A desulfurization wastewater heat softening method

Abstract

The application discloses a desulfurization wastewater heat softening method, comprising the following steps: step one, heat preservation of the desulfurization wastewater conveying process; step two, heat exchange between the desulfurization wastewater and flue gas to increase the temperature; step three, the desulfurization wastewater enters a softening reaction box, sodium hydroxide and sodium carbonate solution are added and continuously stirred, and sufficient reaction is carried out; the method is based on the characteristics that the desulfurization wastewater itself has a high temperature, utilizes the influence of solution temperature, pH value and chloride ion on the solubility of calcium carbonate and magnesium carbonate crystallization hydrate, increases the heat preservation measures of the desulfurization wastewater conveying process, increases a small amount of flue gas waste heat exchange, at a temperature of 55 DEG C-60 DEG C, by adding sodium hydroxide and sodium carbonate, calcium and magnesium ion hardness removal and desulfurization wastewater softening can be simultaneously carried out under low pH value conditions, compared with the existing softening technology, the sodium hydroxide addition amount is reduced, and the acid addition amount required for the acid-base neutralization in the later stage is also reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of flue gas desulfurization and desulfurization wastewater treatment of coal-fired boilers, and particularly relates to a desulfurization wastewater heat softening method. BACKGROUND

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

[0003] According to the DL / T5046-2018 Design Code for Wastewater Treatment of Power Plants, it is proposed that desulfurization wastewater should be treated and reused. When the environmental impact assessment allows, the desulfurization wastewater should be treated and discharged up to the standard. When there is a zero discharge requirement, the desulfurization wastewater should be deeply treated.

[0004] In 2017, the Environmental Protection Department issued the Technical Policy for Pollution Prevention and Control of Thermal Power Plants, which states that desulfurization wastewater should be treated by lime treatment, coagulation, clarification, neutralization and other processes before being reused. It is encouraged to use evaporation drying or evaporation crystallization processes to achieve zero discharge of desulfurization wastewater.

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

[0006] The domestic desulfurization wastewater pretreatment process generally adopts the following flow:

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

[0008] When there is no requirement for near-zero discharge of desulfurization wastewater in the early stage, the desulfurization wastewater can be directly discharged after the above treatment process:

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

[0010] The pH adjustment tank is used to adjust the pH value to between 9-9.5, so that most heavy metals can form hydroxide precipitates for separation.

[0011] In the reaction tank, organic sulfur needs to be added, which is mainly used for the separation of heavy metals such as mercury that can form sulfide precipitates.

[0012] The flocculation tank adds flocculants to form small flocs including fly ash, organic matter, heavy metal hydroxides, etc. by using the electric neutralization, bridging and net capture of the flocculants.

[0013] Before entering the clarification and concentration tank, coagulant aids are added to increase the size of the flocs and form a separation in the clarification and concentration tank, so that the water quality meets the standards for turbidity and heavy metal ion concentration.

[0014] Before discharge, the pH value is adjusted to about 8 using acid.

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

[0016] The most important point is to prevent the scaling of the reverse osmosis membrane surface. The membrane treatment process requires that the calcium and magnesium ion concentration of the pretreated desulfurization wastewater, converted to calcium carbonate, be no more than 200 mg / L or lower.

[0017] To meet the above standards, the treatment method generally used is to adjust the pH directly to 11.0-11.5 in the pH adjustment tank by adding sodium hydroxide or calcium hydroxide, so that magnesium ions precipitate in the form of Mg(OH)2; then add sodium carbonate solution to precipitate calcium ions in the form of CaCO3.

[0018] However, the above adjustment raises several problems:

[0019] 1. Adjusting to a high pH value requires adding more alkaline reagents such as sodium hydroxide or calcium hydroxide; and the product water entering the membrane needs to be adjusted back to a pH of about 8, which requires adding more acidic reagents represented by sulfuric acid.

[0020] 2. Under high pH conditions of 11-11.5, calcium and magnesium ions precipitate in the form of CaCO3 and Mg(OH)2, which meets the requirement that the product water has a calcium and magnesium ion concentration converted to calcium carbonate of no more than 200 mg / L.

[0021] However, the high pH value is in conflict with the optimal pH range of 9-9.5 for the formation of hydroxide precipitates of heavy metals, i.e., under high pH conditions, the heavy metal content of the pretreated desulfurization wastewater increases.

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

[0023] To solve the above problems, some measures are taken in actual production processes:

[0024] For example, heavy metal ions and calcium and magnesium ions are treated by fractional precipitation, an alkaline agent is added first, when the pH is 9-9.5, the heavy metal ions are precipitated and separated; then, the alkaline agent is continuously added, when the pH is 11-11.5, calcium carbonate and magnesium hydroxide are formed and precipitated and separated by flocculation.

