Separation and purification method of coking desulfurization waste liquid and ammonium thiocyanate mixed solution and application

Abstract

This invention provides a method for separating and purifying coking desulfurization wastewater, comprising the following steps: oxidizing the coking desulfurization wastewater to obtain an oxidized liquid, the oxidized liquid comprising ammonium thiocyanate and ammonium sulfate; concentrating the oxidized liquid by a first evaporation at a first temperature, followed by solid-liquid separation to obtain a first separated liquid; concentrating the first separated liquid by a second evaporation at a second temperature, followed by solid-liquid separation to obtain a second separated liquid and a mixed salt, the mixed salt comprising ammonium sulfate and ammonium thiocyanate; diluting the concentration of ammonium thiocyanate in the second separated liquid to 50-70%, and crystallizing and separating the solid and liquid at a third temperature to obtain a mother liquor and ammonium thiocyanate; the third temperature being 10-40℃; wherein 5℃ ≤ the first temperature difference T1 between the first and second temperatures ≤ 20℃, and 40℃ ≤ the second temperature difference T2 between the second and third temperatures ≤ 70℃. This invention can significantly improve the purity of ammonium thiocyanate and ammonium sulfate products, and the method is highly feasible.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of waste liquid treatment, and particularly relates to a separation and purification method and application of coking desulfurization waste liquid and ammonium thiocyanate mixed solution. BACKGROUND

[0002] In the process of wet desulfurization in the coking industry, coking desulfurization waste liquid mainly composed of thiocyanate, sulfate and thiosulfate will be produced. The coking desulfurization waste liquid has the characteristics of high toxicity and high corrosion. If it is directly discharged into the environment, it will destroy the ecological environment and cause resource waste.

[0003] At present, the main treatment technologies of coking desulfurization waste liquid can be divided into two categories: salt extraction treatment technology and salt decomposition treatment technology. The most widely used technology in salt decomposition technology is high-temperature pyrolysis. This technology is widely used due to its low investment and simple operation. However, this method has some disadvantages, such as being greatly affected by seasons and weather, high energy consumption, heavy odor in individual posts, high labor intensity of workers, poor working environment, and certain corrosion of waste liquid on the belt conveyor and support for a long time.

[0004] The salt extraction method mainly extracts components such as ammonium thiocyanate and ammonium thiosulfate in coking desulfurization waste liquid. The main methods include evaporation crystallization, step crystallization, preparation of thiocyanate, membrane separation and ion exchange. Among them, the step crystallization method is the most widely studied. This method is based on the ternary phase diagram of NH4SCN-(NH4)2S2O3-H2O as the theoretical basis. Through the links of activated carbon adsorption decolorization, vacuum evaporation and multi-stage crystallization, different sulfur-containing salt products are obtained. However, due to the complexity of the composition of coking desulfurization waste liquid, the ratio of ammonium thiocyanate and ammonium thiosulfate is constantly changing, which increases the difficulty of operation. At the same time, due to the high solubility of ammonium thiocyanate and ammonium thiosulfate and the small difference, the purity of the main product ammonium thiocyanate is poor, and the recovery rate is also low. There are also reports that it can be purified to 80% or 90%, but it is far from meeting the national industrial salt standard requirements. A large amount of hazardous waste such as mixed salt and sulfur residue is produced in the middle, and the product can only be sold at a very low price or even at a cost, and the sales price is difficult to maintain the cost. And this way of extracting salt cannot completely solve the final disposal problem of coking desulfurization waste liquid.

[0005] However, with the improvement of product purity, it is almost impossible to further purify ammonium thiocyanate and ammonium sulfate products in common technologies, which cannot adapt to the high demand for high-purity ammonium thiocyanate products. SUMMARY

[0006] To solve the technical problem that the purity of ammonium thiocyanate product cannot be further improved in the above-mentioned common technologies, the present application provides a separation and purification method of coking desulfurization waste liquid, comprising the following steps:

[0007] The coking desulfurization waste liquid is oxidized to obtain an oxidized liquid, which includes ammonium thiocyanate and ammonium sulfate.

[0008] The oxidized liquid is concentrated by evaporation at a first temperature, followed by solid-liquid separation to obtain a first separated liquid. The first separated liquid contains 10% to 40% ammonium thiocyanate and 10% to 50% ammonium sulfate by mass fraction.

[0009] The first separated liquid is concentrated by evaporation at a second temperature, followed by solid-liquid separation to obtain the second separated liquid and the mixed salt. The chemical composition of the mixed salt includes ammonium sulfate and ammonium thiocyanate. By mass fraction, the mass fraction of ammonium thiocyanate in the second separated liquid is 70%~80%, and the mass fraction of ammonium sulfate is 7%~11%.

[0010] The concentration of ammonium thiocyanate in the second separation solution is diluted to 50-70%, and crystallization and solid-liquid separation are performed at a third temperature to obtain mother liquor and ammonium thiocyanate; the third temperature is 10-40℃.

[0011] Wherein, the first temperature is greater than the second temperature, and the second temperature difference T2 between the second temperature and the third temperature is 40~70℃.

