Chromium In Stainless Steel Slag Research Articles

Transfer and transformation mechanism of chromium in stainless steel slag in pedosphere

Abstract The trivalent chromium (Cr) leached from stainless steel slag can be oxidized into hexavalent Cr with strong toxicity in the natural storage process, thus causing severe pollution to the surrounding soil, water, and atmosphere. Currently, the toxicity hazards caused by high Cr concentrations in plants, animals, and humans have attracted widespread attention from across the world. In this study, an overview is presented regarding the occurrence mode, leaching mechanism, and influencing factors for the presence of Cr in the soil of stainless steel slag under natural landfilling conditions. Meanwhile, a summary is made for the research progress in Cr absorption, transport, and accumulation in the soil–plant system. Besides, allowing for the toxicity and detrimental effect of Cr(vi) in the soil as well as the application of biological and chemical methods for the remediation of Cr(vi)-contaminated soil, a review is conducted on the approach to recycling Cr from stainless steel slag and the application of chemical remediation and biological methods to remedy Cr-containing soil. Finally, a discussion is conducted about the transfer and transformation behavior of Cr in soil–plant system, the practical application of soil remediation technology and the prospect of research in this field.

Up-Concentration of Chromium in Stainless Steel Slag and Ferrochromium Slags by Magnetic and Gravity Separation

Slags coming from stainless steel (SS) and ferrochromium (FeCr) production generally contain between 1 and 10% Cr, mostly present in entrapped metallic particles (Fe–Cr alloys) and in spinel structures. To recover Cr from these slags, magnetic and gravity separation techniques were tested for up-concentrating Cr in a fraction for further processing. In case of SS slag and low carbon (LC) FeCr slag a wet high intensity magnetic separation can up-concentrate Cr in the SS slag (fraction <150 µm) from 2.3 wt.% to almost 9 wt.% with a yield of 7 wt.%, and in the LC FeCr slag from 3.1 wt.% to 11 wt.% with a yield of 3 wt.%. Different behavior of Cr-containing spinel’s in the two slag types observed during magnetic separation can be explained by the presence or absence of Fe in the lattice of the Cr-containing spinel’s, which affects their magnetic susceptibility. The Cr content of the concentrates is low compared to chromium ores, indicating that additional processing steps are necessary for a recovery process. In the case of high carbon (HC) FeCr slag, a Cr up-concentration by a factor of more than three (from 9 wt.% to 28 wt.%) can be achieved on the as received slag, after a single dry low intensity magnetic separation step, due to the well-liberated Cr-rich compounds present in this slag. After gravity separation of the HC FeCr slag, a fraction with a Cr content close to high grade Cr ores (≥50% Cr2O3) can be obtained. This fraction represents 12 wt.% of the HC FeCr slag, and can probably be used directly in traditional smelting processes.

Immobilization of Chromium in Stainless Steel Slag Using Low Zinc Electric Arc Furnace Dusts

A new synergistic treatment of stainless steel slag and low zinc content electric arc furnace (EAF) dusts is proposed to immobilize harmful chromium in stainless steel slag. The effects of ZnFe2O4 addition on the mineralogic phase and chromium leachability of CaO-SiO2-MgO-Al2O3-Cr2O3 synthetic slag were investigated to explore the feasibility of this method. The mineralogic phases in stainless steel slag were investigated by scanning microscopy equipped with energy-dispersive spectroscopy and X-ray diffraction. The leaching concentration values of chromium and zinc were evaluated according to an alkaline digestion for the hexavalent chromium (US-EPA-3060A) and toxicity characteristic leaching procedure (TCLP, US-EPA-1311) method, respectively. It was found that all synthetic slags mainly contain α-Ca2SiO4, merwinite and spinel phase, in line with the calculation results by FactSage. The crystallization of spinel and merwinite phases was enhanced by the addition of ZnFe2O4 but suppressed the precipitation of α-Ca2SiO4. It was revealed that the leaching concentration of chromium was depressed by adding ZnFe2O4 and was far below the chemical limits defined in the French proposal for a criterion and evaluation methods for waste ecotoxicity (0.1 mg L−1). In addition, the zinc leaching concentration meets the sanitary landfill standard. The proposed synergistic treatment method was further validated on industrial stainless steel slags and EAF dusts. Experimental results indicated that the synergistic treatment method can immobilize chromium effectively and the final slags can be disposed of by sanitary landfilling or recycling as constructional materials. A schematic technologic route diagram of the synergistic treatment was also proposed.

