Method for removing magnesium ions in alkaline sludge

Abstract

The invention belongs to the technical field of industrial sludge treatment, and particularly relates to a method for removing magnesium ions in alkaline sludge. The method for removing the magnesium ions in the alkaline sludge comprises the following steps: (1) dehydrating the alkaline sludge, and mixing the dehydrated alkaline sludge with water to obtain a mixture; (2) carrying out carbonization treatment on the mixture obtained in the step (1) to convert Mg (OH) 2 in the alkaline sludge into Mg (HCO3) 2; (3) carrying out solid-liquid separation on the mixture obtained in the step (2) to obtain primary magnesium-removed sludge and a magnesium-containing carbonized solution; and leaching the primary magnesium-removed sludge to obtain the magnesium-removed sludge. According to the method for removing the magnesium ions in the alkaline sludge, Mg (OH) 2 is selectively converted into soluble magnesium bicarbonate by utilizing low-concentration carbon dioxide, and generated CaCO3 is basically insoluble, so that efficient separation of magnesium and calcium in the sludge is realized; the high-quality sludge product is obtained, and high-purity Mg (OH) 2 can be recycled from the desorption liquid to serve as a flame retardant or a chemical raw material, so that the magnesium resource is recycled.

Description

Technical Field

[0001] This application belongs to the field of industrial sludge treatment technology, specifically relating to a method for removing magnesium ions from alkaline sludge. Background Technology

[0002] The alkaline sludge produced after power plant circulating water undergoes softening and coagulation treatment mainly consists of CaCO3, Mg(OH)2, SiO2, other suspended solids, and trace amounts of heavy metals. Magnesium primarily exists as magnesium hydroxide. When this sludge is considered for use as a cement admixture or brick-making raw material, excessively high magnesium content (especially in the form of magnesium hydroxide) is considered a negative factor. This is because, in the later stages of cement hydration or in humid environments, Mg(OH)2 reacts with SiO2 to form expansive silicates, affecting the long-term dimensional stability of cement products. Furthermore, high magnesium content also affects the sintering performance and strength of bricks.

[0003] Existing technologies have limited research on magnesium removal from alkaline sludge. While direct acid dissolution can dissolve Mg(OH)₂, it also dissolves large amounts of valuable CaCO₃, disrupting the sludge's calcium-based framework and introducing foreign anions, resulting in resource waste and secondary pollution. Simple water washing is almost ineffective against Mg(OH)₂ in solid form.

[0004] Therefore, developing an innovative method that can selectively dissolve and deeply remove magnesium ions from alkaline sludge without affecting its main component CaCO3, and can also realize magnesium resource recovery, is of great significance for enhancing the resource value of alkaline sludge and promoting the circular economy of power plants. Summary of the Invention

[0005] This application provides a method for removing magnesium ions from alkaline sludge, aiming to solve the problem that the calcium-based skeleton of sludge is destroyed by the direct acid dissolution method, thus reducing the quality of sludge reuse. It also makes up for the problem that the direct water washing method has a low removal rate of solid Mg(OH)2.

[0006] This application provides a method for removing magnesium ions from alkaline sludge, comprising the following steps: (1) Dewater the alkaline sludge and mix the dewatered alkaline sludge with water to obtain a mixture; (2) Carbonize the mixture obtained in step (1) to convert Mg(OH)2 in alkaline sludge into Mg(HCO3)2; (3) The mixture obtained in step (2) is subjected to solid-liquid separation to obtain primary magnesium-removing sludge and magnesium-containing carbonized liquid; the primary magnesium-removing sludge is washed to obtain magnesium-removing sludge.

[0007] According to some embodiments of the method for removing magnesium ions from alkaline sludge described in this application, in step (1), the content of MgO in the alkaline sludge is 8%-9% of the dry basis of the alkaline sludge.

[0008] According to some embodiments of the method for removing magnesium ions from alkaline sludge described in this application, the alkaline sludge includes power plant sludge.

[0009] According to some embodiments of the method for removing magnesium ions from alkaline sludge described in this application, in step (1), the water content of the alkaline sludge is reduced to 60%-80% by dewatering treatment.

[0010] According to some embodiments of the method for removing magnesium ions from alkaline sludge described in this application, in step (1), the dehydrated alkaline sludge is mixed with water to obtain a mixture with a solid content of 5%-15%.

[0011] According to some embodiments of the method for removing magnesium ions from alkaline sludge described in this application, the carbonization treatment is: under heating conditions, a mixture of CO2 and N2 gas is introduced into the mixture.

[0012] According to some embodiments of the method for removing magnesium ions from alkaline sludge as described in this application, the volume ratio of CO2 to N2 in the mixed gas is 1:(8-10).

[0013] According to some embodiments of the method for removing magnesium ions from alkaline sludge as described in this application, the flow rate of the mixed gas is 0.08-0.12 L / min.

