JP6251590B2 - Arsenic oxidation method - Google Patents

Description

  The present invention relates to a method for oxidizing arsenic.

Arsenic compounds exist in nature in various forms such as simple substances, arsenides (eg, FeAs), arsenic sulfides (eg: FeAsS), arsenates (eg: FeAsO ₄ ), and arsenic is −3 to +5. In particular, trivalent arsenic and pentavalent arsenic are known.

  It is known that pentavalent arsenic compounds stabilize and form a precipitate of water-insoluble arsenate, such as Fe, Ca, Cu, and rare earths, excluding alkali metals. On the other hand, trivalent arsenic compounds are difficult to form metal ions and stable salts with low solubility, and even if they can be formed, arsenous acid itself is a weak acid, so it easily decomposes and dissolves with acids and alkalis. Resulting in. Therefore, it is difficult to stabilize the trivalent arsenic compound as it is, and it is necessary to convert the trivalent arsenic compound to a pentavalent arsenic compound before the stabilization treatment.

However, it is known that the reaction rate when oxidizing from a trivalent arsenic compound to a pentavalent arsenic compound is very slow. Therefore, industrially, in addition to the method of forcing the equilibrium of the following reaction formula (1) to the right by forming a precipitate with a large excess of Fe ions in the presence of oxygen or air, Instead, a method using a strong oxidizing agent such as hydrogen peroxide or ozone that can promote the oxidation reaction at a higher speed is used.
Fe ³⁺ + AsO ₂ − + H ₂ O + (O) → FeAsO ₄ ↓ + 2H ⁺ (1)

  However, in the method using an excessive amount of Fe ions, in addition to the increase in treatment cost due to the use of an excessive amount of Fe compound, after the reaction, a large amount of goethite (FeOOH), basic iron salts, etc. with respect to the amount of As present. There was a problem that precipitates were produced as by-products and the weight and volume of the As-containing material increased significantly, making storage and processing difficult. Further, in the method using a strong oxidant, it is necessary to store a large amount of strong oxidant, which causes an increase in As treatment costs, and thus commercial use has not progressed.

  Therefore, as another approach, it is required to increase the oxidation rate of the trivalent arsenic compound using a catalyst. However, in the oxidation of inorganic compounds, the difference in redox potential is large and often exceeds the redox potential at which the catalyst itself effectively acts.

  For example, Patent Document 1 proposes converting a divalent iron compound contained in a plating solution into trivalent iron by blowing oxygen in the presence of a platinum catalyst.

JP-A-3-232999

  However, the method of Patent Document 1 cannot be applied to a system that is considered to act as a catalyst poison for platinum group elements such as arsenic and sulfur.

  The present invention has been made in view of the above circumstances, and its object is to oxidize a trivalent arsenic compound to a pentavalent arsenic compound without using an excessive iron compound or an expensive oxidizing agent. Is to provide a method of efficiently proceeding using a catalyst.

  The inventors of the present invention have made extensive studies to solve the above problems, and found that an oxygen-containing gas is supplied in the presence of platinum particles to a trivalent arsenic-containing liquid containing trivalent arsenic, The present invention has been completed.

  Specifically, the following are provided.

  (1) The present invention is an arsenic oxidation method in which an oxygen-containing gas is supplied to a trivalent arsenic-containing liquid containing trivalent arsenic in the presence of platinum particles.

  (2) Moreover, this invention is an arsenic oxidation method as described in (1) whose pH of the said trivalent arsenic containing liquid is 7 or more.

  (3) Further, the present invention is the arsenic oxidation method according to (1) or (2), wherein the carrier on which the platinum particles are fixed is titanium dioxide or zirconium dioxide.

  (4) Further, the present invention is a catalyst in which the platinum particles are supported on a carrier, and the degree of dispersion of the platinum particles when measured according to a CO pulse adsorption method is 20% or less. The method for oxidizing arsenic according to any one of 3).

