JP2024118003A - Method for producing cobalt aqueous solution - Google Patents

Abstract

To provide a method for producing a Co aqueous solution capable of separating Ni and Co from each other and recovering as an aqueous solution with a low Mg concentration.SOLUTION: The method includes: an exchange stage of mixing and contacting a crude Ni aqueous solution containing Co and Mg and an organic solvent containing a Ni-carrying acidic extraction agent, and substituting the Co in the crude Ni aqueous solution and the Ni in the Ni-carrying acidic extraction agent to obtain a high purity Ni aqueous solution and an organic after exchange; a Ni recovery stage of back-extracting the residual Ni in the organic after exchange to a first acidic aqueous solution to obtain a Ni recovery fluid and an organic after Ni recovery; a Co cleaning stage of back-extracting the Mg in the organic after Ni recovery to a Co cleaning liquid to obtain a liquid after Co cleaning and an organic after Co cleaning; and a Co recovery stage of back-extracting the Co in the organic after Co cleaning to a second acidic aqueous solution and recovering a Co aqueous solution. Part of the Co aqueous solution is used as the Co cleaning liquid.SELECTED DRAWING: Figure 2

Description

The present invention relates to a method for producing an aqueous cobalt solution, and in particular to a method for producing an aqueous cobalt solution with a low magnesium concentration by separating nickel and cobalt from each other using an acidic extractant in an aqueous solution containing these metals.

Cobalt is widely used in applications such as special steels and as an alloying element for magnetic materials. For example, in the field of special steels, steel materials containing cobalt are used in the fields of aerospace, generators, and special tools, taking advantage of cobalt's excellent wear resistance and heat resistance, and as an alloying element in magnetic materials, ferromagnetic alloy materials containing cobalt are used in small headphones, small motors, etc. Cobalt is also used as a raw material for the positive electrode material of lithium-ion secondary batteries.

As a mineral resource, cobalt is often found in association with nickel, and most cobalt is produced as a by-product of nickel smelting. Therefore, in the production of cobalt, a technology is required to efficiently separate cobalt from nickel and various other impurity elements. Techniques for separating metals from an aqueous solution containing multiple metal elements include solvent extraction, precipitation, and ion exchange, but of these, solvent extraction is widely used because it can efficiently separate metals using simple equipment.

For example, when recovering cobalt as a by-product in the hydrometallurgy of nickel-containing raw materials, it is common to first subject raw materials containing the valuable metals nickel and cobalt to leaching or extraction treatment using a mineral acid or an oxidizing agent, and then subject the resulting acidic aqueous solution containing nickel and cobalt to solvent extraction to separate and recover cobalt from the aqueous solution containing nickel and cobalt. It is preferable that the cobalt separated from other elements in the above manner has a low concentration of impurities, and in particular, it is required that the concentration of impurities be as low as possible in the aqueous cobalt chloride solution used as a raw material for the positive electrode material of lithium-ion secondary batteries.

Therefore, a technology has been proposed in which impurities are removed from an acidic aqueous solution containing the above-mentioned valuable metals nickel and cobalt using an acidic extractant such as an organic phosphoric acid compound, and these valuable metals are recovered in high purity. For example, Patent Document 1 discloses a technology in which impurities such as cobalt are extracted into an organic phase using an acidic extractant carrying nickel from a crude nickel sulfate solution containing cobalt, and the pH of the organic phase into which the impurities have been extracted is gradually changed to perform back extraction, thereby obtaining a high-purity nickel sulfate solution from which impurities have been removed and recovering cobalt.

Specifically, the technology of Patent Document 1 is composed of an extraction step in which nickel is extracted by contacting a crude nickel sulfate solution containing impurities such as sodium and ammonium with an acidic extractant, preferably under conditions of pH 6.5 to 7.0, to obtain an acidic extractant carrying nickel; a substitution step in which the nickel in the acidic extractant is contacted with a cobalt-rich aqueous solution of crude nickel sulfate containing Co, preferably under conditions of pH 4 to 5, to substitute the cobalt and impurities in the aqueous solution of crude nickel sulfate containing Co for the nickel in the acidic extractant, preferably under conditions of pH 4 to 5, to obtain a purified nickel sulfate solution with high nickel purity; a nickel recovery step in which the acidic extractant containing cobalt and impurities obtained in the substitution step is contacted with an aqueous sulfuric acid solution, thereby back-extracting the nickel remaining in the acidic extractant obtained in the substitution step, preferably under conditions of about pH 4.0; and a cobalt recovery step in which the acidic extractant obtained in the nickel recovery step is contacted with an aqueous hydrochloric acid solution to back-extract cobalt, preferably under conditions of pH 1.4 to 2.0.

