# How can the alloying materials of recycled steels be separated?

As I know, a very considerable part of the currently processed steels (around half of it) is coming from recycling.

But during the steels coming into the recycling process are coming normally from various sources, and thus they are containing very different alloying materials.

But the output of the reprocessed steel must be steel containing alloys exactly in the specified ratios.

Do some type of "separation", or "removal" of the previous alloys of the recycled steel happen? And if yes, how does it work?

• "containing alloys exactly in the specified ratios." In engineering only tolerances are exact - all the rest is exact only to tolerances, and so, for worse quality steels the tolerances for these ratios are quite forgiving.
– SF.
Feb 8 '15 at 9:29
• In metallurgy specifically, alloying elements for standard grades are given fairly generous ranges (tolerances). For example, an AISI 1018 grade steel, which is nominally a plain-carbon steel containing 0.18% w/w carbon, may have carbon in the range of 0.14-0.20% w/w, and Mn in the range of 0.6-0.9% w/w. The reason is that there is a trade-off between precision and speed, and precision loses to speed on the foundry floor because of alloy fading. Feb 8 '15 at 18:58

That is correct, there are a number of unwanted, or tramp, metals (Cu, Sn, Sb, As) that enter the recycling stream from, for example, car bodies that are ground into scrap without removing all the copper wiring, or tin-coated steel cans. Antimony and arsenic tend to creep in from low-quality and low-cost primary iron sources.

The answer to the question is no. Recycled steel is mixed as evenly as possible from varied sources, its composition is measured, and then pure iron is added as needed to dilute the tramp metals to tolerable levels for resale or further processing, such as meeting a specific steel grade for a specific product or application. Stainless steels and other high-alloy grades which are known at recycling time are processed separately due to the value of Ni, Cr, etc.

It is currently uneconomical to reprocess iron to remove tramp elements, and so it simply isn't done at all. Two books mention the process as a regular and economical one: (Minerals, Metals and Sustainability: Meeting Future Material Needs, p. 284, starting at "dilution") and (Steel Production: Processes, Products, and Residuals, starting on p. 104, read until it isn't relevant anymore). The reason it is uneconomical is that the tramp elements react more weakly with oxygen than iron at constant temperature, so to remove them by oxidation would require oxidizing all of the iron first. The reason for this is thermodynamic, and predicated on the fact that among competing reactions, those with the largest decreases in free energy proceed virtually to completion prior to other reactions even starting, especially with large differences in free energy among the competing reactions. To determine which reactions have the largest decreases, an Ellingham diagram may be used.

In the Ellingham diagram below, the horizontal axis is temperature, the vertical axis is change in Gibbs free energy. The lines running across the diagram at various angles correspond to free energy change caused by element oxidation reactions with oxygen, as a function of temperature. In our case, the diagram may be read by choosing a temperature of interest, and reading up from the bottom to find the first element to react with oxygen. For example, if we have steel with Fe, Mn, Sn, and Cu in it, we can see that at 1000K then Mn, Fe (to FeO), Sn and Cu are the order of largest to smallest drop in free energy.

Granted, the temperature of interest is closer to 1900K (above the melting point of iron), but the general trends of each Gibbs free energy change function continue to the right on the diagram and iron remains below the tramp elements Cu, Sn, As and Sb at practical temperatures, and likely to their respective boiling points. As a result, removing tramps from Fe would require oxidizing effectively all of the iron first. And because Sn, Sb, As and Cu are slightly soluble in iron, they require separation via chemical reaction.

One can see the solubility of tramps from their phase diagrams with iron, of which I have posted Sb-Fe below. The diagram has temperature against composition, with each contiguous 2D region composed of either one phase, or a mixture of the two phases to its left and right, which are in equilibrium at that combination of temperature and composition. At the bottom left we see that for small amounts of Sb and room temperature, there is a contiguous region which in this case denotes a single phase, or alpha-Fe (the kind we are familiar with). Because there is Sb present, and it is in a single phase, it must be dissolved in the iron. The same is true, with varying severity, of the other tramps.

(source: himikatus.ru)

As Chris H commented, there is a question also of when other alloying elements are controlled. Generally alloy addition is controlled as close to solidification as possible, to minimize alloy loss.

Scrap is melted in bulk in an electric arc furnace. If the scrap stream is sufficiently mixed, then the tramp concentration may be estimated based on past usage and the primary iron is added prior to chemistry analysis to compensate for the estimate. The bulk is then melted, oxygen is removed via the addition of elements at the bottom of the Ellingham diagram, specifically Ca and Al, and the molten metal is transferred to one or more highly insulated ladles. The Ca and Al rapidly react with oxygen dissolved in the melt to create low-density oxide slag which floats and is removed mechanically. Chemistry is taken after this process, and if the tramps are sufficiently diluted, the metal is transferred to ladles. If not, sufficient primary iron is added to dilute the melt.

