A safety control system and method for evaporation crystallization of nickel sulfate solution

Abstract

The application discloses a safety control system and method for nickel sulfate solution evaporation crystallization, relates to the technical field of safety control of wet metallurgical evaporation crystallization, and solves the problems that existing devices rely on single-point measurement and fixed threshold interlocking, protection hysteresis and false interlocking coexist, and heat exchange surface fouling is difficult to be identified early. The system comprises an MVR evaporation unit, a DTB crystallization unit, an emergency tank, detection instruments and a DCS control system. The DCS control system cross-checks the redundant liquid levels of the evaporation separator, calculates the liquid level change rate and the predicted touch limit time based on a sliding window, realizes the hierarchical protection of pre-regulation and delay confirmation hard interlocking, calculates the fouling index according to the apparent heat transfer coefficient and implements hierarchical disposal, implements multi-temperature point interlocking shutdown and outlet spray temperature reduction on the steam compressor, and transfers the crystal slurry to the emergency tank in an emergency according to a time sequence, thereby improving the operation safety and continuity of the device.

Description

Technical Field

[0001] This invention relates to the field of safety control technology for hydrometallurgical evaporation crystallization, and in particular to a safety control system and method for evaporation crystallization of nickel sulfate solution. Background Technology

[0002] Nickel sulfate is one of the core raw materials for lithium-ion battery cathode materials. With the continuous expansion of the new energy vehicle industry, the demand for battery-grade nickel sulfate is growing rapidly, placing higher demands on the production capacity, energy consumption, and product quality of production equipment. In the nickel sulfate production process, evaporation crystallization is a key step in concentrating the purified nickel sulfate solution and precipitating nickel sulfate hexahydrate crystals. Early equipment mostly adopted multi-effect evaporation crystallization technology, relying on multiple evaporators connected in series to utilize secondary steam, which was energy-intensive and required a large footprint. In recent years, mechanical vapor recompression evaporation technology has gradually become the mainstream. It uses a steam compressor to compress and heat the secondary steam generated by evaporation and then reuse it in the evaporation heater, which can significantly reduce the consumption of live steam. When used in conjunction with a baffle crystallizer, it can also effectively control the crystal particle size. In the industry, these two technologies are usually referred to as MVR evaporation technology and DTB crystallization technology, respectively.

[0003] The operating conditions of nickel sulfate evaporation crystallization equipment are quite harsh. Nickel sulfate solution is corrosive and toxic, classified as a moderately hazardous material and a carcinogenic group 1A. The evaporation operation temperature is above 90℃, and the system includes both negative and positive pressure sections, and is equipped with a high-speed rotating steam compressor. Improper control of the liquid level, temperature, and pressure of critical equipment such as the evaporator separator can lead to equipment burn-out, overpressure, material leakage, and even safety accidents. Existing equipment typically uses a DCS control system to routinely monitor and adjust parameters such as temperature, pressure, and liquid level, with interlocking protection mostly based on single-point measurement and fixed threshold comparison. During nickel sulfate evaporation and crystallization, the liquid level in the evaporator separator fluctuates dramatically due to the combined effects of feed fluctuations, changes in evaporation intensity, and discharge disturbances. The single-point fixed threshold method suffers from delayed protection during rapid level changes, easily leading to heater dry burning, circulating pump cavitation, or overflow and mist entrainment. Furthermore, it lacks the ability to identify instantaneous disturbances and instrument anomalies, frequently resulting in unplanned shutdowns due to false interlocks. The steam compressor lacks comprehensive protection for multiple measuring points, including gearbox bearings, main motor, and main motor bearings, as well as precise temperature reduction measures for the outlet temperature. In emergency shutdown situations, the crystal slurry in the crystallizer and thickener has no reliable destination, easily solidifying and clogging pipes and equipment. Moreover, the corrosiveness of nickel sulfate solution increases significantly at high temperatures and readily crystallizes on heat exchange surfaces. Crystallization and scaling in the heat exchange tubes of the forced circulation evaporator heater are difficult to detect in the early stages. Often, only cleaning after significant deterioration in heat transfer or a significant increase in pressure differential necessitates shutdown. The scouring of crystal-containing salt slurry further exacerbates corrosion and wear, making long-term safe operation of the unit difficult to guarantee.

[0004] Therefore, how to construct a safety control system and method that can protect against and suppress malfunctions and properly handle abnormal operating conditions when key parameters change rapidly, and can also identify and handle crystallization and scaling on the heat exchange surface in the early stage, is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] To address the problems in existing nickel sulfate solution evaporation and crystallization devices, such as interlock protection relying on single-point measurement and fixed thresholds, protection lag and false interlocking coexisting when key parameters such as liquid level change rapidly, insufficient ability to handle abnormal operating conditions, and difficulty in early identification of crystallization and scaling on the heater heat exchange surface, the present invention aims to provide a safety control system and method for nickel sulfate solution evaporation and crystallization to improve the safety and continuity of device operation.

[0006] To achieve the above objectives, this invention provides a safety control system for the evaporation and crystallization of nickel sulfate solution, comprising an MVR evaporation unit, a DTB crystallization unit, an emergency response unit, detection instruments, and a DCS control system. The MVR evaporation unit includes a feed pump, a forced circulation evaporator heater, an evaporator separator, a forced circulation pump, and a steam compressor. Secondary steam generated by the evaporator separator is compressed and heated by the steam compressor and then sent to the forced circulation evaporator heater as a heat source. The DTB crystallization unit includes a DTB crystallizer and a thickener. The concentrated nickel sulfate solution in the evaporator separator is pumped into the DTB crystallizer tank for flash crystallization, and the crystal slurry discharged from the DTB crystallizer tank is sent to the thickener. The emergency response unit includes an emergency tank, which is connected to the DTB crystallizer tank and the thickener via an emergency pipeline equipped with an emergency discharge valve. The detection instruments include at least two redundantly configured level transmitters for detecting the liquid level in the evaporator separator, and... Temperature transmitter, pressure transmitter, and flow transmitter; the DCS control system is configured to: cross-verify the measured values ​​of at least two level transmitters to determine the effective level value; calculate the level change rate of the effective level value based on a sliding time window, and calculate the predicted limit time based on the effective level value, the level change rate, and the corresponding interlock threshold; when the effective level value enters the pre-adjustment zone and the predicted limit time is less than a preset safety time, output a pre-adjustment command for the feed pump, the discharge pump, and / or the forced circulation pump; execute a hard interlock after the effective level value exceeds the interlock threshold and is confirmed by a delay, wherein the forced circulation pump and the discharge pump are interlocked to stop when the effective level value is lower than the low level interlock threshold, and the feed is interlocked to stop when the effective level value is higher than the high level interlock threshold; and upon receiving an emergency stop signal, control the emergency discharge valve to open according to a preset timing sequence to transfer the crystal slurry in the DTB crystallizer and the thickener to the emergency tank. With the above configuration, the liquid level protection is composed of a pre-adjustment layer based on trend prediction and a hard interlocking layer confirmed by time delay. This not only improves the protection lead time under rapidly changing conditions, but also retains a deterministic final line of defense, and provides a reliable destination for the slurry in emergency conditions.

