The technology developed involves the recovery of chromium from the spent tannery liquors and its reuse.
Tanning of hides is carried out with basic chromium sulfate, Cr(OH)SO4, at a pH of 3.5 to 4.0. After tanning the solution is discharged by gravity to a collection pit. The liquor is sieved during this transfer to remove particles and fibers that have come from the hides. The liquor is then pumped to the treatment tank and a calculated quantity of magnesium oxide is added with stirring until the pH reaches at least 8. The stirrer is switched off and the chromium precipitates as a compact sludge of Cr(OH)3. After settling the clear liquid is decanted off. The remaining sludge is dissolved by adding a calculated quantity of concentrated sulfuric acid (H2SO4) until a pH of 2.5 is reached. The liquor now contains Cr(OH)SO4 and is pumped back to a storage tank for reuse (Figure 1.9).
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.
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