A method for resource utilization of ammonium chloride wastewater by using MgO as a chemical chain carrier

Abstract

This invention discloses a method for the resource utilization of ammonium chloride wastewater using magnesium oxide as a chemical chain carrier. The method includes: preheating the wastewater containing ammonium chloride, followed by evaporation, concentration, and crystallization to obtain solid ammonium chloride; then reacting the wastewater in an ammonia release reactor and a chlorine release reactor, which are integrated into a circulating fluidized bed reactor. In the circulating fluidized bed, the magnesium oxide chemical chain carrier circulates between the ammonia release reactor and the chlorine release reactor. The ammonia release reactor operates at a relatively low temperature (300-400°C), where ammonium chloride and the heated magnesium oxide carrier are thoroughly mixed to generate magnesium hydroxide and ammonia gas, achieving the ammonia release reaction. The chlorine release reactor maintains a higher temperature (600-800°C), where magnesium hydroxide pyrolyzes to generate hydrogen chloride gas and solid magnesium oxide, achieving the chlorine release reaction. Simultaneously, the heated magnesium oxide also acts as a heat carrier, meeting the heat requirements of the ammonia release reaction. This method effectively reduces energy consumption and improves efficiency.

Description

Technical Field

[0001] This invention belongs to the field of hydrometallurgical processes, and particularly relates to a method for the resource utilization of ammonium chloride wastewater using MgO as a chemical chain carrier. Background Technology

[0002] In my country's rare earth smelting process, the resulting ammonium chloride wastewater has a concentration of approximately 10%, containing small amounts of impurity ions and oil. It is typically treated using concentration crystallization and calcium oxide stripping methods. The ammonium chloride produced by concentration crystallization is sold as a byproduct, but due to its low price and high recycling costs, it offers no economic benefit. The stripping method involves adding quicklime to the ammonium chloride wastewater and stripping it to obtain ammonia and calcium chloride solution. The ammonia is reused in the rare earth production process, but the byproduct calcium chloride waste salt causes secondary pollution. Neither of these methods can achieve complete recycling of ammonia and chlorine resources. If an economical method is adopted to further decompose ammonium chloride into ammonia and hydrogen chloride, which can be recycled in the rare earth smelting process, it would not only eliminate the costs of ammonia and hydrochloric acid in the original rare earth smelting process but also completely solve the problem of ammonium chloride wastewater discharge from rare earth smelting.

[0003] The decomposition and separation of ammonium chloride are key steps in the aforementioned ammonium chloride wastewater treatment process. Ammonium chloride decomposes into NH3 and HCl gases at 330-350°C. Separating these two gases is very difficult; generally, an acidic or alkaline medium is used to first fix one of the gases, and then the two gases are released sequentially. Magnesium oxide is a suitable solid medium; it can absorb HCl and release NH3 at lower temperatures and regenerate and release HCl at higher temperatures. Patent CN104744209B discloses a moving bed reactor for the chemical loop recycling of ammonium chloride to recover ammonia and hydrogen chloride, achieving low-cost recovery of ammonia and chlorine from ammonium chloride. Patent CN105753016B discloses a multi-tube moving bed reactor for the pyrolysis separation of ammonium chloride to produce ammonia and hydrogen chloride. The chemical chain pyrolysis of ammonium chloride is the core of the patented technology mentioned above. This reaction includes two steps: "ammonia release" and "chlorine release." First, a mixture of ammonium chloride and MgO support is heated to approximately 350 °C. The ammonium chloride and MgO undergo an "ammonia release" reaction to generate magnesium hydroxide and release ammonia, which is then recovered. Next, the support is further heated to approximately 570 °C. The magnesium hydroxide undergoes a "chlorine release" reaction to convert to MgO and release HCl. Hydrogen chloride is recovered to produce hydrochloric acid, and the MgO support is regenerated. Furthermore, the energy consumption for recovering each ton of ammonium chloride is only about 0.15 tons of standard coal. This technology can effectively achieve low-cost, full-resource recovery and utilization of ammonium chloride wastewater. However, the reactor design mentioned in the patent is complex and has high operational requirements. The two-stage circulating fluidized bed proposed in this invention is a more advantageous solution. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a technology for the resource utilization of ammonium chloride wastewater using MgO as a chemical chain carrier. This invention employs ammonium chloride pyrolysis technology with magnesium oxide as the chemical chain carrier and its corresponding pyrolysis reactor technology, which can recover ammonium chloride wastewater to prepare ammonia water and hydrochloric acid, thus realizing the resource utilization of ammonium chloride wastewater.

