JP4635314B2 - Method for producing sodium sulfate - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は硫酸ナトリウムの製造方法に関するものである。さらに詳しくは、不純物の混入を抑制した高純度の硫酸ナトリウムを工業的に経済的に製造する方法に関するものである。
【０００２】
【従来の技術】
硫酸ナトリウムは、染色工業や、ガラス，洗剤，温浴剤等の製造原料に幅広く使用されている基礎化学商品である。この硫酸ナトリウムの殆どは苛性ソーダによる排煙脱硫や人絹製造等で副生する粗硫酸ナトリウム水溶液から種々の処理を施した後、蒸発濃縮で硫酸ナトリウム結晶を晶析させ製造されている。この粗硫酸ナトリウム水溶液はその由来にもよるが、不純物としてＣａ，Ｍｇ等のアルカリ土類金属イオン、Ｆｅ，Ｎｉ，Ｃｒ，Ａｌ等の金属イオン、Ｃｌ，ＮＯ3等のアニオン、そして有機物が含まれる。これら不純物を含む粗硫酸ナトリウム水溶液から高品質の硫酸ナトリウムを得る方法として、種々提案されている。例えば、特開昭５３−５００９１号公報には、Ｆｅ，Ｎｉ等の金属イオンを含む硫酸ナトリウム水溶液を苛性アルカリでｐＨ１０〜１１にしてこれら金属の水酸化物として沈澱除去し、蒸発濃縮して硫酸ナトリウム結晶を得る方法が開示されている。特開平８−３３７４１７号公報には、Ｆｅ，Ｎｉ等の金属イオンをキレート樹脂を用いてｐＨ１．５〜８で吸着除去した後、蒸発濃縮して硫酸ナトリウム結晶を得る方法が開示されている。これらの方法では、確かに金属不純物の除去は可能である。
【０００３】
しかし、前者では、厳密なｐＨ管理とろ過操作が必要であり、しかも、このろ過操作ではろ過性が極めて悪い。さらに、難溶性の水酸化物を形成しない不純物が結晶へ一部移行し、着色等の問題を惹起する。一方、後者では、高価なキレート樹脂を用いており工業的に有利でない。さらに、キレート樹脂は金属イオン種、ｐＨ等によってその除去率は異なり、不純物除去が十分とは言えない。
【０００４】
また、特開昭４９−１２９６９７号公報には、原料硫酸ナトリウム１０水塩結晶にクエン酸、コハク酸、酒石酸の如き有機酸を加え、ｐＨ５．５〜８で加熱融解して硫酸ナトリウム無水塩に転移させ、亜鉛や鉄の不純物を分離精製する方法が開示されている。
【０００５】
しかし、該方法は一旦硫酸ナトリウム１０水塩を生成させる必要があり、操作が煩雑である。また、希望の硫酸ナトリウム無水塩を得るにはｐＨ、温度等の厳密な管理が必要である。更には、これら有機酸は高価であるばかりでなく、硫酸ナトリウム無水塩結晶への移行問題、これを含有する液のＣＯＤ対策の必要性等がある。
【０００６】
【発明が解決しようとする課題】
工業的に有用な硫酸ナトリウムは着色性物質等の各種不純物の混入を極力抑え、高品質を安定に保つことが望まれる。そのためには、晶析時の不純物の混入、とりわけＦｅ，Ｎｉ等の金属不純物の混入を回避する新しい製造技術の開発が望まれている。
【０００７】
本発明は、簡単な操作で高品質な硫酸ナトリウム結晶を効果的、効率的に得ることを目的とする。
【０００８】
【課題を解決するための手段】
本発明者等は、これらの従来の問題点を解決するため、硫酸ナトリウム結晶の晶析方法について鋭意検討した。その結果、アルカリ性で硫酸ナトリウム結晶を晶析させる際、特定の物質を硫酸ナトリウム水溶液に溶存させることにより、これまでの問題点をすべて解決できることを見い出し、本発明を完成するに至った。
【０００９】
即ち、本発明は、珪酸及び／又は縮合燐酸が溶存した硫酸ナトリウムのアルカリ性水溶液から、硫酸ナトリウム結晶を晶析させることを特徴とする硫酸ナトリウムの製造方法であり、硫酸ナトリウム水溶液から硫酸ナトリウム結晶を晶析させるに当り、この硫酸ナトリウム水溶液をアルカリ性に保ち、かつ、珪酸及び／又は縮合燐酸を添加し、これらの添加物の溶解性を十分保持するものである。
【００１０】
以下、本発明を詳細に説明する。
【００１１】
本発明における硫酸ナトリウム水溶液は、アルカリ性であることが必要である。中性付近では鉄等の金属水酸化物が沈澱し、それは極微量であっても硫酸ナトリウム結晶と混在して該結晶は黄色を呈する。また、酸性では不純物の精製効果はかなり低下し、珪酸の溶解度は極めて小さくなる。さらに、酸性では装置材質の腐食の問題もある。
【００１２】
このアルカリ性は特に限定するものではないが、不純物の精製効果を維持するため、温度、夾雑物質の種類，量にもよるが、好ましくはｐＨ９以上であり、また、硫酸ナトリウム水溶液中の硫酸ナトリウム濃度の低下を防止するため、粘度上昇を防止して結晶成長を維持するため及び硫酸ナトリウム結晶に苛性ソーダが包含されるのを防止して純度を維持するために、さらに好ましくはｐＨ１１以上ｐＨ１３以下である。この時不純物の混入は回避でき、結晶成長性の良い白色の高品位な硫酸ナトリウム結晶が安定して得られる。
【００１３】
本発明における硫酸ナトリウム水溶液は、アルカリ性水溶液であれば特に限定するものではなく、例えば、苛性ソーダによる排煙脱硫，人絹製造，海水法臭素製造等で副生する粗硫酸ナトリウム水溶液や、苛性ソーダと硫酸の反応で得られる硫酸ナトリウム水溶液等があげられる。粗硫酸ナトリウム水溶液には、不純物としてＣａ，Ｍｇ等のアルカリ土類金属イオン、Ｆｅ，Ｎｉ等の金属イオン、Ｃｌ，ＮＯ3，ＳＯ3等のアニオン、そして有機物等が含まれることがあるがこれを利用できる。また、苛性ソーダや硫酸にも微量ではあるがこれら不純物が含まれることがある。例えば、製造時の原料やプロセスにおける装置材料からの不純物の混入、これらの保管や輸送中の材質の溶出による微量の金属不純物等の混入である。硫酸には特に材質に由来する鉄の混入がある。これら金属イオンやアニオンのような不純物に対して本発明による効果は著しい。
