GB2091396A - Thermally-degradable taphole plug - Google Patents

Abstract

A furnace taphole plug comprising at least one particulate refractory material, such as, firebrick grog, bonded together by means of a thermally-degradable bonding agent which is such that, in use, the plug disintegrates when heated to a temperature below that of a molten material contained in an associated furnace. Preferably, the bonding agent is a thermally-degradable synthetic resin, for example, a phenolic-based resin which is present in a preferred weight range of 2.5% to 7.5%. Also, the bonding agent is preferably such that the temperature range at which disintegration of the plug occurs is from 450 DEG C to 500 DEG C. In a preferred embodiment, the plug includes a fibrous carbonaceous material, such as paper pulp, which substantially burns out as the plug disintegrates, the desirable range for the paper pulp content by weight being from 2.5% to 10%. The invention also provides a method of manufacturing the plug, wherein a mixture of the plug ingredients is slurried with water, the slurry then being formed into the plugs which are then dried.

Description

SPECIFICATION Taphole plugs This invention relates to taphole plugs and is concerned particularly with taphole plugs used for sealing the tapholes of basic oxygen steel making furnaces. The basic oxygen furnace is used to eliminate carbon and other undesirable elements from iron or steel melts by blowing oxygen on to or through the melts. In the top-blown type of basic oxygen furnace, oxygen is blown on to the surface of the melt through a lance introduced into the top of the furnace; in the bottom-blown type of furnace, oxygen is introduced through a hole in the bottom of the furnace. In either case, the oxygen combines with alloying elements such as silicon in the melt and with some of the iron to form oxides, which enter the layer of slag on top of the melt.The amount of slag is further increased by fluxes, such as burnt lime, fluorospar arid mill scale, which are added in order to adjust the basicity and fluidity of the slag.

The furnace is usually barrel-shaped and is upright during blowing; the furnace is pivotally mounted about a horizontal axis, so that it can be tilted from the upright position in one direction to receive a charge of raw material, such as scrap, and in the opposite direction to discharge its contents through a taphole. The taphole is situated so that it is above the level of the top of the slag when the furnace is in its operative upright position.

When the melt has reached the desired composition, the oxygen supply is cut off and, in the case of a top-blown furnace, the lance is withdrawn. The melt is then discharged into a ladle by turning the furnace down, i.e. by tilting it about its pivot axis, until the taphole is below the surface of the melt and the melt flows through the taphole into the ladle.

As the furnace is turned down, the taphole passes the layer of slag before reaching the molten metal beneath the slag and, if the taphole is open, the slag is discharged into the metalreceiving ladle. The presence of slag in the ladle is undesirable, particularly since the slag contains impurities which were removed from the melt in the furnace. If the steel is being produced to a strict specification, it is imperative to retain the slag in the furnace, so that the steel will not be contaminated.

The usual procedure for retaining the slag in the furnace is to block the taphole temporarily with a thermally-destructible material which when the operation proceeds as desired, remains intact until the taphole is below the bottom of the slag layer and then decomposes, releasing the molten metal.

Wet sacking is widely used for this purpose; three or four wet sacks are rolled into a ball which is then rammed into the taphole, forming a plug which burns away upon making contact with the hot slag and molten metal.

The use of wet sacking is unsatisfactory for a number of reasons. The wet sacks are unpleasant to work with and a certain amount of skill is necessary in ramming them into the taphole, so that they present the desired barrier to the molten steel without leaving any gaps. If they are not rammed in properly, they can fall out. They do not have a clearly defined temperature of decomposition and thus the time for which they will remain intact cannot be predicted with certainty. Consequently, the plug formed by the sacks may burn out too soon, contaminating the melt with slag, or it may burn out too late, delaying the pouring of the melt.

Instead of wet sacks, a wooden plug tapered to fit the taphole can be used. Wooden plugs are not unpleasant to handle and they can usually be relied upon to stay in place, but they take a long time to burn out, delaying the pouring of the melt.

