Chrome tanning process

In conventional chrome tanning processes 20 to 40% of the chrome used is discharged into wastewaters. In the new process 95 to 98% of the waste Cr3+ can be recycled. 

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. 

E. W. Merry, The Chrome Tanning Process. Its Theory, Practical Application and Chemical Control. A. Harvey Publisher, London, 1936. 

Anghelescu-Dogaru, A.G., Popescu, I., Chitanu, G.C. (2004). Maleic Polyelectrolytes as Ecologically Favourable Additives in Chrome Tanning Process. Journal of Environmental Protection and Ecology, Vol. 5, No. 2, (April 2004), pp. (265-270), ISSN 1311-5065... 

Fig. 2. Process flow diagram for the production of chrome-tanned cattle hide leather. Adapted from Ref. 3.

The usual procedure in the manufacture of chrome-tanned leathers is the use of a continuous bate, pickle, and tan method. The hides or skins remain in the dmm from the lime washing through the chrome tanning stages. The entire process usually takes about 22—24 hours. The bate, pickle, and tan can be done faster, but most tanners find that a one-day cycle fits well into production scheduling and results in a quaUty leather. 

Chrome Tanning. The original chrome tanning was a two-bath process. The unhaired hides, delimed and bated, were treated with a solution of sodium bichromate [10588-01-9]. The amount of bichromate used was about 3—5% based on the weight of the hides. The bichromate was absorbed or adsorbed into the hide, the solution drained, and the hides refloated. Sodium bisulfite was added and two important reactions resulted in the formation of a basic chromium and coUoidal sulfur in the hide. This gave a chrome taimage and also helped to fiH the hide with the soHd sulfur. This cmde system, which continued in the industry in some types of leather for over 50 years, is obsolete. 

Miscellaneous. In ore flotation, sodium sulfite functions as a selective depressant. In textile processing, sodium sulfite is used as a bleach for wood (qv) and polyamide fibers and as an antichlor after the use of chlorine bleach. Synthetic appHcations of sodium sulfite include production of sodium thiosulfite by addition of sulfur and the introduction of sulfonate groups into dyestuffs and other organic products. Sodium sulfite is useful as a scavenger for formaldehyde in aminoplast—wood compositions, and as a buffer in chrome tanning of leather. 

Around 1800, the attack of chromite [53293-42-8] ore by lime and alkaU carbonate oxidation was developed as an economic process for the production of chromate compounds, which were primarily used for the manufacture of pigments (qv). Other commercially developed uses were the development of mordant dyeing using chromates in 1820, chrome tanning in 1828 (2), and chromium plating in 1926 (3) (see Dyes and dye intermediates Electroplating Leather). In 1824, the first chromyl compounds were synthesized followed by the discovery of chromous compounds 20 years later. Organochromium compounds were produced in 1919, and chromium carbonyl was made in 1927 (1,2). 

Naturally white, but slightly yellowed fur skins from sheep and lambs are reduction bleached, most commonly with dithionite or/and in combination with hydrogen peroxide in a weakly alkaline medium. A true bleach in the sense of destroying the natural pigments in the hair can only be achieved by means of oxidation with, e g., hydrogen peroxide, catalyzed by iron(n) salts. This exothermic process is very difficult to control and must be monitored carefully. In addition, the skin must not be chrome tanned, as this could cause serious damage. 

Disperse and other synthetic dyes can only be taken up by the hair at elevated temperatures, and for this reason the skins must be chrome-tanned. The shrinking temperature of suede has to be at least 20° C higher then the dyeing temperature. No mordant is required, unlike with oxidation dyes, and killing need be far less intensive, too. In actual fact the operation is more hair cleaning than a killing process. The suede portion is not dyed and can be cleaned very easily with standard washing auxiliaries. 

A method is proposed for using chrome tanned leather wastes produced by the footwear and clothing industry as fillers in polymer matrices. A technique for processing the composite obtained by continuous extrusion is also demonstrated. In order to evaluate the technique, a series of PVC-leather fibre composites was prepared and extruded through a flat die to produce sheets. The process produced a leather-like sheet that could be used in several applications. The influence of the filler content on the processability and final properties of the composite sheets was evaluated. The tests revealed that the sheets were flexible and exhibited suitable water absorption levels for several applications in the footwear and clothing industry. Finally, the tests showed that this composite could be formulated and processed at high productivity levels and at a low cost. 28 refs. 

Oxidation states of chromium - -2, - -3, and -f-6. Oi es of chromium chronate, FeCr204, and crocoite, PbCr04. Chromium metal and its alloys ferrochrome, alloy steels, stainless steel. The aluminothermic process (Goldschrtiidt process). Electrolytic chromium. Chromium trioxide, chromic acid, dichromic acid, potassium chromate, potassium didiromate, sodium chromate, lead chromate. Equilibrium between chromate ion and dichromate ion. Chrome-tanned leather. Chromic oxide (chrome green) chromic ion, chrome alum, chromic chloride, chromic hydroxide, chromite ion. Chromous compounds. Peroxy-chromic acid. 

A very unportant process in converting hide to leather is tanning. In chrome tanned leather, the chromium is presented in a combined form with the hide protein. Though the chromium in leather is usually reported to be chromic oxide (Cr O ), this does not mean that the chromium is present as the oxide. 

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. 

According to the final product, leather requires a certain softness, which is achieved by application of fat. A fat content of 5% is essential gloves, garments, and upholstery may contain up to 15%. Therefore, after dyeing, the leather is oiled. In this process, called fatliquoring, oily fat is emulsified and applied to the leather in a warm bath. The emulsion breaks on contact with the chrome-tanned fibers of the leather, and the fat remains on the leather. The oils used are sulfated or sulfonated plant, animal or fish oils or synthetic products, usually biocide preserved (Heidemann 1990 Podmore 1995 Thorstensen 1995). 

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