Salt manufacturing process utilizing low-grade waste heat

Abstract

The invention relates to a process for producing salt by using low-grade waste heat, which comprises the following steps of: extracting brine from an underground salt mine, refining the brine to obtain refined brine, indirectly heating the refined brine by using the low-grade waste heat, and directly contacting the heated brine with air. The brine is evaporated and concentrated to obtain salt crystals, salt products can be obtained through solid-liquid separation, meanwhile, the temperature of the brine is lowered, the brine is heated through low-grade waste heat, and low-grade waste heat salt manufacturing is achieved through circulating evaporation. Various low-grade waste heat can be utilized, the waste heat resource utilization rate is increased, meanwhile, the salt manufacturing production cost is reduced, and carbon emission is reduced.

Description

Technical Field

[0001] The invention relates to a salt-making process utilizing low-grade waste heat, specifically utilizing various low-grade waste heat to produce industrial sodium chloride, recycling the industrial waste heat, reducing salt production costs, and reducing carbon emissions. Background Art

[0002] There are two main technologies for producing industrial sodium chloride from well salt. After the brine is extracted from the underground salt mine, the brine is refined to obtain refined brine. One process method is to adopt a multi-effect evaporation production process. The first-effect evaporator uses steam to indirectly heat and evaporate the concentrated brine. The secondary steam generated by evaporating and concentrating the brine is used for heating the second-effect evaporation and concentrating the brine. The generated steam is used as the heat source for the next effect. The current technologically advanced multi-effect vacuum salt-making process adopts six effects. After the brine is evaporated and concentrated, a slurry containing salt crystals is obtained. The salt product is obtained by solid-liquid separation using a centrifuge. About 0.7 tons of steam is required for each ton of salt, and about 40kwh of power electricity is required, which has a high energy consumption. The second process method is to adopt an MVR production process to heat the refined brine to obtain brine at a certain temperature. The steam evaporated from the brine is mechanically compressed and pressurized to increase the steam pressure and temperature. After pressurization, the steam heats the brine to evaporate and concentrate to obtain a slurry containing salt crystals. The salt product can be obtained by solid-liquid separation using a centrifuge. About 0.1 tons of steam is required for each ton of salt, and about 185kwh of power electricity is required, which has a high energy consumption.

[0003] A large amount of low-grade heat energy is generated in the process of vacuum salt production and various industrial production. Circulating cooling water is generally used to remove the low-grade heat. The low-grade heat energy is directly or indirectly cooled by circulating water to obtain circulating cooling water with rising temperature. The circulating cooling water is sent back to the cooling tower and the air is in direct contact with the circulating cooling water. After the circulating water contacts the air, the air temperature rises. The rise in air temperature increases the water content in the air, taking away the heat in the circulating cooling water. The wet air is directly discharged into the atmosphere, causing huge energy waste. Summary of the invention

[0004] In view of the above-mentioned problems in the prior art, the inventors of this application conducted in-depth research and found that the use of brine cooling to remove low-grade thermal energy can solve the problem of low-grade thermal energy that needs to be cooled in vacuum salt production or various industrial production processes, while producing salt products, recovering various low-grade waste heat, and reducing carbon emissions.

