A method for oxidizing roasting of a raw material containing chromium and iron to extract chromium

Abstract

The present application belongs to the technical field of inorganic salt production, and particularly relates to a method for oxidizing roasting of a raw material containing chromium and iron to extract chromium, wherein carbon-chromium-iron, alkali, an oxidizing agent and an additive are ground and mixed, the additive is at least one of low-grade chromite, chromium residue and chromium-containing sludge, the mixture is fed into a rotary kiln, then down-flow roasting is carried out at a temperature of less than or equal to 750 DEG C while gradient temperature control is adopted, and then secondary grinding is carried out to obtain roasting clinker; the roasting clinker is leached, and after solid-liquid separation, leaching liquid and leaching residue are obtained. The method has the advantages that carbon-chromium-iron is mixed with an oxidizing agent, an additive and alkali to effectively control the oxidizing roasting process of carbon-chromium-iron, realize efficient extraction of chromium in carbon-chromium-iron and resource utilization of chromium in the additive, and realize low-temperature induction and medium-temperature reaction heat release through gradient temperature control, thereby greatly reducing the oxidizing roasting temperature, strengthening high-temperature roasting, deeply extracting chromium in the additive, and realizing low-temperature and energy-saving roasting treatment of carbon-chromium-iron.

Description

Technical Field

[0001] This invention belongs to the field of inorganic salt production technology, specifically relating to a method for oxidative roasting of chromium- and iron-containing raw materials to extract chromium. Background Technology

[0002] Currently, the main industrial method for producing chromium salts is the calcium-free roasting method. The technical route is as follows: under oxidative roasting conditions, alkalis such as sodium carbonate are used to decompose and oxidize chromite, converting the insoluble trivalent chromium compounds in the chromite into water-soluble sodium chromate. The roasting product is then leached with water to separate chromium, and the resulting sodium chromate solution is acidified, evaporated, and crystallized to prepare sodium dichromate. This sodium dichromate is further used to produce chromic anhydride, chromium green, and other chromium salt products.

[0003] However, in the traditional chromite oxidation roasting process, the roasting temperature is as high as 1100℃~1200℃. Sodium chromate has a low melting point and easily forms low-melting-point eutectics with byproducts such as sodium ferrite and sodium aluminate in the furnace charge. This results in a large amount of liquid phase in the roasting system beforehand. The presence of this liquid phase reduces the oxidation reaction rate and conversion rate of chromium. Furthermore, the liquid phase easily causes material to adhere to the kiln wall, forming rings, which seriously affects the continuity and stability of production. To eliminate the adverse effects of the liquid phase on the production process, 2-3 times the amount of ore in the return slag or limestone / dolomite is usually added as filler to the roasting furnace charge to reduce the volume ratio of the liquid phase in the charge. However, even with the addition of a large amount of filler, the oxidation conversion rate of chromium is still less than 85%. At the same time, the chromium grade in the return slag is relatively low. The addition of a large amount of return slag is equivalent to reducing the overall grade of the ore, reducing the effective contact area between the alkali and chromium, thereby reducing the oxidation rate of chromium and resulting in a low leaching rate.

[0004] CN116590545A discloses a "method to enhance the chromium extraction process of chromite ore without calcium roasting." This method includes the following steps: adding sulfuric acid solution to chromium slag for a leaching reaction to obtain acid-leached slag; using the acid-leached slag as a modified filler, mixing it with chromite ore and sodium carbonate to obtain a mixture; roasting the mixture under air atmosphere to obtain roasted clinker; directly water-quenching or cooling the roasted clinker followed by water leaching, and separating the solid and liquid to obtain sodium chromate solution and chromium filter residue. While this method solves the problems of large chromium slag emissions and high chromium residue in existing chromite ore chromium salt production, the chromium slag still contains residual impurities such as aluminum and magnesium during the acid leaching process. These impurities continuously leach during roasting, affecting the chromium recovery rate, reducing product purity, and causing poor product stability.

[0005] CN118996166A provides "A method for enhancing chromium extraction from chromite ore through calcium-free roasting using ferrochrome alloy leaching residue as an additive," which enables the full extraction of chromium from the ferrochrome alloy leaching residue, improves the chromium conversion rate during chromite roasting, reduces the amount of aluminum sludge produced, and achieves low-temperature, energy-saving roasting treatment of chromite ore. While this method solves the problems of resource utilization of ferrochrome acid leaching residue and efficient chromium extraction from chromite ore, the roasting temperature remains between 600℃ and 1000℃, and the issues of excessively high roasting temperature and a single additive remain.

