CLIAMS:We claim:
1. A process for coating refractories in steel plant by developing microstructures to mitigate corrosion and erosion comprising the following steps:
a) The substrate to be coated is being optimally polished to obtain a porous free surface for coating
b) Coating the polished surface by nanocrystalline material as herein described to achieve improved life of the refractory by slurry deposition technique
c) Baking the coated surfaces at a temperature of 1400/1600°C for 30 hours/1 hour respectively.
d) Repeating the process of coating and baking for at least three cycles to obtain a desired coating thickness of 160-200 microns.
2. A process for coating refractories as claimed in claim 1, wherein nanocrystalline materials are Ceria (CeO2), Magnesia (MgO) and Yttrium Stabilised Zirconia(YSZ).
3. A process for coating refractories as claimed in claim 1, wherein nanocrystalline material is MgO
4. A process for coating refractories as claimed in claim 1, wherein the particle size of the MgO powder is in the range of 150-180nm.
5. A process for coating refractories as claimed in claim 1, wherein the coating thickness of the nanocrystalline MgO on the refractories is in the range of 55-65 microns.
6. A process for coating refractories as claimed in claim 1, wherein the total thickness of the coating after three cycles is 180 microns.
7. A process for coating refractories as claimed in claim 1, wherein the baking temperature applied to the nano coating is 1400°C for at the least 30 hours.
8. A process for coating refractories as claimed in claim 1, wherein the baking temperature applied to the nano coating is 1600°C for 1 hour.
9. A process for coating refractories as claimed in claim 1, wherein the hardness of the refractory lining is increased from about HV 45-49 to HV 775-785 after coating.
10. A process for coating refractories as claimed in claim 1, wherein the slurry deposition technique used is as follows:
- grinding the nanocrystalline materials by mixing 18-22ml of nitric acid to the 10-14gm of the nanocrystalline ceramic powder of CeO2,MgO and YSZ in a mortar and pestle.
- adding desired volume of non-ionic surfactant to the grounded materials for providing binding strength to the grounded materials for adhering to the refractory lining.
11. A process for coating refractories as claimed in claim 10, wherein non-ionic surfactant used in the slurry deposition technique is octyl phenol ethoxylate.
12. A process for coating refractories as claimed in claim 10, wherein the nitic acid used in the slurry deposition technique is prepared in the deionized water.
13. A process for coating refractories as claimed in claim 10, wherein the pH of the nitric acid used in the slurry deposition technique is in the range of 3 to 5.
14. A process for coating refractories as claimed in claim 1, wherein the nanocrystalline coating whenever used, protects the refractory lining from the attack by slag, metal corrosion, erosion and improve hardness of the refractory wall.
,TagSPECI:Field of invention:
The present invention relates to the process for coating Steel Industry Refractories with the nanocrystalline Ceria (CeO2), Magnesia (MgO) and Yttrium Stabilised Zirconia (YSZ) compositions by developing microstructures to mitigate corrosion and erosion.

Background of the invention: Refractory life of the furnace is one of the crucial factors that affect the productivity and cost competitiveness of a steel plant. Refractories are used in a wide variety of units in an integrated steel plant. The major degradation processes that limit refractory life are high temperature corrosion and erosion. The present invention relates to the development of coatings based on ceramic nanocrystals preferably MgO that helps in mitigating erosion. Nanomaterials can be used as feedstock for the coatings to develop microstructures which provide protection against corrosion and erosion.

Prior art
The Patent Application No. US2012/0267812 teaches about the composition useful as mortar or coating refractories. The composition comprising the ceramic refractory particulates made from alumina, one or more rare earth oxides, one or more oxides of a transition metal, the transition metal being Sc, Zn, Ga, Y, Cd, In, Sn, Tl, or a mixture of two or more thereof; an alumina and phosphate containing composition; and water. The composition is useful as a mortar or coating for molten aluminium or steel contact refractories.

Objective: The object of the present invention is the process for coating steel Industry refractories by using nanocrystalline (CeO2, MgO and YSZ) based protective coatings preferably MgO-Coating to protect the refractories against corrosion and erosion.

Summary of the invention
The present invention relates to the development of nanocrystalline material (CeO2, MgO and YSZ) coating preferably MgO coating. Slurry deposition was adopted to get the successful coating of nanomaterials (CeO2, MgO and YSZ) on refractories. In comparison with nanocrystalline YSZ, Ceria and MgO coatings, nanocrystalline Magnesia coating proved to be a better protective coating on refractory bricks. Thermal cycling of steel melt reaction test at 1600°C for 1h & 1650°C for 15 minutes and at 1400°C for 30 hours up to five times performed and number of cycles of coating has been optimised as 3. The nanocrystalline Magnesia coating proved to be a better protective coating for reaction at 1600°C for 1 hour. Thermal cycling of coating has been optimised as 3. The hardness of the uncoated brick significantly increases from 48 to 781 Vicker’s Hardness Number (VHN) after nano MgO coating.