[0025] For example, a floatation process is added for removing fluffy and light Mg(OH)2 flocculation. The process requires adding a floatation barrel and a Roots blower to make the flocculation float and separate by aeration.

[0026] The Chinese invention patent CN105621742A optimizes the general process by adding a magnesium and heavy metal removal tank, a primary calcium removal tank, a secondary calcium removal tank, etc. In the primary sedimentation tank, the pH value of the desulfurization wastewater is adjusted to 11.5. The whole process adds multiple precipitation and separation links, and the device is complex and the dosage of the agent is large.

[0027] The Chinese invention patent CN105565548A mentions that CO2 in flue gas is used to adjust the pH of the wastewater. First, the pH value of the desulfurization wastewater is adjusted to 12.5-14, and then the CO2 in the flue gas is used to adjust the pH value to 11.5-12.5. The patent uses flue gas aeration to replace hydrochloric acid in the traditional desulfurization wastewater treatment process to neutralize the CO2 in the flue gas, 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] To solve the above problems, the present application provides a method. SUMMARY

[0029] In view of the above, to overcome the defects of the prior art, the present application provides a desulfurization wastewater heat softening method which effectively solves the problems raised in the background art.

[0030] To achieve the above object, the present application provides the following technical scheme: a desulfurization wastewater heat softening method, comprising the following steps:

[0031] Step 1: heat preservation during the desulfurization wastewater conveying process;

[0032] Step 2: heat exchange and temperature rise of the desulfurization wastewater and the flue gas;

[0033] Step 3: the desulfurization wastewater enters a softening reaction tank, sodium hydroxide and sodium carbonate solution are added and continuously stirred for sufficient reaction; wherein the pH value is adjusted to 9.5-10 by adding sodium hydroxide; after adding sodium carbonate, the sum of the molar numbers of carbonate and bicarbonate in the solution in the reaction tank is controlled to be 1-1.2 mmol / L greater than the sum of the molar numbers of calcium ions and magnesium ions;

[0034] Step four: solid-liquid separation of the slurry in the reaction box is carried out by using a plate-frame filter press, and the water produced by the filtration is low-turbidity softened desulfurization wastewater, which enters the desulfurization wastewater recycling pretreatment system after passing through an aeration tank.

[0035] Preferably, in step one, the heat preservation of the desulfurization wastewater conveying process is that the desulfurization wastewater pump outlet pipeline, the hydrocyclone, the desulfurization wastewater conveying pipeline and the temporary storage tank are all subjected to heat preservation treatment.

[0036] Preferably, in step two, the heat exchange and temperature rise of the desulfurization wastewater and the flue gas are that the desulfurization wastewater after heat loss is heated to 55-60 DEG C by using the flue gas temperature at the inlet of the desulfurization tower.

[0037] The heating equipment adopts a tubular heat exchanger, the heat exchange medium in the tube is the desulfurization wastewater, and the heat exchange medium outside the tube is the hot flue gas.

[0038] Preferably, in step four, the solid-liquid separation of the slurry in the reaction box is carried out by using a plate-frame filter press, and the water produced by the filtration is low-turbidity softened desulfurization wastewater, which enters the desulfurization wastewater recycling pretreatment system after passing through an aeration tank, and the calcium carbonate, magnesium carbonate crystalline hydrate precipitate and high-turbidity fly ash in the reaction box are separated by using the plate-frame filter press, the filtrate is the softened low-turbidity water, and the filtrate enters the desulfurization wastewater recycling pretreatment system.

[0039] Compared with the prior art, the method has the following beneficial effects:

[0040] 1. Based on the characteristics that the desulfurization wastewater itself has a relatively high temperature, the influence of the solution temperature, pH value and chloride ion on the solubility of calcium carbonate and magnesium carbonate crystalline hydrate is utilized, heat preservation measures are added to the desulfurization wastewater conveying process, a small amount of flue gas waste heat is exchanged, sodium hydroxide and sodium carbonate are added at a temperature of 55-60 DEG C, calcium and magnesium ion hardness removal and desulfurization wastewater softening are simultaneously carried out under low pH value conditions, compared with the existing softening technology, the amount of sodium hydroxide added is reduced, and the amount of acid required for the later acid-base neutralization is also reduced.

[0041] 2. After the desulfurization wastewater in the reaction tank is fully reacted, the desulfurization wastewater can be subjected to solid-liquid separation by using a plate-frame filter press, the calcium and magnesium precipitates formed are removed, other suspended matters such as fly ash contained in the desulfurization wastewater are removed, the turbidity of the desulfurization wastewater is reduced, a desulfurization wastewater pre-sedimentation tank does not need to be arranged, and investment and operation cost are reduced.

