METHOD FOR POLYSULFIDE PRODUCTION IN A KRAFT PULP MILL

Abstract

The invention is related to improved polysulfide production process wherein a specific second filtration process (F.sub.x) is installed before the polysulfide reactor (R.sub.c). According to the inventive method a cross flow filter (F.sub.x) is used as the second filtration process reaching astonishing low levels of residual solids in the white liquor as well as extended availability of the second filtration process. The subsequent polysulfide reactor, either in form of an electrolytic cell or in form of a bed of active carbon, could then also be operated at increased availability. The invention increases the production volume of polysulfide and the retentate from the cross filtering process may be bled out continuously to a process position ahead of a first filtering or clarification stage, capturing most of the increased content of lime mud particles in the retentate and causing less disturbance of the process with a minimum of tanks and pumps.

Claims

1. A method for producing an oxidized white liquor, for use in a kraft pulp mill which comprises the steps of: (a) feeding green liquor to a slaker, adding burnt lime to the green liquor, and passing the liquor from the slaker to a series of causticizing vessels; (b) withdrawing causticized liquor containing calcium carbonate from the series of causticizing vessels and passing the causticized liquor to a first filtering process to obtain an accept flow of white liquor with a first order of suspended solids measured in ppm and a reject flow of lime mud (c) passing the accept flow of white liquor through a second filtering process to obtain a filtered white liquor with a second order of suspended solids measured in ppm, said second order of suspended solids being at least half the order as of the first order of suspended solids and below 5 ppm, said second filtering process comprising an essentially continuously operating cross flow filter, whereby a permeate passing through cross flow filter elements in the cross flow filter forms the filtered white liquor and the discharged flow of retentate (V.sub.1) is bled off and sent back to the causticizing vessels, and (d) passing the filtered white liquor through a polysulfide reactor with addition of an oxygen containing gas for promoting conversion of sulfides to polysulfide.

2. The method defined in claim 1, wherein the cross flow filter has a ceramic filter body including a filter membrane with an effective thickness of 1-100 ?m and a pore size in the range of 0.1-10 ?m.

3. The method defined in claim 1, wherein the polysulfide reactor is a catalytic reactor using one or more beds of active carbon.

4. The method defined in claim 1, wherein the polysulfide reactor is an electrolytic cell.

5. The method defined in claim 1, wherein the first filtering process is carried out in a precoat filter, using the precoat as an active filter media to capture the calcium carbonate particles in the precoat.

6. The method defined in claim 1, wherein said cross flow filer includes at least 2 sets of cross flow filter elements.

7. The method defined in claim 6, wherein said at least 2 sets of cross flow filter elements are installed in series.

8. The method defined in claim 6, wherein said at least 2 sets of cross flow filter elements are installed in parallel.

9. The method defined by claim 1, wherein said oxidized white liquor comprises a polysulfide cooking liquor.

10. The method defined by claim 1, wherein said first filtering process comprises a clarification process.

11. The method defined by claim 3, wherein said catalytic reactor comprises a partial PTFE coating as catalyst therein.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0034] The above and other objects, features and advantages of the invention will become more readily apparent from the following description, reference being made to the figures in which:

[0035] FIG. 1 is a flow-diagram showing a conventional polysulfide production process, using a pre-filter in form of particle bed ahead of the reactor filled with active carbon; and

[0036] FIG. 2 is a flow-diagram showing the inventive polysulfide production process, using a pre-filter in form of a cross flow filter ahead of the reactor filled with active carbon; and

[0037] FIG. 3 shows the principles behind cross flow filters; and

[0038] FIG. 4a, show a ceramic membrane filter body used in the invention, while FIG. 4b show a cross section view A-A in FIG. 4a, and

[0039] FIG. 5 show an embodiment of the cross flow filters arranged in two sets arranged in series; and

[0040] FIG. 6 show an embodiment of the cross flow filters arranged in two sets arranged in parallel.

