Recovery of Chromic Oxide from Leather Chrome Shavings

Abstract

A chrome shaving is a process by which the pelt thickness is adjusted according to its utilization purposes. This chrome shavings which constitute 2.5−5.0 % chromium are mainly disposed at the outside the premises of the tannery and left to dry, posing a hazardous environment. When the chrome shavings are dried, they would be taken by brick - makers and burnt out, a process which will transform chromium III to chromium VI. Chromium VI is 500 times toxic than chromium III and most harmful pollutants in tannery waste. Chromium VI also affects soil quality, more dangerous and carcinogenic. Therefore recovery of chromium VI is very essential. In this study chrome shavings was incinerated at 1000 ºC to transform chromium III to chromium VI. Leaching of chromium VI in the form of dichromate was carried out in a series of 6 batteries and statistically investigated at different levels of solid/solvent ratios, temperatures, degree of mixing and time. Further evaporation was done in a water bath to raise up the concentration from 40% to 90%. From the statistical analysis it is determined that only the degree of mixing at 200rpm is significant, and other factors are fixed at their low levels, temperature of 37 ºC, time of 1 hour and solid/solvent ratio of 1:10. The leachability was found to be 95%. The hexavalent chromium was reduced to basic chromium sulphate. The basic chromium sulphate obtained was of 33% basicity and 21% chromic oxide with conversion of 96.80%. Experiments on the reaction kinetics were performed to determine the reaction order, the reaction rate and conversion. These were found to be first-order reaction and 100% conversion after ageing the chrome liquor for two weeks. Complete reduction was achieved in three hours with pH of 2.5. A chrome tanning trials were performed using pickled sheep skins. The pelts passed the boiling test indicating that the tannage was completed, and then the pelts were neutralized, retanned, fat-liquored and left overnight. The dried pelts were conditioned and the crusts produced were subjected to chemical and physical analysis, with chromic oxide 4.4%, moisture content 14%, ash content 4.6%, shrinkage temperature 100 ºC, fat content 3.4% , tensile strength 171kg/cm2, elongation at break 49% , tear strength 100kg/cm, and load at grain burst 40 kg. The quality of the leather produced using chromium sulphate liquor extracted from chrome shavings was satisfactory with excellent properties.