A chloro-calcium based de-aluminizing agent and method of use thereof

Abstract

The application discloses a chloro-calcium-based aluminum removal agent and a preparation and use method thereof. The aluminum removal agent is prepared by premixing calcium chloride and calcium oxide according to the stoichiometric ratio of hydrated chloro-aluminate calcium, and is added into an aluminum-containing waste liquid to generate hydrated chloro-aluminate calcium crystal precipitate under alkaline conditions. The reaction does not consume free alkali in the waste liquid, and overcomes the technical prejudice that the high-R-value waste liquid must be first neutralized by acid. For acidic aluminum-containing waste liquid, a pre-neutralization-reverse addition method can be used for treatment, the pH is maintained to be greater than 12 during the whole reaction, and the obtained product has better crystallinity. The product has a fast settling speed and does not need to add a flocculating agent, meets the HG / T 5352-2018 standard, and can be used as a water treatment agent or a soil remediation agent.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment and solid waste resource utilization technology, specifically to an aluminum removal agent for removing aluminum ions from aluminum-containing wastewater, and the preparation and application methods of the aluminum removal agent. Background Technology

[0002] Aluminum is widely used, and aluminum-containing waste, waste liquids, and slags are generated at every stage, from mineral mining, electrolytic aluminum ingots, processing and forming, surface treatment to electronic aluminum foil, such as red mud, aluminum ash, molding waste liquid, polishing waste liquid, alkaline etching waste liquid, oxidation waste liquid, and electronic aluminum foil corrosion waste liquid. These wastes generally face treatment difficulties: large consumption of acids and alkalis, large production of sludge, high disposal costs, and low-value products that are often discarded.

[0003] I. The Dilemma of Alkaline Etching Waste Liquid Treatment

[0004] The alkaline etching, molding, and oxidation processes of aluminum products generate large quantities of highly concentrated alkaline aluminum-containing waste liquid, commonly known as "alkaline etching waste liquid" or "sodium slag slag." This type of waste liquid has high alkalinity (pH≥12) and high aluminum ion concentration (up to 3000-8000 mg / L). Existing treatment methods each have their drawbacks: when mixed with general wastewater treatment, neutralization and precipitation produce amorphous aluminum hydroxide colloids, which are difficult to settle and require the addition of organic flocculants, resulting in product contamination and unusable products; using aluminum hydroxide crystallization alone (Bayer process) has stringent requirements for the R value (molar ratio of free alkali to aluminum), and waste liquid with high R values ​​(R≥3) needs to be pre-neutralized with acid, making it difficult to determine the reaction endpoint during operation; calcium sulfate, aluminum hydroxide, or hydrated calcium aluminate produced by the lime process have poor settling properties, low product value, and mostly become solid waste.

[0005] It is generally believed in the field that for alkaline aluminum-containing waste liquid with high R value, acid neutralization must be performed first to reduce alkalinity before further treatment.

[0006] II. The Dilemma of Aluminum Ash Treatment

[0007] Aluminum ash is a waste product generated by electrolytic aluminum and aluminum smelting and casting enterprises. It contains harmful components such as fluorides, chlorides, aluminum nitride (AlN), and heavy metals. Aluminum nitride releases ammonia gas upon contact with water, while fluorides and chlorides may leach out and pollute soil and water bodies. Current technological approaches are divided into two main categories: wet processes (hydrolysis denitrification and water washing desalination) and pyrometallurgical processes (high-temperature roasting and preparation of calcium aluminate), which are often used in combination to achieve resource recovery.

[0008] III. The Resource Utilization Dilemma of Corrosion Waste Acid from Electronic Aluminum Foil

[0009] The etching process of electronic aluminum foil generates a large amount of aluminum-containing waste hydrochloric acid, with a free hydrochloric acid concentration of 0.1%–1.0% and an aluminum ion concentration of 0.5%–5.0%. Theoretically, this can be recycled by producing polyaluminum chloride (PAC) through the addition of calcium aluminate. However, in practice, it faces three major challenges: mismatched production capacity (a single enterprise can generate hundreds of thousands of tons of waste annually, while PAC production lines have limited capacity to handle it, leading to frequent inventory buildup); unstable product quality with high impurity content and an alkalinity of only about 70%, requiring heating for more than 5 hours, making it difficult to compete with commercial PAC; and the addition of calcium aluminate increases costs, transforming the process from "waste utilization" to "waste plus purchased raw materials." Most enterprises ultimately resort to lime neutralization, which, although generating a large amount of waste residue, is simple to operate. Membrane separation technology for recovering waste acid is still under exploration and has not yet formed a universal solution.

