Effect of high magnetic field pre-magnetization on the floc-culation sedimentation of high turbidity clay sewage

One of the major problems that hinders the effective flocculating setting of high turbidity sewage ( HTS) is that the clay particle surface is covered by a “hydration layer” and is electrical charged. This complex surface property of clay particle impedes coagulation and flocculation. In order to improve the sedimentation of HTS and reduce the ad- dition of chemical reagents,high magnetic field was employed to pre-magnetize the HTS before the traditional floccula- tion process. Factors influencing the pre-magnetization and flocculation (PMF),such as magnetic field intensity,mag- netizing time,coagulant and flocculant dosage,were carefully studied by both single factor experiments and orthogonal tests,using a home-made clay HTS. It was observed that the properties of HTS were significantly affected by high-field magnetizing,thus improving clay particle coagulation and flocculation. Compared with the conventional flocculation,the PMF technique had a much better sedimentation effect (after 20 minutes flocculation). The settlement rate of the clay floccules increased by 1. 92 times,the tailing slurry volume decreased by 38. 9% ,and supernatant transmittance in- creased by 54. 1% ,respectively. The optimum magnetic field intensity and magnetizing time were proved as 178-241 mT and 90 s,respectively. Further decrease of the magnetic field intensity would lead to a sharp decrease of floc- culation. The flocculation would not increase with a further increase of magnetic field intensity. To achieve a similar settlement effect,the PMF technique consumes less chemical reagents,compared with the traditional flocculation tech- nique. The coagulant and flocculant dosage was reduced by 75% and 50% ,respectively. In addition,the tailing slurry volume was reduced by 14. 65% ,which is helpful for the subsequent sludge treatment. Careful characterization shows that the zeta potential of clay particles was re-markably reduced by the 90 s pre-magnetization under a 178-242 mT magnetic field. It was found that the high field pre-magnetizing had a remarkable influence on the molecule structure of the hydration shell and the electric double layer on the particle surface. The decrease of the zeta po-tential and the thickness of hydration layer on the particle surface reduced the electrostatic repulsion both between the clay particles and between the clay particles and the flocculant molecule. The reformative surface properties improved the particle co- agulation and the flocculation. As a result,the sedimentation of HTS was improved. This work provides a promising green and efficient method for mineral HTS treatment.