EXTRACTION OF CHROMIUM(III) FROM SPENT TANNING BATHS

Extraction of chromium(III) from a model spent tanning bath of the lea ther industry has been investigated using ammoniated and non-ammoniate d di(2-ethylhexyl) phosphoric acid (D2EHPA) and bis(2,4,4-trimethylpen tyl)phosphinic acid (Cyanex(R) 272). Chromium extraction of 95% by 15% (v/v) D2EHPA, 10% (v/v) isodecanol in kerosene and 86.1% by 15% (v/v) Cyanex(R) 272, 10% (v/v) p-nonylphenol in kerosene was obtained at eq uilibrium pH values of 4.0 and 5.0, respectively. The separation of sm all amounts of iron(III) and aluminium(III) present in the solution al ong with the Cr(III), was also examined and it was found that Cyanex(R ) 272 was a better reagent than D2EHPA. The slow kinetics of extractio n and stripping observed in the case of AI(III) was advantageous for i ts separation from Fe(III) at low pH values. Difficulties faced in the stripping of loaded metals were also studied because only about 80% c hromium recovery by 8 M HCl was obtained from both solvents. The incom plete stripping of the metal may be a result of the formation of a sta ble species in the organic phase and needs further investigation. The Cr(III) can be recovered as chloride from the strip liquor and recycle d for retanning purposes.