Tunnel type lithium carbonate calcination kiln and calcination staged waste heat recovery method

Abstract

This invention relates to the field of lithium salt roasting equipment and waste heat recovery technology, specifically disclosing a tunnel-type lithium carbonate roasting kiln and a method for staged waste heat recovery during roasting. The method includes: diagnosing the optimized state of the hot air injection ratio by analyzing the temperature difference between the brick surfaces at the head and middle of the preheating section; secondly, independently adjusting each flue gas collection hood in the roasting section based on temperature and oxygen content; then, calculating the heat capacity flow ratio of the high-temperature and medium-temperature hot air streams and performing two-stage progressive adjustment; next, detecting the temperature deviation of the brick core at the preheating section outlet to verify the preheating effect; and finally, monitoring the average temperature deviation of the roasting section and outputting a compensation indicator to form a large closed-loop feedforward compensation. This invention converts mass flow rate into heat capacity flow rate based on actual heat, introduces specific heat capacity correction for flue gas components, and balances response speed and adjustment accuracy, effectively improving lithium conversion rate and reducing energy consumption.

Description

Technical Field

[0001] This invention relates to the field of lithium salt roasting equipment and waste heat recovery technology, specifically to a tunnel-type lithium carbonate roasting kiln and a method for recovering waste heat from roasting stages. Background Technology

[0002] The tunnel-type lithium carbonate roasting kiln is the core thermal equipment for extracting lithium carbonate from lepidolite or lithium porcelain stone. Its working process typically includes a preheating section, a roasting section, and a cooling section. The preheating section utilizes the waste heat from the high-temperature flue gas generated in the roasting section and the medium- and low-temperature waste heat recovered in the cooling section to preheat the lithium ore bricks. The roasting section directly supplies heat to cause a solid-phase reaction in the lithium ore to generate soluble lithium salts.

[0003] Existing waste heat recovery systems generally suffer from the following problems: First, the hot air injection ratio in the preheating section often relies on empirical settings, lacking diagnosis and closed-loop adjustment based on the actual temperature distribution of the bricks. This easily leads to deviations in the surface temperature curve of the bricks from the ideal shape, resulting in uneven preheating or thermal shock. Second, the opening control of each flue gas collection hood in the calcination section is crude, usually uniformly adjusted or fixed, failing to identify and eliminate low-temperature ineffective gases and oxygen-rich dilution gases, resulting in a decrease in the quality of the recovered hot air and affecting the subsequent preheating effect. Third, traditional methods adjust the hot air ratio based only on a single parameter such as mass flow rate or temperature, without considering the impact of temperature changes on the actual heat carried by the hot air, and neglecting the difference in specific heat capacity between high-temperature flue gas and air, causing a deviation between heat distribution and actual demand. Fourth, there is a lack of a complete hierarchical linkage control mechanism from preheating diagnosis and calcination regulation to waste heat recovery. The preheating section, calcination section, and waste heat recovery system are independent of each other, easily leading to problems such as excessive extraction of waste heat causing temperature loss in the calcination section, or insufficient preheating affecting lithium conversion rate.

[0004] Therefore, the present invention provides a tunnel-type lithium carbonate roasting kiln and a method for recovering waste heat from roasting stage. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies and solve at least one of the technical problems mentioned in the background art, this invention provides a tunnel-type lithium carbonate roasting kiln and a method for recovering waste heat in roasting stages. Addressing the problem of insufficient optimization of hot air injection ratio, this invention achieves accurate diagnosis of hot air ratio, independent sequential adjustment of flue gas hood, two-stage progressive air distribution based on heat capacity flow rate, terminal verification of preheating effect, and temperature compensation of roasting section through collaborative control, forming a complete closed-loop control system.

[0006] The objective of this invention can be achieved through the following technical solution: a tunnel-type lithium carbonate roasting kiln and a method for recovering waste heat from roasting and grading, comprising the following steps: The average temperature of the bricks at the head and middle of the preheating section is collected, the actual temperature difference is calculated, and the consistency analysis of the core temperature of the bricks at the outlet of the preheating section is performed to determine the preheating gradient coefficient. By comparing the actual temperature difference with the preheating gradient coefficient, it is determined whether there is any deficiency in the optimization of the hot air injection ratio. If present, based on flue gas temperature and oxygen content, each flue gas collection hood is sequentially adjusted along the flue gas flow direction, and the opening value of each hood is output. A flue gas specific heat capacity lookup table is constructed. The mass flow rate and temperature of the high-temperature and medium-temperature hot air are collected respectively. The ratio of high-temperature and medium-temperature heat capacity flow rate is calculated based on the flue gas specific heat capacity lookup table. Combined with the deviation between the actual temperature difference and the preheating gradient coefficient, a two-stage progressive adjustment is performed, and the hot air injection ratio distribution adjustment value is output. By detecting the core temperature of the brick at the outlet of the preheating section and comparing it with the target core temperature, the convergence value of the core temperature deviation is obtained. The temperature of multiple points in the roasting section is collected and the average temperature is calculated. Through comparative analysis, the waste heat extraction suppression trigger value or enhancement trigger value is output as a temperature offset compensation indicator, which is used to adjust the opening of the flue gas collection hood and the opening of the hot air flow regulating valve.

[0007] Furthermore, the process for determining the preheating gradient coefficient is as follows: The average head temperature of lithium ore bricks at the same transverse position on multiple continuous kiln cars in the preheating section and the average middle brick temperature of the same group of kiln cars in the middle of the preheating section were collected, and the actual temperature difference was obtained by calculating the difference. Set multiple sets of different high-temperature hot air and medium-temperature hot air injection ratios, and calculate the corresponding actual temperature difference values ​​under each ratio. Simultaneously, the core temperature of bricks from multiple kiln cars at the preheating section outlet is measured and the standard deviation of the core temperature is calculated. The actual temperature difference corresponding to the minimum standard deviation of the core temperature is taken as the preheating gradient coefficient.

