Descaling method for steam-injection boiler reuse purified sewage

Abstract

The invention relates to the technical field of descaling of steam-injection boilers in oil fields, in particular to a descaling method for recycling and purifying sewage of a steam-injection boiler, which comprises the following steps: on the basis of a hydrochemistry theory, establishing a function relationship of existence states of minerals, on the basis of a hydrolysis reaction, establishing a dynamic model, and on the basis of a process flow of the steam-injection boiler and the dynamic model, calculating a descaling result of the steam-injection boiler. And the scale layer type of the scale is judged, and according to the scale layer type of the scale, based on the dynamic model, the hydrolysis reaction and the field operation parameters, a relation schematic diagram of different types of scale layers is fitted. Based on a dissolution theory based on key factors such as mineral dissolution balance, fluid mixing parameters, gas separation caused by pressure drop or boiling, interaction of organic matters and inorganic matters and the like, boiler scale is eliminated by adopting flushing and soaking methods after the scale type of the boiler tube is pre-judged, so that the maintenance workload is reduced, the production cost is reduced, and the production efficiency is improved. And the running time rate is improved, and the method has a relatively good application prospect in production.

Description

Technical Field

[0001] The invention relates to the technical field of descaling of oil field steam injection boilers, in particular to a descaling method for recycling and purifying sewage in steam injection boilers. Background Art

[0002] With the deepening of heavy oil thermal recovery, the purified wastewater produced by oil fields has been fully recycled. In this process, the treated heavy oil wastewater is used as thermal recovery boiler feed water, which can make full use of the high temperature of thermal recovery heavy oil oil-containing wastewater to achieve comprehensive utilization of thermal energy and recycling of water resources. The water quality of the recycled purified wastewater is complex, except for Ca 2+ Mg 2+ In addition to ions, other ion components of heavy oil purified wastewater increased significantly compared with clean water, among which TDS increased by more than 10 times, Cl - The content of carbon dioxide increased by more than 30 times and the content of silicate increased by more than 6 times. After long-term operation, the boiler inevitably suffered from scaling and other problems.

[0003] During the process of heat exchange and temperature rise of the feed water on the boiler heating surface, certain salts dissolved in the boiler water have a certain solubility. When the solubility limit is exceeded, the boiler water becomes a supersaturated solution of certain salts. At this time, the excess part will precipitate from the boiler water. Once the appropriate conditions are met, it will adhere to the pipe wall to form scale and crystals. The chemical composition of scale is relatively complex. It is usually not a simple compound, but is mainly composed of a certain chemical component. The thermal conductivity of scale is very poor, which makes the heat transfer worse, causing the heat transfer of the heating surface to deteriorate, thereby wasting fuel or causing accidents such as pipe bursts. Necessary methods should be taken to remove the scale. In descaling, judging the type and composition of scale is very important for analyzing the cause of scaling and removing scale.

[0004] Therefore, it is necessary to study a safe and fast boiler scale judgment method and descaling method to ensure the safety of the equipment, achieve energy saving and consumption reduction in production and improve the operating efficiency of the equipment. Summary of the invention

[0005] The present invention provides a descaling method for recycling purified wastewater in a steam injection boiler, which overcomes the deficiencies of the above-mentioned prior art and can effectively solve the problems of short service life of furnace tubes, high fuel consumption, boiler corrosion by scale and high production cost in existing steam injection boilers in oil fields.

[0006] The technical solution of the present invention is achieved by the following measures: A descaling method for recycling and purifying wastewater in a steam injection boiler is carried out according to the following steps: The first step is to establish the functional relationship of the existence state of minerals based on water chemistry theory: ΔG diff =f(Q i , Ki , α i ) Where Q is the reaction entropy, K is the equilibrium constant at a specific temperature and pressure, and α is the ion activation degree; The second step is to establish a dynamic model of the dissolution rate D and influencing factors of the composite precipitate formed by carbonate, sulfate and silicate based on the hydrolysis reaction: D=f(ΔG,P,T,v,g,t,β) Among them, ΔG is the mineral free energy, P is the pressure, T is the temperature, v is the flow rate, g is the gas content, t is the immersion time, and β is the correction parameter; The third step is to determine the scale layer type by combining the process flow and dynamic model of the steam injection boiler; The fourth step is to fit the relationship diagram of the dissolution rate D and pressure P, temperature T, flow rate v, immersion and flushing time t for different types of scale layers according to the type of scale layer, based on the dynamic model, hydrolysis reaction and field operation parameters. By adjusting the flushing and soaking time of the boiler tube scale, the descaling of different types of scale layers can be completed.