[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solutions:

[0006] A method for the resource utilization of ammonium chloride wastewater using magnesium oxide as a chemical chain carrier includes the following steps:

[0007] 1) Wastewater containing ammonium chloride is preheated and then concentrated by evaporation to obtain solid ammonium chloride through crystallization;

[0008] 2) After the solid ammonium chloride is thoroughly mixed with the hot magnesium oxide carrier, it is fed into the ammonia release reactor to produce magnesium hydroxide and ammonia. The separated ammonia is recycled through the ammonia recovery unit, and the solid magnesium hydroxide and magnesium oxide carrier are sent to the chlorine release reactor.

[0009] 3) In the chlorine release reactor, the magnesium oxide carrier containing magnesium hydroxide is fully mixed and heat exchanged with the high-temperature flue gas from the gas boiler. The magnesium hydroxide undergoes a chlorine release reaction to generate hydrogen chloride gas and magnesium oxide solid. The separated hydrogen chloride is recycled through the hydrogen chloride recovery unit, and the solid magnesium oxide chemical chain carrier is recycled to the ammonia release reactor.

[0010] The ammonia release reactor and chlorine release reactor are integrated into a circulating fluidized bed reactor device.

[0011] The aforementioned ammonia release reactor and chlorine release reactor employ a two-stage circulating fluidized bed, which is also the core of this invention. The chemical chain pyrolysis of ammonium chloride occurring in the fluidized bed is a fluidized process that varies along the bed height. Within the fluidized bed reactor, multiple flow regimes—moving bed, bubbling bed, and turbulent bed—coexist from bottom to top, and the existence and size of each regime are determined by heat transfer and reaction conditions. Therefore, the heat transfer and reaction of the material in the fluidized bed affect its size.

[0012] Preferably, the ammonia release reactor is equipped with a gas distributor and a cyclone separator inside;

[0013] The ammonia release reactor is provided with an ammonium chloride inlet and a magnesium chloride carrier inlet on its side wall;

[0014] The ammonia release reactor is provided with a purge gas inlet and a magnesium oxide carrier containing magnesium hydroxychloride outlet at the bottom.

[0015] Preferably, the chlorine release reactor is equipped with a vertically arranged moving bed and a cyclone separator inside;

[0016] The bottom of the moving bed is provided with a high-temperature gas inlet and a magnesium oxide carrier inlet containing magnesium hydroxychloride;

[0017] The sidewall of the chlorine release reactor is provided with a purge gas inlet and a magnesium oxide carrier outlet.

[0018] The inventors have discovered that the operating conditions have a significant impact on the pyrolysis effect of ammonium chloride and the recovery effect of waste gas in this process, therefore, it is necessary to strictly limit the operating parameters.

[0019] Preferably, the concentration of ammonium chloride wastewater is 8-15 wt%.

[0020] As a preferred option, the multi-effect evaporator adopts a three-effect series connection.

[0021] Preferably, the temperature of the ammonia release reactor is controlled at 300~400 °C.

[0022] Preferably, the temperature of the hot magnesium oxide carrier at the outlet of the chlorine release reactor is controlled at 600~800 °C.

[0023] Preferably, the temperature of the high-temperature flue gas at the outlet of the natural gas boiler is controlled at 1600~2400 °C, and the oxygen concentration of the high-temperature flue gas is ≤3%.

[0024] Preferably, the temperature of the preheated air is controlled at 300-500 °C.

[0025] Compared with existing ammonium chloride wastewater treatment technologies, the advantages of this invention are:

[0026] 1) The ammonium chloride wastewater concentration and crystallization technology produces ammonium chloride, which is sold as a byproduct. However, due to the low price of ammonium chloride and the high cost of recycling, there is no economic benefit. This solution can convert ammonium chloride into valuable ammonia water and hydrochloric acid, thereby improving economic benefits.

