JP6286155B2 - Method for producing crystalline iron arsenate from a solution containing pentavalent arsenic - Google Patents
Method for producing crystalline iron arsenate from a solution containing pentavalent arsenic Download PDFInfo
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- JP6286155B2 JP6286155B2 JP2013181303A JP2013181303A JP6286155B2 JP 6286155 B2 JP6286155 B2 JP 6286155B2 JP 2013181303 A JP2013181303 A JP 2013181303A JP 2013181303 A JP2013181303 A JP 2013181303A JP 6286155 B2 JP6286155 B2 JP 6286155B2
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- BMWMWYBEJWFCJI-UHFFFAOYSA-K iron(3+);trioxido(oxo)-$l^{5}-arsane Chemical compound [Fe+3].[O-][As]([O-])([O-])=O BMWMWYBEJWFCJI-UHFFFAOYSA-K 0.000 title claims description 108
- RQNWIZPPADIBDY-UHFFFAOYSA-N arsenic atom Chemical compound [As] RQNWIZPPADIBDY-UHFFFAOYSA-N 0.000 title claims description 82
- 229910052785 arsenic Inorganic materials 0.000 title claims description 70
- 238000004519 manufacturing process Methods 0.000 title claims description 19
- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims description 373
- 229910052742 iron Inorganic materials 0.000 claims description 177
- 238000006243 chemical reaction Methods 0.000 claims description 100
- UQSXHKLRYXJYBZ-UHFFFAOYSA-N Iron oxide Chemical compound [Fe]=O UQSXHKLRYXJYBZ-UHFFFAOYSA-N 0.000 claims description 86
- 239000007788 liquid Substances 0.000 claims description 74
- 239000013078 crystal Substances 0.000 claims description 45
- 239000007864 aqueous solution Substances 0.000 claims description 34
- 238000000034 method Methods 0.000 claims description 30
- 238000001556 precipitation Methods 0.000 claims description 27
- UYZMAFWCKGTUMA-UHFFFAOYSA-K iron(3+);trioxido(oxo)-$l^{5}-arsane;dihydrate Chemical compound O.O.[Fe+3].[O-][As]([O-])([O-])=O UYZMAFWCKGTUMA-UHFFFAOYSA-K 0.000 claims description 19
- 150000001495 arsenic compounds Chemical class 0.000 claims description 18
- 239000007789 gas Substances 0.000 claims description 15
- -1 iron ion Chemical class 0.000 claims description 14
- 230000032683 aging Effects 0.000 claims description 13
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 claims description 11
- 239000001301 oxygen Substances 0.000 claims description 11
- 229910052760 oxygen Inorganic materials 0.000 claims description 11
- 230000001590 oxidative effect Effects 0.000 claims description 10
- 230000008569 process Effects 0.000 claims description 9
- VETKVGYBAMGARK-UHFFFAOYSA-N arsanylidyneiron Chemical compound [As]#[Fe] VETKVGYBAMGARK-UHFFFAOYSA-N 0.000 claims description 8
- 238000007664 blowing Methods 0.000 claims description 8
- JPVYNHNXODAKFH-UHFFFAOYSA-N Cu2+ Chemical compound [Cu+2] JPVYNHNXODAKFH-UHFFFAOYSA-N 0.000 claims description 4
- 229910001431 copper ion Inorganic materials 0.000 claims description 4
- 150000002506 iron compounds Chemical class 0.000 claims description 4
- 230000001376 precipitating effect Effects 0.000 claims 2
- 150000002500 ions Chemical class 0.000 description 76
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 description 69
- 239000002244 precipitate Substances 0.000 description 39
- 150000001875 compounds Chemical class 0.000 description 27
- 230000035484 reaction time Effects 0.000 description 25
- 239000000243 solution Substances 0.000 description 21
- LIKBJVNGSGBSGK-UHFFFAOYSA-N iron(3+);oxygen(2-) Chemical compound [O-2].[O-2].[O-2].[Fe+3].[Fe+3] LIKBJVNGSGBSGK-UHFFFAOYSA-N 0.000 description 18
- DJHGAFSJWGLOIV-UHFFFAOYSA-N Arsenic acid Chemical compound O[As](O)(O)=O DJHGAFSJWGLOIV-UHFFFAOYSA-N 0.000 description 15
- 229940000488 arsenic acid Drugs 0.000 description 15
- 238000010828 elution Methods 0.000 description 15
- 238000002425 crystallisation Methods 0.000 description 14
- 230000008025 crystallization Effects 0.000 description 14
- 229910052595 hematite Inorganic materials 0.000 description 13
- 239000011019 hematite Substances 0.000 description 13
- 239000002243 precursor Substances 0.000 description 13
- 238000000926 separation method Methods 0.000 description 13
- 239000006228 supernatant Substances 0.000 description 12
- 238000007254 oxidation reaction Methods 0.000 description 11
- 239000002245 particle Substances 0.000 description 11
- 238000003723 Smelting Methods 0.000 description 10
- 239000010949 copper Substances 0.000 description 10
- 230000005070 ripening Effects 0.000 description 10
- 229910052802 copper Inorganic materials 0.000 description 9
- 238000005755 formation reaction Methods 0.000 description 9
- 150000003839 salts Chemical class 0.000 description 9
- 238000003756 stirring Methods 0.000 description 9
- DJHGAFSJWGLOIV-UHFFFAOYSA-K Arsenate3- Chemical compound [O-][As]([O-])([O-])=O DJHGAFSJWGLOIV-UHFFFAOYSA-K 0.000 description 8
- 230000002378 acidificating effect Effects 0.000 description 8
- 230000015572 biosynthetic process Effects 0.000 description 8
- 240000001973 Ficus microcarpa Species 0.000 description 7
- 239000002253 acid Substances 0.000 description 7
- 230000007423 decrease Effects 0.000 description 7
- 229940000489 arsenate Drugs 0.000 description 6
- 230000000052 comparative effect Effects 0.000 description 6
- 230000000694 effects Effects 0.000 description 6
- 239000007787 solid Substances 0.000 description 6
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 6
- MYMOFIZGZYHOMD-UHFFFAOYSA-N Dioxygen Chemical compound O=O MYMOFIZGZYHOMD-UHFFFAOYSA-N 0.000 description 5
- 229910001882 dioxygen Inorganic materials 0.000 description 5
- 230000003647 oxidation Effects 0.000 description 5
- 239000002994 raw material Substances 0.000 description 5
- QAOWNCQODCNURD-UHFFFAOYSA-L Sulfate Chemical compound [O-]S([O-])(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-L 0.000 description 4
- 238000002441 X-ray diffraction Methods 0.000 description 4
- 238000010494 dissociation reaction Methods 0.000 description 4
- 229940093920 gynecological arsenic compound Drugs 0.000 description 4
- 239000000543 intermediate Substances 0.000 description 4
- 239000007800 oxidant agent Substances 0.000 description 4
- 238000011084 recovery Methods 0.000 description 4
- 239000000126 substance Substances 0.000 description 4
- DJHGAFSJWGLOIV-UHFFFAOYSA-M arsenate(1-) Chemical compound O[As](O)([O-])=O DJHGAFSJWGLOIV-UHFFFAOYSA-M 0.000 description 3
- 238000004090 dissolution Methods 0.000 description 3
- 239000011790 ferrous sulphate Substances 0.000 description 3
- 235000003891 ferrous sulphate Nutrition 0.000 description 3
- 229910052598 goethite Inorganic materials 0.000 description 3
- AEIXRCIKZIZYPM-UHFFFAOYSA-M hydroxy(oxo)iron Chemical compound [O][Fe]O AEIXRCIKZIZYPM-UHFFFAOYSA-M 0.000 description 3
- BAUYGSIQEAFULO-UHFFFAOYSA-L iron(2+) sulfate (anhydrous) Chemical compound [Fe+2].[O-]S([O-])(=O)=O BAUYGSIQEAFULO-UHFFFAOYSA-L 0.000 description 3
- 229910000359 iron(II) sulfate Inorganic materials 0.000 description 3
- SZVJSHCCFOBDDC-UHFFFAOYSA-N iron(II,III) oxide Inorganic materials O=[Fe]O[Fe]O[Fe]=O SZVJSHCCFOBDDC-UHFFFAOYSA-N 0.000 description 3
- 239000000203 mixture Substances 0.000 description 3
- XKRFYHLGVUSROY-UHFFFAOYSA-N Argon Chemical compound [Ar] XKRFYHLGVUSROY-UHFFFAOYSA-N 0.000 description 2
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 2
- VEXZGXHMUGYJMC-UHFFFAOYSA-M Chloride anion Chemical compound [Cl-] VEXZGXHMUGYJMC-UHFFFAOYSA-M 0.000 description 2
- CWYNVVGOOAEACU-UHFFFAOYSA-N Fe2+ Chemical compound [Fe+2] CWYNVVGOOAEACU-UHFFFAOYSA-N 0.000 description 2
- 229910002588 FeOOH Inorganic materials 0.000 description 2
- 229910002651 NO3 Inorganic materials 0.000 description 2
- NHNBFGGVMKEFGY-UHFFFAOYSA-N Nitrate Chemical compound [O-][N+]([O-])=O NHNBFGGVMKEFGY-UHFFFAOYSA-N 0.000 description 2
- GRYLNZFGIOXLOG-UHFFFAOYSA-N Nitric acid Chemical compound O[N+]([O-])=O GRYLNZFGIOXLOG-UHFFFAOYSA-N 0.000 description 2
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 description 2
- 239000004809 Teflon Substances 0.000 description 2
- 229920006362 Teflon® Polymers 0.000 description 2
- 239000003513 alkali Substances 0.000 description 2
- 238000004458 analytical method Methods 0.000 description 2
- 150000001450 anions Chemical class 0.000 description 2
- 239000011260 aqueous acid Substances 0.000 description 2
- 239000003054 catalyst Substances 0.000 description 2
- 230000003197 catalytic effect Effects 0.000 description 2
- 229910000365 copper sulfate Inorganic materials 0.000 description 2
- ARUVKPQLZAKDPS-UHFFFAOYSA-L copper(II) sulfate Chemical compound [Cu+2].[O-][S+2]([O-])([O-])[O-] ARUVKPQLZAKDPS-UHFFFAOYSA-L 0.000 description 2
- 230000008021 deposition Effects 0.000 description 2
- 238000007323 disproportionation reaction Methods 0.000 description 2
- 230000005593 dissociations Effects 0.000 description 2
- 208000018459 dissociative disease Diseases 0.000 description 2
- 239000012153 distilled water Substances 0.000 description 2
- 238000001914 filtration Methods 0.000 description 2
- 239000011521 glass Substances 0.000 description 2
- XLYOFNOQVPJJNP-UHFFFAOYSA-M hydroxide Chemical compound [OH-] XLYOFNOQVPJJNP-UHFFFAOYSA-M 0.000 description 2
- 238000002354 inductively-coupled plasma atomic emission spectroscopy Methods 0.000 description 2
- 230000007935 neutral effect Effects 0.000 description 2
- 229910017604 nitric acid Inorganic materials 0.000 description 2
- BASFCYQUMIYNBI-UHFFFAOYSA-N platinum Chemical compound [Pt] BASFCYQUMIYNBI-UHFFFAOYSA-N 0.000 description 2
- 229920006395 saturated elastomer Polymers 0.000 description 2
- 241001061225 Arcos Species 0.000 description 1
- 238000004438 BET method Methods 0.000 description 1
- 238000001016 Ostwald ripening Methods 0.000 description 1
- MUBZPKHOEPUJKR-UHFFFAOYSA-N Oxalic acid Chemical compound OC(=O)C(O)=O MUBZPKHOEPUJKR-UHFFFAOYSA-N 0.000 description 1
- 230000001133 acceleration Effects 0.000 description 1
- 230000009471 action Effects 0.000 description 1
- 229910052786 argon Inorganic materials 0.000 description 1
- AQLMHYSWFMLWBS-UHFFFAOYSA-N arsenite(1-) Chemical compound O[As](O)[O-] AQLMHYSWFMLWBS-UHFFFAOYSA-N 0.000 description 1
- 239000007853 buffer solution Substances 0.000 description 1
- 150000001768 cations Chemical class 0.000 description 1
- 238000005119 centrifugation Methods 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 239000013626 chemical specie Substances 0.000 description 1
- 238000010790 dilution Methods 0.000 description 1
- 239000012895 dilution Substances 0.000 description 1
- 230000008034 disappearance Effects 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 239000000706 filtrate Substances 0.000 description 1
- 239000010419 fine particle Substances 0.000 description 1
- 239000001257 hydrogen Substances 0.000 description 1
- 229910052739 hydrogen Inorganic materials 0.000 description 1
- 230000006872 improvement Effects 0.000 description 1
- 239000011261 inert gas Substances 0.000 description 1
- 229910052500 inorganic mineral Inorganic materials 0.000 description 1
- RUTXIHLAWFEWGM-UHFFFAOYSA-H iron(3+) sulfate Chemical compound [Fe+3].[Fe+3].[O-]S([O-])(=O)=O.[O-]S([O-])(=O)=O.[O-]S([O-])(=O)=O RUTXIHLAWFEWGM-UHFFFAOYSA-H 0.000 description 1
- JEIPFZHSYJVQDO-UHFFFAOYSA-N iron(III) oxide Inorganic materials O=[Fe]O[Fe]=O JEIPFZHSYJVQDO-UHFFFAOYSA-N 0.000 description 1
- 229910000360 iron(III) sulfate Inorganic materials 0.000 description 1
- FLTRNWIFKITPIO-UHFFFAOYSA-N iron;trihydrate Chemical compound O.O.O.[Fe] FLTRNWIFKITPIO-UHFFFAOYSA-N 0.000 description 1
- 229910052935 jarosite Inorganic materials 0.000 description 1
- 238000002386 leaching Methods 0.000 description 1
- 150000002632 lipids Chemical class 0.000 description 1
- 239000007791 liquid phase Substances 0.000 description 1
- 239000000463 material Substances 0.000 description 1
- 230000007246 mechanism Effects 0.000 description 1
- 239000012528 membrane Substances 0.000 description 1
- 229910052751 metal Inorganic materials 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 229910021645 metal ion Inorganic materials 0.000 description 1
- 150000002739 metals Chemical class 0.000 description 1
- 239000011707 mineral Substances 0.000 description 1
- 235000010755 mineral Nutrition 0.000 description 1
- 229910052757 nitrogen Inorganic materials 0.000 description 1
- 239000008363 phosphate buffer Substances 0.000 description 1
- XNGIFLGASWRNHJ-UHFFFAOYSA-L phthalate(2-) Chemical compound [O-]C(=O)C1=CC=CC=C1C([O-])=O XNGIFLGASWRNHJ-UHFFFAOYSA-L 0.000 description 1
- 229910052697 platinum Inorganic materials 0.000 description 1
- 239000011148 porous material Substances 0.000 description 1
- 239000012286 potassium permanganate Substances 0.000 description 1
- 239000000047 product Substances 0.000 description 1
- 230000009257 reactivity Effects 0.000 description 1
- 230000009467 reduction Effects 0.000 description 1
- 239000007790 solid phase Substances 0.000 description 1
- 239000012086 standard solution Substances 0.000 description 1
- 239000011550 stock solution Substances 0.000 description 1
- 238000000967 suction filtration Methods 0.000 description 1
- 238000004065 wastewater treatment Methods 0.000 description 1
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- Compounds Of Iron (AREA)
Description
本発明は、砒素を含む水溶液から砒素を分離・回収するための、化学的に安定で、かつ、分離操作時の分離性に優れた結晶性砒酸鉄を得る方法に関する。 The present invention relates to a method for obtaining crystalline iron arsenate that is chemically stable and excellent in separability during a separation operation, for separating and recovering arsenic from an aqueous solution containing arsenic.
