A method for producing polyaluminum chloride and recovering sodium chloride by using sodium tetrachloroaluminate waste salt

Abstract

The present application relates to a method for producing polyaluminum chloride and recovering sodium chloride by using by-products of methyl phosphine dichloride production, and relates to the technical field of water treatment agents. The method comprises the following steps: crushing blocky mixed salt containing sodium tetrachloroaluminate and sodium chloride, dissolving the crushed mixed salt in water, adding sodium metaaluminate, stirring and reacting, filtering, obtaining aluminum hydroxide solid and crude salt aqueous solution; putting the aluminum hydroxide solid into water, adding hydrochloric acid dropwise under heating and stirring to adjust pH, aging, adding an adsorbent, and filtering to obtain a polyaluminum chloride solution; transferring the crude salt water into a flocculation and precipitation tank, adding an oxidizing agent to oxidize phosphorus impurities, adding sodium hydroxide to adjust pH, and adding polyaluminum chloride; obtaining preliminary refined brine after precipitation is completed, and refining the preliminary brine by membrane filtration to obtain refined brine. The present application realizes circular economy, and has the advantages of simple process, cheap and easily available raw materials, safety and environmental protection, high economic benefits, etc.

Description

Technical Field

[0001] This invention relates to the field of chemical technology, specifically to a method for producing polyaluminum chloride from sodium tetrachloroaluminate waste salt and for recovering sodium chloride. Background Technology

[0002] Flocculants are the most commonly used water treatment agents, capable of agglomerating impurities in water into large flocs, thereby separating pollutants from the water. Currently, flocculants on the market are mainly divided into three types: inorganic flocculants, organic flocculants, and microbial flocculants. While organic flocculants require less dosage and are highly efficient, their high-molecular-weight residues can be teratogenic and carcinogenic, limiting their application. Microbial flocculants are still immature and their application is not widespread.

[0003] Polyaluminum chloride (PAC) is a water-soluble inorganic polymer intermediate between Al(OH)3 and AlCl3, with the general chemical formula [Al2(OH)3]. n Cl 6-n ] m Polyaluminum chloride (PAC) possesses advantages such as high charge neutralization and bridging effects on colloids and particulate matter in water, and can powerfully remove trace toxic substances and heavy metal ions. As a new water purification material, PAC is widely used in various wastewater treatment fields, including domestic water and industrial wastewater. Currently, commonly used methods for preparing PAC include the metallic aluminum method, alkali dissolution method, aluminum chloride method, aluminum hydroxide method, and alumina method. All of these methods suffer from drawbacks such as complex processes, expensive raw materials, high energy consumption, high impurity content, and significant safety hazards during production. A cleaner and safer process is needed to produce PAC.

[0004] In the production of methylphosphonic acid, the salting-out reaction in the process design generates a large amount of mixed waste salt consisting of sodium tetrachloroaluminate and sodium chloride (in excess during the reaction). The resulting sodium tetrachloroaluminate dissolves in water, releasing a large amount of heat and producing an acidic solution. It also contains a small amount of phosphine-containing waste, making it hazardous and difficult to handle. Since the advent of related synthesis technologies, sodium tetrachloroaluminate waste salt has been a persistent and insurmountable challenge for the industry.

[0005] Patent CN116022836A discloses a method for recovering sodium tetrachloroaluminate and a method for synthesizing polyaluminum chloride from sodium tetrachloroaluminate. This method involves dissolving sodium tetrachloroaluminate flakes in water, dehydrating and crystallizing to obtain solid aluminum trichloride, then dissolving the aluminum trichloride in water and adding calcium powder to react and obtain a polyaluminum chloride solution. While this process is simple and low-cost, a large amount of sodium chloride is carried out during dehydration and crystallization. Because of the addition of calcium powder, the aluminum content of the produced polyaluminum chloride cannot be guaranteed to meet national standards. Furthermore, the phosphorus-containing wastewater is not treated during the entire process, requiring additional processes or third-party treatment, increasing operating costs.

[0006] Patent CN112794868A discloses a method for treating sodium tetrachloroaluminate generated during the production of methyldichlorophosphine, and a method for recovering sodium tetrachloroaluminate byproducts. This method involves adding sodium tetrachloroaluminate to a sodium hydroxide solution, stirring and reacting, filtering to obtain solid aluminum hydroxide, washing the solid aluminum hydroxide, drying and calcining it to obtain alumina product, mixing the washing liquid with the first filtrate to obtain sodium chloride mother liquor, and concentrating and crystallizing the mother liquor to obtain purified brine after dissolution, precipitation, and membrane treatment. This method is low-cost and has high atom utilization, but the reaction of sodium tetrachloroaluminate with sodium hydroxide generates a large amount of heat, making the process difficult to control, and requiring two high-temperature treatments, resulting in high energy consumption. Summary of the Invention

[0007] To address the above-mentioned technical deficiencies, this invention provides a method for producing polyaluminum chloride and recovering sodium chloride from sodium tetrachloroaluminate waste salt.

