Method and system for preparing lithium carbonate from lithium-containing sodium sulfate solution

Abstract

The invention relates to the technical field of lithium carbonate preparation, in particular to a method and system for preparing lithium carbonate from a lithium-containing sodium sulfate solution, the lithium-containing sodium sulfate solution is pumped to a first MVR evaporation zone after being subjected to impurity removal and mixed with added lithium carbonate seed crystals for first-stage MVR concentration, and first-stage sediment is obtained and flows back to the first MVR evaporation zone; the first-stage concentrated solution enters a lithium precipitation process to react with a sodium carbonate solution to precipitate lithium, then solids are separated out and collected, filtrate enters a cooling process to be subjected to multi-stage cooling to obtain sodium sulfate decahydrate and residual mother liquor, and a sodium sulfate solution generated after the sodium sulfate decahydrate is added with water to be subjected to hot melting is evaporated and centrifuged in a second MVR evaporation area; stirring and washing by a sodium sulfate hot solution which is newly introduced in the next time period to obtain clean sodium sulfate; pumping the residual mother liquor to a third MVR evaporation area, uniformly mixing the residual mother liquor with the added lithium carbonate seed crystal, preheating, evaporating and centrifuging, and separating out a lithium carbonate semi-finished product; and washing with distilled water and collecting.

Description

Technical Field

[0001] The invention relates to the technical field of lithium carbonate preparation, and in particular to a method and a system for preparing lithium carbonate from a lithium-containing sodium sulfate solution. Background Art

[0002] As the most widely used and important basic lithium salt product in the lithium industry, lithium carbonate is widely used in traditional industries such as chemical industry, metallurgy, ceramics, medicine, refrigeration, etc. In the energy field, lithium carbonate is the core material for manufacturing lithium-ion batteries and is widely used in electric vehicles, power tools and portable electronic devices. In the military and aerospace fields, the excellent performance of lithium carbonate makes it an important material for high-energy density batteries and other cutting-edge technologies. In the prior art, there are various methods for preparing lithium carbonate, and sulfate roasting is one of the commonly used processes, which is particularly suitable for processing lithium mica and spodumene. Lepidolite and spodumene are subjected to a series of steps such as roasting and acid leaching to finally generate a sodium sulfate solution containing lithium. Subsequently, high-purity lithium carbonate is prepared through further chemical treatment and evaporation crystallization processes.

[0003] However, in the actual production process, these traditional processes face many challenges. In the process of extracting lithium carbonate from sodium sulfate solution containing lithium using spodumene and lepidolite as raw materials, the prior art often encounters some technical difficulties, such as lithium carbonate is easy to adhere to the equipment wall during evaporation and crystallization, forming scaling, which not only reduces the heat transfer efficiency of the equipment, but also increases the frequency of cleaning and maintenance, resulting in increased downtime, frequent cleaning and maintenance require shutdown operations, and increase production costs. The commonly used method is currently to adopt the mode of evaporation after decarburization, and then use alkali solution to adjust pH after using a large amount of acid decarburization in the raw material, which will cause a large amount of waste of acid-base solution in the process, and due to the increase of excessive sodium carbonate, it will also cause part of lithium carbonate to be difficult to precipitate, reducing product yield. Therefore, how to improve the recovery rate of lithium carbonate and reduce the scaling and maintenance cost of equipment are problems to be solved in the current technical field.

[0004] The information disclosed in this background technology section is only intended to deepen the understanding of the overall background technology of the present invention, and should not be regarded as acknowledging or suggesting in any form that the information constitutes the prior art known to those skilled in the art. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a method for preparing lithium carbonate from a lithium-containing sodium sulfate solution, especially a lithium-containing sodium sulfate solution using lepidolite and spodumene as raw materials, which can effectively solve the problem of lithium carbonate adhering to the wall of the equipment, improve the recovery rate of lithium carbonate, reduce equipment scaling, and reduce maintenance costs, thereby achieving a more efficient and economical processing process.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is: A method for preparing lithium carbonate from a lithium-containing sodium sulfate solution, comprising: S01: After impurities are removed, the lithium-containing sodium sulfate solution is pumped to the first MVR evaporation zone, mixed with the added lithium carbonate seed crystals for primary MVR concentration, and a primary precipitate and a primary concentrated solution are obtained; the primary precipitate is refluxed to the first MVR evaporation zone, and the primary concentrated solution enters the lithium precipitation process; S02: introducing sodium carbonate solution into the lithium precipitation process to react with the primary concentrated solution to precipitate lithium, solids are precipitated and collected after lithium precipitation, and the filtrate enters the cooling process; S03: After multi-stage cooling, the filtrate is cooled to obtain sodium sulfate decahydrate and the remaining mother liquor. The sodium sulfate decahydrate is melted with water and enters the second MVR evaporation zone to wash out the crystallized sodium sulfate. The remaining mother liquor enters the third MVR evaporation zone to purify lithium carbonate. S04: The sodium sulfate solution produced after hot melting of sodium sulfate decahydrate is stirred and washed with the sodium sulfate obtained after evaporation and centrifugation in the second MVR evaporation zone in the previous time period, and then the sodium sulfate obtained after evaporation and centrifugation in the second MVR evaporation zone is stirred and washed with the sodium sulfate hot solution newly introduced in the next time period to obtain clean sodium sulfate; S05: The remaining mother liquor is pumped to the third MVR evaporation zone, mixed evenly with the added lithium carbonate seed crystals, evaporated and centrifuged after preheating to precipitate the lithium carbonate semi-finished product; the precipitated lithium carbonate semi-finished product is washed with distilled water and collected.

[0007] Preferably, in step S01, the lithium-containing sodium sulfate solution is obtained by roasting and acid treatment of lepidolite or spodumene. After roasting and sulfuric acid treatment, the lithium element is converted into soluble lithium sulfate (Li2SO4) and dissolved in water to form a lithium-containing sulfate solution. Specifically, the roasting is carried out at a high temperature of 900°C to 1100°C. The chemical reaction during the roasting process is mainly to convert spodumene or lepidolite into soluble lithium salts, which is convenient for subsequent acid leaching treatment. The roasted ore reacts with sulfuric acid to form soluble lithium sulfate. The specific component contents are shown in Table 1 below: Table 1 Component contents of lithium mica and spodumene solutions before impurity removal element Lepidolite lithium solution results (unit: g / L) Spodumene lithium liquid results (unit: g / L) Lithium Oxide 19.4 21.14 Potassium 23.34 2.5 sodium 36.96 26.5 calcium 0.472 0.82 magnesium 0.316 0.34 chlorine 0.128 0.37 manganese 1.48 0.0064 fluorine 0.646 0.009 pH 6.84 8.8 Preferably, in step S01, the lithium-containing sodium sulfate solution is subjected to impurity removal, including physical filtration to remove insoluble impurities, but not chemical impurity removal. The lithium-containing sodium sulfate solution is transported to a primary filtration device. Larger particles of solid impurities are removed by a screen or a coarse filter cloth. The filtered liquid is further passed through a microporous filter membrane or a multi-layer filter paper to remove suspended matter and fine particles. The filtered clean water infusion is collected and prepared for the next step of concentration and lithium precipitation process.

[0008] Preferably, in step S01, the lithium carbonate seed crystals are concentrated and mixed into the primary precipitate, and then returned to the first MVR evaporation zone for recycling. Specifically in this step, the lithium carbonate seed crystals are a lithium carbonate solution with a raw material content of 1% to 10%. Since the seed crystals are recycled, no secondary addition is required. The lithium carbonate seed crystals added in the present invention can promote the precipitation of lithium carbonate and prevent it from adhering to the equipment wall.

[0009] In industrial production, the solubility of lithium carbonate in the first MVR evaporation zone is measured to be 1.167 g / L, which means that at most 1.167 g of lithium carbonate can be dissolved per liter of solution. The lithium carbonate seeds are added in the present invention mainly to provide crystal nuclei and promote the precipitation of lithium carbonate. These crystal seeds will not be completely dissolved, but exist in the solution as crystal nuclei, so the actual addition amount can exceed its solubility limit.

