JP4194288B2

JP4194288B2 - Method for producing calcium carbonate

Info

Publication number: JP4194288B2
Application number: JP2002097131A
Authority: JP
Inventors: 清金井; 泰徳南里
Original assignee: Nippon Paper Industries Co Ltd
Current assignee: Nippon Paper Industries Co Ltd
Priority date: 2002-03-29
Filing date: 2002-03-29
Publication date: 2008-12-10
Anticipated expiration: 2022-03-29
Also published as: JP2003292320A

Description

【０００１】
【発明の属する技術分野】
本発明は、硫酸塩法またはソーダ法によるパルプ製造工程の蒸解液を製造する苛性化工程において、炭酸カルシウムを製造する方法に関するものであり、さらに詳しくは使用する生石灰の炭酸カルシウム含有率、消和反応及び苛性化反応条件等を特定することにより、従来の苛性化方法では得られなかった、大型の単粒子を特徴とする炭酸カルシウムを製造する方法に関するものである。
【０００２】
【従来の技術】
炭酸カルシウムは製紙用のほか、ゴム、プラスチック、ペイント、シーリング剤、粘着剤、肥料等、工業用原料として重要で、天然の石灰石を乾式或いは湿式で機械粉砕して得られる重質炭酸カルシウムと、化学的方法によって得られる沈降性炭酸カルシウム（合成炭酸カルシウム）がある。
【０００３】
我が国は良質の天然石灰石鉱脈を豊富に有し、古くから採掘して利用されて来た。しかし、その品質はもっぱら鉱脈の品質に依存し、近年要求される高品質の炭酸カルシウムの鉱脈は減少しつつあり、次第により高価な外国産に求められる様に変化の兆しが見えつつあるのが現状である。
【０００４】
そこで注目されつつあるのが、化学的方法で合成される軽質炭酸カルシウム（沈降性炭酸カルシウム）である。この軽質炭酸カルシウムの製造方法としては、（１）石灰の焼成装置その他から得られる二酸化炭素を含有したガスと石灰乳との反応、（２）アンモニアソーダ法における炭酸アンモニウムと塩化カルシウムとの反応、（３）炭酸ナトリウムの苛性化によって水酸化ナトリウムを製造する、石灰乳と炭酸ナトリウムとの反応等が知られている。これらの方法のうち、（２）（３）においては、その主生産物を得る製造法が新たな方法に転換したり、炭酸カルシウムが副産物であることから、その利用方法についてはあまり検討されていない。一方（１）は、反応系が比較的単純（水、消石灰、炭酸ガス）であり、様々な形状の炭酸カルシウムを製造する方法等について広く研究が進み、生産量も順次拡大の傾向に有り、この傾向は今後も続くものと予想されている。しかしながら、この方法は炭酸カルシウムが唯一の生産物であることから製造コストが非常に高くなる為、ユーザーの要望する低コスト化にはそぐわず、安価に提供出来ない状況に有る。
【０００５】
そこで考えられるのが、硫酸塩法又はソーダ法によるパルプ製造工程において、蒸解薬品を回収・再生する苛性化工程で副生する炭酸カルシウムを利用する方法である。
【０００６】
硫酸塩法又はソーダ法によるパルプ製造工程では、木材から繊維素を単離するために水酸化ナトリウムや硫化ナトリウムを溶解した白液を用いて高温、高圧下で蒸解する。繊維素は固相として分離精製してパルプとし、蒸解廃液（黒液）は濃縮燃焼する。その際、木材からの溶出成分は熱源として回収し、薬液中の無機物は炭酸ナトリウム又は硫化ソーダとの混合物を主成分とするスメルトとして回収する。スメルトは弱液と呼ぶ白液成分が一部溶解した炭酸カルシウム洗浄液に溶解して緑液とする。
【０００７】
この緑液と生石灰を混合して、［１］［２］式で示す消和反応と苛性化反応により、炭酸ナトリウムを蒸解薬液に有用な水酸化ナトリウムに転換し、白液を得ると同時に炭酸カルシウムが副生する。従来のパルプ工場では緑液と生石灰をスレーカーと呼ばれる反応槽で混合する為、実際にはこの二段の反応はかなり重複して進行し、消和反応と苛性化反応はほとんど同時に起こる。
【０００８】
ＣａＯ＋Ｈ₂Ｏ → Ｃａ（ＯＨ）₂ ［１］
Ｃａ（ＯＨ）₂ ＋Ｎａ₂ＣＯ₃ → ＣａＣＯ₃ ＋２ＮａＯＨ［２］
ここで生成する炭酸カルシウムは、主生産物である白液を製造する際の副産物であるため、工業用原料として使用した場合、非常に低コストで利用できるばかりでなく、従来閉鎖系である苛性化工程のカルシウム（生石灰、炭酸カルシウム）循環サイクルから炭酸カルシウムを系外に抜き取ることによって、循環石灰の高純度化が達成され、上記［１］［２］の反応性向上や白液の清澄性の向上、さらには廃棄物の低減が期待できる。
【０００９】
しかし、この従来方法では上記［１］［２］の反応が殆ど同時に起こるため、得られる炭酸カルシウムの形状をコントロールする事は難しく、サイコロ状や六角面体などの種々雑多な形状を有し、粒子径も大小様々でほとんどは塊状を呈し、急速な結晶成長によって、緑液由来の狭雑物を包含した純度の低い炭酸カルシウムが生成していた。従ってこれを工業用原料として製造した場合、特に近年要求されつつ有る高品質の炭酸カルシウムの製造には不充分である、等の問題を抱えていた。
【００１０】
このように、従来の方法では炭酸カルシウムの純度が低く、工業用の高品質な炭酸カルシウムを製造することは困難であった。
【００１１】
【発明が解決しようとする課題】
従来の苛性化方法ではスレーカーで生石灰と緑液を直接混合するため、前記［１］［２］の反応がほとんど同時に開始する。従って、炭酸イオン及びカルシウムイオン濃度が高い為、結晶発生や結晶成長が急速に進み、生成する炭酸カルシウム粒子の形状はサイコロ状や六角面体などの種々雑多な形状を有し、粒子径も大小様々でほとんどは比表面積の大きい凝集晶を呈す。更に、急速な結晶成長によって緑液由来の狭雑物を包含し易い傾向に有り、炭酸カルシウムの品位を低下させて来た。又、本来の目的である白液製造の為、白液と炭酸カルシウムの分離、及び分離した炭酸カルシウムに同伴する白液成分を除去する工程が必須になるが、この工程において多大な労力を必要として来た。
【００１２】
そこで、かかる問題点を解決し高品位炭酸カルシウムの製造方法を開発することを本発明の課題とした。
【００１３】
【課題を解決するための手段】
本発明者らは、上記課題を解決するために鋭意研究を重ねた結果、硫酸塩法又はソーダ法によるパルプ製造工程の苛性化工程を利用して、特定量以下の炭酸カルシウムを含有する生石灰を、特定の石灰濃度で、白液、緑液又はそれらの混合物からなる群から選択される液体で連続的に消和反応させる消和工程に引き続き、苛性化槽において、前記消和工程で得られた石灰乳及び又は石灰泥と、硫酸塩法又はソーダ法によるパルプ製造工程の苛性化工程で発生し、従来の操業において白液を製造するのに必要とされると同量の緑液を連続混合し、反応液の滞留時間を特定して苛性化反応を進める事によって、従来の操業において製造すると同等の白液と高品位な炭酸カルシウムを得る苛性化方法を見出した。上記苛性化方法で得られる炭酸カルシウムは形状が単純で比表面積も小さく、平均粒径が大きく、洗浄が容易で粉砕後の白色度が高く、優れた工業原料が得られる。
【００１４】
本発明の消和槽における生石灰の白液又は緑液に対する濃度、特に緑液で消和反応する場合の濃度は、引き続き実施する苛性化反応によって生成する炭酸カルシウムの品質をコントロールする上で非常に重要である。生石灰の白液に対する濃度は高濃度程高い粘性を示すが、低濃度から高濃度に至る実質的に消和反応及び引き続き実施する苛性化反応の実施が可能な範囲であれば、何れの濃度でも設定可能である。生石灰と緑液の場合は高濃度では改良された品質の炭酸カルシウムが得られるが、濃度が従来の苛性化法に近くなるほど改良された炭酸カルシウムの品質が得られなくなり、最も低濃度側では従来の苛性化法と同等の反応条件となってしまう。
【００１５】
本発明の苛性化槽における苛性化反応液の滞留時間は、苛性化反応によって生成する炭酸カルシウムの品位をコントロールする上で非常に重要であり、滞留時間が長くなるほど好ましいが、滞留時間は２時間から１０時間、好ましくは４時間から６時間にコントロールする必要がある。滞留時間が従来の苛性化槽の滞留時間である２時間未満では、苛性化反応が急速に進むため粒子形状は凝集晶を呈し易くなる為、品位や白液との分離性の改善が不充分である。苛性化槽の滞留時間が１０時間を越える場合は、品位や分離性は充分となるが大型の苛性化槽が必要となって経済的に不利となる。
【００１６】
尚、従来の苛性化工程ではスレーカーに続く苛性化槽として２乃至４段の苛性化槽が設置されているが、本開発法の場合の苛性化槽は第１段の滞留時間を規定する方法に関する。従って、第１段の苛性化槽に続く数段の苛性化槽の設置は白液の苛性化率に応じて適宜設置する。
【００１７】
