If the leachate is alkaline, the impurity content is low; if it is neutral, especially acidic, the impurity content is high. A conventional means of purifying impurities is hydrolysis precipitation or addition of a precipitant. Extractants or ion exchange resins are also used in some cases.
First, precipitation and impurities
Precipitation and impurity removal are based on the principle of solubility product. Metal cations such as iron, magnesium, manganese, etc. can often precipitate removed after hydrolysis. Anion 
Alternatively, it can be removed by adding a precipitant. The purification effect depends mainly on the pH value and the type and amount of precipitant.
Impurity | Fe 2 - | Mn 2 + |
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Hydrolyzed pH | 10~12 | 10~12 | 9 to 10 | 9 to 10 | 9.5~11 | 8~9 |
Precipitant | Mg 2 + | Mg 2 + | Mg 2 + , | Ca 2 + | ||
Temperature / °C | 90 | 90 | ||||
Precipitate | Fe(OH) 2 | Mn(OH) 2 | MgCrO 4 | MgSiO 3 | MgNH 4 PO 4 | Ca 3 (PO 4 ) 3 |
2pH below 8, Easy hydrolysis When the pH value is greater than 9, Ca 2 + is easily hydrolyzed to form Ca(OH) 2 , which will reduce the purification efficiency.
Second, solvent extraction
The solvent extractant can effectively extract vanadium into the organic phase, and finally back-extraction to obtain a vanadium-containing solution. At the same time, the original low vanadium solution can be concentrated and enriched. The initial extraction of vanadium is mainly used to extract vanadium from uranium- containing solutions. Many extractant used to extract the vanadium as tributyl sulfate (TBP), bis (2-ethylhexyl phosphoric acid) (D2EHPA), an amine compound. Its representative response is as follows:
For tetravalent vanadium nVO 2 + +m[(HA) 2 ]=(VO) n A 2n (HA) 2 ( m - n ) +2 n h +
Pentavalent vanadium
Where [HA] represents D2EHPA, the extractant concentration is generally 0.4 mol/L, and the pH value is 2. Since D2EHPA has higher selectivity to tetravalent vanadium, a reducing agent such as iron powder, Na 2 S, NaSH, etc. may be added before extraction to reduce the vanadium vanadium to tetravalent. The stripping agent uses dilute sulfuric acid or a 10% Na 2 CO 3 solution.
The use of amyl acetate to extract and separate vanadium and uranium from a mixture of hydrochloric acid and sulfuric acid has high efficiency. When the concentration ratio of HCl/V is 3 or 6 mol/1 mol, an equal volume of concentrated sulfuric acid is added to the pre-extraction solution, and the separation factor of vanadium to uranium can reach 150/1, 1000/1, respectively, so preferential extraction is possible. Vanadium, so that uranium and vanadium can be separated efficiently.
When an amine extractant is used, a secondary amine, a tertiary amine, a quaternary amine extractant can be used. The aqueous medium is HCl, H 2 SO 4 , acid concentration 0.5 mol/L, pH=3, metal 1 g/L, organic phase 0.1 mol/L, diluent is n-octane, and the vanadium partition coefficient is greater than 200. Therefore, it is easy to be extracted.
When an anionic amine extractant is used, it can only extract an anionic vanadate, a pentavalent vanadium ion. For this purpose, the low-valent vanadium should be completely oxidized to pentavalent vanadium by using hydrogen peroxide before extraction. Amine extractant can extract vanadium over a wide pH range, and stripping can be used Ammonia solution, after stripping At higher pH values, it will be converted to ammonium metavanadate crystals and precipitated. The amine extractants commonly used in the industry are tertiary amines (N235) and quaternary amines (N263), and their properties for extracting vanadium are shown in Fig. 1. As can be seen from the figure, the extraction rate of the tertiary amine is highest at pH = 2 to 3, while the quaternary ammonium salt maintains a high extraction rate at pH = 5 to 9.5.
Fig.1 Relationship between extraction rate and pH of vanadium extractant
Third, the ion exchange method
The use of an anion exchange resin can effectively adsorb vanadate. Commonly used resins are Amberlite, IRA-400, IRA-401, IRA-402, IRA-410, IRA-420, and DOWEX-1, DOWEX-2, and the like. They are all highly alkaline, high exchange capacity resins containing chloride ions. The exchange reaction is as follows:
Wherein R represents a resin. The above reaction is reversible. When the solution of Cl - at low concentrations (e.g. less than 1 mol / L), pH 6 to 7.2 carry out the reaction to the right. When the Cl - ion concentration is sufficiently high (for example, up to 4 mol/L), the above reaction will desorb the resin. Will be rinsed back to the solution. The equilibrium formula of the above reaction is as follows:
K=[RV] [Cl] 4 /[S-RV] [V]
Where K-equilibrium constant, for Amberlite IRA-402, K=86;
Vanadium ion concentration on RV-resin;
S-resin total exchange capacity;
[V]-the concentration of vanadium ions in the solution;
[Cl] - Chloride ion concentration in the solution.
If vanadium in the solution exists in a tetravalent state, it is not adsorbed by the above anion exchange resin because of VO 2 + cation. To this end, an oxidizing agent such as NaClO 3 is required to oxidize the tetravalent vanadium to vanadium vanadium for adsorption. Therefore, the leaching of the vanadium-containing resin can also be washed with a reducing agent such as an aqueous SO 2 solution, and the vanadium vanadium is reduced and desorbed from the resin.
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