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CLEAN GREEN & SUSTAINABLE TECHNOLOGY IN IRONMAKING: HISMELT TECHNOLOGY AND ITS RELEVANCE TO INDIA
The Global Steel Industry needs an alternative to conventional ironmaking technology that will surmount the challenge of increasing environmental and cost pressures. Commercial HIsmelt plants will provide an alternative ironmaking route that offers competitive solutions to meet the metallics requirement of the iron and steel industry.
The HIsmelt technology has been proven at Demonstration Plant scale (nominally 100,000 metric tpa production capacity) at the HIsmelt facility at Kwinana, Western Australia. This facility utilised a SRV (Smelt Reduction Vessel) with a hearth diameter of 2.7 metres.
The company now plans commercialisation of the technology by scaling up to a vessel with a a 6-metre hearth diameter, with the intention of progressing to a larger, 8-metre vessel. The 6-metre vessel is expected to produce around 0.8Mtpa of high-grade pig iron and the 8-metre vessel would be expected to produce around 1.5Mtpa of similar product, depending on the configuration and feedstocks.
Rio Tinto Iron Ore has announced that Kwinana, Western Australia, has been selected as the preferred location for the next step in the development of the HIsmelt project, should the project be approved by the Rio Tinto Board. The HIsmelt expansion project has been referred to the Western Australian Environmental Authority (EPA), triggering the formal environmental impact assessment process.
If approved, the project will be based on a 6-metre vessel and will produce approximately 800,000 tpa of high-grade pig iron. This will be predominately marketed as a high quality feedstock for the global EAF sector. The pig iron will contain approximately 96% iron and 4% carbon, with minimal levels of phosphorous, sulphur and other residuals.
The world steel industry is currently under considerable pressure. Prices have been at historical lows since the Asian crisis and environmental pressures are steadily increasing. Although it is possible to speculate on how these effects will play themselves out over the next few years, one thing is fairly certain: economic, environmental and cost pressures will only intensify. “Business as usual” is no longer an option and many steel producers will need to restructure and re-engineer their business to survive.
Blast Furnaces have existed in one form or another for more than a century. In that time they have been developed to their current state of optimisation. Further improvements in the operating capabilities of the Blast Furnace may still occur, however the diminishing returns effect is areality.
In recent times, changing market forces in the steel industry have meant a change from the traditional Blast Furnace/BOF route to the EAF based minimills as illustrated in Figure 1 below.
Since 1996 around 10 new, high quality flat product mini-mills, have been commissioned worldwide adding 15M tonne of new capacity. As well as enjoying lower depreciation charges, these mills have been able to take advantage of the slump in scrap prices to cut production costs. Indeed, it is estimated that, during the year 2000, average Hot Rolled production costs at mini-mills were $25/tonne lower than at integrated mills.
THE REASONS FOR NEW TREND ARE:
• Blast Furnaces require specific types of raw material feeds to enable efficient operation.
• The raw materials must be prepared within tight specifications for the process to work efficiently.
• Blast Furnaces are relatively inflexible and any large reduction in output from the blast furnace results in a subsequent reduction in consistency in metal chemistry and temperature.
• Blast Furnace operation continues to be a major source of environmental emissions through their raw material preparation (coke ovens, pellet and sinter plants).
• NOx, SOx, and dioxin emissions are major concerns.
• The high levels of capital expenditure required to construct new or reline Blast Furnaces detracts from this option. Additionally, ageing coke ovens worldwide will require significant further capital expenditure to meet ever more stringent environmental standards.
Greenfield construction of facilities to produce pig iron in an integrated plant is today almost non-existent. The reasons are obvious with many parts of the integrated industry not even capable of covering cash costs at current prices. Normally, integrated plants have to be built at very large scale to even be marginally economic, whilst having the environmental sword of greenhouse gases and pollution as an increasing longer-term threat.
The situation for EAF producers is different, but no less competitive.
