A method for inhibiting the oxidation of trivalent chromium during the thermal drying process of chromium-containing sludge

Abstract

This invention relates to the field of solid waste disposal, specifically to a method for inhibiting the oxidation of trivalent chromium during the thermal drying process of chromium-containing sludge. This invention provides several inhibitors, including ammonium tripolyphosphate, ammonium citrate, and ammonium oxalate, the molar ratio of which to Cr in the chromium-containing sludge satisfies n(NH4)5P3O 10 The ratios n(C6H5O7(NH4)3) / n(Cr) are 1.5-2.5, n(NH4)2C2O4) / n(Cr) are 3-5, and n((NH4)2C2O4) / n(Cr) are 3-5. The material, after being thoroughly mixed with the inhibitor and chromium-containing sludge, is treated in a dryer and heated for 2 hours. The inhibitory effect stabilizes after this process, achieving the best treatment result. This method is characterized by its simple process, time-saving approach, and significant inhibitory effect, making it a promising pretreatment method for chromium-containing sludge.

Description

Technical Field

[0001] This invention relates to the field of solid waste disposal, and specifically to a method for inhibiting the oxidation of trivalent chromium during the thermal drying process of chromium-containing sludge. Background Technology

[0002] Chromium-containing sludge refers to the solid material remaining after chromium-containing wastewater from certain industrial production processes has undergone treatment such as sedimentation and filtration. The leather tanning and electroplating industries are the main sources of chromium-containing sludge. For example, in the leather tanning industry, trivalent chromium is widely used as a tanning agent due to its unique chemical properties. However, in the entire leather tanning process, approximately 60% of the chromium is utilized, while the remaining chromium is discharged with the wastewater. This chromium then undergoes wastewater treatment, reacting with lime and flocculants to form precipitates, resulting in leather tanning sludge with a high concentration of chromium. Although the chromium in this chromium-containing sludge mainly exists in the form of trivalent chromium, there is a risk of oxidation during subsequent treatment or disposal.

[0003] Chromium in chromium-containing sludge mainly exists in the form of Cr(III) hydrate, including CrOOH, Cr(OH)3, and Fe. x Cr 1-x The form of chromium in solid wastes such as (OH)3 differs significantly from that in incineration fly ash, stainless steel smelting fly ash, and chromium slag. In the latter types of chromium-containing solid wastes, the valence state of chromium is mainly Cr(VI). Therefore, in theory, the treatment of chromium-containing sludge can completely achieve the process of transforming non-toxic chromium-containing sludge into non-toxic products, and no highly toxic Cr(VI) will be generated during the treatment process.

[0004] Currently reported key factors for initiating the solid-phase Cr(VI) formation reaction include temperature, O2, and alkaline substances (such as CaO). Inhibition of the chromium oxidation reaction is also achieved by regulating these three initiating factors. One method is thermal treatment of chromium-containing sludge under anoxic or anaerobic conditions. Studies have shown that incineration of chromium-containing sludge at high temperatures under anoxic (N2:O2 ≥ 9:1) or anaerobic conditions does not oxidize Cr(III) to Cr(VI), and there is good potential for energy recovery from the chromium-containing sludge. However, this treatment method not only has high requirements for equipment and technology, but also suffers from the major drawback of low throughput. Secondly, the formation of Cr(VI) is inhibited by adding substances such as SiO2 to the high-temperature heat treatment system to reduce the alkalinity of alkali metal and alkaline earth metal compounds. The inhibition mechanism utilizes the stronger reaction and binding ability of SiO2 with alkali metal and alkaline earth metal compounds in chromium-containing sludge, allowing SiO2 to react with the alkaline substances in the chromium-containing sludge to form stable compounds (such as CaSiO3), thus preventing the activation effect of alkali metals and alkaline earth metals on Cr(III) oxidation. This is an indirect method of inhibiting Cr(III) oxidation. Medium-high temperature (100-300℃) thermal drying technology using high-temperature steam, flue gas, or heat transfer oil as heat transfer media is widely used in practice due to its higher drying efficiency and better process maturity. However, field applications and research reports have shown that the thermal drying process of chromium-containing sludge is accompanied by the pyrolysis of Cr(III) hydrates, producing some Cr(VI)-containing oxides. Because the temperature during the thermal drying process is insufficient to activate the oxidation of Cr(III) by alkali metals and alkaline earth metals, adding acidic substances such as SiO2 to the system under these conditions has little effect on inhibiting the oxidation of Cr(III).

