Aluminum Alloy Melting

In aluminum alloy melting, many factors affect the metal burning loss rate. The main factors include the type of melting equipment, the structure and condition of the charge, the way scrap is added, and the melting method used.

As mentioned earlier, the metal burning loss is highest in flame furnaces, usually about 1.5% to 3.0%. In electric resistance reverberatory furnaces, the loss is lower, usually about 1.0% to 1.5%. In electromagnetic induction furnaces, because the exposed liquid surface area is smaller for the same melt volume, the metal burning loss is the lowest, usually about 0.4% to 0.6%. Therefore, if the goal is to improve metal recovery, flame furnaces should be avoided as much as possible when melting aluminum alloys.

During aluminum production, process scrap and machining chips must be handled and reused in time. To reduce production costs and improve economic efficiency, many factories purchase outside scrap and increase the scrap ratio in the charge from 10% to 30%, or even more than 50%. In some alloys, finished products are produced entirely from scrap.

In a flame furnace, the flame directly contacts the metal. The furnace temperature is uneven, local areas may become overheated, and there is no protective atmosphere inside the furnace. As a result, metal burning loss is serious. When scrap is used for aluminum alloy melting, the burning loss becomes even greater. This is especially true when a large amount of third-grade scrap is used. In some cases, the burning loss in a gas-fired furnace can reach 15% to 20%.

When melting aluminum alloy in a gas-fired furnace, the gas flow should be reduced at the proper time during charging and melting. The furnace chamber temperature should be kept within the allowed melting temperature range for the alloy. Excessive flame and overly high furnace temperatures should be avoided. At the same time, the furnace atmosphere should be well controlled, keeping the furnace under slight positive pressure and forming a mildly oxidizing atmosphere.

When scrap is remelted, improper charging can also increase metal burning loss. For example, high-magnesium alloy chips or other alloy machining chips will easily burn if they are charged directly into the furnace. During melting, the charge, especially small chips and crushed material, should be prevented from direct contact with the flame.

When using third-grade returns or remelting secondary returns, a molten-bath melting method should be used. First, charge suitable material into the furnace and melt it to form a certain amount of molten metal. Then add third-grade or secondary scrap into the molten bath, so the scrap is submerged in the melt and does not directly contact the flame. After it reaches the required temperature and melts into the bath, stir it thoroughly. If there is a large amount of scrap, add it in batches and stir after each batch melts.

Another method is to spread chips on the furnace bottom and cover them with enough aluminum plates or sheets to prevent direct flame contact. Then heat and melt the charge. This method can also reduce burning loss and is called the sheet-covering method. Production practice shows that both methods are effective in reducing metal loss.

If effective protection is not used, metal burning loss will still increase. For example, using too little covering flux or applying it poorly can intensify metal loss. When solid charge is melted in air, especially when finished alloy is produced from third-grade scrap or when secondary returns are remelted, the charge must be covered with flux as it melts and sinks. The amount of covering flux should be sufficient and may be increased when necessary.

For example, when melting aluminum alloy in a 10 t gas-fired furnace, the covering flux amount is usually 100 to 200 kg per furnace for dry charge, and 800 to 1000 kg per furnace for secondary charge.

The covering flux should be added at the right time. If it is added too early, it may fall into gaps between the metal pieces and fail to protect the surface. If it is added too late, the charge will remain in direct contact with the flame for too long, causing burning loss. Practice shows that processing lump covering flux into powder and drying it before use is very effective for preventing metal burning loss.

In electrolytic aluminum plants, part of the liquid primary aluminum can be used directly, and then solid charge can be added. This reduces the contact area and contact time between the charge and the hot furnace gas, greatly reducing metal burning loss and improving recovery. It also significantly reduces the amount of covering flux required.

During melting, violent stirring that causes the molten metal to surge, or failing to remove dross in time, will also increase metal loss. Stirring and dross skimming should be done slowly to keep the molten surface stable. When a large amount of third-grade scrap is used, more dross is produced and should be skimmed in time. The metal contained in the dross should be returned to the furnace promptly to improve recovery. During centralized dross treatment, flux can be used to recover the aluminum in the dross as mixed aluminum alloy.

According to production practice at Northeast Light Alloy Processing Plant, after adopting the above technical measures in a 10 t gas-fired furnace, satisfactory results were achieved.

In summary, in electrolytic aluminum plants, melting aluminum alloys in electric furnaces, using the residual heat of liquid primary aluminum to process solid charge, and protecting the molten aluminum surface with enough dry covering flux are highly effective ways to reduce metal burning loss. These measures improve the metal recovery rate during melting and reduce flux consumption. They are also among the most effective ways to improve the economic performance of aluminum processing enterprises.