Raising the efficiency of black liquor lignin extraction

The aim of this report was to study possibilities of making the process of extracting lignin from black liquor at pulp and paper mills more efficient in the form of a literature study. Lignin is a commodity that is likely to become more valuable in the future due to work being done by a number of companies in utilizing it as for example a base for the production of renewable fuels and carbon fibre. At the current moment the extraction of lignin from black liquor is quite expensive so it is not yet common on an industrial scale thus it is important to find methods of reducing the costs. One of the more reliable methods of extracting lignin from black liquor is known as acid precipitation and involves utilizing mineral acids to lower the pH of the liquor and induce precipitation of solved lignin into a solid form. Normally carbon dioxide is used in the initial precipitation step where the pH is lowered to 8-9 and then sulfuric acid is added reducing the pH of the lignin further to 2.5-3. Introducing sulfuric acid into the process however will affect the sodium-sulfur balance in the mill and excess sulfur must be purged to avoid reducing the quality of the pulp. Sulfur is commonly purged in the form of dust from the electrostatic precipitator with this dust also containing sodium compounds. This will in turn reduce the sodium content in the mill calling for the demand of make-up sodium which costs quite a bit more than sulfuric acid. By choosing to utilize pre-existing process streams of sufur and carbon dioxide at the mill for use in acid precipitation one can potentially avoid additional chemical costs and avoid upsetting the sulfur balance. Thus the primary goal of this project has been to study the possibility of utilizing carbon dioxide at the mill for acid precipitation and determining a method of separating sulfuric compounds from black liquor and converting them into sulfuric acid. It was found that the required carbon dioxide could be supplied from flue gases
Today it is of great interest to lessen our dependency on fossil fuels. One product of untapped potential is lignin which has already sparked a great amount of interest as a base for the production of renewable fuels, in addition pure lignin has the potential to be used as a base for the production of carbon fibre. One noteworthy source of lignin that is not commonly used yet is in the pulp and paper industry. During the pulping process where cellulose fibres are separated from wood a liquid chemical by-product is formed which is known as black liquor. One major component of black liquor is lignin, this lignin in particular is unique as it is in a solved state and cannot be separated efficiently under normal conditions. By reducing the pH of black liquor through exposure to carbon dioxide and sulfuric acid however it is possible for the lignin to form a solid phase. Through filtration and washing it is possible to produce a solid lignin product that can then be utilized.