Leaching method of chromite

Abstract

The invention relates to a leaching method of chromite. The leaching method comprises the following steps: (1) uniformly mixing pretreated chromite, chlorine salt and a reducing agent, and roasting to obtain a roasted material; and (2) carrying out acid leaching on the roasted material, and then carrying out solid-liquid separation to obtain a leaching solution containing trivalent chromium and trivalent iron and leaching residues. According to the method, pre-roasting of the chromite, the chlorine salt and the reducing agent is combined with acid leaching, the roasting temperature is reduced, the leaching rate of the chromite is increased, high-temperature and high-pressure leaching is not needed, conditions are mild, energy consumption is small, and the process scheme is simple and feasible.

Description

Technical Field

[0001] This invention relates to the field of chromium salt production technology, and more particularly to a leaching method for chromite. Background Technology

[0002] Chromium salts are one of the major series of inorganic chemical products, widely used in metallurgy, leather making, pigments, catalysis, and other fields. Currently, the mainstream production technology for sodium chromate is still the high-temperature roasting method, which mainly uses chromite and sodium carbonate as raw materials. Sodium chromate is produced by roasting at temperatures above 1200℃, followed by sulfuric acid acidification to produce sodium dichromate. The disposal of chromium slag containing hexavalent chromium has always been a difficult and challenging issue for the chromium salt industry.

[0003] Acid leaching of chromite can prevent the formation of Cr(VI) at the source, making it a promising clean production process for chromium salts. For example, CN101979679A discloses a method for treating chromite by sulfuric acid leaching. In a mixture of sulfuric acid and chromite powder, oxidants such as chromic anhydride, potassium chlorate, or ammonium persulfate are added. The reaction is carried out at 120-190℃ and a pressure of 0.2-1.2 MPa. After solid-liquid separation, the filtrate is purified of iron and crystallized to prepare chromium sulfate. The filter residue is directly used as a raw material for ferrosilicon smelting. CN111620370A discloses an electric field-enhanced high-efficiency leaching method for chromite. Chromite is mixed with concentrated sulfuric acid, and an appropriate amount of oxidizing catalyst is added. The leaching process is assisted by an electric field, and through continuous cyclic leaching, the chromium in the chromite is converted into chromium sulfate and leached into the solution. CN109022830A proposes a method for preparing high-chromium products by leaching chromite with hydrochloric acid using a combination of microwave and ultrasonic methods. This method involves adding a small amount of hydrochloric acid, which reacts with impurities such as iron, aluminum, and calcium in chromite at high temperatures. This causes the impurities to dissolve after roasting, while chromium remains in the slag phase. The slag is then further roasted with ferrosilicon alloy at a constant temperature of 1100-1450℃ for 10-40 minutes using microwave to obtain a high-chromium product.

[0004] Existing acid leaching technologies generally require the addition of oxidants and the use of high temperature, high pressure, electric fields, microwaves, and other enhanced reaction processes to achieve chromium leaching or separation. Oxidants generally introduce impurities such as hexavalent chromium, potassium ions, and ammonium ions. Moreover, since the reaction process and the separation process of chromium and iron are not yet clear, the existing acid leaching technology for chromite still needs to be improved. Summary of the Invention

[0005] To solve the above-mentioned technical problems, the present invention pre-roasts chromite before acid leaching. No oxidant is needed during the pre-roasting process, and the leaching rate is improved by chlorination roasting. The process is simple and feasible.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a leaching method for chromite, the leaching method comprising the following steps:

[0008] (1) The pretreated chromite, chloride salt and reducing agent are mixed evenly and then roasted to obtain roasting material;

[0009] (2) The roasted material is acid-leached and then solid-liquid separated to obtain a leachate containing trivalent chromium and trivalent iron and a leachate residue.

[0010] The present invention does not limit the solid-liquid separation described herein, and any method known to those skilled in the art for solid-liquid separation may be used, such as filtration, sedimentation or centrifugation.

[0011] As a preferred technical solution of the present invention, the pretreatment includes mixing chromite with water and then ball milling.

[0012] Preferably, the mass ratio of grinding balls, chromite and water in the ball mill is (0.5-2.0):1:(1.0-3.0), for example, it can be 0.5:1:1.0, 0.8:1:1.5, 1.0:1:2, 1.5:1:2.5 or 2:1:3, etc.

