A method for preparing calcium ferrite by low-temperature decomposition of iron-based solid waste catalytic phosphogypsum.

Abstract

This invention discloses a method for preparing calcium ferrite from iron-based solid waste through low-temperature catalytic decomposition of phosphogypsum, belonging to the field of comprehensive utilization of solid waste. The method involves mixing, granulating, and roasting raw materials, including iron-based solid waste and phosphogypsum, to obtain calcium ferrite-containing sintered material. The roasting process involves feeding the material onto a belt roaster, first performing reduction roasting under a reducing atmosphere, and then performing oxidative roasting under an oxidizing atmosphere. This method fully utilizes the mineral composition characteristics of iron-based solid waste and phosphogypsum from the steel industry. By controlling the temperature and atmosphere during the high-temperature solid-phase reaction, the method achieves the directional transformation of iron and calcium components in the two industrial solid wastes into calcium ferrite-based products, while effectively removing harmful elements, resulting in high-quality calcium ferrite products.

Description

Technical Field

[0001] This invention relates to a method for preparing calcium ferrite, and more particularly to a method for preparing calcium ferrite by low-temperature decomposition of phosphogypsum using iron-based solid waste, belonging to the field of comprehensive utilization of solid waste. Background Technology

[0002] Phosphogypsum is a byproduct of wet-process phosphoric acid production, generating 4-5 tons of phosphogypsum for every ton of phosphoric acid produced. Due to its complex composition and strong acidity (pH 1-3), phosphogypsum is difficult to utilize. Current main uses of phosphogypsum include: (1) preparing cement retarder and cementitious materials; (2) producing gypsum mortar; (3) preparing gypsum products such as gypsum board, gypsum-fired bricks, and gypsum blocks; and (4) other uses, such as gypsum roadbed materials and gypsum-based composite fillers. Although the utilization of phosphogypsum has shown a diversified development trend in recent years, there are still many problems in using it to prepare high-end building materials. Its use as a cement retarder is currently the main way to utilize phosphogypsum, but its disposal capacity is limited. Therefore, it is urgent to develop a key technology for large-scale, harmless, and clean utilization of phosphogypsum, which is of great significance to the healthy and sustainable development of the phosphoric acid chemical industry.

[0003] The steel production process inevitably generates a large amount of iron-based solid waste, with an annual output of up to 80 million tons. This mainly includes sintering dust, blast furnace ash, electric furnace ash, and coke dust. Besides containing high-grade iron, the dust also contains a large amount of carbon. Currently, the main methods for treating iron-based solid waste in the steel industry include: returning it to sintering, smelting it for valuable metals, and landfilling. For iron-based solid waste with low lead and zinc content, the primary method is returning it to the sintering batching system. However, industrial practice shows that the amount of iron-based solid waste added should not be too high (<5%), otherwise the granulation effect of the mixture and the permeability of the sintering process will deteriorate, leading to a deterioration in the quality indicators of the sintered product. Therefore, how to efficiently utilize iron-based solid waste has become a pressing problem for major steel companies.

[0004] Currently, there is no existing technology for the co-utilization of iron-based solid waste and phosphogypsum. If the valuable components such as iron and calcium in these two types of solid waste can be rationally utilized and transformed into bulk high-value-added materials, it will be of great significance to steel and phosphate chemical enterprises. Summary of the Invention

[0005] To address the shortcomings of existing technologies in enabling low-cost, large-scale utilization of phosphogypsum and iron-based solid waste, the present invention aims to provide a method for the low-temperature decomposition of phosphogypsum into calcium ferrite using iron-based solid waste. This method fully utilizes the mineral composition characteristics of both iron-based solid waste and phosphogypsum from the steel industry. By controlling the temperature and atmosphere during the high-temperature solid-phase reaction, the iron and calcium components in the two industrial solid wastes are directionally transformed into calcium ferrite products, while effectively removing harmful elements, resulting in high-quality calcium ferrite products.

