HK40040807A - Dc arc furnace for waste melting and gasification - Google Patents

Description

DC electric arc furnace for melting and gasifying wastes

Cross Reference to Related Applications

This application claims priority from currently pending U.S. provisional application No. 62/572,412, filed on 13/10/2017, which is incorporated herein by reference.

Technical Field

The present subject matter relates to Direct Current (DC) electric arc furnaces for waste vitrification and gasification, and more particularly to a method and apparatus for igniting and restarting a DC arc on non-conductive mixtures of metal oxides, such as those found in waste, and for providing complete melting.

Background

Plasma arc furnaces for converting waste into energy and construction materials have been proposed. More specifically, plasma furnaces have been used to melt inorganic materials in waste and to vaporize organic compounds. Plasma furnaces have several advantages over conventional incineration techniques, such as the ability to process the material independent of the inherent calorific value of the material, the ability to vitrify the inorganic components of the waste into inert slag, and the ability to convert the organic components of the waste into combustible gases, called syngas, consisting mainly of hydrogen and carbon monoxide, thus enabling the production of clean energy from the waste. Several apparatuses and methods have been proposed in connection with the use of plasma furnaces for converting waste into slag and energy.

For example, U.S. Pat. No. 5,280,757 entitled "Munic Solid Water Disposal Process" and issued on 25/1/1994 in the name of Carter et al discloses an apparatus for gasifying Municipal Solid Waste, coal, wood and sludge into intermediate quality gases and inert bulk slag with greatly reduced leachability of toxic elements using a non-transferred plasma torch.

Similarly, U.S. patent No. 4,998,486 entitled "Process and Apparatus for treating hazardous materials in a Plasma filtered Cupola" and issued 3/12 1991 in the name of Dighe et al discloses an Apparatus that also uses a non-transferred Plasma torch to treat hazardous waste in a Cupola furnace so that hazardous materials such as PCBs are volatilized and consumed in a afterburner, while hazardous materials containing heavy metals are melted in the Cupola furnace and converted to a non-leachable solid product.

However, the use of non-transferred plasma torches to vaporize and vitrify waste and other materials has several disadvantages. Due to the extreme temperatures of the plasma gases, water cooling of the non-transferred plasma torch is required. The use of water cooling in the torch reduces the thermal conversion efficiency of the torch. In many cases, the energy loss of the cooling water may reach between 15% and 35% of the electrical energy input to the torch. In addition, since the torch is typically required to protrude through a thick refractory lining wall, additional heat loss can occur from the water cooling body of the torch to such a refractory wall. Finally, in the case of a torch operating in a non-transfer mode, most of the plasma gas escapes into the furnace off-gas rather than treating solid material in the furnace. Therefore, the net efficiency of heat is typically less than 50%.

Another disadvantage of water-cooled non-diverting torches is the risk of water leakage. In some cases, torch water leakage can lead to steam explosion when the high pressure water escaping from a faulty torch hits the overheated slag inside the furnace (Beaudet et al, 2000).

Thus, the use of non-water cooled graphite electric arc furnaces has been proposed for the purpose of gasifying and vitrifying wastes. Graphite arc furnaces have several advantages over plasma furnaces using plasma torches. The graphite electrode is inherently safe compared to furnaces using torches that may generate leaks, as no water cooling is performed. Without water cooling, graphite electrodes are also much more efficient than water-cooled torches, reaching efficiencies approaching 100% in transferring energy from the arc to the bulk waste material to be treated. Graphite arc furnaces may be of the Alternating Current (AC) type or Direct Current (DC) type.

Conventional three-phase AC electric arc furnaces may not generally be used for waste gasification and vitrification purposes. Typically, AC furnaces are open top designs, limiting the ability to control the quality of the syngas produced due to the large amount of air entering the furnace. Three-phase AC furnaces cannot easily transfer current to non-conductive materials such as cold waste glass or burning ash residues. Several approaches to alleviating this problem have been proposed, and in particular some DC furnaces provide a method of switching from a non-transferred arc mode of operation to a transferred arc mode of operation, for example, in U.S. patent No. 5,958,264 entitled "Plasma gas and vision of Ashes" and issued on 28/9 1999 in the name of Tsantrizos et al. Other furnaces may operate in both an AC mode of operation and a DC mode of operation, where AC is used for joule heating of the slag and a DC ARC is used to create an ARC above the melt, such as set forth in U.S. patent No. 5,666,891 entitled "ARC Plasma-filter electro conversion System for water Treatment and Resource Recovery" and issued on 9, 16 1997 in the name of Titus et al.

