FI105933B - A process for preparing white liquor - Google Patents

Description

105933

The present invention relates to a process for the production of white liquor by dissolving a chemical melt obtained by evaporation and combustion of black liquor, causticizing the green liquor formed during leaching, separating the lime mud and burning it into lime, which is used for causticizing the green liquor. The process according to the invention is characterized in that: a) before causticization, the green liquor is removed directly by pressure filtration to obtain a green liquor having a pellet content of less than 20 mg / l and a solid precipitate having a dry solids content of 55-75% by weight; directly separating by pressure filtration while washing and drying the precipitate to obtain a lime sludge product having a dry solids content of 75-85% by weight; and c) transferring the resulting lime sludge product to a storage tank for continuous discharge into a lime kiln where it is incinerated.

The object of the present invention is to simplify the chemical recovery plant and the causticization process of the sulphate pulp mill, thereby improving the quality of the product obtained, i.e. white liquor, and reducing the amount of green liquor slurry and weak liquor produced. In addition, cleaner green liquor and 20 drier lime products are obtained.

In the existing causticization processes of the sulphate pulp mill, the melt from the soda recovery unit is dissolved in the soda solvent in weak liquor and other process waters to form green liquor. The thick green liquor is usually passed through the equalization tank with the green liquor clarifier * «! 1 25 meen. The green liquor clarifier separates the green liquor from the sludge. The clear green liquor is led to the green liquor store, from where it goes to the lime extinguisher. The sludge separated in the green liquor clarifier is introduced by a pumping system into a sediment filter, whereby the resulting precipitate is dumped and the filtrate is discharged into a weak liquor container. The lime burned in the lime extinguisher is quenched with green liquor water and it continues to react as a mixture of lime and white liquor, lime milk. This mixture is introduced into a causticization tank where the causticization reaction is carried to the wood. From the causticization tank, the lime milk is led to a white liquor filter, from which the clear white liquor filtered is taken to a cooking plant. The filtered solid, honey, and a portion of the white liquor are discharged into a lime storage tank and slurried in warm-35 ml water. After filtration, the honey has a dry matter content of about 30 to 40. %. From the storage tank, the lime slurry is led to a lime slurry filter, from which 105933 2 water-washed lime slurry is taken to a lime kiln and incinerated. The dry matter content of the honey after the washing filter is generally at least about 70% but less than 75%. The filtrate obtained by the washing filter is transferred to a weak liquor tank. The burnt lime from the Meesa furnace is taken to a lime store, where the burnt lime an-5 is lifted to a lime extinguisher.

To separate sludge from green liquor, current causticisation processes generally use a green liquor clarifier, which is very bulky and requires high investment costs as well as high drainage costs. In addition, a slurry having a low dry solids content is obtained.

In current causticisation processes, separation of lime sludge and white liquor generally occurs in a tubular filter consisting of suspended slurries of filter cloth covered suspension of lime sludge through which the white liquor is squeezed. The dry matter content of the leaching effluent is about 30-35%. Honey 15 is washed in the next washing step by dilution with water and subsequent thickening. The resulting dry matter content of the lime is about 35%. The honey slurry is filtered and the lime slurry is washed by displacement in a vacuum filter, whereby the leached lime solids content is generally at least about 70% but less than 75%.

For example, a process for separating white liquor and lime as well as green liquor and green liquor slurry in two steps is known from Fl 87 347. The white liquor-containing lime slurry obtained in the first step and the green sludge slurry respectively are diluted with water and possibly stored, and the second filtration step yields pure lime sludge and quite: respectively: 25 dry green liquor slurry and larger quantities of weak liquor.

From Fl 923 127, a method is known for filtering the product formed by causticization and increasing the solids content of the separated solid, whereby the separation of the white liquor and the washing and drying of the lime precipitate take place in one process step. Preferably a pressure filter is used as the filter, whereby the lime mud remaining in the filter is dry. high content, about 75%; the lime slurry can be further dried, for example, with compressed air. It is not mentioned whether the resulting lime slurry can be stored and can be transferred to the incinerator in a steady stream.

Variations in the properties of the filtered, washed and dried lime, the residual alkali and particle size and structure of the dry matter content in known causticisation processes cause changes in the quality of the burnt lime. In addition, fluctuations in lime dosage affect lime burning. With the uniform feeding of the invention, the use of the lime kiln is improved and the variations in the quality of the product, lime, are reduced. Properties of burnt lime particle size affects the lime quenching and causticization reaction.

