US3514520A - Linings of electrolysis,remelting,and similar furnaces,containing molten metals,alone or together with molten salts - Google Patents

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May 26, 1970 R. BACCHIEGA ETAL Flled Jan 29 1968 3,514,520 LININGS 01" ELECTROLYSIS, REMELIING, AND SLMILAR FURNACES, CONTAINING MOLTEN METALS, ALONE OR TOGETHER WITH MOLTEN SALTS 2 Sheets-Sheet l O 8 li 2 \I 6 n 4 fl a I) 7 n 2 /J /1 11 5 4 H 8 j I \l 7 H, 3 /|w 6 1 l V 6 FIG. 2

LININGS OF ELECTROLYSIS, REMELTING, AND SIMILAR FURNACES, CONTAINING MOLTEN METALS, ALONE y 1970 R. BACCHIEGA ETAL 3,514,520

OR TOGETHER WITH MOLTEN SALTS Filed Jan. 29, 1968 2 Sheets-Sheet z:

FIGS

FIG. 4

INVENTORS:

United States Patent 3,514,520 LININGS OF ELECTROLYSIS, REMELTING, AND SIMILAR FURNACES, CONTAINING MOLTEN METALS, ALONE OR TOGETHER WITH MOLTEN SALTS Roberto Bacchiega, Bolzano, and Giorgio Olah de Garab, Milan, Italy, assignors of seventy percent to Montecatiui S.p.A. and thirty percent to Giuseppe de Varda, both of Milan, Italy Filed Jan. 29, 1968, Ser. No. 701,485 Claims priority, application Italy, Feb. 1, 1967, 12,157-A/ 67 Int. Cl. F27d 1/00 US. CI. 1335 J 23 Claims ABSTRACT OF THE DISCLOSURE Disclosed are furnaces for molten metal. These furnaces, made of layers of construction material subject to leakage of the molten metal are protected by layers of silicon carbide in an incoherent state, between layers of the construction material.

Our invention relates to the linings of furnaces for containing molten metals, molten metals and salts, such as electrolysis furnaces, remelting furnaces and the like. In particular our invention relates to furnaces for fused bath electrolysis of aluminum oxide in cryolite, for aluminum production.

More particularly, our invention faces and solves one of the most important problems met in the construction of such furnaces, that is, the problem of the so-called seal of the furnaces. That is the problem of avoiding the seepage of the molten metal through the material constituting the side walls and the bottom of the furnace vats. The problem is particularly serious in the building of the vat of electrolysis furnaces, e.g. the furnaces for the production of aluminum and magnesium. In these furnaces, the inner surface of the vat, that holds the molten metal and the bath, is generally lined by one or more layers of special refractory bricks, prebaked carbon blocks or carbonaceous materials in general. These layers are in direct contact with the fused bath and/or the molten metal. Against these bricks, blocks or carbonaceous material are placed refractory bricks and thermoinsulating bricks. The discontinuities and cracks that may form in the vat are points of easy leakage for the liquid within the vat itself, primarily for the molten metal. The molten metal has a solidification temperature much lower than that of the fused bath in which it is produced.

For instance, in the multicell electrolysis furnaces with suspended bipolar electrodes for aluminum production in contact with the fused cryolite bath, is an inner layer of special refractory material, for instance silicon nitridebonded silicon carbide, often followed by a layer of carbonaceous material. These layers are outwardly followed by one or more layers of thermally insulating material, and encompassed by an iron shell. The electrolysis furnaces of the conventional type used in the fused bath electrolysis, having a similar vat, but these vats are usually lacking an inner lining of refractory material. The carbon, either in blocks or as a ramming, which replaces the cathode in them, is in direct contact with the bath, and/or metal. That is, it is in direct contact with the fused material of the electrolytic furnace. In both the multicell furnaces and in conventional furnaces there is the inconvenience of the outflow escape of the molten aluminum. This drawback occurs with a certain frequency and is rather serious inasmuch as it necessitates shut down of the furnace to rebuild the inside re- 3,514,520 Patented May 26, 1970 fractory lining or the cathodic carbon layer, as the case may be.

