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，旨在回收使用过的石膏。根据我们的研究，当硬化的商业*半水石膏加热到180umﾟC 210min，并被粉碎到106um，以粉/水比0.50的水搅拌时，石膏再次凝固并放热反应。在此条件下获得的干压强度平均为84.7kgf/cm2，仅为典型β-半水石膏的1/3或更低，在180ﾟC下加热210min太长，不适合实际回收应用，因此，我们将用过的石膏加热到700kgf C，这是普通牙科加热炉可以达到的温度水平。每分钟测量用过的石膏的重量变化。在700ﾟC加热10分钟后的重量损失表明，用来捏合石膏粉的水的重量大致相同。A…改进后的粉碎方法粒度小于45微米，捕收率提高到60%左右。当加热粉碎后的再生石膏粉与水按粉水比251.4 43混合时，干压强度平均达到251.4 kgf/cm2，显著高于在180ﾟC加热210min后粉碎至106um或更小的条件下的抗压强度。干压强度约为典型β-半水石膏的70%~80%。干压强度提高的原因可能是粉碎方法的改进提供了与商业*半水石膏几乎相同的粒度，更少的水混合比提供了足够的可处理性。为了进一步缩短加热时间，我们试验了应用电子炉。当300g半水石膏在功率为70%的1600W电烤箱中处理10min时，所产生的质量损失几乎等于捏合所需的加水重量。当石膏被粉碎到小于45微米的粒度，并以粉/水比0.43的水捏合时，石膏在放热反应中重新凝固。然而，当放入电烤箱中的石膏量超过300g时，用水捏合石膏粉后不会重新凝固，因此，电烤箱不宜一次回收大量石膏。较少

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