Reduction of iron from red mud with aluminum dross to capture heavy metals from municipal solid waste incineration fly ash into alloy

Shen H., Lou B., Liu B., Zhang J., Zhang X., Liu J., Zhang R., Chen M., Zhang S.

Municipal solid waste incineration (MSWI) fly ash is a hazardous waste containing heavy metals. Secondary aluminum dross (SAD) is a hazardous waste discharged from aluminum smelting, containing active aluminum nitride (AlN). In this work, heavy metals from MSWI fly ash were reduced into alloy by AlN from SAD, and the slag was manufactured into transparent glass for building. Reduction of iron and zinc was 67 and 100 %, respectively. Reduction mechanism was explored after applying XRD, XRF and thermodynamics analysis. It was found that the reduction reaction was an ion reaction. The AlN and heavy metal oxide transformed into anionic group containing nitrogen and heavy metal cation, after entering slag. The heavy metals were reduced into alloy after electron was transferred from anionic group to cation. In addition, the reduced iron and zinc could merge into alloy, which inhibited evaporation of zinc. Yellow transparent glass was obtained after the reduction process. Yellow was come from titanium oxide, which could not be reduced by AlN. Microhardness, density and water absorption of the transparent glass were 741 HV, 2.86 g·cm−3 and 0.04 %, respectively. Leaching content of Ni, Cu, Zn and Pb of the glass were 0.1,

He D., Zhou F., Meng H., Lu X., Xie H., Wang X., Dong L.

Thermal separation of heavy metals from municipal solid waste incineration fly ash (MSWI FA) is a promising approach for both fly ash purification and heavy metal recovery. However, its commercial viability is hindered by excessive energy consumption. In this study, we introduce carbothermal reduction for MSWI FA treatment as an innovative strategy to enhance heavy metal volatilization and reduce the required heating temperature. By incorporating carbon (FA/Carbon = 1:0.2), the volatilization fractions of Pb, Cd and Zn reached 96.5 %, 100 %, and 63.9 %, respectively, when subjected to 900 °C for 2 h. Remarkably, these volatilization amount surpassed the volatilization fractions attained when raw fly ash (RFA) was independently calcined at 1000 °C for 2 h: Pb (90.2 %), Cd (92.9 %) and Zn (30.2 %). Conversely, Cu volatilization was impeded by carbon inclusion. Comparable trends were shown in washing fly ash (WFA). Notably, concentrated carbothermal reduction led to substantial concentrations of Pb (2.5–7.1 %) and Zn (4.9–20.7 %), rendering them amenable for recycling through metallurgical routes. Intriguingly, higher carbon content (FA/Carbon = 1:0.3) hindered rather than enhanced heavy metal volatilization from RFA. We found that condensation of NaCl and KCl occurred in carbon pores, causing the formation of molten eutectic which trapped the heavy metals at high temperatures. Therefore, more carbon showed more enhancement of heavy metal volatilization from WFA which was free of NaCl and KCl. Ultimately, two comprehensive pathways for the resourceful utilization of fly ash based on carbon thermal reduction were proposed and compared. This study demonstrates carbothermal reduction as an effective approach for simultaneous purification and metal recovery of MSWI FA, exhibiting promising potential for industrial application.

Huang J., Jin Y., Chu X., Shu Z., Ma X., Liu J.

In this study, a new lead (Pb) and chlorine (Cl) recovery process via the thermal co-treatment of Municipal solid waste (MSW) incineration fly ash (FA) and waste cathode-ray tubes (CRT) was developed and the synergistic effects under different CRT ratios, temperatures, and residence times were comprehensively investigated. Thermogravimetric experiments revealed that the co-processing of FA and CRT exhibited a remarkable synergistic effect as evidenced by the considerable increase in mass loss and mass-loss rate when compared with the theoretical values. When the mixtures with 50% CRT addition was treated at 1200 °C for 60 min, Pb removal rate reached the maximum value of 98.67%, and the Cl removal rate considerably increased by 37.32% compared to that with FA treatment alone. Additionally, the Cl content in the residue was < 2%. It was mainly attributed to the volatilization of chlorides, such as PbCl2, NaCl, and KCl. CaCl2 generated from the decomposition of CaClOH in FA was conducive to improve Pb removal in CRT through indirect chlorination and destroying the glass structure in CRT. Co-processing of FA and CRT demonstrates promising potential for several benefits, including the reduction in melting temperature, recovery of Pb and Cl from secondary fly ash, and the reutilization of calcium-rich slag.

