Recycling of chromium

Chaudhary, A., Ganguli, B., and Grimes, S. (2006) The regeneration and recycle of chromium etching solutions using concentrator cell membrane technology. Chemosphere, 62 (5), 841-846. 

Leather making produces a large quantity of effluents, which contain a considerable amount of chromium. Chapter 9 of this book discusses the treatment of wastewater including heavy metals. Section 3.3.2 of this chapter will discuss the analysis of chromium content in solutions and the recycling of chromium briefly. 

Decreased use of chrome by automobile and small-appliance manufacturers and mandated recycling of chromium wastes in the electroplating industry have led to a decline in Na2S04 produced from this source. 

The pollutants of concern are the same as in wet basic oxygen furnaces, but the concentration of metals (primarily lead and zinc, but also arsenic, cadmium, copper, chromium, and selenium) in wastewater is higher because of the higher percentage of scrap charged. Wastewater treatment operations are similar to those for the wet basic oxygen furnaces, including sedimentation in clarifiers or thickeners and recycle of the water.14... 

Medina, B.Y., Torem, M.L., and De Mesquita, L.M.S., Removal of chromium III from liquid effluent streams by precipitate flotation, in Waste Treatment, and Clean Technology, The Global Symposium on Recycling, REWAS 04, Vol. II, Gaballah, I., Mishra, B., Solosabal, R., and Tanaka, M., Eds., Madrid, Spain, September 26-29, 2004. 

The use of heterogeneous catalysts in the liquid phase offers several advantages compared with homogeneous counterparts, in that it facilitates ease of recovery and recycling. A chromium-containing medium-pore molecular sieve (Si Cr > 140 1), CrS-2, efficiently catalyzes the direct oxidation of various primary amines to the corresponding nitro compounds using 70% t-butylhydroperoxide (TBHP).110... 

In another example, a polymer-supported chromium porphyrin complex was supported on ArgoGel Cl and then employed for the ring-opening polymerization of 1,2-cyclohexene oxide and C02 [95], This complex showed higher activity than a C02-soluble equivalent, and the solid nature of the catalyst meant that recycling of the catalyst was much easier. 

It is well known, even from old literature data (ref. 1) that the presence of metal promotors like molybdenum and chromium in Raney-nickel catalysts increases their activity in hydrogenation reactions. Recently Court et al (ref. 2) reported that Mo, Or and Fe-promoted Raney-nickel catalysts are more active for glucose hydrogenation than unpromoted catalysts. However the effects of metal promotors on the catalytic activity after repeated recycling of the catalyst have not been studied so far. Indeed, catalysts used in industrial operation are recycled many times, stability is then an essential criterion for their selection. From a more fundamental standpoint, the various causes of Raney-nickel deactivation have not been established. This work was intended to address two essential questions pertinent to the stability of Raney-nickel in glucose hydrogenation namely what are the respective activity losses experienced by unpromoted or by molybdenum, chromium and iron-promoted catalysts after recycling and what are the causes for their deactivation ... 

Diagenesis of Microbially Reduced Sulfur. Postdepositional transformations play an important role in controlling the extent of recycling of microbially reduced S. Pore water profiles from many freshwater systems clearly show that H2S is a short-lived intermediate in sulfate reduction which does not accumulate in sediments (14.16 41-431. However, the conventional paradigm for sulfur diagenesis, in which H2S is initially immobilized by iron monosulfides that later are diagenetically altered to pyrite and elemental S (e.g., 2Q)> does not apply to all freshwater systems. Instead, organic S and CRS (chromium reducible S, which is believed to represent pyrite + S° after preliminary acid distillation to remove AVS), are important initial endproducts of dissimilatoiy reduction. 

The European Union has been quite visible in recent years with such directives. RoHS (Restriction in the use of Hazardous Substances), WEEE (Waste of Electrical and Electronic Equipment), and REACH (Registration, Evaluation, Authorization, and Restriction of Chemical substances). RoHS places restrictions on use of lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls, and some polybrominated diphenylethers. WEEE targets responsible recycling of electronic equipment. REACH is a new European Community Regulation on chemicals and their safe use... 

Chromium-salen complexes have been used for the reaction between styrene epoxide and scC02 in [C4Ciim][PF6], as illustrated in Scheme 9.15.1601 At low catalyst concentrations, 1-phenyl-1,2-ethanediol was detected as a by-product while at a catalyst loading of ca. 0.35 mol%, 100% selectivity was obtained. Recycling of the catalyst was possible, but the ionic liquid phase needed to be exhaustively purified with volatile organic solvents prior to its reuse. 

Chrome tanning is the most important tanning method in leather production. The effluents from the tannery house contain a considerable amount of chromium. A limit to the chromium discharge is mandated by pollution regulations in almost every country. It is necessary to recycle chrome tanning materials from the effluents. The most common way of recovering the spent chromium salts is by precipitation (Thorstensen, 1993). The pH of the effluents may be raised to the precipitation point of the chromium salts, which precipitate as a hydrated chromium oxide. 

No matter what the source of chromium tannages, either from raw materials or from recycled solutions, care must be taken to maintain the quality of the leather. This can be only achieved by constant chemical analysis and chemical control. The chromium content in the solution can be tested by the following two methods, titration and atomic absorption analysis ... 

This process was formerly the most widely used for the manufacture of butadiene by dehydrogenation. Using a feed containing 95 per cent or more n-C. it produces a mixture of butenes and butadiene in a single step. The butadiene is separated, and the unconverted butenes and butane are recycled. The catalyst, activated alumina containing 18 to 20 per cent weight of chromium oxide, has a life of more than six months. It is placed in a series of horizontal reactors lined with refractory bricks. The inert alumina is mixed with the catalyst to achieve the uniform distribution of the heat required for the reaction and a high heat capacity of the catalyst bed. 

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