Many properties of clays and other ceramic raw materials depend on the type and quantity of the various minerals that constitute them. The identification of such minerals is therefore of primary importance. Yet clear-cut identification is made difficult by the fact that ceramic raw materials rarely consist of pure, well crystallised minerals: instead, several minerals are generally present in appreciable quantities together with many other minor constituents. Given these circumstanc-es, identification of the main phases can be difficult, especially where they are all rather similar.
Sometimes, and this is often the case with clays, a mineral cannot be identified without prior purification and separation. Furthermore, a single clay may contain various minerals and is nearly always associated with significant quantities of quartz, calcareous materials, micas and others.
Also, clayey minerals are made up of very small particles (dimensions as small as 100 Å, that is 10-6 cm, are frequent), making identification even more problematic.
Moreover, clayey raw materials are often characterised by isomorphic substitution, created by the genesis conditions described in the following chapter.
In general, then, analysis methods used for the study of these raw materials must allow for recognition of minerals of non-constant, often intermingled com-position sometimes made up of extremely small particles. Since identification of a mineral depends on its key characteristics, which must necessarily always be the same independently of mine attitude and the surrounding environment, analytical methods which make use of the univocal properties of the individual raw material classes must be employed.
Such properties can be summed up as follows:
properties dependent on the chemical nature of the material
properties dependent on the crystalline aspect of the mineral
properties dependent on the atomic or ionic arrangement in the crystalline
struc- properties dependent on chemical or physical modifications in the mineral causedture by external factors (e.g. enthalpy variations induced by heating or cooling).
Other practical methods for identification or estimation in raw material mineral content exist: for example, when rheological properties such as plasticity, thixotro-py etc. are particularly evident, this suggests the presence of certain clayey miner-als. Similarly, magnetic properties can indicate the presence of ferromagnetic min-erals etc.
While these methods normally provide an overall picture as to the
characteris-tics of the predominant mineral in the mix, they do not identify its individual com-ponents.
An accurate description of the analytical methods used in raw material mineral content recognition is outside the scope of this publication. Nevertheless, there follows an approximate list of the main techniques and the criteria on which they are based because, in the following description of the individual mineral types, ref-erence will frequently be made to characteristic analytical data.
Sampling
Good analysis, whatever the type, where effected on a sample made up of a mix of base components, and especially where that mix is heterogeneous, will require proper preliminary sampling; this allows the necessarily small quantity of material used in analysis to represent what can amount to tons of raw materi-als stored in the body preparation department of a ceramic company. The analy-sed sample, then, must be representative of all the material in question, not just a portion of it.
Beginning with the most general case (i.e. in-quarry sampling, for which there exist specially developed standardised procedures), samples must be taken from var-ious points and different depths along the extraction front; if the material appears to be uniform, selection and conservation of a sample representing about 1% of the total sample may be effected (following mixing and quartering of the taken sam-ples). Where the material is less uniform up to 5% of the taken sample may have to be selected in order to obtain a reasonably analysable sample.
The same procedure applies to shiploads or truckloads of loose material.
The selected preliminary sample is then homogenised further, then quartered again via subsequent formation of flattened heaps and the taking of opposite quar-ters, until a final coarse sample of some 10-12 Kg is obtained. This will be ground and quartered again until a suitable sample of about 3 Kg is obtained. The latter should have an average particle size distribution no greater than 0.5-2 mm and should not be over-ground so as to prevent oxidation of components sensitive to contact with air; for similar reasons, in-sample moisture content should not be com-pletely eliminated during comminution and quartering, and should remain within the 4-12% range so as to prevent alteration or loss of soluble salts. Final drying and grinding will only be effected immediately prior to analysis as a function of the requirements of the analysis itself. The type of comminution can be critical as
Chemical analysis
Determination of the type and quantity of each element in a sample is of great interest to ceramists. Yet in itself, such analysis often carries little significance. The presence or absence of certain elements in certain quantities may certainly help determine behaviour during firing but rarely provides information on the work-ability of the raw material in question (i.e. in grinding, spray drying, pressing, drying) or its physical behaviour.
Nevertheless, properly performed elementary analysis is an exact science while many other techniques for the analysis and determination of technological proper-ties are marred by error and uncertainty. Furthermore, the combination of struc-tural data with constituent data allows for semi-quantitative identification of the minerals in the sample.
