FI108235B - Method and arrangement for processing of sludge - Google Patents

Description

Γ 108235

METHOD AND EQUIPMENT FOR HANDLING MEES -FÖRFARANDE OCH ANORDNING FÖR BEHANDLING AV MESA

The present invention relates to a method and apparatus for treating lime sludge formed at a pulp mill caustic sizing process, wherein the wet lime is dried and separated from the flue gas from the leaching of the lime, and the dried lime is preheated by the flue gas from the lime combustion. the temperature during preheating is adjusted to a certain 10 value in the range of 400-600 ° C.

Honey is a calcium carbonate slurry formed in the causticizing plant of a sulphate pulp mill. 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 upper end of which the lime slurry 15 is fed and moves slowly downwards through the drying, heating and reaction zones. In the drying zone, where heat is transferred from the flue gases to the lime mainly by convection, chains have been used to make the heat transfer more efficient which, when the furnace rotates, are in contact with the lime. 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.

Before feeding the furnace, the lime mud is usually dried by mechanical filtration. The dry matter content of the lime sludge fed into the furnace was approximately 60-70% in the past-25 semis, but due to the development of the lime sludge treatment technology, it is now possible to achieve a dry matter content of up to 80-85%. The dust content of the flue gas in the drum furnace has then been found; will rise significantly. Because of the high solids content of the lime entering the furnace, the lime dries completely in the chain zone, and the chains then become the 30 lime pre-heater. Swinging chains release this kind of lime more easily than wetter, causing the lime to be transported more by the flue gas stream than before. In the chain zone, the temperature may also locally rise to an area (e.g., above 600 ° C) where the lime slurry begins to become lumpy in the chains and the chain zone may become clogged.

2 108235

The honey furnace may have been shortened by replacing the furnace chain system with a crusher dryer. 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, after which a stream of flue gas-5 carries the leached sludge along the pipe. The honey dries and heats up due to 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 as otherwise lime from the filter can be difficult to handle. However, such a slurry pre-treatment apparatus is complex 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 system, 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 suspen-15 sieve dryer and, if necessary, part of the lime slurry can also be fed directly into the furnace, e.g. lean solids content. The chamber has a septum which divides the chamber into two flow channels, one of which is supplied with lime from the filter. The amount of lime slurry entering the dryer can be controlled by changing the ratio of gas volumes flowing through adjacent flow channels by means of a control flap fitted at the top of the bulkhead. This also allows the flue gas outlet temperature to be controlled from the system.

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 heat capacity and, on the other hand, a good temperature economy.

25

The capacity and temperature of the dryer / tumble furnace process can be further improved by connecting two successive slurry drying steps, one of which acts as a dryer and; another as a preheater. Typically, the furnace flue gas is first passed to a slurry pre-heater and then to a dryer. The lime slurry to be treated is fed from the lime sludge filter in a first-stage step to the dryer and in a second step from the dryer to the preheater. Finally, the dried preheated lime is fed to the drum for combustion.

In order to be profitable to preheat the lime slurry, the lime temperature should be raised as high as possible using the energy from the flue gas. In practice, however, the maximum temperature at which lime slurry can be heated is typically in the range of 400 ° C to 600 ° C. This is because, depending on the composition of the lime slurry at a given temperature, lime slurry begins to adhere to the equipment walls. Problems 5 can only be avoided by keeping the lime slurry temperature sufficiently low in the preheater. In addition to the energy content of the flue gases, another natural limit to heat recovery is the dew point of the flue gas, which in practice determines that the lowest temperature at which lime sludge can be pneumatically dried without the risk of moisture clogging is 85-100 ° C. Due to the normal control deviations in the practical process, a safety margin of 30-50 ° C should be left to this temperature. As a result, in the drying phase of the lime slurry, the aim is to keep the temperature above 130-150 ° C, but still so low that too much heat is not lost with the flue gas.

In EP 751916, the lime slurry is prevented from adhering by lowering the temperature of the preheated honey 15 either by feeding a portion of the wet lime directly to the preheating stage or by directing a portion of the flue gas from the lime kiln directly to the lime preheating stage. However, both adjustment methods are difficult to put into practice. The first method has the disadvantage of feeding wet and sticky lime to two separate and distant 2 0 locations. This results in both sophisticated transport and feed equipment and increased need for maintenance. Another problem with the control mode is the large amount of flue gas transfer piping and the fact that maintaining a sufficient flue gas flow in the pipeline may require extreme refinement in extreme situations (i.e. very wet lime or very dry lime).

The object of the present invention is to avoid the problems described above and to provide a method for controlling the temperature of the lime sludge preheating step, which can be implemented in a more technically simple manner than the known solutions.

f · In order to achieve these objects, it is essential for the invention that the pre-heating temperature of 1 to 30 is controlled by circulating a portion of the pre-heated lime to the drying stage of the lime.

The invention also relates to apparatus as defined in the appended claims.

