Blast furnace oxygen enrichment

In lead smelting, the oxygen-enriched blast has been used in lead cupola and slag fuming furnaces. Oxygen enrichment of the copper blast furnace is also common. 

The process of extraction requires first smelting (to obtain the crude metal) and then refining. In smelting, iron ore (usually an oxide) is mixed with coke and limestone and heated, and hot air (often enriched with oxygen) is blown in from beneath (in a blast furnace). At the lower, hotter part of the furnace, carbon monoxide is produced and this is the essential reducing agent. The reduction reactions occurring may be represented for simplicity as ... 

Oxygen enrichment of steel blast furnaces accounts for the greatest use of the gas. Large quantities are also used in making synthesis gas for ammonia and methanol, ethylene oxide, and for oxy-acetylene welding. 

The ISP evolved to fill a very special niche in nonferrous metallurgy because of its capability of treating Lead-zinc concentrates which may also contain appredable amounts of copper. The concentrate is normally-oxidized in a sintering machine to produce a feed for the blast furnace where the zinc oxide is reduced with coke. Some effort has been underway to develop a hot briquetting operation to produce a suitable feed without sintering. Other efforts to improve the economic competitiveness of the process include air preheat and the use of an oxygen-enriched blast to reduce coke consumption. 

Oxygen has been used for enriching the blast supplied to blast furnaces m pig-iron production, with consequent economy in the consumption of coke. This process yields particularly favourable results in the manufacture of ferro-silieon.3... 

Figure 3. Profile of melting furnace (1) upper seal valve (2) lower seal valve (3) inflammable gas outlet (4) drying zone (5) py roly zing zone (6) melting zone— high temperature (7) molten slag (8) oxygen-enriched air inlet—tuyere (9) blast header (10) slag granulation...

Enriching the cupola blast with oxygen has been in practice since the early 1930s.4 Use of oxygen to enrich blast air in iron blast furnaces has an even longer history. More recently, lead and mineral wool cupolas also have been equipped with oxygen... 

The ability to vary oxygen enrichment levels instantaneously also provides the cupola or blast furnace operator with an extremely flexible device to control furnace operations better in spite of unavoidable changes in coke and metallic charge quality, blast humidity and temperature, and cast house demand pattern. Today, hundreds of cupola foundries worldwide use oxygen to increase cupola capacity and/or reduce unit operating cost, and most modem blast furnaces are equipped with oxygen injection. 

Methods used to introduce oxygen into the cupola or blast furnace can be classified into two main categories diffuser enrichment and tuyere injection... 

Effect of Oxygen Enrichment of a Lead Blast Furnace... 

The thermal efficiency of the cold blast cupola furnace can be improved by oxygen enrichment of the combustion air. This gives rise to a higher combustion temperature of the coke. In this way coke consumption can be decreased or higher temperatures of the tapped metal can be... 

To y, almost all European hot blast cupolas inject oxygen through the tuyeres. For cold blast furnaces, the use of oxygen enrichment can be considered as the standard technique. In this case, enrichment of the blast supply is usually applied. The oxygen level of the oxidising air mixture is usually between 22 and 25 % (i.e. an enrichment of 1 % to 4 %). 

None of the furnaces operated with oxygen enriched blast air. Because of the stronger adjusted reduction potential, the lead concentration in the final slag, which is equal to or less than 1.5%, is lower compared to that of blast furnace slags. 

The Herculaneum smelter has three blast furnaces, of which two are operating at any one time. The furnace crucible is built of castable refractory. The furnace sides are built of water-cooled steel jackets. Each furnace has 21 tuyeres on each side, for 42 tuyeres total. Air is supplied to a furnace by one of three blowers capable of supplying 15,000 ftVmin of air. The blast air is directed to the furnace by a series of bustle lines. The air blast is enriched with oxygen that is controlled at each furnace. 

Trials with oxygen enrichment of the furnace blast air were first carried out in the United Kingdom as long ago as 1964. In general, it is considered too expensive to use routinely, but a small amount of enrichment is used in several smelters. At Cockle Creek in 1975 oil was injected into the tuyeres and replaced up to 12% of the coke. There were no adverse results and oil injection continued until the rising cost of oil rendered it uneconomic. Subsequently, in 1986 the Harima smelter became the first ISF to inject solids on a consistent basis. Up to 6% of the lump coke charge was replaced by injected dry quench coke dust. This practice continued until November 1995. 

The use of oxygen enrichment of blast air has also enabled the capacity of the blast furnace to be further increased, and is applied in most operations. 

The tuyeres are fitted through the jackets and are constmcted of copper, often fitted with stainless steel tips to reduce corrosion and erosion. The cold air blast is sufficient to keep the tuyere bodies cool. Jackets at the tuyere level can be refiactory lined on initial installation, but this is gradually replaced by acaetions. The influence of the tuyeres can cause bum out of jackets, particularly with oxygen enrichment of blast air, but this can be overcome by the projection of the tuyere further into the furnace. 

Experience with oxygen enrichment of blast air is illustrated in Figure 5.6 (Fern and Jones, 1980), which shows that furnace capacity is increased by five times the oxygen percentage increase. A two per cent enrichment from 21 per cent to 23 per cent will raise furnace lead production by ten per cent. Blast furnace production is thus directly proportional to the oxygen flow rate given a constant tuyere volume and other furnace parameters. 

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