METAL RECYCLING

Scrap metal can be defined as the combination of waste metal, or any material or product of metallic composition which can be recycled from a previous composition. This could be used vehicle parts, used parts of construction and manufacturing plants, surplus building materials, etc; scrap is of high value if marketed properly. Recycling of metal scraps has received massive attention in recent years because of the inclination of the world towards achieving a green climate. Recycling grossly reduces the environmental impact poised by waste metal.

Scrap metal is largely classified into ferrous and non-ferrous metal. Non-ferrous metals are metals which do not contain iron in them such as aluminum and copper. This can be easily detected by passing them over a strong magnetic field. Iron has magnetic properties, hence every metal containing iron is expected to have magnetic properties. Ferrous metals are metals containing iron in them. Ferrous metals are often compacted and shipped to foundries for reuse while non-ferrous metals undergo further treatment and characterization.

The metal recycling process

The following are the main stages of a metal recycling process:

Collection:

This is the first step in metal recycling. It involves obtaining metal scraps from junk yards, garage etc. Metal scraps are rated above regular materials. Regular materials such as paper, are heaped together and used for land filling. In the US, the largest source of ferrous metal is from scrap vehicles. Other sources includebuilding materials, consumer used metal facets, plant facilities,rail road tracks etc.

Sorting and separation techniques:

Sorting is a very important step in recycling. Effective sorting begets effective recycling especially when dealing with metal scraps. Sorting involves separation of metals from a heap of waste stream or from a mix of scrap metal of different compositions based on shape, size, colour, density, weight, magnetism, electrical conductivity, luminosity, type, form, structure etc.. In automated recycling, machines are fitted with magnets, light beams, sensors and gadgets to separate metals based on the parameters mentioned above. Some sorting and separation techniques include the following

Magnetic separation process

Magnetic separation is used when ferrous metals are to be separated from non-ferrous. Large powerful magnets and electromagnets are employed in this process. Electro magnets are electrically controlled and can be turned off or on. The two popular ways of magnetic separation are drum and belt type. In the former, a big drum is permanently fixed with a magnet underneath it. The waste stream passes through a belt and underneath the drum, the ferrous materials are quickly attracted to the magnet and easily separated. In the belt type system, magnets are attached at various process points of motion such as in between pulleys which the belt travels. However magnetic separation has its limitations as it cannot separate iron and steel from nickel or magnetic stainless steel. It is advised to include manual sorting where inefficiency is observed.

Eddy current separation process

This method separates ferrous metals from waste. An eddy current is inducedon exposure of a conductor to a changing magnetic field. This method exploits the electrical conductivity of non-magnetic metals. This is simply done by passing a magnetic current along the feed stream and using induced eddy currents in the metals. When a feed of waste enters the system, the non-magnetic materials slide down while ferrous metals are deflected sideways due to the interaction of magnetic field and eddy current.

Trommel separators/drum screens

This method separates materials according to their sizes. Waste feed is fed into a very large rotating drum with a perforated bottom. Materials of small sizes fall through these holes while larger particles remain.

Induction sorting

In this process, waste feed is sent through a conveyor belt fitted with sensors at various points beneath it. These sensors have the ability to locate various types of metals which are then separated by a system of fast air jets linked to these sensors.

Near infrared sensors (NIR)

Luminosity is one of the properties associated with metals. Metals readily reflect light when illuminated in the near IR wavelength. NIR sensors can distinguish metals based on the intensity and wavelengths of light they reflect.

X-ray technology

X-ray technology can distinguish metals based on their densities.

Processing:

For further processing, metals are shredded. Shredding is done to increase the surface area of metals. Metals with large surface area require less heat energy to melt. As a result, this saves heating costs as well as costs of cooling heated chambers.

Melting:

Scrap metal is heated in very large furnace. Each metal is melted in a specific furnace designed for it. High temperatures are required for melting metals but this can be reduced with increased surface area. Despite the high energy required in scrap metal processing, it is incomparable to the energy required in extraction and use of metals directly from raw materials. Depending on the size of furnace and degree of heat energy, the process time can vary from mins to hours.

Purification:

Purification is done to eliminate possible contaminants in the material and ensure the final metal is high grade. The most common process is by electrolysis.

Solidifying:

After melting and purification, molten scrap metal is transferred in conveyor belts to cool and solidify forming steel bars and metal beds. They can be easily used for form various products

Metal scrap recycling has become a very important sector of the economy generating massive income in the last decade and this trend is yet to continue. With the need for a green climate, recycling is on the increase. The heat generated and CO2 released in Metal scrap recycling is a lot less compared to that generated from virgin raw materials. Scrap recycle eliminates indiscriminate dumping of metal waste in the environment. It also reduces the strain on our natural resources such as Iron ore.