Chromium-containing Slag Research Articles

PROSPECTS FOR THE EXTRACTION OF CHROMIUM FROM SLAG FERROCHROME PRODUCTION BY ACID METHOD

In the Republic of Kazakhstan, there is a noticeable upward trend in the volume of industrial waste, including from the production of ferrochrome. The problems of man-made pollution of the natural environment are becoming more acute every year, and are beginning to take on a global dimension. Technogenic ferrochrome production waste contains chromium, which is highly toxic and carcinogenic, which poisons water, soil, negatively affects the activity of all living organisms. Slags contain a significant number of valuable components used in various industries, and above all, chromium, which is irrevocably lost during storage. The main task still remains the development of industrial waste disposal processes that reduce the anthropogenic load on the biosphere and ensure the rational use of natural resources. The purpose of the work is to justify the choice of the method of processing the slags of the ferrochrome production. Conclusions: The analysis of the modern scientific and patent literature on acid processing and utilization of chromium-containing slags from the production of refined and high-carbon ferrochrome has been conducted. It should be noted that the described methods of various methods of processing ferrochrome production slags, despite their availability, are characterized by a multi-stage nature and do not allow complete processing of ferrochrome production slags in Kazakhstan. Currently, there is an excess of sulfuric acid production in Kazakhstan, so it becomes advisable to use sulfuric acid as a reagent for leaching chromium from slag from the production of ferrochromium. It can be expected that in sulfuric acid under certain conditions, such as heating, a sufficiently high degree of extraction of chromium (III) will be achieved in slags from the production of high-carbon chromium and in refined ferrochem slags.

Efficient Utilization of Stainless Steel Dust and Chromium-Containing Slag by Carbothermal Direct Reduction: Synergistic Mechanism and Optimization Analysis

Efficient Utilization of Stainless Steel Dust and Chromium-Containing Slag by Carbothermal Direct Reduction: Synergistic Mechanism and Optimization Analysis

Long-term leaching mechanism of chromium-containing slag after vitrification and heat treatment

Vitrification and heat treatment are economical methods for utilizing Cr-containing slag; however, the prepared products have different Cr solidification abilities. Herein, the effects of the two routes on the long-term leaching behavior of Cr-containing slags were studied. The Mg, Al, and Cr contents and heat treatment were found to influence the type and relative content of the mineral phases, which further promoted the Cr leaching behavior of the samples. Among them, the 5Al-3Cr glass and heat-treated samples exhibited relatively higher Cr leaching accumulation. Compared with the heat-treated samples, the glass samples had a better solidification ability, particularly for the high-Cr-containing group. The Cr leaching accumulation decreased by 70%–90%, indicating that the vitrification of the melt can effectively inhibit the formation of the Cr-leachable phase. During the leaching process, the dissolved Ca/Mg/Al ions influence the pH and redox potential of the liquid environment, resulting in the oxidation of Cr3+ to Cr6+. This research showed that vitrification is a safer and more effective route for the utilization of Cr-containing slag.

Advances in microbial remediation of the re-dissolved chromium contaminated sites

Wet detoxification has traditionally been seen as the most promising technology for treating chromium-contaminated sites. However, the addition of chemicals in the wet detoxification process not only increases the cost but also introduces extra pollutants. Moreover, the chromium-containing slag may be re-dissolved in the form of Cr(VI), and the increased concentration of Cr(VI) results in a serious "returning to yellow" phenomenon in the chromium-contaminated sites, causing undesirable secondary pollution. Microbial remediation is a promising technology to address the re-dissolution of chromium-containing slag after wet detoxification, and this article reviews the advances in this area. Firstly, the toxicity, current situation and conventional technologies for treating the chromium-containing slag were briefly summarized. The mechanisms of the inevitable re-dissolution of chromium-containing slag after wet detoxification were summarized. Three main mechanisms, namely bioreduction, biosorption and biomineralization, which are involved in the environmental-friendly and efficient microbial remediation technology, were reviewed. The variation of microbial species and the succession of microbial community during the bioremediation of chromium-contaminated sites were discussed. Finally, future research directions were prospected with the aim to develop long-term, stable and sustainable technologies for remediating the chromium-contaminated sites.