[0006] To achieve the above-mentioned technical objectives, the present invention provides a method for preparing calcium ferrite by low-temperature decomposition of iron-based solid waste catalytic phosphogypsum. The method involves mixing, granulating, and calcining raw materials, including iron-based solid waste and phosphogypsum, to obtain calcium ferrite-containing sintered material. The calcination process is as follows: the material is fed onto a belt calciner and first undergoes reduction calcination at 700–1000°C under a reducing atmosphere, followed by oxidative calcination at 1100–1300°C under an oxidizing atmosphere.

[0007] This invention is the first to propose a high-quality calcium ferrite product obtained by combining iron-based solid waste and phosphogypsum through a high-temperature solid-phase reaction. Iron-based solid waste contains iron oxides and carbon, while phosphogypsum mainly contains calcium sulfate dihydrate. By combining the two and subjecting them to a high-temperature solid-phase reaction, the catalytic effect of the iron oxides and the reducing properties of the carbon in the iron-based solid waste are first utilized to promote the low-temperature reduction and decomposition of the phosphogypsum, yielding calcium sulfide. Simultaneously, under reducing atmosphere and high-temperature conditions, residual phosphate and calcium phosphate in the phosphogypsum are reduced and decomposed into elemental phosphorus, which volatilizes and is removed from the flue gas. Subsequently, under an oxidizing atmosphere, the calcium sulfide is oxidized and desulfurized through a high-temperature reaction, and then directionally converted with iron oxide to form the calcium ferrite product.

[0008] This invention granulates iron-based solid waste and phosphogypsum, which increases the contact area between the iron-based solid waste and phosphogypsum particles and improves the reaction rate. At the same time, the use of a belt roaster for sintering can increase the permeability of the material layer and promote the low-temperature decomposition of phosphogypsum and the oxidative conversion of calcium sulfide.

[0009] As a preferred embodiment, the total iron content of the iron-based solid waste is between 20% and 60% by mass. The iron-based solid waste includes common iron-based solid wastes found in the steel industry, such as sintering dust, blast furnace ash, and electric furnace ash, etc., which contain a carbon content between 5% and 35% by mass. The iron-based solid waste can also be other iron-containing solid wastes; when using such iron-based solid wastes, it is necessary to use an additional carbonaceous reducing agent, such as crushed coke, for example, sulfuric acid slag + crushed coke, etc.

[0010] As a preferred embodiment, the phosphogypsum contains 70% to 95% gypsum dihydrate by mass. Higher purity phosphogypsum makes it easier to obtain high-grade calcium ferrite products. Phosphogypsum can be purified using existing conventional methods, such as conventional flotation for purification and impurity removal.

[0011] As a preferred embodiment, the mass ratio of iron-based solid waste to phosphogypsum is 3:1 to 1:3. A further preferred ratio is (0.5 to 1.5):1. If the proportion of phosphogypsum is too high, the decomposition efficiency of phosphogypsum will be low, making it difficult to fully utilize. If the proportion of phosphogypsum is too low, some iron oxides in the iron-based solid waste will not be fully utilized.

[0012] As a preferred embodiment, the raw material includes a binder; the binder is at least one selected from bentonite, carboxymethyl cellulose, and starch. The binder used is beneficial for the granulation and calcination processes of iron-based solid waste and phosphogypsum. As a preferred embodiment, the mass of the binder is 0.1% to 5% of the total mass of the iron-based solid waste and phosphogypsum.

[0013] As a preferred embodiment, the total carbon content in the raw material is 5-30% by mass. The carbon can originate from the carbon components naturally present in the iron-based solid waste, or the total carbon content in the raw material can be adjusted using coke or similar materials. The total carbon content is crucial for the high-temperature solid-phase reaction between the iron-based solid waste and phosphogypsum, especially during the reduction roasting stage. If the total carbon content is too low, incomplete decomposition of the phosphogypsum will occur, resulting in lower quality of the subsequent calcium ferrite product. More preferably, the total carbon content in the raw material is 8-15% by mass.

[0014] As a preferred embodiment, the CO content in the reducing atmosphere is 5% to 30% by mass. The reducing atmosphere is mainly based on the carbon components in the raw materials generated at high temperatures. Under the preferred reducing atmosphere, the reduction and decomposition process of phosphogypsum can be promoted, and phosphorus removal is also facilitated.

