Ingot casting

With ingot casting, the molten steel from the steelworks solidifies in moulds to form conical blocks, each with a mass of several kilograms up to several tons. The conical shape makes it easier to remove the moulds (which are usually made of nodular cast iron) after the steel has solidified. The solidified blocks are also called ingots, which is why this kind of steelmaking is called ingot casting.

If the ingots are rolled into oblong blocks with a rectangular cross-section, they are also referred to as slabs. Slabs are supplied as semi-finished products to rolling mills for the production of sheet steel, hot rolled strip and cold rolled strip or to forges.

Overall, however, ingot casting is not suitable for mass production. It is mainly limited to high-alloyed steels such as tool steels and rolling bearing steels, which are only produced in small quantities. Today, slabs for mass production are mainly produced using the more efficient continuous casting process (explained in more detail in the following section).

Depending on whether the ingot mold is filled from above or below, a distinction is made between top pouring (top casting or downhill casting) and bottom pouring (bottom casting or uphill casting). With bottom pouring the mold is filled from the bottom via a runner system. This process is mainly used for ingot casting.

Ingot casting is a discontinuous casting and solidification process of steel into ingots (blocks)!

With rimmed steels, gas bubbles rise during solidification and thus provide for flows in the melt. Since the casting block solidifies from the edge, low-melting accompanying and alloying elements are thus conveyed more towards the middle of the block. The result is a chemical segregation of the ingot, also known as ingot segregation (macrosegregation). For example, the last solidified head of the casting block contains up to 5 times higher concentrations of sulphur and phosphorus than the marginal regions.

Conversely, this means that the steel block has only minor impurities on the surface and is also very low in carbon. This tough surface layer also has a high surface quality. This is the reason why deep-drawing sheets are often made of rimmed steels.

In addition, rimmed steels have only small shrinkage values and little blowholes compared to killed steels. The reason for this are the rising gas bubbles, which remain as pores in the casting block after solidification, but for this very reason counteract the reduction in volume during the transition from liquid to solid. The gas pores do not have any further disturbing effect during rolling into sheets, as these are compressed by the high degree of forming and the inner surfaces of the pores “weld” together.

Rimmed casted steel is often used as steel for deep drawing or rolling!

Rimmed steel is out of the question when a high homogeneous distribution of the alloying elements is required, because the ingot then mus not have any macrosegregation. This is particularly the case with alloyed steels. Such steels must be cast as killed steels. Since no gas pores are formed which counteract the contraction of the material during the transition from solid to liquid, however, the shrinkage of the material is greater than with rimmed steels. This leads to large cavities in the head area of the ingot, which must be cut off accordingly in the rolling or forging mill. In this way, about 20% of the casting block is unusable.

For alloyed steels with high demands on a homogeneous composition, steel is casted as killed steel!

Continuous casting

The majority of the steels produced (over 90%) do not solidify as ingot casting but are produced by continuous casting. In continuous casting, the molten steel is cast in copper molds, which are open at the bottom and water-cooled. The steel exits through this opening in a solidified form and is continuously drawn off as an endless strand via guide and drive rollers. On the way from the vertical to the horizontal position, which is more than 15 metres lower, the steel strand also cools down inside. It is sprayed continuously with water and has a relatively tough and solid structure due to its rapid cooling.

With continuous casting (strand casting) steel ist drawn off endlessly!

Only killed steel melts are suitable for continuous casting, as otherwise the gas pores could cause the steel to break. As the melt flows continuously into the mold, no shrinkage cavities occur, as is the case for killed steels in the ingot casting process. Apart from the generally slightly poorer surface finish, continuous casting has many economic advantages compared to ingot casting. For this reason, more than 90 % of semi-finished products are now produced using the continuous casting process.

In the production of slabs using continuous casting, the drawn steel strand is simply cut to length at regular intervals by cutting torches carried along. The slabs produced can have a rectangular cross-section of up to 2000 mm width and 400 mm height, whereby the length of the slabs can in principle be selected as required by the rolling mills.

Depending on the shape of the mold, not only rectangular cross-sections can also be produced, but many others. In contrast, square or circular cross-sections are no longer referred to as slabs but as billets.

Slabs have rectangular cross-sections and are mainly used as semi-finished products for sheet metal production. Billets have square or circular cross-sections and serve as semi-finished products for rod, tube, profile and wire production!

Introduction

In the early days of steel production, the crude steel was cast immediately after refining through oxygen and was ready for sale. Over the course of time, however, the demands on the steels have increased more and more. Especially low concentrations of phosphorus, nitrogen and oxygen have become indispensable nowadays. The required quality can only be guaranteed by further post-treatment procedures.

For historical reasons, therefore, the processes from ore extraction to crude steel after refining through oxygen are counted as primary steelmaking (primary metallurgy); all other refining processes of crude steel to finished (alloyed) steel belong to secondary steelmaking (secondary metallurgy). The crude steels obtained by the direct-reduced iron process are also subjected to such a secondary metallurgical treatment. These include the procedures described in more detail below, such as:

• deoxidation
• vacuum treatment
• remelting treatment
• alloying

For these processes, the fresh crude steel is tipped in the converter and poured into so-called ladles. This is why secondary metallurgy is often referred to as ladle metallurgy.

Secondary steelmaking (ladle metallurgy) is used to refine steels to meet the most diverse requirements!

Deoxidation

In the previous article, the most important equilibrium reactions during refining through oxygen were explained. In particular, the reaction of iron oxide (\(FeO\)) with carbon (\(C\)) to iron (\(Fe\)) and carbon monoxide (\(CO\)) is particularly important for the oxygen content of the later steel:

\begin{align}
\label{1}
&FeO ~&&+~ C &&\rightleftharpoons~ Fe && +~ CO \\[5px]
\end{align}

Since this chemical equation is an equilibrium reaction, oxidized iron (\(FeO\)) always remains dissolved in the melt. According to the equation (\ref{1}), only little oxygen in the form of carbon monoxide can escape from the melt, especially in low-carbon melts. The liquid crude steel then contains a great deal of dissolved oxygen. In the solidified state, in conjunction with sulfur, this can lead to an increased tendency to hot shortness during hot working, since the iron oxide \(FeO\) formed reduces the melting temperature of the already low-melting iron sulfide compound even further.

In addition, the oxygen “boils” out in the form of carbon monoxide when the steel solidifies. This leads to gas inclusions and thus to pores. The boiling out of carbon monoxide is due to the fact that the carbon content in the residual melt increases during solidification (see iron-carbon phase diagram). In order to maintain the chemical equilibrium, according to equation (\ref{1}) carbon dioxide is thus formed more often, which then rises gaseously upwards. Gas pores would also form when a welded joint solidifies again. This is why such oxygen-containing steels are not suitable for welding. For this reason, the oxygen must be removed from the crude steel as far as necessary after refining through oxygen by subsequent deoxidation.

