Currently, metallurgical cobalt-nickel sulfide starting material has been previously cobalt-nickel-cobalt-nickel gradually turned miscellaneous materials, cobalt-nickel oxide ore (containing cobalt, nickel laterite ore) and the like, by a conventional treatment process matting fire, wet-separation The combination is converted into a leaching and purifying total wet process. The sources of cobalt-nickel raw materials are different, and the composition of the leachate is complicated. It is difficult to separate the diamond, nickel and cobalt-nickel from other impurity ions such as calcium and magnesium by precipitation and ion exchange processes. The solvent extraction method has the advantages of good selectivity, high metal recovery rate and fast mass transfer rate. Especially according to the difference in ion properties and the new extractant and extraction system developed by the extraction theory, the extraction effect is optimized. Therefore, fundamentally find the difference in the properties of cobalt and nickel, analyze the existing separation principle of cobalt and nickel, and have guiding significance for the development of new extractants and extraction processes.
First, the difference between cobalt and nickel
Cobalt nickel atomic number is adjacent, which is the fourth cycle of Group VIII element, and only the outer layer d is different in electron number. This difference in properties can be used for extraction separation.
(I) Analysis of the properties of cobalt and nickel by crystal field coordination theory
1. Cobalt-nickel orbital degeneracy
The more common coordination numbers for cobalt and nickel are 4 and 6. When the coordination number is 6, the ligand is octahedral. Due to the different positions between the ligand, degenerate 5 tracks 2 groups, electron ligand plug closest d z 2, d x 2 -y 2 strong effect, higher energy, is 6DQ; and the other d xy The d yz and d zx orbits are much weaker and the energy is lower, which is -4Dq. When the coordination number is 4, the ligand can form a planar square or regular tetrahedral configuration. The molecular weight of the extractant is large, and there is a large steric hindrance between the molecules, so it is generally a regular tetrahedral configuration. Similarly, the tetrahedral field is also degenerate, but contrary to the octahedral field, the d xy , d yz , d zx orbital energy is higher, 1.78Dq, and the orbital energy of d z 2 and d x 2 -y 2 is higher. Low, - 2.67Dq.
2. Cobalt-nickel orbital electronic arrangement
The arrangement of electrons in orbit follows the energy (CFSE) minimum principle, in which pairs of electrons also need to overcome the pairwise energy of energy P or P'. According to this rule, the electronic arrangement and the corresponding energy are as shown in Table 1.
Table 1 Corresponding energy of cobalt-nickel ions with different coordination numbers
It can be seen that the stability of the 6-coordinated regular octahedron is greater than the stability of the 4-coordinated regular tetrahedron. The stability of Ni(II) 6-coordinate octahedron is much greater than that of tetra-coordinate tetrahedron, while the stability of Co(II) 6-coordinate octahedron is only slightly stronger than that of tetra-coordinate tetrahedron. Therefore, Ni(II) in the solution has only 6 coordination sites, while Co(II) 6 or 4 coordination sites can exist.
(2) Valence bond theory
The valence bond theory is L. Pauling is equivalent to the application of hybrid orbital theory proposed in the 1930s to coordination chemistry. According to this theory, when the covalent bond is formed, the orbits whose energy levels are not far apart can constitute a hybrid orbit, and the atomic orbital hybridization can enhance the bonding ability, thereby making the generated "molecule" more stable. When the coordination bond is formed, if the orbit provided by the central ion is the outermost orbit, the complex ion formed is called the outer orbital ion; if the central ion provides a part of the outer outer orbit, the formed complex ion is called the inner rail. Complex ion.
The valence bond theory holds that when the difference between the electronegativity of the central ion and the coordinating atom is large, the external orbital complex ion tends to be generated; when the phase difference is small, the internal orbital complex ion tends to be generated. Generally, when complexed with a more electronegative coordination atom, such as F and O, an external orbital complex ion is often formed; when it is combined with a less electronegative P atom, P, As, etc., an internal rail is formed. The complex ions; when combined with N, Cl, etc., it is possible to form external orbital complex ions and possibly form internal orbital complex ions.