A whole host of chemical analysis techniques exist. In particular, modern in-strumental techniques provide fast, accurate results, the precision of which largely depends on proper selection and preparation of the sample, a must for the attain-ment of complete homogenisation.
In chemically analysing any substance, results are normally expressed in oxides and, as specifically regards ceramic body materials, the main 8 are as follows: silicon dioxide (Silica SiO2), aluminum oxide (Alumina Al2O3), titanium dioxide TiO2, ferric oxide Fe2O3, calcium oxide CaO, magnesium oxide MgO, sodium monoxide Na2O and potassium oxide K2O.
Oxides volatile at 1000 °C (carbon dioxide or carbon monoxide, sulphur oxides such as SO3 and SO2, together with water) are normally indicated as Loss of Igni-tion (L.O.I.). Accurate analysis will rarely see these components account for 100%
of the sample as other elements generally accounting for less than 1% are al-ways present in varying quantities. These include Barium, Strontium and other transition metals such as Copper, Chromium, Manganese and Boron, Lithium etc.
This type of analysis, however, does not indicate how the various elements are combined and this can lead to errors in their technological evaluation: to under-stand this concept just consider the difference between a calcium oxide originating from feldspar rather than lime, or sulphur oxides that fail to take into account the presence of mineral sulphides (e.g. Pyrite).
Before beginning a chemical analysis, then, it is necessary to select the sample carefully as per the above methods. Such samples will, after proper drying at tem-peratures that do not alter the volatile substance content, normally have a weight ranging from a few hundred milligrams up to 1-2 grams; subsequent grinding is effected using high-efficiency milling machines that do not contaminate the sample.
Methods vary depending on the hardness of the sample, ranging from corundum or, even better, agate pestle-and-mortar tools to micro-mills with special hard alloy grinding media.
With the sample weighed (and this is the point at which errors can creep into the entire analysis), a solubilisation method which allows complete homogenisation must be decided on: the sample is usually dissolved in chemical reagents to obtain a
ho-mogeneous liquid solution, or is solubilised in a molten state in a special glass with the resultant solid solution being analysed.
Ceramic materials, unfortunately, being silicate, aluminate and oxide-based, are difficult to solubilise. There exists a vast argument-specific literature indicating hydrofluoric acid HF and other mineral acids, such as nitric acid, HNO3, hydrochlo-ric acid HCl or sulphuhydrochlo-ric acid H2SO4 as appropriate hot liquid solubilisation agents requiring, as might be expected, the use of special containers.
Classical wet-type chemical analysis involved complex, systematic treatment of samples to separate individual components prior to analysis true and proper, mainly effected using (lengthy and complex) gravimetric, colorimetric or complex metric methods (necessarily preceded by accurate preliminary calibration). These methods, while still valid today, have undoubtedly been superseded by the advent of more sophisticated instrumentation capable of extracting immediate analytical re-sults from both solubilised and untreated samples alike.
All instrumental information is ultimately influenced by the initial measurement of the mass of the sample and its proper preparation, which, whatever the method, must provide conditions that are as standardised and homogeneous as possible.
The main aggression and acid solubilisation methods effected on ceramic raw materials are:
Hot acid aggression in open containers using HCl, HNO3 and HClO4-based acid-oxidant mixes: the need to break up the silicate matrix nearly always makes the use of HF indispensable too, thus making it impossible to use standard borosil-icate glass. The use of open containers together with high-temperature solu-bilising acids aids the loss of volatile components.
Hot acid aggression in closed containers (and thus at high pressure). These sys-tems, now becoming more widespread owing to the rapidity of aggression and dissolution, usually employ Teflon containers and programmable microwave heat-ing systems.
Alkaline fusion and subsequent acid solubilisation, normally with HCl. There exists a vast range of alkaline fluxes, to be employed as a function of the desired fusion temperature and process efficacy: in all cases, of course, there is the addi-tion, via the flux, of at least one cation (an aspect difficult to quantify in the unknown specimen). The most widely employed alkaline fluxes are NaKCO3, NaOH, LiBO2 and Li2B4O7, with various salts being added as detachment agents (Lithium halogenides or alkalines in general, mostly complexing agents etc. - see table on following page).
Fusion may be manual or automatic, thus guaranteeing standardised conditions
Whatever the employed instrumental technique, the preventive establishment of various calibration curves will be required via the utilisation of standard solu-tions or solids within the presumed analysis range for the new samples.
