Continuous Copper Melting Furnace

1. Field of the Invention

This invention relates to an improved vertical shaft type furnace construction, and burner design for use therein, which is particularly useful for continuously melting copper pieces such as cathodes.

2. Prior Art

Vertical gas-fired shaft type furnaces for melting metal, such as copper, are well known in the art. Examples of such furnaces are seen in: U.S. Pat. Nos. 3,199,977; 3,701,517; 3,715,203; 3,788,623; and in the prior art patents cited in each of them.

Generally these furnaces have a substantially cylindrical shape and are elongated in a vertical direction. The metal to be melted, such as copper cathode pieces having a low oxide content, is charged into the furnace from an elevated position. The metal drops toward the bottom of the furnace, where a plurality of burners inject hot gases into the melting chamber to cause the metal to melt. The molten metal is drained from the furnace by a suitable outlet near the bottom in order to continuously supply the molten metal to a holding furnace or to a casting operation.

The burners are usually arranged in one or more rows surrounding the lower portion of the furnace, in order to define a melting chamber, and are directly affixed into the furnace walls. Each of a plurality of burners, all fed fuel from one common source, injects a fuel and air mixture into a melting chamber causing a highly turbulent flame to impinge on that metal directly adjacent each burner. Refractory tunnel type burners are known in the art as means for supplying a high temperature blast to a furnace. Typically, the throat mix type of burner is used in the prior art furnaces since they do not experience some of the problems common to a premix type burner such as backfires in the supply manifolds or flameouts, that is, isolation of the flame from the combustion ports. However, the throat mix burners of the prior art have disadvantages also. Throat mix burners must have a very turbulent high velocity flame to ensure adequate mixing of the fuel and air in the short space allotted within the burner. This results in a high operating noise level and very severe service conditions which deteriorate the furnace and burner refractories. When the deterioration reaches a certain state the operating efficiency of the burner and furnace is so adversely affected that reconditioning is required. Specifically, the deterioration has resulted from spalling, slagging, abrasion, or some combination of these. Spalling may be defined as the physical break-down or deformation or crushing of the refractory attributed to thermal or mechanical or structural causes. Slagging is the destructive action that occurs in the refractory due to chemical reactions occuring at the elevated temperatures involved. Abrasion is considered to be the deterioration of the refractory surfaces by the scouring action of solids moving in contact therewith. The solids may be carried by or formed in the combustion gases.

It is generally considered that in the most efficient types of refractory tunnel burners the refractory has good insulting properties, high heat resistance, and a rough interior surface texture. After the burner is lighted the refractory is heated and thereafter serves to maintain ignition. The roughness of the refractory surface causes the gases flowing adjacent thereto to be slightly turbulent and therefore exert a catalytic effect upon and consequently accelerate the combustion process. However, refractories which have good insulating properties and a rough surface also tend to have less resistance to the abrasive effects of the high velocity combustion gases and therefore experience much faster wear than a more dense, smooth refractory, such as silicon carbide. Another disadvantage of prior art burner arrangements is that when the combustion products are not properly mixed within the burner and before entering the furnace they have an uneven, unpredictable effect on the melting process, especially when operated over a varying range of melting rates which is necessary when supplying molten copper to a variable rate continuous casting system.

In summary, the main problem heretofore encountered with the prior art vertical furnace and burner combinations is that it is sometimes metallurgically unsuccessful when adapted to melt copper cathodes and is used to supply molten copper to a continuous casting and rolling process which is intending to produce electrical conductor grade copper bars. Part of the problem is that the molten copper becomes contaminated with unacceptable amounts of impurities. For example, oxygen and sulphur which are easily introduced into the molten metal from the combustion process, have a detrimental effect on the subsequent rolling of the cast copper into bars. Also, slags and metallic contaminants can be introduced into the melt which thereafter have a detrimental effect on the quality or conductivity of the final product. Thus, although vertical furnaces and various types of burners are well known in the art, significant needed improvements therein have been made by the present invention.

It is therefore the main object of this invention to provide an improved vertical furnace and burner structure which is suitable for continuously melting copper and which substantially avoids some disadvantages of prior art furnaces and burners. Another object of the invention is to improve the chemical composition of the product and render the same to more exact control, by increasing the uniformity and predictability of the process. It is another object of this invention to provide an improved refractory tunnel burner in which the combustion of a premixed combustible gas mixture and the operational efficiencies are enhanced and also providing a relatively low operating noise level with good service life.

