Study On Preparation Of Phosphate And Compound Fertilizer By Waste Etching Liquid In The Production Process Of Thin Film Transistor Liquid Crystal Display

In the production process of thin film transistor liquid crystal display(TFT-LCD),several kinds of waste etching liquid are produced, such as waste glass substract thinning etching liquid, waste aluminum etching liquid and waste ITO(Indium Tin Oxide) etching liquid. Direct discharge of the waste etching liquid will lead to pollution of the resources and the environment.This paper used waste aluminum etching liquid, which is one of the waste etching liquid in the production process of thin film transistor liquid crystal display that is of high value for recycling utilization, as raw material for preparation of phosphate and compound fertilizer. The effect mechanism of nitric acid in the preparation process of phosphate and compound fertilizer by adding phosphoric acid and nitric acid in an appropriate ratio either together or separately into the reaction was discussed.The concentrations of acid radical ions in the waste aluminum etching liquid were determined by ion chromatography. The result shows that: the concentrations of phosphoric acid, nitric acid and acetic acid in mass fraction are 51.78%, 4.36% and 11.82%, respectively. The contents of the metallic elements in the waste aluminum etching liquid were analyzed by using the inductively coupled plasma atomic emission spectrometer. The result suggests that the contents of the metallic elements in the waste aluminum etching liquid can meet standard requirements of acid for fertilizer production and this waste aluminum etching liquid can be used as materials of fertilizer production for agricultural utilizationChemical composition and concentration of the experimental phosphorites was analyzed by X-ray fluorescence spectrometer; the available phosphorus was tested by quinoline phosphomolybdate gravimetric method and free acid was determined by volumetric method. Available phosphorous and free acid of the fertilizer products and decomposition rate of phosphorite were analyzed during the preparation process of phosphate and compound fertilizer. The results show that the optimized reaction conditions are as follows: reaction temperature is 85℃, liquid-solid ratio is 0.71 and curing time is 14 days; it is feasible to prepare phosphate and compound fertilizer by waste aluminum etching liquid and low-grade phosphorite in which the contents of phosphorus pentoxide and nitrogen are 22.42% and 0.43% respectively; acetic acid participates in the reaction during the preparation and shows little obvious effect on the productions.The reaction of waste aluminum etching liquid and middle-low grade phosphorite was facilitated in some degree by nitric acid for decomposition of phosphorite, so experiment was conducted to explore the effect mechanism of nitric acid in the preparation process of phosphate and compound fertilizer by using phosphoric acid and nitric acid in an appropriate ratio at the optimized reaction conditions. The result suggests that the optimized reaction conditions are as follows: the concentration of phosphoric acid is 55%, the concentration of nitric acid is 56%, reaction temperature is 70℃ and the way of adding acid into the reaction is to add nitric acid after adding phosphoric acid; NO3- from the nitric acid possess strong oxidization and permeability that can exempt the crystal double salt from the surface of phosphorite particles; meanwhile,phosphoric acid can be produced by part of nitric acid and defluorinated calcium phosphate and the increased amount of concentration of hydrogen ion from both phosphoric and nitric acid can decompose phosphorite more adequately; the way of adding nitric acid after adding phosphoric acid into the reaction in the preparation process of phosphate and compound fertilizer is only suitable for middle-low grade phosphorite with low content of reducing substances.