阅读说明：本技术 含硫酸镁和氯化镁稀土废水处理及资源化利用工艺及系统 (Magnesium sulfate and magnesium chloride containing rare earth wastewater treatment and resource utilization process and system ) 是由 苏德水 杨峰 王春燕 孟凡伟 贾彦涛 何玉田 于 2021-06-11 设计创作，主要内容包括：本发明公开了一种含硫酸镁和氯化镁稀土废水处理及资源化利用工艺及系统,属于废水处理领域。该处理系统经过预处理装置、膜浓缩装置、蒸发装置及分盐装置,最后处理的出水可循环利用,水的利用率100％,实现了含硫酸镁和氯化镁的稀土废水的零排放；本发明的硫酸镁和氯化镁的稀土废水资源化处理工艺避开了互溶和共结晶的难分离特性,采用分步结晶,低成本实现了废水组分资源化利用,大幅降低了母液处理成本,实现了废水废渣零排放；该处理工艺设计合理,易于工业化实施。(The invention discloses a process and a system for treating and recycling waste water containing magnesium sulfate and magnesium chloride rare earth, belonging to the field of waste water treatment. The treatment system passes through the pretreatment device, the membrane concentration device, the evaporation device and the salt separation device, the finally treated effluent can be recycled, the utilization rate of the water is 100 percent, and the zero emission of the rare earth wastewater containing magnesium sulfate and magnesium chloride is realized; the rare earth wastewater resource treatment process for magnesium sulfate and magnesium chloride avoids the difficult separation characteristic of mutual solubility and cocrystallization, adopts fractional crystallization, realizes resource utilization of wastewater components at low cost, greatly reduces the mother liquor treatment cost, and realizes zero discharge of wastewater and waste residues; the treatment process has reasonable design and is easy for industrial implementation.)

1. A process for treating and recycling rare earth wastewater containing magnesium sulfate and magnesium chloride comprises the following steps:

(1) pumping the rare earth wastewater in the raw water tank into an acid-base adjusting tank, adjusting the pH, stirring the rare earth wastewater in an intermediate storage tank, removing some heavy metals, pumping the rare earth wastewater into a vertical flow settler for desilting, overflowing the top of the rare earth wastewater to an oil removal device, and removing oil, and then feeding the rare earth wastewater into a coarse filter to further remove suspended matters;

(2) the wastewater treated by the coarse-effect filter enters a membrane concentration system, and the membrane concentration system adopts an ultrafiltration membrane device, a nanofiltration membrane device and a reverse osmosis membrane device, or adopts the ultrafiltration membrane device and the reverse osmosis membrane device;

when the ultrafiltration membrane device, the nanofiltration membrane device and the reverse osmosis membrane device are adopted, concentrated water of the ultrafiltration membrane device returns to the raw water pool, and produced water enters the nanofiltration membrane device; concentrated water of the nanofiltration membrane device enters an MVR evaporation device, and produced water enters a reverse osmosis membrane device; concentrated water of the reverse osmosis membrane device enters an MVR evaporation device, and produced water reaches the discharge standard for cyclic utilization;

when the ultrafiltration membrane device and the reverse osmosis membrane device are adopted, concentrated water of the ultrafiltration membrane device returns to the raw water pool, and produced water enters the reverse osmosis membrane device; concentrated water of the reverse osmosis membrane device enters an MVR evaporation device, and produced water reaches the discharge standard for cyclic utilization;

(3) the concentrated water entering the MVR evaporation device firstly enters a second-effect evaporator in the MVR two-effect evaporator for evaporation concentration, and enters a MVR first-effect evaporator for continuous evaporation after evaporation concentration;

(4) after the concentration of the feed liquid concentrated by the MVR evaporation device reaches a design value, pumping the feed liquid to a magnesium sulfate crystallization device for cooling crystallization, pumping the feed liquid to a filtering device for filtering, and after filtering, sending magnesium sulfate heptahydrate crystals to a drying device for drying; and (4) the filtered mother liquor enters a mother liquor concentration device to evaporate water again, and then the mother liquor is pumped into a magnesium chloride crystallization device to produce the scraper-shaped magnesium chloride.

2. The process for treating and recycling waste water containing magnesium sulfate and magnesium chloride and rare earth as claimed in claim 1, wherein in the step (1), the rare earth waste water is subjected to acid-base regulation in an acid-base regulation tank, and an automatic pH detection device is arranged in the tank, so that the acid-base amount added can be automatically adjusted by recognizing the acid-base change caused by water quality change; the pH value of the waste water is adjusted to 6.0-10.5.

