Source: DeBecker et al. The mother liquor from the crude ferrous sulfate crystallization contains the bulk of the chromium metal that is then sent to the aging circuit.
Aging and crystallization are conducted at relatively low temperatures. The kinetics are rather sluggish, but the desired purple ammonium chromium alum eventually forms.
The crystal slurry is then filtered, washed, and pumped to the leach circuit. The washed chromium-alum crystals are dissolved in hot water and filtered to produce cell feed. The key to successful, efficient electrodeposition of metallic chromium is ensuring that the proper chemistry is maintained at the cathode. This requires using a diaphragm cell, which separates the anolyte and catholyte, to minimize the migration of chromium VI to the anode and assist in controlling the pH.
If the Ph is too low, hydrogen evolution causes low current efficiency. Thus, control of electrolyte composition and Ph at about 2. The current efficiencies obtained are usually about 45 percent. Figure shows the desired reactions during electrolysis; these reactions are listed below.
The approximate electrolyte composition in both compartments of the diaphragm cell and typical electrode reactions are shown in Figure Other operating data are listed in Table After 72 hours of plating, the brittle chromium metal is removed in pieces from the cathode blanks by hammering.
The metal at this point contains a number of impurities and must be refined by vacuum reduction before final use. A more complete flow sheet for producing electrolytic chromium metal by the chrome-alum process is given in Figure Referred to as the chromium VI or hexavalent process, the chromic-acid electrolyte method is used primarily for surface finishing and electroplating applications. Chromium trioxide, CrO 3 , is obtained from sodium dichromate.
Small, catalyzing additions of halogens or sulfate are essential for electrolysis. The current efficiency of the process is very low e. The excessive hydrogen evolution may result in the precipitation of chromium hydroxides. Passivation of the reactions may then occur, making efficient processing even more difficult.
Some of the cell conditions for chromium electrowinning from chromic acid are given in Table The low overall efficiency obtainable with hexavalent chromium makes it nonviable as an alternative method for the production of bulk, high-purity chromium metal for alloys.
Thus, this method has not been used when large tonnages are required. In this study, the influence of Cr III from tanning, deacidification pH, fatliquors, chrome retanning and vegetable retanning on the formation of Cr VI in leather was analyzed by comparing natural and aged samples. Considering the presence of Cr VI , the supply of chromium during the retanning step had a more significant effect than during the tanning. In the fatliquoring process with sulfites, fish and synthetic fatliquor leather samples contained Cr VI when aged, and the highest concentration detected was The evaluation of Cr VI formation led to recommendations for regulation in the leather industry.
The influence of Chromium supplied by tanning and wet finishing processes on the formation of cr vi in leather. Gutterres; N. The presence of hexavalent chromium in leather and leather products, which is not used in any step of the manufacturing process, has become a concern in the leather industry. This problem can be a strong barrier to the use of the chromium salt as a tanning agent and even cause restrictions in the trade market of leather articles because Cr VI is bioaccumulating, highly toxic, mutagenic and carcinogenic to humans Kolomaznik et al.
According to Basaran et al. In experiments performed by Fuck et al. However, for the majority of the leather samples, Cr VI was not detected, even under more stringent conditions, indicating that tannery practices are attaining the tanning requirements. According to Palop et al. The greatest effect is realized by fatliquoring with sulfated and sulfited fish oils and substances with simple or multiple unsaturated fatty acids free or esterified. On the other hand, vegetable tanning or retanning agents, such as mimosa, quebracho, chestnut and tara, appeared to prevent chromate formation, even when the leathers were exposed to extreme conditions, such as heat and UV radiation Hauber, When the use of vegetable tannin is not possible due to its color interference, a mixture of phenol and antioxidants can be applied.
Gong et al. Therefore, the aim of this work was to study the conditions of Cr VI formation due to the oxidation of Cr III to Cr VI in leather in order to provide recommendations to avoid this problem in the leather industry. The factors studied in these experiments were the amount of chromium supplied in the tanning step and the following influences on the steps of wet finishing: the type of oil used in the fatliquoring step, the pH of deacidification, the effects of chrome retanning and vegetable retanning.
Leather samples were tested both in the natural stage and after the aging treatment. After the tanning step, the finishing of the leather is carried out to improve the characteristics of the leather and ensure the specifications of the articles, such as color, resistance physicomechanical and physicochemical and softness. The leather finishing system includes the wet-finishing, pre-finishing and finishing steps. The wet-finishing step includes deacidification in which the leather pH is adjusted to promote the diffusion of the subsequent agents into the leather , retanning, dying and fatliquoring.
