Desulfurized fly ash composite pellet and preparation method thereof

Abstract

The invention discloses a desulfurized fly ash composite pellet and a preparation method, and belongs to the technical field of metallurgical solid waste recycling. The composite green pellet sequentially comprises an inner core, an outer core, a carbon reducing agent and a binder from inside to outside, the inner core comprises a carbon reducing agent, and the particle size of the carbon reducing agent is 0.2-0.4 mm; the raw materials of the middle layer comprise desulfurized fly ash and iron ore powder; the raw materials of the outer layer comprise a carbon reducing agent and iron ore powder, and the particle size of the carbon reducing agent in the outer layer is smaller than 0.1 mm. According to the method, the desulfurized fly ash and the carbon reducing agent are added into the pellets, and the pellets are designed to be of a multi-layer composite structure, so that the sulfur element in the desulfurized fly ash can be directly and effectively removed in the pellet roasting process, and resource utilization of the desulfurized fly ash serving as a calcium flux in the pellet production process can be achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of metallurgical solid waste resource utilization, and more specifically, relates to a desulfurization ash composite pellet and a preparation method thereof. Background Art

[0002] With the continuous increase in the intensity of controlling and reducing SO 2 emissions in China, flue gas desulfurization has entered a rapid development stage. Along with the successive installation of desulfurization devices, the amount of desulfurization products is increasing, and the comprehensive utilization of desulfurization ash has become an urgent problem to be solved. Since the composition of semi-dry desulfurization ash is extremely complex, consisting of various components such as desulfurizing agents, desulfurization products and fly ash, and the calcium sulfite in desulfurization ash is very unstable during the utilization process, most of it is mainly treated by stacking and discarding, which not only causes waste of resources, but also damages the surrounding environment.

[0003] The use of fluxed pellets is of great significance to the blast furnace ironmaking process. It not only helps to optimize the burden structure, improve the quality of the burden, but also can effectively reduce the energy consumption and emissions during the blast furnace ironmaking process, achieve energy conservation and emission reduction, and at the same time improve the strength and metallurgical properties of pellet ore, which plays an important role in enhancing the blast furnace ironmaking efficiency and economic benefits.

[0004] Semi-dry sintering flue gas desulfurization ash contains substances such as free calcium oxide, calcium sulfite, calcium carbonate, calcium sulfate and calcium hydroxide, and has a very high content of calcium element substances. If the desulfurization ash can be used as a calcium flux for pellets and realize its harmless resource treatment in pellets, it will promote the resource utilization of metallurgical solid waste and industrial waste.

[0005] After retrieval, there have been a small number of studies on adding desulfurization ash to pellets. For example, the patent application case with the Chinese patent application number 2018114889033 discloses a pellet ore and a production method thereof. The pellet ore of this application case is made by pelletizing and roasting raw materials including the following components: 2-4 parts by weight of desulfurization ash mixture; 96-98 parts by weight of iron-containing powder; wherein, the desulfurization ash mixture is composed of desulfurization ash and bentonite, and the content of desulfurization ash in the desulfurization ash mixture is 5-30 wt%. By using desulfurization ash as a raw material for producing pellet ore, this application case can realize the resource utilization of industrial solid waste desulfurization ash, but it does not consider the adverse effects of sulfur in desulfurization ash on the blast furnace.

[0006] For another example, the application case with the Chinese patent application number 202310752772X discloses a method and device for the utilization of desulfurized ash resources. The method of this application case includes: heating and decomposing desulfurized ash using a plasma emitter to obtain sulfur dioxide and calcium oxide-containing materials; recycling the calcium oxide-containing materials to the preparation of ironmaking furnace charge, thereby completing the resource utilization of desulfurized ash. Although this application case considers the influence of S in desulfurized ash, it requires preheating and decomposing desulfurized ash into sulfur dioxide and calcium oxide-containing materials using a plasma emitter to remove sulfur dioxide from desulfurized ash before it can be used in the preparation of furnace charges such as sinter and pellet. Summary of the Invention

[0007] The present invention provides a desulfurized ash composite pellet and a preparation method thereof. By adding desulfurized ash and a carbon reducing agent to the pellet and designing the pellet as a multi-layer composite structure, the sulfur element in the desulfurized ash can be effectively removed directly during the pellet roasting process, so that the resource utilization of desulfurized ash as a calcareous flux in the pellet production process can be realized.