[0013] Preferably, the grinding ball is made of stainless steel, zirconium oxide, aluminum oxide, or agate.

[0014] The size of the grinding balls, the grinding time, and the rotation speed described in this invention can be selected according to the requirements of the grinding particle size, and are not further limited here.

[0015] Preferably, more than 90% of the chromite particles after ball milling are smaller than 200 μm.

[0016] As a preferred technical solution of the present invention, the mass ratio of chromite, chloride salt and reducing agent is 1:(0.5-3.0):(0.5-3.0), for example, it can be 1:0.5:0.5, 1:0.8:1, 1:1:1.5, 1:1.2:2, 1:2.5:2.5 or 1:3:3, etc.

[0017] This invention promotes the decomposition of spinel by disrupting the stable crystal lattice through mixed roasting of chromite and chloride salts. The chlorination process further disrupts the crystal structure, promoting the oxidation of iron to ferric iron (Fe3+), which is then dissolved through acid leaching. The addition of a reducing agent reduces the oxidation of ferric chromium to hexavalent chromium, leaving only a small amount. The presence of hexavalent chromium causes lattice distortion in the spinel structure during acid leaching, reducing structural stability and promoting the acid leaching and dissolution of chromium trioxide generated during chlorination. Simultaneously, controlling the ratio of chloride salts to reducing agent ensures that only a small amount of hexavalent chromium is generated during roasting, minimizing toxicity.

[0018] As a preferred embodiment of the present invention, the chloride salt includes a combination of at least two of sodium chloride, potassium chloride, calcium chloride, magnesium chloride, and ammonium chloride.

[0019] This invention uses a mixture of chloride salts for calcination, and the mixed chloride salts have a lower eutectic point.

[0020] As a preferred embodiment of the present invention, the reducing agent includes carbon powder and / or coal powder.

[0021] As a preferred technical solution of the present invention, the roasting temperature is 500-1000℃, for example, it can be 500℃, 600℃, 700℃, 800℃, 900℃ or 1000℃, etc.

[0022] The addition of chloride salts and reducing agents in this invention lowers the roasting temperature, making it much lower than the 1200℃ required for roasting chromite alone.

[0023] Preferably, the calcination time is 0.5-1.5 hours, for example, 0.5 hours, 0.8 hours, 1 hour, 1.2 hours or 1.5 hours.

[0024] As a preferred embodiment of the present invention, the leaching agent for acid leaching includes hydrochloric acid.

[0025] Preferably, the concentration of the hydrochloric acid is 6 mol / L-12 mol / L, for example, it can be 6 mol / L, 7 mol / L, 8 mol / L, 9 mol / L, 10 mol / L, 11 mol / L or 12 mol / L, etc.

[0026] Preferably, the mass ratio of the leachate to the calcined material is (0.5-4.0):1, for example, it can be 0.5:1, 1.0:1, 2.0:1, 3.0:1 or 4.0:1, etc.

[0027] As a preferred technical solution of the present invention, the acid leaching time is 0.5-2.0h, for example, it can be 0.5h, 0.8h, 1h, 1.5h or 2h.

[0028] Preferably, the acid leaching temperature is room temperature.

[0029] Preferably, the acid leaching is carried out under normal pressure.

[0030] The acid leaching of this invention can achieve a high leaching rate at room temperature, without the need for high temperature and high pressure, under mild conditions, and with low energy consumption.

[0031] As a preferred technical solution of the present invention, the content of trivalent chromium in the leachate is 85%-99% of the total chromium content, for example, it can be 85%, 87%, 90%, 92%, 94%, 96%, 98% or 85%, etc.

[0032] Preferably, the content of ferric iron in the leachate is 95%-99% of the total iron content.

[0033] The chromium and iron ions in the leachate of this invention exist in trivalent form and can be used as raw materials for iron-chromium flow batteries after impurity removal without further oxidation-reduction.

