Bicol University College of Engineering
Legazpi City
Research in Metallurgical Analysis
Clays
Submitted by: GROUP 8 Julius Banez
Renier Villar Sarah Mae Ajon
Dailyn Nivero BSEM – 3
Submitted to: Engr. Victor Florece
I. Introduction
Clay is a common name for a number of fine-grained, earthy materials that become plastic when wet. Chemically, clays are hydrous aluminum silicates, ordinarily containing impurities, e.g., potassium, sodium, calcium, magnesium, or iron, in small amounts.
The term 'clay' has been used in several ways: (1) to designate very fine grained particles of less than 0.004mm diameter, (2) as a rock composed essentially of clay minerals and (3) as a mineral group known as 'clay minerals'. As a rock, it is composed of materials which are earthy in appearance and composed mainly of extremely fine grained mineral particles.
Chemical and Physical Properties
Properties of the clays include plasticity (malleable and capable of being molded into any form when moistened with water), shrinkage under firing and under air drying, fineness of grain, color after firing, hardness, cohesion, and capacity of the surface to take decoration. Most have the ability to soak up ions from a solution and release the ions later when conditions change.
Water molecules are strongly attracted to clay mineral surfaces. When a little clay is added to water, a slurry forms because the clay distributes itself evenly throughout the water. This property of clay is used by the paint industry to disperse pigment (color) evenly throughout a paint. Without clay to act as a carrier, it would be difficult to evenly mix the paint base and color pigment. A mixture of a lot of clay and a little water results in a mud that can be shaped and dried to form a relatively rigid solid.
Another important property of clay minerals, the ability to exchange ions, relates to the charged surface of clay minerals. Ions can be attracted to the surface of a clay particle or taken up within the structure of these minerals. The property of clay minerals that causes ions in solution to be fixed on clay surfaces or within internal sites applies to all types of ions, including organic molecules like pesticides. Clays can be an important vehicle for transporting and widely dispersing contaminants from one area to another.
Individual clay particles are always smaller than 0.004 mm. Clays often form colloidal suspensions when immersed in water, but the clay particles flocculate (clump) and settle quickly in saline water. Clays are easily molded into a form that they retain when dry, and they become hard and lose their plasticity when subjected to heat.
Commoner Varieties
The commoner varieties of clay and clay rocks are china clay, or kaolin; pipe clay, similar to kaolin, but containing a larger percentage of silica; potter's clay, not as pure as pipe clay; sculptor's clay, or modeling clay, a fine potter's clay, sometimes mixed with fine sand; brick clay, an admixture of clay and sand with some ferruginous (iron-containing) matter; fire clay, containing little or no lime, alkaline earth, or iron (which act as fluxes), and hence infusible or highly refractory; shale; loam; and marl.
Formation
Clays and clay minerals occur under a fairly limited range of geologic conditions. The environments of formation include soil horizons, continental and marine sediments, geothermal
fields, volcanic deposits, and weathering rock formations. Most clay minerals form where rocks are in contact with water, air, or steam. Examples of these situations include weathering boulders on a hillside, sediments on sea or lake bottoms, deeply buried sediments containing pore water, and rocks in contact with water heated by magma (molten rock). All of these environments may cause the formation of clay minerals from preexisting minerals. Extensive alteration of rocks to clay minerals can produce relatively pure clay deposits that are of economic interest (for example, bentonite ‹primarily montmorillonite ‹used for drilling muds and clays used in ceramics).
Clay consists of a sheet of interconnected silicates combined with a second sheetlike grouping of metallic atoms, oxygen, and hydroxyl, forming a two-layer mineral such as kaolinite. Sometimes the latter sheetlike structure is found sandwiched between two silica sheets, forming a three-layer mineral such as vermiculite. In the lithification process, compacted clay layers can be transformed into shale. Under the intense heat and pressure that may develop in the layers, the shale can be metamorphosed into slate.
Uses
The plastic clays are used for making pottery of all kinds, bricks and tiles, tobacco pipes, firebricks, and other products. As a building material, it is used in the form of brick , either sun-dried (adobe) or fired. Clays are also of great industrial importance, e.g., in the manufacture of tile for wall and floor coverings, of porcelain, china, and earthenware, and of pipe for drainage and sewage. Highly absorbent, bentonite is much used in foundry work for facing the molds and preparing the molding sands for casting metals. The less absorbent bentonites are used chiefly in the oil industry, e.g., as filtering and deodorizing agents in the refining of petroleum and, mixed with other materials, as drilling muds to protect the cutting bit while drilling. Other uses are in the making of fillers, sizings, and dressings in construction, in clarifying water and wine, in purifying sewage, and in the paper, ceramics, plastics, and rubber industries.
Clay as a Soil
Clay is one of the three principal types of soil, the other two being sand and loam. A certain amount of clay is a desirable constituent of soil, since it binds other kinds of particles together and makes the whole retentive of water. Excessively clayey soils, however, are exceedingly difficult to cultivate. Their stiffness presents resistance to implements, impedes the growth of the plants, and prevents free circulation of air around the roots. They are cold and sticky in wet weather, while in dry weather they bake hard and crack. Clods form very often in clayey soils. Clays can be improved by the addition of lime, chalk, or organic matter; sodium nitrate, however, intensifies the injurious effects. In spite of their disadvantages, the richness of clay soils makes them favorable to the growth of crops that have been started in other soil.
II. Types and Classifications of Clay
On the basis of such qualities clays are variously divided into classes or groups; products are generally made from mixtures of clays and other substances.
Mineral contents of Clay:
The dominant minerals in a clay are usually layer silicates and fine silica (quartz), together with smaller amounts of iron sulphides and oxides, titanium minerals, various carbonates, and organic matter. The layer silicates (phyllosilicates) can be classified into various groups, according to their chemical composition and the layer structure of their crystal lattice. The layers are composed of various combinations of tetrahedral silica sheets and octahedral hydrated aluminium oxide sheets, frequently with appreciable amounts of potassium, calcium, magnesium, sodium, and iron.
Kaolinite Al2Si2O5(OH)4
It is a white mineral consisting of a hydrous silicate of aluminum that constitutes the principal mineral in kaolin.
Figure 1: Authigenic kaolinite plates covering a quartz grain overgrown with authigenic quartz. SEM image of a core sample.
Physical Properties of Kaolinite
Cleavage: {001} Perfect
Color: White, Brownish white, Grayish white, Yellowish white, Grayish green.
Density: 2.6
Diaphaneity: Transparent to translucent
Fracture: Earthy - Dull, clay-like fractures with no visible crystalline affinities, (e.g. howlite).
Habit: Earthy - Dull, clay-like texture with no visible crystalline affinities, (e.g. howlite).
Luminescence: Non-fluorescent.
Luster: Earthy (Dull)
Streak: white
Figure 2: White chalky ammonioalunite on darker kaolinite.
Montmorillonite (Na,Ca)0,3(Al,Mg)2Si4O10(OH)2•n(H2O)
- a soft clayey water-absorbent mineral that is a hydrous aluminum silicate.
Figure 3: Authigenic smectite (montmorillonite) overgrown on pore spaces and authigenicly-overgrown quartz grains in a sandstone. SEM image of a core sample.
