High-concentration iron-based flocculant and its production method

Abstract

According to the present invention, an ultra-high concentration polyferric sulfate solution that cannot be produced by a conventional production method due to a long reaction time can be produced in a short time. When the sulfate ion concentration is [SO4 2‑ and the total iron concentration is [T-Fe] (in molar concentration), the raw materials are adjusted to satisfy the following relationship, and the polyferric sulfate solution is produced by a high-temperature and high-pressure reaction of the raw materials. The molar ratio of sulfate ion to total iron (SO4 2‑ / T-Fe) is at least 1.2, and when the weight concentration of sulfate ion is [SO4 2‑ , [SO4 2‑ is at most 35% by weight.

Description

Technical Field

[0001] The present invention relates to a high-concentration iron-based flocculant for wastewater treatment and a production method thereof. Background Art

[0002] The applicant of this patent application sells wastewater treatment chemicals centered around the initially developed iron-based inorganic polymer flocculant "Polytetsu" (registered trademark) and holds a number of related patents.

[0003] Among these patents, Patent Document 1 describes adding sodium nitrite as a catalyst and an oxidizing agent to ferrous sulfate (FeSO4) as an iron-based raw material, and causing an oxidation reaction to proceed at normal temperature and pressure for about 10 hours, thereby obtaining a solution of polyferric sulfate ([Fe2(OH) n (SO4) 3-n / 2 m , where 0 < n ≤ 2 and m is a natural number).

[0004] However, since this method requires a long reaction time, there is a need to shorten the reaction time by some means.

[0005] Then, the production method of the iron-based inorganic flocculant described in Patent Document 2 is a method using magnetite (Fe3O4) as an iron-based raw material, adjusting the molar ratio of sulfate ions to iron ions, and then reacting at a temperature of 120 to 180 °C in a closed container. This method is a production method aimed at shortening the reaction time by reacting under high temperature and high pressure, but this method still requires a reaction time of 0.8 to 1.5 hours.

[0006] Patent Document 3 discloses a production method of an iron-based flocculant, in which iron(III) oxide (Fe2O3) as an iron-based raw material is dissolved in excessive sulfuric acid to form iron(III) sulfate (Fe2(SO4)3), and then it is partially neutralized with hydrated iron(III) oxide.

[0007] However, since this method consists of two steps, namely the step of dissolving iron(III) oxide in sulfuric acid and the step of partially neutralizing the formed iron(III) sulfate, there is a defect that this production method becomes complicated and it is impossible to efficiently form a polyferric sulfate solution. In the examples, it is described that the reaction needs to be carried out while maintaining a state of heating at 100 °C for about 3 hours.

[0008] Prior Art Documents

[0009] Patent Documents

[0010] Patent Document 1: Japanese Patent Laid-Open No. S51-17516

[0011] ​Patent Document 2: Japanese Patent No. 3379204

[0012] Patent Document 3: Japanese Patent No. 2741137 Summary of the Invention

[0013] Problems to be Solved by the Invention

[0014] As described above, in the conventional technology, although attempts have been made to produce a polyferric sulfate solution by selecting various iron compounds as iron-based raw materials and reacting them in various reaction forms, there are still problems including the generation of a large amount of free sulfuric acid and reaction residues, and there is also a problem of an extended production time for producing a polyferric sulfate solution that can withstand actual use.

[0015] Although it will be described in detail later, it is considered that among iron-based flocculants, the higher the total iron concentration of the flocculant, the higher the characteristics as a flocculant. The applicant of this patent application produces and sells an iron-based inorganic polymer flocculant, "Polytetsu" (R), with a total iron concentration of about 11.0 to 12.5% (referred to as "ordinary product"). Since iron-based inorganic polymer flocculants have high flocculation ability and dewatering performance when the total iron concentration is high, recently, iron-based inorganic polymer flocculants with a total iron concentration of 12.5 or more have been produced and sold as "high-concentration products".

