Hydrogen content of aluminum melt in a 25 t fuel smelting furnace
The 25 t fuel smelting furnace has high combustion heat efficiency, fast melting speed, low pollution, and automatic temperature control. However, due to the high combustion pressure in the furnace gas due to diesel combustion, water reacts with Al to produce H2 (3H2O+2Al=Al2O3+3H2). Hydrogen enters the aluminum melt through physical adsorption, chemical adsorption and diffusion. The water pressure in the gas of the fuel-fired furnace is 7.3×10-3 MPa. The greater the water pressure in the furnace gas, the more hydrogen is absorbed in the aluminum melt. Taking smelting of 6061 aluminum alloy as an example, the hydrogen content in the smelt melted by the fuel flame furnace is as high as 0.296 mL/(100 g Al). Therefore, a degassing device with high efficiency must be prepared on the production line between the holding furnace and the casting machine.
2 How ALPUR Degassing Devices Work
(1) ALPUR is a double-rotor gas refiner, in which a refining gas is blown through a rotor and microbubbles are generated by a rotating nozzle. The rotating nozzle is made of graphite. In the design of the nozzle structure, the uniform ejection of air bubbles is considered. The rotating impeller is used to break up the air bubbles to increase the contact area and contact time between the air bubble and the aluminum melt, and the effect of purifying the aluminum melt is improved. Due to the large melt viscosity of aluminum alloy, if the diameter of the outlet hole is too large, it is easy to stick aluminum and block the pores. According to the flow rate of the gas and the pressure of the molten metal in the molten pool, the diameter of the rotor pore is calculated to be 2 mm to ensure the purification process. effect.
(2) ALPUR itself has two heaters for preheating the vessel prior to injecting the aluminum melt and maintaining or increasing the melt temperature depending on the characteristics of the alloy being cast.
(3) The aluminum melt at the exit of the furnace contains a certain amount of impurities (hydrogen, alkaline metals, slag inclusions) which will be removed in the ALPUR degassing purification unit.
ALPUR's treatment process is based on the principle of gas floatation. The rotating body injects inert gas or a mixture of inert gas and chlorine gas as fine bubbles and evenly diffuses into the melt. These bubbles tend to float upwards, and the purification of the aluminum melt is accomplished by:
1 Hydrogen is dissolved and adsorbed into the bubbles to remove hydrogen; 2 alkaline metals react with chlorine to form chlorine salts to remove basic metals; 3 slag inclusions are trapped by bubbles and larger slags are trapped in collision with bubbles; Smaller slags are trapped by radial cross-sections of the bubbles. The slag is then raised to the surface with air bubbles, see Figure 1.
Fig.1 Schematic diagram of purifying aluminum melt in ALPUR bath
3 ALPUR gas purification system for hydrogen purification equipment
The gas path of the ALPUR dehydrogenation purification device mainly includes an air pan with two chambers.
(1) Inert gas chamber: 1 Inert gas treatment circuit system; 2 Inert protective gas circuit system; 3 Inflatable sealing circuit system; 4 Pneumatic valve control circuit.
(2) Chlorine chamber (when using a mixed gas of inert gas and chlorine): 1 Chlorine circuit system; 2 Wash circuit system.
In the above system, there is a protection system for protecting the aluminum melt against oxidation, a purification system for removing hydrogen and deslagging, a protection system for preventing blockage of the gas path and improving the production efficiency, and a device for preventing the failure of the gas source to maintain the equipment. Not damaged auxiliary system.
4 nozzle speed and gas flow on the effect of purification of aluminum melt
(1) nozzle speed
Rotating nozzles play an important role in the refinement of bubbles and dispersion of bubbles in the aluminum liquid during rotation, but at the same time also cause the liquid surface to churn. As the rotational speed of the nozzle continues to increase, even forming a trumpet negative pressure zone in the center of the aluminum liquid increases the hydrogen absorption, oxidation and slag inclusion in the aluminum liquid. The blades of the spray head not only have a lateral stirring effect on the molten aluminum, but also have the effect of causing the gas-liquid mixture to be sprayed upwards. This spray action exacerbates the agitation of the liquid aluminum. Due to the mechanical action of the blades on the rotating spray head, the liquid aluminum is in a turbulent state. And due to the centrifugal force to make radial movement of aluminum liquid in the nozzle, the gas is sprayed into the aluminum liquid with the liquid aluminum flow. When the rotating speed of the nozzle is low, the nozzle has little stirring effect on the aluminum liquid, and the turbulence degree of the aluminum liquid is relatively low. The mechanical splitting effect of the blade on the gas is small, so the bubble size is larger; when the rotating speed is larger, the liquid aluminum The degree of turbulence is intensified, and the mechanical disintegration effect of the blade on the gas is large. At the same time, the centrifugal force of the aluminum liquid is large, the air bubble diffuses far away with the aluminum liquid, and the bubble has a small bubble formation probability, and the bubble size is small. Therefore, comprehensive consideration of the speed of the refinement of the bubble and the interference of aluminum liquid, in actual production, the nozzle speed should be 250r/min or so is appropriate.
The nozzle blades play an important role in dehydrogenation efficiency. It can collide and break up the air bubbles to make the size of the air bubbles smaller. At the same time, the centrifugal force is used to scatter the air bubbles and evenly disperse, which reduces the probability of combining bubbles and improves the effect of dehydrogenation.
(2) Gas flow
Gas flow is an important parameter for hydrogen removal from aluminum melts. In order to achieve a certain dehydrogenation effect in a molten pool with a capacity of approximately 1.3 t, sufficient gas injection velocity must be available. The greater the gas flow, the higher the dehydrogenation efficiency. However, the air flow rate must not increase unrestrictedly, and the amount of air supply can be adjusted within 1.5 m3/h to 6.5 m3/h. If the air flow is too large, there will be too many air bubbles in the reaction chamber. Relatively speaking, the amount of aluminum liquid to be treated will become less, and the time for gas and liquid contact will be shortened. At the same time, when the air flow is too high, the up-down convection of the aluminum liquid will be aggravated and the air bubbles will increase. The residence time in the aluminum melt is greatly reduced, thereby reducing the dehydrogenation effect. For this reason, in practical production, in order to reduce the influence of up-down convection and improve gas utilization, excessive air flow should be avoided as much as possible. Increasing the gas flow alone to increase the dehydrogenation efficiency is not effective, and the flow of gas should be controlled so that the liquid level of the aluminum does not churn. After field tests, for the 6061 alloy, the gas flow rate was 4 m3/h to 4.5 m3/h when the degassing rate reached more than 50% and the molten aluminum was not generated.
5 actual application effect
The production practice has proved that when the fuel (diesel) flame furnace smelts the aluminum alloy, the furnace in-furnace gas refining and the double-rotor gas purifier (ALPUR) are superimposed to obtain a satisfactory quality of the ingot. The hydrogen content of the 6061 aluminum alloy melt was measured with an ALSCAN liquid hydrogen meter. The results were as follows: 1 The content of the aluminum alloy after smelting was 0.306 mL/(100 g AL); 2 The melt after refining in a conventional furnace gas The hydrogen content was 0.278 mL/(100 g Al); 3 The hydrogen content of the melt after ALPUR double rotor gas refining was 0.136 mL/(100 g Al). Degassing rate of more than 50%, to meet the requirements of the next product.
It should be pointed out that the factors influencing the hydrogen content of aluminum melt are many, there are chemical components, there is the effect of melting temperature and time, there is the influence of air relative humidity, there are aluminum melting furnace baking quality, etc. . The amount of air supplied to the dual-rotor gas-refining device should be adjusted in accordance with their influence on the ingot.
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