Furthermore, the method according to the invention is simpler than the known methods: a significant part of the devices used in the known methods can be omitted. Filtration of the green foliage is carried out directly on one device and the precipitate content of the bright green foliage is less than 20 mg / l; an additional advantage is the high solids content of the separable solid precipitate, 55-75% by weight. The cleaner 10 wings affect the rest of the causticization process advantageously as the amount of harmful solids decreases. Also, separation of white liquor and lime, as well as washing and drying of lime, are performed in a single apparatus, and the resulting lime product has a high solids content of 75 to 85% by weight. White liquor generally has a precipitate content of less than 20 mg / l. Only about 0.8 - 1 m3 / tonne of dry manure is needed. In addition, the residual alkali of the lime slurry can be adjusted, e.g. to less than 0.1 wt.% As Na 2 O. The good quality green foliage used in the causticization reaction allows for a cleaner and more uniform lime mud. Feeding a consistent lime product from the lime tank to the oven simplifies furnace control and reduces the need for lime drying energy and reduces fuel costs. It should also be noted that the yield of white liquor relative to the feed is high.

The invention also relates to a storage tank for lime product useful in the process of the invention.

The method and storage container of the invention will now be described in more detail.

Figure 1 illustrates the process steps of the process of the invention.

Figure 2 shows the filtering and washing steps of a pressure filter useful in the process of the invention.

Figure 3 shows a storage container for a lime product useful in the process of the invention.

In Fig. 1, reference numeral 35 denotes an operating line which brings the soda recovery unit (not shown) to melt in the soda solvent 1. The soda liquor generator with green liquor has a stirrer 2. The soda solvent 35 also terminates with a line 27 for charging weak liquor. Thick green leaves run through the baking solvent. a conduit 3 for green liquor feed equalization tank 4, from where green liquor 4 105933 is automatically fed by a pumping system 6 to a periodically operated pressure filter 8. The equalization reservoir 4 terminates with lime or burnt lime conveyor 36, where necessary to add filtration lime or burnt lime. The equalization tank 4, which has a mixer 5, ends with a filter 5 of an invalid green flap, so-called. pre-filtration return line 9. The equalization tank also terminates with a green wing pump return line 7 which is in use when no green wing feed is provided during the filtration cycle. The pressure filter 8 separates the slurry, i.e. the precipitate 37, from the green liquor. film compression and air. The air is separated from the clear green liquor with an air separator 11.

Alternatively, burnt lime or 15 lime sludge may be added to the green liquor along conveyor 36 to the equalization tank 4. The addition of burnt lime or lime sludge assists the filtration process and increases filtration capacity, which may also be affected by varying the feed pressure of the green liquor.

From the lime quencher 15, a conduit 16 passes through a pumping system 19 along a line 18 to a pressure filter 21 for separating the white liquor and the sludge from the causticisation solution 20 to the causticization tank 17 for the mixture of lime product and white milk, i.e. lime milk. The causticization tank also terminates with a conduit 20 to provide an invalid white liquor filtrate, or pre-filtrate. The filtered white liquor passes along conduit 22 to a cooking plant not shown here. From the pressure filter 21, weak liquor is passed along line 23 ; 25 to a manifold 24 for separating air from the weak liquor and passing the weak liquor through a line 25 to a low liquor container 26, further along the line 27 to a soda solvent 2. The leached, air dried and water washed leach is discharged by conveyor 28 to the lime silo 29 30 in a steady stream to the wheat furnace 31 for feed-30 work. The residual alkali in the leachate separated in the filter can be adjusted. The lime from the honey oven passes along the conveyor 32 to the lime silo 33, from which the lime is fed through a conduit or conveyor 34 to a lime slug 15.

