Huang Luoxing
(Jiangyin Changjiang Gas Separation Equipment Co., Ltd., Jiangyin, Jiangsu 214401)
1 Preamble
The sintering of high-performance MnZn ferrite (high μi and power ferrite) in magnetic materials and the fine pulverization in the production of rare-earth permanent magnet alloys such as NdFeB processes all require high-purity nitrogen for protection against oxidation of the magnets (powder) during the process.
As we all know, MnZn ferrite is generated by Fe, Mn, Zn oxides in high-temperature sintering solid-phase reaction, Mn, Fe is very easy to change the valence, at different temperatures and atmospheres (partial pressure of oxygen) under the conditions of the valence of Mn, Fe is different, in order to make the MnZn ferrite to achieve the required magnetic properties, it must be ensured that the metal ions are in a specific valence and suitable crystal structure, in addition to a suitable crystal structure, the MnZn ferrite can be used for the production of rare-earth permanent magnet alloys such as NdFeB. To make MnZn ferrite achieve the required magnetic properties, it is necessary to ensure that the metal ions are in a specific valence state and suitable crystal structure, in addition to the appropriate formula, the key is to be sintered under the conditions of the balanced atmosphere, and the protective gas is the implementation of the balanced atmosphere of one of the basic conditions of the sintering material. Nitrogen kiln cleaning bin of nitrogen in the oxygen content of hope in the 50 × 10-6 below, so the purity of nitrogen is required in 99.995% or more, and the amount of impurity gas (O2, H2) has a more stringent restrictions: an annual output of about 1,000 tons of MnZn ferrite production line, the general consumption of nitrogen in 100 ~ 120Nm3 / h.
NdFeB and other rare earth permanent magnet alloy of rare earth metals even at room temperature conditions, but also easy to oxidize and lead to rare earth permanent magnet alloy performance reduction, excessive oxidation will make the alloy performance greatly deteriorated. Because 1 part of oxygen can make 6 parts (weight) of the rare earth elements oxidized and lose their role. NdFeB as an example, to produce N45 magnets must ensure that the production process environment of the oxygen content of ≤ 0.01%, the oxygen content of the final product is 0.09 ± 0.02% (mass fraction) [1]. If you use nitrogen as the process environment gas its nitrogen purity must be 99.99% or more.
At present, domestic and foreign large-scale industrial production of rare earth permanent magnet alloy system (fine) powder process are used in a kind of "nitrogen gas flow mill" equipment, it is the use of high-speed nitrogen flow to drive the materials collide with each other to achieve the effect of grinding, the resulting particle size of the powder required to be in the 3 ~ 5μm, there is a large surface area, easy to oxidize, so the nitrogen must be high purity grade. Therefore, the nitrogen gas must be of high purity, and there are strict requirements on the amount of O2, H2 and other impurities. Annual production of about 100 tons of NdFeB production line usually consumes about 60Nm3 / h of high-purity nitrogen.
2 Magnetic materials production with nitrogen technical requirements
From the use of nitrogen, there are four basic parameters need to pay attention to, namely, purity, flow, dew point and pressure, parameter values vary according to the use of different, supply and demand for the *** knowledge, it is necessary to first briefly introduce the concept of the four technical parameters.
2.1 Purity
Purity is an important technical parameter of nitrogen, according to the national standard of nitrogen purity is divided into industrial nitrogen, pure nitrogen and high purity nitrogen three levels, their purity were 99.5% (O2 ≤ 0.5%), 99.99% (O2 ≤ 0.01%) and 99.999% (O2 ≤ 0.001%).
2.2 Flow Rate
It refers to the gas flow process, the amount of gas passing through any cross-section in a unit of time. There are two ways to express the flow rate, namely, volume flow rate and mass flow rate. The former refers to any cross-section of the gas volume through the pipeline, the latter for the passage of the gas mass, in the gas industry are generally used in the volume of flow to m3 / h (or L / h) for the unit of measurement. Because of the gas volume and temperature, pressure and humidity, for easy comparison of the volume flow usually refers to the standard state (temperature of 20 ℃, pressure of 0.101MPa, relative humidity of 65%), the flow rate of this time to Nm3 / h for the unit, "N" that is, the standard state.
