A method for regenerating polluted acid from cryolite defluorination

Abstract

The present invention provides a method for regenerating acid from defluorinated cryolite, comprising the steps of: introducing hydrogen sulfide gas into the waste acid wastewater to generate a sulfidation reaction, and then filtering to obtain sulfide slag and filtrate; electrodialyzing the filtrate to obtain fresh water and dialysis residue, and returning the fresh water to the zinc hydrometallurgical system for water; evaporating the dialysis residue to obtain sulfuric acid solution and distillate; reacting part of the sulfuric acid solution with electrolytic aluminum cathode plate waste to obtain aluminum sulfate solution as a defluoridating agent, and the remaining sulfuric acid solution to prepare concentrated sulfuric acid; adding a defluoridating agent and an adjusting agent to the distillate to defluorinate at 95-98°C for 30-150min, filtering to obtain cryolite and defluorinated water; adding bismuth salt to the defluorinated water and then filtering to obtain two-return water and dechlorination slag, and the two-return water returns to the zinc hydrometallurgical system for water. A method for regenerating acid from defluorinated cryolite provided by the present invention can treat the waste acid wastewater to a reuse standard and is green, environmentally friendly, and economically efficient.

Description

Technical Field

[0001] The invention relates to the technical field of metallurgical environmental protection, and in particular to a method for regenerating cryolite defluorinated contaminated acid. Background Art

[0002] During the zinc hydrometallurgical smelting process, the flue gas produced by roasting zinc concentrate in a fluidized bed furnace can be treated by an acid production process to produce 98% concentrated sulfuric acid products and contaminated acid wastewater. It has the characteristics of high acidity, complex heavy metal composition, and high fluorine and chlorine concentrations, including 10-25g / L of acid, 0.5-5g / L of fluorine, and 0.8-8g / L of chlorine.

[0003] During the zinc electrolysis process in a zinc hydrometallurgical system, fluoride ions corrode the aluminum oxide film on the cathode aluminum plate surface, causing the precipitated zinc to form an alloy with the aluminum metal. This makes the precipitated zinc difficult to strip, increases labor intensity and cathode plate consumption, and leads to increased production costs and reduced product quality. Therefore, zinc hydrometallurgical systems require that the fluoride content in the purified reused water be less than 50 mg / L. When using large-plate electrolysis, the fluoride content in the purified reused water must be below 30 mg / L. Therefore, high-fluoride-content acid wastewater from zinc hydrometallurgical systems cannot be directly returned to the zinc hydrometallurgical system.

[0004] Currently, the treatment processes for acid wastewater reuse as secondary return water often use methods such as selective electrodialysis, air stripping, adsorption, and membrane filtration. Although these methods can, to a certain extent, remove fluorine and chlorine from acid wastewater, meeting industrial secondary return water standards and allowing it to be returned to the zinc hydrometallurgy system for continued use, achieving zero acid discharge, they still present problems such as large equipment investment, high treatment costs, and the generation of secondary slag.

[0005] Therefore, there is an urgent need for a green, environmentally friendly, economical and efficient method for treating contaminated acid wastewater, so that the fluorine content in the contaminated acid wastewater after treatment is lower than 50 mg / L and the chlorine content is lower than 300 mg / L, reaching the industrial secondary return water recovery standard that can be returned to the zinc hydrometallurgical system for continued use. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method for regenerating acid wastewater by defluorination of cryolite, which can treat acid wastewater to meet the reuse standard and is green, environmentally friendly, economical and efficient.

[0007] In order to solve the above technical problems, the present invention provides a method for regenerating contaminated acid by defluorination of cryolite, comprising the following steps:

[0008] Introducing hydrogen sulfide gas into the polluted acid wastewater to cause a sulfidation reaction, followed by filtration to obtain sulfided slag and filtrate;

[0009] The filtrate is electrodialyzed to obtain fresh water and dialysis residue, and the fresh water is returned to the zinc hydrometallurgy system for water use;

[0010] The dialysate is evaporated and stripped to obtain sulfuric acid solution and distillate;

[0011] Part of the sulfuric acid solution is reacted with the electrolytic aluminum cathode plate waste to obtain an aluminum sulfate solution with a pH of 0.5-2.0 as a defluoridating agent, and the remaining sulfuric acid solution is used to prepare concentrated sulfuric acid;

[0012] Add defluoridation agent and regulator to the distillate and defluorinate at 95-98°C for 30-150 minutes, then filter to obtain cryolite and defluorinated water;

[0013] Bismuth salt is added to the defluorinated water for dechlorination and then filtered to obtain secondary return water with a fluorine content of less than 50 mg / L and a chlorine content of less than 300 mg / L and dechlorination slag. The secondary return water is used as water for the zinc hydrometallurgical smelting system.

