METHOD FOR INHIBITING REDUCTION IN CONCENTRATION OF OXIDIZING AGENT IN SULFURIC ACID SOLUTION CONTAINING PERSULFURIC ACID COMPONENT

Abstract

A heterocyclic compound having a structure in which a benzene ring and a nitrogen-containing heterocycle are condensed is added to a sulfuric acid solution that contains a persulfuric acid component such as peroxomonosulfuric acid, peroxomonosulfate, peroxodisulfuric acid, or peroxodisulfate as an oxidant and in which impurities that promote the reduction in the oxidant concentration are present. Here, the heterocyclic compound having a structure in which a benzene ring and a nitrogen-containing heterocycle are condensed is preferably a benzotriazole-based compound. The impurities that promote a reduction in the oxidant concentration are suitably one or more selected from copper ions, iron ions, nitrate ions, and nitrite ions. The method of the present invention of inhibiting a reduction in the oxidant concentration in a sulfuric acid solution containing a persulfuric acid component can inhibit the reduction in the oxidant concentration to a minimum even when impurities that cause a reduction in the oxidant concentration are mixed.

Claims

1. A method of inhibiting a reduction in an oxidant concentration in a sulfuric acid solution that contains a persulfuric acid component as an oxidant and in which an impurity that promotes the reduction in the oxidant concentration is present, the method comprising adding a heterocyclic compound to the sulfuric acid solution in which the impurity that promotes the reduction in the oxidant concentration is present, the heterocyclic compound having a structure in which a benzene ring and a nitrogen-containing heterocycle are condensed.

2. The method of inhibiting a reduction in an oxidant concentration in a sulfuric acid solution that contains a persulfuric acid component according to claim 1, wherein the persulfuric acid component is at least one selected from peroxomonosulfuric acid, peroxomonosulfate, peroxodisulfuric acid, and peroxodisulfate.

3. The method of inhibiting a reduction in an oxidant concentration in a sulfuric acid solution that contains a persulfuric acid component according to claim 1, wherein the heterocyclic compound having a structure in which a benzene ring and a nitrogen-containing heterocycle are condensed is a benzotriazole-based compound.

4. The method of inhibiting a reduction in an oxidant concentration in a sulfuric acid solution that contains a persulfuric acid component according to claim 1, wherein the impurity that promotes the reduction in the oxidant concentration is one or more selected from copper ion, iron ion, nitrate ion, and nitrite ion.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0016] FIG. 1 is a graph illustrating changes in the oxidant concentration when copper ions, iron ions, nitrate ions, and nitrite ions are added to a sulfuric acid solution containing a persulfuric acid component.

[0017] FIG. 2 is a graph illustrating changes in the oxidant concentration of Example 1 and Comparative Example 1.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

[0018] The method of the present invention of inhibiting a reduction in the oxidant concentration in a sulfuric acid solution containing a persulfuric acid component will be described in detail based on the following one or more embodiments.

<Method of Inhibiting Reduction in Oxidant Concentration in Sulfuric Acid Solution Containing Persulfuric Acid Component>

(Sulfuric Acid Solution Containing Persulfuric Acid Component)

[0019] In the present embodiment, the sulfuric acid solution containing a persulfuric acid component is not particularly limited, provided that it contains a persulfuric acid component and is a sulfuric acid solution. Examples of the persulfuric acid component include peroxomonosulfuric acid, peroxomonosulfate, peroxodisulfuric acid, and peroxodisulfate. These may each be appropriately selected and used alone, or two or more types may also be used in combination.

[0020] Examples for use as the sulfuric acid solution containing a persulfuric acid component include a sulfuric acid solution (electrolytic sulfuric acid solution) obtained by electrolyzing a sulfuric acid-containing liquid to generate a persulfuric acid component and a solution obtained by adding hydrogen peroxide to sulfuric acid to generate a persulfuric acid component. In particular, the present invention can be suitably applied to the electrolytic sulfuric acid solution.