本発明の完成によって、生成する炭酸カルシウムの平均粒径が大きく出来る為、白液分離や炭酸カルシウムの洗浄工程の設備負荷が低減するばかりで無く、従来の石灰乳と炭酸ガスの反応によって得られる炭酸カルシウムに比べ、低コストでの製造が可能である。さらに、本発明の技術によって、苛性化工程で生成する炭酸カルシウムの生産性が飛躍的に向上するため、付随効果として、炭酸カルシウムの工程からの抜き取り量が増大し、焼成用キルンの負荷の低減が達成出来る。又、工程から炭酸カルシウムを抜き取る量によってはキルン停止も可能となり、苛性化工程での主生産物である白液の生産コストを大幅に削減できる。
【００１８】
本発明の初段苛性化槽に代えてD.T.B（Draft Tube Buffle Crystallixer）で苛性化を実施しても同様の効果が得られる。但し、苛性化反応工程の特徴として槽内で炭酸カルシウムが析出する工程であるため、装置の接液部のスケール付着が激しく、メンテナンスの面で複雑な装置構成は好ましくない。
【００１９】
【発明の実施の形態】
本発明の消和工程において使用する生石灰は、炭酸カルシウムを主成分とする天然石灰石、及び／又は硫酸塩法またはソーダ法によるパルプ製造工程の苛性化工程において、炭酸ナトリウムを水酸化ナトリウムに転化する際に生成する炭酸カルシウムを焼成したものであればよい。なお、その際の焼成装置に関しては、ベッケンバッハ炉、メルツ炉、ロータリーキルン、国井式炉、ＫＨＤ（カーハーディー）炉、コマ式炉、カルマチック炉、流動焼成炉、混合焼き立炉等の、炭酸カルシウムを生石灰（酸化カルシウム）に転化する装置であれば特に制限されることはない。
【００２０】
苛性化反応に使用する生石灰中の不純物の含量については、特に炭酸カルシウムを着色する成分としてＦe、Ａｌ、Ｍn等の金属元素が問題になるが、用途に合わせて着色成分含量の少ない原料石灰石から得られる生石灰を適宜選択する。あるいは苛性化工程におけるロータリーキルンや流動焼成炉等からの再焼成生石灰の場合には、系外に抜き取られる炭酸カルシウムと系内を再循環する炭酸カルシウムの比率等によって着色成分含量等が変化する為、状況にあわせて苛性化工程のカルシウム循環サイクルに補給する着色成分含量の少ない原料石灰石、あるいはこれを焼成した生石灰の量を調整して使用すればよい。
【００２１】
生石灰は、苛性化工程で発生した炭酸カルシウムをキルン等の焼成炉で脱炭酸して得た生石灰、又は苛性化工程から抜き取る炭酸カルシウムの補給分として外部から供給する生石灰、もしくは補給石灰石と工程を循環する炭酸カルシウムを前記キルンで混合焼成して得た生石灰、及びこれら生石灰を任意の比率で混合した生石灰が使用できる。
【００２２】
生石灰中の炭酸カルシウム含量については、生石灰の重量を基準として０．１〜１０重量％、好ましくは５％以下の場合に良好な結果が得られる。生石灰中の炭酸カルシウムはそれ自身当然の事ながら消和反応及び苛性化反応を起こさず、最終的に工業原料として使用されるまで化学組成的に変化することは無い。しかし、苛性化反応系に混入した炭酸カルシウムは種結晶として結晶成長段階に作用し、新たに生成する炭酸カルシウムを不定形あるいは塊状に誘導する原因になる為、得られる炭酸カルシウムの品位を低下させる。炭酸カルシウム含量で１０重量％を超えると、これらの傾向が顕著になって好ましくない。
【００２３】
一方、０．１重量％以下のものを得るためには、焼成に要するエネルギーが極度に増加する、あるいは焼成装置に特別な工夫を必要とするなどがあり不経済となる。
【００２４】
生石灰の粒度に関しては特に制限はないが、平均粒度０．０１ｍｍ〜１０ｃｍ、好ましくは０．０１ｍｍ〜１０ｍｍのものが消和時の攪拌において均一混合という面で好適である。粒度が０．０１ｍｍ以下の場合は粉砕のためのコストがかかると同時に、粉塵の発生や移送装置でのトラブルの原因となり好ましくない。
【００２５】
消和工程において生石灰の消和に用いる白液、若しくは緑液としては、トータルアルカリ濃度で８０〜１６０ｇ／Ｌ（Ｎａ₂Ｏ換算：以下同様）、好ましくは１００〜１５０ｇ／Ｌで行う必要がある。８０ｇ／Ｌより低い場合は最終白液の濃度が下がる為、蒸解に使用する際に濃度調節を行う必要が出てくる。一方、１６０ｇ／Ｌより高い場合は、苛性化する前の緑液中で炭酸ナトリウム等の結晶が生成し易くなって工程でトラブル発生の原因となる等の理由により、好ましくない。
【００２６】
白液を生産する苛性化工程で操業管理の指標の一つとして、下記［３］式で示される苛性化率が一般に用いられている。因みに、水酸化ナトリウム、炭酸ナトリウムの濃度はＮａ₂Ｏ換算（ｇ／Ｌ）で示す。
【００２７】
苛性化工程では下記［１］［２］式で示す消和反応と苛性化反応の二段の反応が進み、式［２］の苛性化反応は可逆反応であるが、ＣａＣＯ₃の溶解度がＣａ（ＯＨ）₂のそれよりも小さいので反応はかなり右方へ進む。しかし、この反応は反応液の［ＯＨ］濃度によって決まる平衡苛性化率以上には進行することは無く、一般的な苛性化工程に於ける苛性化率は６５〜８５％の範囲である事が多い。
ＣａＯ＋Ｈ₂Ｏ → Ｃａ（ＯＨ）₂ ［１］
Ｃａ（ＯＨ）₂ ＋Ｎａ₂ＣＯ₃ → ＣａＣＯ₃ ＋２ＮａＯＨ［２］
苛性化率（％）＝水酸化ナトリウム濃度／（炭酸ナトリウム濃度＋水酸化ナトリウム濃度） ×１００［３］
我々はこの苛性化率の定義を白液だけに止めず、本発明で言うところの生石灰の消和に使用する緑液を含む消和液に拡大した。なぜなら、苛性化率の低い所謂緑液は、苛性化反応で生成した炭酸カルシウムに同伴する白液成分の一部を洗浄回収した弱液に、ボイラーからのスメルトを溶解したものを言う。従って、現実的な緑液は白液由来の水酸化ナトリウムを含有し、同伴する白液成分の多少が緑液の苛性化率の高低を支配している。又、一般に呼ばれている白液或いは緑液の苛性化率範囲以外であっても、本発明にとって充分に有効な消和液と成り得るからである。
【００２８】
消和工程に用いる白液、若しくは緑液の炭酸ナトリウム濃度と石灰濃度には密接な関係がある。
炭酸ナトリウム濃度の低い白液で消和する場合の生石灰濃度は低濃度から高濃度まで良好な結果が得られる。しかし、炭酸ナトリウム濃度の高い緑液で消和する場合の石灰濃度は比較的高い石灰濃度範囲においてのみ良好な結果が得られる。
【００２９】
本発明者等は、生石灰を消和する際に使用する白液や緑液等の消和液の炭酸ナトリウム濃度と、消和時の石灰濃度の関係について研究を重ねた結果、消和する際の石灰濃度が、消和する前の生石灰を基準として１．０重量％以上、７７．０重量％以下であり、且つ下記（ａ）（ｂ）両条件を同時に満足させる条件で消和することにより、初めて所期の目的に叶う炭酸カルシウムが得られる消和条件である事を発見した。
【００３０】
０≦Ｘ≦消和液の平衡苛性化率（ａ）
１５−０．３Ｘ≦Ｙ≦７７（ｂ）
但し、
Ｘ＝消和液の苛性化率（重量％）
Ｙ＝消和前の生石灰を基準とした時の石灰濃度（重量％）
苛性化率（％）＝水酸化ナトリウム濃度／（炭酸ナトリウム濃度＋水酸化ナトリウム濃度）×１００
消和液が所謂白液であって、苛性化率が６５％以上であれば、例え石灰濃度が１．０％以下であっても何ら問題無く目的が達成出来る。しかし、更に苛性化率の低い所謂緑液の場合は、下記（ｃ）で示す石灰濃度以上、好ましくは下記（ｄ）で示す石灰濃度以上で消和する必要がある。
【００３１】
２０−０．３５Ｘ≦Ｙ≦７７．０（ｃ）
２５−０．４０Ｘ≦Ｙ≦７７．０（ｄ）
但し、Ｘ＝消和液の苛性化率（％）
Ｙ＝消和前の生石灰を基準とした石灰濃度（重量％）
一般的な白液（苛性化率＝６５〜８０％）による消和時の石灰濃度で言えば、消和前の生石灰を基準とした濃度で０．５〜７７重量％、好ましくは３．５〜５０重量％で行う必要がある。７７重量％を超えると生石灰が消和するに必要な水が不足する為、実質的に充分な消和が達成出来ない。又、５０重量％を越えると石灰乳若しくは石灰泥と呼ぶよりもむしろ粉末消石灰と呼ぶにふさわしい状態になって、特殊な消和装置を必要とする他、当然の事ながら初段の苛性化槽にはそれぞれ緑液と同時に粉体状の消石灰を供給する必要があり、特殊な添加装置が必要になる。
【００３２】
従って、現実的にはスラリーの攪拌や、苛性化槽への流下式移送が容易な５０重量％以下で消和する事が望ましい。一方０．５重量％未満では、白液及び炭酸カルシウムの生産性が非常に劣り現実的でない。
【００３３】
消和工程で使用する白液は他の工程から導入することも可能であるが、同一の苛性化工程で生産する白液を使用するのが設備的に都合が良い。消和時の石灰濃度を高く設定すれば自己循環する白液量を少なくできる為、苛性化槽や白液分離などの設備を簡略化出来るため有利である。
【００３４】
同様に、一般的な緑液（苛性化率＝５〜２０％）による消和時の石灰濃度は、消和前の生石灰を基準とした石灰濃度で１５〜７７重量％、好ましくは２０〜５０重量％で行う必要がある。５０重量％を超えると、スラリーの粘度が高すぎて現実的に攪拌が困難となる。一方１５重量％未満では、生成する炭酸カルシウムの品位を損なう為、好ましくない。
【００３５】
消和工程で行う生石灰の消和には、白液と緑液を任意の配合比で混合して用いる事も出来る。緑液の比率を高くする事によって、消和時の石灰濃度を高くする必要があるが、該苛性化工程で生産する白液の自己循環率を低く出来るため、白液分離工程の小型化など設備的に有利である。しかし、緑液の配合比を大きくすると、当然の事ながら炭酸ナトリウム濃度が高くなる為、石灰濃度を高くする必要が有り、高粘度攪拌に適した特殊な攪拌装置の導入が必要となって不経済である。
【００３６】
