Method for recovering formic acid

Abstract

To provide a method for recovering formic acid by which it is possible to obtain a concentrated formic acid aqueous solution containing no salt from formic acid-containing water including formate.SOLUTION: A method for recovering formic acid includes a process of obtaining a desalted formic acid aqueous solution on a distillate side and a salt concentrate on the bottoms side by subjecting formic acid-containing water to pH adjustment to less than pH 4, followed by distillation. After the formic acid-containing water is distilled at pH 4 or above and formate is condensed on the can liquid side, the can liquid is subjected to pH adjustment to less than pH 4, followed by distillation so as to obtain a desalted formic acid aqueous solution on the distillate side and salt concentrate on the can liquid side.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for recovering formic acid, and more particularly to a method for recovering formic acid in the form of an aqueous formic acid solution by separating salts from formic acid-containing water containing formate salts. [Background technology]

[0002] Formic acid is contained in wastewater generated during gas purification when coal is gasified to obtain carbon monoxide and hydrogen gases for fuel, and in intermediate effluent from alkaline thermal hydrolysis of wastewater containing cyanide compounds.

[0003] Formic acid is used as a preservative and antibacterial agent in silage, in the textile industry, in leather tanning, as a snow-melting agent such as sodium formate, as a raw material for fragrances such as formic acid esters, and as a hydrogen carrier.

[0004] Since formic acid is a substance that causes COD, it is decomposed into carbon dioxide using catalytic wet oxidation or biological treatment to meet the COD wastewater standards.

[0005] If formic acid can be recovered and used from wastewater, it will be possible to reduce the burden on wastewater treatment and the CO2 emissions associated with wastewater treatment. In addition, in the case of producing formic acid using fossil fuels as raw materials, it is expected to have the benefit of reducing fossil fuel consumption.

[0006] For applications other than as a snow-melting agent, salt-free formic acid or a formic acid aqueous solution is used. Therefore, when recovering and using formic acid from formic acid-containing water such as wastewater, it is necessary to remove salt from the formic acid-containing water.

[0007] Furthermore, in one example of a method for synthesizing formic acid from CO gas or CO gas, a mixture of formic acid, water, and salts may be produced as the primary synthesis liquid, and a simpler desalination method for purifying formic acid is desired.

[0008] As a method for recovering formic acid from formic acid-containing water, there is an evaporation concentration method in which water in wastewater containing formic acid is evaporated to concentrate and separate the formic acid.

[0009] Formic acid-containing water derived from wastewater usually contains salts in addition to formic acid. Furthermore, under neutral (or alkaline) conditions, most of the formic acid in the water exists in the form of formate ions. Even if the liquid is distilled in this state, the formate ions do not volatilize and therefore do not transfer to the distillate. In other words, even if formic acid-containing water is distilled, the formate ions are concentrated together with the salts and remain in the bottoms. Therefore, it is not possible to separate the formic acid from the salts to obtain an aqueous formic acid solution.

[0010] Patent Document 1 discloses a method for concentrating formic acid by adsorbing formic acid onto an anion exchange resin and regenerating it with acid. This method can concentrate formic acid in water containing a low concentration of formic acid, but it cannot concentrate formic acid to a concentration greater than the ratio of the saturated adsorption amount in the anion exchange resin to the amount of regenerating agent. Furthermore, salts in the regenerating agent that were not involved in the desorption reaction of the adsorbed formate ions are mixed into the concentrated formic acid solution, making it impossible to recover a salt-free aqueous formic acid solution. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Publication No. 2020-15684 Summary of the Invention [Problem to be solved by the invention]

[0012] Conventional methods can concentrate formic acid or formate salts in water, but cannot separate the formic acid from the salts.

[0013] An object of the present invention is to provide a method for recovering formic acid, which can obtain a salt-free aqueous formic acid solution from formic acid-containing water that also contains formate salts. [Means for solving the problem]

[0014] The method for recovering formic acid according to the present invention is as follows.