  (5) Further, the present invention is a catalyst in which the platinum particles are supported on a carrier, and the specific surface area per 1 g of platinum present in the catalyst is determined from the dispersion degree of the platinum particles when measured according to a CO pulse adsorption method. Any one of (1) to (4), wherein the average particle diameter of the platinum particles obtained by calculating and obtaining the diameter when the platinum particles are made into spheres from the specific surface area and the weight of platinum is 5 nm or more. A method for oxidizing arsenic according to claim 1.

  According to the present invention, it is possible to efficiently oxidize trivalent arsenic to pentavalent arsenic without using an excess iron compound or an expensive oxidizing agent.

  Hereinafter, specific embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments, and may be implemented with appropriate modifications within the scope of the object of the present invention. Can do.

  The present invention includes a step of supplying an oxygen-containing gas in the presence of platinum particles to a trivalent arsenic-containing liquid containing trivalent arsenic.

<Trivalent arsenic>
A typical example of the trivalent arsenic compound is arsenous acid. When arsenous acid is oxidized, hydroxide ions are consumed as shown in the following chemical formula (2). Therefore, the reaction is promoted as the pH increases. From this, the pH value of the trivalent arsenic-containing liquid is not particularly limited, but is preferably alkaline, that is, the pH is 7 or more.
AsO ²⁻ + 2OH ⁻ + (O) → AsO ₄ ³⁻ + H ₂ O (2)

<Platinum particles>
The present invention realizes high-speed oxidation of trivalent arsenic ions by activating oxygen with a catalyst. In general, platinum group elements, molybdenum oxides, and the like are known as catalysts that promote oxidation, and examples of platinum group elements include Pd and the like in addition to Pt. However, when used for oxidation of inorganic ions in an aqueous solution, Pd is less effective than Pt and is not very effective. Further, molybdenum oxide exhibits little effect as a catalyst.

  For this reason, in this invention, it is preferable to use Pt as a catalyst. Here, arsenic is generally considered to be a catalyst poison of Pt. However, the present inventor found for the first time that the catalyst poison is limited to the case where an alloy or arsenide is formed with Pt. In addition to the trivalent arsenic compound and pentavalent arsenic compound itself, in these aqueous solutions, It was found that a compound between Pt and As was not formed and the Pt catalyst was not affected. Therefore, as in the present invention, when an oxygen-containing gas is supplied to a trivalent arsenic-containing liquid containing trivalent arsenic in the presence of platinum particles, arsenic does not become a catalyst poison of Pt.

The platinum particles are preferably supported on a carrier. When the pH of the trivalent arsenic-containing liquid is 7 or more, the support is dissolved in titanium dioxide (TiO ₂ ), dioxide dioxide to prevent the supported Pt particles from falling off the support by dissolving in the alkaline region. Zirconium (ZrO ₂ ), magnesium oxide (MgO), an alkaline earth metal oxide, or a composite oxide thereof (eg, ZrO 2 —MgO) is preferable. On the other hand, when the pH of the trivalent arsenic-containing liquid is less than 7, the carrier is dissolved in silicon dioxide (SiO _{2) in} order to prevent the supported Pt particles from falling off the carrier due to dissolution in the acidic region. ), Dialuminum trioxide (Al ₂ O ₃ ), zirconium dioxide (ZrO ₂ ), titanium dioxide (TiO ₂ ), diboron trioxide (B ₂ O ₃ ), or a composite oxide thereof (SiO ₂ —Al ₂ O) ₃ , SiO ₂ —ZrO ₂ ), diatomaceous earth, various zeolites, various phosphates and the like are preferable.

  By the way, it is preferable that the dispersion degree of platinum particles when measured according to the CO pulse adsorption method is 20% or less. As the case where the degree of dispersion is large, both the case where the quality of the platinum particles is too low and the case where the quality of the platinum particles are too high can be considered, but if the quality of the platinum particles is too low, the trivalent arsenic is changed to pentavalent arsenic. It is considered that trivalent arsenic cannot be suitably oxidized because of the lowering of the reaction rate. On the other hand, when the quality of the platinum particles is too high, the form of the platinum particles changes, the platinum ratio in the ionic state increases, the reactivity gradually decreases, and the oxidation efficiency decreases, so that trivalent arsenic cannot be suitably oxidized it is conceivable that.

