The Melting Triangle
In our industry, we all work on continuous improvement. Production supervisors worry about melt rate or throughput. How can we push more pounds through our current furnaces? Energy managers worry about energy efficiency. How can we reduce energy use? Metallurgists worry about melt loss. Oxidized metal is an immense loss that requires more metal to be melted and increases handling of the dross or slag.
This column is on energy; however, we need to understand that these properties are all related. The three points of melt rate, energy efficiency, and melt loss form a triangle. Cost improvements in one property can be negated by cost increases based on the other properties. The goal when making a change is to optimize the system, not just a single property.
The following are some examples of this relationship within the aluminum industry:
• Burners aimed directly at the metal increase the melt rate. Energy efficiency also can improve. However, melt loss is normally worse with flame impingement. Some flame contact can be handled by large charged material, such as sows. Yet, flame contact is bad for light and large scrap as it reaches molten temperatures. Remember, flame temperatures can be 2,500-3,000F (1,370–1,650C). These high flame temperatures increase the melt rate but also increase the oxidation rate. The high flame velocity can shift and move light scrap to the point of fully oxidizing the metal within the combustion gases.
• High fire rates can increase melt rate. After a point, the extra heat is not absorbed by the metal and instead escapes up the flue. Energy efficiency is worse as the flame rate increases without a corresponding increase in melt rate. Melt loss may get worse as the metal temperature increases.
• Oxygen burners are known to increase melt rate with higher flame temperatures. Energy efficiency is greatly improved since the nitrogen in air isn’t heated along with the oxygen. Yet, these higher flame temperatures can increase melt loss. Some oxygen burners are designed to keep a rich atmosphere near the metal surface to reduce oxidation.
• Slow melt rates may improve melt loss with cooler furnace temperatures. Or, slow melt rates mean a longer period for the metal to oxidize. Energy efficiency may or may not improve. The change needs to be tested and analyzed.
• Aluminum has been used as a fuel. For instance, some metalcasters purposely encourage thermiting to provide heat in dross to melt the free aluminum and allow it to drain. Don’t do it. Given current LME costs, the cost of aluminum as fuel is $56 per MMBtu. This is expensive “fuel” compared to natural gas at $4 per MMBtu.
Some changes can help the three properties of the melting triangle:
• Proper air-fuel ratios improve melt rate, energy efficiency, and melt loss. Either regular maintenance of the air-fuel ratio, better controls such as mass-flow with pressure and temperature compensations, or flue gas sensors can optimize the system for all three properties.
• Negative furnace pressure means cold ambient air is pulled into the furnace. This reduces energy efficiency because the cold air can cool charged metal, add excessive O2 to the mixture, or even short-circuit the combustion process. All of this can decrease the melt rate. The extra O2 in the furnace increases oxidation. Improving the furnace pressure system will improve the entire system.
• Reducing hold time may not improve melt rate but it improves throughput. Optimizing non-melt times, such as the door open time, chemistry checks, and maintenance time, or increasing casting rates can all increase throughput. At the same time, the reduced dead time means less energy is used and there is less time for the metal to oxidize.
When you make a change to improve one property, you need to understand how the other properties are affected. More throughput doesn’t help if energy efficiency or melt loss is worse. A more energy efficient furnace may be more expensive to operate if throughput or melt loss is worse. The total cost per pound is critical. Run strictly controlled tests measuring all of these properties before formalizing any change. This means the charge pounds need to be weighed. Energy use must be measured. If possible, measure both pounds out and dross. If you can’t run controlled tests, at least monitor these three properties over a time period such as a day, week, or month.
What is the moral of the story? When you plan a furnace or process change, you need to consider all three properties of melt rate, energy, and melt loss (the melting triangle) to produce the lowest cost material.
Click here to see this story as it appears in the October 2017 issue of Modern Casting