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  • C04B35/63—Preparing or treating the powders individually or as batches ; preparing or treating macroscopic reinforcing agents for ceramic products, e.g. fibres; mechanical aspects section B using additives specially adapted for forming the products, e.g.. binder binders
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Definitions

• This invention relates to a method for manufacturing a ceramic article and to an article formed thereby.
• the invention is particularly, though not exclusively, capable of providing a product for the manufacture of so called ceramic "sanitaryware" articles, which are items such as sinks, baths, shower trays, shower enclosures, lavatories, WC cisterns and other whiteware.
• the first stage in the process is to prepare a body slip, which is a dispersion of largely inorganic mineral materials in water.
• This is normally a two-stage operation as the materials have conflicting requirements.
• Ball clays need high shear treatment to achieve full dispersion. Therefore a fast-running mixer (blunger), with a rotor/stator action, is generally used.
• This stage can be avoided by the use of "slurry" clays, such as china clays and more commonly, slurry ball clays that are supplied ready dispersed in water. High-shear treatment of china clays degrades their properties, so a gentler dispersion action is used at this stage. Fluxes and silica can also be dispersed in this operation.
• Sanitaryware is produced by some form of slip casting.
• slip is poured into a plaster of Paris mould to form the shape of the article.
• the plaster draws water from the slip by capillary action, causing a layer of solids to be deposited on the face of the mould as the water is removed.
• the rheology of the overall body composition for casting must be readily controllable and stable, that is consistent over a reasonable period of time.
• Slip is allowed to stand in the mould until the required thickness of cast (typically 8 mm) has been formed. Excess slip is then drained from the mould. The cast is then left undisturbed in the mould for a period during which further water is drawn from the cast and it becomes firm enough to handle. The cast also contracts slightly during this phase enabling it to be released from the mould.
• required thickness of cast typically 8 mm
• edges of pieces are trimmed and surface pieces are smoothed. Smoothing may be done when the pieces are dry by a sanding operation, but in some jurisdictions this is not permitted as large amounts of potentially inhalable dust (which must be controlled within prescribed legislative limits) can be generated. Alternatively surfaces are sponged smooth whilst still wet. This requires a greater degree of skill, but is held to give superior finish.
• Unevenness in drying can give rise to stresses that can produce cracks in the pieces if excessive. This arises principally during the first of two distinct phases.
• water fills all of the pore space in the cast and separates solid particles.
• the cast is still quite soft and any reduction in moisture content results in shrinkage as the particles are drawn closer together.
• the moisture content is reduced to around 16% -18% (the critical moisture content)
• solid particles are no longer separated and the cast becomes rigid.
• the glaze slip is applied to the unfired body using atomising spray guns. Normally this produces a fired layer about 0.4 mm thick. Thicker coats of up to 1 mm fired thickness can be used to cover minor defects and body colour.
• spraying has been a skilled manual task, with a sprayer taking about a minute to process a single piece, giving it two coats of glaze. Sometimes robot sprayers are employed to coat glaze.
• the maximum temperature for most kilns is 1200 - 1250 °C.
• One exploding piece can destroy 20-30 others through mechanical damage or gross surface contamination by debris.
• Fired pieces have to be inspected and graded immediately after leaving a kiln as some faults can make the pieces dangerous to handle. Typically around 5% of the pieces are scrapped at this stage and around 20-30% require some form of repair.
• Dunts are cracks developing after the body has fused on the cooling part, known as downward part of the firing schedule. They can develop from clay cracks or occur on cooling due to volume changes associated phase changes present in one of the mineral components (e.g. quartz). The glaze edges of these features are extremely sharp and an affected piece can collapse when handled, making this a very dangerous fault for handlers.
• Warpage can occur due to poor handling when wet and soft or due to uneven drying. Some degree of deformation always occurs due to pyroplasticity; this has to be taken into account when the piece is designed.
• Spangling is the term applied to the presence of numerous small pits in the glaze surface, caused by bubbles in the glaze surface.
• Blisters are caused by gas evolution if the body is over fired.
• a disadvantage with the aforementioned process is that it consumes vast quantities of energy in order to make, dry and fire the ceramic articles.
