JP3915682B2 - Method for producing bleached pulp - Google Patents
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【０００１】
【発明の属する技術分野】
本発明は、広葉樹を含むリグノセルロース物質の漂白パルプの製造方法に関する。更に詳しく述べれば、本発明は、広葉樹を含むリグノセルロース物質を蒸解して得られる未漂白パルプをアルカリ酸素漂白し、その後、初段に二酸化塩素漂白段を有する多段漂白工程で処理して漂白パルプを製造する方法に関する。
【０００２】
【従来の技術】
広葉樹を含むリグノセルロース物質を蒸解して得られる未漂白パルプを漂白する手順としては、まず、アルカリ酸素漂白し、その後、塩素で処理し、パルプ中に含有されるリグニンを塩素化し、リグニンに可溶性を付加した後、次いで、アルカリを用いて塩素化リグニンを溶解抽出して、パルプ中からリグニンを分離除去し、最後に、次亜塩素酸塩、二酸化塩素等を使用し、残留する少量のリグニンを分解除去する方法が従来から採られてきた。
【０００３】
しかしながら、近年、パルプの塩素化段からの漂白排水に含まれる有機塩素化合物（以下、ＡＯＸと略す）の環境への影響が懸念され、パルプ漂白に塩素を用いない動きが高まってきている。また、次亜塩素酸塩を用いた場合も、パルプの漂白時にクロロホルムが生成し、環境に悪影響を及ぼす可能性があることから、次亜塩素酸塩をパルプ漂白に使用しない漂白シーケンスが求められてきている。
【０００４】
現在、塩素や次亜塩素酸塩の代替として、オゾン、二酸化塩素、過酸化水素及び過酢酸、過硫酸等の過酸の使用が提案されている。しかしながら、実際には、漂白薬品としての実績があり、取り扱いが比較的容易な二酸化塩素及び過酸化水素が塩素と次亜塩素酸塩の代替として使用されるケースが多くなっている。特に、塩素の代替としては二酸化塩素を使用するケースが圧倒的に多くなっている。ところが、二酸化塩素は塩素に比べて薬品単価が高く、経済的に不利であることから、できるだけ少ない使用量で所望の白色度まで漂白したいという要望があった。
【０００５】
初段の二酸化塩素漂白段において、硫酸でｐＨを３に調整した後、二酸化塩素を添加し、９５℃で１．５〜３時間処理すれば、酸処理と二酸化塩素処理を同時に行うことができ、通常の条件で二酸化塩素漂白を行った場合と比べて二酸化塩素の使用量を削減できることが知られている（例えば、非特許文献１）。しかしながら、ｐＨ３以下の条件で二酸化塩素漂白を行った場合には、一部の二酸化塩素は塩素酸に変換し、十分な漂白効果が得られないという問題があった。
【０００６】
初段の二酸化塩素漂白段において、二酸化塩素漂白の添加を二回に分割し、最初の二酸化塩素添加後に、１分間をｐＨ８．５〜１０で行い、その後、酸を加えてｐＨ２．５で３時間処理した後、再度二酸化塩素を添加し２分間処理し、所望の白色度まで漂白するのに必要な二酸化塩素の使用量を削減する方法が知られている（非特許文献２）。しかしながら、この方法を実施するには操作が煩雑である上に、二回目の二酸化塩素添加後、極短時間で二酸化塩素漂白段を終えてしまうため、添加した二酸化塩素を十分に活用できないという問題点もあった。
【０００７】
後段の二酸化塩素漂白段において、二酸化塩素漂白の開始から５〜１５分間をｐＨ５〜１０で行い、その後、ｐＨ１．７〜４．４で２．５時間、７０℃で処理することにより、二酸化塩素の漂白効率を上げる方法も知られている（非特許文献３）。しかしながら、この方法は論文中にも記述されているように、カッパー価が１０以上のパルプに対しては効果がなく、初段の二酸化塩素漂白段には適用できないと言う問題点があった。
【０００８】
二酸化塩素漂白段にホルマリンなどの還元剤を直接添加し、二酸化塩素から副生する亜塩素酸塩を二酸化塩素に再生し、二酸化塩素の使用量を削減する方法も提案されている（例えば、特許文献１参照）。しかしながら、ホルマリンのような環境への影響を懸念される物質をパルプ漂白に添加することは、環境上好ましいとは言えない。
【０００９】
二酸化塩素の使用量を削減するための間接的な方法としては、蒸解時においてできるだけ脱リグニンを進め、未晒パルプのカッパー価を減少させる方法や、アルカリ酸素漂白工程においてできるだけ脱リグニンを進め、多段漂白工程前のパルプのカッパー価を減少させる方法が多数提案されている。また、キシラナーゼ前処理を行う方法なども知られている（例えば、非特許文献４）。しかしながら、これらの方法を用いて二酸化塩素使用量を低減した場合、特に広葉樹を含むリグノセルロースを原料とした場合、ヘキセンウロン酸が多く残留し、漂白パルプのＫ価が高くなり、退色性が悪化するという問題点があった。
【００１０】
【特許文献１】
米国特許６２３５１５４号明細書（B1 クレーム１、対応日本特許特表２００２−５０６９３５号公報 請求項１）
【非特許文献１】
Dominque Lachenalら、１９９７年ISWPC予稿集、ｐ９５−９８
【非特許文献２】
S.Juutilainenら、１９９９ Tappi Pulping Conference 予稿集ｐ６４５−６５１
【非特許文献３】
G.E.Segerら、Tappi Journal、７５（７）ｐ１７４−１８０、１９９２年
【非特許文献４】
Viikari,Lら、Biotechnology in the Pulp & Paper Industry 第三 回国際会議予稿集 ｐ６７−６９ （１９８６）
【００１１】
【発明が解決しようとする課題】
本発明の目的は、広葉樹を含むリグノセルロース物質を蒸解して得られる未漂白パルプをアルカリ酸素漂白し、その後、初段に二酸化塩素漂白段を有する多段漂白工程で処理して漂白パルプを製造する方法において、二酸化塩素使用量を低減し、かつ退色性が改善される漂白パルプの製造方法を提供することにある。
【００１２】
【課題を解決するための手段】
本発明者らは、かかる現状に鑑み、広葉樹を含むリグノセルロース物質を原料とし、多段漂白工程の初段で二酸化塩素漂白を行い、かつ漂白パルプの退色性を悪化させないで、二酸化塩素の使用量を低減する方法を見出す方法について鋭意研究した結果、初段二酸化塩素段を二段階に分けて設計することが有効であることを見出した。
【００１３】