[0008] Furthermore, the method for determining whether the hot air injection ratio has insufficient optimization is as follows: Calculate the absolute value of the difference between the actual temperature difference and the preheating gradient coefficient. If the absolute value of the difference is greater than the proportional deviation threshold, it is determined that the hot air injection ratio is not optimized enough.

[0009] Furthermore, the process of sequentially adjusting each flue gas collection hood is as follows: Adjust the flue gas flow direction sequentially from the inlet to the outlet of the roasting section, set the initial opening and adjustment cycle, compare the flue gas temperature at the hood with the lower limit temperature of the high-temperature flue gas, and if the flue gas temperature at the hood is lower than the lower limit temperature of the high-temperature flue gas, then set the opening to zero. If the flue gas temperature in the hood is greater than or equal to the lower limit temperature of high-temperature flue gas, the flue gas oxygen content is further compared with the upper limit of flue gas oxygen content. Based on the comparison result, the opening is increased or decreased in a step-by-step manner. After adjusting each hood, wait for a predetermined time before adjusting the next hood. After each round of adjustment, the average opening of all cover positions is calculated, and in the next round, cover positions whose opening deviation exceeds the preset deviation threshold are adjusted first.

[0010] Furthermore, the process of constructing the flue gas specific heat capacity lookup table is as follows: Based on the volume fraction of typical flue gas components, multiple temperature points are set within a temperature range to obtain the isobaric specific heat capacity of each pure gas at each temperature point. The specific heat capacity of the mixed flue gas is calculated by weighting according to the volume fraction, and the values ​​between adjacent temperature points are preprocessed by linear interpolation to obtain a flue gas specific heat capacity lookup table.

[0011] Furthermore, the calculation method for the ratio of high-temperature to medium-temperature heat capacity flow rate is as follows: Substitute the high-temperature hot air temperature value into the flue gas specific heat capacity lookup table to obtain the high-temperature hot air specific heat capacity value. Multiply the high-temperature hot air mass flow rate value with the high-temperature hot air specific heat capacity value to obtain the high-temperature heat capacity flow rate value. Similarly, the temperature of the medium-temperature hot air is collected to calculate the specific heat capacity of the medium-temperature hot air, and then multiplied by the mass flow rate of the medium-temperature hot air to obtain the medium-temperature heat capacity flow rate. The ratio of high-temperature heat capacity flow rate to medium-temperature heat capacity flow rate is calculated. If the medium-temperature heat capacity flow rate is zero, the ratio is directly set to the preset maximum value.

[0012] Furthermore, the two-stage progressive adjustment process is as follows: Set the preheating gradient coefficient and the adjustment dead zone value. The first-stage adjustment uses the comparison between the actual temperature difference value and the preheating gradient coefficient as feedback. Calculate the absolute value of the difference between the actual temperature difference and the preset gradient coefficient. By comparing and analyzing the absolute value of the difference with the dead zone value and the actual temperature difference with the preset gradient coefficient, coarse adjustment step values ​​are made for the opening of the high-temperature side regulating valve and the opening of the medium-temperature side regulating valve.

[0013] Furthermore, the two-stage progressive adjustment process is also as follows: After completing the first stage of adjustment, the opening of the high-temperature side regulating valve and the opening of the medium-temperature side regulating valve are finely adjusted by comparing and analyzing the ratio of high-temperature and medium-temperature heat capacity flow rate with the lower limit and upper limit of heat capacity ratio.

[0014] Furthermore, the calculation method for the core temperature deviation convergence state value is as follows: The core temperature value of the brick is read at the end of the preheating section, and the core temperature deviation value is calculated by the difference between the core temperature value and the target core temperature value. Set a core deviation dead zone value. If the core temperature deviation value is greater than the core deviation dead zone value, the core temperature deviation convergence state value is overheating. If the core temperature deviation value is less than a negative core deviation dead zone value, the core temperature deviation convergence state value is underheating. Otherwise, the core temperature deviation convergence state value is normal.

[0015] Furthermore, the process for determining the temperature offset compensation indication is as follows: Collect temperature values ​​from multiple points along the length of the kiln within the roasting section and calculate the average temperature; then retrieve the lower and upper limits of the roasting section temperature. If the average temperature is lower than the lower limit temperature of the calcination section, the output suppression trigger value is true and the enhancement trigger value is false. If the average temperature is higher than the upper limit temperature of the calcination section, the output enhancement trigger value is true and the suppression trigger value is false. Conversely, both the suppression trigger value and the enhancement trigger value are false. When the suppression trigger value is true, the opening of all flue gas collection hoods and the opening of the high-temperature and medium-temperature flow regulating valves are reduced by a predetermined ratio. When the enhancement trigger value is true, they are increased by the same ratio to obtain the temperature offset compensation indication.

[0016] The beneficial effects of this invention are as follows: By quantitatively comparing the temperature difference between the head and middle brick surfaces of the preheating section, the optimization deficiencies of the hot air injection ratio are identified based on the preheating gradient coefficient, thus avoiding frequent and meaningless adjustments and maintaining system stability. Independent closed-loop regulation based on temperature and oxygen content is implemented for each flue gas collection hood in the roasting section. Low-temperature ineffective gas and oxygen-rich dilution gas are preferentially excluded to ensure that only high-grade flue gas is recovered, thus stabilizing the hot air quality at the inlet of the high-temperature heat medium tank. By converting mass flow rate into heat capacity flow rate based on actual heat and introducing a specific heat capacity correction table based on typical flue gas composition, the accuracy of heat calculation on the high-temperature flue gas side is significantly improved, providing a reliable reference for heat distribution. The two-stage progressive adjustment uses temperature difference feedback for coarse adjustment, which makes the temperature gradient on the brick surface quickly approach the target value; the second stage uses the heat capacity flow rate ratio for verification and fine adjustment, ensuring that the actual heat distribution conforms to thermodynamic constraints and effectively avoiding deviations in the ratio caused by hot air temperature fluctuations. By monitoring the average temperature of the roasting section and outputting compensation indicators, the interference of waste heat extraction on roasting temperature control is fed back to the flue gas hood adjustment and hot air ratio adjustment links, preventing excessive extraction by the waste heat recovery system from causing the roasting section to lose temperature or overheat, and realizing the synergistic optimization of energy cascade utilization and roasting process. The core temperature of the brick at the outlet of the preheating section is used as the final verification indicator of the preheating effect. Combined with the temperature protection of the roasting section, the start and stop of the regulation cycle are determined to ensure that the lithium ore brick reaches a uniform and sufficient thermal preparation state before entering the roasting section, thereby stabilizing and improving the lithium conversion rate. Attached Figure Description

[0017] The invention will now be further described with reference to the accompanying drawings.

