GB2256864A

GB2256864A - Ceramic welding.

Info

Publication number: GB2256864A
Application number: GB9213049A
Authority: GB
Inventors: Kevin Dunderdale; David Kenneth Hurran; Leslie Ernest John Tucker
Original assignee: Coal Industry Patents Ltd
Current assignee: Coal Industry Patents Ltd
Priority date: 1991-06-20
Filing date: 1992-06-19
Publication date: 1992-12-23
Anticipated expiration: 2012-06-19
Also published as: GB9213049D0; GB9113369D0; GB2256864B

Abstract

In a ceramic welding process in which a composition of matter is sprayed at a substrate to form a weld, the composition of matter comprises a mixture of refractory oxide particles and silicon in combination with one or more of magnesium, calcium, zirconium, and chromium.

Description

IMPROVEMENTS IN OR RELATING TO CERAMIC WELDING This invention concerns improvements in or relating to ceramic welding processes and in particular has reference to an improved composition of matter for use therein.

Such processes are well established for repairing the internal refractory structures of coke ovens, glass furnaces and the like and are disclosed for example in British Patents Nos 402 203, 1 330 894 and 2 035 524 and Swedish Patent No 102 283. A ceramic welding process usually involves the conveyance in a carrier gas of a composition of matter in the form of a mixture of powders to a lance, the powder particles being entrained within the lance in an oxygen-containing gas and projected from the lance to a surface where a part of the powder mixture reacts exothermically with oxygen to cause at least partial fusion of the other part of the powder mixture, both with itself and with the surface, so that a ceramic weld is formed.

A composition of matter suitable for use in a ceramic welding process is typically a mixture of refractory oxide and metal and/or metalloid particles in- powder form. A composition of matter and process for in use in forming refractory masses by a ceramic welding process are disclosed in British Patents Nos 2 154 228 and 2 110 200. These earlier patents describe compositions of matter consisting of incombustible refractory material and particles of exothermically oxidisable material which latter have a characteristic average size of below 50 microns and comprising silicon and aluminium, the aluminium being present in an amount up to 12% by weight of the total mixture. Claim 8 of British Patent No 2 154 228 prescribes the aluminium content as being at least 1% by weight of the total mixture.In the Examples of these patents, mixtures of silicon and aluminium are described as being used with refractory oxide particles comprising one or more of the following namely zirconia, magnesia, alumina, silica, sillimanite and mullite.

The above referenced patents refer to the selection of refractory particles and metal particles to provide a weld repair which matches the chemical composition of the substrate refractory to be treated. Similarity of chemical composition is considered by some of those skilled in the art to be of great importance particularly in certain applications. In this respect, our own co-pending British Patent Application No 91 13370.2 (Ref: BCC/P/26) describes and claims a composition of matter for use in a ceramic welding process suitable inter alia for the repair of coke ovens, which composition has been developed to achieve such similarity and indeed other benefits as delimited therein and not to be found in the prior art.

The principal requirements of a weld are durability and process compatibility. Chemical composition is a factor governing process compatibility, but it is not the only one, for it is feasible to form a weld onto a refractory substrate where the weld and substrate differ significantly in composition but where, for example, thermal expansions, thermal conductivities and/or resistance to chemical attack are similar.

It is thus an object of the present invention to provide a composition of matter suitable for a ceramic welding process, the composition being process compatible with the refractory to be repaired.

According to the invention there is provided a composition of matter for use in a ceramic welding process in which the composition of matter is sprayed against a substrate to form a refractory mass in situ on the substrate, the composition comprising a mixture of incombustible refractory material and exothermically oxidisable material in particulate form characterised in that the composition of matter comprises a mixture of refractory oxide particles, and silicon in combination with one or more of magnesium, calcium, zirconium, and chromium.

