Evaporative Recovery Of Basic Chromium Sulphate Fromresidual Chrome

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Hamid, ELsanni
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This research was concerned with the concentration of residual chromium tanning liquor by evaporative techniques and the re-use of this concentrated chromium liquor for tannage after adjustment with a makeup of basic chromium sulphate powder.Twelve samples of the residual chromium liquors were collected randomly from ALNASR and the White Nile Tanneries. They were analysed mainly for Cr2o3 content, concentration of neutral salts (0Be ) and (PH). The average quantity of chromium oxide "Cr2o3" for both tanneries which discharged as waste to the drain was found to be 5.58 g/L. This quantity is equivalent to 32% of the chromic oxide added at the beginning of the tannage. This mean, that more or less one third of the basic chromium sulphate used in tannage is lost as waste. Then two Evaporation experiments were carried out at the White Nile Tannery laboratory by direct heating to determine the optimum evaporation level for maximum chromium oxide recovery. This level of evaporation was found to be about 27% of the water, can be evaporated without much loss of chromium oxide. Then Eight tanning Experiments were carried out at the White Nile Tannery pilot plant in the following manner:- A residual chrome liquor (R1) was taken from the White Nile tannery and it was evaporated to the required level (C1) .Then the chromium oxide in (C1) was determined and was adjusted with fresh chromium powder to maintain the tanning percentage to 7%. Then it was used to tan a delimed, bated, pickled pelt. The residual liquor(R2) from (C1) was again evaporated to (C2). Then the chromium oxide in (C2) was determined and adjusted to the required level with fresh chromium powder. Then it was used to tan a delimed , bated, pickled pelt. This process was repeated eight times untill (C8) and (R9). The average quantity of basic chromium sulphate required to be added as a make-up was found to be 4.9%. That means 2.1% of basic chromium sulphate is saved all through the year, in addition to the following savings:- 1) 4% in the sodium chloride 2) 60-70% in the water 3) 0.1-0.2% in the sulphuric Acid Then four pelts tanned with (C8), (C7), (C6) plus one side from the White Nile Tannery were retanned as usual and they were paste- Dried. Then they were tested for their physical and chemical properties and the results obtained were found to be within the accepted international limits. The economical evaluation of the project was found to be feasible. The annual profit was found to be 33767 US$ and the pay-back period was found to be about 1.3 year. As with regard to the environmental evaluation and since the process is a closed loop process, then appendix (5) proves that the salinity will be reduced by 25 %, the toxicity by 24%, the B.O.D by 1.2% and the C.O.D by 1%, from the total pollution load of the tannery. Hence this process is an eco-friendly process, in addition to the reduction in cost of the end of pipe treatment.
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