Method for preparing dark chromium oxide green with low hexavalent chromium content through low-temperature calcination

Abstract

The invention discloses a method for preparing dark chromium oxide green with low hexavalent chromium content through low-temperature calcination, which comprises the following steps: (1) preparing composite powder of active carbon loaded boric acid: dissolving boric acid in water with the mass being 10-20 times of that of boric acid, then placing active carbon in a boric acid aqueous solution to obtain suspension of active carbon loaded boric acid, and drying the suspension of active carbon loaded boric acid in a drying oven to obtain the composite powder of active carbon loaded boric acid; drying the suspension to obtain composite powder with boric acid loaded on activated carbon; (2) calcining: putting the composite powder with the boric acid loaded by the activated carbon into a rotary kiln, and calcining at 700-800 DEG C for 6-9 hours to obtain a chromium oxide green product; according to the method, the composite powder with the boric acid loaded by the activated carbon is doped in the chromic anhydride calcination process, the calcination temperature is reduced, meanwhile, the obtained chromic anhydride product is greener and more blue in color and higher in color saturation, the chromic anhydride product is a dark green product with excellent color development on the market, and the method is easy to operate and has good industrial application prospects.

Description

Technical Field

[0001] The invention relates to the technical field of chromium salt production, in particular to a method for preparing dark-colored chromium oxide green with low hexavalent chromium content by low-temperature calcination. Background Art

[0002] Chromium oxide green is a green powder. As an inorganic pigment, it can be used for coloring ceramics, artificial leather, and building materials. The chromium oxide green on the market is usually calcined with chromic anhydride. According to different application fields, chromium oxide green is divided into refractory grade chromium oxide green, pigment grade chromium oxide green, and metallurgical grade chromium oxide green. The refractory grade and metallurgical grade mainly focus on the chemical composition and particle size of chromium oxide green, and do not have high requirements for color development. It is usually obtained by high-temperature calcination (>1100℃). The pigment grade has higher requirements for the color development performance of chromium oxide green. The color varies from ultra-light to dark. The corresponding calcination temperature varies from 800℃ to 1200℃. The higher the temperature, the darker the color.

[0003] Water-soluble hexavalent chromium is highly toxic to the human body. 6+ ) must be controlled at an extremely low level, usually not exceeding 0.03% (300ppm), in order to meet environmental protection and human health standards. In order to completely decompose chromic anhydride to meet safety standards, the temperature usually needs to be higher than 800°C, but even if the decomposition temperature is as high as 1000°C, 0.01% (100ppm) of hexavalent chromium will still remain. Therefore, in the actual production process, high-temperature calcination is usually used to ensure that the hexavalent chromium content in chromium oxide green meets the standard requirements.

[0004] The color of chromium oxide green is usually expressed by Lab value, L represents illumination, which is equivalent to brightness. The larger the L value, the lighter the color. a represents the range from red to green, and b represents the range from blue to yellow. The requirements for Lab value of chromium oxide green are as follows: different L values ​​are divided into ultra-light to dark series; the larger the absolute value of a, the better, and the greener the color. The requirements for Lab value of conventional chromium oxide green in the market are: usually |a|≥12; b value is blue or yellow according to user needs, and the b value range is between 10 and 13.

[0005] The applicant of the present invention conducted calcination tests of chromium oxide green at different temperatures. The Lab values ​​of chromium oxide green at different calcination temperatures are shown in Table 1 below.

[0006] Table 1

[0007] Calcination temperature (℃) L value a value b-value 600 40.62 -9.11 9.07 700 40.21 -10.85 9.86 800 39.85 -12.81 11.21 1000 37.79 -12.35 10.77 1150 35.30 -9.89 8.25

[0008] It can be seen from the above table that the brightness (L value) of chromium oxide green gradually decreases with the increase of calcination temperature. The ab value is relatively large in the range of 800-1000℃, which meets the coloring requirements. The temperature is too high or too low and does not meet the application requirements. The calcination temperature is too low, and the chromium oxide green crystals are not fully developed, and its coloring is poor; the calcination temperature is too high, and the chromium oxide green is over-burned, and its coloring is also poor. Usually, ultra-light chromium oxide green is prepared at low temperature, and dark chromium oxide green is prepared at high temperature. However, the energy consumption of preparing dark chromium oxide green at high temperature is high, and the production cost of enterprises is high. However, the preparation of dark chromium oxide green at low temperature does not conform to the calcination law of chromic anhydride.

