'LET'S LOOK AT SAND'

Text by Eileen Barrett and Linda Beskeen.

Graphics by Ron Turner.

Produced by the Mineral I ndustry Manpower and Careers Unit.

We gratefully acknowledge the support towards our work given by British Petroleum plc through the Secondary Science Curriculum Review.

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WHY LOOK AT SAND? Most of us see sand in playgrounds for young children, at the beach, and often beside the road or on a building site. How many people think of sand when they look in the mirror or through a window or knock nails into the walls of their house? It might sound strange at first, but without sand, windows, mirrors, houses, roads and many more of the things which we see and use every day would not exist.

Sand is one of the most important minerals which we mine in our world today. I n Britain it is extracted on a large scale and is used all over the country in buildings, roads and industry. Many people have jobs which are directly related to sand, e.g. in mining, transporting, buildings and roads, glass­making and many others. In the cover picture every object contains sand in one form or another.

In the following pages we will be looking at what sand is made of, how we can find it and how we can use it.

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WHAT IS SAND? Is that title a silly question? Don't we all know what sand is? Most people have seen sand, played with sand and, perhaps, used sand. To describe sand accurately is more difficult.

Geologists try to be precise in their description, which is that sand consists of particles of minerals or rocks of a certain size range between 0.06 and 2.00mm. In practice they do not measure individual grains but determine the 'size' of particles by using a technique of sieving.

Look at your ruler to see what 2mm looks like. The smaller sizes are a Imost too small for the naked eye to see.

It is difficult to avoid sand. There is usually some in your garden soil although the quantity will depend on what rocks the soil was made from. If you live in a house built on clay you will have less sand than if your house is built on other rocks.

By looking at soil in your garden you can begin to find out about the geology below the soil. Sand is one of the particles you will find in soil.

The table below shows the full range of particles (or 'grains') and their sizes.

PARTICLES/GRAINS SIZE (mm)

Boulders >200 Pebbles 200-60 Gravel 60-2 Sand 2-0.06 Silt 0.06-0.002 Clay <0.002

WHAT SIZE ARE THE SAND GRAINS? Some sands may contain mostly small particles and are called-fine-grained sands. Sands which contain mostly large particles are called coarse-grained sands. When sand grains are roughly the same size we say that they are well sorted.

Sorting of sand grains is most common when the grains have been carried by water which drops the larger, heavier grains first and the lighter, smaller grains on top. The amount of sorting can be worked out by sieving the sand to see how much of a sample is left on each sieve. A well sorted sand wi II have most particles in one or two size ranges.

Here are four examples of graphs made up from the results of sieving the sands and clay and weighing the different size ranges.

1 DESERT SAND

2 BEACH SAND

%

3 LAKE CLAY

%

4 GLACIAL WASHOUT

80

60

40

20

N M ...

60 r­

r-

40

-20 I-

r-

N M

40 r-

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I-

N M

40

I

cc ...

I

cc ...

I I

co .. N

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GRAIN SIZE IN mm

... I.C) Lt) I.C) M N

I ...

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GRAIN SIZE IN mm

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o

GRAIN SIZE IN mm

DESERT SAND

BEACH SAND

I I I

CC co oqo N ... ... 0 ~ 0 0 c:i c:i c c::i c:i

LAKE CLAY

CC co ... 0 c:i c:i

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GLACIOFLUVIAL SAND

GRAIN SIZE IN mm

WHAT IS SAND MADE OF? As we have already seen, sand is pieces of rock of a certain size. What minerals or materials make up sand?

If we look closely at the sand on the beach, the individual grains often look white. Some darker specks may be seen, but most of our beach sands are a pale cream or brown colour. This is because the most common mineral in sand is quartz which is a pale coloured mineral.

The particles which make up sand are different for each sand, and they depend on where and how the sand was formed. Some of the most common minerals are listed below:

1 Quartz (silica): This is the main component of most sand. The grains are hard and glassy and vary in colour from white to clear to rusty- red.

2 Mica: This is a shiny mineral which may be black or white. It breaks easily and forms very small particles in most sand deposits. It is a flat, platey mineral.

3 Feldspar: This is a milky white to pink mineral. It is not as hard as quartz, so will form smaller grains or be worn away more quickly.

4 Dark black and green minerals: Many of the minerals on the earth's crust are dark in colour. The harder ones will remain as grains in sand, softer ones will be more worn down.

5 Calcite: Calcium carbonate is most commonly found in bones and shells of living organisms. When these are broken down they may form small fragments which can be part of the sand. Calcium carbonate can be dissolved in rain water which contains carbonic acid and can also form clear or milky white crystals. It is soft and easily worn away.

As well as minerals sand can contain pieces of broken down rock; some common rocks found in sand are:

Slate: Usually dark brown, grey or black. This is usually softer than quartz and may be more rounded.

ii Limestone: A white or grey rock which contains a lot of calcium carbonate. This may form smaller pieces of a sand.

iii Basalt: This is a dark, black rock which is fine grained and hard. This may be broken down to form sand grains.

There are many other minerals and rocks which can be broken down to form a part of a sand. Some of these minerals are valuable and can be separated out for use. Other minerals can be a nuisance because they make it difficult to get a pure silica sand which is needed for many industries.

ROUNDNESS OF SAND GRAINS

When fragments of rock first break apart they have sharp edges. As these are moved down slopes or rolled and washed by water the sharp edges are worn away and the particle becomes rounded.

PATH OF A SAND GRAIN Minerals such as quartz, mica, felspar bound up as part of a rock.

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C] 8)':"; '/' Rock is broken down and " '. . '\ sharp-edged fragments drop (;g':! "':. &8" :;.::- off. These are ANGULAR .\ \ ' . . :.:,',?-" fragments. ~ ?: ,~ \;50'" ~.:: ...... "::."". ':'11

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Running water or winds con­tinue to batter the rock particles. Most sharp edges are smoothed down. The particles are SUB-ROUNDED.

When all the edges are worn away and the surface is very smooth the grains become ROUNDED.

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WHY IS SHAPE IMPORTANT? Some industries need sand grains of a particular shape (see the sections on uses of sand).

CLASSIFICATION OF GRAIN SHAPES

ANGULAR SUB-ANGULAR

SUB-ROUNDED WELL ROUNDED

SUMMARY Sands and sand grains can be very different. The main ways in which they can vary are:

1 2 3 4

Size Sorting Shape Composition (type of minerals)

MAKING SAND As we have already seen, sand is a mixture of grains or particles of a certain size. These grains were originally parts of other rocks which have been broken and worn down. The original rocks may have been formed in a number of ways, but once they are broken down and dropped somewhere else they are known as SEDIMENTARY ROCKS. The three types of rock are:

I GN EOUS = rocks made with fire or heat. These rocks began life in volcanoes or hot molten rock which was pushed up from the earth's interior.

2 METAMORPHIC = rocks which have been changed by heat or pressure.

3 SEDIMENTARY = rocks which are laid down as small fragments which can be cemented or compacted together.

FORMING SEDIMENTARY ROCKS Sedimentary rocks are made up of many fragments of older, broken rocks. These rocks can be broken up in a number of ways, carried away and dumped somewhere else on the earth's surface.

The process of breaking up the rocks is known as WEATHERING. Carrying the broken rocks away is EROSION. Dumping the sediments is called DEPOSITION.

WEATHERING is the process of breaking down rocks which are exposed to the atmosphere.

WEATHERING IS DONE BY:

CHEMICAL WEATHERING

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Chemicals (including water) attacking the rock

BIOLOGICAL WEATHERING

Animals and plants forcing their way between rock crystals or grains, widening cracks and helping to break down the rock.

HEATING

Heating to high temperature in tropical areas can cause the outer skin of rock to expand and later contract. This will eventually lead to the outer skin peeling off.

Rain, snow, hail and mist allow water to fall to the surface of the earth.

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PHYSICAL WEATHERING

Water in cold climates may freeze in cracks in the rock and expand, putting pressure on the rock. This may force cracks to widen and fragments to break off.

Heat and pressure help to break down rocks.

SAND FORMED BY RUNNING WATER

... , ......

As it flows, the water can pick up broken pieces of rock and transport them "in suspension."

Pieces of transported rock knock against each other and are rounded.

On flatter land the river may slow down and suspended particles begin to settle out. On reaching the sea the river drops finer particles. Some may be deposited in a delta, some washed out to sea.

BREAKING UP IS HARD TO DO The process of breaking down hard, solid rocks to form loose fragments of sand requires energy and time. Most of the energy comes from downhill movement of water under gravity. Rocks can be broken by running water in several ways, shown in the diagrams BREAKING AWAY.

BREAKING AWAY

1 The power of running water is enough to push or roll loose particles of rock from the surface of the bedrock. A process called HYDRAULIC ACTION.

