Protective nitrogen gas application in hot-dip galvanizing

As we all know, the hot galvanizing of strip steel is because zinc can form a corrosion-resistant film on the metal surface in a corrosive environment (corrosive media such as H2O, O2, and CO2), thereby extending the service life of the material. Galvanizing has become an effective method for anticorrosion of steel materials. Hot-dip galvanized strip has a beautiful and shiny appearance, so it is very popular and widely used.

Over 160 years, hot-dip galvanizing technology has made great progress. In the long development process of hot-dip galvanizing, the early production efficiency was very low. Until 1931, the Polish Sengimir integrated the annealing process and the hot-dip galvanizing process into one, this world-renowned Sengemir method has truly opened a new era of continuous hot-dip galvanizing of steel strip.

Protective gas for hot-dip galvanizing

Generally, the input of a small amount of hydrogen in high-purity nitrogen will improve the protective effect of the gas. When the leakage rate of the furnace is not too large, the oxygen infiltrating into the furnace reacts with hydrogen, and an appropriate hydrogen-water ratio can prevent the oxidation reaction from occurring.

During the heat treatment, hydrogen and oxygen can react at any temperature to form water. Under this condition, since oxygen is depleted and there are no hydrocarbons in the furnace, only water is the only oxidant. The hydrogen-water ratio is the most important factor in determining the oxidation or reduction potential of the gas in the furnace. In the heating zone of the annealing furnace, adding a small amount of hydrogen is easy to maintain the reducibility of the furnace atmosphere, but it is more difficult in the cooling zone.

If the protective gas of the annealing furnace is N2 and H2, and the moisture content is less than 2% (dew point-18℃), bright annealing of steel can be achieved; in the cooling zone, the moisture content should be reduced to 0.03 (dew point-32℃) or less. For strict processes, when the infiltration of oxygen cannot be controlled, in order to ensure the quality of the product, the hydrogen content in the cooling zone should be increased. Since the nitrogen-hydrogen mixture contains no carbides, decarburization occurs. The decarbonization potential depends on the hydrogen-water ratio (partial pressure ratio) of the furnace atmosphere:

If the ratio of hydrogen to water is high and the amount of ammonia is small, the reaction is very small. The actual carbon burnout depends on the following parameters: gas flow, surface area of the workpiece, temperature and time of heat treatment. For a hydrogen-nitrogen protective atmosphere furnace with a dew point of 42℃ , processing (carbon content 0.8) Φ2mm steel wire, the furnace productivity is 500kg / h, even if all the moisture in the furnace atmosphere participates in the reaction, the gas flow rate is 100m3 / At h, the total decarburization layer of the workpiece per hour is only 0.082um thick

Protective gas plays an important role in the continuous hot-dip galvanizing process of steel strip. H2 can reduce the oxide film on the surface of the strip to active sponge iron, thereby enhancing the binding force between the strip and the coating; N2 is a neutral gas, which can protect the strip from being oxidized in the annealing furnace. Nitrogen also has the safety function of protecting the annealing furnace. The protective gas system is an important part of the hot-dip galvanizing production line, and its investment cost usually accounts for about 10% of the total investment of the entire production line equipment. The reliability of the protective gas system is related to the normal operation of the hot-dip galvanizing production line, and the quality of the protective gas directly affects the quality of the hot-dip galvanized strip.

Hot-dip galvanized protective gas system

The protective gas system is mainly composed of an ammonia decomposition device, a pressure swing adsorption nitrogen production device and a gas distribution system.

Ammonia decomposition atmosphere

The ammonia decomposition atmosphere is an atmosphere obtained by using liquid ammonia as a raw material and heating and decomposing it under the action of a catalyst in an ammonia decomposition furnace.

Ammonia is decomposed to produce a hydrogen-nitrogen mixed gas composed of 75% H2 and 25% N2. After purification, the impurities are O2 <1ppm, residual ammonia <1 ppm, and dew point <60℃. It is similar to pure hydrogen and is a strong reduction Atmospheric atmosphere is also a commonly used protective gas. At 20℃ and 10325Pa, 1kg of liquid ammonia can be gasified to 1.39m3 of gaseous ammonia, and theoretically 2.78m3 of H2 + N2 mixed gas can be obtained after decomposition. The ammonia decomposition atmosphere can not prevent the decarburization of the water vapor present in the furnace gas, so it must be fully dried. Generally, its dew point is below 40 ℃.

Ammonia decomposition cannot be complete, and it often contains a small amount of residual ammonia (0.01% to 0.1%) in the atmosphere. When it is passed into the working furnace, it will cause pyrolysis and generate trace atomic nitrogen, which will cause the metal to undergo slight nitriding. This is not good for steel parts, especially stainless steel strips and wires, which will become brittle and become waste. Therefore, the decomposition rate of ammonia should be increased as much as possible, and purification measures should be taken after the ammonia decomposition.