[0014] Furthermore, the amount of hydrogen sulfide gas added is 1.15-1.45 times the theoretical amount of hydrogen sulfide gas required to convert all heavy metals such as lead, zinc, copper and arsenic in the acid wastewater into corresponding sulfide precipitates during the sulfidation reaction.

[0015] Furthermore, the sulfidation reaction time is 60-100 minutes, the reaction temperature is room temperature, and the sulfided slag obtained by filtration is returned to the zinc concentrate and fed into the boiling furnace for reuse.

[0016] Furthermore, the temperature for the reaction of the sulfuric acid solution with the electrolytic aluminum cathode plate waste is 55-75° C., and the reaction time is 90-300 min.

[0017] Furthermore, the amount of the defluorinating agent added is 1.01-1.06 times the theoretical amount of the defluorinating agent required to convert all F in the distillate into cryolite.

[0018] Furthermore, the adjusting agent is one or two of sodium carbonate, sodium bicarbonate or sodium hydroxide.

[0019] Furthermore, the pH of the defluorinated water is between 6.0 and 8.5.

[0020] Furthermore, the amount of bismuth salt added is 1.02-1.05 times the theoretical amount of bismuth salt required to convert all Cl in the defluorinated water into bismuth oxychloride.

[0021] Furthermore, the fluorine content in the acid wastewater is 0.5-5.0 g / L, and the chlorine content is 0.8-8.0 g / L.

[0022] The present invention provides a cryolite defluorination and waste acid regeneration method, which achieves the purpose of recovering all components of the waste acid wastewater, such as valuable heavy metals copper, lead, zinc, sulfuric acid, water and F, by separating the waste acid wastewater in steps. In addition, the sulfuric acid recovered from the waste acid wastewater can be used as a raw material for preparing a defluoridating agent and reacted with electrolytic aluminum cathode plate waste to prepare the defluoridating agent. This method not only reduces the sodium carbonate decalcification step in the conventional waste acid wastewater treatment process, but also reduces the storage of aluminum cathode plate waste in zinc hydrometallurgy, realizes the reuse of aluminum resources, thereby realizing the harmless comprehensive recycling and utilization of the waste acid wastewater, reduces the impact on the environment, can realize resource recycling and save resources, and is low-carbon, safe, green and environmentally friendly, reduces production costs, and improves economic and social benefits.

[0023] Furthermore, the present invention provides a cryolite defluorination and contaminated acid regeneration method, which can obtain secondary return water with fluorine and chlorine contents of less than 50 mg / L and 300 mg / L after defluorination and chlorination of contaminated acid wastewater with fluorine and chlorine contents of up to 5 g / L and 8 g / L, respectively. The fluorine removal rate reaches 99%, meeting the use standard requirements of zinc hydrometallurgy for recycled and reused secondary return water. This not only reduces the internal production costs of the enterprise such as defluorination and chlorination in the subsequent zinc hydrometallurgy process, but also simultaneously produces cryolite products with higher value, has good economic benefits, and can greatly improve the industry competitiveness of zinc hydrometallurgy enterprises.

[0024] At the same time, the method for regenerating acid from defluorinated cryolite provided by the present invention not only has a reasonable process flow and simple operation, but also the products in the process are basically recycled and reused, basically achieving zero discharge of waste and wastewater. It belongs to green production, meets the requirements of resource recycling and sustainable development, is easy to industrialize, and has very broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 A step diagram of a method for regenerating contaminated acid from defluorinated cryolite provided by an embodiment of the present invention;

[0026] Figure 2 This is a structural flow chart of a method for regenerating contaminated acid by defluorination of cryolite provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0027] See also Figure 1 The embodiment of the present invention provides a method for regenerating contaminated acid from cryolite by defluorination, comprising the following steps:

[0028] Step 1) introducing hydrogen sulfide gas into the contaminated acid wastewater to cause a sulfidation reaction, followed by filtration to obtain sulfided slag and filtrate.

