Huangluoxing

(Jiangyin Changjiang Gas Separation Equipment Co., Ltd., Jiangyin, Jiangsu 2 1440 1)

1 preface

The sintering of high-performance MnZn ferrites (high μi and power ferrites) in magnetic materials and the fine grinding process in the production of rare earth permanent magnet alloys such as NdFeB require high-purity nitrogen to protect magnets (powder) from oxidation in the process.

As we all know, Mn-Zn ferrite is produced by solid-state reaction of oxides of iron, manganese and zinc during high-temperature sintering. Mn and Fe are easy to change valence, and their valence states are different under different conditions of temperature and atmosphere (oxygen partial pressure). In order to achieve the required magnetic properties of MnZn ferrite, it is necessary to ensure that each metal ion in it is in a specific valence state and a suitable crystal structure. In addition to the appropriate formula, the key is to sinter in a balanced atmosphere, and shielding gas is one of the basic material conditions for sintering in a balanced atmosphere. The oxygen content of nitrogen in the cleaning chamber of nitrogen kiln is expected to be below 50× 10-6, so the purity of nitrogen is required to be above 99.995%, and the amount of impurity gases (O2 and H2) is strictly limited: the general consumption of a MnZn ferrite production line with an annual output of about 1000 tons is 100 ~ 65433.

Rare earth metals in rare earth permanent magnet alloys such as NdFeB are easy to oxidize even at room temperature, which will lead to the decline of the properties of rare earth permanent magnet alloys. Excessive oxidation will greatly reduce the properties of the alloy. Because 1 part oxygen can oxidize 6 parts (by weight) of rare earth elements, it has no effect. Taking NdFeB as an example, the oxygen content in the production process environment must be ≤ 0.0 1%, and the oxygen content in the final product is 0.09 0.02% (mass fraction) [1]. If nitrogen is used as the process ambient gas, its nitrogen purity must be higher than 99.99%.

At present, a kind of equipment called "nitrogen flow grinding" is used in the process of milling (refining) of large-scale industrial production of rare earth permanent magnet alloys at home and abroad. It uses high-speed nitrogen flow to drive materials to collide with each other to achieve grinding effect. The particle size of the prepared powder is required to be 3 ~ 5 microns, with large specific surface area and easy oxidation. Therefore, nitrogen must be of high purity, and there are strict requirements on the amount of impurities such as O2 and H2. NdFeB production lines with an annual output of about100t usually consume about 60 nm3/h of high-purity nitrogen.

2 technical requirements for nitrogen in the production of magnetic materials

From the use point of view, nitrogen has four basic parameters to pay attention to, namely purity, flow rate, dew point and pressure. Parameter values are different for different purposes. In order to gain knowledge about * * *, it is necessary to briefly introduce the concepts of four technical parameters.

2. 1 purity

Purity is an important technical parameter of nitrogen. According to national standards, nitrogen can be divided into three grades: industrial nitrogen, pure nitrogen and high purity nitrogen. Their purities are 99.5% (O2 ≤ 0.5%), 99.99% (O2 ≤ 0.0 1%) and 99.999% (O2 ≤ 0.00 1%) respectively.

2.2 process

Refers to the amount of gas passing through any cross section in unit time when gas flows. There are two expressions of flow, namely volume flow and mass flow. The former refers to the volume of gas passing through any section of the pipeline, while the latter refers to the mass of gas passing through. In the gas industry, the volume flow is usually measured in m3/h (or L/h). Because the gas volume is related to temperature, pressure and humidity, for comparison, the volume flow usually refers to the standard state (temperature is 20℃, pressure is 0.10/MPa, and relative humidity is 65%). At this time, the flow rate is nm3/h, and "n" indicates the standard state.

