A method for preparing battery-grade iron phosphate using pyrite slag and wet-process phosphoric acid

Abstract

The present invention discloses a method for preparing battery-grade iron phosphate using pyrite cinder and wet-process phosphoric acid, comprising the following steps: (1) reducing and roasting the pyrite cinder in a tubular atmosphere furnace; (2) leaching the roasted product with sulfur-phosphorus mixed acid and filtering; (3) adding iron powder to the filtrate to completely reduce part of the trivalent iron to divalent iron, then adding sodium sulfide solution to remove heavy metals, adding ammonia water to adjust the pH after the reaction, then adding ammonium bicarbonate, and filtering after the reaction; (4) adding sulfuric acid to the filtrate to adjust the pH, then slowly adding hydrogen peroxide and heating, and filtering after the reaction; (6) washing, drying, roasting, and crushing the filter cake to obtain battery-grade anhydrous iron phosphate. The present invention uses low-value pyrite cinder as an iron source and fertilizer-grade wet-process phosphoric acid as a phosphorus source, fully utilizing the synergistic effect between the raw materials, adopting a mixed impurity removal process to obtain a purified ferrous phosphate solution, and then adopting a one-step method to prepare iron phosphate, thereby greatly reducing the cost of the iron and phosphorus sources.

Description

Technical Field

[0001] The present invention relates to the field of lithium-ion battery materials, and in particular to a method for extracting / purifying low-value iron and phosphorus sources in the preparation process of battery-grade anhydrous ferric phosphate. Background Art

[0002] The technology for producing iron phosphate has matured considerably, with mainstream processes including the ammonia and sodium methods. The ammonia method uses industrial-grade monoammonium phosphate / diammonium phosphate as a phosphorus source, while the sodium method uses industrial-grade phosphoric acid. The majority of the iron source for both methods is ferrous sulfate heptahydrate, a byproduct of titanium dioxide production. With China's vigorous development of new energy industries, iron phosphate, as a precursor for lithium iron phosphate cathode materials, has seen promising growth opportunities. However, with companies in the phosphorus chemical and titanium dioxide industries rapidly expanding their presence, iron phosphate is facing overcapacity. Reducing iron phosphate production costs is crucial to improving market competitiveness. Currently, producing one ton of anhydrous iron phosphate requires 2.2 to 2.5 tons of titanium dioxide slag (ferrous sulfate heptahydrate purity ≥85%), accounting for approximately 7% of the total production cost. This cost is even higher when sourcing from outside the province. For iron phosphate manufacturers in the province, where titanium dioxide production is relatively limited, iron source costs are high, and as iron phosphate production capacity increases, iron supply becomes increasingly difficult to secure. Therefore, expanding iron sourcing channels is crucial for iron phosphate production.

[0003] Pyrite cinder is a byproduct of sulfuric acid production from pyrite. Its iron content is generally 45% to 55%, and it's typically sold at a low price of 100 to 200 yuan per ton. Assuming that 1 ton of sulfuric acid requires 1 ton of pyrite, and 1 ton of pyrite produces 0.6 to 0.8 tons of cinder, domestic pyrite cinder production in 2021 will be 11.1 to 14.8 million tons, enough to produce over 15 million tons of anhydrous iron phosphate. There have been relevant studies on extracting iron from pyrite cinders and preparing ferric phosphate. However, existing patents have one or more problems, such as low iron leaching rate, long iron leaching time, low purity of ferric phosphate, complex preparation process and high cost. Chinese patent CN102730659A (Luling Chemical, 2011) discloses a method for preparing battery-grade ferric phosphate from pyrite cinders. The pyrite cinders are leached with an organic-inorganic acid and then extracted with an organic solvent. The amount of organic acid and organic solvent used is large, and the cost is high. Chinese patent CN108706562A (Wuhan University of Technology, 2018) discloses a method for preparing ferric phosphate from pyrite cinders. The pyrite cinders are leached with sulfuric acid at high temperature for 6 to 12 hours. The acid leaching solution is then used to directly prepare ferric phosphate. The resulting ferric phosphate has a purity of only 90% to 91%, and the product value is low. Chinese patent CN109368610A (Hunan Yacheng, 2018) discloses a method for producing high-iron-phosphorus iron phosphate from pyrite cinders. Alkaline washing is used to remove silicon and aluminum, but the amount of alkali required is large and the wastewater difficult to treat. Furthermore, the inventors, through multiple experiments, found that direct reaction of sulfuric acid with Fe₂O₃ resulted in a very low iron leaching rate (less than 30%), necessitating reduction treatment. The present invention pre-treats the pyrite cinders using reduction roasting followed by acid leaching with a sulfur-phosphorus mixed acid, resulting in a high iron leaching rate. Furthermore, a combination of various impurity removal methods yields a battery-grade anhydrous iron phosphate product. Summary of the Invention

