Lithium carbonate production process
① Roasting and leaching process
Conversion roasting: The spodumene concentrate is manually transported from the concentrate storage to the bucket elevator and then lifted to the concentrate bin. It is then fed to the lithium carbonate rotary kiln tail through a disc feeder and a screw feeder. The concentrate is dried using high-temperature gas in the preheating section at the kiln tail. The concentrate is roasted for crystal conversion at a temperature of about 1200°C in the calcining section, converting from α-type (monoclinic system, density 3150kg/m3) to β-type spodumene (tetragonal system, density 2400kg/m3, i.e. roasted material), with a conversion rate of about 98%.
Acidification roasting: The roasted material is discharged from the kiln head after cooling in the cooling section, and then naturally cooled and ground by ball mill to a particle size of 0.074mm (more than 90%), and then transported to the tailings bin of the acidification roasting kiln. Then, it is added to the acid mixer through the feeder and screw conveyor and mixed with concentrated sulfuric acid (more than 93%) in a certain proportion (the concentrated sulfuric acid is calculated based on the lithium equivalent excess of 35% in the roasted material, and about 0.21t of concentrated sulfuric acid is required per ton of roasted material). After being mixed evenly, it is added to the acidification roasting chamber and sealed acidification roasted at a temperature of about 250-300℃ for 30-60min. The β-type spodumene in the roasted material reacts with sulfuric acid, and the hydrogen ions in the acid replace the lithium ions in the β-type spodumene, so that the Li2O in it combines with SO42- to form water-soluble Li2SO4, and the acidified clinker is obtained.
Slurry leaching and washing: The clinker is cooled and slurried to dissolve the soluble lithium sulfate in the clinker into the liquid phase. In order to reduce the corrosion of the solution to the leaching equipment, the residual acid in the clinker is neutralized with limestone slurry, and the pH value is adjusted to 6.5-7.0. At the same time, most of the impurities such as iron and aluminum are removed. The leaching liquid-solid ratio is about 2.5, and the leaching time is about 0.5h. The leached slurry is filtered and separated to obtain a leachate containing about 100g/L Li2SO4 (27g/L Li2O). The filter cake is the leached slag with a moisture content of about 35%. The leached slag attached liquid contains lithium sulfate. In order to reduce lithium loss, the leached slag is washed by reverse stirring, and the washing liquid is returned to the slurry leaching.
Leachate purification: When the roasted material is acidified and roasted, in addition to the alkali metal that can react with sulfuric acid to produce the corresponding soluble sulfate, other iron, aluminum, calcium, magnesium, etc. also react with sulfuric acid to produce the corresponding sulfate. Although some impurities in the clinker can be removed during the leaching process, the remaining impurities remain in the leachate and need to be further purified to ensure product quality. The leachate purification adopts the alkalization decalcification method. The leachate is alkalized with lime milk (containing CaO100-150g/L) to increase the pH value to 11-12, so that magnesium and iron are hydrolyzed into hydroxide precipitation. Then, sodium carbonate solution (containing Na2CO3 300g/L) is reacted with calcium sulfate to produce calcium carbonate precipitation, thereby removing calcium in the leachate and calcium brought in by the alkalizer lime milk. The alkalization decalcified slurry is separated by liquid-solid separation, and the resulting solution is the purified liquid, with a calcium-lithium ratio of less than 9.6×10-4. The filter cake is calcium slag, which is returned to the slurry for leaching.
Purified liquid evaporation and concentration: Purified liquid cannot be directly used for lithium precipitation or lithium chloride production due to its low lithium sulfate concentration and low lithium precipitation rate. It is necessary to first adjust the purified liquid to pH 6-6.5 with sulfuric acid, and evaporate and concentrate it in a three-effect evaporator to make the lithium sulfate concentration in the concentrated liquid reach 200g/L (containing Li2O 60g/L). The concentrated liquid is separated by filter press, and the filtrate is the finished liquid for use in the next process, and the filter cake is the finished residue returned for slurry leaching.
② Lithium carbonate production section
The finished liquid and soda ash solution (containing Na2CO3 300g/L) are added to the evaporation lithium precipitation tank for evaporation and lithium precipitation (maintaining constant temperature for 2h after boiling). Due to the low solubility of lithium carbonate, it precipitates, and the lithium precipitation rate is about 85%. After lithium precipitation, the centrifuge is used to separate the primary crude lithium carbonate (containing less than 10% filtrate) and the primary lithium precipitation mother liquor while hot.
The mother liquor of lithium precipitation contains a large amount of sodium sulfate and a relatively high lithium sulfate (accounting for about 15% of the total amount). Soda ash solution (containing 300g/L Na2CO3) is added for secondary lithium precipitation to obtain secondary crude product and secondary mother liquor. The mother liquor is neutralized with acid and the pH is adjusted with sodium hydroxide. The by-product anhydrous sodium sulfate and sodium precipitation mother liquor are separated by evaporation, crystallization and centrifugation. The anhydrous sodium sulfate is dried by air flow and packaged to obtain the by-product glauber powder. The sodium precipitation mother liquor is returned to the first mother liquor.
The primary crude lithium carbonate and the secondary crude product attached liquid contain impurities such as Na2SO4. They are then stirred and washed with clean water at about 90°C. The washing liquid is sent to the alkali preparation. After washing, the wet refined lithium carbonate is separated by centrifuge while hot, and then dried by a far-infrared dryer. The iron wire scraps and other debris dropped by the dryer are removed by magnetic separation. Finally, they are crushed by air flow, packaged and put into storage.
This project mainly adds battery-grade lithium carbonate production capacity. From the overall production process, battery-grade lithium carbonate and industrial-grade lithium carbonate are basically the same. The difference lies in the different process control conditions of the evaporation and lithium precipitation sections. That is, when the purified liquid is evaporated and concentrated, the specific gravity of the end-point finished liquid is measured by a hydrometer and the Li2O concentration in the finished liquid is measured by a flame photometer to ensure that the end-point concentration of the finished liquid is within the process requirements; when lithium is precipitated, the electromagnetic flowmeter is used to display the different openings of the regulating valve to control the feeding speed, and the frequency converter is used to adjust the motor speed to control the stirring speed of the stirrer. The above process control conditions are all key technologies of the company.
③ Anhydrous lithium chloride section
The finished liquid obtained in the roasting and leaching section is subjected to a double decomposition reaction with the calcium chloride solution. After the reaction, the CaSO4·2H2O is separated and sent for processing to obtain the CaSO4 product. After separation, a dilute LiCl solution is obtained, and β-type active Al2O3, Na2CO3 and NaOH solutions are added in turn to remove impurities such as SO42-, Ca2+, Mg2+, etc. in the dilute LiCl solution. After evaporation and concentration, the LiCl concentration is increased to 400-500g/L, and then cooled and filtered to separate solid NaCl to obtain a concentrated LiCl solution. The concentrated LiCl solution is transported to a refining kettle, and the company’s self-produced refining agent (the company’s patented technology, inorganic components, and does not contain toxic and harmful heavy metals) is added to carry out a replacement reaction with Na+, and the Na+/LiCl ratio in the reaction endpoint solution is controlled to be less than 30ppm. After separation, a finished LiCl solution is obtained, and finally the finished solution is spray-dried to obtain an anhydrous lithium chloride product with uniform particles.