Method for relieving scaling of discharge pipe of autoclave in metallic nickel extraction process

Abstract

The invention discloses a method for relieving scaling of a discharge pipe of an autoclave in a metal nickel extraction process. According to the method, iron oxide is added into mineral raw materials to promote iron to be hydrolyzed and precipitated on the iron oxide in the acid leaching process, so that scaling of a discharge pipe of the autoclave is relieved. According to the method, under the condition that new impurities are not introduced, the single-time operation period of the high-pressure kettle is remarkably prolonged, the production efficiency of the system is improved, high-pressure kettle scaling, high-flash slag, medium-flash slag and high-pressure leaching slag can be adopted as iron oxide, waste internal recycling is effectively achieved, and the production cost is greatly reduced.

Description

Technical Field

[0001] The invention relates to a method for alleviating scaling of a high-pressure autoclave discharge pipe in a nickel metal extraction process, and belongs to the field of hydrometallurgy. Technical Background

[0002] High-Pressure Acid Leaching (HPAL) is a process used to extract nickel from nickel ore. With increasing resource demands and environmental protection requirements, the role of the HPAL process is becoming increasingly prominent.

[0003] High-pressure acid leaching (HPAL) is an effective method for extracting nickel from low-grade nickel ores, particularly sulfide and oxide ores with low nickel content. This process is particularly suitable for processing lean ores, providing a relatively economical extraction solution. HPAL is commonly used to process laterite ore, a type of nickel ore widely distributed in tropical regions. Many major nickel-producing countries worldwide, such as Indonesia, the Philippines, and New Caledonia, rely on this technology to extract nickel.

[0004] Nickel is widely used in stainless steel production, battery manufacturing, and other high-tech applications. To meet this growing demand, China requires various nickel ore extraction technologies, including HPAL (High Pressure Acid Leaching) (HPAL). China is in need of nickel ore resources, particularly high-grade nickel sulfide and laterite ores. Consequently, companies are actively seeking to improve the economics and recovery rates of the HPAL process. This process is a key means of meeting China's growing nickel demand. As global demand for battery materials and stainless steel increases, the role of high-pressure acid leaching (HPAL) nickel ore is expected to grow even more significant.

[0005] However, the main application object of high pressure acid leaching is low-grade nickel-iron associated ore - limonite. Iron dissolves into soluble iron ions in sulfuric acid environment. The main dissolved form is iron (II) ions (Fe 2+ ) and iron (III) ions (Fe 3+ ). Under the action of high temperature and oxygen, iron (II) ions can be oxidized to iron (III) ions. This process increases the solubility of iron, but also leads to further conversion of iron into insoluble iron oxides or hydroxides.

[0006] Iron hydrolysis precipitates (such as goethite and hematite) can form deposits inside equipment (e.g., reactors, pipes, filters, etc.). These deposits can clog pipes and equipment, hindering material flow and reaction efficiency. Once deposits form, they are often difficult to remove. Equipment blockages may require equipment downtime for cleaning, resulting in production delays and additional maintenance costs. The flow of deposits within the equipment can cause wear, especially on moving parts such as pumps and agitators. This wear shortens equipment life and increases the frequency of repairs and replacements. Deposits can impair heat exchange efficiency in cooling systems, leading to temperature control issues in reactors or other critical equipment, thus impacting production stability and efficiency. Clogged autoclave discharge pipes and high-pressure flash valves can manifest as difficulty discharging the autoclave, even after increasing the valve opening. The only option in this situation is to shut down the autoclave for inspection and clean the scale inside the autoclave and discharge pipe. This increases downtime and maintenance costs, impacting overall production efficiency and economic viability.

[0007] Existing techniques use acidic descaling agents to remove iron oxides, but this method can easily introduce impurities that affect product purity, and secondary separation increases production costs. Therefore, finding a method to prevent or reduce the hydrolysis and precipitation of iron in the autoclave discharge pipe is crucial for improving the economic benefits of nickel smelting companies, thereby reducing equipment losses and increasing the equipment's effective operating cycle. Summary of the Invention

[0008] To address the problems of the prior art, the present invention aims to provide a method for alleviating scaling in the autoclave discharge pipe during nickel extraction. This method effectively reduces scaling in the autoclave discharge pipe without introducing new impurities, thereby improving the operating efficiency of the acid leaching process.

