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INTERNATIONAL CEMENT REVIEW JULY 2013

Buzzi Unicem has progressively installed COSMA material analyser systems in its cement plants in Italy

and the USA. The Festus plant near St Louis, Missouri, is the fourth system that has been installed by the Buzzi Unicem group and the second system at the Festus plant. The first COSMA system at the factory continuously measures the mineralogy of the cement from the cement mills, while the second system reports the clinker mineralogy in real time to the control room. This article focusses on the clinker analyser system and the benefits to production.

The heart of the clinker analysis system is the COSMA analyser. It is a patented X-ray diffraction (XRD) analyser designed to measure a continuous stream of clinker powder, reporting the results in real time. The advantage of this type of analyser is that it is fast and accurate and it reports the full and true clinker mineralogy. As a near-continuous and fully-automated sampling system it provides results that are consistent and available continuously in real time. These results are available immediately in the plant’s control system and are available to the kiln operators and the expert control system. While COSMA has the capability to produce results every minute, it was found that results updated every four minutes were sufficient to make changes in the process readily apparent. The fast analysis and short sampling times mean adjustments to the kiln are made much more reliable and operation becomes smoother, saving fuel and improving quality.

Most cement plants determine oxide analysis using XRF for compliance reasons and for product specifications. Also, Bogue equations are used to calculate ‘hypothetical compounds composition’ because this usually is the only method available for a plant to estimate the mineralogical composition.

This is indirect and can be an inaccurate estimate, particularly when using alternate fuels. The variation is often seen as varying cement strengths or setting times. The COSMA analyser measures the clinker minerals directly (C3S, C2S, C4AF and C3A) so any process- or feed-related changes can be detected quickly and compensated for. If required, these results can be used to adjust the raw material proportions, thereby closing the loop on clinker chemistry. With free lime being reported in real time as well it offers a fast and reliable update for expert systems that are managing the kiln. The analyser system The clinker sample and transport system is usually supplied as a complete turnkey system. However, in this case the plant decided to retain its existing clinker sampler. The circuit is relatively straightforward, and is shown in Figure 1. The key equipment is overlayed on a drawing of the factory to give an idea of where it is located in the plant. Once the

clinker sample is collected it is passed to a jaw crusher and then down to a sample splitter.

For clinker samples, a relatively large amount is required to ensure it is representative and so reducing its volume correctly is important to maintain the representativeness for analysis. While the collection of the clinker sample is discrete, taking about 10kg of sample every few minutes, the crusher, splitter, grinder and COSMA operate with a continuous flow of material passing through. The analyser unit operates with a continuous flow of sample passing the X-ray beam, at around 6kg/h, continuously accumulating data from the sample passing through it, and analysing several thousand times more material than a laboratory-based unit.

After the sample splitter, the clinker is screened and offered to a continuous ring mill (see Figure 2) for grinding to 80 per cent passing 90mm and continuously delivered to the analyser.

The COSMA analyser has a small footprint and is usually located in the

FCT ACTech discusses the benefits of online chemical analysis of clinker at the Buzzi Unicem Festus cement plant in Missouri, USA. This facility uses two online analysers that have been installed by FCT ACTech to improve clinker quality.

by Alexandra Bequette, Buzzi Unicem, USA and Peter Storer, FCT ACTech, Australia

IFestus’ better clinker control

Figure 1: overall layout of the automatic clinker analysis system showing the clinker sample path to the analyser

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plant near the sampling location. However, at Festus it was convenient to install it in the lab area (see Figure 3).

The sampling system is controlled by a small PLC situated in the room beside the analyser, based on requests from the analyser. After analysis the sample is returned pneumatically to a convenient location in the process, in this case it is the cooler. The analyser provides all of the mineral analysis data to the plant via ethernet cable so it is completely unattended. Remote external access is also available for viewing historical data or diagnostics. A local screen is provided which includes full trending of all results as well as the analyser’s status.

The information from the continuous analysis of the clinker is used by the plant in several ways, including providing direct feedback control to the kiln via an expert system, feed forward control to determine where to direct the clinker and, on a longer cycle, feedback control information for the raw mix, along with valuable insight into the kiln operation.

