Technical field of the invention

The invention relates to curing a form-supported refractory coating of an intermediate vessel. Specifically, the invention relates to a method curing a refractory coating of an intermediate vessel with microwave energy. Background of the invention

Varying types of intermediate vessels are used for transferring and containing molten metal. Such intermediate vessels, also known as tundishes or dummy basins, require an inner surface layer that is a refractory coating, which is bound to a more permanent back-up lining.

Different methods for curing a form-supported refractory coating of an intermediate vessel are known. In these methods a mould or a form is mounted inside the intermediate vessel so that an inner surface of the intermediate vessel and an outer surface of the form are opposite to each other and a space is left between the two surfaces for applying refractory coating material. In the known methods the form is heated by using flame or electrical heating elements or by blowing hot air into a cavity of the inner form. Superheated steam and infrared radiators may be utilized for heating the inner form as well . In these solutions air inside the cavity of the form is heated and the heated air in turn heats the structure of the form.

EP1622848 describes a method that applies the heat from back-up lining, either the dense refractory castable or brick lining behind the coating layer, without any external heating devices. The form is typically composed of steel. From the heated structure of the form the heat further transfers to a refractory coating material dosed to the space between the inner surface of the intermediate vessel and outer surface of the form. The heat cures the refractory coating material after which the form can be removed. These known methods based on heating the form have certain disadvantages and limitations. The known methods require relatively high temperatures for the air inside the cavity of the form. Commonly temperatures over 300°C are required for effective curing process, which requires significant amount of energy. Most of the known methods except the one according to EP1622848 are most effective when the back-up lining of the intermediate vessel is relatively cool (less than approximately 150 °C of temperature). The known methods based on heating the form lack capability to control the conduction of the heat to a certain spot or target.

Also methods based on applying earth-moist refractory coating mix to the surface of backup lining are known. In these solutions the refractory coating mass is mounted to the inner surface of the intermediate vessel by using a screw-type mixer and a form made of metal. Resins and curing agents are added to the dry mix refractory coating mass base in solution form, to compose an earth-moist coating mix. In some cases citric acid alone is used as the binder. This causes the earth-moist refractory coating mass to bind also to a relatively cold (under 10 °C of temperature) inner surface of the intermediate vessel and no heating of the form is needed. This type of binder solutions are also applied in foundries for manufactur- ing sand forms and cores.

Using dry or earth-moist installed refractory coating masses often require vibrating or rodding for reaching suitable and acceptable thermomechanical strength for the coating.

Further, EP1622848 describes a coating mass, a dry matter composition and a method for applying a coatings mass onto a relatively cold (under 10 °C of temperature) backup lining surface. This resolution requires also a screw-type mixer and curing time after which the form can be removed is relatively long, approxi- mately 30 minutes.

Also these solutions have certain disadvantages. First of all, the screw-type mixers are expensive tools and cause high investment and maintenance cost. Further, resins or curing agents applied to dry mix refractory coating masses can be harm- ful for both environment and health wise. Finally, vibrating dry installed refractory coating masses requires vibrating motors attached onto the form as well as connecting electrical cables to the form, causes increased noise level and weakens heat insulation. Microwave technique can be applied to heating a variety of materials and processing materials. The microwave technique has been used in several different applications from heating food in a microwave oven to sintering foundry sand forms and cores, and water evaporation (dry-out) of aqueous refractory castables. Microwave technology is replacing some of the traditional heating technology that is based on bringing heat to a surface of the object to be heated and the thermal conductance based heat transfer phenomena is then responsible for transferring the heat into the inner parts of the object. A fundamental feature of the microwave radiation is that certain suitable materials can be warmed with microwave radiation or heated so that the heat is generated at the same time in the whole volume of the object (so called volumetric heating). Thus, the heating energy will be used more effectively than in conduction based heating. Summary of the Invention

The object of the present invention is to provide an energy-efficient solution for curing a refractory coating of an intermediate vessel. Also, the object of the present invention is to provide a solution for curing the refractory coating of the intermediate vessel when a temperature of an inner surface of the intermediate vessel, so called back-up lining or permanent lining, is relatively cold. Relatively cold can be understood as a temperature range under 150 °C in the context of the present invention. Relatively cold can also be understood as a temperature range from 100°C to 150 °C in the context of the present invention.

