Method and apparatus for obtaining strong white liquor and lime mud with low residual alkali level

Abstract

The method and apparatus are for obtaining strong white liquor and lime mud with low residual alkali content from causticized liquor. The first filter (F.sub.WL) receives causticized liquor from causticizing vessels (CT.sub.1-CT.sub.2-CT.sub.3) and is designed as pressurized two stage disc filter with 2 separate vat parts. In the first vat part non-diluted white liquor is obtained as a filtrate from the causticized liquor. The first lime mud cake accumulated on the filter discs in the first vat part is scraped off and diluted before being fed to the second vat part. In the second vat part weak liquor is obtained as a filtrate from the re-slurried first lime mud cake. The second lime mud cake accumulated on the filter discs in the second vat part is washed before being scraped off and diluted before being fed to subsequent lime mud handling.

Claims

1. A method of extracting strong white liquor and lime mud with low residual alkali content from causticized liquor emanating from causticizing vessels in the recovery process of a pulp mill, comprising the following steps in sequence: adding burnt lime to green liquor to start a causticizing reaction to produce a causticized liquor; feeding the causticized liquor to a first part of a vat of a pressurized disc filter wherein the first part of the vat has a first set of filter discs and a separate first filtrate reception system, thereafter extracting strong white liquor from the causticized liquor in the first filtrate reception system by passing the liquid part of the causticized liquor through filter media of the first set of filter discs, while collecting the residual lime particles of the causticized liquor as a first lime cake on the face of the filter media of the first set of filter discs and using no wash water on the formed first lime cake on the filter media, thereafter scraping off at least a part of the first lime cake from the face of the filter media on the first set of filter discs, wherein the dry matter content of the scraped off first lime cake is higher than 50 wt-% and collecting the scraped off first lime cake in a first lime cake collection system while diluting the scraped off first lime cake with a suitable low alkali liquid to a dry matter content of the diluted first lime cake slurry less than 25 wt-%; feeding at least a part of the diluted first lime cake slurry to a second part of the vat of the pressurized disc filter wherein the second part has a second set of filter discs and a separate second filtrate reception system, thereafter extracting weak white liquor from the diluted first lime cake slurry in the second filtrate reception system by passing the liquid part of the diluted first lime cake slurry through filter media of the second set of filter discs, while collecting the residual lime particles of the diluted first lime cake slurry as a second lime cake on the face of the filter media of the second set of filter discs and using wash water on the formed second lime cake on the filter media in order to displace residual alkali in said second lime cake, thereafter scraping off at least a part of the second lime cake from the face of the filter media on the second set of filter discs, wherein the dry matter content of the scraped off second lime cake is higher than 50 wt-% and collecting the scraped off second lime cake in a second lime cake collection system while diluting the scraped off second lime cake with a low alkali liquid to a dry matter content of the diluted second lime cake less than 25 wt-% and passing the diluted second lime cake slurry to at least one lime mud thickening stage before feeding the thickened lime mud to a lime kiln.

2. A method according to claim 1, wherein the method further comprises pressurizing the pressurized disc filter with at least 0.7 bar above atmospheric pressure in both the first and second vat part.

3. A method according to claim 2, wherein the method further comprises pressurizing the pressurized disc filter with at least 1.0 bar above atmospheric pressure in both the first and second vat part.

4. A method according to claim 3, wherein the method further comprises providing the pressurized disc filter with a number of filter discs in the first set of filter discs greater than the number of filter discs in the second set of filter discs.

5. A method according to claim 3, wherein the method further comprises establishing a level of the causticized liquor in the first vat part that is higher than the level of diluted first lime cake slurry in the second vat part, thus obtaining a height difference (H) between liquid surfaces in first and second vat part.

6. A method according to claim 3, wherein the method further comprises using the extracted weak white liquor from the diluted first lime cake slurry as at least part of the suitable low alkali liquid used as dilution liquid diluting the scraped off first lime cake.

7. A method according to claim 3, wherein the method further comprises feeding a part of the diluted first lime cake slurry to a subsequent lime mud thickening stage before feeding the thickened lime mud to a lime kiln.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 Show the principal process stages of a recovery process in the causticizing plant;

(2) FIG. 2 show an alternative of FIG. 1, but with the lime mud thickening stage replaced with a 2-stage lime mud filter according to U.S. Pat. No. 8,002,994;

(3) FIG. 3a show a 2-stage white liquor filter according to the invention that could replace the white liquor filter in FIG. 1 in order to reduce TRS emissions in the flue gases from the lime kiln;

(4) FIG. 3b show a variant of FIG. 3a where the slurry level in first and second vat parts are different, H, with a lower level in 2.sup.nd vat part.