[0008] To achieve the above object, the technical solution provided by the present invention is as follows:

[0009] The present invention provides a desulfurized ash composite pellet. The green pellet of this composite pellet sequentially includes from the inside to the outside:

[0010] An inner core, and the raw materials of the inner core include a carbon reducing agent;

[0011] A middle layer, and the raw materials of the middle layer include desulfurized ash and iron ore powder; and

[0012] An outer layer, and the raw materials of the outer layer include a carbon reducing agent and iron ore powder.

[0013] Since the main components of desulfurized ash are sulfur-containing substances such as calcium sulfite, and the sulfur element will have an adverse impact after entering the blast furnace, which limits the application of desulfurized ash in pellets. In view of the above situation, the present invention adds desulfurized ash internally and adds a carbon reducing agent during the pelletizing process of the pellet, so that the desulfurization process can be completed in advance during the pellet roasting process. Therefore, the successful resource recovery and utilization of desulfurized ash in the pellet process can be realized, which is beneficial to achieving the goal of improving quality and reducing consumption in the steelmaking process and can ensure the smooth operation of the blast furnace.

[0014] It should be noted at the same time that how to effectively ensure the desulfurization effect of the pellet and the normal application performance of the pellet at the same time is another technical problem to be solved when applying desulfurized ash to the pellet. The present invention optimizes the design of the structure of the pellet, that is, designs it as a multi-layer composite structure composed of different components, so that the desulfurization treatment of desulfurized ash can be carried out during the pellet roasting process, reducing the sulfur element in advance and taking it as SO 2It can be released in a form, and at the same time, it can effectively ensure the desulfurization effect even when the amount of reducing agent added is small, avoiding the hardening of pellets caused by excessive addition of reducing agent.

[0015] Furthermore, the particle size of the carbon reducing agent in the core is 0.2 - 0.4 mm, and the particle size of the outer layer carbon reducing agent is less than 0.1 mm.

[0016] The present invention further screens the carbon reducing agent and adopts a grading method. The coarse particle size carbon reducing agent is used as the mother nucleus for pellet preparation, and then the mixture of desulfurized ash and iron ore powder is used as the intermediate layer and wrapped outside the mother nucleus, which is beneficial to increasing the contact area between the desulfurized ash and the carbon reducing agent and improving the desulfurization efficiency; the carbon reducing agent will gradually disappear through combustion reaction with high temperature during pellet roasting. During this process, gas channels will be formed. The mixture of fine particle size carbon reducing agent and iron ore powder is further used as the outer layer mixture and wrapped outside the intermediate layer, so that the actually released SO 2 can escape in the gas channels, avoiding further residue of sulfur elements.

[0017] Furthermore, the addition amount of desulfurized ash is controlled to make the pellet basicity 0.7 - 1. The basicity has an important influence on the pellet properties. Appropriate basicity can greatly improve the pellet properties. When the basicity is too high, the amount of liquid phase generated in the pellet is too much, which hinders the formation of solid phase intercrystals in the pellet and is not conducive to the improvement of pellet strength; when the basicity is too low, the amount of liquid phase generated in the pellet is too little, it is difficult to form an effective bonding phase, the porosity of the pellet increases, and it is easy to powder during the blast furnace reduction process, reducing the blast furnace permeability and being not conducive to the blast furnace operation. The present invention controls the addition amount of desulfurized ash to regulate the pellet basicity to 0.7 - 1, so as to ensure the service performance of the pellet and the normal operation of the blast furnace, and make it meet the requirements of industrial production. More preferably, the pellet basicity is controlled to be 0.8. It should be noted that the basicity calculation in the present invention adopts binary basicity with calcium oxide and silicon dioxide.

[0018] Furthermore, the addition amount of the carbon reducing agent, that is, the mass ratio of the carbon reducing agent to the desulfurized ash added, is controlled to make the mass ratio of C and S in the pellet 1 - 1.5, and more preferably 1.2. If the addition amount of the carbon reducing agent is too small, the desulfurization effect cannot be guaranteed, but if the addition amount is too large, excessive carbon reducing agent will cause hardening in pellet production.

[0019] Furthermore, the particle size of the composite pellet green balls is 11 - 12 mm; and / or the thicknesses of the intermediate layer and the outer layer in the composite pellet green balls are 8 - 9 mm and 2 - 4 mm respectively.