[0007] The following are further optimizations and / or improvements to the above technical solutions: In the above second step, in the functional relationship of the existence state of the mineral, the judgment of the existence state of the mineral includes: when ΔG diff <0, mineral dissolution is unsaturated and minerals do not precipitate; ΔG diff >0, the mineral is supersaturated, the precipitation conditions are met, and the mineral precipitates.

[0008] In the third step, when determining the scale layer type, the specific operation is: determine the scale layer type according to the feed water quality composition and the heating intensity of the feed water on each heating surface and the change process of the working fluid in the process flow of the steam injection boiler.

[0009] The scale layer types of the above scale include: carbonate composite scale and sulfate composite scale deposited in the convection section of the steam injection boiler; silicate composite scale deposited in the radiation section of the steam injection boiler.

[0010] In the above-mentioned carbonate composite scale, the mass content of calcium carbonate is 50% to 95%; in the sulfate composite scale, the mass content of calcium sulfate is 50% to 95%; in the silicate composite scale, the mass content of silicon dioxide is 20% to 25%.

[0011] The present invention is based on dissolution theory based on key factors such as mineral dissolution equilibrium, fluid mixing parameters, gas separation due to pressure drop or boiling, and interaction between organic and inorganic substances. After predicting the type of scaling on the boiler tubes, flushing and soaking methods are adopted to eliminate the scale, thereby reducing the maintenance workload, reducing the production cost, and improving the operating efficiency. It has good application prospects in production. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Attached Figure 1 The figure is a schematic diagram showing the relationship between the carbonate composite scale dissolution rate D and the key technical parameters P, T, v, and t fitted in the present invention.

[0013] Attached Figure 2 The figure is a schematic diagram showing the relationship between the sulfate composite scale dissolution rate D and the key technical parameters P, T, v, and t fitted in the present invention.

[0014] Attached Figure 3 The figure is a schematic diagram showing the relationship between the silicate composite scale dissolution rate D and the key technical parameters P, T, v, and t fitted in the present invention. DETAILED DESCRIPTION

[0015] The present invention is not limited by the following embodiments, and specific implementation methods can be determined according to the technical scheme of the present invention and actual conditions. Unless otherwise specified, the various chemical reagents and chemicals mentioned in the present invention are all known and commonly used chemical reagents and chemicals in the prior art.

[0016] The present invention will be further described below in conjunction with embodiments: Example 1: The descaling method for recycling and purifying wastewater in a steam injection boiler is carried out according to the following steps: The first step is to establish the functional relationship of the existence state of minerals based on water chemistry theory: ΔG diff =f(Q i , K i , α i ) Where Q is the reaction entropy, K is the equilibrium constant at a specific temperature and pressure, and α is the ion activation degree; The second step is to establish a dynamic model of the dissolution rate D and influencing factors of the composite precipitate formed by carbonate, sulfate and silicate based on the hydrolysis reaction: D=f(ΔG,P,T,v,g,t,β) Among them, ΔG is the mineral free energy, P is the pressure, T is the temperature, v is the flow rate, g is the gas content, t is the immersion time, and β is the correction parameter; The third step is to determine the scale layer type by combining the process flow and dynamic model of the steam injection boiler; The fourth step is to fit the relationship diagram of the dissolution rate D and pressure P, temperature T, flow rate v, immersion and flushing time t for different types of scale layers according to the type of scale layer, based on the dynamic model, hydrolysis reaction and field operation parameters. By adjusting the flushing and soaking time of the boiler tube scale, the descaling of different types of scale layers can be completed.

[0017] In the present invention, since the formation of the stratified material of the boiler for purifying sewage reuse is related to the bicarbonate ion, sulfate ion, silicate content in the boiler feed water and the bicarbonate ion, sulfate ion, silicate content in the feed water and the feed water TDS, in the process of generating steam, water is continuously evaporated and concentrated on the wall of the heating surface, and evaporation or bubbles appear locally along the wall surface, and the deposited salts will remain on the surface of the metal, but when the bubbles on the wall burst or leave, the dried salts are in contact with the boiler water, and the unsaturated salts are dissolved in water again, while the supersaturated and insoluble salts adhere to the heating surface, and this process is repeated continuously, and the salts adhered to the wall of the heating surface accumulate more and more. If the metal surface of the heating surface is rough and uneven, it creates conditions for the adhesion of sediments. When the scale layer is not formed by the hard and dense scale layer due to the high heat load intensity on the heating surface, a dynamic model of the dissolution rate D of the composite precipitate formed by carbonate, sulfate and silicate and the influencing factor based on the hydrolysis reaction can be used to guide the descaling work.