[0029] Among them, the dirty acid wastewater is the dirty acid wastewater generated in the zinc hydrometallurgy process with a fluorine content of 0.5 to 5.0 g / L and a chlorine content of 0.8 to 8.0 g / L.

[0030] Hydrogen sulfide gas is introduced into the contaminated acid wastewater, and the hydrogen sulfide gas reacts with heavy metals such as lead, zinc, copper, and arsenic in the contaminated acid wastewater to generate corresponding sulfide precipitates to obtain insoluble zinc-containing sulfide slag.

[0031] In order to completely react heavy metals such as lead, zinc, copper, and arsenic in the contaminated acid wastewater, the amount of hydrogen sulfide gas added is 1.15-1.45 times the theoretical amount of hydrogen sulfide gas required to convert all heavy metals such as lead, zinc, copper, and arsenic in the contaminated acid wastewater into corresponding sulfide precipitates during the sulfidation reaction.

[0032] Furthermore, to ensure a sufficient reaction, the sulfidation reaction time is 60-100 minutes, and the reaction temperature can be room temperature. The filtered zinc-containing sulfided slag is returned to the zinc concentrate and reused in the fluidized bed furnace, achieving resource recycling and conservation.

[0033] Step 2) Electrodialysis is performed on the filtrate to obtain fresh water and a raffinate. Since the fluorine and chlorine content of the fresh water is very low, it meets the water usage standards of the zinc hydrometallurgy system. Therefore, the fresh water obtained by electrodialysis can be directly returned to the zinc hydrometallurgy system for reuse, thereby realizing water reuse and saving resources.

[0034] Step 3) The raffinate obtained by electrodialysis is evaporated and stripped to obtain a sulfuric acid solution and a distillate.

[0035] The mass percentage concentration of the obtained sulfuric acid solution is above 68%. A portion of the sulfuric acid solution with a mass percentage concentration above 68% is used to prepare a defluoridating agent in the next step, and the rest is used to prepare 98% concentrated sulfuric acid. The obtained distillate is enriched to obtain Cl - 、F - The contents of the acidic mixed solutions exceeded 15 g / L and 25 g / L respectively, but the contents of other impurities such as heavy metals in the distillate were already trace amounts.

[0036] In step 4), a portion of the sulfuric acid solution (having a concentration of greater than 68% by mass) obtained in step 3) is reacted with the electrolytic aluminum cathode plate waste to produce an aluminum sulfate solution having a pH of 0.5-2.0. This aluminum sulfate solution serves as the defluoridating agent in the next step. The remaining sulfuric acid solution (having a concentration of greater than 68% by mass) is used to prepare 98% concentrated sulfuric acid.

[0037] Among them, in order to enable the sulfuric acid solution to react fully and quickly with the electrolytic aluminum cathode plate waste, the sulfuric acid solution with a mass percentage concentration of more than 68% is first prepared into a sulfuric acid solution with a molar concentration of 1-2.5 mol / L, and the temperature of the reaction of the sulfuric acid solution with the electrolytic aluminum cathode plate waste is controlled to 55-75°C, and the reaction time is controlled to 90-300 minutes.

[0038] The reaction equation for the preparation of the defluorination agent is as follows:

[0039] 2Al+3H2SO4=Al2(SO4)3+3H2↑

[0040] Step 5) adding the defluorinating agent obtained in step 4) to the distillate obtained in step 3), and adding a regulating agent to carry out defluorination at 95-98° C. for 30-150 minutes. After the reaction is complete, filtration is performed to obtain cryolite and defluorinated water.

[0041] The amount of the defluoridating agent added is 1.01-1.06 times the theoretical amount of the defluoridating agent required to convert all F in the distillate into cryolite.

[0042] The adjusting agent is one or two of sodium carbonate, sodium bicarbonate or sodium hydroxide.

