Low cement refractory

Details Low cement refractory Low cement refractory

1991-03-11

1992-03-24

Dixon, Jr., William R.

Compositions: ceramic

Ceramic compositions

Refractory

106692, 106694, 106816, 106817, C04B 3510

Patent

active

050988735

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

This invention relates to a low cement castable refractory and to a process for making the same.


BACKGROUND OF THE INVENTION

A number of low cement castable refractories have been developed for use within the aluminum industry. These castable refractories consist of a blend of calcined aggregate, cements and fillers which are mixed in various proportions to suit the specific end applications. The calcined aggregate is selected from a range of materials, including tabular alumina, bauxites, kaolins and other clays, and normally contributes up to 95 wt % of the final refractory mixture.
Characteristics and properties of low cement castables compared with conventional castable refractory materials include:
(i) A low moisture requirement during mixing. This results in a fired refractory of higher density and lower porosity.
(ii) Higher cured and fired strengths, particularly in the temperature range of 600.degree. to 1100.degree. C.
(iii) Greater chemical and erosion resistance against molten metals and slags.
(iv) Higher refractories and greater thermal shock resistance.
Even though superior to conventional castables, a major disadvantage of the low cement castables currently available is their susceptibility to chemical attack and erosion. This necessitates a relatively frequent furnace or crucible shutdown/replacement program and hence reduced productivity. Where used as a barrier layer in reduction cells, a gradual breakdown of the refractory over time leads to exposure of the underlying insulation layers to the aggressive cell environment. Ultimately this will result in cell failure and shutdown, but prior to this a gradual increase in cell voltage and hence a resulting loss in energy efficiency.
A further disadvantage of low cement castables presently available is their high cost compared with conventional castable refractories. This is largely an artifact of the aggregate material; premium grade calcined minerals have been used exclusively for these applications.


SUMMARY OF THE INVENTION AND OBJECTS

There is clearly a need for a low cost, chemically resistant low cement castable refractory for use in the aluminium industry and it is an object of the present invention to provide such a castable refractory and a method of providing same.
The invention provides a low cement castable refractory comprising 85 to 96% by weight of a calcined ultrafine bauxite aggregate in which the ultrafine bauxite forming the aggregate is characterised by agglomerated particles of a size of less than 6 um, 2 to 10% by weight of a suitable cement material, 0 to 8% by weight of thixotropic agent and 0 to 2% by weight of suitable deflocculants and plasticizers.
The calcined ultrafine bauxite aggregate is preferably characterised by a particle size of less than 1 um, such as the naturally occurring "ultrafine" bauxite which occurs at specific bauxite deposits, for example, at the Weipa deposit in Northern Australia. Such ultrafine bauxite is usually recovered from settling and drying ponds in the form of large agglomerated lumps. The lumps are crushed to produce aggregate particles in a range of sizes which are then calcined at high temperatures to produce the calcined ultrafine bauxite aggregate. The calcined aggregate is screened and ground to produce a controlled distribution of sized fractions, the following example of which has been found to provide suitable results in the low cement castable described in this invention:
The calcined ultrafine bauxite aggregate differs from conventional calcined refractory grade bauxite aggregates in that it is produced by calcining agglomerated ultrafine bauxite particles. Conventional calcined refractory grade bauxite aggregates are produced by crushing and calcining bauxite ores recovered from the ground in an as-mined form.
The advantage of the calcined ultrafine bauxite aggregate lies in its high microstructural uniformity and low chemical reactivity. Such uniformity cannot be achieved in conventional calcined refractory grade bauxite aggregates because of

REFERENCES:
patent: 4522926 (1985-06-01), Felice
patent: 4568652 (1986-02-01), Petty, Jr.
patent: 4943544 (1990-07-01), McGarry et al.

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