GB922774A - Utilisation of spent digestion liquors obtained in cellulose production - Google Patents

Abstract

Organic compounds present in spent digestion liquors obtained in the production of cellulose from cellulosic materials are partially oxidized <PICT:0922774/III/1> together with a combustible gas and the inorganic compounds, e.g. Na2CO3 with NaCl and NaOH, present in the gas produced are recovered by washing with water and are used to prepare fresh digestion liquor which may be recycled to the digestion vessel together with a proportion of spent liquor, while the washed gas is used as a fuel. The organic compounds may be lignins and the cellulosic material may be wood, straw or bagasse. In Fig. 1, beechwood is comminuted in 1, fed to digestion vessel 2 where the lignin is dissolved with a liquor containing Na2CO3. The treated cellulose is is fed to 3 where it is washed with water. From 2 the spent digestion liquor is fed to vacuum evaporator 4 where it is concentrated to at least 50% solids and then may be fed to burner 5 as well as gas reaction chamber 6 at at least 700 DEG C., where it is injected in a finely divided state through nozzles. Burner 5 is fed with fuel oil and air. The gases from 6 are cooled and washed in 7 supplied with cooling water from plant 3 and the aqueous solution is fed to a causticizer 10, filter 11 and back to 2. A part of the cooling water from 7 may be recycled via a heat exchanger 12. While the gas leaves 7 for a cooling condenser 8 and steam boiler 9. The burner fuel may be gaseous or liquid hydrocarbons with sufficient O2 and/or air and/or steam to give a producer gas having a calorific value 1,000-3,000 Kcal./cm. and to make the oxidation reaction exothermal. The weight ratio air:hydrocarbon:spent liquor may be 3,0-6,0:1,0-2,0:0,2-1,0. The steam used for the oxidation may be derived from the concentration of the spent liquor to less than 50% water, 50-70% solids. 0,1-3,0% Ca compounds, e.g. Ca(OH)2, CaCO3, may be added to the spent liquor to facilitate the oxidation or reduce carbon deposition. CaO may be added to the gas wash water, to precipitate silicates, and further inorganic cellulose solubilizing agents may be added to the wash water prior to use as fresh digestion liquor.