So far, China's rare earth separation plant obtained rare earth oxalate from the organic phase loaded with rare earth, usually adopts high acid back extraction-water washing acid-ammonia water adjustment pH-oxalic acid precipitation rare earth process. The hydrochloric acid back extraction-oxalic acid precipitation process not only has a long process, but also has a high chlorine content in the product.
To simplify the process, to reduce reagent consumption and reduce environmental pollution, Tsung Min Hung like a single-stage small-scale test bench and precipitated rare earths from the organic phase directly P204 load back-extracted praseodymium, neodymium with oxalic acid. Huang Guiwen, et al. used a mixture of oxalic acid and hydrochloric acid to back-precipitate the rare earth from the P507 supported organic phase. However, there have been no reports of amplification experiments at home and abroad. The direct back-extraction of precipitated rare earths from oxalic acid supported Nd and P204 and P507 organic phases and the industrial experiments conducted in Jiangxi Jiujiang Rare Earth Smelter have achieved good results.
First, the theoretical basis and technical key
(1) Theoretical basis
The solubility product of oxalic acid rare earth is small [such as La 2 (C 2 O 4 ) 3 . The solubility products of Ce 2 (C 2 O 4 ) 3 and Pr 2 (C 2 O 4 ) 3 are 2.5×10 -27 , 3.2×10 -26 and 5.3×10 -29 , respectively, and the rare earth element oxalate The solubility is also small (Table 1), so it can be used to back-precipitate rare earths from the rare earth-loaded P204 and P507 organic phases under oxalic acid excess conditions. Exploratory experiments show that the oxalic acid solution can be used to back-precipitate rare earths from the P204 organic phase (back extraction 89.3%-99.6%). The order of back extraction is: light rare earth elements, medium rare earth elements, heavy rare earth elements.
The reaction equation for the back extraction of Nd from oxalic acid supported Nd organic phase is as follows:
2Nd(HA 2 ) 3 +3H 2 C 2 O 4 +XH 2 O→Nd 2 (C 2 O 4 ) 3 ·xH 2 O↓+ 12HA ,
Where: HA is P204 or P507, and the horizontal line represents the organic phase.
Table 1 Solubility of rare earth element oxalate (25 ° C)
Rare earth element oxalate | Solubility / (mg·L -1 ) |
La 2 (C 2 O 4 ) 3 ·10H 2 O | 0.62 |
Ce 2 (C 2 O 4 ) 3 ·10H 2 O | 0.41 |
Pr 2 (C 2 O 4 ) 3 ·10H 2 O | 0.74 |
Nd 2 (C 2 O 4 ) 3 ·10H 2 O | 0.74 |
Sm 2 (C 2 O 4 ) 3 ·10H 2 O | 0.69 |
Gd 2 (C 2 O 4 ) 3 ·10H 2 O | 0.55 |
Y 2 (C 2 O 4 ) 3 ·10H 2 O | 1.00 |
(2) Key technologies
Back-precipitation and precipitation of Nd from Nd-loaded P204 and P507 organic phases with oxalic acid solution involves three-phase-aqueous phase, organic phase and solid phase, and the reaction speed is fast, and it is easy to form fine crystals sandwiched by organic phase and water phase. When the oxalic acid solution is back-extracted to precipitate rare earth, the following problems need to be solved: 1. Appropriate oxalic acid concentration; 2. Suitable contact ratio; 3. No organic phase film is formed on the crystal surface; 4. The crystal is not on the bottom plate of the back extraction tank. Deposition, not crusted in all directions; 5, the organic phase does not contain oxalic acid solution.
(III) Process comparison
The general process and the new process flow are shown in Figure 1.
Figure 1 2 process flow diagram
Second, the test results and discussion
(1) Bench test
When the contact ratio (V O /V a ) is 1/1-2/1, the phase separation time is long and buoyancy is liable to occur, and the test is difficult to carry out. According to the experience of direct back extraction of uranium from TBP with ammonium carbonate, a full countercurrent triple reverse extraction tank was designed. The structure is shown in Figure 2. The contact of the mixing chamber is adjusted according to the requirements of phase separation. test. The mixing chamber has a side length of 50 mm. Single-stage for conditional testing and level 2 for continuous testing.