非鉄製錬のプロセスにおいては、様々な形態の製錬原料を使用し、また、各種製錬中間物が発生する。これらの製錬原料や製錬中間物には有価金属が含まれているが、砒素などの環境上好ましくない元素が含まれている場合も多い。製錬原料や製錬中間物からの砒素の分離・回収には、一般に、製錬原料や製錬中間物から、酸の水溶液により浸出させた砒素化合物を難溶性塩として沈殿・析出させた後に、濾過等の方法により機械的に固液分離する方法が用いられる。したがって、これらの沈殿物には、保管の際の安定性と、分離操作時の良好な分離性が求められる。 In the nonferrous smelting process, various forms of smelting raw materials are used, and various smelting intermediates are generated. These smelting raw materials and smelting intermediates contain valuable metals, but often contain environmentally undesirable elements such as arsenic. For separation and recovery of arsenic from smelting raw materials and smelting intermediates, arsenic compounds leached with an aqueous acid solution from smelting raw materials and smelting intermediates are generally precipitated and deposited as sparingly soluble salts. A method of mechanically separating solid and liquid by a method such as filtration is used. Therefore, these precipitates are required to have stability during storage and good separability during the separation operation.
砒素の化合物は、その酸化数が＋３価もしくは＋５価の状態で存在するが（以下、それぞれ３価もしくは５価と呼称する。）、３価の砒素化合物（亜砒酸塩）は一般的に水溶性のものが多いので、砒素の分離・回収には、通常、水に難溶性の５価の砒素化合物（砒酸塩）を用いる。アルカリ土類の砒酸塩は一般に水に難溶性であるが、これらの塩の沈殿生成はアルカリ性域で行うので、前処理として酸の水溶液で浸出を行う本発明の技術分野には不向きである。酸性域で難溶性を示す砒酸塩には、例えば、砒酸塩鉱物のスコロド石（スコロダイト）がある。スコロダイト（ＦｅＡｓＯ4・２Ｈ2Ｏ）は、３価の陽イオンである鉄（３）イオン（Ｆｅ3+）と３価の陰イオンである砒酸イオン（ＡｓＯ4 3-）が１対１で結合した化学的に安定な化合物であるが、砒酸イオンの対イオンが安価に入手可能な鉄（３）イオンであるため、砒素の分離・回収には、スコロダイト類似の化合物の析出反応を利用することが多い。スコロダイトの析出反応を利用した砒素の処理技術には、例えば、以下のものがある。なお、価数を表す前記の括弧内の数字は、本来、ローマ数字で書き表すべきものである。 Arsenic compounds exist in an oxidation number of +3 or +5 (hereinafter referred to as trivalent or pentavalent, respectively). Trivalent arsenic compounds (arsenite) are generally water-soluble. Therefore, for separation and recovery of arsenic, a pentavalent arsenic compound (arsenate) that is hardly soluble in water is usually used. Alkaline earth arsenates are generally poorly soluble in water, but the precipitation of these salts is carried out in the alkaline region, and is not suitable for the technical field of the present invention in which leaching is performed with an aqueous acid solution as a pretreatment. Examples of arsenates that are sparingly soluble in the acidic region include arborite mineral scorodite. In scorodite (FeAsO 4 · 2H 2 O), iron (3) ion (Fe 3+ ), a trivalent cation, and arsenate ion (AsO 4 3− ), a trivalent anion, are combined in a one-to-one relationship. Although it is a chemically stable compound, the counter ion of arsenate ion is iron (3) ion which can be obtained at low cost, so the precipitation reaction of scorodite-like compound should be used for separation and recovery of arsenic There are many. Examples of the arsenic treatment technology using the scorodite precipitation reaction include the following. In addition, the number in the parenthesis that represents the valence should be written in Roman numerals.
特許文献１（特開２０１０−２８５３２２号公報）には、５価の砒素化合物を含有する水溶液のｐＨを０.８以上３.０以下とし、そこに３価鉄源を添加し、結晶性スコロダイトを得る技術が開示されており、３価鉄源として、鉄（３）イオンおよび、固体状態の水酸化鉄（Ｆｅ(ＯＨ)3）、ゲーサイト（ＦｅＯＯＨ）および酸化鉄（Ｆｅ2Ｏ3）が挙げられている。なお、この技術では、鉄源は全て３価の鉄（３）であり、鉄（２）イオンは使用されていない。
特許文献２（特開２００８−１０５９２１号公報）には、５価の砒素化合物と２価の鉄（２）イオン（Ｆｅ2+）を含有する水溶液に、酸化剤として酸素ガスを吹き込み、スコロダイトの沈殿形成反応を最終的にｐＨが１.２以下で終結させる技術が開示されている。この技術は、スコロダイトに含まれる鉄（３）イオンを、鉄（２）イオンの酸素酸化反応により供給するものである。
特許文献３（特開２０１１−１７８６０２号公報）には、５価の砒素化合物、鉄（３）イオン、および鉄（２）イオンを含有する水溶液のｐＨを１以下とした後、酸化剤として酸素ガスを吹き込み、結晶性ヒ酸鉄を得る技術が開示されている。この技術の場合、結晶性ヒ酸鉄に含まれる鉄（３）イオンは、反応溶液中に最初に添加した鉄（３）イオンおよび、鉄（２）イオンの酸素酸化反応により生成した鉄（３）イオンの両方である。
特許文献４（特開２００９−０１８２９１号公報）には、５価の砒素化合物と鉄（２）イオンを含有する水溶液に、種晶としてスコロダイト、ヘマタイト、ジャロサイト、ゲーサイト等の３価の鉄塩を添加した後、酸化剤として酸素ガスを吹き込み、スコロダイトを生成する技術が開示されている。この技術では、ヘマタイトは、種晶としての効果のみならず、３価の鉄源としての作用も有するとされている。
In Patent Document 1 (Japanese Patent Laid-Open No. 2010-285322), the pH of an aqueous solution containing a pentavalent arsenic compound is adjusted to 0.8 or more and 3.0 or less, a trivalent iron source is added thereto, and crystalline scorodite is added. As a trivalent iron source, iron (3) ions, solid state iron hydroxide (Fe (OH) 3 ), goethite (FeOOH), and iron oxide (Fe 2 O 3 ) are disclosed. Is listed. In this technique, all iron sources are trivalent iron (3), and iron (2) ions are not used.
In Patent Document 2 (Japanese Patent Application Laid-Open No. 2008-105921), oxygen gas as an oxidizing agent is blown into an aqueous solution containing a pentavalent arsenic compound and a divalent iron (2) ion (Fe 2+ ). A technique for finally terminating the precipitation reaction at a pH of 1.2 or less is disclosed. This technique supplies iron (3) ions contained in scorodite by an oxygen oxidation reaction of iron (2) ions.
In Patent Document 3 (Japanese Patent Laid-Open No. 2011-178602), the pH of an aqueous solution containing a pentavalent arsenic compound, iron (3) ions, and iron (2) ions is set to 1 or less, and oxygen is used as an oxidizing agent. A technique for obtaining a crystalline iron arsenate by blowing a gas is disclosed. In this technique, iron (3) ions contained in crystalline iron arsenate are iron (3) ions initially added to the reaction solution and iron (3) produced by the oxygen oxidation reaction of iron (2) ions. ) Both ions.
Patent Document 4 (Japanese Patent Laid-Open No. 2009-018291) discloses trivalent iron such as scorodite, hematite, jarosite, and goethite as a seed crystal in an aqueous solution containing a pentavalent arsenic compound and iron (2) ions. A technique for generating scorodite by adding oxygen gas as an oxidizing agent after adding salt is disclosed. In this technique, hematite is considered to have not only an effect as a seed crystal but also an effect as a trivalent iron source.
しかし、上記の技術はいずれも、問題点を有するものであった。
特許文献１で開示される技術は、鉄源として鉄（３）化合物のみを用いるものであるが、その場合、反応用液中に溶け出した鉄（３）イオンの砒酸イオンとの反応性が乏しく、結晶性の良好な砒酸鉄が得られる低ｐＨ域で、結晶生成速度が極度に遅くなるという問題があった。結晶生成速度を増大させるために、砒酸鉄の生成反応を高ｐＨ域で行うと、結晶の生成速度は速くなるが、得られる結晶の結晶性が悪化し、非晶質化するため、結晶生成反応の最適ｐＨ範囲が非常に狭く、工業上実用性に乏しいという問題があった。
特許文献２の技術の場合には、鉄源として鉄（３）化合物を使用した場合の問題点を解消するために、鉄源として鉄（２）イオンを用い、それを空気酸化により鉄（３）イオンとした後、砒酸イオンと反応させるものであるが、鉄（２）イオンの空気酸化反応を必要とし、反応コストが増大するとともに、酸化反応が進行すると鉄（２）イオン濃度が減少するため、鉄（３）イオンの生成速度が減少するという問題があった。なお、特許文献２に開示された技術には、付随的に、反応系のｐＨが低く、結晶生成速度が遅いという問題、および、気液系の酸化反応を利用するため、反応溶液内の場所により生成物が不均一になるという問題があった。
特許文献３の場合には、反応系のｐＨが低く、かつ、気液系の酸化反応を利用するため、特許文献２と同様の問題があった。
特許文献４の場合には、結晶成長に大量の種晶を必要とする上、反応速度が遅く、生産性が劣るという問題があり、気液系の酸化反応を利用することに関しては、特許文献２と同様な問題点を有していた。また、種晶としてヘマタイトを用いた場合には、得られる結晶がヘマタイトとスコロダイトの混合物であるため、保存安定性に難点があった。
However, all of the above techniques have problems.