  Further, when the specific surface area per 1 g of platinum present in the catalyst is calculated from the dispersion degree of the platinum particles measured according to the CO pulse adsorption method, and the platinum particles are made into spheres from the specific surface area and the weight of platinum. It is preferable that the average particle diameter of the platinum particle obtained by calculating | requiring the diameter of is 5 nm or more. As the case where the average particle size is small, both the case where the quality of the platinum particles is too low and the case where the quality of the platinum particles are too high can be considered, but if the quality of the platinum particles is too low, the trivalent arsenic to the pentavalent arsenic It is considered that trivalent arsenic cannot be suitably oxidized because the reaction rate to the lowers. On the other hand, when the quality of the platinum particles is too high, the form of the platinum particles changes, the platinum ratio in the ionic state increases, the reactivity gradually decreases, and the oxidation efficiency decreases, so that trivalent arsenic cannot be suitably oxidized it is conceivable that.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention does not receive a restriction | limiting at all in these description.

<Preliminary test>
[Preparation of SiO ₂ -supported Pt catalyst]
S iO ₂ to Jinitoroanmin platinum (IV) solution 10ml of a Pt containing 0.05 g (product name: JRC- S IO-8, Catalysis Society reference catalyst) was added 10 g, 2 hours at 75 ° C. with stirring with a stirrer The water was evaporated. Thereafter, it was further vacuum-dried at 80 ° C., and then maintained at 500 ° C. in the atmosphere and calcined for 2 hours to obtain a SiO ₂ -supported Pt catalyst.

[Relationship between oxidation degree and oxidation-reduction potential of arsenic-containing liquid]
To 200 ml of a 0.001 mmol / L HAsO ₂ aqueous solution, 0.5 g of the above-mentioned SiO ₂ -supported Pt catalyst was added, and the mixture was stirred and mixed while blowing oxygen at 57 ml / min while heating for 5 hours while maintaining at 80 ° C. In order to confirm the degree of oxidation from trivalent arsenic to pentavalent arsenic, only trivalent arsenic was selectively extracted and separated by solvent extraction to leave pentavalent arsenic in the extraction residue, and the obtained extraction Each of the liquid and the extraction residual liquid was quantitatively analyzed using ICP, and the oxidation rate was evaluated from the obtained analytical value. As a result, it was confirmed that 23% of the entire arsenic was oxidized from trivalent to pentavalent. It was.

  In addition, the oxidation-reduction potential (ORP) before and after the reaction (the reference electrode is a KCl saturated Ag / AgCl electrode) was measured. As a result, the oxidation-reduction potential before the reaction was 325 mV, and the oxidation-reduction potential after the reaction was 347 mV. From this, it was confirmed that the oxidation state can be grasped from the difference in redox potential before and after the reaction.

The pH of the arsenic-containing aqueous solution was lowered from 4.55 before the reaction to 3.14. This also confirms that the weak acid HAsO ₂ is oxidized to H ₃ AsO ₄ .

<Example 1: Comparison of pH of arsenic-containing liquid>

[Example 1-1]
Trivalent arsenic contained in the arsenic-containing liquid was oxidized using the SiO ₂ -supported Pt catalyst used in the above <preliminary test> in the same manner as in the above <preliminary test>. Table 2 shows the difference in redox potential before and after the reaction when the oxidation reaction time is 5 hours and 6 hours.

[Example 1-2]
[Preparation of TiO ₂ supported Pt catalyst]
The TiO ₂ carrier Jinitoroanmin platinum (IV) solution 10ml containing 0.05g of Pt, to give a TiO ₂ supported Pt catalyst using the same method as described above [Preparation of SiO ₂ supported Pt catalyst].