[0020] Further, the mass ratio of the intermediate layer to the outer layer of iron ore powder is 1:2 to 1:4; and / or the mass ratio of the inner core to the outer layer of carbon reducing agent is 1:4 to 1:9, so that the contact area between the desulfurization ash and the carbon reducing agent can be increased, improving the reaction efficiency; it can also supply heat for the pellet roasting process and improve the roasting effect of the pellets; at the same time, holes will be left during the combustion process of the outer layer carbon reducing agent for SO 2 release.

[0021] Further, the carbon reducing agent includes but is not limited to coke (fixed carbon content is about 80%), biochar, pulverized coal, coke powder and semi-coke.

[0022] Further, the composite pellet raw material further includes bentonite, and the addition amount of bentonite accounts for 0.4% to 0.6% of the total amount of iron ore powder.

[0023] Further, after the composite pellet green balls are roasted at 1100 - 1300 °C, the pellet desulfurization rate reaches more than 90%.

[0024] The present invention also provides a preparation method of the composite pellet as described in any technical solution of the first aspect above, including:

[0025] Using a carbon reducing agent with a particle size of 0.2 - 0.4 mm as the inner core green balls, wrapping an intermediate layer mixture including desulfurization ash and iron ore powder on its surface, and then continuing to wrap an outer layer mixture including iron ore powder and a carbon reducing agent with a particle size less than 0.1 mm on the surface of the intermediate layer to obtain the composite pellet green balls.

[0026] Further, a disk pelletizer is used for pelletizing, the disk rotation speed is 20 - 30 r / min, and / or the disk inclination angle is 40 - 50°.

[0027] Further, it also includes: drying the obtained composite pellet green balls; and / or roasting the dried composite pellets, and the roasting temperature is 1100 - 1300 °C.

[0028] In summary, adopting the technical solution provided by the present invention, compared with the prior art, the following beneficial effects can be achieved:

[0029] (1) By adding desulfurized ash and carbon reducing agent to the pellets simultaneously and optimizing the structure design of the pellets, the present invention makes them form multi-layer composite pellets composed of different components. Thus, not only can the desulfurization treatment of desulfurized ash be realized in advance during the roasting process, enabling the desulfurized ash to be effectively applied to the pellet process, achieving the resource utilization of industrial waste, which is of great significance to the national resource strategic security, but also facilitating the escape of sulfur dioxide generated by reduction from the pellets. At the same time, through the optimization of the structure and components, the desulfurization effect can be effectively guaranteed, and sufficient desulfurization can be achieved without adding too much reducing agent, thus avoiding the agglomeration of pellets caused by adding too much reducing agent.

[0030] (2) By controlling the different particle sizes of the inner core and the outer layer carbon reducing agent, the present invention is conducive to the timely escape of sulfur dioxide gas generated by reduction from the pellets, which is conducive to further improving the mechanical properties and desulfurization effect of the pellets. By further optimizing the design of the pellet basicity, not only the consumption of the original calcareous flux in the steel plant during pellet production is reduced, but also the energy consumption during the roasting process is reduced. At the same time, the application performance of the obtained pellets and the normal operation of the blast furnace can be effectively guaranteed.

[0031] (3) The present invention further optimizes and controls the addition amount of the carbon reducing agent, so as to not only ensure the desulfurization rate of sulfur elements in the desulfurized ash, but also avoid the agglomeration of pellets caused by adding too much reducing agent. In addition, the present invention further optimizes the design of the thickness between each layer in the pellets and the mass ratio of the raw materials of each layer, which is conducive to further ensuring the desulfurization effect and application performance of the pellets. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 is the preparation flow chart of the multi-layer composite pellets in Example 1 of the present invention;

[0033] Figure 2 is the schematic diagram of the microscopic structure of the pellets before and after adding desulfurized ash. DETAILED DESCRIPTION OF THE INVENTION

[0034] The present invention applies desulfurized ash to pellet production, which can replace the calcareous flux and is conducive to the resource recovery and utilization of desulfurized ash. Among them, since desulfurized ash is a by-product of industrial desulfurization process and contains a high content of S element, it may remain in the pellets and enter the blast furnace during subsequent production. In the blast furnace, sulfur reacts with iron and coke to form iron sulfide, which will cause the temperature in the furnace to drop and the combustion to be out of balance, affecting the fluidity of the slag. At the same time, sulfur generates hydrogen sulfide gas in the blast furnace, which not only damages the efficiency of blast furnace ironmaking, but also may lead to brittle iron and reduce the product quality.