[0009] In order to achieve the above technical objectives, the present invention provides a method for alleviating scaling of a high-pressure autoclave discharge pipe in a metallic nickel extraction process. The method comprises adding an iron oxide-containing material to a mineral raw material during an acid leaching process, wherein the mass content of iron in the iron oxide-containing material is not less than 52% of the solid weight of the material, and the dry weight ratio of the iron oxide-containing material to the mineral raw material is 0.2 to 1:100.

[0010] During high-pressure acid leaching of nickel, the autoclave and discharge pipe are susceptible to scaling caused by the hydrolysis of the element Fe. This scaling occurs when Fe dissolves within the autoclave and then hydrolyzes into iron oxide. In actual production, this hard iron oxide scaling forms a hard shell that gradually thickens from the autoclave's feed inlet to the discharge port, and from the top to the bottom. This hard shell effectively protects the autoclave. As production continues, the iron oxide scaling gradually increases, leading to blockage in the autoclave discharge pipe and high-flash valve (autoclave discharge valve), resulting in discharge difficulties.

[0011] When the leaching material first comes into contact with sulfuric acid in the autoclave, Fe reacts with the sulfuric acid and is leached. It is then gradually hydrolyzed into iron oxide under high temperature and pressure until leaching and reaction equilibrium is reached. Because the iron hydrolysis process is more easily carried out on structurally similar substances—in the autoclave leaching system, this substance is hard iron oxide itself—the present invention adds iron oxide (such as hard iron oxide) to the autoclave leaching system. This allows the dissolved Fe element to immediately come into contact with the iron oxide components for hydrolysis, prompting the iron oxide to precipitate on the surface of the flowable particles rather than on the autoclave's inner walls or in the discharge pipe. The newly generated iron oxide then flows out of the autoclave with the slurry, reducing or alleviating discharge pipe scaling and thus resolving discharge pipe blockage issues.

[0012] Controlling the iron oxide addition level within an appropriate range is beneficial for controlling discharge pipe scaling. If the iron oxide addition level is too low, the iron hydrolyzate cannot effectively reach the iron oxide, and the discharge pipe scaling rate is not significantly reduced. Due to the operating principle and internal structure of the autoclave, the autoclave feed port is located in compartment one, and the autoclave discharge pipe is located in compartment seven. The baffle between compartments one and two is the tallest and has the smallest bottom through-hole. Therefore, when the iron oxide addition level is too high, the bottom through-hole of the baffle is easily clogged, reducing the residence time of the slurry in the autoclave, thereby affecting the leaching rate of valuable metals (Ni, Co), significantly affecting production efficiency. Furthermore, an excessively high addition ratio can easily cause rapid scaling and weight gain in the agitator in compartment one of the autoclave, gradually increasing the torque of the agitator motor, and affecting production safety and stability.

[0013] As a preferred solution, during the stable production period of the autoclave, the dry weight ratio of the iron oxide-containing material to the mineral raw material is 0.4-0.8:100.

[0014] As a preferred solution, when the autoclave discharge decreases and the overpressure inside the autoclave increases, an iron oxide-containing material is added to the mineral raw material. The hard scale formed by the iron oxide during the initial acid leaching process protects the autoclave body and is beneficial for production. However, as the system continues to operate, the hard scale gradually increases and can cause blockage in the autoclave's discharge pipe. Adding iron oxide to the mineral raw material can reduce or alleviate scaling in the discharge pipe, preventing serious blockage. In actual production, a decrease in discharge can be determined by observing a rise in the actual liquid level in the autoclave (e.g., a 2%-3% increase above the normal level), while an increase in overpressure can be determined by observing a pressure increase (e.g., a 2%-3% increase above the normal pressure).

[0015] As a preferred solution, iron oxide-containing materials are added to the mineral raw materials 36 to 72 hours after the autoclave is started up; as the system continues to operate, when the autoclave discharge continues to decrease and the overpressure in the autoclave increases, the amount of iron oxide-containing materials added is increased.

[0016] As the production system operates, the amount of fouling in the discharge pipe gradually increases. When the discharge decreases or the overpressure in the kettle increases, increasing the amount of iron oxide added can further alleviate the blockage problem. When the iron oxide addition is increased, a large amount of iron is hydrolyzed early and settles in the kettle. Since the hydrolysis of Fe is basically completed in the tail area, the amount of fouling is significantly reduced.