Effect of Fe2O3 on the size and components of spinel crystals in the CaO−SiO2−MgO−Al2O3−Cr2O3 system

The stability of chromium in stainless steel slag can be enhanced by increasing the spinel crystal size. The effect of Fe2O3 on the size of spinel crystals in the CaO−SiO2−MgO−Al2O3−Cr2O3 system was investigated using lab experiments carried out in a carbon tube furnace. Scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM−EDS) and X-ray diffraction (XRD) were used to analyze the microstructure, components, and the mineral phases of synthetic slags. FactSage 7.1 was used to calculate the crystallization process of the molten slag. The results showed that the addition of Fe2O3 promoted the precipitation of spinel crystals and inhibited the formation of dicalcium silicate. The size of spinel crystals increased from 2.74 to 8.10 µm and the contents of chromium and iron in the spinel varied as the Fe2O3 addition was increased from 0 to 20wt%. Fe2O3 thermodynamically provided the spinel-forming components to enhance the formation of FeCr2O4, MgFe2O4, and Fe3O4. The addition of Fe2O3 increased the fraction of liquid phase in a certain temperature range and promoted diffusion by decreasing the slag's viscosity. Therefore, Fe2O3 is beneficial to the growth of spinel crystals in stainless steel slag.

Experimental Study on Precipitation Behavior of Spinels in Stainless Steel-Making Slag under Heating Treatment

The stability of chromium in stainless steel slag has a positive correlation with spinel particle size and a negative correlation with the calcium content of the spinel. The effect of heating time on the precipitation of spinel crystals in the CaO-SiO2-MgO-Al2O3-Cr2O3-FeO system was investigated in the laboratory. Scanning electron microscopy with energy-dispersive and X-ray diffraction were adopted to observe the microstructure, test the chemical composition, and determine the mineral phases of synthetic slags, and FactSage7.1 was applied to calculate the crystallization process of the molten slag. The results showed that the particle size of the spinel crystals increased from 9.42 to 10.73 μm, the calcium content in the spinel crystals decreased from 1.38 at% to 0.78 at%, and the content of chromium in the spinel crystal increased from 16.55 at% to 22.78 at% with an increase in the heating time from 0 min to 120 min at 1450 °C. Furthermore, the species of spinel minerals remained constant. Therefore, an extension in the heating time is beneficial for improving the stability of chromium in stainless steel slag.

Emission reduction research and formation of hexavalent chromium in stainless steel smelting: Cooling rate and boron oxide addition effects

During the stainless steel smelting process, the control of the slag skimming process affects the formation of hexavalent chromium in the stainless-steel slag. However, the hazardous problem of hexavalent chromium in stainless-steel slag has been a major factor limiting its resource utilization. In this paper, by studying the enrichment form of chromium in stainless-steel slag and the formation mechanism of hexavalent chromium, the cooling system and the content of additives were controlled in the slag skimming process to reduce the formation of hexavalent chromium. Different means of characterization including TG-DSC, phase diagram, FT-IR, XPS, potential-pH equilibrium diagram and HJ 687-2014 method were used to characterize the presence and content change of hexavalent chromium in solid and leachate. Results showed that hexavalent chromium formation mechanism was related with the chromium diffusion in two typical Cr-containing components in stainless-steel slag. A slow cooling rate can reduce the emission of Cr(VI) in stainless-steel slag. A lower addition of boron oxide content made a lower emission of hexavalent chromium.

