THE ALCOA LIQUOR ANALYSER (ALIAN)

Many of the operating strategies of an alumina refinery depend upon a knowledge of the alumina, free caustic and total alkali contents of process liquors. If these strategies are to be effective, liquor analyses must be timely, and of high quality. Imprecise liquor analyses, or a liquor analysis system which is out of control, can affect the plant's productivity and product quality. Although many refineries have installed automated or semi-automatic liquor analysers, these have frequently failed to deliver quality liquor analyses long-term. This is often partly because some of the operator-dependent factors which contribute to variability have not been limited, or an inappropriate analytical technique has been selected.

The Alcoa LIquor ANalyser (ALIAN) was developed with the objective of providing fast, reliable and, above all, precise liquor analyses. By investigating all of the identifiable sources of variability contributing to imprecision in the titrimetric analysis of Bayer liquors, an automated system was designed which met these aims. The system consists of a number of customised titration and sample-handling components under the control of a multi-tasking personal computer. ALIAN is fully automated, largely self-maintaining, and incorporates many of the principles of statistical quality control. Long term, the ALIAN system is capable of analysing most Bayer process liquors in approximately ten minutes, with a 3 standard deviation single-measure precision of better than 0.5%.