A kind of suanite and its preparation method

Abstract

The present invention provides a meyerhofferite and a preparation method thereof, which are prepared by adding carnallite in salt lake brine or old brine in salt lake into a sodium borate solution for reaction. Using carnallite or old brine as by-products in salt lake as raw materials to prepare meyerhofferite solves the technical problems such as low purity, high cost and high energy consumption of meyerhofferite, and meanwhile improves the utilization efficiency of boron and magnesium resources in salt lake, and broadens the high-value utilization ways of magnesium and boron resources in salt lake.

Description

Technical Field

[0001] The present invention relates to the technical field of comprehensive utilization of magnesium and boron in salt lake resources. Specifically, it relates to a sussexite and a preparation method thereof. More specifically, it is a method for preparing sussexite by using salt lake brine or magnesium chloride hexahydrate rich in salt lakes. Background Art

[0002] Boron is a typical non-metallic element. It is located in the second period and the third main group in the periodic table of elements and has unique electron-deficient characteristics. Boron mainly exists in the form of inorganic boric acids and borate salts in nature. In borate salts, boron can form a wide variety of borate compounds with diverse and complex structures in the form of polymeric borate anions. Due to properties such as wear resistance, high strength, flame retardancy, heat resistance, corrosion resistance, and nonlinear optics, borates are widely used in fields such as metallurgy, machinery, building materials, medicine, and pesticides. Magnesium borate is a new type of reinforcing, antifriction, and antiwear and luminescent matrix material. Hydrated magnesium borate is an important chemical raw material and can be used as an additive in industries such as high-grade glass, ceramics, rubber, and plastic polymers, with excellent strengthening and modification effects. It can also be used as a metal protector and a fire retardant.

[0003] There are various forms of magnesium borate salts in nature. The Qinghai-Tibet Plateau salt lake area in China is rich in boron-magnesium minerals. For example, among the 9 borate minerals discovered in the Dachaidan lake area, 5 are magnesium borate salts: inderite (MgO·3B2O3·3H2O), hydroboracite (2MgO·3B2O3·15H2O), korshunite (2MgO·3B2O3·15H2O), zhangite (MgO·2B2O3·9H2O), and sussexite (MgO·3B2O3·7.5H2O). To explore the formation conditions of the above different magnesium borate salts, Academician Gao Shiyang in China has conducted a large number of basic studies on the formation and transformation of magnesium borate salts under low to high temperature conditions, providing an experimental basis for the geochemical origin of boron-magnesium ores in the Qinghai-Tibet Plateau and also providing a new approach for the preparation, properties, and development and utilization of magnesium borate.

[0004] At present, common hydrated magnesium hexaborates include MgO·3B2O3·7.5H2O (sassolite), MgO·3B2O3·7H2O, and MgO·3B2O3·6H2O, etc. When Gao Shiyang et al. studied the processes of evaporation, salt precipitation, concentration, freezing, and dilution with water of magnesium sulfate subtype bittern, as well as the crystallization of MgO·nB2O3-MgCl2-H2O concentrated salt solution, sassolite could be prepared, mainly as a mixture of magnesium hexaborate, and the salt precipitation time was relatively long. Kipcaket et al. proposed a method for synthesizing magnesium hexaborate from different ratios of MgO, B2O3, and H3BO3 in "The Synthesis and physical properties of magnesium borate mineral of admontite synthesized from sodium borates[J], Advances in Materials Science and Engineering, 2014, 2014: 1-9", and the products were also mostly mixtures of magnesium borate. Meral Yildirim et al. studied the synthesis of sassolite from different boron sources such as magnesium chloride hexahydrate (MgCl2·6H2O), borax (Na2B4O7·10H2O), boric acid (H3BO3), and boron oxide (B2O3) under ultrasonic-assisted conditions in "Sonochemical-assisted magnesium borate synthesis from different boron sources[J], Polish Journal of Chemical Technology, 2017, 19(1): 81-88". The reaction temperature was 60°C to 100°C, and the ultrasonic time was 5 to 20 minutes. This study improved the synthesis rate and shortened the reaction time, but the product purity was low. Chinese invention patent CN101696017A prepared activated magnesium oxide using basic magnesium carbonate or magnesium oxide as raw materials, and then used activated magnesium oxide, boric acid, and water as raw materials, and through mixing reaction, crystallization, filtration, washing, and drying, magnesium hexaborate was made according to a certain mass ratio. Chinese invention patent CN101746770A mixed boric acid, magnesium compound, and organic solvent, and carried out process procedures such as azeotropic distillation, solvent removal, and drying to prepare hydrated magnesium borate. Chinese invention patent CN101177274A used industrial boric acid and light-burned magnesium as raw materials, carried out hydrothermal reaction at 100 to 250°C and 0.1 to 3.5 MPa for 6 to 16 hours, and then aged and crystallized at 60 to 95°C for 0.5 to 6 hours. The product was obtained through filtration, washing, and drying. The magnesium borates prepared by the technical solutions adopted in the above existing technologies all have disadvantages such as low product purity and long process flow, and most of the raw materials used are chemical raw materials, which makes the cost and energy consumption of product preparation relatively high, restricting their large-scale use and promotion.

