Chromium Ore Processing Residue Research Articles

Metagenomes from microbial populations beneath a chromium waste tip give insight into the mechanism of Cr (VI) reduction

Dumped Chromium Ore Processing Residue (COPR) at legacy sites poses a threat to health through leaching of toxic Cr(VI) into groundwater. Previous work implicates microbial activity in reducing Cr(VI) to less mobile and toxic Cr(III), but the mechanism has not been explored. To address this question a combined metagenomic and geochemical study was undertaken. Soil samples from below the COPR waste were used to establish anaerobic microcosms which were challenged with Cr(VI), with or without acetate as an electron donor, and incubated for 70 days. Cr was rapidly reduced in both systems, which also reduced nitrate, nitrite then sulfate, but this sequence was accelerated in the acetate amended microcosms. 16S rRNA gene sequencing revealed that the original soil sample was diverse but both microcosm systems became less diverse by the end of the experiment. A high proportion of 16S rRNA gene reads and metagenome-assembled genomes (MAGs) with high completeness could not be taxonomically classified, highlighting the distinctiveness of these alkaline Cr impacted systems. Examination of the coding capacity revealed widespread capability for metal tolerance and Fe uptake and storage, and both populations possessed metabolic capability to degrade a wide range of organic molecules. The relative abundance of genes for fatty acid degradation was 4× higher in the unamended compared to the acetate amended system, whereas the capacity for dissimilatory sulfate metabolism was 3× higher in the acetate amended system. We demonstrate that naturally occurring in situ bacterial populations have the metabolic capability to couple acetate oxidation to sequential reduction of electron acceptors which can reduce Cr(VI) to less mobile and toxic Cr(III), and that microbially produced sulfide may be important in reductive precipitation of chromate. This capability could be harnessed to create a Cr(VI) trap-zone beneath COPR tips without the need to disturb the waste.

Solubility product of chromate analogue of hydrogarnet (Ca3Al2[CrO4]0.12[O4H4]2.88(s)) at 25 °C

Chromium (Cr)-hydrogarnet (Ca3Al2 [CrO4]0.12[O4H4]2.88(s)) is a critical host of hexavalent chromium [Cr(VI)] in cementitious and alkaline solids such as chromium ore processing residue (COPR). Leaching from COPR often results in Cr(VI) contamination of soil and groundwater. Although Cr-hydrogarnet has been synthesized and characterized previously, its solubility product (Ksp) is poorly constrained. Accurate determination of this thermodynamic parameter is critical to quantify the extent of Cr(VI) dissolution from COPR in water. In this contribution, the Ksp of Cr-hydrogarnet was determined by (i) hydrothermal synthesis of the solid at 150 °C in alkaline conditions in two batches; and (ii) batch equilibration experiments over 745 d from undersaturation and over 60 d from supersaturation. For each synthesized batch of solids, Cr-hydrogarnet solubility in ultrapure water at 25 °C in triplicate reactors was measured. Time-dependent aqueous samples were collected, and concentrations of Cr(VI) and co-solutes and pH were measured until a steady state was attained from undersaturation, after which the state was perturbed by the addition of 7× Cr(VI) to trigger the supersaturation experiment. X-ray diffraction and SEM-EDS analyses on the reacted solids confirmed the presence of Cr-hydrogarnet and its constituent elements, respectively. In addition, calcite and aluminum oxyhydroxides were formed as minor secondary phases. The log ion activity products at steady states attained from undersaturation and supersaturation were 72.36 ± 0.86 and 72.75 ± 0.14, respectively. A mean log Ksp of 72.55 ± 0.87 at 25 °C for the reaction Ca3Al2 [CrO4]0.12[O4H4]2.88(s) + 11.52H+ ↔ 3Ca2+ + 2Al3+ + 0.12CrO42− + 11.52H2O was proposed. Furthermore, the new thermodynamic constant was used to analyze data from two previous COPR equilibration studies in water and identify the governing mechanisms of Cr(VI) release.

Immobilization of chromium ore processing residue by alkali-activated composite binders and leaching characteristics.

