CN107098445B - Method for separating magnesium from seawater and recycling magnesium ammonium phosphate in wastewater by using selective electrodialysis - Google Patents

Abstract

A method for separating magnesium from seawater and recycling waste water magnesium ammonium phosphate by utilizing selective electrodialysis belongs to the field of water pollution control and resource regeneration, and comprises the following working procedures: (1) removing suspended and colloidal particles in seawater by adopting a pretreatment method, then carrying out solid-liquid separation, pumping the supernatant into a selective electrodialysis system for Mg²⁺Separation and concentration; (2) adding the polar liquid obtained by the selective electrodialysis system in the step (1) into nitrogen and phosphorus-containing wastewater to perform magnesium ammonium phosphate crystallization precipitation reaction, and adding alkali to control the pH value of the reaction to be stabilized at 8.0-11.0 until the reaction is finished; and then carrying out solid-liquid separation, and recovering the obtained precipitate, namely the recovered magnesium ammonium phosphate product. The method for separating and concentrating magnesium from seawater by using selective electrodialysis provided by the invention provides magnesium salts for recycling waste water magnesium ammonium phosphate, effectively reduces the recycling cost of waste water nitrogen and phosphorus, and achieves the purpose of promoting the recycling of waste water nitrogen and phosphorus resources.

Description

A method to separate magnesium from seawater by selective electrodialysis and use it in wastewater magnesium ammonium phosphate method of recycling

technical field

The invention belongs to the field of water pollution control and resource regeneration, and in particular relates to a method for separating and concentrating magnesium from seawater by electrodialysis, and using magnesium salts for recovery of ammonium magnesium phosphate from wastewater.

Background technique

A large amount of nitrogen and phosphorus in wastewater is one of the main causes of water eutrophication, which has a serious negative impact on the quality of the water environment and has become a major environmental problem. The removal methods of nitrogen and phosphorus in wastewater mainly include biological methods and physical and chemical methods. The biological method has the characteristics of low operating cost and simple operation and management, but the biological method is more suitable for the treatment of low-concentration nitrogen and phosphorus wastewater, while the physical and chemical methods are more advantageous for the treatment of high-concentration nitrogen and phosphorus wastewater. On the other hand, phosphorus is an indispensable nutrient element for all life forms, and it is also an important chemical raw material. Especially the limited resources of phosphate rock in nature make it necessary to realize the recycling of phosphorus resources. The magnesium ammonium phosphate (MgNH ₄ PO ₄ ·6H ₂ O, commonly known as struvite) precipitation method is to add Mg ²⁺ to the sewage containing phosphate and ammonia nitrogen, and the reaction generates insoluble magnesium ammonium phosphate precipitation. The magnesium ammonium phosphate precipitation method can not only remove and recover nitrogen and phosphorus at the same time to reduce the nitrogen and phosphorus load in the wastewater, but also the recovered magnesium ammonium phosphate is an efficient slow-release fertilizer that can be reused in agriculture.

At present, the bottleneck restricting the industrial application of the ammonium-magnesium phosphate method is the high operating cost of the process, and an important factor is the choice of magnesium salt precipitant. The currently commonly used magnesium salt precipitant is MgCl ₂ , and Mg(OH) ₂ , magnesite and MgO are also used as magnesium sources. However, these precipitants limit the popularization and application of the ammonium-magnesium phosphate precipitation method due to the price factor. Magnesium is widely present in seawater and is an important potential source of magnesium in the recovery process of magnesium ammonium phosphate from wastewater. However, the concentration of magnesium in seawater is relatively low. Directly adding seawater to wastewater with magnesium ammonium phosphate precipitation reaction not only dilutes the concentration of nitrogen and phosphorus in wastewater, but also increases the difficulty of recovery of magnesium ammonium phosphate; it also greatly increases the amount of tail water, causing Subsequent tailwater treatment costs. Therefore, developing an economical and feasible method for separating and concentrating magnesium salts from seawater can greatly reduce the recovery cost of magnesium ammonium phosphate from wastewater and achieve sustainable development of the environment and economy.

SUMMARY OF THE INVENTION

The invention provides a method for separating and concentrating magnesium from seawater by selective electrodialysis, providing magnesium salt for the recovery of ammonium magnesium phosphate in wastewater, reducing the cost of nitrogen and phosphorus recovery in wastewater, and achieving the purpose of recycling nitrogen and phosphorus resources in wastewater. The present invention is characterized in that the method steps are:

(1) The suspended and colloidal particles in the seawater are removed by the pretreatment method, and then the solid-liquid separation is performed, and the supernatant is pumped into the set selective electrodialysis system for separation and concentration of magnesium;

(2) The polar liquid obtained by the selective electrodialysis system in step (1) is added to nitrogen and phosphorus-containing wastewater to carry out ammonium magnesium phosphate crystallization precipitation reaction, and the pH value of the reaction is controlled by adding alkali to stabilize at 8.0-11.0 until the reaction ends. ; Solid-liquid separation again, the obtained precipitate is the recovered magnesium ammonium phosphate product;

