Preparation method of dark green chromium oxide green with perfect crystal grain development and uniform crystal grain size

Abstract

The invention relates to a preparation method of dark green chromium oxide green with perfect crystal grain development and uniform crystal grain size, which comprises the following steps: S1, uniformly mixing to prepare a composite salt BaCl2. XKCl according to the molar ratio x = n (BaCl2) / n (KCI) (x = 1.5-2.5); s2, weighing composite salt BaCl2. XKCl according to 0.25%-0.75% of the mass fraction of the chromic anhydride, uniformly mixing the chromic anhydride and the composite salt, adding water, and stirring; s3, the mixture is conveyed into a rotary kiln in a pumping mode, and calcination is conducted through the rotary kiln; s4, carrying out water quenching on the calcined chromium oxide green product; s5, washing the quenched chromium oxide green product with water for 2-4 times to remove the composite salt BaCl2. XKCl so as to ensure the purity of the chromium oxide green product; s6, drying and crushing to obtain a chromium oxide green product; by adding the BaCl2. XKCl composite salt, the dark chromium oxide green product with perfect crystal grain development and uniform crystal grain size is prepared, and the chromaticity parameters L of the product are 37-38, a is-11.5 to-12.5, and b is 12-13; the shape is a flat particle, the grain edge is lubricated, and 90% of the grain size is distributed between 0.5 mu m and 1.5 mu m.

Description

Technical Field

[0001] The invention relates to the technical field of chromium salt production, in particular to a method for preparing dark green chromium oxide green with perfect grain development and uniform grain size. Background Art

[0002] Chromium oxide green is the basis of green pigments, olive green in color, and its trivalent chromium element is the coloring element; it has excellent heat resistance and weather resistance, excellent acid and alkali resistance and is non-toxic, so it has become a traditional green colorant. Due to the differences in raw materials, preparation methods, and reaction conditions, the morphology, particle size, and color parameters of chromium oxide green are different. Comparing the chromaticity and composition of chromium oxide green pigment products from major domestic and foreign manufacturers, it is found that when the chromaticity parameters of chromium oxide green pigments are the same and the types and contents of other impurity elements are similar, the chromaticity parameters are very different. At present, the production of pigment-grade chromium oxide green in my country has reached a certain scale, but the production process and product quality are far behind the international advanced level. Domestic chromium oxide green has the disadvantages of dark color and lack of brightness, which affects its decorativeness and limits its application.

[0003] The production method of chromium oxide green with the largest output in my country is the thermal decomposition method of chromic anhydride, which is also the simplest method. After the decomposition of chromic anhydride, the temperature continues to rise, and chromium trioxide crystals are formed and grown. The crystals have few defects and a complete crystal structure. Chromium oxide green has excellent properties such as deep color (strong tinting power) and high refractive index (hiding power).

[0004] The production of chromium oxide green by thermal decomposition of chromic anhydride is a process of solid→liquid→solid. The solid flake raw material chromic anhydride is converted into a molten state (liquid) while being thermally decomposed at high temperature, and then gradually forms chromium oxide green block solids or large particles through oxygen release and valence changes. However, due to the uneven calcination temperature, the chromium oxide crystal development process is inconsistent, resulting in inconsistent grain size. Usually, increasing the calcination temperature can ensure the complete development of chromium oxide grains, but if the calcination temperature is too high, the color development performance of chromium oxide green will be significantly reduced, limiting its application in the field of pigments. Therefore, developing a method for preparing dark green chromium oxide green with perfect grain development and uniform grain size is an important topic studied by the present invention. Summary of the invention

[0005] The purpose of the present invention is to provide a method for preparing dark green chromium oxide green with perfect grain development and uniform grain size in view of the problems that the chromium oxide green produced by the prior art has inconsistent grain size, dark color tone, and lack of brightness.

[0006] The present invention is achieved through the following technical solutions:

[0007] The method for preparing dark green chromium oxide green with perfect grain development and uniform grain size of the present invention comprises the following steps:

[0008] S1: According to the substance ratio x = n (BaCl 2 ) / n(KCl), where x=1.5-2.5, and the two are mixed evenly to prepare the composite salt BaCl 2 ·xKCl;

[0009] S2: Weigh the composite salt BaCl according to the mass fraction of chromic anhydride 0.25%-0.75% 2 xKCl, mix chromic anhydride and complex salt evenly, add water and stir to obtain a mixture;

[0010] S3: transporting the mixed material to a rotary kiln by pumping, and calcining the mixed material in the rotary kiln;

[0011] S4: quenching the calcined chromium oxide green product with water;

[0012] S5: Wash the quenched chromium oxide green product with water 2 to 4 times to remove the complex salt BaCl 2 xKCl, to ensure the purity of chromium oxide green products;

[0013] S6: Drying and crushing to finally obtain chromium oxide green product.