Physical Properties of Montmorillonite
Cleavage: {001} Perfect
Color: White, Gray white, Yellow, Brownish yellow, Greenish yellow. Density: 2 - 2.7, Average = 2.35
Diaphaneity: Translucent to Opaque
Fracture: Earthy - Dull, clay-like fractures with no visible crystalline affinities, (e.g. howlite).
(e.g. howlite).
Hardness: 1.5-2 - Talc-Gypsum Luminescence: Non-fluorescent.
Luster: Earthy (Dull)
Streak: White
Figure 4: Bright pink chalky massive montmorillonite.
Attapulgite /Palygorskite (Mg,Al)2Si4O10(OH)•4(H2O)
- is a kind of crystalloid hydrous magnesium-aluminum silicate mineral.
Figure 5: Compact massive palygorskite.
Physical Properties of Palygorskite
Cleavage: {110} Good
Density: 2.1 - 2.2, Average = 2.15 Diaphaneity: Translucent
Fracture: Uneven - Flat surfaces (not cleavage) fractured in an uneven pattern.
Habit: Earthy - Dull, clay-like texture with no visible crystalline affinities, (e.g. howlite).
Habit: Massive - Fibrous - Distinctly fibrous fine-grained forms. Hardness: 2-2.5 - Gypsum-Finger Nail
Luster: Earthy (Dull)
Streak: white
Illite (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)]
- any group of clay minerals having essentially the crystal structure of muscovite. Its other names were Gumbelite, Hydromica and Hydromuscovite.
Figure 6: Wispy, authigenic illite crystals lining a pore space in sandstone. SEM image from a core sample.
Physical Properties of Illite
Cleavage: {001} Perfect
Color: White. Density: 2.6 - 2.9, Average = 2.75 Diaphaneity: Translucent
Habit: Aggregates - Made of numerous individual crystals or clusters. Hardness: 1-2 - Between Talc and Gypsum
Luster: Earthy (Dull)
Figure 7: light green Illite with Albite, field of view 6 mm, photo and collection Joachim Esche
Some phyllosilicate particles are composite alternations of different clay minerals. When mica occurs in fine-grained form in a clay it is usually referred to as illiteor clay mica and is similar in composition to muscovite, but there is usually some substitution of hydroxyl ions into the lattice. There are many other clay minerals, including chlorite, vermiculite, and various chain silicates such as pyrophyllite, some of which are valuable industrial materials.
Types of Clay:
Residual clay comes directly from gradual weathering of rock into very fine particles. The particles become mixed with water and material from the surrounding soil. Residual clays are most commonly formed by surface weathering, which gives rise to clay in three ways:
¾ chemical decomposition of rocks, such as granite, containing silica and alumina;
¾ solution of rocks, such as limestone, containing clayey impurities, which, being insoluble, are deposited as clay; and
¾ disintegration and solution of shale.
One of the commonest processes of clay formation is the chemical decomposition of feldspar .
Sedimentary clay is formed when particles of weathered rocks are carried from the place in which they were formed, usually by streams of water, and deposited in another place. It occurs in layers.
Classification of Clay according to their general composition and properties:
1. Kaolin is a fine white clay consisting chiefly of the mineral kaolinite. The purest clays are the china clays and kaolins.
Figure 8: Washed Kaolin Clay
Figure 9: Raw Kaolin Clay
The kaolin group of clay minerals includes kaolinite and halloysite, both of which are hydrated aluminium silicates. Kaolinite has a simple layer structure; a kaolinite layer is composed of one tetrahedral sheet combined with one octahedral sheet. Clays predominantly composed of kaolinite are commercially known as ‘kaolin’.
The name comes from the Chinese “Gaoling,” a reference to a mountain which provided a source of the raw mineral. The Chinese used this mineral to produce their famously fine porcelain, and when European explorers were introduced to Chinese ceramics, many of them remarked on the delicate quality of Chinese ceramic work. This
was made by possible by kaolin, a material that Europeans were not familiar with, and European ceramicists spent centuries trying to replicate the techniques used in China to produce porcelain.
Applications
Today, the secret of Chinese ceramics is out, and manufacturing companies all over the world utilize kaolin in their ceramics. In high concentrations, the mineral produces fine white pieces with a high level of tensile strength, and it can be used to produce several styles of ceramic. Kaolin clay can also be blended with other clays to create specific blends.
The mineral also has an ancient use as a skincare product. Like other clays, kaolin is very absorbent, and it can pull oils and dirt out of the skin. It is commonly used in clay masks or as an additive to baths to sooth the skin, and it is also included in numerous cosmetics. Powdered forms may be dusted on the face to absorb oil and reduce greasiness, while mineralized creams can be used to soothe dry skin or to reduce oiliness, depending on how they are formulated. Companies which carry natural skin care products often stock pure kaolin which people can use to make their own cosmetics and skin care products.
Another historic use of this mineral is as a remedy for gastrointestinal upset. People once ate kaolin because the clay coated the stomach to soothe irritation, and it absorbed bacteria and viruses which caused disease, as well as absorbing loose water, which caused the stools to firm. Some cultures continue to eat clays for stomach pain, and the mineral has also been integrated into many stomach care products, such as the liquid suspensions people take to treat diarrhea.
One of the most widespread uses of kaolin today is in paper manufacturing. The mineral is used to coat and fill paper, and the paper industry demands huge volumes of it annually. Varying levels of kaolin can be used to change the texture and appearance of paper products.
Extraction and Processing
The raw china clay is subject to a refining process including spiral classification, hydrocycloning, deflocculation, bleaching and finally drying, micronizing, air-classification and surface treatment.
The extraction and processing of kaolin in south-west England involves the production of large quantities of arisings much of which has previously been discarded as waste. The arisings produced are of two main types: coarse material comprising sand
(mainly quartz); and rock, otherwise known as ‘stent’. Much of this material is processed and sold as aggregate or stockpiled. Lower quality material is disposed of in large tips or, increasingly, backfiled into pit voids, where sterilisation of unworked reserves will not result. A fine slurry called mica residue is disposed of in large lagoons and abandoned kaolin pits. In the lagoon the mica settles out and the water is pumped off for reuse. Some older mica lagoons have been reworked to recover coarser kaolinite formerly lost in processing.
Figure 11: Hydraulic excavators, bucket-wheel excavators and wheel loaders are used in the excavation process. The unrefined soil is transported to the refinery or to the slagheaps by means of dumper trucks, conveyor belts
or winches.
Kaolin extraction has traditionally been by hydraulic mining in which high pressure jets of water are used to disaggregate the weak, kaolinised granite and dispersed the Kaolinite particles, together with the other components of the granite into a slurry. Ripping, drilling and blasting of the granite are also used to improve yields and unkaolinised material is removed for tipping, although some is processed into aggregate. More recently, ‘dry’ mining has been introduced. This allows more selective extraction and improved yields. The kaolinised granite is extracted by shovel and truck and is transported to a primary screening process to remove large oversize material. The undersize is disaggregated by high pressure jets of water for subsequent processing in the conventional way.
Separation of the fine kaolinite particles from the coarser waste, consisting mainly of quartz, unaltered feldspar and mica, is by a series of wet refining techniques. Additional techniques are used to improve the brightness (whiteness) and particle size of specific grades of clay. These include blending, fine grinding, chemical reductive bleaching and/or the removal of iron-bearing impurities using superconducting magnets. Some clays are also calcined at specific temperatures to give different products.
Finally the clay is dried to a powder or pellets, or supplied in slurry form as a suspension of clay in water.
