While our plant was designed in 1998, the necessity of preblending stacker and reclaimer system was not incorporated based on the ground that run of mine limestone quality is very consistent(CaO standard deviation 0.60 per cent) and silica, iron or alumina oxide are very less in the range of 0.6,0.1,and 0.3 per cent respectively. Our raw meal is four-component limestone, silica rock (75 per cent purity as SiO₂),iron ore (55 per cent purity as Fe₂O₃) and bauxite (48 per cent purity as Al₂O₃). Now since the beginning of this year from the mines receipt limestone CaO is reduced from 54 to 51.5 per cent and at the same time silica, alumina and iron oxide increased to the level of 3.5 to 4.00, 0.8 to 1.1,and 0.25 to 0.35 per cent respectively. The standard deviation for LSF on three random eight-hourly samples we are getting in the range 40 to 90. I would appreciate to have feedback from you which is the level of variation in run of mine limestone making it necessary for installation of preblending stacker and reclaimer system for a one million tonne dry process cement plant.

I do not believe a stacker/reclaimer should be necessary in your situation. With a good software system proportioning the materials into the mill you should be able to achieve less than two per cent standard deviation in kiln feed and clinker LSF. You have another big problem with your mix because it is heterogeneous and therefore of very low reactivity. Do you have problems with high fuel consumption or excursions of high free lime in the clinker. I would recommend strict control of the mineral composition of the clinker and possibly the introduction of Duplex grinding.

We are facing serious problem of build-ups at stage four riser duct starting from kiln housing and at start of mix chamber it becomes to thick and remains thick through out the riser duct so that opening remains only 30-40 per cent. Some coating has also observed at cylindrical part of cyclone and at its roof, but coating at riser duct becomes very tough and hard and similar to sintered material. My first question: is there any special method to get rid off it during operation as normal pocking do not results any way. Is there any chemical treatment, operational technique or physical methods. Secondly, what could we do to prevent such things.

It is no surprise that you have problems. Your instrumentation and control technology is inadequate. I am surprised that you can operate a large modern kiln without coal flow metering equipment to the main burner. I think your problems are caused by an excess of sulphate over alkalis. This results in the very hard coating in the riser duct. The best solution is to increase the alkli content and pass the alkali sulphate out in the clinker. However, with your inadequate coal flow control there is likely to be reductive burning and then there will be a heavy sulphate cycle in the kiln. This is also likely to be contributing to your current problems. You could try installing acoustic cleaners to try to prevent the sulphate build-up in the riser. The high temperature excursions in the precalciner and stage four cyclone are caused by interruption in feed down the preheater tower caused by the build-up. Solve the build-up problem and you will solve the high temperature problem. For the oil compensation you need expert control, however it is doubtful if your general control can be the basis for expert control.