[0016] However, even when producing a flocculant with a high total iron concentration, the maximum total iron concentration is limited to 12.7% (less than 13%), and it is also affected by the above-mentioned problem of extended production time, and a polyferric sulfate solution with a concentration of 13.0% or more cannot be produced.

[0017] Here, unless otherwise indicated as molar concentration, the concentration in the present invention refers to weight %, and [T-Fe] represents the weight concentration of total iron; and [SO4 2- represents the weight concentration of sulfate ions.

[0018] Here, the total iron concentration refers to the concentration of not only the iron dissolved in the raw material liquid but also the iron that is not dissolved in the raw material liquid and exists as a solid (powder, etc.) in the raw material liquid. Since even the iron-based powder present in the raw material liquid contributes to the production reaction of the polyferric sulfate solution, it is reasonable to include the iron-based components that are not dissolved in the raw material liquid in the iron concentration.

[0019] However, in the polyferric sulfate solution produced by the present invention, although the concentration is also expressed as the total iron concentration, it is natural that all the iron is dissolved.

[0020] The present invention has been achieved to solve these problems, and its object is to provide a production method that can produce a polyferric sulfate solution with a high total iron concentration in a short time compared with conventional products.

[0021] Solution for solving problems

[0022] To solve these problems, the present invention is composed of the following technical means.

[0023] (1) A production method of an iron-based flocculant, the iron-based flocculant including a polymeric ferric sulfate solution, the method including reacting a raw material liquid containing ferrous sulfate and sulfuric acid in a closed container under high temperature and high pressure conditions, the raw material liquid satisfying the following conditions:

[0024] The molar ratio of sulfate ion to total iron (SO4 2- / T-Fe) is 1.2 or more; and

[0025] When the weight concentration of sulfate ion is represented by [SO4 2- , [SO4 2- is 35% by weight or less.

[0026] (2) The production method of an iron-based flocculant according to (1), further including adding nitric acid or nitrite as a catalyst to the closed container.

[0027] (3) The production method of an iron-based flocculant according to (1) or (2), wherein the high temperature and high pressure reaction conditions are a temperature of 100 °C or more and a pressure of 0.3 MPa or more.

[0028] (4) An iron-based flocculant, wherein the iron-based flocculant is a high-concentration polymeric ferric sulfate solution with a total iron concentration of 13 to 16% by weight.

[0029] Effects of the invention

[0030] The ultra-high concentration iron-based flocculant of the present invention is characterized in that its concentration is higher than that of the high-concentration iron-based flocculant commercially available from the applicant of the present invention, and it has high flocculation ability and dewatering performance. In addition, since the ultra-high concentration iron-based flocculant has a lower water content than ordinary products, the product transportation cost can be reduced.

[0031] In addition, according to the production method of the iron-based flocculant of the present invention, the production time of more than 10 hours required by the conventional method can be greatly shortened, and the iron-based flocculant can be efficiently produced. Description of the drawings

[0032] Figure 1 Figure 1 Is the area where polymeric ferric sulfate can be produced by high temperature and high pressure reaction.

[0033] Figure 2 Figure 2 Is the concentration shift after filtering the sample with the generated precipitate.

[0034] ​​​​​Figure 3 Figure 3 For the concentration shift after concentrating the sample without forming a precipitate. Detailed implementation mode

[0035] Here, before describing the technical features of the production method of the iron-based flocculant according to the present invention, the inorganic flocculant will first be described.

[0036] Generally, in sewage sludge treatment, the suspended particles and colloidal particles in the sludge are flocculated with a flocculant, and dehydration treatment and solid-liquid separation are carried out. The suspended particles and colloidal particles in sewage sludge usually carry a negative charge on their surfaces and are in a stable state due to the repulsive force of the surface charge and the hydration effect. A flocculant is a chemical that adsorbs on the surfaces of these particles to neutralize the surface charge and weaken the repulsive force between the particles to thereby flocculate the particles.