The main methods used in quantitative ceramic material chemical analysis are based on interaction (of the fluorescent emittance, absorption or emission type) of the sample with electromagnetic radiation. The main methods are:
X-RAY FLUORESCENCE (XRF), in which raw minerals or, preferably, miner-als finely dispersed in an alkaline glass, are bombarded by high-frequency short wavelength radiation. The energy in the radiation induces fluorescent emission owing to the excitation of inner electrons in the orbitals of the elements; these sample-emitted electrons are collected in a special detector. The generated signal is associ-ated with the position of the sample or the detector itself so as to identify the relative intensity of the signal that can be compared with a standard one.
These methods allow for easy quantification of medium-high atomic weight elements as far as the lower Na - F limit. More recently, considerable efforts have Melting temperatures of compounds used in fusion-type decomposition of materials.
(1) After decomposition and transformation of bisulphate in pyrosulphate K2S2O7. (2) Normally introduced in the dihydrate form.
(3) Like PbO after decomposition and outflow of carbon dioxide at 315 °C.
(4) Like PbO after decomposition at 500 °C approx.
Compound Compound Compound
been made to extend sufficiently repeatable element assessment as far as Boron (Fig. 13).
ABSORPTION SPECTROSCOPY (AAS-GFAAS). This technique exploits the energy absorption caused by the presence of atoms placed in the path of one or more monochromatic radiation beams generated by special lamps: in practice the solution to be analysed is injected into a flame, the temperature, geometry and com-position of which ensures that the elements are present in atomic (not ionic) form and provides maximum interaction with the incident radiation. To obtain more ac-curate resolution it is sometimes possible to use fast induction heating as the atom-isation energy source instead of a flame: this is effected in a tube made of graphite or other suitable material in an inert gas flow. Under specific conditions this tech-nique can even be used for direct analysis of easy-volatilisation solid samples; it is also a technique that gives excellent analytical results for any metallic element and can provide superb resolution, in the order of, depending on the element, fractions of parts per million (Fig. 14).
INDUCTIVELY COUPLED PLASMA-ATOMIC EMISSION SPECTROSCO-PY (ICP-AES). Similar to the previous technique, but measures interaction with the incident radiation in terms of emission as opposed to absorption. In this case the sample solution is atomised by the combined action of a standard high temper-ature torch and application of a radio frequency source. With respect to absorption systems, this technique is advantageous in that it is possible to effect sequential analysis of each sample without having to modify the source. Detection limits for each element are generally poorer, yet results are actually better with some ele-ments. In any case it is possible to switch from p.p.m. to p.p.b. fairly easily (Fig. 15).
Other chemical analysis methods pertinent to the search for specific elements (e.g. FLAME PHOTOMETRY in tracking down alkaline elements) do, of course, exist, but a detailed account of these is beyond the scope of this book.
It is, instead, worthwhile pointing out that specific chemical tests can be used to check for the presence of specific elements (carbon, sulphur, fluorine...) or anions (CO32- carbonates, SO42- sulphates ): these employ analytical techniques that are generally simple and efficacious and provide extremely important information for evaluating the applicability of a raw material in a ceramic process.
Fig. 13-13a. Operating principle of X-ray fluorescence (XRF) chemical analysis.
Fig. 15. Operating principle of inductively coupled plas-ma-atomic emission spectroscopy (ICP).
Fig. 13a.
Fig. 14. Operating principle of atomic absorption spectroscopy (AAS).
Optical Systems Torch
(ICP) Generator
Pump/
Nebuliser
SPECTROSONIC SPECTRO AS 300
SPECTROMERC
Mineralogical (or crystallographic) analysis
This type of analysis allows the technician to check for the presence of individ-ual crystal phases in a sample and thus trace its mineral composition, the evaluation of which is of primary importance in defining the technological characteristics of a raw material or the contribution it will make to a body.
A preliminary form of such mineralogical information is provided first by na-ked-eye and then microscopic inspection under reflected, polarised and transmitted light (on naked samples included in the resin): by combining the cited observations with other optical properties (e.g. refractive index) it is possible to obtain good discrimination and recognition results. In the mineralogical analysis of ceramic materials the entire science of optical microscopy should be viewed as a branch in its own respect consisting of numerous techniques (as illustrated by the abundant literature) that achieved maximum diffusion between the 1940s and 60s.The most widespread mineralogical analysis technique now in use is X-RAY DIFFRACTO-METRY (XRD). This can be effected on individual crystals or, more commonly, on powders (Fig. 16).
