Method for producing chromium oxide green from chromium-containing waste sludge

Abstract

This invention provides a method for producing chromium oxide green from chromium-containing waste sludge. The method includes: pulping and washing the chromium-containing waste sludge, separating the solid and liquid phases to obtain cleaned chromium-containing waste sludge; calcining the cleaned chromium-containing waste sludge to obtain calcined material; and pulping and washing the calcined material again, separating the solid and liquid phases, and drying the solid product to obtain chromium oxide green. Based on the source and composition of the chromium-containing waste sludge, this method separates the sodium sulfate component through pulping and washing, decomposes it through calcination to obtain chromium oxide, and then removes impurities through pulping and washing to obtain chromium oxide green. This provides a new resource utilization pathway for the waste sludge generated during the refining of sodium sulfate from chromium-containing sodium sulfate, with high chromium recovery rate and high product added value. The method is simple to operate, low in cost, highly safe, generates virtually no solid waste, and is environmentally friendly.

Description

Technical Field

[0001] This invention belongs to the field of inorganic chromium salt production technology, and relates to a method for producing chromium oxide green using chromium-containing waste sludge. Background Technology

[0002] Chromium oxide green is an important inorganic pigment with advantages such as corrosion resistance, light resistance, abrasion resistance, and chemical resistance. Therefore, it is widely used in metallurgy, ceramics, coatings, rubber, abrasives, catalysis, and building materials. Compared to chromium sulfate, chromium oxide green has a higher added value and has long been in short supply. Besides chromium-containing minerals, various chromium-containing waste materials or substances can also be used as raw materials for the production of chromium oxide green. Different preparation processes are employed depending on the source and composition of the raw materials.

[0003] Chromium-containing sodium sulfate is a byproduct of chromium salt production. Its main component is sodium sulfate, followed by hexavalent chromium and other impurities. The preparation of sodium sulfate from chromium-containing sodium sulfate typically involves reducing the hexavalent chromium to trivalent chromium under acidic or alkaline conditions, followed by the addition of a neutralizing agent and separation, yielding chromium hydroxide sludge. Traditional methods for treating this chromium-containing sludge include two main types: one is mixing it into dry detoxification residue, and the other is adding liquid alkali and oxidizing it at high temperature to recover the trivalent chromium. Of these two methods, the first results in the loss of high-value chromium and increases the amount of solid waste, while the second method, using high-temperature oxidation, is more hazardous and consumes a large amount of raw materials such as liquid alkali and oxidants, leading to higher costs.

[0004] CN 110980773A discloses a method and apparatus for producing sodium sulfate from chromium-containing sodium sulfate. The method involves preparing a solution of chromium-containing sodium sulfate and reducing hexavalent chromium to trivalent chromium under weakly acidic conditions. A precipitant is then added to generate chromium hydroxide from the trivalent chromium under neutral conditions. The neutralization process involves simultaneously adding a liquid alkali solution or soda ash solution and a sodium sulfate solution to a neutralization tank equipped with a stirring device at a certain flow rate. After plate and frame filtration, the solution is evaporated and crystallized, then centrifuged and dehydrated, and finally dried to obtain the sodium sulfate product. The key point of this method is the preparation of sodium sulfate from chromium-containing sodium sulfate. However, it does not specify how to further process the obtained chromium-containing components, nor does it mention how to remove impurities from the chromium hydroxide precipitate and prepare high-purity chromium oxide green.

[0005] CN 106395904A discloses a method for producing chromium oxide green using chromium-containing sodium bisulfate. This method uses chromium-containing sodium bisulfate, a byproduct of chromic anhydride production via the sulfuric acid process, as raw material. After direct reduction with a reducing agent, the pH of the solution is adjusted to neutral. The solution is then filtered and washed using a filter press to obtain a filter cake containing chromium hydroxide. The chromium hydroxide is then calcined to obtain chromium oxide green. The raw material used in this method is waste residue from chromic anhydride production, not chromium-containing waste sludge from the preparation of sodium sulfate using chromium-containing sodium sulfate. The different raw material composition necessitates different separation and impurity removal processes. Therefore, the method does not disclose how the waste sludge generated during sodium sulfate refining is treated.

[0006] In summary, for the process of producing chromium oxide green from chromium-containing waste sludge, especially the chromium-containing waste sludge generated during the refining of sodium sulfate from chromium-containing sodium sulfate, it is necessary to select appropriate production processes based on the composition of the chromium-containing waste sludge, provide new resource utilization pathways for the waste sludge, and obtain high-purity, high-performance chromium oxide green products. Summary of the Invention

[0007] To address the problems existing in the prior art, the present invention aims to provide a method for producing chromium oxide green from chromium-containing waste sludge. The method, based on the source and composition of the chromium-containing waste sludge, separates the sodium sulfate component through pulping and washing, obtains chromium oxide through calcination and decomposition, and then removes impurities through pulping and washing to obtain the chromium oxide green product. This method achieves resource utilization of the waste sludge generated during the refining of sodium sulfate from chromium-containing sodium sulfate, with high chromium recovery rate, low process cost, and high safety.