In the above-mentioned us patent No. 5,666,891, a waste-to-energy conversion system and apparatus for the purpose of converting various waste streams into useful gases and a stable non-leachable solid product is described. In one embodiment, the furnace uses AC joule heating of the molten inorganic portion of the waste in conjunction with DC plasma arc in the gas phase. In this system, the plasma arc furnace and the joule heated melter are formed as a fully integrated unit having a circuit arrangement that allows both the arc plasma and the joule heated portions of the unit to operate simultaneously without interfering with each other. However, the design is complex, requiring multiple power supplies and complex circuit arrangements. There is also a risk that the AC electrodes may freeze in the slag, which makes restarting the furnace very difficult.

For example, the above-mentioned U.S. patent No. 5,958,264 discloses an apparatus for gasifying and vitrifying ashes such as those generated in a hot fuel boiler. The apparatus is a shaft furnace using two or three tiltable electrodes that can be operated in horizontal or vertical position. By changing the position of the electrodes from horizontal to vertical, the arc can be changed from a non-transferred mode to a transferred mode. However, this design has several disadvantages. For example, the electrode channels are not completely sealed and may lead to uncontrolled gasification inside the furnace. Furthermore, the heating efficiency of the slag is low and the slag may freeze in the non-transfer mode: if the slag level is too high, the plasma heat cannot be effectively transferred to the lower layer. Furthermore, the arc voltage is very unstable, depending on the varying composition of the syngas inside the furnace. Furthermore, because the electrodes are angled, the electrodes may generate an arc jet directed at the refractory material, which may cause excessive wear of the refractory material.

Accordingly, it would be desirable to provide an apparatus for gasification and vitrification of waste that ensures substantially complete melting of the slag, substantially avoids slag freezing, and improves the energy transfer from the plasma arc to the waste being processed.

Disclosure of Invention

Accordingly, it would be desirable to provide a novel apparatus for the gasification and vitrification of waste.

In one aspect, embodiments described herein provide an apparatus for gasification and vitrification of waste comprising a plasma arc furnace provided with two movable graphite electrodes, the furnace comprising an air-cooled bottom electrode adapted to divert current entirely through the slag melt, the furnace being sealed at the junction of the furnace's spool and crucible and further provided with a gas-tight electrode seal adapted to control the reducing conditions inside the furnace.

Furthermore, in another aspect, embodiments described herein provide a plasma arc furnace comprising: a bobbin and a crucible; a pair of movable electrodes made of, for example, graphite; an air-cooled bottom electrode adapted to divert the current entirely through the slag melt, the furnace being sealed at the junction of the spool and the crucible of the furnace and further provided with an airtight electrode seal adapted to control the reducing conditions inside the furnace.

Furthermore, in another aspect, embodiments described herein provide a DC arc furnace comprising: a bobbin and a crucible; a pair of movable electrodes made of, for example, graphite; an air-cooled bottom electrode adapted to divert the current entirely through the slag melt, the furnace being sealed at the junction of the spool and the crucible of the furnace and further provided with an airtight electrode seal adapted to control the reducing conditions inside the furnace.

Specifically, a circuit is also provided that is adapted to switch from a transfer mode of heating to a non-transfer mode of heating, thereby enabling the furnace to be restarted in the event that slag freezes.

Drawings

For a better understanding of the embodiments described herein and to show more clearly how the same may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings which show at least one exemplary embodiment, and in which:

FIG. 1 is a general schematic diagram illustrating the principle of operation of a furnace according to an exemplary embodiment;

FIG. 2 is a vertical cross-sectional view of a more detailed furnace, based on the furnace of FIG. 1, according to an exemplary embodiment;

FIG. 3 is a detailed vertical cross-sectional view of an electrode seal according to an exemplary embodiment;

FIG. 4 is a vertical cross-sectional view showing specific details of a bottom anode according to an exemplary embodiment; and

fig. 5a and 5b are schematic diagrams of electrical circuits of the furnace for two modes of operation according to an exemplary embodiment.