[0012] Furthermore, the first temperature difference T1 between the first temperature and the second temperature is 5~20℃.

[0013] Furthermore, after the oxidized liquid is concentrated by the first evaporation and solid-liquid separation, ammonium sulfate is obtained.

[0014] Furthermore, the post-oxidation liquid includes the mixed salt, and the first separated liquid includes the mother liquor.

[0015] Furthermore, the coking desulfurization waste liquid is further subjected to an oxidation treatment step prior to the following: using activated carbon to adsorb and decolorize the coking desulfurization waste liquid.

[0016] Furthermore, the coking desulfurization waste liquid is oxidized to obtain an oxidized liquid, including the following steps: adjusting the pH of the decolorized coking desulfurization waste liquid to 1.0~4.0, and then adding an oxidant to convert the thiosulfate in the coking desulfurization waste liquid into sulfate, sulfur and sulfur dioxide gas.

[0017] Furthermore, the coking desulfurization wastewater is oxidized to obtain an oxidized liquid, and the process further includes the step of adjusting the pH of the oxidized coking desulfurization wastewater to 6-9 to obtain the oxidized liquid.

[0018] Furthermore, the evaporation concentration includes vacuum concentration and atmospheric pressure concentration.

[0019] This invention also provides a method for separating and purifying an ammonium thiocyanate mixture, wherein the ammonium thiocyanate mixture comprises ammonium thiocyanate and ammonium sulfate, and the separation and purification method includes the following steps:

[0020] After the ammonium thiocyanate mixture is concentrated by evaporation at a first temperature, solid-liquid separation is performed to obtain a first separated liquid. The mass fraction of ammonium thiocyanate in the first separated liquid is 10%~40%, and the mass fraction of ammonium sulfate is 10%~50%.

[0021] The first separated liquid is concentrated by evaporation at a second temperature, followed by solid-liquid separation to obtain the second separated liquid and the mixed salt. The chemical composition of the mixed salt includes ammonium sulfate and ammonium thiocyanate. By mass fraction, the mass fraction of ammonium thiocyanate in the second separated liquid is 70%~80%, and the mass fraction of ammonium sulfate is 7%~11%.

[0022] The concentration of ammonium thiocyanate in the second separation solution is diluted to a mass fraction of 50-70%, and then crystallized and separated into solid and liquid components at a third temperature to obtain the mother liquor and ammonium thiocyanate; the third temperature is 10-40℃.

[0023] Wherein, the first temperature is greater than the second temperature, and the second temperature difference T2 between the second temperature and the third temperature is 40~70℃.

[0024] The present invention also provides an application of the separation and purification method described above in the treatment of coking desulfurization wastewater.

[0025] Compared with commonly used technologies, the present invention has at least the following advantages:

[0026] This invention oxidizes ammonium thiosulfate in coking desulfurization wastewater to ammonium sulfate, thus constructing a ternary system of ammonium sulfate, ammonium thiocyanate, and water in coking desulfurization wastewater.

[0027] In this system, the present invention utilizes a separation method of cooling crystallization, combined with the range settings of the first and second temperature differences. On the one hand, based on the solubility difference between ammonium sulfate and ammonium thiocyanate, the separation of the two is more thorough, which is beneficial for the separate purification and impurity removal of ammonium sulfate and ammonium thiocyanate, further improving the purity of the ammonium thiocyanate product. On the other hand, in this ternary system, ammonium thiocyanate has a relatively wide crystallization region at the third temperature. Within this region, ammonium thiocyanate enters the solid phase and is separated, resulting in high-purity crystals. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or common technologies, the drawings used in the description of the embodiments or common technologies will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0029] Figure 1 This is a solubility curve of ammonium sulfate, ammonium thiocyanate, and ammonium thiosulfate in this invention.

[0030] Figure 2 This is a ternary phase diagram of NH4SCN-(NH4)2SO4-H2O at different temperatures in this invention.

[0031] Figure 3 This is a ternary phase diagram of the NH4SCN-(NH4)2SO4-H2O system in this invention at temperatures t1 and t2 (t1 < t2); wherein the phase line at t1℃ is MPN and the phase line at t2℃ is EQF.

[0032] Figure 4 This is a process simulation model diagram constructed based on an actual process in one embodiment of the present invention. Detailed Implementation

[0033] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] Furthermore, the technical solutions of the various embodiments of the present invention can be combined with each other, but only if they are based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

[0035] When numerical ranges are given in the embodiments, it should be understood that, unless otherwise stated in the present invention, both endpoints of each numerical range and any value between the two endpoints may be selected. Unless otherwise defined, all technical and scientific terms used in this invention, as well as the knowledge of common techniques by those skilled in the art and the description of this invention, may be used to implement this invention using any common techniques, devices, and materials similar to or equivalent to those described, used, or made of materials in the embodiments of this invention.

[0036] This invention provides a method for separating and purifying coking desulfurization wastewater, comprising the following steps:

[0037] S1, the coking desulfurization waste liquid is oxidized to obtain an oxidized liquid, wherein the oxidized liquid includes ammonium thiocyanate and ammonium sulfate.

