Huang Luoxing
(Jiangyin Changjiang Gas Separation Equipment Co., Ltd., Jiangyin, Jiangsu 214401, China)
1 Preface
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 (powders) during the process.
As we all know, MnZn ferrite is generated by Fe, Mn, Zn oxides in the 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, to ensure that the metal ions are in the specific valence. In order to make MnZn ferrite achieve the required magnetic properties, it is necessary to ensure that each metal ion is 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 one of the basic material conditions for the implementation of the balanced atmosphere sintering. 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 nitrogen consumption of 100 ~ 120Nm3 / h.
NdFeB and other rare earth permanent magnet alloy of rare earth metals even at room temperature conditions, but also very easy to oxidize and lead to rare earth permanent magnet alloy performance reduction, excessive oxidation will make the alloy performance deteriorated greatly. 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 alloys system (fine) powder process are used in a kind of equipment called "nitrogen gas flow mill", which 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 of nitrogen technical requirements
From the use of nitrogen, there are four basic parameters need to pay attention to, that is, 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 gauge pressure and absolute 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 is zero is called "absolute pressure", the pressure of the gas industry, the pressure of the gas industry, the pressure is zero. 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 of the gas, the lower the dew point, that is, the lower the water content of 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, without special instructions, the dew point referred to are the 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 on the basis of 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 a kind of 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 volume of 40L, rated pressure 15MPa, full storage 6m3, according to different user needs, bottled nitrogen has a different 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 a product of the deep-cooling of air separation of nitrogen, in the standard state, 1m3 liquid nitrogen can be vaporized 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 actually used. 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. The liquid nitrogen is of 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 cooling air separation of nitrogen, 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, but it has a complicated process, a large footprint, high infrastructure costs, the need for specialized maintenance forces, more operators, and a slow production of gas (18-24h), which is suitable for large-scale industrial nitrogen production. 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
Pressure Swing Adsorption (PSA) gas separation technology is an important branch of non-low-temperature gas separation technology, and it 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, the technology is becoming more mature, in the field of small and medium-sized nitrogen production has become a strong competitor of the deep-cooled air separation.
Variable Pressure Adsorption (VPA) is a process that takes air as raw material, uses carbon molecular sieve as adsorbent, utilizes the selective adsorption characteristics of carbon molecular sieve on oxygen and nitrogen in the air, and applies the principle of variable pressure adsorption (adsorption under pressure, desorption under pressure, and regeneration of the molecular sieve), and separates oxygen and nitrogen to produce nitrogen gas at room temperature.
Compared with deep-cooling air separation nitrogen production, variable pressure adsorption nitrogen production has significant features: adsorption separation is carried out at room temperature, simple process, compact equipment, small footprint, easy to start and stop, quick start, fast gas production (generally about 30min), small energy consumption, low operating costs, high degree of automation, easy operation and maintenance, easy to skid, no need for a special foundation, the product can be adjusted within a certain range, the amount of nitrogen production can be adjusted to a certain level, and the amount of nitrogen production can be adjusted to a certain level, and the amount of nitrogen production can be adjusted to a certain range. However, so far, except for the U.S. air supply company with PSA nitrogen technology, without the need of post-purification can be industrialized to produce high-purity nitrogen purity ≥ 99.999% (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, which can be used to produce 99.99% of the nitrogen purity. 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. H2O≤10.7×10-6. The cost of nitrogen production is about 0.5 yuan/m3.
(2) Hydrogenation and deoxidation, dehydrogenation 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 of 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 nitrogen production is 0.6 yuan / m3. -The cost of nitrogen production is about 0.6 Yuan/m3.
In the above three nitrogen purification methods, method (1) is not used because the amount of H2 in the finished nitrogen is too high to meet the requirements of magnetic materials; method (2) meets the purity of the finished nitrogen to meet the requirements of the magnetic material users, but requires a hydrogen source, and there are insecurity factors in the transportation, storage, and use of hydrogen; method (3) the quality of the finished nitrogen to meet the requirements of the magnetic material gas, the process of the gas. The quality of finished nitrogen can fully meet the requirements of magnetic materials, and the process does not use H2, no hydrogenation problems, 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 Combination of membrane separation air separation nitrogen and nitrogen purification device
Membrane separation air separation 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 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 to be around ≤98%. Membrane separation of nitrogen is more suitable for small and medium-sized users with nitrogen purity requirement of ≤98%, which has the best function-price ratio at this time; when the nitrogen purity requirement is higher than 98%, the price is about 30% higher than the same specification of the pressure adsorption nitrogen device, so when the membrane separation of nitrogen and nitrogen purification device are combined to produce high purity nitrogen, the general purity of nitrogen is generally 98%, which will increase the cost of the production of the purification device and the operating cost.
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 nitrogen equipment manufacturers in the nitrogen use of nitrogen on-site nitrogen production, the user to buy the 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 as 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 the cost of nitrogen on the larger the difference, the larger the amount of gas, the more significant the difference. Enterprises choose what kind of nitrogen supply mode, should fully understand 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 "air mill" type and number of nitrogen dosage 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
Vacuum atmosphere furnace for sintering equipment, because the vacuum atmosphere furnace is intermittent 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 tend to be small 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
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 long kiln and the long double-plate kiln direction of the development of a single unit of the amount of gas is generally in the range of 30 ~ 50Nm3 / h. Nitrogen kiln sintering process characteristics determine the continuity of the supply of nitrogen, nitrogen, nitrogen purity, flow, matching nitrogen and nitrogen purity, flow, matching the amount of nitrogen, nitrogen and 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 to supply nitrogen to 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 production scale is not large, usually only one or two kilns, although the cost of nitrogen production site is known to be the lowest, but because of the funds or taking into account the future development and other reasons, most of the decision is to use liquid nitrogen, depending on the situation of the enterprise later. 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. Because compared with the use of liquid nitrogen, 30Nm3 / h nitrogen generation unit annual nitrogen cost savings of about 240,000 yuan, the total investment in equipment in about 400,000 yuan, a year and a half or so to recover the investment in equipment, PSA nitrogen generator life of up to 10 years, 10 years to save nitrogen cost of 2 million yuan.
(2) On-site nitrogen production. Self-purchase of 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 savings can be within one and a half years 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.
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).
① Deep 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 industrial 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, which shows many unique advantages, so more and more by small and medium-sized nitrogen users welcome.
② PSA 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 in the event of normal maintenance of the equipment (eg, oil change of the air compressor and the 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 due to frequent failures and difficulties.
Once an enterprise decides to use on-site nitrogen generation, it should specify the technical requirements, investigate and fully evaluate the suppliers, 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.