Study on defluorination of wet phosphoric acid
Fu Yanan 1, Wang Hua 2, Tang Min 3, Tang Deyuan 1
(1. Department of Chemical Engineering, Guizhou University of Technology; 2 .Institute of Geochemistry, Chinese Academy of Sciences; 3.Physical and Chemical Analysis Center of Guizhou University of Technology, Guiyang, Guizhou 550003)
Abstract: The orthogonal experimental method was used to study the effects of additives, their amounts, addition methods, temperatures, etc. on the wet method. Effects of defluoridation of phosphoric acid and recovery of sodium fluorosilicate. The results show that it is best to use sodium carbonate as the additive. The theoretical dosage of the additive is 110-120. It is better to use dry powder as the adding method and the temperature should be lower.
Keywords: fluorine; purification; recovery; wet phosphoric acid
CLC number: TQ126.3 Document identification code: A
0 Preface
p>
Phosphoric acid is an important raw material for the production of phosphate, and its sources include thermal phosphoric acid and wet phosphoric acid. A comparison of the production of two phosphoric acids shows that the heat energy consumed by thermal phosphoric acid is about 3.4 times that of wet phosphoric acid, and the electricity consumption is about 13.4 times higher, making the phosphate produced after purification of wet phosphoric acid lower in cost, such as for food grade In the production of phosphoric acid, the cost of wet phosphoric acid is about 20-25 lower than that of thermal phosphoric acid. In addition, the phosphorus resource utilization rate of wet phosphoric acid is higher than that of thermal phosphoric acid. For every ton of P2O5 phosphoric acid produced, thermal phosphoric acid costs more Phosphate rock consumption is about 20. Therefore, wet phosphoric acid purification has always received greater attention from people at home and abroad. Existing practice has proven that wet phosphoric acid can replace thermal phosphoric acid through purification, can adapt to different quality requirements, and has lower costs. With the development of wet phosphoric acid purification technology, the proportion of phosphate produced from wet phosphoric acid will gradually increase.
Fluoride is one of the main impurities in wet phosphoric acid. Since fluoride is widely used in industry, during the wet phosphoric acid purification process, the fluoride by-products should be recovered as much as possible, which not only improves process efficiency but also improves resource utilization.
At present, the defluorination of wet phosphoric acid basically adopts chemical methods and concentration methods. This study uses chemical methods to study the defluorination of wet phosphoric acid.
1 Principle
Wet phosphoric acid chemical defluorination is based on the existence form of fluorine and the low solubility of some fluorides. Fluoride and silica are present in phosphate rock, and the P2O5/F ratio is generally between 8-20. When phosphate rock reacts with the slurry, it first generates hydrofluoric acid:
Ca10F2(PO4) 20H =6H3PO4 10Ca2 2HF
Hydrofluoric acid is a weak acid:
(H)(F)/(HF)=10-3.2
But it reacts easily with silicon dioxide:
6HF SiO2=H2SiF6=SiF62- 2H
Fluosilicic acid is a strong acid. It can be seen that fluoride in wet phosphoric acid mostly exists in the form of hydrofluoric acid and fluorosilicic acid. If the content of active silica in the phosphate rock is high, fluosilicic acid is the main form of fluoride.
In the presence of sesquioxide, fluoroaluminic acid and fluoroferric acid may also be present.
Among the salts of fluorosilicic acid, the solubility of alkali metal salts is low. For example, sodium fluorosilicate is slightly soluble in water, and its solubility in water at 298 K is 7.62 g/L, while potassium fluorosilicate is poorly soluble in water, and its solubility in water at 298 K is 1.77 g/L. They are dissolved in acid The solubility in is higher than the solubility in water.
In industrial phosphoric acid, the solubility product of sodium fluorosilicate at 333 K can be expressed as:
(Na )2(SiF62-)=500×10-6
p>Figure 1 Effects of phosphoric acid concentration and temperature on the solubility of sodium fluorosilicate (solid line) and potassium fluorosilicate (dashed line)
The effects of phosphoric acid concentration and temperature on the solubility of sodium fluorosilicate and fluorosilicate The effect of potassium acid solubility is shown in Figure 1.
According to this figure, in phosphoric acid containing 30P2O5, the solubility product of sodium fluorosilicate at 346 K is:
(Na )2(SiF62-)=194×10-6
Comparing the two, industrial phosphoric acid has a higher fluorine content. This is probably due to the extremely slow process of fluoride dissolution reaching a stable equilibrium and the high impurity content in industrial phosphoric acid.
