A sewage treatment process for the harmless disposal of overhaul slag and carbon slag in electrolytic aluminum

Abstract

The present invention provides a sewage treatment process for the harmless disposal of overhaul slag and carbon slag in electrolytic aluminum, which relates to the technical field of disposal and resource utilization of solid waste in electrolytic aluminum, and solves the technical problems of insufficient recovery and treatment of acid-base leaching solutions of existing overhaul slag and carbon slag, low production capacity of frozen crystalline mirabilite, and low product purity. The present invention comprises the following steps: performing fluoride salt directional conversion on the alkali leaching solution and the acid leaching solution to obtain a fluoride salt slurry; performing solid-liquid separation on the fluoride salt slurry to obtain a post-liquid of fluoride salt directional conversion and a process product of water-containing fluoride salt; performing pretreatment on the post-liquid of fluoride salt directional conversion, and then performing solid-liquid separation to obtain water treatment slag and a filtrate; neutralizing and removing aluminum from the filtrate; after neutralizing and removing aluminum, the filtrate enters a nanofiltration or reverse osmosis process to produce fresh water and concentrated water; the fresh water is recycled; and the concentrated water enters a frozen crystallization system to produce mirabilite. By treating the filtrate through nanofiltration or reverse osmosis, the present invention enables ions to be enriched in the concentrated water, improves the production capacity of mirabilite, and realizes zero discharge through the recycling of fresh water and circular use.

Description

Technical Field

[0001] The present invention relates to the technical field of disposal and resource utilization of solid waste in electrolytic aluminum production, and more particularly, to a sewage treatment process for harmless disposal of overhaul slag and carbon slag in electrolytic aluminum production. Background Art

[0002] In the process of aluminum electrolysis production, overhaul slag and carbon slag are the most typical hazardous wastes in the aluminum electrolysis industry, and are all included in the National List of Hazardous Wastes issued by the Ministry of Ecology and Environment of the People's Republic of China. Their main hazardous characteristics include toxicity (T) and reactivity (R). In the harmless disposal processes such as leaching, pressure filtration, and washing of hazardous wastes, if the leachate, filtrate, or wash liquor is not properly disposed of, secondary pollution of hazardous wastes will be caused. Overhaul slag is the waste residue generated during the repair and replacement of the cathode lining of the electrolytic cell in the aluminum electrolysis production process. According to the actual situation of aluminum electrolysis production, overhaul slag is further divided into waste cathode and waste refractory materials.

[0003] Regarding the disposal and comprehensive utilization of resources of overhaul slag in aluminum electrolysis, a large amount of research has been done, which is generally divided into two categories: wet treatment and pyrometallurgical treatment. The wet treatment process mainly includes water leaching method, alkali leaching method, acid leaching method, acid-base combined leaching method, etc. For example, the method for combined treatment and comprehensive utilization of hazardous waste slag in aluminum electrolysis proposed in the invention patent with the publication number CN113426807A is treated by the method of pressurized alkali leaching followed by countercurrent washing, and the electrolyte powder in carbon slag and overhaul slag is treated by the method of leaching with aluminum salt followed by directional conversion of fluoride salt, and the aluminum ash is treated by the method of pressurized alkali leaching followed by sulfuric acid leaching.

[0004] Although it realizes the harmless treatment of the overall overhaul slag in aluminum electrolysis, the acid leaching solution and alkali leaching solution generated during the acid leaching and alkali leaching processes are only simply neutralized to recover precipitated aluminum fluoride, and the supernatant is crystallized to prepare mirabilite. In actual production, the acid leaching solution and alkali leaching solution also contain ions such as iron and silicon, and the directional conversion of fluoride salt cannot directly remove all impurity ions. The yield of mirabilite prepared from the supernatant is low, the energy consumption of freeze crystallization is high, the efficiency is low, and the crystallization solution after freeze crystallization is not pure water and needs to be treated before reuse. Summary of the Invention

[0005] The purpose of the present invention is to provide a sewage treatment process for harmless disposal of overhaul slag and carbon slag in electrolytic aluminum, so as to solve the technical problems of insufficient recovery and treatment of acid-base leaching solutions of existing overhaul slag and carbon slag, low production capacity of mirabilite by freeze crystallization, low product purity, and pollution and waste.

