ' l Figure I gives the schematic flow diagram of a typical closed circuit raw materials grinding in wet process. Cement Grinding Department Advantages or changing open circuit mills to closed circuit operation include: • Increase in mi ll capacity by 15-30 per cent • Reduction in speei fie power consumption by 15-30 per cent • Facil ity of manufacture higher Blaine cemen ts • Better quality of cement Basically closed circuit cement grinding process involves the use of a third generation high efficiency air separator, new dust collectors and a sophisticated control system continually keeping the mill at its most efficient operation. A typical investment cost for one 4-m diameter mjll system is about US$ 1.5m. The payback period is more difficult to determine. Obviously, this is not an attracti ve in vestment on the basis of electric power savings alone. However, a plant may be forced to change to closed circu it operati on for competitive reasons, to be able to sell higher quality cement and higher Blaine cements in the international market place. I caJ:'·[a}' o..,.,.._ ... L.,..,.. Figure 1 Schematic flow diagram of a typical closed circuit raw materials grinding in wet process 7