A need thus exists for a taphole plug which is easy to handle, which can be relied upon to stay in place and to block the taphole completely and which will disintegrate rapidly upon being heated to a temperature which falls within a clearly defined range.

This need is satisfied according to the invention, which provides a taphole plug comprising at least one particulate refractory material bonded together by means of a thermally-degradable bonding agent, the bonding agent being selected so that the plug disintegrates upon being heated to a temperature which is well below the temperature of the molten steel.

In use, as the furnace is turned down, the plug is inserted when the taphole is level with the operators, but still above the level of the slag. As the furnace continues to turn down, the plug is heated first by heat radiated by the slag, then by direct contact with the slag and finally by direct contact with the molten steel. Upon reaching its disintegration temperature, the plug crumbles to a powder which is forced out of the taphole by the metallostatic pressure of the molten steel. The steel is thus released through the taphole into the ladle and turning down of the furnace continues so that the slag remains above the level of the taphole while the steel flows out. The powder formed by disintegration of the plug becomes entrained in the flowing steel and is carried by the steel into the ladle.The powder floats on top of the steel in the ladle and can easily be scooped off when pouring has finished.

The disintegration temperature of the plug must be high enough for the plug to remain intact until it comes into contact with the molten steel and low enough for the plug to disintegrate promptly thereafter. Disintegration temperatures in the range from 4500--5000C have been found to be the most suitable and the bonding agent is therefore preferably selected to give a disintegration temperature for the plug within this range.

According to a preferred feature of the invention, the bonding agent is a thermallydegradable synthetic resin, e.g. a phenolic-based resin. Another preferred feature is the inclusion of a carbonaceous material which substantially burns out as the plug disintegrates.

The preferred composition of the plug, by weight, is: Paper pulp 2.5%-i 0% Resin 2.5%-- 7.5% Kieselguhr 1 0%-30% Vermiculite 1 0%-30% Firebrick grog Balance The preferred method of manufacture is to slurry the mix with water and then vacuum-form the slurry into a plug whose dimensions and shape are such that it will fit the taphole tightly. The plug is then dried out, in the preferred case where the resin is of the heat-curable kind, then stoved to cure the resin. Injection-moulding or pressing can also be used instead of vacuum forming.

The paper pulp or other carbonaceous material facilitates handling of the plug during manufacture, by providing green strength in the unstoved state. Any type of carbonaceous fibre material which burns out at a low enough temperature may be used instead of paper; burnout of the fibre must be complete at the desired disintegration temperature of about 4500--5000C. If the paper content is less than 2.5%, the plug will normally be too brittle. If the paper content is greater than 10%, the plug will normally be too fibrous and lack rigidity.

The resin acts as a binder to provide coherence and strength. The resin must form a thermallydegradable bond which breaks down at the desired disintegration temperature; phenolicbased resins are especially suitable and therefore preferred. In general, resin contents less than 2.5% make the material difficult to bond; resin contents greater than 7.5% give an excessively firm bond which is slow to break down and delays disintegration.

The kieselguhr imparts low density to the plug, making the plug lightweight, to assist handling, and facilitating entrainment of the crumbled plug material in the molten steel and subsequent floating of the material to the surface of the steel in the ladle. The indicated range of 10% to 30% is normally preferred, because kieselguhr contents less than 10% make the material too dense; kieselguhr contents greater than 30% make the crumbled material too sticky, so that it tends to agglomerate like a slag instead of forming a powder on top of the steel in the ladle. Other lowdensity powders can be used in addition to or instead of kieselguhr, e.g. diatomaceous earth or expanded fire-clay grog.

Vermiculite is desirably included because it exfoliates, i.e. it swells upon heating, and thus ensures total disintegration of the plug. Any other material which exfoliates or otherwise expands on heating at comparatively low temperatures may be used, e.g. unexpanded perlite. Vermiculite contents of less than 10% are usually inadequate to ensure total disintegration of the plug.

Vermiculite contents of more than 30% are generally not used, because they give excessive exfoliation and produce an excessive volume of debris which must be removed from the steel in the ladle.

