Raw Slag Research Articles

Utilization of Blast Furnace Slag for Immobilization of Copper Ions from Solution

Disposal of wastes containing metal ions such as Cu(II) ions is serious problem nowadays. Various materials are utilized for thepurpose of immobilization of Cu(II) ions. Attractive type of material is represented by slags – waste from the metallurgical industry.Raw and alkali-activated blast furnace slag were studied for the purpose of immobilization of Cu(II) ions from the aqueoussolution and for disposal of Cu(II) containing wastes. Slags were saturated by Cu(II) ions. Amount of Cu(II) deposited on the rawslag was 6.35 ± 0.12 mg/g and amount deposited on the alkali-activated slag was 151.37 ± 0.95 mg/g. The saturated materialswere thermally treated at 100, 500, and 1000°C. The thermal treatment leads to the slight structural changes in the case of rawslag and to the significant structural changes in the case of alkali-activated slag. Cu(II) ions probably incorporate to the matrix ofmaterials. The materials based on alkali-activated slag exhibit higher stability to the leaching of Cu(II) ions compared to raw slagbased materials when only 0.13% of the total immobilized amount of Cu(II) ions was released to the solutions during the leachingexperiment in the case of alkali activated slag compared to 12% in the case of raw slag. The higher temperature of treatment leadsto more stable material in the case of both initial slags. The studied materials are less stable under the acidic conditions in comparisonwith the neutral and alkaline conditions. Alkali-activated blast furnace slag could be promising material for the Cu(II) ionsimmobilization and for the safe disposal of Cu(II) containing wastes.

Improvement of desulfurization efficiency of Al-rich ladle furnace refining slag with an aqueous carbonation method by hydrothermal or ultrasound pretreatment.

Ladle furnace (LF) slag is a type of steel slag, which has limited applications due to its high S content. Aqueous carbonation is a prospective method for desulfurization of the LF slag. However, the Al-rich LF raw slag has very low desulfurization efficiency with carbonation method. This study investigated the improvement of desulfurization efficiency of the Al-rich LF slag with carbonation method by hydrothermal (HP) or ultrasound pretreatment (UP). The results showed that C3AHX was formed in pretreated slags because of C12A7 hydration, which could remove part of S. After carbonation, most of the C3AHX in pretreated slags produced crystalline CaCO3 bonded by amorphous Al(OH)3 (the other reaction product) in 10min. In the carbonation process, S removal was mainly determined by carbonation efficiency. The S content was reduced to about 0.40% for the pretreated slags from 1.04% in the raw slag. By contrast, the S content was reduced to only 0.93% for slag without pretreatment under the same carbonation conditions. The possible reason of the improvement of desulfurization efficiency by HP or UP was the formation of thin plate-like C3AHx, which increased the surface area available for carbonation reaction. The UP slag presented slightly lower S content than the HP slag because high ultrasound energy increased the reactivity of Ca2SiO4.

Production of raw copper through continuous converting of copper-nickel sulphide materials

The conventional technology of converting copper mattes in horizontal converters has its drawbacks and fails to conform with the current environmental norms and regulations applicable to metallurgical processes. Therefore, it should be replaced with an environmentally sustainable technology. This paper considers the results of a lab and pilot scale study that looked at converting nickel-bearing copper material into raw copper and running nickelbearing slag. It is demonstrated that the continuous converting process can be stable in both three-layer mode (i. e. slag – white matte – copper) and two-layer mode (i. e. slag – copper). With the help of scanning electron microscopy and electron microprobe analysis, it was established that most of copper, nickel and cobalt get wasted in a soluble form. Choice of fluxes produces a significant effect of the process performance. Fluxes were added to maintain the total concentration of SiO2 and CaO in the slag at 32%. At the same time, varying SiO2/CaO ratios were used depending on the operating mode — from pure Fe – Si slag to pure Fe – Ca slag. It is noted that a rising SiO2/CaO ratio leads to a higher recovery of nickel into slag. A pure Fe – Si slag mode is associated with foaming. The results of the lab study were verified through test heats carried out in a pilot Vanyukov furnace. The obtained raw copper has the following composition, wt %: 3.6–4.2 Ni; 0.6–1.2 S; 0.006–0.06 Fe; and it needs to be final converted for better refining.