[0010] The present invention extracts lithium carbonate from a 30 ton / hour lithium-containing sodium sulfate solution, and on this basis, adds 1-10% lithium carbonate, i.e., 300-3000 kg. The energy barrier required for precipitation can be reduced because the newly formed lithium carbonate precipitate is more inclined to adhere to the existing solid particles rather than spontaneously forming new particles. The seed crystal, as the core of the precipitation, accelerates the precipitation process and helps to form larger and more uniform precipitation particles, prevents scaling on the equipment wall, and improves the recovery rate of lithium carbonate. Within the range of 1%-10%, the number of seed crystals is moderate, which can effectively promote crystallization and avoid mutual interference between crystals caused by overcrowding of seed crystals.

[0011] Among them, when the content of lithium carbonate seed is 1%, that is, 300 kg of lithium carbonate is added: 300 kg of lithium carbonate in 30 tons of solution, the volume of the solution is 30,300 liters. Each liter of solution adds 300,000 grams / 30,300 liters = 9.9g / L. When the content of lithium carbonate seed is 10%, that is, 3,000 kg of lithium carbonate is added: the content of lithium carbonate in the system is 90.91 g / L. Both are far higher than the solubility of 1.167 g / L. Some lithium carbonate will exist in the form of seed crystals to help subsequent precipitation.

[0012] Preferably, the lithium carbonate seed is a lithium carbonate solution with a raw material content of 3% to 5%. It has been found through experimental verification that such concentration control allows the solution to be kept in a supersaturated state while avoiding unnecessary waste or handling difficulties caused by excessive concentration.

[0013] When the content of lithium carbonate seed is 3%, 900 kg of lithium carbonate is added to 30 tons of solution. In this state, the amount of lithium carbonate added is 900 kg. In 30 tons of solution, the volume of the solution is 30,900 liters. Each liter of solution increases by 900,000 grams / 30,900 liters ≈ 29.1 g / L. When the content of lithium carbonate seed is 5%, 1,500 kg of lithium carbonate is added to 30 tons of solution. In this state, in 30 tons of solution, the volume of the solution is 31,500 liters. Each liter of solution increases by 1,500,000 grams / 31,500 liters ≈ 47.62 g / L.

[0014] The further preferred content of lithium carbonate seed is 3%, that is, 900 kg of lithium carbonate is added to 30 tons of solution. The growth rate of lithium carbonate crystals is moderate, and the crystal particle size distribution is uniform, which is conducive to subsequent filtration and separation operations. At the same time, this mass fraction is also suitable for MVR evaporation systems in industrial production.

[0015] According to the crystallization kinetics and equipment operation practice, the present invention may not be able to effectively provide a crystallization surface if the concentration of lithium carbonate seeds is too low, and excessive concentration will lead to unnecessary material consumption. The present invention controls the content of the added lithium carbonate seeds to 1% to 10%, which can take into account both crystallization efficiency and material utilization. The added lithium carbonate seeds are mainly used to provide a crystallization surface to prevent scaling. Controlling within the range of 1 to 10% can ensure crystallization efficiency without causing excessive burden on the system.

[0016] In the preparation of lithium-containing sodium sulfate solution using spodumene as raw material, the lithium carbonate contained therein is easily attached to the equipment wall during high-temperature evaporation, resulting in the formation of scale on the lithium carbonate, which requires frequent cleaning and maintenance of the equipment, increases production costs and downtime, affects equipment efficiency, and scaling causes part of the lithium carbonate to fail to be effectively recovered, reducing product yield. The present invention adds lithium carbonate seeds in the first forced evaporation chamber, and the lithium carbonate seeds provide crystallization nuclei, inducing lithium carbonate in the solution to precipitate on the surface of the seeds, which can significantly reduce the adhesion of lithium carbonate on the equipment wall and reduce scaling. After the lithium carbonate seeds are concentrated, they are mixed into the primary precipitation to participate in the next reaction, and the crystallization of the primary MVR concentration reaction of the lithium-containing sodium sulfate solution subsequently introduced is induced, thereby improving the continuity of the reaction.

[0017] Preferably, in step S02, in the lithium precipitation process, the sodium carbonate solution is added in an equimolar amount relative to the primary concentrated solution.

[0018] Preferably, in step S03, multi-stage cooling includes first-stage cooling and second-stage cooling, the first-stage cooling cools the filtrate to 10°C, and the second-stage cooling cools the filtrate to -5°C. Specifically, before the first-stage cooling, circulating cooling water can be used to cool to room temperature, and then forced cooling can be performed, which can save a lot of resources. The present invention gradually reduces the temperature to avoid the problems of thermal shock and reduced efficiency that may occur in a single-stage cooling system, and helps to control temperature changes more stably. The present invention helps to control the crystallization rate in the crystallization process by gradually reducing the temperature, prevents uneven crystallization caused by rapid cooling, and thus improves the purity and quality of the final product. Different types of impurities can be removed at different temperatures to further improve product quality.

[0019] Preferably, in step S04, the entire system is automated for production, and the sodium sulfate obtained after the sodium sulfate solution introduced is evaporated and centrifuged in the second MVR evaporation zone is returned to the third stirring tank, and is stirred and washed with the sodium sulfate solution newly added to the third stirring tank to obtain crystallized sodium sulfate; the newly introduced sodium sulfate solution is continuously added to the second MVR evaporation zone for evaporation and centrifugation to obtain sodium sulfate, which is then returned to the third stirring tank and stirred and washed with the sodium sulfate solution newly added to the third stirring tank in the latter period of time to obtain clean sodium sulfate.

[0020] Preferably, in step S05, lithium carbonate seeds are added in a spiral continuously. Specifically in this step, the lithium carbonate seeds are a lithium carbonate solution with a mass fraction of 3%, and since this part of the seeds is mixed and concentrated with the remaining mother liquor, it needs to be added continuously.

[0021] In this part, 7.5 tons of mother liquor remain, and lithium carbonate is extracted from the 7.5 tons of mother liquor. On this basis, 1-10% lithium carbonate, i.e. 75-750 kg, is added. Within the mass fraction range of 1%-10%, the number of seed crystals is moderate, which can effectively promote crystallization and avoid mutual interference between crystals caused by too dense seed crystals.

[0022] Preferably, 3-5% lithium carbonate is added; experimental verification has shown that such concentration control allows the solution to be kept in a supersaturated state while avoiding unnecessary waste or handling difficulties caused by excessive concentration.

[0023] On this basis, it is therefore related to the crystal seed and the remaining mother liquor mixed and concentrated and then separated from the system, so it is necessary to add continuously, preferably in this step, the lithium carbonate crystal seed is a lithium carbonate solution with a mass fraction of 3%. That is, 225 kg of lithium carbonate is added to 7.5 tons of remaining mother liquor. This concentration control allows to avoid unnecessary waste or handling difficulties caused by excessive concentration while keeping the solution in a supersaturated state. It was found through experimental verification that under this state, the growth rate of lithium carbonate crystals is moderate, and the crystal particle size distribution is uniform, which is conducive to subsequent filtering and separation operations. At the same time, this mass fraction is also suitable for MVR evaporation systems in industrial production.

[0024] The technical problem to be solved by the present invention is to provide a system for preparing lithium carbonate from a lithium-containing sodium sulfate solution, which is used to realize the above-mentioned method for preparing lithium carbonate from a lithium-containing sodium sulfate solution, and can effectively solve the problems of improving the recovery rate of lithium carbonate, reducing equipment scaling, and reducing maintenance costs.

