9. Screening

Screening is the mechanical separation of particles on the basis of size. This separation is accomplished on a scree-

ning surface with uniformly sized apertures. Particles larger than these apertures are rejected and pass over the surface, while those smaller are accepted and pass through. Many terms are applied to this operation including: sifting, sieving and separation.

Most commonly, screens are used in conjunction with size-reduction operations, but they are found in other applications such as washing, dewatering, desliming, dedusting, etc.

Screens are used for processing a wide variety of materials, .ranging from the fineness of talc up to boulders as large as б by 6 ft. Openings in screening surfaces may be as small as 0,005 in, or as large as 8 or 10 in, with some as large as 18 in.

Screens find application in many Industries, including those producing sand and gravel, crushed stone, minerals, ceramics, coal, chemicals, petroleum, and other products.

A screen consists of a plane surface in which,there are apertures or holes of a given size. These holes may be square, rectangular, round or slotted-intermittent or continuous. The surface is usually supported at the edges by a rigid frame to which a vibratory force is applied.' In some screens the force may be applied directly to the screen surface.

Materials to be sized are placed on this surface while it is in motion. For practical considerations, the surface is often inclined, or the line of action of the driving force is inclined slightly from the vertical so that a horizontal component will be available to move the particles across the surface. This permits each particle to be presented to a number of openings, and also makes room for additional feed material to be placed on the screen.

Particles that are much smaller in all dimensions than the openings usually pass through-rapidly and do not travel far across the surface. Particles larger than the openings in two or more dimensions will not pass through, and travel across the surface and pass over the discharge point. When particles are smaller than the openings in two dimensions, they may pass through if their narrow aspect is presented to the opening. The more times they are presented, the greater the probability

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that they will be accepted and pass through. As the size of the particles more nearly matches the size of the openings, the probability of rejection increases.

Screening efficiency is not measured in terms of work, but by a far different set of criteria. For most average screening operations, screening efficiencies of 85 to 95 % are acceptable.

14. to make room - дать /уступить/ место

15. additional - дополнительный

16. discharge point - место выпуска

17. to match - подходить, соответствовать

18. to reject - отбрасывать rejection - отбрасывание

19. efficiency - производительность

20. average - средний

Commentary

1. ranging from ... to - в пределах от ... до

2. as large as 6 by 6 ft - (величиной) до 6x6 футов

3. so that a horizontal component will be available to move the particles across the surface - так что горизонтальная часть будет способствовать продвижения частиц по поверхности

Words and word combinations

Answer the following questions

1. What is screening?

2. Where -is screening accomplished?

3. In what operations are screens used?

4. What are dimensions of openings in screening surfaces?

5. Where do screens find application?

6. What parts does a screen consist of?

7. What may the holes of a screening surface be?

8. What particles pass through the holes of a screening surface, and what particles are rejected?

1. 2. 3. 4. 5. 6. 7.

10 11 12 13

screen - сито, грохот screening - грохочение

aperture - отверстие syn.: opening, hole

at least - no крайней мере dimension - измерение

sifting - просеивание, грохочение

in conjunction with - вместе с

dewatering - обезвоживание, удаление воды

boulder - глыба, крупный кусок

rectangular - прямоугольный . slotted - шлицованный . frame - рама, основа . to incline - наклонять . driving force - движущая сила

Exercises I. Translate the following sentences, paying attention to the Infinitive:

1. Newton was the first to discover the basic laws of motion.

2. The problem to be solved requires some experiments.

3. Materials to be sized are placed on a screening surface.

4. To slow down the oar means to decelerate its motion on the road.

5. To make the car go faster, we press down on the accelerator

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6..The question to be answered requires some time.

7. Where is the article to be translated into English?

8. The material to be used was examined in our laboratory.

9. Where is the mixture to be heated?

10. She was the last learn the news.

II. Translate the following sentences into Russian;

1. The stronger the acid, the greater is the tendency to lose protons.

2. The faster the object moves, the greater is the air resistance.

3. The greater the number of free electrons is a substance, the better that substance is a conductor of electricity.

4. The earlier we get up, the fresher we feel.

5. The more the students read Englisg, the better they speak.

6. The sooner we finish our work, the earlier we can go home