PROCESS TO OBTAIN FULLY OXIDIZED WHITE LIQUOR FOR USE IN THE FIBERLINE OF A KRAFT PULP PROCESS

Abstract

The present application relates to a process to obtain fully oxidized white liquor from white liquor used in a Kraft pulp process. Said fully oxidized white liquor is suitable to be used in alkaline steps of certain stages of the Kraft pulp process fiberline in order to partly or totally substitute the purchased sodium hydroxide used in those stages. The presently disclosed process also allows to purge non-process elements (NPE's) by purging alkaline wash filtrates to a wastewater treatment plant.

Claims

1. A process to obtain fully oxidized white liquor for use in the fiberline of a Kraft pulp process characterized by comprising the following steps: Oxidizing between 1 and 100 m.sup.3/h of white liquor from the Kraft pulp process with oxygen, obtaining fully oxidized white liquor comprising NaOH, Na.sub.2SO.sub.4, Na.sub.2CO.sub.3 and Na.sub.2S.sub.2O.sub.3; Feeding the fully oxidized white liquor to the bleaching stages and/or the delignification stages and/or the alkaline extraction stages of the Kraft pulp process; Purging non-process elements by purging the filtrates from washing steps after the bleaching stages and/or alkaline extraction stages to a Wastewater Treatment Plant; Feeding up to 100% of the alkaline filtrate obtained in the washing steps after the bleaching stages to an evaporation plant; Feeding the alkaline filtrate obtained in the washing steps after the delignification stages to the evaporation plant; and Purging non-process elements from a recovery boiler in the Kraft chemical recovery process of the Kraft pulp process.

2. The process of claim 1, wherein the white liquor is oxidized with 20 to 60 kg of O.sub.2/m.sup.3 of white liquor.

3. The process of claim 1, wherein the oxygen is used as pure oxygen or as a gas mixture comprising oxygen.

4. The process of claim 1, wherein oxygen is used with a purity degree above 70% v/v.

5. The process of claim 1, wherein the fully oxidized white liquor substitutes up to 100% of the amount of purchased NaOH used in the bleaching stages, alkaline extraction stages, delignification stages of the Kraft pulp process and combination thereof.

6. The process of claim 1, wherein the oxidation step occurs at a temperature above 100? C., pressure above 1 barg, and the total Hydraulic Residence Time is at least 30 minutes.

7. The process of claim 1, wherein the oxidation step occurs at a temperature above 100? C.

8. The process of claim 1, wherein the oxidation step occurs at a pressure above 1 barg.

9. The process of claim 1, wherein the total Hydraulic Residence Time is at least 30 minutes.

10. The process of claim 1, wherein up to 100% of the filtrates from washing steps after the bleaching stages and/or alkaline extraction stages are sent to the Wastewater Treatment Plant.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0030] For easier understanding of this application, FIGURES are attached in the annex that represent the preferred forms of implementation which nevertheless are not intended to limit the technique disclosed herein.

[0031] FIG. 1 shows a general schematic representation of a Kraft pulp process including one embodiment of the process disclosed in the present application.

DETAILED DESCRIPTION OF THE INVENTION

[0032] Now, preferred embodiments of the present application will be described in detail with reference to the annexed drawings. However, they are not intended to limit the scope of this application.

[0033] The present application relates to a process to obtain fully oxidized white liquor from the white liquor used in a Kraft pulp process. The FOWL is suitable to be used in the alkaline stages of the fiberline of said Kraft pulp process, such as the bleaching stages, and/or alkaline extraction stages and/or the delignification stages that comprise alkaline steps.

[0034] FIG. 1 shows a general schematic representation of a Kraft pulp process comprising the process of the present application. (A) represents the fiberline and (B) represents the recovery line.

[0035] In general terms, in the Kraft chemical recovery process (C) of a Kraft pulp process, wood chips (1) are mixed with white liquor (2) provided by a white liquor filter (G). The mixture is fed to a digester and after digestion, sent to a washer (D). It results in a pulp (3) that is fed to the delignification stages (H) followed by the bleaching stages (I) producing bleached pulp (4).

[0036] According to the present invention, a fraction of the white liquor (2) is fed to at least one oxidation reactor (K) to produce FOWL (5) which is used in the delignification stages (H) and/or the bleaching stages (I) and/or alkaline extraction stages to partly or totally substitute purchased NaOH used in alkaline steps of these stages. The resulting alkaline wash filtrate (6.1, 6.2) of the delignification stages (H) and/or bleaching stages (I) are fed back to an evaporation plant (E) in the recovery line (B) to return to the Kraft chemical recovery process (C).

[0037] The non-process elements (7.1) will be totally or partly purged to the WWTP through the wash step after the alkaline bleaching stages (I).

[0038] In the recovery boiler (F) of the Kraft chemical recovery process (C) methods that allow removing the non-process elements (7.2) are applied, these non-process elements (7.2) are also discarded to the WWTP.

