The COSMA Mineral Analyser

Posted in: , on 8. Sep. 2010 - 19:47

Continuous On Stream Mineral Analyser, COSMA

COSMA is a unique, patented application of XRD and Rietveld techniques that is only available from FCT ACTech. COSMA has been specifically developed to be located in the plant at either the kiln or mill, and to continuously analyse a stream of material to provide real time analytical results from which automatic control action can be taken. Batch mode analysis is also possible.

COSMA analysis is made possible by the application of a unique combination of innovative technologies.

Firstly, FCT has developed a means of presenting a continuous sample stream suitably prepared for XRD. XRD analysis requires an accurately prepared flat bed of material from which diffracted X-rays can be detected. The sample stage is specially designed to prepare and present cement direct from the cement mill or clinker after milling (or other powdered material) as a flat, continuously moving target bed.

This materials handling system for the XRD sample was devised and developed to allow for a continuous flow of material onto a rotating table that is prepared into a flat bed before passing beneath the X-ray beam.

COSMA uses an X-ray tube and a graphite monochromator (or optional multi-layer mirror) to produce a beam of X-rays at the sample stage. After the analysis zone, the material is continually removed from the rotating table, while new sample is continually added to the table at another point in its rotation. By continuous analysis the requirement for further milling is overcome. For example, the analysis is averaged over many orientations because the sample is continuously moving and renewed, and many more individual particles are presented to the X-ray beam compared to traditional single sample pellets.

Secondly, continuous analysis is made possible through the use of a curved position sensitive detector (PSD) which is able to detect a full range of 120 degrees of 2-theta simultaneously. The PSD produces charge pulses for each diffracted X-ray collected from either end of the detector. The position of the detected X-rays is indicated by the arrival times of the charge pulses at either end of the detector curve. The pulse output from the detector is processed into a multi-channel analyser (MCA) and the MCA output is then calibrated into 2-theta angles in the software.

The detector is stationary and, therefore, more durable and reliable than a scanning goniometer. However, the real advantage in this development is its ability to register a complete diffraction pattern continuously rather than building a diffraction pattern over time from traversing an arc. This means that a complete spectrum of diffracted X-rays is collected relevant to the sample stream flowing through COSMA. This is not possible with a moving goniometer that takes a finite time to scan the diffracted X-ray pattern.

Thirdly, a modified quantitative Rietveld code provides virtually continuous spectral analysis converting the live spectral data to phase composition. Due to the analysis techniques, the instrument is free from requiring standardisation and will only periodically (annually for example) require check calibration. The instrument has been calibrated for the mineral phases of interest after extensive research and electron microprobe analysis, and the calibration of each instrument is preset for each new installation. This is a requirement of the physics of the measurement, not a drawback of the analyser. The calibration accounts for differences in atomic substitution in the various mineral phases, which depend on the raw materials and the process.

COSMA has been designed to be located in the plant, and not in the laboratory. It has been designed to communicate directly with plant PLC and plant control systems. Its data output can be adjusted easily with software but, in the standard form, all mineral analyses of interest are updated every minute, while averaged values are integrated (typically over a 5 - 10 minute period) to give a smoothed result for control action. In addition, trends are displayed over a nominated time period. Batch mode analysis is also possible.

Elemental composition (oxide analyses for example) can be calculated with good accuracy from the mineral composition.

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