FI106642B - Process for treating lime sludge - Google Patents

Description

1 106642

METHOD FOR HANDLING MEES -FÖRFARANDE FÖR BEHANDLING AV MESA

The present invention relates to a process for treating lime sludge from a pulp mill digester, which is thickened in a lime sludge filter and incinerated in a lime kiln.

Honey is a calcium carbonate slurry (formed by sulphates) in the distillery's caustic. For reuse, the lime slurry (CaCO3) is regenerated by burning to calcium oxide (CaO). The combustion usually takes place in a rotary drum furnace, at the top of which the lime slurry is fed, and slowly moves down through the drying, heating and reaction zones. In the drying-15 zone, where heat is transferred from the flue gases to the lime mainly by convection, there are chains used to enhance the heat transfer which come into contact with the lime as the furnace rotates. In order to achieve a good result, the heat treatment of the lime must be slow in the furnace, which requires a long furnace, which naturally requires a lot of space. Other lime sludge calciners are also used in addition to the furnace. Such is for example the fluidized bed reactor. However, the drum oven is by far the most used lime sludge calciner.

25. : The lime sludge is usually dried by mechanical /.·] filtration before being fed into the oven. Previously, the lime slurry fed into the furnace had a dry matter content of about 60-70%, but due to advances in lime slurry technology, it is now possible to achieve a dry matter content of up to 80-85%. The dust content of the drum furnace flue gas has thus been found to increase significantly. Because of the high solids content of the lime that enters the furnace, the lime dries completely in the chain zone and the chains then become a preheater for the lime. Swinging chains loosen- 35 find this kind of lime easier than wetter, resulting in * «* ... more leaching of flue gas than before. In the chain zone, the temperature may also locally rise to <2 106642 (for example, above 600 ° C), where lime mud begins to become lumpy in the chains and the chain zone can become clogged.

The honey furnace may have been shortened by replacing the furnace chain system with 5 dryer crushers. In this system, the lime feed to the furnace is dried with furnace flue gases. In the crusher, wet lime, dried lime and flue gases are mixed, whereupon the flue gas stream conveys the lime through a tube to the time difference. The honey dries and warms by the effect of the heat of the flue gases in this suspension type dryer. From the separator, the lime slurry is led to the furnace feed head and part to the crusher. Dried lime is mixed with wet lime, otherwise lime from leachate may be difficult to handle. However, such a slurry pretreatment apparatus is complex-15 and requires a complex control and regulation system.

Finnish patent 87245 describes a feed furnace for a lime kiln where no crushers are required. In this sys-20 theme, all lime from the filter is fed to the same chamber at the inlet end of the furnace. From this chamber, the lime slurry can be led to a slurry dryer and, if necessary, some of the honeys can also be fed directly into the furnace, e.g. lean solids content. The chamber has a septum which divides the chamber 25 into two flow channels, one of which is fed with lime * *. *. : from the filter. The amount of lime slurry entering the dryer can be adjusted • · · • ·. . ·. by changing the ratio of the gas volumes flowing through adjacent flow channels by means of a * control flap at the top of the bulkhead. This also allows the exhaust gas 30 to be controlled from the system.

FI patent 91005 discloses that when replacing the precoat layer of a lime sludge filter, this wet lime sludge is fed directly into the furnace because it is not desired to be run into the dryer. The dry matter content of wet y [35 lime slurry is significantly lower than the lime slurry obtained from the filter · · * .... Such a very wet • »**”. lime would very likely block the dryer dryer tube.

• * · 3 106642

The above-mentioned suspension type dryer and drum furnace combinations are displacing the solution based on the drum furnace process alone. Said combination achieves, on the one hand, high thermal capacity and, on the other hand, a good temperature saving.

The capacity and temperature of the dryer / tumble furnace process can be further improved by connecting two slurry drying sequentially, one serving as a dryer and the other as a preheater. The furnace flue gas is first directed to the slurry pre-heater and from there to the dryer. The lime sludge to be dried, in turn, is fed from the lime sludge filter in the first stage to the dryer and in the second stage from there to the preheater and finally to the drum for burning.

15

By increasing the suspension pre-heating step, the capacity can be increased by 15-20% and the heat consumption reduced by 4-6%. The drier the lime from the filter is, the greater the savings in heat consumption.

20

The honey in the preheater heats up to 400-600 ° and the gas exiting the furnace in turn typically has a temperature of 650 ° C to 850 ° C, most often above 700 ° C. In the feed of the pre-heated lime, the furnace employs metallic structures which enter into the furnace.

• «; * ♦ '· 25 at high temperatures. The strength of steels begins to · ·. ·: Deteriorate at high temperatures, even if their temperature. . ·. otherwise sustainability would still be reasonable. In addition, hot • · lime tends to adhere to metal surfaces ♦ · «* at these temperatures, which are even hotter than lime.