(3) The pretreatment method for the suspended and colloidal particulate matter in the seawater described in step (1) is a combination of the following methods: coagulation, sedimentation, filtration and membrane filtration;

(4) The selective electrodialysis system described in step (1) consists of electrode chambers on both sides and a chamber sandwiched between the electrode chambers on both sides; the chamber includes one or more chamber units, Each chamber unit consists of an electrode chamber, a concentrate chamber and a feed chamber which are arranged in sequence. The electrode chamber and the concentrate chamber are separated by a monovalent cation selective exchange membrane (MVK). The concentrate chamber and the feed chamber It is separated by anion exchange membrane (SA); if there are multiple chamber units, the material liquid chamber and the polar liquid chamber of the next chamber unit are separated by cation exchange membrane (SK); adjacent to the electrode chambers on both sides The chamber unit and the electrode chamber are separated by a unipolar cation exchange membrane (PC-SC); the electrode chamber adjacent to the material liquid chamber is the cathode chamber, the electrode chamber adjacent to the electrode chamber is the anode chamber, and the anode chamber is the anode chamber. An anode plate connected to the positive electrode of the DC power supply is arranged indoors, and a cathode plate connected to the negative electrode of the DC power supply is arranged in the cathode chamber; the cation exchange layer of the MVK membrane faces the anode chamber during installation;

(5) The operation method of the selective electrodialysis system described in step (1) is as follows: the seawater is circulated and pumped into the material-liquid chamber of the selective electrodialysis system, and the strong electrolyte solution with a concentration of not less than 0.05 mol/L is circulated by the pump into the polar liquid chamber of the selective electrodialysis system, pump the strong electrolyte solution with a concentration of not less than 0.05 mol/L into the concentrated liquid chamber of the selective electrodialysis system, and pump the electrode cleaning solution with a concentration of not less than 0.05 mol/L Circulating pump into the electrode chamber of the selective electrodialysis system; applying direct current to the selective electrodialysis system through the DC power supply for selective electrodialysis, and completing the separation of magnesium in seawater when the conductivity of the material liquid chamber is not lower than 5ms/cm and concentrated;

(6) The material liquid chamber of the selective electrodialysis system described in step (5) is seawater; the strong electrolyte solutions in the polar liquid chamber and the concentrated liquid chamber of the selective electrodialysis system are sodium chloride and potassium chloride , sodium sulfate or potassium sulfate solution; the electrode cleaning solution in the electrode chamber of the selective electrodialysis system is sodium sulfamate or sodium sulfate solution;

(7) The selective electrodialysis system described in step (5) operates at a current density of 10-100 mA/cm ² .

The method proposed by the invention first removes the suspended or colloidal impurities of seawater through pretreatment, and then separates and concentrates Mg ²⁺ through selective electrodialysis to obtain high-concentration Mg ²⁺ -containing concentrated water (1-10 g/L ) for the recovery of ammonium and magnesium phosphate in wastewater, which can greatly reduce the cost of nitrogen and phosphorus recovery in wastewater, and help promote the resource disposal and utilization of nitrogen and phosphorus-containing wastewater.

Description of drawings

Fig. 1 Schematic diagram of selective electrodialysis membrane stack.

Detailed ways

The embodiments described below are only some of the embodiments of the present invention, but not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present invention.

Example 1:

Suspended solid SS 12 mg/L, colloidal particulate matter 6 mg/L, Mg ²⁺ 800 mg/L, Na ⁺ 10 g/L, Cl ^- 12 g/L and SO ₄ ^2- 2000 mg/L in a seawater. A livestock and poultry breeding wastewater, the initial pH value is 7.5, and the concentrations of NH ₄ ⁺ and PO ₄ ^3- are 1080 mg/L and 96 mg/L, respectively. The seawater is pretreated by nanofiltration membrane. The supernatant after nanofiltration is pumped into the set selective electrodialysis system for separation and concentration of Mg ²⁺ , and the polar solution containing Mg ²⁺ 5g/L is obtained in the selective electrodialysis system; then the polar solution is added The livestock and poultry breeding wastewater was subjected to crystallization reaction of magnesium ammonium phosphate, and NaOH was added to control the pH value to be stable at 9.0-9.5; after the reaction, the content of magnesium ammonium phosphate in the recovered precipitate was 99.5%.

