Soda-anthraquinone pulping of Hibiscus sabdariffa (karkadeh) and Calotropis procera from Sudan

Abstract Two potential raw materials ( Hibiscus sabdariffa var. sabdariffa — karkadeh — and Calotropis procera — usher) grown in Sudan were examined to determine their suitability for the production of papermaking pulp. Their anatomical, physical, morphological and chemical characteristics and fibre dimensions are reported. Both species were pulped by the soda-anthraquinone process and the usher also by a chemimechanical process (CMP). The soda-anthraquinone pulping of core alone or the whole stem of karkadeh gave good yields (47–49%) of bleachable pulp at a reasonable alkali charge (22% as NaOH) and 0·13% AQ. The mechanical properties of the whole stem were best and it seems pulping to no rejects is most suitable for whole stem pulping. The soda-AQ pulping of usher with the same alkali level gave acceptable yields (43–46%), with mechanical properties at the lower end for tropical pulpwoods. Although the pulps looked well cooked it was difficult to determine their residual lignin due to the presence of unknown phenolics. The CMP gave a high-yield of pulp (73%), with good properties, suitable for paperboard and corrugated medium. The brightening trials, with hydrogen peroxide, of CMP did not much improve the brightness. Addition of guar gum (0·4% on oven-dry pulp) during beating or treatment of refined pulp with the enzyme Lyftase 40 A (0·4% on oven-dry pulp) gave better mechanical properties at a freeness similar to the control.