The metal casting industry is one of the most energy-intensive manufacturing sectors with the melting process accounting for over half (55%) of its energy consumption. Although its high energy expenses have been a significant concern for metal casters, the industry continues to use melting technologies with poor energy efficiency.
The seemingly simple melting operation – heating metals to turn them into liquids for pouring – is actually a very complex process, involving a series of steps that incur both material and energy losses.
These losses are attributable to several factors: undesired conduction, radiation and convection, stack loss (flue gases), and metal loss. The extent of the losses depends on the furnace design, the fuel used, and the method of imparting heat to the metals.
The low thermal efficiency of current furnaces calls for high-priority actions to improve melting technologies.
The melting of any industrial metal used in manufacturing involves the following steps:
- Preparing the Metal and Loading – removing dirt and moisture and sometimes, preheating the charge material, such as scrap metal or ingot; and introducing solid charge into the furnace system
- Melting the Metal – supplying energy from combustion of fuels, electricity or other sources to raise the metal temperature above its melting point to a pouring temperature
- Refining and Treating Molten Metals – introducing elements or materials to purify, adjust molten bath composition to provide a specific alloy chemistry and/or affect nucleation and growth during solidification
- Holding Molten Metal – maintaining the molten metal in molten state until it is ready for tapping
- Tapping Molten Metal – transferring the molten metal from the furnace to transport ladle
- Transporting Molten Metal – moving the molten metal to the point of use and keeping the metal in molten state until it is completely poured
Material and energy losses during these process steps represent inefficiencies that waste energy and increase the costs of melting operations. Modifying the design and/or operation of any step in the melting process may affect the subsequent steps.
For aluminium die casters a metal loss of 1% of an annual melting output of 5,000 tonnes corresponds to a financial loss of more than 100,000 EUR.
Losing 3 to 7% metal on 5000 tons gives you anual losses of 300.000, to 700.000, euro direct losses per year.
Hence the lost metal means extra costs of 5 to 10 cent per one kilo casting weight. This is a not to be neglected magnitude.
Where can you save more than here! Try to earn 100.000, euro netto profit?
The metal casting industry is one of the most energy-intensive manufacturing sectors with the melting process accounting for over half (55%) of its energy consumption. Although its high energy expenses have been a significant concern for metal casters, the industry continues to use melting technologies with poor energy efficiency. Die casting companies loose like this enermous amounts of money!
Based on our findings, we were driven to explore this “Grand Challenge” and find breakthrough opportunities that might dramatically reduce melting energy and to identify energy-saving areas based on real findings. The potential energy-saving melting technologies posed several technical barriers for implementation which we solved one by one. We convinced our R&D to remove these barriers, as well as to continue further development of the innovative melting methods which will provide the best results in the long term. The key to improving efficiency of melting technologies for the die casting industry resides in developing technologies and logistics improvements that offer large energy savings and require low financial risk.
The seemingly simple melting operation – heating metals to turn them into liquids for pouring – is actually complex, involving a series of steps that incur material and energy losses. These losses are attributable to several factors: undesired conduction, radiation and convection, stack loss (flue gases), and metal loss. The extent of the losses depends on the furnace design, the fuel used, and the method of imparting heat to the metals.
In next table we compare the efficiency and metal loss for different types of furnaces. The low thermal efficiency of current furnaces used in Die casting companies calls for high-priority R&D to improve melting technologies, and generate huge savings.
|Furnace type||capacity||metal||metal loss||effeciency|
|Crucible (Gas)||5kg - 1.5 tons||Aluminum||4-8%||7-19%|
|Crucible (Gas)||5kg - 1.5 tons||Magnesium||4-6%||7-19%|
|Crucible (Gas)||5kg - 1.5 tons||Copper-base||2-4%||7-15%|
|Cupola||50kg/hr - 20 tons/hr||Iron||3-14%||35-40%|
|Direct Arc†||1.5 tons -100 tons||Steel||5-8%||35-45%|
|Immersion (low temperature melting)||800kg/hr||Zinc||N/A||63-67%|
|Induction†||1kg - 50 tons||Aluminum||0.75-1.25%||59-76%|
|Induction†||1kg - 50 tons||Copper-base||2-3%||50-70%|
|Induction†||1kg - 50 tons||Magnesium||2-3%||59-76%|
|Induction†||1kg - 50 tons||Iron||1-2%||50-70%|
|Induction†||1kg - 50 tons||Steel||2-3%||50-70%|
|Reverberatory Electric†||0.5 tons - 125 tons||Aluminum||1-2%||59-76%|
|Reverberatory Electric†||0.5 tons - 125 tons||Zinc||2-3%||59-76%|
|Reverberatory Gas||0.5 tons - 125 tons||Aluminum||3-5%||30-45%|
|Reverberatory Gas||0.5 tons - 125 tons||Zinc||4-7%||32-40%|
|Stack Melter (Gas)||1 ton/hr - 10 tons/hr||Aluminum||1,5-2%||40-45%|
|Our ECO MELTER(Gas)||50kg/hr - 10 tons/hr||Aluminum||<1,5%%||48-60%|
In face of rising energy prices and stricter environmental regulations there is increasing demand for heat treatment plants with greater energy efficiency.
Depending on the furnace size and the process there is always a certain amount of potential energy which can be recovered from the waste heat and re-used. This is especially true for large furnace systems or long process times which allow for huge energy savings that the additional investment has a short pay-back time. The thermal energy from finished charges can also be used to pre-heat cold charges which is also an efficient way of saving energy.
