A process for zero discharge of multi-source wastewater in a steel plant through collaborative treatment

Abstract

The present invention discloses a zero-emission process for the coordinated treatment of multi-source wastewater in a steel mill. This process features zero-emission selective separation of desalted water and coal gas condensate from a softening station, recovery of natronasite from cold-rolling rinsing wastewater using high-pressure nanofiltration concentrate, coordinated treatment of high-pressure reverse osmosis concentrate with desulfurization wastewater and high-salt solid waste ash, and low-cost treatment of ammonia and nitrogen. The coordinated treatment of multi-source wastewater and high-salt solid waste ash significantly reduces the pollutant content in ash washing water, thereby reducing subsequent wastewater treatment costs and wastewater usage, significantly lowering the disposal costs of solid waste and wastewater, and achieving internal consumption and zero-emission of multi-source wastewater.

Description

Technical Field

[0001] The present invention relates to the treatment of wastewater and solid waste from steel plants, and in particular to a zero-emission process for the coordinated treatment of wastewater from multiple sources in steel plants, belonging to the technical field of coordinated resource utilization treatment of wastewater in the steel industry. Background Art

[0002] Currently, steel mills are demanding increasingly high rates of wastewater reuse, and with tightening environmental standards, achieving zero wastewater discharge is imperative. Currently, zero wastewater discharge systems primarily address reverse osmosis brine and coking wastewater from reuse systems, utilizing hardness removal, ultrafiltration, silicon removal, oxidation, and membrane systems to recycle sulfate and chlorine from the wastewater.

[0003] Current zero-discharge wastewater technology focuses on recovering sodium salts, such as sodium sulfate and sodium chloride. However, due to the low added value of sodium salts and the high cost of wastewater evaporation, zero-discharge wastewater projects are primarily loss-making. Furthermore, in the new era, efforts are underway to create a green steel plant characterized by "no solid waste leaving the factory, zero wastewater discharge, and ultra-low flue gas emissions." Environmental protection measures should be coordinated with water, gas, and solid phases. Under this model, the ultimate result is the efficient removal and resource utilization of chlorine throughout the plant. In addition to the previously treated reverse osmosis brine and coking wastewater from the reuse water system, the sources of chlorine also include wet / dry desulfurization wastewater, coal gas condensate, cold rolling rinse water, softening wastewater, and high-salt solid waste. Chinese patent CN111825259A achieves wastewater reuse through softening pretreatment, ultrafiltration, a reverse osmosis concentration system, an ozone oxidation system, nanofiltration, and reverse osmosis membrane salt separation. The concentrated water from the nanofiltration is disposed of together with the coking concentrate station crystallization system, and the reverse osmosis concentrate is treated through fluorine and silicon removal, electrodialysis concentration, and evaporative crystallization to recover sodium chloride. Chinese patent CN112939321A achieves wastewater treatment through softening pretreatment, primary salt concentration, resin adsorption, membrane salt separation, reverse osmosis membrane concentration, electrodialysis, and evaporative crystallization. The primary salt concentration uses reverse osmosis, the produced water is used for industrial fresh water reuse, and the concentrated water enters the resin adsorption system. The membrane salt separation uses two-stage nanofiltration, the concentrated water is used for converter slag flushing, and the fresh water is concentrated through high-pressure reverse osmosis membranes, and finally sodium chloride is obtained through evaporative crystallization.

[0004] In summary, due to the varying characteristics and properties of wastewater from different steel mill processes, existing technologies are often limited to resource-recycling wastewater from a single process. Furthermore, for wastewater with complex components, resource-recycling results are unsatisfactory and the investment costs are high. Currently, the inventors are unaware of any reports on low-cost, coordinated treatment of multi-source wastewater from steel mills to achieve zero emissions. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the present invention aims to increase the added value of zero-emission salt products and expand the coverage of steel plant wastewater treatment. Based on the analysis of the characteristics of chlorine elements in the wastewater of the entire steel plant and the difficulty of treatment, a process for the coordinated treatment of multi-source wastewater from steel plants with zero emission is designed and provided. The process has the characteristics of selective zero-emission of desalted water and coal gas condensate from the softening station, coordinated recovery of natronasite from cold rolling rinsing wastewater by high-pressure nanofiltration concentrate, coordinated treatment of high-pressure reverse osmosis concentrate with desulfurization wastewater and high-salt solid waste ash, and low-cost treatment of ammonia nitrogen.

[0006] To achieve the above objectives, the technical solutions adopted by the present invention are specifically described as follows:

[0007] A zero-emission process for the coordinated treatment of wastewater from multiple sources in a steel plant comprises the following steps:

[0008] 1) The high-quality wastewater is homogeneously mixed with the concentrated brine from the desalted water station to obtain mixed concentrated brine wastewater.

[0009] 2) The mixed concentrated salt wastewater obtained in step 1) is first added with a heavy-removing agent (such as a sulfide or dithiocarbamate heavy-removing agent), a hardness-removing agent (soluble carbonate, such as sodium carbonate or potassium carbonate), and desiliconization treatment (the desiliconization agent is a magnesium agent or an iron salt), and then subjected to secondary reverse osmosis treatment and high-pressure nanofiltration treatment in sequence to obtain high-pressure nanofiltration concentrated water and high-pressure nanofiltration fresh water.

[0010] 3) The high-pressure nanofiltration concentrate obtained in step 2) is mixed with cold-rolling rinsing wastewater to obtain mixed acidic wastewater, which is then heated to undergo a precipitation reaction. After the reaction is complete, solid-liquid separation is performed to obtain natrona and a residual liquid. The natrona is then disposed of in-plant, and the residual liquid is returned to step 1) for homogenization.

[0011] 4) The high-pressure nanofiltration freshwater obtained in step 2) is subjected to high-pressure reverse osmosis treatment to obtain high-pressure reverse osmosis concentrated water and recycled freshwater. The high-pressure reverse osmosis concentrated water is used for washing high-salt solid waste ash, and the recycled freshwater is circulated to any water-requiring process in the steel plant.

[0012] Preferably, in step 1), the high-quality wastewater is desalted water and / or coal gas condensate from a softening station with a conductivity greater than 10,000 μS / cm, preferably desalted water and / or coal gas condensate from a softening station with a conductivity greater than 12,000 μS / cm.

[0013] Preferably, in step 1), the concentrated brine of the desalted water station is neutral concentrated water containing sulfate and chloride ions produced when the steel plant uses reverse osmosis to desalinate the circulating water.

