GB2035524A - Flame spraying refractory material - Google Patents

Abstract

A method and apparatus for flame spraying refractory material to repair cavities 19 in refractory walls. The method comprises supplying to a lance a mixture of one or more finely divided refractory oxides and one or more finely divided metal or metalloid elements which are easily oxidisable to refractory oxides dispersed in a carrier gas itself incapable of supporting oxidisation of said elements, and supplying oxygen to the lance adjacent its outlet. The apparatus comprises a mixture store 8 pressurised by an airline 7, an air driven screw feed 5, and an air transporter line 10 for supplying mixture in suspension to a lance manifold 17. Oxygen is fed to the lance by a feed tube 16. <IMAGE>

Description

SPECIFICATION Flame spraying This invention concerns a method and apparatus for repairing refractory blocks and other substrates, e.g. in furnaces and ovens.

The refractory brickwork or coating in carbonisation ovens or metal processing furnaces, ladles, soaking pits and combustion chambers, e.g. steel blast furnaces, is subject to erosion or cracking by reason of the loads and stresses imposed at very high temperature. The desirability of repairing such ovens or furnaces in situ has been known for many years. It is important in many instances that the oven or furnace cannot be allowed to cool much below operating temperature otherwise serious damage can be done, often amounting to complete loss of the furnace or oven. To overcome this problem, a technique known as 'flame spraying' has been established, which involves spraying molten or sintered refractory particles from a lance into the oven or furnace and onto the area of wall requiring repair, where it builds up.The first commercial system in Britain is believed to have been one involving carrying silica powder in oxygen and supplying, at the tip of a lance, acetylene as fuel. This system is very slow and it takes a long time to build up a significant quantity of refractory, and it will be appreciated that considerable hazards are involved.These hazards will be more fully discussed below.

The patent literature contains a number of proposals, few, if any, of which have reached commercial or even workable states. British Patent Specification No. 1,151,423 discusses prior art proposals of entraining powdered refractory in a stream of fuel gas which is then passed to a burner to be burnt with oxidising gas. Patent Specification No.

1,151,423 proposes that instead of using compressed gas for carrying the powdered refractory, the powder is carried in a liquid fuel, suitably a light fuel oil. Another proposal, although mainly concerned with the apparatus, is in British Patent Specification No.

991,046, also discloses carrying powdered refractory material in oxygen and supplying propane as a fuel gas.

A slightly different approach in prior proposals is to use an easily oxidised metallic element to provide all or a part of the heat required to melt refractory powder. U.S. Patent Specification No. 2,741,822 is concerned with the formation of shaped masses of refractory by oxidation of a mixture of an element such as magnesium, aluminium or silicon with an inert refractory filler such as MgO, A1203 or SiO2. Conveniently, the reaction products of the oxidation of such elements are themselves refractory oxides. More recently, British Patent Specification Nos.

1,330,894 and 1,330,895 relate to a flame spraying apparatus and method using easily oxidisable elements, of a very small average grain size (less than 50cm) and at least one other substance. The powder mixture is preferably carried in oxygen - enriched air or more preferably in oxygen to a lance, after issuing from which the powder mixture is ignited. It is said that it was discovered after much experimental research that the oxidisable elements must be of average particle size less than 50cm, preferably less that 10cm; such small particle sizes and attendant large specific surface areas promote rapid oxidation and release of heat for the melting or surface melting of the other substance.It is said that certain accident hazards are involved and it is preferred to use apparatus having an automatic safety device to establish safe conditions if there is a risk of flashback. It has now been found that such a risk is very real; the flame propagation speed of such a mixture in oxygen is substantially higher than the normal gas velocity, hence flash back is an everpresent danger. Furthermore, it has now been discovered that the very small particle size of the oxidisable element, in addition to increasing the risk of flash-back because of the very high surface area, leads to very early ignition relative to the emission of the particles from the tip of the lance. This early ignition leads to the formation of refractory on or in the lance tip and blocking of the gas flow.Operation of the repairing process is interrupted frequently by flash-back conditions and by blockage of the lance.

The present invention provides a method and apparatus which overcome the above disadvantages. Accordingly, the invention provides a method of flame spraying refractory material which method comprises supplying to a lance a mixture of one or more finely divided refractory oxides and one or more finely divided metal or metalloid elements which are easily oxidisable to refractory oxides, dispersed in a carrier gas itself incapable of supporting oxidation of said element(s) and supplying oxygen to the lance adjacent its outlet.

