What is burning loss in metal smelting?
1. Noun explanation
Aluminum fusion casting involves transforming liquid aluminum into ingots, bars, or other shaped products through a series of steps including batching, stirring, settling, refining, and skimming. During the melting and casting process, various factors such as oxidation, refining, and skimming lead to varying degrees of loss in aluminum and its alloys.
The so-called aluminum casting loss refers to the irrecoverable metal loss and the total metal content in aluminum slag caused by oxidation, volatilization, and interaction with furnace walls and refining agents during the melting process of aluminum and its alloys. Generally speaking, this is colloquially referred to as burning loss.
The general formula for calculating casting loss (burning loss) is: (original aluminum quantity - finished product quantity) ÷ original aluminum quantity × 100%. The higher the casting loss, the lower the amount of finished products. For an aluminum enterprise with an annual output value of 100,000 tons, if the casting loss is reduced by 1 thousandth of a percent without additional investment, it will result in an extra production of 100 tons of aluminum products (i.e., a reduction of 100 tons of burning loss). This would yield significant social and economic benefits. Therefore, how to effectively reduce casting loss is very important. Some customers may ask about the yield rate of the melting furnace. The yield rate essentially consists of two parts: one is the burning loss rate, and the other is the proportion of impurities in the raw materials. These two factors determine the yield rate of the molten aluminum.
2、Causes of Casting Loss
2.1 The primary external manifestations of casting loss can be divided into two parts: one is in the form of pure aluminum ash, and the other is in the form of large chunks of aluminum, defective aluminum, and aluminum dross.
The melting and casting workshop has conducted data statistics, which show that non-recyclable pure aluminum ash accounts for approximately 90% of the casting loss (formed by oxidation burning and slagging), while other factors account for about 10%. Further statistical analysis on these 10% other factors reveals that they mainly consist of secondary burning losses from large chunks of aluminum, defective aluminum, and the aluminum content in aluminum ash (the main raw material of aluminum ash). Therefore, the intrinsic causes of casting loss are primarily due to oxidation burning, secondary burning of defective aluminum, and the aluminum content in aluminum ash.
2.2 The aluminum industry believes that the principle of aluminum oxidation loss can be further understood through the following chemical equation: (4Al + 3O_2 = 2Al_2O_3)
Thermodynamic studies on metal oxidation indicate that the tendency for metals to oxidize, the sequence in which alloy elements oxidize, and the extent of oxidation are all determined by the affinity between the metal and oxygen. These factors also depend on the composition of the alloy, temperature, and pressure conditions. The greater the affinity between a metal and oxygen, the stronger the trend towards oxidation and the higher the degree of oxidation; the higher the temperature, the greater the affinity between metal and oxygen, leading to a stronger trend towards oxidation and a higher degree of oxidation; the lower the decomposition pressure of the oxide, the greater the affinity between metal and oxygen, resulting in a stronger trend towards oxidation and a higher degree of oxidation.
Within the smelting temperature range, aluminum has a strong affinity for oxygen and is easily oxidized. After oxidation, an Al₂O₃ film forms on its surface. When the temperature exceeds 500℃, it becomes metastable r-Al₂O₃. During the transition from this metastable oxide film to a stable one, volume contraction occurs, followed by further oxidation and cracking. As the temperature of the aluminum liquid increases and time passes, the oxide film grows faster, and both the amount of oxidation and the thickness significantly increase.
2.3 The aluminum industry believes that the factors affecting casting loss include:
① Liquid aluminum temperature; ② Intensity of contact between liquid aluminum and oxygen; ③ Aluminum content in dross; ④ Aluminum carried out with skimming; ⑤ Amount of defective aluminum and large chunks of aluminum; ⑥ Other losses caused.
3.Measures to Reduce Casting Loss
3.1 Control the Liquid Aluminum Temperature
Aluminum has a melting point of 660°C. Generally, the casting temperature for primary aluminum is controlled around 730°C or even lower. Alloys with better fluidity have correspondingly lower casting temperatures than primary aluminum, approximately between 710°C and 730°C. For units that directly use liquid aluminum from electrolysis cells, when high-temperature aluminum enters the mixing furnace, it should be promptly mixed with cold materials, such as substandard aluminum, dross, etc. Additionally, some intermediate alloys (industrial silicon) can be added to the furnace in advance to create a pressurized melting state. This not only increases the yield but also reduces the temperature. The surface of the added cold material must be clean without any oil or dirt, otherwise, it might combust and release heat, promoting burnout. In summary, effectively reducing the aluminum liquid temperature to the appropriate casting temperature can significantly mitigate the impact of temperature on casting loss.
3.2 Reduce the Contact Intensity between Liquid Aluminum and Air The greater the contact intensity between liquid aluminum and oxygen, the more severe the oxidative burnout and casting loss.
(1) Minimize the contact time between liquid aluminum and oxygen: ① Under the condition of meeting production needs, convert the liquid aluminum in the furnace into finished products as quickly as possible. Prepare materials and produce within the same shift to avoid prolonged retention of liquid aluminum in the furnace. ② Reasonably arrange the melting and casting equipment to minimize the length of the flow channel, thereby reducing the exposure time of liquid aluminum to air. Additionally, cover the upper part of the flow channel with an aluminosilicate thermal insulation board, which not only provides some insulation but also reduces the oxygen content in the flow channel.