The ultimate disposal of the waste products has been a major problem because of the large areas that are required and the visual impact. However, about 40 abandoned pits have been backfilled where this has not affected researves or the requirements for water holding areas. As the surface extent of workings reach their practical limits surface tipping is giving way in favor of backfilling.
2. Ball clay contains kaolinite and certain micas, and has strong bonding properties. It is a name for a group of plastic, refractory (high-temperature) clays used with other clays to improve their plasticity and to increase their strength.
Ball clays are sedimentary rocks, usually laid down in fresh water, which are composed of a special type of kaolinite known as b-axis disordered, together with clay mica (illite), which gives the clay good plasticity and strength, making it particularly suitable for ceramics. Fireclays have a similar mineral composition to ball clays, but are generally less pure and lack plasticity and strength. Large quantities of fireclay were formerly used in the iron and steel industry, but nowadays their main use is for making bricks and sanitary ware. Halloysite has the same basic layer configuration as kaolinite, but the layers are rolled up into scrolls. Deposits of halloysite are valued for use in high-quality ceramics, such as porcelain.
Ball clay is an extremely rare mineral found in very few places around the world. Its name dates back to the early methods of mining when specialized hand tools were used to extract the clay in rough cube shapes of about 30 cm. As the corners were knocked through handling and storage these cubes became rounded and ‘ball’ shaped. It also is sometimes referred to as plastic clay.
Figure 12: Ball Clay Ore from Turkey
Applications
A vital material in ceramics - Ball clays are used in many different industries, but in particular form a vital component in ceramic manufacturing. Kaolin (‘china clay’) produces a very white color when it is fired, but used alone it is brittle and weak and must be mixed with ball clay to produce a workable, malleable raw material. As a result of their sedimentary origin, raw ball clays have a wide range of colors. However, many of them are valued by the ceramics industry for their white-firing properties, which are determined by the levels of iron and other coloring/fluxing oxides within the clay.
Sanitaryware: A ‘ceramic body’ for sanitaryware typically includes 30% ball clay to provide plasticity and workability, 20% kaolin, 30% feldspar and 20% quartz/silica.
Tableware: Ceramic tableware utilizes ball clay to provide high plasticity and a good white-fired color,
combined with kaolin, feldspar and quartz.
Wall and floor tiles: Combined with talc, feldspar, quartz/silica and kaolin, ball clays are utilized for their plasticity and bonding properties.
Figure 14: Wash
Glazes and engobes: Ball clays are also used in the production of coatings for ceramic products to ensure the perfect finish.
Refractory clays: An ability to resist the effects of extremely high temperatures makes ball clay ideal for use in refractory products such as kiln insulation and furniture.
Construction ceramics: Building materials such as bricks, clay pipes and roof tiles all contain ball clay.
Electrical porcelain insulators: You will find ball clays in the electrical porcelain components that provide insulation from high voltage currents.
Non-ceramic applications: These include the construction industry; horticulture, agriculture and amenity industries; use as fillers and extenders in polymers, adhesives, plastics, sealants, fertilizers and insecticides.
How the Ball Clay Deposits Occurred
A rare coincidence of geological conditions was required to form and preserve the ball clay deposits:
1) suitable kaolinite-rich source rocks largely free of iron oxides;
2) erosion of these rocks into fresh or brackish water 'traps' for the ultra fine particles before they could be washed out to sea;
3) little subsequent erosion or deep burial of the resultant sedimentary deposit.
Ball Clay Production
Over the centuries there has been an evolution in the methods used to extract the valuable seams of ball clay. The methods varied slightly between the three areas of production. They are summarised below and then described in more detail.
Shallow trenches - from the 1600s
Small open pits - the natural development of shallow trenches, which grew in size when pumping techniques improved in the 19th century.
Square pits - a development of small open pits in South Devon, England enabling clays to be worked at a greater depth
Shaft mining (underground) - widely adopted from the second half of the 19th century
Figure 15: Floor Tiles
Inclined shafts (underground) - a variant of shaft mining adopted in North Devon, England from the end of the 19th century until the 1960s
Adit or Inclined tunnel mining (underground) - came into use in the 1930s and adopted in place of most shaft mining from the 1950s to the 1990s
Large scale opencast working - progressively replaced all underground mining during the second half of the 20th century: the only method after 1999.
Figure 17: Methods of Clay Ore Extraction A. Shallow Trenches
The tenant farmers who first found clay under their fields dug it with whatever farm implements came to hand. As time went by special techniques and tools were developed to work the clay. Despite some local variations, they were broadly similar in each of the three production areas.
The basic system was to dig a shallow trench. After removing unwanted overlying material called 'overburden' (or 'head' or 'ridding'), the 'claycutters' cut the exposed floor of clean plastic clay into a criss-cross pattern of 9 inch (23 cm) squares using heavy iron spades with 4 inch (10 cm) wide blades known as 'thirting' (or 'thwirting') irons. Following this, another claycutter used a weighty, ash-handled tool like a wide-bladed pick or mattock called a 'lumper' to undercut each square to a depth of 9 inches (23 cm) and lever out the resultant cube of clay weighing about 36lbs (16.3 kilos): 70 balls made a 'tally' of 221/2 hundredweight (1.14 tonnes). The claycutters dipped their tools into a bucket of water to lubricate the cutting. A tool called a 'poge' - a curved iron spike set into a stout pole - was then used to pitch the cubes up the stepped sides of the pit to the surface and onto a packhorse or cart. A lighter version of the lumper known as a 'tubil' or 'tubal' was used to trim the working. In this way the whole floor area was removed to reveal the next layer for extraction.
B. Small Open Pits
As trenches widened they developed into open pits, which, although of some size considering the manual labour involved, were small by the standards of today. These remained the most economical way of extracting seams of clay that were close to the
Figure 18: An open pit showing cubes of ball clay being cut vertically using a thirting iron (central figure), undercut using a lumper (right figure, partly concealed) and loaded into a wagon using a poge (left figure), c. 1930
surface and not overlain by too much overburden. Neatly terraced slopes were a feature of the best of these pits. However percolating groundwater and rainwater tended to cause open pits to flood, and their depth and area were therefore limited by the capacity of pumps to dispose of the water. Hand operated elm-barrelled pumps with a maximum lift of 15 feet (4.6 metres) were used originally, and although it was possible to have a series of pumps each with such a 'lift', it was not until the introduction of Cornish plunger pumps towards the end of the 19th century that open pits were developed to any great depth.
C. The Square Pit System
The ground water and rain that tended to cause open pits to flood also caused their soft sides to subside. To control subsidence timber began to be used. This developed in South Devon into a system of excavating a sequence of 'square pits' that
were timber lined and braced. After trial and error the optimum size was found to be 18 to 24 feet (5.5 to 7.3 metres) square. These square pits could then be dug to a depth of 50 feet (15.2 metres) with a series of pumping 'lifts' and ladders, but for a long time only the clay within the pit area was worked. About 12 feet (3.7 metres) of solid ground was left unworked between them and the waste from one pit was used to backfill another. To relieve the strenuous labour of manually lifting the clay and waste to the surface, a wooden crane of a type unique to the ball clay industry called a 'crab' would be erected beside the square pit to hoist the clay and waste to the surface in an elm bucket. The crab was a pivoting 'gallows' type crane held in place by two legs called 'tie backs'. Hand winches or horse drawn winches (known as 'whims') were used to raise and lower the buckets.