[0006] Low-grade chromite ore, chromium slag, and chromium-containing sludge are typical bulk solid wastes containing chromium, presenting problems such as low resource utilization, complex composition, and difficult disposal. Low-grade ore resources are being wasted significantly, while chromium slag and chromium-containing sludge contain highly toxic hexavalent chromium, posing a prominent environmental risk. Therefore, carrying out harmless and resource-oriented co-treatment is of great significance and urgent necessity for controlling chromium pollution, improving chromium resource utilization, and promoting the green and sustainable development of related industries.

[0007] Wang Jialiang (Transactions of Nonferrous Metals Society of China, 2015, 25(11): 3820-3826.) studied a new clean production process for sodium chromate by carbon ferrochrome oxidative roasting. The core advantages of this process are its extremely high resource utilization rate and great potential for waste resource recovery. However, this technology still has many problems, mainly reflected in the difficulty of process control due to the complex reaction mechanism, the stringent requirements for equipment materials and operation and maintenance due to the high temperature and strong corrosion environment, the potential for a decline in actual indicators due to the scale-up effect, and the need to verify its overall economic efficiency.

[0008] Therefore, there is an urgent need to propose a method for simultaneously promoting the oxidative roasting of carbon ferrochrome with oxidants and additives to extract chromium. This process has outstanding advantages such as mild reaction conditions, low energy consumption, high chromium extraction efficiency, and easy industrial scale-up, providing a new technical path for the efficient and clean extraction of chromium from carbon ferrochrome. Summary of the Invention

[0009] The technical problem to be solved by this invention is to provide a method for oxidative roasting of chromium-containing and iron-containing raw materials to extract chromium. The method involves mixing carbon ferrochrome with an oxidant, additives, and alkali to effectively control the oxidative roasting process of carbon ferrochrome, thereby achieving efficient extraction of chromium from carbon ferrochrome and resource utilization of chromium in the additives. At the same time, the method achieves low-temperature induction through gradient temperature control and significantly reduces the temperature of oxidative roasting through the exothermic reaction at medium temperature. High-temperature enhanced roasting enables deep extraction of chromium from the additives, thus achieving low-temperature and energy-saving roasting treatment of carbon ferrochrome.

[0010] To achieve this objective, the present invention adopts the following technical solution: A method for extracting chromium from chromium- and iron-containing raw materials by oxidative roasting, the specific steps of which are as follows: (1) After grinding and mixing carbon ferrochrome, alkali, oxidant and additives, the average particle size of the mixture is ≤74μm. It is then fed into a rotary kiln and roasted in the co-current manner at ≤750℃, and the roasting temperature gradient is controlled. After that, a second grinding is carried out to obtain roasted clinker with an average particle size ≤74μm. The additive is at least one of low-grade ferrochrome ore, chromium slag and chromium-containing sludge. (2) The roasted clinker from step (1) is leached in the leachate at 55°C to 90°C for 1 to 5 hours, wherein the liquid-to-solid ratio of the leachate to the roasted clinker is (2 to 9):1m 3 / t, after solid-liquid separation, yields leachate and leachate residue; the chromium content in the leachate residue is <1.7% by mass.

[0011] The present invention provides a method for extracting chromium from ferrochrome carbon through oxidative roasting. This method effectively controls the oxidative roasting process by mixing an oxidant, additives, ferrochrome carbon, and an alkali, achieving efficient extraction of chromium from ferrochrome carbon and resource utilization of chromium in the additives. By controlling the roasting process through gradient temperature control, low-temperature induced oxidation, medium-temperature self-heating roasting, and high-temperature intensified roasting are achieved. This significantly accelerates the deep extraction oxidation reaction of chromium from ferrochrome carbon and additives, significantly increasing the oxidation rate, while simultaneously reducing the oxidative roasting temperature and shortening the roasting time, thus achieving low-temperature, energy-saving roasting treatment of ferrochrome carbon. Subsequently, after leaching the roasted clinker, the chromium content in the leaching residue is significantly reduced.

