PT92341A - PROCESS AND APPARATUS FOR THE TREATMENT OF SUSPENSION OF CALCIUM CARBONATE AS A WASTE OF PAPER MANUFACTURE - Google Patents

Abstract

In a method and using an apparatus for carrying out the method for the treatment of calcium carbonate slurry ("mesa"), which is a re-utilizable residue from the causticisation of sodium carbonate with burnt lime in the manufacture of paper after the sulphate process, the lime sludge or "mesa cake" is conditioned in a mixer (21) with dry, powderous raw material (CaCO3) to a loose, moist mesa pulp which is fed to the lower end of a riser pipe (17) constructed as a flash duct wherein the mesa pulp is suspended, dried and pulverized by means of hot smoke gas from a kiln (1), whereupon the dry, powderous product is separated from the smoke gas in a separator (18) and fed to the kiln (1) for further treatment in same.

Description

-2- 70 225 41100

LL / LN

The present invention relates to a process and an apparatus for the treatment of calcium carbonate slurry ('table') as the caustic residue of green liquor (green 1: 1 color NaoC0.J with quicklime ( CaO) during the manufacture of paper pulp after the sulphate process. The quicklime (CaO) used in the causticisation results from the treatment of a limestone in a furnace usually a rotary kiln. The caustication produces a solution of (NaOH) or white liquor which is used for boiling the wood pulp from which the paper pulp is produced, and a product containing fine particles of calcium carbonate (CaCOâ,ƒ). The residue is washed and filtered off the fine particles of GaCO2 in the form of a filter cake or " table cake " having a typical water content of 30-35% " table cake " is thus fed to a rotary kiln , where it is burned to form r CaO which is reused in the caustication · This cycle is generically referred to as the lime cycle in the sulphate process ·

In a known process and customarily employed to quench the " table cake " the uppermost end of a rotary kiln is provided with a number of chains suspended inside the kiln which serve to promote heat transfer and crushing of the " table cake " added water in order to increase water withdrawal therefrom within the no. The product thus treated in the form of quicklime is removed from the furnace subsequently to be cooled in the cooler of the coupled material after the furnace. The furnace is fed with oil, gas or bark powder and during the treatment the quicklime reaches a temperature within the range of 1000-1200eC · Removal of the large amount of water present in the " through a drying process generates a relatively low outlet temperature of 150-2009C in the flue gases thus reducing stacking to a minimum · The efficiency of the furnace, i.e. the heat consumption during drying By evaporation of water or the calcination of the treated product is quite high and in total the heat economy of the furnace can be said to be quite satisfactory for the purpose. -3- 70 225 41100

LL / LN

In spite of an improved technique for the production and measurement of " table cake " resulted in a percentage of low water in the furnace feed, the expected fuel savings were not met, the flue gas outlet temperatures having been increased and increasing the dust loss of the furnace. The latter is due to the chains spinning in the dry raw material and thus causing it to generate more dust which in turn increases the load imposed on the oven dedusting equipment · The oven reacts in the same way when forced beyond its normal production capacity: increases the temperature of the gases in the chimney and the increase of the dust loss causes that the fuel consumption per unit of quicklime produced increases. Thus, the furnace chain system defines the limitations of increasing the productivity of the furnace and similarly the same is associated with the inconvenience that the chains need to be renewed at regular intervals due to the wear thus determining not only increases in the furnaces. material costs, but also the stopping of the kiln and consequently production stoppages and also an indepen- dent increase in total operating costs. In an alternative known process for further burning " table " the chain system of the rotary kiln is replaced by a dry hammer, for example, of the hammer mill type, which is mounted before the kiln - when viewed in the direction of the feed of the prime matter - thus allowing an increased production for a given Oven size · In such an apparatus the " table cake " wetting agent is often lumpy fed to the seamer together with the hot smoke from the rotary kiln is ground and dried in the dry jackhammer driven to the air dryer and suspended therein as a powder for a separating cyclone therein separated from the smoke, and fed to the rotary kiln as dry raw material. However such a drying apparatus, which is also known in other contexts, for example EP 0041115 is associated with other major drawbacks. The addition of an extra apparatus, such as dry-mixer, which is to be sized to withstand temperatures of up to 700 ° C, assumes the use of. 70 225 41100