An advantage of using silicon in combination with one or more of magnesium, calcium, zirconium and chromium in contrast to employing silicon alone or silicon with aluminium, is that the exothermic reaction of the metal particles with oxygen is easier to initiate and this facilitates the welding process and its operational control at lower substrate temperatures. The ease of ignition of magnesium, calcium, zirconium and chromium relative to aluminium means that coarser particles and/or smaller weight percentages may be used than for aluminium, with conceivably consequential benefits on the overall cost of the welding powder composition.

Preferably the silicon present in the composition comprises the major proportion of the total quantity of metallic elements present. It is thus clear that the composition of matter suitable for the repair of silica refractories and refractories contains a significant amount of silica. Accordingly, silicon is advantageously present in the amount of 10-20% inclusive by weight of the total composition of matter and in some preferred formulations in the amount of 12-16% inclusive by weight.

Preferably, the amounts of magnesium, calcium, zirconium and chromium do not total more than 3% by weight of the total composition of matter. As a further preference, the quantity of these components is 1% or less of the total composition.

By limiting this percentage the advantages of better ignition are realised and any effect of the corresponding metal oxides formed in the process on the properties of the resultant weld is minimised. The use of these components in the ceramic welding process with lower substrate temperatures thus affords control over the reaction during welding, thereby resulting in a better weld quality.

It is common general knowledge that fineness of the metallic components of the composition of matter will promote their reaction with oxygen. In practice, it is preferred to use silicon below 125 microns maximum size which gives an average particle size below 40 microns. In some applications, it is preferred that the other metallic components are of similar size. This is particularly the case where the refractory to be repaired is below about 9000C. For high temperature applications coarser size gradings of magnesium, calcium, zirconium, and chromium may be used up to 1000 microns maximum size and 250 microns average size.

The maximum size and size distribution of the refractory oxide particles of the welding of the composition of matter are not critical, but it should be ensured that the maximum particle size is not over large and that the fine fractions are not excessive, otherwise problems of powder transport and powder classification may arise. In practice, crushing of refractory to below 2 or 3mm top size and to give a minimum of fines is sufficient although in some applications removal of the very fine fraction, say below 125 or 250 microns, may be desirable. Advantageously, the size distribution of the particles may be in accordance with that defined in our co-pending British Patent Application No 91 13365.2.

Preferably the refractory oxide particles comprise one or more of silica, zirconia, magnesia, calcium oxide, chrome oxide, and alumino-silicates.

A composition of matter and a ceramic welding process employing such a composition according to the invention are described below with reference to the following Examples.

The composition of matter herein defined can be employed in different ceramic welding processes. For example, the composition may be transported to the weld site in oxygen or in the alternative in air with oxygen added prior to the weld site. Furthermore, ceramic welding equipment of differing types may be used, for transporting the composition of the invention to the weld site and for conducting the ceramic welding process.

EXAMPLE 1 A composition of matter comprising a mixture was prepared 85% by weight of crushed silica refractory, 14% by weight of silicon powder, and 1% magnesium, the maximum size of the refractory being 2mm. The silicon had a maximum size of 125 microns and an average size of 26 microns. The magnesium had a maximum size of 250 microns and an average size of 140 microns. The mixture was sprayed onto silica brick in an experimental furnace at 10000C using a machine and method essentially as described in British Patent No 2 173 715.

Powder was delivered at 60 kg/hr using an oxidising gas flow of 550 NL/min and an air/oxygen ratio of 1:2. The powder was ejected through a nozzle of 19mm - diameter, the nozzle orifice being positioned 75-100 mm from the substrate refractory. The mixture auto-ignited and produced a good weld which was firmly adhered to the silica brick substrate. Subsequent analysis of the weld and silica brick showed that they were process compatible in terms of similar expansions and conductivities over the operating temperature range, namely 1000-14000C.

EXAMPLE 2 A composition of matter was prepared as in Example 1 but with a mixture containing 0.5% magnesium and 0.5% additional silica refractory. All other conditions were identical to those employed in Example 1. Again welding was effected satisfactorily with a good weld quality in all respects.

