I. Overview
Ultra-fine inorganic powder materials are widely used in chemical, electronics, metallurgy, medicine, food, ceramics and other industries, and they have extremely high requirements on the fineness and purity of materials. Therefore, the finishing of these new materials as a high-tech, high value-added industry has developed rapidly in China in recent years, but in the production technology of the calcination of powder materials, there are currently many problems, such as sandwiching, over-burning, Uneven quality, high energy consumption, etc., which seriously restricts the quality of the final product of Zui and reduces the grade of the product. Therefore, there is an urgent need to develop a device that is completely suitable for superfine powder calcination at home and in the world.
Ultra-fine powder dynamic calcination heating technology can be widely used in the calcination preparation of various fine inorganic non-metallic materials. Including electric porcelain and chemical raw materials BaTiO3, LiMnO2 and LiCoO2 in electronic communication materials, pharmaceutical additives calcium apatite and preparation of catalysts in rare earth and petrochemical industries, even in military and other wider fields. At present, domestic calcining equipment in these fields is mostly imported or borrowed from other industries, but it does not match its raw materials and processes, and it is expensive and the use is not ideal.
Second, the requirements of inorganic non-metallic materials powder materials for calcination technology
For inorganic non-metallic ultrafine powders, magnetic powders, rare earth nano-materials, etc., the calcination characteristics of different varieties are also different, but in general, it has high requirements for uniformity of calcination and temperature control. The purity of the material is required to be above 99.9%. At the same time, some powders have corrosive gas during the calcination process (mostly acidic, such as calcium apatite, lithium cobalt oxide, etc.), and some require atmospheric protection, even high pressure or vacuum heating.
Based on the above factors, when selecting the design, its structure and material should meet the requirements of these characteristics, achieving the versatility of Zui.
2.1 furnace type selection
2.1.1 Heating and heating mode: The calcination of high-grade ultra-fine inorganic powder materials requires high purity, and should avoid the pollution of impurities such as flue gas, which has low relative yield, temperature, atmosphere and pressure. High requirements and other characteristics. At the same time, such manufacturers are mostly in the city, and have high requirements for environmental protection. Therefore, it is feasible to use the electric heating method without pollution and low noise.
In order to overcome the uneven heating caused by the static accumulation heating of the powder in the shuttle kiln and the push-plate kiln, it is prone to defects such as pinning and over-burning. The dynamic furnace type selects the metal cylinder to be heated by rotation, and the scraper is provided in the cylinder body. The material is fully stirred under the action of the rotation of the cylinder and the scraper, and the material moves radially and axially in the cylinder to achieve full, uniform and stable heating. Because the equipment is externally heated by the cylinder, it avoids the interference of the flue gas and waste residue generated by the fuels such as coal, diesel, liquefied gas and natural gas.
2.1.2 Production mode: In order to ensure the reliable control of temperature, pressure and atmosphere, batch production can be used. The material is added from the furnace mouth, the furnace door is closed, and the electric heating is performed. The unloading is carried out by oblique discharge, and the material is completely discharged smoothly by the action of the scraper and the material's own weight; when the pressure atmosphere is not so strict and the output is large, continuous heating and rotary production can be adopted, depending on the furnace body. The inclination of itself is used to ensure the firing cycle, and such a furnace type is relatively inferior to the gap type in sealing effect.
2.1.3 Mechanical rotation: The metal cylinder rotates by chain, gear or belt, and the rolling bearing supports and rotates at both ends of the cylinder.
2.2. Development of rotary resistance furnace
2.2.1 Design technical indicators and parameters Technical parameters Production capacity 50-450kg / times heating mode electric heating (outer Tube heating)
Rated power 30-200kw±10%
Zui high use temperature 1050 ° C
Temperature adjustment range 40-1050 °C
Temperature control accuracy ≤ ± 5 ° C
Work area furnace temperature uniformity ≤ ± 8 ° C
Control mode PID control rotation speed 0-3 rev / min (frequency control)
Third, the structural analysis of the rotary resistance furnace
1. Heating system
Furnace structure: The core of the furnace type is a metal rotary cylinder, which functions to carry materials and heat the materials. In order to obtain a continuous uniform heat of the metal cylinder, a special lightweight circular refractory module is wrapped at a distance from the outside, and a resistance wire is embedded in the side of the metal cylinder. After the electric resistance wire is heated and heated, the cylinder body is mainly heated by radiation, and the cylinder body is rapidly heated, and the inner material is heated and calcined by means of radiation and convection heat transfer.