[0038] Coking desulfurization wastewater refers to the wastewater formed during the wet desulfurization process. Its composition usually includes thiocyanate, sulfate and thiosulfate. It is highly toxic and corrosive. If it is directly discharged into the environment, it will damage the ecological environment and cause waste of resources.

[0039] Although the proportions of thiocyanate, sulfate, and thiosulfate in coking desulfurization wastewater may vary due to differences in the specific wet desulfurization process and raw material feed ratio, they are all applicable to the separation and purification methods for coking desulfurization wastewater described in this application. Therefore, the composition of the coking desulfurization wastewater and the composition of the oxidized liquid obtained from the oxidation of the coking desulfurization wastewater are not limited.

[0040] In some embodiments, by mass fraction, the proportion of ammonium thiocyanate in the coking desulfurization wastewater is 8% to 16%, the proportion of ammonium sulfate is 4% to 12%, and the proportion of ammonium thiosulfate is 2% to 14%.

[0041] For example, by mass fraction, the proportion of ammonium thiocyanate in coking desulfurization wastewater can be 14.82%, the proportion of ammonium sulfate can be 9.99%, and the proportion of ammonium thiosulfate can be 11.25%.

[0042] Oxidation treatment is a pretreatment for vacuum concentration. This operation oxidizes ammonium thiosulfate in coking desulfurization wastewater into ammonium sulfate, sulfur, and a small amount of sulfur dioxide gas. Among these, for example... Figure 1 As shown, the conversion of ammonium thiosulfate to ammonium sulfate increases the solubility difference of the main components in the coking desulfurization wastewater, which is beneficial to the separation and purification of ammonium thiocyanate. Among the oxidation products, sulfur dioxide can be absorbed by the coking desulfurization wastewater, and the absorbed wastewater is returned to the decolorization system for decolorization and adsorption treatment.

[0043] The oxidation reaction equation is as follows:

[0044] (NH4)2S2O3 + oxidizing agent → (NH4)2SO4 + SO2↑ + S↓

[0045] In some embodiments, the oxidation treatment further includes the step of adsorbing and decolorizing the coking desulfurization waste liquid using activated carbon.

[0046] Based on the above, the oxidation treatment may specifically include the following steps: adjusting the decolorized coking desulfurization waste liquid to pH = 1.0~4.0, adding an oxidant to convert the thiosulfate in the coking desulfurization waste liquid into sulfate, sulfur and a small amount of sulfur dioxide gas; adjusting the coking desulfurization waste liquid after the oxidation reaction to pH = 6~9, separating the solid and liquid to obtain the oxidized liquid and sulfur.

[0047] For example, sulfuric acid can be used to adjust the pH of the decolorized coking desulfurization wastewater to 1.0-4.0, and ammonia water can be used to adjust the pH of the coking desulfurization wastewater after the oxidation reaction to 6-9.

[0048] In some embodiments, the proportion of ammonium thiocyanate in the oxidized solution may be 8% to 16% and the proportion of ammonium sulfate may be 12% to 17% by mass fraction.

[0049] For example, by mass fraction, the proportion of ammonium thiocyanate in the oxidized solution can be 14.82%, and the proportion of ammonium sulfate can be 15.28%.

[0050] S2, the oxidized liquid is concentrated by a first evaporation at a first temperature, and then the solid and liquid are separated to obtain a first separated liquid.

[0051] In some embodiments, the oxidized liquid is further concentrated under reduced pressure and separated into solid and liquid components to obtain ammonium sulfate.

[0052] For example, the precipitated ammonium sulfate crystals can be sprayed and washed to remove residual salts from the surface of the ammonium sulfate crystals.

[0053] The ammonium sulfate was concentrated under reduced pressure, causing most of it to crystallize out.

[0054] In some embodiments, the mass fraction of ammonium thiocyanate in the first separation liquid is 10% to 40%, and the mass fraction of ammonium sulfate is 10% to 50%.

[0055] In some embodiments, the first evaporation concentration can be vacuum concentration, and the vacuum degree of the first vacuum concentration can be 0.075~0.085MPa.

[0056] S3, after the first separation liquid is concentrated by the second evaporation at the second temperature, solid-liquid separation is performed to obtain the second separation liquid and the mixed salt, wherein the chemical composition of the mixed salt includes ammonium sulfate and ammonium thiocyanate.

[0057] The second evaporation concentration process causes some of the mixed salt containing ammonium sulfate and ammonium thiocyanate to precipitate out.

[0058] By mass fraction, the ammonium thiocyanate in the second separation liquid accounts for 70% to 80%, and the ammonium sulfate accounts for 7% to 11%.

[0059] In some embodiments, the second evaporation concentration can be vacuum concentration, and the vacuum degree of the second vacuum concentration can be 0.075~0.085MPa.

[0060] S4, the concentration of ammonium thiocyanate in the second separation liquid is diluted to 50-70%, and crystallization and solid-liquid separation are performed at a third temperature to obtain mother liquor and ammonium thiocyanate.

