1156 posts
Cement milling Question 25
We have three 120tph cement mills. All of them are equipped with Sepax separators. We are looking at reducing the power consumption in cement grinding by installing roller presses. For market reasons, any increase in capacity is not necessary. The downstream conveying equipment are also limiting factor for increase in capacity. Under such constraints, in order to use most of the existing equipment but without heavily investing into new ones, which roller press grinding process would be the best for us to achieve our objective?
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1156 posts
Re: Cement milling
There is not a straight-forward answer to this question. There are many possibilities and circuits for combinations of ball mills, roller presses and Sepax separators. You need to make a study of the performance of these various combinations and decide which one is best for your factory and particular constraints. A key questions is whether the circuits are only grinding clinker and cement or whether there are other requirements such as slag, fly ash or pozzolan? As a first suggestion a roll press might be used to pre-crush all the clinker into an intermediate storage which then feeds to two of the existing three ball mills. With pre-crushed clinker the two remaining mills should be able to more than cope with your existing production rate.
1156 posts
Cement milling Question 26
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.
1156 posts
Re: Cement milling
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.