Sodium hydroxide decomposition technology of monazite rare earth concentrate

The monazite rare earth concentrate contains phosphorus , antimony and uranium . In order to recover these valuable components and prevent radioactive elements from dyeing products and the environment, sodium hydroxide decomposition and phosphorus alkali recovery should be included in the process of decomposing monazite from sodium hydroxide. Separation of rare earth and impurities and recovery of uranium and uranium. Figure 1 is a process flow used in the industry.

Figure 1 Process of sodium hydroxide decomposition of monazite rare earth concentrate

1. The chemical reaction of sodium hydroxide to decompose monazite rare earth concentrate

When the monazite is heated to 140-160 ° C in a solution of sodium hydroxide, the following decomposition reaction will occur:

REPO 4 +3NaOH=RE(OH) 3 ↓+Na 3 PO 4 (1)

Th 3 (PO 4 ) 4 +12NaOH=3Th(OH) 4 ↓+4Na 3 PO 4 (2)

U 3 O 8 in the monazite reacts with NaOH and O 2 in the air under agitation:

2 U 3 O 8 +O 2 +6NaOH=3Na 2 U 2 O 7 ↓+3H 2 O (3)

U 3 O 8 is actually a tetravalent and hexavalent complex oxide UO 2 ·UO 3 of uranium. Tetravalent uranium not oxidized by O 2 in NaOH solution reacts with NaOH to form hydroxide:

UO 2 +4NaOH=U(OH) 4 ↓+2Na 2 O (4)

In the case of a large excess of NaOH, U(OH) 4 is dissolved in the lye in the form of uranyl acidate:

U(OH) 4 +OH - =H 3 UO 4 - +H 2 O (5)

At the same time, minerals such as iron , titanium , aluminum , zirconium and silicon are also decomposed by NaOH:

Fe 2 O 3 +2NaOH=2NaFeO 2 +H 2 O (6)

TiO 2 +2NaOH=Na 2 TiO 3 +H 2 O (7)

Al 2 O 3 +2NaOH=2NaAlO 2 +H 2 O (8)

SiO 2 +2NaOH = Na 2 SiO 3 + H 2 O (9)

ZrSiO 4 +4NaOH=Na 2 ZrO 3 ↓+Na 2 SiO 3 +H 2 O (10)

ZrSiO 4 +2NaOH = Na 2 ZrSiO 5 ↓ + 2H 2 O (11)

The decomposition products of iron, titanium, aluminum minerals and quartz are all dissolved in an alkali solution, and are separated from rare earth and barium and sodium diuranate in the presence of poorly soluble hydroxide.

Second, factors affecting the decomposition of concentrate

The reaction of sodium hydroxide to decompose monazite belongs to a solid-liquid heterogeneous reaction. The decomposition reaction is first carried out on the surface of the mineral to form a solid hydroxide film. Due to the compactness of the solid film, the decomposition rate of monazite will be affected by the diffusion rate of NaON in the solid phase membrane, and the relationship between the decomposition rate and the process factors such as temperature, time, NaON concentration and grain size of the concentrate can be used to generate dense The kinetic equation for a solid product is expressed as:

1-2/3x-(1-x)2/3=(2MDc/αρr 0 2 )t (12)

In the formula, after the t time, the reaction fraction of the rare earth mineral (representing the decomposition rate of the rare earth mineral);

Molecular weight of M-monaz mineral;

The density of ρ-monaz minerals;

The concentration of the c-NaOH solution;

r 0 - the original radius of the concentrate particles;

Alpha-chemical calculation factor;

The diffusion coefficient of the D-reactant in solution.

According to the reaction velocity equation of the upper jaw, the influencing factors of the decomposition of monazite rare earth concentrate can be analyzed as follows.

(1) Influence of concentrate size

The decomposition rate (x) in equation (12) is inversely proportional to the square of the concentrate particle size (r o ). It can be seen that the particle size of the concentrate is an important factor affecting the decomposition rate, because the larger the particle size, the smaller the surface area of ​​the concentrate in contact with NaOH, and the slower the reaction rate. In fact, for the dense membrane formed on the surface of the concentrate, the dense membrane hinders the diffusion of NaOH into the deep part of the concentrate. Under this condition, the particle size of the concentrate is larger, and the reaction time is prolonged. The thicker the dense membrane on the surface of the ore, the slower the decomposition reaction, and thus the more incomplete decomposition of the concentrate. The production practice proves that when the particle size of the concentrate is below 0.043 mm, the decomposition rate can reach 98% or more.

