Process Biochemistry 35 (2000) 685 – 691 Influence of oxygen bleaching variables of enzyme-treated soda pulp from wheat straw on the quality of black liquor L. Jime ´nez a *, I. Pe ´rez a , J.C. Garcı ´a a , F. Lo ´ pez b , J. Ariza b a Facultad de Ciencias, Departamento de Ingenierı ´a Quı ´mica, Uniersidad de Co ´rdoba, 14004 Cordoba, Spain b Departamento de Ingenierı ´a Quı ´mica, Uniersidad de Huela, Spain Received 19 May 1999; received in revised form 20 July 1999; accepted 26 September 1999 Abstract Black liquor from the oxygen bleaching of soda pulp from wheat straw pre-treated with the enzyme cartazyme was characterized. A central composite design was used to examine the influence of oxygen bleaching variables (temperature (T ), time (t ), and soda concentration (S )) on some properties of the black liquor (pH, chemical oxygen demand (COD), total carbon (TC), total organic carbon (TOC), total solids (TS), total volatile solids (TVS), total mineral solids (TMS), and colour). Equations that relate the dependent and to the independent variables were derived that predict the former with errors 20%. The least polluting black liquor (pH 7.86, COD 2248 mg/l, TC 671 mg/l, TOC 644 mg/l, TS 2609 mg/l, TVS 2188 mg/l, TMS 884 mg/l, colour 1151 colour units) was obtained by using a low T (90°C), a low S (0.5%) and a short t (15 min) in the oxygen bleaching process. Bleaching under these conditions resulted in pulp and paper sheets the brightness, stretch and double fold number of which were only 10, 13 and 6%, respectively, worse than those obtained under the optimum operating conditions (viz. 90°C, 1.5% soda and 15 min). On the other hand, the viscosity, breaking length and burst index were essentially the same and the yield and tear index were 3 and 2% higher, respectively, than under the optimum conditions. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Black liquor; Soda pulp; Oxygen bleaching; Bleaching variables; Black liquor properties www.elsevier.com/locate/procbio 1. Introduction Pulp obtained from non-wood raw materials ac- counts for only 5–7% of overall pulp production. The chief interest of this type of pulp is that it provides excellent fibres for making paper with special properties and that it is the sole available source of fibrous raw materials in some geographic areas. Non-wood materi- als are also significant because they can help offset the growing shortage of forest wood resources. The world production of this type of pulp has grown at a higher pace than that of wood pulp over the past two decades; in fact, non-wood pulp production has doubled in Latin America and trebled in Africa and the Middle East over this period. Production figures are bound to rise even further in the coming years in response to the increasing demand in highly populated countries that produce vast amounts of agricultural cropping residues and fast-growing plants [1]. This type of pulp is used as such or mixed with others from wood or old paper to obtain a variety of products including paper, board and other lignocellu- losic derivatives. Agricultural cropping residues are highly abundant in Spain, particularly in the southern part (Andalusia). Each year, Spain produces 16 million tons of residues such as cereal straw, olive tree wood, sunflower stalks, vine shoots and cotton stalks that have no useful application at present; about 32.5% of this residual mass is generated in Andalusia [2]. If this amount of potential raw materials was put to use, with an overall yield of 40–50% Spain could obtain four times as much paper and board as it currently does [3]. Spain produces large amounts of wheat straw, over 2 million tons [3], that lack specific application at present. Pulping and bleaching processes produce highly pollut- ing black liquors. A growing environmental concern * Corresponding author. Tel.: +34-957-21-8624; fax: +34-957-21- 8606. E-mail address: iq1jiall@uco.es (L. Jime ´nez) 0032-9592/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII:S0032-9592(99)00130-2