Crushing Question 1
What should be the standard sieve sizes for limestone feeding to Vertical roller mill. We were earlier using Limestone having bond index of 8-9kWh/short tonne. Now we are having limestone with bond index 11-14kWh/ short tonne. Wether any reduction in mill feed size will improve the output of the rawmill. How to calculate finess modulus of crushed limestone and what is the optimum value for VRM.
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Yes, reducing the size of the feed to the raw mill should result in an increase in output. Bond's equations use the size grading where 80 per cent of the feed is passing to characterise the size of the feed to the mill. The optimum size of mill feed is related to the nip angle between the rollers and table. 60mm is quoted as the maximum size.
Crushing Question 2
Why does false air infiltration from the bottom stage of a preheater tower increase tower temperature, but in top stage decrease the tower temperature. The preheater system is SLC-S supplied by Fuller. We understand this is a common phenomenon for any other system like ILC, SLC or SLC.
We have received some suggestions regarding the possible effects of dam ring height from a major cement plant in North America as follows:
"I would suggest that the particle size distribution (PSD) be looked at for both instances. We have found that by changing internal settings in a VRM there are physical changes in the PSD which will impact the performance of the slag. By changing the PSD as little as 2 microns there is a significant change in the performance while not showing up in a simple test of Blaine." "If the dam ring height is increased the clinker and gypsum will stay on the grinding table longer and possibly be better ground when it exits the table and therefore the circulating load in the mill should be lower.
So the two things to look at are the amount of dehydration of the gypsum with lower dam ring height compared to the higher dam ring height and also the particle size distribution of the finished cement. With a lower dam ring there is the chance that there is more over grinding and a higher concentration of superfines."
Please let us know if you find there is a relationship between dam ring height and particle size distribution or gypsum dehydration as this would make an interesting case study for publication.