Detailed Description

With reference to fig. 1 and 2, an embodiment is shown in which the DC arc furnace F comprises two parts: a bobbin 1 and a crucible 2, both of which are refractory-lined to operate at high temperatures. The refractory material used in the crucible 2 should be compatible with the molten silicate-based material and may be made of a high alumina or alumina chrome material. The refractory material used in the bobbin 1 should be compatible with potentially corrosive high temperature gases and may be made of a high alumina or alumina-silica material. Note that the components shown in fig. 2 are part of the furnace F of fig. 1.

In normal operation, the material to be gasified and melted is continuously introduced through one or more feed ports 3 located at the top of the bobbin 1. The material being treated accumulates in the crucible 2 where it forms the top layer of partially treated waste 4. The high temperature (typically above 1400 ℃) in the furnace crucible 2 and the injection of gasification air, oxygen and/or steam separates the organic fraction from the inorganic fraction in the waste. The inorganic part melts into a liquid slag layer 5 floating on top of the molten metal layer 6. The organic portion is converted to a synthesis gas comprising mainly carbon monoxide and hydrogen or a combustion gas comprising mainly carbon dioxide and steam. The gas leaves the furnace through exhaust port 8.

The outer shell of the crucible 2 may be fitted with fins and forced air cooling to minimize refractory erosion. The purpose of the forced air cooling is to move the slag freeze line well inside the layer of liquid slag layer 5 and away from the refractory lining.

A pair of electric arcs 9a and 9b is maintained inside the furnace F. The electric arcs 9a and 9b are partially immersed in the mass of partially treated waste 4 and transferred to the liquid slag layer 5. An electric current is passed through the molten metal layer 6 and the bottom anode 10.

Two power supplies 11a and 11b are used to supply current to maintain the arcs 9a and 9 b. Note that all the components shown in fig. 1 except the power supply portions 11a and 11b are part of the furnace F. The power supplies 11a and 11b are Direct Current (DC) units, such as current control type DC units. An electric current is fed to a pair of electrodes 12a and 12b, typically made of graphite. When sized to the current carrying capacity of graphite (16A/cm2 to 32A/cm2), the graphite does not overheat and does not require water cooling. The use of graphite electrodes 12a and 12b thus solves the problem of water cooling in plasma furnaces and avoids the risk of steam explosion. The use of graphite electrodes 12a and 12b and free-burning arcs 9a and 9b inside the furnace F also ensures very high energy transfer efficiency, since there is no energy loss for water cooling. The graphite electrodes 12a and 12b used may be found in the market, with diameters ranging from a few inches to larger sizes (e.g., 32 inches). The electrodes 12a and 12b are commonly found on the market and are supplied by companies such as SGL Carbon and Graftech/UCAR.

The use of graphite electrodes 12a and 12b simplifies the upscaling process, since the size of the electrodes can be easily increased. The current carrying capacity of the electrodes 12a and 12b is proportional to the cross section of the electrodes or proportional to the square of the diameter of the electrodes. The largest electrodes have a current carrying capacity of 140kA or higher, which makes the largest electrodes suitable for large scale waste treatment applications. For example, a furnace using two 6 inch electrodes may be used to treat 10 tons per day of municipal solid waste, and would require 400kW of power and operate at 2000 amps. On this basis, two 32 inch electrodes would enable the treatment of 700 tons of waste per day in one furnace. In contrast, by using a non-transferred arc plasma torch, it would be necessary to use multiple torches to obtain the same energy. For example, to achieve the same amount of energy transfer, 37 separate plasma torches would be required, at 75% efficiency, using a torch of 1MW total power.

Referring to fig. 2, current is fed to two electrodes 12a and 12b using a pair of electrode clamps 13a and 13b, respectively. Commercially available electrodes include mechanisms for screwing them together using connecting pins. The connecting pin is a threaded connector that enables two lengths of electrode to be connected together. During normal operation of the furnace F, the graphite is gradually eroded by the electric arc 9a/9 b. The electrodes 12a and 12b are mounted on respective moving mechanisms 15a and 15b, and the respective moving mechanisms 15a and 15b slowly move the electrodes 12a and 12b downward in the furnace F as the electrodes 12a and 12b are eroded. The moving mechanisms 15a and 15b provide an up/down feature that also enables the arc voltage to be adjusted. The arc voltage is proportional to the arc length, which is proportional to the distance between the tip of each electrode 12a and 12b and the top of the liquid slag layer 5. Once a length of the electrode 12a/12b has been completely eroded, the connecting pin can be screwed in a new length from outside the furnace F.

