Method for decalcifying by selectively precipitating magnesium ions in high-calcium and high-magnesium wastewater

Abstract

The invention discloses a method for decalcifying by selectively precipitating magnesium ions in high-calcium and high-magnesium wastewater. The method comprises the following steps: pretreating the high-calcium and high-magnesium wastewater, filtering to remove impurities, and adjusting the pH value to obtain a pretreatment solution with the pH value of 9.5-10.5; adding phosphate into the pretreatment liquid to carry out auxiliary precipitation, so that Mg (OH) 2 crystals grow to obtain reaction slurry; introducing the reaction slurry into a sedimentation tank to settle Mg (OH) 2 crystals, and introducing supernate in the sedimentation tank into a decalcification tank; adding 100-200mg / L of polyaluminum chloride and 5-10mg / L of polyacrylamide into the decalcification tank, stirring and reacting, and introducing the reaction liquid into a decalcification sedimentation tank; carrying out solid-liquid separation in the decalcification sedimentation tank to obtain decalcified supernate; and carrying out deep decalcification on the decalcified supernate by using a selective ion exchange method. The method creates conditions for selective precipitation calcium removal and magnesium ion crystallization recovery of the high-calcium and high-magnesium wastewater.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power plant circulating water treatment, in particular to a method for selectively precipitating magnesium ions in high calcium and magnesium wastewater for decalcification. BACKGROUND

[0002] With the deepening of the industrial water-saving and emission reduction target, the industrial circulating cooling water system, as a major water consumer in power plants, its water saving, consumption reduction and resource recycling potential have become the focus of attention. The calcium and magnesium ion concentrations in the long-term circulating and concentrated circulating water in power plants are extremely high, and it also contains suspended solids (silt, corrosion products), carbonates, sulfates, silicates and other impurities. The damage mechanism and removal conditions of different impurities are different.

[0003] The high calcium and magnesium wastewater produced by the long-term circulating and concentrated circulating water in power plants not only causes environmental burden, but also leads to serious waste of magnesium resources. At present, the mainstream high calcium and magnesium wastewater treatment process is mainly chemical precipitation softening method, but this method has the problems of large occupation area and high sludge treatment load, which seriously restricts its application and development. Although some existing technologies can effectively remove calcium ions in circulating water in power plants (the removal rate can reach more than 90%), but in the process of removing calcium and magnesium ions, the high-purity magnesium hydroxide or magnesium carbonate crystal by-products produced are often treated as solid waste, which cannot be reasonably utilized, resulting in significant waste of magnesium resources.

[0004] Therefore, based on the difference in solubility product of calcium and magnesium ions corresponding precipitates, by accurately controlling the reaction conditions and reagent addition strategy, magnesium ions are preferentially formed into insoluble precipitates and separated and removed (while calcium ions are inhibited from co-precipitation), and then the calcium-rich liquid phase is subjected to targeted decalcification treatment, so as to realize efficient separation of calcium and magnesium and deep purification of wastewater, and lay a foundation for magnesium resource recovery thereafter, which has important research significance and application value. SUMMARY

[0005] The present application aims to at least solve one of the technical problems in the related art. The present application provides a method for selectively precipitating magnesium ions in high calcium and magnesium wastewater for decalcification, which utilizes the difference in solubility product of calcium and magnesium ions corresponding precipitates to preferentially form magnesium ions into insoluble precipitates for separation and removal, so as to facilitate subsequent research on magnesium recovery, and the hardness of high calcium and magnesium wastewater can be less than 10 mg / L by using the method.