[0005] According to one aspect of the present invention, there is provided a single-effect salt production process using low-grade waste heat, comprising: (1) After extracting brine from underground salt mines, the brine is refined using the caustic soda-soda method, and 2-8 kg / m³ soda ash (sodium carbonate), preferably 3-6 kg / m³ soda ash, and more preferably 4-5 kg / m³ soda ash, and 0.2-0.8 g / m³ caustic soda (sodium hydroxide), preferably 0.3-0.6 g / m³ caustic soda, and more preferably 0.4-0.5 g / m³ caustic soda are added to the brine to precipitate calcium and magnesium ions, and then 1-10 g / m³, preferably 2-5 g / m³ flocculant (such as polyacrylamide) is added to accelerate the precipitation rate of impurities, and then refined brine is obtained; (2) Using low-grade waste heat to heat brine (e.g., by heat exchange, such as using a heat exchanger) to obtain brine with a temperature higher than the ambient temperature (e.g., above 38°C, such as 38-50°C), and sending the brine to an evaporator (e.g., a spray cooling tower), using air flow to directly contact the hot brine for heat exchange, the brine is concentrated, and salt crystals are precipitated; (3) The brine containing salt crystals enters the thickener, and the crystal slurry is sent to the hydrocyclone. The salt slurry flowing out from the bottom is separated into solid and liquid by a centrifuge to obtain solid salt (i.e., industrial salt product). The mother liquor separated by the centrifuge (or further including the overflow of the hydrocyclone) is returned to the thickener to recover the fine salt crystals that have penetrated the filter cloth by centrifugation; (4) The low-temperature saturated brine overflowing from the thickener is sent to step (2) (i.e., the heat exchanger in step (2)) (for example, by a pump). The low-grade waste heat of the low-temperature saturated brine is used to heat the saturated brine. According to the characteristics of sodium chloride, the solubility of sodium chloride will increase as the temperature rises. The high-temperature brine will not crystallize in the heat exchanger. The saturated brine after heat exchange becomes unsaturated but nearly saturated high-temperature brine, which is sent to the evaporator to directly contact with the air to obtain salt-containing crystallized brine, and industrial salt is produced according to step (3).

[0006] According to a second embodiment of the present invention, there is provided a multi-effect salt production process using low-grade waste heat, which comprises: (1) After extracting brine from underground salt mines, the brine is refined by the caustic soda-soda method, and 2-8 kg / m³ soda ash, preferably 3-6 kg / m³ soda ash, and preferably 4-5 kg / m³ soda ash (sodium carbonate), and 0.2-0.8 g / m³ caustic soda (sodium hydroxide), preferably 0.3-0.6 g / m³ caustic soda, and preferably 0.4-0.5 g / m³ caustic soda are added to the brine to precipitate calcium and magnesium ions, and then 1-10 g / m³, preferably 2-5 g / m³ flocculant is added to accelerate the precipitation rate of impurities, and then refined brine is obtained (the salt mud produced during the refining process can be returned to the underground salt mine); (2) Refined brine is divided into two or more routes, each of which is heated by low-grade waste heat (for example, by direct heat exchange, such as using a heat exchanger, for example, the first route uses waste heat such as light ash furnace gas above 90°C in the soda ash production process for indirect heat exchange, and the remaining one or more routes (such as the second route) are heated by hot air generated by the evaporator of the previous route (such as the first route)), to obtain brine with a temperature higher than the ambient temperature (for example, above 38°C, such as 38~50°C), and the brine of each route is sent to its corresponding evaporator (such as a spray cooling tower), and the brine is concentrated and salt crystals are precipitated by direct contact between air flow and hot brine for heat exchange; (3) The brine containing salt crystals in each route enters the corresponding thickener to obtain a crystal slurry. The salt slurry containing large crystals is selected through a hydrocyclone, and then the solid-liquid separation is carried out through a corresponding centrifuge to obtain solid salt (i.e., industrial salt product). The mother liquor separated by the centrifuge (or further including the overflow of the hydrocyclone) is returned to the corresponding thickener to recover the fine salt crystals that have penetrated through the centrifuge.

[0007] The overflow hot brine from the first thickener is sent to the second evaporator (together with the hot brine) for evaporation and concentration (for example, by a pump). The low-temperature brine produced by the second thickener is circulated to the first thickener for reuse (brine preheating).

[0008] In the present application, the temperature difference between the low-grade thermal energy and the ambient temperature reaches more than 20°C, further more than 30°C, more than 40°C, more than 50°C, more than 60°C, more than 70°C, more than 80°C, more than 90°C, more than 100°C, more than 110°C, more than 120°C, more than 130°C, etc., which can realize secondary and higher utilization of waste heat, control the amount of air entering the evaporator, and use the obtained hot air as a low-grade heat source for heating brine, and utilize the waste heat secondary to improve the thermal efficiency of the low-grade waste heat.

