Impact of Saline Water on Coal Flotation

This project has produced a handbook to assist plant operators to understand the implications of changes in water chemistry in the plant, particularly on flotation.
Large inventories of saline water are accumulating at some mine sites due to discharge restrictions. At the same time there is pressure on the importation of fresh water for use at mine sites. It makes sense to use the saline mine water for all duties and processes where detrimental effects are minimal or where there is beneficial impact on operations. One of the areas where saline water can be used is as process water in the coal preparation plant. The work detailed here is aimed at understanding the major impacts of saline water on coal flotation which is the process most likely to be significantly affected due to its dependence on surface chemistry to function effectively.

Experimental data from batch flotation tests in which no conventional reagent was added clearly showed that flotation recovery generally increases with increasing electrolyte concentration at least for the salts used in these experiments.  For the single salts studied, combustibles recoveries attained in the presence of MgSO4 were slightly higher than those of MgCl2 and CaCl2 and these salts in turn produced much higher flotation recoveries than NaCl at the same concentrations. This is consistent with zeta potential data. When the combustibles recovery data was examined in terms of ionic strength, the results for all the salts were much closer.

Flotation in mixtures of salts produced higher combustibles recoveries than in single salts at the same concentration. The implication is that lower concentrations of several salts as found in saline waters at minesites could produce enhanced flotation recoveries even when individual salts at those concentrations have only minor effect on coal combustibles recovery.

Flotation using conventional collector and frother in the presence of low concentrations of salts such as MgSO4 results in higher combustibles recovery than when the salt is absent. Conversely, lower concentrations of the collector may be required to attain the same combustibles recoveries when the water contains small concentrations of salts.

The flotation kinetics data also show increases in flotation rate with increasing salt concentration.

The concentrations at which maximum recovery is attained are different for the different coal samples. For two coals of similar rank, one was found to be more difficult to float in saline water than the other. Petrographic analysis of the coals showed that the dominant maceral in the more floatable coal was vitrinite and the difficult to float coal was dominated by inertinite. The differences in flotation response between these coals may be attributed to a lower response of inertinite to salt flotation.

Social and commercial requirements mean that many coal preparation plants will be forced to use saline water accumulated from the associated mining operations.  This investigation has identified potential opportunities to decrease the use of collector/frother in the flotation circuit by proactive use of a saline water mixture to optimise a flotation response, especially where divalent cations concentration is high.