Method for efficiently extracting lithium from lepidolite under normal pressure based on synergistic activation and targeted impurity removal

Abstract

The invention belongs to the technical field of hydrometallurgy and comprehensive utilization of resources, and relates to a method for extracting lithium from lepidolite at normal pressure and high efficiency based on synergistic activation and targeted impurity removal. The method comprises the following steps of mechanochemical activation-pre-deconstruction, hydrochloric acid normal-pressure reinforced leaching, fluorine and aluminum synergistic precipitation and recycling, deep removal of iron, calcium and magnesium targeted precipitation and lithium recovery. According to the method disclosed by the invention, the lithium extraction process is optimized, and the lepidolite concentrate is subjected to crystal pre-deconstruction by combining mechanical and chemical activation, so that extremely high reaction activity is created for subsequent acid leaching reaction under normal pressure; then, fluorine, aluminum and iron are removed by accurately controlling reaction conditions, and finally, a composite precipitator is added to deeply remove calcium and magnesium. The lithium extraction and impurity removal method is simple in technological process, high in lithium recovery rate, low in impurity content and wide in applicability.

Description

Technical Field

[0001] This invention belongs to the field of hydrometallurgy and comprehensive resource utilization technology, and relates to a method for efficient lithium extraction from lepidolite under normal pressure based on synergistic activation and targeted impurity removal. Background Technology

[0002] Lithium mica, as an important lithium resource, faces challenges in efficient lithium extraction. Traditional sulfuric acid or sulfate roasting methods are energy-intensive, produce heavy waste gas pollution, and are prone to lithium loss due to the formation of fluorosilicate complex salts. Although the hydrochloric acid method has strong reactivity, direct atmospheric pressure leaching has technical bottlenecks, such as kinetic limitations. The stable aluminosilicate crystal structure of lepidolite is difficult to effectively destroy under atmospheric pressure and moderate acid concentrations, resulting in lithium leaching rates generally below 85%, leading to poor economic efficiency. Another challenge is purification; the presence of Al in the leachate... 3+ Fe 3+ Ca 2+ Mg 2+ and F - High concentrations lead to mutual interference and the formation of stable complexes. Traditional fractional precipitation methods are cumbersome, consume a lot of reagents, and are difficult to remove calcium and magnesium deeply, affecting the purity of the final lithium product.

[0003] Existing technologies have made numerous attempts to improve lithium extraction from lepidolite, but these often encounter new challenges. On the one hand, fluoride-enhanced leaching processes, adopted to overcome leaching rate bottlenecks, inevitably introduce more destructive fluoride pollution and corrosion problems. On the other hand, solvent extraction technologies, borrowed to achieve deep separation of impurities, while achieving acceptable purification results, lack universality in the cost-effective mineral hydrometallurgical industry due to their high investment and operating costs, complex operating units, and potential environmental risks associated with organic solvents.

[0004] Therefore, developing a green and low-cost method that can achieve high lithium leaching rates under normal pressure and achieve deep purification of impurities through a simple process is a technical challenge to be solved in this field. Summary of the Invention

[0005] To address the aforementioned shortcomings, this invention provides a method for efficient lithium extraction from lepidolite under ambient pressure based on synergistic activation and targeted impurity removal. This lithium extraction method features a simple process flow, high lithium recovery rate, low cost, and environmental friendliness, effectively solving the kinetic limitations and purification dilemmas in existing technologies.

[0006] The purpose of this invention is to provide a method for efficient lithium extraction from lepidolite under ambient pressure based on synergistic activation and targeted impurity removal, comprising the following steps: (1) After mixing lepidolite concentrate with a crystal transformation aid, the mixture is subjected to mechanochemical activation in a planetary ball mill to obtain activated powder; in this step, under strong mechanical force, the Na in the crystal transformation aid is activated. + or K+ Driven by highly active sites on the surface of newly formed minerals, lepidolite can be partially embedded into the interlayer structure, undergoing preliminary ion exchange and generating a large number of lattice defects and microcracks. This process pre-deconstructs its stable silicon-oxygen tetrahedral-aluminum-oxygen octahedral layered structure, greatly increasing the specific surface area and chemical activity, and creating a kinetic advantage for subsequent acid leaching.

