JP2012250896A - Method for synthesizing scorodite - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for synthesizing scorodite which is suitable for reducing a specific surface area and suppressing elution of arsenic by lowering a nucleation density and obtaining coarser crystal grains.SOLUTION: Pentavalent arsenic ion, a divalent iron compound and a photocatalyst are mixed together, and heated at 50°C or higher, and simultaneously subjected to ultraviolet irradiation. Hereby, the divalent iron compound is oxidized into a trivalent iron compound to lower a nucleation density, and simultaneously to synthesize scorodite. After the ultraviolet irradiation, the mixture is further heated at 50°C or higher, and simultaneously oxygen is blown into it, to thereby bulk a scorodite crystal.

Description

  The present invention relates to a method for synthesizing scorodite in which arsenic is immobilized so that it is difficult to elute, and more particularly to a method for synthesizing scorodite suitable for suppressing arsenic elution by obtaining coarse crystal grains.

  Smoke ash generated during copper refining contains high concentrations of arsenic. These arsenic are only stored because there is no special demand as a raw material. However, since arsenic during storage may be dissolved in water and contaminate the surrounding environment, arsenic elution must be suppressed. In refining copper, arsenic must be separated and recovered as an impurity.

For this reason, conventionally, the research which fixes so that the arsenic in storage and the arsenic in a solution may not elute has been performed conventionally. In recent years, research on insolubilizing arsenic by synthesizing crystals of scorodite (FeAsO ₄ · 2H ₂ O), which has the lowest solubility among arsenic compounds, has attracted attention. Scorodite is chemically stable and suitable for long-term storage. Since the main component of scorodite is arsenic, arsenic can be immobilized simply by generating scorodite.

The reaction mechanism of scorodite proposed in the past is shown in the following chemical formulas (1) and (2) (see, for example, Patent Documents 1 and 2).
Fe ²⁺ + H ⁺ + 1 / 4O ₂ → Fe ³⁺ +1/2 H ₂ O (1)
Fe ³⁺ + H ₃ AsO ₄ + 2H ₂ O → FeAsO ₄ · 2H ₂ O + 3H ⁺ (2)

Here, the chemical formula (1) is a reaction formula for oxidizing a divalent iron ion to a trivalent iron ion. In this chemical formula (1), Fe ²⁺ generated by dissolution of FeSO ₄ .7H ₂ O is oxidized by the blown oxygen gas to become Fe ³⁺ . In chemical formula (2), Fe ³⁺ and H ₃ AsO ₄ react to produce scorodite (FeAsO ₄ .2H ₂ O). Scorodite is a hardly soluble salt crystal and is easily supersaturated due to its low solubility. In the supersaturated state, scorodite nuclei are easily generated, and the nucleation density is increased. As a result, a large amount of scorodite nuclei are generated, the crystal growth is hindered, and the resulting crystal is miniaturized.

Therefore, in the methods disclosed in Patent Documents 1 and 2, by adding the reaction (1) (Fe ²⁺ → Fe ³⁺ ) in the process of generating crystals, the supersaturation degree of scorodite is finally lowered, The crystal is coarsened by controlling the generation density.

JP 2008-143741 A JP 2008-119690 A

  However, in the disclosed technologies of Patent Documents 1 and 2, since the reaction of the chemical formula (1) proceeds easily and the control of the reaction is not easy, the nucleation density tends to increase, and the scorodite crystals tend not to coarsen. It was. If the obtained scorodite crystals are made finer, the specific surface area becomes large and arsenic may be easily eluted.

  Therefore, the present invention has been devised in view of the above-mentioned problems, and the object thereof is to easily control the reaction of the chemical formula (1), thereby reducing the nucleation density, An object of the present invention is to provide a method for synthesizing scorodite having a small specific surface area and suppressing arsenic elution by obtaining coarser crystal grains.

In order to solve the above-mentioned problems, the present inventor has developed a new method for synthesizing scorodite in which the process of oxidizing iron to Fe ²⁺ → Fe ³⁺ with a photocatalyst by ultraviolet irradiation is performed in order to lower the nucleation density of scorodite crystals. We have studied earnestly.