[0015] As a preferred embodiment, the reduction roasting time is 30–60 minutes. During the reduction roasting process, the strong reducing power and heat storage characteristics of the carbon component in the iron-based solid waste are utilized to reduce and decompose phosphogypsum into CaS. Simultaneously, the iron component in the iron-based solid waste can catalyze the decomposition of phosphogypsum, lowering its decomposition temperature. The preferred reduction roasting temperature is 900–1000℃. Compared to the conventional phosphogypsum decomposition temperature, the decomposition temperature of phosphogypsum is reduced by approximately 200℃ under the reducing and catalytic effects provided by the iron-based solid waste. Under the preferred reduction roasting conditions, the decomposition efficiency of phosphogypsum reaches over 70% or even complete decomposition.

[0016] As a preferred embodiment, the oxygen content in the oxidizing atmosphere is 20% to 40% by mass. Oxidative roasting in an oxidizing atmosphere primarily oxidizes calcium sulfide into highly reactive calcium oxide.

[0017] As a preferred embodiment, the oxidative roasting employs an oxygen-enriched air-blowing method, with a roasting time of 40–90 minutes. Providing an oxygen-enriched atmosphere through this method enhances the oxidative atmosphere in the material layer, promoting the oxidation of CaS to active CaO. Under roasting conditions, calcium oxide reacts with the iron oxide components in the iron-based solid waste to generate calcium ferrite sinter. The preferred oxidative roasting temperature is 1200–1300℃. Under these preferred oxidative roasting conditions, the main components of the calcium ferrite sinter are hemicalcium ferrite, monocalcium ferrite, and dicalcium ferrite.

[0018] The granulation of the present invention is achieved by a cylindrical granulator, a disc granulator, a cold press granulator, or a granule extruder, and the particle size of the granules is 1 to 20 mm.

[0019] The flue gas produced by the oxidative roasting of this invention has a high concentration of sulfur dioxide and can be directly used to prepare sulfuric acid.

[0020] Compared with the prior art, the technical solution of the present invention has the following advantages:

[0021] 1) This invention uses iron-based solid waste and phosphogypsum as raw materials to obtain calcium ferrite products with high added value through high-temperature solid-phase reaction. Compared with the traditional production process of calcium ferrite produced by roasting iron concentrate with quicklime or limestone, this invention can save a lot of iron concentrate and limestone resources, reduce costs, and also consume a lot of iron-based solid waste and phosphogypsum, achieving multiple benefits and truly realizing the resource utilization of solid waste.

[0022] 2) The calcium ferrite product prepared by the present invention from iron-based solid waste and phosphogypsum mainly contains hemicalcium ferrite, monocalcium ferrite and dicalcium ferrite, and the total content of calcium ferrite is more than 80%.

[0023] 3) The process and equipment for preparing calcium ferrite using iron-based solid waste and phosphogypsum is simple and can be achieved using a belt roaster commonly used in the steel industry, which is conducive to its promotion.

[0024] 4) In the process of preparing calcium ferrite from iron-based solid waste and phosphogypsum, sulfur dioxide is released mainly during the oxidative roasting stage. The roasting flue gas is rich in high concentrations of SO2, which can be used for acid production, making it low-carbon and environmentally friendly.

[0025] 5) The key to this invention's preparation of calcium ferrite from iron-based solid waste and phosphogypsum lies in utilizing the iron and carbon components in the iron-based solid waste to promote the low-temperature decomposition of phosphogypsum. Traditional pyrolysis processes for phosphogypsum involve high temperatures and high energy consumption. This invention utilizes the catalytic effect of the iron component and the reducing effect of the carbon component in the iron-based solid waste to achieve low-temperature decomposition of phosphogypsum, reducing the decomposition temperature by more than 200°C compared to traditional processes. Simultaneously, the carbon components inherent in the iron-based solid waste generated by steel enterprises are fully utilized, saving a significant amount of coal resources. Attached Figure Description

[0026] Figure 1 This is the XRD pattern of the final roasted product in Example 3.

