JP2004339628A

JP2004339628A - Method for producing bleached pulp

Info

Publication number: JP2004339628A
Application number: JP2003135886A
Authority: JP
Inventors: Hitoshi Kagawa; 仁志香川; Takahiro Miura; 高弘三浦; Yasushi Ando; 靖安藤; Kazunori Soma; 一徳相馬
Original assignee: Oji Paper Co Ltd
Current assignee: New Oji Paper Co Ltd
Priority date: 2003-05-14
Filing date: 2003-05-14
Publication date: 2004-12-02

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing bleached pulp having extremely improved pulp viscosity and fading characteristics in a method for producing bleached pulp, with which unbleached pulp obtained by digesting a lignocellulose substance is delignified with an alkali and oxygen and then treated by a multistage bleaching process not using an elemental chlorine. <P>SOLUTION: The method for producing bleached pulp comprises delignifying unbleached pulp obtained by digesting a lignocellulose substance, then treating the pulp with an acid at pH 1.5-4.5 at the early stage of a reaction at 30°C-75°C, then subjecting the pulp to ECF (elemental chlorine-free) bleaching by a multistage bleaching sequence for adding chlorine dioxide in ≥0.7 mass % of chlorine dioxide addition ratio based on bone dry pulp at a first-stage chlorine dioxide bleaching stage (DO stage) so as to make ≤15 mmol hexeneuronic acid amount of completely bleached pulp based on bone dry pulp. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、リグノセルロース物質由来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、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脱リグニンし、その後、低温酸性処理を行い、次いで、塩素、次亜塩素酸塩を用いない（ＥＣＦ）二酸化塩素主体の多段漂白工程で処理してなる退色性、及び、パルプ粘度の低下がなく、漂白工程中にシュウ酸カルシウムのスケーリングのない漂白白パルプの製造方法に関する。
【０００２】
【従来の技術】
リグノセルロース物質を製紙原料として多くの用途に使用するためには、蒸解のような化学作用によってパルプ化した後、或いはリファイナー等を用いて機械的作用によってパルプ化した後、得られるパルプを漂白薬品で漂白して白色度を高める必要がある。例えば、クラフトパルプは包装資材のように強度を必要とする用途に使う場合を除いて、通常、アルカリ酸素脱リグニンした後、塩素、次亜塩素酸塩、二酸化塩素、酸素、オゾン、過酸化水素、苛性ソーダ等の漂白剤及び漂白助剤により漂白処理され、パルプに含まれる着色原因物質であるリグニン等が除去された後に漂白クラフトパルプとして使用されるのが一般的である。
【０００３】
未漂白パルプから漂白パルプを製造する場合は、パルプ繊維自体の強度を或る程度維持することが必要であり、そのため、パルプ繊維を構成するセルロース、ヘミセルロース等の炭水化物の分解を最小限に止めるように、過激な条件での一段漂白を避け、漂白薬品と漂白条件を様々に組み合わせて、穏やかな条件での三〜六段の多段漂白法を採用するのが一般的である。
【０００４】
従来から、多段漂白法においては、パルプを最初に塩素で処理し、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、次にアルカリで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、更に次亜塩素酸塩、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し、白色度の高いパルプを得る方法が採られてきた。
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。
【０００５】
現在、塩素や次亜塩素酸塩の代替として、オゾン、酸素、過酸化水素及び過酢酸、過硫酸等の過酸等の酸素系の漂白薬品が注目されている。しかしながら、過酢酸、過硫酸は、脱リグニンに対する選択性が低くパルプ強度を損なう危険性があること、薬品コストが高いこと、あるいは爆発性を有しており取り扱いが困難であること等の理由から一般に普及するまでには至っていない。したがって、現在のところ、塩素や次亜塩素酸塩の代替としては、既に使用実績のある二酸化塩素、アルカリ過酸化水素を主に用いるのが一般的である。特に、塩素漂白−アルカリ抽出の順序で始まる漂白を二酸化塩素漂白及びアルカリ過酸化水素漂白に置き換える実例が多くなってきている。しかしながら、二酸化塩素やアルカリ過酸化水素は、反応漂白機構が塩素と異なることから、特に広葉樹を原料として、酸性で抄紙した場合には、実際に近い条件下では漂白後のパルプの退色性が極端に劣るという問題点があった。更に二酸化塩素主体の漂白シーケンスで、白色度８７％を越えるような漂白パルプを製造する場合、二酸化塩素の添加率が高いため、漂白パルプ中に残留しているヘキセンウロン酸の量が少なく退色性の問題は殆ど無いのに対し、８７％以下の比較的白色度の低い水準では、退色が問題とならない程度まで、ヘキセンウロン酸を除去するのは困難であった。
【０００６】
また、塩素や次亜塩素酸塩を用いない一般的な漂白シーケンス（例えば、Ｄ−Ｅ−Ｄ−Ｐ：Ｄ＝二酸化塩素段、Ｅ＝アルカリ抽出段、Ｐ＝アルカリ過酸化水素段）で漂白したパルプのパルプ中のヘキセンウロン酸量が、絶乾パルプ１ｋｇ当たり１５ｍｍｏｌより高い場合は、パルプの退色性が著しく劣るという問題点があった。
【０００７】
退色性を改善する方法としては、パルプの酸素漂白漂白前又は後にキシラナーゼ処理することが公知（例えば、特開平２−２６４０８７号公報、特開平２−２９３４８６号公報参照。）であり、キシラナーゼ前処理により退色性を改善する提案（例えば、特開平６−１０１１８５号公報参照。）もあるが、処方するコストの割に退色性の改善効果はそれほど大きくないという問題がある。また、高温で酸前処理を行なう方法（例えば、イギリス特許第１０６２７３４明細書、特表平１０−５０８３４６号公報参照。）では、未漂白パルプを漂白段の前に、酸性下で８０℃以上の温度（例えば、イギリス特許第１０６２７３４明細書参照。）あるいは８５〜１５０℃で処理（例えば、特表平１０−５０８３４６号公報参照。）し、その後、多段で漂白し、パルプの退色性が改善されていることが報告されている。
【０００８】
しかしながら、本発明者らが、これらの条件で処理し漂白してみたところ、８５℃を超える温度では退色性は改善されるものの、強度が大きく低下することが判明した。更に、この高温で酸前処理を行なう方法は、酸処理後の白色度低下が大きいため、カッパー価の低下ほどには晒薬品の低減は少なく、パルプ収率の低下、また、排水ＣＯＤの増加、高温・低ｐＨに耐えうる設備が必要等の理由から、設備コストがかかるなどの問題も有していた。
一方、退色性の評価には乾式加熱法（１０５℃で２４時間加熱）を用いている（例えば、特表平１０−５０８３４６号公報参照。）が、実際に則した退色性をみるためには、退色試験を熱・湿度条件（例えば、８０℃、相対湿度６５％）下で行うことが必須であるにもかかわらず、この評価法は熱処理だけの退色評価であるため、我が国の湿度の高い気候条件を考慮すれば、この評価法で製品の退色性を評価することは困難である。
【０００９】
退色の評価として通常用いられるＰＣ価（ポストカラーナンバー）は、退色処理前後の白色度の差から求められる。ＰＣ価の少ないパルプを用いた写真用材料（例えば、特開昭５６−５４４３６号公報参照。）、酸素漂白を含むシーケンスで漂白したパルプを用いている退色に優れた写真用材料（例えば、特開昭６３−３０３１９１号公報参照。）についての報告例があるが、何れも塩素をベースとした漂白によって製造されているパルプを使用している例であり、また、写真用材料という特性から、白色度は９０％以上と高くされていることから、当然ながら漂白パルプの退色性は優れている。しかし、本発明のように、７８％〜８７％という低白色度で、塩素及び次亜塩素酸塩を用いない漂白法によって、パルプ粘度及び強度低下がなく、退色性に優れたパルプを製造しているものではない。