Further, the object of the present invention is to provide an environment-friendly solution for curing the refractory coating of the intermediate vessel. Moreover, the object of the present invention is to provide a solution for curing the refractory coating of the intermediate vessel that reduces the need to use substances harmful for human and environment during or after the curing of the refractory coating. Finally, the object of the present invention is to provide a solution for curing the refractory coating of the vessel that fastens the rotation of intermediate vessels and forms necessary for assembling the refractory coating in their process of use.

The objects of the present invention are fulfilled by providing a method for curing a form-supported refractory coating of an intermediate vessel, the method comprising

- mounting a form comprising at least an outer surface and an inner surface, inside the intermediate vessel comprising an inner surface and an outer sur- face so that the inner surface of the intermediate vessel and the outer surface of the form are positioned opposite to each other and a space is left between the inner surface of the intermediate vessel and the outer surface of the said form; and

- dosing a refractory coating mass essentially in dry existence to the space between the inner surface of the intermediate vessel and the outer surface of the form,

wherein the method further comprises applying microwave energy to the dosed refractory coating mass and removing the form after applying micro- wave energy to the refractory coating mass.

Some advantageous embodiments of the present invention are disclosed in dependent claims. The basic idea of the invention is as follows: An intermediate vessel comprises an inner surface and an outer surface. To enable handling of hot materials such as molten metal inside intermediate vessel, the inner surface of the back-up lining, installed into the intermediate vessel, requires an insulating refractory coating mass to be assembled onto it with the aid of a form. Varying compositions of form- supported refractory coating masses exist. The form or mold is mounted inside the intermediate vessel so that the outer surface of the form and the inner surface of the intermediate vessel are positioned opposite to each other and a space is left between the inner surface of the intermediate vessel and the outer surface of the form. The refractory coating mass is dosed to the space between the inner surface of the intermediate vessel and the outer surface of the form. To cure and harden the refractory coating mass so that it also bounds to the inner surface of the intermediate vessel, microwave energy is applied, thus enabling heating of the coating. Heating the refractory coating mass with microwave energy causes the refractory coating mass to stabilize into a suitable coherence and is being bound to the inner surface of the intermediate vessel.

In one advantageous embodiment of the invention the temperature of the inner surface of the intermediate vessel is between 10 °C and 100 °C when applying the microwave energy. In the context of the present invention the previous tempera- ture range can be considered cold.

In another advantageous embodiment of the invention the temperature of the inner surface of the intermediate vessel is less than 150°C when applying the micro- wave energy. In the context of the present invention such temperature range can be considered relatively cold. However, the present invention may be applied to the intermediate vessels in cases where the temperature of the inner surface is over 150°C. Also, the present invention may be applied to the intermediate vessels in cases where the temperature of the inner surface is between 10°C and 200 °C. If the temperature of the inner surface is less than 100°C, more microwave energy is necessary for curing refractory coating mass.

In the third advantageous embodiment of the invention the form is comprised of a microwave transparent material. Also, the form may comprise one or more isolation structures to prevent spreading of microwave radiation outside the target (while microwave energy is applied to cure the refractory coating mass). Further, the form may comprise a support structure that also guides the microwave radiation.

In the fourth advantageous embodiment of the invention the form is comprised of material acting as a susceptor. The material of form may be, for exemplary purposes only, silicon infiltrated silicon carbide. In the fifth advantageous embodiment of the invention the form is comprised of a microwave transparent material and the mix material or some of its components acting as a susceptor.