The technique disclosed in Patent Document 1 uses only an iron (3) compound as an iron source. In this case, the reactivity of iron (3) ions dissolved in the reaction solution with arsenate ions is low. There was a problem that the crystal formation rate was extremely slow in the low pH range where iron arsenate with poor crystallinity and good crystallinity was obtained. In order to increase the rate of crystal formation, if the iron arsenate formation reaction is performed in a high pH range, the rate of crystal formation increases, but the crystallinity of the resulting crystal deteriorates and becomes amorphous. There was a problem that the optimum pH range of the reaction was very narrow and the industrial utility was poor.
In the case of the technique of Patent Document 2, in order to solve the problem in the case where an iron (3) compound is used as an iron source, iron (2) ions are used as an iron source, and the iron (3) is oxidized by air oxidation. ) Ion and then reacted with arsenate ion, but requires an air oxidation reaction of iron (2) ion, the reaction cost increases, and the iron (2) ion concentration decreases as the oxidation reaction proceeds Therefore, there has been a problem that the production rate of iron (3) ions decreases. In addition, the technique disclosed in Patent Document 2 is accompanied by the problem that the pH of the reaction system is low and the rate of crystal formation is slow, and the oxidation reaction of the gas-liquid system. As a result, the product becomes non-uniform.
In the case of Patent Document 3, since the pH of the reaction system is low and a gas-liquid oxidation reaction is used, there is a problem similar to that of Patent Document 2.
In the case of Patent Document 4, there is a problem that a large amount of seed crystals are required for crystal growth, the reaction rate is slow, and the productivity is inferior. 2 had the same problems. In addition, when hematite is used as a seed crystal, the obtained crystal is a mixture of hematite and scorodite, and thus there is a problem in storage stability.
本発明は、上記の問題点を解決すべくなされたものであり、結晶性砒酸の鉄源として鉄（３）化合物を用い、酸性域において、経済性を有する速度で結晶性の良好な砒酸鉄を得ることを課題としている。 The present invention has been made to solve the above problems, and uses an iron (3) compound as an iron source of crystalline arsenic acid. In the acidic region, iron arsenate having good crystallinity at an economical rate. The challenge is to obtain.
鉄源として３価の鉄（３）イオンを使用する場合、砒酸を含む酸性水溶液に単に鉄（３）イオンを添加しても、低ｐＨ域では、上述の様に、結晶性砒酸鉄の成長は殆ど起こらない。これは、以下の理由による。
砒酸は３塩基酸で、３段解離する弱酸であるが、室温において、第１段の酸解離定数ｐＫa1＝２.２４である。したがって、ｐＨ２.２４以下で優勢な化学種は未解離の砒酸（Ｈ3ＡｓＯ4(aq)）と１段解離した砒酸２水素イオン（Ｈ2ＡｓＯ4 -(aq)）であり、ｐＨがそれよりも１低いｐＨ１.２４以下では、未解離の砒酸（Ｈ3ＡｓＯ4(aq)）が９０％以上を占めることになる。なお、(aq)は、水和していることを意味する。その場合、未解離の砒酸（Ｈ3ＡｓＯ4(aq)）はゼロ電荷なので、鉄（３）イオンとは殆ど反応しない。高温の水溶液中においても、同様の酸解離平衡が存在するものと考えられる。
When trivalent iron (3) ions are used as the iron source, even if iron (3) ions are simply added to an acidic aqueous solution containing arsenic acid, the crystalline iron arsenate grows at a low pH as described above. Hardly happens. This is due to the following reason.
Arsenate is 3-basic acid, is a weak acid that dissociates three stages, at room temperature, an acid dissociation constant pK a1 = 2.24 of the first stage. Therefore, the dominant chemical species at pH 2.24 or lower are undissociated arsenic acid (H 3 AsO 4 (aq)) and one-step dissociated dihydrogen arsenate ion (H 2 AsO 4 − (aq)). If the pH is 1.24 or lower, the undissociated arsenic acid (H 3 AsO 4 (aq)) accounts for 90% or more. In addition, (aq) means that it is hydrated. In that case, undissociated arsenic acid (H 3 AsO 4 (aq)) has zero charge and therefore hardly reacts with iron (3) ions. It is considered that the same acid dissociation equilibrium exists even in a high temperature aqueous solution.
ところが、本発明者等は、この低ｐＨ域においても、砒酸を含む酸性水溶液に予め鉄（２）イオンを添加した後に鉄（３）イオンもしくは酸化鉄（３）を添加すると、酸化剤が存在しなくても、結晶性砒酸鉄の前駆体となる、スコロダイト結晶構造類似のゲル状の析出物が生成することを見出し、本発明を完成した。このゲル状の前駆体の組成は不明であるが、鉄（２）イオンを含むものと推定される。ゲル状の析出物が一旦生成すると、その表面で結晶性砒酸鉄の析出が起こり、反応系のｐＨが低下しても結晶性砒酸鉄の析出が継続することを見出し、本発明を完成するに至った。本発明は、鉄源として鉄（３）化合物を用い、酸性域において、経済性を有する速度で結晶性の良好な砒酸鉄を得ることを目的としている。 However, even in this low pH range, the present inventors added an iron (3) ion or an iron oxide (3) after adding an iron (2) ion to an acidic aqueous solution containing arsenic acid in advance, so that an oxidizing agent is present. Even without this, it was found that a gel-like precipitate similar to a scorodite crystal structure, which becomes a precursor of crystalline iron arsenate, was produced, and the present invention was completed. The composition of this gel-like precursor is unknown, but is presumed to contain iron (2) ions. Once the gel-like precipitate is formed, precipitation of crystalline iron arsenate occurs on the surface, and it is found that the precipitation of crystalline iron arsenate continues even when the pH of the reaction system is lowered. It came. An object of the present invention is to obtain iron arsenate having good crystallinity at an economical speed in an acidic region using an iron (3) compound as an iron source.
なお、前記のゲル状の前駆体の生成、および、ゲル状の前駆体の表面における結晶性砒酸鉄析出の機構については、現時点では不明であるが、鉄（２）イオンが砒酸（Ｈ3ＡｓＯ4(aq)）もしくは砒酸２水素イオン（Ｈ2ＡｓＯ4 -(aq)）の解離反応に対し触媒作用を示すものと推測される。すなわち、水溶液中において鉄（２）イオンと鉄（３）イオンとが共存すると、不均化反応により電子の授受が起こるため、共存する砒酸（Ｈ3ＡｓＯ4(aq)）もしくは砒酸２水素イオン（Ｈ2ＡｓＯ4 -(aq)）の解離反応に影響を及ぼすことが考えられる。ゲル状の前駆体の表面における結晶性砒酸鉄析出反応については、後述するように、反応系の鉄（２）イオンの含有量の増大とともに、生成する結晶性砒酸鉄の量も増大することから、前駆体に含まれる鉄（２）イオンではなく、水溶液中の鉄（２）イオンが触媒作用を示すものと推測される。したがって、反応開始時に反応溶液中に存在する鉄（２）イオン量は殆ど変化せず、鉄源として投入した鉄（３）化合物のみが結晶性砒酸鉄生成に用いられることになる。 The generation of the gel-like precursor and the mechanism of crystalline iron arsenate precipitation on the surface of the gel-like precursor are currently unknown, but the iron (2) ion is arsenic acid (H 3 AsO 4 (aq)) or dihydrogen arsenate ion (H 2 AsO 4 − (aq)) is presumed to exhibit a catalytic action. That is, when iron (2) ions and iron (3) ions coexist in an aqueous solution, electrons are transferred by a disproportionation reaction, so the coexisting arsenic acid (H 3 AsO 4 (aq)) or dihydrogen arsenate ion It is considered that the dissociation reaction of (H 2 AsO 4 − (aq)) is affected. Regarding the crystalline iron arsenate precipitation reaction on the surface of the gel-like precursor, as will be described later, as the content of iron (2) ions in the reaction system increases, the amount of generated crystalline iron arsenate also increases. It is presumed that the iron (2) ions in the aqueous solution, rather than the iron (2) ions contained in the precursor, show a catalytic action. Therefore, the amount of iron (2) ions present in the reaction solution at the start of the reaction hardly changes, and only the iron (3) compound introduced as the iron source is used for the production of crystalline iron arsenate.
上記の目的は、５価の砒素化合物と２価の鉄イオンを含み、ｐＨが１.０〜２.０の水溶液に３価の鉄化合物を添加し、鉄砒素化合物を析出させる、結晶性砒酸鉄の製造方法により達成される。この製造方法において、添加する３価の鉄化合物は３価の鉄イオンであっても、３価の鉄酸化物であっても構わない。 The above object is to add crystalline trivalent iron compound to an aqueous solution containing pentavalent arsenic compound and divalent iron ion and having a pH of 1.0 to 2.0 to precipitate the iron arsenic compound. This is achieved by the iron production method. In this production method, the trivalent iron compound to be added may be a trivalent iron ion or a trivalent iron oxide.
また、本発明の製造方法は、析出した鉄砒素化合物の熟成プロセスをさらに含むことができる。熟成プロセスを経ると、結晶性砒酸鉄の平均粒径が増加し、固液分離時の分離性が増加する。
また、本発明の製造方法は、鉄砒素化合物の析出反応の後期に、酸素を含む酸化性ガスを吹き込むプロセスをさらに含むことができる。酸化性ガスの吹込みにより、残留するゲル状の前駆体を結晶性砒酸鉄へ変換することが出来、当該結晶性砒酸鉄の砒素溶出特性の向上のみならず、固液分離時の分離性が増加する。さらに反応後期における結晶化反応を早めることになるため、結晶化反応終了後の砒素濃度をさらに低減することが出来る。
また、本発明の製造方法は、上記水溶液が２価の銅イオンをさらに含むことができる。反応溶液に２価の銅イオンを共存させると、結晶性砒酸鉄への結晶化反応が反応初期から促進され、当該結晶性砒酸鉄の砒素溶出特性の向上と共に結晶化反応終了後の砒素濃度をさらに低減することが出来る。
また、本発明の製造方法は、上記水溶液が種結晶としてのスコロダイト結晶をさらに含むことができる。反応溶液にスコロダイト結晶を共存させると、結晶性砒酸鉄の平均粒径が増加し、固液分離時の分離性が増加する。
なお、本発明の製造方法においては、熟成、反応後期の酸化性ガスの吹き込み、２価の銅イオンの共存、および、種結晶の共存の各プロセスは、それらを複合して行っても良い。
In addition, the production method of the present invention can further include an aging process of the precipitated iron arsenic compound. Through the aging process, the average particle size of crystalline iron arsenate increases and the separability during solid-liquid separation increases.
The production method of the present invention may further include a process of blowing an oxidizing gas containing oxygen at the latter stage of the precipitation reaction of the iron arsenic compound. By blowing the oxidizing gas, the remaining gel-like precursor can be converted into crystalline iron arsenate, which not only improves the arsenic elution characteristics of the crystalline iron arsenate but also improves the separability during solid-liquid separation. To increase. Furthermore, since the crystallization reaction is accelerated in the later stage of the reaction, the arsenic concentration after the completion of the crystallization reaction can be further reduced.
In the production method of the present invention, the aqueous solution may further contain a divalent copper ion. When divalent copper ions are allowed to coexist in the reaction solution, the crystallization reaction to crystalline iron arsenate is promoted from the beginning of the reaction, and the arsenic concentration after the completion of the crystallization reaction is improved along with the improvement of the arsenic elution characteristics of the crystalline iron arsenate. Further reduction can be achieved.
In the production method of the present invention, the aqueous solution may further contain scorodite crystals as seed crystals. When scorodite crystals are allowed to coexist in the reaction solution, the average particle diameter of crystalline iron arsenate increases and the separability during solid-liquid separation increases.
In the production method of the present invention, the processes of aging, blowing in an oxidizing gas in the late stage of reaction, coexistence of divalent copper ions, and coexistence of seed crystals may be performed in combination.