Effect of basicity and chromium oxide on the viscosity of boron-containing slags

The main diluent for chromium-containing slags is CaF2. However, fluorspar disadvantages make it necessary to search for a replacement. As a substitute, boron oxide allows to improve slag physical properties and environmental situation. The paper presents the experimental study results of the chemical composition and temperature influence on viscosity of slags of CaO–SiO2–Cr2O3 system containing 8% MgO, 3% Al2O3 and 6% B2O3. It was found that a slag free of chromium oxide with basicity of 1.0 has a sufficiently high fluidity of 0.2–0.6 Pa·s in a wide temperature range of 1200–1350°C, due to high concentration of low-melting phases, reaching 22%, and only 11 % high-melting. A slag with 18% Cr2O3 and the same basicity retains a low viscosity of 0.1–1.0 Pa·s but at higher temperature of 1450–1570 °C due to increase in high-melting compound content to 27%. The viscosity of a slag with basicity of 2.5 without chromium oxide is 0.07–1.0 Pa·s in a narrow temperature range of 1650–1700°C and 0.14–1.0 Pa·s for a ‘shorter’ slag with 18% Cr2O3 and same basicity in even narrower range of 1650–1670°C. These slags have the highest high-melting phase content of about 50%.

Relationship between thermophysical properties and structure of CMAS glass melts based on chromium slag

In an attempt to determine the relationship among the network structure, viscosity, electrical conductivity, surface tension, and density of chromium-containing glass melts, the variation in glass melt thermophysical properties of the CaO–MgO–Al2O3–SiO2 quaternary system were investigated by comprehensive physical property analysis and Raman spectroscopy. The results demonstrated that the network structural units Qn (n = 0, 1, 2, 3) coexisted in the glass melts and their mutual transformation caused changes in the depolymerization degree, leading to a range of viscous activation energies, conductive activation energies, surface tension, and melt densities. Additionally, the logarithm of viscosity varied linearly with the logarithm of electrical conductivity (slope: 0.67–1.54). Meanwhile, the surface tension and melt density decreased with temperature. Consideration of these interrelations is indispensable during the development of chromium-containing slag glass ceramics, particularly for pilot and industrial applications.

Transport and fate of hexavalent chromium in slag–soil system

The transport and fate of hexavalent chromium Cr(VI) in slag–soil system, which remain poorly understood, are of great importance for environmental risk assessment and pollution control of Cr(VI) in soil and groundwater. Based on the migration behavior of Cr(VI) in the chromium-containing slag under the effects of rainfall, the transport and fate of Cr(VI) in soil were investigated. Batch and column experiments were undertaken to clarify the Cr(VI) adsorption and transport behavior of aqueous Cr(VI) in soil, respectively. Findings from a combination of experimental and model results clearly suggest that the adsorption of Cr(VI) onto soil is a spontaneous chemical adsorption process. For chromium-soil transport system, the constructed HYDRUS-1D dynamics model is a good simulation for Cr(VI) migration in soil, as the modeling yielded adequate fit for simulated and measured values of Cr(VI) concentration in soil (correlation coefficient R2 = 0.99). The transport modeling then was utilized to assess the transport of Cr(VI) in slag–soil system. Up to the year of 2018, the concentration levels of Cr(VI) in the soil-leaching solution attained 905 mg/L. Due to the thin layer of soil and the high mobility of Cr(VI) in soil, it is no doubt that the Cr(VI) leaching will engender serious pollution of soil and groundwater in the presence of precipitation. The applications and limitations of the HYDRUS-1D were also discussed, such as bacteria transport, more elaborate biogeochemical processes, coupling of vadose zone processes with existing larger scale groundwater flow models.