[0008] The technical solution of the present invention:

[0009] (1) Crush the blocky mixed salt containing sodium tetrachloroaluminate and sodium chloride after the production of methyl dichlorophosphine.

[0010] (2) Dissolve the crushed salt mixture in water in 3 to 5 portions.

[0011] (3) Add sodium aluminate, stir to react, and filter to obtain solid aluminum hydroxide and crude salt solution.

[0012] (4) Add aluminum hydroxide from (3) to water, heat and stir, add hydrochloric acid, add adsorbent, and filter to obtain polyaluminum chloride solution.

[0013] (5) Transfer the crude brine from (3) into a flocculation sedimentation tank, add an oxidant to oxidize the phosphorus-containing impurities, then add sodium hydroxide to adjust the pH, and then add polyaluminum chloride. After the precipitation is complete, a preliminary refined brine is obtained.

[0014] (6) The supernatant is treated with an ion exchange membrane to obtain refined brine.

[0015] The reaction equation of this invention is as follows:

[0016] NaAlCl4+6H2O→AlCl3﹒6H2O+NaCl

[0017] AlCl3+3NaAlO2+6H2O→4Al(OH)3+3NaCl

[0018] 2Al(OH)3+(6-n)HCl→Al2(OH) n Cl 6-n +(6-n)H2O

[0019] Reactions 1 and 2 are exothermic, while reaction 3 is endothermic.

[0020] More preferably, in step (2), the mass ratio of mixed salt to water is 1:2.0~2.4, sodium tetrachloroaluminate is added to water in four batches, and sodium tetrachloroaluminate dissolves in water and releases heat significantly.

[0021] More preferably, in step (3), the mass ratio of sodium aluminate to the mixed salt in step (2) is 0.2~0.6:1, the stirring speed is 40~60 rpm, and the reaction time is 1~2 h.

[0022] More preferably, in step (4), the mass fraction of hydrochloric acid added is 15%, the adsorbent is activated carbon, the mass ratio of hydrochloric acid, water and aluminum hydroxide is 0.18~0.25:1.8~2.4:1, the reaction temperature is 70~85℃, the stirring speed is 50~75rpm, and the reaction time is 1.5~2.5h.

[0023] More preferably, in step (5), the pH needs to be adjusted to 8~10, and the added oxidant is 30% hydrogen peroxide; the mass ratio of polyaluminum chloride, oxidant and crude brine is 0.1~0.3:0.05~0.15:1.

[0024] More preferably, the activated carbon solid waste obtained in step (4) is subjected to harmless treatment before being incinerated in an RTO.

[0025] The precipitate obtained in step (5) is aluminum phosphate, which is packaged as a by-product.

[0026] More preferably, the upstream process for producing methyl dichlorophosphine uses an aluminum-Gergren reagent. To separate the CH3PCl2·AlCl3 complex, excess sodium chloride needs to be added for a salting-out reaction. After the reaction is completed, sodium tetrachloroaluminate by-product salt containing a small amount of phosphine impurities will be produced.

[0027] The present invention has the following beneficial effects:

[0028] (1) This invention achieves efficient recovery of process raw materials by producing polyaluminate mixed salts, which are byproducts of the production of methylphosphine dichloride, into polyaluminate and recovering sodium chloride, thus reflecting the atom economy. Part of the produced polyaluminate can be sold as a product to improve economic benefits, and part can also be used as a flocculant to purify brine. The recovered sodium chloride solution, after concentration and crystallization, meets the requirements of the national standard "QB / T5270-2018" for salt used in ion-exchange membrane caustic soda.

[0029] (2) The refined sodium chloride recovered by this invention can be used as a raw material for chlor-alkali production to prepare chlorine gas and produce phosphorus trichloride as a raw material for methyl dichloride, or it can be used directly to produce methyl dichloride, thus completing the cycle of the original process and realizing the full utilization of elements.

[0030] (3) This invention prepares polyaluminum chloride by reacting sodium tetrachloroaluminate with sodium aluminate, which reduces the introduction of other ions and increases the aluminum content of the product. The latest national standard for polyaluminum chloride, GB / T22627-2022, has raised the requirements for the aluminum content in polyaluminum chloride solution. This process can better ensure that the obtained product meets the national standard.

[0031] (4) Polyaluminum chloride and oxidant completely convert phosphine impurities into aluminum phosphate, which can be used in industry as coatings, refractory adhesives and other materials, avoiding the traditional method of incinerating phosphine waste and turning waste into treasure.