[0012] It is worth noting that this invention has the following innovative points: First, it uses carbon ferrochrome as the main material. Chromium and iron in the alloy exist in a metallic state, exhibiting high reactivity. Under the action of an oxidant, low-temperature induced oxidation can be achieved, allowing the system to initiate the reaction at a relatively low temperature. The presence of a small amount of carbon in the alloy can further promote the initiation of the low-temperature oxidation reaction and enhance the exothermic effect of the system. Second, the oxidation process of chromium and iron is accompanied by a strong self-exothermic effect, which can continuously provide heat for the reaction, significantly reducing the roasting temperature, reducing energy consumption, and shortening the reaction time. Third, the addition of additives improves the fluidity and heat storage capacity of the furnace charge, optimizes the heat transfer efficiency, ensures the uniformity and stability of the temperature field inside the kiln, and forms an active intermediate with sodium carbonate (such as sodium ferrite) to destroy the stable structure of the mineral, significantly promoting the oxidation reaction rate and conversion rate of chromium, thus realizing the resource utilization of chromium.

[0013] Furthermore, in step (1), the carbon ferrochrome includes the following elements by mass content: Cr: 40% to 70%, Fe: 20% to 40%, C: 1% to 10%, with the remainder being unavoidable impurities.

[0014] Furthermore, in step (1), the oxidant is at least one of chromium anhydride, sodium dichromate, and potassium dichromate. Using chromium anhydride, sodium dichromate, and potassium dichromate as oxidants achieves low-temperature induction of the reaction, accelerates the reaction, reduces energy consumption, shortens the reaction time, and avoids the introduction of impurities.

[0015] Furthermore, the mass ratio of the oxidant to carbon ferrochrome is (0.01 to 0.1):1.

[0016] In a further preferred embodiment, the mass ratio of the oxidant to carbon ferrochrome is (0.03 to 0.09):1.

[0017] In this invention, low-grade chromite ore, chromium slag, and chromium-containing sludge are selected as additives to improve the fluidity and heat storage capacity of the furnace charge, optimize the heat transfer efficiency, ensure the uniformity and stability of the temperature field inside the kiln, and at the same time, enable the full extraction of chromium from the additives and improve the conversion rate of chromium in the carbon ferrochrome roasting process, thereby achieving low-temperature energy-saving roasting treatment of carbon ferrochrome.

[0018] Furthermore, in step (1), the mass ratio of the carbon ferrochrome to the additive is (0.5~2):1. The additive includes the following elements by mass content: Cr: 3%~14%, Fe: 3%~13%, Ca: 5%~30%, Mg: 4%~24%, with the balance being O and unavoidable impurities. All elements in the additive are mainly in the form of oxides.

[0019] In a further preferred embodiment, in step (1), the mass ratio of the carbon ferrochrome to the additive is (0.6-1.7):1.

[0020] Furthermore, the calcination temperature gradient and residence time are controlled as follows: Low temperature range: room temperature to 300℃, residence time 10 minutes to 120 minutes; Medium temperature range: 300℃ to 600℃, residence time 30 minutes to 150 minutes; High temperature range: 600℃ to 750℃, residence time 30 minutes to 60 minutes. The calcination temperature gradient and residence time are controlled by controlling the induced draft air volume and kiln speed.

[0021] Further optimization resulted in a total residence time of ≥110 minutes across the low-temperature, medium-temperature, and high-temperature zones.

[0022] Furthermore, in step (1), the alkali is at least one of sodium carbonate, potassium carbonate, sodium hydroxide, and potassium hydroxide.

[0023] Furthermore, the mass ratio of the alkali to carbon ferrochrome is (0.5–3):1.

[0024] In a further preferred embodiment, in step (1), the mass ratio of the alkali to carbon ferrochrome is (0.7-2.8):1.

[0025] Furthermore, in step (1), the roasting is carried out in a mixture of air and oxygen-enriched gas, or in air or oxygen-enriched gas; the oxygen-enriched gas is an atmosphere with an oxygen volume percentage greater than 20%; in step (2), the leaching method is water leaching.

