Froth stability is known to play an important role in determining mineral flotation recovery and selectivity. Overly stable froth causes high entrainment of gangue minerals in flotation and is difficult to handle in downstream processes. On the other hand, unstable froth is associated with the low recovery of valuable minerals. Therefore, correct froth stability is of utmost importance in flotation.
Despite the importance of froth stability, many coal flotation plants encounter overly stable froth problems especially when saline water is used. For example, at a fine coal flotation plant in Queensland, over-frothing occurs due to the formation of stable froth. Residual froth layers on filters cause the high moisture of flotation products. Breaking down the froth with water sprays exacerbates the moisture problem. To meet the moisture requirement in the final coal product of less than 6%, the mass pull of flotation concentrates has to be reduced, resulting in the loss of combustible matter in flotation tailings. Over-frothing in pump boxes, tanks and thickeners causes poor handleability and interferes with the normal plant operation. This is a common problem in coal flotation plants but barely reported in literature.
The objectives of the study are therefore to identify major potential factors contributing to the generation of overly stable froth in coal flotation using saline water with the fine coal flotation plant chosen as a case study, and then to propose possible solutions to overcoming problems resulting from the overly stable froth. Several methods of froth stability measurements have been suggested in the literature. However, controversy exists in the validity and accuracy of these methods and currently there is no specific criterion to quantify froth stability. In order to achieve the objectives of the study, the advantages and disadvantages of these methods were compared against each other by reviewing relevant literature and by experimental testing. The most appropriate method for froth stability measurement in the laboratory identified for this study was the air recovery method and an altered version of this method by determining the vertical movement velocity of froth was employed.
Factors affecting froth stability have been previously identified in the literature, including reagent suite (frother, collector, activator, depressant), particle property (size, shape, hydrophobicity, solid density), operation condition (conditioning of feed, temperature, air flowrate, pH), and water quality. In this study, the ease of study and possibility of application were considered when determining the main factors to be investigated. As a result, coal particle size, process water quality, frother and collector dosage were chosen to be investigated as the procedures required in the experiment are simple and they may be manipulated to overcome the overly stable froth problems in the plant.
The results of this study showed that the stability of froth produced by coal flotation depends on coal particle size, process water quality, and chemical reagent (diesel and MIBC). Fine coal particle size of -106 µm produced higher air recovery than coarser particle size of +150-250 µm and +355-500 µm when either plant process water or de-ionised water was used. It is proposed that fine particles concentrate along the air and solution boundary line, slowing the normal drainage process and therefore stabilising the connection between bubbles and solutions with simultaneous high froth stability. Flotation with plant process water produced higher air recovery than de-ionised water for all the classes of the coal particle size probably due to the compression of electrical double layers around the particles and the reduction of double layer interactions caused by salts resulting in the thinning and rupture of wetting films between particles and bubbles as identified in literature. Flotation in the absence of diesel or MIBC produced lower air recovery than flotation with both diesel and MIBC. Diesel rendered the surface of coal particles more hydrophobic, which strengthened the affinity between coal particles and air bubbles while MIBC inhibited bubble coalescence, producing more stable froth.
Several methods were proposed and tested to reduce froth stability when plant process water was used. The mixture of coarse coal with fine coal (20% +500-710 µm 80% -106 µm), dilution of process water (50% process water 50% de-ionised water) and reduction of reagent usage (no diesel addition or no MIBC addition) all reduced froth stability in flotation. However, the flotation of coal with only diesel and flotation with only MIBC could eliminate overly stable froth issues while achieving acceptable combustible and mineral matter recoveries.
The interaction of chemical reagents with salt concentrations was also studied to provide a guideline to adjust reagent concentrations based on variations in process water conductivity. Central composite rotatable statistical design was conducted and indicated a clear interaction between salts and chemical reagents in determining froth stability and flotation performance. The interaction of salts with MIBC increased froth stability and decreased combustible matter recovery while the interaction of salts with diesel decreased combustible matter content. If the process plant is faced with low or medium salinity water, high or medium frother and collector dosages should be used, respectively to avoid overly stable froth while maintaining good flotation performance. On the other hand, if the plant is faced with high salinity water, low frother and collector dosages should be used.