Formation Of Hexavalent Chromium Research Articles

Reduction of Cr (VI) Formation in Leather with Herbal Extracts

This study investigates the effects of extracts from natural products such as oak bark, and onion peel, on the formation of Hexavalent Chromium (chromium (VI)) or (Cr (VI)) in leather during the finishing process. To enable chromium (VI) formation, finished leather samples were aged under various conditions, including exposure to UV light. The amount of chromium (VI) in each leather sample was measured using a PC UV-Visible spectrophotometer at 540 nm based on the ISO/FDIS 17075 standard method. The results showed that extracts from Quercus cortex, and Allium cepa significantly reduced chromium (VI) formation in the leather under all aging conditions.

Evaluation of the dust and gas cleaning system during meltin of chrome containing refractories in electric arc furnace

The problems of the presence and formation of hexavalent chromium during smelting in electric arc furnaces are discussed. The schemes of gas cleaning systems from chromium-containing gases of electric arc furnaces DS-0,5 and DSP-1,5, The gas cleaning system of furnace DS-0,5 includes a cyclone, two Venturi scrubbers and an electrostatic precipitator. The gas cleaning system of DSP-1,5 furnace consists of a cyclone, regulated and diffuzor scrubbers and two electrostatic precipitators BVKO. It is shown that industrial production of fused chrome-containing refractories can be ensured under safe conditions by using the mentioned dust and gas cleaning systems. Ill. 4. Ref. 4. Tab. 1.

Evidence of hexavalent chromium formation and changes of Cr speciation after laboratory-simulated fires of composted tannery sludges long-term amended agricultural soils

Controlled or accidental fires can impact agricultural soils amended with composted organic materials since high temperatures cause fast organic matter (OM) mineralization and soil properties modifications. During these events, potentially toxic elements (PTEs) associated with OM can be released and change their distribution and speciation thus becoming a threat to the environment and to crops.In this study, we investigated the changes of distribution and speciation of chromium in soils long-term amended with compost obtained from tannery sludges, after simulating fires of different intensity (300, 400 and 500 °C) likely to occur on agricultural soils. A combination of conventional soil chemical analyses and bulk and (sub)micro X-ray analyses allowed the observation of the formation of hexavalent chromium and changes of chromium speciation. Specifically, a strong decrease of Cr-OM associations was found with increasing temperature in favour of Cr-iron (hydr)oxides interactions and CaCrO4 formation. These data provide first evidence that fires can transform OM-stabilized Cr into more mobile, available and toxic Cr-forms potentially accessible for plant uptake, thus posing a risk for the food chain and the environment.

Mitigating the Formation of Hexavalent Chromium in Leather by Using Aloe Barbadensis Miller Mixed with Carrageenan

Aloe barbadensis miller (Abm) mixed with carrageenan has been investigated as an alternative eco benign ingredient in mitigating hexavalent chromium Cr (VI) formation in thermally and photoaged wet blue and leather crust. The effect of post-tanning operations on the formation of Cr (VI) in wet blue and leather crusts due to spontaneous and accelerated ageing caused by exposure to the temperature of 80 °C and UV radiations for 132 hours is also presented. The Cr (VI) content was analysed according to ISO 17075 standard procedure of Diphenyl carbazide and UV-VIS spectrophotometer at 540 nm. The levels of Cr (VI) in retanned wet blue leather were detectably high, while for tanned, dyed and fatliquored crusts, the levels were below detection limit of 0.2247 mg/kg. After ageing, the Cr (VI) content increased to a detectable level, the highest recorded in retanned wet blue, followed by fatliquored crust and with the lowest levels recorded in dyed wet blue leather. In all the aged samples, the levels were remarkably higher than the recommended 3 mg/kg. The levels of Cr (VI) in wet blue leathers processed with Abm/carrageenan were below detection limit of 3.587 mg/kg, even after exposing the samples to accelerated ageing conditions. Abm/carrageenan completely inhibits formation of Cr (VI) in wet blue and leather crusts. Abm/carrageenan will contribute to the eco benign and sustainable production of leather under the superior chrome-tanning technology.

Fused-cast chrome-containing refractories and prospects for their industrial production

Information is given on various types of fused-cast chrome-containing refractories ― chromalumozircon and chromium spinelids. It is shown that the chromium spinelids refractories HPL-85 and HMG are at the same level with the ceramic refractories made of chromium oxide in terms of corrosion resistance in melts of alkali- free borosilicate glass E, mineral  wool,  basalt  fiber, etc. The problems of the presence and formation of hexavalent chromium in the production of chrome- containing materials are discussed. It is shown that the production of fused-cast chrome-containing refractories is possible under safe production conditions. The necessity of creating an industrial production of fused chromium-containing refractories based on domestic developments, taking into account the best foreign achievements, is noted.

