We've been faced with a problem that the collecting efficiency of the electrostatic precipitators lowers remarkably since having started to use sewage sludge as the secondary material. I'm not sure where the cause is from. Is there any possibility of causing such problem by using sewage sludge, and which kind of components that sewage contains can affect the performance of ESP? As the reference, we've been experienced in the same problem when we used TPA sludge(Telephtalic Acid) two years ago.

I have not heard of this phenomenon before. The resistivity of the dust exiting the preheater must be changed when burning these sludges. I hope all the sewage sludge is being burnt in the kiln and the problem is not caused by organic residues entering the ESP. If so then the problem can only be associated with volatiles derived from the sludge entering the ESP with the dust. The most likely source is chlorides from the sludge. You should also check carefully that the problem is not caused by mercury from the sludge. If so then you have a big problem.

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.