Sodium Hydroxide Decomposition Process Technology of Fluorite-Monthite Mixed Rare Earth Concentrate

The fluorocarbon antimony -mononite mixed rare earth concentrate can also be treated by sodium hydroxide solution decomposition process (see Figure 1 for the process flow), but since the fluorine in the concentrate is higher than monazite, the alkali decomposition equipment is seriously corroded, as still The steam jacket used for the decomposition of monazite is heated, the equipment has a short life and the operation is extremely unsafe. The production should be carried out by directly heating the material. A factory inserts three electrodes into a steel decomposition tank, which is a current that passes through a mixture of concentrate and sodium hydroxide, which uses the resistance of the material itself to generate heat and decompose the concentrate. This method is called electric field decomposition. Compared with the jacket heating method, the method has the advantages of high concentrate decomposition rate, low energy consumption, and low alkali consumption. Another characteristic of mixed rare earth concentrates is that the calcium content is higher than that of monazite, and the direct sodium hydroxide decomposition of rare earth yield is very low, and calcium must be removed before decomposition.

Figure 1 Process flow of sodium hydroxide decomposition mixed rare earth concentrate

1. Hydrochloric acid soaked in calcium

In mixed concentrates, CaO in the form of fluorite (CaF 2 ), dolomite (CaCO 3 , MgCO 3 ), and calcite (CaCO 3 ) is about 7%. In alkali decomposition, fluorite is difficult to be decomposed, while other calcium mineral decomposition products easy to form insoluble calcium hydroxide and trisodium phosphate in the role of calcium lye. After the alkali is decomposed, part of the calcium enters the acid dissolution step together with the rare earth hydroxide in the form of calcium fluoride and calcium phosphate. In the acid dissolution process, CaF 2 and Ca 3 (PO 4 ) 2 are decomposed by hydrochloric acid, and HF and H3PO4 appearing in the solution cause RE3+ to form REF3 and REPO4 to precipitate in the slag, resulting in loss of rare earth. The removal of calcium by hydrochloric acid before alkali decomposition can effectively avoid the loss of rare earth. The chemical form of rare earth minerals in the process of leaching and removing calcium by hydrochloric acid has not changed substantially, so this method is also called chemical beneficiation and calcium removal. The chemical reaction of calcium in immersion in mixed concentrates is as follows:

CaF 2 +2HCl=2CaCl 2 +2HCl (1)

CaCO 3 +2HCl=CaCl 2 +H 2 O+CO 2 ↑ (2)

3REFCO 3 +6HCl=2RECl 3 +REF 3 ↓+3H 2 O+3CO 2 (3)

The dissolved rare earth reacts with hydrogen fluoride in the solution to form a hydrogenated rare earth and precipitates in the undecomposed rare earth mineral. Since the REF3 solubility product (Ksp = 8 × 10 -16 ) is smaller than CaF2 (Ksp = 2.7 × 10 -11 ), the chemical reactions shown in the formulas (1), (2), and (3) are continuously performed. The removal rate of calcium can reach more than 90%, and the loss rate of rare earth is not large (2% to 4%).

RECl 3 +3HF=REF 3 ↓+3HCl

The operating conditions for immersion calcium removal by hydrochloric acid are: soaking acidity = 2 mol/L; ratio of mineral acid to solid (liquid: 1:2); temperature = 90 to 95 ° C; time = 3 h. After removing calcium, the rare earth grade of the concentrate increases from 50% to 60% to 60% to 70%, and calcium ≤ 1%. The rare earth loss rate of the hydrochloric acid immersion calcium removal process increases with the increase of the calcium content in the concentrate, and the acid dosage also increases. Therefore, when the mixed rare earth concentrate is treated by immersion in hydrochloric acid and calcium-alkali decomposition process, the concentrate material with high rare earth grade and low calcium content should be selected.

