Over 1,085 million tons of steel are recycled worldwide annually, with over 60 percent of all steel ever manufactured ultimately getting the recycling treatment.
It’s easy to take this for granted, but are you familiar with the details of the process of turning this material from scrap to steel — the recycled steel manufacturing process?
The essential steps undertaken in the recycled steel manufacturing process include collection, sorting, shredding, melting, refining, casting, and rolling. These steps are undertaken chronologically to produce high-quality recycled steel for sustainable construction.
Besides construction, recycled steel is also used in the automotive, health, and food industries.
I’ll dive into the steel recycling process with you in this article. I’ll discuss everything, from steel scrap collection and sorting to quality control practices. Keep reading to understand the entire process from start to finish.
Recycling steel promotes green building by conserving the energy used in virgin steel production and minimizing greenhouse gas emissions and environmental degradation due to primary steel production.
However, the quality of recycled steel depends on the recycling process. You’ll only get high-quality recycled steel by following the right procedure. Such a procedure should ensure the resulting product is contaminant-free for strength and durability.
The process of manufacturing recycled steel from scrap steel entails the following steps:
Recycled steel is made from steel scrap. Therefore, the first step is getting the right steel scrap for recycling.
Scrappers collect steel scraps for recycling at buy-back centers, from designated drop-off points, or in person.
This scrap comes from the following sources:
- Home scraps: These are residual in-house metal leftovers from manufacturing processes. They’re those steel products that don’t reach the market due to manufacturing defects or trimmings. Home scrap doesn’t entail collection because it usually goes straight back into the furnace.
- Industrial (prompt) scrap: Prompt steel scrap also comprises residual in-house leftovers from manufacturing processes. However, the difference between prompt and home scrap is that while the latter is used within the facility from where it was created, the former is taken off-site to a milling plant for processing. Industrial scrap is normally new and clean scrap metal left over from producing sheets, vehicles, and electronics.
- Obsolete scraps refer to the rest of steel waste encompassing end-of-life consumer products and old auto parts. They also include individual household steel appliances that might otherwise end up in landfills.
After collecting the scrap from the above sources, the next step is to sort it for purity.
Sorting is an essential part of steel scrap recycling. It ensures the end product is contaminant-free and suitable for the intended applications.
Sorting involves differentiating the different types of metals and alloys from each other.
Traditionally, manual methods like the use of nitric acid and magnets were common. However, due to technological advancements, there are modern, efficient steel scrap sorting techniques like:
- X-ray Fluorescence (XRF): It’s an analytical technique that uses X-rays to determine the composition of scrap metal. It beams high-energy X-ray radiation into the metal and analyzes the energy emitted, in the form of photons, to determine the scrap metal’s chemistry, including which elements (e.g., other metals) it contains.
- Laser-Induced Breakdown Spectroscopy (LIBS): This uses low-level and optical technology to assess steel scrap for its composition. These devices ablate the metal’s surface using a high-focused laser. They then analyze the features of light wavelengths emitted from the metal surface to determine its composition.
Steel scrap is transported to the recycling mill after sorting.
It’s important to note that scrap steel comes in different shapes and sizes, but processing large and irregular steel scrap is tedious and consumes more energy. Larger pieces make recycling cumbersome by occupying more space, so the scrap is shredded before further processing.
The scrap pieces are fed along a conveyor belt into a shredder, where spinning heavy-duty hammers break down the metal into smaller manageable chunks.
The most commonly used shredders include:
- Hydraulic power systems: It’s the preferred option for heavy-duty shredding due to its ability to manage batch-feeding overloads.
- Electric power systems are ideal for shredding services requiring less room while performing optimally.
After shredding, the scraps undergo separation to ensure the right composition of material is passed to the furnace.
Most recycling firms use the eddy current separation technique to separate the metals.
The eddy current separation technique uses a conveyor belt system and a powerful magnetic field to separate non-ferrous metals from other materials.
The separated steel scrap is transferred into large electric arc furnaces (EAF) for melting.
The furnaces operate using high-voltage electricity to generate temperatures over 3,500°C (6,332°F) to turn metal scraps into liquid.
Oxygen and slag formers, such as magnesium oxide and calcium oxide, are added into the furnace, promoting slag formation over the molten steel. Coal or coke is injected into the slag layer later in the heating operation, which reacts with the iron oxide, causing the slag to foam, increasing the furnace’s thermal efficiency, and forming metallic iron.
The slag creates an envelope on the molten scrap, providing insulation to prevent the content from cooling and solidifying.
It’s worth mentioning that the amount of slag formers, coal, and oxygen added into the furnace depends on a predetermined formula. The wrong proportions of these materials lead to sub-optimal process conditions and impact product quality and emissions from the process.
Refining and purification are necessary to improve the final product’s quality by removing impurities.