In summary, prevent liquid aluminum from being stored in the mixing furnace for an extended period due to various reasons, in order to reduce the contact time between liquid aluminum and oxygen and thus lower casting loss.
(2) Control the Stirring Method of Liquid Aluminum: Whether it is manually stirred with a large rake or mechanically stirred, both methods are carried out with the furnace door open. This not only causes significant fluctuations on the liquid surface and increases the contact area with oxygen but also raises the oxygen content in the furnace. Such conditions inevitably accelerate the aforementioned chemical reactions, resulting in increased burnout. Electromagnetic stirring can be performed in a closed state with minimal surface fluctuations, effectively avoiding these disadvantages. Additionally, it reduces the entry of moisture from the air into the furnace, lowering the probability of liquid aluminum absorbing hydrogen.
(3) Control the Height of Bubble Blowing During Liquid Aluminum Refining: The general refining method involves manually adding refining agents directly into the furnace, followed by stirring for refining. However, for the production of some alloys, nitrogen gas blowing is required (which takes longer, up to about 30 minutes). This will inevitably result in a certain height of bubbles that spread horizontally and vertically, causing significant fluctuations in the liquid aluminum. Therefore, **adjust the nitrogen pressure to control the bubble height within 10-15mm.
3.3 Proper Selection and Use of Refining Agents to Ensure Complete Separation of Dross from Aluminum During the melting process of aluminum and its alloys, apart from inherent impurities, aluminum readily reacts with oxygen to form alumina or sub-oxides, resulting in a layer of dross on the surface of the molten aluminum. This dross has a certain wettability with the aluminum melt, and a considerable amount of the melt is mixed within it. Therefore, a refining agent is needed to alter their wettability, increase the surface tension at the interface between the dross and aluminum, and facilitate the separation of the two. The fluxes used for aluminum and its alloys generally consist of chlorides and fluorides of alkali metals and alkaline earth metals. Their main components include KCl, NaCl, NaF, CaF₂, Na₃AlF₆, Na₂SiF₆, etc. However, the composition content varies significantly, leading to different effects. Apart from using fluxes produced by flux manufacturers, **it is necessary to adjust the proportion of flux components based on the composition of the aluminum alloy being melted. At the same time, strictly control the refining process conditions, such as the amount of flux used, the contact time and area between the flux and the melt, stirring conditions, temperature, etc. Using refining agents can effectively reduce the aluminum content in the dross and lower casting losses.
3.4 Effective Treatment of Aluminum Dross Produced During the Melting and Casting Process Aluminum dross is an inevitable part of the melting and casting process. Despite taking relevant measures, a certain proportion of metallic aluminum will inevitably be carried out. It needs to be effectively treated rather than being sold directly to other entities. The simplest and most economical method can involve repeatedly grinding the aluminum dross with a roller, followed by screening, thereby effectively recovering part of the aluminum beans.
3.5 Reduce the slope of the skimming slag in the mixing furnace to fully remove the aluminum dross from outside the furnace. The size of the slope of the skimming slag in the mixing furnace directly affects the amount of aluminum dross that can be skimmed off. If the slope is too steep, most of the slag cannot be removed, leading to a significant accumulation of aluminum dross and aluminum deposits. This makes it difficult to timely recover the slag and aluminum deposits during furnace cleaning. Therefore, under the premise of ensuring the capacity of the mixing furnace, it is advisable to reduce the slope of the skimming slag as much as possible.
3.6 Strictly control the quality of skimming to prevent liquid aluminum from being carried out. The current skimming operation is primarily manual, using a large rake to remove the aluminum dross from the furnace door. During this process, in addition to requiring careful operation by personnel to avoid carrying out liquid aluminum as much as possible, the design of the large rake also needs to be considered. It is recommended to make several rows of small round holes on the surface of the rake. This allows any liquid aluminum trapped in the dross to flow back into the furnace. Otherwise, if too much liquid aluminum is carried out and returned to the furnace, it can cause burning loss.
3.7 Reduce the amount of defective aluminum and large chunks of aluminum. During the production process, operate strictly according to the technical requirements to ensure that each furnace produces a qualified batch. Especially during the production of primary aluminum, avoid producing defective aluminum such as flash, burrs, ripples, and incorrect weights as much as possible. Additionally, towards the end of the production cycle, try to push the liquid aluminum in the trough into the mold to form qualified products, thereby reducing the amount of large chunks of aluminum.
3.8 Effectively deal with the already produced defective aluminum and other materials. For defective aluminum, large chunks of aluminum, as well as aluminum slag and aluminum beans produced for various reasons, adopt a suitable charging sequence into the mixing furnace. If necessary, perform waste recycling operations first to avoid unnecessary burnout.
In summary, although casting losses are inevitable during the melting and casting process, by controlling the temperature of the aluminum liquid, reducing the contact intensity between the aluminum liquid and air, controlling the aluminum content in the dross, and reducing the amount of defective aluminum, significant effects can be achieved in effectively reducing casting losses during the melting and casting process. This will undoubtedly bring considerable economic benefits to the enterprise.