[0037] The iron-based flocculant is a typical inorganic flocculant, and the positively charged iron ions neutralize the negative charges on the surfaces of suspended substances such as suspended particles and colloidal particles to carry out the flocculation effect. Therefore, the iron-based flocculant always shows a flocculation effect as long as iron ions exist, and due to the improved flocculation ability for suspended substances, a higher iron ion concentration can reduce the addition amount of the flocculant.

[0038] In order for the iron ions in the flocculant to exist stably, a certain amount of negative ions must exist. In the case of the iron-based flocculant, sulfate ions usually play such a role. When the amount of negative ions has a suitable molar ratio relationship with the amount of iron ions, the iron-based flocculant becomes stable, but in the case of excessive negative ions or when the amount is insufficient, the iron-based flocculant becomes unstable and results in deposition as crystals or the like.

[0039] Then, in the case of sewage sludge treatment by using such an iron-based flocculant, the iron ions are adsorbed on the surfaces of the suspended particles and colloidal particles and are recovered as a solid component, but the sulfate ions ultimately remain in the treated water.

[0040] Therefore, since the treated water becomes strongly acidic, in order to discharge the treated water into a river, the treated water needs to be neutralized with a large amount of neutralizing agent; this is considered one of the factors for increasing the cost of sewage sludge treatment. That is, as a characteristic required for the iron-based flocculant, it is required that the total iron concentration ([T-Fe]) contained in the flocculant is high and the sulfate ion concentration ([SO4 2- ) is low.

[0041] In the product of the polyferric sulfate solution using ferrous sulfate as a raw material, the following chemical reactions are considered to have occurred.

[0042] ​m[2FeSO4+(1 - n / 2)H2SO4+1 / 2O2+(n - 1)H2O]→[Fe2(OH) n (SO4) 3-n / 2 m

[0043] Among them, 0 < n ≤ 2, and m is a natural number.

[0044] The present invention provides a method for forming a solution with a high [T-Fe] within a short time for an iron-based flocculant composed of the above-mentioned polyferric sulfate solution, and an iron-based flocculant produced therefrom.

[0045] In the present invention, when ferrous sulfate (FeSO4) is used as a solid raw material and the oxidation reaction is carried out under high temperature and high pressure conditions, the relationship between the total iron concentration and the sulfate ion concentration of the added raw material liquid is set within a specific range. The present invention makes the molar ratio of sulfate ion to total iron (SO4 2- / T-Fe) be above a specific value, and the sulfate ion concentration [SO4 2- be below a specific value, obtaining an extremely remarkable effect that the reaction can be completed within a short time that cannot be estimated by conventional techniques. Moreover, a polyferric sulfate solution with an extremely high total iron concentration ([T-Fe]) that cannot be produced by conventional techniques can be produced.

[0046] More specifically, the present invention is characterized in that a raw material liquid containing ferrous sulfate and sulfuric acid is reacted under high temperature and high pressure conditions, and the raw material liquid satisfies the following conditions.

[0047] The molar ratio of sulfate ion to total iron (SO4 2- / T-Fe) is 1.2 or more.

[0048] When the weight concentration of sulfate ion is represented by [SO4 2- , [SO4 2- is 35 wt% or less.

[0049] The present inventors newly discovered that when the total iron concentration of ferrous sulfate and the concentration of sulfate ion have such a relationship, a polyferric sulfate solution with an ultra-high concentration can be obtained within a short time without generating precipitates.

[0050] (High temperature and high pressure reaction)

[0051] The method described in Patent Document 1 is a conventional production method carried out by the present inventors. In this method, it is conceivable that the reaction proceeds under normal temperature and pressure with the three-phase interrelation of solid phase, liquid phase and gas phase. This is because during the reaction, a yellowish-brown gas generated by NO x and the odor of NO x are perceived. ​

[0052] However, in the method of the present invention, even when the autoclave is opened after the reaction is completed, the odor of NO cannot be perceived. x Therefore, in the high-temperature and high-pressure reaction of the present invention, it is presumed that a reaction related to the solid phase and the liquid phase occurs, in which FeSO4·7H2O as a solid raw material is dissolved in sulfuric acid liquid, and an oxidation reaction occurs.