[0007] (2) The activated powder obtained in step (1) is reacted with hydrochloric acid under normal pressure for 2-3 hours, and a selective scale inhibitor is added to the reaction system simultaneously. After the reaction is completed, the solid and liquid are separated to obtain the leachate. In this step, the activated minerals react with hydrochloric acid, and lithium, aluminum, iron, potassium and other minerals enter the solution in ionic form, while releasing F. - And silicic acid, that is, to dissolve lithium and impurities; then in an acid leaching environment, the dissolved silicic acid readily reacts with F. - Na + / K + The reaction produces insoluble fluorosilicates (such as Na₂SiF₄), which coat the surface of unreacted mineral particles, hindering mass transfer. This is the main reason for the low leaching rate of conventional methods. However, the added scale inhibitors, such as citric acid or sodium chloroacrylate, preferentially react with silica molecules or the generated SiF₆. 2- By combining steric hindrance and electrostatic repulsion, the nucleation and growth of fluorosilicate are inhibited, preventing the densification of the solid fluorosilicate film, thereby keeping the reactive channels on the mineral surface open and ensuring a full reaction under normal pressure.

[0008] (3) Slowly add calcium source to the leachate obtained in step (2), and precisely control the pH to 3.5-4.5. Stir the reaction for 0.5-1 h, and separate the solid and liquid to obtain filtrate A; in this step, by controlling the pH to within 3.5-4.5, Al 3+ It begins to slowly hydrolyze to produce positively charged Al(OH)₂. 2+ Al(OH) 2+ Hydroxyl-bridged polymers; meanwhile, Ca 2+ With F - The reaction produces CaF2 precipitate; while the newly formed, highly surface-active CaF2 particles become Al. 3+ The ideal nucleation matrix for hydrolysis products is formed when the two substances are tightly bound together through electrostatic adsorption and co-precipitation, forming CaF2-Al(OH)3 composite flocs. This synergistic effect not only significantly improves the removal efficiency of fluorine but also transforms harmful fluorine and aluminum into stable solid products that can be used as metallurgical additives.

[0009] (4) Adjust the pH of the filtrate A from step (3) to 5.0-5.5 with alkali, so that Fe 3+ After complete hydrolysis and precipitation, the solution was filtered to obtain filtrate B. In this step, the pH was controlled between 5.0 and 5.5, and the Fe... 3+The hydrolysis and precipitation are complete, forming Fe(OH)3 colloids or crystals, which are then removed.

[0010] (5) Heat the filtrate B obtained in (4) to 60-80℃, add the composite precipitant, stir the reaction for 0.5-1.5h, filter, and obtain a high-purity lithium chloride solution; in this step, heating the filtrate B to 60-80℃ increases the ionic activity and precipitation rate, while the phosphate ions in the composite precipitant can form Ca3(PO4)2 with very low solubility (Ksp=2.0×10⁻⁶) with calcium and magnesium ions respectively. -29 ) and Mg3(PO4)2 (Ksp≈1.0×10 -24 However, magnesium precipitation using phosphate alone is slow and may form a gel-like substance. Oxalate (C2O4) 2- ) and Ca 2+ It produces extremely insoluble CaC₂O₄·H₂O (Ksp = 2.3 × 10⁻⁶). -9 It is a highly efficient reagent for removing calcium, but it is not effective against Mg. 2+ Poor performance (MgC2O4, Ksp≈8.6×10) -5 The key to this step is that phosphate and oxalate have a synergistic precipitation effect, PO4 3- The addition of [a substance] alters the ion balance and precipitation interface properties in the solution, inducing the formation of a denser and more easily filterable mixed crystalline precipitate of (Ca,Mg)3(PO4)2 and CaC2O4, thereby achieving [the desired effect] on Ca [precipitate]. 2+ and Mg 2+ Synchronous, deep, and efficient removal.

[0011] (6) After concentrating the high-purity lithium chloride solution obtained in step (5), lithium chloride product is obtained, or sodium carbonate is added to precipitate it into battery-grade lithium carbonate.