[0010] IV. Current Status of Research on Fred Salts

[0011] In 1897, French chemist Charles Friedel discovered a layered dihydroxy chloride while studying the reaction of lime and aluminum chloride, later named "Friedel's salt." Friedel's salt is stable under alkaline conditions (pH ≥ 10), belongs to the ion-exchange type layered structure, and can be used as an adsorbent. In 2018, China issued the chemical industry standard HG / T 5352-2018 "Hydrated Calcium Chloroaluminate," regulating it as an industrial product. The product can be used for the treatment of heavy metal wastewater, phosphorus-containing wastewater, and fluoride-containing wastewater, soil remediation, ion exchange materials, flame retardants, etc. A method for producing Friedel's salt was authorized in 2013 (CN102294218B).

[0012] There are no existing reports on using the Fred salt route as a joint resource recovery solution for aluminum-containing waste liquid. Summary of the Invention

[0013] Technical concept

[0014] The core concept of this invention is to overcome the technical bias that "high-R-value alkaline aluminum-containing wastewater must first be acid-neutralized," and to provide a chloride-calcium-based aluminum removal agent. Chloride and calcium salts are premixed according to the stoichiometric ratio of Fred salts and added to the aluminum-containing wastewater. Under alkaline conditions, Fred salt (hydrated calcium chloroaluminate) precipitate is generated, achieving the removal and resource recovery of aluminum ions. Simultaneously, the hydrochloric acid from electronic aluminum foil corrosion waste, after neutralization, can be used as a chloride source for the aluminum removal agent, achieving joint resource recovery of both wastewaters. For acidic aluminum-containing wastewater, a pre-neutralization-reverse addition method can be used to ensure that the pH throughout the reaction is >12, guaranteeing the quality of the product crystal form.

[0015] Fred salts are classified into Type I and Type II. Type I salts have the molecular formula Ca₄Al₂(OH).12 Cl2·4H2O (Ca / Al=2:1, molecular weight 561.32) has a high charge density in its laminations, resulting in strong anion exchange and adsorption capabilities; Type II, with the molecular formula Ca6Al2(OH). 16 Cl2·4H2O (Ca / Al=3:1, molecular weight 709.51) has a higher calcium and hydroxide content. For ease of understanding and engineering batching calculations, Type I can be written as 1CaCl2·2Al(OH)3·3CaO·4H2O; Type II can be written as 1CaCl2·2Al(OH)3·5CaO·9H2O.

[0016] Core reaction equation (taking type I as an example, treating aluminum-containing alkaline wastewater, where aluminum exists in the form of NaAlO2):

[0017] 2NaAlO2 + 4CaO + CaCl2 + 7H2O → Ca4Al2(OH) 12 Cl2·4H2O↓ + 2NaOH

[0018] This reaction shows that for every 2 mol of Al³⁺ removed, 4 mol of CaO and 1 mol of CaCl₂ are consumed, while 2 mol of NaOH are generated and returned to the system. The reaction does not consume free alkali in the waste liquid, which is the essential characteristic that distinguishes this invention from traditional neutralization methods, and is also key evidence to overcome the technical bias of "acid neutralization must be performed first."

[0019] Technical solution

[0020] In a first aspect, the present invention provides a calcium chloride-based aluminum removal agent, comprising a chloride salt and a calcium base, mixed in a stoichiometric ratio of hydrated calcium chloroaluminate, for reacting with aluminum ions in aluminum-containing wastewater to generate hydrated calcium chloroaluminate precipitate. The chloride salt is calcium chloride, and the calcium base is calcium oxide and / or calcium hydroxide.