Chromium ore processing residue (COPR) is classified as hazardous solid waste because of the leachable Cr(VI). Cementitious materials are often used to solidify and stabilize heavy metals. However, most of them focus on the leaching concentration of particles after solidification and stabilization and lack research on leaching characteristics. This study investigated the leaching characteristics of heavy metals in three simulated environments (HJ557-2010, HJ/T299-2007, TCLP) after immobilizing COPR with composite binders. Industrial solid waste coal fly ash and lead-zinc smelting slag are used to prepare composite binders through alkali activation technology. Compressive strength, particle leaching toxicity, acid neutralization capability, and semi-dynamic leaching test are used to evaluate the performance of the solidified body. The solidified body can be applied to building materials or treated as general industrial waste. Heavy metals are mainly released from the matrix by surface washing at a low rate. The analysis results, including XRD, FTIR, and SEM-EDS, show that chemical binding and physical encapsulation are the main immobilizing mechanisms to realize the coordinated disposal of Zn and Cr(VI).

Water/acid/reductive biomass leaching of Cr(VI) in chromium ore processing residue (CORP) for safe discharge of effluent and to recover Cr(III) and gypsum

The hydrolyzed biomass, a green and renewable resource, was used as a reducing agent to achieve the harmless treatment of chromium ore processing residue (COPR). This paper proposes a detoxification process of two-stage sulfuric acid leaching accompanied by biomass reduction of hexavalent chromium and a one-step combined leaching and reduction process. The orthogonal tests and single factor tests allowed to propose the most suitable operating conditions of different stages such as biomass hydrolysis, two-stage acid leaching, reduction of the acid leachate and combined leaching and reduction. The Cr(VI) content in both the leach residue and reduction solution reached the discharge criterion. The FTIR and XPS measurements showed that the reducing sugars produced by the hydrolysis of biomass reacted with the leached Cr(VI) to generate chromium sulfate adsorbed on the surface of COPR. In addition, XRD, SEM-EDS showed that the gypsum was generated after COPR detoxification, which can be used for resource utilization, providing a new idea and reference for the harmless treatment of COPR producing Cr(III) and gypsum.

Collaborative resource utilization of hazardous chromium ore processing residue (COPR) and C-bearing dust during limonitic laterite sintering process

Much chromite ore processing residue (COPR), which is defined as hazardous solid waste due to virulent Cr (Ⅵ), is produced from chromium salt industry. C-bearing dust from steel industry is a kind of typically solid waste with poor surface activity. The iron ore sintering process has the potential on large-scale harmless utilization of COPR and C-bearing dust. This study proposes a new approach of co-utilizing COPR and C-bearing dust during limonitic laterite sintering process. The negative influence of COPR and C-bearing dust on permeability was offset by the fuel attribute of extra C from C-bearing dust. The results indicate that, while 6% COPR and 8% C-bearing dust were added, the tumble index and yield of the sinters were basically equal to those of the blank samples. The toxic Cr (Ⅵ) was reduced to Cr (Ⅲ) which was concentrated in high alumina spinel phases during the sintering process. The obtained sinters were identified as nontoxic, and no secondary waste was generated throughout the clean utilization process.

A detoxification and value-added process for chromium ore processing residue (COPR) and Fe-C-bearing dust: Direct reduction-magnetic separation

Chromium ore processing residue (COPR), containing much virulent hexavalent chromium [Cr(VI)], is a by-product derived from chromate production. Fe-C-bearing dust from steel industry is very difficult to be utilized effectively because of its complex composition and unstable properties. The dust was co-processed with COPR by a direct reduction-magnetic separation process in this study. Through the strong reducing ability of solid carbon in Fe-C-bearing dust, the compounds of Cr and Fe were reduced to (Cr, Fe)7C3 alloy during the reduction roasting process and then selectively recovered by magnetic separation. In addition, the Pb and Zn compounds were reduced and volatilized into flue gas at high temperature. Under the optimal conditions, the results indicated that Cr-containing iron powder with 72.59 % Fe and 6.34 % Cr was obtained, and the recoveries of Fe and Cr were respectively 79.1 % and 76.9 %. The volatilization rates of Pb and Zn met 97.47 % and 99.95 %, respectively. The non-magnetic tailings were determined to be nontoxic and could be used as high-quality raw materials for building.