  As a result, a new method for synthesizing scorodite by mixing a pentavalent arsenic ion, a divalent iron compound, and a photocatalyst and irradiating with ultraviolet rays while heating at 50 ° C. or higher was invented.

  The invention according to claim 1 is characterized in that scorodite is synthesized by mixing a pentavalent arsenic ion, a divalent iron compound and a photocatalyst, and irradiating with ultraviolet rays while heating at 50 ° C. or higher.

  The invention according to claim 2 is characterized in that, in the invention according to claim 1, oxygen is blown in while heating at 50 ° C. or higher after the ultraviolet irradiation.

According to the present invention, in order to reduce the nucleation density of scorodite crystals capable of confining harmful arsenic, the process of oxidizing iron to Fe ²⁺ → Fe ³⁺ is performed with a photocatalyst by ultraviolet irradiation. Thereby, the nucleation density can be lowered and the scorodite crystal can be coarsened.

It is a flowchart of the experiment of the synthesis method of the scorodite to which the present invention is applied. It is a figure which shows the identification result of the powder X-ray diffraction performed in step S15. It is a figure of the CCD optical microscope photograph image | photographed in step S15. It is a figure which shows the experimental result about the diameter and time change of the grown scorodite.

  Hereinafter, a method for synthesizing scorodite will be described in detail as an embodiment of the present invention.

  In the present invention, scorodite is synthesized by mixing pentavalent arsenic ions, a divalent iron compound, and a photocatalyst, and irradiating with ultraviolet rays while heating at 50 ° C. or higher. At this time, oxygen may be blown while heating at 50 ° C. or higher after ultraviolet irradiation.

  The reaction mechanism in the synthesis of scorodite in the present invention is based on the following chemical formulas (A) and (B).

Fe ²⁺ + H ⁺ + 1 / 4O ₂ → Fe ³⁺ +1/2 H ₂ O (A)
Fe ³⁺ + H ₃ AsO ₄ + 2H ₂ O → FeAsO ₄・ 2H ₂ O + 3H ⁺ (B)

This chemical formula (A) is a reaction formula for oxidizing a divalent iron ion to a trivalent iron ion. In the present invention, the process of the oxidation reaction (Fe ²⁺ → Fe ³⁺ ) of the chemical formula (1) is performed by a photocatalyst. In chemical formula (B), Fe ³⁺ and H ₃ AsO ₄ react to produce scorodite (FeAsO ₄ .2H ₂ O). Scorodite is a hardly soluble salt crystal and is easily supersaturated due to its low solubility. In general, in the supersaturated state, nuclei of scorodite are easily generated and the nucleation density is increased. However, the present invention intends to reduce the degree of supersaturation by using a photocatalyst as described above. In particular, as a photocatalyst, for example, TiO ₂ (anatase type) is used, and the nucleation density can be easily controlled by reducing the supersaturation degree of scorodite only by controlling the amount of TiO _{2 and} the amount of ultraviolet irradiation.

  Next, each raw material in the scorodite synthesis method to which the present invention is applied will be described.

Pentavalent Arsenic Ion In order to synthesize scorodite based on the above chemical formula (B), pentavalent arsenic ion (As ⁵⁺ ) is required. The pentavalent arsenic ion does not usually exist as an ion but exists as a compound (H ₃ AsO ₄ ) in the above-described chemical formula (B). However, this compound H ₃ AsO ₄ is a solid at normal temperature, but when mixed with water, it becomes a solution and ionizes to exist as AsO ₄ ³⁻ . As an example, H ₃ AsO ₄ is exemplified as a compound, but the compound is not limited to this, and may be replaced with a compound containing other arsenic.

  For compounds composed of arsenic ions other than pentavalent, it is necessary to execute a pentavalent treatment operation in advance. For example, in the case of trivalent arsenic ions, a process of oxidizing to pentavalent arsenic ions using hydrogen peroxide, a photocatalyst, or the like is executed. In this oxidation treatment, for example, oxidation may be performed with hydrogen peroxide. The concentration of this arsenic ion is, for example, 30 g / L, preferably 50 g / L. The higher the concentration, the higher the arsenic precipitation rate.

