Adsorbent assisted drying of spiral and flotation coal fines

Abstract

Collieries inevitably have ever-increasing wet coal fines, which cannot be sold since it is difficult to handle, has a consequently lower heating value and relates to higher transportation costs. Recently, due to the energy industry leaning toward greener technologies, more emphasis has been placed on drying these coal fines as opposed to merely discarding them. Adsorbent assisted drying is one such proposed method that has proven promising in previous studies. This study investigated the drying of spiral and flotation coal filter cakes using activated alumina as an adsorbent. The spiral product has a larger particle size (d50:434 μm) than the flotation product (d50:21 μm), and its effects were evaluated. Spiral product was also compared to spiral tailings to see the mineral content's effect on how the coal is dewatered. The experiments were done on a laboratory-scale rotating bed with the coal being dried in a cascading motion inside cylindrical vessels. The adsorbent-to-coal mass ratio, adsorbent particle size and adsorbent state (new, used or regenerated) were varied to study each variable's effect on the drying performance. The coal's heating values were measured at significant intervals during drying to do an energy balance on the process, determining the feasibility thereof from an economical and energy perspective. Supplementary experiments included the use of molecular sieves to dewater flotation product. The study found that the spiral and flotation coal fines could be dried swiftly to a market specified moisture content of 10% w/w within two minutes’ contact sorption, experiencing overall moisture reductions in the vicinity of 91%. The majority of the coals’ moisture reduction occurred in the first two minutes of dewatering, signifying the technology’s fast dewatering. The spiral and flotation product experienced 17% and 30% increases in calorific value due to moisture reduction. The main driving force for optimal drying was the contact surface area between the adsorbent and coal particles. Drying efficiencies can be optimised using either smaller adsorbent particle sizes or higher adsorbent-to-coal mass ratios or both. Nevertheless, larger adsorbent particle sizes and some lower adsorbent-to-coal mass ratios still showed sufficient dewatering of the coal fines. Spent and regenerated adsorbents were able to deliver adequate drying of the coal fines. Fresh adsorbents’ dewatering performance was followed closely by regenerated adsorbents’ performance, which showed considerable improvement over the spent adsorbents’ performance. Spent adsorbents were able to dry spiral product until the fifth cycle of use and flotation product until the second cycle of use, without regeneration. The spent adsorbent moisture loads confirmed that regenerated adsorbents need not be bone-dry to dewater coal efficiently in continuous use.

The regenerated adsorbents dewatered the spiral and flotation product to market specifications within at least 4 and 6 minutes, irrespective of adsorbent particle size. The spiral product and tailings’ dewatering behaved similarly to one another. Although similar in dewatering behaviour, the spiral tailings experienced slightly higher moisture reductions than the spiral product, suggesting that the mineral matter in coal tailings did not retain moisture stronger than the carbon-containing coal products. Spiral and flotation coal vastly differed in particle size and initial moisture contents, yet their dewatering behaved very similarly, experiencing almost identical moisture desorption rates and moisture reductions. Flotation reagents were thus found to have little to no effects on flotation coal’s dewatering. Flotation coal experienced less weak drying effects than spiral coal when using lower adsorbent-to-coal mass ratios and larger adsorbent particle sizes attributed to flotation coal’s ultrafine particle size, which aided with good contact surface area between the coal and adsorbents. Molecular sieves showed extraordinarily fast dewatering, significantly faster than activated alumina, having the coal dewatered to near bone-dry moisture contents within two minutes. However, spent molecular sieves showed poor coal dewatering, while its regenerated version showed meagre improvement suggesting molecular sieves’ ineligibility for industrial use. The regeneration of spent adsorbents showed sufficient moisture load reductions for activated alumina and molecular sieves. The main driving force for optimal regeneration times is the bed mass to regeneration column area ratio, which stressed the importance of intelligent regeneration column design. Qualitative observations showed the spiral and flotation coal having affinities for caking and forming lumps, which inhibit dewatering performance, signifying the necessity for suitable materials handling before dewatering. Capillary pore blinding of the adsorbents was encountered, which may have played a role in the adsorbents’ drying performance during continuous use without regeneration. Activated alumina was found to be a robust adsorbent with little to no particle breakage or attrition encountered during drying and regeneration. Molecular sieves showed brittleness and severe particle breakage during operation. The adsorbent assisted drying of spiral and flotation coal fines were found to be an energy-positive and economically feasible process on a high level, while the use of activated alumina as adsorbent proved suitability for industrial application. A concept design of a continuous adsorbent assisted drying plant was draughted and described.