A high oxygen content in the steel leads to the formation of gas bubbles during solidification and thus to pores. Crude steel must therefore be deoxidized before solidification!

Depending on the degree of deoxidation, a distinction is made between rimmed, semi-killed and killed steel (the gas bubbles are “killed” so speak). The degree of deoxidation depends on the choice of deoxidants, which must all have a stronger affinity to oxygen than iron.

In the case of rimmed casted steels, only manganese (\(Mn\)) is added, which not only binds the oxygen but also converts the sulphur sulphide (\(FeS\)) contained in the steel into manganese sulphide (\(MnS\)), which reduces the danger for red and hot shortness:

\begin{align}
\label{unberuhigung}
&FeO &&~+~ Mn  &&\rightleftharpoons~ Fe && +~ MnO \\[5px]
&FeS &&~+~ Mn  &&\rightleftharpoons~ Fe && +~ MnS \\[5px]
\end{align}

In the case of half-killed steel, silicon is added acording to the following chemical equilibrium equation:

\begin{align}
\label{halbberuhigung}
&2~FeO ~&&+~ Si  &&\rightleftharpoons~ 2 Fe && +~ SiO_2 \\[5px]
\end{align}

In the case of killed steel, aluminium is also used, whereby the reaction takes place almost completely in favour of the aluminium oxide. Almost all the oxygen remains bound in the crude steel in the form of aluminium oxide and does not gas out during solidification:

\begin{align}
\label{vollberuhigung}
&3~FeO ~&&+~ 2 Al &&\rightarrow~ 3 Fe && +~ Al_2O_3 \\[5px]
\end{align}

Not only oxygen but also other gases such as nitrogen (from the air) are absorbed by pig iron during refining through oxygen. The nitrogen content must also be subsequently brought below the required maximum value, as steels containing nitrogen tend to age.

Ageing is the embrittlement of steel over time. The cause of embrittlement is that nitrogen can be dissolved at relatively high temperatures but remains practically insoluble at room temperature. Thus, the (forcibly dissolved) nitrogen will precipitate in the steel structure over time as iron nitride. These precipitates hinder in particular the dislocation movement necessary for deformation – the steel becomes brittle.

Steels that should not show any signs of aging must therefore be low in nitrogen. Nitrogen uptake was a major problem, especially in the early days of steel production, when nitrogen-containing air was still being used for refining through oxygen. This is why the refining process is today carried out with technically pure oxygen, whereby nitrogen is always absorbed.

At this point, aluminium, which is used for the production of killed steel, offers additional advantages. The added aluminium combines with the dissolved nitrogen to form aluminium nitride and thus makes it harmless to ageing:

\begin{align}
&FeN  ~&&+~ Al &&\rightarrow~ Fe ~&&+~ AlN \\[5px]
\end{align}

The aluminium nitrides also promote nucleation during solidification and retard grain growth so that fine-grain steels suitable for welding can be produced (weldable fine grain steels).

Killed steels with aluminium are particularly suitable for welding (weldable fine-grain steels)!

However, killed steel can have a disadvantage regarding the surface quality, which is why in some cases rimmed steel is used, e.g. for the production of deep drawing sheets (more on this in the article here). Casting killed steel can also lead to reinforced interior cavities (closed cavities), which have to be separated later and thus represent rejects.

Vacuum treatment (degassing)

In a similar way to nitrogen, other gases, such as hydrogen, have an embrittlement effect in steel (called hydrogen embrittlement). In principle, the dissolved gases should therefore be removed as far as possible from the liquid crude steel.

To achieve this, the property is used that the solubility of gases depends on the external pressure. The lower the ambient pressure, the fewer gases can be dissolved in a liquid. Degassing of the liquid crude steel can therefore be achieved by exposing it to a strong negative pressure (“vacuum“). This is exactly what vacuum treatment does. The gases such as hydrogen, nitrogen and oxygen, some of which are no longer soluble, rise in the melt and can be slagged.

Unwanted gases can be removed from the liquid crude steel with a vacuum treatment!

Since some of the oxygen in crude steel is also degassed during vacuum treatment, deoxidation also occurs. A strong decarburization effect can also be achieved by the vacuum treatment, since the chemical equilibrium shifts to the right according to the equation (\ref{1}). As a result, the carbon can no longer be dissolved to the same extent in the melt and rises as carbon monoxide. Such fine decarburization is necessary for some stainless steels, some of which have carbon concentrations of only 0.01% (e.g. X1NiCrMoCu25-20-5).

Remelting process

The electro-slag remelting process (ESR process) provides even greater purity than degassing by vacuum treatment. For this purpose, the crude steel is first cast into a cylindrical form and solidified. The solidified steel cylinder then serves as a melting electrode for a high-voltage source. The second electrode is formed by a mould containing a slag bath consisting of lime and alumina. The slag also serves as electrical resistance.

The crude steel electrode is now approached from above the mould electrode and an arc is ignited. The steel cylinder begins to melt due to the high temperatures and drips onto the liquid slag. When passing through the slag, the undesirable substances are removed from the liquid steel. The cleaned steel then solidifies in the water-cooled mould, while the slag moving upwards with the solidification front of the steel.

Electro-slag remelting can also be carried out under vacuum, so that a degassing effect occurs at the same time. This combined process is then referred to as the vacuum arc remelting (VAR), which is used for particularly high-quality stainless steels.

Alloying

An important task of secondary steelmaking is the adjustment of the steel composition (adjustment of the analysis). This is done by adding alloying elements such as titanium, chromium, nickel, molybdenum, vanadium, tungsten, etc. In the electric steel process, high-melting elements may already be added directly in the arc furnace or induction furnace.

After analysis checks, the liquid steel is finally ready to be casted and processed into semi-finished products or castings (cast steel). The liquid steel is cast either as ingot casting (discontinuous process) or as continuous casting (continuous process). These processes are described in more detail in the following article From steel to semi-finished products.

Introduction

Due to its high carbon content and relatively high concentrations of phosphorus and sulphur, pig iron is generally very brittle and is not suitable for forging or welding. Therefore, the pig iron must be post-treated to produce the actual steel with its typical forging and welding properties. This aftertreatment takes place in steelworks (steel mill) whose main task is to reduce carbon to the desired level of less than 2 % and to remove the contaminants such as sulphur, phosphorus and nitrogen as far as possible (and, to a certain extent, silicon and manganese).