The usual chemicals used in the retanning step are chromium salts, vegetable tannins and synthetic tannins. Commercial fatliquor preparations are mixtures of oil emulsions known as lickers. The most important group of fatliquor agents includes anionic products, such as sulfated, sulfited and sulfoclorated oils Gutterres, In the experiments, tanned hide samples were submitted to wet-finishing and aging of the obtained leather samples was conducted to simulate the possible chromium oxidation over time by exposure to high temperatures.
This work began with tanning in the laboratory of two whole bovine hides acquired from a tannery in the pickled stage acidified to pH 2. The variation of the chromium was tested to examine whether the presence of a higher chromium content would lead to higher hexavalent chromium formation in the leather due to an eventual nonfixation of the excess chromium in the collagen structure. On storage in 0. The product from reduction in perchloric acid was relatively resistant to depolymerization.
When these results are compared to those observed with the other acids Figures , some significant differences are noted. As chloride, fluoride and sulfate form very much stronger complexes with Cr III than do trifluoromethanesulfonate, perchlorate and nitrate, this is not an unexpected result. The results reported here on the reduction of Cr VI in solutions of several oxidizing acids, not involving a conventional reducing species such as chloride, suggest that "acid", i.
Archundia et al. Thus, the role of the different anions present, which depend on the acid used, should contribute importantly, as they form complexes pseudo-esters whose stability may influence the overall kinetics of the acid-reduction process and the resulting product distributions.
The results indicate that low concentrations of Cr VI are reduced by concentrated acids forming hexaaquochromium III and other complexes of Cr III , which can be separated by chromatographic methods. The appearance of Cr III species bonded to one, two or even three anions suggests the direct participation of the anion in the electron transfer process.
Thus, acidic Cr VI standard solutions and procedures for the determination of chromium in metal alloys 49 and biological materials, 50 which are based on acid dissolution of the sample in HClO 4 or another strong acid, followed by Cr VI determination using spectrophotometric or titrimetric procedures, must be used with caution due to this complicating factor of acid-induced reduction. Abrir menu Brasil. Journal of the Brazilian Chemical Society.
Abrir menu. Introduction The acid reduction of Cr VI is a little known aspect of chromium chemistry even though the reduction of Cr VI by hydrochloric acid has been known since the discovery of the element by Vauquelin in Reaction of trace Cr VI with nitric acid solutions A set of acidic solutions of Cr VI with concentrations from 10 -7 to 10 -3 mol L -1 were prepared in nitric acid whose concentration varied from 2 to 10 -5 mol L Speciation by open column ion exchange chromatography Ion exchange columns were prepared by placing 0.
Results and Discussion The reaction of trace Cr VI with hydrochloric acid The cation exchange chromatogram of the products formed when a high specific activity 51 Cr VI solution was placed in concentrated hydrochloric acid, and stored for 60 min, is shown in Figure 1. Weeks, M. Wiberg, K. Cainelli, G. Fendorf, S. Archundia, C. Total Environ. Pezzin, S. Acta , 77 , Khan, Z. Chem , 23 , Pavel, J. Gil, M. He then discovered that the green colouration of emeralds was also due to chromium. Atomic data.
Glossary Common oxidation states The oxidation state of an atom is a measure of the degree of oxidation of an atom. Oxidation states and isotopes. Glossary Data for this section been provided by the British Geological Survey. Relative supply risk An integrated supply risk index from 1 very low risk to 10 very high risk. Recycling rate The percentage of a commodity which is recycled.
Substitutability The availability of suitable substitutes for a given commodity. Reserve distribution The percentage of the world reserves located in the country with the largest reserves. Political stability of top producer A percentile rank for the political stability of the top producing country, derived from World Bank governance indicators.
Political stability of top reserve holder A percentile rank for the political stability of the country with the largest reserves, derived from World Bank governance indicators. Supply risk. Relative supply risk 6. Young's modulus A measure of the stiffness of a substance.
Shear modulus A measure of how difficult it is to deform a material. Bulk modulus A measure of how difficult it is to compress a substance. Vapour pressure A measure of the propensity of a substance to evaporate. Pressure and temperature data — advanced. Listen to Chromium Podcast Transcript :. You're listening to Chemistry in its element brought to you by Chemistry World , the magazine of the Royal Society of Chemistry.