[0012] This invention first mixes lepidolite concentrate with a crystal transformation aid and performs a mechanochemical activation reaction in a planetary mill. This effectively embeds the aid ions into and disrupts the layered structure of lepidolite, achieving pre-crystal deconstruction and enhancing the reaction activity and kinetics for subsequent acid leaching. Then, hydrochloric acid is added to enhance leaching under normal pressure, while a scale inhibitor is added to effectively suppress the formation of fluorosilicate complex salts and prevent the encapsulation of unreacted nuclei. Next, a calcium source and alkaline solution are added, and by precisely controlling the reaction conditions, especially pH, fluorine, aluminum, and iron are removed. Finally, a composite precipitant is added to the filtrate to remove calcium and magnesium. This method has a simple process flow and a high lithium recovery rate.

[0013] Preferably, in step (1) of the above technical solution, the crystal transformation aid is at least one of sodium chloride and potassium chloride; the mass ratio of lepidolite concentrate to crystal transformation aid is 100:1-5.

[0014] Preferably, in step (1) of the above technical solution, the particle size D90 of the activated powder is ≤15μm.

[0015] Preferably, in step (2) of the above technical solution, the concentration of hydrochloric acid is 5-8 mol / L, and the solid-liquid ratio of the activated powder to hydrochloric acid is 1:3-5 g / mL.

[0016] Preferably, in step (2) of the above technical solution, the scale inhibitor is citric acid or sodium polyacrylate, and its addition amount is 0.5-2% of the mass of the lithium mica concentrate. The scale inhibitor added in this technical solution preferentially combines with fluoride ions and dissolved silica in the reaction system, which can effectively inhibit the formation of fluorosilicate complex salts and avoid their encapsulation of unreacted nuclei, thereby significantly improving lithium leaching kinetics and final leaching rate.

[0017] Preferably, in step (2) of the above technical solution, the reaction temperature is 90-100℃.

[0018] Preferably, in step (3) of the above technical solution, the calcium source is Ca(OH)2 or CaCl2. In this technical solution, adding a calcium source and controlling its pH between 3.5 and 4.5 can effectively control the progression of the reaction sequence. Under these conditions, fluoride ions in the reaction system preferentially react with calcium ions to form CaF2 precipitate. Simultaneously, some aluminum ions hydrolyze to form Al(OH)3, which co-precipitates with CaF2, greatly enhancing the fluoride removal efficiency. Furthermore, this precipitate, after separation, can be utilized as a metallurgical auxiliary material.

[0019] Preferably, in step (5) of the above technical solution, the composite precipitant is a mixture of sodium phosphate and sodium oxalate in a molar ratio of 1:0.5-1.5. In this technical solution, sodium phosphate and sodium oxalate are used as a mixed precipitant, which can form a mixed precipitate of phosphate and oxalate with very low solubility with calcium and magnesium ions in the reaction system, far exceeding the effect of a single precipitant, and can achieve deep and simultaneous removal of calcium and magnesium.

[0020] Advantages compared to existing technologies: 1. This invention achieves a lithium leaching rate of over 96% under normal pressure through the synergistic effect of crystal conversion aids and selective scale inhibitors, overcoming the bottleneck of low efficiency in traditional normal pressure processes and greatly improving leaching efficiency.

[0021] 2. This invention achieves efficient removal and resource recovery of fluorine through fluorine-aluminum synergistic precipitation. It uses a composite precipitant to reduce the concentration of calcium and magnesium impurities to below 1 mg / L in one step, solving the problem of co-removal. The process is short and efficient.

[0022] 3. The entire process of this invention is carried out under normal pressure, eliminating the need for expensive high-pressure or calcination equipment. Furthermore, the fluorine is fixed as a resource-recoverable product, avoiding pollution, making it environmentally friendly and low-cost.

[0023] 4. This invention employs deep purification at the front end, ensuring the high purity of the final lithium product. It can directly obtain battery-grade lithium carbonate with a main content of ≥99.5%, resulting in excellent product quality.

[0024] 5. The lithium extraction and impurity removal method of this invention has a simple overall process flow, and can be completed under normal pressure through the synergistic effect of various additives. It has a high lithium recovery rate, low impurity content, and wide applicability. It can be used to prepare high-purity lithium chloride products or battery-grade lithium carbonate. Attached Figure Description

[0025] Figure 1 This is a process flow diagram of the method for efficient lithium extraction from lepidolite under normal pressure according to the present invention. Detailed Implementation

[0026] The above-described technical features of the present invention and the technical features specifically described below (such as in the embodiments) can be combined with each other to form new or preferred technical solutions. However, the present invention is not limited to these embodiments, nor are these embodiments limited in any way.