以上、本発明においては、５価の砒素化合物を含む酸性水溶液に、予め２価の鉄イオンを添加した後、鉄源として３価の鉄化合物を添加することにより、酸性域において、経済性を有する速度で結晶性の良好な砒酸鉄を得ることが出来る。 As mentioned above, in this invention, after adding a bivalent iron ion beforehand to the acidic aqueous solution containing a pentavalent arsenic compound, by adding a trivalent iron compound as an iron source, economical efficiency is obtained in an acidic region. Iron arsenate with good crystallinity can be obtained at a rate of
［砒素化合物］
本発明が処理対象とする砒素含有液（被処理液）としては、非鉄製錬の過程で発生するものを始めとして、いかなるものでも使用することが可能である。本発明においては、被処理液中の３価の砒素は予め全て酸化して、５価の砒酸イオンの形態にしておくことが好ましい。砒素の酸化については、例えば特開２００９−２４２２２１号公報に開示されている方法を始めとして、公知のいかなる方法を用いても構わない。なお、本発明の実施例のモデル被処理液では、砒素源として５価の砒酸を用いている。
本発明においては、被処理液中の砒素濃度は特に規定するものではないが、鉄（２）化合物を添加した状態で、砒素として２０ｇ／Ｌ以上、好ましくは５０ｇ／Ｌ以上とする。砒素濃度が２０ｇ／Ｌ未満と低いと、結晶性砒酸鉄の生成速度が遅く、粒子径が小さくなるので、好ましくない。また一度に処理される砒素量が少ないので、経済的な観点から好ましくない。従って、砒素として２０ｇ／Ｌ以上、好ましくは５０ｇ／Ｌ以上と高い程好ましい。
被処理液中に溶解可能な砒素濃度の上限は、共存する金属イオンや他のアニオンの溶解量等、被処理液の発生の経緯によって変化する量であり、鉄（２）化合物を添加した状態で、砒酸鉄以外の沈殿物が発生しない様に、適宜調整する。
[Arsenic compounds]
As the arsenic-containing liquid (liquid to be treated) to be treated by the present invention, any liquid can be used including those generated in the process of non-ferrous smelting. In the present invention, it is preferable that all of the trivalent arsenic in the liquid to be treated is previously oxidized to form pentavalent arsenate ions. As for the arsenic oxidation, any known method such as the method disclosed in Japanese Patent Application Laid-Open No. 2009-242221 may be used. In the model liquid to be treated according to the embodiment of the present invention, pentavalent arsenic acid is used as the arsenic source.
In the present invention, the concentration of arsenic in the liquid to be treated is not particularly specified, but arsenic is 20 g / L or more, preferably 50 g / L or more in a state where an iron (2) compound is added. An arsenic concentration as low as less than 20 g / L is not preferable because the production rate of crystalline iron arsenate is slow and the particle size becomes small. Moreover, since the amount of arsenic processed at a time is small, it is not preferable from an economical viewpoint. Therefore, the higher the arsenic is, the higher the value is 20 g / L or more, preferably 50 g / L or more.
The upper limit of the arsenic concentration that can be dissolved in the liquid to be treated is an amount that varies depending on the process of generation of the liquid to be treated, such as the dissolved amount of coexisting metal ions and other anions. Then, adjust appropriately so that precipitates other than iron arsenate are not generated.
［鉄（２）化合物］
本発明においては、被処理液中に鉄（２）イオンを共存させることが必須である。鉄（２）イオンそれ自身は、上述の様に、結晶性砒酸鉄に一部取り込まれるが、基本的には触媒として作用するものであり、添加した鉄（２）イオンの大部分は、結晶性砒酸鉄の析出後も、水溶液中に残存する。鉄（２）イオンの供給源としては、硫酸塩、硝酸塩、塩化物等、水に可溶性の塩であれば、いずれを用いても構わないが、価格および入手の容易さから、硫酸塩を用いるのが好ましい。
本発明においては、被処理液中に共存させる鉄（２）イオン濃度は特に規定するものではないが、５ｇ／Ｌ〜７５ｇ／Ｌが好ましい。５ｇ／Ｌ未満では、結晶性砒酸鉄の粒子径が小さくなるので、好ましくない。また、７５ｇ／Ｌを超えると、鉄（２）イオン添加の効果が飽和する。より好ましくは、鉄（２）イオン濃度を２０ｇ／Ｌ〜７５ｇ／Ｌとする。
本発明においては、鉄（３）化合物を水溶液で添加する場合には、被処理液の容積が時間と供に増大する。この場合、鉄（３）化合物の水溶液を全て添加した時点で、上述の鉄（２）イオン濃度の好ましい範囲になる様に調整する。
鉄（２）イオンを含む化合物は、水に溶解した液体状態で被処理液に添加しても、固体状態で被処理液に添加して、その中で溶解しても、いずれでも構わない。
なお、鉄（３）化合物を添加すると結晶性砒酸鉄が析出するので、公知の撹拌手段を用いて、被処理液を強撹拌する。
[Iron (2) compound]
In the present invention, it is essential for iron (2) ions to coexist in the liquid to be treated. As described above, iron (2) ions themselves are partially incorporated into crystalline iron arsenate, but basically function as a catalyst. Most of the added iron (2) ions are crystalline. Remains in the aqueous solution even after precipitation of the ferrous iron arsenate. As a supply source of iron (2) ions, any salt can be used as long as it is soluble in water, such as sulfate, nitrate, chloride, etc., but sulfate is used because of its price and availability. Is preferred.
In the present invention, the concentration of iron (2) ions coexisting in the liquid to be treated is not particularly specified, but is preferably 5 g / L to 75 g / L. If it is less than 5 g / L, the particle diameter of crystalline iron arsenate becomes small, which is not preferable. Moreover, when it exceeds 75 g / L, the effect of iron (2) ion addition is saturated. More preferably, the iron (2) ion concentration is 20 g / L to 75 g / L.
In the present invention, when the iron (3) compound is added as an aqueous solution, the volume of the liquid to be treated increases with time. In this case, when all of the aqueous solution of the iron (3) compound is added, the iron (2) ion concentration is adjusted so as to be within a preferable range.
The compound containing iron (2) ions may be added to the liquid to be treated in a liquid state dissolved in water, or may be added to the liquid to be treated in a solid state and dissolved therein.
In addition, since crystalline iron arsenate precipitates when an iron (3) compound is added, the liquid to be treated is vigorously stirred using a known stirring means.
［鉄（３）化合物］
本発明においては、５価の砒素化合物と鉄（２）イオンが共存する被処理液中に鉄（３）化合物を添加することにより、結晶性砒酸鉄を析出させる。鉄（３）化合物としては、硫酸塩、硝酸塩、塩化物等の塩類と、Ｆｅ2Ｏ3・ｎＨ2Ｏの化学式で表される鉄（３）の酸化物、含水酸化物のいずれを用いても良い（以下、含水酸化物も含めて酸化物と呼称する）。また、これらの鉄（３）塩は、イオン解離した液体状態で添加しても、固体状態で添加してもいずれでも構わない。
上述の塩類は、いずれを用いても構わないが、経済的な観点からは、硫酸塩の使用が好ましい。鉄（３）塩を被処理液に溶解するとイオン解離するので、砒酸鉄の析出反応は以下の化学式（１）または（２）で表され、いずれにしてもプロトンが放出されるので、反応系のｐＨは低下する。反応系のｐＨが低下すると、砒酸鉄析出の反応速度が低下するが、反応自体は継続するので、ｐＨ調整を行わなくても良いし、反応速度を増加させるためにｐＨ調整を行っても構わない。
Ｈ3ＡｓＯ4(l)＋Ｆｅ3+(l)→ＦｅＡｓＯ4(s)＋３Ｈ+(l) …（１）
Ｈ2ＡｓＯ4 -(l)＋Ｆｅ3+(l)→ＦｅＡｓＯ4(s)＋２Ｈ+(l) …（２）
鉄（３）の酸化物は、ｎ＝０の場合は酸化物Ｆｅ2Ｏ3あり、ヘマタイト、マグヘマイト等、ｎ＝１の場合はオキシ水酸化物ＦｅＯＯＨであり、ゲーサイト、レピドクロサイト等、ｎ＝３の場合は、水酸化物Ｆｅ(ＯＨ)3であり、バーナライト、不定形水酸化物等が存在するが、本発明の鉄（３）源としてはいずれを用いても良い。鉄（３）の酸化物を固体で被処理液に添加した場合の溶解反応は以下の（３）式で表されるので、砒酸鉄の析出反応は以下の化学式（４）または（５）で表される。鉄（３）イオンが未解離の砒酸と反応する場合には反応系のｐＨは変化しないが、１段解離の砒酸２水素イオンと反応する場合には、反応系のｐＨは上昇する。
１／２Ｆｅ2Ｏ3(s)＋３／２Ｈ2Ｏ→Ｆｅ3+(l)＋３ＯＨ-(l) …（３）
Ｈ3ＡｓＯ4(l)＋１／２Ｆｅ2Ｏ3(s)→ＦｅＡｓＯ4(s)＋３Ｈ2Ｏ …（４）
Ｈ2ＡｓＯ4 -(l)＋１／２Ｆｅ2Ｏ3(s)→ＦｅＡｓＯ4(s)＋ＯＨ-(l)＋２Ｈ2Ｏ …（５）
本発明においては、鉄（３）の酸化物としてマグネタイトを用いることも可能である。すなわち、マグネタイト（Ｆｅ3Ｏ4）はＦｅＯ・Ｆｅ2Ｏ3であり、結晶中に含まれるＦｅの２／３が鉄（３）の酸化物である。なお、鉄源としてマグネタイトを用いると、反応溶液中に鉄（２）イオンが蓄積する。
[Iron (3) compound]
In the present invention, crystalline iron arsenate is deposited by adding an iron (3) compound to a liquid to be treated in which a pentavalent arsenic compound and iron (2) ions coexist. As the iron (3) compound, any one of salts such as sulfate, nitrate, chloride, etc., and iron (3) oxide or hydrous oxide represented by the chemical formula of Fe 2 O 3 .nH 2 O is used. (Hereinafter referred to as oxides including hydrated oxides). Further, these iron (3) salts may be added in a liquid state in which ions are dissociated or in a solid state.
Any of the above-mentioned salts may be used, but use of a sulfate is preferable from an economical viewpoint. When iron (3) salt is dissolved in the liquid to be treated, the ions are dissociated. Therefore, the precipitation reaction of iron arsenate is represented by the following chemical formula (1) or (2), and in any case, protons are released. The pH decreases. When the pH of the reaction system decreases, the reaction rate of iron arsenate precipitation decreases. However, since the reaction itself continues, it is not necessary to adjust the pH, or the pH may be adjusted to increase the reaction rate. Absent.
H 3 AsO 4 (l) + Fe 3+ (l) → FeAsO 4 (s) + 3H + (l) (1)
H 2 AsO 4 − (l) + Fe 3+ (l) → FeAsO 4 (s) + 2H + (l) (2)
The oxide of iron (3) is the oxide Fe 2 O 3 when n = 0, hematite, maghemite, etc., and the oxyhydroxide FeOOH when n = 1, such as goethite, lipid docrosite, etc. In the case of n = 3, hydroxide Fe (OH) 3 is present, and burnerite, amorphous hydroxide and the like are present, but any of them may be used as the iron (3) source of the present invention. The dissolution reaction when the oxide of iron (3) is added to the liquid to be treated as a solid is represented by the following formula (3). Therefore, the precipitation reaction of iron arsenate is represented by the following chemical formula (4) or (5). expressed. When iron (3) ions react with undissociated arsenic acid, the pH of the reaction system does not change, but when it reacts with one-stage dissociated dihydrogen arsenate ions, the pH of the reaction system rises.
1 / 2Fe 2 O 3 (s) + 3 / 2H 2 O → Fe 3+ (l) + 3OH − (l) (3)
H 3 AsO 4 (l) + 1 / 2Fe 2 O 3 (s) → FeAsO 4 (s) + 3H 2 O (4)
H 2 AsO 4 − (l) + 1 / 2Fe 2 O 3 (s) → FeAsO 4 (s) + OH − (l) + 2H 2 O (5)
In the present invention, it is also possible to use magnetite as the iron (3) oxide. That is, magnetite (Fe 3 O 4 ) is FeO · Fe 2 O 3 , and 2/3 of Fe contained in the crystal is an oxide of iron (3). When magnetite is used as the iron source, iron (2) ions accumulate in the reaction solution.
