English
Created on 11.17
Study on high efficiency selective flocculation flotation of coal gasification fine slag with anionic polyacrylamide
This study addresses the challenge of the reduced sorting efficiency and compromised product quality due to fine particle entrainment during the flotation separation of coal gasification fine slag (CGFS). We propose a selective flocculation flotation method utilizing anionic polyacrylamide (APAM) to achieve high-efficiency separation of residual carbon and inorganic minerals in CGFS. Through examining the particle distribution characteristics of residual carbon and minerals, we systematically investigated the impact of APAM addition on flotation performance. In addition, a detailed analysis of the physicochemical properties of the flotation products and the interaction mechanisms between the APAM and mineral surfaces was conducted to elucidate the underlying flocculation mechanisms. The results show that, compared with traditional flotation processes, APAM-based selective flocculation flotation significantly enhances the residual carbon recovery, by 14.05 %. Characterization techniques including zeta potential measurements, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) revealed that the amide and carboxylic acid groups of APAM interact with mineral surfaces via hydrogen bonding, facilitating floc formation and stabilization. These findings highlight the substantial advantages of APAM for the flotation separation of residual carbon and minerals in CGFS, offering a robust scientific basis for the selection and optimization of flocculants in practical applications.
In recent years, the clean and efficient utilization of coal has become increasingly important under the dual imperatives of energy structure transformation and environmental protection . Central to this effort is coal gasification technology, which significantly enhances the efficiency of coal utilization and pave the way for diverse new applications of coal such as the production of clean gaseous and liquid fuels and the synthesis of fine chemicals . Among the various coal gasification methods, entrained-flow gasification has gained prominence due to its exceptionally high carbon conversion rate, broad feedstock adaptability and efficient production benefits . This technology enables the effective conversion of coal into syngas through reaction with gasification agents—primarily oxygen and steam—under high-temperature and high-pressure conditions, thereby providing a stable gas source for downstream product synthesis . However, downside of this process is the production of coal gasification fine slag (CGFS), a by-product consisting of a complex mixture of unreacted residual carbon and inorganic minerals . The high residual carbon content in CGFS renders direct combustion both inefficient and harmful to the environment . Additionally, the physical and chemical properties of CGFS constrain its direct use in construction materials and backfilling operations, as it fails to meet national and industrial standards for waste resource utilization and emissions control . The rapid development of China's coal gasification industry and the concomitant increase in production capacity of large gasification units has caused the production of CGFS to surge, bringing to the fore pressing challenges related to its treatment and disposal. Traditional methods such as stockpiling and landfilling not only consume extensive land resources but also pose long-term ecological risks, such as soil and groundwater contamination .
The separation of residual carbon and inorganic minerals in CGFS is crucial for its safe, large-scale utilization. Physical separation methods, including gravity separation, sieving and froth flotation, are increasingly seen as attractive options due to their environmental sustainability and cost-effectiveness . Gravity separation exploits the significant density difference between residual carbon and mineral particles to achieve effective component separation . Recent advancements include Lv et al. preliminary enrichment of residual carbon in CGFS using an inflatable-inclined liquid-solid fluidized bed, which significantly enhanced the purity of the residual carbon products . Liu et al. found that residual carbon particles in CGFS predominantly fall within the particle size range of 75–180 μm . Froth flotation, which leverages the differences in hydrophobicity of mineral surfaces, has proven effective in separating residual carbon and mineral particles . Liu et al. demonstrated that under optimal flotation conditions, residual carbon could be enriched to a purity of 59.01 % and an ash content of 37.64 %, reflecting the method’s utility in processing CGFS . Dong et al. analyzed flotation products and identified the inorganic minerals in CGFS as mostly smooth-surfaced, dense alumino-silicate with minor iron-containing oxides, while residual carbon was found to occur primarily in the form of porous flocs . Particle size variation, a critical factor influencing flotation efficacy, was shown to significantly impact recovery. Specifically, an increase in recovery from 39.04 % to 50.93 % was achieved by reducing the fine residue particle size, which underscores the importance of particle size optimization . Moreover, flotation chemicals have greatly enhanced separation precision, with Xuan et al. innovatively employing composite capture agents prepared from naphthenic acid and kerosene . This approach not only improved the flotation effectiveness but also reduced the required reagent dosage, eliminating the energy barrier between high-concentration enriched carbon particles while retaining it in enriched ash components . Thus, froth flotation represents a promising physical separation method for the efficient removal of unburned carbon from gasification slag .