【００１４】
本発明における硫酸ナトリウムのアルカリ性水溶液には、珪酸及び／又は縮合燐酸が溶存する。珪酸としては、例えば、オルト珪酸、メタ珪酸又はこれらの塩等があげられ、また、縮合燐酸としては、例えば、ピロ燐酸、トリポリ燐酸、トリメタ燐酸、テトラメタ燐酸、ヘキサメタ燐酸又はこれらの塩等があげられる。塩はナトリウム、カリウム等いずれでもよいが、ナトリウムが硫酸ナトリウムのカチオンと共通であり、好ましい。珪酸と縮合燐酸では、珪酸が入手容易で取り扱い易く、硫酸ナトリウム結晶への不純物の混入を防止する作用が大きく好ましい。また、後述するように金属不純物、特に鉄の系外パージに対しても珪酸が有利である。本発明における珪酸及び／又は縮合燐酸による効果の理由の一つとしては、金属イオンに対するマスキング作用が考えられる。硫酸ナトリウム水溶液に溶存する珪酸及び／又は縮合燐酸の濃度は特に限定するものではないが、不純物を十分に分離精製できるため、珪酸はＳｉＯ2として、縮合燐酸はＰ2Ｏ5として、好ましくは１００重量ｐｐｍ以上、より好ましくは３００重量ｐｐｍ以上、さらに好ましくは６００重量ｐｐｍ以上であり、一方、珪酸、縮合燐酸に関する費用の増加防止、硫酸ナトリウム結晶の品質維持のため、好ましくは２０００重量ｐｐｍ以下である。
【００１５】
また、添加する珪酸及び／又は縮合燐酸の状態は特に限定するものではなく、予め水に溶解した水溶液でも粉末状でもよい。なお、水溶液状が取り扱い易いために好ましい。これらは直接晶析時に加えてもよいし、予め原料の粗硫酸ナトリウム水溶液や苛性ソーダに加えておいてもよい。
【００１６】
硫酸ナトリウム結晶を晶析させる方法は、特に限定するものではなく、例えば、硫酸ナトリウム水溶液を蒸発濃縮する方法、濃厚な苛性ソーダと濃厚な硫酸の反応による方法、塩化ナトリウムや苛性ソーダを硫酸ナトリウム水溶液に添加する塩析方法等があげられる。苛性ソーダと硫酸を直接混合して反応晶析する場合、その希釈熱、反応熱を水分蒸発に有効利用できる。この熱は苛性ソーダ及び硫酸が高濃度ほど大きくなり好ましい。苛性ソーダは水溶液でも通常市販されている約５０重量％品でもよいが、濃度の高いものほど好ましい。硫酸は通常市販されている９８重量％のものでも希釈されたものでもよいが、高濃度ほど好ましい。晶析方式は、回分式でも連続式でもよいが、生産性、運転操作性等の面から連続式が有利である。また、晶析装置はスラリーの均一流動ができる装置が好ましく、完全混合型のＭＳＭＰＲ、完全混合又は分級型のＤＰ（Ｄｏｕｂｌｅ−Ｐｒｏｐｅｌｌｅｒ）、ＤＴＢ（Ｄｒａｆｔ−Ｔｕｂｅ−Ｂａｆｆｌｅｄ）、ＦＣ（Ｆｏｒｃｅｄ−Ｃｉｒｃｕｌａｔｉｏｎ）等いずれも適用できる。
【００１７】
晶析する硫酸ナトリウム結晶は７水塩、１０水塩等の結晶水を有しても、無水塩でもよいが、硫酸ナトリウム含量が高く、流動性、貯蔵安定性に優れる硫酸ナトリウム無水塩結晶が好ましい。
【００１８】
晶析温度は特に限定するものではないが、好ましい硫酸ナトリウム無水塩が安定に存在する析出温度がよく、結晶成長性、不純物精製の面から高い温度が望ましく、好ましくは４０℃以上、さらに好ましくは７０℃以上であり、一方、エネルギーコストの増加防止と装置材料コストの増加防止のため、１２０℃以下が好ましい。また、蒸発・濃縮による晶析では、効率よく実施するため、例えば、減圧下、４０〜１００℃で行ってもよい。
【００１９】
また、晶析する硫酸ナトリウム結晶のスラリー濃度は特に限定するものではないが、５〜４０重量％が好ましく、安定運転ができ、結晶成長性に優れた硫酸ナトリウム結晶が得られ、また、この範囲では強制攪拌することでスラリーの均一流動が図れ、取り扱い性も良く、スラリー移送も容易である。さらに、１０〜３０重量％がより好ましく、前記効果はより顕著になる。
【００２０】
この晶析操作は１段でも、また、２段以上の多段でも良い。晶析して得られた硫酸ナトリウム結晶のスラリーは、ろ過し、硫酸ナトリウム結晶のろ過ケークと硫酸ナトリウムのろ過液を得る。ろ過は遠心分離機、加圧ろ過機、減圧ろ過機等が適用でき、回分式、連続式いずれでも良い。好ましくは、連続遠心ろ過機である。この時、水で洗浄しても良い。
【００２１】
ろ過ケークは流動乾燥機、フラッシュ乾燥機、バンド乾燥機、パドルドライヤー等で乾燥し、製品となる。この時、連続式でも回分式でも構わない。
【００２２】
硫酸ナトリウムのろ過液は、通常晶析工程に循環する。この際、該ろ過液の一部又は全部に酸を加え中和し、沈澱した不純物を分離した液を晶析工程に循環してもよい。この場合には、系内の不純物蓄積を防げ、より好ましい態様となる。酸は特に限定するものではなく、硫酸、塩酸等の鉱酸が適用できるが、硫酸が硫酸ナトリウムの共通アニオンであり好ましい。中和時のｐＨは晶析時の硫酸ナトリウム水溶液のｐＨより低いのが好ましく、ｐＨ８〜１２がより好ましい。この時、金属イオンを主とする沈澱不純物は一部の珪酸又は縮合燐酸と共沈する。特に、珪酸の場合はろ過性の良いフロック状の沈澱物を形成するという別の効果もある。この点からも珪酸が縮合燐酸よりもより好ましい。また、中和後の硫酸ナトリウム水溶液中には珪酸又は縮合燐酸の殆どが溶存したまま晶析工程に循環される。そのため、これらの補充量が少量で済み経済的である。これらの点から、中和時ではｐＨ９．５から１１．５がさらに好ましい。中和後、スラリーは、静定分離又は固液分離等で分離し、上澄液又は分離液を晶析工程に循環する。この循環液により晶析時のスラリー濃度を調整でき、結晶成長性及び運転操作性の面で有利となる。
