Acetic acid from black liquor

Acetates. Anhydrous iron(II) acetate [3094-87-9], Fe(C2H302)2, can be prepared by dissolving iron scraps or turnings in anhydrous acetic acid ( 2% acetic anhydride) under an inert atmosphere. It is a colorless compound that can be recrystallized from water to afford hydrated species. Iron(II) acetate is used in the preparation of dark shades of inks (qv) and dyes and is used as a mordant in dyeing (see Dyes AND DYE intermediates). An iron acetate salt [2140-52-5] that is a mixture of indefinite proportions of iron(II) and iron(III) can be obtained by concentration of the black liquors obtained by dissolution of scrap iron in acetic acid. It is used as a catalyst of acetylation and carbonylation reactions. 

The production of wood pulp from lignocellulosic materials by treatment with various chemical liquors, particularly the neutral sulfite semichemical process and the kraft or sulfate process, gives residual black liquors. These contain salts that carry acetic acid and formic acid equivalent to 5% or more of the dry weight of the wood. 

Liquors from Neutral Semichemical Pulping and from Kraft Pulping. In making neutral sulfite semichemical pulp, the black liquors may have about 10 parts of water to 1 part of total solids, of which about one-third is sodium acetate and sodium formate. After evaporation to about 1 part solids to 1 part water, sulfuric acid is added to the concentrate to free the acetic and formic acids. When the concentrate is extracted with acetone, the mixed acids are obtained, the acetone is separated for recycle, and the acids are concentrated and refined. The raffinate is stripped and is passed to the usual furnace to be burned for recovery of the inorganic salt values. This process gives a smelt of sodium sulfate, which may be used in the kraft process as chemical makeup. The loss of the fuel value of the acetic and formic acids is practically negligible. 

Hence, a part of the black liquors (10-50%) of a kraft plant may be processed to free, and subsequently to recover, the acetic acid as described by the addition of sulfuric acid and the formation of sodium sulfate. This new sodium sulfate from the sulfuric acid is present in the raffinate from the extractor it is thus the makeup for the balance of the liquors (50-90% ) that can be processed conventionally without recovery of the acetic and formic acids. The liquors from the raffinate are added to the other liquors before they go to the drier and the furnace. This part of the liquors (10-50%) recovers that much of the volatile acids that would otherwise be lost in the burner as salts. 

Biggs Jr WA, Wise JT, CookWR, Baxley WH, Robertson JD, Copenhaver JE. Commercial production of acetic and formic acids from NSSC black liquor. Tappi 1961 44 385-92. 

Lignin can be extracted from sugar cane bagasse by the acetosolv process with 93% acetic acid, 0.2% HCl, 6.8% water, (w/w) at 109 °C after 3 h pulping treatment. The black liquor is concentrated under reduced pressure and lignin is precipitated in water at 70 °C and purified by dissolution in acetone [148]. 

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