This week an element that adds sparkle and value to minerals, through the colourful characteristics of its compounds. In the Western world, the colourful history of chromium begins, suitably enough, at the far end of the visible spectrum with a red-orange mineral that was named "Siberian red lead" by its discoverer, the 18th-century geologist Johann Lehmann. It was in the middle of this surge of discovery, over 35 years after Siberian red lead was first found that the French chemist Louis Vauquelin showed that this mineral, now known as crocoite, contained a previously unknown chemical element.
It took Vauquelin several steps to isolate chromium. First he mixed the crocoite solution with potassium carbonate to precipitate out the lead. Then he decomposed the lemon yellow chromate intermediate in acid, and finally removed the compounded oxygen by heating with carbon - leaving behind elemental chromium. The name for this new element was debated among his friends, who suggested "chrome" from the Greek word for colour because of the colouration of its compounds. Although he objected to this name at first because the metal itself had no characteristic colour, his friends' views won out.
When Vauquelin exhibited his pale grey metal to the French Academy of Sciences, he commented on the metal's brittleness, resistance to acids and incapability of being melted. He thought these properties made it overly difficult to work with and thus limited its applications as a metal. He did suggest, however, that chromium's compounds would be widely used as beautiful, brilliantly coloured pigments. A browse through images of chromium compounds on Wikipedia shows a whole spectrum of colours: dark red chromium VI oxide, orange-red lead chromate, bright yellow sodium chromate, brilliant chrome green that's chrome III oxide , light blue chromium II chloride, and violet anhydrous chromium III chloride.
The last of these compounds shows an amazing property when hydrated. Its colour changes between pale green, dark green and violet depending on how many of the chromium ion's six coordination sites are occupied by chloride rather than water. Of all these pigments, one of them stands out. I'm a chemist who was born, raised and schooled in the Midwestern United States, so the iconic yellow school buses in North America were familiar sights. Chrome yellow, also known as "school bus yellow", was adopted in for all U.
However, the presence of both toxic lead and hexavalent chromium of Erin Brockovitch fame has led to it being largely replaced by a family of azo dyes, known as Pigment Yellows, though chrome yellow is still used in some marine and industrial applications.
Of all chromium's natural occurrences, my favourites are gemstones, where a trace of the element adds a blaze of colour. As corundum, beryl, and crysoberyl, these metal oxides are colourless and obscure minerals. But add a dash of chromium, and they become ruby, emerald and alexandrite. The chemist's tool of crystal-field theory, which models the electronic structure of transition metal complexes, provides a surprisingly accurate way of describing and predicting the source and variability of colour in chromium's compounds.
In ruby - which is aluminium oxide with a few parts per thousand of the aluminium ions are replaced by chromium III ions - the chromium atoms are surrounded by six oxygen atoms.
This means that the chromium atoms strongly absorb light in the violet and yellow-green regions. We see this as mainly red with some blue, giving, in the best cases, the characteristic pigeon-blood colour of the finest rubies. So, when more chromium is added to aluminium oxide, the octahedral environment around the chromium becomes distorted and the two bands of absorption shift towards the red. My next gem, the emerald, in an oxide of silicon, aluminium and beryllium.
It has the same substitution of a chromium ion for an aluminium ion and a similar distorted octahedral arrangement of oxygen around chromium, giving emeralds their characteristic green colour, like that from green sapphires. Of the chromium gemstones, alexandrite is the most fascinating to me. Its stones are strongly pleochroic. That is, they absorb different wavelengths depending on the direction and polarisation of the light that's hitting them.
So, depending on a gem's orientation, alexandrite's colour ranges from red-orange to yellow and emerald green. Its colour also changes depending on whether it is viewed in daylight or under the warm red tones of candlelight.
When moved from daylight to candlelight, the best specimens turn from a brilliant green to a fiery red. Lesser gems turn from dull green to a turbid blood red. Outside this rainbow of chromium compounds, chromium helps prevent a particularly undesirable colour: rust brown. The alloyed chromium reacts with oxygen to form a transparent nanoscopic layer of oxide that forms a barrier to further oxygen penetration and so prevents the ruddy, flaky products of iron oxidation.
Given these widespread uses of chromium complexes, it should come as no surprise when I tell you that under one-half of a per cent of chromium produced is chromium in its elemental form. So, to some extent, Vauquelin's prediction from two centuries ago about the limited usefulness of elemental chromium was spot on.
On the other hand, the first picture in my mind for chromium after gemstones, of course is when it is in its metallic form, such as for the mirrored corrosion and wear-resistant "chrome" surfaces of ball bearings and the shiny silvery trim on car parts. So it's shiny and colourful as well as corrosion and wear resistant.
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