[0026] The beneficial effects of this invention are: (1) The co-current roasting process with the burner and feed inlet at the same end is adopted. Carbon ferrochrome, oxidant, additives and alkali are used as raw materials. The addition of additives effectively controls the carbon ferrochrome oxidation roasting process, improves the fluidity and heat storage capacity of the furnace charge, optimizes the heat transfer efficiency, and ensures the uniform and stable temperature field inside the rotary kiln. This realizes self-heating roasting and efficient extraction of chromium from carbon ferrochrome and resource utilization of chromium from additives. (2) Low-temperature induced oxidation is achieved by adding an oxidant, allowing the system to initiate the reaction at a lower temperature. The roasting process is controlled by a gradient temperature control method. The exothermic reaction at medium temperature significantly reduces the temperature of the oxidative roasting, while the high-temperature enhanced roasting allows for deep extraction of chromium from the additives, significantly accelerating the oxidation reaction and increasing the oxidation rate. At the same time, the temperature of the oxidative roasting is reduced, and the time of the oxidative roasting is shortened, achieving low-temperature energy-saving roasting treatment of carbon ferrochrome. After leaching the roasted clinker, the chromium (calculated as Cr2O3) content in the leaching residue is <1.7% by mass. Detailed Implementation

[0027] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0028] In the following examples and comparative examples, the oxygen-enriched gas is a gas with an oxygen volume percentage greater than 20%.

[0029] Example 1 This embodiment provides a method for extracting chromium from raw materials containing chromium and iron through oxidative roasting, including the following steps: (1) Carbon ferrochrome, sodium hydroxide, chromium anhydride-sodium dichromate mixture and low-grade chromium ore-chromium slag mixture are ground and mixed in a mass ratio of 1:1.8:0.07:1. The average particle size of the mixture is ≤65μm. The mixture is then fed into a rotary kiln for co-current roasting in air. The roasting temperature gradient is controlled as follows: low temperature section: room temperature to 300℃, residence time 40 minutes; medium temperature section: 300℃ to 600℃, residence time 90 minutes; high temperature section: 600℃ to 750℃, residence time 40 minutes. The roasting temperature gradient and residence time are controlled by controlling the induced draft air volume and the rotary kiln speed; then a second grinding is performed to obtain roasted clinker with an average particle size ≤55μm; the carbon ferrochrome includes the following elements by mass content: Cr: 50%, Fe: 35%, C: 7%, with the balance being unavoidable impurities; the low-grade chromite-chromium slag mixture includes the following elements by mass content: Cr: 3%, Fe: 5%, Ca: 25%, Mg: 8%, with the balance being O and unavoidable impurities; (2) The roasted clinker from step (1) is soaked in water at 85°C for 1.5 hours, and after solid-liquid separation, leachate and leachate residue are obtained; the liquid-solid ratio of water to roasted clinker is 8:1m. 3 / t.

[0030] Example 2 This embodiment provides a method for extracting chromium from raw materials containing chromium and iron through oxidative roasting, including the following steps: (1) Carbon ferrochrome, potassium hydroxide, chromium anhydride-potassium dichromate mixture and chromium-containing sludge were ground and mixed in a mass ratio of 1:2.3:0.05:1.25. The average particle size of the mixture was ≤68μm. The mixture was fed into a rotary kiln and roasted in a co-current manner in an oxygen-enriched gas. The roasting temperature gradient was controlled as follows: low temperature section: room temperature to 300℃, residence time 20 minutes; medium temperature section: 300℃ to 600℃, residence time 30 minutes; high temperature section: 600℃ to 750℃, residence time 60 minutes. The roasting temperature gradient and residence time are controlled by controlling the induced draft air volume and the rotary kiln speed; then a second grinding is performed to obtain roasted clinker with an average particle size ≤68μm; the carbon ferrochrome includes the following elements by mass content: Cr: 45%, Fe: 40%, C: 9%, with the balance being unavoidable impurities; the chromium-containing sludge includes the following elements by mass content: Cr: 9%, Fe: 7%, Ca: 10%, Mg: 12%, with the balance being O and unavoidable impurities; (2) The roasted clinker described in step (1) is soaked in water at 60°C for 3 hours, and after solid-liquid separation, leachate and leachate residue are obtained; the liquid-solid ratio of water to roasted clinker is 7:1m. 3 / t.