DETAILED DESCRIPTION OF THE INVENTION

(5) In FIG. 3a is shown a 2-stage white liquor filter 10 according to the invention that could replace the white liquor filter in FIG. 1 in order to reduce TRS emissions in the flue gases from the lime kiln. The causticized green liquor KGL is fed by a slurry pump SP.sub.1 to a first vat part 10a separated from a second vat part 10b by a dividing wall 20. The wall 20 may as shown extend up to the bottom part of the shaft 30 and thus form a common gas phase above the wall 20. A liquid level of causticized green liquor KGL is established below a lime mud chute opening 41a. The entire filter is pressurized by mill air PMA and a differential pressure over the filter elements is provided by a compressor C connected with the suction side to liquor separation tanks 51 and 52 and feeds pressurized gas to the gas phase of the filter as shown. In the first vat part is the shaft 30 rotated by the drive means M and thus also the filter discs connected to the shaft. The strong white liquor is filtered through the filter discs 33a and led to a first filtrate reception system, consisting of a first filtrate channel 31 in the shaft and a liquor separation tank 51. The lime mud cake accumulated on the filter discs 33a is scraped off by scraper blades 34a located immediately above the lime mud chute opening 41a, and the scraped off lime mud cake, typically holding a dry matter content between 65-85%, is simultaneously diluted by adding dilution water reaching a dry matter content well below 25 wt-%. The dilution water added at the entry of the lime chute opening 41a may be weak white liquor (WWL) or hot water HW or condensate or a mixture thereof, and may optionally be added with an amount of recirculated volume RE.sub.1 of diluted lime mud cake in order to keep flow velocity high, avoid settling and provide better mixing in reception chutes. The diluted lime mud slurry LM.sub.1 is then immediately forwarded to a second vat part 10b by a slurry pump SP.sub.2. During the white liquor separation in the first vat part, no washing water is added on the lime mud cake formed on the filter discs, which results in non diluted strong white liquor.

(6) In the second vat part 10b, the liquor phase with its residual content of alkali is filtered through the filter discs 33b and led to a second filtrate reception system, consisting of a second filtrate channel 32 in the shaft and a liquor separation tank 52. The lime mud cake accumulated on the filter discs 33b is first washed by a wash nozzle arrangement WN before being scraped off by scraper blades 34b located immediately above the lime mud chute opening 41b. The wash water is preferably any low alkaline wash water and preferably hot water HW. The scraped off lime mud cake, typically holding a dry matter content between 65-80 wt-%, is simultaneously diluted by adding dilution water reaching a dry matter content well below 25 wt-%. The dilution water added at the entry of the lime chute opening 41b may preferably be hot water HW or any other process liquid that do not contain alkali or sulfur that may increase TRS emissions in flue gases from the lime kiln, and may optionally be added with an amount of recirculated volume RE.sub.2 of diluted lime cake in order to keep flow velocity high, avoid settling and provide better mixing in reception chutes. The diluted lime mud slurry LM.sub.2 is then forwarded to the subsequent process stage, preferably a lime mud thickening stage, by slurry pump SP.sub.3.

(7) As indicated in FIG. 3a could also a shunt line SL controlled by a shunt valve SL.sub.V be connected to the second vat part 10b, enabling a by-pass of the diluted first lime mud cake slurry, by-passing the filter discs 33b of the second vat part. This by pass is preferably opened intermittently if liquid level in vat part 10b becomes excessive or if any other overloading of process capacity occurs. The main objective that most of the lime mud is thickened and thickened/washed in 2 stages may still be obtained even if some 5-10 wt-% of lime mud flow is by-passed the second stage.

(8) As indicated in the table of FIG. 1 is typically a strong white liquor SWL obtained with an effective alkali concentration (counted as NaOH) of about 115 g/l (same as in the causticized liquor), as no washing water is used for the lime cake formed in first vat part. As the lime cake scraped off in first vat part contains residual alkali, typically some 5-10 wt-%, could weak liquor be used as dilution liquor at least in part. As indicated is typically a weak white liquor WWL obtained from second stage with an effective alkali concentration of about 15 g/l, i.e. only some 13 wt-% of the concentration in the strong white liquor. However, in the second vat part is preferably only hot water or similar low alkali liquids used as washing liquid in order to displace residual alkali from the lime cake formed in the second vat part.