つまり、初段二酸化塩素段の前半部を“漂白パート”、後半部を“ヘキセンウロン酸分解パート”として考え、以下のような条件に設定することにより実現できることを見出した。まず、初段二酸化塩素漂白段に添加する二酸化塩素の全量を開始時に一括添加し、１〜１５分間反応させ、二酸化塩素による漂白作用を最大限に引き出す。その後、洗浄することなく酸を添加してｐＨ２．０〜３．５にすることにより、反応初期で生成したクロライトを活性化させてヘキセンウロン酸を分解させる。さらに高温にし、ヘキセンウロン酸の酸加水分解も同時に行う。このように、初段二酸化塩素漂白段を設計することにより、初段二酸化塩素漂白段において、十分な漂白作用とヘキセンウロン酸の分解の両立が可能となり、本発明を完成するに至った。
【００１４】
本発明は、以下の各発明を包含する。
（１）広葉樹材を含むリグノセルロース物質を蒸解して得られる未漂白パルプをアルカリ酸素漂白し、その後、初段に二酸化塩素漂白段を有する多段漂白工程で処理して漂白パルプを製造する方法において、該二酸化塩素漂白段の開始時に二酸化塩素の全量を一括添加し、８０〜１００℃の条件で１〜１５分間滞留させた後、洗浄することなく酸を添加してｐＨ２．０〜３．５とし、さらに、８０〜１００℃の条件で６０〜２４０分間処理して二酸化塩素漂白段を行うことを特徴とする漂白パルプの製造方法。
【００１５】
（２）前記二酸化塩素漂白段において、酸を添加する直前の反応ｐＨが４．０〜６．５、好ましくはｐＨ４．５〜５．５になるように、二酸化塩素漂白段の開始時に酸又はアルカリを添加することを特徴とする前記（１）項記載の漂白パルプの製造方法。
【００１６】
（３）前記多段漂白工程の最終段に二酸化塩素漂白段を有し、かつ該最終段二酸化塩素漂白段の終了時のｐＨが４．５〜６．５の範囲で行われることを特徴とする前記（１）項又は（２）項に記載の漂白パルプの製造方法。
【００１７】
（４）前記最終段二酸化塩素漂白段後の洗浄排水を、初段二酸化塩素漂白段の開始時の希釈水として使用することを特徴とする前記（３）項記載の漂白パルプの製造方法。
【００１８】
（５）前記多段漂白工程において、初段の二酸化塩素漂白段に続いて、酸素及び過酸化水素を添加したアルカリ抽出段が行われることを特徴とする前記（１）項〜（４）項のいずれか１項に記載の漂白パルプの製造方法。
【００１９】
【発明の実施の形態】
本発明で用いられるリグノセルロース物質は、好適には広葉樹材１００％のものであるが、広葉樹材が含まれれば、針葉樹材を含んでいても良く、非木材と呼ばれるものを含んでいても良く、広葉樹材を含むこと以外は特に限定するものではない。本発明に使用されるパルプを得るための蒸解法としては、クラフト蒸解、ポリサルファイド蒸解、ソーダ蒸解、アルカリサルファイト蒸解等の公知の蒸解法を用いることができるが、パルプ品質、エネルギー効率等を考慮すると、クラフト蒸解又はポリサルファイド蒸解が好適に用いられる。
【００２０】
例えば、広葉樹材１００％のリグノセルロースをクラフト蒸解する場合、クラフト蒸解液の硫化度は５〜７５％、好ましくは１５〜４５％、有効アルカリ添加率は絶乾木材質量当たり５〜３０質量％、好ましくは１０〜２５質量％、蒸解温度は１３０〜１７０℃で、蒸解方式は、連続蒸解法或いはバッチ蒸解法のどちらでもよく、連続蒸解釜を用いる場合は、蒸解液を多点で添加する修正蒸解法でもよく、その方式は特に問わない。
【００２１】
蒸解に際して、使用する蒸解液に蒸解助剤として、公知の環状ケト化合物、例えば、ベンゾキノン、ナフトキノン、アントラキノン、アントロン、フェナントロキノン及び前記キノン系化合物のアルキル、アミノ等の核置換体、或いは前記キノン系化合物の還元型であるアントラヒドロキノンのようなヒドロキノン系化合物、さらには、ディールスアルダー法によるアントラキノン合成法の中間体として得られる安定な化合物である９，１０−ジケトヒドロアントラセン化合物等から選ばれた１種或いは２種以上が添加されてもよく、その添加率は通常の添加率であり、例えば、木材チップの絶乾質量当たり０．００１〜１．０質量％である。
【００２２】
本発明では、公知の蒸解法により得られた未漂白化学パルプは、洗浄、粗選及び精選工程を経て、公知のアルカリ酸素漂白法により脱リグニンされる。本発明に使用されるアルカリ酸素漂白法は、公知の中濃度法或いは高濃度法がそのまま適用できるが、現在、汎用的に用いられているパルプ濃度が８〜１５質量％で行われる中濃度法が好ましい。
【００２３】
前記中濃度法によるアルカリ酸素漂白法において、アルカリとしては苛性ソーダ或いは酸化されたクラフト白液を使用することができ、酸素ガスとしては、深冷分離法からの酸素、ＰＳＡ(Pressure Swing Adsorption）からの酸素、ＶＳＡ(Vacuum Swing Adsorption)からの酸素等が使用できる。前記酸素ガスとアルカリは中濃度ミキサーにおいて中濃度のパルプスラリーに添加され混合が十分に行われた後、加圧下でパルプ、酸素及びアルカリの混合物を一定時間保持できる反応塔へ送られ、脱リグニンされる。
【００２４】
酸素ガスの添加率は、絶乾パルプ質量当たり０．５〜３質量％、アルカリ添加率は０．５〜４質量％、反応温度は８０〜１２０℃、反応時間は１５〜１００分間、パルプ濃度は８〜１５質量％であり、この他の条件は、公知のものが適用できる。本発明では、アルカリ酸素漂白工程において、上記アルカリ酸素漂白を連続して複数回行い、できる限り脱リグニンを進めるのが好ましい実施形態である。アルカリ酸素漂白が施されたパルプは次いで洗浄工程へ送られる。パルプは洗浄後、多段漂白工程へ送られる。
【００２５】
本発明の多段漂白処理工程では、必ず初段は二酸化塩素漂白が行われる。本発明の初段二酸化塩素漂白段では、初段二酸化塩素漂白段に添加される二酸化塩素は必ず開始時に全量が一括添加され、処理温度は８０〜１００℃、好ましくは８５〜９５℃であり、１〜１５分間、好ましくは５〜１０分間滞留処理される。その後、洗浄することなく、ｐＨ調整用の酸が添加され、ｐＨ２．０〜３．５、好ましくはｐＨ２．５〜３．０、８０〜１００℃、好ましくは８５〜９５℃の条件で６０〜２４０分間、好ましくは９０〜１８０分間処理して初段二酸化塩素漂白段が行われる。
【００２６】