[0007] As a preferred embodiment of the present invention, the mutual verification includes: when the deviation between the measured values ​​of the at least two level transmitters is not greater than a preset deviation limit, taking the average value of each measured value as the effective level value; when the deviation is greater than the preset deviation limit, triggering an instrument deviation alarm, and taking the lower value among the measured values ​​when the level is in the low level range and the higher value among the measured values ​​when the level is in the high level range as the effective level value, so that the protection judgment still leans towards the safe direction when the instrument is abnormal.

[0008] As a preferred embodiment of the present invention, the liquid level change rate is obtained by least-squares linear fitting of the liquid level sampling sequence within the sliding time window; when the effective liquid level value is in the low-side pre-adjustment zone and the liquid level change rate is negative, the predicted limit time is the ratio of the difference between the effective liquid level value and the low liquid level interlocking threshold to the absolute value of the liquid level change rate; when the effective liquid level value is in the high-side pre-adjustment zone and the liquid level change rate is positive, the predicted limit time is the ratio of the difference between the high liquid level interlocking threshold and the effective liquid level value to the liquid level change rate; the adjustment amplitude of the pre-adjustment command is determined according to the ratio of the predicted limit time to the preset safety time, the smaller the predicted limit time, the larger the adjustment amplitude, so that the adjustment intensity is adapted to the urgency of the liquid level approaching the interlocking threshold.

[0009] As a preferred embodiment of the present invention, the low liquid level interlock threshold is 1600mm, the high liquid level interlock threshold is 3700mm, the low-side pre-adjustment zone is 1600~2000mm, the high-side pre-adjustment zone is 3300~3700mm, the preset safety time is 60~180s, the confirmation time for the delayed confirmation is 2~5s, and the hard interlock reset is provided with a liquid level hysteresis of not less than 100mm to avoid repeated operation of the interlock near the threshold.

[0010] As a preferred embodiment of the present invention, the temperature transmitter includes a gearbox bearing temperature measuring point, a main motor temperature measuring point, a main motor bearing temperature measuring point, and an outlet steam temperature measuring point installed on the steam compressor; the DCS control system is further configured to: interlock and stop the steam compressor when any one of the gearbox bearing temperature, main motor temperature, and main motor bearing temperature exceeds the corresponding shutdown threshold; a spray desuperheating device is installed on the outlet pipe of the steam compressor, and the DCS control system determines the feedforward spray water volume based on the outlet steam flow rate of the steam compressor, the enthalpy difference between the actual state enthalpy of the outlet steam and the target state enthalpy of the desuperheating, and the enthalpy difference between the target state enthalpy of the desuperheating and the spray water specific enthalpy, and performs feedback correction on the spray water volume based on the deviation between the measured value and the set value of the outlet steam temperature, so that the compressor outlet steam is maintained at a moderate superheat, taking into account both heat exchange stability and equipment safety.

[0011] As a preferred embodiment of the present invention, it further includes a vacuum unit connected to the vapor phase space of the evaporator separator; the pressure transmitter is at least installed in the vapor phase space of the evaporator separator, the shell side of the forced circulation evaporator heater, and the inlet and outlet of the steam compressor; the DCS control system sets thresholds for each pressure measuring point according to three levels: early warning, alarm, and interlock; the DCS control system performs constant pressure control on the vapor phase pressure of the evaporator separator by adjusting the pumping volume of the vacuum unit; when the deviation of the vapor phase pressure from the set value reaches the first-level deviation limit, an early warning is issued; when it reaches the second-level deviation limit, an alarm is issued; after the absolute vapor phase pressure exceeds the interlock limit and remains there for a preset time, the steam compressor is stopped and the feed is cut off; safety valves are installed on the shell side of the forced circulation evaporator heater and the evaporator separator, thereby forming a complete pressure protection chain from constant pressure regulation, graded alarms to interlock shutdown and mechanical release.

[0012] As a preferred embodiment of the present invention, the DCS control system is further configured to periodically perform online scaling identification: The actual heat load is calculated based on the circulating liquid flow rate and inlet / outlet temperature of the forced circulation evaporator heater; the logarithmic mean temperature difference is calculated based on the saturation temperature corresponding to the shell-side pressure and the inlet / outlet temperature of the circulating liquid; the apparent heat transfer coefficient is obtained from the actual heat load, heat exchange area, and logarithmic mean temperature difference; and a scaling index is calculated using a benchmark heat transfer coefficient calibrated under clean conditions as a reference. When the scaling index exceeds a first scaling threshold, the flow rate of the salt slurry in the heat exchange tubes of the forced circulation evaporator heater is increased to the upper limit of a preset flow rate window. When the scaling index exceeds a second scaling threshold, the evaporation load is automatically reduced, and hot condensate is introduced into the circulation loop for online hot washing. The scaling index and the tube-side pressure difference of the forced circulation evaporator heater constitute a dual criterion; if either exceeds the corresponding shutdown judgment limit, a shutdown cleaning prompt is issued, thereby achieving early quantitative identification and online treatment of crystallized scaling on the heat exchange surface.

