WO2004052803A1 - Articles ceramiques et fabrication - Google Patents

Articles ceramiques et fabrication Download PDF

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Publication number
WO2004052803A1
WO2004052803A1 PCT/GB2003/005327 GB0305327W WO2004052803A1 WO 2004052803 A1 WO2004052803 A1 WO 2004052803A1 GB 0305327 W GB0305327 W GB 0305327W WO 2004052803 A1 WO2004052803 A1 WO 2004052803A1
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Prior art keywords
slip
composition
slip composition
casting
ctn
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PCT/GB2003/005327
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English (en)
Inventor
John Michael Adams
Nigel Glasson
Neil Robert Forbes
Brian Roger Waters
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Imerys Minerals Limited
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Priority to AU2003290237A priority Critical patent/AU2003290237A1/en
Publication of WO2004052803A1 publication Critical patent/WO2004052803A1/fr

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    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B33/00—Clay-wares
    • C04B33/02—Preparing or treating the raw materials individually or as batches
    • C04B33/04—Clay; Kaolin
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B28—WORKING CEMENT, CLAY, OR STONE
    • B28B—SHAPING CLAY OR OTHER CERAMIC COMPOSITIONS; SHAPING SLAG; SHAPING MIXTURES CONTAINING CEMENTITIOUS MATERIAL, e.g. PLASTER
    • B28B1/00—Producing shaped prefabricated articles from the material
    • B28B1/26—Producing shaped prefabricated articles from the material by slip-casting, i.e. by casting a suspension or dispersion of the material in a liquid-absorbent or porous mould, the liquid being allowed to soak into or pass through the walls of the mould; Moulds therefor ; specially for manufacturing articles starting from a ceramic slip; Moulds therefor
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B33/00—Clay-wares
    • C04B33/28—Slip casting
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B33/00—Clay-wares
    • C04B33/36—Reinforced clay-wares
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
    • C04B35/622—Forming processes; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
    • C04B35/626—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
    • C04B35/62605—Treating the starting powders individually or as mixtures
    • C04B35/62625—Wet mixtures
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
    • C04B35/622—Forming processes; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
    • C04B35/626—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
    • C04B35/62605—Treating the starting powders individually or as mixtures
    • C04B35/62625—Wet mixtures
    • C04B35/6263—Wet mixtures characterised by their solids loadings, i.e. the percentage of solids
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/02—Composition of constituents of the starting material or of secondary phases of the final product
    • C04B2235/30—Constituents and secondary phases not being of a fibrous nature
    • C04B2235/34—Non-metal oxides, non-metal mixed oxides, or salts thereof that form the non-metal oxides upon heating, e.g. carbonates, nitrates, (oxy)hydroxides, chlorides
    • C04B2235/349—Clays, e.g. bentonites, smectites such as montmorillonite, vermiculites or kaolines, e.g. illite, talc or sepiolite
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/02—Composition of constituents of the starting material or of secondary phases of the final product
    • C04B2235/50—Constituents or additives of the starting mixture chosen for their shape or used because of their shape or their physical appearance
    • C04B2235/52—Constituents or additives characterised by their shapes
    • C04B2235/528—Spheres
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/02—Composition of constituents of the starting material or of secondary phases of the final product
    • C04B2235/50—Constituents or additives of the starting mixture chosen for their shape or used because of their shape or their physical appearance
    • C04B2235/54—Particle size related information
    • C04B2235/5418—Particle size related information expressed by the size of the particles or aggregates thereof
    • C04B2235/5436—Particle size related information expressed by the size of the particles or aggregates thereof micrometer sized, i.e. from 1 to 100 micron
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/60—Aspects relating to the preparation, properties or mechanical treatment of green bodies or pre-forms
    • C04B2235/602—Making the green bodies or pre-forms by moulding
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/60—Aspects relating to the preparation, properties or mechanical treatment of green bodies or pre-forms
    • C04B2235/602—Making the green bodies or pre-forms by moulding
    • C04B2235/6027—Slip casting
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/60—Aspects relating to the preparation, properties or mechanical treatment of green bodies or pre-forms
    • C04B2235/61—Mechanical properties, e.g. fracture toughness, hardness, Young's modulus or strength

Definitions

  • the present invention relates to improving the cracking resistance of green bodies for use in the preparation of cast ceramic articles, e.g. whiteware articles such . as tableware, sanitaryware and the like.
  • the invention also relates to green bodies having improved cracking resistance, and to a method for testing slip compositions for use in the slip casting of green bodies, to determine whether green bodies prepared from the slip composition are likely to have a desired level of cracking resistance.
  • Ceramic articles e.g. tableware for use in the home and in the catering industry, or sanitaryware such as wash-basins, shower trays, baths, lavatories, urinals, bidets and the like, are generally formed from a wet composition which comprises a blend of various particulate ingredients which include kaolinitic clays (i.e. clays which contain the mineral kaolinite), such as kaolin or china clays and/or ball clays.
