A method for preparing rare earth oxide powder
By using oxalic acid as a precipitant and an airflow sedimentation reactor in the preparation of rare earth oxide powder, the problems of high cost and small specific surface area in ammonia nitrogen wastewater treatment were solved, and the efficient preparation of rare earth oxide powder with a large specific surface area was achieved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- GANZHOU ZHANHAI NEW MATERIAL TECHNOLOGY CO LTD
- Filing Date
- 2026-03-13
- Publication Date
- 2026-05-26
AI Technical Summary
In existing methods for preparing rare earth oxide powders, the use of ammonia or ammonium bicarbonate as precipitants generates a large amount of ammonia nitrogen wastewater, resulting in high environmental treatment costs. Meanwhile, the rare earth oxide powders produced by using oxalic acid as a precipitant have a small specific surface area.
Oxalic acid was used as a precipitant, and a gas flow precipitation reactor was used for precipitation reaction. Gas disturbance promoted the exchange and mixing of substances to prepare rare earth oxalate precursors with smaller particle size. Subsequently, calcination was used to obtain rare earth oxide powder with a large specific surface area.
It effectively reduced the generation of ammonia nitrogen wastewater, increased the specific surface area of rare earth oxide powder, and achieved higher catalytic activity and adsorption capacity.
Smart Images
Figure CN121823636B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of rare earth oxide technology, and in particular to a method for preparing rare earth oxide powder. Background Technology
[0002] Rare earth oxides are an important class of functional materials with excellent optical, electrical, magnetic, and catalytic properties, and have been widely used in chemical, metallurgical, textile, and permanent magnet materials industries. Among them, rare earth oxides with large specific surface area have become core raw materials for high-end catalysis, adsorption separation, and other applications due to their greater number of surface active sites, stronger adsorption capacity, and higher catalytic activity.
[0003] Currently, the conventional industrial process for preparing rare earth oxide powders involves: adding a precipitant to a rare earth salt solution to generate a precursor slurry; subjecting the precursor slurry to solid-liquid separation, washing, and drying; calcining at high temperature; and then crushing to obtain the final rare earth oxide powder. In the precipitation process, ammonia or ammonium bicarbonate are typically used as precipitants, along with surfactants such as polyethylene glycol, to prepare rare earth oxide powders with large specific surface areas. For example, in related technologies, polyethylene glycol surfactants are added to rare earth nitrate or chloride solutions, followed by the dropwise addition of precipitants such as ammonia or ammonium bicarbonate to induce a precipitation reaction. The resulting precipitate, after washing, filtration, and calcination, yields a product with a specific surface area of up to 10 m². 2 / g or more of rare earth oxide powder.
[0004] However, the above methods use ammonia water and ammonium salts as precipitants, which generate a large amount of ammonia nitrogen wastewater during the reaction process. The high concentration of ammonia nitrogen pollutants and the difficulty in treatment make the wastewater treatment to meet discharge standards very costly, significantly increasing the overall production cost. Using oxalic acid as a precipitant can reduce the generation of ammonia nitrogen wastewater at its source and reduce environmental protection pressure. However, oxalic acid and rare earth ions easily form oxalate rare earth precipitates with large particle sizes and dense crystal structures, resulting in a smaller specific surface area of the rare earth oxide powder. Summary of the Invention
[0005] In view of this, the present invention provides a method for preparing rare earth oxide powder. The present invention uses oxalic acid as a precipitant and employs an airflow sedimentation reactor instead of a traditional mechanically stirred reactor for the precipitation reaction, thereby obtaining rare earth oxalate precursors with smaller particle sizes, and ultimately obtaining rare earth oxide powders with a large specific surface area.
