Ash Leachate Research Articles

Selective Recovery of Rare Earth Elements from Coal Fly Ash Leachates Using Liquid Membrane Processes.

Coal combustion residues and other geological waste materials have been proposed as a resource for rare earth elements (REEs, herein defined as the 14 stable lanthanides, yttrium, and scandium). The extraction of REEs from residues often generate acidified leachates that require highly selective separation methods to recover the REEs from other major soluble ions in the leachates. Here, we studied two liquid membrane processes (liquid emulsion membranes, LEM, and supported liquid membranes, SLM) and compared them to standard solvent extraction techniques for selective recovery and concentration of REEs from a leachate of coal fly ash. All separation methods involved an organic solution of di(2-ethylhexyl)phosphoric acid dissolved in kerosene or mineral oil and an acid strippant solution of 5 M nitric acid for the liquid-based separations. The LEM configuration, which separated REEs by immersing an acid-in-oil emulsion in the ash leachate, resulted in similar recovery percentages of individual REEs as the conventional solvent extraction approach. The recovery of REEs in the SLM configuration, which involved the impregnation of the solvent in a hydrophobic membrane, was slower than the LEM process. However, the SLM process was notably more selective for the heavy (and higher value) REEs, while the conventional extraction and LEM processes were more selective for the light REEs. A flux-based model of the extraction processes suggested that recovery rates were limited by REE affinity for the solvent chelator in the SLM, while the rates of REEs separation via LEM were limited by diffusive mass transfer across the liquid membrane. Altogether, these results help to identify specific steps in the recovery process that future work should target in the development of scalable liquid membrane separations for REE recovery.

The study on determination of Fluoride in Aluminum Ash Leachate

The content of fluoride ion in aluminum ash leaching solution was measured by using the ion selective electrode method accurately. The measure conditions were also studied. Theoretical and experimental results show that pH, temperature, instrument performance and coexisting ions have an important influence on the measure of fluoride ions in aluminum ash leaching solution. Hydrochloric acid was added to adjust pH closing to the neutral and adding 10 mL total ionic strength adjustment buffer solution, Controlling the pH between 5-6. The temperature was consistent with the standard solution, and the measurement result was the most accurate. The fluoride ion content in the aluminum ash leaching solution reaches 446mg/L, far exceeding the national standard.

Rare earth elements and yttrium in coal ash from the Luzhou power plant in Sichuan, Southwest China: Concentration, characterization and optimized extraction

Coal ash is considered as a promising alternative resource for rare earth elements and yttrium (REE + Y or REY) recovery. Coal ash samples (fly ash and bottom ash), as well as corresponding feed coals, collected from the Luzhou coal-fired power plant in Sichuan, southwestern China were analyzed for their chemical and mineralogical characteristics. The fly ash was further tested for its feasibility of REE extraction. The feed coals are characterized as a low-volatile bituminous, medium‑sulfur, and high-ash coal, sourced from the Guxu coalfield (Late Permian Longtan Formation) in Sichuan, SW China. REE and Y, along with high-field-strength elements Zr(Hf) and Nb(Ta), and Li, F, Sc, V, Cr, Co, Cu, Zn, Ga, Ge, Se, Sr, Cd, In, Sn, Hg, Th, U, are all enriched in the feed coal. The mineralogical compositions of the feed coals are dominated by kaolinite and illite/smectite (I/S) mixed layers, followed by the carbonate minerals including calcite, siderite, ankerite, and minor amounts of anatase and jarosite.The combustion of the feed coals produces a Class F fly ash, characterized by an aluminosilicate composition with a low CaO content. The bottom ash is compositionally similar to the fly ash, except that it is more enriched in high-density elements (Fe and Mn), and more depleted in volatile elements. REY are enriched in both the fly ash and bottom ash, although heavy REY (HREY) are notably more enriched in the fly ash. Mineralogical analysis shows that the fly ash consists of >70% amorphous glass and <30% mineral phases such as mullite, quartz and iron oxides. The bottom ash is composed of ~60% amorphous glass but with a more complex mineralogical composition than the fly ash. Dissolution of the fly ash and bottom ash with 4% hydrofluoric acid (HF) showed that ~90% of all the REY are associated with the amorphous glass in the fly ash, whereas <50% of REY in the bottom ash is contained in the glassy component. An alkaline-acid-combined (NaOH-HCl) sequential leaching process was employed to test the extractability of REY in the Luzhou fly ash. The experimental variables, including extractant concentration, liquid-to-solid ratio, leaching temperature and leaching time, were optimized by an orthogonal array design. The optimal NaOH leaching conditions results in 41.10% of active silica removal from the fly ash and 39.43% of REY enrichment. The HCl leaching of desilicated fly ash achieves 88.15% of REY extraction efficiency under the optimal conditions, which is a dramatic increase as compared to the same leaching of raw fly ash. Even though the extraction efficiency may vary between different coal ashes from various sources, a complete REY extraction procedure from fly ash is suggested for future use.

