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Coal Preparation

Technical and Safety Comparison of DP Cell and Nucleonic Density Gauges

Coal Preparation » Environmental Improvement

Published: June 06Project Number: C13050

Get ReportAuthor: Andrew Vince, Ian Brake, Ben Cronin | BHP Billiton Mitsubishi Alliance

In Australian coal processing practice the measurement of slurry density is of critical to medium importance, depending on the application. A number of means are used for measuring density – traditionally nuclear and differential pressure methods. In recent times nuclear methods using Cs137 nucleonic gauges have been commonly applied by designers whatever the duty, and yet nuclear techniques are the subject of increasingly onerous statutory regulation by government that have to be managed by plant operators.

Mining industry practice and legislation relies on risk assessment prior to design and use of any technique, and nuclear techniques pose a level of compliance and health risk not seen in the alternatives. In a legal regime of making risk “as low as reasonably achievable”, it seemed incongruous that some plants use only nuclear sources, some use only differential pressure methods, and some use a combination of both. All plants appear to operate well whichever technique is used and there is often little science to the choice.

Why not just get rid of nuclear sources for density and replace with DP or other methods and save all the risk and regulatory compliance effort? The answer is that accurate density measurement is critical for dense medium circuit performance when a significant level of near gravity exists, and that there has been no technical comparison of the two methods.

This project set out to measure the accuracy of DP and Cs 137 methods on the one density “tester leg” at the Goonyella CPP. The conclusion of a detailed assessment by Dr Andrew Vince was that both methods had similar precision, the calibration methods available were much less precise than the instruments, but that both were also subject to bias. The results could not conclude that either was better than the other, but did conclude laboratory measurement at the site or the slurry sampling was problematic. This was the completion of the original ACARP Project.

4Differential Pressure cells are also inherently subject to bias due to flow rate past the cells due to dynamic rather than static pressure effects. A second study was requested and approved to use a student thesis project engineering test rig at James Cook University Townsville. This fixed price project built a test rig for pumping magnetite past DP and Cs137 measurement systems at variable speed but the rig was not sufficiently robust to deliver useful results.

BMA then privately funded two exercises. A Network query to BHP Billiton operations in many plants across many industries worldwide showed that Cs 137 methods are subject to elemental bias which can be severe – particularly for clays. One coal site in Queensland reported large changes in Cs137 measured slurry density in a tailings thickener underflow as pits changed. It is also widely known in the laterite nickel industry where the ore is principally clay. The fundamental physics for this is referenced.

Secondly a request for other methods showed some plants moving to Coriolis based measurement, or fixed DP probes useful for tank density. A test report on applying a Coriolis based sensor is included. Another ACARP project has taken this further.

The clear conclusion from the work is that all methods, nuclear, DP, Coriolis all have advantages and technical limitations. A qualitative matrix is presented showing options for various coal processing measurement points.

Only nuclear methods have their unique safety and compliance workload issues. For any new installation, a review of the technical task, the ability to design systems that suit non nuclear instruments and the accuracy required should be performed as part of a risk assessment. Without such a review there is no technical basis for simply assuming nuclear methods are the only way of accurately measuring slurry density – they are not. This type of thinking is a profound change to the “one type fits all” approach to fitting nuclear density gauges. 

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