[0009] Preferably, the low-grade waste heat is one or more of hot wastewater and hot tail gas generated in vacuum salt production, industrial production and the like, and the temperature is generally 80-150°C, and further for example 90-120°C.

[0010] In step (2), since the air is heated by the brine and its temperature rises, its saturated humidity increases with the temperature. Therefore, during the convection between the air and the brine, part of the water vapor will be taken away, causing the brine to evaporate part of its water and be concentrated. At the same time, the temperature of the brine drops, and salt crystals will precipitate from the supersaturated brine.

[0011] In order to reduce power consumption and equipment investment, vacuum salt making, industrial production and other processes can directly use circulating brine for heat exchange. On the one hand, it can meet the cooling needs of vacuum salt making, industrial production and other processes, and at the same time, it can heat the brine with low-grade waste heat and reduce heat loss in the intermediate medium heat exchange process. Since the brine has been refined, there is less scaling in the heat exchange equipment, which can improve the heat exchange efficiency.

[0012] Low-temperature waste heat salt-making evaporator equipment can use a spray cooling tower. The hot brine enters the cooling tower in the form of droplets and comes into direct contact with the air to increase the gas-liquid exchange area and improve the evaporation and cooling effects.

[0013] The multi-effect salt evaporator equipment can also use a spray cooling tower to directly contact the brine with the air in the form of droplets to increase the gas-liquid exchange area and improve the evaporation and cooling effects. By controlling the amount of air, the air and brine leaving the evaporator have a certain temperature difference with the environment. The warm air coming out of the evaporator and the overflow liquid of the thickener enter the shell and tube heat exchanger, and the hot air heats the brine to achieve secondary or more utilization of waste heat.

[0014] The saturated brine containing crystals coming out of the evaporator enters the thickener, which on the one hand reduces the supersaturation of the brine and on the other hand makes the salt crystals grow, making it easier for the centrifuge to separate the solid and liquid.

[0015] The bottom flow from the thickener is saturated brine containing crystals, which enters the hydrocyclone. The separated slurry containing large particles enters the centrifuge for solid-liquid separation. The filter cake obtained is industrial salt. The overflow of the hydrocyclone and the mother liquor of the centrifuge are returned to the thickener to recover the salt crystals.

[0016] In the present application, because the saturated humidity of the air increases with the temperature, the direct contact of the air with the salt water with higher temperature can increase the temperature in the air, increase the amount of water carried away by the air, and realize the evaporation of the brine.

[0017] Advantages of the present invention: This application effectively utilizes low-grade thermal energy that is difficult to utilize in the prior art in vacuum salt production or various industrial production processes that require cooling, while producing salt products, recovering various low-grade waste heat, reducing carbon emissions, and significantly reducing the cost of the salt production industry, which has important economic value and environmental significance. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a single-effect salt production process that utilizes low-grade waste heat.

[0019] Figure 2 It is a multi-effect salt production process utilizing low-grade waste heat. DETAILED DESCRIPTION

[0020] The present invention is further described below by way of embodiments with reference to the accompanying drawings.

[0021] like Figure 1 As shown, a single-effect salt production process using low-grade waste heat is provided, which comprises: (1) After extracting brine from underground salt mines, the brine is refined using the caustic soda-soda method. 3-6 kg / m³ soda ash and 0.3-0.6 g / m³ caustic soda are added to the brine to precipitate calcium and magnesium ions. 2-5 g / m³ flocculant is then added to accelerate the precipitation rate of impurities to obtain refined brine. (2) Using low-grade waste heat to heat the brine through heat exchangers or other methods to obtain brine with a temperature higher than the ambient temperature, and then sending the brine to an evaporator (such as a spray cooling tower) to exchange heat with the hot brine through direct contact with air flow, so that the brine is concentrated and salt crystals are precipitated; (3) The brine containing salt crystals enters the thickener, and the resulting crystal slurry is separated by a hydrocyclone to select the salt slurry containing large crystals, and then separated by a centrifuge to obtain solid salt (i.e., industrial salt product). The mother liquor separated by the centrifuge (and the overflow of the hydrocyclone) is returned to the thickener to recover the fine salt crystals that penetrate the filter cloth during centrifugation; (4) The low-temperature saturated brine overflowing from the thickener is sent to the heat exchanger in step (2) (for example, by a pump), and the low-grade waste heat of the low-temperature saturated brine is used to heat the saturated brine. According to the characteristics of sodium chloride, the solubility of sodium chloride will increase as the temperature rises, and the high-temperature brine will not crystallize in the heat exchanger. The saturated brine after heat exchange becomes unsaturated but nearly saturated high-temperature brine, which is sent to the evaporator and directly contacts with the air to obtain salt-containing crystallized brine, and industrial salt is produced according to step (3).