Selection of electric heating element: Since the furnace body zui has a high use temperature of 10500C, since the heating surface of the furnace body is large and cylindrical, the metal electric heating element, such as iron-chromium-aluminum alloy electric heating wire, is selected, and the high temperature of the furnace is 1300-14000C. The resistivity is 1.4 ohms / mm 2 / m, which is higher, so that its position in the furnace is small and the amount is relatively small. However, the strength of the iron-chromium-aluminum alloy is not too high, and the tensile strength is 2 kg/mm ​​2 at 10000 C. If the over-burning is easily deformed and collapsed, it can be overcome in the firing control and the structure of the furnace. At the same time, the alloy has poor processability, is hard and brittle, and has poor weldability. This shortcoming can be overcome by changes in field installation technology.
Resistance furnace power determination: Calculated by full-load operation of resistance furnace (zui large charge quantity 450kg/time, zui high use temperature 10500C), according to heat balance equation ∑Q income=∑Q expenditure, estimate electric heater to release heat Q electricity= 564.48 × 106 joules.
Theoretical power N = 10-6KW = 78.4KW. Considering that this rotary kiln is produced periodically, the safety factor is 1.5.
The actual power N is = 1.5 × N rational = 117.6 KW, taking 120 KW.
N = 1.5 × N = 120.9 KW. Take 120KW.
Zui finally established the resistance furnace power of 120KW.
Connection of electric heating body: It adopts three-phase and star connection, divided into three groups, each group has a power of 40KW. The establishment of the size of the electric heater:
Through formula
To calculate, d is the diameter of the electric heating body, L is the length,
The zui finally establishes d=5.5mm, L=17.5m, and the electric heating body is wound into a spiral and evenly distributed in the furnace.
For the selection of metal liner materials, in order to ensure the normal operation of the equipment at 10500C, the choice of metal liner material is very important, and must be carried out from the properties of high temperature strength, thermal stability, chemical resistance, high temperature volume stability of the material. Assessment. At the same time, in the machining process, especially in the welding process, the interface and the weld must be strictly required and specially treated to make it stable at high temperatures. The furnace selects a kind of heat-resistant alloy material, and its various properties are relatively easy to meet the requirements. After several performance indexes tests in the laboratory, the effect is relatively ideal.
Manufacture of refractory material: The refractory material used in the furnace is cylindrical, and its inner diameter is up to 1000mm. Through the joint research with the refractory fiber factory, the special-shaped parts such as the second largest size are produced in China for the first time, and the installation is successful. design effect.
2. Mechanical transmission:
Rotation of the metal cylinder: In order to ensure sufficient and uniform heating of the powder, the rotation of the cylinder is necessary. It is determined that the rotation speed is adjustable from 0 to 3 rpm. The adjustable speed motor is selected and the frequency converter is added in the control cabinet.
Inclined furnace device: In order to ensure smooth discharge, the furnace body must be tilted to a certain angle. Its power adopts a new type of electric device, which overcomes the shortcomings of complicated hydraulic equipment, many operating procedures and easy oil leakage.
The furnace body is tilted as a whole, and the total weight after feeding is 3 tons. According to the principle of leverage, the push rod should provide thrust, considering that the metal cylinder needs to rotate when unloading. The resulting torque was an electric push rod with a thrust of 3.5 tons.
3. Control System
Electrical control is an indispensable control part of the electrical equipment of this equipment. It bears the automatic control of the operation and protection of various actions of the rotary furnace, and can automatically control and protect the heating curve of the furnace electric heating body. All the operation and control function buttons of the electric control cabinet operation panel are corresponding, and there are prompt light displays to realize the operation at a glance.
The electric control cabinet shall have the following functions: control the rotation of the heated metal cylinder, the rotation speed and the positive and negative rotation; control the electric heating body to meet the requirements of the firing curve; and operate and limit the control of the jacking and falling of the electric push rod.
4. Trial run
After the equipment is installed, after commissioning and drying, the test can be carried out. Considering that the furnace body should have strong adaptability, a variety of inorganic powders can be calcined. Different powders have different requirements for the heating curve, atmosphere and pressure of the calcination. After the Zui, the equipment should be unfilled. Carry out the test at no-load operation. The furnace and wall temperatures were measured using a thermocouple in the center of the furnace and a high temperature optical thermometer. After about 90 minutes of heating, the furnace can reach 1000 °C.
