1.Drying: The moisture content of spodumene purchased by the project is usually 8%. In order not to affect the subsequent roasting process, it is necessary to dry the material. The drying is carried out in the rotary drying kiln, the drying temperature is about 250℃, the duration is about 15min, and the hot air is in direct contact with the material. After drying, the moisture content of spodumene is about 6.5%, and the drying process can reduce the gas amount in the subsequent roasting process. The drying heat source comes from the waste heat of the tail gas of the rotary kiln (in order to achieve energy saving and consumption reduction, a part of the hot flue gas of about 250℃ of the exhaust heat of the rotary roasting kiln is transferred to the drying kiln by the fan). 

2.Roasting: After drying, the pyroxene enters the high-temperature roasting chamber of the rotary kiln through the end of the kiln (coke oven gas is used as fuel, and a spray gun is set in the middle of the tunnel kiln for direct combustion). The rotary kiln uses coke oven gas as fuel to conduct high-temperature roasting of raw materials through direct heating. The controlled sintering temperature is 1100~1200℃, and the roasting time lasts about 2h.

After the completion of roasting, the raw material of spodumene changes from α-type spodumene to β-type spodumene, which makes the structure of lithium insoluble in acid into the structure of soluble in acid, and the conversion rate is 95-98%. The transformation process equation is: α-Li2O•Al2O3•4SiO2→β-Li2O•Al2O3•4SiO2

3.Cooling: When the material is roasted at high temperature, due to the melting of some materials, the sintered material will have a certain strength, and it needs to be cooled, and it will be cooled to about 90℃ by forced air cooling into the cooling kiln. The cooling time after roasting is 1.5 hours. 

4. Crushing: the cooled clinker is transmitted to the crusher using a closed chain bucket conveyor for crushing to prevent large pieces of clinker from entering the subsequent process and crushing to about 10mm particle size, and the clinker is input into the vertical mill system through a closed belt conveyor.

5.Frinding: In order to facilitate the subsequent leaching operation, so that the clinker is more fully dissolved, the crushed clinker needs to be further ground. The material is fed into the grinding disk of the vertical grinding system by the belt weighing scale. Under the action of centrifugal force, the material moves towards the perimeter of the grinding disk. When the material passes between the grinding disc and the grinding roll, it is squeezed and ground under the action of pressing force and is crushed into powder. The ground material powder particles continue to move outward, and finally throw to the rim of the mill; Depending on the negative pressure formed by the main exhaust fan, the slightly hot desiccant gas enters the mill from the vertical mill inlet and passes through the outer edge of the mill disk from bottom to top. The material powder particles are carried up by the slightly hot desiccant gas, and the heavy coarse material powder particles in the separator collide with the blade baffle of the separator and return to the grinding disk for further grinding. The light fine material powder particles are collected through the separator by the powder collector; The separator device at the upper part of the vertical mill sieves the ground material, and the material powder with unqualified fineness is separated from the swirl along the inner wall of the inner cone and returned to the mill plate for further grinding. The particle size of the material after grinding is below 100 mesh to enter the next process.

6.Acidification: crushed to less than 100 mesh β type spodumene and 98% concentrated sulfuric acid in the mixing machine mixing acidification (ratio of 2.5-3.5: 1) The mixture is transported to 2 acidizing kilns by feeding screw conveyor. The acidizing kilns are heated by hot blast furnace burning coke oven gas, and heated by external indirect heating method. The reaction temperature is controlled to be 250~300℃, and the duration is about 10min. Li2O•Al2O3•4SiO2 +H2SO4→Li2SO4 + H2O•Al2O3•4SiO2 

During the salinization reaction of the material, there is a resin impurity removal between the aluminosilicate core of spodumene and sulfuric acid, and the lithium in spodumene is replaced by hydrogen as a result of the exchange, but the mineral structure is not actually damaged. The acidified material completed by the salinization reaction is sent to the cooling kiln by the screw conveyor, which is cooled to below 80℃ by indirect water cooling, and then enters the subsequent leaching process. Acid roasting device design is divided into 4 parts: raw material unit, acid roasting unit, cooling storage unit, heating unit and so on. 

(1) Raw material unit ① fine calcined spodumene charging, weighing and measuring process. The β-spodumene in the fine roasting bin enters the unit, is weighed and measured by the closed weighing feeder, and is fed into the double helix mixer. ② concentrated sulfuric acid transport, measurement process. The concentrated sulfuric acid is piped into the metering tank from the sulfuric acid tank of the factory, and then pumped to the double helix mixer through the sulfuric acid feeding pump.

(2) Acidizing and roasting unit 

①β spodumene fine powder (100 mesh) is exported from the bottom of the fine powder silo and flows into the measuring scale. 

② The measurement generally uses spiral scale, rotor scale, Coriolis scale, etc. This project uses Coriolis scale. 