Figure 2 Schematic diagram of three opposite extraction tanks
1. Effect of contact on phase separation and stripping rate
Test conditions: c(Nd)=0.138mol/L in the organic phase, c(oxalic acid)=0.426mol/L in the stripping agent, and the two phases were mixed for 15 min. The test results are shown in Table 2. It can be seen that the contact has a great influence on the phase separation, but the effect on the stripping rate is not obvious.
Table 2 Effect of contact on phase separation and stripping rate
Vo:Va | Phase separation time / min | c(Nd 3 + )/(mmol·L -1 ) in lean organic phase | Stripping rate /% |
2:1 | Floating | 13.60 | 90.1 |
1:1 | >7 | 0.67 | 99.5 |
1:2 | 0.8 | 0.30 | 99.8 |
1:3 | 0.5 | 0.16 | 99.9 |
2. Effect of contact time on stripping rate
In the organic phase, c(Nd) = 0.138 mol/L, and in the stripping agent, c (oxalic acid) = 0.32 mol/L, and the contact ratio (Vo/Va) = 1/2. The test results are shown in Table 3. It can be seen that after 12 minutes of mixing, the stripping rate is substantially unchanged. The test was selected for a mixing time of 15 min.
Table 3 Effect of mixing time on é’• stripping rate
Mixing time / min | c B /(mol·L -1 ) in the mother liquor | c(Nd 3 + )/(mmol·L -1 ) in lean organic phase | Stripping rate /% | |
Nd 3 + | C 2 O 4 2 - | |||
3 | Trace | 0.23 | 2.3 | 98.30 |
6 | Trace | 0.22 | 0.94 | 99.30 |
9 | Trace | 0.22 | 0.34 | 99.785 |
12 | Trace | 0.22 | 0.69 | 99.50 |
15 | Trace | 0.22 | 0.56 | 99.59 |
18 | - | 0.22 | 0.11 | 99.92 |
3. Effect of oxalic acid concentration on stripping rate and phase separation
In the organic phase, c(Nd)=0.062 mol/L, the contact ratio (Vo/Va)=1/2, and the mixing time was 15 min. The test results are shown in Table 4. It can be seen that the back extraction rate of Nd increases as the concentration of oxalic acid increases, and the phase separation time decreases as the concentration of oxalic acid increases.
Table 4 Effect of oxalic acid concentration on stripping rate and phase separation
C(H 2 C 2 O 4 )/(mol·L -1 ) | c(Nd 3 + )/(mmol·L -1 ) in lean organic phase | Phase separation time / min | Stripping rate /% | |
Stripping agent | Precipitating mother liquor | |||
0.05 | 0.020 | 33.0 | Hard | 46.8 |
0.10 | 0.055 | 2.30 | Hard | 96.3 |
0.20 | 0.152 | 0.24 | 5 | 99.6 |
0.28 | 0.226 | 0.50 | 2 | 99.2 |
4. Effect of temperature on phase separation and stripping rate
In the organic phase, c(Nd)=0.062 mol/L, c (oxalic acid)=0.4 mol/L in the stripping agent, the contact ratio (Vo/Va)=1/2, and the mixing time was 15 min. The test results are shown in Table 5. It can be seen that the elevated temperature favors phase separation and the effect on the stripping rate is not significant. Because the temperature is too high, the volatilization loss of the organic phase is increased, so the control temperature in the process production is about 20 °C.
Table 5 Effect of temperature on phase separation and stripping rate
Temperature / °C | Phase separation time / s | c(Nd 3 + )/(mmol·L -1 ) in the organic phase | Stripping rate /% |
20 | 70 | 0.29 | 99.5 |
31 | 60 | 0.18 | 99.7 |
36 | 56 | 0.11 | 99.8 |
42 | 47 | 0.08 | 99.9 |
5, continuous bench test
In the organic phase, c(Nd)=0.065-0.069mol/L, and c (oxalic acid)=0.32-0.42mol/L in the stripping agent, continuous operation for 48h. The test results show that under various conditions, after 2 stages of back extraction, the c(Nd) in the lean organic phase is less than 0.50mmol/L, and the rare earth stripping rate is greater than 95% (some up to 99.9%). As long as the water phase is maintained as a continuous phase, the phase separation time is short, no solids accumulate on the inclined plate in the tank body, and only a small amount of solid bonds around the tank body.
(2) Semi-industrial test
In order to verify the results of the bench test, a semi-industrial test for back-extracting Nd from P507-loaded organic phase with oxalic acid was carried out at a plant in Jiujiang, Jiangxi, using a modified full countercurrent three-phase extraction tank (200 mm side length of the tempering chamber).