Production of Fe–Ti Alloys from Mixed Slag Containing Titanium and Fe2O3 via Direct Electrochemical Reduction in Molten Calcium Chloride

High-purity intermetallic β-Ti (FeTi4) and FeTi alloys were prepared via molten salt electrolysis from a titanium-containing waste slag and Fe2O3 mixture using molten CaCl2 salt as the electrolyte. The mixed slag powders were pressed into a pellet that served as a cathode, while a graphite rod served as an anode. The electrochemical process was conducted at 900 °C with a cell voltage of 3.1 V under an inert atmosphere. The formation process of the alloys and the influence of the Ti:Fe atomic ratio on the product were investigated. With an increased proportion of Ti, the phase of the product changed from FeTi/Fe2Ti to FeTi/FeTi4, and different structures were observed. At a Ti:Fe ratio of 1.2:1 in the raw slag, an alloy with a sponge-like morphology and a small amount of FeTi4 were obtained. During the initial stages of electrolysis, a large amount of intermediate product (CaTiO3) was formed, accompanied by an abrupt decrease in current and increase in particle size. The current then increased and Fe2Ti alloy was gradually formed. Finally, as the reaction process extended inside the pellet, the current remained stable and the product mainly contained FeTi and FeTi4 phases. The observed stages, i.e., CaTiO3(TiO2) → Fe2Ti(Ti) → FeTi(FeTi4), were consistent with the thermodynamic analysis.

Setting behaviours and early-age microstructures of alkali-activated ground granulated blast furnace slag (GGBS) from different regions in China

Fourteen raw slag samples were collected from different steel companies from China, using which setting, mechanical and early-age microstructural behaviours of alkali-activated slag (AAS) cements were investigated. To assess the reaction process between ground granulated blast furnace slag (GGBS) and activator, isothermal calorimetry tests were conducted on three selected AAS pastes for 50 h, while TG, FTIR, and Raman tests were conducted to characterise the early microstructural changes. Main findings include that the setting behaviour of AAS pastes depends primarily on the fineness and chemical compositions of GGBS, and statistical analysis results indicate that three of the constituents namely, SiO2, MgO and K2O, have a strong impact compared to the rest constituents. It is also found that commonly-used parameters to describe characteristics of GGBS are not sufficient to predict setting behaviours of AAS paste made by different GGBSs. Furthermore, the 28-d compressive strength of AAS mortars cannot be considered as a reliable indicator to assess its reaction potential at early age, at least, has no relationship to its setting behaviour. The early age microstructural analysis indicates that one possible reason behind the huge different setting and early age compressive strengths of AAS pastes is the variation in the releasing speed of Ca ions.

Utilisation of lead–zinc mill tailings and slag as paste backfill materials

Paste backfilling is an incipient underground mine backfill technology in India. It facilitates maximum use of mill tailings with enhanced stability of the underground workings and minimises rehandling of water, as well as provides bulk disposal of mining solid waste. Binder type and dosage plays an important role in paste backfill performance. This paper highlights environmentally friendly utilisation of solid wastes like lead–zinc mill tailings and lead–zinc smelter fuming furnace slag (FFS) as paste backfilling for an underground metalliferous mine. Various experiments were conducted to study the effect of use of FFS as a fractional replacement for ordinary portland cement (OPC) in paste backfilling. The physico-chemical properties of both the lead–zinc mill tailings and fuming furnace slag (FFS) have been examined. In the first set of experiments, raw slag (FFS) was used for paste backfill preparation and experimentation for uniaxial compressive strength (UCS) development, whereas in the second set of experiments, FFS was crushed to -75 μm (80 wt%) and used for the study. Multiple regression analysis of strength development was also conducted up to fifth order. The regression analysis is in accordance with the strength development and justifies the significance of OPC, crushed fuming furnace slag (CFFS) and waste chemistry on the strength gain with curing time. Use of crushed fuming furnace slag as OPC replacement in paste backfill showed encouraging results of strength development in contrast to raw FFS. Also, the economic analysis revealed that the paste backfilling cost per tonne reduced significantly with slag replacement in the binder phase.