• Stable supply of high-quality virgin iron units is a major issue, especially for those with plans to penetrate further into flat products. Some years ago DRI was considered the way of the future, with 20-50% or so in the charge mix considered optimal. Now EAF operators tend to regard DRI in a less optimistic light2 and are becoming interested in 30-50% hot metal as a complementary feed. This gives a significant boost in terms of EAF productivity and operating cost, hence hot metal in the charge mix has a high value-in-use.
In the face of the increasing environmental and cost pressures, the steel industry needs an alternative to conventional ironmaking technology. A recent article by CP Manning and RJ Freuhan3 reviews what has happened in the steel industry in the last few decades, where it is now, and what may be the future.
In summary they characterise the ideal process for iron unit production should include the following attributes:
• Very high efficiency with respect to energy and materials usage.
• Great flexibility in feed materials.
• Reduced capital costs. • Operation flexibility.
HIsmelt has broken through key development barriers and is now in a position to offer competitive solutions. The HIsmelt process has been developed to substantially address the issues indicated above. It produces high quality pig iron from fine iron ores and steel plant wastes utilising non-coking coals. The process is flexible in operation and provides a premium grade product that has a high value in use (VIU) to integrated and electric arc steelmakers.
HISMELT PROCESS DESCRIPTION
Direct smelting of iron ore is an alternative process for pig iron production that is currently coming of age. The HIsmelt Direct Ironmaking Process, after a 20-year development phase, is rapidly emerging as a viable alternative to the traditional blast furnace route. Rio Tinto, together with Nucor Steel, Mitsubishi and Shougang Steel as JV partners, is now in the process of building a 0.8 Mt/a plant .
HIsmelt uses iron ore and coal fines directly by injecting them into a molten bath at high velocity. Smelting gases (mainly CO) are released from the bath and burned in the topspace by hot, oxygen-enriched air. A fountain of metal and slag erupts from the bath and, as droplets and splashes traverse through the topspace, they carry heat back to the bath to sustain the process. This “heat pump” is the heart of the HIsmelt process.
PRINCIPLE OF HISMELT PROCESS
The Hismelt process involves high velocity injection of solids (coal,iron ore and flux) into a molten iron bath at around 14500c. Injection is arranged such that significant penetration of solids into the iron bath is achieved leading to the dissolution of carbon into the metal and reduction of iron ore via the over all reaction:
3Ciron + Fe2O3 = 2Feiron + 3 CO (1)
This reaction is highly endothermic and, if the process is to be sustained ,an external supply of heat is needed. Carbon monoxide ( plus hydrogen) released from the bath provides the fuel for generating heat. Hot blast (12000C, oxygen enriched air) is injected into the top space via a central swirl lance and combustion takes place to burn the bath gases to carbon monoxide and water.
2CO + O2 = 2CO2 (2)
2H2 + O2 = 2H2O
Theoretically one would like to achieve total combustion of this bath gas but , in practice, post-combustion of around 50-60% is typically achieved. Post-comustion is defined as the ratio of the volumetric concentration of combusting species , viz:
PC(%) =100(CO2 + H2O) / (CO + CO2 + H2 +H2O) (3)
Smelting occur in the melt where the oxygen potential is low, where as heat generation occure in the top space where oxygen potential is relatively high. The key to the process is moving heat from the combustion region down to the smelting region without compromising the oxygen potential in either zone.
When CO and H2 are released from the smelting in the bath, the rate of release is such that a violent eruption of liquid is produced. Metal and slag are thrown upward forming a gas-permeable fountain with high surface area for heat transfer. Hot combustion gases pass through this fountaion and, in doing so, transfer heat to the droplets of slag and metal which in turn deliver this heat to the bath.
Metal leaves the vessel continuously via an overflow forehearth( which is effectively a liquid metal manometer seal), whereas slag is tapped periodically through the sidewall of the vessel via a water-cooled slag notch.