3 Pebbles carried by the stream bang into each other and break. Sharp edges are broken off first, making the pebble move round. A process called ATTRITION.

2 Eddies or small whirlpools spin pebbles which break other particles away from the bedrock. A process called ABRASION.

4 The river and its load scour away the base of a river bank. Eventually the top collapses. A process called UNDERCUTTING.

MOVING ALONG

Some rock chemicals can be dissolved and carried by the river. This is the SOLUTION load.

Small, light particles can be held up in the water or suspended. This is the SUSPENDED load.

Larger particles may bounce along the bottom, a movement known as SALTATION.

The largest pebbles are rolled along the river bed. This is the BED LOAD.

LOSING WEIGHT Rivers drop solid particles when they do not have enough energy to carry them. This usually happens when the river slows down. The largest, heaviest particles are dropped first and are not usually carried very far. Sand-sized particles will be dropped later, either on top or further downstream. This order of deposition forms graded bedding, i.e. larger particles lower in the bed or layer of rock, finer particles on top.

WHEN DOES THE RIVER LOSE ENERGY? Rivers flow faster on steeper slopes. When it reaches gentler slopes the river loses energy and may drop some material.

GENTLE SLOPE­DEPOSITION PREDOMINANT

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3 When it meets the sea the river will slow down and deposits build up to form a delta. This will only happen if the sea current is not strong enough to move away all of the river sands. Delta sands have characteristic layers which cross-cut each other.

GRADED BEDDING:

STEEP SLOPE EROSION PREDOMINANT

2 Rivers turning around a bend move more slowly on the inside of the bend. Material is dropped on the inside bend and eroded from the outside bend. The deposit is called a slip-off slope or point bar .

DEPOSITION FORMING A 'POINT BAR'

CROSS-BEDDING IN A DELTA DEPOSIT

SAND FROM THE SEA The coastline is the meeting point for sea and land. The power of the moving sea, especially during storms, is enough to wear away the hardest rocks on the surface of the earth.

Waves at the coastline can be an efficient battering ram, helping to reduce the mighty cliffs to grains of sand.

HYDRAULIC ACTION

A wave approaches the uneven cliff face.

The wave hits the cliff and traps air in the cracks, compressing it slightly.

As the wave retreats, air is released quickly with a burst of energy. Repeating this many times will loosen pieces of rock.

2 ABRASION

SMALL ROCKS

Some waves are strong enough to pick up pebbles and hurl them at the cliff face. This acts as a hammer, breaking off more of the cliff face.

3 ATTRITION Pebbles which are knocked against each other by the sea crack and break. Eventually they become worn down to sand grain size.

4 CHEMICAL ACTION Sea water contains many salts and other chemicals. Water alone is capable of dissolving limestone rocks when it is in contact with carbon dioxide in the air. The chemicals in solution in the sea can react with some rocks and minerals to dissolve them.

WHERE DO WE FIND MARINE SANDS?

In deep water (below low watermark)

I n seas where the offshore current moves the firmer material away from the coast, sands may line the bottom of the sea.

At the edge of the continental shelf where material slumps down the slope, sands are deposited, called GREYWACKE.

ATTHE COAST

SAND IN COVES

SAND BANKS OR SHOALS OF SAND IN SHALLOW SEAS.

SAND IN SPITS OR BARS.

COAST BOULDERS, PEBBLES

LAND

DESERT SANDS

Are deposited by:

WIND

SHALLOW SEA SAND

LIMESTONE

Fine-grained sediments which are carried by gusts of air, mostly over short distances. -

Grains are usually well rounded and referred to as 'millet seed' sands.

A CROSS-SECTION SHOWING MARINE DEPOSITION

CONTINENTAL SHELF SANDS

WATER

DEEP SEA CLAY, SHALES

Fine and coarse grained sediments, not very well rounded because floods do not last very long.-

Large, angular fragments mixed with sands.

ALLUVIAL FANS At the foot of the hillslopes and the mouths of the canyon lies the broken material carried by the flash floods.

2 SAND DUNES Strong winds can pick up the fine material, carry it along, and drop it when the wind dies down. An obstacle in the path of the wind may cause it to lose energy. Sand builds up on the windward side and spreads forward at the edges. A crescent­shaped deposit or dune is formed.

Where there is one strong wind and a weaker cross­wind, the sand dunes are arranged in ridges known as seifs.

STREAMS FLOWING AWAY FROM ICE (CALLED FLUVIO-GLACIAL STREAMS)

Glaciers move down valleys when the ice builds up enough. On their route they pick up fragments of all sizes and can grind some rocks down.

As the glacier melts, the streams which flow from the melting ice carry the sand and later deposit it.

These washed, glacial sands are usually made of hard, resistant grains and have a high percentage of silica.

These sands are called fluvio-glacial (which means that they came from glacial rivers or streams).

When ice melts, mostly in Spring and Summer, the glacial sands are then carried by fast running water. The smaller clay sized particles are easily washed out and this sand has grains of the most regu lar size.

THE ROCK CYCLE We have seen the main ways in which sand is formed. Loose sand may eventually become covered by other layers of particles and may be compressed. Loose or compact it is still referred to as a rock, a sedimentary rock.

Sedimentary rocks can only be formed by the break up of other rocks, sedimentary igneous or metamorphic. In time the sedimentary rocks themselves may be changed to form metamorphic rocks.

A flow chart to draw the main rock types and processes:-

ROCK FROM THE INTERIOR OF THE EARTH =

1r

ROCK WHICH HAS BEEN BAKED OR SQUEEZED TO FORM

METAMORPHIC ROCK

IGNEOUS ROCK

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WATER WIND ICE

TO GIVE

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SEDIMENTARY ROCK

OLD SEDIMENTARY ROCKS MAY BE CHANGED TO FORM

LOOKING AT QUARTZ

Sand is used in three main industries. The pure sands are used to make many types of glass, whilst the more mixed types of sands are used for building and construction work. The foundry industry is the other major user, also requiring a high proportion of silica in their sand. Sand is mostly made up of the mineral quartz.

We could describe quartz as natural silica. Quartz is a mineral made of the two most common elements on earth. They are silicon and oxygen, forming Si02 .

Minerals can be compared using a number of tests. Some of the tests which geologists use are shown below:

THE PROPERTIES OF QUARTZ Colour - different minerals have different colours. Natural quartz can be clear, it can be white (as seen in quartz veins) or it can contain other minerals which give it bright colours. These are some examples:

a brown quartz is called smokey quartz. b purple quartz is called amethyst - used for

necklaces etc. c pink quartz is called rose quartz - used for

jewelry. d layers of quartz from agate - e.g. tiger eye.

2 Light - in Roman times windows did not have glass in them. The Romans, however, had discovered that alabaster could be cut into thin sheets, and that light could be let

through it. Alabaster was used to fill the windows of those who could afford it.

Although alabaster would let light pass through it, it was not transparent. In the same way that it is difficult to see what is happening on the other side of frosted glass used in bathrooms.

When light cannot pass through a material, e.g. wood, we say it is OPAQUE

When light will pass through but we cannot see exactly what is happening on the other side we say it is TRANSLUCENT

When light can pass through and enable us to see what is happening on the other side we say it is TRANSPARENT

Quartz can be either translucent or transparent.

3 Lustre - the surface of minerals can vary a great deal. Materials which do not let light shine through them may have a dull surface known as MATT. They may appear glassy (VITREOUS) or they may shine like metals (METALLIC). Descriptions like this can help describe quartz or sand grains. Quartz is usually vitreous.

4 Hardness - Minerals can be measured to see how hard they are. A scale developed by a geologist called Mohs gives minerals a number one to ten according to how hard they are.

MOHS' SCALE OF HARDNESS

NO. TEST

1

2 2% can be scratched by a fingernail

3

4 can be scratched by a copper coin

5 5% can be scratched with a steel knife blade

6

7 Minerals harder than 6 will scratch glass

8

9

10

MINERAL EXAMPLE

Talc

Gypsum

Calcite

Fluorite

Apatite

Feldspar

Quartz

Obsidian

Corundum

Diamond

5 Density - This is an approximate measure of weight, compared to the amount of substance present. The density of quartz is 2.65 grammes per cm3 . Density is expressed as mass/volume.

6 Fracture - Some minerals will split easily along certain lines or planes. Quartz does not usually split easily to form flat sides. Instead it breaks leaving curved surfaces and a jagged edge.

Stone Age man recognised this when he used flints to make tools. F lints are nodules or lumps of quartz which have formed under the sea in chalk rocks. Some of the uses of flint are shown below:

CONCHOIDAL FRACTURE

7

8

9

Acid test - Some minerals will react readily with acid. The most common mineral to react with dilute hydrochloric acid is calcite (calcium carbonate). Calcium carbonate is found in limestone rocks, formed from the bones and shells of millions of tiny decomposed organisms. Calcium carbonate can also grow to form crystals which can look very similar to quartz. Quartz, however, is very resistant to acid and wi II not redct. Calcite crystals will fizz when acid is added to them, giving off carbon dioxide.

