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Exploring the potential of Hospital Medical Waste Incinerator (HMWI) bottom ash (BASH) as a promising raw material for the production of autoclaved bricks

Bekiari Konstantina-Maria

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Summary

In developed countries, the abandonment and uncontrolled disposal of bottom ash from hazardous medical and other (bio)-waste incinerators are the major cause of soil and groundwater contamination, which can endanger both the environment and public health. The present diploma thesis provides a detailed characterization and novel insights into the valorization of Hospital Medical Waste Incineration Bottom Ash (HMWI BASH) towards the production of pressed autoclaved bricks (a.k.a. “dense monoliths”). The HMWI BASH was provided by Hospital Medical Waste Incineration Plant / HMWIP (a.k.a. “APOTEFROTIRAS S.A.”) operated by the joint venture HELECTOR S.A. – ARSI S.A. (ELLACTOR Group). “APOTEFROTIRAS S.A.” is located near Athens, Greece. The raw material received by the incineration facility was classified into seven (7) granulometric fractions (i.e., +8mm, -8+4mm, -4+2mm, -2+1mm, -1+0.500mm, -0.500+0.250mm and -0.250mm) and characterized separately to identify mineralogical and chemical variations among the fractions which may affect their valorization potential.Powder X-ray diffraction data (PXRD) of the HMWI BASH raw sample and its individual fractions revealed a high amorphous content, especially in the raw one and its coarser fractions, gradually decreasing towards the finer fractions. The raw sample and its fractions consisted of high temperature crystalline phases, namely gehlenite (Ca2Al(AlSi)O7), perovskite (CaTiO3) and cristobalite (SiO2), encapsulated by the amorphous matrix. Bulk chemical analysis by means of wavelength dispersive X-ray fluorescence (WD-XRF) along with inductively coupled plasma optical emission spectroscopy (ICP-OES) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) showed that the studied samples contain a variety of heavy metals and metalloids, some of them being present at relatively higher concentrations (e.g., Cr, Ni, Cu, etc.), and others accumulated in smaller quantities (e.g., Co, Sn, V, As, etc.). Lead is present in all the studied samples (~3-9 ppm), while cadmium has been detected in some of the fraction (~1-3 ppm) by LA-ICP-MS. As it has been revealed by ICP-OES, it is noteworthy to mention the presence of gold (Au) in the studied raw material (15 ppm). It seems that gold is preferably accumulated in the coarser fraction (13 ppm) and less in the finest ones (8-10 ppm). Besides, rare earth elements (including 15 lanthanides, Sc and Y) content was found to be significantly increased in the finer fractions. The SEM-EDS observations revealed the presence of various metals-containing (sub)-micro-sized particles -also containing heavy metals such as Cr and Cu- embedded in a Si-Na-containing amorphous matrix, such as Ca-Ti-oxides (perovskites according to PXRD), etc.For the production of the pressed autoclaved bricks, the HMWI BASH material was used in its initial form (i.e., HMWI BASH raw sample), along with a 1:1 mixture of its -0.500+0.250mm and -0.250mm fractions, due to their lower amorphous and higher gehlenite content resulting in six different formulations, for comparison. Three mix designs were employed for the production of the bricks. The mix designs incorporated the aforementioned two HMWI BASH samples (IP100 mix design), the metakaolin MK (IP_MK mix design) and an ordinary Portland cement OPC (BLD mix design). Different dense monoliths were produced from each formulation. All the dense monoliths were cured for 24 hours under the same hydrothermal conditions in an autoclave. The produced autoclaved bricks were subjected to mineralogical investigation, compressive strength test assessment, and finally were undergone leaching process according to the EN 12457/2 standard. The mechanical performance and leaching properties of the bricks containing the HMWI BASH raw sample, were superior to the mixture of finer fractions, making the sieving process unnecessary for this application. The addition of metakaolin proved detrimental to the strength of the bricks. Among the six formulations used in the thesis, the IP-based bricks, being produced using the HMWI BASH raw sample, are more preferable. The IP-100-based dense monoliths were found to contain Sb and As both exceeding the EN 12457/2 limits by 14% and 15%, respectively. In contrast, the BLD-based bricks showed the best performance in immobilizing potentially toxic elements as the amount of heavy metals and metalloids is ranged within the EN 12457/2 limits. The latter might be attributed to the solidification facilitated by the OPC.

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