Recycling Of Slag Research Articles

Effect of slag recycling on bead geometry, dilution and microstructure in submerged arc cladding of stainless steels

The submerged arc welding (SAW) process generates a large quantity of slag, which, when disposed-of, harms the environment and wastes valuable ingredients. A technology was developed to recycle the SAW slag generated in stainless steel cladding, enabling its reuse as flux. Bead geometry and dilution play crucial roles in weld cladding. Dilution determines the minimum number of layers needed to achieve the desired chemistry, while bead width dictates the fewest passes required to cover the entire surface area. Additionally, bead geometry influences the mechanical strength of the clad layers. The microstructure and solidification mode also significantly affect clad quality. Therefore, understanding the impact of recycled slag on bead geometry, dilution, and microstructure is essential. This paper presents a systematic investigation into the effects of process variables and recycled slag on bead geometry and dilution in clad welding. It includes a comparison of microstructures obtained using fresh flux versus recycled slag. The results indicate that increasing the welding current (I) by 37.5% enhances weld penetration (p), bead width (w), reinforcement (r), and dilution (d) by 50.9%, 14.4%, 69.5%, and 5.5%, respectively. Conversely, increasing the travel speed (V) by 90%, decreases p, w, and r by 19.29%, 11.9%, and 25.3%, respectively; however, dilution (d) increases by 17.6%. On the other hand, increasing the open-circuit voltage (V) by 26.7% increases bead width (w) and dilution (d) by 10.4% and 5.4%, respectively, while reinforcement (r) decreases by 7.5%. Both single and multilayer claddings prepared with fresh flux and recycled slag exhibit similar microstructures.

Influence of polyethylene terephthalate (PET) flakes coating on the microstructure and mechanical properties of alkali activated composites

The objective of this research is the improvement of the mechanical strengths of the alkali activated slag composites with polyethylene terephthalate (PET) flakes content. The main drawback when large PET flakes (2–9 mm) are used in these composites is reduced adhesion between plastic surface and binding matrix. Therefore, the PET flakes were coated with two types of adhesives – based on polyvinyl acetate or sodium silicate solution, and then covered with fly ash, waste glass or slag powders. The microstructure (assessed by scanning electron microscopy) of interfacial transition zone between the PET flakes coated with waste glass and slag powders is denser than for uncoated PET flakes; this explains the improvement of the compressive strength of these mortars especially after longer curing times (28 days). The flexural strengths of the mortars with PET flakes are higher than for reference mortar (with sand aggregate) and the apparent density of the alkali activated composites with PET flakes are in general smaller than for the reference mortar. Thus, the coating of PET flakes improves the mechanical behaviour of mortars, preserving also a low apparent density. These results can lead to a wider recycling of PET waste, slag and waste glass powder, promoting circular economy and reducing the CO2 footprint without compromising the critical mechanical strength of mortars.

Development of a sustainable stabilized macadam road base using steel slag as supplementary cementitious material

As the main waste product of iron and steel industry, steel slag possesses considerable cementitious activity, making it a promising alternative to cement in Cement Stabilized Macadam (CSM). However, CSM was inevitably exposed to groundwater and rainwater when served as the pavement base course, leading to concerns over poor early strength and potential pollutant leakage, which are the main factors that may hinder the widespread utilization of steel slag in CSM base. To address these issues, this study investigated the feasibility of using steel slag powder to produce CSM. Steel Slag Powder-Cement Stabilized Macadam (SSCSM) samples were prepared and the hydration products, microstructure, mechanical properties, water damage resistance and heavy metal ions leaching behavior were investigated. The results show that the nucleation effect of steel slag powder accelerates the early hydration, but the C-S-H produced by hydration is not sufficient to form a stable hydration product network, so the microstructure of SSCSM is looser than that of CSM. The addition of steel slag powder improved the shrinkage performance of SSCSM, and the mechanical properties and heavy metal ion leaching concentration of SSCSM meet the engineering application requirements when the steel slag powder replacement level does not exceed 30 %. It was also found that the addition of steel slag powder promoted the development of pores in SSCSM samples after dry-wet cycles, resulting in the reduction of water damage resistance. Compared with conventional CSM, the use of SSCSM can not only reduce 4 % of raw material cost and 23.5 % of equivalent CO2 emission, but also mitigate the heavy metal ions leaching risk associated with steel slag, making it an effective and sustainable solution for steel slag recycling.