Zhang Z., Zhao C., Rao Y., Yu C., Luo Z., Zhao H., Wang X., Wu C., Wang Q.

Incineration is currently the most common method of treating municipal solid waste. Municipal solid waste incineration fly ash (MSWI FA) contains a high concentration of toxic heavy metals (HMs), making it a hazardous waste. A series of detoxification treatments are required to reduce the toxicity of fly ash. Furthermore, the environmental risk of MSWI FA after treatment is becoming a cause of concern. This paper reviews the primary ash properties, pH, liquid-solid ratio, and other factors (microorganism, type of leaching agents, etc.) that affect the leaching of HMs from MSWI FA, compares and summarizes the most widely applied solidification/stabilization (S/S) techniques. In particular, models and methods for the environmental risk assessment and prediction of HMs are classified and described in detail. Finally, the inadequacy of current S/S techniques for MSWI FA is pointed out, which may be useful for upcoming studies on this topic.

Shen H., Liu B., Liu Y., Zhang J., Zhang B., Zhang X., Liu J., Zhang S.

Red mud is a waste discharged from alumina production by Bayer process. It contains iron and titanium resources. Secondary aluminum dross (SAD) is a waste discharged from aluminum smelting and processing. It contains hazardous reactive aluminum nitride (AlN). In this work, iron and titanium in red mud were reduced by AlN in SAD by pyrometallurgy, and slag was manufactured into glass ceramics. Recovery rates of iron and titanium were 97% and 75%, when addition amount of red mud was double of SAD. The reduced iron and titanium were in form of metal iron and titanium nitride (TiN). Reduction process consisted of two steps: AlN, iron and titanium entered aluminosilicate network of melt; AlN reduced iron and titanium. Density, water absorption and microhardness of the glass ceramics were 3.10 g cm−3, 0.03% and 1195 HV. Utilizing 1 ton of SAD to replace carbon for iron reduction could reduce carbon emissions by 407 kg. In addition, co-disposing 400 kg red mud and 200 kg SAD could gain profit of 293 dollars. This work realized the total recycling of both red mud and SAD.

Tang J., Dong S., Feng Q., Su M., Wei Y., Liang J., Zhang H., Huang L., Kong L., Wang N., Xiao E., Liu Y., Tang X., Xiao T.

While municipal solid waste incineration (MSWI) fly ash is classified as hazardous waste , it can also serve as an urban mining source for numerous precious metals. Of particular interest are antimony (Sb) and zinc (Zn); the former of which is a strategic and critical metal that is being rapidly depleted, putting society at high risk for supply shortages. In this work, a two-step leaching method for recovering Sb and Zn from MSWI fly ash is proposed. Furthermore, the leaching behavior and adsorption mechanism of Sb in the MSWI fly ash waste stream were also investigated. Results from the first constant pH leaching tests (CPLT) showed that under diluted acidic condition, the maximum amount of Sb released from fly ash was ∼20%. In addition, at pH 4.0, 67% of the fly ash was dissolved, while 79.3% and 12.1% of the Zn and Sb, respectively, were recovered. After optimizing and executing a second Sb leaching procedure (6 M HCl solution at 60 °C), >80% of the Sb was recovered. Thus, the proposed two-step leaching process, consisting of extraction followed by decontamination using a magnetic HAP@CoFe 2 O 4 adsorbent, can eliminate the Sb in fly ash effluent with a removal efficiency >95%. Moreover, this process produces less toxic products and lowers the effluent residue concentration. As such, the two-step process described herein is suggested for Sb and Zn recovery from fly ash; as it not only enables precious metal recovery, but also aids in treating secondary waste streams produced from urban mining. • An integrated method for fly ash and secondary waste treatment was proposed. • Both Zn and Sb have great separation factors by pH-dependent leaching at pH 4. • >80% of Sb in residue was leached out using 6 M HCl at 60 °C. • Heavy metals in effluent were fast and effectively removed by HAP@CoFe 2 O 4. • Treated residue and effluent as by-products were more stable and less toxic.

Shen H., Liu B., Zhang J., Liu J., Zhang S.