[0013] As a preferred embodiment of the present invention, the preset flow rate window is 2.0~3.0 m / s; the heat exchange tubes of the forced circulation evaporator heater, the flow-through components of the forced circulation pump, and the slurry conveying pipeline are made of TA2 titanium, and the wall thickness of the heat exchange tubes is not less than 1.5 mm; the evaporator separator, the shell of the forced circulation evaporator heater, and the DTB crystallizer are made of 2205 duplex stainless steel, and the thickener, the emergency tank, and the storage and transportation equipment for mother liquor and condensate are made of 316L stainless steel; the discharge ports and sampling ports of the equipment and pipelines in the system all adopt a double valve shut-off structure, and the equipment and pipelines with an external surface temperature exceeding 60°C are equipped with a heat insulation layer to take into account the requirements of corrosion resistance, wear resistance, leakage prevention, and scalding prevention.

[0014] As a preferred embodiment of the present invention, the effective volume of the emergency tank is not less than 1.1 times the sum of the effective volumes of the DTB crystallizer and the thickener. An agitator is installed in the emergency tank, and a flushing branch controlled by a flushing water valve is connected to the emergency pipeline. The preset timing sequence includes: stopping the steam compressor and cutting off the feed; after a delay, stopping the forced circulation pump and the discharge pump; opening the emergency discharge valve corresponding to the DTB crystallizer and starting the agitator; and opening the emergency discharge valve corresponding to the thickener after the liquid level in the DTB crystallizer drops to a set low value. During the crystal slurry transfer process, the DCS control system monitors the flow rate of the emergency pipeline. If the flow rate is lower than the lower limit within a preset time after any emergency discharge valve is opened, the pipeline is determined to be blocked, and the flushing water valve is automatically controlled to pulse-flush the emergency pipeline with condensate water to ensure the smooth flow of the crystal slurry transfer channel under accident conditions.

[0015] This invention also provides a safety control method for the evaporation and crystallization of nickel sulfate solution. The method is implemented using the safety control system for the evaporation and crystallization of nickel sulfate solution described in any of the above technical solutions, and includes: acquiring the liquid level of the evaporator separator through redundantly configured level transmitters and performing mutual verification to determine an effective liquid level value; calculating the liquid level change rate of the effective liquid level value based on a sliding time window, and calculating a predicted limit time based on the effective liquid level value, the liquid level change rate, and a corresponding interlocking threshold; performing pre-adjustment on the feed pump, discharge pump, and / or forced circulation pump when the effective liquid level value enters the pre-adjustment zone and the predicted limit time is less than a preset safety time; and performing a hard interlock after the effective liquid level value exceeds the interlocking threshold and is confirmed by a delay. The system monitors the gearbox bearing temperature, main motor temperature, and main motor bearing temperature of the steam compressor in real time. If any of these exceed the corresponding shutdown threshold, the steam compressor is interlocked and stopped. The spray water volume of the spray desuperheating device is controlled by feedforward and feedback based on the outlet steam temperature. Constant pressure control is implemented for the vapor phase pressure of the evaporator separator. The vapor phase pressure, shell-side pressure of the forced circulation evaporator heater, and inlet and outlet pressures of the steam compressor are monitored and handled in three levels: early warning, alarm, and interlock. The scaling index of the forced circulation evaporator heater is calculated periodically, and the system implements graded strategies to increase the slurry flow rate, reduce the load for online hot washing, or prompt for shutdown cleaning. Upon receiving an emergency shutdown signal, the crystal slurry in the DTB crystallizer and thickener is transferred to the emergency tank according to a preset sequence.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] This invention employs a graded protection mechanism for the liquid level in the evaporator separator, combining redundant mutual verification, sliding window rate fitting, and predicted limit time judgment. While the liquid level is still within the pre-adjustment zone, it implements graded pre-adjustments for the feed, discharge, and circulation based on the changing trend, ensuring that the lead time for protection actions matches the urgency of liquid level changes. This effectively prevents heater dry burning, circulating pump cavitation, and overflow and mist entrainment under rapidly changing conditions such as feed fluctuations and sudden changes in evaporation intensity. Mutual verification selection, over-limit delay confirmation, and reset hysteresis jointly suppress false interlocks caused by single-meter failures and instantaneous disturbances, reducing unplanned shutdowns. Hard interlocks, as an independent final line of defense, still retain deterministic protection capabilities. In case of emergency shutdown, the slurry is transferred to the emergency tank in a pre-set sequence, and with flow monitoring and pulse flushing, the potential for slurry consolidation and blockage in pipelines and equipment is eliminated.

[0018] This invention calculates the apparent heat transfer coefficient of the forced circulation evaporator heater in real time using the circulating liquid side heat load and the logarithmic mean temperature difference, and normalizes it to a scaling index based on a cleanliness standard. Combined with the tube-side pressure difference, it forms a dual criterion, enabling the early quantitative identification of crystalline scaling inside the heat exchange tubes. In the graded treatment, the salt slurry flow rate is first increased to inhibit deposition, and then the load is reduced and hot condensate is used for online hot washing to dissolve the crystalline scale. The treatment process is coordinated with liquid level prediction and regulation, which can delay the development of scaling and extend the operating cycle without stopping the machine. The flow rate window of 2.0~3.0 m / s takes into account both the requirements of preventing deposition and preventing titanium tube erosion.

[0019] This invention incorporates multi-point interlocking shutdown protection for the steam compressor, measuring the gearbox bearing temperature, main motor temperature, and main motor bearing temperature. It also controls the outlet spray cooling using enthalpy balance feedforward and temperature feedback to maintain moderately superheated steam at the compressor outlet, ensuring the safe operation and stable heat exchange of the core moving equipment in the MVR system. Pressure is organized in three levels: early warning, alarm, and interlock, and is integrated with the constant pressure control of the vacuum unit and the safety valve release, forming a complete pressure protection chain. The partitioned configuration of TA2 titanium, 2205 duplex stainless steel, and 316L stainless steel, along with measures such as dual-valve sealing and heat insulation to prevent scalding, enhances the reliability and intrinsic safety of the system during long-term operation in high-temperature and highly corrosive environments. Attached Figure Description

[0020] Figure 1 This is a system architecture diagram of a preferred embodiment of the present invention;

[0021] Figure 2 This is a flowchart of a preferred embodiment of the present invention.