When Co(II) and Ni(II) form an external orbital complex ion, if it is a 4-coordinate, it is a sp 3 hybrid, a tetrahedral configuration; if it is a 6-coordinate, it is a sp 3 d 2 hybrid. Octahedral configuration. Therefore, when Co(II) generates internal orbital complex ions, it is easily oxidized to Co(III), while Ni(II) is relatively stable and difficult to oxidize.
It can be known from the above coordination theory that: 1) Co(II) is stably present in the solution when cobalt is supported by the orbital type; Co(III) is stably present in the solution when it is coordinated by the internal orbital type; 2) No matter which Coordination, the stability of Ni(II) in solution is higher than that of Ni(III); 3) Co(II) combines with the more electronegative coordination atom to form tetracoordinate complex, stability The complex is higher than Ni(II); 4) Ni(II) combines with the less electronegative coordination atom to form a hexacoordination complex with higher stability than the corresponding complex of Co(II).
Second, the extraction and separation of cobalt and nickel
(1) Extraction and separation of phosphorus (phosphine) from cobalt and nickel
Solvent extraction method is one of the important methods for the separation of cobalt and nickel. The separation effect is good, the metal yield is high, the material liquid is adaptable, and the process is easy to control automatically. With the development of new extractants, extraction systems and the gradual improvement of extraction theory, solvent extraction has become more and more widely used in cobalt-nickel hydrometallurgy.
According to the theory of crystal field coordination, Ni(II) in solution is stable when it is 6 coordination, while Co(II) is close to 4 or 6 coordination. It can exist at the same time and can be converted under certain conditions. . At present, the widely used phosphorus extractant is used to separate cobalt and nickel.
At present, phosphorus (phosphine)-based extractants for cobalt and nickel separation mainly include P204, P507 and Cyanex272, which have large differences in the extraction of cobalt and nickel. According to reports, when extracting cobalt and nickel with P204, P507 and Cyanex272, the pH values ​​of the semi-extraction were 0.53, 1.43 and 1.93, respectively. Obviously, the ability to extract and separate cobalt and nickel is gradually enhanced. This difference is due to the extraction capacity and spatial structure of the three extractants, as shown in Table 2.
Table 2 Comparison of three phosphorus (phosphine) extractants
Pk a indicates the strength of the ability of the extractant to bind metal ions. Obviously, P204 has the strongest binding ability to metal ions. The R-P-R' bond angle in the structural formula of organophosphorus (phosphine) extractants can be used to measure the size of steric hindrance. When the coordination compound of the octahedral configuration is generated, the larger the ∠RPR', the larger the steric hindrance between the different phosphoric acid substituents, and the more unfavorable the formation of the octahedral configuration. Therefore, when the binding ability of the extractant is lowered and the steric hindrance is increased, the octahedral configuration is difficult to form, and other small molecules such as water molecules are easily involved in the coordination. In the tetrahedral configuration, two organophosphorus are simultaneously located in one central ion, and four Os are in a plane perpendicular to each other. The interaction between the ligands is low, and the RPR' has little effect on the tetrahedral configuration.
The total reaction formula for the separation of cobalt and nickel by organophosphorus extraction can be expressed as:
Under the condition of a large excess of extractant, when M is Co, n=2; when M is Ni, n=3. For saturated extraction, no matter cobalt or nickel, n=1. Cobalt extracts include both tetrahedral and octahedral configurations, while nickel has only an octahedral configuration. The tetrahedral extract has a lower water content than the octahedral extract and has a higher lipophilicity, so the cobalt preferentially enters the organic phase.
From P204, P507 to Cyanex272, the acidity gradually weakens and the steric hindrance gradually increases. Nickel extracts always maintain an octahedral configuration, and the difficulty of the extractant to form a 6-coordinate with nickel increases, so the nickel distribution ratio decreases. However, the cobalt extract can be converted to a tetrahedral configuration, compensating for the effect of reduced acidity of the extractant and increased steric hindrance on the distribution ratio. The distribution ratio of nickel is reduced, while the distribution ratio of cobalt is basically unchanged, and the separation effect of cobalt and nickel is getting better and better. Therefore, cobalt and nickel separation can be better achieved by using an extractant having weaker extraction bonding strength and greater steric hindrance.
