GB869800A - Improvements in or relating to the recovery of alkali metal hydroxides and salts from the residual liquors from the production of cellulose - Google Patents

Abstract

Alkali metal hydroxides, carbonates, bicarbonates, or other alkali metal salts are produced from alkaline extraction liquors obtained in the production of cellulose by evaporating the undiluted liquors to a concentration of at least 320 grams of p organic matter per litre, adding an acidic substance, e.g. CO2, SO2 or H2S, preferably at 80-85 DEG C. until the isoelectric point of the solution is reached, whereby lignin and other organic matter, together with alkali metal carbonate, bicarbonate, or other alkali metal salt corresponding to the particular acidic substance employed, are precipitated, filtering off the precipitate, drying and igniting it to destroy the lignin and other organic matter leaving an ash consisting of alkali metal carbonate or other salt, and, if desired, causticising the ash to form an alkali metal hydroxide. The acidic substance may also be sulphuric acid. The ignition may be performed in a boiler arranged to make use of the heat of combustion. The organic matter may be mixed, before burning, with a 'black liquor' obtained by concentrating the undiluted liquor to a solids content of from 50 to 60% by volume. Alkali extraction wash liquor obtained by washing the cellulose with water may be mixed with a concentrated alkaline extraction liquor from which organic matter and other precipitated matter have been removed by filtration, the combined liquors treated with carbon dioxide and heated to 90 DEG C., the precipitated matter mixed with the first precipitated matter and burnt, the alkali metal carbonate obtained dissolved in the residual combined liquor, and the latter treated with quicklime to produce an alkali metal hydroxide solution. In treating alkali-extraction liquors obtained in the treatment of straw (containing silica), the carbon dioxide treatment is effected at 70-80 DEG C., whereby the silica is precipitated with the lignin. The precipitate is washed with a soda solution and the lignin dissolved out, leaving a silica residue. After concentration, the lignin may be recovered by a further CO2 treatment.ALSO:Alkali metal hydroxides, carbonates, bicarbonates, or other alkali metal salts are produced from alkaline extraction liquours obtained in the production of cellulose by evaporating the undiluted liquors to a concentration of at least 320 grams of organic matter per litre, adding an acidic substance, e.g. CO2, SO2 or H2S, preferably at 80-85 DEG C., until the isoelectric point of the solution is reached, whereby lignin and other organic matter, together with alkali metal carbonate, bicarbonate, or other alkali metal salt corresponding to the particular acidic substance employed, are precipitated, filtering off the precipitate, drying and igniting it to destroy the lignin and other organic matter leaving an ash consisting of alkali metal carbonate or other salt, and, if desired, causticising the ash to form an alkali metal hydroxide. The acidic substance may also be sulphuric acid. The ignition may be performed in a boiler arranged to make use of the heat of combustion. The organic matter may be mixed, before burning, with a 'black liquor' obtained by concentrating the undiluted liquor to a solids content of from 50 to 60% by volume. Alkali extraction wash liquor obtained by washing the cellulose with water may be mixed with a p concentrated alkaline extraction liquor from which organic matter and other precipitated matter have been removed by filtration, the combined liquors treated with carbon dioxide and heated to 90 DEG C., the precipitated matter mixed with the first precipitated matter and burnt, the alkali metal carbonate obtained dissolved in the residual combined liquor, and the latter treated with quicklime to produce an alkali metal hydroxide solution. In treating alkali-extraction liquors obtained in the treatment of straw (containing silica), the carbon dioxide treatment is effected at 70-80 DEG C., whereby the silica is precipitated with the lignin. The precipitate is washed with a soda solution and the lignin dissolved out, leaving a silica residue. After concentration, the lignin may be recovered by a further CO2-treatment.