Theoretical basis for wet process phosphoric acid production

1. Chemical reaction of wet process phosphoric acid production

Sulfuric acid, nitric acid or hydrochloric acid decomposition of phosphate ore obtained referred to as wet process phosphoric acid, the phosphoric acid decomposition method Phosphoric acid is used in the production of wet process phosphoric acid the basic method. Sulfuric acid decomposes phosphate rock to form phosphoric acid solution and insoluble calcium sulfate crystal. The total chemical reaction formula is as follows:

Ca ₅ F(PO ₄ ) ₃ +5H ₂ SO ₄ +5nH ₂ O=3H ₃ PO ₄ +5CaSO ₄ ·nH ₂ O↓+HF

In fact, the reaction is carried out in two steps. The first step is to pre-decompose the phosphate rock and the circulating slurry or the phosphoric acid of the return system. The phosphate rock is first dissolved in an excess of phosphoric acid solution to form monocalcium phosphate:

Ca ₅ F(PO ₄ ) ₃ +7H ₃ PO ₄ =5Ca(H ₂ PO ₄ ) ₂ +HF

The purpose of pre-decomposition is mainly to prevent direct reaction of phosphate rock with concentrated sulfuric acid, avoiding the formation of calcium sulfate film on the surface of phosphate rock particles and hindering further decomposition of phosphate rock, and also conducive to the reduction of calcium sulfate supersaturation.

The second step is that the above-mentioned monocalcium phosphate slurry reacts with a slight excess of sulfuric acid to form a calcium sulfate crystal and a phosphoric acid solution:

Ca(H ₂ PO ₄ ) ₂ +H ₂ SO ₄ +nH ₂ O=CaSO ₄ ·nH ₂ O↓+2H ₃ PO ₄

Calcium sulphate can be precipitated from a phosphoric acid solution in three different water and crystalline forms depending on the phosphoric acid concentration, temperature, and free sulfuric acid concentration in the phosphoric acid solution. Therefore, depending on the production conditions, calcium sulfate dihydrate (CaSO ₄ · n2H ₂ O), calcium sulfate hemihydrate (CaSO ₄ · 1/2H ₂ O) and anhydrous calcium sulfate (CaSO ₄ ) can be produced, so The n in the formula CaSO ₄ ·nH ₂ O may be equal to 2, 1/2 or 0. Correspondingly, three basic methods are produced, namely, the dihydrate method, the hemihydrate method, and the anhydrate method. The HF formed in the reaction and the SiO ₂ introduced in the phosphate ore form H ₂ SiF ₆ .

6HF+SiO ₂ =H ₂ SiF ₆ +2H ₂ O

A small amount of H ₂ SiF ₆ will react with SiO ₂ to form SiF ₄ .

2H ₂ SiF ₆ + SiO ₂ = 3SiF ₄ ↑ + 2H ₂ O

It can be seen that the fluorine in the gas phase mainly exists in the image of SiF ₄ , and absorbs water to form an aqueous solution of fluorosilicic acid and precipitates a precipitate of silica gel.

3SiF ₄ +(n+2)H ₂ O=2H ₂ SiF ₆ +SiO ₂ ·nH ₂ O↓

The following reactions occur in the iron , aluminum , sodium, potassium and other impurities in the phosphate rock:

(Fe,Al) ₂ O ₃ +2H ₃ PO ₄ =2(Fe,Al)PO ₄ ↓+3H ₂ O

(Na,K) ₂ O+H ₂ SiF ₆ =(Na,K) ₂ SiF ₆ ↓+H ₂ O

Magnesium is mainly present in carbonates, and carbonates in phosphate rock, such as dolomite, calcite, etc., are first decomposed by sulfuric acid and emit CO ₂ .

CaCO ₃ +MgCO ₃ +2H ₂ SO ₄ =CaSO ₄ ↓+2H ₂ O+2CO ₂ ↑+MgSO ₄

The resulting magnesium salts all enter the carbonic acid solution, which will have an adverse effect on the quality of the phosphoric acid and post-processing.