[0006] The embodiments of the present invention are achieved by the following technical solutions:

[0007] A sewage treatment process for harmless disposal of overhaul slag and carbon slag in electrolytic aluminum includes the following steps:

[0008] Step 1: Perform fluoride salt directional conversion on the overhaul slag, carbon slag mixture alkali leaching solution and acid leaching solution to obtain a fluoride salt slurry;

[0009] Step 2: Perform solid-liquid separation on the fluoride salt slurry obtained in Step 1 to obtain the post-liquid of fluoride salt directional conversion and the process product hydrated fluoride salt;

[0010] Step 3: Pretreat the post-liquid of fluoride salt directional conversion obtained in Step 2, and then perform solid-liquid separation to obtain water treatment slag and filtrate;

[0011] Step 4: Neutralize and remove aluminum from the filtrate obtained in Step 3;

[0012] Step 5: After neutralizing and removing aluminum in Step 4, the filtrate enters nanofiltration or reverse osmosis to produce fresh water and concentrated water;

[0013] Step 6: The fresh water obtained in Step 5 is recycled; the concentrated water obtained in Step 5 enters the freeze crystallization system to produce mirabilite.

[0014] With this technical solution, when the fluoride salt slurry is subjected to solid-liquid separation to obtain the post-liquid of fluoride salt directional conversion with high impurity ion content, if it enters nanofiltration or reverse osmosis without pretreatment, it will cause membrane fouling. Therefore, the post-liquid of fluoride salt directional conversion needs to be pretreated to remove impurity ions, and then solid-liquid separation is performed. The obtained water treatment slag enters other processes for resource utilization. The filtrate after impurity removal needs to be further neutralized and de-aluminized to meet the water quality requirements for entering nanofiltration or reverse osmosis; after entering nanofiltration or reverse osmosis, fresh water and concentrated water are produced; the fresh water is used for processes such as countercurrent washing of hydrated fluoride salt and recycled; the concentrated water enters the freeze crystallization system to produce mirabilite, and the frozen liquid is used for processes such as alkali leaching and slurry preparation of overhaul slag and carbon slag mixture. Thus, the present invention solves the treatment process of sewage generated during the harmless treatment of electrolytic aluminum overhaul slag and carbon slag, and realizes recycling without external discharge.

[0015] Preferably, in Step 1, the pH value of the alkali leaching solution is 13.0 - 13.5, the pH value of the acid leaching solution is 1.5 - 2.0, and after neutralizing the alkali leaching solution and the acid leaching solution, the pH value is adjusted to 6 - 7.

[0016] Preferably, in Step 3, the specific operation of the pretreatment is: adding sodium hydroxide solution to the post-liquid of fluoride salt directional conversion with a pH value of 6 - 7 to increase the pH value to 11.0 - 11.5, and at the same time adding hydrogen peroxide with a content of 5‰, and aerating for 2h - 3h.

[0017] Preferably, in Step 3, the water treatment slag enters other processes for resource utilization.

[0018] Preferably, in Step 4, the specific operation of neutralizing and removing aluminum is: adding concentrated sulfuric acid to adjust the pH value of the filtrate to 7 - 8.

[0019] Preferably, in the fifth step, the obtained fresh water is used for countercurrent washing of the hydrated fluoride salt.

[0020] Preferably, in the fifth step, the water quality requirements for the filtrate after neutralization and aluminum removal and before nanofiltration or reverse osmosis are as follows: pH value 7-8, Al 3+ <10 mg / l, Fe 3+ <10 mg / l, F - <10 mg / l, total hardness <300 mg / l.

[0021] Preferably, in the fifth step, the content of sodium sulfate in the feed water for nanofiltration or reverse osmosis is 60 g / l - 70 g / l, the feed water temperature is 10°C - 30°C, the pH value is 6 - 8, the solid content of the feed water is 1000 - 3000 ppm, the TDS of the produced fresh water <5000 ppm, and the sodium sulfate in the concentrated water ≥150 g / l.

[0022] Preferably, in the sixth step, the frozen post-liquid produced during the production of mirabilite is used for alkali leaching and pulping of the mixture of overhaul slag and carbon slag.

[0023] Preferably, in the sixth step, the feeding conditions for the freeze crystallization are as follows: pH value 7 - 8, feeding flow rate 25 m 3 / h - 30 m 3 / h, the concentration of sodium sulfate is 11.6% - 13.4%, the content of other impurities <2.8%, and the feeding temperature <40°C.

