The only way to change the clinker size will be to change the kiln feed chemistry or the operation of the kiln. To have less fines in the clinker you should try to increase the flux content by reducing the silica modulus. Alternatively, adjust the rotational speed of the kiln by slowing slightly to increase the residence time.

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 & power consumption. I want to ask you is this normal to get different output & power consumption? How much influence does the wear rate of table & rollers have on output & 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 involved justments 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.

I would like to know how to estimate the quantity of grinding media of each size required for a ball mill using the data; mill size, number of chambers and type of material to be ground. Kindly explain me what formula is to be used.

The total quantity of media required for each chamber is determined by calculating the internal volume of the chambers. A rule of thumb is to change the first chamber to 30 per cent of this volume and the second chamber to 27 per cent. You can use 4.6t/m³ as the density of grinding media. As for the individual ball size gradings this has to be determined by conducting a series of axial tests in the mill while you are building up the media charge.

What would be difference in terms of clinker burning and combinability of a raw meal containing sand of low quantity (below three per cent) compared with a raw meal containing of high quantity (about six per cent) sand. In both cases the raw meal residue was kept below 14 per cent on +90mm.

You really need to analyse the composition of the sieve fractions, ie +125 micron, 45 to 125 micron and sub 45 micron for SiO₂ and CaO. I expect the silica is concentrating in the coarser fractions and that this becomes more severe as the amount of sand in the mix increases. So even if the raw mixes have the same +90 micron residue I expect that the burnability of the higher silica raw mix is more difficult.

In our raw mix, iron ore is used as an additive. Its purity is around 94 per cent and it is in the proportion of 1.5 per cent in the raw meal. There is an iron rich material that is a collection of bag house fines from a steel plant. In this Fe₂O₃ content is around 50 per cent and also contains around 20 per cent zinc oxide. We want to know the effect of ZnO, on the burning process, brick life and on the quality of the finished product OPC, in case if use this material in our raw mix.

You will need up to three per cent of the steel plant bag house fines to replace your iron oxide additive due to the relative Fe₂O₃ contents of the two materials. That means the ZnO content of the raw mix will be about 0.6 per cent and almost 1 per cent in clinker. The ZnO will mineralise the clinker formation, reducing the fuel consumption of the kiln and will be incorporated in the clinker minerals, C₃S, C₃A and C₄AF. The lower burning zone temperatures should mean that the refractory brick life is improved. You can expect that the hydration of the final cement will be retarded therefore setting times will increase and early strengths will be lower. Perhaps the longer setting time will also be an advantage.
My recommendation would be to adjust the raw mix chemistry to raise the LSFand AM and compensate for these changes. This will reduce the amount of baghouse fines required and therefore the ZnO additions. The mineralising effect of the ZnO will allow a higher LSF kiln feed to be combined in the kiln. At the same time C₃S and C₃A content in clinker and cement will be increased offsetting the retardation effects of the ZnO.

I would like to ask about variable chloride values in clinker which can vary from 0.003 to 0.05. Also what will happen if the burner is operated in a reducing atmosphere and how to ensure that the main burner is always operating in an oxidising condition.

The first thing you must do is tie down the source of the variable chloride input to your kiln. This must be in one of the raw materials or the fuel. If one of the raw materials perhaps you can solve the problem by selective quarrying. Operating the burner in a reducing manner will not affect the chloride recirculation but will greatly enhance the sulphate recirculation. If you have any incidence of CO at the kiln inlet then you are operating ina reducing manner and are likely to have sulphate based problems. The way to avoid this is to adjust the flow of fuel and primary air to the burner.

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.