Coal Preparation » Dewatering
This tailings dewatering study was undertaken to provide a link between the fundamental tailings properties and observable bulk characteristics produced by a solid bowl centrifuge. Samples were taken from both a commercial and pilot scale unit operated at separate mine sites. This afforded the opportunity to analyse comparative samples from an existing belt press filter and investigate the impact of flocculent addition.
The commercial scale SBC unit was sampled across two campaigns and showed that cake moisture was relatively insensitive to changes in load and only marginally sensitive to flocculent dosing. The lowest achievable cake moisture was 32% at low polymer dosing and rose to 38% at high dosing rate. The solids content in the centrate effluent was observed to change significantly with flocculent from 12% (low dosing) down to 0.8% (high dosing).
The pilot SBC unit samples were suppled from a different mine site and encompassed 7 different seams, of which 4 samples produced cake moistures under 45% (considered “good”) and 3 samples produced cake moistures in the range 45-60% (considered as “poor” dewatering). These samples were then selectively analysed to determine if any fundamental characteristics could be used as predictors for dewatering behaviour. The XRD results showed that all pilot feed samples were rich in SiO2 and kaolinite, but could not identify significantly different mineralogy between tailings of “good” and “poor” dewatering properties. It was observed that the 3 tailings samples with cake moistures had lower NaCl:clay ratios and this suggested that the presence of dissolved salts may impact on the settling/bed compaction within the SBC unit.
The greatest observable discrimination between tailings samples of good and poor cake moistures was the difference in particle size distribution between feed and cake. Lower moisture cakes (<45%) contained a lower amount of ultrafine particles below 10μm than the feed- indicating that these particles had been flocculated to larger size. Higher moisture cakes (>45%) contained significantly more ultrafine materials (relative to the feed). This was considered due to the degradation of particles undergoing high shearing conditions. Results from the SEM-EDS confirmed the presence of a number particle types but specifically observed agglomerated particles consisting of smaller (<5μm) alumina-silicate particles. These particles were considered to be the most likely to breakdown in high shearing conditions to produce ultrafine clay particles. The elemental composition of these agglomerated particles indicated that they contained a number of trace compounds (Na, Cl, K, Fe, S, Mg) suggesting a complex mineralogy.
A laboratory method was developed to capture dewatering characteristics during accelerated gravity. The intention was to provide a controlled procedure to compare settling rates and bed compaction, ultimately providing a cake moisture indicative of the pilot unit. The method was able to replicate cake moistures within ±3% for tailings with good dewatering potential, but could not replicate the higher cake moistures from the 45-60% results. This was considered to be due to the lack of high shearing conditions that cause the degradation in particle size. Further work is needed in replicating this aspect of the solid bowl centrifuge.
Comparative samples from the pilot SBC unit and the existing Belt Press Filter indicated that similar cake moistures and effluent solids content could be obtained from tailings with “good” dewatering
potential. However, the SBC unit produced drier cakes (8-12% lower moisture) from 2 difficult tailings feed samples. This indicated that the SBC unit may be better suited for problematic feeds over a Belt Press Filter unit. Other comparative samples involved the use of a different flocculent (high molecular weight polymer) and the addition of a coagulant. Both the high MW polymer and the coagulent produced marginally drier cakes (3% lower moisture).