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Title:
BRUSH-ARC FURNACES AND A METHOD OF PROCESSING ORES
Document Type and Number:
WIPO Patent Application WO/2012/032421
Kind Code:
A1
Abstract:
A brush-arc furnace 10 comprises a vessel 12. The vessel defines a chamber 24. The chamber holds a burden 28 comprising a body 30, preferably comprising a ferrochrome alloy, and a layer 32 on the body comprising slag. The layer has an upper surface 34. At least two electrodes 22.1, 22.2, each having a diameter d, extend into the chamber to a level between a first limit I1 above the surface 34 and a second limit I2 below the surface. The first limit is smaller than 0.5d and the second limit is smaller than 0.3d. An AC power supply 26 or DC power supply 126 drives the at least two electrodes into brush-arc operation.

Inventors:
GREYLING, Hendrik, Willem (Farm 375, Bankfontein, 1055 Middelburg, ZA)
GREYLING, Frederik, Petrus (53 Coetzee Street, 1050 Middelburg, ZA)
Application Number:
IB2011/052428
Publication Date:
March 15, 2012
Filing Date:
June 02, 2011
Export Citation:
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Assignee:
GREYLING, Hendrik, Willem (Farm 375, Bankfontein, 1055 Middelburg, ZA)
GREYLING, Frederik, Petrus (53 Coetzee Street, 1050 Middelburg, ZA)
International Classes:
F27B3/08; C21C5/52
Domestic Patent References:
Foreign References:
DE4342511A1
Other References:
W.S. Steinberg: "Development of a control strategy for the open slag bath furncase at Highveld Steel and Vanadium Corporation Ltd., MEng dissertation, University of Pretoria, Pretoria,", December 2008 (2008-12), XP000002658237, page 13 - page 16
None
Attorney, Agent or Firm:
DM KISCH INC. (P O Box 8, 2146 Sandton, 78121, ZA)
Download PDF:
Claims:
Claims

1. A brush-arc furnace comprising:

- a vessel defining a chamber holding a burden comprising a body and a layer on the body comprising slag, the layer having an upper surface;

- at least two electrodes, each having a diameter d;

- each of the at least two electrodes extending into the chamber to a level between a first limit above the surface and a second limit below the surface;

- the first limit being smaller than 0.5d and the second limit being smaller than 0.3d; and

- a power supply for driving the at least two electrodes.

2. A brush-arc furnace as claimed in claim 1 wherein the first limit is smaller than 0.3d and the second limit is smaller than 0.2d.

3. A brush-arc furnace as claimed in claim 1 or claim 2 wherein the power supply comprises an AC power supply.

4. A brush-arc furnace as claimed in claim 3 comprising three electrodes, each having a diameter of d.

5. A brush-arc furnace as claimed in claim 4 wherein the power supply comprises three single phase transformers connected in a delta configuration to the three electrodes.

6. A brush-arc furnace as claimed in claim 1 or claim 2 wherein the power supply comprises a DC power supply for driving the at least two electrodes as anode and cathode respectively.

7. A brush-arc furnace as claimed in any one of claims 1 to 6 comprising at least one feed port for charging raw materials into the chamber and at least one outlet for gasses.

8. A brush-arc furnace as claimed in any one of claims 1 to 7 wherein the vessel is cylindrical, comprises a steel shell having a bottom opposite a roof and wherein the steel shell is lined on an inside thereof with a refractory lining.

9. A brush-arc furnace as claimed in claim 8 wherein at least one of the steel shell and burden in the vessel is at earth or ground potential.

10. A brush-arc furnace as claimed in any one of claims 8 and 9 wherein the electrodes extend in an axial direction into the chamber through suitable openings in the roof.

11. A brush-arc furnace as claimed in any one of claims 8 to 10 wherein the roof is water-cooled.

12. A brush-arc furnace as claimed in any one of claims 8 to 1 wherein the electrodes are axially adjustable. 3. A brush-arc furnace as claimed in any one of claims 1 to 12 wherein each electrode comprises one of a self-baking electrode and a pre- baked graphite electrode.