Stability of Chromium in Stainless Steel Slag during Cooling

The chromium elution behavior of stainless steel (SS) slag depends highly on the chromium distribution, and the molten modification process proved to effectively improve the chromium enrichment in stable phases. However, the phase transformation and variation of chromium stability during the subsequent cooling process is still poorly understood. In this work, the phase composition and chromium distribution of SS slag from different quenching temperatures were experimentally studied, and the stability of chromium-bearing phases was evaluated using standard leaching tests. The results indicated that dicalcium silicate and spinel phases had formed in the molten slag at 1600 °C, while the dicalcium silicate disappeared and the phases of merwinite and melilite precipitated when the temperature decreased from 1600 to 1300 °C (at a rate of 5 °C/min). During this cooling process, the chromium migrated from other phases into the spinel, significantly suppressing the chromium elution. The leaching results also demonstrated that the potential chromium-bearing phases of glass, dicalcium silicate and merwinite are unstable and are presumably the main source of chromium release. The treated SS slag meets the requirements for the utilization of chromium-bearing slag in the cement and brick industries.

Effect of Lime on Stability of Chromium in Stainless Steel Slag

The stabilization of chromium is of great importance to the use of stainless steel slag (SSS), and the influence of lime on the stability of chromium is currently unclear. In this work, the variation of phase transformation and chromium distribution with slag basicity (CaO/SiO2) were investigated experimentally, and the leaching ability of chromium was evaluated. Results showed that chromium-bearing phases were glass, dicalcium silicate (C2S), spinel, and periclase, while the degree of enrichment of chromium in these phases was found to be closely related to the basicity. The optimal basicity obtained in this research was 1.5, with the chromium mainly present in the stable spinel and exhibiting the lowest leaching ability. The product layer structure of unmelted lime was studied as well, showing a periclase layer and a Ca2SiO4 layer. Some CaCr2O4 had formed in the periclase layer, which is potentially hazardous for the environment and living organisms.

Growth behavior of spinel in stainless steel slag during cooling process

Spinel is an appropriate phase for sequestrating chromium in stainless steel slag to prevent chromium from polluting, and the crystal size of spinel has a significant effect on the stability of chromium and the extraction efficiency of spinel. The enrichment behavior of chromium in spinel during cooling process was investigated, and the growth process of spinel was studied based on the crystal size distribution theory. The results showed that the enrichment degree of chromium in spinel increased from 88.5% to nearly 100% during the cooling process from 1773 to 1573 K at a rate of 5 K/min. The mean diameter of spinel in stainless steel slag had an obvious growth in the cooling process, and a rapid growth of spinel occurred during the cooling process from 1623 to 1523 K. Leaching results indicated that the leaching amount of chromium could reduce to less than 0.01 mg/L by controlling the cooling condition. The growth mechanism of spinel was investigated and proposed to consist of two stages: in the first stage, the spinel was in the surface-controlled growth with a decaying nucleation rate from 1773 to 1573 K, and in the second stage, the growth mechanism was transformed to supply-controlled Ostwald ripening below 1573 K.

Effects of Boron Oxide Addition on Chromium Distribution and Emission of Hexavalent Chromium in Stainless-Steel Slag

Stainless-steel slag is considered a hazardous waste because it contains significant levels of hexavalent chromium. In the present study, the effect of boron oxide addition on the chromium distribution in Cr-bearing phase and emission of hexavalent chromium has been investigated using 2, 4, and 8 wt % addition at three different temperatures. Thermogravimetric differential scanning calorimetry (TG-DSC) and the binary phase diagram of CaO-Cr2O3 were used to find out the temperature range for the formation of hexavalent chromium. Besides, the treated samples were characterized using X-ray diffraction (XRD), scanning electron microscope-energy dispersive spectrometer (SEM-EDS), NIH ImageJ software and thermodynamic package FactSage 6.4, respectively. Additionally, emission of hexavalent chromium in the treated samples was detected used modified HJ 687–2014 and GB/T 15555.4–1995 after three high-temperature treatments. Finally, the research showed that chromium was enriched in the stable spinel phase and a lo...