【００２３】
ここで、硫酸ナトリウム結晶のろ過ケークの処理について、より好ましい態様を示す。製品である硫酸ナトリウムは、基礎化学商品であり、広く利用範囲を拡大するため、これを水に溶解した時ｐＨ６〜８の中性であることが好ましい。そのため、本発明では、好ましくは硫酸ナトリウム結晶のろ過ケークに水又は硫酸ナトリウム水溶液を加えてスラリーとし、これに酸を添加して再度ろ過する。ろ過ケークに添加する液は硫酸ナトリウム水溶液が好ましく飽和濃度に近いほど該ケークの溶解は抑制され効率的である。また、添加する酸は特に限定するものではなく、硫酸、塩酸等の鉱酸が適用できるが、硫酸が好ましい。酸を添加することによりアルカリ性溶液から晶析した硫酸ナトリウム結晶に微量包含した水酸化ナトリウムを中和でき、製品を水に溶かした時のｐＨを６〜８に安定して調節できる。晶析時の硫酸ナトリウム水溶液中の苛性ソーダ濃度、硫酸ナトリウム結晶の形状と大きさ、ろ過ケークの付着ろ過液率にもよるが、製品の水への溶解ｐＨを維持するため、酸を添加したスラリーの好ましいｐＨは２〜６、さらに好ましいｐＨは３〜５である。このスラリーは遠心分離、加圧ろ過、減圧ろ過等により容易に固液分離して得られる。得られたろ過ケークは乾燥する。乾燥は、熱風乾燥、流動乾燥、気流乾燥等いずれでもよい。
【００２４】
【実施例】
以下に、実施例により本発明をより詳細に説明するが、本発明はこれらによってなんら限定されるものではない。
【００２５】
実施例１
オイルバスにテフロン製攪拌槽型蒸発晶析器をセットし、これに珪酸ソーダをＳｉＯ2として２５０重量ｐｐｍ添加した鉄：８重量ｐｐｍ、塩化ナトリウム：１重量％、苛性ソーダ：２．７重量％を含むｐＨ１２．２の１５重量％粗硫酸ナトリウム水溶液を５５５重量部／Ｈｒで連続して導入し、常圧下、１０５℃で蒸発晶析した。水の蒸発量は３１０重量部／Ｈｒであり、晶析スラリーを５分間毎に一定量ずつ、１時間当たり２４５重量部抜き出した。この時の結晶見掛け滞在時間は３時間であり、析出物は硫酸ナトリウム無水塩でスラリー濃度は２３重量％であった。鉄を含め不純物の析出はなく、晶析ろ過液は無色透明で、組成は硫酸ナトリウム：１５重量％、ＳｉＯ2：７５０重量ｐｐｍ、苛性ソーダ：７．８重量％、鉄：２４重量ｐｐｍであった。硫酸ナトリウム無水塩の結晶成長は良く、形はダイヤモンド状で平均粒径は２０５μｍであった。次に、該スラリーを遠心分離して硫酸ナトリウム無水塩のろ過ケークを得た。遠心分離はすこぶる良好でケークは白色であった。次に、該ろ過ケークに飽和硫酸ナトリウム水溶液を加えて２０重量％スラリーとし、１０重量％硫酸でスラリーｐＨ３．５とした。これを再度遠心分離し、そのろ過ケークを１１０℃で熱風乾燥し、硫酸ナトリウムを得た。この硫酸ナトリウムの純度は９９．９％と高く、鉄含量は０．３重量ｐｐｍと非常に低く、また、この５％水溶液のｐＨは６．３と好ましい値を得た。
【００２６】
硫酸ナトリウム結晶の晶析における結晶成長性及び品質に関する結果を表１に示す。
【００２７】
【表１】
また、晶析スラリーを遠心分離して得たろ過液は攪拌機付テフロン製容器に入れ、８０℃で、９８重量％硫酸を加え、ろ過液中の苛性ソーダを中和しｐＨ１０にした。この時、溶液中に鉄等の金属水酸化物と一部の珪酸がフロックを形成して共沈した。このフロックのろ過性は極めてよく、ろ過助剤を用いる必要もなく、鉄：０．２重量ｐｐｍ、ＳｉＯ2：６３５重量ｐｐｍのろ液を得、先の晶析のフィード液調製の原料とした。
【００２９】
比較例１
粗硫酸ナトリウム水溶液に珪酸ソーダを添加しない以外はすべて実施例１と同様の操作を行い、硫酸ナトリウム結晶の晶析を行った。この時、晶析スラリーは赤褐色を呈し、水酸化第二鉄の析出を認めた。析出物は硫酸ナトリウム無水塩であったものの、結晶成長は劣り、平均粒径１８０μｍであった。この硫酸ナトリウム無水塩には析出した水酸化第二鉄が付着し、黄色を呈した。結晶中の鉄濃度は８．３重量ｐｐｍと高く、晶析時に鉄は精製できなかった。硫酸ナトリウム結晶の晶析における結晶成長性及び品質に関する結果を表１に合わせて示す。
【００３０】
実施例２
硫酸ナトリウム水溶液にピロ燐酸ソーダをＰ2Ｏ5として５００重量ｐｐｍ添加した鉄：８重量ｐｐｍ、塩化ナトリウム：１重量％、苛性ソーダ：０．０４重量％を含むｐＨ１０．２の１５重量％粗硫酸ナトリウム水溶液を使用する以外はすべて実施例１と同様の操作を行い、硫酸ナトリウム結晶の晶析を行った。析出物は平均粒径２１０μｍのダイヤモンド状の硫酸ナトリウム無水塩であり、鉄を含め不純物の析出はなく、また、晶析ろ過液は無色透明であった。次に、該スラリーを遠心分離し、１１０℃で乾燥し、流動性の良い白色の硫酸ナトリウムを得た。その純度は９９．９％と高く、鉄含量は０．３重量ｐｐｍと非常に低く、高品質であることが判った。硫酸ナトリウム結晶の晶析における結晶成長性及び品質に関する結果を表１に合わせて示す。