[0018] Embodiment 2: As an optimization of the above embodiment, in the second step, in the functional relationship of the existence state of the mineral, the judgment of the existence state of the mineral includes: when ΔG diff <0, mineral dissolution is unsaturated and minerals do not precipitate; ΔG diff >0, the mineral is supersaturated, the precipitation conditions are met, and the mineral precipitates.

[0019] Embodiment 3: As an optimization of the above embodiment, in the third step, when judging the scale layer type, the specific operation is: judging the scale layer type according to the water quality composition of the feed water and the heating intensity of the feed water on each heating surface and the change process of the working fluid in the process flow of the steam injection boiler.

[0020] Embodiment 4: As an optimization of the above embodiment, the scale layer types include: carbonate composite scale and sulfate composite scale deposited in the convection section of the steam injection boiler; silicate composite scale deposited in the radiation section of the steam injection boiler.

[0021] Due to HC03 in the water 2- Ion hydrolysis produces C02 and C03 - , 0H - When the plasma combines with the carbon calcium ions in the feed water, carbonate scale mainly composed of CaCO3 is generated. When the water temperature rises, the solubility of certain calcium and magnesium salts such as CaSO4, Mg(OH)2, and Ca(OH)2 in water decreases, and after reaching a supersaturated state, scale is gradually formed.

[0022] As the temperature continues to rise, some calcium and magnesium salts decompose under heat to form insoluble precipitates: Ca(HCO3)2=CaSO4↓+CO2↑+H2O Mg(HC03)2=MgS04↓+C02↑+H20 When the pH value of boiler water is high, it will further hydrolyze into insoluble hydroxide precipitates: MgC03+H20=Mg(OH)2↓+C02↑ Therefore, it can be judged that carbonate composite scale and sulfate composite scale are mainly deposited in the convection section of the steam injection boiler, and silicate composite scale is mainly deposited in the radiation section of the steam injection boiler.

[0023] Example 5: As an optimization of the above examples, in the carbonate composite scale, the mass content of calcium carbonate is 50% to 95%; in the sulfate composite scale, the mass content of calcium sulfate is greater than 50%; in the silicate composite scale, the mass content of silicon dioxide is 20% to 25%.

[0024] During the operation of the on-site steam injection boiler, the system pressure difference (the value of the working fluid outlet pressure minus the working fluid inlet pressure) is used to judge the scaling situation. Since the steam injection boiler is affected by the back pressure (oil well demand pressure) during the on-site operation, the system pressure difference will change greatly. In the actual on-site operation, when the operating parameters do not change much, the system pressure difference increase of more than 0.5MPa is usually used as a sign of system scaling. When the system pressure difference increases by more than 2MPa, it must be flushed. Since the steam injection boiler must be shut down for maintenance after running for a period of time, in order to reduce the loss of steam injection, it is generally flushed when the boiler is shut down. During operation, the increase of 2MPa in the system pressure difference is regarded as an abnormal situation and the load is reduced for flushing. During flushing, different types of scale layers can be descaled according to the descaling method for reuse and purified sewage in steam injection boilers of the present invention.

