The Quarrying - Part 1 Many products of our daily life are associated with china clay. Certainly one thinks primarily of porcelain, the "white gold", the raw material for quality tableware and art products.. But china clay is also used in many industries e.g. as filler in paper production, as pigments and brighteners in the plastics, paint and rubber industry. Special types of china clay or specially treated clays are also used in the cosmetic and pharmaceutical industry. The main component of china clay is kaolinite, a clay mineral, which usually forms very thin, small flocky crystals. Commonly china clay occurs in loose or dense fine flaky and earthy aggregates. Usually it is white, but impurities change its colour and then it tends to reddish, brownish or bluish. Only in the 18th century the name kaolin was used in Europe. It is derived from the Chinese village named "Gao ling," which means "white hill". For a long time over here the raw material was known as Weißton or Passauer Erde. AUTHOR: GERHARD ZIMA, FOTOS: IMERYS Article from: inside acat 5/2007, page 1 of 5
China clay is obtained in opencast mining. It is washed out of the rock with water cannons under high pressure. The largest deposits of china clay are on tectonic fault lines. Some 300 million years ago large land masses collided and huge masses of rock escaped from the interior of the earth. Granite was formed from the masses after cooling. Granite consists of feldspar, quartz and mica. Under the influence of water and carbonic acid feldspar is transformed to china clay. Depending on the place of its origin and its deposition, a distinction is made between primary (place of formation and deposition are the same) and secondary (china clay was deposited on another site) deposits. Noteworthy deposits are in England, the USA, Germany, Japan, China and India. In Austria china clay is mined in Upper Austrian in Tragwein and in Lower Austria in Aspang. The worldwide annual production of china clay is about 50 million tons. Based on this data the global reserves are sufficient for about 300 years. China clay is obtained in opencast mining. First the humus and the dead rock are removed, which are backfilled to the abandoned mines and afterwards the area is recultivated. Depending on the kind of soil, the rock is blasted or loosened by bulldozers with fangs. Following that, the china clay is washed out from the rock with water cannons under high pressure. The amount of china clay is only about ten per cent; the rest consists of rocks, sand and mica. AUTHOR: GERHARD ZIMA, FOTOS: IMERYS Article from: inside acat 5/2007, page 2 of 5
is moved to the centre and the slurry is added at the centre of the surface. The clear water runs over the edges and is pumped back to the mine. A faster deposition is achieved by additives. But china clay can also be kept in continuous suspension to be separated from accessory minerals. The purified china clay is stored in tanks. To achieve the wanted specification different qualities are mixed. To increase the yield of china clay the coarser particles are ground At the lowest point of the mine the washed china clay comes together with the fine fractions of sand and mica and is pumped to the sand separation. The separation is done with a spiral screen, the sand settles and is discharged with a spiral. The fine particles remain in suspension and run over on the lower side. There the sand is applied across a large surface for drying. A part is classified and used for the construction materials industry. Before the finer particles are pumped for further processing in pipelines to the stationary systems they are separated from coarser particles (mainly mica) using hydro cyclones. The first step is the concentration in so-called thickeners, which are large round containers with a downward slope and an outlet. By slowly rotating screens the sediment material AUTHOR: GERHARD ZIMA, FOTOS: IMERYS Article from: inside acat 5/2007, page 3 of 5
and recycled. For special applications the preprocessed kaolin is ground. The ratio of length to thickness of the lamellae is reduced, which has an impact on the gloss, which is important for example in paper coating. Iron-containing minerals are removed with extremely strong electromagnets. By bleaching with sodium hydrosulphite the insoluble iron compounds are converted to watersoluble iron sulphate, which is removed with the water in the dewatering. To increase whiteness the china clay is treated with ozone to destroy organic impurities. For extremely fine kaolin types the oversize material is separated by centrifuge. Abrasive minerals can be separated by a multi-stage flotation. In this process an For extremely fine kaolin types the oversize material is separated by centrifuge additive is added, which helps the china clay particles to be fixed to the air bubbles of the injected air. The finished, refined and in accordance with the specifications tested china clay slurries are further concentrated in thickeners. Afterwards the water obtained is removed with filter presses. The water content of the filter cake is about 30 percent. With the alternatively used tube presses, which operate with higher pressure a water content of less than 20 percent can be achieved. Before drying in rotary tube dryers the filter cake is crushed. To prevent the formation of lumps in the dryer some of the already dried material is added. As the drying is a very energy-intensive process efficient tray dryers or fluidized bed dryers are used. Partly the hot air is generated by gas and oil burners, partly it is recovered from the exhaust air of the gas turbines from the own power generation. A large part of the china clay goes on sale with a moisture content between eight and twelve percent, but as not all consumers can use this quality, part of the china clay is further dried. This drying is carried out with hot air in mills which are equipped with rotors to crush the clay pieces. The agglomerates are just broken and not further crushed. The distribution of the granule is the same as in liquid preparation. AUTHOR: GERHARD ZIMA, FOTOS: IMERYS Article from: inside acat 5/2007, page 4 of 5
China clay mining sites are returned to nature Finally, china clay is packed for sale in paper bags of 25 kg, big bags of 500 or 1,000 kilos or loose in silos. Moreover, for special applications china clay can be also used in liquid form the so-called "slurry". When china clay is heated up to approximately 450 C, the crystal water is split off and the crystal structure changes. If the temperature increases further, further transformations take place. With the change of crystal structure the properties of the china clay change. For applications in plastics, paints and rubber china clay it is calcined at about 1,100 C, for applications in ceramics it has to be heated up to 1500 C. After cooling the briquettes have to be crushed and milled again. A special form of calcination is the so-called "flash calcining In this process the china clay is heated up very rapidly. Causing small hollows and edges to be formed within the china clay particles, which causes a higher opacity in coating applications. If the china clay mines are exploited, they are re-filled with rocks, sand and mica, covered with humus and re-cultivated. By the time the mining area is transformed into a new landscape that can be used for agriculture as well as for recreational activities like sports field or playground or it is simply returned to nature. In our next issue we want to continue with the applications of china clay AUTHOR: GERHARD ZIMA, FOTOS: IMERYS Article from: inside acat 5/2007, page 5 of 5