Optimizing multisteps mechanical-chemical fractionation of wheat straw components

Wheat straw was refined into fibres, hemicellulosic sugars and lignin oligomers by means of a multi-stage fractionation treatment: mechanical separation of the botanical components followed by one or two stage chemical fractionation process. Yellow winter wheat straw was initially separated into two fractions by milling in a Disc Mill: (a) chips, containing mostly internodes and (b) meal, consisting mainly of ground leaves and nodes. The internode fraction (63% of the whole straw) contained 8% more cellulose, 9% more lignin and 10% less ash compared with the unfractionated material. Disc mill fractionation was particularly effective with respect to the separation of the non-lignocellulosic components, i.e. protein, extractives, etc. Wheat straw internodes as well as whole straw were treated with acidic or alkaline aqueous ethanol in one or two stages; two-stage treatment involved the introduction of a pretreatment step with dilute acid improving, both, the production of hemicellulose derived sugars, and the aqueous-ethanol delignification to follow. The fibre yield in the case of internode chemical fractionation was generally higher in comparison with the one of the whole straw. The fibrous fractionation residues from internodes were in all cases enriched in cellulose compared with whole straw fibres. In spite of the higher initial lignin content of the internodes, delignification was more extensive than in the case of whole straw. Moreover, variations in the residual lignin composition were found, reflecting the intensity of the chemical fractionation process, as well as the origin of the raw material. (C) 1998 Elsevier Science B.V.