Secondary aluminum dross (SAD) and pickling sludge were hazardous wastes . In this work, aluminum nitride (AlN) from SAD was used to reduce heavy metals from pickling sludge by pyrometallurgy . Reaction mechanism was studied after exploring transformation of heavy metals and AlN during reaction process. It was found that reduction reaction between heavy metals and AlN was homogeneous reduction reaction. When mixture was heated to 1000 °C, heavy metals and AlN were distributed independently, and reduction reaction had not occurred. When mixture was heated to 1300 °C, heavy metals and AlN entered aluminosilicate network, and homogeneous reduction reaction occurred. Half of the heavy metals were reduced into metallic state and mixed with the dross. When the mixture was heated at 1400 °C for 1 h, metal block separated from dross. Reduction rates of Fe, Cr and Ni were 94%, 88% and 100%. Like water solution, aluminosilicate melt could dissolve heavy metal oxides , aluminum compound (AlN, Al 4 C 3 , etc.) and silicon compound (Si 3 N 4 , SiC, etc.). This work proposed a novel homogeneous reaction environment, aluminosilicate melt ‘solution’ environment, which had a significant effect not only on heavy metals reduction, but also on other high temperature oxidation-reduction. • AlN in SAD was used to reduce heavy metals by pyrometallurgy. • Reduction rates of Fe, Cr and Ni were 94%, 88% and 100%. • Reaction mechanism was studied by exploring AlN and heavy metals transformation. • Reaction between AlN and heavy metals was homogeneous reduction reaction. • Aluminosilicate melt ‘solution’ was proposed for homogeneous reaction.

Shen H., Liu B., Liu Y., Zhang J., Liu J., Zhang S.

• Fe, Cr and Ni were reduced by AlN in SAD. • Reduction rates of Fe, Cr and Ni were up to 97, 91 and 100% • Using a ton SAD for chromium metallurgy could reduce carbon emission of 3961 kg. Secondary aluminum dross (SAD) is a hazardous solid waste due to containing aluminum nitride (AlN). In this work, AlN was used to reduce heavy metals in pickling sludge by pyrometallurgy. Reduction extent of Fe, Cr and Ni was up to 97, 91 and 100%. However, it was found that some AlN was oxidized by oxygen. AlN oxidization could be decreased by shortening melting process. The AlN oxidization would be restricted after aluminosilicate was melted. Reduction extent of heavy metals increased from 51% to 86%, when treatment temperature was increased from 1350 to 1400 °C. Meanwhile, AlN oxidization could be exacerbated by increasing CaF 2 and soda. The CaF 2 and soda could corrode the alumina protective layer on AlN and promote AlN oxidization. Reduction extent of heavy metals decreased from 48% to 19% after adding soda. The AlN oxidization could be restrained by cutting down the air during melting process. After covering the crucible, reduction extent of heavy metals was increased from 48% to 92%, and reduction extent of Fe, Cr and Ni was increased from 58, 18 and 83% to 97, 91 and 100%. After evaluating environment benefit of AlN reducing heavy metals, it was found that utilizing a ton SAD replacing carbon to reduce heavy metals in pickling sludge could reduce carbon emissions of 279 kg. In addition, SAD could be used as reductant to replace Al for chromium metallurgy. Using a ton SAD to replace Al for chromium metallurgy could reduce carbon emission of 3961 kg.

Zhao X., Yang J., Ning N., Yang Z.

Sufficient attention should be attached to the large amount of fly ash containing high levels of toxic heavy metals generated after municipal solid waste incineration. Because heavy metals could be leached out of the fly ash under specific conditions, it is necessary to stabilize the heavy metals in fly ash before landfill disposal. Processing technologies of incineration fly ash include solidification/stabilization technology, thermal treatments, and separation processes. This study reviewed the current treatment technologies of municipal solid waste incineration (MSWI) fly ash, with the main focus on the treatment of heavy metals in fly ash with chemical stabilization. Chemical stabilization processes involve chemical precipitation of heavy metal and chelation of heavy metals. In multiple studies, chemical stabilization technology has shown practical feasibility in terms of technology, economy, and effect. In addition, the combination of two or more stabilization agents broadens the general applicability of the agents to heavy metals and reduces the cost. The application of joint processing technology realizes the remove of soluble salt from fly ash. To minimize pollutants while increase their usable value, effective use of waste and co-disposal of several kinds of wastes have gradually become the research hotspots. New developments in chemical stabilization are progressively moving towards the sustainable direction of harmlessness and resource utilization of MSWI fly ash.

Liu X., Han Y., He F., Gao P., Yuan S.