It can also be seen from the figure that at a certain temperature, when the phosphoric acid concentration is lower than 35P2O5, the solubility of potassium fluorosilicate is lower than that of sodium fluorosilicate, and when the phosphoric acid concentration is higher than 35P2O5, the solubility of fluorosilicate is lower than that of sodium fluorosilicate. The solubility of potassium silicate is higher than that of sodium fluorosilicate. Under a certain phosphoric acid concentration, the solubility of fluorosilicate increases as the temperature increases.
Sodium fluoroaluminate is a complex fluoride, slightly soluble in water, the solubility in water at 298 K is 0.04175, and increases with the increase of pH value, and increases with the concentration of F- and Na And decline. Fluoride is similar.
The solubility of calcium fluoride in water is extremely low, only 0.0163 g/L. It is slightly soluble in cold dilute acid and can react with hot sulfuric acid to generate hydrogen fluoride gas.
Among the above-mentioned fluorides, due to raw material factors, the most commonly used method for wet phosphoric acid defluorination is to use sodium fluorosilicate as a by-product.
2 Experiment
2.1 Additives
The additives commonly used for wet phosphoric acid defluorination are sodium chloride, sodium sulfate and sodium carbonate. The former two are cheap, but introduce chlorine and sulfate radicals, while the latter is expensive, but has no pollution to wet phosphoric acid. Sodium sulfate and sodium carbonate are now used for research.
2.2 Raw materials
Wet-process phosphoric acid is provided by Xifeng Heavy Calcium Plant of Guizhou Kailin Group. It is a dihydrate wet-process phosphoric acid with a fluorine content of 2.04.
2.3 Experimental plan
The experiment is arranged using the orthogonal table L4(23). In addition to additives, the amount, method of addition and temperature are also considered. The additive dosage is 80 and 120 of the theoretical dosage. The experimental temperatures are 296 K and 313 K.
2.4 Analysis method
Fluorine is analyzed using the fluoride ion selective electrode method.
3 Experimental results and discussion
The experimental results are shown in Table 1 to Table 4. Under suitable conditions, other impurities do not precipitate much, so that the purity of the by-product sodium fluorosilicate can be reaches more than 97, and the phosphorus yield can be more than 97.
Table 1 Effect of additives and their amounts
Additives
Additive amount/
Fluoride removal rate/
Sodium fluorosilicate yield/
Na2CO3
80
77.48
56.1
Na2CO3
p>120
82.81
62.2
Na2SO4
80
73.41
52.5
Na2SO4
120
81.09
61.5
Table 2 Influence of additive addition methods
Additive addition method
Additive dosage/
Fluoride removal rate/
Sodium fluorosilicate yield/
Dry powder
80
77.48
56.1
Dry powder
120
82.81
62.2
Solution
80
64.53
51.8
Solution
120
77.26
61.8
Table 3 Effect of temperature
Temperature/K
Additive dosage/
Fluoride removal rate/
Sodium fluorosilicate yield/
296
80
77.48
56.1
296
120
82.81
62.2
313
80
71.26
53.1
313
120
80.73
61.4
3.1 Additives and their dosages
Carry out variance analysis on Table 1, and the results are listed in Table 4. It can be seen that the additives have an impact on the defluorination effect, and the variance analysis from Table 4 to Table 6 shows that the additive dose has an impact on both the defluorination effect and the yield of sodium fluorosilicate.
As far as additives are concerned, the defluorination effect of sodium carbonate is better than that of sodium sulfate, and the defluorination effect increases as the amount of addition increases.
The following reaction occurs between sodium carbonate and fluorosilicic acid:
Na2CO3 H2SiF6=Na2SiF6 CO2 H2
Table 4 Variance analysis table
Variance Source
Sum of Squares of Deviations
Degrees of Freedom
Sum of Mean Squares
Fj
Significance
Additive
8.381/4.622
1
8.381/4.622
6.07/2.20
(*)/-
Added dosage
42.315/57.002
1
42.315/57.002
30.65 /27.11
(*)/(*)
Error
1.381/2.102
1
1.381 /2.102
Note: The numerator is the corresponding value of the fluoride removal rate, and the denominator is the corresponding value of the sodium fluorosilicate yield.