[0014] Preferably, the chromaticity parameters of the chromium oxide green product of the present invention are L=37-38, a=-11.5--12.5, and b=12-13.

[0015] Preferably, the rotary kiln described in the present invention is a countercurrent type, and the fuel gas used by the rotary kiln is natural gas. The countercurrent burner is located at the kiln tail, which is beneficial to chromic anhydride materials with high water content. Hot air enters the kiln head from the kiln tail and is discharged, which satisfies the requirement of moisture discharge of the materials in the low temperature zone.

[0016] Preferably, the calcination temperature in the present invention is 950° C. to 1050° C., which is the highest temperature of the material passing through the kiln tail detected by an infrared non-contact thermometer.

[0017] Preferably, the mass ratio of water to chromic anhydride and composite salt in S2 of the present invention is 100:(15-25).

[0018] The thermal decomposition of chromic anhydride produces the following reaction. The melting point of chromic anhydride is 196°C. It decomposes at 450-490°C to produce Cr 2 O 3 , then nucleation, grains continue to grow, forming chromium oxide grains. BaCl 2 The melting point of is 960℃, the melting point of KCl is 770℃, and the melting point of BaCl is 2· The melting point of the BaCl₂·xKCl composite salt (x = 1.5 - 2.5) is about 660 °C. Therefore, the composite salt mainly acts on the growth stage of chromium oxide grains, providing a liquid phase environment to promote the growth of grains and making the grain growth complete.

[0019] 4CrO 3 =2Cr 2 O 3 +3O 2

[0020] BaCl 2 · The melting point of the BaCl₂·xKCl composite salt (x = 1.5 - 2.5) is closer to the decomposition temperature of chromic anhydride, making the starting temperature for the composite salt to turn into a liquid phase lower. As the rotary kiln rotates continuously, the liquid phase and chromium oxide grains are mixed more evenly, resulting in better consistency in grain size.

[0021] Usually, solid chromic anhydride has a flaky structure, with relatively large particle sizes, a loose particle packing structure, uneven distribution, and large fluctuations in calcination temperature. In the present invention, due to the selection of the composite salt BaCl₂·xKCl, when it is mixed with chromic anhydride in an aqueous solution to form a slurry, chromic anhydride can be mixed more evenly with the composite salt BaCl₂·xKCl. Chromic anhydride is put into the rotary kiln in the form of mud, and the feeding method is stable. Due to the continuous rotation of the rotary kiln, agglomeration of chromic anhydride particles is avoided, the particle distribution is more uniform, and the calcination temperature is more uniform. 2 ·xKCl and chromic anhydride are mixed in an aqueous solution to form a slurry, and chromic anhydride can be mixed more evenly with the composite salt BaCl₂·xKCl. Chromic anhydride is put into the rotary kiln in the form of mud, and the feeding method is stable. Due to the continuous rotation of the rotary kiln, agglomeration of chromic anhydride particles is avoided, the particle distribution is more uniform, and the calcination temperature is more uniform. 2 ·xKCl. A dark green chromium oxide green product with well-developed grains and uniform grain size is prepared. The chromaticity parameters of the product are L = 37 - 38, a = -11.5 - -12.5, b = 12 - 13. The morphology of chromium oxide grains is flat granular, with smooth grain edges, and 90% of the grain sizes are distributed between 0.5 μm and 1.5 μm.

[0022] In the present invention, the chromium oxide green material after calcination in the rotary kiln is water-cooled to below 100 °C, and the temperature of the cold water used is 15 - 25 °C, and the cold water washing is carried out 2 - 4 times. Through washing, the salt components in the chromium oxide green product can be removed, and water-soluble hexavalent chromium in the product can be further removed.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] The present invention uses chromic anhydride as a raw material and prepares chromium oxide green by calcination in a rotary kiln by adding the BaCl₂·xKCl composite salt (x = 1.5 - 2.5). 2 ·xKCl composite salt (x = 1.5 - 2.5). A dark green chromium oxide green product with well-developed grains and uniform grain size is prepared. The chromaticity parameters of the product are L = 37 - 38, a = -11.5 - -12.5, b = 12 - 13. The morphology of chromium oxide grains is flat granular, with smooth grain edges, and 90% of the grain sizes are distributed between 0.5 μm and 1.5 μm.

