Fine Coal Agglomeration using a Novel Economic Binding Agent

In this project a novel economic binding agent was investigated for use in the agglomeration of fine coal. The novel binding agent was a high internal phase (HIP) water-in-oil emulsion and it was to substitute the pure oil traditionally used in coal agglomeration processes. The substitution was made to decrease the amount of organic liquid required to agglomerate a coal feed as the cost of the oil makes the agglomeration process economically unfeasible.

In Stage 1 of the project it was established that the HIP emulsion could agglomerate coal as the particles grew from a mean size of 125 μm to a peak mean size of 430 μm after 40 s of agitation time. It was also found that the HIP emulsion could successfully agglomerate a fine coal feed with 2 - 3 times less organic liquid than was required to agglomerate the same coal feed using pure diesel oil. This result was encouraging but there were indications that degradation of the HIP emulsion was occurring during the agglomeration process. To improve these results, the stability of the HIP emulsion had to be improved and an investigation into methods of achieving improved stability was carried out in Stage 3.

Stage 2 of the project involved a beneficiation study. In this study, the HIP emulsion was used to agglomerate coal from a mineral rich feed which had an ash content of 57 wt%. The agglomerates formed with the HIP emulsion consistently had an ash value of about 8 wt% and a slight increase in ash was observed as the yield increased. The selectivity of the HIP emulsion was comparable to pure diesel which, when tested with the same coal feed, achieved similar product ash values.

As aforementioned, Stage 3 was used to investigate methods by which the stability of the HIP emulsion could be improved. In the initial part of Stage 3, a microniser was introduced as a means of processing the HIP emulsion. This processing was considered to impart greater stability as it further refines (reduces the size of) the internal droplets within the emulsion; thus achieving a condition for stability. A number of commissioning tests were carried out using the microniser and, after some time, it was established that the most efficient way to process the HIP emulsion would be a two stage process. Initially, the microniser would be used to refine a dilute HIP emulsion, as to create a fraction of sub-micron water droplets. Following this, further water would be added using hand-held beaters and an overhead agitator to concentrate the HIP emulsion. This two stage mixing regime would create polydispersed water droplets within the emulsion, which is a condition which greatly enhances emulsion stability. It was found that this mixing regime improved the 'long term' stability of the HIP emulsion, but did not improve the results observed to date.

In the second part of Stage 3, a second variation on the HIP emulsion composition was carried out in an attempt to improve the stability of the HIP emulsion. This variation was the inclusion of a salt into the aqueous, dispersed phase of the emulsion. The inclusion of salt into emulsion through the use of a 3wt% aqueous sodium chloride solution as the internal water phase was found to improve the results achieved thus far, extending the reduction in the organic liquid requirement to a 5.5 fold decrease, as compared to pure diesel.

Therefore, it can be stated that if the HIP emulsion binder was used to agglomerate one tonne of waste coal, assuming a mineral matter content of 50 wt%, the organic liquid required would be approximately 20 L. This figure can be compared to the 110 L of organic liquid that would be required using conventional oil agglomeration. It should be noted that in this study diesel oil, which is reasonably well defined, was used due to the need to make the present study reproducible, to relate this work to previous work. The emulsifier used in this study is worth double the price of diesel, however, alternative products are available at a price close to diesel. Further funding has been secured, and alternatives are being assessed. Clearly, waste oils will be more cost effective, but the findings based on such oils will be more variable.