【００１０】
一方、酸性領域で過酸化水素処理を行なう方法が、数件提案されている（例えば、特許文献１、特許文献２、非特許文献１参照。）が、有機又は無機錯化物併用における過酸化水素の漂白の効果を示しただけであり、パルプシートの退色性との関係に関する記載はない。また、未晒パルプを酸性下で有機又は無機錯化物併用下で過酸化水素処理する方法（特許文献１）にしても、酸素晒後にさらに同様な処理を行うことを示唆する記述は見当らない。さらに酸性領域で過酸化水素を用いた場合、パルプ粘度が低下するといった改善すべき点があった。（例えば、本発明者らの先願である特願２００１−２２７２７４号。）。
【００１１】
一方、塩素酸塩、硫酸、メタノールを原料とする二酸化塩素製造設備から排出され、工場廃液の組成及び量は、一般的に、セスキ芒硝が二酸化塩素製造量１トンに対し１．２〜１．４トン、塩素酸ナトリウムが二酸化塩素製造量１トンに対し０．５〜５ｋｇ、塩化ナトリウムが二酸化塩素製造量１トンに対し０．１〜０．５ｋｇの割合となっており、凝集沈殿処理槽、漂白工程等に利用されたる以外は廃棄処理されていた。そのため、通常、元素状塩素を使用しないＥＣＦ漂白法では、例えば、Ｄ−Ｅ／Ｏ−Ｐ−Ｄのような通常のＥＣＦシーケンスで一日当たり、生産量１０００トン製造している場合、二酸化塩素の使用量は、５〜１５トン必要であり、それに付随して発生する廃液中のセスキ芒硝量は一日当たり６〜２１トン排出されることとなり、二酸化塩素設備からの排出量はかなりの量になる。塩素酸塩をメタノール還元して二酸化塩素を製造する方法は、日本カーリット社のＲ−８法や保土谷エンジニアリング社のＳＶＰ−ＬＩＴＥ法がある。
【００１２】
二酸化塩素製造設備からの廃液処理方法としては、アルミニウム又はアルミニウム化合物を添加し、硫酸アルミニウムを製造する方法（例えば、特開昭５１−９６７９６号公報参照。）、蒸解薬液に利用する方法（例えば、特開昭５２―１０７３０２号公報参照。）、バイポーラ膜と陽イオン交換膜とより構成した二室式電気透析装置にて硫酸塩を硫酸とアルカリに分解し、硫酸を二酸化塩素製造設備で再利用する方法（例えば、特開平５−５８６０１号公報参照。）等があるが、何れも製造コストや設備コストが嵩み、実際の殆どのパルプ製造工場では、工場廃液の凝集沈殿処理槽のｐＨ調整、漂白工程のｐＨ調整等に利用する以外は廃棄するしかないのが現状である。
【００１３】
【特許文献１】
特公昭６３―２０９５３号公報
【特許文献２】
Ｗ０７９／００６３７
【非特許文献１】
１９８５，ＷｏｏｄａｎｄＰｕｌｐｉｎｇＣｈｅｍｉｓｔｒｙＳｙｍｐｏｓｉｕｍ，ＨａｎｓＵｌｒｉｃｈＳｕｓｓ等
【００１４】
【発明が解決しようとする課題】
本発明者等は、元素状塩素を使用せずに漂白処理した漂白パルプを酸性抄紙したたときに起きる退色、パルプ粘度の低下、漂白工程中に発生する蓚酸カルシウムのいずれにも効果がある漂白方法を見出した。
【００１５】
【課題を解決するための手段】
上記目的を達成することができる本発明は、以下の発明を包含する。
（１）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た後、反応初期ｐＨ１．５〜４．５、温度３０℃〜７５℃で酸処理を行い、その後の第一段目の二酸化塩素漂白段での二酸化塩素添加率を対絶乾パルプ当り、０．７質量％以上添加する多段漂白シーケンスでＥＣＦ漂白し、漂白パルプ中のヘキセンウロン酸量を絶乾パルプ１ｋｇ当たり１５ｍｍｏｌ以下にする漂白パルプの製造方法。
【００１６】
（２）前記低温酸処理においてセスキ芒硝を対絶乾パルプ当たり０．０１質量％〜１０質量％添加する（１）記載の漂白パルプの製造方法。
【００１７】
（３）前記セスキ芒硝が二酸化塩素製造設備から廃液として排出されるセスキ芒硝含有廃液である（１）または（２）のいずれか１項に記載の漂白パルプの製造方法。
【００１８】
（４）前記漂白パルプの白色度が７８％〜８７％である（１）〜（３）のいずれか１項に記載の漂白パルプの製造方法。
【００１９】
（５）前記漂白パルプが広葉樹パルプであることを特徴とする（１）〜（４）のいずれか１項に記載の漂白パルプの製造方法。
【００２０】
（６）前記（１）〜（５）のいずれか１項に記載の方法で製造された漂白パルプを主成分とする酸性紙。
【００２１】
【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、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、クラフト蒸解法、又はポリサルファイド蒸解法が好適に用いられる。例えば、木材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重量当たり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、連続蒸解法ある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。
【００２２】
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、その添加率は木材チップの絶乾重量当たり０．００１〜１．０質量％である。
【００２３】
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脱リグニン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脱リグニン法は、公知の中濃度法ある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。
【００２４】
前記中濃度法によるアルカリ酸素脱リグニン法において、アルカリとしては苛性ソーダある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（ＰｒｅｓｓｕｒｅＳｗｉｎｇＡｄｓｏｒｐｔｉｏｎ）からの酸素、ＶＳＡ（ＶａｃｕｕｍＳｗｉｎｇＡｄｓｏｒｐｔｉｏｎ）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。
【００２５】
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、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、アルカリ酸素脱リグニン工程において、上記アルカリ酸素脱リグニン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。アルカリ酸素脱リグニン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、酸処理工程へ送られる。
【００２６】
本発明における酸処理の反応温度は３０〜７５℃が好ましい。温度が３０℃未満の場合に退色性は十分に改善されず、一方、７５℃を超える温度では、退色性は改善されるものの、使用する蒸気コストが著しく増加する。また、８５℃を越えるような温度では、漂白パルプの粘度及び強度が著しく低下する。反応初期ｐＨは１．５〜４．５がよい。反応初期ｐＨが１．５未満であると、パルプ強度への悪影響が大きく、また上記の温度下で１．５未満のｐＨでは耐久性のあるライニングを見出すことが難しく、あっても非常に高価なものになり、実際的ではない。ｐＨが４．５より大きいと、ヘキセンウロン酸の除去効果が少なくなると同時に工程内のシュウ酸カルシウムのスケーリングが激しくなる。また、酸処理は、反応初期ｐＨと反応終了ｐＨは、略同等となるのが特徴的である。
【００２７】
また、酸処理のリテンションは、その効果とパルプ繊維へのダメージを考えると３０〜３００分程度がよく、処理濃度は、一般的な工程内濃度であれば、制限はないが、８〜１５％の中濃度法、又は２５〜４０％の高濃度法が好ましい。本発明で酸性領域を維持するために用いられる酸は、無機酸、有機酸のいずれを併用してもよい。酸処理時のｐＨは、１．５〜４．５であり、具体的には、硫酸、硝酸、塩酸、亜硫酸、亜硝酸あるいは二酸化塩素発生設備から排出されるセスキ芒硝等の無機酸が使用できる。セスキ芒硝以外の酸には、硫酸が入手と取り扱いが容易であるため好適に用いられる。その他、酸処理については一般的な処方が用いられる。一方、芒硝あるいは、セスキ芒硝の添加率は、絶乾パルプ当り、０．０１〜１０質量％がよく、好ましくは、０．１〜５質量％がよい。
【００２８】
更に酸処理においては、キレート剤を添加してもよい。キレート剤の種類は、Ｆｅ^２＋、Ｃｕ^２＋、Ｍｎ^２＋等の金属イオンを封鎖できるものであればＥＤＴＡ（エチレンジアミンテトラ酢酸）、ＤＴＰＡ（ジエチレントリアミンペンタ酢酸）、ＤＴＰＭＰ（ジエチレントリアミンペンタメチレンホスホン酸）、ＰＨＡＳ（ポリ−α−ヒドレキシアクリル酸塩）等何でもよく、各種キレート剤を混合使用してもよい。また、キレート剤の添加率は、一般的に絶乾パルプに対し０．００１質量％〜５質量％の範囲で添加される。
【００２９】
本発明の酸処理においては、酸素含有ガスあるいは窒素含有ガスを用いて加圧することもできる。処理時に加圧のために用いられる酸素含有ガスとしては、深冷分離法からの酸素、ＰＳＡからの酸素、ＶＳＡからの酸素等のように工業規模での利用が可能で、現在、アルカリ酸素脱リグニンに使用されている酸素純度が８５容量％以上の酸素或いは酸素含有ガス、前記モレキュラーシーブを用いた酸素製造設備を用いて酸素の含有量を２１容量％を超えて調整された酸素含有ガス、前記酸素純度が８５容量％以上の酸素含有ガスと空気を混合して製造される酸素富化ガス、酸素含有量が２０容量％以上の空気等を挙げることができ、これらの中から適宜選択して用いることができる。過酸化水素処理時の酸素含有ガス、あるいは窒素含有ガスによる酸処理時の加圧圧力は０．０５〜０．９ＭＰａ（ゲージ圧力）であり、好ましくは０．１５〜０．７ＭＰａである。