[0027] Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, the formulations involved in the following examples are all commercially available products and can be purchased from the market.

[0028] The present invention will be further described in detail below with reference to embodiments: Example 1 A method for efficient lithium extraction from lepidolite under normal pressure, the method comprising the following steps: (1) Mechanochemical activation-pre-deconstruction: Take 100g of lithium mica concentrate (Li2O content is 2.8%), mix it with 3g of sodium chloride, and grind it into activated powder with a particle size D90=12μm in a planetary ball.

[0029] (2) Enhanced leaching with hydrochloric acid at atmospheric pressure: The activated powder was added to 400 mL of hydrochloric acid (concentration 6 mol / L) and 1 g of citric acid was added as a scale inhibitor. The reaction was carried out at atmospheric pressure and 95 °C for 2.5 h. After filtration, the lithium leaching rate in the leachate was measured to be 96.2%.

[0030] (3) Fluorine-aluminum co-precipitation and resource utilization: Lime milk was added to the leachate to adjust the pH to 4.0, and the mixture was stirred for 30 min and then filtered to obtain fluorine-aluminum co-precipitate. Solid-liquid separation was performed to obtain filtrate A. (4) Deep iron removal: The pH of filtrate A was adjusted to 5.2 with NaOH to remove Fe. 3+ After complete hydrolysis and precipitation, the iron was removed by filtration, yielding filtrate B. (5) Targeted precipitation of calcium and magnesium: Filtrate B was heated to 70°C, and a composite precipitant consisting of 5g Na3PO4 and 3g Na2C2O4 was added. After stirring for 1 hour, the mixture was filtered to obtain a high-purity lithium chloride solution. The Ca and Mg contents were analyzed and found to be 0.8 mg / L and 0.5 mg / L, respectively.

[0031] (6) Lithium recovery: After evaporating and concentrating the high-purity lithium chloride solution, sodium carbonate solution is added, and after precipitation, washing and drying, battery-grade lithium carbonate is obtained. The lithium carbonate content is ≥99.5%.

[0032] Example 2 A method for efficient lithium extraction from lepidolite under normal pressure, the method comprising the following steps: (1) Mechanochemical activation-pre-deconstruction: Take 100g of lithium mica concentrate (Li2O content of 2.0%) from another source with a lower grade, mix it with 2g of sodium chloride, and grind it into activated powder with a particle size D90=15μm in a planetary ball.

[0033] (2) Enhanced leaching with hydrochloric acid at atmospheric pressure: The activated powder was added to 350 mL of hydrochloric acid (7 mol / L) and 0.8 g of sodium polyacrylate was added as a scale inhibitor. The reaction was carried out at atmospheric pressure and 98 °C for 3 h. After filtration, the lithium leaching rate in the leachate was measured to be 94.5%.

[0034] (3) Fluorine-aluminum co-precipitation and resource utilization: Add calcium chloride solution and a small amount of Ca(OH)2 slurry to the leachate, control the pH to 3.8, stir for 30 min and filter to obtain fluorine-aluminum co-precipitate, and separate the solid and liquid to obtain filtrate A. (4) Deep iron removal: The pH of filtrate A was adjusted to 5.2 with NaOH to remove Fe. 3+ After complete hydrolysis and precipitation, the iron was removed by filtration, yielding filtrate B. (5) Targeted precipitation of calcium and magnesium: Filtrate B was heated to 70°C, and a composite precipitant consisting of 4g Na3PO4 and 3.5g Na2C2O4 was added. After stirring for 1 hour, the mixture was filtered to obtain a high-purity lithium chloride solution. The Ca and Mg contents were analyzed and found to be 1.2mg / L and 0.8mg / L, respectively.

[0035] (6) Lithium recovery: After evaporating and concentrating the high-purity lithium chloride solution, sodium carbonate solution is added, and after precipitation, washing and drying, battery-grade lithium carbonate is obtained. The lithium carbonate content is ≥99.5%.

