| Résumé : |
In a recent article Smith notes that emphasis on environment, health, and energy conservation has led to new studies to increase the efficiency of chromium usage in a tannery. He cites four basic concepts for increasing efficiency, these being improved uptake, recycling chrome liquors, precipitation and recovery of chrome for reuse and alkaline incineration. Utilising various techniques, all of the first three are practical commercially, while the fourth is being studied seriously from both chrome recovery and solid waste disposal perspectives. Since the first disclosure of a practical chrome tanning process in 1858 there have been literally hundreds of publications about the chemistry and technology of chrome tanning. It is estimated that about half relate to improved chrome uptake, but they can be classified into four main categories. These are control of pH, selection of masking agents, control of time and temperature, and control of float (or concentration).
pH control does not necessarily mean increasing the pH at the end of chrome tanning by adding more neutralising agent. It does include employing more modern techniques such as adding magnesium oxide, a sparingly soluble basifying agent, for control of neutralisation ; or a combination of conventional and self-basifying chrome ; or even a chelated chrome added immediately following the bate. Selection of masking agents can be very critical. The more conventional formate and acetate masking can lead to more rapid arome penetration, while the more complex, polybasic acids which affect the size of the chrome complex can decrease penetration but improve fixation. Time, temperature and control of float are all rational and interrelated. Since chrome tanning is both a physical (diffusion) and a chemical (reaction) process which ultimately leads to an equilibrium situation, any condition which achieves equilibrium more rapidly or at equilibrium increases the quantity of total fixed chrome relative to the total unfixed chrome will improve uptake. The conditions are, of course, low float, higher temperature and longer running times.
It is now possible to add a fifth category to the techniques for improved chrome uptake. Although currently there is insufficient evidence to establish the mechanism of action, it is apparent that a class of chemicals has been discovered, broadly defined as amine salts, which shifts the equilibrium of chrome between stock and float such that more chrome is exhausted in the stock and less remains in the exhaust liquor. This paper describes the results of our laboratory programme to determine the effect of process variations and define an optimum process for tannery trials. The results of confirmatory tannery trials are also reported. |
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Improved chrome utilisation in chrome tanning in JOURNAL OF THE SOCIETY OF LEATHER TECHNOLOGISTS & CHEMISTS (JSLTC), Vol. 66 (Année 1982)