[0031] Example 3 This embodiment provides a method for extracting chromium from raw materials containing chromium and iron through oxidative roasting, including the following steps: (1) Carbon ferrochrome, potassium carbonate, potassium dichromate and low-grade chromium ore-chromium-containing sludge mixture were ground and mixed in a mass ratio of 1:1.2:0.09:0.8. The average particle size of the mixture was ≤55μm. It was then fed into a rotary kiln for co-current roasting. The roasting temperature gradient was controlled as follows: low temperature section: room temperature to 300℃, residence time 60 minutes; medium temperature section: 300℃ to 600℃, residence time 150 minutes; high temperature section: 600℃ to 750℃, residence time 30 minutes. The roasting temperature gradient and residence time were controlled accordingly. The process involves controlling the induced draft air volume and the rotary kiln speed; followed by a second grinding to obtain roasted clinker with an average particle size ≤70μm; the carbon ferrochrome comprises the following elements by mass content: Cr: 60%, Fe: 30%, C: 5%, with the balance being unavoidable impurities; the low-grade chromium ore-chromium-containing sludge mixture comprises the following elements by mass content: Cr: 5%, Fe: 11%, Ca: 15%, Mg: 16%, with the balance being O and unavoidable impurities; oxidative roasting is carried out in oxygen-enriched gas; (2) The roasted clinker from step (1) is soaked in water at 55°C for 4.5 hours, and after solid-liquid separation, leachate and leachate residue are obtained; the liquid-solid ratio of water to roasted clinker is 6:1m. 3 / t.

[0032] Example 4 This embodiment provides a method for extracting chromium from raw materials containing chromium and iron through oxidative roasting, including the following steps: (1) Carbon ferrochrome, potassium carbonate-sodium hydroxide mixture, potassium dichromate-sodium dichromate mixture and chromium slag were ground and mixed in a mass ratio of 1:2.8:0.02:1.7. The average particle size of the mixture was ≤72μm. The mixture was then fed into a rotary kiln for co-current roasting. The roasting temperature gradient was controlled as follows: low temperature section: room temperature to 300℃, residence time 10 minutes; medium temperature section: 300℃ to 600℃, residence time 90 minutes; high temperature section: 600℃ to 750℃, residence time 55 minutes. The temperature gradient and residence time are controlled by adjusting the induced draft air volume and rotary kiln speed; a second grinding is then performed to obtain calcined clinker with an average particle size ≤40μm; the carbon ferrochrome comprises the following elements by mass: Cr: 70%, Fe: 20%, C: 1%, with the balance being unavoidable impurities; the chromium slag comprises the following elements by mass: Cr: 14%, Fe: 9%, Ca: 30%, Mg: 4%, with the balance being O and unavoidable impurities; oxidative roasting is carried out in air; (2) The roasted clinker from step (1) is soaked in water at 75°C for 4 hours, and after solid-liquid separation, leachate and leachate residue are obtained; the liquid-solid ratio of water to roasted clinker is 9:1m.3 / t.

[0033] Example 5 This embodiment provides a method for extracting chromium from raw materials containing chromium and iron through oxidative roasting, including the following steps: (1) Carbon ferrochrome, sodium carbonate, sodium dichromate and low-grade chromite were ground and mixed in a mass ratio of 1:0.7:0.08:0.6. The average particle size of the mixture was ≤60μm. The mixture was then fed into a rotary kiln for co-current roasting. The roasting temperature gradient was controlled as follows: low temperature section: room temperature to 300℃, residence time 80 minutes; medium temperature section: 300℃ to 600℃, residence time 120 minutes; high temperature section: 600℃ to 750℃, residence time 40 minutes. The roasting temperature gradient and residence time were controlled accordingly. The method involves controlling the induced draft air volume and the rotary kiln speed; followed by a second grinding to obtain calcined clinker with an average particle size ≤65μm; the carbon ferrochrome comprises the following elements by mass content: Cr: 65%, Fe: 25%, C: 3%, with the balance being unavoidable impurities; the low-grade chromite-chromium slag mixture comprises the following elements by mass content: Cr: 7%, Fe: 13%, Ca: 5%, Mg: 20%, with the balance being O and unavoidable impurities; oxidative roasting is carried out in air; (2) The roasted clinker from step (1) is soaked in water at 60°C for 3.5 hours, and after solid-liquid separation, leachate and leachate residue are obtained; the liquid-solid ratio of water to roasted clinker is 3:1m. 3 / t.

