A method for recycling and regenerating phosphorus-iron slag resources after lithium extraction by chlorination from waste lithium iron phosphate batteries

Abstract

This invention discloses a method for recycling and regenerating iron phosphate slag from spent lithium iron phosphate batteries after lithium extraction via chlorination. The method includes: defluorinating the lithium-extracted iron phosphate slag with a carrier gas at 500-600℃; absorbing the fluorine-containing gas through ammonia spray to obtain an ammonium fluoride solution; adding the obtained ammonium fluoride solution and hydrochloric acid to the defluorinated slag to prepare a leaching system with a hydrogen ion concentration of 0.20-0.6 mol / L, controlling the fluorine-aluminum molar ratio at 10:1-100:1, selectively leaching aluminum and copper impurities, achieving a total iron phosphate solubility ≤20%, and filtering to obtain a mixed iron phosphate-carbon filter residue; calcining the mixed filter residue at 650-800℃ in an aerobic atmosphere to remove carbonaceous impurities and achieve the transformation of iron phosphate from the olivine type to the hexagonal crystal type; and directly obtaining battery-grade iron phosphate product after cooling. This invention achieves self-recycling of fluorine resources and has advantages such as a short process, low acid consumption, high retention of iron phosphate crystal structure, and significantly reduced costs.

Description

Technical Field

[0001] This application belongs to the field of waste lithium-ion battery resource recycling technology, specifically involving a method for recycling and regenerating phosphorus iron slag from waste lithium iron phosphate batteries after lithium extraction by chlorination. Background Technology

[0002] With the rapid development of the new energy vehicle and energy storage industries, the number of waste lithium iron phosphate batteries has surged year by year, making their resource recycling a necessity for the industry. Currently, the mainstream recycling process for waste lithium iron phosphate batteries is the lithium extraction process, which extracts lithium resources from the batteries preferentially through wet or pyrometallurgical methods. This process generates a large amount of by-product iron phosphate slag. The main components of this slag are crystalline or amorphous iron phosphate, mixed with impurities such as fluorine-containing organic matter, conductive carbon powder, graphite, elemental copper, and aluminum foil fragments. The crystalline iron phosphate in this slag may still retain its original physical morphology and crystal structure, thus possessing core value for direct recycling.

[0003] Current technologies for regenerating iron phosphate from iron phosphate slag generally employ the traditional route of strong acid complete dissolution-purification-crystallization precipitation: First, strong acids such as concentrated hydrochloric acid and concentrated sulfuric acid are used to completely dissolve the iron phosphate in the iron phosphate slag, and the solid impurities such as fluorine-containing organic matter and carbon powder that are insoluble in acid are removed by filtration; then, neutralizing agents, complexing agents or precipitating agents are added to the acid-soluble mother liquor to remove metal impurities such as aluminum and copper that are soluble in acid and fluorine; finally, the purified iron phosphate ions are recrystallized and precipitated by adjusting the pH and heating and aging, and then calcined to obtain battery-grade iron phosphate.

[0004] However, traditional techniques have many insurmountable flaws: First, the complete dissolution of ferric phosphate requires a large amount of strong acid, which is expensive and generates a large amount of acidic wastewater, making subsequent treatment difficult and environmental costs high. Secondly, the process is lengthy, involving multiple liquid-solid separation, pH adjustment, purification and precipitation operations, which are complicated, energy-intensive and have a long production cycle. Third, phosphorus and iron are lost at high rates during the dissolution-precipitation process, resulting in a low overall recovery rate. Fourth, the aluminum removal process requires the purchase of additional fluoride salts such as sodium fluoride and ammonium fluoride, which further increases production costs. Moreover, the original fluorine resources in the phosphorus iron slag cannot be effectively utilized, which can easily cause secondary pollution.

[0005] Therefore, developing a combined defluorination and aluminum removal process that overturns the traditional fully dissolved route, achieves self-circulation of fluorine resources, and retains high levels of iron phosphate has become an urgent need in the field of waste lithium iron phosphate battery recycling. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by proposing a method for recycling and regenerating phosphorus slag from waste lithium iron phosphate batteries after lithium extraction via chlorination. This method couples high-temperature carrier gas defluorination, ammonia absorption to prepare ammonium fluoride, fluorine resource self-circulation, acid-controlled selective impurity removal, low-loss retention of solid-phase original crystalline iron phosphate, and aerobic calcination to crystallize the slag. This results in a revolutionary defluorination and aluminum removal combined process that overturns the traditional all-dissolution route, achieves fluorine resource self-circulation, and high retention of iron phosphate. It features a short process, low cost, and low acid consumption.