The loss of dust is relatively high, the cyclone separator being unable to separate the relatively large portion of the fine fraction resulting from the crushing of the "table cake" in the dry-type jackhammer, and being the ducts connecting the rotary kiln and the dry presser exposed to cakes because the dust from the rotary kiln becomes viscous at temperatures within the range of 600-700 ° C which is the normal temperature range at the exhaust gas. Finally, in use the dry-mixer absorbs a certain amount of false air inside the system and generates a relatively large pressure loss in the equipment itself thereby increasing the energy consumption of the apparatus.

Accordingly, the object of the present invention is to provide a process and apparatus for remedying the aforementioned drawbacks of the art heretofore known. The object is achieved by a process for the treatment of calcium carbonate suspension ("table"), wherein the "table cake" is mixed with an appropriate amount of pulverulent pre-dried raw material (CaCO3) a typical but non-limiting amount being 5 ~ 2θ in a blender to form a loose wettable pulp and subsequently subjected to evaporation drying in suspension using hot furnace smoke gas thereby forming a dry powdery feedstock product (CaCO)) which is fed into the furnace for further heat treatment, subsequent to being separated from the flue gas.

According to a preferred embodiment of the process the drying and pre-heating of the wet and powdery table pulp is carried out in two steps; the pulp is subjected to initial drying in suspension using hot smoke gas at about 500--600 ° C in a rapid evaporation conduit in the form of a lifting tube and then after separation of the smoke gas from the thus dried product is suspended in the tube which connects the furnace and its cyclone separator for preheating to 500-600 ° C before the dry powder product is again separated from the flue gas and fed into the furnace for further treatment therein. 70 225 41100 LL / LN _5_

Further the object is achieved with an apparatus for carrying out a aforesaid process, characterized in that it comprises a lifting tube constructed for rapid drying of an added wet feedstock ("table") which is suspended in the hot furnace smoke gas in the lifting tube (quick-drying) and which - when viewed in the direction of the movement of the smoke gas - is mounted immediately after a rotary kiln, a separator mounted after the lifting tube for separating the dry powder product from the smoke gas, a dividing door ra to convey a separate product undercurrent through a tube for the entrance of the furnace and for transporting a further minor undercurrent through a tube for a mixing mixture to mix with the added raw material in the form of a cake of lime mud filter (" table cake ") containing water and means for feeding the pulp of " table " moisture produced in the mixer to the lower end of the lift tube.

According to a preferred embodiment of the apparatus, the drying and preheating are carried out in two steps: a first step comprising a lifting tube constituted as a quick drying duct, mounted after a cyclone S is mounted immediately after the furnace rotating a material inlet in the lift tube and a separating cyclone mounted after the lift tube; and a second step comprising the hoisting tube connecting the furnace to its separating cyclone an inflow of material in the hoisting tube to the resultant powdery dry stool material resulting from the cyclone of the first layer, and the separate cyclone mounted immediately after oven.

Thus, when compared to the prior art, it is characterized by moving mechanical parts or units such as the chain system or the dry-mixer, which are both very wearable and maintenance-demanding, are replaced by a stationary construction by far simpler: a riser tube in the form of a rapid evaporation duct for instant suspension drying and spraying of a table pulp " wet. This allows for increased production in a kiln of a given length, a reduction of the specific fuel consumption in the kiln and a reduction of dust from the plant. The invention will now be described in more detail with reference which are partially diagrammatic examples of the prior art technique and partly of wind embodiments, in which: figure 1 shows a known and commonly used rotary kiln installation with an inner runner system, figure Figure 2 shows another known rotary kiln installation on the front on which a dry crusher is mounted, Figure 3 depicts an embodiment of the invention comprising drying and preheating the table pulp " in one step, and Figure 4 represents a further embodiment of the invention, comprising suspension drying and two step preheating ·

