One of the biggest deterrents against use of waste heat recuperators and boilers in aluminum smelting furnaces is the use of chemicals to make molten metal flow.
Many of these compounds contain chlorine or fluorine, which react with the hydrogen in the metal to form hydrogen chloride or hydrogen fluoride. In the presence of water, both compounds form highly corrosive acids.
Below, we will talk about the opportunities that exist for the successful use of waste heat boilers and metal recuperators in aluminum melting furnaces.
Metal treatment techniques
The three main reasons to treat aluminum are:
1. To remove the dissolved hydrogen in the metal (degassing)
2. To remove dirt and other impurities in the metal
3. To remove all or part of the magnesium
To achieve these results, a wide variety of metals and processes are used:
• Filtering the molten metal from the furnace through a filtering chamber, a bed of solid material (usually alumina) before it reaches its destination. The filter bed removes solid impurities from the metal. Some filters are also equipped to degas the metal by bubbling nitrogen or argon through it.
• The gas flow bubbles nitrogen, chlorine or a mixture of both through the molten metal. This can be done in the melting furnace chamber, in a separate holding/refining furnace, or in a channel attached to the smelting furnace.
• Nitrogen removes solid impurities and hydrogen gas by mechanical washing without producing corrosive by-products.
• Chlorine removes impurities, hydrogen and magnesium by chemical action. In addition, at the same time elements such as hydrogen chloride, small amounts of aluminum chloride and magnesium chloride are created from the metal surface.
Careless use of gas can also release excess chlorine. Comparatively, chlorine is a much more effective and cleaner degasser than nitrogen. However, it is more expensive and involves certain environmental problems.
Nitrogen mixtures with up to 30% chlorine, known as combined flows, are used in an attempt to combine the best characteristics of both gases. The mixture is much more effective than pure nitrogen, and at the same time does not produce such high concentrations of toxic and corrosive fumes.
Due to environmental problems, the use of chlorine has decreased considerably in recent years. Some plants have completely replaced it with flux or nitrogen filtration. Others have reduced their use by switching to combined flows or devising ways to do the same job with less gas. One of these methods is to add the salts and generate the fluxing in an open pit, rather than in the main furnace chamber.
The restricted volume of the open well provides better gas-to-metal contact. In a properly designed installation, the melting gases are collected in an extractor hood instead of passing through the furnace ducts.
Solid fluxing involves treating the metal with a mixture of salts. The salts used will depend on the purpose of the flux.
Degassing fluxes generally contain hexachloroethane as the main ingredient. They may also contain sodium chloride, potassium chloride, potassium fluoborate, potassium fluoride and cryolite. These salts also remove solid impurities and magnesium. Their use produces hydrogen chloride and sometimes hydrogen fluoride.
Slag or burr (metal cleaning) fluxes are generally also based on hexachloroethane and may additionally contain sodium carbonate, sodium nitrite, cryolite, sodium chloride and potassium chloride. Hydrogen chloride is the main by-product. Some mixtures may also produce hydrogen fluoride.
Magnesium removal is usually carried out with the help of aluminum fluoride. Its intended by-product is magnesium fluoride, which is mostly trapped in the slag or burrs, but inevitably some hydrogen fluoride (gas) is formed.
Operational cycles
The fluxing cycles of the melt vary and can influence the viability of using boilers and waste heat recuperators.
• Fusion-fluxing cycle. The furnace is loaded with cold metal. The load melts, flows and is removed from the furnace. Then the furnace is loaded again. Because fluxing is carried out in the furnace itself, fluxing gases or fumes enter the combustion system, but only during a short period of the furnace cycle.
• Fusion-holding cycle. Two or three furnaces are used: a fusion furnace, where the cold load decomposes, and one or two holding furnaces, in which the load is removed. Fluxing is carried out in the holding furnaces. The melting furnace extraction system does not contain or ever see fluxing gases or fumes. On the other hand, the holding furnace’s exhaust system carries fluxing gases a large part of the time.
• Continuous charging cycle. A single furnace is continuously charged. Due to the nature of this operation, fluxing is also continuous. Fluxing can be carried out in the main furnace chamber or in a separate open pit attached to the furnace.
Formation and action of corrosive products
Corrosion is a potential problem only with those flows that contain chlorine and fluorine. Whether solid or gaseous, these flows react with molten metal to form aluminum chloride and magnesium chloride. When these compounds come into contact with water vapor, they react to form hydrogen chloride, hydrogen fluoride, aluminum oxide and magnesium oxide.
Aluminum chloride is the worst problem, since it sublimes at approximately 180°C (356°F) and easily enters the combustion system. It is also highly hygroscopic and reacts quickly with water vapor.
Dry hydrogen chloride and hydrogen fluoride are not particularly corrosive to carbon or stainless steels. However, once they come into contact with liquid water, they form highly corrosive hydrochloric and hydrofluoric acids, which attack carbon steel and most stainless alloys.
Above 110°C (230°F), hydrochloric acid will not condense on metal surfaces. Hydrofluoric acid is still vapor above 120°C (248°F). Theoretically, corrosion should not be a problem above these temperatures, and most recuperators and waste heat boilers operate well above these temperatures. In practice, however, all furnaces and heat recovery systems are turned off or cooled from time to time, and that is when corrosion problems can occur.
Aluminum chloride also represents a prevalent hazard. Even if the extraction system is completely purged of fluxing gases, aluminum chloride may have accumulated in the heat recovery device and in the ducts. As soon as it comes into contact with water vapor (from atmospheric moisture or combustion products), hydrochloric acid will form.
Theoretically, the normal temperatures in the flue gases in aluminum smelting furnaces provide a window on the corrosion behavior of metals exposed to hydrogen chloride and hydrogen fluoride. In practice, however, fluctuations and/or variations in temperature during normal operations make this window risky to use. Consequently, heat recovery devices are exposed to a high risk of corrosion failure each time the furnace combustion system transports chloride or fluoride gases.
Various options
Due to the wide variety of fluxing and fusion practices in use, each potential application has to be evaluated on its own merits. Figure 1 divides this evaluation process into a logical series of steps.
Waste heat boilers and metal recuperators can be applied to aluminum melting furnaces if halogen-free metal treatment methods are used, or if halogen flux gases do not pass through the furnace combustion system. If the furnace flue is used to continuously extract fluxing dross, heat recovery equipment should not be used.
If fluxing is performed intermittently in the furnace, heat recovery equipment may be used if measures are taken to avoid it during fluxing operations. This requires the installation of a double flue system and isolation valves in the heat recovery device. This may be difficult, but it is perfectly possible.
At Nutec Bickley we are experts in the installation and customization of energy recovery systems for industrial furnaces. Come to us if you are interested in a complete solution or if you need to update your current equipment.
