I work at a plant 4500tpd, based on FLS design, with a five-stage preheater and precalciner, Our problem is repeated blockage in cyclone stage five (some four times at period of five months). We calculated the sulphate/alkali ratio before each blockage, we found it between 1.45 to 1.7. As we know the normal value should be less than 1.2. The LSF between 92 and 105, also the analysis result of a sample from the blockage material are: SiO₂=19.5, Al₂O₃ =4.76, Fe₂O₃ =3.63, CaO=62.1, SO₃=2.31, K₂O=1.37, Na₂O=0.93. Can you advise if the values of SO₃/AlK ratio to cause the blockage in Cyclone 5 or is it another reason?

From the information you have sent there seem to be two problems, (i) the SO₃/Alkali ratio between 1.45 and 1.7 is very high and would be expected to cause very hard dense coatings, (ii) the variability of the LSF from 92 to 105 is much too high and would be expected to cause major process and quality variability. However. the samples from the blockage do not show very high SO levels and have an SO₃/alkali ratio of 0.87 which should not cause any problems. The other cause could be premature calcination in the cyclone 5 due to variable LSF. This would happen with the change to low LSF, easy burning material.

We wish to know about oxygen enrichment in the cement kilns/precalciners. Please brief us about the expected merits and demerits. Are there any cement plants using this technique? If yes, then why is it not used commercially on regular basis in most of the cement plants.

This is a big question! The merit is that the output of the kiln can be boosted by up to 15 per cent, the demerit is that the oxygen is expensive and therefore the costs of producing the additional clinker are higher. If market demand exists with good prices for cement then oxygen enrichment can be a good option. There are a number of kilns in the USA using oxygen enrichment of either the main burner or the precalciner. It is worth noting that CEMEX USA consider oxygen enrichment of precalciner firing to be their technology for which they have lodged a patent. You can find papers on oxygen enrichment in the proceedings of the IEEE conference in Vancouver 2001.

Thanks for your reesponse on our cracking problems. We have ordered a new cone and in the mean time we have welded the effected part and have used dye penetration test for finding the magnitude of crack. But what in I am interested is the reasons;thats why such cracks develop, either due to misappropriate operating, startup /shutdown procedures. improper jackingup the mill at the startup time, overdue servicing of trunion etc. Or only due to matelurgical/ casting failure. Furthermore please let me have the diagnoses procedure/test for the above.

The main reasons why these cracks occur are due to the high stresses that the mill end castings are subjected to and therefore metal fatigue. Most suppliers have taken steps to reduce metallurgical or casting failures as much as possible, but pre-delivery inspection and testing would be one step. Many problems are caused insufficient care by the maintenance department when welding, grinding the mill shell, or cutting bolts/drilling holes for fixing bolts. Of course there are steps you can take to during operation to mitigate against these cracks developing, (i) broken wear protection plates must be replaced immediately, (ii) temperature monitoring. Ultimately there is no alternative to frequent and rigorous inspection.

Could you please advise us with optimum length for preheater cyclones inner pipes for increasing cyclones separation efficiency and increasing decarbonation degree. Because we have kiln capacity 5000tpd, kiln length 100m, kiln diameter 5.0m, preheater two-string, four stage cyclones, calciner system and bypass system.

Varying the length of the dip tube in a preheater cyclone will have two effects: (i) the cyclone collection efficiency will be improved which is a good thing, however (ii) the pressure drop across the cyclone and the whole preheater will be increased. If your kiln runs with the induced draft fan close to maximum speed then the increased pressure drop will create a restriction on the process. Because of these conflicting effects it is not possible for me to tell you an "optimum" length for the dip tube in a cyclone. This depends on other factors in the process and the kiln, and also whether you want to push the kiln to the highest output possible.