[0043] The reaction equation for generating cryolite for fluorine removal in this process is as follows:

[0044] Al2(SO4)3+6Na2CO3+12HF=2Na3AlF6↓+6CO2↑+3Na2SO4+6H2O

[0045] Al2(SO4)3+12NaHCO3+12HF=2Na3AlF6↓+12CO2↑+3Na2SO4+12H2O

[0046] Al2(SO4)3+12NaOH+12HF=2Na3AlF6↓+3Na2SO4+12H2O

[0047] During the reaction process of adding defluorination agent and adjusting agent to the distillate, the high concentration of F in the distillate - Participate in chemical reaction and transform into cryolite, which not only removes F in distillate - The purpose is to produce cryolite products with higher value at the same time, which has good economic benefits and can greatly improve the industry competitiveness of zinc hydrometallurgical enterprises.

[0048] Furthermore, not only is no calcium ion added in the process of the present invention, but also carbonate radicals are introduced during the process due to the addition of adjusting agents such as sodium carbonate, thereby achieving the effect of removing calcium from the acid wastewater.

[0049] As a preferred embodiment of the present invention, in this industrial production, sodium carbonate is preferably used because it is cheap. Sodium bicarbonate can also be used generally, but caustic soda, i.e., sodium hydroxide, is rarely used.

[0050] Through this process, in the process of producing cryolite, F in the distillate -The fluorine content in the defluorinated water is less than 50 mg / L.

[0051] Furthermore, the pH of the obtained defluorinated water is between 6.0 and 8.5.

[0052] Step 6) adding bismuth salt to the defluorinated water for dechlorination. To maximize the removal of chlorine from the defluorinated water, the amount of bismuth salt added is controlled to be 1.02-1.05 times the theoretical amount of bismuth salt required to convert all Cl in the defluorinated water into bismuth oxychloride.

[0053] After full dechlorination, secondary return water and dechlorination slag are obtained by filtration. The fluorine content of the secondary return water is less than 50 mg / L and the chlorine content is less than 300 mg / L, which meets the standard requirements for water used in zinc hydrometallurgy system. In this way, the secondary return water can be returned to the zinc hydrometallurgy system for reuse.

[0054] Among them, the dechlorination slag is regenerated into bismuth hydroxide, which can be used as bismuth salt and added to the defluorination water for dechlorination, thereby realizing the repeated reuse of resources.

[0055] The present invention provides a method for defluorinating and regenerating contaminated acid with cryolite, which applies cryolite preparation to the treatment of contaminated acid wastewater with zero discharge and resource regeneration. Compared with the defluorination of calcium hydroxide or aluminum sulfate in conventional zinc hydrometallurgy, the application system, purpose, defluorination effect and the residual fluorine content in the liquid after defluorination are different.

[0056] The present invention provides a method for regenerating contaminated acid by defluoridation of cryolite, which does not require additional equipment and utilizes residual materials generated in a zinc smelting system as a defluoridation raw material. At the same time, the step of adding sodium carbonate for decalcification in the existing process is reduced. The method is simple to operate, does not require additional investment costs, has low production costs, and has broad prospects for industrial application.

[0057] At the same time, the present invention provides a method for regenerating acid from defluorinated cryolite, which is applied in actual production, improves the recovery and utilization rate of heavy metals, valuable metals copper, lead, zinc, sulfuric acid, water and other resources, turns aluminum cathode plate waste produced by zinc smelting into treasure, and realizes the rational utilization of dilute sulfuric acid produced in the acid production process, realizes the optimal utilization of resources, saves resources, is low-carbon, safe, green and environmentally friendly, reduces production costs, and improves economic and social benefits.

[0058] The following examples illustrate the method for regenerating contaminated acid from cryolite by defluorination provided by the present invention.

[0059] Example 1

[0060] For a kind of acid wastewater with fluorine and chlorine mass-volume concentrations of 5000mg / L and 8000mg / L respectively, and containing heavy metals such as Zn, Cd, Pb, Cu, and As, according to Figure 2The process includes the following steps:

[0061] The first step of pretreatment is to introduce 1.45 times the amount of hydrogen sulfide gas into the acid wastewater, and the supernatant after reacting at room temperature for 60 minutes is subjected to electrodialysis, and high-fluorine chlorine dilute acid is evaporated and stripped to obtain zinc-containing sulfide slag, fresh water, sulfuric acid solution with a mass percentage concentration of 72.63%, and distillate, respectively.

