Patent Description:
Various positive electrode materials have been developed as a positive electrode material of a lithium-ion battery as a secondary battery. A nickel-cobalt-manganese (NCM)-based positive electrode material referred to as a ternary system and a nickel-cobalt-aluminum (NCA)-based positive electrode material referred to as a nickel-base have been recently gathering attention.

The positive electrode material containing nickel like the NCM-based positive electrode material and the NCA-based positive electrode material are manufactured by processing an aqueous solution containing a salt of a metal, such as nickel, with an alkali and performing a burning process on the obtained metallic hydroxide. The metal salt used as the raw material includes chloride (nickel chloride) and sulfate (nickel sulfate). The use of the chloride generates a chlorine gas in the burning process, and therefore a firing furnace is likely to be corroded. Therefore, the sulfate is generally used as the metal salt.

The sulfate is, for example, manufactured by nickel smelting using a nickel oxide ore as a raw material. The nickel-oxidized ore usually contains a cobalt oxide. Therefore, the nickel smelting obtains an acidic aqueous solution of sulfuric acid containing nickel and cobalt. There may be a case where this acidic aqueous solution of sulfuric acid contains magnesium as impurities.

Manufacturing the positive electrode material with the acidic aqueous solution of sulfuric acid containing the magnesium as the raw material results in containing the magnesium as the impurities in the positive electrode material. A lithium-ion battery using the positive electrode material possibly deteriorates battery properties, such as a charge/discharge capacity. Accordingly, preliminarily removing the magnesium from the acidic aqueous solution of sulfuric acid is desired.

Patent Document <NUM> discloses that a solvent extraction method separates and recovers a nickel sulfate aqueous solution and a cobalt sulfate aqueous solution from an acidic aqueous solution of sulfuric acid containing nickel and cobalt. Use of a monothiophosphinic acid compound as an extractant allows obtaining a cobalt sulfate aqueous solution that hardly contains magnesium. However, the magnesium is contained in the nickel sulfate aqueous solution. Thus, selectively separating the magnesium from the acidic aqueous solution of sulfuric acid containing the nickel and the magnesium is difficult. Patent Document <NUM> discloses a method for preparing high purity flame retardant from solution containing cobalt of the cobalt and nickel industry. Patent Document <NUM> discloses a method to produce a high purity cobalt sulphate solution.

Patent Document <NUM> discloses a method for recovering magnesium from low-concentration nickel-cobalt from a tailings bioleaching solution.

In consideration of the circumstances, an object of the present invention is to provide a solvent extraction method that allows selectively separating magnesium from an acidic aqueous solution of sulfuric acid containing nickel, cobalt, and magnesium.

A solvent extraction method according to the invention as defined in claim <NUM> includes: bringing an acidic aqueous solution of sulfuric acid containing nickel, cobalt, and magnesium in contact with an organic solvent to selectively extract the magnesium from the acidic solution into the organic solvent; and using the organic solvent produced by diluting an extractant made of bis(<NUM>-ethylhexyl) hydrogen phosphate with a diluent. A concentration of the extractant in the organic solvent is set to <NUM> volume% or more and <NUM> volume% or less and a pH of the acidic aqueous solution of sulfuric acid is set to <NUM> or more and <NUM> or less.

Further aspects of the invention are defined by the subject-matter of dependent claims <NUM> and <NUM>.

The present invention allows selectively separating the magnesium from the acidic aqueous solution of sulfuric acid containing the nickel, the cobalt, and the magnesium.

Next, one embodiment of the present invention will be described.

A solvent extraction method of this embodiment brings an acidic aqueous solution of sulfuric acid containing nickel, cobalt, and magnesium in contact with an organic solvent to extract the magnesium into the organic solvent. While the nickel and the cobalt are caused to remain in a water phase, the magnesium is extracted to an organic phase to selectively separate the magnesium.

A device used for the solvent extraction is not specifically limited. A solvent extraction device includes a mixer-settler extractor.