In the insulation stage of 1000 °C, the temperature of the metal furnace wall and the center of the furnace is basically the same, that is, the uniformity of the furnace temperature in the working area can be controlled within ±5 °C, so that the design requirements can be realized. At the same time, it is found in the operation that the heating and heat preservation of the furnace body can be easily controlled by adjusting the input power and the electric heating body, and the firing curve of the materials of different properties can be easily adjusted accordingly. The equipment should be specially designed for the furnace door to meet the atmospheric and pressure requirements for the special inorganic powder during calcination.
5. Comparison of performance of similar equipment at home and abroad
At present, there is no furnace type at the same time as the furnace structure at home and abroad. Considering that many domestic manufacturers use electrothermal push-plate kiln to complete the calcination of inorganic non-metal powder, electronic powder, magnetic powder, rare earth nano and other materials. A simple comparison of the two devices is as follows:
Furnace type rotary resistance furnace electric heating plate kiln production mode intermittent continuous power 120kw 240-300kw
Zui high use temperature 1050 ° C 1000 ° C
Work area furnace temperature uniformity ≤ ± 8 ° C ≤ ± 10 ° C
The output is 1200kg/day ≥1000kg/day. Whether the kiln furniture is used, there is no material in the furnace, the radial axial movement, the stirring and stacking, the static calcination product quality is complete, the uniform is easy to appear over-burning, sandwiching, etc.
As can be seen from the above table, the rotary resistance furnace can be set to batch production or continuous production, so the production method and time schedule are flexible, the production organization is also easy, and the maintenance is convenient. The electric heating plate kiln is difficult to overcome in the production of materials such as over-burning and sandwiching, and it also consumes certain kiln furniture. These problems no longer exist in the rotary kiln production, and at the same time, temperature, pressure and atmosphere. The rotary resistance furnace is relatively easy to protect. Therefore, the rotary resistance furnace has incomparable advantages over the push-plate kiln, and has the prospect of popularization and application.
6. Energy-saving performance Compared with the performance of the same output calcining equipment, it has the following characteristics in terms of energy saving:
To produce 800kg per day
As a comparison of the continuous furnace type, due to the turning, spreading and flowing of the powder in the dynamic combustion technology, especially for the powder with a smaller particle size, it is easier to reach the required reaction temperature in a short time and Gas emission, its heating cycle is doubled compared with static enthalpy loading. Compared with its loading capacity and heating power, the energy saving effect will reach more than 65%. These indicators are of great significance for the current power shortage phenomenon in China.
7, application prospects
In recent years, as ultrafine powders and nano-sized powder materials are widely used in ceramics and even wider fields, the calcination of ultrafine powder materials is particularly important, and it can be applied to special ceramic raw materials, electric porcelain, and chemical industry. Raw materials and electronic device raw materials, even the calcination preparation of medicines and food additives, especially for the powder deep processing products with higher requirements at present, the dynamic heating technology is selected to finely control, and the precise atmosphere environment control is realized, which is to realize the production of high-grade powder products. The feasible way.
Fourth, the conclusion
As mentioned above, in the calcination of high-grade inorganic non-metallic ultrafine powders, there is no completely suitable furnace type in China. They are all modified by the equipment of other industries, and they are put into use. The defects overcome have seriously affected the quality of the products. Therefore, the special equipment designed for the calcination of ultrafine inorganic powders overcomes the problems that have been exposed in the production of other kiln types, so its application prospect is very broad. Although the rotary calcining equipment has been separated from the calcination of powder in the traditional industry, it is difficult for the new industry and the new market to be promoted in the early stage. In order to open such a gap in the high-tech field and widely use it, the use of professional technology It will carry out sufficient material argumentation and structural design, and will be continuously modified and improved in the application, which will play a very important role in the equipment grade and production level of the deep processing products of the powder industry.
