Sulfuric acid roasting decomposition technology of fluorocarbon antimony-monazed mixed rare earth concentrate

The sulfuric acid roasting method is divided into two processes of low temperature (below 300 ° C) calcination and high temperature (about 750 ° C) calcination according to the calcination temperature. The main difference is that the two processes: high temperature firing process to concentrate the hardly soluble thorium generated pyrophosphate thorium during leaching and undecomposed mineral into the slag together with the discarded residue (due to excessive radiation should be housed ); the barium in the concentrate during the low-temperature roasting process produces soluble barium sulfate, which enters the leachate together with the rare earth during the leaching process, and is further separated. Since the high-temperature calcined product consumes less chemical raw materials in the leaching and purification process, has a short process flow and relatively high economic efficiency compared with low-temperature roasting, and is widely used by manufacturers.

1. Decomposition reaction of sulfuric acid roasting process

The concentrated sulfuric acid and the mixed rare earth concentrate were uniformly stirred, and the differential thermal changes at different temperatures were tested on a differential thermal (DTA) instrument, and six distinct endothermic reaction peaks were found (see Fig. 1). The decomposition reactions corresponding to each peak are as follows.

The first endothermic peak (181 ° C), the peak width is about 150 ~ 300 ° C, mainly in the minerals of fluorocarbonate, phosphate, fluorite , iron minerals and other reactions with concentrated sulfuric acid:

2REFCO ₃ +3H ₂ SO ₄ =RE ₂ (SO ₄ ) ₃ +2HF↑+2CO ₂ ↑+2H ₂ O↑ (1)

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

CaF ₂ +H ₂ SO ₄ =CaSO ₄ +2HF↑ (3)

Fe ₂ O ₃ +3H ₂ SO ₄ =Fe ₂ (SO ₄ ) ₃ +3H ₂ O↑ (4)

Reaction of reaction product HF with SiO ₂ in minerals:

SiO ₂ +3HF=SiF ₄ ↑+2H ₂ O↑ (5)

In this temperature range, there is also a reaction of dehydration of phosphoric acid into pyrophosphoric acid, and pyrophosphoric acid and barium sulfate react to form insoluble pyrophosphate.

2H ₃ PO ₄ =H ₄ P ₂ O ₇ +H ₂ O↑ (6)

Th(SO ₄ ) ₂ +H ₂ P ₂ O ₇ =ThP ₂ O ₇ +2H ₂ SO ₄ (7)

The reaction tendency to form yttrium pyrophosphate increases with increasing temperature, and when the baking temperature exceeds 200 ° C, the amount of ThP 2 O 7 is significantly increased.

Fig.1 Differential heat curve of concentrated sulfuric acid roasting in mixed concentrate (DTA)

The chemical reaction corresponding to the second endothermic peak (328 ° C) is mainly the decomposition reaction of sulfuric acid:

H ₂ SO ₄ =SO ₃ ↑+H ₂ O↑ (8)

The third endothermic peak (400 ° C) is the reaction of decomposing ferric sulphate into basic ferric sulphate and pyrophosphoric acid:

Fe ₂ (SO ₄ ) ₃ =Fe ₂ O(SO ₄ ) ₂ +SO ₃ ↑ (9)

H ₄ P ₂ O ₇ =2HPO ₃ +H ₂ O (10)

The fourth endothermic peak (622 ° C) and the fifth endothermic peak (645 ° C) partially overlap, indicating that there are at least two chemical reactions at the calcination temperature of 600-700 ° C, but the currently determinable reaction is alkali Sulfate decomposition reaction:

Fe ₂ O(SO ₄ ) ₂ =Fe ₂ O ₃ +2SO ₃ ↑ (11)

The sixth endothermic peak appears at 800 ° C, at which temperature the rare earth sulfate will decompose the basic rare earth sulfate. When the calcination temperature exceeds 1000 ° C, the basic ferric sulfate is further decomposed into rare earth oxide:

RE ₂ (SO ₄ ) ₃ =RE ₂ O(SO ₄ ) ₂ +SO ₃ ↑ (12)

RE ₂ (SO ₄ ) ₃ =RE ₂ O ₃ +2SO ₂ ↑ (13)