[0007] This invention provides a method for recycling and regenerating phosphorus slag from spent lithium iron phosphate batteries after lithium extraction via chlorination. The method includes the following steps: First, the phosphorus iron slag from waste lithium iron phosphate batteries after preferential chlorination for lithium extraction is subjected to high-temperature carrier gas defluorination and fluorine resource recovery treatment. Specifically, the phosphorus iron slag after chlorination for lithium extraction is placed in a reactor, and a high-temperature carrier gas is introduced for defluorination. The volatilized fluorine-containing gas is absorbed by ammonia spray to achieve fluorine resource recovery, preparing an ammonium fluoride solution. The phosphorus iron slag after chlorination for lithium extraction is obtained after preferential gas-solid selective chlorination for lithium extraction from waste lithium iron phosphate battery black powder, followed by leaching of lithium chloride with an aqueous solution. The iron phosphate in the phosphorus iron slag retains the olivine crystal structure and original morphology. The high-temperature carrier gas defluorination treatment is carried out at a holding time of 20 minutes to 2 hours, and the defluorination temperature is 500℃ to 600℃. The carrier gas is a non-oxidizing gas containing crackable H-containing gases, and is mixed with one or more of nitrogen, argon, carbon dioxide, and helium. The gas is a mixture of one or more of the following: methyl methacrylate gas, methyl ethyl methacrylate gas, ethyl carbonate gas, and dimethyl carbonate gas; or a mixture of hydrogen and methane; or a mixture of methanol and methane; or a mixture of ethyl carbonate gas, ethane, and water vapor; after high-temperature carrier gas defluorination treatment, the residual fluorine content in the phosphorus iron slag is <0.045%; the preferential gas-solid selective chlorination extraction of lithium from waste lithium iron phosphate battery black powder refers to the chemical reaction of lithium iron phosphate in waste lithium iron phosphate battery black powder with chlorine as an oxidant at a temperature of 0℃-200℃ to generate iron phosphate and lithium chloride, with a reaction time of 5 minutes-120 minutes, and the molar ratio of chlorine to iron in the waste lithium iron phosphate battery black powder is 0.50-0.60.

[0008] Secondly, solid-phase retention selective leaching was carried out on the defluorinated ferrophosphate slag. Specifically, this step involved adding the ammonium fluoride solution and hydrochloric acid prepared in the previous step to the defluorinated ferrophosphate slag to prepare a leaching system with a hydrogen ion concentration of 0.20-0.6 mol / L. The molar ratio of fluorine to aluminum in the leaching system was controlled at 10:1-100:1, and the liquid-solid ratio at 5:1-20:1. Selective leaching was performed under these conditions to fully dissolve the aluminum and copper impurities in the ferrophosphate slag into the liquid phase, while controlling the total solubility of ferric phosphate to ≤20% and maintaining the olivine crystal structure and main morphology of the solid-phase ferric phosphate. After filtration, a defluorinated and impurity-removed ferric phosphate-carbon mixed filter residue was obtained. The selective leaching temperature was 40-95℃, the leaching time was 1-4 h, and the physical enhancement method was mechanical stirring or ultrasound for selective leaching. The aluminum content in the defluorinated and impurity-removed ferric phosphate-carbon mixed filter residue was <0.030%, and the copper content was <0.0010%.

[0009] Finally, the iron phosphate-carbon mixed filter residue is decarburized and its crystal structure controlled by aerobic calcination to obtain battery-grade iron phosphate. Specifically, the iron phosphate-carbon mixed filter residue is calcined at a high temperature in an aerobic atmosphere (650℃-800℃) for 2-5 hours. The aerobic atmosphere consists of air, oxygen, or a mixture of oxygen with a volume fraction greater than 20% and an inert gas. Carbonaceous impurities are thoroughly removed, and the residual carbon content of the iron phosphate after aerobic calcination decarburization is <0.10%. Simultaneously, the crystal structure of iron phosphate is transformed from olivine to hexagonal. After cooling, battery-grade iron phosphate product is directly obtained.