108235 4

The main principle of the present invention is that excess energy is transferred from the preheating stage of the lime slurry bound to the preheated lime dust back to the drying stage of the lime slurry. The hot sludge directed to the drying stage raises the temperature in the drying stage 5 and at the same time the temperature of the gas leaving this stage rises compared to the situation without recycling the sludge. The temperature of the leach stream from the drying stage to the preheating stage is also slightly higher than from the circulation of the lime to the drying stage. However, thanks to the control measure according to the invention, the amount of lime flow in the preheating step has increased to such an extent that the total effect 10 is cooling the preheating step. More energy is required to preheat a larger amount of lime than in the case where preheated lime is not recycled for drying and further preheating. The method of controlling the temperature of the invention in the preheating step is simpler than known methods. In the invention, wet lime sludge can be fed to a single location in the process, and the division of 15 preheated lime sludge into two streams can be done by a single adjusting member. At its simplest, the adjustment can be constructed by installing a manifold or other distributor on the leachate outlet from the lime preheating stage, which divides the preheated leach into the fraction passing to the incinerator and back to the fraction passing to the leaching drying stage.

20

It is advantageous for the operation of the method according to the invention that the drying and preheating devices of the lime slurry, unlike the previous solutions, are arranged in parallel and not on top of each other. In this way, the slurry flow required for temperature control can flow downward from the preheating stage separator under gravity directly into the slurry dryer 2 without a separate conveyor. This also assists in the implementation of the process since now, for example, the lime sludge filters need not be placed as high as if the aforementioned devices were to be overlapped.

•

The invention can be applied to a variety of lime sludge incinerators, which may be a rotary kiln, fluidized bed furnace, slurry furnace, or any other device suitable for incinerating / calcining the sludge.

The invention will now be described in more detail with reference to the accompanying drawings in which 5 108235

Figure 1 shows a schematic diagram of an apparatus according to the invention which may be used to carry out the method according to the invention, and Figure 2 shows a technical solution for implementing the method according to the invention.

5

Figure 1 illustrates a lime sludge drying / preheating system in connection with a lime sludge incinerator, such as a rotary kiln 1. Said system includes a step of preheating the lime slurry comprising a riser 2 and a separator cyclone 3, and a drying step comprising a riser 5, a separator cyclone 7, a feed unit 8 for supplying the dried product to the riser 2 and 10 to feed a preheated lime slurry from the separator cyclone 3. drain line 14 from the drying stage separation cyclone 7.

The flue gas of the combustor flows from the combustor to the preheating pipe 2 and further through the separation-15 cyclone 3 to the drying riser 5 and to the drying phase separation cyclone 7. From the last separation cyclone, the

The wet treated lime slurry is guided from the lime slurry filter (not shown) through a feeder 6 to a drying riser 5 which rises vertically upwards and is then bent horizontally and tangentially connected to the upper end of the separator 7. The honey has a dry solids content typically of 70-85%.

The wet lime slurry fed by the feeder 6 is suspended in the flue gas in the drying riser pipe 5 and enters with the flue gas into the cyclone 7, 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 cyclone 7: via a feed pipe 8 to a pre-heating stage riser 2. In this pipe, the lime mixes with the hot flue gas from the furnace while warming the gas. 30 The preheated lime slurry is separated from the flue gas entering the drying riser 5 in the separation cyclone 3. The flue gas is led from the separation cyclone 3 along line 11 to the drying tube 5.

In the embodiment of Figure 1, the preheater 2, 3 is so-called. kiertoleijuesilämmitin. The lime sludge separated from the flue gas 6 108235 is discharged from the lower part of the separation cyclone 3 into the pipe 4 into the lower part of the preheating pipe 2. Part of the dry, preheated lime flows back into the preheating pipe 2 with the flue gas. The circulating lime mud forms larger particles in the hot flue gas stream from the furnace as a result of particle collisions. A sufficiently large portion of these particles float at the bottom of the preheater, from where the preheated net flow is dropped into the combustor via line 10. The lime dust separated in the filter 15 of the flue gas line 14 is typically recycled through the line 17 to a process, e.g. a dryer 5 or a preheater 2.

Instead of a circulating fluidized bed heater, there may be a suspension pre-heater in which the preheated lime slurry is mixed with the flue gas in the riser, separated from the flue gas and then fed directly to the combustion system without the above-described recirculation.

According to the invention, part of the preheated lime 15 separated from the flue gas in the separation cyclone 3 is led to a drying step. To this end, means are provided in the bottom portion of cyclone 3 for dividing the leach stream into the fraction going to the combustor and back to the fraction going to the drying stage of the lime. A preferred partitioning arrangement is shown below in Figure 2.

