WO2004013360A1

WO2004013360A1 - Salt furnace

Info

Publication number: WO2004013360A1
Application number: PCT/AU2003/000981
Authority: WO
Inventors: Kerry John Polkinghorne
Original assignee: Australian Magnesium Operations Pty Ltd
Priority date: 2002-08-06
Filing date: 2003-08-04
Publication date: 2004-02-12
Also published as: AU2002950585A0; TW200403345A

Abstract

A salt furnace is receivable within a vessel which contains molten magnesium/magnesium alloy. The salt furnace comprises a body for isolating a salt melt within the salt furnace from molten magnesium/magnesium alloy in the vessel and through which the salt melt is heated by the molten magnesium/magnesium alloy, access means for introducing magnesium bearing material into the salt melt, and transfer means for transferring molten metal from above the salt melt in the salt furnace into the vessel.

Description

SALT FURNACE

Field of the Invention

The present invention relates generally to magnesium foundry practice and, more particularly, to a salt furnace operable within a vessel containing molten magnesium/magnesium alloy.

Background Art US Patent No. 4385931 relates to continuous refining of magnesium by the precipitation of impurities in the form of a sludge and to a refining furnace for use in the method. The refining furnace has a series of consecutively arranged precipitation chambers which are separated from one another by vertical partition walls which do not extend to the bottom of the furnace. A layer of salt melt is located in the bottom of the furnace through which adjacent chambers are in communication with one another. Adjacent chambers are also in communication with one another through an aperture in the intervening partition which is located above the level of the salt melt. Raw molten magnesium is charged into a precipitation chamber towards the salt layer beneath the molten metal. Impurities from the magnesium settle as a sludge in the salt layer and refined magnesium rises and overflows into an adjacent chamber through the aperture in the intervening partition. The process is repeated through subsequent chambers and magnesium is progressively refined until it is removed from the final chamber. Australian Patent No. 650290 extends the teaching of US 4385931 by providing a method for continuous refining of magnesium substantially in accordance with US 4385931 in combination with a process for remelting (and subsequent refining) of scrap and return metal. Remelting is achieved by pumping salt melt over the scrap which is held in a basket in the furnace. The resulting molten metal flows down into a chamber of the remelting and refining furnace for refining substantially in accordance with the teaching of US 4385931.

The electrolytic production of magnesium metal and magnesium foundries produce various waste materials which pose handling and disposal difficulties and contain magnesium values. Such materials include cell sludge from magnesium production, furnace dross, furnace skimmings and material attached to equipment removed from magnesium vessels (such as pumps) in magnesium foundries. Furnace dross and furnace skimmings are typically placed in a sow where they are allowed to burn producing an undesirable fume and loss of magnesium values. It would be advantageous to be able to treat such material to improve the working environment and to recover magnesium values therefrom.

Summary of the Invention

The present invention provides a salt furnace receivable within a vessel containing molten magnesium/magnesium alloy, the salt furnace comprising:

(a) a body for isolating a salt melt within the salt furnace from molten magnesium/magnesium alloy in the vessel and through which the salt melt is heated by the molten magnesium/magnesium alloy, (b) access means for introducing magnesium bearing material into the salt melt, and

(c) transfer means for transferring molten metal from above the salt melt in the salt furnace into the vessel. Preferably, the salt furnace is arranged to be removably received within the vessel which can be facilitated by raising and lowering the salt furnace with a hoisting arrangement. Preferably, the salt furnace further comprises a support member extending outwardly from an upper portion of the body and arranged to rest on the rim of an aperture in a lid of the vessel .

The vessel may be any metallurgical vessel which holds molten magnesium/magnesium alloy including furnace crucibles and furnaces of the kind described in US 4385931 and AU 650290.

The salt melt will typically be formed from salts of the kind used in cells for the electrolytic production of magnesium from anhydrous magnesium chloride, for example, salt mixtures containing magnesium chloride (MgCl₂) , calcium chloride (CaCl₂) , sodium chloride (NaCl) and calcium fluoride (CaF₂) . A particularly preferred salt melt contains 50% by weight NaCl, 26% by weight CaCl₂, 22% by weight MgCl₂ and 2% by weight CaF₂.