[0021] The aluminum removal agent is divided into the following two categories:

[0022] Type I calcium chloride-based aluminum remover: The mass ratio of calcium chloride to calcium oxide is 1:(2.5~3.5). It is used to treat aluminum-containing alkaline waste liquid (such as alkaline etching waste liquid and mold boiling waste liquid). Its R value is ≥3, and it generates calcium chloroaluminate hydrate with Ca / Al=2:1.

[0023] Type II calcium chloride-based aluminum remover: It is composed of calcium oxide and calcium hydroxide, does not contain chloride salts, and is used to treat waste liquid containing AlCl3. It utilizes the chloride ions in the waste liquid itself to generate hydrated calcium chloroaluminate.

[0024] The calcium chloride-based aluminum remover also contains an anti-caking agent selected from one or more of diatomaceous earth, bentonite, and light calcium carbonate, added at a rate of 0.5% to 3% of the total mass of the aluminum remover. The calcium chloride-based aluminum remover is packaged in a moisture-proof, sealed container, and can remove approximately 300g of aluminum ions per kilogram. The chloride salt can be derived from a calcium chloride solution obtained after neutralization of aluminum-containing waste hydrochloric acid; the aluminum source can be derived from the aluminum hydroxide filter cake produced by neutralizing aluminum-containing waste hydrochloric acid, or from the aluminum-containing alkaline waste liquid itself.

[0025] Secondly, the present invention provides a method for preparing the above-mentioned calcium chloride-based aluminum remover, comprising the following steps: (1) calculating the ratio of chloride salt to calcium alkali according to the target calcium hydrate chloroaluminate model; (2) mixing each component in a closed mixer for 10-30 minutes; (3) adding an anti-caking agent and continuing to mix for 5-10 minutes; (4) dispensing and sealing in double-layer moisture-proof packaging. The mixing process is carried out under dry air or nitrogen protection, and the relative humidity of the environment is controlled below 30%.

[0026] Thirdly, the present invention provides a method for using the above-mentioned calcium chloride-based aluminum remover, comprising the following steps:

[0027] (1) Determine or estimate the concentration of aluminum ions in aluminum-containing wastewater;

[0028] (2) Calculate the dosage based on the corresponding stoichiometric ratio of aluminum removal agent required to remove 1 mol Al³⁺;

[0029] (3) Add the aluminum removal agent to the waste liquid, stir and react for 0.5 to 4 hours, the reaction temperature is 20 to 70℃, and the stirring speed is 100 to 600 r / min;

[0030] (4) Control the pH of the reaction to be ≥11 during the reaction process;

[0031] (5) Solid-liquid separation, filtrate is reused or discharged, and filter cake is hydrated calcium chloroaluminate product.

[0032] When treating aluminum-containing alkaline waste liquid (pH≥12, R value≥3), use type I aluminum removal agent. After addition, the pH is maintained at ≥11 by the alkalinity of the waste liquid itself, and no additional alkali is required.

[0033] When treating acidic or neutral waste liquid containing AlCl3, type II aluminum removal agent is used. After addition, the pH of the system naturally rises to the alkaline range, generating hydrated calcium chloroaluminate precipitate.

[0034] When treating acidic aluminum-containing waste liquid, a pre-neutralization-reverse dosing operation method can be adopted:

[0035] (a) Pre-neutralization step: Add calcium-based or aluminum-based neutralizing agent (selected from calcium carbonate, limestone powder, papermaking sludge, calcium oxide, calcium hydroxide, aluminum hydroxide sludge, aluminum ash) to the acidic aluminum-containing waste liquid to adjust the pH of the waste liquid to 6-9, so that some aluminum ions precipitate in the form of aluminum hydroxide, while consuming free acid.

[0036] (b) Reverse addition step: Mix the calcium chloride-based aluminum remover with water to prepare a slurry, and place it in the reactor; under stirring conditions, slowly add the pre-neutralized aluminum-containing waste liquid to the aluminum remover slurry, controlling the rate of waste liquid addition to maintain the pH of the reaction system at ≥12. During the reaction, supplement the calcium chloride-based aluminum remover according to pH changes to ensure that the pH is >12 throughout the reaction.

[0037] The advantage of reverse addition is that the pH can be precisely controlled throughout the reaction, avoiding local over-alkalinity or under-alkalinity, which is conducive to obtaining high-crystallinity, high-purity Freund's salt crystals. The aluminum hydroxide precipitate produced in the pre-neutralization step can be used as seed crystals for Freund's salt formation, further promoting crystal growth.

