Thermal softening method for desulfurization wastewater

Abstract

The invention discloses a thermal softening method for desulfurization wastewater. The thermal softening method comprises the following steps: step 1, carrying out heat preservation in a desulfurization wastewater conveying process; step 2, carrying out heat exchange and heating on the desulfurization wastewater and the flue gas; 3, the desulfurization wastewater enters a softening reaction box, sodium hydroxide and a sodium carbonate solution are added, continuous stirring is conducted, and a full reaction is conducted; according to the method, on the basis of the characteristic that the temperature of the desulfurization wastewater is relatively high, the influence of solution temperature, pH value, chloride ions and the like on the solubility of calcium carbonate and magnesium carbonate crystalline hydrate is utilized, heat preservation measures in the desulfurization wastewater conveying process are added, a small amount of flue gas waste heat is added for heat exchange, and sodium hydroxide and sodium carbonate are added at the temperature of 55-60 DEG C; under the condition of low pH value, calcium and magnesium ion hardness removal and desulfurization wastewater softening can be carried out at the same time, and compared with an existing softening technology, the adding amount of sodium hydroxide is reduced, and meanwhile, the adding amount of acid needed by acid-base neutralization in the later period is also reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of flue gas desulfurization and desulfurization wastewater treatment of coal-fired boilers, and specifically relates to a desulfurization wastewater thermal softening method. Background Art

[0002] Desulfurization wastewater refers to the wastewater generated after the flue gas of a coal-fired boiler is treated according to the principle of the limestone gypsum method.

[0003] According to DL / T5046-2018 Power Plant Wastewater Treatment Design Specification, desulfurization wastewater should be treated and reused. When environmental impact assessment allows, it should be treated and discharged in compliance with standards. When there is a zero emission requirement, desulfurization wastewater should be deeply treated.

[0004] In 2017, the Ministry of Environmental Protection issued the "Technical Policy for Pollution Prevention and Control of Thermal Power Plants", which stated that desulfurization wastewater should be reused after being treated with lime treatment, coagulation, clarification, neutralization and other processes. It is encouraged to adopt treatment processes such as evaporation drying or evaporation crystallization to achieve the goal of not discharging desulfurization wastewater.

[0005] Desulfurization wastewater has the following characteristics: the pH value is generally 4-6.5, containing a large amount of fly ash, calcium, magnesium, chloride, sulfate, heavy metal ions, etc., and has high non-carbonate hardness. Generally, the calcium and magnesium ion concentration of desulfurization wastewater produced by the limestone gypsum method is converted into calcium carbonate in the amount of 7000mg / L-20000mg / L, or even higher.

[0006] Domestic desulfurization wastewater pretreatment process generally adopts the following process:

[0007] Pre-sedimentation tank - pH adjustment tank - reaction sedimentation tank - flocculation tank - clarification and concentration tank - final neutralization / oxidation tank - effluent tank - reuse or discharge.

[0008] In the early days when there was no requirement for near-zero discharge of desulfurization wastewater, desulfurization wastewater could be discharged directly after the above treatment process:

[0009] Among them, the purpose of the pre-sedimentation tank is to remove the impact of the high turbidity desulfurization wastewater generated by the hydrocyclone on the subsequent process. Before entering the desulfurization wastewater treatment system, preliminary sedimentation is carried out to reduce the turbidity to below 1000NTU;

[0010] The pH adjustment box is used to adjust the pH value between 9-9.5, so that most heavy metals have the conditions to form hydroxide precipitation and separation;

[0011] In the reaction box, organic sulfur needs to be added, mainly for the separation of heavy metals such as mercury that can form sulfide precipitation;

[0012] The flocculant is added to the flocculation box, and the electrical neutralization, bridging and net-catching effects of the flocculant are used to form small flocs including fly ash, organic matter, heavy metal hydroxides, etc.

[0013] Before entering the clarification and concentration tank, coagulant aids are added to enlarge the flocs and form separation in the clarification and concentration tank so that the turbidity and heavy metal ion concentration of the produced water meet the standards.

[0014] Before discharge, the pH value needs to be adjusted to about 8 with acid.

