Kiln feed Question 1

I have a question with regards to the sulphur cycle and a method that I adopt to mitigate the effects. Usually in the raw mix we usually run 0.07 per cent sulphur all of a sudden the value increases to 0.12 per cent and the operations are upset. What I have done is to add sodium carbonate in the kiln feed bin to force an increase in the alkali content in the internal cycle. I usually add approximately 100-150kg. What do you think about this? I have often read that sodium carbonate should not be added to the burning zone. I don't know why they say this. All that I think of is that the sodium reduces the viscosity of the liquid melt, making it more penetrating towards the brick!

TXI have published various papers on their patented Cemstar process. The addition of steel slag or air cooled blast furnace slag boosts the output of the kiln with virtually no increase if fuel consumption as the slags have already effectively been clinkered and contains trace elements that mineralise the combination of the kiln feed into the clinker in the kiln. More product with the same amount of fuel burnt inevitably dilutes the NO_x emissions per tonne of clinker produced. The mineralising effect may also lead to lower burning zone temperature, and less thermal NO_x formation in the flame. The real crux of the technology is adjusting the chemistry of the kiln feed to maintain the desired clinker mineralogy when the slags are being added t the kiln inlet. With steel slag the iron oxide input is increased and has to be compensated by increasing the alumina and silca modulii. With air cooled blast furnace slag the material is deficient in lime therefore the LSF of the kiln feed has be increased. This inevitably increases the demand for high CaCO₃ content in limestone and is often the limiting factor.

I'm surprised that a rise from 0.07 to 0.12 per cent sulphur in the raw mix causes upset conditions. These are low values compared to many kilns whose companies add much more in the petroleum coke they use to fire their kilns. The sodium carbonate will dissociate in the kiln and the Na₂O will combine with the sulphur (as sodium sulphate) and pass out in the clinker. Your strategy is sound and based on known principles of the alkali cycle. There is no doubt that alkali penetration is one potential cause of refractory failure but at the levels you are using there should be no problem. There is wealth of material on the alkali cycle. The best reference I know is S.Sprung, "Technological problems in pyroprocessing cement clinker: cause & solution" published by Beton-Verlag.