[0005] On the other hand, salt lake brine is rich in magnesium ions. Currently, most of this is discharged remotely back to the salt lake, where it evaporates under sunlight to form bischofite. With the continuous expansion of potassium chloride production, the amount of salt lake brine and the byproduct bischofite produced by this treatment method has also increased year by year, resulting in a waste of magnesium resources and a serious impact on the natural ecological balance of salt lakes. Therefore, how to improve the utilization rate of salt lake brine or bischofite and alleviate the environmental problems caused by salt lake magnesium resources has become a technical problem that needs to be solved urgently. Summary of the Invention

[0006] In view of the technical problems existing in the prior art, the purpose of the present invention is to provide a method for preparing boraxite, which uses bischofite or old brine, a by-product of salt lakes, as raw materials to prepare boraxite, thereby solving the technical problems of low purity, high cost and high energy consumption of boraxite, while improving the utilization efficiency of boron, magnesium and boron resources in salt lakes, and broadening the high-value utilization path of magnesium and boron resources in salt lakes.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing boraxite, wherein salt lake bischofite or salt lake brine is added to a sodium borate solution for reaction, wherein the mass ratio of magnesium ion content to boron content (calculated as B2O3) is 2.5 to 24:1.

[0008] Preferably, the sodium borate solution is prepared by adding H3BO3 and NaOH as raw materials, heating and stirring to react after adding deionized water; the heating temperature is 25-80°C.

[0009] Preferably, the molar ratio of H3BO3 to NaOH is 2 to 7:1; and the mass of deionized water is 5 to 15 times the mass of H3BO3.

[0010] Preferably, the salt lake brine is diluted and then added to the sodium borate solution, the dilution ratio is 1-4 by volume of water to the salt lake brine, and the pH is adjusted to 4-7.

[0011] Preferably, the volume of the salt lake brine is 1 to 4 times the volume of the sodium borate solution.

[0012] Preferably, the salt lake bischofite or the diluted salt lake brine is added to the sodium borate solution, wherein the salt lake brine is added all at once or in equal amounts in batches.

[0013] Preferably, the magnesium ion content in the salt lake brine is 100-120 g / L, and the boron content in terms of B2O3 is 5-40 g / L.

[0014] Preferably, the purity of the salt lake bischofite is 90-98%.

[0015] Preferably, the product after the reaction of the bischofite in the salt lake or the old bittern in the salt lake with the sodium borate solution crystallizes out, and the inderite is obtained after being treated by the processes of solid-liquid separation - solid-phase washing - drying.

[0016] Preferably, the washing liquid in the solid-phase washing process is one or two of deionized water or ethanol, and the mass ratio of the washing liquid to the solid phase is 6:1 - 15:1; in the drying process, the drying temperature is 25 - 70 °C, and the drying time is 0.5 - 10 h.

[0017] The inderite prepared by the above preparation method is a hexagonal flaky fine crystal, and the crystal grain size is 5 - 10 μm.

[0018] The old bittern in the salt lake is also called bitter brine, bittern, old brine. It is the mother liquor remaining in the salt pond after seawater or salt lake water is used for salt production. After evaporation and cooling, magnesium chloride crystals are precipitated, and the resulting solid is bischofite. Specifically, the main components of the old bittern in the salt lake include: magnesium ions, sodium ions, potassium ions, sulfate ions, chloride ions, and B2O3, etc. The chemical formula of bischofite in the salt lake water is MgCl2·6H2O, which is a pure substance without other components.