[0022] like Figure 2 As shown, a multi-effect salt production process using low-grade waste heat is provided, which comprises: (1) After extracting brine from underground salt mines, the brine is refined using the caustic soda-soda method. 3-6 kg / m³ soda ash and 0.3-0.6 g / m³ caustic soda are added to the brine to precipitate calcium and magnesium ions. 2-5 g / m³ flocculant is then added to accelerate the precipitation rate of impurities to obtain refined brine. (2) The refined brine is divided into two or more routes, each of which is heated by low-grade waste heat (for example, by heat exchange through a heat exchanger, for example, the first route uses waste heat from light ash furnace gas at a temperature above 90°C in the soda ash production process for indirect heat exchange, and the remaining one or more routes (for example, the second route) are heated by hot air generated by the evaporator of the previous route (for example, the first route)), to obtain brine with a temperature higher than the ambient temperature, and the brine of each route is sent to its corresponding evaporator (for example, a spray cooling tower), and the brine is concentrated by direct contact with the hot brine through air flow and heat exchange, and salt crystals are precipitated; (3) The brine containing salt crystals in each route enters the corresponding thickener. The crystal slurry obtained by the thickener passes through the hydrocyclone of each route to select the salt slurry containing large crystals, and then passes through the corresponding centrifuge for solid-liquid separation to obtain solid salt (i.e. industrial salt product). The mother liquor separated by the centrifuge (and the overflow of the hydrocyclone) is returned to the corresponding thickener of each route to recover the fine salt crystals that have penetrated the filter cloth through the centrifuge.

[0023] The overflow hot brine from the first thickener is sent to the second evaporator (together with the hot brine) for evaporation and concentration (for example, by a pump). The low-temperature brine produced by the second thickener can be circulated to the first thickener for reheating.

[0024] Example 1: Single-effect salt production process using low-grade waste heat in summer like Figure 1 As shown in the figure, sodium chloride saturated brine mined from underground is added with soda ash and caustic soda. 3-6 kg / m³ soda ash and 0.3-0.6 g / m³ caustic soda are added to the brine to precipitate calcium and magnesium ions. Then 2-5 g / m³ flocculant is added to accelerate the precipitation rate of impurities. The temperature of refined brine is generally 30°C. Refined brine and low-grade waste heat steam (steam temperature is generally 80-150°C) are indirectly heat exchanged in the heat exchanger. For example, in summer, the ambient temperature is 28°C and the air humidity is 100%. The hot brine temperature is above 38°C and is sent to the evaporator. The evaporator adopts a spray cooling tower. The tower uses a high-efficiency low-pressure centrifugal atomization device to spray water into tiny droplets, which increases the heat transfer area and improves the cooling effect. Compared with the traditional packed cooling tower, the air-water contact specific surface area is increased by more than 10%, and the air-water ratio reaches 1.0-1.3, making the cooling effect more significant. The hot brine is dispersed into small brine droplets in the spray cooling tower. The cooling tower blades rotate to draw air from the bottom of the tower into the tower. The air is in direct contact with the hot brine droplets. The brine temperature drops to 28°C, close to the ambient temperature, and the air temperature rises to 34°C. The specific heat of saturated brine is 3.29KJ / kg, and the specific gravity is 1.33kg / L. The heat loss when the brine temperature drops by 10°C is about 35kJ / kg (calculation: 3.29*1.33*10=43.75kJ / kg), and about 1.358m of air is required. 3 (Specific gravity of air: 1.29kg / m 3 , the enthalpy of saturated moist air at 28℃ is 90.48kJ / kg dry air, and the enthalpy of saturated moist air at 34℃ is 123.72kJ / kg dry air. Calculation: 43.75 / ((123.72-90.48) / 1.29)=1.698m 3 ), the air takes away about 0.0138kg of water (the water vapor in saturated humid air at 28℃ is 0.024435kg, and the water vapor in saturated humid air at 34℃ is 0.03495kg, calculated as (0.03495-0.024435) / 1.29*1.698=0.0138kg), and more water will evaporate if the air humidity is taken into account. If the ambient temperature is 28℃, the water volume in the brine can be taken away by the evaporator by 0.0138kg / L brine for every 10℃ increase in the temperature of the recovered brine.