Red mud (RM) is the major waste material with strong alkaline discharged which is during the alumina extraction process. The global stock of RM has exceeded 4 billion tons and its disposal as a solid waste has always been a thorny environmental problem. However, RM is widely considered to be a potential resource due to its high content of valuable metal components such as iron. High-iron RM is rich in iron and can potentially become a valuable resource if the iron can be extracted effectively. It is of great research value and profound significance to recover iron from high-iron RM. This paper systematically reviews the iron recovery methods for resource utilization of high-iron RM, and divides the technology of iron recovery from high-iron RM into three aspects: physical separation method, pyrometallurgy method (reduction smelting and reduction roasting) and hydrometallurgy method (acid leaching). The basic principles and effect of iron extraction of the above technologies are summarized respectively, and the advantages and disadvantages of different technologies are compared. It is pointed out that the feasibility and economic cost are the main factor restricting the industrial application of these technologies. Therefore, it is of great significance to overcome various problems and difficulties, and develop innovative processes and technologies, which can realize the recycling and utilization of iron in high-iron RM and realize the reduction of RM emission at the same time. • The hazards caused by red mud and trends in red mud research were presented. • The research status of iron recovery methods from high-iron red mud is reviewed. • The advantages and disadvantages of several iron recovery methods are compared. • Technologies suitable for industrial production needs to be studied in the future.

Shen H., Liu B., Shi Z., Zhao S., Zhang J., Zhang S.

Secondary aluminum dross (SAD) from aluminum industry is classified as a hazardous solid waste due to containing aluminum nitride (AlN). In this work, AlN was first used to reduce heavy metals by pyrometallurgy. The reduction rates for iron, chromium and nickel were up to 90%, 80% and 100%, respectively. However, the reduction from AlN and oxygen oxidization of AlN occurred simultaneously. AlN which formed solid solution with alumina could reduce heavy metals, while the rest was oxidized by oxygen. In addition, the reduction rates for iron and chromium could be increased with increasing CaF 2 from 6.7 to 9.0 wt%. CaF 2 could decreased viscosity of molten slag, which favored the ion migration, and then increased the reduction rates. After the reduction, glass ceramics were manufactured from the molten slags. The bending strength, microhardness and alkali resistance of the glass ceramics were up to 77 MPa, 1011 HV and 98.7%, respectively. According to XRD and SEM results, glass ceramics with CaAl 2 SiO 6 crystal phase, crosslinked network structure grains and smaller pores exhibited better bending resistance. In addition, glass ceramics with CaAl 2 SiO 6 crystal phase possessed the highest microhardness and alkali resistance. After this process, hazardous pickling sludge and SAD were totally recycled. • Hazardous pickling sludge and SAD were totally recycled by pyrometallurgy. • AlN in SAD was first used to reduce heavy metals. • Reduction rates for iron, chromium and nickel were 90%, 80% and 100%. • Bending strength and microhardness of the glass ceramics were 77 MPa and 1011 HV.

Al-Ghouti M.A., Khan M., Nasser M.S., Al Saad K., Ee Heng O.O.

Incineration has emerged as one of the acceptable ways to treat municipal solid waste (MSW) due to its potential in reducing the mass and volume of the waste. However, it produces two major by-product residues, namely MSW-bottom ash (MSW-BA) and MSW-fly ash (MSW-FA). These residues have gained great attention to their hazardous nature and potential to be reused and recycled. In this paper, the physicochemical characterizations of the MSW-BA and the MSW-FA were performed, followed by a systematic investigation of metals extraction from MSW-BA and MSW-FA. Various extracting agents were used to investigate the possibility to extract 21 metals including cadmium (Cd), vanadium (V), chromium (Cr), and lead (Pb). It was revealed that some metals were present in a high amount in the MSW-BA while other metals were higher in the MSW-FA. Moreover, the energy-dispersive X-ray spectroscopy results revealed that the MSW-BA was dominated by oxygen (O) 55.4 ±0.6 wt%, silicon (Si) 22.5 ±0.3 wt%, and calcium (Ca) 18.5 ±0.2 wt%. On the other hand, the MSW-FA was enriched with Ca 45.2 ±0.5 wt%, and O 40.3 ±0.4 wt%. From the scanning electron microscopy, the MSW-BA was observed as flaky with an irregular surface that consisted of large pores, while, the MSW-FA was present as agglomerated particles and had a bimodal distribution. Moreover, Fourier transform infrared spectroscopy revealed that Al-Fe-OH, Al-Al-OH, Si-O, C-O, and C-H were some of the major functional groups present in the ashes. The F-tests concluded that the metal extraction from the MSW-BA and MSW-FA were significantly affected by the acid type. it is concluded that nitric acid and phosphoric acid were the best-suited acid for the MSW-BA while sulfuric acid and phosphoric acid for the MSW-FA. More than 11 wt% of Cd and 9 wt% of Cu were extracted from MSW-BA while 6 wt% of Pb and 4.5 wt% of V were extracted from the MSW-FA. The present methodology is an interesting development in metal extraction from the MSW-BA and the MSW-FA, which can develop in a cost-effective and sustainable option to utilize MSW.

