Our facility operates two Humboldt air-swept ball mills for raw meal production. Combined capacity is 100tph. We would like to optimise the raw meal circuit, in particular, mill loading. Unfortunately, there appears to be very little information on the optimisation of air-swept mills. How can mill throughput be improved, utilising existing equipment? The separator is a mechanical classifier, with adjustable vanes. Current reside set point is 15 per cent on 90 micron. We currently use sonic ears to determine mill loading, with mill and fan current also available. There is also a proposal to implement a grit return weigh system to determine the recirculating load. Would this be worthwhile addition, with any positive process benefits? In summary, we wish to determine the most cost effective means of attaining optimal material levels in the milling circuit, with the intention of automating the operation sometime in the future.

If you are adequately controlling the kiln feed residues to 15 per cent I am not sure what the grit return weigher is going to tell you. Will you be able to remotely adjust the vanes of the mechanical separator from the control room while the mill is running? If not I would suggest periodic sampling and measuring residues around the circuit is all you need to do to determine the rejects rate. What do you mean by "optimise" the raw mills? Do you need more raw mix output? Is there sufficient drying capacity in the mills? I presume they are swept by preheater exhaust gases?

How does the content of chromium have influence in quality and performance of grinding balls? Is it better a low content of this metal to improve hardness in balls?

If the grinding balls contain chromium then they are forged, hard grinding balls. If the balls don't contain chrome then they are softer mild steel and the wear rate will be higher. Balls containing chrome cost more but last significantly longer and maintenance of the optimum media loading in the mill is easier. Many cement companies use soft media for raw grinding as the wear on the steel simply increases the iron content of the raw mix slightly. Most cement companies use hard media for cement grinding as the wear rate on soft media when grinding clinker is too high and the mills need continuous recharging.

I am making studies for cement plant and would like to make a following question? Is possible to control the weight in the ball mill (cement or raw mill plus balls) in order to save energy? My view point is if you have any chamber unbalanced in ball mill, the motor drive can be working in a forced way and spent much more energy than the necessary? Is possible to use the balls to correct that balance, because today we have in the cement plant a different weight in each chamber?

The weight of grinding balls in the mill and the power drawn by the mill main motor are directly proportional to one another. Therefore it is possible to reduce the power drawn by the mill by reducing the ball charge. However, this will then result in the output of the mill falling due to less grinding balls being present to grind the clinker and generate surface area. As with all these things there is an optimum ball charge and output which corresponds to the lowest unit energy consumption.

In order to optimise the performance of a ball mill, is there any suggested air velocity through the mill that we should use? Our cement plant has some very old ball mills. One in particular is a 4.572m x 13.9m two compartment mill, closed-circuit, with a Sepax separator 2S-375 separator. The separator and mill are vented by the same dust collector. We are currently running a high mill outlet ventilation on the mill, greater than -4" W.C. What is an acceptable mill ventilation static pressure range for this type mill circuit?

1.5 to 2 m/s air velocity through the mill is normal for a closed circuit mill.

In our plant, we have a typical problem of high wear rate of mill liners. The basic reason identified is the percentage of free silica in silica (one of the components of raw meal) is very high about 50 per cent and the consumption of this material is also very high (around 14 per cent) due the very high quality of limestone (low grade limestone, clay or shale is not available in near vicinity) and thus making kiln feed free silica around 9 to 10 per cent. We are of the opinion that the recirculation of the O-Sepa is causing a cummulative effect on the percentage of free silica inside the mill, which is the reason why we see that the total life time of the diaphragm plates is continually reducing (from six months to four months to three months). We would like to know, are you aware of any recent developments in design which can provide an arrangements to remove this high density coarse grain material out of the mill to avoid high abrasion of mill liners and diaphragm plates?

The silica will inevitably build up in the recirculating load of the mill as this is the hardest component of the raw mix. The limestone is preferentially ground leaving the silica in the rejects. Unfortunately I don't know of any means to avoid this.

Our plant has four cement mills (closed circuit with first generation seprator) - 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.

I have two questions: What is the effect of NO_x and SO_x in controlling the burning zone temperature and what other parameters like kiln torque, kiln inlet oxygen or if anything else affects the kiln operation.
Secondly, how to improve the ventilation in ball mills, as we observe the coating formation on the balls is on the high side.

NO_x and kiln torque are both good indicators of the temperature in the burning zone of the kiln. This is key to ensuring stable operation and good combination of the clinker. Kiln inlet oxygen is a good indicator of the combustion conditions in the kiln where a small excess of oxygen is necessary to ensure burning in oxidising conditions. SO_x rises if there is reductive burning and CO present in the burning zone therefore this reinforces the kiln inlet oxygen measurement.
To improve the ventilation in the ball mill you need to increase the speed of the induced draft fan. However if you are suffering coating of the media this is more likely related to temperature control in the mill. This is a function of the clinker temperature, ventilation and the cooling with water injection.

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.