Cement and Energy

circulating air separators, it is possible to achieve an energy utilisation of 28 to 30% when grinding PZ35F Portland cement from c linker of average grindability. On the plant engineering side it is possible to achieve appreciably higher energy utilisation values onl y by the use of separators of greater separating efficiency by preliminary size reduction of the material in machines with high pressure comminution grinding process. Optimisation Potential • Upgrading of existing ball mill circuit including separator 20-40% • Installation of (high impact crusher) HIC-pregrinding system 15-30% Availahility of grinding plants Example: Operating Production hours (h) (tfh) 5000 100 5000 110 6000 100 6000 110 !\Jill Vrntilation Yearly Production (tfy) 500 000 550 000 600000 660000 The air used for ventilation of the mill has to be dedusted, either in an electrostatic precipitator or in a bag filler. Operational experience has shown that considerable problems can arise with regard to the operation of the mill , as well as with regard to the dedusting efficiency if the ven tilation rate or the dedusti ng eq uipment is not properly adjust~d or selected. Optimisation potential - overall potential : up to 40 per cent The optimum ventilation rate depends mainl y on the following criteria: • Maximum air speed inside the mill • Heat input by clinker and heat input by grinding • Minimum or maximum dew point as dictated by dedusting equipment The optimum grinding efficiency depends to a great extent on the speed of the air drawn through the mill. The maximum mill ventilation rate is therefore also influenced by the air speed inside the tube mill, which should be kept within certain limits to prevent unduly coarse particles from being swept out of the mill. Taking into account the free cross-section above the ball charge, the following air speeds can be assumed : Closed circuit mill : Open circuit mill : Spec1fte 1.7 H~~f-+-t---+-: air rate (m3/kg G.l +-f~~*f----lr--+-i cem) z.a 1.a u 1.0 1.0 Mill dlilmotor {m) 1.2- 1.5m/s 1.0 - 1.2m/s A. ClOsed e~rcuil mill (V= 1.2·1.Smls) B Open • CirCUli moll (v• I .O·I.2rnl&) Mill venting air rate as a function of mill diameter Higher speeds can be considered if the dedusting system is provided with a static separator. In this case the coarse particles are fed to the bucket elevator and the dust extracted from ~he dust collector can be added directly to the final product. It can be seen that the air available for ventilation of the mill drops considerably with increasing mill diameter. However, it is not advisable to ventilate ttie mill at the maximum possible rate. Instead, the rate should be adjusted to suit the prevailing clinker temperature and the desired cement temperature. 11