There is no straightforward answer to your questions. For raw mix the optimum particle size distribution depends on the mineral composition of your raw materials. If you have a high content of quartz then you must have a very tight fine particle size distribution with low content of oversize particles where the quartz will concentrate. On the other hand with a homogenous raw mix with no quartz it is not beneficial to have too small a particle size distribution as this will increase the dust losses from the top stage of the preheater. Similarly with coal there is no hard and fast rule. It is said that the 90 micron residue should not be more than 50 per cent of the volatile content of the coal. Increasing the fineness beyond that is counter-productive. For cement the optimum particle size distribution depends on the cement performance characteristics that your customers have come to expect. Ring formation is caused by the formation of liquids in the kiln at a particular position. These penetrate the refractory lining and cause a localised thickening of the coating. Fe₂O₃ can be a cause of these problems and I am not surprised that raising the alumina modulus has solved the problem. Many cement factories operate with higher alumina modulus than 1.7 and I would recommend keeping the kiln feed mix design with the higher alumina modulus. A snowman is the formation of a large build-up on the first grate of the cooler where the clinker falls from the kiln rather than a build-up in the kiln.

We are analysing particle size distribution of raw meal, coal mill product and cement mill product through CELAS PSA having facility to determine 0.7 to 400 micron. Q1. What is the best particle distribution (on various micron sizes) of raw mill in terms of best mill performance, burnability and kiln performance and which plant in the world is producing? Q2. What is the best particle distribution(on various micron sizes) of coal mill in terms of best mill and kiln performance? Our coal ash is 28. Q3. What is the best particle distribution( on various micron sizes) of cement mill product to have optimum power consumption, best strength and good performance in concrete.

The composition of the coal has affected the melt content of your clinker. This is affecting the kiln torque because stickiness in the charge in the kiln has changed and is not climbing the rotating walls of the same extent. This also explains the increased density of the clinker. However, you are not suffering any other operational problems which is a good result with such a major change in operations. Congratulations!

We have a problem related with high sand usage to make the consistent raw material mix. Sand addition for silica correction is roughly five per cent into raw material results roughly three per cent quartz content in the raw meal over 45 micron residue. Consequently the overall effect is frequent free lime formation in the clinker due to high quartz content in the raw meal, negatively effecting quality and high heat consumption in burning. In order to tackle with the problem following activities are already done
1. The burner pipe is replaced by a new design burner pipe with high momentum to improve burning and have stronger flame.
2. New coal dosing system is installed to have stable and accurate kiln fuel feed. Also with this system, we started to grind petcoke and other types of coal separately to facilitate burning. However the problem still persists.