Desulphurization of Hot Metal
Desulphurization of Hot Metal
Removal of sulphur from hot metal is called desulphurization of hot metal. Sulphur is a desirable element in steel when good machinability is required from the steel product. However it is an unwanted element in most of the applications of steel due to the following reasons.
- Sulphur affects both internal and surface quality of steel
- Sulphur contributes to the steel brittleness and when it exists in sulphide phase it acts as a stress raiser in steel products.
- It forms undesirable sulphides which promotes granular weakness and cracks in steel during solidification.
- It has adverse effect on the mechanical properties.
- It lowers the melting point and intergranular strength and cohesion of steel.
Unlike other impurities which are removed from the hot metal by oxidation in the oxygen converter, the most economic method of removing sulphur from the hot metal is by reduction either in the transfer ladle or in the charging ladle, before it is charged in the converter. A number of technologies have been developed for the external desulphurization of hot metal but all of them have the basic requirement of a reagent and a method of mixing. The difference between the technologies used is the properties of the reagents, the effectiveness of the reagent to remove sulphur and the effectiveness of the mixing method to get the reagent into solution. . Also the effectiveness of hot metal desulphurization is inversely proportional to the desulphurization reagent injection rate. The most popular desulphurizing process today is deep injection of desulphurizing agent in the hot metal.
Dip lance process is the most economical, effective and reliable method of desulphurization hot metal. It consists of pneumatic injection of fine grained desulphurization reagent into the hot metal with high dosing precision via a dispensing vessel and a refractory lined lance. For each reagent, one separate dispensing vessel is used. All the vessels are identical. Nitrogen gas is normally used as a carrier gas for the desulphurization reagent. The reagent transfer in the injection line is under dense flow conditions. The dense flow conditions maximize reagent delivery as well as reduce abrasion wear of injection lines. The injection of desulphurization reagents through deeply submerged lance causes an intimate mixing of the desulphurization reagent with the hot metal. The process allows the use of several desulphurization reagents, such as lime, calcium carbide and magnesium, which remove the sulphur in the hot metal by chemical reaction and convert it to the slag. Sulphur rich slag generated during the process is removed immediately after completion of the reagent reaction. The most common method is to tilt the ladle and rake the slag off with the help of a slag raking machine.
Hot metal sulphur content is reduced in charging ladle or transfer ladle worldwide by this method. For controlling the operating cost, a combination of dip lance method with mathematical process control and flexible control of the desulphurization plant is adopted. This combination provides a range of possible process technological variations. One of these possibilities is to vary the injection rate (kg/min.) to suit the production requirements. Another possibility is to inject different desulphurization reagents during the process of desulphurization. The desulphurization reagents can be injected singly, simultaneously or with a time lag. Accordingly the process variations are known as mono injection, co injection or multi injection. Dip lance method can reliably reduce the sulphur content of hot metal to figures as low as 0.001 %. A typical flow sheet of a desulphurization plant with two dispensing vessels is shown at Fig 1.
Fig 1 Typical flow sheet of desulphurization plant with two dispensing vessels
The most commonly used desulphurizing reagents are lime (CaO), calcium carbide (CaC2) or magnesium (Mg).
- Lime – Its low cost and easy availability make it an attractive reagent. But it has got some critical disadvantages. During the process of desulphurization, lime particles are continuously being covered by two precipitates namely calcium sulphide (CaS) and calcium silicate (CaSiO4). These compounds impede the desulphurizing reaction by surrounding the lime and forming thick barriers at the lime – hot metal interface. In order to reduce this growth, the grain size of the lime is to be restricted to 45 micrometer maximum. The desulphurizing reaction with lime takes place as per equation: 2CaO + 2S =2 CaS + O2.
- Calcium carbide – Calcium carbide was once most used desulphurizing reagent but now it is less prevalent. Complicated material handling procedures as well as stringent environment requirements associated with the disposal of slag have negatively influenced its use. Calcium carbide is also subject to layer formation similar to lime, which impedes the desulphurizing reaction. The desulphurizing reaction with calcium carbide takes place as per equation: CaC2 + S = CaS + 2C.
- Magnesium – Magnesium has a high affinity for both oxygen and sulphur. Unlike lime, magnesium is not accompanied by oxygen when it is injected into the hot metal, therefore it can rapidly react with sulphur to form magnesium sulphide. Magnesium in solution that does not react with any oxygen in the hot metal, thus removing excess oxygen. Mono injection process with magnesium reagent is less common because of the violent nature of the reaction and the relatively complicated equipment requirement. Magnesium is the only one of the three common desulphurization reagents that is soluble in hot metal and reacts with sulphur in solution. The desulphurizing reaction with magnesium takes place as per equation: Mg + S = MgS. Due to low boiling point (1090 deg C), magnesium vapourizes as it enters the hot metal. This vapour is under high pressure which is directly related to solubility. Once in ladle, the magnesium vapour forms bubbles which rise through the hot metal, dissolve and react with sulphur in solution, forming magnesium sulphide (MgS). This magnesium sulphide then floats on the top of the ladle and settles in the slag layer, which is skimmed off. The lime that is injected with the magnesium assists in dissolution by reducing the diameter of the bubbles as well as providing precipitation sites for the MgS.
There are some important issues with respect to the desulphurizing reagents. All desulphurizing reagents are not equal in their ability to remove sulphur. Magnesium although more expensive, has approximately 20 times the capacity of removing sulphur as lime. Calcium carbide has eight times more potential to remove sulphur than lime. However, if injected into hot metal on its own, it must be blended with volatiles in order to increase the agitation of the bath. Pre blending of different desulphurizing reagents such as magnesium-lime or magnesium- calcium carbide is not useful since blended reagents are prone to segregation during transport and storage besides individual injection rates of desulphurizing reagents gets sacrificed.
Important issues in desulphurization of hot metal
- During the desulphurizing process, the generation of slag is proportional to the amount of reagent added to the hot metal. Also during the process, some hot metal gets trapped in the slag and gets pulled out of transfer ladle during the slag rimming. This amount is around 1 % for the co-injection process. Desulphurization slag contains about 50 % iron.
- The loss of heat during the desulphurizing process is an important factor since it reduces the sensible heat of the hot metal sent to the converters. The three primary sources of heat loss are radiation from the surface of the hot metal, addition of cold reagents and introduction of cold injection lances into the hot metal. The largest temperature loss occurs during injection rather than skimming. A temperature loss of 30 deg C is expected during the desulphurization process.
- Desulphurizing process donot have any major effect on the refractory lining life of the hot metal ladle since the treatment time is small.
- Both reagent injection and slag skimming operation generate fumes which are to be collected and dedusted prior to their release to the environment. The captured fumes are typically cleaned in a pulse jet type bag house designed for metallurgical operations.