2.3 pressure

Pressure can be divided into gauge pressure and absolute pressure. In engineering, the pressure greater than atmospheric pressure with atmospheric pressure as the starting point (zero point) is called gauge pressure, and when the pressure is zero, it is called absolute pressure. In the gas industry, unless otherwise specified, its pressure refers to gauge pressure, and the unit is MPa. In many calculations, "absolute pressure" is often used, and there is the following relationship between them:

Absolute pressure = gauge pressure+atmospheric pressure

2.4 dew point

Refers to the temperature at which water in a gas changes from unsaturated steam to saturated steam. When unsaturated steam becomes saturated steam, very fine dewdrops will appear. The temperature at which dew appears is called "dew point", which represents the moisture content in the gas. The lower the dew point, the less the water content in the gas, and the drier the gas. Dew point is related to pressure, so there are atmospheric dew point (atmospheric dew point) and pressure dew point. Atmospheric dew point refers to the condensation temperature of water at atmospheric pressure, while the dew point under pressure refers to the condensation temperature of water at this pressure. There is a conversion relationship between them (refer to the conversion table). For example, the pressure dew point at 0.7 MPa is 5℃, and the corresponding atmospheric pressure (0. 10 1 MPa) dew point is -20℃. In the gas industry, all dew points are atmospheric dew points unless otherwise specified.

The above briefly introduces the significance of several parameters of gas. According to its technological requirements, nitrogen can put forward specific parameters for magnetic materials:

(1) nitrogen flow. The determination of flow rate mainly depends on the type, quantity and production process of nitrogen-using equipment. Taking the nitrogen kiln for MnZn ferrite sintering as an example, the long kiln is different from the short kiln, the single kiln is different from the double kiln, the compact sintering is different from the non-compact sintering, and the kiln adjustment level is different, so the gas consumption is very different. In addition, when determining the amount of nitrogen, an appropriate margin should be left.

(2) Nitrogen purity. According to the production process, magnetic materials generally require high-purity nitrogen-purity ≥99.995%, O2 and H2 content within a certain range.

(3) pressure. Determine the minimum pressure of nitrogen according to the equipment and process, and then adjust the pressure to the required pressure through the pressure regulating valve.

(4) Dew point. Water vapor is also an oxidizing gas, so it should be limited. For nitrogen used for magnetic materials, as long as the dew point is ≤-60℃, that is, the moisture in nitrogen is ≤ 10.7× 10-6, the process requirements can be met.

High purity nitrogen source

There are three kinds of high-purity nitrogen sources to meet the needs of magnetic materials:

3. 1 bottled nitrogen

The cylinder volume is 40L, the rated pressure is 15MPa, and the full gas storage capacity is 6m3. According to different requirements of users, the purity of bottled nitrogen is 99.5%, 99.99% and 99.999%, and the purity of nitrogen for magnetic materials is ≥ 99.995%. It is the product of low temperature air separation and is filled with a film press. According to the regulations, the nitrogen bottle is painted with black paint, marked with yellow paint "nitrogen", and marked with "purity" and inspection mark. Due to the different supply and demand in different places, the price of bottled high-purity nitrogen varies greatly, ranging from18 to 90 yuan/bottle, that is, the price of nitrogen is 3 ~ 15 yuan /m3.

3.2 liquid nitrogen

Liquid nitrogen is the product of nitrogen production by cryogenic air separation. Under standard conditions, 1m3 liquid nitrogen can be gasified into 643m3 nitrogen, but the actual utilization rate is generally about 95%, that is, the actual available nitrogen of 1m3 liquid nitrogen is about 665438+ 100m3. At present, the average price of liquid nitrogen in the market is about 1000 yuan /m3, so the unit price of nitrogen is1000 yuan /m3.

When using liquid nitrogen, the user must be equipped with a liquid nitrogen storage tank, a vaporizer corresponding to the flow rate and a pressure regulating valve corresponding to the pressure. Liquid nitrogen has high purity, stable quality, generally guaranteed supply and convenient use.

3.3 On-site Nitrogen Production

On-site nitrogen production means that nitrogen users buy nitrogen production equipment to produce nitrogen. At present, there are three types of industrial nitrogen production at home and abroad: cryogenic air separation nitrogen production, pressure swing adsorption nitrogen production and membrane separation nitrogen production.

Low temperature air separation for nitrogen production

It is a traditional air separation technology with a history of more than 90 years. It is characterized by large gas production and high nitrogen purity, and can be directly applied to magnetic materials without further purification. However, its technological process is complex, it occupies a large area, the capital construction cost is high, it needs special maintenance force, there are many operators, and the gas production is slow (18 ~ 24h). It is suitable for large-scale industrial nitrogen production, and the cost of nitrogen production is 0.7 yuan/ton.

