Process & quality optimisation in cement plant

Various process instruments have made the job of quality control relatively easy in cement plants. Dr Bibekananda Mohapatra takes into the details of various instruments and the parameters that can be controlled. This is second part of the article. Part 1 was published in the last edition.

The relationship between the burning condition of cement clinker and the ultimate cement properties is well established and being so, determination of burning condition is important in controlling the cement quality. Examination of a large number of cement clinkers from different plants and laboratory clinkers demonstrate that the ratio between the diffraction lines, corresponding to "d" 2.78+ (2?=32.2) and "d"2.74+ (2?=32.7) bears a distinct relationship with the clinkering process. The characteristic of these two lines (d values) is that both of them are combined products of alite (C3S) and belite (C2S) and in case of perfect crystallisation, they bear a ratio of 1 to 1.1 only, termed as "C" index [Lea, 1970]. From the examination of industrial clinkers from different plants, it is observed that to achieve normal cement properties, the "C" index must not be above 1.5 [Goswami and Panda, 1986; Goswami et.al, 1991; Goswami et.al, 2016]. Thus "C" index reflects the degree of crystallisation, and the higher the "C" index, lesser is the degree of crystallisation.

Accordingly, there exists a distinct relationship between the "C" index, burning condition and degree of crystallisation of cement phases. As the burning condition determines the cement quality, the "C" index can be used directly for predicting the cement properties. For example, during the production of the clinker with "C" index ranging from 1.6 to 3.7 (Table 3), the kiln atmosphere was very dusty, and the cement produced showed abnormal properties. Cement from kiln-2 and 3 was of normal quality, while that of the kiln-3 was the best product. This provides a good relationship between "C" index and clinker quality.

Table 3: ‘C’ indices of cement clinkers fromdifferent sources

Evaluation of clinker (potential vis-a-vis actual cement phases) Mineralogy of the cement clinker determines cement properties. It is well known that very often cement clinkers having almost identical chemical composition produce cement of varying properties. This generally happens because of changes in mineralogy due to the presence (absence) of some minor constituents, and mineralisers, a trace of which can alter the mineralogy of the clinker, which was not detected by the widely known oBogueo calculation, based on major oxide constituents.

It is generally observed that clinker phases determined by XRD and optical microscopy were different in comparison to Bogue calculations, indicating that the quality of clinker entirely depends on the phase composition, and could be determined more precisely only by quantitative XRD. Although, XRD and microscopic analysis produce almost similar values, the former is having number of advantages over the later [Goswami et.al, 1992]

During the use of petcoke as a prime fuel in cement plants, along with the main mineral phases of the clinker like C3S, C2S, C3A, C4AF, various other associated minerals are characterized by XRD like anhydrite (CaSO4), aphthitalite (3K2SO4. Na2SO4), arcanite (K2SO4), calcium langbeinite (K2SO4 2CaSO4) and thenardite (Na2SO4).

Overall sulphur content of the clinker increased in proportion with the amount of sulphur in the fuel. Reactivity of clinker also depends on the nature of sulphate bearing phase. The higher sulphur present in the clinker causes fine clinkerization, alite and belite grain size and changing the reactivity.

Higher amount of alkalis (Na2O & K2O), coming from the use of alternative raw materials and fuel during the production of Portland clinker, led to the formation of orthorhombic polymorphic form of C3A instead of cubic (detected by XRD), leading to the different reactivity and hydration characteristics. C3Aorthorhombic is more reactive results in shortening in setting time, problem in reactivity and difficulty in controlling rheology. C3Acubic/ C3A orthorhombic influences the water consumption of the cement and higher water consumption by C3A orthorhombic.

Microscopic evaluation of Portland clinker is another way to investigate quality of clinker, and cement properties by analysing the morphology (mineralogy as well as granulometry) of clinker and interpreting the OM images as given below in Table 4. Images of major grains observed in Optical microscopy are shown in Fig 4. Optical micrographs of some clinker samples are given in Fig 5.

Table 4: Microscopic images and their influence

Therefore, quantitative estimation of clinker phases through microscopy and XRD is very important in controlling and monitoring the quality of raw material as well as clinker. Monitoring and identifying mineral and morphological features can effectively control the process conditions.