Production, Refining, Fabrication and Recycling of Light Metals. Proceedings of the International Symposium on Production, Refining, Fabrication and Recycling of Light Metals, Hamilton, Ontario, August 26–30, 1990

348 Pages·1990·12.96 MB·English

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ACTA METALLURGICA
CANADIAN METALLURGICAL QUARTERLY
MATERIALS RESEARCH BULLETIN
MINERALS ENGINEERING
SCRIPTA METALLURGICA
PROCEEDINGS OF THE INTERNATIONAL SYMPOSIUM ON
PRODUCTION, REFINING, FABRICATION AND
RECYCLING OF LIGHT METALS
HAMILTON, ONTARIO, AUGUST 26-30, 1990
Production, Refining,
Fabrication and Recycling
of Light Metals
Editors
Michel Bouchard
Universite du Quebec ä Chicoutimi
Chicoutimi, Quebec
Pierre Tremblay
Alcan International Ltee,
Jonquiere, Quebec
Symposium organized by the Light Metals Section of
The Metallurgical Society of CIM
29th ANNUAL CONFERENCE OF METALLURGISTS OF CIM
29e CONFERENCE ANNUELLE DES METALLURGISTES DE UICM
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Foreword
In conjunction with the 29th Annual Conference of Mertallurgists of CIM, The Light Metals
Section of The Metallurgical Society of CIM, has organized an International Symposium
of unprecedented scope, on the Production, Refining, Fabrication and Recycling of Light
Metals.
The symposium features over thirty papers from engineers and scientists of seven countries,
reporting recent developments in the production, refining, fabrication and recycling of
aluminum and magnesium.
Technical papers are being presented by all the primary aluminum and magnesium producers
in Canada. These contributions are complemented by exceptional papers from members of
the Canadian and international academia and from other companies involved in the light
metals industries, in Canada and abroad.
For the first time, the symposium will formally broach on subjects fast becoming hallmarks
of our times. The session on composites comprises several papers on this developing high-
potential sector of the aluminum industry. Also, much to the credit of the light metals in-
dustries, sesssions on environmental protection and recycling will be presented.
The success of the Symposium is due to the engineers and scientists reporting their findings.
We wish to thank them for taking precious time to document their work appropriately and
making the publication of these proceedings possible. It is only fitting that we dedicate this
book to all of them.
Michel Bouchard
Pierre Tremblay
Editors
3
Selection of monolithic castables for cathode barriers
D.V. Stewart and A.T. Tabereaux
Reynolds Metals Company, Manufacturing Technology Laboratory,
Extractive Metallurgy Department, P.O. Box 1200, Sheffield, Alabama 35660, U.S.A.
Abstract
Bath penetration into the cathode begins immediately after the
start-up of an alumina reduction cell and is particularly high
during the first weeks of operation. Later on, bath penetration
is considerably reduced but continues throughout the life of the
cathode. The extent of the bath penetration and its composition
will depend upon the cell design and lining materials used. In
this work, commercially available, low-water castable
refractories were evaluated and compared in laboratory tests as a
possible monolithic barrier material in cathodes for protecting
the low thermal conductivity insulating materials. A monolithic
castable barrier would eliminate the inherent weakness of the
mortar seams present in the commonly-used brick barriers in
protecting against penetration and degradation bv crvolitic
salts, bath, and aluminum metal. Installation and effectiveness
of low-water castable barriers in plant trials are discussed.
Keywords
Cathode refractories, castable, monolithic; cryolite resistance
and physical and chemical properties.
Introduction
In the production of primary aluminum, one of the maior costs is
the electrical power requirement. It is the desire of aluminum
producers to reduce the electrical energy consumption per unit of
metal produced by various methods and to save thermal energy by
improving the thermal insulation in the cathodes.
Corrosive cryolitic bath components, and even aluminum metal,
penetrate and saturate the cathode lining, destroying cathode
refractories and, thus, degrading the lower cathode thermal
insulation during the reduction cell's lifetime.