3. The process for treating and recycling the waste water containing the magnesium sulfate and the magnesium chloride and the rare earth according to claim 2, wherein in the step (1), the acid-base regulation is carried out by adding slaked lime in an acid-base regulation tank, the intermediate storage tank is provided with a stirring or aeration device, and the waste water is fully separated out of supersaturated calcium sulfate in the intermediate storage tank and a vertical flow settler to oxidize and precipitate part of heavy metal ions.

4. The process for treating and recycling wastewater containing magnesium sulfate and magnesium chloride and rare earth as claimed in claim 1, wherein in the step (3), the ratio of magnesium sulfate to magnesium chloride is controlled to be 2: 1-8: 1.

5. the process for treating and recycling the magnesium sulfate and magnesium chloride-containing rare earth wastewater as claimed in claim 4, wherein the ratio of magnesium sulfate to magnesium chloride is controlled by returning the bottom precipitate of the mother liquor concentration device in the step (4) to a double-effect evaporator of the MVR evaporation device.

6. The process for treating and recycling rare earth wastewater containing magnesium sulfate and magnesium chloride according to claim 1, wherein in the step (4), the ratio of magnesium sulfate to magnesium chloride in the wastewater from the magnesium sulfate crystallization device is controlled to be 1: 1-1: 2, controlling the ratio of magnesium sulfate to magnesium chloride in the wastewater of the mother liquor concentration device to be 1: 2-1: 4.

7. a treatment and resource utilization system for rare earth wastewater containing magnesium sulfate and magnesium chloride is characterized by comprising a pretreatment unit, a membrane concentration unit, an evaporation unit and a salt separation unit which are sequentially arranged;

the system comprises an evaporation unit, a heat exchanger and a heat exchanger, wherein the evaporation unit adopts an MVR evaporation device, and comprises a second-effect evaporator and a first-effect evaporator;

the heat exchange media of the second-effect evaporator and the first-effect evaporator are respectively communicated with the preheating heat exchanger, and exchange heat with the concentrated wastewater of the membrane concentration unit in the preheating heat exchanger for condensation.

8. The system for treating and recycling wastewater containing magnesium sulfate and magnesium chloride and rare earth as claimed in claim 7, wherein the pretreatment unit comprises:

the waste water is subjected to acid-base regulation in the acid-base regulation tank, and an automatic pH detection device is arranged in the tank, so that the acid-base amount added can be automatically adjusted by recognizing the acid-base change caused by water quality change;

the intermediate storage tank is connected with the acid-base adjusting tank, and a stirring or aeration device is arranged in the tank; the bottom is connected with a sludge tank;

the vertical flow settler is connected with the intermediate storage tank through a pump and a flowmeter; the top part overflows to the oil removal device, and the bottom part is connected with a sludge pool;

the oil removal device is connected with the vertical flow settler, and the floating foam on the top part flows back to the sludge tank;

and the coarse filter is connected with the oil removal device through a pump.

9. The system for treating and recycling wastewater containing magnesium sulfate and magnesium chloride and rare earth as claimed in claim 7, wherein the membrane concentration unit comprises an ultrafiltration membrane device, a nanofiltration membrane device and a reverse osmosis membrane device, or comprises an ultrafiltration membrane device and a reverse osmosis membrane device.

10. The system for treating and recycling waste water containing magnesium sulfate and magnesium chloride and rare earth as claimed in claim 7, wherein the salt separation unit comprises a magnesium sulfate crystallization device, a filtering device, a mother liquor concentration device and a magnesium chloride crystallization device which are connected in sequence.

Technical Field

The invention belongs to the field of water pollution treatment of environmental engineering, and particularly relates to a process and a system for zero discharge treatment and resource utilization of magnesium sulfate and magnesium chloride of rare earth wastewater containing magnesium sulfate and magnesium chloride.

Background

Rare earth is a non-renewable important strategic resource, has great influence on national safety and national economic development, and is increasingly widely applied to various departments of national economy. Through development for many years, the rare earth industry in China is continuously enlarged, the problem of environmental pollution in the rare earth production process is increasingly prominent, the healthy development of the industry is seriously influenced, the huge environmental pollution is always an unsolved soft rib, and the environmental protection problem is gradually raised to the core of the development of the industry.

At present, the methods adopted by various processes for treating the rare earth smelting magnesium sulfate wastewater at home and abroad are summarized as a chemical method, an ion exchange method, a distillation concentration method, a membrane technology treatment method and the like. The methods play a good role in treating rare earth smelting, but have the defects of complex structure, large investment, large occupied area, high operating cost, high energy consumption, difficult maintenance and operation, poor sewage purification effect, discharge after reaching the standard, incapability of recycling water and other resources, incapability of achieving final treatment, incapability of recycling magnesium sulfate and magnesium chloride and the like to different degrees.