[0004] The present invention aims to solve three core technical problems: first, pre-treating pyrite slag to maximize the iron leaching rate during acid leaching; second, removing impurities in the acid leaching solution that affect the subsequent ferric phosphate product (including impurities introduced by fertilizer-grade wet-process phosphoric acid) to the greatest extent possible; and third, producing a qualified battery-grade anhydrous ferric phosphate product in the shortest possible process flow.

[0005] The technical solution of the present invention is a method for preparing battery-grade iron phosphate by using pyrite slag and wet-process phosphoric acid, which mainly comprises the following steps:

[0006] (1) reducing and roasting the pyrite slag in a tubular atmosphere furnace;

[0007] (2) leaching the calcined product with sulfuric and phosphoric acid mixture and filtering;

[0008] (3) adding iron powder to the filtrate to reduce part of the trivalent iron to divalent iron, then adding sodium sulfide solution, reacting for a period of time, adding ammonia water to the filtrate to adjust the pH, and then adding ammonium bicarbonate, and adding flocculant polyacrylamide after a period of reaction, stirring and filtering;

[0009] (4) adding sulfuric acid to the filtrate to adjust the pH, then slowly adding hydrogen peroxide, heating after the addition, and filtering after the reaction;

[0010] (5) Washing, drying, calcining, and crushing the filter cake to obtain battery-grade anhydrous iron phosphate.

[0011] In the above-mentioned method for preparing battery-grade iron phosphate, the reducing agent used in the reduction roasting in step (1) is one of graphite, coal powder, sucrose, glucose, carbon monoxide, and hydrogen. When one of graphite, coal powder, sucrose, and glucose is used as the reducing agent, the molar ratio of pure carbon to iron in pyrite slag is 0.5-1:1, and the roasting temperature is 800-1000°C. Preferably, the molar ratio of pure carbon to iron is 0.7-0.8, and the roasting temperature is 850-900°C. When one of carbon monoxide and hydrogen is used as the reducing agent, the roasting temperature is 600-900°C, preferably 600-650°C. The reduction roasting time is 0.5-3h, preferably 1-1.5h.

[0012] In the above-mentioned method for preparing battery-grade iron phosphate, in step (2), fertilizer-grade wet-process phosphoric acid is used, the phosphoric acid is used at a phosphorus-iron molar ratio of 1.39-1.40:1, and the phosphoric acid concentration is 20%-25%. The molar ratio of sulfuric acid to iron is 0.3-0.4:1, and the sulfuric acid concentration is 20%-25%. The acid leaching temperature is 60-90°C, and the acid leaching time is 0.5-3 hours. Preferably, the temperature is 75-80°C, and the time is 1.5-2 hours.

[0013] In the above-mentioned method for preparing battery-grade iron phosphate, the amount of iron powder added in step (3) is controlled by adding until bubbles begin to appear in the solution. Sodium sulfide is added to a concentration of 13%, and the amount of sodium sulfide solution used is 4% to 6% of the mass of the acid leaching solution. The concentration of ammonia water is 10% to 20%, and the pH value is adjusted to an end point of 3.5 to 4.1 using ammonia water. The amount of ammonium bicarbonate added is 0.3% to 1% of the mass of the solution, preferably 0.5% to 0.7%.

[0014] In the above-mentioned method for preparing battery-grade iron phosphate, in step (4), sulfuric acid is used to adjust the pH to 1.5-2.1, preferably 1.70-1.80. The concentration of hydrogen peroxide is 5%-8%. The molar ratio of hydrogen peroxide to iron is 0.6-0.65:1. The hydrogen peroxide is added for 10-15 minutes, and after addition, the temperature is raised to 90-95°C, and the reaction time is 2.5-3 hours.

