JP3366523B2 - Method for producing low nitrogen basic ferric sulfate solution - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the production of a basic ferric sulfate solution by dissolving an iron salt in sulfuric acid and oxidizing the same, particularly as an oxidizing agent. The present invention relates to a method for producing a low-nitrogen-basic ferric sulfate solution that uses nitrogen oxide and reduces residual nitrogen. BACKGROUND OF THE INVENTION In an urban sewage treatment plant and an industrial wastewater treatment plant in a factory, a basic ferric sulfate solution is widely and commonly used as a flocculant in a water purification process due to an effective water purification effect. Have been. [0003] Such a basic ferric sulfate solution contains an iron salt (ferrous chloride, ferrous sulfate, ferrous nitrate, iron carbonate, iron hydroxide, etc.). It is produced by dissolving in a sulfuric acid solution and oxidizing it. In the step of oxidizing the ferrous iron contained into trivalent iron, it was very advantageous to use an inexpensive nitrogen oxide as the oxide from the viewpoint of reducing the production cost. However, when such a nitrogen oxide is used as an oxide, the produced basic ferric sulfate solution contains 1
A large amount of nitrogen remains in the order of thousands of mg per liter (unit). If this solution is used as a flocculant in the above-mentioned water purification process, this nitrogen is discharged in large quantities together with the wastewater after the water purification. Would be. [0005] This may cause a great environmental destruction, which has been a problem. Therefore, a method that does not use nitrogen oxide as an oxidizing agent has been proposed. For example, as shown in 1986 Patent Publication No. 286229 “Method for producing iron-based flocculant”, as a direct oxidation method using oxygen, air or the like, or as an oxidizing agent, hydrogen peroxide, manganese dioxide, sodium chlorate, And the like. [0006] However, the direct oxidation method requires a long time for the reaction and is not only inefficient in production, but also requires a large-scale facility, resulting in an increase in the unit cost of the product due to a large capital investment. was there. When hydrogen peroxide or manganese dioxide is used as the oxidizing agent, the oxidizing agent is expensive, and the manganese dioxide is oxidized to ozone during the tertiary treatment after coagulation, and the waste liquid turns reddish purple. There is a problem that a decoloring step is required for color development. Further, when sodium chlorate is used as the oxidizing agent, sodium chlorate is also used as a raw material for explosives and the like, and there is a problem in handling the same. Therefore, the present invention has been made in view of such a problem, and the purpose thereof is to use inexpensive nitrogen oxide as a primary oxidizing agent,
By using a two-stage oxidizing method that uses an oxidizing agent that does not contain nitrogen as the secondary oxidizing agent, the production cost is low, and low-nitrogen basic ferric sulfate with a low residual nitrogen content in a simple and small facility. It is intended to provide a method for producing a solution. [0009] In order to achieve the above object, a method for producing a low-nitrous-basic ferric sulfate solution according to the present invention comprises the following steps. I have. First, as a first step, a sulfuric acid solution is prepared in an amount such that the molar ratio of iron and sulfuric acid of a predetermined amount of iron salt to be dissolved in the next step is 1 or more and less than 1.5, and dissolved in this solution. A mother liquor is prepared by preliminarily adding and mixing an amount of nitrogen oxide that reacts with less than 100% of ferrous iron contained in the iron salt. In other words, in other words, the added nitrogen oxide is
Keep the amount of ferrous iron to a level that cannot be oxidized. Next, as a second step, iron salts are added little by little to the prepared mother liquor while stirring. At this time, the oxidation-reduction potential temporarily decreases, but since ferrous iron is oxidized to trivalent iron, the potential increases to near the original potential again. This reaction decomposes the nitrogen oxides and separates the nitrogen from the liquid, so that the nitrogen in the liquid decreases. In the primary oxidation reaction, the amount of the oxidizing agent in the liquid is considerably reduced, and the reaction is continued by gradually adding the iron salt while stirring, and at the end of the primary oxidation, the nitrogen in the liquid is The minutes can be reduced to a considerable extent. As described above, the inventor of the present application has found that by controlling the oxidation-reduction potential in the primary oxidation reaction process, the amount of residual nitrogen in the liquid can be freely controlled. Then, when the monitored oxidation-reduction potential drops to a predetermined potential, the entire amount of the remaining iron salt is introduced and dissolved. Further, as a third step, an oxidizing agent containing no nitrogen is added to the solution prepared in the previous step to oxidize the remaining ferrous iron (secondary oxidation). As the oxidizing agent used here, aqueous hydrogen peroxide, manganese dioxide, sodium perchlorate, and the like are selected. The basic ferric sulfate solution obtained by the above-mentioned process is compared with a basic ferric