The leaching mechanism of heavy metals (Ni, Cd, As) in a gasification slag during acidification

The gasification slag by acidification can leach abundant heavy metals. In this paper, the fate of heavy metals (Ni, Cd, and As) in the raw slag and the acidified slag that treated by HAc and HCl was systematically investigated combined with Density Functional Theory (DFT) calculations. The results show that the content of Ni and Cd is reduced with an increasing acid concentration and meets the regulatory standards by 7 M HAc and 3 M HCl, respectively. Most of Ni combined with gehlenite is released as gehlenite dissolves during acid treatment, whereas Cd in combination with gehlenite and iron compounds is hard to release at lower HAc concentrations. Unexpectedly, the content of As tends to elevate at a higher concentration of HAc, which is due to the increase in the content of Ca by new Ca-compound formation and the higher binding capacity of Ca to As according to DFT results. Additionally, if the acid-base ratio reaches about 2.0 by acid treatment, there would be a maximum leaching rate. It is recommended that acid concentration should be controlled to avoid a secondary risk of heavy metals.

Applying steel slag leachate as a reagent substantially enhances pH reduction efficiency for humidification treatment.

A cost-effective, easy-to-implement, and sustainable approach is needed to mitigate the production of alkaline leachate from steel slags that are reused or disposed in the environment. To address this issue, a humidification treatment process, which is operated by wetting a stack of steel slag using aqueous reagents and letting atmospheric CO2 to be passively diffused into the slag pores to induce slag carbonation reaction, was previously developed. In this study, we demonstrate that the leachate of raw steel slag can be recycled and used as a humidification reagent to substantially enhance the treatment efficiency as well as to enable operating the process with neither synthetic chemical consumption nor wastewater discharge. In a 24-h study, a 0.61-unit reduction in slag pH is achieved using a raw slag leachate as a reagent, which is substantially greater than a 0.28-unit reduction using deionized water. The net amount of CaCO3 produced during an extended humidification duration of 4weeks is increased by 2.7-fold when the leachate is used instead of deionized water. A series of systematically designed experiments demonstrates that the pH (11.0) and ionic strength (0.0048) are the two major characteristics of the raw slag leachate that contribute to the enhanced efficiency of humidification treatment. With further demonstration at larger scales in follow-up studies, the novel humidification process that utilizes the leachate generated on-site as a reagent is expected to be a feasible alternative for alkali waste treatment prior to its reuse or disposal.

Effect of modified basic oxygen furnace slag on the controlled release of nitrate nitrogen and the functional microbial community in soil

The specific surface area and active adsorption sites of basic oxygen furnace (BOF) slag increase after BOF modification. The addition of modified BOF slag to the soil may enable the control of nitrate nitrogen (NO3–N) leaching and also affect the functional microflora in the soil. In this study, soil column leaching experiments were conducted to explore the effects of adding modified slag to the soil on the controlled release of NO3–N and the main functional microbial communities involved in nitrification and denitrification processes. The experimental design included seven column groups: a soil control group (CT); soil groups with 2.5%, 5%, and 10% raw slag (S1, S2, S3); and soil groups with 2.5%, 5%, and 10% modified slag (MS1, MS2, MS3) that were subjected to three cycles of leaching, each of which were comprised of five leaching treatments. After the three cycles of leaching, significantly less NO3–N had leached from the modified slag group compared to the CT and the raw slag groups (P < 0.05). Although both slag treatments increased soil pH and decreased the oxidation reduction potential of the soil leaching solution, the addition of modified slag had less effect on soil pH than the addition of raw slag. During column leaching, the group with modified slag had a higher gene abundance of functional microflora compared with the group with raw slag. Similarly, the modified slag group had a higher diversity and richness of denitrifying bacteria, ammonia-oxidizing archaea, and ammonia-oxidizing bacteria than the raw slag group. In conclusion, the addition of modified slag to soil effectively decreased the NO3–N leaching and had relatively little effect on the functional microbial community in the soil.

Grinding Kinetics of Slag and Effect of Final Particle Size on the Compressive Strength of Alkali Activated Materials

This study aims to model grinding of a Polish ferronickel slag and evaluate the particle size distributions (PSDs) of the products obtained after different grinding times. Then, selected products were alkali activated in order to investigate the effect of particle size on the compressive strength of the produced alkali activated materials (AAMs). Other parameters affecting alkali activation, i.e., temperature, curing, and ageing time were also examined. Among the different mathematical models used to simulate the particle size distribution, Rosin–Rammler (RR) was found to be the most suitable. When piecewise regression analysis was applied to experimental data it was found that the particle size distribution of the slag products exhibits multifractal character. In addition, grinding of slag exhibits non-first-order behavior and the reduction rate of each size is time dependent. The grinding rate and consequently the grinding efficiency increases when the particle size increases, but drops sharply near zero after prolonged grinding periods. Regarding alkali activation, it is deduced that among the parameters studied, particle size (and the respective specific surface area) of the raw slag product and curing temperature have the most noticeable impact on the compressive strength of the produced AAMs.