[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:
[0007] A method for preparing rare earth oxide powder includes the following steps:
[0008] A gas flow sedimentation reactor is provided; the gas flow sedimentation reactor includes a reaction chamber, a raw material input pipe, and a gas input pipe; the reaction chamber includes a lower part and an upper part, the upper part having a double-layer sidewall structure, the double-layer sidewall structure including an outer sidewall and an inner sidewall, the outer sidewall being provided with a gas inlet, and the inner sidewall being provided with oblique gas holes; a slurry outlet is provided at the top of the upper part; an umbrella-shaped baffle is provided inside the reaction chamber; the raw material input pipe and the gas input pipe are connected, and the gas input pipe is connected to the lower part of the reaction chamber;
[0009] A first gas is introduced through a gas input pipe, and raw materials are introduced through a raw material input pipe. The first gas guides the raw materials into the reaction chamber for a precipitation reaction to obtain a reaction solution. The raw materials include a rare earth salt solution and an oxalic acid solution. The concentration of the oxalic acid solution is 2 to 3 times that of the rare earth salt solution. The flow rates of the rare earth salt solution and the oxalic acid solution are 200 L / h or higher. The pressure of the first gas is 0.5 MPa or higher.
[0010] The second gas is introduced through the gas inlet, and the reaction liquid is discharged from the slurry outlet under the swirling action of the second gas to obtain rare earth oxalate precursor.
[0011] The rare earth oxalate precursor was calcined to obtain rare earth oxide powder.
[0012] Preferably, the raw material input pipe is a sleeve, which includes an inner pipe and an outer pipe; the inner pipe is used to introduce a rare earth salt solution or an oxalic acid solution, and the outer pipe is used to introduce a rare earth salt solution or an oxalic acid solution, and the solutions introduced into the inner pipe and the outer pipe are different.
[0013] Preferably, the lower and upper parts of the reaction chamber are both cylindrical structures; the number of the oblique vents is 3 to 6, and the oblique vents are evenly distributed on the inner sidewall.
[0014] Preferably, the rare earth salt in the rare earth salt solution is one or more of rare earth hydrochloride and rare earth nitrate.
[0015] Preferably, the concentration of the rare earth salt solution is 0.1~0.5 mol / L.
[0016] Preferably, the flow rate of the rare earth salt solution and the oxalic acid solution is 200~500 L / h.
[0017] Preferably, both the first gas and the second gas are compressed air.
[0018] Preferably, the pressure of the first gas is 0.5~0.8MPa, and the pressure of the second gas is 0.5~0.8MPa.
[0019] Preferably, the calcination temperature is 700~900℃ and the time is 2~10h.
[0020] Preferably, the specific surface area of the rare earth oxide powder is 20 m². 2 / g or more.
[0021] This invention provides a method for preparing rare earth oxide powder, comprising the following steps: providing an airflow precipitation reactor; the airflow precipitation reactor includes a reaction chamber, a raw material input pipe, and a gas input pipe; the reaction chamber includes a lower part and an upper part, the upper part having a double-layer sidewall structure, the double-layer sidewall structure including an outer sidewall and an inner sidewall, the outer sidewall being provided with a gas inlet, and the inner sidewall being provided with oblique vents; a slurry outlet being provided at the top of the upper part; an umbrella-shaped baffle being provided inside the reaction chamber; the raw material input pipe and the gas input pipe being connected, and the gas input pipe being connected to the lower part of the reaction chamber; and then... A first gas is introduced through a gas input pipe, and raw materials are introduced through a raw material input pipe. The first gas guides the raw materials into the reaction chamber for precipitation reaction, yielding a reaction liquid. The raw materials include a rare earth salt solution and an oxalic acid solution. The concentration of the oxalic acid solution is 2 to 3 times that of the rare earth salt solution. The flow rates of the rare earth salt solution and the oxalic acid solution are above 200 L / h. The pressure of the first gas is above 0.5 MPa. A second gas is introduced through a gas inlet, and under the swirling action of the second gas, the reaction liquid is discharged from the slurry outlet to obtain a rare earth oxalate precursor. The rare earth oxalate precursor is calcined to obtain rare earth oxide powder. This invention uses oxalic acid as a precipitant and employs an airflow sedimentation reactor for the precipitation reaction. When gas is introduced into the liquid, a large number of bubbles are generated. The generation, movement, and collapse of these bubbles cause disturbances in the system. These disturbances promote the exchange and mixing of substances within the system, thus achieving a stirring and dispersing effect. The airflow sedimentation reactor used in this invention amplifies this effect, resulting in a more uniform and complete precipitation reaction. This leads to the production of rare earth oxalic acid precursors with smaller particle sizes, which can then be calcined to obtain rare earth oxide powders with a large specific surface area. The results of the examples show that the specific surface area of the rare earth oxide powders prepared by this invention reaches 20.586~52.499 m². 2 / g, and the particle size is uniform. Attached Figure Description