A method for high value-added utilization of BOF slag: Towards slag recycling and phosphorus recovery

Basic oxygen furnace (BOF) slag is a bulk solid waste generated in steelmaking process, containing large amounts of valuable components. However, its utilization ratio is not high in China, resulting in serious environmental pollution and resource waste. To realize the high value-added utilization of BOF slag, selective leaching was adopted to separate the P-bearing dicalcium silicate from slag. The leaching ratios of Si and P were much higher than those of Fe and Mn. The oxidized BOF slag exhibited a superior leaching of P. 85.6 % of P and 68.0 % of Si was separated from BOF slag while the leaching ratio of Fe was negligible in the HCl solution at pH 3. Almost all the dicalcium silicate was removed by leaching whereas the dicalcium ferrite remained. The tailing containing 46.1 % Fe2O3 and 32.9 % CaO can be reused as a steelmaking feedstock. The leachate was treated to extract phosphates by chemical precipitation. Approximately 98 % of phosphate ions was precipitated in the leachate when the pH increased to 8. Some silica gel was removed from the precipitate via alkaline wash and a phosphate containing 28.4 % P2O5 was recovered, which could be used as a phosphorus fertilizer. This study provided a method for BOF slag recycling in steelmaking process and phosphorus recovery.

Characteristics and modification mechanism of recycled waste silty clay slurry as the shield slag conditioner

The construction process of earth pressure balance shield (EPBS) generates a large amount of shield slag. In China, the conventional methods for treating shield slag involve crude landfill and stockpiling, resulting in increased cost and significant environmental issues. To expand the recycling of shield slag, this study proposes to use waste silty clay to completely replace bentonite as a conditioner in coarse-grained strata. Firstly, the physical properties of silty clay slurries are investigated with four additives. There are six modified silty clay slurries with good improved performance. Then, analyzing the rheological properties of six modified silty clay slurries, and verifying the feasibility of replacing bentonite slurry with modified silty clay slurry. Next, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and Zeta potential were used to analyse the micromechanisms, molecular structures and molecular forces of 14 % bentonite slurry, silty clay slurry with a soil-water ratio of 1:1, sodium carbonate 3 % and sodium silicate 0.1 % in a silty clay slurry with a soil-water ratio of 1:1. Finally, the slump test was carried out to verify the improvement effect of the modified silty clay slurry on the shield slag. The results show that sodium carbonate modified silty clay slurry has a higher interlayer water content and greater ion exchange capacity. When the soil-water ratio is 1:1 and the concentration of sodium carbonate is 3 %, the apparent viscosity and shear force of the modified silty clay slurries are between those of bentonite slurries with a concentration of 10 % and 14 %. Additionally, the modified silty clay slurry showed a stable microstructure and good residue amendment effect, making it suitable as a soil conditioner during EPBS construction in coarse-grained strata. This study offers a viable approach for recycling EPBS slurry.

A Novel Fabrication of Hematite Nanoparticles via Recycling of Titanium Slag by Pyrite Reduction Technology.

