WO2015191426A1

WO2015191426A1 - Magnesium oxide filter made from burned magnesium oxide

Info

Publication number: WO2015191426A1
Application number: PCT/US2015/034636
Authority: WO
Inventors: Feng Chi; Leonard S. Aubrey; Lucas DAMOAH
Original assignee: Porvair Plc
Priority date: 2014-06-10
Filing date: 2015-06-08
Publication date: 2015-12-17

Abstract

An improved process for forming a porous MgO filter for molten metal, and a filter formed thereby is provided. The process comprises: calcining magnesium hydroxide at a temperature of over 1000°C to no more than 1500°C thereby forming hard burned MgO powder; forming a slurry comprising a ceramic portion comprising the hard burned MgO powder and a carrier solvent; impregnating a precursor foam with the slurry to form a ceramic coated foam; heating the ceramic coated foam to remove the carrier solvent and volatilize the precursor foam thereby forming a green ceramic filter; and sintering the green ceramic filter thereby forming the porous MgO filter.

Description

MAGNESIUM OXIDE FILTER MADE FROM HARD BURNED MAGNESIUM OXIDE

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to pending U.S. Provisional Patent Appl. No. 62/010,1 15, filed June 10, 2014, which is incorporated herein by reference.

BACKGROUND

[0002] The present invention is related to an improved method for forming a magnesium oxide filter. More specifically, the present invention is related to a method of forming a magnesium oxide filter comprising sintering of hard burned magnesium oxide.

[0003] Magnesium oxide has been found to be suitable for use in many applications. Based on the Ellingham free energy diagram, magnesium oxide is one of the most stable oxides and it is physically and chemically stable at high

temperatures with a very high melting point temperature of about 2853°C. Due to excellent refractory and corrosion resistant properties, magnesium oxide products are used in super alloys, nuclear applications and in the steel industry.

[0004] Magnesium oxide is produced by the calcination of magnesium hydroxide according to the following reaction,

Mg(OH)₂ → MgO + H₂O

The properties of MgO powder are very much dependent on calcining temperature wherein subtle ranges of temperature can provide different, and unexpected, results when used in subsequent products.

[0005] MgO powder calcined at the higher temperatures, such as above 1500°C, does not sinter well. When using this powder for making MgO filters, sintering therefore requires very high temperature or the incorporation of a sintering aid such as Li₂O or ΤΊΟ2. Titanium dioxide has been used as a sintering aid at above 1500°C for making MgO filters, but titanium dioxide is detrimental to refractory properties and the sintered product is more susceptible to corrosion thereby rendering the MgO product unsuitable for the application where high corrosion resistance is required. MgO calcined above 1500°C has found significant use as protective and refractory linings for use in equipment for handling molten steel.

[0006] MgO powder calcined below 1000°C is reactive with water, forming magnesium hydroxide easily. When using this MgO powder as the raw material for making MgO filter using an aqueous process, the MgO powder is not stable in water, reacts with water and forms large amounts of Mg(OH)₂. It is not desirable to make MgO filter using large amounts of Mg(OH)₂ because of the very high firing shrinkage and the difficulties of achieving high density. Therefore, this type of MgO powder is not suitable for use in an application requiring the formation of a slurry, especially, an aqueous slurry of powdered MgO. Therefore, MgO calcined at below 1000°C is not considered suitable for use in the formation of a porous ceramic filter. MgO powder calcined below 1000°C has been found suitable for use in many applications such as processing of plastics, rubber, paper and the like or for steel boiler applications, adhesives and for acid neutralization.

[0007] In spite of the problems related to MgO_s there is still an ongoing desire to utilize porous sintered MgO filters for filtration of molten metal, particularly aluminum and aluminum alloys. This desire has never been satisfactorily fulfilled. The skilled artisan has therefore been thought to be bound between two inoperative options. One option is to utilize MgO powder calcined at high temperatures to formulate a slurry, yet the material is non-reactive thereby requiring a sintering aid for sintering, yet the sintering aid is detrimental for many uses. The other option is to utilize MgO powder calcined at lower temperatures, yet these materials have a high water reactivity, thereby precluding their use in an aqueous environment. The present invention provides a method for the formation of porous sintered MgO foam filters, which lacks the previous limitations and allows for the formation of a MgO foam filter for filtration of molten metal, particularly aluminum and aluminum alloys, which was previously not considered readily available.