[0022] Figure 1 This is a schematic diagram of the airflow sedimentation reactor in this invention, where the top view is a top view, the lower left view is a front view, and the lower right view is a side view; Figure 1 In the middle: 1-reaction chamber, 2-gas input pipe, 3-inner layer pipe, 4-outer layer pipe, 5-gas inlet, 6-slanted gas hole, 7-slurry outlet, 8-umbrella-shaped baffle;
[0023] Figure 2This is a particle size distribution diagram of the yttrium oxide powder prepared in Example 1;
[0024] Figure 3 This is a particle size distribution diagram of the dysprosium oxide powder prepared in Example 2;
[0025] Figure 4 This is a particle size distribution diagram of the samarium oxide powder prepared in Example 3. Detailed Implementation
[0026] This invention provides a method for preparing rare earth oxide powder, comprising the following steps:
[0027] A gas flow sedimentation reactor is provided; the gas flow sedimentation reactor includes a reaction chamber 1, a raw material input pipe and a gas input pipe 2; the reaction chamber 1 includes a lower part and an upper part, the upper part has a double-layer sidewall structure, the double-layer sidewall structure includes an outer sidewall and an inner sidewall, the outer sidewall is provided with a gas inlet 5, and the inner sidewall is provided with an oblique gas hole 6; a slurry outlet 7 is provided at the top of the upper part; an umbrella-shaped baffle 8 is provided inside the reaction chamber 1; the raw material input pipe and the gas input pipe 2 are connected, and the gas input pipe 2 is connected to the lower part of the reaction chamber 1;
[0028] A first gas is introduced through gas input pipe 2, and raw materials are introduced through raw material input pipe. The first gas guides the raw materials into reaction chamber 1 for precipitation reaction to obtain a reaction solution. The raw materials include rare earth salt solution and oxalic acid solution. The concentration of the oxalic acid solution is 2 to 3 times that of the rare earth salt solution. The flow rate of the rare earth salt solution and oxalic acid solution is 200 L / h or higher. The pressure of the first gas is 0.5 MPa or higher.
[0029] The second gas is introduced through the gas inlet 5, and the reaction liquid is discharged from the slurry outlet 7 under the action of the swirling action of the second gas to obtain rare earth oxalate precursor.
[0030] The rare earth oxalate precursor was calcined to obtain rare earth oxide powder.
[0031] First, the structure of the airflow sedimentation reactor used in this invention will be described.
[0032] Figure 1 This is a schematic diagram of the airflow sedimentation reactor described in this invention, where the top view is a top view, the lower left view is a front view, and the lower right view is a side view. The following diagram is in conjunction with... Figure 1 Please provide a detailed explanation.
[0033] The airflow sedimentation reactor of the present invention includes a reaction chamber 1. In the present invention, the reaction chamber 1 includes a lower part and an upper part, both preferably cylindrical in shape; the upper part has a double-layer sidewall structure, comprising an outer sidewall and an inner sidewall. A gas inlet 5 is provided on the outer sidewall, and oblique air holes 6 are provided on the inner sidewall; the number of oblique air holes is preferably 3 to 6, more preferably 6, and the oblique air holes are evenly distributed on the inner sidewall; in a specific embodiment of the present invention, the six oblique air holes are arranged tangentially along the circumferential direction of the reaction chamber 1. A slurry outlet 7 is provided at the top of the upper part of the reaction chamber 1; an umbrella-shaped baffle 8 is provided inside the reaction chamber 1; the umbrella-shaped baffle 8 includes an umbrella part and a support rod, the support rod is fixed to the bottom of the reaction chamber 1, and the opening end of the umbrella part faces downward; the height of the inclined air hole 6 matches the position of the umbrella-shaped baffle 8, so that the gas entering through the inclined air hole 6 can form a vortex on the upper part of the umbrella-shaped baffle 8, thereby driving the reaction liquid to be discharged upward; specifically, the height of the inclined air hole 6 can be slightly higher than the opening end of the umbrella part.
