Green liquor clarification using sedimentation tank during white liquor preparation

Abstract

The invention relates to a method for clarification of raw green liquor in a sedimentation tank. According to the invention is a part of dregs separated in the sedimentation tank recirculated back into the inflow of raw green liquor, and preferably after passing the dregs through at least one turbulence generator (30, 31) that could break up larger dregs particles into smaller dregs particles, and thus create larger total surface on the dregs particles, improving sedimentation rate in the sedimentation tank. In a preferred embodiment is the recirculated dregs added into the flow of raw green liquor before a flocculant is added into the flow of raw green liquor and mixed recirculated dregs.

Claims

1. A method for clarification of raw green liquor, comprising: feeding the raw green liquor to a sedimentation tank through a mixing chamber with a multitude of lower outlets that provide for an even distribution of the raw green liquor at low flow velocity in a lower half of the sedimentation tank via a rotating distributor sweeping over a cross section of the sedimentation tank, said sedimentation tank being equipped with an underflow outlet in a bottom of the sedimentation tank for settled dregs and an overflow outlet in an upper part of the sedimentation tank for clarified green liquor; recirculating a part of the dregs fed out from the underflow outlet, back to infeed of raw green liquor; causing the part of the dregs fed out from the underflow outlet, to pass through a first turbulence generator; causing the part of the dregs having passed through the first turbulence generator, to pass through a second turbulence generator that is directly connected to the first turbulence generator via a pipe; and feeding (1) the raw green liquor and (2) the part of the dregs having passed through the second turbulence generator, to the mixing chamber of the sedimentation tank, wherein the first turbulence generator and the second turbulence generator are configured to break up some of the dregs into smaller fractions, such that a total exposed area of the dregs is increased.

2. A method for clarification of raw green liquor according to claim 1, wherein at least one of the first turbulence generator or the second turbulence generator is a pump.

3. A method for clarification of raw green liquor according to claim 2, wherein the part of the dregs fed out from the underflow outlet passes through at least one centrifugal pump with chopping action on a pumped flow of dregs.

4. A method for clarification of raw green liquor according to claim 1, wherein the recirculated part of the dregs has a volume ratio versus a volume of raw green liquor in the range of 5-30%.

5. A method for clarification of raw green liquor according to claim 1, wherein a flocculant is added to a flow of raw green liquor.

6. A method for clarification of raw green liquor according to claim 5, wherein a flocculant is added to the flow of raw green liquor after having been subjected to a turbulence generator and before being fed to the mixing chamber of the sedimentation tank.

7. A method for clarification of raw green liquor according to claim 1, wherein a flocculant is added to a flow of raw green liquor and recirculated dregs in the mixing chamber under the influence of a rotating turbine mixer.

8. A method for clarification of raw green liquor according to claim 7, wherein a total amount of flocculant added to a gross flow of raw green liquor and recirculated part of dregs is in the range of 1-5 ppm.

9. A method for clarification of raw green liquor according to claim 8, wherein the flocculant is a polymer which comprises polyamine or polyacrylamide.

10. A method for clarification of raw green liquor according to claim 1, further comprising: causing a mixture of (1) a flow of raw green liquor and (2) the part of the dregs having passed through the first turbulence generator, to pass through the second turbulence generator before being fed to the mixing chamber of the sedimentation tank.

Description

SHORT DESCRIPTION OF THE DRAWINGS

(1) In the following schematic drawings are details numbered alike in figures, and details identified and numbered in one figure may not be numbered in other figures in order to simplify figures, where:

(2) FIG. 1 is a standard set-up with a sedimentation tank for green liquor clarification in the recovery island;

(3) FIG. 2 is a detailed cross-sectional view of the sedimentation tank shown in FIG. 1, sold by Valmet as OptiClear™;

(4) FIG. 3 is an inclined second view of the OptiClear™ as shown in FIG. 2;

(5) FIG. 4 is similar view as shown in FIG. 3, but with the inventive modification; and

(6) FIGS. 5a-5g show the results on green liquor clarity simulating different operational set-ups of the OptiClear™ system.

DETAILED DESCRIPTION OF THE INVENTION

(7) In FIG. 1 is shown a standard set-up with a sedimentation tank for green liquor clarification in the recovery island. The raw green liquor is emanating from a conventional smelt dissolving tank (not shown) where smelt from a recovery boiler is dissolved in some process liquid. The raw green liquor can be stored in an equalizing tank, EQ tank, before being added to the sedimentation tank, here in form of an OptiClear™ sedimentation tank, where dregs are separated in an underflow outlet in the bottom and clear green liquor is fed to an intermediate green liquor storage tank. The dregs from the sedimentation tank are fed to a dregs filter where residual alkaline liquor is washed out as a weak liquor and final dregs with high dryness are obtained ready to be disposed in a land fill. In next step the green liquor is sent to the first stage of conversion to white liquor in the form of a slaker where burnt lime is added to the green liquor. The burnt lime is typically obtained from a lime kiln (not shown). As the process in the slaker is exothermic, most often the green liquor is passing a green liquor cooler ahead of the slaker, thereby avoiding excess temperature in the slaker. Once the burnt lime is added the conversion to white liquor starts, and as the process is time consuming, several causticizing vessels are arranged in series. Once the green liquor has been converted to white liquor, the process liquor is sent to a white liquor filter, where lime mud is separated from the process liquor and a clear white liquor is obtained.

