Article

Particle density and the plummet balance

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Abstract

Soil suspensions are usually characterised by the percentage weights of particles settling with velocities between defined limits, such as silt and clay. If percentages were measured sufficiently accurately by both the pipette method and a plummet balance, dry densities of fractions could be calculated and hence their composition inferred. The accuracy and precision of the pipette method were confirmed by the mass balance of particles in peptised suspensions of a red-brown earth subsoil and a kaolinite. Percentages indicated by a plummet balance were corrected for the actual densities of fractions measured separately. Residual differences from the pipette values showed that the balance was accurate but lacked the precision needed to deduce densities to 0.01 mg/mm3. Values of the density of clay fractions were deduced from the means of published percentages obtained by the two methods where replicate suspensions were used and also from the mean difference based on a comparison using 199 soil samples. For the latter, an underestimate of clay percentages indicated by the balance had first to be allowed for. Clay-deduced densities were in the range 2.80–2.83 mg/mm3, compared with 2.84 mg/mm3 measured on the separated subsoil clay. The values are similar, probably because the clays are derived from mica. The density of the separated subsoil silt was 2.64 mg/mm3, less than quartz, because of the presence of feldspar. Such results indicate the potential use of the method if the precision of the balance is improved.

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... Two broad techniques for soil particle size analysis (PSA) have been developed using Stokes's Law. These are the sieve-pipette method that measures a weight concentration and the sieve-hydrometer (or sieve plummet balance) method that measures the suspension density [3]. The differences between these two approaches have been extensively studied (e.g. ...
... The differences between these two approaches have been extensively studied (e.g. [3][4][5][6]). ...
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Slurries formed by the neutralization of tailings from acid-extracted ground rock contain sulphates. As the slurries dry out, sulphate precipitates, and the solid sulphates present dehydrate. The dehydration of pure calcium and magnesium sulphates was determined over a range of relative humidities at 20°C and after oven-drying. The results were used to help determine the amounts and forms of sulphates present in two samples of tailings after air-drying and oven-drying. One sample had been neutralized with lime and the other with calcined magnesite. Using the two samples of tailings, it was shown that values of solids density agreed with values calculated in the usual way from measurements on sulphate-free samples. -from Authors
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