[0042] 3. Compared with the conventional desulfurization wastewater zero discharge pretreatment, the method does not need multiple sedimentation tanks of the staged precipitation method, and does not need a floating barrel and a Roots blower required for removing magnesium hydroxide by using the floating method. BRIEF DESCRIPTION OF DRAWINGS

[0043] The accompanying drawings are included to provide a further understanding of the application, and are incorporated in and constitute a part of the specification, illustrate embodiments of the application, and are included to provide a further understanding of the application, and are made a part of the specification.

[0044] In the drawings:

[0045] Figure 1 A flow chart of a desulfurization wastewater heat softening method of the application is shown in the figure. DETAILED DESCRIPTION

[0046] The technical solutions in the embodiments of the application will be clearly and completely described below with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments of the application. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the application.

[0047] Embodiment one, by Figure 1 The application relates to a desulfurization wastewater heat softening method, which comprises the following steps:

[0048] Step one: heat preservation in the desulfurization wastewater conveying process;

[0049] Step two: heat exchange and temperature rise of the desulfurization wastewater and flue gas;

[0050] Step three: the desulfurization wastewater enters a softening reaction box, sodium hydroxide and sodium carbonate solution are added and continuous stirring is carried out, and sufficient reaction is carried out; wherein the pH value is adjusted to 9.5-10 by adding sodium hydroxide; after adding sodium carbonate, the sum of the molar numbers of carbonate and bicarbonate in the solution in the reaction box is controlled to be 1-1.2 mmol / L larger than the sum of the molar numbers of calcium ions and magnesium ions;

[0051] Step four: solid-liquid separation of the slurry in the reaction box is carried out by using a plate-and-frame filter press, the water produced by pressure filtration is low-turbidity softening desulfurization wastewater, and the low-turbidity softening desulfurization wastewater enters a desulfurization wastewater recycling pretreatment system after passing through an aeration tank.

[0052] In step one, heat preservation in the desulfurization wastewater conveying process is that the desulfurization wastewater pump, the water outlet pipeline, the liquid cyclone separator, the desulfurization wastewater conveying pipeline and the temporary storage tank are all subjected to heat preservation treatment.

[0053] In step two, heat exchange and temperature rise of the desulfurization wastewater and flue gas is that the desulfurization wastewater after heat loss is heated to 55-60 DEG C by using the flue gas temperature at the inlet of the desulfurization tower.

[0054] The heating equipment adopts a tubular heat exchanger, the heat exchange medium in the tube is the desulfurization wastewater, and the heat exchange medium outside the tube is hot flue gas.

[0055] In step four, the plate and frame filter press is used to separate the slurry in the reaction box, the water produced by the filter press is low turbidity softened desulfurization wastewater, which is treated by the aeration tank and then enters the desulfurization wastewater recycling pretreatment system, the plate and frame filter press is used to separate the calcium carbonate, magnesium carbonate crystal hydrate precipitate and high turbidity fly ash in the reaction box, the filtrate is softened low turbidity produced water, and then enters the desulfurization wastewater recycling pretreatment system.

[0056] Introduction of the principles of the application:

[0057] 1. Na (OH) 2 is added to remove heavy metals

[0058] The wastewater contains complex heavy metal components, and the removal effect of heavy metals is an important indicator of desulfurization wastewater treatment.

[0059] Na (OH) is added to the desulfurization wastewater to adjust the pH to 9-10, and after sufficient reaction, most of the heavy metal ions can form hydroxide precipitates. The reaction occurring in this process is shown in formula (1).

[0060] M 2+ +2Na (OH) = M (OH) 2↓+Na + (1)

[0061] Wherein M represents a heavy metal ion.

[0062] 2. Add carbonated calcium and magnesium ions

[0063] Ca 2+ +Na2CO3 = CaCO3↓+2Na + (2)

[0064] Mg 2+ +H2O+CO3 2+ +OH - → xMgCO3·yMg(OH)2·zH2O (3)

[0065] Mg 2+ +2OH-=Mg(OH)2↓ (4)

[0066] xMgCO3·yMg(OH)2·zH2O is basic magnesium carbonate, x is 3-5, y is approximately 1, and z is between 3-7. When y=0, xMgCO3·zH2O is magnesium carbonate crystal hydrate, x=1, and z=3-7.

[0067] Reaction formula (3) is the reaction of magnesium ions at a pH of 9-10.

