Recausticizing to Recover Sodium Hydroxide in Black Liquor from Biomass Deacetylation

Biomass deacetylation is an important process to break down industrial effluent into its components for alternative product recovery. Maximizing the efficiency of this process is central to techno-economic analysis; in alkaline deacetylation, black liquor production is economically limited by the cost of sodium hydroxide. Recausticizing was explored as a route to maximize sodium hydroxide recovery from black liquor. A conventional recausticizing reaction, standard practice in the pulping industry, was applied along two pretreatment pathways. Reagent concentration was varied in the use of calcium hydroxide in experimental samples and sodium acetate in control samples. Treatment methods were atmospheric reduction and high-temperature oxidation.

It was demonstrated that a model black liquor, 1M sodium acetate solution, was not effectively oxidized under standard recausticizing conditions, yielding on average 5% of available sodium in the form of sodium hydroxide after recausticizing. In addition, it was determined that the model standard recaustizing conditions (calcium hydroxide interaction at 90C for 30 minutes) yielded 16% of original sodium as titratable sodium hydroxide in black liquor. The atmospheric reduction of sodium acetate solution and black liquor with CO2 was determined not to have a significant effect on recausticizing yield. High-temperature oxidation in a Parr reactor showed the most promise as a means of liquor pretreatment, increasing sodium hydroxide yields by twofold where applied.