Ultra-fine inorganic powder materials are widely used in chemical, electronics, metallurgy, medicine, food, ceramics and other industries, and they have extremely high requirements on the fineness and purity of materials. Therefore, the finishing of these new materials as a high-tech, high value-added industry has developed rapidly in China in recent years, but in the production technology of the calcination of powder materials, there are currently many problems, such as sandwiching, over-burning, Uneven quality, high energy consumption, etc., which seriously restricts the quality of the final product of Zui and reduces the grade of the product. Therefore, there is an urgent need to develop a device that is completely suitable for superfine powder calcination at home and in the world.
Ultra-fine powder dynamic calcination heating technology can be widely used in the calcination preparation of various fine inorganic non-metallic materials. Including electric porcelain and chemical raw materials BaTiO3, LiMnO2 and LiCoO2 in electronic communication materials, pharmaceutical additives calcium apatite and preparation of catalysts in rare earth and petrochemical industries, even in military and other wider fields. At present, domestic calcining equipment in these fields is mostly imported or borrowed from other industries, but it does not match its raw materials and processes, and it is expensive and the use is not ideal.
Second, the requirements of inorganic non-metallic materials powder materials for calcination technology
For inorganic non-metallic ultrafine powders, magnetic powders, rare earth nano-materials, etc., the calcination characteristics of different varieties are also different, but in general, it has high requirements for uniformity of calcination and temperature control. The purity of the material is required to be above 99.9%. At the same time, some powders have corrosive gas during the calcination process (mostly acidic, such as calcium apatite, lithium cobalt oxide, etc.), and some require atmospheric protection, even high pressure or vacuum heating.
Based on the above factors, when selecting the design, its structure and material should meet the requirements of these characteristics, achieving the versatility of Zui.
2.1 furnace type selection
2.1.1 Heating and heating mode: The calcination of high-grade ultra-fine inorganic powder materials requires high purity, and should avoid the pollution of impurities such as flue gas, which has low relative yield, temperature, atmosphere and pressure. High requirements and other characteristics. At the same time, such manufacturers are mostly in the city, and have high requirements for environmental protection. Therefore, it is feasible to use the electric heating method without pollution and low noise.
In order to overcome the uneven heating caused by the static accumulation heating of the powder in the shuttle kiln and the push-plate kiln, it is prone to defects such as pinning and over-burning. The dynamic furnace type selects the metal cylinder to be heated by rotation, and the scraper is provided in the cylinder body. The material is fully stirred under the action of the rotation of the cylinder and the scraper, and the material moves radially and axially in the cylinder to achieve full, uniform and stable heating. Because the equipment is externally heated by the cylinder, it avoids the interference of the flue gas and waste residue generated by the fuels such as coal, diesel, liquefied gas and natural gas.
2.1.2 Production mode: In order to ensure the reliable control of temperature, pressure and atmosphere, batch production can be used. The material is added from the furnace mouth, the furnace door is closed, and the electric heating is performed. The unloading is carried out by oblique discharge, and the material is completely discharged smoothly by the action of the scraper and the material's own weight; when the pressure atmosphere is not so strict and the output is large, continuous heating and rotary production can be adopted, depending on the furnace body. The inclination of itself is used to ensure the firing cycle, and such a furnace type is relatively inferior to the gap type in sealing effect.
2.1.3 Mechanical rotation: The metal cylinder rotates by chain, gear or belt, and the rolling bearing supports and rotates at both ends of the cylinder.
2.2. Development of rotary resistance furnace
2.2.1 Design technical indicators and parameters Technical parameters Production capacity 50-450kg / times heating mode electric heating (outer Tube heating)
Rated power 30-200kw±10%
Zui high use temperature 1050 ° C
Temperature adjustment range 40-1050 °C
Temperature control accuracy ≤ ± 5 ° C
Work area furnace temperature uniformity ≤ ± 8 ° C
Control mode PID control rotation speed 0-3 rev / min (frequency control)
Third, the structural analysis of the rotary resistance furnace
1. Heating system
Furnace structure: The core of the furnace type is a metal rotary cylinder, which functions to carry materials and heat the materials. In order to obtain a continuous uniform heat of the metal cylinder, a special lightweight circular refractory module is wrapped at a distance from the outside, and a resistance wire is embedded in the side of the metal cylinder. After the electric resistance wire is heated and heated, the cylinder body is mainly heated by radiation, and the cylinder body is rapidly heated, and the inner material is heated and calcined by means of radiation and convection heat transfer.
