Fixing chromium and lead in ceramic materials : a petrological approach to inertization and recycling of toxic industrial waste

Industrial wastes rich in toxic heavy metals are produced in high amounts yearly. As other hazardous wastes, they require special, expensive disposal, and they may represent a serious environmental and health issue in case of heavy metal dispersion. Petrology may offer useful tools to effectively inertize hazardous industrial wastes and eventually recycle them back in industry. Mineral phases like oxides and aluminosilicates, occurring in natural rocks and employed in the ceramic industry, have a high potential for long-term bonding a wide variety of refractory as well as low-melting heavy metals. These resistant phases may then be used to incorporate a wide range of hazardous metal components in waste during inertization treatments. Experimental work was done on petrological models like the MgO-SiO2-Al2O3 (MAS) system and its little known extensions to Cr2O3 (MASCr) as well as to the low-melting PbO (PMASCr). Experiments were planned with different bulk compositions and on a wide range of temperature in order to test feasibility and efficiency of a petrology-based inertization of highly chromiferous and Cr-Pb-rich industrial wastes, as those from tanneries and from galvanic processes. Run products exploring the refractory MASCr system between 1250° and 1560°C showed that the addition of Cr contributes to stabilize the refractory, Cr-rich phases of the MAS system and to lower the thermal minimum of the system by approximately 100°C. Different Cr-bearing phases are dominant in the different portions of the system, from spinels in the most Mg-rich bulk compositions to sapphirine and mullite in the least Mg-rich ones. Glass occurs in all runs and is Cr-poor. Cr2O3 content of Cr-hosting phases in the run products may vary between 100 wt% in pure eskolaite to 60 wt% maximum in spinel, 30 wt% in sapphirine down to 12-23 wt% in mullite. Spinels and sapphirine and the most abundant and most interesting phases in the view of inertization. Experiments in the unknown PMASCr system were planned (a) to cope with compositions of galvanic sludge (Crtot + Pb oxides > 30 wt%, molar Cr:Pb about 1:1), and (b) to induce simultaneous crystallization of Pb-feldspar and Cr-bearing spinel between 950°C and 1050°C, from subsolidus and from melt conditions (after short high-temperature treatment at 1350°C). The planned association of Pb feldspar and Cr-rich spinel (with Cr2O3 up to 60 wt%) occurs in all runs, even at low temperature, and in association with terms of the eskolaite-corundum solid solution. Spinel and Pb feldspar are the most abundant phases crystallized except in the runs treated at high temperature, where Cr-poor, Pb-bearing glass can overcome Pb feldspar. Cr and Pb are therefore completely separated in distinct phases. Both groups of experiments are highly encouraging for waste-oriented applications: heavy metals like Cr and Pb can be efficiently bondend in crystalline phases and they are sharply fractionated, thereby allowing further recovery of the economically interesting Cr-rich phases for recycling and safe disposal of the remaining inertized waste.