CN109095732B - Process for preparing high-purity magnesium hydroxide based on magnesium-method desulfurization wastewater - Google Patents
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Abstract
The invention provides a process for preparing high-purity magnesium hydroxide based on magnesium-method desulfurization wastewater, which comprises the steps of pretreating the desulfurization wastewater through the processes of aeration, electric flocculation, oil removal, ammonia nitrogen removal, COD removal and the like in a pretreatment stage, separating most of calcium ions and part of magnesium ions in a salt adding and separating stage by adding dibasic acid, separating monovalent ions and divalent ions by a nanofiltration technology to obtain a high-magnesium-ion-concentration solution, adding sodium hydroxide in a magnesium recovery stage to obtain magnesium hydroxide precipitate, filtering, drying, dehydrating and the like to obtain a magnesium hydroxide product with high purity, wherein the purity of the magnesium hydroxide product is 99.83% by determination; the method has the advantages of short process flow, economy and environmental protection, and realizes the reutilization of resources in the desulfurization wastewater treatment process.
Description
Technical Field
The invention relates to the technical field of wastewater treatment, in particular to a process for preparing high-purity magnesium hydroxide based on magnesium-method desulfurization wastewater.
Background
In the water treatment system of the thermal power plant, the reliability of the magnesium desulfurization wastewater treatment system equipment has been more and more emphasized. The magnesium resource is abundant in China, the wet magnesium flue gas desulfurization technology is researched and developed for more than ten years in China, the reliability of the technology, the economical efficiency of construction and operation and the feasibility of recycling of desulfurization byproducts are widely accepted, and the wet magnesium flue gas desulfurization technology has a wide development prospect. The wet magnesium method flue gas desulfurization process is characterized by that the hydrated slurry of magnesium oxide is used as magnesium hydroxide as main component, and said magnesium hydroxide is used as absorbent to absorb sulfur dioxide in flue gas, and its desulfurization process principle is basically identical to that of calcium oxide wet desulfurization process, the flue gas can be radially fed into reaction tower from lower portion of desulfurization reaction tower, and can be contacted with magnesium hydroxide slurry in the course of raising in the reaction tower, and the sulfur dioxide in the flue gas can be reacted with magnesium hydroxide to produce magnesium sulfite SO as to obtain the desulfurization effect, at the same time the SO contained in the flue gas is2Harmful gases such as HCl and HF are also absorbed in the absorption tower and dissolved into the slurry. With the consumption of magnesium hydroxide in the desulfurizing tower, the pH value is reduced, fresh slurry is required to be continuously supplemented into the desulfurizing tower, and a large amount of desulfurization waste water is generated after desulfurization. The magnesium desulfurization wastewater contains a large amount of gypsum, fly ash and sulfate suspended particles, the concentration of chloride ions is more than 20000Mg/L, and cations in the magnesium desulfurization wastewater are mainly Mg2+Also contains a small portion of Ca2+And a small amount of heavy metal ions, and secondary pollution can be brought to the environment by direct discharge, while the technical means of zero discharge of the magnesium desulfurization wastewater is expensive, the technique is not mature, and the common power plant can not bear the technique. If the ions contained in the magnesium desulfurization wastewater are recycled in a reasonable mode, the environmental protection obligation is completed, and meanwhile, obvious economic benefits are brought to enterprises, which has great significance to environmental protectors.
The magnesium desulfurization wastewater contains a large amount of magnesium ions and can be used as a raw material for preparing magnesium hydroxide. The magnesium hydroxide has the advantages of alkalescence, large buffering property, good thermal decomposition property, large activity, strong adsorption capacity, no toxicity and no smoke, shows excellent performance in various aspects, is widely applied to the fields of acid wastewater treatment, ceramic materials, inorganic flame retardants, flue gas desulfurization and the like, and besides general varieties, the magnesium hydroxide powder has a large number of special and compound new products and is wide in application field.