This is the chemical reaction:

2 HCI + CaC0 3

Streak - The back of a porcelain tile like those used in bathrooms or kitchens provides a useful geological tool. When minerals are scratched along the unglazed back they may leave a mark. The colour can help to distinguish some minerals which may look very similar.

Crystal shape - Look at the diamond shape on the edge of this playing card. Most people know what a diamond shape is. This is a commonly seen crystal shape.

DOG·TOOTH SPAR

QUARTZ

CALCITE

RHOMBOHEDRAL CLEAVAGE

HALITE

Another common shape is a cube.

Different minerals produce different crystal shapes. The shape of the crystal helps us decide which mineral it is.

The shape of any crystal depends on the arrangement of the atoms or ions within it.

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THE ARRANGEMENT

OF SILICON •

AND OXYGEN 0 ATOMS IN QUARTZ

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WHAT DO THE PROPERTIES TELL US? The properties of quartz depend on the elements of which it is made, and the way in which these elements are bound together.

Quartz is made of two elements, silicon and oxygen. There is one atom of silicon for every two atoms of oxygen.

The atoms in quartz are held together very strongly, which gives the mineral its particular properties. The pattern in which the atoms are held together affects the shape of the crystal and therefore it will affect the way in which light passes through and the planes along which the crystal breaks.

The strength of the bonds which hold the quartz crystal together make this a hard mineral, difficult to scratch. It will also not easily dissolve in water, and does not react with di lute acid.

Two other minerals, halite and calcite, are the same colour as quartz and can look like quartz. Both of these minerals, however, have a system of bonding wh ich is more easi Iy broken and therefore they are easily scratched and will react more easily with water and/or acid.

QUARTZ IN THE EARTH The elements which make up quartz are among the most abundant in the earth's crust. The most abundant elements are shown in the table below.

% in Earth's

Element Crust

Oxygen 46.6 Silicon 27.7 Aluminium 8.1 Iron 5.0 Calcium 3.6 Sodium 2.8 Potassium 2.6 Magnesium 2.0 Others 1.4

These elements are not always easy to see or identify since they are often a part of a complicated mineral within a rock.

The most abundant minerals in the earth's crust are those which contain oxygen and silicon in some form (the silicate minerals!' If you look at most common rocks they will contain mainly the two important elements oxygen and silicon.

Some common silicate minerals:

QUARTZ FELDSPAR MICA

J (FOUND IN GRANITE)

HORNBLENDE] OLlVINE AUGITE

(FOUND IN BASALT)

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MAJOR SANDS AND SANDSTONES IN BRITAIN There are many deposits of sands in Britain. Some of the deposits are fairly recent in geological time (i.e. within the last two million years), others are much older and have been compressed to form a hard sandstone.

SANDS AND SANDSTONES IN BRITAIN

GEOLOGICAL PERIOD AGE

Recent (Less than 10,000 years old)

Pleistocene 1.6 million -10,000 years old

Tertiary 65 million-1.6 million years old

Cretaceous 140-65 million years old

Jurassic 195-140 million years old

Triassic 230-195 million years old

Permian (New 280-230 million red sandstone) years old

Carboniferous 345-280 million years old

Devonian (Old 395-345 million red sandstone) years old

The geological map shows which rock types are found in an area. Since there are many layers of rock on top of one another, the surface layer is the one shown.

Geologists sometimes have a problem trying to decide when a loose sediment can be called a rock. Some surface sands are not shown on geological maps which show the solid geology. They may however be shown on some maps where they are called drift. The sand industry would need to use both of these types of geological map.

SAND TYPES FOR INDUSTRY Sands from rivers, coasts and deserts often have a great mixture of substances within them.

DEPOSITS

Coastal sands and river sands still being formed.

Glacial sands and sands washed out from glaciers.

Delta and river sands and sandstones.

Rivers, estuaries, deltas shallow seas.

Some shallow seas, some estuarine and deltaic deposits.

Red desert sand and shallow lake deposits.

Red desert sands.

Delta sands, hardened into sandstones and gritstones.

Coastal and shallow seas and some tropical river sands.

They may be suitable for use in the building industry, but are often not pure enough for the glass industry which needs as high a proportion of silica as possible. This is found in the quartz grains which are left when most other minerals have been washed away. The sand most suitable for this is glacial sand and this is most commonly used (eg near Congleton, Cheshire).

Some river sands may be pure enough, especially if they have been well sorted by the river.

Sand with a high proportion of quartz is called silica sand.

A GEOLOGICAL MAP SHOWING AREAS OF SI L1CA SAND M I N ED TODAY.

A MAP TO SHOW AREAS OF SAND & GRAVEL EXTRACTED IN ENGLAND AND WALES TODAY.

WHAT IS SANDSTONE? Sandstone differs from sand in that the sand grains are held together.

HOW CAN THE GRAINS BE HELD TOGETHER? Loose grains may become compacted with time until they are so well fitted that they become a solid rock. In other cases water may seep between the grains and slowly dry out. As it dries the water may deposit any dissolved materials. This will fill the gaps between the sand grains, holding them together. We call this a cement.

".: :: .......... .

.....

1 Layers of loose sand bui Id up. , . .. .... . .... : .. " .

2 As more layers build up over the sand it is compressed and the grains move closer

tOgether., "

3 Grains may be compressed enough to hold together as a tightly packed rock. We call this compaction.

How can we tell the difference?

a In sandstones formed only by compression, grains are all close together and touching.

b In sandstones held together by cement the sand grains do not always interlock or touch each other. If the cement is weak these can be broken apart easily.

COMMON CEMENTS:

a Iron oxide is commonly found in cements which gives them a red colour.

b Calcium carbonate is often found as a cement which suggests that the rock has been under the sea. Calcium carbonate fizzes with dilute hydrochloric acid.

c Silica is sometimes found as a cement. This is usually very hard and pale in colour. It does not react with hydrochloric acid.

4 Water moves down through rock.

5

,

As the water evaporates the dissolved solids are left behind (precipitated out) and form a cement between the sand grains.

SANDSTONE ~ QUARTZITE

Under intense heat, usually as a result or earth movement sandstones may be changed or METAMORPHOSED. The quartz grains knit tightly together to form a hard rock where individual grains cannot be seen. The other minerals usually form separate crystals within the rock.

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WHERE TO DIG - EXPLORATION

A geologist will need to explore an area before a decision is made to dig out the sand, or sandstone. As much as possible should be known about the type of sand, where it is and how much there is.

There are several types of investigation which can be made;

The first stage is to find out which rocks are at the surface, the area they cover and the angle at which they dip into the ground. This will help to know what is happening below ground.

Another way to find out about rocks underground is to drill a bore hole. This means that a sample of rock from a known depth can be removed and examined.

With this knowledge a map or model of an area can be built up.

WHAT HAPPENS UNDERGROUND? The bore holes will give a sample of different rocks and the depths and extent of each rock type.

When the geologists have gained enough inform­ation, the managers of the company have to decide whether it is worth mining the sand. If they go ahead the sand will be extracted by one of the following methods.

excavation of loose material dredging from rivers, lagoons, estuaries or off coastal shores open cast quarrying underground mining

There are two main types of mine. One type is an opencast mine, where material is dug out at the surface creating a large 'hole' in the ground. This may also be called quarrying or open-pit mining.

The other main type of mining is to work under­ground in tunnels, digging rock out of the inside of the earth's crust. The hole created may be as big or bigger than many quarries, but it is not seen at the surface.

Most sand is mined from the surface in opencast pits. This is for a number of reasons. Firstly some sands have not been cemented together to form sandstone. It is therefore almost impossible and very dangerous to try to dig a tunnel in loose sand. This sand is always dug out from the surface. Another reason is that underground mining is much more expensive than opencast mining. Underground mining, therefore, is only done where deposits of a good enough quality cannot be found anurface. There is only one underground sand mine in Britain today.

This is the mine at Loch Aline in Scotland. The sandstone there is a long way below the surface. It would be uneconomic to remove the large quantities of hard rock above this sandstone. Since the sandstone only 'outcrops' on the steep hillside above the loch, it is removed by underground mining.

The sand is particularly white and pure. It also has an ideal grain size distribution for use in making glass.

For these reasons, it is worth while mining the sandstone. The sand, when processed, is only used for specialist glass manufacture as it is of such high quality.

