Pratik Swarup Dash

Chemical Leaching of High-Ash Indian Coals for Production of Low-Ash Clean Coal

It is difficult to reduce the ash content in Indian coals below a certain level by conventional physical beneficiation techniques due to their poor washability characteristics. In the present work, the effect of aqueous alkali leaching at elevated temperature, followed by acid washing on the removal of mineral matter, was evaluated for different captive coals of Tata Steel. The research study revealed that the ash content of all these coals could be reduced by more than 50% using this method. The degree of demineralization improved by increasing the reaction time, alkali concentration, and temperature, and by reducing the coal particle size. No significant change was observed in the alkali content, whereas silica, alumina, and phosphorous content reduced significantly after the final acid treatment. The mechanism of demineralization was evaluated by analyzing the coal samples before and after the alkali and acid treatments using X-ray diffraction (XRD) techniques. Overall, this study provides an insight into possible alternative methods of beneficiation for removal of ash from physically beneficiated high-ash Indian coals.

Use of waste plastics in cokemaking at Tata Steel

Abstract Disposal of waste plastics is a serious global issue, and hence it is imperative to explore methods to recycle them for efficient utilisation. The coke making process is considered to be a promising area to which the thermal decomposition of waste plastics is applicable, because the process involves coal carbonisation at a high temperature under a reducing atmosphere. As an outcome of the research work carried out on a laboratory scale followed by plant trials at Tata Steel, it was found that waste plastics recycling process using stamp charged coke ovens is feasible to recover coke, tar, light oil and gas from general waste plastics mixed in coal by carbonisation in coke ovens. It has also been established in the laboratory that waste plastics up to 1% can be used in coke making without any deterioration in coke strength. Plant trials have also indicated that co-carbonisation of waste plastics with coal is a possible solution for its disposal.

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.

Prediction of coke quality using adaptive neurofuzzy inference system

Abstract The production rate, quality and cost of hot metal production through the blast furnace route depend mainly on the quality of the metallurgical coke. Coke reactivity index and coke strength after reaction (CSR) are the most important parameters used for the assessment of the high temperature properties of coke. Many coke plants and blast furnaces around the world use CSR as a specification just as important as cold strength, size and chemistry. The present work aims to fulfil the need for a model that will predict the coke CSR from coal blend characteristics. In this work, the functional relationship between the coal blend properties (ash, volatile matter, average vitrinite reflectance, crucible swelling number, total reactives, vitrinite distribution V9–V13 and basicity index) and the corresponding coke CSR has been mapped using an adaptive neurofuzzy inference system (ANFIS). The ANFIS model is formulated with different sets of coal blend properties as input variable, and the singular value decomposition and QR factorisation based techniques have been employed for model reduction. It has been found that the developed ANFIS model predicts the CSR with reasonable accuracy.

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.

Prediction of Coke CSR from Coal Blend Characteristics using Various Techniques: A Comparative Evaluation

At Tata Steel, laboratory tests are carried out to see the suitability of the coal blends using imported coals from new sources in coke making and for evaluating the quality of coke produced. The present work is aimed to fulfill the need of a model that will predict the coke properties from coal blend characteristics so that optimization of coal blends for producing desired quality of stamp charged coke could be done easily, quickly, and with a lesser number of carbonization tests in a 7 kg test oven. CSR is predicted with reasonable accuracy from 8 coal blend characteristics (ash, volatile matter, average vitrinite reflectance, crucible swelling number, total reactives, total inerts, vitrinite distribution [V9–V13], and Basicity Index), using different statistical analysis tools (MLR and PCR) and the ANN technique. ANN using the MLP network was found to be most suitable technique for coke properties prediction followed by PCR and MLR.

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.

Laboratory scale investigation on maximising utilisation of carbonaceous inerts in stamp charging to improve coke quality and yield

Abstract Stamp charging has been established as a versatile technology which not only improves the coke properties that can be obtained from a given coal blend, but also broadens the coal base for cokemaking, permitting the use of inferior coals without impairing the coke quality. To produce high strength coke at least cost, Tata Steel ventured into stamp charging in combination with 40–45% low ash imported coals. Research carried out by R&D, Tata Steel revealed that stamp charging has the potential power to digest even non-softening carbonaceous inerts, if the coal blend is suitably designed. Addition of such inerts would enhance the carbon content in the resultant coke. However, the use of these inerts calls for judicious selection of other components of the coal blend and optimisation of the blend proportion, or otherwise these inerts may have an adverse impact on the coke quality. Addition of such materials would also contribute to higher coke yield, since the volatile matter contents of these materials are very low. A quantum jump in the coke yield can be envisaged only if the addition of these low ash carbonaceous inerts is substantial (>10%). Therefore, it is imperative to design the base stamp charge coal blend so that it has enough fluidity to assimilate these infusible inerts and still produce coke with high coke strength after reaction. The present paper describes the scope of utilising low ash carbonaceous inerts such as fluid coke and anthracite optimally in the stamp charging blend by replacing the present imported semisoft coal with high fluidity imported coals. The laboratory scale experimentation and the results obtained with respect to improvement in coke properties and yield have been described. The results are also supported by the microscopic investigation of the coke samples.