New weldable 316L stainless flux-cored wires with reduced Cr(VI) fume emissions: part 2—round robin creating fume emission data sheets

Welding fumes have been found to be carcinogenic and stainless steel welders may be at higher risk due to increased formation of hexavalent chromium (Cr(VI)). The slag-shielded methods, identified to generate most airborne particles and Cr(VI), would potentially be most harmful. With ever-stricter limits set to protect workers, measures to minimize human exposure become crucial. Austenitic stainless steel flux-cored wires of 316L type have been developed with the aim to reduce the toxicity of the welding fume without compromised usability. Collected particles were compared with fumes formed using solid, metal-cored, and standard flux-cored wires. In part 1, the new wires were concluded to have improved weldability, to generate even less Cr(VI) in wt.-% than with solid wire and to be less acute toxic in cultured human bronchial epithelial cells as compared to standard flux-cored wires. In part 2, two additional institutes created fume emission datasheets for the same wires for correlation with the fume data obtained in part 1. The reported values showed large variations between the three laboratories, having a significant effect on the standard deviation. This is suggested to be the result of different welding parameters and various ways to collect and analyze the fume. More stringent specifications on parameter settings and fume collection would be required to increase the accuracy. This means that at present, it may not be possible to compare fume data on datasheets from two different wire producers and care should be taken in interpretation of values given in the available literature. Nevertheless, the laboratories confirmed the same trends for Cr(VI) as presented in part 1.

Enhancing mechanism of improved slag resistance of Al2O3-spinel castables added with pre-synthesized (Al,Cr)2O3 micro-powder

In order to enhance the slag resistance of Al2O3-spinel castables, (Al,Cr)2O3 is added into Al2O3-spinel system as a pre-synthesized micro-powder. Firstly, (Al,Cr)2O3 micro-powder is synthesized by sintering under reduction conditions to prevent the formation of hexavalent chromium. The Al2O3-spinel castables are prepared using tabular alumina, fused spinel, α-Al2O3 micro-powder, calcium aluminate cement and (Al,Cr)2O3 micro-powder as the raw materials. The bulk density, porosity, mechanical properties, and slag resistance of the samples are tested. Afterward, the effects of (Al,Cr)2O3 micro-powder (0–3 wt%) on the slag resistance and microstructures of the Al2O3-spinel castables are assessed by X-ray diffraction (XRD) and energy-dispersive (SEM-EDS) analysis. The results show that the addition of (Al,Cr)2O3 micro-powder can could inhibit the deteriorating effects of Cr3+ on the mechanical properties of the samples. The microstructure results also shows that with the addition of the (Al,Cr)2O3 micro-powder, a secondary solid solution of Ca(Al,Cr)12O19 formed, causing the unit cell to become larger. In the slag erosion area, CA6 crystals formed with network-like interwoven structures, high density, and greater thickness. These characteristics significantly reduce the erosion and permeability indices of the castables, and improve the slag erosion resistance of the material.

Prevention of Hexavalent Chromium Formation by Iron-ion-added Electrolyte in Electrochemical Machining of Chromium-containing Steel (2nd report)

Electrochemical machining (ECM) of Cr-containing materials such as stainless steel produces hexavalent chromium, which is often considered a problem in ECM. In this study, using the relationship of the standard electrode potential, we investigated whether the formation of hexavalent chromium itself could be suppressed during ECM and developed the method of preventing the formation of hexavalent chromium using divalent iron ions in the electrolyte. If the electrolyte contains much easily oxidized ion, the oxidation of trivalent chromium is prevented by the oxidation of the ion. To supply divalent iron ions to the electrolyte, an iron-ion-supplying device composed of iron chips was fabricated. In our first report, it was shown that the formation of hexavalent chromium can be prevented by this method, but it was also found that it is difficult to maintain the prevention effect. In this report, a method to continuously prevent the formation of hexavalent chromium is described. It was found that the required amount of Fe2+ ions could be supplied to the electrolyte by temporarily acidifying the electrolyte. And it was shown to be possible to carry out ECM with a neutral electrolyte while preventing a decrease in Fe2+ ion concentration by neutralizing the acidic electrolyte before supplying it to the machining tank.

