The Australian coal industry is treating clayey deposits as a result of the depletion of high quality resources and flotation plays an important role in recovering combustible matter from these low quality coals. Some coal preparation plants float particles with a top size of 710 µm, which is referred to as coarse flotation, while some coal preparation plants float particles with a top size of 150 µm, which is referred to as fine flotation. Despite a high concentration of combustible matter in the feed of both fine and coarse coal flotation, combustible matter recovery is often low, with high quality combustion products being difficult to obtain. This has been attributed to associated clay minerals which may have an adverse effect on coal flotation. Coal flotation in Australia is also complicated by the use of saline water. Due to scarcity of fresh water and stringent regulations on the quality of discharged water, coal preparation plants have used recycle water with a higher concentration of electrolytes compared to fresh water. Despite a number of studies that have been conducted to investigate the effect of saline water on mineral flotation, effective ways to solve many problems encountered in coal preparation plants using saline water are not currently available. The major objective of this study is to understand the interaction of clay minerals with saline water and its effect on fine and coarse coal flotation based on coal samples supplied from the Australian coal industry.
This study initially aimed to determine the recovery of clay minerals in fine coal flotation where mechanical entrainment through water films between air bubbles is an important mechanism. This is of particular interest given the colloidal size of clay particles and the inhibition of bubble coalescence by electrolytes. The flotation of fine coal samples containing clay minerals was examined in both de-ionised water, and artificial saline water having high conductivity. As expected, in both de-ionised water and saline water, the recovery of either combustible matter or mineral matter was higher at smaller size fractions, and saline water enhanced their recoveries. The degree of entrainment (ENT) was then calculated on a size-by-size basis using a standard model which integrates the water as a reference to define the classification effect of the drainage of entrained particles in the froth phase. It was interesting that the calculated ENT was about 1.5 and 2.2 for particles smaller than 38 µm when fresh water and saline water were used, respectively. This was unexpected since ENT was often less than 1, the maximum ENT of liberated gangue minerals in flotation. Obviously, entrapment attributed to the recovery of mineral matter in coal flotation besides entrainment.
To further evaluate the entrapment, pulp rheology measurements were performed firstly on coal suspensions, and secondly on the suspensions of ash after the combustible matter was combusted. It was found that ash suspensions in the absence of coal particles showed Newtonian behaviour in both de-ionised water and saline water, whilst coal suspensions exhibited non-Newtonian and shear-thinning behaviour indicating the occurrence of particle aggregations probably resulting from the hydrophobic force between coal surfaces. Saline water enhanced coal aggregations due to significant increase in viscosity and shear stress compared to de-ionised water. This is because electrolytes compress electrical double layers and therefore reduce the electrical double layer repulsive forces. It stands to reason that aggregates forming in coal flotation promoted the entrapment of mineral matter. To quantify the recovery of mineral matter in coal flotation through entrainment and entrapment, the flotation of ash after the combustion was performed and the calculated ENT reflected the true entrainment, while the degree of entrapment (ENP) was the difference in ENT in the presence and absence of coal particles. Indeed the true ENT value was smaller than 1 across the size range. The smaller the particle size, the greater the entrainment. Saline water increased the entrainment across the size range compared to de-ionised water and this increase was more pronounced for particles smaller than 38 µm. Meanwhile, ENP was insignificant for particles greater than 38 µm in both de-ionised water and saline water. For particles smaller than 38 µm, entrapment was significant and even greater than ENT in both de-ionised and saline water. Compared to fresh water, saline water significantly increased the entrapment of particles smaller than 38 µm.
The attention of this study was then turned to identify the surface chemistry effects on fine coal flotation using saline water. Industry observations strongly suggest that slime coating may occur on fine coal particle surfaces depressing coal flotation, but rigorous evidence is not available. In this study, a range of techniques were used to determine the surface properties of two coal samples which displayed low floatability. Firstly, Visual MINTEQ 3.0 was applied for solution speciation modelling calculations to predict possible hydrophilic precipitates on the coal surface in the flotation system. It was found that the coal flotation circuits using process water having medium conductivity were super saturated with respect to a range of carbonates, silicates, and aluminates, as their saturation indices were greater than zero. In particular, hydroxyapatite, hercynite, and greenalite had high saturation indices. When precipitation was allowed in the modelling, the predicted precipitates were chrysotile, calcite, dolomite, and aragonite with a concentration ranging from 3.6-4.5 mmol/L. The calculations were also conducted by replacing process water with deionized water, and no precipitation occurred in both flotation circuits. Apparently, electrolytes in the process water promoted these precipitates. The precipitations are hydrophilic and likely to have a negative effect on the flotation.
Cryo-SEM, an in situ technique, was then used to detect clay slime coatings on the coal surface during flotation. For comparison, flotation concentrate and tailings samples were measured by SEM images taken in backscattered electron mode (BSE), and EDS elemental analysis on the randomly chosen particles. Clay minerals were small, and their shapes were difficult to identify by SEM images. The SEM image showed that the coal surface was clean in the flotation concentrate, but in the tailings, it was extensively covered by small bumps suggesting that slime coatings occurred on coal particles that entered flotation tailings. On the coal particle in the flotation concentrate, a very strong signal from C element was detected and other clear signals detected were from O and Au. In contrast, on the particle in flotation tailings, the signal from C was very weak, but clear signals from Si, O, Al, and Mg were detected further confirming that slime coatings that occurred on coal particles in flotation tailings.
XPS was also used to measure the oxidation on the coal surface. Of particular interest to this study was the XPS C spectrum showing the oxidized and unoxidized carbon chemical state. On the two coal samples examined, about 19 at.% C and 11 at.% C associated with the oxidation species. While the coal oxidation is not strong, it is likely that hydrogen bonding between clay minerals and hydrophilic carbon oxidation products enhances the adsorption of clay minerals.
Another challenge accompanying the use of saline water in the Australian coal industry is a wide variation in water quality in terms of salinity, types and concentrations of ions from plant to plant which is normally summarised by giving the minimum, medium and maximum values of the concentration of the major ions. In this study, the effect of minimum, medium and maximum conductivity saline water on the flotation of fine coal and coarse coal containing clay minerals were investigated. It was found that water quality had a more pronounced effect on fine coal flotation than coarse coal flotation despite similar mineral compositions and clay mineral types present. In fine coal flotation, both combustible recovery and mineral matter recovery were low when low conductivity saline water was used. An increase in water conductivity significantly increased the recovery of combustible and mineral matter. This is because electrolytes in saline water increased froth stability resulting in fine entrainment, and also enhanced the aggregation of fine coal particles resulting in fine entrapment. Similarly, the froth stability was increased in coarse coal flotation with an increase in water conductivity with a simultaneous increase in fine entrainment. However, the water conductivity did not affect the aggregation of coarse coal particles and therefore the fine entrapment through coal particle aggregates. This highlights the major difference of the effect of water conductivity on fine and coarse coal flotation.
The interaction of saline water and clay minerals and its effect on froth stability and coarse coal flotation were further studied given the importance of froth stability in coarse particle flotation. Flotation tests on a typical coal sample having a low content of clay minerals and its mixture with another coal sample having a high content of clay minerals were examined in de-ionised water and saline water with medium conductivity. It was found that froth stability was higher in the flotation with a higher concentration of clay minerals in both deionised water and saline water, corresponding to increased combustible matter recovery and mineral matter recovery. A synergistic interaction was found between saline water and clay minerals in stabilising the froth and recovering combustible matter and mineral matter in flotation. This is because saline water promoted the formation of network structures of clay minerals that entered the flotation concentrate and altered the froth property and coarse coal flotation behaviour.