【００３０】
また、多段漂白処理工程においてオゾン漂白段を有する場合には、酸素を含有するその排ガスも好適に使用することができる。本発明の過酸化水素処理段に使用される窒素含有ガスとしては、窒素ガス含有率が９５％以上のガスであればいかなるガスでもよいが、経済的見地から、アルカリ酸素脱リグニンに使用される深冷分離法からの酸素、ＰＳＡからの酸素、ＶＳＡからの酸素等の酸素ガスを製造する際に副生する窒素含有ガスが好適に用いられる。
【００３１】
本発明においては、酸処理工程後に、酵素処理工程を設けることも可能である。前記酵素処理工程で使用される酵素は、パルプと反応させることにより、ＪＩＳＰ８２０６で測定されるパルプの過マンガン酸カリウム価が低下するものであれば、いかなる酵素でもよい。例えば、キシラナーゼ、リグニンパーオキシダーゼ、マンガンパーオキシダーゼ、ラッカーゼ等が知られいるが、勿論これらの酵素でもよく、未だ知られていない酵素でも該当する酵素であればよいことは言うまでもない。また、これらの酵素は単独で用いてもよく、あるいは複合、混合して、さらには複数回に分けて使用することもできる。これらの酵素のうち、キシラナーゼと呼ばれるキシラン分解酵素は、漂白促進効果も同時に有しており、好適に用いられる。
【００３２】
本発明の多段漂白処理工程では、初段は二酸化塩素漂白段（Ｄ０段）を用いる。また、二段目にはアルカリ抽出段（Ｅ）が用いられ、三段目以降には、二酸化塩素、アルカリ過酸化水素等の組み合わせが好適に用いられる。本発明の初段の二酸化塩素漂白段に用いられる二酸化塩素は、当業者にとって公知の多くの二酸化塩素発生法より得られる二酸化塩素から選ぶことができるが、好適には、塩素を副生しない発生法から得られる二酸化塩素が用いられる。本発明の初段の二酸化塩素段での添加率は、対絶乾パルプ当り、０．７質量％以上であれば退色性への問題は無くなるが、添加率が高い場合、漂白コストが嵩むため、０．７％〜３．０％程度が好適である。二酸化塩素段のｐＨは２〜６、好ましくは２．５〜４であり、ｐＨを調整するために任意の酸又はアルカリを補助的に添加することも可能である。また、二酸化塩素処理時間、パルプ濃度等のその他の二酸化塩素漂白条件は、全て公知の条件を採用することができる。
【００３３】
本発明の二酸化塩素漂白段に続くアルカリ抽出段では、当業者にとって公知の多くのアルカリ化合物を使用することができるが、苛性ソーダが最も使用しやすく、好適に使用される。本発明のアルカリ抽出段では、酸素及び／又は過酸化水素を併用することもできる。その他、本発明のアルカリ抽出段は、公知の条件で行うことができる。
【００３４】
本発明の多段漂白工程で用いられる二酸化塩素漂白段、アルカリ抽出段に続く三段目以降の漂白段では、塩素及び次亜塩素酸塩以外の漂白薬品であれば如何なる漂白薬品を用いてもよいが、二酸化塩素、アルカリ過酸化水素、オゾン、過酸等の一般的な漂白薬品が好適に用いられる。三段目以降の段数も特に限定されるわけではないが、エネルギー効率、生産性等を考慮すると、合計で三段あるいは四段で終了するのが好適である。
【００３５】
本発明に用いられる薬品としては、塩素及び次亜塩素酸塩を除く、酸（Ａ）、二酸化塩素（Ｄ）、アルカリ（Ｅ）、酸素（Ｏ）、アルカリ過酸化水素（Ｐ）、オゾン（Ｚ）、酵素（Ｅｚ）、有機過酸等の公知の漂白剤と漂白助剤を挙げることができる。漂白シーケンスとしては、酸素脱リグニン後に、例えば酸処理段（Ａ）から始まるシーケンスとしては、Ａ−Ｄ−Ｅ／Ｏ−Ｄ、Ａ−Ｄ−Ｅ／Ｏ−Ｐ−Ｄ、Ａ−Ｄ−Ｅ／Ｏ−Ｄ−Ｄ、Ａ−Ｄ−Ｅ／Ｏ−Ｄ−Ｐ、Ａ−Ｄ−Ｅ／ＯＰ−Ｄ、Ａ−Ｄ−Ｅ／Ｏ−Ｚ−Ｄ等、及び酵素を含むＡ−Ｅｚ−Ｄ−Ｅ／Ｏ−Ｄ、Ａ−Ｅｚ−Ｄ−Ｅ／Ｏ−Ｐ−Ｄ、Ａ−Ｅｚ−Ｄ−Ｅ／Ｏ−Ｄ−Ｄ、Ａ−Ｅｚ−Ｄ−Ｅ／Ｏ−Ｄ−Ｐ、Ａ−Ｅｚ−Ｄ−Ｅ／ＯＰ−Ｄ、Ａ−Ｅｚ−Ｄ−Ｅ／Ｏ−Ｚ−Ｄ等も挙げることができる。
【００３６】
本発明において、未漂白パルプをアルカリ酸素漂白し、その後、反応初期ｐＨ１．５〜４．５、温度３０℃〜７５℃で酸処理を行って、その後、第１段目の二酸化塩素漂白段（Ｄ０段）での二酸化塩素添加率を対絶乾パルプ当り、０．７質量％以上添加する多段漂白シーケンスでＥＣＦ漂白し、漂白パルプ中のヘキセンウロン酸量が絶乾パルプ１ｋｇ当たり１５ｍｍｏｌ以下にすれば、退色性が改善される理由については今後の研究を待たなければならないが、ヘキセンウロン酸が酸性紙の完成パルプ中に多く残留していると、湿度が高く、かつ温度も高い条件下では、これが色素団に変化し、白色度が低下すると考えている。したがって、針葉樹に比べ、ヘキセンウロン酸の含有量の多い広葉樹の方が退色しやすい。
【００３７】
一方、本発明により、漂白工程のスケーリングが防止できる理由は、低温の酸処理により漂白工程外へカルシウムイオンを排出できるようになるためである。一般的に漂白工程でのシュウ酸カルシウムスケーリングは、シュウ酸アニオンを多く含むアルカリ性（あるいは酸性）工程液にカルシウムイオンを多く含む酸性（あるいはアルカリ性）工程液が混合される場所で、ｐＨが、４〜９の範囲で変化する場所に発生するとされている。また、カルシウムイオンのモル濃度とシュウ酸イオンのモル濃度の積がシュウ酸カルシウムの溶解度積４×１０^−９をこえるとシュウ酸カルシウムスケールが発生することからも低温の酸処理によるカルシウム除去は、スケーリング防止効果が高い。
【００３８】
本発明の漂白パルプを用いて、酸性紙を調製する方法は、漂白パルプをビーターでＣＳＦ（カナダ標準ろ水度、カナディアンスタンダードフリーネス）３５０ｍｌ〜５５０ｍｌ程度に叩解し、その後、硫酸バンド約２．５％、ロジンサイズ剤（例えば、サイズパインＥ、荒川化学工業製）約０．５％、タルク（例えば、イライト、日本タルク社製）約２０％、歩留向上剤（例えば、パーコール１８２、協和産業製）約０．０２％の順に配合し、常法にて坪量６４ｇ／ｍ^２程度の酸性紙を抄造する方法がある。酸性紙の抄紙に際しては，本発明の漂白パルプによる酸性紙が有する優れた特性を損なわない範囲で他の漂白パルプを混合使用することはもちろん可能である。
【００３９】
また、本発明の漂白パルプを用いて、中性紙を調製する方法は、漂白パルプをビーターでＣＳＦ３５０ｍｌ〜５５０ｍｌ程度に叩解し、その後、カチオン化澱粉（例えば、エースＫ１００、王子コーンスターチ製）約０．５％、硫酸バンド約０．５％、ＡＫＤ（例えば、ＳＰＫ９０２、荒川化学工業製）約０．０５％、軽質炭酸カルシウム（例えば、ＴＰ１２１、奥多摩工業製）約２０％、歩留向上剤（例えば、パーコール１８２、協和産業製）約０．０２％の順に配合し、常法にて坪量６４ｇ／ｍ^２程度の中性紙を抄造する方法がある。
本発明の退色性改善効果は、酸性紙の場合に大きいが、本発明で処理したパルプを中性紙に用いて何ら問題はない。
【００４０】
【実施例】
以下に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。また、特に示さない限り、カッパー価の測定、パルプ中のヘキセンウロン酸量の測定、パルプ白色度の測定、パルプの退色性の評価、パルプ粘度の測定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。
【００４１】
１．パルプのカッパー価の測定
カッパー価の測定は、ＪＩＳＰ８２１１に準じて行った。
【００４２】
２．パルプ中のヘキセンウロン酸量の定量
５００ｍｌのＳＵＳ製容器に十分にイオン交換水で洗浄したパルプを絶乾パルプ５ｇ量り取って入れ、蟻酸−蟻酸ナトリウムバッファー１０ｍｍｏｌ／ｌ溶液を用いてトータル３００ｍｌとした。その後、ＳＵＳ製容器内を窒素ガスで置換し、油恒温槽内で、１１０℃、５時間処理した。ＳＵＳ容器を流水冷却後、処理後のパルプ懸濁液を洗浄液を含めて５００ｍｌにメスアップした後、ろ過して、液をＨＰＬＣ（高速液体クロマトグラフィー）にて分析し、２−ｆｕｒｏｉｃａｃｉｄと５−ｃａｒｂｏｘｙ−２−ｆｕｒａｌｄｅｈｙｄｅを定量した。定量に際し、算出式、参考文献は、以下のものを使用した。
【００４３】
測定サンプル中の２−ｆｕｒｏｉｃａｃｉｄの定量値をａ（ｎｇ／μｌ）、５−ｃａｒｂｏｘｙ−２−ｆｕｒａｌｄｅｈｙｄｅの定量値をｂとした。
１）２−ｆｕｒｏｉｃａｃｉｄ量（ｍｍｏｌ／ｋｇ）＝ａ×（５００／１０００）／（１０×１０^−３）／１１２．０８
２）５−ｃａｒｂｏｘｙ−２−ｆｕｒａｌｄｅｈｙｄｅ量（ｍｍｏｌ／ｋｇ）＝ｂ×（５００／１０００）／（１０×１０^−３）／１４０．１
３）ヘキセンウロン酸量（ｍｍｏｌ／ｌ）＝２−ｆｕｒｏｉｃａｃｉｄ量＋５−ｃａｒｂｏｘｙ−２−ｆｕｒａｌｄｅｈｙｄｅ量
【００４４】
参考文献：著者Ｖｕｏｒｉｎｅｎ，Ｔ．
ＳｅｌｅｃｔｉｖｅｈｙｄｒｏｌｙｓｉｓｏｆｈｅｘｅｎｕｒｏｎｉｃａｃｉｄｇｒｏｕｐｓａｎｄｉｔｓａｐｐｌｉｃａｔｉｏｎｉｎＥＣＦａｎｄＴＣＦｂｌｅａｃｈｉｎｇｏｆｋｒａｆｔｐｕｌｐｓ
ＩｎｔｅｒｎａｔｉｏｎａｌＰｕｌｐＢｌｅａｃｈｉｎｇＣｏｎｆｅｒｅｎｃｅ，Ａｐｒｉｌ１４−１８，１９９６，Ｐ４３−５１
【００４５】
３．漂白パルプの白色度の測定
漂白パルプを離解後、パルプスラリーに硫酸バンドを対パルプ３．０％加え、Ｔａｐｐｉ試験法Ｔ２０５ｏｓ−７１（ＪＩＳＰ８２０９）に従って坪量６０ｇ／ｍ^２のシートを作製した。その後、ＪＩＳＰ８１２３に従ってパルプの白色度を測定した。
【００４６】
４．パルプの退色性評価
白色度測定用パルプシートを８０℃、相対湿度６５％の条件下で、４８時間の退色させ、退色前後のパルプ白色度から下式に従いＰＣ価を算出し、評価した。
ＰＣ価＝１００×［｛（１−退色後白色度）^２／（２×退色後白色度）｝−｛（１−退色前白色度）^２／（２×退色前白色度）｝］
【００４７】
５．漂白パルプの粘度の測定
パルプ粘度の測定は、Ｊ．ＴＡＰＰＩ４４に準じて行った。
【００４８】
６．Ａ段後パルプとＤ０段後パルプ搾液中の全カルシウム濃度及びシュウ酸イオン濃度の測定