[0027] Figure 2 The image shown is the XRD pattern of the final calcined product in Example 6. Detailed Implementation

[0028] The following examples are intended to further illustrate the present invention, but not to limit the scope of protection of the claims of the present invention.

[0029] Example 1

[0030] This embodiment uses phosphogypsum, a byproduct of a phosphate chemical plant, with a calcium sulfate dihydrate content of 92.2%. The iron-based solid waste is blast furnace ash from an ironmaking plant, with a total iron content of 51.3% and a carbon content of 27.5%. 1 kg of phosphogypsum, 1 kg of blast furnace ash, and 20 g of carboxymethyl cellulose were mixed and granulated using a cylindrical granulator, with the particle size controlled at 2–5 mm. The mixed granules were placed in a small laboratory roasting furnace and heated to 950°C using electric heating for 1 hour. At this time, due to the gasification of the carbon components, the roasting furnace exhibited a strong reducing atmosphere, with a carbon monoxide content of approximately 16%. Subsequently, the roasting furnace was heated to 1200°C, and an oxidizing gas with a 35% oxygen concentration was introduced at a flow rate of 1 L / min, continuing roasting for another hour. The final roasted product was cooled to room temperature by air. The content of complex calcium ferrite in the roasted product was about 81%, and the specific chemical composition was CaO 23.29%, Fe2O3 58.42%, SiO2 10.96%, and other impurities 7.33%. Among them, most of the iron and calcium existed in the form of complex calcium ferrite.

[0031] Table 1. Results of multi-element chemical analysis / %

[0032]

[0033] Example 2

[0034] This embodiment uses phosphogypsum, a byproduct of a phosphate chemical plant, with a calcium sulfate dihydrate content of 88.9%. The iron-based solid waste is sulfuric acid slag from a chemical plant, with a total iron content of 58.9%. The reducing agent is crushed coke from a steel plant, with a fixed carbon content of 69%. 1.2 kg of phosphogypsum, 0.8 kg of sulfuric acid slag, 400 g of crushed coke, and 20 g of carboxymethyl cellulose were mixed and granulated using a pellet extruder. The granules were cylindrical with a diameter of 3 mm and a length of 2–5 mm. The mixed granules were placed in a small laboratory roasting furnace and heated to 980°C using electric heating. At this temperature, due to the gasification of carbon components, the roasting furnace exhibited a strong reducing atmosphere, with a carbon monoxide content of approximately 15%. After roasting the mixture for 40 minutes, the furnace temperature was raised to 1230°C, and an oxidizing gas with a 20% oxygen concentration was introduced at a flow rate of 1 L / min, continuing roasting for 1.5 hours.

[0035] The final roasted product was cooled to room temperature by air. The content of complex calcium ferrite in the roasted product was about 86%, and the chemical composition was 31.8% CaO, 54.2% Fe2O3, 7.5% SiO2, and 6.5% other impurities. Most of the iron and calcium existed in the form of complex calcium ferrite.

[0036] Example 3

[0037] This embodiment uses phosphogypsum, a byproduct of a phosphate chemical plant. After flotation purification to remove impurities, the phosphogypsum dihydrate has a purity of 98.6%. The iron-based solid waste is dust from a sintering plant, with a total iron content of 50.7%, a calcium oxide content of 12.9%, and a carbon content of 6%. The reducing agent is coke from a steel plant, with a fixed carbon content of 69%. 1 kg of purified phosphogypsum, 1 kg of sintering plant dust, 200 g of coke, and 50 g of bentonite are mixed and granulated using a disc pelletizer, producing spherical particles with a size of 5–8 mm. The mixed particles are placed in a small laboratory roasting furnace and heated to 950°C using electric heating. At this temperature, due to the gasification of carbon components, the roasting furnace exhibits a strong reducing atmosphere, with a carbon monoxide content of approximately 13%. After roasting the mixture for 40 minutes, the furnace temperature is raised to 1280°C, and an oxidizing gas with a 30% oxygen concentration is introduced at a flow rate of 1.2 L / min, continuing roasting for another hour.

