Method for handling spent wash solution of a lignin-recovery process

Abstract

A method for handling spent wash solution produced in the washing of lignin that has been precipitated and then separated from spent alkaline pulping liquor. At least part of the spent wash solution is introduced into the chemical recovery cycle at a location in that part of the cycle which starts at, and includes, the smelt dissolving stage and ends at, and includes, the pulping stage. In preferred embodiments, input of other water and / or aqueous solution into the chemical recovery cycle after the recovery boiler is adjusted so that, as a consequence of the introduction of the spent wash solution, the amount of water in the white liquor going to the pulping line is not significantly increased. Among others, two preferred locations for the introduction of the spent wash solution are (1) the smelt dissolver and (2) the location where unwashed lime mud is diluted.

Description

TECHNICAL FIELD[0001]Lignin may be removed from spent alkaline pulping liquor (black liquor) by first acidifying the liquor and then separating the resulting lignin precipitate. The separated material is usually washed in order to obtain a solid product comprised primarily of lignin and having a reasonably low level of impurity. At least one washing stage is usually undertaken under acidic conditions. The present method deals with the handling of the spent wash solution which is typically in the form of one or more filtrates. The lignin product may be utilized as a fuel, as a material or as a chemical in a range of potential applications.PRIOR ART[0002]The most commonly applied method for recovering reasonably pure lignin from spent alkaline pulping liquor, commonly termed black liquor, involves the following Steps:[0003]1. Acidification of the liquor to a pH typically in the range 9-10 (value at 25° C.) using an acid such as carbon dioxide (CO2) or sulphuric acid (H2SO4)[0004]2. Se...

AI Technical Summary

Benefits of technology

The new method described in this patent allows for the reuse of part of the washing solution used in the preparation of lime mud. This solution does not contain any harmful impurities that could affect the quality of the final lime mud product. In addition, the use of this solution helps to reduce the amount of water used in the process. The technical benefit of this method is to provide an efficient and environmentally friendly solution for the recycling of washing solutions in the lime mud preparation process.

Problems solved by technology

The cost of evaporating water from the spent wash solution in the evaporation plant is significant.

The extra evaporation load resulting from the input of water in the fresh wash solution may restrict application of a lignin-recovery process.

Another detrimental aspect of the known method of handling the spent wash solution is that the costs and difficulties are in direct proportion to the amount of fresh wash solution employed in the washing process.

So, as demands on the purity of lignin product are tightened up, the detrimental aspects of the known method of handling the spent wash solution become more pronounced.

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