Ceramic composition and material comprising said ceramic composition as part of a heat recovery unit

Abstract

The invention relates to a ceramic composition and a material comprising said ceramic composition in the form of a coating and a steel substrate. Furthermore, the invention relates to the process to obtain said material and its use as part of a heat recovery unit.

Claims

1. A ceramic composition characterized in that it comprises a weight percent with respect to the end ceramic composition expressed in terms of the following equivalent oxides: between 54% and 66% of SiO.sub.2, between 12% and 20% of Cr.sub.2O.sub.3, between 5% and 12% of Na.sub.2O, and between 5% and 12% of ZrO.sub.2.

2. The ceramic composition according to claim 1, which comprises up to 10% of an oxide selected from the list consisting of Al.sub.2O.sub.3, B.sub.2O.sub.3, BaO, CaO, CoO, K.sub.2O, Li.sub.2O, MnO.sub.2, TiO.sub.2 or any combination thereof.

3. A material characterized in that it comprises the ceramic composition according to claim 1 deposited on a substrate of steel.

4. The material according to claim 3, wherein the ceramic composition has a thickness of between 100 m and 160 m.

5. The material according to claim 3, wherein a porosity of the ceramic composition is higher than 10%.

6. The material according to claim 3, wherein the substrate is a tube and wherein the ceramic composition is deposited on at least one of the following surfaces selected from an inner surface of the tube or an outer surface of the tube.

7. The material according to claim 6, wherein the substrate is a tube and wherein the ceramic composition is deposited on the outer surface of the tube.

8. A process to obtain the material according to claim 3 comprising the following steps: a) preparing a suspension of the ceramic composition comprising in weight percent the following oxides: between 54% and 66% of SiO.sub.2, between 12% and 20% of Cr.sub.2O.sub.3, between 5% and 12% of Na.sub.2O, and between 5% and 12% of ZrO.sub.2, in a weight percent of between 65% to 75% and a solvent in a weight percent of between 25% to 35%, the suspension having an average particle size of between 40 m and 200 m, b) preatreating at least one surface of a substrate of steel, c) depositing the suspension obtained in step (a) onto the pretreated surface of step to form a coating (b), (d) drying the coating obtained in step (c), and (e) sintering the coating obtained in step (d) at temperatures between 700 C. and 1000 C.

9. The process according to claim 8, wherein the solvent of step (a) is water.

10. The process according to claim 8, wherein step (b) is performed by shot blasting.

11. The process according to claim 8, wherein step (c) is performed by electrophoretic deposition (EPD), dipping, waterfall glazing or spraying.

12. The process according to claim 11, wherein step (c) is performed by spraying.

13. The process according to claim 8, wherein step (e) is performed at temperatures between 850 C. and 950 C.

14. A heat recovery unit working at metal temperatures between 400 C. and 750 C. comprising the material according to claim 3.

15. The heat recovery unit according to claim 14 selected form a boiler or an incinerator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1. Optical microscope image of substrate and ceramic coating.

(2) FIG. 2. Mapping results of the chemical composition of the ceramic coating analyzed by SEM-EDX technique.

(3) FIG. 3. Visual inspection of specimens after six thermal cycles at 480 C.

(4) FIG. 4. Comparison between ceramic coated tubes and not ceramic coated tubes after two years' service in an urban waste incinerator.

EXAMPLES

Example 1

(5) A suspension was obtained by grinding a mixture of commercial silicate of Keracoat SL RT100 (64%-66%), clay (3%-3.7%), borax (0.15%), sodium nitrite (0.15%) and water (30%-31.7%). A smooth and homogeneous paste is formed and it is sieved to obtain an average particle size between 40 m and 200 m. The ceramic composition is hereby expressed in terms of the following equivalent oxides: 61% of SiO.sub.2, 16% of Cr.sub.2O.sub.3, 6% of Na.sub.2O, 7% of ZrO.sub.2 2% B.sub.2O.sub.3, 2% Al.sub.2O.sub.3 and 3% TiO.sub.2.

(6) This suspension was deposited onto the outer surface of a stainless steel tube. The outer surface of the tube was pretreated by shot blasting to guarantee a clean surface to get a good bonding of the ceramic coating to the steel substrate. The suspension was sprayed onto the pretreated surface and completely dried by convection at controlled temperature of 85 C. Finally, sintering was performed at 900 C.

(7) FIG. 1 shows the results of the coating thickness measurements. Thickness values of 111.0 m, 114.4 m and 119.7 m can be observed by optical microscope as it is shown in FIG. 1.

(8) The ceramic coating provides a strong increase in hardness to the tube, Vickers hardness (HV) is 724 in the coated layer and 155 in the substrate.

(9) The chemical composition of the ceramic composition has been analyzed using SEM-EDX technique. Mapping results showing the main chemical elements present in the coating are shown in FIG. 2.

(10) SEM qualitative analysis: Na, Mg, Ca, Al, Cr, Si, Zn and Zr oxides.

(11) The thermal resistance of the ceramic coating deposited onto the steel substrate have been checked applying six thermal cycles at 480 C. during 1 hour, after visual inspection there is no signs of defects or deterioration on the coating (See FIG. 3).

(12) The roughness has been measured after six thermal cycles at 480 C. during 1 hour on the substrate and on the coating. Roughness on the coating area is 0.9 mm RA and on the substrate 6.13 m RA.

(13) Furthermore, a series of stainless steel tubes coated with the above mentioned ceramic coating have been exposed to real conditions being part of a reheater of an urban waste incinerator for 2 years. The working conditions at this urban waste incinerator were: Steam temperature: 300 C. Pressure: 170 bar Fumes temperature: 850 C. Inner fluid media of the tubes: steam Outer fluid media of the tubes: waste to energy fumes Other exposition: alkaline ashes

(14) A chemical analysis shows that the main components of the ashes are: silica, aluminum, iron and calcium and that the secondary components are titanium, magnesium, sodium, potassium or phosphate and in very small quantities barium, strontium, rubidium and heavy metals such as zinc, copper, lead, chromium, nickel or cadmium.

(15) FIG. 4 shows (from the left to the right): a coated tube of the present invention installed two years ago and afterwards washed by hand, a coated tube of the present invention just installed, and two tubes (without coating) installed wo years ago and shot blasted.

(16) As shown in FIG. 4 the ashes are removed very easily from the coated tubes of the invention, furthermore, it can be noted that the coated layer is not deteriorated.