Alkali decomposition in a hot ball mill is an effective method to solve the decomposition rate of particle size. For example, in a sealed hot ball mill, the monazite concentrate with a particle size of 0.5 to 1.5 mm is decomposed with NaOH, the NaOH concentration is 50%, the reaction temperature is 175 ° C, and the mineral surface is generated by the impact and friction of the steel ball during the decomposition process. The hydroxides are falling, and new surfaces are constantly exposed. At 4.5 to 6 hours, the monazite is almost completely decomposed. However, the loss of the hot ball mill, as well as the power consumption and small production capacity, limit the application of this method.

(2) Influence of reaction temperature and NaOH concentration

The diffusion of the reactants in the liquid phase and in the dense solid phase membrane is involved in the solid-liquid reaction in which the dense membrane is formed. In the initial stage of the decomposition reaction, the dense film on the surface of the concentrate is incomplete or very turbid. At this time, the diffusion is mainly carried out in the liquid phase, and the reaction temperature is increased to increase the diffusion coefficient in the liquid phase, thereby increasing the reaction rate. However, as the reaction time increases, the thickness of the dense film in the decomposition process of the monazite rare earth concentrate increases continuously. The diffusion speed is changed from the diffusion control in the liquid phase to the diffusion rate in the dense membrane. At this time, the reaction temperature is increased. The diffusion coefficient in the solid phase has little effect. If the reaction temperature is too high, it will cause the local temperature of the reactor to overheat and dehydrate the rare earth and barium hydroxides, reducing their solubility in the inorganic acid, resulting in rare earth in the acid dissolution process. The yield is reduced.

The determination of the reaction temperature is related to the concentration of NaOH. Since the concentration of NaOH is related to the boiling point of its solution, it is shown in Table 1.

Table 1 Relationship between concentration of sodium hydroxide solution and boiling point

NaOH/%

37.58

48.30

60.13

69.97

77.53

Boiling point / °C

125

140

160

180

200

In order to obtain a high decomposition rate and maintain the fluidity of the material during the decomposition process, the concentration of NaOH used in the production is 55% to 60%, and the amount of NaOH is more than 2 to 3 times the theoretical calculation. If the concentration of NaOH is too high, the viscosity of the lye will increase, the fluidity will deteriorate, and the material will crystallize in the conveying pipeline, which will affect the smooth progress of production. In addition, the higher the concentration of NaOH, the more uranium enters the sodium phosphate, complicating the extraction process of sodium phosphate. According to the data in Table 1, the temperature corresponding to this should be 140-150 ° C. Above this temperature, the lye is in a boiling state, which easily causes overflow. Sometimes in production, in order to increase the reaction speed and shorten the reaction time, solid sodium hydroxide is added to the atmospheric pressure batch reaction tank to increase the concentration of NaOH in the solution. At the end of the decomposition operation, water is diluted to dilute the alkali solution to facilitate the transportation of the material.

(3) Influence of reaction time and stirring intensity

It can be known from formula (12) that the decomposition rate is proportional to the reaction time, and prolonging the reaction time increases the decomposition. However, as previously analyzed, when the particle size of the mineral is large, the surface of the concentrate is dense with the extension of the reaction time. The thicker the film, the slower the decomposition reaction. Increasing the stirring strength can increase the contact chance between the solid and liquid phases, and has a certain effect on the peeling of the hydroxide film formed on the surface and promoting the angular reaction. Another important function of agitation in production is to maintain the uniformity and fluidity of the alkali decomposition slurry, and to some extent prevent the material from forming a bottom and overflowing in the alkali decomposition tank.

In summary, the process of decomposing the monazite rare earth concentrate by sodium hydroxide is a process of converting a rare earth phosphate which is hardly soluble in the alkali liquid into another rare earth hydroxide which is insoluble in the alkali liquid. When the concentrate size is 0.043 mm, the NaOH concentration is 55% to 60%, and the equivalent temperature and a certain stirring strength, the decomposition rate can reach 97% or more.

3. Extracting rare earth from decomposition products

Decomposed by sodium hydroxide is an alkali-soluble cake composed of rare earth, barium and most uranium hydroxide precipitates and undecomposed minerals, and an alkali-soluble slurry composed of soluble salts of phosphorus and other impurities and excess NaOH. . In order to recover rare earth from the alkali-soluble cake, it is necessary to remove the alkali-soluble substance by water washing, the hydrochloric acid-dissolved hydroxide and the rare earth chloride solution purification process.