The invention also provides an apparatus for flame spraying refractory material, which apparatus comprises a material store, feeding means for feeding material from said store into a first gas stream to disperse said material in said gas stream, a lance comprising a first feed tube for the gas stream containing dispersed material and an oxygen feed tube, the outlet of the oxygen feed tube being adjacent the first feed tube outlet.

Preferably, the mixture of refractory oxide(s) and metal or metalloid element(s) are such that the chemical composition after oxidation is substantially identical to that of the fire brick, furnace lining, refractory block etc.

which is being repaired. Depending on the use of the refractory, a considerable range of refractory oxides are used in practice, varying from acidic to basic bricks blocks or coatings.

The refractories normally used are composed principally of silica, alumina or magnesite, with smaller amounts of other refractory oxides such as ZrO2 and ZrSiO4 or complex oxides such as spinels. It is relatively simple matter to analyse the material which is to be repaired, and to establish by simple trial and error the proportion of oxidisable element necessary to yield a satisfactory flame spray and to yield an identical final composition. The reactivity of the mixture can, of course, be modified by varying the proportion of inert material, that is the proportion of refractory oxide. The mixture may contain small amounts of other components if desired for specific purposes and may, for example, include silicon carbide particles to give a material highly resistant to abrasion and/or of higher thermal conductivity.On the other hand, however, it has been found in practice that the repair, although of the same chemical composition as the brick etc. to which it bonds, is of different physical characteristics and is usually more resistant to wear or generally tougher that the parent brick.

A convenient source of refractory oxide is crushed firebrick or refractory of the same composition as it is wished to repair. For example, in the repair of carbonisation ovens or blast furnaces the so called semi-silica or silica firebricks are used. Such material is generally available on the site or plant at which the repair is desired. Powdered silicon, aluminium and magnesium are all commercially available in a variety of nominal particle sizes. In general, the particle size of the mixture is not critical, but it is preferred to use the oxidisable element in a size range of up to 1 52 m (- 100 BS Mesh); not only is the cost less but the slower release of heat from larger particles is believed to contribute to the avoidance of the problem of blocking the lance tip.When using powdered silicon and aluminium, a suitable particle size is - 100 BS mesh, and the refractory oxide is conveniently of larger particle size than the elemental parti cle size, for example - 0.8 mm, although larger particle sizes are envisaged also.

As the result of experimental work by the Applicants, it was found that the mixture of refractory and oxidisable element(s) does not combust in air. If air is used as the carrier gas, therefore, a very much safer method results, without the danger of flash-back to the material store. Air is the preferred carrier gas because of its cheapness and its inherent content of oxygen, so that less oxygen needs to be supplied to complete oxidation. Other gases which do not support combustion of the mixture, such as an inert gas, e.g. nitrogen, may, however, be used in an alternative.

The quantity of mixture carried in air is suitably of the order of 0.5 Kg in 50 - 100 1, per minute.

The quantity of oxygen supplied, preferably ranges from 2:1 to approximately 4:1 by volume, oxygen to air or inert gas, depending on the repair zone temperature.

The apparatus according to the invention preferably includes a feeding screw driven by a compressed air motor; part of the compressed air used in preferred embodiments is bled to drive the motor. It is also preferred to pressurise the material store, which is conveniently a hopper mounted above a screw feed, to prevent back pressure causing blow back of material. If the material bridges in the material store, conventional handling means such as vibrators may assist even feeding.

Preferably the oxygen feed pipe is connected to an annular manifold fitted around the first feed tube, so that the material dispersed in the first gas stream emerges from the lance with a surrounding 'skin' of oxygen.

Other gas flow configurations may also be used, however.

For safety, when handling oxygen, it is preferred that all tubes or fittings in contact with oxygen or oxygen - rich gas are not of mild steel but are of non-arsenical copper, stainless steel or brass.

Having attained a suitable mixture composition and carrier gas and oxygen flow rates the jet or spray issuing from the lance may be easily ignited by contact with a hot wall of the oven of furnace or by contact with a flame.

The jet or spray is immediately converted into a flame spray of refractory material and contact of the flame spray on eroded areas or fissures in the oven or furnace causes surface melting and the build up of molten or sintered particles which impinge thereon. A very strong bond between the oven or furnace wall and the built up refractory material results.

Preferably, the flame spray is moved slowly across the area to be repaired to avoid slumping because of too localised build up of refractory material.

