Study on the mechanism and thermodynamics of potassium leaching from phosphorus-potassium associated ore

Abstract

This paper has mainly focused on the technology of transforming potassium minerals from phosphate-potassium associated minerals into soluble potassium by acid hydrolysis, and discussed the leaching mechanism of potassium in the process of leaching. As it is shown from the research, in the acid leaching system, [H⁺] = 8.71 mol L^–1, m_NH4F:m_{phosphorushboxpotassiumassociatedore} = 0.25, the leaching rate of potassium reaches 74% at 413 K for 2 h. And the release of potassium ion is the result of the interaction of “oxidation” and “ion exchange”. It was found from the Eh-pH diagram of K-(Si)-(P)-(F)-H₂O system that, when P-F or Si-F appeared in K-H₂O system, the K⁺ stable region became narrow, and the excess fluorine was not conducive to the stability of K⁺ in aqueous solution.

Keywords

Phosphorus-potassium associated ore potassium leaching mechanism thermodynamics

1 Introduction

Phosphorus-potassium associated ore is a resource mineral associated with phosphate rock [1, 2] and potassium-bearing ore [3, 4].

There are huge reserves of phosphorus and potassium associated minerals in China. Take the large-scale phosphorus-potassium associated mineral deposits in Yichang, Hubei and other places as an example. The reserves are 3 billion tons, mainly concentrated in the two mining areas of Zhongping and Taiyangshan in Yiling area, Yichang. The mineral composition of P₂O₅ + K₂O is about 14%, and Al₂O₃ + Fe₂O₃ is about 16%, and acid insoluble (AI) is 53% (the main component is SiO₂) [5].

Due to its low grade, phosphorus-potassium associated ore is currently only used as a phosphate rock. For the scarcity of potassium resources in China and the occurrence characteristics of phosphorus-potassium associated minerals, it is of great significance to study the extraction technology of potassium minerals from phosphorus-potassium associated minerals for its comprehensive utilization.

The current process of changing insoluble potassium ore into water-soluble/citric-soluble potassium salt can be divided into four categories: high temperature sintering method, hydrothermal method, microbial method and acid hydrolysis method [6, 7].

The high temperature sintering method is that potassium feldspar, additives (mainly calcium salt) and coal are used as raw materials, which are crushed and pelletized and roasted in a high-temperature kiln to generate citric-soluble potassium salt. Haseli[8] reports the application of a eutectic NaCl-CaCl₂ salt system for the extraction of potassium from ultrapotassic microsyenite. Approximately 70% of K⁺ in the deposit was extracted at 650°C. The salt as a melting agent offers a reduction in the reaction temperature due to its lower melting temperature when compared to pure salts (NaCl or CaCl₂).

The hydrothermal method is to destroy the silicon-aluminum tetrahedron structure of feldspar by high temperature and high pressure to achieve the purpose of potassium leaching. Analcime single crystals were successfully synthesized from natrolite syenite powder (K₂O 10.89%) and 92.6% of potassium was extracted simultaneously by means of soda roasting followed by alkali-hydrothermal method. K⁺ leaching is found to be as follows: 175°C for 4 h in 0.5 mol·L^–1 NaOH [9].

The microbial method refers to the direct or indirect interaction of microbial strains with potassium feldspar, which destroys the crystal lattice of minerals and release soluble potassium. Peng Mi and Yuan Wengong [10] isolated a silicate bacterium YS6 from samples from the Yanshan Hongsheke bauxite mining area. This strain has strong potassium-dissolving activity and phosphorous-dissolving activity on mica, which are 322.3% and 608.7% higher than those of general Bacillus.

The acid hydrolysis method uses the synergistic promotion principle of strong inorganic acid and cosolvent to decompose minerals rich in potassium salts into water-soluble potassium salts. Sun Xuefei [11] established a co-leaching reaction system of phosphate rock, potash feldspar and phosphoric acid. The reaction temperature is 250°C for 3h, the leaching rate of phosphorus and potassium can be above 95% . Lan Fangqing [12] studied the process of extracting potassium from potassium feldspar-fluorite-sulfuric acid-fluorosilicic acid system. The results show that the optimal process conditions for potassium extraction in this system are: m(fluorite): m(potassium feldspar) = 0.35 : 1, the dosage of 60% acid and 10% fluorosilicic acid are respectively m(H₂SO₄): m(potassium feldspar) = 1.35 : 1, m(H₂SiF₆): m(potassium feldspar) = 0.162 : 1, the reaction temperature is 120°C, and the reaction time is 3.5 h. Under these conditions, the extraction rate of potassium reaches 97.2% .