Each square pit produced a few hundred tonnes of the several types of clay through which the pit was sunk. Most were worked for just a few months until incoming water became too much for the hand pumps. Whilst shallow open pits remained the principal means of extracting stoneware clays, the square pit system was used to win the more valuable potters' clays that were typically found at a greater depth.
D. The Evolution of Underground Mining: Vertical Shafts
In South Devon the square pit system evolved by stages into true underground mining. First the claycutters started to rob a small area of the seam adjacent to the bottom of the pit - giving the pit a bell-like shape. Then short timbered levels were driven out from the base of the pit. This focused the operation on extracting a particular seam and on minimising the amount
Figure 19: Diagram showing timber supports for a square pit Figure 20: Head of shaft mine showing a 'crab' crane and an elevated timber ramp known as a 'high back'. The pivoted arm with a ring at the end, called a 'mouse', is a safety device to prevent the bucket being 'overwound', c. 1950
of material to be dug to reach it, which meant changing from digging a wide square pit to digging a vertical shaft wide enough simply to accommodate the bucket, ladders and pump lines.
By the 1870's underground shaft mining was enabling large quantities of deeper clays to be worked in all three production areas. By the end of the century the availability of Cornish pumps did away with the depth limitations of hand pumps. As a result, shafts could then be used to extract potters' clays at a depth of 50 to 150 feet (15 to 46 metres) - the greatest depth reached being 200 feet (61 metres) - whilst square pits and open pits remained in use to work the shallower stoneware clays.
Typical cross section dimensions of the shafts were 9 feet x 4 ½ feet (2.75 x 1.37 metres) and 13 feet x 6 feet (3.96 x 1.83 metres). The horizontal frames of larch supporting the sides of the shafts were separated vertically by timber 'studdles' between which boards and 'vraiths' or 'wreathes' of sedge grass or heather were rammed to hold back the sands through which the shafts were frequently driven. The shafts were divided into two compartments, one for hoisting by means of a crab and one for the access ladders and pump lines.
Having reached the desired seam and established a sump at the base of the shaft two side drives (tunnels) supported by closely set green round larch timbers would be driven up to 100 feet (30 metres) in opposite directions from the base of the shaft through the level 'strike' of the seam. Traditionally, the seam would then be worked in a 'fan' shape by cutting further timbered drives from the base of the shaft, predominantly on the 'rise' side (where the seam sloped upwards). The drives were usually only 5 ½ feet (1.68 metres) high and 4 feet (1.22 metres) wide. Clay would squeeze through the timber supports, and a
worked out drive would soon collapse under pressure from the ground above when the timber was withdrawn for re-use. The life of a shaft was rarely more than two to three years.
Because of the poor ventilation, normally only four miners at a time worked underground. A miner at each of two 'headings' cut the clay with a tubil. Another miner in each heading barrowed the clay down the drive to the base of the shaft using a heavy-duty elm wheelbarrow, typically carrying about 3 hundredweight (136 kg). The miners worked in what were, by modern standards, appallingly unsafe conditions - by candlelight and sometimes ankle deep in water. In many mines there was the ever-present threat of a sudden water inrush and the ignition of 'fire damp' methane gases. Nevertheless, records of serious accidents (as distinct from singed eyebrows!) are surprisingly rare.
The 'top-ganger' - who was in charge of the gang working in the mine - operated the crab at the top of the shaft. The buckets carrying about 7 hundredweight (317 kg) of mined clay or waste were hoisted by wire ropes connected via overhead pulleys to
steam engines or, later, DC electric motors in remote winch or drum houses that served several mines. Water wheels and turbines powered some Cornish pumps and hoists. The ganger controlled his rope hoist by means of a lever and long pull wire connected to a drum in the winch house, which he could engage against a continuously revolving shaft. Clay from the hoisted bucket was discharged into wagons. A 'top trammer' was responsible for the haulage of the wagons (by wire ropes from the winch house) up elevated timber ramps known as 'high backs' and the discharge of the clay and waste into separate heaps. The screeching of the ubiquitous wire ropes in their pulleys was the characteristic noise of the ball clay works.
Several companies established their own sawmills (replacing earlier saw pits) for the specialised cutting of the considerable quantities of mining timber that were required. One company even became involved in forestry. Skills such as those of the blacksmith (who forged the claycutters' tools), carpenter and rope splicer were essential in support of the claycutters. The claycutters' work - both in the pits and underground - was extremely arduous. Considerable physical strength was required to extract and man handle the clay, often in soaking wet conditions. A miner's pay depended on the tonnage produced by him and his gang (so-called 'piecework'), but the pay was generally good so that, for generations, sons followed their fathers down the mines. Strength of character, prodigious cyder consumption and powerful tug of war teams were characteristic of the claycutters - as was rheumatism!
E. Inclined Shaft Mines
By the end of the 19th century shafts in North Devon were being sunk with a steep incline of 75 degrees from the horizontal, enabling wagons to run underground and be hauled to the surface on rails in the shafts. The tunnels of the North Devon Clay Co. were lined with bricks made at the adjacent Marland brickworks.
F. Underground Mining and Opencast Workings
Figure 22: This photograph shows the inclined shafts at Peters Marland, North Devon c. 1900, showing head frames, mining wagons and stockpiles of clay adjacent to the three foot gauge railway.
In the early 1900's steam generated DC electricity was introduced. Rail-mounted wagons hauled by wire ropes and discharging from 'high backs' were being widely used in open pits.
In the 1920's and 1930's some of the clay companies experimented with new working techniques and equipment. Advances were made by The Devon & Courtenay Clay Co. in the technique of sinking shafts through waterlogged sands, using techniques developed in the trenches of the First World War. This permitted hitherto inaccessible reserves to be worked. Devon & Courtenay in South Devon and the Meeth Clay Company in North Devon drove the first primitive 'adit' mines to follow seams from the bottom of existing pits. Steel arches were introduced in Meeth to support the main drives. In 1929 The Newton Abbot Clays introduced the first steam excavator. Draglines were also introduced, potentially allowing the removal of greater thicknesses of
overburden.
In the early 1930's, Newton Abbot Clays, followed by Devon & Courtenay and The North Devon Clay Co. introduced the first (very heavy!) hand-held pneumatic spaders to replace the thirting spade in the cutting of clays in open pits. Newton Abbot Clays experimented with a china clay-type 'sky tip' for waste (using an inclined railway), and, in 1937, introduced the first 2-3cu yard Muir Hill dumpers in their open pits.
Although the benefits of draglines, mechanical excavators, dumpers, pneumatic spaders and adits would later prove to be enormous, the depressed state of the industry between the Wars meant that little progress had been made in their adoption by 1946. In that year a Board of Trade Inquiry reported that: '….it would be difficult to find any industry in this country where there has been so marked unawareness and such lack of initiative on the part of many producers to modern industrial change'. One of the Board of Trade's principal recommendations to increase productivity was to fit rubber tyres to the wheelbarrows used underground! The industry's problems were compounded by a shortage of manpower as a result of many able miners having been called up for military service, and the difficulty of recruiting
ex-servicemen into such a backward industry.
This situation was soon to change remarkably, so that by 1969 one of the clay companies, Watts, Blake, Bearne & Co., was to be a recipient of the Queen's Award for technical innovation in underground mining.
