Study on Pulsed Discharge Fragmentation in Water and Electrostatic Separation for Low-grade Magnesite
Author:Cao Yun Xia
Supervisor:li guo feng
With the excessive consumption of high-grade Magnesite resources in China,the effective utilization of low-grade Magnesite resource has become an inevitable choice.Mineral components liberation and impurities removal are prerequisites of resource utilization of low-grade Magnesite.The mechanical crushing and chemical flotation are the common pre-existing mature methods nowadays.However,there are several technical limitations to improve the mineral utilization efficiency of these methods.Additionally,dust and water pollution issues are unavoidable during the process.To meet the current severe resource and environmental policies,exploring environment friendly solutions and improving present methods are needed.In this thesis,an improved method has been studied to achieve resource utilization of low-grade Magnesite,which includes both pulsed discharge fragmentation in water and electrostatic separation.The key problems in the study include Magnesite mineral components liberation effect by pulsed discharge,direct impact pressure characteristics in discharge fragmentation region,separation dynamics low and its influence factors of particles in electrostatic separator.And they are researched and validated based on experimental study,with theoretical analysis and numerical simulation.In addition,modern image processing is an important analyzing method in the research.This research can provide theoretic and practical foundation for Magnesite resource utilization.In this research,the major contents and results are as followed:(1)Experimental study on Magnesite fragmentation characteristics by high-voltage pulsed discharge in water and liberation status of fragmented products.An experimental platform of high-voltage pulsed discharge fragmentation in water is built and relative experiments are performed.Results indicate that the plasma discharge channel generally impacts on the boundary of different mineral components.A pressure direct passive measurement method in discharge impact region is proposed.With 4μF capacity and 10mm discharge gap,Magnesite can be effectively fragmented by impact pressure at the discharge voltage-40kV.The liberation status of mineral components is analyzed by several methods,including microscopic observation,SEM,crystal structure analysis,etc.The granularity of effective liberation between Magnesite and gangue minerals is 0～600μm.(2)Analytical study on the Magnesite pulsed discharge fragmentation.According to the pulse voltage rise time,plasma discharge channel impact position and crack morphology of fragmented products,Magnesite is fragmented by both electrohydraulic effect and electrical discharge effect.Based on the characteristics of plasma discharge channel,the energy released in the discharge channel ranges from 1276J to 1464J,the range of fragmentation energy is 213-244J.Thus,the impact pressure on the Magnesite surface is calculated in the range of 131～139MPa.The pressure load applied on the surface of Magnesite is obtained by pressure passive measurement and numerical simulation.The pressure load has a good agreement with the theoretical calculation.The electrohydraulic impact pressure generated by pulsed discharge exceeds the compressive strength of raw Magnesite,thus determining the effectiveness of electrohydraulic effect.The electrical field distribution among Magnesite and gangue minerals is also simulated in pulsed discharge fragmentation.The electrical field strengths in gangue minerals are 1.1 and 1.2 times than Magnesite,when they are Dolomite and Quartz respectively.The electrical field strength difference on the boundaries of mineral components provides condition of Magnesite electrical discharge fragmentation.(3)Experimental study on electrostatic separation of Magnesite particles by pulsed discharge fragmentation.Magnesite particles with diameter of 0～150μm and 150～600μm are separated in a roll-type high-voltage electrostatic separator,which are obtained by pulsed discharge fragmentation and mechanical crushing.The particle size distribution before and after separation is analyzed by image processing method.The particle granularity distribution is uniform for 0～150μm pulsed discharge fragmented particles,and the average particle diameter is bigger.Conversely,the average particle diameter is smaller for pulsed discharge fragmented particles with a size range of 150～600μm Comparing with the particles by mechanical crushing,there is more concentrate separated from pulsed discharge fragmented particles after electrostatic separation.And there is a higher MgO content of the concentrate.The concentrate reaches the high-grade Magnesite level.Results corroborate the effectiveness of pulsed discharge fragmentation and electrostatic separation.(4)Study on separation dynamics low of separated particles.Force analysis of charged particles on the roller surface is performed.The coupling relationship of departure degree,departure position electrical field strength and particle diameter is obtained.Particles begin to depart from the roller at departure degree 76°.The bigger particle departs in the position with a smaller departure degree.The analysis result agrees with the size distribution of particles in different collecting regions in separation experiment.For different minerals with the same granularity,Quartz,Dolomite and Magnesite depart in sequence.The dynamic model is built based on the force analysis.Particle movement trajectories are simulated by numerical analysis.Simulation analysis also determines the influence of departure position,particle charge and mineral species on the trajectories.The actual particle movement trajectories are restructured by sequence images.The dynamic model is modified based on the actual trajectories.The separated movement law of particles is revealed by the theoretic analysis.