[0008] To achieve this objective, the present invention adopts the following technical solution:

[0009] This invention provides a method for producing chromium oxide green from chromium-containing waste sludge, the method comprising the following steps:

[0010] (1) The chromium-containing waste sludge is pulped and washed with water, and the solid and liquid are separated to obtain the cleaned chromium-containing waste sludge.

[0011] (2) The chromium-containing waste sludge obtained in step (1) is calcined to obtain calcined material;

[0012] (3) The calcined material obtained in step (2) is pulped and washed again. After solid-liquid separation, the solid product is dried to obtain chromium oxide green product.

[0013] In this invention, the recovery and resource utilization of chromium from chromium-containing waste sludge, especially the chromium-containing waste sludge generated during the refining of sodium sulfate from chromium-containing mirabilite, is addressed. This waste sludge has a high sulfate content and requires initial pulping and washing. Through stirring and agitation, the sulfate components in the chromium-containing waste sludge are dissolved to remove as much sulfate as possible. Then, calcination is performed to decompose the chromium hydroxide in the waste sludge into chromium oxide components. After further pulping, washing, and solid-liquid separation, the solid product is dried to obtain chromium oxide green. This method, based on the source of the chromium-containing waste sludge, employs appropriate processes to produce chromium oxide green, providing a new resource utilization pathway for chromium-containing waste sludge. The product has high added value, and the method is simple to operate, low in cost, highly safe, and generates virtually no solid waste, making it environmentally friendly.

[0014] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. The technical objectives and beneficial effects of the present invention can be better achieved and realized through the following technical solutions.

[0015] As a preferred technical solution of the present invention, the chromium-containing waste sludge in step (1) is the chromium-containing waste sludge generated during the refining of sodium sulfate with chromium-containing mirabilite.

[0016] Preferably, the process for refining sodium sulfate containing chromium sulfate includes: passing SO2 into a chromium sulfate solution to reduce hexavalent chromium, adjusting the pH, and then performing solid-liquid separation to obtain chromium hydroxide-containing waste sludge.

[0017] In this invention, since chromium in chromium-containing sodium sulfate mainly exists in the hexavalent form, trivalent chromium is obtained after reduction. The pH is adjusted to near neutral to form chromium hydroxide precipitate, which is then filtered through a plate and frame filter to obtain chromium-containing waste sludge.

[0018] As a preferred technical solution of the present invention, the chromium-containing waste sludge mainly comprises chromium hydroxide and sodium sulfate; in addition, the chromium-containing waste sludge also contains soluble impurities such as aluminum, iron, vanadium, silicon, and calcium, in relatively small amounts.

[0019] Preferably, the moisture content of the chromium-containing waste sludge is 40–50 wt%, such as 40 wt%, 42 wt%, 44 wt%, 45 wt%, 46 wt%, 48 wt%, or 50 wt%, but not limited to the listed values; other unlisted values ​​within this range also apply. The wet-basis chromium content, calculated as chromium trioxide, is 25–32 wt%, such as 25 wt%, 26 wt%, 27 wt%, 28 wt%, 30 wt%, 31 wt%, or 32 wt%, but not limited to these values. The values ​​listed apply to all other values ​​not listed within the same range; the content of wet-based sodium sulfate is 20–25 wt%, such as 20 wt%, 21 wt%, 22 wt%, 23 wt%, 24 wt%, or 25 wt%, but is not limited to the listed values, and all other values ​​not listed within the same range apply; the content of other impurities is 5–8 wt%, such as 5 wt%, 5.5 wt%, 6 wt%, 6.5 wt%, 7 wt%, 7.5 wt%, or 8 wt%, etc.

[0020] Preferably, before the chromium-containing waste sludge is pulped and washed, it is first filtered and separated, and the resulting filtrate is returned to the sodium sulfate production process.

[0021] Preferably, the filtration separation includes plate and frame filtration.

[0022] In this invention, since the solid content in chromium-containing waste sludge is limited, it still contains liquid components. In order to reduce the amount of water used during pulping, plate and frame filtration can be performed first to separate the sulfate-containing liquid solution, thereby reducing the treatment steps and costs of chromium-containing waste sludge.

[0023] As a preferred technical solution of the present invention, the pulping and washing in step (1) involves adding chromium-containing waste mud to a pulping tank and then using hot water for pulping and washing.

[0024] Preferably, the temperature of the hot water is 70-80℃, such as 70℃, 72℃, 74℃, 75℃, 76℃, 78℃ or 80℃, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0025] Preferably, the pulping and washing in step (1) includes two stages of pulping and washing, namely, primary pulping and washing and secondary pulping and washing.