[0034] Example 6 This embodiment provides a method for extracting chromium from raw materials containing chromium and iron through oxidative roasting, including the following steps: (1) Carbon ferrochrome, potassium carbonate, chromium anhydride and low-grade chromium ore-chromium-containing sludge mixture were ground and mixed in a mass ratio of 1:2:0.03:1.1. The average particle size of the mixture was ≤50μm. It was then fed into a rotary kiln for co-current roasting. The roasting temperature gradient was controlled as follows: low temperature section: room temperature to 300℃, residence time 40 minutes; medium temperature section: 300℃ to 600℃, residence time 60 minutes; high temperature section: 600℃ to 750℃, residence time 50 minutes. The roasting temperature gradient and residence time were controlled by controlling the induced draft. Air volume and rotary kiln speed; then a second grinding is performed to obtain roasted clinker with an average particle size ≤50μm; the carbon ferrochrome includes the following elements by mass content: Cr: 40%, Fe: 40%, C: 10%, with the balance being unavoidable impurities; the low-grade chromite-chromium slag mixture includes the following elements by mass content: Cr: 11%, Fe: 3%, Ca: 20%, Mg: 24%, Al: 6%, Si: 10%, with the balance being O and unavoidable impurities; oxidative roasting is carried out in oxygen-enriched gas; (2) The roasted clinker described in step (1) is soaked in water at 90°C for 2 hours, and after solid-liquid separation, leachate and leachate residue are obtained; the liquid-solid ratio of water to roasted clinker is 5:1m. 3 / t.

[0035] Example 7 This embodiment provides a method for oxidative roasting of chromium-containing raw materials to extract chromium. Except for adjusting the mass ratio of carbon ferrochrome to additives in step (1) to 0.5:1, all other conditions are the same as in Example 1.

[0036] Example 8 This embodiment provides a method for oxidative roasting of chromium-containing raw materials to extract chromium. Except for adjusting the mass ratio of carbon ferrochrome to additives in step (1) to 2.0:1, all other conditions are the same as in Example 1.

[0037] Example 9 This embodiment provides a method for oxidative roasting of chromium- and iron-containing raw materials to extract chromium. Except for adjusting the roasting temperature gradient in step (1) to the following: low temperature range: room temperature to 300°C, residence time 10 minutes; medium temperature range: 300°C to 600°C, residence time 30 minutes; high temperature range: 600°C to 750°C, residence time 30 minutes, all other conditions are the same as in Example 1.

[0038] Example 10 This embodiment provides a method for oxidative roasting to extract chromium from chromium and iron-containing raw materials. Except for adjusting the roasting temperature gradient in step (1) to the following: low temperature range: room temperature to 300°C, residence time 120 minutes; medium temperature range: 300°C to 600°C, residence time 150 minutes; high temperature range: 600°C to 750°C, residence time 60 minutes, all other conditions are the same as in Example 1.

[0039] Example 11 This embodiment provides a method for oxidative roasting of chromium-containing and iron-containing raw materials to extract chromium. Except for the mass ratio of carbon ferrochrome to alkali in step (1) being 1:3, all other conditions are the same as in Example 1.

[0040] Example 12 This embodiment provides a method for oxidative roasting of chromium-containing raw materials to extract chromium. Except for the mass ratio of carbon ferrochrome to alkali of 1:0.5 in step (1), all other conditions are the same as in Example 1.

[0041] Example 13 This embodiment provides a method for oxidative roasting of chromium-containing raw materials to extract chromium. Except for the mass ratio of carbon ferrochrome to oxidant of 1:0.01 in step (1), all other conditions are the same as in Example 1.

[0042] Example 14 This embodiment provides a method for oxidative roasting of chromium-containing raw materials to extract chromium. Except for the mass ratio of carbon ferrochrome to oxidant of 1:0.1 in step (1), all other conditions are the same as in Example 1.

[0043] Comparative Example 1 This comparative example provides a method for oxidative roasting of chromium-containing and iron-containing raw materials to extract chromium, except that no additives are added in step (1); all other conditions are the same as in Example 1.