【００３１】
実施例３
硫酸ナトリウム水溶液に珪酸ソーダをＳｉＯ2として２００重量ｐｐｍ及びピロ燐酸ソーダをＰ2Ｏ5として４００重量ｐｐｍ添加する以外はすべて実施例１と同様の操作を行い、硫酸ナトリウム結晶の晶析を行った。析出物は平均粒径２０５μｍのダイヤモンド状の硫酸ナトリウム無水塩であり、鉄を含め不純物の析出はなく、また、晶析ろ過液は無色透明であった。次に、該スラリーを遠心分離し、１１０℃で乾燥し、流動性の良い白色の硫酸ナトリウムを得た。その純度は９９．９％と高く、鉄含量は０．３重量ｐｐｍと非常に低く、高品質であることが判った。硫酸ナトリウム結晶の晶析における結晶成長性及び品質に関する結果を表１に合わせて示す。
【００３２】
実施例４
実施例１と同様の装置を用いて、珪酸ソーダを添加したＳｉＯ2３３５重量ｐｐｍの４８重量％苛性ソーダ水溶液を１２９重量部／Ｈｒ、鉄８０重量ｐｐｍの９８重量％硫酸６０重量部／Ｈｒ、そして実施例１で得た中和処理ろ液２８２重量部／Ｈｒを別々に連続して晶析装置に導入して、ｐＨ１２．６の１９重量％粗硫酸ナトリウム水溶液として、蒸発晶析を行った。操作条件は絶対圧３５０Ｔｏｒｒ、８５℃で蒸発水量は５０重量部／Ｈｒであった。そして、スラリー濃度２９重量％の晶析スラリーを５分間毎に一定量ずつ１時間当たり４２１重量部抜き出した。結晶見掛け滞在時間は２時間、晶析スラリーのろ過液はｐＨ１２．８で、苛性ソーダ濃度５重量％、硫酸ナトリウム濃度１７重量％、無色透明で、また、鉄等不純物の析出はなかった。得られた結晶は、硫酸ナトリウム無水塩で、その成長は良く、平均粒径２１０μｍのダイヤモンド状結晶であった。このスラリーを遠心分離し、１１０℃乾燥して得られた硫酸ナトリウムは、流動性良好で、純度は９９．９％と高く、鉄含量は０．４重量ｐｐｍと低く、白色で高品質であった。硫酸ナトリウム結晶の晶析における結晶成長性及び品質に関する結果を表１に合わせて示す。
【００３３】
比較例２
実施例１と同様の装置を用いて、４８重量％苛性ソーダ水溶液を２６１重量部／Ｈｒ、鉄３０重量ｐｐｍの９８重量％硫酸１５５重量部／Ｈｒを別々に連続して晶析装置に導入し、常圧下、１００℃で、硫酸ナトリウム結晶の晶析を行った。蒸発水量は２５重量部／Ｈｒであり、スラリー濃度３０重量％の晶析スラリーを５分間毎に一定量ずつ１時間当たり３９１重量部抜き出した。この時の結晶見掛け滞在時間は２時間、晶析スラリーのろ過液はｐＨ１１．８の苛性ソーダ：０．５重量％、鉄：１７重量ｐｐｍ、硫酸ナトリウム３０重量％、赤褐色を呈し、鉄が水酸化第二鉄として析出した。このスラリーを遠心分離して得られた結晶は硫酸ナトリウム無水塩であり、強い黄色を呈した。該結晶の平均粒径は１８５μｍであった。このろ過ケークを１１０℃で乾燥し、硫酸ナトリウムを得たが、その鉄含量は、１３．０重量ｐｐｍと高く、製品にはできなかった。硫酸ナトリウム結晶の晶析における結晶成長性及び品質に関する結果を表１に合わせて示す。
【００３４】
比較例３
実施例１と同様の装置を用いて、４８重量％苛性ソーダ水溶液を２７３重量部／Ｈｒ、鉄３０重量ｐｐｍの９８％硫酸１６５重量部／Ｈｒを別々に連続して晶析装置に導入し、常圧下、１００℃で、硫酸ナトリウム結晶の晶析を行った。蒸発水量は３０重量部／Ｈｒであり、スラリー濃度３５重量％の晶析スラリーを５分間毎に一定量ずつ１時間当たり４０８重量部抜き出した。この時の結晶見掛け滞在時間は２時間、晶析スラリーのろ過液は硫酸濃度０．５重量％（ｐＨ２．４）、硫酸ナトリウム濃度３１重量％、無色透明で鉄の析出はなかった。しかし、このスラリーを遠心分離して得られた結晶は硫酸ナトリウム無水塩であったものの、黄色を呈し鉄の結晶内移行が示唆された。該結晶の平均粒径は１７０μｍで、このろ過ケークの１１０℃乾燥品は、黄色でその鉄含量は２６．５重量ｐｐｍと高く、製品にはできなかった。硫酸ナトリウム結晶の晶析における結晶成長性及び品質に関する結果を表１に合わせて示す。
【００３５】
【発明の効果】
以上、詳細に説明したように、本発明では、簡単な操作で不純物の分離精製を実施でき、経済的、工業的であり、産業上極めて有益である。以下、本発明の効果を列記する。
【００３６】
（１）硫酸ナトリウム結晶への不純物の混入を抑制できる。
【００３７】
（２）硫酸ナトリウム結晶の結晶成長は良く、粒径は大きく、流動性が良い。
【００３８】
（３）アルカリ性水溶液の晶析であることにより装置材質の耐食性が向上する。
【００３９】
（４）晶析ろ過液を中和する操作で不純物を系外へ容易にパージできる。
【００４０】
（５）晶析ろ過液の循環が可能で、原料の原単位が低減できる。
【００４１】
（６）硫酸ナトリウム結晶に酸を添加してろ過することで水に溶解した時のｐＨが６から８の中性硫酸ナトリウムが得られる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing sodium sulfate. More specifically, the present invention relates to a method for industrially and economically producing high-purity sodium sulfate in which contamination of impurities is suppressed.