QUARRYING OF SANDSTONE

REMOVAL OF TOPSOIL

DRILLING HOLES FOR EXPLOSIVES

LOADING BROKEN ROCK INTO TRUCKS

TRANSPORT OF BROKEN ROCK TO THE PRIMARY CRUSHER

FIRST STAGE OF CRUSHING READY FOR PROCESSING

UNDERGROUND MINING OF SAND

DRILLING HOLES FOR EXPLOSIVES

AFTER THE BLAST

LOADING THE BROKEN ROCK

TAKING ROCK TO THE SURFACE

CRUSHING READY FOR PROCESSING

Miners and quarry workers use special technical terms to describe their operation. They need to use several factors to calculate how much rock will be removed in one explosive blast. The amount of explosive required also has to be calculated.

The diagram below shows a quarry BENCH prepared for breakage. The front and sides of the bench which are vertical are known as quarry FACES. They are F R E E faces as they are free to move when blasting occurs.

The three dimensions needed to calculate the mass (or tonnage) of rock to be removed are:

BENCH HEIGHT BENCH LENGTH BURDEN (the distance between one row of drill holes and the next row or the main quarry face.

DRILLHOLE--O~/o:, ______ -----=:0 ~ 0 0 0

C)~v.O~o~o~ ---~-~-~ . __ C)~o

The number of holes and the distance between the holes are also important for calculating the quantity of explosives.

Miners use some of the same technical terms in underground operations.

The term FACE is used for the vertical wall which will be drilled. Usually there are no other FREE FACES in an underground mine, unlike those in a quarry. This means that a special pattern of drilling has to be carried out so that one section of the rock (near the centre) is blasted first. The detonators on the explosives are timed so that this central section breaks first. In doing so, it creates a hole with a 'free face' so that the surrounding rock can move into a space as it breaks.

DRILL HOLES

----

The 'tunnel' with the face to be drilled at the end is known as the HEADING.

I n many mines some of the rock is left unbroken to support the roof and these supports are known as PILLARS.

DREDGING SAND AND GRAVEL Sand may be removed from river estuary beds, or from shallow coastal areas. This is called DREDGING.

2

A floating dredger can pump loose material from the floor of the river or sea, into the hold of a sand barge.

Other types of dredger may scoop material off the floor, using a scraper or bucket system. I n this diagram a ladder dredger has a line of buckets continually moving. They are hardened and can dig harder material than a pump.

EXCAVATING I n some areas sand in quarries is loose enough to be scooped up without being loosened in any way.

1 A bulldozer may be used to scoop up sand.

2 A line excavator may be used, where a bucket is dragged through the sand to pick up the material.

HEADMAST &.

HOIST

READY TO SELL? Most sand or sandstone is not ready to sell in the condition it is in when dug from the ground. For different uses it has to be PROCESSED. Some details of processing are given in the chapters which deal with the different uses of sand.

THE USES OF SAND By far the greatest amount of sand used in the world today is in the construction industry.

Our houses, roads, hospitals, schools, bridges, tunnels, factories, power stations and all buildings containing mortar, cement mixtures and concrete use sand. Sand is mined or dredged along with gravel from open pits all over the country. The quality of this type of sand is not as important as that of the silica sand used in more specialised products. Many types of sand can be used and since such large amounts are used the buyer needs to keep his transport costs low. Carrying these quantities to the user is expensive and therefore distances are kept as short as possible. Looking at the map of Britain you will see that sand is mined for construction all over the country. Wherever there is a suitable deposit and permission is given, sand quarries will open up to provide the local needs.

The types of sand used in construction can be river sands, glacial sands, desert sands and even coastal sands. About 8% of the cost of materials for a new house is spent on sand.

The other major uses are in glass manufacture and in foundries for making moulds for metal casting.

For both these uses processing is more com­plicated. The sand must contain a very high percentage of quartz and impurities should be few.

GLASS

GLASS G lass is used in many way in our houses, as windows, drinking glasses, and heat resistant dishes for cooking.

It is used for spectacle lenses and other optical instruments. A most important modern use is in optical fibres used in telecommunications.

WHAT IS GLASS? The substance we know as 'glass' is the most common example of a type of structure known as glass.

It is often described as a super-cooled liquid. This means that the liquid cooled very quickly. As it did so the atoms or molecules within it could not order themselves properly so they remained disordered as they would be in the liquid. The difference is that, in the liquid the molecules would move constantly but in a glass they are 'frozen.'

The diagrams below show (in two dimensions only) the difference in structures between quartz and soda glass.

SILICA PARTICLES IN A QUARTZ CRYSTAL

• =Si

0=0

SILICA AND METAL PARTICLES IN ORDINARY GLASS

~ IS METAL ION USUALLY Na+ $

Many substances exist in glass-like forms. Some plastics like perspex are glasses at room temperature. Rubber, if cooled in liquid nitrogen becomes brittle like glass and shatters when hit with a hammer.

NON-CRYSTALLINE REGION

CRYSTALLINE REGION OF A PLASTIC POLYMER

PROPERTIES OF GLASS When glass is heated it does not change suddenly from solid to liquid at a fixed temperature. I nstead it gradually softens over a wide range of temperature. This is because some of the bonds break more easi Iy than others.

This property is very useful as, when it is soft it can be worked by bending, moulding and blowing.

HEATING SILICA CRYSTALS

C 1 LIQUID ~ SOLID CRYSTALLINE SILICA 1--- SILICA

g ~--------------------------I -o 1750

TEMPERATURE °c

HEATING SODA- LIME -SILICA GLASS

Cl :::> o -'

t w Z o N I­u. o Cl)

t Cl

-' o

I I I I I I I I

I I I I ~SOFT .,.j~ LIQUID I GLASS I GLASS

I I I I

Cl) ~ ______ ~~ ______ ~ ______ ~~

o 650 1200

2

TEMPERATURE °C

Glass is brittle. This is mainly an unhelpful property causing glass to break when dropped or hit. The reason is that glass is usually imperfect and has tiny cracks an he surface. When stress is appl ied some of the bonds break at the tip of a crack. There is even more stress at the new tip and more bonds break.

t t

THE STRESS CONCENTRATION AT THE TIP OF A SURFACE CRACK.

Even this brittleness could be turned to good use. A small file mark on a glass tube will cause weakness. If the two ends of the tube are grasped and pulled downwards, a clean break should result.

THE STORY OF GLASSMAKING The earliest known glass has been found in the ancient world of the Phoenicians, who lived in ancient Mesopotamia. Glass dating from about 5000-3000 BC has been found there, and glass from about 1500 BC in Egypt.

One of the great writers of the Roman times was a man called Pliny. Pliny was a well-travelled man with an interest in places, people and animals. One of his stories concerns the discovery of glass, which he thought originated with the Phoenicians.

Some sailors who had camped on a beach for a night's rest, left their cooking pots resting on their cargo of soda, around the fire. By the morning the sand and soda had been heated by the fire and fused together to form glass.

;\'., ;' I "'--"

//! ·'1·· .. · / I /.

/}74~6~J r 0.-/

i

?- ,-- ~I . ;}I~'e{f-!j! 'Kj't---, - '".'

&1:~f))y:,;r;.," ......... c.//.· .•.....•....... : •.. This seems a good story but glass had been in existence for about three thousand years by then, and its origins were further away than Pliny had thought.

With the spread of the Roman Empire, glass making skills spread into the central and western Mediterranean. Areas such as Turkey and Syria had learnt the ski lis earlier since they were nearer to Mesopotamia.

The countries which were part of the Roman Empire learnt the skill which was used for making bowls, vases and mosaics. The colouring of glass was done using chemicals which produce different colours. Examples are:

copper and cobalt to produce blue; iron to produce green and brown; copper to produce red; tin oxide to produce white.

Most sand has some iron oxide in it, which gives it the well known golden or red colour. This iron oxide is not easily washed out, and most older glass is tinted with green or brown as a result of the natural iron found within it.

HOW WAS IT USED? From about 3000 BC until 50 BC the technique of glass blowing was not known. I nstead a mould was used which was dipped into glass to form the designed shape.

THE MOULD OR 'CORE' PROCESS

Pot formed by spreading molten glass around a mould or core, made of sand and clay and straw.

A solid shape is moulded out of clay and straw and a metal or wooden rod is bedded into it.

2

. ) \ 1.1 \ 1 I 1 ~,I I •. \ \ , • / ,j /~J I •

.\ I /1 !1;:7 J \'\ ", .\ I 11 1/( /1 '~'\\ .11 II

FIRE ~ \ \ [\ , ", I '"~I,, ,,1,,1, tf '/ ,d :-.J~t\\\': I h:,lij/II// '~,,:(,\~\ 1\ \:. 1I),?~10"I~'1 (~--.; '1~ J '" ,\' I} '\ 11 ~ -

\lj\ ~~II~ J/ltr; !ilA-M:1 ,,// ~01/~t!:-' ::,.',~

The core is dipped into the molten glass and coated with a layer of glass.

