The present invention relates to a method of producing white Iiquor by dissolving chemical smelt obtained from evaporating and burning of black Iiquor, by causticizing the green liquor formed in the dissolution, by separating lime mud (calcium carbonate) and burning it to lime (caicium oxide) to be used in causticizing of green liquor The method of the invention is characterized in that a) before causticizing suspended solids are directly removed from green iiquor by pressure filtration to get green Iiquor with a suspended solids i e dregs content of less than 20 mg/l, b) lime mud (calcium carbonate) is separated directly by pressure filtration while the lime mud cake is simultaneously washed and dried to pro¬ duce a lime mud product with a dry soiids content of 75-85% by weight and c) the lime mud product is led to a storage silo from where it is dis- charged and fed as an even flow to a lime kiln and burned to lime (calcium oxide)

It is an object of the invention to provide an improved method by simplifying the soda recovery plant and the causticizing process of a sulphate pulp mill to improve the quality of the product white Iiquor and to decrease the amount of green liquor dregs and resulting weak iiquor In addition purer green iiquor and a drier lime mud product is obtained

In present causticizing processes employed at sulphate puip mills the smelt from a recovery boiler is dissolved in a smelt dissolver in weak Iiquor and other process waters to give green Iiquor Green iiquor is usually led via an equalizing tank to a green Iiquor clarifier In the green iiquor clarifier dregs are separated Clarified green iiquor is led to a green Iiquor tank and then to a lime slaker The dregs separated in the green liquor clarifier are led by a pumping system to a dregs filter and the resulting dregs cake is dumped and the filtrate is led to a weak Iiquor tank In the lime slaker burnt lime is slaked with green Iiquor to form a lime mud-white iiquor mixture lime milk This mix¬ ture is led to a causticizing tank where the causticizing reaction is comDleted From the causticizing tank nme milk is led to a white liquor filter from wnere fil¬ trated clarified white liquor is led to a digester The separated soiids lime mud and part of the white Iiquor are led to a iime mud silo and suspended in warm water After filtration the dry solids content of iime mud is about 30-40°/c From the silo the nme mud suspension is led to a lime mud filter from where tne lime

mud washed with water, is led to a iime kiln and burned After the filter the dry solids content of lime mud is generally at least about 70% The filtrate ob¬ tained from the wash filter is led to a weak Iiquor tank Burnt lime obtained from the lime kiln is led to iime storage from where the burnt iime is fed to the lime slaker

Present causticizing processes generally use a large green Iiquor clarifier to separate dregs from green liquor, which involves major investment and emptying costs Moreover, the dregs suspension produced has a low dry solids content In present causticizing processes, lime mud and white Iiquor are usually separated in a tube filter comprising perforated tubes that are covered with filter cloth and hang in the lime mud suspension and through which white Iiquor is pressed The dry solids content of the removed iime mud is about 30- 35% In the next wash step lime mud is washed by diluting with water and subsequent concentration The dry soiids content of the produced lime mud is about 35% The lime mud suspension is filtered and the lime mud washed by displacement in a vacuum filter, generally giving the removed iime mud a dry solids content of at least about 70%, but less than 75%

A method of separating white liquor and lime mud, and green Iiquor and green Iiquor slurry, in two steps is known for example from publication Fl 87,347 The lime mud containing white Iiquor obtained in the first step, and correspondingly the green liquor slurry, is diluted with water and possibly stored and in the second filtering step pure iime mud and correspondingly relatively dry green Iiquor waste is obtained, and quite large amounts of weak Iiquor

Publication Fl 923, 127 discloses a method of filtering the causticiz¬ ing product and of increasing the dry solids content of the separated suspen¬ sion the separation of white iiquor and the washing and drying of the lime mud usually taking place in one process step A pressure filter is preferably used as filter the time mud dry solids content after filtration being high, about 75% The lime mud can then be dried by e g pressurized air No mention is made whether tne resulting lime mud can be stored or if it can be transferred to a kiln as an even flow

