We have a problem in determining which residue is correct, either the raw meal or the kiln feed. In fact we do get about 13 per cent residue on 90 micron for raw meal and when it comes down to kiln feed it is about two per cent higher. Could you be kind enough to explain why this phenomenon takes place and what actions could we attempt to resolve the differences.

The kiln feed normally contains the dust from the precipitator in addition to the raw meal and therefore the two samples are quite distinct. The kiln feed is normally finer than the raw meal but this depends on the exact circuit used on the particular factory. Your situation is different and might be explained by drying the raw materials in the preheater exhaust prior to precipitation. In terms of quality control the raw meal is the residue you should be controlling. The kiln feed sample is within the external dust recirculation circuit. If you send more details of the circuit I can explain better.

If air cooled slag is used as a raw material for kiln , replacing uncalcined calcium carbonate, will the calcium silicates, aluminates, etc. revert back to free state, so they can reform clinker crystals of cement design proportions? Will the existing sicates go to C₂S state with the increased temperature and time. In other words could this slag combined with correction proportions of silica, iron, alumina, calcium be used as a primary raw material for a kiln feed? If there is a reversion of the state of the compound will there be a requirement for additional heat (as opposed to the exothermic heat released to compound formation) ? Would there be any big colour changes if air cooled slag was used as a primary raw material.

The process of adding air-cooled slag to the kiln inlet has been patented by TXI under the name Cemstar. The minerals in the slag do not convert back to the free state. They are already intermediate states in the formation of Portland cement clinker and all that is required is the completion of exothermic combination to C₃S. No additional heat is required, also there are no reported changes in the colour of the cement subsequently made from the clinker.