[0039] In the context of the present invention, purchased NaOH is also known as fresh NaOH.

[0040] The process to obtain fully oxidized white liquor for use in the fiberline of a Kraft pulp process comprises the following steps: [0041] Oxidizing between 1 and 100 m.sup.3/h of white liquor (2) from the Kraft pulp process with oxygen, obtaining fully oxidized white liquor (5) comprising NaOH, Na.sub.2SO.sub.4, NaCO.sub.3 and Na.sub.2S.sub.2O.sub.3; [0042] Feeding the fully oxidized white liquor (5) to the bleaching stages (I) and/or alkaline extraction stages and/or the delignification stages (H) of the Kraft pulp process; [0043] Purging non-process elements (7.1) by purging the filtrates from washing steps after the bleaching stages (I) and/or alkaline extraction stages to a Wastewater Treatment Plant; [0044] Feeding up to 100% of the alkaline filtrate (6.2) obtained in the washing steps after the bleaching stages (I) to an evaporation plant (E); [0045] Feeding the alkaline filtrate (6.1) obtained in the delignification stages (H) to the evaporation plant (E); [0046] Purging non-process elements (7.2) from a recovery boiler (F) in the Kraft chemical recovery process (C) of the Kraft pulp process.

[0047] In one embodiment, less than 5 g of Na.sub.2S.sub.2O.sub.3 per liter of fully oxidized white liquor (5) are produced.

[0048] In one embodiment, white liquor is oxidized with 20 to 60 kg of O.sub.2/m.sup.3 of white liquor.

[0049] The amount of alkaline filtrate (6.2) obtained in the bleaching stages (I) and/or alkaline extraction stages that is fed to the evaporation plant (E) varies according to the concentration of non-process elements.

[0050] The amount of alkaline filtrate (6.1) obtained in the delignification stages (H) that is fed to the evaporation plant (E) is 100% due to the high concentration of chemical oxygen demand (COD).

[0051] In one embodiment, up to 100% of the fully oxidized white liquor (5) is fed to the bleaching stages (I) and/or alkaline extraction stages and/or the delignification stages (H).

[0052] In one embodiment, oxygen is used in the process as pure oxygen or as a gas mixture comprising oxygen.

[0053] For the purposes of the present application, pure oxygen is considered an oxygen stream with a purity degree above 70% O.sub.2 v/v.

[0054] The FOWL totally or partially substitutes up to 100% of the amount of purchased NaOH used in the delignification stages of the Kraft pulp process.

[0055] The FOWL substitutes up to 100% of the amount of purchased NaOH used in the bleaching stages and/or alkaline extraction stages of the Kraft pulp process.

[0056] The oxidation step of the process makes use of at least one oxidation reactor (K). A plurality of reactors can be used for the purpose of the present invention.

[0057] The FOWL produced is an aqueous solution comprising NaOH, Na.sub.2SO.sub.4, NaCO.sub.3 and less than 5 g of Na.sub.2S.sub.2O.sub.3 per liter of FOWL, in which the presence of sulfide ions is undetectable by Ionic Chromatography and the extent of conversion of total sulfur to sulfate ions is above 90%.

[0058] In one embodiment, during the oxidation step the temperature is above 100? C., the pressure is above 1 barg, and the total Hydraulic Residence Time (HRT) is at least 30 minutes.

[0059] The sodium makeup of the recovery line of the Kraft pulp process is donesolely or in part, i.e., up to 100%,in the delignification stages (H) by using purchased NaOH.

[0060] The sodium makeup of the recovery line of the Kraft pulp process is donesolely or in partin the recovery boiler (F) as Na.sub.2CO.sub.3 and/or Na.sub.2SO.sub.4.

[0061] The non-process elements, such as K.sup.+ or Cl.sup.? but not exclusively, are purged from the recovery line through the use of the FOWL.

[0062] The totality or a fraction, i.e., up to 100% of the filtrates from washing steps after the bleaching stages and/or alkaline extraction stages are sent to the WWTP-thus discharging the recovery line from non-process elements.

[0063] This description is of course not in any way restricted to the forms of implementation presented herein and any person with an average knowledge of the area can provide many possibilities for modification thereof without departing from the general idea as defined by the claims. The preferred forms of implementation described above can obviously be combined with each other. The following claims further define the preferred forms of implementation.

[0064] While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

[0065] The singular forms a, an and the include plural referents, unless the context clearly dictates otherwise.

[0066] Comprising in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing i.e. anything else may be additionally included and remain within the scope of comprising. Comprising is defined herein as necessarily encompassing the more limited transitional terms consisting essentially of and consisting of; comprising may therefore be replaced by consisting essentially of or consisting of and remain within the expressly defined scope of comprising.

[0067] Providing in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

[0068] Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

[0069] Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

[0070] All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.