Figure 2 illustrates the principle of a pressure filter useful in the process according to the invention in different stages. In step 1, the slurry is fed through a manifold and feed lines into closed and sealed filter plate pack chambers, whereupon the solids collect on the filter cloth and form a cake. The filtrate penetrates the cloth and flows to the filtrate space and from there through the outlet side of the plate of the tube manifold. Film extrusion is used for filtration of green liquor but not for filtration of white liquor. Thereafter, in step 2, the filtrate remaining in the chambers is pressed through the fabric to the filtrate side by means of membranes and pressurized water of the chambers by passing water through the tubes and bottom plates of the plates into the space above the rubber membranes. An alternative to film extrusion is air drying, which, if necessary, replaces the first film extrusion in step 2 and removes the second film extrusion in step 4. Alternative air drying is used to filter lime sludge. 10 at the end of the pressing, in step 3 is washed cake remaining on the cloth with water, whereby the water lifts the press diaphragms against the chamber plates, and the wash filtrate passes through the cake and the cloth to the manifold. The wash water is fed to the filter chambers along the same manifold as the slurry. The precipitate from the green liquor filtration is not washed, while the lime cake from the white liquor filtration 15 is washed with warm water. After washing the cake, in step 4, the cake is pressed with a second film squeeze into the filtrate side of the remaining washing liquid and fed to the air separator of the weak liquor tank. The second film compression is similar to the first film compression. After the second film pressing, in step 5, the cake is dried with compressed air 20 coming from the manifold. As the air fills the chambers, it lifts the clamping membranes and the water in the chambers is discharged to the filtrate side. After air drying, in step 6, the filter plate pack is opened up and emptied by moving the filter cloth forward. The cake will drain on the conveyors on either side of the filter.

> ·: 25 The structure of the storage tank for the honey product and the unloader for the lime product are of considerable importance for the success of the storage. A well-resolved solution for a substance that is so difficult to handle is important for the continuity of the process. The structure of the storage tank is further illustrated in Figure 3.

The honey product storage tank consists of a top and 30 unloading sections beneath it. The length and width of the top are determined by the characteristics of the lime product. The longitudinal walls of the upper portion open downward at an angle of 1-2; the upper part is insulated. Preferably, the longitudinal * walls of the discharge section open upwardly at an angle of 15-30, so that normal operation of the lime load does not interfere with the operation of the discharge. Discharge end discharge wall (beak) Auke-35 aa down 30 - 60 angles. The discharge opening in the nose is curved in the nose. The other end of the discharge section is closed. The lower part is coated with a 6 105933 release liner based on product characteristics; suitable coatings include epoxy and polyfluoroethylene plastics. The walls of the discharge section have rounded corners.

The manure storage is continuous and the lime slurry can be discharged with the desired steady stream of 5 out of the silo with the right unloader solution. If necessary, the discharge of the lime storage can be stopped even when the storage is full and the leaching can be resumed after several days.

For example, in a test run of about 20 m3 of lime stock, the lime stock was run at 76-80 wt% dry litter with the discharge on top of 10, after which the unloader was stopped and allowed to stand in the tank for 3 days. Despite the winter conditions (-10 ° C), even after the standstill, the lime could be discharged in a steady stream. The dry matter content of the leached lime was 78-80% by weight.

The method according to the invention has the following advantages over the known 15 methods: - lower space and structural requirements - less serviceable equipment - higher solids slurry solids content and less green liquor slurry production , mm. while the temperature of the filtered white liquor remains high 25 - higher lime dry solids content - more stable use of lime kiln thanks to residual alkali lime content and adjustable solids content - lower white liquor pellet content - lower amount of weak liquor per cubic meter of white liquor g

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Claims (4)

105933 Patent claim my other A process for the preparation of white liquor by dissolving black liquor in a chemical obtained by evaporation and combustion of black liquor, causticizing the green liquor formed in leaching 5, separating the lime precipitate and burning it into lime to be used for causticizing the green liquor, with a pellet content of less than 20 mg / l and a solid precipitate with a dry matter content of 55 to 75% by weight, 10 (b) The sludge precipitate is directly separated by pressure filtration and the precipitate is washed and dried to give a 75 85 wt.%, And c) transferring the resulting lime product to a storage container, where it is discharged and metered in a steady stream to a lime kiln for incineration. The lime mud product storage container useful in the method of claim 1, comprising a downwardly opening upper portion and an upwardly opening discharge portion at the lower end, and a downwardly opening discharge outlet having a discharge opening. Storage container according to Claim 2, characterized in that the longitudinal walls of the upper part are open downwards and the longitudinal walls of the discharge part are upwards open, and the discharge part is internally disengaged. Storage container according to Claim 3, characterized in that the release coating is a fluorocarbon polymer. > · 1. i · «105933