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ISO 26845:2008 — Chemical Analysis of Refractories — General Requirements for Wet Chemical, AAS and ICP-AES Methods
General requirements for chemical analysis of refractory products using wet chemical, AAS, and ICP-AES
1. Scope and Analytical Framework for Refractory Analysis
ISO 26845:2008 specifies the general requirements for the chemical analysis of refractory products and raw materials using wet chemical methods, atomic absorption spectrometry (AAS), and inductively coupled plasma atomic emission spectrometry (ICP-AES), prepared by ISO/TC 33, Refractories. It serves as a companion standard to four specific product standards: ISO 10058 (magnesite and dolomite), ISO 20565 (chrome-bearing refractories), ISO 21079 (alumina-zirconia-silica refractories containing 5% to 45% ZrO2), and ISO 21587 (aluminosilicate refractories). The standard provides an alternative to X-ray fluorescence (XRF) methods when XRF instrumentation is unavailable or does not meet precision requirements, particularly important for laboratories in developing regions or for arbitration analysis where primary methods are preferred.
The analytical scope covers major and minor element oxides including SiO2, Al2O3, Fe2O3, CaO, MgO, Cr2O3, ZrO2, TiO2, MnO, K2O, Na2O, and P2O5, as well as loss on ignition (LOI). The standard defines apparatus requirements (including spectrometers, platinumware, ion-exchange columns, and furnaces), reagent specifications, sampling protocols referring to ISO 8656-1 and ISO 5022, sample preparation procedures (lithium tetraborate fusion and acid digestion), and methods for expressing and validating test results with proper statistical treatment.
| Analytical Technique | Typical Applications | Detection Limits |
|---|---|---|
| Wet chemical (gravimetric/titrimetric) | SiO2 by gravimetry, Al2O3 by EDTA titration, CaO and MgO by complexometric titration | ~0.1% – 50% (major constituents) |
| Flame AAS (FAAS) | Fe2O3, MnO, Cr2O3, CaO, MgO, K2O, Na2O with N2O/C2H2 flame for refractory elements | ~0.01% – 5% (minor/trace) |
| ICP-AES | Full elemental suite including ZrO2, TiO2, P2O5, simultaneous multi-element | ~0.001% – 10% (trace to minor) |
2. Apparatus, Reagents, and Sample Preparation
The standard details a comprehensive list of laboratory apparatus including AAS and ICP-AE spectrometers meeting ISO 6955, molecular absorption spectrometers per ISO 6286, platinum crucibles (20 mL or 30 mL) and dishes (75 mL or 150 mL), cation-exchange resin columns (strongly acidic, 8% DVB, 75-150 um particle size) for separation, electric muffle furnaces capable of exceeding 1150 degree C, sand baths, steam baths, magnetic stirrers, ashless filter paper, and various volumetric glassware meeting ISO 1042 Class A requirements. All reagents must meet recognised analytical grade standards (ISO 6353 series). Special attention is given to the preparation of standard solutions for AAS and ICP-AES calibration, including matrix matching for complex refractory compositions.
Sample preparation is critical as refractory materials are notoriously difficult to dissolve due to their high-temperature stability (often fired above 1500 degree C during manufacture). The standard specifies lithium metaborate/tetraborate fusion at 1000-1100 degree C as the primary dissolution method in platinum crucibles, with acid digestion (HF/HClO4 or HF/H2SO4) as an alternative for certain materials like high-silica products where fusion may introduce contamination. Loss on ignition is determined gravimetrically at 1025 degree C +/- 25 degree C to constant mass, with the ignited sample mass used as the basis for all subsequent oxide calculations to ensure results sum to approximately 100%.
Platinumware requires special care: samples containing phosphorus, sulphur, heavy metals or easily reduced elements can attack platinum at high temperatures, forming low-melting-point alloys and causing mass loss or perforation. Separate platinum crucibles should be reserved for different sample types to prevent cross-contamination. Regular monitoring of crucible mass is essential.
3. Engineering Design Insights for Refractory Quality Control
The multi-technique approach specified in ISO 26845 reflects the compositional complexity of refractory materials. Silica (typically 30-95% of the sample) is determined gravimetrically after double dehydration with HCl or HClO4, ensuring complete precipitation of SiO2. Alumina is determined by EDTA back-titration or ICP-AES after chromatographic separation from interfering elements. For chrome-bearing refractories, the presence of chromium (as Cr2O3 at levels from 5% to 35%) requires specialised dissolution protocols using perchloric acid oxidation to ensure complete conversion to Cr(VI) without volatilisation losses. The cation-exchange resin column procedure described in Clause 4.3.4 is particularly valuable for removing interfering cations (Fe, Al, Ti, Zr) before AAS or ICP-AES determination of alkali and alkaline earth elements.
From a quality control perspective, the standard mandates reporting to two decimal places for oxides >1% and to one significant figure for those below. Acceptance follows a defined procedure: if duplicate determination difference exceeds the repeatability limit, a third determination is required and the median reported. The standard references ISO 5725-1 and ISO 5725-2 for statistical treatment of precision data. For complete analysis validation, the sum of all determined oxides plus LOI should approach 100%, typically within 99.5% to 100.5% for well-executed analyses.
For laboratories performing routine refractory analysis, implementing ISO 26845 alongside the appropriate product standard (ISO 10058, 20565, 21079, or 21587) ensures complete analytical coverage from raw material certification through to finished product quality control.
When switching from XRF to wet chemical/AAS/ICP-AES methods per ISO 26845, laboratories should expect longer analysis times (2-4 hours vs. 30 minutes for XRF) but gain the ability to analyse non-routine samples and achieve better accuracy for trace elements. The absolute nature of wet chemical methods makes them ideal for reference material certification.
4. Frequently Asked Questions
Q1: Can ISO 26845 be used for all refractory types?
The standard is designed for four specific refractory families (magnesite/dolomite, chrome-bearing, alumina-zirconia-silica, and aluminosilicate). For carbon-containing refractories or silicon carbide, the general principles may be adapted but specific procedures are not covered.
The standard is designed for four specific refractory families (magnesite/dolomite, chrome-bearing, alumina-zirconia-silica, and aluminosilicate). For carbon-containing refractories or silicon carbide, the general principles may be adapted but specific procedures are not covered.
Q2: What is the advantage of wet chemical over XRF?
Wet chemical methods provide absolute (primary) measurements without matrix-matched calibration standards, ideal for reference material certification and arbitration. XRF is faster but requires extensive calibration and is more susceptible to matrix effects.
Wet chemical methods provide absolute (primary) measurements without matrix-matched calibration standards, ideal for reference material certification and arbitration. XRF is faster but requires extensive calibration and is more susceptible to matrix effects.
Q3: How is loss on ignition (LOI) used?
LOI determined at 1025 degree C accounts for water, carbonates, organic matter, and oxidation state changes. The ignited mass is the reference basis for reporting oxide concentrations.
LOI determined at 1025 degree C accounts for water, carbonates, organic matter, and oxidation state changes. The ignited mass is the reference basis for reporting oxide concentrations.
Q4: What are the critical maintenance considerations for platinumware?
Avoid heating platinum with easily reducible elements (P, S, Pb, Sn, Bi), use separate crucibles for different sample types, monitor crucible mass regularly, and clean with fused potassium pyrosulfate or boiling HCl rather than mechanical abrasion.
Avoid heating platinum with easily reducible elements (P, S, Pb, Sn, Bi), use separate crucibles for different sample types, monitor crucible mass regularly, and clean with fused potassium pyrosulfate or boiling HCl rather than mechanical abrasion.