CORES OF GLASS

3 Coloured strips of molten glass may be added for decoration.

FINISHED GLASS

DISCARDED CORE

4 The glass is cooled and the core is picked out.

At the end of the Roman Empire the trade of glass-making almost disappeared in Western Europe. In Turkey and the Middle East the interest in fancy glass remained and was developed. Many colours and designs were produced, the most elaborate being the Mosque Lamps of the 14th century.

Meanwhile in Venice on the north east coast of Italy, the most well known glass centre in Europe provided fine decorated glasses which supplied the houses and palaces of the kings, queens and nobles of many countries.

1st CENTURY B.C.

The glass makers of Venice were carefully watched and their secrets were "guarded. The people who made glass were not allowed to give the formulae to anyone outside their community. The amount of fuel was restricted and furnaces could only be used at certain times. These strict laws meant that there was enough fuel to keep the furnaces going. By keeping a careful watch on the amount of glass produced glass was always a rare and expensive luxury for customers.

The Venetians discovered the recipe for clear glass in the late 15th century. They called this glass 'Cristallo,' and many people tried to copy it. The clearness was caused by adding a small amount of manganese which reacted to take away the other colours.

IRAQ 3000 - 2000 B.C.

Once the secret was out, glassworks were set up all over Europe. In Britain a Venetian called Jacopo Verzelini was the first to improve the old fashioned methods of the British glassmakers, taking over a London glasshouse in 1572.

Glass windows before this time were green or brown. From 1572 onward clear glass was made and the beginnings of our modern glass industry commenced.

When you look at old houses you will notice that their window panes are much smaller than our modern ones. Large glass windows have been very popular this century because people know how to make them.

Originally, however, panes of glass were made by taking a large ball of glass from the furnaces to a rod.

The glass would be held near the furnace mouth and rotated on the end of the rod until it formed a large open bowl shape.

Eventually, the heat of the fire and the spinning molten glass would make the bowl shape flash out into a large, flat circle.

The glass would be cooled gently and gradually cut into panes. Where the rod had been broken off from the centre a humpy, circular knot was left, known as a crown. This would be the cheaper part of the glass and it is this that we see in many houses.

CROWN TABLE MARKED FOR CUTTING

FURNACE

SAND FOR GLASS Choosing a sand for glass is important. By the time the sand reaches the glass making factory it must consist of a minimum of 98.5% silica. There must be very few impurities in the silica which could give colour to the glass. Even a trace of iron can give a greenish colour to a bottle and no one wants green tinted mi Ik.

%

100 90 80

0.09 0.125

You will notice that most of the sand is between 125 and 5001lm. This is the range of particle size used to make glass. The importance of having a fairly even grain size for glass sand is seen at the furnace. It is much easier to get an evenly melted mixture when the particles are all the same size. If some larger particles were included, the time and temperature of the furnace would have to change to prevent a lumpy mixture from forming.

The same principle applies to boiling potatoes! If small potatoes were boiled in a pot with large potatoes, the small ones would be ready long before the large ones. You would either have hard large potatoes and well cooked small potatoes or mushy small potatoes if you waited for the large ones to cook.

A well-sorted sand - most particles are about the same size

The other factor is the grain size of the sand. Grains which are too large will not melt and they cause imperfections in the glass. Small sizes would melt easily but they get blown about in the furnace by the jets of burning gas in air.

The bar chart below shows the sizing required.

0.50 0.71 1.00 mm

A poorly-sorted sand - the grains are a mixture of different sizes

PROCESSING SAND FOR GLASS The process must achieve two main objectives -

1 Size separation to remove oversize and undersize grains.

2 I mpurity removal. The major impurities are clay (which can be washed off) and the oxides of iron and chromium which colour the glass.

Different sands need more or less treatment according to the impurities. It is important to realise that the impurity level must be small in any 'glass sand' and the size distribution must be near to the one shown in the bar chart. Many sands would cost too much to treat.

I n genera I the older sandstone needs more powerfu I conditions to remove impurities. Younger sands are often unconsolidated so do not have to be drilled and blasted like the older ones.

Below is a flowsheet which applies to a sand with clay and iron oxide impurities. Some deposits need less processin~ than is shown here.

WATER SANDSTONE

MILL

PRODUCTION OF SAND

WATER SPRAY

o 0 --..... ~.... /f'I'\,- /1'1\\ ~ //, \", /11 \'

SAND AND CLAY SLURRY ................... : ... _---,

SCREENING

HYDROCYCLONE

DAMP SAND

SURGE HOPPER

DRIER

LEACHING

--0] 150.c]J ____ Q.U.EN ..... CH TANK

LEACHING REACTOR

HYDROCYCLONE

STOCKPILE (WHITE SAND)

•

CLAY SLURRY TO WASTE TREATMENT

STOCKPILE (RED SAND)

Some of the equipment is described in more detail below.

MILL After the rock has been crushed it goes to the MI LL. As the mill rotates, the sandstone lumps of rock hit against each other. There are also some large steel balls, in the mill to help in this process. Eventually all the sand grains are separated and the mixture of sand grains and clay is washed out for size separation.

HEAVY METAL LINERS

SCREENS

CRUSHING BY IMPACT AND BY STEEL BALLS

Screens are just sieves and these take out any large or OVERSIZE sand grains.

..s;:;;)~~"l~ (')~ v~OVERSIZE . f1 l): 0·» • D SCREEN 1

•• I.. <> o. .' (\. I ') Q 0 .\) "~ ~.!

L\ a '. ... ," .) 0', !l '".' OVERSIZE ... ____ a. ___ ~,Q_._~ __ _ • •.• .• . SCREEN 2

.» .'. • '" , <#, ~ '. , . .. . '" .. . . . . ... .." '" .

. "sie j;i:;;,:b;-;;..;; ;.; ;,; I.;.·; ;;:;.::.:; 'j :-0,'" '. '. : •

HYDROCYCLONE This removes most of the clay particles which are lighter (as they are smaller) than the sand grains. Sand and clay in water are pumped into the cyclone. The mixture is forced to rotate by centrifugal action .and the heavier sand grains move to the outside. They then leave at the small end of the conical section. Clay and other fine particles exit with most of the water at the top_

HYDROCYCLONE

SLURRY

SAND PARTICLES' AND WATER

SMALL PARTICLES

WET FEED IN

IN WATER MAINLY CLAY

HYDROSIZER This does a further sorting. The sand, clay and water mixture is fed in at the top. Less dense particles rise to the top in the water. Denser sand

e :··

LAMEXTABLE This is a suction filter. It is rather like a rotating Buchner funnel. Steam is also passed through to assist drainage and to heat the sand to 30oC.

. ' .' ••• : "0' ( ..

grains settle more rapidly and are taken out through a pipe in the bottom. Sorting occurs in the same way as it does in geological processes in water.

SLURRY IN

- v .' ;, • '0 ~.~ .Q"

:.. ... ; .' U '.J

0 Cl

... 'J

.J'

•

CLAY MOVES UP OVER WEIR TO

~ EFFLUENT PLANT

HEAVIER SAND SLURRY OUT HERE

SAND TO LEACH PLANT

() ((\Vlli))_~" __ " ___ _

'-==-_____ <=:J ___ _ --- STEAM INLET

DRIER A fluidized bed drier is used to remove the last trace of water and to heat the sand.

WATER AND VACUUM

WHOLE TABLE ROTATES

LEACHING REACTOR Sulphuric acid is added to the hot dry sand. The acid reacts with iron oxide on the surface of the sand grains. Rotation of the reactor assists removal of impurities from the grains as they rub against each other.

TIME IN REACTOR VESSEL -1% HRS.

SAND FROM LAMEX TABLE

IRON CONTENT APPROX. 0.25%

THE LEACHING PLANT

CHIMNEY

o

FLUIDIZED BED

DUST REMOVAL

(5-6 % OF FEED)

IN (WET)

SAND OUT

(HOT, DRY)

70% SULPHURIC ACID & WATER

(ACID FEED)

After the acid leaching, the acid is washed off. The washings can be tested to show that iron (Ill) IONS are present in the solution. Some possible tests are given below:

1

2

To 2cm depth of solution in a test tube, add 2M sodium hydroxide solution, until a precipitate of iron (Ill) hydroxide appears.

Add to a second sample a solution of potassium hexacyano ferrate (11). A deep blue colour proves iron (Ill) to be present.

3 Add to a third sample a solution of potassium thiocyanate. A 'blood-red' colour indicates the presence of iron (Ill).

It should be noted that some plants did try to use hydrochloric acid for leaching the iron oxide but it was too corrosive on the pipe work. It is also a more expensive acid but it could be used at a lower temperature.

HOT AIR (GAS HEATED)

Sand from Loch Aline only needs washing and sizing after crushing as this is from a very pure sandstone.

