Thermal treatment of solid waste in view of recycling: Chromate and molybdate formation and leaching behaviour

Abstract

Elevated Cr and Mo concentrations are often found in leachates of thermally treated solid waste, but there is no general explanation for this so far. Therefore, we studied the leaching behaviour after thermal treatment as a function of heating temperature and residence time for two types of solid waste: contaminated sludge and bottom ash from municipal solid waste incineration. The leaching behaviour of both waste streams was compared with experiments on synthetic samples, allowing deduction of a general mechanism for Cr and Mo leaching. Cr and Mo showed a similar leaching behaviour: after an initial increase, the leaching decreased again at higher temperatures. Oxidation of these elements from their lower oxidation states to chromate and molybdate at temperatures up to 600 °C was responsible for the increased leaching. At higher temperatures, both Mo and Cr leaching decreased again owing to the formation of an amorphous phase, incorporating the newly formed chromate and molybdate salts, which prevents them from leaching.

In-depth mineralogical quantification of MSWI bottom ash phases and their association with potentially toxic elements

“…FeO x was more concentrated in the bigger size fractions of BA (fraction M) due to its mechanical resistance and contained the majority of Zn, Cu and Ni (Table 4), which led to the higher contents of these PTEs released from M. Interestingly, in the F1 extraction solutions (initial pH 2.85) from both fractions, no antimony was detected. Although, anti-mony from MSWI bottom ash is reported to be leachable at acidic pH (Verbinnen et al, 2014). Furthermore, the leaching analysis according to column leaching test showed the presence of leach-Fig.…”

Recovery of phosphorus from sewage sludge ash: Influence of incineration temperature on ash mineralogy and related phosphorus and heavy metal extraction

“…It is therefore difficult to link the lab experiments to the sudden mineralogy change predicted by the thermochemical calculations. Verbinnen et al [67] reported a similar decrease when leaching Cr with water from contaminated sludge incinerated at temperatures ranging between 400 and 900 °C. They suggested that the highly soluble chromates present at low temperatures will start to form a silicate melt as the incineration temperature increases, in this way preventing the Cr from leaching [67].…”

Recycling of spent adsorbents for oxyanions and heavy metal ions in the production of ceramics

“…After 30 min, the produced aggregates were removed from the furnace and allowed to cool at room temperature. The temperature and residence time were chosen based on the results from earlier research (Verbinnen et al, 2013a(Verbinnen et al, , 2014, in which was shown that 30 min at 1100 °C was optimal for stabilizing oxyanion forming compounds like Cr and Mo (Figure 1), and for obtaining lightweight aggregates. Other toxic elements as given in Table 1, are stabilized at lower temperatures (Table 1).…”

“…This indicates that Mo is effectively stabilized in the matrix of the sludge during heat treatment. Indeed, Mo is incorporated in a glassy phase at temperatures of 1100°C (Verbinnen et al, 2014), preventing it from leaching. The elevated temperature is needed to form aggregates with high mechanical strength and to effectively stabilize Mo and Cr, but is far lower than the temperatures that are reported for similar waste streams that are stabilized by full vitrification of the materials (typically 1400 -1500 °C, Li et al, 2007;Bingham and Hand, 2006;Kavouras et al, 2003).…”

“…Contaminated sludge and sediments have often been used to produce ceramics (Alonso-Santurde et al, 2008;Gonzalez-Corrochano et al, 2012). From earlier research on recycling of sludge from soil cleaning for the production of ceramics (Verbinnen et al, 2014), it appeared that upon heating inorganic sludge, the leaching concentrations of the toxic elements Ni, Cu, Zn, As, Cd and Pb could be reduced (Table 1). This was also reported for other waste streams that were used to produce ceramics (Chang et al, 2007;Xu et al, 2008;Quijorna et al, 2012), and is related to removal of organic acids responsible for the formation of mobile organo-metallic complexes, and/or a change in pH, and/or incorporation of the heavy metals in stable phases from which they do not leach easily (Xu et al, 2008).…”