Coal Preparation » Fine Coal
Evidence is presented which illustrates that subjecting coal flotation feed to ultrasonic radiation can result in yield improvements of the order of 15%. The effect is most marked with coals containing large amounts of material in the - 0.05 mm size range. This type of size distribution is common in classifying cyclone overflow material. The technique is likely to be most beneficial with poorly floating coals.
The effect results in reductions in the necessary reagent addition (frother and collector) to achieve a given yield. Some evidence is presented which leads to the tentative conclusion that such treatment also results in lower product ashes without any significant reduction in yield.
The mechanism by which this yield improvement comes about results from the passage of compression waves through the slurry. These compression waves produce local pressure fluctuations, which, in turn, determine the local solubility of dissolved gases within the liquid. As the liquid experiences a low pressure perturbation, the local value of the solubility decreases resulting in the precipitation of extremely small bubbles referred to as picobubbles. These picobubbles are believed to be of the order of 0.03 mm in diameter. These bubbles are therefore an order of magnitude finer than the ~0.5 mm bubbles produced in many of the new flotation technologies. The thermodynamics of the system dictate that the stability of these picobubbles is enhanced by the presence of the hydrophobic coal particles so that adherence is promoted. This adherence promotes the subsequent adherence of the larger bubbles normally encountered in a commercial flotation cell.
In the present work, picobubble precipitation has been effected through the use of acoustic power but experimental evidence is presented which suggests that this can be achieved more efficiently and very cheaply with the aid of a properly designed orifice plate type generator. It is considered that the implementation of such a measure could readily be evaluated at plant scale. A successful outcome could result in significant cost savings in terms of reagent consumption in flotation plants. It is recommended that consideration be given to conducting an appropriate trial.
An unanticipated observation was that the application of acoustic power resulted in a significant decrease in the stability of froth concentrates. This finding could have application in the treatment of plant water streams containing high residual frother concentrations.
- Ultrasonic treatment of fine coal flotation feeds can lead to 15% yield increase.
- The effect is most appropriate for the treatment of -0.05 mm material in dilute suspension, ie. classifying cyclone overflow streams.
- Implementation of this technology leads to reduced collector and frother requirements.
- The use of an orifice cavitation plate is superior to an acoustic transducer.
- Implementation costs are likely to be low.
- ?Acoustic treatment should have benefits with respect to the control of residual frothing reagents in plant water streams.