[0027] 2) Compared with the ammonium chloride and calcium oxide stripping technology, the stripping method involves adding quicklime to ammonium chloride wastewater and stripping it to obtain ammonia water and calcium chloride solution. The by-product calcium chloride waste salt will cause secondary pollution to the environment. However, this solution can achieve the complete resource utilization of ammonium chloride.

[0028] 3) Compared with conventional bipolar membrane electrodialysis technology for treating ammonium chloride wastewater, the trace amounts of high-valence ions and other pollutants in the ammonium chloride wastewater can seriously affect the membrane life. The ammonium chloride wastewater entering the bipolar membrane device needs to undergo strict purification, and the high cost of the membrane leads to a significant increase in the wastewater purification cost. In contrast, the solution of this invention has lower energy consumption.

[0029] 4) In summary, this solution has both environmental and economic benefits in the field of ammonium chloride wastewater recovery technology. Attached Figure Description

[0030] Figure 1 Simplified process flow diagram for the resource utilization of ammonium chloride wastewater;

[0031] Figure 2 Simplified structural diagram of a two-stage circulating fluidized bed reactor. Detailed Implementation

[0032] As attached Figure 1 As shown in the figure, this is a simplified process flow diagram of the technical solution of the present invention. After preheating (stream 01), the ammonium chloride wastewater is concentrated and evaporated to remove water in a multi-effect evaporator (or MVR, E-101) to obtain solid ammonium chloride (stream 02). The ammonium chloride is sent to the ammonium chloride pyrolysis reactor (R-201) to fully contact with the circulating hot magnesium oxide and undergo the following ammonia release reaction:

[0033] (1)

[0034] After the ammonia release reaction has fully occurred, the generated magnesium hydroxide is separated by a two-stage cyclone separator and sent to the chlorination reactor R-201. The separated NH3 gas phase is heated by the ammonia waste boiler E-401 to generate steam, and then sent to the ammonia recovery unit (ammonia tower D-401), where the ammonia gas is absorbed by pure water to obtain ammonia water.

[0035] (2)

[0036] After the chlorine release reaction has fully occurred, the generated magnesium oxide solid is sent to the R-201 ammonia release reactor via a multi-stage cyclone separator. The reaction gas is then sent to the E-202 reactor for heat recovery, and further sent to the hydrochloric acid waste boiler E-301 to recover heat and generate steam. Finally, it is sent to the hydrochloric acid recovery unit (hydrochloric acid tower D-301) where hydrogen chloride is absorbed by pure water to obtain hydrochloric acid.

[0037] Figure 2 A simplified structural diagram of a two-stage circulating fluidized bed reactor is shown below. Figure 2 As shown, the ammonia release reactor and the chlorine release reactor are integrated in a circulating fluidized bed reactor device. Two carrier channels are provided between the ammonia release reactor and the chlorine release reactor. The hot magnesium oxide carrier circulates between the ammonia release reactor and the chlorine release reactor through the carrier channels. A solid flow rate controller is provided on the carrier channels.

[0038] The ammonia release reactor is equipped with a gas distributor and a cyclone separator inside;

[0039] The ammonia release reactor is provided with an ammonium chloride inlet and a magnesium chloride carrier inlet on its side wall;

[0040] The ammonia release reactor is equipped with a purge gas inlet and a magnesium oxide carrier outlet containing magnesium hydroxychloride at its bottom. Ammonium chloride and magnesium chloride enter the ammonia release reactor through the ammonium chloride inlet and the magnesium chloride carrier, respectively. The ammonia release reaction takes place in the reactor. Under the action of the purge gas, the ammonia gas and small solid particles rise and pass through a gas distributor, entering a cyclone separator for thorough gas-solid separation. The ammonia gas is then recycled in an ammonia recovery unit. The small solid particles separated by the cyclone separator and the larger solid particles that did not enter the cyclone separator enter the chlorine release reactor through the carrier outlet.