Preferably, the salt furnace generally takes the form of an elongate receptacle. The access means may take the form of an open top of the receptacle. Preferably however, the access means comprises an aperture formed in a roof of the salt furnace. Preferably, the aperture can be open and closed by a hatch or the like in the roof. The body may be of any shape but preferably takes the form of a base from which upwardly extends walls . The base may be of any shape (eg. circular, triangular, pentagonal etc . ) but for ease of manufacture is preferably square or rectangular. The receptacle may be formed of any material which is operable at the temperature and in the chemical ■ environment within a molten magnesium vessel, which is capable of transferring heat from the molten metal to the salt, and which is capable of holding the salt melt. Preferred materials include mild steel and materials which exhibit high corrosion resistance, strength and stability at high temperatures. Portions of the salt furnace above the level of the melt may be refractory lined on the inside and/or on the outside of the salt furnace.

The transfer means may take the form of a weir or, more preferably, an aperture formed in the body of the salt furnace above the level of molten metal in the vessel so that as molten metal in the salt furnace rises atop the salt melt it flows from the salt furnace into the vessel. The aperture may be formed with closing means such as a gate valve by which it may be opened and closed as desired. The transfer means preferably includes lifting means for raising the salt melt and molten metal above the salt melt to a level within the salt furnace where the molten metal can flow into the vessel. Preferably, the salt furnace has a working portion and a reservoir portion arranged such that downward displacement of salt melt in the reservoir portion causes upward displacement of salt melt in the working portion. Preferably, the reservoir portion and the working portion are in fluid communication below a barrier which otherwise separates the reservoir portion from the working portion. Preferably, the reservoir portion is arranged for the downward displacement of salt melt therein by introduction of a gas into the reservoir portion. The gas is preferably an inert gas such as argon.

Brief Description of Drawings

A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a side elevation of a salt furnace of the present invention, and

Figure 2 is a plan view of the salt furnace of Figure 1.

Drawing Related Description

The salt furnace 10 generally comprises a 495 mm x 390 mm base 12, two 495 mm x 1700 mm walls 14a and 14b, two 390 mm x 1700 mm walls 16a and 16b, a roof 17 and a 50 mm wide flange 18 which are formed from 4 mm mild steel plate and welded together. The flange 18 has four lifting rings 20 by which the salt furnace 10 can be lowered by an overhead chain block or the like into a magnesium processing vessel (not shown) through a rectangular aperture in a lid of the vessel which is sized to pass the salt furnace until the flange 18 rests on the rim of the aperture in the lid of the vessel to support the salt furnace 10 in its working position.

The salt furnace 10 is sized to hold 380 kg of a salt mixture which when molten occupies the salt furnace 10 to the level marked A which is 1195 mm above the base 12. The salt mixture is preferably 50% by weight NaCl, 26% by weight CaCl₂, 22% by weight MgCl and 2% by weight CaF₂ - The salt mixture is introduced into the salt furnace 10 through opening 22 in roof 17 which can be closed by hinged hatch 23 which is illustrated in a partially open position. The salt mixture may be introduced as a solid and then melted by heat transferred from molten magnesium in the vessel (not shown) . Alternatively, the salt mixture may be pumped into the salt furnace 10 through the opening 22 in the molten state.

A barrier 24 of 4 mm mild steel is welded between walls 14a and 14b and extends from the roof 17 to a point B which is 10 mm from the base 12 and 100 mm from wall 16a. The barrier 24 generally divides the salt furnace 10 into a reservoir portion 26 and a working portion 28. A nozzle 30 and regulator (not shown) are arranged to be coupled to a source of argon gas (not shown) for delivery of argon into the upper portion 32 of reservoir portion 26 above salt melt level A. A length of 50 mm ID mild steel pipe 34 is welded around an aperture 36 in wall 16b for transfer of molten metal from the salt furnace 10 into the vessel (not shown) .

In use, molten salt occupies the salt furnace 10 to level A and is heated by heat transferred through the base 12 and walls 14 and 16. Typically, the molten salt will have a temperature in the order of 520°C - 660°C but the actual temperature will depend on the amount of heat transferred to the salt. Magnesium bearing material such as dross and skimmings from the vessel (not shown) are placed into the salt melt in working portion 28 through opening 22. Impurities from the material drop to the bottom of the working portion 28 as a sludge and, due to the difference in density between molten metal and the salt melt, the molten metal floats to the top of, and forms a layer on, the salt melt. A grid/basket (not shown) may be located in the working portion 28 to capture sludge. The volume within the salt furnace 10 will progressively increase as magnesium bearing material is added until the molten metal layer reaches the aperture 36 through which it flows via pipe 34 into the vessel (not shown) . It is desirable to effect transfer of molten metal prior to such a volume increase to avoid oxidation of the molten metal in the salt furnace and this can be facilitated by introducing sufficient argon into upper portion 32 via nozzle 30 to downwardly displace salt melt in reservoir portion 26 under barrier 24 and upwardly in working portion 28 until the molten metal layer atop the salt melt in working portion 28 reaches aperture 36.

