GB664811A - Improvements in the recovery of lignin from black liquor - Google Patents

Abstract

<PICT:0664811/IV (b)/1> Lignin material is obtained from black liquor resulting from the digestion of cellulosic fibres in an alkaline cooking liquor by carbonating the liquor with carbon dioxide-containing gases, accelerating the carbonation by providing continuously a large and constantly changing interface between liquid and gas, the area of liquid surface exposed at any time relative to the liquor volume in the carbonation vessel and the speed with which the liquid surface is changed in contact with the gas during carbonation in the vessel being such that the carbonation is rapidly completed, and while the carbonated liquor has a temperature above the melting point of the lignin material which is present as a distinct flowable viscous liquid layer, flowing the layer of lignin material from the liquor. The separated lignin material may be allowed to cool and solidify and, if desired, stored for subsequent processing such as washing in dilute mineral acid to give a gritty water-insoluble product, but preferably the lignin is redissolved in hot water and precipitated by adding acid. If a product of higher melting point is desired, it may be oxidized during carbonation or subsequently, e.g. by bubbling air through a solution of lignin. When the black liquor contains sodium sulphide, it is subjected to rapid oxidation with air prior to carbonation. In a cyclic system, including the digestion of wood chips in known manner, weak black liquor, which may be concentrated in a multiple effect evaporator 13, Fig. 1 (not shown), from tank 12 is passed through a separating tower 15 and successively through spray towers 16, 17, 18, wherein provision is made for recirculating a larger volume of liquor through the sprays than that entering tower 16 from tower 15. Flue gas from duct 20 is passed through the spray towers countercurrently to the black-liquor and into the top of tower 15 where it impinges on the surface of the liquor contained in the trays therein while passing downwardly through the tower and out to stack. Carbonated liquor is withdrawn continuously from tower 18 and passed through a heater 27 to a continuous separator 28 wherein acid precipitated lignin coagulates and settles to the bottom of the tank as a viscous liquid, the hydrostatic pressure of the accumulating aqueous layer in compartment 29 forcing the lignin to rise in compartment 30 and overflow through outlet 33. Similarly the aqueous component overflows through outlet 32 into tank 37, from which it is passed to an evaporator and sprayed into a furnace in conventional manner for recovery of sodium salts, gases of combustion being used for carbonation and the heat values therefrom being used to generate the steam required in the process. The separated lignin is then dissolved in hot water at 35 and passed into storage tank 40 from which it is withdrawn to 41 where lignin is precipitated by addition of sulphuric acid, steam being introduced if the temperature is too low. The change is then passed into a storage tank 43 and pumped to rotary vacuum filters 44 where the lignin is separated and washed, the primary filtrate being returned to separator 28 and the secondary filtrate to the sewer. The lignin from 44 may be further purified by washing with water and be used in the pasty condition or in powder form obtained after drying. The recovered sodium salts may be causticized and re-used for making the digestion liquor.ALSO:After removal of lignin material from black liquor, resulting from the digestion of cellulosic fibres in an alkaline cooking liquor, by rapidly carbonating the liquor and separating the molten lignin therefrom (see Group IV (b)), sodium salts are recovered from the resulting liquor in known manner by spraying the liquor into a furnace, dissolving the melt in water, causticizing with lime and decanting off the clear liquid.