Coal Preparation » Fine Coal
In this project a process for the continuous, selective agglomeration and recovery of fine coal was designed and investigated. The agglomeration was achieved using a binder consisting of a highly concentrated water in oil emulsion. The binder, which was developed in a previous ACARP project, comprised 95% aqueous salt solution, 2.5% kerosene, and 2.5% sorbitan monooleate. Due to the speed with which the emulsion binder achieved agglomeration, the continuous process required only a high shear zone created by a simple flow constriction to form the agglomerates. Initially a partially closed ball valve was used as the flow constriction, however this was later replaced by an orifice plate.
The project involved two phases. Phase I focussed on the flow constriction and the influence of feed flowrate on the performance of the emulsion binder under the continuous conditions. Then, Phase II focussed on the development of a continuous method for product recovery. Throughout all of the work, high ash, ultra-fine coals were used as the feed for the experiments.
In Phase I, it was found the performance of the continuous agglomeration system improved as the feed flowrate and, thus, pressure drop across the flow constriction increased. At the highest feed flowrate examined, which was 128 L/min (equating to a throughput of 460 t/m2h), a combustible recovery of 79.5% at a product ash% of 8.2% was achieved at an organic liquid dosage of 1.6 wt%. It was also found that an orifice plate was as effective as the partially closed ball valve in providing a high shear zone for agglomeration.
After investigating a number of different separation devices in Phase II, a simple trough-style open tank combined with a downcomer arrangement was selected for use. The tank allowed for the low density agglomerates to disengage from the strong downwards flow and rise quickly to the surface of the liquid where they were recovered. The operating capacity of the trough was investigated by evaluating the influence of feed flowrate on the separation performance of the unit. It was found that at low feed flowrates of 20 - 40 L/min the trough achieved an excellent separation. At 40 L/min, for example, a combustible recovery of 75% was achieved at a product ashes of 7.4% when the overflow rate was held at the minimum required to recover only the agglomerates. However, at higher flowrates there was increased scatter in the results, especially in the reject ash and hence combustible recovery, due mainly to some small agglomerates reaching the tailings. However, the ash% of the product remained satisfactory, provided the overflow rate was minimized. Overall the performance of the trough was satisfactory up to about 100 L/min.
Based on the findings in this report it appears that the flow constriction combined with the trough and downcomer provides an effective and robust method for fine coal agglomeration and recovery.