Turning toxic industrial waste to something useful : thermal treatments of Cr-bearing sludges from tanneries for inertization and recycling

Leather treatment in the tanneries may employ high dosages of chromium. Beside the smell in the immediate neighbourhood, the production of great quantities of Cr-rich waste waters makes this type of industrial activity as one of the worst for the environment. Tannery waste waters and their solid sediment (sludge) may be variably treated before being disposed: more frequently lime is added in order to oxidate proteins; attempts are done to recover tanning Cr. Only one tannery in Scandinavia owns a pyrolysis plant where Cr is recovered as Cr-Fe alloy and a safely disposable vitrified and unleachable slag is produced. We started a series of thermal treatments on waste sludge from tanneries. Our aim is to turn the inorganic part of the sludge into a harmless ceramic slag. Then this material can possibly be recycled, e.g. in road construction or in concrete. The composition of the inorganic fraction of the tannery sludge includes variable amounts of Ca (depending on treatment with lime), Na, Cr and Fe (both in the % range), with subordinate Mg, Mn, Al, P, Ti, Zn, etc.. We prepared series of mixtures of variably treated waste with natural additives such as kaolinitic clay, magnesite, silica (quartz) and in some cases alumina. We planned to induce crystallization of chemically stable Ca-Al-Mg silicates and oxides immobilizing Cr, Fe and the other metal components. The ratio of waste to additives was always between 1:1 and 3:2. We used sludge in four different conditions: sludge with or without lime treatment and with simple dehydration at 120°C or heated at 550°C (decomposition of organics). The mixed bulk compositions were planned to correspond to three different mineral assemblages containing Ca-silicates and spinel: gehlenite + spinel, diopside + spinel and anorthite + spinel. The mixtures were pressed in cylindric pellets and heated in a vertical furnace up to 1300°C in several runs according to different thermal schemes. The thermal schemes comprise (a) rapid heating, (b) dwell at the maximum temperature variable between one to four hours and (c) slow cooling (to prevent glass formation).The runs lasted less than 1 day in order to simulate rapid industrial-scale treatment, and at first they were done in moderately reducing atmosphere determined by flux of a mixture of air and CO2 (50 cc/min each) to prevent oxidation of Cr. The run products were characterized for their phase assemblages by x-ray powder diffraction techniques, by scanning electron microscope (SEM) equipped with EDS for chemical microanalysis and by x-ray data refinement methods (Rietveld). The run products are characterized by dominant granular texture, fine to very fine grain size, high porosity and disequilibrium textures due to short residence time at high temperature. The phase assemblages obtained contain a range of Ca-bearing silicates such as melilites (gehlenite-åkermanite), diopside-type pyroxene, anorthite and grossular-type Ca-garnet.They occur as fine to coarse grains and/or reaction rims around newly-formed phases or unreacted/refractory grains. Unreacted and refractory phases are represented by quartz, periclase from decomposition of magnesite or corundum (when added).The porous matrix to the coarser grains may also include aggregates of very fine-grained Cr-Fe(-Zn)-rich Mg-spinels, undetermined lamellar Fe-Cr phases and phosphates. Occasionally disseminated micro-pellets of Cr-Fe-Ti-Al alloys occur. The best results in terms of textures and phase assemblages significant for effective inertization are those for bulk compositions involving sludges pre-heated at 550°C (i.e. without organic component) and with longer residence time (e.g. 2 to 4 hours) at high temperature: better degree of crystallization, coarser textures, greater abundance of stable phases and minimal gaseous emissions.