Japanese Patent Application Publication No. 10-310437

As described above, by extracting an aqueous solution containing nickel and cobalt using an acidic extractant such as an organic phosphoric acid compound, it is possible to separate and recover nickel and cobalt separately, and to suppress the inclusion of impurities in the nickel and cobalt recovered in the aqueous solution. However, when an organic phosphoric acid compound is used as the acidic extractant, the order of ease of extraction is iron (Fe) > zinc (Zn) > copper (Cu) > manganese (Mn) > cobalt (Co) > calcium (Ca) > magnesium (Mg) > nickel (Ni). Therefore, if magnesium is contained as an impurity in the aqueous solution containing nickel and cobalt, magnesium will be contained in the recovered aqueous nickel solution or cobalt solution, or both. In this case, in order to keep the magnesium concentration of both the recovered aqueous nickel solution and the recovered aqueous cobalt solution low, it is necessary to install, for example, a multi-stage mixer settler, which causes a problem of high equipment costs.

As described above, the present invention has been made in consideration of the problems associated with the conventional solvent extraction method in which an acidic extractant is used on an aqueous solution containing nickel and cobalt to separate the nickel and cobalt from each other and recover them separately. The object of the present invention is to provide a method for producing an aqueous cobalt solution that uses an acidic extractant on an aqueous solution containing nickel and cobalt to separate the nickel and cobalt from each other and recover them as an aqueous solution with a low magnesium concentration.

In order to achieve the above object, the method for producing a cobalt aqueous solution according to the present invention comprises an exchange stage in which a crude nickel aqueous solution containing cobalt and magnesium is mixed and contacted with an organic solvent containing a nickel-supported acidic extractant to replace the cobalt in the crude nickel aqueous solution with the nickel in the nickel-supported acidic extractant to obtain a high-purity nickel aqueous solution and an exchanged organic solution; a nickel recovery stage in which nickel remaining in the exchanged organic solution is stripped into a first acidic aqueous solution to obtain a nickel recovery solution and a nickel recovery organic solution; a cobalt washing stage in which magnesium in the nickel recovery organic solution is stripped into a cobalt washing solution to obtain a cobalt washing solution and a cobalt washing organic solution; and a cobalt recovery stage in which cobalt in the cobalt washing organic solution is stripped into a second acidic aqueous solution to recover a cobalt aqueous solution, and a part of the cobalt aqueous solution is used for the cobalt washing solution.

According to the present invention, by using an acidic extractant for an aqueous solution containing nickel and cobalt, it is possible to separate the nickel and cobalt from each other and recover an aqueous cobalt solution with a low magnesium concentration.

FIG. 1 is a block flow diagram showing a method for producing an aqueous cobalt solution having a low magnesium concentration according to an embodiment of the present invention. FIG. 2 is a process flow diagram of the solvent extraction step included in the block flow diagram of FIG. 1.

The embodiment of the method for producing a cobalt aqueous solution with a low magnesium concentration according to the present invention will be described in detail below. As shown in FIG. 1, the method for producing a cobalt aqueous solution according to the embodiment of the present invention uses nickel-cobalt mixed sulfide (MS: Mixed Sulfide) as a raw material. The nickel-cobalt mixed sulfide is obtained by removing impurities such as iron from the leachate obtained by high pressure acid leaching (HPAL: High Pressure Acid Leaching) of nickel oxide ore such as low-grade limonite ore, and then injecting hydrogen sulfide gas into the leachate to cause a sulfurization reaction. A crude nickel sulfate aqueous solution is obtained by leaching this nickel-cobalt mixed sulfide under high temperature and high pressure conditions in the pressure leaching process. The iron content of this crude nickel sulfate aqueous solution is removed in the iron removal process, and a crude nickel sulfate aqueous solution containing at least cobalt is obtained as the Fe-free final solution.