• Tunnel kilns are usually used in the mass production of ceramic articles. Tunnel kilns may be in excess of 50 metres long and consume Tera Joules every day. The temperature profile varies through the kiln in a controlled way in order to provide a controlled rate of heating and cooling. One reason why this is done is due to the thermal properties of the material: namely it cannot be fired (or cooled) too quickly otherwise articles could undergo thermal shocks and may fracture.
• Ceramic articles are loaded onto kiln-cars, which then pass through the kiln. Close control needs to be exercised over the firing process in terms of temperature and airflow. The vast majority of kilns are fired by natural gas.
• An aim of the present invention is to overcome the aforementioned problems.
• Another aim of the invention is to develop a vitreous china body or bodies that vitrifies at lower firing temperatures than current bodies.
• a further aim is to lower raw material costs.
• a method for manufacturing a ceramic article comprising the steps of: producing a slip by mixing at least 70%, by weight, of a mixture of filler and milled clay, with 30% by weight, or less, of fluxing agent; forming said slip into pieces; allowing the pieces to dewater, for example, by use of a porous mould material; and firing said pieces so as to produce finished ceramic articles, characterised in that the fluxing agent includes milled glass derived from recovered glass containers.
• Fillers may include materials such materials as quartz.
• milled glass may be mixed with another fluxing agent as an additive or bulking agent.
• Waste recycling companies have been seeking outlets for reclaimed glass for some time.
• Recycled container glass is quite consistent in terms of viscosity vs temperature behaviour, comparing well with good quality mineral raw materials, and it is relatively rich in alkali elements like sodium or potassium -. the elements that lower melting, firing temperatures.
• Tests have indicated that recovered glass can be used as a suitable substitute for, or additive to, expensive mineral fluxes eg feldspar or nepheline syenite, in the manufacture of ceramic articles. Therefore a cheap replacement to mineral is effectively provided.
• recovered glass flux reduces significantly the amount of energy required to fire the ceramic articles.
• ground waste glass can be blended with smaller amounts of existing fluxes such as nepheline syenite.
• bodies into which the clay and flux mix are formed are suitable to be slip cast so that they are compatible with current production moulds and production systems.
• any shrinkage of a body is similar to that of currently produced bodies.
• Most preferably any shrinkage of the body is less than + 1% variation of currently produced bodies.
• Firing deformation is advantageously as similar to, or possibly slightly less than, currently produced bodies. A limiting value for this is believed to be in the range 80 - 100% of current deformation.
• An unexpected benefit of the invention is that less hydrogen fluoride gas is emitted. Hydrogen fluoride is a harmful gas. Hydrogen fluoride, and other hydrogen halides, is emitted as a result of firing the clay. As the temperature is now lower, due to the fluxing agent, less hydrogen halides are emitted.
• a deflocculating agent may be used in order to assist and/or promote this mixing. Adjustment of rheology may be achieved by inclusion of acetic acid or by substitution of calcium, magnesium or aluminium sulphate, or other flocculating substances.
• the percentage weight of clay lies in the range between 50% to 60%, with a corresponding percentage weight of fluxing agent in the range from 30% to 5% and sand in the range of 10-35%.
• the fluxing agent includes crushed coloured glass, comprising any mixture of flint, green, brown and other coloured glass.
• the glass is preferably glass that has been milled or ground to an average particle size of 100 ⁇ m or less. Most preferably the average particle size is 75 ⁇ m or less.
• crushed glass An unexpected benefit of using crushed glass is that the ceramic article can be fired in less time than was previously possible. Thus for particular ceramic articles, such as pottery and chinaware a faster throughput is achievable. However, scheduling of other processes like casting will need to be changed.
• crushed glass as a fluxing agent has also been found to improve the control of the firing of the ceramic article, by making it more predictable.
• Another unexpected advantage with the invention therefore is that the previous practice of constantly having to reset and change firing times and conditions, due to variations in content of fluxing agent, is no longer required. This saves time and money and provides a more consistent quality in end product.
• a further advantage of the method of manufacturing a ceramic article is that it has been found that there is reduction in emissions of gasses such as Hydrogen Fluorides (HF), when the method is compared with existing manufacturing processes.
• the reduced emissions may be a corollary of firing at lower kiln temperatures and/or over a shorter firing cycle.