[0044] Comparative Example 2 This comparative example provides a method for oxidative roasting of chromium-containing and iron-containing raw materials to extract chromium. Except for the fact that the roasting temperature gradient was not controlled in step (1), all other conditions are the same as in Example 3.

[0045] Comparative Example 3 This comparative example provides a method for oxidative roasting of chromium-containing and iron-containing raw materials to extract chromium. Except for the absence of an oxidant in step (1), all other conditions are the same as in Example 5.

[0046] Comparative Example 4 This comparative example provides a method for oxidative roasting of chromium-containing and iron-containing raw materials to extract chromium. Except for the countercurrent roasting method in step (1), the other conditions are the same as those in Example 6.

[0047] After testing the clinker, leachate, and leaching residue obtained from the above embodiments and comparative examples, the oxidation rate of chromium after roasting in step (1), the leaching rate of chromium after step (2), and the chromium content (calculated as Cr2O3) in the leaching residue were calculated. The results are shown in Table 1.

[0048] Table 1. Chromium oxidation rate, leaching rate, and chromium content in leaching residue for each example and comparative example. As shown in Table 1: (1) The carbon ferrochrome oxidation roasting method for chromium extraction provided in Examples 1-6 of the present invention uses carbon ferrochrome, alkali and additives to achieve efficient extraction and recovery of chromium from carbon ferrochrome and additives. The oxidation rate of chromium is >95%, the leaching rate of chromium is >99%, and the mass content of chromium (calculated as Cr2O3) in the leaching residue is <1.7%.

[0049] (2) Comparing Example 1 and Example 7-8, it can be seen that when the amount of additive is too high, some of the chromium in the additive is not fully oxidized, resulting in a decrease in the overall oxidation rate; when the amount of additive is too low, some materials are not completely oxidized due to the small amount of ring formation during the roasting process, resulting in a decrease in the oxidation rate.

[0050] (3) Comparing the comprehensive examples 1 and 9-10, it can be seen that when the temperature gradient in different temperature sections of the kiln is large and the residence time is short, the self-heating roasting time of the reaction is short and the roasting is insufficient, thus reducing its oxidation rate. When the temperature gradient in different temperature sections of the kiln is small and the residence time is long, the low-temperature reaction is fully started, the self-heating roasting time is sufficient, and the high-temperature strengthening is thorough, so its oxidation rate is higher.

[0051] (4) A comparison of Examples 1 and 11-12 shows that when the amount of alkali added is too high, although it will not affect the oxidation rate and leaching rate, the remaining alkali will leach into the liquid phase, ultimately affecting the purity of the sodium chromate product. When the alkali content is too low, the overall oxidation rate will be low due to incomplete oxidation.

[0052] (5) A comparison of Examples 1 and 13-14 shows that when the amount of oxidant added is too high, it is beneficial to improve the conversion rate and leaching rate of chromium, but it increases the cost of chromium extraction. When the oxidant content is too low, the conversion rate decreases due to the slow start-up reaction.

[0053] (6) As can be seen from the comparison between the examples and Comparative Example 1, when no additives are added, the flowability and heat storage capacity of the furnace charge become worse, the temperature field inside the kiln becomes unstable, which is not conducive to the efficient extraction and recovery of chromium in carbon ferrochrome, the oxidation rate of chromium in the roasted clinker decreases, and the chromium content (calculated as Cr2O3) in the leaching residue increases.

[0054] (7) As can be seen from the comparison between the examples and Comparative Example 2, when the temperature gradient inside the kiln is not controlled, the reaction of each reaction stage of the material is insufficient, which is not conducive to the efficient extraction and recovery of chromium in carbon ferrochrome. The oxidation rate of chromium in the roasted clinker decreases and the chromium content (calculated as Cr2O3) in the leaching residue increases.

[0055] (8) As can be seen from the comparison between the examples and Comparative Example 3, when no oxidant is added, it is difficult for the material to induce reaction at low temperature, the reaction temperature increases, the material oxidation is insufficient, which is not conducive to the efficient extraction and recovery of chromium in carbon ferrochrome, the oxidation rate of chromium in the roasted clinker decreases, and the chromium content (calculated as Cr2O3) in the leaching residue increases.