The above-mentioned de-Fe final solution is then treated in a solvent extraction step using an acidic extractant to produce a high-purity nickel sulfate aqueous solution and a cobalt aqueous solution. This solvent extraction step is composed of an extraction stage, a washing stage, an exchange stage, a Ni recovery stage, a Co washing stage, a Co recovery stage, and a stripping stage, and in each stage, a general mixer-settler type solvent extraction apparatus (hereinafter simply referred to as a mixer-settler) having a structure in which a mixing tank equipped with a stirrer for performing solvent extraction and a separation tank for phase separation into an organic phase and an aqueous phase by gravity are integrated can be used. In particular, it is preferable to connect a plurality of mixer-settlers in series to perform a multi-stage countercurrent continuous extraction process. As the acidic extractant used in this solvent extraction step, it is preferable to use a general organic phosphoric acid-based acidic extractant, and in particular, it is more preferable to use mono-2-ethylhexyl 2-ethylhexyl phosphonate having the structural formula shown in Chemical Formula 1 below as the above-mentioned acidic extractant.

The above-mentioned solvent extraction step will be specifically described with reference to Fig. 2. First, in the extraction stage, the post-wash solution supplied from the next washing stage is mixed and contacted with an organic solvent containing an acidic extractant, preferably under conditions of pH 6.0 to 7.0, to extract mainly nickel contained in the post-wash solution into the organic phase by, for example, the extraction reaction shown in the following formula 1 (R represents the acidic extractant with H of the hydroxyl group removed), while impurities such as sodium and ammonia, which have lower extraction rates than nickel, are separated and removed by remaining in the aqueous phase. This results in an extracted organic solution containing an acidic extractant carrying nickel (hereinafter also referred to as Ni-loaded acidic extractant).
[Formula 1]
2(R-H)+Ni ²⁺ →R-Ni-R+2H ⁺

Next, in the washing stage, the post-extraction organic phase extracted as an organic phase from the above extraction stage is washed by mixing and contacting it with a washing liquid containing nickel, thereby transferring fine droplets of aqueous phase that remain in suspension in the post-extraction organic phase and are not separated from the organic phase in the previous extraction stage, which are called entrainment, to the washing liquid side. Furthermore, impurities such as sodium and ammonia remaining in the post-extraction organic phase are removed by replacing them with nickel in the washing liquid. For this washing liquid, it is preferable to use an aqueous nickel sulfate solution with a low impurity concentration, for example, a high-purity aqueous nickel sulfate solution extracted from the subsequent exchange stage or a solution obtained by diluting the dehydrated mother liquor produced in the manufacturing process of high-purity nickel sulfate crystals with water. The post-washing liquid is supplied to the extraction stage.

Next, in an exchange stage consisting of, for example, four consecutive mixer settlers, the washed organic phase extracted as an organic phase from the above-mentioned washing stage is introduced from the mixer settler on the most upstream side in the flow direction of the organic phase, and the de-Fe final solution is introduced from the mixer settler on the most downstream side in the flow direction, and these organic phases and aqueous phases are brought into multi-stage countercurrent contact with each other, preferably within the pH range of 4.0 to 5.0 of the aqueous phase. As a result, nickel contained in the Ni-loaded acidic extractant as the washed organic phase and impurities such as cobalt contained in the de-Fe final solution are replaced by, for example, an exchange reaction of the following formula 2, and a high-purity nickel sulfate aqueous solution is produced in the aqueous phase. This high-purity nickel sulfate aqueous solution is shipped as it is as a product, or is transferred to a crystallization facility, where it is concentrated and crystallized by heating to produce nickel sulfate crystals. Meanwhile, the exchanged organic phase containing cobalt is obtained in the organic phase.
[Formula 2]
R-Ni-R+Co ²⁺ →R-Co-R+Ni ²⁺

As described above, in the exchange stage, cobalt is extracted using a Ni-loaded acidic extractant that has already extracted nickel in the previous extraction stage, making it possible to produce a high-purity nickel sulfate aqueous solution without using a neutralizing agent for adjusting the pH. This makes it possible to prevent contamination of the high-purity nickel sulfate aqueous solution with sodium, which is a problem when using NaOH as a neutralizing agent, for example. In addition, nickel is back-extracted in proportion to the extraction of cobalt, making it possible to increase the nickel concentration in the high-purity nickel sulfate aqueous solution.

