Better Energy Efficiency Through Better Recovery
When we look at the energy efficiency for a metals plant, we look at the energy per lb. (or ton or kg). If you look at BTU per input pound, you are determining the energy efficiency of the equipment. If you look at the BTU per output pound, you are determining the real energy efficiency of the plant. The difference is the amount of metal that drops out of the process along the way. This can include melt loss, planned and unplanned losses, and scrap.
Melt Loss—If you are melting metal, these are losses due to the oxidation of the metal called dross or slag. This loss may not be your greatest loss based on the percentage of input metal but it can be your greatest cost since this metal can’t be recovered once the metal is oxidized. Instead, you have to buy more metal to replace it.
Numbers are hard to find and there is confusion between % dross and % melt loss in the literature. However, 2% melt loss in aluminum is considered average. The range can vary from 0.5–15% depending on the scrap type and process. Better control of the process can reduce this loss. Things such as reducing oxygen in the furnace with better seals or a lower air/fuel ratio may help. Some technologies such as vortex feed can pull the charge under the melt quicker to reduce oxidation.
Look for opportunities throughout the molten process. High molten metal drops and turbulence can also increase melt loss.
Recovery Losses—We all know there are losses through the process. Metal may not be actually lost through oxidation, but the metal is lost to your first-time-through process and the energy is wasted. This metal either has to be remelted or sent to someone who remelts it.
Planned recovery losses include:
• Metal during the casting process. This may include gates, runners, and trough skulls.
• Ingots, rods, billets, and sheet are cast longer than needed. Ends tend to be cracked, uneven, and may have a slightly different alloy or metallurgical properties.
Extra metal is cut off “just in case”.
• Castings are made larger “just in case” to handle heat treat distortions or process variations like cooling temperatures.
• Metal is machined to a final product. The chips or pieces are remelted. By the way, metal chips typically are more easily oxidized than larger pieces when remelted.
Unplanned Recovery Losses—Stuff happens. There are spills and splashes. Incorrect alloys can mean that metal is poured off to be reused. More metal is melted than expected due to communication errors. These all cause losses where the metal must be remelted.
Within the metals industry, 34% recovery losses are considered typical. Analyze how much metal is “lost” at different points in your process and the cost of that loss. The loss includes both the energy and labor that went into making that material. Work through the most costly losses with a team or other method. Don’t be satisfied with the status quo and the addition of metal “just in case.” Try to control your process better, use computer solidification modeling where appropriate, and look for new technologies to allow near-net-shape.
Scrap—Scrap can occur throughout the process. Castings, ingots, and rods can crack causing the entire piece to be scrapped. The metallurgical properties of the product could be incorrect. If machining is inaccurate, the final piece may need to be scrapped. And worst case, pieces are returned from the customer.
Scrap can be remelted, but you’ve already put energy into that material. The metal industry average is a 5% scrap rate, which includes most metal types. This can be reduced by identifying the top scrap causes and working the problem by using continuous improvement teams or with an engineering project.
With all these, that means an average of 41% of the metal that is melted is “lost” through the process and the yield is only 59% (Fig.1). It is not unusual to melt twice as much metal as what is ultimately needed! All this metal uses energy, but that metal is not seen by the customer. By taking the time to improve your melt loss, scrap rates, and recovery, you can improve your plant’s energy efficiency.
Click here to see this story as it appears in the January 2019 issue of Modern Casting