[0009] China's invention patent publication number CN 112158884A discloses a method for improving the green color of chromium oxide, wherein dichromate is mixed with a reducing agent and then roasted in an oxidizing atmosphere, the reducing agent is an ammonium salt and / or sulfur, and the amount of reducing agent added is 1 to 2 times the theoretical amount required for complete reaction with dichromate, and the color improver is selected to include any one or a combination of at least two of boric acid, borax, starch, glucose, urea, sulfate or fluoride. Through the above method, a chromium oxide green product with color parameters of L = 42.2-44.2, a = -17.8-16.9, b = 15.3-18.4 is successfully prepared, and the performance reaches the standard of Lanxess Chemical. This technical solution encompasses the current method of improving the green color of chromium oxide, but the amount of reducing agent used is large, and the monitoring of oxygen concentration in the production process is difficult; in addition, after using the color improver, the calcination temperature can be increased, and the Lab value is increased, and the L value is increased to more than 42, which belongs to ultra-light high-brightness chromium oxide green.

[0010] China's invention patent publication number CN 102557136A discloses a method for improving the green color of chromium oxide to solve the problem of darkening the green color of chromium oxide prepared by thermal decomposition of traditional chromic anhydride. Additives are added at 2-5% of the mass of chromic anhydride and mixed evenly with chromic anhydride, and then put into a rotary kiln for thermal decomposition reaction. The additive is one of ammonium chloride, potassium fluoride or sodium fluoride. Finally, it is calcined at 1050-1150°C to prepare color parameters of L=34-36, a=-12-14, and b=11-4. This patent optimizes the additives and prepares a dark chromium oxide green product under high temperature calcination. The product has bright color, good tinting power and is relatively saturated. However, the calcination temperature is high and the product energy consumption is high.

[0011] The Chinese invention patent publication number CN 108529677B discloses a method for producing low-sulfur metallurgical grade chromium oxide green, wherein chromic anhydride is put into a rotary kiln to start roasting, and during the process of putting in chromic anhydride, one or more of starch, sawdust powder, and coal powder are evenly blown into the rotary kiln through a feeding system independently set up from the rotary kiln. Based on the principle that a redox reaction occurs during the instantaneous contact between a strong oxidizing substance and a reducing fuel at high temperature and a large amount of heat is released, it can not only effectively improve the desulfurization efficiency, but also the released heat directly acts on the thermal decomposition process of chromic anhydride to achieve energy-saving effects. This patent is applied to the production of metallurgical grade chromium oxide green, and the purpose of energy saving and consumption reduction is achieved by adding reducing raw materials, but the influence of reducing fuels on the color-developing properties of chromium oxide green is not considered.

[0012] At present, the methods for reducing the hexavalent chromium content are to treat the calcined chromium oxide green. The technical difficulty is relatively low, but a large amount of water is consumed, the wastewater disposal cost is high, the operation is difficult, the environmental pressure is high, and the production cost is high. For example, the Chinese invention patent publication number CN 114436331A discloses a method for preparing an ultra-light, bright green, low hexavalent chromium content chromium oxide green pigment, which is rinsed with cold water or warm water, and the water-soluble hexavalent chromium is reduced to 10-50ppm. The Chinese invention patent publication number CN 112194183A discloses a method for producing chromium oxide green with low soluble chromium and zero hexavalent chromium, which uses a reducing machine uric acid to treat the calcined chromium oxide green, and the soluble chromium in the obtained product is reduced to less than 20ppm. The Chinese invention patent publication number CN108793252B discloses a method for reducing the hexavalent chromium content in chromium oxide green, and a reducing agent is added to the chromium oxide green with a discharge temperature of 60 to 400°C during the production process to obtain a chromium oxide green product.

[0013] In summary, how to prepare dark-colored, low-hexavalent chromium content chromium oxide green by low-temperature calcination to meet the market demand for high-performance, environmentally friendly pigment-grade chromium oxide green products and greatly reduce production energy consumption is an important topic studied in the present invention. Summary of the invention

[0014] The purpose of the present invention is to provide a method for preparing dark chromium oxide green with low hexavalent chromium content by low-temperature calcination in view of the problems that the calcination temperature is high when chromium anhydride is used to prepare chromium oxide green in the prior art, and the content of hexavalent chromium in the chromium oxide green product is high. The method reduces the calcination temperature by adding a composite powder of activated carbon loaded with boric acid during the calcination of chromium anhydride. At the same time, the obtained chromium anhydride product has a greener and bluer color and a higher color saturation, and is a dark green product with excellent color development on the market. The method is simple to operate and has good industrial application prospects.