[0015] [1] A method for recovering formic acid, comprising the steps of adjusting the pH of formic acid-containing water containing formate salts to less than pH 4 and then distilling the water to obtain a desalted formic acid aqueous solution in the distillate and a salt-concentrated solution in the bottoms.

[0016] [2] The method for recovering formic acid described in [1] above, in which the pH is adjusted to below 3.6.

[0017] [3] The method for recovering formic acid according to [1], wherein the pH of the formic acid-containing water is adjusted to less than 4 by adding an acid.

[0018] [4] The method for recovering formic acid according to any one of [1] to [3], wherein the formic acid-containing water is distilled at a pH of 4 or higher, and formate salt is concentrated in the bottoms, and the bottoms are then adjusted to a pH of less than 4 and distilled to obtain a desalted aqueous formic acid solution as the distillate and a salt-concentrated solution as the bottoms. [Effects of the Invention]

[0019] In the present invention, water containing formic acid is acidified and distilled, thereby obtaining a salt-free aqueous formic acid solution as the distillate and an aqueous salt solution as the bottoms.

[0020] Furthermore, in the preceding stage of the distillation, a primary distillation is carried out at a pH of neutral or higher to obtain a bottoms liquid in which formate salts are concentrated, and this liquid is acidified and distilled to obtain a salt-free aqueous formic acid solution having a high concentration of formic acid.

[0021] When formic acid is recovered from wastewater containing formic acid by the method of the present invention, COD treatment equipment for treating COD derived from formic acid becomes unnecessary, and wastewater treatment costs can be reduced. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a flow diagram of a method for recovering formic acid according to an embodiment. [Figure 2] FIG. 1 is a flow diagram of a method for recovering formic acid according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0023] In one embodiment of the method for recovering formic acid of the present invention, as shown in FIG. 1, formic acid-containing water (formic acid-containing raw material solution) is acidified and distilled to obtain a salt-free aqueous formic acid solution as a distillate, and a salt-containing aqueous solution as a bottoms product.

[0024] The formic acid-containing water (formic acid-containing raw material liquid) preferably has a formic acid concentration of about 0.01 to 10 wt%, particularly about 0.1 to 5 wt%, as formic acid. Examples of such a formic acid-containing raw material liquid include coal gasification wastewater and cyanide-containing wastewater.

[0025] The pH of the formic acid-containing water during distillation is preferably less than 4, particularly 3.6 or less, and more particularly 1 to 3.6, in view of the dissociation constant of formic acid.

[0026] The pH can be adjusted by adding an agent (acid) such as sulfuric acid or hydrochloric acid, or by contacting the solution with an H-type cation exchange resin to exchange cations such as sodium ions in the solution for hydrogen ions.

[0027] To adjust the pH by adding a chemical, the acid may be added in a stirring tank, or line mixing may be used in which the acid is injected into a pipe and then stirred with a line mixer.

[0028] In the method of contacting an H-type cation exchange resin for pH adjustment, the resin used may be any cation exchange resin.

[0029] The distillation may be any of batch, semi-batch, continuous, etc. The distillation is preferably carried out at the boiling point of the formic acid-containing water, for example, at about 100°C under atmospheric pressure, or at 100°C or lower under reduced pressure.

[0030] In one embodiment of the present invention, as shown in Fig. 2, an alkaline agent such as NaOH is added to the formic acid-containing raw material solution as needed to adjust the pH to 4 or higher, particularly 7 to 10, followed by primary distillation, whereby formate is concentrated into the bottoms, and the bottoms are acidified by adding an acid as described above and distilled. This method provides a salt-free aqueous formic acid solution with a high formic acid concentration.

[0031] If the liquid contains volatile substances such as methanol or ethanol, they will be removed in the primary distillation step and separated from the formic acid. [Example]

[0032] Example 1 (Distillation under Acidic Conditions at pH 2) Using a rotary evaporator, distillation under reduced pressure was carried out on a formic acid-containing water containing formic acid and sodium chloride at a pH of 2 as shown below.