[0025] If the sodium chloride concentration of refined brine is 310g / L, the amount of water evaporated to produce one ton of salt is about 2.23 tons. In summer, the brine temperature rises by 10℃, and the brine circulation volume required is 161.6m 3 (Calculation: 2.23 / 0.0138=161.6m 3 ), it is necessary to recover 7070MJ of waste heat (calculated as 43.75*161.6*1000=7070MJ), which can save about 241Kg of standard coal.

[0026] The brine containing salt crystals enters the thickener, and the resulting crystal slurry is passed through a hydrocyclone to select the salt slurry containing large crystals. After solid-liquid separation by a centrifuge, solid salt (i.e., industrial salt product) is obtained. The mother liquor separated by the centrifuge is returned to the thickener to recover the fine salt crystals that have penetrated the filter cloth through the centrifuge.

[0027] The low-temperature saturated brine overflowing from the thickener is pumped to the above-mentioned heat exchanger, and the low-grade waste heat of the low-temperature saturated brine is used to heat the saturated brine. According to the characteristics of sodium chloride, the solubility of sodium chloride will increase as the temperature rises, and the high-temperature brine will not crystallize in the heat exchanger. The saturated brine after heat exchange becomes unsaturated but nearly saturated high-temperature brine, which is sent to the evaporator and directly contacted with the air to obtain salt-containing crystallized brine, and industrial salt is produced according to the above steps.

[0028] Example 2: Single-effect salt production process using low-grade waste heat in winter like Figure 1 As shown, soda ash and caustic soda are added to the sodium chloride-containing brine mined from underground to produce calcium carbonate and magnesium hydroxide from calcium and magnesium ions. Refined brine is obtained after clarification. The refined brine (about 25°C) and low-grade waste heat steam (the steam temperature is generally 80-150°C) are indirectly heat exchanged in a heat exchanger. For example, if the ambient temperature in winter is 6°C and the air humidity is 100%, the hot brine temperature is above 38°C and is sent to the evaporator. The evaporator adopts a spray cooling tower. The hot brine is dispersed into small brine droplets in the spray cooling tower. The cooling tower fan blades rotate to draw air from the bottom of the tower into the tower. The air is in direct contact with the hot brine droplets. The brine temperature drops to 6°C, which is close to the ambient temperature. The air temperature rises to 30°C. The specific heat of saturated brine is 3.29KJ / kg, and the specific gravity is 1.33kg / L. The heat loss when the brine temperature drops by 32°C is about 140kJ / kg (calculation: 3.29*1.33*32=140.02kJ / kg), and about 1.358m of air is required. 3 (Specific gravity of air: 1.29kg / m 3 , the enthalpy of saturated moist air at 6℃ is 20.77kJ / kg dry air, and the enthalpy of saturated moist air at 30℃ is 100.57kJ / kg dry air. Calculation: 140 / ((100.57-20.77) / 1.29)=2.263m 3), the air takes away about 0.038kg of water (6℃ saturated humid air contains 0.005868kg of water vapor, 30℃ saturated humid air contains 0.027558kg of water vapor, calculation: (0.027558-0.005868) / 1.29*2.263=0.038kg), if the air humidity is taken into account, more water will evaporate. If the ambient temperature is 6℃, the temperature of the recovered waste heat brine increases by 32℃, and the amount of water in the brine can be taken away by the evaporator by 0.038kg / L brine.

