Method for reducing the content of sulfur in aluminum fluoride

Abstract

The present application relates to a method for reducing the content of sulfur in aluminum fluoride, which comprises the following steps: uniformly mixing crude aluminum fluoride powder containing sulfate impurities with ammonium hydrogen fluoride powder to obtain a mixture; placing the mixture in a reactor and performing programmed temperature calcination under a protective atmosphere; discharging the sulfur and fluorine-containing volatile gas generated by calcination after treatment by an absorption system; cooling the calcined material to room temperature; washing the cooled material with water or blowing inert gas to remove residual ammonium salt or dust, and then drying to obtain a low-sulfur aluminum fluoride product. The in-situ HF generated by the decomposition of NH4HF2 is used for gas-solid reaction, which can effectively attack and convert stable sulfate impurities existing in various forms, has high desulfurization efficiency and is deep and thorough; the reaction conditions are mild, the main phase of the product is stable, the process flow is simple, and the energy consumption is low.

Description

Technical Field

[0001] This invention relates to the field of inorganic fluoride refining, and more specifically to a method for deep removal of sulfur impurities from aluminum fluoride through solid-phase reaction. Background Technology

[0002] Aluminum fluoride (AlF3) is a core flux in aluminum electrolysis, and its impurity content directly affects electrolysis efficiency, environmental emissions, and equipment lifespan. Among these impurities, sulfur (usually in the form of sulfate SO42-) is particularly important. 2- Sulfur (in its natural form) is a highly hazardous impurity element: under high-temperature electrolysis conditions, sulfur-containing compounds decompose to produce sulfur dioxide (SO2), causing severe air pollution; simultaneously, sulfur corrodes the lining of the electrolytic cell and steel components, reducing current efficiency, and may enter the molten aluminum, affecting metal quality. Therefore, the national standard GB / T4292-2017 strictly stipulates that the sulfate content in aluminum fluoride must not exceed 0.6%.

[0003] Currently, when recovering and preparing aluminum fluoride from fluorine-containing waste (such as recycled cryolite), sulfate systems (such as aluminum sulfate solution) are often used for leaching or conversion. Although this process can efficiently extract fluorine and aluminum, it inevitably leads to a large amount of sulfate ions being doped into the intermediate products or the final aluminum fluoride product, forming stubborn sulfur impurities that make it difficult to meet high-grade product standards.

[0004] Existing methods for reducing the sulfur content of aluminum fluoride mainly include high-temperature calcination decomposition and water washing. For example, CN120004300A discloses a method for reducing the content of fine powder and metal impurities in aluminum fluoride products. This method uses ammonium bifluoride solution and aluminum hydroxide as raw materials to prepare an aluminum fluoride solution; sodium sulfide solution is added to the aluminum fluoride solution for purification, resulting in a secondary aluminum fluoride solution; the secondary aluminum fluoride solution is crystallized and calcined; the calcined material is crushed and sieved to remove fine powder, yielding a semi-finished aluminum fluoride product; the semi-finished fluoride powder is then purified using magnetic separation equipment to remove metal impurities, and the aluminum fluoride powder is purified again to obtain the final aluminum fluoride product. This method removes metal impurities from the produced aluminum fluoride product through sulfide precipitation, removes fine powder through crushing and sieving, and finally removes residual metal impurities from the crushed aluminum fluoride product through magnetic separation. CN121181013A discloses a method for preparing recycled aluminum fluoride from fluorine-containing solid waste obtained through aluminum electrolysis. This method utilizes carbonaceous additives to address the problem of excessive sulfate residue in aluminum fluoride products. Its mechanism is based on the reduction of carbon at high temperatures. The carbonaceous additives react with calcium sulfate byproducts, first converting them into calcium sulfide and releasing carbon dioxide (CaSO4 + 2C = CaS + 2CO2). Subsequently, the generated CaS can further react with the remaining CaSO4 in the system to produce calcium oxide and sulfur dioxide gas (3CaSO4 + CaS = 4CaO + 4SO2). The essence of this series of reactions is to recycle the stably bound sulfate ions (S... 6+The sulfur in the solid phase is reduced to gaseous SO2 and escapes (2CaSO4+C=2CaO+2SO2+CO2), thereby effectively removing the sulfur element in the solid phase in gaseous form, and thus effectively removing sulfate ions.

