METHOD AND APPARATUS FOR THE TREATMENT OF CALCIUM CARBONATE SLURRY AS RESIDUE FROM THE MANUFACTURE OF PAPER

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 separa­ tor (18) and fed to the kiln (1) for further treatment in same.

Description: The invention relates to a method and an apparatus for the treatment of calcium carbonate slurry ("mesa") as residue from the causticisation of green liquor (Na₂CO₃) with burnt lime (CaO) during the manufacture of paper pulp after the sulphate process. The burnt lime (CaO) used in the causticisation results from the treatment of a calcareous raw material in a kiln, usually a rotary kiln. Causticisation produces a sodium hydroxide solution (NaOH) or white liquor, which is used for the boiling of the wood pulp from which the paper pulp is produced, and a residue product containing fine calcium carbonate (CaCO₃) particles. The residue is washed and filtered thus separating the fine CaCO₃-particles in the form of a filter cake or "mesa cake" having a typical water content of 30-35%. The mesa cake is then fed to the rotary kiln where it is burnt to form CaO which is re-utilized in the causticisation. This cycle is generally referred to as the lime cycle of the sulphate process. In a known and commonly employed method for the reburning of mesa cake the uppermost end of a rotary kiln is provided with a number of chains suspended inside the kiln serving to promote the heat transfer and the crushing of added wet mesa cake so as to enhance the removal of water from same within the kiln. The product thus treated in the form of burnt lime is removed from the kiln subsequent to being cooled off in the material cooler coupled after the kiln. The kiln is fuelled with oil, gas or bark powder and during treatment the burnt lime reaches a temperature within the range of 1000-1200°C. The removal of the large amount of water present in the mesa cake through a drying process generates a relatively low exit temperature of 150-200°C in the flue gases thus reducing the stack loss to a minimum. The efficiency of the kiln, i.e. the calorific consumption during the drying away of water or the calcining of the treated product, is quite high and, on the whole, the heat economy of the kiln may be said to be quite satisfactory for the purpose. Although an improved technique for the production and dewatering of mesa cake has resulted in a lower percentage of water in the kiln feed, the expected fuel economies have not been fulfilled, the flue gas exit temperatures having been increased and the dust loss of the kiln being augmented. The latter is due to the chains whirling up the dry raw material and thus causing it to generate more dust, which, in turn, increases the load imposed on the dedusting equipment of the kiln. The kiln reacts in the same way when forced beyond its normal production capacity: rising flue gas temperature and augmented dust loss cause the fuel consumption per unit burnt lime produced to increase. Thus, the kiln chain system clearly defines the limitations of increasing kiln productivity and, similarly, it is associated with the inconvenience that the chains need renewal at regular intervals due to wear, thus entailing not only increases in the material costs proper but also closing down of the kiln and consequently production halts and hence an undesirable increase in the total operational costs. In an alternative known method for reburning of mesa the rotary kiln chain system is replaced by a dryer crusher, e.g. of the hammer mill-type, which is mounted before the kiln - as seen in the movement direction of the raw material feed - thus permitting an increased production for a given kiln size. In such an apparatus the sticky, frequently lumpy, wet mesa cake fed to the dryer crusher together with hot smoke from the rotary kiln, is ground and dried in the dryer crusher and conveyed by the dryer air and suspended therein as a powder to a separator cyclone, and separated therein from the smoke and fed to the rotary kiln as dry raw material. However, such drying apparatus, which is also known in other contexts, see e.g. EP-A-0 041 113, is associated with other major drawbacks. The adding of an extra apparatus such as the dryer crusher, which is to be dimensioned to endure temperatures of up to 700°C, presupposes the use of special steel and the apparatus requires maintenance. The dust loss is relatively high, the separator cyclone being unable to separate the relatively large portion of fine fraction resulting from the crushing of the mesa cake in the dryer crusher, and the ducts connecting the rotary kiln and the dryer crusher being exposed to cakings, due to the dust from the rotary kiln becoming sticky at temperatures within the range of 600-700°C which is the normal temperature range at the smoke gas outlet. Finally, in use the dryer crusher takes a certain amount of false air into the system and generates a relatively great pressure loss over the equipment proper thus increasing the power consumption of the apparatus. Therefore, it is the object of the invention to provide a method and an apparatus to remedy the above-mentioned drawbacks of the hitherto known technique. The object is achieved through a method for the treatment of calcium carbonate slurry ("mesa"), wherein the mesa cake is admixed with a suitable amount of pre-dried, powderous raw material (CaCO₃), a typical but non-limiting amount being 5-20%, in a mixer in order to form a loose, moist mesa pulp and subsequently subjected to flash drying in suspension using hot kiln smoke gas thus forming a dry, powderous raw material product (CaCO₃) which is fed to the kiln for further heat treatment subsequent to having been separated from the smoke gas. According to a preferred embodiment of the method, drying and pre-heating of the powderous, moist mesa pulp is effected in two steps; the pulp is subjected to initial drying in suspension using hot smoke gas of about 