[0018] Figure 1 This is a flowchart of a tunnel-type lithium carbonate roasting kiln and a method for recovering waste heat from roasting and grading, according to the present invention. Figure 2 This is a logic diagram of a tunnel-type lithium carbonate roasting kiln and a method for recovering waste heat from roasting stages in this invention. Detailed Implementation

[0019] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments.

[0020] Example: Please refer to Figure 1 As shown, the tunnel-type lithium carbonate roasting kiln and the method for recovering waste heat from roasting and grading according to the present invention specifically include the following steps: Step 1: Collect the average temperature of the bricks at the head and middle of the preheating section, calculate the actual temperature difference, and conduct a consistency analysis of the core temperature of the bricks at the preheating section outlet to determine the preheating gradient coefficient. By comparing and analyzing the actual temperature difference with the preheating gradient coefficient, determine whether there is any insufficient optimization in the hot air injection ratio. It should be noted that: In a tunnel-type lithium carbonate roasting kiln, the preheating section and the roasting section are two functional zones arranged sequentially along the kiln car's travel direction. Together, they complete the heating and roasting process of the lithium ore bricks. Specifically: The preheating section is located in the entrance area of ​​the tunnel kiln, immediately after the feeding end. The main function of the preheating section is to use the waste heat from the high-temperature flue gas in the calcination section and the medium- and low-temperature waste heat recovered in the cooling section to gradually preheat the lithium ore bricks loaded on the kiln car. During this stage, the temperature difference between the surface and core of the brick gradually decreases, and moisture and some volatile components are removed. The brick is then thermally prepared to enter the calcination section. The heat source of the preheating section mainly comes from the high-temperature hot air and medium-temperature hot air returned by the waste heat recovery system, rather than direct combustion. The roasting section, located after the preheating section, is the core reaction area of ​​the tunnel kiln. The roasting section raises the temperature inside the kiln to a high-temperature range through direct heating by burners or electric heating. In this high-temperature range, lepidolite or lithium ceramic stone in the lithium ore undergoes a solid-phase reaction, and lithium is converted into soluble lithium salt (the precursor of lithium carbonate), thus completing the core chemical reaction of pyrometallurgical roasting. A flue gas collection hood is installed at the top of the roasting section to extract high-temperature flue gas and send it to the waste heat recovery system. The roasting section requires precise temperature control. Too low a temperature will lead to a decrease in lithium conversion rate, while too high a temperature will cause over-firing of the bricks or waste of energy. The preheating section is the preparatory area before the calcination section. Its function is to steadily raise the temperature of the bricks to near the reaction temperature, so as to prevent the bricks from directly entering the high-temperature calcination section and causing cracks or uneven heating due to thermal shock. The high-temperature flue gas generated in the calcination section is drawn to the heat exchanger, and the heated hot air is returned to the preheating section for use, forming a cascade utilization of energy. The temperature of the two sections is controlled in a coordinated manner, which directly affects the lithium conversion rate and product quality of the final clinker. In step one, the calculation process for the actual temperature difference includes: High-temperature hot air refers to hot air that originates from flue gas in the roasting section, is heated by a heat exchanger, and is then reused in the preheating section. The temperature range is 600℃-900℃. Medium-temperature hot air refers to hot air that originates from air in the cooling section, is then recovered after heat exchange. The temperature range is 200℃~400℃. Three infrared temperature sensors are installed in the head area of ​​the preheating section, corresponding to the lithium ore brick surface at the same transverse position on the three continuous kiln cars. The temperature value is collected every 30 seconds to obtain the surface temperature value of the three head bricks. Three infrared temperature sensors were also installed in the middle area of ​​the preheating section to collect the surface temperature of the bricks in the middle of the same group of kiln cars. Calculate the average of the surface temperatures of the three head bricks to obtain the average head brick temperature. Calculate the average of the three middle brick temperatures to obtain the average middle brick temperature. Subtract the average head brick temperature from the average middle brick temperature to obtain the actual temperature difference. In step one, the calculation process of the preheating gradient coefficient includes: The preheating gradient coefficient is determined as follows: During the stable operation of the kiln, the hot air injection ratio is set to several different values, the standard deviation of the core temperature of the brick at the outlet of the preheating section is measured, and the actual temperature difference value corresponding to the minimum core temperature standard deviation is taken as the preheating gradient coefficient. Among them, stability is defined as the average temperature fluctuation of the roasting section not exceeding ±10℃ and the kiln car travel speed fluctuation not exceeding ±5%, and lasting for at least 1 hour; For example, at least 5 different ratios of high-temperature hot air to medium-temperature hot air injection are selected, such as: |Test No.|High Temperature Hot Air Ratio|Medium Temperature Hot Air Ratio|; |1|30|70|; |2|40|60|; |3|50|50|; |4|60|40|; |5|70|30|; Perform the following sub-steps for each group of proportions: The control system sets the opening of the high-temperature side regulating valve and the opening of the medium-temperature side regulating valve to the target ratio and maintains stable operation for at least 30 minutes to fully establish the temperature field of the preheating section. Read the values ​​of the three infrared temperature sensors at the head of the preheating section and calculate the average temperature of the bricks at the head. Read the values ​​of the three infrared temperature sensors in the middle of the preheating section and calculate the average temperature of the bricks in the middle. Calculate the ratio between the average temperature of the bricks at the head and the average temperature of the bricks in the middle to obtain the actual temperature difference under each ratio. At the end of the preheating section near the entrance of the firing section, select 10 kiln cars (or more, to ensure statistical validity) that are running continuously. Each kiln car has a sheathed thermocouple pre-embedded in the brick body on top. When the kiln car passes the fixed detection position, the core temperature of the bricks in each kiln car is read in sequence. Calculate the average and standard deviation of the core temperature of this group. After completing all test groups, compare the standard deviation of the core temperature of each group and find the test group with the smallest standard deviation of core temperature. The corresponding actual temperature difference value is the preheating gradient coefficient. This group represents the most uniform core temperature of the brick at the outlet of the preheating section with the smallest fluctuation. It is understandable that the physical meaning of the preheating gradient coefficient is: when the surface temperature difference between the head and middle of the preheating section is at a certain value (i.e., the preheating gradient coefficient), the preheating section heats the brick most evenly and the temperature dispersion of the brick core is minimal, thus providing the best thermal preparation state for the calcination section. Too large or too small a temperature difference will lead to poor core temperature consistency and affect the final lithium conversion rate. In step one, the process of determining whether there are any optimization deficiencies in the hot air injection ratio includes: The actual temperature difference is compared with the preheating gradient coefficient, and the absolute value of the difference is calculated. A proportional deviation threshold is set, which is twice the zero value of the preheating gradient coefficient. If the absolute value of the difference is greater than the proportional deviation threshold, it is determined that the hot air injection ratio optimization is insufficient; otherwise, it is determined that the optimization is normal. It should be noted that the purpose of judging whether there is insufficient optimization in the hot air injection ratio in the preheating section is as follows: as a pre-diagnostic step in the entire control process, it identifies whether the distribution of hot air in the head and middle of the preheating section causes the preheating temperature curve of the lithium ore brick to deviate from the ideal shape. By comparing the deviation between the actual temperature difference value and the preheating gradient coefficient, it determines whether to trigger subsequent adjustment actions, avoids unnecessary control intervention, and maintains system stability. Step 2: If present, based on the flue gas temperature and oxygen content, sequentially adjust each flue gas collection hood along the flue gas flow direction and output the opening value of each hood position. In step two, the process of sequentially adjusting each flue gas collection hood along the flue gas flow direction includes: Adjust the flue gas flow direction sequentially from the inlet to the outlet of the roasting section. The initial opening of each flue gas collection hood is set to 50%, and the adjustment cycle of each hood position is set to 90 seconds. Only one hood position is allowed to be in adjustment state at the same time. An adjustment is performed starting from the first hood position near the inlet of the roasting section: the flue gas temperature value is compared with the lower limit temperature of high-temperature flue gas, and if it is lower than the lower limit temperature of high-temperature flue gas, the opening is set to zero; The method for determining the lower limit temperature of high-temperature flue gas is as follows: Select a representative hood position in the roasting section, gradually reduce the flue gas temperature, measure