Identification of extraction methods for the production of humic acids from black liquor
Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2006.
South Africa has a variety of sources for humic substances that may be utilised for the large-scale production of specialised fertilizers. Humic substances are dark coloured fully decomposed remains of plant and animal organic matter. Amongst the sources that were investigated in this study, a 5-weeks old compost (C5W), a 6-weeks old compost (C6W), a compost prepared by Medallion Mushroom farm for the production of mushrooms (CB1), a compost produced as waste material after mushroom production (CB2), recycled water used in compost (i.e. CB1) preparation for mushroom production (“Goodie water,” GW) and black liquor from pulp industry (BL), black liquor showed the best potential for the large scale production of humic acids in South Africa. The quantity and quality of humic acids in these materials were determined by using the basic acid-precipitation method and E6/E6 ratio analyses of the products. Extracting humic acids from black liquor was optimised by comparing three possible variations to the standard humic acid-precipitation method (HAp), namely, lignin extraction/humic acid precipitation method (LHAp), humic acid precipitation/acid treatment method (HApA) and lignin extraction/humic acid precipitation/acid treatment method (LHApA). The method of extraction had a substantial impact on the quantity and quality of humic acids produced. Humic acid precipitation method followed by humic acid precipitation/acid treatment method (HApA) demonstrated high quantities of humic acids compared to lignin extraction/humic acid precipitation method (LHAp) and lignin extraction/humic acid precipitation/acid treatment method (LHApA). Analyses such as Fourier Transform Infrared (FTIR) spectroscopy, Ultraviolet Visible Spectroscopy (UV-VIS), High Performance Liquid Chromatography (HPLC) and Elemental Analysis demonstrated that the quality of humic acids produced from black liquor was similar to those reported previously. The type of material (e.g. softwood, hardwood and bagasse) used for black liquor production significantly affected the elemental and molecular composition of humic acids produced. The quality of humic acids produced was affected by both lignin extraction and acid treatment. However, chemical analyses indicated that the features of the humic acids extracted from black liquor correlated with their lignin origin. Humic acids from hardwood and softwood black liquor had a low nitrogen and high sulphur content compared to those extracted from black liquor produced from bagasse material. Although chemical and instrumental analysis of humic acids was quite reasonable for all humic acids produced, LHAp method was favoured for the extraction of humic acids from pine liquor due to the high quantity of humic acids produced (low S and ash contents). The humic acids extracted by means of HAp method and HApA had a positive effect on plant growth, when extracted from bagasse and Eucalyptus black liquors. Humic acids extracted by HAp from pine liquor had an inhibitory effect on plant growth. A possible process flow for the large-scale production of humic acids from black liquor was formulated. The possible process flow indicated that with the amounts of black liquor produced from pulp industry production of humic acids in South Africa is a possibility. Nevertheless, there is still more that needs to be done to investigate the cost effectiveness of this production prior its implementation. HAp method was most suitable for the production of humic acids from Eucalyptus and bagasse liquors in terms of quantity and plant response but LHAp method was more suitable for the production of humic acids from pine liquor in terms of both quantity and quality.