このような観点から生石灰の消和に用いる消和液として、白液と緑液との混合比の調節によって消和液の苛性化率を調節し、更に石灰濃度を調節すれば、白液分離性や洗浄性等炭酸カルシウムの品質をバランス良く調整する事も可能である。
【００３７】
尚、消和工程で生石灰から調整する石灰乳に代えて、水酸化カルシウムを白液若しくは緑液、又はこれらの混合液に分散し、本方法と同じ濃度に調整した石灰乳を使用することも可能である。この場合、消石灰が持ち込む水によって若干ではあるが白液のアルカリ濃度が低下する。
【００３８】
消和時における生石灰と白液、若しくは緑液等との混合には、一般的な攪拌羽根式、ポンプ式、押し出し機類、捏和機類、混練機類の中から、混合時のスラリー粘度にあわせて適宜選定して使用すれば良い（昭和６３年３月１８日丸善株式会社発行、化学工学便覧参照）。
【００３９】
従来、一般に操業されている苛性化工程では、緑液温度は回収ボイラーからの溶融した高温のスメルトを熱源として９０℃前後に制御する事が多く、苛性化温度は生石灰の消和熱発生によって昇温し、沸騰温度を越えない温度として１００℃前後に制御することが多い。従って苛性化後、生成した炭酸カルシウムを分離した白液の温度は９０℃前後である事が多い。
【００４０】
本発明の場合、消和工程の温度は、特に純度の低い生石灰の場合、消和温度が低いと充分な消和反応が進み難く、更に後段の苛性化反応速度も高温ほど有利である事などから、少なくとも１００℃以上で操業することが好ましい。一方、１０５℃より高くする場合には消和装置で沸点を超えるため、加圧型の消和装置や苛性化装置等を必要とするため不経済である。
【００４１】
更に、消和槽における滞留時間は、未反応の生石灰が残留する事の無い充分な消和時間を設定する必要が有る。但し、生石灰の消和反応は非常に速く、特に限定する項目ではないが、従来の一般的なスレーカーの滞留時間である１０〜３０分間で充分であった。
【００４２】
本発明の苛性化方法で最も重要な制御因子の一つに初段苛性化槽の滞留時間が挙げられる。この滞留時間の制御は初段苛性化槽の容量によって実施でき、初段苛性化槽に流入する石灰乳と緑液から成る苛性化反応液の流量と初段苛性化槽の容積から求められる。この滞留時間は一般的な苛性化法では一槽当たりの滞留時間が1時間から２時間で、多段の苛性化槽から成る事が多い。本発明の大きな特徴として２時間から１０時間が規定される。この滞留時間は１槽当たりの滞留時間で、長時間ほど平均粒子径の大きい炭酸カルシウム粒子に成長する為、希望する粒子径に応じて適宜選択する。また滞留時間を長く設定することから、必ずしも多段の苛性化槽の設置を必要とせず、１槽でも充分な苛性化率が得られる事を確認している。
【００４３】
該石灰乳と緑液の混合には、前記消和工程と同様の混合機が使用できるが、苛性化反応液は消和工程の石灰乳に比べ濃度低下によって粘度が低下するため、一般的な攪拌羽根式、ポンプ式、等の攪拌機が使用可能となる。
【００４４】
初段苛性化槽の他の制御因子に温度条件がある。従来、一般に操業されている苛性化工程の温度は、回収ボイラーからの溶融した高温のスメルト、苛性化工程の生石灰の消和反応による発熱等を熱源とし、高温ほど苛性化反応速度が速い事などから、少なくとも９０℃以上で操業されることが多い。しかしながら、白液の生産と同時に炭酸カルシウムを製造する本発明の場合、苛性化反応温度の選択によって炭酸カルシウム粒子の粒子径が制御できることを明らかにした。
【００４５】
初段苛性化槽の温度については、反応温度が３０〜１０５℃で行う必要がある。１０５℃より高くする場合には、消和装置や苛性化槽で沸騰点を超えるため、加圧型の消和装置や苛性化装置等を必要とするため不経済である。
【００４６】
苛性化反応温度が３０〜１０５℃の場合、高温では低温に比べ小型の炭酸カルシウム粒子に成長する傾向が見られ、希望する粒子径に応じて適宜選択する。
一方３０℃より低い場合には、目標とする平均粒子径の炭酸カルシウムが得られない訳ではないが反応速度が極端に遅くなる為、容積の大きな苛性化槽を必要とする。さらに、反応前の緑液の冷却工程に大型の冷却装置が必要になり、又白液を高温の蒸解に使用するには白液の加熱が必要で、経費がかさみ不経済である。
【００４７】
白液と炭酸カルシウムの分離、及び炭酸カルシウムに同伴する白液成分の洗浄除去方法は、通常の固液分離装置、例えばクラリファイヤーによる重力沈降分離方式、オリバーフィルターに代表される減圧濾過分離方式、及びリーフ濾過機に代表される加圧濾過分離方式、遠心力を利用した遠心濾過方式等、用件が達成できれば何れの分離装置も利用できる。これらの分離装置における炭酸カルシウムの分離特性は炭酸カルシウムの粒子径が重要な因子であることは自明であり、本発明の大きな特徴である大型の粒子はこれら分離装置や洗浄装置の負荷を大幅に低下できる。一方、従来の苛性化法で得られる炭酸カルシウム粒子は粒子表面の形状が複雑な凹凸から成るが、本発明で得られる炭酸カルシウムの粒子表面は単純で凹凸も少ない。従って、比表面積が小さく、これも分離洗浄性の有利な理由となっている。
【００４８】
一方、分離回収した炭酸カルシウムの粒径は、そのままでは大粒径なため工業用原料として使用するには充分な品質が得られない場合が多い。従って、目的とする品質要求に応じて平均粒子径を０．２〜１０μｍに調整する事が望ましい。粒径の調整はサンドミルに代表される媒体攪拌型粉砕機、ボールミル粉砕機、等の粉砕機が利用できる。
【００４９】
本発明によって得られる炭酸カルシウムは、従来の苛性化工程で得られた炭酸カルシウムに比べて、結晶成長に伴って結晶内に取り込まれる狭雑物が少なく、加えて洗浄性に優れ、白色度に優れた特徴を与える。このことは、製紙用のほか、ゴム、プラスチック、ペイント、シーリング剤、粘着剤等にも有益に使用可能である。
【００５０】
この様な苛性化反応条件下に生成した炭酸カルシウムは、工業原料としての優れた品質ばかりでなく、主生産物である白液が工業的規模で生産が可能になる為、軽質炭酸カルシウムを安価に提供できる。
【００５１】
【作用】
本発明のメカニズムについては充分に解明されてはいないが、現象論的には原料の一つである生石灰の炭酸カルシウム含有量、及び消和反応時の石灰濃度、消和反応で使用する白液、緑液又はその混合物等消和液のトータルアルカリ濃度、及び炭酸ナトリウム濃度が、苛性化反応によって生成する炭酸カルシウム結晶の品質に大きく影響を与える事。その外、消和反応、苛性化反応の際の温度や苛性化反応時の石灰乳と緑液の混合条件、及び初段の苛性化槽における滞留時間によって、生成する炭酸カルシウムの品質が大きく変化する事を確認している。
【００５２】
これらの現象から推察すると、従来一般に行なわれて来た苛性化方法では、緑液とこの緑液を苛性化して白液を製造するに必要な生石灰をスレーカーで同時に混合し、生石灰の消和反応と苛性化反応を同時に進行させる方式であった。この様に消和反応と苛性化反応が同時に進行する為、それぞれの反応に適した反応条件を同時に制御する事が不可能であった。
【００５３】
そこで改良法として、消和反応と苛性化反応を分離し、夫々の反応条件を個別に設定した。即ち、スレーカーでは炭酸イオンを含まない例えば水、若しくは実質的に生成した水酸化カルシウムとは苛性化反応をほとんど起こさない例えば白液、或いは消和反応に必要な水を充分に持ち込みながら生石灰に対し圧倒的に少ない炭酸イオン量の例えば緑液による高濃度消和等、生石灰を適正な条件で充分に消和反応を起こさせた後、この消和石灰と苛性化反応すべき量の緑液を改めて添加混合し、適正な苛性化条件で反応する事によって、蒸解に必要な白液を製造すると同時に、工業原料として有用な高品質炭酸カルシウムを生産可能にした。
【００５４】
更に、苛性化反応条件として最初の苛性化反応を起こす反応槽での滞留時間を規定し、生成する炭酸カルシウム結晶粒子の大きさや形状を制御する事が可能になり、白液分離性や洗浄性を改良した。以上述べたように、消和反応と苛性化反応を分離した事によって、それぞれの反応条件を精度良く制御出来る事がメカニズムを考える上で重要な意味を持つ。
【００５５】
消和工程では生石灰の消和反応が進行して消石灰への転化が起こり、固体の消石灰の一部は溶解する。ところが、従来の苛性化法では緑液が持ち込む水と生石灰に消和反応が起こるが、固体の生石灰の表層は消石灰に転化し、その一部は溶解する。しかし、炭酸イオンが多量に存在する為、溶解した消石灰は急速に苛性化反応を起こすため、固体表面に炭酸カルシウムを形成する。従って、粒子内部の生石灰の新たな消和反応と消石灰の溶解を阻害する。この傾向は反応が進むほど生石灰や消石灰を取り巻く炭酸カルシウム層が厚くなり、益々強くなる。この様な反応が生石灰と消石灰が駆逐されるまで継続する為、結果的に凝集晶と呼ばれる小型の結晶粒子が集合状に結合した二次粒子を形成する。
【００５６】
一方、白液による消和反応や緑液による高濃度消和反応のように、混入する炭酸ナトリウム量を制限する事によって、消石灰粒子表面の炭酸カルシウムの生成は抑えられ、生石灰の充分な消和反応が達成可能となる。従って、次工程の苛性化工程では均一な苛性化反応が起き易くなり、従来の苛性化法と同等の滞留時間では微細な炭酸カルシウム粒子が生成する。
【００５７】