[0015] The present invention is achieved through the following technical solutions:

[0016] A method for preparing dark-colored chromium oxide green with low hexavalent chromium content by low-temperature calcination of the present invention comprises the following steps:

[0017] (1) preparing a composite powder of activated carbon loaded with boric acid: dissolving boric acid in water, wherein the mass of the water is 10-20 times the mass of the boric acid, placing activated carbon in an aqueous solution of boric acid to obtain a suspension of activated carbon loaded with boric acid, and drying the suspension to obtain a composite powder of activated carbon loaded with boric acid;

[0018] (2) Calcination: Put the composite powder of activated carbon loaded with boric acid into a rotary kiln and calcine it at 700-800°C for 6-9h to obtain a chromium oxide green product. The composite powder of activated carbon loaded with boric acid can be mixed evenly with chromic anhydride by a mixer before being put into the rotary kiln for calcination; or the composite powder of activated carbon loaded with boric acid can be fed evenly and continuously from the feeding port at the head of the rotary kiln. The calcination temperature of the present invention is the highest temperature of the material passing through the tail of the kiln measured by an infrared non-contact thermometer, and the calcination time is the time from the material entering the rotary kiln to the material leaving the rotary kiln. A retaining ring is set at the tail of the rotary kiln, and the height of the retaining ring is 30%-40% of the diameter of the rotary kiln. The calcination time of the material is controlled by controlling the height of the retaining ring and the rotation speed of the rotary kiln.

[0019] The chromic anhydride calcining time of the present invention is the time from when the material enters the rotary kiln to when it leaves the rotary kiln, and the calcining time is 6 to 9 hours. A material retaining ring is arranged at the end of the rotary kiln, and the height of the material retaining ring is 30% to 40% of the kiln diameter. The calcining time of the material is controlled by controlling the height of the material retaining ring and the rotating speed of the rotary kiln.

[0020] Preferably, the mass of the boric acid in the present invention is 0.5% to 1.5% of the chromic anhydride, and the mass of the activated carbon is 4% to 8% of the chromic anhydride.

[0021] The morphology of wood activated carbon includes powder, column, crushed, and spherical. The present invention prefers spherical wood activated carbon. Chromic anhydride has a flake structure. Spherical wood activated carbon and flake chromic anhydride are mixed and easily piled up to form a loose structure, which is conducive to air flow heat transfer, making the temperature of the material more uniform and further improving the thermal efficiency.

[0022] Preferably, the particle size of the activated carbon in the present invention is 300-800 meshes. This is because if the activated carbon particle size is too fine, it is easy to agglomerate, and it is difficult to mix it evenly with chromic anhydride; if the activated carbon particle size is too coarse, it is easy to locally burn and generate a lot of heat, and the local temperature is too high. In addition, the activated carbon has a developed pore structure, a large specific surface area and rich surface chemical groups. The surface area of ​​each gram of activated carbon is 500-1500m 2 The porous structure of activated carbon helps it absorb oxygen produced by the decomposition of chromic anhydride, thus allowing it to burn fully.

[0023] Preferably, the rotary kiln described in the present invention has a diameter of 1.5 to 3.0 m and a length of 30 to 50 m. The gas used in the rotary kiln is natural gas, and the heating methods of the rotary kiln are divided into downstream and countercurrent types. The downstream burner is located at the kiln head of the rotary kiln, and the countercurrent burner is located at the kiln tail. The present invention preferably uses a rotary kiln with a downstream calcination method. After the material is put into the kiln head of the rotary kiln, the material contacts the flame. After the material absorbs heat (chromic anhydride melts), the material temperature rises and gradually flows to the kiln tail. The material temperature further rises. Generally, the temperature of the material is the highest at 1 / 3 of the distance from the kiln tail. The countercurrent calcination method is conducive to the chromic anhydride material mixed with activated carbon to exert a higher thermal efficiency. The loose stacking structure of activated carbon and chromic anhydride has a higher heat transfer efficiency at the flame temperature.