[0035] Therefore, when the present invention applies desulfurized ash to the pellet process, with the help of the carbon reducing agent, the sulfur element is reduced in advance and in the form of SO 2is released in the form of, so that the released SO can be recycled through the activated carbon flue gas treatment system in the pelletizing process 2 to realize the harmless treatment of desulfurized ash in the pelletizing process.

[0036] However, if the desulfurized ash is directly mixed evenly with other pellet materials and pelletized, the following problems will occur: (1) The addition amount of desulfurized ash accounts for a relatively small proportion of the total mass of the pellets. The contact between the carbon reductant and the desulfurized ash in the pellets is insufficient, which will affect the sulfur removal efficiency; (2) The calcareous flux improves the pellet properties by improving the pellet alkalinity. Adding too much desulfurized ash also means increasing the S element in the green pellets. To remove the sulfur element, it is necessary to further increase the ratio of the carbon reductant. However, excessive addition of the carbon reductant will cause agglomeration during the pellet roasting process; (3) Sulfur will be released in the form of SO 2 and the consolidation of the pellets during the roasting process will make the pellets dense, thus restricting the release of SO 2 and may cause sulfur residue to appear in the pellets again.

[0037] Based on the above problems, the present invention optimizes the structure of the pellets and designs a composite multi-layer pellet. The coarser-grained carbon reductant is used as the nucleus in the preparation of green pellets, and then a mixture of desulfurized ash and iron ore powder is added for coating to increase the contact between the carbon reductant and the desulfurized ash and carry out an efficient desulfurization reaction; a mixture of a finer-grained carbon reductant and iron ore powder is further coated on the outer layer, which can provide a better reduction effect and create a path for the release of SO 2 to avoid the re-residue of S.

[0038] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which this invention belongs; the term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0039] To further understand the content of the present invention, the present invention will be described in detail below in conjunction with specific embodiments. It should be noted that due to limited space, only some embodiments are listed here. The protection scope of the present invention is not limited to the following specific embodiments. For example, the numerical range of each parameter in the present invention not only includes the numerical values clearly described as the range limits, but also includes all individual numerical values or sub-ranges covered within the said range, just as if each numerical value and sub-range were clearly described.

[0040] Example 1

[0041] The desulfurized ash composite pellets of this embodiment have green pellets that sequentially include a core, an intermediate layer, and an outer layer from the inside out. The core raw materials include a carbon reducing agent with a particle size of 0.2 mm. The intermediate layer raw materials include desulfurized ash and iron ore powder. The outer layer raw materials include a carbon reducing agent and iron ore powder, and the particle size of the carbon reducing agent in this layer is less than 0.1 mm. At the same time, the pellet raw materials also contain bentonite, and the addition amount of bentonite is 0.5% of the total amount of iron ore powder.

[0042] Specifically, the chemical compositions of the iron ore powder and desulfurized ash in this embodiment are shown in Table 1 and Table 2 below, respectively, and the mass ratio of desulfurized ash to iron ore powder is 1:16, so as to control the pellet basicity to 0.8. In this embodiment, coke (with a solid content of 80%) is used, and its addition amount is controlled according to the molar ratio of C and S elements of 1.2, and the mass ratio of the core carbon reducing agent to the outer layer carbon reducing agent is 1:9.

[0043] Table 1 Chemical composition of iron ore powder

[0044]

[0045] Table 2 Chemical composition of desulfurized ash

[0046]

[0047] Combined Figure 1 As shown, the preparation method of the desulfurized ash composite pellets of this embodiment includes the following steps:

[0048] 1) Pour carbon reducing agent with a diameter of 0.2 mm as mother balls on a disk pelletizer with a disk diameter of 1000 mm, a rotation speed of 23 r / min, and an inclination angle of 47°.

[0049] 2) Mix desulfurized ash, iron ore powder, and bentonite in a mixer for five minutes to make them uniform, and then add them to the disk pelletizer to make the mother balls continue to grow to a particle size of 9 mm.