[0022] The components include: 1. Feed pump; 2. Forced circulation evaporator heater; 3. Evaporator separator; 4. Forced circulation pump; 5. Steam compressor; 6. Spray desuperheating device; 7. Discharge pump; 8. DTB crystallizer; 9. Crystal slurry pump; 10. Thickener; 11. Emergency tank; 12. Vacuum unit; 13. DCS control system; 14. Level transmitter; 15. Temperature transmitter; 16. Pressure transmitter; 17. Flow transmitter; 18. Safety valve; 19. Emergency discharge valve; 20. Flushing water valve; 21. Feed regulating valve. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the following description is provided in conjunction with... Figure 1 and Figure 2 Taking a battery-grade nickel sulfate production unit as an example, the specific embodiments of the present invention will be further described in detail. It should be understood that the embodiments described herein are for illustrative purposes only and are not intended to limit the scope of protection of the present invention.

[0024] See Figure 1 The safety control system for nickel sulfate solution evaporation and crystallization provided in this embodiment includes an MVR evaporation unit, a DTB crystallization unit, an emergency response unit, detection instruments, and a DCS control system 13. The MVR evaporation unit includes a feed pump 1, a forced circulation evaporator 2, an evaporator separator 3, a forced circulation pump 4, a steam compressor 5, and a spray desuperheating device 6. The nickel mass concentration of the nickel sulfate solution from the upstream purification process is 120±3 g / L. It is fed into the circulation pipeline by the feed pump 1 through the feed regulating valve 21. After mixing with the circulating liquid delivered by the forced circulation pump 4, it flows from bottom to top through the heat exchange tubes of the forced circulation evaporator 2. After being heated by the shell-side steam, it enters the evaporator separator 3 for vapor-liquid separation. The secondary steam generated at the top of the evaporator separator 3 is defoamed and then enters the steam compressor 5 for compression and heating. The superheated steam after compression is conditioned by the spray desuperheating device 6 and returned to the shell side of the forced circulation evaporator 2 as a heating source. The condensate is collected into the condensate system, and a portion of it is conditioned to serve as the spray water source for the spray desuperheating device 6 and the system flushing water source. The concentrated liquid at the bottom of the evaporator separator 3 is maintained by the forced circulation pump 4. The absolute operating pressure of the evaporator separator 3 is about 65 kPa, and the temperature of the liquid in the tank is 90~93℃. When the nickel mass concentration in the concentrated liquid increases to 190~210 g / L and is still in the unsaturated region at this temperature, it is sent to the DTB crystallization unit by the discharge pump 7, thereby ensuring that crystallization only occurs in the crystallizer and not on the heat exchange surface.

[0025] The DTB crystallization unit includes a DTB crystallizer tank 8, a crystal slurry pump 9, and a thickener 10. The DTB crystallizer tank 8 is equipped with a guide tube, baffles, and a stirring paddle. After the concentrate enters the tank, it undergoes adiabatic flash evaporation under an operating absolute pressure of 16-25 kPa, causing the liquid temperature to drop to 55-65°C and become supersaturated, precipitating nickel sulfate hexahydrate crystals. The stirring paddle in the guide tube drives the internal circulation of the crystal slurry, allowing the crystals to grow repeatedly in the supersaturated zone. Fine crystals are separated with the mother liquor in the baffle clarification zone and returned to the evaporation system for digestion, thus controlling the crystal particle size. The crystal slurry at the bottom of the tank is pumped by the crystal slurry pump 9 into the thickener 10 for thickening. The underflow is sent downstream for centrifugal separation and drying processes to obtain nickel sulfate hexahydrate crystal products. The overflow mother liquor from the thickener 10 is returned to the evaporator separator 3. The vacuum unit 12 is connected to the vapor phase space of the evaporator separator 3 and the DTB crystallizer tank 8, used to maintain system vacuum and remove non-condensable gases. The emergency response unit includes an emergency tank 11, which is connected to the bottom of the DTB crystallizer 8 and the thickener 10 via an emergency pipeline equipped with an emergency discharge valve 19. The emergency pipeline is also connected to a condensate flushing branch controlled by a flushing water valve 20.

[0026] The monitoring instruments are arranged as follows: two redundant level transmitters 14 are installed on the evaporator separator 3; level transmitters 14 are also installed on the DTB crystallizer 8, thickener 10, and emergency tank 11; temperature transmitters 15 are installed at measuring points such as the gearbox bearing of the steam compressor 5, the stator winding of the main motor, the main motor bearing, and the outlet pipe, as well as the tube-side inlet and outlet of the forced circulation evaporator heater 2 and inside the evaporator separator 3; pressure transmitters 16 are installed in the vapor phase space of the evaporator separator 3, the shell side of the forced circulation evaporator heater 2, and the inlet and outlet of the steam compressor 5; flow transmitters 17 are installed in the feed line, circulation line, spray water line, and emergency line. Safety valves 18 are installed on the shell side of the forced circulation evaporator heater 2 and on the evaporator separator 3, respectively. The set pressure of the safety valves 18 is 0.25 MPa, corresponding to the equipment design pressure of 0.3 MPa. All the above instrument signals are connected to the DCS control system 13. The DCS control system 13 provides centralized indication, automatic adjustment, trend recording and over-limit alarm for the process parameters of the entire unit. The alarms are organized in a hierarchical manner of first-level early warning, second-level alarm and third-level interlock. The interlock circuit is set with first-out memory and event sequence recording. All interlocks adopt over-limit delay confirmation and reset hysteresis strategy to suppress malfunctions caused by instantaneous disturbances.

[0027] The liquid level protection of evaporator separator 3 adopts a graded mechanism combining mutual verification, rate prediction, pre-adjustment, and hard interlocking. The measured values ​​of the two liquid level transmitters 14 are recorded as follows: and When the deviation between the two satisfies At that time, take the effective liquid level value. Deviation limit Take 50mm; when the deviation exceeds When the liquid level is triggered, an instrument deviation alarm is established. If the liquid level is in the low liquid level range, the lower of the two values ​​is used; if it is in the high liquid level range, the higher of the two values ​​is used as the valid liquid level value. This ensures that the protection judgment is always biased towards the safe direction. If the deviation exceeds the limit for 60 seconds, an instrument fault is determined, prompting manual verification, and the protection function is maintained by a reliable instrument. The DCS control system 13 samples the valid liquid level value at a period of 1 second. Within a sliding time window of 30 seconds, the sampling sequence is fitted with least squares linearly to obtain the liquid level change rate.