[0005] Patent CN 107606624 A discloses a method for inhibiting Cr(III) oxidation during the incineration of tannery sludge. The main components of the provided inhibitor are (NH4)2SO4 and NH4HSO4, and the treatment temperature is between 500-1200℃. This method is energy-intensive, and fly ash is generated during the high-temperature incineration process. This fly ash may contain Cr(VI) that has not been inhibited by (NH4)2SO4 and NH4HSO4. Summary of the Invention

[0006] To address the problems existing in the current technology, this invention provides a method for inhibiting the oxidation of trivalent chromium during the thermal drying process of chromium-containing sludge. By adding one of ammonium tripolyphosphate, ammonium citrate, and ammonium oxalate as an inhibitor, these substances can preferentially react with Cr(III) hydrate in the range of 100-300℃ to generate stable compounds, thereby preventing the oxidation of Cr(III). This is a direct method for inhibiting the oxidation of Cr(III). The fixed trivalent chromium compound is very stable and will not decompose or be oxidized again.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] The method for inhibiting the oxidation of trivalent chromium during the thermal drying process of chromium-containing sludge is carried out according to the following steps:

[0009] Step 1: Add inhibitors to the chromium-containing sludge. These inhibitors include ammonium tripolyphosphate ((NH4)5P3O4). 10 Ammonium citrate (C6H5O7(NH4)3) and ammonium oxalate ((NH4)2C2O4).

[0010] Furthermore, the amount of inhibitor used is such that the added inhibitor satisfies the molar ratio of the inhibitor to Cr in the chromium-containing sludge as n(NH4)5P3O 10 / n(Cr)=0.5-2.5, nC6H5O7(NH4)3 / n(Cr)=1-5, n(NH4)2C2O4 / n(Cr)=1-5.

[0011] Furthermore, the optimal range for the amount of inhibitor used satisfies: n(NH4)5P3O 10 / n(Cr)=1.5-2.5,nC6H5O7(NH4)3 / n(Cr)=3-5,n(NH4)2C2O4 / n(Cr)=3-5。

[0012] Step 2: Mix the inhibitor and chromium-containing sludge thoroughly for 1 hour.

[0013] Step 3: Place the mixture in a dryer for further processing.

[0014] Furthermore, the mixture is placed in a dryer and treated at 150-300℃ until the inhibitory effect tends to stabilize, with a treatment time of 2 hours.

[0015] The main mechanism of this invention is the transformation of the Cr(III) hydrate's intrinsic form during the thermal drying process of chromium-containing sludge. Taking Cr(OH)3 as a reactant as an example, when there is no O2 involved, after the Cr-O bond breaks during dehydration, it forms a Cr=O bond with the adjacent Cr-O bond, and the adjacent Cr atoms are connected through Cr-O-Cr bonds, forming Cr2O3 with the unchanged valence state. When O2 is involved in the dehydration process, after the Cr-O bond breaks, it may be occupied by the O atoms after O2 dissociation, forming a Cr=O bond. Since other Cr-O bonds are not broken, the adjacent Cr atoms cannot form a Cr-O-Cr bond, so a Cr=O bond may be formed, ultimately forming Cr(VI) oxide CrO3. Inorganic acids and organic acids, such as oxalic acid, citric acid, and polyphosphoric acid, are commonly reacted with Cr(III) hydrate during thermal drying. Taking oxalic acid as an example... Figure 2Because oxalic acid readily dissociates into H+. + Attacks the Cr-O bond and interacts with OH - The reaction combines to form H2O and C2O4. 2- Further, it reacts with Cr ions to form Cr2(C2O4)3, which has better thermal stability. Furthermore, the reactions between oxalic acid, citric acid, and polyphosphoric acid and Cr(OH)3 show a greater tendency to react compared to the dehydration reaction of Cr(OH)3. The (NH4)5P3O proposed in this invention... 10 The three inhibitors, C6H5O7(NH4)3 and (NH4)2C2O4, will gradually hydrolyze into NH3·nH2O and the corresponding acids—H2C2O4, C6H8O7, and H3PO4, after being incorporated into chromium-containing sludge. As mentioned above, the H2O released from these acids... + It will attack the Cr-O bond during the thermal drying process and react with OH. - The reaction combines to form H2O, while C2O4... 2- C6H5O7 3- and PO3 2- It reacts with Cr ions to form stable Cr(III) compounds such as Cr2(C2O4)3, C6H5CrO7, and Cr2(PO4)3, which are not oxidized throughout the entire thermal drying temperature range. Therefore, it can inhibit the oxidation of Cr(III) during the thermal drying process of chromium-containing sludge.

