Raw Meal Preparation and Grinding Questions and Answers

Answer: A

It is the product of the raw mill and the feed to the preheater, ground fine enough for the chemical reactions in the kiln to go to completion. Clinker, cement and fuel are each quite distinct materials at different points in the process.

Answer: B

Correct answer: impact and attrition from tumbling steel balls

The mill rotates fast enough to carry the charge up the shell and let it fall back, so the balls break material by impact and grind it by rubbing against one another as they roll. Both mechanisms operate together, and the ball size distribution is chosen to give the right balance between them along the mill.

Answer: C

Bed compression is a far more efficient way of breaking particles than impact, which is why these mills have largely displaced ball mills for raw grinding. They also dry the feed readily using hot gas drawn from the kiln.

Answer: D

Both are true and the reason gives the decisive advantage: electricity is one of the largest costs in a cement works, so a saving of a third or more on the raw grinding circuit is worth a great deal over the life of the plant.

Answer: A

Raw grinding, coal grinding and cement grinding together account for the great majority of the electrical demand of a plant. This is why mill selection and circuit design receive so much attention when a works is designed or upgraded.

Answer: B

Only material fine enough to pass the separator leaves the circuit, so nothing is ground further than it needs to be. This avoids the wasted energy of over-grinding part of the charge while the rest is still too coarse.

Answer: C

Both are true and the reason gives the mechanism: in an open circuit everything receives the same treatment, so fine particles are ground needlessly while coarse ones may still leave undersize, and energy is wasted at both ends.

Answer: D

It divides the mill discharge into a fine product that leaves the circuit and a coarse reject that returns for further grinding. The cooler, preheater and kiln all belong to the pyroprocessing side of the plant rather than to grinding.

Answer: A

Correct answer: making a sharper cut between fine and coarse material

A poor separator returns fine material to the mill for regrinding and lets coarse material through into the product. A sharp cut does neither, so the mill grinds only what still needs grinding and the product is more uniform. Both effects reduce the energy per tonne, which is where the efficiency gain comes from.

Answer: B

Correct answer: 150 per cent

The circulating load is the returned material as a fraction of the finished product: \[ \frac{500 - 200}{200} = \frac{300}{200} = 150\ \mathrm{per\ cent}. \] Dividing the mill throughput by the product instead gives 250 per cent, which is the standard slip - the ratio is of the returned material, not the total passing through.