HIsmelt Process of Ironmaking
HIsmelt Process of Ironmaking
HIsmelt process is an air based direct smelting technology which is simple yet innovative. The process is for the production of liquid iron (hot metal) using iron ore fines or any other appropriate ferrous feed material. The smelting is carried out in a molten iron bath using coal as the reductant and energy source material. The principal raw materials required for the process are iron ore fines, coal and fluxes (limestone and dolomite).
HIsmelt is short for ‘high intensity smelting’. It is a direct smelting process for making liquid iron straight from the iron ore. The process has been developed to treat iron ore fines with minimum of pre-treatment, making the process more flexible in terms of the quality of iron ore it can treat. The process allows the use of non coking coal and iron ore fines with significant impurities. The main product of the process is liquid iron or hot metal which can be used in steel melting shop or can be cast in pig casting machine to produce pig iron. The by-product of the process is slag and the off gas.
The driving force for this alternative ironmaking is (i) the ability to utilize cheaper and more abundant raw materials such as non-coking coals and non-agglomerated ores, (ii) smaller economic plant sizes, (iii) competitive capital and operating costs, (iv) reduced environmental problems through the elimination of coke ovens and sinter / pellet plants, and (v) flexibility of operation. The HIsmelt process is very flexible since it can use a wide range of ferrous feeds, including steel plant wastes and high phosphorus ore. The process can use ores which are minus 6 mm in size, which is the normal sinter plant feed, and can also process ores which are typical pellet feed, that is 80 % finer than 40 micrometers without any change in the iron yield in the process. Coals ranging from coke breeze to a 38 % volatile non-coking coal can be used.
Main features of the process
The HIsmelt process is simple and has demonstrated a high level of reliability. The basic mechanism of the HIsmelt process is the reduction and smelting of iron-bearing ores with dissolved carbon in the metal bath. This is achieved by the injection and partial combustion of coal directly into the bath and by transferring the heat generated by post combustion of the evolved gases from the bath with an oxygen enriched hot air blast back to the bath. Oxygen enrichment of the hot air blast (upto 30 % oxygen content) has been successfully utilized to increase the operating intensity of the vessel, resulting in the expected increase in productivity. The overall reactions and heat transfer mechanism provide sufficient energy to maintain the reduction reactions and the heat for smelting of the iron and slag.
The process occurs within a vertical Smelt Reduction Vessel (SRV) under pressure which is the core of the HIsmelt process (Fig 1). The SRV has a refractory lined hearth and water cooled upper shell. The process is carried out in this vessel. The refractory hearth contains the molten iron bath and liquid slag. A thick slag layer is situated above the metal bath. Iron ore fines, coal and fluxes are injected directly into the melt in the SRV. Upon contact with the iron bath, dissolution of the carbon in the coal occurs, which reacts with the oxides in the iron bearing feeds, forming carbon monoxide (CO). Rapid heating of the coal also results in cracking of the coal volatiles releasing hydrogen.
Fig 1 Smelt reduction vessel of HIsmelt
A fountain of molten material, consisting largely of slag, erupts into the top space by the rapid expulsion of the CO, hydrogen and nitrogen carrier gas from the molten bath. Hot air at 1,200 deg C is blown into the top space through a water-cooled lance. The CO and hydrogen is post combusted with oxygen of the hot air blast. The heated metal and slag fall back into the bath providing the energy for direct smelting of the iron ore. Ejected slag coats the water-cooled panels, which reduces the energy loss.
The off gas from the process is partially cooled in a membrane tubed hood. The sensible and chemical energy in the off gas can then be used to effect some preheating, pre-reduction and / or calcination of the metallic feed and fluxes. The off gas is then cleaned in a scrubber and used as fuel for the hot blast stoves or in a co-generation plant.
The vessel is equipped with a forehearth for continuous tapping of hot metal. This maintains an optimum bath level within the SRV and provides a clean product stream without the need for external slag and metal separation. Slag is tapped periodically through a water cooled notch.
HIsmelt process has a number of unique features which sets it apart from the other direct smelting processes. These features are given below.