Ａ処理及びＤ０処理後のパルプを２００ＭＥＳＨワイヤ−上で１０％→１２％まで絞り、搾液中の全カルシウム濃度は、ＪＩＳ−Ｋ０１０２に従って、ＩＣＰ発光分光分析法にて測定した。また、シュウ酸イオン濃度は、一般的なイオンクロマトグラフ法（カラムＩＯＮＰＡＣＡＳ１４）にて測定した。
【００４９】
実施例１
工場製広葉樹の蒸解−アルカリ酸素脱リグニン後のクラフトパルプ（白色度５０．５％、カッパー価１０．０）の絶乾質量８０．０ｇを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セスキ芒硝を２．０％、硫酸を０．３３％添加し、温度が６０℃の恒温槽に１８０分間浸漬して、酸処理を行った（以下、Ａ段と略す）。Ａ段の反応初期ｐＨは３．０であった。得られたパルプを２００ＭＥＳＨワイヤーで受け、約１２％まで脱水後、イオン交換水で３％に希釈した後、ブフナーロートを用いて脱水・洗浄し、Ａ段後パルプを得た。次いで、Ａ後のパルプを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、絶乾パルプ質量当たり二酸化塩素を０．７５％添加し、温度が７０℃の恒温水槽に６０分間浸漬して初段の二酸化塩素段（以下、Ｄ０段と略す）の漂白を行った。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。
【００５０】
Ｄ０段後のパルプを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、苛性ソーダを絶乾パルプ質量当たり１．２％加え、Ｄ０段と同様にして温度７０℃で１６５分間処理し、アルカリ抽出段（以下、Ｅ段と略す）を行った。得られたパルプをイオン交換水で希釈してパルプ濃度を３％に調整した後、ブフナーロートを用いて脱水・洗浄し、Ｅ段後パルプを得た。
続いて、Ｅ段後パルプを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０．２５％添加し、Ｄ０段と同様にして温度７０℃で３００分間処理し、二段目の二酸化塩素漂白段（以下Ｄ１段と略す）の漂白を行った。得られたパルプをイオン交換水で３％に希釈し、ブフナーロートを用いて洗浄、脱水し、白色度が８４．２％の漂白パルプを得た。得られた漂白パルプのヘキセンウロン酸量、４８時間後の漂白パルプシートのＰＣ価、漂白パルプの粘度、Ａ段後及びＤ０段後のパルプ搾液中の全カルシウム濃度及びシュウ酸イオン濃度を測定し、表１に示した。
【００５１】
実施例２
アルカリ酸素脱リグニン後、クラフトパルプのＤ０段の二酸化塩素添加率を０．９０％、Ｄ１段の二酸化塩素添加率を０．１５％とした以外は実施例１と同様の操作を行った。Ａ段の反応初期ｐＨは、３．０であり、多段漂白後のパルプ白色度は８４．８％であった。得られた漂白パルプのヘキセンウロン酸量、４８時間後の漂白パルプシートのＰＣ価、漂白パルプの粘度を測定し、表１に示した。
【００５２】
実施例３
アルカリ酸素脱リグニン後、クラフトパルプのＡ段の硫酸添加率を０．５３％とした以外は実施例１と同様の操作を行った。Ａ段の反応初期ｐＨは、２．１であり、多段漂白後のパルプ白色度は８４．１％であった。得られた漂白パルプのヘキセンウロン酸量、４８時間後の漂白パルプシートのＰＣ価、漂白パルプの粘度、Ａ段後及びＤ０段後のパルプ搾液中の全カルシウム濃度及びシュウ酸イオン濃度を測定し、表１に示した。
【００５３】
実施例４
アルカリ酸素脱リグニン後、クラフトパルプのＡ段の硫酸添加率を０．２３％とした以外は実施例１と同様の操作を行った。Ａ段の反応初期ｐＨは、３．９であり、多段漂白後のパルプ白色度は８４．１％であった。得られた漂白パルプのヘキセンウロン酸量、４８時間後の漂白パルプシートのＰＣ価、漂白パルプの粘度、Ａ段後及びＤ０段後のパルプ搾液中の全カルシウム濃度及びシュウ酸イオン濃度を測定し、表１に示した。
【００５４】
実施例５
アルカリ酸素脱リグニン後、クラフトパルプのＡ段の処理温度を４０℃とした以外は実施例１と同様の操作を行った。Ａ段の反応初期ｐＨは、３．０であり、多段漂白後のパルプ白色度は８３．９％であった。得られた漂白パルプのヘキセンウロン酸量、４８時間後の漂白パルプシートのＰＣ価、漂白パルプの粘度、Ａ段後及びＤ０段後のパルプ搾液中の全カルシウム濃度及びシュウ酸イオン濃度を測定し、表１に示した。
【００５５】
実施例６
アルカリ酸素脱リグニン後、クラフトパルプのＡ段の処理温度を７５℃とした以外は実施例１と同様の操作を行った。Ａ段の反応初期ｐＨは、３．０であり、多段漂白後のパルプ白色度は８４．１％であった。得られた漂白パルプのヘキセンウロン酸量、４８時間後の漂白パルプシートのＰＣ価、漂白パルプの粘度を測定し、表１に示した。
【００５６】
実施例７
アルカリ酸素脱リグニン後、クラフトパルプのＤ１段の二酸化塩素添加率を０．１０％とした以外は実施例１と同様の操作を行った。Ａ段の反応初期ｐＨは、３．０であり、多段漂白後のパルプ白色度は７８．８％であった。得られた漂白パルプのヘキセンウロン酸量、４８時間後の漂白パルプシートのＰＣ価、漂白パルプの粘度を測定し、表１に示した。
【００５７】
比較例１
アルカリ酸素脱リグニン後、クラフトパルプのＤ０段の二酸化塩素添加率を０．６５％、Ｄ１段の二酸化塩素添加率を０．３０％とした以外は実施例１と同様の操作を行った。Ａ段の反応初期ｐＨは、３．０であり、多段漂白後のパルプ白色度は８３．０％であった。得られた漂白パルプのヘキセンウロン酸量、４８時間後の漂白パルプシートのＰＣ価、漂白パルプの粘度を測定し、表１に示した。
【００５８】
比較例２
アルカリ酸素脱リグニン後、クラフトパルプのＡ段の硫酸添加率を０．１０％とした以外は実施例１と同様の操作を行った。Ａ段の反応初期ｐＨは、５．０であり、多段漂白後のパルプ白色度は８３．８％であった。得られた漂白パルプのヘキセンウロン酸量、４８時間後の漂白パルプシートのＰＣ価、漂白パルプの粘度を測定し、表１に示した。
【００５９】
比較例３
アルカリ酸素脱リグニン後、クラフトパルプのＡ段の反応温度を８５℃とした以外は実施例１と同様の操作を行った。Ａ段の反応初期ｐＨは、３．０であり、多段漂白後のパルプ白色度は８４．３％であった。得られた漂白パルプのヘキセンウロン酸量、４８時間後の漂白パルプシートのＰＣ価、漂白パルプの粘度を測定し、表１に示した。
【００６０】
比較例４
アルカリ酸素脱リグニン後、クラフトパルプのＡ段処理を行なわない以外は実施例１と同様の操作を行った。多段漂白後のパルプ白色度は８３．５％であった。得られた漂白パルプのヘキセンウロン酸量、４８時間後の漂白パルプシートのＰＣ価、漂白パルプの粘度、Ａ段後及びＤ０段後のパルプ搾液中の全カルシウム濃度及びシュウ酸イオン濃度を測定し、表１に示した。
【００６１】
【表１】

【００６２】
表１の実施例１〜７と比較例１を比較すると明らかなように、Ｄ０添加率が０．７％より低い場合、漂白パルプ中のヘキセンウロン酸量が多く、４８時間後のＰＣ価が高い事から、退色性に劣る。また、１〜７と比較例２を比較すると明らかなようにＡ段の処理ｐＨが４．５より高いと漂白パルプ中のヘキセンウロン酸量が多く、４８時間後のＰＣ価が高い事から、退色性に劣る。更に、実施例１〜７と比較例３を比較すると明らかなように、７５℃を越える高温で酸処理を行なうと退色性には優れるものの、漂白パルプの粘度が低下する。一方、酸処理を全く行なわない場合、やはりヘキセンウロン酸量が多くなり、ＰＣ価が高く、退色性に劣ったパルプとなる。
【００６３】
【発明の効果】
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脱リグニンした後、反応初期ｐＨ１．５〜４．５、温度３０℃〜７５℃で酸処理を行って、その後第一段目の二酸化塩素漂白段（Ｄ０段）での二酸化塩素添加率を対絶乾パルプ当り、０．７質量％以上添加する多段漂白シーケンスでＥＣＦ漂白し、漂白パルプ中のヘキセンウロン酸量が絶乾パルプ１ｋｇ当たり１５ｍｍｏｌ以下とすることで、前記漂白パルプ作成したシートは８０℃、相対湿度６５％の恒温度かつ恒湿度条件で４８時間処理したＰＣ価は、１０．０以下となった。本発明により漂白パルプの退色性を著しく改善すると共にパルプ粘度を大幅に向上させることが可能となった。また、漂白工程内のシュウ酸カルシウムスケールを防止するが可能となった。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing bleached pulp derived from lignocellulosic materials. More specifically, the present invention provides that the unbleached pulp obtained by digesting the lignocellulosic material is subjected to alkaline oxygen delignification, followed by a low-temperature acid treatment, and then without the use of chlorine or hypochlorite (ECF The present invention relates to a method for producing bleached white pulp which does not decrease in pulp viscosity and does not undergo scaling of calcium oxalate during the bleaching process, which is performed in a multi-stage bleaching process mainly comprising chlorine dioxide.