However, if the concentration of cobalt and impurities contained in the post-wash organic is too high, when the concentration of impurities in the aqueous phase is reduced by the exchange reaction in the exchange stage composed of the four mixer settlers, the cobalt and impurities are less likely to migrate from the aqueous phase to the organic phase. Therefore, it is preferable to extract a portion of the post-stripping organic regenerated in the stripping stage preceding the extraction stage, bypass the extraction stage and the subsequent washing stage, and use it as a dilution liquid for the post-wash organic introduced from the mixer settler on the most upstream side in the flow direction of the organic phase in the exchange stage. In addition, since the efficiency of the exchange reaction can be further increased in the aqueous phase with a high concentration of cobalt and impurities, it is preferable to extract a portion of the post-wash organic and introduce it into the third mixer settler from the most upstream side in the flow direction of the exchange stage.

Next, in the Ni recovery stage, the exchanged organic phase extracted from the exchange stage as an organic phase is mixed and contacted with a first acidic aqueous solution, preferably made of dilute sulfuric acid and having a pH of about 4.0, to back-extract nickel remaining in the exchanged organic phase into the first acidic aqueous solution on the aqueous phase side. The nickel-containing Ni recovery solution thus obtained is then reused, for example, in the nickel sulfate manufacturing process.

Next, in the _CO cleaning stage, the entrainment of the organic phase after Ni recovery, which is extracted as an organic phase from the Ni recovery stage, is replaced with a cobalt cleaning solution, so that the magnesium contained in the entrainment of the organic phase after Ni recovery is distributed to the aqueous phase. Since the cobalt cleaning solution is an aqueous cobalt solution obtained in the subsequent Co recovery stage, the magnesium extracted into the organic phase after Ni recovery and the cobalt in the cobalt cleaning solution are replaced by an exchange reaction, and the magnesium in the organic phase is transferred to the aqueous phase. As a result, magnesium, which is more easily back-extracted than cobalt in the subsequent Co recovery stage, is separated and removed in advance, so that a high-purity aqueous cobalt solution with a low concentration of magnesium impurities can be obtained in the Co recovery stage. The cobalt concentration of the organic phase after Ni recovery is about 3.0 to 6.0 g/L.

In the above Co cleaning stage, it is preferable to set the pH of the cobalt cleaning solution on the aqueous phase side to 2.0 or more and 4.5 or less, and in particular, the lower limit of the pH is more preferably 3.0. If the pH of this cobalt cleaning solution is less than 2.0, the amount of cobalt back-extracted into the aqueous phase becomes too large, and conversely, if the pH exceeds 4.5, magnesium becomes difficult to back-extract into the aqueous phase. In addition, in the above Co cleaning stage, it is preferable to set the volume ratio O/A of the organic phase supply flow rate O to the aqueous phase supply flow rate A to 0.1 or more and 30 or less, and more preferably 0.5 or more and 1.0 or less. If this volume ratio O/A is less than 0.1, the aqueous phase flow rate becomes excessive, the residence time in the settler section becomes short, and oil-water separation is caused, so that the entrainment of the organic after Co cleaning extracted as an organic phase from the Co cleaning stage increases, and as a result, the decrease in the magnesium concentration in the organic after Co cleaning is inhibited. Conversely, if the volume ratio O/A exceeds 30, the flow rate of the cobalt cleaning solution is too low, and the cleaning effect, i.e., the effect of replacing the organic entrainment with the cobalt cleaning solution after Ni recovery, cannot be fully achieved.

When using the crude cobalt chloride aqueous solution described below as the Co cleaning solution, the cobalt concentration is preferably about 80 g/L, and the lower the magnesium concentration of the Co cleaning solution, the more desirable it is to increase cleaning efficiency. When using the crude cobalt chloride aqueous solution described above, the magnesium concentration is about 0.080 to 0.090 g/L. The liquid temperature in the Co cleaning stage is preferably 10°C to 50°C, and considering the distribution of magnesium between the organic phase after cobalt cleaning and the aqueous phase after cobalt cleaning (liquid after cobalt cleaning), it is desirable to have a temperature of 40°C or higher.