[0016] This invention differs fundamentally from existing methods that inhibit Cr(III) oxidation by adding SiO2 to the system to reduce its alkalinity. First, the reaction temperature differs. Inhibiting Cr(III) oxidation by adding SiO2 to reduce alkalinity typically occurs in high-temperature systems around 800°C; the thermal drying temperature of this invention is typically in the range of 100-300°C. Second, the inhibition mechanisms differ. Adding acidic substances like SiO2 aims to preferentially react with alkaline substances in the system that can activate Cr(III) oxidation, and to stably bind them, preventing Cr(III) from being activated for oxidation—an indirect method of Cr(III) oxidation inhibition. The inhibition mechanism of this invention is the H+ released from the added inhibitor. + The anions can directly act on Cr(III) hydrate and form stable Cr(III) compounds, which is a method to directly inhibit Cr(III) oxidation.

[0017] The beneficial effects of this invention are as follows: The method for inhibiting chromium oxidation during the thermal drying process of chromium-containing sludge proposed in this invention has simple operation steps; the chromium-containing sludge can be directly mixed with the inhibitor for thermal drying treatment; the inhibition effect is obvious, and the addition of the inhibitor satisfies n((NH4)5P3O 10With a ratio of 2.5 for Cr(OH)3 and a treatment temperature of 300℃ for 2 hours, the inhibition rate of Cr(III) oxidation can reach 98.95%. In summary, the inhibitor component described in this invention is simple, has a significant inhibitory effect, and has great potential application prospects. Attached image description:

[0018] Figure 1 (a) Differences in the products of Cr(OH)3 dehydration reaction without O2 and (b) with O2 and possible mechanisms.

[0019] Figure 2 Oxalic acid preferentially combines with Cr(OH)3 to replace the dehydration process and inhibit the formation of Cr(VI).

[0020] Figure 3 The color of chromium-containing sludge after treatment with different inhibitors and the leaching color.

[0021] Figure 4 Oxidation rate of trivalent chromium in chromium-containing sludge at different temperatures.

[0022] Figure 5 The inhibitory effects of different inhibitors on the oxidation of trivalent chromium in chromium-containing sludge. Detailed Implementation

[0023] The present invention will be further described below with reference to embodiments. These embodiments are intended to illustrate the present invention and not to further limit the present invention.

[0024] Example 1

[0025] The laboratory simulation of the thermal drying process of chromium-containing sludge was employed. Analytical grade Cr(NO3)3·9H2O was dissolved in deionized water, and the pH was adjusted to 7.5 ± 0.5 with NaOH. The resulting precipitate was repeatedly centrifuged and washed three times to prepare amorphous Cr(OH)3. The amorphous Cr(OH)3 was then reacted with CaO according to the n(Cr...)... 3+ ):n(Ca 2+ The chromium-containing sludge was prepared by mixing the chromium-containing sludge in a 1:2 ratio. The simulated chromium-containing sludge was then heated at 200℃ for 2 hours to simulate the thermal drying process of chromium-containing sludge. The hexavalent chromium in the dried product was leached using the method provided in the "Identification Standard for Hazardous Waste - Leaching-Toxicity Identification". The results showed that the leaching concentration of hexavalent chromium was 12.2-60.26 mg / L, and the oxidation rate of trivalent chromium was 5.2-25.6%, far exceeding the national standard for hazardous waste identification categories.

[0026] According to the inhibitor provided by the present invention, the added component is selected to be (NH4)5P3O. 10 The inhibitor was mixed with simulated chromium-containing sludge in a ball mill for 1 hour. The amount of inhibitor added was adjusted to ensure that the mixture contained...