Layers of materials are used in the cathode to control the rate
that cryolite constituents penetrate the bottom cathode
refractories to prevent this diffusion process from happening too
rapidly. Materials which have been used as physical and chemical
barriers in industrial cells include: super duty fireclay and
chamotte bricks, ceramic tiles, metal plates, graphite foil,
glass, and various chemical barriers. In an effort to address
the problem of selecting refractories for application as the
cathode barrier, an investigation was undertaken to evaluate
commercially available, low-water castable refractories.
4 PRODUCTION, FABRICATION AND RECYCLING OF LIGHT METALS
FIGURE 1
PREBAKE CARBON
SIDE WALL BLOCK
REDUCTION CELL CATHODE
CROSS SECTION VIEW
□ m □
0-2% 2-4% 4-6% *e%
TEMPERATURE ISOTHERMS & CRYOLITIC SALT
CONTENT OF ALUMINA INSULATED CATHODE
PRODUCTION, FABRICATION AND RECYCLING OF LIGHT METALS 5
CRITERIA FOR SELECTING REFRACTORIES
FOR CATHODE BARRIERS
1. Maximum Service Temperature
• Cathode operating temperatures
• Freeze plane for cryolitic bath components
2. Resistance To Attack By Cryolitic Bath Components and Aluminum
at Service Temperature
• Density and permeability (porosity & pore size
distribution)
• Chemical composition
3. Structure Strength, or Load Bearing Capability
• Strength at operating temperatures
• Cold crushing strength
4. Volume Stability
• Thermal expansion
• Chemical reation & expansion
5. Insulation
• Thermal conductivity
6. Installation Limitations
• Sizing tolerances
• Equipment necessary
• Curing and drying considerations
7. Material Availability
8. Labor Availability
o Training, experience, supervision
9. Cost considerations
Materials received by the user for cathode construction should be
evaluated to ensure that thev are within statistical control of
established refractory specifications.
Maximum Refractory Service Temperature
• Refractories should have sufficient structural and chemical
stability over the entire cathode temperature range during
cell startup and operations.
• Refractory layer should not lose strength, disintegrate,
change weight nor dimension during cell startup and
operation.
• Refractory's softening or solidus temperature should not be
reached, even if penetrated by cryolitic bath components.
6 PRODUCTION, FABRICATION AND RECYCLING OF LIGHT METALS
MAXIMUM RECOMMENDED
SERVICE TEMPERATURE
REFRACTORY (°F) (°C)
Hoganus 3236 1780
Densecast 50
National Refractory 3200 1760
Kricon 32
Carborundum 3000 1650
ND-8 4
Plibrico 3100 170
3100 Special
Clayburn 2800 1538
Kilcast 46
National Refractory 2800 1538
Kricon 28
North American Refractories 230 1260
Refracrete 23
AP Green 3169 1743
SM Brick
Clayburn 2772 152
Kilgard Brick
Pamas 2650 1450
PMS Brick
COMMENTS:
• The maximum recommended service temperatures of the
castables are equal to that of the three fireclay bricks
used in cathodes.
• The maximum recommended service temperatures of all
castables and bricks are much higher than the 900 to 1000°C
temperature isotherms experienced in cathodes during startup
and operations.
CATHODE OPERATING TEMPERATURES/ISOTHERMS
• It is advisable, but not always possible, to keep the main
body of refractory lining below the freezing isotherms of
most bath components.
• The ideal location for the castable barrier layer is between
a top layer of protective refractory (bricks or vibrated
powdered alumina) and bottom layer of insulation.
• This position may provide a sufficient temperature gradient,
(about 750 to 850°C) to prevent contact with the low
temperature eutectic corrosive cryolitic constituents for
some period of time.
PRODUCTION, FABRICATION AND RECYCLING OF LIGHT METALS 7
• The absolute worst situation for a castable barrier would be
to locate it directly below the cathode blocks.