A divalent iron compound may be used as the iron compound raw material. Examples of the divalent iron compound include FeSO ₄ .7H ₂ O. In addition, the divalent iron compound needs to be soluble in the arsenic solution. The addition amount must be Fe / As = 0.5 to 2 molar ratio, and desirably, the molar ratio is 1 or more and the iron is mixed so as to be more than arsenic. Since the precipitation rate of arsenic becomes higher as the amount of iron is more than that of arsenic, the preparation is appropriately performed in consideration of the precipitation rate and the raw material cost of the iron compound. Moreover, when mixing an arsenic ion and an iron compound, it is necessary to adjust so that pH of a reaction liquid may become 2 or less.

A photocatalytic photocatalyst is a substance that exerts a strong oxidizing power from its surface when irradiated with light such as ultraviolet rays. This photocatalyst has a function of oxidizing a divalent iron compound to a trivalent iron compound. Examples of this photocatalyst are, for example, anatase-type titanium dioxide, but are not limited to these, and any substance that exhibits an oxidizing power that causes an iron oxidation reaction (Fe ²⁺ → Fe ³⁺ ) can be used. Can be substituted. Further, since the oxidizing power exhibited by the photocatalyst is different, the amount of photocatalyst added, the intensity of ultraviolet light to be irradiated, and the like are appropriately adjusted in order to exhibit the desired oxidizing power. As this photocatalyst, finer TiO ₂ may be used, which is particularly advantageous when weight separation is used for separation after crystal synthesis.

  Fine particles such as anatase-type titanium dioxide are desirable because they can be physically separated from the scorodite after the synthesis reaction by coarsening the scorodite.

  FIG. 1 is a flowchart of an experiment of a scorodite synthesis method to which the present invention is applied. Incidentally, this flowchart is an example, and may be replaced with other raw materials or processing operations.

First, in step S11, an arsenic solution and a hydrogen peroxide solution are mixed. In this arsenic solution, As ₂ O ₃ was dissolved in 40 ml of pure water to make a solution with an arsenic concentration of 100 g / L. By the way, this As ₂ O ₃ was purified from smoke ash. When As ₂ O ₃ is dissolved in pure water, trivalent arsenic ions (As ³⁺ ) are obtained, but in order to synthesize scorodite, pentavalent arsenic ions (As ⁵⁺ ) are required. Therefore, for the purpose of oxidation, hydrogen peroxide water is added in step S11. The amount of hydrogen peroxide added is 5.4 ml with respect to 40 ml of the arsenic solution having the above-mentioned concentration. After mixing the arsenic solution and the hydrogen peroxide solution, the mixture is heated to 80 ° C. and stirred for 8 hours.

Next, it transfers to step S12 and 0.5009g of anatase type titanium dioxide is added as a photocatalyst. Then the process proceeds to step S13, the addition of FeSO ₄ · 7H ₂ O as an iron compound. The amount of FeSO ₄ .7H ₂ O added is 11.1216 g so that the molar ratio of As to Fe is Fe / As = 0.77. Note that the steps S12 and S13 may be reversed in order or performed simultaneously.

Next, the process proceeds to step S14, and the mixed solution is heated to 90 ° C. and then irradiated with ultraviolet rays while stirring. At this time, the wavelength of the ultraviolet light is 365 nm and the intensity is 810 μw / cm ² . The ultraviolet lamp irradiates from a position about 50 mm away from the solution containing the solution. The stirring and ultraviolet irradiation time is 48 hours, but from the viewpoint of collecting experimental samples, samples are taken from the mixing liquid 1, 6, 12, 24, and 36 hours after the start of ultraviolet irradiation. ing.