[0017] As a preferred option, the dry weight ratio of the initially added iron oxide-containing material to the raw mineral material is 0.2-0.3:100. Within 36-72 hours of autoclave operation, a small amount of iron oxide scale forms, protecting the autoclave's inner walls and reducing internal corrosion and wear from slurry impact. After 36-72 hours of operation, adding a small amount of iron oxide can significantly reduce the scaling rate and maintain the autoclave's effective volume at its maximum. Excessive initial iron oxide addition can easily clog the bottom openings of the reactor baffle, reducing the slurry's residence time in the autoclave, thereby affecting the leaching rate of valuable metals (Ni, Co), and thus production efficiency. Furthermore, excessive initial iron oxide addition can easily lead to rapid scaling and weight gain in the agitator compartment of the autoclave, gradually increasing the torque of the agitator motor and compromising production safety and stability.

[0018] As a preferred solution, the material containing iron oxides is iron oxide slurry.

[0019] As a preferred solution, the solid content of the iron oxide slurry is 15-25 wt%.

[0020] As a preferred solution, the iron oxide slurry is prepared by mixing iron oxide slag gravel with water.

[0021] As a preferred solution, the iron oxide slag gravel comprises at least one of autoclave scale, high flash slag, medium flash slag, and acid leaching residue. Because autoclave scale, high flash slag, medium flash slag, and high pressure leaching residue primarily consist of hard iron oxide, which originates from the acid leaching process, adding this slag to the system does not affect the original reaction system and can also enable waste recycling, significantly reducing production costs.

[0022] As a preferred solution, the particle size of the iron oxide slag gravel does not exceed 38 μm.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] (1) Simply by adding iron oxide, the scaling of the autoclave discharge pipe can be greatly reduced, the blockage of the high flash valve can be effectively avoided, the operation cycle of the autoclave can be improved, the maintenance frequency can be reduced, and the production system efficiency can be significantly improved. No new impurities are introduced, and there is no impact on the entire acid leaching system.

[0025] (2) Iron oxides can be recycled by using production waste materials such as autoclave scale, high flash slag, medium flash slag, and acid leaching residue to achieve internal circulation in the system, so that the production waste materials can be "self-digested" and production costs can be greatly reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a process flow chart of the present invention.

[0027] Figure 2 Schematic diagram of the front, middle and tail areas of the autoclave. DETAILED DESCRIPTION

[0028] The present invention will be further described in detail below in conjunction with specific embodiments, but the protection scope of the present invention is not limited to the contents described above.

[0029] Example 1

[0030] A method for alleviating scaling of an autoclave discharge pipe by using a flowable slurry, comprising the following steps:

[0031] The autoclave is the final trial autoclave used in the high-pressure acid leaching production workshop. Its internal structure is consistent with the actual production equipment structure. The temperature in the high-pressure acid leaching stage is maintained at 245~248℃, the pressure in the autoclave is 4.4±0.05MPa, and the concentrated sulfuric acid used in production is 98% industrial sulfuric acid.

[0032] (1) Select two groups of experimental autoclaves of the same specifications and flush the inside of the autoclaves. Label them A and B respectively. The sections from the feed port to the discharge port in the autoclave are named as the front end, middle, and tail areas in the order of 30%, 40%, and 30% (e.g. Figure 2 ).

[0033] (2) The autoclaves in group A were subjected to conventional high-pressure acid leaching without any other treatment. The autoclaves in group B used high-pressure acid leaching slurry, which was added with high-pressure acid leaching slag slurry (the dry basis content of the slurry was 15%) at a ratio of 0.4:100 of the dry basis weight of the high-pressure acid leaching slag to the mineral raw materials. The particle size of the high-pressure acid leaching slag was below 38 μm and the mass content of Fe element was 52%.

[0034] (3) Two groups of autoclaves simultaneously carried out high-pressure acid leaching continuous leaching experiments (the raw materials were continuously fed, that is, the high-pressure acid leaching residue of group B was continuously added to the mineral raw materials according to the above ratio to form the initial material of acid leaching slurry), and various parameters were linked during the experiment.

[0035] (4) After 360 hours of continuous leaching experiment, the scaling conditions in the two kettles were compared.

[0036] After 360 hours of continuous leaching, iron oxide scale formed in the autoclave of Group A. The scale thickness increased with the position within the autoclave from compartments 1 to 7, coating the inner walls of the autoclave. Scaling was evident in the discharge pipe of the autoclave, consistent with actual production conditions. The scale thickness in the front end of the autoclave of Group B increased slightly compared to that of Group A; the scale thickness in the middle area of ​​Group B decreased slightly compared to that of Group A; and the scale thickness in the tail area and discharge pipe was significantly reduced compared to that of Group A.