[0028]

[0029] in , The first in the window Each sampling time and its corresponding effective liquid level value , This represents the average time and liquid level within the window. This represents the number of sampling points within the window. In this embodiment, the normal operating liquid level of the evaporator separator 3 is 2200~3200 mm, and the low liquid level interlock threshold is... The high liquid level interlock threshold is 1600mm. The effective liquid level is 3700mm, with 1600~2000mm designated as the low-side pre-adjustment zone and 3300~3700mm as the high-side pre-adjustment zone. When the effective liquid level is within the low-side pre-adjustment zone and... At that time, the predicted trigger time is based on Calculation; when the effective liquid level is in the high-side pre-adjustment zone and At that time, according to Calculation. Preset safety time. Take 120 seconds, when Start pre-adjustment at the time: In the 80~120s range, the operating frequency of the discharge pump 7 is reduced by 10% and the opening of the feed regulating valve 21 is increased by 3%~5% in the low-side operation condition. In the high-side operation condition, the opening of the feed regulating valve 21 is symmetrically reduced and the frequency of the discharge pump 7 is increased. For values ​​in the 40-80s range, the adjustment range is doubled. For cycles shorter than 40 seconds, in low-side operation, the discharge pump 7 is reduced to the lowest stable frequency, and the feed regulating valve 21 is opened to the upper limit of the allowable range. In high-side operation, the feed regulating valve 21 is closed to the lower limit of the allowable range. Each pre-adjustment command has amplitude and rate of change limits to avoid impacting the cyclic operation.

[0030] When the effective liquid level exceeds the interlock threshold and remains above it for 3 seconds, a hard interlock is activated: When the effective liquid level is below 1600mm, the interlock stops the forced circulation pump 4 and the discharge pump 7 to prevent cavitation of the forced circulation pump 4 and dry burning of the equipment. Simultaneously, the interlock stops the steam compressor 5 to cut off the heat source input, and feed is maintained to promote a rise in liquid level. When the effective liquid level is above 3700mm, the interlock closes the feed regulating valve 21 and stops the feed pump 1 to prevent material overflow and secondary steam contamination from mist. The hard interlock reset setting is 100mm of liquid level hysteresis; that is, the operator can only confirm the reset after the liquid level has risen above 1700mm or fallen below 3600mm and stabilized. The pre-conditioning layer and the hard interlock layer are independent of each other. If the pre-conditioning fails or the disturbance is too rapid, the hard interlock still acts as the last line of defense.

[0031] The safety protection of the steam compressor 5 covers three aspects: body temperature, outlet steam status, and operating conditions. Regarding body temperature, an alarm sounds when the gearbox bearing temperature reaches 85℃, and an interlock shutdown occurs when it reaches 95℃. The main motor temperature measuring point is located on the stator winding; an alarm sounds when it reaches 130℃, and an interlock shutdown occurs when it reaches 140℃. Similarly, an alarm sounds when the main motor bearing temperature reaches 80℃, and an interlock shutdown occurs when it reaches 90℃. Upon any shutdown condition being met, the steam compressor 5 trips and the feed regulating valve 21 is interlocked and closed. The forced circulation pump 4 runs for a delayed period to remove the residual heat from the forced circulation evaporator heater 2. Regarding the outlet steam status, the secondary steam temperature at the top of the evaporator separator 3 is approximately 88℃. After being compressed to an absolute pressure of 110~120 kPa by the steam compressor 5, the uncooled outlet superheated steam temperature can reach over 135℃. To ensure stable heat exchange and avoid localized overheating in the shell side, the spray cooling device 6 cools the outlet steam to 110~115℃, corresponding to a superheat of 5~10℃. The spray water volume is controlled by a combination of feedforward and feedback, with the feedforward water volume determined based on enthalpy balance.

[0032]

[0033] in The spray water mass flow rate is expressed in kg / h. The mass flow rate of steam at the outlet of steam compressor 5, as measured by flow transmitter 17, is expressed in kg / h. The specific enthalpy of superheated steam is determined based on the measured outlet temperature and pressure. Specific enthalpy for the target cooling state under export pressure The specific enthalpy of the spray water is expressed in kJ / kg. The spray water is taken from steam condensate at a temperature of 60-80℃ to avoid introducing impurities from external water sources that could affect product purity. The feedback loop uses the deviation between the measured and set outlet temperature as input to perform PID correction on the opening of the spray water regulating valve. The proportional coefficient is 0.5-2, and the integral time is 30-120s. The feedforward calculated opening and the feedback correction are superimposed and output to the actuator. A high-level alarm is triggered when the outlet temperature reaches 130℃, and an interlock shutdown is triggered when it reaches 140℃ and remains there for 5 seconds, serving as a fallback protection in case of spray failure. The inlet flow rate and inlet / outlet pressure ratio of the steam compressor 5 are continuously monitored by the DCS control system 13. When the operating point approaches the surge boundary and the margin is less than 10%, the anti-surge bypass is automatically activated, returning some outlet steam to the inlet to maintain stable operation.

[0034] Pressure protection is organized according to constant pressure regulation and graded handling. The vapor phase pressure setpoint of the evaporator separator 3 is 65 kPa absolute pressure. The DCS control system 13 forms a constant pressure loop by adjusting the pumping volume of the vacuum unit 12. When the pressure deviates from the setpoint by 5 kPa, a first-level warning is issued; when it reaches 8 kPa, a second-level alarm is issued, prompting a check of the vacuum unit 12 and the non-condensable gas discharge status. When the vapor phase absolute pressure rises above 90 kPa or falls below 45 kPa and remains below for 10 seconds, a third-level interlock is executed, stopping the steam compressor 5 and closing the feed regulating valve 21. The shell-side gauge pressure of the forced circulation evaporator heater 2 alarms when it reaches 0.15 MPa, and interlocks to stop the steam compressor 5 when it reaches 0.2 MPa. The safety valve 18 opens at 0.25 MPa as a last mechanical venting protection. The low inlet pressure alarm of the steam compressor 5 is used to indicate insufficient evaporation intensity or vacuum abnormality. The inlet and outlet pressure signals also serve as anti-surge judgment.

[0035] To address the high-temperature, strong corrosive properties of nickel sulfate solution and its tendency to crystallize and scale on heat exchange surfaces, the DCS control system 13 periodically performs online scaling detection on the forced circulation evaporator heater 2. Detection is performed at 5-minute intervals, and only during quasi-steady-state periods when the evaporation load fluctuation does not exceed ±10% of the rated value. The actual heat load is determined based on the enthalpy rise on the circulating liquid side.