[0015] For processes that require recycling of desulfurization wastewater, taking the process combining membrane concentration technology and evaporation crystallization technology as an example, higher pretreatment requirements are proposed on the basis of the above.

[0016] Among them, the most important point is that in order to prevent scaling on the surface of the reverse osmosis membrane, the membrane treatment process requires that the calcium and magnesium ion concentration of the desulfurization wastewater after pretreatment be converted into calcium carbonate not more than 200 mg / L or lower.

[0017] In order to achieve the above standards, the treatment method generally adopted is: in the pH adjustment tank, by adding sodium hydroxide or calcium hydroxide, the pH is directly adjusted to 11.0-11.5, so that the magnesium ions are precipitated in the form of Mg(OH)2 precipitates; then sodium carbonate solution is added to precipitate calcium ions in the form of CaCO3 precipitates.

[0018] However, the above adjustments will lead to several problems:

[0019] 1. To adjust the pH value to a high value, more alkaline agents such as sodium hydroxide or calcium hydroxide need to be added; and the pH value of the produced water entering the membrane needs to be adjusted back to around 8, so more acidic agents such as sulfuric acid need to be added;

[0020] 2. Under the high pH value of 11-11.5, calcium and magnesium ions are conducive to precipitation in the form of CaCO3 and Mg(OH)2 precipitates, so that the produced water meets the requirement that the calcium carbonate amount converted from calcium and magnesium ion concentration is not more than 200 mg / L;

[0021] However, the high pH value is in conflict with the optimal pH range of 9-9.5 for heavy metal ions to form hydroxide precipitation, that is, under high pH conditions, the heavy metal content in the pretreated desulfurization wastewater will increase.

[0022] 3. Although Mg(OH)2 is a precipitate, the flocs formed are fluffy and light, so they often float on the water surface, affecting the normal operation of the clarification and concentration tank and the water quality of the produced water.

[0023] In order to solve the above problems, some measures will be taken in the actual production process:

[0024] For example, heavy metal ions and calcium and magnesium ions are treated by graded precipitation. First, an alkaline agent is added to precipitate and separate the heavy metal ions when the pH is 9-9.5. Then, the alkaline agent is continued to be added to form calcium carbonate and magnesium hydroxide when the pH is 11-11.5, and then flocculation is used to precipitate and separate them.

[0025] For example, a flotation process is added to remove the fluffy and light Mg(OH)2 flocs. This process requires the addition of a flotation barrel and a Roots blower to separate the flocs through aeration.

[0026] China's invention patent CN105621742A optimizes and adds the following to the general process: magnesium removal and weight removal tank, primary decalcification tank, secondary decalcification tank, etc. In the embodiment, the pH value of the desulfurization wastewater is adjusted to 11.5 in the primary sedimentation tank. The whole process adds multiple precipitation and separation links, the device is complex and the dosage of reagents is large.

[0027] Chinese invention patent CN105565548A mentions using CO2 in flue gas to adjust the pH of wastewater. First, the pH value of desulfurized wastewater needs to be adjusted to 12.5-14, and then the carbon dioxide in flue gas is used to adjust the pH value to 11.5-12.5. This patent uses flue gas aeration to change the traditional desulfurized wastewater treatment process of using hydrochloric acid to adjust the pH value of desulfurized wastewater to using CO2 in flue gas for neutralization, thereby reducing the equipment cost of the system and improving the safety of the system. However, a large amount of lime milk, sodium sulfate and other agents are still consumed.

[0028] In order to solve the above problems, this invention method is proposed. Summary of the invention

[0029] In view of the above situation, in order to overcome the defects of the prior art, the present invention provides a method for thermal softening of desulfurization wastewater, which effectively solves the problems raised by the background technology.

[0030] To achieve the above object, the present invention provides the following technical solution: a method for thermal softening of desulfurization wastewater, comprising the following steps:

[0031] Step 1: Insulation of desulfurization wastewater transportation process;

[0032] Step 2: Desulfurization wastewater and flue gas exchange heat to increase temperature;

[0033] Step 3: The desulfurized wastewater enters the softening reaction box, and sodium hydroxide and sodium carbonate solutions are added and stirred continuously for full reaction;

[0034] Step 4: Use a plate and frame filter press to separate the solid and liquid of the reaction box slurry. The water produced by filtration is low-turbidity softened desulfurization wastewater, which enters the desulfurization wastewater recycling pretreatment system after passing through the aeration tank.