3.3.2 Combination of pressure swing adsorption nitrogen production and nitrogen purification device

Pressure swing adsorption gas separation technology is an important branch of non-cryogenic gas separation technology, which is the result of people's long-term efforts to find a simpler air separation method than cryogenic method. In 1970s, Essen Mining Company of West Germany successfully developed carbon molecular sieve, paving the way for the industrialization of nitrogen production by pressure swing adsorption air separation. In the past 30 years, this technology has developed rapidly and matured day by day, and has become a strong competitor of cryogenic air separation in small and medium-sized nitrogen production field.

Nitrogen production by pressure swing adsorption takes air as raw material and carbon molecular sieve as adsorbent. By using the characteristics of selective adsorption of oxygen and nitrogen in air by carbon molecular sieve and the principle of pressure swing adsorption (pressure adsorption, decompression desorption, molecular sieve regeneration), oxygen and nitrogen are separated at room temperature to produce nitrogen.

Compared with nitrogen production by cryogenic air separation, nitrogen production by pressure swing adsorption has obvious characteristics: adsorption separation is carried out at room temperature, the process is simple, the equipment is compact, the floor space is small, the start-up and shutdown are convenient, the gas production is fast (usually about 30 minutes), the energy consumption is low, the operating cost is low, the automation degree is high, the operation and maintenance are convenient, the skid-mounted operation is convenient, no special foundation is needed, the purity of product nitrogen can be adjusted within a certain range, and the nitrogen yield is ≤ Except that American Air Supplies Company can industrially produce high-purity nitrogen with a purity of ≥ 99.999% without post-purification (the import price is very high), domestic and foreign counterparts can only produce general nitrogen with a nitrogen purity of 99.9% (that is, O2 ≤ 0. 1%), and individual enterprises can produce 99.99% pure nitrogen (O2). It is technically possible to produce higher purity nitrogen from PSA, but the production cost is too high for users to accept, so it is necessary to add a post-purification device to produce high purity nitrogen by non-low temperature nitrogen production technology. At present, there are three nitrogen purification methods (industrial scale):

(1) hydrodeoxygenation method. Under the action of catalyst, the residual oxygen in ordinary nitrogen reacts with the added hydrogen to generate water, and the reaction formula is 2h2+O2 = 2h2o, and then the water is removed by post-drying, thus obtaining high-purity nitrogen, the main components of which are N2 ≥ 99.999%, O2 ≤ 5× 10-6, and H2 ≤1500× 60. The cost of nitrogen production is about 0.5 yuan /m3.

(2) hydrodeoxygenation and hydrogen removal. The method is divided into three stages, the first stage is hydrodeoxygenation, the second stage is dehydrogenation, and the third stage is dehydration to obtain high-purity nitrogen, which consists of N2≥99.999%, O2 ≤ 5× 10-6, H2 ≤ 5× 10-6 and H2O ≤/kloc. The cost of nitrogen production is about 0.6 yuan /m3.

(3) Carbon deoxidation method. Under the action of carbon-supported catalyst (at a certain temperature), the residual oxygen in nitrogen reacts with the carbon provided by the catalyst itself to generate CO2. Reaction formula: C+O2 = CO2. Then removing CO2 and H2O to obtain high-purity nitrogen with the following composition: N2≥99.999%%, O2 ≤ 5× 10-6, CO2 ≤ 5× 10-6, H2O ≤10.7×/kloc-0-6. The cost of nitrogen production is about 0.6 yuan /m3.

Among the above three nitrogen purification methods, the (1) method is not adopted because the H2 content in the finished nitrogen is too high to meet the requirements of magnetic materials. Methods (2) The purity of finished nitrogen meets the requirements of users of magnetic materials, but hydrogen source is needed, and there are unsafe factors in the transportation, storage and use of hydrogen; Method (3) The quality of finished nitrogen can completely meet the gas requirements of magnetic materials. H2 is not used in this process, and there are no problems caused by hydrogenation. There is no H2 in nitrogen, and the quality of finished nitrogen is not affected by the fluctuation of total nitrogen. Therefore, compared with other nitrogen purification methods, the quality of nitrogen is more stable, and it is the most suitable nitrogen purification method for magnetic materials industry.

