Metals and Plastics: Properties, Processing and Recycling

Metals

Metals: materials obtained from minerals through a transformation process.

• Extraction: depending on its nature, the metal is found together with other substances, so it must be separated from them. The ore is made up mostly of useful material and partly of waste.

• Obtaining: crush, grind and purify the ore, which can then be distinguished.

- Mineral concentration: separate mechanically.

- Reduction in a furnace: melt and separate the desired metal from the rest.

- Refining: remove impurities.

• Transformation: process used to convert metals into industrial metal products.

Pure metals: formed by a single chemical element.

Alloys: mixtures of metals, formed by a base metal and other metals.

Ferrous metals: iron and its alloys, such as steel.

Non-ferrous metals: metals without iron, such as copper, aluminium, zinc, chromium and titanium. Their study is part of metallurgy.

Properties: resistance, hardness, plasticity and toughness.

Ferrous Metals

Obtaining:

• Reduction: melt iron ore with limestone, which forms slag, and coke to remove the carbon.

• Refining: remove carbon.

Cast iron: magnetic, hard, brittle, resistant to compression, not plastic, used for sewer covers.

Steel: magnetic, hard, tenacious, resistant, rust-prone unless painted or coated, or made resistant to corrosion with other elements. Includes carbon steel and alloy steels (iron, carbon and other elements), and stainless steel (chromium and nickel).

Non-Ferrous Metals

Copper: reddish, electrical conductor, hard, poorly resistant, malleable and resistant to corrosion. Brass: yellow shiny (copper + zinc). Bronze: orange-brown shiny (copper + tin).

Aluminium: abundant but difficult to separate from its components, shiny silver colour, electrical and heat conductor, ductile, malleable, and resistant to corrosion (except salt water).

Techniques

• Casting: melt the metal and pour it into a mould.

• Plastic deformation: deform the metal by applying force.

- Forging: heat the metal and shape it with hammer blows.

- Stamping: place the metal inside a mould and apply force.

• Machining: carve the metal (lathe, milling machine, etc.).

Metal Waste

• Natural resources: spend energy and water.

• Contamination: pollution of the environment.

• Waste: must be properly managed and recycled.

Plastics

Plastics are among the materials most used in modern times. Polymers are formed by macromolecules, which are giant molecules.

Obtaining: through synthesis processes, mainly from petroleum.

Thermoplastics: can be melted and reshaped several times without losing their properties.

Thermosetting plastics: once shaped, they cannot be melted again; they can only be obtained once.

Elastomers: elastic materials.

Properties: plasticity, lightness, toughness, flexibility, low conductivity and resistance to chemical and atmospheric agents.

Techniques

• Bending: heat the plastic and bend it into shape.

• Moulding and casting: pour molten plastic into a mould.

• Extrusion: plastic granules are fed into a screw that moves them forward while they are heated until they melt; the molten plastic is then forced through a die.

• Blow moulding: containers are made by extruding a tube, then inflating it with compressed air inside a mould.

• Plastic injection: molten plastic is injected into a closed mould.

Recycling

Plastics can be remelted and reshaped, incinerated, or treated by pyrolysis (heating without oxygen to obtain new products from plastics).