[0019] In the present invention, the old bittern in the salt lake or bischofite in the salt lake water is used as a raw material and reacted with sodium borate. The specific reaction mechanism includes:

[0020]

[0021]

[0022]

[0023]

[0024]

[0025]

[0026] It can be seen from the above mechanism that sodium pentaborate solution is prepared by the reaction of boric acid and sodium hydroxide, and polyborate ions such as B3O3(OH)4 - , B5O6(OH)4 - and B(OH)4 - ions are generated. Adding bischofite can promote the polycondensation reaction of B3O3(OH)4 - ions to generate B6O7(OH)7 2- ions, and these ions combine with magnesium ions to form inderite and crystallize out.

[0027] The technical effects of the technical solution of the present invention:

[0028] 1. By adopting the technical solution of the present invention, salt lake brine or salt lake bischofite, a by-product of the salt lake, is used as a raw material to react with sodium borate, which not only solves the technical problems of complex process, high cost and high energy consumption of bischofite, but also improves the utilization efficiency of salt lake magnesium and boron resources, increases the utilization value of salt lake magnesium and boron resources, and broadens the high-value utilization path of salt lake magnesium and boron resources.

[0029] 2. By adopting the technical solution of the present invention, salt lake by-products are used as raw materials, which reduces production costs, reduces energy consumption in the production process, and simplifies the preparation process, making it suitable for large-scale use and promotion.

[0030] 3. By adopting the technical solution of the present invention, the by-products of the salt lake are fully utilized, the discharge of old brine from the salt lake back to the salt lake is alleviated, which causes serious damage to the natural ecological balance of the salt lake, and the magnesium resources can be fully utilized to promote its transformation in a high-value direction.

[0031] 4. The composition of the product of boraxite prepared by adopting the technical solution of the present invention is close to the theoretical value and has the advantages of high purity and low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 The XRD diagram of the solid phase of the pyroxenite prepared in Examples 1 and 2 of the present invention.

[0033] Figure 2 This is an SEM image of the pyroxenite prepared in Example 3 of the present invention.

[0034] Figure 3 This is a thermogravimetric analysis diagram of the pyroxenite prepared in Example 4 of the present invention. DETAILED DESCRIPTION

[0035] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention are clearly and completely described. Obviously, the described embodiments are part of the embodiments of this application, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0036] The present invention provides a method for preparing boraxite, which is prepared by adding salt lake bischofite or salt lake brine into a sodium borate solution for reaction; wherein the mass ratio of the magnesium ion content to the boron content (calculated as B2O3) in the salt lake brine is 2.5-24:1, preferably 20-24:1.

[0037] In a preferred embodiment, the sodium borate solution is prepared by adding H3BO3 and NaOH as raw materials, heating and stirring to react after adding deionized water; the heating temperature is 25-80°C.

[0038] In a preferred embodiment, the molar ratio of H3BO3 to NaOH is 2 to 7:1; and the mass of deionized water is 5 to 15 times the mass of H3BO3.

[0039] In a preferred embodiment, the salt lake brine is diluted and then added to the sodium borate solution, the dilution ratio is 1-4 by volume of water to the salt lake brine, and the pH is adjusted to 4-7.

[0040] In a preferred embodiment, the volume of the salt lake brine is 1 to 4 times the volume of the sodium borate solution.

[0041] In a preferred embodiment, equal amounts of salt lake bischofite or diluted salt lake brine are added to the sodium borate solution in batches.

[0042] In a preferred embodiment, the magnesium ion content in the salt lake brine is 100-120 g / L, and the boron content is 5-40 g / L in terms of B2O3.

[0043] In a preferred embodiment, the product of the reaction between salt lake bischofite or the salt lake brine and sodium borate solution is crystallized and precipitated, and then subjected to a solid-liquid separation-solid phase washing-drying process to obtain the boraxite.

[0044] In a preferred embodiment, the washing liquid in the solid phase washing process is one or both of deionized water and ethanol, and the mass ratio of the washing liquid to the solid phase is 6:1 to 15:1; in the drying process, the drying temperature is 25 to 70°C, and the drying time is 0.5 to 10 hours.