2.3 Pressure
Pressure and absolute pressure pressure points, engineering greater than atmospheric pressure and atmospheric pressure as the starting point (zero) to indicate the pressure is called "gauge pressure", the pressure is zero is called "absolute pressure", in the gas industry, if there is no pressure, the pressure of the gas industry, the pressure of the gas industry, the pressure of the gas industry, the pressure of the gas industry, the pressure of the gas industry is called "absolute pressure", and the pressure of the gas industry. In the gas industry, without special instructions, the pressure refers to the gauge pressure, its unit is MPa, in many calculations, often with the "absolute pressure", the relationship between them is as follows:
Absolute pressure = gauge pressure + atmospheric pressure
2.4 Dewpoint
It refers to the water in the gas from the saturated water vapor into saturated water vapor. saturated water vapor into saturated water vapor temperature. When the unsaturated water vapor into saturated water vapor, there is a very fine dewdrops appear, dewdrops appear when the temperature is called the "dew point", which characterizes the water content in the gas, the lower the dew point, that is, the lower the water content in the gas, the gas is drier. The lower the dew point, the less water is in the gas and the drier the gas is. The dew point is related to pressure, so there are atmospheric pressure dew point (atmospheric pressure dew point) and dew point under pressure. Atmospheric pressure dew point refers to the condensation temperature of water under atmospheric pressure, while the dew point under pressure refers to the condensation temperature of water under the pressure, the two have a conversion relationship (you can check the conversion table), such as pressure 0.7MPa when the pressure dew point of 5 ℃, then the corresponding atmospheric pressure (0.101MPa) dew point is -20 ℃. In the gas industry, if there is no special instructions, the dew point referred to are atmospheric pressure dew point.
The above introduction to the significance of several parameters of the gas, magnetic materials with nitrogen can be based on its process requirements, the parameters of the specific indicators:
(1) nitrogen flow. Flow rate is determined mainly based on the type of nitrogen equipment, the number of equipment and production process. MnZn ferrite sintering with nitrogen kiln, for example, long kiln and short kiln, single-plate kiln and double-plate kiln, densification sintering and not densification sintering, adjusting the level of kiln, etc. There are large differences in the amount of gas. In addition, in determining the amount of nitrogen, there should be an appropriate margin.
(2) nitrogen purity. According to the production process to determine, for magnetic materials generally require high-purity nitrogen - purity ≥ 99.995%, O2 and H2 content in a certain range.
(3) Pressure. According to the equipment and process to determine the minimum pressure value of its nitrogen, and then use the regulator to adjust to the process required pressure.
(4) Dew point. Water gas is also an oxidizing gas, of course, should be limited. For magnetic materials with nitrogen, usually as long as the dew point ≤ -60 ℃, that is, the nitrogen in the water content ≤ 10.7 × 10-6 can meet the process requirements.
3 high-purity nitrogen source
To meet the use of magnetic materials, high-purity nitrogen source has the following three types to choose from:
3.1 Bottled nitrogen
Cylinder capacity of 40L, rated pressure 15MPa, the full storage of 6m3, according to the user's needs are different, bottled nitrogen has a purity of 99.5%, 99.99% and 99.999% of the points! The purity of nitrogen for magnetic materials is ≥99.995%, which is the product of deep-cooling air separation, and it is filled by membrane pressure machine. According to the regulations, nitrogen cylinders are coated with black paint and have yellow paint "Nitrogen" logo, in addition, there is a label indicating its "purity" and qualified inspection. Due to the different supply and demand situation, the price of bottled high-purity nitrogen varies greatly from 18 to 90 yuan / bottle, that is, the price of nitrogen is 3 to 15 yuan / m3.