[0005] However, while high-temperature treatment (>800℃) can decompose some sulfates, it is extremely energy-intensive and may lead to the thermal decomposition of aluminum fluoride itself and fluorine loss, resulting in poor economic efficiency and product yield. Conventional water washing has limited effectiveness in removing sulfate ions encapsulated within particles or forming stable complexes with aluminum. Therefore, there is an urgent industrial need to develop a technology that can efficiently and deeply desulfurize aluminum fluoride under relatively mild conditions without damaging the aluminum fluoride matrix. Summary of the Invention

[0006] In view of the current state of the prior art, especially the problems of high sulfur content in aluminum fluoride products and high energy consumption or poor effectiveness of traditional desulfurization methods, the purpose of this invention is to provide a method for reducing the sulfur content in aluminum fluoride. This invention involves mixing sulfur-containing crude aluminum fluoride with ammonium bifluoride (NH4HF2) and then calcining it at a medium-to-low temperature. The active hydrogen fluoride (HF) produced by the decomposition of ammonium bifluoride reacts with sulfate ions in a solid-gas reaction, converting stable sulfur impurities into volatile sulfur-fluorine compounds (such as SF4, SOF2, etc.) for removal, thereby achieving deep desulfurization of the aluminum fluoride product.

[0007] The technical solution for achieving the above-mentioned objectives can be summarized as follows:

[0008] A method for reducing the sulfur content in aluminum fluoride includes the following steps:

[0009] S1. Mixing: Mix the crude aluminum fluoride powder containing sulfate impurities with ammonium bifluoride powder evenly to obtain a mixture.

[0010] S2, Calcination desulfurization: The mixture from step S1 is placed in a reactor and calcined under a protective atmosphere with programmed temperature increase;

[0011] S3. Exhaust gas treatment and product cooling: The sulfur- and fluorine-containing volatile gases generated during calcination are treated by an absorption system before being discharged. The calcined material is then cooled to room temperature.

[0012] S4. Post-processing: The cooled material is washed with water or purged with inert gas to remove residual ammonium salts or dust, and then dried to obtain low-sulfur aluminum fluoride products.

[0013] According to the present invention, preferably, the mass fraction of sulfate in the crude aluminum fluoride in S1 (as SO42-) 2- (Calculated) is 0.8%-5.0%;

[0014] Preferably, the purity of the ammonium bifluoride in S1 is ≥99%;

[0015] Preferably, the dry basis mass ratio of crude aluminum fluoride to ammonium bifluoride in S1 is 1:(0.1~0.5), and more preferably 1:(0.2~0.35).

[0016] According to the present invention, preferably, the protective atmosphere in S2 is a nitrogen atmosphere, an argon atmosphere, or a weakly reducing atmosphere, and more preferably, the weakly reducing atmosphere is a mixture of carbon monoxide and an inert gas (such as nitrogen or argon), wherein the volume fraction of carbon monoxide is 10% to 15%;

[0017] Preferably, the programmed heating calcination in S2 is: heating to the target temperature at a rate of 2~10℃ / min and holding at that temperature;

[0018] Preferably, the target calcination temperature in S2 is 250℃~450℃, and more preferably 300℃~400℃.

[0019] Preferably, the heat preservation time in S2 is 1h to 6h, and more preferably 2h to 4h.

[0020] According to the present invention, preferably, the tail gas treatment in S3 adopts multi-stage alkaline absorption, and the preferred alkaline solution is NaOH or Ca(OH)2 solution.

[0021] According to the present invention, preferably, the water washing in S4 is carried out with room temperature or warm water for a short time (<10 min) and is rapid; preferably, the liquid-to-solid ratio of the water washing is controlled at (2~5) mL / g;

[0022] Preferably, the drying temperature in S4 is 80℃~120℃, and the drying time is 1h~3h.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] 1. High and thorough desulfurization efficiency: Utilizing in-situ HF generated from the decomposition of NH4HF2 for a gas-solid reaction, it effectively attacks and transforms stable sulfate impurities existing in various forms, achieving a desulfurization rate of over 95%. The sulfur content of the product is easily controlled within international premium grade (SO4). 2- ≤0.3%).

[0025] 2. Mild reaction conditions and stable main phase of product: The core desulfurization reaction is carried out at a low temperature (<450℃), which is far below the direct thermal decomposition temperature of sulfate. This avoids the adverse effects of high temperature on the crystal structure of aluminum fluoride, resulting in high product yield and low fluorine loss.

[0026] 3. Simple process flow and low energy consumption: The main process consists of one mixing and one medium-low temperature calcination, without the need for complex high-temperature equipment or multiple repeated treatments, and the energy consumption is significantly lower than that of traditional high-temperature desulfurization processes.