[0068] Reaction (4) is a reaction that proceeds gradually after the pH value is greater than 10.5, and as the pH reaches about 11.5, it can proceed fully, so that magnesium ions are separated in the form of magnesium hydroxide precipitate.

[0069] The pH control range of this invention corresponds to reaction formulas (2) and (3). At this time, under the condition that the sum of the molar number of carbonate and bicarbonate is greater than the sum of the molar number of calcium and magnesium ions by 1-1.2 mmol / L, calcium ions are mostly present in the form of calcium carbonate precipitate; magnesium ions are mainly in the form of magnesium carbonate hydrate, which are crystalline precipitates of different shapes.

[0070] Calcium ions form calcium carbonate precipitate, and magnesium ions form magnesium carbonate hydrate crystal precipitate. After separation, a softening effect is achieved.

[0071] 3. Temperature effect:

[0072] Within the range of 20-60℃

[0073] Calcium carbonate precipitation: As the temperature increases, the solubility of calcium carbonate decreases, and the amount of precipitation increases;

[0074] Magnesium carbonate hydrate crystallization: As the temperature increases, the solubility of magnesium carbonate hydrate decreases, and the precipitation of crystals increases.

[0075] The solubility of hydrated calcium carbonate and magnesium carbonate crystals decreases further with increasing temperature, leading to the formation of more precipitates and further enhancing the softening effect.

[0076] Because magnesium carbonate compounds exist in various crystalline forms, and the quality of desulfurization wastewater varies greatly due to factors such as the composition of the desulfurizing agent, the control of the desulfurization process, and the composition of the flue gas, the effectiveness of desulfurization wastewater in removing calcium and magnesium ions should be determined based on the actual water samples from different users, through corresponding reaction experiments to determine the dosage of reagents and temperature control parameters.

[0077] Example:

[0078] The desulfurization wastewater used in the experiment had the following concentrations: calcium ion concentration: 4200 mg / L; magnesium ion concentration: 800 mg / L; pH=6.0; chloride ion concentration: 10000 mg / L; and solution temperature: 55℃.

[0079] After following the experimental steps of this invention, the hardness ion content of the filtered water after settling was measured.

[0080] When the pH was adjusted to 10.0, the sodium carbonate addition was 12000 mg / L, the solution temperature was 55℃, and the stirring reaction time was greater than 5 minutes, the magnesium ion content in the filter press permeate decreased to approximately 10 mg / L, and the calcium ion content decreased to approximately 15 mg / L. This translates to a calcium carbonate content of approximately 80 mg / L, which meets the softening requirements for general desulfurization wastewater pretreatment where the dissolved calcium and magnesium ion concentrations (converted to a calcium carbonate concentration of less than 200 mg / L) are insufficient.

[0081] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0082] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which 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 during the transport of desulfurization wastewater; Step 2: Heat exchange between desulfurization wastewater and flue gas to raise the temperature; Step 3: The desulfurization wastewater enters the softening reaction tank, sodium hydroxide and sodium carbonate solutions are added and continuously stirred to allow for a full reaction; sodium hydroxide is added to adjust the pH value to 9.5-10; after adding sodium carbonate, the sum of the molar numbers of carbonate and bicarbonate ions in the reaction tank solution is controlled to be 1-1.2 mmol / L greater than the sum of the molar numbers of calcium and magnesium ions. Step 4: Use a plate and frame filter press to separate the solid and liquid components of the slurry in the reaction tank. The filter press water is low-turbidity softened desulfurization wastewater, which enters the desulfurization wastewater recycling pretreatment system after passing through the aeration tank.

2. The method for thermal softening of desulfurization wastewater according to claim 1, characterized in that: In step one, the insulation of the desulfurization wastewater transportation process involves insulating the inlet and outlet pipes of the desulfurization wastewater pump, the hydrocyclone separator, the desulfurization wastewater transportation pipe, and the temporary storage tank.

3. The method for thermal softening of desulfurization wastewater according to claim 1, characterized in that: In step two, the desulfurization wastewater and flue gas exchange heat to raise the temperature by using the temperature of the flue gas at the inlet of the desulfurization tower to heat the desulfurization wastewater after heat loss to 55℃-60℃. The heating equipment uses a tubular heat exchanger, in which the heat exchange medium inside the tube is desulfurization wastewater and the heat exchange medium outside the tube is hot flue gas.

4. The method for thermal softening of desulfurization wastewater according to claim 1, characterized in that: In step four, a plate and frame filter press is used to separate the solid and liquid components of the slurry in the reaction tank. The filter press product 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 crystal hydrate precipitates and high-turbidity fly ash in the reaction tank. The filtrate is softened low-turbidity product water, which enters the desulfurization wastewater recycling pretreatment system.

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