[0025] In order to achieve the above object, the technical solution adopted by the present invention is: A system for preparing lithium carbonate from a lithium-containing sodium sulfate solution, comprising: A storage tank area, including a feed tank, for collecting and storing the lithium-containing sodium sulfate solution after physical filtration to remove insoluble impurities and conveying it to the first MVR evaporation area; The first MVR evaporation zone includes a first forced evaporation chamber connected to the discharge port of the feed tank, a first heat exchanger connected to the discharge port of the first forced evaporation chamber, and a cyclone connected to the feed port of the first forced evaporation chamber. A first stirring tank is also arranged between the discharge port of the first heat exchanger and the feed port of the cyclone, and the liquid outlet of the cyclone is connected to the lithium precipitation process; Preferably, in the first MVR evaporation zone, the lithium-containing sodium sulfate solution is pumped into the first forced evaporation chamber for evaporation and concentration after impurities are removed, the shell side of the first heat exchanger is connected to the external fresh steam pipeline, the feed port of the first heat exchanger tube side is connected to the discharge port of the first forced evaporation chamber, the discharge port of the first heat exchanger tube side is connected to the first stirring tank, and the cyclone is used to separate the evaporated solid and liquid parts, wherein the solid part is refluxed to the first forced evaporation chamber, and the liquid part flows to the lithium sinking tank; Lithium carbonate seeds are added to the first stirring tank, pumped to the cyclone and then into the first forced evaporation chamber, where a primary precipitate is generated. The primary concentrated liquid generated in the first forced evaporation chamber flows back to the cyclone and then flows to the lithium precipitation tank. This process uses MVR (mechanical vapor recompression) technology to recover and utilize steam energy to concentrate the solution. Through forced evaporation and cyclone separation, preliminary concentration and solid-liquid separation are achieved, which improves the concentration efficiency. The recycled lithium carbonate seeds added can prevent lithium carbonate from adhering to the equipment wall, making the entire production process more stable, reducing the risk of equipment scaling and clogging, and extending the service life of the equipment.

[0026] The lithium precipitation process area includes a plurality of lithium precipitation tanks connected to the liquid outlet of the cyclone, and a plurality of second stirring tanks connected to the lithium precipitation tanks. Sodium carbonate solution is added in an equal molar ratio in the second stirring tanks and pumped into the lithium precipitation tanks. The discharge port of the lithium precipitation tank is connected to the first filter to obtain and collect lithium carbonate solids, and the remaining filtrate enters the next process; Preferably, the lithium precipitation tank is used to precipitate lithium salts, and an equimolar ratio of sodium carbonate solution is added to the second stirring tank and mixed evenly to promote the lithium precipitation reaction. The first filter is used to collect and compress the precipitated lithium carbonate solid. In this process, lithium is precipitated from the lithium-containing sodium sulfate solution by adding sodium carbonate solution to generate lithium carbonate solid, and excess liquid is removed by compression and filtration to obtain a preliminary lithium carbonate product. The larger the amount of sodium carbonate solution, the lower the crystallization rate of the precipitated lithium carbonate due to the increase in liquid, so the amount of sodium carbonate should not be too high. In the present invention, the sodium carbonate solution and the primary concentrated solution are increased in an equimolar ratio to achieve the best purification effect.

[0027] The cooling process area includes a first cooling device and a second cooling device connected in sequence, and a dissolving kettle arranged at the discharge ports of the first cooling device and the second cooling device, the discharge port of the dissolving kettle is connected to the second MVR evaporation area, the liquid outlet of the first cooling device is connected to the second cooling device, and the liquid outlet of the second cooling device faces the third MVR evaporation area; Preferably, the cooling process further includes a pre-cooling device arranged between the first cooling device and the first filter, the pre-cooling device is a plate heat exchanger arranged in series, which is used to initially cool the filtrate to room temperature, the first cooling device includes a first cooling crystallization tank, a first forced heat exchange component and a first centrifuge, which are used to cool the filtrate to 10°C, and the second cooling device includes a second cooling crystallization tank, a second forced heat exchange component and a second centrifuge, which further cools the filtrate to -5°C; The first centrifuge is arranged at the discharge port of the first cooling crystallization tank, the liquid outlet of the first centrifuge is connected to the second cooling crystallization tank, the discharge port of the first centrifuge is connected to the dissolution kettle, the second centrifuge is arranged at the discharge port of the second cooling crystallization tank, the discharge port of the second centrifuge is connected to the dissolution kettle; the liquid outlet of the second centrifuge faces the third MVR evaporation zone. This process promotes the crystallization of sodium sulfate decahydrate by gradually reducing the temperature of the filtrate, controls the cooling rate, and ensures the stability and efficiency of the precipitation process.

[0028] The second MVR evaporation zone includes a third stirring tank connected to the discharge port of the dissolving kettle, and a second forced evaporation chamber and a second heat exchanger connected thereto, for further evaporating and concentrating the solution. A plurality of preheaters are provided at the liquid outlet of the third stirring tank, and the output end of the preheater is connected to the second forced evaporation chamber. The discharge port of the second forced evaporation chamber is connected to a third centrifuge for separating evaporated solids and liquids. The liquid outlet of the third centrifuge is connected to the second forced evaporation chamber, and the discharge port of the third centrifuge is connected to the third stirring tank. A fourth centrifuge is provided at the discharge port of the third stirring tank, and the discharge port of the fourth centrifuge is connected to a dryer. The purity and dryness of the sodium sulfate are ensured by further processing the separated solids.

[0029] Preferably, in the second MVR evaporation zone, the liquid inlet of the dissolving kettle is connected to the condensed water tank, sodium sulfate decahydrate enters the dissolving kettle from the discharge ports of the first cooling device and the second cooling device to be hot-melted to obtain a sodium sulfate solution, the liquid outlet of the dissolving kettle is connected to the third stirring tank, and the liquid outlet of the third stirring tank is provided with a plurality of plate preheaters, the hot-melted sodium sulfate solution is preheated by the plurality of plate preheaters, and then enters from the feed port of the second forced evaporation chamber; The air inlet of the second forced evaporation chamber is connected to the shell side of the second heat exchanger, the shell side of the second heat exchanger is connected to the external fresh steam pipeline, and the feed inlet and the discharge port of the tube side of the second heat exchanger are both connected to the second forced evaporation chamber; The discharge port of the second forced evaporation chamber is provided with a third centrifuge, and the third centrifuge returns the liquid to the second forced evaporation chamber, and the obtained sodium sulfate enters the third stirring tank through the discharge port of the third centrifuge; after being stirred and washed by the newly entered sodium sulfate solution, clean sodium sulfate is obtained; and then it is centrifuged from the fourth centrifuge and enters the dryer for drying.

[0030] The third MVR evaporation zone includes a mother liquid tank connected to the liquid outlet of the second cooling device, and a third forced evaporation chamber and a third heat exchanger which are connected to each other. The liquid outlet of the mother liquid tank is provided with a plurality of preheaters, and the output end of the preheater is connected to the third forced evaporation chamber. The discharge port of the third forced evaporation chamber is connected to the third heat exchanger. The discharge port of the third heat exchanger is provided with a second filter. The second filter receives the semi-finished lithium carbonate outputted through the third heat exchanger and washes it. The discharge port of the second filter outputs the finished lithium carbonate. The liquid outlet of the second filter is connected to the washing liquid tank and the centrifugal liquid tank.

[0031] Preferably, in the third MVR evaporation zone, the mother liquid tank is connected to the liquid outlet of the second cooling device, lithium carbonate seeds are added to the mother liquid tank in a spiral and continuously, and are mixed with the remaining mother liquid and then preheated before entering the third forced evaporation chamber; The shell side of the third heat exchanger is connected to the external fresh steam pipeline, the feed port of the third heat exchanger tube side is connected to the third forced evaporation chamber, and the discharge port of the third heat exchanger tube side is connected to the second filter; After 4-5 times concentration in the third heat exchanger, the material is output to the second filter to separate the lithium carbonate semi-finished product, and the lithium carbonate finished product is obtained after washing with distilled water. The washing liquid is output to the washing liquid tank for collection for the preparation of sodium carbonate solution; the filtered centrifugal liquid is returned to the centrifugal liquid tank for collection.