The high temperature sintering method has high energy consumption, numerous greenhouse gas emissions, and single product structure [13], so the technology has not been promoted. In the hydrothermal process, the equipment has been operating under high temperature, high pressure and high corrosion conditions for a long time, causing serious corrosion and potential safety accidents. In the microbial methods, the steps of bacterial culture and reproduction are cumbersome, the batch conversion cycle is long, the efficiency is low, and it occupies a large amount of working capital and turnover land. Fluoride additives in acid hydrolysis can destroy the stable structure of potash feldspar and effectively reduce the reaction temperature, thus improving the leaching efficiency of potash, and in addition, phosphorus-potassium ore has the inherent advantage–the associated fluorine in the phosphate rock. So, with this method, there is no need or only a small amount of supplementary fluorine additives in the leaching process. In summary, the acid hydrolysis method has the most prospects for industrial promotion.

This paper focuses on the study of the efficiency and thermodynamics of potassium leaching in the process of acid hydrolysis of phosphorus-potassium associated ore, and explores the mechanism of efficient extraction of soluble potassium, in order to provide theoretical support for optimizing the new technology of efficient preparation of water-soluble potassium and realizing the efficient utilization of phosphorus- potassium associated ore resources.

2 Materials and methods

2.1 Associated phosphorus and potassium ore

The samples of associated phosphorus and potassium ore of middle-low grade used in the experiments were obtained from Yichang, Hubei Province, China. The major chemical components of the associated ore are shown in Table1.

Both results of Table 1 and Fig. 1 indicated that the major mineral phases of the associated phosphorus and potassium ore were apatite(Ca₅(PO₄)₃F), triplite((FeMn)₂PO₄F)[14,15, 14,15] and potassium feldspar(KAlSi₃O₈) (Fig. 1), and impurities were albite(NaAlSi₃O₈), quartz(SiO₂) et al. [16, 17].

Table 1
Chemical composition of the associated phosphorus and potassium ore (wt.%)

Components Mass fraction Components Mass fraction

P₂O₅ 21.36 Fe₂O₃ 2.45

K₂O 4.7 MgO 1.37

SiO₂ 26.46 CO₂ 1.46

CaO 29.18 F 2.3

Al₂O₃ 7.74 Na₂O 0.56

Components	Mass fraction	Components	Mass fraction
P₂O₅	21.36	Fe₂O₃	2.45
K₂O	4.7	MgO	1.37
SiO₂	26.46	CO₂	1.46
CaO	29.18	F	2.3
Al₂O₃	7.74	Na₂O	0.56

Fig. 1

XRD pattern of the Ca₅(PO₄)₃F-KAlSi₃O₈ associated mineral.

2.2 Reagents and equipments

Reagents (analytically pure): sulfuric acid (H₂SO₄), ammonium fluoride (NH₄F).

Equipments: (XMB-67, 200^*240) rod mill, (2XZ2) vacuum filter, (101-4) electrothermal drying oven, (FA2104N) electronic balance and (TAS990) atomic absorption spectrophotometer.

2.3 Methods

To obtain optimal technological conditions. First, the sample ore is crushed by rod mill for further testing and experimentation until 95.64 wt.% the associated phosphorus and potassium ore is less than 0.074 mm. Then the ore, sulfuric acid and ammonium fluoride were mixed in the teflon crucible proportionally and reacted 2 h at 413 K. Finally, the samples were cooled to room temperature. The materials in teflon crucible were transferred to volumetric flask and diluted to 50 mL with water.

The sample was then filtered and the filtrate was analyzed by Flame Atomic Absorption Spectrometry (FAAS) to determine the K⁺ content[18]. The structure of filter residue was analyzed by using XRD. $The dissolution rate of potassium (wt . % =$ $\frac{Amount of K_{2} O in leaching solution}{Amount of K_{2} O in phosphorusand potassium} \times 100 %$

2.4 Characterizations

The chemical composition of the Ca₅(PO₄)₃F-KAlSi₃O₈ associated mineral was determined by using X-Ray Fluorescence (XRF) on an Axios advanced spectrometer with a SST-mAX X-ray tube (4.0 kW).

The structure of the Ca₅(PO₄)₃F-KAlSi₃O₈ associated mineral was observed by XRD (RU-200B/D/MAX-RB, Rigaku Corporation, Japan) using Cu Kα radiation (λ= 0.154 nm) (40 kV, 50 mA) over the scanning range 2θ= 5–70° with a step width of 2°/min.