SUMMARY OF THE INVENTION

[0008] It is an object of the invention to provide a method for forming a ceramic foam filter of MgO.

[0009] It is another object of the invention to provide a method for forming a ceramic foam filter of MgO wherein the filter can be prepared in a slurry without sintering aids and without the undesirable formation of hydroxides of magnesium.

[0010] These and other advantages, as will be realized, are provided in a process for forming a porous MgO filter for molten metal, and a filter formed thereby, wherein the process comprises:

calcining magnesium hydroxide at a temperature of over 1000°C to no more than 1500°C thereby forming hard burned MgO powder;

forming a slurry comprising a ceramic portion comprising the hard burned MgO powder and a carrier solvent;

impregnating a precursor foam with the slurry to form a ceramic coated foam;

heating the ceramic coated foam to remove the carrier solvent and volatilize the precursor foam thereby forming a green ceramic filter; and

sintering the green ceramic filter thereby forming the porous MgO filter.

DESCRIPTION [0011] The instant invention is specific to a method for forming a magnesium oxide porous filter using MgO calcined at over 1000°C to no more than 1500°C, which is referred to herein as "hard burned" magnesium oxide. More specifically, the present invention is related to a method of forming a sintered magnesium oxide filter that has limited reactivity and sufficient strength to withstand the demands associated with the filtration of molten metal passing there through, yet the filter can be prepared in an aqueous slurry without significant magnesium hydroxide formation.

[0012] Hard burned magnesium oxide powder has limited reactivity and is stable in water for several days. The limited reactivity provides a narrow processing window for making magnesium oxide ceramic products if water has to be used in the process. Hard burned magnesium oxide can also be properly sintered between 1500°C and 1600°C without using sintering aids to achieve sufficient strength for the application.

[0013] The hard burned magnesium oxide is used to form a porous ceramic filter that is particularly suitable for filtering molten aluminum or aluminum alloys. The process, which will be further explained herein, includes the formation of a slurry comprising hard burned magnesium oxide, impregnation of a volatilizable polymer foam and heating. The heating removes any liquids, volatilizes any organics and ultimately sinters the MgO, thereby forming a sintered MgO replica of the foam.

[0014] The ceramic slurry is prepared by mixing the desired ingredients together to form an aqueous suspension of particles. The slurry preferably has rheology characteristics such that the slurry flows easily with applied stress such as during the impregnation of the slurry into the foam precursor, but does not flow when the stress is removed. Such slurry has an inherent high yield stress and thixotropic characteristics.

[0015] In the preparation of the material of this invention, the starting ingredients preferably have a high content of MgO with a particle size of less than 44 microns. It is preferable for the particle size to be at least 2 microns. Below about 2 microns the particles become difficult to handle in a manufacturing environment. Above about 44 microns the properties are less desirable.

[0016] The invention preferably utilizes an aqueous slurry with 40-82 wt. % solids fraction wherein solids fraction, or ceramic portion, is greater than 80 wt. % MgO powder, preferably greater than 90 wt. % MgO powder and more preferably greater than 95 wt. % MgO powder. The aqueous slurry preferably comprises adjuvants for controlling various properties. Particularly preferred adjuvants include surfactants, rheology modifiers, anti-foamants, sintering aids, solvents, dispersants, pore formers and the like. The slurry can be defined as having a solid phase and a carrier phase wherein the solid phase includes the ceramic precursors and the carrier phase includes solvents and adjuvants. Water is the preferred solvent or carrier.

[0017] Sintering aids can be used. A particularly suitable sintering aid is magnesium hydroxide. While not limited by theory, it is believed that magnesium hydroxide decomposes to fine magnesium oxide as a freshly formed and reactive component thereby serving to activate sintering. An excess of magnesium hydroxide oxide is undesirable, as it will release water at high temperature and forms undesirable porosity. It is preferable that the solids content comprise no more than 20 wt. % sintering aid. Other sintering aids such as Y2O3 may also be used, as Y2O3 is one of the most stable oxides with a high melting temperature. [0018] A precursor foam material is impregnated with the slurry such that the interstitial struts of the foam are covered with the slurry to form a ceramic coated foam. It is preferable that the foam be compressed and then allowed to return to the natural shape thereby improving the ability of the slurry to enter into the inner struts of the foam. A ceramic coated foam is thereby formed which is dried and sintered to form the porous ceramic filter.

