Method for reducing content of hexavalent chromium in chromium oxide green

Abstract

The invention discloses a method for reducing the content of hexavalent chromium in chromium oxide green, which comprises the following steps: S1, calcining the chromium oxide green at a high temperature of 1200-1300 DEG C, and cooling to 300-450 DEG C to obtain high-temperature chromium oxide green; and S2, according to the mass ratio of 390-400: 1-1.05, adding the high-temperature chromium oxide green in the step S1 and a reducing agent into a hexavalent chromium removal device, and reacting to obtain a chromium oxide green product. According to the method, the reducing agent is added into the high-temperature chromium oxide green, then the reduction reaction is carried out in the rotary drum, and hexavalent chromium in the chromium oxide green is reduced into trivalent chromium, so that the content of hexavalent chromium in the chromium oxide green is reduced, and the recovery rate of metal chromium is improved; the hexavalent chromium can be removed from the source on the basis of not changing the chromium oxide green components, so that the hexavalent chromium is prevented from entering the chromium oxide green product, the investment is less, and the cost is low.

Description

Technical Field

[0001] The invention relates to the technical field of chromium salt production, and in particular to a method for reducing the content of hexavalent chromium in chromium oxide green. Background Art

[0002] Chromium oxide green, also known as chromium trioxide (Cr203), is a green powder with metallic luster and magnetism. It is used to smelt metallic chromium and chromium carbide. However, chromium oxide green usually contains hexavalent chromium. The presence of hexavalent chromium will cause the following problems: (1) Hexavalent chromium will not only increase the content of water-soluble substances, but also make the product yellow, affecting its use; (2) Hexavalent chromium compounds are highly toxic chemicals that can enter the human body through the digestive tract, respiratory tract, skin and mucous membranes. Long-term or short-term contact or inhalation may cause cancer and affect human health; (3) Hexavalent chromium can persist in the environment and has a high bioaccumulation capacity and hazard; (4) Hexavalent chromium compounds may also cause hereditary genetic defects and have serious impacts on the ecosystem. Therefore, hexavalent chromium compounds are included in the "List of Toxic and Hazardous Water Pollutants" and are strictly regulated and controlled to improve water environment quality, prevent and control water environment risks, reduce the adverse effects of toxic and hazardous substances on the water environment and human health, and protect public health and environmental safety.

[0003] At present, the method for reducing the hexavalent chromium content in chromium oxide green is to mix a small amount of boric acid into chromic anhydride and then roast it, or add a small amount of boric acid to the chromium oxide green containing hexavalent chromium and then roast it, then put the chromium oxide green obtained by thermal decomposition of chromic anhydride into a closed container previously filled with urea while hot and cool it, wash the chromium oxide green to remove water-soluble substances, suspend it in urea solution, and finally heat treat it at 200-280°C, and grind the chromium oxide green obtained by thermal decomposition of chromic anhydride with a small amount of ethanol in a vibration mill to obtain a chromium oxide green product. Although this method can reduce the hexavalent chromium content in chromium oxide green, it has the problems of complex operation, large amount of additives, and high production cost. Another method for reducing the hexavalent chromium content in chromium oxide green is a slurry washing process, which mainly includes the steps of slurrying, reduction, washing, filtering, drying and the like at room temperature. There are problems such as complicated procedures, high operating costs and large equipment investment. Moreover, when slurry washing chromium oxide green is used to remove hexavalent chromium at room temperature, a trace amount of hexavalent chromium cannot be completely washed out, which ultimately results in the chromium oxide green product still containing hexavalent chromium, affecting product performance. In addition, a certain amount of chromium oxide green will be lost while removing chromium, further increasing production costs.

[0004] In addition, Chinese patent application number 02119561.7 discloses a method for producing chromium trioxide by continuous chromic acid thermal decomposition. The method uses chromic acid as raw material, adds return material and additives, and mixes the materials evenly under the control of a microcomputer. The materials are quantitatively added to the fuel injection end of the heat-insulating rotary kiln. After a violent thermal decomposition reaction, the chromic acid is converted into chromium trioxide. After being kept warm in the rotary kiln for a period of time, the chromium trioxide product is obtained after cooling and crushing. Although the method solves the problems of low main content, high residual hexavalent chromium and water-soluble salt, and low thermal efficiency of the traditional chromic acid thermal decomposition method, it has high requirements for equipment and operation, and the additives and return materials also increase the energy consumption in the production process, so it is not suitable for promotion and use. Summary of the invention

[0005] The invention provides a method for reducing the hexavalent chromium content in chromium oxide green, aiming at removing the hexavalent chromium in the chromium oxide green by adopting a simple treatment process, thereby improving the performance of the chromium oxide green product while reducing the production cost, and reducing the pollution of the environment caused by the hexavalent chromium.