  • kaolinitic clays i.e. clays which contain the mineral kaolinite
  • kaolin clay will be used herein to refer to all such kaolinitic clays
  • fluxing materials such as china stone, feldspar or nepheline syenite - and at least one silica-containing material - such as quartz or flint - are also included in such compositions.
  • the composition will also contain a substantial proportion of ground, calcined, animal bone, especially from cattle, or bone ash.
  • the composition may also include minor proportions of other ingredients such as calcium carbonate, dolomite, talc, etc.. The proportions of the various ingredients used in the composition vary according to the properties required in the fired ceramic article.
  • Ceramic tableware and sanitaryware can be formed from such compositions by a slip casting process.
  • the clays and other ingredients of the composition are mixed with a quantity of water, optionally with one or more additives, e.g. one or more dispersing agents, to form a fluid suspension, slurry or "slip" composition.
  • the slip is poured into a porous mould where a shaped article is formed by a process which is similar to that by which a filter cake is formed in a filter press.
  • Partial dewatering of the shaped article occurs as water passes from the composition into or through the porous walls of the mould, until the article is sufficiently formed, in a dry and firm state, to be removed from the mould.
  • the mould is often made of plaster or a similar material, but polymer moulds and pressure casting are being used on an increasing scale.
  • the shaping process is normally completed by cutting and/or working of the unfired shaped body, e.g. to form any desired edge or surface configuration or ornamentation.
  • the unfired shaped body is dried before firing one or more times in a kiln at a suitable temperature, to produce the desired ceramic article. Glazes and decoration may also be applied at this stage.
  • Shaped bodies at all stages between initial forming in the mould and completion of the firing stage will be referred to herein as "green bodies”.
  • An important parameter of a slip composition is its casting rate, which as used herein refers to the rate of increase of the square of the thickness of the partially dewatered shaped article in a standard mould as the partial dewatering takes place, expressed in mm 2 /min.
  • the rate of increase of the square of the thickness is in practice relatively constant over the desired time period (typically about one hour), whereas the rate of increase of the thickness itself is normally not constant.
  • the casting rate of a particular slip composition will vary according to the conditions used, and is therefore measured according to a standard test method. Generally speaking, the industry desires slip compositions to have a easting rate that maximises production and reduces costs.
  • green bodies have the potential to crack (i) in the mould, (ii) on removal from the mould, (iii) during or immediately after cutting or working, (iv) during drying and (v) during firing.
  • the fundamental cause of cracking is usually the accumulation of stresses within the green body due to drying of the clay, and may only appear after the process of firing.
  • the term "cracking resistance” and like terms, used herein, refers to a resistance to cracking of the green body material in any one or more of the above stages (i) to (iv).
  • a slip composition to increase the cracking resistance of a green body prepared from the composition will reduce the casting rate of the slip composition when forming the green body.
  • Such adjustments include, for example, adjustments to the viscosity, slip thixotropy (time dependent increase in viscosity) and slip density. These adjustments generally focus on increasing the packing density of the ceramic particles in the green body.
  • Particular clay parameters that affect particle pa ⁇ king include: mineral composition, mineral lattice disorder, organic content, associated trace elements and ions, particle size, particle size distribution, particle shape, etc..
  • the fluid component of the slip composition can also be adjusted using dispersants/deflocculants and water soluble materials (e.g. salts), again with a view to altering the permeability of the green body.
  • slip casting process will be represented by a balance struck between casting rate and cracking resistance.
  • a casting rate of 1 mm 2 /min corresponds to a time of about 60 minutes in the mould, to form a green body wall thickness of around 8mm (8 /60 « 1).
  • the CTN of a green body material can be measured in a test apparatus by calculating the modulus of rupture (MoR) and plasticity (peak deformation) of 6mm thick bars of the composition after casting, demoulding and drying for varying periods of time, and subsequently measuring the moisture content of the bars in mass % (Moisture cast ). Knowing the initial moisture content of the slip (Moisture s ii p ), the CTN is calculated by the following formula:
  • CTN MoR x Plasticity/(Moisture SHp - Moisture oast ).
  • CTN cmc [MoR cmc x Plasticity cmc ]/[(Moisture SHP - cmc) x 0.33].
  • the present invention is based on our surprising finding that a particular quantitative and reproducibly measurable relationship exists between the CTN parameter of the partially dewatered slip composition and the casting rate (CR) parameter of the slip composition (or between CTN and any other parameter, such as slip permeability, which is correlatable with CR), across a range of casting rates, so that a mathematical relationship can be established between CTN and CR (or other, CR-correlatable, parameter) across that range of casting rates or across the corresponding range of values of the other, CR-correlatable, parameter, which defines slip compositions which provide test green bodies having a good balance between cracking resistance and casting rate (or other, CR-correlatable, parameter), particularly slip compositions which provide green bodies which have a desired level of cracking resistance at a suitable casting rate or within a suitable range of casting rates.
  • CR casting rate
  • the relationship between CTN and CR can therefore be measured and the results compared with the established mathematical relationship.
