LABORATORY-SCALE MEDIUM-CONSISTENCY OZONE BLEACHING SYSTEM

A laboratory design for medium consistency ozone bleaching using high- shear mixer is outlined. Ozone is compressed to a high-pressure in ord er to reduce the phase ration of gas to fiber suspension and inject a larger quantity of ozone. Using the designed system the bleachability of oxygen bleached kraft pulp was studied. Ozone bleaching at medium c onsistency followed by an alkaline extraction stage showed considerabl e improvement in delignification and brightness gain with high selecti vity. Optimum operating conditions to perform ozone bleaching at mediu m consistency with the high-shear mixer were low temperatures and pH 2 to 3. Low temperatures and low pH reduced ozone decomposition and inc reased available ozone for bleaching. An increase in speed of mixing f rom 1200 rpm to 2400 rpm while bleaching 10% consistency pulp resulted in better delignification and increased viscosity loss together with an increase in freeness loss. Bleaching 5% consistency pulp at 2400 rp m mixing speed had higher selectivity and lower specific consumption w ith comparable delignification as 10% consistency pulp at the same mix ing speed.