Chemical demineralization of a Turkish high ash bituminous coal

Abstract

Demineralization of coal prior to usage offers some technical and cost-effective advantages since one of the most common limitations on coal processes results from high mineral matter content. In this work aimed at reducing the mineral matter content of high ash, low sulfur Amasra bituminous coal from Turkey, since physical methods were of limited use for reducing the ash content of bituminous coals, demineralization was studied using different acids (HF, HCl, HNO₃ and H₂SO₄) alone and 0.5 N aqueous NaOH in combination with either one or two of the acids. The use of extraction with 0.5 N NaOH followed by leaching with 10% HCl was the optimum approach for the chemical cleaning of the coal under study, the maximum degree of demineralization obtained being 46.78%.

Introduction

High ash and/or high sulfur coals are unsuitable for efficient use in carbonization, combustion, gasification or liquefaction. Due to the energy situation and the gradual depletion of high quality coal reserves of the world, more attention is being given to demineralization and/or desulfurization of low grade high ash and/or high sulfur coals to obtain environmentally acceptable clean fuels. The possibility for and the methods of upgrading by demineralization depends on the structure and composition of the minerals in coal, which again depends on the place and time of their deposition and formation 1, 2.

Mineral matter in coal exists essentially in three forms: true minerals, dissolved salts in pore water, and the elements associated with the hydrocarbonaceous matrix [3]. The major minerals in coals are silicates or shales (kaolinite type), quartz and/or sandstone, pyrites and carbonates such as siderites and ankerites [1]. The coal ash results from the mineral matter in both the organic and inorganic parts of coal and only a small fraction volatilizes during ashing. There are some macrocomponents whose concentrations generally exceed 1 wt.% in ash. These are firstly Al, Ca, Fe, S and Si and secondly, K, Mg, Na, P and Ti. It seems that excluding Si, which is almost entirely of mineral origin, the other inorganic elements have a mixed abundance in mineral and organic matter in a proportion which depends on the structure of coalification area [4]. High ash (>20%, weight) coals show increased contents of H, N, O, S and minerals such as quartz, illite, calcite, pyrite and gypsum. Their ashes are enriched in SiO₂, Fe₂O₃ and K₂O [5].

The effects of treatments on various forms of mineral matter may be expected to be different. In many instances, some demineralization can be effected by grinding to liberate the minerals and then by carrying out a separation based on differences in physical properties of minerals and carbonaceous part of coal [6]. However, a simple physical separation may not be adequate when mineral grains are tiny and strongly bound to the surrounding carbonaceous material, as in some low grade deep-mined bituminous coals; grinding fine enough to liberate mineral grains may be impractical. While some demineralization can be achieved for many fuels by simple physical processes, chemical cleaning is known as being the only solution to obtain clean demineralized coals [6].

Chemical processes, in general, are likely to be more expensive than physical processes in terms of both capital and operating costs. The methods used for chemical cleaning of coals include among others, acid leaching, alkali leaching under high pressure and at elevated temperatures, leaching by molten caustic baths 7, 8. Demineralization can be attributed to the dissolution of water- and acid-soluble parts (generated during reaction) of the mineral matter present in coal, and to the conversion of the non-water soluble portion into soluble parts due to oxidation of mineral matter such as pyrite. When sulfur is present as mineral pyrite, demineralization is also accompanied by desulfurization [9]. It has been found that using dilute to moderately concentrated NaOH solution, kaolin is converted into crystalline sodium derivatives. The solubility of this sodium-alumino-silicate derivative is not very high in alkali solutions but it is fairly soluble in dilute acids. This forms the basis for coal cleaning using dilute alkali followed by washing with acid. Chemical cleaning of coal may be simple when it operates under mild conditions but the cost of chemicals needed, particularly the acid and alkali, would make the process unattractive. Thus, several efficient methods of regeneration, recovery and recycling of both acid and alkali have been developed [1].

In Turkey there are 1.3 billion tons of bituminous coal reserves that are high in ash and low in sulfur. At present, there are only four coal preparation plants in operation for improving the quality of the run-of-mine bituminous coal. In these plants, the top size of run-of-mine coal is controlled only by crushing to meet the specifications of metallurgical plants [10]. So, demineralization of turkish bituminous coal prior to usage should be under consideration with more emphasis.