The sulphur has a particularly detrimental effect. It leads to the formation of iron sulfide, which forms a low-melting mixture in combination with oxygen. The iron sulfide segregates at the grain boundaries of the austenite grains during subsequent hot forming in the range of 800 °C to 1000 °C and thus leads to embrittlement of the microstructure. Since the fracture surface of a steel broken at these temperatures is red-hot, this type of fracture is also known as red shortness. From a temperature of 1200 °C the iron sulfide inclusions even begin to melt and thus also lead to breakage. This is then referred to as hot shortness.

In order to avoid red and hot shortness, steels must therefore generally be low in sulphur (exception: free machining steel). In addition, manganese is usually added, which then binds the sulphur to itself in the form of manganese sulphide and is therefore not present in the harmful form of iron sulphide.

In the following sections, the reduction of sulphur by targeted desulphurisation and the reduction of carbon content by refining through oxidation are therefore discussed in more detail.

Pig iron is too brittle to be used as a construction material due to its high carbon and sulphur content! Pig iron must therefore be desulphurised and decarburised!

Desulfurization

The refining of pig iron into steel is carried out in liquid form. For this purpose, the pig iron produced in the ironworks is first collected in large mixing containers (called pig iron mixers) with a capacity of up to 1800 tons. Several pig iron tappings are mixed in it to compensate for differences in composition and thus provide the steelworks with a constant quality.

With manganese-rich pig iron, pre-desulfurization already takes place without any further action, as the sulfur has a greater affinity to manganese than to iron. Ferromanganese additives enhance this effect. During this so-called manganese desulfurization, the iron sulfide (\(FeS\)) forms manganese sulfide (\(MnS\)) in the pig iron melt:

\begin{align}
\label{manganentschwefelung}
&FeS ~+~ Mn \rightleftharpoons~ Fe ~+~ MnS ~~~~~~ \text{(exothermic)}  \\[5px]
\end{align}

The manganese sulphide, which is insoluble in liquid pig iron, is deposited as slag on the melt. In principle, however, it is not possible to remove all the sulphur from the melt because the reaction equilibrium is shifted to the left at high temperatures (which are necessary to keep the pig iron liquid).

For this reason, calcium oxide or calcium carbide is added to the pig iron before it is transported to or in the steel mill and thus desulfurized (called lime desulfurization). The iron sulfide (\(FeS\)) contained in liquid pig iron essentially reacts with the calcium oxide (\(CaO\)) or calcium carbide (\(CaC_2\)) to form calcium sulfide (\(CaS\)). Magnesium oxide (\(MgO\)) has the same desulfurizing effect. High temperatures in particular favour this form of desulphurisation. The chemical equations for this are as follows:

\begin{align}
\label{kalkentschwefelung}
&FeS ~+~ CaO  &&\rightleftharpoons~ FeO && +~ CaS \\[5px]
&FeS ~+~ CaC_2 &&\rightleftharpoons~  Fe &&+~ CaS ~+~ 2~C  \\[5px]
&FeS ~+~ MgO  &&\rightleftharpoons~ FeO &&+~ MgS \\[5px]
\end{align}

The calcium or magnesium sulphide formed is then bound in a basic slag. Basically, only slags made of basic compounds can bind acidic substances such as sulphur (and phosphorus). Depending on the process, the sulfur content in pig iron can be reduced to up to 0.001 %.

The largely desulphurised pig iron is further processed in the steel mill itself. If it is not an integrated steelworks, the pig iron may have to be transported several kilometres from the smelter to the steel mill. This usually takes place in so-called torpedo cars, which are located on railway tracks.

This means of transport owes its name to its elongated torpedo shape. The capacity of such a torpedo car is about 300 tons. The refractory lined wagons allow the pig iron to cool only minimally on its way to the steelworks (approx. 10 °C per hour). The still molten steel can then be further processed in the steelworks.

Decarburization (basic oxygen steelmaking)

Once the molten pig iron has arrived in the steelworks after desulfurization, it is further processed into (crude) steel. At first one makes use of that the accompanying elements, such as phosphorus, which are still present in undesirable amounts, but also elements such as carbon, silicon and manganese, which are present in excessive concentrations, have a greater affinity to oxygen than iron.

This offers the possibility of burning the accompanying elements in the liquid pig iron relatively simply with the supply of oxygen. This causes the unwanted substances to oxidise and are bound or gasified in a slag-forming layer. In contrast to pig iron production in the blast furnace, the combustion process at this point does not involve carbon, as this is to be partially removed from the pig iron, among other things.

In the past, fresh air was blown into the pig iron melt for oxidation. In the course of time, different refined processes have developed, many of which are no longer up to date. Therefore, the most important process, the oxygen converter process (basic oxygen steelmaking), is described in more detail below.

In the oxygen converter process, the pig iron is first filled into huge crucibles with a capacity of approx. 300 t, so-called converters. Oxygen is then blown through a water-cooled copper lance onto the pig iron melt. This process was first developed in the Austrian cities of Linz and Donawitz, which is why it is also known as the Linz–Donawitz-steelmaking (LD-steelmaking).

The carbon and undesirable elements are oxidized out of the pig iron by the injected oxygen. The melt is mixed by the high blowing pressure and the violent oxidation that sets the pig iron melt in motion. With the aid of lime fluxes, the oxides formed are then bound in a basic slag or converted to the gaseous state.

The oxidation of the accompanying elements takes place via the formation of liquid iron oxide (\(FeO\)), which is formed by oxygen (\(O_2\)) on the pig iron (\(Fe\)):

\begin{align}
\label{eisenoxidation}
&Fe ~+~ O_2  &&\rightleftharpoons~ FeO ~+~ O \\[5px]
\end{align}

The oxidized iron then reacts with the accompanying elements, whereby these themselves are oxidized due to the greater oxygen affinity, while the iron oxide is reduced again. The chemical equilibrium equations of the most important accompanying elements are:

\begin{align}
\label{konverterverfahren}
&FeO ~&&+~ C  &&\rightleftharpoons~ Fe && +~ CO \\[5px]
5~&FeO &&~+~ 2~P  &&\rightleftharpoons~ 5~Fe && +~ P_2O_5 \\[5px]
&FeO &&~+~ Mn  &&\rightleftharpoons~ Fe && +~ MnO \\[5px]
2~&FeO &&~+~ S  &&\rightleftharpoons~ 2~Fe && +~ SO_2 \\[5px]
\label{konverterverfahren_ende}
2~&FeO &&~+~ Si  &&\rightleftharpoons~ 2~Fe && +~ SiO_2 \\[5px]
\end{align}

The extremely exothermic oxidation of the accompanying elements ultimately resembles a combustion process. The temperature of the pig iron melt rises from 1250 °C to over 1600 °C. For this reason, approx. 20 % iron scrap is added for cooling in order to protect the converter lined with stones from excessive temperatures. Iron ore or sponge iron (from the direct reduced iron process) can also be used for cooling.