[0038] The final roasted product was air-cooled to room temperature. The content of complex calcium ferrite in the roasted product was approximately 80% (the vast majority of iron and calcium existed in the form of complex calcium ferrite). The XRD pattern of the product is shown below. Figure 1 The chemical compositions are shown in Table 2.

[0039] Table 2 Results of multi-element chemical analysis / %

[0040]

[0041] Example 4 (Comparative Example)

[0042] This embodiment uses phosphogypsum, a byproduct of a phosphate chemical plant, with a calcium sulfate dihydrate content of 92.2%. The iron-based solid waste is blast furnace ash from an ironmaking plant, with a total iron content of 51.3% and a carbon content of 27.5%. 1 kg of phosphogypsum, 1 kg of blast furnace ash, and 20 g of carboxymethyl cellulose were mixed and granulated using a cylindrical granulator, with the particle size controlled at 2–5 mm. The mixed granules were placed in a small laboratory roasting furnace and heated to 850°C using electric heating. At this point, due to the gasification of the carbon components, the roasting furnace exhibited a strongly reducing atmosphere, with a carbon monoxide content of approximately 16%. Subsequently, the roasting furnace was heated to 1200°C, and an oxidizing gas with a 35% oxygen concentration was introduced at a flow rate of 1 L / min, continuing roasting for 1 hour.

[0043] The final roasted product was cooled to room temperature by air. The content of compound calcium ferrite in the roasted product was about 60% (a large portion of the iron and calcium did not exist in the form of compound calcium ferrite). The chemical composition is shown in Table 3.

[0044] Table 3. Results of multi-element chemical analysis / %

[0045]

[0046] This embodiment uses the same raw material types and proportions as Example 1. Compared with Example 1, the first-step roasting temperature is different. As can be seen from the chemical composition of the final product in Table 3, the product still contains 5.86% SO3, indicating that CaSO4 is not completely decomposed in the first step. The XRD pattern of the product shows that after lowering the roasting temperature, the decomposition rate of CaS phase in phosphogypsum drops sharply, and the main phase in the roasted product is still CaSO4. In contrast, the main phase of the first-step roasted product in Example 1 is CaS. It can be seen that the first-step reduction decomposition temperature is crucial for the decomposition of phosphogypsum.

[0047] Example 5 (Comparative Example)

[0048] This embodiment uses phosphogypsum, a byproduct of a phosphate chemical plant, with a calcium sulfate dihydrate content of 88.9%. The iron-based solid waste is sulfuric acid slag from a chemical plant, with a total iron content of 58.9%. The reducing agent is crushed coke from a steel plant, with a fixed carbon content of 69%. 1.2 kg of phosphogypsum, 0.8 kg of sulfuric acid slag, 100 g of crushed coke, and 20 g of carboxymethyl cellulose were mixed and granulated using a pellet extruder. The granules were cylindrical with a diameter of 3 mm and a length of 2–5 mm. The mixed granules were placed in a small laboratory roasting furnace and heated to 980°C using electric heating. At this temperature, due to the gasification of carbon components, the roasting furnace exhibited a weakly reducing atmosphere, with a carbon monoxide content of approximately 3.5%. After roasting the mixture for 40 minutes, the furnace temperature was raised to 1230°C, and an oxidizing gas with a 20% oxygen concentration was introduced at a flow rate of 1 L / min, continuing roasting for 1.5 hours.

[0049] The final roasted product was cooled to room temperature by air. The content of compound calcium ferrite in the roasted product was about 57% (a large portion of the iron and calcium did not exist in the form of compound calcium ferrite). The chemical composition is shown in Table 4.

[0050] Table 4. Results of multi-element chemical analysis / %

[0051]

[0052] Compared with Example 2, it can be seen that this example uses the same types and proportions of raw materials as Example 2, but the amount of reducing agent is less. It can be seen that under the same calcination temperature and time, the decomposition rate of calcium sulfate in phosphogypsum is very low. Most of the phases of the calcined product are CaSO4, and a small part is CaS. The purity of the final calcium ferrite product is only 57%, of which the SO3 content is as high as 11%. It can be seen that the amount of reducing agent used in the decomposition process of phosphogypsum is crucial to the decomposition of phosphogypsum.