F0.25(1,1)=5.83
Table 5 Variance analysis table
Source of variance
Sum of squared deviations
Degrees of freedom
Sum of mean squares
Fj
Significance
Additive addition method
85.56/5.52
1
85.56/5.52
6.25/1.45
(*)/-
Additional dosage
81.54/64.80
1
81.54/64.80
5.96/17.04
(*)/(*)
Error
13.69/3.80
1
13.69/3.80
Note: The numerator is the corresponding value of fluoride removal rate , the denominator is the corresponding value of sodium fluorosilicate yield. F0.25(1,1)=5.83
Table 6 Variance analysis table
Source of variance
Sum of squared deviations
Degrees of freedom
Sum of mean squares
Fj
Significance
Temperature
15.76/3.61
1
15.76/3.61
4.41/2.98
-/-
Additional dosage
52.13 /51.84
1
52.13/51.84
14.59/42.84
(*)/(*)
Error
3.57/1.21
1
3.57/1.21
Note: The numerator is the corresponding value of the fluorine removal rate, and the denominator is fluorine Corresponding value of sodium silicate yield. F0.10(1,1)=39.86
Due to the release of carbon dioxide gas, not only will the impurities in the acid not be increased, but also fluoride will be brought out with the release of gas under certain conditions. gas, thereby increasing the defluorination effect. For other sodium salts such as sodium chloride and sodium sulfate, the reaction is:
2NaCl H2SiF6=Na2SiF6 2HCl
Na2SO4 H2SiF6=Na2SiF6 H2SO4
It can be seen that except for fluorine Except for sodium silicate, the rest remains in the phosphoric acid, increasing the impurities in the phosphoric acid.
The effect of additive dosage on fluoride removal rate and sodium fluorosilicate yield is shown in Figure 2 and Figure 3. It can be seen from the figure that as the additive dosage increases, it is beneficial to the precipitation of sodium fluorosilicate and the fluoride removal rate. and the yield of sodium fluorosilicate both increase. However, if too much is added, not only will too many impurities be introduced, which will increase the complexity of subsequent purification processes, but the fluorine removal rate and the yield of sodium fluorosilicate will also increase slowly. Therefore, the additive dosage should be appropriate, generally slightly higher than the stoichiometric amount.
Figure 2 The relationship between fluoride removal rate and additive dosage
Figure 3 The relationship between sodium fluorosilicate yield and additive dosage
3.2 Addition method
Additives can be added in two ways: dry powder and solution. Perform variance analysis on Table 2 and the results are listed in Table 5. It can be seen that the addition method has an impact on the fluoride removal rate, but has little impact on the yield of sodium fluorosilicate.
Using dry powder addition can maintain the P2O5 concentration of the solution, but the dry powder should be of sufficient fineness and the stirring should be strong enough. Otherwise, the reaction effect will be inferior to that of solution addition.
Using solution addition, if the stirring intensity is sufficient, it will be easy to mix evenly with the solution to be treated, and the reaction effect will be better. However, the water brought into the solution will dilute the solution to be treated, and the P2O5 concentration will decrease.
When adding dry powder, the temperature of the reactant system is slightly higher than when adding solution, which is beneficial to the analysis of gas.
The data in Table 2 shows that dry powder addition is better than solution addition. Therefore, dry powder addition should be used.
3.3 Temperature
Temperature has little effect on the defluorination effect and the yield of sodium fluorosilicate, see Table 6. As can be seen from Figure 1, due to the solubility of sodium fluorosilicate It decreases as the temperature decreases, so a lower temperature is better.
4 Conclusion
Defluorination of wet phosphoric acid is one of the important steps in its industrial application. In the process of wet phosphoric acid defluoridation, recovering sodium fluorosilicate as much as possible not only increases by-products and improves economic benefits, but is also related to comprehensive utilization, improved resource utilization, and sustainable development of cleaner production. This study used an orthogonal experimental design to arrange experiments, and obtained the results of the effects of additives, additive amounts, addition methods, temperatures, etc. on the wet phosphoric acid defluorination rate and sodium fluorosilicate yield. The research results show that it is best to use sodium carbonate as the additive. The theoretical dosage of the additive is 110-120. It is better to use dry powder as the adding method and the temperature should be lower.
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Study of Fluorine Purification of Wet-Process Phosphoric Acid
FU Ya-nan1, WANG Hua2, TANG Min3, TANG De-yuan1
(1.Department of Chemical Engineering, GUT, Guiyang 550003, China;
2.Geochemical Institute, Chinese Academy of Sciences, Guiyang 550003, China;
3.Physics and Chemistry Research Center, GUT, Guiyang 550003, China )
Abstract: This paper studies the effect of additive and its quantity, additive fashion, and temperature on removal ratio of fluorine and yield of fluosilicate sodium.The result shows that it is better to use sodium carbonate as additive , in 110-120 of its theoretical quantity, in dry powder form, and at a lower temperature.
Key words: fluorine; purification; recover; wet-process phosphoric acid
Collect Date of manuscript: 2001-05-17
Fund project: Guizhou Province 1998 Science and Technology Fund Project, Qianji Hezi No. 198