Method for treating sulfate sewage through electric flocculation coupling double salt method

Abstract

The invention relates to a method for treating sulfate sewage, in particular to a method for treating sulfate sewage through an electric flocculation coupling double salt method. The invention aims to solve the technical problems of large dosage and high treatment cost of high-concentration sulfate sewage in the traditional double salt method. The method comprises the following three steps: step 1, a pretreatment reaction stage: adjusting the pH value of sewage by using lime milk, and removing part of heavy metal ions and sulfate ions in the sewage; the second step is a calcium-aluminum double salt method treatment stage, aluminum salt and lime milk are added to react with calcium ions and sulfate ions in the sewage, the sulfate ions in the sewage can be effectively removed in the process, and ettringite with high economic value is generated; 3, the sewage is filtered and then conveyed to an electric flocculation treatment device, and sludge generated by electric flocculation can be reused in the second stage after being treated. According to the process, electric flocculation and a calcium-aluminum double salt method process are coupled, and resource utilization of high-concentration sulfate sewage is achieved.

Description

Technical Field

[0001] The invention relates to a method for treating sulfate sewage. Background Art

[0002] At present, the treatment technologies for high-concentration sulfate wastewater can be roughly divided into three categories according to the treatment methods: thermal treatment technology, membrane treatment technology and chemical treatment technology. Thermal treatment is to evaporate the wastewater. Common evaporation technologies mainly include multi-stage flash evaporation technology, mechanical vapor recompression technology and multi-effect evaporation technology. Membrane treatment technology is to separate brine through the selective permeability of the membrane. Common membrane treatment technologies for high-concentration sulfate wastewater include nanofiltration and reverse osmosis technology. Compared with chemical treatment technology, these two technologies have the disadvantages of small water treatment volume and high equipment operation and maintenance costs. At present, the chemical treatment technology for high-concentration sulfate wastewater mainly uses the double alkali method for treatment, that is, by using lime milk slurry to convert sulfate ions into gypsum, but gypsum is slightly soluble, and wastewater is easy to form scale deposited on the surface of the equipment, and the effluent has the defects of high residual sulfate ion content (>2000mg / L), high hardness and high pH. At present, the calcium aluminum double salt method is introduced to treat slightly supersaturated calcium sulfate wastewater, and its reaction precipitation is mainly calcium sulfonate (calcium aluminum sulfate mineral), a cement expansion agent. This technology can be used to realize the resource treatment of slightly supersaturated calcium sulfate wastewater, but the cost of using aluminum sources limits the promotion of this technology. Summary of the invention

[0003] The present invention aims to solve the technical problems of large dosage and high treatment cost of high-concentration sulfate wastewater in traditional double salt method, and provides a method for treating sulfate wastewater by coupling electric flocculation with double salt method.

[0004] The method for treating sulfate wastewater by electro-flocculation coupled with double salt method of the present invention is carried out according to the following steps:

[0005] 1. Pretreatment: Add lime milk to the high-concentration sulfate wastewater to be treated, stir for 30 to 50 minutes, and collect the generated sludge at the bottom of the container;

[0006] The molar ratio of calcium ions in the lime milk to sulfate ions in the high-concentration sulfate wastewater to be treated is (1-2):1;

[0007] 2. Double salt treatment: Continue to add lime milk to the above pretreated sewage, stir for 5min to 6min, then add aluminum source, stir for another 20min to 40min, and collect the generated sludge at the bottom of the container;

[0008] The ratio of the sum of the molar amount of calcium ions in the lime milk added in step 2 and the molar amount of calcium ions remaining in the sewage after the pretreatment in step 1 to the molar amount of sulfate ions remaining in the sewage after the pretreatment in step 1 is 2:(1-1.1);

[0009] The ratio of the molar amount of sulfate remaining in the wastewater after the pretreatment in step 1 to the molar amount of aluminum ions in the aluminum source added in step 2 is 3:(2-2.2);

[0010] 3. Electro-flocculation treatment: Filter the wastewater treated in step 2 first, and then introduce it into the electro-flocculation reactor, start the electro-flocculation treatment and stir at the same time, the treatment time is 30min to 40min, part of the sludge generated in this stage will be transferred to the upper layer by the bubbles generated in the electro-flocculation process, and the floating sludge will be collected by the scraper; another part of the sludge will be collected at the bottom of the reactor; the two parts of the sludge will be dehydrated together to obtain aluminum hydroxide, which will be returned to the double salt method treatment stage as an aluminum source; the wastewater after electro-flocculation treatment will meet the discharge standards.

