Second-Stage Refining and Technology Advances Second-Stage Refining and Technology Advances
The recognition that the stirring being provided by the topblown
jet of a BOF furnace toward the end of the refining process was inadequate, together with the development of the
Savarde-Lee shrouded tuyere (Ref 4), triggered a remarkable change in the technology of these oxygen-blown vessels.
The tuyere development work made possible and practical the bottom blowing of low-pressure oxygen at high flow rates
through a series (typically eight) of tuyeres set in the bottom of the furnace. Each tuyere consists of a central pipe for the
oxygen jet and an annular space for injecting a hydrocarbon (such as methane) to form a solid mushroom of steel (Fig. 6).
This mushroom protects the refractory base from the fluxing effects of FeO and has allowed the revived use of the
Bessemer vessel of 1856 (Ref 5), except that pure oxygen rather than air is injected.

The first North American licensee named this process the quick-quiet basic oxygen process, or Q-BOP. The bottomblown
oxygen jets provide better mixing, lower turndown carbons (of the order of 0.01 wt% C), higher yields (less FeO in
slag), and shorter processing times (for example, 14 versus 17 min/blow). One drawback, however, is higher levels of
turndown hydrogen in the steel. This is caused by the endothermic cracking of the methane that is needed for the
formation of the protective thermal accretions, or mushrooms (Ref 6). Higher levels of dissolved hydrogen can be
deleterious for heavy-section products such as pipeline steels and ship plate products; postrefining stir with argon is
sometimes favored for steels with these applications.
Another feature of these bottom-blown vessels is the need to inject a fine powdered lime simultaneously with oxygen.
Top charging of lime particles or lumps in a similar manner to BOF operations leads to unacceptable foaming and
slopping.
A wide variety of other processes have been spawned that take advantage of some features of both top- and bottom-blown
vessels. In the Kawasaki basic oxygen process (K-BOP) operation, 30% of the oxygen is soft blown from a multihole
lance set high above the steel bath, with the remainder injected through the base of the vessel using shrouded tuyere
technology. This allows low turndown carbons (of the order of 0.02 to 0.04% C), together with higher scrap-melting
capabilities (for example, 33% versus 30% of the charge). Other similar technologies, such as the German Kloeckner
metallurgy scrap (KMS) process, are also in use.
The improved scrap-melting capability of such vessels is enabled by the burning of a higher proportion of effluent carbon
monoxide to carbon dioxide within the upper reaches of the vessel itself. Part of the attendant heat can be usefully
transferred back to the metal bath, allowing more scrap to be melted. Because scrap generally represents a less expensive
source of iron units versus hot metal from the blast furnace, such operations can be profitable, even though they are more
technically complex to operate.
Practically all BOF (or oxygen-blown method (OBM) or Linze-Donovitz (LD) method) steelmaking operations in North
America now use bottom-blown gas injections to at least stir the steel bath. For example, nitrogen, argon, or carbon
dioxide can be blown through submerged injector ports, plugs, or nozzles of various proprietary designs. The Sumitomo
top and bottom blowing (STB) process, in which CO2/N2 mixtures are bottom blown at about 5% of the flow of the topblown
oxygen in a BOF-like vessel, is a good example of this concept. The STB process increases yields and lowers
turndown carbons, thus approaching the performance of Q-BOP vessels.

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