  • an indicative or diagnostic test for likely good cracking resistance, or for likely good balance between cracking resistance and casting rate (or other, CR-correlatable, parameter) can be achieved using simple test methods which are available in the laboratory.
  • the present invention can, for example, be used to test a slip composition for cracking resistance within a factory's available casting rate range, to improve an existing slip composition recipe, and to engineer slip compositions for particular applications and processes.
  • CR 2Ppvuk
  • CR is the casting rate
  • P is the pressure
  • p is the permeability
  • v is the volume cast per unit volume filtrate
  • u is the temperature correction to interpolate to standard temperature
  • k is the constant to establish the conversion of units depending on how the factors are expressed.
  • CTN and permeability (p) which represents slip compositions which provide test green bodies having a good balance between cracking resistance and casting rate or permeability is as follows:
  • p slip permeability expressed in appropriate units (preferably cm 2 x 10 "14 ) and A is the appropriate constant of proportionality.
  • Slip compositions which provide test green bodies having CTNs which obey this relationship can be considered to be likely to provide, in the factory, green bodies which are likely to have a desired level of cracking resistance at a suitable casting rate or permeability or within a suitable range of casting rates or permeabilities.
  • a slip composition for use in a slip casting process comprising an aqueous suspension of a kaolin clay and other optional ingredients, wherein the slip composition has a casting rate (CR) under a test method substantially as described herein, and a resultant test green body has a cracking tolerance number at the critical moisture content (CTN orac ) under a test method substantially as described herein, the CTN cmc and CR obeying the relationship
  • the said first aspect of the present invention correspondingly includes slip compositions in which the measured parameter for the right hand side of the relationship is another parameter (e.g. slip permeability) which is quantitatively correlatable with (e.g. proportional to) the CR parameter, the said measured parameter being used in the relationship subject to an appropriate correlation operator (e.g. multiplication by an appropriate constant of proportionality).
  • another parameter e.g. slip permeability
  • the measured parameter e.g. slip permeability
  • an appropriate correlation operator e.g. multiplication by an appropriate constant of proportionality
  • CR or "casting rate” will, where the context requires, be understood to include parameters which are correlatable with CR and are used in relation to the mathematical relationship subject to an appropriate correlation operator.
  • the first aspect of the present invention includes a shaped green body prepared from the slip composition and a fired ceramic article prepared from the shaped green body, as well as preparative methods for the slip composition, the shaped green body and the fired ceramic article.
  • the preparative method for the slip composition comprises admixing the ingredients of the composition to form an aqueous suspension thereof.
  • the preparative method for the shaped green body comprises partially dewatering the slip composition in a slip casting apparatus and recovering the shaped green body from the apparatus.
  • the preparative method for the ceramic article comprises firing the shaped green body, optionally after normal pre-firing processing steps such as trimming, hole punching, sponging, decorating, conditioning or glazing.
  • a slip composition comprising an aqueous suspension of a kaolin clay and other optional ingredients, the method comprising:
  • the procedure of adjusting the slip composition in the method of the second aspect of the invention will generally focus on increasing the packing density of the ceramic particles in the green body.
  • This can be modified in the slip by modifying the viscosity, thixotropy and density.
  • Particular adjustments that may be made will be to "those clay parameters that affect particle packing, including: mineral composition, mineral lattice disorder, organic content, associated trace elements and ions, particle size, particle size distribution, particle shape, etc..
  • the fluid component of the slip composition can also be adjusted using dispersants/deflocculants and water soluble materials (e.g. salts), again with a view to altering the permeability of the green body.
  • a slip composition comprising an aqueous suspension of a kaolin clay and other optional ingredients, and green bodies prepared therefrom, to determine whether green bodies prepared from the slip composition in a slip casting process are likely to have a desired level of cracking resistance, the method comprising:
  • ln e CR is the natural logarithm of the casting rate expressed in mm 2 /min, a positive determination being representative of a likelihood of a desired level of cracking resistance.
  • test method substantially as described herein means a test method which includes at least the following standard aspects:
  • test methods are used in both the CR (or permeability) and the CTN test methods.
  • the standard density we have selected is the density which provides a viscosity of 5 poise (0.5 Pa.s) at full deflocculation, as measured by a Brookfield Synchro-Lectric RNF constant rate shear viscometer using a No. 3 spindle at a spindle speed of 20 rpm.
  • the actual viscosity of a particular slip composition complying with this standard may be 5 poise or may be greater than 5 poise, depending on the amount of any deflocculating agent present. It will be appreciated that the viscosity of slip compositions of the type with which this invention is concerned rises when the composition is left to stand. The viscosity of 5 poise is as measured immediately after the composition is left to stand.
  • the standard tong term thixotropy we have selected is a N ⁇ o (apparent viscosity after standing for 60 minutes) of approximately 60 poise (6 Pa.s), as measured by a Brookfield Synchro-Lectric RNF constant rate shear viscometer using a No. 3 spindle at a spindle speed of 2 rpm.