In the sixth advantageous embodiment of the invention the refractory coating mass is dosed in granular form. The refractory coating mass may be relatively dry mass by its composition while it is applied to the surface of back-up lining. Relatively dry means hereby that the coating mix does not contain water in liquid existence just prior to applying onto back-up lining, but, may contain a tiny amount of dust suppressants that are composed of hydrocarbons in liquid form. Further, one or more susceptor substances may be added to the refractory coating mass for boosting heat production. One or more susceptor substances may be added to the refractory coating mass for boosting heat production under microwave radiation. Such susceptor substances comprise at least one of the following: essentially hydrous chloride, sulfate, phosphate and carbonate salts of Mg, Ca, Na and K, hydroxides, essentially earth-alkali and iron hydroxides, hydrous natural or man-made minerals, graphite, metal powders, essentially fine aluminum powder, essentially fine ferrous powder, iron-bearing spinel powders, essentially magnetite powder, sugars, resins, phenolic resin, starch derivative substances and Silicon carbide. Further scope of applicability of the present invention will become apparent from the detailed description given hereafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

Brief description of the drawings The present invention will become fully understood from the detailed description given herein below and accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein

Fig. 1 shows an exemplary cross-sectional representation of an intermediate vessel and an inner form; and

Fig. 2 shows an exemplary flow chart representing main method steps according to the invention.

Detailed description

In the following description, considered embodiments are merely exemplary, and one skilled in the art may find other ways to implement the invention. Although the specification may refer to "an", "one; or "some" embodiment(s) in several locations, this does not necessarily mean that each such reference is made to the same em- bodiment(s), or that the feature only applies to a single embodiment. Single feature of different embodiments may also be combined to provide other embodiments.

Figure 1 shows an exemplary cross-sectional representation of an intermediate vessel (10-1 1 ) and an inner form (13-15). An intermediate vessel (10-1 1 ) comprises an inner surface (1 1 ) and an outer surface (10). To enable handling of hot materials such as molten metal with the intermediate vessel (10-1 ), the inner surface (1 1 ) of the intermediate vessel (10-1 1 ), also known as back-up lining, requires a refractory coating mass to be assembled onto it.

Assembling the refractory coating mass to the inner surface (1 1 ) of the intermediate vessel (10-1 1 ) is assisted with a form (13-15). The form (13-15) comprises an outer surface (14). The form (13-15) may also comprise an outer surface (14) and an inner surface (13). The form (13-15) is mounted inside the intermediate vessel (10-1 1 ) so that the outer surface (14) of the form (13-15) and the inner surface (1 1 ) of the intermediate vessel (10-1 1 ) are positioned opposite to each other and a space (12) is left between the inner surface (1 1 ) of the intermediate vessel (10- 1 1 ) and the outer surface (14) of the form (13-15). Suitable supporting structures and/or tools may be used for mounting the form (13-15) inside the intermediate vessel (10-1 1 ). The thickness of the coating may vary between 30 and 120 mm, depending on the metallurgical process related. Further, the form (13-15) can be comprised of a microwave transparent material. Advantageously, the form (13-15) may comprise one or more isolation structures to prevent spreading of microwave radiation outside the target to which microwave energy is applied to cure the refractory coating mass for curing it. The form (13- 15) may, more advantageously, also comprise a support structure (15) that also guides the microwave radiation.

More advantageously, the form can be comprised of material acting as a susceptor. The material of form may be, for exemplary purposes only, silicon infiltrated silicon carbide. The form (13-15) may comprise isolation to prevent spreading of mi- crowave radiation outside the target to which microwave energy is applied to cure the refractory coating mass for curing it. The form (13-15) may, more advantageously, also comprise a support structure (15) that also guides the microwave radiation. Finally, the form (13-15) can be comprised of a microwave transparent material or the form can be comprised of material acting as a susceptor. The form (13-15) may comprise isolation to prevent spreading of microwave radiation outside the target to which microwave energy is applied to cure the refractory coating mass for curing it. The form (13-15) may, more advantageously, also comprise a support structure (15) that also guides the microwave radiation.