[0021] When the sulfuric acid solution containing a persulfuric acid component is an electrolytic sulfuric acid solution, for example, the sulfuric acid concentration may be preferably 60 to 87 wt % and particularly preferably 70 to 83 wt %. The oxidant concentration in the initial state may be 2 g/L or more and particularly 3 to 20 g/L depending on the intended use. If the oxidant concentration is less than 2 g/L, the influence of the reduction in the oxidant concentration may not be so large, while if the oxidant concentration exceeds 20 g/L, the production itself may not be economical and the oxidizing ability may not be problematic even when the oxidant concentration is reduced to a certain degree.

(Impurities that Promote Reduction in Oxidant Concentration)

[0022] Impurities that promote the reduction in the concentration of an oxidant mixed in the sulfuric acid solution containing the persulfuric acid component as described above are not particularly limited, provided that the impurities react with and consume the oxidant component in the sulfuric acid solution containing a persulfuric acid component. Examples of such impurities include copper ions, iron ions, nitrate ions, and nitrite ions, which are likely to be mixed as impurities because they are often used industrially. Among these, when copper ions and iron ions, particularly copper ions, are mixed as impurities, the reduction in the oxidant concentration can be suitably inhibited.

[0023] The concentration of the impurities, which promote the reduction in the oxidant concentration as described above, in the sulfuric acid solution containing a persulfuric acid component may be about 5000 ppm or less and particularly about 3000 to 1 ppm. If the impurity concentration exceeds 5000 ppm, the amount of impurities is too large, and the effect of inhibiting the reduction in the oxidant concentration in the solution may not be sufficiently exhibited. The lower limit of the impurity concentration is not particularly limited, but if it is less than 1 ppm, the reduction in the oxidant concentration is not large and it is difficult to confirm the effect, which may not be preferred.

(Oxidant Concentration Reduction Inhibitor)

[0024] In the present embodiment, a heterocyclic compound having a structure in which a benzene ring and a nitrogen-containing heterocycle are condensed is used as an oxidant concentration reduction inhibitor. Examples of the heterocyclic compound having a structure in which a benzene ring and a nitrogen-containing heterocycle are condensed include 1,2,3-benzotriazole, 5-methylbenzotriazole, 4-methylbenzotriazole, 5,6-dimethylbenzotriazole, benzoimidazole, and 5,6-dimethylbenzoimidazole, among which benzotriazole compounds typified by 1,2,3-benzotriazole and 5-methylbenzotriazole may be particularly suitable.

[0025] The addition concentration of the oxidant concentration reduction inhibitor may be 0.01 times or more and further preferably 0.05 times or more with respect to the molar concentration of impurities that cause a reduction in the concentration of an oxidant contained in the solution. The upper limit of the addition amount may be preferably 1 times or less and particularly preferably 0.5 times or less with respect to the molar concentration of impurities that cause a reduction in the oxidant concentration because if it is too large, the oxidant concentration reduction inhibitor may affect the effect of treatment such as surface treatment with a sulfuric acid solution containing a persulfuric acid component.

[0026] The method of adding the oxidant concentration reduction inhibitor as described above is not particularly limited, provided that it is possible to achieve a state in which the required concentration is dissolved in the sulfuric acid solution containing a persulfuric acid component. The oxidant concentration reduction inhibitor may be added and dissolved in a state of solid (powder) in the sulfuric acid solution containing a persulfuric acid component or may otherwise be added in a state of liquid by preliminarily dissolving the oxidant concentration reduction inhibitor in a solution. For example, when a sulfuric acid-containing liquid is electrolyzed to generate a persulfuric acid component, the heterocyclic compound may be added to the solution after electrolysis, or a sulfuric acid solution in which the heterocyclic compound is preliminarily dissolved may be electrolyzed. When the persulfuric acid component is generated by a method of adding persulfate to sulfuric acid or a method of adding hydrogen peroxide to sulfuric acid, the heterocyclic compound may be added to a sulfuric acid solution containing the generated persulfuric acid component, or a sulfuric acid solution containing a persulfuric acid component can be generated by using sulfuric acid, hydrogen peroxide, or the like to which the heterocyclic compound is preliminarily added.