[0027] 4. Environmentally friendly and highly resource-efficient: The main byproducts of the reaction are volatile sulfur and fluorine compounds and ammonia, which can be completely converted into harmless or recyclable chemicals (such as Na2SO4, NaF, etc.) through the supporting tail gas absorption system, realizing the targeted removal and resource utilization of sulfur, without generating secondary solid waste. Attached Figure Description

[0028] Figure 1 This is a process flow diagram of the method for reducing the sulfur content in aluminum fluoride according to the present invention.

[0029] Figure 2 The image shows the XRD pattern of the crude high-sulfur aluminum fluoride raw material used in Example 1.

[0030] Figure 3 The image shows the XRD pattern of the low-sulfur aluminum fluoride product obtained in Example 1. Detailed Implementation

[0031] This invention provides a method for reducing the sulfur content in aluminum fluoride. The method involves mixing crude aluminum fluoride containing sulfur with ammonium hydrogen fluoride (NH4HF2) and then calcining it at a medium-low temperature. The active hydrogen fluoride (HF) produced by the decomposition of ammonium hydrogen fluoride reacts with sulfate ions in a solid-gas reaction, converting stable sulfur impurities into volatile sulfur-fluorine compounds (such as SF4, SOF2, etc.) for removal, thereby achieving deep desulfurization of aluminum fluoride products.

[0032] The present invention provides a method for reducing the sulfur content in aluminum fluoride, comprising the following steps:

[0033] S1. Mixing: Mix the crude aluminum fluoride powder containing sulfate impurities with ammonium bifluoride powder evenly to obtain a mixture.

[0034] S2, Calcination desulfurization: The mixture from step S1 is placed in a reactor and calcined under a protective atmosphere with programmed temperature increase;

[0035] S3. Exhaust gas treatment and product cooling: The sulfur- and fluorine-containing volatile gases generated during calcination are treated by an absorption system before being discharged. The calcined material is then cooled to room temperature.

[0036] S4. Post-processing: The cooled material is washed with water or purged with inert gas to remove residual ammonium salts or dust, and then dried to obtain low-sulfur aluminum fluoride products.

[0037] According to the present invention, sulfur in crude aluminum fluoride exists mainly in the form of sulfate ions. In one or more preferred embodiments, the mass fraction of sulfate ions in the crude aluminum fluoride in S1 (as SO42-) 2- The purity of the ammonium bifluoride in S1 is ≥99%.

[0038] According to the present invention, the mixing of crude aluminum fluoride and ammonium bifluoride needs to ensure that sufficient active fluorine is provided to completely convert sulfate. In one or more preferred embodiments, the dry basis mass ratio of the crude aluminum fluoride to ammonium bifluoride in S1 is 1:(0.1~0.5), more preferably 1:(0.2~0.35).

[0039] According to the present invention, the S2 calcination process is carried out in a protective atmosphere to prevent excessive oxidation of ammonium bifluoride or the product. In one or more preferred embodiments, the protective atmosphere in S2 is a nitrogen atmosphere, an argon atmosphere, or a weakly reducing atmosphere, preferably a mixed atmosphere of carbon monoxide and an inert gas (such as nitrogen or argon), wherein the volume fraction of carbon monoxide is 10% to 15%.

[0040] According to the present invention, the calcination temperature in S2 needs to be controlled within a suitable range. Too low a temperature will result in incomplete reaction, while too high a temperature may lead to a crystal transformation of aluminum fluoride or excessive decomposition and loss of ammonium bifluoride. In one or more preferred embodiments, the target calcination temperature in S2 is 250℃~450℃, more preferably 300℃~400℃. Preferably, the programmed temperature rise calcination in S2 involves heating to the target temperature at a rate of 2~10℃ / min and holding at that temperature; the holding time in S2 is 1h~6h, preferably 2h~4h. Calcination in S2 can be carried out using conventional equipment, such as a tubular furnace, rotary kiln, or fluidized bed reactor that can be sealed and connected to a tail gas treatment system.

[0041] According to the present invention, the tail gas treatment in S3 employs multi-stage alkaline absorption to ensure that sulfur- and fluorine-containing waste gases are completely absorbed and fixed, avoiding secondary pollution. In one or more preferred embodiments, the alkaline solution is NaOH or Ca(OH)2 solution. The room temperature mentioned in this invention refers to 25℃±5℃.

