Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
2000-09-01
2001-09-04
Marcantoni, Paul (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
C501S100000, C501S101000, C501S155000
Reexamination Certificate
active
06284689
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to refractory materials and is particularly concerned with refractory materials that are recycled from otherwise waste materials.
DESCRIPTION OF THE RELATED ART
Increasing environmental awareness has fostered a move to recycling of otherwise “waste” material. Users of refractory products have begun to show such awareness, and are attempting recycle residual refractory materials.
For example, U.S. Pat. No. 4,140,745 to Hassler describes a process using superheated steam to recover magnesia from used refractory articles that contain magnesia. Hassler first hydrates magnesia to magnesium hydroxide, then carbonizes the hydroxide to magnesium carbonate, and finally decomposes the carbonate to reform magnesia. The recovery process chemically changes magnesia into a soluble magnesium compound, which can be leached from used refractory articles, and reconstitutes magnesia after separation. In any separation process, a significant quantity of original material can be lost or rejected, thereby reducing yield. Hassler's multi-step process also decreases commercial throughput.
In the steel industry, a significant quantity of refractory material finds use in recycling operations. Such refractory materials often comprise alumina, magnesia, graphite or their combinations, and are hereafter described as “magnesia-graphite” compositions. In one example of recycling, magnesia-graphite bricks that are removed from furnace linings at the end of the lining's normal life have been recycled by removing front-face contamination acquired in service and crushing to suitable particle sizes. The recycled brick could then be used as a component in new production. Recently, the use of brick recycled in this manner causes cracking of product primarily during the curing process of organic binder systems such as phenol-formaldehyde resins and pitches. Cracking is coincident with the increased use of reactive metal powders, notably aluminum powder, which act as antioxidants to protect readily oxidizable components, such as graphite, carbon black and a binder system. In service, reactive metal antioxidants form carbides and to some extent nitrides, both of which can undergo an expansile hydration reaction with water or condensation products of the usual binder systems. Expansile hydration can then cause cracking.
Reactive metal powders will likely remain important antioxidants in magnesia-graphite articles because of the improved performance such antioxidants offer. A treatment process is required to stabilize carbides and nitrides of such antioxidants in magnesia-graphite waste products before the waste can be reused. The process should hydrate contaminants, such as carbides and nitrides, but should not hydrate magnesia or alumina or cause other undesirable competing reactions. The process should also be rapid enough to be commercially feasible but retain high overall yield. Preferably the process should comprise a single step. In the case of magnesia-graphite refractories, temperatures should be high enough to accelerate the reaction but not so high as to oxidize carbonaceous components and reduce overall yield of the process.
SUMMARY OF THE INVENTION
The objective of the present invention is to provide a process for the treatment of otherwise waste refractory materials, thereby enabling their recycling and subsequent use in fresh batches of materials. Unlike prior art, the present invention avoids chemically changing magnesia, alumina and graphite.
One aspect of the invention describes a single-step process for the treatment of waste material comprising applying superheated steam to particulate feedstock formed from the reclaimed material while maintaining the feedstock material at an elevated temperature. Superheated steam, which means steam at a temperature above the boiling point of water, may be introduced into a flow of particulate feedstock, for example, by injecting the superheated steam into the particulate material. Temperature should be maintained in a range disfavoring the hydration of magnesia and alumina or the oxidation of graphite. A preferred temperature range is from about 100° C. to 500° C.
A further aspect of the invention describes prehydating hydratable contaminants in reclaimed material to produce a stabilized refractory aggregate. The stabilized aggregate is less prone to expansile hydration and reduces cracking in new product made from reclaimed material. Examples of hydratable contaminants include, for example, carbides and nitrides of reactive metals, and especially aluminum carbide and aluminum nitride.
Another aspect of the invention teaches exposing waste material, which is reclaimed from refractory lining after the end of the lining's useful life, to steam at an elevated temperature. The waste material includes magnesia-graphite scrap containing contaminants such as aluminum carbide, aluminum nitride and aluminum oxynitride. Steam can stabilize such contaminants against further reaction with water or steam during re-use of the scrap in subsequent production. Stabilization occurs without significant reduction in yield and is desirable in both resin-bonded pressed shapes and monothilics.
In a further aspect of the invention, the time during which the particulate feedstock material is exposed to superheated steam is described as depending the feedstock's mean particle size and temperature, but largely by the level of hydratable contaminants. Five millimeters is a typical mean particle size for virgin magnesia-graphite used in pressed shapes. This size represents a sensible upper mean particle size limit. A finer size reduces the diffusion path of steam to contaminants but also finds contaminants closer to the surface of the particles. Excessively fine particles can create magnesia dust, which might not benefit from stabilization and might itself hydrate to an unusable material. Use of excessively fine material could also limit its use as a feedstock for typical magnesia-graphite products. A composition that included excessive fines would have a lower magnesia-to-graphite ratio than normal because of excessive hydration of magnesia in the fines.
One embodiment of the invention exposes the particulate feedstock to superheated steam for not less than ten minutes but not greater than ninety minutes. Time of exposure is preferably between 15-45 minutes. Adequate treatment of a particulate feedstock with a mean particle size of less than 4 mm may be achieved by treating the feedstock with superheated steam for 40 minutes at 400° C.
Another aspect of the invention shows the particulate feedstock being vibrated or other agitated while superheated steam is being applied around the substantially the entire periphery of a substantial number of particles. Vibrating or agitating the particulate feedstock can provide fresh surfaces for reaction within the particulate material and reduce the duration of the treatment process. Preferably, a suitable vessel, such as, for example, a rotating reactor or rotary swept multiple level vessel, contains the particulate feedstock during the process.
REFERENCES:
patent: 4140745 (1979-02-01), Hassler et al.
patent: 100445 (1973-09-01), None
Marcantoni Paul
Vesuvius Crucible Company
Williams James R.
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