As can be seen by the above reaction: 1 main component, concentrate bastnaesite, monazite, fluorite, iron, silica and the like can be decomposed at 300 deg.] C before the sulfuric acid, is converted into a soluble rare earth minerals sulfate, This facilitates the recovery of rare earths during the leaching process; 2. The strontium (Th ₃ (PO ₄ ) ₄ ) in the presence of phosphate is first decomposed by sulfuric acid into soluble sulfate before 300 ° C, and then the decomposition products of sulfate and H3PO4 Pyrophosphoric acid and metaphosphoric acid react to form poorly soluble ThP ₂ O ₇ and Th(PO ₃ ) ₄ . When the calcination temperature is higher than 250 ° C, the reaction tendency of barium sulfate to form a poorly soluble compound increases, and the amount of leachate remaining at the time of leaching increases, whereas when it is below 200 ° C, the tendency of barium sulfate to form a poorly soluble compound decreases, and when leaching The amount of rare earth entering the solution increases. In industrial production, the process route should be determined based on the chemical form and solubility properties of rhodium in the calcined product. In order to prevent radioactive elements from damaging the health of workers and environmental pollution, it is hoped that the first process (leaching) process after the decomposition of concentrates will separate and recover the strontium; 3. Increasing the calcination temperature is beneficial to the decomposition of rare earth minerals, but Excessive temperature (above 800 °C) rare earth sulfate will decompose into basic sulfuric acid rare earth, and even rare earth oxide, which will reduce the leaching rate of rare earth, which is unfavorable for the recovery of rare earth.

Second, factors affecting the decomposition of concentrate

The roasting process of the rare earth concentrate is carried out in a rotary kiln. The rare earth concentrate mixed with concentrated sulfuric acid is continuously added from the tail of the rotary kiln, and moves toward the kiln head as the kiln body rotates. The rotary kiln is an internal heat type, and the heavy oil combustion chamber is arranged at the kiln head. The combustion gas directly heats the material by radiation, and the calcination reaction gas and the combustion gas are discharged from the kiln tail and sent to the purification system through the exhaust fan. The temperature in the kiln gradually rises from the kiln tail to the kiln head. According to the reaction process of the material in the kiln, the kiln body can be roughly divided into a low temperature zone (kiln tail part) with a temperature range of 150-300 ° C; a medium temperature zone (kiln body part), a temperature range of 300-600 ° C; and a high temperature zone (kiln Head part), the temperature range is 600 ~ 800 °C. According to the above decomposition reaction, the main function of the low temperature zone is the decomposition of rare earth minerals by sulfuric acid, and the chemical reaction belongs to the solid-liquid-gas heterogeneous reaction; however, since the porous film is formed on the surface of the concentrate particles during the reaction, the diffusion is caused. The process is relatively simplified. For the sake of discussion, it is assumed that the amount of sulfuric acid is large, the acid concentration in the reaction process is constant, the resistance caused by the liquid-solid phase diffusion membrane is extremely small, that is, the diffusion step can be neglected, and the decomposition reaction rate is mainly controlled by the chemical reaction step. The calcination reaction kinetic equation can be expressed by the following formula.

1-(1-x) ^1/3 =(kc _o /ργ ₀ )t (14)

The reaction fraction of the X-rare earth mineral in the formula (or the rate of decomposition of the concentrate);

Ρ-concentrate density;

K-chemical reaction rate constant;

c _o - initial concentration of sulfuric acid;

r _O - the particle size of the concentrate;

T-reaction time.

The factors affecting the decomposition of rare earth concentrates in the sulfuric acid roasting process using the kinetic equation are discussed below.

(1) Effect of roasting temperature

The reaction kinetics of concentrated sulfuric acid calcined mixed rare earth concentrate is limited by the chemical reaction rate. According to the Arrhenius formula, the chemical reaction rate constant K is related to the reaction temperature T.

K=Z·e ^{( -E/RT)} (15)

a constant in the formula where Z- is independent of reactant concentration and temperature;

E-activation energy;

K-Arrhenius Tungsten formula reaction rate constant, K = kc _o / ργ ₀ ;

T-temperature;

R-gas constant.

