Coal Preparation » Fine Coal
The purpose of this study was to address the problems associated with excess frother in the process water of coal preparation plants. The goal was to establish an approach for recovering up to 50% of the frother from the flotation tailings. The frother concentrate would then be recycled back to the flotation feed. The Reflux Flotation Cell offers significant hydrodynamic advantages over conventional cells, so is seen as offering key advantages in addressing this problem.
In this foam extraction application it is nevertheless essential to appreciate key differences to particulate flotation. Particulate flotation is irreversible in nature, meaning that once the particles have adhered to the bubbles, the fine particles do not detach (or de-adsorb), even if the local concentration of coal particles around the bubbles declines to negligible levels. Foam fractionation, or ion flotation, however, is fundamentally different to particulate flotation given the surfactant adsorption at the bubble surface is reversible. Here, the bubble surface concentration of surfactant is directly proportional to the bulk concentration of surfactant in the liquid. If the bulk concentration decreases, the bubble surface concentration also decreases.
In extracting the frother, a high surfactant recovery corresponds to a low surfactant tailings concentration. Hence, in a well-mixed system in which the tailings concentration and bulk concentration are the same, the bubble surface concentration must also be low, and therefore the extraction rate correspondingly low. This in turn demands a relatively low feed flux (flow rate per unit area). Hence, a strong hydrodynamic advantage is essential in extracting frother.
Single-stage RFC experiments were initially conducted over a wide range of conditions using the model surfactant, Cetyl Trimethyl Ammonium Bromide (CTAB), a cationic surfactant. This reagent was very easy and inexpensive to analyse, and hence provided an ideal initial approach.
In summary, this research has demonstrated that the RFC produced a four-fold improvement in the feed flux over conventional flotation systems, for the same surfactant recovery and upgrade. This outcome was consistent with the original hypothesis that the RFC offers a hydrodynamic advantage over conventional systems. Stronger performance would be likely if bubble coalescence could be prevented. However, bubble coalescence is a natural consequence of the extraction process due to the reduction in the frother concentration.
It is concluded that the prospects for economic frother extraction from flotation tailings are relatively weak, with a stage-wise recovery of 14% at a feed flux of only 0.7 cm/s. At this feed flux up to six stages would be required for an overall recovery of 50%, and upgrade of only 1.8, making the proposition difficult to justify.