Formation of hexavalent chromium in Cr-containing steelmaking slag

Purpose: In present work, the mineral phases in chromium-containing slags were identified, and the formation mechanisms of Cr(VI) in Cr2O3-containing mineral phases were discussed. Design/methodology/approach: In steelmaking slag containing chromium, chromium is mainly present as spinel phases such as MgCr2O4 and (Mg,Fe)Cr2O4. When these Cr(III) oxides are oxidized to Cr(VI) oxide, the Cr(VI) is easily eluted from the slag due to its high solubility in water. Findings: The Cr (VI) adversely affects the human health and the environment. In this study, the influence of cooling rate, quenching temperature and oxidation/reduction condition during cooling on the Cr(VI) amount eluted from synthesized MgCr2O4, (Mg,Fe)Cr2O4 and CaCr2O4 were investigated. Research limitations/implications: The formation of Cr(VI) oxide in MgCr2O4 and CaCr2O4 compounds during heating under air was considered to be indispensable. The amounts of Cr(III) and Cr(VI) dissolved from MgCr2O4 were smaller than those from CaCr2O4. Since the formation of CrO3 in MgCr2O4 started at around 1400 K during cooling, slag should be cooled rapidly from the high temperature above 1400 K, or cooled in inert atmosphere in order to minimize Cr(VI) formation. FeO in (Mg,Fe)Cr2O4 solid solution suppressed Cr(VI), Cr(III) and Mg elution. Originality/value: The development of prevention method of Cr(VI) formation in the slags containing chromium is urgent in order to utilize the slags to land-fill and civil engineering works.

Durability and environment evaluation of an eco-friendly cement-based material incorporating recycled chromium containing slag

Abstract This study investigates the durability of cement-based materials incorporating recycled chromium containing slag (CCS), including chloride permeability, carbonation resistance, freeze-thaw cycle and microstructure development. Additionally, the environmental impacts on the cement-based material with the addition of CCS are evaluated by toxicity characteristic leaching procedure (TCLP), carbon dioxide (CO2) emission and energy consumption. The obtained results show that the durability properties of chromium containing slag concrete (CCSC) are improved based on the addition of 20–25 wt% CCS and 5–10 wt% silica fume (SF), and the hexavalent chromium (Cr(VI)) can be well immobilized in the cement-based product. Furthermore, the incorporation of recycled CCS can significantly reduce the energy consumption and CO2 emission of cement-based materials. Based on this study, it is possible to utilize the recycled CCS to produce a clean and durable eco-friendly construction product.

An efficient method of chromium extraction from chromium-containing slag with a high silicon content

An efficient process was proposed for extracting chromium from chromium-containing slag after desilication with aqueous NaOH solution. The effect of the NaOH concentration, liquid-to-solid mass ratio, and leaching temperature on the leaching rate of silicon was investigated. The results showed that 83.78% of the silicon, 41.87% of the aluminum, and 46.42% of the vanadium were removed from the slag under suitable conditions. The kinetic analysis results showed that the leaching rate of silicon is controlled by a chemical reaction under two regimes due to the two silicon-bearing minerals presented in the slag. The first stage is limited by the reaction between NaOH and SiO2, whereas the second stage is subject to the reaction between NaOH and 2MgO·2Al2O3·5SiO2. The corresponding apparent activation energies were calculated to be 67.27kJ/mol and 35.82kJ/mol, respectively. The extraction rate of chromium from the leaching residue using lime-free roasting reached 98.41%, which was highly superior to the 9.85% obtained from the slag without desilication.

Influence of Silica on the Lime-Free Roasting of Chromium-Containing Slag

The evolution of silica during the lime-free roasting and their roles were investigated using chromium-containing slag with high content of ferric oxide and silica. The siliceous salt phases were analyzed by means of X-ray diffraction, scanning electron microscope, and energy-dispersive X-ray spectroscopy. The reaction rule involved with silicon-containing compounds in the roasting process was also discussed. The results showed that both oxidation of Cr2O3 and leaching of Na2CrO4 were influenced by the siliceous salts. It suggested that roasting temperature and the amount of alkali act as important factors for the phase transformation of silicon. NaFe(SiO3)2 was formed in the temperature which is best for the production of Na2CrO4, while excess amount of alkali could decompose acmite into NaFeO2 and Na2SiO3. It was observed that a large amount of insoluble NaFe(SiO3)2 took shape during the lime-free roasting when the temperature was above 1273 K (1000 °C). The extracting rate of chromium from chromium-containing slag roasted at 1373 K (1100 °C) was only 29.99 pct and almost no silicon was leached out.