[0006] To this end, an embodiment of the present application provides a method for selectively precipitating magnesium ions in high calcium and magnesium wastewater for decalcification, comprising the following steps: The high calcium and magnesium wastewater is pretreated and filtered to remove impurities, and then the pH is adjusted to obtain a pretreated liquid with a pH of 9.5-10.5; The pretreatment liquid is introduced into a reaction tank to add phosphate for auxiliary precipitation to make Mg(OH)2crystal grow, and a reaction slurry is obtained after reaction; wherein the addition amount of the phosphate is 0.5-1.0 times of the concentration of magnesium ions in the pretreatment liquid; The reaction slurry is introduced into a precipitation tank to make the Mg(OH)2crystal settle, and the supernatant in the precipitation tank is introduced into a decalcification tank; 100-200 mg / L of polyaluminum chloride and 5-10 mg / L of polyacrylamide are added into the decalcification tank, and after stirring reaction, the reaction liquid is introduced into a decalcification precipitation tank; The decalcification precipitation tank is subjected to solid-liquid separation to obtain decalcification supernatant; and the decalcification supernatant is subjected to deep decalcification by selective ion exchange method, so that the hardness of the treated decalcification supernatant is not more than 10 mg / L.

[0007] In some embodiments, the pretreatment process of the high-calcium and high-magnesium wastewater includes solid-liquid separation, water quality balance adjustment, and impurity ion treatment.

[0008] In some embodiments, the impurity ion treatment of the high-calcium and high-magnesium wastewater includes treatment of iron ions and silicon ions; wherein the iron ions are removed by activated carbon; the addition amount of the activated carbon is 50-100 mg / L, and the stirring reaction time is 30-40 min; and / or, the silicon ions are removed by polyaluminum chloride, and the addition amount of the polyaluminum chloride is 100-150 mg / L.

[0009] In some embodiments, the concentration ratio of calcium ions to magnesium ions in the high-calcium and high-magnesium wastewater is not more than 2.

[0010] In some embodiments, after the high-calcium and high-magnesium wastewater is pretreated, lime milk is used to adjust the pH; and the stirring speed is 200-250 r / min.

[0011] In some embodiments, the phosphate includes sodium phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, or disodium hydrogen phosphate.

[0012] In some embodiments, the pretreatment liquid is introduced into the reaction tank to add the phosphate, and after 40-60 min of reaction, the reaction slurry is introduced into the precipitation tank.

[0013] In some embodiments, the reaction slurry is introduced into the precipitation tank and stays for 2.5-3.5 h; and the obtained precipitate is dehydrated and processed.

[0014] In some embodiments, the polyaluminum chloride and the polyacrylamide are added to the decalcification tank, and the reaction is stirred for 15-30 min; the surface load of the decalcification sedimentation tank is 1.0-1.5 m³ / (m²·h); and the concentration of calcium ions in the decalcification supernatant is not more than 80 mg / L.

[0015] In some embodiments, the decalcification supernatant is subjected to decalcification treatment by using a weak acid cation exchange resin at an empty bed flow rate of 10-15 m / h.

[0016] Additional aspects and advantages of the present application will be made apparent from the following description. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 is a flow chart of a method for selectively precipitating magnesium ions for decalcification of high-calcium and high-magnesium wastewater according to an embodiment of the present application. DETAILED DESCRIPTION

[0018] Embodiments of the present application are described in detail below with reference to examples shown in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to explain the present application, and should not be construed as limiting the present application.

[0019] To this end, an embodiment of the present application proposes a method for selectively precipitating magnesium ions for decalcification of high-calcium and high-magnesium wastewater, as shown in Figure 1 comprising the following steps: S1: After the high-calcium and high-magnesium wastewater is filtered to remove impurities after pretreatment, the pH is adjusted to obtain a pretreated liquid with a pH of 9.5-10.5; S2: The pretreated liquid is introduced into a reaction tank to add phosphates for auxiliary precipitation to grow Mg(OH)2crystals, and a reaction slurry is obtained after the reaction; wherein the addition amount of phosphates is 0.5-1.0 times the concentration of magnesium ions in the pretreated liquid; S3: The reaction slurry is introduced into a sedimentation tank to make the Mg(OH)2crystals settle, and the supernatant in the sedimentation tank is introduced into a decalcification tank; 100-200 mg / L of polyaluminum chloride and 5-10 mg / L of polyacrylamide are added to the decalcification tank, and after stirring and reaction, the reaction liquid is introduced into a decalcification sedimentation tank; S4: The decalcification supernatant is obtained after solid-liquid separation of the decalcification sedimentation tank; and the decalcification supernatant is subjected to deep decalcification by using a selective ion exchange method, so that the hardness of the treated decalcification supernatant is not more than 10 mg / L.