In the Figures, like reference numerals are used for the identical components of the apparatus. The known apparatus shown in Figure 1 has a rotary kiln 1 with a cooler, here a planetary cooler 2 for cooling the product treated in the kiln, an outlet 3, an oil burner, gas or tree bark powder and a smoke chamber 5 · By means of a lifting tube 10 the furnace smoke gas is led, via a fan 6, to a smoke gas filter 7 after which the clean smoke is fed to a chimney 8. The lime mud or "table cake" is fed directly to the oven through an inlet 9 and dried and crushed at the upper end of the oven by means of a stream system 11 mounted inside the fire prior to further heat treatment in the oven

In the other known apparatus shown in Figure 2, the chain system inside the oven has been replaced by a dry wiper 12, for example of the hammer mill type, assembled before the kiln 1 · dry air in the form of hot furnace smoke gas, enters the dry jackhammer through the pipe 10 to which the lime mud is fed, or " table " at 9 and crushed and suspended in the drying air by the dry crusher, whereupon the suspension through a lifting tube 13 and fed to a 70 720 41100

LL / LN separator 14, here a cyclone separator, in which the pulverulent dry feedstock is separated from the smoke gas and through a tube 15 fed the furnace 1 for further treatment thereof. From the separator 14 the smoke gas is conducted to the carbon gas filter 7 and to the chimney 8. If necessary a mixer (not shown) is used to mix the " table top " with solids, may here be connected to the inlet 9 to facilitate the feed of the feedstock into the dry brittle

In the embodiment of the invention shown in figure 3 the lime mud, or " table cake " is fed to an inlet 9 of a mixer 21 in which the " table cake " containing pulp is charged with powdered dry feedstock which is fed into the mixer from the cyclone separator 18 via the pipe 19, the partition door 20 and the pipe branch 191 1. The table pulp " in the mixer is fed to the inlet 9 'of the lower part of a lifting tube 10 constructed as a rapid evaporating tube 17 and in direct communication with the smoking chamber 5 of the rotary kiln and is suspended, dried and pulverized in the furnace smoke gas. In the cyclone separator 18, the dry powder material is separated from the smoke gas and through the tube 19 the partition door 20 and a branch pipe 19 'thereof is fed to the furnace 1 for further treatment. A minor undercurrent of the separated dry pulverulent feedstock in the separating cyclone 18 is, as described above, fed through the pipe branch 19 'into the mixer 21. From the cyclone separator 18 the carbon gas is, via a outlet pipe 16, led to the filter 7 and to the chimney 8. The dust separated from the smoke gas is withdrawn through the outlets 22 and 221. If desired the dust can be supplied to the mixer 21 via a branch pipe 22 ' for additional conditioning of " table cake " in the mixer. Figure 4 shows a further embodiment of the invention having a two-step drying of the table pulp " and wetted from the mixer 21 In this embodiment the riser pipe, " rapid evaporator line "

LL / LN after a separator cyclone 18 fitted immediately after for for 1. Table pulp " and enters the lower end of the rapid evaporation tube 25 through an inlet 9 ', and is suspended dry and sprayed in the quick drying tube after which the pulverulent feedstock is dried and separated from the carbon gas in the cyclone separator 26 and through the partition door 24 and the conduit 27 'fed to the lift tube 10 which connects the furnace 1 to the cyclone separator 18. The pulverulent feedstock is subjected to conditional preheating in the lift tube 10 and separated of the smoke gas in the cyclone 18 after which the raw material is fed into the furnace through the partition door 20, a minor part of the dried raw material being simultaneously fed through the tube branch 19 'to the mixer 21 to be conditioned on it.