[0034] As a preferred embodiment of the present invention, the leaching method includes the following steps:

[0035] (1) Chromite is mixed with water and then ball-milled. The mass ratio of grinding balls, chromite and water in the ball mill is (0.5-2.0):1:(1.0-3.0). More than 90% of the chromite particles after ball milling are smaller than 200μm. The chromite, chloride salt and reducing agent after ball milling are mixed evenly and then roasted at 500-1000℃ for 0.5-1.5h to obtain roasted material. The mass ratio of chromite, chloride salt and reducing agent is 1:(0.5-2.0):(0.5-3.0). The chloride salt includes any one or a combination of at least two of sodium chloride, potassium chloride, calcium chloride, magnesium chloride and ammonium chloride.

[0036] (2) Hydrochloric acid with a concentration of 6mol / L-12mol / L is mixed with the calcined material at a mass ratio of (0.5-4.0):1 and then acid-leached at room temperature for 0.5-2.0h. After solid-liquid separation, leachate containing trivalent chromium and trivalent iron and leach residue are obtained.

[0037] Compared with the prior art, the present invention has at least the following beneficial effects:

[0038] (1) The present invention decomposes the spinel structure at a lower temperature by chlorination roasting with mixed molten salt, which reduces the decomposition temperature of spinel by more than 200°C compared with the roasting of chromite alone.

[0039] (2) The formation of a small amount of hexavalent chromium on the surface of the roasted material during the chlorination roasting process effectively improves the leaching rate of the acid leaching process;

[0040] (3) The solution obtained after leaching the roasted material is mainly a leachate containing chromium chloride and ferric chloride. After deep purification and impurity removal, it can be used as a raw material for iron-chromium flow batteries, with broad application prospects.

[0041] (4) The process of this invention is simple, with less "three wastes" emissions and good product prospects, providing an economical and effective way to cleanly extract chromium from chromite at low cost. Detailed Implementation

[0042] To facilitate understanding of the present invention, the following embodiments are provided. Those skilled in the art should understand that these embodiments are merely illustrative and should not be construed as limiting the scope of the invention.

[0043] Example 1

[0044] This embodiment provides a leaching method for chromite, the leaching method comprising the following steps:

[0045] (1) Chromite is mixed with water and then ball-milled. The mass ratio of grinding balls, chromite and water in the ball mill is 1:1:2. More than 90% of the chromite particles after ball milling are smaller than 200μm. The chromite, chloride salt and carbon powder after ball milling are mixed evenly and then roasted at 800℃ for 2h to obtain roasted material. The chloride salt is sodium chloride and ammonium chloride mixed in a mass ratio of 1:1. The mass ratio of chromite, chloride salt and reducing agent is 1:1.5:1.2.

[0046] (2) Mix 10 mol / L hydrochloric acid with the calcining material at a mass ratio of 1.5:1 and then leach at room temperature for 2.0 h. After filtration, leachate containing trivalent chromium and trivalent iron and leach residue are obtained.

[0047] Example 2

[0048] This embodiment provides a leaching method for chromite, the leaching method comprising the following steps:

[0049] (1) Chromite is mixed with water and then ball-milled. The mass ratio of grinding balls, chromite and water in the ball mill is 0.5:1:3. More than 90% of the chromite particles after ball milling are smaller than 200μm. The chromite, chloride salt and carbon powder after ball milling are mixed evenly and then roasted at 500℃ for 3h to obtain roasted material. The chloride salt is calcium chloride and potassium chloride mixed in a mass ratio of 1:1. The mass ratio of chromite, chloride salt and reducing agent is 1:2:3.0.

[0050] (2) Mix 6 mol / L hydrochloric acid with the calcined material at a mass ratio of 4:1, then leach at room temperature for 0.5 h and filter to obtain a leachate and leaching residue containing trivalent chromium and trivalent iron.

[0051] Example 3

[0052] This embodiment provides a leaching method for chromite, the leaching method comprising the following steps:

[0053] (1) Chromite is mixed with water and then ball-milled. The mass ratio of grinding balls, chromite and water in the ball mill is 2.0:1:1.0. More than 90% of the chromite particles after ball milling are smaller than 200μm. The chromite, chloride salt and carbon powder after ball milling are mixed evenly and then roasted at 1000℃ for 0.5h to obtain roasted material. The chloride salt is ammonium chloride and potassium chloride mixed in a mass ratio of 1:1. The mass ratio of chromite, chloride salt and reducing agent is 1:0.5:0.5.