[0033] <Preparation of formic acid-containing water> An aqueous solution containing 10 g / L of sodium formate and 100 mg / L of sodium chloride (Na concentration 4500 mg / L, Cl concentration 52 mg / L, pH 4.9) was adjusted to pH 2 with hydrochloric acid and used as the stock solution for distillation (formic acid-containing water).

[0034] <Distillation operation> 300 mL of the formic acid-containing water was placed in a rotary evaporator and distilled at a temperature of 50° C. and a pressure of 60 kPa for about 70 minutes. The distillate was cooled to 10° C., and 110 mL of distillate was obtained.

[0035] <Results / Discussion> The water qualities of the raw solution, the bottoms after distillation, and the distillate are shown in Table 1. As shown in Table 1, an aqueous formic acid solution with a Na concentration and Cl concentration of 10 mg / L or less was obtained as the distillate. The formic acid concentration in the distillate was almost the same as that of the raw solution and the bottoms.

[0036] [Table 1]

[0037] [Comparative Example 1] (Distillation at pH 4.9 without adding acid) Vacuum distillation of formic acid-containing water was carried out under the same conditions as in Example 1, except that a stock solution prepared without adding hydrochloric acid was used. That is, the concentrations of sodium formate and sodium chloride in this formic acid-containing water stock solution were the same as in Example 1, and the pH was 4.9.

[0038] <Results / Discussion> The water qualities of the raw solution, the bottoms after distillation, and the distillate are shown in Table 2. As shown in Table 2, the formic acid concentration in the distillate was 130 mg / L, and most of the formic acid remained in the bottoms together with sodium chloride, so it was not possible to separate the formic acid from the salt.

[0039] [Table 2]

[0040] [Example 2] (Pre-distilled at alkaline pH (7.9), then distilled at pH 2.2)

[0041] <Experimental conditions> <Pre-distillation (primary and secondary concentration)> An aqueous solution of formic acid with a sodium formate concentration of 10 g / L (after dissolving the sodium formate reagent, the pH was adjusted to 7.9 by adding 8 μL of 5% sodium hydroxide solution per 1 L) was concentrated 8 times by vacuum distillation at 50°C and 60 kPa using a rotary evaporator, and the bottom product was used as the primary concentrated solution.

[0042] This primary concentrate was concentrated five times by vacuum distillation at 50°C and 60 kPa using a rotary evaporator, and the bottom product was used as a secondary concentrate.

[0043] <Distillation at pH=2.2> Approximately 430 mL of 36% hydrochloric acid was added per liter to this second concentrated solution to adjust the pH to 2.2, producing pH-adjusted water with the quality shown in Table 3. 300 mL of this pH-adjusted water was distilled at 50°C and 60 kPa under reduced pressure to obtain 220 mL of distillate. This distillate (third distillate) was collected as formic acid-enriched water.

[0044] Table 3 shows the water qualities of the primary concentrate (primary bottoms), secondary concentrate (secondary bottoms), tertiary concentrate (tertiary bottoms), and the recovered formic acid-enriched water (tertiary distillate).

[0045] [Table 3]

[0046] <Consideration> As shown in Table 3, an aqueous formic acid solution was obtained as recovered water that was substantially free of salt and had a higher formic acid concentration than the formic acid concentration in the original solution.

Claims

1. A method for recovering formic acid, comprising a process of adjusting the pH of formic acid-containing water containing formate salts to less than pH 4 and then distilling the water to obtain a desalted aqueous formic acid solution in the distillate and a salt-concentrated solution in the bottoms.

2. 2. The method for recovering formic acid according to claim 1, wherein the pH is adjusted to 3.6 or less.

3. 2. The method for recovering formic acid according to claim 1, wherein the pH of the formic acid-containing water is adjusted to less than 4 by adding an acid.

4. 4. The method for recovering formic acid according to claim 1, wherein the formic acid-containing water is distilled at a pH of 4 or higher, a formate is concentrated in a bottoms solution, and the bottoms solution is then adjusted to a pH of less than 4 and distilled, thereby obtaining a desalted aqueous formic acid solution in the distillate solution and a salt concentrate in the bottoms solution.

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