[0034] The airflow sedimentation reactor of this invention includes a raw material input pipe and a gas input pipe 2, which are connected. The gas input pipe 2 is connected to the lower part of the reaction chamber 1. The raw material input pipe is preferably a sleeve, which includes an inner pipe 3 and an outer pipe 4. The inner pipe 3 is used to introduce a rare earth salt solution or an oxalic acid solution, and the outer pipe 4 is used to introduce the same solution. The solutions introduced into the inner pipe 3 and the outer pipe 4 are different. In a specific embodiment of this invention, it is preferable to introduce a rare earth salt solution into the inner pipe 3 and an oxalic acid solution into the outer pipe 4.
[0035] The preparation method of rare earth oxide powder is described in detail below.
[0036] In this invention, a first gas is introduced through a gas input pipe 2, and raw materials are introduced through a raw material input pipe. The first gas guides the raw materials into the reaction chamber 1 for precipitation reaction to obtain a reaction liquid. In this invention, the raw materials include a rare earth salt solution and an oxalic acid solution; the rare earth salt in the rare earth salt solution is preferably one or more of rare earth hydrochloride and rare earth nitrate; the rare earth salt is preferably one or more of yttrium salt, dysprosium salt, and samarium salt; in a specific embodiment of this invention, the rare earth salt is yttrium chloride, dysprosium nitrate, or thiocyanate; the concentration of the rare earth salt solution is preferably 0.1~0.5 mol / L, specifically 0.1, 0.2, or 0.5 mol / L; the concentration of the oxalic acid solution is 2~3 times the concentration of the rare earth salt solution, specifically 2, 2.5, or 3 times; the solvent for both the rare earth salt solution and the oxalic acid solution is water, which will not be elaborated further; the flow rate of the rare earth salt solution and the oxalic acid solution is 200 L / h or higher, preferably 200~500 L / h, specifically 200, 300, or 500 L / h; the flow rates of the rare earth salt solution and the oxalic acid solution are preferably the same.
[0037] In this invention, the first gas is preferably compressed air; the pressure of the first gas is 0.5 MPa or higher, preferably 0.5~0.8 MPa, specifically 0.5, 0.6 or 0.8 MPa. The first gas rapidly introduces the rare earth salt solution and oxalic acid solution into the reaction chamber 1, and the liquid flow swirls below the umbrella-shaped baffle 8, making the precipitation reaction more uniform and the reaction more complete.
[0038] In this invention, a second gas is introduced through gas inlet 5. Under the swirling action of the second gas, the reaction liquid is discharged from slurry outlet 7 to obtain rare earth oxalate precursor. In this invention, the second gas is preferably compressed air; the pressure of the second gas is preferably 0.5~0.8 MPa, specifically 0.5, 0.6, or 0.8 MPa. After the second gas is introduced through gas inlet 5, it enters the reaction chamber 1 through oblique gas holes 6, forming a vortex above the umbrella-shaped baffle 8 to discharge the reaction liquid from slurry outlet 7 into the reaction chamber 1. Preferably, the discharged reaction liquid is filtered, and the resulting solid product is washed and dried to obtain the rare earth oxalate precursor. In a specific embodiment of this invention, it is preferable to first introduce the first gas and the second gas, and then introduce the rare earth salt solution and the oxalic acid solution.
[0039] After obtaining the rare earth oxalate precursor, the present invention calcines the rare earth oxalate precursor to obtain rare earth oxide powder. In the present invention, the calcination temperature is preferably 700~900℃, specifically 710℃, 850℃ or 900℃; the calcination time is preferably 2~10h, specifically 2, 6 or 10h; the calcination can be carried out under air conditions.
[0040] In this invention, the specific surface area of the rare earth oxide powder is 20 m². 2 / g or more, preferably 20~60m 2 / g, in the examples 20.586, 28.137 or 52.449m 2 / g, median particle size D 50 The size ranges from 0.5 to 2 μm.
[0041] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.
[0042] The structure of the airflow sedimentation reactor used in the following embodiments is as follows: Figure 1 As shown, there are 6 oblique pores.
