Abstract:
An apparatus and a method for melting solid metals, including iron, are provided. The apparatus includes a vertical shaft furnace through which the metal is fed. The shaft furnace is mounted on a horizontal induction furnace containing a molten pool of metal. Metal solids are charged at the top of the vertical shaft furnace down onto a refractory pedestal in the molten metal pool or suspended magnetically in the vertical shaft above gas burners. The metal is melted by contact with the molten metal pool. The combination of oxygen fuel burner preheating and induction melting creates an extremely efficient melting unit that can process metal at a much lower cost than conventional systems while preventing the oxidation problems incurred by apparatuses which melt metal using combustion only. Because of the compact nature and the low stack velocity of the apparatus, almost all charged materials will convert into molten metal or slag, making the cleaning of the vertical shaft effluent to meet government air quality standards much easier than with conventional methods.

Description:
FIELD OF THE INVENTION 
     This invention relates to methods and devices for processing metals, and in particular, methods and devices for melting steel and/or iron objects efficiently. 
     BACKGROUND OF THE INVENTION 
     Conventional and commercially available melting systems include reverberatory, crucible, open hearth, cupola, electric induction, electric channel, electric arc, fuel assisted electric arc, conventional cupola, gas/oil fired vertical shaft furnaces with water-cooled grates and fuel-fired rotary drum furnaces. While all of these systems can and do operate to melt and superheat metal, there is room for improvement of each of these systems with respect to cost of installation, adaptability to a wide selection of fuels, cost of operation, cost of maintenance, flexibility of operation, equipment size, control of metallurgical properties, degree of potential pollution, installation cost of pollution controls and cost of operation of pollution controls. 
     Cupola melting units are efficient, low-cost melting devices especially in high ton-per-hour (over 50) units; however, they have a number of drawbacks. Using coke as the primary fuel, cupola operations are undesirably dependent on coke prices and availability. Because of the sulphur content of the coke, cupola melted irons can have excessive sulphur levels that are a problem in the production of ductile irons. Cupola off-gases are especially toxic and expensive to clean in order to meet governmental air quality standards for sulphur and particulate emissions. Cupolas are difficult to operate on an intermittent basis. When stopped, the delicate temperature equilibriums necessary for effective operation are destroyed and metallurgical control is made extremely difficult. 
     It is known to melt a charge in a vertical shaft furnace by feeding the charge into the top of the shaft furnace to form a column of charge in the furnace, and then melting the column from below with a flame. See, e.g., U.S. Pat. No. 5,224,985 to Kullik et al., U.S. Pat. No. 4,877,449 to Khinkis, U.S. Pat. No. 4,097,028 to Langhammer and U.S. Pat. No. 1,713,543 to Machlet. 
     U.S. Pat. No. 5,560,304 to Duchateau et al. discloses a furnace which employs a gas oxygen burner on a rotating barrel furnace; however this furnace is large and operates in a batch mode rather than continuously. 
     Vertical shaft furnaces with water-cooled grates operate successfully, but also have a number of drawbacks. The water-cooled pipes or grates have the potential for damage and leaking in such an environment. Water and water vapor are extremely detrimental to metallurgical properties of metal in the molten, superheated state of such a furnace. Further, the flame temperature and the metal temperature difference and therefore the efficiency of the system suffers as the operator attempts go above 2550° F. Metal in the molten condition at such temperatures is also highly susceptible to undesired oxidation. Metal is then guided into some form of electric furnace for superheating to temperatures required for treatment, pouring, processing or casting. This method is inefficient from an energy perspective and further exposes the molten metal to oxidizing conditions. Silicon losses can go as high as 1% of the charge weight, a loss that can be very expensive as the lost silicon must be replaced. Oxidized irons also exhibit inferior metallurgical properties. 
     Electric melting, whether it be induction, induction channel or arc, is an efficient and effective way to process metal. Recovery of ingredients and additives is nearly 100%, as there is little oxidation since the metal charges are preheated primarily for removing water and for a small assist in melting efficiency. Arc furnaces can be fitted with fuel burners to assist in the melting down of the charge and are highly efficient melters. However, installation of dust collectors, which is required for electric furnaces, is a major expense. Further, installation of a large electric furnace is very costly. In terms of overall efficiency from a macroeconomic perspective, electric furnaces are not necessarily very efficient, since only 22% of the coal burned in a power plant is typically converted to electric power, which can then be used to power an electric furnace. The remainder is wasted heat and excessive production of CO 2  gases. 
     Thus, there has been a need for more efficient furnaces for melting metals, which are less costly to install and maintain. 
     All references cited herein are incorporated herein by reference in their entireties. 
