Method for stabilizing of a bleaching stage for a polysaccharide material using an alkaline waste liquid

Abstract

This invention relates to a method for bleaching of a polysaccharide material that includes a hydrogen peroxide bleaching stage, wherein before the end of the hydrogen peroxide bleaching stage the polysaccharide material is treated with a hydroxy carboxylic acid solution that is obtained from a viscose process press lye.

Description

[0001] Method for stabilizing of a bleaching stage for a polysaccharide material using an alkaline waste liguid

[0002] This invention relates to a method for bleaching of a polysaccharide material that includes a hydrogen peroxide bleaching stage, wherein before the end of the hydrogen peroxide bleaching stage the polysaccharide material is treated with a hydroxy carboxylic acid solution that is obtained from a viscose process press lye.

[0003] Prior Art

[0004] Bleaching processes are widely applied in many industries. For example, many bleaching processes are applied on cellulosic materials, such as in the manufacture of paper and of cellulosic pulp for the manufacture of textile fibers. In these bleaching processes complexation of transition metal ions is very important, especially in the bleaching processes involving hydrogen peroxide stages (“P-stages”).

[0005] If the transition metal ions are not complexed, so-called Fenton chemistry occurs and damages the pulp or paper severely, resulting in lower product quality due to reduced molecular weight and inferior mechanical properties. At the same time, transition metal ions can catalytically degrade hydrogen peroxide, so that cellulose damage is accompanied by additional hydrogen peroxide consumption. "Inactivation” of the transition metal ions by complexing agents is therefore necessary to avoid these degradation reactions. Currently, EDTA (ethylenediaminetetraacetic acid) is the most widely used chelating / complexing agent.

[0006] EDTA is considered an environmental pollutant due to its persistency, which means that it is very difficult to be degraded in the environment. In several jurisdictions worldwide, EDTA will be regulated or even banned in the near future.

[0007] In the article by Kolodyhska, D.: “Application of a new generation of complexing agents in removal of heavy metal ions from different wastes”, Environ Sci Pollut Res 20, 5939-5949 (2013), the replacement of EDTA by aminopolycarboxylic acids was investigated. However, those compounds are not derived from natural sources nor from waste streams.

[0008] DTPA (Diethylenetriamine pentaacetate) is another widely used synthetic chelating / complexing agent.

[0009] In the article by M. Sankari and R. Aksela: “Environmental Friendly Chelating Agents” in Pulp&Paper Industry - Mill Scale Experiences, Professional Papermaking 1 , 34-37 (2011 ), the replacement of EDTA with the biodegradable chelating agent “Fennobio” was tested. However, this compound is also not derived from natural sources nor from waste streams. Other biodegradable complexing agents that also contain raw materials from natural sources are e.g. methylglycine diacetic acid (MGDA), commercially available under the trade name Trilon® M from BASF / Germany, and tetrasodium glutamate diacetate (GLDA). However, both agents are produced with synthetic and / or problematic raw materials. For example according to patent application CN105732408A GLDA is produced using critical substances like cyanides.

[0010] It is generally known to re-use liquid waste streams of a process step in another process step in order to reduce the consumption of virgin raw materials. As an example, EP 2405056 A1 discloses a paper pulp manufacture process wherein the press lye from a cold-caustic extraction (CCE) treatment of a cooked dissolving wood pulp is used in the cooking step of a paper pulp manufacture process in order to reduce the consumption of virgin sodium hydroxide. By this measure the polysaccharide yield of the cooking process is increased. The addition of the press lye is done far before the bleaching of the so prepared dissolving wood pulp.