(1) Washing and separating alkali-soluble substances

The water washing process belongs to the liquid and solid separation process. In order to facilitate the separation of liquid and solid, before clarification, the alkali solution is first diluted with water and aged at 70 ° C for 6-7 hours to agglomerate the solid particles and increase the sedimentation rate. After the solution is clarified, the supernatant is discharged from the middle of the washing tank (the siphon method can also be used). Because the concentration of NaOH and Na 3 PO 4 in the alkali solution is very high, usually 10 times the amount of water in the production is used in the production, and the solution is heated to 60-70 ° C, and the washing process is repeated 7 to 8 times under the action of stirring. In order to achieve the requirements of P 2 O 5 <1% and pH=7-8 in the washing liquid. The NaOH and Na 3 PO 4 concentrations in the previous washes are high and can be used to recover NaOH and Na 3 PO 4 .

(2) Hydrochloric acid dissolved rare earth hydroxide

The concentrated hydrochloric acid is slowly added to the thick slurry of the washed hydroxide, and the rare earth, cerium and uranium will be dissolved in the hydrochloric acid solution:

RE(OH) 3 +3HCl=RECl 3 +2H 2 O (13)

Th(OH) 4 +4HCl=ThCl 4 +4H 2 O (14)

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

In the acid dissolution process, Na 2 U 2 O 7 is also decomposed by hydrochloric acid, and is present in a solution of U 4 + and UO 2 2 + .

NaOH in the decomposition process, cerium is decomposed into trivalent phosphate hydroxides oxygen while in contact with a portion of the trivalent cerium and air is further oxidized to tetravalent hydroxide. Ce 4 + has a strong oxidizing property in an acidic solution, and Cl - can be oxidized, and the form of chlorine gas escapes from the solution.

Ce(OH) 4 +4HCl=CeCl 3 +4H 2 O+1/2(Cl 2 ) (16)

The tetrabasic hydrazine has a low basicity, and hydrolysis starts at a pH of 0.7 to form a Ce(OH) 4 precipitate. In order to improve the recovery rate of hydrazine in production, the reaction acidity is controlled within the range of pH=1.5-2.0, and a small amount of H 2 O 2 is added to reduce the tetravalent cerium to trivalent to promote the complete dissolution of Ce(OH) 4 .

(3) Purification of rare earth chloride solution

When the hydrochloric acid is dissolved, the impurities in the hydroxide thick slurry, iron, antimony, uranium and a trace amount of radium enter the rare earth chloride solution. According to the basic solubility product principle, according to the data in test (17) and Table 2, the pH value of the solution is adjusted to hydrolyze iron, strontium and uranium into hydroxide precipitates and remove them from the solution.

10 -14 /(K sp [RE(OH) 3 ])/[RE 3 + ] 1/3 <[H + ]<10 -14 /(K' sp [Me(OH) n ]/[Me n + ]) 1/n (17)

Where Me- represents Fe, Th, U;

The solubility product of K' sp -Me(OH)n.

Table 2 RE(OH) 3 , Th(OH) 4 , Fe(OH) 3 , Fe(OH) 2 precipitation pH and solubility product

hydroxide

Ce(OH) 3

Th(OH) 4

Fe(OH) 3

Fe(OH) 2

UO 2 (OH) 2

U(OH) 4

Solubility product Ksp

1.6×10 -20

4.0×10 -45

3.0×10 -39

8.0×10 -16

1.1×10 -22

1.0×10 -45

Precipitating pH

6.83~8.03

4.15

3.68

9.61

6.17

9.25

Degree of precipitation

Start to precipitate

Complete precipitation

Complete precipitation

Complete precipitation

Complete precipitation

Complete precipitation

As can be seen from Table 2, if the pH is controlled to about 4.5, Th 4 + and Fe 3 + can be completely removed, but Fe 2 + remains in the solution. To this end, an appropriate amount of H 2 O 2 may be added to the solution to oxidize Fe 2 + to Fe 3 + and then removed by hydrolysis.

