Study On The Treatment And Recovery Of Waste Water From Liquid Crystal Display Etching Processing

In the process of thin film transistor liquid crystal display（TFT-LCD）production,various of waste water will be formed and discharged, such as glass substract thinningetching waste water, Al etching waste water,ITO（Indium Tin Oxide）) etching wastewater and so on. Because of abundant valuable resources contained in such waste water,the treatment should be based on the recycling methods In this study, two processes forthe treatment and recovery of useful constitutes in glass thinning and Indium-tin oxide（ITO） etching waste water were built and investigated, respectively.The accumulation of fluosilicic acid（H₂SIF₆）in the thinning process of the glasssubstract was one of the main reasons which caused the deterioration of the thinningsolution and the discharge of such waste solution. This study concentrated on theremoval of H₂SiF₆from the used glass thinning solution without destroying the basicconstituents of the solution. Due to the low solubility of the alkali fluosilicate salts，sodium salt and potassium salt were tentatively employed as the precipitation agentsto react with H₂SiF₆and form Na₂SiF₆or K₂SiF₆sediment. It was found that excesssodium or potassium metal salts could remove H₂SiF₆efficiently by chemical deposit.Comparing with K₂SiH6, Na₂SiF₆was more readily to be settled. Moreover, thesodium salt was much cheaper. Therefore, sodium chloride appeared to be thepotential candidate for the removal of H₂SiF₆from the exhausted glass thinningsolution. The removal efficiency of H₂SiF₆was over90%with mole ratio ofNaCl/H₂SiF₆is2and10%of H₂SiF₆concentration in the simulate solution. Theremoval efficiency of H₂SiF₆for actual liquid waste water was just60%with moleratio of NaCl/H₂SiF₆is2，but if increased the mole ratio of NaCl/H₂SiF₆to2.8，theresult was that90%H₂SiF₆would be removed。In additiona, Indium is a kind of important metal resource and widely used incomputer, electronics and aerospace fields etc. Indium consumption ability indirectly reflects a nation’s scientific and technological level. But the reserves of indiumresources are low without any independent mineral deposits. Primary indium sourcehas failed to meet the market demand and regeneration indium becomes the hot spotof the market. In this paper, extracting method was employed to recover indium fromITO waste water. Extraction parameters including extractant concentration, A/O ratio（Aqueous Phase/Oil Phase） and acidity were tested to choose the optimal condition.The results demonstrated that P204performed much better than P507for indiumextraction. It was found that high acidity was adverse to the extraction of indium fromITO etching waste water by P204. Though extraction efficiency of indium showednegative correlation, increase of A/O ratio could improve indium concentration inorganic pHase. HCl was selected as the stripping solution. A/O ratio was found havingno significant impact on the strip of indium. However, change of A/O ratio couldincrease the concentration of indium in stripping solution. After extraction andstripping, tin could be efficiently separated from indium. The optimum extractionconditions were as follows: For extraction process, the concentration of P204was25%with sulfonated kerosene as diluent solution; A/O ratio was2/1; the time for extractionwas5min and the operation was carried out at room temperature. For strippingprocess, the concentration of stripping solution was3mol/L; the stripping time was5min while the A/O ratio was controlled from1/4to1/8; the operation was also carriedout at room temperature.On the ground of the above studies, a pilot plant for the treatment of glassthinning waste water was built and installed in a Hangzhou-based company, and theresults showed that H₂SiF₆could be selectively removed and the treated solution couldbe reused for glass etching.