- The process uses the metal bath as the primary reaction medium. Other direct smelting processes normally top-feed the ore and coal, with smelting through char (plus a small amount of metal) in the slag layer. Dissolved carbon in metal is a more readily available reductant than char in slag, since the latter requires an intermediate gas-phase (CO). In other words, HIsmelt process achieves significantly faster smelting rates by using carbon in a more active (i.e. dissolved) form.
- Another differentiating factor is the degree of mixing in the melt. Injecting feed materials directly into metal generates large volume of ‘deep’ gas. This creates a strong buoyancy-driven upward plume which in turn causes rapid turnover of liquid. It has been calculated that this turnover is of the order of tons per second. Under these conditions, there is very little potential for establishing significant temperature gradients (greater than 20 deg C to 30 deg C) in the liquid phase and the system operates with an (essentially) isothermal melt. The rapid mass turnover promotes good heat transfer from the top space to the bath without significant over-heating of individual liquid droplets. Implications are significant for hearth refractories in the slag-line region, since good mixing leads to the bricks being exposed to low FeO and uniform (low) temperature.
- The method of solid injections using high speed lances ensure that the capture efficiency in the melt is high and even ultra fines can be used directly.
- The ‘natural’ 5 % to 6 % FeO level in the slag in conjunction with the metal carbon at 4 % creates conditions for strong partition of phosphorus from metal to slag. Typically around 80 % to 90 % of phosphorus goes to slag (Fig 2).
- Coal performance has virtually no dependence on particle morphology, since the coal is ground fine for injection.
Fig 2 Phosphorus removal conditions in SRV and SRV of HRDF
Historical process development
The origin of the HIsmelt process is traced back to the bottom blown oxygen converter process (OBM) and the evolution of the combined blowing steel making process developed by Klöckner Werke at their Maxhütte steel works. CRA (now Rio Tinto) formed a joint venture in 1981 with Klöckner Werke to pursue the steelmaking and smelting reduction technologies. Trials were conducted in a 60 tons OBM converter to demonstrate the fundamentals of the smelt reduction process. The successful testing of the smelt reduction concept led to a small scale pilot plant (SSPP) of capacity around 12,000 tons per annum located at the Maxhütte steel works. The design of the SSPP was based on a horizontal rotating SRV which used bottom tuyeres for injection of coal, fluxes and iron ore. The SSPP operated from 1984 to 1990 and proved the viability of the technology.
Next stage of the process development was the HIsmelt Research and Development Facility (HRDF) constructed at Kwinana, Western Australia. Construction of the HRDF commenced in 1991. HRDF was having a design capacity of 100,000 tons per annum. The main objective of the HRDF was to demonstrate the process and engineering scale up of the core plant and to provide operating data for commercial evaluation. The original SRV configuration for Kwinana was a direct scale up of the SRV of SSPP and was based on a horizontally shaped vessel capable of rotation through 90 degrees. The horizontal vessel was operated from October 1993 to August 1996. Whilst scale-up of the process was successfully demonstrated, the complexity of engineering a horizontal vessel limited its commercial viability.
To overcome this deficiency a design was developed for water cooled vertical vessel. Design and engineering for the vertical SRV (Fig 2) was completed in 1996. The main improvements incorporated into the design included a stationary vertical vessel, top injection of solid raw materials, a simplified hot air blast lance, a forehearth for continuous tapping of hot metal and water cooled panels to overcome refractory wear problems.
HRDF vertical smelt reduction vessel was commissioned in the first half of 1997 and operated through to May 1999. The vertical vessel demonstrated major improvements in terms of refractory wear, reliability, availability, productivity and simplicity in design. This vessel addressed all the key requirements for a successful direct smelting iron making technology – combining a high level of technical achievements with simple engineering concepts and plant technology. This stage of operation confirmed that the process was ready to be scaled up to level of a commercial plant.
