Tanning chrome

Chromenes Chrome-nickel alloys Chrome-nickel steels Chrome nitrate Chrome ore Chrome oxide greens Chrome plating Chrome tanning Chrome-tin... 

The manufacture of leather foUows the same general steps for a great variety of leathers (Fig. 2). The largest category of hides taimed is catde hides. Of the cattle hides chrome tanning of unhaired hides is by far the dominant system used throughout the world. The tanning of other types of hides and skins requires variations in the systems used for cattle hides (3). 

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

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). 

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. 

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. 

The chrome-tanned leather is removed from the dmm and wmng to remove the absorbed tanning solution. The leather is then inspected for quaUty of the grain and other characteristics of importance for the leather being made. In large tanneries where very uniform hides are worked, the leather may be trimmed and spHt to the desired thickness. Hides spHt before tanning need no splitting at this point. 

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. 

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. 

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. 

Chrome-tanned leather has chromium bonded to the leather fibers. This chromium can act as a mordant for acid dyes resulting in fast colors and intense shading at the surface of the leather. 

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). 

The chrome tanning is one step in a compHcated series of leather operations leading from the raw hide to the finished products. Chrome tanning is the most important tannage for all hides except heavy catde hides, which are usually vegetable tanned. In heavy shoe uppers and soles, a chrome tanned leather is frequently given a vegetable retan to produce chrome retan leather. 

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 Oeko-Tex label tests for and restricts a high number of chemicals in the product. It should be noted that chrome-tanned leather cannot receive the Oeko-Tex Standard 100 certificate due to stringent demands on chromium. Since around 85-90% of all leather in the world is tanned with chromium, the certificate is only valid for a minor part of the leather production. 

Chrome refractories, 21 518 Chrome-tanned leather, 9 225 Chrome yellow, 6 523, 554, 555t color, 7 332 for green, 6 555t... 

Chromates and dichromates are used in industry as oxidising agents, for example in the coal tar industry, in the leather industry (chrome tanning), and in the dye industry as mordants. Some chromates are used as pigments, for example those of zinc and lead. Chromates and dichromates are poisonous. 

Final rinse from chrome tanning (two samples) ND... 

This facility is a re-tanning and finishing operation that receives chrome tanned hides from other plants which it re-tans, re-colors and surface finishes. This facility was visited on February 5, 1979 when 21 air samples were collected at all stages of the operation. There was no reported use of dimethylamine sulfate or any other amines in this facility. 

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. 

See refe. 1208 and 1209. Technical specifications are available from manufacturers, e.g. Yorkshire Chemicals (Selby, UK), Chrome Tanning Products and Auxilliaries , Technical Bulletin, 1984. 

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