初段苛性化槽内では、炭酸カルシウム結晶核の発生と既に発生した結晶自身が結晶核になって結晶成長が同時に進む。初段苛性化槽には消石灰がカルシウムイオンの供給元として、又緑液が炭酸イオンを供給し、炭酸カルシウムの過飽和度が高いと結晶核発生が優勢となって粒子径は微細に、逆に過飽和度が低いほど結晶成長が優勢になる。従って、苛性化槽の滞留時間が短い程炭酸カルシウムの過飽和度が高いため結晶核発生が優勢となって粒子径は微細になり、逆に初段苛性化槽内の滞留時間が長いと槽内で過飽和度が低くなるため結晶成長が優勢になって結晶粒子が大型になる。
【００５８】
本発明の場合、初段苛性化槽の滞留時間が短い場合は、生成する炭酸カルシウムの平均粒子径は小さく、また滞留時間を長くする程大型化の傾向が見られ、滞留時間に関わらず何れもほとんどが一次粒子で凝集晶を示さず、粒子表面が単純な形状を呈す特徴がある。
【００５９】
本発明の炭酸カルシウムの特徴は、一つには結晶核発生に対し結晶成長を優先させて狭雑物を少なくした高品質炭酸カルシウムであり、二つには白液分離と洗浄性に優れた炭酸カルシウムにある。
【００６０】
【実施例】
以下に本発明を実施例および比較例をあげてより詳細に説明するが、当然ながら本発明は実施例のみに限定されるものではない。
【００６１】
《試験法》
▲１▼ アルカリ分析法： TAPPI６２４ｈｍ−８５， TAPPI６２５ｈｍ−８５あるいはこれに準じて測定した。
▲２▼ 生石灰平均粒度：ＪＩＳＲ９００１−１９９３に準じ、乾式操作にて測定した。
▲３▼ 生石灰中の炭酸カルシウム含量：金属中炭素分析装置(堀場製作所ＥＭＩＡ−１１０)により炭素含量を測定し、その量より炭酸カルシウム含量を算出した。
▲４▼ 炭酸カルシウム平均粒子径：生成物を水洗濾過し、水で希釈後、レーザー回折式粒度分布計(シーラス社製モデル７１５)で重量平均粒子径を測定した。
▲５▼ 濾過速度：リーフテスター（宮本製作所製ＶＲ−２３）を使用し、差圧：０．７Ｋｇ／ｃｍ²、ケーキ厚：２０ｍｍの濾液通過速度を求めた。
【００６２】
［比較例１］
攪拌機（攪拌速度４５０ｒｐｍ、ＫｙｏｅｉＰｏｗｅｒＳｔａｉｒｒｅｒＴｙｐｅＰＳ−２Ｎ）、及び加熱用のマントルヒーターを備え、流入管及びオーバーフロー管付セパラブルフラスコを２セット、直列に連結して苛性化反応装置とした。＃１フラスコ（容積１２５ｍＬ）を消和槽、＃２フラスコ（容積５００ｍＬ）を苛性化槽とした。特に断らない限り以下の実施例についても同様の反応装置を用いた。
【００６３】
消和槽とした＃１フラスコに、白液（組成：Ｎａ₂ＣＯ₃＝２１ｇ／Ｌ、Ｎａ₂Ｓ＝２５ｇ／Ｌ、ＮａＯＨ＝８３ｇ／ＬいずれもＮａ₂Ｏ換算値、苛性化率＝８０％、特に断らない限り以下の実施例について同様）を毎時２５０ｍＬ注入し、同時に炭酸カルシウム含有率１．３％の工業用特号生石灰（平均粒度：０．４ｍｍ、宇部マテリアルズ社製）を毎時３０ｇ加え、消和反応させた。従って、消和槽の滞留時間は約０．５時間である。
【００６４】
苛性化槽とした＃２フラスコにおいて、＃１フラスコからのオーバーフローによって流入する石灰乳に、緑液（組成：Ｎａ₂ＣＯ₃＝９５ｇ／Ｌ、Ｎａ₂Ｓ＝２５ｇ／Ｌ、ＮａＯＨ＝１２ｇ／ＬいずれもＮａ₂Ｏ換算値、苛性化率＝１１％、特に断らない限り以下の実施例についても同様）を毎時２５０ｍＬ注入した。従って、苛性化槽での滞留時間は約１．０時間である。
【００６５】
反応系が充分安定した後、＃２フラスコ出口の反応液のリーフテストによる濾過速度測定結果は３．５ｍ／Ｈｒであった。
試験終了時の＃１と＃２フラスコの温度はそれぞれ１０２℃、及び９９℃であった。生成した炭酸カルシウムは白液を濾過分離回収した後、水道水で充分にろ過洗浄し、各種試験に供した。生成した炭酸カルシウムは立方体状の結晶が多数集合した凝集晶を呈し、平均粒子径８．２μｍであった。
【００６６】
この例に見られるように、初段の苛性化槽の滞留時間が１５分以下好ましくは５分程度であれば、一次粒子が分散した米粒状炭酸カルシウムを生成するが、滞留時間が概ね３０分から９０分の間では、上記のように不特定形状の凝集晶を形成することを確認している。
【００６７】
尚、反応によって得られた白液の苛性化率は６７％で、苛性化槽での滞留時間が短かいために苛性化反応が不充分と思われた。
［実施例１］
消和槽とした＃１フラスコに供給する生石灰を炭酸カルシウム含有量５．５％、平均粒度２ｍｍの苛性化キルン焼成生石灰に変更し、苛性化槽容積を１０００ｍLとした以外は、比較例１と同条件で苛性化実験を行なった。従って、苛性化槽での滞留時間は約２．０時間である。試験条件及び結果を第１表に示す。回収した炭酸カルシウムは平均粒子径５．３μｍであった。また、電子顕微鏡観察の結果、平均長径３．６μｍ、平均短径１．２μｍである米粒状炭酸カルシウムと、平均長径１０μｍ、平均短径５μｍである炭酸カルシウムの一次粒子が主体であったが、一部には分散の不充分な凝集晶も散見された。
【００６８】
［実施例２］
＃１フラスコに添加する生石灰を炭酸カルシウム含有量８．５％、平均粒度１．６ｍｍの苛性化キルン焼成生石灰に変更し、苛性化槽容積を３０００ｍLとした以外は、実施例２と同条件で苛性化反応試験を行なった。従って、苛性化槽での滞留時間は約５．８時間である。試験条件及び結果を第１表に示す。回収した炭酸カルシウムは平均粒子径９．２μｍであった。また、電子顕微鏡観察の結果、平均長径１０μｍ、平均短径５μｍである炭酸カルシウム一次粒子の比率が、実施例１に比べ明らかに増加傾向にあった。
【００６９】
［実施例３］
＃１フラスコで生石灰を消和する消和液の苛性化率を白液と緑液の混合によって調節した。更に反応系内の炭酸ナトリウム増加に見合う生石灰流量を補足するとともに、苛性化槽容積を５０００ｍLとした以外は、比較例１と同条件で苛性化試験を行なった。従って、苛性化槽での滞留時間は約９．８時間である。消和液組成、その他試験条件と試験結果を第１表に示す。回収した炭酸カルシウムは平均粒子径１０．７μｍであった。また、電子顕微鏡観察の結果、平均長径１５μｍ、平均短径７μｍである炭酸カルシウム一次粒子の比率が、実施例２に比べ更に増加傾向のあることを確認した。
【００７０】
［実施例４］
＃２フラスコの温度を６０℃に制御し、苛性化槽容積を３０００ｍLとした以外は、比較例１と同条件で苛性化試験を行なった以外は、比較例１と同一条件で苛性化試験を行った。従って、苛性化槽での滞留時間は約５．９時間である。消和液組成、その他試験条件と試験結果を第１表に示す。回収した炭酸カルシウムは平均粒子径８．５μｍで、電子顕微鏡観察の結果、平均長径４．５μｍ、平均短径１．０μｍである紡錘状炭酸カルシウムと、平均長径１０μｍ、平均短径５μｍである炭酸カルシウムの一次粒子が主体であった。
【００７１】
［実施例５］
＃１フラスコに注入する消和液を緑液に変更し、消和反応を生石灰基準で１９．４％とした。又＃２フラスコの滞留時間を約７．７％以外は比較例１と同条件で苛性化反応試験を行なった。従って、苛性化槽での滞留時間は約７．７時間である。試験結果を第１表に示す。また、電子顕微鏡観察の結果、平均長径１２μｍ、平均短径８μｍである炭酸カルシウム一次粒子と平均長径２μｍ、平均短径０．８μｍである米粒状炭酸カルシウム一次粒子の混合物であることが観察された。
【００７２】
【表１】

【００７３】
【発明の効果】
従来の苛性化方法の場合、スレーカーで生石灰と緑液を混合する。従って、消和反応と苛性化反応がほとんど同時に進み、両反応に適した反応条件を個々に設定する事は困難であり、苛性化反応終了後に白液から分離した炭酸カルシウムは立方体を主体にした結晶がランダムに結合した凝集晶を形成する。一方、結晶の発生速度や成長速度も速く、反応系内に共存する異物も狭雑物として結晶に取り込まれる。
【００７４】
以上の状況から、白液からの炭酸カルシウムの分離や同伴する白液成分の洗浄除去が困難な炭酸カルシウム結晶が生成し、分離洗浄設備に多大な労力を必要としているばかりでなく、生成した炭酸カルシウムを苛性化工程から抜き取って工業用原料として活用を考える時，結晶に取り込まれた異物が原因となって高品質炭酸カルシウムとしての応用範囲が限られてしまう。
【００７５】
そこで、これらの課題を解決する手段として本発明に示したように、消和反応と苛性化反応条件を規定して、苛性化工程で蒸解用薬品である白液の製造の副産物である炭酸カルシウムを高品質工業用原料として提供することを可能にした。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing calcium carbonate in a causticizing step for producing a cooking liquor in a pulp production step by a sulfate method or a soda method, and more particularly, a calcium carbonate content rate of a lime used, The present invention relates to a method for producing calcium carbonate characterized by large single particles, which cannot be obtained by a conventional causticizing method, by specifying reaction and causticizing reaction conditions.