[0014] Preferably, in step 3), the cold rolling rinsing wastewater is wastewater with a pH value less than 2.5, containing FeCl 3 and HCl, generated in the rinsing section of the cold-rolled strip pickling process, and preferably wastewater with a pH value less than 2.

[0015] Preferably, in step 3), the pH of the mixed acidic wastewater is 2-4, preferably 2-3. The heating of the mixed acidic wastewater for precipitation reaction is specifically heating the mixed acidic wastewater to 80-100° C. for 1-8 hours, preferably heating to 85-95° C. for 2-5 hours.

[0016] Preferably, in step 3), before heating the mixed acidic wastewater for precipitation reaction, a soluble iron salt (preferably ferric chloride) is added to adjust the molar ratio of iron ions to sulfate ions in the mixed acidic wastewater to 1:0.4-0.8, preferably 1:0.5-0.7.

[0017] Preferably, in step 4), the high-pressure reverse osmosis concentrated water is used for the water washing treatment of high-salt solid waste ash as follows:

[0018] 4a) The high-salt solid waste ash is washed by mixing the high-pressure reverse osmosis concentrated water with the desulfurization wastewater. After solid-liquid separation, filter cake and ash washing wastewater are obtained. The filter cake is comprehensively disposed of in the factory, and the ash washing wastewater is sent to the next process.

[0019] 4b) The ash washing wastewater is adjusted to acidic using desulfurization wastewater or cold rolling rinsing wastewater, and then heated to carry out precipitation reaction. After the reaction is completed, solid-liquid separation is performed to obtain impurity-removed wastewater and slag phase. The slag phase is comprehensively disposed of in the factory, and the impurity-removed wastewater enters the next process.

[0020] 4c) The impurity removal wastewater is first treated with iron-carbon micro-electrolysis. A mixed reagent is then added to the treated wastewater to adjust its alkalinity. A precipitation reaction is performed to remove heavy particles and hard substances. After solid-liquid separation, high-salt wastewater and residue are obtained. The residue is then disposed of in a comprehensive manner within the plant, and the high-salt wastewater proceeds to the next process.

[0021] 4d) The high-salinity wastewater is first heated, concentrated and crystallized, and solid-liquid separation is performed to obtain sodium chloride and a primary filtrate. The primary filtrate is then cooled and crystallized, and solid-liquid separation is performed to obtain potassium chloride and a secondary filtrate. Finally, the secondary filtrate is mixed with the high-salinity wastewater and circulated for heating and salt precipitation treatment.

[0022] Preferably, in step 4a), the desulfurization wastewater is obtained by first subjecting wet desulfurization wastewater to evaporation concentration to obtain suspended matter wastewater, then adding an ammonia nitrogen precipitant to the suspended matter wastewater for precipitation treatment, and finally subjecting the wastewater to solid-liquid separation. The evaporation concentration comprises concentrating the wet desulfurization wastewater by 3-6 times using multi-stage evaporation concentration equipment. The ammonia nitrogen precipitant is a soluble ferrous salt and a soluble sulfite. The wet desulfurization wastewater is wastewater generated during flue gas treatment using a limestone / gypsum process.

[0023] Alternatively, the desulfurization wastewater is the wastewater from the activated carbon process that has been diverted and treated. Specifically, 1 / 4 to 1 / 2 of the activated carbon process wastewater is mixed with high-pressure reverse osmosis concentrated water to wash the high-salt solid waste ash, and the remaining wastewater is used to adjust the ash washing wastewater to acidity. The activated carbon process wastewater is the acidic flue gas scrubbing wastewater generated by the activated carbon adsorption method for gas scrubbing.

[0024] Preferably, the evaporation concentration of the wet desulfurization wastewater is a multi-stage evaporation concentration process with gradually decreasing evaporation temperature and gradually increasing vacuum degree. Preferably, the evaporation temperature of the first stage evaporation concentration is 80-100°C and the vacuum degree is -40 to -5 kPa. The evaporation temperature of the second stage evaporation concentration is 50-80°C and the vacuum degree is -70 to -40 kPa. The evaporation temperature of the third stage evaporation concentration is 30-50°C and the vacuum degree is -100 to -70 kPa.

[0025] Preferably, the soluble ferrous salt is one or more of ferrous chloride, ferrous sulfate, and elemental iron powder. The soluble sulfite is one or more of sodium sulfite, sodium bisulfite, and potassium sulfite. Preferably, the soluble ferrous salt and the soluble sulfite are added sequentially.

[0026] Preferably, the hot steam generated by evaporation and concentration is used to heat the mixed acidic wastewater in step 3) and / or is used to heat the ash washing wastewater in step 4b).

[0027] Preferably, in step 4b), adjusting the acidity of the ash washing wastewater is to adjust the pH of the ash washing wastewater to 2-4, preferably 2-3. Heating the ash washing wastewater for precipitation reaction is specifically heating the ash washing wastewater to 80-100° C. for 1-8 hours, preferably heating to 85-95° C. for 2-5 hours.

[0028] Preferably, in step 4b), before heating the ash washing wastewater for precipitation reaction, a soluble iron salt is added to adjust the molar ratio of iron ions, sulfate ions and ammonia nitrogen in the ash washing wastewater to 1:0.4-0.8:0.2-0.6, preferably 1:0.5-0.7:0.3-0.5.

[0029] Preferably, in step 4c), before the impurity removal wastewater is subjected to iron-carbon micro-electrolysis, the pH of the impurity removal wastewater is adjusted to 3-5, preferably 3.5-4. Preferably, the alkali is sodium hydroxide and / or potassium hydroxide.

[0030] Preferably, in step 4c), the mixed reagent is composed of sodium hydroxide and / or potassium hydroxide, sodium carbonate and / or potassium carbonate, sodium sulfide and / or potassium sulfate, and a heavy capture agent (preferably a xanthate heavy capture agent or a dithiocarbamate heavy capture agent). Wherein: the amount of sodium hydroxide and / or potassium hydroxide added is such that the pH of the impurity removal wastewater is 7-9, preferably 7.5-8. The amount of sodium carbonate and / or potassium carbonate added is 3-10 g / L, preferably 4-8 g / L. The amount of sodium sulfide and / or potassium sulfide added is 1-7 g / L, preferably 1.5-6 g / L. The amount of the heavy capture agent added is 1-8 g / L, preferably 2-5 g / L.