前記ｐＨ調整用の酸が添加される前のｐＨは４．０〜６．５、好ましくはｐＨ４．５〜６．５、さらに好ましくはｐＨ５．０〜６．０であり、必要に応じて、二酸化塩素漂白段開始時にｐＨ調整用のアルカリ、或いは酸が添加される。二酸化塩素の添加率は、絶乾パルプ質量当たり二酸化塩素として０．２〜２．０質量％であり、酸素漂白後パルプの性状や漂白パルプの目標白色度によって決められる。ｐＨ調整用のアルカリ及び酸は特に限定されるのもではないが、取り扱いのし易さや経済的な面から、苛性ソーダ及び硫酸、塩酸或いは硝酸が好適に使用される。パルプ濃度に関しては特に限定されるものではないが、操作性の点から好適には８〜１５質量％で行われる。
【００２７】
本発明では、初段二酸化塩素段の次段以降の漂白段に関しては特に限定されるものではないが、次段はアルカリ抽出段、最終段は二酸化塩素漂白段とするのが好ましい実施形態である。次段をアルカリ抽出段とする場合する場合の条件としては、アルカリ添加率は絶乾パルプ質量当たり０．５〜３質量％、反応温度は６０〜１２０℃、反応時間は１５〜１２０分間、パルプ濃度は８〜１５質量％である。好適には、アルカリ抽出段に酸素ガスが添加される。酸素ガスの添加率は、絶乾パルプ質量当たり０．１〜３質量％である。さらに好適には、過酸化水素も添加される。過酸化水素の添加率は、絶乾パルプ質量当たり０．０５〜２質量％である。
【００２８】
本発明では、初段二酸化塩素漂白段の終了時のｐＨが３．５以下であるため、初段二酸化塩素漂白段後のパルプ中の重金属含有量が著しく少なくなり、重金属によって誘発される過酸化水素、酸素、オゾンのような酸素系薬品の分解率が低くなるので、より効果的に酸素系薬品を反応させることができるという特徴がある。
【００２９】
本発明の多段漂白工程では、最終段は二酸化塩素漂白段とするのが好ましい実施形態である。最終段の二酸化塩素漂白段の条件は、好ましくはｐＨは４．５〜６．５、さらに好ましくはｐＨ５．０〜６．０であり、反応温度５０〜９０℃、反応時間３０〜２４０分間、パルプ濃度は８〜１５％であり、ｐＨを調整するために、必要に応じて、酸、アルカリが添加される。ｐＨ調整用のアルカリ及び酸は特に限定されるのもではないが、取り扱いのし易さや経済的な面から、苛性ソーダ及び硫酸、塩酸或いは硝酸が好適に使用される。二酸化塩素の添加率は、絶乾パルプ質量当たり０．０５〜１質量％であり、分割添加することも可能である。
【００３０】
本発明においては、最終段を二酸化塩素漂白段とした場合、最終段二酸化塩素漂白段後の洗浄排水を、初段二酸化塩素漂白段の開始時の希釈水として使用することも可能である。前記最終段二酸化塩素漂白段後の洗浄排水を、初段二酸化塩素漂白段の開始時の希釈水として使用するメリットとしては、最終段二酸化塩素段の洗浄排水に含まれるクロライトを有効利用できる点が挙げられる。本発明では、好ましくは、最終二酸化塩素漂白はｐＨ４．５〜６．５の範囲で行われるため、ほとんどの廃二酸化塩素はクロライトの形態で残留している。クロライトは、ｐＨ４．５〜６．５の範囲では反応性が乏しいが、ｐＨが低くなるとリグニンやヘキセンウロン酸との反応性が高まるため、初段二酸化塩素段の希釈水としてパルプに添加された洗浄排水中のクロライトは、硫酸添加後、リグニンやヘキセンウロン酸と反応し、有効利用されることになる。
【００３１】
【実施例】
以下、実施例及び比較例を挙げて本発明をより具体的に説明するが、勿論本発明はこれらの実施例によって限定されるものではない。以下に示す実施例１〜３と比較例１〜３は、工場製酸素漂白後広葉樹クラフトパルプをＤ１−Ｅ／Ｏ−Ｄ２シーケンスで漂白したものであり、実施例４と比較例４は、Ｄ１−Ｅ／ＯＰ−Ｄ２シーケンスで漂白したものでる。
また、特に示さない限り、カッパー価の測定、過マンガン酸カリウム価（Ｋ価）の測定、パルプ白色度の測定、パルプの退色性の評価は、それぞれ以下の方法で行った。なお、実施例及び比較例における薬品の添加率は絶乾パルプ質量当たりの質量％示す。
【００３２】
１．パルプのカッパー価の測定
カッパー価の測定は、ＪＩＳ Ｐ ８２１１に準じて行った。
【００３３】
２．パルプの過マンガン酸カリウム価（Ｋ価）の測定
過マンガン酸カリウム価の測定は、ＪＩＳ Ｐ ８２０６に準じて行った。
【００３４】
３．パルプ白色度の測定
漂白パルプを離解後、Ｔａｐｐｉ試験法Ｔ２０５ｏｓ−７１（ＪＩＳ Ｐ ８２０９）に従って坪量６０ｇ／ｍ2のシートを作製し、ＪＩＳ Ｐ ８１２３に従ってパルプの白色度を測定した。
【００３５】
４．パルプの退色性評価
漂白パルプを離解後、硫酸アルミニウムを加え、ｐＨ４．５に調整した後、坪量６０ｇ／ｍ2のシートを作製し、送風乾燥機にて乾燥させた。このシートを８０℃、相対湿度６５％の条件下で、２４時間退色させ、退色前後の白色度から下式に従いＰＣ価を算出し、評価した。
実施例１
工場製酸素漂白後広葉樹クラフトパルプ（材配合；ユーカリ材８０％、アカシア材２０％配合、白色度；４９．５％、カッパー価；１２．０）を絶乾質量で７０ｇ採取し、プラスチック袋に入れ、イオン交換水を用いてパルプ濃度を１０％に調整した後、絶乾パルプ質量当たり二酸化塩素を０．７％を添加し、温度が９０℃の恒温水槽に１０分間浸漬した。この時のパルプスラリーのｐＨは４．０であった。その後、洗浄することなく、即座に硫酸を絶乾パルプ質量当たり０．６％添加し、再度、温度が９０℃の恒温水槽に１８０分間浸漬し、Ｄ１段の漂白を行った。Ｄ１段終了時のパルプスラリーのｐＨは３．０であった。得られたパルプをイオン交換水で３％に希釈した後、ブフナーロートで脱水、洗浄した。Ｄ１段後パルプのＫ価は、２．５であった。
【００３７】
Ｄ１段後のパルプにイオン交換水を加えて、パルプ濃度を１０％に調整した後、絶乾パルプ質量当たり苛性ソーダを１．０％添加し、ステンレス製２リットル容の間接加熱式オートクレーブに入れ、ゲージ圧力が０．１５ＭＰａとなるように純度が９９．９％の市販の圧縮酸素ガスで加圧し、７０℃で２０分間反応させた。その後、パルプスラリーをオートクレーブから取り出し、プラスチック袋に移した後、温度が７０℃の恒温水槽に７０分間浸漬し、Ｅ／Ｏ段の抽出を行った。得られたパルプをイオン交換水で３％に希釈した後、ブフナーロートで脱水、洗浄した。
【００３８】
続いて、Ｅ／Ｏ段後のパルプを絶乾質量で６０．０ｇ、プラスチック袋に入れ、イオン交換水を用いてパルプ濃度１０％に調整した後、絶乾パルプ質量当たり二酸化塩素を０．２％と苛性ソーダを０．０５％添加し、温度が７０℃の恒温水槽に１８０分間浸漬し、Ｄ２段の漂白を行った。Ｄ２段終了時のパルプスラリーのｐＨは５．５であった。得られたパルプをイオン交換水で３％に希釈した後、ブフナーロートで脱水、洗浄した。得られた漂白パルプの白色度は８５．０％、Ｋ価は１．０、ＰＣ価は１．５であった。