Disclosure of Invention

The invention aims to provide a treatment process and a treatment system for efficiently treating zero discharge of rare earth wastewater containing magnesium sulfate and magnesium chloride and recycling the magnesium sulfate and the magnesium chloride.

According to the first aspect of the invention, the process for treating and recycling the waste water containing magnesium sulfate and magnesium chloride rare earth comprises the following steps:

(1) pumping the rare earth wastewater in the raw water tank into an acid-base adjusting tank, adjusting the pH, stirring the rare earth wastewater in an intermediate storage tank, removing some heavy metals, pumping the rare earth wastewater into a vertical flow settler for desilting, overflowing the top of the rare earth wastewater to an oil removal device, and removing oil, and then feeding the rare earth wastewater into a coarse filter to further remove suspended matters;

(2) the wastewater treated by the coarse-effect filter enters a membrane concentration system, and the membrane concentration system adopts an ultrafiltration membrane device, a nanofiltration membrane device and a reverse osmosis membrane device, or adopts the ultrafiltration membrane device and the reverse osmosis membrane device; the milipore filter device mainly used filters the colloid in the waste water, receives the divalent salt that the filter membrane device mainly used filtered in the waste water, and reverse osmosis membrane device mainly used filters the monovalent salt in the waste water.

When the ultrafiltration membrane device, the nanofiltration membrane device and the reverse osmosis membrane device are adopted, concentrated water of the ultrafiltration membrane device returns to the raw water pool, and produced water enters the nanofiltration membrane device; concentrated water of the nanofiltration membrane device enters an MVR evaporation device, and produced water enters a reverse osmosis membrane device; concentrated water of the reverse osmosis membrane device enters an MVR evaporation device, and produced water reaches the discharge standard for cyclic utilization;

when the ultrafiltration membrane device and the reverse osmosis membrane device are adopted, concentrated water of the ultrafiltration membrane device returns to the raw water pool, and produced water enters the reverse osmosis membrane device; concentrated water of the reverse osmosis membrane device enters an MVR evaporation device, and produced water reaches the discharge standard for cyclic utilization;

(3) the concentrated water entering the MVR evaporation device firstly enters a second-effect evaporator in the MVR two-effect evaporator for evaporation concentration, and enters a MVR first-effect evaporator for continuous evaporation after evaporation concentration;

(4) after the concentration of the feed liquid concentrated by the MVR evaporation device reaches a design value, pumping the feed liquid to a magnesium sulfate crystallization device for cooling crystallization, pumping the feed liquid to a filtering device for filtering, and after filtering, sending magnesium sulfate heptahydrate crystals to a drying device for drying; and (4) the filtered mother liquor enters a mother liquor concentration device to evaporate water again, and then the mother liquor is pumped into a magnesium chloride crystallization device to produce the scraper-shaped magnesium chloride.

In the step (1), acid-base adjustment is carried out on the rare earth wastewater in an acid-base adjusting tank, and an automatic pH detection device is arranged in the tank, so that the acid-base amount added can be automatically adjusted due to acid-base change caused by water quality change; the pH value of the waste water is adjusted to 6.0-10.5, preferably 7.5-9.5.

In the step (1), acid-base adjustment is carried out by adding slaked lime in an acid-base adjustment tank, the rare earth wastewater containing magnesium sulfate and magnesium chloride stays in an intermediate storage tank for more than 12 hours, and the intermediate storage tank can be formed by connecting a plurality of tanks in series or in parallel. Preferably, the multiple tanks are connected in series, the stirring or aeration device is installed in the middle storage tank, the top overflows to the vertical flow settler, supersaturated calcium sulfate is fully separated out from the wastewater in the middle storage tank and the vertical flow settler, and part of heavy metal ions are oxidized and precipitated to prevent the clear liquid after sedimentation from returning to the turbid state. And sludge water at the bottoms of the acid-base adjusting tank, the intermediate storage tank and the vertical flow settler is periodically discharged to a sludge tank.

In a specific case, the vertical flow settler can select inclined tube sedimentation or conical sedimentation, and can also adopt conical and inclined tube combined sedimentation, preferably conical and inclined tube combined sedimentation.

In a specific situation, the rare earth wastewater containing magnesium sulfate and magnesium chloride is settled and deoiled in a deoiling device, and a clapboard, an air pump and a scraper blade are arranged in the device, so that floating oil, heavy oil and a part of emulsified oil in the wastewater can be removed; the oil removing mode in the oil removing device can select air flotation oil removing, gravity oil removing, chemical oil removing, electrochemical oil removing, filtering oil removing and the like, and air flotation oil removing is preferred.