In the open pits, diesel powered face shovels, dumpers and lighter
pneumatic spaders became commonplace during the late 1940's and early 1950's, relieving the workers of the hardest physical labour. Newton Abbot Clays - which had the problem of bringing clay out of constricted, steep-sided pits - installed 'Blondin' suspended wire rope systems to lift the clay out, but the company suffered a disaster in
Figure 23: Extracting clay from an open pit in North Devon in the 1930s. The traditional system of cutting clay into cubes is still being used, but pneumatic spaders have been introduced to replace thirting irons for making vertical cuts. Figure 24: Ruston Bucyrus 19‐RB face shovel loading a Dennis Pax lorry from a sockpile at White Pit, Preston Manor works, Kingsteignton, c. 1950.
1961 when one of its pits flooded, and the Blondins were subsequently replaced by dumpers.
In the 1960's bucket wheel excavators were introduced but were phased out in favour of hydraulic excavators that had the power and accuracy, in the hands of an experienced driver, to extract the most plastic seams to within a few inches of the bottom of the seam - minimising waste.
Underground, the lighter (but still heavy!) pneumatic spaders also became standard in the late 1940's, largely replacing the tubil.
G. Adit Mining
'Adit' or 'Inclined Tunnel' mining, which had been introduced in the 1930's, largely replaced vertical shaft mining during the 1950's and 1960's. Pairs of tunnels in South Devon and single tunnels in Dorset were driven into the ground at a shallow angle following a particular seam from close to its outcrop to a considerable depth. A retreat method of working was then followed. Rails were laid in the drives so that wagons winched from a surface gantry could be used to haul the clay underground - replacing wheelbarrows - up to the surface.
To maintain ground support over the years and in view of the increased tunnel size, the main drives were supported either by squared timbers or by steel arches or rings with timber backing boards. During the 1960's, leading up to the Queen's Award for technical innovation, the adit system was greatly improved with the introduction in South Devon of hydraulic mining machines (that greatly reduced the use of pneumatic spaders), of submersible pumps and of steel wagons with automatic tipping at the surface.
The longest adit drives extended underground for over half a mile (0.9 kms) to a depth of up to 450ft (137 metres). Individual adits in South Devon sustained an annual production of up to 25,000 tonnes for much of their 25-year lives.
The inherent greater safety of twin adit tunnels compared with a single mine shaft was augmented by the implementation of the 1954 Mines and
Quarries Act, the banning of smoking and naked lights underground (ending the miners' traditional candle-heated
Figure 25: Open pit mining at WBB's Southacre pit, Preston Manor, Kingsteignton, c. 1960, using pneumatic spaders and 3 cu. Yard Muir Hill dumpers running on timber railway sleeper roads. Figure 26: WBB mining machine operated by Ivor Basset in an adit tunnel supported by steel arches. Figure 27: WBB mining machine operated by Ivor Basset in an adit tunnel supported by steel arches. The machine had a cutting boom with rotating knives and a loading conveyor feeding finely cut clay into a wagon in the foreground, c. 1967.
fried breakfast) and the training of mines rescue and first aid teams.
H. Underground Mining Gives Way to Opencast Working
Due to its labour- and timber-intensive nature and the high cost of safety measures, underground mining was very expensive. As the mechanisation of opencast working progressed, underground mining became increasingly uneconomic, except to extract seams of the highest value that lay too deep for current opencast workings to reach.
Having been the principal means of working for most of the industry's history, underground production was completely replaced by opencast working in North Devon by the mid 1970's and reduced steadily in South Devon and Dorset during the 1980's and 90's when the company sawmills were closed, until the last ball clay mines closed in 1999. Nowadays all production is from progressively larger and deeper pits using powerful hydraulic excavators and dumpers.
Figure 28: Systematic opencast working in Westbeare Pit, North Devon in 1989 using hydraulic excavators and articulated dumpers running on movable concrete sleeper roads. The clay seams are being individually selected and taken by dumper to separate lump clay storage bays.
Processing
Traditionally, ball clays were sold 'as dug' in lumps or 'balls' - the 'potters'' clays usually being 'weathered' for several months in outside heaps. The clay producers did some very crude selection and mixing of what they perceived to be good and bad examples of individual clay types. The potters often bought in a variety of clays and mixed them together to their 'secret' formulae to make the pottery body they wanted.
This procedure continued virtually unchanged until the widespread adoption in the 1950's of the 'shredding' of clay into small pieces - a process first introduced in the1930's that originally used mobile turnip cutting machines (see photograph, left). Shredding makes handling much easier and, most importantly, enables the clay
producers to blend together up to 20 or more different seams of clay, often from different production areas. This has helped them to compensate for the natural variation in individual seams and to produce blends that are consistent and meet their customers' specifications, especially for faster casting and faster firing.
The development of powdering during the 1950's facilitated the sale of ball clays into non-ceramic applications such as rubber, fertilisers and animal feeds. Shredded clay is fed into an 'Atritor' mill together with hot air. The mill contains rotating shafts with pegs that break up the clay pieces. The hot air stream dries the feed clay from its natural 15-18% moisture down to a 2% moisture powder. The product is
passed through an air classifier to remove any coarse particles and is then either bagged in paper sacks or delivered in bulk powder tankers.
The desire to produce controlled products for particular applications whilst optimising the use of marginal clays led in the 1970's to the development of ball clay refining using automated computer-controlled process equipment. Clays with too much lignite are made into a wet slurry and the excess lignite particles are removed by fine screens (sieves); clays with too much quartz sand are powdered and the excess silica removed by air separation. The resultant refined slurry and powder are mixed together into a paste, extruded in 'noodles' and dried for bulk handling. The product is also sold in a liquid or 'slurry' form.
For many years ball clay companies produced 'prepared bodies'. These are the various mixtures of minerals (such as ball and china clays, silica, feldspar etc.) that normally the potter prepares and then shapes, decorates and fires. WBB also produced 'calcined' clays - pelletised ball clays fired in a
rotary kiln and then ground down and incorporated in a pottery body. Having already been fired, calcined clays reduce the expansion and contraction of the body when it is fired as a finished ceramic.
At the beginning of the 21st century, about 75% of ball clay production is sold in shredded and blended form, almost 10% in powdered form and over 15% is refined. Less than 1% is sold 'as dug'. Process control has become an essential skill of ball clay production. Employment Relationships Figure 30: Filling 50 kg‐paper sacks with clay that has been dried and pulverised to a fine powder, Figure 31: The East Golds processing site at Newton Abbot in 2000 showing ball clay refining plants and product storage in the foreground, powdering plant at upper centre and a lignite processing plant for horticultural applications on the upper right.
Enormous changes in working methods occurred in the ball clay industry in the second half of the 20th century - especially during the 1960's. Highly regarded 'productivity agreements' provided for these changes to be accompanied by the adoption of progressive employment practices, for which the companies became well known. These included the replacement of piecework and overtime pay systems by 'staff' conditions of employment for all employees, with fixed rates of pay and hours of work, pensions and sick pay. They were combined with systematic training and great emphasis both on health and safety and on employee communication and
involvement. The success of these arrangements depended on the high degree of trust that developed between management and employees.