In order to adjust the plasma power, the voltage is kept constant by adjusting the height of the electrodes 12a and 12 b. The current set point is given to the power supplies 11a and 11b with their own current control means. Power is a function of voltage multiplied by current. The temperature of the liquid slag layer 5 can be controlled by adjusting the plasma power. Plasma power can also be used to compensate for the energy requirements of endothermic reactions, such as pyrolysis reactions.

The bobbin 1 and the crucible 2 are made of two different parts. The crucible 2, which is removable from the spool 1, is provided with wheels 19 and can be lowered onto a track to be rolled up for refractory maintenance. Once maintenance is complete, the crucible 2 is replaced and the crucible 2 can be moved up and held in place using a series of tie rods 18. A series of nuts 20 on each tie rod 18 are used to lift and hold the crucible 2 in place.

Two tap holes 16 and 17 are provided to extract excess liquid slag and liquid metal from the respective liquid slag layer 5 and molten metal layer 6 of the furnace F. As more waste is fed to the furnace F, the molten inorganic material merges into the existing liquid slag layer 5. Over time, and as the waste material continues to be fed into the furnace F, the height of the liquid slag layer 5 will increase. Non-oxidized metal, which is denser than the oxidized part, will accumulate below the slag layer 5 in the liquid molten metal layer 6. The upper tap hole 16 is thus used for extracting the oxidizing slag from the liquid slag layer 5, while the lower tap hole 17 is used for extracting the metal from the molten metal 6.

Referring again to fig. 1, the oven F is completely enclosed to prevent any unwanted air from entering the oven F. Oxygen from the air can cause excessive combustion of the furnace waste and can reduce the quality of the syngas produced. A seal 14 is provided between the spool 1 and the crucible 2. The seal 14 may be made of graphite or high temperature refractory paper. Two electrode seals 14a and 14b are provided, the two electrode seals 14a and 14b preventing air from entering around the electrodes 12a and 12 b.

A detailed view of the electrode seals 14a and 14b is provided in fig. 3. Each electrode 12a/12b passes through a metal tube 21. There is a bottom plate 22 welded to the tube 21, the bottom plate 22 enabling the tube 21 to be mounted to the top of the refractory material 7 of the spool 1 via a nut 24 and a threaded rod 23 cast in the refractory material 7, the nut 24 and threaded rod 23 serving to hold the tube 21 in place with its plate 22. Attaching the electrode sealing tube 21 to the refractory material 7 instead of to the steel housing of the bobbin 1 insulates the electrodes 12a and 12b from each other and from the housing.

As described in detail below, the top flange 25 is welded to the tube 21 and is used to attach the second free moving tube 21a with a set of threaded rods, nuts and washers. Several layers of graphite rope 26 arranged on top of the fire-resistant rope 29 are used to seal the gap between the outer tube 21 and the electrodes 12a/12 b. When the seal is eroded by movement of the electrodes 12a/12b, the seal may be tightened around the electrodes 12a/12b by tightening four nuts 27 (two such nuts 27 are shown herein) around the electrodes 12a/12 b. A set of beveled washers 28 is used to prevent the nut 27 from loosening during operation. The use of fire-resistant rope 29 avoids the use of any water cooling around the seal.

As shown in fig. 4, the bottom anode 10 provides a current return path for the power used to power the arcs 9a and 9 b. The bottom anode 10 is air-cooled to avoid any risk of contact between the liquid slag and the water in case of crucible failure and thus to prevent steam explosion. The design eliminates the use of cooling water.

The bottom anode 10 is provided with one or more electrodes, which are conductive rods 31 made of metal or graphite, said conductive rods 31 being embedded in the refractory lining 30 of the crucible 2. The number of electrodes and the size of the cross-section depend on the current carrying capacity requirements of the electrodes. The conductive rod 31 may be in direct contact with the liquid slag layer 5 or in contact with the conductive plate 37. The conductive plate 37 may be made of graphite or metal such as iron or steel. In the case of metal plate 37, the metal plate typically melts during furnace operation. To ensure that the electrode itself does not melt, the electrode is externally cooled using cooling fins 33.

The conductive rods 31 are connected to copper rods 32. The copper bars 32 are mounted to the conductor bars 31 and herein in an aligned relationship. The copper bar 32 has a male thread machined and the conductor bar 31 has a female thread machined so that the conductor bar 31 and the copper bar 32 can be threadedly assembled together. Shoulders on the bars 31 and 32 ensure good electrical contact between the two parts. Copper is used for the rods 32 to provide high electrical and thermal conductivity, while a high melting point metal or graphite is used for the current conducting rods 31 to minimize the electrode melting effect near the liquid slag layer 5.

