Study on Extraction of Phosphorus and Potassium from Low-grade Phosphorus Potassium Ore and Phosphoric Acid

I. Introduction

Potassium is one of the essential elements for plant growth and development. It can enhance photosynthesis and play an important role in improving crop quality. Phosphorus application can make plants grow well and improve drought resistance and cold resistance ^[1] . China is a country with relative potassium deficiency. The self-sufficiency rate of potassium fertilizer is less than 30%, and it needs to be imported to make up for the domestic shortage ^[2] . Potassium ore has a low grade, a large number of associated components, and the distribution is extremely uneven. Therefore, the development and utilization of China's existing low-grade phosphate rock mine is one of the effective ways to improve the self-sufficiency rate of potassium fertilizer in China.

The main component of a phosphorus and potassium phosphate rock ore and potassium feldspar ^[3]. Potassium feldspar is stable in nature, and Si, Al and O are stable tetrahedral network structures. Apart from hydrofluoric acid, they are hardly decomposed by acid and alkali under normal temperature and pressure ^[4] . At home and abroad, a variety of processes have been studied for the use of potassium feldspar to prepare potassium fertilizer. The combination can be divided into: blast furnace smelting method, autoclave method, open leaching and closed thermostat method, thermal decomposition water immersion method, thermal method for bismuth soluble potassium. , acid decomposition method, sintering method, low temperature decomposition method, microbial method, etc. [引引. The insoluble potassium is changed into soluble or bismuth-soluble potassium. The current research methods have the disadvantages of serious pollution, high energy consumption, complicated process and many tailings residues.

Quartz and K-feldspar in phosphate-potassium ore are similar in physical properties, chemical composition and structural structure. Phosphorus-mineral minerals are various in variety, fine in size, irregularly embedded in each other, and the degree of crystallization of colloidal phosphate is low and contains various kinds of packages. Body, these factors determine that it belongs to difficult minerals ^[6] . The conventional method of flotation of SiO ₂ in the phosphorite ore is the hydrofluoric acid method. Since hydrofluoric acid is extremely toxic, it is necessary to explore a fluorine-free flotation method.

The raw material of this research comes from the Yiling area of ​​Yichang, Hubei Province. This mine is associated with phosphate-rich ore, and the phosphorus and potassium grades are relatively low. There is no economic value for the use of phosphate or potash alone. In this experiment, the phosphorus-potassium ore powder was firstly enriched by a fluorine-free and acid-free reverse flotation process. Then, according to the mechanism of ion exchange reaction, the ore powder was mixed with agricultural grade phosphoric acid and leached at low temperature. The dissolution rate of potassium was 95. %the above. The use of phosphate acid phosphate potassium ore, greatly reducing the acid hydrolysis temperature, but also facilitate the adjustment of N, P, K ratio in the later compound fertilizer. Compared with the traditional phosphorus and potassium extraction process of phosphorus and potassium, the method reduces energy consumption and optimizes reaction conditions. In this experiment, the reaction mechanism of each stage was also studied, and the main reaction was subjected to thermodynamic analysis.

Second, the experimental part

(1) Raw materials and instruments

The composition of the phosphate ore minerals is shown in Table 1.

Instruments: MBS3245 rod mill, XSHF ₂ -3 wet sampler, XFD-63 single tank flotation machine, 2XZ2 vacuum filter, PL203 electronic balance, DZF-6050 drying oven, etc.

(2) Test methods and test methods

After the phosphate rock is ground to a certain particle size by the crushing rod, some quartz in the phosphorus-bearing associated ore is removed by flotation under the condition of no fluorine and acid. After the flotation, the phosphorus and potassium concentrate is passed through a 0.074mm standard sieve, and then mixed with phosphoric acid in a certain ratio, firstly acidified in a constant temperature water bath at 80 ° C for a period of time, and then placed in a muffle furnace at a suitable temperature for a period of time. The heat source was turned off, allowed to cool to room temperature, and continued to mature for 1 h. Add water leaching, filtering, volume, mass filtrate fraction K ₂ 0 ^[7] was analyzed by sodium tetraphenylborate volumetric method.

(3) Test procedure

The test flow is shown in Figure 1.

Fig.1 Process of preparing compound fertilizer by low temperature co-impregnation of phosphorite ore and phosphoric acid

Third, the results and discussion

(1) Enrichment of phosphorus and potassium in phosphate ore

According to preliminary estimates, there are more than 20% of free SiO _{2 in the} ore powder. If part of the quartz can be removed, the grades of potassium and phosphorus will be greatly improved. In this experiment, phosphorus and potassium were enriched by flotation, NaOH was used as activator, diamine was used as collector , the pH of the ore was adjusted to about 8, and the flotation process is shown in Figure 2.