Non-dispersive selective extraction of germanium from fly ash leachates using membrane-based processes

ABSTRACTNon-dispersive selective extraction of Ge(IV) tartrates was carried out from simulated fly ash solutions containing heavy metals through supported liquid membranes (SLM). The optimum transport was obtained using a PTFE membrane containing Alamine 336 5%v/v in the condition of tartaric acid 2.76 mmol/L and HCl 1 mol/L in feed and strip phases, respectively. Under this condition, a hollow fiber (HF) SLM experiment was conducted. The results showed that this system could transport germanium from the feed to the strip phase so much faster than the flat sheet (FS) SLM system. The rate of transport through HFSLMs is comparable to dispersive extractions.Abbreviation: A: Effective area of the membrane (cm2); BDL: Below detection limit; Cf,: Feed concentration (mg/L); Ct,: Initial concentration (mg/L); D: Distribution coefficient; ELM: Emulasion liquid membranes; FSSLM: Flat sheet supported liquid membrane; HF: Hollow fiber; LLX: Liquid-liquid extraction; P: Permeation coefficient (m/h); PTFE: Poly tetra fluoro ethylene; PVDF: Polyvinylidene difluoride; T: Tartrate species (C4H4O6); T: Temperature (K); %T:Germanium transport effeciency; V: Volume of feed solution (cm3); t: time (h); η: Viscosity (P.s (c.P)); κ: Boltzmann constant; r: Ionic radius of species; τ: Tortuosity of the membrane; ρ: Porosity; α: Separation factor.

An economical lysimeter setup for fly ash leachate studies

Coal combustion in power plants produces a large amount of fly ash, which can be stored in monofill until further use. The quality of the inorganic leachate produced during storage can be studied by constructing a customised lysimeter setup as per the need of application and pertaining to principles described in ASTM D-4874. Factors affecting the concentration of heavy metals in fly ash leachate are the liquid to solid ratio (L/S ratio), pH of the leachant water, and properties of fly ash. Based on standards and literature survey, an economical lysimeter setup for fly ash leachate studies is developed and results are investigated. The heavy metal concentrations in the leachate extracted are complying with results of laboratory and field analysis concentrations documented in existing literature. Studies performed using the proposed economical setup would provide accelerated results for obtaining trace element concentrations, by which decisions of safe monofilling of fly ash is possible.

Assessment of groundwater contamination risk due to fly ash leaching using column study

Fly ash contains various elements such as copper (Cu), cadmium (Cd), cobalt (Co), chromium (Cr), iron (Fe), manganese (Mn), sodium (Na), nickel (Ni), lead (Pb), zinc (Zn) potassium (K), and calcium (Ca) that may contaminate the surrounding environment due to leaching. In the present study, Open Column Percolation Test (OCPT) was used to understand the leaching behaviour of various elements from fly ash. The columns were filled with fly ash and underlying soil layer(s) of different particle size. The initial concentrations of different toxic elements for all combinations of column varied from BDL to 0.04 mg/L for Cu, BDL to 0.01 mg/L for Cd, BDL to 0.33 mg/L for Co, BDL to 0.01 mg/L for Cr, BDL to 1.10 mg/L for Fe, 0.32 to 2.23 mg/L for Mn, 0.06 to 0.71 mg/L for Ni, BDL to 0.17 mg/L for Pb, and 1.76 to 13.1 mg/L for Zn. Similarly, initial concentrations of ions ranged from 0.08 to 105.0 mg/L for Na, 2.0 to 87.80 mg/L for K, and 3.90 to 345.0 mg/L for Ca. The concentrations of most of the elements decreased with decreasing particle size and with increasing depth of the underlying soil layers. The study proposed an approach to find a suitable combination of different soil layers below fly ash with proper ratio of fly ash and soil layers to protect the leaching of elements from fly ash. This study will help to design suitable soil liners for safe disposal or utilization of fly ash.