[0025] Description of the drawings

[0026] Figure 1 is the scanning electron microscope image of the chromium oxide green prepared in Example 1 of the present invention;

[0027] Figure 2 This is a scanning electron microscope image of chromium oxide green prepared in Comparative Example 2 of the present invention;

[0028] Figure 3 This is a scanning electron microscope image of chromium oxide green prepared in Comparative Example 3 of the present invention;

[0029] Figure 4 This is a scanning electron microscope image of chromium oxide green prepared in Comparative Example 4 of the present invention. DETAILED DESCRIPTION

[0030] 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.

[0031] Example 1

[0032] The present embodiment provides a method for preparing dark green chromium oxide green with well-developed grains and uniform grain size. The present embodiment uses a rotary kiln with a diameter of 1.8 m and a length of 30 m. The heating method of the rotary kiln is countercurrent, and the burner is located at the end of the rotary kiln. Natural gas is used as fuel. The continuous feeding amount of chromic anhydride is 1.0 ton / hour. The composite salt BaCl is weighed according to the mass fraction of chromic anhydride of 0.5%. 2 ·2KCl, the continuous feeding amount of the composite salt is 5kg / hour, the mass ratio of water to chromic anhydride and composite salt is 100:20, the continuous feeding amount of water is 5.025 tons / hour, the water, chromic anhydride and composite salt are mixed evenly, and then transported to the rotary kiln by pumping. The maximum temperature of the material at the end of the kiln is 1000℃ measured by an infrared non-contact thermometer, and the calcination time of the material from the kiln head to the kiln tail is 6h. The calcined chromium oxide green product is water quenched, the material is cooled to below 100℃, the temperature of cold water is 20℃, and the cold water is washed 3 times, dried and crushed to finally obtain the chromium oxide green product. The SEM image of the chromium oxide green product is shown in the attached Figure 1 From the attached Figure 1 It can be seen that in the microstructure of chromium oxide green products, the grains are fully developed, the morphology is flat granular, the grain edges are smooth, the grain size is uniform, and the average particle size is about 1μm.

[0033] Weigh 1.0 g of the chromium oxide green product obtained in this example with an analytical balance. Pour the weighed sample onto a grinding disc, add 12 - 15 drops of castor oil, approximately 0.5 g, and use a scraping rod to evenly scrape out a uniform color patch on the test paper. Place the test paper on a high-temperature panel at 70 °C to let it heat evenly. After 30 - 45 minutes, take it down and use a color difference meter to measure the Lab value of the sample. The test results are shown in Table 1 below. Wet grind the chromium oxide green powder with a ball mill for 4 h, take the suspension, and use a laser particle size analyzer to conduct particle size tests. By ball milling, the agglomerated chromium oxide grains are dispersed, and through the particle size test, the results of the grain size distribution can be accurately obtained. The test results are shown in Table 1 below.

[0034] Table 1

[0035] serial number L value a value b-value <![CDATA[D 10 / μm]]> <![CDATA[D 50 / μm]]> <![CDATA[D 90 / μm]]> Example 1 37.62 -12.1 12.5 0.64 1.03 1.41 Example 2 37.78 -11.7 12.3 0.55 0.95 1.32 Example 3 37.35 -11.8 12.1 0.71 1.10 1.48 Example 4 37.75 -11.6 12.2 0.54 0.93 1.33

[0036] As can be seen from Table 1 above, the dark green chromium oxide green product prepared in Example 1 of the present invention has chromaticity parameters of L = 37.62, a = -12.1, and b = 12.5. From the particle size test results, it can be known that the morphology of the chromium oxide grains is flat granular, the grain edges are smooth, the grain sizes of the chromium oxide green product are uniform, and they are in the range of 0.64 - 1.41 μm.

[0037] Example 2

[0038] A preparation method of dark green chromium oxide green with perfect grain development and uniform grain sizes. In this example, weigh the composite salt BaCl 2 ·2.5KCl according to 0.25% of the mass fraction of chromic anhydride. The continuous feeding amount of the composite salt is 2.5 kg / hour, and the mass ratio of water to chromic anhydride and the composite salt is 100:20. The continuous feeding amount of water is 5.0125 tons / hour. The other conditions are the same as those in Example 1.

[0039] Weigh 1.0 g of the chromium oxide green product prepared in this example with an analytical balance. Pour the weighed sample onto a grinding disc, add 12 - 15 drops of castor oil, approximately 0.5 g, and use a scraping rod to evenly scrape out a uniform color patch on the test paper. Place the test paper on a high-temperature panel at 70 °C to let it heat evenly. After 30 - 45 minutes, take it down and use a color difference meter to measure the Lab value of the sample. The test results are shown in Table 1. Wet grind the chromium oxide green powder with a ball mill for 4 h, take the suspension, and use a laser particle size analyzer to conduct particle size tests. The test results are shown in Table 1.