[0010] Compared with the prior art, the present invention has the following beneficial technical effects: (1) Short process: In the traditional wet process, the recovery of ferric phosphate from ferrophosphate slag involves a long process of complete dissolution, multiple precipitation, multiple filtration and impurity removal, recrystallization, and calcination. The present invention only includes three process steps: defluorination and fluorine resource utilization, one-time acid control dissolution, one-time filtration and impurity removal, and calcination.

[0011] (2) Low acid and alkali consumption: This invention is based on solid-phase retention selective impurity removal. By controlling the acid, it achieves high selective dissolution of impurities and low solubility of iron phosphate. Therefore, the acid consumption is reduced by more than 70% compared with the traditional wet process total dissolution route.

[0012] (3) Low cost: Compared with the traditional wet process, the present invention has a significant advantage in low cost. The short process flow of the present invention reduces fixed input costs and process costs, the low acid consumption reduces reagent costs, and the resource-based internal recycling of fluorine further reduces material costs. These combined factors enable the present invention to achieve the recovery and regeneration of iron phosphate at a rate more than 60% lower than that of traditional methods. Attached Figure Description

[0013] Figure 1 This is a process flow diagram of a method for recycling and regenerating phosphorus iron slag from waste lithium iron phosphate batteries after lithium extraction by chlorination. Detailed Implementation

[0014] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way.

[0015] Example 1 This embodiment provides a method for recycling and regenerating phosphorus-iron slag from spent lithium iron phosphate batteries after lithium extraction via chlorination. The process flow of this method is shown in the attached figure. Figure 1 As shown, the specific steps are as follows: (1) Prioritize chlorination to extract lithium and obtain lithium extraction residue. First, waste lithium iron phosphate battery black powder is loaded into a chlorination reactor. Chlorine is used as an oxidant to react chemically with the lithium iron phosphate in the battery black powder to generate iron phosphate and lithium chloride. The reaction time is controlled at 100℃ for 60 minutes. The molar ratio of chlorine to iron in the battery black powder is 0.55, and finally chlorinated clinker is obtained. The chlorinated clinker is leached, washed, and filtered with aqueous solution to obtain lithium extraction residue and lithium chloride-containing brine. The iron phosphate in the lithium extraction residue retains the olivine-type orthorhombic crystal structure and retains the physical morphology of the initial raw material lithium iron phosphate cathode material.

[0016] (2) High-temperature carrier gas defluorination and fluorine resource utilization treatment. The lithium-extracted iron phosphate slag is placed in a defluorination reactor, the oxygen in the defluorination reactor is removed, the temperature is raised to 550°C, the defluorination carrier gas is continuously introduced and kept at the temperature for 60 minutes, and the defluorination tail gas is quickly discharged from the defluorination reactor; the fluorine-containing gas in the volatilized defluorination tail gas is absorbed by ammonia water spraying to prepare ammonium fluoride solution; the defluorination carrier gas is a non-oxidizing gas, which contains crackable H-containing gas and nitrogen gas with a volume content of 20%; it is a mixed gas of crackable H-containing ethane and water vapor; after the lithium-extracting slag of waste lithium iron phosphate batteries is defluorinated and fluorine resource utilization treatment, the defluorinated iron phosphate slag is obtained, with a residual fluorine content of 0.030%.

[0017] (3) Solid-phase retention selective impurity removal. The ammonium fluoride solution prepared in the previous step was added to hydrochloric acid to prepare an acid etching solution system, wherein the hydrogen ion concentration in the acid etching solution was 0.40 mol / L, the molar ratio of fluorine to aluminum in the acid etching solution was 30:1, and the defluorinated ferrophosphate slag was added to the acid etching solution for selective leaching to remove impurities. The liquid-solid ratio was controlled at 10:1, the selective leaching temperature was 75℃, the leaching time was 2h, and the physical strengthening method was mechanical stirring. Under these conditions, selective leaching was carried out so that the aluminum and copper impurities in the ferrophosphate slag were fully dissolved into the liquid phase, the solubility of ferrophosphate was 10.6%, and the olivine crystal structure and main morphology of the solid phase ferrophosphate were maintained. After filtration, the defluorinated and impurity-removed ferrophosphate-carbon mixed filter residue was obtained. In the defluorinated and impurity-removed ferrophosphate-carbon mixed filter residue, the aluminum content was 0.021% and the copper content was <0.0006%.