Variations in properties of filtered washed and dried lime slurry i e dry soliαs content, residual alkali and particle size and structure cause changes in the quality of burnt lime in known causticizing processes Addition-

ally variations in iime slurry dosage affect lime burning The even feed of the invention improves the use of a lime kiln and reduces variations in product quality, i e burnt lime The properties of burnt lime, e g particle size affect lime slaking and the causticizing reaction The method of the invention is also simpler than known methods a significant part of the equipment used in known methods can be omitted Green iiquor is filtered directly in one device, and the suspended solids, i e dregs content of clarified green Iiquor is less than 20 mg/l, a further advantage being the high dry solids content, 55-75% by weight, of the separated dregs Cleaner green liquor affects advantageously the rest of the causticizing proc¬ ess as the amount of harmful suspended solids reduces White Iiquor and lime mud are also separated and lime mud washed and dried with a single device the dry solids content of the lime mud product being high 75-85% by weight The dregs content of white iiquor is generally less than 20 mg/l Only about 0 8-1 m ³ per ton of dry lime mud of wash water is needed Moreover, the resid¬ ual alkali of the iime mud can be adjusted, for example to less than 0 1 % by weight calculated as Na ₂ O Good quality green Iiquor used in the causticizing reaction produces a purer and more homogeneous lime mud Feeding a ho¬ mogeneous lime mud product from a lime mud storage silo to a kiln simplifies kiln adjustment and reduces the need for drying energy thereby lowering costs It may be mentioned that the yield of white Iiquor is high

The invention also relates to a storage silo for a lime mud product applicable to the method of the invention

In the following the method and the storage silo of the invention will be described in greater detail

Figure 1 shows the process steps of the method of the invention Figure 2 shows filtration and washing steps in a pressure filter ap¬ plicable to the method of the invention

Figure 3 shows a lime mud storage silo applicable to the method of the invention

In Figure 1 , numeral 35 refers to a take-off line for feeding smelt to a smelt dissolver 1 from a recovery unit (not shown) The smelt dissolver for producing green Iiquor comprises an agitator 2 A line 27 for weak Iiquor also ends in the smelt dissolver A line 3 for thick green Iiquor leads from the smelt dissolver to an equalizing tank 4 for green Iiquor feed, from where green Iiquor is fed by means of a pumping system 6 automatically to an intermittently oper-

ating pressure filter 8 A conveyor 36 for lime mud or burnt lime ends in the equalizing tank 4 and can be employed to add lime mud or burnt lime used as additive in the filtration. A return line 9 for filtrated green Iiquor. a prefiltrate, ends in the equalizing tank 4, which comprises an agitator 5. A return line 7 for pumping green Iiquor aiso ends in the equalizing tank and is used when no green Iiquor is fed in a filtering cycle. The pressure filter 8 separates sus¬ pended soiids, i.e. dregs 37, from green Iiquor. Filtrated clarified green Iiquor is conveyed via a line 10 to an air separator 11 for green Iiquor, from where the green Iiquor is then led via a line 12 to a storage tank 13 for green Iiquor From the storage tank the green Iiquor is led via a line 14 to a iime slaker 15. The green Iiquor dregs in the filter 8 are dried by diaphragm pressing and air Air is separated from clarified green Iiquor by the air separator 11.

An alternative is to add burnt iime or lime mud to green Iiquor via the conveyor 36 to the equalizing tank 4 Adding burnt lime or lime mud con- tributes to the filtration and adds to filtrate capacity, which can also be influ¬ enced by changing the feeding pressure of green Iiquor.

A line 16 for a mixture of lime mud product and white liquor, lime milk, leads from the lime slaker 15 to a causticizing tank 17. Lime milk is con¬ veyed from the causticizing tank by means of a pumping system 19 via a line 18 to a pressure filter 21 which separates white Iiquor and iime mud A ime 20 for feeding white Iiquor filtrate, a prefiltrate, also ends in the causticizing tank Filtered white Iiquor flows via a line 22 to a digester (not shown) From the pressure filter 21 weak iiquor is led via a line 23 to an air separator 24 where air is separated from weak iiquor and weak Iiquor is led via a line 25 to a weak iiquor tank 26, and then via a line 27 to the smelt dissolver 2. The lime mud that was separated, air dried and washed with water in the pressure filter 21 , is led by a conveyor 28 to a lime mud silo 29 from where the lime mud is dis¬ charged and fed by a conveyor 30 as an even flow to the inlet of a iime kiln 31 The residual alkali of the lime mud separated in the filter can be adjusted. The burnt lime is conveyed from the lime kiln via a conveyor 32 to a iime silo 33 and from there the burnt lime is fed via a line or a conveyor 34 to the lime slaker 15.

