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Innovations in online analyser use – part two

World Cement,

About the analysers

When the plant was commissioned in 2003, it included an online analyser on the belt line feeding the raw mill, to aid in raw mix proportioning. The analyser and the associated raw mix proportioning software were part of the raw material equipment package supplied by Polysius (now thyssenkrupp). The analyser was supplied by Sodern in France. Very new at the time, the product is known as the CNA, Controlled Neutron Analyser. Like other elemental analysers, it employs prompt gamma neutron activation analysis (PGNAA); unlike most others, however, its higher-energy neutrons permit a second type of analysis from a phenomenon known as inelastic scatter, enabling the analysis of more elements. It also uses an electronic generator rather than a radioisotope for its neutron flux; this ensures a constant flux level while in use as well as a means by which the source can be turned off when not in use. This latter attribute is particularly beneficial for quarry applications, since most quarries operate only 40 – 80 hours per week, permitting the neutron generators to turn off automatically when not in use, and thus extending the source life.

In 2007 the plant added a second CNA analyser to improve quarry quality management. The choice of the CNA was driven, to a large extent, by the success realised from the CNA which was used to control raw mix. As pointed out by Tom Messer, Branford Plant Manager, “on-line analysers have been an essential part of our quality control strategy since the plant’s beginning and their contribution has been significant.” This second analyser is located downstream of the portable crusher which is located in the quarry, and is used to control stockpile chemistry as explained later in this article. In 2010 the CNA became a PANalytical product, and is now in its fifth generation.

Between them, the two CNA analysers have now provided nearly 20 years of reliable service, with availability close to 98%. The products have undergone electronics upgrades over the years and when the neutron generators reach the end of their life, which can be as much as eight years in quarry applications, the replacement of the generators is performed by plant personnel.

Quarry management

Most Florida cement plants obtain their limestone from a sponge-like deposit, some of which is found on the surface, and the rest is essentially underwater, beneath the surface layer – Branford is no exception. The calcium content is highest in the underwater limestone and, when mixed with the surface limestone in proper ratios, yields an LSF compatible with target raw mix composition.

Adding to that mining challenge at SAC are issues of land use restrictions, tightening gas stack emission limits, and depleting reserves. Taken together, these phenomena made it clear that an online analyser in the quarry would not only enhance real-time quality control, but would lead to better reserve optimisation. That led the plant to add the quarry analyser in 2007, an investment that has proven itself in the years to follow. Ronaldo Dos Santos, the Corporate Mining Manager, explained, “without the CNA the turnaround time of the chemical results was often undependable, having to take numerous samples off of the crushed limestone belt conveyor and always trailing the process. With the CNA we have been able to reduce waste material and reduce the LSF variability in the raw meal, which in turn leads to lower energy costs.”

The team at SAC went one step further by creating a system in which the quarry operators are in charge of the short-term blending requirements. The blending method involves front-end loaders loading differing proportions of limestone from either the top rock pile or the underwater rock pile into the mobile crusher.

The proportion of bucket loads from each pile is dictated by readouts from the quarry CNA. The analyser analysis results are shown on a 3 in. x 4 in. monitor inside the loaders (see Figure 3), and corrections are then made in real time by adjusting the ratio of top rock to underwater rock if the trends are out of spec. The monitor shows a four-hour trend line of LSF and SO3, as well as the cumulative average LSF and SO3 for the limestone stockpile currently being built. The monitor display was designed by plant personnel and draws its information from the plant’s PI data manager over the plant’s wireless network.The target LSF for the pile is approximately 320, and for each stockpile the quality control department provides an expected blending ratio of top rock to underwater rock, based upon quarry and core samples. When the cumulative LSF deviates from the target range, the loader operators begin to adjust the initial blending ratio as necessary to return to target. In addition, If SO3 spikes beyond the allowable limits, the loader operators can also respond by seeking out alternative top rock and underwater rock pile sources with known low SO3.

The plant’s quality control manager, Claudia Urrutia, is integrally involved in the process. She said, “One of our main goals is to simplify the hourly and daily decisions our employees have to make, and the analyser helps immensely in the process. We know that by getting the chemistry right early in the process, the quality control downstream is made that much simpler.”

This is part two of a three-part article written for World Cement’s May issue and abridged for the website. Subscribers can read the full May issue by signing in, and can also catch up on-the-go via our new app for Apple and Android. Non-subscribers can access a preview of the April 2016 issue here.

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