[0024] The technical solution of the embodiment of the present invention has at least the following advantages and beneficial effects:

[0025] 1. The present invention treats the filtrate after the directional conversion of fluoride salts by nanofiltration or reverse osmosis, so that ions are enriched in the concentrated water, improving the production capacity of mirabilite, and fresh water can be produced for reuse;

[0026] 2. The present invention pre-treats the filtrate to remove impurity ions and neutralizes and removes aluminum before the filtrate enters nanofiltration or reverse osmosis, preventing the clogging of the filter membrane, and facilitating subsequent treatment and improving the production capacity of mirabilite;

[0027] 3. Sodium hydroxide, oxidant hydrogen peroxide and sulfuric acid added during the pre-treatment process of the present invention do not introduce new impurities. Except for generating water, sodium ions and sulfate ions finally enter the concentrated water to prepare mirabilite. The added reagents are completely reused to prepare mirabilite while removing impurities during pre-treatment, without waste and having no negative impact on other recycling processes;

[0028] 4. All the materials used in all steps of the present invention are recycled and not discharged;

[0029] 5. The process of the present invention is easy to control and is convenient for industrial production. Description of the Drawings

[0030] To more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the embodiments. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation of the scope. For those of ordinary skill in the art, without creative efforts, other relevant drawings can also be obtained based on these drawings.

[0031] Figure 1 It is a schematic diagram of the sewage treatment process flow for the harmless treatment of overhaul slag and carbon slag in electrolytic aluminum provided in Embodiment 1 of the present invention. Specific embodiments

[0032] Embodiment 1

[0033] A sewage treatment process for the harmless treatment of overhaul slag and carbon slag in electrolytic aluminum. According to the embodiments of the present invention, the process method includes:

[0034] Step 1: Perform directional conversion of fluorides on the mixed alkali leaching solution and acid leaching solution of the overhaul slag and carbon slag mixture in a certain proportion to obtain a fluoride slurry.

[0035] Step 2: Perform solid-liquid separation on the fluoride slurry to obtain the post-liquid of the directional conversion of fluorides and the process product hydrated fluoride.

[0036] Step 3: Adjust the pH value of the post-liquid of the directional conversion of fluorides, oxidize to remove iron, silicon, reduce hardness, etc., and then perform solid-liquid separation. The water treatment slag obtained enters other processes for resource utilization.

[0037] Step 4: Neutralize and remove aluminum from the filtrate obtained in Step 3.

[0038] Step 5: After neutralizing and removing aluminum in Step 4, the filtrate enters nanofiltration or reverse osmosis to produce fresh water and concentrated water.

[0039] Step 6: Use the fresh water for countercurrent washing of the product and recycle it; send the concentrated water to a freezing crystallization system to produce mirabilite, and use the post-freezing liquid for alkali leaching and slurry preparation of the overhaul slag and carbon slag mixture.

[0040] In this embodiment, in Step 1, first perform pressure alkali leaching on the overhaul slag. The pH value during the alkali leaching process is 13.0. After solid-liquid separation, first wash the alkali leached slag with the subsequent recycled fresh water, and then perform acid leaching on the alkali leached slag. The pH value during the acid leaching process is 1.5. Adjust the pH value of the alkali leaching solution and the acid leaching solution to 6.5 after neutralization to form a precipitate and obtain a fluoride slurry.

[0041] In this embodiment, in Step 2, the post-liquid of the directional conversion of fluorides has high impurity ion content and high hardness: Al 3+ ≥100mg / l, Fe 3+ ≥50mg / l, F- ≥100 mg / l, total hardness ≥ 15000 mg / l. It can be seen that without pretreatment, the liquid after the directional conversion of fluorine salts cannot enter nanofiltration or reverse osmosis.

[0042] In this embodiment, in step three, the pH value of the liquid after the directional conversion of fluorine salts is 6.5. Sodium hydroxide solution is added to increase the pH value to 11.0. At the same time, hydrogen peroxide with a content of 5‰ is added according to the amount of the liquid after the directional conversion of fluorine salts, and aeration is carried out for 2 h. Fe 2+ is converted to Fe 3+ , F - , SiO4 2- , Fe 3+ and other ions form precipitates, and the total hardness decreases.

[0043] In this embodiment, in step three, sodium hydroxide solution is added to increase the pH to 11.0. Al 3+ is converted to AlO 2- , and no precipitate will be formed. After solid-liquid separation, concentrated sulfuric acid is added to the filtrate to adjust the pH of the filtrate to 7. Under neutral conditions, AlO 2- is converted to aluminum hydroxide precipitate to further neutralize and remove aluminum.

[0044] In this embodiment, in step five, the water quality for reverse osmosis is: pH value is 7, Al 3+ < 10 mg / l, Fe 3+ < 10 mg / l, F- < 10 mg / l, total hardness < 300 mg / l.

[0045] In this embodiment, in step five, the sodium sulfate content in the nanofiltration or reverse osmosis inlet water is 70 g / l, the inlet water temperature is 25 °C, the pH value is 7, the inlet water solid content is about 2000 ppm, and the fresh water recovery rate is 50% (fresh water production is 15 m 3 / h, concentrated water production is 15 m 3 / h).