[0061] The concentration of ammonium thiocyanate in the second separation liquid is diluted to a mass fraction of 50-70%, allowing the liquid phase to enter the ammonium thiocyanate crystallization zone. This, combined with subsequent cooling and crystallization operations, allows pure ammonium thiocyanate crystals to precipitate.

[0062] In some embodiments, 5℃≤the first temperature difference T1 between the first temperature and the second temperature≤20℃, and 40℃≤the second temperature difference T2 between the second temperature and the third temperature≤70℃.

[0063] This invention utilizes the ternary phase diagram principle of salt separation. Based on the NH4SCN-(NH4)2SO4-H2O ternary system, it specifically designs the evaporation and concentration temperature, pressure, and cooling crystallization temperature. It adjusts the water-soluble components (parts) of an aqueous solution containing ammonium thiocyanate and ammonium sulfate to the crystallization region of the material to be separated, allowing a specific component to enter the solid phase for separation. This, in turn, increases the concentration ratio of other components in the aqueous solution, thus obtaining an aqueous solution containing the specific component. When the concentration of this component reaches a certain range, high-quality crystals can be obtained through cooling.

[0064] The desulfurization waste liquid from coking plants, after decolorization and oxidation, is a ternary system of NH4SCN-(NH4)2SO4-H2O after elimination. The separation and recycling process of this system is studied based on the principle of ternary phase diagram.

[0065] like Figure 2 The figure shows the ternary phase diagram of NH4SCN-(NH4)2SO4-H2O at different temperatures. As can be seen from the figure, the solubility of NH4SCN gradually decreases with decreasing temperature, and its crystallization region gradually increases with decreasing temperature. Therefore, low temperature helps the crystallization and precipitation of NH4SCN. The solubility of (NH4)2SO4 does not change much with temperature, and its crystallization region is larger at high temperature. Therefore, high temperature helps the precipitation of (NH4)2SO4.

[0066] For ease of explanation, from Figure 2 Choose any two phase lines from the ternary phase diagram shown for analysis, as follows: Figure 3The diagram shows the ternary phase diagram of the NH4SCN-(NH4)2SO4-H2O system at temperatures t1 and t2 (t1 < t2). Point O represents the composition (%) of the waste liquid after oxidation. After evaporation and dehydration at temperature t2, the liquid phase enters the (NH4)2SO4 crystallization region (FQC) at that temperature, at which point (NH4)2SO4 precipitates (enters the solid phase). Filtering away the solid (NH4)2SO4 yields a solution with composition Q. However, considering practical operation, to ensure that the solid phase point Q is reached, some mixed salt needs to precipitate. The liquid phase after filtering the mixed salt is point Q. However, upon cooling, because point Q is in the t1 ternary phase region, mixed salt will precipitate, which is why the ammonium thiocyanate produced by the salt extraction process cannot yield pure NH4SCN. To ensure that the liquid phase only enters the NH4SCN crystallization region, a small amount of water needs to be added at t2. According to phase diagram theory, the new system point must be on the line connecting points A and Q, as shown by point R. This moves the liquid phase point from point Q to point R, and then the temperature is lowered to t1. At this point, it can cross the QR line segment and precipitate in the region of BMP in the three-phase diagram, yielding over 99% ammonium thiocyanate crystals. The remaining mother liquor is composed of P. Through the above steps, (NH4)2SO4 and NH4SCN are initially separated. The small portion of mixed salt and the remaining mother liquor P can be recycled.

[0067] Back Figure 2 In fact Figure 2 Including the NH4SCN-(NH4)2SO4-H2O phase line at different temperatures, i.e. Figure 2 This includes two or more phase wires. Figure 2 It includes 5 phase lines: the diamond symbol corresponds to the first curve, the triangle symbol corresponds to the second curve, the five-pointed star symbol corresponds to the third curve, the circle symbol corresponds to the fourth curve, and the square symbol corresponds to the fifth curve.

[0068] In some more specific examples, the first curve can be the phase line at 20°C, the second curve can be the phase line at 40°C, the third curve can be the phase line at 55°C, the fourth curve can be the phase line at 75°C, and the fifth curve can be the phase line at 85°C.

[0069] In some more specific examples, the first curve can be the phase line at 15°C, the second curve can be the phase line at 35°C, the third curve can be the phase line at 50°C, the fourth curve can be the phase line at 70°C, and the fifth curve can be the phase line at 80°C.

[0070] In some more specific examples, the first curve can be the phase line at 23°C, the second curve can be the phase line at 43°C, the third curve can be the phase line at 58°C, the fourth curve can be the phase line at 78°C, and the fifth curve can be the phase line at 88°C.

[0071] In some more specific examples, the first curve can be the phase line at 30°C, the second curve can be the phase line at 50°C, the third curve can be the phase line at 65°C, the fourth curve can be the phase line at 85°C, and the fifth curve can be the phase line at 95°C.

[0072] Therefore, t1 and t2 can be selected from different phase lines according to their respective needs, as long as t1 and t2 meet the condition of having a certain temperature difference. That is, t2-t1 is greater than 40℃.