A lift 23 drives the " table cake " conditioner from the mixer to the inlet 9 '. During the start-up phase of the apparatus, when the temperature in the lift tube 10 is not high enough for the additional preheating of the raw material, the raw material can be driven directly into the furnace inlet 5 through the partition door 24 and the tubes 27 "and 19 '. Further treatment of the smoke gas and filter dust after treatment in the separator cyclone 26 is identical to that described in connection with Figure 3. In both embodiments of the invention the consistency " table cake " before the cake is fed into the lift tube 17 or 25 can be regulated through the partition door 20 by changing the amount of dry raw material added to the condition of the " table cake " in the blender 21. Similarly, as compared to the prior art, both embodiments produce a greater specific production for a given rotary kiln size, a reduced radiation loss of the kiln produced per quicklime produced, a gas outlet temperature lower smoke and less dust loss all of which contribute to a reduction in heat consumption. The two-step process according to Figure 4 is superior to the one-step process shown in figure 3, yielding an optionally even greater specific production for a furnace 70 225 41100

LL / LN-9- of a given size and allowing adjustment by the partition door 2o to the lowest possible exhaust gas temperature for satisfactory drying and to be effected in the lift tube, thereby reducing the heat consumption even more specific.

Claims (2)

A process for the production of lime sludge ("table") as the residue of caustication of green liquor (NagCO) with lime (GaO) ) in the manufacture of paper pulp according to the sulphate process and wherein the quicklime (CaO) used for the caustlcation results from the treatment of a calcareous raw material in a furnace and wherein the caustlcation partially produces a solution of sodium hydroxide (white liquor), partially a calcium carbonate residue containing the latter fine CaCO particles which are separated from the residue to form a " table cake " to be fed to the kiln for the fresh production of quicklime, characterized in that a quantity of powdery pre-dried raw material (CaCO)) is added to " table cake " in a blender to form a loose, moist pulp and the " table cake " is subjected to rapid drying in suspension with the furnace fumes, thus forming without any further mechanical treatment a dry and pulverulent raw material product (CaCO3) which is, after having been separated from the flue gas in a separator, fed to the furnace for further heat treatment therein. 2. A process according to claim 1, characterized in that the amount of powdery dry feedstock, with which the " tabletop cake " is mixed, constituting 50-50% of the total material and preferably 5-20% thereof. A process according to claim 1, characterized in that the drying and preheating of the table pulp " loose and wet in two steps; the pulp being initially dried in hot carbon gas suspension in a lifting tube constructed with a rapid evaporation conduit, and after the product so dried has been separated from the flue gas, it is again resuspended in the gas in the raising tube by ligating the furnace and its cyclone separator for further heating before the dry and pulverulent product is again separated from the flue gas and fed into the furnace for further treatment therein. 2. An apparatus for carrying out the process according to claim 1, characterized in that it comprises a riser pipe (17) constructed for rapid drying of an added wet particulate material (" table ") which is suspended in the flue gas (quick drying) and which - as seen in the direction of the movement of the smoke gas - is mounted immediately after a rotary kiln (1), a separator (18) positioned immediately after the lifting tube (17) for separating the dry and pulverulent product from the smoke gas 'a dividing door (20) for conveying an undercurrent of the separated product through a tube (19') for the entrance of the furnace and for transporting another undercurrent through from a tube 19M to the mixer 21 to mix the latter with added raw material in the form of lime mud containing water and means 9 'for transporting the powder of "; loose table produced in the mixer to the lower end of the hoisting tube (17). The apparatus of claim 4, having means for carrying out in two steps the rapid drying and the preheating of the added wet and moist table pulp , characterized in that it comprises in a drying step comprising a lifting tube (25) constructed as a quick dryer and mounted after a cyclone separator (18) which is mounted immediately after the rotary kiln (1), a material inlet (91) in the hoisting tube 25 and a separator cyclone 26 mounted immediately after the hoisting tube and in a pre-drying step comprising the hoisting tube 10 connecting to the rotary kiln 1 and its separating cyclone 18) a material inlet (27 ') in the lift tube for separate powder material and a separator cyclone (18) mounted immediately after the furnace. 0. Lisbon, By P.L. SMIDTH & Co- A / S