Fecal sludge as a fuel: characterization, cofire limits, and evaluation of quality improvement measures.

In many low-income cities, a high proportion of fecal sludge, the excreta and blackwater collected from onsite sanitation systems such as pit latrines, is not safely managed. This constitutes a major danger to environmental and human health. The water, sanitation, and hygiene sector has recognized that valorization of treated fecal sludge could offset the upfront cost of treatment by using it as a fuel source. The few quantitative studies on fecal sludge fuel published to date have focused on heating value, moisture, ash fraction, and heavy metals. However, other factors impacting fuel utility, specifically ash speciation, have not been adequately quantified for fecal sludge. This study contributes to closing that gap and shows the value of more detailed quantification. It first characterizes fecal sludge samples from Colorado and Uganda, confirms that the fuel is better if cofired with other biomass, and outlines a framework for determining safe cofire ratios. Second, the study evaluates two methods for improving fecal sludge as a fuel: carbonization and ash leaching. Carbonization of fecal sludge did not improve fuel quality, but leaching showed promise in ash reduction.

Effects of electrostatic precipitators ash leachate (EPAL) from recovery boilers on the biological treatment of effluent of kraft pulp mills

The electrostatic precipitators ash leachate (EPAL) from recovery boilers of kraft pulp mills is generated in processes dedicated to the removal of potassium and chloride salts seeking to avoid fouling and corrosion in pipes and equipment. However, the EPAL is rich in salts and has high electrical conductivity. Whenever it is sent to the effluent treatment plant (ETP), it can cause negative impacts to the biological process. It may jeopardize particularly the microbial community and the sludge bio-flocculation, causing sludge losses in the secondary clarifiers, reducing the overall efficiency of the effluent treatment plant. The objective of this research was to evaluate the influence of EPAL from the recovery boilers added to a typical effluent from a bleached eucalyptus kraft pulp mill in gradually increased doses and in sudden doses (shock loads) in the efficiency of the biological treatment. Three biological sequential batch reactors (R1, R2, R3) with a volume of 1000 ml each were tested. The Reactor R1 was operated as a reference and fed only with typical effluent from the pulp mill; Reactor R2 was fed with mill effluent with gradually increased doses of EPAL in fixed proportions until reaching a real industrial condition; Reactor R3 was fed with mill effluent and with sudden doses of EPAL. The reactors were operated in similar conditions, i.e., the concentration of dissolved oxygen (DO) was always kept higher than 2 mg L−1, COD:N:P ratio equal to 250:5:1 and hydraulic retention time of 12 h. Depending on the dose which was applied, the EPAL inhibited microbiological activity, reduced the efficiency of COD removal of the biological treatment (10% when EPAL was gradually applied and 15% in sudden doses) and increased the sludge carryover in the sedimentation phase (148% when EPAL was gradually applied and 170% in sudden dosages). Furthermore, the reduced efficiency and suspended solid carryover were more significant when sudden doses were applied compared to the application of gradual doses of the EPAL.

Graphene oxide wrapped melamine sponge as an efficient and recoverable adsorbent for Pb(II) removal from fly ash leachate.

Lead is one of the most toxic elements, which has been well recognized for its negative effect on the environment and human beings. But, preliminary methods such as chemical precipitation, membrane separation etc. and commonly used adsorbents based on adsorption technology were found to be expensive and inefficient. In this study, we modify the surface of melamine sponge (MS) with polydopamine (PDA) and then coat with glutathione/graphene oxide (GG) as the adsorbent (MS@GG) to removal Pb(II) from aqueous solutions and fly ash leachate. The maximum adsorption capacity of MS@GG was calculated to be 349.7 mg Pb/g GG, and the reaction reached equilibrium in 30 min which were both higher than raw GG material and most previously reported adsorbents due to active sites on the surface of GG, as well as the unique macroporous and hydrophilic structure of MS. Meanwhile, based on its easy separation, by using HCl as the regeneration agent, the materials revealed good reproducibility. In addition, when MS@GG was applied for the removal of Pb(II) in fly ash leachate, the removal efficiency reached up to 99.24%, indicating that the novel MS@GG was the promising candidate adsorbent material for Pb(II) removal.