A joint venture was formed in 2002 between the Rio Tinto (60 %), Nucor Corporation (25 %), Mitsubishi Corporation (10 %), and Shougang Corporation (5 %) for the purpose of constructing and operating an 800,000 tons per annum HIsmelt plant. Located in Kwinana, Western Australia, the merchant pig iron facility was designed and engineered with a 6 meter hearth diameter SRV. Construction of the plant was started in January 2003. Cold commissioning commenced in the second half of 2004 while the hot commissioning was carried out in second quarter 2005. The plant had achieved a production rate of 80 tons of hot metal per hour in early 2008 with a coal rate of 810 kg per ton of hot metal.
Due to the economic conditions in 2008, the Kwinana plant was closed and in 2014 some of the Kwinana equipment was transferred from Australia to China. The new HIsmelt plant is located near Shouguang Port in Shandong Province, and is owned and operated by Molong Petroleum Machinery Limited, a private steel company. The Molong HIsmelt plant started up in 2016 and has produced hot metal at a cost lower than the local mini blast furnaces previously used by Molong. The hot metal ladles are delivered by the road vehicles to the Molong basic oxygen steelmaking plant located 40 km south of the HIsmelt plant. In 2017, Molong purchased the HIsmelt intellectual property from Rio Tinto for the licensing of the technology to other users in China and overseas.
The HIsmelt process, depicted in Fig 1, involves high-velocity injection of solid materials (coal, iron ore and fluxes) into a molten iron bath at around 1450 deg C. The basic mechanism of the process is the reduction and smelting of the iron bearing ores with the dissolved carbon in the bath. The process uses high velocity injection of coal and ore into the melt through downwardly angled water cooled injection lances. Injected coal after heating and devolatilization dissolves to maintain around 4 % carbon in the molten metal and replenish the carbon used in the reduction reaction. Injected iron ore fines are injected deep into the bath where they are reduced instantly on contact with carbon dissolved in the bath for smelting to take place. This reduction reaction produces iron and CO. The lower part of the SRV is maintained at low oxygen potential to allow this reduction reaction to occur and the reaction kinetics balance out at around 5 % to 6 % of FeO (iron oxide) in the slag.
Reaction gas (CO) and coal devolatilization products which are generated from deep within the bath form a fountain (splash) of mostly slag and some metal. Heat supply to maintain the necessary thermal balance comes by the combustion of reaction gas (mostly CO) in the upper part of the SRV. Oxygen enriched (typically 35 %) hot blast at 1,200 deg C is introduced through a top lance and efficiently burns the gases generated within the bath and releases large amounts of energy. This combustion occurs in the relatively oxidizing region in the upper section of the SRV. The heat transfer between the upper (oxidizing) regions to the lower (reducing) region is achieved in such a way that the oxygen potential gradient is maintained. This is done through large amounts of liquid splash moving between the two regions. Liquid slag and metal splash acts as a carrier of the heat.
Injection of the materials is arranged such that significant penetration of solids into the iron bath is achieved leading to the dissolution of carbon into the metal and the reduction of iron ore through the overall reaction given by the equation 3[C]iron + Fe2O3 = 2[Fe]iron + 3CO. This reaction is highly endothermic and, if the process is to be sustained, an external supply of heat is needed. CO plus hydrogen released from the bath provides the fuel for generating this heat. Hot blast (oxygen-enriched air at 1,200 deg C) is injected into the top space through a central swirl lance and combustion takes place to burn the bath gases to carbon dioxide and water as per equations 2CO + O2 = 2CO2 and 2H2 + O2 = 2H2O.
Theoretically it is desired to achieve total combustion of this bath gas but, in practice, post-combustion of around 50 % to 60 % is typically achieved. Post-combustion (PC) is defined as the ratio of the volumetric concentration of combusting species as given by the equation PC (%) = 100(CO2 = H2O) / (CO + CO2 + H2 + H2O).
Smelting occurs in the melt where the oxygen potential is low, whereas heat generation occurs 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 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 (splash) with high surface area for heat transfer. Hot combustion gases pass through this fountain 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 through an overflow forehearth (which is effectively a liquid metal manometer seal), whereas slag is tapped periodically through the side-wall of the vessel through a water-cooled slag notch.