The method for producing high-purity magnesium hydroxide by using the magnesium desulfurization wastewater as the raw material has the natural advantage of low raw material cost, not only creates economic benefits, but also reduces environmental pollution. Compared with the traditional raw material for preparing magnesium hydroxide, the magnesium-method desulfurization wastewater has complex components, contains a large amount of sodium ions, potassium ions, calcium ions and a small amount of sulfate radicals, carbonate radicals, silicates and other substances besides magnesium ions, and must remove impurity components which have adverse effects on preparation reaction, finished product purification and other process operations in the process of preparing high-purity magnesium hydroxide by utilizing the magnesium-method desulfurization wastewater, in particular to an impurity removal process which is simple to operate and low in cost. The magnesium hydroxide product particles have larger surface energy and surface polarity and are easy to agglomerate. The invention aims to solve the problems and provides a method for preparing high-purity magnesium hydroxide by utilizing magnesium desulfurization wastewater.
Disclosure of Invention
In order to solve the technical problems, the invention provides a process for preparing high-purity magnesium hydroxide based on magnesium desulfurization wastewater, which purifies the salt in the magnesium desulfurization wastewater through three stages, so that the magnesium desulfurization wastewater can be recycled, and economic and environment-friendly benefits are achieved.
The technical scheme adopted by the invention for solving the technical problems is as follows:
the invention comprises the following steps:
(1) a pretreatment stage:
pretreating the desulfurization wastewater to oxidize reductive ions and remove COD and ammonia nitrogen at the same time;
(2) adding medicine and separating salt:
adding sodium carbonate and sodium hydroxide into the pretreated wastewater to obtain suspension containing magnesium hydroxide and calcium carbonate precipitates, and aging and separating the precipitates and supernatant;
(3) and (3) magnesium recovery stage:
the supernatant mainly contains magnesium sulfate and sodium chloride solution, and is subjected to nanofiltration, and the cation on the water production side is mainly Na+The positive ion at the concentrated water side is mainly Mg2+(ii) a And adding sodium hydroxide into the concentrated water side to obtain magnesium hydroxide precipitate, separating the precipitate from supernatant, filtering, drying and dehydrating the precipitate to obtain a high-purity magnesium hydroxide product. In the process, softened wastewater is led to a nanofiltration system for separation of monovalent and divalent ions, and cations on a water production side are mainly Na+The positive ion at the concentrated water side is mainly Mg2 +And the water producing side is directly sent to a NaCl recycling system.
As a further improvement of the invention, the pretreatment stage comprises an aeration process, an electric flocculation process, an oil removal process, an ammonia nitrogen removal process and a COD removal process. The aeration process adopts blast aeration, when the desulfurization wastewater flows through the aeration oxidation tank, the bottom of the aeration oxidation tank is provided with a fan which can carry out blast aeration, on one hand, the suspended matter state of the raw water is maintained, the COD of the raw water is reduced, and the magnesium desulfurization wastewater is aerated to carry out the NH in the magnesium desulfurization wastewater3N and small molecular organic matters are blown off into the atmosphere, and reducing ion sulfite ions in the magnesium desulphurization wastewater can be oxidized into sulfate ions.
As a further improvement of the invention, the flocculating agent in the electric flocculation process is an anionic polyacrylamide flocculating agent. The electrocoagulation process can further remove COD in the raw water. Because the concentration of suspended matters in the desulfurization wastewater is higher, the desulfurization wastewater is subjected to flocculation treatment firstly. The flocculation can effectively remove suspended particles such as limestone, gypsum particles, silicon dioxide and the like in the tail end wastewater. The flocculant is anionic polyacrylamide, the dosage of the flocculant is 20mg/L, the stirring time is 30min, and the temperature of a reaction system is 50-60 ℃.
As a further improvement of the invention, the oil removing process adopts oil removing resin as a filter medium. Oil substances in the magnesium desulfurization wastewater can foul and block a nanofiltration membrane treatment system, so that the nanofiltration membrane treatment system cannot normally and stably operate, and therefore the oil substances need to be removed in a pretreatment stage. The oil removing reactor adopts oil removing resin as a filter medium, utilizes lipophilic groups in the resin to collect and adsorb emulsified oil and oil droplets in magnesium desulfurization wastewater, and when an oil layer on the surface of the resin reaches a certain thickness, the oil droplets are separated from the resin and float to the water surface under the repulsion action of water flow power and hydrophilic groups in the resin, so that the aim of oil-water separation is fulfilled.
