Valoración de la propuesta del entorno virtual de aprendizaje para la

Steel production through an integrated steel plant involves three primary product steps. First, the heat source used to melt iron ore is produced; followed by the melting and reduction of iron ore in the blast furnace; finally the molten metal is reacted with oxygen to form the low carbon compositions of steel. The steps can be conducted at one facility through an on-site power station and collocation of BF and BOS facilities. However power is often supplied from off-site producers. A schematic of a modern integrated mill is shown in figure 2.1.

2.2.1 Coke Making

Coke is the solid carbon fuel used to melt and reduce iron ore. Coke production begins with pulverised bituminous coal. The coal is fed into a coke oven, sealed and heated to around 1573 K for 14 to 36 hours. Coke is produced in a batch process, with multiple ovens operating simultaneously to offer a constant supply of material

Crude Steel

Figure 2.1: A schematic of the modern integrated steel mill [7]. to the BF.

Heat is frequently transferred from one oven to another, reducing energy re- quirements [8]. After the coking process is finished, the coke is moved to a quenching tower where it is cooled with water spray. Once cooled the coke is moved directly to an iron making furnace or into storage.

2.2.2 Ironmaking

During ironmaking, iron ore, coke, heated air and limestone or other fluxes are fed into the BF. The heated air causes the coke to combust, which provides heat and the carbon source for iron production. Limestone or variants (e.g. dolomite) are added to react with and remove the acidic impurities from the molten iron. The limestone-impurity mixture floats on the top of the liquid metal forming a slag and can be skimmed off during the continuous process.

Sintering products may also be added to the furnace. Sintering is the process in which solid wastes are combined into a porous mass which can be added to the BF. The wastes can include iron ore fines, pollution control dust, coke breeze, water treatment sludge and flux. Sintering plants help reduce solid waste by combusting waste products and capturing trace levels of iron present in the mixtures. Sintering plants are not used at all steel production sites [6] [9]. The process of iron making has issues of phosphorus carry over from the originating ore as well as coke additions.

2.2.3 Basic Oxygen Steelmaking

Molten iron from the BF is sent to the BOF. The BOF gives utility for the final refinement of the iron into steel. High purity oxygen is blown into the furnace via a top lance at supersonic speeds. The oxygen combusts with elements such as carbon and silicon in the molten iron. Further fluxes are fed into the BOF to offer a partitioning oxide phase for the removal of impurities such as phosphorus, manganese and titanium etc. Further to refining in the BOF, liquid steel often undergoes alloying steps within the ladle to deliver the required compositions.

The resulting steel is most often cast into slabs, beams or billets. Further shaping of the metal may be conducted within steel foundries, which re-melt the steel and pour the liquid into moulds, or at rolling facilities with both hot and cold conditions, depending on the desired product shape and properties.

Slag is a significant by-product of the BOF. Slag is essentially a slurry of metal oxides in liquid state. The most common components within slag of the steel industry are CaO, F eO (in variable oxidation states), Al2O3,SiO2, M gO,Cr2O3, T iO2,P2O5andM nO. The ratio of these oxides affects the melting point, viscosity,

and impurity partitioning capability along with other processing factors such as foam stability and refractory wear. Of the varying properties the slags basicity is often considered one of the more important features; the basicity is effectively a measure of free oxygen, meaning network forming oxides such asSiO2 reduce basicity whereas

network breaking oxides such asCaO increase basicity. The two selected examples

are in fact the most influential and are thus combined to calculate the binary basicity (%CaO/%SiO2), the quickest and most common way to understand how a slag is

likely to behave during a process or reaction. With the likely high levels of F eO

as an additive in the sintering process [10]. However due to its relatively low iron content compared to raw materials, recently there has been significant activity in its use as carbon sinks, road materials and water purification systems [11].

Hot gases are a further by-product of the BOF, with modern furnaces being equipped with air pollution control equipment able to contain and cool the gases. The gas is quenched and cooled using water and cleaned of suspended metals and other solids. The process produces air pollution control dust [10] and water treat- ment plant sludge [12].

Steel Production from Scrap

Steelmaking from scrap involves melting of scrap, removing impurities and casting into a desired shape. The EAF is often used, and melts with the use of electrical energy in the presence of oxygen. The process is a direct method to produce desired products (as opposed to the multiple steps required through the BF-BOF route) offering the potential to be economically viable on a much smaller scale. Frequently mills producing steel with EAF technology are referred to as mini-mills. In addition it is possible to utilise a portion of scrap within the BOF. The key reasons for doing so are temperature control and the fluctuation in raw material price on a global scale; whereas scrap prices are often dictated within smaller geographic regions due to oversupply and government preference through legislation for internal recycling.

2.2.4 Steel Forming and Finishing

After molten steel is released from either the BOF or EAF, it must be formed into its final shape and finished to prevent corrosion. Traditionally steel was poured into a conventional ingot and stored until a specific final product was required. However current practices favour continuous casting where the steel is poured directly into semi-finished shapes. Continuous casting saves time by: reducing the steps required to produce the desired shape; it increases length versatility of product; and lends itself well to the modern just-in-time manufacturing approach to production and acquisition.

After cooling within the mould further shaping is conducted through hot or cold forming. Hot forming consists of heated steel being passed between rollers until it reaches its desired thickness. The process lends itself to the production of slabs,

strips, bars or plates from the steel.

Cold forming is used to produce wire, tube, sheet and strip. This process involves the steel being passed between rollers without being heated to reduce thick- ness. The steel is then heated in an annealing furnace to improve the ductile prop- erties. Cold rolling is more time consuming, but is used because products taking this route have better mechanical properties, better machinability and can more easily be manipulated into challenging sizes and thinner gauges [8]. After rolling is completed the steel pieces are finished to prevent corrosion and improve properties of the metal [8] [13]).