After 20 minutes of oxygen supply, the decarburized melt is then called crude steel. For further refinement, the crude steel is poured into ladles. This further treatment is also referred to as secondary metallurgy and is dealt with in the following chapter.

With basic oxygen steelmaking (oxygen converter process), oxygen is blown onto the liquid melt and carbon and other elements are oxidized! Scrap is added for cooling!

Direct reduced iron (DRI) process

In addition to the process route via the blast furnace process, there is another option for extracting crude steel from iron ores using the so-called direct reduced iron (DRI) process (secondary route). The starting point for the efforts to find an alternative to the blast furnace process was the relatively high process expenditure. Just think of the complex coke production, hot-blast production and the high demands placed on blast furnace materials due to the enormous temperatures and not least the harmful waste gases. Particularly in the wake of climate change, it is necessary to look for alternatives that produce less CO2 than is currently the case in the blast furnace process.

For these reasons, the DRI-process was developed, but in terms of productivity it still stands far behind the blast furnace process.

The heart of the direct reduction is the shaft furnace, which is filled from the top with lump ore and pellets. Carbon monoxide and hydrogen are used to dissolve out the oxygen in the iron ores. In contrast to the blast furnace process, however, these reduction gases do not result from the combustion of coke. Instead, these reducing agents are produced from natural gas by catalysis outside the shaft furnace.

The process gases carbon monoxide (\(CO\)) and hydrogen (\(H_2\)) are blown into the furnace at temperatures of approx. 1000 °C and flow through the iron ores. The reduction of iron oxides takes place according to the following chemical equations:

\begin{align}
\label{direktreduktionsverfahren}
&Fe_2O_3 ~&&+~ 3 CO  &&\rightarrow~ 2 Fe && +~ 3 CO_2 \\[5px]
&Fe_2O_3 ~&&+~ 3 H_2 &&\rightarrow~ 2 Fe && +~ 3 H_2O \\[5px]
\end{align}

In contrast to the blast furnace process, the direct-reduced iron process operates at temperatures of up to 1000 °C. The iron ores are therefore not melted! This also applies to the gangue contained in the ore, which is why the iron ores used must be relatively low in gangue from the outset.

The reduction gases only dissolve oxygen from the iron ores. This causes the iron ores to crack on the surface. The appearance resembles a porous sponge, which is why the deoxidized and thus strongly ferrous iron ore is also called sponge iron.

Sponge iron is a highly ferrous ore from the direct reduced iron process!

In order to prevent excessive re-oxidation of the sponge iron in the ambient air, it must be cooled down in the lower region of the shaft furnace. Carburisation as in the blast furnace process does not occur in the DRI-process, as coke is not used as a reducing agent. Thus the carbon content of the sponge iron remains relatively low at around 2 %. Slag-forming aggregates (flux) are also not used in the shaft furnace. Impurities are removed by the subsequent electric steel process. The sponge iron is melted into the actual crude steel. This process will be discussed in more detail in the next section.

In the direct-reduced iron (DRI) process, the iron ores are reduced directly to sponge iron by gaseous reducing agents!

Electro-steel process

In the electric steel process, the sponge iron obtained from the direct reduced iron process is used for crude steel making. In general, scrap is also added to this process as well as pig iron from the blast furnace process. These components are then mixed in a special oven. While the pig iron is already liquid, the sponge iron and the added scrap must be melted down.

In the electric steel process, this is done with the aid of electrodes that ignite a hot arc in the furnace (electric arc process). By turning the furnace, the slag can first be poured off and by turning it again in the other direction, the liquefied crude steel.

For smaller quantities of steel, the necessary heat of fusion can also be generated in special induction furnaces (electric induction process). Due to the relatively high oxygen content in the iron scrap (oxidized iron), the melt is oxidized, so that the melt is refined in a certain way. Aggregates also bind the undesirable substances in a slag.

The electric steel processes are characterized by extremely high process temperatures of over 3000 °C. This also makes it possible to melt high-melting alloying elements in scrap such as tungsten, molybdenum and niobium. In principle, all types of steel can be produced with the electric steel process, although the electric steel process is usually reserved for high-alloyed steels due to the high costs involved (e.g. tool steels).

After crude steel has finally been obtained from the solid iron sponge using the electric steel process, the refined after-treatment is also carried out using secondary metallurgical methods.

Combustion process

The iron in the processed ores must be dissolved out by chemical processes. This requires an element that binds oxygen more strongly than iron. This is exactly what carbon can do as a reducing agent. The carbon is fed to the blast furnace in the form of coke (pyrolyzed coal) from the coking plant.

At sufficiently high temperatures, the carbon can participate directly in the reduction of the iron (direct reduction). At lower temperatures, the reduction takes place indirectly with the help of the carbon monoxide gas formed during combustion (indirect reduction). In both cases the iron oxide is reduced to iron.

The use of carbon as a reducing agent is not only due to its high availability but as a gaseous substance carbon monoxide has the advantage that it can penetrate the porous ores well and react with the iron oxides. For this reason, the iron ores are also processed as porous as possible in the form of sinter and pellets.

Carbon not only provides the necessary heat to melt the iron ore but also serves in a special way as a reducing agent to reduce the iron oxides to iron!

The individual chemical processes in the blast furnace are described in more detail in the following sections.

To generate the necessary heat and the reducing carbon monoxide gas (\(CO\)), the coke is burnt in the lower part of the blast furnace with the supply of the hot-blast. The carbon (\(C\)) contained in the coke initially reacts with the atmospheric oxygen in the hot-blast (\(O_2\)) according to the following chemical equation to carbon dioxide (\(CO_2\)):

\begin{align}
\label{kohlendioxid}
& C + O_2  \rightarrow CO_2 ~~~ \text{(exotherm)} \\[5px]
\end{align}

This combustion is very strongly exothermic, i.e. heat is released – as is usual in a combustion process. This leads to a temperature increase of up to 2000 °C. The carbon dioxide gas (\(CO_2\)) reacts with the carbon contained in the coke (\(C\)) due to the very high temperatures and forms the carbon monoxide gas (\(CO\)) as the reducing agent:

\begin{align}
\label{kohlenmonoxid}
& CO_2 + C  \rightleftharpoons 2~CO ~~~ \text{(endotherm)} \\[5px]
\end{align}

The formation of carbon monoxide is endothermic, i.e. with energy absorption, so that the temperature of the gas drops to about 1700 °C. An excessive drop in temperature must be avoided, however, as the reaction of carbon dioxide to carbon monoxide only takes place at sufficiently high temperatures.