[0014] According to some embodiments of the method for removing magnesium ions from alkaline sludge described in this application, the molar ratio of the amount of carbon dioxide introduced to the amount of magnesium ions in the alkaline sludge is (1.8-2.2):1.

[0015] According to some embodiments of the method for removing magnesium ions from alkaline sludge as described in this application, the heating temperature is 30-50°C.

[0016] According to some embodiments of the method for removing magnesium ions from alkaline sludge described in this application, the method further includes the steps of mixing a magnesium-containing carbonized liquid and a magnesium ion adsorbent, filtering, and obtaining an adsorbent loaded with magnesium ions and a magnesium removal liquid.

[0017] According to some embodiments of the method for removing magnesium ions from alkaline sludge as described in this application, the magnesium ion adsorbent includes SBA15 mesoporous molecular sieve and / or polystyrene microspheres.

[0018] According to some embodiments of the method for removing magnesium ions from alkaline sludge described in this application, the temperature for mixing the magnesium-containing carbonized liquid and the magnesium ion adsorbent is 20-30°C, and the time is 60-180 min.

[0019] The beneficial effects of this application include: the method for removing magnesium ions from alkaline sludge described in this application utilizes low-concentration carbon dioxide to selectively convert Mg(OH)2 into soluble magnesium bicarbonate, while the generated CaCO3 is basically insoluble, thus achieving efficient separation of magnesium and calcium in the sludge.

[0020] The method for removing magnesium ions from alkaline sludge described in this application not only yields high-quality sludge products (MgO content can be reduced to below 2%, meeting the requirements of high-quality building materials), but also recovers high-purity Mg(OH)2 from the desorption liquid, which can be used as a flame retardant or chemical raw material, thus realizing a closed-loop recovery of magnesium resources. Detailed Implementation

[0021] The embodiments of the present invention are described in detail below. These embodiments are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0022] In this invention, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to a specific feature, structure, material, or characteristic described in connection with that embodiment or example, which is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0023] This application provides a method for removing magnesium ions from alkaline sludge, comprising the following steps: (1) Dewater the alkaline sludge and mix the dewatered alkaline sludge with water to obtain a mixture; (2) Carbonize the mixture obtained in step (1) to convert Mg(OH)2 in alkaline sludge into Mg(HCO3)2; (3) The mixture obtained in step (2) is subjected to solid-liquid separation to obtain primary magnesium-removing sludge and magnesium-containing carbonized liquid; the primary magnesium-removing sludge is washed to obtain magnesium-removing sludge.

[0024] The method for removing magnesium ions from alkaline sludge described in this application first dewaters the alkaline sludge to remove chloride ions and other water-soluble impurities contained in the alkaline wastewater. Then, low-concentration carbon dioxide is used to selectively convert Mg(OH)2 into soluble magnesium bicarbonate, while the generated CaCO3 is essentially insoluble, achieving efficient separation of magnesium and calcium from the sludge.

[0025] In some embodiments of this application, in step (1), the MgO content in the alkaline sludge is 8%-9% of the dry basis of the alkaline sludge; for example, 8%, 8.5%, 9%, etc.

[0026] In some embodiments of this application, the alkaline sludge includes power plant sludge.

[0027] In some embodiments of this application, in step (1), the water content of the alkaline sludge is reduced to 60%-80% by dewatering treatment; for example, 60%, 68%, 70%, 75%, 80%, etc.

[0028] In some embodiments of this application, in step (1), the dehydrated alkaline sludge is mixed with water to obtain a mixture with a solid content of 5%-15%, such as 5%, 8%, 10%, 12%, 15%, etc.

[0029] In some embodiments of this application, the carbonization treatment involves introducing a mixture of CO2 and N2 gas into the mixture under heating conditions. Introducing a mixed gas instead of directly introducing carbon dioxide allows for precise "fine-tuning" of the reaction pH, accurately controlling and terminating the pH of the entire system within the range of 8.5-9.5. This ensures that only Mg(OH)2 dissolves, while CaCO3 remains intact in the sludge. Directly introducing CO2 would cause the pH to drop rapidly; when the pH drops to 7.5-8.0, CaCO3 would dissolve to form Ca(HCO3)2, damaging the carbon-based framework.

[0030] In some embodiments of this application, the volume ratio of CO2 to N2 in the mixed gas is 1:(8-10). This volume ratio is designed because, on the one hand, high concentrations of CO2 pose a risk of suffocation when reacting in a closed container, and the mild acidification reaction can reduce equipment corrosion and maintenance costs. On the other hand, in actual industrial applications, the CO2 content in power plant flue gas is approximately 8%-15% (depending on fuel and combustion efficiency), with the remainder mainly being N2. The laboratory simulation of the actual composition of power plant flue gas can provide a reference for the subsequent direct utilization of power plant flue gas to achieve "waste treatment with waste". In some embodiments of this application, the flow rate of the mixed gas is 0.08-0.12 L / min, such as 0.08 L / min, 0.09 L / min, 0.1 L / min, 0.12 L / min, etc.

