PROCESS AND APPARATUS FOR WHITE LIQUOR OXIDATION

Abstract

In a process for oxidation of white liquor, in particular of white liquor used in a process of production of paper or cellulose, the oxygen required for the oxidation is supplied to the reactor or the reactors in which the oxidation is carried out at least partially in the form of oxygen-containing nanobubbles. Due to the relatively long lifetime of the nanobubbles this very efficiently provides oxygen also for oxidation reactions in the white liquor proceeding at different rates.

Claims

1. A process for oxidizing white liquor, in which white liquor is contacted with oxygen in a reactor and hence sulfur compounds in the white liquor are oxidized, wherein the oxygen required for the oxidation is supplied to the reactor at least partly in the form of oxygen-containing nanobubbles.

2. The process as claimed in claim 1, wherein the oxygen-containing nanobubbles are supplied at least partly by generating nanobubbles in a feed for the white liquor or for a fluid to be fed into the white liquor, for example for fresh water, that has flow connection to the reactor.

3. The process as claimed in claim 1, wherein the oxidation of the white liquor proceeds in multiple stages each conducted in a separate reactor and the oxygen required is fed to one of the reactors or multiple reactors at least partly in the form of oxygen-containing nanobubbles.

4. The process as claimed in claim 1, wherein the white liquor that has been treated with the oxygen is used as alkali source in bleaching stages of a chemical pulp bleaching operation.

5. An apparatus for oxidizing white liquor, comprising a reactor in which white liquor is contacted with oxygen and hence sulfur compounds in the white liquor are oxidized, wherein the reactor and/or a feed for the white liquor or for an aqueous fluid to be supplied to the reactor that has flow connection to the reactor has an assigned introduction device for introduction of oxygen in the form of nanobubbles.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0025] The drawing is intended to elucidate a working example of the invention. The sole drawing (FIG. 1 1) shows a flow diagram for a white liquor oxidation in which the treated white liquor is subsequently sent to a bleaching process.

DETAILED DESCRIPTION

[0026] FIG. 1 shows the feeding of treated white liquor into a process 1 for bleaching of chemical pulp, as used, for example, during the production of cellulose fibers. During the bleaching process 1, an aqueous chemical pulp suspension 2 containing not only chemical pulp but also fractions of lignin passes through multiple stages that proceed in succession, two of which are shown here, namely an alkaline oxygen delignification 3 and an oxygen-enhanced peroxide bleaching 4. Further bleaching stages, for instance an oxygen-enhanced extraction, may likewise be present, but are not shown here.

[0027] In the oxygen delignification 3, the chemical pulp suspension 2 is treated with oxygen in an alkaline environment in one or more reactors at high temperatures. This removes significant proportions of the lignin still present in the suspension by reaction with oxygen. For reasons of clarity, just one process step for oxygen delignification 3 is shown here in abstract form; the oxygen delignification 3 may, however, be effected either in a single reactor or—as is customary in modern bleaching processes—in multiple stages in multiple reactors connected in series.

[0028] The oxygen delignification 3 requires an alkaline medium having a pH of about pH=11 at a temperature between 80° C. and 105° C. The alkaline medium is achieved by the supply of an alkali to the reactor(s), as elucidated in detail below. The suspension here has an average consistency of, for example, 10% to 14%. Oxygen or an oxygenous gas is introduced into the reactor(s). In the comparatively unusual case nowadays of a one-stage oxygen delignification, the treatment is effected at a pressure of, for example, 7 to 8 bar in the feed and 4.5 to 5.5 bar in the output from the (single) reactor. The treatment time (retention time) here is, for example, 50 to 60 min. In the case of a two-stage oxygen delignification, there is generally a difference in pressure and reaction time in the two reactors. In the first stage, for example, a customary pressure is a pressure of 7 to 10 bar and a customary retention time is 10 to 15 minutes, and in the second stage a pressure of 3 to 5 bar with a retention time of about 1 h.

[0029] In the peroxide bleaching 4, the suspension is supplied, as a further bleaching agent, with a peroxide, especially hydrogen peroxide (H.sub.2O.sub.2), although the efficiency of this process step can also be significantly improved by addition of oxygen (“PO”, oxygen-enhanced peroxide bleaching). The treatment is effected in a reactor, for example at atmospheric pressure and a temperature of, for example, between 85° C. and 90° C. or under an elevated pressure at temperatures of, for example, between 100° C. and 110° C. The peroxide bleaching 4 is also effected in alkaline medium which is produced by supply of an alkali, as likewise elucidated in detail below. The suspension 5 of bleached chemical pulp produced in the bleaching stages 3, 4 is subsequently sent to further process steps that are of no interest here.