[0002]
[Prior art]
In order to use the lignocellulosic material as a raw material for papermaking in many applications, the pulp obtained after pulping by a chemical action such as digestion or pulping by a mechanical action using a refiner or the like is used as a bleaching chemical. It is necessary to bleach to increase the whiteness. For example, kraft pulp is usually subjected to alkali oxygen delignification, then chlorine, hypochlorite, chlorine dioxide, oxygen, ozone, hydrogen peroxide, unless used for applications that require strength, such as packaging materials. It is generally used as bleached kraft pulp after bleaching treatment with a bleaching agent such as caustic soda and a bleaching aid to remove lignin and the like, which are coloring agents contained in the pulp.
[0003]
In the case of producing bleached pulp from unbleached pulp, it is necessary to maintain the strength of the pulp fiber itself to a certain degree, so that decomposition of carbohydrates such as cellulose and hemicellulose constituting the pulp fiber is minimized. In general, one-stage bleaching under extreme conditions is avoided, a bleaching chemical and bleaching conditions are variously combined, and a three- to six-stage multistage bleaching method under mild conditions is generally employed.
[0004]
Conventionally, in the multi-stage bleaching method, pulp is first treated with chlorine, lignin contained in the pulp is chlorinated, after adding solubility to lignin, then chlorinated lignin is dissolved and extracted with alkali, A method has been adopted in which lignin is separated and removed from the pulp, and a small amount of the remaining lignin is decomposed and removed by using hypochlorite, chlorine dioxide and the like to obtain pulp having high whiteness.
However, in recent years, there has been a concern about the effect of an organic chlorine compound (hereinafter abbreviated as AOX) contained in the bleaching wastewater from the pulp chlorination stage on the environment, and movements not using chlorine for pulp bleaching have been increasing. Also, when hypochlorite is used, chloroform is generated during pulp bleaching, which may have an adverse effect on the environment.Therefore, a bleaching sequence that does not use hypochlorite for pulp bleaching is required. Is coming.
[0005]
At present, as a substitute for chlorine and hypochlorite, oxygen-based bleaching chemicals such as ozone, oxygen, hydrogen peroxide, and peracids such as peracetic acid and persulfuric acid are attracting attention. However, peracetic acid and persulfuric acid have low selectivity for delignification, which may impair pulp strength, high chemical costs, or have explosive properties and are difficult to handle. It has not yet become widespread. Therefore, at present, as a substitute for chlorine and hypochlorite, chlorine dioxide and alkali hydrogen peroxide, which have already been used, are mainly used. In particular, there is an increasing number of instances in which bleaching, which begins with a chlorine bleach-alkali extraction sequence, is replaced by chlorine dioxide bleach and alkaline hydrogen peroxide bleach. However, chlorine dioxide and alkaline hydrogen peroxide have a different reaction bleaching mechanism than chlorine, so the bleaching of pulp after bleaching under extreme conditions is extremely high, especially when hardwood is used as the raw material and acidified. There was a problem that it was inferior. Furthermore, when a bleaching pulp with a brightness of more than 87% is produced in a bleaching sequence mainly composed of chlorine dioxide, the amount of hexeneuronic acid remaining in the bleached pulp is small due to a high chlorine dioxide addition rate, and the bleaching property is low. While there was almost no problem, it was difficult to remove hexenuronic acid to a level where discoloration was not a problem at a relatively low whiteness level of 87% or less.
[0006]
In addition, bleaching is performed using a general bleaching sequence that does not use chlorine or hypochlorite (for example, DEPD: D = chlorine dioxide stage, E = alkali extraction stage, P = alkali hydrogen peroxide stage). When the amount of hexenuronic acid in the pulp of the pulp obtained is higher than 15 mmol per 1 kg of the absolutely dried pulp, there is a problem that the discoloration of the pulp is extremely poor.
[0007]
As a method for improving the bleaching property, it is known that pulp is treated with xylanase before or after oxygen bleaching and bleaching (for example, see JP-A-2-26487 and JP-A-2-293486). (For example, see Japanese Patent Application Laid-Open No. 6-101185), but there is a problem that the effect of improving the discoloration is not so great for the cost of prescribing. In the method of performing acid pretreatment at a high temperature (for example, see British Patent No. 1062734, Japanese Patent Application Laid-Open No. 10-508346), unbleached pulp is heated to 80 ° C. or more under acidity before the bleaching stage. It is treated at a temperature (for example, see British Patent No. 1062734) or at 85 to 150 ° C. (for example, see Japanese Patent Application Laid-Open No. 10-508346), and then bleached in multiple stages to improve the bleaching property of the pulp. Have been reported.
[0008]
However, when the present inventors performed processing and bleaching under these conditions, it was found that at temperatures exceeding 85 ° C., although the fading property was improved, the strength was greatly reduced. Further, in the method in which the acid pretreatment is performed at this high temperature, the whiteness after the acid treatment is greatly reduced. Therefore, as the Kappa number decreases, the amount of the bleached chemicals decreases, the pulp yield decreases, and the wastewater COD increases. In addition, there is also a problem that equipment cost is high because equipment that can withstand high temperature and low pH is required.
On the other hand, a dry heating method (heating at 105 ° C. for 24 hours) is used for the evaluation of the fading property (see, for example, Japanese Patent Application Laid-Open No. 10-508346). Although it is essential to perform the fading test under heat and humidity conditions (for example, 80 ° C. and a relative humidity of 65%), since this evaluation method is a fading evaluation based on only heat treatment, the high humidity in Japan is high. Considering climatic conditions, it is difficult to evaluate the fading properties of products using this evaluation method.
[0009]
The PC value (post-color number) usually used for the evaluation of fading is obtained from the difference in whiteness between before and after fading processing. A photographic material using pulp having a low PC value (see, for example, JP-A-56-54436) and a photographic material excellent in fading using pulp bleached in a sequence including oxygen bleaching (for example, There are reports on the use of pulp manufactured by chlorine-based bleaching, and from the characteristics of photographic materials, Since the whiteness is as high as 90% or more, the bleached pulp naturally has excellent fading. However, as in the present invention, pulp excellent in bleaching property without a decrease in pulp viscosity and strength and having a low whiteness of 78% to 87% and having no decrease in pulp viscosity and strength is produced by a bleaching method without using chlorine and hypochlorite. Is not what it is.
[0010]
On the other hand, several methods for performing a hydrogen peroxide treatment in an acidic region have been proposed (for example, see Patent Document 1, Patent Document 2, and Non-Patent Document 1). Only the effect of the bleaching of the pulp sheet, but there is no description of the relationship with the bleaching property of the pulp sheet. Further, even in a method of treating unbleached pulp with hydrogen peroxide under an acidic condition in combination with an organic or inorganic complex (Patent Document 1), there is no description suggesting that the same treatment is performed after oxygen exposure. Furthermore, when hydrogen peroxide was used in the acidic region, there was a point to be improved such that the pulp viscosity was reduced. (For example, Japanese Patent Application No. 2001-227274, which is a prior application of the present inventors.).
[0011]
On the other hand, the composition and amount of factory waste liquid discharged from chlorine dioxide production equipment using chlorate, sulfuric acid, and methanol as raw materials are generally as follows. 4 tons, 0.5 to 5 kg of sodium chlorate per 1 ton of chlorine dioxide production, and 0.1 to 0.5 kg of sodium chloride per 1 ton of chlorine dioxide production. , Except for the bleaching process. Therefore, in an ECF bleaching method that does not use elemental chlorine, for example, when a production volume of 1,000 tons per day is produced by a normal ECF sequence such as DE / O-PD, chlorine dioxide is not used. The amount of use is 5 to 15 tons, and the amount of sesqui sodium sulfate in the waste liquid accompanying the discharge is to be discharged 6 to 21 tons per day, and the amount of discharge from chlorine dioxide equipment is considerable. . Methods for producing chlorine dioxide by reducing chlorate with methanol include the R-8 method of Carlit Japan and the SVP-LITE method of Hodogaya Engineering.
[0012]
As a method for treating waste liquid from a chlorine dioxide production facility, a method of adding aluminum or an aluminum compound to produce aluminum sulfate (see, for example, JP-A-51-96796), and a method of using a cooking chemical liquid (for example, Japanese Patent Application Laid-Open No. 52-107302), sulfates are decomposed into sulfuric acid and alkali by a two-chamber electrodialysis apparatus composed of a bipolar membrane and a cation exchange membrane, and the sulfuric acid is reused in a chlorine dioxide production facility. (For example, refer to Japanese Patent Application Laid-Open No. 5-58601), but all of them increase production costs and equipment costs, and in most actual pulp production plants, adjust the pH of the coagulation and sedimentation treatment tank of the factory waste liquid. At present, there is no choice but to discard except to use it for pH adjustment in the bleaching process.
[0013]
[Patent Document 1]
JP-B-63-20953 [Patent Document 2]
W079 / 00637
[Non-patent document 1]
1985, Wood and Pulp Chemistry Symposium, Hans Ulrich Suss, etc.