In a specific case, the selection form of the coarse filtration in the coarse filter can be a fiber ball filter, and one or a combination of several filter devices of a grating filter, a titanium rod filter, a micro-filter device, a quartz sand filter, an activated carbon filter and the like can be selected.

In the specific case, the antisludging agent of the membrane concentration device is selected from a saturated calcium sulfate resistant type, and specifically, an organic phosphonic acid type antisludging corrosion inhibitor or an organic phosphonate type antisludging corrosion inhibitor can be selected.

In the step (3), the secondary steam from the second-effect evaporator enters an MVR compressor to raise the temperature and the pressure, the saturation temperature of the compressed secondary steam is raised, and the compressed steam is sent to a first-effect heater to heat the material; secondary steam generated by the first-effect evaporator returns to the first-effect evaporator to be used as a circulating heat exchange medium; in the process of heating materials, steam is condensed into water, the water is sent into the preheating heat exchanger by the condensate pump to exchange heat with the waste water after membrane concentration, the waste water after membrane concentration is preheated, and the condensed water is discharged out of the system after the temperature is reduced.

Preferably, in the step (3), the ratio of magnesium sulfate to magnesium chloride in the MVR evaporation device is controlled to be 2: 1-8: 1, preferably 2.5:1 to 6: 1. Under the specific condition, the control of the returned calcium sulfate crystal seeds in the MVR evaporation device, the returned crystal seed amount: the amount of wastewater is 1: 5-1: 20, preferably 1: 10-1: 15. the calcium sulfate crystal seed source can adopt added calcium hydroxide, calcium sulfate or mother liquor at the bottom of a crystallization device for precipitation. Preferably, the ratio of magnesium sulfate to magnesium chloride is controlled by returning the bottom precipitate of the mother liquor concentration unit of step (4) to the two-effect evaporator of the MVR evaporation unit.

In the step (4), the magnesium sulfate crystallization device is connected in series with the crystal growth device by adopting a flash evaporation device, the temperature of the flash evaporation material outlet is controlled below 48 ℃, and the vacuum degree is controlled above-78 KPa. The proportion of magnesium sulfate and magnesium chloride in the wastewater entering the crystal growth device from the flash evaporation device is controlled to be 1: 1-1: 2, preferably magnesium sulfate: magnesium chloride 1: 1.5-1: 2; the ratio of magnesium sulfate to magnesium chloride in the wastewater entering the mother liquor concentration device from the crystal growth device is controlled to be 1: 2-1: 4, preferably magnesium sulfate: magnesium chloride 1: 2.5-1: 4. the temperature in the mother liquor concentration device is controlled below 65 ℃, so as to prevent the generation of magnesium sulfate monohydrate. The crystal growth device and the magnesium chloride crystallization device can adopt normal temperature and normal pressure crystallization.

Aiming at the characteristic of difficult separation of magnesium sulfate and magnesium chloride mutual solubility and cocrystallization in the rare earth wastewater recycling treatment process containing magnesium sulfate and magnesium chloride, the invention realizes fractional crystallization by controlling the concentration ratio of magnesium sulfate and magnesium chloride in an evaporation and crystallization device. At the end of the high-proportion concentration, since magnesium sulfate has the highest solubility (about 35%) at 75 ℃, and then decreases with increasing temperature, when 100 ℃ (about the one-effect discharge temperature) is reached, only about 30%, at this time, the principle theory requires that the magnesium sulfate content at the end of the concentration is greater than 34.83% and at most 40%. Therefore, magnesium sulfate monohydrate can be separated out firstly from the part higher than 30%, and the magnesium sulfate monohydrate can not be converted into magnesium sulfate heptahydrate under the existing process conditions to take away corresponding moisture, so that the water content in the mother liquor is higher than that in the phase diagram theory, the magnesium sulfate content is higher than that in the mother liquor at the end point of the phase diagram theory, and the mother liquor is unqualified. According to the method, firstly, the magnesium sulfate and the magnesium chloride in the MVR evaporation device are controlled in a reasonable proportion by adding the crystal seeds, then the magnesium sulfate heptahydrate is preferentially crystallized by controlling the proportion of the magnesium sulfate and the magnesium chloride in the wastewater of the magnesium sulfate crystallization device, and then the proportion of the magnesium sulfate and the magnesium chloride in the mother liquor concentration device is controlled, so that the magnesium chloride hexahydrate is crystallized in the magnesium chloride crystallization device.

According to a second aspect of the invention, the system for treating and recycling the waste water containing the magnesium sulfate and the magnesium chloride and the rare earth comprises a pretreatment unit, a membrane concentration unit, an evaporation unit and a salt separation unit which are sequentially arranged.

