Aluminum oxidation wastewater quality-divided treatment method for extracting aluminum through crystallization and preserving phosphorus and alkali

Abstract

The invention discloses an aluminum oxidation wastewater quality-divided treatment method for extracting aluminum through crystallization and preserving phosphorus and alkali. Comprising the following steps: (1) closed-loop recycling of a brought-out liquid: recycling and concentrating a tank liquid brought out by a workpiece and recycling the tank liquid to an original process tank in a closed-loop manner; (2) selective crystallization regeneration: adding a crystallizing agent into the aluminum-containing process tank liquid to selectively crystallize and separate out aluminum ions while retaining phosphate radicals, sulfate radicals or alkalinity, and returning mother liquid to an original tank; and (3) performing water quality treatment on the cleaning water: performing segmented treatment on the discharged low-concentration cleaning water according to pH, and respectively recovering calcium sulfate, aluminum hydroxide and phosphate precipitate. According to the method, aluminum taking, phosphorus keeping and alkali keeping are achieved through aluminum taking through crystallization, the industrial technology prejudice that waste water is taken out, and neutralization is needed for aluminum removal is broken through, solid waste is greatly reduced, agent consumption is reduced, alum by-products with market value are produced, and the potential of promoting the industry to transform from passive pollution control to active income creation is achieved.

Description

Technical Field

[0001] This invention relates to the fields of environmental protection and wastewater treatment technology, specifically to a method for the differentiated treatment of aluminum oxidation wastewater by crystallization to extract aluminum while preserving phosphorus and alkali. Background Technology

[0002] Aluminum surface treatment (aluminum oxidation) includes processes such as polishing, oxidation, and sealing. Acid polishing uses a large amount of phosphoric acid, oxidation uses sulfuric acid, and alkaline polishing uses sodium hydroxide. During production, a certain amount of the bath solution is carried out with the workpiece when it is removed (called "carry-out solution"). Subsequent water washing will generate cleaning wastewater containing acid, phosphorus, and aluminum.

[0003] Currently, the common wastewater treatment method in the aluminum oxidation industry is to collect all wastewater (including carryover liquid and washing water) by mixing it together, and then add lime for neutralization and precipitation. The principle is that calcium in the lime reacts with phosphate ions to form hydroxyapatite (for phosphorus removal) and with sulfate ions to form calcium sulfate (for neutralization), while aluminum ions hydrolyze to form aluminum hydroxide precipitate. This method is considered simple and effective and has been used for decades.

[0004] However, there are several real problems in this field that have long been overlooked:

[0005] First, the carry-over solution is treated as "wastewater" rather than "chemical solution." The tank solutions carried over between processes actually contain high concentrations of phosphoric acid, sulfuric acid, or alkali, making them perfectly usable for reuse. However, the industry habitually discharges them into the wastewater system. To control the aluminum ion concentration in the tank solutions, sometimes more solutions are intentionally carried over and mixed with large amounts of cleaning water for further treatment. This not only wastes expensive chemical raw materials but also increases the burden on wastewater treatment. A company producing 100,000 tons of aluminum profiles annually can generate tens of tons of phosphoric acid carry-over each year, all of which becomes sludge. Everyone knows this, but very few people actually calculate the costs and benefits—because the assumption that "carry-over solution is wastewater" has become the default.

[0006] Secondly, mixed treatment turns recyclable resources into mixed waste. Phosphoric acid, sulfuric acid, aluminum, and alkali react with each other in the mixed wastewater, and after lime precipitation, it eventually becomes a grayish-black sludge containing calcium sulfate, hydroxyapatite, aluminum hydroxide, and excess lime. This sludge is produced in large quantities and cannot be separated for reuse, so it can only be landfilled or stockpiled. Phosphoric acid and aluminum are both valuable resources—phosphoric acid is an important raw material for phosphate chemical industry, and aluminum is the source of metallic aluminum. However, in this industry, about 90,000 tons of aluminum and a large amount of phosphoric acid are thus mixed into the sludge every year. It's not that there is no technology to recycle them, but that the existing "mixing + precipitation" approach has blocked the possibility of separate recycling from the very beginning.