[0037] From the situation of the autoclave in group B, it can be seen that when the flowable slag slurry is added according to the ratio of slag gravel: dry ore = 0.4:100, the scaling rate in the tail area of ​​the autoclave and the discharge pipe is reduced by 21.3% (1-98.2 / 124.9), the overall scaling rate of the autoclave is reduced by 8.8%, and the scaling rate in the front and middle areas of the autoclave is slightly reduced.

[0038] The scale in the kettle was cleaned and weighed, and the results are shown in Table 1.

[0039]

[0040] Example 2

[0041] A method for alleviating scaling of an autoclave discharge pipe by using a flowable slurry, comprising the following steps:

[0042] (1) Two groups of experimental autoclaves of the same specifications were selected and flushed inside the autoclaves, which were labeled A and B respectively. The areas from the feed port to the discharge port in the autoclave were named as the front end, middle area, and tail area in the order of 30%, 40%, and 30%.

[0043] (2) The autoclaves in group A were subjected to conventional high-pressure acid leaching without any other treatment. The autoclaves in group B used high-pressure acid leaching slurry, which was added with high-pressure acid leaching slag slurry (the dry basis content of the slurry was 20%) according to the ratio of high-pressure acid leaching slag to the dry basis weight of the mineral raw materials of 0.6:100, wherein the particle size of the high-pressure acid leaching slag was less than 38 μm and the Fe content was 52%.

[0044] (3) Two groups of autoclaves were used to conduct high-pressure acid leaching continuous leaching experiments simultaneously, and all parameters were linked during the experiment.

[0045] (4) After 360 hours of continuous leaching experiment, the scaling conditions in the two kettles were compared.

[0046] The autoclave is the final trial autoclave used in the high-pressure acid leaching production workshop. Its internal structure is consistent with the actual production equipment structure. The temperature during the high-pressure acid leaching stage is maintained at 245~248℃, and the pressure inside the autoclave is 4.4±0.05MPa. The concentrated sulfuric acid used in production is 98% industrial sulfuric acid.

[0047] After 360 hours of continuous leaching, iron oxide scale formed in the autoclave of Group A. The scale thickness increased with increasing location within the autoclave from compartments 1 to 7, coating the entire autoclave wall. Scaling was evident in the autoclave discharge pipe, consistent with actual production conditions. Compared to Example 1, increasing the proportion of flowable slurry added increased scale at the front end of the autoclave and further decreased scale at the rear end.

[0048] It can be seen that when the flowable slag slurry is added at a ratio of slag gravel: dry ore = 0.6:100, the scaling rate in the tail area of ​​the autoclave and the discharge pipe is reduced by 26.3%, while the scaling rate in the front and middle areas of the autoclave remains basically unchanged.

[0049] The scale in the kettle was cleaned and weighed, and the results are shown in Table 2.

[0050]

[0051] Example 3

[0052] A method for alleviating scaling of an autoclave discharge pipe by using a flowable slurry, comprising the following steps:

[0053] (1) Two groups of experimental autoclaves of the same specifications were selected and flushed inside the autoclaves, which were labeled A and B respectively. The areas from the feed port to the discharge port in the autoclave were named as the front end, middle area, and tail area in the order of 30%, 40%, and 30%.

[0054] (2) The autoclaves in group A were subjected to conventional high-pressure acid leaching without any other treatment. The high-pressure acid leaching slurry used in the autoclaves in group B was added with high-pressure acid leaching slurry (the dry basis content of the slurry was 25%) according to the ratio of high-pressure acid leaching slag to mineral raw material dry weight of 0.6:100, wherein the particle size of the high-pressure acid leaching slag was less than 38 μm and the Fe content was 52%.

[0055] (3) Two groups of autoclaves were used to conduct high-pressure acid leaching continuous leaching experiments simultaneously, and all parameters were linked during the experiment.

[0056] (4) After 360 hours of continuous leaching experiment, the scaling conditions in the two kettles were compared.

[0057] The autoclave is the final trial autoclave used in the high-pressure acid leaching production workshop. Its internal structure is consistent with the actual production equipment structure. The temperature in the high-pressure acid leaching stage is maintained at 245~248℃, the pressure in the autoclave is 4.4±0.05MPa, and the concentrated sulfuric acid used in production is 98% industrial sulfuric acid.