[0053] Example 6 (Comparative Example)

[0054] This embodiment uses phosphogypsum, a byproduct of a phosphate chemical plant. After flotation purification to remove impurities, the phosphogypsum dihydrate has a purity of 98.6%. The iron-based solid waste is dust from a sintering plant, with a total iron content of 50.7%, a calcium oxide content of 12.9%, and a carbon content of 6%. The reducing agent is coke from a steel plant, with a fixed carbon content of 69%. 1 kg of purified phosphogypsum, 1 kg of sintering plant dust, 200 g of coke, and 50 g of bentonite are mixed and granulated using a disc pelletizer, producing spherical particles with a size of 5–8 mm. The mixed particles are placed in a small laboratory roasting furnace and heated to 900°C using electric heating. At this temperature, due to the gasification of carbon components, the roasting furnace exhibits a strong reducing atmosphere, with a carbon monoxide content of approximately 13%. After roasting the mixture for 40 minutes, the furnace temperature is raised to 1100°C, and an oxidizing gas with a 30% oxygen concentration is introduced at a flow rate of 1.2 L / min, continuing roasting for another hour. The XRD pattern of the final roasted product is shown in Figure 5, and the chemical composition is shown in Table 5. Due to the low final oxidative roasting temperature, the amount of calcium ferrite decreased, and the product was mainly composed of Fe2O3 phase. Some of the Fe2O3 phase was not converted into calcium ferrite.

[0055] Table 5. Results of multi-element chemical analysis / %

[0056]

Claims

1. A method for preparing calcium ferrite by low-temperature decomposition of iron-based solid waste catalytic phosphogypsum, characterized in that: Raw materials, including iron-based solid waste and phosphogypsum, are mixed, granulated and roasted to obtain calcium ferrate sinter. The roasting process is as follows: the material is fed onto a belt roaster and first undergoes reduction roasting at 900~1100℃ in a reducing atmosphere, and then undergoes oxidative roasting at 1200~1300℃ in an oxidizing atmosphere. The total carbon content in the raw material is 8-15% by mass. The CO mass content in the reducing atmosphere is 5% to 30%.

2. The method for preparing calcium ferrite by low-temperature decomposition of iron-based solid waste catalytic phosphogypsum according to claim 1, characterized in that: The total iron content of the iron-based solid waste is between 20% and 60% by mass; The phosphogypsum contains 70% to 95% gypsum dihydrate by mass.

3. The method for preparing calcium ferrite by low-temperature decomposition of iron-based solid waste catalytic phosphogypsum according to claim 1 or 2, characterized in that: The mass ratio of the iron-based solid waste to phosphogypsum is 3:1 to 1:

3.

4. The method for preparing calcium ferrite by low-temperature decomposition of iron-based solid waste catalytic phosphogypsum according to claim 1, characterized in that: The raw material includes a binder; the binder is at least one of bentonite, carboxymethyl cellulose, and starch.

5. The method for preparing calcium ferrite by low-temperature decomposition of iron-based solid waste catalytic phosphogypsum according to claim 4, characterized in that: The mass of the binder is 0.1% to 5% of the total mass of iron-based solid waste and phosphogypsum.

6. The method for preparing calcium ferrite by low-temperature decomposition of iron-based solid waste catalytic phosphogypsum according to claim 1, characterized in that: The reduction calcination time is 30-60 minutes.

7. The method for preparing calcium ferrite by low-temperature decomposition of iron-based solid waste catalytic phosphogypsum according to claim 1, characterized in that: The oxygen content in the oxidizing atmosphere is 20% to 40% by mass.

8. The method for preparing calcium ferrite by low-temperature decomposition of iron-based solid waste catalytic phosphogypsum according to claim 1, characterized in that: The oxidation roasting is carried out using an oxygen-enriched blast furnace method, and the roasting time is 40~90 minutes.

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