[0025] Example 6: The descaling method for recycling and purifying wastewater in a steam injection boiler is carried out according to the following steps: The first step is to establish the functional relationship of the existence state of minerals based on water chemistry theory: ΔG diff =f(Q i , K i , α i ) Where Q is the reaction entropy, K is the equilibrium constant at a specific temperature and pressure, and α is the ion activation degree; The second step is to establish a dynamic model of the dissolution rate D and influencing factors of the composite precipitate formed by carbonate, sulfate and silicate based on the hydrolysis reaction: D=f(ΔG,P,T,v,g,t,β) Among them, ΔG is the mineral free energy, P is the pressure, T is the temperature, v is the flow rate, g is the gas content, t is the immersion time, and β is the correction parameter; The third step is to combine the process flow and dynamic model of the steam injection boiler to determine the scale layer type: carbonate composite scale and sulfate composite scale are deposited in the convection section of the steam injection boiler; silicate composite scale is deposited in the radiation section of the steam injection boiler; The fourth step is to fit the relationship between carbonate composite scale dissolution rate D and pressure P, temperature T, flow rate v, immersion and flushing time t according to the scale layer type, based on the dynamic model, hydrolysis reaction and field operation parameters, as shown in Figure 1 As shown, from Figure 1 It can be seen that the main influencing factor of carbonate composite scale is temperature. According to the dynamic model, when the temperature is greater than 100°C, the dissolution rate does not change much, and the pressure and other factors have little effect. The longer the immersion time, the better. However, as time goes by, the dissolution rate does not change much and it is easy to cause under-scale corrosion. Therefore, the immersion time is generally not more than 3 days. Intermittent flow rate changes can flush the scale layer after immersion. It needs to be implemented in combination with the following on-site conditions: (1) During boiler maintenance and normal shutdown, after the automatic purge is completed, manually start the plunger pump for flushing. When the outlet temperature of the radiation section drops below 100°C, stop the pump and close the convection section inlet and radiation section outlet valves to ensure that the furnace tubes are filled with water. When the temperature drops to 40°C, it can be discharged.

[0026] (2) Combined with daily water quality monitoring data, the pressure difference of the boiler convection section, radiation section, and superheating section system is calculated. When the pressure difference changes by more than 0.5 MPa compared to normal, flushing can be carried out when the boiler is shut down for maintenance or equipment repair. The temperature should be controlled between 60°C and 150°C during flushing, and the flushing time should be no less than 30 minutes. If the pressure difference is still large, the flushing time needs to be extended and the furnace tubes should be soaked for more than 8 hours before flushing.

[0027] (3) For boilers that have been out of service for a long time and are determined to be mainly carbonate composite scale, the temperature should be controlled below 150°C during soaking. The higher the temperature, the better. The soaking time should be no less than 24 hours. Multiple soaking and flushing methods can be used to reduce the system pressure difference.

[0028] Example 7: The descaling method for recycling and purifying wastewater in a steam injection boiler is carried out according to the following steps: The first step is to establish the functional relationship of the existence state of minerals based on water chemistry theory: ΔG diff =f(Q i , K i , α i ) Where Q is the reaction entropy, K is the equilibrium constant at a specific temperature and pressure, and α is the ion activation degree; The second step is to establish a dynamic model of the dissolution rate D and influencing factors of the composite precipitate formed by carbonate, sulfate and silicate based on the hydrolysis reaction: D=f(ΔG,P,T,v,g,t,β) Among them, ΔG is the mineral free energy, P is the pressure, T is the temperature, v is the flow rate, g is the gas content, t is the immersion time, and β is the correction parameter; The third step is to combine the process flow and dynamic model of the steam injection boiler to determine the scale layer type: carbonate composite scale and sulfate composite scale are deposited in the convection section of the steam injection boiler; silicate composite scale is deposited in the radiation section of the steam injection boiler; The fourth step is to fit the relationship between the dissolution rate D of the sulfate composite scale and the pressure P, temperature T, flow rate v, immersion and flushing time t according to the scale layer type, based on the dynamic model, hydrolysis reaction and field operation parameters, as shown in Figure 2 As shown, from Figure 2 It can be seen that the main influencing factor of sulfate composite scale is temperature. The lower the temperature, the more sulfate scale layer will be hydrolyzed and fall off. According to the dynamic model, when the temperature is greater than 80°C, the dissolution rate does not change much, and the pressure and other factors have little effect. The longer the soaking time, the better. However, over time, the dissolution rate does not change much and it is easy to cause under-scale corrosion. Therefore, the soaking time is generally not more than 3 days. Intermittent flow rate changes can flush the scale layer after soaking. It needs to be implemented in combination with the following on-site conditions: (1) During boiler maintenance and normal shutdown, after the automatic purge is completed, manually start the plunger pump for flushing. When the outlet temperature of the radiation section drops below 100°C, stop the pump and close the convection section inlet and radiation section outlet valves to ensure that the furnace tubes are filled with water. When the temperature drops to 40°C, it can be discharged.

