Brisse, Andre H. (Pittsburgh, PA)
Griffiths, David K. (Penn Hills Twp., PA)
1. A vessel for refining a molten metal bath contained therein, said vessel having an opening in its upper portion and at least one tuyere extending vertically through the bottom wall of said vessel for introducing refining gas to said bath, each tuyere having a hydraulic diameter from 1/25th to 1/13th of the bath depth and having a total cross-sectional area of all such refining gas tuyeres in cm2 from one to three times the bath size in metric tons.
2. A vessel according to claim 1 in which each refining gas tuyere has a hydraulic diameter from about 2.5 centimeters to 7.5 centimeters.
3. A vessel according to claim 1 in which each refining gas tuyere has a hydraulic diameter from about 4.0 to 6.5 centimeters.
4. A vessel according to claim 1 wherein each refining gas tuyere is surrounded by a larger concentric outer tube which forms with said tuyere an annular passage for simultaneous injection of a jacket gas into the molten metal bath.
In the bottom blown oxygen steelmaking process, which is known as the Q-BOP process, a vessel has a removable bottom or plug which contains one or more tuyeres through which oxygen and other gases or particulate matter such as lime or other flux is blown into the vessel. Each tuyere comprises an oxygen inlet tube surrounded by a larger concentric inlet tube for the simultaneous injection of a jacket gas, which does not react, or reacts only slowly with the molten metal in the bath and the material from which the nozzle is constructed. This jacket gas acts as a coolant reducing the rate of reaction between the molten metal and the oxygen adjacent to tuyere, preventing rapid erosion of the tuyere, reducing the rate of erosion of the lining in the vessel bottom. Thus, the furnace lining and the tuyere wear at the same rate.
South African Pat. No. 691,280 teaches that the total cross-sectional area of oxygen delivering tuyere pipes in square centimeters should be approximately equal to the weight of pig iron charged into the converter in metric tons. The same reference teaches that the greatest allowable tuyere diameter for vertically mounted tuyeres should not exceed 1/35 of the depth of the bath, assuming an oxygen pressure of about 5 to 10 atmospheres.
We have discovered that not only can larger tuyeres be used in a bottom blow process, but by increasing the diameter of the tuyeres to 1/15 of the bath depth, the number of tuyeres required in a given vessel is only one-third the number required if the diameter is 1/35 of the bath depth. This results not only in a reduced requirement in number of tuyeres, but also in a reduced number of piping connections for both oxygen and jacketing gas since fewer tuyeres are required. This, in turn, results in a decreased manpower requirement for maintenance and periodic replacement of parts.
It is the principal object of our invention to provide improved apparatus for refining molten metal in a Q-BOP steelmaking vessel.
It is another object of our invention to provide apparatus having increased tuyere size over what was heretofore possible.
These and other objects will be more readily apparent with reference to the following detailed specification and the appended drawing in which:
The single figure is a vertical, cross-sectional view of a Q-BOP steelmaking vessel.
A bottom blow oxygen steelmaking vessel 10 has a removable bottom 12, comprising a bottom plate 14, and one or more generally upstanding tuyeres 16, which are surrounded by refractory material 18. The bottom plate 14 is fastened to the furnace by bolts 20. The sides of the refractory portion of the removable bottom do not contact the refractory lining 22 of the vessel, but sufficient clearance is provided around the bottom for inserting a refractory gunning mixture 24 to provide a metal and slag tight seal.
Tuyere 16 is a dual concentric tuyere composed of an inner tube 30 and an outer tube 34. Tube 30 is spaced from tube 34 by spacers 36, which may be weld beads, spiral wound wire, or any other suitable means for maintaining concentricity. The inner or central tube 30 delivers oxygen and lime to the molten metal bath. The annular space 38 between the central tube 30 and the outer tube 34 delivers a jacketing gas which is, in this case, natural gas.
We have found that increasing the diameter of a standard circular, cross-sectional tuyere to about 1/15 of the bath depth apparently reduces the height of the jet of gas in the bath and thus reduces the tendency of spitting to occur in the bath. We have also found that the total cross-sectional area of all oxygen delivering tuyeres in square centimeters may be from one to three times the bath size in metric tons.
The tuyere need not be a standard circular tuyere, but may have any of a number of cross-sectional shapes such as square, rhombic, rectangular, ellipse, oval, pointed ellipse, or any other desired shape. To define a common parameter of such shapes, we must turn to the terminology of fluid mechanics, wherein the hydraulic radius of a non-circular duct is the fluid-filled area divided by the inside perimeter of the duct. The hydraulic diameter equals four times the hydraulic radius. As above, a tuyere of any cross section having a hydraulic diameter up to about 1/13 the bath depth is operable in our invention.
The following examples illustrate the operability of our invention.
Circular oxygen tuyere pipes 2.86 centimeters in diameter have been employed in a Q-BOP vessel in which the bath depth was approximately 28 inches (71.1 centimeters). The hydraulic diameter was 1/25 the bath depth. Six tuyeres, having a total cross-sectional area of the oxygen pipe of 38.5 square centimeters, were used to blow oxygen into a bath of 18.2 metric tons. Thus, the cross-sectional area of the oxygen-delivering tuyeres was 2.11 times the bath size in metric tons.
Circular oxygen tuyere pipes having an internal diameter of 2.323 inches (5.9 centimeters) were employed in a Q-BOP vessel in which the bath depth was 32 inches (81.3 centimeters). The hydraulic diameter was 1/14 the bath depth. Two tuyeres having a total cross-sectional area of the oxygen pipe of 52.8 square centimeters were used to blow oxygen into a bath of 23 metric tons. The cross-sectional area of the oxygen-delivering tuyeres was 2.3 times the bath size in metric tons. Oxygen flow was 1,400 scfm (2352 Nm3 /hr.) at 15 psig.
Our invention comprehends the broad range of hydraulic diameters from about 1 inch to 2.85 inches or 2.5 centimeters to 7.5 centimeters. However, we prefer a hydraulic diameter in the range of about 1.6 to 2.5 inches or 4.0 to 6.5 centimeters. We prefer a large hydraulic radius since it will accommodate low pressures. Low oxygen pressures require large tuyeres to obtain sufficient oxygen throughput to operate the refining process. Of course the pressure must be sufficient to overcome the ferrostatic head of molten metal in the vessel. We have found that the oxygen pressure must be maintained at a minimum of 1 atmosphere and can be as high as 15 atmospheres. The oxygen throughput must be at least 85 normal cubic meters per hour per square centimeter of oxygen tuyere cross section.
The tuyere pipes may be installed at an angle with respect to the axis of the Q-BOP converter vessel to improve mixing or impart rotation to the bath thus, minimizing sloshing and bath instability. In this case, tuyeres of even larger cross-sectional area may be used. For instance, the tuyere diameter may be increased about 20 percent for nozzles inclined 30° to the vessel's vertical axis. The tuyere pipes may also be installed in the sidewall of the converter beneath the surface of the molten metal bath.
From the foregoing it is readily apparent that we have invented an improved Q-BOP vessel for refining molten metal in which tuyeres extending through the bottom wall of the vessel have a hydraulic diameter no greater than 1/13 of the bath depth and a total cross-sectional area of the oxygen delivering portion of such tuyeres in square centimeters from 1 to 3 times the bath size in metric tons.