The development of these progressive employment practices created an environment in which the workforce was willing to respond positively to the changes taking place and to develop their skills through the training opportunities offered. As the physical demands on the workforce diminished, the roles of maintenance fitters and electricians became more important. Production increasingly required the close collaboration of multi-skilled teams of geologists, drillers, surveyors and quality control chemists working closely with process engineers, ceramists and technical sales personnel to assess how best to fulfil customers' needs with the complex sequence of clay seams in the ground and use of the appropriate processing facilities.
Ball Clay Transport
The history of ball clay is bound up with the development of canals, railways and shipping which have all played a vital part in transporting clays economically to customers.
Packhorses and, later, horses and carts were the only methods of getting clay away from the pits until the advent of railways and lorries. However, it was only practical to use packhorses and carts over short distances. Fortunately, each of the ball clay deposits is reasonably close to a port from which the clay could be transported by vessel to such ports as London,
Bristol and Liverpool and Runcorn on the Mersey (for the Staffordshire potteries via the Bridgewater Canal and Josiah Wedgwood's Trent & Mersey Grand Trunk Canal), as well as continental European ports.
Figure 32: An X‐ray diffractometer being used to indentify the minerals, such as kaolinite, illite and quartz, in a sample of ball clay. This and other chemical and physical tests are carried out to monitor the properties of each seam of clay Figure 33: Amos Hewings with J. Vallance's horse and cart at Teignbridge loaded with ball clay en route to Teignbridge clay cellars on the Stover Canal, 1906.
In South Devon the port for ball clay was Teignmouth. The first recorded shipment from Teignmouth was to Plymouth in 1691, shortly after William of Orange was proclaimed king in Newton Abbot in 1688. Originally, packhorses carried the clay from the pits to Hackney quay at the head of the Teign estuary where it was either loaded directly into small sailing vessels or taken by barge to larger sailing vessels moored in Teignmouth harbour.
The high cost of the packhorse journey was greatly reduced by the construction of the Stover Canal from the Teign estuary to Teigngrace by James Templer II in 1790-1792 and of the Hackney Canal from Hackney quay into Kingsteignton for Lord Clifford in 1843. Clay cellars were built on both canals for the storage of clay prior to shipment. At
about this time the Earl of Devon's 'Devon Wharf' (now the 'Town Quay') in Newton Abbot started to be used to load barges with clay from his newly opened Decoy pits. The barges had characteristic square 'Viking' sails but were latterly towed in the estuary by the steam tug 'Kestrel' and finally by the paraffin engine tug 'Heron'.
The construction of the Moretonhampstead branch railway line in 1867, with sidings at Teignbridge and East Golds, enabled the railway to be used increasingly to supply domestic customers. However, the introduction of lorries, enabling clay to be carried far more economically than by cart and barge, led to use of the canals ceasing in the 1930's and of the railway in the 1980's.
The main outlets for ball clay in North Devon were Bideford Quay and, for a period, Fremington Quay on the Taw estuary. The first recorded shipments of tobacco pipe clay from Bideford were in the 1650's. However, after the initial century or more of activity, the high cost of the packhorse journey from the pits at Peters Marland (relative to transport costs in South Devon and Dorset) seems to have led to the closure of the works in the early 19th century. The situation was improved by the construction in 1827 of Lord John Rolle's Canal up to Torrington. It was transformed in 1881 when the eminent railway engineer, J.B. Fell, commissioned by Marland Brick & Clay Works Ltd., completed a 3-foot (91cm) gauge light railway from Peters Marland to Torrington - including a remarkable wooden viaduct over the River Torridge.
Figure 34: Barges at the Teignbridge clay cellars on the Stover Canal waiting to be moved to Teignmouth, c. 1920.
Figure 35: Falke loading clay from barges at Teignmouth in the 1920s
Figure 36: Three foot gauge North Devon railway at Peters Marland in the 1920s, alongside the headgear of inclined shafts. The Fletcher Jennings locomotive came from a breakwater scheme at St Helier in 1908
Eventually, in 1925, a standard gauge railway was built along the course of the narrow gauge one, enabling clays from both Peters Marland and Meeth to be shipped
out of the rail-connected Fremington Quay until the closure of the line to clay traffic in 1982. Now, once again, Bideford Quay is regularly used for clay shipments.
The harbour at Poole was used for the shipment of ball clays from Dorset. Until the 19th century the clay was carried by packhorse or cart to Wareham Quay on the river Frome or a loading point on the edge of Poole Harbour. From there it was barged to Poole where it was transferred to sea-going vessels for shipment to Runcorn, London and other ports. In 1805-6 Benjamin Fayle built the first railway in Dorset: a pioneering cast iron 'plateway' along which horses hauled wagons of clay from his pits at Norden to Middlebere Pier. In 1907 'Fayle's Tramway', a 3 foot 9 inch (114 cm) gauge railway from Newton to Goathorn Pier using steam locomotives, was extended to Norden and the plateway was closed. The other producers, Pike Brothers, operated a 2 foot 8 inch (81 cm) gauge railway from Furzebrook to Wareham Quay. From its arrival in 1884 the main line LSWR line was used to transport clay to domestic customers. The private narrow gauge railways continued until 1954 when the two companies were amalgamated and transferred all local movements to road transport.
Nowadays ball clay for European and Mediterranean markets is generally hauled by lorry to the ports of Teignmouth, Bideford or Poole, whilst clay for other parts of the world is generally shipped in containers that are filled at the clay works and transported by lorry to deep-sea container ports such as Southampton, Felixstowe and Thamesport.
Ball Clay and the Environment
Old ball clay workings often show an astonishing biodiversity that is far greater than existed on the farmland before the ball clay working began. When - in the past - the old, shallow
Figure 37: Poole Quays, clay being loaded into vessels for the Mersey and other ports. Late 19th century. Figure 38: Clay being loaded into the hold of a vessel at Teignmouth c. 1985. The lorry tips the clay onto an enclosed conveyor to minimise dust. For many years Teignmouth has been the major port for ball clay shipments.
workings were finished the land was either restored to farmland or it was abandoned. In the latter case the pits themselves quickly filled with water and the surrounding areas soon re-vegetated. This resulted in the creation of havens for many species of increasingly rare flora and fauna - notably dragonflies and butterflies. Several of the old workings have become nature reserves managed by the Devon Wildlife Trust and have been the subject of TV films by Peter Scott and Andrew Cooper. Others have become ponds and lakes for amenity, coarse fishing and boating. The best examples are the famous 'Blue Pool' near Wareham and Decoy Lake in Newton Abbot, which also have nature trails open to visitors.
Since the merging of the 15 ball clay companies into two by the end of the 1960's, workings have been rationalised and fewer, much larger, pits have been developed. The industry was amongst the first to take measures to mitigate the inevitable impact that its workings have on the environment, and tip design and landscaping continue to improve within the framework of long term plans agreed with the local planning authorities.
In far-sighted recognition of ball clay's national importance and scarcity, special planning safeguards against the sterilisation of ball clay reserves by housing and other development have been in place since 1953. However the balance to be struck between the interests of working an internationally rare and useful mineral where it lies and preserving the existing local e
n
vironment has become an increasingly prominent issue, especially in Dorset, where the deposits lie in areas with some of the highest landscape and nature designations.
With annual ball clay production at the beginning of the 21st century running at over one million tonnes, and with the enormous deposits, particularly in the Bovey Basin, able to sustain production for at least another 50 years, this debate is set to continue.