[0011] The amount of lime milk added in the pretreatment stage of step 1 needs to be determined according to the concentration of sulfate ions in the wastewater to be treated. The reactions occurring in the pretreatment stage are mainly:

[0012] Ca 2+ +SO4 2- =CaSO4

[0013] The main purpose of this stage is to reduce the high concentration of sulfate ions to a critical saturated state and adjust the pH of the sewage to 12-14. This stage can convert part of the sulfate ions into usable gypsum (CaSO4·2H2O) and separate the heavy metal ions in the sewage.

[0014] In the double salt treatment stage of step 2, the dosage of lime milk and aluminum source (sodium aluminate and aluminum hydroxide) needs to be determined according to the concentration of calcium ions and sulfate ions in the pretreated sewage. The reactions in the double salt treatment tank are mainly:

[0015] 6Ca 2+ +3SO4 2- +2[Al(OH)6] 3- +26H2O=3CaO·Al2O3·3CaSO4·32H2O

[0016] The bottoms of the above two stages of the tanks are designed to be conical to facilitate the enrichment of sludge in the tanks.

[0017] In the electro-flocculation treatment process of step three, in order to realize the reuse of electro-flocculation sludge, the electrodes used in the electro-flocculation device are all aluminum plates, which can produce aluminum hydroxide; compared with the traditional flocculation method, electro-flocculation has the advantages of no need for drug administration, high degree of automation, small footprint, small amount of sludge produced and no secondary pollution; this stage can deeply treat the residual calcium ions and sulfate ions in the sewage, and use the hydroxyl free radicals with strong oxidizing effect produced by electro-flocculation to mineralize the difficult-to-degrade organic matter; the electro-flocculation process will produce hydrogen, which has a certain flotation effect, which helps to make the water quality uniform, and therefore the electro-flocculation flocs will float on the upper layer of the water body; the sludge (upper and bottom) produced in this stage is mainly active aluminum hydroxide, so it can be reused as an aluminum source in the double salt treatment stage, thereby achieving zero discharge of sludge and reducing the cost of using aluminum salt agents.

[0018] The beneficial effects of the present invention are:

[0019] 1. The method of the present invention can achieve deep treatment of high-concentration sulfate wastewater to meet the wastewater reuse standard;

[0020] 2. The present invention can realize resource utilization of calcium ions and sulfate ions in high-concentration sulfate wastewater by adding corresponding reagents;

[0021] 3. The sludge produced by the electro-flocculation treatment device can be effectively reused in the double salt treatment process, which can effectively achieve the cost control of aluminum sources;

[0022] 4. The main components of the sludge produced in the pretreatment stage and the double salt treatment stage are gypsum and calcium sulfonate, both of which have high economic value; the cost of the method of the present invention mainly comes from the cost of adding reagents, the market price of calcium oxide (lime milk) is 420 yuan / t, and the unit price of sodium aluminate (aluminum source) is 2650 yuan / t; the cost of the reagent required for treating 1 ton of wastewater by this process is 14.74 yuan; but treating one ton of wastewater can produce 9.313kg of desulfurized gypsum and 9.074kg of calcium sulfonate. According to the unit price of desulfurized gypsum of 480 yuan / t and the unit price of concrete expansion agent (calcium sulfonate is the main raw material) of 1400 yuan / t, each ton of wastewater treated can obtain a profit of 17.17 yuan, and as the electrocoagulation process continues, the aluminum source required in the subsequent treatment process will be further reduced. In summary, this process can realize the resource utilization of desulfurized wastewater and has high economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a schematic diagram of the device used in the method for treating sulfate wastewater by electro-flocculation coupled with double salt method in Experiment 1;

[0024] Figure 2 This is the XRD image of the sludge obtained after pretreatment in Experiment 1;

[0025] Figure 3 This is the XRD image of the sludge obtained after double salt treatment in Experiment 1;

[0026] Figure 4 This is the XRD image of the sludge obtained after electrocoagulation treatment in Experiment 1. DETAILED DESCRIPTION

[0027] Specific implementation method 1: This implementation method is a method for treating sulfate wastewater by electro-flocculation coupled with double salt method, which is carried out according to the following steps:

[0028] 1. Pretreatment: Add lime milk to the high-concentration sulfate wastewater to be treated, stir for 30 to 50 minutes, and collect the generated sludge at the bottom of the container;

[0029] The molar ratio of calcium ions in the lime milk to sulfate ions in the high-concentration sulfate wastewater to be treated is (1-2):1;

[0030] 2. Double salt treatment: Continue to add lime milk to the above pretreated sewage, stir for 5min to 6min, then add aluminum source, stir for another 20min to 40min, and collect the generated sludge at the bottom of the container;

[0031] The ratio of the sum of the molar amount of calcium ions in the lime milk added in step 2 and the molar amount of calcium ions remaining in the sewage after the pretreatment in step 1 to the molar amount of sulfate ions remaining in the sewage after the pretreatment in step 1 is 2:(1-1.1);

[0032] The ratio of the molar amount of sulfate remaining in the wastewater after the pretreatment in step 1 to the molar amount of aluminum ions in the aluminum source added in step 2 is 3:(2-2.2);

[0033] 3. Electro-flocculation treatment: Filter the wastewater treated in step 2 first, and then introduce it into the electro-flocculation reactor, start the electro-flocculation treatment and stir at the same time, the treatment time is 30min to 40min, part of the sludge generated in this stage will be transferred to the upper layer by the bubbles generated in the electro-flocculation process, and the floating sludge will be collected by the scraper; another part of the sludge will be collected at the bottom of the reactor; the two parts of the sludge will be dehydrated together to obtain aluminum hydroxide, which will be returned to the double salt method treatment stage as an aluminum source; the wastewater after electro-flocculation treatment will meet the discharge standards.

[0034] Specific implementation method 2: This implementation method is different from specific implementation method 1 in that the concentration of sulfate in the high-concentration sulfate wastewater to be treated in step 1 is 4000 mg / L to 10000 mg / L. Other aspects are the same as those of specific implementation method 1.

[0035] Specific implementation method 3: This implementation method is different from specific implementation method 1 or 2 in that the bottom of the container used in step 1 and step 2 is a conical structure, and the lower part is connected to the sludge collection tank. The rest is the same as specific implementation method 1 or 2.

[0036] Specific embodiment 4: This embodiment is different from specific embodiments 1 to 3 in that the aluminum source in step 2 is sodium metaaluminate and aluminum hydroxide, or a mixture of both. Other aspects are the same as those of specific embodiments 1 to 3.

[0037] Specific implementation method 5: This implementation method is different from specific implementation method 4 in that in step 3, the sewage treated in step 2 is first filtered through a filter membrane and then introduced into the electric flocculation reactor. The rest is the same as specific implementation method 4.

[0038] Specific implementation example 6: This implementation example is different from specific implementation example 5 in that the filtration membrane described in step 3 is a microfiltration membrane. The rest is the same as specific implementation example 5.

[0039] Specific embodiment 7: This embodiment differs from specific embodiment 6 in that the pore size of the microfiltration membrane in step 3 is 0.45 μm. The rest is the same as specific embodiment 6.

[0040] Specific embodiment eight: This embodiment differs from specific embodiment seven in that the temperature of the sludge dehydration treatment in step three is 80° C. to 90° C. The rest is the same as specific embodiment seven.

[0041] Specific embodiment 9: This embodiment differs from specific embodiment 8 in that: in step 3, the electrocoagulation process uses a pulse current with a current density of 20 mA / cm 2 ~40mA / cm 2 . The rest is the same as the eighth specific implementation method.

[0042] Specific embodiment 10: This embodiment differs from specific embodiment 9 in that the electrode plates used in the electrocoagulation process in step 3 are aluminum plates, and the distance between adjacent electrode plates is 0.5 cm. Other aspects are the same as specific embodiment 9.