[0054] (2) Mix 12 mol / L hydrochloric acid with the calcining material at a mass ratio of 0.5:1, and then perform acid leaching at room temperature for 0.5 h. After filtration, leachate containing trivalent chromium and trivalent iron and leach residue are obtained.

[0055] Example 4

[0056] This embodiment provides a leaching method for chromite, which is the same as that in Embodiment 1 except that the roasting temperature is 400°C.

[0057] Example 5

[0058] This embodiment provides a leaching method for chromite, which is the same as that in Embodiment 1 except that the roasting temperature is 1100℃.

[0059] Example 6

[0060] This embodiment provides a leaching method for chromite, which is the same as that in Example 1 except that the mass ratio of chromite, chloride salt and reducing agent is 1:1:4.0.

[0061] Example 7

[0062] This embodiment provides a leaching method for chromite. Except for the mass ratio of chromite, chloride salt and reducing agent being 1:1:0.2, the leaching method is the same as in Example 1.

[0063] Example 8

[0064] This embodiment provides a leaching method for chromite, which is the same as that in Example 1 except that the mass ratio of chromite, chloride salt and reducing agent is 1:4:1.2.

[0065] Example 9

[0066] This embodiment provides a leaching method for chromite. The leaching method is the same as in Example 1, except that the mass ratio of chromite, chloride salt and reducing agent is 1:0.2:1.2.

[0067] Comparative Example 1

[0068] This comparative example provides a leaching method for chromite, which is the same as that in Example 1 except that no chloride salt is added during the roasting process.

[0069] Comparative Example 2

[0070] This comparative example provides a leaching method for chromite, which is the same as in Example 1 except that no carbon powder is added during the roasting process.

[0071] Comparative Example 3

[0072] This comparative example provides a leaching method for chromite, which is the same as in Example 1 except that chloride salts and carbon powder are not added during the roasting process.

[0073] Comparative Example 4

[0074] This comparative example provides a leaching method for chromite, which adopts the leaching method of Example 1 disclosed in CN101979679A.

[0075] Test methods

[0076] The chromium and iron content in the leachate and leaching residue obtained in Examples 1-9 and Comparative Examples 1-4 was measured by inductively coupled plasma atomic emission spectrometry (ICP-AES), and the leaching rates of chromium and iron were calculated. The test results are shown in Table 1.

[0077] Test Results

[0078] Table 1

[0079] Examples and Comparative Examples Chromium leaching rate (%) Iron leaching rate (%) Example 1 99.2 99.0 Example 2 86.9 86.8 Example 3 93.6 92.1 Example 4 68.2 66.7 Example 5 99.1 98.2 Example 6 82.4 81.9 Example 7 90.8 88.5 Example 8 99.6 99.3 Example 9 60.2 65.3 Comparative Example 1 55.2 53.8 Comparative Example 2 48.5 47.1 Comparative Example 3 2.5 2.3 Comparative Example 4 93.4 92.3

[0080] The test results show that:

[0081] (1) As can be seen from Examples 1 to 3, the present invention can reduce the decomposition temperature of chromite and improve the leaching effect by combining pre-roasting with acid leaching. The leached chromite ions mainly exist in the trivalent form, which is beneficial to industrial applications.

[0082] (2) As can be seen from Examples 1 and 4-5, the present invention can fully decompose chromite while reducing energy consumption by further optimizing the roasting temperature. When the roasting temperature in Example 4 is too low, the chromium leaching rate decreases to 68.2% and the iron leaching rate decreases to 66.7%. When the roasting temperature in Example 5 is too high, the energy consumption increases while the leaching rate does not increase further. As can be seen from Examples 1 and 6-9, the present invention can achieve better leaching effect and further improve the leaching rate by further optimizing the ratio of chromite, chloride salt and reducing agent. When too much reducing agent is added in Example 6, the leaching rates of both chromium and iron decrease. When too little reducing agent is added in Example 7, the leaching rates of chromium and iron decrease while the content of hexavalent chromium in the leached chromium increases significantly to 28%. When the addition of chloride salt is further increased in Example 8, the increase in chromium leaching rate and iron leaching rate is small. When the amount of chloride salt added in Example 9 is too low, the leaching rates of chromium and iron decrease significantly.