Next, in the Co recovery stage, the organic solution after cobalt washing, which is extracted as an organic phase from the Co washing stage, is mixed and contacted with a second acidic aqueous solution adjusted to about pH 1.0, preferably made of hydrochloric acid, to back-extract the cobalt contained in the organic solution after cobalt washing into the second acidic aqueous solution on the aqueous phase side. A part of the crude cobalt chloride aqueous solution containing cobalt thus recovered is used as a cobalt washing solution in the preceding Co washing stage, and the remainder is turned into a high-purity aqueous cobalt chloride solution by removing impurities such as zinc, manganese, copper, and cadmium in a solution purification process as necessary. This high-purity aqueous cobalt chloride solution is used as a raw material for the positive electrode material of secondary batteries as it is, or is supplied to an electrowinning process to produce high-quality electrolytic cobalt. Finally, in the back extraction stage, the organic solution after cobalt recovery, which is extracted as an organic phase from the Co recovery stage, is mixed and contacted with dilute sulfuric acid to back-extract impurities such as iron into the dilute sulfuric acid.

As mentioned above, by using an acidic extractant for the solvent extraction, hydrogen ions are involved in the extraction reaction, and the extraction rate can be changed by changing the pH. The extraction rate differs depending on the metal, with the metals most likely to be extracted in the following order: Fe>Zn>Cu>Mn>Co>Ca>Mg>Ni. Therefore, by lowering the pH in the order of the organic phase flow, which is the extraction stage, washing stage, exchange stage, Ni recovery stage, Co washing stage, Co recovery stage, and back extraction stage, different metals can be separated in each stage.

[Example 1]
An organic solvent was prepared by diluting mono-2-ethylhexyl 2-ethylhexyl phosphonate (product name: PC88A), an organophosphate acidic extractant manufactured by Daihachi Chemical Industry Co., Ltd., to 20% by volume using a saturated hydrocarbon (product name: Teclain N-20) manufactured by ENEOS Corporation as a diluent. In solvent extraction using an organic solvent containing this acidic extractant, the removal effect of the Co washing stage, which is a process for separating and removing magnesium impurities from the cobalt-containing acidic extractant, was confirmed.

Specifically, in an extraction facility that produces a high-purity nickel sulfate aqueous solution according to the solvent extraction process shown in Figure 2, the post-Ni recovery organic solution discharged from the Ni recovery stage was sampled and divided into seven 100 mL plastic containers as the initial organic phase, each containing 30 mL of the sample. The crude Co chloride aqueous solution discharged from the Co recovery stage of the extraction facility was sampled and divided into seven 30 mL portions, each of which was charged as the initial aqueous phase into the seven plastic containers containing the initial organic phase, to prepare samples 1 to 7.

During this preparation, the starting aqueous phase was divided into seven portions, and six of them, used for samples 2 to 7, were pH-adjusted using dilute hydrochloric acid and a dilute aqueous sodium hydroxide solution so that each had a different pH value within the range of pH 2.1 to 5.1. For comparison, the starting aqueous phase of sample 1 was used as is without pH adjustment.

Seven plastic containers containing the liquid samples 1 to 7 prepared as described above were each strongly stirred with a magnetic stirrer to mix and contact the initial organic phase and initial aqueous phase for extraction, and then the mixture was left to stand to separate into the final organic phase and final aqueous phase. The liquid temperature was left to run without any special adjustment, and was approximately 20°C, which is roughly the same as room temperature. The metal element concentrations in the organic and aqueous phases before and after this extraction process were analyzed with an X-ray fluorescence analyzer, and the results are shown in Tables 1 to 7 below, along with the pH values of the aqueous phase before and after. Furthermore, the results of the separation performance of cobalt and magnesium in the extraction process performed on samples 1 to 7 are shown in Table 8 below.

As shown in Table 8 above, the ratio A of the magnesium concentration (g/L) to the cobalt concentration (g/L) in the initial organic phase divided by the ratio B of the magnesium concentration (g/L) to the cobalt concentration (g/L) in the final organic phase is 3.36 to 8.75, which shows that magnesium can be effectively removed from the organic solvent having the acidic extractant in the Co washing stage under all conditions. However, in sample 6, while cobalt is extracted, magnesium is not back-extracted. Therefore, from the viewpoint of back-extraction of magnesium from the acidic extractant, a pH of less than 5.0 is preferable, and a pH in the range of 2.0 to 4.5 is more preferable.