14. A brush-arc furnace as claimed in any one of claims 7 to 13 wherein the at least one feed port comprises means for controlling at least one of the rate and volume of raw material fed into the chamber and wherein the at least one outlet comprises means for controlling at least one of the rate and volume of hot gas escaping from the chamber.

15. A brush-arc furnace as claimed in any one of claims 1 to 14 wherein the body of the burden comprises a ferrochrome alloy.

16. A method of processing ores comprising the steps of:

- feeding the ores into a chamber of a furnace, so that the chamber holds a burden comprising a body and a layer comprising slag on the body, the layer having an upper surface; and

- energizing at least two electrodes extending into the furnace to operate in brush-arc manner.

17. A method as claimed in claim 16 wherein the at least two electrodes are energized utilizing an AC voltage.

18. A method as claimed in claim 16 wherein the at least two electrodes are energized utilizing a DC voltage. 9. A method of processing ores as claimed in any one of claims 16 to 18 wherein the body of the burden comprises a ferrochrome alloy.

20. A method of converting a submerged arc AC furnace into a DC brush-arc furnace, the method comprising the steps of:

- replacing a set of three electrodes with a pair of two electrodes;

- causing the electrodes to extend into a chamber of the furnace to a level between a first limit above a surface of a burden in the chamber and a second limit below the surface of the burden; and

- connecting the pair of electrodes to a DC voltage.

Description:
Title: Brush-arc furnaces and a method of processing ores

INTRODUCTION AND BACKGROUND

This invention relates to brush-arc furnaces, more particularly for use in the production of ferrochrome and to a method of processing ferrochrome ores.

Submerged arc AC furnaces are known in the art. A known submerged-arc AC furnace comprises a generally cylindrical vessel comprising a closed roof through or from which three Soderberg electrodes extend axially into a chamber defined by the vessel. The electrodes have a diameter of d and typically extend to between 0.5d and 2d into a burden held in the chamber. The electrodes are connected to three single-phase furnace transformers, alternatively to a single three phase transformer that acts as AC power supply to the furnace. The vessel is provided with a refractory lining to provide protection against high reaction temperatures caused by the electric arcs created by the furnace electrodes. In use and during processing, raw materials comprising in general a combination of metallic ores, reductants and fluxes are fed into the chamber on a continuous basis, utilizing devices such as feed chutes extending through the roof. Molten alloy and molten slag (waste material) are periodically removed through one or more refractory lined tap holes in the refractory lined vessel. Hot gases emanating from the reaction in the furnace vessel are drawn off via one or more off-take ducts extending through the roof. In the known submerged-arc AC furnaces, it is generally required to have a permeable burden comprising raw materials to ensure that gasses generated during the process can flow to the top of the burden and therefore the raw materials are required to comprise a relatively large portion of lumpy ores and/or pelletized fine ores. In addition, a large percentage of carbon fed into the furnace needs to be in the form of coke and char to ensure the formation of a permeable coke bed inside the burden. These requirements place an unnecessary restriction on the shape and size of the raw materials to be processed.

OBJECT OF THE INVENTION

Accordingly, it is an object of the present invention to provide an alternative furnace and a method of processing ores with which the applicants believe the aforementioned disadvantages may at least be alleviated or which may provide useful alternatives for the known furnaces and methods.

SUMMARY OF THE INVENTION

According to the invention there is provided a brush-arc furnace comprising:

a vessel defining a chamber holding a burden comprising a body and a layer on the body comprising slag, the layer having an upper surface; at least two electrodes, each having a diameter d;

each of the at least two electrodes extending into the chamber to a level between a first limit above the surface and a second limit below the surface;

the first limit being smaller than 0.5d and the second limit being smaller than 0.3d; and

- a power supply for driving the at least two electrodes.