The selective electrodialysis system set up in the above steps is composed of electrode chambers on both sides and a chamber sandwiched between the electrode chambers on both sides, and the size of each pair of membranes used is 80×80 mm; the chambers include 5 Chamber unit, each chamber unit is composed of an electrode chamber, a concentrate chamber and a feed chamber arranged in sequence. The electrode chamber and the concentrate chamber are separated by a monovalent cation selective exchange membrane (MVK), and the concentrate chamber The material-liquid chamber is separated by an anion-exchange membrane (SA); the material-liquid chamber and the electrode chamber of the next chamber unit are separated by a cation-exchange membrane (SK); the chamber units adjacent to the electrode chambers on both sides are separated from The electrode chamber is separated by a unipolar cation exchange membrane (PC-SC); the electrode chamber adjacent to the material liquid chamber is the cathode chamber, the electrode chamber adjacent to the electrode chamber is the anode chamber, and the anode chamber is provided with a direct current connection. An anode plate connected to the positive pole of the power supply, and a cathode plate connected to the negative pole of the DC power supply is arranged in the cathode compartment; the cation exchange layer of the MVK membrane faces the anode compartment during installation.

The operation mode of the selective electrodialysis system in the above steps is as follows: the seawater is circulated and pumped into the material-liquid chamber of the selective electrodialysis system, and the 0.5 mol/L sodium chloride solution is circulated and pumped into the polar liquid chamber of the selective electrodialysis system. , the 0.5 mol/L sodium chloride solution was pumped into the concentrated liquid chamber of the selective electrodialysis system, and the 1 mol/L sodium sulfamate solution was pumped into the electrode chamber of the selective electrodialysis system; Selective electrodialysis was carried out by applying direct current to the selective electrodialysis system, and the current density was controlled to be 30 mA/cm ² ; the separation and concentration of magnesium in seawater were completed when the conductivity of the material-liquid chamber was 6 ms/cm.

Claims (2)

1. a method utilizing selective electrodialysis to separate magnesium from seawater and be used for the recovery of waste water magnesium ammonium phosphate, it is characterised in that the method steps are: (1) adopt the pretreatment method to remove the suspended and colloidal particulate matter in the seawater, then solid-liquid separation, the supernatant is pumped into the set selective electrodialysis system to carry out the separation and concentration of Mg ²⁺ ; the described pretreatment The method is a combination of the following methods: coagulation, precipitation, and filtration; the selective electrodialysis system is composed of electrode chambers on both sides and a chamber sandwiched between the electrode chambers on both sides; the chamber includes one or more Chamber unit, each chamber unit consists of an electrode chamber, a concentrate chamber and a feed chamber which are arranged in sequence. The electrode chamber and the concentrate chamber are separated by a monovalent cation selective exchange membrane. The concentrate chamber and the feed chamber The chamber is separated by an anion exchange membrane; if there are multiple chamber units, the material liquid chamber and the polar liquid chamber of the next chamber unit are separated by a cation exchange membrane; the chamber units adjacent to the electrode chambers on both sides are separated from each other. The electrode chambers are separated by a unipolar cation exchange membrane; the electrode chamber adjacent to the material liquid chamber is the cathode chamber, the electrode chamber adjacent to the electrode chamber is the anode chamber, and the anode chamber is provided with a positive electrode connected to the DC power supply. The anode plate, the cathode chamber is provided with a cathode plate connected to the negative electrode of the DC power supply; the cationic exchange layer of the monovalent cation selective exchange membrane faces the anode chamber during installation; (2) adding the polar liquid obtained by the selective electrodialysis system in the step (1) into the nitrogen-containing phosphorus waste water to carry out the crystallization precipitation reaction of magnesium ammonium phosphate, and the pH value of the reaction is controlled to be stable at 8.0-11.0 by adding alkali until the reaction finishes; Then solid-liquid separation, the obtained precipitate is the recovered magnesium ammonium phosphate product; The material liquid chamber is seawater, the strong electrolyte solution in the polar liquid chamber and the concentrated liquid chamber is sodium chloride, potassium chloride, sodium sulfate or potassium sulfate solution, and the electrode cleaning solution in the electrode chamber is sodium sulfamate or sulfuric acid sodium. 2. a kind of method that utilizes selective electrodialysis to separate magnesium from seawater and is used for the recovery of waste water magnesium ammonium phosphate according to claim 1, it is characterized in that, described selective electrodialysis system operation method is as follows: by seawater circulation Pump into the material liquid chamber of the selective electrodialysis system, circulate and pump the strong electrolyte solution with a concentration of not less than 0.05mol/L into the polar liquid chamber of the selective electrodialysis system, and pump the strong electrolyte solution with a concentration of not less than 0.05mol/L into the liquid chamber of the selective electrodialysis system. The solution is circulated and pumped into the concentrate chamber of the selective electrodialysis system, and the electrode cleaning solution with a concentration of not less than 0.05mol/L is circulated and pumped into the electrode chamber of the selective electrodialysis system; DC power is applied to the selective electrodialysis system through a DC power supply. Selective electrodialysis is carried out, the current density is controlled to be 10-100 mA/cm ² , and the separation and concentration of magnesium in seawater are completed under the condition that the conductivity of the material-liquid chamber is not lower than 5 ms/cm.

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