Utilization Of Waste Foundry Sand From Investment Casting In Refractories

Investment casting generates an average of 1.5 tons of used foundry sand per ton of casting output.The current reclamation rate of this sand is 30-60 wt%.A large amount of waste foundry sand?WFS?that cannot be reclaimed is disposed in landfills,which occupies land and harms the environment,especially groundwater and soil.Although some progress on the utilization of WFS from sand casting has been made,few studies investigate the utilization of WFS from investment casting.Therefore,it is necessary to find an appropriate way to dispose the WFS based on its individual characteristics.In this thesis,the characteristics of WFS are studied and the effects of various sintering temperatures on compression specimens composed of WFS are analyzed.In addition,the adverse environmental effects of disposing WFS in landfills are evaluated.Zircon and calcined clay are taken as raw materials to clarify the interaction between the main components of WFS.Based on the results obtained,WFS is utilized in dense and lightweight refractories.The main conclusions of this work are as follows:?1?The main phases of the WFS are mullite,cristobalite and zircon.The zircon exists in the form of independent grains as well as bonded with the matrix.Sintering at1500°C yields m-ZrO₂ and t-ZrO₂ as the breakdown products of zircon,and the cristobalite melts completely when the sintering temperature is increased to 1550°C.?2?Fe₃O₄,TiO₂,and regions of Cr/Mn enrichment are found in the WFS.Further,the concentration of Hg in the WFS leaching solution obtained through toxicity characteristic leaching procedure?TCLP?is beyond the standard limit.?3?The phase compositions and physical properties of calcined clay are significantly affected by the addition of zircon.Mullite,cristobalite,and zircon are the main phases when the compression specimens composed of zircon and calcined clay are sintered at 1500°C.The melting of cristobalite from calcined clay is accelerated by the addition of zircon.Due to the framework structure formed by the zircon and mullite,the shrinkage of the calcined clay during sintering is suppressed.The modulus of rupture?MOR?at room temperature and high temperature?1100°C?are improved with the addition of zircon,and the fracture mode of zircon grains is predominantly transgranular in both.For the hot elastic modulus measurement,stabilization of the modulus is improved by the addition of zircon.The zircon has a shielding effect for 2.6-6.7?m infrared rays,and therefore decreases the thermal conductivity at high temperatures.?4?Mullite-zirconia and cordierite-zirconia composites are prepared with WFS by adding alumina powder and magnesium oxide,respectively.The ZrO₂ in the mullite-zirconia composite is monoclinic.The sintering temperature is the determining factor in the formation of cordierite,and excess magnesia results in an increase in the coefficient of thermal expansion.?5?In the preparation of lightweight aggregates,when 80 wt%WFS is added,the aggregates exhibit a uniform microstructure.Further,their thermal conductivity in the range of 200-1000°C is 0.27-0.38 W·?m·K?^-1.For the lightweight bricks,the thermal conductivity in the range of 200-1000°C is 0.13-0.92 W·?m·K?^-1 after adding 55 wt%WFS.The concentrations of specific heavy metal elements in the leaching solution of the aggregates and bricks are within the standard limit.