Raising the efficiency of black liquor lignin extraction

Sammanfattning: 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 leaving both the lime kiln and recovery boiler. As the carbon dioxide needed must be of a high purity amine-based chemical absorption was the chosen method of separation. Unfortunatly this also introduced a high steam cost due to the rather low concentration of carbon dioxide in the process streams, this coupled with the reasonably low costs of purchasing carbon dioxide from an outside source lead to the feasibility of this method being questionable. From a number of separation methods it was found that the thermal treatment of black liquor was an efficient method of separating volatile organic sulfur compounds. The treatment itself involves exposing the black liquor to elevated temperatures during alkaline conditions leading the the formation of sulfuric off-gases. In addition the viscosity of the liquor is reducing causing additional benefits such as increased recovery boiler capacity. The conversion of the sulfuric compounds was done through the wet sulfuric acid process and a preliminary simulation in Aspen was done to clarify that the process was reasonable. From the results it was found that even in a worst-case scenario enough sulfuric acid could be produced to cover a theoretical 70 mass% extraction of lignin. The big question mark over the project was the thermal treatment of the liquor. If future work was done it should include studies on how high the heat demand for thermal treatment is and how varying parameters such as retention time and alkali charge effect off-gas formation.