[0007] Third, the inefficiency of calcium-based phosphorus removal has been tolerated for a long time. Wastewater contains a large amount of free sulfuric acid. When lime is added, it preferentially reacts with the sulfuric acid to form calcium sulfate. Calcium sulfate is slightly soluble in water and quickly coats the surface of lime particles, preventing further reaction within the lime. To achieve phosphorus removal, companies have to add excessive amounts of lime (usually 1.5-2 times the theoretical amount), leading to further expansion of sludge and persistently high operating costs. Industry technicians are well aware of the shortcomings of this process, but because "it's been used this way all along" and "there's no better way," they have accepted it.

[0008] Some technologies have attempted to overcome these difficulties. For example, the industry has proposed using "acid retardation" technology to treat acid polishing recovery liquid. The principle is to use anion exchange resin to adsorb free acid, causing aluminum ions to flow out as salt, thus separating the acid from the aluminum. This method has been proven feasible in laboratories or small-scale trials, but due to various reasons, there are few practical applications, and it has not been widely adopted in the industry. The reasons are as follows: First, the resin has poor tolerance to high concentrations of aluminum ions, is easily contaminated and clogged, and requires frequent regeneration; second, the acid retardation process requires additional acid and water for elution, generating new diluted waste liquid; third, this method is only applicable to acid systems and cannot solve the problems of alkaline polishing tanks and oxidation tanks, let alone achieve the system goal of "retaining phosphorus and alkali." Therefore, acid retardation technology has so far failed to change the mainstream treatment pattern of "mixing + lime" in the aluminum oxidation industry.

[0009] At the same time, the aluminum oxidation industry faces increasing pressure from rigid demand: environmental policies are imposing increasingly stringent requirements on solid waste disposal, and the cost of sludge landfill is rising year by year; the prices of chemical raw materials (phosphoric acid, sulfuric acid, alkali) are fluctuating wildly, and enterprises urgently need to reduce unit consumption; in some regions, the total phosphorus emission standard has been raised to below 0.5 mg / L, challenging the stability of the traditional lime process. These rigid demands combined are forcing the industry to seek new technological paths.

[0010] Conversely, if we could achieve a solution without mixing or separating the components—by separately recovering and concentrating the carryover liquid for reuse, selectively removing aluminum from high-concentration bath solutions without damaging phosphoric acid and alkali, and separately recovering low-concentration washing water through staged precipitation—then the previously wasted phosphoric acid, sulfuric acid, alkali, and aluminum could all become recyclable resources, significantly reducing sludge volume and substantially lowering reagent costs. This is precisely the solution the industry needs but has been unable to implement. Summary of the Invention

[0011] Consul General

[0012] This invention stems from a seemingly simple technical insight: during aluminum oxidation, the continuous accumulation of aluminum ions in the bath solution is a necessary byproduct of the process, and the traditional approach is to dilute or remove these aluminum ions by draining the waste liquid. This invention takes the opposite approach—it doesn't drain the bath solution, but only removes the aluminum ions. This simple "crystallization and aluminum removal" operation solves a major problem that has long plagued the industry: waste of carryover liquid, resource contamination, and large amounts of sludge.

[0013] Through this simple operation, the present invention achieves a closed loop of "minimal carryover, recycling, and efficient utilization": the carryover liquid is recovered and concentrated instead of being discharged; aluminum in the bath is selectively removed while acids, phosphorus, and alkalis are retained; and components in the washing water are recovered in stages instead of being mixed into sludge. The ultimate effect is cost reduction and efficiency improvement—a significant decrease in reagent consumption, a substantial reduction in solid waste production, and even the production of marketable alum by-products. This is not merely a process improvement, but has the potential to change the industry landscape: aluminum oxidation companies are transforming from "pollutant generators" to "resource recyclers," and wastewater treatment is shifting from a "cost center" to a "profit center."

[0014] Technical solution

[0015] To achieve the above objectives, the present invention comprises the following three technical modules:

[0016] Module 1: Closed-loop reuse of outflowing liquid

[0017] A recovery tank is installed after the aluminum polishing and oxidation processes to collect and concentrate the bath liquid carried out by the workpieces, and then return the concentrated recovery liquid to the original process tank. Specifically, the carry-out liquid is concentrated in situ by aeration and rinsing to accelerate water evaporation and cooling, combined with the return of concentrated water generated by the countercurrent rinsing in the later stage. Then, the heating process of the process tank itself (100~115℃ for acid polishing tank and about 70℃ for alkali polishing tank) is used to further evaporate water from the returned recovery liquid, allowing the effective components to be retained. Through this closed loop, the carry-out liquid is transformed from "wastewater" into "chemical solution source", significantly reducing the amount of new chemical solution added and the subsequent wastewater treatment load.