[0002]
[Prior art]
Calcium carbonate is important for industrial materials such as rubber, plastics, paints, sealants, adhesives, fertilizers, etc., as well as heavy calcium carbonate obtained by mechanically pulverizing natural limestone dry or wet, There is precipitated calcium carbonate (synthetic calcium carbonate) obtained by chemical methods.
[0003]
Japan has abundant high-quality natural limestone veins and has been mined and used for a long time. However, the quality depends solely on the quality of the veins, and the high-quality calcium carbonate veins required in recent years are declining, and gradually there are signs of change as required for more expensive foreign products. Currently.
[0004]
Accordingly, light calcium carbonate (precipitated calcium carbonate) synthesized by a chemical method is attracting attention. The light calcium carbonate production method includes (1) a reaction between carbon dioxide-containing gas obtained from a lime baking apparatus and the like and lime milk, (2) a reaction between ammonium carbonate and calcium chloride in the ammonia soda method, (3) The reaction of lime milk and sodium carbonate, etc., in which sodium hydroxide is produced by causticization of sodium carbonate, is known. Among these methods, in (2) and (3), the production method for obtaining the main product has been changed to a new method, and since calcium carbonate is a byproduct, its utilization method has not been studied much. Absent. On the other hand, (1) has a relatively simple reaction system (water, slaked lime, carbon dioxide), and extensive research has been conducted on methods for producing various shapes of calcium carbonate. This trend is expected to continue. However, since this method is very expensive because calcium carbonate is the only product, it is not suitable for cost reduction requested by the user and cannot be provided at low cost.
[0005]
One conceivable method is to use calcium carbonate produced as a by-product in the causticizing process for recovering and regenerating cooking chemicals in the pulp manufacturing process by the sulfate method or the soda method.
[0006]
In the pulp manufacturing process by the sulfate method or the soda method, in order to isolate fibrin from wood, cooking is performed at high temperature and pressure using white liquor in which sodium hydroxide or sodium sulfide is dissolved. Fibrin is separated and purified as a solid phase into pulp, and cooking waste liquid (black liquor) is concentrated and burned. At that time, the eluted component from the wood is recovered as a heat source, and the inorganic substance in the chemical solution is recovered as a smelt mainly composed of a mixture with sodium carbonate or sodium sulfide. Smelt is dissolved in a calcium carbonate cleaning solution in which a white liquor component called a weak solution is partially dissolved to form a green solution.
[0007]
This green liquor and quicklime are mixed to convert sodium carbonate into sodium hydroxide, which is useful as a cooking chemical, by the soothing and causticizing reactions shown in the formulas [1] and [2]. Calcium is a by-product. In conventional pulp mills, green liquor and quicklime are mixed in a reaction tank called a slaker. In practice, this two-stage reaction proceeds considerably overlappingly, and soaking and causticization occur almost simultaneously.
[0008]
CaO + H ₂ O → Ca (OH) ₂ [1]
Ca (OH) ₂ + Na ₂ CO _Three → CaCO _Three + 2NaOH [2]
The calcium carbonate produced here is a by-product in producing white liquor, which is the main product, so when used as an industrial raw material, it is not only available at a very low cost, but also caustic which is a conventional closed system. By extracting calcium carbonate out of the system from the calcium (quick lime, calcium carbonate) circulation cycle in the crystallization process, high purity of the circulating lime is achieved, and the above-mentioned [1] and [2] are improved in reactivity and clarified white liquor. Improvement and further reduction of waste.
[0009]
However, since the reactions [1] and [2] occur almost simultaneously in this conventional method, it is difficult to control the shape of the obtained calcium carbonate, and it has various shapes such as dice and hexahedron. The diameters varied widely and most were massive, and rapid crystal growth produced low-purity calcium carbonate that included green liquor-derived constrictions. Therefore, when this is produced as an industrial raw material, it has problems such as inadequate for the production of high-quality calcium carbonate which has been demanded particularly in recent years.
[0010]
Thus, in the conventional method, the purity of calcium carbonate is low, and it is difficult to produce high-quality calcium carbonate for industrial use.
[0011]
[Problems to be solved by the invention]
In the conventional causticizing method, quick lime and green liquor are directly mixed by a slaker, so the reactions [1] and [2] start almost simultaneously. Therefore, since the carbonate and calcium ion concentrations are high, crystal generation and crystal growth proceed rapidly, and the generated calcium carbonate particles have various shapes such as dice and hexahedron, and the particle sizes vary widely. Most of them exhibit aggregated crystals with a large specific surface area. Furthermore, the rapid growth of crystals tends to easily include green liquor-derived impurities, and the quality of calcium carbonate has been lowered. In addition, for the purpose of producing white liquor, which is the original purpose, it is essential to separate the white liquor from calcium carbonate and to remove the white liquor components accompanying the separated calcium carbonate, but this process requires a great deal of labor. Came as.
[0012]
Accordingly, an object of the present invention is to solve such problems and develop a method for producing high-grade calcium carbonate.
[0013]
[Means for Solving the Problems]
As a result of intensive studies to solve the above-mentioned problems, the present inventors made use of a causticizing step of a pulp manufacturing process by a sulfate method or a soda method, to obtain quick lime containing calcium carbonate of a specific amount or less. In the causticizing tank, obtained in the decontamination step, following the decontamination step of continuously decontaminating with a liquid selected from the group consisting of white liquor, green liquor or a mixture thereof at a specific lime concentration. Lime milk and / or lime mud and the same amount of green liquor that is generated in the causticizing process of the pulp manufacturing process by the sulfate method or soda method and is required to produce white liquor in conventional operations A causticizing method was obtained by mixing and specifying the residence time of the reaction liquid to advance the causticizing reaction to obtain a white liquor and high-grade calcium carbonate equivalent to those produced in the conventional operation. The calcium carbonate obtained by the above causticizing method has a simple shape, a small specific surface area, a large average particle size, easy washing, high whiteness after pulverization, and excellent industrial raw materials can be obtained.
[0014]
The concentration of quicklime in the septic tank of the present invention with respect to the white liquor or green liquor, particularly the concentration in the case of dehydration reaction with the green liquor, is very important for controlling the quality of calcium carbonate produced by the subsequent causticizing reaction. is important. The concentration of quicklime in white liquor shows higher viscosity as the concentration increases, but any concentration is acceptable as long as it can substantially perform the decontamination reaction and the subsequent causticization reaction from low to high concentrations. It can be set. In the case of quicklime and green liquor, calcium carbonate of improved quality can be obtained at high concentrations, but the quality of improved calcium carbonate cannot be obtained as the concentration becomes closer to the conventional causticization method, and the conventional solution is used at the lowest concentration side. The reaction conditions would be equivalent to the causticizing method.
[0015]
The residence time of the causticizing reaction solution in the causticizing tank of the present invention is very important for controlling the quality of calcium carbonate produced by the causticizing reaction, and the residence time is preferably longer, but the residence time is 2 hours. From 10 to 10 hours, preferably from 4 to 6 hours. If the residence time is less than 2 hours, which is the residence time of a conventional causticizing tank, the causticization reaction proceeds rapidly and the particle shape tends to form aggregated crystals, so the improvement in quality and separability from white liquor is insufficient. It is. When the residence time of the causticizing tank exceeds 10 hours, the quality and separability are sufficient, but a large causticizing tank is required, which is economically disadvantageous.
[0016]
In the conventional causticizing process, 2 to 4 stages of causticizing tanks are installed as causticizing tanks following the slaker, but the causticizing tank in the case of this development method is a method of defining the first stage residence time. About. Accordingly, several stages of causticizing tanks following the first stage causticizing tank are appropriately installed according to the causticization rate of the white liquor.
[0017]
By completing the present invention, the average particle size of the calcium carbonate to be produced can be increased, so that not only the equipment load in the white liquor separation and calcium carbonate washing process is reduced, but also obtained by the conventional reaction between lime milk and carbon dioxide. Compared to calcium carbonate, it can be manufactured at low cost. Furthermore, the productivity of calcium carbonate produced in the causticizing process is greatly improved by the technology of the present invention, and as an accompanying effect, the amount of calcium carbonate extracted from the process is increased and the load on the kiln for firing is reduced. Can be achieved. Further, depending on the amount of calcium carbonate extracted from the process, the kiln can be stopped, and the production cost of white liquor, which is the main product in the causticizing process, can be greatly reduced.