The copper bar 32 is connected to a copper plate 34. The copper plate 34 is held to the crucible 2 by a T-shaped metal support 35 embedded in the refractory material of the crucible 2. The copper plate 34 is bolted to the T-shaped support 35. The fact that the support 35 is embedded in the refractory material without contact with the metal housing ensures that the entire bottom anode 10 remains electrically floating and not at the same potential as the grounded crucible housing.

The copper bars 32 are connected in parallel. Copper plate 34 is connected to the DC cable by lugs 38. Cooling fins 33 made of copper or aluminum are used to maximize the heat transfer surface to the copper bars 32.

Forced air cooling is used to cool the fins 33. A plenum 36 is provided to force air around the fins 33 to circulate. A low pressure blower (not shown) is used to feed cooling air to the plenum 36. The plenum chamber 36 is held to the bottom of the crucible 2 by a set of bolts threaded into the crucible housing. The plenum 36 may be provided with baffles (not shown) to ensure optimal distribution of air to the cooling fins 33.

As illustrated in fig. 5a and 5b, a circuit and method for switching between a transferred arc mode of operation and a non-transferred arc mode of operation in a furnace is provided.

The transfer mode of operation is shown in fig. 5 a. In the transfer mode of operation, current is transferred between each cathode 12a and 12b to the bottom anode 10. The current for the left circuit is provided by the power supply PS 111 a. The contactor CON3 is closed and the contactor CON1 remains open. The current for the right circuit is provided by power supply PS 211 b. The contactors CON2 and CON4 are closed.

The non-transfer mode of operation is shown in fig. 5 b. In the non-transfer mode of operation, current is transferred between the cathode 12a and the electrode 12b, which acts as a cathode. A separate power supply PS 111 a is used to drive the arc. In this case, the contactors CON2, CON3, and CON4 are open, and the contactor CON1 is closed.

Methods for restarting the oven F in case of a process anomaly (upset) and for switching between a non-transfer mode of operation and a transfer mode of operation are also provided. In case of a process anomaly and the liquid slag layer 5 is frozen, the transfer mode to the bottom anode 10 is not possible, since the frozen slag will not conduct electricity. In this case, a conductive material such as graphite powder or metal dust may be fed between the electrodes 12a and 12 b. The electrodes 12a and 12b are lowered to contact the conductive material. Once the circuit has been initiated, the non-transferred mode of operation may be used to slowly move the electrodes 12a and 12b upward and create an arc between the electrodes 12a and 12 b. It is desirable to switch to the transfer mode of operation quickly because this mode is more efficient in transferring energy to the bulk of the waste to be treated. In this case, referring to fig. 5b, the contactor CON3 is closed and the current through the wire next to the contactor CON3 is monitored using an ammeter. Once the current begins to pass through the wire, CON1 opens, forcing all power to be transferred and passed through the bottom electrode 10. Once the transferred arc mode has stabilized, the power supply PS 211 b is energized and the contactors CON2 and CON4 are closed, returning to the normal transferred operation mode.

To stabilize the arc in the transferred arc mode of operation, a hollow electrode may be used and a plasma of forming gas is injected into the electrode. The gas is preferably a monatomic gas (e.g., argon or helium) or a mixture of monatomic gases.

Although the above description provides examples of embodiments, it will be appreciated that some features and/or functions of the described embodiments may be susceptible to modification without departing from the spirit and principles of operation of the described embodiments. Thus, what has been described above is intended to be illustrative of embodiments and not restrictive, and it will be understood by those skilled in the art that other variations and modifications may be made without departing from the scope of embodiments of the invention as defined in the appended claims.

Reference to the literature

[1]G.O.Carter and A.Tsangaris,“Municipal Solid Waste DisposalProcess”,United States of America Patent No.5,280,757,January 25^th1994.

