Smelting Reduction Technologies

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Smelting reduction usually produces hot metal from ore in two steps. Ores are partly reduced in the first step and then final reduction and melting takes place in the second stage. Some of the most common process in use are described below :

The coal reduction process (COREX) , was developed by Voest-Alpine industries (VAI) and DVAI .This is a two-stage operation in which DRI from a shaft furnace like that used in the Midrex and HYL process is charged into a final smelter-gasifier. Reducing gas for the shaft furnace is produced by partial combustion of coal with oxygen in the fluidized bed of the smelter-gasifier.

The energy needed to complete the reduction of the DRI and produce the hot metal and slag is provided by the partial combustion. The liquid products are tapped periodically and partially spent off gas from the shaft reducer is exported along with excess gas produced in the smelter-gasifier. The smelter-gasifier operates at 3-5 bars and comprises an upper fluidized bed zone at approximately 1500°C (2730°F) and a lower melting and liquid collection zone at approximately 1550°C (2820°F). Coal and limestone are injected into the freeboard above the fluidized bed zone where they are heated rapidly to 1000-1200°C (1830-2190°F). The volatile matter is driven off and shattered fixed carbon particles fall into the gasification zone where a gas with high oxygen content is injected through blast furnace-type tuyeres to burn the carbon to carbon monoxide. The exothermic combustion provides the energy to complete the reduction of the hot DRI and to melt the slag and hot metal. The gas leaving the smelter-gasifier is cooled to 800-900°C (1470-1650°F) and cleaned in a hot cyclone to recycle entrained fines. A portion of the clean gas is then introduced into the shaft furnace as reducing gas containing more than 94% CO plus H2. The remaining gas is mixed with the cleaned offgas from the shaft furnace and the mixture used as export fuel gas.

COREX uses approximately one tonne of coal per tonne of hot metal, with approximately 45% of the total energy input used in ironmaking and the rest going to export fuel gas. The hot metal produced has carbon and silicon contents similar to blast furnace hot metal; however, the sulfur content is much higher because nearly all of the sulphur in the coal enters the slag and hot metal. In this connection, organic sulphur in the coal gasifies and is absorbed by the DRI and returned to the smelter-gasifier as iron sulphide.

The Direct Iron Smelting Reduction ( DIOS) was developed by the Japan Iron and Steel Federation (JISF) , the Centre of Coal utilisation and a consortium of eight Japanese steelmakers. The DIOS system has three fluidised furnaces .Iron ore is preheated in the first of two fluidized bed reactors in series and pre-reduced to 15-25% in the second reactor using cleaned offgas from the smelter. Dust removed from the smelter, off-gas and fines removed from the gases leaving the fluidized bed reactors are injected back into the smelter. In addition, a small amount of coal fines, of the order of 50 kg per tonne of hot metal production, is injected into the smelter offgas to cool the offgas and provide additional CO and H2 for pre-reduction. Most of the coal, 600-700 kg per tonne of hot metal, is gravity-fed into the smelter. Oxygen is injected into the smelter for combustion of primary coal and for post-combustion. The oxygen lance is designed to provide both high velocity oxygen for carbon oxidation (hard blow) and lower velocity oxygen for post-combustion in the freeboard (soft blow) simultaneously. The aim post-combustion is approximately 40%, and with pre-reduction to 20%, the coal consumption is roughly 700-800 kg per tonne of hot metal depending on coal type.

The Ausmelt process was developed by Ausmelt Ltd. Australia . Lump ore or ore fines are fed continuously into a converter along with lump coal and flux. Fine coal , oxygen and air are injected through a top lnce to allow submerged combustion. The degree of oxidation and reduction is controlled by adjusting fuel to air and coal ratios as well as the proportion of fine coal injected down the lance. All reactions are completed in a single reactor.

The Hismelt process was developed by CRA ltd., Australia and Midrex Corporation , United States .Coal is injected through bottom tuyers into a molten bath. Carbon is rapidly dissolved and reacts with oxygen from incoming iron ore to from carbon monoxide and iron. This reaction is endothermic and therefore to keep the process going additional heat has to be supplied .This is achieved by reacting carbon monoxide released from the bath with oxygen from top injection of air. Reacted hot gasses exit the vessel and are used in a fluidised bed to pre- heat and pre-reduce incoming ore.

The ROMELT process was developed by Moscow institute of steel and alloys in Russia . A major feature is there is no pre-reduction process step. The smelter has a water-cooled roof and sidewalls in contact with slag and conventional refractories in contact with the metal. A mixture of air and oxygen is injected through two rows of tuyeres. Coal and ore are fed by gravity. The system, simple and robust. ROMELT consumes more energy than other smelting processes due to the lack of pre-reduction and extensive water cooling.

In plasma-based smelting reduction processes, the reactions take place in a coke-filled shaft furnace with tuyeres spaced symmetrically around the lower part of the furnace. The shaft is completely filled with coke.Plasma generators and equipment for injection of metal oxides mixed with slag forming material and possibly reductants are attached to the tuyeres. In front of each tuyere a cavity is formed inside the coke column where reduction and smelting take place. At regular intervals the produced slag and metal are tapped from the bottom of the shaft furnace. In the case of iron ore smelting the off-gas from the furnace, consisting mainly of carbon monoxide and hydrogen, can be used for pre reduction of the ore.In other applications of the process, such as reclaiming of alloying metals from baghouse dust, the produced gas is utilised as a fuel gas.If the raw material contains metals with high vapour pressures, for example zinc and lead, these metals leave the furnace with the off-gas which is then passed through a condensor where the metals are recovered from the gas.

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