Investigation of Reaction Mechanism on the Lime-Free Roasting of Chromium-Containing Slag

The lime-free roasting process of trivalent chromium-containing slag was investigated. The effect of Fe and liquid phase on the conversion reaction of chromium was discussed. The oxidation of trivalent chromium depends greatly on the diffusion of Na+ and O-2. Both the raw material Na2CO3 and the intermediate product NaFeO2 serve as the carriers of Na+. The Na+ diffusion is improved by the binary liquid phase of Na2CrO4-Na2CO3, whereas excess liquid phase results in a low conversion rate of chromium by hindering the diffusion of oxygen towards the reaction interface. With the increasing of liquid volume, the controlled step of chromium oxidation changes from Na+ diffusion to oxygen diffusion. The mechanism study showed that the volume of liquid phase increased while raising the reaction temperature or prolonging the reaction time. Based on the role of both liquid phase and Fe, the oxidation process of chromium was summarized as a three-stage model: the Na+ diffusion-controlled stage, the O-2 diffusion-controlled stage, and the oxidation reaction halted stage. (C) The Minerals, Metals & Materials Society and ASM International 2015

Bioreduction of Chromate by an Isolated Bacillus anthracis Cr-4 with Soluble Cr(III) Product

Microbial Cr(VI) reduction is a significant process in detoxification of Cr(VI) pollution. In this study, a new Cr(VI)-reducing bacterial strain, Cr-4, was isolated from soil around the chromium-containing slag. The analysis of the 16S ribosomal RNA (rRNA) gene sequence revealed that the newly isolated strain was closely related to Bacillus anthracis. The response to Cr(VI) stress and reduction capacity of the isolate were investigated. Cell growth decreased with the increase of Cr(VI) concentration. Cell morphology varied and cell growth was inhibited remarkably in the presence of 125 mg/L Cr(VI). The strain grew well and removed Cr(VI) effectively at a Cr(VI) concentration lower than 50 mg/L. Cr(VI)-reducing activity was inhibited by Zn2+, while significantly stimulated by Cu2+. The activity of Cr(VI) reduction by cell-free extract was demonstrated. Total chromium analysis and the energy-dispersive X-ray analysis (EDAX) spectrum revealed that Cr(VI) removal was caused mainly by microbial reduction rather than by biosorption and the main part of the reduced Cr(III) existed as soluble form in solutions.

Draft Genome Sequence of Bacillus subtilis QH-1, a Chromium-Reducing Bacterial Strain Isolated in Qinghai Province, China

Bacillus subtilis strain QH-1, a chromium-reducing bacterial strain, was isolated from a soil sample from a chromium-containing slag heap. The draft genome sequence of this bacterium is 4,034,036 bp in length, with a G+C content of 43.71%, and it is predicted to contain 4,082 protein-coding genes.

Bioreduction of Cr(VI) by Bacillus sp. QH-1 isolated from soil under chromium-containing slag heap in high altitude area

A chromium-reducing strain QH-1, identified as Bacillus sp., was isolated from soil under chromium-containing slag heap in Qinghai high altitude area, China. The strain was found to resist 200 mg/L Cr(VI), and Cr(VI) negatively affects the metabolic activity of the cells, as well as the cell morphology of Bacillus sp. QH-1. The reduction efficiency of Cr(VI) at concentrations of Cr(VI) 25 mg/L, 50 mg/L, 100 mg/L and 200 mg/L were 99.48 %, 65.99 %, 23.22 % and 6.99 %, respectively, decreasing with increasing initial Cr(VI) concentration. This indicates that the toxicity of Cr(VI) increased with concentration. Energy dispersive X-ray analysis revealed that there was insoluble Cr(III) generated during Cr(VI) reduction. In order to apply strain QH-1 to remove Cr(VI) from groundwater, factors of concentration of electron donors (glucose) and temperature were investigated in a synthetic medium. The results demonstrated that glucose could promote the reduction of Cr(VI) by this strain, and the general trend of Cr(VI) reduction increased with temperature within the range of 4 to 37 °C. Cr(VI) was reduced effectively at 25 °C and 37 °C, and all of Cr(VI) was reduced after 96 h at 37 °C, while the reduction was slow at 4 °C and 15 °C, and it almost ceased after about 120 h. These results could be potentially useful for the bioremediation of Cr(VI) in groundwater.

