PEROXIDE BLEACHING OF MECHANICAL PULP USING DIFFERENT TYPES OF ALKALI

In this work, peroxide bleaching of mechanical pulp made from spruce w as studied. The aim was to find an alkali that would replace sodium hy droxide and silicate and yet give pulp with sufficient brightness and mechanical strength, and possibly also be more economical and more fri endly to the environment. Bleaching was performed at high consistency (20%) at 60-degrees-C in a small laboratory reactor; some experiments were subsequently performed in a larger pilot reactor. The chosen alka lis were tested both with and without silicate. Optimum alkali charges and bleaching times were determined in terms of the increase in brigh tness. After bleaching, the amount of residual peroxide, COD(Cr), cati onic demand, UV absorbance at 280 nm and sugar content of the residual liquor were measured. The optical properties, freeness and mechanical strength of the pulp were tested. The results showed that CaO, Ca(OH) 2, NaHCO3, MgO and Mg(OH)2 could be used in place of sodium hydroxide. The two Mg compounds gave the highest brightness increases (about 2 u nits higher than with sodium hydroxide and silicate), even without sil icate. With Mg compounds, the COD(Cr) Of the residual liquor fell to a bout half its normal value. The mechanical strength suffered somewhat (about 20%). One reason for the good results with the Mg alkalis could be their low solubility, which might keep the pH at a suitable value. The economy of the process did not suffer from use of the new alkalis .