[0073] For example, when t1 is the first curve, t2 can be the fifth curve and the fourth curve; when t1 is the second curve, t2 can be the fifth curve.

[0074] Through the above steps, (NH4)2SO4 and NH4SCN were initially separated. The small amount of mixed salt and the remaining mother liquor P can be recycled. Part of the mixed salt and mother liquor P were dissolved in the oxidized solution, evaporated at temperature t2, and dehydrated to point H, reaching the (NH4)2SO4 crystallization zone (FQC). The solid (NH4)2SO4 was removed by maintaining the temperature, resulting in a solution with concentration Q. To ensure that the solid phase point Q is reached, some mixed salt needs to precipitate. The liquid phase after filtering the mixed salt is at point Q. Subsequent operations are the same as above, forming a cycle along HQRP. Each cycle yields some pure solid (NH4)2SO4 and NH4SCN, as well as some mixed salt and mother liquor with concentration P.

[0075] However, if coking desulfurization waste liquid is directly used for recycling, the whole process is more complicated because the coking desulfurization waste liquid is a quaternary system of NH4SCN-(NH4)2S2O3-(NH4)2SO4-H2O. At the same time, since the solubility difference between ammonium thiosulfate and ammonium thiocyanate is very small, the purity of the obtained ammonium thiocyanate and ammonium thiosulfate is not high, and a large amount of mixed salt is generated, resulting in a low sulfur salt recovery rate.

[0076] In the embodiments of this application, by oxidizing ammonium thiosulfate to ammonium sulfate, the sulfur form in the coking desulfurization wastewater is controlled, simplifying it into two sulfur salts with greater solubility: ammonium sulfate and ammonium thiocyanate, forming a ternary system of NH4SCN-(NH4)2SO4-H2O. Based on the drawing and analysis of the NH4SCN-(NH4)2SO4-H2O ternary phase diagram, a high-purity ammonium sulfate product can be obtained through a single concentration, and a high-purity ammonium thiocyanate product can be obtained through a second concentration and dilution-cooling crystallization. The recovery rate of sulfur salts and the purity of the product are greatly improved.

[0077] The NH4SCN-(NH4)2SO4-H2O ternary phase diagram is a chart used to study the solubility of NH4SCN, (NH4)2SO4, and H2O. It can be drawn by conducting solubility experiments on the three substances at different temperatures, or it can be generated using simulation software. This study uses simulation software capable of simulating both steady-state and dynamic systems, utilizing its property database, unit operation modules, and system implementation methods to construct the NH4SCN-(NH4)2SO4-H2O ternary phase diagram.

[0078] For example, this can be done using Aspen Plus software from Aspen Tech. In this example, the software retrieves basic physical property parameters from a database and combines them with specific equations of state and operational modules to obtain relevant data on energy flow and material flow. Simultaneously, the software's built-in property data estimation system can estimate missing parameters based on the molecular structure of the input data, enabling simulation of new substances not included in the database and simulating the evaporation and crystallization of a water-salt system.

[0079] The unit operations involved in the separation and purification process of NH4SCN and (NH4)2SO4 mainly include mixing, heating, cooling crystallization, and separation. These unit operations can be represented by the following unit operation modules: mixer / splitter unit, heat exchanger unit, solid-liquid separation unit, gas-liquid separation unit, etc. The selected unit operation modules are HeaterX (heat exchanger), Mixer (mixer), Sep (separator), and Flash (flash evaporator), etc. Based on the actual process, a system is established as follows: Figure 4 The process simulation model shown sets parameters such as feed components, inlet and outlet temperatures, and pressures. After operation, it obtains the mass of each component and the component streams at those temperatures and pressures. Based on the mass fraction of each substance in the stream (solid or liquid), Origin plots are generated, ultimately yielding the ternary phase diagrams of NH4SCN-(NH4)2SO4-H2O at different temperatures, as shown below. Figure 3 As shown.

[0080] In some embodiments, the first temperature difference T1 is 5~20℃, and the second temperature difference T2 is 40~70℃.

[0081] The third temperature refers to the temperature at which crystallization is cooled. In some embodiments, the third temperature can be 10~40°C.

[0082] The above-described method for separating and purifying coking desulfurization wastewater is based on the solubility characteristics of the NH4SCN-(NH4)2SO4-H2O ternary system at 70℃ and 15℃. Accordingly, the first temperature is 70℃, the second temperature is 55℃, and the third temperature is 15℃. The first temperature difference is 15℃, and the second temperature difference is 40℃.

[0083] In this embodiment, the purities of ammonium sulfate and ammonium thiocyanate were obtained as 99.2% and 99.5%, respectively, and the recovery rate of the entire system was 91%.

[0084] In some other embodiments, recovery can also be performed based on the solubility characteristics of the NH4SCN-(NH4)2SO4-H2O ternary system at other temperatures.

