Integrated cereal straw valorization by an alkaline pre-extraction of hemicellulose prior to soda-anthraquinone pulping. Case study of barley straw

Hemicellulose alkaline extraction was integrated prior to the well-known soda-anthraquinone (soda-AQ) pulping process. The resulting integrated process is called the integrated soda pulping (ISP) process. By the means of central compositely designed experiments, the ISP process was optimized in order to produce a good quality pulp from barley straw along with a significant amount of xylan-containing hemicellulose. The identified pre-extraction parameters to be optimized were the NaOH charge (dry straw basis), operating temperature, and extraction time. For comparison purposes, a reference soda-AQ pulp was also produced under the same cooking conditions. The optimized hemicellulose pre-extraction conditions were found with a 13% NaOH charge, at 60°C and for 2 h. By operating the pre-extraction stage under these conditions, 57% of the barley straw hemicellulose was recovered and was usable as a substrate for further chemical transformations. The strength properties of the ISP pulp at 40°SR (Shopper-Riegler beating degree) remain at a very good level for a cereal straw pulp; they were: breaking LENGTH = 9800 m, tear INDEX = 5.40, burst INDEX = 5.70. Only a very slight difference was detected between the ISP pulp and the reference soda-AQ pulp. A possible improvement regarding cereal straw black liquor processing and papermaking was suggested after analysis of the silica in the ISP black liquor and analysis of pentosans in the ISP pulp.