The content of potassium in the filtrate was measured by Flame Atomic Absorption Spectrometry (TAS-990, Persee Corporation, China) by using K-element-light with 404.41 nm wavelength.

3 Experimental conclusions

The mineral particle size is –0.074 mm, accounting for 95.64 wt.%, m_NH4F:m_{Ca5 (PO4) 3F}_hboxKAlSi3O8 _{associated mineral} = 0.35, [H⁺] = 2.37, 4.35, 6.03, 7.46, 8.71, 9.80, 10.76, 11.61, 12.37, 13.06 mol L^–1, reacted 2 h at 413 K.

The effect of [H⁺] on potassium leaching rate is shown in Fig. 2.

Fig. 2

Relationship between sulfuric acid content and dissolution rate of potassium under different reaction time.

It can be seen from Fig. 2 that the leaching rate of potassium is increasing with the increase of [H⁺]. When [H⁺] = 8.71–10.76 mol L^–1, the leaching rate of potassium is stable at about 68%, with [H⁺] continues to increase, the potassium leaching rate shows a downward trend. As the amount of sulfuric acid increases, [H⁺] tends to be saturated, and the potassium leaching rate reaches the highest value, however, excessive SO₄^2– combines with Ca²⁺ to form CaSO₄, which is adsorbed on the surface of minerals to form a passive film, which hinders the reaction of minerals and sulfuric acid, reduces the precipitation rate of soluble potassium, and the potassium leaching rate shows a downward trend.

The mineral particle size is –0.074 mm, accounting for 95.64 wt.%, [H⁺] = 8.71 mol·L^–1, m_NH4F:m_{Ca5 (PO4) 3F}_hboxKAlSi3O8_{associated mineral} = 0.025, 0.050, 0.100, 0.150, 0.200, 0.250, 0.300, 0.350, reacted 2 h at 413 K.

The effect of the ratio of m_NH4F:m_{associated phosphorus and potassium ore} on the potassium leaching rate is shown in Fig. 3.

Fig. 3

Effect of m_NH4F:m_{associated phosphorus and potassium ore} on dissolution rate of potassium.

It can be seen from Fig. 3 that as the ratio of m_NH4F:m_{associated phosphorus and potassium ore} increases, the potassium leaching rate first increases and then decreases; when m_NH4F: m_associated _{phosphorus and potassium ore} = 0.25, the potassium leaching rate peaks at 74% . The added F^– and free H⁺ generate HF. The strong oxidizing property of HF causes the oxidation and fracture of the Si-O and Si-Al bonds in the potassium feldspar silica tetrahedron, and K⁺ is released into the solution. But as the ratio increases, the excess F^– react with K⁺ in the solution to form K₂SiF₆ precipitation, and [K⁺] decreases.

4 Characterization analysis

X-ray diffraction analysis (XRD) was performed on the filter residue after the reaction with [H⁺] of 6.03, 8.71, 10.76, and 13.06 mol·L^–1. The spectrum is shown in Fig. 4.

It can be seen from Fig. 4 that the diffraction intensity of potassium feldspar decreases with the increase of [H⁺]. When [H⁺] is not high, H⁺ is not enough to destroy the crystal lattice structure of the potassium feldspar mineral, with the increase of [H⁺], the diffraction intensity of calcium sulfate decreases and the anorthite diffraction intensity increases. Potassium leaching changes from the “oxidation” mode to the “oxidation”+“ion exchange” combination mode[19 –22].

Fig. 4

XRD patterns of leaching residues with different sulphuric acid dosage.

X-ray diffraction analysis (XRD) was performed on the filter residue after the reaction of m_NH4F:m_{associated phosphorus and potassium ore} at 0.1, 0.25, 0.3 and 0.35. The spectrum is shown in 5.

It can be seen from Fig. 5 that the diffraction intensity of potassium feldspar decreases with the increase of the ratio of m_NH4F:m_{associated phosphorus and potassium ore}, which is conducive to the “oxidation” of potassium leaching. As the ratio of m_NH4F:m_{associated phosphorus and potassium ore} increases, the anorthite diffraction intensity decreases, and the increase in the amount of ammonium fluoride will hinder the “ion exchange”.

Fig. 5

XRD patterns of leaching residues with different ammonium fluoride ratio.