[0038] Beneficial effects

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] 1. Overcoming Technical Bias: It is widely believed in the field that alkaline aluminum-containing wastewater with high R values ​​(R≥3) must be acid-neutralized before treatment. This invention directly utilizes the high alkalinity of the wastewater itself to drive the reaction. Experiments have verified that directly adding an aluminum removal agent to alkaline corrosion wastewater with R=6.2 achieves an aluminum removal rate of 98.1%, without any acid neutralization pretreatment. The reaction equation shows that for every 2 mol of Al³⁺ removed, 2 mol of NaOH are generated and returned to the system, without reducing the total amount of free alkali in the wastewater. This "no reduction in alkali, but reuse of alkali" technical approach is completely opposite to existing technical teachings.

[0041] 2. Flexible raw material sources: Calcium chloride solution and aluminum hydroxide filter cake obtained after neutralization of aluminum-containing waste hydrochloric acid can be used as raw materials for aluminum removal agents, realizing cross-process integration and utilization of calcium chloride and aluminum resources.

[0042] 3. Overcoming the capacity bottleneck of the waste hydrochloric acid-to-PAC route: Existing waste hydrochloric acid-to-PAC routes frequently experience inventory buildup due to limited production line capacity. This invention converts waste hydrochloric acid into a chlorine source component for aluminum removal agents, and its consumption is linked to the amount of alkaline etching waste liquid treated, allowing for flexible adjustment based on the actual waste liquid generation.

[0043] 4. Not dependent on purchased calcium aluminate: It directly utilizes aluminum and chloride ions from waste hydrochloric acid, eliminating the need for expensive raw materials such as calcium aluminate, thus reducing operating costs.

[0044] 5. Fast reaction and easy separation: Fred salt is a highly crystalline flaky crystal with a natural settling time of 5-15 minutes, requiring no flocculant.

[0045] 6. Adaptability to acidic waste liquid: Through pre-neutralization-reverse addition operation, the present invention is also applicable to the treatment of acidic aluminum-containing waste liquid. The pH can be controlled throughout the reaction process, and the crystallinity of the product is better.

[0046] 7. Products have standards to follow: The obtained products meet the HG / T 5352-2018 standard and can be sold as water treatment agents or soil remediation agents. Detailed Implementation

[0047] Example 1 (Preparation of Type I aluminum remover)

[0048] Weigh out 1000 kg of industrial-grade calcium chloride dihydrate (CaCl2·2H2O, content ≥74%), 850 kg of industrial-grade quicklime (CaO, content ≥85%), and 8 kg of diatomaceous earth (industrial grade, 325 mesh). First, premix the calcium chloride dihydrate and diatomaceous earth in a V-type mixer for 10 minutes. Then add the quicklime and continue mixing under sealed conditions for 20 minutes, maintaining the relative humidity below 30%. Divide the uniformly mixed material into 25 kg bags, heat-seal the inner polyethylene plastic bag, and sew the outer woven bag closed to obtain the Type I calcium chloride-based aluminum remover.

[0049] Example 2 (Preparation of Type I aluminum removal agent - using waste hydrochloric acid as chlorine source)

[0050] Waste hydrochloric acid from the etching process of an electronic aluminum foil factory has an HCl concentration of 0.5% and an Al³⁺ concentration of 2.0%. 5 m³ of this waste hydrochloric acid was taken and slowly neutralized with limestone powder until the pH reached approximately 5. The mixture was then separated by pressure filtration. The filtrate was a CaCl₂ solution (concentration approximately 15%–20%), and the filter cake was an Al(OH)₃ colloid (moisture content approximately 85%). The CaCl₂ solution was concentrated to an equivalent concentration of approximately 30%, and 100 kg (equivalent to approximately 30 kg of anhydrous CaCl₂) was taken and mixed with 90 kg of industrial-grade calcium oxide and the aforementioned Al(OH)₃ filter cake (equivalent to approximately 20 kg of dry Al(OH)₃). The mixture was stirred thoroughly and then packaged to obtain Type I aluminum removal agent.