(II) Coordination of 4-coordinate anion of Co(II)
The ligand with higher electronegativity has weaker coordination ability, and preferentially forms the outer rail type 4 coordination sp 3 hybridization. Since Co(II) preferential Ni(II) forms a 4-coordination, a suitable electron-positive ligand is selected to control the appropriate concentration, and Co(II) can be preferentially combined to increase the extraction and separation of cobalt and nickel. .
1. SCN - selective coordination
SCN- has a large electronegativity, and forms a stable tetrahedral anion complex Co(SCN) 4 2- with Co 2+ at a certain concentration, and hardly forms a stable complex with Ni 2+ . Therefore, in this system, cobalt exists as a complex anion, nickel exists as a hydrated cation, and cobalt can be selectively extracted from a nickel-containing solution using MIBK, an amine, a quaternary ammonium extractant:
The capacity of quaternary ammonium salt extraction cobalt is proportional to the concentration of SCN - in the organic phase and is suitable for the extraction of cobalt from low concentration cobalt solutions. However, cobalt loaded organic phase is required NH 3 - NH 4 HCO 3 solution was back-extracted, and back-extracted cobalt liquid ammonia requires special equipment for recycling, greater production costs.
2. Selective coordination of Cl -
When Cl - mass concentration is 200-250 g/L, about 90% of Co(II) exists in the form of CoCl 4 2- , and metal ions such as Cu 2+ , Fe 3+ and Zn 2+ also form an anionic CuCl 4 . 2- , FeCl 4 - , ZnCl 4 2- , while Ni 2+ is still present in the form of hydrated cation [Ni(H 2 O) 6 2+ ]. The amine (ammonium) type extractant can be used to extract the complex anion to achieve separation from nickel.
The separation effect of the process is good, the extractant is low in price, and the chlorination of cobalt sulfide and nickel ore is smoothly connected. The nickel and cobalt plants established in the 1960s and 1970s mostly adopt this system. More representative ones are: Canada Eagle Bridge Company in the nickel plant in Kristiansund, Norway, using a tertiary amine to separate cobalt and nickel from the chloride system; domestic Chengdu Electrometallurgical Plant, Fuzhou Smelter, etc. all use chloride system to N235 Extraction and separation of cobalt and nickel.
(3) Cobalt oxidation to internal orbital complex ions
According to the valence bond theory, when Co(II) and Ni(II) are combined with a lower electronegative ion, Co(II) is easily oxidized to form a very stable internal orbital Co(III) complex ion. If the ion is oleophilic, a stable extract is formed which is preferentially extracted, and if it is hydrophilic, it is not extracted.
1. Ammonia-ammonium system
NH 3 can form Co(II) with the outer rail type complex Co(NH 3 ) 6 2+ . Due to a 3d electron transition to 5s orbital, the complex is easily oxidized to a more stable internal orbital complex Co. (NH 3 ) 6 3+ , see Table 3.
Table 3 Stability constants of cobalt and nickel-ammonia complexes (18 to 25 ° C, i = 0.1)
In the ammonia-ammonium system, the concentration of ammonia and the solution potential are controlled to ensure that cobalt and nickel in the solution exist in the form of Co(NH 3 ) 6 3+ and Ni(NH 3 ) 6 2+ , respectively . Since the stability constant of Co(NH 3 ) 6 3+ is 10.26 times that of Ni(NH 3 ) 6 2+ , a chelating extractant with a stronger nickel than NH 3 can be substituted for Ni (NH 3 ). Selective extraction of nickel by NH 3 in 6 2+ .
In 1987, Australia's Queensland Company used Henkel's LIX84-I extractant to selectively extract nickel from air-oxidized cobalt and nickel-containing ammonia solutions, and then back-extracted with a sulfate solution to obtain a nickel sulfate solution. High quality cathode nickel is obtained by electrowinning. The remaining cobalt in the solution was precipitated with H 2 S to give a CoS product.