Flotation results: The mass fractions of phosphorus and potassium in raw ore, concentrate and tailings were measured by energy spectrometer. The results are shown in Table 2.

Table 2 Mass fraction of K ₂ O and P ₂ O ₅ before and after flotation %

It can be seen from Table 2 that the mass fraction of phosphorus and potassium in the concentrate is slightly higher than that of the ore; the ratio of phosphorus and potassium in the tailings is much lower than that of the ore. This result indicates that flotation can remove part of the quartz in the phosphorite ore, and it is feasible to carry out flotation pretreatment of the phosphorite ore.

(II) Exploration of acid hydrolysis reaction of phosphorus and potassium mine

The preliminary exploration results show that when the phosphate rock is mixed with phosphoric acid and directly put into the muffle furnace, the phosphorus and potassium dissolution rate increases as the amount of phosphoric acid increases. When the amount of phosphoric acid reaches a certain value, the dissolution rate of phosphorus and potassium increases slowly, which may be caused by partial phosphoric acid volatilization under higher temperature conditions. The test is carried out in an open vessel where the temperature rises and evaporation of the water vapor removes some of the phosphoric acid.

This test was carried out in two steps. In the first step, the phosphate rock is acidolyzed, and the reaction is controlled to be below 100 ° C, and H ₃ P0 _{4 is} substantially non-volatile. After a period of reaction, the phosphoric acid is converted to calcium dihydrogen phosphate. In the second step, the acid hydrolysis product was transferred to a 250 ° C muffle furnace, at which time Ca ^{2+ was} displaced with potassium feldspar (KAlSi ₃ 0 ₈ ), and the insoluble potassium became soluble potassium ions.

1. Effect of acid hydrolysis process of potassium phosphate on potassium dissolution rate

Experiment 1: 3 g of ore powder was uniformly mixed with 12 mL of 24.56% phosphoric acid, firstly acidified in a water bath at 80 ° C for 5 d, and then placed in a muffle furnace at 250 ° C for a period of time, and the product was leached with a water bath of 80 ° C.

Experiment 2: 3 g of ore powder was uniformly mixed with 12 mL of 24.56% phosphoric acid, and the mixture was directly placed in a muffle furnace at 250 ° C for a certain period of time without reaction in a water bath, and the product was leached with a water bath of 80 ° C.

The experimental results are shown in Figure 3. Figure 3 shows that the dissolution rate of potassium in phosphoric acid leaching conditions is much higher than that in the absence of acid leaching, indicating that phosphoric acid leaching is beneficial to increase the dissolution rate of potassium. As the reaction time prolongs, the dissolution rate of potassium increases. When the reaction is more than 3 hours, the reaction product is sintered into a block, which is not conducive to leaching. Therefore, the optimal ion exchange reaction time was chosen to be 3 h.

2. Exploration of acid hydrolysis time of phosphorus and potassium mine

The acid hydrolysis reaction temperature of phosphoric acid and collophanite is initially set at 80 ° C. In order for the two to fully react, the acid hydrolysis reaction time must be prolonged. 3 g of ore powder was uniformly mixed with 10 mL of 24.56% phosphoric acid, firstly acidified in a water bath at 80 ° C for a period of time, then placed in a muffle furnace and reacted at 250 ° C for 3 h, and the product was leached with a water bath of 80 ° C. The results are shown in Figure 4.

It can be seen from Fig. 4 that the potassium dissolution rate increases with the prolongation of the acid hydrolysis time, indicating that the longer the acid hydrolysis time, the more calcium ions participating in the displacement reaction in the solution, and the higher the potassium dissolution rate. When the acid hydrolysis time is short, the potassium dissolution rate is relatively low, which may be an excess of phosphoric acid after the acid hydrolysis reaction, and the excess phosphoric acid is largely volatilized at a higher temperature of the ion exchange reaction. When the acid hydrolysis time reached 5d, the potassium dissolution rate almost no increase, indicating that the calcium in the phosphate rock was basically dissolved during the acid hydrolysis process, so the optimal acid hydrolysis time was determined to be 5d.

