A method for reducing water-soluble hexavalent chromium in ordinary portland cement by passing inert gas in the cooling section

Abstract

The application discloses a method for reducing water-soluble hexavalent chromium in ordinary Portland cement by introducing inert gas in the cooling section, relates to the technical field of cement clinker preparation, and comprises the following steps: first, preheating, decomposing and high-temperature calcining cement raw materials according to a conventional process to prepare high-temperature cement clinker; and then, in the cooling stage of the cement clinker out of the kiln, nitrogen, carbon dioxide or mixed inert gas is introduced into the cooling area to control the cooling atmosphere and reduce the oxygen partial pressure of the system, so that the oxygen volume fraction in the cooling section is controlled to be below 5%, and the oxidation conversion of trivalent chromium to water-soluble hexavalent chromium in the high-temperature cooling process is inhibited from the source. The application does not need to introduce any chromium-reducing additives or chemical reducing agents, avoids the adverse effects of additional materials on the cement setting time, strength development and long-term performance, and eliminates the risk of re-oxidation and rise of hexavalent chromium in the storage and use process.

Description

Technical Field

[0001] This invention relates to the field of cement clinker preparation technology, specifically a method for reducing water-soluble hexavalent chromium in ordinary silicate cement by introducing an inert gas in the cooling section. Background Technology

[0002] With the rapid development of the cement industry, co-processing solid waste in cement kilns has become an important technological approach to achieving the resource utilization of bulk industrial solid waste and the green and low-carbon development of the cement industry. However, compared with traditional natural raw materials such as limestone and clay, most industrial solid wastes have higher heavy metal content, especially chromium. As the proportion of co-processing continues to increase, the total amount of chromium entering the cement raw material system gradually increases, leading to an increase in the hexavalent chromium content in cement clinker and final cement products, posing challenges to the environmental safety and compliance of cement products.

[0003] Chromium exists primarily in cement systems in two valence states: trivalent and hexavalent. Trivalent chromium is relatively chemically stable, has low solubility, and weak migration, posing less harm to the environment and human health. Hexavalent chromium, however, possesses strong oxidizing properties and water solubility, exhibiting high biotoxicity. It readily migrates through water bodies or through skin contact, causing adverse effects on human health, soil, and the ecological environment. Therefore, the content of water-soluble hexavalent chromium in cement has become one of the important indicators for assessing the environmental safety of cement products.

[0004] Currently, the main technical means to reduce the water-soluble hexavalent chromium content in cement are concentrated in the later stages of cement production. This typically involves adding reducing agents during the final grinding stage to reduce the already formed hexavalent chromium to trivalent chromium. However, this method is an end-of-pipe treatment approach, generally suffering from high addition costs, strong process dependence, diminishing reduction effects over time, and the susceptibility of hexavalent chromium to re-oxidation, making long-term stable control of hexavalent chromium difficult. Furthermore, the addition of reducing agents may have some adverse effects on cement setting time, strength development, and product stability.

[0005] Furthermore, the formation of hexavalent chromium in cement is closely related to the oxidizing atmosphere during clinker calcination and cooling, especially during the high-temperature cooling stage after clinker exits the kiln. If the environment is oxygen-rich, trivalent chromium is easily re-oxidized to water-soluble hexavalent chromium. Therefore, simply reducing hexavalent chromium in the later stages of cement production is insufficient to fundamentally solve the problem of its formation and reversion.

[0006] Based on this, a method is now provided to reduce water-soluble hexavalent chromium in ordinary silicate cement by introducing inert gas in the cooling section, which can eliminate the drawbacks of existing methods. Summary of the Invention

[0007] The purpose of this invention is to provide a method for reducing water-soluble hexavalent chromium in ordinary silicate cement by introducing an inert gas in the cooling section, so as to solve the problems in the background art.

[0008] To achieve the above objectives, the present invention provides the following technical solution: A method for reducing water-soluble hexavalent chromium in ordinary Portland cement by introducing an inert gas in the cooling section includes the following steps: S1: Cement raw meal is preheated, decomposed and calcined at high temperature in sequence to obtain high-temperature cement clinker; S2: During the cooling stage after the cement clinker exits the kiln, inert gas is introduced into the clinker cooling area so that the inert gas comes into contact with the cement clinker to replace the oxygen-containing atmosphere in the cooling section environment, reducing the oxygen volume fraction in the local atmosphere of the cooling section to below 5%, forming a low-oxygen atmosphere environment, so that the clinker completes the cooling process in the low-oxygen atmosphere environment. S3: After the clinker is cooled, the cooled clinker is ground to prepare ordinary silicate cement.