Eliminating the Formation of Hexavalent Chromium in Chrome Stripping Operations Using Pulse Reverse Processes

Chromium electrodeposits are commonly applied to aeronautical and automotive components as protective coatings from harsh and/or corrosive operational environments. Through normal use, these coatings may become damaged. During repair and refurbishment of these components, the parts are stripped of the existing chromium deposit before being recoated and reinstalled into various systems. Traditionally, the chrome stripping process has been facilitated by applying a potential bias to a workpiece in an alkaline electrolyte per MIL-STD-871B. While the alkaline pH may help to protect the underlying steel surface from corrosion, it preferentially favors the formation of the hexavalent chromium species, which is a carcinogen and an environmental toxin. In the conventional stripping process, hexavalent chromium ions build up in the stripping electrolyte over time, resulting in decreased stripping rates, high energy consumption and possible damage to the part. To minimize these effects, the bath must be purified or discarded and replaced. Prior to waste disposal all hexavalent chromium in the electrolytes must be reduced to trivalent form as the trivalent species is non-toxic, non-hazardous and more stable compared to the hexavalent species. Therefore, if the oxidation of chromium metal can be limited to the trivalent species during stripping operations, air quality regulations will be easier to meet and working conditions personnel would be greatly improved. Faraday is working to develop an electrolytic stripping process intended as a drop-in replacement for the conventional stripping process. The process seeks to eliminate the formation of hexavalent chromium while maintaining the material properties of the underlying substrate. Faraday will present work that demonstrates removal of chrome from high strength steel substrates using a weak acid electrolyte that results in the dissolution of the metal to its trivalent state. The effect of pulse and pulse/reverse electric fields on the efficacy of the stripping process will be presented in comparison to operation under constant voltage electric fields. The stripping process based on use of a weak acid electrolyte in conjunction with pulse/pulse reverse electric fields was demonstrated for flat and round specimens in a pilot scale cell using a simple oxalic acid electrolyte. In addition, studies were conducted to assess how age of the oxalic acid electrolyte impacts the stripping process using constant voltage electric fields. Faraday also developed a process for the recycling of spent alkaline stripping electrolytes loaded with hexavalent chromium ions. This process relies on the use of catalytic Au-Pd-Cu cathodes and pulse reverse process conditions. Results of the process in bench scale validation studies and lessons in the scaling of this technology are discussed. Acknowledgement: Funding for this work is gratefully acknowledged from Air Force SBIR Contract Number FA8222-16-C-0006.

Corrosion mechanisms of magnesia-chrome refractories in copper slag and concurrent formation of hexavalent chromium

Magnesia-chrome refractories have been essential materials for application as wear lining in copper production furnaces for many years. Formation of hexavalent chromium (Cr(VI)) has been a major concern despite its admirable corrosion resistance in copper slag. Magnesia-chrome refractory specimens (9, 12, 15 and 18 wt% Cr2O3) with addition of Al2O3 and TiO2 were fabricated and subsequent corrosion tests using copper slag was conducted in air. The corrosion mechanism between magnesia-chrome refractories fabricated and synthetic copper slag were investigated by means of X-ray diffraction (XRD) and scanning electron microscope (SEM), and concurrent formations of Cr(VI) were assessed effectively by leaching test. All the magnesia-chrome specimens prepared are composed of periclase and composite spinel Mg(Al,Cr,Ti)2O4 phases after firing at 1700 °C. Densification of the specimens decreases with the increase in Cr2O3 content due to the increased volume expansion, while TiO2 addition can improve their densification as mass transfer is enhanced during the sintering process. The corrosion resistance of the specimens decreases slightly with the Cr2O3 content due to the increased apparent porosity, which would result in penetration of more corrosive slag at the initial stage of the corrosion test while a newly formed (Mg,Cu)(Cr,Fe)2O4 spinel dense layer between the slag and penetration layer effectively hinders further penetration of slag and well protects the specimens. The Cr(VI) in the specimens prepared increases obviously with the Cr2O3 added content, while the TiO2 addition can suppress the formation of Cr(VI) effectively and the copper slag can cause the formed Cr(VI) to be reduced further during the corrosion process. For all the specimens, concentrations of Cr(VI) in the leachates is much lower than the US-EPA limit of 100 mg/kg but exceeds the European limit of 2 mg/kg after the corrosion test.

Hexavalent chromium release from leather over time natural ageing vs accelerated ageing according to a multivariate approach.