[0024] Preferably, the greenness parameters of the chromium oxide product in the present invention are: L: 36 to 38; a: -12 to -13.5; b: 11 to 12.5.

[0025] Preferably, the content of hexavalent chromium in the chromium oxide product of the present invention is 100-200 ppm.

[0026] The reason why the present invention chooses to put the composite powder of activated carbon loaded with boric acid into the rotary kiln is that the amount of boric acid added is relatively low. If it is directly added to chromic anhydride, it is difficult to disperse evenly. The present invention loads boric acid on activated carbon and uses activated carbon as a carrier. The density of activated carbon is relatively small, usually 0.3g-0.6g / cm 3 , while the density of chromic anhydride is about 2.7g / cm 3 , under the same mass, the volume of activated carbon is larger, and when activated carbon and chromic anhydride are mixed in the rotary kiln, the two can fully contact.

[0027] The chemical reaction occurring in the rotary kiln of the present invention is as follows. The thermal decomposition of chromic anhydride occurs as follows:

[0028] 4CrO 3 =2Cr 2 O 3 +3O 2

[0029] Activated carbon has strong reducing properties. It reacts with oxygen produced by the thermal decomposition of chromic anhydride, further promoting the decomposition of chromic anhydride. At the same time, the combustion of activated carbon generates a lot of heat. The ignition point of activated carbon is about 344.31℃, but when the temperature exceeds 200℃, activated carbon can start to spontaneously combust. The combustion of activated carbon further promotes the decomposition of chromic anhydride. Through the above process, the thermal decomposition temperature of chromic anhydride can be reduced and energy can be saved. The density of activated carbon is smaller than that of reducing agents such as sucrose and starch. It mixes more fully with chromic anhydride. In the decomposition process of chromic anhydride, activated carbon participates more fully, and the heat released by combustion is more uniform.

[0030] Compared with the prior art, the method of the present invention is a process for preparing chromium oxide green by calcining with a rotary kiln using chromic anhydride as a raw material. Only by low-temperature calcination at 700 to 800°C, a dark green color-grade chromium oxide green product is obtained, and its chromaticity parameters are: L: 36 to 38; a: -12 to -13.5; b: 11 to 12.5. In addition, the water-soluble hexavalent chromium content of the chromium oxide green product is 100 to 200 ppm, which is lower than the conventional pigment-grade chromium oxide green products on the market. The process of the present invention has the advantages of energy saving and consumption reduction, convenient operation, easy implementation, and little environmental pollution.

[0031] Instruction Manual

[0032] Figure 1 is a scanning electron microscope image of chromium oxide green prepared in Example 1 of the present invention;

[0033] Figure 2 This is a scanning electron microscope image of chromium oxide green prepared in Example 2 of the present invention;

[0034] Figure 3 This is a scanning electron microscope image of chromium oxide green prepared in Example 3 of the present invention;

[0035] Figure 4 This is a scanning electron microscope image of chromium oxide green prepared in Example 4 of the present invention;

[0036] Figure 5 This is a scanning electron microscope image of chromium oxide green prepared in Comparative Example 1;

[0037] Figure 6 This is a scanning electron microscope image of chromium oxide green prepared in Comparative Example 2;

[0038] Figure 7 This is a scanning electron microscope image of chromium oxide green prepared in Comparative Example 3. DETAILED DESCRIPTION

[0039] In order to better explain the present invention and facilitate understanding of the technical solution of the present invention, it is now further described in conjunction with the drawings and embodiments. It should be understood that the specific embodiments of the present invention are only for illustrative purposes and are not intended to limit the present invention.

[0040] Example 1

[0041] A method for preparing dark-colored, low-hexavalent chromium content chromium oxide green by low-temperature calcination in this embodiment, the diameter of the rotary kiln selected in this embodiment is 2.3m, the length is 36m, the heating mode of the rotary kiln is divided into downstream type, the burner is located at the kiln head of the rotary kiln, and natural gas is used as fuel. The continuous feeding amount of chromic anhydride is 1.25 tons / hour, the input amount of boric acid is 0.0125 tons / hour, the input amount of water is 0.1875 tons / hour, the input amount of activated carbon is 0.075 tons / hour, wherein the particle size of the activated carbon is 500 mesh, the boric acid is dissolved in water, and the activated carbon is placed in a boric acid aqueous solution to obtain a suspension of activated carbon loaded with boric acid, the suspension is dried to obtain an activated carbon loaded with boric acid composite powder, and the addition amount of the activated carbon loaded with boric acid composite powder is 1.25*(1.0+6.0)%=0.0875 tons / hour. The composite powder of activated carbon loaded with boric acid is uniformly fed into the rotary kiln head from the feeding port, and the uniformly mixed materials are calcined at 750°C for 7 hours to obtain the chromium oxide green product. The SEM image of the product is shown in the attached Figure 1 .