This drawback is fully or at least greatly eliminated by our invention for the construction of the furnaces of the above type for the electrolytic production or the remelting of aluminum.

We have surprisingly found that silicon carbide in an incoherent state, that is, in the form of powder or granules, without a binder, even in a very thin layer and in commercial grain size, constitutes a barrier practically unsurmountable for molten metal, particularly for molten aluminum even when mixed with an electrolytic bath, for instance with molten cryolite salts. It therefore suifices to interpose layers of this material between layers constituting the vats of the furnaces in question and/or by stuffiing the silicon carbide into the interstices between bricks and/or between the'carbon blocks, in order to eliminate the leaking through of the aluminum and the ensuing damage.

In the actual operation, the build up of these layers is carried out by spreading the incoherent silicon carbide between layers on the bottom of the furnace vats and by pouring it into interstices purposely left in the side walls of the vats. Layers of 1 cm. or more are definitly efficient, very thin layers e.g. of 1 mm. and more, exert the same surprising effect. In particular, in the. case of multicell electrolysis furnaces with bipolar electrodes, with carbon vats fully lined with refractory bricks of special material, such as for instance silicon nitridebonded silicon carbide, the powdery or granulated silicon carbide layer is preferably interposed between the layer of said refractory bricks forming the internal lining and the layer of carbonaceous material adjacent to it. In conventional electrolysis furnaces with cathodic vats, the layer of powdery or granulated silicon carbide is arranged all around and beneath the layer of carbon blocks or rammed carbon, being careful, however, not to insulate electrically the cathodic iron bars. It is quite feasible to so place the silicon carbide since experience shows that the zones most exposed to leakages of the aluminum, and thus most needful of protection inside the furnaces, are those zones located at the ledge of the vat bottom, that is, the zones joining the bottom to the side walls. For instance, in the remelting furnaces for aluminum, the protective layer of silicon carbide powder or granules is placed against, i.e. on the outside of, the first refractory brick layer, which is in direct contact with the molten metal.

The drawings schematically illustrate two embodiments of our invention. A non-limiting example will be given with respect to the drawing, wherein:

FIG. 1 shows a longitudinal cross section along the longitudinal axis of the vat in a conventional aluminum electrolysis furnace operating on monopolar electrodes (anodes) and with a cathodic bottom;

FIG. 2 shows in detail, on a larger scale, a transverse cross section of the cathodic vat of FIG. 1;

FIG. 3 shows a longitudinal cross section along the longitudinal axis of the vat of a multicell furnace; and

FIG. 4 shows a transverse cross section of the vat of FIG. 3.

In FIGS. 1 and 2, the outer iron shell is 1 while 2 is the prebaked anthracitous carbon blocks. The side walls of the vat, also of prebaked anthracitous are 3. An anthracitous ramming baked in situ is shown at 4. Number 5 marks the iron cathode bars. For greater clearness the interposed silicon carbide powder or granules 6 are not drawn to scale, that is, they show an apparently greater thickness than necessary. In the conventional electrolysis furnaces for the production of aluminum, of the type illustrated by FIGS. 1 and 2, it is also advisable to place the silicon carbide powder or granules also between carbon block and carbon block of the bottom in the lowest part of it, that is, in the zones 7 and, covering the silicon carbide with a mass of carbonaceous ramming 4 so that the container shows the required continuity for the carbon parts. At 8 we see the peripheral joint between carbon block and ramming, while at 9 are the foundation and the ledges of clay bricks.

In FIGS. 3 and 4, the silicon carbide powder or granules 10 is between the internal lining 11 of special refractory and the carbon layer 12 (ramming or blocks), which constitues the vat proper. This in turn is within external insulating layer 13 (refractory and/or thermal insulant), which is within the iron shell 14.