Test conditions: oxalic acid concentration 0.4mol/L, oxalic acid concentration in sedimentation mother liquor 0.2mol/L; precipitation mother liquor recycling rate 80%; contact comparison: first-stage mixing chamber Vo/Va=1/3-1/5, 2-stage mixing chamber Vo/Va=1/2-1/3; stirring speed 200r/min; room temperature. The organic phase has a c(Nd) of 0.0533-0.1114 mol/L.
1. Influence of contact time on stripping rate
The effect of the joint time (10-40min) on the stripping rate shows that the stripping rate increases with increasing contact time. In the test range, the total stripping rate is greater than 97% (97.36%-99.03%).
2, washed filter cake in addition to impurities
The anti-extraction agent is prepared with industrial oxalic acid, and 80% of the precipitated mother liquor is recycled, and it is inevitable that non-rare earth impurities accumulate and contaminate the product. In order to improve the quality of the product, the filter cake was washed twice with a 10 g/L oxalic acid solution. The test results are shown in Table 6.
Table 6 Washing filter cake removal test results
sample | ψ B /10 -5 | ||||
CaO | Fe 2 O 3 | PbO | NiO | ||
1 | Before washing | 5.8 | 24.0 | 4.0 | 1.0 |
After washing | 5.0 | 2.6 | 4.0 | 1.0 | |
2 | Before washing | 2.7 | 11.0 | 1.8 | 1.5 |
After washing | 2.3 | 2.3 | 1.0 | 1.0 | |
3 | Before washing | 3.7 | 66.0 | 1.0 | 1.0 |
After washing | 3.5 | 2.6 | 1.0 | 1.0 | |
4 | Before washing | 16.0 | 110.0 | 11.0 | 1.0 |
After washing | 10.0 | 10.0 | 5.7 | 1.0 | |
5 | Before washing | 75.0 | 120.0 | 1.5 | 1.0 |
After washing | 6.7 | 10.0 | 1.5 | 1.0 | |
average | Before washing | 20.6 | 66.2 | 3.9 | 1.0 |
After washing | 5.5 | 5.5 | 2.6 | 1.0 | |
Table 6 shows that washing the filter cake twice with oxalic acid solution has better iron removal effect, because in the acidic solution, oxalic acid reacts with ferric ions to form soluble iron oxalate complex, while calcium and lead interact with oxalic acid. An oxalate precipitate is formed and enters the product.
3. Accumulation of impurities after recycling of mother liquor
In the test, the recycling mother liquor circulation rate was 80%, that is, 80% of the precipitate mother liquor was used for each time the stripping agent was prepared. Non-rare earth impurities in such stripping agents will accumulate, and the test results are shown in Table 7. It can be seen that after 80% of the mother liquor is recycled for 7 times, only the iron content of the stripping agent is increased.
Table 7 Mass concentration of non-rare earth impurities in stripping agent
Cycles | ÏB/(mg·L -1 ) | ||
CaO | Fe 2 O 3 | PbO | |
0 | 0.21 | 0.59 | 0.08 |
1 | 0.04 | 25 | 0.06 |
3 | 0.04 | 26 | 0.06 |
7 | 0.04 | 30 | 0.067 |
4. Effect of oxalic acid back extraction precipitation on the quality of extractant in organic phase
In order to investigate the performance of P507 when the rare earth was precipitated by direct back-extraction of rare earth from the N-loaded P507 organic phase with oxalic acid, the concentration of P507 in the lean organic phase was periodically determined. The results are shown in Table 8. It can be seen that direct back extraction of rare earth with oxalic acid solution does not change the properties of P507.
Table 8 P507 concentration in lean organic phase
Measurement number | (P507)/(mg·L -1 ) |
Background | 1.66 |
1 | 1.65 |
2 | 1.65 |
3 | 1.66 |
4 | 1.66 |
5 | 1.67 |
5, material balance
The material balance result is calculated by multiplying the content of Nd 2 O 3 in the organic phase by the volume of the organic phase flowing into the tank and the mass of Nd 2 O 3 obtained after the filter cake is burned. The results are shown in Table 9. Among them, the yield is more than 100% because it is difficult to completely separate the products obtained per day.