[0032] (5) This invention treats phosphine-containing impurities by adding an oxidant, which reduces energy consumption and avoids the generation of phosphine-containing waste gas compared to the traditional process of oxidizing phosphine-containing impurities at high temperatures. Hydrogen peroxide also consumes hydrogen ions during hydrogen oxidation, increasing the basicity of polyaluminum chloride solution when treating crude brine, thus improving the coagulation and flocculation effects of the product and reducing the cost of using the product. At the same time, the reduction of hydrogen peroxide only produces water, without affecting the purity of the brine and aluminum phosphate.

[0033] (6) This invention solves the problem of waste salt treatment in the production of methylphosphine chloride, and has the advantages of simple process flow, simple operation, low energy consumption, safe and controllable, and high economic benefits. It improves product value while solving safety and environmental protection problems. Attached Figure Description

[0034] Figure 1 This invention provides a flowchart for the production of polyaluminum chloride from sodium tetrachloroaluminate waste salt and the recovery of sodium chloride. Detailed Implementation

[0035] The technical solutions of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0036] The process for obtaining the raw material sodium tetrachloroaluminate waste salt in the examples and comparative examples is as follows: Using an aluminum Grignard reagent, to separate the CH3PCl2·AlCl3 complex, excess sodium chloride is added for a salting-out reaction. After the reaction is complete, sodium tetrachloroaluminate byproduct salt containing a small amount of phosphine impurities is produced. Its specific composition is as follows:

[0037] Table 1

[0038]

[0039] Note: Because methyl dichlorophosphine is easily oxidized, when waste salt is stored in a warehouse, methyl dichlorophosphine will oxidize into various forms of phosphorus-containing organic impurities.

[0040] The examples and comparative examples contain 68% NaAlCl4, 28% NaCl, and 4% organophosphorus impurities.

[0041] Example 1

[0042] A method for producing polyaluminum chloride and recovering sodium chloride from sodium tetrachloroaluminate waste salt includes the following steps:

[0043] (1) Put 500 kg of a mixture of sodium tetrachloroaluminate and sodium chloride into a crusher and crush it.

[0044] (2) Add 1100 kg of water to the dissolving vessel, add the crushed mixed salt into the water in four batches to dissolve, and dissolve completely after 1 hour. After dissolving, the water temperature rises to 90℃.

[0045] (3) Add 60 kg of sodium aluminate, stir at 45 rpm, and the reaction ends after 1.5 h. The slurry is then pumped to a plate and frame filter press for filtration. 372 kg of wet filter cake is obtained and transferred to the polyaluminum synthesis reactor. The crude brine is transferred to the flocculation sedimentation tank.

[0046] (4) Add 750 kg of water to the polyaluminum synthesis reactor, add 73 kg of 15% hydrochloric acid dropwise over 0.5 h, start stirring at 65 rpm, heat to 80 °C, and add activated carbon; after 1.8 h, cool and discharge the material into a plate and frame filter press, transfer the filtrate to a polyaluminum chloride temporary storage tank, and incinerate the filter cake in an RTO.

[0047] (5) Add 112 kg of 30% hydrogen peroxide solution to the flocculation sedimentation tank. After 3 hours, add 68 kg of 30% sodium hydroxide solution to adjust the pH to 8.2. Add 210 kg of polyaluminum chloride solution. After standing for 5 hours, transfer the supernatant to the membrane treatment device to prepare refined brine. Transfer the lower sediment to the centrifuge, dry it, and package it as aluminum phosphate product. Combine the centrifugal filtrate and the supernatant for membrane removal treatment.

[0048] The final yield was 953 kg of polyaluminum chloride, 1182 kg of refined brine, and 21 kg of aluminum phosphate.

[0049] Example 2

[0050] A method for producing polyaluminum chloride and recovering sodium chloride from sodium tetrachloroaluminate waste salt includes the following steps:

[0051] Based on Example 1, the amount of sodium aluminate added in step (3) was changed to 80 kg, and finally 1050 kg of polyaluminum chloride product, 1299 kg of refined brine and 25 kg of aluminum phosphate were obtained.

[0052] Example 3

[0053] Based on Example 1, the amount of sodium aluminate added in step (3) was changed to 100 kg, and finally 1110 kg of polyaluminum chloride product, 1352 kg of refined brine and 28 kg of aluminum phosphate were obtained.

[0054] Example 4

[0055] A method for producing polyaluminum chloride and recovering sodium chloride from sodium tetrachloroaluminate waste salt includes the following steps:

[0056] Based on Example 1, the amount of sodium aluminate added in step (3) was changed to 120 kg, and finally 1011 kg of polyaluminum chloride product, 1394 kg of refined brine and 27 kg of aluminum phosphate were obtained.