• The higher temperature gradient (900°C) and almost certain
direct contact with corrosive bath constituents diffusing
through the carbon cathode blocks would attack the
monolithic castable refractory rapidly and increase chemical
reaction kinetics.
• It should be noted that the temperature gradient on the hot
face side of the castable barrier increases insulation in
the layer of low density, low thermal conductivity
insulation below the barrier material.
2. Density/permeability
2a. Density and apparent porosity
• Generally, as the bulk density of a refractory increases the
apparent porosity decreases.
• Porosity is an indication of how permeable the refractory is
to vapor.
• Less porosity means less area in the pore structure for
corrosion attack to start and proceed in refractory.
• Refractory is less permeable if it has a lower porosity and
finer pore size distribution.
• The more continuous pore phase generally leads to a greater
permeability.
• Reduction of the permeability in refractory relates to the
ability of the refractory to prevent penetration of
corrosive materials (gases and/or liquids) that leads to
chemical change in the material and/or redistribution of the
material's chemical and physical makeup which alters the
structure, and may lead to expansive growth.
• However, too high density with very low porosity in a
refractory may result in poor refractory thermal shock
properties.
WATER BULK APARENT
ADDED, DENSIT3Y, POROSITY,
REFRACTORY (%) (g/cm ) (%)
ND-8 4 5 2.68 16.5
Kricon 32 6 2.51 16.5
Kilcast 46 7 2.36 15.2
Kricon 28 5 2.35 15.8
3100 Special 9 2.29 20.4
Densecast 50 9 2.21 21.8
Refracrete 2 3 12 2.02 28.0
AP Green SM Brick - 2.30 10.0
Kilgard Brick - 2.21 14.6
Pamas PMS Brick - 2.20 14.0
8 PRODUCTION, FABRICATION AND RECYCLING OF LIGHT METALS
COMMENTS:
• Higher density and lower porosity are related to the amount
of mix water used in the castables. By using the minimum
amount of water there will less void space after curing.
• By comparison, the commercial fireclay bricks had the lowest
3
density, 2.2 to 2.3 g/cm , and the lowest porosity-due to
their higher fired and pressed state.
2b. Pore size distribution
• Castables usually have a finer pore size than bricks, with
the pore size increasing as the firing temperature
increases.
• Measured porosity and pore structure in the castables are
associated with the porosity connected to the surface of the
material.
APPARENT PORE SIZE DIAMETER
REFRACTORY POROSITY (PORE VOLUME, %
(%) >SHOWN MICRONS)
50 40 30 20 10 5 1
Densecast 50 21.8 2 3 4 6 10 15 17
3100 Special 20. 4 13 17 18 19 28 30 35
Kricon 32 16. 5 16 17 19 22 26 29 35
ND-8 4 16. 5 13 14 14 15 20 28 38
Kilgard Brick 14.6 11 11 13 15 21 43 62
COMMENTS:
• Desecast 50 had the finer porosity structure of the
castables measured, and started out with a higher mix water,
9%.
• Kilgard brick had a coarser pore structure with fewer
percentage of pores in the range <1 to 5 microns than the
castables.
2c. Chemical composition
• Generally, a higher alumina content provides more resistance
to cryolite attack in the cathode, and correspondingly a
higher silica content is more prone to attack by cryolite.
MAJOR CHEMICAL COMPONENTS
REFRACTORY A1203 Si02 CaO Fe203 Ti02
(%) (%) (%) (%) (%)
3100 Special (69.3 24.1 1.5 0.8 1.3)
ND 84 61.1 33.3 0.5 1.1 0.7
Kricon 32 59.2 33.3 1.2 1.1 1.7
Densecast 50 51.4 41.2 — 1.4 1.0
Kilcast 46 (45.9 48.5 1.1 2.0 1.4)
Kricon 28 (45.6 47.7 1.5 1.3 1.8)
Refracrete 23 (32.6 49.4 14.3 0.9 1.6)