[0031] Preferably, the high-salt solid waste ash includes one or more of sintered electric field ash, blast furnace bag ash, rotary kiln surface cooling ash, and waste incineration fly ash, preferably sintered electric field ash.

[0032] Preferably, the washing of the high-salt solid waste ash is a three-stage countercurrent washing treatment. Specifically, the high-salt solid waste ash is first washed with a first-stage water, and dehydrated by a first-stage filter press to obtain a first-stage filtrate and a first-stage filter residue, and the first-stage filtrate is the ash washing wastewater. The first-stage filter residue enters the second-stage water washing, and the water source of the second-stage water washing is the third-stage filtrate and high-pressure reverse osmosis concentrated water and desulfurization wastewater. After the second-stage water washing, it is dehydrated by a second-stage filter press to obtain a second-stage filtrate and a second-stage filter residue, and the second-stage filtrate is discharged to the first-stage water washing for recycling. The second-stage filter residue enters the third-stage water washing, and the water source of the third-stage water washing is industrial water. After the third-stage water washing, it is dehydrated by a third-stage filter press to obtain a third-stage filtrate and a third-stage filter residue, and the third-stage filtrate is discharged to the second-stage water washing for recycling, and the third-stage filter residue is comprehensively disposed of in the factory.

[0033] Preferably, in step 4d), the high-salinity wastewater is treated using a multiple-effect countercurrent evaporation device having 2-6 stages, preferably 3-4 stages. The high-salinity wastewater is heated to 80-100° C., preferably 90-95° C. The primary filtrate is cooled to below 60° C., preferably 20-55° C., by flash evaporation or heat exchange.

[0034] Preferably, the hot steam generated by the multiple-effect countercurrent evaporation device is used to heat the mixed acidic wastewater in step 3) and / or is used to heat the ash washing wastewater in step 4b).

[0035] In the existing technology, for the resource recovery treatment of multi-source wastewater from different processes in steel mills, the existing technology can often only perform simple resource recovery treatment on a certain type of wastewater. In order to achieve standard discharge or zero discharge of wastewater leaving the factory, it is often necessary to establish multiple sets of different wastewater treatment equipment to meet the treatment needs of wastewater discharged from different processes of the steel mill, resulting in high investment costs for wastewater treatment and low added value of by-products.

[0036] In this application, gas condensate primarily originates from condensate generated during the transportation of blast furnace, converter, and coking gas. All contain a certain amount of sulfate and chloride ions, but their specific water quality characteristics are closely related to drainage time and gas transportation distance. Demineralized water from softening stations primarily originates from the concentrated water produced when ion exchangers remove chlorine and sulfate from wastewater. General gas condensate and demineralized water from softening stations are often biochemically treated alongside biodegradable wastewater. However, an analysis of the water quality of the coal gas condensate and the desalted water from the softening station in the steel plant revealed that both streams were high-concentration wastewater containing a large amount of chloride ions, with the highest concentration being 200 g / L. Such high-salt wastewater entering the biochemical system is bound to cause biological poisoning. Therefore, the present invention selectively separates and discharges the coal gas condensate and the desalted water from the softening station, that is, first uses real-time conductivity detection to separate the water with a concentration greater than 10,000 μS / cm (preferably greater than 12,000 μS / cm), and then, after pretreatment and membrane concentration and salt separation, participates in high-value recovery of natantrazol and coordinated ash washing.

[0037] In the present invention, coal gas condensate containing high salt concentrations, demineralized water from the softening station, and concentrated brine from the demineralizing station are mixed and then subjected to pretreatment followed by membrane concentration to separate the salts, ultimately yielding high-pressure nanofiltration concentrate containing high sulfate concentrations and high-pressure reverse osmosis concentrate containing high chloride ion concentrations. The nanofiltration concentrate is typically recovered using evaporation, crystallization, and freezing to recover sodium sulfate. However, this process suffers from high energy consumption and low added value of the sodium sulfate. Considering that cold-rolling rinse water contains a large amount of trivalent iron, iron and sulfate react with sodium to form natron. Furthermore, cold-rolling rinse water is a highly acidic wastewater, providing unique conditions for this reaction. Therefore, the waste heat from the evaporation system (a wet desulfurization wastewater evaporation and concentration system and a high-salt wastewater countercurrent evaporation and dilute salt system) is used to heat the mixed wastewater (a mixture of high-pressure nanofiltration concentrate and cold-rolling rinse wastewater) to 80-100°C, achieving high-value recovery of sulfate from the nanofiltration concentrate while simultaneously consuming the cold-rolling rinse water. Generally, the reaction can be promoted by adding a soluble iron salt (preferably ferric chloride) to adjust the molar ratio of iron ions to sulfate ions in the mixed wastewater to 1:0.4-0.8 (preferably 1:0.5-0.7).

[0038] In the present invention, high-pressure reverse osmosis concentrated water is mainly composed of sodium chloride. Studies have shown that the use of this high-pressure reverse osmosis concentrated water in the washing of high-salt solid waste ash can reduce the alkalinity of the ash washing water and make it close to neutral. Therefore, the present invention uses high-pressure reverse osmosis concentrated water mixed with acidic wastewater from other processes to carry out coordinated ash washing (high-salt solid waste ash) treatment, which is beneficial to improve the water quality of the ash washing wastewater. Generally, when the desulfurization wastewater is mainly wet desulfurization wastewater, it is concentrated (generally concentrated 3-6 times) before washing. Since the concentrated wet desulfurization wastewater has a strong acidity, after mixing with high-pressure reverse osmosis concentrated water, on the one hand, it can reduce the solution of the ash washing water, making the ash washing water weakly acidic, thereby preventing the formation of stable [TlCl4 - On the other hand, since the concentrated wet desulfurization wastewater contains a large amount of sulfite, its addition will cause thallium to be weakly reduced, thereby destroying the complex, which is conducive to the removal of thallium, thereby achieving source suppression of thallium. When the desulfurization wastewater is mainly activated carbon wastewater, similarly, since activated carbon wastewater has a strong acidity, when it is used for ash washing, on the one hand, it can reduce the solution of the ash washing water, making the ash washing water acidic, thereby preventing the formation of stable [TlCl4 - On the other hand, since acidic washing wastewater contains thiosulfate, its addition will also facilitate the removal of thallium. Therefore, by co-processing high-pressure reverse osmosis concentrated water, desulfurization wastewater (wet desulfurization wastewater or activated carbon wastewater) with high-salt solid waste ash, the water quality of ash washing water can be greatly improved and the dissolution of pollutants can be reduced. The main composition of high-pressure reverse osmosis concentrated water and desulfurization wastewater is similar to that of ash washing water, and co-processing can significantly reduce the water consumption of high-salt solid waste.