[0015] In the above-mentioned method for preparing battery-grade iron phosphate, a three-stage reslurry countercurrent washing method is adopted in step (5), and the volume flow ratio of washing water to slurry volume is 1 to 1.1:1. The drying temperature is 200 to 250°C, the drying time is 1 to 2 hours, and the roasting temperature is 600 to 700°C, and the roasting time is 1.5 to 2 hours.

[0016] The present invention uses a reduction roasting method to reduce the high-valent iron in the pyrite slag into low-valent iron (mainly FeO, a small amount of Fe3O4). When the roasted product is subjected to acid leaching, the iron leaching rate is above 96% by controlling the process parameters. The key to the present invention is the removal of impurities (including impurities brought in by fertilizer-grade wet-process phosphoric acid) in the acid leaching solution. First, Fe 3+ All reduced to Fe 2+ , in Fe 2+ Impurities are removed under the system, because Fe 3+ It begins to precipitate at pH>1.5, and Fe 2+ Precipitation begins when pH>6.5. Another purpose of adding iron powder is to remove inert metal impurities such as copper. The purpose of adding sodium sulfide is to remove heavy metal impurities such as arsenic and lead in the acid leaching solution. In addition, it also has a certain effect on the removal of manganese. The purpose of step (3) adjusting the pH to 3.5-4.1 with ammonia water is to remove impurities such as aluminum and titanium. 3+ It begins to precipitate when pH>3.3. The removal principle of titanium is the adsorption of ferrous phosphate colloid. 4+ Because it carries more positive charges, the removal effect is obvious. The purpose of adding ammonium bicarbonate is to remove Ca in the acid leaching solution. 2+ .

[0017] Compared with the existing preparation method of battery-grade anhydrous ferric phosphate, the present invention has the following advantages:

[0018] (1) Pyrite slag is used as the iron source, which has low iron source cost. Currently, the iron content of Guizhou Phosphate Group's pyrite slag (existing in the form of Fe2O3) is 45% to 50%, and the selling price is 80 to 100 yuan per ton. The iron leaching rate of the present invention is ≥96%, and the comprehensive iron yield when preparing iron phosphate is ≥92%.

[0019] (2) Wet-process phosphoric acid is used as the phosphorus source, which has low phosphorus source cost. The existing process uses industrial-grade phosphoric acid or industrial-grade monoammonium phosphate as the phosphorus source. One ton of anhydrous ferric phosphate consumes about 0.85 tons of industrial-grade phosphoric acid or industrial-grade monoammonium phosphate. Fertilizer-grade wet-process phosphoric acid has a greater cost advantage than the above-mentioned phosphorus sources.

[0020] (3) A mixed impurity removal process of iron and phosphorus sources is adopted to avoid the problem of a long process flow caused by separate impurity removal. In addition to being a phosphorus source, phosphoric acid also plays a role in leaching iron. Through the combined impurity removal process of the present invention, impurities such as copper, aluminum, titanium, calcium, manganese, arsenic, and lead in the iron and phosphorus sources can be removed. Other impurities such as sodium, sulfate, potassium, and magnesium can be removed by multi-stage re-slurry washing of the iron phosphate filter cake, and finally a qualified battery-grade iron phosphate product is obtained.

[0021] (4) The iron content in wet-process phosphoric acid (P2O5 concentration of 25% to 30%) is generally 0.2% to 0.6%. The present invention removes harmful impurities (aluminum, calcium, manganese, arsenic, lead, etc.) in wet-process phosphoric acid that are harmful to subsequent iron phosphate products while retaining the beneficial element iron. Through the selective impurity removal process, the value of wet-process phosphoric acid is maximized.

[0022] Figures in the specification

[0023] Figure 1 It is a process flow chart of the present invention. DETAILED DESCRIPTION

[0024] The main chemical composition of the pyrite slag used in the present invention is shown in Table 1.