[0018] (4) Aerobic calcination decarburization and crystal form control. The above-mentioned iron phosphate-carbon mixed filter residue was placed in an aerobic atmosphere for calcination at a high temperature of 700℃ for 3 hours. The aerobic atmosphere was air. The residual carbon content of iron phosphate after aerobic calcination decarburization was 0.03%. At the same time, the crystal form of iron phosphate changed from olivine to hexagonal. After cooling, battery-grade iron phosphate product was obtained directly.

[0019] Example 2 This embodiment provides a method for recycling and regenerating phosphorus-iron slag from spent lithium iron phosphate batteries after lithium extraction via chlorination. The process flow of this method is shown in the attached figure. Figure 1 As shown, the specific steps are as follows: (1) Prioritize chlorination to extract lithium and obtain lithium extraction residue. First, waste lithium iron phosphate battery black powder is loaded into a chlorination reactor. Chlorine is used as an oxidant to react chemically with the lithium iron phosphate in the battery black powder to generate iron phosphate and lithium chloride. The reaction time is controlled at 0°C for 120 minutes. The molar ratio of chlorine to iron in the battery black powder is 0.6, and finally chlorinated clinker is obtained. The chlorinated clinker is leached, washed, and filtered with aqueous solution to obtain lithium extraction residue and lithium chloride brine. The iron phosphate in the lithium extraction residue retains the olivine-type orthorhombic crystal structure and retains the physical morphology of the initial raw material lithium iron phosphate cathode material.

[0020] (2) High-temperature carrier gas defluorination and fluorine resource utilization treatment. The lithium-extracted iron phosphate slag is placed in a defluorination reactor, the oxygen in the defluorination reactor is removed, the temperature is raised to 500℃, the defluorination carrier gas is continuously introduced and kept at the temperature for 120 minutes, and the defluorination tail gas is quickly discharged from the defluorination reactor; the fluorine-containing gas in the volatilized defluorination tail gas is absorbed by ammonia water spraying to prepare ammonium fluoride solution; the defluorination carrier gas is a non-oxidizing gas, which contains crackable H-containing gas, and the non-oxidizing gas is mixed with nitrogen gas with a volume content of 30%; the mixed gas of crackable H-containing gas ethane and water vapor; after the lithium-extracting slag of waste lithium iron phosphate batteries is defluorinated and fluorine resource utilization treatment by high-temperature carrier gas, the defluorinated iron phosphate slag is obtained, and its residual fluorine content is 0.023%.

[0021] (3) Solid-phase retention selective impurity removal. The ammonium fluoride solution prepared in the previous step was added to hydrochloric acid to prepare an acid etching solution system, wherein the hydrogen ion concentration in the acid etching solution was 0.20 mol / L, the molar ratio of fluorine to aluminum in the acid etching solution was 10:1, the defluorinated iron phosphate slag was added to the acid etching solution for selective leaching to remove impurities, the liquid-solid ratio was controlled at 5:1, the selective leaching temperature was 40℃, the leaching time was 4h, and the physical strengthening method was mechanical stirring; under these conditions, selective leaching was carried out so that the aluminum and copper impurities in the iron phosphate slag were fully dissolved into the liquid phase, the solubility of iron phosphate was 13.5%, the olivine crystal structure and main morphology of the solid phase iron phosphate were maintained, and after filtration, iron phosphate-carbon mixed filter residue after defluorination and impurity removal was obtained; in the iron phosphate-carbon mixed filter residue after defluorination and impurity removal, the aluminum content was 0.018% and the copper content was <0.0005%.

[0022] (4) Aerobic calcination decarburization and crystal form control. The above-mentioned iron phosphate-carbon mixed filter residue was placed in an aerobic atmosphere for calcination at a high temperature of 800℃ for 2 hours. The aerobic atmosphere was air. The residual carbon content of iron phosphate after aerobic calcination decarburization was 0.02%. At the same time, the crystal form of iron phosphate changed from olivine to hexagonal. After cooling, battery-grade iron phosphate product was obtained directly.

