My Company is planning to install a New Cement Grinding Plant with a capacity of 150tph, The Supplier is insisting to have separate mill ventilation bag filter, and the separator fines to be collected through another bag filter. My idea is to have only one bag filter in common while mill ventilation duct is connected to a static grit separator before this bag filter, and the separator fines are collected through a cyclone collection. I understand the advantages and disadvantages of both systems but what I'm not able to understand is why they are insisting on this.
I also don't know the reason why the supplier is insisting on two bag filters. There are many cement mills operating with a single filter and a circuit layout similar to the one you envisage. If you refer the supplier to me I might be able to understand their reasoning. At this stage I cannot.
I have a few questions on a ospa separator for a finish mill we are operating:
1. We have a mill draft fan that is not a frequency drive and with the damper completely closed the lowest mill exit presser we can run is about -1.8. The problem being controlling mill temps, there are four dampers on the separator that are being use to cool the mill temp. From what I can see this disrupts the air flow in the separator and causes problems with the cement quality. Here's the question would it be better to introduce some false air in to the mill draft fans exit draft, lowering the draft on the mill, then opening the damper and controlling the mill draft from 0 to where ever we need it.
I need to understand your milling circuit better to make an informed comment. However it sounds to me that the four separator dampers are being opened to cool the cement temperature. This will contribute to cooling the mill exit temperature but only because the reject returns from the O-Sepa will be at a lower temperature. For sure there is a danger of affecting the efficiency of separation by this mode of operation. I don't see the advantage of introducing false air after the mill draft fan except to control the temperature of the air entering the filter. You don't want to reduce the ventilation and air flow through the mill as this will make controlling temperatures more difficult. Key questions: (i) do you have water injection on the mill? and (ii) have you developed a heat balance model of the mill? The latter will allow you to run some what-if scenarios and identify the best way forward. Question clarification We do have a water injection system, we have one spray on the feed end of the mill. We have not done a heat balance on the mill. The reason for introducing false air to the mill draft fan is the fan is to big. The air is actually introduced after the dust collector. And is only introduced to lower the efficiently of the fan, the fan damper is always closed, this allows the operator some control with the damper. The way the damper is made when it is closed it still allows air though the damper, so what ever mill exit pressure you have with the damper closed, that's what your stuck with. I don't like the damper control and would like to have a frequency drive on the fan or a smaller fan to lower the power cost. As an example of how the mill is being controlled. (This is just one scenario).... the blaine is high so the Lab tech will ask the mill operator to lower the separator speed. The operator then notices that the mill temperature is rising and opens one of the dampers on the separator this changes the air flow in the separator. It does begin to cool the mill off but the false air introduced to the separator lowers the differential pressure on the separator and allows more of the course material to return to the mill. Further answer I certainly agree that the separator drafting should not be the primary response to control mill temperature. That is the tail wagging the dog. The fresh air damper into the separator should be at a constant opening for a particular grade of cement. The amount open needs to be determined by trials but once the optimum is identified it should not be changed as this changes the separation characteristics. I also agree that adjusting the mill drafting is a better response. You need to be careful of this as well as it affects the amount of material pulled out of the mill with the ventilation air and the particle size distribution of the cement. Again trials to identify the optimum is the only way forward. Water injection is normally on the outlet of the mill first with injection into the inlet (feed) end when the outlet water injection reaches its maximum. The outlet water injection rate can be controlled in a PID loop with the mill outlet temperature and this loop works well on lots of cement mills. If you decide to go this route make sure you have good atomisation nozzles on the outlet water injection lance.