Table 9 Material Balance Results
Running time /d | Organic phase flow / (mL·min -1 ) | Organic phase Ï(Nd 3 + )/(g·L -1 ) | Yield /% | Remarks |
2 | 100 | 18.725 | ||
3 | 100 | 18.725 | ||
4 | 100 | 18.725 | 86 | |
5 | 100 | 18.725 | 92.04 | |
6 | 100 | 18.725 | 95.01 | |
7 | 100 | 18.725 | 108.00 | |
8 | 200 | 18.725 | 77.24 | |
9 | 200 | 18.725 | 99.91 | |
10 | 300 | 18.725 | 90.92 | |
12 | 300 | 8.95 | 94.58 | |
13 | 400 | 8.95 | 139.19 | Clearing solid |
6, product quality comparison
The average mass fraction of impurities in the 5 batches of products obtained from the 7 batches of products and the conventional process is shown in Table 10.
Table 10 The mass fraction of impurities in Nd 2 O 3 obtained by the old and new processes
Impurity | Conventional process* | New Technology** |
La 2 O 3 | 0.67 | 0.02 |
Ce 2 O 3 | <0.01 | 0.01 |
Pr 2 O 3 | 0.045 | 0.042 |
Sm 2 O 3 | 0.030 | 0.011 |
Y 2 O 3 | 0.0037 | 0.019 |
Nd 2 O 3 /REO | 99.26 | 99.89 |
CaO | 0.0019 | 0.0062 |
Fe 2 O 3 | 0.026 | 0.024 |
SiO 2 | 0.003 | 0.004 |
Cl - | 0.0244 | 0.01 |
* Wash the filter cake 5 times with 20 g/L oxalic acid tray; ** Wash the filter cake twice with 10 g/L oxalic acid tray.
Compared with the conventional process, the main advantages of the new process are: 1. Simplified the process, reducing the consumption of hydrochloric acid, ammonia, steam and electricity; 2. Reducing the chlorine content of the product, thereby reducing the corrosion and environment of the burning equipment. Pollution. The main disadvantage of the new process is the high level of extractant in the product.
Third, economic benefits
The cost comparison of the two processes is shown in Table 11. It can be seen that with the new process, for every 1kg of Nd 2 O 3 produced, the material consumption cost can be reduced by 7.86 yuan, which can also save equipment investment, energy consumption and labor costs.
Table 11 Comparison of two process costs for producing Nd 2 O 3 (calculated to produce 1 kg of Nd 2 O 3 )
Material name | Unit price / (yuan·kg -1 ) | Conventional process | New Technology | ||
Dosage / kg | Fee/yuan | Dosage / kg | Fee/yuan | ||
Raw material Nd 2 O 3 | 100 | 1.0466 | 104.66 | 1.019 | 101.98 |
Industrial hydrochloric acid | 0.45 | 4.25 | 1.91 | 0 | |
Industrial hydrochloric acid | 2.20 | 0.38 | 0.84 | 0 | |
Industrial oxalic acid | 4.60 | 1.70 | 7.82 | 1.365 | 6.28 |
Pure water | 5.66×10 -3 | 19 | 0.11 | 25 | 0.14 |
steam | 46×10 -3 | 20 | 0.92 | 0 | |
total | 116.26 | 108.40 | |||
Fourth, the conclusion
(1) Directly back-extracting and precipitating rare earth from Nd-loaded P204 and P507 organic phases with oxalic acid solution, and the purity of Nd2O3 obtained is 99.9%, which meets the requirements.
(2) The reverse extraction order of various rare earth elements is: light rare earth element, medium rare earth element, heavy rare earth element.
(3) The results of the semi-industrial test show that the process of directly back-precipitating and precipitating Nd from the P507 organic phase with oxalic acid solution can save two processes compared with the process of adjusting the pH-oxalic acid precipitation Nd with hydrochloric acid-ammonia acid-ammonia water. The yield is increased by 2.5%, and 4,250 kg of hydrochloric acid, 380 kg of liquid ammonia, and 20 kg of steam are saved per t product. The cost of materials is reduced by 7,860 yuan per 1 t Nd 2 O 3 produced.
(4) The self-developed design of the full countercurrent three opposite extraction tank has a simple structure, stable operation, strong adaptability, no solid accumulation on the inclined plate, and no crusting in the tank.
(5) In industrial production, it is necessary to increase the water-poor organic phase and the slurry section to reduce the phase entrainment loss.
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