[0036]

[0037] in The mass flow rate of the circulating liquid is expressed in kg / s and is measured by flow transmitter 17. For the specific heat capacity of the circulating liquid, the concentrated nickel sulfate solution in this embodiment is taken as 3.0 kJ / (kg·℃); , These are the circulating liquid temperatures at the inlet and outlet of the second-side tube of the forced circulation evaporator heater, respectively, in °C. The shell-side vapor undergoes approximately isothermal condensation; the saturation temperature corresponding to the shell-side pressure is used. Logarithmic mean temperature difference

[0038]

[0039] The apparent heat transfer coefficient is calculated to be...

[0040]

[0041] in The heat exchange area of ​​the forced circulation evaporator heater 2 is expressed in m². A 10-minute moving average was used to suppress measurement noise. The average value was calculated based on 72 hours of continuous stable operation after equipment commissioning or chemical cleaning. Average value calibration cleaning benchmark Defining the scaling index

[0042]

[0043] The larger the value, the thicker the scale buildup on the inner wall of the heat exchange tube. And after 30 minutes, a first-level treatment is initiated. The DCS control system 13 increases the operating frequency of the forced circulation pump 4, raising the flow rate of the salt slurry in the heat exchange tubes to 2.6~3.0 m / s, using shearing action to suppress grain deposition and issue an early warning; when The system performs secondary treatment, automatically reducing the evaporation load to below 70% of the rated value. It then opens the flushing water valve 20, introducing 80-90°C hot condensate into the circulation loop at 5%-10% of the circulation volume. Online hot washing lasts 20-40 minutes. This process increases the circulating liquid temperature and reduces supersaturation, promoting the dissolution of scale on the pipe walls. Changes in the amount of water introduced for hot washing are simultaneously incorporated into the feed and discharge regulation by the aforementioned liquid level rate prediction mechanism. These two mechanisms work in tandem to prevent liquid level interlocking from being triggered during the hot washing process. After hot washing, the system recalculates. If the pressure difference between the tube side of the forced circulation evaporator heater 2 and the pressure difference between the tube side and the pressure difference is still not lower than 0.25 or reaches 0.08 MPa, it is recommended to arrange a planned shutdown for chemical cleaning. When the tube-side pressure difference reaches 0.12 MPa, the interlock will reduce the device to the minimum stable load and issue an audible and visual alarm. The scaling index and tube-side pressure difference form a dual criterion for cross-verification, which can effectively distinguish between heat transfer deterioration and instrument drift. During normal operation, the flow velocity of the salt slurry in the heat exchange tube is controlled within a window of 2.0~3.0 m / s, with a target value of 2.2~2.5 m / s. The lower limit of the flow velocity is used to prevent grain deposition from inducing scale growth, and the upper limit is used to limit the scouring and erosion of the titanium heat exchange tube by the crystalline salt slurry. This flow velocity is automatically maintained by the frequency-driven forced circulation pump 4 based on the feedback from the flow transmitter 17.

[0044] The emergency response logic is activated upon receiving an emergency shutdown signal. Triggering sources include manual emergency shutdown buttons in the control room and on-site, plant-wide power outages, and situations where the steam compressor 5-interlock trips and cannot be restored in a short time. The DCS control system 13 and its interlocking circuits are powered by an uninterruptible power supply to ensure that the response sequence can still be executed during power outages. At time T0, stop the steam compressor 5, close the feed regulating valve 21, and stop the feed pump 1; at T0+30s, stop the forced circulation pump 4 and the discharge pump 7, and use the 30s of circulation to remove the residual heat of the forced circulation evaporator heater 2; at T0+40s, open the emergency discharge valve 19 corresponding to the DTB crystallizer 8 and start the agitator of the emergency tank 11. The crystal slurry in the DTB crystallizer 8 is discharged into the emergency tank 11 by the level difference and the pressure difference inside and outside the tank; after the liquid level in the DTB crystallizer 8 drops below 300mm, close its emergency discharge valve 19 and open the emergency discharge valve 19 corresponding to the thickener 10 to continue the transfer; after all the transfer is completed, open the flushing water valve 20 and flush the emergency pipeline in the forward direction with 0.3~0.5MPa condensate water for 30~60s to prevent the residual crystal slurry from solidifying. During the transfer process, the DCS control system 13 monitors the readings of the flow transmitter 17 on the emergency pipeline. If the flow rate is still lower than 30% of the design flow rate within 60 seconds after any emergency discharge valve 19 is opened, the pipeline is considered blocked. Automatic pulse flushing with a 5-second on / 5-second off cycle is executed. If no flow is established after three consecutive cycles, an alarm is issued and the flushing water valve 20 remains closed, requiring manual intervention. The emergency tank 11 is made of 316L stainless steel, with an effective volume not less than 1.1 times the sum of the effective volumes of the DTB crystallizer 8 and the thickener 10. A bottom-mounted agitator with steam tracing is installed inside the tank to maintain the crystal slurry at 50-60°C and continuously stir to prevent the nickel sulfate hexahydrate crystals from settling and solidifying. The vent pipe at the top of the tank is connected to an acid mist absorption device. The tank is equipped with level and temperature monitoring. After the device is restored, the crystal slurry in the tank is pumped back to the DTB crystallizer 8 for reprocessing.

[0045] In terms of intrinsic safety design, the parts in contact with high-temperature, high-flow-rate crystalline salt slurry, including the heat exchange tubes of the forced circulation evaporator heater 2, the flow-through components of the forced circulation pump 4, and the slurry conveying pipes and elbows, are all made of TA2 titanium. The wall thickness of the heat exchange tubes is not less than 1.5mm, with a margin for erosion and wear compared to conventional specifications. Elbows and other areas with concentrated erosion are thickened or replaced with wear-resistant bushings. The shells and tube boxes of the evaporator separator 3 and the forced circulation evaporator heater 2, which are subjected to long-term high-temperature corrosion above 90℃, and the DTB crystallizer 8 are made of 2205 duplex stainless steel. The thickener 10, the emergency tank 11, and the storage and transportation equipment for mother liquor and condensate are in a low-temperature general corrosive environment and are made of 316L stainless steel. All system equipment and pipeline discharge and sampling ports adopt a dual-valve structure with a root valve and an end shut-off valve, and the end is equipped with a plug or blind flange. The pump area is equipped with a dike and a collection ditch to collect the leakage liquid. Equipment and pipelines whose external surface temperature may exceed 60°C are equipped with a heat insulation layer, and the external surface temperature of the heat insulation layer does not exceed 60°C, which takes into account both preventing scalding and reducing heat loss.