Advantages of mineralogical analysisOne of the first observations made with the online data is how the clinker minerals vary with freelime.

In the cement kiln system, the finely-ground raw materials are calcined and then combine at temperatures around 1450˚C to form the clinker minerals. When the blend and fineness of the raw materials is on target, the sintering reactions in the presence of liquid fluxing minerals form crystals of alite (C3S), along with smaller quantities of belite (C2S), ferrite (C4AF) and aluminates (C3A). Measurement of the residual free lime in the clinker has traditionally been used as an indicator of how well the kiln reactions have progressed. When the free lime is in the range 1-1.5 per cent the reactions are generally considered complete. If the free lime result is less than this amount then the clinker is considered ‘over-burnt’ and excess fuel has been used (increased costs) and the resulting clinker is often harder to grind as a consequence and therefore requires extra power (cost) for grinding into cement. Should the free lime result be higher than the target, then the clinker would be considered as ‘under-burnt’ and the full potential of the product would not have been achieved, while larger amounts of free lime in cement can cause quality concerns, which is to be avoided.

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Figure 2: a continuous ring mill grinds the clinker to <90µm ready for analysis

Driving progress

Contactemail: fi vesfcb@fi vesgroup.comTel. +33 (0)3 20 43 76 07

COSMA™, the system which optimizes the production effi ciency and product quality while reducing operating costs.

COSMATM, the Continuous On-Stream Mineral Analyser

On-line X-Ray diffraction

Mineral phases

Continuous analysis

Product consistency

Mill and kiln control

Automated quality & process control

Clinker, Cement & Mineral applications

Real-time

Fully automated operation

Data trends

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JULY 2013 INTERNATIONAL CEMENT REVIEW

INTERNATIONAL CEMENT REVIEW JULY 2013

In reality, while every effort is made to ensure consistency, there are variations in the chemistry of the raw materials, particle size and homogeneity, combined with kiln operational variations that mean occasionally there are short periods of time when the reactions may not complete as required.

By reviewing the real time clinker analysis from the kiln (see Figure 4) it can be seen that a high free lime excursion of around two per cent is associated with lower alite and higher belite content. The higher than usual free lime was a result of under-burning because the raw materials had not spent enough time at high temperature to allow the reactions to use up all of the available lime, hence an increase in the unreacted or free lime. There are many potential reasons for this, but it could have been as a consequence of fuel rate or heat value changes for example.

It is also possible to have high free lime excursions associated with the correct levels of alite and belite in the clinker. For example, when the raw material lime saturation factor (LSF) is higher than usual, it is possible to have high levels of alite without using up all of the available lime. In this case, increasing the burning time or temperature will do little to reduce the free lime, but can lead to over-burnt clinker which would be less reactive and harder to grind. While every attempt is made to ensure that the LSF of the raw materials is correct before entering the kiln system, in practice it is difficult to do this perfectly all the time. Coal ash content and composition can vary so on occasions the clinker may contain excess lime which

is unused and is measured as free lime. A situation like this is shown in Figure 5.

While these two situations are easily understood by cement process engineers,

it is clear that more complicated combinations of similar events may arise, with each situation needing to be dealt with on its own merits. The chemists and engineers at Festus have vastly improved their understanding of clinkering reactions since having the mineralogy results available in real time. The integration of COSMA data into the expert system has further improved the process.

ConclusionThe use of the continuous online clinker analyser has provided new information about the reactions inside the cement kiln, previously only dreamt of. At Festus the analysis from the COSMA analyser has been incorporated into the plant expert system for improved kiln control. Engineering studies are continuing to find new ways to use the data to improve the quality and reduce production costs. ___I

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Figure 3: COSMA clinker analyser (centre) with the sampling system control PLC, left

Figure 5: free lime levels (orange) increase as a result of excess lime in the raw mix rather than under-burning

Figure 4: a rapid increase in clinker free lime (orange) is associated with under burning of the kiln feed. A reduction in alite and increase in belite confirms incomplete reactions