[0018]
The same effect can be obtained by carrying out causticization with DTB (Draft Tube Buffle Crystallixer) instead of the first stage causticizing tank of the present invention. However, as a characteristic of the causticizing reaction step, calcium carbonate is precipitated in the tank, so that the scale adheres to the wetted part of the apparatus, and a complicated apparatus configuration is not preferable in terms of maintenance.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
The quicklime used in the soaking process of the present invention converts sodium carbonate into sodium hydroxide in the causticizing process of the pulp production process by the natural limestone mainly composed of calcium carbonate and / or the sulfate process or the soda process. What is necessary is just to calcinate the calcium carbonate produced | generated in the case. Regarding the firing apparatus at that time, carbon dioxide such as Beckenbach furnace, Melz furnace, rotary kiln, Kunii type furnace, KHD (Kerhardy) furnace, Koma type furnace, Kalmatic furnace, fluidized firing furnace, mixed baking furnace, etc. If it is an apparatus which converts calcium into quicklime (calcium oxide), it will not be restricted in particular.
[0020]
Regarding the content of impurities in quicklime used for the causticizing reaction, metal elements such as Fe, Al, Mn are particularly problematic as components for coloring calcium carbonate. The obtained quicklime is appropriately selected. Or in the case of recalcined quicklime from a rotary kiln or fluidized calciner in the causticizing process, because the coloring component content etc. changes depending on the ratio of calcium carbonate extracted outside the system and calcium carbonate recirculated inside the system, etc. Depending on the situation, the amount of raw material limestone with a low coloring component content to be replenished to the calcium circulation cycle of the causticizing process or the amount of quick lime obtained by calcining this may be used.
[0021]
Quick lime is a quick lime obtained by decarboxylation of calcium carbonate generated in the causticizing process in a kiln or other baking furnace, or quick lime supplied from the outside as a supplement for calcium carbonate extracted from the causticizing process, or a supplemental limestone and process. Quick lime obtained by mixing and calcining circulating calcium carbonate in the kiln, and quick lime obtained by mixing these quick limes in an arbitrary ratio can be used.
[0022]
As for the calcium carbonate content in quicklime, good results are obtained when it is 0.1 to 10% by weight, preferably 5% or less, based on the weight of quicklime. As a matter of course, calcium carbonate in quicklime does not cause a soothing reaction and a causticizing reaction, and does not change in chemical composition until it is finally used as an industrial raw material. However, calcium carbonate mixed in the causticizing reaction system acts as a seed crystal in the crystal growth stage, and causes newly formed calcium carbonate to be induced in an indeterminate form or in a lump shape, thus reducing the quality of the obtained calcium carbonate. . When the calcium carbonate content exceeds 10% by weight, these tendencies become remarkable, which is not preferable.
[0023]
On the other hand, in order to obtain a material having a content of 0.1% by weight or less, the energy required for firing is extremely increased, or a special device is required for the firing apparatus, which is uneconomical.
[0024]
Although there is no restriction | limiting in particular regarding the particle size of quicklime, The thing with an average particle diameter of 0.01 mm-10 cm, Preferably 0.01 mm-10 mm is suitable in terms of uniform mixing in the stirring at the time of decontamination. When the particle size is 0.01 mm or less, the cost for pulverization is high, and at the same time, generation of dust and troubles in the transfer device are undesirable.
[0025]
The white liquor or green liquor used for the dehydration of quicklime in the dehydration process is 80 to 160 g / L (Na ₂ O conversion: the same applies hereinafter), preferably 100 to 150 g / L. When it is lower than 80 g / L, the concentration of the final white liquor is lowered, so that it is necessary to adjust the concentration when used for cooking. On the other hand, when it is higher than 160 g / L, it is not preferable because crystals such as sodium carbonate are easily generated in the green liquor before causticizing, causing trouble in the process.
[0026]
A causticizing rate represented by the following formula [3] is generally used as one of the indicators of operation management in the causticizing process for producing white liquor. Incidentally, the concentration of sodium hydroxide and sodium carbonate is Na ₂ Shown in O conversion (g / L).
[0027]
In the causticizing step, a two-step reaction of a decontamination reaction and a causticizing reaction represented by the following formulas [1] and [2] proceeds, and the causticizing reaction of formula [2] is a reversible reaction. _Three Solubility of Ca (OH) ₂ Because it is smaller than that, the reaction goes to the right. However, this reaction does not proceed beyond the equilibrium causticization rate determined by the [OH] concentration of the reaction solution, and the causticization rate in a general causticization process is in the range of 65 to 85%. Many.
CaO + H ₂ O → Ca (OH) ₂ [1]
Ca (OH) ₂ + Na ₂ CO _Three → CaCO _Three + 2NaOH [2]
Causticization rate (%) = sodium hydroxide concentration / (sodium carbonate concentration + sodium hydroxide concentration) × 100 [3]
We extended the definition of this causticization rate not only to white liquor, but to the dehydration solution containing green liquor used for the decontamination of quicklime as referred to in the present invention. This is because the so-called green liquor having a low causticization rate is obtained by dissolving smelt from a boiler in a weak liquor obtained by washing and recovering a part of the white liquor component accompanying the calcium carbonate produced by the causticization reaction. Therefore, the realistic green liquor contains sodium hydroxide derived from white liquor, and the amount of the accompanying white liquor component dominates the level of causticization of the green liquor. Moreover, even if it is outside the causticizing rate range of white liquor or green liquor which is generally called, it can be a sufficiently effective decontamination solution for the present invention.
[0028]
There is a close relationship between the concentration of sodium carbonate and lime in the white liquor or green liquor used in the soaking process.
Good results can be obtained from a low concentration to a high concentration of quicklime when the white liquor with a low sodium carbonate concentration is used. However, good results can be obtained only when the lime concentration in the green liquor with high sodium carbonate concentration is relatively high.
[0029]
As a result of repeated research on the relationship between the sodium carbonate concentration of a dehydrating solution such as white liquor and green liquor used to dehydrate quick lime and the lime concentration at the time of decontamination, the present inventors The lime concentration is 1.0% by weight or more and 77.0% by weight or less based on the quick lime before squeezing, and squeezing is performed under the conditions satisfying both of the following conditions (a) and (b). , For the first time, it was discovered that the conditions are soothing to obtain calcium carbonate that fulfills the intended purpose.
[0030]
0 ≦ X ≦ Equilibrium causticization rate of decontamination solution (a)
15-0.3X ≦ Y ≦ 77 (b)
However,
X = Causticizing rate of decontamination solution (wt%)
Y = lime concentration based on quick lime before waking (% by weight)
Causticization rate (%) = sodium hydroxide concentration / (sodium carbonate concentration + sodium hydroxide concentration) × 100
If the soaking liquid is a so-called white liquor and the causticizing rate is 65% or more, the object can be achieved without any problems even if the lime concentration is 1.0% or less. However, in the case of a so-called green liquor having a lower causticization rate, it is necessary to reduce the lime concentration as shown in (c) below, preferably above the lime concentration shown in (d) below.
[0031]
20−0.35X ≦ Y ≦ 77.0 (c)
25-0.40X ≦ Y ≦ 77.0 (d)
However, X = causticization rate of decontamination solution (%)
Y = lime concentration based on quick lime before waking (% by weight)
Speaking of the lime concentration at the time of dehydration with a general white liquor (causticization rate = 65 to 80%), the concentration is 0.5 to 77% by weight, preferably 3.5 based on the quick lime before the dehydration. It is necessary to carry out at -50% by weight. If it exceeds 77% by weight, the water required for the quick lime to subside is insufficient, so that substantially sufficient sublimation cannot be achieved. In addition, if it exceeds 50% by weight, it will be suitable to be called powdered slaked lime rather than lime milk or lime mud. It is necessary to supply powdered slaked lime simultaneously with the green liquor, and a special addition device is required.
[0032]
Therefore, in reality, it is desirable to eliminate at 50 wt% or less, which is easy to stir the slurry and flow down to the causticizing tank. On the other hand, if it is less than 0.5% by weight, the productivity of white liquor and calcium carbonate is very poor and not practical.
[0033]
The white liquor used in the decontamination process can be introduced from other processes, but it is convenient in terms of equipment to use the white liquor produced in the same causticizing process. Setting the lime concentration during soaking to a high level is advantageous because the amount of white liquor that circulates can be reduced, and facilities such as a causticizing tank and white liquor separation can be simplified.
[0034]
Similarly, the lime concentration at the time of dehydration with a general green liquor (causticization rate = 5 to 20%) is 15 to 77% by weight, preferably 20 to 50%, based on the lime before the dehydration. It is necessary to carry out by weight%. If it exceeds 50% by weight, the viscosity of the slurry is too high and stirring becomes difficult in practice. On the other hand, if it is less than 15% by weight, the quality of the produced calcium carbonate is impaired, which is not preferable.
[0035]
For the quickening of quicklime that is performed in the soaking process, a white liquor and a green liquor can be mixed and used in an arbitrary mixing ratio. By increasing the ratio of green liquor, it is necessary to increase the concentration of lime at the time of soothing, but since the self-circulation rate of white liquor produced in the causticizing process can be lowered, the size of the white liquor separation process is reduced. It is advantageous in terms of equipment. However, if the blending ratio of green liquor is increased, the concentration of sodium carbonate naturally increases, so it is necessary to increase the lime concentration, and it is not necessary to introduce a special stirring device suitable for high viscosity stirring. It is an economy.
[0036]
From this point of view, as a decontamination solution used for decontamination of quicklime, the causticization rate of the decontamination solution is adjusted by adjusting the mixing ratio of the white liquor and the green liquor. It is also possible to adjust the quality of calcium carbonate, such as the properties and cleanability, with a good balance.
[0037]
In addition, instead of lime milk prepared from quick lime in the sautéing process, calcium hydroxide may be dispersed in white liquor or green liquor, or a mixture thereof, and lime milk adjusted to the same concentration as this method may be used. Is possible. In this case, the alkali concentration of the white liquor is slightly reduced by the water brought in by the slaked lime.
[0038]
For mixing quicklime with white liquor or green liquor during soaking, slurry viscosity at the time of mixing can be selected from general stirring blades, pumps, extruders, kneaders, and kneaders. It may be selected and used as appropriate (see Maruzen Co., Ltd., March 18, 1988, refer to Chemical Engineering Handbook).
