Processing waste waters from vat esp. indigo dyeing - comprises flocculation with magnesium and/or calcium salts, decantation, reducing dye in sediment and recycling

摘要

Waste waters from vat- esp. indigo-dyeing processes are worked up by: (a) flocculating the vat dye quantitatively by adding Ca- and/or Mg-salts, esp. CaCl2 and/or MgCl2, to waste waters and setting pH 12-13f (b) allowing flakes to settle; (c) sepg. sediment sludge from decantate; (d) oxidising dithionite, sulphite and further oxidisable S cpds. in decantate to sulphate, by adding H2O2. Method further comprises (e) neutralising the sulphate-contg. decantate and discharging the neutralised decantate; (f) reducing the dye obtd. in sepd. sediment sludge back to the original vat dye, by adding reducing agents, esp. Na2S2O4 + NaOH soln.; (g) adding filtering auxiliaries to the suspension obtd. and filtering, and (h) recycling the vat-dye, obtd. as filtrate, to vat-dyeing process. The process is economical and non-polluting. Added chemicals can be regenerated. 100% vat dye is sepd. as a re-usable material.