Probing the structural role of Cr in stabilized tannery wastes with X-ray absorption fine structure spectroscopy.

The effective stabilization of tannery sludge wastes is explored using X-Ray Absorption Fine Structure (XAFS) spectroscopies. Solidification of the Cr-rich waste was realized via vitrification of the incinerated sludge with silica and flux agents. It is demonstrated that the effective reduction of Cr(VI) and the structural role of Cr are strongly modulated by the chemical composition of the waste. Eskolaite microcrystallites are embedded in the silica matrix of all vitrified samples and the extent of microcrystalline formation is strongly related to the glass basicity. Both Cr(VI) and Cr(III) species are identified, corresponding to Cr(VI)O4 glass formers and Cr(III)O6 network modifiers. The toxic Cr(VI) prevails only in the glasses with the highest basicity index and lowest waste content, nevertheless it is safely incorporated and immobilized in the silica matrix. However, the detected abundance of Cr(VI) increases glass basicity and as a result, glass polymerization is hindered. Thermal treatment, a process that leads to glass-ceramics transforms almost all Cr(VI) to Cr(III), while eskolaite formation is promoted concurrently. Nevertheless, microcrystalline growth proceeds mainly via depletion of Cr(III) from the silica matrix and not from the reduced Cr(VI); yet, Cr-removal from the glass matrix does not impair the chemical stability of the devitrified products.

Vitrification of incinerated tannery sludge in silicate matrices for chromium stabilization.

The vitrification process was applied for the stabilization and solidification of a rich in chromium ash that was the by-product of incineration of tannery sludge. Six different batch compositions were produced, based on silica as the glass former and sodium and calcium oxides as flux agents. As-vitrified products (monoliths) were either composed of silicate matrices with separated from the melt Eskolaite (Cr2O3) crystallites or were homogeneous glasses (in one case). All as-vitrified products were thermally treated in order to transform them to partially crystallized, i.e. devitrified products. Devitrification is an important part of the work since studying the transformation of the initial as-vitrified products into glass-ceramics with better properties could result to stabilized products with potential added value. The devitrified products were diversified by the effective crystallization mode and separated crystal phase composition. These variations originated from differences in: (a) batch composition of the initial as-vitrified products and (b) thermal treatment conditions. In devitrified products crystallization led to the separation of Devitrite (Na2Ca3Si6O16), Combeite (Na4Ca4Si6O18) and Wollastonite (CaSiO3) crystalline phases, while Eskolaite crystallites were not affected by thermal treatment. Leaching test results revealed that chromium was successfully stabilized inside the as-vitrified monoliths. Devitrification impairs chromium stabilization, only in the case where the initial as-vitrified product was a homogeneous glass. In all other cases, devitrification did not affect successful chromium stabilization.

Incineration of tannery sludge under oxic and anoxic conditions: study of chromium speciation.

A tannery sludge, produced from physico-chemical treatment of tannery wastewaters, was incinerated without any pre-treatment process under oxic and anoxic conditions, by controlling the abundance of oxygen. Incineration in oxic conditions was performed at the temperature range from 300°C to 1200°C for duration of 2h, while in anoxic conditions at the temperature range from 400°C to 600°C and varying durations. Incineration under oxic conditions at 500°C resulted in almost total oxidation of Cr(III) to Cr(VI), with CaCrO4 to be the crystalline phase containing Cr(VI). At higher temperatures a part of Cr(VI) was reduced, mainly due to the formation of MgCr2O4. At 1200°C approximately 30% of Cr(VI) was reduced to Cr(III). Incineration under anoxic conditions substantially reduced the extent of oxidation of Cr(III) to Cr(VI). Increase of temperature and duration of incineration lead to increase of Cr(VI) content, while no chromium containing crystalline phase was detected.