• a mixture suitable for producing a ceramic article comprising at least 80% of a sand and clay mixture and 20% or less of fluxing agent, characterised in that the flux agent comprises a mixture of conventional flux and milled glass.
• a mixture suitable for producing a ceramic article comprising: at least 90%, by weight, of clay and milled sand and 10% by weight, or less, of recovered glass fluxing agent.
• a particularly advantageous embodiment has been found to comprise at least 90%, by weight, of milled clay and 10% by weight, or less, of fluxing agent, including crushed glass.
• Figures 1-4 are graphs showing results of apparent porosities in various trials using different mixtures of materials.
• Figure 5 is a graph showing firing results of different mixtures of materials. Detailed Description of Preferred Embodiments
• Sands that contain small quantities of feldspars, as impurities are known to have a significant fluxing effect on bodies they are incorporated in.
• the use of more feldspathic sands was investigated with a view to the use of kaolin processing waste.
• Samples of the bodies were made up as slips at approximately 1820 g/l, with fluidity values of 300 - 310 degrees and 1 -minute thixotropy values of 30-35 degrees. Most samples were produced by slip casting in plaster of Paris moulds. Samples for firing deformation trials were formed by extruding slip that had been flocculated and dewatered to produce a plastic body.
• silica Four types were used in the trials:
• CMO 305 a feldspathic sand provided by English China Clay (ECC) as an example of the waste material from china clay production.
• the glass had a deflocculating effect on the slips that contained it; this means the suspensions became more fluid or lower in viscosity in time. In some cases a fine adjustment of fluid properties by a small addition of plaster (0.01-0.03% weight) is necessary.
• plaster 0.01-0.03% weight
• inorganic compounding capable of trapping the alkali ions leached from the glass powder can be used.
• the glass body shows slightly less deformation at T1, but slightly more at T2. This indicates a slightly narrower firing range has been produced.
• the body containing the CMO 305 sand showed a general marked increase in deformation, both at T1 and T2, indicating that no body type would be unsuitable unless some additional technique was employed to reduce deformation.
• the two slips showed very similar viscosity development, with values between 7500 and 8000 MPa being reached after 60 minutes standing.
• Tables 4 and 5 list the casting properties of these two body samples. Some differences were noted, in particular faster casting and firmer casts and slightly greater shrinkage from the glass/feldspar body.
• the firing deformation test was carried out using an industrial "sag bar” test run at T1. This showed the glass/feldspar-fluxed body to give less deformation than the base at this temperature, as in the previous Pyroplastic Index tests.
• the required firing temperature was achieved with a composition of:
• Figure 6 shows the response to firing temperature and indicates a potential drawback in that the useable firing range is reduced in this body.
• Table 6 shows that the deformation values for this body were also more sensitive to firing temperature, being lower than the "base" body at T1 but higher at T2. This behaviour appears to be related to the amount of glass used in the body.
• feldspathic sands and glass can both be regarded as using an increased proportion of alkalis in the body.
• the glass probably has a secondary effect in that it has already received significant heat treatment and can be regarded in a similar manner to adding a frit to a glaze.
• the said firing cracks are known as dunting. Overall, it appears to be possible to produce bodies by this route that can vitrify at approximately 1160 °C (T1), have a firing range of approximately 40 °C and retain similar deformation values to current types. This would represent a reduction in firing temperature of 50-60 °C over the base bodies that have been used.
• T1 1160 °C
• the spodumene/feldspar fluxes were effective in reducing firing temperature without an increase in alkali content, provided a sufficient proportion was used.
• This type of flux system has previously been shown to reduce firing deformation at moderate levels of substitution. This behaviour could either be employed alone or in combination with a markedly increased alkali content flux system to restore deformation to normal levels.
• the finely milled glass can be added into the clay processing stream of the clay at a number of points.
• the nature of where in the clay processing stream the glass is added is reflected in the manufacture and method of addition as identified below:
• An advantage of this method is that the material can be easily handled at the factory with no extra capital needed. On simple plants the tempered powder can be handled by a front-end loader. As the colour of the body of the article is of low concern, because all that is seen in the fired product is a white opaque surface glaze, full advantage of the properties of the replacement feldspar can be taken. This may also be applicable to coloured glass.

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