[0085] In some embodiments, the solubility characteristics at 75°C and 25°C are used for recovery. Correspondingly, the first temperature is 75°C, the second temperature is 60°C, and the third temperature is 15°C. The first temperature difference is 15°C, and the second temperature difference is 45°C. The specific salt separation principle is similar to that described above for the solubility curves of the NH4SCN-(NH4)2SO4-H2O ternary system at 70°C and 15°C, and will not be repeated here.

[0086] In this embodiment, the purities of ammonium sulfate and ammonium thiocyanate were obtained to be 99.4% and 99.6%, respectively, and the recovery rate of the entire system was 92%. This embodiment has the advantages of high product purity, low energy consumption, and short processing time.

[0087] For example, the solubility characteristics of the solubility curves at 65℃ and 20℃ are used for recovery. Correspondingly, the first temperature is 65℃, the second temperature is 60℃, and the third temperature is 20℃. The first temperature difference is 5℃, and the second temperature difference is 40℃. The specific salt separation principle is similar to that described above for the solubility curves of the NH4SCN-(NH4)2SO4-H2O ternary system at 70℃ and 15℃, and will not be repeated here.

[0088] In this embodiment, the purities of ammonium sulfate and ammonium thiocyanate were obtained to be 99.0% and 99.2%, respectively, and the recovery rate of the entire system was 90.3%. This embodiment has the advantages of safe operation and ease of implementation.

[0089] This invention utilizes oxidation treatment to oxidize ammonium thiosulfate in coking desulfurization wastewater into ammonium sulfate, thereby constructing a ternary system of ammonium sulfate, ammonium thiocyanate, and water in coking desulfurization wastewater, and obtaining high-purity ammonium sulfate and ammonium thiocyanate products.

[0090] In this system, on the one hand, the solubility difference between ammonium sulfate and ammonium thiocyanate allows for more thorough separation, which is beneficial for the separate purification and impurity removal of ammonium sulfate and ammonium thiocyanate, further improving the purity of the ammonium thiocyanate product. In contrast, the solubility difference between unoxidized ammonium thiosulfate and ammonium thiocyanate is small, often resulting in mutual interference and inclusion of components in the ammonium thiosulfate and ammonium thiocyanate products, leading to the loss of ammonium thiocyanate.

[0091] On the other hand, in this ternary system, ammonium thiocyanate has a relatively wide crystallization region between 15 and 35°C. Within this region, ammonium thiocyanate can be separated into the solid phase and high-purity crystals can be obtained. In contrast, in the ternary system of ammonium thiosulfate, ammonium thiocyanate, and water before oxidation, the crystallization region of ammonium thiocyanate is relatively small. Even at the crystallization critical point, a certain amount of ammonium thiosulfate will be present, and impurities cannot be avoided.

[0092] In addition, compared with ammonium thiosulfate, ammonium sulfate has higher value, wider sales channels, and greater market demand, thus its production has higher economic benefits.

[0093] In some embodiments, the mixed salt obtained in step S3 can be added to the post-oxidation liquid for further primary and secondary concentration. The mother liquor obtained in step S4 can be added to the first separation liquid in step S2 for further secondary concentration. The above cycle can increase the sulfur salt recovery rate in the entire purification system by more than 40%, with a recovery rate as high as 92%.

[0094] For example, the mother liquor obtained in step S4 can also be added to the post-oxidation liquid to continue participating in the first-stage concentration.

[0095] For example, the washing solution obtained from washing ammonium thiocyanate in step S4 can also be added to the post-oxidation solution for primary concentration.

[0096] Compared with commonly used technologies, the present invention has at least the following advantages:

[0097] 1. This invention oxidizes ammonium thiosulfate in coking desulfurization wastewater to ammonium sulfate, thus constructing a ternary system of ammonium sulfate, ammonium thiocyanate, and water in coking desulfurization wastewater.

[0098] In this system, the present invention utilizes a separation method of cooling crystallization, combined with the range settings of the first and second temperature differences. On the one hand, based on the solubility difference between ammonium sulfate and ammonium thiocyanate, the separation of the two is more thorough, which is beneficial for the separate purification and impurity removal of ammonium sulfate and ammonium thiocyanate, further improving the purity of the ammonium thiocyanate product. On the other hand, in this ternary system, ammonium thiocyanate has a relatively wide crystallization region at the third temperature. Within this region, ammonium thiocyanate enters the solid phase and is separated, resulting in high-purity crystals.

[0099] 2. Ammonium sulfate is valuable, widely sold, and in high market demand, making its preparation economically viable. Furthermore, the improved purity achieved in this invention, building upon the existing high grade, signifies its promising future applications in high-end, fine chemical industries.

[0100] 3. This invention further simplifies the process based on commonly used technologies, improving product quality and reducing process costs while reducing the number of concentration stages.

[0101] 4. This invention eliminates the alcohol precipitation process commonly used in technology, effectively avoiding the safety hazards and environmental burden caused by organic solvents.

[0102] This invention also provides a method for separating and purifying an ammonium thiocyanate mixture, wherein the ammonium thiocyanate mixture comprises ammonium thiocyanate and ammonium sulfate, and the separation and purification method includes the following steps:

[0103] S10, after the ammonium thiocyanate mixture is concentrated by the first evaporation at the first temperature, the solid and liquid are separated to obtain the first separated liquid.