As the organic solvent, a solvent produced by diluting an extractant with a diluent is employed. Alkylphosphonic acid ester is used as the extractant. The alkylphosphonic acid ester includes bis(<NUM>-ethylhexyl) hydrogen phosphate (D2EHPA), <NUM>-ethylhexyl hydrogen -<NUM>-ethylhexylphosphonate (PC-88A), and Diisooctylphosphinic acid (CYANEX272). According to the invention, the bis(<NUM>-ethylhexyl) hydrogen phosphate is used as the extractant.

As long as the extractant can be dissolved, the diluent is not specifically limited. As examples of the diluent, a naphthene-based solvent and an aromatic-based solvent can be employed.

The alkylphosphonic acid ester is one kind of an acid extractant. Taking only an action as the acid extractant into consideration, an extraction reaction is a pure acid-base reaction. An amount of substance of the extractant contributing to the extraction reaction is determined according to a concentration and a valence of an element to be extracted in the aqueous solution. In a case where the extraction reaction to all the elements is the acid-base reaction, a separation factor between the elements does not depend on the concentration of the extractant in the organic solvent.

Note that the alkylphosphonic acid ester also acts as a chelate extractant. The alkylphosphonic acid ester contains phosphorus and oxygen in molecules. In addition to the acid-base reaction, an element that forms a coordinate bond with phosphorus or oxygen is extracted by formation of a chelate compound. Increasing the concentration of the extractant in the organic solvent promotes the formation of the chelate compound. Among the elements in the aqueous solution, an element that facilitates the formation of the chelate compound increases an extraction rate compared with that of an element that is less likely to form the chelate compound.

A formation trend of the chelate compound by nickel, cobalt, and magnesium is in the order of nickel > cobalt ≈ magnesium. That is, the nickel preferentially forms the chelate compound. As the concentration of the extractant increases, the extraction of the nickel is promoted compared with that of the magnesium, and therefore a separation factor of the magnesium relative to the nickel (hereinafter referred to as a "Mg/Ni separation factor") decreases. Meanwhile, the formation trends of the chelate compound of the cobalt and the magnesium are approximately the same. Accordingly, the concentration of the extractant hardly affects the separation factor of the magnesium relative to the cobalt (hereinafter referred to as a "Mg/Co separation factor"). Accordingly, as the concentration of the extractant decreases, the Mg/Ni separation factor can be increased, thereby ensuring efficiently separating the magnesium chemically.

However, the decrease in the concentration of the extractant reduces a reaction volume, resulting in decrease in the extraction rate of the magnesium. Industrially, some extent of the magnesium extraction rate needs to be maintained. Therefore, the concentration of the extractant is adjusted to ensure maintaining a desired magnesium extraction rate.

Appropriately adjusting the concentration of the extractant allows increasing the extraction rate of the magnesium while the extraction rates of the nickel and the cobalt are suppressed to be low. Specifically, the concentration of the extractant is adjusted to be from <NUM> to <NUM> volume%. This allows selectively separating the magnesium from the acidic aqueous solution of sulfuric acid containing the nickel, the cobalt, and the magnesium.

In the acidic aqueous solution of sulfuric acid processed by the solvent extraction method according to this embodiment, the magnesium concentration decreases. Therefore, the acidic aqueous solution of sulfuric acid can be used as, for example, a raw material of a positive electrode material, such as an NCM-based positive electrode material and an NCA-based positive electrode material.

Note that the solvent extraction method according to this embodiment extracts the magnesium as impurities in the organic solvent. In a case where the nickel and the cobalt, which are the obj ective metals, are extracted in the organic solvent, the obj ective metals in the organic phase need to be back-extracted in a water phase in a post-process. In a case where a large amount of the objective metals is contained in the acidic aqueous solution of sulfuric acid, a large amount of an agent used for the back extraction, such as alkali and acid, is required. In contrast to this, since the objective metals remain in the water phase in this embodiment, the operation of back-extracting the objective metals is unnecessary. Additionally, since the magnesium extracted in the organic phase is a trace, usage of the agent used for the back extraction of the magnesium can be reduced.

