Chrome-tanned materials

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. 

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. 

Coordination complexes of titanium(III), titanium(IV), zirconium(IV), iron(m) and the lanthanides all have tanning properties, particularly if they are stabilised by the addition of appropriate ligands. None of these, however, give leathers with the properties of chrome-tanned materials. 

Deliming and Bating. The limed hides have a pH around 12. Because chrome tanning is done at pH 2—4, the lime must be removed for pH adjustment. In addition, the undesirable materials in the hide, ie, both natural and the degradation products from the unhairing, must be removed (7,9). 

Modem chrome-tanning methods are weU controUed and employ an extensive knowledge of the chemistry of the system. The most common chromium-tanning material used is basic chromium sulfate [12336-95-7] Cr(0H)S04, made by the reduction of sodium bichromate with sulfur dioxide or by sulfuric acid and a sugar. 

Chrome-tanning is the crosslinking of the collagen carboxyl groups in animal hides with the help of chromium(III) compounds. This results in a material with increased temperature stability and reduced swelling capacity. 

Based on the production processes of leather, this chapter will discuss the chemical tests which allow identification of leather from its synthetic substitutes and analyses of tanning materials. Some tests of important leather properties, such as pH, fat, chrome and ash content will also be described. The azo dye tests will be illustrated here since many countries have already adopted mandatory regulations... 

Conversely, when the technique of chrome tanning was being developed in the late nineteenth century, it was found that the leather produced would dry out as a hard, stiff, inflexible material. It was only by the addition of fatty lubricating products in the form of an emulsion, in what became known as the fat liquoring process, and by mechanically working the skin that a useable product could be made. In a similar way, the production of alum-tawed skins involved the use of such fatty materials as egg yolk or olive oil and mechanical softening procedures. 

The predominant tanning technique in the world is chrome tannage (about 85%). As a multi-purpose material chrome tanned leather can be transferred to leather with different properties (to different leather types) by further processing. However, a significant quantity of vegetable tanned leather is manufactured, too. Part-tanned chrome-free - often termed wet white - leather is also produced, in small but growing quantity. 

The material obtained by the above-stated processing of waste buffing dust from chrome tanned leather is potentially applicable on cemented soles, wrapping sheets, lagging and noise damping materials, sealing materials, antivibration pads, and so on. 

Glue is derived from collagen, the major proteinaceous component of animal and fish skins, as well as tendons and the proteinaceous matrix of bones. In recent times the important commercial raw materials for making animal glue come from tanneries, which supply limed splits and chrome tanned pieces, and from slaughter houses, which furnish fleshings, hide pieces and bones. 

From among the methods mentioned above, iron has been determined with the use of Chrome Azurol S - in waters [176], Bromopyrogallol Red - in magnetic Fe-Co-Ni films [177], sulphanilic acid - in blood plasma [129] and in plants [109], Tiron - in geological materials [114], in aluminium alloys and copper [115], 2,2 -diquinoxalyl - in niobium oxide [128], PAN - in alloys and biological samples [79] and in waste waters [178], TAN - in geological samples [83], 5-Br-PADAP - in biological samples (by derivative spectrophotometry) [91] and in copper alloys [179], and morin - in copper-chromium and nickel-chromium alloys [122]. 

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