[Problems to be solved by the invention]
The present inventors have found that bleaching is effective for any bleaching, bleaching pulp viscosity, and calcium oxalate generated during the bleaching process that occur when bleached pulp bleached without using elemental chlorine is subjected to acidic papermaking. Found a way.
[0015]
[Means for Solving the Problems]
The present invention that can achieve the above object includes the following inventions.
(1) Unbleached pulp obtained by digesting a lignocellulosic substance is subjected to alkali oxygen bleaching, and then acid-treated at an initial reaction pH of 1.5 to 4.5 and a temperature of 30 ° C to 75 ° C. ECF bleaching in a multi-stage bleaching sequence in which 0.7% by mass or more of chlorine dioxide was added to the bleached pulp at a chlorine dioxide bleaching stage, and the amount of hexeneuronic acid in the bleached pulp was reduced to 15 mmol or less per kg of the absolutely dried pulp. To produce bleached pulp.
[0016]
(2) The method for producing bleached pulp according to (1), wherein sesqui sodium sulfate is added in an amount of 0.01% by mass to 10% by mass based on absolutely dry pulp in the low-temperature acid treatment.
[0017]
(3) The method for producing bleached pulp according to any one of (1) and (2), wherein the sesqui sodium sulfate is a waste liquid containing sesqui sodium sulfate discharged as waste liquid from a chlorine dioxide production facility.
[0018]
(4) The method for producing bleached pulp according to any one of (1) to (3), wherein the bleached pulp has a whiteness of 78% to 87%.
[0019]
(5) The method for producing bleached pulp according to any one of (1) to (4), wherein the bleached pulp is hardwood pulp.
[0020]
(6) An acidic paper mainly containing bleached pulp produced by the method according to any one of (1) to (5).
[0021]
BEST MODE FOR CARRYING OUT THE INVENTION
The lignocellulosic substance used in the present invention is not particularly limited. As a cooking method for obtaining the pulp used in the present invention, known cooking methods such as kraft cooking, polysulfide cooking, soda cooking, and alkali sulphite cooking can be used, but pulp quality, energy efficiency, etc. are taken into consideration. Then, the kraft cooking method or the polysulfide cooking method is suitably used. For example, when wood is kraft cooked, the sulphidity of the kraft cooking liquor is 5 to 75%, preferably 15 to 45%, and the effective alkali addition rate is 5 to 30% by mass, preferably 10 to 25% by mass of the absolutely dry wood. %, The cooking temperature is 130-170 ° C., the cooking method may be either continuous cooking method or batch cooking method. When using a continuous cooking furnace, a modified cooking method in which cooking liquor is added at multiple points may be used. The method is not particularly limited.
[0022]
In the cooking, a known cyclic keto compound such as benzoquinone, naphthoquinone, anthraquinone, anthrone, phenanthroquinone and a nucleus-substituted product such as an alkyl or amino of the quinone-based compound, or the quinone-based compound is used as a cooking aid in the cooking liquor to be used. Hydroquinone-based compounds such as anthrahydroquinone, which is a reduced form of the compound; and 9,10-diketohydroanthracene compounds, which are stable compounds obtained as intermediates in anthraquinone synthesis by the Diels-Alder method. Species or two or more species may be added, and the addition rate is 0.001 to 1.0% by mass based on the absolute dry weight of the wood chips.
[0023]
In the present invention, the unbleached chemical pulp obtained by the known digestion method is subjected to washing, roughing and selective steps, and then delignified by the known alkaline oxygen delignification method. As the alkali oxygen delignification method used in the present invention, a known medium concentration method or a high concentration method can be applied as it is, but the medium concentration method currently used generally at a pulp concentration of 8 to 15% by mass is used. Is preferred.
[0024]
In the alkali oxygen delignification method by the above-mentioned medium concentration method, caustic soda or oxidized kraft white liquor can be used as an alkali, and oxygen from cryogenic separation method, PSA (Pressure Swing Adsorption) can be used as an oxygen gas. Of oxygen, oxygen from VSA (Vacuum Swing Adsorption) and the like can be used. The oxygen gas and alkali are added to a medium-concentration pulp slurry in a medium-concentration mixer, and after sufficient mixing is performed, the mixture is sent to a reaction tower capable of holding a mixture of pulp, oxygen, and alkali for a predetermined time under pressure, and subjected to delignification. Is done.
[0025]
The addition rate of oxygen gas is 0.5 to 3% by mass per absolutely dry pulp mass, the alkali addition ratio 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 from 8 to 15% by mass, and other known conditions can be applied. In the present invention, in the alkali oxygen delignification step, it is a preferred embodiment that the above alkali oxygen delignification is continuously performed a plurality of times, and delignification is advanced as much as possible. The pulp subjected to the alkali oxygen delignification is then sent to a washing step. After washing, the pulp is sent to an acid treatment step.
[0026]
The reaction temperature of the acid treatment in the present invention is preferably from 30 to 75C. When the temperature is lower than 30 ° C., the fading property is not sufficiently improved, while when the temperature is higher than 75 ° C., the fading property is improved, but the steam cost to be used is significantly increased. At a temperature exceeding 85 ° C., the viscosity and strength of the bleached pulp are significantly reduced. The initial pH of the reaction is preferably 1.5 to 4.5. If the initial pH of the reaction is less than 1.5, the pulp strength is greatly adversely affected, and if the pH is less than 1.5 at the above temperature, it is difficult to find a durable lining, and even if it is very expensive, And is not practical. When the pH is higher than 4.5, the effect of removing hexenuronic acid is reduced, and the scaling of calcium oxalate in the process becomes severe. The acid treatment is characterized in that the initial pH of the reaction and the pH at the end of the reaction are substantially equal.
[0027]
Further, the retention of the acid treatment is preferably about 30 to 300 minutes in consideration of its effect and damage to the pulp fiber. The treatment concentration is not limited as long as it is a general concentration in a process, but is 8 to 15%. The medium concentration method or the high concentration method of 25 to 40% is preferred. The acid used for maintaining the acidic region in the present invention may be any of an inorganic acid and an organic acid. The pH at the time of the acid treatment is 1.5 to 4.5, and specifically, an inorganic acid such as sulfuric acid, nitric acid, hydrochloric acid, sulfurous acid, nitrous acid, or sesqui sodium sulfate discharged from chlorine dioxide generating equipment can be used. . As an acid other than sesqui sodium sulfate, sulfuric acid is preferably used because it is easy to obtain and handle. In addition, a general recipe is used for the acid treatment. On the other hand, the addition ratio of sodium sulfate or sesqui sodium sulfate is preferably 0.01 to 10% by mass, and more preferably 0.1 to 5% by mass, based on the absolutely dried pulp.
[0028]
Further, in the acid treatment, a chelating agent may be added. Type chelating ^agents, Fe ^{^2+,} Cu ^{^2+,} as long as it can sequester metal ions ^{Mn 2+,} etc. EDTA (ethylenediaminetetraacetic acid), DTPA (diethylenetriaminepentaacetic acid), DTPMP (diethylene triamine pentamethylene phosphonic acid), PHAS ( Poly-α-hydroxy acrylate), and various chelating agents may be mixed and used. The addition rate of the chelating agent is generally in the range of 0.001% by mass to 5% by mass based on the absolutely dry pulp.
[0029]
In the acid treatment of the present invention, the pressure can be increased using an oxygen-containing gas or a nitrogen-containing gas. The oxygen-containing gas used for pressurization during the treatment can be used on an industrial scale, such as oxygen from cryogenic separation, oxygen from PSA, oxygen from VSA, etc. Oxygen or oxygen-containing gas having an oxygen purity of 85% by volume or more used for lignin, an oxygen-containing gas whose oxygen content has been adjusted to exceed 21% by volume using an oxygen production facility using the molecular sieve, An oxygen-enriched gas produced by mixing oxygen-containing gas having an oxygen purity of 85% by volume or more with air, air having an oxygen content of 20% by volume or more, and the like can be mentioned. Can be used. The pressure applied during the acid treatment with the oxygen-containing gas or the nitrogen-containing gas during the hydrogen peroxide treatment is 0.05 to 0.9 MPa (gauge pressure), and preferably 0.15 to 0.7 MPa.
[0030]
In the case where an ozone bleaching step is provided in the multi-stage bleaching step, the exhaust gas containing oxygen can also be suitably used. As the nitrogen-containing gas used in the hydrogen peroxide treatment stage of the present invention, any gas may be used as long as the nitrogen gas content is 95% or more, but from an economic viewpoint, it is used for alkaline oxygen delignification. A nitrogen-containing gas by-produced when producing oxygen gas such as oxygen from cryogenic separation, oxygen from PSA, and oxygen from VSA is preferably used.
[0031]
In the present invention, an enzyme treatment step can be provided after the acid treatment step. The enzyme used in the enzyme treatment step may be any enzyme as long as it reacts with pulp to reduce the potassium permanganate value of the pulp measured by JIS P 8206. For example, xylanase, lignin peroxidase, manganese peroxidase, laccase, and the like are known. Of course, these enzymes may be used, and it is needless to say that enzymes that are not yet known may be applicable enzymes. In addition, these enzymes may be used alone, or may be used in combination, mixed, or divided into a plurality of times. Among these enzymes, a xylan-degrading enzyme called xylanase has a bleach accelerating effect at the same time and is preferably used.
[0032]
In the multi-stage bleaching process of the present invention, the first stage uses a chlorine dioxide bleaching stage (D0 stage). In the second stage, an alkali extraction stage (E) is used, and in the third and subsequent stages, a combination of chlorine dioxide, alkali hydrogen peroxide or the like is suitably used. The chlorine dioxide used in the first chlorine dioxide bleaching stage of the present invention can be selected from chlorine dioxide obtained from many chlorine dioxide generation methods known to those skilled in the art. The chlorine dioxide obtained from is used. The addition rate in the first stage chlorine dioxide stage of the present invention is not more than 0.7% by mass per absolutely dry pulp, but there is no problem on the bleaching property. However, when the addition ratio is high, the bleaching cost increases. About 0.7% to 3.0% is preferable. The pH of the chlorine dioxide stage is from 2 to 6, preferably from 2.5 to 4, and it is also possible to supplementally add any acid or alkali to adjust the pH. Known chlorine dioxide bleaching conditions, such as chlorine dioxide treatment time and pulp concentration, can be all known conditions.