[0026] Preferably, after the first-stage pulping and washing, the solid phase is filtered through a plate and frame filter, and then subjected to a second-stage pulping and washing. After single-chamber plate and frame filtration, the cleaned chromium-containing waste sludge is obtained.

[0027] In this invention, the single-chamber plate and frame filter is more advanced than the traditional plate and frame filter, as it can press the filter cake, reduce the moisture content of the filter cake, reduce the heat consumption when the filter cake is calcined in the kiln, and save energy.

[0028] As a preferred technical solution of the present invention, the solid-liquid ratio of chromium-containing waste sludge and hot water in the primary pulping and washing process is 1:0.8 to 1:1, for example, 1:0.8, 1:0.85, 1:0.9, 1:0.95 or 1:1, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0029] Preferably, the wet basis content of sodium sulfate in the chromium-containing waste sludge after primary pulping and washing is 14-18 wt%, such as 14 wt%, 14.5 wt%, 15 wt%, 15.5 wt%, 16 wt%, 17 wt%, or 18 wt%, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0030] Preferably, the solid-liquid ratio of chromium-containing waste sludge and hot water in the secondary pulping and washing process is 1:0.8 to 1:1, such as 1:0.8, 1:0.85, 1:0.9, 1:0.95 or 1:1, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0031] Preferably, after the secondary pulping and washing, the wet basis content of sodium sulfate in the chromium-containing waste sludge is below 9-12 wt%, such as 9 wt%, 9.5 wt%, 10 wt%, 10.5 wt%, 11 wt%, 11.5 wt%, or 12 wt%, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0032] As a preferred technical solution of the present invention, the calcination in step (2) is carried out in a rotary kiln.

[0033] Preferably, the calcination temperature in step (2) is 850 to 1100°C, such as 850°C, 900°C, 950°C, 1000°C, 1050°C or 1100°C, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0034] In this invention, the calcination temperature is selected according to the color requirements of the product. Different calcination temperatures will affect the color performance of the chromium oxide green product. If the calcination temperature is too low, the waste sludge will not react completely, and the content of the finished chromium oxide green product will be too low, failing to reach a content of more than 92%. If the calcination temperature is too high, it will result in energy waste.

[0035] Preferably, the calcination time in step (2) is 4 to 6 hours, such as 4 hours, 4.5 hours, 5 hours, 5.5 hours or 6 hours, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0036] Preferably, the calcination atmosphere in step (2) is a reducing atmosphere.

[0037] As a preferred technical solution of the present invention, the pulping and washing in step (3) involves adding the calcined material that has not been cooled to a pulping tank and then using hot water for pulping and washing, while simultaneously completing water quenching and cooling.

[0038] In this invention, the calcined material is pulped and washed again. At this time, in addition to chromium oxide components, the calcined material also contains soluble sulfates and impurities such as aluminum, vanadium, iron, and silicon. Pulping can increase the content of chromium oxide components.

[0039] Preferably, the temperature of the hot water is 70-80℃, such as 70℃, 72℃, 74℃, 75℃, 76℃, 78℃ or 80℃, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0040] Preferably, the solid-liquid ratio of the calcined material to the hot water is 1:1 to 6:1, such as 1:1, 2:1, 3:1, 4:1, 5:1 or 6:1, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0041] Preferably, after pulping and washing, the mixture is filtered through a single-chamber plate and frame filter to obtain a chromium oxide filter cake.

[0042] As a preferred technical solution of the present invention, the drying method in step (3) is hot air drying.

[0043] Preferably, the drying process uses the high-temperature exhaust gas generated during calcination as a heat source.

[0044] Preferably, the temperature of the high-temperature exhaust gas is 700-900℃, such as 700℃, 720℃, 750℃, 780℃, 800℃, 820℃, 850℃, 880℃ or 900℃, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0045] Preferably, the drying time in step (3) is 0.2 to 0.5 h, such as 0.2 h, 0.25 h, 0.3 h, 0.35 h, 0.4 h, 0.45 h or 0.5 h, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0046] As a preferred technical solution of the present invention, the composition of the chromium oxide green product in step (3) includes chromium oxide green and sodium sulfate; in addition, it also contains a small amount of impurities such as aluminum, iron, silicon, and vanadium.

[0047] Preferably, the chromium oxide green content in the chromium oxide green product is 92-97 wt%, such as 92 wt%, 93 wt%, 94 wt%, 95 wt%, 96 wt%, or 97 wt%, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0048] In this invention, the chromium aluminum oxide product can be used in pigments, refractory materials, and metallurgy.