[0032] The beneficial effects of the present invention are: The present invention adopts a seed circulation method to avoid lithium carbonate scaling, does not require evaporation after decarbonization, and only uses a physical filtration method to remove insoluble impurities in the lithium-containing sodium sulfate solution impurity removal step, without involving chemical impurity removal, thereby avoiding acid-base adjustment, and the amount of alkali used can be moderately excessive. The operation is simple, and the amount of acid and alkali can be greatly reduced, the amount of decarbonization heating steam can be reduced, and the material cost can be reduced. On this basis, the cleaning and maintenance frequency of the equipment after scaling is reduced, thereby reducing downtime and maintenance costs.

[0033] In the present invention, since the crystal seeds induce a more effective crystallization process, most of the lithium carbonate will be precipitated on the crystal seeds during the first forced evaporation process. The lithium carbonate precipitate formed in this process is not raised and kept circulating, ensuring that the concentration of lithium carbonate in the primary concentrated solution entering the lithium precipitation process is high. In the lithium precipitation process, the primary concentrated solution that has not been extracted with lithium carbonate and has not been adjusted in pH is subjected to lithium precipitation, and 95% of lithium carbonate solids can be raised in one stage, thereby improving the crystallization yield of lithium carbonate. Then, in the cooling process, the remaining mother liquor discharged from sodium sulfate decahydrate is further evaporated and concentrated to achieve a total yield of lithium carbonate of 97.55%. The entire system in the present invention is continuously and automatically produced, which can further improve production efficiency and product consistency.

[0034] (3) In conventional technology, sulfuric acid is used to adjust the pH to 4, and then liquid caustic soda is used to adjust the pH to 7 before evaporation to precipitate lithium, with an excess of 5% to 10% sodium carbonate. In the lithium precipitation process of the present invention, sodium carbonate is added in an equal molar ratio to the first-level concentrated liquid whose pH is not adjusted, and the filtrate after lithium precipitation is frozen to obtain sodium sulfate decahydrate, and further adopts sodium sulfate decahydrate hot melt lithium washing technology, which not only simplifies the operation but also improves the recycling efficiency of sodium sulfate. On this basis, the remaining mother liquor is directly evaporated to obtain lithium carbonate. The amount of sodium carbonate and sulfuric acid is saved, and the material cost is greatly reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0036] Figure 1It is a schematic flow diagram of a method for preparing lithium carbonate from a lithium-containing sodium sulfate solution of the present invention; Figure 2 It is a partial schematic diagram of points A and B in the system for preparing lithium carbonate from a lithium-containing sodium sulfate solution of the present invention; Figure 3 It is a partial schematic diagram of points B and C in the system for preparing lithium carbonate from a lithium-containing sodium sulfate solution of the present invention; Figure 4 It is a partial schematic diagram of points C and D in the system for preparing lithium carbonate from a lithium-containing sodium sulfate solution of the present invention; Figure 5 It is a partial schematic diagram of points D and E in the system for preparing lithium carbonate from a sodium sulfate solution containing lithium according to the present invention; Figure numerals: 10, storage tank area; 11, feed tank; 20, first MVR evaporation area; 21, first forced evaporation chamber; 22, first heat exchanger; 23, cyclone; 24, first stirring tank; 30, lithium precipitation process area; 31, lithium precipitation tank; 32, second stirring tank; 33, first filter; 40, cooling process area; 41, pre-cooling device; 42, first cooling device; 43, second cooling device; 44, dissolving kettle; 421, first cooling crystallization tank; 422, first forced heat exchange component; 423, first centrifugal machine; 431, second cooling crystallization tank; 432, second forced heat exchange component; 433, second centrifuge; 50, second MVR evaporation zone; 51, third stirring tank; 52, second forced evaporation chamber; 53, second heat exchanger; 54, preheater; 55, third centrifuge; 56, fourth centrifuge; 57, dryer; 60, third MVR evaporation zone; 61, mother liquor tank; 62, third forced evaporation chamber; 63, third heat exchanger; 64, preheater; 65, second filter; 66, washing liquid tank; 67, centrifugal liquid tank. DETAILED DESCRIPTION

[0037] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0038] It should be noted that when an element is referred to as being "fixed to" another element, it may be directly on the other element or there may be a central element. When an element is considered to be "connected to" another element, it may be directly connected to the other element or there may be a central element at the same time. The terms "vertical", "horizontal", "left", "right" and similar expressions used herein are for illustrative purposes only and do not represent the only implementation method.

[0039] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0040] Example 1

[0041] like Figures 2~5 A system for preparing lithium carbonate from a lithium-containing sodium sulfate solution is shown, comprising: like Figure 2 As shown, the storage tank area 10 includes a feed tank 11, which is used to collect the lithium-containing sodium sulfate solution after impurities are removed and transport it to the first MVR evaporation area 20; like Figure 2 As shown, the first MVR evaporation zone 20 includes a first forced evaporation chamber 21 connected to the discharge port of the feed tank 11, a first heat exchanger 22 connected to the discharge port of the first forced evaporation chamber 21, and a cyclone 23 connected to the feed port of the first forced evaporation chamber 21. A first stirring tank 24 is also provided between the discharge port of the first heat exchanger 22 and the feed port of the cyclone 23, and the liquid outlet of the cyclone 23 is connected to the lithium precipitation process; Specifically, in the first MVR evaporation zone 20, the lithium-containing sodium sulfate solution is pumped to the first forced evaporation chamber 21 for evaporation and concentration after impurities are removed, the shell side of the first heat exchanger 22 is connected to the external fresh steam pipeline, the feed port of the tube side of the first heat exchanger 22 is connected to the discharge port of the first forced evaporation chamber 21, the discharge port of the tube side of the first heat exchanger 22 is connected to the first stirring tank 24, and the cyclone 23 is used to separate the evaporated solid and liquid parts, wherein the solid part flows back to the first forced evaporation chamber 21, and the liquid part flows to the lithium precipitation tank 31; Lithium carbonate seeds are added to the first stirring tank 24, pumped to the cyclone 23 and then into the first forced evaporation chamber 21, where a primary precipitate is generated. The primary concentrated liquid generated in the first forced evaporation chamber 21 flows back to the cyclone 23 and then flows to the lithium precipitation tank 31. This process uses MVR (mechanical vapor recompression) technology to recover and utilize steam energy to concentrate the solution. Preliminary concentration and solid-liquid separation are achieved through forced evaporation and cyclone separation, which improves the concentration efficiency. The recycled lithium carbonate seeds added can prevent lithium carbonate from adhering to the equipment wall, making the entire production process more stable, reducing the risk of equipment scaling and clogging, and extending the service life of the equipment.

[0042] Please continue to refer to Figure 3, a lithium precipitation process area 30 includes a plurality of lithium precipitation tanks 31 connected to the liquid outlet of the cyclone 23, and a plurality of second stirring tanks 32 connected to the lithium precipitation tank 31, a sodium carbonate solution is added in a equimolar ratio in the second stirring tank 32 and pumped into the lithium precipitation tank 31, and the discharge port of the lithium precipitation tank 31 is connected to the first filter 33 to obtain and collect lithium carbonate solid, and the remaining filtrate enters the next process; Specifically, the lithium precipitation tank 31 is used to precipitate lithium salts, and the second stirring tank 32 is added with an equimolar ratio of sodium carbonate solution and mixed evenly to promote the lithium precipitation reaction. The first filter 33 is used to collect and compress the precipitated lithium carbonate solid. This process adds sodium carbonate solution to precipitate lithium from the lithium-containing sodium sulfate solution to generate lithium carbonate solid, and removes excess liquid by compression and filtration to obtain a preliminary lithium carbonate product. The greater the amount of sodium carbonate solution, the lower the crystallization rate of the precipitated lithium carbonate due to the increase in liquid, so the amount of sodium carbonate should not be too high. In the present invention, the sodium carbonate solution and the primary concentrated solution are increased in an equimolar ratio to achieve the best purification effect.