Treatment of soda ash chromite ore processing residue by Waste-Molasses-Based Ball Milling: A new strategy for disposal of waste with waste

The current technology of soda ash Chromium Ore Processing Residue (COPR) detoxification consumes a large amount of acids (as in the wet reduction process) and requires high temperature, along with the generation of carbon dioxide (as in the dry reduction process), and thus, is not cost-effective and environmentally-friendly. Waste molasses (WM) is waste biomass consisting of various reducing sugars, a cost-effective reducing agent. According to the idea of treating waste with waste, this study put forward a ball milling + WM method to detoxify leached-soda ash COPR (e.g., dried COPR containing 1622 ± 28 Cr(VI)/kg). Through response surface experiments, it was determined that under the best conditions (i.e., the milling speed = 600 rpm, the milling time = 2 h, WM dosage = 7.5% of the dry mass of COPR, and the ball-to-powder weight ratio, BPR = 12), the reduction efficiency of Cr(VI) reached 98.35%. The Toxicity Characteristic Leaching Procedure (TCLP) total Cr concentration reached 1.82 mg/L, meeting the standard (total Cr < 5 mg/L) of the US Environmental Protection Agency (USEPA). The associated reaction mechanism was determined as: under the action of mechanical energy, the structure of Cr(VI)-embedded matrixes was destroyed, and the internal, otherwise non-exchangeable, Cr(VI) was exposed and underwent redox reactions with WM under alkaline conditions. In brief, WM-based ball milling to detoxify COPR is cost-effective and provides a new reference for the disposal of waste with waste.

Sufficient extraction of Cr from chromium ore processing residue (COPR) by selective Mg removal

Chromium ore processing residue (COPR) is a hazardous waste generated during the production of chromate. Currently, approximately 10% of Cr2O3 cannot be extracted after chromite sodium roasting and remains in COPR, wasting valuable Cr resources. In this study, Mg was selectively removed by using (NH4)2SO4 roasting in combination with H2SO4 leaching. The results showed that the selective removal of 79.55% Mg from COPR could be achieved under the optimum (NH4)2SO4 roasting conditions (80 mmol (NH4)2SO4, 800 °C, 2 h). During the subsequent sodium roasting and acid leaching stages, the Cr extraction rate was 84.63% for the COPR direct roasting and 95.39% for the Mg removal residue roasting. The increased Cr extraction efficiency is attributed to the transformation of Mg-rich spinel and diopside (the Mg & Cr coexisting phases) in COPR converted into easily extractable (Fe,Cr)2O3 and Cr2O3 after the Mg treatment. This study investigated that the phase transformation of the Cr host phases is crucial for the sufficient extraction of Cr and provides inspiration for the development of efficient and practical Cr extraction techniques. Moreover, the method can be extended to the effective extraction of Cr from other Cr-containing wastes.

Unsaturated characteristics and transport of hydrated chromium ore processing residue

A mining tail cover system for controlling toxic substances (e.g. chromium (VI) (Cr6+)) in chromium ore processing residue (COPR) requires unsaturated transport modelling with inputs of water retention curves (WRCs) to evaluate the system. This study experimentally characterised the unsaturated properties of unweathered grey-black (GB) and weathered hard-brown (HB) COPR before and after hydration associated with mineralogical changes and estimated chromium transport by employing numerical modelling with the properties. X-ray diffraction (XRD) tests were also performed before and after testing to quantify the mineralogical compounds of materials. The XRD results showed that the brownmillerite decreased with the degree of the hydration. The relationship between the fitting parameters of WRC models and variables, such as the degree of hydration and specific gravity, was investigated. The WRC of GB COPR shows a bimodal shape because of the hydration process during the test. The hydration process transformed the chemical and mineralogical composition of GB COPR, thus influencing its WRC characteristics. Numerical simulations parameterised by WRC properties were conducted to compare unsaturated transport between GB and HB COPR through a sandy cover. HB COPR had a longer transportation time than GB COPR, revealing the importance of considering water retention characteristics in remediation plans.