An enormous quantity of titanium slag has caused not merely serious environment pollution, but also a huge waste of iron and sulfur resources. Hence, recycling iron and sulfur resources from titanium slag has recently been an urgent problem. Herein, hematite nanoparticles were fabricated through a pyrite reduction approach using as-received titanium slag as the iron source and pyrite as the reducing agent in an nitrogen atmosphere. The physicochemical properties of the hematite nanoparticles were analyzed using multiple techniques such as X-ray diffraction pattern, ultraviolet-visible spectrophotometry, and scanning electron microscopy. The best synthesis conditions for hematite nanoparticles were found at 550 °C for 30 min with the mass ratio of 14:1 for titanium slag and pyrite. The results demonstrated that hematite nanoparticles with an average particle diameter of 45 nm were nearly spherical in shape. The specific surface area, pore volume, and pore size estimated according to the BET method were 19.6 m2/g, 0.117 cm3/g, and 0.89 nm, respectively. Meanwhile, the fabricated hematite nanoparticles possessed weak ferromagnetic behavior and good absorbance in the wavelength range of 200 nm-600 nm, applied as a visible light responsive catalyst. Consequently, these results show that hematite nanoparticles formed by the pyrite reduction technique have a promising application prospect for magnetic material and photocatalysis.

A low-carbon cement based on silicomanganese slag and granulated blast furnace slag: Hydration process, microstructure, mechanical properties and leaching risks

The calcium content of silicomanganese slag is only about 50 % of that in granulated blast furnace slag but the manganese content is high (>10 wt%), which limits the wide application of silicomanganese slag in cementitious materials. The present study provides a low-carbon cementitious material with an above-average silicomanganese slag content, which provides a new method for the recycling of silicomanganese slag and other solid wastes. The mechanical properties, hydration mechanisms, and pore structures of low-carbon cementitious materials with different silicomanganese slag and granulated blast furnace slag ratios were investigated using carbide slag as alkali-activator and flue gas desulfurization gypsum as sulfate-activator. The feasibility of silicomanganese slag as a significant replacement for granulated blast furnace slag in cementitious materials was explored, and the risk of manganese leaching was assessed. Silicomanganese slag has slower hydration rate in the early stage, but the reaction accelerates in a later stage and the gap with granulated blast furnace slag decreases. The 3, 7, and 28-d compressive strengths of mortars consisting of 40 % silicomanganese slag, 40 % granulated blast furnace slag, 12 % flue gas desulfurization gypsum, and 8 % carbide slag reached 28.7, 41.2, and 49.9 MPa, respectively. When the silicomanganese slag content is 80 %, the leaching toxicity test results showed ≤0.1 mg/L, and jouravskite was present in the hydration products, which indicated a low risk of manganese leaching in normal service environments.

Recycling of iron and steel slag for carbon reduction and low‐environment load application

AbstractThe high‐value utilization of blast furnace slag (BFS) and steel slag (SS) as a valuable resource in the field of carbon reduction represents a green revolution, and also is an indispensable path toward breaking through resource and environmental constraints and achieving high‐quality, sustainable development through solid waste utilization in the steel industry. Achieving resource recycling while harnessing the untapped latent energy of resources and exploring their carbon sequestration capabilities has become a crucial avenue for further valorization through waste utilization. BFS and SS discharged from iron‐making or steel‐making furnaces carry a significant amount of latent heat, especially the calcium oxide component in SS, which gives it a unique advantage in the field of comprehensive BFS and SS utilization and carbonation‐based SS utilization. This article discusses the current research status of low‐carbon‐waste‐heat utilization in the production of microcrystalline glass, cementitious materials, functional adsorbents, and other products through front‐end modification of molten BFS and SS. This report also provides an overview of carbon capture by utilizing BFS and SS, offering insights into the research directions for subsequent heat recovery, online quality adjustment, high‐value utilization, and carbon sequestration using BFS and SS in the steel industry.

Understanding Chromium Slag Recycling with Sintering-Ironmaking Processes: Influence of Cr2O3 on the Sinter Microstructure and Mechanical Properties of the Silico-Ferrite of Calcium and Aluminum (SFCA).