The optimization of froth flotation technology is crucial for enhancing the efficiency of resource utilization and reducing production costs. However, several fundamental challenges remain to be addressed. In the context of CGFS flotation, issues such as fine mud coverage, water entrainment, and the non-selective flotation of continuous particles significantly impact the flotation efficiency and product quality . For instance, water molecules adsorbed in residual carbon pores form clusters, a phenomenon that varies with pore structure. Specifically, a more developed pore structure correlates with increased flotation difficulty and higher reagent consumption . Pre-grinding treatment, combined with secondary flotation processes, has also been demonstrated to enhance the decarburization efficiency of CGFS. This approach effectively promotes the dissociation of residual carbon and mineral particles, thereby exposing more hydrophobic surfaces and reducing the amount of flotation reagent required . Furthermore, the application of emulsified capture agents has shown promise in addressing the issue of high ash fine particle entrainment and improving the recovery rate of combustibles . This method positively impacts the flotation process by reducing the dispersed particle size of the agent and enhancing the contact angle and hydrophobicity of the residual carbon surface . Ultrasonic pretreatment technology offers a novel solution to the challenges associated with the flotation of tightly connected carbon-ash particles . This technology not only facilitates the effective separation of fine ash particles from CGFS but also improves the quality of the resulting enriched carbon, which exhibits a high low-level heat generation and a more developed pore structure . Despite these advancements, the presence of oxygen-containing functional groups in CGFS reduces its hydrophobicity. This, combined with its well-developed pore structure and high specific surface area, results in high chemical consumption during flotation, thus making the process more expensive.
To address this issue, Zhang et al. proposed an innovative pore sealing method which effectively prevents the trapping agent from entering pore channels through coal dust adhesion, while simultaneously increasing the adsorption sites for the trapping agent. This method holds promise for enhancing residual carbon recovery during flotation . Additionally, the selective dispersive flocculation technique provides an alternative optimization strategy by manipulating the interactive forces between adsorbed particles . By constructing a hydrophilic layer on ash particles surfaces, this technique enables a transition from carbon-ash selective to carbon-carbon selective agglomeration, which improves separation efficiency and reduces reagent consumption . Among them, high molecular weight flocculants such as polyacrylamide (PAM), polymeric aluminum chloride (PAC), polyacrylovinyl alcohol (PVA) and sodium polyacrylate (PAAS), have been extensively utilized due to their excellent flocculation properties . These flocculants typically contain a high density of hydrophilic groups (e.g., amino and hydroxyl groups) along their molecular chains, which enable them to facilitate the flocculation of fine-grained minerals while inhibiting the flotation of unwanted mineral components. Mechanistic studies have demonstrated that the carboxyl groups resulting from the hydrolysis of PAM can selectively adsorb onto the mineral surfaces, significantly influencing the flotation behavior of minerals . Xia et al. observed that in the flotation separation of a coal slurry-kaolin system, anionic polyacrylamide (APAM) effectively flocculated the kaolin fraction, thereby inhibiting its flotation and enhancing the yield of refined coal products . Furthermore, polyacrylamide, with its hydrophobic characteristics, exhibits substantial advantages in flocculation performance by enabling selective flocculation through hydrophobic interactions. This results in improved recovery of target minerals and overall product quality. Thus, the incorporation of flocculants containing hydrophobic groups can significantly enhance the efficiency of the combined flocculation-flotation process, particularly in the treatment of microfine-grained minerals, such as gasification slag .
The present study aims to investigate the potential application of the low-cost and widely used substance anionic polyacrylamide (APAM) as a flocculant in the flotation process, with a particular focus on its effectiveness in addressing fine particle entrainment issues. Through systematically evaluating the impact of APAM addition on the flotation products and analyzing their physicochemical properties, the authors seek to elucidate the mechanisms underlying APAM action within the flotation system. To achieve this, advanced characterization techniques—including zeta potential measurements, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were used to thoroughly analyze the interaction mechanisms between the APAM and mineral surfaces. Ultimately, this research is expected to enhance the understanding of APAM behavior in flotation systems and offer a theoretical foundation for the efficient separation of fine-grained minerals.
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