Precipitation and separation of lignin from kraft black liquor
The separation of lignin from black liquor is an attractive option to consider for pulp mills. In modern pulp mills, lignin extraction offers the opportunity of withdrawing an energy surplus in the form of a solid biofuel. The introduction of lignin separation can de-bottleneck pulp mills in which pulp production is limited by recovery boiler capacity. Separated lignin can be used within the mill, e.g. by replacing fuel oil in the lime kiln or fired in a power boiler, or externally e.g. in CHP plants. Today, only half of the wood material (i.e. the cellulose) is used for the production of valuable products. In a more futuristic perspective, separated lignin can be used as a raw material for the production of green chemicals and/or materials replacing oil as a raw material, thus increasing the value created from the wood material.
In this thesis, the extraction of lignin in pulp mills, in particular by precipitation from black liquor, was studied with the focus being on quantifying separation properties of the lignin material. After precipitation, the lignin is filtered and washed. Laboratory studies showed that precipitation and filtration is a rather straight-forward process if the right conditions in terms of pH and temperature are used during precipitation. While lignin precipitated from different softwood kraft black liquors taken from the evaporators showed comparable filtration properties, lignin from membrane concentrated black liquors proved to be more difficult to filter.
Direct washing of lignin filter cakes proved to be difficult; problems with plugging and uneven washing, in addition to large yield losses, were encountered. These problems were found to be caused by large gradients in pH and ionic strength in the lignin filter cake during washing. A novel washing method was proposed and developed to solve these problems. This process was investigated in laboratory, bench and pilot scale equipment with excellent results in terms of final purity of the lignin product and very low yield losses during separation. Filtration and washing characteristics were quantified and found to be similar in the three different scales that were studied. During the pilot scale trial, more than 8 tonnes of lignin could be produced using the novel separation method, with high levels of purity (<0.5 % w sodium), dry solids content (>60%) and calorific value (LHV 25.4 MJ/kg).