[0045] The technical solution of the present invention is further described in detail below through specific embodiments.

[0046] Example 1

[0047] This embodiment provides a method for preparing pyroxenite, which specifically includes the following steps:

[0048] 1. Weigh 7.80g H3BO3 and 1.02g NaOH, add 40.0g deionized water, heat and stir until fully dissolved and react to obtain sodium borate solution.

[0049] 2. Add 120 mL of salt lake brine diluted 1 times to the sodium borate solution in step 1 at a time, wherein the magnesium ion in the salt lake brine is 100 g / L, B2O3 is 5 g / L, and the pH value is 5.0. After the salt lake brine is completely dissolved, react with magnetic stirring in a constant temperature water bath at 25°C for 2 hours to crystallize out a solid.

[0050] 3. Solid-liquid separation is carried out on the product in step 2, and the solid phase is washed twice with water slurry with a liquid-solid ratio of 6:1. After washing, the product is dried at 40 °C for about 4 hours to obtain the suanite product.

[0051] Example 2

[0052] The difference between this example and Example 1 is that the magnesium ion in the old brine of the salt lake is 120 g / L and B2O3 is 5 g / L.

[0053] Example 3

[0054] The difference between this example and Example 1 is that the magnesium ion in the old brine of the salt lake is 100 g / L and B2O3 is 40 g / L.

[0055] Example 4

[0056] The difference between this example and Example 1 is that the magnesium ion in the old brine of the salt lake is 120 g / L and B2O3 is 40 g / L.

[0057] Example 5

[0058] The difference between this example and Example 1 is that in step 2, the old brine of the salt lake is completely dissolved in the sodium borate solution, and the reaction is carried out with magnetic stirring in a constant temperature water bath at 40 °C for 2 hours to crystallize out solids.

[0059] Example 6

[0060] The difference between this example and Example 1 is that in step 2, the old brine of the salt lake is completely dissolved in the sodium borate solution, and the reaction is carried out with magnetic stirring in a constant temperature water bath at 60 °C for 2 hours to crystallize out solids.

[0061] Example 7

[0062] The difference between this example and Example 1 is that in step 2, the old brine of the salt lake is completely dissolved in the sodium borate solution, and the reaction is carried out with magnetic stirring in a constant temperature water bath at 80 °C for 2 hours to crystallize out solids.

[0063] Example 8

[0064] The difference between this example and Example 3 is that in the sodium borate solution prepared in step 1, 120 mL of the old brine of the salt lake diluted 4 times is added in 4 equal amounts at intervals of 2 minutes each. Among them, the magnesium ion in the old brine of the salt lake is 100 g / L and B2O3 is 40 g / L.

[0065] Example 9

[0066] This example provides a method for preparing suanite, and the specific steps include:

[0067] 1. Weigh 7.80 g of H3bO3 and 1.02 g of NaOH, add 100.0 g of deionized water, heat and stir to fully dissolve them to prepare a sodium borate solution.

[0068] 2. Add 200 mL of the old brine from the salt lake diluted 4 times in 4 equal portions to the sodium borate solution in Step 1 at intervals of 2 minutes each. The concentration of magnesium ions in the old brine from the salt lake is 120 g / L, the concentration of B2O3 is 40 g / L, and the pH value is 7.0. After complete dissolution of the two, carry out a constant-temperature magnetic stirring reaction at 40 °C for 15 hours and then crystallize to precipitate a solid.

[0069] 3. Carry out solid-liquid separation on the product in Step 2, and wash the solid phase with water and ethanol twice with a liquid-solid ratio of 15:1. Dry the washed product at 40 °C for 6 hours to obtain a product of inderite.

[0070] Example 10

[0071] This example provides a method for preparing inderite, and the specific steps include:

[0072] 1. Weigh 7.80 g of H3bO3 and 1.02 g of NaOH, add 78.0 g of deionized water, heat and stir to fully dissolve them to prepare a sodium borate solution.

[0073] 2. Add 150.0 g of bischofite from the salt lake with a purity of 90% in 10 equal portions to the sodium borate solution in Step 1 at intervals of 2 minutes each. After complete dissolution of the two, carry out a constant-temperature magnetic stirring reaction at 80 °C for 20 hours and then crystallize to precipitate a solid.

