APPARATUS AND METHOD OF TREATING BLACK LIQUOR

Abstract: An apparatus and method of drying black liquor to a dry solids content of about 100 % in which water is evaporated from the black liquor by direct heat exchange contact with hot smelt from a recovery furnace in which the dried black liquor is burned.

Description: APPARATUS AND METHOD OF TREATING BLACK LIQUOR FIELD OF THE INVENTION The present invention relates to an apparatus and a process for the treatment of spent pulping liquor known as black liquor. More particularly the present invention relates to a process for the treatment of black liquor to achieve a high dry solids content of the black liquor before it is introduced into a recovery furnace. BACKGROUND OF THE INVENTION It is customary in the pulp and paper industry when producing chemical pulp to burn the used pulping liquor, i.e. the black liquor in a chemical and heat recovery furnace. In a conventional recovery furnace the process chemical is recovered by injecting black liquor at a dry solids content of about 65% into the furnace. As the liquor enters the furnace, it rapidly dries and burns under reducing conditions above the hearth whereby a smelt is formed which is discharged from the furnace. Oxygen is added in form of secondary air to burn the gases generated at a higher location in the furnace. The furnace is provided with heat exchange surfaces to recover heat and generate steam. It is desirable that as much water as possible is removed from the black liquor before it is introduced into the recovery furnace as evaporation of the residual water of the black liquor in the furnace causes cooling and slowing of the combustion process. This involves, however, difficult handling problems. Black liquor goes into a semi-solid state at a dry solids content of about 60% and at about 80% DS it has a viscosity exceeding 500 cP at 100°C. It is the prime object of the present invention to provide a method for concentrating black liquor to a high dry solids content, preferably to about 100%. Earlier attempts to reduce the water content of the black liquor which is introduced into the recovery furnace have been made. U.S. Patent 4,347,220 discloses a method in which sawdust is mixed with partially concentrated black liquor and dried by heated air. It has also been proposed to evaporate water from black liquor by mixing the black liquor which has been previously concentrated by conventional evaporating methods with superheated steam under pressure. The released vapor is removed, heated and recycled. When the water content of the black liquor has been reduced by this treatment 0 - 20%, the black liquor is exhausted from the apparatus through a nozzle, whereby, as the pressure decreases, the remaining water vaporizes and a particulate, dry and non-sticky substance which can be burned in a furnace is produced. Such a method is described in the Swedish patent specification 119215. DISCLOSURE OF THE INVENTION According to the present invention smelt from the recovery furnace is used to evaporate water from partially concentrated black liquor. The black liquor is preferably dried to about 100% so as to form a dry, non-sticky, particulate material which can be easily handled. The dry black liquor can, if desired, be formed into pellets. No auxiliary material such as sawdust is needed. The invention is described in more detail in the following with reference to the accompanying drawing, in which a preferred embodiment of the invention is illustrated. BRIEF DESCRIPTION OF THE DRAWINGS The drawings shows schematically a system for carrying out the process according to the invention. DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENT Black liquor which has been concentrated in conventional evaporators by indirect heat exchange with steam to a dry solids content of about 75% is supplied through a line 2 to a vessel 4. Hot smelt which runs out of a boiler 6 is supplied through a line 8 to the same vessel. The smelt and the liquor are conducted to rotating discs 10 and 11, disposed in the upper part of the vessel. By the rotational movement of the discs droplets are produced, which are thrown out and fall down through the vessel mixed with each other. By the time the droplets reach the bottom of the vessel, the sensible heat in the smelt has evaporated the water from the black liquor. The vapor generated thereby is removed through a line 12 connected to the top of the vessel and is used in the evaporators. The dry material which collects in the bottom part of the vessels in the form of pebbles is removed by a screw 14 and conveyed by an air stream produced by a fan 16 to a cyclone separator 18 through a line 20. Air which has been preheated by contact with the dried black liquor and separated from it, is removed from the cyclone through a line 22 connected to the top of the cyclone and introduced into the boiler as secondary air. The particulate material is removed at the bottom of the cyclone through a line 24 and introduced into the boiler by means of primary air supplied through a line 26. The dried black liquor is burned in the boiler is a manner known per se whereby part of the smelt so produced is used to dry the black liquor and part of the smelt is removed from the system to recover pulping chemicals. Example: Flow of black liquor at 75% DS 2 000 kg/ton pulp Water to be evaporated 500 kg/ton pulp Heat required 270 000 kcal/ton pulp Smelt required at a temperature of 900°C and having a specific heat of 0.3 1 200 kg/ton pulp Smelt produced 550 kg/ton pulp Total smelt flow 1 750 kg/ton pulp It should be understood that the preferred embodiments and example desired above are for illustrative purposes only and are not to be construed as limiting the scope of the invention which is properly delineated in the appended claims.