[0049] As a preferred technical solution of the present invention, the method includes the following steps:

[0050] (1) The chromium-containing waste sludge is pulped and washed. The chromium-containing waste sludge is generated during the refining of sodium sulfate with chromium-containing sodium sulfate. Its main components include chromium hydroxide and sodium sulfate. The moisture content of the chromium-containing waste sludge is 40-50 wt%, the wet basis chromium content is 25-32 wt% based on chromium trioxide, and the wet basis sodium sulfate content is 20-25 wt%. The pulping and washing is carried out by adding the chromium-containing waste sludge to a pulping tank and then using hot water for pulping and washing. The temperature of the hot water is 70-80℃.

[0051] The pulping and washing process includes primary pulping and washing and secondary pulping and washing. After primary pulping and washing, the solid phase is filtered through a plate and frame filter, and then subjected to secondary pulping and washing. After single-chamber plate and frame filtration, the washed chromium-containing waste sludge is obtained. In the primary pulping and washing process, the solid-liquid ratio of chromium-containing waste sludge to hot water is 1:0.8 to 1:1, and the wet basis content of sodium sulfate in the chromium-containing waste sludge after primary pulping and washing is 14 to 18 wt%. In the secondary pulping and washing process, the solid-liquid ratio of chromium-containing waste sludge to hot water is 1:0.8 to 1:1, and the wet basis content of sodium sulfate in the washed chromium-containing waste sludge is 9 to 12 wt%.

[0052] (2) The chromium-containing waste mud obtained after cleaning in step (1) is calcined. The calcination is carried out in a rotary kiln. The calcination temperature is 850-1100℃ and the time is 4-6h. The calcination atmosphere is a reducing atmosphere to obtain calcined material.

[0053] (3) The calcined material obtained in step (2) is subjected to pulping and washing again. The pulping and washing is carried out by adding the calcined material to a pulping tank and then using hot water for pulping and washing, while simultaneously completing water quenching and cooling. The temperature of the hot water is 70-80℃, and the solid-liquid ratio of the calcined material to the hot water is 1:1 to 6:1. After pulping and washing, the material is filtered through a single-chamber plate and frame filter to obtain a chromium oxide filter cake, which is then dried. The drying method is hot air drying, using the high-temperature tail gas of 700-900℃ generated during calcination as the heat source. The drying time is 0.2-0.5h to obtain a chromium oxide green product. The composition of the chromium oxide green product includes chromium oxide green and sodium sulfate, wherein the content of chromium oxide green is 92-97wt%.

[0054] Compared with the prior art, the present invention has the following beneficial effects:

[0055] (1) The method of the present invention separates the sodium sulfate component by pulping and washing according to the source and composition of chromium-containing waste sludge, obtains chromium oxide by calcination and decomposition, and then removes impurities by pulping and washing to obtain chromium oxide green product. This provides a new way to utilize the waste sludge generated in the process of refining sodium sulfate containing chromium mirabilite. The chromium recovery rate is high, the product purity can reach 92-97 wt%, and the added value is high.

[0056] (2) The method described in this invention is simple to operate, low in cost, high in safety, and generates virtually no solid waste, making it green and environmentally friendly. Attached Figure Description

[0057] Figure 1 This is a process flow diagram of the method for producing chromium oxide green using chromium-containing waste sludge provided in Embodiment 1 of the present invention. Detailed Implementation

[0058] To better illustrate the present invention and facilitate understanding of its technical solutions, the present invention is further described in detail below. However, the following embodiments are merely simplified examples of the present invention and do not represent or limit the scope of protection of the present invention. The scope of protection of the present invention is determined by the claims.

[0059] The specific embodiments of this invention provide a method for producing chromium oxide green using chromium-containing waste sludge, the method comprising the following steps:

[0060] (1) The chromium-containing waste sludge is pulped and washed with water, and the solid and liquid are separated to obtain the cleaned chromium-containing waste sludge.

[0061] (2) The chromium-containing waste sludge obtained in step (1) is calcined to obtain calcined material;

[0062] (3) The calcined material obtained in step (2) is pulped and washed again. After solid-liquid separation, the solid product is dried to obtain chromium oxide green product.

[0063] The following are typical but non-limiting embodiments of the present invention:

[0064] Example 1:

[0065] This embodiment provides a method for producing chromium oxide green using chromium-containing waste sludge, and the process flow diagram of the method is shown below. Figure 1 As shown, it includes the following steps:

[0066] (1) The chromium-containing waste sludge is pulped and washed. The chromium-containing waste sludge is generated during the refining of sodium sulfate with chromium-containing sodium sulfate. The composition includes chromium hydroxide and sodium sulfate. The moisture content of the chromium-containing waste sludge is 45wt%, the wet basis chromium content is 28wt% based on chromium trioxide, and the wet basis sodium sulfate content is 22wt%. The pulping and washing is carried out by adding the chromium-containing waste sludge to a pulping tank and then using hot water for pulping and washing. The temperature of the hot water is 75℃.