[0042] According to the present invention, in step S4, water washing or inert gas purging is used to remove residual ammonium salts or dust. In one or more preferred embodiments, the water washing in step S4 uses room temperature or warm water and is a short (<10 min) rapid wash, designed to dissolve any trace amounts of NH4 that may remain. + Salt. Preferably, the liquid-to-solid ratio during water washing is controlled at (2~5) mL / g, and the warm water temperature is 30-50℃. After water washing and drying, aluminum fluoride product can be obtained. Preferably, the drying temperature in S4 is 80℃~120℃, and the drying time is 1h~3h.

[0043] The core reaction mechanism of this invention is as follows:

[0044] Ammonium bifluoride decomposes upon heating to produce highly reactive hydrogen fluoride (HF). HF gas then undergoes a fluorination-reduction reaction with sulfates doped in aluminum fluoride, releasing sulfur (S). 6+It is converted into low-valent volatile sulfur and fluorine compounds. The main reaction pathway can be represented as follows:

[0045]

[0046]

[0047] The volatile product SF4 is carried out of the reaction system by the carrier gas and safely treated by the tail gas absorption system (such as alkaline absorption), thereby achieving the complete removal of sulfur from solid aluminum fluoride.

[0048] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0049] Example 1

[0050] S1: Take 100g of crude aluminum fluoride powder with a sulfate content of 2.1% (mass fraction) and mix it thoroughly with 30g of ammonium bifluoride (purity 99.5%) in a glove box (dry air environment) in a mixing tank for 30 minutes.

[0051] S2: Load the mixture into a quartz boat and place it in the constant temperature zone of a tube furnace. Purge with nitrogen as both carrier and protective gas at a flow rate of 100 mL / min. Increase the temperature to 350 °C at a rate of 5 °C / min and hold at this temperature for 3 hours. Pass the exhaust gas into two connected 2 mol / L NaOH solution absorption bottles.

[0052] S3: After the heat preservation is completed, allow the material to cool naturally to below 80°C under a nitrogen atmosphere before removing it.

[0053] S4: Wash the material with 300mL of 40℃ warm water by rapid stirring for 5 minutes, and immediately vacuum filter. Place the filter cake in an oven and dry it at 100℃ for 2 hours to obtain a white powder product.

[0054] Ion chromatography analysis showed that the sulfate content of the product was reduced to 0.08%, far below the national standard requirement of 0.6%; XRD analysis showed that the product was pure phase β-AlF3 with no sulfur-containing peaks; the main aluminum fluoride content reached 99.1%.

[0055] Example 2

[0056] S1: Take 100g of crude aluminum fluoride with a sulfate content of 3.8% and mix it evenly with 40g of ammonium bifluoride.

[0057] S2: Under argon protection, dynamically calcined at 380℃ for 2.5 hours in a rotary kiln. The exhaust gas is treated in a lime slurry absorption tower.

[0058] S3: After cooling, nitrogen gas is used to purge and remove the adsorbed trace gases.

[0059] S4: Do not wash with water, dry directly at 90℃ for 3 hours.

[0060] The product was tested and found to contain 0.15% sulfate and 98.7% aluminum fluoride.

[0061] Example 3

[0062] S1: Take 100g of crude aluminum fluoride with a sulfate content of 1.2% and mix it evenly with 18g of ammonium bifluoride.

[0063] S2: Static calcination at 300℃ for 5 hours under a nitrogen atmosphere.

[0064] S3 / S4: After cooling, wash with 200mL of 50℃ warm water for 3 minutes, and then dry to obtain the product.

[0065] The product was found to contain 0.05% sulfate and 99.3% aluminum fluoride.

[0066] Comparative Example 1

[0067] 100g of crude aluminum fluoride from the same batch as in Example 1 was taken and calcined directly in air at 350°C for 3 hours without adding any reagents. After cooling, the product was analyzed. The sulfate content was 1.95%, indicating a weak desulfurization effect, proving that simple medium-temperature heat treatment cannot effectively decompose sulfates.

[0068] Comparative Example 2

[0069] Take 100g of crude aluminum fluoride from the same batch as in Example 1, wash it directly with 500mL of hot water (80℃) for 1 hour without calcination, then filter and dry. The sulfate content of the product is 1.88%, proving that physical washing is ineffective in removing bound sulfate ions.

[0070] Experimental Example 1: Analysis of Sulfur Content and Main Content of Products

[0071] The sulfate content and aluminum fluoride content of the products obtained in Examples 1-3 and Comparative Examples 1-2 were analyzed, and the results are shown in Table 1.