[0025] Table 1 Main chemical composition of pyrite slag (unit: %)

[0026] name <![CDATA[Fe2O3]]> <![CDATA[SiO2]]> <![CDATA[SO3]]> CaO <![CDATA[Al2O3]]> MgO <![CDATA[TiO2]]> <![CDATA[K2O]]> <![CDATA[P2O5]]> content 67.30 13.50 5.04 2.38 2.35 0.64 0.17 0.47 0.10

[0027] The main chemical composition of the wet-process phosphoric acid used in the present invention is shown in Table 2.

[0028] Table 2 Main chemical composition of wet-process phosphoric acid (unit: %)

[0029] name <![CDATA[P2O5]]> <![CDATA[Fe2O3]]> <![CDATA[Al2O3]]> MgO <![CDATA[SiO2]]> CaO <![CDATA[SO4 2- ]]> <![CDATA[K2O]]> As content 48.10 0.58 0.95 1.85 0.21 0.28 4.36 0.083 0.0045

[0030] Example 1

[0031] 200g of pyrite slag powder and 14.1g of graphite powder were weighed and mixed in a small high-speed universal grinder. The mixture was then transferred to a square corundum crucible and placed in a tubular atmosphere furnace (Φ100mm). After evacuation, nitrogen was continuously introduced and the temperature was increased (10°C / min). The end temperature was set to 850°C and the temperature was maintained for 1.5 hours. After cooling to 200°C, the mixture was removed and cooled naturally. The calcined product was placed in a 2L glass-jacketed reactor. 345.1g of wet-process phosphoric acid, 483.1g of pure water, and 263.2g of 20% sulfuric acid were added. The reactor was heated with jacketed circulating water at 80°C for 1.5 hours and filtered after completion of the reaction.

[0032] Transfer the filtrate to a 2000ml beaker and place the beaker in a constant temperature water bath at 45°C. Add 23g of reduced iron powder to the beaker (addition time 5min), and the color of the solution changes from light red to green. Then add 53g of 13% sodium sulfide solution (addition time 5min), react for 15 minutes, add 12% ammonia water, adjust the pH end point to 3.6, add 3.6g of ammonium bicarbonate, continue the reaction for 0.5h, then add 15g of 0.1% polyacrylamide, stir for 2min and filter.

[0033] The filtered filtrate was transferred to a 2L glass jacketed reactor, stirred, and the pH was adjusted to 1.82 with industrial-grade sulfuric acid. 664.7 g of 6% hydrogen peroxide was slowly added over a 12-minute period. The jacketed circulating water was turned on and the temperature of the external thermostat was set to 90°C. After aging for 2 hours and 26 minutes, the slurry color was observed to change from yellow to white. After further aging for 0.5 hours, the slurry was re-pulped with pure water (keeping the amount of pure water added similar to the volume of the filtrate), and then filtered. The above process was repeated three times (washing water was used in stages). The filter cake was transferred to a tray and evenly spread out, placed in a 200°C oven to dry for 2 hours, taken out and cooled, and then crushed. It was transferred to a square corundum crucible and the crucible was placed in a 680°C box-type resistance furnace (with an exhaust port on the top) and calcined for 2 hours. After cooling, 295.5 g of battery-grade anhydrous iron phosphate was obtained. The comprehensive recovery rate of iron is 92.03%, the recovery rate of phosphorus is 84.98%, and the product indicators of anhydrous ferric phosphate are shown in Table 3.

[0034] Example 2

[0035] 200g of pyrite slag powder and 45.3g of glucose were weighed and mixed in a small high-speed universal grinder. The mixture was then transferred to a square corundum crucible and placed in a tubular atmosphere furnace (Φ100mm). After evacuation, nitrogen was continuously introduced and the temperature was raised (10°C / min). The end temperature was set to 1000°C and the temperature was maintained for 1 hour. After cooling to 200°C, the mixture was removed and cooled naturally. The calcined product was placed in a 2L glass-jacketed reactor, and 346.6g of wet-process phosphoric acid, 318.8g of pure water, and 263.2g of 25% sulfuric acid were added. The reactor was heated with jacketed circulating water at 90°C for 0.5h and filtered after the reaction.