As a further improvement of the invention, zeolite with strong selective adsorption capacity to ammonia nitrogen is adopted in the ammonia nitrogen removal process. The chemical reaction formula of the zeolite at the ammonia nitrogen adsorption stage is as follows:
Z·M+nNH3-H→Z·nNH4+M (1)
wherein Z is zeolite; m is zeolite heavy metal cation; n is the number of nuclear charges;
as a further improvement of the invention, modified activated carbon is adopted as filling in the COD removal process, and ozone is filled for pre-oxidation before reaction. The COD removing reactor is filled with modified activated carbon, and O is added before entering the reactor3And carrying out pre-oxidation. The process is to use O3A method of combining oxidation and a biological activated carbon filter, which is to mix O3The chemical oxidation, the physical and chemical adsorption of the activated carbon and the biological oxidation degradation are integrated into a whole. The main purpose is to remove trace organic components and reducing salts in the wastewater. O is carried out before biological activated carbon adsorption3Pre-oxidizing, namely primarily oxidizing organic components and other reducing salts in the water to reduce the organic load of the biological activated carbon filter; on the other hand, O3The pre-oxidation can convert part of the organic matters which are difficult to biodegrade into substances which are easy to biodegrade, thereby improving the biodegradability of the inlet water of the biological activated carbon filter.
As a further improvement of the method, sodium carbonate is added in the salt adding and separating stage to remove a small amount of calcium ions, then sodium hydroxide is added to adjust the pH value to 7.5-8.5, and simultaneously a small amount of magnesium ions are precipitated, after aging, the bottom precipitate is sent to a tubular membrane system to be filtered, and then sent to a plate-and-frame filter press to be dried and dehydrated, so that calcium carbonate and magnesium hydroxide precipitate are obtained. The chemical reactions that occur at this stage are as follows:
Ca2++CO3 2-→CaCO3↓ (2)
Mg2++2OH-→Mg(OH)2↓ (3)
the slurry mixed liquid in the reaction tank is filtered out through a tubular membrane filtering system, and then is connected with a frame filter press for treatment, the water is recycled, and the final product is CaCO3And Mg (OH)2The mixture can be recycled to a desulfurization system.
Compared with the prior art, the invention has the following technical effects:
according to the invention, the double alkali is added to precipitate calcium ions, and the calcium carbonate solid product of the mixed oil with a small amount of magnesium hydroxide is obtained after tubular membrane separation, wherein the calcium carbonate is an important raw material in the desulfurization process, and can be used as a medicament to be recycled in the desulfurization process, so that the calcium ion treatment can be realized, and the cost can be reduced.
The magnesium hydroxide is an important chemical raw material, and the traditional preparation process is characterized in that the magnesium hydroxide is prepared by taking the water chestnut ore as a raw material, the process is complex, and the water chestnut ore is used as a non-renewable resource and cannot be exploited infinitely.
According to the invention, the desulfurization wastewater is purified step by step and softened step by step, so that the finally prepared fresh water has excellent quality, completely meets the standard of production water, can be used as the production water for backflow, and greatly saves water resources. The method converts calcium and magnesium ions in the desulfurization wastewater into byproducts with economic value in the process of softening the desulfurization wastewater, thereby realizing the comprehensive utilization of resources.