Immobilization of Cr (VI) in stainless steel slag and Cd, As, and Pb in wastewater using blast furnace slag via a hydrothermal treatment

The immobilization of hexavalent chromium in stainless steel slag using blast furnace slag as the immobilizing agent and by performing a hydrothermal treatment was investigated. The results showed that there was no immobilization in the absence of the blast furnace slag. On the other hand, the hexavalent chromium in stainless steel slag could be immobilized through the hydrothermal reaction when blast furnace slag was used at 250 °C for 24 h. A leaching test was performed to evaluate the degree of immobilization of hexavalent chromium in the products formed by the hydrothermal reaction. It was found that the degree of immobilization was very high. Based on the results obtained, the immobilization mechanism of hexavalent chromium in stainless steel slag, resulting from the hydrothermal treatment of blast furnace slag, could be elucidated. Finally, the immobilization of cadmium, lead, and arsenic using blast furnace slag as the immobilization agent was also studied while focusing on the effects of the hydrothermal treatment.

Effect of Al2O3 modification on enrichment and stabilization of chromium in stainless steel slag

Spinel phase is considered to be the optimal phase for stabilization of chromium in stainless steel slag. In order to restrain chromium leaching from slag for the effective environmental protection, Al2O3 was utilized for the modification treatment, and the effects on the enrichment and stabilization of chromium were investigated. The mineral phases and the existence state of chromium in slag with various Al2O3 contents at different basicities (wCaO/wSiO2) were analyzed by scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) and X-ray diffraction (XRD). The results showed that chromium mainly existed in the glass and spinel phases at basicity of 1.0 and 1.5. As the slag basicity increased to 2.0, chromium was also found in periclase phase. Al2O3 in the stainless steel slag reacted with MgO and Cr2O3, which could generate the Al-rich Mg(Crx Al1–x)2O4 solid solution. Moreover, the addition of Al2O3 was favorable to reduce the solubility of chromium oxide in liquid phase and suppress the precipitation of periclase phase. The experimental results demonstrated that Al2O3 modification has a positive influence on the enrichment and the stabilization of chromium in the stainless steel slag.

Crystallization Behaviour of Chromium in Stainless Steel Slag: Effect of Feo and Basicity

Crystallization Behaviour of Chromium in Stainless Steel Slag: Effect of Feo and Basicity

Crystallization Behaviour of Chromium in Stainless Steel Slag: Effect of Feo and Basicity

The synthetic stainless steel slag systems with different basicity and different FeO contents were prepared using reagent powders, the crystallization of which was experimented by cooling from melt to 1400°C in N2 atmosphere. The crystallization in the collected slag was studied using X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). It was found that an amorphous structure in slags with FeO was easier to be formed in low basicity (CaO/SiO2) of 1.0 than the basic slags, while in slag systems with high basicity of 1.25 or 1.5, well crystalline phases including spinel and merwinite were formed, irrespective of FeO content. The results also showed that higher FeO content and lower basicity improved the precipitation of Cr2O3 into Mg(Al,Fe,Cr)2O4 spinel solid solution.

Reduction Behavior of Chromium Oxide in Molten Slag Using Aluminum, Ferrosilicon and Graphite.

To reduce chromium in stainless steel slag and simultaneously recover chromium as a ferroalloy for the steel making industry, the direct smelting reduction of chromium oxide in molten slag was investigated by small furnace experiments. Thermodynamic equilibrium calculations were also carried out to suggest the limit of reduction of Cro x in the slags. Aluminum, ferrosilicon and graphite were used as reductants under conditions of 1 723-1 823 K and various slag compositions. In the experiments, the formation of high Al 2 O 3 solid phase in the upper and middle parts of slag prevented uniform reduction when aluminum was added as a reductant, although FeO and Cro x were reduced drastically to a low content compared with the cases of ferrosilicon and graphite except for the lower part of slag melt. The low content of Cro x by aluminum was about 0.01 mass% which was about one order higher than calculated thermodynamic equilibrium one. The reductions of FeO and Cro x by aluminum were promoted to the lower part of slag with the increase in temperature and CaO/SiO 2 ratio because of the increase in fluidity of slag. A liquid metal containing chromium was formed and precipitated on the bottom of slag during the reduction. The formation behavior of metal drop was observed directly by an X-ray fluoroscopy technique.