Multitudinous components recovery, heavy metals evolution and environmental impact of coal gasification slag: A review

Resource utilization of decarbonized coal gasification slag in soil quality improvement: New insights into microbial community composition and environmental risk assessment

Decarbonized coal gasification slag (DCGS) is a coal-based solid waste generated from raw coal through the processes of gasification and decarbonization. However, the excessive production of DCGS has caused large-scale environmental problems and seriously affected the sustainable development of coal chemical enterprises. It's urgent to explore a high-value utilization approach. Here, a field trial was conducted to evaluate the feasibility of soil amendment using DCGS in a sandy soil. The 16S rRNA gene sequencing, soil quality approach and partial least squares path modeling were used to assess the responses of soil properties and relative forage value (RFV) of Leymus chinensis to DCGS addition in soil-plant-microbe system. Results showed that DCGS addition significantly increased soil pH, soil organic carbon (22.4 %), alkaline phosphatase (ALP) enzyme activity (16.5 %) and α-diversity of bacterial communities (1.37 %). Soil microbial biomass CNP in DCGS1, DCGS2, DCGS3 and DCGS4 treatments were 10.7 %, 21.3 %, 44.8 % and 69.1 % higher than control check (CK) treatment, respectively. Our study emphasized the β-diversity of bacterial communities and topological parameters of microbial co-occurrence networks were significantly altered after DCGS addition. Ultimately, higher soil quality and RFV of Leymus chinensis were obtained in DCGS addition treatments rather than the CK treatment (p < 0.01). Moreover, soil pH and p_Methylomirabilota were identified as the crucial factors affecting soil quality, while soil ALP and p_Entotheonellaeota were key factors affecting RFV of Leymus chinensis according to Mantel test. Our result further evidenced that there were relatively low ecological risk level after DCGS addition (Ecological Risk Index < 150), thus DCGS addition was considered as a potential method in improving soil quality. Taking into account the impact of DCGS addition on soil microbial community, soil quality, and ecological safety, the recommended application rate for sandy soil is 60 t·ha-1 (DCGS3). Our findings elucidate that soil amendment with DCGS not only enhance soil quality and RFV of Leymus chinensis, but also provide potential possibility for safe and environmentally friendly utilization of DCGS. These findings deepened our understanding of sustainable development and efficient management of DCGS.

Environmental impact and carbon recovery in coal gasification slag after Separation-Oxidation-Acid washing (SOA) process

Coal gasification slag (CGS) is a challenging solid waste due to the presence of highly toxic heavy metals, which pose significant risks to environmental and human health. CGS cannot be freely reused or disposed of, creating considerable obstacles to solid waste resource utilization. This study presents a novel method for heavy metal removal from CGS through a separation-oxidation-acid washing (SOA) process, which effectively recycles residual carbon (RC) while minimizing the risk of heavy metal leakage. The chemical morphology, leaching patterns, and environmental risks of heavy metals in CGS following the SOA process were investigated, demonstrating effective separation and removal. The ignition loss (LOI) in CGS exceeded 45% after treatment, significantly surpassing the original value. Removal rates for Sb, Pb, and As from coal gasification fine slag (CGFS) were 88.6%, 75.88%, and 79.35%, respectively, while rates for Sb, As, and Ni from coal gasification coarse slag (CGCS) were 56.65%, 63.24%, and 50.8%. The risk assessment codes (RAC) for Cu, Co, and Sb in CGFS were reduced to low-risk levels of 3.18%, 10.30%, and 25.21%, respectively, and the relative leaching ratios (RLR) for Co (0.162%), Cu (0.006%), and Ni (0.180%) in CGCS exhibited notable reductions, suggesting that the applied process significantly mitigates the environmental risk and leaching potential of these heavy metals. Overall, this study contributes to the clean production of coal gasification enterprises by offering an environmentally friendly strategy for heavy metal removal and enhancing resource utilization through RC recovery.

Speciation distribution and leaching behavior of heavy metals in coal gasification fine ash: Influence of particle size, carbon content and mineral composition

In this study, the occurrence and distribution of heavy metals in coal gasification fine ash (CGFA) with different particle sizes were investigated to ensure safer disposal and utilization strategies for CGFA. These measures are critical to sustainable industrial practices. This study investigates the distribution and leachability of heavy metals in CGFA, analyzing how these factors vary with particle size, carbon content, and mineral composition. The results demonstrated that larger CGFA particles (>1 mm) encapsulated up to 70 % more heavy metals than smaller particles (<0.1 mm). Cr and Zn were present in higher concentrations in larger CGFA particles, whereas volatile elements such as Zn, Hg, Se, and Pb were found in relatively higher contents in finer CGFA particles. At least 70 % of Hg in CGFA was present in an acid-soluble form of speciation, whereas Cd, Zn, and Pb were mostly present in a reducible form of speciation, which could be attributed to the presence of franklinite. More than 40 % of Cd and Zn in fine CGFA particles exist in an acid-soluble form. With the exception of CGFA_1.18, Se in CGFA mainly existed in an oxidizable form at a ratio of 60 %-80 %. This could be attributed to the presence of bassanite particles as well as the higher affinity of Se for S. In contrast, Cr, Cu, and As were mostly present in residual speciation forms owing to their parasitism in quartz, sillimanite, and amorphous Fe solid solution in CGFA. Additionally, the study revealed that there was no significant relationship between heavy metal content, leaching behavior, and carbon content in CGFA. Based on combined analyses using toxicity characteristic leaching procedure (TCLP) leaching concentrations and risk assessment code (RAC) results, it is recommended to focus on the environmental risks posed by Cd, Cr, Pb, Zn, and Hg in CGFA during their modification and utilization processes.

