Alkaline sulphite/anthraquinone (AS/AQ) pulping of rice straw and TCF bleaching of pulps

Alkaline sulphite-anthraquinone (AS/AQ) pulping of rice straw from North eastern Iran and totally chlorine free (TCF) bleaching of resulting pulps was investigated. In the AS/AQ pulping experiments, the active alkali charge was chosen between 14 and 18% on oven dry rice straw and the alkali ratio, Na(2)SO(3)/NaOH, was varied from 60/40 to 30/70. While a constant cooking time at maximum temperature of 60 minutes was applied, the maximum temperature was varied between 155 and 160 degrees C. AQ dose and liquor to straw ratio were kept constant at 0.1% on o.d. straw and 4:1, respectively. Reference soda and soda/AQ pulps were produced using different chemical charges. AS/AQ pulps with kappa numbers below 7 were easily produced while Soda/AQ and soda cooks carried out under similar conditions yielded kappa numbers of around 10. The AS/AQ process gave higher yield and the pulps had higher brightness than soda and soda/AQ pulps. The soda pulps had higher tensile strength-but the AS/AQ pulps had higher tear strength. TCF bleaching of AS/AQ pulp was perk formed in two sequences, OQ(OP) and OQ(OP)P. Delignification degree of over 50% was obtained and after the (OP) stage a brightness of 80% ISO was achieved. The sequence OQ(OP)P, with 2 or 4% H(2)O(2) in the final P stage improved brightness to 85.5 and 87% ISO, respectively. These results confirm that rice straw AS/AQ pulp is easy to bleach compared to soda/AQ and soda pulps which require OQ(OP)QP or even an OQ(OP)QQP sequence in order to reach target brightness of 80% ISO. Bleaching led to only small losses in tensile and burst strength but enhanced tear strength. The results of this study indicate that AS/AQ pulping together with TCF bleaching is able to produce high quality pulp from rice straw for writing and printing paper.