[0029] (2) Combined with daily water quality monitoring data, the pressure difference of the boiler convection section, radiation section, and superheating section system is calculated. When the pressure difference changes by more than 0.5MPa compared with the usual, flushing can be carried out when the boiler is shut down for maintenance or equipment repair. The temperature should be controlled at no more than 80℃ during flushing, and the flushing time should be no less than 30 minutes. If the pressure difference is still large, the flushing time needs to be extended and the furnace tubes should be soaked for more than 8 hours before flushing.

[0030] (3) For boilers that have been out of service for a long time and are judged to be mainly sulfate scale, the temperature should be controlled below 80°C during soaking. The lower the temperature, the better. The soaking time should be no less than 24 hours. Multiple soaking and flushing methods can be used to reduce the system pressure difference.

[0031] Example 8: The descaling method for recycling and purifying wastewater in a steam injection boiler is carried out according to the following steps: The first step is to establish the functional relationship of the existence state of minerals based on water chemistry theory: ΔG diff =f(Q i , K i , α i ) Where Q is the reaction entropy, K is the equilibrium constant at a specific temperature and pressure, and α is the ion activation degree; The second step is to establish a dynamic model of the dissolution rate D and influencing factors of the composite precipitate formed by carbonate, sulfate and silicate based on the hydrolysis reaction: D=f(ΔG,P,T,v,g,t,β) Among them, ΔG is the mineral free energy, P is the pressure, T is the temperature, v is the flow rate, g is the gas content, t is the immersion time, and β is the correction parameter; The third step is to combine the process flow and dynamic model of the steam injection boiler to determine the scale layer type: carbonate composite scale and sulfate composite scale are deposited in the convection section of the steam injection boiler; silicate composite scale is deposited in the radiation section of the steam injection boiler; The fourth step is to fit the relationship between the silicate composite scale dissolution rate D and the pressure P, temperature T, flow rate v, immersion and flushing time t according to the scale layer type, based on the dynamic model, hydrolysis reaction and field operation parameters, as shown in Figure 3 As shown, from Figure 3 It can be seen that the main influencing factors of silicate composite scale (silicate composite scale is generally composed of silicate and part of soluble composite silicate scale layer, and the present invention mainly removes soluble composite silicate) are temperature and pressure. The solubility of soluble composite silicate is most affected by temperature. Generally, the solubility is very small when the temperature is below 80°C, and the solubility increases sharply when it is greater than 80°C. At the same time, pressure has a greater impact on silicate scale. The dissolution rate increases sharply at 9MPa. Other factors have less impact. The longer the immersion time, the better. However, with the passage of time, the dissolution rate does not change much and under-scale corrosion is prone to occur. Therefore, the immersion time is generally not more than 3 days. Intermittent flow rate changes can flush the scale layer after immersion. It needs to be implemented in combination with the following on-site conditions: (1) Combined with daily water quality monitoring data, the pressure difference of the boiler convection section, radiation section, and superheating section system is calculated. When the pressure difference changes by more than 0.5MPa compared with the usual, flushing can be carried out when the boiler is shut down for maintenance or equipment repair. The temperature during flushing should be controlled between 80℃ and 150℃, and the flushing time should be no less than 30 minutes. If the pressure difference is still large, the flushing time needs to be extended and the furnace tubes should be soaked for more than 8 hours before flushing.

[0032] (2) When the boiler is out of service for a long time, the temperature should be controlled above 80°C during soaking. The higher the temperature, the better. The soaking time should be no less than 24 hours. Multiple soaking and flushing methods can be used to reduce the system pressure difference.

[0033] (3) For boilers that have been out of service for a long time and are judged to be mainly silicate scale, the soaking temperature should be controlled at above 100°C and the soaking time should be no less than 24 hours. The pressure should be kept between 9MPa and 12MPa during flushing. Multiple soaking and flushing methods can be used to reduce the system pressure difference. If the pressure conditions are not met, pressure change pulsation flushing can be used.

[0034] The beneficial effects of the present invention are as follows: (1) Cost savings The scale of the boiler tubes for reusing heavy oil purification water in domestic oil fields is generally removed by pickling. After adopting the present invention, the pickling cycle can be extended from 2 to 3 years to 5 years.

[0035] The boiler pickling cycle has been extended from 2 to 3 years to more than 5 years. Taking the heavy oil company as an example, which cleans 20 boilers annually, the acid cleaning cost of each boiler is 38,000 yuan, which can save 760,000 yuan annually. If Xinjiang Oilfield adopts this method to slow down the scaling trend of boilers, it can save more than 4 million yuan annually.

