Extractive Metallurgy and Steelmaking

Extraction process applications

Each extraction process is tied to a characteristic product and a characteristic stage of metal production, so the first task is to recognise which metal a process serves. Fused salt electrolysis of alumina dissolved in molten cryolite is the Hall-Héroult route to aluminium, whereas carbothermal reduction of iron oxide by carbon monoxide and carbon in the blast furnace is the basis of ironmaking. Oxidation refining removes carbon and other impurities from molten pig iron in a converter, which places it in steelmaking, while matte converting oxidises the iron sulphide in copper matte to blister copper. Red mud, the insoluble iron-rich residue left after digestion of bauxite in hot concentrated caustic soda, is a by-product of the Bayer process and therefore of aluminium production, not of iron, titanium or copper. Likewise, the COREX process gives molten pig iron, the Bayer process gives alumina, matte smelting gives copper, and the MIDREX process gives direct reduced iron from pellets using natural gas, so each process must be paired with its own product.

Oxidation sequence in BOF

In the basic oxygen furnace, the solutes in the hot metal are oxidised in an order set by their affinity for oxygen and by their concentration. Silicon has the highest affinity for oxygen among the common solutes and is removed first, forming silica that enters the slag, while carbon is removed as carbon monoxide only after silicon and manganese have begun to oxidise. Manganese and phosphorus follow, with phosphorus transferred to a basic slag. Sulphur is removed at the slowest rate because the oxidising conditions of the blow do not favour desulphurisation, so sulphur is normally controlled by hot metal pretreatment rather than by the converter itself.

Steelmaking unit processes

A steelmaking route is built from unit processes, each with its own equipment and function, and examination questions test the link between a term and its unit. The submerged entry nozzle is the refractory delivery tube of continuous casting, the ladle furnace provides electric heating to correct the temperature of molten steel, the raceway zone is the region in front of the blast furnace tuyeres where coke burns with the hot blast, and oxygen lancing is the refining step of the LD converter. In the same way, Corex is a melter-gasifier based smelting-reduction process, Midrex uses a natural gas reformer to generate reducing gas, an electric arc furnace often retains a hot heel of liquid steel to speed the melting of the new charge, and an electromagnetic stirrer in the continuous casting mould homogenises the melt.

Blast furnace material balance

A blast furnace material balance starts from the iron balance, because the iron in the hot metal fixes the quantity of ore required. The iron in a given mass of hot metal is the hot metal mass multiplied by the iron fraction, and the Fe_2O_3 needed is found by multiplying the iron mass by the ratio of molar masses, Fe_2O_3 to 2Fe, which is 160/112. The ore mass is the Fe_2O_3 requirement divided by its weight fraction in the ore, and the slag is the sum of the gangue carried in with the ore and the ash carried in with the coke. If a fraction of the iron is lost to slag, the total iron input must be raised by dividing the iron in the metal by one minus that loss fraction, while carbon and silicon in the hot metal do not enter the iron balance.

Role of nickel in steel

Nickel is an austenite stabiliser because it has the face-centred cubic structure and enlarges the austenite field in the iron-rich part of the phase diagram. It does not form stable carbides and dissolves mainly in the matrix, so it is not a carbide former, and it lowers the martensite start temperature M_s, which makes retained austenite more stable at room temperature. When nickel is added to a low alloy steel, it retards the diffusional transformations to ferrite, pearlite and bainite, so hardenability increases, and at the same time it depresses M_s. The correct combination is therefore increased hardenability with a lowered M_s, and statements claiming a decrease in hardenability or a rise in M_s are wrong.

Ellingham diagram for metal oxides

The Ellingham diagram plots the standard Gibbs energy of formation of oxides per mole of oxygen against temperature, so that oxides can be compared on a common basis. A metal reduces the oxide of a less stable metal whenever its own oxide has a more negative standard Gibbs energy of formation, because the coupled reaction then has a negative \Delta G^\circ. Using this rule at 1000 K, titanium can reduce manganese oxide, whereas silicon cannot reduce titanium dioxide and vanadium cannot reduce silica. The melting point of a metal can be found from the Ellingham expressions for the solid and liquid states. Subtracting the liquid-state equation from the solid-state equation gives the standard free energy change of fusion, which vanishes at the melting point, and the T\ln T terms cancel in this subtraction, so the melting point is the root of a linear equation in T.

Hydrometallurgical operations

Hydrometallurgy recovers metals by aqueous chemistry at comparatively low temperature, and its central unit operation is leaching, in which the wanted metal is dissolved from the ore into solution by reagents such as sulphuric acid or cyanide. Roasting, smelting and sintering are pyrometallurgical operations carried out at high temperature, and zone refining is a physical purification based on localised melting, so none of these is a hydrometallurgical operation. Students should distinguish leaching, which is a solution step, from the high-temperature steps that precede or replace it.

Kroll process for titanium

Commercial titanium metal is produced by the Kroll process, in which titanium tetrachloride is reduced by molten magnesium under an inert atmosphere, giving a porous titanium sponge with magnesium chloride as the by-product: \mathrm{TiCl_4} + 2\mathrm{Mg} \rightarrow \mathrm{Ti} + 2\mathrm{MgCl_2}. The sponge is subsequently melted to produce ingot. Reduction of titanium dioxide by hydrogen is not practical because the oxide is too stable, and hydride-based and direct oxide routes serve only specialised products rather than bulk titanium.

Blast furnace top pressure practice

The solution loss reaction \mathrm{C + CO_2 = 2CO} increases the number of moles of gas. By Le Chatelier's principle, raising the total pressure shifts this equilibrium toward the side with fewer gas molecules, so carbon gasification is suppressed. High top pressure therefore retards the solution loss reaction and favours a lower coke rate, whereas bell-less tops and rotating chutes control burden distribution, and pulverised coal injection works by substituting for coke.

Mould flux in continuous casting

Mould flux is the powder or liquid layer spread on the steel meniscus in the continuous casting mould, and it performs several functions at once. It lubricates the interface between the solidifying shell and the copper mould, which reduces friction and the risk of sticking, and it insulates the meniscus, which reduces heat loss from the top surface of the melt. The molten flux also absorbs and dissolves non-metallic inclusions that float to the surface, so inclusion control is one of its roles. Solidification shrinkage, by contrast, is governed by the density change on freezing and by the cooling conditions, and it is not controlled by the flux.

Desulfurization of molten steel by slag

The desulfurizing index L_S is the ratio of the sulphur content in the slag to the sulphur content in the metal at equilibrium, L_S = (\%S)/[\%S]. A mass balance links the two phases, because the sulphur removed from the metal equals the sulphur taken up by the slag, so the slag sulphur content follows from the mass of metal, the change in its sulphur content, and the mass of slag. Both contents must be expressed on the same basis, usually mass per cent, before the ratio is taken, and the final index is then a dimensionless number.