[0029] If the sodium chloride concentration of refined brine is 310g / L, the amount of water evaporated to produce one ton of salt is about 2.23 tons. The brine temperature rises by 32℃ in winter, and the brine circulation volume required is 58.68m 3 (Calculation: 2.23 / 0.038=58.68m 3 ), 8216MJ of waste heat can be recycled (calculated as 140*58.68*1000=8216MJ), which is about 280Kgce.

[0030] The brine containing salt crystals enters the thickener, and the resulting crystal slurry passes through a hydrocyclone to select the salt slurry containing large crystals. After solid-liquid separation in a centrifuge, solid salt (i.e., industrial salt product) is obtained. The mother liquor separated by the centrifuge (as well as the overflow of the hydrocyclone) is returned to the thickener to recover the fine salt crystals that penetrate the filter cloth during centrifugation.

[0031] The low-temperature saturated brine overflowing from the thickener is pumped to the above-mentioned heat exchanger, and the low-grade waste heat of the low-temperature saturated brine is used to heat the saturated brine. According to the characteristics of sodium chloride, the solubility of sodium chloride will increase as the temperature rises, and the high-temperature brine will not crystallize in the heat exchanger. The saturated brine after heat exchange becomes unsaturated but nearly saturated high-temperature brine, which is sent to the evaporator and directly contacted with the air to obtain salt-containing crystallized brine, and industrial salt is produced according to the above steps.

