Utilization of wood-decay fungi for biokraft pulping of softwood

[摘要] English: The forest products industry is one of the most important earners of foreignexchange for South Africa. The major focus of the industry is the production of pulpwith an annual capacity of 2,4 million tons. Wood from plantations of exotic trees isthe most important source of fibre, but other fibre sources are also used.Biotechnology can play a significant role in the industry to produce high valueproducts at lower cost and could reduce the environmental impact of conventionalprocesses. Biopulping is potentially the most important of these biotechnologicalprocesses, because it can influence all downstream operations. The aim of this studywas, therefore, to develop a biopulping process for the treatment of softwood at theSappi Ngodwana kraft mill in Mpumalanga.Initially, 278 strains of wood-decay fungi were collected from various naturalhabitats. This collection represents a diversity of fungal families and included speciesthat have not previously been recorded from South Africa. The first step in selectingsuitable fungal strains for biopulping was to characterize different groups on the basisof the enzymes that they produce and their oxidase reactions. The suitability of thesestrains for the pre-treatment of softwood chips prior to kraft pulping was subsequentlyassessed by evaluating their influence on kappa number, yield and strength propertiesof pulp. Seven of the strains tested were able to reduce the kappa number of pulpsignificantly, without having a significant influence on the pulp yield. These strainswere more efficient than strains of Phanerochaete chrysosporium and Ceriporiopsissubvermispora that have been patented for other biopulping applications. Treatmentof wood with strains of Peniophora sp., an unidentified specie as well as two strainsof Stereum hirsutum resulted in pulp with improved strength properties.The envisaged biopulping process aimed at treating wood chips in outside chipstorage with a biopulping fungus. The aim of one study was to investigate conditionssuch as temperature, moisture, CO2 and microbial populations that develop in a chippile, and to determine the suitability of the chip pile for colonization by biopulpingfungi. High temperatures and high moisture levels were observed in some areas ofthe chip pile, which suggested that part of the pile was unsuitable for colonization bymesophilic fungi. It will, therefore, be necessary to manage the chip pile to maintaina suitable environment for biopulping.Problems were experienced with poor colonization of freshly chippedsoftwood by biopulping fungi. The effect of contaminating microbes and inhibitorycompounds present in wood was, therefore, studied. It was found that the inhibitionof biopulping fungi by α-pinene and by contaminating microbes were both veryimportant. The inhibition by microbes, as well as by extractives, was mitigated by ashort steam treatment of wood chips. Steaming for ten minutes under atmosphericpressure could be an economical method to improve colonization by biopulping fungi.Alternatively biopulping fungi with good competitive ability and tolerance tomonoterpenes could be selected.Pinus patula wood chips were pre-treated with a selected strain of Stereumhirsutum to determine the optimal conditions for the kraft pulping of pre-treatedsoftwood and to do an economic evaluation of the process. Chips were pulped on asmall scale under various pulping conditions. Lignin content, yield, and viscosity ofthe pulp as well as alkali consumption were evaluated. The results were used todevelop models for biokraft pulping. This study showed that biopulping can reducethe kappa number of pulp or reduce the pulping time. Pulp yield losses wererelatively small when pulps with low kappa number were produced. Increased alkaliconsumption was, however, an important factor in the economic evaluation.

[发布机构] University of the Free State