• I would assume, and you may be able to confirm, that as well as adding iron, other major alloying elements will be controlled at roughly the same process point - certainly the carbon will have to be controlled. Feb 8 '15 at 17:42
• Might a foundry, after examining the concentrations of tramp metals, sometimes select alloys that could make use of them, or do foundries usually decide what alloy to produce before they melt the scrap, and then simply add whatever is needed to yield the pre-selected alloy within specified tolerance? Apr 2 '15 at 20:35
• Good question that I don't have a definite answer to. I imagine most foundries would have an alloy design in mind prior to executing the process. I also think certain tramp metals above their respective tolerances are unacceptable in structural applications because they reduce mechanical properties below required levels. Because the foundry does not necessarily know final applications of billets, etc., it would then be up to the customer to select the alloy they need. If a customer stated that tramp metals are OK, that might be possible, depending on risk of contamination. Apr 2 '15 at 21:41

To the best of my knowledge, such separation of components is not attempted.

I have a friend who at one time worked for Lukens Steel in Coatesville, PA. His job was to write computer software that kept track of the composition of all of the scrap steel they had in their yards and to come up with the correct proportions of which kinds of scrap to use for any new melts. Obviously, this implies that they did a fairly comprehensive analysis of all incoming scrap and sorted similar alloys into separate piles.

To concur with the David Tweed & starrise, it is uneconomic to separate the individual metals in steel alloys.

To do so would first require the alloys to be crushed and ground to the size of the crystal grains within the alloys. Then some form of mineral/crystal selection process would need to be devised to segregate and separate the wanted from the unwanted, such as: froth flotation; maybe heavy media; possibly gravity separation methods such as shaking tables or spirals (but I doubt these would be successful as gravity separation methods rely of significant density & weight differences); though magnetic separation, as used in the mineral sands industry might be an option for some alloys. Even after this, there will always be a waste or tailings division where the really difficult alloy crystals will be collected in a dump.

Crushing, grinding and separation all cost money. These costs and a profit have to come from the steel alloys being recycled into there individual metals.

As of early February 2015, the value of a selection of metals is:

• Gold USD 1233.30 per oz, USD 39.6515 per g or USD 39 651 510.84 per tonne (yes, 39.651 million dollars per tonne)
• Platinum USD 1220 per oz or USD 39 223 905.97 per tonne (39.2239 M$/t) • Silver USD 16.68 per oz or USD 536 274.38 per tonne (0.536 274 M$/t)

• Cobalt USD 29 500 per tonne
• Nickel USD 14 965 per tonne
• Lead USD 1850 per tonne
• Steel Billet USD 500 per tonne

For the aptly named precious metals, Au, Pt & Ag, the price source was Kitco. The price source for the base metals, Co, Ni, Pb & steel billet was LME.

Iron ore is currently selling for approximately USD 65 per tonne as stated on Index Mundi and Y Charts. That's for an average grade of 60 per cent iron. The open cut iron mines in Australia and Brazil, operated by Rio Tinto, BHP-Billiton & Vale are quite happy to produce iron ore at that price. LKAB is likewise happy to produce magnetite iron ore from the Kiruna underground mine in Sweden for that price.

Macrobusiness has an article about the possibility of iron ore prices going down to USD 30 per tonne in 2015.

At prices such as 0.536 to 39.6 million dollars a tonne it is easy to see why the precious metals are recycled. But at USD 500 a tonne for steel billet and USD 65 per tonne for iron ore there is no incentive to separate the alloying metals from steel alloys.

• Given the difference in price between say nickel and steel, your analysis doesn't rule out the economic case for extracting the alloying elements - though they would end up being put in again. Separation could also be achieved in a molten state, and/or by chemical means if it were economical to do so. Feb 8 '15 at 17:45
• Gravity separation doesn't work for iron because of solubility and thermodynamic considerations. The threshold for unacceptable tramp concentrations is lower than the solubility of those elements at room temperature. The only ways to separate dissolved tramp metals is by a chemical process, which is uneconomical for Fe, or by distillation, which is practically impossible due to the extreme boiling point of the metals involved, let alone the economics. Feb 8 '15 at 18:54

First the scrap is separated at the source ; for example cast iron generally only contains Si and Mn. High vapor pressure elements boil off or collected in the flux/slag : eg, Zn, Pb, Sn, Bi, An ,,,,Aluminum oxidizes and goes into the slag. Steels do pick up Cr, Ni, Mo, and Cu residuals , generally these are advantageous ; they all add to hardenability except Cu. ( Cu is important in atmospheric corrosion resistance). V and Nb and W are present in very small amounts so insignificant. , And Co , expensive and has specialized applications so it is also separated at the scrape source ; Co scrape is easy to identify; medical prosthesis and jet engine hot section blades and vanes ,also in some high speed tools -again separated at the scrape source. Ni alloys and austenitic stainless are separated at the source as they are not ferromagnetic. Magnetic Martensitic and ferritic stainless ( typically 13 % Cr) can be separated at the scrape source . The separation of steels at the sources is done because all the alloy elements are worth more than carbon steel . There must be books available on this ; it is a major factor in the steel industry. An example of what happens in the real world ; grade A 516 carbon steel plate is the workhorse of industry but when a thick section with high strength is ordered , "somehow" the Cr, Mo, Ni residuals are high enabling acceptable heat-treatment results.