# Separation of Conospheres from Lignite Fly Ash Using Acetone-Water Mixture
## 1 Introduction
The advantage of the wet separation method is the capability of obtaining low-density, intact cenospheres directly from the separation process: nonetheless, the availability of land and water is a major concern for large-scale production. Another problem is the issue of dissolution of hazardous materials into water sources [35; 36]. A further drawback is the need for an additional drying step before further use. In terms of the material quality (particularly for class C fly ash with a high calcium content), crystals are formed on the particle surface and then become hardened in the drying stage, consequently limiting their use for further applications. The dry separation is an alternative method aiming to overcome the problems of the wet separation. With this method, the chemical composition remains unchanged. In addition, there is no drying stage needed, thus avoiding energy consumption issue: nonetheless, this method requires advanced technology involving pneumatic separation, such as an air classifier, to efficiently classify the particles [37]. Air classification is an operation that separates dispersed solid particles based on their differences in size, geometrical shape, and density in the air stream. Focusing herein on the traditional and facile process, in the wet separation, water is a widely used medium to collect lightweight cenosphere particles from fly ash due to simplicity and convenient availability [29]. Cenospheres are glass spheres with a thin wall and are generally regarded as a lighter material than water with a relative density of less than 1.0 g/cc: thus, they float on the water surface and can be separated by skimming off the floating layer. Cenospheres of different densities lighter and heavier than water have been found to vary in their physico-chemico-mineralogical properties [18]. Tailoring appropriate density ranges of the medium specific to the large fraction of cenospheres to be collected has come into focus. Different liquid densities have been examined to enhance the recovery percentage in terms of the mass fractions of cenospheres collected from fly ash. Different ranges of density of the prepared mixtures have been reported in the literature. These include acetone (0.789 g/cc)-water and acetone-triethanol amine lauryl sulfate (TEALS) [23], water-zinc chloride (1.10-1.80 g/cc), lithium metatungstate (LMT) (1.5 g/cc)-water [24], a carbon tetrachloride-dibromomethane mixture (2.2 g/cc), dibromomethane (2.5 g/cc), and di-iodomethane(3.3 g/cc) [33]. To some extent, single-liquid flotation seems to be more likely not to give efficient separation of all cenosphere fractions from total fly ash. Two types of liquids with difference densities are normally adopted specifically to meet particular purposes, such as obtaining controllable particle size ranges and narrow-range density. For instance, in a water-LMT mixture, water was employed for separating the floaters of density of less than 1.0 g/cc, while LMT was specifically used for separating the floaters of density of greater than 1.0 g/cc but less than 1.5 g/cc [24]. Dilution of the solution could be prepared by mixing heavy liquids in different proportions: heavy cenospheres with a density range between 1.0 and 3.0 g/cc could be obtained [24]. Using water as a medium, cenosphere separation from low-calcium class F fly ash is mostly seen in the literature [22; 24; 27]. However, for class C fly ash, the use of water for cenosphere separation appears to have become of disadvantageous concern: when mixed with water, the soaked layer rapidly hardens and cannot be used further for other purposes [24]. The high calcium composition of class C fly ash inevitably leads to the formation of crystals, with a significant amount on the particle surface. Thus, special attention has been given to the use of nonaqueous solvents as liquid medium with the purpose of inhibiting the crystalline formation, which deteriorates the quality and recovery yield of cenospheres.