[0011] The viscose process is a method of producing man-made cellulosic fibers. The process uses pulp from wood or other cellulose sources to produce long, shiny, silky fibers. During viscose fiber production, pulp flakes and sheets are soaked in caustic soda (steeping) for obtaining alkali cellulose. Excess lye is removed before shredding and aging, whereby press lyes are produced as a side stream. These press lyes are a mixture of by-products of the viscose process in aqueous NaOH of different concentrations. These by-products are dissolved hemicelluloses, various hydroxy carboxylic acids (mostly in the form of their sodium salts), carbonyl compounds, and low-molecular weight hydroxy acids, hydroxy ketones and hydroxy aldehydes. These hydroxy carboxylic acids include glucoisosaccharinic acid, glycolic acid, lactic acid and xyloisosaccharinic acid as the most abundant hydroxy carboxylic acids (see e,g DJ Mozdyniewicz, G Schild and H Sixta (2014) Alkaline steeping of dissolving pulp. Part II: Soluble compounds in the press lye. Cellulose 21 :2889-2900, DOI: 10.1007 / s10570-014-0291 -6). Presently these press lyes are either incinerated or re-used as an alkali source in kraft pulping. For the purposes of the present invention the term “viscose process” shall include a modal process, as well, as the modal process in principle contains the same steps, e.g. alkalization to form alkali cellulose, xanthation of the alkali cellulose, fiber forming and regeneration of the cellulose, however with different concentrations of the various chemicals involved.

[0012] Yehia Fahmy et al, Cellulose Chem. Technol., 13, 673-675 (1979), investigated the use of the hemicellulose-containing press lye from viscose fiber production as a binder in papermaking. They found that the retentionability and binding capacity was low, however their idea could be worthwhile due to economic reasons. The addition of the press lye was done during paper-forming, i.e. long after any bleaching step of the cellulosic material.

[0013] Problem

[0014] There is a growing demand for possible, economically feasible alternatives for the complexation of transition metal ions in bleaching processes. In view of the prior art described herein above the problem to be solved consisted in finding a chelating / complexing agent with high efficiency that does not show the downsides of the described prior art.

[0015] Description Surprisingly it was found that the organic compounds contained in the press lyes from a viscose fiber process can be used very efficiently to replace conventional complexing agents such as EDTA and DTPA in pulping / bleaching applications.

[0016] It is therefore an object of the present invention to provide a method for bleaching of a polysaccharide material that includes a hydrogen peroxide bleaching stage, wherein before the end of the hydrogen peroxide bleaching stage the polysaccharide material is treated with a hydroxy carboxylic acid solution that is obtained from a viscose process press lye and hence this hydroxy carboxylic acid solution contains two or more different hydroxy carboxylic acids from the group containing glucoisosaccharinic acid, glycolic acid, lactic acid and xyloisosaccharinic acid. In particular, this hydroxy carboxylic acid solution contains glucoisosaccharinic acid as a main compound. The hydrogen peroxide bleaching stage is often also called the „P-stage“. According to the invention the polysaccharide mostly will be cellulose. However, in principle the invention is also applicable to hydrogen peroxide bleaching of other polysaccharides, such as, but not limited to starch, alginate, chitin, pectin and carrageenan. The viscose process press lye for the purposes of the present invention shall mean a press lye from the alkalization stage of a viscose fiber or modal fiber production process. Preferably the treatment according to the invention is performed at a consistency (i.e. the content of solids in the slurry), of between 6 and 15%(w / w), even more preferably of between 8 and 12%(w / w). However, depending on the specific operation mode of a peroxide bleaching stage, the treatment according to the invention may also be performed at a low consistency of between 3 and 4%(w / w) or at a high consistency of between 30 and 40%(w / w). The treatment temperature preferably is between 30 and 100°C, more preferably between 60-90°C. The treatment time preferably is between 0.5 and 3.0 h, more preferably between 0.6 and 1.8 h. The hydrogen peroxide concentration is preferably between 3 and 30 kg per ton ODP, more preferably between 5 and 28 kg per ton ODP. The alkaline hydroxy carboxylic acid salt solutions are preferably the solutions of oligosaccharides and their decomposition products, typically with a polymerization degree of less or equal to 5, e.g. isosaccharinic acid, xyloiso-saccharic acid, lactic acid, etc.; furthermore beta saccharides may be present, such as e.g. glucan, xylan, mannan, arabinan, rhamnan and / or galactan in aqueous alkaline solutions containing NaOH.

[0017] A press lye may be taken from the alkalization stage of a commercial-scale viscose or modal fiber production process, where the pulp had been alkalized with an aqueous solution of sodium hydroxide, usually at a concentration of about 18.5% (w / w) of sodium hydroxide. This press lye could in principle even be used directly in the complexation step without further pre-treatment. However, the mechanically filtered lye usually is afterwards purified using different industrial scale membrane purification methods, to remove dissolved cellulose and hemicellulose degradation products and to recover the sodium hydroxide. A membrane pretreatment step usually is advantageous because the recovery of sodium hydroxide from the press lye for further use in the viscose process is economically beneficial and therefore preferred.