[0006] To achieve its purpose, the present invention adopts the following technical solution: A method for reducing the hexavalent chromium content in chromium oxide green comprises the following steps: S1: calcining chromium oxide green at a high temperature of 1200-1300° C. and then cooling to 300-450° C. to obtain high-temperature chromium oxide green; S2: adding the high-temperature chromium oxide green and the reducing agent in step S1 into a hexavalent chromium removal device at a mass ratio of 390-400:1-1.05, and obtaining a chromium oxide green product after reaction. Furthermore, in step S2, the reducing agent is added in excess relative to the hexavalent chromium ions in the chromium oxide green.

[0007] Furthermore, the amount of the reducing agent added in step S2 is 1.05 to 1.1 times the theoretical amount of hexavalent chromium ions in the reduced chromium oxide green.

[0008] Furthermore, in step S2, the reducing agent is one or more of sodium sulfite, sodium pyrosulfite, urea, sodium sulfide or hydroxylamine hydrochloride.

[0009] Furthermore, in step S2, the reaction time of the chromium oxide green and the reducing agent in the hexavalent chromium removal device is 15 to 20 minutes.

[0010] Furthermore, the hexavalent chromium removal device used in the method for reducing the hexavalent chromium content in chromium oxide green comprises a frame, a start switch is provided on the frame, a rotating drum driven by a transmission shaft is provided on the top of the frame; a first transmission part is provided on the top of the frame, and the output end of the first transmission part is connected to the transmission shaft; a feeding port is provided on the top of the rotating drum, and a cover plate is provided above the feeding port, and the cover plate is driven to open and close by a flip cylinder provided on the top of the rotating drum, and the flip cylinder is controlled by a time relay; a reducing agent storage silo and a chromium oxide green storage silo are provided above the rotating drum, a first metering screw driven by a second transmission part is provided at the bottom of the reducing agent storage silo, and a second metering screw driven by a third transmission part is provided at the bottom of the chromium oxide green storage silo, and the discharge ports of the first metering screw and the second metering screw are both aligned with the feeding port at the top of the rotating drum, and the bottoms of the first metering screw and the second metering screw are respectively connected to the top of the frame through connecting rods; a discharging scraper driven by a fourth transmission part is provided below the rotating drum; The start switch and the time relay are electrically connected to the signal input end of the PLC programmable controller respectively, and the signal output end of the PLC programmable controller is electrically connected to the flip cylinder, the first transmission part, the second transmission part, the third transmission part and the fourth transmission part respectively.

[0011] Furthermore, both ends of the transmission shaft of the hexavalent chromium removal device are supported by bearing seats.

[0012] Further, the rotating drum is supported by a first bracket, and the first transmission part is supported by a second bracket.

[0013] Furthermore, in the hexavalent chromium removal device, the fixed end of the turning cylinder and the rotating drum, and the movable end of the turning cylinder and the cover plate are respectively connected through pins.

[0014] Furthermore, the output end of the first transmission part is connected to the transmission shaft via a coupling.

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1. The method for reducing the hexavalent chromium content in chromium oxide green provided by the present invention comprises adding a reducing agent to high-temperature chromium oxide green, and then performing a reduction reaction in a rotating drum to reduce the hexavalent chromium in the chromium oxide green to trivalent chromium, thereby reducing the hexavalent chromium content in the chromium oxide green and improving the recovery rate of metallic chromium.

[0016] 2. This method utilizes the waste heat after calcination in the production process of chromium oxide green, and can significantly reduce the hexavalent chromium content from the source without changing the composition of chromium oxide green, thereby preventing hexavalent chromium from entering the chromium oxide green product. Moreover, this method does not require subsequent treatment and will not affect the quality and performance of the chromium oxide green product, requiring little investment and low cost.

[0017] 3. The method for reducing the hexavalent chromium content in chromium oxide green provided by the present invention, in order to ensure complete reaction, the reducing agent is added in excess relative to the hexavalent chromium ions in the chromium oxide green, ensuring that the hexavalent chromium is removed as much as possible, so that the water-soluble content of the chromium oxide green product is reduced, the problem of yellowing of the product color is solved, the performance of the chromium oxide green product is improved, and the pollution caused by hexavalent chromium to the environment is reduced, thereby protecting public health and environmental safety.