(III) Effect of phosphoric acid dosage on experimental results

The acid hydrolysis reaction temperature was maintained at 80 ° C for 5 days; the ion exchange reaction temperature was 250 ° C for 3 hours. Take 3g of mineral powder and change the amount of phosphoric acid. The result is shown in Figure 5. As can be seen from Fig. 5, as the amount of phosphoric acid increases, the dissolution rate of potassium increases. When increasing the amount of phosphoric acid solution out of the increased K ^+, P and K Ore other metal ions ^{Na +, Al 3+, Cu 2+} , Fe 3+, Ca 2+, Mg 2+ , also consume phosphoric acid and phosphate to The form is present in the solution. At the same time, the increase of the amount of phosphoric acid increases the acid reactivity, thereby accelerating the ion diffusion rate and the decomposition rate of the collophosphate. When the amount of phosphoric acid reaches 12 mL, the dissolution rate of phosphorus is substantially unchanged, and the dissolution rate of potassium is slowly increased. Considering the reduction in energy consumption, the optimum amount of phosphoric acid is chosen to be 12 mL.

(4) Effect of ion exchange reaction temperature on experimental results

The acid hydrolysis reaction temperature was maintained at 80 ° C for 5 days; 3 g of ore powder was used, and the amount of phosphoric acid was 12 mL; the ion exchange reaction temperature was changed, and the reaction time was 3 h, and the results are shown in Fig. 6.

As seen from Fig. 6, as the reaction temperature increases, the dissolution rate of potassium increases. When the temperature rises, the viscosity of the phosphoric acid decreases, the diffusion rate of calcium ions in the phosphoric acid solution increases, and the ion exchange reaction speed increases, which is favorable for the reaction. When the temperature reached 250 ° C, the potassium dissolution rate increased slowly. The higher the temperature, the higher the cost, so choose the optimum temperature to be 250 °C.

The solid residue after the above reaction was analyzed by a spectrometer. The residue contained a small amount of phosphorus and potassium, indicating that most of the potassium in the ore was dissolved. It is estimated that the dissolution rate of phosphorus can reach over 95%. The amount of calcium and silicon in the residue is basically equal to that of the original ore, indicating that the calcium ion produced by the decomposition of phosphate rock is replaced by potassium feldspar to form anorthite.

The main components of the phosphoric acid potassium phosphate and phosphoric acid co-impregnation reaction products are calcium dihydrogen phosphate, potassium dihydrogen phosphate, free phosphoric acid and calcium feldspar, etc., and the soluble components and insoluble residues are separated by water leaching, and the basic ammonia nitrogen compound can be used. The leachate is neutralized or the leachate is added with a certain amount of potassium-containing compound (such as K ₂ S0 ₄ ) to be compounded, and the filtrate is evaporated, concentrated, and dried to obtain a nitrogen, phosphorus and potassium compound fertilizer.

Fourth, the feasibility analysis of the reaction process

(1) Analysis of the reaction process

The low-temperature co-impregnation reaction of phosphorus-potassium ore and phosphoric acid is mainly carried out in two steps. The first step is the acid hydrolysis reaction of colloidal phosphate rock, and the second step is the displacement reaction of calcium ion and potassium feldspar produced by acid leaching. The main principles are as follows:

First step: Ca ₅ (P0 ₄ ) 3F + 7H ₃ P0 ₄ + 5H ₂ 0 = 5Ca (H ₂ P0 ₄ ) ₂ · 2H ₂ 0 + HF

Second step: Ca ²⁺ +2KAlSi ₃ 0 ₈ =CaAl ₂ Si ₂ 0 ₈ +2K ⁺ +4Si0 ₂ ↓

24HF+2KAlSi ₃ 0 ₈ +8H ⁺ =2K ⁺ +2A1 ³⁺ +6SiF ₄ ↑+16H ₂ 0

The ion exchange reaction of Ca ²⁺ with potassium feldspar plays a major role in the second step reaction.

At the initial stage of the acid hydrolysis of the phosphate rock, the reaction proceeds only on the surface of the particles, and then under the action of phosphoric acid, the reaction site rapidly broadens and expands, and the reaction penetrates into the inner layer of the particles. In the middle stage of the reaction, the internal surface area has a great influence on the decomposition of the colloidal phosphate. The phosphoric acid in the solution has a dissociation equilibrium under certain conditions, and the dissociated hydrogen ions, anions and phosphate molecules all diffuse into the particles. In the later stage of the reaction, the soluble fraction of the phosphate rock particles is decomposed by phosphoric acid and is present in the form of phosphate in the solution ^[8] .