Drawings
FIG. 1 is a process flow diagram of the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The invention comprises the following steps:
(1) a pretreatment stage:
pretreating the desulfurization wastewater to oxidize reductive ions and remove COD and ammonia nitrogen at the same time;
(2) adding medicine and separating salt:
adding sodium carbonate and sodium hydroxide into the pretreated wastewater to obtain suspension containing magnesium hydroxide and calcium carbonate precipitates, and aging and separating the precipitates and supernatant;
(3) and (3) magnesium recovery stage:
carrying out nanofiltration on the supernatant liquid which mainly comprises magnesium sulfate and sodium chloride solution, wherein the water producing side is the sodium chloride solution, and the concentrated water side is the high-concentration magnesium sulfate solution; and adding sodium hydroxide into the concentrated water side to obtain magnesium hydroxide precipitate, separating the precipitate from supernatant, filtering the precipitate by a tubular membrane, and drying and dehydrating to obtain a high-purity magnesium hydroxide product. In the process, softened wastewater is led to a nanofiltration system for separation of monovalent and divalent ions, and cations on a water production side are mainly Na+The positive ion at the concentrated water side is mainly Mg2+. In the process, the supernatant after the magnesium hydroxide precipitate is separated is refluxed to a nanofiltration system for secondary nanofiltration so as to improve the yield.
The pretreatment stage comprises an aeration process, an electric flocculation process, an oil removal process, an ammonia nitrogen removal process and a COD removal process. The aeration process adopts blast aeration, when the desulfurization wastewater flows through the aeration oxidation tank, the bottom of the aeration oxidation tank is provided with a fan, the blast aeration can be carried out, on one hand, the suspended matter state of raw water is maintained, the COD of the raw water is reduced, on the other hand, the magnesium desulfurization wastewater is aerated, NH3-N and micromolecule organic matters in the magnesium desulfurization wastewater can be blown off to the atmosphere, and on the other hand, reducing ions such as sulfite ions in the magnesium desulfurization wastewater can be oxidized into sulfate ions.
The flocculating agent in the electric flocculation process is an anionic polyacrylamide flocculating agent. The electrocoagulation process can further remove COD in the raw water. Because the concentration of suspended matters in the desulfurization wastewater is higher, the desulfurization wastewater is subjected to flocculation treatment firstly. The flocculation can effectively remove suspended particles such as limestone, gypsum particles, silicon dioxide and the like in the tail end wastewater. The flocculant is anionic polyacrylamide, the dosage of the flocculant is 20mg/L, the stirring time is 30min, and the temperature of a reaction system is 50-60 ℃.
As a further improvement of the invention, the oil removing process adopts oil removing resin as a filter medium. Oil substances in the magnesium desulfurization wastewater can foul and block a nanofiltration membrane treatment system, so that the nanofiltration membrane treatment system cannot normally and stably operate, and therefore the oil substances need to be removed in a pretreatment stage. The oil removing reactor adopts oil removing resin as a filter medium, utilizes lipophilic groups in the resin to collect and adsorb emulsified oil and oil droplets in magnesium desulfurization wastewater, and when an oil layer on the surface of the resin reaches a certain thickness, the oil droplets are separated from the resin and float to the water surface under the repulsion action of water flow power and hydrophilic groups in the resin, so that the aim of oil-water separation is fulfilled.
The process for removing ammonia nitrogen adopts zeolite with strong selective adsorption capacity to ammonia nitrogen. The chemical reaction formula of the zeolite at the ammonia nitrogen adsorption stage is as follows:
Z·M+nNH3-H→Z·nNH4+M (1)
wherein Z is zeolite; m is zeolite heavy metal cation; n is the number of nuclear charges;
modified activated carbon is used as filling in the COD removing process, and ozone crystal form pre-oxidation is filled before reaction. The COD removing reactor is filled with modified activated carbon, and O is added before entering the reactor3And carrying out pre-oxidation. The process is to use O3A method of combining oxidation and a biological activated carbon filter, which is to mix O3The chemical oxidation, the physical and chemical adsorption of the activated carbon and the biological oxidation degradation are integrated into a whole. The main purpose is to remove trace organic components and reducing salts in the wastewater. O is carried out before biological activated carbon adsorption3Pre-oxidation ofThe organic components and other reducing salts in the water can be primarily oxidized to reduce the organic load of the biological activated carbon filter; on the other hand, O3The pre-oxidation can convert part of the organic matters which are difficult to biodegrade into substances which are easy to biodegrade, thereby improving the biodegradability of the inlet water of the biological activated carbon filter.