[0046] See Figure 2 The safety control method based on the above system includes the following steps: Step S1, the liquid level of the evaporator separator 3 is collected and cross-checked through redundant liquid level transmitters 14 to determine the effective liquid level value; Step S2, the liquid level change rate is obtained by least squares fitting of the effective liquid level value based on the sliding time window, and the predicted limit time is calculated in combination with the interlocking threshold; Step S3, when the effective liquid level value enters the pre-adjustment zone and the predicted limit time is less than the preset safety time, graded pre-adjustment is performed; when the effective liquid level value exceeds the interlocking threshold and is confirmed after a delay, hard interlocking is performed; Step S4, the gearbox bearing temperature, main motor temperature, and main motor bearing temperature of the steam compressor 5 are monitored in real time, and over-limit interlocking shutdown is performed. Simultaneously, the outlet spray cooling is controlled by enthalpy balance feedforward plus temperature feedback; in step S5, constant pressure control is implemented on the vapor phase pressure of the evaporator separator 3, and the shell pressure of the forced circulation evaporator heater 2 and the inlet and outlet pressures of the steam compressor 5 are handled according to three levels: early warning, alarm, and interlock; in step S6, the scaling index of the forced circulation evaporator heater 2 is calculated periodically, and the flow rate is increased, the load is reduced, online hot washing or shutdown cleaning prompts are executed according to the graded strategy; in step S7, when an emergency shutdown signal is received, the crystal slurry in the DTB crystallizer 8 and the thickener 10 is transferred to the emergency tank 11 according to the preset sequence and the pipeline is flushed. Steps S1 to S3 constitute continuous protection in the liquid level dimension, steps S4 and S5 constitute protection in the moving equipment and pressure dimension, step S6 addresses the slow variable of scaling and deterioration, and step S7 addresses the handling of accident conditions. Each step is executed in parallel by the DCS control system 13, which together constitutes a multi-layered safety system with predictive early warning, automatic adjustment and hard interlock progressively.

[0047] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any equivalent substitutions, parameter adjustments, or structural modifications made by those skilled in the art to the above embodiments without departing from the concept of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A safety control system for the evaporation and crystallization of nickel sulfate solution, characterized in that, Includes MVR evaporation unit, DTB crystallization unit, emergency response unit, detection instruments and DCS control system; The MVR evaporation unit includes a feed pump, a forced circulation evaporation heater, an evaporation separator, a forced circulation pump, and a steam compressor. The secondary steam generated by the evaporation separator is compressed and heated by the steam compressor and then sent to the forced circulation evaporation heater as a heat source. The DTB crystallization unit includes a DTB crystallization tank and a thickener. The concentrated nickel sulfate solution in the evaporator is pumped into the DTB crystallization tank for flash crystallization, and the crystal slurry discharged from the DTB crystallization tank is sent into the thickener. The emergency response unit includes an emergency tank, which is connected to the DTB crystallizer and the thickener via an emergency pipeline equipped with an emergency discharge valve. The detection instruments include at least two redundant level transmitters for detecting the liquid level of the evaporator separator, as well as a temperature transmitter, a pressure transmitter and a flow transmitter. The DCS control system is configured to: cross-verify the measured values ​​of at least two level transmitters to determine the effective level value; calculate the level change rate of the effective level value based on a sliding time window, and calculate the predicted limit time based on the effective level value, the level change rate, and the corresponding interlock threshold; when the effective level value enters the pre-adjustment zone and the predicted limit time is less than a preset safety time, output a pre-adjustment command for the feed pump, the discharge pump, and / or the forced circulation pump; execute a hard interlock after the effective level value exceeds the interlock threshold and is confirmed by a delay, wherein the forced circulation pump and the discharge pump are interlocked to stop when the effective level value is lower than the low level interlock threshold, and the feed is interlocked to stop when the effective level value is higher than the high level interlock threshold; and upon receiving an emergency stop signal, control the emergency discharge valve to open according to a preset timing sequence to transfer the crystal slurry in the DTB crystallizer and the thickener to the emergency tank.

2. The safety control system for nickel sulfate solution evaporation and crystallization according to claim 1, characterized in that, The mutual verification includes: when the deviation between the measured values ​​of the at least two level transmitters is not greater than a preset deviation limit, the average value of each measured value is taken as the effective level value; when the deviation is greater than the preset deviation limit, an instrument deviation alarm is triggered, and when the level is in the low level range, the lower value among the measured values ​​is taken, and when the level is in the high level range, the higher value among the measured values ​​is taken as the effective level value.

3. The safety control system for nickel sulfate solution evaporation and crystallization according to claim 1, characterized in that, The liquid level change rate is obtained by performing least-squares linear fitting on the liquid level sampling sequence within the sliding time window. When the effective liquid level value is in the low-side pre-adjustment zone and the liquid level change rate is negative, the predicted trigger time is the ratio of the difference between the effective liquid level value and the low liquid level interlock threshold to the absolute value of the liquid level change rate. When the effective liquid level value is in the high-side pre-adjustment zone and the liquid level change rate is positive, the predicted trigger time is the ratio of the difference between the high liquid level interlock threshold and the effective liquid level value to the liquid level change rate. The adjustment range of the pre-adjustment command is determined according to the ratio of the predicted trigger time to the preset safety time, and the smaller the predicted trigger time, the larger the adjustment range.

4. The safety control system for nickel sulfate solution evaporation and crystallization according to claim 3, characterized in that, The low liquid level interlock threshold is 1600mm, the high liquid level interlock threshold is 3700mm, the low-side pre-adjustment zone is 1600~2000mm, the high-side pre-adjustment zone is 3300~3700mm, the preset safety time is 60~180s, the confirmation time for the delayed confirmation is 2~5s, and the hard interlock reset is set with a liquid level hysteresis of not less than 100mm.