The evaporation unit adopts an MVR evaporation device and comprises a second-effect evaporator and a first-effect evaporator, an MVR compression pump is connected between the second-effect evaporator and the first-effect evaporator, secondary steam generated by the second-effect evaporator enters the first-effect evaporator after being pressurized and heated by the MVR compression pump to serve as a heat exchange medium, and secondary steam generated by the first-effect evaporator returns to the first-effect evaporator to serve as a circulating heat exchange medium.

The heat exchange media of the second-effect evaporator and the first-effect evaporator are respectively communicated with the preheating heat exchanger, and exchange heat with the concentrated wastewater of the membrane concentration unit in the preheating heat exchanger for condensation.

In a specific case, the preprocessing unit includes:

the waste water is subjected to acid-base regulation in the acid-base regulation tank, and an automatic pH detection device is arranged in the tank, so that the acid-base amount added can be automatically adjusted by recognizing the acid-base change caused by water quality change;

the intermediate storage tank is connected with the acid-base adjusting tank, and a stirring or aeration device is arranged in the tank; the bottom is connected with a sludge tank;

the vertical flow settler is connected with the intermediate storage tank through a pump and a flowmeter; the top part overflows to the oil removal device, and the bottom part is connected with a sludge pool;

the oil removal device is connected with the vertical flow settler, and the floating foam on the top part flows back to the sludge tank;

and the coarse filter is connected with the oil removal device through a pump.

In a specific case, the vertical flow settler adopts a combination of a taper and an inclined tube for settlement.

In a specific situation, the combination form of the membrane concentration unit can be an ultrafiltration membrane system, a nanofiltration membrane system and a reverse osmosis membrane system, or an ultrafiltration membrane system, a primary nanofiltration membrane system, a secondary nanofiltration membrane system and a reverse osmosis membrane system, or an ultrafiltration membrane system and a high-pressure reverse osmosis membrane system, preferably the ultrafiltration membrane system, the nanofiltration membrane system and the reverse osmosis membrane system.

In a specific case, the salt separation unit comprises a magnesium sulfate crystallization device, a filtering device, a mother liquor concentration device and a magnesium chloride crystallization device which are connected in sequence. The magnesium sulfate crystallization device comprises a flash evaporation device and a crystal growth device which are connected in series.

In a specific situation, the system for treating and recycling the rare earth wastewater containing magnesium sulfate and magnesium chloride further comprises a sludge treatment unit, wherein the sludge treatment unit comprises a sludge tank and a plate-and-frame filter press, and the sludge tank is respectively connected with the bottoms of the acid-base adjusting tank, the intermediate storage tank and the vertical flow settler and is connected with the top of the oil removal device; the plate-and-frame filter press is connected with the top of the intermediate storage tank through a pump and used for returning filter-pressing wastewater to the intermediate storage tank.

Compared with the prior art, the invention has the beneficial effects that:

(1) the finally treated effluent can be recycled, the utilization rate of the water is 100 percent, and the zero discharge of the rare earth wastewater containing magnesium sulfate and magnesium chloride is realized; the resource treatment process of the rare earth wastewater containing magnesium sulfate and magnesium chloride avoids the difficult separation characteristic of mutual dissolution and cocrystallization, realizes fractional crystallization by controlling the concentration ratio of magnesium sulfate and magnesium chloride in an evaporation and crystallization device, realizes resource utilization of wastewater components at low cost, greatly reduces the treatment cost of mother liquor, and realizes zero discharge of wastewater and waste residues; the treatment process has reasonable design and is easy for industrial implementation.

(2) The system can recover magnesium sulfate in the wastewater, the purity of the recovered magnesium sulfate can reach more than 98.5 percent, and the recovered magnesium sulfate can be used for other production; the system can recover magnesium chloride in the wastewater, the purity of the magnesium chloride reaches over 75 percent, and the recovered magnesium chloride can also be used for other production.

(3) The heat source of the system is MVR compression and primary steam generation, so that energy recycling is realized, and consumption is reduced.

(4) The effluent finally treated by the system can be recycled, the utilization rate of water is improved, and the discharge amount of wastewater is reduced.

(5) The final effluent of the system can reach the first-level sewage comprehensive discharge standard, and no secondary pollution is caused to the environment in the treatment process.

Drawings

FIG. 1 is a flow chart of the rare earth wastewater treatment system containing magnesium sulfate and magnesium chloride and the resource utilization process.

Fig. 2 is a schematic view of the structure of the evaporation apparatus of the present invention.