Recycling steel slag from municipal wastewater treatment plants into concrete applications – A step towards circular economy

Municipal wastewater is a major source of the release of phosphorus into the marine environment, and its excessive release could negatively impact marine flora and fauna. In this study, steel slag a by-product from steel industry was used to remove phosphorus content from the municipal wastewater. The Langmuir isotherm model revealed a maximum adsorption capacity of 3.8 mg phosphate/g of steelmaking slag. The results showed that the phosphate removal efficiency of granular slag was about 90%. The wastewater contained a significant amount of iron and manganese content that was filtered out by the slag granules. X-ray fluorescence and x-ray diffraction analysis show that the wastewater treatment process changes the chemical phase composition of slag in addition to the absorption/adsorption of various chemicals within its pore structure. To recycle this waste slag after the water treatment process, that significantly altered its chemical and mineralogical composition, it was used in concrete as a replacement of conventional coarse aggregates. The results show that the treated slag aggregates provide compressive strength improvement of 32.5% and 18% at 7 days and 8.2 and 16.8% improvement at 28 days compared to that of raw slag and conventional aggregates, respectively. In addition, due to the pozzolanic nature of slag both the raw and treated slag aggregates showed a seamless monolithic bond at the interfacial transition zone between the slag and the cement matrix. This study demonstrates that the waste slag generated from the wastewater treatment process has a great potential to be used as a replacement of conventional aggregates in concrete applications.

Local Ca-structure variation and microstructural characteristics on one-part activated slag system with various activators

Material characteristics including local Ca-structure of activated slag with various activators (Mg-, Ca-, and Ba-based) were investigated. The use of an activator with less bound water increased compressive strength regardless of the types of main element in an activator even at an identical molar ratio, but this effect diminished as the atomic number of the main elements in the activators increased. A heavier activator at an identical molar ratio produced a higher compressive strength at 28 days, but Ba-based activation showed dormant period for 3 days due to the late participation of Ba-activator in slag reaction. It was firstly found out that the Mg-based system negligibly changed local Ca-structure of the raw slag whereas the Ba-based system has larger radial distance and smaller coordination number of Ca-O bond than those of Ca-based system. This is explained by increased degree of slag dissolution by using activators with heavier main elements.

Transformation of industrial steel slag with different structure-modifying agents for synthesis of catalysts

Abstract Industrial slag containing Si, Al, Ca, Fe, Mg and Ti was used as source for synthesis of novel catalytic materials upon variation of the treating agents (sodium hydroxide, ethylenediaminetetraacetic acid disodium salt dihydrate - sodium hydroxide and tetraethylammonium hydroxide - sodium hydroxide mixtures) and synthesis parameters (solution concentration, synthesis temperature and time). Physico-chemical properties of the raw slag and the catalysts were evaluated by nitrogen physisorption, scanning electron microscopy, energy dispersive X-ray analysis, temperature programmed desorption of CO2 and NH3, transmission electron microscopy, and X-ray powder diffraction. Catalytic activity of the most promising slag-based material was investigated in analytical pyrolysis of wood biomass. Synthesized materials exhibited highly crystalline phases of CaCO3, Ca(OH)2, Ca2SiO4, SiO2, Al2O3, Fe2O3,and TiO2 and showed high basicity due to the presence of large amounts of basic components. Treatment with surfactants had a significant influence on formation of acid sites. Modification of slag textural properties and internal porous structure by such treatment resulted in formation of pores and channels depending on the leaching agent. Upon variation of a leaching agent the specific surface area of the raw slag could be changed from 19 m2/g up to 80 m2/g, what turned out to be important in creation of an effective catalyst for biomass pyrolysis. Slag treatment improved its catalytic activity, which was evidenced by variations of the liquid yield and composition of pinewood derived compounds in non-catalytic and catalytic pyrolysis using original slag and the catalysis synthesized on its basis.