[0002]
[Prior art]
Sodium sulfate is a basic chemical product widely used in the dyeing industry and manufacturing raw materials such as glass, detergents and warm baths. Most of this sodium sulfate is produced by subjecting various treatments to crude sodium sulfate aqueous solution produced as a by-product in flue gas desulfurization with caustic soda and human silk production, and then crystallizing sodium sulfate crystals by evaporation concentration. Although this crude sodium sulfate aqueous solution depends on its origin, impurities include alkaline earth metal ions such as Ca and Mg, metal ions such as Fe, Ni, Cr and Al, anions such as Cl and NO 3 , and organic substances. It is. Various methods for obtaining high-quality sodium sulfate from a crude sodium sulfate aqueous solution containing these impurities have been proposed. For example, in Japanese Patent Laid-Open No. 53-50091, a sodium sulfate aqueous solution containing metal ions such as Fe and Ni is adjusted to pH 10 to 11 with caustic, precipitated and removed as a hydroxide of these metals, evaporated and concentrated to sulfuric acid. A method for obtaining sodium crystals is disclosed. JP-A-8-337417 discloses a method for obtaining sodium sulfate crystals by evaporating and concentrating metal ions such as Fe and Ni at a pH of 1.5 to 8 using a chelate resin. With these methods, it is possible to remove metal impurities.
[0003]
However, the former requires strict pH control and filtration operation, and the filtration performance is extremely poor. Further, impurities that do not form a hardly soluble hydroxide partially migrate to the crystal, causing problems such as coloring. On the other hand, the latter uses an expensive chelate resin and is not industrially advantageous. Further, the removal rate of the chelate resin varies depending on the metal ion species, pH, etc., and it cannot be said that the removal of impurities is sufficient.
[0004]
Japanese Patent Laid-Open No. 49-129697 discloses that an organic acid such as citric acid, succinic acid and tartaric acid is added to a raw material sodium sulfate decahydrate crystal and heated and melted at pH 5.5 to 8 to form sodium sulfate anhydrous salt. A method for transferring and separating and purifying impurities of zinc and iron is disclosed.
[0005]
However, in this method, it is necessary to once generate sodium sulfate decahydrate, and the operation is complicated. In addition, in order to obtain the desired sodium sulfate anhydrous salt, it is necessary to strictly control pH, temperature and the like. Furthermore, these organic acids are not only expensive, but also have a problem of migration to anhydrous sodium sulfate crystals, the need for measures against COD of liquids containing them.
[0006]
[Problems to be solved by the invention]
Industrially useful sodium sulfate is desired to keep contamination of various impurities such as coloring substances as much as possible and to keep high quality stable. For this purpose, it is desired to develop a new manufacturing technique that avoids the mixing of impurities during crystallization, especially the mixing of metal impurities such as Fe and Ni.
[0007]
An object of the present invention is to obtain high-quality sodium sulfate crystals effectively and efficiently with a simple operation.
[0008]
[Means for Solving the Problems]
In order to solve these conventional problems, the present inventors diligently studied a method for crystallizing sodium sulfate crystals. As a result, when crystallization of alkaline sodium sulfate crystals was performed, it was found that a specific substance can be dissolved in an aqueous sodium sulfate solution to solve all the problems so far, and the present invention has been completed.
[0009]
That is, the present invention is a method for producing sodium sulfate characterized by crystallizing sodium sulfate crystals from an alkaline aqueous solution of sodium sulfate in which silicic acid and / or condensed phosphoric acid is dissolved. In crystallization, this aqueous sodium sulfate solution is kept alkaline, and silicic acid and / or condensed phosphoric acid is added to sufficiently maintain the solubility of these additives.
[0010]
Hereinafter, the present invention will be described in detail.
[0011]
The aqueous sodium sulfate solution in the present invention needs to be alkaline. In the vicinity of neutrality, a metal hydroxide such as iron precipitates, and even if it is a trace amount, it is mixed with sodium sulfate crystals and the crystals are yellow. In addition, when it is acidic, the purification effect of impurities is considerably reduced, and the solubility of silicic acid is extremely small. Furthermore, there is a problem of corrosion of the device material when it is acidic.
[0012]
Although the alkalinity is not particularly limited, it is preferably pH 9 or higher, and the sodium sulfate concentration in the aqueous sodium sulfate solution is dependent on the temperature and the type and amount of contaminants in order to maintain the purification effect of impurities. More preferably, the pH is 11 or more and 13 or less in order to prevent the increase in viscosity, to maintain the crystal growth by preventing the increase in viscosity, and to maintain the purity by preventing the inclusion of sodium hydroxide in the sodium sulfate crystal. . At this time, mixing of impurities can be avoided, and white high-quality sodium sulfate crystals with good crystal growth are stably obtained.
[0013]
The sodium sulfate aqueous solution in the present invention is not particularly limited as long as it is an alkaline aqueous solution. For example, a crude sodium sulfate aqueous solution by-produced in flue gas desulfurization using caustic soda, human silk production, seawater bromine production, etc., or caustic soda and sulfuric acid. Examples thereof include an aqueous sodium sulfate solution obtained by the above reaction. The crude sodium sulfate aqueous solution may contain impurities such as alkaline earth metal ions such as Ca and Mg, metal ions such as Fe and Ni, anions such as Cl, NO 3 and SO 3 , and organic substances. Can be used. Caustic soda and sulfuric acid may contain a small amount of these impurities. For example, it may be contamination of impurities from raw materials at the time of manufacturing or equipment materials in the process, or a trace amount of metal impurities due to elution of materials during storage or transportation. In particular, sulfuric acid contains iron from the material. The effect of the present invention is remarkable for impurities such as metal ions and anions.