Description of the reference numerals

1. The device comprises a raw water pool, 2, an acid-base adjusting tank, 3, an intermediate storage tank, 4, a vertical flow settler, 5, a sludge pool, 6, an oil removing device, 7, a membrane water collecting pool, 8, a coarse effect filtering device, 9, an ultrafiltration membrane device, 10, a nanofiltration membrane and reverse osmosis membrane device, 11, a lime bin, 12, a plate-and-frame filter press, 13, an evaporator water storage tank, 14, a preheating device, 15, a two-effect evaporator, 16, an MVR compressor, 17, a one-effect evaporator, 18, a magnesium sulfate crystallizing device, 19, a filtering device, 20, a mother liquor concentrating device, 21, a drying device, 22, a magnesium chloride crystallizing device, 24, a seed crystal pool, 25 and a stirring tank.

Detailed Description

The invention is further explained below with reference to the drawings and examples. Advantages and features of the present invention will become more apparent as the description proceeds, but the examples are exemplary only, and do not limit the scope of the present invention in any way. The devices or components used in the embodiments may take any conventional construction known in the art, unless otherwise specified.

The proportions of the materials described in the context of this specification are by weight unless otherwise indicated.

Referring to fig. 1, the magnesium sulfate and magnesium chloride containing rare earth wastewater treatment and resource utilization system mainly comprises a pretreatment unit, a membrane concentration unit, an evaporation unit and a salt separation unit.

The pretreatment unit includes: the device comprises a raw water pool 1, an acid-base adjusting tank 2, an intermediate storage tank 3, a vertical flow settler 4, an oil removal device 6 and a coarse-effect filtering device 8. Wherein, intermediate storage tank 3 establishes ties through a plurality of storage tanks, and last storage tank effluent overflows to next storage tank bottom through the top, forms supersaturation calcium sulfate backward flow circulation, and some heavy metals fully form salt precipitation, pump into vertical flow settler 4, subsides the overflow of top supernatant to deoiling device 6 through the pipe chute, and deoiling device 6 further removes suspended solid and oils, overflows to membrane catch basin 7, gets into coarse effect filter device 8 subsequently and further removes the suspended solid. The treatment system of the invention also comprises a sludge treatment unit, and the sludge treatment unit comprises a sludge tank 5 and a plate-and-frame filter press 12. The sludge tank 5 is respectively connected with the bottoms of the acid-base adjusting tank 2, the intermediate storage tank 3 and the vertical flow settler 4 and is connected with the top of the oil removing device 6; the plate-and-frame filter press 12 performs filter pressing operation on the sludge in the sludge tank 5, and the obtained calcium sulfate filter cake is recycled. The plate-and-frame filter press 12 is connected with the top of the intermediate storage tank 3 through a pump and used for returning filter-pressing wastewater to the intermediate storage tank 3.

The membrane concentration unit comprises an ultrafiltration membrane device 9, a nanofiltration membrane and a reverse osmosis membrane device 10, wherein concentrated water of the ultrafiltration membrane device 9 returns to the raw water pool 1, and produced water enters the nanofiltration membrane and reverse osmosis membrane device 10. Concentrated water passing through the nanofiltration membrane and the reverse osmosis membrane respectively enters the water storage tank 13 of the evaporator, the TDS of the concentrated water can reach over 120000mg/L, and the produced water reaches the discharge standard for recycling.

The evaporation unit includes: an evaporator water storage tank 13, a preheating device 14, a two-effect evaporator 15, an MVR compressor 16 and a one-effect evaporator 17. The concentrated magnesium sulfate and magnesium chloride wastewater firstly enters a second-effect evaporator 15 of the MVR two-effect evaporator for evaporation concentration, and enters a MVR first-effect evaporator 17 for continuous evaporation after evaporation concentration. And when the concentration of the feed liquid reaches the designed value, the evaporated feed liquid is conveyed to a salt separation device by a material transfer pump for cooling and crystallization.

The evaporation device in the invention adopts a specially designed MVR two-effect evaporation device, referring to fig. 2, the MVR two-effect evaporation device mainly comprises an evaporator water storage tank 13, a preheating device 14, a seed crystal pool 24, a stirring tank 25, a two-effect evaporator 15 and a one-effect evaporator 17, an MVR compression pump 16 is connected between the two of the one of the two of the one of the two of the one of the two of the one of the two of the one of the two of the one of the two of the. The heat exchange mediums of the second-effect evaporator 15 and the first-effect evaporator 16 are respectively communicated with the preheating device 14.