[0057] Example 5

[0058] Based on Example 1, the amount of 30% hydrogen peroxide solution added in step (5) was changed to 0 kg. In step (5), the amount of 30% sodium hydroxide solution was increased to 80 kg to obtain the same pH. The sedimentation tank was completely clarified after standing for 8 hours. Other aspects were the same as in Example 1.

[0059] The final yield was 965 kg of polyaluminum chloride product, 1195 kg of purified brine, and 10 kg of aluminum phosphate. An increased content of organophosphorus compounds was also detected in the obtained brine.

[0060] Example 6

[0061] Based on Example 1, in step (5), 150 kg of potassium permanganate was added instead, and the time required for precipitation was 6 hours.

[0062] The final yield was 962 kg of polyaluminum chloride, 1183 kg of refined brine, and 30 kg of aluminum phosphate. The aluminum phosphate obtained from precipitation contained sludge.

[0063] Example 7

[0064] Based on Example 1, in step (5), 73 kg of sodium hypochlorite was added instead, and the time required for precipitation was 7 h.

[0065] The final yield was 958 kg of polyaluminum chloride product, 1176 kg of refined brine (the brine had a distinct odor), and 33 kg of aluminum phosphate, which contained a large amount of sludge.

[0066] The above examples lead to the conclusion that, to effectively remove phosphine-containing impurities, ensure product quality, increase product yield, and save costs, the optimal mass ratio of sodium aluminate, hydrogen peroxide, and waste salt in actual production is approximately 0.2~0.24:0.256:1. Hydrogen peroxide also consumes hydrogen ions during oxidation, increasing the basicity of the polyaluminum chloride solution, enhancing the coagulation and flocculation effects of the product, and reducing the product's usage cost. In other words, using hydrogen peroxide as an oxidant achieves the best flocculation effect and yields the highest quality byproducts when treating phosphine impurities.

[0067] The technical specifications of the polyaluminum chloride product prepared according to this invention are as follows:

[0068] Table 2

[0069]

[0070] Technical specifications for recovering wet sodium chloride:

[0071] Table 3

[0072]

[0073] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for producing polyaluminum chloride and recovering sodium chloride from sodium tetrachloroaluminate waste salt. Its features are, Includes the following steps: (1) Crush the blocky mixed salt containing sodium tetrachloroaluminate and sodium chloride after the production of methyldichlorophosphine; (2) Dissolve the crushed salt mixture in water in four separate batches; (3) Add sodium aluminate, stir to react, and filter to obtain solid aluminum hydroxide and crude salt aqueous solution; (4) Add aluminum hydroxide from step (3) into water, heat and stir, add hydrochloric acid with a mass fraction of 15%, add activated carbon as an adsorbent, the mass ratio of hydrochloric acid to sodium tetrachloroaluminate in step (1) is (0.18~0.25):1, the reaction temperature is 75~85℃, the stirring speed is 50~75rpm, the reaction time is 2~2.5h, and filter to obtain polyaluminum chloride solution; (5) Transfer the crude brine from step (3) into a flocculation sedimentation tank, add hydrogen peroxide solution as an oxidant to oxidize the phosphorus-containing impurities, then add sodium hydroxide to adjust the pH, and then add polyaluminum chloride solution. After precipitation is complete, a preliminary refined brine is obtained. (6) The preliminarily refined brine is treated with an ion-exchange membrane to obtain refined brine; The production of polyaluminum chloride from waste sodium tetrachloroaluminate and the recovery of sodium chloride were completed.

2. The method for producing polyaluminum chloride and recovering sodium chloride from sodium tetrachloroaluminate waste salt according to claim 1, characterized in that, In step (2), the mass ratio of sodium tetrachloroaluminate to water is 1:(1.8~2.2), and sodium tetrachloroaluminate is added to the water in batches.

3. The method for producing polyaluminum chloride and recovering sodium chloride from sodium tetrachloroaluminate waste salt according to claim 1, characterized in that, In step (3), the mass ratio of sodium aluminate to sodium tetrachloroaluminate is (0.1~0.3):1, the stirring speed is 40~60 rpm, and the reaction time is 1~1.5 h.

4. The method for producing polyaluminum chloride and recovering sodium chloride from sodium tetrachloroaluminate waste salt according to claim 1, characterized in that, In step (5), the pH is adjusted to 8-10.

5. The method for producing polyaluminum chloride and recovering sodium chloride from sodium tetrachloroaluminate waste salt according to claim 1, characterized in that, The solid waste obtained in step (4) is subjected to harmless treatment and then incinerated in an RTO.

Images