[0043] Example 1
[0044] A 0.1 mol / L yttrium chloride solution and a 0.3 mol / L oxalic acid solution were prepared. Compressed air at 0.8 MPa was first introduced into the gas inlet pipe 2 and gas inlet 5 of the gas flow sedimentation reactor. Then, the yttrium chloride solution was introduced through the inner pipe 3, and the oxalic acid solution was introduced through the outer pipe 4. The flow rates of both the yttrium chloride and oxalic acid solutions were 500 L / h. The reaction liquid flowing out of the slurry outlet 7 was connected to a filter. After separating the mother liquor water, the solid product was washed and dried to obtain a rare earth oxalate precursor. The rare earth oxalate precursor was calcined at 710 °C for 10 h to obtain yttrium oxide powder. The multi-point BET specific surface area of the yttrium oxide powder was determined to be 52.499 m² by nitrogen adsorption static volumetric method. 2 / g, median particle size D measured by laser particle size analyzer 50 It is 1.39 μm, D 10 The particle size is 0.68 μm, D 25 The particle size is 0.94 μm, D 75 The particle size is 2.07 μm, D 90 The particle size is 2.90 μm, D 99 The particle size is 4.62 μm, and the particle size distribution diagram is as follows. Figure 2 As shown.
[0045] Example 2
[0046] A 0.5 mol / L dysprosium nitrate solution and a 1 mol / L oxalic acid solution were prepared. Compressed air at 0.5 MPa was first introduced into the gas inlet pipe 2 and gas inlet 5 of the gas flow sedimentation reactor. Then, the dysprosium nitrate solution was introduced through the inner pipe 3, and the oxalic acid solution was introduced through the outer pipe 4. The flow rates of both the dysprosium nitrate solution and the oxalic acid solution were 200 L / h. The reaction liquid flowing out of the slurry outlet 7 was connected to a filter. After separating the mother liquor water, the solid product was washed and dried to obtain a rare earth oxalate precursor. The rare earth oxalate precursor was calcined at 900℃ for 2 h to obtain dysprosium oxide powder. The multi-point BET specific surface area of the dysprosium oxide powder was measured to be 20.586 m² by nitrogen adsorption static volumetric method. 2 / g, median particle size D measured by laser particle size analyzer 50 It is 0.57μm, D 10 The particle size is 0.435 μm, D 25 The particle size is 0.493 μm, D 75 The particle size is 0.645 μm, D 90 The particle size is 0.745 μm, D 99 The particle size is 0.819 μm, and the particle size distribution diagram is as follows. Figure 3 As shown.
[0047] Example 3
[0048] A 0.2 mol / L samarium chloride solution and a 0.5 mol / L oxalic acid solution were prepared. Compressed air at 0.6 MPa was first introduced into the gas inlet pipe 2 and gas inlet 5 of the gas flow sedimentation reactor. Then, the samarium chloride solution was introduced through the inner pipe 3, and the oxalic acid solution was introduced through the outer pipe 4. The flow rates of both samarium chloride and oxalic acid solutions were 300 L / h. The reaction liquid flowing out of the slurry outlet 7 was connected to a filter. After separating the mother liquor water, the solid product was washed and dried to obtain a rare earth oxalate precursor. The rare earth oxalate precursor was calcined at 900℃ for 2 h to obtain samarium oxide powder. The multi-point BET specific surface area of the samarium oxide powder was determined to be 38.137 m² by nitrogen adsorption static volumetric method. 2 / g, median particle size D measured by laser particle size analyzer 50 It is 1.93μm, D 10 The particle size is 0.82 μm, D 25 The particle size is 1.22 μm, D 75 The particle size is 2.98 μm, D 90 The particle size is 4.25 μm, D 99 The particle size is 6.98 μm, and the particle size distribution diagram is as follows. Figure 4 As shown.
[0049] Comparative Example 1
[0050] Other conditions were the same as in Example 1, except that compressed air was omitted, and a conventional stirred reactor was used with a stirring speed of 200 r / min to obtain yttrium oxide powder. The multi-point BET specific surface area of the yttrium oxide powder was determined to be 3.503 m² / g by nitrogen adsorption static volumetric method. 2 / g.