3.2 Liquid Nitrogen
Liquid Nitrogen is the product of the deep-cooling of air separation of nitrogen, in the standard state, 1m3 liquid nitrogen can be gasified into 643m3 of nitrogen, but the use of the actual utilization of the general in the 95% up and down, that is, 1m3 liquid nitrogen can be utilized in practice. The actual utilization rate of 1m3 liquid nitrogen is about 610m3, and the average price of liquid nitrogen in the current market is about 1,000 yuan/m3, and the unit price of nitrogen is 1.67 yuan/m3.
When using liquid nitrogen, the user must be equipped with liquid nitrogen tanks and flow corresponding to the gasifier and pressure corresponding to the pressure regulator, etc.
Liquid nitrogen has high purity and stable quality. Liquid nitrogen is of high purity, stable quality, generally guaranteed supply and easy to use.
3.3 On-site nitrogen production
On-site nitrogen production refers to the nitrogen users to purchase their own nitrogen equipment to produce nitrogen, at present at home and abroad, there are three types of industrial-scale nitrogen production: namely, deep-freezing air nitrogen, variable pressure adsorption nitrogen and membrane separation nitrogen.
3.3.1 Deep-cooling air separation of nitrogen
It is a traditional air separation technology, which has a history of more than ninety years, and is characterized by large gas production, high purity of nitrogen products, which can be directly applied to magnetic materials without re-purification; however, it is a complex process, covers a large area, has a high cost of infrastructure, and requires specialized maintenance force, more operators, and slow gas production (18-24h). It is suitable for large-scale industrial nitrogen production, nitrogen cost of 0.7 yuan / m3 or so.
3.3.2 Combination of Pressure Swing Adsorption and Nitrogen Purification Device
Variable Pressure Adsorption (Pressure Swing Adsorption, PSA for short) gas separation technology is an important branch of non-low-temperature gas separation technology, which is the result of people's long-term efforts to find a simpler method of air separation than the deep cooling method. In the seventies, West Germany Essen Mining Company successfully developed carbon molecular sieve, which paved the way for the industrialization of PSA air separation of nitrogen. Over the past thirty years the technology has developed rapidly and become more mature, and has become a strong competitor of deep-cooling air separation in the field of small and medium-sized nitrogen production.
Variable pressure adsorption nitrogen production is based on air as raw material, using carbon molecular sieve as adsorbent, using carbon molecular sieve on the oxygen and nitrogen selective adsorption characteristics of the air, the use of variable pressure adsorption principle (adsorption under pressure, desorption under pressure and regeneration of the molecular sieve), and the separation of oxygen and nitrogen at room temperature to make nitrogen production.
Variable pressure adsorption of nitrogen and deep-cooling air separation of nitrogen, compared with the significant features: adsorption and separation is carried out at room temperature, the process is simple, the equipment is compact, the footprint is small, it is convenient to start and stop, the start-up is rapid, the production of gas is fast (generally about 30min), the energy consumption is small, the running cost is low, the degree of automation is high, it is convenient to operate and maintain, it is easy to skid, there is no need to specialize in the foundation, the product can be adjusted to the purity within a certain range, and the amount of nitrogen production can be adjusted to a certain level. However, so far, except for the U.S. air supply company with PSA nitrogen technology, without the need for back-end purification can be industrialized to produce high purity nitrogen ≥ 99.999% purity (high import price), domestic and foreign counterparts generally PSA nitrogen technology can only produce nitrogen purity of 99.9% of the general nitrogen (i.e., O2 ≤ 0.1%), and individual enterprises can produce 99.99% of the nitrogen purity (i.e., O2 ≤ 0.5%), which can be used to produce 99.99% of the nitrogen. Can produce 99.99% pure nitrogen (O2 ≤ 0.01%), higher purity from PSA nitrogen technology is possible, but the production cost is too high, the user is also very difficult to accept, so the use of non-low-temperature nitrogen technology to produce high-purity nitrogen must be added to the back stage purification device. There are three methods of nitrogen purification (industrial scale):
(1) Hydrogenation and deoxidation method. Under the action of catalyst, the residual oxygen in the nitrogen and hydrogen added to the chemical reaction to generate water, the reaction formula: 2H2 + O2 = 2H2O, and then remove the water through the backstage drying, and obtain the following main components of high-purity nitrogen: N2 ≥ 99.999%, O2 ≤ 5 × 10-6, H2 ≤ 1500 × 10-6, H2O ≤ 10.7 × 10-6, H2 ≤ 1500 × 10-6, H2 ≤ 1500 × 10-6, H2 ≤ 1500 × 10-6, H2 ≤ 1500 × 10-6, H2 ≤ 1500 × 10-6. H2O≤10.7×10-6. The cost of nitrogen production is about 0.5 yuan/m3.