Figure 2 shows an exemplary flow chart representing main method steps according to the invention. References to elements of Figure 1 are made. Process for curing a refractory coating of the intermediate vessel (10-1 1 ) is started in step 20. To enable handling of hot materials such as molten metal with the intermediate vessel (10-1 1 ), the inner surface (1 1 ) of the intermediate vessel (10-1 1 ), so called back-up lining, requires a refractory coating mass to be assembled onto it. In step 21 the form (13-15) is mounted inside the intermediate vessel (10-1 1 ) to enable assembling the refractory coating to the inner surface (1 1 ) of the intermediate vessel (10-1 1 ). The form (13-15) is arranged inside the intermediate vessel (10-1 1 ) so that the inner surface (1 1 ) of the intermediate vessel (10-1 1 ) and the outer surface (14) of the form (13-15) are arranged opposite to each other. The form (13-15) is arranged inside the intermediate vessel (10-1 1 ) so that the inner surface (1 1 ) of the intermediate vessel (10-1 1 ) and the outer surface (14) of the form (13-15) are arranged opposite to each other and a space (12) is left between the inner surface (1 1 ) of the intermediate vessel (10-1 1 ) and the outer surface (14) of the form (13-15).

In step 22 the refractory coating mass is dosed to the space between the inner surface (1 1 ) of the intermediate vessel (10-1 1 ) and the outer surface (14) of the form (13-15). The refractory coating mass is dosed to the space between the in- ner surface (1 1 ) of the intermediate vessel (10-1 1 ) and the outer surface (14) of the form (13-15) in order to be bound to the inner surface (1 1 ) of the intermediate vessel (10-1 1 ) to form a layer of refractory coating for the intermediate vessel (10-1 1 ). The composition of one or more binder substances of the refractory coating mass may be essentially inorganic. Also, the composition of one or more binder substances of the refractory coating mass may be essentially organic. Further, the composition of one or more binder substances of the refractory coating mass may be essentially organic or inorganic. Moreover, one or more susceptor substances may be added to the refractory coating mass for boosting heat production. Such susceptor substances comprise at least one of the following: essentially hydrous chloride, sulfate, phosphate and carbonate salts of Mg, Ca, Na and K, hydroxides, essentially earth-alkali and iron hydroxides, hydrous natural or man-made minerals, graphite, metal powders, essentially fine aluminum powder, essentially fine ferrous powder, iron-bearing spinel powders, essentially magnetite powder, sugars, resins, phenolic resin, starch derivative substances and Silicon carbide.

Advantageously, the refractory coating mass may be applied in granular existence. The refractory coating mass may be essentially dry mass by its composition.

In step 23, in order to cure the refractory coating mass so that it bounds to the inner surface (1 1 ) of the intermediate vessel (10-1 1 ), microwave energy is applied to the dosed refractory coating mass. The dosed refractory coating mass is heated and cured with microwave energy in order to cure the refractory coating mass. The dosed refractory coating mass is heated with microwave energy. Heating the dosed refractory coating mass with microwave energy causes the said refractory coating mass to stabilize into a suitably firm coating and is being bound to the in- ner surface (1 1 ) of the intermediate vessel (10-1 1 ). After the refractory coating mass is stabilized into the suitably firm refractory coating and is bound to the inner surface (1 1 ) of the intermediate vessel (10-1 1 ), the form (13-15) can be removed.

The temperature of the inner surface (1 1 ) of the intermediate vessel (10-1 1 ) is, advantageously, between 10 °C and 100 °C when applying the microwave energy to it. In the context of the present invention such temperature range can be considered cold. However, the temperature of the inner surface (1 1 ) of the intermediate vessel (10-1 1 ) may be less than 150°C when applying the microwave energy. In the context of the present invention such temperature range can be considered relatively cold. Further, the present invention may be applied to the intermediate vessels in cases where the temperature of the inner surface is over 150°C. This may shorten the time needed for curing the refractory coating. The temperature of the inner surface (1 1 ) of the intermediate vessel (10-1 1 ) may even be between 10 °C and 200 °C when applying the microwave energy. The colder the surface of the back-up lining, then the more microwave energy is necessary for curing the coating.

