Other decomposition process technology of monazite rare earth concentrate

Under the calcination conditions of 200 to 240 ° C, concentrated sulfuric acid can effectively decompose monazite. The rare earth, lanthanum and uranium in the decomposition products are present as RE ₂ (SO ₄ ) ₃ , Th(SO ₄ ) ₂ and UO ₂ SO _{4 , respectively} . Sodium carbonate is mixed with monazite and calcined at a temperature of 600 to 700 ° C to decompose monazite. Whether it is alkali decomposition or sulfuric acid roasting decomposition method, there are disadvantages such as large amount of waste water and high consumption of chemical raw materials. In response to these problems, mechanical ball milling and calcium oxide fluxing decomposition methods have appeared in recent years, but these two methods are still in the experimental research stage.

I. Mechanical ball milling decomposition of monazite rare earth concentrate

The principle of mechanical ball milling to decompose monazite minerals is to mix substances that have been decomposed into monazite (such as CaO, CaCl ₂ , NaOH, etc.) with monazite in a ball mill . Under high-speed rotation conditions, the ball-powder-ball and ball-powder-container collide, and the monazite and disintegrator powder undergoes repeated pulverization and cold welding processes, while the process is being performed on the welded surface. A decomposition reaction has occurred. The mechanical ball milling method can decompose the essence of monazite rare earth concentrate. In the ball mill, the mechanical energy generated by high-speed rotation is transmitted to the chemical reaction substance through collision, and is converted into chemical energy, so that the decomposition reaction speed is improved.

(1) Mechanical ball milling NaOH decomposition of monazite rare earth concentrate

The decomposition test was carried out in a ball mill containing 3.7 L of steel balls. The rotation speed of the ball mill was 100 r·min ^-1 , and the particle size of the concentrate was about 180 μm. The raw materials used in the test were monazite used in the production of a rare earth smelter. The chemical composition of the ore is shown in Table 1.

Table 1 Raw material components used in the test Unit: % (mass)

The following results were obtained in the experiment. 1. The decomposition rate increases with the increase of the amount of alkali, but when the amount of alkali is 1.6 times of the theoretical amount (0.65 times of the concentrate), the decomposition rate has reached 98.82%. 2. When the temperature rises, the decomposition rate increases. When the amount of alkali is 1.84 times of the theoretical amount (0.75 times of the concentrate), the decomposition rate can reach 99.23% within 3 hours when the temperature is 160 °C. 3. The amount of alkali is 2.08 times of the theoretical amount (0.85 times of concentrate), the temperature is 160 °C, the holding time has a significant effect on the decomposition rate, but when the amount of alkali is high, it can be achieved even within 2 hours. Higher decomposition rate. 4. According to the results of the conditional test, comprehensive tests were carried out on several different raw materials in Table 1. The results showed that the decomposition rate of other mines averaged 99.44% except for the lower decomposition rate of Korean mines, indicating that the process is correct. The application of raw materials should be strong. Meanwhile, when using trisodium phosphate plant return crystalline alkali decomposition rate up to 99.77%.

(2) Mechanical ball milling of CaO and CaCl2 to decompose monazite rare earth concentrate

15 steel balls (15 mm in diameter) were placed in a sealed steel tank, and then monazite, CaO, CaCl _{2 were} charged according to the experimental requirements, and decomposition tests were carried out under different atmospheres. After the end of the test, the monazite phase was analyzed by X-ray diffraction, and the phase disappeared and the monazite was completely decomposed. In the experiment, the western coastal sand monazite concentrate was selected. The purity of monazite was 98.5%, the rare earth content was 50.1%, the strontium content was 7.1%, and the concentrate particle size was about 0.1 mm. The following results were obtained in the experiment. 1. In a argon atmosphere, the mass ratio of the ball to the material is 15:1, and monazite: CaO:CaCl ₂ = 3: ₂ : ₂ (molar ratio). When the ball milling time reaches 12h, the monazite completely decomposes, and the decomposition products are rare earth oxides, cerium oxide, rare earth oxychloride, chloroapatite and undecomposed minerals. 2. Under the air atmosphere, other conditions are the same as in the argon atmosphere. Only when the ball milling time reaches 24h or more, the monazite is completely decomposed. The decomposition product differs from the argon atmosphere in that no rare earth oxychloride is produced.

Second, calcium oxide plus flux roasting decomposition of monazite rare earth concentrate

The monazite has high thermal stability and does not decompose at 1700 ° C in the air. Although CaO can decompose monazite at 700 ° C, the decomposition rate is very low and has no industrial significance. The addition of NaCl or other low melting point molten salts such as NaCl-CaCl ₂ , NaCl-KCl, NaCl-CaCl ₂ -KCl, etc. during the decomposition process can increase the decomposition rate and lower the decomposition temperature. The experimental study on the decomposition of monazite by CaO-NaCl showed that the decomposition rate of monazite increased from 5% to 78% compared with that without NaCl. The main reasons are as follows: 1. NaCl exists in the liquid phase, promotes the mass transfer process between the solid phase reactions, and increases the reaction rate; 2. NaCl participates in the decomposition reaction, and the solid single phase becomes a liquid phase. The phase reaction accelerates the reaction rate, thereby increasing the ability of CaO to decompose REPO ₄ . In addition to the formula (1), in addition to the formula (1), there is a reaction formula (2) in which NaCl and CaO act together to decompose monazite with the participation of NaCl. It should be noted that while the decomposition of the reaction proceeds, Ce ₂ O ₃ in the decomposition product is simultaneously oxidized to CeO ₂ , and the reaction is as shown in the formula (3).

3CaO+2REPO ₄ =Ca ₃ (PO ₄ ) ₂ +RE ₂ O ₃ (1)

15CaO+3NaCl+10REPO ₄ =3Ca ₅ Cl(PO ₄ ) ₃ +Na ₃ PO ₄ +5RE ₂ O ₃ (2)

Ce ₂ O ₃ +(1/2)O ₂ (air)=2CeO ₂ (3)

In the test range where the calcination temperature is 670 to 870 ° C and the CaO addition amount is 5% to 45%, the decomposition rate of monazite increases with the increase of temperature, CaO addition amount, and NaCl addition amount. When the calcination temperature is 870 ° C, and the CaO and NaCl addition amounts are 45%, the decomposition rate of monazite reaches 78.49%.