[0056] (9) As can be seen from the comparison between the examples and Comparative Example 4, when countercurrent roasting is adopted (the burner is placed at the tail of the rotary kiln and the hot air enters the rotary kiln from the tail of the rotary kiln), the material low-temperature induced start-up reaction time is long, the medium-temperature self-heating roasting process is difficult to control, the material reaction effect is poor, and the dust is easily carried into the high-temperature zone, which increases the dust removal load at the kiln tail, which easily causes material blockage and accumulation. The kiln has poor permeability, which is not conducive to oxidation reaction and sodium salt diffusion, and the roasting is incomplete.

[0057] The above are merely specific embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various modifications and variations. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for extracting chromium from chromium- and iron-containing raw materials through oxidative roasting, characterized in that: Detailed steps as follows: (1) After grinding and mixing carbon ferrochrome, alkali, oxidant and additives, the average particle size of the mixture is ≤74μm. It is then fed into a rotary kiln and roasted in the co-current manner at ≤750℃, and the roasting temperature gradient is controlled. After that, a second grinding is carried out to obtain roasted clinker with an average particle size ≤74μm. The additive is at least one of low-grade ferrochrome ore, chromium slag and chromium-containing sludge. (2) The roasted clinker from step (1) is leached in the leachate at 55°C to 90°C for 1 to 5 hours, wherein the liquid-to-solid ratio of the leachate to the roasted clinker is (2 to 9):1m 3 / t, after solid-liquid separation, yields leachate and leachate residue; the chromium content in the leachate residue is <1.7% by mass.

2. The method for extracting chromium from chromium- and iron-containing raw materials by oxidative roasting according to claim 1, characterized in that: In step (1), the carbon ferrochrome includes the following elements by mass content: Cr: 40% to 70%, Fe: 20% to 40%, C: 1% to 10%.

3. The method for extracting chromium from chromium- and iron-containing raw materials by oxidative roasting according to claim 1, characterized in that: In step (1), the oxidant is at least one of chromium trioxide, sodium dichromate, and potassium dichromate, and the mass ratio of the oxidant to carbon ferrochrome is (0.01-0.1):

1.

4. The method for extracting chromium from chromium- and iron-containing raw materials by oxidative roasting according to claim 3, characterized in that: The mass ratio of the oxidant to carbon ferrochrome is (0.03~0.09):

1.

5. The method for extracting chromium from chromium- and iron-containing raw materials by oxidative roasting according to claim 1, characterized in that: In step (1), the mass ratio of carbon ferrochrome to additives is (0.5-2):

1. The additives include the following elements by mass content: Cr: 3%-14%, Fe: 3%-13%, Ca: 5%-30%, Mg: 4%-24%, with the balance being O and unavoidable impurities. All elements in the additives exist mainly in the form of oxides.

6. The method for extracting chromium from chromium- and iron-containing raw materials by oxidative roasting according to claim 5, characterized in that: In step (1), the mass ratio of carbon ferrochrome to additives is (0.6-1.7):

1.

7. The method for extracting chromium from chromium- and iron-containing raw materials by oxidative roasting according to claim 1, characterized in that: The calcination temperature gradient and residence time are controlled as follows: Low temperature zone: room temperature to 300℃, residence time 10 minutes to 120 minutes; Medium temperature zone: 300℃ to 600℃, residence time 30 minutes to 150 minutes; High temperature zone: 600℃ to 750℃, residence time 30 minutes to 60 minutes; The total residence time of the low temperature zone, medium temperature zone and high temperature zone is ≥110 minutes.

8. The method for extracting chromium from chromium- and iron-containing raw materials by oxidative roasting according to claim 1, characterized in that: In step (1), the alkali is at least one of sodium carbonate, potassium carbonate, sodium hydroxide, and potassium hydroxide, and the mass ratio of the alkali to carbon ferrochrome is (0.5-3):

1.

9. The method for extracting chromium from chromium- and iron-containing raw materials by oxidative roasting according to claim 8, characterized in that: In step (1), the mass ratio of the alkali to carbon ferrochrome is (0.7-2.8):

1.

10. The method for extracting chromium from chromium- and iron-containing raw materials by oxidative roasting according to claim 1, characterized in that: In step (1), the roasting is carried out in a mixture of air and oxygen-enriched gas, or air or oxygen-enriched gas; the oxygen-enriched gas is an atmosphere with an oxygen volume percentage greater than 20%; in step (2), the leaching method is water leaching.

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