本発明の場合、得られる砒酸鉄の結晶性は、鉄（３）化合物の添加速度に依存して変化する。鉄（３）をイオン状態で添加する場合には、添加速度が大き過ぎると、前駆体の発生速度が速くなり、得られる析出物がゲル状になり易く、結晶状になるまでに熟成時間を長く要する。また、添加速度が少な過ぎると、反応に長時間を要するので、経済的ではない。なお、処理の操作性を考えると、鉄（３）源として塩類を用いる場合の好適な速度は、所定量の鉄（３）源を１〜６時間で添加終了できるように調整することが好ましい。以上を総合的に考慮すると、鉄（３）の添加速度は、具体的には、処理液１Ｌに対して０.０４〜４.０ｇ／ｍｉｎが好ましく、０.２５〜０.６７ｇ／ｍｉｎがより好ましい。
また、鉄（３）源として酸化物の固体を用いる場合には、酸化物の溶解反応を経由するので、好適な範囲は、０〜３時間で酸化物の全量を添加する様に調整する。ここで、０時間とは、所定量の鉄（３）源を反応開始時に一括添加することを意味し、添加された酸化物は、酸の作用により徐々に溶解し、鉄（３）イオンの供給源となる。
上記の説明は、鉄（３）化合物の添加速度を一定とするものであるが、本発明の砒酸鉄の析出反応は、砒素濃度の高い反応初期には反応速度が大きく、砒素濃度が減少する反応後期には小さくなる。そのため、被処理液への鉄（３）化合物の添加速度を多段にしても構わない。
その場合、鉄（３）源の添加前期の添加速度は、添加後期の添加速度より早め、添加開始から１〜２時間時点で、被処理液中の砒素の５０〜８０％が反応するように調整することが好ましい。これは、後述の熟成時間の確保の観点から好ましい。
鉄（３）化合物の添加量は、被処理液中の砒素と化学量論比の砒酸鉄を形成する量の１〜１.１倍の量にすることが好ましい。鉄（３）化合物を大過剰に添加しても、反応後期の析出反応速度は大きくならない上、被処理液中に鉄（３）化合物が大量に残存すると、排水処理上の問題が発生する。
In the present invention, the crystallinity of the obtained iron arsenate varies depending on the addition rate of the iron (3) compound. When iron (3) is added in an ionic state, if the addition rate is too high, the precursor generation rate will increase, the resulting precipitate will tend to be in a gel state, and a ripening time will be required until it becomes crystalline. It takes a long time. On the other hand, if the addition rate is too low, the reaction takes a long time, which is not economical. In consideration of the operability of the treatment, it is preferable to adjust the suitable speed when using salts as the iron (3) source so that the addition of a predetermined amount of iron (3) source can be completed in 1 to 6 hours. . Considering the above comprehensively, the addition rate of iron (3) is specifically preferably 0.04 to 4.0 g / min, and preferably 0.25 to 0.67 g / min with respect to 1 L of the treatment liquid. More preferred.
In addition, when an oxide solid is used as the iron (3) source, since a dissolution reaction of the oxide is performed, a preferable range is adjusted so that the entire amount of the oxide is added in 0 to 3 hours. Here, 0 hour means that a predetermined amount of iron (3) source is added all at once at the start of the reaction, and the added oxide is gradually dissolved by the action of the acid, and the iron (3) ion A source of supply.
In the above explanation, the addition rate of the iron (3) compound is made constant, but the iron arsenate precipitation reaction of the present invention has a high reaction rate at the beginning of the reaction with a high arsenic concentration, and the arsenic concentration decreases. It becomes smaller in the late reaction. Therefore, the addition rate of the iron (3) compound to the liquid to be treated may be multistage.
In that case, the addition rate in the first stage of addition of the iron (3) source is faster than the addition rate in the second half of the addition so that 50 to 80% of arsenic in the liquid to be treated reacts at 1 to 2 hours from the start of the addition. It is preferable to adjust. This is preferable from the viewpoint of securing the aging time described later.
The amount of the iron (3) compound added is preferably 1 to 1.1 times the amount that forms iron arsenate in a stoichiometric ratio with arsenic in the liquid to be treated. Even if the iron (3) compound is added in a large excess, the deposition reaction rate in the latter stage of the reaction does not increase, and if a large amount of the iron (3) compound remains in the liquid to be treated, a problem in wastewater treatment occurs.
［ｐＨ］
本発明においては、反応系のｐＨは、砒酸鉄の析出速度および析出形態に影響を及ぼす重要な因子である。本発明においてｐＨは、以下で定義される。
本明細書に記載のｐＨの値は、ＪＩＳ Ｚ８８０２に基づき、ガラス電極を用い、ｐＨ標準液として、酸性域ではシュウ酸塩およびフタル酸塩緩衝液を、中性域では中性りん酸塩緩衝液を用いて、３点校正したｐＨ計により測定した値をいう。
また、本明細書に記載のｐＨは、温度補償電極により補償されたｐＨ計の示す測定値を、反応温度条件下で直接読み取った値である。
本発明においては、５価の砒素化合物と２価の鉄イオンが共存する被処理液のｐＨを１.０〜２.０とする。ｐＨが１.０未満では、結晶性砒酸鉄の析出速度が小さいので、経済的に不利である。ｐＨが２.０を超えると、前駆体の発生速度が速くなり、得られる析出物がゲル状になり易いので好ましくない。
なお、上述の様に、被処理液に、鉄（３）化合物を鉄（３）イオンで添加する場合には、反応の進行とともに被処理液のｐＨが低下するが、反応開始の時点で反応系のｐＨが前記の範囲内であれば、その後ｐＨがその範囲外になっても、問題はない。
[PH]
In the present invention, the pH of the reaction system is an important factor that affects the precipitation rate and precipitation form of iron arsenate. In the present invention, pH is defined as follows.
The pH values described in this specification are based on JIS Z8802, using glass electrodes, oxalate and phthalate buffer solutions in the acidic range, and neutral phosphate buffer in the neutral range as pH standard solutions. The value measured with a pH meter calibrated using a liquid at three points.
The pH described in the present specification is a value obtained by directly reading a measured value indicated by a pH meter compensated by a temperature compensation electrode under reaction temperature conditions.
In the present invention, the pH of the liquid to be treated in which the pentavalent arsenic compound and the divalent iron ion coexist is set to 1.0 to 2.0. If the pH is less than 1.0, the precipitation rate of crystalline iron arsenate is small, which is economically disadvantageous. If the pH exceeds 2.0, the rate of generation of the precursor is increased, and the resulting precipitate tends to be gelled, which is not preferable.
As described above, when an iron (3) compound is added to the liquid to be treated as iron (3) ions, the pH of the liquid to be treated decreases as the reaction proceeds. If the pH of the system is within the above range, there is no problem even if the pH is subsequently outside that range.
［処理条件］
本発明においては、反応温度は特に規定するものではないが、９０〜１００℃が好ましい。反応温度が９０℃未満では、結晶性砒酸鉄の析出に長時間を要するので、経済的に不利となる。反応温度が１００℃を超えると、オートクレーブ等の高圧反応設備が必要となり、設備費用が高額となり、エネルギーコストも増大するので好ましくない。より好ましい範囲は９０〜９５℃である。
反応時間は、被処理液中に含まれる砒素濃度と鉄（３）化合物の添加速度とに依存するものであるが、後述する熟成プロセスを含めて７時間以内になる様に条件設定することが好ましい。
上述の様に、処理を開始すると沈殿物が発生するので、処理中は、被処理液を強撹拌する。
本発明においては、被処理液に鉄（２）イオンを共存させるが、撹拌に伴う空気の巻き込みによる鉄（２）イオンの酸化は、それ程多くはないので、特に雰囲気制御をする必要はない。酸化防止のために雰囲気を制御する場合でも、露点管理する等、厳密なものである必要はなく、撹拌による大気の巻き込みを防止する程度で足りる。具体的には、被処理液に窒素、アルゴン等の不活性ガスを吹き込み、そのガスにより被処理液上部をシールすれば良い。
[Processing conditions]
In the present invention, the reaction temperature is not particularly limited, but is preferably 90 to 100 ° C. If the reaction temperature is less than 90 ° C., it takes a long time to deposit crystalline iron arsenate, which is economically disadvantageous. When the reaction temperature exceeds 100 ° C., a high-pressure reaction facility such as an autoclave is required, which increases the cost of the facility and increases the energy cost. A more preferable range is 90 to 95 ° C.
The reaction time depends on the arsenic concentration contained in the liquid to be treated and the rate of addition of the iron (3) compound, but the conditions can be set to be within 7 hours including the aging process described later. preferable.
As described above, since a precipitate is generated when the treatment is started, the liquid to be treated is vigorously stirred during the treatment.
In the present invention, iron (2) ions coexist in the liquid to be treated, but the oxidation of iron (2) ions due to entrainment of air accompanying stirring is not so much, so there is no need to control the atmosphere. Even when the atmosphere is controlled to prevent oxidation, it is not necessary to strictly control the dew point or the like, and it is sufficient to prevent the air from being engulfed by stirring. Specifically, an inert gas such as nitrogen or argon is blown into the liquid to be processed, and the upper part of the liquid to be processed may be sealed with the gas.
［熟成］
本発明においては、５価の砒素化合物と鉄（２）イオンが共存する被処理液中に鉄（３）化合物を添加することにより、結晶性の砒酸鉄を析出させるが、結晶性砒酸鉄の析出後に、結晶の熟成プロセスを設けると、結晶性砒酸鉄が粗大化するので、結晶性砒酸鉄の分離性をさらに高めることが出来る。熟成プロセスにおいては、微細析出物の消失やファセットの成長が観察されることから、オストワルド熟成に類似の現象が生起しているものと考えられる。
熟成プロセスは、被処理液への鉄（３）化合物の添加が終了した後、析出物の分散した被処理液をそのまま保持することにより行う。また、熟成プロセスを加速するために、被処理液を加温しても構わない。
なお、鉄（３）化合物を多段で添加する場合、反応後期において結晶性砒酸鉄の粗大化も起こるので、結晶性砒酸鉄の析出と同時に、結晶性砒酸鉄の熟成も起こっているものと考えられる。
[Aging]
In the present invention, crystalline iron arsenate is precipitated by adding an iron (3) compound to a treatment liquid in which a pentavalent arsenic compound and iron (2) ions coexist. If a crystal ripening process is provided after the precipitation, the crystalline iron arsenate is coarsened, so that the separability of the crystalline iron arsenate can be further enhanced. In the ripening process, the disappearance of fine precipitates and the growth of facets are observed, and it is considered that a phenomenon similar to Ostwald ripening occurs.
The aging process is performed by holding the liquid to be treated in which precipitates are dispersed as it is after the addition of the iron (3) compound to the liquid to be treated is completed. Moreover, in order to accelerate the aging process, the liquid to be treated may be heated.
In addition, when iron (3) compound is added in multiple stages, the coarsening of crystalline iron arsenate also occurs in the late stage of the reaction. Therefore, it is considered that the ripening of crystalline iron arsenate occurs simultaneously with the precipitation of crystalline iron arsenate. It is done.
［酸化性ガス］
本発明においては、結晶性砒酸鉄の析出反応の後期に、酸素を含む酸化性ガスを吹き込むことにより、結晶性砒酸鉄の溶出特性と固液分離時の分離性を向上させ、結晶化反応終了後の砒素濃度をさらに低減することが出来る。
酸化性ガスとしては、純酸素ガス、空気等、酸素を含むガスを使用する。ここで、反応の後期とは、被処理液に溶存する砒素の８０％以上が析出した時点を意味する。なお、反応開始時から酸化性ガスを吹き込むと、反応開始時に反応溶液中に添加した鉄（２）イオンが酸化するので、好ましくない。
酸化性ガスの吹込みにより、結晶性砒酸鉄の溶出特性と固液分離時の分離性が向上する理由に関しては、反応後期に残留する前駆体が酸化され結晶性砒酸鉄へ転換されることに起因するものと推定される。
さらに、結晶化反応終了後の砒素濃度をさらに低減する理由に関しては、酸化性ガスを吹き込むことにより未反応の砒素と鉄（３）酸化物や鉄イオン（３価、２価）による結晶化反応が同時に進行し結晶化反応が完結するためと推定される。
[Oxidizing gas]
In the present invention, the elution characteristics of crystalline iron arsenate and the separability during solid-liquid separation are improved by blowing oxygen-containing oxidizing gas at the latter stage of the precipitation reaction of crystalline iron arsenate, and the crystallization reaction is completed. The subsequent arsenic concentration can be further reduced.
As the oxidizing gas, a gas containing oxygen such as pure oxygen gas or air is used. Here, the latter stage of the reaction means the time when 80% or more of arsenic dissolved in the liquid to be treated is deposited. Note that it is not preferable to blow an oxidizing gas from the start of the reaction because iron (2) ions added to the reaction solution at the start of the reaction are oxidized.
The reason why the blowing of oxidizing gas improves the elution characteristics of crystalline iron arsenate and the separability during solid-liquid separation is that the precursor remaining in the late stage of the reaction is oxidized and converted to crystalline iron arsenate. It is presumed to be caused.
Furthermore, the reason for further reducing the arsenic concentration after the completion of the crystallization reaction is that the crystallization reaction between unreacted arsenic and iron (3) oxide or iron ion (trivalent or divalent) by blowing an oxidizing gas. It is presumed that the crystallization proceeds simultaneously and the crystallization reaction is completed.