Phase equilibrium and conversion kinetics of calcium sulfate in CaSO ₄ -H ₃ PO ₄ -H ₂ O and CaSO ₄ -H ₃ PO ₄ -H ₂ SO ₄ -H ₂ O system

Calcium sulfate (CaSO ₄ ·2H ₂ O) has only one crystal form; calcium sulfate hemihydrate (CaSO ₄ ·1/2H ₂ O) has two crystal forms of α-form and β-form; anhydrous calcium sulfate (CaSO ₄₎ There are three crystal forms: anhydrate I, anhydrate II, anhydrate III. However, the crystal forms associated with the wet process phosphoric acid production process are only dihydrate, α-hemihydrate and anhydrate II. Some of their physical constants and theoretical chemical compositions are listed in Table 1.

Table 1 Some physical constants and theoretical chemical compositions of calcium sulfate crystals

(II) Phase equilibrium of calcium sulfate in CaSO ₄ -H ₃ PO ₄ -H ₂ O ternary system

Figure 1 is a phase equilibrium diagram of a CaSO ₄ -H ₃ PO ₄ -H ₂ O ternary system, which may also be referred to as a conversion multi-temperature diagram or a schematic representation of the conversion of different crystal forms of calcium sulphate at different temperatures.

Figure 1 Phosphoric acid concentration % P ₂ O ₅

AB line: dihydrate Thermodynamic equilibrium curve of anhydrate;

CD line: dihydrate Semi-aqueous metastable equilibrium curve

In Figure 1, the AB line is dihydrate. Thermodynamic equilibrium curve of anhydrate, dotted line CD is dihydrate Semi-aqueous substance metastable equilibrium curve. These two curves divide Figure 1 into three regions, namely regions I, II, and III. The following four conclusions can be obtained thermodynamically from Figure 1.

1. In the CaSO ₄ -H ₃ PO ₄ -H ₂ O system, calcium sulfate has only two stable crystal forms: dihydrate region I and anhydrate region II, III.

2. In the three regions, the conversion order of calcium sulfate crystals is shown in Table 2.

3. On the AB line, the dihydrate has the same stability as the anhydrate, and can exist at the same time in the solution and is in equilibrium. On the CD line, the only stable solid phase is an anhydrate.

4. At 80 ° C, the theoretical maximum concentration of dihydrated phosphoric acid is about 33% P ₂ O ₅ . It can be seen from Fig. 1 that when the phosphoric acid concentration is higher than 33% P ₂ O ₅ , the first precipitated hemihydrate will be directly converted into an anhydrate, and the dihydrate crystal will not be obtained, so that the dihydrate flow cannot be achieved.

The above analysis of the ternary phase diagram of CaSO ₄ -H ₃ PO ₄ -H ₂ O provides a theoretical basis for the production of wet process phosphoric acid.

Table 2 Conversion sequence of calcium sulfate crystals

(III) Phase equilibrium of calcium sulfate in CaSO ₄ -H ₃ PO ₄ -H ₂ SO ₄ -H ₂ O quaternary system

The results for the ternary system ₂ O CaSO ₄ -H ₃ PO ₄ -H only when the reaction slurry phase with Ca ^{2 +} The concentration is meaningful when it exists in the amount of the same substance, but in the production of wet-process phosphoric acid, the sulfuric acid is excessive, that is, there are a large amount in the system. exist. The use of ternary phase diagrams for analysis produces large deviations, so it is necessary to study the CaSO ₄ -H ₃ PO ₄ -H ₂ SO ₄ -H ₂ O quaternary system. Figure 2 is a phase diagram of this quaternary system. This picture is incomplete and only shows the semi-hydrate Part of the dihydrate conversion process. The curve in Figure 2 is the transfer trajectory of the equilibrium point at a given H ₂ SO ₄ concentration (expressed as SO ₃ content, %). The metastable zone of the hemihydrate above the line, the stable zone of the dihydrate under the line, and the result of the ternary system when SO ₃ = 0%.