     SUMMARY OF THE INVENTION 
     The invention addresses the foregoing deficiencies of the prior art by providing a method for melting a metal, the method comprising: 
     providing an apparatus comprising: 
     a vertical shaft furnace having a top portion, an intermediate portion, a bottom portion and at least one shaft furnace heating device preferably selected from the group consisting of a gas burner and electric arc generator; and 
     a horizontal induction furnace in fluid communication with, and at least partially below, said shaft furnace; 
     feeding said metal into said top portion of said shaft furnace to form a column of said metal within said shaft furnace; 
     preheating metal in a bottom of said metal column with said at least one shaft furnace heating device to a temperature below a melting temperature of said metal; 
     contacting said preheated metal with a molten pool of metal in said induction furnace to melt said metal; and 
     removing from said apparatus at least a portion of said molten pool of metal. 
     The invention also provides an apparatus for melting a metal by the foregoing method, and an exhaust fumes treatment apparatus for treating the fumes generated in the method. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in conjunction with the following drawings in which like reference numerals designate like elements and wherein: 
     FIG. 1 is a partial cross-sectional view of an embodiment of a melting apparatus according to the invention; 
     FIG. 2 is a partial cross-sectional view of another embodiment of a melting apparatus according to the invention; 
     FIG. 3 is a partial cross-sectional view of another embodiment of a melting apparatus according to the invention; and 
     FIG. 4 is a cross-sectional view of an exhaust gas recycling system according to the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The apparatus of the present invention is essentially a fuel fired vertical shaft furnace married to a horizontal electric induction furnace, whereby the efficiencies of each are maximized in the melting of metal without adversely affecting the metallurgical properties of the charged material. 
     Melting is accomplished using electrical energy. The preheated melted column sits in a shallow bath of molten metal supported on a pedestal or suspended magnetically in the shaft. At that point, almost eighty percent (80%) of the energy required for the melting process is in the charge. Therefore, the relatively more expensive electrical energy is preferably used where it is most effective and where fuel systems are least effective. This combination allows the apparatus of the invention to operate at less than half of the cost of electric melt systems and substantially less than other systems. Because of its relatively small size, it is less expensive and easier to install than other systems. By using a short stack design, the invention can be easily shut down if metal is not needed without affecting metallurgical properties or process efficiency. 
     Dusts or additives can be easily injected into the melted metal using the apparatus of the invention. Carbon, in the form of coke or coal, can be added in the stack for carbon pickup by the charged metal or to assist in controlling oxidizing conditions. 
     By providing the induction furnace on its side, there is a highly efficient transfer of the electrical energy into the metal. Furthermore, keeping the induction furnace constantly filled with molten metal prevents the furnace refractory from being exposed to air and therefore enables the induction furnace of the inventive apparatus to operate up to a year without having to be relined as often as conventional induction furnaces. 
     The combustion system can use special combinations of fuel and oxygen to create a reducing atmosphere and stratified temperature gradient that allows high temperature preheating without oxidation of silicon and reduced oxidation of iron during processing. 
     As shown in FIGS. 1-3, preferred embodiments of an apparatus  1  for performing the method of the invention comprise a vertical shaft furnace  2  having a top portion  3 , an intermediate portion  4  and a bottom portion  5 . Bottom portion  5  adjoins, and is in fluid communication with, a horizontal induction furnace  6 . Induction furnace  6  is at least partially below shaft furnace  2 , and extends horizontally away from an axis defined by shaft furnace  2 . 
     In the embodiment of FIG. 1, shaft furnace  2  tapers inwardly from bottom portion  5  to top portion  3  at a taper angle of about 5°. In the embodiment of FIG. 2, shaft furnace  2  flares outwardly below the intermediate portion  4 , in trumpet-like fashion. Shaft furnace  2  can be constructed of any material known to be durable when subjected to the temperatures employed in the method of the invention discussed below, such as, e.g., aluminum oxide, magnesium oxide and refractory carbons. 
     Shaft furnace  2  is substantially vertical and induction furnace  6  is substantially horizontal. For present purposes, “vertical” means that the central axis of the shaft furnace is within ±10° of being perfectly plumb, and “horizontal” means that the central axis of the induction furnace is within ±10° of being perfectly parallel to the ground. 
     In embodiments of the method of the invention, a metal  7  is fed through an inlet  8  in the top portion of shaft furnace  2 . The apparatus and method are particularly suited for melting ferrous objects, more particularly, steel and iron objects. 
     Metal  7  passes downward through shaft furnace  2 , filling the internal cavity of shaft furnace  2  to form metal column  77 , until metal  7  encounters resistance sufficient to oppose the force of gravity pulling metal  7  downward. 