[0058] After 360 hours of continuous leaching, iron oxide scale formed in the autoclave of Group A. The scale thickness increased with increasing location within the autoclave from compartments 1 to 7, coating the entire autoclave wall. Scaling was evident in the autoclave discharge pipe, consistent with actual production conditions. Compared to Examples 1 and 2, increasing the proportion of flowable slurry added increased scale at the front end of the autoclave and further decreased scale at the rear end.

[0059] The scale deposits within the autoclave were cleaned and weighed, and the results are shown in Table 3. When the proportion of flowable slurry added was further increased, the amount of scale deposited in the front end of the autoclave increased significantly, while the amount of scale deposited in the rear end of the autoclave decreased significantly. With the increased slurry proportion, a large amount of iron oxide was hydrolyzed prematurely and settled within the autoclave. This macroscopic manifestation was a significant increase in scale deposits in the front end of the autoclave, while the amount of scale deposited in the rear end of the autoclave decreased significantly due to the near-complete hydrolysis of Fe.

[0060] When flowable slurry is added at a ratio of slag gravel: dry ore = 0.75:100, the scaling rate in the tail area of ​​the autoclave and the discharge pipe is reduced by 42%, the scaling rate in the front area of ​​the autoclave is accelerated, and the scaling rate in the middle area remains basically unchanged. The increased scaling in the front and middle areas of the autoclave has little impact on production. In actual production, after the autoclave has been running for a certain period of time, scaling will be obvious in the tail area of ​​the autoclave, especially at the connection between the tail area of ​​the autoclave and the autoclave discharge pipe, where the inner diameter of the container suddenly narrows. Scaling of the same thickness has little effect on the autoclave itself, but it is very easy to cause blockage in the discharge pipe with a smaller inner diameter. The macroscopic manifestation is difficulty in discharging the autoclave and a significant increase in the opening of the high flash valve. Since the leaching and hydrolysis of Fe are both carried out in the autoclave, increasing the slurry ratio will not cause scaling and blockage in the autoclave feed pipe. When the autoclave discharge pipe is blocked, the autoclave is difficult to discharge, and the high flash valve opening increases significantly, increasing the slurry ratio can effectively alleviate the blockage of the autoclave tail and the discharge pipe, thereby extending the autoclave working cycle.

[0061]

[0062] Example 4

[0063] A method for alleviating scaling of an autoclave discharge pipe by using a flowable slurry, comprising the following steps:

[0064] (1) Retain, crush and screen the autoclave scale and high flash slag and medium flash slag cleaned during the maintenance of the high-pressure acid leaching system, and use them to make flowable slurry (iron oxide slurry), in which the particle size of the slag gravel does not exceed 38μm.

[0065] (2) After the autoclave maintenance is completed, the normal start-up process is carried out.

[0066] (3) After the autoclave is heated, no flowable slag is added within 72 hours to promote a small amount of scaling in the autoclave to protect the autoclave. After 72 hours, flowable slag slurry is delivered to the low-temperature preheater feed trough at a ratio of dry weight slag gravel: dry ore = 0.47:100, where the slag slurry mass concentration is 20%.

[0067] (4) After the flowable slurry is mixed with the acid leaching raw material, it stays in the low-temperature preheater feed trough for about 2 hours and then enters the low-temperature preheater.

[0068] (5) The flowable slurry and acid leaching raw materials are heated in a preheater and continuously transported to the autoclave for high-pressure acid leaching continuous production (wherein the normal actual liquid level in the autoclave is 80% of the container height and the pressure in the autoclave is 4.4 MPa).

[0069] (6) After acid leaching, the slurry is neutralized, washed, and thickened to obtain the bottom flow acid leaching slurry.

[0070] (7) Carry out random inspection on a batch of bottom flow acid leaching slurry.

[0071] (8) If the sampling results show that some heavy Fe elements are ≥52%, about 55%, the batch will be sent to the vibrating screen to make flowable slurry; if the conditions are not met, the tailings will be sent for treatment.