[0020] In step S1, the high-calcium-magnesium wastewater undergoes pretreatment filtration to remove impurities. This process includes solid-liquid separation, water quality equalization, and impurity ion treatment. The high-calcium-magnesium wastewater is filtered through a grid assembly for impurity filtration. The initial high-calcium-magnesium wastewater contains large particles such as hair and silt, large suspended solids, gravel, as well as suspended solids such as grease, scum, and some colloids. Before selective treatment of calcium and magnesium ions in the high-calcium-magnesium wastewater, pretreatment is necessary to remove macroscopic impurities, creating optimal conditions for the subsequent efficient, economical, and stable ion recovery unit. In this embodiment, the grid assembly includes a first grid, a second grid, and a multi-media filter layer arranged sequentially upstream and downstream; the spacing between the first grid, the second grid, and the multi-media filter layer decreases sequentially. For example, the spacing of the first grid is 5-20 mm and the spacing of the second grid is 1-10 mm. In some embodiments, the multi-media filter layer includes sand particles with a particle size of 0.5-1.2 mm, coal slag with a particle size of 2.0-5.0 mm and stones with a particle size of 1-2 mm, wherein the high-calcium and magnesium wastewater passes through the coal slag, sand particles and stones in sequence.

[0021] In particular, the multi-media filter layer of this application is composed of cinders, stones, and sand, wherein the cinders, stones, and sand are arranged sequentially as a cinder layer, a stone layer, and a sand layer. The particle size of the cinders is 2.0-5.0 mm, the particle size of the stones is 1-2 mm, and the particle size of the sand is 0.5-1.2 mm. The density of the cinders is less than that of the sand, and the density of the sand is less than that of the stones. For example, the density of the cinders is 0.6-1.0 g / cm³, the density of the sand is 2.0-2.5 g / cm³, and the density of the stones is not less than 2.65 g / cm³. According to the flow direction of the high-calcium-magnesium wastewater, the wastewater passes through the cinder layer, the sand layer, and the stone layer sequentially.

[0022] The coal slag layer has the lowest density and the coarsest particles. As the first line of defense for high-calcium-magnesium wastewater flowing through the multi-media filtration layer, it can intercept most of the larger suspended solids in the wastewater. Because of the coarse particles and large pores of the coal slag, the contaminants in the high-calcium-magnesium wastewater can penetrate to deeper areas, resulting in high dirt-holding capacity and preventing rapid surface clogging. The sand layer, located downstream of the coal slag layer, is used to intercept medium-sized suspended solids. Since the coal slag layer has removed large particles, the flow velocity of the wastewater is more stable when it reaches this layer, and the smaller pores provided by the sand layer allow for further fine filtration. The stone layer, as the final barrier, maintains its high density and remains stable at the bottom after backwashing.

[0023] The wastewater filtered by the bar screen assembly undergoes preliminary sedimentation in a pre-sedimentation unit. This pre-sedimentation unit is a circular, lower cone structure with an internal stirring device, and its cone bottom naturally forms a sludge compression zone. The settled sludge is continuously squeezed and concentrated at the cone bottom, resulting in a high solids content, facilitating subsequent sludge dewatering and reducing sludge volume. In this embodiment, the supernatant from the pre-sedimentation unit enters an equalization tank equipped with an aeration device for aeration and homogenization. This allows the supernatant entering the equalization tank at different times to remain for 6-12 hours, balancing water quality and quantity and reducing hardness fluctuations; achieving true water quality homogenization and stabilizing the effluent concentration.