The Widespread Use of Ball Clay
Earliest Uses Figure 39: The Blue Pool at Furzebrook, near Wareham a tourist attraction for over 60 years based on a ball clay pit started by Pike Brothers c.1840. Image courtesy Miss Jenner Barnard Figure 40: Visit by pupils of All Saints Marsh Primary School, Newton Abbot to WBB's Preston Manor Works, Kingsteignton in Industry Year, 1986
It is generally accepted that Dorset ball clays - and probably Devon ball clays too - have been used since Roman times to make crude pottery. However it was the introduction of tobacco to England in the 16th century by Sir Walter Raleigh and the need for a suitable clay with which to make tobacco pipes that led to the start of the modern ball clay trade.
Although the highly plastic ball clays were ideal for tobacco pipe manufacture, their expansion and contraction during firing made them difficult to control in tableware manufacture. Most pottery was made with easy-to-use local coloured clays. By the 17th century it was common for jugs, bowls and other tableware made with these clays to be covered with either a thick white glaze (as in Delft ware) or a white clay slip coating. From at least the 1650's potters in Bideford were using a white slip of North Devon ball clay and scratching designs through the white slip, exposing the coloured body beneath. Shipments of Dorset tobacco pipe clays from Poole were significant by the 1630's and were the port's most important cargo for most of the 17th and 18th centuries, especially to London and many south coast ports. By 1662 the trade had become sufficiently important for an Act to be passed forbidding the export of pipe clays to foreign countries. Shipments of North Devon clay through Bideford were also important in this period, especially to pipe manufacturers in Bristol, but shipments of South Devon clays seem to have been relatively small until the middle of the 18th century.
Important 18th Century Developments
Whilst the Chinese learnt how to make fine white porcelain many centuries ago, it was only in the 18th century that European potters learnt how to make good quality white-bodied pottery. They had to overcame the difficulties of using white firing plastic 'tobacco pipe' clays, and had to both discover and learn how to use china clays with little plasticity.
It was the achievements in this area by the famous early potters in Stoke-on-Trent such as Wedgwood, Astbury and Spode that caused the demand for ball clays to take off - along with the demand for china clays. They all needed ball clays from Devon and Dorset - as well as china clays from Cornwall and Devon - to make their fine 'cream wares', 'Queen's Ware' and so on.
A typical recipe for such pottery could have included equal quantities of ball clay, china clay, flint (a form of silica) and Cornish Stone (a source of feldspar). Between 1765 and 1785 - at the same time as the industrial revolution in the manufacture of pottery and the associated 'canal mania' - the annual shipments from South Devon quadrupled to almost 10,000 tons.
Coade Stone
One of the lesser-known early applications of ball clay was in the production of a high-grade ceramic known as Coade Stone. This was first produced in London in 1770
Figure 41: Josiah Wedgwood's most famous achievement in 'Queens Ware' was the 952 piece dinner and dessert service with 'Frog' crests made for the Empress Catherine the Great of Russia in 1774. [Image courtesy of the Trustees of the Wedgwood Museum, Barlaston, Staffordshire (England).]
by a Mrs Coade from Lyme Regis. It was an architectural ceramic of high artistic and technical quality that has been found to be an exceptionally durable, artificial 'stone' for building decoration and statuary. Examples include friezes on Buckingham Palace, fan vaulting in St George's Chapel, Windsor and the Lion Statue on Waterloo Bridge. A recent detailed scientific analysis of the 'stone' has confirmed that ball clays from Devon or Dorset were the major component, together with pre-fired clay. Mrs Coade died in 1825 and production had ceased by about 1840.
3. Fireclay is basically kaolinite with some iron oxides, magnesia, and alkalies. It can resist high temperatures. Because of the abundant supply of fireclay and its comparative cheapness, the refractory bricks made out of it are the most common and extensively used in all places of heat generation.
Fireclays are sedimentary mudstones that occur as the ‘seatearths’ that underlie almost all coal seams. Seatearths represent the fossil soils on which coal-forming vegetation once grew and are distinguished from associated sediments by the presence of rootlets and the absence of bedding. Fireclays are, therefore, mainly confined to coal-bearing strata and are commonly named after the overlying coal seam. The term ‘fireclay’ was derived from their ability to resist heat and their original use in the manufacture of refractories for lining furnaces. Today the term ‘fireclay’ is used to describe seatearths that are of economic interest, irrespective of their refractory properties. They are mainly used in the manufacture of structural clay products, principally high-quality facing bricks. Fireclays are typically thin (normally <1 m, although rarely >3m) and are composed of the clay minerals kaolinite and hydrous mica (illite), together with fine-grained quartz in varying proportions; kaolinite is the key component. Typically these three minerals make up some 90% of the rock and their relative proportions, together with the amount of and type of impurities present (carbon, sulfur and iron), greatly influences their ceramic properties. Fireclays are similar in basic composition to ball clays. However, because of their much greater geological age they are not as plastic as ball clays and they are also not as light-firing, because of a higher iron content. These are two principal properties for which ball clays are valued.
Applications
Fire clay is a type of clay which is used in the production of heat resistant clay items, such as the crucibles used in metals manufacturing. This type of clay is commonly mined from areas around coal mines, although other natural deposits are also available as potential sources, with many nations having deposits of clays suitable for use in high temperature applications. Fire clay can also be refined and treated to make it suitable for specialty applications.
This type of clay has a very high fusion and melting point. Once it is worked and fired, it will hold up to extremely high temperatures such as those found in kilns, furnaces, and retorts, in addition to the high temperatures of some production lines. Fire clay can also be used to create fire resistant chimney and flue liners, and fire resistant pads for safety, as seen when a hearth in front of a fireplace is made with fire
clay to reduce the risk of fire. In many settings where there are concerns about fire, fire clay can be used.
This clay contains high percentages of alumina and silica, with minimal amounts of trace impurities. It tends to be pale to creamy yellow in color, due to the balance of minerals in the clay, although it can also be colored for various applications. Although fire clay will grow sooty with use, underneath the layer of soot, the clay will remain intact. Damage to fire clay can occur when it sustains trauma or when temperatures climb outside its safety range.
Numerous companies make bricks and other products out of fire clay. They specify the tolerances of their products so that people know which settings they are appropriate for, and to help potential consumers make the best purchasing choices. These companies can also make custom products by request for people who need objects of a particular shape and size. For example, a metalworker might need a custom crucible for a project.
Like other clays, this clay is highly malleable in raw form. It can be molded, extruded, shaped by hand, and stamped. Various additives can be mixed in to make it more coarse, and it can be ground to be smoother. Slip and scraps for fire clay manufacturing can also be recycled, as long as they are not fired, and worked into new batches of clay for use. Fire clay also shrinks after it has been molded and during the firing process, which is something to be aware of when working with raw clay.
Figure 43: Fireclay Bricks Extraction and Processing
Fireclay extraction is not normally commercially viable on its own and almost all production (except that recovered from existing stockpiles) is as a co- or by-product of opencast coal production. However, only a small proportion of opencast coal sites will have associated fireclay recovery. Fireclays (except that beneath the lowest excavated coal) form part of the over-burden in opencast coal operations and have to be removed whether they are marketed or not. Where fireclay is recovered for sale it must be worked carefully to ensure there is no contamination with associated rocks. Under favorable conditions fireclay can be worked down to bed thicknesses of 0.3 m.