[2]S.V.Dighe,R.F.Taylor,R.J.Steffen and M.Rohaus,“Process andApparatus for Treatment of Excavated Landfill material in a Plasma FiredCupola”,United States of America Patent No.4,998,486,March 12^th1991

[3]R.A.Beaudet et al.,“Evaluation of Demonstration Test Results ofAlternative Technologies for Demilitarization of Assembled Chemical Weapons-ASupplemental Review,Committee on Review and Evaluation of AlternativeTechnologies for Demilitarization of Assembled Chemical Weapons”,NationalResearch Council,(2000)

[4]P.G.Tsantrizos,M.G.Drouet and A.Alexakis,“Plasma Gasification andVitrification of Ashes,United States of America Patent No.5,958,264,September28^th1999

[5]C.H.Titus,D.R.Cohn and J.E.Surma,“Arc Plasma-Melter ElectroConversion System for Waste Treatment and Resource Recovery”,United States ofAmerica Patent No.5,666,891,September 16^th1997

Claims (35)

1. A furnace for the gasification of waste, the furnace being fully enclosed and sealed to control the gasification environment.

2. A furnace that is fully air cooled to avoid any risk of water leakage and steam explosion due to water cooling circuit failure.

3. A circuit enables an oven to operate in both a non-transferred arc mode of operation and a transferred arc mode of operation, and to switch between the non-transferred mode and the transferred mode.

4. A method of operation restarts an arc in the event of a process anomaly.

5. A plasma arc furnace, comprising: a bobbin and a crucible; a pair of movable electrodes made of, for example, graphite; an air-cooled bottom electrode adapted to divert current entirely through the slag melt, the furnace being sealed at the junction of the spool and the crucible of the furnace and further provided with an air-tight electrode seal adapted to control reducing conditions inside the furnace.

6. The plasma arc furnace of claim 5 wherein an electrical circuit is provided that is adapted to switch from a transfer mode of heating to a non-transfer mode of heating, thereby enabling the furnace to be restarted if slag freezes.

7. A DC electric arc furnace, comprising: a bobbin and a crucible; a pair of movable electrodes made of, for example, graphite; an air-cooled bottom electrode adapted to divert current entirely through the slag melt, the furnace being sealed at the junction of the spool and the crucible of the furnace and further provided with an air-tight electrode seal adapted to control reducing conditions inside the furnace.

8. The DC arc furnace of claim 7, wherein both the spool and the crucible are refractory-lined to operate at high temperatures; the refractory material used in the crucible is for example compatible with molten silicate-like materials and can be generally made of high alumina or alumina chrome materials; the refractory materials used in the spools are, for example, compatible with potentially corrosive high temperature gases and can generally be made of high alumina or alumina-silica materials.

9. The DC arc furnace of any of claims 7 and 8, wherein the material to be gasified and melted is introduced into the furnace generally continuously through at least one feed opening located at the top of the spool.

10. The DC arc furnace of any of claims 7 to 9, wherein the material being processed is adapted to accumulate in the crucible, forming a top layer of partially processed waste there.

11. The DC electric arc furnace of any of claims 7 to 10, wherein the high temperature in the crucible, typically above 1400 ℃, and the injection of gasification air, oxygen and/or steam, separate the organic part from the inorganic part of the waste, wherein the inorganic part melts into a liquid slag layer floating on top of the layer of molten metal; and wherein the organic fraction is converted into a synthesis gas comprising mainly carbon monoxide and hydrogen or a combustion gas comprising mainly carbon dioxide and water vapour, said synthesis gas being adapted to leave the furnace through a vent.

12. The DC arc furnace of any of claims 7 to 11, wherein the outer shell of the crucible is fitted with fins and forced air cooling adapted to move slag freeze lines well inside the layer of liquid slag layer 5 and away from the refractory lining.

13. The DC arc furnace of any of claims 7 to 12, wherein a pair of electric arcs is maintained inside the furnace and is partially submerged in the volume of partially processed waste and transferred to the liquid slag layer, an electric current passing through the layer of molten metal and the bottom anode.

14. DC arc furnace according to any of claims 7 to 13, wherein a pair of power supplies are adapted to provide current to maintain the arc, the power supplies being Direct Current (DC) units, such as current-controlled ones; wherein an electric current is fed to the pair of electrodes, typically made of graphite.

15. The DC arc furnace of any of claims 7 to 14, wherein a pair of electrode clamps is used to feed current to the two electrodes.

16. DC arc furnace according to any of claims 7 to 15, wherein the electrodes comprise connection pins, typically threaded connectors, to enable two lengths of electrode to be connected together so that once a length of electrode has eroded it can be screwed in a new length from outside the furnace using the above-mentioned connection pins.

