Generally, there are two major methods of separating unburned carbon out of other residues such as ash and minerals, which are wet and dry techniques. Dry techniques includes sieving and triboelectrostatic separations while floatation and density separation, are categorized as wet separating methods.
188.8.131.52 Sieving Method
In order to prepare coal ash for further preparation, sieving is most preferable method. Sieving method able to characterize bulk material which is in different forms and sizes. This characterization has an essential role in both physical and chemical properties. This method is typically used due to its simplicity, low cost investment and chances to separate the particle size fractions. For an example, in a research conducted by D.Oz, H. Koca, S. Koca (2004), which used floatation method in their work, sieved the coal ash prior to the floatation.
184.108.40.206.1 Mechanism of Sieving
Sieving shakers are equipped with sieving trays and holes in between so that the particles able to pass through. During this sieving process, the apertures of every sieve are being compared with the particles. The ratio of the particle size to the sieve openings and particles’ orientation are being analysed to determine the chances of a particle to pass through the sieving mesh.
220.127.116.11 Tribo-electrostatic Separation
One of the advanced technique used in the separation methods is tribo-electrostatic separation. In this method, triboelectric effect is being used when particles of two different levels of non-conducting particles are keep in contact with electric charge. In an experiment carried out by R. Dwari, K. H. Rao (2008), tribo-electrostatic method was used to analyse dry coal beneficiation. The process was carried out in a newly built fluidised bed tribo-charger which works on the basis of polarity of particle charge generated. From the experiment, they concluded that the magnitude of particles was relatively high which indicates great efficiency.
18.104.22.168.1 Mechanism of Tribo-electrostatic Separation
This equipment works based on triboelectric effect by electrifying the two different levels of non-conducting particles. Their trajectories is affected by the charge in the electric field. A separator consisting of two electrodes, where one of them is grounded. The electrodes are covered by insulating materials. This is to prevent the particles from recharging (Mach, F., K?s, P., Karban, P., Doležel, I, 2012).
Figure 6 Trajectories of Particles
Flotation is one of the separation route based on differences in the physicochemical properties of interfaces. The mechanism of flotation can take place either in a liquid–gas, a liquid–liquid, liquid–solid or solid–gas interface. According to D. Oz, H. Koca and S. Koca (2004), ash containing loss on ignition (LOI) of just 7.88% is able to separate by means of dry sieving and floatation technique. A combination of several techniques is used in order to obtain a higher purity of unburned carbon. By using this technique, D. Oz, H. Koca, S. Koca proved that they recovered concentrate of 41.65% of LOI content with 68.51% recovery.
However, in a recent experiment conducted by Z. Lu, M.M. Maroto-Valer and H.H. Schobert (2010), a series of froth floatation was carried out with the objective of separating the individual inorganic particles from the fly ash carbons. From this research, they verified that there are significant effects on the development of the surface area with increasing carbon burn-off levels as well as increasing concentration of the catalytic particles being used. Apart from that, inorganic particles which are meant to separate by froth floatation process was proven to produce a quite high LOI% ranging from 82.4% up to 90.7%. This results in the huge recovery percentage with an average of 90.5%. The high percentage of recovery validates that froth floatation is most effective method to be used for detaching carbon particles from coal ash compared to conventional floatation method.
In froth flotation, the separation occurs on a gas–liquid interface. During this process, the hydrophobic materials are selectively separated from hydrophilic. The hydrophobic particles, which possibly be molecular, colloidal, or even macro-particulate in size, are selectively adsorbed or attached to and remain on the surface of gas bubbles rising through suspension. Then they are separated from the suspension in the form of froth.
22.214.171.124.1 Mechanism of Floatation
If the particulates to be floated are completely free from other phases, the floatation process able to run effectively. In froth floatation, air bubbles are formed by injected air into a moving stream of an aqueous slurry comprising mixtures of particles. The bubbles formed will then collide and binds to those particles which have attained hydrophobicity. It will rise to the surface and develop as the product, froth. The froth will be collected in the concentrate launder. Meanwhile, the remaining hydrophilic particles remain in the pulp and will be drained out as tailings. A simple representation of the process is shown in Figure 6 below. The size of floatation cell differs from a laboratory scale model to commercial cell. For the laboratory model, the cell is able to accommodate a volume of about 2 litres while for commercial usage, the volume can rise up to 200,000 litres (Yoon. R.H., 2000)
Figure 7 Schematic Diagram of a Floatation Cell
In a similar research conducted by Fan M, Tao D, Honaker R, Luo Z (2010), they analysed the effect of the generation of the nano-bubble in the froth floatation set up. As previous researches shown that the probability of attachment and at the same time decrease the chances of detachment can be improved by the presence of the microbubble or nano-bubble, Fan M et al. studied its effect on different set up which are pilot scale, laboratory scale and also specially designed pilot scaled floatation column. The varied parameters were flow rate fraction between the nano-bubble and conventional sized bubble generators, superficial air velocity, collector dosage, both frother and feed solid concentrations, feed rate and etc. As a result, they proved that the presence of the nano-bubbles in both mechanical cells and column flotations increased the carbon recovery up by 8% – 27% while the separation selectivity index was inclined up to 34%. The highest yield, 98.4% was achieved by using coal ash which was in the range of 355~180 micron.
126.96.36.199.2 Chemicals of Floatation
To guarantee that the floatation process is highly selective and efficient besides producing optimum conditions, floatation chemicals are being used. They can be categorized into three major groups based on their role which are collectors, frother and also modifier, also known as regulator.
Collector is the surfactant which capable of creating the surface to possess hydrophobic characteristics. It usually own at least one non-polar group and usually belongs to a hydrocarbon group. Due to the attraction between the polar and non-polar groups, the collector tends to absorb onto the particles, positioned towards the bulk solution and eventually transmit hydrophobicity to the particles. Furthermore, collector also have a role to control the characteristics of gas-liquid interface.
As most of the collectors are non-ionic, slightly soluble compound which undergone mono-hydroxylation process are added to stimulate a desired froth stability. Three-phase foam which is generated when the bubbles selectively collected and risen to the surface is known as froth. It is also the desired concentrate or product of the floatation process (Schramm L.L., 2005). These additives are known as frothers.
Apart from that, some organic or inorganic reagents acts as modifying or regulating agent in a floatation process. They further optimizes the separation process by modifying the properties of the solution. For an instance, copper sulphate is added in the floatation of sphalerite with xanthate in industry at high pH values in order to generate low solubility products (Schramm L.L., 2005).
188.8.131.52 Density Separation
Density separation is also known as gravity separation which can be used to separate particles of two different densities. Dense particles is to be separated from light particles for instance, extracting unburned carbon from the coarse fraction coal ash. Coal ash such as fly ash which has high LOI value is preferably to separate by using this method. Hollow cenospheres which has low specific gravity able to discrete by density separation method.
184.108.40.206.1 Mechanism of Density Separation
A hydraulic classification device which equipped with pyramidal discharging units is density separator. All over the upper unit, a rising current of water is being introduced by means of pre-determined flow injection. These injections are installed so that water is directly flow straight to bottom.