GB2103959A - Repairing refractory substrates - Google Patents

Abstract

An apparatus for flame spraying a mixture of refractory and easily combusted metal particles includes a material store (2), which holds the mixture in a state of full or partial fluidisation and means (3, 4, 5) for entraining mixture in a jet of relatively inert gas, such as air. The entrained mixture is supplied to a lance (14) where it is mixed with sufficient oxygen to combust. <IMAGE>

Description

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

The refractory linings, e.g. brickwork or coatings in carbonisation ovens, especially coke ovens, or metal processing furnaces, ladles kilns, soaking pits and combustion chambers, is subject to erosion or cracking by reason of the abrasion, 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 discussed 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, which 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 part of the heat required to melt refractory powder. U.S. Patent Specification No.

2,741,922 is concerned with the formation of shaped mass of refractory by oxidation of a mixture of an element such as magnesium, aluminium or silicon with an inert refractory filler such as MgO, Al203 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 50,us) 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 30 um, preferably less than 10 um; 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 flash-back. 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 flashback is an ever-present danger. Furthermore, 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, may lead 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.

Although this method and apparatus has reached commercial use, it is found that operation of the repairing process is interrupted frequently by flash-back conditions and by blockage of the lance, despite the use of specialised and experienced teams of operators.

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 holding in a store, a mixture of finely divided refractory oxide and one or more finely divided metal or metalloid elements which are easily combustible to refractory oxides, in a state of partial or full fluidisation by a relatively inert gas, metering said mixture into a relatively inert carrier gas by entrainment of the mixture in a jet of said carrier gas, supplying the resulting mixture dispersed in the carrier gas to a lance and supplying sufficient oxygen to the lance adjacent an outlet for the mixture to permit combustion of the elements in the mixture.

The invention also provides an apparatus for flame spraying refractory material, comprising a material store, means for providing partial or full fluidisation of material in said store, means for metering material from said store by entrainment in a jet of relatively inert carrier gas, means for conveying the resulting mixture to a lance and means for supplying the lance with oxygen adjacent an outlet for the material in the carrier gas.

The jet of carrier gas may be projected between axially-aligned input and output pipes positioned in the material store and these are preferably mounted above the base of the store and are preferably positioned substantially centrally in the store. It is especially preferred that the gap between the input and output pipes should be adjustable, in order to vary the uptake of material in the carrier gas according to the requirements of the operator of the equipment. Other arrangements for the provision of the jet may, however, be used, and these include in particular a venturi device in which material is fed at one or more points at right angles to the jet. Such a venturi device may be mounted external of the store.

Although the material, transported in the relatively inert carrier gas, may be fed directly to the lance, it may be advantageous to admix the material/gas mixture with a second supply of relatively inert gas before it is fed to the lance.

Suitably, 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 found 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 ZrSiO4 and ZrO2 or complex oxides such as spinels. It is a 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 mixture may contain small amount of other components if desired for specific purposes and may, for example, include 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 than the parent refractory.

A convenient source of refractory oxide is crushed firebrick or refractory of the same composition as it is wished to repair. For example, in carbonisation ovens 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 criticai, but it is preferred to use the oxidisable element in a size range up to 152,um; 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 particle size, for example -0.8 mm, although larger particle sizes are envisaged also, especially if this gives a closer match to the crack size and the physical properties of the refractory to be repaired.

Although the result of experimental work by the Applicants showed that the mixture of refractory and oxidisable element(s) does not combust in air.

even greater safety and reliability can be achieved by using a more inert gas, according to the invention, to transport the material. Suitable relatively inert gases include nitrogen, helium, carbon dioxide, oxygen-depleted air, air and mixtures thereof. If such a relatively inert gas is used to partially or fully fluidise the material in the store, there is little danger of dust-type explosions despite the reactivity of the metal or metalloid particles. Full fluidisation is not necessary to achieve the benefits of improved supply and metering of material, providing there is sufficient gas flow through the material to facilitate movement of the material.