Robust route to efficient extraction of Cr from different Cr-containing hazardous wastes by capitalizing on a reduction–chlorination–volatility technique

We efficiently extract Cr from four different Cr-containing wastes through a phase transformation process caused by a reduction–chlorination–volatility route. In particular, almost 100% of Cr could be recovered from chromium ore processing residue.

Long-Term Leaching Behavior of Chromite Ore Processing Residue as Backfill Material and the Propagation of Chromium in the Surrounding Soil

Abstract Leaching characteristics of chromium ore processing residue (COPR) for the long term (simulating a 20-year leaching scenario) were investigated using laboratory flow-through column testing...

Ultrastrong Anion Affinity of Anionic Clay Induced by Its Inherent Nanoconfinement.

Layered double hydroxide (LDH), the only anionic clay in the environment, plays a key role in natural ion transportation. The ion retention effect of LDHs was traditionally attributed to ion exchange with low affinity. Here, we demonstrated an ultrastrong interaction between anions and LDHs induced by their inherent nanoconfinement using chromium ore processing residue (COPR) that contained several Cr(VI)-bonded LDHs as a probe. Hydrocalumite (Ca/Al-Cl LDH) was verified as the primary phase for Cr(VI) retention through two types of interactions such as ion exchange and Cr-Ca coordination. More significantly, the confined spacing between two layers of hydrocalumite provided spatial restriction and shielding effects to the intercalated Cr(VI), which enhanced Cr-Ca coordination by shortening the bonding distance and modulating the binding angle to achieve the lowest bonding energy. Such enhancement boosted Cr(VI) affinity up to 3.2 × 105 mL/g, which was 1-3 orders of magnitudes higher than ion exchange. The universality of this mechanism was verified using another Mg/Al-Cl LDH and various anions. This study broke the traditional awareness of low ion affinities of LDHs limited by single ion exchange and disclosed an essential mechanism for unexpected ion retention effects of anionic clays in nature.

Removal of hexavalent chromium by hyporheic zone sediments in an urbanized estuary.

More than 2 million tons of chromium ore processing residue (COPR) waste was disposed of in Hudson County of New Jersey, which was known as the center of the production of chromate in the 20th century. The Cr(VI) removal experiments were conducted with the hyporheic zone (HZ) sediments collected along the shore of an urbanized estuary located in and near Hudson County to investigate the natural remediation of Cr(VI). Fine-grained and organic-rich Passaic River sediments showed the highest removal capacity for Cr(VI), whereas the lowest removal of Cr(VI) occurred in coarse-grained and organic-poor sediments from Newark Bay. In general, Cr(VI) removal increased with higher amounts of sediment organic matter, sulfur, and silt and clay fractions, as well as lower pH conditions. The removal of hexavalent chromium in organic-rich sediments is attributed mainly to the reduction of Cr(VI) to Cr(III), resulting in less reversible immobilization of Cr(VI), while reversible adsorption could also remove Cr(VI). The results suggest that the organic-rich, fine-grained HZ sediments can act as a natural reactive barrier for the remediation of Cr(VI) transport from subsurface to surface water in the estuary. Further research is needed to understand the long-term mobility of Cr along the urban estuary.

