1. Comparison of Process Flow between Primary Aluminum and Recycled Aluminum
Primary aluminum starts from bauxite mining and is refined into alumina through the Bayer process. It is then reduced to liquid aluminum through Hall Elute electrolysis and finally cast into primary aluminum ingots. Recycled aluminum directly uses various types of aluminum waste as raw materials, bypassing mining and electrolysis processes. The core processes include feeding preheating, melting, refining, insulation, casting, sawing, and homogenization heat treatment.
Differences in production processes between primary aluminum and recycled aluminum
Waste is divided into internal waste and external waste. Internal waste comes from the production process of our factory, such as casting risers, sawing edge scraps, and rolling edges. These types of waste have clean composition and generally do not require complex pretreatment. They can be directly remelted in the furnace; External waste comes from scrapped products on the consumer end, such as automotive components, aluminum cans, cables, and household appliance aluminum parts, which need to be sorted and uncoated before they can be put into the furnace. The refining process after melting includes alloying and blending components, flux slag removal, rotary degassing, ceramic filtration, and removal of molten hydrogen and non-metallic inclusions; Afterwards, the melt enters the insulation furnace for static temperature control, completing semi continuous casting to obtain aluminum plate blanks. Some of the blanks also need to undergo long-term homogenization heat treatment to eliminate casting segregation and improve subsequent rolling processing performance.
The complete material energy input and output process of SAPA factory is shown in the following figure. The system boundary includes the entire process of preheating furnace, melting furnace, insulation furnace, casting, sawing, and homogenization heat treatment; The input includes raw aluminum ingots, internal and external waste materials, flux, propane fuel, electricity, oxygen, and argon gas; The output includes aluminum plate blanks, aluminum slag, various process wastes, and energy loss carried by flue gas and cooling water.
2. Main types of melting furnaces
The research focuses on comparing induction furnaces widely used in industry with oxygen fuel furnaces. (1) Induction furnace: relying on electromagnetic induction to directly generate Joule heat inside the metal to melt aluminum material, without the need for fuel combustion, with minimal smoke emissions, and the melt is not easily contaminated by combustion products; The main energy loss comes from the heat dissipation of the furnace wall, the heat carried away by the slag removal aluminum slag, and the large amount of heat carried away by the cooling water of the induction coil.

2) Oxygen fuel furnace: using propane as fuel, without air to assist combustion, using pure oxygen to assist combustion. The combustion of ordinary air fuel will bring in a large amount of nitrogen, which is heated and carried away with a large amount of heat by the exhaust gas; Oxygen fuel combustion does not contain a large amount of nitrogen in the flue gas, which can significantly reduce the total flow rate of the flue gas and minimize heat loss; The equipment is equipped with electromagnetic stirring EMS to enhance melt heat transfer and improve melting rate. The main heat losses of oxygen fuel furnaces come from high-temperature flue gas, furnace walls, cooling water, and aluminum slag.

The factory is also equipped with preheating furnaces, which are divided into fuel direct combustion preheating furnaces and flue gas waste heat preheating furnaces; The flue gas waste heat preheating furnace recovers oxygen - the high-temperature flue gas heat of the fuel furnace dries and preheats the waste materials into the furnace without the need for additional fuel consumption, which is an important energy-saving measure. All insulation furnaces are gas-fired, maintaining the temperature of the melt and completing refining and slag removal; All homogenization heat treatments are carried out using electric heating furnaces.
3. Research accounting methods
Following the first law of thermodynamics, the total input mass is equal to the total output mass; Input total energy=effectively utilized heat+various types of heat losses. [Q {in}=Q {out}=Q {useful}+Q {loss}] Heat loss is subdivided into: furnace wall radiation+convective heat dissipation loss, flue gas sensible heat loss, heat carried away by cooling water, temperature drop heat loss during transportation, and sensible heat carried away by aluminum slag and process waste. Simultaneously calculating radiative heat transfer and convective heat transfer for furnace wall heat dissipation; The heat loss of flue gas is calculated based on the enthalpy difference of each component in the flue gas; The enthalpy values of materials at different temperatures are calculated using the Thermo Calc thermodynamic software TCAL1 database to obtain the enthalpy values of aluminum and alloys at different temperatures, which are used for heat calculation during transportation, heating, and cooling processes.
Following ISO14040 and ISO14044 standards, adopting a Cradle to Gate boundary, excluding the subsequent use and scrap recycling stages of the product.
Carbon emissions include direct and indirect emissions:
Direct emissions: The combustion of propane and diesel within the factory produces greenhouse gases;
Indirect emissions: emissions from upstream primary aluminum ingot production, waste pretreatment, power production, and auxiliary material preparation.
The electricity used by the factory comes from Vattenfall nuclear power in Sweden, with a carbon emission factor of only 6.5 gCO ₂ eq/kWh; Native aluminum ingots are sourced from the EAA database of the European Aluminum Association; The burden of pre-treatment for returned waste materials within the internal factory is calculated as 0; External recycling waste is included in the carbon emissions of crushing and sorting treatment; The contribution of oxygen, cooling water, and argon is very low and is ignored in the calculation. The carbon footprint is equal to the consumption of various materials multiplied by the corresponding carbon emission factors, and accumulated to obtain CO ₂ equivalent per ton of product.