[0046] In this embodiment, in step five, after reverse osmosis, the concentrations of Al 3+ , Fe 3+ , F- and other ions in the fresh water are close to 0, and the TDS is 3000 ppm; the concentrations of Al 3+ , Fe 3+ , F - and other ions in the concentrated water are close to 0, and Na + , SO4 2- are enriched in the concentrated water, and the sodium sulfate is 160 g / l, which is beneficial for the next step of the concentrated water entering the freeze crystallization to produce mirabilite (Na2SO4·10H2O).

[0047] In this embodiment, in step six, the pH value of the feed for freeze crystallization is 7, the feed flow rate is 25 m 3 / h, the feed concentration (sodium sulfate) is 13.4%, other impurities < 2.8%, the feed temperature is 25 °C, and the output of mirabilite from freeze crystallization is 152 kg per ton of filtrate.

[0048] As can be seen from the above embodiments, by using the sewage treatment process for harmless disposal of overhaul slag and carbon slag in electrolytic aluminum of the present invention, after the post-treatment of the liquid after the directional conversion of fluorine salts is qualified, it enters nanofiltration or reverse osmosis. The concentrations of ions such as Al 3+ 、Fe 3+ 、F - in the fresh water are close to 0, and the TDS < 5000 ppm. It can be used for countercurrent washing and recycling of products. The concentrated water produced enters freeze crystallization to produce mirabilite, and the liquid after freeze crystallization returns to the alkali leaching and pulp adjustment of overhaul slag and carbon slag. The sewage treatment rate is 100%, the treatment qualification rate is 100%, and the recycling rate is 100%. Thus, it can be seen that the sewage treatment process for harmless disposal of overhaul slag and carbon slag in electrolytic aluminum of the present invention realizes the treatment and recycling of sewage during the harmless disposal and comprehensive utilization of overhaul slag and carbon slag in electrolytic aluminum without external discharge.

[0049] Example 2

[0050] The difference between this embodiment and Embodiment 1 is only that in this embodiment, concentrated sulfuric acid is added to the filtrate to adjust the pH of the filtrate to 8. Under neutral conditions, AlO 2- is converted into aluminum hydroxide precipitate to further neutralize and remove aluminum.

[0051] In this embodiment, the concentration of Al 3+ in the filtrate entering reverse osmosis is 0.28 mg / l, and the recovery rate of fresh water from reverse osmosis is 48.5% (fresh water production is 14.55 m 3 / h, concentrated water production is 15.45 m 3 / h), and the concentration of Al 3+ in the concentrated water is 0.53 mg / l.

[0052] Example 3

[0053] The difference between this embodiment and Embodiment 1 is only that in step two, the content of impurity ions and hardness in the liquid after the directional conversion of fluorine salts is high: Al 3+ ≥100 mg / l, Fe 3+ ≥50 mg / l, F - ≥100 mg / l, total hardness ≥ 15000 mg / l. The liquid after the directional conversion of fluorine salts is not pretreated and cannot enter nanofiltration or reverse osmosis.

[0054] In this embodiment, the sodium sulfate concentration in the liquid after the directional conversion of the fluorine salt is 78 mg / l. Since other impurity ions cannot enter the nanofiltration or reverse osmosis and directly enter the freeze crystallization to prepare mirabilite, the mirabilite output is about 75 kg / t filtrate.

[0055] Example 4

[0056] The difference between this embodiment and Example 1 is only that in Step 3, calcium hydroxide is added to adjust the pH value to 11.0; in Step 4, dilute hydrochloric acid is added to adjust the pH value to 7.

[0057] In this embodiment, the sodium sulfate concentration in the concentrated water is 152 g / l, and about 142 kg of mirabilite is produced per ton of filtrate.

[0058] Specifically, compared with the aluminum removal step in Example 1 in Example 2, sulfuric acid is added to adjust the pH value to 8, and AlO 2- is not completely converted into aluminum hydroxide precipitate. When entering the reverse osmosis, the concentration of impurity ions is relatively high, the fresh water output is low, the sodium sulfate content in the concentrated water is low, and the mirabilite output is relatively low;

[0059] In Example 3, no pretreatment is carried out compared with Example 1, reverse osmosis or nanofiltration is not carried out, and the filtrate is directly subjected to freeze crystallization. The mirabilite output is low, and the impurity content in the liquid after freezing is high;

[0060] In Example 4, sodium hydroxide is used for pretreatment and concentrated sulfuric acid is used for neutralizing and removing aluminum compared with Example 1; in Example 4, calcium hydroxide and dilute hydrochloric acid are used for adjustment treatment, introducing calcium ions and chloride ions, increasing the content of impurity ions, and the amount of dilute hydrochloric acid to be added is greater than that of concentrated sulfuric acid, introducing more water into the system, reducing the sodium sulfate content in the concentrated water, and reducing the mirabilite output.

