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.

Your idea is a very good one. There are plants that do exactly what you propose. This has the advantage that the chemistry of the kiln feed is more stable because it is not destabilised by slugs of ESP dust being added to the homogenisation silo when the raw mill is not running. I am not sure that you will get more output but the kiln operation and clinker quality should be more stable.

NO_x control approaches applicable to the cement industry may be grouped in three categories: process modifications, combustion control, and NO_x reduction controls. One process modification that can be quantified is the CemStar process, which is a small addition of steel slag to the raw kiln feed. Can you provide more information on CemStar?

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.