[0072] Table 1

[0073]

[0074] As shown in Table 1, the method of the present invention can significantly reduce the sulfur content in aluminum fluoride, and the sulfur content in the treated product is far below the national standard requirement of 0.6%.

[0075] Experimental Example 2: Phase and Structure Analysis

[0076] XRD analysis was performed on the raw materials and products of Example 1, and the results are as follows: Figure 2 ,Figure 3 As shown. Figure 2 The raw material exhibits weak diffraction peaks of sulfur-containing impurities such as ammonium aluminum sulfate, in addition to the characteristic peaks of aluminum fluoride. Figure 3 The results show that after processing by the method of the present invention, all sulfur-containing impurity phase diffraction peaks in the XRD pattern of the product completely disappeared, and only sharp and pure β-AlF3 diffraction peaks were observed, indicating that sulfur impurities were effectively removed without damaging the main structure of aluminum fluoride.

[0077] Experimental Example 3: Analysis of Exhaust Gas Absorption Products

[0078] Composition analysis of the NaOH absorption solution in Example 1 revealed a high concentration of SO3. 2- SO4 2- The presence of F⁻ ions confirmed that sulfur was absorbed and fixed in the expected form (SF4 hydrolysis products), verifying the reliability of the reaction pathway.

[0079] The above results fully demonstrate that the method of the present invention can efficiently and deeply remove sulfur from aluminum fluoride under mild conditions, resulting in excellent product quality, environmentally friendly process, and extremely high industrial application value.

Claims

1. A method for reducing the sulfur content in aluminum fluoride, comprising the following steps: S1. Mixing: Mix the crude aluminum fluoride powder containing sulfate impurities with ammonium bifluoride powder evenly to obtain a mixture. S2, Calcination desulfurization: The mixture from step S1 is placed in a reactor and calcined under a protective atmosphere with programmed temperature increase; S3. Exhaust gas treatment and product cooling: The sulfur- and fluorine-containing volatile gases generated during calcination are treated by an absorption system before being discharged. The calcined material is then cooled to room temperature. S4. Post-processing: The cooled material is washed with water or purged with inert gas to remove residual ammonium salts or dust, and then dried to obtain low-sulfur aluminum fluoride products.

2. The method for reducing the sulfur content in aluminum fluoride according to claim 1, characterized in that, The mass fraction of sulfate in the crude aluminum fluoride described in S1 is 0.8%-5.0%; Preferably, the purity of the ammonium bifluoride in S1 is ≥99%.

3. The method for reducing the sulfur content in aluminum fluoride according to claim 1, characterized in that, The dry basis mass ratio of crude aluminum fluoride to ammonium bifluoride in S1 is 1:(0.1~0.5), preferably 1:(0.2~0.35).

4. The method for reducing the sulfur content in aluminum fluoride according to claim 1, characterized in that, The protective atmosphere described in S2 is a nitrogen atmosphere, an argon atmosphere, or a weakly reducing atmosphere.

5. The method for reducing the sulfur content in aluminum fluoride according to claim 4, characterized in that, The weakly reducing atmosphere is a mixture of carbon monoxide and inert gas, wherein the volume fraction of carbon monoxide is 10% to 15%.

6. The method for reducing the sulfur content in aluminum fluoride according to claim 1, characterized in that, The programmed calcination described in S2 is as follows: heating to the target temperature at a rate of 2~10℃ / min and holding at that temperature; Preferably, the target calcination temperature in S2 is 250℃~450℃, and the holding time is 1h~6h.

7. The method for reducing the sulfur content in aluminum fluoride according to claim 1, characterized in that, The target calcination temperature in S2 is 300℃~400℃, and the holding time is 2h~4h.

8. The method for reducing the sulfur content in aluminum fluoride according to claim 1, characterized in that, In S3, the exhaust gas treatment adopts multi-stage alkaline absorption, and the preferred alkaline solution is NaOH or Ca(OH)2 solution.

9. The method for reducing the sulfur content in aluminum fluoride according to claim 1, characterized in that, The water washing described in S4 uses room temperature or warm water and is a short-time, rapid washing process; preferably, the liquid-to-solid ratio of the water washing is controlled at (2~5) mL / g.

10. The method for reducing the sulfur content in aluminum fluoride according to claim 1, characterized in that, The drying temperature in S4 is 80℃~120℃, and the drying time is 1h~3h.

Images