3.3.3 Membrane separation and air separation for nitrogen production are combined with nitrogen purification device.

Nitrogen production by membrane separation and air separation is also a new branch of non-low temperature nitrogen production technology, which is a new nitrogen production method developed rapidly abroad in the 1980 s and has been popularized and applied in China in recent years

Nitrogen production by membrane separation takes air as raw material. Under a certain pressure, oxygen and nitrogen are separated to produce nitrogen by using the different permeation rates of oxygen and nitrogen in hollow fiber membranes. Compared with the above two nitrogen production methods, it has the characteristics of simpler equipment structure, smaller volume, no switching valve, simpler operation and maintenance, faster gas production (within 3 minutes) and more convenient capacity expansion. However, the hollow fiber membrane has stricter requirements on the cleanliness of compressed air, and the membrane is prone to aging and failure, difficult to repair and needs to be replaced. Nitrogen production by membrane separation is more suitable for small and medium-sized users with nitrogen purity ≤ 98%, and the effect is the best at this time. When the purity of nitrogen is required to be higher than 98%, its price is about 30% higher than that of the pressure swing adsorption nitrogen plant with the same specification. Therefore, when membrane separation nitrogen production and nitrogen purification device jointly produce high-purity nitrogen, the purity of ordinary nitrogen is generally 98%, which will increase the production cost and operation cost of purification device.

In addition to the above-mentioned three methods for producing high-purity nitrogen on site, in recent years, another nitrogen supply mode has emerged, that is, users rent nitrogen-making equipment for on-site production or nitrogen-making equipment manufacturers are using nitrogen to make nitrogen on site, and users buy gas and pay by quantity. Due to different gas sources, the price is about 1.0 ~ 1.4 yuan /m3. Although the cost of nitrogen production per unit is higher than that of on-site nitrogen production with self-purchased equipment, it has less one-time investment, convenient use and no risk to users, but this method is suitable for occasions with large gas consumption, otherwise it will increase the rental cost. See table 1 for a summary of nitrogen unit prices of various high-purity nitrogen sources.

4 Selection of nitrogen supply mode

The above-mentioned high-purity nitrogen sources can meet the gas requirements of magnetic materials in terms of nitrogen quality, but the cost of nitrogen is quite different. The greater the fuel consumption, the greater the difference. On the basis of fully understanding the characteristics of various gas supply modes, enterprises should comprehensively consider the nitrogen supply mode and scale according to their own products, production technology, production scale, gas equipment type, quantity, capital situation and development planning.

4. 1 NdFeB production line

The main nitrogen making equipment used in NdFeB production line is "air mill". According to the production scale, the type and quantity of "jet mill" are determined, and the amount of nitrogen is determined accordingly. At present, except a few domestic production enterprises use bottled nitrogen, some other enterprises use liquid nitrogen, and some use PSA to make nitrogen on site.

4.2 Mn-Zn ferrite production line

4.2. 1 vacuum atmosphere furnace

Vacuum atmosphere furnace is used as sintering equipment. Because the vacuum atmosphere furnace runs intermittently, it usually takes 24 hours as a production cycle. The gas consumption of a single unit is not large, and the gas consumption is relatively concentrated in a short time. This kind of enterprises tend to produce small scale, and almost all use bottled nitrogen, which is flexible and convenient to use. Although the unit price of nitrogen is the highest among all kinds of nitrogen supply methods, the total gas consumption is limited, so it is affordable economically.

Nitrogen kiln

Nitrogen kiln is used as sintering equipment. Because nitrogen kilns are continuously operated equipment, they consume a lot of gas. Moreover, from the trend, the new nitrogen kilns of various enterprises are developing towards long kilns and long double-plate kilns, and the gas consumption of a single kiln is generally 30 ~ 50 nm3/h ... The technological characteristics of nitrogen kiln sintering determine the continuity of gas supply, the high purity of nitrogen, the matching of nitrogen content, the stability of nitrogen purity, flow rate and pressure, and the low cost of nitrogen use as the basic requirements for nitrogen kiln gas supply. Obviously, it is not suitable to use bottled nitrogen. At present, domestic enterprises mainly use liquid nitrogen and on-site nitrogen production.