The incident X-radiation falling on the sample, properly filtered so as to confer monochromaticity, interacts with the crystal lattice, giving rise to related diffrac-tion images, via the equadiffrac-tion Bragg nλ = 2dsinϑ, at the inter-lattice distance d of the crystal as a function of the diffraction angle. Since a wide range of X-ray sources can be used (the most common is provided by Ni-filtered Cu anticathodes emitting CuKα = 1.541 Å only) it is correct to list every interaction set of a particular crystal lattice with the list of the active lattice distances expressed in Angstroms (Å).
Indexes and data bases are available for all crystalline substances (PDF, see the examples in tables 5 and 6). These are updated by an international controlling body.
This reference material allows recognition of crystal types in natural and artificial samples, while recent software improvements now provide more modal manage-ment of diffractometry data, providing quantitative evaluations with regard to the presence of individual minerals.
Of course, when working with powders it is essential that the sample be high-ly representative of the whole. There should be no preferential orientation, easihigh-ly caused by bi-directionally developed crystals: this is why preparation and laying out of the sample are so important. Milling must be carried out as efficiently as possible, taking care not to alter the structural characteristics of the sample, es-pecially where clayey, so as to aid homogenisation of all the phases and ensure casual orientation of all crystalline faces; where analysis with a spinning sample
Fig. 16. X-ray diffrac-tometry analysis equip-ment (left) and the oper-ating principle on which it is based (top).
and simple and, on sufficiently simple matrices, leads to fairly easy interpretation of the gathered data, allowing moreover through combination with the verified pres-ence of certain crystal phases, attribution of a chemical formula and combination with quantitative chemical analysis so-called rational analysis of a raw material or body (i.e. approximation of its composition expressed in standard minerals). It will therefore be possible to evaluate a ceramic body not only in terms of its oxide-based chemical composition, but also its mineralogical composition, expressed in Quartz, Kaolinite, Illite, Calcite, Dolomite, Albite (Sodium feldspar), Microcline (Potassium feldspar) etc. (see Fig. 17 for example).
Thermal analyses
As the heading suggests, there is more than one way of analysing samples via the parameter variations that occur as a function of an increase (or decrease) in temperature. Variations in weight, temperature, heat evolved or absorbed, dimen-sions, gaseous substances emitted etc. can all be registered as a function of a partic-ular temperature gradient. Each of these techniques has a corresponding type of analysis, as listed below:
TGA (ThermoGravimetricAnalysis),
Tab. 5. PDF chart containing crystallographic data for quartz (SiO2).
Fig. 17. Example of body chart with rational XRF + XRD analysis.
Porous single firing, coloured body dry grinding, pressing
DTA (DifferentialThermalAnalysis), DSC (DifferentialScanningCalorimetry),
TMA (ThermoMechanicalAnalysis) or DIL (Dilatometry), EGA (EvolvedGasAnalysis).
These types of analysis are not only an obvious method for observing and pre-dicting the behaviour of an individual raw material or a body during drying, firing or cooling: they are also an excellent aid in determining the mineralogical composi-tion of a material, as the observed effects closely correlate with the crystalline struc-ture and the phase transformations of various minerals. Since the presence of min-erals and salts such as dolomite, carbonates, sulphates, sulphides and fluorides etc.
(Figs. 18-22) can be recognised, such analyses can also help identify chemical pa-rameters.
The instrumentation used for thermal measurements essentially consists of a measuring head (which houses the appropriate physical form of the sample and transforms variations in such sample into amplifiable, manageable electrical sig-nals) and a heating system. The latter generally uses electrical elements which must
Fig. 18. Comparison between the most significant (appropriately schematised) DTA curves represent-ing clayey minerals: A = kaolinite; B = metahalloysite; C = Na-montmorillonite; D = Ca-montmo-rillonite; E = vermiculite; F = sepiolite; G = polygorskite.
Fig. 20. Examples of dilatometric curves. Unfired samples undergo irreversible transformation (curve a) and fired ones, in cooling, essentially return along the curve followed during heating (curve b).
be extremely stable, homogeneous and programmable so as to ensure perfect repeatability of measurement.
It is often possible to combine several such analyses with just one instrument (e.g. TGA + DTA, TGA + DSC, DTA + EGA or TGA +DTA + EGA), allowing the user to obtain a wealth of information from just one measurement.
To provide the reader with a short summary of the operating principle
To provide the reader with a short summary of the operating principle