Reagents for Desulphurization of Hot Metal
Reagents for Desulphurization of Hot Metal
Sulphur (S), present in solid steel as iron sulphide (FeS) inclusions, has several detrimental effects on steel processing and on steel’s physical properties. During deformation, the iron sulphide inclusions act as crack initiation sites and zones of weakness. Such inclusions from sulphur adversely affect steel’s toughness, ductility, formability, weldability, and corrosion resistance. An increase in manganese (Mn) however, helps prevent formation of iron sulphide, which is highly detrimental to steel’s hot workability and also leads to severe cracking. Sulphur is such an undesirable element in steel that its removal is desired.
The ever increasing requirements to steel properties and the growing demand for steel qualities and quantities with lowest sulphur contents of down to 0.001 %, has made it necessary for the steel makers to carry out the desulphurization of hot metal. Presently hot metal is regularly being desulphurized to below 100 ppm, and in some steel plants, to 10 ppm. Besides the increased requirements of steel quality, other reasons which necessitate desulphurization of hot metal are reduced scrap quality and increasing cost of high quality iron ores.
In the desulphurization process, powdered desulphurization reagents are injected into the hot metal through an immersed lance using an inert carrier gas such as argon or nitrogen, as shown in Fig 1. Since desulphurization is a diffusion-controlled reaction, and related to the reactive surface area available for reaction, the desulphurization reagents are to be as fine grained as possible. However, flowability is reduced with very fine grains and hence it is necessary to find an optimum between efficiency and conveying ability. In order to obtain good flow characteristics, normally a fluxing agent is added during the grinding operation, so that pneumatic transport during injection does not pose any problems.
Fig 1 Injection of desulphurization reagents in hot metal ladle
During the process of desulphurization, powdered reagent, in dense phase, is pneumatically transported and injected into the hot metal through a submerged lance. A jet is created at the outlet of the lance which penetrates into the hot metal (liquid iron) until its momentum is dissipated. In this process, gas bubbles rising through the liquid enhance mixing, promote chemical reactions, and minimize temperature and chemical inhomogeneities in the liquid metal. Also, the stirring caused in the injection process improves the top-slag desulphurization. The bubbles forming in the liquid rise upward due to buoyancy, and the kinetic energy at the nozzle exits. A number of complex phenomena take place during the injection process.
The reagents react chemically with the dissolved sulphur in the hot metal and form a sulphide which rises and is captured in the slag layer which is covering the hot metal. Sodium carbonate (Na2CO3), lime (CaO), calcium carbide (CaC2), and magnesium (Mg) are the powdered reagents normally used for the desulphurization of the hot metal.
Over time, several desulphurization reagents have been developed for efficiently carrying out the desulphurization of hot metal in a cost effective manner. The major types of reagents used for the desulphurization of hot metal are described below.
Sodium carbonate, also known as soda was a popular agent for the desulphurization of hot metal during the 1960s. Sodium carbonate was popular because of its easy application method. The sodium carbonate is delivered in paper bags and, according to the requirements; number of bags are placed in the empty ladle bottom before pouring the hot metal. This method of desulphurization is easy to use in every plant.
However, there were considerable problems associated with the use of the sodium carbonate. Sodium carbonate has a melting temperature of around 851 deg C. It is stable until 1200 deg C, which is lower than the temperature of the hot metal. So once sodium carbonate is in contact with the hot metal, it decomposes to Na2O and CO2 as per the equation Na2CO3 = Na2O + CO2. Na2O can be reduced by dissolved carbon into sodium (Na) gas. Both Na2O and sodium gas can react with sulphur intensively according to the following reactions.
Na2O (l) + S ? Na2S (l) + O
Na (g) + S ? Na2S (l)
Sodium carbonate can promote separation of hot metal from slag and iron losses in slag are lower. However, presently sodium carbonate alone is no longer being used as desulphurization reagent. This is because it gives a violent gas evolution. The decomposition of the carbonate in contact with the hot metal produces pungent sodium oxide fumes which are harmful to the human health. Further, sodium carbonate slag is soluble in water. This results into leaching and contamination of land.