Mineralogy, chemical composition and leachability of ash from biomass combustion and biomass–coal co-combustion

Abstract Ash samples from biomass combustion or co-combustion with coal were analysed. The aim of this study of ash was to determine its mineral and chemical composition, and the chemical composition of solutions obtained during one-step water extraction. Besides the chemical analysis, X-ray diffraction (XRD) and scanning electron microscopy coupled with energy dispersive spectrometry (SEM-EDS) were applied. The mineral and chemical composition of ash samples differ strongly. The content of heavy metals in the ash is generally low, but in some samples the limits of the content of some elements determined for fertilizers or soil amendments are exceeded. The relatively poor correlation between the concentration in leachate and bulk content in ash indicates that numerous elements are present in different forms in the studied samples. The results indicate that the potential use of biomass ash, or ash from biomass–coal co-combustion, requires complex studies that explore ash and leachates.

Sodium fluoride assisted acid leaching of coal fly ash for the extraction of alumina

Coal fly ash, generated in thermal power plants was processed through sulphuric acid leaching for the extraction of alumina. Both fly ash and alkali treated fly ash residue, obtained after partial removal of silica, were leached in sulphuric acid medium. Solubilisation of alumina in the acidic medium was found to be more efficient while processing the alkali treated fly ash residue as compared to the fly ash. The effect of addition of sodium fluoride (NaF) on leaching of alumina was examined at varied acid concentrations, temperatures, and time of leaching. A marked improvement in leaching recovery for alumina was observed due to the addition of NaF in the acidic leaching medium. XRD pattern of the leach residue confirmed that the alumina bearing phase i.e. mullite got dissociated in the presence of fluoride ions releasing alumina to the leaching medium for which a significant improvement in leaching efficiency was observed. A process flowsheet has been developed for the extraction of alumina from fly ash and production of a by-product, calcium silicate, for the regeneration of NaOH to make the process industrially viable.

Selenium, Sulphur, Trace Metal, and BTEX Levels in Soil, Water, and Lettuce from the Croatian Raša Bay Contaminated by Superhigh-Organic-Sulphur Coal

This paper elaborates soil, water, and lettuce contamination status with respect to selenium, sulphur, trace metals, and BTEX (benzene, toluene, ethylbenzene, and xylenes) in a coal-based area (Raša Bay, Adriatic Sea, Croatia). A local coal-fired power plant polluted soil with S, Se, Cd, and PAHs due to the combustion of domestic superhigh-organic-sulphur coal. The locality is dotted with waste from coal mining/separation, coal combustion, former metal factories, untreated municipal and coal mine effluents, along with various harbour activities, which contribute to environmental contamination. The methodology involved ICP-MS and GC-MS for the measurement of trace elements and BTEX, respectively, while soil sulphur was determined with Eschka’s mixture. The max values of the analysed trace elements in soil (mg/kg) are reported: Hg 1.14, Cd 3.29, V 624, Se 10.3, Pb 872, Cr 1860, Zn 6580, Cu 1850, and U 25.2. According to ecological indices, these values fall into the category of an extremely high level of soil pollution. Elevated total Se values in surface water are ascribed to leaching of seleniferous coal, ash, and coal-polluted soil. Levels of BTEX in water samples were very low (0–0.83 µg/L). The data provide basic information on the inorganic and organic contamination status of the Raša Bay area.

The Purification of Electroplating Industry Effluents with Agricultural Waste

The article suggests the usage of ash from sunflower husk combustion, which is constantly considered to be production waste, as a treating agent for electroplating industry wastewater purification. The chemical and mineral composition of ash waste has been researched by means of X-ray diffraction method with an X-ray fluorescent spectrometer. The structure of waste has been studied with a high-resolution scanning electron microscope. The ash contains calcium, magnesium and potassium oxides, so this waste can be used as a chemical treating agent in purifying water from heavy metals. The acid leaching of ash has been performed to determine the degree of its components transition. The trial purification of real electroplating wastes with the ash has been carried out. The purification efficiency for nickel and copper amounts to 62.0 and 48.0 % at the ash consumption 3.0 g/dm3.