[0043] like Figure 4 As shown, the cooling process zone 40 includes a first cooling device 42 and a second cooling device 43 connected in sequence, and a dissolving kettle 44 arranged at the discharge ports of the first cooling device 42 and the second cooling device 43, the discharge port of the dissolving kettle 44 is connected to the second MVR evaporation zone 50, the liquid outlet of the first cooling device 42 is connected to the second cooling device 43, and the liquid outlet of the second cooling device 43 faces the third MVR evaporation zone 60; The cooling process also includes a pre-cooling device 41 disposed between the first cooling device 42 and the first filter 33. The pre-cooling device 41 is a plate heat exchanger disposed in series, which is used to initially cool the filtrate to room temperature. The first cooling device 42 includes a first cooling crystallization tank 421, a first forced heat exchange component 422 and a first centrifuge 423, which are used to cool the filtrate to 10°C. The second cooling device 43 includes a second cooling crystallization tank 431, a second forced heat exchange component 432 and a second centrifuge 433, which further cools the filtrate to -5°C. The first centrifuge 423 is arranged at the discharge port of the first cooling crystallization tank 421, the liquid outlet of the first centrifuge 423 is connected to the second cooling crystallization tank 431, the discharge port of the first centrifuge 423 is connected to the dissolution kettle 44, the second centrifuge 433 is arranged at the discharge port of the second cooling crystallization tank 431, the discharge port of the second centrifuge 433 is connected to the dissolution kettle 44; the liquid outlet of the second centrifuge 433 faces the third MVR evaporation zone 60. This process promotes the crystallization and precipitation of sodium sulfate decahydrate by gradually reducing the temperature of the filtrate, controlling the cooling rate, and ensuring the stability and efficiency of the precipitation process.

[0044] like Figure 5As shown, the second MVR evaporation zone 50 includes a third stirring tank 51 connected to the discharge port of the dissolving kettle 44, and a second forced evaporation chamber 52 and a second heat exchanger 53 connected thereto, for further evaporating and concentrating the solution. A plurality of preheaters 54 are provided at the liquid outlet of the third stirring tank 51, and the output end of the preheater 54 is connected to the second forced evaporation chamber 52. The discharge port of the second forced evaporation chamber 52 is connected to a third centrifuge 55 for separating evaporated solids and liquids. The liquid outlet of the third centrifuge 55 is connected to the second forced evaporation chamber 52, and the discharge port of the third centrifuge 55 is connected to the third stirring tank 51; a fourth centrifuge 56 is provided at the discharge port of the third stirring tank 51, and the discharge port of the fourth centrifuge 56 is connected to a dryer 57; the purity and dryness of the sodium sulfate are ensured by further processing the separated solids.

[0045] In the second MVR evaporation, the liquid inlet of the dissolving kettle 44 is connected to the condensed water tank, and sodium sulfate decahydrate enters the dissolving kettle 44 from the discharge ports of the first cooling device 42 and the second cooling device 43 to be hot-melted to obtain a sodium sulfate solution. The liquid outlet of the dissolving kettle 44 is connected to the third stirring tank 51, and the liquid outlet of the third stirring tank 51 is provided with a plurality of plate preheaters 54. The hot-melted sodium sulfate solution is preheated by the plurality of plate preheaters 54, and then enters from the feed port of the second forced evaporation chamber 52; The air inlet of the second forced evaporation chamber 52 is connected to the shell side of the second heat exchanger 53, the shell side of the second heat exchanger 53 is connected to the external fresh steam pipeline, and the feed inlet and the discharge port of the tube side of the second heat exchanger 53 are both connected to the second forced evaporation chamber 52; The discharge port of the second forced evaporation chamber 52 is provided with a third centrifuge 55, and the third centrifuge 55 refluxes the liquid into the second forced evaporation chamber 52, and the obtained sodium sulfate enters the third stirring tank 51 through the discharge port of the third centrifuge 55; after being stirred and washed by the newly entered sodium sulfate solution, clean sodium sulfate is obtained; and then it is centrifuged from the fourth centrifuge 56 and enters the dryer 57 for drying.

[0046] like Figure 5 As shown, the third MVR evaporation zone 60 includes a mother liquid tank 61 connected to the liquid outlet of the second cooling device 43, and a third forced evaporation chamber 62 and a third heat exchanger 63 which are connected to each other. The liquid outlet of the mother liquid tank 61 is provided with a plurality of preheaters 64, the output end of the preheater 64 is connected to the third forced evaporation chamber 62, the discharge port of the third forced evaporation chamber 62 is connected to the third heat exchanger 63, the discharge port of the third heat exchanger 63 is provided with a second filter 65, the second filter 65 receives the semi-finished lithium carbonate outputted through the third heat exchanger 63 and washes it, the discharge port of the second filter 65 outputs the finished lithium carbonate, and the liquid outlet of the second filter 65 is connected to the washing liquid tank 66 and the centrifugal liquid tank 67.

[0047] Specifically, in the third MVR evaporation zone 60, the mother liquid tank 61 is connected to the liquid outlet of the second cooling device 43, and lithium carbonate seeds are spirally added to the mother liquid tank 61, mixed with the remaining mother liquid, and then preheated and enter the third forced evaporation chamber 62; The shell side of the third heat exchanger 63 is connected to the external fresh steam pipeline, the feed port of the tube side of the third heat exchanger 63 is connected to the third forced evaporation chamber 62, and the discharge port of the tube side of the third heat exchanger 63 is connected to the second filter 65; After being concentrated 4 to 5 times in the third heat exchanger 63, the material is output to the second filter 65 to separate the lithium carbonate semi-finished product, and the lithium carbonate finished product is obtained after being washed with distilled water. The washing liquid is output to the washing liquid tank 66 for collection for preparing the sodium carbonate solution; the filtered centrifugal liquid is returned to the centrifugal liquid tank 67 for collection.

[0048] The present invention adopts three-stage MVR evaporation. After the raw material is subjected to the first-stage MVR evaporation to obtain the first-stage MVR concentrated liquid and enter the lithium precipitation process, 95% of the lithium carbonate finished product is obtained. The lithium precipitation filtrate enters the cooling process. In order to improve the overall efficiency of the cooler, three-stage cooling is adopted. The first-stage cooling adopts the lithium liquid raw material and the circulating cooling water cooler, the second-stage cooling adopts the refrigerant direct cooling forced circulation cooler to cool to 10°C, and the third-stage cooling adopts the refrigerant direct cooling forced circulation cooler to cool to -5°C. Cooling and freezing are both continuously automated. The sodium sulfate decahydrate produced by freezing is melted and then enters the second-stage MVR evaporation system to obtain sodium sulfate solid. The obtained sodium sulfate solid can be stirred and washed with the sodium sulfate solution after the sodium sulfate decahydrate is melted, which can prevent lithium accumulation from causing lithium loss and prevent the evaporator efficiency from decreasing. The frozen filtrate is evaporated and concentrated in the three-stage MVR evaporation system and centrifuged to generate a lithium carbonate semi-finished product. The centrifuged liquid is extracted or sent to the phosphate lithium precipitation system. The lithium carbonate semi-finished product is washed with distilled water in the filter, and the washing liquid is collected separately to prepare the sodium carbonate solution.

[0049] The present invention is composed of three-stage MVR + continuous cooling and freezing. The MVR evaporation process is more energy-efficient and more stable than the ordinary evaporation process. The freezing adopts Freon continuous direct cooling crystallization. First, it uses cooling water to cool to below 40°C, then freezes to 10°C, and finally freezes to -5°C. The freezing saves more than 40% of power consumption. In addition, other lithium raw materials can enter this system after removing calcium and magnesium to produce battery-grade lithium carbonate. Compared with the prior art, the present invention can not only use lithium liquid of spodumene and lithium mica as raw materials, but also add other lithium-containing solutions to this system. Although there will be no scaling effect, the system and related methods can significantly improve the crystallization rate of lithium carbonate, and due to the saving of materials, the material cost can be greatly reduced, and the economic benefits are obvious.