[0039]
Conventionally, in the causticizing process that is generally operated, the temperature of the green liquor is often controlled at around 90 ° C. using the molten high-temperature smelt from the recovery boiler as the heat source, and the causticizing temperature is increased by the generation of heat from the quicklime. The temperature is often controlled to around 100 ° C. as a temperature that does not exceed the boiling temperature. Therefore, after causticization, the temperature of the white liquor from which the produced calcium carbonate is separated is often around 90 ° C.
[0040]
In the case of the present invention, especially in the case of quick lime having a low purity, the temperature of the saponification process is low, and if the saponification temperature is low, it is difficult for a sufficient saponification reaction to proceed. Therefore, it is preferable to operate at least at 100 ° C. or higher. On the other hand, when the temperature is higher than 105 ° C., the boiling point is exceeded by the soaking device, which is uneconomical because a pressure-type soaking device or a causticizing device is required.
[0041]
Furthermore, it is necessary to set the residence time in the soaking tank to a sufficient soaking time in which unreacted quicklime does not remain. However, the quick lime soothing reaction is very fast and is not particularly limited, but 10 to 30 minutes, which is the residence time of a conventional general slaker, was sufficient.
[0042]
One of the most important control factors in the causticizing method of the present invention is the residence time of the first stage causticizing tank. This residence time can be controlled by the capacity of the first stage causticizing tank, and can be determined from the flow rate of the causticizing reaction liquid consisting of lime milk and green liquor flowing into the first stage causticizing tank and the volume of the first stage causticizing tank. In a general causticizing method, this residence time is 1 to 2 hours per tank, and it is often composed of a multi-stage causticizing tank. As a major feature of the present invention, 2 to 10 hours are specified. This residence time is a residence time per tank, and grows into calcium carbonate particles having a larger average particle size as the time is longer. Therefore, the residence time is appropriately selected according to the desired particle size. Moreover, since the residence time is set long, it is not always necessary to install a multi-stage causticizing tank, and it has been confirmed that a sufficient causticizing rate can be obtained even with one tank.
[0043]
The mixing of the lime milk and the green liquor can use the same mixer as in the decontamination process, but the causticizing reaction liquid has a lower viscosity due to a decrease in concentration compared to the lime milk in the decontamination process. A stirrer such as a stirring blade type or a pump type can be used.
[0044]
Another control factor of the first stage causticizing tank is the temperature condition. Conventionally, the temperature of the causticizing process that is generally operated is based on the high-temperature smelt melted from the recovery boiler and the heat generated by the decalcification reaction of quick lime in the causticizing process. Therefore, it is often operated at least at 90 ° C or higher. However, in the present invention in which calcium carbonate is produced simultaneously with the production of white liquor, it has been clarified that the particle size of the calcium carbonate particles can be controlled by selecting the causticizing reaction temperature.
[0045]
About the temperature of a first stage causticizing tank, it is necessary to perform reaction temperature at 30-105 degreeC. When the temperature is higher than 105 ° C., the boiling point is exceeded in the simmering device or the causticizing tank, so that a pressure-type sunk device or a causticizing device is required, which is uneconomical.
[0046]
When the causticizing reaction temperature is 30 to 105 ° C., a tendency to grow into small calcium carbonate particles is seen at a high temperature as compared with a low temperature, and it is appropriately selected according to the desired particle diameter.
On the other hand, when the temperature is lower than 30 ° C., calcium carbonate having a target average particle diameter cannot be obtained, but the reaction rate becomes extremely slow, so that a causticizing tank having a large volume is required. Furthermore, a large cooling device is required for the green liquor cooling step before the reaction, and heating of the white liquor is necessary to use the white liquor for high-temperature cooking, which is expensive and uneconomical.
[0047]
Separation of white liquor and calcium carbonate, and washing and removal method of white liquor components accompanying calcium carbonate, ordinary solid-liquid separation device, for example, gravity sedimentation separation method by clarifier, vacuum filtration separation method represented by Oliver filter, Any separation device can be used as long as the requirements can be achieved, such as a pressure filtration separation system represented by a leaf filter, a centrifugal filtration system using centrifugal force, and the like. It is obvious that the calcium carbonate particle size is an important factor in the separation characteristics of calcium carbonate in these separation devices, and the large particles, which is a major feature of the present invention, greatly increase the load on these separation devices and cleaning devices. Can be reduced. On the other hand, the calcium carbonate particles obtained by the conventional causticizing method are composed of irregularities having complicated particle surface shapes, but the calcium carbonate particles obtained by the present invention are simple and have few irregularities. Therefore, the specific surface area is small, which is also an advantageous reason for the separation and cleaning properties.
[0048]
On the other hand, the separated and recovered calcium carbonate has a large particle size as it is, so that in many cases, sufficient quality for use as an industrial raw material cannot be obtained. Therefore, it is desirable to adjust the average particle size to 0.2 to 10 μm according to the desired quality requirement. The particle size can be adjusted by using a pulverizer such as a media stirring type pulverizer represented by a sand mill, a ball mill pulverizer, or the like.
[0049]
The calcium carbonate obtained by the present invention is less confined in the crystal as the crystal grows than the calcium carbonate obtained in the conventional causticizing process, and also has excellent detergency and whiteness. Gives excellent characteristics. This can be beneficially used not only for papermaking but also for rubber, plastic, paint, sealing agent, adhesive and the like.
[0050]
Calcium carbonate produced under such causticization conditions not only has excellent quality as an industrial raw material, but also white liquor, which is the main product, can be produced on an industrial scale. Can be provided.
[0051]
[Action]
Although the mechanism of the present invention has not been fully elucidated, the calcium carbonate content of quick lime, which is one of the raw materials, and the lime concentration during the dehydration reaction, the white liquor used in the deconversion reaction The total alkali concentration and sodium carbonate concentration of the soaking liquid such as green liquor or a mixture thereof greatly affect the quality of calcium carbonate crystals produced by the causticizing reaction. In addition, the quality of the calcium carbonate produced varies greatly depending on the temperature during the causticization reaction, the temperature during the causticization reaction, the mixing conditions of lime milk and green liquor during the causticization reaction, and the residence time in the first stage causticization tank. I have confirmed that.
[0052]
Inferring from these phenomena, in the conventional causticizing method, the green liquor and the quick lime required to produce the white liquor by causticizing the green liquor are simultaneously mixed with a slaker, and the quick lime dehydration reaction And the causticization reaction proceeded simultaneously. In this way, since the soothing reaction and the causticizing reaction proceed simultaneously, it was impossible to control the reaction conditions suitable for each reaction at the same time.
[0053]
Therefore, as an improved method, the soaking reaction and the causticizing reaction were separated, and the respective reaction conditions were set individually. That is, the slaker does not contain carbonate ions, for example, water, or substantially does not cause causticization reaction with calcium hydroxide, for example, white liquor, or water that is necessary for the dehydration reaction, while bringing enough water to quicklime. After causing a quick dehydration reaction of quick lime under appropriate conditions, such as high-concentration decontamination with an overwhelmingly small amount of carbonate ions, for example, green liquor, the amount of green liquor to be causticized with this dehydrated lime By adding and mixing again and reacting under appropriate causticizing conditions, white liquor required for cooking was produced, and at the same time, high-quality calcium carbonate useful as an industrial raw material could be produced.
[0054]
Furthermore, as the causticizing reaction conditions, the residence time in the reaction vessel that causes the first causticizing reaction is specified, and the size and shape of the generated calcium carbonate crystal particles can be controlled. Improved. As described above, by separating the soaking reaction and the causticizing reaction, it is important to consider the mechanism that each reaction condition can be accurately controlled.
[0055]
In the slaked process, the slaked reaction of quick lime proceeds and conversion to slaked lime occurs, and a part of solid slaked lime is dissolved. However, in the conventional causticizing method, the water is brought into the green liquor and quick lime occurs, but the surface layer of solid quick lime is converted to slaked lime, and a part of it is dissolved. However, since a large amount of carbonate ions are present, the dissolved slaked lime rapidly causes a causticizing reaction, so that calcium carbonate is formed on the solid surface. Therefore, the new soaking reaction of the quick lime inside the particles and the dissolution of the slaked lime are inhibited. This tendency becomes stronger as the reaction proceeds, and the calcium carbonate layer surrounding quicklime and slaked lime becomes thicker. Since such a reaction is continued until quicklime and slaked lime are driven out, as a result, secondary particles in which small crystal particles called aggregated crystals are combined in an aggregate form are formed.
[0056]
On the other hand, by limiting the amount of sodium carbonate that is mixed in, such as white liquid and high-concentration green liquid, the formation of calcium carbonate on the surface of slaked lime particles can be suppressed, and quick lime can be sufficiently eliminated. The reaction can be achieved. Accordingly, a uniform causticizing reaction is likely to occur in the next causticizing step, and fine calcium carbonate particles are generated with a residence time equivalent to that of the conventional causticizing method.
[0057]
In the first stage causticizing tank, the generation of calcium carbonate crystal nuclei and the already generated crystals themselves become crystal nuclei, and crystal growth proceeds simultaneously. In the first stage causticizing tank, slaked lime is the source of calcium ions, and green liquor is supplied with carbonate ions. When the supersaturation degree of calcium carbonate is high, the generation of crystal nuclei becomes dominant and the particle size becomes finer. The lower the degree, the more dominant the crystal growth. Therefore, the shorter the residence time in the causticizing tank, the higher the supersaturation degree of calcium carbonate, so the generation of crystal nuclei becomes dominant and the particle size becomes finer. Conversely, if the residence time in the first stage causticizing tank is long, Since the degree of supersaturation is low, crystal growth becomes dominant and crystal grains become large.
[0058]
In the case of the present invention, when the residence time of the first stage causticizing tank is short, the average particle diameter of the generated calcium carbonate is small, and as the residence time is lengthened, the tendency to increase in size is seen. Most of them are primary particles and do not show aggregated crystals, and the particle surface is characterized by a simple shape.
