A review of chemical leaching of coal by acid and alkali solution

Abstract

Low rank or low-grade (LRC or LGC) coals are most abundant distribute around the several regions in the world. The contribution of low-rank coal is a significant role in the energy sectors and chemical feedstock to the industries. The hard coal reserves are gradually depleting, and the mining operation at deeper coal seam with greater difficulties as well as the cost of exploration is so high, which has a significant issue for plant economics. Therefore, the low-grade coal can be used as an alternative energy source to minimize these problems. Low-rank coals are usually associated with high mineral matter and moisture content, which exerts substantial impacts on their consumption including pyrolysis, liquefaction, gasification and combustion process. In order to understand the essential treatment of coal for efficient removal of mineral matters and improve coal properties by beneficiation techniques are crucial to developing advanced technologies. The present article provides a comprehensive overview of the various processes concerning demineralization of coal by chemical beneficiation technique. It has been found from the study; the degree of demineralization was greater in chemical beneficiation compared to physical beneficiation. It is because the chemical reagents are attacked to the interior of coal which removes the inorganic materials and finely dispersed minerals from the coal matrix. The chemical methods have separated all types of minerals from the coal matrix. However, the separation of minerals by the physical method depends on the mineral properties. Chemical beneficiation is an appropriate method to reduce both organic and inorganic mineral constituents from the LGC by leaching method. The chemical reagents are diffusing to the interior of coal matrix through the pores and subsequently dissolute the minerals. Throughout the study challenges, the chemical cleaning of low-grade coal has been efficient techniques for reducing the minerals to a minimum level that can be upgraded to high rank coal.

Froth flotation beneficiation and physiochemical characterization of coal from Achibo-Sombo-Dabaso area, southwestern Ethiopia

“…Therefore, to solve these problems different researchers have tried to develop different beneficiation technology to minimize the impurity of coal and enhance the energy density ( Mishra et al., 2015 ; Liu et al., 2018a , Liu et al., 2018b ; Behera et al., 2018 ). Coal beneficiation is the process of improving the fixed carbon content to increase the calorific value of the coal by removing the mineral matters.…”

“…Coal beneficiation is the process of improving the fixed carbon content to increase the calorific value of the coal by removing the mineral matters. It involves the combustion behavior, reducing slagging, fouling characteristics, and minimizing particulate pollutants and gas emissions ( Behera et al., 2018 ; Meshram et al., 2015 ).…”

“…Different physical, chemical and physiochemical beneficiation techniques are used for upgrading low-rank coals ( Behera et al., 2018 ; Barma, 2019 ; Qu et al., 2015 ). The Froth flotation beneficiation technique is the most cost-effective and less expensive physiochemical beneficiation to separate finely dispersed minerals or mineral materials based on the hydrophobicity of coal particles and mineral matters.…”

Leaching of coal by trioxoboric acid for coal cleaning

“…Chemical enrichment by leaching of low-grade coals is a suitable method to reduce both organic and inorganic mineral components. Chemical leaching is the most effective method for removing undesirable impurities in coal in order to obtain ultra-clean coal products with an ash value of less than 1% (Behera et. al.…”

Influence of coal treatments on the Ni loading mechanism of Ni-loaded lignite char catalysts

“…Acid treatment demineralized some minerals in coal such as carbonates, Fe 2 O 3 , and sulphides; however, it could not leach the clay-bearing minerals. 9 For alkali treatment, alkali solutions such as NaOH could dissolve silica and alumina and other silica-alumina clay-bearing minerals in coal and convert them into sodium silicate and sodium aluminate, which were leached out into the solution. 10 For the alkali followed by acid treatment, the combination of alkali and acid treatments was one of the great methods for ash removal in coal because the acid treatment could dissolve sodalites and sodium-aluminosilicates that were formed after the alkali treatment.…”