The purpose of milling a ceramic powder is chiefly to
Answer: A
A fine and well-dispersed powder sinters at a lower temperature and gives a more uniform microstructure. Milling changes the physical state of the powder rather than its chemistry, and in practice it usually reduces purity slightly through wear of the media.
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Assertion (A): Milling a ceramic powder often lowers its chemical purity slightly.
Reason (R): The grinding media and mill lining wear and contribute their own material to the batch.
Answer: B
Both are true and the reason gives the mechanism: every collision removes a little of the media, and over a long grind that contamination becomes measurable, which is why the media are chosen to match the powder wherever possible.
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Grinding media for a high-purity alumina powder would normally be made of
Answer: C
Correct answer: alumina of the same purity as the powder
Milling media wear, and whatever they are made of ends up in the powder. For a high-purity product the only acceptable contaminant is more of the same material, so the media are made of it - which is why alumina and zirconia media exist despite costing far more than steel.
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State whether the following is true or false.
Contamination from grinding media is always harmful to a ceramic.
Answer: D
Where the media are of the same composition as the powder the wear is simply added to the batch. It is only harmful when it introduces something foreign, which is why media selection is treated as part of the process design.
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Wet milling is generally preferred to dry milling for fine ceramic powders because it
Answer: A
A liquid separates the particles and carries away the heat, so grinding continues to a finer size than dry milling reaches. The penalty is that the slurry must afterwards be dried, which costs energy and risks re-agglomeration.
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Assertion (A): Wet milling reaches a finer particle size than dry milling but is not always chosen.
Reason (R): The resulting slurry has to be dried again, which costs energy and can undo the dispersion achieved.
Answer: B
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the mechanism. Wet milling disperses the particles and prevents them re-agglomerating, so it reaches a finer size; but the water must then be removed, and ordinary evaporation draws the particles back together as it recedes. The cost and that risk are why dry milling is still used where the size demanded permits it.
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A deflocculant added during wet milling acts to
Answer: C
By giving the particles a like surface charge or a steric barrier it stops them flocculating, so a higher solids content can be handled at a workable viscosity. Sodium silicate and various polyacrylates are common choices.
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State whether the following is true or false.
Adding a deflocculant to a ceramic slip increases its viscosity.
Answer: D
Correct answer: False
A deflocculant does the opposite: it puts a like charge on the particles so they repel rather than flocculate, and the slip becomes markedly more fluid at the same solids content. That is its purpose - a higher solids loading at a workable viscosity means less water to remove later.
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A slip is 70 per cent solids by mass. How much dry powder is present in 40 kg of slip?
Answer: A
Correct answer: 28 kg
\[ 0.70 \times 40 = 28\ \mathrm{kg}. \] The remaining 12 kg is the liquid that must be removed by drying, which is why a high solids content is worth achieving - every kilogram of water is energy spent evaporating it and a risk of agglomeration as it goes.
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An attrition mill differs from a conventional ball mill in that it
Answer: B
Correct answer: stirs the media with an internal agitator
An attritor holds the media in a stationary tank and stirs them with arms on a shaft, so the energy is delivered directly rather than by tumbling the whole shell. That gives far higher energy density, so the same fineness is reached in a fraction of the time - hours rather than days.
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