Chromium slag is a solid waste of chromium salt production, which contains highly toxic Cr(VI) and significant amounts of valuable metals, such as Fe and Cr. Recycling chromium slag as a raw sintering material in sintering-ironmaking processes can simultaneously reduce toxic Cr(VI) and recover valuable metals. A micro-sintering experiment, compressive strength test, microhardness test, and first-principles calculation are performed to investigate the influence of Cr2O3 on the sintering microstructure and mechanical properties of the silico-ferrite of calcium and aluminum (SFCA) in order to understand the basis of the sintering process with chromium slag addition. The results show that the microstructure of SFCA changes from blocky to interwoven, with further increasing Cr2O3 content from 0 wt% to 3 wt%, and transforms to blocky with Cr2O3 content increasing to 5 wt%. Cr2O3 reacts with Fe2O3 to form (Fe1-xCrx)2O3 (0 ≤ x ≤ 1), which participates in forming SFCA. With the increase in Cr doping concentrations, the hardness of SFCA first decreases and then increases, and the toughness increases. When Cr2O3 content increases from 0 wt% to 3 wt%, the SFCA microhardness decreases and the compressive strength of the sintered sample increases. Further increasing Cr2O3 contents to 5 wt%, the SFCA microhardness increases, and the compressive strength of sintered sample decreases.

Comprehensive recycling of slag from the smelting of spent automotive catalysts

Spent automotive catalyst is a valuable source of platinum group metals (PGMs), making them crucial secondary resources. Currently, the main process for recovering PGMs from these catalysts is metal capture method. The smelting process would produce a significant amount of residual slags containing various components like SiO2, Al2O3, CaO, Fe2O3, Na2O, as well as high levels of heavy metals including Cr, Pb, and Ni. These slags are considered either solid waste or hazardous waste and usually disposed of in landfills, which can have detrimental effects on the environment. Furthermore, the smelting slags also contain approximately 2–5 % rare earth elements and trace amounts of precious metal elements. In this study, vacuum carbothermal reduction is employed to pretreat the smelting slags. This method not only successfully removes the heavy metal element lead, but also efficiently achieves the recovery of rare earths, and the enrichment of platinum group metals by mineral phase transformation of cerium-zirconium solid solution and in-situ iron capture. The lead removal rate exceeded 99 %, and the grade of precious metals in the enrichment process was approximately 12 times higher than the original material, with a recovery rate more than 90 %. The leaching rate of rare earths also increased from less than 50 % to more than 95 % after vacuum carbothermal reduction. The effect of vacuum carbothermal reduction pretreatment on the smelting slag phase was systematically studied, and the mechanism behind the increasing rare earth leaching rate due to this pretreatment was further analyzed.

Desulfurization Reaction according to Ladle Slag Recycling Method in Shaft-Type EAF Operation

Desulfurization Reaction according to Ladle Slag Recycling Method in Shaft-Type EAF Operation

Foaming and Physico-Mechanical Properties of Geopolymer Pastes Manufactured from Post-Metallurgical Recycled Slag.

This paper presents a research program aimed towards developing a method of producing lightweight, porous geopolymer composites for the construction industry based on industrial wastes. A direct method involving the addition of chemicals is currently most commonly used to produce the porous mineral structure of a geopolymer matrix. This relies on a reaction in a highly alkaline environment of the geopolymer to produce a gas (usually hydrogen or oxygen) that forms vesicles and creates a network of pores. This paper demonstrates the feasibility of producing a slag-based geopolymer paste foamed with aluminum powder, taking into account different parameters of fresh paste production: the mixing duration, its speed and the timing of foaming agent addition. The foaming process of the fresh paste in terms of the volumetric changes and temperature development of the fresh paste during the curing of the material are observed. After hardening, the physical properties (density and porosity) as well as the mechanical parameters (compressive strength and work of damage) are determined for the nine manufactured foamed pastes. Image analysis software was used to assess the porosity distribution of the material across the cross-section of the samples. The results enabled the design of the mixing procedure to be adopted during the manufacture of such composites.

