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.

We are about to lose our traditional sand supplies for our raw mix. Now we are using 78-80 per cent limestone, 15-17 per cent clay, 1-2 per cent iron ore and 2-4 per cent sand. Now we are thinking to use low grade limestone and no sand. Our management says to use iron dust through kiln firing system in place of using in the raw mill grinding. Mix design is iron dust 0.5 per cent limestone 70 per cent, low grade limestone 18.5 per cent and clay 11 per cent. My point is any raw mix is made to be through raw mill grinding and only fuel through firing system. Let me know your opinion and suggestion.

As regards the enquiry, I have no experience of anyone firing iron ore through the burner. I presume they plan to use an insufflation pipe and not simply feed through the coal mill as this could lead to a number of problems. Many plants around the world insufflate kiln dust successfully without major quality problems but care has to be taken to ensure the dust can be properly absorbed into the raw mix and, of course, the chemistry of the dust is very similar to the kiln feed chemistry. I assume the plant is wet process and would suggest looking at back end insufflation as a possible alternative to ensure thorough mixing of the iron ore. If it is dry process then there is some comparable experience in Malaysia at the Rawang plant where they introduce their secondary raw material - oil shale - to get the benefit of the fuel value and thus rely on the precalciner and kiln to do the raw meal blending. To insufflate iron ore as a separate component will, in my view, be far from ideal and the following problems may be encountered:- * the kiln feed mix would be deficient in flux and hence difficult to burn and less able to absorb the iron * there would be the risk of chemical reduction of the iron ore in the flame to ferrous state which can increase the basicity of the mix by iron replacing calcium and hence artificially raising the LSF making the mix difficult to burn * there would be the possibility of increased coating and build-up

We are about to lose our traditional sand supplies for our raw mix. Now we are using 78-80 per cent limestone, 15-17 per cent clay, 1-2 per cent iron ore and 2-4 per cent sand. Now we are thinking to use low grade limestone and no sand. Our management says to use iron dust through kiln firing system in place of using in the raw mill grinding. Mix design is iron dust 0.5 per cent limestone 70 per cent, low grade limestone 18.5 per cent and clay 11 per cent. My point is any raw mix is made to be through raw mill grinding and only fuel through firing system. Let me know your opinion and suggestion.