Studies of Vaporization of Chromium from Thin Slag Films at Steelmaking Temperatures in Oxidizing Atmosphere

In the present work, the volatilization of chromium from thin chromium-containing slag film surfaces was studied in oxidizing atmosphere in the temperature range 1673 K to 1873 K (1400 °C to 1600 °C). The slag films on alumina rings were exposed to air or pure oxygen and the loss of Cr from the post-experiment sample films was examined by SEM/WDS analysis. The mass loss of the samples was also monitored during the heat-treatment. The results indicate that chromium loss increased with increase in temperature and oxygen partial pressure was found also to be relatively less as the sample thickness increased. The implications of chromium escape from slags during the tapping of stainless steel slags are discussed.

Evaluation of Chromium Pollution on Soil Surrounding Chromium-Containing Slag Heap

In this paper, the soil pollution condition of a typical chromium-containing slag heap in the North China plain were analyzed on the basis of previous data and the test during the field investigation. The hexavalent chromium content contour map was drawn. The regional soil chrome pollution level was evaluated by using single factor index method and potential ecological harm index method. The results of the single factor index method show that soil surrounding chromium-containing slag heap was seriously polluted. However, the results of the potential ecological harm index method evaluation show a slight pollution, which indicates that the area of chromium slag is not significantly harmful to ecology of the surrounding soil.

Microbial Detoxification of Hexavalent Chromium from Chromium-Containing Slag

In this paper,treatment and recovery of hexavalent chromium(Cr(Ⅵ)) from chromium-containing slag by bacteria was studied.The work showed that a bacterial strains of Ch-1 isolated from chromium-containing slag can efficiently accelerate Cr(Ⅵ) leaching and remove it.Moreover,bacteria Ch-1 can effectively change the structure of chromium-containing slag by Scanning Electron Microscopy(SEM) and the Energy Dispersive X-Ray(EDX),which is benefitial to Cr(Ⅵ) leaching and its detoxification by changing Cr(Ⅵ) to Cr(Ⅲ).The effect of detoxification by bacteria of Ch-1 is obvious,the toxicity in leached slag was 3.3 μg/g lower than the national standard of 5 μg/g,and the recovery rates of hexavalent chromium are more than 90%.

Static and Dynamic Leaching of Chromium(VI) from Chromium-Containing Slag

Because the slag can be a major source of soil and groundwater pollution, understanding the release and leaching behavior of hexavalent chromium [Cr(VI)], from chromium-containing slag is critical. We report Cr(VI) behavior after static and dynamic leaching from chromium-containing slag deposited at a ferro-alloy manufacturing plant in Hunan province, China. Bench-scale experiments indicate that the main factors influencing the dissolution release of Cr(VI) include the solid-liquid ratio, soak time, agitation, acidity, particle size, and water dynamic flow. Based on the parameters obtained from the dynamic leaching experiment, a Cr(VI) release and leaching model was developed and applied to the study area. The model can quantitatively predict the extent of Cr(VI) pollution of soil and groundwater under atmospheric precipitation.

Identification and hexavalent chromium reduction characteristics of Pannonibacter phragmitetus

A hexavalent chromium [Cr(VI)] reducing bacterial strain was isolated from chromium-containing slag. It was identified as Pannonibacter phragmitetus based on physiological, biochemical characteristics and 16S rRNA gene sequence analysis. This bacterium displayed great Cr(VI) reduction capability. The Cr(VI) could be completely removed in 24 h under anaerobic condition when the initial concentration was 1,917 mg L(-1), with the maximum reduction rate of 562.8 mg L(-1) h(-1). The Cr(VI) reduction rate increased with the increase of Cr(VI) concentration. P. phragmitetus was able to use many carbon sources such as lactose, fructose, glucose, pyruvate, citrate, formate, lactate, NADPH and NADH as electron donors, among which the lactate had the greatest power to promote the reduction process. Zn(2+), Cd(2+) and Ni(2+) inhibited, while Cu(2+), Pb(2+), Mn(2+) and Co(2+) stimulated the reduction. The optimum pH and temperature for reduction were 9.0 and 30 °C, respectively. The results indicated that this strain had great potential for application in the bioremediation of chromate-polluted soil and water systems.