［銅（２）イオン］
本発明においては、反応溶液に銅（２）イオンを共存させることにより、結晶性砒酸鉄への結晶化反応が反応初期から促進され、当該結晶性砒酸鉄の溶出特性の向上と共に結晶化反応終了後の砒素濃度をさらに低減することが出来る。
本発明においては、銅（２）イオン濃度は特に規定するものではないが、０.１ｇ／Ｌ以上で効果が現れ、濃度の上昇とともに効果が高まっていく。しかし同時に結晶性砒酸鉄粒子が微細化していくので後述の種結晶を添加し結晶化反応を行うことで、結晶性砒酸鉄粒子の微細化を阻止することが出来る。
銅（２）イオンの共存により、結晶性砒酸鉄への結晶化反応が反応初期から促進される理由については、現時点で不明であるが、前駆体の表面での鉄（２）イオンと鉄（３）イオンとの不均化反応による砒酸（Ｈ3ＡｓＯ4(aq)）から砒酸２水素イオン（Ｈ2ＡｓＯ4 -(aq)）への解離反応の触媒として作用することに起因するものと推定される。
また、反応溶液に銅（２）イオンを共存させると、鉄（３）酸化物の溶解が促進されるので、鉄源として鉄（３）の酸化物を用いる時に共存させることが好ましい。
[Copper (2) ion]
In the present invention, by coexisting copper (2) ions in the reaction solution, the crystallization reaction to crystalline iron arsenate is promoted from the beginning of the reaction, and the elution characteristics of the crystalline iron arsenate are improved and the crystallization reaction is completed. The subsequent arsenic concentration can be further reduced.
In the present invention, the copper (2) ion concentration is not particularly specified, but the effect appears at 0.1 g / L or more, and the effect increases as the concentration increases. However, since the crystalline iron arsenate particles are refined at the same time, the refinement of the crystalline iron arsenate particles can be prevented by adding a seed crystal to be described later and performing a crystallization reaction.
The reason why the crystallization reaction to crystalline iron arsenate is promoted from the beginning of the reaction due to the coexistence of copper (2) ions is unknown at this time, but iron (2) ions and iron ( and due to acting as a catalyst for the dissociation reaction to - ((aq H 2 AsO 4 )) 3) arsenic acid (H 3 AsO 4 (aq) ) from the arsenate 2 hydrogen ions by disproportionation reaction with ion Presumed.
Moreover, since coexistence of copper (2) ions in the reaction solution promotes dissolution of iron (3) oxide, it is preferable to coexist when iron (3) oxide is used as the iron source.
［種結晶］
本発明においては、反応溶液に種結晶としてスコロダイト結晶を添加することにより、得られる結晶性砒酸鉄が粗大化するので、結晶性砒酸鉄の分離性をさらに高めることが出来る。本発明においては、種結晶の添加量は特に規定するものではないが、１０〜２０ｇ／Ｌで効果が飽和する。
鉄源として鉄（３）の酸化物を用いた場合、酸化物の溶解に時間を要し、反応にタイムラグが生ずるので、種結晶の添加が結晶性砒酸鉄の粗大化に有効である。
なお、種結晶として使用するスコロダイト結晶は、本発明をはじめとして、砒素化合物の回収により得られた結晶を使用することが可能であるが、結晶性砒酸鉄の粗大化のためには、可能な限り結晶性の良好なものを使用することが好ましい。
[Seed crystal]
In the present invention, the crystalline iron arsenate obtained is coarsened by adding scorodite crystals as seed crystals to the reaction solution, so that the separability of crystalline iron arsenate can be further enhanced. In the present invention, the amount of seed crystal added is not particularly limited, but the effect is saturated at 10 to 20 g / L.
When an iron (3) oxide is used as the iron source, it takes time to dissolve the oxide and a time lag occurs in the reaction. Therefore, the addition of a seed crystal is effective for coarsening of crystalline iron arsenate.
As the scorodite crystal used as a seed crystal, it is possible to use a crystal obtained by recovering an arsenic compound including the present invention, but it is possible to coarsen crystalline iron arsenate. It is preferable to use a material having good crystallinity.
［固液分離］
結晶性砒酸鉄の析出反応、または結晶性砒酸鉄の析出および熟成の終了した後、濾過、遠心分離等の公知の固液分離手段を用いて固相を分離する。残存する液相には、処理の際に添加した鉄（２）イオンの大部分が含まれているので、本発明を用いた砒素の分離・回収のための鉄（２）源として再利用することが可能である。
[Solid-liquid separation]
After the precipitation reaction of crystalline iron arsenate, or the precipitation and ripening of crystalline iron arsenate, the solid phase is separated using a known solid-liquid separation means such as filtration or centrifugation. Since the remaining liquid phase contains most of the iron (2) ions added during the treatment, it is reused as an iron (2) source for separation and recovery of arsenic using the present invention. It is possible.
［砒素、鉄（２）および鉄（３）の分析］
被処理液の上澄み液は、メンブレンフィルター（孔径０.４５μｍ、φ４７ｍｍ）で吸引ろ過後、１００μＬを１００ｍＬのメスフラスコに分取、６０％硝酸０.７７ｍＬを加え、０.１ｍｏｌ／Ｌ硝酸溶液となるように蒸留水で希釈し１００ｍＬとした。この液をＩＣＰ−ＡＥＳ分析装置（ＳＰＥＣＴＲＯ製 Ａｒｃｏｓ）で測定し、全砒素濃度および全鉄濃度を定量した。被処理液中の砒素はすべて添加形態の５価とみなし、全砒素濃度を砒素（５）濃度として扱った。
また鉄（２）は、３００ｍＬのコニカルビーカーに上記吸引ろ過後のろ液原液〜１０倍希釈を１０ｍＬ分取、（１＋１）硫酸１０ｍＬを加え、全量がおよそ５０ｍＬになるように蒸留水で希釈、ホットプレート上で７０℃まで加熱撹拌、０.０２ｍｏｌ／Ｌの過マンガン酸カリウム水溶液で滴定し（終点は淡紅色）濃度を求めた。
鉄（３）濃度は、ＩＣＰ−ＡＥＳの全鉄濃度から鉄（２）濃度を差し引き求めた。
[Analysis of arsenic, iron (2) and iron (3)]
The supernatant of the liquid to be treated was filtered with suction through a membrane filter (pore size 0.45 μm, φ47 mm), and 100 μL was dispensed into a 100 mL volumetric flask, 0.77 mL of 60% nitric acid was added, and 0.1 mol / L nitric acid solution and Diluted with distilled water to make 100 mL. This solution was measured with an ICP-AES analyzer (Arcos manufactured by SPECTRO), and the total arsenic concentration and the total iron concentration were quantified. All arsenic in the liquid to be treated was regarded as pentavalent in the added form, and the total arsenic concentration was treated as the arsenic (5) concentration.
In addition, iron (2) is 10 mL of 10 to 10 dilution of the filtrate stock solution after suction filtration in a 300 mL conical beaker, 10 mL of (1 + 1) sulfuric acid is added, and diluted with distilled water so that the total amount is about 50 mL. The mixture was heated and stirred to 70 ° C. on a hot plate, and titrated with a 0.02 mol / L potassium permanganate aqueous solution (the end point was light red) to determine the concentration.
The iron (3) concentration was obtained by subtracting the iron (2) concentration from the total iron concentration of ICP-AES.
［砒酸鉄の構造観察］
走査電子顕微鏡はＨＩＴＡＣＨＩ製ＳＵ６６００を使用した。試料には白金蒸着をおこない、加速電圧１５ｋＶで観察した。Ｘ線回折装置はＲＩＧＡＫＵ製ＲＩＮＴ−Ｖを使用した。Ｘ線管球にはＣｕのＫα線を用い、印加電圧４０ｋＶ、電流３０ｍＡとした。
[Structural observation of iron arsenate]
The scanning electron microscope used was SU6600 manufactured by HITACHI. The sample was subjected to platinum deposition and observed at an acceleration voltage of 15 kV. RINT-V made by RIGAKU was used as the X-ray diffractometer. A Cu Kα ray was used as the X-ray tube, and the applied voltage was 40 kV and the current was 30 mA.
［実施例１］
モデル被処理液として、（砒素（５）７０ｇ＋鉄（２）４８ｇ）／Ｌの水溶液を５４５ｍＬ準備した。被処理液の初期ｐＨは、酸やアルカリでの調整行わず、１.５となった。被処理液の温度を９５℃とし、Ａｒガスを流量７００ｍＬ／ｍｉｎで吹き込み、テフロン（登録商標）被覆撹拌棒を用いて１０００ｒｐｍで撹拌を行いながら、被処理液に鉄（３）イオンを添加した。鉄（３）イオン源としては、硫酸第二鉄を用い、鉄として１６０ｇ／Ｌの水溶液で添加した。鉄（３）イオンを含む水溶液の添加速度は、添加開始から７０ｍｉｎまでは２ｍＬ／ｍｉｎ、７０ｍｉｎから４２０ｍｉｎまでは０.１ｍＬ／ｍｉｎとした。被処理液に添加された鉄（３）イオンの全量は２８.０ｇであり、最終的な被処理液の体積は、７２０ｍＬとなった。また、反応終了時のｐＨは０.１３であった。
反応終了後、被処理液を静置し、上澄み液中の砒素（５）、鉄（３）および鉄（２）の濃度を測定したところ、それぞれ３.６ｇ／Ｌ、１５.６ｇ／Ｌ、および３４.３ｇ／Ｌであった。この結果は、処理前に被処理液中に含まれていた砒素（５）の９３％、添加した鉄（３）イオンの６０％が結晶性砒酸鉄の沈殿として分離されたことを意味する。この結果から、析出した結晶性砒酸鉄はほぼ量論比に近いものであることが判る。また、反応終了後の鉄（２）イオン量は、初期の９４％であり、反応によりその量が殆ど変化していない。
図１に反応時間１０ｍｉｎ、６０ｍｉｎおよび４２０ｍｉｎで得られた沈殿物のＳＥＭ写真を示す。反応時間１０ｍｉｎでは、沈殿物はゲル状の前駆体であるが、反応時間６０ｍｉｎおよび４２０ｍｉｎでは、結晶性砒酸鉄が生成していることが観察される。
図２に反応時間４２０ｍｉｎで得られた沈殿物のＸ線回折図形を示す。この条件で得られた沈殿物は、ほぼスコロダイト構造の結晶性砒酸鉄である。
[Example 1]
As a model solution, 545 mL of an aqueous solution of (arsenic (5) 70 g + iron (2) 48 g) / L was prepared. The initial pH of the liquid to be treated was 1.5 without adjustment with acid or alkali. The temperature of the liquid to be treated was 95 ° C., Ar gas was blown at a flow rate of 700 mL / min, and iron (3) ions were added to the liquid to be treated while stirring at 1000 rpm using a Teflon-coated stir bar. . As an iron (3) ion source, ferric sulfate was used, and iron was added in an aqueous solution of 160 g / L. The addition rate of the aqueous solution containing iron (3) ions was 2 mL / min from the start of addition to 70 min and 0.1 mL / min from 70 min to 420 min. The total amount of iron (3) ions added to the liquid to be treated was 28.0 g, and the final volume of the liquid to be treated was 720 mL. The pH at the end of the reaction was 0.13.
After completion of the reaction, the liquid to be treated was allowed to stand, and the concentrations of arsenic (5), iron (3), and iron (2) in the supernatant were measured to find 3.6 g / L, 15.6 g / L, And 34.3 g / L. This result means that 93% of arsenic (5) and 60% of added iron (3) ions contained in the liquid to be treated before the treatment were separated as crystalline iron arsenate precipitates. From this result, it can be seen that the precipitated crystalline iron arsenate is almost close to the stoichiometric ratio. Moreover, the iron (2) ion amount after completion | finish of reaction is 94% of the initial stage, and the amount has hardly changed by reaction.
FIG. 1 shows SEM photographs of the precipitates obtained at reaction times of 10 min, 60 min and 420 min. At the reaction time of 10 min, the precipitate is a gel-like precursor, but at the reaction times of 60 min and 420 min, it is observed that crystalline iron arsenate is formed.
FIG. 2 shows an X-ray diffraction pattern of a precipitate obtained at a reaction time of 420 minutes. The precipitate obtained under these conditions is crystalline iron arsenate having a substantially scorodite structure.
