Coal-derived power comprises over 39% of the world’s power production. Therefore, a mass volume of coal combustion byproducts are generated and shifted the extra burden onto the economy and environment. Circulating fluidized bed combustion (CFBC) has been found to be a clean and ultimate technology for Korea’s coal-fired power plants to have effective power generation from low-grade imported coal with reduced emissions. Efforts have been made to broaden the utilization of CFBC coal ash, and to promote sustainable development of CFBC technology. Investigations provided numerous evidences for coal ash to be a potential deposit for rare earths reclamation. However, the basic characteristics and the methods of rare earth mining from the CFBC bottom ash lack detailed understanding and are poorly reported. This study highlighted an insight of the CBFC bottom ash with respect to REEs concentration. Moreover, agents were tested as a means for leaching REEs from Samcheok CFBC bottom ash. The leaching tests were performed in relation to variations in concentration, time and temperature. The results were applied to identify suitable processes to leach REEs from the ash and clarify the potential valuation of CFBC bottom ash. The leaching conditions attained by ANOVA analysis for hydrochloric concentration, temperature, and time of 2 mol L⁻¹, 80 °C, and 12 h, were found to provide a maximum extraction of yttrium, neodymium and dysprosium of 62.1%, 55.5% and 65.2%, respectively.

In this study, we utilized lime mud waste from paper mills to synthesize calcium hydroxide (Ca(OH)₂) nanoparticles (NPs) and investigate their application for the removal of phosphorus from aqueous solution. The NPs, composed of green portlandite with hexagonal shape, were successfully produced using a precipitation method at moderately high temperature. The crystal structure and characterization of the prepared Ca(OH)₂ nanoparticles were analyzed by field emission scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction. The effects of Ca(OH)₂ NP dosage and contact time on removal of phosphorus were also investigated. The results show that the green portlandite NPs can effectively remove phosphorus from aqueous solution. The phosphorus removal efficiencies within 10 min are 53%, 72%, 78%, 98%, and 100% with the different mass ratios of Ca(OH)₂ NPs/phosphorus (CNPs/P) of 2.2, 3.5, 4.4, 5.3, and 6.2, respectively. Due to the efficient phosphorus removal, the calcium hydroxide nanoparticles (CNPs) could be a potential candidate for this application in domestic or industrial wastewater treatment.

In recent years, heat treatment has attracted attention as a means to improve the color sorting technology with the aim of improving the quality of low-grade limestone. The crucial stage in this technique is to evaluate the breakage characteristics of the sample materials after color sorting. In this study, the breakage characteristics of samples showing color differences after heat treatment and of the original raw material were investigated using a laboratory ball mill. The grinding was characterized using the population balance model. Specific rates of breakage and the primary breakage distribution were experimentally determined by first-order kinetic plots and the BII method. The breakage parameters were also back-calculated by employing a simplex method. Grinding of the three materials indicated first-order kinetics, and the experimental results were well described by the model with parameters obtained by back-calculating.

The effects of alkaline extraction on the behavior of rare earth elements in coal ashes were investigated in the present study. Independent variables are the concentration of extractant and particle size of coal ashes. Sodium hydroxide was used as an extractant, and the molarity of the solvents varied from 1.0 to 7.0 M. The coal ashes used here were fly and bottom ashes. Fly and bottom ashes were classified into four samples by particle size, with each categorized at both 45 and 300 µm. The particle size distribution and crystalline characteristics of coal ashes, leaching, and the contents of rare earth elements in coal ashes after alkaline extraction were investigated, and the effects of independent variables on the content of rare earth elements in coal ashes were discussed. The test results showed that the dissolution of amorphous phase in coal ashes mainly occurred when the molarity was not more than 3 M, while dissolution and precipitation such as geopolymerization and the formation of zeolite occurred simultaneously when the molarity was more than 5 M. The dissolution and formation of precipitates such as geopolymer in the present study affected the variation of the rare earth element contents in the ashes. Besides, the ashes being smaller in size was more favorable for an increase in the rare earth elements in coal ashes.

Blast-furnace slag (BFS) has been used as a feedstock for CO₂ sequestration by indirect mineral carbonation to produce calcium carbonate precipitates and solid residues. The most-abundant elements in these residues, Si and Al, are usually considered to be impurities that need to be removed in acid-dissolution processes involving BFS. The co-production of value-added materials from these residues is an attractive option for strengthening the economic competitiveness of mineral carbonation methods. In view of this, we separated the Si and Al, as their hydrated forms, during the dissolution of BFS in acetic acid prior to carbonation. During the sol-gel processing of Si-Al nanoparticles, a catalyst is usually required during the hydrolysis and subsequent condensation processes. In this study, only condensation occurs because the low-concentrations of acetic acid used facilitate in-situ hydrolysis during the dissolution process. Aging was carried out not only to structurally arrange the Si and Al but also to oxidize the marginal Fe(II) to reddish Fe(III). Silica-alumina nanoparticles (78% Si and 22% Al) were prepared by a simple sol-gel route at ambient pressure. These nanoparticles were amorphous and below 20 nm in size. Fourier-transform infrared (FT-IR) studies reveal that the nanoparticles consist of Si–O–Si and Si–O–Al bonds. ²⁷Al nuclear magnetic resonance (NMR) spectroscopy reveals a significant resonance corresponding to tetra-coordinated Al inside the particle framework.

In the pulp and paper industry, the white liquor obtained from the causticizing green liquor in the smelting process mostly contains NaOH and Na₂S. These chemicals are returned to the digester for reuse in the pulping process. The lime mud (LM) material is obtained from the reaction of the causticization process in paper industries. It is mainly composed of CaCO₃; it appears with a green color with a high moisture content; and it has a small proportion of impurities such as non-process elements, for example Fe, Na, Mg, Al, Si, P, and S oxides and other toxic metals. This lime mud has poor whiteness with less efficiency due to its contaminated with impurities. The recycling or reutilizing process for lime mud and solid wastes are minimizing its toxic effect on the environment. The present study proposed to improve the whiteness of the waste lime mud by the calcination and hydration process at high temperatures and reutilizing it for hand sheets, making the process improve the paper brightness. In this study, we used a lime mud sample for calcination at 1000 °C and 1200 °C for 2 h and hydration at different times (3–24 h) with different temperatures (30–80 °C) and measured the powder whiteness and hand sheet brightness. The results indicated that after the calcination and hydration process, the lime mud sample whiteness was improved and that re-utilization with pulp for making hand sheets also can improve the paper brightness. It can be concluded that waste lime mud sample purification and the re-utilization process are more advantageous in paper industries.