Combined with the analysis of Figs. 4 5, the initial reaction only proceeds on the surface of the mineral. The cosolvent NH₄F reacts with sulfuric acid to produce HF, (NH₄)₂SO₄ and other products, and HF is further hydrolyzed to produce (HF)₂, HF₂^– and F^–[22], destroying the silico-alumina-oxy tetrahedral structure, and K⁺ is released from mineral inclusions. In the middle of the reaction, the activation points involved in the reaction increase. When the reaction is heated to a certain temperature, the activation energy of the reaction increases, and the feldspar structure framework expands to cause K⁺ to have a certain fluidity[23, 24], and the free Ca²⁺ radius is smaller than K⁺, it is easy to release the activated K⁺ from the potassium feldspar structure to form anorthite, and ion exchange participates in the reaction[25].

5 Thermodynamics research

The Eh-pH diagram is based on the fact that a given equilibrium can be affected by pH and Eh in the solid-liquid equilibrium system. The equilibrium can be expressed in a form of composition and compositional concentration, showing the relationship between potential Eh and pH in two-dimensional plane (or possibly three-dimensional or multidimensional spaces).

In this paper, HSC chemistry software simulation was used to study the potassium ion leaching exploration in single system and multi-system. Around the regional change of “soluble substances” in EH pH diagram, analyze the influence of main substances on the optimal potassium dissolution rate.

The products of the main concern factors in the potassium leaching system in the aqueous solution and the thermodynamic data of each product at 298.15 k are shown in Table 2 [26, 27].

Table 2
Thermodynamic data of the main attention factors under 298.15 k [24, 25]

Number Substance ΔG/kJ·mol^–1 Number Substance ΔG/kJ·mol^–1

1 K 0 13 HPO₃ –1027.71

2 K₂O –320.969 14 H₃PO₂ –655.199

3 KOH –448.793 15 H₃PO₃ –1019.19

4 K⁺ –282.281 16 H₃PO₄ –1342.37

5 Si 0 17 HPO₃^2– –974.002

6 SiO₂ –923.213 18 HPO₄^2– –1289.03

7 HSiO^3– –1152.11 19 H₂PO^2– –643.841

8 SiO₃(OH)^3– –1326.3 20 H₂PO^3– –1008.96

9 F^– –331.42 21 HPO₄^2– –1289.03

10 HF –349.374 22 H₂PO^3– –1008.96

11 HF^2– –683.055 23 H₂PO^4– –1330.16

12 P 0 24 PO₄^3– –1218.55

Number	Substance	ΔG/kJ·mol^–1	Number	Substance	ΔG/kJ·mol^–1
1	K	0	13	HPO₃	–1027.71
2	K₂O	–320.969	14	H₃PO₂	–655.199
3	KOH	–448.793	15	H₃PO₃	–1019.19
4	K⁺	–282.281	16	H₃PO₄	–1342.37
5	Si	0	17	HPO₃^2–	–974.002
6	SiO₂	–923.213	18	HPO₄^2–	–1289.03
7	HSiO^3–	–1152.11	19	H₂PO^2–	–643.841
8	SiO₃(OH)^3–	–1326.3	20	H₂PO^3–	–1008.96
9	F^–	–331.42	21	HPO₄^2–	–1289.03
10	HF	–349.374	22	H₂PO^3–	–1008.96
11	HF^2–	–683.055	23	H₂PO^4–	–1330.16
12	P	0	24	PO₄^3–	–1218.55

Assuming that the total concentration of K and P in the aqueous solution is 0.1 mol·L^–1, the total concentration of the Si aqueous solution is 0.01 mol L^–1, the total concentration of F in the aqueous solution is 1 mol·L^–1; p_H2 = p_O2 = 1.013×10⁵ Pa. The E_h-pH diagram of the unit system is shown in Fig. 6.

Fig. 6

E_h-pH diagrams of the unit system.

Figure 6 (a), (b), (c), (d) respectively show the E_h-pH diagrams of K-H₂O, P-H₂O, Si-H₂O and F-H₂O systems. The upper and lower dotted lines represent the O₂/H₂O and H₂O/H₂ stable regions. The results show that: when pH = 0 –14, the elements K, P, and F are all ionic. In the Si-H₂O system (Fig. 6 (c)), the stable area of silicon in the aqueous solution is dominated by solid SiO₂, and under high alkaline conditions (pH > 11.3), silicon is dominated by the soluble ion SiO₃(OH)₃^–. Element Si is unstable in the region above –1.0 V_SHE and pH = 0, far below the Eh value of the water stability line, and is easily oxidized to form SiO₂[20 , 28–31].