[0009] Based on the above technical solutions, the present invention also provides the following optional technical solutions: In one alternative: the inert gas in step S2 is selected from nitrogen, carbon dioxide, or a mixture of gases in any proportion thereof.

[0010] In one alternative: the initial temperature of the inert gas introduced into the cement clinker cooling section in step S2 is 1200-700℃.

[0011] In one alternative: the inert gas is introduced into the cement clinker cooling section at a final temperature of 300–100°C in step S2.

[0012] In one alternative: the clinker cooling equipment is a grate cooler, a rotary cooler, or other device suitable for cooling cement clinker.

[0013] In one alternative: In step S2, ordinary air is first used to cool the high-temperature cement clinker after it exits the kiln. After the clinker temperature drops to 1200-700℃, inert gas is then introduced into the clinker cooling area to complete the subsequent cooling process.

[0014] In one alternative: In step S2, the oxygen volume fraction in the local atmosphere of the clinker cooling section is maintained below 5% by adjusting the inert gas flow rate.

[0015] In one alternative: the inert gas in step S2 is introduced through the air inlet, side air outlet or a dedicated gas inlet located in the cooling area of ​​the clinker cooling equipment.

[0016] In one alternative approach: the high-temperature calcination process in step S1 includes the following calcination regime: (1) First, raise the temperature from room temperature to 950℃, and the heating process lasts for 65 minutes; (2) Keep warm at 950℃ for 30 minutes; (3) Raise the temperature to 1450℃ and continue the heating process for 50 minutes; (4) Finally, keep warm at 1450℃ for 30 minutes.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention regulates the atmosphere conditions during the clinker cooling process by introducing inert gas during the clinker cooling stage, thereby reducing the oxygen partial pressure in the cooling environment and effectively inhibiting the oxidation and transformation of trivalent chromium in the clinker to hexavalent chromium during the high-temperature cooling process, thus reducing the formation of water-soluble hexavalent chromium in cement from the source.

[0018] Compared with existing technologies that reduce chromium content by adding chemical reducing agents in the later stages of cement production, this invention does not require the introduction of any chromium-reducing additives or chemical reducing agents. This avoids the adverse effects of added materials on cement setting time, strength development, and long-term performance, while also eliminating the risk of hexavalent chromium re-oxidizing and rebounding during storage and use.

[0019] The method of this invention only controls the cooling atmosphere in the cement clinker cooling section, without changing the existing cement clinker calcination system and the main cement production process. It can be directly implemented with existing clinker cooling equipment, has strong process adaptability, low implementation cost, safe and controllable operation process, and has no significant adverse effects on the mineral phase composition of clinker and the mechanical properties of cement. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the structure of the present invention. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0022] In one embodiment, such as Figure 1 As shown, a method for reducing water-soluble hexavalent chromium in ordinary Portland cement by introducing inert gas in the cooling section includes the following steps: S1: Cement raw meal is preheated, decomposed and calcined at high temperature in sequence to obtain high-temperature cement clinker; S2: During the cooling stage after the cement clinker exits the kiln, inert gas is introduced into the clinker cooling area so that the inert gas comes into contact with the cement clinker to replace the oxygen-containing atmosphere in the cooling section environment, reducing the oxygen volume fraction in the local atmosphere of the cooling section to below 5%, forming a low-oxygen atmosphere environment, so that the clinker completes the cooling process in the low-oxygen atmosphere environment. S3: After the clinker is cooled, the cooled clinker is ground to prepare ordinary silicate cement.

[0023] The chemical composition and heavy metal content of the cement raw materials used in the following embodiments and comparative examples of the present invention are shown in Table 1 and Table 2, respectively:

[0024] Table 1 Chemical composition of cement raw meal (%)

[0025] Table 2 Heavy metal content of cement raw materials (mg / kg) All examples and comparative examples used the same calcination regime: temperature was raised from room temperature to 950°C for 65 minutes; held at 950°C for 30 minutes; temperature was raised to 1450°C for 50 minutes; and held at 1450°C for 30 minutes. After cooling, the clinker was ground using the same method to prepare ordinary Portland cement, and the water-soluble hexavalent chromium content, 3-day compressive strength, standard consistency water requirement, initial setting time, and final setting time of the cement were tested according to national standard methods.

[0026] Comparative Example 1 The conventional cement clinker cooling process was adopted. No atmosphere control measures were taken during the entire cooling process after the clinker exited the kiln. Ordinary air was directly used as the cooling medium to complete the clinker cooling.

