Evaporation of Acidic Effluents from ECF Bleaching: Effect of Feed pH on the Quality and Re-Useability of the Condensate, 1997 Pulping Conference Proceedings

The effect of feed pH on the condensate and concentrate quality upon evaporation of acidic effluents from ECF bleaching, was investigated in this study, in view of the re-useability of the condensates in the final chlorine dioxide stage of the bleaching process. The experiments were carried out in laboratory conditions. The results indicate that it is advantageous to neutralise the feed pH of the Do and Q effluents, before evaporation. Neutralisation of the feed prevents essentially the transfer of inorganic and organic compounds into the condensate during evaporation. Further addition of alkali had no clear positive additional effect on condensates quality. Negative impacts can also be partly avoided by evaporating acidic effluents only up to a 30% level of total solids of the concentrate. In the concentrate the main precipitate compound in the very acidic condition was calcium sulphate. When the pH of the concentrates was increased, the predominant compounds were calcium oxalate hydrates and, while at higher total solids, the sodium compounds Na2SO4 and NaCl were dominant. The quality of the condensates was good enough for re-use in the final chlorine dioxide stage of the ECF bleaching process. No appreciable effects on pulp quality or chemical consumption in the final D stage were observed.