(2) Hydrogenation and deoxygenation method. This method is divided into three levels, the first level of hydrogen deoxygenation, the second level of dehydrogenation, the third level of dehydrogenation, to obtain the following composition of high-purity nitrogen: N2 ≥ 99.999%, O2 ≤ 5 × 10-6, H2 ≤ 5 × 10-6, H2O ≤ 10.7 × 10-6, nitrogen production cost of 0.6 yuan / m3. The cost of nitrogen production is about 0.6 yuan/m3.
(3) Carbon deoxygenation. Under the action of carbon-loaded catalyst (at a certain temperature), the residual oxygen in the nitrogen and the carbon provided by the catalyst itself react to produce CO2, the reaction formula: C + O2 = CO2, and then after the latter stage in addition to CO2 and H2O to obtain the following composition of high-purity nitrogen: N2 ≥ 99.999%, O2 ≤ 5 × 10-6, CO2 ≤ 5 × 10-6, H2O ≤ 10 × 10-6, H2O ≤ 10 × 10-10-6, and the cost of production is 0.6 yuan / m3. O2≤5×10-6, CO2≤5×10-6, H2O≤10.7×10-6. The cost of nitrogen production is about 0.6 yuan/m3.
The above three nitrogen purification methods, method (1) because the finished nitrogen in the amount of H2 is too high to meet the requirements of magnetic materials, so do not use; method (2) the finished nitrogen purity in line with the requirements of the magnetic material users, but the need for hydrogen source, and hydrogen in the transportation, storage, use of all the existence of insecurity; method (3) the quality of the finished nitrogen to meet the requirements of the magnetic material gas, process The quality of finished nitrogen can fully meet the requirements of magnetic materials, and the process does not use H2, without the problems brought by hydrogenation, no H2 in nitrogen and the quality of finished nitrogen is not affected by the fluctuation of universal nitrogen, so compared with other nitrogen purification methods, the quality of nitrogen is more stable, and it is the most suitable for a nitrogen purification method for the magnetic materials industry.
3.3.3 Membrane separation air separation of nitrogen and nitrogen purification device combination
Membrane separation air separation of nitrogen is also a new branch of non-low temperature nitrogen technology, is a new method of nitrogen production in the 80's developed rapidly abroad, in the domestic popularization and application of the matter in recent years.
Membrane separation of nitrogen is to use air as raw material, under a certain pressure, the use of oxygen and nitrogen in the hollow fiber membrane in the different permeability rate to make oxygen, nitrogen separation of nitrogen production. Compared with the above two nitrogen production methods, it has the features of simpler equipment structure, smaller volume, no switching valve, easier operation and maintenance, faster gas production (within 3min), more convenient to increase the capacity, etc. However, the hollow fiber membrane has more stringent requirements on compressed air cleanliness, and the membrane is easy to aging and failing, which is difficult to repair and needs to be replaced by a new membrane, and the membrane separation is more suitable for the small and medium-sized users who require the nitrogen purity of about ≤98%. At this time, it has the best function-price ratio; when the purity of nitrogen is higher than 98%, the price is about 30% higher than that of the same specification of the variable pressure adsorption nitrogen device, therefore, when high purity nitrogen is produced by the combination of membrane separation nitrogen and nitrogen purification device, the purity of general nitrogen is generally 98%, which will increase the cost of the purification device and the cost of production and operation.