[0027] Test results: The water-soluble hexavalent chromium content in the cement was 14.28 mg / kg; the 3-day compressive strength was 24.15 MPa; the standard consistency water requirement was 25.8%; the initial setting time was 115 min; and the final setting time was 200 min. The results indicate that under conventional air cooling conditions, the clinker is in an oxygen-rich environment during the high-temperature cooling stage, leading to a vigorous oxidation conversion of trivalent chromium to hexavalent chromium, resulting in a high level of water-soluble hexavalent chromium content in the cement.

[0028] Example 1 Using the inert gas atmosphere control method of the present invention, after the clinker exits the kiln and enters the cooling equipment, carbon dioxide gas is continuously introduced into the clinker bed through a dedicated gas inlet in the cooling area until the clinker temperature drops to 100°C and the gas supply is stopped, so that the entire cooling process of the clinker is in a low-oxygen environment with an oxygen volume fraction of less than 5%.

[0029] Test results: The water-soluble hexavalent chromium content in the cement was 11.42 mg / kg, a decrease of 19.9% ​​compared to Comparative Example 1; the 3-day compressive strength was 23.98 MPa, the standard consistency water requirement was 25.7%, the initial setting time was 113 min, and the final setting time was 198 min. The results indicate that introducing carbon dioxide throughout the process can significantly inhibit the formation of hexavalent chromium without significantly adversely affecting the mechanical and setting properties of the cement.

[0030] Example 2 The difference between this embodiment and Embodiment 1 is that: after the clinker exits the kiln, it is first cooled by ordinary air. When the clinker temperature drops to 700°C, carbon dioxide gas is introduced into the cooling area until the clinker temperature drops to 100°C and then the introduction is stopped.

[0031] Test results: The water-soluble hexavalent chromium content in the cement was 13.55 mg / kg, a decrease of 5.1% compared to Comparative Example 1, but higher than that in Example 1; the 3-day compressive strength was 24.02 MPa, the standard consistency water requirement was 25.8%, the initial setting time was 114 min, and the final setting time was 199 min. The results indicate that segmented carbon dioxide introduction has little impact on cement performance, but its chromium-reducing effect is weaker than that of continuous introduction, suggesting that the timing of inert gas introduction has a significant impact on the inhibition of hexavalent chromium.

[0032] Example 3 The difference between this embodiment and Embodiment 1 is that carbon dioxide gas is introduced immediately after the clinker exits the kiln until the clinker temperature drops to 700°C, at which point the introduction is stopped, and the subsequent cooling process uses ordinary air cooling.

[0033] Test results: The water-soluble hexavalent chromium content in the cement was 12.05 mg / kg, a decrease of 15.6% compared to Comparative Example 1, and a further decrease compared to Example 2; the 3-day compressive strength was 24.10 MPa, the standard consistency water requirement was 25.7%, the initial setting time was 115 min, and the final setting time was 200 min. The results indicate that establishing a low-oxygen environment at the initial stage of cooling can effectively inhibit the main formation stage of hexavalent chromium, with a better chromium reduction effect than later introduction, and without adversely affecting the cement performance.

[0034] Example 4 In this embodiment, nitrogen is used as an inert gas. Nitrogen is continuously introduced through the side air vent of the cooling equipment immediately after the clinker exits the kiln until the clinker temperature drops to 100°C and then the introduction is stopped. This ensures that the entire cooling process of the clinker is carried out in a low-oxygen environment with an oxygen volume fraction of less than 5%.

[0035] Test results: The water-soluble hexavalent chromium content in the cement was 11.84 mg / kg, a decrease of 17.1% compared to Comparative Example 1; the 3-day compressive strength was 24.07 MPa, the standard consistency water requirement was 25.8%, the initial setting time was 114 min, and the final setting time was 199 min. The results indicate that using nitrogen as an inert gas can effectively inhibit the formation of hexavalent chromium without significantly affecting the cement's performance. Example 5 The difference between this embodiment and embodiment 4 is that: after the clinker exits the kiln, it is first cooled with ordinary air. When the clinker temperature drops to 700°C, nitrogen gas is then introduced until the clinker temperature drops to 100°C and then the introduction of nitrogen gas is stopped.

[0036] Test results: The water-soluble hexavalent chromium content in the cement was 13.85 mg / kg, which was 3.0% lower than that in Comparative Example 1 but higher than that in Example 4; the 3-day compressive strength was 24.12 MPa, the standard consistency water requirement was 25.8%, the initial setting time was 115 min, and the final setting time was 200 min. The results further verified the principle that the later the inert gas is introduced, the worse the chromium reduction effect, and that the staged introduction of nitrogen gas had no significant impact on the cement performance.