[0042] From the attached Figure 1 It can be seen that in the microstructure of chromium oxide green products, the grains are well developed, the morphology is flat granular, the grain edges are smooth, and the grain size is 0.4-1.0μm. The grains are agglomerated to form a lamellar structure, which retains the morphology of some flake chromic anhydride.

[0043] The Lab value of the chromium oxide green obtained in this example and the soluble hexavalent chromium content in the product are shown in Table 2 below.

[0044] Table 2

[0045] serial number L value a value b-value Soluble hexavalent chromium content Example 1 37.1 12.9 12.1 150ppm Example 2 36.2 12.1 11.5 105ppm Example 3 37.6 13.2 12.4 189ppm Example 4 37.0 12.7 11.6 130ppm

[0046] Example 2

[0047] A method for preparing dark-colored, low-hexavalent chromium content chromium oxide green by low-temperature calcination in this embodiment, the diameter of the rotary kiln selected in this embodiment is 1.5m and the length is 30m, the heating mode of the rotary kiln is divided into downstream type, the burner is located at the kiln head of the rotary kiln, and natural gas is used as fuel. The continuous feeding amount of chromic anhydride is 1.25 tons / hour, the input amount of boric acid is 0.00625 tons / hour, the input amount of water is 0.0625 tons / hour, the input amount of activated carbon is 0.1 tons / hour, wherein the particle size of the activated carbon is 500 mesh, the boric acid is dissolved in water, and the activated carbon is placed in a boric acid aqueous solution to obtain a suspension of activated carbon loaded with boric acid, and the suspension is dried to obtain an activated carbon loaded with boric acid composite powder, and the addition amount of the activated carbon loaded with boric acid composite powder is 1.25*(0.5+8.0)%=0.10625 tons / hour. The composite powder of activated carbon loaded with boric acid is uniformly fed into the rotary kiln head from the feeding port, and the uniformly mixed material is calcined at 800°C for 6 hours to obtain the chromium oxide green product. The SEM image of the product is shown in the attached Figure 2 .

[0048] From the attached Figure 2 It can be seen that in the microstructure of the chromium oxide green product, the grains are well developed, the grain size is larger than that of Example 1, and the grain size is 0.5-1.1 μm. The grain surface contour is clear.

[0049] The Lab value of the chromium oxide green obtained in this example and the soluble hexavalent chromium content in the product are shown in Table 2 above.

[0050] Example 3

[0051] A method for preparing dark-colored, low-hexavalent chromium content chromium oxide green by low-temperature calcination in this embodiment, the diameter of the rotary kiln selected in this embodiment is 3.0m, the length is 50m, the heating mode of the rotary kiln is divided into downstream type, the burner is located at the kiln head of the rotary kiln, and natural gas is used as fuel. The continuous feeding amount of chromic anhydride is 1.25 tons / hour, the input amount of boric acid is 0.01875 tons / hour, the input amount of water is 0.28125 tons / hour, the input amount of activated carbon is 0.05 tons / hour, wherein the particle size of the activated carbon is 500 mesh, the boric acid is dissolved in water, and the activated carbon is placed in a boric acid aqueous solution to obtain a suspension of activated carbon loaded with boric acid, the suspension is dried to obtain an activated carbon loaded with boric acid composite powder, and the addition amount of the activated carbon loaded with boric acid composite powder is 1.25*(1.5+4.0)%=0.06875 tons / hour. The composite powder of activated carbon loaded with boric acid is uniformly fed into the rotary kiln head from the feeding port, and the uniformly mixed material is calcined at 700°C for 9 hours to obtain the chromium oxide green product. The SEM image of the product is shown in the attached Figure 3 .

[0052] From the attached Figure 3It can be seen that in the microstructure of the chromium oxide green product, the grains are well developed, the morphology is flat granular, the grain size is 0.3-1.0 μm, and the particles are more densely agglomerated than those in other embodiments.

