Reduction of Ash Content in Raw Coal Using Acids and Alkali

Ash, an inorganic matter present in coal is amenable for dissolution using suitable reagents. Thus the dissolution of ash and its subsequent removal reduces the release of many toxic elements into the environment by coal based industries. Removal of ash also enhances the calorific value. In the present investigation an attempt has been made to reduce the ash content of raw coal obtained from nearest thermal power by using hydrochloric acid, sulfuric acid and sodium hydroxide. A series of leaching experiments were conducted on coal of different size fractions by varying the parameters like concentration, temperature and time of leaching. The results indicate that it is possible to remove nearly 75% of ash from coal sample by leaching.


Introduction
India is endowed with vast deposits of coal, but the Indian coals possess very high ash content 1 .The composition and properties of ash in coal depend on its type and origin.Different coal samples exhibit different leaching behaviors as the elemental composition of ash varies from one sample to another.The pH and time of leaching have been found to influence the leaching behaviors 2 .Some coking coals with reduced ash content have been found to be suitable for graphitization which is industrially very important 3 .
World wide production of coal is estimated to be more than 550 million tons 4 .Many potentially hazardous elements are released in to the environment.Their impact on the environment including that of ground water has been assessed [5][6][7] .In order to reduce the emission of toxic elements in to the atmosphere during combustion different chemical pretreatments like acid leaching to decrease metal contents have been reported by Seferinoghi et al and Sakanishi et al. 8,9 Mehtap Paul et al 10 have suggested combination of bacteriological oxidation followed by acid leaching for different types of coals.J.Y.Carbon et al 11 have evaluated the behaviour of coal in seawater to leach out trace elements.
In the present investigation, an attempt has been made to reduce ash content from raw coal using hydrochloric acid, sulfuric acid and sodium hydroxide.A series of experiments were conducted by varying the parameters like concentration of leachants, time and temperature of leaching.

Experimental
Coal samples, for the experiment, were obtained from the Bellary Thermal Power Station.The sample consisting of larger sized lumps, was crushed by using Jaw and Roll crushers followed by grinding.The ground sample was sieved and separated into three different size fractions; (i) 2.3 mm to 1.0 mm; (ii) 1.0 mm to 0.15 mm; (iii) < 0.15 mm All the samples, including feed coal, were analyzed for their moisture content, volatile matter, ash and fixed carbon.The results are tabulated in Table 1.The chemicals used for leaching were of laboratory grade and distilled water was used prepare solutions.
Accurately weighed (10 g) coal sample along with 100 mL leachant were taken in a conical flask fitted with a water cooled condenser.Leaching experiments were conducted on magnetic stirrer with heating facility.Following parameters were varied during leaching.1.5 M, 3.0 M and 6.0 M 4. Size fractions : Size -1, Size-2 and Size-3 After leaching, the contents of the flask were filtered and the residue was washed with water or dilute acid.The residue was subjected to ash analysis and the leaching conditions were optimized by conducting a series of experiments.

Results and Discussion
The analyses of feed coal and different size fractions show that the ash content in the sample is very high.The results in the Table 1 indicate that size fraction 3 has more ash content.As many as 240 leaching experiments were conducted by using hydrochloric acid, sulfuric acid and sodium hydroxide.The results are tabulated in Tables 2, 3 and 4

Effect of concentration of leachants on leaching
Reduction of ash content in the leached coal sample by acids may be due to acid soluble components like carbonates, iron oxide and sulfides.The results indicate that a maximum of57% of ash can be leached by using 6.0 M hydrochloric acid at boiling temperature for a period of 60 minutes.Whereas under the similar conditions, only 40% ash reduction was possible with sulfuric acid.Sodium hydroxide has been found to be better leachant than the acids.Up to 75% ash reduction was possible with sodium hydroxide.This may be due to the presence of more amounts of acid insoluble silicates, aluminates and pyrites in coal in the form of ash which easily react with alkali rather than acids.In all the leaching experiments involving alkali, the residue was washed by using dilute acids.This acid was responsible for the removal of some of the acid soluble inorganic matter.Following chemical reactions have been proposed.

Effect of particle size on leaching
It is evident from the results that leaching of ash increases with decrease in size of particles.This may be due to the fact that reduction in size of coal would lead to increased liberation of ash particles from lump.Also decrease in size increases the surface area which enhances interaction between leachant and ash particles.

Effect of temperature and time on leaching
The experiments were done at room temperature, 70 o C and 100 o C. The results show that increase in temperature increases leaching.Probably, increase in temperature favored the reaction between ash particles and leachants.Increase in temperature must have enhanced activation energy also.At 100 o C, maximum leaching has been possible.Three time periods of 15, 30 and 60 minutes were selected for conducting leaching experiments.It is obvious from the results that leaching increases with increase in time.As the residence of time of coal with leachants increases the reaction between ash particles and leachants is favored

Conclusion
Though various physical and physicochemical methods are available for reducing ash quantities in coal chemical methods have an advantage over them.Various toxic elements can be leached out into solution by using different leachants.The results indicate that sodium hydroxide is a better leachant than hydrochloric acid and sulfuric acid.As the size of the sample decreases, leaching is favoured, because of the increased surface area.Leaching has been found to increase with increase in temperature, concentration and time of leaching.

Table 4 .
Effect of sodium hydroxide on leaching of ash