Different industries need different qualities of sand. Some examples are given below:

Opthalmic glass G lass tableware, cut glass, etc Refractory sand Foundry sand Glass sand - for colourless window

99.7% silica 99.6% silica

97% silica 96% silica

glass, etc 92-95.5% silica Building sand Not specified, but must have

hard minerals

In opthalmics the glass needs to be very pure in order to give a clear view to the human eye, which is very sensitive to colour.

MODERN GLASS MANUFACTURE New advances in glassmaking have mostly been concerned with trying to produce larger sheets of flat glass.

For one hundred years from 1832, flat glass was made using a cylinder method. This involved blowing the glass into a cylinder shape and cutting it down one side. The glass was then reheated and allowed to flatten out.

THE CYLINDER OF GLASS BEFORE IT IS CUT

The process used to make this type of flat glass was a modern version of a similar system thought to have been used by the Romans in Britain.

As you can imagine, changing the shape of a cylinder in this way was difficult. There were many broken sheets of glass.

What was needed was a new method of making flat glass which was less likely to break.

Two methods were found for this at about the same time, in 1910. Both methods involved catching a strip of molten glass and slowly drawing it out of the furnace. In Belgium this was done by pulling the glass vertically. In America it was pulled vertically for a short time and then bent around onto horizontal rollers.

MAKING SHEET GLASS A bait dipped into the molten glass is used to start a continuously rising ribbon of glass. The rate of drawing determines thickness. Small rollers grip the edges of the glass keeping the width constant. Annealing takes place in the tower and the glass is cut off to required lengths at the top.

WAREHOUSE

ANNEALING

DRAWING

FLOAT GLASS Sheet glass has a brilliant surface and polished plate glass is very flat. A process was produced to combine the best of these and to avoid a lot of tedious grinding and polishing.

The process involves floating the glass on a bath of molten tin. This gives the smooth surface and bright finish without further treatment.

MAKING FLOAT GLASS Molten glass flows from the tank in a continuous ribbon to float on the surface of liquid tin at a carefully controlled temperature. The parallel brilliant surfaces produced make grinding and polishing unnecessary. After annealing, the glass is cut off to required lengths.

The metal chosen as a base on which to allow the glass to form was tin. Tin melts at 2320 C but does not boil until it is heated to 22600 C. This means that tin is a liquid between these two temperatures. This makes it ideal for use in the glass factory as it will be fairly easy to melt, but will not give off

any poisonous fumes. It is, therefore, much safer than most metals at high temperatures.

Tin is an expensive metal compared with lead which was also considered. The problem with lead is that it evaporates (like water in the atmosphere) more than tin. This means that it was likely to condense as droplets on the glass (like water on a cooler window) and cause imperfections.

Although a lot of metal was needed to set up the float bath, once it was working, very little more needed to be bought.

Heat zone Fire polishing Cooling zone zone

14000C

Float bath

ANNEALING When any glass article is first produced, it cools quickly. This results in even more internal disorder than in the final usable product. The disorder is so great that there is a great deal of stress in the article. If a milk bottle from the production line is tapped gently it will shatter immediately. This is obviously very dangerous.

A simple process of reheating and slow cooling is used. This is known as ANNEALING. Heating allows the atomic particles to move around into a more stable structure.

Slow cooling prevents any further stresses being set up and the article is now quite strong.

FROM SAND TO GLASS The skill of making glass has been known to man for about 5000 years. It is one of the oldest known skills still in use by modern man.

We have seen that glass is made using a very pure silica sand. If you try to melt pure quartz you would find that it needed to be heated to a temperature of 1610 degrees centigrade. Such high temperatures were difficult to achieve in those days.

a Controlled atmosphere

Warehouse Annealing lehr ....i!! ... I I

o 6000 C

By adding burnt ashes from wood or plants the melting temperature of the silica could be brought down to about 900 degrees centigrade. The burnt ashes form a compound known as soda ash.

Soda ash and silica made into glass have one great disadvantage: the product can dissolve in water and is therefore known as waterglass. The waterglass compound can be hardened if limestone is added to the molten mixture, forming the type of glass we know today.

RAW MATERIALS FOR GLASS MANUFACTURE Although the main ingredient for glass manufacture is silica sand, the production of soda-glass involves the addition of limestone and soda-ash. Other chemicals are added in small amounts for special purposes. Some of these for instance are for adding or removing colour. Recycled glass known as cullet is also added.

It would save on transport costs if the manufacturing plants were close to the raw materials. This is rarely the case in glass manufacture although the industry near St. Helens is close to a number of the raw materials, in particular to the sand.

The map below shows the source of limestone. There is no naturally occurring soda-ash in Britain but it is manufactured from salt (sodium chloride). The location of the salt is shown on the map, in addition to the major glass sand deposits.

Limestone is Carboniferous limestone. The purest comes from the Buxton area of Derbyshire. This is amongst the best quality limestone in the world. It is transported to Cheshire for glass manufacture. I n addition this pure limestone is needed for manufacturing soda-ash.

Soda-ash. The chemical name for soda-ash is sodium carbonate. It is found naturally in salt flats near Trona in California. The name trona is sometimes given to the mineral. I n other parts of the world it is found as dried-up lakes.

Egypt, Kenya and Botswana have similar deposits. I n Wyoming it is an underground deposit.

Most soda-ash now used in Britain is manufactured in Cheshire using the salt deposits there.

KEY

~.: LIMESTONE

• SAND

• SALT

THE AMMONIA-SODA PROCESS FOR THE MANUFACTURE OF SODIUM CARBONATE

The overall reaction is:

sodium solid chloride + calcium ~ solution carbonate

sodium calcium carbonate + chloride solution solution

To make salt and limestone react is not easy. Salt in the sea does not react with chalk or limestone cliffs.

Ammonia has to be involved as well as salt and limestone to make the reaction work. Limestone is heated to produce carbon dioxide gas.

CaC03 (s) ~ CaO(s) + CO2 (g) solid solid gas

The overall reaction produces sodium hydrogen carbonate (as used in baking powder).

Salt + ammonia + carbon dioxide + water ~ sodium hydrogen carbonate + ammonium chloride

The sodium hydrogen carbonate is crystallised out, filtered and heated to produce soda ash.

2NaHC03 (s) ~ Na2 C03 (s) + CO2 (g) + H2 0(g) 150°C

FOUNDRY SAND We have seen that the melting point of silica is 16100C. How can this be of use to us?

The melting point of some common metals is as follows:

Iron Copper Aluminium

All of these metals have a melting point which is lower than that of silica.

Steels are ALLOYS of iron with other elements. The alloys of different compositions have melting temperatures within the range 15800 C to 16500C.

Some typical steel compositions are shown below:

Element Fe C Si

Plain carbon steel 97.2 1.4 0.3

Stainless steel 10.7 0.06 0.42

Cutting steel 97.0 0.37 0.34

The diagram above shows a selection of metal objects:-

Mn

1.0

1.43

1.16

All of them are made of metal and all of them have been shaped, when molten, in a mould. The mould must be strong enough to resist the heat of the molten metal and still retain its shape. Here again, two important properties of silica sand are its grain size and melting point.

% Composition

S P Cr Ni Mo Ti V AI Cu

0.05 0.05

0.02 0.02 18.2 9.0 0.15

0.37 0.05 0.12 0.12 0.02 0.01 0.02 0.08 0.34

There are different qualities of foundry sands with different ranges of grain sizes. However, they all must have the common property of being well­sorted sands. This means that, when sieved, most of the grains should remain on three or four adjacent sieves.

A bar chart for a medium grain size sand is shown below.

100 90 80 70 60 50 40 30 20 10

1000 710 500 355 250 212 150

I n Section 3 we saw that sand can be compacted or cemented together to form sandstone.

In a similar way silica sand can be made into a mould by adding something which holds the grains together. This can be done using certain types of clay. Today foundry moulds are often made by using clean silica sand held together with a resin. A resin can also be used to produce a smooth finish on the mould.

Casting these metal objects is the job of the foundry industry.

This industry needs at least 95% si lica in its sand and a medium sized grain of sand. The shape of the grains is important, since the rounded grains flow more easily and provide a smoother surface finish.

PROCESSING SAND FOR FOUNDRIES After washing to remove clay, sand is put into a large furnace where air is pumped at high pressure through it. The hot air not only dries the sand but separates the different grains of sand according to their size - this makes five different types of sand.

00 90 80 70 WT% 60 OF SAND

50 IN RANGE

40 30 20 10

106 75 MESH SIZE (MICRONS)

N.B. 1000 MICRONS (Ilm) = 1mm

Sf/lit/ '0' tile COIIIII'"ctloll lilt/lilt"

Sand is one of the most important mineral products in Britain.