[0061] In the description of this specification, the description with reference to terms such as "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples. In addition, without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0062] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A sewage treatment process for the harmless treatment of overhaul slag and carbon slag in electrolytic aluminum, characterized in that: It includes the following steps: Step 1: Directionally convert the alkali leaching solution and acid leaching solution of the overhaul slag and carbon slag mixture to obtain a fluoride salt slurry; Step 2: Perform solid-liquid separation on the fluoride salt slurry obtained in Step 1 to obtain the post-liquid of the directional conversion of fluoride salts and the water-containing fluoride salts as the process product; Step 3: Pretreat the post-liquid of the directional conversion of fluoride salts obtained in Step 2, and then perform solid-liquid separation to obtain water treatment slag and filtrate; Step 4: Neutralize and remove aluminum from the filtrate obtained in Step 3; Step 5: After neutralizing and removing aluminum in Step 4, the filtrate enters nanofiltration or reverse osmosis to produce fresh water and concentrated water; Step 6: The fresh water obtained in Step 5 is recycled; the concentrated water obtained in Step 5 enters the freeze crystallization system to produce mirabilite.

2. The sewage treatment process for harmless treatment of overhaul slag and carbon slag in electrolytic aluminum according to claim 1, wherein: In Step 1, the pH value of the alkali leaching solution is 13.0 - 13.5, the pH value of the acid leaching solution is 1.5 - 2.0, and the pH value is adjusted to 6 - 7 after neutralizing the alkali leaching solution and the acid leaching solution.

3. The sewage treatment process for harmless disposal of overhaul slag and carbon slag in electrolytic aluminum according to claim 1 or 2, characterized in that: In Step 3, the specific operation of the pretreatment is: add sodium hydroxide solution to the post-liquid of the directional conversion of fluoride salts with a pH value of 6 - 7 to increase the pH value to 11.0 - 11.5, and at the same time add an oxidant and aerate for 2h - 3h.

4. The sewage treatment process for harmless disposal of overhaul slag and carbon slag in electrolytic aluminum according to claim 1 or 2, characterized in that: In Step 3, the water treatment slag enters other processes for resource utilization.

5. The sewage treatment process for harmless disposal of overhaul slag and carbon slag in electrolytic aluminum according to claim 1 or 2, characterized in that: In Step 4, the specific operation of neutralizing and removing aluminum is: add concentrated sulfuric acid to adjust the pH value of the filtrate to 7 - 8.

6. The sewage treatment process for harmless disposal of overhaul slag and carbon slag in electrolytic aluminum according to claim 1 or 2, characterized in that: In Step 5, the fresh water obtained is used for countercurrent washing of the water-containing fluoride salts.

7. The sewage treatment process for harmless treatment of overhaul slag and carbon slag in electrolytic aluminum according to claim 1 or 2, characterized in that: In the fifth step, the water quality requirements for the filtrate after neutralization and aluminum removal before nanofiltration or reverse osmosis are as follows: pH value 7-8, Al 3+ < 10 mg / l, Fe 3+ < 10 mg / l, F - < 10 mg / l, total hardness < 300 mg / l.

8. The sewage treatment process for harmless disposal of overhaul slag and carbon slag in electrolytic aluminum according to claim 1 or 2, characterized in that: In Step 5, the content of sodium sulfate in the feed water for nanofiltration or reverse osmosis is 60g / l - 70g / l, the feed water temperature is 10°C - 30°C, the pH value is 6 - 8, the solid content of the feed water is 1000 - 3000ppm, the TDS of the fresh water < 5000ppm, and the sodium sulfate in the concentrated water ≥ 150g / l.

9. The sewage treatment process for harmless disposal of overhaul slag and carbon slag in electrolytic aluminum according to claim 1 or 2, characterized in that: In Step 6, the frozen post-liquid produced from the production of mirabilite is used for alkali leaching and slurry preparation of the overhaul slag and carbon slag mixture.

10. The sewage treatment process for harmless disposal of overhaul slag and carbon slag in electrolytic aluminum according to claim 1 or 2, characterized in that: In Step 6, the feeding conditions for freeze crystallization are as follows: the pH value is 7 - 8, the feeding flow rate is 25 m 3 - 30 m 3 / h, the sodium sulfate concentration is 11.6 - 13.4%, the content of other impurities is < 2.8%, and the feeding temperature is < 40°C.

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