  • test methods provide for interpolation of the parameter under test to the value that would be found in a slip composition having the standard long term thixotropy.
  • a slip composition of the standard density which has been determined to exhibit the actual standard long term thixotropy, although more typically it may be difficult or impossible to formulate such a composition with sufficient accuracy.
  • Standard Mould Material for CR measurement
  • the standard mould material we have selected is plaster of Paris that has been prepared by mixing 100 parts of dry powdered plaster into 70 parts of water over a period of one to three minutes, and then leaving the slurry to stand for one to four minutes before pouring into the mould former.
  • the plaster is left to set for at least 20 minutes, before the mould former is removed. After drying in air, the plaster moulds are checked using a slip with a known casting rate, and any moulds which are found to produce results which disagree with the standard value are rejected.
  • the standard casting temperature and air pressure we have selected are a casting temperature of 20°C and atmospheric pressure. If the temperature and pressure conditions used are not at the standard conditions, the parameter under test is interpolated to the value that would be found under the standard casting conditions.
  • the standard permeability (pressure filtration) conditions we have selected use a Whatman Hardened Ashless 541 filter paper, a cell radius of 3.87 cm (giving a filter cake area of 47 cm 2 ), a standard temperature of 20°C and a standard gas (air) pressure of 90 psi (620 kPa), using 200ml of slip composition for a standard filtration time period of 30 minutes.
  • a suitable baroid apparatus for these standards is a Baroid FANN Filter Press - Series 300.
  • the thickness (h) of the resultant filter cake is measured in cm (conveniently measured by measuring the wet cake weight, the cake density and the cell (piston) area and dividing the weight by the product of the density and the piston area), and the permeability of the slip composition (p) in 10 "14 cm 2 is calculated according to the following formula:
  • u is the temperature correction
  • k is the constant to establish the conversion of units (2.76 x 10 "9 )
  • t is the filtration time in minutes
  • the CTN parameter of a cast green body varies with the moisture content of the green body. It is therefore necessary to define a standard moisture content for the CTN measurements in this invention.
  • the standard moisture content we have selected is the critical moisture content, which is the moisture content at which shrinkage of the drying green body stops and the final dimensions of the green body are established.
  • this critical moisture content is typically around 14 weight % water, although it will vary from case to case, and will therefore be measured in each case so that the CTN parameter can be expressed as the CTN cmo .
  • the critical moisture content can be measured in a number of ways. It may, for example, be measured in the way described in the cfi Ber.DKG prior art document referred to above, namely by reading the moisture content at the point of intersection (inflection) of the best-fit lines of the MoR, plasticity (peak deformation) or shrinkage of the green body, plotted against moisture content.
  • the plot of plasticity against moisture content does not always yield straight best-fit lines in practice, and in this case the best-fit lines may be curved.
  • the assessment of the point of inflection may in that case be a matter of judgement.
  • An alternative way of measuring the critical moisture content uses test bars of the green body material which have been allowed to dry to the extent that shrinkage has stopped.
  • the measured casting rate will preferably correspond to a casting rate within the range of casting rates within which a particular industrial or commercial process under consideration is intended to be performed. Most preferably, the casting rate lies within the normal industrially used range of casting rates, for example between about 0.5 and about 2.5 mm 2 /min, e.g. about 1 mm 2 /min.
  • a casting rate of 1 mm 2 /min under the standard conditions described above corresponds to a slip permeability of approximately 12 to 15 x 10 "14 cm 2 , e.g. approximately 12 to 14 x 10 "14 cm 2 , under the standard conditions described above.
  • the methods according to the present invention may be easily conducted in a laboratory.
  • the recipe of the slip composition established to be suitable for forming a cracking resistant green body in a particular industrial or commercial scale process is relayed from the laboratory to the factory for use, and may conveniently be stored on a suitable data storage medium for future use and reuse.
  • the present invention enables known slip compositions, modified versions of known slip compositions, and newly engineered slip compositions, to be prepared and tested in the laboratory for cracking resistance, prior to being applied -to an industrial slip casting apparatus, avoiding down time at the factory while compositions are tested on an industrial slip casting scale.
  • Figure 1 illustrates a typical deflocculation curve established as part of the test method described herein;
  • Figure 2 illustrates a set of typical thixotropy curves established as part of the test method described herein;
  • Figure 3 illustrates a typical graph of the casting rate (CR) of a standard slip composition under the test method described, plotted against the N 6 o thixotropy of the slip composition;
  • Figure 4 illustrates a typical graph of the cast strength of a standard test green body under the test method described herein, plotted against moisture content of the test green body;
  • Figure 5 illustrates a typical graph of the peak deformation of a standard test green body under the test method described herein, plotted against moisture content of the test green body
  • Figure 6 illustrates a typical graph of the CT ⁇ plotted against CR for the standard slip composition under the test method described, showing the boundary between relatively good and relatively poor cracking resistance.
  • the kaolin clay may comprise one or more kaolin clays of primary or secondary origin.