[0039] In the present invention, research has shown that due to the raw materials and reaction characteristics of steel mills, ammonia nitrogen in wet / dry desulfurization wastewater, gas condensate, demineralized water from softening stations, and high-salt solid waste ash all contain varying concentrations of ammonia nitrogen. This ammonia nitrogen will eventually accumulate in the ash washing water, thereby affecting the quality of the recovered salt product. To remove ammonia nitrogen, the present invention provides a new two-stage deamination technology. Stage 1: After the wet desulfurization wastewater is concentrated, the ammonia nitrogen concentration reaches 10-30 g / L. For precipitation and removal of high-concentration ammonia nitrogen, the ferrous ammonium sulfite method and the magnesium ammonium phosphate method can be used. In the ferrous ammonium sulfite method, ammonia nitrogen combines with ferrous iron and sulfite ions at a solution pH of 5-8 to form a ferrous ammonium sulfite precipitate. Stage 2: Because cold-rolled rinsing water contains a large amount of trivalent iron, iron and sulfate ions react with potassium, sodium, and ammonia nitrogen to form jarosite, sodium ferrosite, and ammonium ferrosite. Therefore, cold rolling mill rinsing wastewater can be added to the ash washing wastewater, and the waste heat from the evaporation system can be used to heat the wastewater to 80-100°C to remove ammonia nitrogen from the wastewater. Similarly, in the second stage of ammonia nitrogen removal, the precipitation reaction of ammonia nitrogen can be promoted by adding additional ferrous ions and sulfite ions.

[0040] In the present invention, since the high-salt solid waste ash is high-potassium and low-sodium ash, the potassium-sodium ratio in its conventional water washing solution is generally greater than 4 (preferably greater than 5). According to the principle of potassium-sodium variable temperature salt separation, it is suitable for downstream evaporation, that is, by temperature variable evaporation, potassium-sodium salt phase diagram analysis, after the high-potassium and low-sodium solution is concentrated by evaporation, potassium salt will inevitably be precipitated first. Therefore, the salt separation method for high-salt solid waste ash washing water is generally downstream evaporation. That is, the solution is a process of gradually cooling during the evaporation process. At the multi-effect outlet, potassium salt is discharged first. This evaporation method can cause pollutants to precipitate out along with the precipitation of potassium, which can reduce the quality of potassium. At the same time, the subsequent sodium salt precipitation requires two-stage evaporation, which increases investment and consumes a lot of energy. Therefore, the present invention is by introducing high-pressure osmotic concentrated water (mainly containing sodium chloride), with desulfurization wastewater (containing sodium) as high-salt solid waste ash washing water, and then can make the potassium-sodium content ratio in ash washing wastewater approach 1:1, under such conditions, according to phase diagram simulation and experimental verification, it is suitable for countercurrent evaporation, namely by after concentrating, preferentially isolate sodium salt, can make evaporation process can be adjusted to countercurrent evaporation, namely at the outlet of an effect, first discharge sodium salt.And then separate out sylvite by cooling down, this evaporation mode makes residual pollutant separate out along with the precipitation of sodium (because countercurrent evaporation only concentrates through one section, its pollutant mainly enters sodium salt), can not enter into sylvite, is conducive to improving the quality of potassium.Simultaneously whole evaporation only utilizes one section evaporation system, is applicable to the variation of different evaporation amounts, is more suitable for the applicability of raw material, and investment is lower.

[0041] In the present invention, impurity removal is also performed by iron-carbon micro-electrolysis (the time of the iron-carbon reaction should be not less than 20 min). 3+ Tl + It is easier to remove. Generally speaking, it can be pretreated by oxidation. Iron-carbon micro-electrolysis has the effect of synergistic weight removal and oxidation. After passing through iron-carbon, Tl can be changed into a form that is easier to remove. At the same time, a large number of metal ions in the ash washing water will be replaced by elemental iron, thereby achieving removal, and producing a large amount of ferrous and trivalent iron in the solution. In addition, iron-carbon is also conducive to the efficient removal of fluoride ions in wastewater. Furthermore, since acidic flue gas washing wastewater also contains sulfite, iron-carbon will release ferrous iron. Use alkali (such as sodium hydroxide) to adjust the ash washing wastewater to weak alkalinity. When the solution is adjusted to weak alkalinity, ammonia nitrogen will react rapidly with sulfite and ferrous iron to form ammonium ferrous sulfite precipitate, thereby achieving deep removal of ammonia nitrogen. At the same time, under weak alkaline conditions, the removal of trivalent iron and some calcium and magnesium ions can be achieved. The purpose of adding sodium carbonate is to remove calcium and magnesium. The purpose of adding sodium sulfide and heavy capture agent is to achieve deep removal of trace heavy metals.

[0042] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0043] 1. This method treats high-salinity wastewater from steel mills by quality and grade. The main treatment targets include demineralized water from softening stations, gas condensate, concentrated brine from demineralized water stations, cold rolling rinsing wastewater, and desulfurization wastewater. Compared with traditional zero-emission technologies, the treatment targets are more comprehensive.

[0044] 2: The present invention treats high-salt wastewater and high-salt solid waste ash in a coordinated manner, significantly reducing the pollutant content in the ash washing water, thereby reducing the subsequent wastewater treatment costs, while reducing the amount of wastewater used and greatly reducing the disposal costs.

[0045] 3. This invention addresses the low added value of sulfate and utilizes the acidic nature and high content of trivalent iron in cold-rolled rinse water to achieve high-value recovery of sulfate ions from concentrated water via high-pressure nanofiltration using an iron alum process. Furthermore, this characteristic of cold-rolled rinse water can be used to remove ammonia nitrogen and sulfate ions from the ash washing water, thereby improving the purity of the salt product. This method offers the advantages of low cost and simple operation.

[0046] 4: The present invention combines the analysis of the characteristics of ash washing water produced by the coordinated ash washing of high-salt solid waste and wastewater, and realizes the coordinated oxidation of thallium and weight removal treatment based on iron-carbon micro-electrolysis pretreatment, which greatly reduces the wastewater treatment process. At the same time, based on the characteristics and process design of individual wastewater, it realizes the coordinated resource treatment of multi-source wastewater and achieves zero wastewater discharge.