(9) In FIG. 3b is essentially the same 2-stage disc filter shown as in FIG. 3a, but with some modifications on how they are designed and operated. In this embodiment is shown that the liquid levels in first vat part 10a and second vat part 10b are different, i.e. that the liquid surface in first vat part 10a with causticized liquor in the first vat part that is higher than the level of diluted first lime cake slurry in the second vat part, thus obtaining a height difference H between liquid surfaces in first and second vat part. There are some process advantages with this design. As the liquid level is higher in the first vat part, so the effective filtering area for strong white liquor is increased, as a larger part of the filter discs are immersed in the causticized green liquor. The higher level also gives an increased differential pressure over the discs and thus an increase in hydraulic capacity. One option is as disclosed in FIG. 3b to establish a liquid level that is above the filtrate reception channel 31 in the shaft 30, which results restricted passage of the gas phase through the filter discs and down to the liquor separation tank 51. This will reduce operating costs for the compressor C, and as the first stage is optimized for extracting strong white liquor, rather than high dryness in the lime cake, an optimum filtering capacity for strong white liquor is obtained. On the other hand, with a lower liquid level in second vat part 10b, an increased gas flow from the gas phase, through the filter discs and down to the liquor separation tank 52 is obtained. The pressurization of the entire gas phase in this design apply full pressure of the liquid level in first vat part but less gas flow, while the major part of the gas flow is established in second vat part where the displacement, i.e. washing and thickening, is sought for. As a consequence, displacement of residual alkali and wash liquid will be improved obtaining a lower level of residual alkali in the lime mud cake in the second vat part. As indicated in FIG. 3b, the dividing wall 20 in the pressure vessel is extended upwards, and a sealing is arranged around the drive shaft preventing slurries in the separate vats from migrating into the opposite vat. Such a sealing may also be provided with sealing liquid, preferably weak white liquor. Optionally the dividing wall 20 may extend all the way upwards, such that each gas phase in the separate vats are separated and may be pressurized independently from the other, this however requires more expensive design and complex control of the pressurizing means.

(10) Tests Between Different Systems.

(11) Different types of filtering systems according to options a)-d) have been studied as of effects on TRS emissions in flue gases from lime kiln. (TRS=Totally Reduced Sulphur). In this comparison, the same quantity and quality of causticized slurry was used, having a White Liquor content of 90 wt-% with an effective Alkali level of about 115 g/l as well as a lime mud content of 10 wt-% with normal softwood/hardwood filterability properties. a) WLF.sub.P+LMT b) WLF.sub.P&W+LMt c) WLF.sub.P&W+LMT+LMT [according to U.S. Pat. No. 8,002,994, wherein both LMT stages are integrated in same filter apparatus] d) WLF.sub.P+WLF.sub.P&W+LMT [according to the invention described above wherein WLF.sub.P and WLF.sub.P&W is integrated in same filter apparatus]

(12) All filters above are disc filters with the same filter area per filter. WLF.sub.P is a 1 stage pressurized white liquor filter without washing; WLF.sub.P+W is a 1 stage pressurized white liquor filter with washing; LMT is a 1 stage thickening filter of the vacuum type with washing.

(13) Lime mud filters, with or without washing are conventionally vacuum type filters, while many white liquor filters are pressurized in order to maintain temperature of the white liquor and avoid boiling in the filtrate. The maximum vacuum level in atmospheric filters is practically found at some 0.2-0.3 bar (absolute) pressure, i.e. a differential pressure over filter surface of about 0.7-0.8 bar. At these low pressures in filtrates (i.e. strong or weak liquor), the liquid may start to boil if the temperature is close to boiling point. At 0.21-0.28 bar (absolute) pressure the water boils at 60.8 C. and 67.2 C. respectively. Normally the temperature is decreased well below the boiling point by adding colder dilution liquids, but this decreases filterability as viscosity decreases with temperature. In pressurized filters on the other hand, temperature in the filtrate can be kept high with no boiling as a result and thereby minimize the loss in temperature.

(14) Option a) resulted in a residual alkali content of 6% in the lime mud from the WLF.sub.P filter at 75% dry matter content. The residual alkali content in the lime mud after the LMT thickening filter was about 0.25% at 75% dry matter content. The TRS level in the flue gases from the lime kiln was found to be in the range 7-15 mg/m.sup.3.

(15) Option b) resulted in a somewhat lower residual alkali content of 3.5% in the lime mud from the WLF.sub.P&W filter at 70% dry matter content due to washing effect. The residual alkali content in the lime mud after the LMT thickening filter was about 0.1% at 75% dry matter content. The TRS level in the exhaust gases from the lime kiln was found to be in the range 3-10 mg/m.sup.3.