Ｄ１段における二酸化塩素添加率、Ｄ１段後パルプのＫ価、漂白パルプの白色度、Ｋ価及びＰＣ価を表１に示した。
【００３９】
実施例２
実施例１のＤ１段において、二酸化塩素添加時に同時に苛性ソーダを絶乾パルプ質量当たり０．１％添加し、硫酸の添加率を絶乾パルプ質量当たり０．７％に替えた以外は実施例１と同様の操作を行った。Ｄ１段における硫酸添加直前のｐＨは５．０であり、Ｄ１段終了時のｐＨは３．０であり、Ｄ１段後のＫ価は２．４、漂白パルプの白色度は８６．０％、Ｋ価は０．９、ＰＣ価は１．４であった。
Ｄ１段における二酸化塩素添加率、Ｄ１段後パルプのＫ価、漂白パルプの白色度、Ｋ価及びＰＣ価を表１に示した。
【００４０】
実施例３
実施例２のＤ１段における希釈水を実施例１で採取したＤ２段洗浄濾液に替え、苛性ソーダの添加率を絶乾パルプ質量当たり０．１５％に替えた以外は実施例２と同様の操作を行った。Ｄ１段における硫酸添加直前のｐＨは５．２であり、Ｄ１段終了時のｐＨは３．０であり、Ｄ１段後パルプのＫ価は２．３、漂白パルプの白色度は８６．８％、Ｋ価は０．８、ＰＣ価は１．３であった。
Ｄ１段における二酸化塩素添加率、Ｄ１段後パルプのＫ価、漂白パルプの白色度、Ｋ価及びＰＣ価を表１に示した。
【００４１】
実施例４
実施例２のＥ／Ｏ段において、過酸化水素を絶乾パルプ質量当たり０．２％添加したこと以外は実施例２と同様の操作を行った。漂白パルプの白色度は８８．０％、Ｋ価は０．８、ＰＣ価は１．２であった。Ｄ１段における二酸化塩素添加率、Ｄ１段後パルプのＫ価、漂白パルプの白色度、Ｋ価及びＰＣ価を表１に示した。
【００４２】
比較例１
実施例１のＤ１段開始時に同時に硫酸を絶乾パルプ質量当たり０．６％添加し、途中で硫酸を添加しなかった以外は実施例１と同様の操作を行った。Ｄ１段開始１０分後及びＤ１段終了時のｐＨは３．０、Ｄ１段後パルプのＫ価は２．６、漂白パルプの白色度は８３．８％、Ｋ価は１．１、ＰＣ価は２．０であった。
Ｄ１段における二酸化塩素添加率、Ｄ１段後パルプのＫ価、漂白パルプの白色度、Ｋ価及びＰＣ価を表１に示した。
【００４３】
比較例２
実施例１のＤ１段の途中で硫酸を添加しなかった以外は、実施例１と同様の操作を行った。Ｄ１段後パルプのＫ価は３．５、漂白パルプの白色度は８４．１％、Ｋ価は２．０、ＰＣ価は４．４であった。
Ｄ１段における二酸化塩素添加率、Ｄ１段後パルプのＫ価、漂白パルプの白色度、Ｋ価及びＰＣ価を表１に示した。
【００４４】
比較例３
実施例１のＤ１段における反応温度を７０℃に替えた以外は、実施例１と同様の操作を行った。Ｄ１段後パルプのＫ価は４．１、漂白パルプの白色度は８２．８％、Ｋ価は２．６、ＰＣ価は６．０であった。
Ｄ１段における二酸化塩素添加率、Ｄ１段後パルプのＫ価、漂白パルプの白色度、Ｋ価及びＰＣ価を表１に示した。
【００４５】
比較例４
比較例２のＥ／Ｏ段に絶乾パルプ質量当たり過酸化水素を０．２％を添加したこと以外は比較例２と同様の操作を行った。Ｄ１段後パルプのＫ価は３．５、漂白パルプの白色度は８４．９％、Ｋ価は２．０、ＰＣ価は４．２であった。Ｄ１段における二酸化塩素添加率、Ｄ１段後パルプのＫ価、漂白パルプの白色度、Ｋ価及びＰＣ価を表１に示した。
【００４６】
【表１】
表１の実施例１、２と比較例１〜３を比較することから明らかなように、広葉樹材を含むリグノセルロース物質を蒸解して得られる未漂白パルプをアルカリ酸素漂白し、その後、初段に二酸化塩素漂白段を有する多段漂白工程で処理して漂白パルプを製造する際に、初段二酸化塩素漂白段の開始時に二酸化塩素の全量を一括添加し、８０〜１００℃の条件で１〜１５分間滞留させた後、洗浄することなく酸を添加してｐＨ２．０〜３．５とし、さらに、８０〜１００℃の条件で６０〜２４０分間処理して二酸化塩素漂白段を行えば、白色度が高く、Ｋ価が低く、退色性のよい漂白パルプが得られることが分る。さらに、初段の二酸化塩素漂白段において酸を添加する直前のｐＨを４．５〜６．０に調整すればさらに効果が高くなることが分る。
【００４８】
また、表１の実施例２と３を比較することから明らかなように、最終二酸化塩素漂白段を終ｐＨが４．５〜６．５になる条件で行い、この段の洗浄濾液を初段二酸化塩素漂白段の希釈水として使用すれば漂白効率が向上することが分る。
一方、実施例４と比較例４を比較することから明らかなように、前記条件で初段二酸化塩素漂白段を行った場合には、アルカリ抽出段に酸素及び過酸化水素を添加した際の漂白効率も高くなることもわかる。
【００４９】
【発明の効果】
広葉樹材を含むリグノセルロース物質を蒸解して得られる未漂白パルプをアルカリ酸素漂白し、その後、初段に二酸化塩素漂白段を有する多段漂白工程で処理して漂白パルプを製造する方法において、該二酸化塩素漂白段の開始時に二酸化塩素の全量を一括添加し、８０〜１００℃の条件で１〜１５分間滞留させた後、洗浄することなく酸を添加してｐＨ２．０〜３．５とし、さらに、８０〜１００℃の条件で６０〜２４０分間処理して二酸化塩素漂白段を行うことにより、効率良く漂白でき、かつ退色性の改善された漂白パルプを製造することが可能となった。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing bleached pulp of lignocellulosic material including hardwood. More specifically, in the present invention, unbleached pulp obtained by digesting lignocellulosic material including hardwood is subjected to alkaline oxygen bleaching, and then treated in a multi-stage bleaching process having a chlorine dioxide bleaching stage in the first stage to produce bleached pulp. It relates to a method of manufacturing.