[0032] Example 3: Multi-effect salt production process using low-grade waste heat in summer like Figure 2As shown, sodium chloride-containing brine mined from underground is added with soda ash and caustic soda, 3-6 kg / m³ soda ash and 0.3-0.6 g / m³ caustic soda are added to the brine to precipitate calcium and magnesium ions, and 2-5 g / m³ flocculant is added to accelerate the precipitation rate of impurities, so that calcium and magnesium ions produce calcium carbonate and magnesium hydroxide, and refined brine is obtained after clarification. Half of the refined brine is indirectly heat exchanged with waste heat such as light ash furnace gas above 90°C in the soda ash production process in heat exchanger 1, and the other half is sent to heat exchanger 2 to be heated by the hot air generated by evaporator 1. The light ash calcining furnace gas no longer needs cooling circulating water, thereby reducing the power consumption and water consumption of salt making. For example, if the ambient temperature in summer is 28℃ and the air humidity is 100%, the hot brine temperature is above 78℃ and sent to evaporator 1. Evaporator 1 adopts a spray cooling tower. The hot brine is dispersed into small brine droplets in the spray cooling tower. The cooling tower fan blades rotate to draw air from the bottom of the tower into the tower. The air is in direct contact with the hot brine droplets. The brine temperature drops to 48℃ and the air temperature rises to 58℃. The specific heat of saturated brine is 3.29KJ / kg and the specific gravity is 1.33kg / L. The heat loss when the brine temperature drops by 30℃ is about 131.27kJ / kg (calculation: 3.29*1.33*30=131.27kJ / kg). The first evaporation requires about 1.358m of air. 3 (Specific gravity of air: 1.29kg / m 3 , the enthalpy of saturated moist air at 28℃ is 90.48kJ / kg dry air, and the enthalpy of saturated moist air at 58℃ is 417.72kJ / kg dry air. Calculation: 131.27 / ((417.72-90.48) / 1.29)=0.5175m 3 ), the air takes away about 0.0455kg of water (the water vapor contained in saturated moist air at 28℃ is 0.024435kg, and the water vapor contained in saturated moist air at 58℃ is 0.1379kg, calculation: (0.1379-0.024435) / 1.29*0.5175=0.0455kg), saturated moist air at 58℃ heats the brine, and after heating 1kg of brine, the air temperature drops to 38℃, and the brine temperature can rise from 28℃ to about 52℃ (calculation: the thermal enthalpy of saturated moist air at 58℃ is 417.72kJ / kg dry air, and the thermal enthalpy of saturated moist air at 38℃ is 151.6kJ / kg dry air, calculation: (417.72-151.6) / 1.29*0.5175 / 3.29 / 1.33 / 1+28=52.39℃). Mix the brine cooled to 48℃ and the brine heated to 52℃ by moist air to get 2kg of 50℃ brine. The brine will evaporate again and the ambient temperature will drop to 28℃. The heat loss of 2kg brine when the temperature drops by 22℃ is about 35kJ / kg (calculation: 3.29*1.33*22*2=192.58kJ / kg), which requires about 1.358m3 of air. 3(Specific gravity of air: 1.29kg / m 3 , the enthalpy of saturated moist air at 28℃ is 90.48kJ / kg dry air, and the enthalpy of saturated moist air at 34℃ is 123.72kJ / kg dry air. Calculation: 192.58 / ((123.72-90.48) / 1.29)=7.474m 3 ), the air takes away about 0.0138kg of water (the water vapor content of saturated humid air at 28℃ is 0.024435kg, and the water vapor content of saturated humid air at 34℃ is 0.03495kg, calculation: (0.03495-0.024435) / 1.29*7.474=0.0609kg), the amount of water in the brine that can be taken away by the secondary evaporator is 0.00609kg / L brine, and the total amount of water taken away by the first evaporation and the second evaporation is 0.1064kg (calculation: 0.0455+0.0609=0.1064kg) If the sodium chloride concentration of refined brine is 310g / L, the amount of water evaporated to produce one ton of salt is about 2.23 tons. In winter, the brine temperature rises by 50℃, and the brine circulation volume required is 58.68m 3 (Calculation: 2.23 / 0.1064=20.959m 3 ), 4585MJ of waste heat needs to be recovered (calculated as 3.29*1.33*50*20.959*1000=4585.5MJ). Two-effect evaporation can greatly improve the thermal efficiency. If the temperature is high and three-effect or above is used, the thermal efficiency can be further improved.

[0033] The two routes of brine containing salt crystals enter the corresponding thickener 1 and thickener 2, and the obtained crystal slurry is respectively passed through hydrocyclones 1 and 2 to select the salt slurry containing large crystals, and then passed through the corresponding centrifuges 1 and 2 for solid-liquid separation to obtain solid salt (i.e. industrial salt product). The mother liquor separated by the centrifuge (and the overflow of the hydrocyclone) is returned to the corresponding thickeners 1 and 2 to recover the fine salt crystals that penetrate the filter cloth by centrifugation; The overflow hot brine of the thickener 1 can be used for heating the hot brine in the evaporator 2, and the low-temperature brine overflowing from the thickener 2 is returned to the first circuit for circulation heating.

[0034] The above is a detailed description of the present invention. For those skilled in the art, according to the concept of the present invention, there will be changes and adjustments in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention, and any changes made according to the concept of the present invention are within the scope of protection of the present invention.