Under the condition of pH>2, U 4 + and UO 2 2 + in the solution begin to undergo primary hydrolysis to form U(OH) 3+ and UO 2 (OH) + ; with increasing pH, U(OH) 3+ Further hydrolysis to a colloidal polymerized hydroxide [U(OH) 4 ]n, while UO 2 (OH) + requires a higher pH to form uranium and polyuranium hydroxide precipitation. The colloidal uranium hydroxide is adsorbed on the surface of the particles of iron hydroxide and barium hydroxide to precipitate.

In the production practice, the concentrated slurry of hydroxide after washing or rare earth carbonate is used, and the pH value of the acid leaching solution is adjusted from 1 to 2 to about 4.5, and a small amount of coagulant is added to rapidly condense the hydrolyzed product in a suspended state. precipitation. The slag obtained by clarification and filtration contains higher radioactive element cerium, which can be used as a raw material for extracting cerium or sealed, and the filtrate can be used for producing mixed crystal chlorinated rare earth or extracting and separating the raw material. This production process is called in the industry. The "smelling of hydrochloric acid" and the slag thus obtained are referred to as "good slag".

The solubility products of radium and strontium sulfate are 4.2×10 -11 and 1.10 -10 , respectively, and are poorly soluble substances. Moreover, the radium ion radius (1.42 Å) and the iridium ionic radius (1.38 Å) have small differences, and under the condition of coexistence of two ions, a homogeneous co-precipitation can be formed. According to this principle, the addition of ammonium sulfate and cesium chloride to a hot rare earth chloride solution (70 to 80 ° C) can remove trace amounts of radium in the solution by the carrier action of BaSO 2 crystal.

(4) Preparation of mixed rare earth products from rare earth chloride solution

The purified rare earth chloride solution can be used as a raw material for the separation of rare earths to separate the single rare earths into the extraction plant. Crystalline mixed rare earth chloride and mixed rare earth carbonate can also be prepared as needed.

1. Preparation of crystalline rare earth chloride

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 RECl3·nH2O product is obtained by cooling. In order to increase the speed of evaporation in production, it is usually concentrated under reduced pressure. When the degree of 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 by about 14 °C.

2. Preparation of rare earth carbonate

Adding ammonium hydrogencarbonate (either solid or liquid) to a rare earth chloride solution containing 40 to 60 g/L of REO will produce a rare earth carbonate precipitate according to reaction formula (18). The precipitated rare earth carbonate is washed with water to remove the adsorbed sulfate, and the obtained RECl 3 ·nH 2 O product is prepared by filtration.

2RECl 3 +3NH 4 HCO 3 =RE 2 (CO 3 ) 3 +3NH 4 Cl3+HCl (18)

4. Recovery of antimony, uranium and rare earth from slag

The main chemical components in the slag are rare earth, barium, uranium hydroxides and small amounts of silicates and undecomposed minerals. After the slag is washed with water to remove chloride ions (Cl - <0.6 g / L), the rare earth, strontium and uranium are usually dissolved by dissolving nitric acid. The dissolution reaction is an exothermic reaction in which a large amount of heat is released to the solution during the dissolution to raise the temperature. If the concentrated slag is directly dissolved by concentrated nitric acid, the temperature of the solution can be sharply raised to above 120 °C. This is beneficial to the agglomeration of the silica gel produced by the dissolution of silicon. At the same time, the addition of polyacrylamide can accelerate the aggregation of the silica gel and increase the clarification effect of the solution. The main chemical composition of the insoluble residue is rutile (TiO 2), ilmenite (FeO · TiO 2), zircon (ZrSiO 4), quartz (SiO 2) and other undecomposed mineral isolated by filtration or Remove. The main chemical reactions in the acid dissolution process are:

RE(OH) 3 +3HNO 3 =RE(NO 3 ) 3 +3H 2 O (19)

Th(OH) 4 +4HNO 3 =Th(NO 3 ) 4 +4H 2 O (20)

Na 2 U 2 O 7 +6HNO 3 =2UO 2 (NO 3 ) 2 +2NaNO 3 +3H 2 O (21)

A trace amount of radium in the solution is removed by adding a small amount of (NH 4 ) 2 SO 4 and Ba(NO 3 ) 2 .

In addition to the nitric acid solution after radium, usually TBP (tributyl phosphate extractant) - Coal Oil (diluent) consisting of organic solvent extraction and separation of rare earth, thorium, uranium. Figure 2 is an extraction separation process used in production.

Figure 2 TBP-kerosene extraction separation RE/Th/U process

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