[0059]
The features of the calcium carbonate of the present invention are high-quality calcium carbonate in which the crystal growth is given priority over the crystal nucleus generation and the number of contaminants is reduced, and the second is excellent in white liquor separation and detergency. Located in calcium carbonate.
[0060]
【Example】
The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is naturally not limited to the examples.
[0061]
《Test method》
(1) Alkaline analysis method: TAPPI 624hm-85, TAPPI 625 hm-85, or a similar measurement.
(2) Quick lime average particle size: Measured by dry operation according to JIS R 9001-1993.
(3) Calcium carbonate content in quicklime: The carbon content was measured by a carbon-in-metal analyzer (Horiba Seisakusho EMIA-110), and the calcium carbonate content was calculated from the amount.
(4) Calcium carbonate average particle size: The product was washed and filtered with water, diluted with water, and then the weight average particle size was measured with a laser diffraction particle size distribution meter (Model 715 manufactured by Cirrus).
(5) Filtration rate: Leaf tester (Miyamoto Seisakusho VR-23) is used, differential pressure: 0.7Kg / cm ² Cake thickness: The filtrate passage speed of 20 mm was determined.
[0062]
[Comparative Example 1]
A causticizing reaction apparatus was prepared by connecting two sets of an inflow pipe and an overflow pipe separable flask in series, equipped with a stirrer (stirring speed: 450 rpm, Kyoei Power StylerType PS-2N) and a heating mantle heater. The # 1 flask (with a volume of 125 mL) was used as a decontamination tank, and the # 2 flask (with a volume of 500 mL) was used as a causticizing tank. Unless otherwise noted, similar reactors were used for the following examples.
[0063]
To the # 1 flask used as a septic tank, add white liquor (composition: Na ₂ CO _Three = 21 g / L, Na ₂ Both S = 25 g / L and NaOH = 83 g / L are Na ₂ O-converted value, causticization rate = 80%, unless otherwise specified, 250 mL per hour was injected at the same time, and at the same time industrial special quicklime with a calcium carbonate content of 1.3% (average particle size: 0.4 mm) , Manufactured by Ube Materials Co., Ltd.) was added at 30 g per hour to cause a mild reaction. Therefore, the residence time of the septic tank is about 0.5 hours.
[0064]
In the # 2 flask used as the causticizing tank, the green liquor (composition: Na ₂ CO _Three = 95 g / L, Na ₂ Both S = 25 g / L and NaOH = 12 g / L are Na ₂ O conversion value, causticization rate = 11%, unless otherwise specified, 250 mL / hour was injected. Accordingly, the residence time in the causticizing tank is about 1.0 hour.
[0065]
After the reaction system was sufficiently stabilized, the filtration rate measurement result by leaf test of the reaction solution at the outlet of the # 2 flask was 3.5 m / Hr.
The temperatures of the # 1 and # 2 flasks at the end of the test were 102 ° C. and 99 ° C., respectively. The produced calcium carbonate was filtered and recovered from white liquor, then sufficiently filtered and washed with tap water, and subjected to various tests. The produced calcium carbonate had agglomerated crystals in which many cubic crystals were aggregated, and had an average particle diameter of 8.2 μm.
[0066]
As can be seen from this example, if the residence time in the first stage causticizing tank is 15 minutes or less, preferably about 5 minutes, rice granular calcium carbonate in which primary particles are dispersed is produced, but the residence time is approximately 30 minutes to 90 minutes. In minutes, it was confirmed that aggregate crystals having an unspecified shape were formed as described above.
[0067]
The causticization rate of the white liquor obtained by the reaction was 67%, and the causticization reaction was considered insufficient due to the short residence time in the causticizing tank.
[Example 1]
Comparative Example 1 except that the quick lime supplied to the # 1 flask as a septic tank was changed to causticized kiln calcined quick lime with a calcium carbonate content of 5.5% and an average particle size of 2 mm, and the causticizing tank volume was 1000 mL. The causticization experiment was conducted under the same conditions. Therefore, the residence time in the causticizing tank is about 2.0 hours. The test conditions and results are shown in Table 1. The recovered calcium carbonate had an average particle size of 5.3 μm. Moreover, as a result of observation with an electron microscope, rice granular calcium carbonate having an average major axis of 3.6 μm and an average minor axis of 1.2 μm and primary particles of calcium carbonate having an average major axis of 10 μm and an average minor axis of 5 μm were mainly. In some cases, agglomerated crystals with insufficient dispersion were observed.
[0068]
[Example 2]
# 1 The quick lime added to the flask was changed to causticized kiln calcined quick lime with a calcium carbonate content of 8.5% and an average particle size of 1.6 mm, and the same conditions as in Example 2 except that the causticizing tank volume was 3000 mL. A causticization test was conducted. Accordingly, the residence time in the causticizing tank is about 5.8 hours. The test conditions and results are shown in Table 1. The recovered calcium carbonate had an average particle size of 9.2 μm. Moreover, as a result of observation with an electron microscope, the ratio of primary particles of calcium carbonate having an average major axis of 10 μm and an average minor axis of 5 μm was clearly on an increasing trend as compared with Example 1.
[0069]
[Example 3]
The causticization rate of the deodorizing liquid that dehydrates quicklime in the # 1 flask was adjusted by mixing white liquor and green liquor. Further, a causticizing test was performed under the same conditions as in Comparative Example 1 except that the flow rate of quick lime commensurate with the increase in sodium carbonate in the reaction system was supplemented and the causticizing tank volume was set to 5000 mL. Therefore, the residence time in the causticizing tank is about 9.8 hours. Table 1 shows the composition of the soaking solution, other test conditions, and the test results. The recovered calcium carbonate had an average particle size of 10.7 μm. Further, as a result of observation with an electron microscope, it was confirmed that the ratio of primary particles of calcium carbonate having an average major axis of 15 μm and an average minor axis of 7 μm was further increased as compared with Example 2.
[0070]
[Example 4]
The causticization test was performed under the same conditions as in Comparative Example 1 except that the # 2 flask temperature was controlled at 60 ° C. and the causticizing tank volume was 3000 mL, except that the causticizing test was performed under the same conditions as in Comparative Example 1. went. Therefore, the residence time in the causticizing tank is about 5.9 hours. Table 1 shows the composition of the soaking solution, other test conditions, and the test results. The recovered calcium carbonate has an average particle size of 8.5 μm, and as a result of electron microscope observation, spindle calcium carbonate having an average major axis of 4.5 μm and an average minor axis of 1.0 μm, and an average major axis of 10 μm and an average minor axis of 5 μm. The primary particles were mainly calcium.
[0071]
[Example 5]
The sanitizing liquid poured into the # 1 flask was changed to a green liquid, and the sacrificial reaction was 19.4% based on quick lime. Further, a causticizing reaction test was conducted under the same conditions as in Comparative Example 1 except that the residence time of the # 2 flask was about 7.7%. Therefore, the residence time in the causticizing tank is about 7.7 hours. The test results are shown in Table 1. Moreover, as a result of electron microscope observation, it was observed that the mixture was a mixture of calcium carbonate primary particles having an average major axis of 12 μm and an average minor axis of 8 μm and rice granular calcium carbonate primary particles having an average major axis of 2 μm and an average minor axis of 0.8 μm. .
[0072]
[Table 1]

[0073]
【The invention's effect】
In the case of the conventional causticizing method, quicklime and green liquor are mixed with a slaker. Therefore, the soothing reaction and causticizing reaction proceed almost simultaneously, and it is difficult to individually set reaction conditions suitable for both reactions, and the calcium carbonate separated from the white liquor after the causticizing reaction is mainly composed of cubes. Aggregated crystals in which the crystals are randomly bonded are formed. On the other hand, the generation rate and growth rate of crystals are high, and foreign substances coexisting in the reaction system are also taken into the crystals as impurities.
[0074]
From the above situation, calcium carbonate crystals are difficult to separate from white liquor and accompanying white liquor components are difficult to wash and remove. When calcium is extracted from the causticization process and considered to be used as an industrial raw material, the scope of application as high-quality calcium carbonate is limited due to foreign substances incorporated into the crystals.
[0075]
Therefore, as shown in the present invention as a means for solving these problems, calcium carbonate, which is a by-product of the production of white liquor that is a cooking chemical in the causticizing process, defines the conditions for the soothing reaction and causticizing reaction. Can be provided as a high-quality industrial raw material.

Claims

A method of producing calcium carbonate in a causticizing step of a pulp manufacturing process by a sulfate method or a soda method,
In the septic tank, the quicklime which is generated in the causticizing process and / or is introduced from outside the process and contains 0.1 to 10% by weight of calcium carbonate based on the weight of the quicklime, and the Lime generated in the causticizing process and / or introduced from outside the process and mixed with a liquid selected from the group consisting of white liquor, green liquor or a mixture thereof, and dehydrated with stirring or kneading. In the causticizing tank and the causticizing tank for preparing milk or lime mud, the lime milk or lime mud introduced from the sacrificing tank and generated in the causticizing process and / or introduced from outside the process, A causticizing step of adding and mixing a predetermined amount of green liquor necessary for producing a white liquor, and performing a causticizing reaction at a reaction temperature of 30 to 105 ° C. with a residence time of 2 hours to 10 hours,
Tona is,
In the said decontamination process, the lime density | concentration at the time of deodorizing quick lime with the liquid selected from the group which consists of white liquor, green liquor, or those mixtures is 1.0 weight% on the basis of the quick lime before dehydrating. Above, it is 77.0 weight% or less, and following formula (a) and (b) :
0 ≦ X ≦ equilibrium causticization rate of decontaminating solution (a)
15-0.3X ≦ Y ≦ 77.0 (b)
However, X = causticization rate of decontamination solution (%)
Y = lime concentration based on quick lime before waking (% by weight)
The method for producing calcium carbonate, which satisfies the above .