[0040] As can be seen from Table 1 above, the chromaticity parameters of the chromium oxide green product prepared in Example 2 of the present invention are L=37.78, a=-11.7, and b=12.3. The particle size test results show that the grain size of the chromium oxide green product is uniform, but compared with Example 1, due to the reduction in the amount of composite salt added, the grain size is overall smaller, ranging from 0.55 to 1.32 μm.

[0041] Example 3

[0042] The present embodiment provides a method for preparing dark green chromium oxide green with well-developed grains and uniform grain size. In the present embodiment, the composite salt BaCl is weighed according to the mass fraction of chromic anhydride of 0.75%. 2 1.5KCl, the continuous feeding amount of the composite salt is 7.5kg / hour, the mass ratio of water to chromic anhydride and composite salt is 100:20, the continuous feeding amount of water is 5.0375 tons / hour, and the other conditions are the same as those in Example 1.

[0043] The chromium oxide green product prepared in this example was weighed with an analytical balance to obtain 1.0 g of sample, poured onto a grinding disc, added with 12 to 15 drops of castor oil, about 0.5 g, and scraped out a uniform color block on the test paper with a color scraping stick at a uniform speed, and the test paper was placed on a high-temperature panel at 70°C to allow it to be evenly heated, removed after 30-45 minutes, and the Lab value of the sample was tested using a colorimeter, and the test results are shown in Table 1. The chromium oxide green powder was wet-milled for 4 hours using a ball mill, and the suspension was taken out and the particle size was tested using a laser particle size analyzer, and the test results are shown in Table 1.

[0044] As can be seen from Table 1 above, the chromaticity parameters of the chromium oxide green product prepared in Example 3 of the present invention are L=37.35, a=-11.8, and b=12.1. The particle size test results show that the chromium oxide green product has a uniform grain size, but compared with Example 1 and Example 2, the amount of composite salt added is the largest, and the grain size is larger overall, ranging from 0.71 to 1.48 μm.

[0045] Example 4

[0046] The present embodiment provides a method for preparing dark green chromium oxide green with well-developed grains and uniform grain size. In the present embodiment, the composite salt BaCl is weighed according to the mass fraction of chromic anhydride of 0.25%. 2 ·2KCl, the continuous feeding amount of the composite salt is 2.5kg / hour, the mass ratio of water to chromic anhydride and composite salt is 100:20, the continuous feeding amount of water is 5.0125 tons / hour, and the other conditions are the same as those in Example 1.

[0047] The chromium oxide green product prepared in this example was weighed with an analytical balance to obtain 1.0 g of sample, poured onto a grinding disc, added with 12 to 15 drops of castor oil, about 0.5 g, and scraped out a uniform color block on the test paper with a color scraping stick at a uniform speed, placed the test paper on a high-temperature panel at 70°C to allow it to be evenly heated, removed after 30 to 45 minutes, and tested the Lab value of the sample using a colorimeter, as shown in Table 1. According to the methods of GB9760-88 and GB20785-2006, the chromium oxide green powder was wet-milled for 4 hours using a ball mill, and the suspension was taken and tested for particle size using a laser particle size analyzer, as shown in Table 1.

[0048] As can be seen from Table 1 above, the chromaticity parameters of the chromium oxide green product prepared in Example 4 of the present invention are L=37.75, a=-11.6, and b=12.2. The particle size test results show that the chromium oxide green product has a uniform grain size, but relative to Example 2, the proportion of KCl in the composite salt increases, and the grain size is generally larger.

[0049] Comparative Example 1-2

[0050] In order to verify the effect of different calcination temperatures on the chromium oxide green product, the calcination temperatures of Comparative Examples 1-2 were 950°C and 1050°C respectively, and the rest of the experimental data were the same as those of Example 1. The SEM image of the chromium oxide green product prepared in Comparative Example 2 is shown in the attached figure. Figure 2 From the attached Figure 2 It can be produced that with the increase of calcination temperature, the size of chromium oxide grains increases and the grain development is further improved.

[0051] Weigh 1.0 g of the chromium oxide green product prepared in Comparative Example 1-2 with an analytical balance, pour the weighed sample on a grinding disc, add 12 to 15 drops of castor oil, about 0.5 g, and use a scraping stick to uniformly scrape out a uniform color block on the test paper. Place the test paper on a high-temperature panel at 70°C to allow it to be evenly heated, remove it after 30-45 minutes, and use a colorimeter to test the Lab value of the sample. The test results are shown in Table 2. The chromium oxide green powder is wet-milled using a ball mill for 4 hours, and the suspension is taken out. The particle size is tested using a laser particle size analyzer. The test results are shown in Table 2.

