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.

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 have recently conducted some raw meal burnability testing on samples with varying residue and C₃S. Results showed little dependence on these two variables. Following further investigation I was told burnability can be highly dependent on 45 micron insoluble residue. What are your thoughts on this? I am still reluctant to believe the low dependence on C₃S because in the kiln I am still observing significant fuel variations with changing C₃S.

The classical burnability index takes into account both the chemical composition (ie LSF, C₃S etc) and also the fineness and mineral composition of the kiln feed. The theory is that large quartz (SiO₂) grains and lime (CaO) grains are very difficult to combine in the kiln, and this is fairly well established. You can get the formula from the proceedings of the IEEE 2002 in Jacksonville.