[0053] Thus, it is conceivable that due to the reaction under high-temperature conditions, the dissolution of FeSO4·7H2O as a solid raw material is more likely to occur; and due to the reaction under high-pressure conditions, the partial oxygen pressure increases, and the amount of dissolved oxygen in the liquid phase increases, whereby the dissolved oxygen directly contributes to the oxidation of nitrite ions NO 2- and Fe 2+ and greatly promotes the oxidation reaction of iron ions.

[0054] (Reaction temperature and reaction pressure)

[0055] The temperature in the container needs to be adjusted within the range of 100 to 150 °C.

[0056] If the reaction temperature is lower than 100 °C, the oxidation reaction of ferrous sulfate will not proceed sufficiently. If it is higher than 150 °C, yellow precipitates are confirmed, and X-ray analysis shows that the precipitates are Fe(OH)SO4.

[0057] Although specific experimental data are omitted, the present inventors have confirmed that the higher the reaction pressure, the more efficiently the reaction proceeds. Considering the reaction mechanism of the above high-temperature and high-pressure reaction, this fact can be said to be natural.

[0058] Therefore, the reaction pressure of the present invention can be set in consideration of actual conditions such as product cost, and it is sufficient to be 0.3 MPa or more.

[0059] (Catalyst)

[0060] In order to promote the reaction for forming the above-mentioned polyferric sulfate solution, it is preferable to use a catalyst. Preferred catalysts for promoting the reaction include nitric acid and nitrites; nitrites include sodium salts, potassium salts of nitrous acid, etc. From the viewpoints of the function of promoting the reaction and cost, nitric acid is preferred.

[0061] [Experiment 1]

[0062] As the high-temperature and high-pressure reaction conditions, the inventors of the present invention set the reaction temperature at 110 °C, the reaction pressure at 0.30 MPa, and the reaction time at 10 minutes, and prepared a raw material liquid containing various concentrations of ferrous sulfate and sulfuric acid. Nitric acid as a catalyst was added thereto and a high-temperature and high-pressure reaction was carried out. Then, after the reaction time, it was checked whether precipitates were formed.

[0063] [Experiment 2]

[0064] In addition, as the high-temperature and high-pressure reaction conditions, the reaction temperature was set at 120 °C, the reaction pressure was 10.00 MPa, and the reaction time was 10 minutes. A raw material solution containing various concentrations of ferrous sulfate and sulfuric acid was prepared. Nitric acid was added thereto as a catalyst and a high-temperature and high-pressure reaction was carried out. Then, after the reaction time, it was checked whether a precipitate was formed.

[0065] The experimental results of whether a precipitate was formed are summarized in Tables 1 and 2.

[0066] In the case of Experiment 1 conducted under the conditions of a reaction temperature of 110 °C and a reaction pressure of 0.30 MPa, and in the case of Experiment 2 conducted under the conditions of a reaction temperature of 120 °C and a reaction pressure of 10.00 MPa, it was found that exactly the same results were given in terms of the formation of a precipitate. That is, the results in Tables 1 and 2 are the same as those of Experiments 1 and 2.

[0067] In the case of the total iron concentration [T-Fe] and the total sulfuric acid concentration [SO4 2- shown in Table 1, a polyferric sulfate solution was formed without forming a precipitate, and it is an example of the present invention; and in the case shown in Table 2, it was confirmed that a precipitate was formed, and it is a comparative example of the present invention.

[0068] [Table 1]

[0069] Addition concentration without forming a precipitate

[0070]

[0071] [Table 2]

[0072] Addition concentration for forming a precipitate

[0073]

[0074] (Specific area)

[0075] These results are summarized in Figure 1 . The area occupied by the ○ mark is the area where a polyferric sulfate solution is formed without forming a precipitate. This area is the area specified by the present invention and is hereinafter referred to as the "specific area". The [T-Fe] and [SO4 2- represented by the white ○ marks included in the specific area are the raw material compositions of the examples of the present invention. By reacting each of the compositions under high-temperature and high-pressure conditions, a reddish-brown polyferric sulfate solution can be obtained.