17. The DC arc furnace of any of claims 7 to 16, wherein the electrodes are mounted on respective moving mechanisms adapted to move the electrodes slowly downwards in the furnace F as they are progressively eroded by the arc.

18. The DC arc furnace of claim 17 wherein the movement mechanism provides an up/down feature that also enables the arc voltage to be adjusted.

19. The DC arc furnace of any of claims 7 to 18, wherein, for adjusting the plasma power, the voltage is kept constant by adjusting the height of the electrodes; wherein a current set point is given to the power supply section provided with a current control device; wherein the temperature of the liquid slag layer is adapted to be controlled by adjusting the plasma power; and wherein the plasma power is adapted to compensate for the energy requirements of the endothermic reaction, such as the pyrolysis reaction.

20. The DC arc furnace of any of claims 7 to 19, wherein the spool and the crucible are made of two different parts, wherein the crucible is adapted to be detached from the spool.

21. The DC arc furnace of claim 20 wherein the crucible is provided with wheels and is adapted to be lowered onto a track using, for example, a series of tie rods and to be raised to a return position; the crucible is provided with a series of nuts on each pull rod 18 which are normally used to lift and hold the crucible in place.

22. The DC electric arc furnace of any one of claims 7 to 21, wherein a pair of upper and lower tap holes are provided to extract excess oxidizing slag and liquid metal from the respective layers of liquid slag and molten metal of the furnace, respectively.

23. The DC arc furnace of any of claims 7 to 22, wherein the furnace is substantially completely enclosed to prevent any unwanted air from entering the furnace; wherein a sealing member is provided between the spool and the crucible, the sealing member being made of, for example, graphite or high temperature refractory paper.

24. The DC arc furnace of any of claims 7 to 23, wherein an electrode seal is provided around each of the two electrodes and outside the bobbin.

25. The DC arc furnace of claim 24, wherein each electrode extends through an outer tube fixed to the refractory of the spool, for example via a nut and a threaded rod cast in the refractory, the nut and the threaded rod being used to hold the tube in place.

26. The DC arc furnace of claim 25, wherein a layer of graphite rope arranged on top of the refractory rope is used to seal the gap between the outer tube and the electrode.

27. The DC arc furnace of any of claims 25 to 26, wherein a moving tube is provided on top of the layer of graphite rope and is adapted to be lowered on the layer of graphite rope using, for example, a set of threaded rods, nuts and washers, so that when the seal is eroded by movement of the electrode, the seal can be tightened around the electrode by lowering the moving tube relative to the layer of graphite rope.

28. The DC arc furnace of any of claims 7 to 27, wherein the bottom anode provides a current return path for power used to power the arc; wherein the bottom anode is air-cooled to avoid any risk of contact between the liquid slag and water in case of crucible failure and thereby prevent steam explosion.

29. The DC arc furnace of any of claims 7 to 28, wherein the bottom anode is provided with one or more electrodes, which are electrically conductive rods, typically made of metal or graphite, embedded in the refractory lining of the crucible; wherein the electrically conductive rod is, for example, in direct contact with the liquid slag layer or in contact with an electrically conductive plate made of, for example, graphite or a metal such as iron or steel.

30. The DC arc furnace of claim 29, wherein the electrode of the bottom anode is externally cooled using, for example, cooling fins to avoid melting of the electrode, since the metal plate normally melts during furnace operation.

31. The DC arc furnace of any of claims 29 to 30, wherein the electrically conductive rod is generally threadedly connected to a copper rod in aligned relation; wherein shoulders are typically defined on the collector bars to ensure good electrical contact between the collector bars.

32. The DC arc furnace of claim 31 wherein the copper rods are connected together with a copper plate 34, the copper plate 34 being held to the crucible by a T-shaped metal support embedded in the refractory material of the crucible.

33. The DC arc furnace of any of claims 31 to 32, wherein the copper rods are connected in parallel and the copper plates are connected to a DC cable by lugs; and wherein the cooling fins are made of copper or aluminum to maximize the heat transfer surface to the copper bar.

34. The DC arc furnace of any of claims 30 to 33, wherein the cooling fins are cooled using forced air cooling, a plenum being provided to force air circulation around the cooling fins.

35. The DC arc furnace of claim 34, wherein a low pressure blower is provided to feed the cooling air to the plenum; wherein the plenum chamber is typically held to the crucible bottom by a set of bolts threaded into the crucible housing; and wherein the plenum is provided with, for example, baffles to better distribute air to the cooling fins.