[0036] Transfer the filtrate to a 2000ml beaker and place the beaker in a constant temperature water bath at 45°C. Add 21.6g of reduced iron powder to the beaker (addition time 5min), and the color of the solution changes from light red to green. Then add 46g of 13% sodium sulfide solution (addition time 5min), react for 15min, add 17% ammonia water, adjust the pH end point to 3.9, add 5.2g of ammonium bicarbonate, continue the reaction for 0.5h, then add 15g of 0.1% polyacrylamide, stir for 2min and filter.

[0037] The filtered filtrate was transferred to a 2L glass jacketed reactor, stirred, and the pH was adjusted to 1.71 with industrial-grade sulfuric acid. 526.9 g of 8% hydrogen peroxide was slowly added over a 10-minute period. The jacketed circulating water was turned on and the temperature of the external thermostat was set to 95°C. After aging for 2 hours and 3 minutes, the slurry color was observed to change from yellow to white. After aging for 45 minutes, the filter cake was re-slurried with pure water (keeping the amount of pure water added similar to the volume of the filtrate), and then filtered. The above process was repeated three times (washing water was used in stages). The filter cake was transferred to a tray and spread evenly, placed in a 250°C oven to dry for 1 hour, taken out and cooled, and then crushed. It was transferred to a square corundum crucible and the crucible was placed in a 700°C box-type resistance furnace (with an exhaust port on the top) and calcined for 1.5 hours. After cooling, 294.0 g of battery-grade anhydrous iron phosphate was obtained. The comprehensive recovery rate of iron is 92.27%, the recovery rate of phosphorus is 84.20%, and the product indicators of anhydrous ferric phosphate are shown in Table 3.

[0038] Example 3

[0039] Weigh 200g of pyrite slag powder into a square corundum crucible, and place the crucible in a tubular atmosphere furnace (Φ100mm). After evacuation, introduce carbon monoxide to atmospheric pressure, then continue to introduce carbon monoxide at a rate of 6mL / min (the gas at the outlet of the tubular furnace is exhausted to the outside through a fume hood). Then, begin heating (10℃ / min), set the end temperature to 650℃, hold the temperature for 1h, cool to 200℃, and then remove and cool naturally. The calcined product is placed in a 2L glass jacketed reactor, and 345.8g of wet-process phosphoric acid, 485g of pure water, and 269.2g of 25% sulfuric acid are added. The reactor is heated with jacketed circulating water at 75℃ for 2h and filtered after the reaction is completed.

[0040] Transfer the filtrate to a 2000ml beaker and place the beaker in a constant temperature water bath at 45°C. Add 23g of reduced iron powder to the beaker (addition time 5min), and the color of the solution changes from light red to green. Then add 55g of 13% sodium sulfide solution (addition time 5min), react for 15min, add 15% ammonia water, adjust the pH end point to 4.0, add 6.9g of ammonium bicarbonate, continue the reaction for 0.5h, then add 15g of 0.1% polyacrylamide, stir for 2min and filter.

[0041] The filtered filtrate was transferred to a 2L glass jacketed reactor, stirred, and the pH was adjusted to 1.92 with industrial-grade sulfuric acid. 824.2 g of 5% hydrogen peroxide was slowly added over a 15-minute period. The jacketed circulating water was turned on and the temperature of the external thermostat was set to 94°C. After aging for 2 hours and 37 minutes, the slurry color was observed to change from yellow to white. After aging for 0.5 hours, the slurry was re-slurried with pure water (keeping the amount of pure water added similar to the volume of the filtrate), and then filtered. The above process was repeated three times (the washing water was used in stages). The filter cake was transferred to a tray and spread evenly, placed in a 220°C oven to dry for 2 hours, taken out and cooled, and then crushed. It was transferred to a square corundum crucible and the crucible was placed in a 630°C box-type resistance furnace (with an exhaust port on the top) and calcined for 2 hours. After cooling, 295.7 g of battery-grade anhydrous iron phosphate was obtained. The comprehensive recovery rate of iron is 92.13%, the recovery rate of phosphorus is 85.15%, and the product indicators of anhydrous ferric phosphate are shown in Table 3.