Selection of electric heating element: Since the furnace body zui has a high use temperature of 10500C, since the heating surface of the furnace body is large and cylindrical, the metal electric heating element, such as iron-chromium-aluminum alloy electric heating wire, is selected, and the high temperature of the furnace is 1300-14000C. The resistivity is 1.4 ohms / mm 2 / m, which is higher, so that its position in the furnace is small and the amount is relatively small. However, the strength of the iron-chromium-aluminum alloy is not too high, and the tensile strength is 2 kg/mm ​​2 at 10000 C. If the over-burning is easily deformed and collapsed, it can be overcome in the firing control and the structure of the furnace. At the same time, the alloy has poor processability, is hard and brittle, and has poor weldability. This shortcoming can be overcome by changes in field installation technology.
Resistance furnace power determination: Calculated by full-load operation of resistance furnace (zui large charge quantity 450kg/time, zui high use temperature 10500C), according to heat balance equation ∑Q income=∑Q expenditure, estimate electric heater to release heat Q electricity= 564.48 × 106 joules.
Theoretical power N = 10-6KW = 78.4KW. Considering that this rotary kiln is produced periodically, the safety factor is 1.5.
The actual power N is = 1.5 × N rational = 117.6 KW, taking 120 KW.
N = 1.5 × N = 120.9 KW. Take 120KW.
Zui finally established the resistance furnace power of 120KW.
Connection of electric heating body: It adopts three-phase and star connection, divided into three groups, each group has a power of 40KW. The establishment of the size of the electric heater:
Through formula
To calculate, d is the diameter of the electric heating body, L is the length,
The zui finally establishes d=5.5mm, L=17.5m, and the electric heating body is wound into a spiral and evenly distributed in the furnace.
For the selection of metal liner materials, in order to ensure the normal operation of the equipment at 10500C, the choice of metal liner material is very important, and must be carried out from the properties of high temperature strength, thermal stability, chemical resistance, high temperature volume stability of the material. Assessment. At the same time, in the machining process, especially in the welding process, the interface and the weld must be strictly required and specially treated to make it stable at high temperatures. The furnace selects a kind of heat-resistant alloy material, and its various properties are relatively easy to meet the requirements. After several performance indexes tests in the laboratory, the effect is relatively ideal.
Manufacture of refractory material: The refractory material used in the furnace is cylindrical, and its inner diameter is up to 1000mm. Through the joint research with the refractory fiber factory, the special-shaped parts such as the second largest size are produced in China for the first time, and the installation is successful. design effect.
2. Mechanical transmission:
Rotation of the metal cylinder: In order to ensure sufficient and uniform heating of the powder, the rotation of the cylinder is necessary. It is determined that the rotation speed is adjustable from 0 to 3 rpm. The adjustable speed motor is selected and the frequency converter is added in the control cabinet.
Inclined furnace device: In order to ensure smooth discharge, the furnace body must be tilted to a certain angle. Its power adopts a new type of electric device, which overcomes the shortcomings of complicated hydraulic equipment, many operating procedures and easy oil leakage.
The furnace body is tilted as a whole, and the total weight after feeding is 3 tons. According to the principle of leverage, the push rod should provide thrust, considering that the metal cylinder needs to rotate when unloading. The resulting torque was an electric push rod with a thrust of 3.5 tons.
3. Control System
Electrical control is an indispensable control part of the electrical equipment of this equipment. It bears the automatic control of the operation and protection of various actions of the rotary furnace, and can automatically control and protect the heating curve of the furnace electric heating body. All the operation and control function buttons of the electric control cabinet operation panel are corresponding, and there are prompt light displays to realize the operation at a glance.
The electric control cabinet shall have the following functions: control the rotation of the heated metal cylinder, the rotation speed and the positive and negative rotation; control the electric heating body to meet the requirements of the firing curve; and operate and limit the control of the jacking and falling of the electric push rod.
4. Trial run
After the equipment is installed, after commissioning and drying, the test can be carried out. Considering that the furnace body should have strong adaptability, a variety of inorganic powders can be calcined. Different powders have different requirements for the heating curve, atmosphere and pressure of the calcination. After the Zui, the equipment should be unfilled. Carry out the test at no-load operation. The furnace and wall temperatures were measured using a thermocouple in the center of the furnace and a high temperature optical thermometer. After about 90 minutes of heating, the furnace can reach 1000 °C.