[0062] The second step is to prepare the defluorination agent: under the conditions of T=75℃ and t=90min, use 1mol / L sulfuric acid to dissolve the electrolytic aluminum cathode plate waste to prepare a defluorination agent with pH=2.0.

[0063] The third step is to remove fluorine with cryolite: add 1.06 times of the defluoridation agent and the adjusting agent sodium carbonate to the distillate and remove fluorine at T=95℃, t=30min. The defluoridation endpoint pH=6.0. Filter to obtain cryolite and defluoridated water.

[0064] The fourth step is bismuth salt dechlorination: 1.02 times the activated bismuth salt is added to the defluorinated water for dechlorination, and the qualified secondary return water that can be returned to the zinc hydrometallurgy system is filtered to obtain the fluorine and chlorine mass-volume concentration content of 41 mg / L and 160 mg / L respectively.

[0065] The water in the contaminated acid wastewater is returned to the zinc smelting wet process system in the form of secondary return water and is recycled as water resources. The output sulfuric acid solution with a mass percentage concentration of 71.63% and waste aluminum cathode material are used to prepare defluoridation agent. The resource utilization rate of sulfuric acid is 99.26%. The heavy metal valuable metals zinc, lead and cadmium are recovered in the form of sulfides as zinc concentrate returned to the boiling furnace, with a recovery rate of 99.51%. Fluorine is prepared into cryolite for sale, with a recovery rate of 99.18%.

[0066] Example 2

[0067] For a kind of acid wastewater with fluorine and chlorine mass-volume concentrations of 500mg / L and 800mg / L respectively, and containing heavy metals such as Zn, Cd, Pb, Cu, and As, according to Figure 2 The process includes the following steps:

[0068] The first step of pretreatment is to introduce 1.15 times the amount of hydrogen sulfide gas into the acid wastewater, and the supernatant after reacting at room temperature for 100 minutes is subjected to electrodialysis, and high-fluorine chlorine dilute acid is evaporated and stripped to obtain zinc-containing sulfide slag, fresh water, sulfuric acid solution with a mass percentage concentration of 75.19%, and distillate, respectively.

[0069] The second step is to prepare the defluorination agent: under the conditions of T=55℃ and t=300min, use 2.5mol / L sulfuric acid to dissolve the electrolytic aluminum cathode plate waste to prepare a defluorination agent with pH=0.5.

[0070] The third step is to remove fluorine with cryolite: add 1.01 times of the defluoridating agent and the adjusting agent sodium bicarbonate to the distillate and remove fluorine at T=98℃, t=150min. The defluoridation end point pH=8.5. Filter to obtain cryolite and defluoridated water.

[0071] The fourth step is bismuth salt dechlorination: 1.05 times the activated bismuth salt is added to defluorinated water for dechlorination, and filtered to obtain qualified secondary return water with fluorine and chlorine mass-volume concentrations of 4.5 mg / L and 138 mg / L respectively, which can be returned to the zinc hydrometallurgical smelting system.

[0072] The water in the contaminated acid wastewater is returned to the zinc smelting wet process system in the form of secondary return water and is recycled as water resources. The output sulfuric acid solution with a mass percentage concentration of 75.17% and waste aluminum cathode material are used to prepare defluoridation agent. The resource utilization rate of sulfuric acid is 99.31%. The heavy metal valuable metals zinc, lead and cadmium are recovered in the form of sulfides as zinc concentrate returned to the boiling furnace, with a recovery rate of 99.48%. Fluorine is prepared into cryolite for sale, with a recovery rate of 99.10%.

[0073] Example 3

[0074] For a kind of acid wastewater with fluorine and chlorine mass-volume concentrations of 4052mg / L and 6284mg / L respectively, and containing heavy metals such as Zn, Cd, Pb, Cu, and As, according to Figure 2 The process includes the following steps:

[0075] The first step of pretreatment is to introduce 1.30 times of hydrogen sulfide gas into the acid wastewater, and the supernatant after reacting at room temperature for 80 minutes is subjected to electrodialysis, and high-fluorine chlorine dilute acid is evaporated and stripped to obtain zinc-containing sulfide slag, fresh water, sulfuric acid solution with a mass percentage concentration of 70.82%, and distillate, respectively.