The main product of the process is hot metal. Hot metal is tapped continuously through an open forehearth and is free of slag. Typical temperature of hot metal is around 1,420 deg C to 1,450 deg C and typical composition of the hot metal is carbon – 4.4 % +/- 0.15 %, silicon – less than 0.01 %, manganese – less than 0.02 %, phosphorus – less than 0.02 % +/- 0.01 %, and sulphur – 0.1 % +/- 0.05 %
The relative oxidizing atmosphere and the low temperature slag in the SRV results in as much as 90 % to 95 % of the phosphorus in the feed materials partitioning to the slag (Fig2). Hence this process has flexibility to use high phosphorus containing ores.
Since the slag is batch tapped through a slag notch, the hot metal is slag free. The hot metal can be treated in a hot metal desulphurization plant to bring down the sulphur level of the hot metal to less than 0.05 %. Typical flow sheet of the HIsmelt process is shown in Fig 3.
Fig 3 Typical flow sheet of HIsmelt process
The off gas from the SRV is cooled from 1500 deg C to 800 deg C by a radiative boiler hood, partially cleaned in hot cyclones and then further cooled to 200 deg C through a convective boiler system. The radiative and convective boilers generate saturated steam which flows to a steam drum located on an off gas boiler.
The SRV off gas exiting the convective boiler is quenched, scrubbed and cooled through water sprays and circulated through ducts for use as fuel for the hot blast stoves and the off gas boiler. In the off gas boiler, the excess SRV off gas is combusted and superheats the saturated steam from the radiative, convective and off gas boilers. This superheated steam flows to steam turbines which generate sufficient power to operate the HIsmelt plant, ancillaries, and send excess power to the local grid. The exhaust gasses from the off gas boiler and stoves are scrubbed with a lime slurry to remove the sulphur in the gas before being vented to atmosphere.
By-products of the process are slag and off gas. Slag is formed by fluxing the gangue in the iron ore and ash in the coal with lime and dolomite. Slag can be granulated or directed into pits for its further processing. It can then be used as a raw material for a variety of purposes such as cement manufacture, road base, or soil conditioning. Off gas from the process typically has a post combustion degree of 50 % to 60 %. These gases exit from the top of the SRV at high temperatures and have energy values similar to the blast furnace gas. It is cleaned, cooled and used as a fuel and for power generation.
HIsmelt process is highly flexible. The highly responsive nature of the process means that it converts iron ore, coal and flux to metal, slag and energy almost instantaneously. The process capabilities allow for raw material feed rates to be changed very efficiently without affecting product quality. This operating flexibility maximizes productivity, as it is easy to maintain a steady state operating window. Unlike blast furnaces, the HIsmelt process can be started, stopped or idled with ease. Fig 4 shows the layout of the HIsmelt plant.
Fig 4 Layout of the HIsmelt plant
The environmental benefits of the HIsmelt process are considerable. By reducing the demand for coke, sinter and pellets, and improving the energy efficiency of the iron making process, it reduces emissions of greenhouse gases and other damaging environmental pollutants such as SOx, NOx and dioxins. The improvements from the process are (i) reduction in CO2 emissions by 20 %, (ii) reduction of SOx emissions by 90 %, (iii) reduction in NOx emissions by 40 %, and (iv) no toxic emissions with the emissions of dioxins and furans at a nil level. The operating conditions within the SRV preclude the formation of dioxins. Further there is big reduction in dust emissions and specific wate consuption. The process by recycling of the plant waste further helps in the environment protection.
The projected advantages of the process are (i) low cost raw materials since ferrous feed agglomeration and coke making processes are eliminated, (ii) flexible feed stocks since the process operates on a wide range of iron ore fines and waste oxide materials, together with high and low volatile coals, (iii) production flexibility, (iv) better product quality, (v) minimized total energy consumption, (vi) simplified engineering and process configuration, (vii) environmental emissions well below the industry, (viii) lower capital cost, and (ix) lower operational cost.