The at least two electrodes are driven into brush-arc operation, so that a short and somewhat diffused arc is maintained between the electrode and the burden.

The first limit may be smaller than 0.4d and preferably is smaller than 0.3d. The second limit may be smaller than 0.2d.

In some embodiments, the power supply may comprise an AC power supply and the furnace may comprise three electrodes, each having a diameter of d.

The AC power supply may comprise three single phase transformers connected in a delta configuration to the three electrodes. In other embodiments, the power supply may comprise a DC power supply for driving the at least two electrodes as anode and cathode respectively.

The vessel may comprise at least one feed port for charging raw materials into the chamber and at least one outlet for gasses. The at least one feed port and the at least one outlet may be provided in a roof of the vessel.

The vessel may be cylindrical in shape and may comprise a steel shell having a bottom opposite the roof and the steel shell may be lined on an inside thereof with a refractory lining.

At least one of the steel shell and the burden in the vessel may be kept at earth or ground potential.

The electrodes may extend in an axial direction into the chamber through suitable openings in the roof.

The roof may be water-cooled.

The electrodes may be axially adjustable.

Each electrode may comprise a so-called self-baking electrode, such as the type of electrode known in the art under the name Soderberg electrodes. Alternatively, each electrode may comprise a pre-baked graphite electrode.

The at least one feed port may comprise means for controlling at least one of the rate and volume of raw material fed into the chamber and the at least one outlet may comprise means for controlling at least one of the rate and volume of hot gas escaping from the chamber.

Further according to the invention there is provided a method of processing ores, the method comprising the steps of:

feeding the ores into a chamber of a furnace, so that the chamber holds a burden comprising a body and a layer on the body comprising slag, the layer having an upper surface; and energizing at least two electrode extending into the chamber to operate in brush-arc manner.

The at least two electrodes may be energized utilizing an AC voltage, alternatively the at least two electrodes may be energized utilizing a DC voltage.

The body of the burden may comprise a ferrochrome alloy. Yet further included within the scope of the present invention is a method of converting a conventional submerged arc furnace into a DC brush-arc furnace, the method comprising the steps of:

replacing a set of three electrodes with a pair of two electrodes; causing the electrodes to extend into a chamber of the furnace to a level between a first limit above a surface of a burden in the chamber and a second limit below the surface of the burden; and

connecting the pair of electrodes to a DC voltage.

BRIEF DESCRIPTION OF THE ACCOMPANYING DIAGRAMS

The invention will now further be described, by way of example only, with reference to the accompanying diagrams wherein:

figure 1 is a sectional view through an example embodiment of an

AC brush-arc furnace;

figure 2 is an electrical circuit diagram of the furnace in figure 1 ;

figure 3 is a sectional view through an example embodiment of DC brush-arc furnace; and

figure 4 is an circuit diagram of the furnace in figure 3.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

An example embodiment of an AC brush-arc furnace is generally designated by the reference numeral 10 in figures 1 and 2. The furnace 10 comprises a vessel, in this example embodiment, a circular cylindrical vessel 12 comprising a steel shell 14 lined with a layer 16 or more of refractory bricks. The furnace comprises a roof 18 and an opposed bottom 20. Three electrodes 22.1 , 22.2, and 22.3 extend through the roof in an axial direction into a chamber 24 defined by the vessel. The electrodes have a diameter d. The electrodes are connected to an AC power supply 26 (shown in figure 2).

The chamber holds a burden 28 comprising a body 30 comprising a ferrochrome alloy and a layer 32 comprising slag on the body. The layer has an upper surface 34.