Claims

1. A single-effect salt production process using low-grade waste heat, characterized in that include: (1) After extracting brine from underground salt mines, the brine is refined by the caustic soda-soda method, and 2-8 kg / m³ soda ash, preferably 3-6 kg / m³ soda ash, and preferably 4-5 kg / m³ soda ash, and 0.2-0.8 g / m³ caustic soda, preferably 0.3-0.6 g / m³ caustic soda, and preferably 0.4-0.5 g / m³ caustic soda are added to the brine to precipitate calcium and magnesium ions, and then 1-10 g / m³, preferably 2-5 g / m³ flocculant is added to accelerate the precipitation rate of impurities, and then refined brine is obtained; (2) Using low-grade waste heat to heat brine to obtain brine with a temperature higher than the ambient temperature (e.g., above 38°C, such as 38-50°C), and sending the brine to the evaporator. Using air flow to directly contact the hot brine for heat exchange, the brine is concentrated and salt crystals are precipitated; (3) The brine containing salt crystals enters the thickener, and the crystal slurry is sent to the hydrocyclone. The salt slurry flowing out from the bottom is separated into solid and liquid by a centrifuge to obtain solid salt, i.e. industrial salt product. The mother liquor separated by the centrifuge is returned to the thickener to recover the fine salt crystals that penetrate the filter cloth during centrifugation. (4) The low-temperature saturated brine overflowing from the thickener is sent to step (2), and the low-grade waste heat of the low-temperature saturated brine is used to heat the saturated brine. According to the characteristics of sodium chloride, the solubility of sodium chloride will increase as the temperature rises, and the high-temperature brine will not crystallize in the heat exchanger. The saturated brine after heat exchange becomes unsaturated but nearly saturated high-temperature brine, which is sent to the evaporator and directly contacts with the air to obtain salt-containing crystallized brine, and industrial salt is produced according to step (3).

2. A multi-effect salt production process using low-grade waste heat, characterized in that include: (1) After extracting brine from underground salt mines, the brine is refined by the caustic soda-soda method, and 2-8 kg / m³ soda ash, preferably 3-6 kg / m³ soda ash, and preferably 4-5 kg / m³ soda ash, and 0.2-0.8 g / m³ caustic soda, preferably 0.3-0.6 g / m³ caustic soda, and preferably 0.4-0.5 g / m³ caustic soda are added to the brine to precipitate calcium and magnesium ions, and then 1-10 g / m³, preferably 2-5 g / m³ flocculant is added to accelerate the precipitation rate of impurities, and then refined brine is obtained; (2) The refined brine is divided into two or more routes, each of which is heated by low-grade waste heat to obtain brine with a temperature higher than the ambient temperature (for example, above 38°C, such as 38~50°C). The brine in each route is sent to its corresponding evaporator, and the air flow is used to directly contact and exchange heat with the hot brine, so that the brine is concentrated and salt crystals are precipitated; (3) The brine containing salt crystals in each route enters the corresponding thickener to obtain the crystal slurry which is sent to the hydrocyclone. The salt slurry flowing out from the bottom is then separated into solid and liquid by the corresponding centrifuge to obtain solid salt, i.e. industrial salt product. The mother liquor separated by the centrifuge is returned to the corresponding thickener to recover the fine salt crystals that penetrate the filter cloth during centrifugation.

3. The salt production process according to claim 1 or 2, characterized in that: Step (2) utilizes low-grade waste heat to heat the brine using a heat exchanger.

4. The salt production process according to claim 1 or 2, characterized in that: Step (2) The evaporator uses a spray cooling tower.

5. The salt production process according to claim 1 or 2, characterized in that: The overflow of the hydrocyclone is returned to the thickener together with the mother liquor separated by the centrifuge to recover the fine salt crystals that penetrate the filter cloth during centrifugation.

6. The salt production process according to claim 1 or 2, characterized in that: Low-grade waste heat is one or more of hot wastewater and hot tail gas generated during vacuum salt production and industrial production.

7. The salt production process according to claim 1 or 2, characterized in that: The temperature difference between low-grade waste heat and ambient temperature is more than 20°C.

8. The salt production process according to claim 1 or 2, characterized in that: The evaporator equipment adopts a spray cooling tower.

9. The salt production process according to claim 2, characterized in that: The refined brine is divided into two routes. The first route uses the waste heat of the light ash furnace gas above 90°C in the soda ash production process for indirect heat exchange, and the second route uses the hot air generated by the evaporator of the first route for heating.

10. The salt production process according to claim 9, characterized in that: The overflow hot brine from the first thickener is sent to the second evaporator for evaporation and concentration together with the hot brine. The low-grade waste heat of the low-temperature brine generated by the second thickener is used as brine preheating.

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