Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
2000-08-30
2003-04-15
Sample, David (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
C501S110000, C501S117000, C501S119000, C501S120000, C501S121000, C501S122000, C501S132000
Reexamination Certificate
active
06548435
ABSTRACT:
The invention concerns a basic dilatant refractory, free-flowing refractory castable material and precast shapes produced therefrom, such as bricks, prefabricated elements, insulating refractory products and/or functional products such as pocket block, porous plugs, and sleeves.
Refractory castable materials are defined as refractory materials introduced or shaped by casting. The appropriate consistency of the castable materials is achieved by mixing dry components with to mixing water or mixing solution.
The refractory castable material can solidify by hydraulic setting of the calcium aluminate cements without heating, by chemical bonding or micropowderbonding without and with heating, and by sinter processes at operating temperatures. Examples for chemical bonding are a phosphate bonding, water glass bonding, microsilica bonding, or bonding arising when metal powders are used. Micropowder bonding is chiefly a result of the operation of London-van der Waals attraction forces. Refractory castable materials exhibiting more than one bonding type at once are advantageous for numerous applications, as they show the desired strength over a wide temperature range.
Refractory castable materials have been known for a long time. Depending on their chemical composition and on the raw materials used, one distinguishes non-basic and basic refractory castable materials. Alumina refractory castable materials and zircon-containing refractory castable materials are among the nonbasic ones. Basic raw materials for basic refractory castable materials are magnesia, magnesiochromite, chromium ore, chromium oxide and spinel, for instance MgCr
2
O4. Different metal oxides, metal carbides, metal powders or carbon supports are used as refractory additives.
Traditional refractory castable materials lined with the aid of vibration technology are thixotropic. Thixotropy implies a decrease in apparent viscosity with the time of load at constant shear velocity or increase in apparent viscosity (thixotropic rigidification) with decreasing shear rate. Compaction aids such as pneumatic or electrical vibrators must be used for fluidification and compaction of a thixotropic castable material, since this castable material is half-dry and stiff after mixing with water. An inhomogeneous distribution of coarse and fine grains, or demixing, is obtained when a thixotropic refractory castable material is fluidified by an overdose of mixing water, which may be necessary for a lining of narrow fissures and/or complex shapes. An overdose of water will moreover produce a decrease in physical test parameters, such as open porosity and strength. Such monolithic refractory linings have a lower resistance against infiltration and corrosion when used in metallurgical equipment.
Using mechanical compaction aids such as vibrators and/or bottle vibrators has the following disadvantages:
The compacted refractory castable material does not always possess optimum homogeneity, that is, shrinkage cavities may be present;
Problems when lining narrow fissures and complex shapes, since bottle vibrators have a reduced radius of action;
physical stress for operators.
A thixotropic refractory castable material on the basis of magnesite preferred for the lining of steel foundry ladles is know from EP 0,248,171 B1. The bonding of this castable material consists of boric acid, alkali polyphosphates and calcium hydroxide. Vibrators are used in order to achieve sufficient density during lining.
A water containing, refractory castable material on the basis of MgO and having a carbon content between 3 and 10% by weight, a dispersing agent in an amount between 0.1 and 2.0% by weight, and a reactive silicic acid in an amount between 1 and 10% by weight is described in DE 195,18,468 A1. According to this document (see column 2, lines 4 to 7) the silicic acid is decisive for preventing a hydration of the sintered magnesia. This castable material, too, which is used for a monolithic lining of metallurgical melting vessels, and preferably of the slag region of foundry ladles, is compacted by vibration technology.
The use of an oxide-type micropowder dispersed in a nonaqueous dispersion medium is known from EP 0,573,029 B1. This fine-grained suspension on the basis of MgO, Al
2
O
3
, Cr
2
O
3
and/or TiO
2
: can be used to produce refractory ceramic materials and castings of high density and strength. The risk of hydration of MgO is reduced since nonaqueous solvents are used to prepare the suspension. Tests have shown that the castings can be made by dry pressing from a castable material containing 85% by weight of coarse-grained matrix material in the grain fraction between 1.0 &mgr;m and 3 mm as well as 15% by weight of a finely divided, previously dispersed MgO micropowder. The raw density and strength of the green compact and of fired castings is distinctly higher than that known in the art. The use of nonaqueous MgO suspensions for the preparation of refractory castable materials is little appropriate on account of the environmental to impacts and of safety risks during the drying and heating of lined devices.
The use of basic aqueous suspensions having a high concentration of MgO micropowder could be advantageous for the production of castings, for instance by dry pressing or ramming, if it was possible the prepare such suspensions. However, it would be necessary not to have an excessive degree of hydration of the MgO-based materials in the suspension.
A process for the preparation of magnesia of low hydration is further known from the document EP 0,448,156 A1. However, it is not possible to produce a refractory castable material with free-flowing properties by this process, even when hydration is low and a large amount of mixing water is used. In this context the reader is referred to the reference examples cited at the end of the description.
Free-flowing alumina refractory castable materials are a relatively new development. In contrast to the thixotropic refractory castable materials, mechanical compaction aids are not needed in order to achieve an appropriate consistency and physical test parameters comparable with those of the thixotropic castable materials. Free flow is promoted by dilatant properties of the castable material. Dilatancy is defined as a decrease in viscosity occurring with decreasing shear rate, here in the sense of so-called rheological dilatancy.
A non-basic refractory, hydraulically bonded refractory castable material is described in EP 0,525,394 B1. This refractory castable material on the basis of alumina raw materials is composed as follows, and exhibits free flow when 3.5 to 7.0 parts by weight of mixing water are added for each 100 parts by weight of solids:
65 to 87% by weight of a refractory matrix material on the basis of Al2O3, ZrO2 and/or Cr2O3 with a grain size between 0.1 and 10.0 mm;
7.0 to 22.0% by weight of a reactive refractory component on the basis of Al2O3, ZrO2 and/or Cr2O3 with a grain size between 0.1 and 10 &mgr;m:
0.5 to 10.0% by weight of a hydraulic binder with an Al2O3 content above 68% by weight;
0.2 to 6.5% by weight of one or several stabilizing additives and/or additives promoting the water retention of the refractory castable material.
Decisive for the desired flow properties of the free-flowing alumina refractory castable material are the adjustment of the grain size ranges in the refractory matrix material and the reactive refractory component as well as the adjustment of the mixing water with the 5 additives named.
The flowability of a free-flowing refractory castable material is rated according to the flow value. The flow value Fo is determined immediately after mixing with the mixing water with a vibration-free consistency test, and using the formula:
Fo
=[(dm,mm−100 mm)/100 mm]×100,%
where dm is the mean diameter of the sample after a certain time of flow, and 100 mm is the lower diameter of the truncated cone.
The consistency test of the free-flowing refractory castable materials is performed at present, most often using a truncated cone of 100×70&
Ostrolenk Faber Gerb & Soffen, LLP
Sample David
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