The low-temperature co-impregnation of phosphorus-potassium ore with phosphoric acid can promote the transformation of the crystal structure of K-feldspar in the phosphorite ore, the stability of the crystal structure is reduced, and the activity is improved, which is beneficial to the extraction of potassium. When the system reaches a certain temperature, the degree of K ⁺ deviates from the original position, the KO bond breaks, the free K ^{+ is} formed, the Ca ²⁺ radius is small, and there is strong polarization, and Ca ²⁺ will enter the cavity formed by the ring. In the middle, occupy some positions of the lattice nodes in the K-feldspar crystals, destroy the order of the original particle arrangement of the K-feldspar, and form an intermediate solid solution. The potassium feldspar lattice is distorted, the crystal structure is incomplete, and it is in an unstable state, and the reaction ability is greatly enhanced.

Potassium feldspar crystals change from monoclinic system to triclinic system. Because feldspar skeleton does not adapt to the flexibility of tetrahedral rotation, Ca ²⁺ only slightly increases the average value of TOT angle, and the skeleton expansion is also very small. the binding force of the silicon crystal framework tetrahedra aluminum oxide backbone is weak compared with a certain fluidity, so that the replacement of Ca ²⁺ K ^+, silicates frame structure of the process unit fELDSPAR - sialon The tetrahedron is not destroyed and its entire skeleton is not destroyed.

(2) Reaction thermodynamic analysis

A Gibbs function is obtained by calculating or looking up a table to obtain a standard molar generation Gibbs function of each reactant and product. The Gibbs free energy ΔrG _m ^θ is a criterion for judging the direction in which a chemical reaction proceeds. When ΔrG _m ^θ <0, the reaction can proceed spontaneously; when ΔrG _m ^θ =0, the reaction reaches equilibrium; when ΔrG _m ^θ >0, the reaction cannot proceed spontaneously.

ΔrG _m ^θ is the change in free energy of both the reactant and the product in a standard state. It can only determine the direction of change under certain conditions, and in reality, the reactants and products are not necessarily in a standard state. But according to the isothermal form:

ΔrG _m = ΔrG _m ^θ + RTInQ The ΔrG _m value was calculated by experiments to determine the direction in which the reaction proceeded. The results are shown in Table 3.

It can be seen from Table 3 that in the range of 298 to 523 K, the ΔrG _T ^{θ of the} main reaction is negative, indicating that the reaction can proceed spontaneously. The degree of spontaneous decomposition of the acid hydrolysis reaction decreases with increasing temperature; the ion exchange reaction increases spontaneously with increasing temperature, which is consistent with the conclusion of Experiment 2.4.

V. Conclusion

a. The process uses reverse flotation to pretreat the phosphate rock, overcomes the serious corrosion problem of the positive flotation process equipment, and improves the equipment operation rate; the flotation under alkaline conditions overcomes the environmental pollution of the acid medium. The results of the flotation showed that the grades of potassium and phosphorus were several percentage points higher than before the flotation.

b. The optimal reaction conditions of the process are: acid hydrolysis reaction temperature 80 ° C, reaction time 5 d, phosphoric acid dosage 12 mL (/3 g phosphorus potassium ore); ion exchange reaction temperature 250 ° C, reaction time 3 h. Under these conditions, the dissolution rates of phosphorus and potassium can reach more than 95%.

c. The experimental results show that the low-temperature co-impregnation of phosphorite ore with phosphoric acid can promote the transformation of the crystal structure of K-feldspar in the phosphorite ore, the stability of the crystal structure is reduced, and the activity is improved, which is beneficial to the extraction of potassium. The presence of phosphoric acid in the system accelerates the dissolution and diffusion of the solid phase and greatly reduces the reaction temperature. The FTIR results show that the phosphate rock in the phosphate-potassium ore is decomposed by phosphoric acid to form calcium ions, and the calcium ions are replaced with potassium feldspar in the phosphorite ore to form anorthite. The basics of feldspar in the whole reaction process. The constituent unit Si-Al-O tetrahedral structure is not destroyed.

d. The main components of the co-impregnation reaction of potassium ore and phosphoric acid are calcium dihydrogen phosphate, potassium dihydrogen phosphate, free phosphoric acid and anorthite. The soluble components and insoluble residues are separated by water leaching, and the soluble phosphorus and potassium components are obtained by filtration. Leaching solution. The leaching solution thus obtained contains soluble phosphorus and potassium and excess free phosphoric acid in the reaction, and the pH of the leaching solution is low, and can not be directly applied to the farmland as a fertilizer, and the leaching solution can be carried out with a basic ammonia nitrogen compound. with. At the same time, a certain amount of potassium-containing compound (such as K ₂ S0 ₄ ) is added to the leach solution for compounding, and the filtrate is evaporated, concentrated, and dried to obtain a nitrogen, phosphorus and potassium compound fertilizer.

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