Main Process Flow
Ores containing rubidium elements do not exist independently in nature; they often coexist with lithium-containing ores. The concentrate obtained from the beneficiation of the raw ore undergoes 17 steps to produce lithium carbonate, along with by-products of lithium phosphate and rubidium-cesium salts. Specifically, the process includes: raw material drying → raw material mixing → roasting → ball mill water leaching → impurity removal → evaporation concentration → resin impurity removal → lithium precipitation → stirring and washing → decarbonization → salt solution evaporation crystallization → nitrate freezing → secondary lithium precipitation → lithium phosphate precipitation → carbonization → rubidium salt extraction.
Raw material drying
The moisture content of Rubidium -lithium mica after washing is usually between 15-20%. To avoid affecting the subsequent roasting process, the material needs to be dried. The drying is carried out in a drying kiln at a temperature of approximately 250°C for about 15 minutes, with direct contact between the hot air and the material. After drying, the moisture content of the Rubidium-lithium mica concentrate is about 6.5%. Additionally, the drying process can reduce the amount of gas needed for the subsequent roasting process. The heat source for drying comes from the waste heat of the rotary kiln exhaust gas, and the collected dust from drying is reused in the roasting process.
Step 1
Raw material mixing
Rubidium-lithium mica is mixed with calcium oxide, calcium sulfate, end calcium carbonate slag, impurity-removing slag, sodium sulfate, and potassium sulfate, and is then sent into a mixer via a belt conveyor to ensure thorough and uniform mixing of lithium-rubidium mica with the various additives. During the roasting of Rubidium-lithium mica, toxic fluoride gases are produced. Therefore, calcium oxide must also be added to the raw material ratio. Calcium oxide serves to absorb fluoride elements, forming calcium fluoride. Additionally, calcium oxide has a high melting point, which helps to prevent the raw materials in the furnace from sintering or forming glass bodies due to high temperatures. After the mixing of ingredients is complete, the raw materials are hermetically conveyed to a rotary kiln for the next roasting section. Calcium sulfate can be added during mixing, which not only provides calcium elements to absorb fluorides but also provides the sulfate ions needed for the reaction.
Step 2
Roasting
After the raw and auxiliary materials are fully mixed, they enter the high-temperature roasting chamber of the roasting rotary kiln for roasting. The raw materials are subjected to high-temperature roasting through direct heating, with the controlled sintering temperature being 900~1100°C. The duration from feeding to discharging is approximately 2 hours. The rotary kiln operates continuously for 24 hours. It is known that the melting point of sodium sulfate is 884°C, the melting point of potassium sulfate is 1067°C, and the melting point of rubidium-lithium mica is 903°C. Calcium sulfate will also enter a molten state under the fluxing conditions of calcium oxide.
Step 3
Cooling
During high-temperature roasting, some of the material melts, resulting in sintered material with a certain strength. This material needs to be cooled. The material enters the cooling kiln, where forced cooling is carried out by blowing in natural air through a blower fan, reducing the temperature to around 90°C. The cooling time after roasting is approximately 1.5 hours
Step 4
Crushing and Fine Grinding
After cooling, the material may be sintered into lumps and needs to be finely ground before leaching. First, it is initially crushed to a particle size of about 10 mm using a horizontal crusher. Then, it is further ground to a fineness of about 100-150 mesh using a vertical grinding mill, conveyed by a belt.
Step 5
Water Leaching
After the clinker has been finely ground and meets the required standards, it is conveyed to the leaching tank by a screw conveyor. At the same time, leaching wash water is added to the tank (with a liquid-to-solid ratio of approximately 0.85:1) for slurry mixing. A large amount of water is added in this step, allowing the rubidium sulfate and lithium sulfate in the clinker to dissolve in the water solution. The solution undergoes liquid-solid separation through a belt filter, and the resulting leachate proceeds to the next step. The filtered residue is washed with condensate water to obtain the leaching residue, which is returned to the mixing step. The leaching water is recycled and reused.
Step 6
Impurity Removal
The leachate enters the purification reactor to begin impurity removal. In addition to Li+, Na+, and SO4^2-, the leachate also contains a certain amount of Fe3+ and Al3+. This process mainly removes Al3+ and Fe3+ cations from the leachate. After impurity removal, the main components are rubidium, lithium, potassium, sodium, and sulfate ions. A sufficient amount of quicklime is added to the leachate to adjust the pH value of the solution. Due to the high solubility of LiOH, the first precipitates to form are water-insoluble substances like CaF2, ferric hydroxide, and aluminum hydroxide. When the pH is greater than 6, Al3+ in the solution precipitates in the form of Al(OH)3. When the pH is greater than 10, iron ions precipitate in the form of Fe(OH)3. Subsequently, the solution undergoes solid-liquid separation through a plate-frame filter press to remove the precipitates. The impurity residue produced in this step mainly consists of calcium fluoride, along with some aluminum hydroxide, calcium sulfate, and ferric hydroxide.
Step 7
Evaporation and Concentration of Salt
After purification, the main cations in the solution are rubidium, lithium, calcium, magnesium, potassium, and sodium ions. The concentrations of rubidium sulfate and lithium sulfate are relatively low. An evaporator is used to evaporate and concentrate the solution, increasing the concentrations of rubidium and lithium ions to improve the efficiency of subsequent processes. The condensed water produced by evaporation is sent to the leaching process for use as wash water. The evaporation concentration temperature is around 90°C-105°C.
