A System and Method for the Production of High Strength Materials

Abstract

The invention provides a process for manufacturing ceramics and refractories comprising the steps of producing a porous powder comprising nanograin sized particles wherein the particles have a Young’s modulus value that is smaller in value compared to the same crystalline material; compacting and processing the powder such that the powder forms a stable homogeneous composite; and sintering the composite for a time and temperature to lead to uniform shrinkage of the composite to make a dense homogenous material.

Claims

1. A process for manufacturing ceramics and refractories comprising the steps of: (a) producing a porous powder comprising nano-grain sized particles with a Young’s modulus less than 10% of that of a same crystalline material; (b) compacting and processing the powder such that the powder forms a stable-homogeneous composite; and (c) sintering the composite for a time and temperature to lead to uniform shrinkage of the composite to make a denser homogenous material to a required specification of density and strength.

2. The process of claim 1 wherein the powder comprises particles with a size distribution of between 0.1 to 100 microns.

3. The process of claim 2 wherein the powder comprises particles with a size distribution of between 1 to 20 microns.

4. The process of claim 1, wherein porosity of the particles is between 0.4 to 0.7.

5-6. (canceled)

7. The process of claim 1 wherein step (b) additionally comprises the steps of: (b1) maximizing bulk density of the powder by shaking the powder in a device; and (b2) applying pressure to produce the homogeneous composite wherein conditions are chosen to limit growth of a nano-grain size of the particles during this process.

8. The process of claim 7 wherein temperature conditions are controlled to limit the growth of the nano-grain size of the particles.

9. The process of claim 8 wherein the composite does not expand or fragment when pressure is released.

10. The process of any one of claim 7, wherein a shape of the device is designed for as specific use of the homogeneous material, including the use of shapes formed by additive manufacturing techniques.

11. The process of claim 1 wherein the steps (b) and (c) occur simultaneously.

12. The process of claim 1, wherein the porous powder is magnesium oxide.

13. The process of claim 12, wherein porous magnesium oxide powder in produced by flash calcination of ed-magnesium carbonate or magnesium hydroxide, and cooled by flash quenching.

14. (canceled)

15. The process of any one of claim 1, wherein the porous powder is aluminium oxide.

16. The process of any one of claim 1, wherein the porous powder is silicon carbide.

17. The process of claim 1, wherein the powder comprises at least one material comprising nano-grain particles with a Young’s modulus less than 10% of that of the same crystalline material.

18. The process of claim 15, wherein the porous aluminium oxide powder is produced by flash calcination of aluminium hydroxide or gibbsite, and preferably cooled by flash quenching.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0030] FIG. 1 depicts a flowchart illustrating a process for manufacturing ceramics and refractories using a flash calcining process according to an example embodiment.

[0031] FIG. 2 depicts a schematic cross-sectional drawing of a calciner reactor according to an example embodiment;

DESCRIPTION OF THE INVENTION

[0032] Preferred embodiments of the invention will now be described by reference to the nonlimiting examples.

[0033] The process of a preferred embodiment of the present invention comprises the steps of producing a porous powder comprising nano-grain sized particles. The particles of the powder are designed to have a Young’s modulus value that is smaller in value compared the same crystalline material. This is the property of the material that enables the particles to deform under pressure to eliminate the pores described above without the need for large scale change of the grain size distribution. The powder is treated to form a stable homogeneous composite, and sintered for a time and temperature to lead to uniform shrinkage of the composite to make a dense homogenous material. The conditions of pressure, and temperature are selected to minimise the coarsening of the nano-grain size and eliminate all other pores as far as possible to maximise grain to grain contact. The porous powder preferably comprises particles with a size distribution of between 1 to 20 microns, with a porosity between 0.4% to 0.7% and a Young’s modulus less than 10% of that of the crystal value of the same crystalline material.

[0034] A preferred embodiment of the invention is a process for manufacturing ceramics and refractories using flash calcination. A flow chart of an exemplary process is outlined in FIG. 1. The example embodiments described using flash calcination provide a continuous calcination system and method. The described embodiments provide a system and process that takes advantage of both the faster chemical kinetics engendered by the catalytic effect of superheated steam in association with a small particle size, and the use of the superheated steam for gas phase heat transfer. As shown in steps outlined FIG. 1, feedstock particles move in a granular flow through a vertical reactor segment by forces such as steam, gravity or a centrifugal force. Horizontal forces are thereby imparted on these particles passing through the reactor segment in a vertical direction. As the particles flow inside the reactor segment, heat is provided to the particles via heat transfer through the walls of the reactor segment. A superheated gas may be introduced into the reactor segment to create conditions of a gas-solid multiphase system. Gas products can be at least partially flushed from the reactor segment under the flow of the superheated/gas.

[0035] At the same time, however, the described embodiments are designed such that the dominant mechanism of heat transfer is from the walls of the calciner directly to the particles as a result of two major factors. That is, the heat transfer arising from the strong interaction of the particles with the gas engendered by the large centrifugal forces acting on the particles and resultant friction with the gas that is imparted to the walls of the reactor tube, and the heat transfer arising from the radiation heating of the particles. The granular flow through the helical tube is significantly slower than through an equivalent straight tube, and this not only generates the friction required for the above first mechanism for heat transfer, but also controls the transit time through the reactor to allow the heat transfer to take place efficiently. Thus, a helical tube can process a higher throughput than a linear tube of the same diameter and length.

