10. Direct experimental evidence: from the model mixture to the metal-insulation interface
The historical patent specification describes the reaction pathway. Four primary studies confirm it experimentally at different levels and close the chain of evidence from the model experiment to the realistic interface.
| Study | Constellation investigated | Central finding | Significance |
|---|---|---|---|
| Paoletti (2002) | Cr₂O₃ with CaO in synthetic air and in inert atmosphere; thermobalance (TG-DSC) and XRD. | Rapid mass increase between 500 and 850 °C; XRD detects CaCrO₄ after 850 °C. In inert atmosphere only trivalent Ca(CrO₂)₂. [50] | Early direct phase evidence of CaCrO₄ from Cr₂O₃ and CaO; oxygen demonstrated as an essential prerequisite. The work was carried out in the context of municipal waste incineration; for grate ash and the pilot plant see section "Paoletti 2002" under combustion ashes. |
| Verbinnen et al. (2013) | Cr₂O₃ with CaO, NaOH or KOH under thermal treatment. | With NaOH, 83% at 500 °C and 89% at 700 °C were leached as Cr(VI) after 60 minutes. CaO also opens up the chromate formation pathway. | Demonstrates the thermal-oxidative formation pathway with calcium and sodium compounds and the role of oxygen. |
| Mao et al. (2015) | Chromium-containing sludge with CaO, 200-1,200 °C. | Between 500 and 900 °C, leachable CaCrO₄ forms; at 900 °C the maximum Cr(III) oxidation was observed after one hour. | Demonstrates the direct role of CaO and the pronounced temperature dependence. |
| Sayano et al. (2015) | Direct contact of Cr-containing steels with Ca-Si-O or Na-containing insulation materials; up to 5,000 h at 773/873 K. | Yellowish reaction products were identified by XRD as CaCrO₄ and Na₂CrO₄ respectively. The amount of Cr(VI) depends on metal, temperature and time. | Direct experimental evidence at the interface between chromium-containing metal and thermal insulation material. |
Verbinnen et al.: the role of basic calcium and sodium compounds
Verbinnen, Billen, Van Coninckxloo and Vandecasteele investigated the temperature dependence of Cr(III) oxidation in synthetic mixtures of Cr₂O₃ and alkali or alkaline earth compounds. The study clearly separates a point that is often blurred: in the relevant temperature range, Cr₂O₃ is not readily oxidised to Cr(VI) by atmospheric oxygen alone. The basic calcium, sodium or potassium component opens up the chromate formation pathway.
The experiments yielded maximum Cr(VI) leaching in the range of 600 to 800 °C. Under inert atmosphere, the Cr(VI) leaching of an industrial sludge fell from 24.2 mg/kg to 0.42 mg/kg after 30 minutes at 700 °C. The role of oxygen is thus not merely postulated thermodynamically but demonstrated experimentally.
The quantitative data are to be read as leaching findings with the respective temperature and holding time; they are not general conversion values for industrial insulation. [9]
Mao et al.: CaO forms CaCrO₄ in the temperature window from 500 to 900 °C
Mao, Gao, Deng and other authors investigated chromium-containing sludge in the presence of CaO over the range from 200 to 1,200 °C. They distinguish two relevant reaction ranges:
• Between 200 and 400 °C, a Cr(VI)-containing intermediate oxide (Cr₂O₃+x) formed in chromium hydroxide/CaO mixtures.
• Between 500 and 900 °C, Cr(III) reacted directly with CaO to form leachable CaCrO₄. At 900 °C, maximum oxidation was observed after one hour; extending the holding time did not significantly reduce the amount of calcium chromate in this range.
The study thus confirms that calcium oxide is not merely a favourable background component but a direct reaction partner in the formation of calcium chromate.
Sayano et al.: direct evidence at the contact between hot component and insulation material
The work of the Japanese authors Sayano, Kanno, Inagaki, Takahashi and Yoshida is particularly significant for the assessment of industrial components with high-temperature insulation. What was investigated was not a sludge or a loose laboratory powder mixture, but the direct contact between Cr-containing metals and thermal insulation materials.
The authors placed, among others, SUS304 with 18.2% Cr as well as low- and high-chromium steels on calcium-silicate and sodium-containing insulation materials. The calcium-silicate block insulation contained 35% CaO; a sodium-containing insulation material 7.3% Na₂O. After heat treatment in air, yellowish reaction products were detected at the contact surfaces:
• For the calcium-containing insulation material, XRD analysis identified CaCrO₄; for the sodium-containing insulation material, Na₂CrO₄.
• For SUS304 and calcium-silicate insulation material, 160 mg Cr(VI) per m² of contact surface was measured after 1,000 hours at 873 K (approximately 600 °C).
• After 5,000 hours at 873 K, the amount of Cr(VI) in the uncoated SUS304/Ca-Si-O system was 940 mg/m²; with the additional calcium-containing, self-hardening coating insulation material it was 3,890 mg/m².
The authors also show the mass transport: chromium diffuses from the metal into the insulation material, oxidises and reacts with the basic insulation component. An Al₂O₃-based barrier layer only about 3-4 µm thick suppressed this chromium diffusion and almost completely prevented Cr(VI) formation. This also makes the preventive logic clear: it is not the subsequent treatment of chromates already formed, but the prevention of reaction contact or the substitution of basic oxides that prevents the pathway by which the substance is formed.
Consequence of the overall picture
The four studies leave no room for the representation that the formation of calcium and sodium chromate on chromium-containing hot components is merely an untested possibility. The reaction has been
1. chemically described (Udy; Verbinnen),
2. experimentally demonstrated as temperature-dependent (Paoletti; Mao; Verbinnen) and
3. directly observed at the metal-insulation interface with the identified target products CaCrO₄ and Na₂CrO₄ (Sayano).
Sources cited in this section
- 9.Verbinnen, B.; Billen, P.; Van Coninckxloo, M.; Vandecasteele, C.: Heating Temperature Dependence of Cr(III) Oxidation in the Presence of Alkali and Alkaline Earth Salts and Subsequent Cr(VI) Leaching Behavior. Environmental Science & Technology 2013, 47, 5858-5863. DOI: 10.1021/es4001455. Submitted study
- 50.Paoletti, Francesca: Behavior of oxyanions forming heavy metals in municipal solid waste incineration. Dissertation, University of Stuttgart; Forschungszentrum Karlsruhe, Wissenschaftliche Berichte FZKA 6755, 2002. In particular summary pp. II and V, pp. 39-41, Table 6.3 (p. 78).