[0036] (2) Furnace tube service life Pickling will make the furnace tube thinner. When it is not enough to bear the pressure, the furnace tube has to be replaced. The present invention can reduce the number of cleaning times and extend the service life of the furnace tube by more than double.

[0037] (3) Fuel consumption Timely removal of pipe wall attachments can extend the service life of the equipment and save fuel. By using the soaking and flushing method during equipment maintenance intervals, each boiler can save about 50,000 cubic meters of natural gas per year (equipment operation is calculated based on 250 days / year).

[0038] In summary, the present invention is based on the dissolution theory based on key factors such as mineral dissolution equilibrium, fluid mixing parameters, gas separation due to pressure drop or boiling, and interaction between organic and inorganic substances. After predicting the type of scaling on the boiler tubes, flushing and soaking methods are adopted to eliminate the scale, thereby reducing the maintenance workload, reducing the production cost, and improving the operating efficiency. It has good application prospects in production.

[0039] The above technical features constitute the embodiments of the present invention, which have strong adaptability and implementation effect. Non-essential technical features can be added or reduced according to actual needs to meet the requirements of different situations.

Claims

1. A descaling method for recycling and purifying wastewater in a steam injection boiler, characterized in that Follow the steps below: The first step is to establish the functional relationship of the existence state of minerals based on water chemistry theory: ΔG diff =f(Q i 、K i 、α i ) Where Q is the reaction entropy, K is the equilibrium constant at a specific temperature and pressure, and α is the ion activation degree; The second step is to establish a dynamic model of the dissolution rate D and influencing factors of the composite precipitate formed by carbonate, sulfate and silicate based on the hydrolysis reaction: D=f(ΔG,P,T,v,g,t,β) Among them, ΔG is the mineral free energy, P is the pressure, T is the temperature, v is the flow rate, g is the gas content, t is the immersion time, and β is the correction parameter; The third step is to determine the scale layer type by combining the process flow and dynamic model of the steam injection boiler; The fourth step is to fit the relationship diagram of the dissolution rate D and pressure P, temperature T, flow rate v, immersion and flushing time t for different types of scale layers according to the type of scale layer, based on the dynamic model, hydrolysis reaction and field operation parameters. By adjusting the flushing and soaking time of the boiler tube scale, the descaling of different types of scale layers can be completed.

2. The descaling method for recycling and purifying wastewater in a steam injection boiler according to claim 1, characterized in that In the second step, in the functional relationship of the mineral's existence state, the judgment of the mineral's existence state includes: when ΔG diff <0, mineral dissolution is unsaturated and minerals do not precipitate; ΔG diff >0, the mineral is supersaturated, the precipitation conditions are met, and the mineral precipitates.

3. The descaling method for recycling and purifying wastewater in a steam injection boiler according to claim 1 or 2, characterized in that In the third step, when judging the scale layer type, the specific operation is: judge the scale layer type according to the water quality composition of the feed water and the heating intensity of the feed water on each heating surface and the change process of the working fluid in the process flow of the steam injection boiler.

4. The descaling method for recycling and purifying wastewater in a steam injection boiler according to claim 1 or 2, characterized in that The scale layer types include: carbonate composite scale and sulfate composite scale deposited in the convection section of the steam injection boiler; silicate composite scale deposited in the radiation section of the steam injection boiler.

5. The descaling method for recycling and purifying wastewater in a steam injection boiler according to claim 3, characterized in that The scale layer types include: carbonate composite scale and sulfate composite scale deposited in the convection section of the steam injection boiler; silicate composite scale deposited in the radiation section of the steam injection boiler.

6. The descaling method for recycling and purifying wastewater in a steam injection boiler according to claim 4, characterized in that In carbonate composite scale, the mass content of calcium carbonate is 50% to 95%; in sulfate composite scale, the mass content of calcium sulfate is 50% to 95%; in silicate composite scale, the mass content of silicon dioxide is 20% to 25%.

7. The descaling method for recycling and purifying wastewater in a steam injection boiler according to claim 5, characterized in that In carbonate composite scale, the mass content of calcium carbonate is 50% to 95%; in sulfate composite scale, the mass content of calcium sulfate is 50% to 95%; in silicate composite scale, the mass content of silicon dioxide is 20% to 25%.

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