[0041] The chlorine release reactor is equipped with a vertically arranged moving bed and a cyclone separator inside;

[0042] The bottom of the moving bed is provided with a high-temperature gas inlet and a magnesium oxide carrier inlet containing magnesium hydroxychloride;

[0043] The chlorine release reactor has a purge gas inlet and a magnesium oxide carrier outlet on its side wall. The solid carrier entering the chlorine release reactor is mixed with high-temperature gas and enters a moving bed for chlorine release reaction. It gradually moves upward and is then separated by a cyclone separator to obtain hydrogen chloride gas and magnesium oxide solid. The hydrogen chloride gas is recovered and reused through a hydrogen chloride recovery unit, while the solid magnesium oxide carrier is recycled into an ammonia release reactor under the action of purge gas.

[0044] The present invention will be further described below with reference to embodiments.

[0045] Example 1: Concentration of Ammonium Chloride Wastewater

[0046] Using MVR technology to concentrate ammonium chloride with low-pressure steam at 125 °C, it takes 3.4 tons of low-pressure steam to treat 1 ton of wastewater containing 10 wt% ammonium chloride, which is about 99 yuan per ton.

[0047] Example 2: Pyrolysis of Ammonium Chloride

[0048] As attached Figure 2 As shown, ammonium chloride and preheated hot magnesium oxide carrier (temperature 650 °C) from the chlorine-releasing fluidized bed are fully mixed and heat exchanged in the ammonia-releasing fluidized bed reactor. After entering the fluidized bed, ammonium chloride reaches the reaction temperature (temperature 350 °C) and undergoes a pyrolysis reaction (Equation 1) to produce ammonia (gas) and magnesium hydroxide (solid). After passing through two stages of cyclones, the reaction products are separated into magnesium hydroxide and magnesium oxide, which are then sent to the chlorine-releasing fluidized bed reactor via a moving bed, while the ammonia is sent to the ammonia recovery unit.

[0049] After magnesium hydroxide and hot magnesium oxide carrier enter the chlorination fluidized bed reactor, they are fully mixed and heat exchanged with high-temperature flue gas (temperature 2000 °C) in the riser, and the chlorination reaction of magnesium hydroxide (Equation 2) occurs. After passing through two-stage cyclones, the magnesium oxide is separated and sent to the ammonia fluidized bed reactor via a moving bed for heat exchange, while the hydrogen chloride is sent to the hydrogen chloride recovery unit.

[0050] Calculations show that to process 1 ton of ammonium chloride waste, approximately 14 tons of magnesium oxide would be required for circulating heat exchange.

[0051] Example 3: Overall Process Overview, Material and Energy Accounting

[0052] The process of this invention is described and calculated in detail throughout. Taking the treatment of 3500 kg / hr of ammonium chloride wastewater containing 10 wt% ammonium chloride as an example, the process diagram is attached. Figure 1 The entire process comprises five units: wastewater concentration, natural gas boiler, circulating fluidized bed pyrolysis reaction, ammonia recovery, and hydrochloric acid recovery. The Aspen Plus program was used to simulate this process.

[0053] The main flow parameters and specific material balance data for the entire process are detailed in Table 1. The energy consumption estimates for each major energy-consuming and capacity-generating device in the process are listed in Table 2.

[0054] Table 1 Material Balance Sheet for the Resource Utilization of Ammonium Chloride Wastewater

[0055]

[0056] Table 2 Process Energy Consumption Indicators and Cost Estimates

[0057]

[0058] Magnesium oxide serves as the carrier for the chemical loop, with a thermal magnesium oxide carrier quantity of 4890 kg / hr. The ammonia release reactor is estimated to be 7 m high and 1.1 m in diameter, with a gas linear velocity of 0.1 m / s. The chlorine release reactor is estimated to be 20 m high and 1.2 m in diameter for the straight section, which contains a variable-velocity fluidized bed. From bottom to top, the fluidized bed exhibits bubbling, turbulence, and rapid fluidization sections, with a linear velocity of 0.2 m / s in the rapid fluidization region.

[0059] 350 kg / hr of ammonium chloride waste can produce 684.0 kg / hr of ammonia water containing 16.1 wt% ammonia and 804.2 kg / hr of hydrochloric acid containing 29.65% hydrogen chloride, and the energy consumption for treating each ton of ammonium chloride wastewater is only 108.6 yuan.

[0060] Unless otherwise specified, the raw materials and equipment used in this invention are all commonly used in the field; unless otherwise specified, the methods used in this invention are all conventional methods in the field.

