Coal Preparation » Fine Coal
A frother is traditionally added to the flotation feed slurry prior to the introduction of air. There is the possibility of the frother being added to the gas-phase as an aerosol rather than via direct addition to the slurry. This is an approach which has received very little attention in the research arena, but there are a few publications that suggest that reagent addition as an aerosol could lead to a marked reduction in reagent consumption.
The objective of this project was to assess the impact of adding frother to the gas-phase rather than the slurry phase in coal flotation. The aim is to decrease costs while maintaining or improving metallurgical performance and minimise the impact of frothing in other sections of the plant due to the lower concentration.
A generic flotation column and a pilot scale Jameson cell were used to determine the magnitude of any potential change in flotation performance. The commonly used frother, MIBC was added either into the slurry or via atomising nozzles at a number of different concentrations and the impact in terms of flotation performance and frother usage assessed.
It was determined from a study of the structure of the two-phase flow produced by the two modes of frother addition and confirmed by metallurgical performance analysis that for column flotation where the slurry and air bubbles are in counter-current flow, there is no advantage in adding the frother as an aerosol over adding it to the slurry. The aerosol frother addition produces fine bubbles at the sparger. In counter-current flows, when the fine bubbles with adsorbed frother molecules/species encounter the liquid without any frother, there is a net desorbing flux of frother molecules/species from the bubble-liquid interface into the bulk liquid. There is an overall transport of frother from the interface to the bulk solution, leading eventually to depletion of frother and bubble coalescence.
For the Jameson cell, a yield increase of the order of 6 percentage points is achieved with a product ash increase of 0.5%, by aerosol frother addition compared to frother addition to the slurry. Conversely, the frother dosage may be reduced by a third while maintaining process performance by adding the frother as an aerosol. Large scale confirmation is necessary.