These days, sodium carbonate is included as a part of many desulphurization mixtures. It is also added into the hot metal at the end of the desulphurization process for mono-injection with other desulphurization agents. It is still also been injected into the hot metal as a third component at the end of a desulphurization treatment.
Lime is the most important calcium source which is used in hot metal desulphurization. Lime for desulphurization is made by the calcination of limestone in shaft or rotary kilns, then ground to the required size. Very fine grains are unsuitable for injection. Typical composition of lime is CaO – 92 % to 98 %, MgO – 0.5 % to 2.0 %, SiO2 – 0.1 % to 1.0 %, S – 0.1 % maximum, CO2 – 2.0 % maximum, chemically combined moisture – 1.5 %, and ignition loss – 2.5 %.
Typical grain size distribution of lime suitable for hot metal desulphurization is at Fig 2. Lime has problematic conveying characteristics in the granulation size necessary for hot metal desulphurization, and is also adversely affected by moisture, leading to nozzle blockages and/or irregular flow characteristics. To counteract these tendencies the lime grains are generally are surface treated with special oils.
Fig 2 Typical grain size distribution of lime
Lime is the cheapest hot metal desulphurization reagent which is being used. However, it has a lower effectiveness. It also produces large quantity of slag and, since iron losses are directly proportional to the generation of slag, this adversely affects the cost. These slags have a lower viscosity than those generated with calcium carbide injection and they decompose more readily, thus they can be more easily treated without having to add fluxing agents. A further advantage is the fact that lime is not subject to dangerous goods regulations and there are no transport and handling problems. Presently lime is primarily used in combination with other reagents, principally magnesium. The mechanism of desulphurization with lime is described below.
During desulphurization, the sulphur in the hot metal is reduced into the slag phase accompanied with an oxidation of the metal bath. A general reaction can be written as S+ MeO ? MeS + O where S and O is dissolved sulphur and oxygen in the hot metal respectively and MeO and MeS is sulphide and oxide in the slag respectively.
Usually calcium is added to react with sulphur, either directly injected into the melt or as an important component in the slag. The most common and cheap reagent for the removal of sulphur is lime. The reaction for sulphur removal with lime can be written as CaO (s) + S ? CaS (s) + O. The Gibbs free energy for this reaction is 110 000 – 31,1T (J/mol) where T is the temperature. The equilibrium constant K for the reaction is Ao*Acas/As*Acao which is equal to Ao*Acas/Fs*(%S)*Acao. Here Ao and Acas are the activities of dissolved oxygen and sulphur in the hot metal, Acas and Acao are the activities of CaS and CaO in the slag, Fs is the activity coefficient and % S is the dissolved sulphur. Hence, the content of sulphur can be calculated as %S = Ao*Acas/Fs.K*Acao.
The activity of CaO can be said to be Acao=1 if the slag is saturated with CaO. The activity coefficient of sulphur (Fs) can be calculated by the Wagner expression. The oxygen activity can be measured while the activity of CaS is more difficult to get. The equilibrium content of sulphur is based on reaction CaO (s) + S ? CaS (s) + O.
Understanding the desulphurization mechanism of lime enables more efficient usage and higher yield. Oeters et al 1973 made a fundamental study on the mechanism of lime for desulphurization of hot metal. They reported that an outer layer of CaS was formed together with 2CaO.SiO2 on the surface of CaO according to reactions CaO + S = CaS + O and 2CaO + Si + 2O = 2CaO*SiO2. The first reaction proceeds by diffusion of the reactants (S and O) through the CaS layer. On the other hand, diffusion of reactants through the di-calcium silicate layer is slow and therefore limits the desulphurization. Not only di-calcium silicate has been found around lime particles used for desulphurization, but also tri-calcium silicate.
Takahashi et al. dipped lime rods in hot metal and found that CaS was only formed in the presence of slag. They reasoned that the formation of di-calcium silicate stopped the CaS formation and that solid CaO has to go into the slag in order to significantly contribute to desulphurization.
