Decarburization reaction is major steelmaking reactions so taking carbon from hot metal to produce liquid steel, this is a major reactions. However, before carbon is taken out, the other impurities are removed in advance. This is called hot metal pre-treatment. The first element we need to remove is sulfur. Once hot metal is delivered to steelmaking plant, this hot metal is charged into ladle, then sulfur in the hot metal is react with oxygen and iron in slag to form sulfide anions. So in this way the sulfur in hot metal is transferred from hot metal to slag phase for this reaction the oxygen and iron is necessary and this oxygen and iron is usually provided by lime, so lime is very effective desulfurization flux dependant on plant instead of using lime some other fluxes like sodium carbonate or magnesium or calcium carbide are used. These fluxes are injected inside, hot metal through submerged lance with a carrier gas like nitrogen, then this flux react with hot metal to take sulfur out of hot metal. Or this flux is just simply spread onto the hot metal, but they are strongly agitated by putting this type of Kanbara. Once implanted is put inside a metal that it rotates at high speed. This is quite successful in these days, and this is called Kanbara reactor. I will simply call this as KR process. In this graph shows increasing reaction temperature increases desulfurization so at high temperature sulfur removal is accelerated. This line shows desulfurization ratio without agitation, however using the type of converter reactor. With strong agitation, then desulfurization relationship could be improved significantly. This is due to quite intense mixing inside, hot metal accelerating mass transport inside liquid, liquid Metal. One sulfur is removed into slag then this slag is no more used, and it should be removed shown like this. After then this desulfurized hot metal is transferred for further reactions. The next reaction is to remove silicon and phosphorus so this is simply desiliconization or dephosphorization. The hot metal is charged into this type of vessels called converter and in order to take a silicon and phosphorus out of hot metal we need oxygen so oxygen lance is lowered. Just over liquid metal and oxygen is blown at high speed then the oxygen actually burns the iron Silicon and phosphorus so iron becomes iron oxide. Silicon becomes silicon oxide and phosphorus becomes phosphorus oxide. Silicon and phosphorus may react with oxygen gas directly or they may react with iron oxide which was oxidized already and the silicon or phosphorus they react with this iron oxide to form silicon oxide or phosphorus oxide. Silicate and phosphorus, and iron should stay inside slag. However in particular the phosphorus oxide is not stable. They need to stay in slag so it will to catch this phosphorus phosphate anions. We also add lime to catch this phosphate anion then the added lime and already formed silicon oxide iron oxide and little amount of this phosphate they form steelmaking slag. So this is how silicon and phosphate are removed from hot metal. Of course still during the step some amount of carbon is oxidized from hot metal. Once this reaction is finished it the slag contains lime silica and or phosphorus if this slag stays together with liquid steel, then phosphorus is reverse it into liquid iron so this slag are quickly removed. Once slag is removed then next reaction is carried out the next reaction is decarburization. This is main reactions so this slide shows the schematic operation steps of the steelmaking process this steelmaking process is called Basic Oxygen Furnace, BOF. First, the old ferrous scrap is charged into this vessel called converter. Then the hot metal is charged over this scrap, then the oxygen lance is lowered and oxygen is blown to initiate the decarburization reactions. So carbon evolves in forms of carbon monoxide and then leave out of hot metal through this step the hot metal is now converted into liquid steel, so after some time the carbon content in hot metal decreases to become liquid steel. Then carbon concentration and temperature is measured during reactions once these factors approach at the desired value then this reaction is stopped and liquid steel is tapped into ladle and the used slag is also removed from this reaction vessel. This slide shows the process description in terms of reactions. There are several reactions take place. As you see in this slide most of the reaction is oxidation reactions. Iron oxidizes to form iron oxide of course this is in liquid form. Silicon, phosphorus, and even manganese they oxidize to form silicon oxide, phosphorus oxide, and maganese oxide. They may react with oxygen directly which has been delivered by oxygen lance or they may react with the iron oxide which is formed here. So this silicon phosphorus manganese become oxidized and iron is reduced again. In these days due to use of local ore, some other tramp elements like titanium, vanadium, and chromium are also introduced in hot metal. Then during the blowing of oxygen gas these elements are also oxidized and they also enter into slag phase and separately, the carbon in hot metal react with either oxygen gas or the iron oxide and slag and it is oxidized to form carbon monoxide. This view of steelmaking process is commercially successful, there are two reasons. The first reason is this is very energy economic process. The old reactions shown before, they are actually of the oxidation reaction oxidation reactions means they exothermic after reaction heat is produced by themselves. So it means during the steelmaking reactions. There is no need of external heating devices. Heat required for these reactions generated by themselves so temperature of hot metal for when first charge it was about 1350 degrees celsius. But after these steelmaking reactions. They increased quickly to 1650 or even 1700 degrees celsius. Oxidation reaction is quite exothermic for example if you oxidize one mol of silicon, it generates about 900 kJ. In case of carbon, one mol of carbon generates approximately 110 kJ. 110 kJ means, this amount of heat could increase temperature of one kilogram of iron by 400 degrees celsius, this is huge amount of heat. So in hot metal there are silicon, carbon, and some other elements they oxidize they produce it this heat is used to increase temperature of liquid steel. Secondly for the success of this BOF steelmaking process is extremely rapid reaction rate, once oxygen lance provides the oxygen gas it blows very fast over mach 1. Then it forms CO gas which forms the bubbles here and which makes the emergence with liquid slag and also this high-speed oxygen gas make it very intense mixing inside this vessel. Even it forms spitting of liquid small droplets so in this part this is a three phase mixtures, the gas, slag and metal phase they are all mixed and the droplet enters into emulsion it contains carbon is react with iron oxide in the slag phase here. Then it forms CO get so this CO is attached to small liquid droplets, then the upper density of this droplet is very low and they fall down back to this liquid metal, but very slowly it means they stay with these gases and slag it means reaction chance becomes long. All these small droplets increase the surface area per given mass of steel reaction area becomes increased enormously so it also contributes to increase reaction rate. So very rapid reaction, and rapid mass transfer is possible in this view of steelmaking process. Once this is finish it then the hot metal which contained high amount of carbon and some other impurities and at low temperature it is converted into steel with low carbon low impurities and having high temperature. So carbon level is lowered, so we now get liquid steel from iron and temperatures has been increased so this liquid steel has enough here to stay at the liquid phase until it is cast. On the other hand, the oxygen level has been increased this needs to be treated. Subsequently, which will be discussed in next lecture, now we get liquid steel.