[0002]
[Prior art]
The procedure for bleaching unbleached pulp obtained by digesting lignocellulosic material including hardwood is first bleached with alkali oxygen, then treated with chlorine, chlorinated lignin contained in the pulp, soluble in lignin Then, chlorinated lignin is dissolved and extracted using alkali to separate and remove lignin from the pulp. Finally, hypochlorite, chlorine dioxide, etc. are used, and a small amount of residual lignin is removed. The method of decomposing and removing has been conventionally employed.
[0003]
However, in recent years, there is concern about the environmental impact of organochlorine compounds (hereinafter abbreviated as AOX) contained in the bleaching effluent from the pulp chlorination stage, and there is an increasing trend of not using chlorine for pulp bleaching. In addition, when hypochlorite is used, chloroform is generated during pulp bleaching, which may adversely affect the environment, so a bleaching sequence that does not use hypochlorite for pulp bleaching is required. It is coming.
[0004]
At present, the use of peracids such as ozone, chlorine dioxide, hydrogen peroxide, peracetic acid and persulfuric acid has been proposed as an alternative to chlorine and hypochlorite. However, in fact, chlorine dioxide and hydrogen peroxide, which have a track record as bleaching chemicals and are relatively easy to handle, are often used as substitutes for chlorine and hypochlorite. In particular, the use of chlorine dioxide as an alternative to chlorine is overwhelmingly increasing. However, since chlorine dioxide has a higher chemical unit price than chlorine and is economically disadvantageous, there has been a demand for bleaching to a desired whiteness with the least amount of use possible.
[0005]
In the first stage of chlorine dioxide bleaching, after adjusting the pH to 3 with sulfuric acid, if chlorine dioxide is added and treated at 95 ° C. for 1.5 to 3 hours, acid treatment and chlorine dioxide treatment can be performed simultaneously, It is known that the amount of chlorine dioxide used can be reduced as compared with the case where chlorine dioxide bleaching is performed under normal conditions (for example, Non-Patent Document 1). However, when chlorine dioxide bleaching is performed under conditions of pH 3 or less, there is a problem that a part of chlorine dioxide is converted into chloric acid and a sufficient bleaching effect cannot be obtained.
[0006]
In the first stage chlorine dioxide bleaching stage, the addition of chlorine dioxide bleaching was divided into two parts, and after the first chlorine dioxide addition, 1 minute was carried out at pH 8.5-10, and then acid was added for 3 hours at pH 2.5. After the treatment, a method is known in which chlorine dioxide is added again and treated for 2 minutes to reduce the amount of chlorine dioxide used to bleach to the desired whiteness (Non-Patent Document 2). However, in order to carry out this method, the operation is complicated and the chlorine dioxide bleaching stage is completed in a very short time after the second chlorine dioxide addition, so that the added chlorine dioxide cannot be fully utilized. There was also a point.
[0007]
In the subsequent chlorine dioxide bleaching stage, chlorine dioxide bleaching is carried out at pH 5-10 for 5 to 15 minutes from the start of chlorine dioxide bleaching, and then treated at 70 ° C. for 2.5 hours at pH 1.7-4.4. A method for increasing the bleaching efficiency is also known (Non-patent Document 3). However, as described in the paper, this method has a problem that it is not effective for pulp having a kappa number of 10 or more and cannot be applied to the first stage chlorine dioxide bleaching stage.
[0008]
There has also been proposed a method in which a reducing agent such as formalin is directly added to the chlorine dioxide bleaching stage to regenerate chlorite by-produced from chlorine dioxide into chlorine dioxide, thereby reducing the amount of chlorine dioxide used (for example, patents). Reference 1). However, it cannot be said that it is environmentally preferable to add a substance such as formalin, which is concerned about environmental influences, to pulp bleaching.
[0009]
As an indirect method to reduce the amount of chlorine dioxide used, delignification is promoted as much as possible during cooking, and the kappa number of unbleached pulp is reduced, or delignification is promoted as much as possible in the alkaline oxygen bleaching process. Many methods have been proposed to reduce the kappa number of the pulp before the bleaching step. Moreover, the method etc. which perform a xylanase pretreatment are also known (for example, nonpatent literature 4). However, when the amount of chlorine dioxide used is reduced by using these methods, particularly when lignocellulose containing hardwood is used as a raw material, a large amount of hexeneuronic acid remains, the K value of bleached pulp increases, and the fading property deteriorates. There was a problem.
[0010]
[Patent Document 1]
US Pat. No. 6,235,154 (B1 claim 1, Japanese Patent Special Publication 2002-506935 gazette claim 1)
[Non-Patent Document 1]
Dominque Lachenal et al., 1997 ISWPC Proceedings, p95-98
[Non-Patent Document 2]
S. Juutilainen et al., 1999 Tappi Pulping Conference Proceedings p645-651
[Non-Patent Document 3]
GESeger et al., Tappi Journal, 75 (7) p174-180, 1992 [Non-Patent Document 4]
Viikari, L et al. Biotechnology in the Pulp & Paper Industry 3rd International Conference Proceedings p67-69 (1986)
[0011]
[Problems to be solved by the invention]
An object of the present invention is to produce bleached pulp by subjecting unbleached pulp obtained by cooking lignocellulosic material containing hardwood to alkaline oxygen bleaching and then treating it in a multistage bleaching step having a chlorine dioxide bleaching stage in the first stage. Is to provide a method for producing bleached pulp in which the amount of chlorine dioxide used is reduced and the fading property is improved.
[0012]
[Means for Solving the Problems]
In view of the present situation, the present inventors use a lignocellulosic material containing hardwood as a raw material, perform chlorine dioxide bleaching in the first stage of the multistage bleaching process, and reduce the amount of chlorine dioxide used without deteriorating the bleaching of bleached pulp. As a result of diligent research on how to find a reduction method, it was found that it is effective to design the first stage chlorine dioxide stage in two stages.