The firebrick grog provides thermal insulation.

Any type of refractory grog which provides adequate insulation and is compatible with the method of manufacture may be used. The insulation is necessary to prevent excessively rapid heating of the plug and thus ensure that the plug does not disintegrate while it is still at the level of the slag.

In addition to the firebrick grog or other particulate refractory material and the phenolicbased resin or other thermally-degradable bonding agent, the composition used to make the taphole plug of the invention thus preferably includes a combustible fibrous material, a lightweight refractory material for reducing the density of the composition and a thermally-expandable disintegration agent.

Claims (23)

1. A furnace taphole plug comprising at least one particulate refractory material bonded together by means of a thermally-degradable bonding agent, wherein the bonding agent is such that, in use, the plug disintegrates when heated to a temperature below that of a molten material contained in an associated furnace.

2. A taphole plug according to claim 1 wherein the bonding agent is a thermally-degradable synthetic resin.

3. A taphole plug according to claim 2, wherein the bonding agent is a phenolic-based resin.

4. A taphole plug according to claim 2 or 3, wherein the resin content by weight is in the range from 2.5% to 7.5%.

5. A taphole plug according to any preceeding claim, wherein the bonding agent is such that disintegration of the plug occurs at a temperature in the range from 4500C to 5000 C.

6. A taphole plug according to any preceding claim including a fibrous carbonaceous material which substantially burns out as the plug disintegrates.

7. A taphole plug according to claim 6, wherein the carbonaceous material comprises paper pulp.

8. A taphole plug according to claim 7, wherein the paper pulp content by weight is in the range from 2.5% to 10.0%.

9. A taphole plug according to any preceeding claim including at least one material which swells upon heating, thereby facilitating disintegration of the plug.

10. A taphole plug according to claim 9, wherein said at least one material which swells upon heating comprises vermiculite.

11. A taphole plug according to any preceeding claim including at least one of kieselguhr, diatomaceous earth and expandable fireclay grog.

12. A taphole plug according to claim 11, wherein the kieselguhr content by weight is in the range from 10% to 30%.

13. A taphole plug according to any preceeding claim, wherein said at least one particulate refractory material comprises firebrick grog.

14. A taphole plug according to any preceeding claim comprising, by weight: Paper pulp 2.5% to 10% Bonding agent 2.5% to 7.5% Kieselguhr 10% to 30% Vermiculite 10% to 30% Refractory material Balance

1 5. A furnace taphole plug substantially as hereinbefore described.

1 6. A method of manufacturing a furnace taphole plug according to any preceeding claim, which method comprises slurrying with water a mixture of the plug ingredients, forming the slurry into at least one plug and then drying said plug.

17. A method according to claim 16, wherein the bonding agent comprises a heat-curable synthetic resin and said at least one dried plug is stoved to cure the resin.

18. A method according to claim 16 or 17, wherein the slurry is shaped into said at least one plug by vacuum-forming, injection-moulding or pressing.

1 9. A method of manufacturing a furnace taphole plug substantially as hereinbefore described.

20. A furnace taphole plug when made by a method according to any of claims 16 to 1 9.

21. A method of pouring molten metal from a furnace vessel provided with a taphole, which method comprises inserting a plug in the taphole, when the latter is above the level of the slag surface of the molten metal, turning down the vessel such that the plug is heated firstly by heat radiated by the slag and, secondly, by direct contact with the slag and, finally, by direct contact with the molten metal, the plug being formulated such that, upon reaching its disintegration temperature, it crumbles to a powder which is forced from the taphole by the metallostatic pressure of the molten metal in the vessel, whereby the molten metal is released through the taphole into an associated ladle, and continuing the turning down of the vessel so that the slag remains above the level of the taphole while the molten metal flows therethrough.

22. A method according to claim 21, wherein the plug is formulated so that the powder, which is forced from the taphole by the pressure of the molten metal, floats on top of the metal in the ladle.

23. A method according to claim 21 or 22, wherein the plug is according to any of claims 1 to 15and20.