[0031] In some embodiments of this application, the molar ratio of the amount of carbon dioxide introduced to the magnesium ions in the alkaline sludge is (1.8-2.2):1, for example, 1.8:1, 2:1, 2.2:1, etc.

[0032] In some embodiments of this application, the heating temperature is 30-50℃, such as 30℃, 40℃, 45℃, 50℃, etc. 30-50℃ is the optimal balance point between reaction rate (requiring heating) and gas solubility / product stability (requiring cooling). Increasing the temperature can significantly accelerate the reaction rate of Mg(OH)2 and CO2, but when the temperature is higher than 60℃, CO2 gas is difficult to dissolve into the slurry, and bubbles will escape rapidly, resulting in a sharp decrease in gas utilization and reaction efficiency. At the same time, Mg(HCO3)2 is easily decomposed into insoluble MgCO3·3H2O or basic magnesium carbonate at >60℃, resulting in "reverse dissolution" precipitation. Therefore, it is necessary to control the temperature within this range.

[0033] In some embodiments of this application, the method further includes the steps of mixing the magnesium-containing carbonization liquid and the magnesium ion adsorbent, filtering, and obtaining an adsorbent loaded with magnesium ions and a magnesium-removing liquid. The magnesium ion adsorbent is used to remove Mg from the magnesium-containing carbonization liquid. 2+ Absorption can be performed from complex background ions rich in Ca. 2+ Na + K + Deep capture of Mg in isocationic solutions 2+ The obtained magnesium removal solution can be used to rinse the magnesium-removed sludge to further remove residual soluble magnesium. The saturated magnesium ion adsorbent can be desorbed using dilute hydrochloric acid solution. The pH of the desorbed solution can be adjusted to 10-11 by adding NaOH, and high-purity Mg(OH)2 precipitate can be recovered. The adsorbent after desorption treatment can be recycled and reused.

[0034] The magnesium ion adsorbent yields high-purity Mg(OH)2, which can be used as a flame retardant or chemical raw material, realizing a closed-loop recycling of magnesium resources.

[0035] In some embodiments of this application, the magnesium ion adsorbent comprises SBA15 mesoporous molecular sieve and / or polystyrene microspheres.

[0036] In some embodiments of this application, the temperature for mixing the magnesium-containing carbonization liquid and the magnesium ion adsorbent is 20-30°C, and the time is 60-180 min, such as 60 min, 80 min, 120 min, 150 min, 180 min, etc.

[0037] The technical solution of this application will be further described below with reference to specific embodiments.

[0038] Example 1 Alkaline sludge (initial MgO content of 8.5% dry basis) produced by the wastewater treatment station of a power plant's circulating water system was dewatered to a moisture content of 70% and then mixed with water to form a slurry with a solid content of 10%.

[0039] Take 1 L of the above slurry into a pressurized reactor, and introduce a mixed gas containing 10% CO2 diluted with N2 at a flow rate of 0.1 L / min. Control the reaction temperature at 40℃ and monitor it online with a pH meter. Stop the gas supply when the pH of the system stabilizes at 9.0. The total reaction time is about 60 minutes. At this time, the molar ratio of carbon dioxide introduced to MgO in the alkaline sludge is 2:1. After the reaction was completed, solid-liquid separation was performed by vacuum filtration to obtain primary magnesium-removed sludge and magnesium-containing carbonized liquid.

[0040] Take 500 mL of the above magnesium-containing carbide solution (Mg 2+ (Concentration approximately 520 mg / L) 3.0 g of aminophosphonic acid-functionalized SBA-15 mesoporous molecular sieve was added, and adsorption was carried out at 25 °C with stirring at 150 rpm for 120 minutes.

[0041] After adsorption, solid-liquid separation was performed to obtain purified effluent. ICP-OES analysis showed that the effluent contained Mg. 2+ The concentration dropped to 8 mg / L.

[0042] The primary magnesium-removing sludge was rinsed with 200 mL of the above-mentioned purified effluent and dewatered again. The MgO content of the final sludge product was measured to be 2.8%, which meets the requirements of high-quality cement admixtures for magnesium content (usually MgO < 3%).

[0043] Example 2 The method for removing magnesium ions from alkaline sludge described in Example 2 differs from that in Example 1 only in that the volume ratio of carbon dioxide to nitrogen in the mixed gas introduced during the implementation of the method for removing magnesium ions from alkaline sludge described in Example 2 is 2:8.

