Removal chromium electrolytes

SchiffI H, Weidmann P, Weiss M, et al. Dialysis treatment of acute chromium intoxication and comparativeefficacy of peritoneal versus hemodialysis In chromium removal. Miner Electrolyte Metab. 1982 7 28-35... 

Stainless steel develops a passive protective layer (<5-nm thick) of chromium oxide [1118-57-3] which must be maintained or permitted to rebuild after it is removed by product flow or cleaning. The passive layer may be removed by electric current flow across the surface as a result of dissinulat metals being in contact. The creation of an electrolytic cell with subsequent current flow and corrosion has to be avoided in constmction. Corrosion may occur in welds, between dissimilar materials, at points under stress, and in places where the passive layer is removed it may be caused by food material, residues, cleaning solutions, and bmshes on material surfaces (see CORROSION AND CORROSION CONTROL). 

At the end of the 72-h cycle, the cathodes are removed from the cells, washed in hot water, and the brittie deposit, 3—6 mm thick, is stripped by a series of air hammers. The metal is then cmshed by roUs to 50-mm size and again washed in hot water. The metal contains about 0.034% hydrogen and, after drying, is dehydrogenated by heating to at least 400°C in stainless steel cans. Composition limits for electrolytic chromium are shown in Table 4. 

If the chromium (II) chloride is prepared by the Holah-Fackler procedure from electrolytic chromium and hydrochloric acid, a three-necked round-bottomed flask may be used as a reaction vessel, and ammonia may be bubbled directly into the chromium(II) chloride solution using a T-joint in the nitrogen line. Bubbling of the ammonia through the liquid avoids the necessity for stirring. When the above procedure was used, it was necessary to centrifuge the final filtered solution at 2700 r.p.m. for 15 minutes in order to remove all the platinum catalyst. 

The Lewis ENVIRO-CLEAN process removes and recovers metals such as chromium, copper, nickel, mercury, lead, zinc, iron, and cadmium and has effectively demonstrated that it can treat a matrix of multiple metals in a single stream with positive results. The process treats wastes from wood preserving, metal finishing, mining, surface and groundwaters. The two-step process uses granular-activated carbon and electrolytic metal recovery to yield a salable metallic by-product. 

Electrolytic methods have been applied to the treatment of other metal waste streams generated in the electroplating or metal finishing industries. Pollution engineering processes have been designed and implemented for the removal of hexavalent chromium, trivalent chromium, nickle, copper, zinc and cadmium.Besides the Edwards patent, there seems to be no documentation of electrolytic methods for removal and recovery of mercury metal from waste streams. 

PBE can be used for the destruction of organic compounds, regeneration ofhexavalent chromium, production of hydrogen peroxide, but their major application is in metal removal from dilute solutions. In this situation, the choice of small-sized particles is tempting for the increase of process efficiency nevertheless, a compromise must be considered since very fine particles also decrease the bed permeability causing problems to the electrolyte flow. 

The removal of six-valent chromium from an electrolyte is a special problem. Its toxic compounds (e.g. CrC>42 ) remain in the dissolved state, though a fraction of chromium is adsorbed by the sludge. To transform dissolved chromium into sludge, substances converting Cr(VI) into Cr(III) are added to the spent electrolyte. Na2SC>3, NaHSC>3, and FeS04 are commonly used as the reductants. By introducing barium salts into the electrolyte, Cr(VI) can be directly precipitated in the form of insoluble BaCrC>4. A fraction of Cr(VI) is reduced in the electrolyte by the reaction with Fe2+ and N02- yielded by the cathodic reaction (5). 

Good cooling is necessary to prevent the transformation of violet to green chromium (III) sulfate [the latter is not as readily reduced to chromium (II) sulfate]. The excess SO3 is driven off with a fast stream of air. The last traces of SO3 must be removed Iqr brief boiling. The solution, whose volume is now about 50 ml., is transferred to the clay cell. The anodic electrolyte is 2 N H3SO4. Electrolysis proceeds at a current density of 0.13 amp./in. , that is, at a current of 4.6 amp. The reduction takes 12 hours, but up to 24 hCurs may be required if a great deal of green chromium (III) sulfate is present. The course of the reduction can be followed... 

One concern is that Cr(III) and Al(III) compounds are both capable of forming octahedral complexes, and the introduction of these ions into an aqueous electrolyte will interfere with conversion of the hydrous alumina into the aluminum hydroxide film by bonding to the active film sites. Therefore, similar to their application in hexavalent chromate conversion coatings, fluoride ions are used to remove aluminum oxide and hydroxide films on the substrate surface before forming trivalent chromium conversion coatings. ... 

The synthesis of anhydrous metal halides is often a complex process. In thermal dehydration reactions complete removal of water is often complicated by decomposition of the compound. The reactions of metals with gaseous hydrogen halides or with halogens usually takes place at high temperatures and requires apparatus that may not be avaUable routinely. For example, CrBra can be obtained by the action of Br2 on chromium metal at a temperature of 750 °C.53 An alternative method has been proposed for the preparation of this compound that involves the anodic dissolution of chromium in the presence of bromine with a platinum cadiode immersed in benzene connected by a salt bridge to a chromium anode immersed in MeOH/Br2. The electrolytic cell is represented by Eq. 7.7 ... 

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