In addition to the above three high-purity nitrogen on-site gas production methods, in recent years there has been a leasing of nitrogen supply, namely, by the user leasing nitrogen equipment on-site gas production or by the nitrogen equipment manufacturers in the nitrogen use of nitrogen on-site nitrogen production, the user to buy gas, according to the amount of payment. Because of the different amount of gas supply, the price of 1.0 to 1.4 yuan / m3 or so. Although the unit cost of nitrogen production is higher than the self-purchase of equipment on-site nitrogen production, but the one-time investment is small, easy to use, the user has no risk, but this way is suitable for occasions with a large amount of gas, otherwise, the rental cost will increase. Various high-purity nitrogen source nitrogen unit price summary in Table 1.
4 Nitrogen supply mode selection
The above high-purity nitrogen source from the nitrogen quality, can meet the magnetic material gas requirements, but in the cost of nitrogen on the larger differences in the amount of gas, the greater the difference is more significant. Enterprises choose what kind of nitrogen supply mode, should be in a full understanding of the characteristics of each gas supply mode on the basis of the enterprise's products, production processes, production scale, gas equipment type, number, capital status, development planning and other comprehensive consideration of nitrogen supply mode and nitrogen supply scale.
4.1 NdFeB production line
NdFeB production line main nitrogen equipment for the "air mill", according to the scale of production to determine the type and number of "air mill", the amount of nitrogen will be determined accordingly. At present, except for a very small number of domestic production enterprises with very small production scale, and the use of bottled nitrogen, the other enterprises use liquid nitrogen, some use PSA on-site nitrogen production.
4.2 MnZn ferrite production line
4.2.1 Vacuum atmosphere furnace
The vacuum atmosphere furnace as the sintering equipment, because the vacuum atmosphere furnace is a batch operation, generally 24h for a production cycle, a single unit of gas is not large, and non-continuous balanced use of gas, but a relatively centralized, short-term use of gas is more such enterprises are often not large in scale. Almost all of them use bottled nitrogen, which is flexible and convenient. Although the unit price of nitrogen in a variety of nitrogen supply is the highest, but because the total amount of gas is limited, so the economy can still afford.
4.2.2 Nitrogen kiln
The nitrogen kiln for sintering equipment, because the nitrogen kiln is a continuous operation of the equipment, the amount of gas is more, and from the trend, the new nitrogen kiln enterprises are to the direction of the long kiln and the long double-plate kiln development, a single unit of the amount of gas is generally in the range of 30 to 50 Nm3 / h. Nitrogen kiln sintering process characteristics determine the continuity of the supply of nitrogen, nitrogen, nitrogen purity, flow, matching of nitrogen and nitrogen purity, flow, and matching of the amount of nitrogen. The process characteristics of nitrogen kiln determine the continuity of gas supply, high purity of nitrogen, matching of nitrogen quantity, stability of nitrogen purity, flow rate and pressure, and low cost of nitrogen, which are the basic requirements of nitrogen kiln gas supply, and it is obvious that the use of bottled nitrogen is no longer suitable. At present, there are two main ways of nitrogen supply adopted by domestic enterprises, namely, liquid nitrogen and on-site nitrogen production.
(1) Liquid nitrogen. Those who use liquid nitrogen, when the enterprise is established, the general scale of production is not large, usually only one or two kilns, although they know that the cost of on-site nitrogen production is the lowest, but due to the funds or consider the future development and other reasons, most of them decide to use liquid nitrogen first, and then depending on the enterprise situation. Once the enterprise expansion or financial situation allows, from the perspective of reducing production costs, most of them will switch to on-site nitrogen production, but if the enterprise funds allow and no expansion plan in the past two years, I think that a single kiln with gas consumption of more than 30Nm3 / h, or self-purchase of PSA nitrogen equipment nitrogen production is preferred. Compared with the use of liquid nitrogen, 30Nm3 / h nitrogen generation unit can save about 240,000 yuan of annual nitrogen charges, the total investment in equipment at about 400,000 yuan, about one and a half years to recover the investment in equipment, PSA nitrogen generator life of up to 10 years, within 10 years, can save nitrogen charges of 2 million yuan.