In the 1970s, calcium carbide was introduced as a reagent for hot metal desulphurization. Calcium carbide is made from coke and lime in an electric arc furnace. The coarse fraction is used as base material for acetylene gas generation while the finer fraction is used for preparation of desulphurization reagent. Initially carbide mixture having around 50 % calcium carbide with 10 % calcium carbonate as a gas separator (the balance was mainly lime), was used for desulphurization of hot metal in the torpedo ladle. With the introduction of transfer ladle treatment, carbide mixture having around 70 % CaC2 with 5 % gas separators is being used since higher gas flow rates can be obtained. This has resulted into greater efficiency of calcium carbide.
Further, since the use of calcium carbonate causes an increase in oxygen activity, high volatile coals are replacing limestone as the gas separator material, thus increasing desulphurization efficiency. Also during the injection into the hot metal, carbon comes out from the calcium carbonate and become bound into the slag. This when tilting the slag onto the dump, the finely distributed graphite combusts resulting into excessive smoke.
Compared to the feathery structure of lime, calcium carbide has a tetragonal crystal lattice and crushes in a sharp-edged material and hence, conditioning of calcium carbide with auxiliary flow materials is usually not necessary. Calcium carbide is rarely used in its pure condition. It is used mostly as mixture of several components. Since all components have a similar density and are ground together with the calcium carbide, segregation can largely be excluded so that the desulphurization effect is effectively constant during injection.
Calcium carbide is a hazardous material and reacts with water to form acetylene, a combustible gas. By adding coals with a high proportion of volatile components, carbide-based desulphurization agents are possibly pyrophoric. Hence, calcium carbide based reagents need to be stored in a dry place and under nitrogen. At temperatures above 700 deg C and in the presence of nitrogen, calcium cyanamide is formed. This reaction (known as azotation reaction) is exothermic and localized heating can be enough to start the process. To avoid this, hot metal desulphurization plants are usually equipped with argon purging facilities.
During injection calcium carbide splits into calcium and carbon. The carbon separates as kish graphite into the slag and increases its viscosity. Hence, for minimizing the iron losses, fluxing agents can be added to the calcium carbide. The fluxing agents can be sodium carbonate, fluorspar, or any alkali fluorides. By the addition of potassium cryolite, good results have been achieved with regards to reduction in the iron losses.
The use of fluxing agents also has some disadvantages. These fluoride based agents decompose during contact with hot metal, and with silicon they can form silicon tetra fluoride (SiF4) which is a gaseous, corrosive substance leading to corrosion in the fume exhaust system. The use of alkalis as fluxing agents also means that contact with water in the slag dump results in the formation of heavy metal-rich contaminated water. This causes considerable environmental issues.
In comparison with lime, calcium carbide mixture is hazardous substance and is covered under dangerous goods regulations. These results into the necessity of several safety devices such as measuring facilities for determining the acetylene, oxygen and moisture content in the silo system and facilities for argon purging.
The mechanism of desulphurization with calcium carbide was first studied by Talballa et al, According to their results, calcium carbide, which is solid at the temperature of liquid iron (1350 deg C), partially dissociates into calcium vapour and a layer of graphite. This calcium vapour then reacts with sulphur dissolved in iron to form a layer of calcium sulphide along with the graphite layer. The layers of graphite and calcium sulphide progressively thicken to form a barrier to calcium diffusion vapour which slows the rate. The reaction between calcium carbide and the sulphur dissolved in the hot metal is given by the equation CaC2 + S = CaS + 2C.
Chiang and Irons showed that the reaction was controlled by the diffusion of sulphur through the particle boundary layer, rather than the diffusion of calcium vapour in the product layer. Their experimental results confirmed that the reaction can be described as a first-order, diffusion controlled reaction. In a further work, Irons examined mixing as a possible controlling step in the desulphurization process. He concluded that mixing was always fast enough not to be rate controlling. Therefore, the reaction is controlled by diffusion through the boundary layers around the particles.