5. The safety control system for nickel sulfate solution evaporation and crystallization according to claim 1, characterized in that, The temperature transmitter includes temperature measuring points for the gearbox bearing, main motor, main motor bearing, and outlet steam of the steam compressor. The DCS control system is further configured to interlock and stop the steam compressor when any one of the gearbox bearing temperature, main motor temperature, or main motor bearing temperature exceeds the corresponding shutdown threshold. A spray desuperheating device is installed on the outlet pipe of the steam compressor. The DCS control system determines the feedforward spray water volume based on the outlet steam flow rate of the steam compressor, the enthalpy difference between the actual state enthalpy of the outlet steam and the target state enthalpy of the desuperheating, and the enthalpy difference between the target state enthalpy of the desuperheating and the spray water, and corrects the spray water volume based on the deviation between the measured value and the set value of the outlet steam temperature.

6. The safety control system for nickel sulfate solution evaporation and crystallization according to claim 1, characterized in that, It also includes a vacuum unit that is connected to the vapor phase space of the evaporator separator; The pressure transmitter is installed at least in the vapor phase space of the evaporator separator, the shell side of the forced circulation evaporator heater, and the inlet and outlet of the steam compressor. The DCS control system sets thresholds for each pressure measuring point in three levels: early warning, alarm, and interlock. The DCS control system controls the vapor phase pressure of the evaporator separator by adjusting the pumping volume of the vacuum unit. When the deviation between the vapor phase pressure and the set value reaches the first-level deviation limit, an early warning is issued. When the deviation reaches the second-level deviation limit, an alarm is issued. When the absolute vapor phase pressure exceeds the interlock limit and continues for a preset time, the steam compressor is stopped and the feed is cut off. Safety valves are installed on the shell side of the forced circulation evaporator and on the evaporator separator.

7. The safety control system for nickel sulfate solution evaporation and crystallization according to claim 1, characterized in that, The DCS control system is also configured to periodically perform online scaling identification: calculate the actual heat load based on the circulating liquid flow rate and circulating liquid inlet and outlet temperatures of the forced circulation evaporator heater, calculate the logarithmic mean temperature difference based on the saturation temperature corresponding to the shell-side pressure and the circulating liquid inlet and outlet temperatures, obtain the apparent heat transfer coefficient from the actual heat load, heat exchange area and the logarithmic mean temperature difference, and calculate the scaling index with the reference heat transfer coefficient calibrated under clean conditions as a reference. When the scaling index exceeds the first scaling threshold, the flow rate of the salt slurry in the heat exchange tube of the forced circulation evaporator heater is increased to the upper limit range of the preset flow rate window. When the scaling index exceeds the second scaling threshold, the evaporation load is automatically reduced, and hot condensate is introduced into the circulation loop for online hot washing. The scaling index and the tube-side pressure difference of the forced circulation evaporator heater constitute a dual criterion. When either of them exceeds the corresponding shutdown judgment limit, a shutdown cleaning prompt is issued.

8. The safety control system for nickel sulfate solution evaporation and crystallization according to claim 7, characterized in that, The preset flow rate window is 2.0~3.0 m / s; The heat exchange tubes of the forced circulation evaporator heater, the flow-through components of the forced circulation pump, and the slurry conveying pipeline are made of TA2 titanium material, and the wall thickness of the heat exchange tubes is not less than 1.5mm. The shells of the evaporator separator, the forced circulation evaporator heater, and the DTB crystallizer are made of 2205 duplex stainless steel, while the thickener, the emergency tank, and the storage and transportation equipment for mother liquor and condensate are made of 316L stainless steel. The system's equipment and pipeline discharge and sampling ports all adopt a double-valve shut-off structure, and equipment and pipelines with an external surface temperature exceeding 60°C are equipped with a heat insulation layer.

9. The safety control system for nickel sulfate solution evaporation and crystallization according to claim 1, characterized in that, The effective volume of the emergency tank is not less than 1.1 times the sum of the effective volumes of the DTB crystallizer and the thickener. An agitator is installed in the emergency tank, and a flushing branch controlled by a flushing water valve is connected to the emergency pipeline. The preset timing sequence includes: stopping the steam compressor and cutting off the feed, stopping the forced circulation pump and the discharge pump after a delay, opening the emergency discharge valve corresponding to the DTB crystallizer and starting the agitator, and opening the emergency discharge valve corresponding to the thickener after the liquid level in the DTB crystallizer drops to a set low value. During the crystal slurry transfer process, the DCS control system monitors the flow rate of the emergency pipeline. If the flow rate is lower than the lower limit within a preset time after any of the emergency discharge valves is opened, the pipeline is determined to be blocked, and the flushing water valve is automatically controlled to pulse flush the emergency pipeline with condensate.

10. A safety control method for the evaporation and crystallization of nickel sulfate solution, characterized in that, The method is implemented using a safety control system for nickel sulfate solution evaporation and crystallization as described in any one of claims 1 to 9, comprising: The liquid level of the evaporator separator is collected by redundant liquid level transmitters and cross-verified to determine the valid liquid level value. The liquid level change rate of the effective liquid level value is calculated based on the sliding time window, and the predicted trigger time is calculated based on the effective liquid level value, the liquid level change rate and the corresponding interlocking threshold. When the effective liquid level value enters the pre-adjustment zone and the predicted limit time is less than the preset safety time, pre-adjustment is performed on the feed pump, discharge pump and / or forced circulation pump; hard interlock is performed after the effective liquid level value exceeds the interlock threshold and is confirmed by a delay. The temperature of the gearbox bearing, main motor, and main motor bearing of the steam compressor are monitored in real time. If any one of them exceeds the corresponding shutdown threshold, the steam compressor is stopped by interlock. The spray water volume of the spray desuperheating device is controlled by feedforward and feedback based on the outlet steam temperature. The vapor phase pressure of the evaporator is controlled under constant pressure, and the vapor phase pressure, the shell side pressure of the forced circulation evaporator heater, and the inlet and outlet pressures of the steam compressor are monitored and handled in three levels: early warning, alarm, and interlock. Periodically calculate the scaling index of the forced circulation evaporator heater and implement graded strategies to prompt for increasing the slurry flow rate, reducing the load for online hot washing, or shutting down for cleaning. Upon receiving an emergency shutdown signal, the crystal slurry in the DTB crystallizer and thickener is transferred to the emergency tank according to a preset sequence.

Images