[0044] Example 3 The method for removing magnesium ions from alkaline sludge described in Example 3 differs from that in Example 1 only in that the volume ratio of carbon dioxide to nitrogen in the mixed gas introduced during the implementation of the method for removing magnesium ions from alkaline sludge described in Example 3 is 3:7.

[0045] Example 4 The method for removing magnesium ions from alkaline sludge described in Example 4 differs from that in Example 1 only in that the volume ratio of carbon dioxide to nitrogen in the mixed gas introduced during the implementation of the method for removing magnesium ions from alkaline sludge described in Example 4 is 4:6.

[0046] Example 5 The method for removing magnesium ions from alkaline sludge described in Example 5 differs from that in Example 1 only in that the heating temperature during the carbonization process in the method for removing magnesium ions from alkaline sludge described in Example 5 is 30°C.

[0047] Example 6 The method for removing magnesium ions from alkaline sludge described in Example 6 differs from that in Example 1 only in that the heating temperature during the carbonization process in the method for removing magnesium ions from alkaline sludge described in Example 6 is 35°C.

[0048] Example 7 The method for removing magnesium ions from alkaline sludge described in Example 7 differs from that in Example 1 only in that the heating temperature during the carbonization process in the method for removing magnesium ions from alkaline sludge described in Example 7 is 45°C.

[0049] The results of the methods for removing magnesium ions from alkaline sludge described in Examples 1-7 of this application are shown in Table 1.

[0050] Table 1

[0051] Table 1 shows that as the volume ratio of carbon dioxide to nitrogen increases, the pH of the reaction system decreases, leading to a reduction in the formation of Mg(HCO3)2 and a gradual increase in the magnesium oxide content in the demagnesified sludge. As the reaction temperature gradually increases, the magnesium oxide content in the demagnesified sludge initially decreases, reaching its lowest value at 40℃, and then begins to rise again. Therefore, the lowest magnesium oxide content in the demagnesified sludge is achieved when the volume ratio of carbon dioxide to nitrogen is 1:9 and the reaction temperature is 40℃.

[0052] Although the above embodiments have been shown and described, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Any changes, modifications, substitutions and variations made to the above embodiments by those skilled in the art are within the protection scope of the present invention.

Claims

1. A method for removing magnesium ions from an alkaline sludge, characterized in that, The method comprises the following steps: (1) performing dewatering treatment on the alkaline sludge, and mixing the dewatered alkaline sludge with water to obtain a mixture; (2) performing carbonization treatment on the mixture obtained in step (1) to convert Mg(OH)2 in the alkaline sludge into Mg(HCO3)2; (3) performing solid-liquid separation on the mixture obtained in step (2) to obtain primary demagnesium sludge and a magnesium-containing carbonization liquid, and performing leaching treatment on the primary demagnesium sludge to obtain demagnesium sludge.

2. The method of removing magnesium ions from an alkaline sludge according to claim 1, wherein, In step (1), the content of MgO in the alkaline sludge is 8%-9% of the dry basis of the alkaline sludge; And / or, the alkaline sludge comprises power plant sludge.

3. The method of removing magnesium ions from an alkaline sludge according to claim 1, wherein, In step (1), the dewatering treatment reduces the water content of the alkaline sludge to 60%-80%; And / or, in step (1), the dewatered alkaline sludge is mixed with water to obtain a mixture with a solid content of 5%-15%.

4. The method of removing magnesium ions from an alkaline sludge of claim 1, wherein, The carbonization treatment is: under heating conditions, a mixed gas of CO2 and N2 is introduced into the mixture.

5. The method of removing magnesium ions from an alkaline sludge according to claim 4, wherein, The volume ratio of CO2 to N2 in the mixed gas is 1:(8-10). And / or, the flow rate of the mixed gas introduced is 0.08-0.12 L / min.

6. The method of removing magnesium ions from an alkaline sludge according to claim 4, wherein, The amount of carbon dioxide introduced and the molar ratio of magnesium ions in the alkaline sludge are (1.8-2.2):

1.

7. The method of removing magnesium ions from an alkaline sludge according to claim 4, wherein The temperature of the heating is 30-50℃.

8. The method of removing magnesium ions from an alkaline sludge of claim 1, wherein, Further comprising the step of mixing the magnesium-containing carbonization liquid with a magnesium ion adsorbent, filtering to obtain a magnesium ion-loaded adsorbent and a magnesium-removed liquid.

9. The method of removing magnesium ions from an alkaline sludge according to claim 8, wherein, The magnesium ion adsorbent comprises SBA15 mesoporous molecular sieve and / or polystyrene microspheres.

10. The method of removing magnesium ions from an alkaline sludge of claim 8, wherein, The temperature for mixing the magnesium-containing carbonization liquid with the magnesium ion adsorbent is 20-30℃, and the time is 60-180 min.

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