[0033]
In the alkali extraction stage following the chlorine dioxide bleaching stage of the present invention, many alkali compounds known to those skilled in the art can be used, but caustic soda is most easily used and is preferably used. In the alkali extraction stage of the present invention, oxygen and / or hydrogen peroxide can be used in combination. In addition, the alkali extraction stage of the present invention can be performed under known conditions.
[0034]
In the chlorine dioxide bleaching stage used in the multi-stage bleaching step of the present invention, any bleaching agent other than chlorine and hypochlorite may be used in the third and subsequent bleaching stages following the alkali extraction stage. However, common bleaching chemicals such as chlorine dioxide, alkaline hydrogen peroxide, ozone, and peracid are preferably used. Although the number of stages after the third stage is not particularly limited, it is preferable that the process be completed in three or four stages in consideration of energy efficiency, productivity, and the like.
[0035]
The chemicals used in the present invention include acids (A), chlorine dioxide (D), alkalis (E), oxygen (O), alkali hydrogen peroxide (P), ozone (excluding chlorine and hypochlorite). Z), enzymes (Ez), known bleaching agents such as organic peracids, and bleaching aids. As the bleaching sequence, for example, the sequence starting from the acid treatment stage (A) after oxygen delignification is ADE / OD, ADE / OPD, ADE. / ODD, ADE / ODP, ADE / OP-D, ADE / OZD, etc., and A-Ez- containing enzymes DE / OD, A-Ez-DE / OPD, A-Ez-DE / ODD, A-Ez-DE / O-DP, A-Ez-DE / OP-D, A-Ez-DE / O-ZD, etc. can also be mentioned.
[0036]
In the present invention, unbleached pulp is subjected to alkaline oxygen bleaching, followed by acid treatment at an initial reaction pH of 1.5 to 4.5 and a temperature of 30 to 75 ° C., and then a first chlorine dioxide bleaching stage ( D0 stage), the ECF bleaching is performed in a multi-stage bleaching sequence in which 0.7% by mass or more of chlorine dioxide is added to the absolute dry pulp, and the amount of hexenuronic acid in the bleached pulp is 15 mmol or less per kg of absolute dry pulp. However, it is necessary to wait for further study on the reason why the bleaching property is improved.However, when hexeneuronic acid remains in the finished pulp of acidic paper in a high humidity and at a high temperature, this may not be possible. It is thought that the chromophore changes and the whiteness decreases. Therefore, compared to conifers, broadleaf trees with a higher hexenuronic acid content are more likely to fade.
[0037]
On the other hand, the reason that the scaling of the bleaching step can be prevented by the present invention is that calcium ions can be discharged out of the bleaching step by low-temperature acid treatment. In general, calcium oxalate scaling in the bleaching step is performed in a place where an alkaline (or acidic) process solution containing a large amount of oxalate anions and an acidic (or alkaline) process solution containing a large amount of calcium ions are mixed, and the pH is set at 4 or less. It is supposed to occur at places that change in the range of ９9. Further, if the product of the molar concentration of calcium ions and the molar concentration of oxalate ions exceeds the solubility product of calcium oxalate of 4 × 10 ⁻⁹ , calcium oxalate scale is generated. High scaling prevention effect.
[0038]
The method for preparing acidic paper using the bleached pulp of the present invention is as follows. Bleached pulp is beaten with a beater to about 350 ml to 550 ml of CSF (Canadian Standard Freeness, Canadian Standard Freeness). %, Rosin sizing agent (for example, Size Pine E, manufactured by Arakawa Chemical Industries) about 0.5%, talc (for example, Illite, manufactured by Nippon Talc) about 20%, retention agent (for example, Percoll 182, Kyowa Sangyo) ) Of about 0.02%, and an acid paper having a basis weight of about 64 g / m ² is formed by a conventional method. When making acidic paper, it is of course possible to mix and use other bleached pulp within a range that does not impair the excellent properties of the acid bleached pulp of the present invention.
[0039]
The method for preparing neutral paper using the bleached pulp of the present invention is as follows: beaten the bleached pulp with a beater to about 350 ml to 550 ml of CSF, and then prepare a cationized starch (for example, Ace K100, manufactured by Oji Cornstarch) of about 0%. 0.5%, sulfate band about 0.5%, AKD (for example, SPK902, manufactured by Arakawa Chemical Industries) about 0.05%, light calcium carbonate (for example, TP121, manufactured by Okutama Industry) about 20%, retention aid ( For example, there is a method of mixing about 0.02% in the order of Percoll 182 (manufactured by Kyowa Sangyo Co., Ltd.) and making a neutral paper having a basis weight of about 64 g / m ² by an ordinary method.
The effect of the present invention for improving the fading property is great in the case of acidic paper, but there is no problem when the pulp treated in the present invention is used for neutral paper.
[0040]
【Example】
Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples. However, needless to say, the present invention is not limited to these Examples. Unless otherwise indicated, the measurement of the kappa number, the measurement of the amount of hexenuronic acid in the pulp, the measurement of the pulp brightness, the evaluation of the bleaching property of the pulp, and the measurement of the pulp viscosity were performed by the following methods, respectively. In addition, the addition rate of the chemicals in Examples and Comparative Examples is shown by mass% per absolutely dry pulp mass.
[0041]
1. Measurement of kappa number of pulp Kappa number was measured according to JIS P 8211.
[0042]
2. Determination of the amount of hexeneuronic acid in pulp 5 g of absolutely dried pulp was weighed and washed with ion-exchanged water in a 500 ml SUS container, and a total of 300 ml was prepared using a 10 mmol / l solution of formic acid-sodium formate buffer. Thereafter, the inside of the SUS container was replaced with nitrogen gas, and the container was treated at 110 ° C. for 5 hours in an oil thermostat. After cooling the SUS container with running water, the pulp suspension after the treatment was diluted to 500 ml including the washing solution, filtered, and the solution was analyzed by HPLC (high performance liquid chromatography). -Carboxy-2-furaldehyde was quantified. The following calculation formulas and references were used for quantification.
[0043]
The quantitative value of 2-furic acid in the measurement sample was defined as a (ng / μl), and the quantitative value of 5-carboxy-2-furaldehyde was defined as b.
1) Amount of 2-furic acid (mmol / kg) = a × (500/1000) / (10 × 10 ⁻³ ) /112.08
2) 5-carboxy-2-furaldehyde amount (mmol / kg) = b × (500/1000) / (10 × 10 ⁻³ ) /140.1
3) Amount of hexeneuronic acid (mmol / l) = Amount of 2-furoic acid + Amount of 5-carboxy-2-furaldehydide
References: Author Vuorinen, T .;
Selective hydrolysis of hexenuronic acid groups and it's applications in ECF and TCF bleaching of craft pulses.
International Pulp Breaking Conference, April 14-18, 1996, P43-51.
[0045]
3. Measurement of whiteness of bleached pulp After disaggregating the bleached pulp, a sulfuric acid band was added to the pulp slurry by 3.0% with respect to the pulp, and a sheet having a basis weight of 60 g / m ² was prepared according to Tappi test method T205os-71 (JIS P 8209). . Thereafter, the whiteness of the pulp was measured according to JIS P 8123.
[0046]
4. Evaluation of bleaching property of pulp The pulp sheet for whiteness measurement was bleached for 48 hours under the conditions of 80 ° C. and 65% relative humidity, and the PC value was calculated and evaluated from the pulp brightness before and after bleaching according to the following formula.
PC value = 100 × [{(1−whiteness after bleaching) ² / (2 × whiteness after bleaching)} − {(1−whiteness before bleaching) ² / (2 × whiteness before bleaching)}]
[0047]
5. Measurement of viscosity of bleached pulp Measurement of pulp viscosity is described in J. Mol. Performed according to TAPPI 44.
[0048]
6. Measurement of total calcium concentration and oxalate ion concentration in pulp after A-stage and pulp after D0-stage Pulp after A treatment and D0 treatment was squeezed from 10% to 12% on 200 MESH wire, The calcium concentration was measured by ICP emission spectroscopy according to JIS-K0102. The oxalate ion concentration was measured by a general ion chromatography method (column IONPAC AS14).
[0049]
Example 1
80.0 g of absolutely dry mass of kraft pulp (whiteness 50.5%, kappa number 10.0) after digestion-alkali oxygen delignification of factory hardwood was put into a plastic bag, and pulp concentration was adjusted using ion-exchanged water. After being adjusted to 10%, 2.0% of sesqui sodium sulfate and 0.33% of sulfuric acid were added per absolutely dried pulp mass, and immersed in a thermostat at a temperature of 60 ° C. for 180 minutes to perform an acid treatment (hereinafter, referred to as acid treatment). , A stage). The initial pH of the reaction in stage A was 3.0. The obtained pulp was received with a 200 MESH wire, dehydrated to about 12%, diluted with ion-exchanged water to 3%, and then dehydrated and washed using a Buchner funnel to obtain a pulp after A-stage. Next, the pulp after A is put into a plastic bag, the pulp concentration is adjusted to 10% using ion-exchanged water, 0.75% of chlorine dioxide is added per absolute dry pulp mass, and the temperature is set to a constant temperature water bath at 70 ° C. It was immersed for 60 minutes to bleach the first chlorine dioxide stage (hereinafter abbreviated as D0 stage). The obtained pulp was diluted to 3% with ion-exchanged water, and then dehydrated and washed with a Buchner funnel.
