Coal Preparation » Process Control
Sampling from the discharge of high capacity vibrating screens, especially multi-slope screens, is subject to sampling bias, due to the stratification that occurs on the vibrating screen deck. The stratification in the vibrating bed translates to a stratified falling stream at the discharge end of the screen, with smaller particles at the back of the falling stream and large particles at the front of the falling stream.
The characteristics of a heterogeneous material like coal and associated mineral matter vary significantly with particle size. The processes employed in coal preparation vary in type and efficacy according to the particle size distribution. If samples do not correctly reflect the source material size distribution, they will be biased and the resulting analytical data will not reflect the actual physical nor chemical characteristics of the source material.
Prior published data has demonstrated that a bias appears to exist for samples collected from high capacity multi-slope screens.
This project measured the bias for one coal preparation plant by comparing the desliming screen overflow size distribution to that for 'reference' coal preparation plant feed samples collected using the recognised stopped-belt technique. The study procured twenty sets of comparison samples in order to ensure statistical relevance.
Two forms of sampling device were employed for the vibrating screen discharge.
A conventional square aperture scoop was demonstrated to exhibit a significant coarse bias, in that the resulting samples exhibited a significantly coarser size distribution compared to the reference samples.
A new 'collapsible' sample scoop was evaluated in parallel, and its samples were found to exhibit no bias.
Use of the collapsible scoop is recommended for all future sampling from vibrating screens for bed depths in excess of 100mm and particle top size in excess of 16mm.