[0027] n((NH4)5P3O 10 The ratio of Cr(OH)3 to n(Cr(OH)3) is 2.5. The mixed material is then treated in a muffle furnace at a heating rate of 5℃ / min, a treatment temperature of 300℃, and a treatment time of 2 hours. The hexavalent chromium in the treated material is leached using the method provided in the "Identification Standard for Hazardous Waste: Leaching - Toxicity Identification". The oxidation rate of trivalent chromium after treatment decreases from 25.1% to 0.26%, and the selected inhibitor achieves an inhibition efficiency of 98.95% for trivalent chromium oxidation.

[0028] Example 2

[0029] According to the inhibitor provided by the present invention, the added component is selected to be (NH4)5P3O. 10 The inhibitor was mixed with simulated chromium-containing sludge in a ball mill for 1 hour. The amount of inhibitor added was adjusted to ensure that the mixture contained...

[0030] n(NH4)5P3O 10 The ratio of Cr(OH)3 to Cr(OH)3 is 1.5. The mixed material was then treated in a muffle furnace at a heating rate of 5℃ / min, a treatment temperature of 300℃, and a treatment time of 2 hours. The hexavalent chromium in the treated material was leached using the method provided in the "Identification Standard for Hazardous Waste: Leaching - Toxicity Identification". The oxidation rate of trivalent chromium decreased from 25.1% to 1.51%, and the selected inhibitor achieved an inhibition efficiency of 94.2% on the oxidation of trivalent chromium.

[0031] Example 3

[0032] According to the inhibitor provided by the present invention, the added component is selected to be (NH4)5P3O. 10 The inhibitor was mixed with simulated chromium-containing sludge in a ball mill for 1 hour. The amount of inhibitor added was adjusted to ensure that the mixture contained...

[0033] n(NH4)5P3O 10 The ratio of Cr(OH)3 to Cr(OH)3 is 2.5. The mixed material was then treated in a muffle furnace at a heating rate of 5℃ / min, a treatment temperature of 250℃, and a treatment time of 2 hours. Hexavalent chromium in the treated material was leached using the method provided in the "Identification Standard for Hazardous Waste: Leaching - Toxicity Identification". The oxidation rate of trivalent chromium decreased from 25.6% to 5.12%, and the selected inhibitor achieved an inhibition efficiency of 80% for trivalent chromium oxidation.

[0034] Example 4

[0035] According to the inhibitor provided by the present invention, the added component is selected as (NH4)2C2O4. The inhibitor is mixed with simulated chromium-containing sludge in a ball mill for 1 hour. The amount of inhibitor added is sufficient to ensure that the mixture contains...

[0036] n((NH4)2C2O4) / n(Cr(OH)3)=5. The mixed material was treated in a muffle furnace at a heating rate of 5℃ / min, a treatment temperature of 250℃, and a treatment time of 2 hours. Hexavalent chromium in the treated material was leached using the method provided in the "Identification Standard for Hazardous Waste: Leaching - Toxicity Identification". The oxidation rate of trivalent chromium decreased from 25.6% to 1.00%, and the selected inhibitor achieved an inhibition efficiency of 96.1% on the oxidation of trivalent chromium.

[0037] Example 5

[0038] According to the inhibitor provided by the present invention, the added component is selected as (NH4)2C2O4. The inhibitor is mixed with simulated chromium-containing sludge in a ball mill for 1 hour. The amount of inhibitor added is sufficient to ensure that the mixture contains...

[0039] n((NH4)2C2O4) / n(Cr(OH)3)=3. The mixed material was treated in a muffle furnace at a heating rate of 5℃ / min, a treatment temperature of 250℃, and a treatment time of 2 hours. Hexavalent chromium in the treated material was leached using the method provided in the "Identification Standard for Hazardous Waste: Leaching - Toxicity Identification". The oxidation rate of trivalent chromium decreased from 25.6% to 2.30%, and the selected inhibitor achieved an inhibition efficiency of 91% for trivalent chromium oxidation.

[0040] Example 6

[0041] According to the inhibitor provided by the present invention, the added component is selected as (NH4)2C2O4. The inhibitor is mixed with simulated chromium-containing sludge in a ball mill for 1 hour. The amount of inhibitor added is sufficient to ensure that the mixture contains...