The advantage of ammonia decomposition atmosphere is that the preparation process is simple and it is easy to obtain a pure and stable atmosphere. We use a nickel-based catalyst containing more than 14% nickel, with a compressive strength of> 300N / grain, a space velocity of> 1000h-1, and an operating temperature of 800-850 ° C, and residual ammonia <100ppm. If the decomposition temperature is too low, overload production will cause the catalyst activity to decline or fail.

The space velocity test of nickel-based catalyst for ammonia decomposition shows that changing the space velocity has little effect on the decomposition effect. The reaction tube is the core of the ammonia decomposition furnace. The purifier uses high-purity nitrogen to purify and regenerate, which improves the purification effect. Two sets of purifiers are set, one works and the other is regenerated. Alternate rotation work and regeneration, automatic operation with PLC programmable controller.

PSA nitrogen production

Pressure swing adsorption gas separation technology is an important branch of non-cryogenic gas separation technology. PSA uses air as the raw material and carbon molecular sieve as the adsorbent. Using the principle of pressure swing adsorption, the selective adsorption of O2 and N2 by carbon molecular sieve is used to achieve the separation of oxygen and nitrogen to obtain nitrogen.

Compared with cryogenic nitrogen production, PSA nitrogen production has significant characteristics. Adsorption is performed at normal temperature without involving adiabatic problems. The process is simple, the device is compact, the operation and maintenance are simple, the startup is fast, and the gas production is fast (15 ~ 30min). The nitrogen purity of the product can be arbitrarily adjusted according to process requirements, and it has obvious advantages for hot-dip galvanizing that requires continuous supply of protective gas.

Compressed air is used as the raw material for nitrogen production. Because piston air compressors are noisy and need frequent maintenance, screw air compressors should be used.

Carbon molecular sieve is the key of nitrogen generator. The carbon molecular sieve we choose has excellent adsorption performance and the cost is not high.

Valves are also the key to nitrogen generators. They require fast switching speeds, good airtightness, and long life. Generally, the valve switching life is required to be more than one million times. In fact, our valves can be used for several years and are very reliable.

The structure of the adsorption tower (including the diameter-to-height ratio), air distribution, and the way of packing and compacting the carbon molecular sieve also greatly affect the performance of the nitrogen generator. The compacting of the carbon molecular sieve and the control of the gas flow rate are very important, otherwise frequent pressure changes in the adsorption tower will cause the carbon molecular sieve to move, wear quickly and become powder and useless.

The valve switching is automatically controlled by PLC programmable controller, and the working cycle can be adjusted according to the performance and process requirements of the carbon molecular sieve.

In order to ensure the long-term continuous production of galvanized lines, the configuration of the nitrogen generators adopts the scheme of two uses and one preparation or one use and one preparation.

Nitrogen purification device

The protection gas for hot galvanizing requires high purity. A nitrogen purification device is equipped with a pressure swing adsorption nitrogen generator, which is catalyzed by hydrogenation.

The method of oxygen and adsorption drying is simple and reliable, and the purification effect is good. It is easy to reduce the impurity oxygen in nitrogen to less than 1 ppm and the dew point to below -60°C. Its purity exceeds the cryogenic air separation. In order to meet the requirements of galvanized wire, the following optimization measures are taken:

The nitrogen purification device uses a high-efficiency palladium catalyst, which can purify the nitrogen purity from 99.5% to more than 99.9999%.

The nitrogen purification device adopts a hydrogenation catalytic deoxidation method. For the addition of hydrogen, low-pressure ammonia decomposition should be used, and the working pressure of nitrogen purification should be increased by 0.5 to 0.7MPa through hydrogen pressurization.

Water vapor has a strong oxidizing property in the high temperature furnace, so the moisture in the protective gas should be fully removed.

The nitrogen purification device is equipped with trace oxygen, trace water, and nitrogen-to-hydrogen analyzers to monitor nitrogen purity online. In addition, the valve switching and regeneration temperature control of the entire system can be automatically operated using a PLC programmable controller.

Besides hydrogen purification of 99.5% purity nitrogen. Now more and more customers are also using carbon purifier to get 99.9999% purity nitrogen. Its process is similar to hydrogen purification. After the 99.5-99.9% purity nitrogen is produced, one oxygen remover filled with carbon is equipped. With the help of catalyst, oxygen in nitrogen reacts with carbon to generate CO2, in this way, oxygen is removed and 99.9995-99.9999% purity nitrogen is produced. 

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