Raw Meal Preparation and Grinding Questions and Answers

Answer: C

Correct answer: about 40 hours

\[ \frac{5{,}000}{3{,}000} = 1.67\ \mathrm{days} = 40\ \mathrm{hours}. \] That is comfortably longer than a routine raw mill outage, which is the point of the capacity: the kiln keeps running on stored meal while the mill is stopped for maintenance.

Answer: D

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. Blending itself needs only a few hours of meal, but a kiln is far more costly to stop and restart than a mill, so the silo is sized to carry the kiln through a mill outage. The extra capacity buys operational independence rather than better blending.

Answer: A

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: the separator returns the coarse fraction for regrinding, and specific energy is quoted per tonne. Statement 3 is the reverse of the case - an open circuit must grind everything until the coarsest particle is fine enough, which overgrinds most of the material, and closing the circuit is what avoids that.

Answer: B

Correct answer: True

A vertical roller mill grinds between rollers and a rotating table while hot gas passes up through it, carrying the moisture away and sweeping the ground material to the integral separator above. Grinding, drying and classification therefore happen in one machine, which is a large part of why the type has displaced the ball mill for raw grinding.

Answer: C

Analysing the meal as it is produced allows the feeder proportions to be corrected before a large quantity has been made wrongly. Sampling further upstream or downstream would either miss the mill's own effect or come too late to act on.

Answer: D

Correct answer: False

Once the meal has entered the preheater the chemistry is fixed: nothing can be added or removed, and what is burnt is what was fed. Every correction must be made upstream at the raw mill, which is why the analyser and the proportioning control there are the critical part of the plant.

Answer: A

Correct answer: will not react completely during burning in the kiln

The reactions in the burning zone are between solid particles and depend on diffusion over the distance a particle spans, so a coarse grain of silica cannot combine fully in the time available. The result is uncombined lime in the clinker, and the residue specification exists to keep the coarse fraction small enough to react.

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. Lime combines with silica by solid-state diffusion, and a large quartz grain cannot be reached to its centre in the time and temperature the kiln provides. The lime it should have taken up remains as free lime, which is why free lime in the clinker is read as evidence about the meal as well as about the burning.

Answer: C

Once the particles are small enough to react completely, further reduction achieves nothing chemically while consuming a great deal of electricity. Fineness is therefore optimised against a target rather than pushed as far as the mill allows.

Answer: D

Correct answer: 4 million kilowatt hours

\[ (22 - 18) \times 1{,}000{,}000 = 4{,}000{,}000\ \mathrm{kWh\ a\ year}. \] Four kilowatt hours a tonne looks small until it is multiplied by the annual tonnage, and it is that multiplication which justifies the capital spent on separators, roller presses and grinding aids.