③ The outlet of the measuring scale is connected with the screw conveyor, and the speed of the screw conveyor is controlled by the reading of the Coriolis scale, so as to control the amount of spodumene powder. 

④ The screw conveyor outlet is connected with the mixer. 

⑤The mixer generally uses a single screw mixer, screw belt mixer, double screw mixer. 

⑥Double screw mixer is used in this project. 98% sulfuric acid from the storage tank through the fluorine lined magnetic pump, into the mixer.

⑦Set the flow meter and regulating valve on the sulfuric acid pipeline. 

⑧ The mass flow of sulfuric acid is controlled by the mass flow of spodumene. 

⑨ Sulfuric acid is sprayed into the mixer through the multi-point nozzle, and the spodumene fine powder is mechanically mixed, the mixture is called mixed acid. 

⑩ The acid mixture is then fed into the acidizing kiln through the kiln inlet screw and the kiln tail cover.

(3) Acidified clinker cooling storage unit the acidified material from the furnace tail is spirally sent to the cooling kiln to be cooled below 80 ° C. The cooling kiln adopts the form of indirect cooling by chilled water, and the cooled acidified clinker is sent to the wet process section of the plant. 

(4) Heating unit The heating system of this project adopts the form of hot air circulation system supplied by coke oven gas hot blast stove (hot air circulation). The coke oven gas sent by the pipe and the preheated air sent by the air fan are burned in the hot blast furnace, and the hot gas generated by the combustion is mixed with the circulating hot gas sent by the high temperature circulating fan at the end of the hot blast furnace. The hot air pipe is controlled by the valve and enters the jacket of the acidizing kiln to heat the furnace body. Most of the hot flue gas circulates in the heating system, and some of it is drained by the chimney after preheating the air. Each air duct in the system is equipped with adjusting doors, which can realize the different requirements of heating temperature in each section of the furnace body. The flue is provided with a regulating valve to adjust the balance of the system wind pressure. Operating conditions and control parameters: the control reaction temperature is 250℃~300℃, the dosage of sulfuric acid is determined according to the amount of active component Li2O in the ore, and the free acid content in the mixed acid is maintained to be 20-22%, thus determining the ratio of material to acid is about 2.5-3.5, and the residence time of the material in the baking furnace is about 30min. The content of free acid in acid clinker is 2-6%, under normal pressure.

7. Water immersion: the acidified and cooled spodumene acid clinker contains about 2~6% sulfuric acid, which is sent to the beater tank by the buried scraper conveyor through the acid-cooked silo, and the effluent and condensate water are recycled with lithium slag liquid and resin according to about 1.6: 1 About the liquid solid ratio into acidic acid clinker slurry (acid clinker cooling temperature of about 40℃, through the feeding channel into the closed leaching tank below the liquid level), then into the neutralization tank, and then add limestone, quick lime to neutralize the residual sulfuric acid. Limestone and quicklime are fed by the feeder through the buried scraper conveyor into the neutralization tank, and the pH meter in the neutralization tank controls the speed of the feeder, and then controls the amount of limestone and quicklime added. When the pH reaches 6~6.5, the feeding is stopped, and the neutralization reaction lasts for 20 minutes. After the completion of leaching, lithium enters the liquid phase (neutral slurry) from the solid phase, and the leaching rate of lithium reaches about 98%. The neutralized slurry is fed to the filter press by the slurry pump for subsequent operations. Reaction equation: CaCO3 +H2SO4→CaSO4 + CO2↑+H2O CaO+H2SO4→CaSO4 +H2O Operating conditions and control parameters: leaching temperature 40℃, liquid-solid ratio 1.6:1, leaching time 2~3h, atmospheric pressure.

8. Solid-liquid separation: After the neutral slurry is pumped into the filter press, the wet lithium slag, lithium sulfate filtrate and lithium slag lotion are obtained through filtration, washing, pressing, blowing, unloading and other processes.

Filtration: The neutral slurry is pumped into the filter press for solid-liquid separation. When the flow rate of the slurry pump decays to about 5m3/h, the filter press stops feeding. The filtrate out is mainly lithium sulfate filtrate, which is temporarily stored in the filtrate tank and then enters the impurity removal process. 

Washing: The lithium content in the filter residue obtained from the separation of solid waste is still high, and the lithium sulfate solution in the lithium residue is washed online through the condensate water to wash out as much as possible. The leaching solution is lithium residue lotion, which is returned to the leaching process for acid clinker mixing. 

Press: The lithium residue after washing is pressed, and the filtrate is pressed out and returned to the leaching process for acid clinker mixing. Drain: The lithium residue after the press is finished is drained by 0.7MPa wet air, then drained and returned to the leaching process for acid clinker mixing. The moisture content of the drained lithium residue is less than 20%. 

Slag discharge: The leaching slag completed by press filtration is treated as solid waste by the belt conveyor.