[0104] S20, after the first separation liquid is concentrated by the second evaporation at the second temperature, solid-liquid separation is performed to obtain the second separation liquid and the mixed salt, wherein the chemical composition of the mixed salt includes ammonium sulfate and ammonium thiocyanate;

[0105] S30, the concentration of ammonium thiocyanate in the second separation liquid is diluted to 50-70%, and crystallization and solid-liquid separation are performed at a third temperature to obtain mother liquor and ammonium thiocyanate; the third temperature is 10-40℃;

[0106] Wherein, the first temperature is greater than the second temperature, and the difference between the second temperature and the third temperature is greater than 40°C.

[0107] It should be noted that the ranges of the first, second, and third temperatures are as described above and will not be repeated here.

[0108] This invention provides an application of the separation and purification method described in any one of the above claims in the treatment of coking desulfurization wastewater.

[0109] This invention provides an application of the separation and purification method as described in any one of the claims in the treatment of coking desulfurization wastewater.

[0110] To facilitate a further understanding of the present invention by those skilled in the art, the following examples are provided:

[0111] Example 1

[0112] For the desulfurization wastewater from oxidized coking plants, the ternary phase diagram principle is based on the elimination of NH4SCN-(NH4)2SO4-H2O. The phase diagram was compiled using experimental data from laboratory conditions under specific temperature and pressure, as shown below. Figure 2 , Figure 3 As shown.

[0113] Based on the experimental ternary phase diagram, the waste liquid composition was simulated using analytical grade ammonium sulfate and ammonium thiocyanate to verify the ternary phase diagram.

[0114] 158.74 g of ammonium thiocyanate and 161.74 g of ammonium sulfate were weighed and dissolved in pure water. The solution was then diluted to volume in a 1000 mL volumetric flask. 600 mL of the liquid was taken and concentrated under reduced pressure at 78℃ (vacuum degree -0.080 MPa). Solid precipitated. The concentration was further increased, and the mixture was filtered after maintaining the temperature to obtain the first separated liquid and crystalline particles. The mass fraction of ammonium sulfate in the first separated liquid was approximately 12.16%, and the mass fraction of ammonium thiocyanate was approximately 23.86%. After spraying, 31.04 g of ammonium sulfate product with a purity of 99.51% was obtained from the crystalline particles.

[0115] The first separation liquid was further distilled. When a large amount of solid appeared, it was centrifuged to obtain a mixed salt and a second separation liquid. At this time, the mass fraction of ammonium sulfate in the second separation liquid was about 7.69%, and the mass fraction of ammonium thiocyanate was about 73.08%. Distillate was added to the second separation liquid to dilute it to an ammonium thiocyanate mass fraction of 62.01%. The mixture was cooled to 20°C, filtered, and centrifuged by spraying to obtain 32.47g of ammonium thiocyanate product with a purity of 99.64%.

[0116] The mixed salt, the last remaining mother liquor, and the spray washing liquid are returned to the next round of oxidation liquid. After at least 5 rounds of evaporation and concentration, the total recovery rate of sulfur salt is 93%.

[0117] Example 2

[0118] Using desulfurization wastewater from a coking plant in Tangshan as raw material, the contents of (NH4)2S2O3, (NH4)2SO4, and NH4SCN were 120.38 g / L, 106.95 g / L, and 158.62 g / L, respectively. After decolorization, oxidation, neutralization, and filtration, the oxidized liquid was obtained, with the contents of (NH4)2S2O3, (NH4)2SO4, and NH4SCN being 0.72 g / L, 163.54 g / L, and 158.62 g / L, respectively.

[0119] 600 mL of liquid was taken and concentrated under reduced pressure at 70℃ (vacuum degree -0.075 MPa), resulting in solid precipitation. Concentration was continued, followed by filtration at the same temperature to obtain the first separated liquid and crystalline particles. The mass fraction of ammonium sulfate in the first separated liquid was approximately 14.63%, and the mass fraction of ammonium thiocyanate was approximately 22.88%. After spraying, 36.61 g of ammonium sulfate product with a purity of 99.2% was obtained from the crystalline particles.

[0120] The first separation liquid was further distilled. When a large amount of solid appeared, it was centrifuged to obtain a mixed salt and a second separation liquid. At this point, the mass fraction of ammonium sulfate in the second separation liquid was approximately 9.38%, and the mass fraction of ammonium thiocyanate was approximately 71.09%. Distillate was added to the second separation liquid to dilute it to an ammonium thiocyanate mass fraction of 64.1%. The mixture was cooled to 15°C, filtered, and centrifuged after spraying to obtain 30.57 g of ammonium thiocyanate product with a purity of 99.81%.

[0121] The mixed salt, the last remaining mother liquor, and the spray washing liquid are returned to the next round of oxidation liquid. After at least 5 rounds of evaporation and concentration, the total recovery rate of sulfur salt is 92%.