[0047] 5. The potassium salt recovered by the present invention is of high quality. On the one hand, pollutants such as ammonia nitrogen, thallium, and sulfate can be removed at low cost through wastewater pretreatment, preventing contamination of the potassium salt. On the other hand, by introducing sodium-containing wastewater in conjunction with ash washing, the potassium-sodium ratio in the ash washing water changes, making countercurrent evaporation suitable, preventing pollutants from entering the potassium salt, thereby increasing the value of the salt product. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 This is an outline diagram of the process for the coordinated zero-discharge treatment of multi-source wastewater in a steel plant according to the present invention.

[0049] Figure 2 This is a detailed diagram of the process for coordinated zero-emission treatment of multi-source wastewater in a steel plant according to the present invention. DETAILED DESCRIPTION

[0050] The technical solutions of the present invention are illustrated below with examples, and the scope of protection requested by the present invention includes but is not limited to the following embodiments.

[0051] Example 1

[0052] A zero-emission process for the coordinated treatment of wastewater from multiple sources in a steel plant comprises the following steps:

[0053] 1) The high-quality wastewater is homogeneously mixed with the concentrated brine from the desalted water station to obtain mixed concentrated brine wastewater.

[0054] 2) The mixed concentrated salt wastewater obtained in step 1) is first treated with deweighting and dehardening agents and desiliconization, and then subjected to secondary reverse osmosis treatment and high-pressure nanofiltration treatment in sequence to obtain high-pressure nanofiltration concentrated water and high-pressure nanofiltration fresh water.

[0055] 3) The high-pressure nanofiltration concentrate obtained in step 2) is mixed with cold-rolling rinsing wastewater to obtain mixed acidic wastewater, which is then heated to undergo a precipitation reaction. After the reaction is complete, solid-liquid separation is performed to obtain natrona and a residual liquid. The natrona is then disposed of in-plant, and the residual liquid is returned to step 1) for homogenization.

[0056] 4) The high-pressure nanofiltration freshwater obtained in step 2) is subjected to high-pressure reverse osmosis treatment to obtain high-pressure reverse osmosis concentrated water and recycled freshwater. The high-pressure reverse osmosis concentrated water is used for washing high-salt solid waste ash, and the recycled freshwater is circulated to any water-requiring process in the steel plant.

[0057] Example 2

[0058] Example 1 was repeated, except that in step 1), the high-quality wastewater was demineralized water from a softening station and coal gas condensate with a conductivity greater than 10,000 μS / cm.

[0059] Example 3

[0060] Example 1 was repeated, except that in step 1), the high-quality wastewater was demineralized water from a softening station and coal gas condensate with a conductivity greater than 12000 μS / cm.

[0061] Example 4

[0062] Example 3 was repeated, except that in step 3), the cold rolling rinsing wastewater was wastewater with a pH value less than 2.5 containing FeCl3 and HCl generated in the rinsing section of the cold-rolled strip pickling process.

[0063] Example 5

[0064] Example 3 was repeated, except that in step 3), the pH of the mixed acidic wastewater was 3. The step of heating the mixed acidic wastewater to perform precipitation reaction specifically comprises heating the mixed acidic wastewater to 90° C. for 4 hours.

[0065] Example 6

[0066] Example 5 was repeated, except that in step 3), before heating the mixed acidic wastewater for precipitation reaction, ferric chloride was added to adjust the molar ratio of iron ions to sulfate ions in the mixed acidic wastewater to 3:2.

[0067] Example 7

[0068] Example 6 was repeated, except that in step 4), the high-pressure reverse osmosis concentrated water was used for the water washing treatment of the high-salt solid waste ash as follows:

[0069] 4a) The high-salt solid waste ash is washed by mixing the high-pressure reverse osmosis concentrated water with the desulfurization wastewater. After solid-liquid separation, filter cake and ash washing wastewater are obtained. The filter cake is comprehensively disposed of in the factory, and the ash washing wastewater is sent to the next process.

[0070] 4b) The ash washing wastewater is adjusted to acidic using desulfurization wastewater or cold rolling rinsing wastewater, and then heated to carry out precipitation reaction. After the reaction is completed, solid-liquid separation is performed to obtain impurity-removed wastewater and slag phase. The slag phase is comprehensively disposed of in the factory, and the impurity-removed wastewater enters the next process.

[0071] 4c) The impurity removal wastewater is first treated with iron-carbon micro-electrolysis. A mixed reagent is then added to the treated wastewater to adjust its alkalinity. A precipitation reaction is performed to remove heavy particles and hard substances. After solid-liquid separation, high-salt wastewater and residue are obtained. The residue is then disposed of in a comprehensive manner within the plant, and the high-salt wastewater proceeds to the next process.

[0072] 4d) The high-salinity wastewater is first heated, concentrated and crystallized, and solid-liquid separation is performed to obtain sodium chloride and a primary filtrate. The primary filtrate is then cooled and crystallized, and solid-liquid separation is performed to obtain potassium chloride and a secondary filtrate. Finally, the secondary filtrate is mixed with the high-salinity wastewater and circulated for heating and salt precipitation treatment.

[0073] Example 8

[0074] Example 7 was repeated, except that in step 4a), the desulfurization wastewater was obtained by first evaporating and concentrating the wet desulfurization wastewater by 5 times to obtain suspended matter wastewater, then adding ferrous chloride and sodium sulfite to the suspended matter wastewater for precipitation treatment, and finally performing solid-liquid separation to obtain wastewater.

[0075] Example 9

[0076] Example 8 was repeated, except that the wet desulfurization wastewater was evaporated and concentrated as follows: the evaporation temperature of the first stage evaporation and concentration was 80-100°C and the vacuum degree was -40 to -5 kPa. The evaporation temperature of the second stage evaporation and concentration was 50-80°C and the vacuum degree was -70 to -40 kPa. The evaporation temperature of the third stage evaporation and concentration was 30-50°C and the vacuum degree was -100 to -70 kPa.

[0077] Example 10

[0078] Example 9 was repeated except that the hot steam generated by evaporating and concentrating the wet desulfurization wastewater was used to heat the mixed acidic wastewater.

[0079] Example 11

[0080] Example 10 was repeated, except that in step 4b), the adjusting the ash washing wastewater to acidic was to adjust the pH of the ash washing wastewater to 3, and the heating of the ash washing wastewater for precipitation reaction was to heat the ash washing wastewater to 90° C. for 4 h.