[0053] The Lab value of the chromium oxide green obtained in this example and the soluble hexavalent chromium content in the product are shown in Table 2 above.

[0054] Example 4

[0055] A method for preparing dark-colored, low-hexavalent chromium content chromium oxide green by low-temperature calcination in this embodiment, the diameter of the rotary kiln selected in this embodiment is 2.0m, the length is 40m, the heating mode of the rotary kiln is divided into downstream type, the burner is located at the kiln head of the rotary kiln, and natural gas is used as fuel. The continuous feeding amount of chromic anhydride is 1.25 tons / hour, the input amount of boric acid is 0.01875 tons / hour, the input amount of water is 0.421875 tons / hour, the input amount of activated carbon is 0.1 tons / hour, wherein the particle size of the activated carbon is 500 mesh, the boric acid is dissolved in water, and the activated carbon is placed in a boric acid aqueous solution to obtain a suspension of activated carbon loaded with boric acid, and the suspension is dried to obtain an activated carbon loaded with boric acid composite powder, then the addition amount of the activated carbon loaded with boric acid composite powder is 1.25*(1.5+8.0)%=0.11875 tons / hour. The composite powder of activated carbon loaded with boric acid is uniformly fed into the rotary kiln head from the feeding port, and the uniformly mixed material is calcined at 700°C for 9 hours to obtain the chromium oxide green product. The SEM image of the product is shown in the attached Figure 4 .

[0056] From the attached Figure 4 It can be seen that in the microstructure of chromium oxide green products, the grains are fully developed and the grains are more densely packed. The layered structure of the grains is more obvious, and more chromic anhydride flake structures are retained.

[0057] The Lab value of the chromium oxide green obtained in this example and the soluble hexavalent chromium content in the product are shown in Table 2 above.

[0058] It can be seen from Table 2 above that the green parameters of the chromium oxide green product obtained by calcining using the methods of Examples 1-4 of the present invention are: L: 36-38; a: -12--13.5; b: 11-12.5, and the content of hexavalent chromium is 100-200 ppm, which meets the market demand for high-performance, environmentally friendly, pigment-grade chromium oxide green products.

[0059] Comparative Examples 1-3

[0060] In order to verify the influence of the composite powder of activated carbon loaded with boric acid added in the process of calcining chromic anhydride on the chromium oxide green product, three comparative examples are carried out in the present invention, wherein comparative example 1 is a blank test, in which chromic anhydride is directly calcined; comparative example 2 is to add 500-mesh spherical wood activated carbon, and the mass ratio of activated carbon to chromic anhydride is 6:94 at the feeding port of the rotary kiln head, and the feeding is uniformly and continuously fed; comparative example 3 is to add boric acid, in which the mass ratio of boric acid to chromic anhydride is 1:99, and the boric acid and chromic anhydride are mixed evenly before feeding.

[0061] The diameter of the rotary kiln is 2.3m and the length is 36m. The burner is located at the head of the rotary kiln and uses natural gas as fuel. The continuous feeding amount of chromic anhydride is 1.25 tons / hour. The maximum temperature of the material passing through the kiln tail is 750℃ measured by an infrared non-contact thermometer, and the time for the material from the kiln head to the kiln tail is 7.5h. The calcined material is air-cooled to 100℃ and crushed to 300℃. Weigh 1.0g of sample with an analytical balance, pour the weighed sample on the grinding disc, add 12 to 15 drops of castor oil, about 0.5g, and use a scraping stick to scrape out a uniform color block on the test paper at a uniform speed. Place the test paper on a high-temperature panel at 70℃ to allow it to be evenly heated. After 30 to 45 minutes, remove it and use a colorimeter to test the Lab value of the sample. According to the methods of GB9760-88 and GB20785-2006, the hexavalent chromium in chromium oxide green is extracted and analyzed. The experimental results are shown in Table 3 below.

[0062] Table 3

[0063] serial number L value a value b-value Soluble hexavalent chromium content Comparative Example 1 40.02 -11.64 10.51 345ppm Comparative Example 2 36.8 -9.89 9.51 163ppm Comparative Example 3 41.2 -12.82 12.03 324ppm

[0064] The chromium oxide green products prepared in the above comparative examples 1-3 were characterized by SEM. The SEM images thereof are shown in the attached Figure 5-7 shown.