the hot air temperature at the heat exchanger outlet, and when the hot air temperature is lower than 250 degrees Celsius, take the flue gas temperature value at this hood position as the lower limit temperature of high-temperature flue gas. This value is usually between 750 and 850 degrees Celsius and is fixed after on-site calibration. If the temperature is greater than or equal to the lower limit of the high-temperature flue gas temperature, then the flue gas oxygen content value is further compared with the upper limit of the flue gas oxygen content. If the oxygen content is greater than the upper limit of the flue gas oxygen content, the opening is reduced by one step value (ten percent of the fully open range of the valve). If the oxygen content is less than or equal to the upper limit of the flue gas oxygen content, the opening is increased by one step value. The method for determining the upper limit of flue gas oxygen content is as follows: at the same hood position, by changing the opening of the air inlet to introduce different amounts of air, measure the decrease in flue gas temperature and the heat exchanger outlet hot air temperature rise efficiency. When the oxygen content exceeds a certain value, the hot air temperature rise efficiency drops to less than 85% of the initial value. Take this oxygen content value as the upper limit, which is usually 5% to 7%. After adjustment, wait 60 seconds to allow its effect on adjacent positions to be fully transmitted and stabilized, then move to the second position for adjustment, and so on until the last position. After completing one round, start the next round of adjustment from the first cover position. After each round, calculate the average opening value of all cover positions and the deviation between the opening value of each cover position and the average opening value. Obtain the opening deviation of each cover position. Cover positions with opening deviations exceeding 15% are given priority for adjustment in the next round. Output the opening values ​​of each cover position in sequence. It should be noted that the function of independently controlling the opening of each flue gas collection hood in the roasting section is to: independently adjust each flue gas collection hood in a closed loop to ensure that only high-grade flue gas is recovered, while controlling excessive air mixing; by comparing two independent conditions, temperature and oxygen content, low-temperature ineffective gas and oxygen-rich dilution gas are treated respectively, stabilizing the hot air quality at the inlet of the high-temperature heat medium tank, and providing reliable input for subsequent heat capacity flow calculation. Step 3: Construct a flue gas specific heat capacity lookup table, collect the mass flow rate and temperature of the high-temperature and medium-temperature hot air respectively, calculate the high-temperature and medium-temperature heat capacity flow rate ratio based on the flue gas specific heat capacity lookup table, and combine the deviation between the actual temperature difference and the preheating gradient coefficient to perform two-stage progressive adjustment and output the hot air injection ratio distribution adjustment value. In step three, the process of constructing the flue gas specific heat capacity lookup table includes: A specific heat capacity lookup table is pre-established based on typical flue gas components (75% nitrogen, 8% carbon dioxide, 7% water vapor, and 10% oxygen) in the standard air specific heat capacity lookup table, specifically as follows: Determine the volume fraction baseline for the flue gas components. Take nitrogen as 75%, carbon dioxide as 8%, water vapor as 7%, and oxygen as 10%, with the sum of these four components being 100%. Determine the temperature range and interval of the specific heat capacity table, set the lower limit of the temperature to fifty degrees Celsius and the upper limit to one thousand degrees Celsius, and set a temperature point every ten degrees Celsius, for a total of ninety-six temperature points; Obtain the isobaric specific heat capacity values ​​of four pure gases at various temperature points; Calculate the specific heat capacity of the mixed flue gas at each temperature point: multiply the specific heat capacity of nitrogen at that temperature point by the nitrogen gas integral of 0.75 to obtain the nitrogen contribution value; multiply the specific heat capacity of carbon dioxide at that temperature point by the carbon dioxide volume fraction of 0.08 to obtain the carbon dioxide contribution value; multiply the specific heat capacity of water vapor at that temperature point by the water vapor volume fraction of 0.07 to obtain the water vapor contribution value; multiply the specific heat capacity of oxygen at that temperature point by the oxygen volume fraction of 0.10 to obtain the oxygen contribution value; add the four contribution values ​​together to obtain the specific heat capacity of the mixed flue gas at that temperature point. Record each temperature point and its corresponding specific heat capacity of the mixed flue gas in tabular form to form the original specific heat capacity table. Linear interpolation preprocessing is performed on the data in the original specific heat capacity table. If a temperature value that is not a multiple of 10 degrees Celsius is encountered in actual application between two adjacent temperature points, the corresponding specific heat capacity value is calculated according to the distance ratio between the temperature value and the two endpoint temperature values. The interpolation method is as follows: Suppose the temperature T to be checked is located between T1 and T2, and T1 and T2 are adjacent multiples of 10 degrees Celsius. The specific heat capacity values ​​are C1 and C2, respectively. Then the specific heat capacity value corresponding to the temperature T to be checked is C1 + [(T-T1)×(C2-C1)] / 10. The correspondence between the processed temperature points and specific heat capacity values ​​is stored as a callable two-dimensional array, thus completing the establishment of a specific heat capacity lookup table based on typical flue gas components; In step three, the calculation process for the high-temperature to medium-temperature heat capacity flow rate ratio includes: A mass flow meter and a temperature sensor are installed on the inlet pipe of the high-temperature heat medium tank to collect the mass flow rate and temperature of the high-temperature hot air. The same two parameters, the mass flow rate and temperature of the high-temperature hot air, are also collected on the inlet pipe of the medium-temperature heat medium tank. The specific heat capacity of the high-temperature hot air is obtained by linear interpolation between the high-temperature hot air temperature value and the two closest temperature points in the specific heat capacity lookup table. If the temperature exceeds the lookup range, the specific heat capacity value of the nearest endpoint is used. Multiply the high-temperature hot air mass flow rate by the high-temperature hot air specific heat capacity to obtain the high-temperature heat capacity flow rate. Similarly, the temperature value of the medium-temperature hot air is collected to calculate the specific heat capacity value of the medium-temperature hot air. Since the medium-temperature hot air comes from the air heat exchange in the cooling section and its composition is close to that of air, the mass flow rate value of the medium-temperature hot air is multiplied by this specific heat capacity value to obtain the medium-temperature heat capacity flow rate value. The ratio of high-temperature heat capacity flow rate to medium-temperature heat capacity flow rate is calculated by proportionally comparing the high-temperature heat capacity flow rate value with the medium-temperature heat capacity flow rate value. If the medium-temperature heat capacity flow rate value is zero, the high-temperature heat capacity flow rate ratio is directly set to one hundred to obtain the high-temperature heat capacity flow rate ratio. It is understandable that the physical meaning of the ratio of high temperature to medium temperature heat capacity flow rate is: the ratio of the heat transfer capacity of high temperature flue gas hot air and medium temperature hot air under a unit temperature change per unit time, that is, the ratio of the heat transfer capacity of the two hot air streams. In step three, the two-stage progressive adjustment process includes: The first point to clarify is that the first level of regulation specifically refers to: The first level uses the actual temperature difference between the average temperature of the bricks at the head and the average temperature of the bricks in the middle of the preheating section as the feedback value, and compares it with the target preheating gradient coefficient of the preheating section. When the actual temperature difference is too low, it indicates that the preheating at the head is insufficient or the preheating in the middle is excessive. The temperature difference is widened by increasing the opening of the high temperature side and decreasing the opening of the medium temperature side. Conversely, the adjustment is reversed. After the first-level adjustment is completed, the second-level verification is performed based on the ratio of high-temperature and medium-temperature heat capacity flow rate. If the ratio of high-temperature and medium-temperature heat capacity flow rate exceeds the target range, the opening is finely adjusted. The purpose is to ensure that the actual heat ratio of the two hot air injections is always within a reasonable range, and to avoid the heat capacity flow rate ratio deviating from the preset range due to temperature fluctuations. The final output head and middle hot air injection ratio distribution adjustment value is directly used to drive the flow regulating valve to perform opening changes. Call the average temperature of the head brick, the average temperature of the middle brick, the actual temperature difference, and the preheating gradient coefficient; call the ratio of high temperature to medium temperature heat capacity flow rate; and call the pre-calibrated lower limit and upper limit of heat capacity ratio. The method for determining the lower and upper limits of the heat capacity ratio is as follows: Under the stable operating conditions of the kiln, the ratio of high-temperature and medium-temperature heat capacity flow rate under different hot air injection ratios is measured respectively. At the same time, the heating rate of the bricks at the head and middle of the preheating section is monitored. The range of the ratio corresponding to the ratio of the head heating rate to the middle heating rate being 0.8 to 1.2 times is taken as the target range. The lower limit is usually 1.1 to 1.3, and the upper limit is usually 1.7 to 1.9. Obtain the current opening