According to Research Gate, impurities like sulfur and phosphorus reduce the material’s weldability, toughness, and fatigue strength.
Other impurities like zinc, tin, and lead can impact with recycled steel’s surface quality.
Steel recycling companies use different methods to refine and purify molten steel scrap. The two most common methods include:
- Electrolysis: This uses electricity, water, and metal to extract impurities from the molten steel. A direct electric current is passed through the molten steel scrap for dissolving and separating pure steel from contaminants like tin, zinc, and dirt.
- Magnetism involves passing a powerful magnet over the sludge to separate ferrous from non-ferrous metals.
Casting and rolling are the processes that solidify molten metal into semi-finished billets and, later, the final product.
Traditional casting and rolling involved pouring the molten metal into molds to form ingots. However, modern casting and rolling use the more efficient continuous casting and rolling method.
The continuous casting process entails the following steps:
- Feeding the molten steel into the mold: A tundish above the mold feeds molten steel into the mold at a controlled rate for cooling.
- Primary cooling: The liquid steel from the tundish is fed into a water-cooled copper mold to create a solidified outer shell. This shell is strong enough to sustain the strand shape before proceeding to secondary cooling.
- Secondary cooling: There is a containment section below the mold with a secondary cooling zone. The molten steel strand from the primary cooling zone flows into the containment zone through the secondary cooling zone. The steel is sprayed with water and air as it flows through these components to solidify the strand more.
- Cutting: The casting system has a severing unit that uses mechanical shears or a cutting torch to cut the solidified recycled steel strands into pieces based on the required product.
Steel rolling involves plastically deforming the steel strands from the casting step by passing them between rollers in a hot rolling mill. This shapes the steel strands into the required profiles and sizes.
The main products of rolling include:
- Steel sheets
The basic principle of steel rolling is subjecting the strands to high compressive force between the rollers.
The compressive force plastically deforms the strands by affecting their grain structure and reducing their thickness, forming them into the desired shapes and sizes.
As with any metal production, quality control in steel recycling is vital to ensure good quality and reliable recycled steel.
Consistently good quality recycled steel safeguards the company’s reputation and increases customers’ trust.
Quality control in steel recycling is undertaken through the following tests:
- Metallurgical testing: This entails evaluating the properties of recycled steel by subjecting it to mechanical, chemical, and thermal testing. Companies use these tests to assess the product’s composition and structure.
- Hardness value testing: This evaluates the recycled steel’s ability to withstand compression under different conditions.
- Tension testing: This evaluates the material’s strength and ability to withstand external tensile forces without breaking.
- Fatigue testing: This determines the steel’s behavior under fluctuating loads.
- Ultrasonic testing: This uses high-frequency sound waves to detect and locate shrinkages, cracks, and internal voids, affecting the steel’s integrity.
Scrap metal identification is essential in sorting different metal types for reuse.
Scrap metals are classified into two categories:
- Ferrous scrap metals: These are the most common types in recycling yards. They contain iron and, as a result, are magnetic. Examples include mild steel, carbon steel, wrought, and cast iron.
- Non-ferrous scrap metals: These are more malleable than their ferrous counterparts and are non-magnetic because they don’t contain iron. Examples include copper, aluminum, lead, bronze, nickel, tin, and titanium.
Magnets are used to identify scrap metals based on their magnetic differences. Metals that stick to the magnet are ferrous, while those that don’t are non-ferrous.
Steel scrap quality for recycling is crucial since it determines the final product’s characteristics and value.
Using low-quality steel scrap with impurities makes recycling more difficult and less efficient. It will require more processing to remove the impurities to ensure a good quality product fit to manufacture sustainable building materials.
That said, the scrap quality standard for steel recycling is based on the amount of foreign material.
A rule of thumb is that the total amount of foreign material in iron and steel for recycling must not exceed 2 percent by weight.
The foreign materials in question include:
- Residues like dust and sludge
- Poor conductors of electricity
- Non-ferrous metals
- Non-metallic combustible materials
The stepwise process of recycling steel involves collecting, sorting, shredding, melting, refining, casting, and rolling. These steps must be undertaken in that order to ensure quality recycled steel.
The 3 R’s of recycling are reduce, reuse, and recycle, as discussed below:
- Reduce: Reduce waste generation through initiatives like multi-use.
- Reuse: Repurposing and reusing materials elsewhere instead of discarding them as waste.
- Recycle: Transforming an end-of-life product into recycled raw materials to manufacture new items.
The recycled steel manufacturing process requires precise steps to be followed to successfully produce high-quality recycled steel.
The above five steps are crucial and must be undertaken in order.
Finally, before recycling scrap steel, ensure it is well sorted, and any foreign materials are separated and removed so they don’t exceed 2 percent by weight for the best results.