[0019] The ceramic filter has a primary porosity imparted by the macro structure of the precursor foam. The ceramic forms an exoskeleton of the precursor foam and is a replicate thereof. The primary pore size is typically 3 to 100-ppi and more preferably 40-70 ppi. The filter pore size is dictated principally by the starting pore size of the precursor foam used in the process. For effective aluminum alloy filtration, typical primary pore sizes are between 10 and 70 pores per linear inch. However, each application will have a filter pore size requirement depending upon final end product quality requirements. Pore size is typically referred to in the art as the number of pores in a linear dimension, such as pores per inch wherein a higher ppi value has a smaller cell diameter.

[0020] The foam precursor material could be any type of material that has sufficient resilience to recover its original shape after compression. Polyurethane foam is a particularly suitable material for demonstration of the invention.

[0021] The ceramic coated foam is heated to remove any solvent, heated to volatize any organics and heated to sinter the MgO thereby forming a ceramic replica of the foam precursor. The heating can be done sequentially in separate steps using separate furnaces with a cool-down between each step or continually as a process wherein the temperature is increased over time with a predetermined temperature profile as a function of time. The steps of drying the green ceramic filter are generally performed in a convection-type dryer at a temperature of between 100°C and 200°C for a duration of between 15 minutes and 6 hours. Shorter durations are desirable for process economics and high manufacturing rates.

[0022] Sintering is generally performed in a continuous furnace at a temperature greater than 1000°C over 1 -3 hours, with peak temperatures maintained for 15 minutes to one hour. Sufficient time and temperature must be provided to achieve the desired strength and corrosion resistance properties of the material. More preferably, sintering occurs at temperatures of at least 1500°C to preferably no more than 1 ,650°C wherein a bond between grains is formed thereby creating the strength and corrosion resistance characteristics that are desired in the final product.

Sintering at or above the melting point of MgO is not suitable, nor economical as the replicated foam structure is irreversibly damaged.

[0023] The ceramic foam material has an open cell structure with a distribution of connected voids that are surrounded by webs of ceramic material. Such a structure is commonly used for molten metal filtration and is known in the industry as ceramic foam.

[0024] The ceramic foam filter is shown to be resistant to chemical attack by molten aluminum or aluminum alloys, particularly Al-Li alloys, under typical use conditions. Filters prepared as described herein are also suitable for use in filtering magnesium or magnesium alloys, reactive nickel-base super alloys or steel.

[0025] The ceramic foam filter is lightweight with a preferred density of about 0.33-0.55 g/cc. The density of porous ceramic materials is typically reported as a relative density. A relative density is the ratio of measured density to theoretical density wherein theoretical density assumes no voids. [0026] The Modulus of Rupture (MOR) is a common test used to test the strength of ceramic materials. In the test, a test bar with a nominal size of 30.5x5x5 cm (12x2x2 inches) is broken in four-point bending method with an outer span of 9" and an inner span of 3". The maximum force required to break the test bar is recorded and the MOR is calculated as:

MOR = PL/Wt²

Where P is the breaking load, L is the outer span, W the part width, and t the part thickness. For the ceramic foam filter of this invention, the MOR is greater than 50 psi at a relative density of less than 10% theoretical.

[0027] Air drop pressure was measured on a pressure drop machine where air with certain flow rate and pressure flows through the filter. The difference in air pressure between the entrance and the exit is defined as the air pressure drop and is consider a suitable parameter to represent the permeability of the filter.