[0050] 3) Mix the remaining fine-particle-size carbon reducing agent, iron ore powder, and bentonite in a mixer for five minutes to make them uniform, and then add them to the disk pelletizer to make the mother balls continue to grow to a particle size of 12 mm; then continue to roll for 2 minutes to compact and make the mother balls uniform, thus obtaining multi-layer composite green pellets.

[0051] 4) Put the obtained green pellets into an electric heating drying oven and dry them at 100 °C (measure the moisture content of the green pellets), and detect the compressive strength and bursting temperature of the green pellets, and conduct roasting at 1250 °C in a horizontal tube furnace, and then conduct roasting strength detection.

[0052] Example 2

[0053] The main difference between this embodiment and Embodiment 1 is that in this embodiment, layered pelletizing is carried out, but the particle size distributions of the inner core carbon reducing agent and the outer layer carbon reducing agent are the same, and the particle size is 0.2 mm for both.

[0054] Comparative Example 1

[0055] The main difference between this comparative example and Embodiment 1 is that in this comparative example, desulfurized ash is not added to the pellet raw materials, but traditional slaked lime is added as a calcareous flux, and when preparing pellets, all the raw materials are directly mixed evenly and then pelletizing treatment is carried out.

[0056] Comparative Example 2

[0057] The main difference between this comparative example and Embodiment 1 is that in this comparative example, desulfurized ash is added, but no carbon reducing agent is added.

[0058] Comparative Example 3

[0059] The main difference between this comparative example and Embodiment 1 is that in this comparative example, desulfurized ash and carbon reducing agent are added, but all the pellet raw materials are directly mixed evenly and then pelletizing treatment is carried out.

[0060] Comparative Example 4

[0061] The main difference between this comparative example and Embodiment 1 is that in this comparative example, the mass ratio of desulfurized ash to iron ore powder is adjusted to 1:34 to control the pellet basicity to 0.4.

[0062] Comparative Example 5

[0063] The main difference between this comparative example and Embodiment 1 is that in this comparative example, the pellet basicity is controlled to 0.6.

[0064] Comparative Example 6

[0065] The main difference between this comparative example and Embodiment 1 is that in this comparative example, the pellet basicity is controlled to 1.2.

[0066] Comparative Example 7

[0067] The main difference between this comparative example and Embodiment 1 is that in this comparative example, the addition amount of the carbon reducing agent is controlled according to the molar ratio of C to S being 0.8.

[0068] Comparative Example 8

[0069] The main difference between this comparative example and Embodiment 1 is that in this comparative example, the addition amount of the carbon reducing agent is controlled according to the molar ratio of C to S being 1.8.

[0070] The performance parameters of the pellets prepared in Examples 1 and 2 and Comparative Examples 1-8 are shown in Table 3 below. From the data in the table, it can be seen that in Comparative Example 1, when traditional slaked lime was used as the pellet calcium flux and directly mixed evenly with all other raw materials for pelletizing, the strength indexes of the green pellets could meet the requirements for charging into the blast furnace. In Comparative Example 2, when desulfurized ash was added to the pellets to replace slaked lime but no carbon reducing agent was added, the strength indexes of the pellets could still meet the requirements for charging into the blast furnace, but at this time the desulfurization rate was only 5.7%. The S element brought by the desulfurized ash remained in the pellets and would have an extremely adverse impact on the operation of the blast furnace after entering the blast furnace.

[0071] In Comparative Example 3, by further adding a carbon reducing agent to the pellets, the desulfurized ash could be reduced during the roasting process, enabling SO 2 to be released during the roasting process. After the pelletizing experiment was completed, the performance of the pellets could meet the requirements of industrial production, and the strength of the roasted pellets did not show an obvious decrease. By comparing the roasted pellet raw materials with the green pellet raw materials using XRF, the removal rate of S reached 64% at this time, showing a significant improvement compared to Comparative Example 2, but still could not meet the production requirements of the blast furnace.