Factor effect of operating conditions on the leaching of fly ashes from a pulp and paper mill

The non-process-elements (NPE's) in the pulp and paper production, in particular chloride (Cl−) and potassium (K+), eventually accumulate in a sodium sulfate recovery cycle of a paper plant, especially on boiler fly ashes (flue gas cleaning). This accumulation often leads to fouling and corrosion. Thus, in order to reduce such problems, a leaching unit is typically integrated aiming to remove Cl− and K+ and recover sodium sulfate (Na2SO4) from boiler fly ashes. In this context, this research is focused on investigating the effects of temperature, ash concentration and pH on the separation efficiency (Cl− and K+ removal and Na2SO4 recovery) of recovery boiler fly ashes using design of experiments in a devised laboratory setup. As expected from the literature, the results obtained reveals that concentration has the most significant effect on the response variables followed by temperature; whereas pH, not mentioned in this application so far, also has a significant effect. The optimum operating condition, which yields 84 wt% of Na2SO4 recovery and 97 wt% of Cl− and K+ removal, is achieved at 60 °C, 50 wt% of ash and pH 9.0. For practical applications, this optimal condition reduces the accumulation of NPE‘s and mitigates the risks of fouling and corrosion.

Cr(VI) removal using different reducing agents combined with fly ash leachate: A comparative study of their efficiency and potential mechanisms

Remediation of high concentrations of Cr(VI) in wastewater involves its chemical reduction to Cr(III), a product with low toxicity that can be easily removed. To date, NaBH4 has rarely been used to reduce Cr(VI). This article reports a comparative study of Cr(VI) removal by NaBH4 and five sulfur-based reducing agents (FeSO4, Na2S2O5, NaHSO3, Na2S2O3, and Na2SO3). The potential mechanisms of Cr(VI) removal by these six reducing agents with and without fly ash leachate (FAL) are also discussed. The results revealed that the reduction and subsequent removal of Cr(VI) are influenced by the hydrolysis and ionization of the reducing agents in solution. Thus, the reduction reaction was significantly enhanced when Na2S2O5 and NaHSO3 were added in excess of 600 mg L−1. Combined with FAL, smaller amounts of NaBH4 were required to reduce Cr(VI) to Cr(III) at pH 3.0 compared to those with the other reducing agents. NaBH4 combined with FAL at a dose of 100 mg L−1 afforded a total Cr (CrT) removal of 96.32% within 20 min, a value much higher than that obtained with the other reducing agents. The catalytic mechanism of NaBH4 for such a FAL-catalyzed Cr(VI) reduction system is similar to that of acid catalysis via the hydrolysis of the Fe(III) and Al(III) species in FAL. Improvement of the CrT removal was also observed via Cr(VI) entrapment in the structure of Fe(III) and Al(III) metal hydroxides. These results indicate that relatively low loadings of NaBH4 combined with FAL show great promise for Cr(VI) pollution remediation.

Low environmental impact process for germanium recovery from an industrial residue

This paper focused on the germanium recovery from an Integrated Gasification with Combined Cycle fly ash (IGCC FA). The global process comprised the leaching of the IGCC fly ash with an aqueous solution containing tartaric acid, retention of the germanium-tartaric chelate onto a conventional anionic resin (IRA-900) and elution of germanium. The last step consisted of the germanium precipitation which was accomplished with tannic acid. The leaching experiments were performed with tartaric aqueous solutions and with the raffinate from the subsequent ion-exchange (IX) step. Raffinate from the IX step can be reintroduced in the process as leaching solution. When the contact between FA and leaching solution was maintained for 3 h at pH = 1, the Ge leaching efficiency achieved 86%. The effect of pH, resin dosage and tartaric acid dosage on the Ge retention onto the IRA-900 resin was investigated employing a centered composite rotatable design (CCRD) for experimental design and analysis of results. The retention of Ge-tartaric acid complex onto IRA-900 was optimized using design-expert software and the optimum predicted efficiency and loading capacity were 90–98% and 3.0–3.5 mg·g−1. The experimental affinities that IRA-900 showed for the elements extracted (leached) were: Ge >> Sb > V > Ni > As > Sn > B > Zn. Different eluting solutions were tested, and more than 90% of Ge elution was achieved with 2 M HCl. Final solutions contained Ge, Sb and V. Precipitation tests focused on the optimum pH for Ge precipitation adding tannic acid to the elution solutions. A 99.3% of Ge was precipitated from eluting solution, adding 33.3 g of tannic acid per g of Ge. V also precipitated in some extent along with Ge but the rest of elements practically remained in solution.