The lime slurry to be recycled to the drying stage is led along line 12 to the lower portion of the drying riser 5, where lime slurry is mixed with the flue gas from line 11. In this way, heat energy is transferred from the preheating stage to the lime drying stage. The introduction of preheated lime into the drying stage raises the temperature of the flue gas in line 14 and the leachate in line 8 compared to the situation where the lime slurry is not recycled from the pre-heating stage to the drying stage. The amount of lime slurry fed from line 8 to the preheating stage is greater than without said recycling. The greater amount of lime in the preheating tube 2 requires more energy to achieve the same temperature for the lime than without recirculation, resulting in a cooling effect of the preheating step. By controlling the amount of sludge recycled, the pre-heating step 2 can maintain a certain temperature of 400-600 ° C. The amount of recycled lime sludge is in the range of 1-500%, typically 10-500%, preferably 50-200%, of the amount of dry sludge solids fed through the feeder 6.

7, 108235

Lines 8, 10 and 12 have barrier feeders 9, 18, 13, respectively, for adjusting the amount of lime slurry flowing in each line.

Fig. 2 shows an apparatus for controlling a desired amount of preheated lime slurry 5 to a drying step. The base tube 31 of the preheating cyclone (cyclone 3 in FIG. 1) is provided with an oblique base 32, from which a line 34 provided with an adjustable closing member, such as a valve 33, is directed to the dryer. A tube 36 having a smaller diameter than the bottom tube and leading to the combustion device is disposed in the center of the bottom tube and about its vertical axis 35. As the solid separates on the walls of the cyclone, the solute first enters 10 in line 34 with an adjustable barrier 33. After line 34 is filled (barrage strangulation), solids begin to flow into the conduit 36 leading to the incinerator. 34 and 36 and flow control.

The invention is not limited to the exemplary embodiment, but its various details may vary within the scope of the inventive idea defined in the appended claims. For example, the dryer and preheater can also be connected differently with respect to the flue gas flow. For example, flue gas can be introduced into the dryer directly from the furnace.

In order to divide the preheated lime slurry into two partial streams, an arrangement may be used, for example, instead of the solution shown in Figure 2, in which gas is blown to the upper end of the product tube from the base of cyclone (Figure 1, cyclone 3). the separated sludge is returned to the cyclone near its central axis and the flue gas leaving the cyclone therein at line 11. There is thus no need for a separate drop line 2 5 to supply the preheated lime to the drying step. Honey is divided into furnace and drying fractions by changing the size and possibly the direction of the gas flow.

«

I I

«

The solution of Figure 2 can also be replaced by an arrangement in which the pre-heated lime sludge 30 is divided into two fractions by a so-called rotational speed. with an overcut screw. The screw is placed in the outlet line of the preheating stage separator (line 4 in Figure 1), along which the lime slurry is led to combustion. The screw is always full. The part of the lime that overflows flows into the combustion while the screw feeds the rest of the lime back to the drying stage.

8 108235

The foregoing is limited to some solutions for dividing preheated lime into two different fractions, and the invention is in no way limited to these solutions, but extends to other methods.

Claims (6)

A method of treating the mesa which arises at the cellulose pulp mill in the causticization plant, wherein the white mesa method is dried by means of the flue gas derived from mesa burning and separated from the flue gas and the dried mesa is preheated by the flue gas derived from mesa gas the flue gas and the measurement gas into a combustion device and the temperature at the preheating is set to a certain value within the range 400 - 600 ° C, characterized in that the temperature at the preheating is adjusted by bringing a portion of the preheated mesh to circulate to the drying step of the knife. 2. A method according to claim 1, characterized in that the preheated mesh amount caused to circulate is 1-500%, typically 10-500% of the dry matter amount of the white mesh to be treated. 15 Devices for applying the method according to any of the preceding claims, which means comprise at least one first riser (5) connected to the knife burning device (1) for drying the knife, by the flue gas of the burning device; a feeding device (6) for feeding the white web into the first riser (5); a first separator (7) connected to the first riser (5) in which the dried, powdery web is separated from the flue gas; a second riser (2) connected to the firing device (1) in which the dried web is preheated with flue gas; a first feed tube (8) for guiding the dried web from the first separator (7) to the second rising tube (2) for preheating; a second separator (3) connected to the second riser (2) in which the preheated knife is separated from the flue gas; a second feed tube (4) for conducting the preheated mesh from the second separating device (3) to the firing device (1), characterized in that the devices comprise means (36) for distributing the preheated mesh into at least two fractions, of the first fraction being passed through the second feed tube (4) to the firing device (1) and the second fraction being led to the drying step. Devices according to claim 3, characterized in that the distributing means are formed by a damper arranged in connection with the second feeding tube. Devices according to claim 3 or 4, characterized in that the preheater (2, 3) and the wiper (5, 7) are arranged in parallel so that the second blade fraction is conducted by utilizing gravity from the distribution means along a line (12) to the first one. 10 (5). Apparatus according to claim 3, characterized in that the second separator is a cyclone (3), between which the upper part and the first riser (5) are arranged a flue gas line (11), and the distribution means are formed by a pipe arranged in conjunction with With the bottom of the cyclone and whereby gas is biased so that the preheated knife fraction to be led to the drying step flows there together with the flue gas which emits from the cyclone. «