Waste management strategies for cleaner recycling of spent batteries: lead recovery and brick production from slag

Recycling of spent lead-acid batteries is leading to the production of slag which may be classified as hazardous waste because of its toxicity or lead (Pb) content. The leaching behavior of metals in the raw and solidification/stabilization (S/S) applied slag samples is evaluated with the toxicity characterization leaching procedure (TCLP). Then, TCLP results are compared to the threshold values given in USEPA Method 1311: 1992 and EU standard EN12457-4 2002 which suggest different pH values for TCLP. Raw slag sample were classified as hazardous waste according to USEPA and as non-hazardous according to EU-EN. The blocks which are produced by S/S applied slag with cement (slag ratio 25%—B25L and 50%—B50L) are classified as non-hazardous according to USEPA Method. However, the overall toxicity of blocks for B50L is classified as highly acute toxic (TU > 100 class 4). Therefore, B25L can be reused in the construction since it is not toxic and classified as high-quality concrete. It is suggested that appropriate waste management should be as follows: (1) spraying the acid solution to slag heap and collecting drained water in coagulation/flocculation tank, (2) recovery of lead (%3.3–4.0 Pb) and significant amount of iron (%18–25 Fe) from dried sludge and return to furnace and (3) S/S to slag for safer disposal to landfill or reuse of blocks as construction materials. Moreover, the results showed that toxicity test is a more convenient criterion to designate hazardous waste since it is directly reflecting the hazardous characteristics of waste.

Performance assessment of ferrochrome slag as partial replacement of fine aggregate in concrete

This study introduces the possible utilization of water cooled ferrochrome slag, an industrial waste from ferrochrome industry in concrete production. Traditional sand was substituted with six percentages (0% for reference mix, 10%, 20%, 30%, 40% and 50%) of ferrochrome slag by weight in all concrete mixtures. Leaching, micro-structural and mechanical properties, these three aspects were explored for the qualification of ferrochrome slag concrete. Different tests for the physical and mechanical performance of the concrete were done and the outcome demonstrated that substitution of sand by ferrochrome slag reduced a little in the performance of the strength and water absorption increases along with the increase of rate of substitution. However, the concrete having 30% ferrochrome slag shows almost comparable results with the reference mix. Leachability characteristics were also observed by pH dependence leaching test method for the five concrete samples and one raw slag sample. Moreover, micro-structural study of reference mix and concrete mix with 10–50% ferrochrome slag was performed by utilizing SEM techniques. This experimental study concludes that that ferrochrome slag can be used in the production of concrete as a partial substitution of traditional sand with no unfavourable ecological, micro-structural and mechanical effects.

Structural Characteristics and Hydration Kinetics of Oxidized Steel Slag in a CaO-FeO-SiO2-MgO System

Abstract Although steel slag exhibits cementitious properties, the addition of steel slag in cement is still limited due to both the presence of excess iron oxides and instability of free lime and periclase. This paper proposes a method for oxidizing molten slag in air, aiming at extraction of superfluous wustite and stabilization of free lime and periclase. Mineralogical characteristics of raw slag and modified products were examined using X-ray diffraction (XRD), scanning electron microscopy equipped with backscattered electron imaging (SEM-BEI), energy dispersive spectrometry (EDS) and thermogravimetric analysis (TGA) with differential scanning calorimetry (DSC). Thermodynamic calculations were performed to aid to discuss the experimental results. The results indicate that non-magnetic wustite and periclase are transformed into magnetic spinel (magnetite/magnesioferrite) after oxidation. Temperature has a significant effect on the formation of spinel. The mass fraction of free lime decreases from 3.54 wt.% to 0.96 wt.% as a result of the conversion from free lime to calcium ferrite.