Next, the examples will be described. Those examples relating to extractant concentrations of <NUM> to <NUM> volume% or a pH of <NUM> and <NUM> do not form part of the invention.

First, an acidic aqueous solution of sulfuric acid containing nickel, cobalt, and magnesium was prepared as a raw solution. A nickel concentration is <NUM>/L, a cobalt concentration is <NUM>/L, and a magnesium concentration is <NUM>/L in the acidic aqueous solution of sulfuric acid.

Next, an extractant was diluted with a diluent to prepare an organic solvent. Bis(<NUM>-ethylhexyl) hydrogen phosphate (BAYSOLVEX D2EHPA manufactured by LANXESS Corporation) was used as the extractant. A naphthene-based solvent (Teclean N20 manufactured by JXTG Nippon Oil & Energy Corporation) was used as the diluent. Six kinds of organic solvents whose concentrations of the extractants were different were prepared. The concentrations of the extractants in the respective organic solvents are <NUM> volume%, <NUM> volume%, <NUM> volume%, <NUM> volume%, <NUM> volume%, and <NUM> volume%.

<NUM> of the raw solution and <NUM> of the organic solvent were put in a <NUM> beaker and stirred for <NUM> minutes. During the stirring, a sulfuric acid or sodium hydroxide aqueous solution was added, and a pH of a water phase (acidic aqueous solution of sulfuric acid) was adjusted to be any of <NUM>, <NUM>, <NUM>, and <NUM>. Note that a final additive amount of the sulfuric acid and the sodium hydroxide aqueous solutions was <NUM> or less.

After ending the stirring, the mixed liquid was left for phase separation and the water phase (acidic aqueous solution of sulfuric acid) and an organic phase (organic solvent) were each recovered. A nickel concentration, a cobalt concentration, and a magnesium concentration of the water phase and the organic phase were analyzed by ICP optical emission spectrometer. Respective masses of the nickel, the cobalt, and the magnesium in the organic phase were obtained from analysis values. Extraction rates of the nickel, the cobalt, and the magnesium were each calculated by dividing the mass in the organic phase by a mass in the raw solution. Distribution ratios of the nickel, the cobalt, and the magnesium were each calculated by dividing the concentration in the organic phase by the concentration in the water phase. Then, the distribution ratio of the magnesium was divided by the distribution ratio of the cobalt to obtain a Mg/Co separation factor. The distribution ratio of the magnesium was divided by the distribution ratio of the nickel to obtain a Mg/Ni separation factor.

Table <NUM> and <FIG> show the extraction rate of the nickel. Table <NUM> and <FIG> show the extraction rate of the cobalt. Table <NUM> and <FIG> show the extraction rate of the magnesium. Table <NUM> and <FIG> show the Mg/Co separation factor. Table <NUM> and <FIG> show the Mg/Ni separation factor.

As seen from <FIG>, as the pH increases, the extraction rate of each metallic element increases. In the case of the nickel and the cobalt, as the pH increases, the extraction rate increases in an accelerated manner. Meanwhile, in the case of the magnesium, especially setting the concentration of the extractant to <NUM> volume% or more asymptotically increases the extraction rate as the pH increases. Accordingly, it is predicted that the low pH improves separation efficiency of the magnesium from the nickel and the cobalt.

The separation factors back up the above-described prediction. As seen from <FIG>, the lower the pH is, the higher the Mg/Co separation factor becomes. Additionally, as seen from <FIG>, as the pH decreases, the Mg/Ni separation factor tends to be high. Accordingly, it can be said that the decrease in pH facilitates separating the magnesium from the nickel and the cobalt.

As seen from <FIG>, the concentration of the extractant hardly affects the Mg/Co separation factor. Meanwhile, as seen from <FIG>, the Mg/Ni separation factor depends on the concentration of the extractant. The lower the concentration of the extractant is, the higher the Mg/Ni separation factor becomes. That is, as the concentration of the extractant lowers, the separation of the magnesium from the nickel and the cobalt is facilitated.