[0037] Example 6 The difference between this embodiment and embodiment 4 is that nitrogen gas is introduced immediately after the clinker exits the kiln and is stopped when the clinker temperature drops to 700°C. The subsequent cooling process uses ordinary air cooling.

[0038] Test results: The water-soluble hexavalent chromium content in the cement was 12.55 mg / kg, a decrease of 12.1% compared to Comparative Example 1, and lower than that in Example 5; the 3-day compressive strength was 24.05 MPa, the standard consistency water requirement was 25.7%, the initial setting time was 113 min, and the final setting time was 198 min. These results again demonstrate that the earlier and longer the inert gas is introduced, the more significant the inhibitory effect on hexavalent chromium formation, without causing significant fluctuations in the various performance indicators of the cement.

[0039] Data shows that the earlier the inert gas is introduced and the longer the duration, the more significant the inhibitory effect on the formation of hexavalent chromium; carbon dioxide and nitrogen have similar chromium-reducing effects and neither has a significant adverse effect on the physical and mechanical properties of cement.

[0040] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for reducing water-soluble hexavalent chromium in ordinary Portland cement by passing an inert gas in a cooling section, characterized by, Includes the following steps: S1: Cement raw meal is preheated, decomposed and calcined at high temperature in sequence to obtain high-temperature cement clinker; S2: During the cooling stage after the cement clinker exits the kiln, inert gas is introduced into the clinker cooling area so that the inert gas comes into contact with the cement clinker to replace the oxygen-containing atmosphere in the cooling section environment, reducing the oxygen volume fraction in the local atmosphere of the cooling section to below 5%, forming a low-oxygen atmosphere environment, so that the clinker completes the cooling process in the low-oxygen atmosphere environment. S3: After the clinker is cooled, the cooled clinker is ground to prepare ordinary silicate cement.

2. A method for reducing water-soluble hexavalent chromium in ordinary Portland cement by passing inert gas in the cooling section according to claim 1, characterized in that, The inert gas in step S2 is selected from nitrogen, carbon dioxide, or a mixture of gases in any proportion thereof.

3. The method according to claim 1, wherein the inert gas is introduced in the cooling section to reduce the water-soluble hexavalent chromium in the ordinary Portland cement. In step S2, the initial temperature of the inert gas introduced into the cement clinker cooling section is 1200-700℃.

4. The method according to claim 1, wherein the inert gas is introduced in the cooling section to reduce the water-soluble hexavalent chromium in the ordinary Portland cement. In step S2, the inert gas is introduced into the cement clinker cooling section at a temperature of 300–100°C.

5. The method according to claim 1, wherein the inert gas is introduced in the cooling section to reduce the water-soluble hexavalent chromium in the ordinary Portland cement. The clinker cooling equipment is a grate cooler, a rotary cooler, or other device suitable for cooling cement clinker.

6. The method for reducing water-soluble hexavalent chromium in ordinary silicate cement by introducing inert gas in the cooling section according to claim 1, characterized in that, In step S2, ordinary air is first used to cool the high-temperature cement clinker after it exits the kiln. After the clinker temperature drops to 1200-700℃, inert gas is then introduced into the clinker cooling area to complete the subsequent cooling process.

7. The method for reducing water-soluble hexavalent chromium in ordinary silicate cement by introducing inert gas in the cooling section according to claim 1, characterized in that, In step S2, the oxygen volume fraction in the local atmosphere of the clinker cooling section is maintained below 5% by adjusting the inert gas flow rate.

8. The method for reducing water-soluble hexavalent chromium in ordinary silicate cement by introducing inert gas in the cooling section according to claim 1, characterized in that, The inert gas in step S2 is introduced through the air inlet, side air outlet or dedicated gas inlet in the cooling area of ​​the clinker cooling equipment.

9. The method for reducing water-soluble hexavalent chromium in ordinary Portland cement by introducing inert gas in the cooling section according to claim 1, characterized in that, The high-temperature calcination process in step S1 includes the following calcination regime: (1) First, raise the temperature from room temperature to 950℃, and the heating process lasts for 65 minutes; (2) Keep warm at 950℃ for 30 minutes; (3) Raise the temperature to 1450℃ and continue the heating process for 50 minutes; (4) Finally, keep warm at 1450℃ for 30 minutes.

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