(8) In FIG. 2 is a detailed cross section view of the sedimentation tank shown, sold by Valmet as OptiClear™. The raw green liquor is fed through an inlet ending up in a central mixing chamber that prevents the turbulence from the inflow of raw green liquor to affect the upper sedimentation volume where clear green liquid accumulates. The stationary mixing chamber contains a turbine mixer, seen in the 50% cut-out section of the mixing chamber wall, having at least three impeller blades. During in-feed of raw green liquor is a flocculant/polymer added into the raw green liquor, a first part of the flocculant added already in the feed pipe ahead of the mixing chamber, but also as indicated in several addition positions, here three positions, at different height positions in the mixing chamber. After being subjected to the mixing action of the turbine mixer, the added flocculant/polymer and the raw green liquor are gently dispersed over the entire cross section of the sedimentation tank by rotating distributor arms with an indicated downwardly directed outflow. The sedimentation starts directly below the distributor, and the dregs with higher density settle towards the bottom while the green liquor ascends upwardly due to lower density. As the green liquor reaches the upper surface, it flows into a collecting channel running around the outer periphery of the sedimentation tank, and is fed out from the collecting channel in an outlet as clear green liquor. The dregs settling to the bottom are subjected to the sweeping action of a rotating arm provided with a number of rakes that scrape the dregs towards a center outlet, having a further cone scraper to the keep the flow of dregs alive, preventing plug formation, as it finally is fed out in a dregs outlet.

(9) In FIG. 3 is shown an inclined second view of the OptiClear™ sedimentation tank for green liquor, and the system is designed for a production capacity of about 2 m.sup.3/m.sup.2, h, i.e. 2 m.sup.3 of clarified green liquor can be obtained for each square meter of cross section area in the sedimentation tank. As also shown here is the polymer/flocculant added both in the in-feed pipe to the mixing chamber, as well as into the mixing chamber, and the amount of flocculant may lie in the order of 1-5 ppm.

(10) In FIG. 4 is a similar view as shown in FIG. 3, but with the inventive modification. The modification consists of adding a pipe for circulating a part of the dregs back to the inflow of raw green liquor. The recirculated flow of dregs can preferably pass a first turbulence generator, here in form of a centrifugal pump 30, that can introduce a mechanical disintegration of larger dregs particles into smaller fractions. Preferably the centrifugal pump 30 has an open impeller with exposed impeller vanes that clash into larger dregs particles and cut them into pieces. Once these dregs particles are disintegrated into smaller fractions, they are reintroduced into the flow of raw green liquor and start as sedimentation surfaces for dry matter in the raw green liquor.

(11) In a later position of the feed pipe, after the introduction of the dregs with reduced particle size, the mixed flow of raw green liquor and particles with reduced size can be subjected to a second turbulence generator that at least provides with a mixing effect, but preferably adds an additional reduction of the particle size of the dregs. As the first turbulence generator, this second turbulence generator can also be a centrifugal pump 31 that introduces a mechanical disintegration of larger dregs particles into smaller fractions. Preferably the centrifugal pump has an open impeller with exposed impeller vanes that clash into larger dregs particles and cut them into pieces.

(12) Once the recirculated dregs with reduced size have passed one or two turbulence generators 30 and 31, the flocculant/polymer is added into the flow, and this last phase mixing is done in order to obtain as much particle growth on the surfaces of the reduced dregs particles, before the flocculant is added.

(13) As shown in FIG. 4, also the turbine mixer 32 in the mixing chamber is adding a final mixing effect before the mixture of raw green liquor and reintroduced dregs particles with reduced size, with any addition of flocculant, are fed out into the lower half of the sedimentation tank.

(14) Tests

(15) In FIGS. 5a to 5g are shown different tests on green liquor clarity obtained from different operational set ups of the OptiClear™ system. The test results are shown as photos in grey-scale that clearly demonstrates the prior processing results with processing results of the invention.

(16) In these tests are different liquids from a recovery process in a Japanese Paper Mill used for simulation of the clarification process, where:

(17) #1a: Raw green Liquor from an Equalizing tank ahead of the OptiClear sedimentation tank, (total suspended solids/TSS 810 mg/l);

(18) #1b: Raw green Liquor from an Equalizing tank ahead of the OptiClear sedimentation tank, (total suspended solids/TSS 721 mg/l);

(19) #1c: Raw green Liquor from an Equalizing tank ahead of the OptiClear sedimentation tank, (total suspended solids/TSS 658 mg/l);

(20) #2: Clarified green Liquor from the overflow outlet of the OptiClear sedimentation tank in a process where 3 ppm of flocculant was used;

(21) #3: Dregs from the dregs outlet of the OptiClear sedimentation tank using flocculant to the OptiClear sedimentation tank, and thus containing residual flocculant in dregs; and

(22) #4; Dregs from the dregs outlet of the OptiClear sedimentation tank using no flocculant to the OptiClear sedimentation tank, and thus no residual flocculant in dregs.