value of the high-temperature side flow control valve and the opening value of the medium-temperature side flow control valve. Set the initial value of both to 50%. The actual temperature difference is compared with the preheating gradient coefficient. One-tenth of the preheating gradient coefficient is set as the adjustment dead zone value. The absolute value of the difference between the actual temperature difference and the preheating gradient coefficient is calculated. If the absolute value of the difference is less than or equal to the adjustment dead zone value, the first-level adjustment is not performed and the second-level verification is directly entered. If the absolute value of the difference is greater than the dead zone value and the actual temperature difference is less than the preheating gradient coefficient, then the opening of the high-temperature side regulating valve will be increased by one coarse adjustment step value based on the current value, and the opening of the medium-temperature side regulating valve will be decreased by one coarse adjustment step value. The coarse adjustment step value is five percent of the valve's full opening range. If the actual temperature difference is greater than the preheating gradient coefficient, then reverse the operation: decrease the coarse adjustment step value on the high temperature side and increase the coarse adjustment step value on the medium temperature side. Wait ninety seconds after each adjustment to stabilize the temperature field. Secondly, it should be noted that the second-level condition is as follows: After completing the first-level adjustment and waiting for 90 seconds, compare the ratio of high-temperature to medium-temperature heat capacity flow rate with the lower limit and upper limit of the heat capacity ratio. If the ratio of high-temperature to medium-temperature heat capacity flow rate is less than the lower limit of the heat capacity ratio, increase the opening of the high-temperature side regulating valve by one fine adjustment step value based on the current value, and decrease the opening of the medium-temperature side regulating valve by one fine adjustment step value. If the ratio of high-temperature to medium-temperature heat capacity flow rate is greater than the upper limit of the heat capacity ratio, then the opening of the high-temperature side regulating valve will be reduced by one fine adjustment step value, and the opening of the medium-temperature side will be increased by one fine adjustment step value. The fine adjustment step value is two percent of the valve's fully open range. If the ratio of high temperature to medium temperature heat capacity flow rate is between the lower limit and the upper limit of the heat capacity ratio, no fine-tuning will be performed. The final high temperature side opening value and low temperature side opening value are the adjustment values ​​for the hot air injection ratio in the middle of the head, which are directly sent to the two flow regulating valves for execution. It should be noted that the purpose of calculating the heat capacity flow rate ratio of the high-temperature and medium-temperature hot air streams and adjusting the hot air injection ratio in two progressive stages is to convert the mass flow rate into a heat capacity flow rate based on actual heat, thus eliminating the influence of temperature changes on heat measurement. Introducing a specific heat capacity correction for flue gas components improves the accuracy of calculations on the high-temperature flue gas side, outputting a dimensionless ratio that directly reflects the multiple relationship of heat carried by the two hot air streams. This provides a precise thermal reference for the second-stage verification. The first stage uses temperature difference feedback for coarse adjustment, allowing the temperature gradient on the brick surface to quickly approach the target value. The second stage uses the heat capacity flow rate ratio for verification and fine-tuning, ensuring that the actual heat distribution conforms to thermodynamic constraints and avoiding deviations in the heat ratio due to hot air temperature fluctuations. The two-stage series connection balances response speed and adjustment accuracy. Step 4: By detecting the core temperature of the brick at the preheating section outlet and comparing it with the target core temperature, the convergence value of the core temperature deviation is obtained. In step four, the process of determining the convergence state value of the core temperature deviation includes: At the end of the preheating section near the entrance of the roasting section, a sheathed thermocouple is pre-embedded in the lithium ore brick on the top of each kiln car, with the thermocouple inserted to a depth of half the thickness of the brick. When the kiln car moves to the fixed detection position, the temperature value of the brick core measured by the thermocouple is read through the sliding contact line; The target core temperature value is called. The method for determining the target core temperature value is as follows: at the outlet of the preheating section, the lithium conversion rate of the subsequent roasting section corresponding to different core temperatures is measured, and the average temperature at which the lithium conversion rate reaches its peak is taken, which is usually 480 to 520 degrees Celsius. Call the core deviation dead zone value, which is one percent of the target core temperature value, usually plus or minus five degrees Celsius; Subtract the target core temperature value from the core temperature value of the brick to obtain the core temperature deviation value. If the core temperature deviation value is greater than the core deviation dead zone value, the core temperature deviation convergence state value at the preheating section outlet is set to overheating; if it is less than the negative core deviation dead zone value, it is set to underheating; otherwise, it is set to normal. It should be noted that the purpose of judging the temperature deviation of the brick core at the preheating section outlet is to output the final verification index of the preheating effect, directly assess whether the brick core temperature has reached the target. The core temperature is more representative of the overall preheating state of the brick than the surface temperature, and is used to judge whether the adjustment has made the preheating sufficient and uniform, and to decide whether the adjustment cycle needs to be continued. Step 5: Collect the temperature at multiple points in the roasting section and calculate the average temperature. Through comparison and analysis, output the waste heat extraction suppression trigger value or enhancement trigger value as a temperature offset compensation indicator, which is used to adjust the opening of the flue gas collection hood and the opening of the hot air flow regulating valve. In step five, the output process of the temperature offset compensation indicator includes: A thermocouple is installed every two meters along the length of the kiln in the roasting section, for a total of ten thermocouples. Temperature values ​​are collected at each point, and the sum of all temperature values ​​is divided by ten to obtain the average temperature value of the roasting section. Call the lower limit temperature and upper limit temperature of the roasting section. The method for determining the lower limit temperature of the roasting section is as follows: gradually reduce the temperature of the roasting section and measure the lithium conversion rate of the clinker. When the lithium conversion rate drops to 90% of the normal value, take that temperature value as the lower limit, which is usually 740 to 760 degrees Celsius. The method for determining the upper limit temperature of the calcination section is as follows: gradually increase the temperature, and when the lithium conversion rate no longer increases and obvious melting traces appear on the surface of the brick, take that temperature as the upper limit, which is usually 1,180 to 1,220 degrees Celsius. The average temperature of the roasting section was compared with the lower and upper limits of the temperature: If the average temperature is lower than the lower limit temperature, the waste heat extraction suppression trigger value is set to true, and the waste heat extraction enhancement trigger value is set to false. If the average temperature value is higher than the upper limit temperature, the waste heat extraction enhancement trigger value is set to true and the suppression trigger value is set to false. If the value falls between these two, both trigger values ​​are set to false. Output the temperature offset compensation indicator for the calcination section, which is a two-Boolean value. In step five, the process of adjusting the opening degree of the flue gas collection hood and the hot air flow regulating valve includes: Feedback linkage rules: When the waste heat extraction suppression trigger value is true, the opening of the air intake of all flue gas collection hoods is uniformly reduced by 15% of the current opening value, and the opening of the two flow control valves on the high-temperature side and the medium-temperature side is each reduced by 10% of the current opening value, and the cycle begins. When the waste heat extraction enhancement trigger value is true, the opening of all air intakes is increased by 15% of the current opening value, and the opening of the two flow control valves is increased by 10% of the current opening value. The adjustment range in a single instance shall not exceed 30% of the fully open range of the valves. Cyclic control mechanism: At the end of a complete adjustment cycle, if the preheating section outlet core temperature deviation convergence value is normal and both trigger values ​​are false, the current valve openings remain unchanged, and monitoring continues. If the status value is overheated or underheated, or if any trigger value is true, the cycle is repeated, and the repetition interval is the cycle time. After three consecutive cycles are determined to be normal, adjustment is paused, monitoring continues, and the adjustment cycle is restarted when an abnormality reappears. It should be noted that the function of judging the average temperature deviation of the roasting section and outputting the compensation indication is to: monitor the overall temperature level of the roasting section, determine whether the amount of waste heat extraction interferes with the roasting temperature control, and output the extraction adjustment command by comparing the average temperature with the upper and lower limit protection values, and feed it back to steps two and four to form a large closed-loop feedforward compensation, so as to prevent the waste heat recovery system from excessively extracting heat, which would cause the roasting section to lose temperature or overheat.

