Cement and Energy

Dedusting aspects Determination of the optimum mill ventilation rate is directly related to the dedusting equipment, i.e. an increased mill ventilation rate in conjunction with grinding aids can onl y prove beneficial if the dedusting equipment is adjusted to suit these new conditions. For dedusting cement mills, either bag filters or electrostatic precipitators are employed. Bag filters have been successfully used for years with mills up to about specific dedusting air rates for a bag filter and an electrostatic precipitator were calculated as a function of the mill diameter. From the graph given alongside, it can be deduced that with a bag filter a larger mill ventilation air rate than for an electrostatic precipitator can be accepted. . It can therefore be concluded that the optimum mill ventilation rate is not only dictated by the mill size and the required heat output, but also depends on the dedusting equi pment 3.5m in diameter. Large mills have often been equipped used. with electrostatic precipitators and can be maintained without major problems. However, the efficiency drops drastically as soon as the water injection rate has to be reduced because of low clinker temperatures (e.g. when starting up mills). Since uncontrolled dust emission is often no lon ger acceptable by the authorities, some large mi lls have been equipped with hag filters. They, in turn, are sensiti ve to high dew-points (condensation) which can also reduce the dedusting efficiency. The dedusting equipment for a cement mill has therefore 'to be selected very carefull y. Special attention in this connection has to be paid to the sensitivity with regard to fluctuation of the dev.: point (maximum and minimum water injection rate) and the operating and investment costs. If water is injected into the mill , the limiting factor as regards .the dedusting air rate is the dew-point.With a bag fi lter the dew-point should normally not exceed 50° C, whereas for an electrostatic precipitator a minimum dew– point of about 50 ° C has to be observed to maintain normal dedusting efficiency. Based on these assumptions and considering the limits set by the mill size, the 1.0 0.1 Sl)l!dtlo 0.6 venltlatlon (m*g o.• com\ 0,1 M l,O 1,0 j ,Q Mitt dlamolor Jm) Specific ventilation rate as a function of dedusting equipment and mill diameter A. B~U llit~t (ffitl~ tli!W PI!IMI) ~: ttll<li!O• llftllc proclpiiAI~t (ml~. dow I)Oitlll Based on the latter consideration the following approximate values for a rough estimate of the mill ventilation rate can be assumed : Mill diameter Bag Filter Electrostatic precipitator · 2.0- 3.5m 0.90m 3 /kg cement 0.45m 3 /kg cement 3.5 - 6.0m 0.35 - 0.90m 3 /kg. 0.20 - 0.45m 3 /kg. The generally lower investment costs of a bag filter are to a certai n extent compensated by the higher specific power consumption (pressure drop) and the higher cost of manpower required for its maintenance. However, it is believed that the latter disadvantage of a bag filter can be overcome by adequate design, as weil as by selecting suitable filter media. The higher ventilation rate of a bag filter allows fuH advantage to be taken of the application of grinding aids. Practical experience has shown that a pre-separator in front of the dust collector is needed to eliminate oversize particles in the filter dust, the pre-separator should preferably be adj ustable. So far, the best results have been obtained with static separators wi th adjustable vanes. Experience has also shown that the active collecting surface of both the electrostatic precipitator and the bag filter should be increased to compensate for the decreasing dedusting efficiency caused by the addition of grinding aids. 12 .. ...