[0018] 4. When the hexavalent chromium removal device of the present invention is used to remove hexavalent chromium from chromium oxide green, it is only necessary to add chromium oxide green and reducing agent to the chromium oxide green storage bin and reducing agent storage bin respectively, and then the PLC programmable controller can control the rotation of the second transmission part and the third transmission part, and then control the first metering screw and the second metering screw to add materials in proportion. After the chromium oxide green and the reducing agent all enter the rotating drum, the PLC programmable controller controls the rotation of the first transmission part to drive the rotating drum to rotate for reduction reaction. After the set reaction time is reached, the time relay transmits the signal to the PLC programmable controller, and the PLC programmable controller receives the signal to control the operation of the flip cylinder and then open the cover plate. The material flows out from the feeding port of the continuously rotating rotating drum, and is finally scraped out by the discharging scraper. The device as a whole can automatically complete the feeding and discharging of materials, with a high degree of intelligence, simple process and stable operation, low operating cost and strong practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a schematic diagram of the structure of the hexavalent chromium removal device of the present invention; Figure 2 It is a schematic diagram of the structure of the discharging scraper of the present invention; In the figure: 1-frame; 2-rotating drum; 3-turning cylinder; 4-cover plate; 5-first metering screw; 6-reducing agent storage silo; 7-chromium oxide green storage silo; 8-second metering screw; 9-first transmission part; 10-bearing seat; 11-discharging scraper; 12-feeding port; 13-first bracket; 14-second bracket; 15-transmission shaft; 16-second transmission part; 17-third transmission part; 18-connecting rod; 19-fourth transmission part. DETAILED DESCRIPTION

[0020] The present invention will be further described below in conjunction with specific embodiments: The present invention provides a method for reducing the hexavalent chromium content in chromium oxide green, comprising the following steps: S1: calcining chromium oxide green at high temperature and cooling it to obtain high temperature chromium oxide green; S2: adding the high-temperature chromium oxide green and the reducing agent in step S1 into the hexavalent chromium removal device in proportion, and obtaining a chromium oxide green product after reaction.

[0021] The hexavalent chromium removal device comprises a hollow frame 1, on which a start switch is provided, and a rotating drum 2 driven by a transmission shaft 15 is connected to the top of the frame 1 through a first bracket 13, a feeding port 12 is provided on the top of the rotating drum 2, a cover plate 4 is provided above the feeding port 12, and the cover plate 4 is driven to open and close by a flip cylinder 3 provided on the top of the rotating drum 2, wherein the flip cylinder 3 is controlled by a time relay, and the fixed end of the flip cylinder 3 is connected to the rotating drum 2, and the movable end of the flip cylinder 3 is connected to the cover plate 4 through a pin shaft. A first bracket 13 is provided on one side of the top of the frame 1, and a second bracket 14 is provided on the other side, and the first bracket 13 and the second bracket 14 are arranged in a colinear manner; a first transmission part 9 is connected to the top of the frame 1 through the second bracket 14, and the output end of the first transmission part 9 is connected to the transmission shaft 15 through a coupling, and both ends of the transmission shaft 15 are supported by bearing seats 10. A reducing agent storage silo 6 and a chromium oxide green storage silo 7, both of which are conical, are provided above the rotating drum 2. A first metering screw 5 driven by a second transmission part 16 is provided at the bottom of the reducing agent storage silo 6. A second metering screw 8 driven by a third transmission part 17 is provided at the bottom of the chromium oxide green storage silo 7. The discharge ports of the first metering screw 5 and the second metering screw 8 are both aligned with the feeding port 12 at the top of the rotating drum 2. The bottoms of the first metering screw 5 and the second metering screw 8 are respectively connected to the top of the frame 1 through hollow connecting rods 18. A discharging scraper 11 driven by a fourth transmission part 19 is provided below the rotating drum 2.

[0022] The start switch and the time relay are electrically connected to the signal input end of the PLC programmable controller respectively, and the signal output end of the PLC programmable controller is electrically connected to the flip cylinder, the first transmission part, the second transmission part, the third transmission part and the fourth transmission part respectively.

[0023] When removing hexavalent chromium from chromium oxide green, it is only necessary to add chromium oxide green and reducing agent into chromium oxide green storage silo 7 and reducing agent storage silo 6 respectively, and then the PLC programmable controller can control the second transmission part 16 and the third transmission part 17 to rotate, and then control the first metering screw 5 and the second metering screw 8 to add materials in proportion. After the chromium oxide green and the reducing agent all enter the rotating drum 2, the PLC programmable controller controls the first transmission part 9 to rotate and drive the rotating drum 2 to rotate for reduction reaction. After the set reaction time is reached, the time relay transmits the signal to the PLC programmable controller, and the PLC programmable controller receives the signal to control the operation of the flip cylinder 3 and then open the cover plate 4. The material flows out from the feeding port of the continuously rotating rotating drum 2, and is finally scraped out by the discharging scraper 11.