  • Kaolinitic clays were formed in geological times by the weathering of the feldspar component of granite.
  • Primary kaolin clays are those which are found at the site at which they were formed, and are generally present in a matrix of undecomposed granite which must be separated from the clay during the refining process for the clay.
  • Secondary kaolin clays which are alternatively known as sedimentary kaolin clays, are those which were flushed out in geological times from the matrix in which they were formed, and were deposited in an area remote from their site of formation, generally in a basin formed in the surrounding strata.
  • Kaolin clays are generally found in association with relatively small proportions of impurities, such as mica, feldspar, quartz, titanium compounds and the like, and may also include a trace of smectite clays.
  • the kaolin clay may comprise one or more ball clays, or a mixture of one or more ball clays with one or more kaolin clays.
  • Ball clays are sedimentary clays which are very finely divided, in that they have a particle size distribution such that the particles predominantly have an equivalent spherical diameter smaller than 2 ⁇ m.
  • ball clays tend to have a higher proportion of impurities than kaolin clays, and to be less white in colour.
  • the impurities present in ball clays may include significant proportions of fine silica, together with minor amounts of compounds of iron and titanium and also organic matter such as lignite.
  • the kaolin clay used in the present invention may have been subjected to known preliminary processing or refining steps, e.g. steps selected from degritting, washing, magnetic separation of impurities and one or more particle size separation steps.
  • the kaolin clay used in the present invention may suitably have been subjected to beneficiation, e.g. mechanical beneficiation.
  • a slip composition for use in the present invention may suitably contain, in addition to the kaolin clay in suspension, other suspended and dissolved ingredients which may be selected from conventional ingredients by one skilled in the art, for example selected from: fluxing materials such as china stone, feldspar or nepheline syenite; at least one silica-containing material such as quartz or flint; ground, calcined, animal bone, especially from cattle, or bone ash; sodium carbonate; sodium silicate; calcium carbonate; dolomite; talc; water soluble salts (e.g.
  • the proportions of the various ingredients used in the composition will vary according to the properties required in the fired ceramic article. The selection of suitable additional ingredients and their proportions will be well within the abilities of one skilled in this art.
  • the composition suitably comprises an aqueous slip, (slurry) having a solids content of at least about 65% by weight, more typically about 70 to about 75% by weight.
  • the slip composition may suitably comprise kaolin clay at about 10 to about 35 weight % of the solids content, ball clay at about 15 to about 35 weight % of the solids content, deflocculant at less than about 1 weight % of the solids content, flux at about 15 to about 35 weight % of the solids content, and filler at about 15 to about 35 weight % of the solids content.
  • the slip composition consists essentially of the above ingredients in an aqueous suspension, with less than about 10 weight %, more particularly less than about 5 weight %, of other solid components.
  • the slip composition may be essentially free, or entirely free, of wollastonite.
  • the slip composition used for establishment of the CTN/CR relationship should be of a composition which is generally comparable with the composition of the slip composition whose cracking resistance or balance between cracking resistance and casting rate is under investigation. Typical differences in the compositions will include minor differences in the nature and/or proportions of the components, differences in the pre-working and pre-treatment of the kaolin clays used, and minor differences in the water content or solids content.
  • the range of options for adjustment of a particular type of slip composition for use in a slip casting process is well known in the art, and will be readily available to one of ordinary skill in this art.
  • the ingredients are formulated together by conventional processing and mixing methods, which are well known in the art, to form the slip composition. These mixing methods may, for example, include as an option formulating a particulate solids mixture of ingredients prior to adding the solids mixture to water to form the slip composition.
  • At least one parameter of the slip composition may be adjusted in a way which is known potentially to improve cracking resistance of a green body cast therefrom.
  • the density of the slip composition may be adjusted without substantially affecting the viscosity or the thixotropy of the slip composition.
  • Such an adjustment may take place before, during and/or after a step of suspending the kaolin clay and/or other ingredients in the aqueous medium to form a slip composition. Addition and removal of different materials may be effected simultaneously or sequentially, if desired.
  • Each slip formulation contains china clay, ball clay, a deflocculant such as sodium carbonate and sodium silicate, a filler such as quartz powder, and a fluxing agent such as nepheline syenite or feldspar powder, in suspension in water.
  • the amount of slip composition prepared for each formulation is generally about 3kg dry weight, which is enough to enable evaluation of the formulation at different viscosities. In the example illustrated in Figures 1 and 2 of the drawings, four viscosities are tested, referenced as A, B, C and D.
  • the ball clay must first be blunged in water to expose the surfaces of the ball clay particles. To do this, the ball clay(s) is/are first crushed or milled and passed through a coarse sieve. The moisture content of the ball clay(s) is then measured in a conventional manner. The amount of ball clay and water required to achieve an approximately 60 wt% solids slip containing the equivalent of about lOOOg dry of ball clay is then calculated. To make up the ball clay slip at 60 weight % solids, initially a small amount of anhydrous sodium carbonate (e.g.