The following examples outline engineering alternatives for heat recovery we use in our systems:
The principle of the counterflow heat exchanger is to use the hot exhaust gas coming from the furnace to pre-heat the cold fresh air channelled into the furnace. In many cases, there is no need anymore for a separate fresh air preheating unit. Such a system is recommended if the process requires continuous air exchange in the furnace chamber, such as when tempering silicone, or during drying processes that are covered by the EN 1539 industrial standard.
Large gas-heated heat-treatment furnaces are especially advantageous for the installation of recuperator burners. Recuperator burners also use hot exhaust gas; to pre-heat the combustion air. Depending on the furnace model and the process, substantial energy savings of as much as 35% can be realized by using recuperator burners so that there is a short pay-back time for the additional purchase costs.
Our stack/shaft melters are available with production throughputs ranging from 150 kg per hour to 5000 kg per hour. The effective use of the residual melting heat rising through the stack to preheat newly added charge material and the recuperation of exhaust heat for the preheating of the burner air, greatly increases furnace efficiency. The stack furnace design can operate continuously and, therefore, is highly productive. Low flue temperatures result in reduced NOx emissions, reducing pollution control equipment requirements. With no wet bath charging, the stack melter furnace enhances operator safety.
Large doors provide easy access to the bath. The furnace design options can permit many possible options and solutions, like integrated filtering of the melt, degassing of the melt.
The stack melter’s energy savings, greater production and reduced emissions control costs, offer to all aluminum casters a quick payback on their investment.
Optional chip melting technology is available.
Reduced energy consumption even when melting large-volume or thin-walled material as a result of optimal use of the energy in the shaft.
Extremely low metal loss regardless of the charge material as a result of minimizing the oxygen content in the furnace atmosphere by means of near-stoichiometric combustion and self-sealing doors.
Without a clear understanding of all the contributing factors, cost cutting efforts in one area of the melting operation may adversely affect other costs. Additionally, potential improvements may be overlooked or prioritized incorrectly.
A metal casting melt department can measure and target five main areas for cost reduction:
- melt loss;
- maintenance and consumables;
Calculating Melt Loss
Any time you melt metal, losses will occur due to oxidation. Furnace practices and equipment have a major effect on these losses.
Ideally, each load charged to a furnace and each casting run should be accurately weighed. At least two out of three measurements should be made within the department: pounds in and pounds out or pounds dross. Estimations, such as conveyor loads or average charge loads, may be used in some cases. If these estimates are used, they must be rechecked periodically, as changes will occur over time.
The calculation of melt loss (pounds in minus pounds out, divided by pounds in) depends on how dross is handled. Metal casters can sell dross (equation 1) or send it out so the aluminum can be recovered for a tolling cost (equation 2). (Note: The tolling costs in the second equation should include the costs of shipping dross out and returning the recovered aluminum. All weights are in pounds; costs are in dollars per pound.)
Eq. 1. Total Cost = % Melt Loss x Aluminum Cost – % Melt Loss x Dross Price
Eq. 2. Total Cost = % Melt Loss x Aluminum Cost + % Melt Loss x Toll Cost – % Melt Loss x % Metal Recovery x Aluminum Cost
Calculating Energy Costs
Equipment, maintenance and furnace practices all affect energy costs. While a basic cost, it is only sporadically measured within a melt department. At a minimum, the department needs a gas meter. Since each furnace is different, each process should be measured. Meters must be maintained and read at least once a month. Be aware of internal conventions, such as reporting data by calendar months, billing months or fiscal months. Ensure the time period between meter readings coincides with the measurement of the pounds and includes both melting and holding energy usage. Contact your natural gas company to obtain the heat content of the natural gas you are purchasing.
A furnace may have a combustion blower, damper blower, floor blower, baghouse blower, conveyor motors and a pump motor, and their electric costs also should be measured. Other gases used at the furnace, including nitrogen, chlorine and oxygen, can be included within this section.
Natural Gas Cost = Gas Usage (m³/month or mcf/month) x Heat Content (kilojoule/cubic meter m³ or BTU/cf) x 1,000 (m³/1000m³) x Euro per m³ / Euro In / 1,000,000 (KJoule/m³)
Electricity Cost = Electric Usage (kWh/month) x Euro per kWh / Pounds In
Total Energy Cost = Natural Gas Cost + Electricity Cost
Calculating Labor Costs
The labor required for the melting operation includes anyone who is charging, skimming, cleaning or casting from the furnace. Labor is sometimes reduced by automatic charging or casting equipment.
Labor Cost = Annual Labor Cost / Annual Pounds In
Calculating Maintenance and Consumables Costs
The largest consumable expense is refractory. Refractory life can be detrimentally affected by operator practice, equipment and furnace design. Other consumable items in a furnace can include pump parts, doors and thermocouples. Labor for maintenance work should be included within this calculation.
Maintenance/Consumable Cost = Average Annual Cost / Annual Pounds In
Deprecation of the furnace, along with related charging equipment, pumps, baghouses, etc., should be included. The values used can vary from facility to facility. For instance, many furnaces that have been fully depreciated are still running, while depreciation might alternatively be charged to the melt department for equipment that has long been removed from service. (Note: Service life should be measured in years.)
Depreciation Cost = (Furnace Cost / Service Life + Pump Cost / Service Life + Charging Equipment Cost / Service Life) / Annual Pounds In