500-600°C in a flash duct in the form of a riser pipe and then, after separation from the smoke gas of the product thus dried, is suspended in the riser pipe connecting the kiln and its separator cyclone for pre-heating up to 500-600°C prior to the dried, powderous product being again separated from the smoke gas and fed to the kiln for further treatment in same. Moreover, the object is achieved with an apparatus for carrying out the above method, characterized in comprising a riser pipe constructed for flash drying of an added, moist raw material ("mesa") which is suspended in hot kiln smoke gas in the riser pipe (flash drying) and which - as seen in the smoke gas flow direction - is mounted immediately after a rotary kiln, a separator mounted after the riser pipe for the separation of dry, powderous product as residue from the smoke gas, a splitting gate for conveying a substream of the separated product through a pipe towards the kiln inlet and for conveying another minor substream through a pipe to a mixer for mixing with added raw material in the form of a filter cake of lime sludge (water-containing "mesa cake"), and means for feeding the loose, moist mesa pulp produced in the mixer to the lowermost end of the riser pipe. According to a preferred embodiment of the apparatus the drying and the preheating are effected in two steps; a first step comprising a riser pipe constructed as a flash duct mounted after a separator cyclone mounted immediately after the rotary kiln, a material inlet in the riser pipe and a separator cyclone mounted after the riser pipe; and a second step comprising the riser pipe connecting the kiln and its separator cyclone, a material inlet in the riser pipe for the separated dry, powderous material resulting from the first step cyclone, and the separated cyclone mounted immediately after the kiln. Thus, as compared to the prior art technique the invention is characterized in that the movable mechanical parts or units, such as the chain system or the dryer crusher, which are both very wear- and maintenance-demanding, are replaced by a far more simple stationary construction: a riser pipe in the form of a flash duct for instantaneous drying in suspension and pulverizing of a loose, moist mesa pulp. This enables an increased production in a kiln of a given length, a reduction of the specific fuel consumption of the kiln and a reduction of the dust loss from the plant. The invention will now be described more in detail with reference to the drawings, which are diagrammatical examples partly of the prior art technique and partly of embodiments of the invention, wherein Figure 1 shows a known and commonly used rotary kiln plant with an interior chain system, Figure 2 shows another known rotary kiln plant at the front of which a dryer crusher is mounted, Figure 3 illustrates an embodiment of the invention comprising suspension drying and preheating of mesa pulp in one step, and Figure 4 illustrates another embodiment of the invention comprising suspension drying and preheating in two steps. In the figures identical reference numerals are used for identical apparatus components. 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-, gas or bark powder-fired burner 4, and a smoke chamber 5. Through a riser pipe 10 the kiln smoke gas is led through a fan 6 to a smoke gas filter 7, whereupon the cleansed smoke is fed to a stack 8. Lime sludge or mesa cake is fed directly to the kiln through an inlet 9 and dried and crushed at the uppermost end of the kiln by means of a chain system 11 mounted in the interior of the kiln prior to the further heat treatment in the kiln. In the other known apparatus shown in Figure 2 the interior kiln chain system has been replaced by a dryer crusher 12, e.g. of the hammer mill-type, mounted before the kiln 1. Drying air in the form of hot kiln smoke gas enters the dryer crusher through the pipe 10 to which the lime sludge or mesa is fed at 9 and crushed and suspended in the drying air by the dryer crusher whereupon the suspension through a riser pipe 13 is fed to a separator 14, here a cyclone separator, in which the dried, powderous raw material is separated from the smoke gas and through a pipe 15 fed to the kiln 1 for further treatment in same. From the separator 14 the smoke gas is led to the smoke gas filter 7 and the stack 8. If necessary a mixer (not shown) for mixing the mesa cake with solids may here be connected to the inlet 9 to facilitate the feeding of the raw material into the dryer crusher. In the embodiment of the invention shown in figure 3 lime sludge or mesa cake is fed to an inlet 9 of a mixer 21 in which the water-containing mesa cake is mixed with dry, powderous raw material which is fed to the mixer from the cyclone separator 18 via the pipe 19, the splitting gate 20 and the pipe branch 19''. The loose, moist mesa pulp produced in the mixer is fed to the inlet 9' of the lowermost part of a riser pipe 10 constructed as a flash pipe 17 and in direct communication with the smoke chamber 5 of the rotary kiln and is suspended, dried and pulverized in the kiln smoke gas. In the separator cyclone 18 the powderous, dried raw material is separated from the smoke gas and through the pipe 19, the splitting gate 20 and a pipe branch 19' it is fed to the kiln 1 for further treatment. A minor substream of the powderous, dried raw material separated in the separator cyclone 18 is as above described fed through the pipe branch 19'' to the mixer 21. From the separator cyclone 18 the smoke gas is via an outlet pipe 16 led to the filter 7 and the stack 8. Separated dust from the smoke gas is removed through outlets 22 and 22''. If desired, dust may be fed to the mixer 21 through a pipe branch 22'' for further conditioning of the mesa cake in the mixer. Figure 4 shows another embodiment of the invention comprising a two-step drying of the loose, moist mesa pulp from the mixer 21. In this embodiment the "flash-duct" riser pipe is placed after a separator cyclone 18 mounted immediately after the kiln 1. Loose, moist mesa pulp enters at the lowermost end of the flash pipe 25 through the inlet 9', and is suspended, dried and pulverized in the flash pipe, whereupon the powderous, dried raw material is separated from the smoke gas in the separator cyclone 26 and via the splitting gate 24 and the conduit 27' fed to the riser pipe 10 connecting the kiln 1 and the separator cyclone 18. The powderous raw material is subjected to a further preheating in the riser pipe 10 and separated from the smoke gas in the cyclone 18 whereupon the dried raw material is fed to the kiln via the splitting gate 20, a minor part of dried raw material being simultaneously fed via the pipe branch 19'' to the mixer 21 for being conditioned in same. An elevator 23 conveys the conditioned mesa cake from the mixer to the inlet 9'. During the upstart phase of the apparatus, when the temperature in the riser pipe 10 is not sufficiently high for the further preheating of the raw material, the raw material may be conveyed directly to the kiln inlet 5 via the splitting gate 24 and the pipes 27'' and 19'. The further treatment of smoke gas and filter dust after treatment in the separator cyclone 26 is identical to the one described in connection with Figure 3. In both embodiments of the invention the mesa cake consistency prior to the cake being fed into the riser pipe 17 or 25 may be regulated via the splitting gate 20 by changing the amount of dry raw material added for the conditioning of the mesa cake in the mixer 21. Similarly, as compared to the prior art technique both embodiments yield a higher specific production for a given rotary kiln size, a reduced radiation loss from the kiln per unit produced of burnt lime, a lower smoke gas exit temperature and a smaller dust loss, all of which are factors contributing to a reduction of the calorific consumption. The two-step method according to Figure 4 is superior to the one-step method shown in Figure 3 in yielding an optionally even higher specific production for a kiln of a given size and in permitting adjustment by means of the splitting gate 20 to the lowest possible smoke gas exit temperature for a satisfactory drying to be effected in the riser pipe, thus reducing the specific calorific consumption even more.

Claims: A method for the treatment of lime sludge ("mesa") as residue from the causticisation of green liquor (Na₂CO₃) with burnt lime (CaO) in the manufacture of paper pulp according to the sulphate process and wherein the burnt lime (CaO) used for the causticisation results from the treatment of a calcareous raw material in a kiln, and wherein the causticization produces partly a sodium hydroxide solution (white liquor), partly a calcium carbonate residue, the latter containing the fine CaCO₃ particles which are separated from the residue to form a "mesa cake" to be fed to the kiln for renewed production of burnt lime, characterized in that a suitable amount of pre-dried, powderous raw material (CaCO₃) is added to the mesa cake in a mixer to form a loose, moist pulp and in that the mesa cake is subjected to a flash drying in suspension with kiln smoke gas, thus forming without any further mechanical treatment a dried, powderous raw material product (CaCO₃) which is after being separated from the smoke gas in a separator fed to the kiln for further heat treatment in same. A method according to claim 1, characterized in that the amount of dried, powderous raw material with which the mesa cake is mixed constitutes 5-50% of the total material amount and preferably 5-20% thereof. A method according to claim 1, characterized in that the drying and the preheating of the loose, moist mesa pulp is effected in two steps; the pulp being initially dried in suspension with hot smoke gas in a riser pipe constructed as a flash duct and after the thus dried product having been separated from the smoke gas, being again resuspended in the gas in the riser pipe connecting the kiln and its separator cyclone for further heating, before the dried, powderous product being again separated from the smoke gas and fed to the kiln for further treatment in same. An apparatus for carrying out the method according to claim 1, characterized in that a riser pipe (17) constructed for flash drying of an added moist raw material ("mesa") which is suspended in hot kiln smoke gas in the riser pipe (flash drying) and which - as seen in the movement direction of the smoke gas - is mounted immediately after a rotary kiln (1), a separator (18) positioned immediately after the riser pipe (17) for the separation of dry, powderous product from the smoke gas, a splitting gate (20) for conveying a substream of the separated product through a pipe (19') to the kiln inlet and for conveying another substream through a pipe (19'') to a mixer (21) for mixing the latter with added raw material in the form of lime sludge (water-containing "mesa cake"), and means (9') for conveying the loose mesa pulp produced in the mixer to the lower end of the riser pipe (17). An apparatus according to claim 4 having means to carry out in two steps the flash drying and the preheating of the added loose, moist mesa pulp, characterized in a drying step comprising a riser pipe (25) constructed as a flash dryer and mounted after a separator cyclone (18) which is mounted immediately after the rotary kiln (1), a material inlet (9') in the riser pipe (25) and a separator cyclone (26) mounted immediately after the riser pipe, and in a predrying step comprising the riser pipe (10) connecting the rotary kiln (1) and its separator cyclone (18), a material inlet (27') in the riser pipe for separated, powderous material, and a separator cyclone (18) mounted immediately after the kiln.

Inventors: THEIL, JORGEN
Applicants: F.L. SMIDTH & CO. A/S
Application year: 1989
Main classification: D21C11
IPC codes: D21C11