In the potassium leaching system, K forms a binary, ternary, and quaternary system with Si, P, and F. The compounds formed between the elements are K₂PF₆ and K₂SiF₆, and their standard Gibbs function free energies are –2417.33 kJ·mol^–1 and –3023.35 kJ·mol^–1, respectively.

At 298.15 k and normal pressure, the activity of K is 0.1 mol·L^–1, and the activities of Si, P and F are 0.01 mol·L^–1, 0.1 mol·L^–1 and 1 mol·L^–1. The E_h-pH diagrams of binary, ternary and quaternary systems are shown in Fig. 7.

Fig. 7

E_h-pH diagrams K system.

Figure 7 (a), (b) and (c) show the E_h-pH of the K-Si-H₂O, K-P-H₂O and K-F-H₂O binary systems, respectively, indicating that potassium ions will not form complexes when contacting with the elements Si, P or F alone.

In the K-F-P-H₂O ternary system (Fig. 7(d)), under high acid conditions, K⁺ and PF₆^– produce KPF₆, and the stable area of K⁺ is reduced. In the K-F-Si-H₂O ternary system (Fig. 7(e)), the K₂SiF₆ stable zone is composed of vertical –0.8 < V_SHE < 2.2 and horizontal –3.7 < pH < 6.4, K⁺ is unstable when the pH is above –1.0V_SHE and pH = 0∼6.3, and it is easy to transform into K₂SiF₆. Fig. 7(f) (shows the Eh-pH of the K-Si-P-F-H₂O system) compared with Fig. 7(d) and Fig. 7(e), it is found that when the element P is present, the stable region of K₂SiF₆ changes from the vertical –1.0 < V_SHE < 2.0 and horizontal –3.9 < pH < 6.3 extended to the vertical –3.0 < V_SHE < 5.0 and horizontal –3 < pH < 7.2 regions, and the stable area of K⁺ was greatly reduced[20 , 28–31].

Comparing Fig. 7(a) with Fig. 7(e), Fig. 7(b) and Fig. 7(d), it can be seen that when element F is present, K⁺ generates complex precipitation, which causes the main area of K⁺ to narrow. The presence of excess fluorine is not conducive to the stability of K⁺ in the aqueous solution.

6 Conclusion

The optimal reaction conditions for potassium leaching: [H⁺] = 8.71 mol·L^–1, m_NH4F:m_{associated phosphorus and potassium ore} = 0.25, reacting at 413 K for 2 h, the potassium leaching rate is 74% .

In the leaching system, when the free F^– and H⁺ contact with the surface of the feldspar, the Si-O-Al structure is destroyed and K⁺ is released, and the potassium leaching is mainly in the form of oxidation. At the same time, when the reaction system is heated to a certain temperature, K⁺ has a certain fluidity due to the expansion of feldspar structure framework. Ca²⁺ replaces K⁺ in potassium feldspar to form anorthite, and “ion exchange” participates in the reaction. And the release of potassium ions is the result of the combined action of “oxidation” and “ion exchange”.

When P appears in the K-Si-F-H₂O system, the stable area in the plot for K₂SiF₆ expands, and the stable area in the plot for K⁺ shrinks. When P-F or Si-F appears in the K-H₂O system, the KPF₆/K₂SiF₆ stable region will be formed, and the K⁺ stable region will be narrowed. Excessive presence of fluorine is not conducive to the stability of K⁺ in aqueous solution.

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, Metal removal from silicon surfaces in wet chemical systems, J. Journal of Electronic Materials 24 (1995), 397–404. DOI: 10.1007/BF02659705.

Study on the mechanism and thermodynamics of potassium leaching from phosphorus-potassium associated ore

Abstract

Keywords

1 Introduction

2 Materials and methods

2.1 Associated phosphorus and potassium ore

Table 1 Chemical composition of the associated phosphorus and potassium ore (wt.%) Components Mass fraction Components Mass fraction P2O5 21.36 Fe2O3 2.45 K2O 4.7 MgO 1.37 SiO2 26.46 CO2 1.46 CaO 29.18 F 2.3 Al2O3 7.74 Na2O 0.56

2.3 Methods

2.4 Characterizations

3 Experimental conclusions

References

Table 1
Chemical composition of the associated phosphorus and potassium ore (wt.%)

Components Mass fraction Components Mass fraction

P₂O₅ 21.36 Fe₂O₃ 2.45

K₂O 4.7 MgO 1.37

SiO₂ 26.46 CO₂ 1.46

CaO 29.18 F 2.3

Al₂O₃ 7.74 Na₂O 0.56