The rare earth wastewater after passing through the pretreatment device and the membrane concentration device firstly enters an evaporator water storage tank 13, is subjected to heat exchange by a preheating device 14, is uniformly mixed with the crystal liquid in a crystal seed tank 24 in a stirring tank 25, then enters a secondary evaporator 15 for further evaporation concentration, is automatically sent to a primary evaporator 17 for continuous evaporation concentration by a material transfer pump after the concentration of the material liquid reaches a design value according to the real-time calculation result of a computer, is sent to a magnesium sulfate crystallization device 18 for crystallization by the material transfer pump after the concentration of the material liquid reaches the design value again, and enters a filtering device 19 for separating magnesium sulfate and mother liquid after the crystallized material liquid reaches a certain requirement.

The secondary steam from the second-effect evaporator 15 enters an MVR compressor 16 to raise the temperature and the pressure. After the secondary steam is compressed, the saturation temperature can be raised to about 105 ℃, and the compressed steam is sent into a first-effect heater 17 to heat the material. In the process of heating the materials, steam with the saturation temperature of about 105 ℃ is condensed into water, the water is sent into the preheating device 14 by a condensate pump to exchange heat with the wastewater, and the temperature is reduced to about 45 ℃ and then is discharged out of the system.

The salt separating unit comprises: a magnesium sulfate crystallization device 18, a filtration device 19, a mother liquor concentration device 20, a drying device 21 and a magnesium chloride crystallization device 22. The magnesium sulfate crystal-containing material in the magnesium sulfate crystallization device 18 is conveyed to a filtering device 19 by a conveying pump for filtering, the filtered magnesium sulfate heptahydrate crystal is conveyed to a drying device 21 for drying, and the product is packaged and sold. The filtered mother liquor mainly contains magnesium sulfate and magnesium chloride, and is sent to a mother liquor concentration device 20 to separate most of magnesium sulfate, and then the evaporated water is pumped into a magnesium chloride crystallization device 22 to produce scraping magnesium chloride.

Example 1

Resource utilization of rare earth wastewater of magnesium sulfate and magnesium chloride produced by certain northern rare earth plant of inner Mongolia steelworks.

The water quality indexes of the rare earth wastewater coming from magnesium sulfate and magnesium chloride produced by certain northern rare earth plant of inner Mongolia ladle steel works are shown in Table 1

TABLE 1 Water quality of magnesium sulfate and magnesium chloride wastewater raw water

Item

pH

MgSO4

Ca2+

Al3+

Fe2+

Cl-

F-

SS

Oil dirt

Temperature of

Dimensionless

g/L

g/L

g/L

g/L

g/L

g/L

mg/L

mg/L

℃

Numerical value

3.5-5.5

＞40

0.7-1.2

＜0.1

＜0.1

＜7

＜0.05

＜100

＜300

30-60

1. Pretreatment device

Lime and water were mixed according to 1: 10 to obtain slaked lime, adding the slaked lime into the magnesium sulfate and magnesium chloride waste water, and adjusting the pH value of the mixed waste water to 6.5-10.5. Preferably 7.5-9.5.

The intermediate device is formed by connecting a plurality of storage tanks in series, the volume of each storage tank is 20-70% of the daily water treatment amount, an aeration device is arranged in each storage tank, the effluent of the previous storage tank overflows to the bottom of the next storage tank through the top to form supersaturated calcium sulfate circulation, some heavy metals are fully precipitated, and bottom mud water is periodically discharged to a sludge tank.

The vertical flow settler can select inclined tube sedimentation or conical sedimentation, and can also adopt conical and inclined tube combination sedimentation, preferably conical and inclined tube combination sedimentation. The turbidity of the overflow water is reduced to below 10NTU, and the SDI is reduced to below 10.

The supersaturated calcium sulfate is fully separated out from the waste water containing magnesium sulfate and magnesium chloride in an intermediate device and a vertical flow settler, and part of heavy metal ions are oxidized and precipitated, so that the phenomenon of turbidity returning of clear liquid after settling is prevented.

The waste water containing magnesium sulfate and magnesium chloride is settled and degreased in a degreasing device, a clapboard, an air pump and a scraper are arranged in the device to further remove SS and greasy dirt, and the content of the greasy dirt in the clear liquid is reduced to below 20 mg/L.