Step 8
Resin Impurity Removal
The concentrated mother liquor needs to undergo further deep impurity removal using chelating resin to remove cations like calcium and magnesium. Chelating resin is a type of cross-linked functional high polymer material that can form multi-coordinated complexes with metal ions. The functional atoms on the chelating resin will coordinate with cations like calcium and magnesium, forming stable structures similar to small molecule chelates. After a period of use, which is related to the efficiency of front-end impurity removal and generally around 28 hours, the resin is replaced every three years. When the resin becomes saturated, it needs to be regenerated using a regenerating agent (sulfuric acid). After regeneration, the resin can continue to be used following transformation (by adding sodium hydroxide).
Step 9
Primary Lithium Precipitation
In the alkali solution blending reaction kettle, a sodium carbonate solution is prepared using lithium carbonate wash water and caustic soda, with a sodium carbonate concentration of around 20%. The prepared caustic soda solution is pumped into the lithium precipitation reaction kettle. The mother liquor, after resin impurity removal, is then added to the sodium carbonate solution, where lithium sulfate reacts with sodium carbonate to form lithium carbonate. According to the physicochemical properties of the product, the solubility of Li2CO3 decreases as the temperature rises. Therefore, the solution temperature is raised to 90-95°C in the lithium precipitation kettle through indirect heating by steam coils, to precipitate as much Li2CO3 as possible. The mother liquor after lithium precipitation enters the centrifuge to obtain crude lithium carbonate and primary lithium precipitation mother liquor; the crude lithium carbonate then enters the stirring and washing process.
Step 10
Stirring and Washing
The crude lithium carbonate obtained from the lithium precipitation process is washed with pure water at 95°C. This washes away impurity ions adsorbed on the surface of the lithium carbonate, improving the purity of the product. The washing solution is filtered through a centrifuge, and the filtered wet lithium carbonate enters the carbonation purification process. The generated lithium carbonate wash water is returned to the lithium precipitation process to prepare the caustic soda solution.
Step 11
Decarbonization
The mother liquor from the lithium precipitation is pumped into a decarbonization reactor. 98% concentrated sulfuric acid is added to the reactor for decarbonization, with the solution's pH controlled between 6-7. The generated carbon dioxide is vented out through the exhaust port in the reactor. Then, sodium hydroxide is added to adjust the pH of the mother liquor to around 8-9. The decarbonized mother liquor is then pumped into an MVR (Mechanical Vapor Recompression) for evaporation and concentration.
Step 12
Salt Solution Evaporation and Crystallization
The decarbonized mother liquor is pumped into an MVR (Mechanical Vapor Recompression) evaporator for evaporation and crystallization. By controlling the evaporation temperature (around 85-95°C), potassium salt (potassium sulfate) and sodium salt (mainly sodium sulfate, with a small amount of potassium sulfate) are sequentially crystallized. Sodium sulfate has larger particles and is separated by a double-push centrifuge to obtain sodium sulfate salt. Fine particles of potassium sulfate are obtained through plate-frame filtration after passing through the double-push centrifuge. The filtered mother liquor proceeds to the next step. Sodium salt is returned as an auxiliary material to the front-end mixing process; the condensed water is recycled to the front-end water reuse section.
Step 13
Nitrate Freezing Crystallization
Taking advantage of the significant change in the solubility of sodium sulfate with temperature, the concentrated mixed mother liquor is pumped into a freezing tank for freezing treatment (-5 to 5°C). This results in the crystallization of decahydrate sodium sulfate. The crystallization of decahydrate sodium sulfate not only reduces the amount of sodium sulfate in the solution but also significantly consumes the water in the solution. The decahydrate is then separated by a centrifuge and transported to a re-dissolution kettle. After heating and dissolving, it undergoes centrifugal filtration. The separated sodium sulfate can be sold as a by-product; the separated freezing mother liquor proceeds to the secondary lithium precipitation process.
Step 14
Secondary Lithium Precipitation
The centrifuged freezing mother liquor is pumped into the lithium precipitation reactor and reacts with the caustic soda solution to precipitate lithium. It then undergoes liquid-solid separation through a centrifuge, resulting in crude lithium carbonate and secondary lithium precipitation mother liquor. During the secondary lithium precipitation, when the concentrations of rubidium and cesium are low, neither rubidium sulfate nor cesium sulfate will react with sodium carbonate. The crude lithium carbonate proceeds to the stirring and washing stage. The secondary lithium precipitation mother liquor goes into the decarbonization stage. After several cycles, if the rubidium and cesium in the secondary lithium precipitation mother liquor are enriched to a certain extent, they proceed to the lithium phosphate precipitation process, rather than going back to the decarbonization stage.
Step 15
Lithium Phosphate Precipitation
The secondary lithium precipitation mother liquor with high concentrations of rubidium and cesium is pumped into the lithium precipitation reactor. Phosphoric acid and sodium hydroxide are added to the reactor, and the reaction is carried out at 80°C for a certain period. After that, solid-liquid separation is performed using a plate-frame filter press, resulting in solid lithium phosphate and lithium phosphate precipitation mother liquor (rubidium and cesium mother liquor). The lithium phosphate then undergoes washing, drying, and packaging to become the final product.
Step 16
Rubidium and Cesium Extraction
By adding an extraction agent, rubidium and cesium elements are dissolved into the organic phase. Separation is then carried out between the aqueous and organic phases. The raffinate, which is the aqueous phase, still contains a certain concentration of lithium. After accumulating to a certain amount, it can be recycled back to the precipitation process of the lithium carbonate production line for further lithium recovery, without being discharged externally. The organic phase undergoes back-extraction to recover rubidium and cesium salts. In this step, dilute sulfuric acid is added to replace rubidium and cesium from the extraction agent. The recovered solution is concentrated and crystallized by steam heating, and rubidium and cesium salts (rubidium sulfate, cesium sulfate) are precipitated in crystal form. The extraction agent separated during the back-extraction process is recycled for use in the extraction step, without being discharged externally.