[0013]
In other words, it was found that the first half of the first stage chlorine dioxide stage can be realized by setting the following conditions, considering the first half as a “bleaching part” and the second half as a “hexeneuronic acid decomposition part”. First, the total amount of chlorine dioxide added to the first stage chlorine dioxide bleaching stage is added all at once at the start and reacted for 1 to 15 minutes to maximize the bleaching action by chlorine dioxide. Thereafter, acid is added without washing to adjust the pH to 2.0 to 3.5, thereby activating chlorite generated at the initial stage of the reaction to decompose hexeneuronic acid. At higher temperatures, hexeneuronic acid is simultaneously hydrolyzed. Thus, by designing the first stage chlorine dioxide bleaching stage, it was possible to achieve both sufficient bleaching action and decomposition of hexeneuronic acid in the first stage chlorine dioxide bleaching stage, and the present invention was completed.
[0014]
The present invention includes the following inventions.
(1) In a method for producing bleached pulp by subjecting unbleached pulp obtained by digesting lignocellulosic material containing hardwood to alkaline oxygen bleaching, and then treating it in a multistage bleaching step having a chlorine dioxide bleaching stage in the first stage, At the start of the chlorine dioxide bleaching stage, all of the chlorine dioxide is added all at once, and after 1 to 15 minutes of residence at 80 to 100 ° C., acid is added without washing to pH 2.0 to 3.5. Furthermore, the manufacturing method of the bleached pulp characterized by performing a chlorine dioxide bleaching stage by processing for 60 to 240 minutes on 80-100 degreeC conditions.
[0015]
(2) In the chlorine dioxide bleaching stage, at the start of the chlorine dioxide bleaching stage, the acid or the reaction solution is adjusted so that the reaction pH immediately before adding the acid is 4.0 to 6.5, preferably pH 4.5 to 5.5. The method for producing bleached pulp according to (1) above, wherein an alkali is added.
[0016]
(3) The final stage of the multi-stage bleaching step has a chlorine dioxide bleaching stage, and the pH at the end of the final stage chlorine dioxide bleaching stage is 4.5 to 6.5. The manufacturing method of the bleached pulp as described in said (1) term or (2) term.
[0017]
(4) The method for producing bleached pulp according to (3) above, wherein the washing waste water after the final stage chlorine dioxide bleaching stage is used as dilution water at the start of the first stage chlorine dioxide bleaching stage.
[0018]
(5) In any of the above (1) to (4), in the multi-stage bleaching step, an alkali extraction stage to which oxygen and hydrogen peroxide have been added is performed following the first chlorine dioxide bleaching stage. The method for producing bleached pulp according to claim 1.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
The lignocellulosic material used in the present invention is preferably 100% hardwood material, but if hardwood material is included, it may contain softwood material or may contain non-wood material. There is no particular limitation except that hardwood is included. As the cooking method for obtaining the pulp used in the present invention, known cooking methods such as kraft cooking, polysulfide cooking, soda cooking, alkali sulfite cooking, etc. can be used, considering pulp quality, energy efficiency, etc. Then, kraft cooking or polysulfide cooking is suitably used.
[0020]
For example, when kraft cooking 100% hardwood wood lignocellulose, the degree of sulfation of the kraft cooking liquor is 5 to 75%, preferably 15 to 45%, the effective alkali addition rate is 5 to 30% by weight per mass of absolutely dry wood, Preferably, 10 to 25% by mass, cooking temperature is 130 to 170 ° C., and the cooking method may be either a continuous cooking method or a batch cooking method. When a continuous cooking kettle is used, a cooking solution is added at multiple points. A cooking method may be used, and the method is not particularly limited.
[0021]
In cooking, as a cooking aid for the cooking liquor used, known cyclic keto compounds, for example, benzoquinone, naphthoquinone, anthraquinone, anthrone, phenanthroquinone, and nuclear substitutes such as alkyl and amino of the quinone compounds, or the quinone Selected from hydroquinone compounds such as anthrahydroquinone, which is a reduced form of the above compounds, and 9,10-diketohydroanthracene compounds which are stable compounds obtained as intermediates in the anthraquinone synthesis method by the Diels-Alder method 1 type or 2 types or more may be added, The addition rate is a normal addition rate, for example, is 0.001-1.0 mass% per the absolute dry mass of a wood chip.
[0022]
In the present invention, the unbleached chemical pulp obtained by a known cooking method is delignified by a known alkaline oxygen bleaching method after washing, roughing and fine selection steps. As the alkaline oxygen bleaching method used in the present invention, a known medium concentration method or high concentration method can be applied as it is, but currently, a medium concentration method in which the pulp concentration is generally used at 8 to 15% by mass. Is preferred.
[0023]
In the alkali oxygen bleaching method by the intermediate concentration method, caustic soda or oxidized kraft white liquor can be used as the alkali, and oxygen gas from the cryogenic separation method, from PSA (Pressure Swing Adsorption) Oxygen, oxygen from VSA (Vacuum Swing Adsorption), etc. can be used. The oxygen gas and alkali are added to a medium-concentration pulp slurry in a medium-concentration mixer and mixed sufficiently, and then sent to a reaction tower capable of holding a mixture of pulp, oxygen, and alkali for a certain period of time under pressure, and delignified. Is done.
[0024]
The oxygen gas addition rate is 0.5 to 3% by mass per bone dry pulp mass, the alkali addition rate is 0.5 to 4% by mass, the reaction temperature is 80 to 120 ° C., the reaction time is 15 to 100 minutes, and the pulp concentration Is 8 to 15% by mass, and other known conditions can be applied. In the present invention, in the alkali oxygen bleaching step, it is a preferred embodiment that the alkali oxygen bleaching is continuously carried out a plurality of times and delignification proceeds as much as possible. The pulp that has been subjected to alkaline oxygen bleaching is then sent to a washing step. After washing, the pulp is sent to a multistage bleaching process.
[0025]
In the multistage bleaching process of the present invention, the first stage is always subjected to chlorine dioxide bleaching. In the first stage chlorine dioxide bleaching stage of the present invention, the chlorine dioxide added to the first stage chlorine dioxide bleaching stage is always added all at once at the start, and the processing temperature is 80 to 100 ° C, preferably 85 to 95 ° C. The residence treatment is performed for 15 minutes, preferably 5 to 10 minutes. Thereafter, an acid for adjusting the pH is added without washing, and the pH is 2.0 to 3.5, preferably pH 2.5 to 3.0, 80 to 100 ° C, preferably 85 to 95 ° C. The first stage chlorine dioxide bleaching stage is performed by treatment for 240 minutes, preferably 90 to 180 minutes.