Arroyo F., Luna-Galiano Y., Leiva C., Vilches L.F., Fernández-Pereira C.

More and more by-products are being used in certain materials, especially in the construction industry. Natural construction materials contain amounts of heavy metals and radionuclides , but when by-products are used in these kinds of materials, this could lead to a growth in their concentrations and have a negative impact on public health.In this paper, red mud was used as a raw material (as a clay substitute) to manufacture fired bricks. Physical, mechanical, radiological and heavy metal leaching properties of fired bricks with a replacement ratio of up to 80 wt% of clay to red mud are discussed. In addition, the effect of different sintering temperatures (1173K and 1373K) was analyzed, and results showed that the higher the temperature produced, the higher the mechanical strength.To environmentally characterize materials, they were subjected to two different leaching tests: a batch test for raw materials and a monolithic test for the bricks, respectively. The results obtained were compared with the limits stated for several heavy metals by the European Landfill Directive. Results showed that red mud gives leachate concentration values for Cr higher than the limits stated for non-hazardous by-products. Bricks do not exhibit the same problem in the samples containing a high RM proportion and manufactured at a low sintering temperature (1173K), although in the case of V, a high concentration is observed.The contents of radionuclides such as Ra-220, Th-232 and K-40 of the final construction materials were analyzed and compared with different indexes. This paper indicates the maximum amounts of RM that can be used to replace clay for the manufacture of fired bricks without environmental risk. • Fired brick utilized red mud instead of traditional clay in a high ratio have been studied. • The temperatures and content of red mud on the properties of bricks were studied. • Bricks with 80%wt of RM and fired at 1100 °C present a compressive strength higher than 16 MPa. • No leaching and radiological problems according to different test and limits have been found with a high RM content.

Liu S., Li Q., Wang Z., Yang F., Lu X.

Stainless steel sludge contained large amounts of heavy metals like Cr. These sludge are now mainly disposed by landfills, occupying a large amount of land and polluting the environment. The objective of the paper is to explore the synergistic reduction of Cr and Fe, and the phase transition during the process. Find out the forming condition of Fe–Cr–C alloy. Study the growth and migration trend of low melting point Fe–Cr–C droplets. Thus we could find a way to recover heavy metals from the sludge with less energy and make the tailings harmless to the environment. Through X-Ray Diffraction and Energy Dispersive Spectrometer analysis, the results showed that chromium mainly presented in the FeCr2O4 phase before it was reduced, and the reduction product was Fe–Cr–C metal carbide. By observing samples reduced at different temperatures with metallographic microscope, it could be found that the melted metal carbides flowed under the surface tension to form larger droplets at temperatures around 1250 °C. On the basis, the method of semi-melting sludge reduction at a temperature of 1275 °C which was lower than the existing method of 1350 °C, was first proposed. Under this condition, the average diameter of the obtained Fe–Cr–C particles was 2.7 mm, which was easy to separate from the tailings. The yield of Fe and Cr reached 86.7% and 71.3% respectively, which means 0.22 ton of metal particles could be recovered per ton of sludge, and could be used in steel making. The obtained non-magnetic tailings with a Cr content of less than 0.04% was harmless, and the remained magnetic powder could be used as a nucleating agent for the reduction of sludge.

Zinoveev D., Grudinsky P., Zakunov A., Semenov A., Panova M., Valeev D., Kondratiev A., Dyubanov V., Petelin A.

Red mud is a by-product of alumina production from bauxite ore by the Bayer method, which contains considerable amounts of valuable components such as iron, aluminum, titanium, and scandium. In this study, an approach was applied to extract iron, i.e., carbothermic reduction roasting of red mud with sodium and potassium carbonates followed by magnetic separation. The thermodynamic analysis of iron and iron-free components’ behavior during carbothermic reduction was carried out by HSC Chemistry 9.98 (Outotec, Pori, Finland) and FactSage 7.1 (Thermfact, Montreal, Canada; GTT-Technologies, Herzogenrath, Germany) software. The effects of the alkaline carbonates’ addition, as well as duration and temperature of roasting on the iron metallization degree, iron grains’ size, and magnetic separation process were investigated experimentally. The best conditions for the reduction roasting were found to be as follows: 22.01% of K2CO3 addition, 1250 °C, and 180 min of duration. As a generalization of the obtained data, the mechanism of alkaline carbonates’ influence on iron grain growth was proposed.