EXAMPLE A multicell furnace for the electrolytic production of aluminum, fitted with a protective layer according to the invention and with vats built according to FIGS. 3 and 4, was kept in operation for several months. After this period, a cracking of the bottom was noticed. Upon dismantling of the furnace itself, a certain leakage of electrolytic bath was ascertained. This, however, had solidified between the carbon and the external insulation, while the molten aluminum, quite surprisingly, had not passed through the granulated silicon carbide layer.

Further similar tests were conducted on conventional aluminum electrolysis furnaces operating with monopolar electrodes in cathodic vats. Still other tests were carried out with remelting furnaces, even of faulty construction. The results ineach case fullyconfirmed the perfect effectiveness of our invention.

Blank testswere carried out contemporaneously to the tests using siilcon carbide layers according to this invention. By blank tests wer mean trials under equal condiitions, but omitting the silicon carbide layer. These blank tests had severe metal leakages, clearly showing the protective effect against such leakages was a result of the silicon carbide.

We claim:

1. In furnaces for molten metal which comprise vats made of layers of construction material subject to the penetration and leakage of the molten metal, the improvement which comprises providing such vats with protective layers of silicon carbide in an incoherent state, placed between layers of said construction material.

2. The furnaces of claim 1, wherein the silicon carbide is a powdery state.

3. The furnaces of claim 1, wherein the silicon carbide is in a granulated state.

4. The furnace of claim 1, wherein the silicon carbide is used without binders.

5. The furnaces of claim 1, wherein the silicon carbide is blended with other materials.

6. The furnaces of claim 1, wherein the silicon carbide used is of a commercial granulometric size.

7. The furnace of claim 1, wherein the vats are lined with layers of carbonaceous material in direct contact with the molten mass within said vat.

8. The furnace of claim 1, wherein the vat comprises layers of carbonaceous material and layers of special refractory material, interposed between the layers of carbonaceous material and the molten mass within the furnace -vat.

9. The furnace of claim 8, wherein the special refractory material is silicon carbide bonded with silicon nitride.

10. The furnace of claim 1, wherein the furnaces are electrolysis furnaces.

11. The furnaces of claim 10, wherein the furnaces are fused bath electrolysis.

12. The furnaces of claim 10, wherein the furnaces are for the electrolytic production of metals.

13. The furnaces of claim 12, wherein the furnaces are for the production of aluminum.

14. The furnaces of claim 12, wherein the furnaces are for the production of magnesium.

15. The furnaces of claim 10, wherein the furnaces have monopolar electrodes and a cathodic bottom.

16. The furnaces of claim 10, wherein the furnaces are multicell furnaces with bipolar electrodes.

17. The furnaces of claim 1, wherein the furnaces are melting furnaces.

18. The furnaces of claim 1, wherein the furnaces are remelting furnaces.

19. The furnaces of claim 1, wherein silicon carbide is placed against the outer face of the first relatively thin interior layer of coherent material of the vat, said furnace contains baths of molten salts besides the metal, and the silicon carbide forms a layer placed internally with respect to the freezing isothermic surface of said bath.

20. The furnaces of claim 1, wherein silicon carbide constitutes one uninterrupted layer over the whole extension of the walls and of the bottom of the vat to be protected.

21. The furnaces of claim 1, wherein said protective layer of silicon carbide forms, on the bottom of the vat, layers of such discontinuities as neither to insulate the bottom nor to impair the electrolytic function thereof.

22. The furnaces according to claim 21, wherein said protective silicon carbide layers are located in the zones most prone to leakages of metal.

23. The furnaces according to claim 21, wherein the protective silicon carbide layers are prevailingly located in the joint areas between bottom and side Walls of the vat.

References Cited UNITED STATES PATENTS 3,256,173 6/ 1966 Schmitt et al 204-243 3,321,392 5/1967 McMinn et al 204243 3,412,195 11/1968 Mumper 1335 X B. A. GILHEANY, Primary Examiner R. N. ENVALL, Assistant Examiner U.S. Cl. X.R. 204-243; 266-43

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