[0021] The technical solution and advantages of this application are as follows: The average temperature of the bricks at the head and middle of the preheating section is collected, the actual temperature difference is calculated, and a consistency analysis of the core temperature of the bricks at the preheating section outlet is performed to determine the preheating gradient coefficient. By comparing the actual temperature difference with the preheating gradient coefficient, it is determined whether there are any optimization deficiencies in the hot air injection ratio. If so, based on the flue gas temperature and oxygen content, each flue gas collection hood is sequentially adjusted along the flue gas flow direction, and the opening value of each hood is output. A flue gas specific heat capacity lookup table is constructed, and the mass flow rate and temperature of the high-temperature and medium-temperature hot air streams are collected respectively. Based on the flue gas specific heat capacity lookup table, the ratio of high-temperature to medium-temperature heat capacity flow rate is calculated. Combined with the deviation between the actual temperature difference and the preheating gradient coefficient, a two-stage progressive adjustment is performed, and the hot air injection ratio distribution adjustment value is output. By detecting the core temperature of the brick at the preheating section outlet and comparing it with the target core temperature, the core temperature deviation convergence value is obtained. The temperature of multiple points in the firing section is collected and the average temperature is calculated. Through comparison and analysis, the waste heat extraction suppression trigger value or enhancement trigger value is output as a temperature offset compensation indicator, which is used to adjust the opening of the flue gas collection hood and the opening of the hot air flow regulating valve. This invention diagnoses the optimized state of the hot air injection ratio by analyzing the temperature difference between the brick surfaces at the head and middle of the preheating section. Secondly, it independently adjusts each flue gas collection hood in the calcination section based on temperature and oxygen content. Then, it calculates the heat capacity flow rate ratio of the high-temperature and medium-temperature hot air streams and performs a two-stage progressive adjustment. Next, it detects the temperature deviation of the brick core at the preheating section outlet to verify the preheating effect. Finally, it monitors the average temperature deviation of the calcination section and outputs a compensation indicator, forming a large closed-loop feedforward compensation. This invention converts mass flow rate into heat capacity flow rate based on actual heat, introduces specific heat capacity correction for flue gas components, and balances response speed and adjustment accuracy, effectively improving lithium conversion rate and reducing energy consumption.