[0027] The method of the present invention of inhibiting the reduction in the oxidant concentration in the sulfuric acid solution containing a persulfuric acid component has been described above, but the present invention is not limited to the above embodiment and various modifications can be carried out. For example, the sulfuric acid solution containing a persulfuric acid component may not have to be composed only of the persulfuric acid component and the sulfuric acid solution, and may contain other acids such as phosphoric acid and/or a chemical liquid component if they do not reduce the oxidant concentration.

EXAMPLES

[0028] The present invention will be specifically described with reference to the following Examples and Comparative Examples. Note, however, that the present invention is not limited to these descriptions.

Reference Example

[0029] A confirmation test was conducted to confirm the reduction in the oxidant concentration by using a test liquid generated by electrolyzing 78 wt % sulfuric acid (H.sub.2SO.sub.4) so that the oxidant concentration would be 7 to 8 g/L as S.sub.2O.sub.8. The oxidant concentration was measured by an iodine titration method. The iodine titration method refers to a method that includes adding KI to a small amount of a sampled test liquid to liberate I.sub.2, titrating the I.sub.2 with a Na.sub.2S.sub.2O.sub.3 standard solution to obtain the amount of I.sub.2, and obtaining the oxidant concentration from the amount of I.sub.2.

[0030] To 200 mL of the test liquid heated to 60° C., 1 mg/L of copper ions (Cu.sup.2+), 1000 mg/L of iron ions (Fe.sup.2+), 10 mg/L of nitrate ions (NO.sup.3−), or 1 mg/L of nitrite ions (NO.sup.2−) was added. Stirring was continued with a stirrer while keeping the temperature of the test liquid constant, and the oxidant concentration in the test liquid was measured every hour until the lapse of 3 hours. The results are illustrated in FIG. 1. In addition, for comparison, the results of measuring the oxidant concentration in the test liquid every hour until the lapse of 3 hours are also illustrated in FIG. 1 for the test liquid without addition of these ionic components.

[0031] As apparent from FIG. 1, it has been confirmed that the oxidant concentration in the test liquid is kept constant in the case of no addition, while when copper ions, iron ions, nitrate ions, or nitrite ions are added to the sulfuric acid solution containing the persulfuric acid component as an oxidant, the oxidant concentration in the test liquid is reduced in a short time.

Examples 1 to 11 and Comparative Examples 1 to 6

[0032] To 200 mL of the test liquid heated to 60° C. and used in the above Reference Example, 1,2,3 benzotriazole, 5-methylbenzotriazole, benzoimidazole, 1-hydroxyethane-1,1-diphosphonic acid (HEDP), phosphoric acid, or ethylenediaminetetraacetic acid-tetrasodium (EDTA-4Na) was added and dissolved as an oxidant concentration reduction inhibitor as listed in Tables 1 and 2 so as to have a concentration listed in Tables 1, 2, and 3. Subsequently, copper ions, iron ions, or nitrate ions were added as impurity components causing a reduction in the oxidant concentration so as to have a concentration listed in Tables 1, 2, and 3. Stirring was continued with a stirrer while keeping the temperature of the test liquid constant, and the oxidant concentration in the test liquid was measured after the lapse of 3 hours. The results are listed in Tables 1, 2 and 3 together with the oxidant concentration reduction inhibition rate. In addition, for Example 1 and Comparative Example 1, the oxidant concentration in the test liquid was measured every hour until the lapse of 3 hours. The results are illustrated in FIG. 2.

[0033] In Tables 1 to 3, the oxidant concentration reduction inhibition rate is calculated in accordance with the following equation.