[0050] Example 2

[0051] like Figure 1A method for preparing lithium carbonate from a lithium-containing sodium sulfate solution extracted from a spodumene raw material is shown, comprising: S01: After impurities are removed, the lithium-containing sodium sulfate solution is pumped to the first MVR evaporation zone, mixed with the added lithium carbonate seed crystals for primary MVR concentration, and a primary precipitate and a primary concentrated solution are obtained; the primary precipitate is refluxed to the first MVR evaporation zone, and the primary concentrated solution enters the lithium precipitation process; Specifically, the lithium-containing sodium sulfate solution is obtained by roasting and acid treatment of spodumene. After roasting and sulfuric acid treatment, the lithium element is converted into soluble lithium sulfate (Li2SO4) and dissolved in water to form a lithium-containing sulfate solution. The specific component contents include: lithium oxide: 21.14g / L; potassium: 2.5g / L; sodium: 26.5g / L; calcium: 0.82g / L; magnesium: 0.34g / L; chlorine: 0.37g / L; manganese: 0.0064g / L; fluorine: 0.009g / L; pH value is 8.8. In this step, the lithium-containing sodium sulfate solution is decontaminated, including physical filtration to remove insoluble impurities, but not chemical decontamination. At 30 tons / hour, the composition of lithium liquid after impurity removal is as follows: lithium: 0.7453%, lithium sulfate: 5.9025%; potassium sulfate: 0.1667%; sodium: 12g / L, sodium sulfate: 3.4775%; pH value is 9.37; COD: 65mg / L; TDS: 9.5467%.

[0052] On this basis, the lithium carbonate seeds are concentrated and mixed into the primary precipitate, and returned to the first MVR evaporation zone for recycling. Specifically in this step, the lithium carbonate seeds are lithium carbonate solutions with a lithium content of 3.5%. Since the seeds are recycled, no secondary addition is required.

[0053] The present invention adopts a seed circulation method to avoid lithium carbonate scaling, does not require evaporation after decarbonization, and only uses a physical filtration method to remove insoluble impurities in the lithium-containing sodium sulfate solution impurity removal step, without involving chemical impurity removal, thereby avoiding acid-base adjustment, and the alkali dosage can be moderately excessive. The operation is simple, and calculated based on the lithium raw material ratio, the amount of 98% sulfuric acid is reduced by 2.28%, 0.684 tons / hour. The amount of 30% liquid alkali is reduced by 3.6%, 1.08 tons / hour. The decarbonization heating steam is reduced by 1.77 tons / hour. Calculated based on 98% sulfuric acid at 600 yuan / ton, 30% liquid alkali at 900 yuan / ton, and steam at 153 yuan / ton, the material cost is reduced by 1,653 yuan / hour.

[0054] In the present invention, the cleaning and maintenance frequency of equipment after scaling is reduced, thereby reducing downtime and maintenance costs. On this basis, since the crystal seeds induce a more effective crystallization process, most of the lithium carbonate will be precipitated on the crystal seeds during the first forced evaporation process, and the lithium carbonate precipitate formed in this process is not proposed and kept circulating, ensuring that the concentration of lithium carbonate in the primary concentrated solution entering the lithium sinking process is high, and the primary concentrated solution that is not extracted by lithium carbonate is not adjusted for pH in the lithium sinking process. Lithium is sunk in the primary concentrated solution, and 95% of lithium carbonate solids can be proposed in one stage, which improves the crystallization yield of lithium carbonate. Then in the cooling process, the remaining mother liquor discharged from sodium sulfate decahydrate is further evaporated and concentrated to achieve a total yield of 97.55% of lithium carbonate, and the whole system is continuously automated in the present invention, which can further improve production efficiency and product consistency.

[0055] S02: introducing sodium carbonate solution into the lithium precipitation process, reacting with the primary concentrated solution to precipitate lithium, solid precipitates and is collected after lithium precipitation, and the filtrate enters the cooling process; sodium carbonate solution is added in equal moles relative to the primary concentrated solution.

[0056] In conventional technology, sulfuric acid is used to adjust the pH to 4, and then liquid caustic soda is used to adjust the pH to 7 before evaporation to precipitate lithium, with an excess of 5% to 10% sodium carbonate. In the lithium precipitation process of the present invention, sodium carbonate is added in an equal molar ratio to the primary concentrated solution whose pH is not adjusted. After freezing the filtrate after lithium precipitation, the remaining mother liquor is directly evaporated to obtain lithium carbonate. 0.682 tons of sodium carbonate are saved per hour, and 0.638 tons of 98% sulfuric acid are saved per hour. Calculated at 600 yuan per ton of 98% sulfuric acid and 2,600 yuan per ton of sodium carbonate, the material cost is reduced by 2,156 yuan per hour. However, freezing increases the operating electricity cost by 495 yuan per hour. Net operating cost savings are 1,661 yuan per hour.

[0057] S03: After multi-stage cooling, the filtrate is cooled to obtain sodium sulfate decahydrate and the remaining mother liquor. The sodium sulfate decahydrate is melted by adding water and enters the second MVR evaporation zone, and the remaining mother liquor enters the third MVR evaporation zone; The traditional method uses hot water to dissolve sodium sulfate decahydrate and then conducts secondary evaporation to obtain anhydrous sodium sulfate. 4.238 tons of sodium sulfate are produced per hour, with a lithium content of 0.075%. The present invention obtains sodium sulfate decahydrate by freezing the filtrate after lithium precipitation, and further adopts sodium sulfate decahydrate hot melt lithium washing technology, 3.267 tons of sodium sulfate per hour, with a lithium content of 0.0003%; 0.01687 tons of lithium carbonate are recovered per hour; based on 85,000 yuan per ton of lithium carbonate, a benefit of 1,434 yuan per hour is created. This improvement not only simplifies the operation, but also improves the recycling efficiency of sodium sulfate.

[0058] Specifically, the multi-stage cooling includes a first stage cooling and a second stage cooling, the first stage cooling cools the filtrate to 10°C, and the second stage cooling cools the filtrate to -5°C. Specifically, before the first stage cooling, circulating cooling water can be used to cool to room temperature, and then forced cooling can be performed, which can save a lot of resources. The present invention gradually reduces the temperature to avoid the problems of thermal shock and reduced efficiency that may occur in a single-stage cooling system, which helps to control temperature changes more stably.

[0059] S04: The sodium sulfate solution produced after hot melting of sodium sulfate decahydrate is stirred and washed with the sodium sulfate obtained after evaporation and centrifugation in the second MVR evaporation zone in the previous time period, and then the sodium sulfate obtained after evaporation and centrifugation in the second MVR evaporation zone is stirred and washed with the sodium sulfate hot solution newly introduced in the next time period to obtain clean sodium sulfate; S05: The remaining mother liquor is pumped to the third MVR evaporation zone, mixed evenly with the added lithium carbonate seed crystals, evaporated and centrifuged after preheating, and concentrated 5 times to precipitate the lithium carbonate semi-finished product; the precipitated lithium carbonate semi-finished product is washed with distilled water and collected.

[0060] Specifically, lithium carbonate seeds are added in a spiral continuously. Specifically in this step, the lithium carbonate seeds are lithium carbonate solution with a lithium content of 3%. Since this part of the seeds is mixed and concentrated with the remaining mother liquor, it needs to be added continuously.

[0061] The present invention fully considers the influence of material composition, dirt and other factors on the power consumption of the system in the actual operation engineering of the system, and a certain margin is left for the design parameters and equipment configuration. In addition, 1.707 tons of sodium carbonate are required per hour. Based on 2,600 yuan per ton of sodium carbonate, the cost of lithium carbonate is 5,240.57 yuan per ton (excluding refined drying and grinding of lithium carbonate). One ton of lithium carbonate consumes 1.47 tons of sodium carbonate, which saves about 0.73 tons of sodium carbonate, 1.19 tons of sulfuric acid, and 1.95 tons of decarbonization steam relative to traditional devices. Sodium sulfate reduces the loss of lithium carbonate by 134.96 tons per year (calculated based on 8,000 hours). Lithium carbonate is calculated at 85,000 yuan per ton. Annual cost savings are 37.98 million yuan.