[0083] (3) As can be seen from Example 1 and Comparative Examples 1-4, the present invention can promote the destruction of chromite lattice and the leaching of chromium iron by adding chloride salt and reducing agent. When chloride salt is lacking, the lattice of chromite cannot be fully destroyed during roasting. When carbon powder is lacking, a large amount of hexavalent chromium is produced during chlorination roasting, which greatly increases the toxicity. When both are lacking, the chromium leaching rate drops sharply to 2.5% and the iron leaching rate drops sharply to 2.3%. When the existing technology is used to add oxidant for acid leaching in Comparative Example 4, the leaching rate is still lower than that in Example 1 even under heating and pressure conditions.

[0084] In summary, this invention controls the formation of hexavalent chromium by pre-roasting chromite with chloride salts and reducing agents combined with acid leaching, thereby reducing the roasting temperature and toxicity during the roasting process. The synergistic effect of pre-roasting and acid leaching improves the leaching rate of chromite, and the resulting leachate contains chromium and iron in trivalent form, which facilitates further industrial applications.

[0085] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A leaching method for chromite, characterized in that, The leaching method includes the following steps: (1) The pretreated chromite, chloride salt and reducing agent are mixed evenly and then roasted to obtain roasting material; (2) The roasted material is acid-leached and then solid-liquid separated to obtain a leachate containing trivalent chromium and trivalent iron and a leachate residue.

2. The leaching method according to claim 1, characterized in that, The pretreatment includes mixing chromite with water and then ball milling it. Preferably, the mass ratio of grinding balls, chromite, and water in the ball mill is (0.5-2.0):1:(1.0-3.0); Preferably, more than 90% of the chromite particles after ball milling are smaller than 200 μm.

3. The leaching method according to claim 1 or 2, characterized in that, The mass ratio of chromite, chloride salt and reducing agent is 1:(0.5-3.0):(0.5-3.0).

4. The leaching method according to any one of claims 1-3, characterized in that, The chloride salt includes a combination of at least two of sodium chloride, potassium chloride, calcium chloride, magnesium chloride, and ammonium chloride.

5. The leaching method according to any one of claims 1-4, characterized in that, The reducing agent includes carbon powder and / or coal powder.

6. The leaching method according to any one of claims 1-5, characterized in that, The roasting temperature is 500-1000℃; Preferably, the roasting time is 0.5-3.0 h.

7. The leaching method according to any one of claims 1-6, characterized in that, The leaching agent for acid leaching includes hydrochloric acid; Preferably, the concentration of the hydrochloric acid is 6 mol / L-12 mol / L; Preferably, the mass ratio of the leachate to the calcining material is (0.5-4.0):

1.

8. The leaching method according to any one of claims 1-7, characterized in that, The acid leaching time is 0.5-2.0 hours; Preferably, the acid leaching temperature is room temperature.

9. The leaching method according to any one of claims 1-8, characterized in that, The content of trivalent chromium in the leachate is 85%-99% of the total chromium content; Preferably, the content of ferric iron in the leachate is 95%-99% of the total iron content.

10. The leaching method according to claims 1-9, characterized in that, The leaching method includes the following steps: (1) mixing chromite with water and then ball milling, wherein the mass ratio of grinding balls, chromite and water in the ball mill is (0.5-2.0):1:(1.0-3.0), and more than 90% of the chromite particles after ball milling are smaller than 200μm. After the ball milling, the chromite, chloride salt and reducing agent are mixed evenly and then roasted at 500-1000℃ for 0.5-1.5h to obtain roasted material, wherein the mass ratio of chromite, chloride salt and reducing agent is 1:(0.5-3.0):(0.5-3.0), and the chloride salt includes any one or a combination of at least two of sodium chloride, potassium chloride, calcium chloride, magnesium chloride and ammonium chloride; (2) Hydrochloric acid with a concentration of 6mol / L-12mol / L is mixed with the calcined material at a mass ratio of (0.5-4.0):1 and then acid-leached at room temperature for 0.5-2.0h. After solid-liquid separation, leachate containing trivalent chromium and trivalent iron and leach residue are obtained.