Coal Preparation

Commercial Laboratory Scale Float-Sink Testing Using Stabilised Suspension

Coal Preparation » Gravity Separation

Published: May 12Project Number: C18038

Get ReportAuthor: Shenggen Hu and Andrew Taylor | CSIRO

The objective of this project was to carry out commercial lab scale float-sink testing using the stabilised suspension of submicron zirconium dioxide developed in ACARP project C15057.


A float-sink separator has been developed for carrying out float-sink testing using opaque suspensions. In this separator, floats can be quickly and easily separated from sinks by using a separation plate without causing back-mixing.


To compare the washability data from the two methods when the number of repeated tests or the mass of sample is increased, float-sink testing on six parallel samples was carried out using both stabilised suspensions and organic heavy liquids. Variations among washability curves can be reduced to an acceptable level when the mass of combined sub-samples satisfies the rule on minimum sample masses as specified in Australian Standard AS 4156.1. The combined washability curve of six sub-samples obtained from the stabilized suspension is almost identical to that from organic heavy liquids except in the density range lower than 1.4. The small difference was caused by water filling the pore structure of coal particles and also the different drying methods used.


Water can penetrates more pore spaces than organic liquids due to its smaller molecular size, leading to a slightly higher apparent relative particle density, particularly for coals having a RD less than 1.3. Coals with a lower RD are more porous than those with a higher RD.


As a significant degree of uncertainty was involved in air dried masses due to uncontrolled air temperature, humidity and air flowrate, the mass of floats and sinks was determined after drying in an oven at a temperature between 1010C to 105 0C for 24 hours. Consequently, washability results obtained by stabilised suspensions using thermal drying have a relatively high precision.


Although the washability data obtained from aqueous suspensions would better reflect what occurs in coal preparation plants, both stabilised suspension and organic heavy liquids would give equally good predictions of process efficiencies, yields and separation cutpoints as the difference in washability curves occurs only at the lowest end of density range. In term of the evaluation of coal resources, the accuracy in the determination of the amount of coal in situ using the washability data obtained from aqueous suspensions will depend on the moisture content of the coal in situ. For dry coals, the amount of coal with a relative density lower than 1.3 would be slightly underestimated due to the reason that the apparent relative density of porous coal is higher in aqueous suspensions than that in organic liquids. For the prediction of yield, there should be no significant difference between the two methods.


Operating procedures for preparing stabilised suspensions and carrying out float-sink testing using the suspension were developed and validated.


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