[0076] On the other hand, the case of reacting under high temperature and high pressure by using a raw material composition represented by ▲ outside a specific region is equivalent to a comparative example of the present invention. In any case of using these compositions, precipitation was confirmed; and in the samples in the region where the molar ratio of sulfate ion to total iron (SO4 2- / T-Fe) is less than 1.2, the precipitate was determined to be jarosite.

[0077] The present inventors specified a specific region from the following two conditions.

[0078] First, the upper limit of the region can be set such that the weight concentration of sulfate ion [SO4 2- is 35 wt% or less.

[0079] Then, the lower limit of the region can be specified by an upward-slanting straight line to the right. Each of the straight lines is a straight line, which represents the relationship where the molar ratio of sulfate ion to total iron (SO4 2- / T-Fe) is 1.2 or more. In the figure, the vertical axis and the horizontal axis are the weight concentration of sulfate ion and the weight concentration of total iron, respectively.

[0080] The specific region of the raw material composition of [T-Fe] and [SO4 2- specified by the present invention represents the region where the formation of polyferric sulfate solution can proceed stably under high temperature and high pressure conditions.

[0081] The technical significance of the specific region can be confirmed by the following additional experiments 1 and 2.

[0082] (Additional Experiment 1)

[0083] [T-Fe] and [SO4 2- with concentrations of 14% and 28% respectively in the sample (hereinafter, labeled as (14.0:28.0)), and Figure 1 the (15.0:30.0) sample in

[0084] generated precipitates. The samples were filtered; and the concentrations of the solutions after removing the precipitates were measured, and the compositions of the solutions were (12.8:27.2) and (14.5:29.8) respectively. Figure 2 Show the content.

[0085] (Additional Experiment 2)

[0086] According to Figure 1, both samples (15.0:32.0) and (15.0:34.0) are samples without precipitate formation. These samples were kept for one month in three environments: (i) in a desiccator at 50 °C, (ii) in the laboratory at approximately 20 °C, and (iii) in an incubator at 10 °C; thereafter, the changes in the samples were observed. As a result, a precipitate was observed only in the sample (15.0:34.0) at (i) 50 °C in the desiccator.

[0087] It is considered that the reason is attributed to the following.

[0088] For samples (15.0:32.0) and (15.0:34.0) kept in a desiccator at 50 °C for one month, [T-Fe] and [SO4 2- were measured again, and the respective measured values were (16.0:34.0) and (16.0:36.0). Figure 3 Show the content.

[0089] Although the samples kept in the desiccator were concentrated by evaporation of moisture, even for samples that were already concentrated samples, no precipitate was detected in samples where the concentration relationship of [T-Fe] and [SO4 2- was within the region specified in the present invention. However, as a result of concentration, samples in which the concentration relationship of the two deviated from the above region due to concentration formed a precipitate.

[0090] Therefore, only the sample (15.0:34.0) under condition (i) formed a precipitate.

[0091] (Reaction time)

[0092] The production method using the conventional technique described in Patent Document 1 is a method of oxidizing ferrous sulfate at normal temperature and normal pressure; and even when designing a catalyst, an oxidant, etc., this method can only obtain a solution with a maximum total iron concentration ([T-Fe]) of about 12.5%, and it is a method with a reaction time of more than 16 hours.

[0093] By adopting the high-temperature and high-pressure method, the present invention has successfully greatly shortened the reaction time.

[0094] In Figure 1 In the examples shown, the reaction was completed within 30 minutes for all samples from a high-concentration solution with a total iron concentration of 12.5% to an ultra-high-concentration solution with a total iron concentration as high as 16%. It is natural that the reaction time depends on the total iron concentration; and in a solution with a total iron concentration of 12.5%, the reaction was completed within 7.5 minutes, and even in a solution with a total iron concentration of 16%, the reaction was completed within 30 minutes. Here, the completion of the reaction was judged by measuring the concentration of divalent iron in the sample solution.