[0035] Preferably, in step 1, the heat preservation of the desulfurization wastewater transportation process is to perform heat preservation treatment on the desulfurization wastewater pump inlet and outlet pipes, the hydrocyclone separator, the desulfurization wastewater delivery pipe, the temporary storage tank, etc.

[0036] Preferably, in the step 2, the desulfurized wastewater is heated by heat exchange with the flue gas, and the desulfurized wastewater after heat loss is heated to 55°C-60°C by utilizing the flue gas temperature at the desulfurization tower inlet;

[0037] Among them, the heating equipment adopts a tubular heat exchanger, in which the heat exchange medium inside the tube is desulfurized wastewater, and the heat exchange medium outside the tube is hot flue gas.

[0038] Preferably, in the step three, the desulfurization wastewater enters a softening reaction box, and sodium hydroxide and sodium carbonate solutions are added and continuously stirred for sufficient reaction, wherein sodium hydroxide is added to adjust the pH value to 9.5-10, and sodium carbonate is added to control the sum of the molar numbers of carbonate and bicarbonate in the reaction box solution to be greater than the sum of the molar numbers of calcium ions and magnesium ions by 1-1.2 mmol / L after the addition of sodium carbonate.

[0039] Preferably, in step four, a plate and frame filter press is used to separate the solid-liquid slurry in the reaction box, and the water produced by filtration is low-turbidity softened desulfurization wastewater, which enters the desulfurization wastewater recycling pretreatment system after passing through the aeration tank. The plate and frame filter press is used to separate the calcium carbonate and magnesium carbonate crystalline hydrate precipitates and high-turbidity fly ash in the reaction box by filtration, and the filtrate is softened low-turbidity produced water, which enters the desulfurization wastewater recycling pretreatment system.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] 1. The method of the present invention, based on the characteristic of high temperature of desulfurized wastewater itself, utilizes the influence of solution temperature, pH value, chloride ion, etc. on the solubility of calcium carbonate and magnesium carbonate crystalline hydrate, increases the heat preservation measures in the desulfurized wastewater transportation process, increases a small amount of flue gas waste heat heat exchange, and adds sodium hydroxide and sodium carbonate at a temperature of 55°C-60°C. It can remove the hardness of calcium and magnesium ions at the same time under low pH conditions to soften the desulfurized wastewater. Compared with the existing softening technology, the amount of sodium hydroxide added is reduced, and the amount of acid added required for the later acid-base neutralization is also reduced;

[0042] 2. After the desulfurization wastewater has fully reacted in the reaction box, the solid-liquid separation can be carried out by plate and frame filter pressing. While removing the formed calcium and magnesium precipitates, other suspended matter such as fly ash contained in the desulfurization wastewater will be removed, reducing the turbidity of the desulfurization wastewater. It is no longer necessary to set up a desulfurization wastewater pre-sedimentation tank, reducing investment and operating costs;

[0043] 3. Compared with the conventional zero-discharge pretreatment of desulfurization wastewater, the method of the present invention does not require multiple sedimentation tanks of the graded sedimentation method, and does not require the flotation barrel and Roots blower required for the flotation method to remove magnesium hydroxide. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The accompanying drawings are used to provide further understanding of the present invention and constitute a part of the specification. They are used to explain the present invention together with the embodiments of the present invention and do not constitute a limitation of the present invention.

[0045] In the attached picture:

[0046] Figure 1 A schematic diagram of a process for thermal softening of desulfurization wastewater according to the present invention; DETAILED DESCRIPTION

[0047] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments; based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0048] Embodiment 1, by Figure 1 The present invention relates to a method for thermal softening of desulfurization wastewater, comprising the following steps:

[0049] Step 1: Insulation of desulfurization wastewater transportation process;

[0050] Step 2: Desulfurization wastewater and flue gas exchange heat to increase temperature;

[0051] Step 3: The desulfurized wastewater enters the softening reaction box, and sodium hydroxide and sodium carbonate solutions are added and stirred continuously for full reaction;

[0052] Step 4: Use a plate and frame filter press to separate the solid and liquid of the reaction box slurry. The water produced by filtration is low-turbidity softened desulfurization wastewater, which enters the desulfurization wastewater recycling pretreatment system after passing through the aeration tank.