When the baking temperature T is raised, the reaction rate constant K is increased to increase the decomposition rate X. In the high-temperature enhanced sulfuric acid roasting process, in order to strengthen the decomposition reaction of rare earth minerals, the rare earth is converted into soluble sulfate, while the non-rare earth elements such as antimony, phosphorus, iron and calcium are in the form of pyrophosphate and insoluble sulfate. Usually, the reaction temperature is controlled at 300-350 ° C, the kiln temperature (ie, low temperature zone) is controlled at about 250 ° C, and the kiln head temperature (high temperature zone) is controlled between 680 and 750 ° C. If the temperature is too low, the decomposition rate is slow, the decomposition is incomplete, and the ruthenium is dispersed in the solution and the leaching slag during leaching, which is inconvenient to recover; when the baking temperature is higher than 800 ° C, the rare earth sulphate is decomposed into insoluble RE ₂ O ( SO ₄ ) ₂ and RE ₂ O ₃ enter the slag upon leaching, resulting in a decrease in the recovery of rare earth. For the purpose of roasting process for further recovery of strontium into the solution during leaching, it is necessary to reasonably select the roasting completion to prevent the temperature from being too high, and the pyrophosphate is retained in the slag, the temperature is too low, and the decomposition of the rare earth mineral is incomplete. The decomposition rate is too low.

(2) Effect of sulfuric acid dosage on decomposition rate

Sulfuric acid is used as a reactant to infiltrate around the concentrate particles before the reaction. When the concentration of the surrounding sulfuric acid c _{O is} higher, the decomposition rate x is larger. Therefore, the amount of sulfuric acid added is generally excessively calculated in production. In fact, the amount of sulfuric acid is related to the concentrate grade. The lower the grade of the concentrate, the more acid is consumed, because the fluorite, iron ore and other impurities in the mineral consume sulfuric acid. In addition, the loss caused by the decomposition of sulfuric acid at the calcination temperature must also be considered.

(3) Effect of roasting temperature

From the kinetic expression of the calcination reaction of sulfuric acid and the formula of Arrhenius, it can be visually seen that the decomposition rate x increases with the increase of temperature T. However, it should be noted that too long a time will prolong the production cycle and reduce the processing capacity of the rotary kiln. From the previous decomposition reaction of sulfuric acid, it is known that in the low temperature region, the region where the rare earth mineral decomposes, prolonging the decomposition time is beneficial to the improvement of the decomposition rate, and for the middle and high regions, the prolongation of time causes the decomposition of sulfuric acid and the rare earth insoluble compound. The formation, and thus the increase in sulfuric acid consumption and the decrease in the yield of the rare earth. This means that it is important to control the length of each temperature section of the rotary kiln.

(4) Influence of concentrate size

Due to the strong ability of sulfuric acid to impregnate minerals and the porosity of solid products, the diffusion rate of reactants and products is large. Therefore, the concentration of concentrate sulfuric acid roasting process is more relaxed, generally less than 200 mesh. However, if the particle size is too large, the surface area of ​​the concentrate will be reduced, and the reaction rate and decomposition rate will be lowered.

Third, the leaching rate and purification of rare earth

The products calcined in the rotary kiln differ according to the different chemical properties of the calcination temperature, and thus the leaching and purification processes adopted are also different. The high-temperature intensification roasting method is used, and impurities such as barium, calcium, iron and phosphorus in the calcined product are all present in a poorly soluble compound, and are left in the slag during leaching, so as to facilitate separation from the rare earth and simplify the purification process of the leachate. For the low-temperature calcination product, in the industrial production, the rare earth sulfuric acid double salt is firstly dissolved in water and the acid solution is separated from the impurities such as iron and calcium, and then the solvent is extracted or the hydrochloric acid solution is used to separate the ruthenium (see Fig. 2); For the high temperature calcined product, a small amount of phosphorus, iron and ruthenium in the leachate is removed by neutralizing the residual acid with MgO and adding FeCl ₃ (see Fig. 3).

Figure 2 Principle of the principle of extracting rare earth from sulfate solution by double salt of sulfuric acid

Fig. 3 Process flow of high temperature sulfuric acid roasting mixed rare earth concentrate and pretreatment principle

In view of the current industrial application of high-temperature roasting process to decompose mixed rare earth concentrates, the paper will mainly describe the leaching and purification process of high-temperature roasting products.