[0052] Table 2

[0053] serial number L value a value b-value <![CDATA[D 10 / μm]]> <![CDATA[D 50 / μm]]> <![CDATA[D 90 / μm <!-- 4 -->]]> Comparative Example 1 37.95 -12.1 12.5 0.45 0.92 1.28 Comparative Example 2 37.15 -11.6 12.1 0.68 1.05 1.45

[0054] By analyzing the Lab value and particle size test results of comparative examples 1-2, within the range of 950°C to 1050°C, as the sintering temperature increases, the color of chromium oxide green becomes darker and the color development performance becomes worse (the absolute values ​​of a value and b value become smaller). As the calcining temperature increases, the size of chromium oxide grains increases and the grain development becomes more perfect.

[0055] Comparative Examples 3-4

[0056] In order to verify the composite salt BaCl 2 Effect of xKCl on chromium oxide green products, comparative examples 3-4 using KCl and BaCl 2 Calcination at 1000 °C, KCl and BaCl 2 The addition amount is 0.5% of the mass fraction of chromic anhydride, and the other experimental parameters are the same as those in Example 1.

[0057] The SEM images of the chromium oxide green products prepared in Comparative Examples 3-4 are shown in the attached Figure 3 and attached Figure 4 From the attached Figure 3 and 4 It can be seen that the addition of KCl or BaCl alone 2 When the grain size distribution range is large, there are a certain amount of small particles, the chromium oxide grain morphology is flat granular, and the grain edge is not clear, indicating that some grains are not fully developed.

[0058] Weigh 1.0g of the chromium oxide green product prepared in Comparative Example 3-4 with an analytical balance, pour the weighed sample on a grinding disc, add 12-15 drops of castor oil, about 0.5g, and use a color 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°C to allow it to be evenly heated, remove it after 30-45 minutes, and use a colorimeter to test the Lab value of the sample. The test results are shown in Table 3. The chromium oxide green powder was wet-milled for 4 hours using a ball mill, and the suspension was taken out for particle size testing using a laser particle size analyzer. The test results are shown in Table 3.

[0059] Table 3

[0060] serial number L value a value b-value <![CDATA[D 10 / μm]]> <![CDATA[D 50 / μm]]> <![CDATA[D 90 / μm]]> Comparative Example 3 38.12 -11.01 10.94 0.24 0.96 1.67 Comparative Example 4 36.96 -10.78 11.18 0.21 1.03 1.35

[0061] By analyzing the Lab values ​​and particle size test results of Comparative Examples 3-4, adding KCl or BaCl alone 2 When KCl is added alone, the color of chromium oxide green becomes lighter, while when BaCl is added alone, the color of chromium oxide green becomes lighter. 2 The color of chromium oxide green becomes darker. 2 The particle size distribution becomes wider.

Claims

1. A method for preparing dark green chromium oxide green with well-developed grains and uniform grain size, characterized in that: The following steps are involved: S1: According to the molar ratio x = n (BaCl2) / n (KCl), where x = 1.5-2.5, the two are mixed evenly to prepare the composite salt BaCl2·xKCl; S2: weigh the composite salt BaCl2·xKCl according to the mass fraction of chromic anhydride of 0.25%-0.75%, mix the chromic anhydride and the composite salt evenly, add water and stir to obtain a mixture; S3: transporting the mixed material to a rotary kiln by pumping, and calcining the mixed material in the rotary kiln; S4: quenching the calcined chromium oxide green product with water; S5: Wash the quenched chromium oxide green product with water for 2 to 4 times to remove the complex salt BaCl2·xKCl and ensure the purity of the chromium oxide green product; S6: Drying and crushing to finally obtain chromium oxide green product.

2. The method for preparing dark green chromium oxide green with well-developed grains and uniform grain size according to claim 1, characterized in that: The chromaticity parameters of the chromium oxide green product are L=37~38, a=-11.5~-12.5, and b=12~13.

3. The method for preparing dark green chromium oxide green with perfect grain development and uniform grain size according to claim 1, characterized in that: The rotary kiln is a countercurrent type.

4. The method for preparing dark green chromium oxide green with well-developed grains and uniform grain size according to claim 1, characterized in that: The calcination temperature is 950°C to 1050°C.

5. The method for preparing dark green chromium oxide green with perfect grain development and uniform grain size according to claim 1, characterized in that: The mass ratio of water to chromic anhydride and composite salt in the S2 is 100:(15-25).

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