[0024] Specific examples and chromium oxide green product test results are shown in Table 1: Table 1 Test results of chromium oxide green products Among them, the amount of reducing agent added in Example 1 is 1.1 times the theoretical amount of hexavalent chromium ions in the reduced chromium oxide green, the amount of reducing agent added in Examples 2 to 3 is 1.08 times the theoretical amount of hexavalent chromium ions in the reduced chromium oxide green, and the amount of reducing agent added in Examples 4 to 6 is 1.05 times the theoretical amount of hexavalent chromium ions in the reduced chromium oxide green.

[0025] As can be seen from Table 1, the method of the present invention is used to remove hexavalent chromium in chromium oxide green. The hexavalent chromium in chromium oxide green is reduced to trivalent chromium by reacting chromium oxide green with a reducing agent, thereby removing the hexavalent chromium in chromium oxide green and obtaining a chromium oxide green product with a very low hexavalent chromium content (≤5ppm). The removal effect is good, the pollution caused by hexavalent chromium to the environment is reduced, and public health and environmental safety are protected.

Claims

1. A method for reducing the hexavalent chromium content in chromium oxide green, characterized in that: The following steps are involved: S1: calcining chromium oxide green at a high temperature of 1200-1300° C. and then cooling to 300-450° C. to obtain high-temperature chromium oxide green; S2: adding the high-temperature chromium oxide green and the reducing agent in step S1 into a hexavalent chromium removal device at a mass ratio of 390-400:1-1.05, and obtaining a chromium oxide green product after reaction.

2. A method for reducing the hexavalent chromium content in chromium oxide green according to claim 1, characterized in that: In step S2, the reducing agent is added in excess relative to the hexavalent chromium ions in the chromium oxide green.

3. A method for reducing the hexavalent chromium content in chromium oxide green according to claim 2, characterized in that: The amount of reducing agent added in step S2 is 1.05 to 1.1 times the theoretical amount of hexavalent chromium ions in the reduced chromium oxide green.

4. A method for reducing the hexavalent chromium content in chromium oxide green according to claim 3, characterized in that: In step S2, the reducing agent is one or more of sodium sulfite, sodium pyrosulfite, urea, sodium sulfide or hydroxylamine hydrochloride.

5. A method for reducing the hexavalent chromium content in chromium oxide green according to claim 4, characterized in that: In step S2, the reaction time of chromium oxide green and reducing agent in the hexavalent chromium removal device is 15 to 20 minutes.

6. The method for reducing the hexavalent chromium content in chromium oxide green according to any one of claims 1 to 5, characterized in that: The hexavalent chromium removal device comprises a frame (1), a start switch is provided on the frame (1), a rotating drum (2) driven by a transmission shaft (15) is provided at one end of the top of the frame (1); a first transmission part (9) is provided at the other end of the top of the frame (1), and an output end of the first transmission part (9) is connected to the transmission shaft (15); a feeding port (12) is provided at the top of the rotating drum (2), A cover plate (4) is provided above the feeding port (12), and the cover plate (4) is driven to open and close by a flip cylinder (3) provided at the top of the rotating drum (2), and the flip cylinder (3) is controlled by a time relay; a reducing agent storage bin (6) and a chromium oxide green storage bin (7) are provided above the rotating drum (2); a first metering screw (5) driven by a second transmission part (16) is provided at the bottom of the reducing agent storage bin (6), and a second metering screw (8) driven by a third transmission part (17) is provided at the bottom of the chromium oxide green storage bin (7), and the discharge ports of the first metering screw (5) and the second metering screw (8) are both aligned with the feeding port (12) at the top of the rotating drum (2), and the bottoms of the first metering screw (5) and the second metering screw (8) are respectively connected to the top of the frame (1) via a connecting rod (18); a discharging scraper (11) driven by a fourth transmission part (19) is provided below the rotating drum (2); The start switch and the time relay are respectively electrically connected to the signal input end of the PLC programmable controller, and the signal output end of the PLC programmable controller is respectively electrically connected to the turning cylinder (3), the first transmission part (9), the second transmission part (16), the third transmission part (17) and the fourth transmission part (19).

7. A method for reducing the hexavalent chromium content in chromium oxide green according to claim 6, characterized in that: Both ends of the transmission shaft (15) of the hexavalent chromium removal device are supported by bearing seats (10).

8. A method for reducing the hexavalent chromium content in chromium oxide green according to claim 7, characterized in that: The rotating drum (2) is supported by a first bracket (13), and the first transmission part (9) is supported by a second bracket (14).

9. A method for reducing the hexavalent chromium content in chromium oxide green according to claim 8, characterized in that: In the hexavalent chromium removal device, the fixed end of the turning cylinder (3) and the rotating drum (2), and the movable end of the turning cylinder (3) and the cover plate (4) are respectively connected via pins.

10. A method for reducing the hexavalent chromium content in chromium oxide green according to claim 9, characterized in that: The output end of the first transmission part (9) is connected to the transmission shaft (15) via a coupling.

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