[0043] The present invention is verified by the following tests:

[0044] Experiment 1: This experiment is a method for treating sulfate wastewater by electro-flocculation coupled with double salt method. The device used is as follows: Figure 1 As shown,

[0045] The outlets of the lime milk dosing tank 3 and the aluminum source dosing tank 4 are both connected to the first reactor 1. A first agitator 2 is arranged in the first reactor 1. The bottom of the first reactor 1 is a conical structure. The bottom of the first reactor 1 is connected to the first sludge collection tank 13. A first valve 15 is arranged between the bottom of the first reactor 1 and the first sludge collection tank 13. The liquid outlet of the first reactor 1 passes through the filter membrane 11 (microfiltration membrane, pore size is 0.45 μm), the second valve 5 and the lift pump 10 and the inlet of the electric flocculation reactor 9 in sequence. The liquid port is connected, a plurality of aluminum plates 14 are arranged in the electric flocculation reactor 9, and the spacing between adjacent aluminum plates 14 is 0.5 cm. A second agitator 8 is also arranged in the electric flocculation reactor 9. The bottom of the electric flocculation reactor 9 is a conical structure. The bottom of the electric flocculation reactor 9 is connected with the second sludge collection tank 12. A third valve 6 is arranged between the bottom of the electric flocculation reactor 9 and the second sludge collection tank 12; a scraper 16 is arranged above the electric flocculation reactor 9, and a third sludge collection tank 7 is also arranged outside the electric flocculation reactor 9;

[0046] The specific steps are as follows:

[0047] 1. Pretreatment: The high-concentration sulfate wastewater to be treated is introduced into the first reactor 1, and then lime milk is added to the first reactor 1 through the lime milk dosing tank 3 to adjust the pH to about 11, and the first agitator 2 is started to stir for 40 minutes, and the sulfate and magnesium ions in the wastewater will be converted into calcium sulfate and magnesium hydroxide precipitation, and the first valve 15 is opened. Due to the effect of gravity, the generated sludge is collected in the first sludge collection tank 13 at the bottom, and then all the sludge is taken out. This sludge is gypsum, which can be converted into desulfurized gypsum with certain economic value after treatment;

[0048] Figure 2 This is the XRD image of the sludge obtained after pretreatment. From the standard card PDF#72-0596, it can be seen that this sludge is mainly calcium sulfate;

[0049] The water quality parameters of the high-concentration sulfate wastewater to be treated are as follows: pH is 5.2, TOC is 137.85 mg / L, sulfate ion concentration is 7846 mg / L, calcium ion concentration is 493.87 mg / L, and magnesium ion concentration is 481.52 mg / L;

[0050] The molar ratio of calcium ions in the lime milk to sulfate ions in the high-concentration sulfate wastewater to be treated is 1.5:1;

[0051] 2. Double salt treatment: After the pretreatment in step 1, the concentration of calcium sulfate in the sewage is in a critical saturation state, and lime milk is continuously added to the pretreated sewage in the first reactor 1, and the first agitator 2 is started to stir for 5 minutes, and then the aluminum source sodium aluminate is added to the first reactor 1 through the aluminum source dosing tank 4, and then stirred for 30 minutes, and the calcium ions and sulfate ions in the sewage react with the aluminum salt to form calcium sulfate; the first valve 15 is opened, and the generated sludge is collected in the first sludge collection tank 13 at the bottom due to gravity;

[0052] The ratio of the molar amount of the sum of the calcium ions in the lime milk added in step 2 and the remaining calcium ions in the sewage after the pretreatment in step 1 to the molar amount of the sulfate ions remaining in the sewage after the pretreatment in step 1 is 2:1;

[0053] The ratio of the molar amount of sulfate remaining in the wastewater after the pretreatment in step 1 to the molar amount of aluminum ions in the aluminum source added in step 2 is 3:2;

[0054] After treatment in step 2, the concentration of sulfate ions in the sewage was 568.90 mg / L, the concentration of calcium ions was 380.2 mg / L, and the pH was 7.2;

[0055] Figure 3 This is the XRD image of the sludge obtained after double salt treatment. From the standard card PDF#72-0646, it can be seen that this sludge is mainly ettringite;

[0056] 3. Electrocoagulation treatment: Open the second valve 5, and the wastewater treated in step 2 passes through the filter membrane 11, the second valve 5 and the lifting pump 10 in turn into the electrocoagulation reactor 9, and the electrocoagulation treatment is started and the second agitator 8 is started for stirring at the same time. The treatment time is 30 minutes. The electrocoagulation process uses a pulse current with a current density of 30 mA / cm 2 ; Part of the sludge generated in this stage will be transferred to the upper layer by the bubbles generated during the electro-flocculation process, and the floating sludge will be collected by the scraper 16 into the third sludge collection tank 7; another part of the sludge is collected at the bottom of the reactor, and the third valve 6 is opened. Due to the effect of gravity, the sludge is collected in the second sludge collection tank 12 at the bottom; the two parts of the sludge are dehydrated at 80-90°C to obtain aluminum hydroxide, which can be returned to the double salt treatment stage as an aluminum source; the sulfate ion concentration in the sewage after electro-flocculation treatment is 83.37 mg / L, the calcium ion concentration is 74.08 mg / L, the TOC concentration is 30 mg / L, and the pH is 6.8, achieving standard discharge.