[0076] The second step is to prepare the defluorination agent: under the conditions of T=60℃ and t=200min, use 2.0mol / L sulfuric acid to dissolve the electrolytic aluminum cathode plate waste to prepare a defluorination agent with pH=1.5.

[0077] The third step is to remove fluorine with cryolite: add 1.04 times of the defluoridation agent and the adjusting agent sodium hydroxide to the distillate and remove fluorine at T = 97 ° C, t = 100 min. The defluoridation end point pH = 7.5. Filter to obtain cryolite and defluoridated water.

[0078] The fourth step is bismuth salt dechlorination: 1.03 times the activated bismuth salt is added to defluorinated water for dechlorination, and filtered to obtain qualified secondary return water that can be returned to the zinc hydrometallurgy system with a fluorine and chlorine mass-volume concentration of 12.8 mg / L and 149 mg / L respectively.

[0079] The water in the contaminated acid wastewater is returned to the zinc smelting wet process system in the form of secondary return water and is recycled as water resources. The output sulfuric acid solution with a mass percentage concentration of 70.75% and waste aluminum cathode material are used to prepare defluoridating agent. The resource utilization rate of sulfuric acid is 99.48%. The heavy metal valuable metals zinc, lead and cadmium are recovered in the form of sulfides as zinc concentrate returned to the boiling furnace, with a recovery rate of 99.36%. Fluorine is prepared into cryolite for sale, with a recovery rate of 99.68%.

[0080] Finally, it should be noted that the above specific implementation methods are only used to illustrate the technical solutions of the present invention and are not limiting. Although the present invention has been described in detail with reference to examples, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A method for regenerating acid from defluorinated cryolite, characterized in that: The steps include: Introducing hydrogen sulfide gas into the polluted acid wastewater to cause a sulfidation reaction, followed by filtration to obtain sulfided slag and filtrate; The filtrate is electrodialyzed to obtain fresh water and dialysis residue, and the fresh water is returned to the zinc hydrometallurgy system for water use; The dialysate is evaporated and stripped to obtain sulfuric acid solution and distillate; Part of the sulfuric acid solution is reacted with the electrolytic aluminum cathode plate waste to obtain an aluminum sulfate solution with a pH of 0.5-2.0 as a defluoridating agent, and the remaining sulfuric acid solution is used to prepare concentrated sulfuric acid; Add defluoridation agent and regulator to the distillate and defluorinate at 95-98°C for 30-150 minutes, then filter to obtain cryolite and defluorinated water; Add bismuth salt to the defluorinated water for dechlorination and filter to obtain secondary return water with a fluorine content of less than 50 mg / L and a chlorine content of less than 300 mg / L and dechlorination slag. The secondary return water is used as water for the zinc hydrometallurgical smelting system. The adjusting agent is one or two of sodium carbonate, sodium bicarbonate or sodium hydroxide.

2. The method for regenerating cryolite defluorination waste acid according to claim 1, wherein: The amount of hydrogen sulfide gas added is 1.15-1.45 times the theoretical amount of hydrogen sulfide gas required to convert all heavy metals such as lead, zinc, copper and arsenic in the waste acid wastewater into corresponding sulfide precipitation during the sulfidation reaction.

3. The method for regenerating cryolite defluorination waste acid according to claim 2, wherein: The sulfidation reaction time is 60-100 minutes, the reaction temperature is room temperature, and the sulfided slag obtained by filtration is returned to the zinc concentrate and fed into the boiling furnace for reuse.

4. The method for regenerating cryolite defluorination waste acid according to claim 1, wherein: The temperature for the reaction of the sulfuric acid solution with the electrolytic aluminum cathode plate waste is 55-75° C., and the reaction time is 90-300 minutes.

5. The method for regenerating cryolite defluorination waste acid according to claim 1, characterized in that: The amount of the defluorinating agent added is 1.01-1.06 times the theoretical amount of the defluorinating agent required to convert all F in the distillate into cryolite.

6. The method for regenerating acid from defluorinated cryolite according to claim 5, wherein: The pH of the defluorinated water is between 6.0 and 8.

5.

7. The method for regenerating cryolite defluorination waste acid according to claim 1, characterized in that: The amount of bismuth salt added is 1.02-1.05 times the theoretical amount of bismuth salt required to convert all Cl in the defluorinated water into bismuth oxychloride.

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