Oxidant concentration reduction inhibition rate (%)=(R.sub.1−R.sub.M)/(100−R.sub.M)×100

[0034] (In the equation, R.sub.M represents an oxidant residual rate under a condition in which the oxidant concentration reduction factor is added and the inhibitor is not added, and R.sub.1 represents an oxidant residual rate under a condition in which the oxidant concentration reduction factor of the same type as in the condition for obtaining R.sub.M is added and the inhibitor is added.)

TABLE-US-00001 TABLE 1 Example Example Example Example Example Example 1 2 3 4 5 6 Oxidant Copper ions (mg/L as Cu.sup.2+) 1 1000 1000 1000 1000 1 concentration Iron ions (mg/L as Fe.sup.2+) 0 0 0 0 0 0 reduction Nitrate ions (mg/L as NO.sup.3−) 0 0 0 0 0 0 factor Nitrite ions (mg/L as NO.sup.2−) 0 0 0 0 0 0 Inhibitor 1,2,3 benzotriazole (mg/L) 1000 100 500 1000 0 0 5-methylbenzotriazole (mg/L) 0 0 0 0 1000 0 Benzoimidazole (mg/L) 0 0 0 0 0 1000 Oxidant concentration reduction inhibition 97.5 92.2 98.6 100.0 88.8 49.4 rate (%)

TABLE-US-00002 TABLE 2 Example Example Example Example Example 7 8 9 10 11 Oxidant Copper ions (mg/L as Cu.sup.2+) 0 0 0 0 0 concentration Iron ions (mg/L as Fe.sup.2+) 100 1000 0 0 0 reduction Nitrate ions (mg/L as NO.sup.3−) 0 0 1 10 0 factor Nitrite ions (mg/L as NO.sup.2−) 0 0 0 0 1 Inhibitor 1,2,3 benzotriazole (mg/L) 1000 1000 1000 1000 1000 5-methylbenzotriazole (mg/L) 0 0 0 0 0 Benzoimidazole (mg/L) 0 0 0 0 0 Oxidant concentration reduction inhibition 83.3 52.5 69.1 30.3 77.3 rate (%)

TABLE-US-00003 TABLE 3 Comparative Comparative Comparative Comparative Comparative Comparative Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Oxidant Copper ions (mg/L as Cu.sup.2+) 1 1 1 0 0 0 concentration Iron ions (mg/L as Fe.sup.2+) 0 0 0 100 100 0 reduction Nitrate ions (mg/L as NO.sup.3−) 0 0 0 0 0 1 factor Nitrite ions (mg/L as NO.sup.2−) 0 0 0 0 0 0 Inhibitor HEDP (mg/L as PO.sub.4) 1000 0 0 1000 0 0 Phosphoric acid (mg/L as PO.sub.4) 0 1000 0 0 0 1000 EDTA-4Na (mg/L) 0 0 1000 0 1000 0 Oxidant concentration reduction inhibition 4.9 −19.8 0.0 15.8 −1145.5 −10.0 rate (%)

[0035] As apparent from Tables 1 to 3, in Examples 1 to 11 in which the benzoazole-based inhibitor was added to the test liquid to which copper ions, iron ions, nitrate ions, or nitrite ions were added as the oxidant concentration reduction factor, the effect of inhibiting the reduction in the oxidant concentration was recognized. In particular, the reduction inhibition rate in Examples 1 to 5 in which 1,2,3 benzotriazole or 5-methylbenzotriazole was added as an inhibitor to the test liquid containing copper ions was high. On the other hand, in Comparative Examples 1 to 6 in which HEDP, phosphoric acid, or EDTA-4Na was added to the test liquid to which copper ions, iron ions, nitrate ions, or nitrite ions were added as the oxidant concentration reduction factor, the effect of inhibiting the reduction in the oxidant concentration was low, or the reduction was rather promoted.

[0036] Furthermore, as apparent from FIG. 2, it has been confirmed that, in Example 1, the oxidant concentration exhibits an approximately constant value for 3 hours, and the reduction in the oxidant concentration due to the presence of copper ions can be inhibited, whereas in Comparative Example 1, the oxidant concentration is reduced significantly in a short time.