[0057] Figure 4 This is the XRD image of the sludge obtained after electrocoagulation treatment. From the standard card PDF#74-1119, it can be seen that this sludge is mainly aluminum hydroxide.

Claims

1. A method for treating sulfate wastewater by electro-flocculation coupled with double salt process, characterized in that The method for treating sulfate wastewater by electro-flocculation coupled with double salt method is carried out in the following steps:

1. Pretreatment: Add lime milk to the high-concentration sulfate wastewater to be treated, stir for 30 to 50 minutes, and collect the generated sludge at the bottom of the container; The molar ratio of calcium ions in the lime milk to sulfate ions in the high-concentration sulfate wastewater to be treated is (1-2):1; 2. Double salt treatment: Continue to add lime milk to the above pretreated sewage, stir for 5min to 6min, then add aluminum source, stir for another 20min to 40min, and collect the generated sludge at the bottom of the container; The ratio of the sum of the molar amount of calcium ions in the lime milk added in step 2 and the molar amount of calcium ions remaining in the sewage after the pretreatment in step 1 to the molar amount of sulfate ions remaining in the sewage after the pretreatment in step 1 is 2:(1-1.1); The ratio of the molar amount of sulfate remaining in the wastewater after the pretreatment in step 1 to the molar amount of aluminum ions in the aluminum source added in step 2 is 3:(2-2.2); 3. Electrocoagulation treatment: The wastewater treated in step 2 is filtered first, and then introduced into the electrocoagulation reactor, and electrocoagulation treatment is started and stirred at the same time. The treatment time is 30min to 40min. Part of the sludge generated in this stage will be transferred to the upper layer by the bubbles generated in the electrocoagulation process, and the floating sludge will be collected by the scraper; Another part of the sludge is collected at the bottom of the reactor; the two parts of sludge are dehydrated together to obtain aluminum hydroxide, which is returned to the double salt treatment stage and used as an aluminum source; the wastewater after electrocoagulation treatment is discharged in compliance with the standards.

2. The method for treating sulfate wastewater by electro-flocculation coupled with double salt method according to claim 1, characterized in that The concentration of sulfate in the high-concentration sulfate wastewater to be treated in step 1 is 4000 mg / L to 10000 mg / L.

3. The method for treating sulfate wastewater by electro-flocculation coupled with double salt method according to claim 1, characterized in that The bottom of the container used in step 1 and step 2 is a conical structure, and the lower part is connected to the sludge collection tank.

4. The method for treating sulfate wastewater by electro-flocculation coupled with double salt method according to claim 1, characterized in that The aluminum source described in step 2 is one or a mixture of sodium metaaluminate and aluminum hydroxide.

5. The method for treating sulfate wastewater by electro-flocculation coupled with double salt method according to claim 1, characterized in that In step three, the wastewater treated in step two is first filtered through a filter membrane and then introduced into an electrocoagulation reactor.

6. The method for treating sulfate wastewater by electro-flocculation coupled with double salt method according to claim 5, characterized in that The filtration membrane described in step three is a microfiltration membrane.

7. The method for treating sulfate wastewater by electro-flocculation coupled with double salt method according to claim 6, characterized in that The pore size of the microfiltration membrane described in step 3 is 0.45 μm.

8. The method for treating sulfate wastewater by electro-flocculation coupled with double salt method according to claim 1, characterized in that The temperature of the sludge dehydration treatment in step three is 80°C to 90°C.

9. The method for treating sulfate wastewater by electro-flocculation coupled with double salt method according to claim 1, characterized in that In step 3, the electrocoagulation process uses a pulse current with a current density of 20 mA / cm 2 ~40mA / cm 2 .

10. The method for treating sulfate wastewater by electro-flocculation coupled with double salt method according to claim 9, characterized in that The electrode plates used in the electrocoagulation process in step 3 are aluminum plates, and the distance between adjacent electrode plates is 0.5 cm.

Images