30

The object of the present invention is to avoid the problems described above in connection with the supply of lime sludge to the furnace, whereby the lime sludge is pretreated with the flue gas leaving the lime furnace before being fed to the furnace. More particularly, it is an object of the invention to provide a method whereby the adhesive of the drum furnace * · * ... has conditions under which the adherence of the lime slurry to metal surfaces and the wear of these surfaces can be effectively achieved. 106642 prevent.

To achieve these purposes, it is essential to the invention that a portion of the lime sludge is dried and preheated by the flue gas of the honey-5 furnace, separated from the flue gases and fed to the lime furnace feed head, and a portion of the lime is fed directly to the lime furnace feed head

10 When wet lime slurry is fed directly into the furnace after drying and preheating, it is mixed with the slurry from the slurry pre-heater in the furnace feed chamber to evaporate the water contained in the wet lime lime. Evaporation absorbs a great deal of energy, which effectively cools the structures of the feed end of the lime kiln 15 and also slightly reduces the temperature of the flue gas. Thus, the temperature of the furnace feed head structures is reduced to about 550 ° C so that adhering to the lime slurry surfaces caused by too high a temperature and weakening of the strength of the furnace feed head structures, such as the feed chamber, can be substantially prevented.

The wet lime sludge comes from the lime sludge filter, where it is thickened.

How much of this sludge is fed directly into the furnace after the drying and preheating stages depends essentially on the furnace load. When the oven load is high, that is «« · • ·. ·. operating close to the rated capacity of the furnace, the flue gas temperature • · ♦ is 650-850 ° C. The temperature is high enough for preheating and drying of the lime sludge, but at the same time the hot gas • · * heats the metal structures of the furnace feed end. Correspondingly, the temperature of the pre-heated lime lime is 400-600 ° C, even higher than 600 ° C, if the leach sludge content from the filter is very high, e.g. above 85% (w / w). According to the invention! the amount of wet lime directly fed into the furnace is 0.5-30%, preferably 3-20% of the total amount of lime slurry fed to the furnace ./ * 35. The amount of lime slurry to be fed closer to the drying stage depends on the size of the furnace, the load and the dry matter content of the lime from the filter. The higher the dry solids content of wet lime, the more it must be fed directly into the furnace past the slurry dryer to provide enough water to evaporate and thereby cool the furnace feed head structures. The cooling 5 is thus of a local nature, that is, it is directed only to the feeder head structures and is not significantly influenced, for example, by the gas temperature. If necessary, the cooling of the structures can be enhanced by spraying water directly into the structures.

10

The honey can be dried and preheated by the flue gas in one suspension dryer. Alternatively, the lime slurry may be pre-treated in two steps. It is dried by the lime furnace flue gas in the first slurry dryer, the dried lime furnace 15 is heated by the flue gas in the second slurry dryer and fed to the furnace. There may be more than two slurry dryers, although usually only one or two slurry dryers are used in calcining lime sludge.

The invention will now be explained in more detail with reference to the accompanying drawing, which shows a schematic diagram of an apparatus which may be used to carry out the 'method according to the invention.

The figure illustrates the principle of an embodiment of a method according to the invention for a heater / dryer / oven system. Ku- • «.

The main components of the apparatus shown in the drawing are the feed screw 1, ', * 1 rupture screw or bypass screw 2, the furnace feed chamber 3, the lime kiln 4, the drying tube 5, the dryer separation cyclone 6, the preheat-30 separation tube 7 and the preheater separation cyclone.

The lower part 9 of the feeding chamber 3 of the honey furnace 4 has a feeding coil 13 for feeding the lime slurry into the furnace. The upper part 10 of the feed chamber is: '! * Connected to the lower end of the heating pipe 7. The tube 7 rises' 35 vertically upwards and is then bent horizontally and connected tangentially to the upper end of the separator 8. Correspondingly, the drying pipe 5 rises from the upper end of the separator 8 106642, and then joins its horizontal portion tangentially to the upper end of the preheater separator 6.

The wet lime stream is introduced by a transfer conveyor 11 into a feed screw 5, which is connected to the dryer tube 5 and dispenses the portion of the wet lime to be dried to the vertical part of the dryer tube. Part of the lime supplied to the feed screw 1 passes through the drop tube 12 to the loot screw 2.

The honey furnace flue gas flows from the furnace feed chamber 3 to the preheating pipe 7, and further through the separator 8 to the drying pipe 5 and to the preheater separator 6. The flue gas is discharged from the separator 6 through the pipe 14.