[0061] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, alterations, and equivalent transformations made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for resource utilization of ammonium chloride wastewater using magnesium oxide as a chemical looping carrier, characterized by, Includes the following steps: 1) Wastewater containing ammonium chloride is preheated and then concentrated by evaporation to obtain solid ammonium chloride through crystallization; 2) After the ammonium chloride solid is thoroughly mixed with the hot magnesium oxide carrier, it is fed into the ammonia release reactor to produce magnesium hydroxide and ammonia. The separated ammonia is recycled through the ammonia recovery unit, and the solid magnesium hydroxide and the remaining magnesium oxide carrier are sent to the chlorine release reactor. 3) In the chlorine release reactor, the magnesium oxide carrier containing magnesium hydroxide is fully mixed and heat exchanged with the high-temperature flue gas from the gas boiler. The magnesium hydroxide undergoes a chlorine release reaction to generate hydrogen chloride gas and magnesium oxide solid. The separated hydrogen chloride is recycled through the hydrogen chloride recovery unit, and the solid magnesium oxide chemical chain carrier is recycled to the ammonia release reactor. The ammonia release reactor and chlorine release reactor are integrated into a single circulating fluidized bed reactor device; Two carrier channels are provided between the ammonia release reactor and the chlorine release reactor. The hot magnesium oxide carrier circulates between the ammonia release reactor and the chlorine release reactor through the carrier channels. A solid flow rate controller is provided on the carrier channels. The ammonia release reactor is equipped with a gas distributor and a cyclone separator inside; The ammonia release reactor is provided with an ammonium chloride inlet and a magnesium chloride carrier inlet on its side wall; The bottom of the ammonia release reactor is provided with a purge gas inlet and a magnesium oxide carrier outlet containing magnesium hydroxychloride; The chlorine release reactor is equipped with a vertically arranged moving bed and a cyclone separator inside; The bottom of the moving bed is provided with a high-temperature gas inlet and a magnesium oxide carrier inlet containing magnesium hydroxychloride; The sidewall of the chlorine release reactor is provided with a purge gas inlet and a magnesium oxide carrier outlet.

2. The method for resource utilization of ammonium chloride wastewater using magnesium oxide as a chemical looping carrier according to claim 1, characterized in that The temperature of the ammonia release reactor is 300-400 °C; The temperature of the chlorine release reactor is 600-800 °C.

3. The method for resource utilization of ammonium chloride wastewater using magnesium oxide as a chemical looping carrier according to claim 1, characterized in that, In step (1), the evaporation, concentration and crystallization adopts multi-effect evaporation or mechanical vapor recompression evaporation energy-saving technology.

4. The method for resource utilization of ammonium chloride wastewater using magnesium oxide as a chemical chain carrier as described in claim 1, characterized in that, In step (2), the ammonia from the ammonia release reactor is passed through the ammonia absorption tower to obtain ammonia water, and the remaining tail gas is sent to the waste gas treatment unit for unified treatment.

5. The method for resource utilization of ammonium chloride wastewater using magnesium oxide as a chemical chain carrier as described in claim 1, characterized in that, In step (3), in the gas-fired boiler, the outlet high-temperature flue gas temperature is 1600~2400 °C, and the oxygen content in the outlet high-temperature flue gas is ≤3%.

6. The method for resource utilization of ammonium chloride wastewater using magnesium oxide as a chemical chain carrier as described in claim 1, characterized in that, In step (3), in the hydrogen chloride recovery unit, the hydrogen chloride from the ammonia release reactor is converted into hydrochloric acid through the hydrogen chloride absorption tower, and the remaining tail gas is sent to the waste gas treatment unit for unified treatment.

7. The method for resource utilization of ammonium chloride wastewater using magnesium oxide as a chemical chain carrier as described in claim 1, characterized in that, The circulating mass of the thermal magnesium oxide carrier is 10 to 15 times the mass of the ammonium chloride to be treated.

8. The method for resource utilization of ammonium chloride wastewater using magnesium oxide as a chemical chain carrier as described in claim 1, characterized in that, The air intake temperature of the air boiler is 300~500 °C.

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