European restriction limits the hexavalent chromium content to not more than 3 mg/kg in leather products. Owing to the evolution of hexavalent chromium content in leathers over time, it's difficult to guarantee the products will be harmless for consumers. The designing of an accelerated and representative artificial ageing procedure is therefore highly desirable. This article proposes a thorough study of the influence of storage conditions on the formation of hexavalent chromium in 4 bovine leathers. A factorial design of experiment was built with the following factors: temperature of 40-80 °C, relative humidity of 20-50%, presence or not of UV and exposure duration of 24-48 h. The results of these artificial ageings demonstrate that a high temperature, a dry atmosphere and the presence of light favor the formation of hexavalent chromium and that synergistic effects operate between temperature/humidity, temperature/UV and humidity/UV. At the same time, the leathers were subjected to a natural ageing for 12 months with a weekly hexavalent chromium analysis. The principal component analysis of the artificial ageing tests combined with the natural ageing tests, show that the artificial ageings 40 °C-50%RH-UV-48 h and 40 °C-20%RH-UV-24 h best simulate a natural ageing in the tannery, whatever the leather studied.

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.

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...

Effect of Cr2O3 addition on corrosion mechanism of refractory castables for waste melting furnaces and concurrent formation of hexavalent chromium

Cr2O3 containing refractories are used as the linings of various waste melting furnaces. Formation of hexavalent chromium (Cr(VI)) has been a major concern despite its admirable corrosion resistance properties. Al2O3-CaO-Cr2O3 refractory castables (0, 4%, 8%, and 12wt% Cr2O3) were fabricated in reducing condition to inhibit Cr(VI) formation while subsequent corrosion tests using waste slag was performed in air atmosphere to simulate the operating conditions. The corrosion mechanism between Cr2O3-containing corundum castables and synthetic waste slag were investigated by XRD and SEM-EDS, whereas concurrent formations of Cr(VI) were assessed effectively by leaching tests. Corrosion of castables (0wt% Cr2O3) occurs via dissolution of Al2O3 and CA6 phases, and formation of Ca2(Al,Mg)(Al,Si)2O7 and (Ca,Na)(Al,Si)2Si2O8, which were significantly restricted by the formation of Al2O3-Cr2O3 (III) solid solution (4%, 8%, and 12wt% Cr2O3). Most of the specimens exhibited less than 5mg/L (EPA limit) of Cr(VI). Only specimen M3 (12wt% Cr2O3) from slag-refractory interface (stage C) leached out 5.41mg/L of Cr(VI) which is above the EPA limit. Specimen M2 (8wt% Cr2O3) exhibits optimum properties in terms of corrosion and Cr(VI) formation.

Pulse Reverse Electrolytic Stripping of Chrome Plate

A strong desire exists from an environmental, health and safety perspective to eliminate hexavalent chromium during electroplating of protective hard chrome coatings used to enhance component wear resistance. However, toxic hexavalent chromium waste streams are also generated during repair and refurbishment of chrome components as the parts are electrolytically stripped of the existing chrome before being recoated and reinstalled into various systems. Furthermore, the chromate formed during stripping accumulates in the electrolyte, interfering with the process efficiency and damaging the base metal. Faraday is working to develop an electrolytic stripping process intended as a drop-in replacement for the conventional stripping process. The process seeks to eliminate the formation of hexavalent chromium while maintaining the material properties of the underlying substrate. Faraday will present work that demonstrates removal of chrome from high strength steel substrates using a weak acid electrolyte that doesn’t produce hexavalent chromium or adversely affect the substrate properties. Furthermore, the effect of pulse and pulse/reverse electric fields on the efficacy of the stripping process will be presented.

Formation of Hexavalent Chromium through an Ash Drying Process

Formation of Hexavalent Chromium through an Ash Drying Process

Mechanism of chromium oxidation by alkali and alkaline earth metals during municipal solid waste incineration

The influence of alkali and alkaline earth metals on the formation of hexavalent chromium (Cr(VI)) during municipal solid waste (MSW) incineration has drawn much attention due to the high toxicity of Cr(VI). In the present work, the distribution of Cr(VI) in MSW incineration ash residues sampled from various plants was measured. The results show that the oxidation of chromium in combustion process is determined by the content of free CaO (f-CaO) in the MSW and is facilitated by alkali metal compounds at lower temperatures in the flue gas cooling process. In some cases, the content of f-CaO is low during MSW incineration, and the oxidation of Cr2O3 in the case of low mole ratio of Ca and Cr was investigated. The mixture of CaO and Cr2O3 was heated at temperatures ranging from 973 to 1373K and the oxidation of Cr2O3 was detected by determining the consumption of oxygen in the carrier gas using an online mass spectrometer. According to the results, more chromium is oxidized in the thermally treated mixture at 1173K than at higher temperatures. At 1173K, part of chromium is oxidized to form CaCrO4 and the unoxidized chromium remains in the form of Cr2O3. In contrast, Cr2O3 is prior to form Ca3(CrO4)2 at higher temperatures and the unoxidized chromium is mostly transformed into CaCr2O4. Compared with Cr2O3, CaCr2O4 is more easily oxidized by CaO or K2CO3, indicating that more Cr(VI) would be formed during MSW incineration when trivalent chromium (Cr(III)) is transformed into CaCr2O4. Measures should be taken to suppress the formation of CaCr2O4 such as the control of the operating temperature and CaO addition.