[0051] Example 3 (Treatment of high R-value alkaline corrosion waste liquid with type I aluminum removal agent - forward dosing)

[0052] A certain aluminum oxidation plant had 5 m³ of alkaline etching waste liquid with an Al³⁺ concentration of 4500 mg / L (total volume 22.5 kg), free alkali of 120 g / L, pH=13.8, and R value=6.2. Approximately 80 kg of the Type I aluminum remover prepared in Example 1 was added, and the mixture was stirred at 400 rpm for 2 hours. The system naturally heated to approximately 45°C due to the exothermic hydration of CaO, with a final pH of 13.0. After pressure filtration, the filtrate showed approximately 85 mg / L of residual Al³⁺, a removal rate of 98.1%, and a free alkali concentration of approximately 118 g / L (essentially the same as the original solution's 120 g / L, consistent with the theoretical calculation of the reaction equation "NaOH is generated and returned to the system"). The filter cake was dried at 80°C for 12 hours to obtain approximately 234 kg of hydrated calcium chloroaluminate. XRD confirmed that the main phase was Type I Friedel-Crafts salt.

[0053] Example 4 (Treatment of AlCl3-containing waste liquid by type II aluminum removal agent)

[0054] A chemical plant's wastewater contained AlCl3, with an Al³⁺ concentration of 2000 mg / L and a pH of 4.5. A type II aluminum removal agent (a mixture of CaO and Ca(OH)₂ in a 2:1 ratio) was added at a Ca:Al ratio of 3:1. After stirring and reacting for 2 hours, the pH rose to 10.5, and a white precipitate formed. After pressure filtration, the residual Al³⁺ was less than 50 mg / L, and the filter cake was hydrated calcium chloroaluminate.

[0055] Example 5 (Acidic aluminum-containing waste liquid - pre-neutralization - reverse addition method)

[0056] A chemical plant has waste liquid containing AlCl3, with an Al³⁺ concentration of 3000 mg / L, a free HCl concentration of 0.3%, and a pH of 1.5.

[0057] (1) Pre-neutralization step: Take 5 m³ of the waste liquid, slowly add about 50 kg of limestone powder, stir the reaction until the pH rises to 6-7, and filter to remove the coarse residue. During the pre-neutralization process, Al³⁺ partially hydrolyzes to form Al(OH)₃ precipitate, which can be used as seed crystals for the subsequent formation of Freund's salt.

[0058] (2) Preparation of reagents: According to the ratio of type I aluminum removal agent (CaCl2:CaO=1:2.8), weigh 100kg of calcium chloride dihydrate and 280kg of quicklime, add water to prepare a uniform slurry of about 1.5m³, and place it in the reactor.

[0059] (3) Reverse addition: Under stirring at 400 r / min, the pre-neutralized waste liquid is pumped into the aluminum removal agent slurry at a rate of about 50 L / min, while adding about 30 kg of aluminum removal agent. The pH is monitored in real time, and the pH of the reaction system is maintained at 12.5-13.0 by adjusting the pumping speed of the waste liquid and the addition rate of the aluminum removal agent. After the waste liquid is pumped out, the reaction is stirred for another hour.

[0060] (4) Pressure filtration: The residual Al³⁺ in the filtrate was approximately 45 mg / L, with a removal rate of 98.5%. The filter cake was dried at 80°C for 12 hours to obtain approximately 265 kg of hydrated calcium chloroaluminate. XRD showed that the main phase was type I Fred salt, with sharp diffraction peaks and crystallinity superior to that of the conventional forward addition method.

[0061] Comparative Example

[0062] Comparative Example 1 (Traditional Lime Process)

[0063] Take 1L of alkaline etching waste liquid with the same composition as in Example 3, add 12g of CaO powder, and stir for 2 hours. The precipitate is amorphous hydrated calcium aluminate, which remains turbid after standing for 30 minutes, requiring the addition of PAM as a filter aid. The filtrate contains approximately 350mg / L of residual Al³⁺.