In the insulation stage of 1000 °C, the temperature of the metal furnace wall and the center of the furnace is basically the same, that is, the uniformity of the furnace temperature in the working area can be controlled within ±5 °C, so that the design requirements can be realized. At the same time, it is found in the operation that the heating and heat preservation of the furnace body can be easily controlled by adjusting the input power and the electric heating body, and the firing curve of the materials of different properties can be easily adjusted accordingly. The equipment should be specially designed for the furnace door to meet the atmospheric and pressure requirements for the special inorganic powder during calcination.
5. Comparison of performance of similar equipment at home and abroad
At present, there is no furnace type at the same time as the furnace structure at home and abroad. Considering that many domestic manufacturers use electrothermal push-plate kiln to complete the calcination of inorganic non-metal powder, electronic powder, magnetic powder, rare earth nano and other materials. A simple comparison of the two devices is as follows:
Furnace type rotary resistance furnace electric heating plate kiln production mode intermittent continuous power 120kw 240-300kw
Zui high use temperature 1050 ° C 1000 ° C
Work area furnace temperature uniformity ≤ ± 8 ° C ≤ ± 10 ° C
The output is 1200kg/day ≥1000kg/day. Whether the kiln furniture is used, there is no material in the furnace, the radial axial movement, the stirring and stacking, the static calcination product quality is complete, the uniform is easy to appear over-burning, sandwiching, etc.
As can be seen from the above table, the rotary resistance furnace can be set to batch production or continuous production, so the production method and time schedule are flexible, the production organization is also easy, and the maintenance is convenient. The electric heating plate kiln is difficult to overcome in the production of materials such as over-burning and sandwiching, and it also consumes certain kiln furniture. These problems no longer exist in the rotary kiln production, and at the same time, temperature, pressure and atmosphere. The rotary resistance furnace is relatively easy to protect. Therefore, the rotary resistance furnace has incomparable advantages over the push-plate kiln, and has the prospect of popularization and application.
6. Energy-saving performance Compared with the performance of the same output calcining equipment, it has the following characteristics in terms of energy saving:
To produce 800kg per day
As a comparison of the continuous furnace type, due to the turning, spreading and flowing of the powder in the dynamic combustion technology, especially for the powder with a smaller particle size, it is easier to reach the required reaction temperature in a short time and Gas emission, its heating cycle is doubled compared with static enthalpy loading. Compared with its loading capacity and heating power, the energy saving effect will reach more than 65%. These indicators are of great significance for the current power shortage phenomenon in China.
7, application prospects
In recent years, as ultrafine powders and nano-sized powder materials are widely used in ceramics and even wider fields, the calcination of ultrafine powder materials is particularly important, and it can be applied to special ceramic raw materials, electric porcelain, and chemical industry. Raw materials and electronic device raw materials, even the calcination preparation of medicines and food additives, especially for the powder deep processing products with higher requirements at present, the dynamic heating technology is selected to finely control, and the precise atmosphere environment control is realized, which is to realize the production of high-grade powder products. The feasible way.
Fourth, the conclusion
As mentioned above, in the calcination of high-grade inorganic non-metallic ultrafine powders, there is no completely suitable furnace type in China. They are all modified by the equipment of other industries, and they are put into use. The defects overcome have seriously affected the quality of the products. Therefore, the special equipment designed for the calcination of ultrafine inorganic powders overcomes the problems that have been exposed in the production of other kiln types, so its application prospect is very broad. Although the rotary calcining equipment has been separated from the calcination of powder in the traditional industry, it is difficult for the new industry and the new market to be promoted in the early stage. In order to open such a gap in the high-tech field and widely use it, the use of professional technology It will carry out sufficient material argumentation and structural design, and will be continuously modified and improved in the application, which will play a very important role in the equipment grade and production level of the deep processing products of the powder industry.
Dental Examination Kits are tools that provide dental treatment. The dental examination kits include dental mirror, dental probe, Tweezers, etc.
The dental examination kits are used to examine,treat, restore and remove teeth. Different components could be offered as customers requirements.
The dental examination kits are for single use and disposable use, which could prevent the infect from different patients.It is clean and safe and easily to operate.
Dental Examination Kits
Dental Examination Kits,Disposable Dental Examination Kits,Dental Examination Instruments,Dental Examination Set
Hebei Orient Imp. & Exp. Co. Ltd. , https://www.orientmedcare.com