[0014]
Silicic acid and / or condensed phosphoric acid is dissolved in the alkaline aqueous solution of sodium sulfate in the present invention. Examples of silicic acid include orthosilicic acid, metasilicic acid, or salts thereof, and examples of condensed phosphoric acid include pyrophosphoric acid, tripolyphosphoric acid, trimetaphosphoric acid, tetrametaphosphoric acid, hexametaphosphoric acid, or salts thereof. It is done. The salt may be any of sodium, potassium, etc., but sodium is preferred because it is common with the cation of sodium sulfate. Of silicic acid and condensed phosphoric acid, silicic acid is easily available and easy to handle, and has a large effect of preventing impurities from being mixed into sodium sulfate crystals. Further, as will be described later, silicic acid is also advantageous for out-of-system purging of metal impurities, particularly iron. One of the reasons for the effect of silicic acid and / or condensed phosphoric acid in the present invention is considered to be a masking action on metal ions. The concentration of silicic acid and / or condensed phosphoric acid dissolved in the sodium sulfate aqueous solution is not particularly limited. However, since impurities can be sufficiently separated and purified, silicic acid is SiO 2 and condensed phosphoric acid is P 2 O 5 , preferably 100 Weight ppm or more, more preferably 300 ppm by weight or more, and still more preferably 600 ppm by weight or more. On the other hand, in order to prevent an increase in costs related to silicic acid and condensed phosphoric acid, and to maintain the quality of sodium sulfate crystals, preferably 2000 ppm by weight or less. is there.
[0015]
Further, the state of the silicic acid and / or condensed phosphoric acid to be added is not particularly limited, and it may be an aqueous solution dissolved in water or powdered. An aqueous solution is preferable because it is easy to handle. These may be added directly at the time of crystallization, or may be added in advance to a raw crude sodium sulfate solution or caustic soda.
[0016]
The method for crystallizing sodium sulfate crystals is not particularly limited. For example, a method of evaporating and concentrating sodium sulfate aqueous solution, a method of reacting concentrated caustic soda with concentrated sulfuric acid, adding sodium chloride or caustic soda to the aqueous sodium sulfate solution The salting-out method to perform is mention | raise | lifted. When reaction crystallization is performed by directly mixing caustic soda and sulfuric acid, the heat of dilution and heat of reaction can be effectively used for water evaporation. This heat is preferred as the concentration of caustic soda and sulfuric acid increases. Caustic soda may be an aqueous solution or a commercially available product of about 50% by weight, but a higher concentration is preferred. Sulfuric acid may be 98% by weight or a commercially available sulfuric acid, but a higher concentration is preferred. The crystallization method may be a batch method or a continuous method, but a continuous method is advantageous in terms of productivity, operation operability, and the like. Further, the crystallizer is preferably an apparatus capable of uniformly flowing the slurry, and is a fully mixed MSMPR, a completely mixed or classified DP (Double-Propeller), DTB (Draft-Tube-Buffled), and FC (Forced-Circulation). Any of these can be applied.
[0017]
Crystallized sodium sulfate crystals may have crystal water such as heptahydrate, 10 hydrate, or anhydrous salts, but sodium sulfate anhydrous crystals with high sodium sulfate content and excellent fluidity and storage stability preferable.
[0018]
Although the crystallization temperature is not particularly limited, the precipitation temperature at which the preferred sodium sulfate anhydrous salt is stably present is good, and a high temperature is desirable from the viewpoint of crystal growth and impurity purification, preferably 40 ° C. or more, more preferably On the other hand, it is preferably 120 ° C. or lower in order to prevent an increase in energy costs and an increase in device material costs. Moreover, in order to carry out the crystallization by evaporation / concentration efficiently, it may be carried out, for example, at 40 to 100 ° C. under reduced pressure.
[0019]
Further, the slurry concentration of the sodium sulfate crystal to be crystallized is not particularly limited, but it is preferably 5 to 40% by weight, a stable operation is possible, and a sodium sulfate crystal excellent in crystal growth property is obtained. Then, by forced stirring, a uniform flow of the slurry can be achieved, the handleability is good, and the slurry can be easily transferred. Furthermore, 10 to 30% by weight is more preferable, and the effect becomes more remarkable.
[0020]
This crystallization operation may be performed in one stage or in multiple stages including two or more stages. The slurry of sodium sulfate crystals obtained by crystallization is filtered to obtain a sodium sulfate crystal filter cake and a sodium sulfate filtrate. For the filtration, a centrifugal separator, a pressure filter, a vacuum filter or the like can be applied, and either a batch type or a continuous type may be used. Preferably, it is a continuous centrifugal filter. At this time, you may wash | clean with water.
[0021]
The filter cake is dried with a fluid dryer, a flash dryer, a band dryer, a paddle dryer or the like to become a product. At this time, a continuous type or a batch type may be used.
[0022]
The sodium sulfate filtrate is usually circulated in the crystallization process. At this time, a part or all of the filtrate may be neutralized by adding an acid, and the separated liquid may be circulated in the crystallization step. In this case, accumulation of impurities in the system can be prevented, which is a more preferable embodiment. The acid is not particularly limited, and a mineral acid such as sulfuric acid and hydrochloric acid can be applied. However, sulfuric acid is preferable because it is a common anion of sodium sulfate. The pH during neutralization is preferably lower than the pH of the aqueous sodium sulfate solution during crystallization, and more preferably pH 8-12. At this time, precipitation impurities mainly composed of metal ions coprecipitate with some silicic acid or condensed phosphoric acid. In particular, in the case of silicic acid, there is another effect of forming a floc-like precipitate having good filterability. In this respect, silicic acid is more preferable than condensed phosphoric acid. Further, most of silicic acid or condensed phosphoric acid is circulated in the crystallization step while being dissolved in the neutralized sodium sulfate aqueous solution. Therefore, these replenishing amounts are small and economical. From these points, pH 9.5 to 11.5 is more preferable at the time of neutralization. After neutralization, the slurry is separated by static separation or solid-liquid separation, and the supernatant or separated liquid is circulated in the crystallization step. This circulating liquid can adjust the slurry concentration during crystallization, which is advantageous in terms of crystal growth and operational operability.
