Wet gunning has the problem that the machine or the gunning hose may get clogged by already moistened refractory material, particularly when not in continuous use; hence the process has to be carefully controlled. Also, the equipment requires more intensive cleaning and is not considered an efficient operation for applications less than 400 kg. With dry gunning, blockages in the conveying hose can be blown free by compressed air only. For optimum gunning and refractory performance the gunning material is essentially the same as the original refractory (plus binder material), but with a size distribution of 4 mm maximum. The applied thickness is typically in the range 10–30 mm and a vessel can normally be used again after 3–5 minutes.
In this article we will try to understand the basics of Gunning and Spraying, two frequently used methods for enhancing the refractory lining life of a furnace or refractory lining repair and maintenance. Also, differences between Gunning and Spraying, preferential situations for either of these two methods in EAF, BOF, melting furnaces, steel ladles, tundish lining, etc.
There are instances when steel plants are found switching over from one method to other for refractory lining repair and maintenance depending upon their perceived and actual benefits obtained. But well-documented published data of such experiences, which can be of immense help for others, are either scanty or sparsely available. In our article Tundish Lining Refractories: An assessment of their suitability on basis of actual experiences in Steel Plants it has been tried to put together some such experiences made by others and were presented in some recently held different seminars and conferences on Refractories.
Difference between GUNNING and SPRAYING
The principles, procedure and differences between Gunning and Spraying can be understood by the diagram (flow chart) below:
The gunning repair is a well-proven procedure by which refractory material can be applied quickly and cheaply. Initially these were alumino-silicate based and later converted to basic type magnesite based to assist with metallurgical practice.Gunning or gunnable refractories are used for the hot repair of ladles and melting furnaces as well as relining or cold repair of the back lining. There are two basic methods: dry and wet gunning. With dry gunning the material is discharged from the machine with a maximum of 5% moisture and then fed to the gunning nozzle by an air stream where the required water, typically 5–10%, is added. With wet gunning the gunning material is moistened with water in a mixer and then pumped through a hose by means of an eccentric screw or a piston pump. At the end of the line the material is dispersed with compressed air and, if necessary, an additional liquid bonding/hardening agent can be added (shotcreting).
Standard gunning refractories are magnesia, alumina or silica based refractory materials normally a monolithic applied on the areas that encounter severe wear out such as trunnions, scrap impact area and the slag line. These materials differ in price so the optimum choice depends on many factors such as steel grade, slag composition and the vessel used. A shooter type of gun is used for the gunning process to encounter hostile environment of the process. Consumption of gunned refractory is also extremely varied, ranging from 0.2-2 kg per ton steel depending on conditions of the vessel and environment.
The most commonly faced problems of using a gunning machine or in the process of gunning are -
Whereas when the material is applied through spraying it has the following benefit -
Gunning materials or gunnable refractories, refractory castables are transported through flexible hoses to the installation position, where the materials are wetted and projected through a handheld nozzle at the target area. Dry gunning allows operating farther away from the feed station, though wet gunning or shotcreting offers a faster output rate.
Since homogeneous mixing is possible in spraying (before the product is applied), the incorporation of special chemical additives can help to improve thermal stability properties of the lining and also impart good flexibility.
Mixing of Water and Gunnable Refractory Materials
Three essential requirements for a good gunning repair of refractory lining are optimal moistening, homogeneous mixing of the gunning material with water, and a high quality gunning machine that guarantees even conveying. With pre-moistening a share of the gunning water is added some meters away from the gunning nozzle and relies on the turbulence within the conveyance for premixing. This approach, however, is very susceptible to operational problems. For instance adding too much water in advance may lead to clogging, particularly with quick binding systems where hardening starts inside the hose. Too much water reduces refractory quality and hence refractory lifetime. Also, the water can dissociate in the liquid steel to hydrogen, which can be detrimental to some steel grades (micro-cracks). With standard mixing heads (for dry gunning) the water is jetted through radial borings and, in order to compensate for inadequate initial moistening, the operator often works with a surplus of water so that the dust is minimized. This, however, often leads to use of incorrect water/ cement ratio, resulting in reduced refractory durability. Improvements can be achieved by use of pressure increasing pumps so that the water jet becomes sharper and more readily reaches the centre of the nozzle cross-section. Optimum nozzle-vessel distance is quite varied; between 40 and 1,000 mm depending on application.