GB2173582A - Injection lance - Google Patents

Abstract

An injection lance, for introducing flux mixtures into baths of molten metal, has inner and outer spaced metal tubes 14, 13 with between them a tubular sheath 15 of refractory material that is spaced from both the inner and outer tubes by air gaps. The outer tube may be of high temperature metal with a refractory coating. The tubes can be maintained in coaxial relationship by an end cap at the inlet end of the lance and by rammed ceramic fibre insulation introduced at both ends, the outer tube at the discharge end being chamfered and containing a body of rammed insulation in which the inner tube is buried, with the terminal portion of the flow passage through the lance at the discharge end being provided as a passage formed through the rammed insulation. <IMAGE>

Description

SPECIFICATION Injection lance This invention relates two injection lances for the introduction of chemical substances into baths of molten metal.An example is the injection of a drossing-offflux into molten aluminium or its alloys.

When fluxes consisting of appropriate mixtures of chloride and fluoride salts are introduced into molten aluminium or aluminium alloys, a series of chemical reactions take place and as a resultthe alloy is cleansed of non-metallic inclusions, and degassed (hydrogen is removed). The resultant oxide dross on the surface of the melt contains only a very small amount of entrapped aluminium metal. The preferred method ofintroduc- tion is to inject the substances of the flux mixture, in a stream of inert gas such as nitrogen, directly beneath the surface ofthe molten metal by means of an injection lance.

The flux injection lances available atthe presenttime are usually made of: 1) cast iron; 2) mild steel; 3) mild steel clad with ceramicfibre; 4) graphite-silicon carbide or graphite alone.

Each of the above materials suffers from serious disadvantages. The materials all possess relatively high thermal conductivity and the internal temperature ofthe lance reaches equilibrium with the molten metal in which it is immersed, usually ata level about30'C below the metal temperature, during thetimetaken forflux injection (10 to 15 minutes). Lances constructed from mild steel or cast iron, if not coated with a suitable refractory coating,will actually lose material into solution inthe aluminium alloy, thereby undesirably increasing the iron content of the aluminium alloy which can give rise to excessive shrinkage and, in extreme cases, render the alloy out of specification.

Moreover, the high internal temperature of the lance imposes severe restrictions on the formulation and effectiveness of the injected flux, in that the melting or fusion point of the flux mixture may need to be selected at an otherwise disadvantageously high temperature solely in order to try and prevent the flux fusing in the lance. As soon as the flux starts to fuse in the lance a blockage will occur in an extremelyshorttime,thereby requiring the lance to be withdrawn from the melt, eitherto be unblocked, which can take several minutes,or to be replaced with another lance which, if made from the same material, could itselfthen block.However, raising thefluxfusion pointtemperatureto preventthis happening tends to restrictthe effectiveness ofthe flux mixture being used since, in general,the higherthefusion point of the flux the lower is the chemical reactivity, and the more limited the scope of the possible beneficial chemical functions of the flux. The overall effectiveness of the drossing-off operation isthus impaired.

It is an object of the present invention to overcome these disadvantages.

According to the present invention, an injection lance is provided comprising inner and outer spaced coaxial tubes, with a thermally insulating layeror layers interposed between them, the innertube having its extremity at the nozzle or discharge end ofthe lance contained within refractory material and buried some distance in from the lance nozzle opening.

By the use of such a construction, the constraint of a high internal lance temperature can be avoided.

Instead, an equilibrium with the molten alloy at a temperature of some 300 to 350 below that of the alloycan be achieved.

In the preferred form, the space between the inner and outertubes contains a tubular sheath of refractory material, with air gaps provided between the refractory sheath and both the inner and outertubes. The inner and outertubes may be of metal,the outer tube being preferablyofa high temperature metal, such as titanium alloy, and coated with an outer skin or coating of refractory material.

One arrangement in accordance with the invention will now be described by way of example with reference to the accompanying drawing, which shows an injection lance embodying the invention in diagrammatic longitudinal section.

In the drawing,the lance 11 is onetotwo metres long with an obliquely sloped or chamfered end face 12 at its discharge end. It comprises an outer metallic tube 13, a coaxial inner metallic tube 14, and an intermediate refractory sheath ortube 1 5 disposed coaxially between the outer and inner metallictubes, with outer and inner annular air gaps 16,17 between, respectively, the outertube 13 and the intermediate tube 15, andthe intermediate tube 15 and the innertube 14. At the back end of the lance, the outer and intermediate tubes 13, 15terminate art a flanged end plate 18, only the innertube 14 passing through the end plate for connection to the supply of flux mixture and inert gas.

Adjacent the discharge end ofthe lance, the outerannularairgap 16 is plugedwith a rammed plug 19of rammable insulating material, such as ceramic fibre insulation. The innerairgap 17 is plugged at both ends by means of plugs of insulating material 20,21 which can also be of ceramic fibre. Atthe discharge end,the intermediate tube or sheath 15 is square-ended, as is also the inner tube 14which projects somewhat beyond the intermediate tube 15 and the end plug 20 closing the air gap 17, butthre end ofthe outer tube 13 is cuton the chamfer and to build upthe nozzle of the lance the interior of this chamfered end is filled with rammable insulation 22, which again can beceramicfibre insulation.The end ofthe innertube 14 is entirely sheathed by this rammed insulation 22 and, if desired, the tube end can be somewhat enlarged and threaded or serrated to provide an effective key between the insulation and the tube end.