Second, sodium hydroxide decomposition

After addition of hydrochloric acid immersion calcium rare earth concentrate at 60% to 65% sodium hydroxide solution and heated to a three-phase AC electrodes 160 ~ 165 ℃, rare earths, thorium, an acid soaking process occurs undecomposed fluorite The following chemical reactions:

REFCO 3 +3NaOH=RE(OH) 3 ↓+Na 2 CO 3 +NaF (4)

REPO 4 +3NaOH=RE(OH) 3 ↓+Na 3 PO 4 (5)

Th 3 (PO 4 ) 4 +12NaOH=3Th(OH) 4 ↓+4Na 3 PO 4 (6)

CaF 2 +2NaOH=Ca(OH) 2 ↓+2NaF (7)

3Ca(OH) 2 +2Na 3 PO 4 =Ca 3 (PO 4 ) 2 ↓+6NaOH (8)

At the same time, impurities such as iron and antimony in the concentrate react with sodium hydroxide to form corresponding hydroxides.

During the alkali decomposition process, the trivalent hydroxide of cerium will be further oxidized to tetravalent hydroxide, and its chemical reaction formula is as follows:

2Ce(OH) 3 +H 2 O+(1/2)O 2 =2Ce(OH) 4 (9)

After the decomposition is completed, in addition to the hardly soluble substances such as RE(OH) 3 , Th(OH) 4 , Ca (OH) 2 , Fe(OH) 3 , etc., there is excessive NaOH and decomposition in the precipitate (alkali cake). The soluble salts of NaF and Na3PO4 are highlighted. In industrial production, according to the solid-liquid ratio 1: (10 ~ 12), wash the precipitate with water of 60 ~ 70 ° C 6 ~ 7 times (washed to water pH = 8 ~ 9), remove soluble salts from it, so that the rare earth hydroxide is preliminary Purification. The decomposed waste lye still contains a lot of sodium hydroxide and can be recycled.

Third, hydrochloric acid dissolution

The washed precipitate (alkali cake) is dissolved in an acid-soluble tank by adding hydrochloric acid, so that the rare earth hydroxide is converted into a chloride into solution and separated from the undecomposed mineral and insoluble impurities. Also dissolved in the rare earth hydroxide are Th(OH) 4 , Fe(OH) 3 , Fe(OH) 2 , and the chemical reaction is as follows:

RE(OH) 3 +3HCl=RECl 3 +3H 2 O (10)

Ce(OH) 4 +4HCl=CeCl 3 +4H 2 O+(1/2)Cl 2 (11)

Th(OH) 4 +4HCl=ThCl 4 +4H 2 O (12)

Ca(OH) 2 +2HCl=CaCl 2 +2H 2 O (13)

Fe(OH) 2 +2HCl=FeCl 2 +2H 2 O (14)

Fe(OH) 3 +3HCl=FeCl 3 +3H 2 O (15)

The rare earth concentration of the solution after acid dissolution is generally controlled at REO=200-300 g/L, and the acidity of the solution is at pH=1~2. There is usually a small amount of rare earth mineral in the acid-soluble slag. In order to fully recover the rare earth, it is returned to alkali decomposition after washing with water.

Fourth, the purification of rare earth chloride solution

The reaction of formula (10) to (15) with hydrochloric acid dissolution process can be known, the acid-soluble rare earth chloride solution to obtain RE 3 +, in addition, also contains Th 4 +, Fe 3 +, Fe 2 +, based on their melting The difference between the pH and the pH of the hydrolysis can be removed one by one from the solution.

V. Recovery of rare earth from slag

The slag contains undissolved concentrates such as antimony and iron, and the rare earth content (REO) in the slag is more than 10%. In this regard, the rare earth is generally recovered by a method of completely dissolving sulfuric acid. Its main dissolution reaction is as follows:

2REPO 4 +3H 2 SO 4 =RE 2 (SO 4 ) 3 +2H 3 PO 4 (16)

2REF 3 +3H 2 SO 4 =RE 2 (SO 4 ) 3 +6HF (17)

Th(OH) 4 +2H 2 SO 4 =Th(SO 4 ) 2 +4H 2 O (18)

2Fe(OH) 3 +3H 2 SO 4 =Fe 2 (SO 4 ) 3 +4H 2 O (19)

The solution eluted by sulfuric acid is precipitated by separation of iron and the like by the double salt of sulfuric acid, and the double salt of sulfuric acid of rare earth and lanthanum is converted into hydroxide by a sodium hydroxide solution at 90 ° C, and then the rare earth and lanthanum are separated by a hydrochloric acid solution process.

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