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 ₂SO₃/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°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 performed 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% H₂O₂ 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.