However, as seen from <FIG>, the lower the concentration of the extractant is, the lower the extraction rate of the magnesium becomes. When the extraction rate of the magnesium is excessively low, it is not realistic to industrially separate the magnesium from the acidic aqueous solution of sulfuric acid.

Therefore, it is preferred that the concentration of the extractant is set to be <NUM> volume% or more and the pH of the acidic aqueous solution of sulfuric acid is set to be <NUM> or more, the concentration of the extractant is set to be <NUM> volume% or more and the pH of the acidic aqueous solution of sulfuric acid is set to be <NUM> or more, or the concentration of the extractant is set to be <NUM> volume% or more and the pH of the acidic aqueous solution of sulfuric acid is set to be <NUM> or more. Doing so allows obtaining the extraction rate of the magnesium of <NUM>% or more. Additionally, it is more preferred that the concentration of the extractant is set to be <NUM> volume% or more and the pH of the acidic aqueous solution of sulfuric acid is set to be <NUM> or more, or the concentration of the extractant is set to be <NUM> volume% or more and the pH of the acidic aqueous solution of sulfuric acid is set to be <NUM> or more. Doing so allows obtaining the extraction rate of the magnesium of <NUM>% or more.

As seen from <FIG>, from a perspective of separating the magnesium from the cobalt, the pH of the acidic aqueous solution of sulfuric acid is preferably set to be <NUM> or less. Doing so allows obtaining the Mg/Co separation factor of <NUM> or more.

As seen from <FIG>, from a perspective of separating the magnesium from the nickel, it is preferred that the concentration of the extractant is set to be <NUM> volume% or less and the pH of the acidic aqueous solution of sulfuric acid is set to be <NUM> or less, or the concentration of the extractant is set to be <NUM> volume% or less and the pH of the acidic aqueous solution of sulfuric acid is set to be <NUM> or less. Doing so allows obtaining the Mg/Ni separation factor of <NUM> or more. Additionally, it is more preferred that the concentration of the extractant is set to be <NUM> volume% or less and the pH of the acidic aqueous solution of sulfuric acid is set to be <NUM> or less, or the concentration of the extractant is set to be <NUM> volume% or less and the pH of the acidic aqueous solution of sulfuric acid is set to be <NUM> or less. Doing so allows obtaining the Mg/Ni separation factor of <NUM> or more.

In conclusion, the concentration of the extractant is set to be <NUM> to <NUM> volume% and the pH of the acidic aqueous solution of sulfuric acid is set to be <NUM> to <NUM>, or preferably the concentration of the extractant is set to be <NUM> to <NUM> volume% and the pH of the acidic aqueous solution of sulfuric acid is set to be <NUM> to <NUM>. Doing so allows obtaining the extraction rate of the magnesium of <NUM>% or more, the Mg/Co separation factor of <NUM> or more, and the Mg/Ni separation factor of <NUM> or more.

It is preferred that the concentration of the extractant is set to be <NUM> to <NUM> volume% and the pH of the acidic aqueous solution of sulfuric acid is set to be <NUM> to <NUM>, or the concentration of the extractant is set to be <NUM> to <NUM> volume% and the pH of the acidic aqueous solution of sulfuric acid is set to be <NUM> to <NUM>. Doing so allows obtaining the extraction rate of the magnesium of <NUM>% or more, the Mg/Co separation factor of <NUM> or more, and the Mg/Ni separation factor of <NUM> or more.

Claim 1:
A solvent extraction method comprising:
bringing an acidic aqueous solution of sulfuric acid containing cobalt, magnesium and nickel in contact with an organic solvent to selectively extract the magnesium from the acidic aqueous solution into the organic solvent; and
using the organic solvent produced by diluting an extractant made of bis(<NUM>-ethylhexyl) hydrogen phosphate with a diluent, wherein
a concentration of the extractant in the organic solvent is set to <NUM> volume% or more and <NUM> volume% or less and a pH of the acidic aqueous solution of sulfuric acid is set to <NUM> or more and <NUM> or less.