What is the best method to charge a ball mill ?

The normal method to charge balls into the ball mill is to position the doors at the top of the mill and remove them. Then to pour the barrels of grinding balls into the mill using an overhead crane. The exact grading of media required is identified by performing an axial test taking samples along the axis of mill. You want the desired fineness to be reached at the exit of the mill and adjust the ball grading to achieve this. For a new mill you need to rely on the experience of the commissioning engineer to determine the initial ball grading. It is normal practice to begin commissioning with only 65 per cent of the balls in the mill and bring this up to target by additions with monitoring between additions by axial testing.

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.

We are using compressed air from Twin Lobe blowers for extracting fly ash from a 400t and 40t capacity hoppers equipped with open airslides at the bottom floor of the hopper. Please let us know whether the compressed air from the twin lobe blowers need to heated since at times flow problem occurs due to clogging.

If the fly ash is calcareous then there is the possibility of hydraulic reaction between the fly ash and any water present in the compressed air. My suggestion would be to install dryers on the compressed air supply.

Our plant has four cement mills (closed circuit with first generation separator) - we use about 20 per cent slag, the output is 100tph ,the dimension of every mill is 15.5 x 4.4, the liners in the first chamber are lifting and in second chamber classifier. Are there any ways to increase mill production? How about for a raw mill?

There are numerous ways to increase the production from your cement mills. In the short term use of grinding aids combined with optimisation of the ball charge and drafting would be the best method. Longer term (and at greater cost) you could upgrade the separators to third generation or install a roll press to pre-crush the clinker ahead of the mills. With regard to raw mills you can again use grinding aids or install pre-grinding equipment.

We are producing sulphate resistant cement clinker. We believe from the analysis of the clinker that we could also produce oilwell cement as per API standard. Do you have any comments (analysis attached).
Secondly, our ALM has been reduced from 1.25 to 0.70 while keeping LSF as 0.90 and SIM 2.25 we are facing following problems:
1.Ring in inlet causing inlet spillage
2.Thick coating at 27-28m hampering production How can we overcome these problems?

Some classes of oil well cement have low C₃A content and are coarsely ground. The key to producing oil well cements is the testing that is required to achieve the certification. You need to investigate the costs of installing the equipment to conduct this testing on a regular basis and then apply for certification to produce oil well cements. The ring and coating problems are caused by the high Fe₂O₃ content of the SR raw mix. I suggest you increase the silica modulus of the raw mix.

Are there any recognised ways to increase raw mill production?

The first stage is to maximise the output with the existing mill. You should conduct axial tests and recirculating load tests to make sure that the ball charge and lining are optimal. The next stage might be to conduct trials with grinding aids to try to boost the output of the mill by up to 15 per cent. Beyond that you will need to consider modifications to the milling circuit by adding further equipment. There are two options to do that: (i) add a pregrinding facility such as a hammer mill or roll press, or (ii) add a regrind mill. Adding further equipment will be significantly more expensive than introducing grinding aids but has the potential to boost the output to higher levels.

How can we reduce the clinker size -5mm >35 per cent (existing),we want to reduce to under 20 per cent. Our plant is a 1Mta capacity unit. Fuel is pet coke 70 per cent balance Indian coal.