Figure 2 shows the principle of a pressure filter applicable to a method of the invention in different steps, in step 1 slurry is fed via a distribu- tor line and feeding lines to the chambers of a closed and sealed filter plate pack, whereby the solids gather on the filter cloth forming a cake The filtrate

penetrates the cloth and flows to the filtrate space and from there via a tube in the plate to the collector line at the discharge side Diaphragm pressing is used in filtering green Iiquor but not in filtering white Iiquor After this in step 2, the filtrate remaining in the chambers is pressed by means of chamber dia- phragms and pressure through the cloth to the filtrate side by leading pressing water through the tubes in the plates and the bottom plates to a space above the rubber diaphragms An alternative to diaphragm pressing is air drying which can be used to replace the first diaphragm pressing in step 2 and to eliminate the second diaphragm pressing in step 4 Air drying is used as an alternative in filtering lime mud

At the end of the pressing, in step 3, the cake remaining on the cloth is washed with water whereby the water lifts the press diaphragms against the chamber plates, and the wash filtrate penetrates the filtrate side of the cake and the cloth to the collector line Wash water is fed to the filter chambers via the same distπbutor line as the slurry The dregs cake formed in filtering green Iiquor is not washed, whereas the lime mud cake formed in white iiquor filtration is washed with warm water After the cake has been washed, in step 4 the washing liquid remaining in the cake is pressed from it by a second diaphragm pressing to the filtrate side and is led to the air sepa- rator of the weak Iiquor tank The second diaphragm pressing is similar to the first one After the second diaphragm pressing, in stage 5, the cake is dried with pressure air coming from the distributor line When air fills the chambers, it lifts the pressing diaphragms up and the water in the chambers is removed to the filtrate side After air drying, in step 6, the filter plate pack is opened to an upper position and emptied by moving the filter cloth forward The cake is discharged to conveyors on both sides of the filter

The structure of the storage silo and the discharger for lime mud product piav a significant role in successful storage For continuous process¬ ing it is vital to choose the right solution for a substance that is difficult to han- die The structure of the storage silo is shown in detail in Figure 3

A storage silo for lime mud product consists of an upper part and a discharge part below it The properties of the lime mud product determine the length and breadth of the upper part The longitudinal walls of the upper part open downwards at an 1 -2E angle and the upper part is thermally insulated The longitudinal walls of the discharge part open upwards advantageously at a 15-30E angle for the normal pressure of the lime mud load not to disturb the

operation of the discharger The wall (spout) of the discharge end of the dis¬ charge part opens downwards at a 30-60E angle The discharge opening in the spout is arched at spout level The other end of the discharge part is closed The lower part is overlaid with an antiadhesive coating determined by the properties of the lime mud product, suitable coatings include epoxy resm and polyfluoroethylene The wall angles of the discharge part are rounded

The lime mud storage operates continuously and iime mud can be discharged at a desired even flow from the silo by using the right discharger If needed the discharge of lime mud storage can be interrupted even with full storage and lime mud discharge can be resumed even after several days

In a test run with an about 20m ³ lime mud storage the lime mud storage was filled with iime mud having a dry solids content of 76-80% by weight while the discharger was on and then the discharger was halted and the lime mud was left in the silo for three days In spite of winter conditions (- 10EC) it was possible to discharge the lime mud as an even flow after the halt The discharged lime mud had a dry soiids content of 78-80% by weight

The method of the invention offers following advantages compared with known methods

- less requirements on space and structure - less equipment to be maintained

- higher dry solids content of green Iiquor slurry and less green liq¬ uor dregs waste

- lower dregs content in clear green Iiquor

- lower amount of circulating process impurities due to efficient di- rect filtration of green Iiquor

- less energy needed as the temperature of e g the filtrated white Iiquor remains high

- higher dry solids content of lime mud

- more even use of lime kiln due to adjustable residual alkali and dry solids content of lime mud

- lower suspended solids content in white Iiquor

- less weak Iiquor per m ³ of white iiquor

- less drying energy needed in lime kiln