Figure 2 Balance diagram of CaSO ₄ -H ₃ PO ₄ -H ₂ SO ₄ -H ₂ O quaternary system

Figure 2 shows that when the concentration of H ₂ SO4 is increased, the hemihydrate The equilibrium point of the dihydrate will vary with low phosphoric acid concentration and temperature. The A value and B value at different temperatures have been found in Table 3.

Table 3 A and B values ​​at different temperatures

Application (3-3) and Table 3 can more accurately explain and explain the practical problems in production. If the dihydrate process is used to produce 22% P ₂ O ₅ phosphoric acid, the dihydrate obtained according to formula (3-3) at a reaction temperature of 80 ° C The limit of the equilibrium point of the hemihydrate conversion process [SO ₃ ] shall be:

[SO ₃ ]=(-0.885×22+25.4)%=5.93%

Obviously, if the SO ₃ concentration at the equilibrium point exceeds this value, it will enter the metastable region of the hemihydrate and the dihydrate crystals will not be obtained. However, if the temperature is lowered, the SO ₃ concentration at the equilibrium point will increase accordingly, which is the reason that a higher SO ₃ concentration can be allowed at lower temperature conditions. In the actual dihydrate production, the liquidus SO ₃ concentration is much lower than the above calculated value. Even with medium-grade phosphate ore with high impurity content, the high-limit control range of liquid-phase SO ₃ concentration is actually about 4% at a phosphoric acid concentration of 22% P ₂ O ₅ and a temperature of 80 ° C. At 0.05 m/mL, it is significantly lower than the value of 5.93% [SO ₃ ] calculated by the above formula, so that it is no problem to form stable dihydrate crystals and smoothly realize the production of the dihydrate method. In addition, the results of the quaternary system can also explain the hemihydrate in the recrystallization process based on the different sulfuric acid concentrations in the sulfuric acid and phosphoric acid mixed acid. The conversion process of dihydrate.

(IV) Conversion kinetics of CaSO ₄ -H ₃ PO ₄ -H ₂ O system

1. The practical significance of the study of transformation kinetics

The CaSO ₄ -H ₃ PO ₄ -H ₂ O system equilibrium diagram introduced above only discusses the relationship between the order of mutual conversion of calcium sulfate in different aqueous waters in aqueous phosphoric acid solution with solution concentration and temperature, but only understands these thermodynamics. The results of the study are not enough. Because the discussion of thermodynamics does not involve the conversion rate of calcium sulphate crystallization, while kinetics studies the reaction rate of conversion, it has important practical significance for actual production.

It has been found that the conversion rate in the aqueous solution of calcium sulphate crystals is fast only at the instant after crystallization, and the slowness can be continued for several months without reaching complete conversion. This difference in conversion speed provides an important theoretical basis for the choice of wet-process phosphoric acid production methods.

In the production of dihydrate, as can be seen from Figure 1, the choice of process conditions (mainly phosphoric acid concentration and reaction temperature) seems to be in the region I, because in the region I, the dihydrate is the only stable crystal form, but the actual This area cannot be selected because the temperature of the phosphoric acid solution that needs to be maintained in the area I is very low (below 40 ° C). This low temperature is not only detrimental to the decomposition of phosphate rock and the crystallization of calcium sulfate, but also removes a large amount of heat of reaction. It is also difficult to achieve in the industry. Therefore, the process conditions for the production of the dihydrate process are actually selected in the region II. The phosphoric acid concentration range is generally 20% to 30% P ₂ O ₅ and the reaction temperature is 60 to 80. °C, however, it can be seen from Fig. 1 that the dihydrate is not in a stable state in the region II, and is in a metastable state, and the crystal form in which the region is in a stable state is an anhydrate. Why is it possible to industrially produce dihydrogen in the region II where the dihydrate is metastable? The following study of transformation kinetics will answer this question.