METAL LI FE ROUS £41m (1.2%)

COAL £2,051 m

(60"k)

CONSTRUCTIONAL RAW MATERIALS

£960m (28.0%)

NON-PETROLEUM MINERALS (1984)

Sand and gravel make up a large proportion of the constructional raw materials portion of the above pie-chart.

This is because most of the sand produced is used for building of one sort or another. Looking at any large town or city in Britain we can see plenty of evidence of new buildings.

More sales means more money being paid into the industry and often means more people can be employed.

So much sand is used in making concrete and cement in building, that it is heavy and bulky to transport. The nearer it is to the site where it is used, the cheaper the transport cost will be. Building sand is therefore, not often carried far from the area where it is extracted.

The exceptions to this will be where the sand is of a special quality or colour that may be particularly desirable.

Building sand is not exported because of its low value and is not imported because we have plenty available in Britain.

Two types of building sand are called sharp sand and soft sand.

Sharp sand has sharp edges. It is unlikely to have been carried far by water or wind. Otherwise the particles would have been rounded. Soft sand has rounded grains. It has been transported by water) or wind for some distance.

(Sharp Sand particle x 10)

~.: ...

.,lo' . .. 0'; .:, : .~ .~ ..... ,

(Soft Sand particle x 10) y ..... . . ','

.... :~.~::.}'

Sharp sand is useful in a concrete mix since it will hold cement together more easily.

Soft sand is used for bricklaying in mortar as it flows more easily. This makes it easier to place and level the bricks. However sharp sand is used for 'pointing' as this gives a harder resistant finish.

The colour of sand can be important. Sometimes a particular colour of mortar is required to give a pleasing finish to a building. This might mean that the sand has to be carefully selected for colour.

SAND IN CONCRETE

Concrete is a mixture of sand, gravel and cement mixed with water. The sand and gravel together are called aggregate (a mixture of rock particles which are hard, strong and durable).

The proportions of sand and gravel and cement may vary according to what the concrete is to be used for.

It is best to mix this and let it dry out slowly in damp cool conditions but not cold. If the temperature is too low the water will freeze forming crystals which later melt and form a crumbly cement matrix which is weak.

GRAVEL STOCK PILES

If the concrete dries out too quickly the chemical reaction which causes setting can not take place and the concrete is not as strong as it should be.

PROCESSING SAND AND GRAVEL Processing usually involves:

a) Removal of clay by washing b) Screening of the sand into several size ranges c) Crushing of 'oversize' gravel in a cone crusher.

The crushed gravel is returned to the screening section.

d) Different sizes travel by conveyor to bins or stockpi les.

EXTRACTION OF SAND AND GRAVEL

SAND, GRAVEL CLAY, WATER RETURN TO SCREENS

f', " ,."

"

1IIIIIIIIilillIlIlIlIl.! .......... ~ CLAY IN OVERFLOW . TO WASTE LAGOON

SAND AND WATER

WATER

Probably the most well-known use of silicon is in silicon 'chips', those important components of computers, calculators and many components of rockets, aeroplanes.

In quantity the greatest use of silicon is as an alloying element. Silicon, in a form known as FERROSI L1CON is added to iron to produce a steel with specific properties.

Only when processes were developed to produce hyperpure silicon 99,9999999% did the silicon chip industry begin to develop.

HOW IS SI LlCON EXTRACTED Quartz contains the elements silicon and oxygen. If the quartz is pure, the problem is how to remove the other element, oxygen, from quartz and leave silicon.

The standard method of producing an element from its oxide is to use another element to remove the oxygen. The other element should be relatively cheap and plentiful for the process to be economic. The element used is the REDUCING AGENT.

The first successful extraction of silicon in 1824 made use of potassium, a highly reactive metal.

If we look at the affinity for oxygen of some elements at 250 C, the order obtained is shown below.

Highest affinity for oxygen.

calcium magnesium barium aluminium titanium sodium potassium SI L1CON tin zinc iron CARBON lead copper mercury

Lowest affinity for oxygen. silver

The elements aluminium and titanium, near the top of the list occur as oxides in the Earth's crust, as does silicon. This is because of their high affinity for oxygen.

Luckily, at higher temperatures these affinities can change and bring about a different order than the one given.

An element which will remove oxygen from silicon oxide must have a higher affinity for oxygen than silicon.

I n any case, most reactions need a high temperature for the rate of reaction to be fast enough to be practical. Very little would happen if CALCIUM were mixed with silicon oxide at 250 C.

At Bunsen burner temperatures, about 8000 C maximum, magnesium is higher in oxygen affinity than silicon. I n the laboratory therefore, magnesium can be used to produce silicon from quartz.

Compare the price of silicon as extracted from its oxide with the price of magnesium.

METAL magnesium silicon

PRICE PER TONNE £2,500 £ 800

To use an expensive metal such as magnesium to extract silicon would be uneconomic.

However, at much higher temperatures, carbon, a much used and much cheaper reducing agent finds itself higher in the oxygen affinity table than silicon. Carbon is the element used to extract silicon in industry.

THE SILICON SMELTER A mixture of carbon (as coal and wood) and lump quartz is fed into the smelter from above.

Heat is provided electrically in the carbon-arc furnace. A number of electrodes are electrically heated to maintain the required temperature of the furnace.

The quartz must be in the form of 'lumps' rather than sand for two reasons.

2

The small sand particles would prevent effective circulation of gases.

The reaction takes time so the quartz lumps must fall through the furnace at a controlled rate.

Temperatures of 170Q-18000 C must be maintained.

One reaction in the furnace is a direct reaction between silica and carbon:

Si02 (s) + 2C(s) ~ Si(1) + 2CO(g)

However, two solids do not react well together. Other reactions provide more of the major reducing agent which is carbon monoxide, a gas.

Molten silicon is tapped from the bottom of the furnace. This is led into moulds to cool and solidify.

SUBMERGED ARC FURNACE

WHERE ARE THE SILICON SMELTERS OF THE WORLD? The table below shows some of the major silicon producing countries in order of quantity produced in one year.

U.S.A. Norway USSR France Yugoslavia Portugal Canada S. Africa China

Si licon smelters are situated either near the quartz/ quartzite rock source or where a cheap source of energy is available.

A country like Norway can produce hydro electric power at about 1/5 of the cost of electricity in Britain.

TRAMCAR

FURNACE MIXBIN

RAW MATERIALS

SKIP HOIST

FLAT CAST SILICON METAL

SILICON FOR CHIPS Metallurgical grade silicon from the smelter is 98.5% Si. This is reacted with chemicals in a complicated series of reactions then converted back to silicon which is now over 99% pure.

This is still not pure enough. A process called zone refining is used. This is like carrying out a series of fractional crystallisation stages but can be carried out continuously.

The process relies on melting a bar of the elementa little at a time. The bar is passed through an induction coil which heats up and melts that section of the bar.

As the element is heated it melts. After leaving the coil, it solidifies but the impurities are left behind. If this is repeated several times impurities are gradually eliminated.

, .. . ~: .. ,-.- ... ;'.'.": " ", .. " " ........ ." .. : ....... ": " .... " .... ,,'" .. , .. " ... " ... , .... " -.' , .. , .. " .. , .. ,,:,,-... ~ :~ ... ' .... ' .. -.. , ... -... , ... '-.. -: .. " ... "' .. :. ...... ........... - .............. , ............... : .... , .. , ..... " ..... , ' ... .. . '"'''''''''' ....... , .. , ..... , .. , ... ', ......... " ',.' ....... -- ........... , .. , ... . ... ',,"""" .. , "" ..... " ' ........... " ........... -_ .... "" ... ',,,,,,-, ... , .. , .. "',.,,,,,',,

IMPURITIES COLLECT

GROWING A SINGLE CRYSTAL For transistors, the whole transistor must be made from a single crystal of silicon (or germanium). The transistor cannot function properly unless it is made in this way.

The usual way a molten substance solidifies is to form many nuclei from which crystals grow.

The sequence is shown below:

STAGE 1 2

4)

!J ;,) ~ 0

LIQUID -b 9

~ ~ ~

• 0

NUCLEI FORM SMALL DENDRITES GROW

3

~ Y-'-'\, "I ,-

~~

0 J~~

DENDRITES GET LARGER

PURE ELEMENT EMERGES

GRAIN BOUNDARY

SOLIDIFICATION COMPLETE

To prepare a single crystal of silicon, the zone­refined rod is melted and kept at a temperature just above its melting point in a crystal-pulling furnace. A seed crystal, held just below its melting point is brought into contact as the liquid freezes on it. The whole procedure is carried out either in an inert atmosphere or a high vacuum. Single crystals 3 or 4cm in diameter can be grown in this way.