[0062] Example 3

[0063] like Figure 1 A method for preparing lithium carbonate from a lithium-containing sodium sulfate solution extracted from a lepidolite raw material is shown, comprising: S01: After impurities are removed, the lithium-containing sodium sulfate solution is pumped to the first MVR evaporation zone, mixed with the added lithium carbonate seed crystals for primary MVR concentration, and a primary precipitate and a primary concentrated solution are obtained; the primary precipitate is refluxed to the first MVR evaporation zone, and the primary concentrated solution enters the lithium precipitation process; Specifically, the lithium-containing sodium sulfate solution is obtained by calcining and acid treatment of lithium mica. After calcination and sulfuric acid treatment, the lithium element is converted into soluble lithium sulfate (Li2SO4) and dissolved in water to form a lithium-containing sulfate solution. The specific component contents include: lithium oxide: 19.4g / L; potassium: 23.34g / L; sodium: 36.96g / L; calcium: 0.472g / L; magnesium: 0.316g / L; chlorine: 0.128g / L; manganese: 1.48g / L; fluorine: 0.646g / L; pH value is 6.84. In this step, the lithium-containing sodium sulfate solution is impurity-removed, including physical filtration to remove insoluble impurities, excluding chemical impurity removal. At 23 tons / hour, the components of the lithium liquid after impurity removal are as follows: sodium sulfate about 9.8428%, potassium sulfate about 4.4838%, lithium 9.0127g / L, lithium sulfate 6.1532%.

[0064] On this basis, the lithium carbonate seed crystals are concentrated and mixed into the primary precipitate, and then returned to the first MVR evaporation zone for recycling. Specifically in this step, the lithium carbonate seed crystals are a lithium carbonate solution with a lithium content of 3%. Since the seed crystals are recycled, no secondary addition is required.

[0065] The present invention adopts a seed circulation method to avoid lithium carbonate scaling, does not require evaporation after decarbonization, and only uses a physical filtration method to remove insoluble impurities in the lithium-containing sodium sulfate solution impurity removal step without involving chemical impurity removal, thereby avoiding acid-base adjustment, and the alkali dosage can be moderately excessive, and the operation is simple.

[0066] In the present invention, the cleaning and maintenance frequency of equipment after scaling is reduced, thereby reducing downtime and maintenance costs. On this basis, since the crystal seeds induce a more effective crystallization process, most of the lithium carbonate will be precipitated on the crystal seeds during the first forced evaporation process, and the lithium carbonate precipitate formed in this process is not proposed and kept circulating, ensuring that the concentration of lithium carbonate in the primary concentrated solution entering the lithium sinking process is high, and the primary concentrated solution that is not extracted by lithium carbonate is not adjusted for pH in the lithium sinking process. Lithium is sunk in the primary concentrated solution, and 95% of lithium carbonate solids can be proposed in one stage, which improves the crystallization yield of lithium carbonate. Then in the cooling process, the remaining mother liquor discharged from sodium sulfate decahydrate is further evaporated and concentrated to achieve a total yield of 97.55% of lithium carbonate, and the whole system is continuously automated in the present invention, which can further improve production efficiency and product consistency.

[0067] S02: introducing sodium carbonate solution into the lithium precipitation process, reacting with the primary concentrated solution to precipitate lithium, solid precipitates and is collected after lithium precipitation, and the filtrate enters the cooling process; sodium carbonate solution is added in equal moles relative to the primary concentrated solution.

[0068] In conventional technology, sulfuric acid is used to adjust the pH to 4, and then liquid caustic soda is used to adjust the pH to 7 before evaporation and lithium precipitation, with an excess of 5% to 10% sodium carbonate. In the lithium precipitation process of the present invention, sodium carbonate is added in an equal molar ratio to the first-stage concentrated liquid whose pH is not adjusted, and the filtrate after lithium precipitation is frozen, and the remaining mother liquor is directly evaporated to obtain lithium carbonate.

[0069] S03: After multi-stage cooling, the filtrate is cooled to obtain sodium sulfate decahydrate and the remaining mother liquor. The sodium sulfate decahydrate is melted by adding water and enters the second MVR evaporation zone, and the remaining mother liquor enters the third MVR evaporation zone; The traditional method is to dissolve sodium sulfate decahydrate in hot water and then perform secondary evaporation to obtain anhydrous sodium sulfate. The present invention obtains sodium sulfate decahydrate by freezing the filtrate after lithium precipitation, and further adopts the sodium sulfate decahydrate hot melt lithium washing technology to improve the recovery efficiency of sodium sulfate.

[0070] Specifically, the multi-stage cooling includes a first stage cooling and a second stage cooling, the first stage cooling cools the filtrate to 10°C, and the second stage cooling cools the filtrate to -5°C. Specifically, before the first stage cooling, circulating cooling water can be used to cool to room temperature, and then forced cooling can be performed, which can save a lot of resources. The present invention gradually reduces the temperature to avoid the problems of thermal shock and reduced efficiency that may occur in a single-stage cooling system, which helps to control temperature changes more stably.

[0071] S04: The sodium sulfate solution produced after hot melting of sodium sulfate decahydrate is stirred and washed with the sodium sulfate obtained after evaporation and centrifugation in the second MVR evaporation zone in the previous time period, and then the sodium sulfate obtained after evaporation and centrifugation in the second MVR evaporation zone is stirred and washed with the sodium sulfate hot solution newly introduced in the next time period to obtain clean sodium sulfate; S05: The remaining mother liquor is pumped to the third MVR evaporation zone, mixed evenly with the added lithium carbonate seeds, evaporated and centrifuged after preheating, and concentrated 4 times to precipitate the lithium carbonate semi-finished product; the precipitated lithium carbonate semi-finished product is washed with distilled water and collected. Specifically, the lithium carbonate seeds are added continuously by spiral. Specifically in this step, the lithium carbonate seeds are a lithium carbonate solution with a lithium content of 3%. Since this part of the seeds is mixed and concentrated with the remaining mother liquor, it needs to be added continuously.

[0072] Due to the difference between lithium mica and spodumene raw materials, lithium mica can also extract potassium sulfate in subsequent operations. Therefore, the centrifugal liquid of the lithium carbonate semi-finished product extracted after step S05 is decarbonized and enters the fourth-stage low-temperature MVR, and then concentrated about 4 times again and enters the independent refrigeration system to cool to -5°C, and then the secondary lithium precipitation is carried out separately, and the secondary lithium precipitation filtrate goes to the decarbonization kettle. This design scheme fully considers the influence of factors such as material composition and dirt on the power consumption of the system in the actual operation engineering of the system, and a certain margin is left for the design parameters and equipment configuration. In addition, 1.419 tons / hour of sodium carbonate and 0.082 tons / hour of sulfuric acid are required. According to 2,300 yuan / ton of sodium carbonate and 500 yuan / ton of sulfuric acid, the cost of lithium carbonate is 5,629 yuan / ton (excluding refined drying and grinding of lithium carbonate).

[0073] Those skilled in the art should understand that the present invention is not limited to the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, and these changes and improvements fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for preparing lithium carbonate from a lithium-containing sodium sulfate solution, characterized in that: include: S01: After the lithium-containing sodium sulfate solution is impurity-removed, it is pumped to the first MVR evaporation zone, mixed with the added lithium carbonate seed crystals for primary MVR concentration, and a primary precipitate and a primary concentrated solution are obtained; the primary precipitate is refluxed to the first MVR evaporation zone, and the primary concentrated solution enters the lithium precipitation process; S02: introducing sodium carbonate solution into the lithium precipitation process to react with the primary concentrated solution to precipitate lithium, solids are precipitated and collected after lithium precipitation, and the filtrate enters the cooling process; S03: After multi-stage cooling, the filtrate is cooled to obtain sodium sulfate decahydrate and residual mother liquor. The sodium sulfate decahydrate is melted with water and enters the second MVR evaporation zone, and the residual mother liquor enters the third MVR evaporation zone; S04: the sodium sulfate solution generated after the sodium sulfate decahydrate is hot-melted is used to stir and wash the sodium sulfate obtained after evaporation and centrifugation in the second MVR evaporation zone in the previous time period, and then the sodium sulfate obtained after evaporation and centrifugation in the second MVR evaporation zone is stirred and washed with the sodium sulfate hot solution newly introduced in the next time period to obtain clean sodium sulfate; S05: The remaining mother liquor is pumped to the third MVR evaporation zone, mixed evenly with the added lithium carbonate seed crystals, evaporated and centrifuged after preheating to precipitate the lithium carbonate semi-finished product; the precipitated lithium carbonate semi-finished product is washed with distilled water and collected.