Adsorption Characteristics and Transport Behavior of Cr(VI) in Shallow Aquifers Surrounding a Chromium Ore Processing Residue (COPR) Dumpsite

This study explored the stratigraphic distribution and soil/shallow aquifer characteristics surrounding a chromium ore processing residue (COPR) dumpsite at a former chemical factory in China. Total Cr levels in top soils (5–10 cm) nearby the COPR dumpsite were in the range of 8571.4–10711.4 mg/kg. Shallow aquifers (1–6 m) nearby the COPR dumpsite showed a maximum total Cr level of 9756.7 mg/kg. The concentrations of Cr(VI) in groundwater nearby the COPR dumpsite were 766.9–1347.5 mg/L. These results display that the top soils, shallow aquifers, and groundwater of the study site are severely polluted by Cr(VI). Then, three aquifers (silt, clay, and silty clay), respectively, collected from the depth of 1.4–2.4 m, 2.4–4.8 m, and 4.8–11.00 m were first used to evaluate the adsorption characteristics and transport behavior of Cr(VI) in shallow aquifers by both batch and column experiments. The adsorption of Cr(VI) on tested aquifers was well described by pseudo-second-order equation and Freundlich model. The adsorption capacities of Cr(VI) on three aquifers followed the order: clay &gt; silty clay &gt; silt. The kinetics proved that Cr(VI) is not easily adsorbed by the aquifer mediums but transports with groundwater. Thermodynamics indicated that Cr(VI) adsorption on tested aquifers was feasible, spontaneous, and endothermic. Cr(VI) adsorption on tested aquifers decreased with increasing pH. Furthermore, the transport of Cr(VI) in adsorption columns followed the sequence of clay &lt; silty clay &lt; silt. Desorption column experiments infer that the Cr(VI) adsorbed on aquifers will desorb and release into groundwater in the case of rainwater leaching. Therefore, a proper treatment of the COPR and a comprehensive management of soils are vital to prevent groundwater pollution.

Optimisation of metal extraction from chromium ore processing residue in New Jersey, USA

The feasibility of extracting metals from chromium (Cr) ore processing residue (COPR) was investigated. COPR samples collected from four sites in New Jersey, USA (sites A, B, C and D) were mixed with 15% carbon by weight with different percentages of sand to neutralise the basic oxide in COPR and heated under a reducing environment to extract metals. At 15% and higher sand additions, a pool of melted metal was formed underneath the slag for some test batches. In this research, knowing the chemical composition of COPR and the depth of melt, the chemical engineering tools of phase and viscosity diagrams were used to compute the type and amounts of additives needed to optimise and compute the temperature and duration of melt to achieve maximum metal separations. With optimisation, as much as 30% by weight of metal was extracted from the initial mass of COPR collected from sites C and D when those melting mixtures contained 20% or more sand by weight. On average, the metal phase contained ∼90% of combined amounts of iron (Fe), chromium and titanium (Ti).

Complete Reduction of Hazardous Cr(VI) in Chromium Ore Processing Residue Dump Site

Complete Reduction of Hazardous Cr(VI) in Chromium Ore Processing Residue Dump Site

Modifications to EPA Method 3060A to Improve Extraction of Cr(VI) from Chromium Ore Processing Residue-Contaminated Soils.

It has been shown that EPA Method 3060A does not adequately extract Cr(VI) from chromium ore processing residue (COPR). We modified various parameters of EPA 3060A toward understanding the transformation of COPR minerals in the alkaline extraction and improving extraction of Cr(VI) from NIST SRM 2701, a standard COPR-contaminated soil. Aluminum and Si were the major elements dissolved from NIST 2701, and their concentrations in solution were correlated with Cr(VI). The extraction fluid leached additional Al and Si from the method-prescribed borosilicate glass vessels which appeared to suppress the release of Cr(VI). Use of polytetrafluoroethylene vessels and intensive grinding of NIST 2701 increased the amount of Cr(VI) extracted. These modifications, combined with an increased extraction fluid to sample ratio of ≥900 mL g-1 and 48-h extraction time resulted in a maximum release of 1274 ± 7 mg kg-1 Cr(VI). This is greater than the NIST 2701 certified value of 551 ± 35 mg kg-1 but less than 3050 mg kg-1 Cr(VI) previously estimated by X-ray absorption near edge structure spectroscopy. Some of the increased Cr(VI) may have resulted from oxidation of Cr(III) released from brownmillerite which rapidly transformed during the extractions. Layered-double hydroxides remained stable during extractions and represent a potential residence for unextracted Cr(VI).

