Raw Meal Preparation and Grinding Questions and Answers

Answer: A

Correct answer: a fall in grinding efficiency and mill output

Coating is a layer of fine material adhering to the balls and the liners, and it cushions each impact so the energy is absorbed rather than delivered to a particle. Output falls and the specific energy rises. Excess moisture and excessive fineness both promote it, and grinding aids are the usual remedy.

Answer: B

Correct answer: 20 kilowatt hours per tonne

\[ \frac{3{,}000\ \mathrm{kW}}{150\ \mathrm{t/h}} = 20\ \mathrm{kWh\ per\ tonne}. \] The units divide out directly because kilowatts are kilowatt hours per hour. Raw grinding at 20 kWh per tonne is typical for a ball mill circuit; a vertical roller mill would do it for rather less.

Answer: C

Correct answer: the difference in the reciprocals of the square roots of the sizes

\[ W = 10 W_i\left(\frac{1}{\sqrt{P_{80}}} - \frac{1}{\sqrt{F_{80}}}\right). \] Bond’s law sits between Kick’s, which uses the ratio of the sizes, and Rittinger’s, which uses the reciprocals directly, and it fits the intermediate size range that industrial grinding actually works in.

Answer: D

The cost rises considerably faster than in proportion, which is what Bond's relationship expresses. This is precisely why closed circuits and sharp separators are worth the capital they cost. Every avoidable micrometre of over-grinding is paid for in electricity.

Answer: A

Squeezing the material between two rolls at high pressure cracks the particles and creates fissures that make subsequent grinding easier. Fitting one ahead of an existing ball mill is a common way of raising capacity without replacing the mill.

Answer: B

Correct answer: True

A roller press pre-crushes the feed, so the ball mill receives material that is already much finer and does less work per tonne. Circuit capacity can rise by half or more for a fraction of the cost of a new mill, which is why the arrangement is a standard upgrade for an existing plant.

Answer: C

Grinding limestone is abrasive work and liners, rollers and balls all wear steadily. Replacement means both the cost of the parts and the loss of production while the mill is stopped, so wear rates are watched carefully.

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. A raw mill stopped unexpectedly may stop the kiln behind it, and a kiln stoppage costs a great deal in lost production and in the fuel needed to bring it back. Liners are therefore measured and replaced at a planned outage, well before they would actually fail.

Answer: A

Correct answer: pneumatic conveying or an air slide

Raw meal is a fine dry powder that fluidises readily, so it is moved on a cushion of low-pressure air down a slight slope in an air slide, or blown through a pipe. Both are enclosed, which matters for a material that would otherwise dust badly, and both have no moving parts in contact with an abrasive powder.

Answer: B

Passing low-pressure air through a porous membrane makes the powder behave much like a fluid, which allows both gentle conveying and thorough mixing. The kiln, crusher and cooler all handle coarse material for which the technique is useless.