GB2256866A

GB2256866A - Improvements in or relating to ceramic welding process

Info

Publication number: GB2256866A
Application number: GB9213098A
Authority: GB
Inventors: Kevin Dunderdale; Robert Geoffrey Everitt; 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: GB9213098D0; ZA924435B; GB2256866B; GB9113365D0

Abstract

In a ceramic welding process a composition of matter therefor includes a mixture of refractory particles and oxidisable metal particles wherein the harmonic mean size of the refractory particles is between 300 and 1000 microns, and the size range spread factor of the refractory particles is between 0.4 and 1.1 inclusive.

Description

IMPROVEMENTS IN OR RELATING TO CERAMIC WELDING PROCESS This invention concerns improvements in or relating to ceramic welding processes and in particular has reference to the size characteristics of the powder elements of the composition of matter employed in such processes.

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, 2 110 200, 2 170 191 and our co-pending British Patent Applications Nos 91 3370.2 and 91 13369.4 (Ref: BCC/P/26 and BCC/P/27).

British Patent No 2 170 191 specifies the size characteristics of the refractory particles and the oxidisable metal particles by various properties including one term which approximates to average size and another term which relates to size distribution. The language of the principal claims of 2 170 191 can be simplified by translation into more readily understandable expressions to convey the meaning that the average size of the refractory particles must be greater than the average size of the oxidisable particles and that the spread of sizes of the refractory particles should be above a certain minimum.The former of these two requirements is plainly obvious and merely reproduces the disclosures of earlier patents in the ceramic welding field, for example British Patent No 1 330 894 which gives maximum average sizes for refractories and oxidisable particles as 500 microns and 50 microns respectively. A wide distribution of sizes, giving a high size range spread, is said in 2 170 191 to be of benefit in that it contributes to the formation of durable welds which are less porous and contain less cracks than welds produced using refractory particles of more homogeneous size distribution.

In view of the benefits said to be derived from the operation of the process and use of a composition of matter as defined in British Patent No 2 170 191, we have found, surprisingly, that high quality, durable refractory masses can be formed by employing refractory particles of more uniform size distribution than is suggested to be the case by this earlier patent. Moreover, the removal of the coarse fractions of the refractory particles assists the flow of the composition of matter through the transport system, enables a smoother finish to the resultant weld to be achieved and reduces the proportion of conveyed material which does not form part of the weld.

The elimination of the fine size fractions of the refractory particles is also beneficial in aiding the flow of matter, but has added benefits in regard to reducing the extent of silica dust which becomes airborne during handling, to reducing the dust cloud at the reaction zone and hence to reducing the proportion of conveyed material which does not form part of the weld, and to enhancing the exothermic reactions of the oxidisable particles by not stifling these reactions in a similar manner to stone dust which is used in extinguishing fires in underground mines.

An object of the present invention is to provide an improved ceramic welding process and a composition of matter therefor.

According to the invention there is provided a ceramic welding process in which a composition of matter comprising a mixture of refractory particles and oxidisable metal particles is projected with an oxygen-containing gas through a lance to a surface where oxidisable particles react exothermally to cause at least partial fusion of the other particles of the mixture, both with themselves and with the surface, so that a ceramic weld is formed, characterised in that the harmonic mean size (as defined herein) of the refractory particles is between 300 and 1000 microns inclusive, and the size range spread factor (as defined herein) of the refractory particles is between 0.4 and 1.1 inclusive.

The harmonic mean size is defined as

where Wi is the weight or percentage of material in the ith size fraction and Si is the mean size of the ith fraction.

The size range spread factor is defined as follows:

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 used to denote the % proportion by weight of refractory particles which will pass a screen having a mesh of that size.

The harmonic mean size is a useful term to adopt to describe particle size distributions since it is widely known and in processes such as ceramic welding where the chemical reactions are surface dependent, the harmonic mean size is relevant in that it is defined as the diameter of a particle of average surface area.

The refractory oxides which can be used in the invention include silica, alumina, zirconia, magnesia, calcium oxide, chrome oxide, alumino-silicates and mixtures thereof. The percentage by weight of refractory oxides in the composition of matter should be at least 80%. There should be 10-20% by weight of oxidisable metal particles which can comprise silicon, aluminium, zirconium, magnesium, calcium, chromium and mixtures thereof.

It is common general knowledge that the oxidisable metal particles should be fine in order to promote their reaction with oxygen. In practice, commercially available metal particles of this kind with a maximum size of 125 microns (but with a size distribution down to sub-micron size) are commonly used and are very satisfactory.

The invention also provides a composition of matter for a ceramic welding process, the composition comprising a mixture of refractory particles and. oxidisable metal particles characterised in that the harmonic mean size as defined herein of the refractory particles is between 300 and 1000 microns inclusive, and the size range spread factor as defined herein of the refractory particles is between 0.4 and 1.1 inclusive.

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 may be transported 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.

A ceramic welding process and a composition of matter according to the invention are now illustrated by the following Examples.

EXAMPLE 1 A composition of matter comprising a mixture was prepared with 84% by weight of crushed silica refractory and 16% by weight silicon. The refractory had a harmonic mean size of 600 microns and a size range spread factor of 0.95. The silicon was of 125 microns maximum size.

The mixture was sprayed onto silica refractory brick 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 19 mm diameter, the nozzle being positioned 75-100 mm from the substrate refractory.

The mixture auto-ignited and produced a good weld which was firmly adhered to the substrate.

EXAMPLE 2 The experiment of Example 1 was repeated but with a composition of matter comprising a mixture of refractory particles sized between lmm and 250 microns to give an harmonic mean size of 400 microns and a size range spread factor of 0.6. Again, the welding operation was easy to initiate and control, and a good dense durable weld was formed.

Claims

1. A ceramic welding process in which a composition of matter comprising a mixture of refractory particles and oxidisable metal particles is projected with and oxygen-containing gas through a lance to a surface where oxidisable particles react exothermally to cause at least partial fusion of the other particles of the mixture, both with themselves and with the surface, so that a ceramic weld is formed, characterised in that the harmonic mean size as defined herein of the refractory particles is between 300 and 1000 microns inclusive, and the size range spread factor as defined herein of the refractory particles is between 0.4 and 1.1 inclusive.

2. A process according to claim 1 in which the harmonic mean size is between 400 and 700 microns inclusive.

3. A process according to claim 1 or 2 in which the size range spread factor is between 0.5 and 1.0 inclusive.

4. A process according to any one of the preceding claims in which the refractory oxide comprises one or more of silica, alumina, zirconia, magnesia, calcium oxide, chrome oxide and alumino-silicates.

5. A process according to any one of the preceding claims in which the refractory oxide comprises at least 80% by weight of the total composition of matter.

6. A process according to any one of the preceding claims in which the oxidisable metal particles comprise 10-20% by weight of the total composition of matter.

7. A process according to any one of the preceding claims in which the oxidisable metal particles comprise one or more of silicon, aluminium, zirconium, magnesium, calcium and chromium.

8. A ceramic welding process substantially as hereinbefore described with reference to the Examples.

9. A composition of matter for use in the ceramic welding process according to any one of the preceding claims.