[0023]
Here, a more preferable aspect is shown about the process of the filter cake of a sodium sulfate crystal | crystallization. Sodium sulfate, which is a product, is a basic chemical product and has a neutral pH of 6 to 8 when dissolved in water in order to broaden the range of use. Therefore, in the present invention, water or an aqueous solution of sodium sulfate is preferably added to a sodium sulfate crystal filter cake to form a slurry, and an acid is added to the slurry, followed by filtration again. The solution added to the filter cake is preferably a sodium sulfate aqueous solution, and the closer the saturation concentration is, the more efficient the dissolution of the cake is. The acid to be added is not particularly limited, and mineral acids such as sulfuric acid and hydrochloric acid can be applied, but sulfuric acid is preferred. By adding an acid, sodium hydroxide contained in a trace amount in sodium sulfate crystals crystallized from an alkaline solution can be neutralized, and the pH when the product is dissolved in water can be stably adjusted to 6-8. Depending on the concentration of sodium hydroxide in the sodium sulfate aqueous solution at the time of crystallization, the shape and size of the sodium sulfate crystals, and the attached filtrate rate of the filter cake, a slurry with acid added to maintain the pH of the product dissolved in water The preferred pH is 2-6, and the more preferred pH is 3-5. This slurry can be obtained by solid-liquid separation easily by centrifugation, pressure filtration, vacuum filtration or the like. The resulting filter cake is dried. Drying may be any of hot air drying, fluidized drying, airflow drying, and the like.
[0024]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited thereto.
[0025]
Example 1
A Teflon-type evaporation crystallizer made of Teflon was set in an oil bath, and iron: 8 wt ppm, sodium chloride: 1 wt%, and caustic soda: 2.7 wt% added with 250 wt ppm of sodium silicate as SiO 2. A 15 wt% crude sodium sulfate aqueous solution having a pH of 12.2 was continuously introduced at 555 parts by weight / hr, and evaporated and crystallized at 105 ° C. under normal pressure. The amount of water evaporated was 310 parts by weight / hr, and 245 parts by weight of crystallization slurry was withdrawn per hour at a constant rate every 5 minutes. At this time, the apparent crystal residence time was 3 hours, the precipitate was anhydrous sodium sulfate, and the slurry concentration was 23% by weight. Precipitation of impurities including iron is no crystallization filtrate was colorless and transparent, the composition is sodium sulfate: 15 wt%, SiO 2: 750 ppm by weight, sodium hydroxide: 7.8 wt%, iron: was 24 ppm by weight . Crystal growth of anhydrous sodium sulfate was good, the shape was diamond-like, and the average particle size was 205 μm. The slurry was then centrifuged to obtain an anhydrous sodium sulfate filter cake. Centrifugation was very good and the cake was white. Next, a saturated sodium sulfate aqueous solution was added to the filter cake to make a 20 wt% slurry, and the slurry pH was adjusted to 3.5 with 10 wt% sulfuric acid. This was centrifuged again, and the filter cake was dried with hot air at 110 ° C. to obtain sodium sulfate. The purity of this sodium sulfate was as high as 99.9%, the iron content was very low as 0.3 ppm by weight, and the pH of this 5% aqueous solution was 6.3, which was a preferable value.
[0026]
Table 1 shows the results relating to crystal growth and quality in crystallization of sodium sulfate crystals.
[0027]
[Table 1]
The filtrate obtained by centrifuging the crystallization slurry was put in a Teflon container with a stirrer, and 98 wt% sulfuric acid was added at 80 ° C to neutralize caustic soda in the filtrate to pH 10. At this time, metal hydroxide such as iron and a part of silicic acid co-precipitated in the solution by forming flocs. The filterability of this floc is very good, there is no need to use a filter aid, and a filtrate of iron: 0.2 wt ppm and SiO 2 : 635 wt ppm is obtained and used as a raw material for the preparation of the feed liquid of the previous crystallization. .
[0029]
Comparative Example 1
All operations were performed in the same manner as in Example 1 except that sodium silicate was not added to the crude sodium sulfate aqueous solution to crystallize sodium sulfate crystals. At this time, the crystallization slurry was reddish brown, and precipitation of ferric hydroxide was observed. Although the precipitate was anhydrous sodium sulfate, the crystal growth was inferior and the average particle size was 180 μm. Deposited ferric hydroxide adhered to this sodium sulfate anhydrous salt and exhibited a yellow color. The iron concentration in the crystals was as high as 8.3 ppm by weight, and iron could not be purified during crystallization. The results relating to crystal growth and quality in crystallization of sodium sulfate crystals are also shown in Table 1.
[0030]
Example 2
Sodium sulfate aqueous solution containing 500 ppm by weight of sodium pyrophosphate as P 2 O 5 : iron: 8 ppm by weight, sodium chloride: 1% by weight, caustic soda: 15% by weight crude sodium sulfate having a pH of 10.2 Except for using an aqueous solution, the same operation as in Example 1 was performed to crystallize sodium sulfate crystals. The precipitate was diamond-like sodium sulfate anhydrous with an average particle diameter of 210 μm, no impurities including iron were precipitated, and the crystallization filtrate was colorless and transparent. Next, the slurry was centrifuged and dried at 110 ° C. to obtain white sodium sulfate having good fluidity. The purity was as high as 99.9%, and the iron content was as low as 0.3 ppm by weight. The results relating to crystal growth and quality in crystallization of sodium sulfate crystals are also shown in Table 1.
[0031]
Example 3
The same procedure as in Example 1 except that the addition of 400 ppm by weight of sodium silicate in the aqueous solution of sodium sulfate and 200 ppm by weight and pyrophosphate soda as SiO 2 as P 2 O 5, crystallization was carried out in the sodium sulfate crystals . The precipitate was diamond-like sodium sulfate anhydrous with an average particle diameter of 205 μm, no impurities including iron were precipitated, and the crystallization filtrate was colorless and transparent. Next, the slurry was centrifuged and dried at 110 ° C. to obtain white sodium sulfate having good fluidity. The purity was as high as 99.9%, and the iron content was as low as 0.3 ppm by weight. The results relating to crystal growth and quality in crystallization of sodium sulfate crystals are also shown in Table 1.