  In addition, if the temperature of the mixing temperature in this step S14 is 60 degreeC or more, the intended reaction shown by Chemical formula (A) and (B) will advance, If it is desirably 80 degreeC, the reaction is suitable. Will progress. Moreover, if the reaction time of mixing temperature is 30 minutes or more in this step S14, reaction shown by the chemical formula (A) and (B) mentioned above will advance. Since the scorodite crystal itself has been generated immediately after the start of the reaction, and the scorodite crystal grows with time and the arsenic precipitation rate also increases, the reaction time is preferably 12 to 48 hours. The time for setting these is appropriately determined in consideration of the production cost, the ultraviolet irradiation amount, the arsenic concentration in the arsenic solution, the arsenic precipitation rate, and the like.

  Next, it transfers to step S15 and a mixed solution is filtered and air-dried. As a result, coarse scorodite remains on the filter paper. In this experiment, the scorodite remaining on the filter paper was isolated, identified by powder X-ray diffraction, and observed with a CCD optical microscope.

In FIG. 2, the identification result of the powder X-ray diffraction performed in this step S15 is shown. The circles in FIG. 2 are peaks corresponding to the crystal structure of scorodite. From the result of this diffraction peak, the formation of scorodite could be confirmed, and the disappearance of As ₂ O ₃ as a raw material could be confirmed. As a result, it was found that scorodite can be synthesized even with As ₂ O ₃ purified from smoke ash. It was also found that scorodite synthesis can be realized by utilizing oxidation by photocatalyst.

FIG. 3 is a CCD optical micrograph taken in step S15. It was confirmed that coarse scorodite crystals of about 30 μm were formed. In this photograph, scorodite crystal grains of about 10 μm are also confirmed. Further, the fine particles of several μm are TiO ₂ .

  FIG. 4 shows experimental results on the diameter and time change of the grown scorodite. The horizontal axis is the time from the start of the reaction, and the vertical axis is the average value of the crystal grain size measured by microscopic observation. For comparison, the data disclosed in the conventional method (FUJITA T. Novel atmospheric scorodite synthesis by oxidation of ferrous sulfate solution.Part I; Hydrometallurgy Vol. 90, No2-4, p92-102 (2008)) This is shown as “Comparative Example”. In the comparative example, scorodite rapidly coarsened up to 3 hours from the start of the reaction, fine crystals were formed, and the average particle size was lowered. On the other hand, in the present invention, scorodite was coarse and about 5 μm 1 hour after the start of the reaction, but became 15 μm 24 hours after the start of the reaction, and became coarse crystal grains of 30 μm after 48 hours.

Thus, according to the present invention, in order to reduce the nucleation density of the scorodite crystal capable of confining harmful arsenic, the process of oxidizing iron from Fe ^{2+ to} Fe ³⁺ is performed with a photocatalyst by ultraviolet irradiation. Thereby, the nucleation density can be lowered and the scorodite crystal can be coarsened. Further, it is possible to perform weight separation with high efficiency by coarsening the scorodite crystal.

  The present invention is not limited to the above-described embodiment. For example, oxygen may be blown while heating at 50 ° C. or higher after the ultraviolet irradiation in step S14. The inflow of oxygen is performed at a rate of 40 ml / min, but is not limited thereto. As a result, it was found that the crystal size of scorodite can be increased to 50 μm after 24 hours from the inflow of oxygen. In addition, after 48 hours from the inflow of oxygen, the crystal size of scorodite reached 60 μm. The oxygen blowing time may be 30 minutes or longer, and preferably about 12 to 48 hours. Similarly, the oxygen blowing time may be appropriately determined in consideration of the production cost, the ultraviolet irradiation amount, the arsenic concentration, the arsenic precipitation rate, and the like.

  Thus, by introducing a step of blowing oxygen after the ultraviolet irradiation, the scorodite crystal can be further coarsened. According to this method, the obtained scorodite crystal can have a width up to a particle size of 30 to 80 μm.

Claims (2)

A method for synthesizing scorodite, comprising mixing a pentavalent arsenic ion, a divalent iron compound, and a photocatalyst, and synthesizing scorodite by performing ultraviolet irradiation while heating at 50 ° C. or higher. The method for synthesizing scorodite according to claim 1, wherein oxygen is blown in after the ultraviolet irradiation while heating at 50 ° C or higher.