İNŞaat mühendiSLİĞİ anabiLİm dali

Recycling of Al-Rich Anodizing Sludge and the Development of Mullite Based Ceramics
During the anodizing process, which is carried out to increase resistance against corrosion and abrasion and to lengthen physical life of aluminum extrusion products by producing controlled oxide layer, aluminum rich sludge names as “anodizing sludge” is produced at high tonnages. Despite that anodizing sludge is classified as nonhazardous waste, costs of disposal process should be lowered since its annual production amount is considerably high. The sludge contains solid matter that contains 15 to 30% inorganic compounds by weight. wt 90% of the solid matter is boehmite (AlOOH) and the remaining portion is thenardite (Na2SO4) and barite (BaSO4).
Sodium removal procedure that includes leaching, filtering and drying steps was applied to anodizing sludge containing approximately wt 3% sodium as it prevents mullite formation at high temperatures. The cycle was repeated until sodium was completely removed. The sodium free powder obtained was transformed to alpha alumina (α-Al2O3) phase by calcining at 1400°C for 1 hour at a heating rate of 5°C/min. α-Al2O3 powder produced was mixed with kaoline, diatomite and clay at proportions 15%, 28% and 15%, respectively. Then, it was shaped by dry press, slip casting and extrusion, and sintered for 1, 3 and 5-hour periods between 1450°C and 1550°C. This procedure was repeated with commercial α-Al2O3 powder (Code A-16) purchased from Alcoa Company instead of α-Al2O3 powder produced by leaching and tempering anodizing sludge (kaoline, diatomite and clay proportions were kept constant).
When α-Al2O3 powder obtained by pretreatment of anodizing sludge is utilized, production of ceramic material containing 100% mullite was achieved. The material obtained was 12.35 to 17.39% smaller with10% weight loss. Furthermore, relative density value was between 63.9% and 78.8% and porosity was between 0.72% and 26.08%. Strength value was between 53.12 and 84.42 MPa and crystallite height was between 927 and 2082.6 Å.
Mullite ceramics produced using commercial Alcoa A-16 α-Al2O3 were observed to shrink 7.13 to 8.18% with a weight loss of 8%. Relative density was between 66.8 and 70.3%, porosity was between 0.81% and 6.36%, strength was between 72.18 and 79.51 MPa and crystallite height was between 1115,2 and1782,9 Å.
In conclusion, it was found that if appropriately treated and mixed with natural mineral additives such as clay, kaoline and diatomite, anodizing sludge, one of the wastes of aluminum industry, can be utilized in the production of mullite-based ceramic materials, which are used in many areas such as ceramic filters, ignition elements, high-temperature furnaces, jet motors, rolls used in firing kilns, laboratory equipment, roof ceramic furnaces and steam turbines.