Too low a temperature would again mean the back reaction (indicated by the double arrow in the chemical equation) of the carbon monoxide to carbon dioxide. But precisely this carbon monoxide is necessary for the reduction of iron oxides and must be produced in sufficient quantities.

Boudouard reaction

The reaction of carbon dioxide and carbon to carbon monoxide and its reverse reaction is temperature-dependent and is described by the so-called Boudouard reaction.

The situation can be illustrated by the evaporation of water in a closed room filled with air. Depending on the temperature, the air can absorb a certain amount of water. If the air is completely saturated with water, it cannot absorb water any further and a remainder of liquid water remains in the room. Over time, a balance between the water that is still liquid and the water contained in the air will develop.

While the liquid water can evaporate briefly at a local point due to statistical fluctuations (“reaction”), water is again condensed locally from the supersaturated air at another point (“reverse reaction”). Globally, however, an equilibrium between these phases or between the reaction and its reverse reactionof liquid and air-bound water will occur. One also speaks of a dynamic equilibrium.

However, this dynamic equilibrium can be shifted in favour of or at the expense of the liquid water, depending on the temperature. At high temperatures, the air can absorb more water, so that more water evaporates and the residual liquid content decreases. The equilibrium thus shifts in the direction of the evaporation process. If the temperature is lowered, however, part of the water bound in air condenses and the liquid content of the ground rises. The equilibrium then shifts in the direction of the condensation process (this effect can also be seen very clearly on cooled beverage cans, where the water contained in the air condenses and water pearls form).

In the same way, there is also a temperature-dependent equilibrium between the reaction of carbon dioxide to carbon monoxide and the corresponding reverse reaction of carbon monoxide to carbon dioxide. The figure above shows the ratio between carbon monoxide and carbon dioxide at the corresponding equilibrium states at different temperatures. It becomes obvious that at temperatures above 1000 °C carbon dioxide disintegrates almost exclusively in favour of carbon monoxide.

At high temperatures, carbon reacts predominantly with carbon dioxide to form carbon monoxide (as an important reducing agent). At low temperatures, carbon monoxide mainly decomposes to carbon dioxide and carbon!

Preheating

The combustion of coke produces hot gases consisting mainly of carbon monoxide, carbon dioxide, nitrogen and (gaseous) water. These hot gases rise up in the blast furnace and cool down. From a maximum of 2000 °C at the level of the hot-blast nozzles (tuyere), the temperature decreases to around 200 °C up to the throat. At this temperature, the filled charge are first dried and preheated. This zone of the blast furnace is therefore called the preheating zone.

In the preheating zone the charge is preheated and dried!

Indirect reduction zone

A further lowering of the iron ores in the blast furnace leads to the temperature zone of 400 °C to 800 °C. In this region, the so-called indirect reduction of iron ores to iron by carbon monoxide takes place.

The carbon monoxide gas (\(CO\)) is oxidized to carbon dioxide (\(CO_2\)) by absorbing the oxygen contained in the iron oxide (\(O\)). The indirect reduction can be divided into several intermediate reactions, whereby the iron content in the ores continuously increases and ultimately pure iron is produced.

First, the weakly ferrous hematite (\(Fe_2O_3\)) is reduced by the carbon monoxide (\(CO\)) to more ferrous magnetite (\(Fe_3O_4\)). Then the magnetite is reduced to so-called wüstite (\(FeO\)) before it is finally reduced to iron (\(Fe\)):

\begin{align}
\label{indirekt}
3~Fe_2O_3 &~+~ CO  &&\rightarrow~ 2~Fe_3O_4 &&+ ~CO_2 ~~~~~~ \text{(hematite} \rightarrow \text{magnetite)} \\[5px]
Fe_3O_4 &~+~ CO  &&\rightarrow~ 3~FeO &&+ ~CO_2 ~~~~~~ \text{(magnetite} \rightarrow \text{wüstite)} \\[5px]
FeO &~+~ CO &&\rightarrow~ Fe &&+ ~CO_2 ~~~~~~ \text{(wüstite} \rightarrow \text{iron)} \\[5px]
\end{align}

Only the last chemical reaction of wüstite to iron is exothermic and supplies heat. However, the heat released is sufficient to supply heat for the first two endothermic reactions. The overall balance thus remains exothermic. Nevertheless, the temperature in this indirect reduction zone of 400 °C to a maximum of 800 °C is too low to melt the reduced iron! The consistency of the iron is therefore still solid.

The unmelted iron ores still contain non-reduced iron oxides. An unfavourable accumulation of ores at the contact points can also make gas flow difficult and thus leave non-reduced iron ore behind. At the latest, however, when the melting process occurs in the deeper and hotter zones of the blast furnace, iron ores can be almost completely reduced.

In the indirect reduction zone, the iron oxide contained in the iron ores is reduced by carbon monoxide gas, whereby the iron ores are not melted!

Direct reduction zone

If the iron ores, which have not yet been reduced, move further down in the blast furnace, then the carbon can also directly reduce the iron oxides contained in the ore due to the high temperatures. The carbon (\(C\)) is oxidized to carbon monoxide (\(CO\)) by the absorption of oxygen (\(O\)). Due to the direct reduction of iron oxides by carbon, one also speaks of a direct reduction.

The direct reduction takes place in the temperature zone between approx. 800 °C to approx. 1600 °C. The direct reduction can again be divided into intermediate steps in which the iron content in the ores increases in each case:

\begin{align}
\label{direkt}
3 Fe_2O_3 &~+~ C &&\rightarrow~ 2~Fe_3O_4 &&+ ~CO ~~~~~~ \text{(hematite} \rightarrow \text{magnetite)} \\[5px]
Fe_3O_4 &~+~ C &&\rightarrow~ 3~FeO &&+ ~CO ~~~~~~ \text{(magnetite} \rightarrow \text{wüstite)} \\[5px]
FeO &~+~ C &&\rightarrow~ Fe &&+ ~CO ~~~~~~ \text{(wüstite} \rightarrow \text{iron)} \\[5px]
\end{align}

All these reactions are endothermic. The heat required for this is provided by the (exothermic) combustion of the coke. The silicon, manganese and phosphorus compounds (and many other compounds) contained in the gangue are also reduced endothermically. The heat required for this must also be generated by burning of coke. In order to keep the heat input as low as possible for energetic reasons, as little gangue as possible should be contained in the ores. This makes the ore processing described above necessary.