The construction is such that the extreme end of the inner metal tube 14 is buried some Sto 30 mm within the refractory material 22, the terminal portion 23 ofthe bore through which the flux mixture discharges into the molten metal bath being formed by the rammed refractory material. This isto preventthe heat ofthe molten metal being conducted along the inner tu be. The material ofthe inner tube is notcritical and itcan be of mild steel.

The inner air gap 17 around the innertube 14 may be, say,3 to 5 mm.The intermediate refractorytubeor sheath 15, which may be, say, 28 to 30 mm in diameter, can be constructed using a variety of refractory materials, such as ceramidfibre, or ceramicfibre paper rolled into a tube, foamed refractory or a refractory.

aggregate, perlite, vermiculite, and so forth. So long as the sheath is strong enough to be self-supporting underthe forces transmitted to it during use ofthe lance, mechanical robustness is not a prime requirement since it is protected by the inner and outertubes. The quter airgap 16 surrounding thesheath 15 map be, say, 10 to 12 mm.

The outermetaltube 13 has a protective refractory coating applied to it by either plasma- orflame-spraying.

The metal itself can be mild or stainless steel, a cupro-nickel or nimonic alloy, titanium or a titanium alloy, zirconium ortantalum; butthe preferred material is atemperature resistant or refractory metal such as titanium alloy. The refractory coating on the metal should have a thermal coefficient of expansion compatible with thatofthe metal or alloy onto which it is sprayed, and is should also be chemically compatible withthe molten metal in which the lance isto be immersed.Some ceramic coatings that have a similar coefficientdf expansion to that of titanium and its alloys, and could be suitable for immersion into molten aluminium alloys, are as follows: Al203- MgO Spinel; CrO3; CeO2; -ZrO2-SrO; TiO2- SrO; 2Ti02- MgO; Cr203-MnO; Cr203-FeO; Al203-tiO2Ni0; Al203-NiO; Al203-CoO; TiO2; Zr; Nb; TiC; Cr3C2 86.6% Cr; Al203-ZnO; Al203.

By the use of an injection lance according to this invention, the internal lance temperature is readily kept down to a level at which a flux mixturewith a comparatively lowfusion temperature, say around 580 C, can be - injected'into molten metal ata considerably highertemperature,around 750'C for molten aluminium,without risk of blocking the lance. Using one of the lances available hitherto, blocking would have taken place under these conditionswitnin three to four minutes. The accompanying Table showsthe results oftemperaturetrials usingourimproved lance.

As a consequence, the range of metal treatments possible has been expanded asfollows: a) degassing (removal of H2); b) removal ofnon-metallicinclusions (Al203, MgO etc.); c) grain refinementwith Ti - B - Zr - P; d)-modification of Al Si alloys 11 to 13% (Na); e) refinement of aluminium and silicon alloys 15-25%; f) removal of magnesium, sodium, calciumfrom Al alloys, separately orcombined; g)fluxwashing with very low melting point flux mixes.

Temperature trials Metal temperature atstart 760'C Finish 740'C Time Temp. Time Temp. Time Temp. REMARKS Secs. "C. Secs. "C. Secs. "C.

0 17 10 200 20 336 Temperature was 10 38 20 206 30 339 measured using 20 42 30 213 40 342 C.Al-Alumel T/C 30 47 40 220 50 344 down centre tube 40 51 50 226 11.00 347 11/2-2inches from 50 56 6.00 232 10 348 bottom of lance.

1.00 60 10 239 20 350 Normal duration 10 63 20 245 30 349 of test is 20 67 30 251 40 348 10 minutes.

30 70 40 257 50 353 40 73 50 263 12.00 362 50 76 7.00 269 10 367 2.00 79 10 275 20 370 10 81 20 280 30 372 20 85 30 285 40 374 30 90 40 291 50 375 40 95 50 296 13.00 377 50 102 8.00 300 3.00 108 10 304 10 115 20 307 20 123 30 312 30 130 40 316 40 137 50 319 50 144 9.00 322 4.00 151 10 325 10 159 20 327 20 165 30 329 30 172 40 331 40 179 50 331.

50 186 10.00 332 5.00 192 10 334

Claims (9)

1. An injection lance for the introduction of chemical substances into baths of molten metal, comprising inner and outer spaced coaxial tubes, with a thermally insulating layer or layers interposed between them,the innertube having its extremity at the nozzle or discharge end of the lance contained within refractory material and buried some distance in from the lance nozzle opening.

2. A lance according to Claim 1,wherein the space between the inner and outer tubes contains a tubular sheath of refractory material, with air gaps provided between the refractory sheath and both the inner and outertubes.

3. A lance according to Claim 2,wherein the inner and outertubes are of metal.

4. A lance according to Claim 3, wherein the outertube is of high temperature metal and has an outer refractory coating.

5. A lance according to Claim 2 Claim 3 or Claim 4, wherein the gap between the refractory sheath and the innertube is plugged at both ends with insulating material.

6. Alance according to anyone ofthe preceding claims,whereinthe end ofthe outertube atthedischarge end ofthe lance iscutto a chamfer and the chamfered end ofthistube contains afilling of rammed refractory material in which the end of the inner tube is buried, the terminal portion of the central bore ofthe lance through which the chemical substances are discharged being formed by the rammed refractory material.

7. A lance according to Claim 6, wherein the rammed refractory material is composed of ceramicfibres.

8. A lance according to anyone ofthe preceding claims, wherein atthe inlet end ofthe lance the end ofthe outer tube is closed by a flanged end cap having a central aperture through which the inner tube passes.

9. An injection lance substantially as described with reference to the accompanying drawings.