［実施例２］
初期の鉄（２）濃度を７ｇ／Ｌとした以外は実施例１と同一の条件で結晶性砒酸鉄生成を行った。
図３に反応時間１０ｍｉｎ、６０ｍｉｎおよび４２０ｍｉｎで得られた沈殿物のＳＥＭ写真を示す。いずれの反応時間においても、結晶性砒酸鉄の生成が観察される。得られた沈殿物は、ほぼスコロダイト構造の結晶性砒酸鉄であった。
[Example 2]
Crystalline iron arsenate was produced under the same conditions as in Example 1 except that the initial iron (2) concentration was 7 g / L.
FIG. 3 shows SEM photographs of the precipitates obtained at reaction times of 10 min, 60 min and 420 min. Formation of crystalline iron arsenate is observed at any reaction time. The obtained precipitate was crystalline iron arsenate having a substantially scorodite structure.
［実施例３］
初期の鉄（２）濃度を２３ｇ／Ｌとした以外は実施例１と同一の条件で結晶性砒酸鉄生成を行った。
図４に反応時間１０ｍｉｎ、６０ｍｉｎおよび４２０ｍｉｎで得られた沈殿物のＳＥＭ写真を示す。いずれの反応時間においても、結晶性砒酸鉄の生成が観察される。得られた沈殿物は、ほぼスコロダイト構造の結晶性砒酸鉄であった。
[Example 3]
Crystalline iron arsenate was produced under the same conditions as in Example 1 except that the initial iron (2) concentration was 23 g / L.
FIG. 4 shows SEM photographs of the precipitates obtained at reaction times of 10 min, 60 min and 420 min. Formation of crystalline iron arsenate is observed at any reaction time. The obtained precipitate was crystalline iron arsenate having a substantially scorodite structure.
［実施例４］
初期の鉄（２）濃度を７５ｇ／Ｌとした以外は実施例１と同一の条件で結晶性砒酸鉄生成を行った。
図５に反応時間１０ｍｉｎ、６０ｍｉｎおよび４２０ｍｉｎで得られた沈殿物のＳＥＭ写真を示す。いずれの反応時間においても、結晶性砒酸鉄の生成とファセットの成長が観察される。得られた沈殿物は、ほぼスコロダイト構造の結晶性砒酸鉄であった。
[Example 4]
Crystalline iron arsenate was produced under the same conditions as in Example 1 except that the initial iron (2) concentration was 75 g / L.
FIG. 5 shows SEM photographs of the precipitates obtained at reaction times of 10 min, 60 min and 420 min. At any reaction time, crystalline iron arsenate formation and facet growth is observed. The obtained precipitate was crystalline iron arsenate having a substantially scorodite structure.
［実施例５］
モデル被処理液として、（砒素（５）５１ｇ＋鉄（２）５６ｇ）／Ｌの水溶液を７２０ｍＬ準備した。被処理液の初期ｐＨは、酸やアルカリでの調整行わず、１.６となった。被処理液の温度を９５℃とし、Ａｒガスを流量７００ｍＬ／ｍｉｎで吹き込み、テフロン（登録商標）被覆撹拌棒を用いて１０００ｒｐｍで撹拌を行いながら、被処理液にヘマタイト（Ｆｅ2Ｏ3）を添加した。添加するヘマタイトには、５０％粒子径（Ｄ５０）が１８.１４μｍ、ＢＥＴ法により測定した比表面積が８.２９６ｍ2／ｇのものを用いた。ヘマタイトの添加速度は、添加開始から７０ｍｉｎまで５７２ｍｇ／ｍｉｎ（鉄（３）に換算すると４００ｍｇ／ｍｉｎ）とし、７０ｍｉｎでヘマタイトの添加を止め、その後４２０ｍｉｎまで同一の被処理液中で沈殿物の熟成を行った。被処理液に添加された鉄（３）イオンの全量は２８ｇである。また、熟成ステップ終了時のｐＨは２.１８であり、反応開始時点よりわずかに上昇した。
ヘマタイトの添加終了直前の６０ｍｉｎでの被処理液の上澄み液中の砒素（５）、鉄（３）および鉄（２）の濃度を測定したところ、それぞれ１３.０ｇ／Ｌ、３.１ｇ／Ｌ、および５１.４ｇ／Ｌであった。この結果は、処理前に被処理液中に含まれていた砒素（５）の７４％、添加した鉄（３）イオンの９２％が結晶性砒酸鉄の沈殿として分離されたことを意味する。この結果から、析出した結晶性砒酸鉄はほぼ量論比に近いものであることが判る。また、熟成ステップ終了後の鉄（２）イオン量は、初期の１０７％であり、反応によりその量が殆ど変化していない。熟成ステップ終了後に、上澄み液中の砒素（５）、および鉄（３）の濃度を測定したところ、それぞれ１.１ｇ／Ｌ、および２.７ｇ／Ｌであった。したがって、熟成ステップ中においても、わずかであるが砒酸鉄の析出が起こっていることが判る。
図６に反応時間６０ｍｉｎおよび４２０ｍｉｎで得られた沈殿物のＳＥＭ写真を示す。反応時間６０ｍｉｎでは、多量のゲル状の前駆体の生成が見られ、熟成ステップ終了後の４２０ｍｉｎでは、結晶性砒酸鉄のファセットが成長していることが観察される。
図７に熟成ステップ終了後に得られた沈殿物のＸ線回折図形を示す。得られた沈殿物は、ほぼスコロダイト構造の結晶性砒酸鉄であるが、残留した未溶解ヘマタイトの微小なピークも見られる。
[Example 5]
As a model treatment liquid, 720 mL of an aqueous solution of (arsenic (5) 51 g + iron (2) 56 g) / L was prepared. The initial pH of the liquid to be treated was 1.6 without adjustment with acid or alkali. The temperature of the liquid to be treated was 95 ° C., Ar gas was blown at a flow rate of 700 mL / min, and hematite (Fe 2 O 3 ) was added to the liquid to be treated while stirring at 1000 rpm using a Teflon (registered trademark) coated stirring rod. Added. As the hematite to be added, one having a 50% particle diameter (D50) of 18.14 μm and a specific surface area measured by the BET method of 8.296 m 2 / g was used. The addition rate of hematite is set to 572 mg / min (400 mg / min when converted to iron (3)) from the start of addition to 70 min, the addition of hematite is stopped at 70 min, and then ripening of the precipitate in the same treated liquid until 420 min Went. The total amount of iron (3) ions added to the liquid to be treated is 28 g. The pH at the end of the ripening step was 2.18, which was slightly increased from the start of the reaction.
The concentration of arsenic (5), iron (3), and iron (2) in the supernatant of the liquid to be treated in 60 minutes immediately before the end of the addition of hematite was measured, and the results were 13.0 g / L and 3.1 g / L, respectively. And 51.4 g / L. This result means that 74% of arsenic (5) and 92% of added iron (3) ions contained in the liquid to be treated before the treatment were separated as crystalline iron arsenate precipitates. From this result, it can be seen that the precipitated crystalline iron arsenate is almost close to the stoichiometric ratio. Further, the amount of iron (2) ions after completion of the ripening step is 107% of the initial stage, and the amount hardly changes due to the reaction. After completion of the aging step, the concentrations of arsenic (5) and iron (3) in the supernatant were measured and found to be 1.1 g / L and 2.7 g / L, respectively. Therefore, it can be seen that the iron arsenate precipitates slightly even during the ripening step.
FIG. 6 shows SEM photographs of the precipitates obtained at reaction times of 60 min and 420 min. In the reaction time of 60 min, a large amount of gel-like precursor is generated, and in 420 min after the ripening step, it is observed that facets of crystalline iron arsenate are growing.
FIG. 7 shows an X-ray diffraction pattern of the precipitate obtained after completion of the aging step. The obtained precipitate is crystalline iron arsenate having a substantially scorodite structure, but a minute peak of residual undissolved hematite is also observed.
［比較例１］
初期に被処理液に共存させる鉄（２）イオンの濃度を０ｇ／Ｌ、すなわち無添加とした以外は実施例１と同じ条件で処理を行った。反応終了後の砒素（５）および鉄（３）の濃度はそれぞれ３０.３ｇ／Ｌ、および２９.９ｇ／Ｌであった。この結果は、処理前に被処理液中に含まれていた砒素（５）の３５％、添加した鉄（３）イオンの２３％が砒酸鉄の沈殿として分離されたことを意味する。なお、この条件下では、反応時間１０ｍｉｎでは前駆体の生成は観察されず、沈殿物も回収されなかった。
図８に反応時間６０ｍｉｎおよび４２０ｍｉｎで得られた沈殿物のＳＥＭ写真を示す。なお、反応時間１０ｍｉｎでは、沈殿物は回収されなかった。沈殿物はいずれも、微小粒子の凝集体であった。
[Comparative Example 1]
The treatment was performed under the same conditions as in Example 1 except that the concentration of iron (2) ions coexisting in the liquid to be treated at the beginning was 0 g / L, that is, no addition. The concentrations of arsenic (5) and iron (3) after completion of the reaction were 30.3 g / L and 29.9 g / L, respectively. This result means that 35% of arsenic (5) and 23% of added iron (3) ions contained in the liquid to be treated before the treatment were separated as iron arsenate precipitates. Under these conditions, no precursor was observed and no precipitate was recovered at a reaction time of 10 min.
FIG. 8 shows SEM photographs of precipitates obtained at reaction times of 60 min and 420 min. In addition, precipitate was not collect | recovered in reaction time 10min. All of the precipitates were aggregates of fine particles.
［実施例６］
モデル被処理液として、６０％砒酸溶液と硫酸第一鉄とを用いて、（砒素（５）４５ｇ＋鉄（２）１６.８ｇ）／Ｌの水溶液を８００ｍＬ準備し、これに鉄（３）源としてヘマタイト（Ｆｅ2Ｏ3）を投入し、４枚邪魔板付き２段ディスクタービン羽で撹拌しながら昇温し、９５℃になった時点から３６０ｍｉｎ間、砒酸鉄の析出反応を行った。本実施例では、ヘマタイトとして、粒子径（Ｄ５０）が２７.１μｍ、比表面積６.８ｍ2／ｇのものを用いた。なお、この条件は、Ｆｅ（２）／Ａｓ（５）のモル比が０.５、Ｆｅ（３）／Ａｓ（５）のモル比が０.８、全Ｆｅ／Ａｓ（５）のモル比が１.３である。なお、Ｆｅ（３）の投入量は、原料のヘマタイト（Ｆｅ2Ｏ3）を化学分析して決定した。反応は、大気開放条件下、１０００ｒｐｍで撹拌を行いながら行い、被処理液のｐＨが１.７を超過しない様に制御した。反応終了後、被処理液を静置し、上澄み液中の砒素（５）、鉄（３）および鉄（２）の濃度を測定したところ、それぞれ３.３ｇ／Ｌ、０.１ｇ／Ｌ、および１２.４ｇ／Ｌであった。
図９に反応時間３６０ｍｉｎで得られた本実施例の沈殿物のＳＥＭ写真を示す。図９では、スコロダイト状の結晶性砒酸鉄の生成が観察される。本実施例で得られた砒酸鉄結晶は、環境庁告示１３号で規定される方法で測定した砒素の溶出値が０.０５ｍｇ／Ｌであり、耐砒素溶出性に優れたものであった。なお、図９〜１３の場合、それぞれの写真の下部中央にある白いバーの長さが１μｍに相当する。
[Example 6]
Using a 60% arsenic acid solution and ferrous sulfate as a model process liquid, 800 mL of an aqueous solution of (arsenic (5) 45 g + iron (2) 16.8 g) / L was prepared, and an iron (3) source was added thereto. As hematite (Fe 2 O 3 ) was added, the temperature was increased while stirring with a two-stage disk turbine blade with four baffle plates, and the iron arsenate precipitation reaction was performed for 360 minutes after reaching 95 ° C. In this example, hematite having a particle diameter (D50) of 27.1 μm and a specific surface area of 6.8 m 2 / g was used. This condition is that the molar ratio of Fe (2) / As (5) is 0.5, the molar ratio of Fe (3) / As (5) is 0.8, and the molar ratio of total Fe / As (5). Is 1.3. The amount of Fe (3) input was determined by chemical analysis of raw material hematite (Fe 2 O 3 ). The reaction was carried out with stirring at 1000 rpm under open air conditions, and the pH of the liquid to be treated was controlled so as not to exceed 1.7. After completion of the reaction, the liquid to be treated was allowed to stand, and the concentrations of arsenic (5), iron (3), and iron (2) in the supernatant were measured to find 3.3 g / L, 0.1 g / L, And 12.4 g / L.