The reduced admixtures in the blast furnace then react with the added flux in the charge and are bound in the resulting slag.

In the direct reduction zone, carbon takes part directly in the reduction of iron oxides!

Carburizing zone

Although the carbon reduces the iron oxides and produces iron, the iron formed is partially enriched with carbon itself. This is done either by the carbon monoxide gas or, at higher temperatures, directly by the carbon. This produces iron carbide \(Fe_3C\), also known as cementite.

The enrichment of solid iron with carbon in a temperature range between 900 °C and 1200 °C is also referred to as carburisation. The corresponding chemical reactions are as follows:

\begin{align}
\label{aufkohlen}
3 Fe &~+~ 2 CO  &&\rightarrow~ Fe_3C ~+~CO_2  \\[5px]
3 Fe &~+~ C &&\rightarrow~ Fe_3C  \\[5px]
\end{align}

By absorbing the carbon, the solidification point of the iron is reduced from 1536 °C to approx. 1200 °C (see iron-carbon phase diagram).

In the carburizing zone, the reduced iron is enriched with carbon and forms the intermediate compound iron carbide (\(Fe_3C\), cementite)! The carburization lowers the solidification temperature of the iron!

Melting zone

Due to the reduction of the solidificatoin point by diffused carbon into the reduced iron, it begins to melt in the hotter zones of the blast furnace. Now the inner ore layers also come into contact with the carbon or the carbon monoxide gas and can be reduced to iron and then carburized. Gradually all the iron ore is reduced, carburized and melted. The absorbed carbon remains dissolved in the molten iron. This is the reason why pig iron has a relatively high carbon content.

In the melting zone the iron begins to melt and forms strongly carbon-containing pig iron!

Tapping zone

The molten pig iron finally collects in the hearth of the blast furnace together with the slag, which is also liquid. Due to the higher density of the pig iron, the pig iron can be cut off at the lowest point of the blast furnace (called tapping). It has a high carbon content of about 4.5 %. Further accompanying elements are listed below, although the composition may vary considerably depending on the type of pig iron.

• 4.5 % carbon
• 2.5 % silicon
• 1.5% manganese
• 0.5 % phosphorus
• 0.1 % sulphur

Other elements such as titanium, copper, etc. are also contained in small quantities in the pig iron.

When the liquid pig iron solidifies, the silicon content in particular determines whether the dissolved carbon precipitates in its pure form as graphite (\(C\)) in the microstructure or as iron carbide (\(Fe_3C\), cementite).

Graphite precipitation is preferred for high silicon concentrations. Graphite formation is also favoured by a relatively low manganese content and slow cooling. With a relatively low silicon content, on the other hand, cementite precipitation is preferred when the pig iron solidifies. This in turn is favoured by relatively high manganese concentrations and faster cooling rates.

Since at high temperatures and long annealing times the formed cementite would decompose back into its constituents (iron and carbon), this form of solidification is also called metastable solidification in the thermodynamic sense. In contrast, graphite is a thermodynamically stable compound that has survived even with long annealing times. This type of solidification is therefore also referred to as stable solidification.

Due to the graphite in the stable solidified microstructure, the fracture surface of the pig iron appears matt-grey. Therefore the solidified pig iron is also called grey pig iron. The fracture surface of a metastable solidified pig iron, on the other hand, has a whitish shine and is therefore also called white pig iron.

With stable solidification, carbon precipitates in pure form as graphite (grey pig iron)! With metastable solidification, the iron carbide compound cementite is formed (white pig iron)!

The grey pig iron (“cast pig iron”) serves as a raw material for foundries to produce various types of cast iron or cast iron materials. It is usually cast in small ingots and offered to foundries for melting down with other materials (charging). However, the demand for grey pig iron accounts for only a small part. More than 90 % of the pig iron is beeing tapped as white pig iron (“steel pig iron”) and processed in liquid form in ironworks to the actual steel by special after-treatment processes. These processes are described in more detail in the following articles.

Charging

After the ground ores have been processed in forms of sinter and pellets (see article iron ore mining and dressing), they are stored in bunkers in the iron works and mixed with lump ore as required. Before this mixture is then fed to the blast furnace together with coke as fuel, additives are added (also called fluxes). These mixture is referred to as charge. The feeding of charge and coke into the top of the blast furnace in layers is then referred to as charging.

The mixture of iron ore and flux is called charge, which is fed into the blast furnace in layers together with coke (called charging)!

Even if the iron content in the ores could be greatly increased by the processing, undesirable substances will still be present. For this reason, additives (fluxes) such as limestone (\(CaCO_3\)) or  quicklime (\(CaO\)) are added to the charge. These fluxes react in the blast furnace with the gangue and the coke ash. A liquid mixture is formed which binds the undesirable elements. This mixture is also called slag.

Since the liquid slag has a lower density than the iron, it floats on the melt and can then be separated relatively easily. In addition, the slag forms a passivation layer to prevent excessive oxidation of the molten iron by the ambient air.

Slag binds the undesirable substances (gangue) and protects the liquid iron from excessive oxidation!

About 20 % of the total tapped mass from the blast furnace is accounted for by the slag alone! Slag, however, is by no means a pure waste product but serves as a raw material for cement production. In this case the slage ist solidified and granulated and is referred to as ground-granulated blast furnace slag (GGBFS). The blast furnace slag contains large quantities of calcium oxide (\(CaO\)), silicon dioxide (\(SiO_2\)), aluminium oxide (\(Al_2O_3\), alumina) and magnesium oxide (\(MgO\)).

Ground-granulated blast furnace slag is used as a raw material for cement production!

Coking plant

Coke, which consists mainly of carbon, not only supplies the necessary heat by burning but also serves in a special way as a reducing agent so that the iron can be removed from the ores! The chemical processes are discussed in more detail in the chapter on blast furnace processes. Modern blast furnaces require around 350 kg of coke to melt one tonne of pig iron. With a daily production of 10,000 tons, this results in a coke input of 3,500 tons per day!

For this reason, the coke required (also called metallurgical coke) is usually produced directly in the iron works, in so-called coking plants. Coke production is essentially based on heating coal in the absence of air by temperatures above 1000 °C (pyrolysis). The air seal prevents the coal from burning; this is only supposed to happen in the blast furnace. Due to the high temperatures, the undesirable compounds in the coal, such as sulphur, are gasified and then removed.

Coke is a carbon-rich coal produced by pyrolysis in coking plants!