sulfate solution produced by oxidizing only with nitrogen oxide, and the total amount of nitrogen in the solution (total nitrogen amount) Can be manipulated to an arbitrary amount from the order of several thousand ppm, and has an aggregation effect equivalent to that of a conventional ferric polysulfate solution. Next, several examples in which the present invention is specifically implemented will be described, and two examples of a comparative example will also be given to show a comparison with a conventional process. Example 1 75% sulfuric acid, 60% nitric acid and water are mixed and prepared in the proportions shown in Table 1. The solution is heated to 60 ° C. (reaction temperature is preferably about 50 to 90 ° C.), and a small amount of 98.5% pure ferrous sulfate / heptahydrate crystals are stirred and stirred until the oxidation-reduction potential drops to 620 mV. And a primary oxidation reaction is performed. Next, the remaining crystals are charged and dissolved. Hydrogen peroxide is added to the obtained solution, and the addition is stopped when the oxidation-reduction potential is 700 mV. Then, dilution water was added so that the iron concentration in the solution was 160 g / L to produce a basic ferric sulfate solution. EXAMPLE 2 A basic ferric sulfate solution was produced by using the mixed solution and the production method of Example 1 with an oxidation-reduction potential of primary oxidation of 600 mV. EXAMPLE 3 A basic ferric sulfate solution was produced by using the mixed solution and the production method of Example 1 while setting the oxidation-reduction potential of the primary oxidation to 580 mV. Comparative Example 1 75% sulfuric acid, 60% nitric acid and water are mixed and prepared in the proportions shown in Table 1. While stirring this solution, 6
The mixture was heated to 0 ° C., and ferrous sulfate / heptahydrate crystals having a purity of 98.5% were added until the oxidation-reduction potential dropped to 700 mV.
Dilution water was added so that the iron concentration in the solution was 160 g / L to produce a basic ferric sulfate solution. Comparative Example 2 75% sulfuric acid and water are mixed and prepared in the proportions shown in Table 1. The solution is heated to 60 ° C. while stirring, and all ferrous sulfate / heptahydrate crystals having a purity of 98.5% are charged. To this solution, hydrogen peroxide was added as an oxidizing agent to the solution obtained in the previous step until the oxidation-reduction potential increased to 700 mV, and dilute water was added so that the iron concentration of the solution was 160 g / L, and basic sulfuric acid was added. A ferrous solution was prepared. The analytical values of the basic ferric sulfate solutions prepared in the above Examples and Comparative Examples are as shown in Table 2. Further, even if the basic ferric sulfate solution produced in the above embodiment is used for municipal sewage treatment or industrial waste liquid treatment, the total nitrogen in the wastewater is an amount that can sufficiently satisfy the regulation value. When a flocculation test was performed, a flocculating effect equivalent to that of a commercially available basic ferric sulfate-based flocculant (ferric polysulfate solution) was obtained. With the above configuration, the present invention has the following effects. Even if inexpensive nitrogen oxides are used as the primary oxidizing agent, the amount of nitrogen in the liquid can be reduced by separating the nitrogen in the liquid as much as possible by the above reaction, thereby improving the quality of the product and reducing the production cost. It is measured and very economical. Further, the production method of the present invention can be realized only by partially changing the conventional facilities for producing a basic ferric sulfate solution using nitrogen oxide as an oxidizing agent. Therefore, a large capital investment is not required, and a low nitrogen basic ferric sulfate solution that does not cause an increase in product cost can be produced, thereby providing an inexpensive and effective flocculant that does not destroy the environment. be able to. [Table 1] [Table 2] The iron content in the above analysis values was determined by ICP emission spectrometry, Fe ²⁺
The amount is based on the potassium permanganate titration method and the total nitrogen amount is based on JIS K0102.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Rokuro Matsumoto 2-8 Kanda Surugadai, Chiyoda-ku, Tokyo (56) References JP-A-8-48526 (JP, A) JP-A-55-104925 (JP, A) JP-A-8-253326 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C01G 49/14 CA (STN)

Claims (1)

(57) [Claims 1] A method for producing a low nitrogen basic ferric sulfate solution, which comprises the following steps. (A) In a sulfuric acid solution having a molar ratio of iron to sulfuric acid of a predetermined amount of iron salt to be dissolved in the next step of 1 to less than 1.5, 100% of divalent iron contained in the dissolved iron salt is added. A first step of preparing a mother liquor by adding and mixing a nitrogen oxide that reacts to less than (B) The iron salt is added little by little to the mother liquor prepared in the first step while stirring, and when the monitored oxidation-reduction potential drops to a predetermined potential, the remaining amount of the iron salt is introduced and dissolved. Second step. (C) a third step of oxidizing the ferrous iron remaining in the solution after the second step with an oxidizing agent containing no nitrogen.