[0018] Module 2: Selective Crystallization Regeneration (Aluminum Extraction, Phosphorus Preservation, and Alkali Preservation)

[0019] Different selective crystallization strategies are adopted for different process bath solutions:

[0020] • Acid polishing tank (containing phosphoric acid): Ammonium sulfate or sodium sulfate is added to the tank solution, utilizing Al... 3+ With SO4 2- NH4 + / Na + The selective formation of double salts (ammonium aluminum sulfate / sodium aluminum sulfate) causes aluminum ions to crystallize out, while phosphate ions hardly participate in the reaction and remain intact in the mother liquor. After solid-liquid separation, the phosphoric acid-rich mother liquor is returned to the acid polishing tank for continued use.

[0021] • Oxidation tank (sulfuric acid system): Similarly, add ammonium sulfate or sodium sulfate to selectively precipitate aluminum ammonium sulfate / aluminum sodium sulfate crystals, and the sulfate ions are retained in the mother liquor and returned to the oxidation tank.

[0022] • Alkali polishing tank: Add aluminum hydroxide crystal nuclei to induce dissolved aluminum ions to selectively crystallize in the form of aluminum hydroxide. The alkalinity is not affected, and the mother liquor is returned to the alkaline polishing tank.

[0023] The core of this module lies in "extracting aluminum" without "damaging phosphorus / acid / alkali", thus achieving non-destructive regeneration of the bath solution.

[0024] Another layer of technical logic in this invention lies in maximizing the value of the product. From the perspective of elemental fate, the stable form of sulfur is sulfate, and the stable form of aluminum is alumina or a water purification agent. Traditional methods convert sulfur into calcium sulfate (gypsum) and aluminum into aluminum hydroxide, both of which are mixed in the sludge and cannot be utilized. This invention generates ammonium aluminum sulfate / sodium aluminum sulfate double salt (alum), allowing sulfur and aluminum to simultaneously enter into a single valuable product, thus obtaining both. Alum itself is a mature industrial product (raw material for water purification agents, tanning agents for leather, etc.) with direct market value. This contrasts sharply with traditional methods where sulfur and aluminum are separately turned into mixed sludge, both losing their utilization value.

[0025] Module 3: Cleaning Water Sediment Deposits

[0026] For low-concentration cleaning water that cannot be reused, the pH value is controlled in stages to precipitate calcium sulfate (pH 2-3), aluminum hydroxide (pH 6-7), and hydroxyapatite or aluminum phosphate (pH 9-10) sequentially. The precipitated aluminum hydroxide can be reused as an alkali to neutralize free sulfuric acid in acid polishing / oxidation wastewater, and further participate in the crystallization cycle of the second module.

[0027] Synergistic effect

[0028] The three modules support each other and form a closed loop: the first module provides concentrated recovery liquid to the second module, reducing the amount of crystallization to be processed; the second module regenerates the tank solution, extending its lifespan; and the third module treats the final effluent and recovers aluminum hydroxide to feed back into the second module. Through their synergy, the three modules achieve near-zero discharge of aluminum oxidation wastewater and tiered resource recovery.

[0029] Beneficial effects

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] 1. Change the mindset that "carryover liquid is wastewater" and use it as a source of pharmaceutical solutions for recycling and reuse, thereby reducing pollutants at the source and saving chemical raw materials.

[0032] 2. Breaking through the technical prejudice that "aluminum removal must be neutralized", a selective crystallization strategy is adopted to "remove aluminum while preserving phosphorus and alkali", allowing phosphoric acid, sulfuric acid, and alkalinity to be recycled indefinitely.

[0033] 3. Avoid resource mixing caused by mixed treatment, achieve separate recycling of calcium sulfate, aluminum hydroxide and phosphate, and significantly reduce solid waste production.

[0034] 4. The recovered aluminum hydroxide is reused as an alkali agent, realizing "waste treatment with waste" and further reducing the addition of external reagents.

[0035] 5. Optimize product design from the perspective of element fate: enable sulfur and aluminum to be converted into alum (ammonium aluminum sulfate / sodium aluminum sulfate) with market value, realizing the resource-based upgrading of "turning waste into wealth".

[0036] 6. Overall, it significantly reduces treatment costs and secondary pollution, resulting in good environmental and economic benefits.