[0081] Example 12

[0082] Example 11 was repeated, except that in step 4b), before heating the ash washing wastewater for precipitation reaction, the molar ratio of iron ions, sulfate ions, and ammonia nitrogen in the ash washing wastewater was adjusted to 3:2:1 by adding soluble iron salt.

[0083] Example 13

[0084] Example 12 was repeated, except that in step 4c), the mixed reagent was composed of sodium hydroxide and potassium hydroxide, sodium carbonate and potassium carbonate, sodium sulfide and potassium sulfate, and a dithiocarbamate heavy-capturing agent. The amount of sodium hydroxide and potassium hydroxide added was such that the pH of the impurity-removed wastewater was 8. The amount of sodium carbonate and potassium carbonate added was 7 g / L. The amount of sodium sulfide and potassium sulfide added was 3.5 g / L. The amount of the dithiocarbamate heavy-capturing agent added was 3.6 g / L.

[0085] Example 14

[0086] Example 13 was repeated, except that the high-salt solid waste ash included sintered electric field ash.

[0087] Example 15

[0088] Example 14 is repeated, except that the washing of the high-salt solid waste ash is a three-stage countercurrent washing treatment. Specifically: the high-salt solid waste ash is first washed with a first-stage water, and dehydrated by a first-stage filter press to obtain a first-stage filtrate and a first-stage filter residue, and the first-stage filtrate is the ash washing wastewater. The first-stage filter residue enters the second-stage water washing, and the water source of the second-stage water washing is the third-stage filtrate and high-pressure reverse osmosis concentrated water and desulfurization wastewater. After the second-stage water washing, it is dehydrated by a second-stage filter press to obtain a second-stage filtrate and a second-stage filter residue, and the second-stage filtrate is discharged to the first-stage water washing for recycling. The second-stage filter residue enters the third-stage water washing, and the water source of the third-stage water washing is industrial water. After the third-stage water washing, it is dehydrated by a third-stage filter press to obtain a third-stage filtrate and a third-stage filter residue, and the third-stage filtrate is discharged to the second-stage water washing for recycling, and the third-stage filter residue is comprehensively disposed of in the factory.

[0089] Example 16

[0090] Example 15 was repeated, except that in step 4d), a three-stage multi-effect countercurrent evaporator was used to treat the high-salinity wastewater. The high-salinity wastewater was heated to 95° C. The primary filtrate was cooled to below 60° C. by flash evaporation or heat exchange.

[0091] Example 17

[0092] Example 16 was repeated except that the hot steam generated by the multiple-effect countercurrent evaporation device was used to heat the ash washing wastewater.

[0093] Example 18

[0094] Example 17 was repeated, except that in step 4c), the time for the iron-carbon micro-electrolysis treatment of the impurity-removing wastewater was not less than 20 minutes.

[0095] Example 19

[0096] Example 18 was repeated, except that in step 4c), after the mixed reagent was added to the impurity removal wastewater, the time for the heavy and hard removal precipitation reaction of the impurity removal wastewater was not less than 10 minutes.

[0097] Application Example 1

[0098] The process described in Example 6 was used to carry out collaborative resource treatment of desalination wastewater from the steel plant softening station, gas condensate, concentrated brine from the desalination station, and cold rolling rinsing wastewater:

[0099] Conduct conductivity tests on the desalted water and coal gas condensate water from the softening station respectively, and mix the high-quality desalted water and high-quality coal gas condensate water with conductivity greater than 12000μS / cm to obtain high-quality wastewater; the low-quality desalted water and low-quality coal gas condensate water with conductivity less than 12000μS / cm can be sent for conventional biochemical treatment.

[0100] High-quality wastewater and concentrated brine from the desalting station are stirred and mixed in a homogenizing tank for homogenization; a deweighting agent (sodium sulfide), a hardness removing agent (sodium carbonate or potassium carbonate), and a desiliconizing agent (ferric chloride) are then added to the homogenizing tank for weight removal, hardness removal, and desiliconization; after solid-liquid separation, the filtrate is sequentially subjected to secondary reverse osmosis treatment and high-pressure nanofiltration treatment to obtain high-pressure nanofiltration concentrated water and high-pressure nanofiltration fresh water; the high-pressure nanofiltration fresh water is further subjected to high-pressure reverse osmosis treatment to obtain recyclable reused fresh water and high-pressure reverse osmosis concentrated water for washing high-salt solid waste ash;

[0101] Cold-rolling rinse wastewater is added to the high-pressure nanofiltration concentrate, raising the pH of the mixed acidic wastewater to 3. The mixed acidic wastewater is then heated to 90°C using steam for 4 hours. After the reaction is complete, solid-liquid separation is performed to obtain high-purity natronaite, and the filtrate is returned to the homogenization tank for recycling. The resulting natronaite is then transported to the coking wastewater oxidation treatment stage, where it serves as a Fenton oxidant to catalyze the degradation of refractory organic pollutants.

[0102] Application Example 2

[0103] The process described in Example 18 is used to collaboratively treat zero-emission wastewater from multiple sources in a steel plant and to collaboratively treat high-salt solid waste ash:

[0104] First, the high-pressure reverse osmosis concentrated water obtained in Example 1 and part of the activated carbon flue gas washing wastewater were mixed to obtain acidic mixed water, and then the acidic mixed water was used to wash 100 kg of high-salt solid waste ash (a mixture of sintering power plant ash and blast furnace bag ash, the potassium content of the mixed ash was detected to be about 27.6%, and the sodium content was about 5.1%) in a three-stage countercurrent water wash. After filtration, a filter cake and about 350 L of ash washing wastewater (wherein the potassium-sodium content ratio was about 5.7) were obtained. The filter cake was returned to the sintering process for batching and disposal; then, the ash washing wastewater was first added to the ash washing wastewater. Cold rolling rinsing wastewater is added to adjust the pH of the ash washing wastewater to 3, and ferric chloride is added to the ash washing wastewater so that the molar ratio of iron ions, sulfate ions, and ammonia nitrogen in the ash washing wastewater is close to 3:2:1; hot steam is then introduced into the ash washing wastewater to heat the ash washing wastewater to 90°C and continue the reaction for 4 hours. After the reaction is completed, pressure filtration is performed to obtain impurity-removed wastewater and a slag phase, which is then transported to the sintering batch. The ammonium resources in the slag phase can be used to reduce the source of sintering nitrogen oxides, and the sulfate in the slag can be converted into sulfur dioxide and absorbed;