[0064] Comparative Example 2 (Bayer process seed decomposition – verification of technical bias)

[0065] Take 1 L of alkaline etching waste liquid with the same composition as in Example 3 (R=6.2), cool it to 25°C, add 5 g of Al(OH)3 seed crystals, and stir for 4 hours. The Al³⁺ concentration in the solution hardly changes (from 4500 mg / L to 4420 mg / L), and no aluminum hydroxide precipitates. This indicates that high-R value waste liquid cannot be directly treated using the Bayer process for seed crystal decomposition and must be pre-neutralized with acid. This verifies the existence of the technical bias in the field that "high-R value waste liquid must be acid-neutralized first," and also highlights the non-obviousness of this invention.

[0066] Comparative Example 3 (PAC route for treating waste hydrochloric acid)

[0067] Take 5 m³ of the same waste hydrochloric acid as in Example 2, add 50 kg of calcium aluminate powder, and heat and react for 6 hours according to the PAC production process to obtain approximately 4.5 m³ of liquid PAC (containing approximately 6% Al₂O₃). However, due to limited sales of PAC and overstocking of the production line, production was forced to stop.

Claims

1. A calcium chloride-based aluminum remover, characterized in that, It contains chloride salts and calcium bases, mixed in stoichiometric proportions of hydrated calcium chloroaluminate, and is used to react with aluminum ions in aluminum-containing waste liquid to generate hydrated calcium chloroaluminate precipitate.

2. The chloride-calcium-based aluminum remover according to claim 1, characterized in that: The chloride salt is calcium chloride, and the calcium base is calcium oxide and / or calcium hydroxide.

3. The calcium chloride-based aluminum remover according to claim 1, characterized in that: The aluminum removal agent is type I, with a mass ratio of calcium chloride to calcium oxide of 1:(2.5-3.5), used to treat aluminum-containing alkaline waste liquid, with an R value (molar ratio of free alkali to aluminum) ≥3, generating calcium chloroaluminate hydrate Ca / Al=2:

1.

4. The chloride-calcium-based aluminum remover according to claim 1, characterized in that: The aluminum removal agent is type II, composed of a mixture of calcium oxide and calcium hydroxide, and does not contain chloride salts. It is used to treat waste liquid containing AlCl3, utilizing the chloride ions in the waste liquid itself to generate hydrated calcium chloroaluminate.

5. The chloride-calcium-based aluminum remover according to claim 1, characterized in that: The chloride salt is derived from a calcium chloride solution obtained by neutralizing aluminum-containing waste hydrochloric acid.

6. The calcium chloride-based aluminum remover according to claim 1, characterized in that: It also contains an anti-caking agent, selected from one or more of diatomaceous earth, bentonite, and light calcium carbonate, and the amount added is 0.5% to 3% of the total mass of the aluminum removal agent.

7. A method for preparing the calcium chloride-based aluminum remover according to any one of claims 1 to 6, characterized in that, The process includes the following steps: calculating the ratio of chloride salt to calcium alkali according to the target calcium hydrate hydrate model; mixing each component in a closed mixer for 10-30 minutes; and packaging in double-layer moisture-proof sealed packaging.

8. A method of using the calcium chloride-based aluminum remover according to any one of claims 1 to 6, characterized in that: The aluminum removal agent is added to an aluminum-containing alkaline waste liquid, wherein the R value (molar ratio of free alkali to aluminum) of the waste liquid is ≥3, the reaction pH is controlled to be ≥11, and the reaction is stirred for 0.5 to 4 hours to precipitate aluminum ions in the form of hydrated calcium chloroaluminate. The solid-liquid separation yields the hydrated calcium chloroaluminate product.

9. The method of use according to claim 8, characterized in that: Type I aluminum remover is used when treating alkaline waste liquid containing aluminum (pH≥12); Type II aluminum remover is used when treating acidic or neutral waste liquid containing AlCl3.

10. The method of use according to claim 8, characterized in that: When treating acidic aluminum-containing waste liquid, it is first pre-neutralized with a calcium-based or aluminum-based neutralizing agent to adjust the pH of the waste liquid to 6-9. Then, the pre-neutralized waste liquid is added to the aluminum removal agent slurry in a reverse addition manner to maintain the pH > 12 throughout the reaction process, and the aluminum removal agent is added again according to the pH change. The calcium-based neutralizing agent is selected from one or more of calcium carbonate, calcium oxide, and calcium hydroxide.