Development of Escherichia coli-based gene expression profiling of sewage sludge leachates.

The impact of municipal waste on pathogenic micro-organisms released into the environment is a public health concern. This study aims to evaluate the effects of sewage sludge and antibiotic contaminants on stress response, virulence and antibiotic resistance in a pathogenic Escherichia coli. The effects of sewage sludge leachates on uropathogenic E.coli CFT073 were determined by monitoring the expression of 45 genes associated with antibiotic/metal resistance, stress response and virulence using RT-qPCR. The E.coli gene expression was validated using subinhibitory concentrations of tetracycline and ciprofloxacin. E. coli exposed to sewage sludge or sewage sludge+fly ash leachates altered the expression of five antibiotic and metal resistance, three stress response and two virulence-associated genes. When antibiotics were combined with sludge or sludge+fly ash the antibiotic-associated gene expression was altered. E. coli treated with two sludge leachates had distinct gene expression patterns that were altered when the sludge leachates were combined with tetracycline, although to a lesser extent with ciprofloxacin. The E.coli multigene expression analysis is a potential new tool for assessing the effects of pollutants on pathogenic microbes in environmental waters for improved risk assessment.

Dissolution and Leaching of Fly Ash in Nitric Acid Using Automated Scanning Electron Microscopy

Abstract This article studies the physical and chemical changes in two Class F fly ashes exposed to 3 % HNO3 solution for 15 min. Automated scanning electron microscopy was used to simultaneously determine the physical and chemical properties of thousands of individual particles before and after being exposed to the solution. Exposure for 15 min in the HNO3 solution resulted in the selective dissolution of particles containing over 30 % CaO content but did not cause significant physical or chemical changes in other particles for both fly ashes. In addition, the leaching of sulfur was observed from all particles. Two possible mechanisms are discussed. This work reveals that fly ash particle dissolution is not homogenous and is dependent on the chemical consistency of the particle. This work provides new insights into fly ash dissolution in aqueous solutions and characterization.

Removal of silicon from CFB-derived fly ash leachate in the context of phosphorus recovery

High concentrations of dissolved silica in the acid leachate impose two major challenges on precipitation based recovery of phosphorus (P). Firstly, co-precipitation of colloidal silica in the acidic regimes decreases the purity and value of precipitated P-products. In addition, silica scaling on internal surfaces of equipment is also a problematic issue in industrial operations. Therefore, removal of dissolved silica prior to P-recovery process minimizes the risks of Si-contamination in P-products and Si-scaling. In the present study, silica removal was achieved by accelerated silica polymerization with higher acidity and ionic strength of mineral acid, which also assisted the leaching of CFB-derived fly ash, by aging of the leachate solution. The effects of acid concentration, temperature, and stirring on silica removal rate in acid leachate were also investigated. Higher concentrations of H+ and Cl− and higher temperature significantly reduce the silica removal time. HCl was more suitable for improving leachability of phosphorous and precipitating silica in comparison with HNO3. The silica removal method discussed in this work has been shown to be capable of removing >98% of dissolved silica from the acid leachate solution.

Application of nanofiltration for Rare Earth Elements recovery from coal fly ash leachate: Performance and cost evaluation

This work evaluated nanofiltration (NF) as a potential step in the recovery process of Rare Earth Elements (REEs) from leachates of coal combustion fly ashes. A pre-treatment step, by pH adjustment and microfiltration (MF), has been studied to separate REEs by major elements. The individual and combined effects of applied pressure (12–24 bar) and NF feed acidity (pH 1.5–3.5) on rejection of six critical REEs and permeate flux have been investigated via response surface methodology (RSM). The resulting model equations were used to develop a cost model for the recovery chain, in order to select the optimum NF operating conditions. The optimization of the REE recovery chain, including pre-treatment and NF, was done with respect to the objective of maximizing the difference between NF concentrate economic value and treatment cost. NF with an appropriate MF pre-treatment has been effective in concentrating REEs from fly ash leachate, reaching the maximum potential gain at the optimum operating conditions of 12 bar and pH 3.5.