Recycling Perspectives of Electric Arc Furnace Slag in the United States: A Review

This article presents a comprehensive review of electric arc furnace (EAF) slag recycling in the United States, examining its classification and the associated challenges and opportunities of its industrial use. The study affirms EAF slag's nonhazardous status. The main challenges identified in EAF slag applications include substantial variations in composition and volume instability during/after hydration. Analysis of the U.S. recycling practices reveals that EAF slag is predominantly reused, with minimal landfill disposal. However, its prevalent use as a low value‐added aggregate in construction applications underscores the industry's ongoing challenge to get additional value from EAF slag recycling. Despite these challenges, the study highlights a great potential for increased value extraction from EAF slag recycling. Beyond conventional applications as a clinker material for the cement industry, the review explores modern technologies for steelmaking slag recycling, revealing options for recovering valuable metals such as Cr, V, Mo, and Fe through methods such as leaching, reduction, and oxidation.

Steelmaking Slag Recycling as Raw Material and Its Effect on Burning Temperature of Portland Cement Clinker Production

Steelmaking Slag Recycling as Raw Material and Its Effect on Burning Temperature of Portland Cement Clinker Production

A novel approach for extracting lithium from overhaul slag by low temperature roasting – water leaching

Overhaul slag, as one of the hazardous wastes generated in the aluminum industry, which can pollute the environment and waste resources when arbitrarily stockpiled or landfilled. Scientific and standardized recycling of overhaul slag with both valuable and pollutant characteristics can avoid environmental pollution, and greatly alleviate the global resource shortage crisis. In this paper, an innovative process of sulfuric acid roasting and water leaching is proposed, which can efficiently and environmentally recover F, Al and Li from overhaul slag. The effects of roasting parameters including H2SO4 dosage, roasting temperature and time were systematically investigated. The optimal conditions have been established as: m(H2SO4)/m(overhaul slag) of 1.4:1, roasting temperature of 360 ℃, and time of 2.0 h. Under this conditions, the lithium was converted to NaLi(SO4) and the aluminum was converted to NaAl(SO4)2, during which the HF volatilization rate reached 99.4 %. The roasted clinker was then subjected to water leaching for the extraction of lithium and aluminum. About 99.5 % of lithium and 72.57 % of aluminum was extracted by the leaching process under the optimal conditions. After the leaching solution was treated by pH adjustment, roasting and water leaching, lithium and aluminum could be effectively separated, and the leaching rates of lithium and aluminum were 99.34 % and 0.12 %, respectively. The process proposed in this paper is not limited to the treatment of overhaul slag, but can also be applied to the processing of other aluminum industrial wastes and has a wide application potential.

Recycling of steel slag in sustainable bituminous mixtures: Self-healing performance, mechanism, environmental and economic analyses

The accumulation and disposal of steel slag (SS) is a growing problem with economic impact and environmental burden. This work aims to use SS to partially replace limestone to prepare sustainable self-healing steel slag bituminous mixtures (SSBM) to reduce maintenance costs and reduce greenhouse gas emissions. The damage-healing test, the microwave heating test of SSBM was carried out, and CT scanning was performed on SSBM before and after damage-healing. Cost-benefit analysis (CBA) was performed on the SSBM. The self-healing performance of SSBM is far better than traditional mixtures. The scale evolution of voids is the essential reason for the increase of fracture resistance of SSBM after healing. In the pre-crack (void) stage, the self-healing theory was improved based on the morphological repair theory of void interface. CBA results show that SSBM is economically and ecologically efficient. This work is helpful to the reuse of SS and the development of sustainable pavement.

