Editorial Feature

Steel Recycling Principles and Practice

Steel is a ferrous metal and is an alloy of iron and other elements. The most common element that is combined with iron is carbon. Other alloying elements that are used include manganese, chromium, vanadium, and tungsten.

The earliest known production of steel is a piece of ironware that was excavated from Anatolia (Kaman-Kalehoyuk) and is dated to be 4,000 years old. Now obsolete, the Cementation process is an expensive method used to produce steel; however with the invention of the Bessemer process during the mid-19th century, steel soon became an inexpensive mass-produced material.

Of all ferrous alloys, steel is the most useful and widely used for its hardness, flexibility, and tensile strength. Steel has better durability and can be given a sharper edge than softer wrought iron yet able to resist shock and tension much better than cast iron.

Types of Steel

Many variations of steel are available. Below is a brief look at the key types of steel:

  • Crucible steel – One of the early forms of steel (the 1740s) was made from blister steel that was melted in clay crucibles and refined by adding a special flux to remove fine slag particles that the cementation process could not remove. It was of high quality but expensive.
  • High carbon steel - has a carbon content ranging from 0.50 to 1.05%. Mainly used to manufacture drills, taps, dies, springs, and other machine tools and hand tools that are heat-treated after fabrication to develop a hard structure.
  • Spring steel - is a low alloy, medium carbon steel, or high carbon steel with very high yield strength such that it can return to its original shape despite significant bending or twisting.
  • Alloy steel – To meet a variety of specifications such as hardness, toughness, and machinability, manufacturers use various alloying elements like chromium, manganese, molybdenum, nickel, tungsten, and vanadium
  • Stainless steel - The most popular type of steel, manufactured by adding nickel and chromium to steel.
  • High-speed steel - A self-hardening steel alloy that can withstand elevated temperatures without becoming soft.
  • Tool steel - Particular steel that is used in the manufacture of tools such as chisels, cutters, woodturning tools, shear blades, blacksmith’s tools, and razors, where high hardness is required to maintain a sharp cutting edge.
  • Advanced high-strength steels (AHSS)- A modern steel type used in nearly every new vehicle design as AHSS makes the vehicle lighter, thus enhancing safety, improving fuel economy, and reducing lifetime greenhouse gas emissions.

Properties of Steel

The key properties of steel are listed below:

  • High strength/weight ratio makes it an ideal choice for use in the construction of high-rise buildings, long-span bridges, structures located on soft ground, and structures located in highly seismic areas
  • Ductility and easy to repair predictable material properties
  • Steel structures can be erected quite easily and rapidly with good quality workmanship, resulting in quick dividends
  • Steel is highly suitable for prefabrication and mass production
  • Steel can be reused after a structure is disassembled
  • Good fatigue strength and high compressive and tensile strengths
  • Exposure to constant air and water causes corrosion of the steel structures and hence requires regular painting
  • The excellent temperature resistance and economic properties

Manufacturing Process of Steel

The cementation process was widely replaced after the invention of the Bessemer converter, which made it possible for high tonnage production of steel for ships, railroads, bridges, and large buildings during the mid-nineteenth century. However, this steel was brittle as there were many impurities (mainly phosphorus and sulfur) in the steel.

Today about 70% of global steel production is dependent on coal. Steel is manufactured by the chemical reduction of iron ore using either the integrated steel manufacturing process or the direct reduction process.

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In the conventional integrated steel manufacturing process, the iron from the blast furnace is converted to steel in a basic oxygen furnace (BOF). Steel can also be made in an electric arc furnace (EAF) from scrap steel and also from direct reduced iron. BOF is typically used for high-tonnage production of carbon steels, while the EAF is used to produce carbon steels and low-tonnage specialty steels.

Applications of Virgin Material Steel

Steel and its variations are widely used in a number of fields and its applications keep increasing year after year. The key applications are listed below:

  • Automotive industry - Manufacture of chassis, engine parts, transmissions, and exhaust systems. About 55 to 70% of the car’s weight is made up of steel
  • Rail transport- trains and for the rails and infrastructure
  • Construction products, surgery equipment, and shipbuilding
  • Electronic gadgets such as refrigerators and washing machines
  • Important engineering and construction materials in the world
  • In solar panel roofing system (used to convert solar energy to electricity)
  • Modern solar heating systems for large buildings and warehouses

Environmental Impacts of Steel

As steel is an alloy of iron and carbon, it requires a lot of energy to produce heat during the production process. Most of the electricity is generated by burning fossil fuels, which releases greenhouse gases into the atmosphere, thus contributing to global warming. However, the most beneficial feature of steel is that it is 100% recyclable and is long-lasting, thereby making it an ideal environmental performer to remain in use for many decades.

Stainless steel products help man to maintain a healthier life and a cleaner environment. Some examples are detailed below:

  • New catalytic converters use stainless steel for the holding and transportation of catalytic substances so as to improve its energy efficiency
  • Stainless steel converters decrease pollution from motor vehicles, generator sets, forklifts, mining equipment, trucks, buses, and trains
  • Stainless steel condensing boilers provide a 100% efficiency rating
  • Stainless steel pipes in drinking systems keep the water clean and quality standards high. It helps prevent the formation of any medium on which bacteria can grow
  • Newer versions of diesel filters use stainless steel to improve the removal of smoke and soot particles from the exhaust gas of a diesel engine.

Recycling Process

Steel can be recycled over and over again without losing any of its properties. An interesting fact is that recycling 1 t of steel helps to save 1.8 barrels of oil, 10.9 million Btu's of energy, 642 kWh of energy, and 2.3 m3 of landfill space.

Steel recycling can be performed by following some of these steps:

  • Sorting – Magnets can be used to attract the steel away from other materials in a recycling facility
  • Shredding - Shredders are designed with rotating magnetic drums that extract iron and steel from other metals and materials
  • Media separation- by using electrical currents, high-pressure air flows, and liquid floating systems, further separation of steel from other materials can be achieved
  • Shearing - Hydraulic machinery is used to cut thick heavy steel recovered from railways and ships by exerting enormous pressure on the items. Sometimes gas and plasma arch are used

Steel is then melted and reshaped before being used in the production of a variety of commodities ranging from bikes and new cans to paper clips.

Applications of Recycled Steel

Recycled steel can be used for the same applications as steel produced from virgin material. Products that are made of recycled steel include:

  • Electrical appliances
  • Automobiles and other vehicles
  • Office supplies
  • Hardware such as bolts, nuts, screws, etc.
  • Construction materials
  • Cans and containers

Sources and Further Reading

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