[0042] Table 3 Iron phosphate product indicators of Examples 1 to 3

[0043] name Example 1 Example 2 Example 3 A downstream customer indicator Fe (%) 36.30 36.14 36.31 36.0～36.6 P (%) 20.80 20.8 20.87 20.6～21.0 Fe / P 0.968 0.963 0.965 0.960～0.980 Na (ppm) 17 19 25 ≤50 Mg (ppm) 22 23 23 ≤250 K (ppm) 9 11 8 ≤50 Ca (ppm) 26 25 29 ≤50 Zn (ppm) 23 22 26 ≤30 Cu (ppm) Not detected Not detected Not detected ≤20 Al (ppm) 27 35 32 ≤150 Mn (ppm) 3 5 5 ≤200 Ni (ppm) Not detected Not detected Not detected ≤10 Pb (ppm) 25 25 23 ≤30 S (ppm) 292 339 230 ≤350 Ti (ppm) 2 2 3 ≤150 Moisture (ppm) 2306 2664 2289 ≤5000 pH 2.98 3.07 3.14 2.7～3.6 <![CDATA[Particle size D 50 μm]]> 3.52 3.27 3.23 ≤5 <![CDATA[Specific surface area m 2 / g]]> 4.47 4.82 4.35 4～8 <![CDATA[Tap density g / cm 3 > 0.816 0.831 0.802 ≥0.6

Claims

1. A method for preparing battery-grade iron phosphate by using pyrite slag and wet-process phosphoric acid, characterized in that The steps include: (1) reducing and roasting the pyrite slag in a tubular atmosphere furnace; (2) leaching the calcined product with sulfuric and phosphoric acid mixture and filtering; (3) adding iron powder to the filtrate to reduce part of the trivalent iron to divalent iron, then adding sodium sulfide solution, reacting for a period of time, adding ammonia water to the filtrate to adjust the pH, and then adding ammonium bicarbonate, and adding flocculant polyacrylamide after a period of reaction, stirring and filtering; (4) adding sulfuric acid to the filtrate to adjust the pH, then slowly adding hydrogen peroxide, heating after the addition, and filtering after the reaction; (5) washing, drying, calcining, and crushing the filter cake to obtain battery-grade anhydrous ferric phosphate; The reducing agent used in the reduction roasting in step (1) is one of graphite, coal powder, sucrose, glucose, carbon monoxide, and hydrogen. When one of graphite, coal powder, sucrose, and glucose is used as the reducing agent, the molar ratio of pure carbon to iron in pyrite slag is 0.5 to 1:1, and the roasting temperature is 800 to 1000° C.; when one of carbon monoxide and hydrogen is used as the reducing agent, the roasting temperature is 600 to 900° C., and the reduction roasting time is 0.5 to 3 h. In step (2), the phosphoric acid adopts fertilizer-grade wet-process phosphoric acid, the phosphoric acid dosage is a phosphorus-iron molar ratio of 1.39-1.40:1, and the phosphoric acid concentration is 20%-25%; the sulfuric acid-iron molar ratio is 0.3-0.4:1, and the sulfuric acid concentration is 20%-25%; the acid leaching temperature is 60-90° C., and the acid leaching time is 0.5-3 h; In step (3), the amount of iron powder added is controlled by adding until bubbles begin to appear in the solution, and sodium sulfide is added to a concentration of 13%, with the amount of sodium sulfide solution being 4% to 6% of the mass of the acid leaching solution; In step (3), the concentration of ammonia water is 10% to 20%, and the pH value is adjusted to 3.5 to 4.1; the amount of ammonium bicarbonate added is 0.3% to 1% of the mass of the solution; In step (4), the pH is adjusted to 1.5-2.1 with sulfuric acid, the concentration of hydrogen peroxide is 5%-8%, the molar ratio of hydrogen peroxide to iron is 0.6-0.65:1, the hydrogen peroxide is added for 10-15 minutes, and the temperature is raised to 90-95° C. after the addition, and the reaction time is 2.5-3 hours; In step (5), a three-stage repulping countercurrent washing method is adopted, the volume flow ratio of washing water to slurry volume is 1-1.1:1, the drying temperature is 200-250°C, the drying time is 1-2h, the roasting temperature is 600-700°C, and the roasting time is 1.5-2h.

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