[0026]
The pH before adding the acid for pH adjustment is 4.0 to 6.5, preferably pH 4.5 to 6.5, more preferably pH 5.0 to 6.0, and if necessary, At the start of the chlorine dioxide bleaching stage, an alkali for adjusting pH or an acid is added. The addition rate of chlorine dioxide is 0.2 to 2.0% by mass as chlorine dioxide per mass of absolutely dry pulp, and is determined by the properties of the pulp after oxygen bleaching and the target whiteness of the bleached pulp. The alkali and acid for pH adjustment are not particularly limited, but caustic soda and sulfuric acid, hydrochloric acid or nitric acid are preferably used from the viewpoint of ease of handling and economical aspect. Although it does not specifically limit regarding a pulp density | concentration, It is 8-15 mass% suitably from the point of operativity.
[0027]
In the present invention, the bleaching stage subsequent to the first stage chlorine dioxide stage is not particularly limited, but a preferred embodiment is that the next stage is an alkali extraction stage and the last stage is a chlorine dioxide bleaching stage. As the conditions when the next stage is an alkali extraction stage, the alkali addition rate is 0.5 to 3% by mass per mass of the dry pulp, the reaction temperature is 60 to 120 ° C., the reaction time is 15 to 120 minutes, the pulp The concentration is 8 to 15% by mass. Preferably, oxygen gas is added to the alkali extraction stage. The addition rate of oxygen gas is 0.1-3 mass% per mass of dry pulp. More preferably, hydrogen peroxide is also added. The addition rate of hydrogen peroxide is 0.05-2 mass% per mass of dry pulp.
[0028]
In the present invention, since the pH at the end of the first stage chlorine dioxide bleaching stage is 3.5 or less, the heavy metal content in the pulp after the first stage chlorine dioxide bleaching stage is significantly reduced, hydrogen peroxide induced by heavy metals, Since the decomposition rate of oxygen-based chemicals such as oxygen and ozone is lowered, the oxygen-based chemicals can be reacted more effectively.
[0029]
In the multi-stage bleaching process of the present invention, the final stage is preferably a chlorine dioxide bleaching stage. The conditions of the final stage chlorine dioxide bleaching stage are preferably pH 4.5 to 6.5, more preferably pH 5.0 to 6.0, reaction temperature 50 to 90 ° C., reaction time 30 to 240 minutes, The pulp concentration is 8 to 15%, and an acid and an alkali are added as necessary to adjust the pH. The alkali and acid for pH adjustment are not particularly limited, but caustic soda and sulfuric acid, hydrochloric acid or nitric acid are preferably used from the viewpoint of ease of handling and economical aspect. The addition rate of chlorine dioxide is 0.05 to 1% by mass with respect to the mass of the absolutely dry pulp, and can be added in divided portions.
[0030]
In the present invention, when the final stage is a chlorine dioxide bleaching stage, the washing waste water after the final stage chlorine dioxide bleaching stage can be used as dilution water at the start of the first stage chlorine dioxide bleaching stage. The advantage of using the washing waste water after the final stage chlorine dioxide bleaching stage as dilution water at the start of the first stage chlorine dioxide bleaching stage is that chlorite contained in the washing waste water of the final stage chlorine dioxide bleaching stage can be used effectively. Can be mentioned. In the present invention, the final chlorine dioxide bleaching is preferably carried out in the range of pH 4.5 to 6.5 so that most of the waste chlorine dioxide remains in the form of chlorite. Chlorite has poor reactivity in the range of pH 4.5 to 6.5, but since the reactivity with lignin and hexeneuronic acid increases when pH falls, washing added to the pulp as dilution water for the first stage chlorine dioxide stage Chlorite in the wastewater reacts with lignin and hexeneuronic acid after sulfuric acid addition and is effectively used.
[0031]
【Example】
EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated more concretely, of course, this invention is not limited by these Examples. Examples 1-3 and Comparative Examples 1-3 shown below are bleached hardwood kraft pulp after factory oxygen bleaching in a D1-E / O-D2 sequence. Example 4 and Comparative Example 4 are D1 -Bleached with E / OP-D2 sequence.
Unless otherwise indicated, the following methods were used to measure the kappa number, the potassium permanganate value (K value), the pulp whiteness, and the pulp fading. In addition, the addition rate of the chemical | medical agent in an Example and a comparative example shows the mass% per absolute dry pulp mass.
[0032]
1. Measurement of pulp kappa number The kappa number was measured according to JIS P8211.
[0033]
2. Measurement of potassium permanganate value (K value) of pulp The potassium permanganate value was measured according to JIS P 8206.
[0034]
3. Measurement of Pulp Whiteness After the bleached pulp was disaggregated, a sheet having a basis weight of 60 g / m 2 was prepared according to Tappi test method T205os-71 (JIS P 8209), and the whiteness of the pulp was measured according to JIS P 8123.
[0035]
4). Evaluation of Pulp Fading After bleaching the bleached pulp, aluminum sulfate was added to adjust the pH to 4.5, and then a sheet having a basis weight of 60 g / m 2 was prepared and dried with a blow dryer. This sheet was faded for 24 hours under the conditions of 80 ° C. and 65% relative humidity, and the PC value was calculated from the whiteness before and after the fade according to the following formula and evaluated.
Example 1
70g of hardwood kraft pulp after factory oxygen bleaching (mixed material; 80% eucalyptus wood, 20% acacia wood, whiteness; 49.5%, kappa number; 12.0) was collected in an absolutely dry mass and put into a plastic bag. Then, after adjusting the pulp concentration to 10% using ion-exchanged water, 0.7% of chlorine dioxide was added per mass of dry pulp, and immersed in a constant temperature water bath at 90 ° C. for 10 minutes. The pH of the pulp slurry at this time was 4.0. Then, without washing, 0.6% sulfuric acid was immediately added per mass of the dry pulp, and again immersed in a constant temperature water bath having a temperature of 90 ° C. for 180 minutes to perform D1 stage bleaching. The pH of the pulp slurry at the end of stage D1 was 3.0. The obtained pulp was diluted to 3% with ion-exchanged water, then dehydrated and washed with a Buchner funnel. The K value of the pulp after the D1 stage was 2.5.
[0037]
Add ion-exchanged water to the pulp after stage D1 and adjust the pulp concentration to 10%, then add 1.0% caustic soda per mass of absolutely dry pulp, put it in a 2 liter stainless steel indirectly heated autoclave, The pressure was increased with a commercially available compressed oxygen gas having a purity of 99.9% so that the gauge pressure was 0.15 MPa, and the mixture was reacted at 70 ° C. for 20 minutes. Thereafter, the pulp slurry was taken out from the autoclave, transferred to a plastic bag, and then immersed in a constant temperature water bath having a temperature of 70 ° C. for 70 minutes to extract the E / O stage. The obtained pulp was diluted to 3% with ion-exchanged water, then dehydrated and washed with a Buchner funnel.