[0018] According to a preferred embodiment of the present invention the alkaline hydroxy carboxylic acid salt solution contains between 5 and 250 g / l NaOH, more preferably between 30 and 180 g / l NaOH. Lower NaOH concentrations would result in higher efforts for transportation of the solution. Higher NaOH concentrations could only be obtained by water removal like evaporation which also be economically disadvantageous. For the purposes of the present invention it is most desirable to use the alkaline hydroxy carboxylic acid salt solution as it is obtained from the previous process step.

[0019] Preferably the alkaline hydroxy carboxylic acid salt solution contains between 2 and 150 g / l TOC, preferably between 10 and 70 g / l TOC. Lower TOC contents would not give sufficient complexation effect. Higher TOC contents are usually not available from press lyes according to the invention, among others because they would have a too high viscosity.

[0020] In a preferred embodiment of the present invention the ratio (w / w) of TOC to NaOH in the alkaline hydroxy carboxylic acid salt solution is between 0,01 and 2,00, more preferably between 0,10 and 1 ,20, especially preferred between 0,20 and 0,70. For the purpose of the present invention it is favorable to have as much organic compounds as possible in relation to sodium hydroxide, however there are these physical and economic limitations.

[0021] The hydroxy carboxylic acid solution may be obtained from a viscose process press lye in a separation step before the bleaching stage by a process step wherein NaOH is removed from the press lye in a way that increases the share of organic compounds in relation to NaOH.

[0022] In a preferred embodiment of the present invention this separation step contains a membrane filtration step, for example a nanofiltration, an osmose or a diffusion dialysis. In a membrane filtration step two streams are obtained: the permeate and the retentate. According to the invention the permeate, i.e. the sodium hydroxide-rich stream, may be re-used in a viscose fiber production process. The retentate, i.e. the stream containing the organic compounds, is used to prepare the alkaline hydroxy carboxylic acid salt solution for the bleaching step according to the invention. Preferably the permeate of the membrane filtration step is re-used in a viscose fiber production process.

[0023] In a preferred embodiment of the present invention the treatment with the alkaline hydroxy carboxylic acid salt solution takes place before the polysaccharide material is treated with the hydrogen peroxide in the hydrogen peroxide bleaching stage. Before an unbleached polysaccharide enters a bleaching step it may be optionally washed. In this embodiment of the invention the alkaline hydroxy carboxylic acid salt solution may be added either before (Option C in Fig. 1 ) or after (Option B in Fig. 1) that prebleaching washing step.

[0024] In another preferred embodiment of the present invention the treatment with the alkaline hydroxy carboxylic acid salt solution takes place while the polysaccharide material is treated with the hydrogen peroxide in the hydrogen peroxide bleaching stage (Option A in Fig. 1). For this embodiment the separation step is not needed. Preferably the ratio (w / w) of the alkaline hydroxy carboxylic acid salt solution (measured as TOC) to the polysaccharide material (ODP) is between 0,1 and 20,0 kg TOC / 1 ODP, preferably between 0,2 and 15,0 kg TOC / 1 ODP, more preferably between 0,5 and 10,0 kg TOC / 1 ODP.

[0025] Preferably the polysaccharide material is a cellulosic material.

[0026] In a preferred embodiment of the present invention the cellulosic material is a dissolving wood pulp. The dissolving wood pulp may be produced according to a sulfite process or a kraft process. The method according to the invention will be applicable in both process types. Dissolving wood pulps are the preferred cellulosic raw material for the production of man-made cellulosic fibers such as viscose, modal, lyocell or cellulose acetate fibers and also for other cellulose derivates such as carboxymethyl cellulose.

[0027] In another preferred embodiment of the present invention the cellulosic material is a paper pulp. Paper pulp is mainly characterized in that it is used for paper-making. It is mainly produced according to a kraft process.

