某TFT-LCD廠濕蝕刻機台酸性製程排氣的白煙去除之研究

Abstract

光電業TFT-LCD廠中濕蝕刻機台使用之酸性化學品鋁蝕刻液在製程中會產生含有硝酸、醋酸、磷酸之廢氣，廢氣經由酸性排氣風管抽至廠務端之酸性濕式洗滌塔洗滌去除酸性物質後，再經由煙囪排至大氣中。洗滌塔入口的硝酸濃度約為8~30 ppm，以pH約為7及導電度約為3 ms/cm之洗滌水在洗滌過程中會產生大量的白煙，經檢測後發現硝酸為造成白煙之主要成份。 本研究首先測試各種改善洗滌效率之方法，或加裝硬體設備試圖改善白煙問題，但結果發現效果有限。最後發現以不更換洗滌水的方式提高其水中溶入醋酸及硝酸廢氣之濃度，且洗滌水pH值控制在7左右，結果發現當水中硝酸根約在2.3 wt%以上及總陰離子濃度合計達約26 wt%以上或是硝酸根約在3.5 wt%以上及總陰離子濃度合計達約17 wt%以上，導電度達72 ~ 90 ms/cm，洗滌塔出口相對濕度會降至80%以下，白煙現象可以完全消失，且酸性排氣的去除效率仍可達90%以上並未因此而降低。 若以溶解精鹽(NaCl)提高鹽類溶質百分比至洗滌水中氯離子約為 7 wt%，可降低洗滌塔出口相對濕度至80%~ 90%之間，但尚無法完全消除白煙，須再洗滌醋酸及硝酸廢氣使水中溶入硝酸及醋酸至總陰離子濃度25 wt%以上，洗滌塔出口相對濕度會降至80%以下，白煙現象方可以完全去除。 本研究推論白煙消失的原因是因為含有高濃度硝酸根離子、醋酸根離子及氯離子之洗滌水蒸氣壓變低，使得洗滌塔出口的相對濕度由90~100%降低至80%以下，因而使得硝酸無法反應及成長成可見之白煙微粒。研究發現白煙完全消失的導電度操作範圍為洗滌水達到最大值之後的期間，此時若持續進行洗滌時，白煙仍可完全消除，但導電度會降低；當水中溶質過濃時需要進行少量排換水，使導電度保持在72 ~ 90 ms/cm及洗滌塔出口相對濕度至80%以下, 即可維持無白煙之情況。

In TFT-LCD manufacturing factories, the chemicals that are used in the wet-etching equipments generate acidic waste gases consisting of nitric、acetic and phosphoric acids. The acidic gases are introduced into the acidic wet scrubber via the acidic exhaust duct for treatment before being discharged into the atmosphere from the stack. The inlet concentration of nitric acid at the scrubber is about 8 ~ 33 ppm. In the scrubbing process using scrubbing water of pH about 7 and conductivity of about 3 mS/cm, dense white smoke is generated. After sampling and analysis, nitric acid is found to be the major component in the white smoke. In this study, many methods to improve the efficiency or additional hardwares to eliminate white smoke were found to be ineffective. Finally, we found that the white smoke would disappear completely by keeping the sccrubber water running until the concentration of the nitric anion of 3.5 wt% and the total anion of 17wt%, or the nitric anion of 2.3 wt% and the total anion of 26 wt% were reached, and when the conductivity of 72 and 90 mS/cm、pH of about 7 was maintained in the scrubbing water. During this smokeless period, the relative humidity in the outlet of scrubber was found to be below 80%. The removal efficiency of the acidic gas was not changed and still maintained over 90 %. If NaCl was used to raise the Cl- concentration of the scrubbing water , it was found that the relative humidity at the scrubber outlet was reduced to 80%~ 90% but the white smoke was not eliminated completely. It was found that continuous scrubbing until the total anion concentration reached above 25 wt% and the relative humidity at the scrubber outlet was reduced below 80 %, the white smoke could disappear completely. This study has hypothesized that high concentration of nitrate, acetic and chorine ions in the scrubbing water lowers the water vapor pressure which in turn reduces the relative humidity of the scrubber outlet from 90~100% to below 80%. As a result, nitric acid can’t absorb water to grow into visible white smoke particles. This study found that the operation range of the conductivity in the smokeless condition is after the period when the maximum conductivity value is reached. Continuing the scrubbing operation will reduce the conductivity value of the scrubbing water until when the solute concentration is too high and draining and making-up a small amount of the scrubber water becomes necessary. During this period, keeping the conductivity of the scrubbing between 72 ~ 90 ms/cm and the relative humidity of the scrubber outlet below 80% can maintain the smoke-less condition.