[0038]
Subsequently, the pulp after the E / O stage is put into a plastic bag with an absolute dry mass of 60.0 g, adjusted to a pulp concentration of 10% using ion-exchanged water, and then the chlorine dioxide per absolute dry pulp mass is 0.2. % And caustic soda 0.05% were added and immersed in a constant temperature water bath at a temperature of 70 ° C. for 180 minutes to perform D2 stage bleaching. The pH of the pulp slurry at the end of stage D2 was 5.5. The obtained pulp was diluted to 3% with ion-exchanged water, then dehydrated and washed with a Buchner funnel. The whiteness of the obtained bleached pulp was 85.0%, the K value was 1.0, and the PC value was 1.5.
Table 1 shows the chlorine dioxide addition rate in the D1 stage, the K value of the pulp after the D1 stage, the whiteness of the bleached pulp, the K value, and the PC value.
[0039]
Example 2
In Example D1, Stage 1 and Example 1 except that 0.1% of caustic soda was added per mass of dry pulp at the same time as chlorine dioxide was added, and the addition rate of sulfuric acid was changed to 0.7% per mass of dry pulp. The same operation was performed. The pH immediately before the addition of sulfuric acid in the D1 stage is 5.0, the pH at the end of the D1 stage is 3.0, the K value after the D1 stage is 2.4, the whiteness of the bleached pulp is 86.0%, The K value was 0.9 and the PC value was 1.4.
Table 1 shows the chlorine dioxide addition rate in the D1 stage, the K value of the pulp after the D1 stage, the whiteness of the bleached pulp, the K value, and the PC value.
[0040]
Example 3
The diluted water in the D1 stage of Example 2 was replaced with the D2 stage washing filtrate collected in Example 1, and the same operation as in Example 2 was performed except that the addition rate of caustic soda was changed to 0.15% per absolute dry pulp mass. went. The pH immediately before the addition of sulfuric acid in the D1 stage is 5.2, the pH at the end of the D1 stage is 3.0, the K value of the pulp after the D1 stage is 2.3, and the whiteness of the bleached pulp is 86.8%. The K value was 0.8 and the PC value was 1.3.
Table 1 shows the chlorine dioxide addition rate in the D1 stage, the K value of the pulp after the D1 stage, the whiteness of the bleached pulp, the K value, and the PC value.
[0041]
Example 4
In the E / O stage of Example 2, the same operation as in Example 2 was performed except that 0.2% of hydrogen peroxide was added per mass of dry pulp. The whiteness of the bleached pulp was 88.0%, the K value was 0.8, and the PC value was 1.2. Table 1 shows the chlorine dioxide addition rate in the D1 stage, the K value of the pulp after the D1 stage, the whiteness of the bleached pulp, the K value, and the PC value.
[0042]
Comparative Example 1
The same operation as in Example 1 was performed except that 0.6% of sulfuric acid was added at the same time as the start of stage D1 of Example 1 and the sulfuric acid was not added midway. The pH at the start of D1 stage 10 minutes and at the end of D1 stage is 3.0, K value of pulp after D1 stage is 2.6, whiteness of bleached pulp is 83.8%, K value is 1.1, PC value Was 2.0.
Table 1 shows the chlorine dioxide addition rate in the D1 stage, the K value of the pulp after the D1 stage, the whiteness of the bleached pulp, the K value, and the PC value.
[0043]
Comparative Example 2
The same operation as in Example 1 was performed except that sulfuric acid was not added during the D1 stage of Example 1. The K value of the pulp after the D1 stage was 3.5, the whiteness of the bleached pulp was 84.1%, the K value was 2.0, and the PC value was 4.4.
Table 1 shows the chlorine dioxide addition rate in the D1 stage, the K value of the pulp after the D1 stage, the whiteness of the bleached pulp, the K value, and the PC value.
[0044]
Comparative Example 3
The same operation as in Example 1 was performed except that the reaction temperature in the D1 stage of Example 1 was changed to 70 ° C. The K value of the pulp after stage D1 was 4.1, the whiteness of the bleached pulp was 82.8%, the K value was 2.6, and the PC value was 6.0.
Table 1 shows the chlorine dioxide addition rate in the D1 stage, the K value of the pulp after the D1 stage, the whiteness of the bleached pulp, the K value, and the PC value.
[0045]
Comparative Example 4
The same operation as in Comparative Example 2 was performed except that 0.2% of hydrogen peroxide per mass of dry pulp was added to the E / O stage of Comparative Example 2. The K value of the pulp after the D1 stage was 3.5, the whiteness of the bleached pulp was 84.9%, the K value was 2.0, and the PC value was 4.2. Table 1 shows the chlorine dioxide addition rate in the D1 stage, the K value of the pulp after the D1 stage, the whiteness of the bleached pulp, the K value, and the PC value.
[0046]
[Table 1]
As is clear from comparing Examples 1 and 2 and Comparative Examples 1 to 3 in Table 1, the unbleached pulp obtained by digesting the lignocellulosic material containing hardwood is subjected to alkaline oxygen bleaching, and then to the first stage. When producing bleached pulp by processing in a multistage bleaching process having a chlorine dioxide bleaching stage, the whole amount of chlorine dioxide is added at the start of the first stage chlorine dioxide bleaching stage and stays at 80-100 ° C for 1-15 minutes Then, acid is added without washing to adjust the pH to 2.0 to 3.5, and further, if the chlorine dioxide bleaching stage is performed by treating at 80 to 100 ° C. for 60 to 240 minutes, the whiteness is high. It can be seen that bleached pulp having a low K value and good color fading can be obtained. Further, it can be seen that the effect is further enhanced by adjusting the pH immediately before the addition of the acid to 4.5 to 6.0 in the first stage chlorine dioxide bleaching stage.
[0048]
Further, as apparent from the comparison between Examples 2 and 3 in Table 1, the final chlorine dioxide bleaching stage was performed under the condition that the final pH was 4.5 to 6.5, and the washing filtrate of this stage was used as the first stage dioxide dioxide. It can be seen that the bleaching efficiency is improved when used as a dilution water in the chlorine bleaching stage.
On the other hand, as is clear from comparison between Example 4 and Comparative Example 4, when the first stage chlorine dioxide bleaching stage was performed under the above conditions, the bleaching efficiency when oxygen and hydrogen peroxide were added to the alkali extraction stage It can also be seen that it becomes higher.
[0049]
【The invention's effect】
In a method for producing bleached pulp by subjecting unbleached pulp obtained by digesting lignocellulosic material containing hardwood to alkaline oxygen bleaching and then treating it in a multistage bleaching step having a chlorine dioxide bleaching stage in the first stage, the chlorine dioxide At the start of the bleaching stage, all of the chlorine dioxide is added at once, and after 1 to 15 minutes of residence at 80 to 100 ° C., acid is added without washing to pH 2.0 to 3.5, By performing the chlorine dioxide bleaching stage by treating for 60 to 240 minutes at 80 to 100 ° C., it becomes possible to produce a bleached pulp that can be efficiently bleached and has improved fading.
Claims (5)
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