SPECIFIC DESCRIPTION AND PRIOR ART

[0041] Prior Art Process

[0042] In FIG. 1 is disclosed a flow diagram in a conventional polysulfide production process, using a pre-filter F in form of particle bed ahead of the reactor R filled with active carbon. The white liquor production process starts with a green liquor tank GL that feed the green liquor to a slaker SL, wherein burnt lime is added under agitation. The slaker mixes the lime into the green liquor and separates larger pieces of grits from the liquor. The even mixture of lime and green liquor is then fed to a train of causticizing vessels CV1.fwdarw.CV2.fwdarw.CV3.fwdarw.CV4.fwdarw.CV5 wherein the causticizing process converts the green liquor to white liquor. The white liquor from the causticizing vessels contains a large amount of lime mud particles, the majority of the particle content being calcium carbonate particles that needs to be separated in a subsequent filtering or clarification process. In this embodiment is shown a white liquor filter WLF that may be either a disc filter or a tube filter that may or may not be using a formation of lime mud precoat on the filter surface, which results in low residual content of lime mud particles in the filtrate. Still, the residual content of lime mud particles may be in the order of some 10-100 ppm after this filtration or clarification. The filtered white liquor is sent to a White Liquor storage tank WLST. Before sending this white liquor to the polysulfide reactor it is further sent to a second filter F. This filter F uses granular filtration and may contain coarse as well as fine granular particles, conventionally anthracite granules that settles with the coarse granules in bottom and fine granules in top. During passage the calcium carbonate particles in the white liquor is penetrating the particle bed and is captured in the bed. The white liquor obtained from the second filtration is sent to a second filtered white liquor storage tank FL-WL-ST, and typically contains less than 5 ppm of calcium carbonate particles. The filtered white liquor is then suitable to be sent to a polysulfide reactor R, where the polysulfide concentration is increased by a catalytic reaction in a bed of active carbon while adding oxygen O.sub.2. The final polysulfide is sent from reactor R to a polysulfide tank, or oxidized white liquor tank OWL-ST.

[0043] The disadvantage with this conventional process is that the filter bed in filter F needs to be regenerated and all calcium carbonate particles captured in the bed needs to be removed in order to prevent the filter to become blocked over time.

[0044] The method of regeneration implemented is to wash the entire particle bed vigorously with the already filtered white liquor to such an extent that the entire particle bed is fluidized. This is done by starting pump P.sub.5 and opening valve V.sub.3, while closing valve V.sub.1, and introducing the pressurized white liquor into the bottom of the particle bed and flushing out the liquid via an open valve V.sub.4, while valve V.sub.2 is shut, with its content of calcium particles into a storage tank WaL-ACC capable of storing the entire volume of wash liquid used. Once the washing process is ended, the wash liquid used, with its content of calcium particles flushed out, is sent back to the process, and preferably before the first white liquor filter WLF. In order not to disturb the process, the wash liquid is returned as a small flow over an extended time.

[0045] The Inventive Process

[0046] The inventive process is shown in FIG. 2, and the process steps are similar to that shown in FIG. 1 up to the second filtration of the white liquor in a filter F.sub.X. This filter F.sub.X uses a cross flow filter instead. The white liquor subjected to filtration in the cross flow filter F.sub.X is recirculated via pump P.sub.6 over the filter at a volume rate that is at least 2 times larger, preferably 4-10 times larger than the feed rate of white liquor from the white liquor storage tank WL-ST. This keeps the flow velocity high in the cross filter flushing off any particles from the surface of the membrane filter element. The volume rate of discharged permeate is the same as the feed flow, and permeate in form of filtered white liquor obtained from the second filtration is sent to filtered white liquor storage tank FL-WL-ST. This white liquor typically contains less than 5 ppm of calcium carbonate particles, and typically at a level of 0-1 ppm and 3 ppm at the most, as the discharged flow is only composed of the permeate. The filtered white liquor is then suitable to be sent to a polysulfide reactor R, where the polysulfide concentration is increased by a catalytic reaction in a bed of active carbon while adding oxygen O.sub.2. The final polysulfide is sent from reactor R to a polysulfide tank, or oxidized white liquor tank OWL-ST.

[0047] In chemical engineering cross flow filtration is a type of filtration. Cross flow filtration is different from dead-end filtration in which the feed is passed through a membrane or bed, the solids being trapped in the filter and the filtrate being released at the other end. Cross-flow filtration gets its name because the larger part of the feed flow travels tangentially across the surface of the filter, rather than into the filter. The principal advantage of this is that the filter cake (which can blind the filter) is substantially washed away during the filtration process, increasing the length of time that a filter unit can be operational. It can be a continuous process, unlike batchwise dead-end filtration.

[0048] In FIG. 3 the principle function of a cross flow filter element is shown in a cross section view. A feed flow P.sub.IN of the suspension containing particles is fed to a membrane filter body F.sub.B which allows clean liquid to pass as a permeate P.sub.per, while the retentate flow P.sub.OUT still contain most, if not all, particles contained in the suspension. In FIG. 4a is shown a typical elongated cylindrical membrane filter body, and in FIG. 4b a cross section A-A in FIG. 4a is shown. The membrane body is made of a porous material and contains a number of flow passages for the suspension flow, and permeate is bled out from the filter body.

