The Lead Blast Furnace

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. 

Details of blast furnace performance and operation are covered in Chapter 5. 

In 1960 the first standard commercial scale Imperial Smelting Furnace (ISF) was constructed at Swansea in the UK, as an adaptation of the lead blast furnace, to simultaneously produce zinc and lead. The furnace operated with a hot top to retain zinc in the vapour phase. The top was sealed and gases passed through a lead splash condenser to strip zinc from the gas phase into a lead-zinc bullion which could be cooled for separation of crude zinc and lead metals. The ratio of zinc to lead production from these units is generally more than 2 1, and lead production from the standard unit is close to 40 000 t/a. Thirteen plants were constructed around the world but due to unfavourable economics a number of these have now closed. Details are given in Chapter 6. 

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The lead blast furnace operates at a lower temperature than the iron blast furnace, die temperature at the tuyeres being around 1600K as opposed to 1900K in the ironmaking furnace (see p. 333) and this produces a gas in which die incoming air is not completely reduced to CO and N2, as much as one per cent oxygen being found in the hearth gas. 

The development of electrostatic precipitators soon led to new applications, including the separation of metal oxide fumes. This was followed by various metal manufacturing processes such as the lead blast furnace, ore roaster, and reverberatory furnace. Electrostatic gas cleaning was soon applied also in cement kilns and in several exotic applications, such as recovering valuable metals from exhaust gases. 

There have been a number of attempts to model the lead blast furnace, notably Lumsden (1971), MadeUn, Sanchez and Rist (1990), as well as descriptions of the prcxrss chemistry by Willis (1980) and Oldwright and Miller (1936). 

As illustrated in Figure 5.2 accretions are a key feature of the lead blast furnace and must be adequately managed to maintain operation. Accretion formation is complex and as indicated above, results from reactions of volatile components such as zinc and lead sulfide as weU as fusion of the charge and solidification in cooler regions of the furnace. Analysis has been reported by Oldwright and Miller (1936), Ruddle (1957) and Polyvyannyi et al (1971). 

The lead blast furnace consists of a rectangular shaft with sidewalls made up from a series of water-cooled hollow steel jackets. Refiactory lining is unnecessary since an accretion layer forms on the inner surface to protect the steel from attack. 

Lumsden, J, 1971. The physical chemistry of the lead blast furnace, in Proceedings Metallurgical Chemistry Symposium (ed O Kubaschewski), pp 533-548 (National Physical Laboratory, HMSO London). 

Oldwright, G L and Miller, V, 1936. Smelting in the lead blast furnace, Trans AIME, 121 82-105. 

Ruddle, R W, 1957. Difficulties Encountered in Smelting in the Lead Blast Furnace, 55 p (Institution of Mining and Metallurgy London). 

The ISF was developed from the standard lead blast furnace, but with evolutionary change to a more intensive operation, more akin to the iron blast furnace conditions at the tuyere level. The standard ISF design had an upper shaft cross-section of 17.2 m and a cross-section at tuyere level or hearth area of 13.2 m. Data for the standard ISF is compared with the lead blast furnace in Table 6.1 and demonstrates the higher intensity operation. 

The design of a lead smelter depends to a significant extent on the nature of the feed materials processed, particularly the grade of the concentrates. In simple terms this is due to the large possible variation in the sulfur to lead ratio in feed materials and hence the size of the sinter plant required, which is dictated by the sulfur burning capacity. Usually lead sinter has a relatively common lead composition at around 45 per cent and hence the lead blast furnace sizing is not so critically dependent on the nature of the feed. For this reason the cost estimates provided are based on a standard concentrate feed of 60 per cent Pb and 20 per cent S content. Capacity is standardised at 100 000 tonnes per annum (t/a) lead production, representing the median capacity smelter. 

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