[0037] 7. Change the industry landscape: It can shift the wastewater treatment process of aluminum oxidation enterprises from "meeting emission standards by inputting costs" to "recovering resources and creating value", and has the potential to promote the transformation of the industry from "passive pollution control" to "active revenue generation". Detailed Implementation

[0038] Example 1

[0039] A method for the differentiated treatment of aluminum oxidation wastewater by crystallization to extract aluminum while preserving phosphorus and alkali, comprising the following specific steps:

[0040] Step 1: Carryover liquid recovery and concentration

[0041] A recovery tank is installed after each of the acid polishing, oxidation, and alkali polishing tanks to collect the bath liquid carried out by the workpiece. The bottom of the recovery tank is equipped with an aeration device that continuously blows in air (air-to-water ratio 3:1-5:1), which accelerates water evaporation while stirring to prevent sedimentation and lowers the bath liquid temperature to below 40℃. A three-stage water washing tank is installed after the recovery tank, using a counter-current washing method: fresh water is added to the third-stage water washing tank, with the water flow direction opposite to the workpiece movement direction, while the concentrated water generated in the first-stage water washing tank overflows into the recovery tank. Through this step, more than 80% of the carry-out liquid can be recovered and concentrated to 70%-80% of its original volume.

[0042] Step 2: Intermittent reuse of recovered liquid

[0043] The concentrated recovery solution accumulated in the recovery tank is intermittently added to the corresponding process tank using a metering pump. The process tanks are heated during normal operation (40-50℃ for acid polishing tanks, 20-30℃ for oxidation tanks) to further evaporate the water in the added recovery solution, allowing the effective components to remain. The addition frequency is 1-2 times per shift, with each addition amounting to 1%-3% of the tank liquid volume.

[0044] Step 3: Selective crystallization regeneration

[0045] The aluminum ion concentration in each process tank is tested weekly. When the aluminum ion concentration reaches 18-25 g / L, crystallization treatment is initiated.

[0046] • Acid polishing bath solution: Pump a portion of the bath solution into a separate crystallization reaction tank, and stir slowly (30-50 rpm) according to Al. 3 + :SO4 2- Ammonium sulfate is added at a molar ratio of 1:1.5, cooled to 10-15℃, and reacted for 2-4 hours to produce ammonium aluminum sulfate crystals. After centrifugation, the crystals are sold as a byproduct (purity can reach over 95%, which can be used as a raw material for water purification agents), while the mother liquor is returned to the acid polishing tank.

[0047] • Oxidation tank solution: Using the same method, sodium sulfate is added to generate sodium aluminum sulfate crystals, and the mother liquor is returned to the oxidation tank.

[0048] • Alkali polishing tank solution: Add 0.1%-0.5% of aluminum hydroxide crystal nuclei to the tank solution, stir slowly for 1-2 hours to precipitate aluminum hydroxide crystals, and return the mother liquor to the alkali polishing tank after separation.

[0049] Step 4: Rinse away water and sediment.

[0050] The low-concentration cleaning water discharged from each washing tank is collected and introduced into a multi-stage pH adjustment tank for treatment in the following three stages:

[0051] • First stage (acidic stage): Adjust the pH to 2.5-3.0, add calcium chloride, precipitate calcium sulfate, and separate to obtain gypsum.

[0052] • Second stage (neutral stage): Adjust the pH to 6.2-6.8, precipitate aluminum hydroxide, and after separation, part of it is recycled to neutralize the free acid in the acid salvage tank in step 3.

[0053] • Third stage (alkaline stage): Adjust the pH to 9.0-9.5, add a small amount of lime to precipitate hydroxyapatite, and ensure that the total phosphorus is ≤0.5mg / L to meet the emission standard.

[0054] After the above treatment, the final effluent is clear and meets the standards. Compared with the traditional lime method, the solid waste production in this embodiment is reduced by about 70%, and the consumption of phosphoric acid and alkali is reduced by about 60% and 50%, respectively.