2. Membrane concentration device

The water quality indexes of the magnesium sulfate and magnesium chloride rare earth wastewater produced by a certain northern rare earth plant of the inner Mongolia steelworks after pretreatment are shown in Table 2

TABLE 2 Water quality after pretreatment of magnesium sulfate and magnesium chloride wastewater

Item

pH

MgSO4

Ca2+

Al3+

Fe2+

Cl-

F-

SS

Oil dirt

Turbidity of water

Dimensionless

g/L

g/L

mg/L

g/L

g/L

mg/L

mg/L

mg/L

NTU

Numerical value

8.5-9.5

＞40

0.7-1.2

＜5.0

＜0.2

＜7

＜0.5

＜1

＜20

＜10

And (3) further filtering the wastewater with the turbidity lower than 10 by using an ultrafiltration device, allowing the wastewater to enter a nanofiltration device, allowing nanofiltration concentrated water to enter an evaporation device, allowing nanofiltration produced water to enter a reverse osmosis device, allowing the reverse osmosis produced water to be recycled as reuse water, and allowing the reverse osmosis concentrated water to enter the evaporation device. The TDS of the concentrated water entering the MVR evaporation device can reach over 120000 mg/L.

3. MVR evaporation plant and divide salt device

Concentrated magnesium sulfate and magnesium chloride wastewater firstly enters a second-effect evaporator of an MVR two-effect evaporator for evaporation concentration, and enters an MVR first-effect evaporator for continuous evaporation after evaporation concentration. And when the concentration of the feed liquid reaches the designed value, the evaporated feed liquid is conveyed to a flash evaporation system by a material transfer pump for cooling crystallization, and then is conveyed to a crystal growth device for crystallizing at normal temperature to obtain magnesium sulfate.

And the secondary steam from the MVR secondary evaporator enters an MVR compressor to raise the temperature and the pressure. After the secondary steam is compressed, the saturation temperature can be raised to about 105 ℃, and the compressed steam is sent into a one-effect heater to heat materials. In the process of heating the materials, steam with the saturation temperature of about 105 ℃ is condensed into water, the water is sent into a preheater by a condensate pump to exchange heat with the raw material liquid, and the temperature is reduced to about 45 ℃ and then is discharged out of the system.

The concentration of the feed liquid is calculated by a computer in real time according to the proportion and the boiling point of magnesium sulfate and magnesium chloride, and the evaporated feed liquid is conveyed to a magnesium sulfate crystallization device by a material transfer pump after reaching a design value.

Controlling the proportion of magnesium sulfate and magnesium chloride in an MVR evaporation device, wherein the ratio of magnesium sulfate: magnesium chloride ═ 2: 1-8: 1, preferably 2.5:1 to 6: 1. The ratio of magnesium sulfate to magnesium chloride can be controlled by the mother liquor after the crystallization device.

The magnesium sulfate crystallization device consists of a flash evaporation system and a crystal growth system. The outlet temperature of the flash evaporation material is controlled below 48 ℃, and the vacuum degree is controlled above-78 KPa. Preferably, the outlet temperature is controlled below 40 ℃ and the vacuum degree is controlled above-85 KPa.

Controlling returned calcium sulfate crystal seeds in the MVR evaporation device, wherein the returned crystal seed amount is as follows: the amount of wastewater is 1: 5-1: 20, preferably 1: 10-1: 15. calcium hydroxide and calcium sulfate can be added to the calcium sulfate crystal seed source, and the mother liquor is precipitated at the bottom of the mother liquor concentration device after the crystallization device. The position for adding the calcium sulfate returning crystal seeds is at the inlet of the double-effect evaporator, and the calcium sulfate returning crystal seeds are preferably precipitated at the bottom of a mother liquor concentration device after the crystallization device.

The proportion of magnesium sulfate and magnesium chloride in the wastewater entering the crystal growth device from the flash evaporation device is controlled to be 1: 1-1: 2, preferably magnesium sulfate: magnesium chloride 1: 1.5-1: 2; the ratio of magnesium sulfate to magnesium chloride in the wastewater entering the mother liquor concentration device from the crystal growth device is controlled to be 1: 2-1: 4, preferably magnesium sulfate: magnesium chloride 1: 2.5-1: 4.

and (3) conveying the magnesium sulfate-containing crystal material in the magnesium sulfate crystallization device to a filtering device through a conveying pump for filtering, conveying the filtered magnesium sulfate heptahydrate crystal to a drying device for drying, and packaging to serve as a product for sale. And (3) the filtered mother liquor mainly contains magnesium sulfate and magnesium chloride, and the mother liquor is sent to a mother liquor concentration device to separate most of magnesium sulfate, and then the evaporated water is pumped into a magnesium chloride crystallization device again to produce scraping magnesium chloride. The crystal growth device and the magnesium chloride crystallization device can adopt normal temperature and normal pressure crystallization.

The purity of the magnesium sulfate heptahydrate prepared by the method is more than 98.5 percent, and the main impurity is calcium sulfate.

The purity of the magnesium chloride hexahydrate prepared by the method is over 75 percent, and the main impurity is magnesium sulfate.

The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and improvements can be made without departing from the principle of the present invention, and these modifications and improvements should also be considered as the protection scope of the present invention.