[0023] Example 3 This embodiment provides a method for recycling and regenerating phosphorus-iron slag from spent lithium iron phosphate batteries after lithium extraction via chlorination. The process flow of this method is shown in the attached figure. Figure 1 As shown, the specific steps are as follows: (1) Prioritize chlorination to extract lithium and obtain lithium extraction residue. First, waste lithium iron phosphate battery black powder is loaded into a chlorination reactor. Chlorine is used as an oxidant to react chemically with the lithium iron phosphate in the battery black powder to generate iron phosphate and lithium chloride. The reaction time is controlled at 200℃ for 5 minutes, and the molar ratio of chlorine to iron in the battery black powder is 0.5, finally obtaining chlorinated clinker. The chlorinated clinker is leached, washed, and filtered with aqueous solution to obtain lithium extraction residue and lithium chloride-containing brine. The iron phosphate in the lithium extraction residue retains the olivine-type orthorhombic crystal structure and retains the physical morphology of the initial raw material lithium iron phosphate cathode material.

[0024] (2) High-temperature carrier gas defluorination and fluorine resource utilization treatment. The lithium-extracting phosphate slag is placed in a defluorination reactor, the oxygen in the defluorination reactor is removed, the temperature is raised to 600℃, the defluorination carrier gas is continuously introduced and kept at the temperature for 20 minutes, and the defluorination tail gas is quickly discharged from the defluorination reactor; the fluorine-containing gas in the volatilized defluorination tail gas is absorbed by ammonia water spraying to prepare ammonium fluoride solution; the defluorination carrier gas is a non-oxidizing gas, which contains crackable H-containing gas, and the non-oxidizing gas is mixed with nitrogen gas with a volume content of 10%; the mixed gas of crackable H-containing ethane and water vapor; after the lithium-extracting slag of waste lithium iron phosphate batteries is defluorinated and fluorine resource utilization treatment by high-temperature carrier gas, the defluorinated phosphate slag is obtained, and its residual fluorine content is 0.018%.

[0025] (3) Solid-phase retention selective impurity removal. The ammonium fluoride solution prepared in the previous step was added to hydrochloric acid to prepare an acid etching solution system, wherein the hydrogen ion concentration in the acid etching solution was 0.60 mol / L, the molar ratio of fluorine to aluminum in the acid etching solution was 100:1, the defluorinated ferrophosphate slag was added to the acid etching solution for selective leaching to remove impurities, the liquid-solid ratio was controlled at 20:1, the selective leaching temperature was 95℃, the leaching time was 1h, and the physical strengthening method was mechanical stirring; under these conditions, selective leaching was carried out so that the aluminum and copper impurities in the ferrophosphate slag were fully dissolved into the liquid phase, the solubility of ferrophosphate was 16.3%, the olivine crystal structure and main morphology of the solid phase ferrophosphate were maintained, and after filtration, the defluorinated and impurity-removed ferrophosphate-carbon mixed filter residue was obtained; in the defluorinated and impurity-removed ferrophosphate-carbon mixed filter residue, the aluminum content was 0.015% and the copper content was <0.0004%.

[0026] (4) Aerobic calcination decarburization and crystal form control. The above-mentioned iron phosphate-carbon mixed filter residue was placed in an aerobic atmosphere for calcination at a high temperature of 650℃ for 5 hours. The aerobic atmosphere was air. The residual carbon content of iron phosphate after aerobic calcination decarburization was 0.04%. At the same time, the crystal form of iron phosphate changed from olivine to hexagonal. After cooling, battery-grade iron phosphate product was obtained directly.

[0027] The foregoing provides a detailed description of a method for recycling and regenerating phosphorus-iron slag from spent lithium iron phosphate batteries after lithium extraction via chlorination. Specific examples have been used to illustrate the principles and implementation methods of this application. The descriptions of these embodiments are merely illustrative of the core ideas of this application. It should be noted that those skilled in the art can make various improvements and modifications to this application without departing from its principles, and these improvements and modifications also fall within the scope of protection of the claims.