2. A method for preparing lithium carbonate from a lithium-containing sodium sulfate solution according to claim 1, characterized in that: In step S01, the lithium-containing sodium sulfate solution is obtained by calcining and acid-treating lepidolite or spodumene, and the lithium-containing sodium sulfate solution is subjected to impurity removal, including physical filtration to remove insoluble impurities, but excluding chemical impurity removal.

3. A method for preparing lithium carbonate from a lithium-containing sodium sulfate solution according to claim 1, characterized in that: In step S01, the lithium carbonate seed crystals are concentrated and mixed into the primary precipitate, and then returned to the first MVR evaporation zone for recycling. The lithium carbonate seed crystals are lithium carbonate solutions with a raw material content of 1%-10%.

4. A method for preparing lithium carbonate from a lithium-containing sodium sulfate solution according to claim 1, characterized in that: In step S02, in the lithium precipitation process, the sodium carbonate solution is added in an equimolar amount relative to the primary concentrated solution.

5. A method for preparing lithium carbonate from a lithium-containing sodium sulfate solution according to claim 1, characterized in that: In step S03, the multi-stage cooling includes a first stage cooling and a second stage cooling, wherein the first stage cooling cools the filtrate to 10°C, and the second stage cooling cools the filtrate to -5°C.

6. A method for preparing lithium carbonate from a lithium-containing sodium sulfate solution according to claim 1, characterized in that: In step S05, the lithium carbonate seed crystals are added continuously in a spiral.

7. A system for preparing lithium carbonate from a lithium-containing sodium sulfate solution, wherein lithium carbonate is prepared by the method of claims 1 to 6, characterized in that: include: The storage tank area includes a feed tank for collecting the lithium-containing sodium sulfate solution after impurities removal and transporting it to the first MVR evaporation area; The first MVR evaporation zone includes a first forced evaporation chamber connected to the discharge port of the feed tank, a first heat exchanger connected to the discharge port of the first forced evaporation chamber, and a cyclone connected to the feed port of the first forced evaporation chamber. A first stirring tank is further arranged between the discharge port of the first heat exchanger and the feed port of the cyclone. The liquid outlet of the cyclone is connected to the lithium precipitation process; A lithium precipitation process area includes a plurality of lithium precipitation tanks connected to the liquid outlet of the cyclone, and a plurality of second stirring tanks connected to the lithium precipitation tanks, wherein a sodium carbonate solution is added in an equal molar ratio in the second stirring tanks and pumped into the lithium precipitation tanks, and the discharge port of the lithium precipitation tanks is connected to a first filter to obtain and collect lithium carbonate solids, and the remaining filtrate enters the next process; a cooling process zone, comprising a first cooling device and a second cooling device connected in sequence, and a dissolving kettle arranged at the discharge ports of the first cooling device and the second cooling device, wherein the discharge port of the dissolving kettle is connected to the second MVR evaporation zone, the liquid outlet of the first cooling device is connected to the second cooling device, and the liquid outlet of the second cooling device faces the third MVR evaporation zone; The second MVR evaporation zone includes a third stirring tank connected to the discharge port of the dissolving kettle, and a second forced evaporation chamber and a second heat exchanger connected thereto, a plurality of preheaters are provided at the liquid outlet of the third stirring tank, the output end of the preheater is connected to the second forced evaporation chamber, a third centrifuge is connected to the discharge port of the second forced evaporation chamber, a liquid outlet of the third centrifuge is connected to the second forced evaporation chamber, and a discharge port of the third centrifuge is connected to the third stirring tank; a fourth centrifuge is provided at the discharge port of the third stirring tank, and a dryer is connected to the discharge port of the fourth centrifuge; The third MVR evaporation zone includes a mother liquid tank connected to the liquid outlet of the second cooling device, and a third forced evaporation chamber and a third heat exchanger connected thereto. The liquid outlet of the mother liquid tank is provided with a plurality of preheaters, the output end of the preheater is connected to the third forced evaporation chamber, the discharge port of the third forced evaporation chamber is connected to the third heat exchanger, the discharge port of the third heat exchanger is provided with a second filter, the second filter receives the semi-finished lithium carbonate outputted through the third heat exchanger and washes it, the discharge port of the second filter outputs the finished lithium carbonate, and the liquid outlet of the second filter is connected to a washing liquid tank and a centrifugal liquid tank.

8. A system for preparing lithium carbonate from a lithium-containing sodium sulfate solution according to claim 7, characterized in that: In the first MVR evaporation zone, the lithium-containing sodium sulfate solution is pumped into the first forced evaporation chamber after impurities are removed, the shell side of the first heat exchanger is connected to the external fresh steam pipeline, the feed port of the first heat exchanger tube side is connected to the discharge port of the first forced evaporation chamber, and the discharge port of the first heat exchanger tube side is connected to the first stirring tank. Lithium carbonate seeds are added into the first stirring tank, pumped to the cyclone and then into the first forced evaporation chamber, a primary precipitate is generated in the first forced evaporation chamber, and the primary concentrated liquid generated in the first forced evaporation chamber flows back to the cyclone and then flows to the lithium precipitation tank.

9. A system for preparing lithium carbonate from a lithium-containing sodium sulfate solution according to claim 7, characterized in that: In the second MVR evaporation zone, the liquid inlet of the dissolving kettle is connected to the condensed water tank, and sodium sulfate decahydrate enters the dissolving kettle from the discharge ports of the first cooling device and the second cooling device to be hot-melted to obtain a sodium sulfate solution, and the liquid outlet of the dissolving kettle is connected to the third stirring tank, and the liquid outlet of the third stirring tank is provided with a plurality of plate preheaters, and the hot-melted sodium sulfate solution is preheated by the plurality of plate preheaters and then enters from the feed port of the second forced evaporation chamber; The air inlet of the second forced evaporation chamber is connected to the shell side of the second heat exchanger, the shell side of the second heat exchanger is connected to the external fresh steam pipeline, and the feed inlet and the discharge port of the tube side of the second heat exchanger are both connected to the second forced evaporation chamber; The discharge port of the second forced evaporation chamber is provided with a third centrifuge, and the third centrifuge refluxes the liquid into the second forced evaporation chamber, and the obtained sodium sulfate enters the third stirring tank through the discharge port of the third centrifuge; After being stirred and washed with the newly introduced sodium sulfate solution, clean sodium sulfate is obtained; Then, the product is centrifuged in the fourth centrifuge and then enters the dryer for drying.

10. A system for preparing lithium carbonate from a lithium-containing sodium sulfate solution according to claim 7, characterized in that: In the third MVR evaporation zone, the mother liquid tank is connected to the liquid outlet of the second cooling device, lithium carbonate seeds are added to the mother liquid tank in a spiral and continuously, and are mixed with the remaining mother liquid and then preheated before entering the third forced evaporation chamber; The shell side of the third heat exchanger is connected to the external fresh steam pipeline, the feed port of the tube side of the third heat exchanger is connected to the third forced evaporation chamber, and the discharge port of the tube side of the third heat exchanger is connected to the second filter; After 4-5 times concentration in the third heat exchanger, the material is output to the second filter to separate the lithium carbonate semi-finished product, and the lithium carbonate finished product is obtained after cleaning. The cleaning liquid is output to the washing liquid tank for collection for preparing the sodium carbonate solution; the filtered centrifugal liquid is returned to the centrifugal liquid tank for collection.

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