Applications in Industrial Systems

By way of example Scarlett et al. and Manias et al. have described an online XRD analyser designed for use in monitoring the abundance of the major phases in finished Portland cement. The instrument is placed at the exit of an operational cement mill and passes some 30 kg h of material on a continuous basis while obtaining XRD data and quantitative phase abundances every 1 to 2 min. Portland cement is a mineralogically complex material consisting of some 10 to 15 phases of interest that vary in concentration from 60 wt.% down to 0.2wt.%. The physical and chemical characteristics of the component 

To produce the most reliable analyses under these conditions, especially for the minor phases, Rietveld-based analytical conditions need to be developed using the following steps  

1 Alkali Sulfates. Since the silicate phases (C3S and C2S), present in total at up to 85 wt.%, normally dominate clinker samples, all minor phases are significantly concentrated in Residue 2. This includes the important alkali sulfate phases which can (i) affect setting times and final strength, and (ii) be used to assess kiln operating conditions. Since these are normally present at a total of about 0.5 wt.% in clinker, and are often distributed across several Na and K sulfate phases, they are not easily identified in raw clinker XRD patterns. However, their presence may be more easily detected in the XRD pattern of Residue 2. By optimizing the parameters of the alkali sulfates from Residue 2 data, and then constraining them in the on-line analysis system, these phases can be measured at the 0.5wt.% level (Madsen, Scarlett and Storer 2001, unpublished results) even when rapidly collected on-line data is used. 

For Portland cement, the removal of the major phases to leave a residue in which the minor phases are concentrated relies on a chemical extraction process. Clearly, this approach will not be suitable for all phase systems. In some cases, concentration of minor phases can be achieved by magnetic, density or grain size 

The value in using diffraction based methods for the determination of phase abundance arises from the fact that diffraction information is derived directly from the crystal structure of each phase rather than from secondary parameters such as measurement of total chemistry. However, the methodology of quantitative phase estimation is fraught with difficulties, many of which are experimental or derive from sample related issues. Hence it is necessary to verify diffraction based phase abundances against independent methods. In those circumstances where this is not possible, the QPA values should be regarded only as semi-quantitative. While such values may be useful for deriving trends within a particular system, they cannot be regarded as an absolute measure. 

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