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John Binner - Week 5 - Coggle Diagram
John Binner - Week 5
Refractories
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Used in steel, aluminium, glass and cement production
Refractories must resist
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Catalytic heat (heat arising from molecular reactions, e.g. recombination of dissociated oxygen molecules arising from shock conditions)
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Refractory types
Fireclay - contains between 25-45% alumina and 50-80 % silica. Higher alumina content leads to higher temperature performance. Applications = regenerators, ovens, kilns
High alumina - contains 45-95 % alumina - good resistance to slag and spalling, good load bearing capacity and may be used typically up to 1850 C
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Chromite-magnesite - contain more chromite than magnesite - basic in nature, they may be used up to 1700 C. Resistant to thermal shock and applications include basic oxygen steel making.
SiC - contains >85% SiC and offers high thermal conductivity and resistance to thermal spalling - inert to acidic slags
Blast furnace
A blast furnace is used for smelting iron, but can be used for other metals such as lead or copper. The term 'blast' comes from the combustion air being forced in at high pressure
Different refractories are used at different places within the blast furnace depending on the temp - much hotter at bottom of furnace and must withstand the weight of the molten metals
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Glass furnace
There are a range of different furnace designs and sizes for producing glass, ranging from pot furnaces (glass is melted in pots so no contact with refractories) to tank furnaces, some of which produce 100's of tonnes a day
The raw material enter at one end of the melting zone and are often heated using gas flames - though electricity can also be used. The glass flows through the throat (which ensures only fully melted) into the working end, where it cools to the right temperature and hence viscosity for extraction and secondary forming
Cement kiln
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Castables are powders that are mixed with water and then pumped onto a surface, where they are left to cure for several days
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Cement is mixed by heating a mixture of finely ground clay, limestone and sand in a rotating kiln at 1450 C. This forms cement clinker, which is extracted, cooled and finely ground to form cement powder.
Castable linings are patchable - down time is minimal as problem area can simply be cut out and replaced. Despite the initial costs of castables and bricks being the same, the installation and repair of castables is much cheaper, making it the more attractive option when designing a furnace.
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Replacing bricks is much more complex especially in arches, the structural integrity relies on all of the bricks being in place.
Furnace refractories
As an example of a protective viscous layer, the boundary layer can easily be 1 cm thick in glass melting furnaces where silicate slags are present. This is due to the high slag velocities (top of the melt) and low fluid velocities (deeper in the melt).
The thick layer helps to protect the refractory linings, which would need replacing much more frequently otherwise. To avoid the boundary layers being disturbed, glass making furnaces are designed so that turbulence is avoided.
With the exception of HF, most ceramic materials, particularly advanced ceramics, are resistant to ambient temperature corrosion. Since HF is effective at attacking many ceramics, it is often used as an etchant for revealing the microstructure. It is very dangerous however. High temperature corrosion of oxide ceramics is often encountered however, where the ceramic is in contact with either a molten ceramic or a molten metal
The corrosion rate increases as the temperature increases, thus processes should be operated at the lowest practical temperature - this also saves energy. For etching, raising the temperature reduces the time required - but can make the etchant more dangerous.
Grain boundaries are regions of high local energy so are attacked preferentially, even if there are no second phases. This is why etching usually reveals grain boundaries. When glassy grain boundaries are present, they usually etch very quickly. Finally, corrosion rates are usually faster when when forced convection or stirring is used, i.e. when the liquid is flowing. This is a result of disturbing the reaction products, preventing the build up of a protective boundary layer.
The etchants used for etching ceramics are usually very dangerous- typical etchants used are commonly boiling phosphoric acid, concentrated hydrochloric acid or nitric acid or most dangerous of all, hydrofluoric acid.
Gas solid reactions
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Depending on the ceramic composition, the temperature, the initial surface condition and the environment, oxidation can increase, decrease or not affect strength
Low temperature oxidation or a short sharp high temperature oxidation, can actually result in an increase in strength by blunting surface cracks.
Prolonged high temperature oxidation goes past this stage, resulting in the formation of pits which significantly reduce strength.
Liquid- solid reactions
The kinetics of a liquid solid reaction can be as important as whether or not the reaction occurs at all.
In common with all corrosion reactions, the process is usually only serious if reaction products can escape since this exposes fresh surface for the reactants to attack (this is why rust is so damaging to iron-based products - it readily falls off the substrate)
Fortunately, with ceramics, the reactions products are often viscous and so remain on the surface and act as a boundary layer, forcing diffusion through it to be the rate controlling process.
Chemical resistance
Largely determined by
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The reaction kinetics between the ceramic and its surrounding environment at the exposure temperature.
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Strongly bonded ionic and covalent ceramics have much better resistance and are less easily attacked. Resistant to most acids and bases and can be used in contact with molten metals or glasses.
Oxides tend to be more stable than non-oxides - O2 will react with any exposed surface at elevated temperatures.
For Si bearing ceramics, at low oxygen partial pressures, SiO gas forms. This is active oxidation. Oxide formation is approx. linear and continuous - the component can be completely consumed (the oxide layer is not protective).
Such conditions are not common, but can occur in non-oxygen environments, e.g. closed cycle gas turbines or nuclear reactors, where oxygen is picked up as an impurity.
At greater oxygen partial pressures, sufficient O2 is present to form a protective oxide layer at the surface. This is passive oxidation, and as expected for a diffusion controlled process, the oxidation rate increases at higher temperatures. Formation of SiO2 is initially rapid, but decreases as the thickness of the oxide layer increases so that oxidation is controlled by oxygen diffusion through the silica layer - this yields parabolic kinetics.
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