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Measure Twice, Mix Once

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World Cement,

Claudio Piccino, Thermo Fisher Scientific, talks about the benefits of in-stream raw material analysis and what plants should consider when implementing cross-belt online elemental analysers into their process.

Cement production involves the use of a wide range of raw materials, which need to be mixed in the right proportions to produce a high quality end product. Minute-by-minute analysis of these components allows accurate process control, helping to reduce downtime, increase throughput, minimise energy and material waste, lower costs, and, ultimately, improve clinker. Cross-belt online elemental analysers – based on technologies such as prompt gamma neutron activation analysis (PGNAA) – enable reliable analysis of bulk raw materials, and can help to ensure the mix of the raw materials going into the kiln is correct and consistent. These technologies have been widely used in process control applications, and can provide rapid, accurate, and actionable results.

Real-time elemental analysis from quarry to kiln

The production of high quality cement is dependent on the precise mixing of calcium, silicon, aluminium, and iron with other raw materials, such as limestone, shale, clay, and chalk. These ingredients are crushed and sorted into separate raw mill hoppers, and then fed into a kiln by conveyor belts to produce clinker. Post-kiln, the clinker is cooled and goes through a final grinding stage before it is ready to ship. Portland cement – the most common type of cement – is formulated in a variety of strengths and colours depending on its intended use. Manufacturers therefore need to closely monitor and control the chemical composition and particle size of each stream to ensure the desired properties of the final product. The homogeneity of the mix is also important in keeping costs down, as kiln feed material with highly variable chemistry requires more fuel to properly react, and more energy is needed at the finishing mill to grind over-reacted clinker. Optimising the consistency of material flows can also help to increase throughput and extend the lifetime of the kiln.

The robust control of cement composition safeguards key performance characteristics, including strength. This quality control should begin at the quarry and extend through mixing and the kiln to clinker blending and milling. It relies on having timely and precise information about the composition of raw materials and blends. Noncontact, nondestructive analytical technologies – such as PGNAA – are well suited to online analysis of these bulk materials.

All about PGNAA

PGNAA works by bombarding materials with neutrons, which interact with the elements present, leading to the emission of secondary gamma rays. Each of the elements within a sample produces a unique set of characteristic prompt gamma rays, creating a ‘fingerprint’ for that specific element. PGNAA analysers can be situated directly on the conveyor belt, and penetrate the entire raw material cross-section, providing minute-by-minute, uniform measurement of the entire material stream, not just a sub-sample. This is in contrast to surface analysis technologies – such as X-ray fluorescence (XRF), X-ray diffraction (XRD), and other spectral analysis technologies – which only measure limited depths and surface areas, and so may not be representative of the entire amount of material on the belt. Due to the penetrating measurements with PGNAA, sample errors are reduced, and the high frequency of analysis helps to minimise variation in material quality.

PGNAA online analysers detect and ‘read’ the gamma rays using scintillation detectors. These detectors are made up of a high purity crystalline structure that produces photons proportional in energy to that of the gamma rays it is exposed to. A photon detector coupled to the crystal converts the pulses of light into electrical signals that are amplified and processed by sophisticated high speed electronic circuits, yielding a composite energy spectrum. Analysis of the spectrum can determine information about specific elements, since each element has a different tendency to interact with neutrons. Elements with a high neutron cross-section are easier to measure than those with a low neutron cross-section, and there must be enough of the element to interact with the neutrons. The ‘threshold of detection’ for an element is a function of the percentage of the element in the material being analysed, and the tendency for that element to interact with neutrons. PGNAA is a particularly useful technique for cement manufacturers, because of its ability to directly measure key compounds of interest – including calcium, silicon, aluminium, and iron – as well as other important elements and compounds, such as magnesium oxide, sodium oxide, titanium dioxide, potassium oxide, sulfur trioxide, chromium, and chlorine. It can also be used to calculate industry standard parameters, such as lime saturation factor (LSF), tricalcium silicate, silica modulus, and iron modulus, amongst others.

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