The use of a first supply of relatively inert gas to meter the material, and a second supply of relatively inert gas to further disperse the material and to transport it to the lance is designed to give excellent control and flexibility to operation of the method of the invention. In particular, it facilitates igniting a combustion flame at the lance using a low flow rate of material, which can then be increased to a suitable flow rate for repair of the substrate. It also facilitates the transport of relatively coarse particles.

The quantity of mixture carried in the relatively inert gas is typically of the order of 0.1 kg in 50 litres of gas at operating pressure, and a flow rate of about 0.1 kg per minute is typical, which is convenient for the repair of carbonisation ovens, for example, but higher rates may be used although control of placing of the mixture by the operator may be adversely affected.

The gases used may be supplied from cylinders or tanks, or by means of lines; for example, a compressed air line may be used provided the air is dry enough to avoid caking problems in the material store or elsewhere in the apparatus.

The quantity of oxygen supplied for the above preferred quantity of mixture in gas is approximately 2:1 by volume, oxygen to total carrier gas, depending on the repair zone temperature. Pure oxygen need not be used, however, provided that sufficient oxygen is supplied to support continuous combustion of the elements.

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 be used, however, and it may be advantageous to induce a swirl flow pattern.

For safety, when handling oxygen, it is preferred that all metal tubes of 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 or 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 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 significant 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, or by forced air or gas cooling.

The invention will now be described by way of an example with reference to the accompanying schematic drawing which shows an apparatus according to the invention.

A charge of welding powder 1, is held in a material store 2, which can be easily opened to recharge with powder. Near, but spaced apart from the base, are compressed nitrogen input and output lines 3 and 4, which are axially-aligned and define between their ends a gap 5. The gap can be adjusted in length by a simple mechanical arrangement mounted on the outside of the container, so that the feed rate of welding powder suspended in gas in the output line can be adjusted by an operator.

Cylinders of compressed nitrogen 6, and compressed oxygen 7, which are conveniently mounted on a portable support, which may or may not be used also as a support for the material store, feed through reducing valves 8, and flow indicators, such as rota meters 9, nitrogen and oxygen lines 10 and 11. Both lines are controlled by valves 12, 13 adjacent the lance which is generally indicated by 14, so that the lance operator can have essentially complete control over the gas feed rates. The oxygen line feeds directly into a hand-held mixing chamber 15, which is part of the lance. A part of the nitrogen feed is taken off and fed to a chamber 1 6 under the store 2, passing through a large number of apertures in the base to partially fluidise the charge 1.The jet of nitrogen picks up a controlled quantity of welding powder and carries it in dispersion through output line 4 to the mixing chamber 1 5. Removably fitted to the mixing chamber is a lance 17 which can be chosen according to the duty required and the accessibility of the place in the oven, furnace, kiln, ladle or the like, to be repaired.

It has been found in practical tests in repairing high and low (ca 6000 C) temperature substrates that an operator can carry out effective repairs after only a few minutes training. After the reducing valves 8 are turned on, adjustment of valve 12 in the nitrogen line varies the powder delivery rate to the mixing chamber 1 5.

Subsequent adjustment of valve 13 in the oxygen line enables the operator to optimise the combustion behaviour of the powder in the flame spraying operation. The rota meters 9 serve to indicate the flow rates of the nitrogen and oxygen to the operator to enable him to monitor operation. The rota meters are, of course, chosen to indicate clearly in their middle range, flow rates which are generally found to be suitable, although clearly the flow rates of nitrogen and oxygen are not the same.

An important feature associated with the function and location of these valves is that, in the event or anticipation of an emergency, the operator can instantaneously shut down the process.

For a typical repair to a cracked or eroded semisilica firebrick wall of an oven chamber the weld powder consists of: by weight 9% Aluminium by weight 31% Silicon and 60% crushed semi-silica fire brick The metallic elements are crushed to 10% minus 100 BSS and the firebrick to less than 0.8 mm.

The reactivity of the powder can be modified by varying the proportion of semi-silica in the blend.

Another suitable weld powder consists of 12% silicon, 4% aluminium and 84% silica.