[0065] Depend on Figure 5-7 By comparison, Figure 6 Only adding activated carbon can make the chromium trioxide grains more complete and the grain distribution more uniform. Figure 7 Adding only boric acid can refine the grains, but the grains are not fully developed and the surface of the particles is covered by glass phase. From the test results in Table 2 above, it can be seen that after adding activated carbon, the content of soluble hexavalent chromium in the chromium oxide green product is greatly reduced.

[0066] Comparative Examples 4-6

[0067] In order to verify the influence of different activated carbon morphologies on the experimental results, the present invention conducted three groups of comparative examples, wherein comparative example 4 used 500-mesh spherical activated carbon, comparative example 5 used powdered activated carbon, comparative example 6 used columnar activated carbon, and the rest of the experimental conditions were the same as those in Example 1. The hexavalent chromium in the chromium oxide green prepared in comparative examples 4-6 was tested respectively, and the experimental results obtained are shown in Table 4 below.

[0068] Table 4

[0069] serial number L value a value b-value Soluble hexavalent chromium content Comparative Example 4 37.1 12.9 12.1 150ppm Comparative Example 5 38.0 12.3 11.4 188ppm Comparative Example 6 37.4 12.5 11.8 155ppm

[0070] As can be seen from Table 4, compared with spherical activated carbon, the structure formed by powdered and columnar activated carbon and chromic anhydride is denser, which ultimately leads to poor color development performance of chromium oxide green products and a high content of soluble hexavalent chromium.

[0071] Comparative Examples 7-9

[0072] In order to compare the effects of different calcination times on the quality of chromium oxide green products, the present invention conducted three groups of comparative examples, wherein the calcination time of comparative example 7 was 7.5 h, the calcination time of comparative example 8 was 6 h, and the calcination time of comparative example 8 was 9 h, and the remaining experimental conditions were the same as those of Example 1. The hexavalent chromium in the chromium oxide green prepared in comparative examples 7-9 was tested respectively, and the experimental results obtained are shown in Table 5 below.

[0073] Table 5

[0074] serial number L value a value b-value Soluble hexavalent chromium content Comparative Example 7 37.1 12.9 12.1 150ppm Comparative Example 8 37.3 12.7 11.8 180ppm Comparative Example 9 37.0 12.6 11.8 115ppm

[0075] It can be seen from Table 5 above that within the calcination time range of 6 to 9 hours, the longer the calcination time, the lower the soluble hexavalent chromium content. The Lab value changes slightly, and the performance of Comparative Example 7 is the best. Compared with the calcination time of 9 hours, the calcination time of 7.5 hours has lower energy consumption and moderate hexavalent chromium content.

[0076] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing dark-colored, low-hexavalent chromium content chromium oxide green by low-temperature calcination, characterized in that: The following steps are involved: (1) preparing a composite powder of activated carbon loaded with boric acid: dissolving boric acid in water, wherein the mass of the water is 10-20 times the mass of the boric acid, placing activated carbon in the boric acid aqueous solution to obtain a suspension of activated carbon loaded with boric acid, and drying the suspension to obtain a composite powder of activated carbon loaded with boric acid; (2) Calcination: The composite powder of activated carbon loaded with boric acid is placed in a rotary kiln and calcined at 700-800 °C for 6-9 h to obtain the chromium oxide green product.

2. The method for preparing dark-colored chromium oxide green with low hexavalent chromium content by low-temperature calcination according to claim 1, characterized in that: The mass of the boric acid is 0.5% to 1.5% of the chromic anhydride, and the mass of the activated carbon is 4% to 8% of the chromic anhydride.

3. The method for preparing dark-colored chromium oxide green with low hexavalent chromium content by low-temperature calcination according to claim 1, characterized in that: The activated carbon is spherical wood activated carbon with a particle size of 300-800 meshes.

4. The method for preparing dark-colored chromium oxide green with low hexavalent chromium content by low-temperature calcination according to claim 1, characterized in that: The rotary kiln has a diameter of 1.5-3.0 m and a length of 30-50 m.

5. The method for preparing dark-colored chromium oxide green with low hexavalent chromium content by low-temperature calcination according to claim 1, characterized in that: The green parameters of the chromium oxide product are: L: 36~38; a: -12~-13.5; b: 11~12.

5.

6. The method for preparing dark-colored chromium oxide green with low hexavalent chromium content by low-temperature calcination according to claim 1, characterized in that: The content of hexavalent chromium in the chromium oxide product is 100-200 ppm.

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