[Oxidation of arsenic-containing liquid]
Subsequently, NaOH was added to 200 ml of a 0.001 mmol / L HAsO ₂ aqueous solution, and 4 mg of the above TiO ₂ -supported Pt catalyst was added to a solution adjusted to pH 14 and maintained at 80 ° C. for 5 hours. Oxygen was mixed while blowing 57 ml / min. Then, after measuring the difference in oxidation-reduction potential before and after the reaction, the oxidation reaction was continued for 6 hours. Table 2 shows the difference in redox potential before and after the reaction when the oxidation reaction time is 5 hours and 6 hours.

[Comparative Example 1-1]
Trivalent arsenic contained in the arsenic-containing liquid was oxidized by the same method as in Example 1-1 except that no Pt catalyst was used. Table 2 shows the difference in redox potential before and after the reaction when the oxidation reaction time is 5 hours and 6 hours.

[Comparative Example 1-2]
Trivalent arsenic contained in the arsenic-containing liquid was oxidized by the same method as in Example 1-2 except that no Pt catalyst was used. Table 2 shows the difference in redox potential before and after the reaction when the oxidation reaction time is 5 hours and 6 hours.

  From Table 2, it was confirmed that trivalent arsenic can be suitably oxidized by supplying an oxygen-containing gas in the presence of platinum particles to a trivalent arsenic-containing liquid containing trivalent arsenic (Example). In particular, it was confirmed that trivalent arsenic can be oxidized more suitably by making the trivalent arsenic-containing liquid alkaline (Example 1-2).

  On the other hand, even if oxygen-containing gas was supplied to the trivalent arsenic-containing liquid containing trivalent arsenic, it was confirmed that trivalent arsenic could hardly be oxidized in the absence of a catalyst (Comparative Example). This means that only trivalent arsenic is selectively extracted and separated by the solvent extraction method to leave pentavalent arsenic in the extraction residual liquid, and the obtained extraction liquid and the extraction residual liquid are quantified using ICP. The results of the analysis and the evaluation of the oxidation rate from the analytical values obtained were also supported. As a result of the analysis, the oxidation rate of trivalent arsenic was less than 2% below the lower limit of detection.

<Example 2: Comparison of catalyst amount and oxygen blowing amount>

[Example 2-1]
Trivalent arsenic contained in the arsenic-containing liquid was oxidized by the same method as in [Example 1-2] above. Table 4 shows the difference in redox potential before and after the reaction when the oxidation reaction time is 5 hours and 6 hours.

[Example 2-2]
The arsenic-containing liquid was prepared in the same manner as in Example 1-1 except that the amount of the TiO ₂ -supported Pt catalyst was 40 mg and the amount of oxygen blown into the trivalent arsenic-containing liquid was 200 ml / min. The trivalent arsenic contained in was oxidized. Table 4 shows the difference in redox potential before and after the reaction when the oxidation reaction time is 2 hours.

From Table 4, it is confirmed that when the amount of TiO ₂ carrier catalyst is increased to 40 mg and the oxygen blowing amount is increased to 200 ml / min, the reaction rate further increases, and the difference in redox potential after 2 hours is 260 mV. (Example 2-2). This state is an equilibrium state.

<Example 3: Comparison of Pt quality in catalyst>

[Preparation of TiO ₂ -supported Pt catalyst according to Examples 3-1 to 3-6]
Add TiO ₂ carrier (manufactured by Wako Pure Chemical Industries, Ltd.) to 10 ml of dinitroammineplatinum (IV) aqueous solution containing 0.05 g of Pt so that the Pt quality becomes the value shown in Table 5 with respect to 100 parts by mass of the catalyst. Then, the water was evaporated at 75 ° C. for 2 hours while stirring with a stirrer. Thereafter, further dried in vacuo at 80 ° C., followed by calcination over 500 ° C. maintained for 2 hours in air to obtain a TiO ₂ supported Pt catalyst according to Example 3-1 to 3-6.

