Alkaline sulfite-anthraquinone (AS/AQ) pulping of wheat straw and totally chlorine free (TCF) bleaching of pulps

Alkaline sulfite-anthraquinone (AS/AQ) pulping of wheat straw and totally chlorine free (TCF) bleaching of resulting pulps was investigated. Wheat straw was supplied by the Ferdosii University Experimental Station in Northeastern Iran, cleaned and chopped into about 3-5 cm long pieces. In the AS/AQ pulping experiments, the active alkali charge on oven dry wheat straw, heating time to maximum cooking temperature, AQ dose and liquor to straw ratio were kept constant at 16%, 60 min, 0.1% and 4/1, respectively. The alkali ratio, Na(2)SO(3)/NaOH, calculated as NaOH, was varied from 20/80 to 80/20. Cooking time at maximum temperature was varied from 30 to 90 min and maximum cooking temperature was adjusted between 152 and 160 degrees C. Reference soda and soda/AQ pulps were produced. AS/AQ and soda/AQ pulps with kappa numbers below 15 were produced easily. Cooking of corresponding soda pulps under comparable conditions stopped at a kappa number of 25. The characteristics of unbleached AS/AQ, soda/AQ and soda pulps were compared. AS/AQ and soda/AQ pulps had similar tensile strength but tear was in favor to AS/AQ. The latter had much higher yield and brightness. TCF bleaching of AS/AQ pulp was performed in O Q (OP) or O Q (OP) P sequences. Oxygen delignification (O) resulted in a delignification degree of approximately one third. Followed by a chelating treatment (Q) and a subsequent (OP) stage under drastic conditions a brightness of 82.7% ISO was achieved in an O Q (OP) sequence. The sequence O Q (OP) P, with 2% H(2)O(2) in the final P stage, improved brightness from 77.2% ISO after the OP stage to 83.6% ISO. Bleaching led to only small losses in tensile and burst strength but enhanced tear strength. Short beating in a Jokro mill was sufficient to attain good pulp strength. The results of this study indicate that AS/AQ pulping and TCF bleaching is a convincing alternative to produce high-quality pulp from wheat straw for writing and printing paper. (C) 2008 Elsevier B.V. All rights reserved.