[0032]
Example 4
Using the same apparatus as in Example 1, 129 parts by weight of 48 wt% sodium hydroxide aqueous solution of SiO 2 335 ppm by weight with the addition of sodium silicate / Hr, 98 wt% of iron 80 ppm by weight sulfuric acid 60 parts by weight / Hr Then, The neutralized filtrate 282 parts by weight / Hr obtained in Example 1 was separately and continuously introduced into a crystallizer, and evaporated and crystallized as a 19% by weight aqueous sodium sulfate solution having a pH of 12.6. The operating conditions were an absolute pressure of 350 Torr, 85 ° C., and the amount of evaporated water was 50 parts by weight / Hr. Then, 421 parts by weight of crystallization slurry having a slurry concentration of 29% by weight per hour was extracted every 5 minutes. The apparent crystal residence time was 2 hours, the filtrate of the crystallization slurry had a pH of 12.8, a caustic soda concentration of 5% by weight, a sodium sulfate concentration of 17% by weight, colorless and transparent, and no precipitation of impurities such as iron. The obtained crystal was anhydrous sodium sulfate, and the growth thereof was good, and it was a diamond-like crystal having an average particle diameter of 210 μm. The sodium sulfate obtained by centrifuging this slurry and drying at 110 ° C. has good fluidity, high purity of 99.9%, low iron content of 0.4 ppm by weight, white color and high quality. It was. The results relating to crystal growth and quality in crystallization of sodium sulfate crystals are also shown in Table 1.
[0033]
Comparative Example 2
Using the same apparatus as in Example 1, 261 parts by weight of 48% by weight aqueous caustic soda solution and 155 parts by weight of 98% sulfuric acid / Hr by 30 ppm by weight of iron were separately and continuously introduced into the crystallizer, Crystallization of sodium sulfate crystals was performed at 100 ° C. under normal pressure. The amount of evaporated water was 25 parts by weight / Hr, and 391 parts by weight of crystallization slurry having a slurry concentration of 30% by weight was withdrawn at a constant rate every 5 minutes. The apparent crystal residence time at this time was 2 hours, and the filtrate of the crystallization slurry was caustic soda having a pH of 11.8: 0.5% by weight, iron: 17% by weight, sodium sulfate 30% by weight, reddish brown, and iron was hydroxylated. Deposited as ferric iron. The crystals obtained by centrifuging this slurry were anhydrous sodium sulfate and exhibited a strong yellow color. The average particle size of the crystals was 185 μm. This filter cake was dried at 110 ° C. to obtain sodium sulfate, but its iron content was as high as 13.0 ppm by weight and could not be made into a product. The results relating to crystal growth and quality in crystallization of sodium sulfate crystals are also shown in Table 1.
[0034]
Comparative Example 3
Using the same apparatus as in Example 1, 273 parts by weight of 48% by weight aqueous caustic soda solution and 165 parts by weight of 98% sulfuric acid / Hr with 30 ppm by weight of iron were separately and continuously introduced into the crystallizer. Crystallization of sodium sulfate crystals was performed at 100 ° C. under pressure. The amount of evaporated water was 30 parts by weight / Hr, and 408 parts by weight of crystallization slurry having a slurry concentration of 35% by weight was withdrawn at a constant rate every 5 minutes. At this time, the apparent crystal residence time was 2 hours, the filtrate of the crystallization slurry had a sulfuric acid concentration of 0.5% by weight (pH 2.4), a sodium sulfate concentration of 31% by weight, was colorless and transparent, and there was no precipitation of iron. However, although the crystals obtained by centrifuging this slurry were anhydrous sodium sulfate, it was yellow, suggesting the transfer of iron into the crystals. The average particle size of the crystals was 170 μm, and the 110 ° C. dry product of this filter cake was yellow and its iron content was as high as 26.5 ppm by weight, which could not be made into a product. The results relating to crystal growth and quality in crystallization of sodium sulfate crystals are also shown in Table 1.
[0035]
【The invention's effect】
As described above in detail, according to the present invention, impurities can be separated and purified by a simple operation, which is economical and industrial, and is extremely useful industrially. The effects of the present invention are listed below.
[0036]
(1) Impurities can be prevented from being mixed into sodium sulfate crystals.
[0037]
(2) The crystal growth of sodium sulfate crystals is good, the particle size is large, and the fluidity is good.
[0038]
(3) Corrosion resistance of the device material is improved by crystallization of an alkaline aqueous solution.
[0039]
(4) Impurities can be easily purged out of the system by the operation of neutralizing the crystallization filtrate.
[0040]
(5) The crystallization filtrate can be circulated, and the basic unit of the raw material can be reduced.
[0041]
(6) By adding an acid to sodium sulfate crystals and filtering, neutral sodium sulfate having a pH of 6 to 8 when dissolved in water can be obtained.
Claims (8)
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| KR101016846B1 (en) * | 2009-02-25 | 2011-02-22 | 이엔비나노텍(주) | Apparatus for manufacturing nanoporous silica, sodium sulfate and hydrogen fluoride using fast reaction nozzle, and method for manufacturing nanoporous silica using fast reaction nozzle |
| KR101014984B1 (en) | 2010-12-02 | 2011-02-16 | 이엔비나노텍(주) | Method for manufacturing sodium sulfate and hydrogen fluoride using fast reaction nozzle |
Citations (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS5265195A (en) * | 1975-11-26 | 1977-05-30 | Showa Denko Kk | Method for production of neutral sodium sulfate |
| JPS5280276A (en) * | 1975-12-27 | 1977-07-05 | Nippon Chem Ind Co Ltd:The | Treatment of solution resulted from sodium sulfitegypsum exhaust gas d esulfurization |
| JPS5383994A (en) * | 1976-12-02 | 1978-07-24 | Ajinomoto Co Inc | Production of neutral anhydrous sodium sulfate |
| JP2002087814A (en) * | 2000-09-12 | 2002-03-27 | Tosoh Corp | Neutral sodium sulfate composition and its manufacturing method |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS5265195A (en) * | 1975-11-26 | 1977-05-30 | Showa Denko Kk | Method for production of neutral sodium sulfate |
| JPS5280276A (en) * | 1975-12-27 | 1977-07-05 | Nippon Chem Ind Co Ltd:The | Treatment of solution resulted from sodium sulfitegypsum exhaust gas d esulfurization |
| JPS5383994A (en) * | 1976-12-02 | 1978-07-24 | Ajinomoto Co Inc | Production of neutral anhydrous sodium sulfate |
| JP2002087814A (en) * | 2000-09-12 | 2002-03-27 | Tosoh Corp | Neutral sodium sulfate composition and its manufacturing method |
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