[0072] At the same time, it can be seen from Comparative Examples 4-6 that when the pellet basicity is not reasonably controlled, it will also have an adverse impact on the pellet performance. Specifically, when the pellet basicity is low, its bursting temperature drops significantly, and its roasting strength decreases significantly. When the pellet basicity is high, the indexes of the green pellets can still meet the requirements, but the bursting temperature still decreases, and the roasting strength decreases more significantly;

[0073] In Comparative Examples 7 and 8, the indexes of the pellets did not change significantly and could meet the requirements for charging the pellets into the furnace. However, at this time, the desulfurization rate in Comparative Example 7 was 85.6% because the carbon reducing agent was insufficient and the desulfurization reaction was not complete at this time; while in Comparative Example 8, the desulfurization rate was only 93.2% because the carbon reducing agent was excessive and a desulfurization side reaction occurred at this time, fixing the S element back in the pellets.

[0074] In Example 2, by carrying out layered pelletizing, but since the particle size distributions of the inner core carbon reducing agent and the outer layer carbon reducing agent were the same, more heat was generated inside the pellets during the roasting process, which was beneficial to the progress of the desulfurization reaction, and the desulfurization rate increased significantly, reaching 90.4%.

[0075] In Example 1, since desulfurized ash and carbon reductant were added to the pellets simultaneously and the layered pelletizing process was adopted, not only the mechanical properties of the obtained pellets can be improved, but also the desulfurization rate of the pellets can be further increased. The desulfurization rate is as high as 90.4%, meeting the production requirements for the pellets to enter the blast furnace. In Example 2, the particle size grading of the carbon reductant was further optimized, with different particle sizes for the inner core and outer layer of the carbon reductant, so that the mechanical properties and desulfurization rate of the pellets can be further improved. The desulfurization rate is as high as 99.3%, showing a relatively small difference compared with not using desulfurized ash as the calcareous flux. In addition, combined with Figure 2 It can also be seen that after adding desulfurized ash (compared with the pellets made of iron ore powder and bentonite), the formation of iron oxide intercrystals inside the pellets can be significantly increased, thereby improving the pellet strength.

[0076] Table 3 Comparison of the properties of the pellets obtained in Example 1 and Comparative Examples 1-9

[0077]

[0078]

[0079] Example 3

[0080] The desulfurized ash composite pellets of this example are mainly different from those of Example 1 in that: in this example, the pellet basicity is controlled at 0.7.

[0081] Example 4

[0082] The desulfurized ash composite pellets of this example are mainly different from those of Example 1 in that: in this example, the pellet basicity is controlled at 1.0.

[0083] Example 5

[0084] The desulfurized ash composite pellets of this example are mainly different from those of Example 1 in that: in this example, the particle size of the inner core carbon reductant is 0.3 mm, the particle size of the outer layer carbon reductant is less than 0.1 mm, and the mass ratio of the inner core carbon reductant to the outer layer carbon reductant is 1:4. The addition amount of desulfurized ash is controlled to make the pellet basicity 0.9, and the addition amount of the carbon reductant (using biochar) is controlled according to the mass ratio of C:S of 1:1. At the same time, the pellet raw materials also include bentonite, and the addition amount of bentonite is 0.4% of the total amount of iron ore powder.

[0085] The preparation method of the desulfurized ash composite pellets of this example includes the following steps:

[0086] 1) Pour the carbon reductant with a diameter of 0.3 mm as the mother balls on a disk pelletizing machine with a disk diameter of 1000 mm, a rotation speed of 23 r / min, and an inclination angle of 47°.

[0087] 2) Mix the desulfurized ash, iron ore powder, and bentonite in a mixer for five minutes to make them uniform, and then add them to a disk pelletizer to continue growing the mother pellets until the particle size reaches 8.5 mm;

[0088] 3) Mix the remaining fine-particle-size carbon reductant, iron ore powder, and bentonite in a mixer for 6 minutes to make them uniform, and then add them to a disk pelletizer to continue growing the mother pellets until the particle size reaches 11 mm; then continue to roll for 2 minutes to make the mother pellets compact and uniform, thus obtaining multi-layer composite pellet green balls;

[0089] 4) Put the obtained green balls into an electric heating drying oven and dry them at 110 °C (measure the moisture content of the green balls), and then roast them in a horizontal tube furnace at 1100 °C to obtain the finished composite pellets.