Solidification and Stabilization of Chromium Bearing Waste

Objectives: Heavy metal contaminated land is increasingly becoming an important environmental, health, economic, and planning issue in the World. The unplanned disposal of heavy metal containing wastes, has resulted in a multi fold increase in environmental issues. Around 2.2 Lakhs tons of Chromium Ore Processing Residue(COPR) is left untreated at Ranipet SIPCOT Industrial complex. Solidification technique is proposed for the safe disposal of COPR in this study. Methods/ Analysis: Solidification process curtails migration of contaminants from the Cr(VI) bearing waste and reduces the toxicity of Cr(VI). In the present study ceramic composite are used as binding material. Mixture of COPR and ceramic composite in different ratios are verified at 1000 & 1200 0 C. The resultant composite has more strength and are bound like solid matrix, thereby avoid leaching of metal ions trapped in the middle. Findings: It is observed that more than 98% fixation is possible at specific composition. Leaching ability at 1200 0 C is very low than it observed for 10000 C. Compressibility strength also depends upon the temperature. Composition of the composite influences compressibility test. Novelty /Improvement: The vitrified composite of COPR and ceramic at 1200o C has acceptable non leachable character with porcelain tiles of appearance. It may be used in construction activity. Keywords: Ceramic Solidification, Chromium(VI), Chromium Ore Processing Residue, Leaching ability

Transport of hexavalent chromium in the vadose zone by capillary and evaporative transport from chromium ore processing residue

Column tests were conducted to evaluate the transport of Cr from underlying chromium ore processing residue (COPR) through the vadose zone in capped and uncapped scenarios and to determine if an asphalt cover system is a viable containment strategy. Four large (140 mm diameter) columns and one small (50 mm diameter) column were filled with COPR underlying a silty sand. Wet–dry cycling (representing precipitation events) induced suction gradients in the uncapped columns. At termination, the uncapped columns contained 10–120 mg/kg Cr on the surface (background Cr level, 8.3 mg/kg), and the capped columns had Cr concentrations equal to background levels, indicating that the capped surface limited Cr migration to the surface. The smaller column was constructed to study the evaporative transport of Cr by simulating a heat source (i.e., sunshine) to increase suction. Soil specimens from the surface of the small-diameter, uncapped column were as high as 410 mg/kg Cr(VI) after 4 weeks. A series of numerical analyses was conducted to interpret and validate the experimental results and to quantify Cr(VI) transport through composite asphalt caps. Through column testing and numerical analysis, in a controlled condition, Cr was found to migrate from COPR in the vadose zone by evaporation and capillary rise, but a controlled, capped condition effectively limited Cr migration.

Role of an organic carbon-rich soil and Fe(III) reduction in reducing the toxicity and environmental mobility of chromium(VI) at a COPR disposal site

Cr(VI) is an important contaminant found at sites where chromium ore processing residue (COPR) is deposited. No low cost treatment exists for Cr(VI) leaching from such sites. This study investigated the mechanism of interaction of alkaline Cr(VI)-containing leachate with an Fe(II)-containing organic matter rich soil beneath the waste. The soil currently contains 0.8% Cr, shown to be present as Cr(III)(OH)3 in EXAFS analysis. Lab tests confirmed that the reaction of Cr(VI) in site leachate with Fe(II) present in the soil was stoichiometrically correct for a reductive mechanism of Cr accumulation. However, the amount of Fe(II) present in the soil was insufficient to maintain long term Cr(VI) reduction at historic infiltration rates. The soil contains a population of bacteria dominated by a Mangroviflexus-like species, that is closely related to known fermentative bacteria, and a community capable of sustaining Fe(III) reduction in alkaline culture. It is therefore likely that in situ fermentative metabolism supported by organic matter in the soil produces more labile organic substrates (lactate was detected) that support microbial Fe(III) reduction. It is therefore suggested that addition of solid phase organic matter to soils adjacent to COPR may reduce the long term spread of Cr(VI) in the environment.