     In the embodiment depicted in FIG. 1, resistance is provided by water-cooled magnets  9  embedded within intermediate portion  4  of the shaft furnace  2 . Magnets  9  are separated from metal  7  by water-cooled plates  79 . Magnets  9  attract and prevent metal  7  from falling unimpeded into induction furnace  6 . The metal column  77  formed above magnets  9  is heated from below by at least one burner  10 . Burner  10  preferably burns an oil or gas and oxygen mixture, and is located sufficiently below the column of metal  7  to avoid contacting the column of metal  7  with flame  11 . This arrangement allows for uniform heating of the charge without direct flame impingement. 
     Instead of burner  10 , a plasma arc generator  300  can be used to heat metal  7 , as shown in FIG.  3 . Plasma arc generators are described, e.g., in U.S. Pat. Nos. 4,309,170; 3,673,375; 3,194,941; 3,147,330; and 2,922,869. 
     In the embodiment of FIG. 1, metal  7  at the bottommost portion of metal column  77  is heated by burner  10  sufficiently to lose enough of its magnetic properties to fall further down shaft furnace  2  past burner  10  and into a pool  12  of molten metal  7  in induction furnace  6 . It is preferred to heat metal at the bottommost portion of metal column  77  to a temperature below its melting point. This helps to prevent undue oxidation of metal  7 . In the case of iron, it is preferred to heat the metal column bottom to about 1500° F., at which temperature pieces of iron drop from the column bottom and fall into pool  12 . In general, it is preferred to heat the bottom of metal column  77  to a temperature within about 100° F. of the melting temperature of said metal. 
     It is preferred to maintain pool  12  at a temperature approximately at or just above the melting point of metal  7 . Raising the temperature of pool  12  significantly above the melting point of metal  7  serves no useful purpose and is an energy drain on the system. In the case of iron, it is preferred to heat pool  12  to a temperature of about 2750° F. Pool  12  is heated with coreless induction coils  13 , or the like, which are preferably water-cooled. 
     In addition to heating the bottommost portion of the metal column, heat and combustion gases rise through the shaft furnace, preheating distal portions of the metal column. The preferred temperature of the gas as it leaves the shaft furnace through outlet  14  is about 1600° F. A temperature probe (not shown) can be used to monitor this exhaust temperature. 
     By using at least one oxygen burner  10  designed to process any type of fuel including methanes, oils and carbons, the vertical shaft furnace  2  operates with a reducing atmosphere and a high flame temperature at the bottom of the shaft. As the gases rise through the charge burden (i.e., metal column  77 ), preheated air or more oxygen can be injected into the stream to complete combustion of the gases higher in shaft furnace  2 , thereby recovering the heat potential without oxidizing the metal. Heat can be recaptured by an air-to-air preheater in outlet  14  and reinjected into the upper regions of shaft furnace  2  to complete combustion of the reducing gas where, with the lower temperatures of the metal charge, the resulting oxidizing atmosphere will not attack metal  7  in the charge. 
     The heating method controls the rate of metal oxidation and the condition of elements, such as silicon and manganese, present in the metal matrix. By careful selection of fuels, conditions within shaft furnace  2  can be created whereby elements can be reduced from their oxides (e.g., sand, SiO 2 , can be reduced to silicon) . Alternative fuels and additives may be injected into the lower combustion area of shaft furnace  2  to regulate and control the metallurgical properties of the melted metal  7 . 
     By primarily using oxygen in the lower part of shaft furnace  2  rather than air, no nitrous oxides will be formed in this process. Furthermore, shaft gas volumes are only 20% of what they would be with air combustion units. The resulting low shaft gas velocity allows small particles to be charged without being blown back out of the shaft. 
     Slag  15  generated by the process can be separated from the molten pool of metal  7  through a slag outlet  16  at the base of shaft furnace  2 . Most of the dust and dirt is converted to slag  15 , removed from slag outlet  16  and can be sold for use in construction of roads or in making concrete. The apparatus  1  can be designed to have an almost pure CO 2  output, which allows for inexpensive conversion of the gas stream to marketable byproducts, such as limestone or baking soda, as discussed in further detail below. 
     FIG. 2 depicts an embodiment in which most of metal column  77  is supported above pool  12  by pedestal  17 , which is supported by induction furnace  6  and submerged in pool  12 , at least intermittently. The bottommost portion of metal column  77  is submerged in pool  12  and slag  15 , which melts the bottommost portion of metal column  77 . As the bottommost portion of metal column  77  melts, metal  7  in metal column  77  advances downward continuously. The depth of the molten metal covering the bottommost portion of metal column  77  is varied according to the melt rate and whether more energy is desired from the fuel or electric systems. Preferably, about 2 to about 12 inches of molten metal cover the bottommost portion of metal column  77 . 
