Elemental Analysis of Cement quality control of cement - from crushing/grinding to elemental analysis

To produce high-quality cement, the mineralogical and chemical composition of raw materials as well as intermediate and finished products has to be determined. At each stage of the production, samples have to be taken, processed and analysed to ensure quality control without gaps.

RETSCH offers a range of instruments suitable for sample preparation during the complete production process, from the quarrying of the raw materials to the final product. The typical sample preparation process involves preliminary size reduction, sample division and fine size reduction before the sample can be submitted to further analyses.

Elemental analyzers using combustion technology are ideally suited for the fast, accurate and reliable determination of carbon and sulphur in cement. Apart from size reduction, no further sample preparation is needed.ELTRA’s elemental analyzers measure the concentrations of carbon and sulphur from ppm levels up to 100%.

Fine grinding

Now the part sample can be ground to sizes of 100 μm and smaller, either in a ball mill or in a vibratory disc mill such as RETSCH’s RS 200. This mill is typically used for sample preparation to XRF analysis and delivers grind sizes down to 100 μm and smaller in as little as 30 to 40 seconds. The grinding results of the RS 200 are highly reproducible. The pulverised sample can now be analyzed by XRF or elemental analysis. Solid, high-quality pellets are an important precondition for reliable and meaningful XRF analysis.

With the PP 40, RETSCH offers a pellet press which produces strong pellets with a smooth surface.

Measurement of carbon and sulphur concentrations

A reliable determination of the carbon and sulphur content in cement is an essential part of the quality control process. If the sulphur content is too high, the cement could be destroyed as a result of the chemical reaction of sulphuric acid. The carbon content is usually certified as CO2 and sulphur content is given as SO3. In general, elemental analysers combust the sample and measure the released gases. Inorganic samples with a low carbon and sulphur content such as cement are usually combusted in an induction furnace at temperatures up to 2200°C. Table 1 shows typical results for CO2 and SO3 concentrations in a cement sample and also in wood and coal which are both used as fuels in many cement factories.

The results were obtained with ELTRA’s CS-i analyzer, with very low standard deviations. The system works reliably and quickly with a typical measurement time of only 60 to 120 seconds.

Sample preparation and analysis

As secondary fuels are often soft and elastic materials, primary size reduction has to be effected with cutting and shearing forces such as are applied in cutting mills. Here, the same grinding procedure as for hard and brittle material has to be followed: Pre-cutting followed by sample division and fine grinding. Since the secondary fuels usually are very heterogeneous, the sampling process becomes evident for correct analytical results. RETSCH’s heavy-duty cutting mill SM 300 easily handles inhomogeneous sample materials with different breaking properties and is especially safe to operate. It is available with heavy-metal-free grinding tools for neutral-to-analysis sample preparation which makes it ideal for processing secondary fuels. With the three exchangeable cutting rotors and its high powered drive, textile and leather parts, plastic caps and ductile metal cans are cut up just as effectively as abrasive electronic scrap and non-metallic car-shredding parts. As there are limits to this cutting mechanism, i.e. steel and iron parts should be separated by a magnetic separator before the cutting process. The final fineness is mainly determined by replaceable bottom sieves with defined aperture sizes. Depending on the sample material, final particle sizes < 2 mm can be achieved. A cyclone increases the material throughput and helps to dissipate the grinding heat. For the fine grinding down to 200 μm, the Ultra-Centrifugal Mill ZM 200 from RETSCH is the ideal tool. It grinds extremely fast and thanks to the patented cassette system it is very easy to clean which allows for high sample throughput. The sample size again is determined by an exchangeable sieve. Experience shows that aperture size of 0.5 or 1 mm is fully adequate for obtaining analytical fineness. Very elastic material like rubber has to be embrittled with liquid nitrogen before pre cutting and final grinding to avoid melting of the sample.

Due to their different combustion behaviour (compared to inorganic material), secondary fuels cannot be processed in an induction furnace. The most suitable choice is a measurement with a resistance furnace at a temperature of 1,300°C. As cement factories use different types of fuels with different calorific values, it is essential to measure the carbon content as a perfect indicator for the calorific value of a material. Sulphur, on the other hand, is an important parameter for processing the combustion gas desulphurisation. Whereas coal has a rather high sulphur content of up to 5%, it is only around 0.02% for secondary fuels such as wood or biomass. With ELTRA elemental analyzers samples of greatly varying sulphur content can be examined precisely and reliably.