[0036] Example 3 A method for efficient lithium extraction from lepidolite under normal pressure, the method comprising the following steps: (1) Mechanochemical activation-pre-deconstruction: Take 100g of lithium mica concentrate (Li2O content is 2.8%), mix it with 5g of potassium chloride, and grind it into activated powder with a particle size D90=10μm in a planetary ball.

[0037] (2) Enhanced leaching with hydrochloric acid at atmospheric pressure: The activated powder was added to 500 mL of hydrochloric acid (concentration 5 mol / L) and 1 g of citric acid was added as a scale inhibitor. The reaction was carried out at atmospheric pressure and 100 °C for 2 h. After filtration, the lithium leaching rate in the leachate was measured to be 97.0%.

[0038] (3) Fluorine-aluminum co-precipitation and resource utilization: Lime milk was added to the leachate to adjust the pH to 4.0, and the mixture was stirred for 30 min and then filtered to obtain fluorine-aluminum co-precipitate. Solid-liquid separation was performed to obtain filtrate A. (4) Deep iron removal: The pH of filtrate A was adjusted to 5.5 with NaOH to remove Fe. 3+ After complete hydrolysis and precipitation, the iron was removed by filtration, yielding filtrate B. (5) Targeted precipitation of calcium and magnesium: Filtrate B was heated to 70°C, and a composite precipitant consisting of 4g Na3PO4 and 4g Na2C2O4 was added. After stirring for 1 hour, the mixture was filtered to obtain a high-purity lithium chloride solution. The Ca and Mg contents were analyzed and found to be 0.7 mg / L and 0.4 mg / L, respectively.

[0039] (6) Lithium recovery: After evaporating and concentrating the high-purity lithium chloride solution, sodium carbonate solution is added, and after precipitation, washing and drying, battery-grade lithium carbonate is obtained. The lithium carbonate content is ≥99.5%.

[0040] Comparative Example 1 A method for high-efficiency lithium extraction from lepidolite under normal pressure. The difference between this method and Example 1 is that step (1) is as follows: take 100g of lepidolite concentrate (Li2O content is 2.8%) and grind it into powder with a particle size D90=12μm in a regular ball mill. The rest is the same as in Example 1.

[0041] Test results: During the leaching reaction, an increase in system viscosity was observed, and the filtration speed was significantly slower than in Example 1. The final lithium leaching rate was only 85.5%, mainly because the minerals were not decomposed during simple ball milling, resulting in low chemical activity, which is unfavorable for subsequent acid leaching.

[0042] Comparative Example 2 A method for high-efficiency lithium extraction from lepidolite under normal pressure. The method differs from Example 1 in that no scale inhibitor is added in step (2), while the rest is the same as in Example 1.

[0043] Test results: During the leaching reaction, an increase in system viscosity was observed, the subsequent reaction was inhibited, and agglomeration occurred. The filtration speed was significantly slower than in Example 1. The final lithium leaching rate was only 90.5%, and the leaching residue showed severe agglomeration, mainly because the dissolved silica readily reacts with F. - Na + / K + The reaction produces insoluble fluorosilicates, which coat the surface of unreacted mineral particles, hindering mass transfer and affecting the leaching rate.

[0044] Comparative Example 3 A method for high-efficiency lithium extraction from lepidolite under normal pressure. The method differs from Example 1 in that step (1) is: take 100g of lepidolite concentrate (Li2O content is 2.8%) and grind it into powder with a particle size D90=12μm in a regular ball mill. In step (2), no scale inhibitor is added. The rest is the same as in Example 1.

[0045] Test results: During the leaching reaction, an increase in system viscosity was observed, and the filtration speed was significantly slower than in Example 1. The final lithium leaching rate was only 76.4%, and the leaching residue showed severe agglomeration. This comparative example further demonstrates that without both mechanochemical activation and scale inhibition measures, lithium leaching kinetics significantly slowed down, and the leaching rate was easily reduced due to complex salt encapsulation. This indicates that the mechanochemical activation and scale inhibition measures of this invention have a synergistic effect, and their combined action can significantly improve lithium leaching kinetics and the final leaching rate.

[0046] Comparative Example 4 A method for efficient lithium extraction from lepidolite under normal pressure. The method differs from Example 1 in that sodium oxalate is used instead of the composite precipitant in step (5), while the rest is the same as in Example 1.

