Studies On Treating Of Alkali Weight-Reduction Wastewater

At present, the poly (ethylene terephthalate) (PET) fiber is an important material, out of which textile can be made. With the development of the modern science and technology, Alkali weight-reduction process is designed to give the PET textile a silky property, as well as to improve the dyeing capability by treating the PET textile, and meanwhile its performance is ameliorated greatly and new higher qualities are endowed. The wastewater, brought from Alkali weight-reduction process, contains disodium terephthalate salt (TA-Na₂) of some concentration; it is the difficulty and keystone to the wastewater-treating. Therefore, the study catches researcher's eye greatly and widely. The recovery of terephthalic acid (TA) from alkali weight-reduction wastewater is great of importance to reducing environment pollution and the full use of resource. The paper has researched how to recover TA of high purity from the wastewater to lay foundation for the future industrial application.In order to improve the purity of recovered TA, purifying has been researched respectively with three methods to remove the impurities: coagulation-sedimentation, carbon absorption and acid eduction. When the coagulation-sedimentation is. adopted to remove the impurities, the paper has examined the impacts that all kinds of factors have on the performances of recovered products (including ash content, 340 run Transmittance, Recovery rate, acid value, DMF chroma, IR spectrum, TGA spectrum, and so on), and researched the mechanism of the coagulation-sedimentation. The analysis has shown that the pH of wastewater, and the using and amount of coagulant have great impacts on the performances of recovered TA, and the quality of recovered TA can be improved obviously if under a proper condition. When three kinds of wastewater of different concentration are tested to remove the impurities in the way of coagulation-sedimentation, the best technical way of coagulation-purification is settled down.When carbon absorption is adopted to remove the impurities, the amount of the carbon has great effects on the way of absorption-purification. The function of impurities-removal will be limited with too little amount of carbon, while too much amount will lower the recovery rate. With proper amount of carbon, the time of absorption will also influence the outcome of purification a lot, that is, maximum impurities will be removed if through proper time. The analyses of the recovered TA's quality index, IR spectrum and TGA spectrum show that, under the perfect condition, the performance of recovered TA has been improved significantly after the purification of carbon absorption. When the wastewaterquality has been changed, the amount of the carbon and the time of absorption have the same tendency of influences on the performances of recovered TA. With the rising of the concentration of CODa, TA, and impurities in the wastewater, both the amount of carbon and the time of absorption will be increased correspondingly.The paper has also researched the influences that crystal conditions of acid eduction from alkali weight-reduction wastewater have on the quality of recovered TA. The research shows that if the reaction conditions of acid eduction are under control, then the crystals of great particle size will be formed. Thus, it not only has a purification effect, but also favors dehydrating washing-up. The technics conditions for recovering TA have been researched through the orthogonal experiment. The orthogonal experiment and analysis of variance show that when acid precipitating, all of the temperature of the wastewater, the concentration of the acid added, the mixing speed of acid-adding, the speed of acid-adding, and alkalinity of the wastewater have effects on the results of the experiments. This research also probe into the mechanism of crystal in acid eduction through the system mensuration and analyses of the main performances of recovered TA.Increasing of the temperature of the wastewater and the speed of mixture, and the decreasing of the acid concentration will reduce the water retention percentage and resistance of recovered TA, and enlarge the crystal grains, improve the purity of the crystal, which is verified through the analyses of scanning electron microscope and particle distribution. However, there are floating ranges best for all these three aspects mentioned above, not boundless. After the crystals have been separated out, proper mature time will reduce the impurities in the recovered products and enlarge the grains precipitated from recovery. The analyses of the performances of the TA formed under the best experiments conditions show that the TA performances have been improved evidently.The paper has also researched some experiments of acid eduction to three wastewater of different concentration, and surveyed the influence that the changing of the wastewater quality has on acid eduction ? And the experiments show that after the wastewater quality has been changed, the temperature is the factor which has the most obvious influence of the same tendency on acid eduction. Little change almost happens to the optimal values of the concentration of the acid added and the speed of adding acid. The smaller the values of the speed of adding acid and the concentration of the acid added are, the better the performances of the recovered TA will be. As far as the mixing speed, one of the influencing factors, is concerned, with the rising of percentage of CODa in the wastewater, the best mixing speed is also be reduce. With the rising of the concentration of the CODcr, TA and impurities in the wastewater, the performances of the TA recovered under the best conditions is also be levelled down.The F values in the orthogonal experiment of three kinds of wastewater show that with the concentration of the CODcr in the wastewater rising, the impact that each influencing factor has on the experiments decreases. Thus, it can be concluded that when the concentration of the wastewater increases, the influencing factors which have obvious impact on the experiments will be reduced gradually. It is even harder to control theinfluencing factors when acid precipitating.Through coagulation-sedimentation, carbon absorption or acid eduction to purify the wastewater will improve the quality of the recovered TA. However, if only one way of purifying is used, some impurities will be left with the recovered TA, which doesn't meet the requirements of the practicality. Therefore, in view of adopting the purifying process united with above-mentioned ways to have better effects on the results, the paper has done some systematic research on the recovering processes. The flow is as follows: first to purify the wastewater through coagulation-sedimentation; then put wastewater through carbon absorption; at last, to purify and recover TA though acid precipitating. The result shows that the TA recovered from the wastewater of lower concentration has reached the standards of industrial TA. When the wastewater is in high concentration., the TA is recovered with some impurities, which is still qualified for the industrial standard TA by recrystal in acid eduction. This process of recovery is more consummate, efficient, and feasible, and will realize the reclamation of recovered TA.This paper discusses the treatment of alkali weight-reduction wastewater, and shows that only recovering the useful resource of TA from the wastewater can meet the policy of recycling economy and sustainable developments, and meanwhile solve the difficulties of dealing with the wastewater, and recover resources.