[0067] The pulping and washing process includes primary pulping and washing and secondary pulping and washing. After primary pulping and washing, the solid phase is filtered through a plate and frame filter, and then subjected to secondary pulping and washing. After single-chamber plate and frame filtration, the washed chromium-containing waste sludge is obtained. In the primary pulping and washing process, the solid-liquid ratio of chromium-containing waste sludge to hot water is 1:0.9, and the sodium sulfate content in the chromium-containing waste sludge after primary pulping and washing is 16wt%. In the secondary pulping and washing process, the solid-liquid ratio of chromium-containing waste sludge to hot water is 1:0.9, and the sodium sulfate content in the washed chromium-containing waste sludge is 10wt%.

[0068] (2) The chromium-containing waste sludge obtained after cleaning in step (1) is calcined. The calcination is carried out in a rotary kiln. The calcination temperature is 950°C and the time is 5 hours. The calcination atmosphere is a hydrogen reducing atmosphere to obtain calcined material.

[0069] (3) The calcined material obtained in step (2) is directly subjected to pulping and washing to cool down. The pulping and washing is carried out by adding the calcined material to the pulping tank and then using hot water for pulping and washing. The temperature of the hot water is 75°C, and the solid-liquid ratio of the calcined material to the hot water is 4:1. After pulping and washing, the material is filtered through a single-chamber plate and frame filter to obtain chromium oxide filter cake, which is then dried. The drying method is hot air drying, using the high-temperature tail gas of 800°C generated during calcination as the heat source. The drying time is 0.25 hours to obtain chromium oxide green product.

[0070] In this embodiment, by using the above method to treat chromium-containing waste sludge, chromium oxide green product can be prepared with a purity of 96.5 wt%.

[0071] Example 2:

[0072] This embodiment provides a method for producing chromium oxide green using chromium-containing waste sludge, the method comprising the following steps:

[0073] (1) The chromium-containing waste sludge is pulped and washed. The chromium-containing waste sludge is generated during the refining of sodium sulfate with chromium-containing sodium sulfate. The composition includes chromium hydroxide and sodium sulfate. The moisture content of the chromium-containing waste sludge is 50 wt%, the wet basis chromium content is 25 wt% based on chromium trioxide, and the wet basis sodium sulfate content is 20 wt%. The pulping and washing is carried out by adding the chromium-containing waste sludge to a pulping tank and then using hot water for pulping and washing. The temperature of the hot water is 70°C.

[0074] The pulping and washing process includes primary pulping and washing and secondary pulping and washing. After primary pulping and washing, the solid phase is filtered through a plate and frame filter, and then subjected to secondary pulping and washing. After single-chamber plate and frame filtration, the washed chromium-containing waste sludge is obtained. In the primary pulping and washing process, the solid-liquid ratio of chromium-containing waste sludge to hot water is 1:0.8, and the sodium sulfate content in the chromium-containing waste sludge after primary pulping and washing is 14 wt%. In the secondary pulping and washing process, the solid-liquid ratio of chromium-containing waste sludge to hot water is 1:1, and the sodium sulfate content in the washed chromium-containing waste sludge is 9 wt%.

[0075] (2) The chromium-containing waste mud obtained after cleaning in step (1) is calcined. The calcination is carried out in a rotary kiln. The calcination temperature is 850°C and the time is 6 hours. The calcination atmosphere is a hydrogen reducing atmosphere to obtain calcined material.

[0076] (3) The calcined material obtained in step (2) is directly subjected to pulping and washing to cool down. The pulping and washing is carried out by adding the calcined material to the pulping tank and then using hot water for pulping and washing. The temperature of the hot water is 70°C, and the solid-liquid ratio of the calcined material to the hot water is 2:1. After pulping and washing, the material is filtered through a single-chamber plate and frame filter to obtain chromium oxide filter cake, which is then dried. The drying method is hot air drying, using the high-temperature tail gas of 900°C generated during calcination as the heat source. The drying time is 0.2 hours to obtain chromium oxide green product.

[0077] In this embodiment, by using the above method to treat chromium-containing waste sludge, chromium oxide green product can be prepared with a purity of 95.2 wt%.

[0078] Example 3:

[0079] This embodiment provides a method for producing chromium oxide green using chromium-containing waste sludge, the method comprising the following steps:

[0080] (1) The chromium-containing waste sludge is pulped and washed. The chromium-containing waste sludge is generated during the refining of sodium sulfate with chromium-containing sodium sulfate. The composition includes chromium hydroxide and sodium sulfate. The moisture content of the chromium-containing waste sludge is 40 wt%, the wet basis chromium content is 32 wt% based on chromium trioxide, and the wet basis sodium sulfate content is 25 wt%. The pulping and washing is carried out by adding the chromium-containing waste sludge to a pulping tank and then using hot water for pulping and washing. The temperature of the hot water is 80°C.