(23) Test 1

(24) This test simulates the clarification results from a process using only raw green liquor with an addition of flocculant. The beaker is filled with 500 ml of #1a liquid, and 3 ppm of flocculant (Kurita PA-322 K) is added. The mixture is agitated at 200 rpm during 30 seconds and is then allowed to settle during 60 seconds. The picture shown in FIG. 5a clearly indicates that a large amount of dregs particles are dispersed in the entire liquid volume above the lower volume of settled dregs.

(25) Test 2

(26) This test simulates the clarification results from a process using prior art recirculation of clarified green liquor at a recirculation rate of 50%. The beaker is filled with 250 ml of #1a liquid, and 250 ml of #2 liquid, and 3 ppm of flocculant (Kurita PA-322 K) is added. The mixture is agitated at 200 rpm during 30 seconds and is then allowed to settle during 60 seconds. The picture shown in FIG. 5b clearly indicates that a larger amount of dregs particles are dispersed in the entire liquid volume above the lower volume of settled dregs, compared to Test 1.

(27) Test 2 was also modified with a smaller recirculation rate, i.e. 20% and a larger recirculation rate, i.e. 60%. The lower circulation rate of 20% resulted in slightly better clarification, but it was still clearly worse than in the no recirculation test. The higher recirculation rate of 60% had the lowest clarification effect than all the other tests with recirculation, as shown in FIG. 5c. The conclusion from Test 1 and Test 2 is that recirculation of clarified green liquor should be avoided.

(28) Test 3 with Inventive Underflow Recirculation

(29) In FIG. 5d is a mixture of 40 ml of liquid #3 added into 500 ml of liquid #1a, with an addition of 3 ppm of flocculant (Kurita PA-322 K). The mixture is agitated at 200 rpm during 30 seconds and is then allowed to settle during 60 seconds. The picture shown in FIG. 5d clearly indicates that best clarification compared to Test 1 and Test 2.

(30) Test 4 with Inventive Underflow Recirculation

(31) In FIG. 5e is 25 ml of liquid #3 added into 500 ml of liquid #1b, i.e. with a TSS of 721 mg/l instead of the 810 mg/l in liquid #1a, with an addition of 3 ppm of flocculant (Kurita PA-322 K). The mixture is agitated at 200 rpm during 30 seconds and is then allowed to settle during 60 seconds. The picture shown in FIG. 5e clearly indicates same order of clarification as in Test 3, even if the rate of dregs recirculated is somewhat smaller.

(32) Test 5 with Inventive Underflow Recirculation and Repeated Turbulence and Late Flocculant Addition

(33) In FIG. 5f is 38 ml of liquid #3 (dregs with residual flocculant) added into 450 ml of liquid #1c, i.e. with a TSS of 658 mg/l, with an addition of only 1 ppm of flocculant (Kurita PA-322 K). The mixture is pre-agitated without flocculant addition at 200 rpm during 30 seconds and is then allowed to settle during 60 seconds. Then the mixture is subjected to further agitation after addition of the flocculant at 200 rpm during 30 seconds and is then allowed to settle during 60 seconds. The picture shown in FIG. 5e clearly indicates same order of clarification as in Test 3, even if the rate of dregs recirculated is somewhat smaller.

(34) Comparative Test 6 Recirculating Dregs without Residual Flocculant in Dregs.

(35) In FIG. 5g is 120 ml of liquid #4 (dregs without residual flocculant) added into 450 ml of liquid #1c, i.e. with a TSS of 658 mg/l, with an addition of only 1 ppm of flocculant (Kurita PA-322 K). The mixture is pre-agitated without flocculant addition at 200 rpm during 30 seconds and is then allowed to settle during 60 seconds. Then the mixture is subjected to further agitation after addition of the flocculant at 200 rpm during 30 seconds and is then allowed to settle during 60 seconds. The picture shown in FIG. 5g clearly indicates that numerous dregs particles are dispersed in the upper liquid volume, but still better clarification than tests without recirculation of dregs.

(36) Conclusions from Tests

(37) The prior art practice to recirculate clarified green liquor has a negative effect on dregs sedimentation. Hence, recirculation of clarified green liquor should not be recommended.

(38) The recirculation of sedimentary dregs into raw green liquor has positive effect on dregs sedimentation and improves clarification in the sedimentation processes.

(39) The recirculation of sedimentary dregs into raw green liquor where said dregs contain residuals of flocculants improves the clarification process further, and enables lower charge of fresh flocculant to the sedimentation process and thus reduce operational costs.

(40) Subjecting the raw green liquor and recirculated dregs for a turbulence effect improves the clarification process further.

(41) The best mode of operation is recirculating dregs from a sedimentation process using flocculant addition, where said dregs have been subjected to a turbulence effect that reduces the particle size and mixing the dregs particles reduced in size with the raw green liquor before addition of flocculant. The dregs recirculated will have a larger total active surface upon which dissolved solids can precipitate on, and with a later addition of flocculant it will speed up the sedimentation process further on dissolved solids that are still dissolved in the liquor.