2. Conversion kinetics of hemihydrate to dihydrate and anhydrate at 80 °C

In the production of dihydrate method, since the activation energy required for the formation of crystal nucleus is the least, the first precipitation is the semi-aqueous substance in the unstable state, and then converted into the metastable dihydrate and finally converted to stable. Anhydrate. Therefore, it is instructive to study the conversion kinetics between them at 80 °C.

Semi-aqueous substance at 80 ° C The conversion equilibrium point of the dihydrate occurs when the solution contains 33% P ₂ O ₅ , so when the solution concentration is higher than 33% P ₂ O ₅ , the conversion sequence is hemihydrate. Dihydrate. Below 33% P ₂ O ₅ , the conversion sequence is semi-hydrate Dihydrate Anhydrate. The conversion rate of hemihydrate to dihydrate at 80 °C is shown in Table 4. As can be seen from Table 4, the conversion of the hemihydrate to the dihydrate at 80 ° C was carried out rapidly in a phosphoric acid solution having a phosphoric acid concentration of 10% to 25% P ₂ O ₅ . When the concentration of phosphoric acid is 10% P ₂ O ₅ , it can be completely converted within 1 h; when the concentration is 18% P ₂ O ₅ , about 2 h; when the concentration is 25% P ₂ O ₅ , it is about 6-7 h. As the concentration of phosphoric acid increases, the conversion time increases accordingly. As the metastable dihydrate formed by the conversion continues to be converted into a stable anhydrate, it is extremely slow. When the phosphoric acid concentration (mass fraction) was 30% P ₂ O ₅ , 19.6% P ₂ O ₅ and 12.75% P ₂ O ₅ , the required complete conversion times were 10 days, 19 days, and 78 days, respectively.

The above complete conversion time is the sum of the time required for the crystallization conversion latent period and the actual conversion period. For example, the conversion of hemihydrate to dihydrate is used. The so-called conversion potential means that the crystallization of hemihydrate remains unchanged during this period, mainly because of the germination of the new phase, and the transformation process has not actually begun. The actual conversion period means that the hemihydrate crystals begin to transform into dihydrate crystals, and the crystal water content changes with time (Table 4).

Table 4 Conversion rate of semi-aqueous to dihydrate at 80 °C

The above analysis shows that under the actual production conditions of the dihydrate method (phosphoric acid concentration is generally 20% to 50% P ₂ O ₅ , reaction time is 5 to 6 hours), the hemihydrate to the dihydrate can achieve complete conversion, so from the semi-aqueous From the relative conversion rate of the dihydrate to the dihydrate and the anhydrate, it is seen that in the region II of Fig. 1, although the dihydrate is in the metastable state, the hemihydrate is converted into the dihydrate quickly. And the dihydrate is converted to a stable anhydrate which is extremely slow (can be expressed as a semi-aqueous substance) Therefore, it can be considered that the dihydrate which is thermodynamically metastable is relatively stable from the viewpoint of kinetics, so that the production of the dihydrate method can be smoothly achieved under this condition.

Since the conversion time of the calcium sulfate crystal in the phosphoric acid solution is related to the phosphoric acid concentration and the reaction temperature, it is also related to the sulfuric acid concentration of the solution and the reflow operation. Excessive sulfuric acid in actual production will greatly promote the conversion of the hemihydrate to the dihydrate. At the same time, since the acid hydrolysis reaction tank adopts a continuous production of a large amount of circulating slurry (recycled), the new crystal can grow on the basis of the original crystal, so that the conversion process of the hemihydrate to the dihydrate can be greatly accelerated. Therefore, due to the action of excess sulfuric acid and slurry, the reaction time of actual production can be further shortened.

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