Non-clay Raw Materials Questions and Answers

Answer: A

It is the natural mineral from which zirconia is produced and is itself a very effective opacifier in glazes. Its low thermal expansion also makes it useful in refractories subject to thermal shock. Zircon sand is also used directly in foundry moulds.

Answer: B

Slag cannot penetrate a lining that it will not wet, so magnesia-carbon bricks resist attack far better than magnesia alone. The drawback is that graphite oxidises readily and must be protected from air at temperature. Antioxidant additions are made for exactly that purpose.

Answer: C

Both are true and the reason gives the benefit that justifies the risk: resistance to slag penetration extends lining life considerably, and antioxidants are added to slow the oxidation that would otherwise consume the carbon.

Answer: D

Correct answer: 3.4 kg

\[ 4 \times 0.85 = 3.4\ \mathrm{kg}. \] The remaining 0.6 kg is the silica, which is offered as a distractor. Alumina content is the primary classification of an aluminosilicate refractory, and 85 per cent places this brick well above the mullite composition of about 72 per cent, so it will contain free corundum as well as mullite.

Answer: A

Melting and recrystallising gives large dense crystals that are harder and less porous than a sintered grain. Fused alumina is used for abrasives and for the coarse fraction of refractory mixes. Sintered grades are preferred where reactivity during firing matters.

Answer: B

Correct answer: True

Both are essentially the same compound. Fused alumina is melted in an electric arc furnace and solidified, giving large, dense, blocky crystals suited to abrasives and heavy refractories. Sintered alumina is formed from a fine powder and densified below the melting point, giving a fine-grained microstructure with much higher strength. The difference in properties comes entirely from grain size and porosity, not from composition.

Answer: C

Being neutral in the acid-base scheme it can be placed between an acid and a basic lining without a reaction zone forming. Concerns over hexavalent chromium have reduced its use considerably. Neutral alternatives are chosen wherever they will serve.

Answer: D

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason is the cause of the decline. Chrome-bearing refractories can form hexavalent chromium compounds in service under alkaline and oxidising conditions, and hexavalent chromium is a recognised carcinogen. That makes the spent lining a hazardous waste with expensive disposal, so magnesia-spinel and other chrome-free linings have displaced chromite in most applications.

Answer: A

It is among the hardest materials known and has a low density, which matters greatly where the armour must be worn or carried. The cost is high and it is difficult to sinter, which limits it to applications that justify both.

Answer: B

Correct answer: high thermal conductivity with electrical insulation

Those two properties normally go together only in metals, which conduct both, so a material that conducts heat like a metal and blocks current like a ceramic is unusual and valuable. Aluminium nitride is used as a substrate and heat spreader in power electronics, where the heat must be carried away from the device and the current must not be. Its thermal expansion also matches silicon reasonably well.