[0024] Impurity ion treatment primarily targets iron and silicon ions. First, activated carbon is used to remove iron ions, with an addition amount of 50-100 mg / L. The reaction is stirred for 30-40 minutes to prevent co-precipitation of iron and magnesium ions, which would affect the purity of the precipitate. Then, polyaluminum chloride (PAC) is used to remove silicon ions (silicate), with an addition amount of 100-150 mg / L to remove silicate ions and ensure the selectivity of the subsequent magnesium ion precipitation reaction.

[0025] In the pretreatment stage of this application, the high-calcium and magnesium wastewater is pretreated and filtered to remove suspended solids, colloids, iron ions, and silicon ions, thus avoiding affecting the selective precipitation effect of magnesium ions and the operation of subsequent filtration equipment.

[0026] In this embodiment, the calcium-to-magnesium ion concentration ratio of the high-calcium-magnesium wastewater does not exceed 2. After pretreatment, a pH adjuster (such as lime slurry or sodium hydroxide) is added, and the solution is adjusted at a stirring speed of 200-250 r / min to obtain a pretreated solution with a pH of 9.5-10.5. Calcium hydroxide (lime slurry) or sodium hydroxide is used as a precipitant. By precisely controlling the pH value, magnesium ions are preferentially converted into Mg(OH)2 precipitate, while inhibiting the formation of CaCO3 precipitate from calcium ions. When the pH of the pretreated solution is <9.5, magnesium ion precipitation is incomplete; when the pH is >10.5, calcium ion co-precipitation is easily caused, reducing selectivity. Therefore, it is necessary to precisely control the dosage of lime slurry (or sodium hydroxide) to ensure efficient precipitation of magnesium ions while minimizing calcium ion precipitation.

[0027] In step S2, the pretreated liquid is passed into the reaction tank and phosphate is added to assist precipitation, promoting the growth of Mg(OH)2 crystals. After the reaction, a reaction slurry is obtained. The pretreated wastewater enters the reaction tank, the stirring device is turned on, and then phosphate is added to the reaction tank. The phosphate includes sodium phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, or disodium hydrogen phosphate. The amount of phosphate added is 0.5-1.0 times the magnesium ion concentration in the pretreated liquid, acting as an auxiliary precipitant to promote the growth of Mg(OH)2 crystals and improve precipitation separation efficiency.

[0028] In step S3, after adding phosphate to the reaction tank and reacting for 40-60 minutes, the resulting reaction slurry is passed into a sedimentation tank. In this embodiment, the sedimentation tank is an inclined tube sedimentation tank. The reaction slurry is held for 2.5-3.5 hours to allow the Mg(OH)2 crystals to settle completely. The supernatant in the sedimentation tank is passed into a decalcification tank for decalcification. The precipitate is dewatered by a plate and frame filter press (filter cake moisture content ≤65%) and can be recycled.

[0029] In this embodiment, 100-200 mg / L of polyaluminum chloride and 5-10 mg / L of polyacrylamide are added to the decalcification tank; after adding polyaluminum chloride and polyacrylamide, the mixture is stirred and reacted for 15-30 minutes; the surface loading of the decalcification sedimentation tank is 1.0-1.5 m³ / (m²·h), achieving efficient solid-liquid separation.

[0030] In step S4, after solid-liquid separation in the decalcification sedimentation tank, a decalcified supernatant is obtained, and the calcium ion concentration in the decalcified supernatant does not exceed 80 mg / L. The decalcified supernatant is then subjected to deep decalcification using a selective ion exchange method. For example, the decalcified supernatant is subjected to a selective ion exchange method using a weak acid cation exchange resin, such as an acrylic acid-based carboxylic acid resin. That is, an acrylic acid-based carboxylic acid resin tower is used to perform decalcification treatment at an empty bed flow rate of 10-15 m / h, so that the hardness of the treated decalcified supernatant does not exceed 10 mg / L.