Molten metal within the vessel will typically be protected from oxidation by being blanketed with a cover gas. If desired, cover gas can also be directed into the salt furnace 10 or the salt furnace 10 can be arranged for diffusion of cover gas from the vessel, for example, ^• through holes in the walls 14 and 16.

Examples

Example 1 - Treatment of Dross

Dross was treated in the salt furnace illustrated in Figures 1 and 2 which was located within a magnesium furnace. Dross on the surface of the molten magnesium in the magnesium furnace was periodically skimmed and placed in the salt furnace. As compared with placement of dross in a sow, an immediate elimination of fume was apparent when dross was treated in accordance with Example 1 and the presence of metal floating on the surface of the molten salt in the salt furnace was apparent. Example 2 - Pump Cleaning

A molten metal pump used for pumping molten magnesium from the magnesium furnace of Example 1 was removed from the magnesium furnace and cleaned in the salt furnace of Example 1 by placing the pump covered in molten magnesium into the salt furnace on removal from the magnesium furnace. Burning of the molten magnesium and consequent emission of fume ceased immediately the pump was placed in the salt furnace. On subsequent removal of the pump from the salt furnace, the pump was found to be substantially free of magnesium and a layer of molten magnesium was observed on the surface of the salt melt in the salt furnace.

Example 3 - Recovery of Dross

The magnesium furnace and salt furnace were ' arranged as in Example 1 with the magnesium furnace full of pure magnesium. Both furnaces were maintained at operating temperature for 2 hours prior to commencement of dross recovery. Thereafter, the magnesium furnace was skimmed every 15 minutes with the skimmed dross transferred immediately to the salt furnace. To artificially enhance dross formation in the magnesium furnace, the lid of the magnesium furnace was removed 5 minutes prior to each skimming and magnesium in the magnesium furnace was not protected by a cover gas throughout the skimming operation. The magnesium furnace was skimmed nine times with 4 - 6 scoops of dross removed from the magnesium furnace each time it was skimmed. The loss of mass from the magnesium furnace was measured to be

50 kg and minimal fuming was evident over the entire skimming operation.

Molten magnesium was periodically recovered from the surface of the salt in the salt furnace before it reached the level of pipe 34 and was thus prevented from flowing back into the magnesium furnace. The molten magnesium was recovered by ladling from the salt furnace into ingot moulds. A total of 46 kg of magnesium was recovered which represented a 92 % recovery of dross.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as

"comprises" or "comprising" is used in an inclusive sense, ie. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

It is to be clearly understood that although prior art publications are referred to herein, this reference does not constitute an admission that any of these documents forms part of the common general knowledge in the art in Australia or in any other country.

Claims

1. A salt furnace receivable within a vessel containing molten magnesium/magnesium alloy, the salt furnace comprising:

(c) transfer means for transferring molten metal from above the salt melt in the salt furnace into the vessel .

2. A salt furnace as claimed in claim 1 arranged to be removably received within the vessel.

3. A salt furnace as claimed in claim 2, the salt furnace further comprising a support member extending outwardly from an upper portion of the body and arranged to rest on the rim of an aperture in a lid of the vessel.

4. A salt furnace as claimed in any one of the preceding claims wherein the access means comprises an aperture formed in a roof of the salt furnace.

5. A salt furnace as claimed in any one of the preceding claims wherein the transfer means comprises an aperture formed in the body of the salt furnace for passage of molten metal therethrough.

6. A salt furnace as claimed in any one of the preceding claims further comprising lifting means for raising salt melt within the salt furnace.

7. A salt furnace as claimed in any one of claims 1 - 5 wherein the salt furnace has a working portion and a reservoir portion arranged such that downward displacement of salt melt in the reservoir portion causes upward displacement of salt melt in the working portion.

8. A salt furnace as claimed in claim 7 wherein the reservoir portion and the working portion are in fluid communication below a barrier which otherwise separates the reservoir portion from the working portion.

9. A salt furnace as claimed in claim 7 or claim 8 wherein the reservoir portion is arranged for the downward displacement of salt melt therein by introduction of a gas into the reservoir portion.

10. A salt furnace substantially as herein described with reference to the accompanying drawings .