[Example 2]
The organic phase after Ni recovery discharged from the Ni recovery stage of the extraction equipment similar to that of Example 1 was sampled and divided into 20 mL portions as the initial liquid organic phase in eight 100 mL plastic containers. Meanwhile, the crude Co chloride aqueous solution discharged from the Co recovery stage of the above extraction equipment was sampled and divided into four portions, which were diluted with water so that the Co concentrations were different within the range of 5 to 40 g/L, and used as the initial liquid aqueous phase.

60 mL each was taken from each of the four types of starting aqueous phases with adjusted Co concentrations in this way, and the pH was adjusted to within the range of 3.5 to 3.8 using a diluted aqueous sodium hydroxide solution. The samples were then placed in four of the eight plastic containers containing the starting organic phases to prepare samples 8 to 11, and the liquid temperature of these samples was adjusted to approximately 20°C, which is almost the same as room temperature. Similarly, 60 mL each was taken from each of the four types of starting aqueous phases in the remaining four of the eight plastic containers, and the pH was adjusted to within the range of 3.5 to 3.8 using a diluted aqueous sodium hydroxide solution. The samples were then placed in the remaining four of the eight plastic containers to prepare samples 12 to 15, and the liquid temperature of these samples 12 to 15 was adjusted to 50°C by immersing them in a thermostatic bath.

Then, the initial organic phase and the initial aqueous phase were mixed and contacted in the same manner as in Example 1 to carry out the extraction process, and then the mixture was allowed to stand to separate into a final organic phase and a final aqueous phase. The organic phase and the aqueous phase were analyzed for metal element concentration before and after this extraction process in the same manner as in Example 1, and the results are shown in Tables 9 to 16 below, along with the pH value of the initial aqueous phase. Furthermore, the results of the separation performance of cobalt and magnesium in the extraction process carried out on samples 8 to 15 are shown in Table 17 below.

As shown in Table 17 above, the ratio A of the magnesium concentration (g/L) to the cobalt concentration (g/L) in the initial organic phase divided by the ratio B of the magnesium concentration (g/L) to the cobalt concentration (g/L) in the final organic phase is 3.79 to 4.94, and it can be seen that under all conditions, magnesium can be efficiently removed from the organic solvent having an acidic extractant in the Co washing stage. In addition, the lower the initial aqueous phase cobalt concentration, the higher the washing effect of magnesium in the initial organic phase, but it is thought that the effect of the aqueous phase cobalt concentration is small when the initial aqueous phase pH is in the range of 3.5 to 3.8. In addition, comparing the liquid temperatures of 20°C and 50°C, the higher the washing temperature, the higher the separation efficiency of magnesium and cobalt. Incidentally, samples 1 to 7 correspond to 1.0 in O/A, and samples 8 to 15 correspond to 0.33 in O/A, and it is thought that there is no effect of O/A with a difference of this magnitude.

Claims (4)

an exchange stage in which a crude nickel aqueous solution containing cobalt and magnesium is mixed and contacted with an organic solvent containing a nickel-supported acidic extractant to replace the cobalt in the crude nickel aqueous solution with the nickel in the nickel-supported acidic extractant, thereby obtaining a high-purity nickel aqueous solution and an exchanged organic solvent;
a nickel recovery stage in which nickel remaining in the post-exchange organic is stripped into a first acidic aqueous solution to obtain a nickel recovery solution and a post-nickel recovery organic;
a cobalt wash stage in which magnesium in the nickel-recovery organic is stripped into a cobalt wash solution to obtain a cobalt wash solution and a cobalt wash organic;
a cobalt recovery stage for recovering an aqueous cobalt solution by back-extracting the cobalt in the organic solvent after the cobalt washing into a second acidic aqueous solution;
A method for producing an aqueous cobalt solution, comprising using a part of the aqueous cobalt solution as the cobalt cleaning solution. The method for producing the cobalt aqueous solution according to claim 1, characterized in that the pH of the cobalt cleaning solution is 2.0 or more and 4.5 or less. The method for producing an aqueous cobalt solution according to claim 1, characterized in that the second acidic aqueous solution used in the cobalt recovery stage is an aqueous hydrochloric acid solution, and the aqueous cobalt solution is an aqueous cobalt chloride solution. The method for producing an aqueous cobalt solution according to any one of claims 1 to 3, characterized in that the acidic extractant is mono-2-ethylhexyl 2-ethylhexylphosphonate.

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