Hot metal pretreatment is a prerequisite for converter steel makers in order
to attain the twin objectives of ensuring cost efficiency and
optimising productive capacity
Hot metal pretreatment is a process which is performed after the tapping of a blast furnace (BF) and before decarburisation in a basic oxygen furnace (BOF). In most cases, this process lowers the impurity content of hot metal. Hot metal desulphurisation, de-siliconisation, and de-phosphorisation are processes that are included in this category. Among the various reactions that take place, de-sulfurisation plays the most important role in hot metal pretreatment. This process was first used in the 1960s, and it
still is the predominant hot metal pretreatment method used in most steel-making shops. Hot metal de-phosphorisation began in the 1980s in Japan. The principal advantage of this process was cost reduction. Recently, when combined with the recycling of decarburised slag, it has also made reduced slag generation possible. Hot metal de-siliconisation was developed to adjust the slag volume in order to minimise the cost of refining.
Pretreatment & Cost-efficiency The Nellikuru Process – pioneered by Nellikuru Innovations – is an innovative way of hot metal pretreatment in steel-making, promising positive EBITDA to manufacturers. Chemical reagents are added to the hot metal at any convenient location in the existing layout before the hot metal reaches the steel plant. The aim is to reduce the impurity levels in hot metal as per the requirements of the steel melting shop and thereby achieve a significant reduction in the cost of production either in the BF or the Corex route.
The main advantages of the Nellikuru process are:
a) No capital expenditure (CAPEX),
b) Better flexibility as the re-agents can be added as per the hot metal chemistry either in the transfer ladle at the steel melting shop or in the hot metal ladle at blast furnaces itself,
c) BF operators can control hot metal chemistry at the shortest possible time, and
d) Manufacturers can claim reduction in CO2 emissions as a result of less oxygen blowing in steel making
EBITDA Gains For Steel Makers So, with the ever-increasing demand for higher grade steels, how important is hot metal pretreatment in converter steel making? Sabukutty Dominic, CEO, Nellikuru Innovations, told Steel 360: “Hot metal pre-treatment has to be viewed in an altogether different perspective. We produce high quality steels even today with the same available raw materials and techniques. So, hot metal pretreatment mainly plays an important role in overall cost reduction and productivity improvement.”
Explaining how hot metal pretreatment leads to significant EBITDA gains for steel makers, Dominic said, “The major aim of hot metal pretreatment is to improve the EBITDA. Over the years, the quality of hot metal has been improved with the introduction of bigger blast furnaces and better process control. However, Indian steel makers have not optimised to the level that we should have, which can only be achieved through better pre-treatment techniques.” “We are the first start-up company in India under the government’s Make-in-India programme in the realm of research in the iron and steel industry,” said Dominic. “We have successfully demonstrated the benefits of hot metal pretreatment to three of our customers. Although we cannot disclose the names as per the standard ‘non disclosure agreement’, Nellikuru is optimising the productive systems in all the three steel plants,” he added.
Purging Hot Metal Of Impurities
Explaining the significance of desiliconisation and de-phosphorisation of hot metal in achieving the desired quality, he said, “For primary steel makers, hot metal de-siliconisation is an effective way of increasing EBITDA. Indian primary steel makers should try to achieve an average hot metal silicon content of 0.3- 0.4% before charging into their steel converters to maximise the overall steelmaking efficiency such as lesser slag volume, lesser blowing and thereby maximum production. If we want to derive the real benefits, we should maintain consistent hot metal quality.” He added: “These days, the annual hot metal silicon variation in the best operating blast furnaces in India is in the range of around 0.2%. Since the variation is low compared to that in the past, special efforts are lacking in most steel plants to maintain a constant silicon level in the hot metal feed to the converters. Since it is not possible to control this fluctuation in a BF, hot metal desiliconisation through innovative methods is the only solution.
Optimisation of converter efficiency mainly depends on the silicon level in hot metal and there is enough scope of improvement in this field, said Dominic, adding, “We have developed a unique technique to reduce hot metal silicon in ladles or in the blast furnace main runner. For economical de-phosphorisation of hot metal, we have to achieve a hot metal silicon content which is as low as possible, preferably less than 0.1%. De-siliconisation is very important in this respect too.”
Ensuring Effective De-siliconisation Dwelling on the advantages of the dip lance process and pneumatic injection of re-agents in hot metal pretreatment, Dominic averred, “For de-sulphrisation of hot metal, the dip lance process and pneumatic injection of re-agents are very effective as the quantity of re-agents is very less and it facilitates better contact of the impure elements with the chemical re-agents.” However, for de-siliconisation of hot metal this technique is not suitable as the quantity of re-agents and the slag formed as a result of chemical reaction are considerably more. Moreover, the process of oxygen blowing to reduce the silicon is as good as a steel-making process itself – it is neither economical nor user-friendly. “We have developed a unique technique to reduce the silicon in hot metal without oxygen blowing and the dip lance process, which is economical and user-friendly,” he added. Asked how important is slag collection and treatment after de-sulphurisation of hot metal and whether Nellikuru Innovations provides the requisite expertise, Dominic replied: “Deslagging after de-sulphrisation is very important to reduce the sulphur reversal. Hence, it should be removed at the earliest, after de-sulphrisation. However, we are currently not providing any expertise in this regard as there already exists established techniques in the market.”
Improving Metal Yield
Highlighting the future projects of Nellikuru Innovations, the veteran engineer and expert said, “We have developed another product for hot metal insulation in ladles, which can totally eliminate the age-old practice of covering the hot metal with rice husk, fly ash and similar materials which are added after filling the ladles or after closing the cast.” “Our product is added at the bottom of the ladle just before taking the hot metal in it,” he continued, adding: “The moment our re-agent comes into contact with the hot metal, a thin insulation layer is formed over the metal at the bottom of the ladle itself. As the metal rises, the insulation layer also floats over it and remains intact.” According to Dominic, the salient features of the process are:
- No ladle jamming at the bottom or metal ring jam.
- Better refractory life, as it preheats and gives an insulation coating before the refractory comes into contact with the liquid metal. Hence, the refractory joints are well protected.
- Lesser temperature drop as the liquid metal is completely insulated from the ladle bottom itself. The maximum temperature drop happens while filling the ladle as a result of turbulence at the metal surface.
- Negligible fugitive emissions from ladle during filling. Even in torpedo ladles, Nellikuru Innovations switches off the de-dusting fans.
- The process ensures better yield improvement. With the use of conventional ladle covering materials, secondary nucleation of fine iron particles happens with the foreign particles such as fly ash, rice husk, the trapped graphite particles released from the metal at the metal surface and sand particles from the runners, which is generally termed as ‘kish’. With the material used by Nellikuru, the metal is completely covered and external contamination and oxidations are almost eliminated. Hence, with respect to the final product, the improvement in metal yield is more than 1% for open ladles.