[0122] Comparative Example 1

[0123] Based on Example 2, the first separation liquid was further distilled. When a large amount of solid appeared, it was centrifuged to obtain a mixed salt and a second separation liquid. At this time, the mass fraction of ammonium sulfate in the second separation liquid was 9.38%, and the mass fraction of ammonium thiocyanate was 71.09%. The second separation liquid was further evaporated and concentrated, and centrifuged after being cooled to 15°C to obtain ammonium thiocyanate product with a purity of 90.4%. The mixed salt, the last remaining mother liquor, and the spray washing liquid were returned to the next round of oxidation liquid. After at least 5 rounds of evaporation and concentration, the total recovery rate of sulfur salt was about 62%.

[0124] The above technical solutions of the present invention are merely preferred embodiments of the present invention and do not limit the patent scope of the present invention. All equivalent structural transformations made under the technical concept of the present invention using the contents of the present invention specification and drawings, or direct / indirect applications in other related technical fields, are included in the patent protection scope of the present invention.

Claims

1. A method for separating and purifying coking desulfurization waste liquid, characterized in that, The method comprises the steps of: oxidizing the coking desulfurization waste liquid to obtain an oxidized liquid, wherein the oxidized liquid comprises ammonium thiocyanate and ammonium sulfate; concentrating the oxidized liquid at a first temperature by first evaporation, and then performing solid-liquid separation to obtain a first separated liquid, wherein the mass fraction of ammonium thiocyanate in the first separated liquid accounts for 10-40%, and the mass fraction of ammonium sulfate accounts for 10-50%; concentrating the first separated liquid at a second temperature by second evaporation, and then performing solid-liquid separation to obtain a second separated liquid and mixed salt, wherein the chemical composition of the mixed salt comprises ammonium sulfate and ammonium thiocyanate, the mass fraction of ammonium thiocyanate in the second separated liquid accounts for 70-80%, and the mass fraction of ammonium sulfate accounts for 7-11%; adding water to the second separated liquid to dilute the concentration of ammonium thiocyanate to 50-70%, and then performing crystallization at a third temperature and solid-liquid separation to obtain mother liquor and ammonium thiocyanate, wherein the third temperature is 10-40℃; wherein the first temperature is greater than the second temperature, and the second temperature difference T2 between the second temperature and the third temperature is 40-70℃.

2. The separation and purification method according to claim 1, characterized by, The first temperature difference T1 between the first temperature and the second temperature is 5-20℃.

3. The separation and purification method according to claim 1, characterized by, The oxidized liquid is concentrated by the first evaporation and solid-liquid separation, and ammonium sulfate is further obtained.

4. The separation and purification method according to claim 1, characterized by, The oxidized liquid comprises the mixed salt, and the first separated liquid comprises the mother liquor.

5. The separation and purification method according to claim 1, characterized by, Before the oxidation treatment of the coking desulfurization waste liquid, the method further comprises the step of: using activated carbon to adsorb and decolorize the coking desulfurization waste liquid.

6. The separation and purification method according to claim 1, characterized by, The coking desulfurization waste liquid is oxidized to obtain an oxidized liquid, and the method comprises the steps of: adjusting the decolorized coking desulfurization waste liquid to pH=1.0-4.0, and then adding an oxidizing agent to convert thiosulfate in the coking desulfurization waste liquid into sulfate, sulfur and sulfur dioxide gas.

7. The separation and purification method according to claim 6, characterized by, The coking desulfurization waste liquid is oxidized to obtain an oxidized liquid, and the method further comprises the step of: adjusting the coking desulfurization waste liquid after the oxidation reaction to pH=6-9 to obtain the oxidized liquid.

8. The separation and purification method according to claim 1, characterized by, The evaporation concentration comprises reduced pressure concentration and normal pressure concentration.

9. A method for separating and purifying an ammonium thiocyanate mixed solution, characterized by comprising the steps of: The ammonium thiocyanate mixed solution comprises ammonium thiocyanate and ammonium sulfate, and the separation and purification method comprises the following steps: concentrating the ammonium thiocyanate mixed solution at a first temperature by first evaporation, and then performing solid-liquid separation to obtain a first separated liquid, wherein the mass fraction of ammonium thiocyanate in the first separated liquid accounts for 10-40%, and the mass fraction of ammonium sulfate accounts for 10-50%; concentrating the first separated liquid at a second temperature by second evaporation, and then performing solid-liquid separation to obtain a second separated liquid and mixed salt, wherein the chemical composition of the mixed salt comprises ammonium sulfate and ammonium thiocyanate, the mass fraction of ammonium thiocyanate in the second separated liquid accounts for 70-80%, and the mass fraction of ammonium sulfate accounts for 7-11%; diluting the concentration of ammonium thiocyanate in the second separated liquid to 50-70%, and then performing crystallization at a third temperature and solid-liquid separation to obtain mother liquor and ammonium thiocyanate, wherein the third temperature is 10-40℃. Wherein, the first temperature is greater than the second temperature, and a second temperature difference T2 between the second temperature and the third temperature is 40-70 ℃.

10. The use of the separation and purification method according to any one of claims 1-8 in the treatment of coking desulfurization waste liquid.

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