(2) On-site nitrogen production. Self-purchase equipment on-site production of high-purity nitrogen, although the one-time investment is larger, but the operating cost is lower (0.7 yuan / m3 or less). It is compared with the use of liquid nitrogen, the same amount of gas, the annual cost savings can be within a year and a half to recover the full investment in equipment. On-site nitrogen production of three technologies - deep cooling air separation nitrogen, PSA nitrogen and membrane separation nitrogen have their own characteristics, and in different nitrogen production and nitrogen purity range has its own advantages, there have been articles [2] dedicated to the three investment value analysis, the conclusion is that the purity of nitrogen is 99.99% or more, nitrogen production of 500Nm3 / h or less. PSA nitrogen (plus purification) can compete with deep cooling air separation.
At present, domestic magnetic materials (MnZn ferrite) production enterprises use on-site nitrogen production and there are two ways of deep-cooled air separation of nitrogen and PSA nitrogen (plus purification).
①Deep cooling air separation nitrogen. This kind of enterprise was established in the 90's before the establishment of a considerable scale, from the economic point of view is not suitable for the use of liquid nitrogen, and at that time the deep cooling air separation nitrogen is the only industrialized nitrogen technology, coupled with the financial conditions can allow, so the use of deep cooling air separation nitrogen. Limited to the production scale at that time, the nitrogen production capacity of the nitrogen equipment were below 200Nm3/h. The energy consumption of the equipment was high and the failure rate was high. The equipment had high energy consumption and high failure rate, and had to be overhauled regularly. Into the mid-1990s, due to the new nitrogen technology - PSA nitrogen in the country's rapid development and popularization of the application, it shows a lot of unique advantages, so more and more by the small and medium-sized nitrogen users welcome.
②PSA nitrogen, PSA nitrogen and nitrogen purification phase combination of high-purity nitrogen using the following process and equipment configuration:
Liquid nitrogen storage tanks are any magnetic material enterprises must be equipped with on-site nitrogen, its role is to ensure the supply of gas when the normal maintenance of the equipment (such as oil change of air compressors and air purification equipment cartridge cleaning or replacement), short-term shutdowns or occasional equipment failure. It serves as a standby measure to ensure the continuity of gas supply during normal maintenance of equipment (such as oil change of air compressor and cleaning or replacement of air purification equipment) or occasional failure of equipment. The quality of high purity nitrogen produced by this process can be fully compared with that of liquid nitrogen. Equipped with liquid nitrogen storage tank, users have no worries about gas supply, and practice has fully proved this. Jiangyin Changjiang Gas Separation Equipment Co., Ltd. has had four sets of PSA high-purity nitrogen generation units since 1997, which have been used by four MnZn ferrite producers in Zhejiang, Jiangxi, Shandong, etc. The equipment is running well, with mature technology and stable quality, and it can fully satisfy the requirements of high-grade magnetic cores; three of these four enterprises originally used liquid nitrogen, and one of them used deep-cooling air separation, which was difficult to repair due to frequent failures, and they switched to the Yangtze River system to produce high-purity nitrogen. The four enterprises, three of which originally used liquid nitrogen and one of which used deep-cooling air separation, have achieved remarkable benefits by switching to CK's nitrogen equipment.
Once an enterprise decides to use on-site nitrogen production, it should specify the technical requirements, investigate and comprehensively evaluate the supplier, and choose the best and cheapest one.
5 Conclusion
(1) Defining the requirements of nitrogen source is the prerequisite for choosing the nitrogen supply method.
(2) Familiarizing with the characteristics of various nitrogen sources is the basis for selecting the nitrogen supply method.
(3) When the amount of nitrogen used is more than 30Nm3/h, it is more economical to choose on-site nitrogen production, and the larger the amount of gas used, the more significant the benefit.
(4) When the amount of nitrogen used is below 500Nm3/h, PSA high-purity nitrogen generation unit on site is the best choice.
References:
[1] Yang Daqi et al. Proceedings of the fourth national exchange of magnetic materials and devices application technology.1999.77.
[2] Zheng Linqiang. Proceedings of the second all-network large and academic exchanges of mechanical industrial gas separation equipment science and technology information variable pressure adsorption sub-network.1999.19.