Moreover, the form (13-15) is comprised of a microwave transparent material. Also, the form (13-15) may comprise an isolation structures to prevent spreading of microwave radiation outside the target while microwave energy is applied to cure the refractory coating mass. The form (13-15) may also comprise a support structure to guide the microwave radiation between the inner surface (1 1 ) of the intermediate vessel (10-1 1 ) and the outer surface (14) of the form (13-15) while microwave energy is applied to cure the refractory coating mass. The support struc- ture to guide the microwave radiation between the inner surface (1 1 ) of the intermediate vessel (10-1 1 ) and the outer surface (14) of the form (13-15) may enable more effective application of the microwave energy. The support structure to guide the microwave radiation between the inner surface (1 1 ) of the intermediate vessel (10-1 1 ) and the outer surface (14) of the form (13-15) may enable more effective application spreading of microwave radiation to the refractory coating mass. The support structure to guide the microwave radiation between the inner surface (1 1 ) of the intermediate vessel (10-1 1 ) and the outer surface (14) of the form (13-15) may enable more effective application spreading of microwave radiation to the refractory coating mass and fasten the curing of the refractory coating mass.

In step 24 the refractory coating mass is stabilized into the suitably firm refractory coating and is bound to the inner surface (1 1 ) of the intermediate vessel (10-1 1 ). The refractory coating mass is stabilized into the suitably firm refractory coating and is bound to the inner surface (1 1 ) of the intermediate vessel (10-1 1 ) after application microwave energy to the heat mass according to step 23. The form (13- 15) can now be removed. The form (13-15) can be removed after applying micro- wave energy to the refractory coating mass. The form (13-15) can be removed after applying microwave energy to the refractory coating mass when the refractory coating mass is stabilized into the suitably firm refractory coating and is bound to the inner surface (1 1 ) of the intermediate vessel (10-1 1 ). In step 25 the curing process is finished. The form (13-15) has been removed and the intermediate vessel (10-1 1 ) is ready to be used with the refractory coating.

Some exemplary test results regarding application of the method according to the invention are herewith provided for varying types of refractory coating masses that may be used for intermediate vessels (10-1 1 ). The exemplary test results describe type of refractory coating mass mix, amount of mix, main binder substance used, applied efficiency, duration of heating with microwave energy and reached Cold Compressive Strength (CCS) after application of microwave energy to the refractory coating mass. These results have been obtained with the following set-up and equipment: Panasonic NN-SD microwave oven with rotating platform and frequency of 2,45 GHz and a small size sample form with outer surface made of plastic and inner surface made of silicone, featuring a miniature size intermediate vessel (10-1 1 ). Temperatures of inner parts for the said sample intermediate vessel have been over 100°C while applying the microwave energy. The said inner parts in general comprise parts between the inner surface (1 1 ) and the outer surface (10) of the intermediate vessel. Surface temperatures for the obtained refractory coating between 80°C and 100°C after application of the microwave energy to the refractory coating mass in manner of step 23 were measured. According to first example ("Example 1 ") olivine-rich refractory coating mass that is applied as a dry mix having composition comprising approximately 56 % of MgO and 1 % of graphite is used. The amount of mix is 500 g, no compacting is applied to the mass and the mix density as installed is approximately 1 .7 g/cm3. The main binder substance is crystalline magnesium sulphate heptahydrate of 6 w-%. Applied efficiency is 1000 W and duration of heating with microwave energy was three (3) minutes. CCS of 0.8 MPa is achieved after application of heating the described refractory coating mass of this Example 1 with microwave energy for three (3) minutes.

According to a second example ("Example 2") olivine-rich refractory coating mass that is applied as a dry mix having composition comprising approximately 56 % of MgO and 1 % of graphite is used. The amount of mix is 500 g, no compacting is applied to the mass and the mix density as installed is approximately 1 .7 g/cm3. The main binder substance is crystalline magnesium sulphate heptahydrate of 6 w-%. Applied efficiency is 500 W and duration of heating with microwave energy was three (3) minutes. CCS of 0.4 MPa is achieved after application of heating the described refractory coating mass of this Example 2 with microwave energy for three (3) minutes.