(16) Option c) resulted in a residual alkali content of 3.5% in the lime mud from the WLF.sub.P&W filter at 70% dry matter content, same as in option b) in this stage. The residual alkali content in the lime mud after the first LMT thickening filter with washing was about 0.5-1% at 65% dry matter content. Finally, the residual alkali content in the lime mud after the second LMT thickening filter was about 0.01-0.05% at 75% dry matter content. The TRS level in the flue gases from the lime kiln was found to be in the range 0.5-3 mg/m.sup.3.

(17) Option d) resulted in a residual alkali content of 6% in the lime mud from the WLF.sub.P filter at 75% dry matter content, same as in option a) in this stage. The residual alkali content in the lime mud after the WLF.sub.P&W filter with washing was however surprisingly low at about 0.25-0.5% at 65% dry matter content. Even though the residual alkali level was 71% higher than after first filter stage in option c) a remarkable wash out of residual alkali was achievable in the WLF.sub.P&W filter. The explanation is that the pressurized 2.sup.nd stage in option d) could have a higher filter load, partly because of higher differential pressure, compared with a vacuum filter, and partly because the viscosity of the lime mud slurry is lower if kept at high temperature which is possible due to the higher pressure. Finally, the residual alkali content in the lime mud after the LMT thickening filter was about half of that obtainable in option c), i.e. 0.005-0.025% at 75% dry matter content. The TRS level in the flue gases from the lime kiln was found to be in the range 0.25-1.5 mg/m.sup.3.

(18) The results from the tests with options a to d are listed in below table;

(19) TABLE-US-00001 Filter Dry Residual TRS Lime Area matter % Alkali, % kiln mg/m.sup.3 Option a) WLF.sub.P A 75 6.sup. Dilute To next filter 10 LMT B 75 0.25 7.0-15 Option b) WLF.sub.P&W A 70 3.5 Dilute To next filter 10 LMT B 75 0.1 3.0-10 Option c) WLF.sub.P&W A 70 3.5 Dilute To next filter 20 LMT (1.sup.st) B1 65 0.5-1.sup. 3.0-10 Dilute To next filter 10 LMT (2.sup.nd) B 75 0.01-0.05 0.5-3.0 Option d) WLF.sub.P A 75 6.sup. Dilute To next filter 15 WLF.sub.P&W A2 65 0.25-0.5 Dilute To next filter 10 LMT (2.sup.nd) B 75 0.005-0.025 0.25-1.5

(20) Test Remarks; All tests done with causticized green liquor, having a lime content of 10 wt-%, and effective alkali level (as NaOH) at about 115 g/l. Filter area B1 is about the same as for A2 (for order of comparison of principles) Filter area of B1<B, and A2<A (less number of filter discs) In all examples is the cake after filter diluted to 10% dry matter content, except for; i. To 1.sup.st LMT filter in option c; ii. To WLF.sub.P&W filter in option d As next filter has less filter area (B1 or A2) and hydraulic load limits lower consistency level.

(21) The tests shows that a remarkable improvement could be obtained by simply arranging the two-stage pressure disc filter directly after the causticizing vessels, instead of arranging the two-stage atmospheric disc filter directly ahead of the lime kiln. Conventionally further lowering of emissions is rather costly when starting at an already low emission level, and costs commonly increase exponentially in proportion to further reductions in emissions. These are well known effects when reducing pollutions in any media. According to the invention a remarkable reduction of TRS emissions in the flue gases from the lime kiln could be obtained, i.e. almost 50% reduction, by replacing an existing white liquor filter with a 2-stage white liquor filter, rather than replacing an existing lime mud wash with a 2-stage lime mud wash.

(22) The tests were made from typical green liquor from softwood pulping processes, and if other silica rich wood material is used in the pulping process, such as annual plants and bamboo etc, then other process parameters would be at hand such as higher residual alkali levels, and lower dry matter content in cakes and order of dilution to next filtering stage. Filterability of silica rich green liquors is lower, and typically the cakes has some 10-15% units lower dryness, i.e. down to about 50% instead of 65% dry matter content in cakes, and cakes are typically diluted to a lesser extent to some 10-15% units higher consistency, i.e. 20-25% instead of 10%, in following filtration.

(23) From this example with silica rich green liquors it is understood that process parameters may change even between green liquors from the same type of origin, i.e. softwood processes and also in comparison with hardwood processes as they all may have different process set up as of cooking and/or bleaching, all contributing to different characteristics of the green liquor. However, the inventive concept is still applicable for those other types of green liquors of different origin, and provide for same improved relative effect in reduced residual alkali levels in lime sent to lime kiln and reduced TRS emissions in flue gases from lime kiln.

(24) While the present invention has been described in accordance with preferred compositions and embodiments, it is to be understood that certain substitutions and alterations may be made thereto without departing from the spirit and scope of the following claims.