(1) liquid nitrogen. When the enterprise was established, the general production scale using liquid nitrogen was very small, generally only one or two kilns. Although they know that the cost of on-site nitrogen production is the lowest, most of them decide to use liquid nitrogen first, and then look at the situation of the enterprise. Once the enterprise capacity expansion or capital situation allows, in order to reduce production costs, metropolis will turn to local nitrogen production. However, if the enterprise funds permit, and there is no capacity expansion plan in the last two years, the author thinks that the gas consumption of a single kiln exceeds 30 nm3/h, and it is best to buy PSA nitrogen production equipment for nitrogen production. Compared with the use of liquid nitrogen, the annual nitrogen production cost of 30 nm3/h nitrogen generator set can save about 240,000 yuan, and the total equipment investment is about 400,000 yuan. The equipment investment can be recovered in about one and a half years. The service life of PSA nitrogen generator can reach 10 year, and the nitrogen cost can be saved by 10 year by 2 million yuan.

(2) On-site nitrogen production. Self-purchased equipment is used to produce high-purity nitrogen on site. Although the one-time investment is large, the operating cost is low (within 0.7 yuan /m3). Compared with the use of liquid nitrogen, the total investment of equipment can be recovered within one and a half years with the same gas consumption and annual cost savings. Three nitrogen production technologies in this field-cryogenic air separation, pressure swing adsorption and membrane separation-have their own characteristics, and each has its own advantages in different nitrogen output and nitrogen purity ranges. The existing article [2] has specifically analyzed the investment value of the three technologies, and considered that the nitrogen purity is above 99.99% and the nitrogen output is within 500 nm3/h, and PSA nitrogen production (plus purification) can compete with cryogenic air separation.

At present, domestic manufacturers of magnetic materials (MnZn ferrite) use two methods to produce nitrogen on site, namely cryogenic air separation and PSA nitrogen production (plus purification).

① Nitrogen production by low temperature air separation. This kind of enterprise was established before the 1990s, and it was quite large when it was established. From the economic point of view, the use of liquid nitrogen is not suitable. At that time, low-temperature air separation was the only industrialized nitrogen production technology in China, and the capital conditions allowed, so low-temperature air separation was adopted. Limited by the production scale at that time, the nitrogen output of nitrogen production equipment was below 200 nm3/h, and the equipment with high energy consumption and high failure rate should be overhauled regularly. Since the mid-1990s, pressure swing adsorption nitrogen production, a new nitrogen production technology, has been rapidly developed and popularized in China, showing many unique advantages, so it is increasingly welcomed by small and medium-sized nitrogen production users.

②PSA nitrogen production. Pressure swing adsorption nitrogen production and nitrogen purification are combined to produce high-purity nitrogen, and the following process flow and equipment configuration are adopted:

Liquid nitrogen storage tank is a necessary equipment for on-site nitrogen production in any magnetic material enterprise. Its function is to ensure the continuity of gas supply during short-term parking when the equipment is in normal maintenance (such as oil change of air compressor, cleaning or replacement of filter element of air purification equipment) or when the equipment is stopped unexpectedly. The quality of high purity nitrogen produced by this process can be completely compared with that of liquid nitrogen. Equipped with liquid nitrogen storage tank, users have no worries about gas supply, which has been fully proved by practice. Since 1997, Jiangyin Changjiang Gas Separation Equipment Co., Ltd. has used four PSA high-purity nitrogen generating units in four manganese-zinc ferrite production enterprises in Zhejiang, Jiangxi and Shandong. The equipment runs well, the technology is mature and the quality is stable, which can fully meet the production requirements of high-grade magnetic cores. Three of these four enterprises used liquid nitrogen and one used cryogenic air separation. Because of frequent failures and difficult maintenance, they switched to the Yangtze River nitrogen production equipment, which achieved remarkable benefits.

Once the enterprise decides to adopt on-site nitrogen production, it is necessary to clarify the technical requirements, inspect and comprehensively evaluate the suppliers, and choose the best and cheapest one.

5 conclusion

(1) Defining the requirements of nitrogen sources is the premise of choosing nitrogen supply methods.

(2) Familiarity with the characteristics of various nitrogen sources is the basis for selecting nitrogen supply methods.

(3) When the nitrogen consumption is above 30 nm3/h, it is more economical to choose on-site nitrogen production, and the greater the gas consumption, the more significant the benefit.

(4) When the nitrogen consumption is less than 500 nm3/h, PSA high-purity nitrogen generator is the best choice on site.

References:

Yang Da, et al. Proceedings of the 4th National Symposium on Application Technology of Magnetic Materials and Devices. 49660.88868688686

Zheng. Proceedings of the second large-scale academic exchange conference on the sub-network of pressure swing adsorption of scientific and technological information of gas separation equipment in mechanical industry 46660.88868688666