[0074] 3. Carry out solid-liquid separation on the product in Step 2, and wash the solid phase with ethanol three times with a liquid-solid ratio of 8:1. Dry the washed product at 70 °C for 0.5 hour to obtain a product of inderite.

[0075] Example 11

[0076] This example provides a method for preparing inderite, and the specific steps include:

[0077] 1. Weigh 7.80 g of H3bO3 and 1.02 g of NaOH, add 80.0 g of deionized water, heat and stir to fully dissolve them to prepare a sodium borate solution.

[0078] 2. Add 60.0 g of bischofite from the salt lake with a purity of 98% in 7 equal portions to the sodium borate solution in Step 1 at intervals of 2 minutes each. After complete dissolution of the two, carry out a constant-temperature magnetic stirring reaction at 60 °C for 20 hours and then crystallize to precipitate a solid.

[0079] 3. Solid-liquid separation is carried out on the product in step 2, and the solid phase is washed with water slurry three times with a liquid-solid ratio of 10:1. The washed product is dried at 25 °C for 10 hours to obtain the suanite product.

[0080] Performance characterization:

[0081] Refer to Figure 1 , from top to bottom in the figure are the XRD diffraction patterns of suanite obtained from Example 1 (crystalline solid phase at 25 °C), suanite obtained from Example 2 (crystalline solid phase at 40 °C), and standard suanite. By Figure 1 analysis, the synthesized crystal has strong peaks and sub-strong peaks at 10.1°, 15.3°, 21.8°, 27.2° and 31.9°. Each diffraction peak corresponds to the standard spectrum of suanite XRD, indicating that the XRD diffraction patterns of Example 1 and Example 2 are in line with the standard diffraction pattern.

[0082] Meanwhile, the solid phase compositions of Example 1 and Example 2 are analyzed. The boron content is determined by the alkali volumetric method in the presence of mannitol and expressed as B2O3; the magnesium content is determined by the EDTA volumetric method and expressed as MgO.

[0083] For Example 1 (crystalline solid phase at 25 °C), the solid phase composition is 10.48 wt% MgO and 54.44 wt% B2O3; for Example 2 (crystalline solid phase at 40 °C), the solid phase composition is 10.50 wt% MgO and 54.42 wt% B2O3; the theoretical composition of suanite is 10.49 wt% MgO and 54.35 wt% B2O3.

[0084] The suanite product prepared in Example 3 is subjected to SEM characterization, refer to Figure 2 . As Figure 2 shown, suanite is hexagonal flaky fine crystals with a crystal grain size of 5 - 10 μm.

[0085] The suanite product prepared in Example 11 is subjected to thermogravimetric analysis, and the thermogravimetric analysis pattern is shown in Figure 3 .

[0086] As Figure 3As shown, in the temperature range of 150 - 229°C, incolemanite loses its crystal water, resulting in an endothermic reaction with an endothermic peak; at 780°C, there is an exothermic valley, which is due to the exothermic reaction caused by the recrystallization of the amorphous substance after the loss of crystal water. In the temperature range of 760 - 830°C, there is an exothermic valley, which is also due to the exothermic reaction caused by the recrystallization of the amorphous substance after the loss of crystal water. This is consistent with the reports in Qu Yihua et al., Incolemanite - A New Borate Mineral [J], Acta Geologica Sinica, 1965, 45(3) and Gao Shiyang et al., The Chemistry of Salt Lake Borates II, Magnesium Hexaborate MgO·3B2O3·7.5H2O Precipitated from Boron - Containing Concentrated Magnesium Chloride Brine [J], Acta Chimica Sinica, 1983, 41(3). Further, the weight loss of incolemanite due to water loss is 35.89%, which is close to the theoretical water content of the compound, 35.16%.

[0087] Table 1 Analysis Table of Solid - Phase Composition of Incolemanite Prepared in Example 1

[0088]

[0089]

[0090] From the results in Table 1, it can be seen that Examples 1 - 11 are examples with different magnesium and boron ratios and different process conditions in the old brine of the salt lake, and solid - phase products of incolemanite are respectively crystallized, and the solid - phase composition content is close to the theoretical value.