[0053] In step one, the thermal insulation of the desulfurization wastewater transportation process is to insulate the desulfurization wastewater pump inlet and outlet pipes, cyclone separator, desulfurization wastewater delivery pipes, temporary storage tanks, etc.

[0054] In step 2, the desulfurized wastewater is heated by heat exchange with the flue gas, and the desulfurized wastewater after heat loss is heated to 55°C-60°C by utilizing the flue gas temperature at the desulfurization tower inlet;

[0055] Among them, the heating equipment adopts a tubular heat exchanger, in which the heat exchange medium inside the tube is desulfurized wastewater, and the heat exchange medium outside the tube is hot flue gas.

[0056] In step three, the desulfurization wastewater enters a softening reaction box, and sodium hydroxide and sodium carbonate solutions are added and continuously stirred for sufficient reaction, wherein sodium hydroxide is added to adjust the pH value to 9.5-10, and sodium carbonate is added to control the sum of the molar numbers of carbonate and bicarbonate in the reaction box solution to be greater than the sum of the molar numbers of calcium ions and magnesium ions by 1-1.2 mmol / L after the addition of sodium carbonate.

[0057] In step four, a plate and frame filter press is used to separate the solid-liquid slurry in the reaction box. The filtered water is low-turbidity softened desulfurization wastewater, which enters the desulfurization wastewater recovery and utilization pretreatment system after passing through the aeration tank. The plate and frame filter press is used to filter and separate the calcium carbonate and magnesium carbonate hydrate precipitates and high-turbidity fly ash in the reaction box. The filtrate is softened low-turbidity produced water, which enters the desulfurization wastewater recovery and utilization pretreatment system.

[0058] Introduction to the principle of the present invention:

[0059] 1. Add Na(OH)2 to remove heavy metals

[0060] Wastewater contains complex heavy metal components, and heavy metal removal effect is an important indicator for desulfurization wastewater treatment.

[0061] Adding Na(OH) to the desulfurization wastewater adjusts the pH to 9-10. After sufficient reaction, most of the heavy metal ions can form hydroxide precipitation. The reaction occurring in this process is shown in formula (1).

[0062] M 2+ +2Na(OH) = M(OH)2↓+Na + (1)

[0063] Wherein M represents a heavy metal ion.

[0064] 2. Add sodium carbonate to remove calcium and magnesium ions

[0065] Ca 2+ +Na2CO3=CaCO3↓+2Na + (2)

[0066]

[0067] Mg 2+ +2OH-=Mg(OH)2↓ (4)

[0068] xMgCO3·yMg(OH)2·zH2O is basic magnesium carbonate, x is 3-5, y is approximately 1, and z is between 3 and 7. When y=0, xMgCO3·zH2O is magnesium carbonate crystalline hydrate, x=1, z=3-7.

[0069] Reaction formula (3) is the reaction of magnesium ions at a pH value between 9 and 10.

[0070] Reaction formula (4) is a reaction that gradually proceeds after the pH value is greater than 10.5, and as the pH reaches about 11.5, the reaction can be fully carried out to separate the magnesium ions in the form of magnesium hydroxide precipitation.

[0071] The pH control range of the present invention corresponds to reaction equations (2) and (3). At this time, under the condition that the sum of the molar numbers of carbonate ions and bicarbonate ions is greater than the sum of the molar numbers of calcium ions and magnesium ions by 1-1.2 mmol / L, calcium ions are mostly present in the form of calcium carbonate precipitates; magnesium ions are mainly present in the form of magnesium carbonate hydrates, which are crystalline precipitates of different shapes.

[0072] Calcium ions form calcium carbonate precipitates, and magnesium ions form magnesium carbonate hydrate crystals. After precipitation and separation, the softening effect is achieved.

[0073] 3. Temperature influence:

[0074] In the range of 20-60℃

[0075] Calcium carbonate precipitation: As temperature increases, the solubility of calcium carbonate decreases and the amount of precipitation increases;

[0076] Crystallization of magnesium carbonate hydrate: As the temperature increases, the solubility of magnesium carbonate hydrate decreases and the precipitation of crystals increases.

