S. K. Sriramoju

Formation of Carbon Nanostructures During Chemical Demineralization of Indian Coals

Formation of carbon nanostructures of diameters less than 50 nm with an aspect ratio of 100 is achieved during the chemical demineralization of Indian coal. Demineralization was carried out using an aqueous solution of sodium hydroxide at 90°C for 5 hours. Structural analysis using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that removal of the inorganic layer, anisotropically embedded in the carbon layers of the coal, had a strong influence on nanostructure formation. The concentration of alkali and the ratio of solid (coal)-to-liquid (alkali reagent) crucially determine the extent of demineralization and thereby affect the nanostructure formation.

Characterization of Chemically Beneficiated Indian Coals

Chemical leaching using alkali and acid can be employed for effective separation of mineral matter from difficult-to-wash Indian coals. In this article, an attempt has been made to characterize these types of coals at different stages of leaching, using x-ray diffraction (XRD), Fourier transform infrared spectroscope (FTIR), scanning electron microscope (SEM), and optical microscope. Although an increase in the concentration of alkali and reaction time positively affects the ash reduction, the reaction rate decreases beyond a certain point due to the formation and simultaneous accumulation of sparingly soluble sodium compounds. The leaching process does not affect the organic macerals.

Maximizing Demineralization during Chemical Leaching of Coal through Optimal Reagent Addition Policy

Abstract The main objective of the optimal reagent addition was to maximize the quantity of product with minimal quantity of feed. In the present study, the optimal addition of reagents during the chemical leaching of coal was computed. Chemical leaching of coal was carried out using aqueous solution of caustic to dissolve and remove the mineral matter. Simulation studies were carried out using the optimal reagent addition for chemical leaching of coal in batch reactors. This was experimentally validated, using the bench-scale reactor setup with hierarchical optimization architecture. Chemical leaching experiments were conducted using West Bokaro coal. Samples collected at various time intervals during the experiment were analyzed. Variations in silica (SiO2) and alumina (Al2O3) concentrations, which were main constituents present in coal ash, were evaluated with respect to time for different concentrations of caustic. The simulation studies for optimal addition were carried out at 6, 8 and 10 intervals. An objective function, required for maximum ash removal, was solved, using sequential quadratic programming (SQP) algorithm to find out the optimum sequence for reagent dosing. An improvement of about 1% (wt) ash reduction on an average was observed with implementation of optimal reagent addition.

Mechanism of a Coal Chemical-Leaching Process and Recovery of Spent Chemicals: A Pilot-Scale Study

ABSTRACT Methods for enriching organic macerals in coal are of great importance, but, due to the yield constraint, physical-beneficiation methods are less effective. Chemical beneficiation using alkali followed by acid has been studied at the pilot stage with a batch of 500 kg of feed coal for three different coal fractions of the physical-beneficiation process using dense media cyclone and froth flotation. The mechanism of silica and alumina reactions during the alkali-leaching process has been derived and found that 150°C is the critical point for precipitation of the unwanted sodalite compound. The composition of sodalite is calculated based on the experimental results. Due to the formation of sodalite, the acid-leaching step is critical for the removal of sodalite and, hence, regeneration of acid is of great importance. A process has been developed for acid regeneration in which the first major impurity, silica, is removed by a polycondensation process and the second major impurity, alumina, is removed by a reaction with sulfuric acid. Optimum operating conditions for the regeneration process have been identified where more than 99% of the silica and 80% of the alumina can be removed from the spent acid. Almost 60% of the spent acid can be regenerated for further use and is recycled.

Parameter Estimation of Kinetic Model Equations for Chemical Leaching of Coal

Abstract Coals are invariably associated with mineral matter, which makes it unsuitable for efficient utilisation. For difficult-to-wash coals, advanced coal beneficiation technologies like chemical leaching methods are under development. In this paper, kinetic equations using different methods have been evolved, and related parameters have been estimated, using the experimental results obtained during coal leaching process. As coal is a heterogeneous rock, three different methods namely (i) parametric estimation through rate equation, (ii) non-linear regression and (iii) parametric estimation through shrinking core model have been developed and validated to check the minimum level of permitted error tolerance. Experiments were designed, using full factorial design with three variables, which are sensitive to the process. Values of activation energy and k0 obtained, using the parametric estimation of rate equation and shrinking core model, are almost in the same range. The order of the reaction for silica and alumina is two, using rate equation method. The parametric data obtained from the polynomial regression method were compared with the actual data. The exponential polynomial provides a better fit for the chemical leaching process of coal.

Upgrading Coal Washery Rejects Through Caustic-acid Leaching

ABSTRACT Physical beneficiation of West Bokaro run-of-mine coal gives 15% ash clean coal with 38% yield. Further lowering the ash content in clean coal may cause significant yield loss as well as generation of enormous quantities of middling and reject coals, which are low value material. West Bokaro coal washery of Tata Steel Ltd produces 0.7 million tons of reject coal annually. This work deals with the effect of caustic-acid leaching route for producing 19.7% ash clean coal with more than 40% yield from the rejects with 60% ash. The clean coal obtained in this process can be utilized in thermal power stations and cement industries.

Optimization study of sodium hydroxide consumption in the coal demineralization process

ABSTRACT Demineralization study on flotation tailings coal with an objective to minimize the consumption of sodium hydroxide was explored. In the present work, the effect of process parameters such as pulp density (PD) of coal slurry, alkali (NaOH) concentration and reaction temperature on the ash content of product coal and NaOH consumption was investigated. Optimum process conditions were selected based on the minimum loss of NaOH and maximum PD (so that reactor volume required per ton of raw coal treatment can be minimized) along with the desired level of ash reduction. Experimental results shows that the consumption of sodium (Na) per unit ash removal increases with increase in alkali concentration for a given PD and at the same time decreases with increase in PD for a given alkali concentration. Furthermore, it was observed that the amount of Na consumption per unit ash removal is directly proportional to the reaction temperature.

Optimization of Process Conditions for Leaching of Middling Coal

ABSTRACT In a typical coal washery, middlings are generated as a byproduct of a dense media cyclone having a size range of -13+0.5 mm and an ash content of 40–45%. Nearly 40% of the feed material is discharged as middlings. Middlings have good carbon values but embedded minerals in it and, hence, it is difficult to wash using conventional physical-cleaning techniques. Here in this article, the optimization of process variables from the lab scale to the pilot scale with a chemical-leaching technique is reported. Almost a 68% ash reduction could be achieved while maintaining the optimum process conditions. Higher reaction temperature, increased alkali concentration, and lesser reaction time favor the leaching process. Moreover, the alumina-to-silica ratio reduced from 0.53 to 0.25 and the phosphorus content significantly reduced by 90%.