[0091] Further, Examples 1 - 4 are examples with different magnesium and boron ratios. In the old brine of the salt lake, the mass ratio of magnesium ion content to boron content (calculated as B2O3) is 2.5 - 24∶1; Examples 1 and 2 are relatively optimal examples, and the mass ratio of magnesium ion content to boron content (calculated as B2O3) is 20 - 24∶1.

[0092] Examples 1 and 5 - 7 are examples under different reaction temperatures respectively. The analysis results of the solid - phase composition at 25°C (Example 1) are better than those under other temperature conditions (40°C, 60°C, 80°C). Obviously, from the test results of the present invention, it can be known that the preparation of incolemanite under the process conditions of 25°C can reduce energy consumption, the reaction conditions are milder, and better test results can be obtained.

[0093] Examples 3 and 8 are comparisons of adding the old brine of the salt lake to boric acid solution in equal amounts - in batches after diluting it by different multiples under the condition of the same magnesium - boron ratio. It can be seen that the purity of incolemanite is improved after diluting it by a higher multiple. The purity of incolemanite prepared in Example 8 is better than that prepared in Example 3.

[0094] In both Example 10 and Example 11, bischofite in salt lake brine was used as the raw material to prepare inderite, and relatively ideal results could also be obtained.

[0095] Therefore, through the analysis of the above test results of XRD diffraction patterns, solid-phase components and thermogravimetric analysis, it shows that the product prepared by adopting the technical scheme of the present invention is inderite, and its purity can be close to the theoretical value.

[0096] From the characterization results of the above examples, it can be seen that by adopting the technical scheme of the present invention, using bischofite in salt lake brine or old salt lake brine as the raw material, inderite is obtained after reacting with sodium borate solution, and it has the characteristics of simple preparation process and low energy consumption. Using old salt lake brine or bischofite in salt lake brine as the raw material in the preparation process has the characteristics of low cost, and can improve the utilization rate of old salt lake brine or bischofite in salt lake brine, and alleviate the technical problems brought by magnesium harm in salt lakes.

[0097] The above are only the preferred embodiments of the present invention, and it does not limit the protection scope of the present invention. For those skilled in the art, various changes and modifications can be made to the present invention. All changes, modifications, substitutions, integrations and parameter changes made to these embodiments by conventional substitutions or by achieving the same functions without departing from the principle and spirit of the present invention fall within the protection scope of the present invention.

Claims

1. A method for preparing pyroxenite, characterized in that: The salt lake bischofite or salt lake brine is added to a sodium borate solution for reaction to prepare the product; The salt lake brine is diluted and added to the sodium borate solution, the dilution ratio is 1-4 by volume of water to the salt lake brine, and the pH is adjusted to 4-7; The volume of the salt lake brine is 1 to 4 times the volume of the sodium borate solution; The salt lake bischofite or the diluted salt lake brine is added to the sodium borate solution; wherein the salt lake brine is added at once or in equal amounts in batches; The sodium borate solution is prepared by adding H3BO3 and NaOH as raw materials, heating and stirring to react after adding deionized water; The heating temperature is 25-80°C; The molar ratio of H3BO3 and NaOH is 2~7:1; the mass of deionized water is 5~15 times the mass of H3BO3.

2. The method for preparing boraxite according to claim 1, wherein: The magnesium ion content in the salt lake brine is 100-120 g / L, and the boron content calculated as B2O3 is 5-40 g / L; the purity of the salt lake bischofite is 90-98%.

3. The method for preparing boraxite according to claim 1, wherein: The product of the reaction between the salt lake bischofite or the salt lake brine and the sodium borate solution is crystallized and precipitated, and then subjected to a solid-liquid separation-solid phase washing-drying process to obtain the boraxite.

4. The method for preparing boraxite according to claim 3, wherein: The washing liquid in the solid phase washing process is one or both of deionized water and ethanol, and the mass ratio of the washing liquid to the solid phase is 6:1-15:1; in the drying process, the drying temperature is 25-70°C and the drying time is 0.5-10 h.

5. A boraxite prepared by the preparation method according to any one of claims 1 to 4; the boraxite is a hexagonal flaky fine crystal with a crystal size of 5 to 10 μm.

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