APPLICATION OF ULTRASONIC ATTENUATION FOR ONLINE MONITORING OF PARTICLE SIZE IN BAYER RED MUD AND HYDRATE SLURRIES

A change of mining area in 2021 for Alcoa’s Wagerup refinery led to a significant increase in total silica in the ore, causing unpredictable periods of elevated +150μm material in the slurry leaving digestion. Limitations in the sand removal building resulted in significant risk to the downstream mud washer vessels. Continuous online analysis of particle size in the red mud stream to clarification was identified as a potential solution to this problem, in particular, analysis of the +150μm fraction. An online ultrasonic particle size analyser, known as an OPUS, was leased from German technology supplier Sympatec GmbH. The instrument was trialled in the Wagerup laboratory over a three-month period. Application of this technology to other materials such as hydrate slurry sizing, while not as successful, was also considered.
An electrically heated accumulation vessel with agitator was used to introduce the slurry to the measuring window of the OPUS by means of a peristaltic pump in a continuous sample circulation loop. Various iterations of equipment configuration were trialled before finding the optimum conditions to present a homogenous, non-aerated sample to the OPUS sensor. After fine-tuning of the software evaluation model, the OPUS was able to provide highly repeatable and accurate analysis of the sample +150μm fraction, as compared to the reference method of sizing analysis by woven sieve. Sample replicates with a standard deviation of less than 0.2% were routinely achieved, with differences between the OPUS and the reference method of less than +/- 1% of the total %+150μm fraction (values shown in Fig. 8 in text). The OPUS was robust with respect to scaling, showing little signal attenuation of the reference liquor sample.
Advancing to field installation of the OPUS instrument for online mud thickener sizing will give Wagerup refinery the ability to assess mud load capacity accurately in real-time. This information will allow operations to optimise sand trap pressure and BSD (bauxite slurry to digestion) flows, maximising tonnage throughput while protecting the mud washer vessels.