[0055] Economic analysis (taking an aluminum profile enterprise with an annual output of 100,000 tons as an example)

[0056] Traditional methods: Approximately 500 tons / year of aluminum-containing sludge (dry basis), with disposal costs of about 500,000 yuan; approximately 80 tons / year of phosphoric acid consumption, with costs of about 800,000 yuan; and approximately 120 tons / year of sulfuric acid consumption, with costs of about 300,000 yuan. After adopting this invention: Sludge volume is reduced by 70%, and disposal costs are reduced to 150,000 yuan; phosphoric acid reuse rate is over 80%, saving approximately 640,000 yuan in phosphoric acid costs annually; approximately 200 tons / year of ammonium aluminum sulfate is produced, generating an additional 300,000 yuan in revenue at 1,500 yuan / ton; the overall annual economic benefit is approximately 1.1 million yuan. Enterprises have shifted from "spending money on pollution treatment" to "generating revenue from pollution treatment," resulting in a substantial change in their business model.

[0057] Recommendations for the treatment of other waste liquids

[0058] Aluminum anodizing production may also involve processes such as sealing, coloring, and chemical stripping. The wastewater generated from these processes varies depending on the type of pollutants, so it is recommended that it be collected separately and treated using targeted methods.

[0059] • Nickel-containing wastewater from sealing: Nickel is a Class I pollutant and must meet standards at the workshop discharge point. It can be treated by chemical precipitation (adjusting the pH to 10-11, adding sodium hypochlorite to oxidize the nickel complex, and then adding a coagulant to precipitate) or ion exchange. The precipitated nickel hydroxide can be disposed of as hazardous waste or recycled.

[0060] • Coloring wastewater: containing dyes, tin salts, etc., can be decolorized by oxidation (such as adding sodium hypochlorite or ozone), reduction (such as adding sodium hydrosulfite), or adsorption (activated carbon) before entering the washing water separation treatment system of this invention.

[0061] • Nitrate-containing stripping waste liquid: The nitrate nitrogen concentration is high, and it can be treated separately by denitrification biological treatment, or the nitric acid can be recovered by evaporation and concentration.

[0062] The above-mentioned waste liquid treatment method is a well-known technology in the field and does not fall within the core scope of this invention. However, it can be implemented in parallel with the main process of this invention to jointly achieve comprehensive treatment of aluminum oxidation production wastewater.

[0063] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for the graded treatment of aluminum oxidation wastewater by crystallization to extract aluminum while preserving phosphorus and alkali, characterized in that, include: (a) Closed-loop reuse of carry-out liquid: The bath liquid carried out by the workpiece in the aluminum oxidation process is recovered, concentrated, and reused in a closed loop back to the process tank that generated the bath liquid; (ii) Selective crystallization regeneration step: Add a crystallizing agent to the process bath containing aluminum ions to selectively precipitate aluminum ions in crystal form, while the phosphoric acid, sulfuric acid or alkalinity in the bath are basically retained. After the crystals are separated, the mother liquor containing phosphoric acid, sulfuric acid, or alkalinity is returned to the original process tank for continued use. (III) Washing water separation treatment steps: The low-concentration washing water discharged is treated in stages according to pH value, and the calcium sulfate, aluminum hydroxide and phosphate precipitates are recovered separately.

2. The method according to claim 1, characterized in that, The recovery and concentration described in step (1) are achieved in the following way: a recovery tank is set up to receive the carry-out liquid, the liquid in the recovery tank is aerated to flush and clean it and promote water evaporation and cooling, the subsequent water washing tank adopts countercurrent cleaning and returns the concentrated water to the recovery tank, and the concentrated recovery liquid is further evaporated by the heating process of the process tank itself.

3. The method according to claim 1, characterized in that, In step (ii): ammonium sulfate or sodium sulfate is added to the acid polishing tank or oxidation tank to generate double salt aluminum ammonium sulfate crystals or aluminum sodium sulfate crystals; aluminum hydroxide crystal nuclei are added to the alkaline polishing tank to generate aluminum hydroxide crystals.

4. The method according to claim 1 or 3, characterized in that, The crystallization described in step (ii) is carried out in the original process tank, or the tank liquid is pumped to an independent crystallization tank and slowly stirred.

5. The method according to claim 1, characterized in that, The aluminum hydroxide obtained from the precipitation in step (iii) is used as an alkaline agent to neutralize the free sulfuric acid in acid precipitate or oxidized wastewater.

6. The method according to claim 1, characterized in that, The segmented treatment described in step (iii) includes: precipitation of calcium sulfate in the acidic segment, precipitation of aluminum hydroxide in the neutral segment, and precipitation of hydroxyapatite or aluminum phosphate in the alkaline segment.

7. The method according to claim 1, characterized in that, The aluminum oxidation process includes one or more of acid polishing, alkali polishing, and oxidation processes.