It is found that combustion of the mixture does not start for some distance from the tip of the lance, where sufficient mixing of the oxygen with the mixture is attained. This prevents any real fouling of the lance tip by build up of refractory, and continuous operation can easily be achieved.

it will be appreciated that if the lance is used for a long time in a high temperature environment, for example deep in a carbonisation oven, it will be advantageous to cool the lance. This may be achieved by circulating water through tubes or coils in thermal contact with the feed tubes.

The invention will now be described with reference to the accompanying schematic drawing which shows an apparatus according to the invention.Line 1 is connected to a source of compressed air, and is split into two lines controlled by valves 2 and 3. The air line controlled by valve 2 leads to an air-driven motor 4, which drives a screw feeder 5. The speed of the motor and hence the speed of the screw feeder can be adjusted using valve 2. The air line controlled by valve 3 is further split into two separate lines 7, controlled by valve 6, and 10. Air line 7 pressurises a hopper or material store 8, which contains a quantity of mixture for use in the process. The hopper 8 is mounted above the screw feeder 5 which carries mixture to a T-junction 20, where the air from line 10 carries the mixture in suspension through flexible hose 11.The hose 11 is connected to a first feed tube 1 2 which forms part of a lance.

A source of compressed oxygen is connected to line 1 3 which is controlled by valve 14, and connected by flexible hose 1 5 to oxygen feed tube 1 6. Oxygen feed tube 1 6 is connected to a manifold 1 7 which also forms part of the lance. Each of the gas lines has conveniently incorporated therewith pressure and/or flow rate gauges or controllers also safety pressure relief valves; it will be possible for an operator to adjust the rate of mixture fed and rates of gas flow to achieve or reproduce desired conditions or automatic control means may be connected to the gauges or controllers.

In a typical operation, a cavity 1 9 in the wall of a semi-silica firebrick in a carbonisation oven requires repair. The mixture 9 consists of:

particle by wt. size range 31% Si metal -100 BS mesh 9% Al metal 60% crushed semi-silica firebrick - 0.8 mm.

Valve 2 is set to deliver approximately 0.5 Kg of mixture per minute, and valve 3 to deliver 50 to 100 1 of air per minute. Oxygen is supplied to the lance at an approximate 4:1 volume ratio to the air supplied, at, or corrected to, the same pressure. The spray issuing from the lance is ignited by contact with the hot oven wall and becomes a flame spray 18, in which the elemental silicon and aluminium oxidise to silica and alumina respectively.

The operator controlling the lance, conveniently one of a two-man team, the other controlling the material and gas flow-rates, plays the flame onto the cavity 1 9 and a deposit of material is rapidly built up to fill the cavity.

Examination of the deposited material shows an identical chemical composition to that of the parent semi - silica firebrick; the physical state is a glassy solid considerably harder than the parent firebrick. There appears to be surface melting of the firebrick and a very strong bond is obtained between the repair and the firebrick. Tests on the bond strength by repeated thermal shock did not lead to any separation or cracking.

Claims (11)

1. A method of flame spraying refractory material which method comprises supplying to a lance a mixture comprising one or more finely divided refractory oxides and one or more finely divided metal or metalloid elements which are easily odixisable to refractory oxides, dispersed in a carrier gas itself incapable of supporting oxidation of said element(s) and supplying oxygen to the lance adjacent its outlet.

2. A method according to claim 1, wherein the refractory oxides are selected from alumina, silica and magnesia and the elements are selected from aluminium, silicon and magnesium.

3. A method according to claims 1 or 2, wherein the carrier gas is air.

4. A method according to claim 3, wherein the volume ratio of oxygen to air is 2:1 to 4:1.

5. A method according to any one of the preceding claims, in which the mixture is constituted to yield after oxidation a refractory of substantially identical chemical composition as that of a refractory which is being repaired.

6. A method according to claim 1, substantially as hereinbefore described.

7. An apparatus for flame spraying refractory material, which apparatus comprises a material store, feeding means for feeding material from said store into a first gas stream to disperse said material in said gas stream, a lance comprising a first feed tube for the gas stream containing dispersed material and an oxygen feed tube, the outlet of the oxygen feed tube being adjacent the first feed tube outlet.

8. An apparatus according to claim 7, wherein the feeding means is a feeding screw driven by a compressed air motor.

9. an apparatus according to claim 7 or 8, wherein the material store is connected to a source of compressed carrier gas.

10. An apparatus according to claim 7, 8 or 9, wherein the oxygen feed pipe is connected to an annular manifold mounted at the outlet of the first feed pipe.

11. An apparatus according to claim 7, substantially as hereinbefore described with reference to the drawing.