In the production process, the wet nickel and cobalt from nickel and cobalt-containing solution to meet certain standards to produce nickel and cobalt products, intermediate must impurity removal (purge) and the valuable metal element separation enrichment step. At present, the solution purification and separation and enrichment methods applied in the nickel-cobalt extraction metallurgy industry mainly include chemical precipitation, solvent extraction and ion exchange.

First, chemical precipitation method

The chemical precipitation method is the most commonly used method for solution impurity removal and separation. The nickel-cobalt extraction metallurgy industry mainly uses hydrolysis precipitation, sulfide precipitation, insoluble salt precipitation and displacement separation.

(1) Hydrolysis precipitation

Hydrolyzed precipitation

The principle of hydrolysis precipitation is that different metal hydroxides have different solubility or solubility products in water, and thus have different pH values ​​for starting precipitation. By controlling the pH value of the solution in the solution, the ions required to be removed from the solution can be hydrogen. Precipitation in the form of oxides sometimes requires the control of oxidation-reduction potential. The solubility product of some metal hydroxides at 25 ° C and the pH of the initial precipitation obtained according to the Eh-pH diagram are listed in Table 1 for reference when designing a hydrolysis precipitation purification scheme. The commonly used hydrolysis and precipitation processes in the industry include oxidizing water to remove iron , and oxidative hydrolysis to separate nickel and cobalt.

Table 1 PK _{SP of} certain metal hydroxides and the lowest pH at which precipitation begins

Iron removal by goethite is also a hydrolysis precipitation process. The main conditions for the formation of goethite (FeOOH) crystals are: low concentration Fe ^{3 +} , pH = 3 to 5, high temperature (≥ 90 ° C). A common method is to first reduce Fe ^{3 +} to Fe ^{2 +} , then neutralize to a desired pH value, and then slowly oxidize Fe ^{2 + at a} high temperature. The precipitate thus obtained is FeOOH instead of Fe(OH) ₃ and is easily filtered. In the production of nickel and cobalt, high nickel ruthenium is commonly used as a reducing agent, and air is used as an oxidant. Another way to form goethite is to add the iron solution to be sprayed in a large-capacity iron-removing solution. Under sufficient agitation, the overall concentration of Fe ^{3 +} is not high (<1g/L). The addition of a neutralizing agent under oxidizing conditions forms FeOOH. Thus, the solution does not need to be reduced first and then oxidized.

(2) Sulfide precipitation

Sulfide precipitation is a common method for separating valuable metals such as nickel, cobalt, copper, etc. The vulcanizing agents are mostly Na ₂ S, NaHS and H ₂ S. Generally, the metal sulfide has a small solubility in water, and is commonly used for precipitating and separating copper from a nickel-cobalt solution, and also for precipitating and separating copper, nickel, and cobalt from a laterite ore leachate. When H ₂ S is used for sulfide precipitation, the equilibrium pH of the sulfide formed depends on the active (concentration) product of the sulfide, the metal ion concentration in the solution, and the ion valence. The equilibrium pH values ​​at which H ₂ S precipitated sulfide at 25 ° C and atmospheric pressure are shown in Table 2.

Table 2 Equilibrium pH (25 ° C and atmospheric pressure) of sulfide formation at different ion concentrations

(3) Insoluble salt (compound) precipitation method

The most commonly used insoluble salt (compound) precipitation method is the removal of iron from the yellow sodium iron slag process. Sodium sulphate is a double salt of two or more sulphates. The test is Na ₂ Fe ₆ (SO ₄ ) ₄ (OH) ₁₂ or Me ⁺ Fe ₃ (SO ₄ ) ₂ (OH) ₆ , Me ₂ ⁺ Fe ₆ (SO ₄ ) ₄ (OH) ₁₂ , has the advantages of good crystallization and easy filtration. In the formula, Me ^{+ is} generally Na ⁺ , K ⁺ , NH ₄ ⁺ or H ₃ O ⁺ , wherein potassium vanadium is the most stable and has the best sedimentation performance.

(four) replacement precipitation

A typical displacement precipitate is an electronegative metal that displaces an electropositive ion from a solution, such as nickel powder to remove copper. Broadly speaking, displacement precipitation also includes the reaction of a solid resting material with a solution in which an element of the solid exchanges with a metal ion in the solution, such as copper precipitated from the solution using Ni ₂ S ₃ .

Second, solvent extraction separation

Solvent extraction is one of the common methods for separating and enriching metal ions. It has a wide range of industrial applications in the field of non-ferrous metal hydrometallurgy, and its application in the nickel-cobalt extraction industry is also maturing.

Solvent extraction is a separation method that utilizes an organic phase to extract a substance from an immiscible liquid phase. The solvent extraction process includes three stages of extraction, washing and stripping. The extraction is to transfer some substances in the aqueous phase to the organic phase, the washing is to return the impurities entering the organic phase to the aqueous phase (washing liquid), and the stripping is to transfer the extracted material (target component) from the organic phase to the aqueous phase. (Regression) for further processing into products. Some extractants require pretreatment (such as saponification, etc.) prior to extraction to ensure extraction conditions.

The key to the solvent extraction process is the choice of extractant. In addition to economic benefits, the basic principles for selecting extractants are:

1. Good selectivity and easy metal separation;

2, good extraction kinetic performance, fast balance;

3, large extraction capacity, less amount of extractant;

4. The solubility in the aqueous phase is small and the chemical stability is good;

5, easy to dissolve with the diluent, after mixing has a good phase separation performance, not easy to produce a third phase.

The application of solvent extraction in nickel-cobalt metallurgy mainly has two aspects: one is to extract and remove the impurity elements from the main metal solution, or conversely, to extract the main metal ions; the other is to separate nickel and cobalt with similar properties.

In industrial production, a multi-stage extraction process is often employed. Due to the different flow modes of the organic phase and the aqueous phase, the multi-stage extraction is further divided into a countercurrent extraction, a cross-flow extraction and a fractional extraction. As shown in Fig. 1, the fractionation extraction is a washing section in which an organic phase is added to the countercurrent extraction.

Figure 1 Extraction process

A-three-stage cross-flow extraction; b-three-stage countercurrent extraction

F-liquid; S-organic phase; E-extract; R-raffinate

In the nickel-cobalt extraction industry, solvent extraction is mainly used for the separation of nickel and cobalt, as well as the separation of impurities such as copper and iron. CYANEX272, P ₅₀₇ or N _{235 is used to} extract cobalt and nickel in sulfuric acid medium. CYANEX272 is a newly developed extractant with a separation factor larger than that of P507. P _{204 is} often used for the extraction separation of impurities (iron, copper, zinc ). Ammonium extractants are commonly used in chlorinated media. Some new extractions for nickel-cobalt separation are under development.

Third, ion exchange

Specific ions can be removed from the solution by adsorption and desorption of the ion exchange resin. The ion exchange method is generally used to treat dilute solutions at low concentrations (eg, concentrations less than 10 ^-6 mol/L). When the solution concentration is high (eg, above 1%), the separation effect by this method is not significant. The main industrial applications of ion exchange are the deep purification of trace impurities, the removal of lead and zinc in nickel-cobalt hydrometallurgy, and the removal of trace amounts of copper.

The research on ion exchange process for nickel-cobalt separation is also active, and some new ion exchange resins with potential industrial applications are proposed.

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