Cement, Energy and Environment

Table-1 Three chemical parameters for cement clinker characteriziation typification A typical raw mix using the average analyses for each of these materials is given in Table-2, Mix 1. The mix requires 83.4 per cent of the raw feed to be Lime Saturation Factor (LSF)= Ca0/2.8 X Si02 + 1.2 X AI20 3+0.65 x Fe203 Silica Ratio (SR)= Si02/(AI203 + Fe203) Alumina to iron ratio = AI20 J!Fe20 3 0.94 to 0.98, SR from 2.2 to 2.8, AF from 1.4 to 2.0. The purpose of the raw materials other than limestone is to bring the total raw mix to the required values for these ratios. In some areas the formulae or names for the parameters may vary but they will be close to those above. An example is given below of a potential quarry site and the steps required to determine its viability for cement production. Characteristics of the limestone A geological report prepared some years earlier had indicated that the limestones proposed for the cement raw material were of two types; oolitic, with a thickness of 3.5m to 9.5m, and, overlying the oolite a bioclastic (shelly) limestone ranging from 12 m to 16.5m thick. The chemical analyses of cored samples from boreholes were not split by limestone type, but comparison of the composition with depth and study of the logs indicated that the oolitic limestone contained a higher proportion of silica than the bioclastic limestone. This was supported by other local information suggesting that the lower part of the deposit had an especially high content of silica. The silica ratio of the limestone increased with depth, reaching levels of 15 and over at the base of some boreholes, thus limiting the usefulness of some of the deposit unless of adequate blending of the two types could be achieved from the outset. Without this, a silica ratio below three in the cement clinker would be difficult to achieve using just the oolite and the locally-available clays. Some typical blends with various combinations of the materials are given in Table-1 , using averages of the compositional data available fore each deposit in the mix calculations. Characteristics of the clay materials The locally-available argillaceous (clayey) deposit was a variable material overlying the limestone, comprising clays and loams with sandy layers and marls. The compositional data available indicated that it would not be possible to make a cement clinker with a silica ratio below 3.5 using only the limestone and these clays. Another material available for reducing the silica ratio was a mudstone. This was characterized by higher alumina content. limestone. With about 46,000 t of limestone available, a plant producing 1Mta would require approximately 1.3 Mta of limestone, giving a life of 35 years for the limestone deposit, assuming the whole deposit could be adequately blended and utilized and making no allowance for moister in the raw materials. Although a usable mix could be made with these materials, the alumina-to-iron ratio of the resulting clinker would always be in excess of 2.5 with no other additives. This would make combination of the raw feed in the kiln difficult and would limit the output of the kiln. The cement would be of a light colour. An additional material with high iron content would be desirable at a level of up to 1.5 per cent of the raw mix. Typically wastes from steel plants are used for this purpose in cement production and the mixes containing iron oxide in Table-2 use a typical composition of such a waste. Table-2 Preliminary examples of mixes from the raw materials Limestone (average) Bioclastic Oolite Argillites Mudstone Ironstone Clinker composition LSF SR AF 83.4 3.9 12.7 0.96 2.5 2.63 82.8 9.3 6.9 0.96 2.5 1.6 79.8 12.1 7.2 1 0.96 2.5 1.6 85.8 1.2 12 1 0.96 2.5 1.6 16