EXAMPLE 3 A composition of matter was prepared as in Example 1 but with a finer grade of magnesium which had a maximum size of 125 microns and an average size of 26 microns. The other parameters remained the same except for the temperature at which the welding was conducted: in this example the host silica brick was at a temperature of 6000C. Again welding was effected satisfactorily with a good weld quality in all respects.

EXAMPLE 4 A composition of matter comprising a mixture was prepared containing 83.5% by weight of crushed silica refractory, 16% by weight of silicon and 0.5% by weight of calcium. The silica and silicon had the same size characteristics as given in Example 1. The calcium had a maximum size of 1 mm and an average size below 250 microns.

All other conditions were identical to those employed in Example 1, excepting that a nozzle of 16 mm diameter was used. .Again welding was effected satisfactorily with a good weld quality in all respects.

EXAMPLE 5 Further experimentation utilising the compositions of matter set forth in the previous Examples was followed but in situ on coke ovens at operating temperature. In these instances, the welds were formed on damaged silica brickwork in order to establish the efficacy of the compositions according to the invention. In all cases, the compositions ignited readily, the welding was easy and controllable, and good quality durable welds were formed. The coke ovens so repaired are now back in operation.

The present invention thus represents a departure from the prior art in terms of the actual composition of matter used in ceramic welding processes, and affords the possibility of carrying out repairs to substrates at lower temperatures with concomitant savings in the preparation of powder components.

Claims

1. A composition of matter for use in a ceramic welding process in which the composition of matter is sprayed against a substrate to form a refractory mass in situ on the substrate, the composition comprising a mixture of incombustible refractory material and exothermically oxidisable material in particulate form characterised in that the composition of matter comprises a mixture of refractory oxide particles, and silicon in combination with one or more of magnesium, calcium, zirconium, and chromium.

2. A composition of matter according to claim 1 in which the silicon is present in the mixture in an amount in excess of the combined total of the other metallic particles.

3. A composition of matter according to claim 1 or 2 in which the silicon is present in the mixture in the range 10-20% inclusive by weight of the total mixture.

4. A composition of matter according to any one of the preceding claims in which the silicon is present in the mixture in the range 12-16% inclusive by weight of the total mixture.

5. A composition of matter according to any one of the preceding claims in which the magnesium, calcium, zirconium, and chromium or mixtures thereof is present in the mixture up to 3% by weight of the total mixture.

6. A composition of matter according to any one of the preceding claims in which the magnesium, calcium, zirconium, and chromium or mixtures thereof is present in the mixture up to 1% by weight of the total mixture.

7. A composition of matter according to any one of the preceding claims in which the maximum size of the silicon is below 125 microns.

8. A composition of matter according to any one of the preceding claims in which the average size of the silicon is below 40 microns.

9. A composition of matter according to any one of the preceding claims in which the maximum size of the magnesium, calcium, zirconium, and chromium is below 125 microns.

10. A composition of matter according to any one of the preceding claims in which the refractory oxide particles comprise one or more of silica, zirconia, magnesia, calcium oxide, chrome oxide, and alumino-silicates.

11. A composition of matter according to any one of the preceding claims in which the maximum size of the refractory oxide is below 3mm and the average size is below 1.5mm.

12. A composition of matter according to any one of the preceding claims in which the maximum size of the refractory oxide is below 2mm and the average size is below imam.

13. A composition of matter according to claims 8 or 9 in which all the refractory oxide particles are above 125 microns.

14. A composition of matter according to claims 8 or 9 in which all the refractory oxide particles are above 250 microns.

15. A composition of matter according to any one of the preceding claims in which the size range spread factor of the refractory particles lies in the range 0.4 to 1.1, the size range spread factor being defined as:

where G80 denotes the 80% grain size of the refractory particles and G20 denotes the 20% grain size of the refractory particles and "% grain size" is denotes the % proportion by weight of refractory particles which will pass a screen having a mesh of that size.

16. A composition of matter for use in a ceramic welding process substantially as hereinbefore described with reference to the Examples.

17. A ceramic welding process employing a composition of matter as claimed in any one of the preceding claims.