Tanning chromium complexes

Control of chromium penetration, essential to permit tannage of the center of the hide, is accompHshed by pH adjustment. At a pH > 3.0 the reactivity of the hide to the chromium complex is greatiy increased. The pH is therefore raised gradually to the desired point by addition of a mild alkah, usually sodium bicarbonate. The chemistry of chrome tanning involves competing reactions that must be controlled for satisfactory results. 

Karthikeyan R, Balaji S, Chandrababu NK, Sehgal PK (2008) Horn meal hydrolysate-chromium complex as a high exhaust chrome tanning agent-pilot scale studies. Clean Techn Environ Policy 10 295-301. doi 10.1007/sl0098-007-0119-2... 

Other companies preferred to precipitate the catalyst finm solutions of the cheaper zinc sulfate and chromium sulfate (chrome tan) with sodium carbonate. An active catalyst, with a long life, could be produced although only about 90% of the metals were recovered. This resulted from the variable pH of the solution and the partial solubihty of the precipitates at about pH 7 during the precipitation procedure. Significant amounts of sulfate and sodium, in the form of chromium complexes, also remained in the final catalyst although this did not appear to result in loss of activity or to cause operating problems. 

The chromium can be stabilized in a limited way to prevent surface fixation by addition of formate ions. The formate displaces the sulfate from the complex and masks the hydroxyl ions from forming the larger higher basicity complexes. This stabilization can then be reversed in the neutralization to a pH of about 4.0 and taimage becomes complete. This simple formate addition has decreased the time of chrome tanning by about 50% and has greatly increased the consistent quaHty of the leather produced. 

Leather Tanning and Textiles. Although chromium (VT) compounds are the most important commercially, the bulk of the appHcations in the textile and tanning industries depend on the abiUty of Cr(III) to form stable complexes with proteins, ceUulosic materials, dyestuffs, and various synthetic polymers. The chemistry is complex and not well understood in many cases, but a common denominator is the coordinating abiUty of chromium (ITT) (see LEATHER Textiles). 

Chromium(III) is a commonly-used crosslinker for preparing profile control gels with polymers having carboxylate and amide functionalities (la,b). Cr(III) is applied in many forms. For example, it can be used in the form of simple chromic salts of chloride and sulfate, or as complexed Cr(III) used in leather tanning (2), or as in situ generated Cr(III) from the redox reaction of dichromate and bisulfite or thiourea. The gelation rate and gel quality depend on which form of Cr(III) is used. 

Leather is an important material in the world economy, being used in footwear, clothing, luggage and upholstery. It is a complex substrate prepared from animal hides and skins by a long series of treatments. After the removal aU organic material and hairs the material is tanned, usually with chromium salts, which causes the protein chains to cross-link. After treating with oils or fats, to improve its handle, the leather is dyed. 

Simple carboxylates of chromium(III) find industrial application as catalysts for the polymerization of to-alkenes854,855 and in the preparation of chrome-tanning solutions.856,857 There seems to be no simple carbonate of chromium(III). Compounds formed on the surface of Cr203, sometimes formulated Cr2(C03)3- H20, are best viewed as carbon dioxide adsorbed on the oxide.858 Carboxylate complexes of chromium(III) will now be considered in terms of the various ligand types. 

The long established use of chromium salts in the tanning of leather has been reflected in interest in the determination of chromium in leather extracts. Generally the leather is leached with acids or complexing agents such as oxine to remove the metal salts. Della Monica and McDowell [189] recommended leather strips be air dried at 80°C for 24 h and ground to pass a 10 mesh screen. The powder was vacuum dried at 70°C for 16 h and then 100 mg refluxed with 25 ml 2M hydrochloric acid. The filtered acid was aspirated into a flame. For chromium a fuel-rich air/acetylene flame or a nitrous oxide/acetylene flame is to be recommended. 

In connection with the chrome tanning of leather it has been found in technical practice that by the addition of neutral salts it is possible to use more basic chrome liquors The added salt prevents the precipitation of hydrated chromium oxide This phenomen has been regarded as due to an actual increase in hydrogen ion concentration It is much more probable, however, that we are dealing with an increase of activity rather than an increase of mere concentration Instead of dealing with the complex system presented in the tanning bath we shall refer briefly to the results obtained by Thomas and Miss Baldwin loc at) in the chemically better defined case of a solution of chromium chloride... 

Hydrolysed, polynuclear Cr complexes are of considerable commercial importance in the dying and tanning industries. In the former the role is that of a mordant to the dye. In leather production it is necessai to treat animat hides to prevent putrefaction and to render them supple when dry. Traditionally, tannin was used, hence the name of the process, but this was superseded towards the end of the nineteenth century by solutions of chromium(lil) sulfate. After soaking in sulfuric acid the hides are impregnated with the Cr solution. This is subsequently made alkaline, when the polynuclear complexes form and bridge... 

The reactions involved in the chrome-tanning process are those of coordination complexes. They involve the interaction between charged carboxyl groups on the collagen macromolecule and polynuclear chromium(III) coordination compounds. The most widely used chrome-tanning material is 33% basic chromium(III) sulfate produced industrially by reducing sodium dichromate with sulfur dioxide. 

Tanning industry is one of the m gor industries in India. Tannery effluents are the most complex industrial wastes which are toxic to plants, animals, and soil naicroorganisms [1]. Taimery wastes contain vegetable tannins in addition to soluble organic matter, suspended solids, chromium, high chloride and sulphide concentration, and high pH (Table 1) [2]. 

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