[0077] The solubility of calcium carbonate and magnesium carbonate hydrate crystals will further decrease with increasing temperature, and more precipitates can be formed, which will further improve the softening effect.

[0078] Since magnesium carbonate compounds exist in a variety of crystalline forms, and the quality of desulfurization wastewater varies greatly due to various factors such as desulfurizer composition, desulfurization process control, flue gas composition, etc. Therefore, the removal effect of desulfurization wastewater on calcium and magnesium ions should be determined based on the actual water samples of different users, and through corresponding reaction experiments to determine the control parameters such as the amount of reagent added and temperature.

[0079] Example:

[0080] The experiment used desulfurization wastewater, calcium ion concentration: 4200 mg / L; magnesium ion concentration: 800 mg / L; pH = 6.0; chloride ion concentration: 10000 mg / L; solution temperature 55°C.

[0081] After operating according to the experimental steps of the present invention, hardness ion detection is performed on the filter press water after standing.

[0082] When the pH is adjusted to 10.0, the amount of sodium carbonate added is 12000mg / L, the solution temperature is 55℃, and the stirring reaction time is greater than 5min, the magnesium ion content of the filter press water is reduced to about 10mg / L; the calcium ion content is reduced to about 15mg / L. The converted calcium carbonate content is about 80mg / L, which can meet the softening effect of the general desulfurization wastewater pretreatment water, where the concentration of calcium and magnesium ions dissolved is converted to a calcium carbonate concentration value of less than 200mg / L.

[0083] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0084] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for thermal softening of desulfurization wastewater, comprising the following steps: Step 1: Insulation of desulfurization wastewater transportation process; Step 2: Desulfurization wastewater and flue gas exchange heat to increase temperature; Step 3: The desulfurized wastewater enters the softening reaction box, and sodium hydroxide and sodium carbonate solutions are added and stirred continuously for full reaction; Step 4: Use a plate and frame filter press to separate the solid and liquid of the reaction box slurry. The water produced by filtration is low-turbidity softened desulfurization wastewater, which enters the desulfurization wastewater recycling pretreatment system after passing through the aeration tank.

2. A desulfurization wastewater thermal softening method according to claim 1, characterized in that: In the step 1, the heat preservation of the desulfurization wastewater transportation process is to perform heat preservation treatment on the desulfurization wastewater pump inlet and outlet pipes, the hydrocyclone separator, the desulfurization wastewater delivery pipe, the temporary storage tank, etc.

3. A method for thermal softening of desulfurization wastewater according to claim 1, characterized in that: In the step 2, the desulfurized wastewater is heated by heat exchange with the flue gas, and the desulfurized wastewater after heat loss is heated to 55°C-60°C by utilizing the flue gas temperature at the desulfurization tower inlet; Among them, the heating equipment adopts a tubular heat exchanger, in which the heat exchange medium inside the tube is desulfurized wastewater, and the heat exchange medium outside the tube is hot flue gas.

4. A desulfurization wastewater thermal softening method according to claim 1, characterized in that: In the step three, the desulfurized wastewater enters a softening reaction box, and sodium hydroxide and sodium carbonate solutions are added and continuously stirred to react fully, wherein sodium hydroxide is added to adjust the pH value to 9.5-10, and sodium carbonate is added to control the sum of the molar numbers of carbonate and bicarbonate in the reaction box solution to be greater than the sum of the molar numbers of calcium ions and magnesium ions by 1-1.2 mmol / L after the addition of sodium carbonate.

5. A method for thermal softening of desulfurization wastewater according to claim 1, characterized in that: In the step 4, the solid-liquid separation is carried out on the reaction box slurry by using a plate and frame filter press, and the water produced by filtration is low-turbidity softened desulfurization wastewater, which enters the desulfurization wastewater recycling pretreatment system after passing through the aeration tank. The calcium carbonate and magnesium carbonate hydrate precipitates and high-turbidity fly ash in the reaction box are separated by filtration by using a plate and frame filter press, and the filtrate is softened low-turbidity produced water, which enters the desulfurization wastewater recycling pretreatment system.

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