[Preparation of TiO ₂ -supported Pt catalyst according to Examples 3-7 to 3-12]
Example 3-7 was prepared in the same manner as in “[Preparation of TiO ₂ -supported Pt catalyst according to Examples 3-1 to 3-6]” except that the TiO ₂ carrier was P25 manufactured by Nippon Aerosil Co., Ltd. A TiO ₂ -supported Pt catalyst according to ˜3-12 was obtained.

[Oxidation of arsenic-containing liquid]
Subsequently, NaOH was added to 200 ml of 1.0 mmol / L HAsO ₂ aqueous solution, and the pH was adjusted to 14 so that the Pt weight of each TiO ₂ -supported Pt catalyst was constant (0.2 mg). The amounts shown in Table 5 and Table 6 were added, and mixing was performed while blowing pure oxygen at 57 ml / min for 140 minutes while maintaining the temperature at 80 ° C. Tables 7 and 8 show the difference in redox potential before and after the reaction.

[Dispersion degree and particle diameter of TiO ₂ -supported Pt platinum particles]
The degree of dispersion of the TiO ₂ -supported Pt platinum particles was measured according to a CO pulse adsorption method which is a known method. The results are shown in Table 7 and Table 8. In addition, the specific surface area per gram of Pt present in the catalyst is calculated from the degree of dispersion, and the diameter when the Pt particles are made into spheres is obtained from the specific surface area and the weight of Pt, and this is defined as the Pt average particle diameter. . The results are shown in Table 7 and Table 8.

  From Tables 7 and 8, it was confirmed that the degree of oxidation of trivalent arsenic differs depending on the quality of Pt contained in 100 parts by mass of the catalyst. In particular, the degree of oxidation of trivalent arsenic is determined based on the degree of platinum particle dispersion measured according to the CO pulse adsorption method or the amount of platinum particle dispersion measured according to the CO pulse adsorption method. The specific surface area per gram is calculated, and from this specific surface area and the weight of platinum, it is correlated with the average particle diameter of the platinum particles obtained by determining the diameter when the platinum particles are spheres. When the average particle diameter is 20% or less, or when the average particle diameter is 5 nm or more, the difference in redox potential is remarkably large. From this, it can be said that trivalent arsenic can be significantly oxidized (Example 3-3, 3-9 to 3-12).

  Moreover, it can be seen from Tables 7 and 8 that the dispersity or the average particle diameter deviates from the preferred range both when the quality of the platinum particles is too low and when the quality of the platinum particles is too high. If the quality of the platinum particles is too low, the reaction rate from trivalent arsenic to pentavalent arsenic is lowered, so that it is considered that trivalent arsenic cannot be suitably oxidized. On the other hand, when the quality of the platinum particles is too high, the form of the platinum particles changes, the platinum ratio in the ionic state increases, the reactivity gradually decreases, and the oxidation efficiency decreases, so that trivalent arsenic cannot be suitably oxidized it is conceivable that.

Claims (4)

Arsenic containing a trivalent arsenic-containing liquid containing trivalent arsenic and supplying oxygen-containing gas in the presence of the platinum particles as a catalyst using platinum particles supported on titanium dioxide or zirconium dioxide as a catalyst as a catalyst. (Except for the method of oxidizing trivalent arsenic by artificially irradiating light) .   The arsenic oxidation method according to claim 1, wherein the trivalent arsenic-containing liquid has a pH of 7 or more. C O pulses degree of dispersion of the platinum particles as measured according adsorption method is 20% or less, oxidation process of arsenic according to claim 1 or 2. And calculating the specific surface area of platinum per 1g present in the catalyst from the dispersion of the platinum particles as measured in accordance with C O pulse adsorption method, and a weight of the specific surface area and platinum, in the case of platinum particles and spheres The arsenic oxidation method according to any one of claims 1 to 3 , wherein an average particle diameter of the platinum particles obtained by determining a diameter is 5 nm or more.