[0047] Test results: The Ca content in the high-purity lithium chloride solution was measured. 2+ Concentration of 15 mg / L, Mg 2+ The concentration is 8 mg / L.

[0048] Comparative Example 5 A method for efficient lithium extraction from lepidolite under normal pressure. The method differs from Example 1 in that sodium phosphate is used instead of the composite precipitant in step (5), while the rest is the same as in Example 1.

[0049] Test results: The Ca content in the high-purity lithium chloride solution was measured. 2+ Concentration of 5 mg / L, Mg 2+ The concentration is 25 mg / L.

[0050] Comparing Example 1 and Comparative Examples 4-5, it can be seen that single oxalate or phosphate precipitants cannot achieve simultaneous and deep removal of calcium and magnesium. Only by using the specific composite precipitant of this invention can the synergistic effect be utilized to achieve the best purification effect, laying the foundation for the preparation of high-purity products.

[0051] In summary, this invention optimizes the lithium extraction process by utilizing the synergistic effect of mechanochemical activation and scale inhibition measures to effectively improve lithium leaching kinetics and final leaching rate. Through precise control of reaction conditions and purification with a specific composite precipitant, impurities are deeply removed. This lithium extraction and impurity removal method has a simple process flow, can be completed under normal pressure, has a high lithium recovery rate, low impurity content, and wide applicability. It can be used to prepare high-purity lithium chloride products or battery-grade lithium carbonate.

[0052] Finally, it should be emphasized that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for efficient lithium extraction from lepidolite under ambient pressure based on synergistic activation and targeted impurity removal, characterized in that, The method includes the following steps: (1) After mixing lepidolite concentrate with crystal transformation aid, it is subjected to mechanochemical activation in a planetary ball mill to obtain activated powder; (2) The activated powder obtained in step (1) is reacted with hydrochloric acid under normal pressure for 2-3 hours, and a selective scale inhibitor is added to the reaction system simultaneously. After the reaction is completed, the solid and liquid are separated to obtain the leachate. (3) Slowly add calcium source to the leachate obtained in step (2) and precisely control the pH to 3.5-4.

5. Stir the reaction for 0.5-1h, separate the solid and liquid, and obtain filtrate A; (4) Adjust the pH of the filtrate A from step (3) to 5.0-5.5 with alkali, so that Fe 3+ After complete hydrolysis and precipitation, the solution was filtered to obtain filtrate B. (5) Heat the filtrate B obtained in (4) to 60-80℃, add the composite precipitant, stir the reaction for 0.5-1.5h, filter, and obtain a high-purity lithium chloride solution; (6) After concentrating the high-purity lithium chloride solution obtained in step (5), lithium chloride product is obtained, or sodium carbonate is added to precipitate it into battery-grade lithium carbonate.

2. The method for efficient lithium extraction from lepidolite under ambient pressure based on synergistic activation and targeted impurity removal according to claim 1, characterized in that, In step (1), the crystal transformation aid is at least one of sodium chloride and potassium chloride; the mass ratio of lepidolite concentrate to crystal transformation aid is 100:1-5.

3. The method for efficient lithium extraction from lepidolite under ambient pressure based on synergistic activation and targeted impurity removal according to claim 1, characterized in that, In step (1), the particle size D90 of the activated powder is ≤15μm.

4. The method for efficient lithium extraction from lepidolite under ambient pressure based on synergistic activation and targeted impurity removal according to claim 1, characterized in that, In step (2), the concentration of hydrochloric acid is 5-8 mol / L, and the solid-liquid ratio of the activated powder to hydrochloric acid is 1:3-5 g / mL.

5. The method for efficient lithium extraction from lepidolite under ambient pressure based on synergistic activation and targeted impurity removal according to claim 1, characterized in that, In step (2), the scale inhibitor is citric acid or sodium polyacrylate, and its addition amount is 0.5-2% of the mass of the lepidolite concentrate.

6. The method according to claim 1, wherein the method is characterized in that, In step (2), the reaction temperature is 90-100℃.

7. The method according to claim 1, wherein the method is characterized in that, In step (3), the calcium source is Ca(OH)2 or CaCl2.

8. The method according to claim 1, wherein the method is characterized in that, In step (5), the composite precipitant is a mixture of sodium phosphate and sodium oxalate in a molar ratio of 1:0.5-1.5.

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