  • up to 1 g in 1000 g of clay may be added to the water, although this is not essential, and then slowly the ball clay is added, with vigorous stirring up to lOOOrpm. If the slurry thickens, small increments of sodium silicate are added to produce a freely moving slip. When all the ball clay has been added, stirring is continued at 1000 rpm until a total work input of 7.5 kWhr/tonne is achieved. The slip is screened through a fine sieve (typically 125 ⁇ m mesh size) to remove contaminants. The slip solids content is measured, and then the slip is covered and allowed to stand overnight.
  • a fine sieve typically 125 ⁇ m mesh size
  • the required amounts of the ball clay slip prepared as described above and water are mixed using a stirrer.
  • the non-plastics are added, followed by the china clay(s).
  • the china clay is added slowly, with sufficient incremental additions of a 50% w/v solution of C 100 sodium silicate to keep the slip moving.
  • the solids of the preferred slip composition can conveniently be grouped as (i) ball clay, (ii) kaolin and (iii) non-plastics (e.g. filler and flux).
  • An example of the proportions of these components out of 100% total solids is approximately 25% by weight ball clay, approximately 28.5% by weight kaolin and approximately 46.5% by weight non-plastics, the slip composition vitrifying at 1200°C to less than 0.5 mass % water absorption.
  • the slip is stirred at 1000 rpm until a work input of 3 to 4 kWhr/tonne is achieved.
  • the slip is agitated, using the laboratory stirrer, until it appears smooth and is circulating freely.
  • the slip may be sieved again at this stage, though a 125 ⁇ m screen.
  • the required number (here: four) of about lOOOg portions of the slip are weighed out into buckets and sealed with lids, and left to age overnight.
  • the weight % solids content of the remaining slip is measured by weighing 2 to 3 g of slip, to 3 decimal places, into a pre-weighed dish. The slip is then dried in an oven at 110°C for a minimum of 30 minutes, removed from the oven, cooled and weighed again. The weight % solids content is calculated in known manner.
  • the procedure for establishing the deflocculation curve is as follows: The contents of a first bucket of the slip composition are stirred until the slip is moving freely and smoothly. The typical density of the slip at this point should be in the range of about 1800 to about 2000 g/1. The stirrer is switched off and the apparent viscosity of the slip measured quickly with a Brookfield Synchro-Lectric RNF constant rate shear viscometer using a No. 3 spindle at 20 rpm. The Brookfield reading is multiplied by 0.5 (50/100) to obtain the viscosity in poise.
  • This viscosity is recorded against the percentage by weight of sodium silicate present in the slip (e.g. 0.1%), to establish the first point on the deflocculation curve, at the highest viscosity (see Figure 1).
  • This slip composition is the standard slip composition for use in the CR and CTN measurements. Adjustment of Portions the Slip Composition to Different Viscosities
  • the slip composition in the first bucket has already been adjusted to minimum viscosity (point A in Figure 1), as described above.
  • the result is a group of four portions of the original slip composition, designated A, B, C and D, having constant density and solids content but differing in their apparent viscosities immediately on standing, generally in the range about 5 to about 10 poise.
  • a slip composition is not considered stable and workable in a slip casting process unless its apparent viscosity reaches a plateau on standing for an appreciable length of time (e.g. between about 30 and about 90 minutes). This plateau viscosity is referred to as the N 6 o or long-term thixotropy (i.e. the viscosity after standing for 60 minutes). According to the thixotropy standard preferred for use in the present invention, the N ⁇ o of the slip composition should be approximately 60 poise.
  • a set of thixotropy curves must first be established using each portion of the slip composition. A typical set of thixotropy curves is illustrated in Figure 2 of the accompanying drawings.
  • the slip portion is re-stirred for at least 15 seconds, and then the change in the Brookfield viscosity over 1 hour using the same apparatus with a spindle speed of 2 rpm is recorded, at 0.5, 1, 5, 10, 20, 30, 40, 50 and 60 minutes.
  • the Brookfield reading is multiplied by 5 (500/100) to obtain the viscosity in poise.
  • the Brookfield thixotropy is reported as a graph of viscosity at 2 rpm versus time (see Figure 2).
  • the V 6 o is the final viscosity value at 2 rpm after 60 minutes.
  • each slip portion is re-stirred in preparation for casting and/or permeability measurement.
  • the test casting method uses four plaster of Paris cone moulds.
  • the contents of each bucket is poured into the moulds, one bucket for each mould, and the moulds left to cast for one hour. At some time during this hour, usually between 20 and 40 minutes, the slip temperature should be checked. Following the casting hour, the excess slip is poured back into the appropriate bucket.
  • the casts are then left to drain, by inverting the respective mould and supporting the inverted mould on a brass bar, for a further hour. Fpllowing the draining hour, each cast is released from its mould.
  • the firmness of the cast is subjectively assessed by touch and the drainage of the cast is subjectively assessed visually.
  • the terms V. Soft, Soft, Slightly Soft and Firm may be used to describe the cast firmness and the cast drainage may be described as V.