[0081] The pulping and washing process includes primary pulping and washing and secondary pulping and washing. After primary pulping and washing, the solid phase is filtered through a plate and frame filter, and then subjected to secondary pulping and washing. After single-chamber plate and frame filtration, the cleaned chromium-containing waste sludge is obtained. In the primary pulping and washing process, the solid-liquid ratio of chromium-containing waste sludge to hot water is 1:1, and the sodium sulfate content in the chromium-containing waste sludge after primary pulping and washing is 18 wt%. In the secondary pulping and washing process, the solid-liquid ratio of chromium-containing waste sludge to hot water is 1:1, and the sodium sulfate content in the cleaned chromium-containing waste sludge is 12 wt%.

[0082] (2) The chromium-containing waste mud obtained in step (1) is calcined. The calcination is carried out in a rotary kiln. The calcination temperature is 1100℃ and the time is 4h. The calcination atmosphere is a hydrogen reducing atmosphere to obtain calcined material.

[0083] (3) The calcined material obtained in step (2) is directly subjected to pulping and washing to cool down. The pulping and washing is carried out by adding the calcined material to the pulping tank and then using hot water for pulping and washing. The temperature of the hot water is 80°C and the solid-liquid ratio of the calcined material to the hot water is 6:1. After pulping and washing, the material is filtered through a single-chamber plate and frame filter to obtain chromium oxide filter cake, which is then dried. The drying method is hot air drying, using the high-temperature tail gas of 700°C generated during calcination as the heat source. The drying time is 0.5 hours to obtain chromium oxide green product.

[0084] In this embodiment, by using the above method to treat chromium-containing waste sludge, chromium oxide green product can be prepared with a purity of 95.7 wt%.

[0085] Example 4:

[0086] This embodiment provides a method for producing chromium oxide green using chromium-containing waste sludge, the method comprising the following steps:

[0087] (1) The chromium-containing waste sludge is pulped and washed. The chromium-containing waste sludge is generated during the refining of sodium sulfate with chromium-containing sodium sulfate. The composition includes chromium hydroxide and sodium sulfate. The moisture content of the chromium-containing waste sludge is 42wt%, the wet basis chromium content is 30wt% based on chromium trioxide, and the wet basis sodium sulfate content is 24wt%. The pulping and washing is carried out by adding the chromium-containing waste sludge to a pulping tank and then using hot water for pulping and washing. The temperature of the hot water is 72℃.

[0088] The pulping and washing process includes primary pulping and washing and secondary pulping and washing. After primary pulping and washing, the solid phase is filtered through a plate and frame filter, and then subjected to secondary pulping and washing. After single-chamber plate and frame filtration, the washed chromium-containing waste sludge is obtained. In the primary pulping and washing process, the solid-liquid ratio of chromium-containing waste sludge to hot water is 1:0.95, and the sodium sulfate content in the chromium-containing waste sludge after primary pulping and washing is 17wt%. In the secondary pulping and washing process, the solid-liquid ratio of chromium-containing waste sludge to hot water is 1:0.85, and the sodium sulfate content in the washed chromium-containing waste sludge is 11wt%.

[0089] (2) The chromium-containing waste sludge obtained in step (1) is calcined in a rotary kiln at a temperature of 1000°C for 4.5 hours in a carbon monoxide reducing atmosphere to obtain calcined material.

[0090] (3) The calcined material obtained in step (2) is directly subjected to pulping and washing to cool down. The pulping and washing is carried out by adding the calcined material to the pulping tank and then using hot water for pulping and washing. The temperature of the hot water is 72°C, and the solid-liquid ratio of the calcined material to the hot water is 5:1. After pulping and washing, the material is filtered through a single-chamber plate and frame filter to obtain chromium oxide filter cake, which is then dried. The drying method is hot air drying, using the high-temperature tail gas of 750°C generated during calcination as the heat source. The drying time is 0.4 hours to obtain chromium oxide green product.

[0091] In this embodiment, by using the above method to treat chromium-containing waste sludge, chromium oxide green product can be prepared with a purity of 94.5 wt%.

[0092] Example 5:

[0093] This embodiment provides a method for producing chromium oxide green using chromium-containing waste sludge, the method comprising the following steps:

[0094] (1) The chromium-containing waste sludge is pulped and washed. The chromium-containing waste sludge is generated during the refining of sodium sulfate with chromium-containing sodium sulfate. The composition includes chromium hydroxide and sodium sulfate. The moisture content of the chromium-containing waste sludge is 48wt%, the wet basis chromium content is 27wt% based on chromium trioxide, and the wet basis sulfur content is 23wt%. The pulping and washing is carried out by adding the chromium-containing waste sludge to a pulping tank and then using hot water for pulping and washing. The temperature of the hot water is 78℃.