     In the present context, the term “continuously” as used in expressions such as “advances downward continuously” is not intended to denote an infinitely long occurrence of the activity being described, but rather is intended to denote that the activity occurs without pause to reload the apparatus between charges. That is, the term “continuously” is used in its broadest sense to distinguish the continuously operative mode of the invention from the batch mode of certain prior art. The term does not preclude periods of inactivity, as long as they are not caused by the need to recharge the apparatus with another batch of metal. 
     Pedestal  17  must be resistant to metal  7 , pool  12  and the temperatures employed in the methods of the invention. It is preferred that pedestal  17  comprise carbon or a refractory material, such as alumina. 
     A pedestal  17  can also be used in the embodiment depicted in FIG. 1 to minimize splashing if metal column  77  inadvertently falls into molten metal pool  12 . 
     In the embodiment of FIG. 2, gas burner  10  is directed at a side of metal column  77  just above pedestal  17  to preheat metal  7  before it enters pool  12  and melts. Preferably, a plurality of gas burners  10  are set with a downward slope to assist penetration of flame  11  into metal column  77 . 
     Molten metal  7  in pool  12  is drawn off from induction furnace  6  through a metal outlet  18 , so as to maintain pool  12  at a relatively constant level as metal  7  falls into pool  12  and melts therein. While the apparatus of the invention is preferably employed in a continuous melt process, a vacuum column (not shown) can be attached to the apparatus to store melted metal until the time for transfer. 
     The apparatus  1  can be mounted on wheel assemblies  19  and/or jacks  20 . 
     As mentioned above, FIG. 3 shows an embodiment in which an arc generator  300  is used to heat metal  7 . Generator  300  comprises an electrode rod  301  partially submerged within metal column  77 . Electrode rod  301  is supported by an electrode holder  302 , which is attached to a hydraulic lift mechanism  303 . Hydraulic lift mechanism  303  enables movement of electrode rod  301  relative to metal column  77 . Electrode rod  301  is in electrical communication with an electrical source  304  through an electrically conductive wire  305 . Another electrically conductive wire  305  places electrical source  304  in electrical communication with electrode pedestal  306 . An arc (not shown) is formed between electrode rod  301  and electrode pedestal  306 . 
     FIG. 3 also depicts a carousel feeding assembly  350 , which is a preferred means for feeding metal  7  into shaft furnace  2 , and is suitable for use with, e.g., the embodiments of the invention depicted in FIGS. 1 and 2 as well as in FIG.  3 . Carousel feeding assembly  350  comprises a conveyor belt assembly  351 , which feeds metal  7  onto rotating funnel  352 . Funnel  352  is rotated to facilitate the movement of metal  7  into inlet  8  at the top portion of shaft furnace  2 . A ring  353  is provided above funnel  352  to help control the flow of metal  7  into shaft furnace  2 . Ring  353  is raised to increase the flow of metal  7  or lowered to decrease the flow of metal  7  by air cylinders  354  attached to ring  353  and to the external walls of exhaust outlet  14 . The air cylinders  354  are controlled by a charge height sensor  355  (see FIG. 4) at the top of shaft furnace  2 . 
     FIG. 4 depicts an exhaust recovery system  400 , which is a preferred means for recycling the exhaust fumes generated in melting metals. The fumes flow through exhaust fumes duct  401  and are cooled by water showers  402  in duct  401 . The fumes then flow through a cooling/filtration device  403  to a fan  404 , which forces the cooled and filtered fumes through valves  405  into at least one tower  406  (the preferred embodiment of two towers  406  is shown in FIG.  4 ). The fumes, which largely consist of carbon dioxide, are sprayed with a reactive fluid from tower showers  407 . The reactive fluid comprises water and at least one reactive agent which reacts with carbon dioxide in the presence of water to form a useful solid compound. Sodium hydroxide, which reacts with carbon dioxide to form sodium bicarbonate, and calcium oxide, which reacts with carbon dioxide to form calcium carbonate, are preferred reactive agents. Water and the product of the reaction in towers  406  are transferred from towers  406  through effluent tubes  408  into a filter press system  409 , from which the solid product of the reaction is separated from water and recovered for use. 
     The exhaust fumes treatment system described above can also be used with melting devices other than the melting device of the invention, and can be adapted for use with any device which generates carbon dioxide-containing fumes or other reactive fumes. 
     While the invention has been described in detail and with reference to specific examples thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.