Panorama Editorials

Editorials

Cooling and Transport of Hot HBI

Millenium Steel:

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Frank Reddemann, AUMUND Fördertechnik, Rheinberg / Germany

Direct reduction has evolved most successfully worldwide over the past years. In this arena new, innovative fields of steel production from various direct reduction plants came into being. One area of this is the handling of hot, direct reduced iron in briquette form - HBI for further utilisation in steel production. During utilisation of HBI two aspects can be of crucial importance to users of this technology: the cooling of HBI after briquetting for further transport or for sales of HBI or the most efficient charging of HBI in the downstream steel production process. For the cooling of HBI as well as for direct charging e.g. in electric arc furnaces, there are very innovative solutions.

Since the beginning of the 21st century utilisation of Hot DRI (HDRI) in steel plants has become ever more popular. As a result the proportion rose from 1.83Mt in 2003 to 6.47Mt in 2010 [1]. The proportion of HBI – 7.21Mt in 2010 represents roughly 10.2% of the total amount of produced DRI, 70.37Mt. DRI production mainly took place where high quality gas and/or high quality iron ore deposits existed. The greatest increases are seen in Asia / Oceania and in the Middle East, in contrast to Latin America where volumes produced have somewhat decreased. Also in Russia a slight increase has been recorded over the last two years. Around 2/3 of the processes employed are gas-based, the rest are spread across coal-based processes and special applications.

For the striving markets in Asia, Middle East and Russia/CIS these processes are becoming ever more interesting, although the reasons for it are most varied. For many Asian countries, especially India and China, utilisation of domestic raw materials is at the forefront. Through the refinement of Direct Reduction technology over the past years production of high quality steel has been increasingly achieved employing available qualities of iron ore. In the Middle East energy sources are available in considerable volumes at very favourable rates which is a significant geographical advantage for energy intensive steel production. In Russia and its bordering CIS States modernisation of the huge metallurgical complexes are in full swing and is strongly supported politically. Particularly with regard to existing high quality ores, the Direct Reduction process has decisive advantages sociopolitically compared with conventional production via the blast-furnace route.

In the development of Direct Reduction it can be observed how annual capacities rise and how the process itself is refined further and improved. Apart from the optimum utilisation of the local ores and the available energy sources, the flexibility of the plant plays a decisive role in the decision for one or the other technical detail or plant component. Especially required are innovative solutions which enable the producers to react quickly and efficiently to constantly changing market conditions. A reduction in energy consumption plays a significant role. But also cutting production times can mean a considerable increase in efficiency and productivity. Reducing media consumption such as water is above all necessary where water is only available in limited quantity.

In spite of the opinion of a few conservative academics, the basic principle of Direct Reduction has, alongside the traditional blast-furnace route, established itself as a modern technology because it was possible to prove that the prevailing conditions in the completed plants made high quality steel more efficiently. A large step was the utilisation of DRI and HBI in the electric arc furnace. The EAF has been very successful in Mini Mills over the past decades and is very widespread nowadays. The initial shortcoming of only being able to produce limited steel qualities using cheap, but hard to classify scrap became a very successful advantage since employing DRI and HBI, if not already at an earlier stage. The combination of DR and EAF today produces very competitive steel qualities. Further improvements are expected in the future through further plant optimisation and an enhanced linkage of the various process stages. Among these on the one hand is the direct linkage of the Direct Reduction plant to the EAF through continuous charging with a hot material conveyor, whereby considerable production increases and energy savings can be achieved. This point has been described many times and should not be focused upon here. A further aspect which has arisen is the handling of hot DRI after briquetting in the form of HBI.

The transport and cooling of hot HBI at the interface between iron production and steelworks has become ever more the subject of interest of many producers over the last few years. Unfortunately not always with the necessary attentiveness of the plant designers as the handling of hot HBI no longer quite belongs to iron production but is also not quite assigned to the steelworks. Not until practical experience was gained in the operation of equipment for the transport and cooling of HBI was it determined that there was considerable optimisation potential. On the other hand practical experience was gained which shows that different methods of cooling and transport of HBI have a decisive influence on both quality and costs. (Fig. 1)

For cooling of HBI various methods and structural concepts are currently in use. The quenching of HBI in the water quench immediately after briquetting is frequently encountered although this method has equally frequently shown itself to have significant disadvantages in practice. It was reported that the water volume in the dipping baths vapourises too quickly and then equipment damage occurs during dry operation. In other versions too great a thermal expansion took place at the boundaries of water quench – bath wall – external wall. Often repairs are necessary whereby availability of  the installation drops.

AUMUND has developed a completely new concept over the last few years which to date has been operated successfully in two installations and which has also been patented. Under the heavy conditions in metallurgical plants the use of quench baths only appears to be linked to great effort and expenditure. Through rapid cooling of the briquettes some undesirable side effects occur: cracks develop in the briquettes, sometimes small particles burst which lead to increased formation of fines. At the point of dipping HBI into the quench bath an unavoidable steam cloud is created, sometimes with considerable dust content. As a result, special concepts are necessary for pipework, dedusting and water preparation in order to avoid steam discharge into the environs and to prevent increased wear.

The new AUMUND concept permits a far smoother cooling of the HBI with water vapour instead of liquid water. The HBI are placed on a metal apron conveyor and are evenly disstributed on the conveyor using special technical conveying devices. Cooling takes place via the formation of water vapour which diverts the heat. Through the aligned thermodynamic model the required volume of vapour can be adjusted to the capacity of the installation. By employing this optimised water vapour the required water volume was drastically reduced so that practically no liquid water is present. That is a decisive factor for plants where only limited quantities of water are available. In some cases the water consumption could be reduced to one tenth. The water vapour is sucked off at the prescribed points and can be used in closed-loop if so desired. With the water vapour an inert protective cloud forms de facto around the HBI, which minimises reoxidation and loss of metallisation. When cooling with the calculated volume of water vapour no shock-like cooling arises - which would otherwise form a steam cloud as occurs with quenching – and no increased levels of dust and fine particles were determined in the completed plants’ off-gases. Where downstream filter and fan installations are fitted, wear is reduced owing to the lower dust content in the off-gas. (Fig. 2)

When designing new plants or refurbishing existing systems, AUMUND know-how can make a valuable contribution. With the thermodynamic model especially developed for this application the volume of the required water vapour can be precisely calculated. It was possible to check the correctness of these calculations in the completed installations. (Fig. 3) In conjunction with in-house calculation software the geometry of the cooling conveyor is ascertained according to the given conditions. Apart from length and width, the end temperature can already be calculated in advance in conjunction with the conveying speed.  For further handling in steelworks or for onward transport to storage for HBI sales, a maximum temperature of 100°C is usually necessary. At the end of the cooling zone the HBI are completely dry and can be transported further using standard belt conveyors. The cooling conveyors are fitted with all the necessary components which permit a safe working environment.



[1] Midrex: Continuing Growth in DRI Production