According to a third example ("Example 3") olivine-rich refractory coating mass that is applied as a dry mix having composition comprising approximately 56 % of MgO. Further, graphite applied in Examples 1 and 2 is replaced with 200M MgO- powder in the dry mix. The amount of mix is 500 g, no compacting is applied to the mass and the mix density as installed is approximately 1 .7 g/cm3. The main binder substance is crystalline magnesium sulphate heptahydrate of 6 w-%. Applied efficiency is 1000 W and duration of heating with microwave energy was three (3) minutes. CCS of 0.5 MPa is achieved after application of heating the described re- fractory coating mass of this Example 3 with microwave energy for three (3) minutes.

According to a fourth example ("Example 4") olivine-rich refractory coating mass that is applied as a dry mix having composition comprising approximately 50 % of MgO. The amount of mix is 500 g, no compacting is applied to the mass and the mix density as installed is approximately 1 .7 g/cm3. The main binder substance is crystalline magnesium sulphate heptahydrate of 5 w-%. Applied efficiency is 1000 W and duration of heating with microwave energy was seven (7) minutes. CCS of 0.7 MPa is achieved after application of heating the described refractory coating mass of this Example 4 with microwave energy for seven (7) minutes.

According to a fifth example ("Example 5") MgO-rich dry installed tun-dish disposable coating mix having composition comprising MgO-content of 82% is used as refractory coating mass. The amount of mix is 500 g, no compacting is applied to the mass and the mix density as installed is approximately 1 .8 g/cm3. The main binder substance is epsom salt 5 w-% in crystal form. Applied efficiency is 1000 W and duration of heating with microwave energy was eight (8) minutes. CCS of 0.8 MPa is achieved after application of heating the described refractory coating mass of this Example 5 with microwave energy for eight (8) minutes.

According to a sixth example ("Example 6") MgO-rich dry installed tun-dish disposable coating mix having composition comprising MgO-content of 82% is used as refractory coating mass. The amount of mix is 500 g, no compacting is applied to the mass and the mix density as installed is approximately 1 .8 g/cm3. The main binder substance is epsom salt 5 w-% in crystal form. As an additive 0.05% atomized aluminium powder is applied. Applied efficiency is 1000 W and duration of heating with microwave energy was four (4) minutes. CCS of 0.8 MPa is achieved after application of heating the described refractory coating mass of this Example 5 with microwave energy for four (4) minutes.

According to a seventh example ("Example 7") MgO-rich dry installed tun-dish disposable coating mix having composition comprising MgO-content of 82% is used as refractory coating mass. The amount of mix is 500 g, no compacting is applied to the mass and the mix density as installed is approximately 1 .7 g/cm3. The main binder substance is phenolic resin of 4 w-%. Applied efficiency is 700 W and duration of heating with microwave energy was eleven (1 1 ) minutes. CCS of 0.8 MPa is achieved after application of heating the described refractory coating mass of this Example 7 with microwave energy for eleven (1 1 ) minutes. The measured surface temperature for the refractory coating is 80°C after application of heating the described refractory coating mass of this Example 7 with microwave energy for eleven (1 1 ) minutes. As becomes clear from the examples presented herewith, the CCS after applying microwave energy is increased while the efficiency is increased. Further, as becomes apparent from the examples presented herewith, the CCS after applying microwave energy is increased while the radiation time is increased. Also, as becomes clear from the examples presented herewith, the CCS after applying mi- crowave energy is increased while the efficiency and radiation time is increased. Furthermore, the acceptable CCS value is obtained in a shorter period of time if additional susceptors, like graphite or aluminum powder, for example, are applied in the mix formula. At least 0.3 MPa CCS value is commonly adequate for ena- bling the form removal and the intermediate vessel's transport and preheating treatment prior to steel casting process in real steelmaking factory conditions.

Some advantageous embodiments according to the invention were described above. The invention is not limited to the embodiments described. The inventional idea can be applied in numerous ways within the scope defined by the claims attached hereto.