Magnesium reagent is an important material used to desulphurize hot metal. It is the material of choice for the desulfurization of hot metal during the steelmaking process The practice of desulphurization of hot metal with magnesium has increased significantly in recent years. Some of the early uses of magnesium for desulphurization included magnesium impregnated coke and magnesium lime briquettes but these have been now replaced by the injection of magnesium which is more controllable and efficient.
For satisfying low sulphur requirements for hot metal to be processed into steel, magnesium reagents in powder or granular form are used to remove sulphur via slag-liquid metal reactions done under reducing conditions, where sulphur is transferred out of the hot metal through the slag-metal interface, and into the slag. Slag is an ionic solution made up of oxides and fluxes.
Magnesium appears to be an expensive desulphurization reagent, but if its effectiveness is taken into consideration, which is around seven times higher than that of calcium carbide then the specific desulphurization cost is undoubtedly lower than the costs incurred with the use of other desulphurization reagents. Further, magnesium has a higher effectiveness at low temperatures. Hence, it is more suitable for carrying out desulphurization in the transfer ladles where the temperatures are usually lower.
The desulphurization of hot metal with magnesium is normally carried out by co-injecting it with lime or calcium carbide. This is because, in contrast to the other typical hot metal desulphurization reagents, magnesium is normally associated with high vapour pressure. The fast addition of magnesium is absolutely necessary, since magnesium melts at 650 deg C and vapourizes at 1090 deg C. These temperatures are much lower than the temperature of hot metal pretreatment. When magnesium is injected into the hot metal, violent vapourization of magnesium is liable to result in a decrease in the desulphurization efficiency and lack of safety in operation. Hence, magnesium is normally used for desulphurization of hot metal along with calcium carbide and high grade injectable lime. Of these three, magnesium gives the best low sulphur level results. Industrial studies have shown different degrees of effectiveness of magnesium addition. Magnesium is reported to be able to take down the sulphur level from the initial sulphur level typically 300 ppm – 600 ppm to 10 ppm – 200 ppm.
The different investigations of hot metal desulphurization with magnesium have shown that significant reduction of sulphur concentrations are achieved with magnesium. Nakanishi has argued that the mass transfer of sulphur in the metal bath was the rate controlling step while Irons and Guthrie believed that the dissolution of magnesium bubble into the hot metal determines the rate of desulphurization. Irons and Guthrie has also developed a model to describe the desulphurization process. They suggested that magnesium is first dissolved in liquid metal and then meet sulphur forming magnesium sulphide (MgS) through heterogeneous nucleation on inclusions already present in the liquid metal according to the reaction Mg + S = MgS. Whether magnesium functions in the form of gas bubbles taking down the sulphur content directly or by increasing the sulphide capacity of the slag is always a question. In some of the studies, the addition of magnesium is accompanied or followed by lime or calcium carbide additions. How the different reagents help each other in desulphurization is still uncertain.
In order to satisfy the requirements of low sulphur in a better way, the present practice is to inject magnesium reagent in powder or granular form into the ladle for the removal of sulphur, which is converted into the ladle slag. In some practices, granules of magnesium alloys are preferred to granules of primary magnesium. The main alloying elements are aluminum, silicon, manganese and zinc. The vapourization point of magnesium alloys is higher than primary magnesium, thus the magnesium component remains liquid and hence chemically active for a longer time
Due to the equilibrium of dissolved sulphur in the hot metal and sulphur in the slag, a calcium based reagent (like lime) is essential when stable low sulphur concentrations are to be reached. This is a major drawback for mono-injection of magnesium which needs to either add a calcium containing material to the slag or ‘undershoot’ to compensate for the high sulphur content later during the steel production.
Magnesium is a much faster desulphurizing agent than lime, about twenty times faster. This means that for fast desulphurization of hot metal, magnesium is required or large quantities of lime. Therefore co-injection has the advantage that its speed can be altered by changing the injection ratio between magnesium and lime.