15 In the drying pipe 5, the honey is carried along with the flue gas to the separator 6 and at the same time dries by the heat of the flue gas. The dried lime slurry is separated from the flue gas and removed from the lower end of the separator 6 along a so-called feed pipe 15. riser, or pre-heating pipe 7. In this pipe, the lime mixes with the hot flue gas from furnace 20, warming while cooling the gas. The preheated lime is separated from the gas entering the drying pipe 5 in the separator 8 and guided from the lower part of the separator via conveyor 16 and feed pipe 17 to the lower part of the furnace • ♦ ·. · "Feed chamber. As disclosed in FI patent application • · ·; 1 · 1; 962777, a protective conduit 18 is provided inside the feed coil 13 to prevent flue gas circulation between the dryer and the furnace, through which the flue gas is directed to flow from the furnace 30 to the feed chamber. lime does not have much contact **! 1 'with the flue gas at the point of injection or when flowing - «· · * · 1 I get into the oven.

»· .1 ··. According to the invention, • · «.1; also wet lime slurry with loot screw 2. The amount of lime slurry is 0.5-30%, typically 3-20% of the total amount of lime slurry fed into the furnace. The wet lime and the dried and preheated lime from the feed tube 17 are mixed to evaporate the water in the wet lime, and the lime mixture takes the feed coil 13 from the feed chamber to the furnace, which effectively cools the feed chamber, which hot flue gas tends to heat up. In this way, the temperature of the surfaces of the feed chamber can be adjusted to a suitable level so that it cannot be trapped by the lime mud at a temperature of about 400-600 ° C. By preventing heating of the feed chamber structures, the stress on them and consequent loss of strength can be reduced.

Cooling can be enhanced by injecting water directly from the inlet end into the furnace structures along the unit 19. Water injection is directed so that its effect is local, that is to say, on the structures and not on the entire area of the inlet.

20

The invention is not limited to the exemplary embodiment, but its various details may vary within the scope of the inventive idea '• s /' as defined in the appended claims. For example, to divide a wet lime slurry into two partial streams, a device other than a so-called.

; robbery screws. To eliminate air and gas leaks, • · · sludge feeders, which may include barrier feeders or other similar structures. The dryer and preheater can also be connected differently with respect to the flue gas flow. For example, the dryer may be supplied with flue gas directly from the furnace.

«I« • · · • «« · · 1 · • · ♦ «» * *

Claims (5)

8 106642 A process for treating lime sludge 5 produced at a pulp mill causticizing plant which is thickened in a lime sludge filter and incinerated in a lime kiln which is dried and preheated by the flue gases prior to combustion, characterized by drying and preheating a and that, to cool the structures of the lime kiln feeder, a portion of the thickened lime is fed directly through the flue gas treatment directly to the lime kiln feeder, where it mixes with dry and preheated leach and 0.5% to 30% of the total amount of lime entering the furnace. • «« «« «· A process according to claim 1, characterized in that the proportion of the leaching bypass of the flue gas treatment is,. : 3-20% of the total amount of lime in the furnace. I « 3. Förfarande enligt patentkravet 1 eller 2, kännetecknat av att mesan prickly and förvärms med rökgaserna i en sus-25 pensionstork. • M • ♦ ar · * 4. 4. Förfarande enligt patentkravet 1 eller 2, kännetecknat • · ί ß av att mesan throat med hjälp av mesaugnens rökgaser i en * * * ... ... första suspensionstork och att den thorkade mesan förvärms 30 med hjälp av rökgaserna i en andra suspensionstork. ♦ · • · · · · · Method according to Claim 1 or 2, characterized in that the lime slurry is dried and preheated by the flue gas 25 in a single dryer. The method according to claim 1 or 2, characterized in that the lime slurry is dried by means of a lime kiln flue gas in a first slurry dryer and that the dried lime slurry. · * 30 is preheated by flue gas in another • · ί ** slurry dryer. • · • · · ··· · · Method according to Claim 1, characterized in that the cooling is enhanced by spraying water 35 into the structures of the feed head. 106642 l.Förfarande för behandling av mesan sora uppstär i en mas-5 safabriks kausticeringsanläggning och som urvattnas i et mesafilter och förbränns i en mesaugn, stem rökgaser an-vänds för att torka och förvärma mes en avörningen urvattnade mesan throats och förvärms med 10 hjälp av mesaugnens rökgaser, avskiljs ur rökgaserna och inmatas i mesaugnens inmatningsände, och att för kylning av constructuktionerna i fiens inmatningsd med groen poke och förvarmma mesan, och mesan som rotten rökgasbehandlingen utgör 0.5-30% av den total game-. den mesa som inmatas i fire. 2. • Förfarande enligt patentkravet 1, kännetecknat av att • · * '. * · 20 mesan som mätäs förbi rökgasbehandlingen utgör 3-20% av • · *. * I den total play mesa som inmate into fire. «« «« 5. Förfarande enligt patentkravet 1, kännetecknat av att kylningen or effektivare genom att spruta vatten pd construktionerna i inmatningsändan. 35