The coking of the coal takes up to 20 hours, whereby the coal loses about a quarter of its original mass. The carbon content increases to approx. 90 %. For economic reasons, however, it is not possible to remove all the sulphur from the coal, leaving around 1% sulphur in the coke. In this way, sulphur in form of iron sulphide (\(FeS\)) enters the pig iron to an undesirable degree. It must be removed later for steel production by special after-treatment processes.

Cowper stove (hot-blast stove)

As with any combustion, the burning of coke in a blast furnace requires oxygen. Oxygen is blown into the blast furnace in the form of air via so-called tuyeres (hot-blast nozzles). The tuyeres are attached to the bosh of the blast furnace. In order to reduce cooling of the blast furnace by the injected air, the air is first heated to over 1000 °C. Hot air is generated in hot-blast stoves, also known as cowper stoves (named after their inventor Edward Alfred Cowper). The heated air is fed to the hot-blast nozzles via a bustle pipe.

Hot-blast is air heated in hot blast stoves, which is blown into the blast furnace via tuyeres!

In principle, hot-blast stoves consist of several brick towers. The inner masonry of these towers is heated alternately. Among other things, the hot but low energetic waste gas of the blast furnace is used (which for this reason is initially mixed with other high energetic fuel gases like natural gas). This gas mixture is then burned inside the hot-blast stoves. The large-area masonry heats up accordingly.

If the desired temperatures of over 1000 °C are reached, the combustion process is interrupted and oxygen-rich ambient air is passed over the heated masonry. The air heats up and can then be fed into the blast furnace as a hot blast. To enable continuous operation of the blast furnace, at least two cowper towers must always be switched alternately to “heating” or “blowing”.

Blast furnace

The actual melting down and removal of the iron from the ores takes place in blast furnaces, which project about 30 m in height and have a diameter of about 10 m. Some blast furnaces even reach heights of over 70 m and diameters of 15 m. The temperatures of up to 2000 °C in the blast furnace require an extremely heat-resistant lining. For this purpose, artificially produced fireclay bricks made of corundum (\(Al_2O_3\)) are used, which must also have a very high carbon monoxide resistance.

In the lower part of the blast furnace with the highest temperatures even special carbon stones are used. The brickwork in this lower section is sometimes more than one metre thick. In addition, the masonry is specially water-cooled by a pipe system. In this way, a blast furnace can run continuously for up to 15 years before it has to be serviced. In principle, blast furnaces are only switched off at these times, otherwise they run around the clock. A blast furnace essentially consists of the components described in more detail below.

Throat

The blast furnace is fed alternately with charge and coke from the top by a special lock system. This part of the blast furnace is also called throat. The top closure is constructed of two separate charging bells which open alternately. First the material to be filled lies on the top bell. As it then opens, the load falls onto the lower, still closed bell. Only when the uppermost bell has closed again, the one underneath is opened and the charge falls into the interior of the blast furnace. This prevents environmentally harmful exhaust particles (blast furnace gases and blast furnace dust) from entering the environment.

In addition to nitrogen, carbon dioxide and small amounts of hydrogen, the blast furnace gas also contains the very toxic carbon monoxide. The blast furnace gases or the blast furnace dust must therefore be specially cleaned and processed. The hot gases are also used to heat up the hot air in cowper stoves and are fed to the coking plant to process the coke. But steam power plants or other industrial plants can also be customers of this hot gas.

Stack

Following the throat is the so-called stack, which has the shape of a truncated cone that widens downwards. This shape prevents the stack from clogging, as the downwardly increasing temperatures also cause the charging to expand.

In the upper part of the stack, the carbon monoxide gas (caused by the combustion of the coke) that rises upwards flows through the iron ores layers. Chemical reactions occur that break down the iron-containing oxides and dissolve pure iron in a form that is not yet liquid.

The coke layers lying between the iron ores must not disintegrate and form a gas-proof layer that impedes the gas flow. The 100 mm coke chunks must therefore be very pressure-resistant.

Belly

The conical shape of the stack finally ends in a cylindrical ring. From there, the still solid components begin to melt and some liquid iron and slag are formed. The sagging of the coke and iron ores from the throat to the melting zone takes several hours. This cylindrical part of the blast furnace is also called belly.

Bosh & hearth

As the volume of iron decreases during melting, the interior of the blast furnace tapers again from the belly. This section of the blast furnace is also known as the bosh. As a result, the bosh has the shape of an inverted truncated cone before the blast furnace becomes cylindrical again. The molten iron, also known as pig iron, collects in this section.

Above the liquid pig iron lies the slag, which is partly drained via water-cooled forms made of bronze or copper. This lowest part of the blast furnace where pig iron and slag accumulate is also called hearth.

The pig iron itself is drained off at regular intervals from a closed hole in the hearth. The piercing of the tap hole to remove the pig iron is called tapping. Together with the remaining slag, the pig iron is then passed to the casting hall via a runner system in the floor. The slag is separated by specially guided line systems.

The draining of liquid pig iron at regular intervals is referred to as tapping!

Introduction

Iron is one of the most important metals in mechanical engineering, as it is present in sufficient quantities on earth. The extraction of iron can therefore be very economical. However, pure iron is not suitable for use as a construction material. It is much too soft in its pure form and has too little strength to meet most mechanical requirements. For this reason, it is necessary to use alloying additives to give the iron its hardness and strength.

It has been shown that carbon is an excellent alloying element. Within certain limits, carbon has a very high strength and hardness increasing effect and is very cheap compared to other alloying elements. Such a compound of iron and carbon is generally referred to as steel, where the carbon content is less than 2 %. The special importance of steel in mechanical engineering is already demonstrated by the daily production of around 4 million tons of steel!

Steel is an alloy of iron and carbon, with a maximum carbon content of 2 %!

Basically, a distinction can be made between two routes by which steel is made today:

• Blast furnace process with subsequent oxygen converter process
• Direct reduced iron process with subsequent electric arc furnace process

In contrast to the direct reduced iron process, the blast furnace process can be operated on a large scale. The blast furnace process accounts for over 80 % of total steel production. Therefore, the blast furnace process will be discussed in more detail in the following.

The flow chart shows the basic steps of how iron ore is used to produce semi-finished products of steel. The individual process steps are explained in more detail in the following articles.

Steel is extracted from iron ores (mainly found in sedimentary rocks) and is also known as ore smelting. The plants for the production of pig iron or steel are called ironworks.

An ironworks (or iron works) is an industrial plant for the production of steel from iron ores!

Before the iron ores can be fed to the ironworks, they must first be mined (ore extraction) and specially processed for the blast furnace process (ore processing). These process steps are described in more detail in the following sections.