Fireclays are then normally stocked by seam, although subsequent blending with other clays (both fireclays and other brick clays) to provide a range of properties, as well as feedstock with consistent and predictable properties, is normal commercial practice. For this reason it is highly desirable that a range of clays are available to the brick maker so that bricks with different colours and textures can be produced.
Fireclays undergo processing other than blending.
Transport
All fireclay is transported by road to brick/pipe manufacturing plants. Because of the higher intrinsic value of fireclay compared with most other brickmaking raw materials, it is sometimes transported considerable distances with up to 120-140 km being reported. Transport is thus an important element in the delivered price of fireclay. The ephemeral nature of supply from most sites means that there is little alternative to road. However, it is desirable, on economic and environmental grounds, that fireclay is sourced from operations that are in close proximity to manufacturing plants.
Manufacturing Process of Fireclay Bricks
Manufacturing of refractory bricks from fire-clay is an interesting feature. The clay mined is stacked in the factory yard and allowed to weather for about a year. For daily production of different types of refractories, this weathered clay is taken and mixed in different percentages with grog.
The mixture is sent to the grinding mill from where it is transferred to the pug mill. In the pug mill a suitable proportion of water is added so as to give it proper plasticity. The mould is supplied to different machines for making standard bricks or shapes. Intricate shapes are made by hand. The bricks thus made are then dried in hot floor driers and after drying they are loaded in kilns for firing. The firing ranges are, of course, different for different grades of refractories. After firing, the kilns are allowed to cool; then the bricks are unloaded. By burning fireclay is converted into a stone-like material, highly resistant to acid, water and most other solutions. While manufacturing high aluminous fire-bricks bauxite is added along with grog in suitable proportions.
4. Common clay contains more impurities than fire clay, and does not have as great resistance to heat. Common clay is a mixture of kaolin, or china clay (hydrated clay), and the fine powder of some feldspathic mineral that is anhydrous (without water) and not decomposed.
Figure 44: Common Clay
Common clays are usually mixtures of clay minerals such as illite, smectite, and kaolinite, together with fine silica and other minor constituents. Interlayered clay minerals also frequently occur. Formations such as the Oxford Clay or the London Clay typify this type of clay. Although widespread, these clay formations are important economically, for they provide the basic material for brickmaking and for heavy clay products such as sewer pipes and clay floor tiles.
Common clays occur in a variety of environments and in many different rock types across all time periods of the geologic record. The source material includes glacial clay, soils, alluvium, loess, shale, weathered and fresh schist, slate, and argillite. Fireclay and kaolin are sometimes considered common clays, particularly when used in the manufacture of structural clay products. Mineralogically, common clays are highly varied, although the most common constituent is usually one of the members of the mica mineral group. Mica clays include illite, sericite, muscovite, and biotite. Other frequently occurring clay mineral components of common clays are kaolinite, smectite, mixed-layer clays, and chlorite. Quartz and other detrital minerals are typically nonclay minerals present in rock mined for common clay.
Production and Markets
Common clays are widely distributed, usually easily located, and are often used in products that do not require elaborate processing. Typically, common clays and shales are dug from open pits, and these pits must be near the processing plants to minimize production costs. Usually both the raw material and the finished products are
heavy and the profit margin is low, so production costs must be controlled. Most products made from these materials are processed and marketed in a similar manner to refractory clays. Common clays and shale require little beneficiation.
Figure 45: Process flow sheet for structural clay products.
Typically they are crushed or ground only before pugging and extrusion. Physical contaminants such as concretions are removed by dry screening. Beneficiation for clays needed in ball clay or kaolin applications may occur in the form of drying or air flotation. Clays used in refractories are often blended to meet product standards.
5. Bentonite consists largely of montmorillonite. Some types that contain sodium swell when mixed with water. Bentonite beds usually form from altered volcanic ash, but other types of rock may also serve as sources. They are valued for use in various applications such as drilling mud, iron ore pelletizing and foundry use, and in civil engineering, as well as for clarifying liquids used in the food and drink industry.
Bentonite is very unusual in the fact that once it becomes hydrated, the electrical and molecular components of the clay rapidly change and produce an "electrical charge". Its highest power lies in the ability to absorb toxins, impurities, heavy metals and other internal contaminants. Bentonite clay's structure assists it in attracting and soaking up poisons on its exterior wall and then slowly draw them into the interior center of the clay where it is held in a sort of repository. To state it another way…" Bentonite is a swelling clay. When it becomes mixed with water it rapidly swells open like a highly porous sponge. From here the toxins are drawn into the sponge through electrical attraction and once there, they are bound.
The term bentonite was first proposed by Knight in 1898 after he head originally named this clay taylorite after the site of the original mine at the Taylor ranch near Rock River, Wyoming. The name bentonite is from the Benton Shale, the formation within which the clay was thought to have occurred. The Benton Shale is named after Fort Benton, Montana, located more than 640 km to the north.
Bentonites are found in both marine and nonmarine environments ranging in age from Jurassic to Pleistocene. These beds can be very extensive geographically, range in thickness from several centimeters to tens of meters, and are usually parallel with the over- and underlying strata. Bentonites also occur as small, lens-shaped deposits, Other less common types of bentonite deposits are those that grade into unaltered host rock.
Bentonites are found in a range of colors, the most common being gray, yellow, olive green, brown, gray-blue, and white. Bentonites have a characteristic soapy texture and waxy appearance. Sodium bentonites often display a typical “popcorn” texture on weathered outcrops. Weathered calcium bentonites have an “alligator skin” texture. History and Use
Most of today’s major uses of bentonite were developed in the 20th century. In the United States bentonite finds major uses in iron ore pelletizing, drilling muds, pet litters, and foundry sands.
Figure 47: Bentonite Clay
Figure 48: Application of Bentonite Clay on the Face to Prevent Acnes
Overall, global consumption follows similar trends to U. S. consumption, with the exception that use in pet litters is less, especially in developing countries. Significant secondary uses include water-proofing and sealing applications, animal feed additives, oil and grease absorbents, agricultural carriers, and filtering, clarifying, and decolorizing agents. Still, smaller specialty uses of bentonite include asphalt emulsions, catalysts, paints, plastics, inks, greases, cosmetics, and pharmaceuticals.
In the United States, mining of natural sodium bentonite began in 1888 on the Taylor Ranch near Rock River, Wyoming, and in 1903 a mine was opened in Upton, Wyoming, within the bentonite beds surrounding the Black Hills.
Significant use of bentonite as a sand bonding agent in foundries and as a drilling mud in the oil industry began in the late 1920s. The suspension and fluid loss control properties of western bentonites make them especially suitable for water-based drilling applications. As a bonding agent in greensand molding, bentonite has the advantage of producing good compressive strengths when wet or dry. The natural sodium bentonites from the western United States also perform well at the elevated temperatures encountered during the pouring of molten iron and steel, resulting in good clay utilization and fewer casting defects.
Figure 49: Bentonite clay is used for foundry sand binders
Extraction and Manufacture
Bentonite is usually quarry mined from deposits that can range anywhere from 100 feet to several thousand feet. This depends on the health and vitality of the land it is processed from and how far a producer will go to find the right clay with the proper characteristics and consistency.
The most common method of mining bentonite is the open pit method. This involves removing overlying material to expose the desired commodity, in this case bentonite. Bulldozers, scrapers, and excavators, and often a combination of these types of equipment, remove the overburden. In a typical bentonite mine the topsoil and subsoil
