Scrap metal is a key part of steel mills' raw materials

We probably understood that the importance of recycling should not be heard only in political or commercial words and proclamations, but in everyday facts, trying to choose products that truly pursue the philosophy of the circular economy, intercepting greenwashing, that misleading form of information that makes you believe that a product is circular but that in reality it is not, or only partially.

We're not just talking about plastic, which is on everyone's lips today, but also metals which, together with glass and paper, form the family of the largest quantities of waste, of which we have to deal with it every day.

How does the separation of metals take place?

The various ferrous and non-ferrous metals that are collected are sent to sorting and recycling centres, which arrange, as a first step, to separate them by type and size.

The first macro separation takes place, in fact, performed by dividing those belonging to the family of ferrous and non-ferrous metals.

To better understand these two divisions we can say that:

Ferrous metals are metals and metal alloys that contain iron, among which, the best known are the steel and cast iron.

Cast iron is obtained from the blast furnace and can subsequently be refined to obtain steel or used in the foundry.

Cast iron is very hard and brittle, has very low resilience, practically zero elongation at break, therefore it cannot be worked plastically, either hot or cold, but can only be worked by fusion.

Steel is obtained by refining cast iron, an operation which consists of decreasing the carbon content to reduce the elements harmful substances, such as sulphur, phosphorus, oxygen, etc., which can derive from the furnace charge materials or from the products of the previous processing phases.

Indeed at the increase in the amount of carbon increase:

- mechanical resistance,

- hardness,

- hardenability,

- castability/fusibility,

- wear resistance

Decreases instead:

- elongation A%

- mechanical resistance

- cold workability and plasticity

- weldability

Furthermore steels are divided into hard, semi-hard and mild, in fact, mild steels have a much lower tensile strength than that of hard steels, however they are more malleable, more ductile and more resistant to impact.

They are easily weldable and workable by machine tools, but are less resistant to wear and corrosion than hard steels.

During the preparation, in the melting phase, it is possible to add ferrous or non-ferrous binders to increase its performance, thus calling these steels alloy or non-alloy.

Let's see what influence binders have in the preparation of steel:

Chromium (Cr)

It is often found in steels, improving hardness, mechanical strength and wear resistance. In quantities greater than 12% it makes stainless steel.

Nickel (Ni)

It is often found together with chromium, improving all the mechanical properties of steel, such as resistance to corrosion, while decreasing thermal expansion and weldability. Nickel is also found in stainless steels in quantities that depend on the chromium content.

Molybdenum (Mo)

It improves hardenability and mitigates the phenomenon of "tempering fragility". Together with chromium and nickel, it creates steels with the best mechanical properties (Rm up to 1200 N/mm2).

Silicon (Si)

It is naturally contained in steel in small quantities (about 0.3%), but if it is intentionally added up to about 2%, it increases the mechanical resistance, to oxidation and above all significantly increases the elasticity. In fact, silicon steels are used to make springs.

Manganese (Mn)

It increases hardness, mechanical strength and resistance to wear. It also greatly improves hardenability but causes the phenomenon of "temper fragility".

Tungsten (W) – Cobalt (Co) – Vanadium (V) – Titanium (Ti)

They are all very hard elements which, added to the steel, give it very high hardness which is maintained even at high temperatures. These mechanical characteristics are found in tool steels.

Lead (Pb) – Sulfur (S)

They are harmful elements for steel because they make it highly fragile. However, they can be found in small quantities because the fragility induced by their presence facilitates the detachment of the chip and favors the workability with machine tools. These steels are called automatic.

Sulfur (S) – Phosphorus (P) – Hydrogen (H) – Nitrogen (N) – Oxygen (O)

They are all harmful elements because they bind chemically with iron or carbon, forming compounds that make steel very brittle. Their presence, therefore, must be minimized.

Regarding non-ferrous materials all those alloys which do not contain iron or contain a negligible fraction of it can be defined as such.

We can list among non-ferrous metals magnesium, copper, zinc, bronze, lead, nickel, brass and aluminum.

Non-ferrous metals combined with other metals can generate a large amount of alloys, with the aim of making improvements in mechanical performance, machinability, corrosion resistance and at the high temperatures of the base metal.

Furthermore, they are also divided into density categories:

Heavy goods weighing more than 5000 Kg. per Mc

Light weight with a weight between 2000 and 5000 Kg. per Mc

The use of non-ferrous metals can be done in their pure state , or in alloys with other elements. Their major peculiarities are characterized by lightness, stainlessness, high electrical and thermal conduction, hardness, a high melting point and malleability.

How are metals recycled?

We have seen that the first operation is to identify the families they belong to and separate them from each other to send the metals for recycling.

This begins with the volumetric reduction of scrap, through mechanical systems that have the purpose, not only of reducing its size, but also to separate any polluting elements present in the scrap itself.

These first treatment plants have incorporated in the line gravitational, eddy current, screens and magnetic separators systems, which have the purpose of to ennoble the treated scrap metal.

This, once selected, is sent to the steel mills for their use together with other materials, which allows the creation of new elements made up of recycled scrap.

The recycling of steel mill slag

In the context of the circular economy the reuse of blast furnace slag has become a very sensitive issue, not only from an economic point of view, due to the increasingly high costs of landfill disposal, but also for environmental reasons.

In fact, the disposal in landfills of this waste which contains heavy metals is a factor of great environmental concern, for which, through their recycling, it is possible to extract the precious metals from the waste ashes.

Once recycled, they are an inert material that is used in cement plant ovens, or for the production of ceramic materials, glass fibers, inert fillers or in flooring road.

Machine translation. We apologize for any inaccuracies. Original article in Italian.