A Study on Recycling Gypsum

用过的石膏被当作医疗废物处理。我们着眼于石膏遵循可逆反应的事实，并以回收利用废旧石膏为目标。根据本研究，将硬化后的商品*-半水石膏加热至180℃，加热210 min后，将其粉碎至106 μ m的粒径，以粉水比0.50的比例与水揉搓，石膏再次凝固并产生一定的放热反应。在此条件下获得的干抗压强度平均为84.7 kgf/cm^2，为典型-半水石膏的三分之一或更低。在180°C的温度下加热210分钟对于实际的回收应用来说太长了，因此我们将使用过的石膏加热到700°C的温度，这是普通牙科炉的温度水平。每分钟测量用过石膏的重量变化。在700℃的温度下加热10分钟后，石膏粉的失重量与揉制石膏粉所用的水的重量相当。一种改进的粉碎方法的粒度小于45 μ m，收集率提高到60%。在粉水比为0.43的条件下，经加热粉碎后的再生石膏粉的干抗压强度平均为251.4 kgf/cm^2，与在180°C温度下加热210分钟，再粉碎至106 μ m以下的条件相比，具有显著的提高。干抗压强度约为典型-半水石膏的70 ~ 80%。干抗压强度提高的原因可能是粉碎方法的改进使石膏的粒度与商品*-半水石膏的粒度几乎相等，而较少的水掺比使其具有足够的处理能力。为了进一步缩短加热时间，我们测试了使用电子烤箱。300g *-半水合石膏在1600w的电子烘箱中，以70%的功率处理10min，其质量损失几乎等于加入的揉制水的重量。当石膏粉碎至小于45 μ m的粒度，并与水按粉水比0.43进行揉搓时，石膏发生放热重凝反应。然而，当放入电子烤箱的石膏量超过300克时，将石膏粉与水揉搓后不复位。因此，使用电子烤箱不应适用于一次回收大量石膏。少

Used gypsum is treated as a medical waste. We focused on the fact that gypsum follows reversible reaction, and aimed to recycle the used gypsum. According to our study, when a hardened commercial *-hemihydrate gypsum was heated to 180ﾟC for 210 minutes, and was pulverized to 106 mum of particle size to knead with water at a powder/water ratio of 0.50, the gypsum set again with some heat generation of exothermic reaction. The dry-compressive strength gained under this condition was 84.7 kgf/cm^2 as average, which was at one third or less of that of typical beta-hemihydrate gypsum.The heating time of 210 minutes at 180ﾟC is too long for practical recycle application, therefore, we heated a used gypsum to a temperature of 700ﾟC which was available temperature level in ordinary dental furnace. The weight change of the used gypsum was measured at every minute.The weight loss after 10 minutes heating at 700ﾟC showed approximatlly the same weight of water applied to knead the gypsum powder. A … More n improved pulverizing method gave less than 45 mum of particle size, and the collecting ratio increased to a level of 60 %. When the heated and pulverized recycled gypsum powder was kneaded with water at a powder/water ratio of 0.43, the dry-compressive strength became 251.4 kgf/cm^2 as average, which was significantly high comparing with that obtained under the condition of heating at 180ﾟC for 210 minutes followed by pulverizing to 106 mum or less of particle size. The value of dry-compressive strength was a level of around 70 to 80 % of that of typical beta-hemihydrate gypsum. Presumable reasons of improved dry-compressive strength are that the improvement of pulverizing method provides nearly equal particle size with that of commercial *-hemihydrate gypsum and that less water mixing ratio provides sufficient treatability.For further shortening the heating time, we tested to apply an electronic oven. When 300 g of *-hemihydrate gypsum was treated in an 1600 W electronic oven with 70 % of power output for 10 min., the resulted mass loss was nearly equal to the added water weight for kneading. When the gypsum was pulverized to less than 45 mum of particle size and was kneaded with water at powder/water ratio of 0.43, the gypsum re-set in exothermic reaction. When, however, the quantitity of gypsum placed in the electronic oven exceed 300 g, it did not re-set after kneading the pulverized gypsum powder with water.Consequently, the use of electronic oven should not be applicable for recycling a large amount of gypsum at a time. Less