EXAMPLES

[0028] A high purity magnesium oxide foam filter was formed using hard burned magnesium oxide. Magnesium oxide slurry was made according to the composition in Table 1 wherein the coarse fraction of MgO powder is hard burned magnesium oxide with a particle size of less than 325 mesh (44 microns) available from Martin Marietta Magnesia Specialties as Magchem 10 -325S. The fine fraction of MgO powder is also hard burned magnesium oxide with an average particle of between 4 to 6 microns. Fine MgO is from air-jet milled MagChem 10 -325S powder. Mg(OH)₂ was obtained from Martin Marietta Magnesia Specialties as MagChem MH 10. The MgO and Mg(OH)₂ powders represent the ceramic portion of the slurry. The other components are solvents used to suspend the ceramic powders or adjuvants used to adjust the properties of the slurry for adequate coverage of the internal struts. Twenty percent of the total MgO was air-jet milled MagChem 10 -325S to improved sintering and particle packing. Hydroxypropyl methylcellulose was obtained from The Dow Chemical Company as METHOCEL K4M and was added as a slurry thickener. Sugar and Production C-Probond latex adhesive were added to improve the structure of the green state filter. Magnesium acetate was added as a rheology modifier and polycarbonate was added as a dispersant.

Table 1 :

[0029] Four 40-ppi foam filters were made using slurry as described in Table 1 . The filters were sintered at a temperature between 1500°C and 1600°C. One filter, arbitrarily numbered filter 4, was cut into bars for mechanical strength measurement Four point bending modulus of rupture testing was used to measure filter strength. Relative filter density was calculated from the dimension and weight of the MOR bars. The resultant values are provided in Table 2.

Table 2:

[0030] The filter has excellent MOR strength at a suitable relative density to function as a filter for molten metal, and particularly, molten aluminum and alloys of molten aluminum.

[0031] The invention has been described with reference to the preferred embodiments without limit thereto. One of skill in the art would realize additional embodiments and improvements which are not specifically set forth herein, but which are within the scope of the invention as more specifically set forth in the claims appended hereto.

Claims

Claimed is:

1 . A process for forming a porous MgO filter for molten metal comprising:

calcining magnesium hydroxide at a temperature of over 1000°C to no more than 1500°C thereby forming hard burned MgO powder; forming a slurry comprising a ceramic portion comprising said hard burned

MgO powder and a carrier solvent;

impregnating a precursor foam with said slurry to form a ceramic coated foam; heating said ceramic coated foam to remove said carrier solvent and volatilize said precursor foam thereby forming a green ceramic filter; and sintering said green ceramic filter thereby forming said porous MgO filter.

2. The process for forming a porous MgO filter for molten metal comprising of claim 1 wherein said slurry further comprises a sintering aid.

3. The process for forming a porous MgO filter for molten metal comprising of claim 2 wherein said sintering aid is Mg(OH)₂.

4. The process for forming a porous MgO filter for molten metal comprising of claim 1 wherein said slurry is an aqueous slurry.

5. The process for forming a porous MgO filter for molten metal comprising of claim 1 wherein at least 80 wt.% of said ceramic portion is said hard burned MgO powder.

6. The process for forming a porous MgO filter for molten metal comprising of claim 5 wherein at least 90 wt.% of said ceramic portion is said hard burned MgO powder.

7. The process for forming a porous MgO filter for molten metal comprising of claim 6 wherein at least 95 wt.% of said ceramic portion is said hard burned MgO powder.

8. The process for forming a porous MgO filter for molten metal comprising of claim 1 wherein said porous MgO filter has a porosity of at least 3 ppi to no more than 100 ppi.

9. The process for forming a porous MgO filter for molten metal comprising of claim 8 wherein said porous MgO filter has said porosity is at least 40 ppi to no more than 70 ppi.

10. The process for forming a porous MgO filter for molten metal comprising of claim 1 wherein said porous MgO filter has a density of at least 0.33 g/cc to no more than 0.55 g/cc.

1 1 . The process for forming a porous MgO filter for molten metal comprising of claim 1 wherein said porous MgO filter has an MOR of at least 50 psi at less than 10% theoretical density.

12. The process for forming a porous MgO filter for molten metal comprising of claim 1 wherein said hard burned MgO powder has a particle size of 2 to 44 microns.

13. The process for forming a porous MgO filter for molten metal comprising of claim 1 wherein said sintering is at a temperature of at least 1000°C to no more than 1650°C.

14. The process for forming a porous MgO filter for molten metal comprising of claim 13 wherein said sintering is at a temperature of at least 1500°C to no more than 1650°C.

15. A filter prepared by the process of any of claims 1 -14.