[0050]
The pulp after the D0 stage was placed in a plastic bag, and the pulp concentration was adjusted to 10% using ion-exchanged water. Then, 1.2% of caustic soda was added to the absolute dry pulp mass. After treating for 165 minutes, an alkali extraction stage (hereinafter abbreviated as E stage) was performed. The obtained pulp was diluted with ion-exchanged water to adjust the pulp concentration to 3%, and then dewatered and washed using a Buchner funnel to obtain pulp after E-stage.
Subsequently, the pulp was placed in a plastic bag after the E-stage, and the pulp concentration was adjusted to 10% using ion-exchanged water. Then, 0.25% of chlorine dioxide was added per absolute dry pulp mass, and the temperature was adjusted in the same manner as in the D0 stage. The mixture was treated at 70 ° C. for 300 minutes to perform bleaching of a second chlorine dioxide bleaching stage (hereinafter abbreviated as D1 stage). The obtained pulp was diluted to 3% with ion-exchanged water, washed and dehydrated using a Buchner funnel to obtain bleached pulp having a whiteness of 84.2%. The hexeneuronic acid content of the obtained bleached pulp, the PC value of the bleached pulp sheet after 48 hours, the viscosity of the bleached pulp, the total calcium concentration and the oxalate ion concentration in the pulp squeezed liquid after the A and D0 stages were measured. The results are shown in Table 1.
[0051]
Example 2
After the alkaline oxygen delignification, the same operation as in Example 1 was performed except that the addition ratio of chlorine dioxide in the D0 stage of the kraft pulp was 0.90% and the addition ratio of chlorine dioxide in the D1 stage was 0.15%. The initial reaction pH of the A-stage was 3.0, and the pulp brightness after the multi-stage bleaching was 84.8%. The hexeneuronic acid content of the bleached pulp obtained, the PC value of the bleached pulp sheet after 48 hours, and the viscosity of the bleached pulp were measured.
[0052]
Example 3
After the alkaline oxygen delignification, the same operation as in Example 1 was performed except that the sulfuric acid addition rate in the A-stage of the kraft pulp was changed to 0.53%. The initial pH of the A-stage reaction was 2.1, and the pulp brightness after the multi-stage bleaching was 84.1%. The hexeneuronic acid content of the obtained bleached pulp, the PC value of the bleached pulp sheet after 48 hours, the viscosity of the bleached pulp, the total calcium concentration and the oxalate ion concentration in the pulp squeezed liquid after the A and D0 stages were measured. The results are shown in Table 1.
[0053]
Example 4
After the alkaline oxygen delignification, the same operation as in Example 1 was performed except that the sulfuric acid addition ratio in the A stage of the kraft pulp was changed to 0.23%. The initial reaction pH of the A-stage was 3.9, and the pulp brightness after the multi-stage bleaching was 84.1%. The hexeneuronic acid content of the obtained bleached pulp, the PC value of the bleached pulp sheet after 48 hours, the viscosity of the bleached pulp, the total calcium concentration and the oxalate ion concentration in the pulp squeezed liquid after the A and D0 stages were measured. The results are shown in Table 1.
[0054]
Example 5
After the alkaline oxygen delignification, the same operation as in Example 1 was performed except that the treatment temperature in the A stage of the kraft pulp was set to 40 ° C. The initial pH of the reaction in stage A was 3.0, and the pulp brightness after multistage bleaching was 83.9%. The hexeneuronic acid content of the obtained bleached pulp, the PC value of the bleached pulp sheet after 48 hours, the viscosity of the bleached pulp, the total calcium concentration and the oxalate ion concentration in the pulp squeezed liquid after the A and D0 stages were measured. The results are shown in Table 1.
[0055]
Example 6
After the alkaline oxygen delignification, the same operation as in Example 1 was performed except that the treatment temperature of the kraft pulp in the A stage was set to 75 ° C. The initial pH of the reaction in stage A was 3.0, and the pulp brightness after multistage bleaching was 84.1%. The hexeneuronic acid content of the bleached pulp obtained, the PC value of the bleached pulp sheet after 48 hours, and the viscosity of the bleached pulp were measured.
[0056]
Example 7
After the alkaline oxygen delignification, the same operation as in Example 1 was performed except that the chlorine dioxide addition rate in the D1 stage of the kraft pulp was changed to 0.10%. The initial reaction pH of the A-stage was 3.0, and the pulp brightness after the multi-stage bleaching was 78.8%. The hexeneuronic acid content of the bleached pulp obtained, the PC value of the bleached pulp sheet after 48 hours, and the viscosity of the bleached pulp were measured.
[0057]
Comparative Example 1
After the alkaline oxygen delignification, the same operation as in Example 1 was performed except that the addition ratio of chlorine dioxide in the D0 stage of the kraft pulp was 0.65% and the addition ratio of chlorine dioxide in the D1 stage was 0.30%. The initial pH of the reaction in stage A was 3.0, and the pulp brightness after multistage bleaching was 83.0%. The hexeneuronic acid content of the bleached pulp obtained, the PC value of the bleached pulp sheet after 48 hours, and the viscosity of the bleached pulp were measured.
[0058]
Comparative Example 2
After the alkaline oxygen delignification, the same operation as in Example 1 was performed except that the sulfuric acid addition rate of the A stage of the kraft pulp was changed to 0.10%. The initial reaction pH of the A-stage was 5.0, and the pulp brightness after the multi-stage bleaching was 83.8%. The hexeneuronic acid content of the bleached pulp obtained, the PC value of the bleached pulp sheet after 48 hours, and the viscosity of the bleached pulp were measured.
[0059]
Comparative Example 3
After the alkaline oxygen delignification, the same operation as in Example 1 was performed except that the reaction temperature in the A stage of the kraft pulp was changed to 85 ° C. The initial reaction pH of the A-stage was 3.0, and the pulp brightness after the multi-stage bleaching was 84.3%. The hexeneuronic acid content of the bleached pulp obtained, the PC value of the bleached pulp sheet after 48 hours, and the viscosity of the bleached pulp were measured.
[0060]
Comparative Example 4
After the alkaline oxygen delignification, the same operation as in Example 1 was performed except that the A-stage treatment of the kraft pulp was not performed. The pulp brightness after multi-stage bleaching was 83.5%. The hexeneuronic acid content of the obtained bleached pulp, the PC value of the bleached pulp sheet after 48 hours, the viscosity of the bleached pulp, the total calcium concentration and the oxalate ion concentration in the pulp squeezed liquid after the A and D0 stages were measured. The results are shown in Table 1.
[0061]
[Table 1]

[0062]
As is clear from the comparison between Examples 1 to 7 and Comparative Example 1 in Table 1, when the D0 addition ratio is lower than 0.7%, the amount of hexenuronic acid in the bleached pulp is large, and the PC value after 48 hours is high. For this reason, it is inferior in fading. As is clear from comparison of 1 to 7 with Comparative Example 2, when the treatment pH in the A stage was higher than 4.5, the amount of hexenuronic acid in the bleached pulp was large, and the PC value after 48 hours was high. Poor sex. Further, as is apparent from a comparison between Examples 1 to 7 and Comparative Example 3, when the acid treatment is carried out at a high temperature exceeding 75 ° C., the bleached pulp has a reduced viscosity although the discoloration is excellent. On the other hand, when no acid treatment is performed, the amount of hexenuronic acid also increases, resulting in a pulp having a high PC value and poor discoloration.
[0063]
【The invention's effect】
The unbleached pulp obtained by digesting the lignocellulosic material is subjected to alkaline oxygen delignification, and then subjected to an acid treatment at an initial reaction pH of 1.5 to 4.5 and a temperature of 30 to 75 ° C., and then the first stage of the oxidation. ECF bleaching was performed in a multi-stage bleaching sequence in which the chlorine dioxide addition rate in the chlorine bleaching stage (D0 stage) was 0.7% by mass or more per absolute dry pulp, and the amount of hexeneuronic acid in the bleached pulp was 15 mmol / kg of absolute dry pulp. By setting it as follows, the PC value of the sheet prepared for the bleached pulp treated at a constant temperature of 80 ° C. and a relative humidity of 65% and a constant humidity of 48 hours was 10.0 or less. According to the present invention, it is possible to remarkably improve the bleaching property of bleached pulp and to significantly improve the pulp viscosity. Further, it became possible to prevent calcium oxalate scale in the bleaching step.

Claims

The unbleached pulp obtained by digesting the lignocellulosic material is subjected to alkaline oxygen bleaching, and then subjected to an acid treatment at an initial reaction pH of 1.5 to 4.5 and a temperature of 30 ° C. to 75 ° C., followed by the first stage chlorine dioxide. Bleached pulp in which ECF bleaching is performed in a multi-stage bleaching sequence in which the chlorine dioxide addition rate in the bleaching step is 0.7% by mass or more per absolutely dry pulp, and the amount of hexeneuronic acid in the bleached pulp is 15 mmol or less per kg of absolutely dry pulp. Manufacturing method.

2. The method for producing bleached pulp according to claim 1, wherein in the acid treatment, sesqui sodium sulfate is added in an amount of 0.01% by mass to 10% by mass per absolutely dry pulp.

The method for producing bleached pulp according to any one of claims 1 to 2, wherein the sesqui sodium sulfate is a waste liquid containing sesqui sodium sulfate discharged as waste liquid from a chlorine dioxide production facility.

The bleached pulp according to any one of claims 1 to 3, wherein the whiteness of the bleached pulp is 78% to 87%.

The method for producing bleached pulp according to any one of claims 1 to 4, wherein the bleached pulp is hardwood pulp.