  • the firmness is reassessed twice more at hourly intervals or until the casts are firm.
  • the wet weight of each cast is then noted, so that the moisture content may be calculated later.
  • the casts are then dried in an oven at 110°C, and left for at least three hours in the oven before the hot cones are carefully removed. Upon cooling, the dry weight of each cast is determined, and the pre-drying moisture content thus calculated.
  • each cone is broken off and discarded.
  • the hollow frustoconical portion is then used as described below, for calculating the relevant parameters of the cast.
  • the thickness of the cast wall of the hollow frustoconical portion of the cast cone is measured using a 0-150mm TESO callipers or a 0-25mm micrometer. This gives the thickness cast in 60 minutes, in mm.
  • the casting rate (CR) expressed in mm 2 /minute at 20°C is calculated according to the formula:
  • the temperature factor provides the correction from the actual cast temperature as ⁇ .-.A determined during casting and the standard temperature of 20°C at which the CR parameter is expressed. Temperature factors for cast temperatures between 19.0°C and 29.0°C are given in the table below:
  • That portion of the slip composition which exhibits a V ⁇ o closest to the standard approximately 60 poise is selected and adjusted with water and deflocculant to achieve a modified slip which has a V ⁇ o of generally 55 to 65 poise, using the data on the rheological properties of the slip composition obtained from the graphs of Figures 1 and 2.
  • Such a slip composition may typically have a Vo of around 6 or 7 poise. The slip solids/moisture and density are noted.
  • bars Approximately 36, 7 to 8cm long, 6mm diameter, rods ("bars") are cast from 6mm maximum diameter plaster of Paris bar moulds using the adjusted slip composition, and the cast bars are demoulded after casting for a period of 1.5 hours.
  • the bars shrink during casting, so that the dimensions of the bars as demoulded are somewhat less than the mould dimensions.
  • the dimensions of the bars are chosen to suit the strength testing apparatus used, and may be varied to suit that apparatus.
  • the number of bars used is not critical, but should be high enough to provide a statistically useful number of tests, whereby anomalies or errors in the tests do not substantially influence the overall CTN measurement.
  • the bending strength at peak (Modulus of Rupture) and maximum flexural deformation (representative of plasticity of the green body) of each bar are measured in conventional manner, using a 2 kg load cell at a crosshead speed of 2mm/min and a span of 60mm over which the bar 10 is supported. The measurements of bending strength at peak and maximum flexural deformation, as well as the moisture content, of each bar are recorded.
  • Both the bending strength and the plasticity (maximum flexural deformation) parameters are dependent on the moisture content of the bar, so that for each parameter
  • a value can be plotted at the various moisture contents, and a best-fit line then applied by statistical analysis, preferably using appropriate statistical software.
  • graphs of the form shown in Figures 4 and 5 of the drawings are obtained, in which typical best-fit lines have been added (if desired, the shrinkage of the green body on drying may alternatively be plotted against moisture content, and best-fit lines
  • the point of intersection (inflection) of the straight best-fit lines should indicate the point of the critical moisture content on the horizontal axis. It' should be noted that the addition of straight best-fit lines can sometimes be difficult, particularly with the deformation plot. The lines may need to be curved to provide the best fit, which can lead to a judgement being required as to the position of the point of
  • the values of bending strength and plasticity used in the calculation of the CTN parameter for each composition are the interpolated values at the critical moisture
  • the critical moisture content is first calculated (see below), and the appropriate value of bending strength and plasticity (peak deformation) is read off the vertical ordinate of the graphs established from the test measurements (e.g. graphs of the type shown in Figures 4 and 5).
  • the critical moisture content (cmc) is defined as the moisture content at which shrinkage on drying ceases.
  • the void fraction or pore volume at the cmc is the same as the void fraction when dry.
  • the voids are filled with water, so a knowledge of the void fraction, obtained from the dry bulk density, allows the cmc to be simply calculated.
  • the bulk density (BD) of five bars of the green body material which have been allowed to dry to the extent that shrinkage has stopped is measured in conventional manner, from the weight and dimensions of the bars, and the average value obtained.
  • the specific gravity (p) of the dry green body material of the bar is a known quantity, or can easily be measured in conventional manner.
  • the specific gravity of the dry green body material can normally be assumed to be 2.65 g.cm "3 .
  • the cmc is calculated using the following equation:
  • the CTN cmc of the green body may be expressed as:
  • the bending strength (or Modulus of Rupture) is expressed in MPa as measured on the Testometric 500AX apparatus
  • plasticity is expressed as the peak flexural deformation before rupture (in mm) as measured on the Testometric 500AX apparatus
  • slip and bar moisture contents are expressed as the respective numeral percent by weight, the bar moisture being - according to the test method used - the critical moisture content.
  • the denominator factor 0.33 is included in the CTN formula stated above as a conversion factor from a volume to a linear expression of shrinkage.
  • the slip composition under investigation should correspond generally in its composition to the slip composition for which the CTN/CR relationship ( Figure 6) was established.