[0072] (9) When the actual liquid level of the material in the autoclave reaches 82% of the container height and the pressure in the autoclave reaches 4.5MPa, the opening of the autoclave discharge valve (high flash valve) is appropriately increased, and the amount of flowable slurry (mass concentration is 20%) added is increased to dry weight slag gravel: dry ore = 0.6:100. This operation can alleviate the blockage of the autoclave discharge pipe and return the actual liquid level of the material in the autoclave and the pressure in the autoclave to normal production levels. As the system continues to operate, when the actual liquid level of the material in the autoclave reaches 82% again and the pressure in the autoclave reaches 4.5MPa again, the amount of flowable slurry added is further increased to dry weight slag gravel: dry ore = 0.8:100. At this time, the blockage problem of the autoclave discharge pipe is alleviated again. The scaling rate in the front and middle areas of the autoclave has increased significantly, and the autoclave should be stopped for maintenance according to the production capacity plan.

[0073] (9) Record the time from the start of the autoclave to the second maintenance operation of the autoclave. The basis for the autoclave to be maintained is: the autoclave discharge pipe is blocked, the high flash valve cannot be opened normally or cannot discharge normally after opening to a large extent.

[0074] This example and Example 1 used different autoclave series in the same process section. The two series of equipment were highly consistent, and production parameters were controlled separately. During autoclave operation, the average Fe content of the waste materials used to produce the flowable slurry (autoclave scale, high-flash slag, medium-flash slag, and acid leaching residue) was 53.7%. The autoclave's designed maintenance cycle is twice per year. This run lasted 225 days, significantly increasing its effective operating period.

[0075] This demonstrates that the present invention utilizes the variability in Fe precipitation through hydrolysis, increasing the total surface area within the autoclave through the use of flowable slurry. This allows some of the precipitated iron oxide to be deposited on the flowable particles, where it is then removed from the system along with the material flow. When the flowable slag contains ≥52% Fe, it is delivered to the system at a ratio of 0.4-0.8 dry slag weight to dry ore weight (100), with the delivery ratio adjusted based on production duration. This significantly improves the autoclave's operating cycle and reduces scale buildup within the autoclave and within the autoclave by 20-25%.

[0076] The above describes the specific embodiments of the present invention in detail, but the present invention is not limited to the above embodiments. Various changes can be made within the knowledge of ordinary technicians in this field without departing from the scope of the present invention.

Claims

1. A method for alleviating scaling of an autoclave discharge pipe in a nickel extraction process, characterized in that: During the acid leaching process, an iron oxide-containing material is added to the mineral raw material, the mass content of iron in the iron oxide-containing material is not less than 52% of the solid weight of the material, and the dry weight ratio of the iron oxide-containing material to the mineral raw material is 0.2~1:

100.

2. The method for alleviating scaling of an autoclave discharge pipe in a nickel extraction process according to claim 1, characterized in that: The dry weight ratio of the iron oxide-containing material to the mineral raw material is 0.4-0.8:

100.

3. The method for alleviating scaling of an autoclave discharge pipe in a nickel extraction process according to claim 1, wherein: When the discharge of the autoclave decreases and the overpressure in the autoclave increases, materials containing iron oxides are added to the mineral raw materials.

4. The method for alleviating scaling of an autoclave discharge pipe in a nickel extraction process according to claim 1 or 3, characterized in that: After the autoclave has been running for 36h~72h, materials containing iron oxides are added to the mineral raw materials. As the system runs, when the discharge of the autoclave decreases and the overpressure in the autoclave increases, the amount of iron oxide-containing materials added is increased.

5. The method for alleviating scaling of an autoclave discharge pipe in a nickel extraction process according to claim 4, characterized in that: The dry weight ratio of the initially added iron oxide-containing material to the mineral raw material is 0.2~0.3:

100.

6. The method for alleviating scaling of an autoclave discharge pipe in a nickel extraction process according to claim 1, characterized in that: The material containing iron oxides is iron oxide slurry.

7. The method for alleviating scaling of an autoclave discharge pipe in a nickel extraction process according to claim 6, characterized in that: The solid content of the iron oxide slurry is 15-25 wt %.

8. The method for alleviating scaling of an autoclave discharge pipe in a nickel extraction process according to claim 6 or 7, characterized in that: The solid in the iron oxide slurry is iron oxide slag gravel.

9. The method for alleviating scaling of an autoclave discharge pipe in a nickel extraction process according to claim 8, characterized in that: The iron oxide slag gravel includes at least one of autoclave scaling, high flash slag, medium flash slag and acid leaching slag.

10. The method for alleviating scaling of an autoclave discharge pipe in a nickel extraction process according to claim 8 or 9, characterized in that: The particle size of the iron oxide slag gravel does not exceed 38 μm.

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