Measuring and testing – Liquid analysis or analysis of the suspension of solids in a... – Content or effect of a constituent of a liquid mixture
Patent
1995-02-08
1996-10-29
Williams, Hezron E.
Measuring and testing
Liquid analysis or analysis of the suspension of solids in a...
Content or effect of a constituent of a liquid mixture
73 32A, 738655, 378 53, G01N 1502, G01N 2310, G01N 2918, G01N 2920
Patent
active
055698441
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a method and apparatus for determining the particle size distribution, the solids content and the solute concentration of a suspension of solids in a solution bearing a solute. Advantageously, the invention is applicable for use in industrial processes such as in the precipitation section of the Bayer process.
BACKGROUND ART
In certain industrial processes, such as the precipitation section of the Bayer process, it is desirable to monitor the particle size distribution of solids in a slurry or suspension under dynamic conditions and to determine the deviation of the actual particle size distribution from that desired. The invention will be herein described with particular reference to the Bayer process but is not limited to that use.
In the precipitation section of the Bayer process alumina is extracted and purified from bauxite. Bauxite is crushed and ground, then digested at elevated temperature (104.degree.-230.degree. C.) and pressure in a strong solution of caustic soda (80-110 g Na.sub.2 O/liter). The residue, known as red mud, is separated from the solution by countercurrent decantation and filtration. After cooling, the solution is supersaturated with respect to alumina. In the precipitation (crystallization) process the supersaturated solution is seeded with recycled alumina trihydrate fines and agitated in large tanks. The productivity of the Bayer process is limited by the slowest step, the precipitation of the alumina trihydrate from the liquor. After precipitation, the alumina is calcined at temperatures up to 1300.degree. C. and sold typically for commercial smelting to aluminium.
The goal of the precipitation section is to maximise alumina yield whilst maintaining product quality (particle size and strength) and seed balance. Control variables in precipitation are temperature, starting alumina/caustic ratio, seed charge, caustic concentration, holding time and impurity levels (particularly organics).
It is desirable to minimise the proportion of fines (minus about 45 microns as well as minus about 20 microns) in product alumina. These fines cause dust problems during handling operations and they cause significant flow and segregation problems in aluminium smelters. Fines are generated in the precipitation, calcining and handling stages of the plant. Improved control of the precipitation stage should produce less fines in precipitator product and a stronger alumina (Sang, J. V., "Factors affecting the attrition strength of alumina products", Light Metals, 1987, 121-127) which will in turn generate less fines in downstream calcining and handling.
The optimisation of alumina precipitation would benefit from the development of suitable on-line particle size analysers to monitor the proportions of both coarse and fine alumina at a number of cut-off points, for example at 20,45,75 and 100 microns. Measurements preferably need to be made under the following plant conditions: caustic concentration 150 to 250 g NaOH/liter; temperature 60.degree. to 80.degree. C.; solids content up to 35 wt %; scale build-up rates of about 10 mm /week; and variable air bubble concentration and size distribution.
At present there are no commercially available on-line particle size monitors capable of this measurement.
The conventional method of measuring particle size distribution is to remove samples from the streams of interest and to perform screen analyses on these samples. A screen analysis involves a series of procedures by which a measurement is made of the proportion of the sample that remains on each of several screens having progressively smaller openings of known size. While this kind of measurement can provide a reasonably accurate determination of particle size distribution above about 45 microns, it is representative only of the particular sample taken, and cannot accurately and reliably indicate either the average condition in the flowstream over a period of time, or the changes that occur between sampling. Also the method is not applicable to fine fractions
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Brock Michael J.
Commonwealth Scientific and Industrial Research Organisation
Williams Hezron E.
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