[0030] The alkali used for production of the alkaline medium in the bleaching stages 3, 4 in the working example disclosed here is white liquor. The white liquor consisting predominantly of sodium sulfide and sodium hydroxide is used in the kraft process for digestion of cell walls and can subsequently be recovered. In the working example according to FIG. 1, for example, recovered white liquor 6 is fed to bleaching stages 3, 4, but it is also possible to branch off a substream of the white liquor intended for digestion and to use it in the manner described here.

[0031] In order to be usable in the bleaching stages 3, 4, the sodium sulfide present in the white liquor that would disrupt the bleaching operation must be removed. For this purpose, the white liquor 6 is sent to a process for white liquor oxidation 7. In the white liquor oxidation 7, the sulfide is converted by supply of oxygen in the form of air, an oxygen-rich gas or pure oxygen (having a purity of 95% by volume or more) to thiosulfate (“partly oxidized white liquor”) and/or to sulfate (“fully oxidized white liquor”). Partly oxidized white liquor is suitable for the bleaching process in the oxygen delignification 3, while fully oxidized white liquor is also usable for peroxide bleaching 4.

[0032] In the working example shown here, the white liquor 6 is first fed to a first reactor 8 in which there is partial oxidation of the white liquor 6. A substream of the partly oxidized white liquor formed is fed via a feed 9 to the oxygen delignification 3. The remaining substream of the partly oxidized white liquor is fed to a second reactor 10 in which there is full oxidation of the white liquor. The fully oxidized white liquor is fed via a feed 11 to the peroxide bleaching 4.

[0033] The oxygen required for the oxidation of the white liquor can be fed directly or indirectly to the reactors 8, 10. According to the invention, at least a portion of the oxygen is introduced in the form of nanobubbles, i.e. bubbles having an average diameter between 20 nm and 1000 nm. In the working example shown here, by way of example, various options for sites where oxygen can be introduced in the form of nanobubbles are shown.

[0034] For example, for partial oxidation of the white liquor, oxygen can be introduced directly into the reactor 8 in the form of nanobubbles via an oxygen feed 12 or via the feeding of oxygen in the form of nanobubbles via an oxygen feed 13 that opens into a feed 14 for white liquor that leads to the reactor 8.

[0035] Owing to the comparatively long lifetime of the nanobubbles, introduction of oxygen via the oxygen feeds 12, 13 is also sufficient for the subsequent full oxidation of the white liquor in the reactor 10. Alternatively, for full oxidation, there is an additional introduction of oxygen in the form of nanobubbles, either directly into the reactor 10 via an oxygen feed 15 or via an oxygen feed 16 that opens into a feed 17 for partly oxidized white liquor that leads to the reactor 10. In addition, the oxygen in the form of nanobubbles may also be introduced into a feed for an aqueous medium, for example fresh water, that opens into the feed 14, 17, but this is not shown here.

[0036] The nanobubbles are produced in each case at the opening of the oxygen feeds 12, 13, 15, 16 into the respective fluid-conducting conduit 14, 17 or the respective reactor 8, 10 at suitable introduction devices 18, 19, 20, 21. All that is required here is that, in operation of the introduction devices 18, 19, 20, 21, this at least with one apparatus that produces the nanobubbles, for example a nozzle or a bubbling system or a section thereof, are surrounded by water or an aqueous fluid, such that the nanobubbles can form in the aqueous phase. The nanobubbles are then entrained by the flow of the respective fluid and hence arrive in the respective reactor 8, 10 for the reaction.

[0037] Incidentally, it is in no way a requirement in the context of the invention that oxygen be introduced exclusively in the form of nanobubbles. It is instead also possible that the introduction of oxygen in the form of nanobubbles is undertaken in addition to other modes of introduction for the oxygen, as known, for example, from the prior art.

[0038] The process of the invention and the apparatus of the invention make it possible to use the oxygen introduced into the white liquor over the course of the various oxidation reactions with significantly higher efficiency than is the case in prior art processes. The small size of the nanobubbles enables uniform distribution of the oxygen in the white liquor, and they constitute a sustainably available oxygen reservoir for the comparatively slow oxidation of the sulfur compounds in the white liquor to sulfate.

LIST OF REFERENCE NUMERALS

[0039] 1. Process for bleaching [0040] 2. Chemical pulp suspension [0041] 3. Alkaline oxygen delignification [0042] 4. Peroxide bleaching [0043] 5. Suspension of bleached chemical pulp [0044] 6. White liquor [0045] 7. White liquor oxidation [0046] 8. Reactor [0047] 9. Feed (for partly oxidized white liquor) [0048] 10. Reactor [0049] 11. Feed (for partly oxidized white liquor) [0050] 12. Oxygen feed [0051] 13. Oxygen feed [0052] 14. Feed (for white liquor) [0053] 15. Oxygen feed [0054] 16. Oxygen feed [0055] 17. Feed (for partly oxidized white liquor) [0056] 18. Introduction device [0057] 19. Introduction device [0058] 20. Introduction device [0059] 21. Introduction device