[0028] In still another preferred embodiment of the present invention the cellulosic material consists of or contains recycled cellulose-containing post-industrial or post-consumer textile material. In this embodiment of the invention the cellulosic material may be either derived solely from a recycled cellulose- containing post-industrial or post-consumer textile material or it may be a mixture of a virgin dissolving wood pulp and a cellulosic material derived from recycled cellulose-containing post-industrial or post-consumer textile by textile recovery technology. Such textile recovery technologies are in principle known by the skilled in the art.

[0029] In a special embodiment of the invention the press lye may be neutralized by adding an acid. By this step the beta-glucans will be precipitated while gamma-glucans would stay dissolved. The precipitated solids may be removed from the press lye before the press lye is further used for treating the polysaccharide material according to the invention. The removal of the precipitated solids would be done by a mechanical separation method chosen from the group containing centrifugation, sedimentation, filtration and flotation. Alternatively the removal of the beta-glucans may also be done by a suitable membrane separation process. However, e.g. in the paper industry the removal of the beta-glucans would be disadvantageous because the betaglucans as sheet-forming material would be lost. Also for other uses of the bleached polysaccharides the removal of the beta-glucans may be economically not feasible.

[0030] BRIEF DESCRIPTION OF THE REFERENCE SIGNS:

[0031] 1 Dissolving wood pulp for viscose process

[0032] 2 Alkalization

[0033] 3 Xanthation

[0034] 4 Press lye from viscose process

[0035] 5 Membrane separation

[0036] 6 Hydroxy carboxylic acid solution from membrane separation (may be either the permeate or the retentate)

[0037] 7 Precipitation (optional)

[0038] 8 Unbleached polysaccharide material

[0039] 9 Polysaccharide material washing (optional)

[0040] 10 Hydrogen peroxide bleaching of the polysaccharide material

[0041] 11 Bleached polysaccharide material

[0042] The invention will now be illustrated by examples. These examples are not limiting the scope of the invention in any way. The invention includes also any other embodiments which are based on the same inventive concept

[0043] Examples

[0044] Measuring methods:

[0045] The content of organic compounds was measured as TOC (“total organic carbon”) in [g / l], according to the Austrian Standard ONORM EN 1484:2019.

[0046] “ODP” means a pulp quantity expressed as oven dried pulp, i.e. without humidity / water. It is measured according to standard ISO 638:2008. Bleaching experiments:

[0047] Eucalyptus kraft pulp from a commercial-scale pulp production process after the delignification step was used for the experiments.

[0048] A press lye was taken from the alkalization stage of a commercial-scale viscose fiber production process, where the pulp had been alkalized with an aqueous solution of 18.5% (w / w) of sodium hydroxide and mechanically filtered. For the experiments press lyes taken from different points in different membrane pretreatment processes were used as described as follows:

[0049] In the experiments the complexation step was performed using the following complexation solutions

[0050] • EDTA: Aqueous solution of 0,5 kg EDTA per ton ODP with pH = 5.5,

[0051] • “PL1” means a process lye obtained from a commercial-scale viscose process containing 34 g / l NaOH and organic compounds (21 ,9 g / l TOC), i.e. TOC:NaOH = 0,64

[0052] • “PL2” means a process lye obtained from a commercial-scale viscose process containing 166 g / l NaOH and organic compounds (14,3 g / l TOC), i.e. TOC:NaOH = 0,086.

[0053] • “PL3” means a process lye obtained from a commercial-scale viscose process containing 153 g / l NaOH and organic compounds (43,3 g / l TOC), i.e. TOC:NaOH = 0,28.

[0054] • As a reference, a P-stage under identical conditions, but without the addition of any complexing agent was performed (Example b).

[0055] The complexation step was performed as follows: The Eucalyptus Kraft pulp after the delignification step and washing step was mixed with the aqueous complexing agent solutions in a stirred vessel in amounts that always resulted in a consistency of 10% (w / w) and an NaOH content of 10 kg NaOH per ton ODP. In the examples where press lyes were used, all NaOH in the mixture originated from the press lye. Due to varying NaOH concentrations of the viscose process lyes used in the following examples, this means that different amounts of the active complexation components were introduced. A peroxide bleaching stage was directly applied to the pulps after the complexation step, without a previous A- or Q-stage, to reveal the influence of the metal ions on pulp degradation and yield loss: The appropriate amount of hydrogen peroxide was added to the mixture of the complexation step. The hydrogen peroxide treatment was performed for 180 min at 80 °C, with 10 % consistency, in a stirred vessel. 10 kg NaOH per ton ODP were used. The hydrogen peroxide concentration was 25 kg per ton ODP. The properties of the treated pulps after bleaching are shown in Table 1 .

