Iron reduction of oxides

The Iron Bla.stFurna.ee, The reduction of iron oxides by carbon in the iron (qv) blast furnace is the most important of all extractive processes, and the cornerstone of all industrial economies. Better understanding of the reactions taking place within the furnace has made possible a more efficient operation through better preparation of the burden, higher blast temperature, and sometimes increased pressure. Furnace capacity has doubled since the 1800s, whereas coke consumption has been reduced by about half The ratio of coke to iron produced on a per weight basis is ca 0.5 to 1. 

In atomization, a stream of molten metal is stmck with air or water jets. The particles formed are collected, sieved, and aimealed. This is the most common commercial method in use for all powders. Reduction of iron oxides or other compounds in soHd or gaseous media gives sponge iron or hydrogen-reduced mill scale. Decomposition of Hquid or gaseous metal carbonyls (qv) (iron or nickel) yields a fine powder (see Nickel and nickel alloys). Electrolytic deposition from molten salts or solutions either gives powder direcdy, or an adherent mass that has to be mechanically comminuted. 

Carbotbermic Reactions Some apparently solid/solid reactions with carbon apparently take place through intermediate CO and CO2. The reduction of iron oxides has the mechanism... 

FIG. 23-43 Reactors for solids, (a) Temperature profiles in a rotary cement lain, (h) A multiple hearth reactor, (c) Vertical lain for lime burning, 55 ton/d. (d) Five-stage fluidized bed lime burner, 4 by 14 m, 100 ton/d. (e) A fluidized bed for roasting iron sulfides. (/) Conditions in a vertical moving bed (blast furnace) for reduction of iron oxides, (g) A mechanical salt cake furnace. To convert ton/d to kg/h, multiply by 907. 

The reduction of iron oxide takes place in several stages in different temperature zones within the fiimace. The reducing agent is CO produced from burning coke. Here are the key reactions ... 

Although this chemistry is complex, the basic process is reduction of iron oxide by carbon in an atmosphere depleted of oxygen. Archaeologists have found ancient smelters in Africa (in what is now Tanzania) that exploited this chemistry to produce iron in prehistoric times. Early African peoples lined a hole with a fuel of termite residues and added iron ore. Chamed reeds and charcoal provided the reducing substance. Finally, a chimney of mud was added. When this furnace was fired, a pool of iron collected in the bottom. 

A certain amount of iron oxide is present in the alumina slag and a certain amount of aluminum is present in the as-reduced iron. Assuming Raoult s law, an aluminum content of 1 wt-% in iron will correspond to A 0.02. The value of aFeG is then 6 10 4. Even a trace of aluminum dissolved in liquid iron results in a practically complete reduction of iron oxide from the slag phase. The assumption that the product phases are pure iron metal and alumina slag thus holds. 

The reason for a previously reported explosion during reduction of iron oxide with carbon monoxide is given as the formation of pentacarbonyliron at temperatures between 0 and 150°C. Suitable heating arrangements and precautions will eliminate this hazard. 

Figure 36. Comparison of rate of reduction of iron oxide with and without co-MSFB (Tan, Yao, Wang, Liu, andKwauk, 1983).

The slag layer plays important roles in heat transfer and the reduction of iron oxide. The role of the slag layer in heat transfer was described in Sec. 2.3. Katayama et al. (1992) proposed that the reduction of iron oxide occurred at the following three locations... 

Indirect employment of hydrocarbons involves a preliminary conversion (e.g., by steam reforming) of hydrocarbons to syngas followed by the reduction of iron oxides with H2 and CO components of the syngas. The following reactions occur during the reduction of... 

As the equilibrium constant for the reduction of iron oxide is on the order of 0.1 (see Table 2.1), traces of water are already sufficient to oxidize a supported iron catalyst. 

Conditions that are conducive to the accumulation and decay of plant residues, and therefore conducive to the formation of coal, are typically associated with water-saturated and reducing environments. Consequently, a large portion of the trace elements associated with the mineral fraction of coal are expected to occur in reduced forms, primarily as sulphides or carbonates. However, because of its abundance compared to sulphur, it is unlikely that the complete reduction of iron oxides to iron sulphide would ever occur. Therefore, the presence of Fe oxides, and trace... 

PIG IRON. Product of blast-furnace reduction of iron oxide in the presence of limestone. About half the ore is converted to iron. Average analysis is 1% silicon, 0.03% sulfur, 0.27% phosphorus, 2.4% manganese, 4.6% carbon, balance iron, Pig iron is the basic raw material for steel and cast iron. In metal terminology a pig is a bar or ingot of cooled metal. 

There is a large class of industrially important heterogeneous reactions in which a gas or a liquid is brought into contact with a solid and reacts with the solid transforming it into a product. Among the most important are the reduction of iron oxide to metallic iron in a blast furnace the combustion of coal particles in a pulverised fuel boiler and the incineration of solid wastes. These examples also happen to be some of the most complex chemically. Further simple examples are the roasting of sulphide ores such as zinc blende ... 

Iron is the work horse of man. It is obtained by the reduction of iron oxide. The reduction is carried out in a blast furnace. The reduction takes place in stages. 

Solids blending Acrylonitrile synthesis Reduction of iron oxide Coal gasification... 

In the blast furnace, the iron oxides Fe203, Fe304, and FeO, are successively reduced to iron by CO produced by the reaction of coke with air. Calculate AG° at 1500 K for the reaction FeO(solid) + CO(gas) -o- Fe(solid) + C02(gas), the final stage of reduction of iron oxide in the blast furnace, using the constant heat capacity approximation. 

Table 2.1 lists equilibrium ratios for the reduction of selected metal oxides [4], while Figure 2.2 provides a complete phase diagram for the reduction of iron oxide at different temperatures [3, 5], In order to reduce bulk iron oxide to metallic iron at 600 K, the water content of the hydrogen gas above the sample must be below a few percent, which is easily achieved. However, in order to reduce Cr2C>3, the water content should be as low as a few parts per billion, which is much more difficult to realize. The data in Table 2.1 also illustrate that, in many cases, only partial reduction to a lower oxide may be expected. Reduction of Mn2C>3 to MnO is thermodynamically allowed at relatively high water contents, but further reduction to manganese is unlikely. 

Consider for instance the reduction of iron oxide to metallic iron by gaseous molecular hydrogen ... 

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