And in the salt adding and separating stage, sodium hydroxide and sodium carbonate are added, then a small amount of calcium ions are removed, the pH value is adjusted to 7.5-8.5, meanwhile, a small amount of magnesium ions are precipitated, and after aging, the bottom precipitate is sent to a tubular membrane system for filtering and then sent to a plate-and-frame filter press for drying and dewatering to obtain calcium carbonate and magnesium hydroxide precipitate. The chemical reactions that occur at this stage are as follows:
Ca2++CO3 2-→CaCO3↓ (2)
Mg2++2OH-→Mg(OH)2↓ (3)
the slurry mixed liquid in the reaction tank is filtered out through a tubular membrane filtering system, and then is connected with a frame filter press for treatment, the water is recycled, and the final product is CaCO3And Mg (OH)2The mixture can be recycled to a desulfurization system.
The technical solution of the present invention will be described in further detail with reference to specific examples.
Example 1:
this example was tested by formulating a wastewater having the same composition as that of a wastewater desulfurized by the magnesium process.
The device used in the simulation experiment comprises a ton barrel for storing desulfurization wastewater, an aeration tank with a blower at the bottom, a flocculation tank for electric flocculation, an oil removal reactor for removing oil, an ammonia nitrogen removal reactor for removing ammonia nitrogen, an ozone generator, a COD removal reactor for removing COD, a calcium sulfate sedimentation tank for producing calcium sulfate, a plate-and-frame filter press for drying and dewatering sediment, a nanofiltration system, Mg (OH)2Sedimentation tank, tubular membrane system and plate and frame filter press. These devices are all commercially available products, and the specific structures and the using methods are all the prior art, which are not described herein.
In the experimental process, the concentration of calcium ions in the simulated magnesium desulfurization wastewater is 400mg/L, the concentration of magnesium ions is 5000mg/L, the concentration of sulfate ions is 12000mg/L, the concentration of chloride ions is 28769mg/L, the concentration of COD is 1550mg/L, the concentration of TDS is 138500mg/L, the concentration of ammonia nitrogen is 2076mg/L, and the pH value is controlled between 5.75 and 6.92.
Leading the drained magnesium desulfurization wastewater 1t into the process system, sampling and measuring at the end of a pretreatment process section, wherein the pretreatment has an obvious effect of removing COD, TDS and ammonia nitrogen. Opening a high-pressure pump at the top end of the ton barrel, introducing the wastewater into a treatment system, opening a fan at the bottom of the aeration tank, continuously aerating at 10L/min, opening an electric flocculation device, wherein the flocculating agent is anionic polyacrylamide, the using amount of the flocculating agent is 20mg/L, the stirring time is 30min, and the temperature of a reaction system is 50-60 ℃.
The ozone generator is opened, the valve is controlled to control the ozone ventilation quantity to be 25m3/L。
In the presence of CaCO3Adding sodium carbonate Na into the sedimentation tank2CO3The dosage is 1325mg/L, Na (OH) is used for adjusting the PH to 7.0-8.5,
the chemical reaction at this stage takes place as follows
Ca2++CO3 2-→CaCO3↓
Mg2++2OH-→Mg(OH)2↓
Standing for 10-15 min after each addition, and after the precipitation reaction is completed, CaCO3And (3) delivering the slurry in the reaction tank to a tubular membrane system for filtering treatment, delivering the filtered slurry to a plate-and-frame filter press for filter pressing, and then taking the filtered slurry as gypsum which can be recycled to a desulfurization system.
And (4) the softened desulfurization wastewater passes through a nanofiltration system to be subjected to salt separation. The NaCl produced water produced at the water producing side after the salt separation is finished is directly sent to a recycling system; the recovery rate of the nanofiltration system is set to be 70 percent, namely the water yield ratio of the produced water at the water production side to the source water and the MgSO (MgSO) produced at the concentrated water side4Mg in concentrated water2+The concentration is increased to 16666.6mg/L, and concentrated water is introduced into Mg (OH)2Adding Na (OH) after the sedimentation tank, adjusting the PH to 11 to generate Mg (OH)2Precipitation, the reaction is as follows:
Mg2++2OH-→Mg(OH)2↓
after the reaction is finished, the mixed solution is introduced into a tubular membrane system for filtration treatment, the water producing side is recycled to a nanofiltration system, and the slurry is filtered by the tubular membrane and then made into solid Mg (OH) by a plate-and-frame filter press2The purity was determined to be 99.83%.