[0056] Table 1 :

[0057] The ability of EDTA (Example c) and viscose press lyes (Examples d to f) to prevent undesired pulp degradation is demonstrated by higher resulting intrinsic viscosity values, compared to the pulp bleached without using of complexing agents (Example b). The application of PL2 (Example e) and PL3 (Example f) led to somewhat higher degradation, compared to PL1 (Example d), due to lower resulting concentration of the active agents. The obtained brightness of all samples was comparable independent of the complexing agent used.

[0058] Further examples of hydroxy carboxylic acid solutions obtained from viscose process press lyes of a commercial-scale viscose process in a separation step wherein NaOH is removed from the press lye in a way that increases the share of organic compounds in relation to NaOH are shown in Table 2. Such solutions will be suitable for the process of the invention, too.

[0059] Table 2:

[0060]

Claims

Claims1. Method for bleaching of a polysaccharide material that includes a hydrogen peroxide bleaching stage with a hydrogen peroxide concentration of between 3 and 30 kg per ton ODP, characterized in that before the end of the hydrogen peroxide bleaching stage the polysaccharide material is treated with a hydroxy carboxylic acid solution that is obtained from a viscose process press lye and hence this hydroxy carboxylic acid solution contains two or more different hydroxy carboxylic acids from the group containing glucoisosaccharinic acid, glycolic acid, lactic acid and xyloisosaccharinic acid, and further characterized in that the alkaline hydroxy carboxylic acid salt solution contains between 5 and 250 g / l NaOH and between 2 and 150 g / l TOC and the treatment is performed at a consistency (i.e. the content of solids in the slurry), of between 6 and 15%(w / w), at a temperature of between 30 and 100°C and a treatment time of between 0.5 and 3.0 h.

2. Method according to claim 1 , wherein the alkaline hydroxy carboxylic acid salt solution contains between 30 and 180 g / l NaOH.

3. Method according to claim 1 , wherein the alkaline hydroxy carboxylic acid salt solution contains between 10 and 70 g / l TOC.

4. Method according to claim 1 , wherein the ratio (w / w) of TOC to NaOH in the alkaline hydroxy carboxylic acid salt solution is between 0,01 and 2,00, preferably between 0,10 and 1 ,20, especially preferred between 0,20 and 0,70.

5. Method according to claim 1 , wherein the hydroxy carboxylic acid solution is obtained from a viscose process press lye in a separation step before the bleaching stage by a process step wherein NaOH is removed from the press lye in a way that increases the share of organic compounds in relation to NaOH.

6. Method according to claim 5, wherein the separation step contains a membrane filtration step, for example a nanofiltration, an osmose or a diffusion dialysis.

7. Method according to claim 6, wherein the permeate of the membrane filtration step is re-used in a viscose fiber production process.

8. Method according to claim 1 , wherein the treatment with the alkaline hydroxy carboxylic acid salt solution takes place before the polysaccharide material is treated with the hydrogen peroxide in the hydrogen peroxide bleaching stage.

9. Method according to claim 1 , wherein the treatment with the alkaline hydroxy carboxylic acid salt solution takes place while the polysaccharide material is treated with the hydrogen peroxide in the hydrogen peroxide bleaching stage.

10. Method according to claim 1 , wherein the ratio (w / w) of the alkaline hydroxy carboxylic acid salt solution (measured as TOC) to the polysaccharide material (ODP) is between 0,1 and 20,0 kg TOC / 1 ODP, preferably between 0,2 and 15,0 kg TOC / 1 ODP, more preferably between 0,5 and 10,0 kg TOC / 1 ODP.11 . Method according to claim 1 , wherein the polysaccharide material is a cellulosic material.

12. Method according to claim 11 , wherein the cellulosic material is a dissolving wood pulp.

13. Method according to claim 11 , wherein the cellulosic material is a paper pulp.

14. Method according to claim 11 , wherein the cellulosic material consists of or contains recycled cellulose-containing post-industrial or post-consumer textile material.

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