Distribution, occurrence, and leachability of typical heavy metals in coal gasification slag

Coal gasification slag (CGS) contains variable amounts of heavy metals, which can negatively impact the environment. The mineral composition, element distribution, occurrence, and leaching characteristics of heavy metals in coal gasification coarse slag (CGCS) and coal gasification fine slag (CGFS) are studied to explain the leaching behavior of heavy metals in CGS. The movable components of heavy metals in CGFS (0.06 %-63.03 %) are significantly higher than those in CGCS (0 %-18.72 %). Leaching Environmental Assessment Framework 1313 data shows that heavy metals Zn, Cr, Cd, As, Pb, Ni, and Cu exhibit high leaching rates at low pH conditions, with Zn leaching concentrations as high as 2.11 mg/L at pH 2. Zn, Cr, and As exhibit obvious amphoteric leaching characteristics, and the leaching concentration of As at high pH (1.34 mg/L) even exceeds that at low pH (1.31 mg/L). Except for Cu, all heavy metals in CGS exceed the class III groundwater standard in some cases. Therefore, evaluation is needed before resource utilization of CGS due to potential leaching of some heavy metals.

Research status of comprehensive utilization of coal-based solid waste (CSW) and key technologies of filling mining in China: A review

Coal-based solid waste (CSW) is the solid waste generated in the process of coal mining, washing and pyrolysis, which is an important industrial solid waste. The comprehensive utilization of CSW is a key link in the process of clean and efficient utilization of coal, and the use of CSW for coal mine filling mining is an important means of "harmless, resourceful and large-scale" utilization. In order to study the research status of comprehensive utilization of CSW and key technologies of filling mining in China, this paper combs and analyzes the current situation of comprehensive utilization of CSW from three parts, namely, physical and chemical properties of CSW, Industry-related policies, and ways and means of comprehensive utilization. It is found that coal mine filling mining is a green disposal method with relatively reliable technical means, low supervision cost and large-scale disposal of CSW in the comprehensive utilization of CSW in China. Furthermore, an analysis was conducted on the current research status of key technologies in the CSW filling and mining process, including the integration of "mining, selection and filling", adsorption and complexation passivation of heavy metals in CSW, the preparation of CSW collaborative filling materials, and monitoring and control of the whole filling process, etc. Based on the above analysis and research, it was pointed out that there were some problems, namely: (1) large output of CSW and low level of comprehensive utilization; (2) high investment and high cost of CSW filling and mining; and (3) imperfect CSW waste filling mining theory and technology. In response to these issues, prospects have been made from the aspects of policy incentive mechanisms, collaborative utilization of CSW with multi-industry links, and the theory and technology of CSW filling mining. This study provided reference and inspiration for the comprehensive utilization of CSW in the world, and provides guidance for the large-scale promotion and application of CSW filling mining methods.

Adsorption performance of mineral-carbon adsorbents derived from coal gasification fine ash: Prepared via low-temperature alkali fusion method

To address the solid waste challenges associated with coal gasification fine ash, this study conducted a low-temperature alkali fusion de-ashing treatment to transform coal gasification fine ash into mineral-carbon adsorbent. The preparation process was simplified without grinding, carbonization and high-temperature (500-800 °C) activation treatment. The results demonstrate a positive linear correlation between the ash removal rate of the samples (measured during the preparation process, i.e., low-temperature alkaline fusion treatment of coal gasification fine ash) and their maximum equilibrium adsorption capacity for methylene blue. For the samples with an ash removal rate of 95.71 %, which exhibit a maximum adsorption capacity of 161.36 mg/g for methylene blue. The adsorption behavior of methylene blue on mineral-carbon adsorbent was a monolayer adsorption on the surface of homogeneous medium, involving both physical and chemical adsorption. The main adsorb rate-controlling steps for the samples with ash removal rates of 27.91-59.33 % and 95.71 % were the intra particle diffusion process and the liquid film diffusion process, respectively. The adsorption mechanism of methylene blue on the surface of mineral-carbon adsorbent involved electrostatic attraction and hydrogen bonding. The aforementioned results demonstrated the potential of coal gasification fine ash as an adsorbent material, providing new options for promoting the resource utilization and high-value applications of coal gasification fine ash.

A Preliminary Study on the Improvement of Gangue/Tailing Cemented Fill by Bentonite: Flow Properties, Mechanical Properties and Permeability

Backfill mining has significant advantages in safe mining, solid waste utilization and ecological environmental protection, but solid waste materials (tailings, gangue and coal gasification slag, etc.), as derivative residues of the chemical and metallurgical industries, contain a large number of heavy metal elements, which is posing great challenges to the underground environment after backfill. In order to study the feasibility of bentonite for reducing the permeability of gangue/tailing sand cemented backfill body, relevant tests were carried out from the basic performance index, flow performance and mechanical properties of paste backfill materials. The test results show that bentonite has a significant effect on the water secretion rate of cemented fillers, and also promotes the improvement of slump and diffusion diameter of backfill slurry. The enhancement effect of mechanical properties in the early stage is not obvious, mainly concentrated in the middle and late stages of specimen curing. With the increase of bentonite content, the 28-day uniaxial compressive strength increased from 7.1 MPa and 7.9 MPa to 8.7 MPa and 9.0 MPa, respectively. Bentonite is filled between the pores of the cemented backfill with its fine particles and water swelling, which can reduce the porosity and permeability of the gangue and tailings cemented backfill. Therefore, on the premise of satisfying the flow and mechanical properties of paste backfill, bentonite can be used to improve the permeability of cemented backfill and reduce the leaching and migration of heavy metal ions.