[0049] Now, this Cross flow filtration technique has been successfully implemented in green liquor filtration in Kraft pulp mills by CleanFlow AB, and the filtering body used with a membrane pore size of 0.1-10 ?m, more preferred 0.1-5 ?m and most preferred 0.2-1.0 ?m, for the green liquor filtration application has been patented in SE,C,533833 (corresponds to pending EP,A,2414585).

[0050] In FIG. 5 a first embodiment of the cross filter configuration is shown. In this embodiment two sets of the membrane filters are arranged in series, and one single pump P.sub.6 is used to circulate the particle containing white liquor through both sets of filter arrangements. P.sub.1 is the feed flow coming from pump P.sub.1 in FIG. 2 and V.sub.1 is the discharged flow of retentate bled off and sent back to the causticizing vessels. The advantage is that one single pump is used to circulate the white liquor trough the filters.

[0051] In FIG. 6 a second embodiment of the cross filter configuration is shown. In this embodiment two sets of the membrane filters are arranged in parallel, and thus two pumps P.sub.6a and P.sub.6b are used to circulate the particle containing white liquor trough each set of filter arrangement. P1 is the feed flow coming from pump P.sub.1 in FIG. 2 and V.sub.1 is the discharge flow of retentate sent back to the causticizing vessels. The advantage is that one filter arrangement may be shut off during cleaning of the filter elements in that filter arrangement, but still an essentially continuous discharge flow of permeate may be obtained.

[0052] Cleaning of the filter elements in the white liquor filtering process is relatively simple as the bulk content of particles in the already filtered white liquor is calcium carbonate that relatively easy dissolves in an acidic cleaning cycle.

[0053] The cleaning process could also be a simple backwashing of the membrane filters, pressurizing the white liquor on the permeate side such that a reversal of flow is obtained over the membrane filters.

[0054] The membranes may also be cleaned in place, or by disconnecting an individual filter body as disclosed in FIGS. 4a and 4b, blinding the connections in the system, and then clean the individual filter body. When cleaning the filters in place with acid also additional tanks and pumps (not shown) with acidic washing liquid are also installed, and could preferably be used in a system as disclosed in FIG. 6 where one set of filters may be cleaned at the time.

SPECIFIC EXAMPLE

[0055] In a specific implementation of the invention the cross flow filter system is designed for a nominal production of 420 m.sup.3/h filtered white liquor, using ceramic membrane filters with a pore size of approximately 0.3-0.8 ?m. The white liquor filtered once before is obtained from a pressurized disc filter operating with a precoat of lime mud that reduce the content of lime mud particles in the white liquor to a level of 20 mg/l, i.e. this is thus the level of particles in the white liquor fed to the cross flow filters. The part of the retentate that is returned back to the causticizing vessels, preferably to last causticizing vessel ahead of the disc filter is, due to the recirculation and bleed off of permeate, experiencing an increase of the content of lime mud particles in the white liquor to a level of 1060 mg/l. The product liquor, i.e. the filtered white liquor has a very low content of lime mud particles, and in the mg/l scale less than 0 mg/l, and as measured in ppm well below 3 ppm. During normal operation varying in between 1-2 ppm. Compared with the nominal production of filtered white liquor, i.e. about 420 m.sup.3/h filtered white liquor, the recirculation rate through the filters is 2400 m.sup.3/h and the recirculated volume is pressurized to about 0.2-0.3 bar. The recirculation volume over the filters is thus in excess of 5.7 times that of the production volume of filtered white liquor

[0056] Above example is from a conventional causticizing plant with its specific lime recovery process, and the residual lime mud particle content in the white liquor may differ both in absolute level (mg/I or ppm) and in distribution of particle sizes, but cross flow filters may be modified using different pore sizes in the membrane adapted for the specific white liquor. However, the combination of a first filter and a subsequent cross flow filter reach astonishing low residual content of solids in the permeate. The reduction of particle content is especially important for the subsequent catalytic process in the polysulfide reactor, as this reactor may extend its availability at least 3-5 fold, before the active carbon bed is blocked, by reducing solids content in the white liquor from 5 ppm and down to 1 ppm. A similar positive effect is also obtained if the polysulfide conversion is done in an electrolytic cell, where deposits may be formed on the cathode depending on residual content of solids.