Claims: What is Claimed is: 1. A method of treating black liquor, characterized in that: a) black liquor is concentrated by evaporation by direct heat exchange contact with smelt discharged from a recovery furnace; b) concentrated black liquor is fed to said recovery furnace; c) said concentrated black liquor is burned in said recovery furnace to produce a smelt which is used in step a). 2. The method of claim 1, characterized in that the black liquor is concentrated to a dry solids content of about 100%. 3. The method of claim 1, characterized in that the black liquor has been concentrated to a dry solids content of about 60-80%, before it is brought into contact with the smelt. 4. The method of claim 3, characterized in that the vapor generated by concentrating the black liquor is used as a heating medium for concentrating said black liquor to a dry solids content of about 60-80%. 5. The method of claim 1, further characterized in that: a) the concentrated black liquor is fed to a particle separator by means of an air flow; b) air and concentrated black liquor are removed separately from the particle separator; c) the concentrated black liquor and primary air are introduced in the recovery furnace; and d) the air removed from the particle separator is introduced as secondary air at a location above that of the primary air. 6. A method of treating black liquor, characterized in that: a) black liquor is fed into a recovery furnace; b) said black liquor is burned in said recovery ' 5 furnace to produce a smelt; c) said smelt is discharged from said furnace; d) said smelt from said furnace and black liquor are introduced into a reaction zone ; and e) water is evaporated from said black liquor 10 introduced into said reaction zone by direct heat exchange with said smelt discharged from said recovery furnace thereby concentrating said black liquor. 7. The method of claim 6, further characterized in that: 15 said concentrated black liquor is discharged from said reaction zone; and said concentrated black liquor is introduced into said recovery furnace. 20 8. The method of claim 1, characterized in that the black liquor has been concentrated to a dry solids content of about 70-75%, before it is brought into contact with the smelt. 25 9. The method of claim 3, characterized in that the vapor generated by concentrating the black liquor is used as a heating medium for concentrating said black liquor to a dry solids content of about 70-75%. 30 10. An apparatus for concentrating black liquor, characterized in that it comprises: a) means for combusting black liquor to produce a smelt; b) a primary air inlet connected to said 35 combustion means; c) a discharge port for said smelt connected to said combustion means; d) evaporator means; e) an inlet port for black liquor and an inlet port for said smelt discharged from said combustion means, said inlet ports being connected to said evaporator means; and f) an outlet port connected to said evaporator means for removal of black liquor therefrom. 11. The apparatus of claim 10, characterized in that it further comprises a cyclone; a conduit connecting the bottom of said cyclone with said primary air inlet; a conduit connecting the top of said cyclone with said combustion means above the level of said primary air inlet; and an inlet port connected to said cyclone for the introduction of black liquor. 12. The apparatus of claim 11, characterized in that said outlet port of said evaporator means is connected to said cyclone inlet port for said black liquor.

Inventors: RYHAM ROLF, RYHAM, ROLF
Applicants: AHLSTROEM OY, A. AHLSTROM CORPORATION
Application year: 1989
Main classification: D21C11
IPC codes: C02F1, D21C11