[0036] The calciner reactor 10 described in FIG. 2 is more generally applicable to calcining minerals other than limestone. A broad statement is that calcination is the chemical process that is activated by heat, and includes dehydration as well as decarbonation, with or without superheated steam. Starting materials can be carbonates, but hydroxides, as in the present invention, also calcine to oxides, and hydrated materials are dehydrated. In many chemical reactions (other than dehydration), superheated steam is quite likely to assist most such processes because the water molecule is a well-established labile ligand to mostly all metal ions, and therefore chemical intermediates Involving water may be engendered by the presence of superheated steam. Even where the catalysis does not occur, there may be advantages in using the process of the described embodiments in which the role of superheated steam, or other injected gases, is principally to promote the transfer of heat to the particles. That is, generally, the fine grinding of feedstock will remove the impact of heat transfer and mass transfer process of decomposition, and enhance the chemical reaction step. The operating conditions of the calciner reactor 10 described can be readily adapted to any calcination process in which the calcination can be accommodated within the residence time of feedstock passing through the system.

[0037] FIG. 2 shows a single segment vertical calciner reactor 10. The feedstock indicated at 11 is produced from rocks and ores that have been dried, crushed and pre-ground. A feedstock size distribution with a mean size in the range of about 40 microns to about 250 microns is achieved by a conventional cyclone system (not shown) with a crusher and grinder (not shown). The feedstock- 11 is collected in a Feedstock Hopper 12 and is mixed with superheated steam 13 in mixer 14 and conveyed pneumatically through a conveyor tube 15 to an injector 16 at the top of the reactor where it is injected into the reactor tube 17. The injector 16 thus functions as both, feeder for the particles into the reactor tube 17, and as an inlet for superheated steam 13 into the reactor tube 17.

[0038] It will be appreciated that additional inlets may be provided along the tube 17 in different embodiments for feeding super-heated steam into the reactor tube 17. The reactor tube 17 is formed into a helix 18, and preferably the helix 18 is formed into a structure which forms a leak proof central column 20. The helix 1 imparts horizontal forces on particles passing through the reactor 10 in a vertical direction. The reaction proceeds in the reaction tube 17 to the desired degree. The superheated steam, the product particles and the reaction gases flow out of the open end 32 of tube 17 and through to the gas-particle separator 19. The reaction tube 17 and the gas-particle separator form a reactor segment in this example embodiment. The gas motion is reversed and the gases are exhausted into the central column 20 by the vortex formed in the separator 19 as a result of the centrifugal forces induced in the helix 18.

[0039] It will be appreciated that additional exhaust openings may be provided along the tube 17 in different embodiments. The exhausted gases in the central column 20 heat the steam 13 and feedstock 11 being conveyed to the injector 16 before the gases are exhausted at the top of the reactor 21, The exhaust gases can be processed by condensing the steam in a condenser 29 and compressing the gas for other uses. The product particles 22 are collected in the hopper 23, and are rapidly cooled using heat exchanger 30, e.g. with the water used to produce the steam. The reactor tube 17 is heated externally by a heat source 24, and the reactor is thermally insulated 25 to minimise heat loss.

[0040] The flow rates of the superheated steam in the calcination process are set so as to maximise the degree of calcination. In FIG. 2, the steam moves in the same direction as the particles, so that the steam has maximum impact on the reaction rate at the top of the reactor 10, and this effect decreases through the reactor 10 as the steam is diluted by the reaction gases and the pressure drops as a result of the friction along the tube 17.

[0041] The temperature of the particles during transportation in a flash calcination process is preferably kept sufficiently low to ensure that both the steam catalysed calcination reaction and the sintering by steam heat is minimised, and the adsorption of steam maximised, while the steam temperature is preferably kept sufficiently high so that the steam does not condense. The travel time of the particles down the gravity feed calciner is between 1 to 15 seconds, preferably about 6 seconds.

[0042] The temperature of the calciner walls is maintained at the desired calcination temperature by heating the outer wall of the reactor tube 17, as shown in FIG. 2. When multiple reactor chambers are used, the average temperatures for each chamber may be different and each chamber may operate with a temperature gradient. There are several means of achieving the external heating, with the design of external heating systems being a known art. The helix 18 provides a large external surface area, and the control of the temperature can provide the system with a uniform thermal load. It is preferable that the thermal load be less than about 50 kW/m.sup.2. Where distributed frameless heating is used, the suppression of pyrolysis can be achieved by feeding a portion of the calciner exhaust into the fuel in the external heating system 24 via a pipe connection 31 coupled to the exhaust 21, to control the rate of production of heat.