Iron ore extraction

Iron does not occur in nature as a pure substance but as a compound in rocks, which can contain up to 50 % iron. These ferrous rocks are also called iron ores. The iron compounds contained therein are mainly iron oxides, but also iron carbonates or iron sulphides. The most important iron oxides include:

• magnetite: \(Fe_3O_4\)
• hematite (haematite): \(Fe_2O_3\)
• siderite: \(FeCO_3\)
• limonite: \(2Fe_2O_3 \cdot H_2O\)

The iron itself is removed from these iron oxides in the subsequent blast furnace process and processed into steel in combination with carbon.

Iron ores are mixtures of ferreous compounds (usually iron oxides) and other undesirable elements!

In addition to the actual iron compounds, however, the iron ore always contains various (undesirable) admixtures, which are not of interest for steel production. These waste rocks are also referred to as gangue and are often oxides such as, for example:

• silicon dioxide
• manganese oxide
• calcium oxide
• magnesium oxide
• aluminium oxide
• phosphorus compounds and
• sulphur compounds.

These unwanted admixtures are also the reason why steel always contains accompanying elements such as silicon, manganese, phosphorus and sulphur as well as other elements. The maximum values to be observed for these accompanying elements are prescribed for steels depending on the steel grade.

The undesirable admixtures in the iron ore are called gangue!

For technical and economic reasons, it makes sense to reduce the gangue to a minimum even before the iron ore actually melts down. Otherwise, an unnecessarily high use of coal or thermal energy in the subsequent blast furnace process would be necessary if too much impurities had to be melted. This means that the mined iron ores need to be specially processed in advance before the blast furnace process.

Regions in which iron ore has formed over millions of years (e.g. through volcanism) and can be mined economically are also referred to as iron ore deposits. Especially many ore deposits can be found in countries such as China, Australia, Brazil, India and Russia. These countries account for around 85 % of the total global iron ore mining volume.

Ore deposits are natural accumulations of ores that can be mined economically!

The iron ore extracted in the deposits, mostly by blasting, is initially very coarse and unwieldy, which gives this untreated rock the name coarse ore. After the coarse ore has been extracted, it is crushed directly at the deposits with the aid of cone crushers or jaw crushers to more manageable sizes of approx. 15 mm. This is also known as lump ore. In addition to lump ore, fine ore (approx. 1 mm) and ground ore (ore powder) (approx. <0.1 mm) can also be obtained by crushing and grinding.

Depending on the grain size of the iron ore, it can be divided into lump ore, fine ore and ground powder!

Lump ore can usually fed directly into the blast furnace process, because the special processing of these ores is relatively complex and expensive, so that it is only to a certain extent economically worthwhile to process this ore. Fine ore and ore powder, on the other hand, are specially processed for the blast furnace process. This ore processing will be discussed in more detail in the next section.

Iron ore processing

After the iron ore has been prepared by crushing and grinding during ore extraction, the ore is actually processed. The aim of this is to reduce the undesirable high proportion of admixtures to a desired minimum in order to increase the iron content. This is done by processes such as flotation or magnetic separation. Subsequently, the milled ores are lumped by sintering or pelletizing in order to optimize chemical reactions in the blast furnace process.

Ore processing (or ore dressing) is the separation of iron ore from gangue in order to optimize the blast furnace process!

In principle, the undesirable gangue can never be completely separated from the iron ores during ore dressing. This means that a certain amount of unwanted elements always enters the blast furnace process. Slag-forming aggregates (and other special processes) are then used to separate these unwanted substances during or after the blast furnace process.

Froth flotation

In froth flotation, the different wettability between the iron compounds and the undesirable gangue is used. While, for example, water is wettening the gangue relatively well, i.e. adheres to them, water tends to roll off the ferrous particles. This effect can finally be used to separate gangue from iron compounds.

In froth flotation, the different wettability of the substances is used for separation!

For this purpose, the ground ores (obtained by crushing and grinding) are mixed with water in froth flotation cells. This aqueous suspension is also called slurry. Gas bubbles are generated in the slurry by air supply or stirrers on the floor. Due to the rather low water wettability of the iron-containing ore powder, the rising gas bubbles adhere relatively well to them. The significantly better wettability of the gangue, however, means that they remain completely wetted with water and gas bubbles hardly adhere to them.

While the ferrous particles are thus floated upwards with the adhering gas bubbles, the gangue in the slurry sinks to the bottom. To prevent the gas bubbles from bursting after ascent and to prevent the iron ore from sinking back to the ground, foam stabilizers are added, which create a relatively stable foam layer on the surface. The fluffy, strongly ferrous foam can then be skimmed off and dried. The gangue remaining in the slurry is pumped off after froth flotation and disposed of.

Magnetic separation

In magnetite-containing rock, there is another possibility of separating gangue and iron ore. As the name magnetite already suggests, this type of iron ore is a magnetic rock. This allows the ground ore to pass relatively easily through magnetic separators, where the ferrous rock is separated from the rest of the gangue (magnetic separation).

In magnetic separators, the effect of magnetism is used to separate ferromagnetic materials from non-magnetic materials!

For this purpose, the ground ore is mixed with water to a mud-like mass and passed over a rotating magnetic roller. The ferrous mud adheres to the rolls and is then stripped off and dried. The separated gangue falls through a separate funnel into a container and is disposed of. In principle, this process is also suitable for the iron ores siderite and hematite, which become weakly magnetic when heated.

Sintering and Pelletizing

After processing the iron ore in froth flotation cells or magnetic separators, the finely ground ores cannot be fed directly to the blast furnace, as the enormous compression due to the charging in the blast furnace would impede gas flow. The ores must therefore be made lumpy so that there are sufficient cavities in the charging column for a good gas flow through. The lumpy pieces are made by sintering and pelletizing.

Baking ground ore into lumpy, porous pieces is necessary to improve chemical reactivity!

During sintering, the fine ores are first mixed with additives and fine coke. This mixture then passes through a funnel onto a circulating moving grate. Ignition flames then set the mixed coke on fire. Due to the high temperatures, the ores “bake” together to a sinter cake (called sintering). Air vents provide a suction effect (“chimney effect”) so that the sinter cake actually bakes together over the entire cross-section. Afterwards, the porous sinter cake is broken to grain sizes of approx. 15 mm by rotating blades. Such sinter plants are usually located directly in the ironworks.

During pelletizing, ore powder is rolled into iron ore green pellets in rotating drums together with water, binding agents and additives. Globules with grain sizes of approx. 15 mm are produced, which are then baked into porous pellets. Pellets are mainly produced by special ore suppliers and then delivered to the ironworks.

The main advantages of sintering or pelleting are the increased controllability of the composition and the accelerated chemical reaction in the blast furnace process due to the porosity (better gas flow).