| Summary: | 碩士 === 國立成功大學 === 資源工程學系碩博士班 === 96 === A huge amount of reserves of sericite deposite is located at about 5 kilometers in the southeast direction of Hsiang-Yang, Lidao village, Haiduan, Taitung county. The recoverable amount is estimated to be about 15 million tons. This sericite deposit belongs to the sericite- pyrophyllite- quartz schist. It is the only one of mine producing mica powders from the schist in Taiwan Province and is an extremely unique natural resource. The annual production at present is about 30,000 tons. It is one of the most economically important industrial minerals in Taiwan Province.
The particle size of Hsiang-Yang sericite is smaller than 400 mesh sizes after water-classification. The mineral compositions are sericite and pyrophyllite as well as a small amount of chlorite. The content of sericite is about 60% which pyrophyllite is 40%. In this study, the processed sericite supplied by Sunshine Mineral Company (as recieved), samples ground using a stirrer beads mill (ground) and classified samples (>10μm and <2μm) were used as the starting material. Hydrothermal reactions was used to synthesize zeolites with the mineralizers, NaOH and Na2SiO3. The reaction conditions investigated were raw material properties, concentration of mineralizers, reaction temperature and the time frame of temperature retention so as to find their effects on the phase and morphology of zeolites produced.
From the results, it is dound that analcime, hydroxycancrinite and hydroxysodalite can be synthesized from Hsiang-Yang sericite by hydrothermal reactions. The temperature must reach above 210℃, so that the hydrothermal reactions can be carried out substantially. The formation of zeolites increases as temperature is increased. At the temperature of 240℃, the XRD pattern of zeolites were maximum indicating rapid reactions.
Using NaOH as the mineralizer, at 240℃, concentration of NaOH can be roughly divided by 1M and 5M into three ranges. In each range, as received, ground, >10μm and <2μm samples all had similar zeolite products. For [NaOH]≦1M, the product is trapezohedron-shaped analcime [NaAlSi2O6•H2O] (besides analcime, the product of <2μm samples also included an unnamed sodium aluminum silicate hydrate [NaAlSi2O6•1.1H2O], with rectangular column. It chemical formula has 0.1 mole more of H2O then analcime); for [NaOH]=2M~4M, the product is short rod-like or needle-shaped hydroxycancrinite [Na8(AlSiO4)6(OH)2•2H2O]; for [NaOH]≧5M, the product is dodecahedral hydroxysodalite [Na8(AlSiO4)6(OH)2]. A close examination of the reaction results, it is found that >10μm samples (pyrophyllite70%, sericite 30%) can synthesize hydroxycancrinite rather easily, which <2μm samples (sericite 75%, pyrophyllite 25%) can synthesize hydroxysodalite.
Using Na2SiO3 ([Na2SiO3]=0.5M~5M) as the mineralizer, at 240℃, the product of as received, ground, >10μm and <2μm samples were all analcime with perfect trapezohedron crystal form.
Under various reaction conditions, it is found that diffraction peaks of pyrophyllite always disappeared at a faster rate than sericite with the showing that pyrophyllite reacts more easily than sericite in the hydrothermal synthesis process. However, at high concentration of mineralizers, the dissolution rates of three ~ two minerals by alkaline approached similar.
The particle size of the zeolite products synthesized from >10μm samples as the largest, followed by as received, and <2μm samples. The smallesr sizes are from ground samples. This shows that the size of the raw materials and that of the products are in proportion. Due to the extremely small sizes of ground samples, not only the synthesized zeolites are small, the reaction can also occur at at lower temperatures and finish reaction in shorter time.
In the NaOH system, the synthesis reactions have an optimal time of reaction. Beyond the optimum, a second phase appeared and is deleterious to the purity of the product. In Na2SiO3 system, analcime is the only phase synthesized. Longer time of temperature retention did not produce second phase, in contrary, helped analcime forming perfect trapezohedron crystal .
In the early stage of nucleation, there were wing-like projections of the analcime crystal planes. As the reaction proceeded, the wing-like projections decreased and finally disappeared resulting in flat crystal plane. This shows that nucleation of analcime occurred at the edges or faces of raw materials and the wing-like projections are unreacted sericite particles, which have not completely dissolved probably due to their large particle sizes and the alkalinity, reaction temperature and time have not reached the optimum.
The zeolite products sizes were measured by the sedimentation method. The median size(d50) were consistent with the average sizes visually counted (over 50 particles) from SEM pictures. However, the particle size distributions were wider than the seemingly mono-sized results of SEM pictures. This might be due to the second phase in the product, more important is that because of the cluster growth of zeolites. The aggregates cannot be broken so their sizes were incorporated into the particle size distributions. This phenomenon was especially obvious for the product of ground samples. The distribution modulus (k) was inversely proportional to the size of the raw materials for the other three samples. If the particle sizes of the raw materials was smaller, distribution modulus was larger representing a narrower particle size distribution or approaching mono-size.
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