Study On Performance Characteristic And Reclaimable Capability Of Used Sodium Silicate Sand Processed By Non-room Temperature

The sodium silicate sands process possess many advantages including being odor free, non-toxic and environmentally friendly, hence it is considered as one of the most promising casting mould sands to allow low environmental impact. However, the difficult problem of the used sodium silicate sand reclamation must be solved to realize the green casting based on the sodium silicate sand process. In the present dissertation, the performance characteristic of used sodium silicate sands processed by non-room temperature was studied first. Afterward, the dry reclamation effectiveness of used sands processed by non-room temperature and the performance of its reclaimed sands were tested through the experiments. Furthermore, the methods for improving the reclaimed property of used sodium silicate sands were discussed from the modified sodium silicate and hardening process.The physical appearance and chemical composition of the used sodium silicate sands processed by heating or freezing were studied systematically. When the heating temperature was 320℃-520℃, the binder film covered on the used sand was broken badly, and the sodium silicate binder was converted into amorphous structure from composite structure with crystalline and amorphous at room temperature, which is helpful to deskinning and reclamation since the materials with amorphous structure are more brittle than that with crystalline structure. Frozen at -10℃- -40℃, air bubbles and ice crystals emerged in the hydrous sodium silicate binder film, and the remnant binder on the surface of the used sands could be changed into amorphous state result in increasing the deskinning rate of used sands.The mechanical properties of sodium silicate sand samples processed by non-room temperature were tested firstly, and the mechanical conditions for deskinning and reclamation of the used sands were deduced. Consequently, the mathematical formula was established to judge the reclaimable capability of the used sands. After analysis, it was found that the key whether the used sands could be reclaimed or not depends on the mechanical property of the remnant binder film covered on the used sands, namely the Young's modulus E and the yielding compression stressσ_s. The bigger the E/σ_s value is, the severer the remnant binder film damaged degree is, and the easier the used sands reclamation is. In the present dissertation, the mechanical property E/σ_s of the hardened sand samples processed by heating or freezing was practical tested firstly, which could reflect the mechanical property of the binder film directly. The E/σ_s values of the sand samples were big if they were heated between 320℃and 520℃or frozen between -10℃and -40℃, the binder film could be removed by colliding or rubbing easily, and the reclaimable capabilities of the used sands were very good. In the condition of freezing, the mechanical properties of the sand samples were affected by their moisture content and freezing temperature greatly. When the moisture content is definite, the E/σ_s values of the sand samples increased as the freezing temperature lowered; and when the freezing temperature is invariant, the E/σ_s values of the sand samples containing 10% moisture are the biggest.A new reclamation method of used sodium silicate sands was invented, which is the "freezing-mechanical" dry reclamation. The dry reclamation effectiveness of used sodium silicate sands processed by non-room temperature was tested using the multi-functional dry reclamation device developed by ourselves. The deskinning rate of "freezing-mechanical" dry reclamation of the used sodium silicate sands using the optimized technical parameters could reach about 43%, which is much bigger than the deskinning rate of the common mechanical dry reclamation(5%-10%) and 120℃-520℃"heating-mechanical" dry reclamation(15%-25%), the used sands was not heated in the process of common mechanical dry reclamation. Using liquid nitrogen or natural sub-zero temperature to freeze the used sodium silicate sands is feasible. The deskinning rate would be about 40% and 32% respectively when using liquid nitrogen cooled the used sands to -40℃and adopting the natural sub-zero temperature cooled the used sands to -10℃--15℃.The effect of processed temperature on the characteristics of the reclaimed sodium silicate sands was studied. The rebind capabilities (strength and live time) of the reclaimed sands were improved when the used sodium silicate sands were heated between 320℃and 520℃, but the collapsibility was deteriorated severely. The collapsibility of the sands reclaimed by "freezing-mechanical" dry reclamation at about -40℃was good. Compared with the common dry reclaimed sands, the rebind strength of "heating-mechanical" (320℃-520℃) reclaimed sands increased about 130%, the live time prolonged approximately 80%-100%, but the remnant strength enhanced about 35%, the collapsibility deteriorated; the rebind strength of "freezing-mechanical" (-40℃) reclaimed sands increased approximately 90%, the live time prolonged about 60%-75%, and the remnant strength reduced about 20%, the collapsibility was very good.The selection principle of materials for modifying sodium silicate was proposed based upon the reactive mechanism of silicate system in the sodium silicate sands at high temperature. The solid mineral powder, liquid compound and nanometer oxide powders were selected to modify the sodium silicate and raw sand. When the sodium silicate and raw sand was modified by the solid mineral powder and corresponding assistant agent, the room temperature strength of the sodium silicate sand increased about 30%-40%, and the remnant strength reduced about 60%-70%. The effectiveness of sodium silicate sand modified by liquid compound was also good, that the remnant strength decreased about 78% and the room temperature increased approximately 16% contrasted with the common sodium silicate sand. However, the effect of sodium silicate modified by nanometer oxide powders was not perfect. Furthermore, the sodium silicate sand modified by solid mineral powder and liquid compound has better recycled characteristics than common sodium silicate sand.The effect of microwave heating hardening process on the reclaiming performance of used sodium silicate sand was researched. In the microwave heating process, when the adding quantity of sodium silicate is 1.5%, the room temperature strength of the sand samples was a little higher than the ones with 3% sodium silicate hardened by ester, and the remnant strength was only 1/7 of the latter ones, which is helpful for improving the reclaiming performance of used sand. But the moisture absorbability of sodium silicate sands hardened by microwave heating is quite high, and the requirements of mould materials in the microwave heating process is very strict. Aimed at the problem of strict requirements for mould materials, a new method for twice heating sodium silicate sand heated by microwave was invented. The invention reduced the requirements of mould materials greatly in the microwave heating process. The common wood mould and plastic mould could be used in the process. The microwave heating efficiency was increased, and the process of sodium silicate sand hardened by microwave heating made a great stride towards the practical application.The effect of ultra-fine powders hardening process on the reclaiming performance of used sodium silicate sand was studied. When the adding quantity of powder is 25% of the sodium silicate addition, compared with the sand samples with 5% sodium silicate hardened by 70μm powders, the room temperature strength of the sand samples with 3% sodium silicate hardened by 5μm ultra-fine powders increased about 27%, and the remnant strength reduced approximately 85%. The sodium silicate sand hardened by ultra-fine powders is not sensitive to the module of the sodium silicate. Contrast to the sand samples using sodium silicate with module 2.3, the 24h strength of sand samples using sodium silicate with high module (2.7) is much closed to its, but the remnant strength of latter ones is lower than that of former ones, which deduced about 58%. Therefore, the process of sodium silicate sand hardened by ultra-fine powders could improve the collapsibility and reclaimable capability of used sodium silicate sand consumedly.