Furnace Temperature Control and Aluminum Alloy Batching

IV. Temperature Control

So far, we have discussed how to avoid damage caused by creep due to excessive stress and repeated bending during design. We have also discussed the influence of design work on local overheating.

In the discussion, the temperature problem of those parts where serious damage to the heat exchanger is caused by overheating has not been mentioned. The level of this temperature depends on the properties of the heat exchanger metal material. These properties include mechanical strength, elongation at high temperature, and corrosion resistance.

Therefore, it seems reasonable to measure the metal temperature at dangerous points and use it to control the waste gas inlet temperature of the preheater. However, the only reliable method for determining the most dangerous point is based on existing damage experience from similar heat exchangers.

Therefore, in engineering practice, the waste gas temperature entering the heat exchanger is preferably used for control. In general, it is almost always necessary to reduce the waste gas temperature. Mixing in cold air or spraying water can reduce the waste gas temperature. If hard water is used, scale will adhere to the surface of the heat exchanger.

The important point is how to mix in cold air or spray water, because serious temperature unevenness must be avoided. Considering temperature unevenness and secondary combustion, a low waste gas inlet temperature is generally maintained.

The maximum waste gas inlet temperature specified by metal heat exchanger manufacturers is usually within the range of 930–1040°C. This range considers variations in design, material, and safety factor.

To keep the metal temperature of the heat exchanger within a safe range, there are two other methods.

One method is to inject cooling air into the flue at high speed at a certain point before the heat exchanger, if space permits. In this way, secondary combustion has already been completed at the place where the pyrometer for control is installed. This preventive method allows the waste gas temperature at the inlet of the heat exchanger to be high, because the safety factor for secondary combustion does not need to be considered.

The second method is used when reducing the heat load. At this time, the fuel amount is reduced, and the amount of air supplied to the burner is also reduced. Unless this second preventive method is used, the metal will overheat because the heat cannot be carried away by the air.

This method keeps the air volume constant and discharges part of the hot air after the heat exchanger. By always maintaining the maximum air velocity, the heat exchanger can operate more safely and durably at the highest possible preheating temperature. However, to ensure that an accurate amount of air is supplied to the burner, an air measuring device must be installed after the heat exchanger. Several methods can be used, and the most commonly used method has already been discussed in Chapter 5. Excess air is discharged through a controlled relief valve.

If there is no accidental interference, as long as the design is correct, the manufacturing is careful, and the maintenance is appropriate, the service life of the heat exchanger can be as long as the interval between major furnace repairs.

For inspection, including tightness testing, whether the heat exchanger is installed below, above, or on the side of the furnace, it must be accessible to personnel. Figure 155 shows a metal heat exchanger installed above a continuous heating furnace.

Aluminum Batching Work

1. What Should the Batching Worker Do First After Starting Work?

After taking over the shift, the batching worker should first arrange the order of aluminum tapping from the electrolytic cells according to the production tasks of the current shift. The batching worker should also contact the electrolysis deputy shift leader and aluminum tapping workers in time, and handle temporary problems occurring in the electrolysis plant promptly.

After understanding the tapping condition of liquid primary aluminum, the batching worker should also understand the inventory status and chemical composition of various raw materials, so that materials can be reasonably selected according to the chemical composition of the finished alloy to be produced.

The temperature of molten aluminum is extremely important to product quality. The temperature of the mixing furnace should be strictly controlled. After batching, the temperature of liquid aluminum should comply with the operating procedure requirements to prevent overheating.

When the mixing furnace is not working, the furnace doors must be closed properly to avoid excessive heat loss. Even during operation inside the furnace, all furnace doors should not be opened at the same time, so as to avoid excessive fluctuation of furnace temperature.

2. What Is the Basis for Batching?

The task of batching is to ensure that the chemical composition of the product meets the specified requirements.

The basis for batching includes the standard chemical composition requirements of the cast product, the pre-analysis report made by the enterprise central laboratory for the liquid primary aluminum in the electrolytic cells to be tapped during the current shift, and the inventory status and chemical composition of various raw materials.

The pre-analysis report of liquid primary aluminum in the tapping cell is the analysis result of the sample taken one day before aluminum tapping. Due to fluctuations in electrolytic process conditions, there may sometimes be errors between the aluminum tapping result and the pre-analysis report.

For a specific casting unit, the furnace charge amount should first be determined according to the ingot specification and size, furnace capacity, and metal burning loss. Then the batching calculation is carried out.

Batching should be carefully calculated according to the pre-analysis report to prepare molten aluminum that meets quality requirements.

The standard chemical composition requirements of products include national standards and enterprise standards. Enterprise standards are established to ensure that product quality meets the requirements of national standards.

In wrought aluminum alloys, there are two major categories: aluminum alloys that cannot be strengthened by heat treatment and aluminum alloys that can be strengthened by heat treatment.

Among wrought aluminum alloys that cannot be strengthened by heat treatment, there are the following aluminum alloy series:

• Industrial high-purity aluminum and industrial pure aluminum
• Clad aluminum
• Al-Mg series anti-rust aluminum
• Al-Mn series anti-rust aluminum

Among wrought aluminum alloys that can be strengthened by heat treatment, there are the following aluminum alloy series:

• Hard aluminum, including Al-Cu-Mg series and Al-Cu-Mn series
• Forged aluminum, including Al-Mg-Si series, Al-Mg-Si-Cu series, and Al-Cu-Mg-Fe-Ni series
• Super-hard aluminum, including Al-Zn-Mg-Cu series

Wrought aluminum and aluminum alloys all require reasonable batching, and the impurity content must also be reasonably controlled.

Under the current condition that high-alumina bricks are widely used as the lining of melting furnaces, even when producing industrial pure aluminum ingots, liquid primary aluminum of the same grade may be used. However, to avoid increased impurity content during melting and reduced quality, liquid primary aluminum of one grade higher should account for 20%–30% of the total furnace charge. Especially when producing industrial high-purity aluminum ingots, liquid primary aluminum of one grade higher should be used. During the melting process, the grade is usually reduced by one level.

Al-Mg alloys include alloy grades such as 5A02 (LF2), 5A03 (LF3), 5A05 (LF5, LF11), 5A06 (LF6), 5B05 (LF10), and 5A12 (LF12). Those with magnesium content greater than 4% are commonly called high-magnesium aluminum alloys.

The hot cracking tendency of high-magnesium aluminum alloy ingots mainly depends on the degree of melt contamination. The minimum critical sodium content that causes cracking is 10^-3%.

Industrial production statistics show that for 5A12 (LF12) alloy flat ingots of 300 mm × 1200 mm, when the sodium content is above 3.5 × 10^-4%, surface cracks are likely to occur on the ingot if no antimony is present. When the sodium content increases, the antimony content must be increased accordingly to prevent cracking.

To improve the stability of the casting process for ingots of this specification, the antimony content should preferably not be lower than 0.013%.

High-magnesium aluminum alloys have high oxidizability. A loose and porous oxide film forms on the ingot surface, which often becomes the cause of cracking. To eliminate the tendency of hot cracking in ingots and improve ingot surface quality, except for alloys with specified antimony content, it is recommended to add 0.001%–0.002% beryllium to the melt, especially in large flat ingots.

Aluminum alloys 3A21 (LF21), 5A02 (LF2), and 6A02 (LD2) have no tendency to form cold cracks.