CONTROLLED UPWARD MOVEMENT

ORIGINAL 'SEED' CRYSTAL

SINGLE SILICON CRYSTAL

SI LlCON FOR SI LlCONES Silicones are polymers and have some properties in common with the older polymers based on the element carbon - known as plastics. Silicones have the general structure:

R R R R R R

'\./ '\./ '\./ Si Si Si

,,/'\. /" /'\./ 00000 0

Where R is a group containing carbon

(Note the strong silicon-oxygen bonds which are important in the structure of silica and silicate minerals)

Silicones are superior to carbon based polymers in several ways. Silicone 'rubbers' do not crack (perish) like carbon-based 'rubbers.'

1 Silicone rubbers have outstandingly con­sistent properties over a temperature range from - 1 OOoC to some 3000 C. These are ideal for certain flexible parts of supersonic aircraft and for sealing doors and windows. They are also used for ducting hot air.

They are more fire-resistant and therefore used as insulants in wiring in ships.

2 Silicone fluids do not mix with other liquids in particular with water. Very good water­proofing agents are made from silicones. The silicones have excellent 'non-stick' properties and are used in coating moulds for rubber tyres, die-casting and even in bread and cake moulds.

3 Silicones control foam in brewery effluents and in sewage works. A dose of silicone anti-foam can cure the 'frothy bloat' in the stomachs of cows and sheep - and in humans too as anti-burping medicine.

4 Silicones are good lubricants - particularly at high temperatures. They resist oxidation and maintain a reasonably high viscosity.

SILlCONES IN THE BODY Only since the mid-1960's have surgeons begun to get around the problem of rejection of natural organs transplanted from one individual to another.

For centuries they have been searching for non­living materials for relatively simple constructional tasks.

Since the first bakelite was produced in 1909, plastics have been used for heart valves and for repairing damaged tissue.

Silicone rubbers are outstanding materials for prostheses (artificial parts of the body).

They are:

- inert

- have low surface attraction for water

- do not corrode

The material is so different from anything the body is used to that the defence system does not recognize it as foreign.

Applications of si licone rubber in surgery.

Brain damage for hydrocephalus.

I n the heart - valve replacement.

- substitute ventricles.

Eye surgery.

Replacement tendons and sheaths to protect transp lanted tendons.

Grafts and sponge silicon - cosmetic surgery (such as breast size increase).

Countlnl tile COlt HOW MUCH SAND IS USED? The amount of sand used in any year in Britain wi 11 depend on the amounts needed for different industries. The industry that uses the most sand is the building industry. At times when there is a lot of money to spend, more building work takes place. At the same time other industries may be doing well and ordering sand for their products.

Sand is found in many places across the world, and is used in large quantities. It is cheaper for people who use sand to buy if from as near as possible to the user. Transport is cheapest by boats, on river, canal or sea, but in Britain canals and rivers are rarely used. Road transport is most common in the U.K., with rail freight second.

Only special, high grade sands would be carried over long distances. A few sands are imported to Britain if they have a specially pure silica content or colour and are suitable for some types of glassmaking. Britain also exports good quality sands to areas where there are not enough silica sands.

The areas producing silica sand in Britain are shown in section 3.

The sand production industry employs about 11,000 people in the U.K. The sand mined from the ground by these people helps to keep many employed in building, glassmaking, foundry work and the other industries that depend on sand or glass.

It is difficult to work out how much this industry puts into the whole economy, but it is a very important raw material.

Envll'onment.' Alpeetl

The demand for sand for many uses in Britain is met by the sand industry. Where is it taken from? Some, a small proportion only, comes from under­ground mining. The rest is removed from open pits, of various shapes, sizes and depth, across the country or dredged from estuaries and coastal waters.

WHAT IS THE EFFECT OF MINING SAND? The most obvious effects produced by open pits, are the holes which are left in the landscape.

In most years more than one hundred million tonnes of sand is taken out of the ground in the U.K. Each tonne of sand would fill up a bath tub.

One hundred million bath tubs take up a lot of space which has to be environmentally restored.

This is not the only problem. Lorry transport, noise and dust created by quarries may also be a nuisance to people who live nearby.

In the last forty years, local authorities have been careful to check the new area which could be used for quarries, and planning permission needs to be given before new areas can be exploited. The permission often has a legal requirement for the companies to return the land to a desired standard when extraction has finished.

When sandstone or crude sands are processed to make better quality sands, the waste products from the plant must be dealt with.

TREATING THE WASTE PRODUCTS

I SETTLING TANK

OVERFLOW OF WATER RECYCLED TO THE PROCESS

THICKENED SLURRY

Natural sand is often mixed with larger and smaller particles of other substances. The fine waste materials are washed out as a slurry, known as tailings.

I n a glass sand plant some water is first removed in a 'thickener' or settling tank.

The waste tailings are often pumped to disused quarry workings where the clay and fine particles can settle out in a pond, known as a TAl LlNGS LAGOON.

SETTLED CLAY WASTE

The industry may wish to re-use the water and still want the clay to settle as quickly as possible.

"SETTLlNG DOWN" To help the clay to settle more quickly, certain chemicals can be added to the wet mixture, in order to make the finer particles join together to form larger particles which will sink more quickly.

Chemicals which attract particles to stick together are called COAGULANTS, or FLOCCULANTS.

r , ......

( 0 () ) COAGULANT

( - ... - OR " ~ o 0) '- / FLOCCULANT '- -

I Larger particles sink more quickly.

Eventually the lagoons will fill up and dry out.

ACID WATER Glass sands need to be clean to make clear glass. Many of our natural sands look golden or even brown or red. When this is the case the grains are usually covered with a thin coating of iron oxide

COATING OF DARK

QUARTZ @O+N OX~ GRAIN I, ,,~:~ .:. ::.:::~ U

The acid solution has to be disposed of when it is no longer useful. If it is allowed to run into the lakes or rivers or seep into the ground it could cause enormous damage to fish, animals and plants. To prevent this another chemical called an alkali is added, to stop the acid from being effective. The acid is NEUTRALISED.

(rust!) left behind as water has dried out in the rock. This coating can be removed with acid. Sulphuric acid is the acid most commonly used in cleaning plants.

DIRTY ACID

(lj... CLEAN + .; .::; ...... QUARTZ ...... . GRAIN

"0:. "0: •

Calcium sulphate occurs naturally as gypsum. It is solid and can be made to settle out more quickly in the same way as the clay particles.

H2S04 (aq) + Ca (OH)2 (aq) -+ CaS04 (s) + 2H 20(1)

sulphuric acid + calcium hydroxide -+ calcium sulphate + water (acid solution) (alkaline solution) (neutral solid)

MAKING THE MOST OF A GRAVEL PIT When the water is clean and clear it may fill up an old sand or gravel pit. Artificial lakes can be very attractive but too much farmland would be lost if all disused pits were left as lakes.

Instead careful planning now goes into the res­toration of the gravel pits, known as reclamation.

I n a typical case the mining or quarrying company will only be allowed to start new work in the

U.K. if an agreement has been made with the Local Planning Authority, showing the area of land to be taken from, and returned to, various types of use.

Some gravel pits must be restored to agricultural land. Others may be converted to water parks or nature reserves. Sometimes they may be used for the disposal of waste.

LOOKING AT BLAXTON QUARRY Five miles east of Doncaster, near the village of Blaxton, lies one of the largest sand and gravel quarries in Britain. The quarry began in 1950 and it continues to be worked to the present day. A lot of time, effort and money has been used to help restore much of this land to a good standard for agriculture, some woodland and a nature reserve. The countryside here is flat and easily flooded, so special care needs to be taken to drain the land properly, especially since much of the restored area wi 11 be below the water table.

In total about 560 hectares of land will have been dug up for sands and gravels.

The map shows the area of working and the details of restoration. The quarrying work has finished in the southern part of the quarry and is now continuing in the area to the north where there are sti II usefu I amounts of sand and gravel.

A nature reserve has been planned, away from the noisy main road, but with a nearby minor road for visitors. With a lake and large wooded area it should attract a variety of birds, plants and insects.

Restoring land to make it good enough for agriculture is a difficult process, and we are learning more about this as we try new methods.

,:::::::::::::::::::,',: EMBANKMENT

Some of the problems dealt with at Blaxton were:-

finding the right depth of topsoil to use. 200mm(8")

2 Some areas had very coarse or very fine material, unsuitable for agriculture.

3 The land left was lower than the water level (water table) in the area and would easily flood.

topsoil was not enough

This was later increased to 250mm (10")

heath land or wetland ~ trees were planted

instead of making farmland.

Special drainage systems were built in

~ and a pump system was installed by the company.

In the future more knowledge of the environment will help people to make more careful, sensible decisions on how to work with nature. There is always more to learn, but modern quarries can be returned to form farmland or nature reserves or lakes and can be made attractive and useful in other ways.

WOODLAND

WATER

A AGRICULTURE

N NATURE RESERVE

a QUARRY USE

1-0\ .. ----1 km --...., .. ~I