[0095] The pulping and washing process includes primary pulping and washing and secondary pulping and washing. After primary pulping and washing, the solid phase is filtered through a plate and frame filter, and then subjected to secondary pulping and washing. After single-chamber plate and frame filtration, the washed chromium-containing waste sludge is obtained. In the primary pulping and washing process, the solid-liquid ratio of chromium-containing waste sludge to hot water is 1:0.85, and the sodium sulfate content in the chromium-containing waste sludge after primary pulping and washing is 15wt%. In the secondary pulping and washing process, the solid-liquid ratio of chromium-containing waste sludge to hot water is 1:0.9, and the sodium sulfate content in the washed chromium-containing waste sludge is 9.5wt%.

[0096] (2) The chromium-containing waste sludge obtained in step (1) is calcined in a rotary kiln at a temperature of 900°C for 5.5 hours in a carbon monoxide reducing atmosphere to obtain calcined material.

[0097] (3) The calcined material obtained in step (2) is directly subjected to pulping and washing to cool down. The pulping and washing is carried out by adding the calcined material to the pulping tank and then using hot water for pulping and washing. The temperature of the hot water is 78°C, and the solid-liquid ratio of the calcined material to the hot water is 3:1. After pulping and washing, the material is filtered through a single-chamber plate and frame filter to obtain chromium oxide filter cake, which is then dried. The drying method is hot air drying, using the high-temperature tail gas of 850°C generated during calcination as the heat source. The drying time is 0.3 hours to obtain chromium oxide green product.

[0098] In this embodiment, by using the above method to treat chromium-containing waste sludge, chromium oxide green product can be prepared with a purity of 95.1 wt%.

[0099] Example 6:

[0100] This embodiment provides a method for producing chromium oxide green using chromium-containing waste sludge. The method is the same as that in Embodiment 1, except that the pulping and washing in step (1) only includes one stage, but the amount of hot water used is the sum of the amounts of hot water used in the two stages of pulping and washing in Embodiment 1.

[0101] In this embodiment, since the chromium-containing waste sludge is only subjected to one-stage pulping and washing before calcination, although the amount of hot water is sufficient, the cleaning effect on sulfates in the chromium-containing waste sludge is slightly weaker than that of two-stage pulping and washing. After washing, the sodium sulfate content in the chromium-containing waste sludge reaches 13wt%, and the purity of the final chromium oxide green product is about 90wt%.

[0102] As can be seen from the above embodiments, the method of the present invention, based on the source and composition of chromium-containing waste sludge, separates the sodium sulfate component through pulping and washing, obtains chromium oxide through calcination and decomposition, and then removes impurities through pulping and washing to obtain chromium oxide green product. This provides a new resource utilization pathway for waste sludge generated during the refining of sodium sulfate, with a high chromium recovery rate, product purity reaching 92-97 wt%, and high added value. The method is simple to operate, low in cost, highly safe, and generates virtually no solid waste, making it green and environmentally friendly.

[0103] The applicant declares that the present invention is illustrated through the above embodiments, but the present invention is not limited to the above detailed methods, that is, it does not mean that the present invention must rely on the above detailed methods to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions for the methods of the present invention, additions of auxiliary steps, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A method for producing chromium oxide green using chromium-containing waste sludge, characterized in that, The method includes the following steps: (1) The chromium-containing waste sludge is pulped and washed with water, and the solid and liquid are separated to obtain the cleaned chromium-containing waste sludge; The chromium-containing waste sludge mainly consists of chromium hydroxide and sodium sulfate; The chromium-containing waste sludge has a moisture content of 40-50 wt%, a wet-based chromium content of 25-32 wt% (calculated as chromium trioxide), and a wet-based sodium sulfate content of 20-25 wt%. The pulping and washing process includes two stages: primary pulping and washing and secondary pulping and washing. The wet basis content of sodium sulfate in the chromium-containing waste sludge after primary pulping and washing is 14-18 wt%. After the secondary pulping and washing, the wet basis content of sodium sulfate in the chromium-containing waste sludge is 9-12 wt%. (2) The chromium-containing waste sludge obtained in step (1) is calcined to obtain calcined material; (3) The calcined material obtained in step (2) is pulped and washed again. After solid-liquid separation, the solid product is dried to obtain chromium oxide green product.

2. The method according to claim 1, characterized in that, The chromium-containing waste sludge mentioned in step (1) is the chromium-containing waste sludge generated during the refining of sodium sulfate with chromium-containing sodium sulfate.

3. The method according to claim 2, characterized in that, The process for refining sodium sulfate containing chromium sulfate includes: passing SO2 into a chromium sulfate solution to reduce hexavalent chromium, adjusting the pH, and then performing solid-liquid separation to obtain chromium hydroxide-containing waste sludge.

4. The method according to claim 1, characterized in that, Before the chromium-containing waste sludge is pulped and washed, it is first filtered and separated, and the resulting filtrate is returned to the sodium sulfate production process.

5. The method according to claim 4, characterized in that, The filtration separation includes plate and frame filtration.