(a) leaching

The rare earth in the calcined product has been converted into a soluble sulfate, and the product contains a small amount of residual sulfuric acid. Generally, it is not necessary to add sulfuric acid when leaching, and it can be directly leached with water. Because the solubility of rare earth sulfate in water is low, the REO at room temperature is only 40g/L for the mixed rare earth group, and it decreases with the increase of temperature. Therefore, in order to ensure the complete leaching of rare earth during leaching, there should be a larger liquid. Solid ratio while keeping the temperature as low as possible. The calcined product should not be stored for a long time after it exits the kiln, otherwise it will form a slow-dissolving aqueous salt. Generally, the hot calcined material is directly added with water to make a slurry, and then pumped into the leaching tank, and leached at a solid-liquid ratio of 1: (10-15) under stirring.

(two) leachate purification

The rare earth concentrate which is calcined at a high temperature can remove most of the poorly soluble non-rare earth impurities during leaching. In order to ensure the full leaching of rare earth, the leaching acidity is generally controlled to be about 0.2mol/L. Under such conditions, the leaching rate of rare earth can reach more than 95%. However, due to the high acidity of leaching, the leaching solution still contains a small amount of calcium, iron and phosphorus. Silicon, aluminum , titanium and trace amounts of antimony affect the subsequent extraction and separation process and the quality of mixed rare earth chloride and rare earth carbonate. The method of removing these impurities in production is as follows.

First, FeCl3 is added to the leachate to adjust Fe/P=2~3, so that phosphorus forms FePO ₄ precipitate:

FeCl ₃ +H ₃ PO ₄ =FePO ₄ +3HCl (16)

Then, MgO is added to the leachate to adjust pH=4.0-4.5 to hydrolyze Fe ₂ (SO ₄ ) ₃ and Th(SO ₄ ) ₂ in the leachate into hydroxide precipitate:

Fe ₂ (SO ₄ ) ₃ +6MgO+3H ₂ SO ₄ =2Fe(OH) ₃ ↓+6 MgSO ₄ (17)

Th(SO ₄ ) ₂ +4MgO+2H ₂ SO ₄ =Th(OH) ₄ ↓+ ₄ MgSO ₄ (18)

The leachate also contains silicic acid and fine particles of calcium sulfate, which makes filtration and washing operations difficult. A small amount of polyacrylamide coagulant can be added to promote coagulation and increase filtration speed.

4. Preparation of mixed rare earth products due to leachate

The purified leachate can be used as a raw material for the separation of rare earths to enter the extraction plant to separate a single rare earth. It is also possible to prepare crystallized mixed rare earth chloride and mixed rare earth carbonate as needed.

(1) Preparation of crystalline rare earth chloride

To prepare crystalline rare earth chloride from a rare earth sulfate solution, it is first necessary to convert the rare earth sulfate solution into a rare earth chloride solution. The conversion method can be generally divided into solid precipitation-hydrochloric acid dissolution and solvent extraction-hydrochloric acid stripping. The latter has the advantages of convenient connection with the prior process and further purification of the rare earth solution and production cost. The rare earth chloride solution generally contains RE-200-280 g/L, and after evaporation, the REO is concentrated to about 450 g/L, and the crystallized RECl ₃ ·nH ₂ O product is obtained by cooling. In order to increase the speed of evaporation in production, it is usually concentrated under reduced pressure. When the vacuum in the evaporation can is maintained at 6 × 10 ⁴ Pa by a water jet, the boiling point of the rare earth chloride solution can be lowered to about 14 °C.

(2) Preparation of rare earth carbonate

To the leach solution containing REO of 40 to 60 g/L, ammonium hydrogencarbonate (solid or liquid) may be added to produce a rare earth carbonate precipitate according to the reaction formula (19). The precipitated rare earth carbonate is washed with water to remove the adsorbed sulfate, and the obtained RE(CO ₃ ) ₃ ·nH ₂ O product is filtered.

RE ₂ (SO ₄ ) ₃ +6NH ₄ HCO ₃ =RE ₂ (CO ₃ ) ₃ +3H ₂ O+3CO ₂ (19)