The above-described embodiments are merely illustrative of the preferred embodiments of the present invention, and do not limit the scope of the present invention, and various modifications and improvements of the technical solutions of the present invention can be made by those skilled in the art without departing from the spirit of the present invention, and the technical solutions of the present invention are within the scope of the present invention defined by the claims.
Claims (3)
1. A process for preparing high-purity magnesium hydroxide based on magnesium-method desulfurization wastewater is characterized by comprising the following steps:
(1) a pretreatment stage:
pretreating the desulfurization wastewater, oxidizing reductive ions, and removing COD and ammonia nitrogen at the same time;
(2) adding medicine and separating salt:
adding a sodium carbonate solution into the pretreated wastewater, then adding a NaOH solution, adjusting the pH to 7.5-8.5 to obtain a suspension containing magnesium hydroxide and calcium carbonate precipitates, and aging and separating the precipitates and a supernatant;
(3) and (3) magnesium recovery stage:
the supernatant mainly contains magnesium sulfate and sodium chloride solution, and is subjected to nanofiltration, and the cation on the water production side is mainly Na+The positive ion at the concentrated water side is mainly Mg2+(ii) a Adding sodium hydroxide into the concentrated water side to obtain magnesium hydroxide precipitate, separating the precipitate from supernatant, filtering, drying and dehydrating the precipitate to obtain a high-purity magnesium hydroxide product;
the pretreatment stage comprises an aeration process, an electric flocculation process, an oil removal process, an ammonia nitrogen removal process and a COD removal process;
the flocculating agent in the electric flocculation process is an anionic polyacrylamide flocculating agent;
the oil removing process adopts oil removing resin as a filter medium;
zeolite with strong selective adsorption capacity on ammonia nitrogen is adopted in the ammonia nitrogen removal process;
modified activated carbon is used as filling in the COD removing process, and ozone is filled for pre-oxidation before reaction.
2. The process for preparing high-purity magnesium hydroxide based on magnesium-method desulfurization wastewater according to claim 1, characterized in that after sodium hydroxide and sodium carbonate are added in the salt adding and separating stage, a small amount of calcium ions are removed, the pH is adjusted to 7.5-8.5, meanwhile, a small amount of magnesium ions are precipitated, and after aging, the bottom precipitate is sent to a tubular membrane system for filtration and then sent to a plate-and-frame filter press for drying and dehydration, so that calcium carbonate and magnesium hydroxide precipitate is obtained.
3. The process for preparing high-purity magnesium hydroxide based on magnesium desulfurization wastewater according to claim 1, wherein the concentrated water subjected to nanofiltration separation is subjected to sodium hydroxide addition to obtain magnesium hydroxide precipitate, the precipitate and the supernatant are separated by a tubular membrane, and the precipitate is filtered, dried and dehydrated to obtain a high-purity magnesium hydroxide product.
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| TWI786413B (en) * | 2019-07-02 | 2022-12-11 | 水善研發有限公司 | System for flue gas desulfurization wastewater treatment |
| CN110386710A (en) * | 2019-07-19 | 2019-10-29 | 北京朗新明环保科技有限公司南京分公司 | A method of epsom salt is produced using heat-engine plant desulfurized waste water |
| CN111533319B (en) * | 2020-05-20 | 2020-12-22 | 北方工业大学 | Device and method for treating COD (chemical oxygen demand) in desulfurization wastewater by ozone oxidation method |
| CN113634224B (en) * | 2020-12-30 | 2023-07-28 | 华北电力大学（保定） | Functional material for synchronously controlling magnesium sulfite and heavy metal ions and desulfurization method |
| CN115814752B (en) * | 2023-01-31 | 2024-04-05 | 成都理工大学 | Composite material of calcium carbonate and nano magnesium oxide and preparation and application methods thereof |
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