[0022] The embodiments of the present invention have been described in detail above, but the content described is only a preferred embodiment of the present invention and should not be considered as limiting the scope of the present invention. All equivalent changes and improvements made in accordance with the scope of the present invention should still fall within the scope of the present invention.

Claims

1. A tunnel-type lithium carbonate roasting kiln and a method for recovering waste heat from roasting and grading, characterized in that: Includes the following steps: The average temperature of the bricks at the head and middle of the preheating section is collected, the actual temperature difference is calculated, and the consistency analysis of the core temperature of the bricks at the outlet of the preheating section is performed to determine the preheating gradient coefficient. By comparing the actual temperature difference with the preheating gradient coefficient, it is determined whether there is any deficiency in the optimization of the hot air injection ratio. If present, based on flue gas temperature and oxygen content, each flue gas collection hood is sequentially adjusted along the flue gas flow direction, and the opening value of each hood is output. A flue gas specific heat capacity lookup table is constructed. The mass flow rate and temperature of the high-temperature and medium-temperature hot air are collected respectively. The ratio of high-temperature and medium-temperature heat capacity flow rate is calculated based on the flue gas specific heat capacity lookup table. Combined with the deviation between the actual temperature difference and the preheating gradient coefficient, a two-stage progressive adjustment is performed, and the hot air injection ratio distribution adjustment value is output. By detecting the core temperature of the brick at the outlet of the preheating section and comparing it with the target core temperature, the convergence value of the core temperature deviation is obtained. The temperature of multiple points in the roasting section is collected and the average temperature is calculated. Through comparative analysis, the waste heat extraction suppression trigger value or enhancement trigger value is output as a temperature offset compensation indicator, which is used to adjust the opening of the flue gas collection hood and the opening of the hot air flow regulating valve.

2. The tunnel-type lithium carbonate roasting kiln and the method for recovering waste heat from roasting stage according to claim 1, characterized in that: The process for determining the preheating gradient coefficient is as follows: The average head temperature of lithium ore bricks at the same transverse position on multiple continuous kiln cars in the preheating section and the average middle brick temperature of the same group of kiln cars in the middle of the preheating section were collected, and the actual temperature difference was obtained by calculating the difference. Set multiple sets of different high-temperature hot air and medium-temperature hot air injection ratios, and calculate the corresponding actual temperature difference values ​​under each ratio. Simultaneously, the core temperature of bricks from multiple kiln cars at the preheating section outlet is measured and the standard deviation of the core temperature is calculated. The actual temperature difference corresponding to the minimum standard deviation of the core temperature is taken as the preheating gradient coefficient.

