Three-stage paper pulp bleaching - giving high whiteness level, by treating with chlorine, hypochlorite or peroxide oxidant and chlorine di:oxide

摘要

In bleaching to a high level of whiteness any paper pulp conventionally pre-oxidised, e.g. with a peroxide or oxygen under pressure in alkaline medium, which results in a reduced amt. of polluting effluent, (a) gaseous Cl2 is injected, followed by addn. of ClO2, without intermediate washing, the amt. of Cl2 used being less than that required by the pulp, the remainder being supplied by the ClO2. Further bleaching takes place at pH 8-12 (10.5-11.5) at 30-60 (40-50) degrees C, in the presence of an oxidising agent of the hypochlorite or peroxide type, and then bleaching is carried out using ClO2, at 60-90 degrees C. Stage (i) is carried out at 30-80 (30-50) degrees C, pref. using a pulp concn. of 3-20 (9-11)%. The amt. of gaseous Cl2 used corresponds to 60-90% of the total chlorine introduced.