In one embodiment of the invention, the gap between the input and output pipes is set to 6.4 mm (- in.) and the feed control valve adjusted to permit a nitrogen flow rate of approximately 50 litres/min. at an input gauge pressure of 1.035 bar (15 p.s.i.). The oxygen supply is then set to deliver 100 litres/min. at gauge pressure 1.104 bar (16 p.s.i.) i.e. slightly positive with respect to that of the nitrogen. Under these conditions, the powder delivery rate is of the order of 100--120 g/min. which given a hopper capacity of 5 kg, offers an active welding operational time of 1 hour. The powder spray issuing from the lance is ignited by contact with a hot (8000C) carbonisation oven wall and becomes a flame spray which, when directed on to the designated area deposits a glassy solid which is chemically bonded to its surface.

The apparatus of the invention is relatively simple and reliable, and does not require skilled operators or skilled routine maintenance, and in particular is believed to be safe in operation.

Highly portable units can be built at relatively low cost, thus permitting individual sites to have their own units for repair of defects before further degradation. This is in sharp contrast with the major commercial flame spraying process, which requires a team of skilled operators. This known process requires great care and many safety precautions because the welding powder is transported in oxygen. Furthermore, because of the cost of the prior process, it is customary to allow a number of defects to accumulate before the necessary repairs are made.

In comparison to known apparatus, the apparatus of the present invention can provide mechanically simple equipment which gives a reliable feed to the lance without causing sieving or size segregation of the feed particles. Moreover a single operator can control all the necessary parameters for successful repair work; this also means that the single operator can easily control all the parameters necessary for safey operation and he does not need to rely upon communication with another operator. Repairs can be effected using the apparatus and method of the invention on cool substrates, if the spray of material is ignited, for example by a separate flame.

Claims (13)

1. A method for flame spraying refractory material, which method comprises holding in a store, a mixture of finely divided refractory oxide and one or more finely divided metal or metalloid elements which are easily combustible to refractory oxides, in a state of partial or full fluidisation by a relatively inert gas, metering said mixture into a relatively inert carrier gas by entrainment of the mixture in a jet of said carrier gas, supplying the resulting mixture dispersed in the carrier gas to a lance and supplying sufficient oxygen to the lance adjacent an outlet for the mixture to permit combustion of the elements in the mixture.

2. A method according to claim 1, wherein the jet of carrier gas is projected between axiallyaligned input and output pipes positioned in the material store.

3. A method according to claim 2, wherein the gap between said input and output pipes is adjusted to vary the uptake of mixture in the carrier gas.

4. A method according to claim 1,2 or 3, wherein the quantity of mixture entrained is approximately 0.1 kg in 50 litres of carrier gas.

5. A method according to any one of the preceding claims, wherein the flow rate of mixture is approximately 0.1 keg per minute.

6. A method according to any one of the preceding claims, wherein the carrier gas is selected from nitrogen, helium, carbon dioxide, oxygen-depleted air, air and mixtures thereof.

7. A method according to claim 6, wherein air is used as the carrier gas and oxygen is supplied to support combustion at an approximate volume ratio of 2 :1, oxygen to air.

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

9. An apparatus for flame spraying refractory material, comprising a material store, means for providing partial or full fluidisation of material in said store, means for metering material from said store by entrainment in a jet of relatively inert carrier gas, means for conveying the resulting mixture to a lance and means for supplying the lance with oxygen adjacent an outlet for the material in the carrier gas.

10. An apparatus according to claim 9, wherein the jet of carrier gas is projected between axiallyaligned input and output pipes positioned in the material store.

11. An apparatus according to claim 10, wherein the gap between the input and output pipes may be adjusted to vary the uptake of material in carrier gas.

12. An apparatus according to any one of claims 9 to 11, wherein the means for providing partial or full fluidisation of material in the store is separately controllable from the supply of carrier gas for entraining the material.

13. An apparatus according to claim 9, substantially as hereinbefore described with reference to the accompanying drawing.