Application Of High Density Sludge Re-circulation Process In Semiconductor Fluoride-containing Wastewater Treatment

The semiconductor industry is the foundation of the development of a national information industry,the lifeblood of the country's economic development,and an important part of the international competitiveness.In recent years,the rapid development of semiconductor technology has brought great convenience to people's lives.The demand for semiconductor products is huge in the fields of Internet,electronic products,and military.the pollution caused by its wastewater is increasingly widespread and serious,threatening human's health and safety.The semiconductor industry wastewater has many characteristics such as various kinds of pollutants,large fluctuation range of water qualities,strong toxicity of pollutants and large emissions.Therefore,the waste water treatment methods and process parameters are different due to different types of waste water treatment process and pollutants.It is necessary to determine the most appropriate treatment process and the best process parameters according to different water quality.In this paper,the practical problems of frequent fluctuations in effluent fluoride value,large chemical consumption and large sludge production in the process of treating fluorine-containing wastewater in a high-density sludge re-circulation process of a semiconductor company are studied,and feasible process optimization solutions are proposed.The sources of fluorine-containing wastewater from a semiconductor company were investigated and divided into three types,including semiconductor machine process fluorine-containing wastewater,production-supporting special waste-washing drainage,and acid-exhaust-washing tower drainage.The samples of influent from different wastewater treatment systems were taken for experiments.By changing the dosage of sulfuric acid,sludge,polyacrylamide(PAM)and adjusting the pH of the fluorine removal reaction,the relationship between fluorine concentration,sulfuric acid concentration,sludge concentration,pH and fluorine removal rate is determined,and suitable fluorine removal parameters were selected.optimum pH range in the fluorine removal process is 6.5~8.The influent of fluorine-containing wastewater treatment system A andB is the three types of wastewater,and the dosage of sulfuric acid and sludge was 757 mg/L and 128 g/L,respectively.The influent of treatment system C is the mixture of semiconductor machine process fluorine-containing wastewater and acid-exhaust-washing tower drainage.The dosage of sulfuric acid and sludge was 757 mg/L and 128 g/L in system C,respectively.Combining the test results with the actual process,the quantitative control of online data is realized.The study showed that sludge re-circulation in the treatment of fluorine-containing wastewater not only enabled the reuse of calcium in the sludge,but also made the sludge precipitation better,and the most important is that the sludge dewatering performance was improved.When the size of the sludge crystal particles is about 10 microns,the dehydration performance is good,the water content can be reduced to 25%,and the SVI is in the range of 0.87~1.05 mL/g,which realized the purpose of sludge reduction.In view of the frequent failure of the effluent from the fluorine-containing wastewater,the polyaluminum chloride(PAC)was added to the effluent of the treatment system.It was found that the hydrolysis of the PAC caused the pH of the solution to drop,resulting in poor fluorine removal and flocculation effects.Sodium hydroxide was added to raise the pH to 6.0 to maintain the fluorine removal and flocculation effect;PAC was added to the mixed water sample of the reaction tank sludge in the fluorine-containing wastewater treatment system,and it was found that the pH of the liquid did not decrease significantly and the effect of removing fluorine was good.It was not necessary to add sodium hydroxide to adjust the pH.We propose a scheme to increase the fluoride ion on-line monitoring in the reaction tank and simultaneously link the PAC dosing to control the excessive discharge of fluorine-containing wastewater.This solution was based on the buffering effect of sludge on pH,and there is no need to set up sodium hydroxide dosing equipment.Due to the high cost of PAC and the low fluorine removal rate,it is further proposed to add a fluorine probe to the on-line monitoring of the No.2 outlet of the reaction tank,and the sulfuric acid dosage varied with the fluoride value of the reaction tank effluent,thereby realizing the variable addition of the sulfuric acid.This process improvement can not only cope with the excessive fluoride caused by unstable pH and fluorine concentration,but also reduce the dosage of sulfuric acid,calcium hydroxide and the emission of sludge.The goals of energy saving and emission reduction are achieved.