[0090] Example 6

[0091] The desulfurized ash composite pellets of this example are mainly different from those of Example 1 in that: in this example, the particle size of the inner core carbon reductant is 0.4 mm, the particle size of the outer layer carbon reductant is less than 0.1 mm, and the mass ratio of the inner core carbon reductant to the outer layer carbon reductant is 1:6. The addition amount of the carbon reductant (using semi-coke) is controlled according to the mass ratio of C and S of 1.5. At the same time, the pellet raw materials also contain bentonite, and the addition amount of bentonite is 0.6% of the total amount of iron ore powder.

[0092] The preparation method of the desulfurized ash composite pellets of this example includes the following steps:

[0093] 1) Pour carbon reductant with a particle size of 0.4 mm as mother pellets onto a disk pelletizer with a disk diameter of 1000 mm, a rotation speed of 25 r / min, and an inclination angle of 47°;

[0094] 2) Mix the desulfurized ash, iron ore powder, and bentonite in a mixer for five minutes to make them uniform, and then add them to a disk pelletizer to continue growing the mother pellets until the particle size reaches 9 mm;

[0095] 3) Mix the remaining fine-particle-size carbon reductant, iron ore powder, and bentonite in a mixer for 4 minutes to make them uniform, and then add them to a disk pelletizer to continue growing the mother pellets until the particle size reaches 12.5 mm; then continue to roll for 2 minutes to make the mother pellets compact and uniform, thus obtaining multi-layer composite pellet green balls;

[0096] 4) Put the obtained green balls into an electric heating drying oven and dry them at 90 °C (measure the moisture content of the green balls), and then roast them in a horizontal tube furnace at 1300 °C to obtain the finished composite pellets.

[0097] Table 4 Performance comparison of the pellets obtained in Examples 3 - 6

[0098]

Claims

1. A desulfurized ash composite pellet, characterized in that: The composite pellet raw ball includes from the inside to the outside: A core, wherein the core raw material includes a carbon reducing agent; An intermediate layer, wherein the intermediate layer raw materials include desulfurized ash and iron ore powder; and The outer layer raw materials include carbon reducing agent and iron ore powder.

2. The desulfurized ash composite pellet according to claim 1, characterized in that: The particle size of the carbon reducing agent in the inner core is 0.2-0.4 mm, and the particle size of the outer layer carbon reducing agent is less than 0.1 mm; and / or the amount of desulfurized ash added is based on controlling the basicity of the pellets to be 0.7-1; and / or the amount of carbon reducing agent added is based on controlling the mass ratio of C to S in the pellets to be 1-1.

5.

3. The desulfurized ash composite pellet according to claim 2, characterized in that: The particle size of the composite pellets is 11-12.5 mm; and / or the thickness of the middle layer and the outer layer of the composite pellets are 8-9 mm and 2-4 mm respectively.

4. The desulfurized ash composite pellet according to claim 2, characterized in that: The mass ratio of the iron ore powder in the middle layer to that in the outer layer is 1:2 to 1:4; and / or the mass ratio of the carbon reducing agent in the inner core to that in the outer layer is 1:4 to 1:

9.

5. The desulfurized ash composite pellet according to any one of claims 1 to 4, characterized in that: The carbon reducing agent includes but is not limited to coke, biochar, coal powder, coke powder and blue charcoal.

6. The desulfurized ash composite pellet according to any one of claims 1 to 4, characterized in that: The composite pellet raw material also includes bentonite, and the added amount of bentonite accounts for 0.4% to 0.6% of the total amount of iron ore powder.

7. The desulfurized ash composite pellet according to any one of claims 1 to 4, characterized in that: After the composite green pellets are calcined at 1100-1300°C, the desulfurization rate of the pellets reaches more than 90%.

8. A method for preparing composite pellets according to any one of claims 1 to 7, characterized in that: include: A carbon reducing agent with a particle size of 0.2 to 0.4 mm is used as the inner core mother ball, and an intermediate layer mixture including desulfurized ash and iron ore powder is coated on its surface. Then, an outer layer mixture including iron ore powder and a carbon reducing agent with a particle size less than 0.1 mm is continuously coated on the surface of the intermediate layer to obtain a composite pellet green ball.

9. The preparation method according to claim 8, characterized in that: A disc pelletizing machine is used for pelletizing, with a disc speed of 20 to 30 r / min and / or a disc inclination of 40 to 50 degrees.

10. The preparation method according to claim 8 or 9, characterized in that: Also includes: Drying the obtained composite pellets; And / or calcining the dried composite pellets at a calcination temperature of 1100-1300°C.

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