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.

I am working in a grinding unit having a vertical roller mill for slag grinding (UBE VRM). Initially this mill is for only slag grinding but we are also able grind clinker. We are getting clinker from various sources. Because of this we are getting different output and power consumption. I want to ask you is this normal to get different output and power consumption? How much influence does the wear rate of table and rollers have on output and power consumption. Also I want to know what are ways to increase the output of the vertical roller mill.

It is normal to get different outputs from the mill with materials from different sources. This will depend on the hardness of individual materials. There are ways to increase the output from a vertical mill. These involve adjustments to the separator and the airflow through the mill. A number of case studies have been reported by Siam Cement where major increases in output were achieved by these means.

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.

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 to 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.

Please advise us that is it possible to use the combination of Ca(OH)₂ and CO₂ and waste of molasses (in sugar and lump sugar factories) for correction of raw material, instead of CaCO₃?

It is possible to use Ca(OH)₂ as a corrective source of CaO. I do not believe you would want to use CO₂ as this is one of the exhaust products from the cement manufacturing process rather than making a contribution to the final product. I have not heard of the use of molasses in cement manufacturing and can envisage two problems with this material: (i) it is likely to be sticky and present handling problems, and (ii) there will be some organic material in the molasses. This organic material will smoulder and burn in the upper stages of the preheater, or in the chain systems of long kilns and will give rise to CO and hydrocarbon emissions in the exhaust gases. To add this material safely to a cement kiln would require some means to add the molasses to the hotter sections of the process such as the precalciner or the kiln inlet of a preheater kiln or some mid-kiln firing system of a long kiln.

I need to know how and why companies use air cooled blast furnace slag in the manufacture of cement.

Air-cooled blast furnace slag can be added to the inlet of the cement kiln in the patented Cemstar process developed by TXI cement in the USA. I know a number of US cement companies are doing this. This is in the manufacture of cement powder. Another use is as an aggregate in the manufacture of concrete.

Could you please to explain the possibility of steel slag in cement industry as raw material if it was added in the kiln inlet ?

Steel slag can certainly be added to the kiln inlet to boost the output from the kiln. This process has been patented by TXI in the USA and is known as Cemstar. The limiting factor is the high iron content of the steel slag, which effectively limits the possible addition rate. This subject is discussed in more detail in the February edition of International Cement Review.

We have a problem related with high sand usage to make the consistent raw material mix. Sand addition for silica correction is roughly five per cent into raw material results roughly three per cent quartz content in the raw meal over 45 micron residue. Consequently the overall effect is frequent free lime formation in the clinker due to high quartz content in the raw meal, negatively effecting quality and high heat consumption in burning. In order to tackle with the problem following activities are already done
1. The burner pipe is replaced by a new design burner pipe with high momentum to improve burning and have stronger flame.
2. New coal dosing system is installed to have stable and accurate kiln fuel feed. Also with this system, we started to grind petcoke and other types of coal separately to facilitate burning. However the problem still persists.

I agree with you that the high quartz content above three per cent over 45 micron is the root of the problem. The new burner and the improved coal dosing can only be advantages but they are unlikely to solve this problem of low reactivity of the kiln feed. There are many white cement factories that are operating with very high silica sand additions. These would be the best references. My suggestion of a remedy would be to introduce duplex grinding where the rejects from the raw mill classifier are separately ground and reintroduced to the raw mix. This is a radical solution and requires process modifications to introduce, however it will solve the problem.