[0105] The pH of the impurity-removed wastewater was adjusted to 3 using sodium hydroxide. The treated water was then passed through an iron-carbon micro-electrolysis reactor for 40 minutes, during which the reactor was regularly aerated and backflushed. After micro-electrolysis, sodium hydroxide was added again to adjust the pH of the treated wastewater to 8. 2.45 kg of sodium carbonate, 1.23 kg of sodium sulfide, and 1.26 kg of a dithiocarbamate-based recapture agent were then added, stirred, and reacted for 30 minutes. After filtration, a high-salinity wastewater (the potassium-to-sodium ratio in the high-salinity wastewater was measured to be approximately 1.1:1) was obtained. Concentration and crystallization were performed in a three-stage multi-effect countercurrent evaporator (the first stage was evaporation at 95°C and a vacuum of -20 kPa; the second stage was evaporation at 75°C and a vacuum of -60 kPa; and the third stage was evaporation at 40°C and a vacuum of -90 kPa). Sodium chloride and a primary filtrate were obtained by centrifugation. The primary filtrate is cooled to below 60°C to precipitate crystals, which are then centrifuged to obtain crude potassium chloride and a secondary filtrate. The secondary filtrate is returned to the countercurrent evaporation inlet for cyclic evaporation. The crude potassium chloride is washed multiple times with a saturated potassium chloride solution and centrifuged to obtain high-purity potassium chloride (purity: 99.95%).

Claims

1. A zero-discharge process for the coordinated treatment of wastewater from multiple sources in a steel plant, characterized by: The process specifically includes the following steps: 1) homogeneously mixing high-quality wastewater with concentrated brine from a desalting station to obtain mixed concentrated brine wastewater; the high-quality wastewater is desalted water from a softening station and / or coal gas condensate with a conductivity greater than 10,000 μS / cm; 2) first adding a de-weighting agent, a de-hardening agent, and a desiliconizing agent to the mixed concentrated salt wastewater obtained in step 1); then sequentially performing a secondary reverse osmosis treatment and a high-pressure nanofiltration treatment to obtain high-pressure nanofiltration concentrated water and high-pressure nanofiltration fresh water; 3) mixing the high-pressure nanofiltration concentrated water obtained in step 2) with the cold rolling rinsing wastewater to obtain mixed acidic wastewater, then heating the mixed acidic wastewater to perform a precipitation reaction, and after the reaction is completed, performing solid-liquid separation to obtain natantria ferroalloy and a residual liquid; the natantria ferroalloy is comprehensively disposed of in the factory, and the residual liquid is returned to step 1) for homogenization treatment; 4) subjecting the high-pressure nanofiltration fresh water obtained in step 2) to high-pressure reverse osmosis treatment to obtain high-pressure reverse osmosis concentrated water and recycled fresh water; the high-pressure reverse osmosis concentrated water is used for washing high-salt solid waste ash, and the recycled fresh water is circulated to any water-requiring process in the steel plant.

2. The process according to claim 1, characterized in that: In step 1), the high-quality wastewater is demineralized water from a softening station and / or coal gas condensate with a conductivity greater than 12,000 μS / cm.

3. The process according to claim 1, characterized in that: In step 1), the concentrated brine of the desalted water station is neutral concentrated water containing sulfate and chloride ions produced when the steel plant uses reverse osmosis to desalinate the circulating water.

4. The process according to claim 1, wherein: In step 3), the cold rolling rinsing wastewater is wastewater with a pH value less than 2.5 and containing FeCl3 and HCl generated in the rinsing section of the cold rolled strip pickling process.

5. The process according to claim 4, characterized in that: In step 3), the cold rolling rinsing wastewater is wastewater with a pH value less than 2 and containing FeCl3 and HCl generated in the rinsing section of the cold rolled strip pickling process.

6. The process according to claim 1, characterized in that: In step 3), the pH of the mixed acidic wastewater is 2-4; and the step of heating the mixed acidic wastewater for precipitation reaction specifically comprises heating the mixed acidic wastewater to 80-100° C. for 1-8 hours.

7. The process according to claim 6, characterized in that: In step 3), the pH of the mixed acidic wastewater is 2-3; and the step of heating the mixed acidic wastewater for precipitation reaction specifically comprises heating the mixed acidic wastewater to 85-95° C. for 2-5 hours.

8. The process according to claim 6, characterized in that: Before heating the mixed acidic wastewater for precipitation reaction, a soluble iron salt is added to adjust the molar ratio of iron ions to sulfate ions in the mixed acidic wastewater to 1:0.4-0.

8.

9. The process according to claim 8, characterized in that: Before heating the mixed acidic wastewater for precipitation reaction, ferric chloride is added to adjust the molar ratio of iron ions to sulfate ions in the mixed acidic wastewater to 1:0.5-0.

7.

10. The process according to claim 1, characterized in that: In step 4), the high-pressure reverse osmosis concentrated water is used for the water washing treatment of high-salt solid waste ash as follows: 4a) The high-salt solid waste ash is washed by mixing the high-pressure reverse osmosis concentrated water with the desulfurization wastewater. After solid-liquid separation, filter cake and ash washing wastewater are obtained. The filter cake is comprehensively disposed of in the factory, and the ash washing wastewater is sent to the next process; 4b) Using desulfurization wastewater or cold rolling rinsing wastewater to adjust the ash washing wastewater to acidity, then heating the ash washing wastewater to carry out precipitation reaction. After the reaction is completed, solid-liquid separation is performed to obtain impurity-removed wastewater and slag phase. The slag phase is comprehensively disposed of in the factory, and the impurity-removed wastewater enters the next process; 4c) first subjecting the impurity removal wastewater to iron-carbon micro-electrolysis treatment; then, adding a mixed reagent to the impurity removal wastewater after micro-electrolysis treatment, adjusting the impurity removal wastewater to alkalinity and performing a precipitation reaction to remove heavy and hard substances from the impurity removal wastewater; after solid-liquid separation, obtaining high-salt wastewater and residue, which are comprehensively disposed of in the plant, and the high-salt wastewater enters the next process; 4d) First, the high-salt wastewater is heated, concentrated and crystallized, and solid-liquid separation is performed to obtain sodium chloride and a primary filtrate; then, the primary filtrate is cooled and crystallized, and solid-liquid separation is performed to obtain potassium chloride and a secondary filtrate; finally, the secondary filtrate is mixed with the high-salt wastewater and circulated for heating and salt precipitation treatment.