[0095] The fact that the reaction can occur in such a short time is an extremely remarkable effect that cannot be anticipated in conventional techniques.

[0096] (Ultra-high concentration solution)

[0097] To confirm the effects attributable to the ultra-high concentration of the polyferric sulfate solution, a flocculation test was conducted on the following Samples A and B. Sample A was a sample having the same total iron concentration as the sample produced by the conventional technique over a time period of more than 16 hours. On the other hand, Sample B was a sample having an ultra-high concentration of total iron produced by the present invention.

[0098] [Table 3]

[0099] Sample [T-Fe] <![CDATA[[SO4 2- > <![CDATA[[SO4 2- / [T-Fe]]]> Sample A 12.7 32.5 1.49 Sample B 14.7 31.5 1.25

[0100] Samples A and B were respectively added to the liquid to be treated, which was colored water of acrylic paint as a simulated liquid. By reducing the addition amount of Sample B, the total iron addition amounts of Samples A and B were made the same, that is, by making the flocculation ability of iron ions the same; and the flocculation abilities of the two were compared.

[0101] The conditions of the flocculation test are shown in Table 4.

[0102] [Table 4]

[0103] Sample Addition amount (μL) Total iron amount (g) pH (after addition) NaOH addition amount (μL) Sample A 380 <![CDATA[6.91×10 -2 > 2.82 340 Sample B 258 <![CDATA[6.91×10 -2 > 3.73 180 Increase or decrease Decrease by 32% Same Decrease by 47%

[0104] Since Sample B was of ultra-high concentration, when the flocculation ability of Sample B was made the same as that of Sample A, the addition amount of Sample B could be reduced by up to 32% compared to Sample A. Then, since in the case of Sample B, [SO4 2- was low as shown in Table 1, the decrease in the pH of the liquid to be treated after the flocculation treatment could be suppressed; therefore, the addition amount of caustic soda for neutralization could be reduced by 47% compared to the case of using Sample A.

[0105] Then, the situation after the formation of flocs was observed; Sample B, which was of ultra-high concentration, had a higher floc formation ability and also a higher floc precipitation rate. Although the iron amount of Sample B was made the same as that of Sample A, Sample B had a higher flocculation ability.

[0106] It is considered that the reason is that Sample B polymerizes more and is favorable for the crosslinking and adsorption of flocs. This is also confirmed by the fact that the ultra-high concentration sample of Sample B has a higher liquid viscosity than Sample A.

[0107] Industrial applicability

[0108] The present invention relates to a flocculant used in the treatment of wastewater such as sewage, and since a flocculant showing high flocculation performance can be produced in a short time, the flocculant can be widely used in the field of wastewater treatment.

Claims

1. A production method of an iron-based flocculant, the iron-based flocculant including a polymeric ferric sulfate solution, The method includes reacting a raw material liquid containing ferrous sulfate and sulfuric acid in the closed container under high temperature and high pressure conditions after adding nitric acid or nitrite as a catalyst to the closed container, m[2FeSO4+(1-n / 2)H2SO4+1 / 2O2+(n-1)H2O]→[Fe2(OH) n (SO4) 3-n / 2 m wherein, 0 < n ≤ 2, and m is a natural number; The reaction conditions of high temperature and high pressure are a temperature above 100 °C and a pressure above 0.3 MPa. The reaction is completed within 30 minutes, and the total iron concentration of the polymeric ferric sulfate solution is 14% - 16%. The raw material liquid meets the following conditions: The molar ratio of sulfate ion to total iron (SO4 2- / T-Fe) is 1.2 or more; and When the weight concentration of sulfate ions is represented by [SO4 2- , [SO4 2- is 35% by weight or less.

2. An iron-based flocculant, wherein the iron-based flocculant is a high-concentration polymeric ferric sulfate solution with a total iron concentration of 14 to 16% by weight, wherein the molar ratio of sulfate ions to total iron (SO4 2- / T-Fe) is 1.2 or more; and when the weight concentration of sulfate ions is represented by [SO4 2- , [SO4 2- is 35% by weight or less.​

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