6. The method according to claim 1, characterized in that, Step (1) involves adding chromium-containing waste mud to a pulping tank and then using hot water for pulping and washing.

7. The method according to claim 6, characterized in that, The temperature of the hot water is 70~80℃.

8. The method according to claim 1, characterized in that, After the first-stage pulping and washing, the solid phase is filtered through a plate and frame filter, and then subjected to a second-stage pulping and washing. After single-chamber plate and frame filtration, the cleaned chromium-containing waste sludge is obtained.

9. The method according to claim 1, characterized in that, The solid-liquid ratio of chromium-containing waste sludge and hot water in the primary pulping and washing process is 1:0.8 to 1:

1.

10. The method according to claim 1, characterized in that, The solid-liquid ratio of chromium-containing waste sludge and hot water in the secondary pulping and washing process is 1:0.8 to 1:

1.

11. The method according to claim 1, characterized in that, The calcination in step (2) is carried out in a rotary kiln.

12. The method according to claim 1, characterized in that, The calcination temperature in step (2) is 850~1100℃.

13. The method according to claim 1, characterized in that, The calcination time in step (2) is 4 to 6 hours.

14. The method according to claim 1, characterized in that, The calcination atmosphere in step (2) is a reducing atmosphere.

15. The method according to claim 1, characterized in that, Step (3) involves adding the calcined material that has not been cooled to a pulping tank and then using hot water for pulping and washing, while simultaneously completing water quenching and cooling.

16. The method according to claim 15, characterized in that, The temperature of the hot water is 70~80℃.

17. The method according to claim 15, characterized in that, The solid-liquid ratio of the calcined material to the hot water is 1:1 to 6:

1.

18. The method according to claim 15, characterized in that, After pulping and washing, the mixture is filtered through a single-chamber plate and frame filter to obtain a chromium oxide filter cake.

19. The method according to claim 1, characterized in that, The drying method described in step (3) is hot air drying.

20. The method according to claim 19, characterized in that, The drying process uses the high-temperature exhaust gas generated during calcination as a heat source.

21. The method according to claim 20, characterized in that, The temperature of the high-temperature exhaust gas is 700~900℃.

22. The method according to claim 1, characterized in that, The drying time in step (3) is 0.2~0.5h.

23. The method according to claim 1, characterized in that, The composition of the chromium oxide green product in step (3) includes chromium oxide green and sodium sulfate.

24. The method according to claim 23, characterized in that, The chromium oxide green product contains 92-97 wt% chromium oxide green.

25. The method according to claim 1, characterized in that, The method includes the following steps: (1) The chromium-containing waste sludge is pulped and washed. The chromium-containing waste sludge is generated during the refining of sodium sulfate with chromium-containing sodium sulfate. Its main components include chromium hydroxide and sodium sulfate. The moisture content of the chromium-containing waste sludge is 40~50wt%, the wet basis chromium content is 25~32wt% based on chromium trioxide, and the wet basis sodium sulfate content is 20~25wt%. The pulping and washing is carried out by adding the chromium-containing waste sludge to the pulping tank and then using hot water for pulping and washing. The temperature of the hot water is 70~80℃. The pulping and washing process includes primary pulping and washing and secondary pulping and washing. After primary pulping and washing, the solid phase is filtered through a plate and frame filter press. The resulting solid phase is then subjected to secondary pulping and washing. After single-chamber plate and frame filtration, the washed chromium-containing waste sludge is obtained. In the primary pulping and washing process, the solid-liquid ratio of chromium-containing waste sludge to hot water is 1:0.8 to 1:1, and the wet basis content of sodium sulfate in the chromium-containing waste sludge after primary pulping and washing is 14 to 18 wt%. In the secondary pulping and washing process, the solid-liquid ratio of chromium-containing waste sludge to hot water is 1:0.8 to 1:1, and the wet basis content of sodium sulfate in the washed chromium-containing waste sludge is 9 to 12 wt%. (2) The chromium-containing waste sludge obtained in step (1) is calcined in a rotary kiln at a temperature of 850~1100℃ for 4~6h in a reducing atmosphere to obtain calcined material. (3) The calcined material obtained in step (2) is pulped and washed again. The pulping and washing is carried out by adding the calcined material to the pulping tank and then using hot water for pulping and washing, and water quenching and cooling are completed simultaneously. The temperature of the hot water is 70~80℃, and the solid-liquid ratio of the calcined material to the hot water is 1:1~6:

1. After pulping and washing, the chromium oxide filter cake is obtained by single-chamber plate and frame filtration and then dried. The drying method is hot air drying, using the high temperature tail gas of 700~900℃ generated during calcination as the heat source, and the drying time is 0.2~0.5h to obtain chromium oxide green product. The composition of the chromium oxide green product includes chromium oxide green and sodium sulfate, wherein the content of chromium oxide green is 92~97wt%.

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