11. The process according to claim 10, characterized in that: In step 4a), the desulfurization wastewater is obtained by first evaporating and concentrating the wet desulfurization wastewater to obtain suspended matter wastewater, then adding an ammonia nitrogen precipitant to the suspended matter wastewater for precipitation treatment, and finally performing solid-liquid separation; the evaporation and concentration is performed by using multi-stage evaporation and concentration equipment to concentrate the wet desulfurization wastewater by 3-6 times; the ammonia nitrogen precipitant is a soluble ferrous salt and a soluble sulfite; and the wet desulfurization wastewater is wastewater generated when treating flue gas using a limestone / gypsum method.

12. The process according to claim 11, characterized in that: The wet desulfurization wastewater is evaporated and concentrated into a multi-stage evaporation concentration in which the evaporation temperature is gradually reduced and the vacuum degree is gradually increased; the soluble ferrous salt is one or more of ferrous chloride and ferrous sulfate; the soluble sulfite is one or more of sodium sulfite, sodium bisulfite and potassium sulfite.

13. The process according to claim 12, characterized in that: The evaporation temperature of the first stage evaporation concentration is 80~100℃, and the vacuum degree is -40~-5kPa; the evaporation temperature of the second stage evaporation concentration is 50~80℃, and the vacuum degree is -70~-40kPa; the evaporation temperature of the third stage evaporation concentration is 30~50℃, and the vacuum degree is -100~-70kPa; the order of adding soluble ferrous salt and soluble sulfite is sequential addition.

14. The process according to claim 12, characterized in that: The hot steam generated by evaporation and concentration is used to heat the mixed acidic wastewater in step 3) and / or is used to heat the ash washing wastewater in step 4b).

15. The process according to claim 10, characterized in that: In step 4b), adjusting the ash washing wastewater to acidity is to adjust the pH of the ash washing wastewater to 2-4; heating the ash washing wastewater for precipitation reaction is specifically heating the ash washing wastewater to 80-100° C. for 1-8 hours.

16. The process according to claim 15, characterized in that: In step 4b), adjusting the ash washing wastewater to acidity is to adjust the pH of the ash washing wastewater to 2-3; heating the ash washing wastewater for precipitation reaction is specifically heating the ash washing wastewater to 85-95° C. for 2-5 hours.

17. The process according to claim 15, characterized in that: Before heating the ash washing wastewater for precipitation reaction, soluble iron salt is added to adjust the molar ratio of iron ions, sulfate ions and ammonia nitrogen in the ash washing wastewater to 1:0.4-0.8:0.2-0.

6.

18. The process according to claim 17, characterized in that: Before heating the ash washing wastewater for precipitation reaction, soluble iron salt is added to adjust the molar ratio of iron ions, sulfate ions and ammonia nitrogen in the ash washing wastewater to 1:0.5-0.7:0.3-0.

5.

19. The process according to claim 10, characterized in that: In step 4c), before the impurity removal wastewater is subjected to iron-carbon micro-electrolysis, the pH of the impurity removal wastewater is adjusted to 3-5 using alkali; The mixed reagent is composed of sodium hydroxide and / or potassium hydroxide, sodium carbonate and / or potassium carbonate, sodium sulfide and / or potassium sulfate, and a heavy capture agent; wherein: the amount of sodium hydroxide and / or potassium hydroxide added is such that the pH value of the impurity removal wastewater is 7-9; the amount of sodium carbonate and / or potassium carbonate added is 3-10 g / L; the amount of sodium sulfide and / or potassium sulfide added is 1-7 g / L; and the amount of the heavy capture agent added is 1-8 g / L.

20. The process according to claim 19, characterized in that: In step 4c), before the impurity removal wastewater is subjected to iron-carbon micro-electrolysis, the pH of the impurity removal wastewater is adjusted to 3.5-4 using alkali; The amount of sodium hydroxide and / or potassium hydroxide added is such that the pH value of the impurity removal wastewater is 7.5-8; the amount of sodium carbonate and / or potassium carbonate added is 4-8 g / L; the amount of sodium sulfide and / or potassium sulfide added is 1.5-6 g / L; and the amount of the heavy capture agent added is 2-5 g / L.

21. The process according to claim 19, wherein: The alkali is sodium hydroxide and / or potassium hydroxide; the recapture agent is a xanthate recapture agent or a dithiocarbamate recapture agent.

22. The process according to claim 4, characterized in that: The high-salt solid waste ash includes one or more of sintering electric field ash, blast furnace bag ash, rotary kiln surface cooling ash, and garbage incineration fly ash; The water washing of the high-salt solid waste ash is a three-stage countercurrent water washing process; Specifically: the high-salt solid waste ash is first washed with primary water, and dehydrated through the primary filter press to obtain primary filtrate and primary filter residue, and the primary filtrate is the ash washing wastewater; the primary filter residue enters the secondary water washing, and the secondary water washing water source is the tertiary filtrate and high-pressure reverse osmosis concentrated water and desulfurization wastewater. After the secondary water washing, it is dehydrated through the secondary filter press to obtain secondary filtrate and secondary filter residue, and the secondary filtrate is discharged to the primary water washing for recycling; the secondary filter residue enters the tertiary water washing, and the tertiary water washing water source is industrial water. After the tertiary water washing, it is dehydrated through the tertiary filter press to obtain tertiary filtrate and filter cake, and the tertiary filtrate is discharged to the secondary water washing for recycling, and the filter cake is comprehensively disposed of in the factory.

23. The process according to claim 22, characterized in that: In step 4d), the high-salt wastewater is treated by a multi-effect countercurrent evaporation device, wherein the number of stages of the multi-effect countercurrent evaporation device is 2-6; the high-salt wastewater is heated to 80-100°C; and the primary filtrate is cooled to below 60°C by flash evaporation or heat exchange.

24. The process according to claim 23, characterized in that: The number of stages of the multi-effect countercurrent evaporation device is 3-4; the heating of high-salt wastewater is to heat the high-salt wastewater to 90-95° C.; the cooling of the primary filtrate is to cool the temperature to 20-55° C. by flash evaporation or heat exchange.

25. The process according to claim 23, characterized in that: The hot steam generated by the multi-effect countercurrent evaporation device is used to heat the mixed acidic wastewater in step 3) and / or is used to heat the ash washing wastewater in step 4b).

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