2. Chelating extraction system
When cobalt and nickel are extracted by a chelating extractant, cobalt poisoning is likely to occur because the formed Co 2+ chelate is easily oxidized to a Co 3+ chelate. The Co 3+ chelate is very stable, difficult to be directly back extracted by the acid, and needs to be back extracted under reducing conditions. However, since the back extraction requires a large amount of reducing agent, and Co 3+ has a certain decomposition effect on the extracting agent, the method has not been applied to a large scale.
(4) Ni(II) 6-coordinate synergistic extraction
The 6-coordinate extract of nickel has higher stability and hydrophobicity, but has higher steric hindrance, so it is necessary to add some co-extraction agent that replaces bound water during the extraction process.
1. Synergistic extraction of acid extractant and non-chelating quinone
The South African Mineral Process Association found that the addition of non-chelating 2-ethylhexyl hydrazine (EHO) to alkyl phosphates (DEH-PA) has a great synergistic effect on nickel, but the effect on cobalt is small. many. The synergistic mechanism is that EHO provides better lone pair electrons than H 2 O or DEHPA, which can be easily substituted to achieve a stable 6-coordination configuration. The naphthenic acid also has a strong synergistic effect with isotridecyl hydrazine. The pH of nickel is shifted to 2.8 to the left, the pH of cobalt is shifted to the left by 1.8, and the pH of cobalt and nickel is extended to 1.2. Cobalt and nickel are completely separated. The acid extractant and the non-chelating extractant synergistically extract cobalt and nickel, and the extraction rate is fast, and there is no problem that the cobalt is oxidized.
2. Synergistic extraction of acid extractant and chelated quinone
A certain amount of LIX63 is added to the phosphoric acid, carboxylic acid and sulfonic acid extractant, which has strong synergistic effect on the extraction of cobalt and nickel, and the stronger the acidity of the acidic extractant, the more the E-pH line shifts to the left. The synergy is stronger. The shortcoming of this system is that the extraction and stripping rate of nickel is slow, and back extraction requires a certain acidity, while LIX63 degrades under strong acidic conditions. These two problems have not been solved in essence, so the system has not been applied in the late 1990s. The anti-degradation alkylpyridine methylamine and DNNS synergistic extraction system developed afterwards showed excellent performance, but the cost was high and industrialization was not achieved.
Australia has developed a synergistic extraction process between a carboxylic acid extractant and a chelating quinone extractant: synergistic extraction with a very weak acid carboxylic acid extractant and a oxindole extractant reduces the oxonium degradation rate; Accelerate the extraction and stripping rates; there is no cobalt poisoning during the extraction process. However, the disadvantage is that the separation coefficient of cobalt and nickel is not too large, and the separation process requires more stages of washing. When nickel is extracted, LIX63 is an extractant and carboxylic acid is a synergist; when cobalt is extracted, carboxylic acid is an extractant and LIX63 is a synergist.
Third, the conclusion
With the gradual depletion of high-quality cobalt-nickel sulfide ore resources, the development and utilization of cobalt-nickel oxidized ore has been paid more and more attention. Pressurized acid leaching and sulfuric acid heap leaching technology have become the mainstream technology of cobalt and nickel hydrometallurgy, so it is urgent to develop The process of extracting cobalt nickel directly from a higher acidity system (DSX), and preferably has an inhibitory effect on calcium, magnesium and the like. The main research directions are: 1) the development of new extractants, especially chelating extractants, which may be the preferred extractant for extracting cobalt and nickel directly from calcium-containing magnesium solutions in the future; 2) developing new extraction systems, along with Extraction theory, especially the development of synergistic theory, study the synergistic effect of extractant to achieve better separation; 3) Develop new extraction equipment, the thermodynamic values ​​of some cobalt and nickel extraction processes are very good, but the kinetics The rate is slow and new extraction equipment is needed to enhance the extraction process.
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