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention, objects, features and advantages thereof will be better understood from the following description taken in connection with the accompanied drawings in which:

FIG. 1 is a partial elevational view of a vertical shaft furnace useful for melting pieces of metal;

FIG. 2 is an enlarged exterior view of the lower portion of the furnace showing the fuel/air manifolds in communication with the burners;

FIG. 3A is a longititudinal sectional view of one burner assembly showing the nozzle mounted to the refractory combustion chamber; and

FIG. 3B is an end view of the burner nozzle as seen from the hot side.

The vertical melting furnace and burner apparatus of the invention is comprised of the major parts: a refractory lined furnace, rows of burners situated around the furnace's lower circumference, manifolds supplying a fuel and air mixture to the burners, and mixers for forming and regulating the combustible mixture.

The furnace of FIG. 1 is vertically elongated, the upper end being open to receive the metal loaded for melting and the bottom end closed forming the furnace floor. The outer metallic wall supports and controls the inner wall which is of a refractory material, such as fire brick, capable of withstanding the temperatures involved in melting copper, for example, and defines the cylindrical melting chamber.

The furnace floor is a "V" shaped trough formed of a refractory material and is inclined such that the molten metal flows by gravity down the sides of the trough and down the trough incline to the lowest point on the furnace floor, where a tap hole 10 is located to drain off the molten metal.

Two or more rows of eight burners substantially are equispaced on the furnace circumference. They communicate with the melting chamber through ports 20 piercing both walls and melt the metal within by direct contact with the streams of hot gases from the novel burners. The burners are affixed to the outer containment by bolting 21 or welding or other means. Their longitudinal axes are inclined at a slight angle from the horizontal and intersect the furnace longitudinal centerline, the lower row of burners being situated such that the bottoms of their refractory tiles are just above the furnace floor. In this configuration the hot products of combustion expelled by the bottom row of burners continuously wash the furnace floor clean of frozen metal and slag.

FIG. 3A shows a flame retention burner of the invention in section. A combustible gaseous fuel and air mixture enters nozzle body 30 under pressure. Nozzle 31 delivers the mixture, ignited by sparkplug 32 or other means, to the combustion chamber and is adapted to avoid backfire into the supply. An annular series of holes 33 formed through the nozzle lip communicates with the cutaway space 34 surrounding the nozzle end downstream and serve to retain the flame at the nozzle. The lip 35 extending from the cutaway outside diameter to the point where the nozzle body necks up to the slightly greater diameter of the combustion chamber 36 adapted to contain flames of high velocity.

The combustion chamber 36 is advantageously cylindrical and straight in size or restricted, formed of refractory tile and allowing substantially complete combustion of the fuel and air mixture such that essentially only products of combustion exit it to contact the metal in the melting chamber. The refractory tile enhances combustion and gives the mixture time to burn completely, allowing greater control over combustion products entering the furnace and making the melting process uniform and predictable, particularly when a wide range of melting rates is required.

Due to the fact that no mixing of fuel and air occurs in the burner structure, the burner of the invention is simple in design and produces a less turbulent flame than the usual throat mix burner, there being no extra turbulence induced at the burner to mix the fuel with air. The lack of mixing turbulence results in two improvements: quieter operation, as the turbulent mixing component of the operational noise is not present, and less refractory wear because the burner output is a flame of less turbulence.

Manifolds 11 deliver the premixed fuel and air to the burners, arranged so that there are relatively few burners per manifold--4 burners per manifold is the preferred embodiment--to prevent flashback into the air and fuel mixture. To increase furnace size more manifolds and burners in the above numerical relation must be added.

A mixing station (not shown) is provided for each manifold. A suitable design is that of a venturi mixer, well known in the prior art (for example U.S. Pat. No. 3,799,195), wherein mixing is accomplished when air under pressure passes through a venturi and fuel is injected into the air stream at the low pressure in the venturi throat. Mixture proportioning is set by proportional inline orifices or valves in the fuel and air supply lines preferably in conjunction with orifice flow measuring equipment, all well known in the art. A most suitable method of controlling the fuel mixture is disclosed in U.S. Pat. No. 4,239,191 assigned to the assignee of the present invention which is incorporated herein by reference.

In the preferred embodiment, the furnace operates under slightly reducing conditions, i.e., 5 to 10 percent excess fuel over stochiometric, as adjusted by the mixers. Due to the fact that the burner design allows essentially complete combustion within the combustion chamber, the melting chamber atmosphere can be closely maintained in the reducing state, avoiding the introduction of excess oxygen to the copper therein.

Without further analysis, we believe the foregoing will fully reveal the essence of the present invention so that others can, by applying current knowledge or reasonable experimentation, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and scope of equivalence of the following claims.