  • the CR and CTN are established for green bodies made from the slip composition under investigation, following the same procedures and test standards as set out above.
  • the point on Figure 6 represented by the combination of the measured CTN cmc and the measured CR under the test conditions is then plotted (e.g. point X), and a determination made as to whether the point lies in the domain Dl of good (or improved) cracking resistance. Since in this illustration the point X lies in the domain Dl, it is concluded that the cracking resistance of green bodies made in an industrial casting process from the slip composition under investigation would be likely to have good cracking resistance.
  • the slip composition under investigation is a modified or adjusted form of an earlier or currently used slip composition, for example one that was found to give a poor resistance to cracking
  • the plotted point X can be compared with a plotted point representing the earlier or currently used composition, so that the achieved improvement can be quantified, and any consequent loss of casting rate analysed against the established casting rate or the acceptable "window" of workable casting rates of the factory.
  • the data from the body evaluations shows that the density of the casting slip prepared from E2 does not differ greatly from the body prepared from El. However, the other properties such as casting rate and cast moisture indicate a difference in the two bodies.
  • the measured CTN of the fully (high energy) mechanically beneficiated material E3 is markedly higher than the required minimum according to the CTN/CR relationship established for this type of composition.

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Abstract

L'invention concerne une composition de barbotine destinée à être employée dans un procédé de coulage en barbotine, contenant une suspension aqueuse de kaolinton et d'autres ingrédients optionnels. Ladite composition de barbotine présente un taux de coulée (CR) selon un procédé de test standard, et une ébauche crue de test résultante présente un indice de tolérance au craquelage au contenu en humidité critique (CTNcmc) selon un procédé de test standard : CTNcmc et CR obéissent à la relation CTNcmc = 29 - 8 lneCR, lneCR étant le logarithme naturel du taux de coulée exprimé en mm2/min. Une relation correspondante entre CTNcmc et la perméabilité de la composition de barbotine, selon un procédé de test standard, peut également être employée. La composition selon l'invention permet également d'améliorer la résistance au craquelage d'ébauches crues préparées à partir de compositions de barbotine, et de mettre en oeuvre un procédé de testage d'une composition de barbotine en ce qui concerne la résistance au craquelage. Lesdits procédés font intervenir la même relation comme indicateur d'une bonne résistance au craquelage.
PCT/GB2003/005327 2002-12-06 2003-12-05 Articles ceramiques et fabrication WO2004052803A1 (fr)

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Publication number Priority date Publication date Assignee Title
RU2608379C1 (ru) * 2015-09-15 2017-01-18 Автономная некоммерческая организация высшего образования "Белгородский университет кооперации, экономики и права" Керамический шликер
CN111592223A (zh) * 2020-06-10 2020-08-28 福建省德化尚本道陶瓷有限公司 一种烧成范围宽、上釉厚度小、釉面效果好的白色亚光釉
CN112777998A (zh) * 2021-01-14 2021-05-11 李仁刚 一种土陶制作酒甄的技术
CN115611612A (zh) * 2022-10-21 2023-01-17 江镇 一种超薄单双面釉高骨瓷板制备方法

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US4880759A (en) * 1986-08-12 1989-11-14 H. C. Spinks Clay Company Inc. Ball clay slurry
US5034448A (en) * 1987-09-17 1991-07-23 The Dow Chemical Company Slip composition for whiteware articles
DE19501530A1 (de) * 1994-01-21 1995-07-27 Ecc Int Ltd Gußkeramische Zusammensetzung
FR2769015A1 (fr) * 1997-09-30 1999-04-02 Thierry Millot Procede de coulage de porcelaine et porcelaine ainsi obtenue

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US4880759A (en) * 1986-08-12 1989-11-14 H. C. Spinks Clay Company Inc. Ball clay slurry
US5034448A (en) * 1987-09-17 1991-07-23 The Dow Chemical Company Slip composition for whiteware articles
DE19501530A1 (de) * 1994-01-21 1995-07-27 Ecc Int Ltd Gußkeramische Zusammensetzung
FR2769015A1 (fr) * 1997-09-30 1999-04-02 Thierry Millot Procede de coulage de porcelaine et porcelaine ainsi obtenue

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N.P.GLASSON ET AL.: "Clay Systems for Improved Performance", CFI/BER. DKG, vol. 78, no. 3, 2001, pages E22 - E27, XP001179601 *

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
RU2608379C1 (ru) * 2015-09-15 2017-01-18 Автономная некоммерческая организация высшего образования "Белгородский университет кооперации, экономики и права" Керамический шликер
CN111592223A (zh) * 2020-06-10 2020-08-28 福建省德化尚本道陶瓷有限公司 一种烧成范围宽、上釉厚度小、釉面效果好的白色亚光釉
CN112777998A (zh) * 2021-01-14 2021-05-11 李仁刚 一种土陶制作酒甄的技术
CN115611612A (zh) * 2022-10-21 2023-01-17 江镇 一种超薄单双面釉高骨瓷板制备方法

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