Thermodynamic study of magnesium (Mg) production by the Pidgeon process: The distribution of MgO and axial thermal field

The magnesium slag of the Pidgeon process is rich in MgO, which restricts their resource utilization. In this paper, the MgO content in the magnesium slag at different positions of the reduction tank was analyzed and the heating model of the reduction furnace was established, which disclosed the distribution of MgO in the magnesium slag and the change of the axial thermal field of the reduction tank with time. Research shows that the content of MgO in the reduction tank firstly increased (from 0.20 m to 0.40 m), and then decreased, finally increased slowly (from 1.20 m to 2.40 m). The MgO content in the magnesium slag within 0.4 m away from the tank mouth is extremely high, which affects the resource utilization of magnesium slag. The MgO residual at different positions in the upper and lower reduction tanks are similar, indicating that the temperature in the reduction furnace is relatively stable. Temperature and heat transfer are the reasons for the uneven distribution of MgO at different positions in the reduction tank. The research results provide a theoretical basis for magnesium smelting and magnesium slag recycling.

Recycling of Silicomanganese Slag and Fly Ash for Preparation of Environment-Friendly Foamed Ceramics.

In order to reduce the manufacturing cost of foamed ceramics and expand the application scope of industrial solid waste, in this study, a new type of environment-friendly foamed ceramics was prepared using direct high-temperature foaming with waste silicomanganese slag (SMS) and fly ash (FA) as raw materials and silicon carbide (SiC) as a foaming agent. The influence of SMS content, SiC content, and sintering temperature on the characteristics and microstructure of the specimen were explored. More concretely, the compressive strength, pore morphology, bulk density, and crystalline composition of the foamed ceramics were discussed. The foaming mechanism was also further analyzed. The results showed that including 20% SMS significantly reduced the melt's viscosity and stimulated bubble expansion. This, in turn, facilitated the creation of a porous structure. Moreover, it was noted that samples containing 20% SMS exhibited an anorthite phase when sintered at 1110 °C, resulting in enhanced compressive strength. The bulk density and compressive strength of the foamed ceramics decreased with an increase in the sintering temperature and SiC content. This trend was primarily attributed to the higher total porosity and the insufficient support of the pore wall to the matrix. The best all-around performance was achieved with 20 wt% SMS, 80 wt% FA as raw material, SiC addition of 1.0 wt%, and a sintering temperature of 1100 °C. Under these conditions, the compressive strength, bulk density, and total porosity of the foamed ceramics were 8.09 MPa, 0.57 g/cm3, and 71.04%, respectively. Taken together, the outstanding porous structure and mechanical properties of this foamed ceramic make it suitable for use as insulation or for building partition materials.

Recovery of low phosphorus iron from steel slag using secondary aluminum dross as the reductant

Iron extraction by carbothermic reduction is a promising technique for the recycling and valorization of steel slag, but phosphorus reduction and dissolution are inevitable and limit the value of the metal products. To solve this issue, in this work, a novel smelting reduction method was proposed to produce low phosphorus iron from steel slag by using secondary aluminum dross (SAD) as the reductant. The roles of modified basicity and SAD/slag in the iron recovery and phosphorus distribution behavior were investigated by various characterization and theoretical approaches, such as ICP-AES, XRD, SEM-EDX, FTIR, and FactSage. Owing to the fluxing action of Al2O3 and the supply of abundant free (O2-), the slag shows a fully melted and highly depolymerized state during reduction even when no slag modification is made, contributing to a high iron recovery (92.8 %). This is an advantage compared to other traditional reduction methods. Besides, the higher basicity under no modification significantly suppresses the reduction of P2O5 by reducing its activity coefficient, resulting in a decrease of 50 % in P content in the separated metals. It is also found that phosphorus recovery presents a power growth law with SAD/slag. The P content in the metals can be as low as 0.16 wt% by decreasing this ratio from 0.28 to 0.26. Although a sacrifice of iron recovery (∼6 %) is required for this purification, it shows great potential in producing high-quality metals from steel slag.

Equilibrium Phase Relationships of CaO-SiO2-TiO2 System with 5 wt% Cr2O3 Addition for Titania-Bearing Slag Recycling

Equilibrium Phase Relationships of CaO-SiO2-TiO2 System with 5 wt% Cr2O3 Addition for Titania-Bearing Slag Recycling