3. The tunnel-type lithium carbonate roasting kiln and the method for recovering waste heat from roasting stage according to claim 2, characterized in that: The method for determining whether the hot air injection ratio is insufficiently optimized is as follows: Calculate the absolute value of the difference between the actual temperature difference and the preheating gradient coefficient. If the absolute value of the difference is greater than the proportional deviation threshold, it is determined that the hot air injection ratio is not optimized enough.

4. The tunnel-type lithium carbonate roasting kiln and the method for recovering waste heat from roasting and grading as described in claim 1, characterized in that: The process of sequentially adjusting each flue gas collection hood is as follows: Adjust the flue gas flow direction sequentially from the inlet to the outlet of the roasting section, set the initial opening and adjustment cycle, compare the flue gas temperature at the hood with the lower limit temperature of the high-temperature flue gas, and if the flue gas temperature at the hood is lower than the lower limit temperature of the high-temperature flue gas, then set the opening to zero. If the flue gas temperature in the hood is greater than or equal to the lower limit temperature of high-temperature flue gas, the flue gas oxygen content is further compared with the upper limit of flue gas oxygen content. Based on the comparison result, the opening is increased or decreased in a step-by-step manner. After adjusting each hood, wait for a predetermined time before adjusting the next hood. After each round of adjustment, the average opening of all cover positions is calculated, and in the next round, cover positions whose opening deviation exceeds the preset deviation threshold are adjusted first.

5. The tunnel-type lithium carbonate roasting kiln and the method for recovering waste heat from roasting stage according to claim 1, characterized in that: The process of constructing the flue gas specific heat capacity lookup table is as follows: Based on the volume fraction of typical flue gas components, multiple temperature points are set within a temperature range to obtain the isobaric specific heat capacity of each pure gas at each temperature point. The specific heat capacity of the mixed flue gas is calculated by weighting according to the volume fraction, and the values ​​between adjacent temperature points are preprocessed by linear interpolation to obtain a flue gas specific heat capacity lookup table.

6. The tunnel-type lithium carbonate roasting kiln and the method for recovering waste heat from roasting stage according to claim 5, characterized in that: The calculation method for the ratio of high-temperature to medium-temperature heat capacity flow rate is as follows: Substitute the high-temperature hot air temperature value into the flue gas specific heat capacity lookup table to obtain the high-temperature hot air specific heat capacity value. Multiply the high-temperature hot air mass flow rate value with the high-temperature hot air specific heat capacity value to obtain the high-temperature heat capacity flow rate value. Similarly, the temperature of the medium-temperature hot air is collected to calculate the specific heat capacity of the medium-temperature hot air, and then multiplied by the mass flow rate of the medium-temperature hot air to obtain the medium-temperature heat capacity flow rate. The ratio of high-temperature heat capacity flow rate to medium-temperature heat capacity flow rate is calculated. If the medium-temperature heat capacity flow rate is zero, the ratio is directly set to the preset maximum value.

7. The tunnel-type lithium carbonate roasting kiln and the method for recovering waste heat from roasting and grading as described in claim 6, characterized in that: The two-stage progressive adjustment process is as follows: Set the preheating gradient coefficient and the adjustment dead zone value. The first-stage adjustment uses the comparison between the actual temperature difference value and the preheating gradient coefficient as feedback. Calculate the absolute value of the difference between the actual temperature difference and the preset gradient coefficient. By comparing and analyzing the absolute value of the difference with the dead zone value and the actual temperature difference with the preset gradient coefficient, coarse adjustment step values ​​are made for the opening of the high-temperature side regulating valve and the opening of the medium-temperature side regulating valve.

8. The tunnel-type lithium carbonate roasting kiln and the method for recovering waste heat from roasting and grading as described in claim 7, characterized in that: The two-stage progressive adjustment process also includes: After completing the first stage of adjustment, the opening of the high-temperature side regulating valve and the opening of the medium-temperature side regulating valve are finely adjusted by comparing and analyzing the ratio of high-temperature and medium-temperature heat capacity flow rate with the lower limit and upper limit of heat capacity ratio.

9. The tunnel-type lithium carbonate roasting kiln and the method for recovering waste heat from roasting stage according to claim 1, characterized in that: The calculation method for the core temperature deviation convergence state value is as follows: The core temperature value of the brick is read at the end of the preheating section, and the core temperature deviation value is calculated by the difference between the core temperature value and the target core temperature value. Set a core deviation dead zone value. If the core temperature deviation value is greater than the core deviation dead zone value, the core temperature deviation convergence state value is overheating. If the core temperature deviation value is less than a negative core deviation dead zone value, the core temperature deviation convergence state value is underheating. Otherwise, the core temperature deviation convergence state value is normal.

10. The tunnel-type lithium carbonate roasting kiln and the method for recovering waste heat from roasting and grading as described in claim 1, characterized in that: The process for determining the temperature offset compensation indicator is as follows: Collect temperature values ​​from multiple points along the length of the kiln within the roasting section and calculate the average temperature; then retrieve the lower and upper limits of the roasting section temperature. If the average temperature is lower than the lower limit temperature of the calcination section, the output suppression trigger value is true and the enhancement trigger value is false. If the average temperature is higher than the upper limit temperature of the calcination section, the output enhancement trigger value is true and the suppression trigger value is false. Conversely, both the suppression trigger value and the enhancement trigger value are false. When the suppression trigger value is true, the opening of all flue gas collection hoods and the opening of the high-temperature and medium-temperature flow regulating valves are reduced by a predetermined ratio. When the enhancement trigger value is true, they are increased by the same ratio to obtain the temperature offset compensation indication.

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