Detailed description

Preparation of Nanocrystalline Magnesia coatings
Nanocrystalline cubic MgO powder with an average particle size of 150-180 nm has been used for coating. Before doing this coating the substrate is polished perfectly to get porous free coating. Each Coating thickness is 55-65 microns and total thickness of the coatings are 160-200 microns. Nanocrystalline MgO is deposited on MgO-C refractories by slurry deposition technique. After coating, it was baked at 1600°C for 1h or at 1400°C for 30h. The coating and baking process was repeated for three cycles to get better adherent coating of sufficient thickness. In order to understand the nature of coating, the hardness measurement is performed on uncoated and nanocrystalline MgO coated MgO-C refractory samples. A significant increase in the hardness is evident after nanocoating from HV 48 to HV 781.

Protective nature of coatings against steel melts
In order to study the protective nature of nanocrystalline MgO coating, steel is allowed to react with the coated and uncoated refractory bricks at 1600°C for 1h and 1650°C for 15min or 1400°C for 30 hrs. in three different cycles i.e. one, three and five. Each cycle involves heating of coated and uncoated refractory bricks to 1600°C and 1650°C in contact with steel melt and holding at these temperatures for 1h and 15min, respectively and then cooling back to room temperature.

The uncoated refractory brick surface after the reaction with molten steel at 1600°C for 1 hr and at 1650°C for 15min indicates significant reaction of the molten steel with refractory brick. Nanocrystalline MgO coated refractory brick surface after the reaction with molten steel at 1600°C for 1h after 1, 3 and 5 cycles indicates no reaction of the molten steel with the coated brick. A similar reaction at 1650°C for 15min has also not shown any reaction of molten steel with coated surface. Similar reaction have been performed with steel making slag and no reaction has been observed between the slag and nanocrystalline MgO coated refractory bricks, for which significant reaction was observed with the uncoated bricks.
Composition analysis of the refractory brick surface after reaction with steel using SEMEDS micro analysis clearly indicates a large amount of Fe contamination on the surface in case of uncoated brick, while the nanocrystalline MgO coated brick showed very little amount of Fe on the surface after the reaction. Even the EDS line scan analysis confirmed these results.

According to the present invention, a process for coating refractories in steel plant by developing microstructures to mitigate corrosion and erosion comprises the following steps:
a) The substrate to be coated is being optimally polished to obtain a porous free surface for coating
b) Coating the polished surface by nanocrystalline material as herein described to achieve improved life of the refractory by slurry deposition technique
c) Baking the coated surfaces at a temperature of 1400/1600°C for 30 hours/1 hour respectively.
d) Repeating the process of coating and baking for at least three cycles to obtain a desired coating thickness of 160-200 microns.
According to an embodiment, a process for coating refractories uses nanocrystalline materials as CeO2, MgO and YSZ.
According to another embodiment, a process for coating refractories uses nanocrystalline material as MgO.
According to further embodiment, a process for coating refractories uses the slurry deposition technique which is as follows:
-grinding the nanocrystalline materials by mixing 18-22ml of nitric acid to the 10-14gm of the nanocrystalline ceramic powder of CeO2,MgO and YSZ in a mortar and pestle.
-adding desired volume of non-ionic surfactant to the grounded materials for providing binding strength to the grounded materials for adhering to the refractory lining.
The non-ionic surfactant used in the slurry deposition technique is octyl phenol ethoxylate.

The invention is further illustrated hereinafter by means of example without limiting the scope of the invention.

Example 1
A coating of nanocrystalline ceramic materials, the said nanocrystalline material is prepared by using slurry deposition techniques. First of all the grinding of nanocrystalline materials is done by mixing 19ml of nitric acid to the 12gm of the nanocrystalline ceramic powder of CeO2,MgO and YSZ in a mortar and pestle. Then desired volume of non-ionic surfactant is added to the grounded materials for providing binding strength to the grounded materials for adhering to the refractory lining. Then coating of the polished surface by nanocrystalline material is performed to 57microns thickness in one cycle and the total thickness of the coatings is 170 microns after 3 cycles. Then Baking of the coated surfaces at a temperature of 1600°C for 1 hour is performed. The process of coating and baking is repeated for at least three cycles to obtain a desired coating thickness of 180 microns.

Example 2
A coating of nanocrystalline ceramic materials, the said nanocrystalline material is prepared by using slurry deposition techniques. First of all the grinding of nanocrystalline materials is done by mixing 18ml of nitric acid to the 11gm of the nanocrystalline ceramic powder of CeO2,MgO and YSZ in a mortar and pestle. Then desired volume of non-ionic surfactant is added to the grounded materials for providing binding strength to the grounded materials for adhering to the refractory lining. Then coating of the polished surface by nanocrystalline material is performed to 62microns thickness in one cycle and the total thickness of the coatings is 190 microns after 3 cycles. Then Baking of the coated surfaces at a temperature of 1400°C for 30 hours is performed. The process of coating and baking is repeated for at least three cycles to obtain a desired coating thickness of 190 microns.