[0043] For example, it is often desirable that the temperature near the base of the calciner reactor 10 is larger than that at the top. Near the injector 16, the CO partial pressure is small, and the reaction rate is faster than at the base, so that for a constant thermal load, the temperature at the top can be lower than the base. This can be achieved by injection of the fuel near the base, so that the flow of gas in the external heater system 24 is in counterflow to the flow of gas and solids in the tube 17. In another such example system, the heat is produced electrically by applying a voltage between an upper portion and a lower portion of the tube 17 with a current supplied to heat the reactor tube 17 by its electrical resistivity.

[0044] In another example system, the heat is produced by burners arrayed around the external surface of the tube 17 so as to produce the desired temperature distribution along the reactor tube 17. In another example system, the heat is provided by a heat exchanger from a heat exchange fluid, such as compressed carbon dioxide. In another example, oxygen is used instead of air. A combination of such systems may be used.

[0045] In one embodiment, the powder of the present invention is produced by flash calcination of a precursor material in which volatile materials are released to develop porosity. In this preferred embodiment, the calcined powder is flash quenched to minimise the grain size.

[0046] In an alternative preferred embodiment, step (b) of the process additionally comprises the steps of: (b1) maximising the bulk density of the powder by shaking the powder in a device; and (b2) applying pressure to produce a homogeneous composite material wherein the conditions are chosen to limit the growth of the nano-grain size of the powder during this process. The shape of the device may be designed for as specific use of the processed material, including the use of shapes formed by additive manufacturing techniques. In another alternative embodiment, the processes of forming and sintering the composite material occur in a single process.

[0047] Preferred powders are magnesium oxide, aluminium oxide, or silicon carbide. A mixture of these powders may also be used.

[0048] The first example embodiment is the production of magnesium oxide ceramics. In this example, the nano-active material is made by the calcination of the mineral magnesite (magnesium carbonate) as the precursor. Steam is produced by the decomposition of magnesium hydroxide. Steam is preferably formed by the reaction of water vapour in the calcination process. In a particularly preferred process, the powder comprises at least one nano-active material. This application is described in the Sceats Horley invention, and is known to produce a material with the desired physical properties of nano-grains of crystals of magnesia (MgO).

[0049] A nanoparticle or ultrafine particle is typically understood as a particle of matter that is between 1 and 100 nanometres (nm) in diameter. Nanoparticles are distinguished from “fine particles”, sized between 100 and 2500 nm, and “coarse particles”, ranging from 2500 to 10,000 nm. Nanoparticles are much smaller than the wavelengths of visible light (400-700 nm), and require an electron microscope to be seen. Dispersions of nanoparticles in transparent media can be transparent. Nanoparticles also easily pass through common filters, such that separation from liquids requires special nanofiltration techniques.

[0050] The properties of nanoparticles very often differ markedly from those of larger particles of the same substance. Since the typical diameter of an atom is between 0.15 and 0.6 nm, a large fraction of the nanoparticle’s material lies within a few atomic diameters from its surface. Therefore, the properties of that surface layer may dominate over those of the bulk material. This effect is particularly strong for nanoparticles dispersed in a medium of different composition, since the interactions between the two materials at their interface also becomes significant.

[0051] The exemplary powder of the present invention can be ground by conventional processes to meet the desired broad particle size distribution that maximises the tapped density of the porous powder. The powder is selected to have a high porosity so that calcination proceeds quickly at low temperature and thermal sintering of the powder is minimised. During the calcination processes there are macropores in the initial magnesite powder that expand as the calcination proceeds, and adjacent grains form necks to produce a stable particle by further expansion of the macropores. Flash quenching of the powder further supresses sintering, and some moisture is introduced to enable the formation of magnesium hydroxide, at about 1% mole/mole as the powder is cooled.

[0052] Nano-indentation of the particles shows that the particle Young’s modulus is about 5% of that of the crystal value. Thus, the nanograin array has the flexibility to re-arrange under modest pressure. In this example, the composite is made by concentrating the powder by tapping and applying sound and ultrasound to maximise the bulk density of the powder. The process further comprises controlling temperature conditions to limit the growth of the grain size of the powder. In another preferred embodiment, the process further comprises the use of additives such that the composite does not significantly expand or fragment when pressure is released.

[0053] In the second step, the powder is put under pressure, of about 1-10 MPa, and the temperature is raised to about 300° C. so activate a binding process which arises from the release of water vapour, and this process activates the MgO to bind the particles under pressure, so that as the pressure is relived and the temperature is reduced, the composite does not expand significantly.

[0054] Microscope analysis and light scattering shows that the composite is substantially uniform, and a comparison of the Small Angle X-ray Scattering of the powder and composite shows that the material gain size has remained on the nano-scale with a small change of the grain size distribution. The composite is heated in a furnace and the densification is measured as a function of temperature and time. The temperature and time are consistent with traditional sintering kinetics, but are significantly lower because the diffusion of material is on the nano-scale, rather than the micron-scale of MgO ceramics and refractories. For example, the sintering temperature is reduced from 1500° C. to 1000° C. and the time is reduced from hours to minutes.

[0055] Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms, in keeping with the broad principles and the spirit of the invention described herein.

[0056] The present invention and the described preferred embodiments specifically include at least one feature that is industrial applicable.