In use, the electrodes extend to between a first limit above the surface 34 and a second limit l 2 below the surface 34. The first limit is preferably smaller than 0.3d and the second limit is preferably smaller than 0.2d. A brush-arc 36 required for work or a reaction in the vessel in the form of a very short and somewhat diffused arc is maintained between the electrodes and the burden. As for all other arcing operations, the length of the brush-arc may vary, depending on application and circumstances. With brush-arc operation, arcing characteristics are independent of burden resistance or porosity, but are determined by the electrode tip position and the applied voltage. The electrodes 22.1 to 22.3 extend through suitable openings in the roof into the chamber 24. The electrodes are adjustable in an axial direction. Each electrode may comprise a so-called self-baking electrode, such as the type of electrode referred to in the art as a S derberg electrode. The vessel may further comprise at least one inlet port (not shown) for raw material to be fed into the vessel and at least one outlet (also not shown) from the chamber for hot gasses. The at least one inlet port and the at least one outlet may be provided in the roof 18 of the vessel. The inlet port and/or outlet may comprise a gate or valve controllable to control the rate and/or volume of material and/or hot gasses, as the case may be. The vessel further comprises a molten metal and slag tapping hole 40 (shown in figure 2). In other embodiments separate, tapping holes (nor shown) for molten metal and for slag may be provided. The tapping holes may be opened and closed by hydraulically powered drill and plugging apparatus. Refractory clay may be used as plugging material.

As shown in figure 2, the electrodes 22.1 to 22.3 are connected to an AC power supply 26. The power supply comprises three single phase transformers 42.1 to 42.3 connected in a delta configuration and to the electrodes 22.1 to 22.3. The AC brush-arc operation will have a substantially higher arc resistance than a comparable AC submerged-arc furnace and it is therefore envisaged that furnace transformers will be replaced when converting an AC submerged-arc furnace to a brush-arc furnace, as hereinafter described.

In figures 3 and 4 there is shown an example embodiment of a DC brush- arc furnace 100. The furnace 100 comprises a circular cylindrical vessel 112 comprising a steel shell 114 lined with a layer 116 or more of refractory bricks. The furnace comprises a roof 118 and an opposed bottom 120. Two electrodes 122.1 and 122.2 extend through the roof in an axial direction into a chamber 124 defined by the vessel. The electrodes have a diameter d. The electrodes are connected to a DC power supply 126 (shown in figure 4).

The chamber holds a burden 128 comprising a body 130 comprising a product and a layer 132 comprising slag on the body. The layer has an upper surface 134.

In use, the electrodes extend to a level between a first limit above the surface 134 and a second limit l 2 below the surface 134. The first limit is preferably smaller than 0.3d. A brush-arc 136 required for work or a reaction in the vessel in the form of a very short and somewhat diffused arc is maintained between the electrodes and the burden. Referring to figure 4, the DC power supply 126 comprises an AC supply 140 connected to a rectifier arrangement, which may comprise an SCR bridge 42, a DC reactor 144 and a polarity switching circuit 146 having an output 148. The output 148 is connected to the electrode pair 122.1 and 122.2 by water-cooled bus tubes 150. The polarity switching circuit is manipulatable or automatically controllable intermittently, preferably periodically, to reverse the polarity of the DC voltage at the output 148, so that the first electrode 122.1 will be driven as anode for a first period of time, and thereafter as cathode and vice versa for the second electrode 122.2.

The invention also extends to a method of converting an existing known submerged arc AC furnace into a DC brush-arc furnace substantially as shown in figures 3 and 4. The method comprises the steps of replacing the set of three electrodes of the existing furnace with an electrode pair. This may be done by removing the electrode 22.3 and moving the electrode 22.1 into line with the electrode 22.2 on the tap hole center line 152 of the vessel 24. The electrode pair is then connected to a DC power supply 126 shown in figure 4, thereby, in use, to drive a first electrode of the pair as an anode and another electrode of the pair as a cathode.

In other embodiments the vessel may be rectilinear in configuration and a plurality of electrode pairs may extend into the chamber. In one preferred form of this embodiment, three pairs of electrodes may be provided, each pair being driven by a respective DC power supply.

It is believed that the brush-arc furnaces and methods according to the invention may be used for processing raw materials, more particularly ferrochrome ores, with at least a reduced requirement for permeable lumpy or pelletized ores and for lumpy coke to form a coke bed.