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Rapid free lime analysis for environment friendly cement production

World Cement,

Kiln operation is at the heart of the cement production process. At the same time, from a chemical perspective, the free lime analysis is at the heart of the kiln operation. Free lime is an important measure of the extent of the clinker producing reactions and an important predictor of the quality of the cement that will be produced. The free lime information allows adjustment of the balance between product quality and fuel consumption and CO2 emission. In a world where we there is increasing environmental focus, not only CO2 emission is important, but also emissions like NOx, SOx are increasingly important and regulated. While free lime analyses do not directly measure these emissions, good burning zone control, aided by free lime analysis, is critical to minimising these and other emissions.

These factors combined with a higher degree of chemical variation of the input materials create the need for tighter and faster process control. For free lime analysis, this means that an analysis once per shift or day that was sufficient in the past is replaced by higher frequency analyses - once per hour or more.A well known technique for gauging the free lime content in clinker is wet chemistry. It is according to the norm and it is a well accepted technique.

However, it has major drawbacks. One of the most important is that wet chemistry is very time consuming. Typically an analysis takes about 30 to 50 min. when done properly. For high frequency process control, hourly analyses will require a significant amount of manpower. The method is difficult and expensive to automate. Moreover, this technique requires some skill of the operator and results are operator dependent so that reproducibility of the results cannot be guaranteed. Lastly, other common minerals in clinker such as portlandite, K2SO4 and MgO interfere and can make the analysis inaccurate.

A more rapid way of probing the free lime content in clinker is with X-ray diffraction (XRD). With typical measurement times of about 5 min. XRD allows high frequency process control. Furthermore, it is an accurate method; results are highly reproducible, operator independent; and automatable. Two practical forms of XRD exist for free lime analysis: 1) full pattern X-ray diffraction; and 2) a free lime channel.

Full pattern X-ray diffractometers for cement processing are typically used in conjunction with an X-ray spectrometer (XRF), such as PANalytical’s CubiX3 Cement and Zetium TWIN. Together they provide the necessary information on the elemental composition (XRF) and the mineralogical composition (XRD). In addition to free lime, full XRD yields also all other minerals that are present in the clinker, such as alite, belite, aluminate and ferrite. As it gives the information of all minerals in the sample, full pattern XRD is also suited for quality assurance of the final cement. The results can then be used for benchmarking against the classification norms like EN-197 and ASTM C150.

Rapid free lime analysis

A free lime channel only focuses on the quantification of free lime in the clinker. Although the amount of mineralogical information is less than with full pattern XRD, its compact design allows full integration in an X-ray spectrometer. An example of such a system is PANalytical’s Cement Edition of Zetium. Here a full WDXRF instrument is combined with the THETA free lime channel. From a chemical point of view, one can analyse the most essential process control parameters with a single instrument with modest floor space and infrastructure requirements.

An important aspect of free lime analysis in clinker is that the surface of powder samples can quickly hydrate with the free lime converting to portlandite. With X-ray analysis one typically probes only the surface of the sample, so that the free lime reported is often less than the actual amount of free lime in the sample. For full pattern X-ray diffraction this is not an issue as the portlandite is also measured.

For a free lime channel, however, surface hydration is critical as only free lime is measured. In order to solve this problem, the THETA free lime channel uses high energy radiation that is transmitted through the sample, probing the complete volume. With this “Total Volume Analysis” the effect of surface hydration is virtually eliminated guaranteeing a more accurate measure of the free lime content, and more effective process control.

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