FIG. 9 shows an SEM photograph of the precipitate of this example obtained at a reaction time of 360 minutes. In FIG. 9, the formation of scorodite-like crystalline iron arsenate is observed. The iron arsenate crystal obtained in this example had an arsenic elution value of 0.05 mg / L measured by the method prescribed in Environment Agency Notification No. 13 and was excellent in arsenic elution resistance. In addition, in the case of FIGS. 9-13, the length of the white bar in the lower center of each photograph corresponds to 1 μm.
［実施例７］
反応時間１８０ｍｉｎ以降は、ガラス管を介し反応容器底部より酸素ガスを１Ｌ／ｍｉｎのペースで吹き込んだ以外は、実施例６と同じ条件で砒酸鉄の析出反応を行った。この場合、反応終了後の上澄み液中の砒素（５）、鉄（３）および鉄（２）の濃度は、それぞれ０.７７ｇ／Ｌ、１.１ｇ／Ｌ、および１２.１ｇ／Ｌであり、得られた砒酸鉄結晶の砒素溶出値は０.０４ｍｇ／Ｌであった。図１０に反応時間３６０ｍｉｎで得られた本実施例の沈殿物のＳＥＭ写真を示す。
[Example 7]
After a reaction time of 180 min, iron arsenate was precipitated under the same conditions as in Example 6 except that oxygen gas was blown from the bottom of the reaction vessel through the glass tube at a rate of 1 L / min. In this case, the concentrations of arsenic (5), iron (3), and iron (2) in the supernatant after completion of the reaction are 0.77 g / L, 1.1 g / L, and 12.1 g / L, respectively. The arsenic elution value of the obtained iron arsenate crystal was 0.04 mg / L. FIG. 10 shows an SEM photograph of the precipitate of this example obtained at a reaction time of 360 minutes.
［実施例８］
モデル被処理液として、６０％砒酸溶液と硫酸第一鉄および硫酸銅とを用いて、（砒素（５）４５ｇ＋鉄（２）８.４ｇ＋銅（２）１.０ｇ）／Ｌの水溶液を８００ｍＬ準備し、これに鉄（３）源として実施例６で用いたもと同一Ｌｏｔのヘマタイト（Ｆｅ2Ｏ3）を添加し、実施例６と同じ手順で砒酸鉄の析出反応を行った。なお、この条件は、Ｆｅ（２）／Ａｓ（５）のモル比が０.２５、Ｆｅ（３）／Ａｓ（５）のモル比が１.０、全Ｆｅ／Ａｓ（５）のモル比が１.２５である。この場合、反応終了後の上澄み液中の砒素（５）、鉄（３）および鉄（２）の濃度は、それぞれ０.０６ｇ／Ｌ、０.６ｇ／Ｌ、および３.０ｇ／Ｌであり、得られた砒酸鉄結晶の砒素溶出値は０.０３ｍｇ／Ｌであった。図１１に反応時間３６０ｍｉｎで得られた本実施例の沈殿物のＳＥＭ写真を示す。
[Example 8]
Using a 60% arsenic acid solution, ferrous sulfate and copper sulfate as a model treatment solution, 800 mL of an aqueous solution of (arsenic (5) 45 g + iron (2) 8.4 g + copper (2) 1.0 g) / L The same lot of hematite (Fe 2 O 3 ) used in Example 6 as an iron (3) source was added thereto, and iron arsenate was precipitated by the same procedure as in Example 6. This condition is that the molar ratio of Fe (2) / As (5) is 0.25, the molar ratio of Fe (3) / As (5) is 1.0, and the molar ratio of total Fe / As (5). Is 1.25. In this case, the concentrations of arsenic (5), iron (3), and iron (2) in the supernatant after the reaction are 0.06 g / L, 0.6 g / L, and 3.0 g / L, respectively. The arsenic elution value of the obtained iron arsenate crystal was 0.03 mg / L. FIG. 11 shows an SEM photograph of the precipitate of this example obtained at a reaction time of 360 minutes.
［実施例９］
銅（２）イオン濃度が０ｇ／Ｌであること以外は実施例８と同一の砒酸鉄析出反応条件において、反応時間１８０ｍｉｎ以降から酸素ガス１Ｌ／ｍｉｎのペースで吹き込んだ場合と吹き込まない場合の２種類の試験を行った。
酸素を吹き込んだ場合の反応終了後の上澄み液中の砒素（５）、鉄（３）および鉄（２）の濃度は、それぞれ０.４９ｇ／Ｌ、０.１ｇ／Ｌ、および４.２ｇ／Ｌであり、得られた砒酸鉄結晶の砒素溶出値は０.０６ｍｇ／Ｌであった。
一方、酸素を吹き込まない場合の反応終了後の上澄み液中の砒素（５）、鉄（３）および鉄（２）の濃度は、それぞれ１.６１ｇ／Ｌ、＜０.１ｇ／Ｌ、および６.４ｇ／Ｌであり、得られた砒酸鉄結晶の砒素溶出値は０.３５ｍｇ／Ｌであった。
本実施例８および実施例９より、水溶液中のＦｅ（２）／Ａｓ（５）のモル比が０.２５のように共存するＦｅ（２）イオン濃度が低い場合には、砒酸鉄結晶の生成速度が低下するため反応を完結させるまでの熟成時間を十分に取る必要があると考えられるが（例えば、本実施例で反応後期に酸素を吹き込まなかった試験が該当）、反応後期に酸素を吹き込むか（本実施例で反応後期に酸素を吹き込んだ試験が該当）、水溶液に銅（２）イオンを共存（実施例８が該当）させることで、同じ反応時間でも得られる結晶性砒酸鉄の溶出特性は向上し、さらに反応後の液中の砒素濃度がさらに低下することが理解される。
[Example 9]
Except that the copper (2) ion concentration is 0 g / L, in the same iron arsenate precipitation reaction conditions as in Example 8, 2 when oxygen gas was blown at a pace of 1 L / min after the reaction time of 180 min A variety of tests were conducted.
The concentrations of arsenic (5), iron (3), and iron (2) in the supernatant after completion of the reaction when oxygen is blown are 0.49 g / L, 0.1 g / L, and 4.2 g / L, respectively. L, and the obtained iron arsenate crystal had an arsenic elution value of 0.06 mg / L.
On the other hand, the concentrations of arsenic (5), iron (3), and iron (2) in the supernatant after completion of the reaction when oxygen is not blown are 1.61 g / L, <0.1 g / L, and 6 respectively. The arsenic elution value of the obtained iron arsenate crystal was 0.35 mg / L.
From Example 8 and Example 9, when the Fe (2) ion concentration in the aqueous solution is low such that the Fe (2) / As (5) molar ratio is 0.25, the iron arsenate crystals It is considered necessary to take sufficient aging time until the reaction is completed because the production rate is reduced (for example, in this example, the test in which oxygen was not blown in the late reaction) The crystalline iron arsenate obtained in the same reaction time can be obtained by injecting (corresponding to the test in which oxygen was injected in the latter part of the reaction in this example) or by coexisting copper (2) ions in the aqueous solution (corresponding to Example 8). It is understood that the elution characteristics are improved and the arsenic concentration in the solution after the reaction is further reduced.
［実施例１０］
モデル被処理液として、６０％砒酸溶液と硫酸第一鉄および硫酸銅とを用いて、（砒素（５）４５ｇ＋鉄（２）１６.８ｇ＋銅（２）３０ｇ）／Ｌの水溶液を８００ｍＬ準備し、これに鉄（３）源として実施例６で用いたものと同一Ｌｏｔのヘマタイト（Ｆｅ2Ｏ3）を添加し、実施例６と同じ手順で砒酸鉄の析出反応を行った。なお、この条件は、Ｆｅ（２）／Ａｓ（５）のモル比が０.５、Ｆｅ（３）／Ａｓ（５）のモル比が１.０、全Ｆｅ／Ａｓ（５）のモル比が１.５である。この場合、反応終了後の上澄み液中の砒素（５）、鉄（３）および鉄（２）の濃度は、それぞれ０.０４ｇ／Ｌ、３.１ｇ／Ｌ、および７.８ｇ／Ｌであり、得られた砒酸鉄結晶の砒素溶出値は０.０３ｍｇ／Ｌであった。尚、当該砒酸鉄結晶の粒子径（Ｄ５０）は３.０μｍであった。
次に、上記試験で得られた砒酸鉄結晶を種晶として１５ｇ／Ｌの濃度で上記水溶液に添加した以外は上記試験と全く同じ条件で砒酸鉄の析出反応を行った。
この場合、反応終了後の上澄み液中の砒素（５）、鉄（３）および鉄（２）の濃度は、それぞれ０.０８ｇ／Ｌ、３.２ｇ／Ｌ、および８.０ｇ／Ｌであり、得られた砒酸鉄結晶の砒素溶出値は０.０８ｍｇ／Ｌであった。尚、当該砒酸鉄結晶の粒子径（Ｄ５０）は１５.５μｍであった。図１２に反応時間３６０ｍｉｎで得られた砒酸鉄結晶のＳＥＭ写真を示す。
以上の結果から、結晶化反応時に種晶を共存させることにより、得られる砒酸鉄結晶粒子の肥大化が達成されることが理解される。
[Example 10]
Using a 60% arsenic acid solution, ferrous sulfate and copper sulfate as a model treatment liquid, prepare 800 mL of an aqueous solution of (arsenic (5) 45 g + iron (2) 16.8 g + copper (2) 30 g) / L. Then, hematite (Fe 2 O 3 ) of the same lot as that used in Example 6 was added as an iron (3) source, and iron arsenate was precipitated by the same procedure as in Example 6. This condition is that the molar ratio of Fe (2) / As (5) is 0.5, the molar ratio of Fe (3) / As (5) is 1.0, and the molar ratio of total Fe / As (5). Is 1.5. In this case, the concentrations of arsenic (5), iron (3), and iron (2) in the supernatant after completion of the reaction are 0.04 g / L, 3.1 g / L, and 7.8 g / L, respectively. The arsenic elution value of the obtained iron arsenate crystal was 0.03 mg / L. The iron arsenate crystal had a particle size (D50) of 3.0 μm.
Next, the iron arsenate precipitation reaction was carried out under exactly the same conditions as in the above test except that the iron arsenate crystal obtained in the above test was added as a seed crystal to the aqueous solution at a concentration of 15 g / L.
In this case, the concentrations of arsenic (5), iron (3), and iron (2) in the supernatant after completion of the reaction are 0.08 g / L, 3.2 g / L, and 8.0 g / L, respectively. The arsenic elution value of the obtained iron arsenate crystal was 0.08 mg / L. The iron arsenate crystal had a particle size (D50) of 15.5 μm. FIG. 12 shows an SEM photograph of iron arsenate crystals obtained at a reaction time of 360 minutes.
From the above results, it is understood that enlargement of the obtained iron arsenate crystal particles can be achieved by making the seed crystal coexist in the crystallization reaction.
［比較例２］
モデル被処理液として、６０％砒酸溶液を用いて、砒素（５）４５ｇ／Ｌの水溶液を８００ｍＬ準備し、これに鉄（３）源として実施例６で用いたものと同一Ｌｏｔのヘマタイト（Ｆｅ2Ｏ3）を添加し、実施例６と同じ手順で砒酸鉄の析出反応を行った。なお、この条件は、Ｆｅ（２）／Ａｓ（５）のモル比が０、Ｆｅ（３）／Ａｓ（５）のモル比が１.０、全Ｆｅ／Ａｓ（５）のモル比が１.０である。本比較例における反応終了後の上澄み液中の砒素（５）濃度は、３６ｇ／Ｌであり、添加したヘマタイト結晶のかなりの部分が反応せずに残存する結果となった。尚、当該沈殿物の砒素溶出値は１４.３ｍｇ／Ｌであった。図１３に反応時間３６０ｍｉｎで得られた本比較例の沈殿物のＳＥＭ写真を示す。
[Comparative Example 2]
As a model treatment liquid, a 60% arsenic acid solution was used to prepare 800 mL of an aqueous solution of 45 g / L of arsenic (5), and the same lot of hematite (Fe as used in Example 6 as an iron (3) source) was prepared. 2 O 3 ) was added and iron arsenate precipitation reaction was carried out in the same procedure as in Example 6. In this condition, the molar ratio of Fe (2) / As (5) is 0, the molar ratio of Fe (3) / As (5) is 1.0, and the molar ratio of all Fe / As (5) is 1. 0.0. The concentration of arsenic (5) in the supernatant after the reaction in this comparative example was 36 g / L, which resulted in a significant portion of the added hematite crystals remaining without reacting. The arsenic elution value of the precipitate was 14.3 mg / L. FIG. 13 shows an SEM photograph of the precipitate of this comparative example obtained at a reaction time of 360 minutes.
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