Study On Neutralization Processing And Comprehensive Utilization Of Metal Acid-pickling Wastewater

With the development of economy,our country's demand for steel increases gradually.Because enterprises mostly use sulfuric acid pickling process in the production,the annual amount of acid-pickling wastewater rises.At present,due to the high processing cost for most of small and medium steel enterprises,the harmful acid-pickling wastewater is discharged directly without effective treatment,which pollutes the environment and leads to the waste of water resources.In this paper,the sulfuric acid-pickling wastewater from a steel factory in Xi'an was used as the research object and the acid neutralization process was optimized by orthogonal experiment.As for the high content of calcium sulfate in neutralization slag,it was studied that the slag was directly used in the building materials and prepared calcium sulfate whiskers by hydrothermal method.Based on researches,the achievements were mainly as follows:The optimum conditions for the treatment of acid-pickling wastewater by neutralization-sedimentation process were as follows: the dosage of lime is 3.20 g/L,the stirring rate is 450 r/min,the neutralization time is 5 min,the dosage of nonionic polyacrylamide is 85 ppm,the settling time is 8 min.Under these conditions,the neutralized slurry could achieve quickly solid-liquid separation,which obtained clarified solution with the pH value of 6.90.The lime neutralization and flocculation-sedimentation process was applied in the treatment of acid-pickling wastewater from a steel factory in Xi'an.As a result,the application effect was well and the treated wastewater could be reused as backwater.The optimum conditions for preparing calcium sulfate whiskers by hydrothermal process were as follows: the raw materials were dihydrate gypsum and the neutralization slag with the mass ratio of 19:1,the slurry concentration was 5%,the pH value was 4,the reaction time was 30 min,the reaction temperature was 140?,the stirring rate was 180 r/min.Under these conditions,hemihydrate calcium sulfate whiskers with good morphology could be prepared.It achieved high value-added use for the neutralization slag.The effects of neutralization slag on the mechanical performance and the setting time for desulphurized building gypsum was investigated.The optimum conditions were as follows: the calcination temperature was 180?,the calcination time was 45 min,the dosage of calcinated neutralization slag was 8%.Under these conditions,the building gypsum meeting the standard of GB9776-2008 grade 2.0 could be obtained.Neutralization slag could be prepared for building gypsum powder by controlling the calcination time,temperature and the slag dosage.The effects of neutralization slag on the mechanical performance for gypsum block was investigated.The optimum preparation conditions were as follows: the ratio of raw materials was gypsum: cement: neutralization slag: slag = 77:10:8:5,the dosage of polycarboxylate superplasticizer was 1.0%,the dosage of citric acid retarder was 0.08%,the dosage of whiskers was 5%.Under these conditions,the 7d tensile strength and compressive strength of gypsum block were 3.85 MPa and 13.91 MPa,respectively,the 28 d compressive strength was up to 31.51 MPa.The study is of great practical value to the harmless treatment and recycling of acid-pickling wastewater in steel factories...