The hazardous hexavalent chromium formed on trivalent chromium conversion coating: The origin, influence factors and control measures

In this paper, the effects of processing parameters and constituents of treating-agent on the presence of hazardous hexavalent chromium on trivalent chromium conversion coating were studied. Results showed that shorter immersion time, lower bath pH value as well as lower working and baking temperatures retarded the presence of hexavalent chromium. In addition, the concentration of hexavalent chromium on conversion coatings prepared by the oxalic acid treating-agent was far greater than those on conversion coatings prepared by formic acid and acetic acid treating-agents. Results also indicated that the concentration of hexavalent chromium on conversion coatings was enhanced due to the addition of bivalent cobalt and nitrate anion in treating-agent, especially for oxalic acid conversion coating. However, the addition of hydroxyl compound d-gluconic acid in treating-agent could reduce the concentration of hexavalent chromium effectively. Moreover, a possible formation mechanism of hexavalent chromium on trivalent conversion coating was proposed. Findings of this study provide a better understanding of the formation of hexavalent chromium on trivalent chromium conversion coating and can facilitate the management of trivalent chromium treating-agents and trivalent chromium fasteners.

Control of Cr<sup>6+</sup> Emissions from Gas Metal Arc Welding Using a Silica Precursor as a Shielding Gas Additive

Hexavalent chromium (Cr(6+)) emitted from welding poses serious health risks to workers exposed to welding fumes. In this study, tetramethylsilane (TMS) was added to shielding gas to control hazardous air pollutants produced during stainless steel welding. The silica precursor acted as an oxidation inhibitor when it decomposed in the high-temperature welding arc, limiting Cr(6+) formation. Additionally, a film of amorphous SiO(2) was deposited on fume particles to insulate them from oxidation. Experiments were conducted following the American Welding Society (AWS) method for fume generation and sampling in an AWS fume hood. The results showed that total shielding gas flow rate impacted the effectiveness of the TMS process. Increasing shielding gas flow rate led to increased reductions in Cr(6+) concentration when TMS was used. When 4.2% of a 30-lpm shielding gas flow was used as TMS carrier gas, Cr(6+) concentration in gas metal arc welding (GMAW) fumes was reduced to below the 2006 Occupational Safety and Health Administration standard (5 μg m(-3)) and the efficiency was >90%. The process also increased fume particle size from a mode size of 20 nm under baseline conditions to 180-300 nm when TMS was added in all shielding gas flow rates tested. SiO(2) particles formed in the process scavenged nanosized fume particles through intercoagulation. Transmission electron microscopy imagery provided visual evidence of an amorphous film of SiO(2) on some fume particles along with the presence of amorphous SiO(2) agglomerates. These results demonstrate the ability of vapor phase silica precursors to increase welding fume particle size and minimize chromium oxidation, thereby preventing the formation of hexavalent chromium.

Stabilization of chromium: An alternative to make safe leathers

In this study, the original causes for hexavalent chromium presence in the leather were first evaluated by ageing of chromium(III) solutions and chrome tanned hide powder (50 °C, UV lightening at 340 nm, 0–36 h). The results showed that the trivalent chromium at instable coordination state was easy to convert into hexavalent chromium in high pH environment, and the probability of the oxidation increased in this order: multi-coordinate chromium, mono-coordinate chromium, and free chromium. For this reason, the process for stabilizing chromium in the leather was designed with the specific material, which was mostly consisted of the reducers and the chelating agents. After treated with the developed process, these leathers were aged (50 °C, UV irradiance as 0.68 W/m 2 at 340 nm, 0–72 h) to estimate chromium(VI) presence. Hexavalent chromium was not found in these treated leathers even if the leathers were aged for 72 h. Moreover, the physical and mechanical properties for the leathers varied little after treating. In a word, an inherent safe and effective process was proved to avoid the formation of hexavalent chromium in the leather.