[0051] Comparative Example 2
[0052] Yttrium oxide powder was prepared under the same conditions as in Example 1, with an oxalic acid concentration of 0.5 mol / L. The multi-point BET specific surface area of the yttrium oxide powder was determined to be 8.588 m² / L by nitrogen adsorption static volumetric method. 2 / g.
[0053] Comparative Example 3
[0054] The compressed air pressure was changed to 0.3 MPa, and other conditions remained the same as in Example 1, yielding yttrium oxide powder. The multi-point BET specific surface area of the yttrium oxide powder was determined to be 6.533 m² using nitrogen adsorption static volumetric method. 2 / g.
[0055] Comparative Example 4
[0056] The flow rates of yttrium chloride solution and oxalic acid solution were changed to 100 L / h to prepare yttrium oxide powder. The multi-point BET specific surface area of the yttrium oxide powder was determined to be 9.681 m² by nitrogen adsorption static volumetric method. 2 / g.
[0057] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.
Claims
1. A method for preparing rare earth oxide powder, characterized in that, Includes the following steps: A gas flow sedimentation reactor is provided; the gas flow sedimentation reactor includes a reaction chamber, a raw material input pipe, and a gas input pipe; the reaction chamber includes a lower part and an upper part, the upper part having a double-layer sidewall structure, the double-layer sidewall structure including an outer sidewall and an inner sidewall, the outer sidewall being provided with a gas inlet, and the inner sidewall being provided with oblique vents; both the lower and upper parts of the reaction chamber are cylindrical structures; the number of oblique vents is 3 to 6, and the oblique vents are evenly distributed on the inner sidewall; a slurry outlet is provided at the top of the upper part; an umbrella-shaped baffle is provided inside the reaction chamber; the raw material input pipe and the gas input pipe are connected, and the gas input pipe is connected to the lower part of the reaction chamber; A first gas is introduced through a gas input pipe, and raw materials are introduced through a raw material input pipe. The first gas guides the raw materials into the reaction chamber for a precipitation reaction to obtain a reaction liquid. The raw materials include a rare earth salt solution and an oxalic acid solution. The concentration of the oxalic acid solution is 2 to 3 times that of the rare earth salt solution. The flow rate of the rare earth salt solution and the oxalic acid solution is 200 L / h or higher. The pressure of the first gas is 0.5 MPa or higher. The first gas guides the rare earth salt solution and the oxalic acid solution into the reaction chamber, and the liquid flow swirls below the umbrella-shaped baffle. The second gas is introduced through the gas inlet, and under the swirling action of the second gas, the reaction liquid is discharged from the slurry outlet to obtain rare earth oxalate precursor; after the second gas is introduced through the gas inlet, it enters the reaction chamber through the oblique gas hole, and forms a vortex above the umbrella-shaped baffle to discharge the reaction liquid from the slurry outlet to the reaction chamber. The rare earth oxalate precursor was calcined to obtain rare earth oxide powder.
2. The preparation method according to claim 1, characterized in that, The raw material input pipeline is a casing, which includes an inner pipeline and an outer pipeline. The inner pipeline is used to introduce rare earth salt solution or oxalic acid solution, and the outer pipeline is used to introduce rare earth salt solution or oxalic acid solution. The solutions introduced into the inner pipeline and the outer pipeline are different.
3. The preparation method according to claim 1, characterized in that, The rare earth salt in the rare earth salt solution is one or more of rare earth hydrochloride and rare earth nitrate.
4. The preparation method according to claim 1 or 3, characterized in that, The concentration of the rare earth salt solution is 0.1~0.5 mol / L.
5. The preparation method according to claim 1, characterized in that, The flow rates of the rare earth salt solution and oxalic acid solution are 200~500 L / h.
6. The preparation method according to claim 1, characterized in that, Both the first gas and the second gas are compressed air.
7. The preparation method according to claim 1 or 6, characterized in that, The pressure of the first gas is 0.5~0.8MPa, and the pressure of the second gas is 0.5~0.8MPa.
8. The preparation method according to claim 1, characterized in that, The calcination temperature is 700~900℃, and the time is 2~10h.
9. The preparation method according to claim 1, characterized in that, The specific surface area of the rare earth oxide powder is 20 m². 2 / g or more.