[0031] The method described in this application achieves a magnesium ion removal rate of ≥94% and a calcium ion removal rate of ≥98% in high-calcium and magnesium wastewater, with the effluent quality meeting the Class I standard of the Integrated Wastewater Discharge Standard (GB8978-1996). Compared with the traditional calcium and magnesium co-precipitation method, the sludge production is reduced by 25%, while simultaneously realizing the resource utilization of sludge, thus achieving both environmental and economic benefits.

[0032] In the description of this application, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc., indicating the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application.

[0033] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0034] In this application, unless otherwise expressly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection, an electrical connection, or a connection that allows communication between components; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise expressly limited. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0035] In this application, unless otherwise expressly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "on top of," and "over" the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0036] In this application, 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 this application. 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.

[0037] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. A method for decalcification of high calcium and magnesium wastewater by selectively precipitating magnesium ions, characterized in that, The method comprises the following steps: The high-calcium magnesium wastewater is pretreated by filtering and removing impurities, and then the pH is adjusted to obtain a pretreated liquid with a pH of 9.5-10.5; The pretreated liquid is introduced into a reaction tank to add phosphate for auxiliary precipitation to grow Mg(OH)2 crystals, and a reaction slurry is obtained after reaction; wherein the addition amount of the phosphate is 0.5-1.0 times of the magnesium ion concentration in the pretreated liquid; The reaction slurry is introduced into a precipitation tank to make the Mg(OH)2 crystals settle, and the supernatant in the precipitation tank is introduced into a decalcification tank; 100-200 mg / L of polyaluminum chloride and 5-10 mg / L of polyacrylamide are added to the decalcification tank, and after stirring and reaction, the reaction liquid is introduced into a decalcification precipitation tank; The decalcification precipitation tank is subjected to solid-liquid separation to obtain a decalcification supernatant; and the decalcification supernatant is subjected to deep decalcification by a selective ion exchange method, so that the hardness of the treated decalcification supernatant is not more than 10 mg / L.

2. The method of claim 1, wherein, The pretreatment process of the high-calcium magnesium wastewater comprises solid-liquid separation, water quality balance adjustment and impurity ion treatment.

3. The method of claim 2, wherein, The impurity ion treatment of the high-calcium magnesium wastewater comprises treatment of iron ions and silicon ions; The iron ions are removed by activated carbon; the addition amount of the activated carbon is 50-100 mg / L, and the stirring reaction time is 30-40 min; And / or, the silicon ions are removed by polyaluminum chloride; the addition amount of the polyaluminum chloride is 100-150 mg / L.

4. The method according to claim 2 or 3, characterized in that, The concentration ratio of calcium ions to magnesium ions in the high-calcium magnesium wastewater is not more than 2.

5. The method of claim 4, wherein, The pH of the pretreated high-calcium magnesium wastewater is adjusted by lime milk; and the stirring speed is 200-250 r / min.

6. The method of claim 4, wherein, The phosphate comprises sodium phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate or disodium hydrogen phosphate.

7. The method of claim 6, wherein, The pretreated liquid is introduced into the reaction tank to add the phosphate and react for 40-60 min, and then the reaction slurry is introduced into the precipitation tank.

8. The method of claim 7, wherein, The reaction slurry is introduced into the precipitation tank and stays for 2.5-3.5 h; and the obtained precipitate is dehydrated and processed.

9. The method of claim 8, wherein, The polyaluminum chloride and the polyacrylamide are added to the decalcification tank and stirred and reacted for 15-30 min; the surface load of the decalcification precipitation tank is 1.0-1.5 m³ / (m²·h); and the calcium ion concentration in the decalcification supernatant is not more than 80 mg / L.

10. The method of claim 1, wherein, The selective ion exchange method for the decalcification supernatant comprises using a weak acid cation exchange resin to perform decalcification treatment at an empty bed flow rate of 10-15 m / h.

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