Coal Wettability & Floatability

Coal Wettability & Floatability

In testing and designing coal flotation circuits, there are still a lot of unanswered questions. It is still not clear whether it is better to work with the use of flotation feed obtained from a plant and then to cope with all types of sample storage problems, or to rely on a coarse coal sample and the fines generated in a standardized way just before the flotation experiment.


The eight samples used in this investigation were received from Line Creek and Fording Coal mines in British Columbia.


The Contact Angle of water on coal was measured on the surface of discs prepared by compressing powdered coal (70% below 38 µm) in a MET-A-TEST mounting press at a pressure of 17.25 MPa (2500 psi) following the procedure described earlier.

Coal oxidation was also studied using a simple procedure. In this test a percent light transmission of the solution obtained by boiling a one gram coal sample in a one normal NaOH solution is measured at 520 nm relative to a standard of one normal NaOH representing 100 percent light transmission.

According to some researchers, coal equilibrium moisture strongly depends on coal hydrophilic sites and is proportional to the product of the coal oxygen content and specific surface area. In our experiments we used as received coal samples and the demineralized samples.

The particle size of the coal samples used was 70% below 38µm. The compacting time was set at 5 min. The columns after being released from the press (i” inner diameter) were strong enough to withstand handling and liquid penetration. For each coal sample at least six columns, varying in height from 7 to 14 mm, were made to check their packing density.


Notes: 1 – on demineralized coals. 2 – on as-received coals

The batch flotation tests were carried out in an Agitair flotation cell of a volume of 4 litre. 250 g coal samples ground below 0.5 mm were used in each test.


The first concentrate was collected during the first 15 seconds and the second during the following 20 seconds, both with the air flow switched off entirely. The third concentrate was collected during the next 30 seconds at a flowrate of 15 l/min, and the fourth was collected over a 2 min period at a flowrate of 30 l/min.

The floatability curves, equivalent of washability curves used to evaluate performance of gravity separation circuits, are always needed in evaluating coal flotation. Our simplified procedure is undoubtedly not as good as complete floatability analysis.

floatability relationship between contact angle and total acdity

floatability relationship between transmittance and total acidity

floatability relationship between contact angle and transmittance




floatability relationship between contact angle






coal wettability and its correlation with floatability