[0042] n((NH4)2C2O4) / n(Cr(OH)3)=5. The mixed material was treated in a muffle furnace at a heating rate of 5℃ / min, a treatment temperature of 300℃, and a treatment time of 2 hours. Hexavalent chromium in the treated material was leached using the method provided in the "Identification Standard for Hazardous Waste: Leaching - Toxicity Identification". The oxidation rate of trivalent chromium decreased from 25.1% to 1.38%, and the selected inhibitor achieved an inhibition efficiency of 94.5% for trivalent chromium oxidation.

[0043] Example 7

[0044] According to the inhibitor provided by this invention, the selected component is C6H5O7(NH4)3. The inhibitor is mixed with simulated chromium-containing sludge in a ball mill for 1 hour. The amount of inhibitor added is such that n(C6H5O7(NH4)3) / n(Cr(OH)3) = 5 in the mixture. The mixed material is then treated in a muffle furnace at a heating rate of 5°C / min, a treatment temperature of 150°C, and a treatment time of 2 hours. The hexavalent chromium in the treated material is leached using the method provided in the "Identification Standard for Hazardous Waste: Leaching-Toxicity Identification". The oxidation rate of trivalent chromium after treatment is reduced from 5.2% to 0.31%, and the selected inhibitor achieves an inhibition efficiency of 94% for the oxidation of trivalent chromium.

[0045] Example 8

[0046] According to the inhibitor provided by this invention, the selected component is C6H5O7(NH4)3. The inhibitor is mixed with simulated chromium-containing sludge in a ball mill for 1 hour. The amount of inhibitor added is such that n(C6H5O7(NH4)3) / n(Cr(OH)3) = 3 in the mixture. The mixed material is then treated in a muffle furnace at a heating rate of 5°C / min, a treatment temperature of 150°C, and a treatment time of 2 hours. The hexavalent chromium in the treated material is leached using the method provided in the "Identification Standard for Hazardous Waste: Leaching-Toxicity Identification". The oxidation rate of trivalent chromium after treatment is reduced from 5.2% to 0.52%, and the selected inhibitor achieves an inhibition efficiency of 90% for the oxidation of trivalent chromium.

[0047] Example 9

[0048] According to the inhibitor provided by this invention, the selected component is C6H5O7(NH4)3. The inhibitor is mixed with simulated chromium-containing sludge in a ball mill for 1 hour. The amount of inhibitor added is such that n(C6H5O7(NH4)3) / n(Cr(OH)3) = 5 in the mixture. The mixed material is then treated in a muffle furnace at a heating rate of 5°C / min, a treatment temperature of 200°C, and a treatment time of 2 hours. The hexavalent chromium in the treated material is leached using the method provided in the "Identification Standard for Hazardous Waste: Leaching - Toxicity Identification". The oxidation rate of trivalent chromium after treatment is reduced from 25.6% to 7.94%, and the selected inhibitor achieves an inhibition efficiency of 69% for the oxidation of trivalent chromium.

[0049] The basic principles of the present invention have been shown and described above. However, those skilled in the art should understand that modifications can be made to the embodiments without departing from the spirit and scope defined by the claims.

Claims

1. A method for inhibiting the oxidation of trivalent chromium during the thermal drying process of chromium-containing sludge, characterized in that, The method steps are as follows: (1) Add an inhibitor to the chromium-containing sludge and mix the inhibitor thoroughly with the chromium-containing sludge; wherein, the inhibitor is ammonium tripolyphosphate ((NH4)5P3O 10 One of the following: ammonium citrate (C6H5O7(NH4)3) and ammonium oxalate ((NH4)2C2O4); The dosage of the inhibitor is: the molar ratio of the inhibitor to Cr in the chromium-containing sludge is: n((NH4)5P3O 10 n(C6H5O7(NH4)3) / n(Cr) = 1.5-2.5, n(NH4)2C2O4) / n(Cr) = 3-5; (2) Place the mixed material in a dryer and heat for 2 hours; the heating temperature in the dryer is 150-300℃.

2. The method for inhibiting the oxidation of trivalent chromium during the thermal drying process of chromium-containing sludge according to claim 1, characterized in that, The mixing time in step (1) is 1 hour.

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