Claims

1. A method for recycling and regenerating phosphorus-iron slag from spent lithium iron phosphate batteries after lithium extraction via chlorination, characterized in that, Includes the following steps: (1) The phosphorus iron slag after lithium chlorination is placed in a reactor and high-temperature carrier gas is introduced for defluorination treatment. The phosphorus iron slag after lithium chlorination is obtained by preferential gas-solid selective lithium chlorination and lithium chloride leaching in aqueous solution after waste lithium iron phosphate battery black powder. The iron phosphate in the phosphorus iron slag maintains the olivine crystal structure. (2) Add ammonium fluoride solution and hydrochloric acid to the defluorinated iron phosphate slag to prepare a leaching system with a hydrogen ion concentration of 0.20-0.6 mol / L. Control the molar ratio of fluorine to aluminum in the leaching system to be 10:1-100:1 and the liquid-solid ratio to be 5:1-20:

1. Under these conditions, selective leaching is carried out to fully dissolve aluminum and copper impurities in the iron phosphate slag into the liquid phase. At the same time, the total solubility of iron phosphate is controlled to be ≤20%. After filtration, iron phosphate-carbon mixed filter residue after defluorination and impurity removal is obtained. (3) The iron phosphate-carbon mixed filter residue is placed in a high-temperature oxygen atmosphere for calcination to completely remove carbonaceous impurities. After cooling, battery-grade iron phosphate product is obtained directly.

2. The method for recycling and regenerating phosphorus-iron slag from spent lithium iron phosphate batteries after lithium extraction via chlorination, as described in claim 1, is characterized in that... The selective leaching temperature in step (2) is 40-95℃, the leaching time is 1-4h, and the physical enhancement method is mechanical stirring or ultrasound for selective leaching.

3. The method for resource recycling and regeneration of phosphorus iron slag after lithium extraction by chlorination from waste lithium iron phosphate batteries according to claim 1 or 2, characterized in that, In the defluorination and impurity removal of the iron phosphate-carbon mixed filter residue after step (2), the aluminum content is <0.030% and the copper content is <0.0010%.

4. The method for recycling and regenerating phosphorus-iron slag from spent lithium iron phosphate batteries after lithium extraction via chlorination, as described in claim 1, is characterized in that... In step (3), the calcination temperature is 650℃-800℃, the calcination time is 2-5h, and the oxygen atmosphere is air or oxygen or a mixture of oxygen with a volume fraction greater than 20% and an inert gas; the residual carbon content of iron phosphate after decarburization by oxygen calcination is <0.10%.

5. The method for recycling and regenerating phosphorus-iron slag from spent lithium iron phosphate batteries after lithium extraction via chlorination, as described in claim 1, is characterized in that... In step (1), the holding time for high-temperature carrier gas defluorination is 20 minutes to 2 hours, and the defluorination temperature is 500℃ to 600℃. The carrier gas is a non-oxidizing gas containing crackable H-containing gas, and is mixed with one or more of nitrogen, argon, carbon dioxide, and helium. The crackable H-containing gas is a mixture of one or more of the following: manganese acid gas, methyl carbonate gas, ethyl methyl carbonate gas, ethyl carbonate gas, and dimethyl carbonate gas, or a mixture of hydrogen and methane, or a mixture of methanol and methane, or a mixture of ethyl carbonate gas, ethane, and water vapor. After high-temperature carrier gas defluorination treatment, the residual fluorine content in the phosphorus iron slag is <0.045%.

6. The method for resource recycling and regeneration of phosphorus iron slag after lithium extraction by chlorination from waste lithium iron phosphate batteries according to claim 1 or 5, characterized in that, In step (1), the selective gas-solid chlorination extraction of lithium from waste lithium iron phosphate battery black powder refers to the chemical reaction between chlorine gas as an oxidant and lithium iron phosphate in waste lithium iron phosphate battery black powder at a temperature of 0℃~200℃ to generate iron phosphate and lithium chloride. The reaction time is 5 minutes to 120 minutes, and the molar ratio of chlorine gas to iron in waste lithium iron phosphate battery black powder is 0.50-0.

60.

7. The method for recycling and regenerating phosphorus-iron slag from spent lithium iron phosphate batteries after lithium extraction via chlorination, as described in claim 1, is characterized in that... The volatile fluorine-containing gas generated during the defluorination treatment of phosphorus iron slag after lithium extraction in step (1) was absorbed by ammonia water spraying to obtain ammonium fluoride solution, and the obtained ammonium chloride solution was added to the phosphorus iron slag after defluorination in step (2).

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