Removal of nickel and methylene blue from aqueous solutions by steel slag as a low cost adsorbent

Nowadays, wastewater from various industries contains a large number of harmful heavy metals and coloring agents, which have to be removed to restore the quality of the environment. In this study, the removal of nickel ions (Ni2+) and methylene blue (MB) from the aqueous solution using steel slag as a low cost adsorbent was investigated. The chemical and mineralogical compositions, as well as the surface area of slag, were analyzed by using X-ray fluorescence spectroscopy, X-ray diffraction, and the Brunauer-Emmett-Teller method (BET). The effect of several important parameters such as contact time, adsorbent dose, pH, temperature, and initial adsorbate concentration on the adsorption process was studied systematically by batch experiments. The adsorption data were well correlated with the Langmuir isotherm model by all samples. The maximum adsorption capacity of the raw slag samples was 36.49 mg/g for Ni2+ and increased from 0.68 to 1.98 mg/g for MB after being acid-activated. The determined thermodynamic parameters indicate that the adsorption of Ni2+ and MB on steel slag is spontaneous in nature, endothermic (for Ni2+), and exothermic (for MB).

Crystallization and Beneficiation of Magnetite for Iron Recycling from Nickel Slags by Oxidation-Magnetic Separation

The iron resources in nickel slag were recycled by oxidation and magnetic separation. The effects of holding time, temperature, air flow rate and basicity on the crystallization of magnetite were investigated systematically. Moreover, the influence of particle size and magnetic flux density on the recovery and grade of iron during the magnetic separation was also explored. Results showed that the magnetite particles were significantly influenced by holding time, and the average diameter size reached about 20 μm after holding for 20 min at 1623 K. The holding temperature obviously affected the microstructure of magnetite phases: with the increase in holding temperature, the shapes of the magnetite particles changed from polyhedral form to skeletal particles. As the air flow rate was increased to 170 mL/min, the magnetite developed into tiny spherical particles due to the strong stirring. It was also found that the crystallization of magnetite was slightly effected by basicity. The iron recovery reduced with the decrease of particle size, while the iron grade first increased to a maximal value of 38 μm, and then decreased. As the magnetic flux density increased, the iron recovery initially increased rapidly, reaching a maximal value at 120 mT, while the iron grade remained almost constant. The final iron recovery and grade were 75.99% and 54.08%, respectively, via multi-step magnetic separation instead of single magnetic separation. Iron in concentrate mainly exists in the form of magnetite and magnesium ferrite, and contents of siderophile elements (Ni, Co) in final concentrate were also higher than that of raw slags.

Phase and morphological transformation stages during carbothermal reduction nitridation process: From coal gasification slag wastes to Ca-α-SiAlON powders

Effective utilization of industrial solid wastes is of great benefit for environmental and economic needs. In order to clarify the theoretical issues and pave the way to synthesize high quality Ca-α-SiAlON powder from coal gasification slag by carbothermal reduction nitridation (CRN), in this study, the theoretical issues including the network structure of the slag as well as the phase and morphological transformation stages of raw slag during CRN process were comprehensively studied at different temperatures (1350, 1400, 1450 and 1500°C). It was found that the CRN process was dominated by nitridation of the Ca-Si-Al-O glass component in slag. With the increase of processing temperature, spherical Ca-Si-Al-O particles were firstly nitrided into porous Ca-Si-Al-O-N spheres, plate-like O-SiAlON grains, and then nitrogen-rich SiAlONs (β-SiAlON, 15R polytypoid and α-SiAlON). At the end of CRN, Ca-α-SiAlON was found to be the only SiAlON phase presenting elongated prismatic morphology. In addition, high concentration Ca-α-SiAlON powder was obtained through a two-step purification process with a conversion rate as high as 45wt%. The obtained results shed light on the preparation of Ca-α-SiAlON products from coal gasification slag and related wastes.

Experimental investigation of surface modified EOF steel slag as coarse aggregate in concrete

An experimental work was carried out to study the effect of Energy Optimizing Furnace (EOF) steel slag as coarse aggregate replacement in concrete. Surface modification of slag was carried out to seal the surface voids of raw slag aggregates. Quarry dust obtained as an extractive waste from the granite stone quarries has been used as a blending material in this work. After several trials, it was found that a mix proportion of 1:6:14 (cement:quarry dust:slag aggregate) was the most suitable mix ratio for the surface modification of the slag aggregates. Various mixes of concrete were prepared with different proportions of modified slag (ranging from 0% to 100%) as replacements for aggregates. Three grades of concrete (20MPa, 30MPa and 40MPa) were used in the investigation and the concrete mixes were evaluated for compressive strength and splitting tensile strength. It was found that the compressive strength improved for 25 percent replacement of natural coarse aggregates. The splitting tensile strength was found to peak at 25 percent replacement of natural aggregates.