复合式摩擦电选机分选机理的研究

【作者】 何家宁；

【导师】 张宗华； Daniel Tao；

【作者基本信息】 昆明理工大学 ， 矿物加工工程， 2007， 博士

【摘要】 电选是利用各种矿物和物料在高压电场内电性的差异而进行分选的一种很有效的物理选矿方法。由于电选具有流程简单、经济、不产生废水、没有环境污染等特点,因此,电选具有与其他物理和化学选矿方法所难以比拟的特点,愈来愈显示出其优越性,日益为许多国家所重视,必然会在世界上更加快速的发展。摩擦电选技术中摩擦荷电是通过接触、碰撞、摩擦的方法使矿粒带电,一种是矿粒与矿粒相互摩擦,使各自获得不同符号之电荷;另一种是矿粒与某种材料摩擦、碰撞使之带电。互相摩擦碰撞带电的根本原因是由于电子的转移。但是,由于摩擦获得的电荷比较少,且受到摩擦处理量的影响,故摩擦电选技术在国内外都未能广泛地应用于生产。根据摩擦电选技术在国内外的研究和应用现状,本文首次提出了矿粒复合带电这一新概念,在此基础上开展复合式摩擦电选机的研究工作,复合式摩擦电选机的结构主要由摩擦荷电装置和复合电场组成。摩擦荷电装置由摩擦溜槽和外加电场构成;复合电场由静电极和多根百叶窗状刀片（或丝状）电晕电极与圆筒接地极构成。矿粒的复合带电方式,兼有摩擦带电和电晕（粒子化）带电的特点,采用这种带电方式来提高矿粒的带电量,增加矿粒在电场中的电场力,从而提高摩擦电选机的分选效果和分选效率。为研究出新型高效环保的复合式摩擦电选机打下了一定的基础。本文研究了矿粒兼有摩擦和电晕（粒子化）荷电的复合带电机理,建立了复合式摩擦电选机的数学模型;研制出了复合式摩擦电选机的实验装置,并利用该实验装置进行了测量矿粒荷电量的实验,从而对本文所建立的复合式摩擦电选机的数学模型进行了实验验证,同时还利用复合式摩擦电选机的实验装置进行了粉煤灰的分选实验,取得了较好的分选效果。从实验结果来看:复合式摩擦电选机作为一种新的技术能被广泛地应用于矿物分选、材料提纯和食品加工等领域。1、在摩擦荷电的过程中,导体矿粒的介电常数大具有较高的能位,容易受到极化,易于给出外层电子而带正电,非导体矿粒的介电常数小能位低,难于极化,易于接受电子而带负电。2、本文研究了矿粒的电晕荷电的机理。复合电场由一根静电极和四根百叶窗状刀片（或丝状）电晕电极与圆筒接地极构成。在高电压作用下,电晕电极周围空气被击穿,正电荷迅速飞向高压负电极,负电荷迅速飞向接地正电极,从而在整个分选空间充满荷电体。对于通以高压直流负电的电晕电极构成的复合电场,不论导体和非导体矿粒均先在电晕电场中获得负电荷,但随着矿粒在复合电场中往前运动,立即受到静电场的作用,导体传走电荷后,受到静电极的感应而带电并吸向静电极方向,非导体则不同,由于所吸附之电荷不能传走,受到静电极的斥力,将矿粒压于接地极,显然两者的运动轨迹很不相同,据此将导体和非导体分开。电晕极与静电极混装强化了静电场的作用,对导体加强了静电极的吸引力,对非导体则加强了斥力,使之紧吸于鼓面。通过对高压电选的两大分选理论,即剩余荷电分选理论和稳态荷电分选理论的分析和研究后发现:剩余荷电分选理论中对矿粒在电晕电场中荷电量的计算理论符合于高压电选的实际分选效果;而稳态荷电分选理论比较适合于解释矿粒在电晕电场中的充电和放电机理。3、本文推导出在考虑外加电场E_F时导体矿粒摩擦荷电量计算公式,并且首次引入了美国的Schein L.B.,Laha M.和Novotny D三位科学家对非导体矿粒摩擦荷电理论新的研究成果,即采用他们的在低密度和高密度限制条件下分别计算非导体矿粒的摩擦荷电量的理论计算公式。在实验中发现在相同的实验条件下,非导体的石英颗粒的荷电量比导体的磷酸盐颗粒的小得多,从实验结果来看:运用不同的理论公式进行导体和非导体矿粒摩擦荷电量的分析计算是正确的。本文还首次在实验中发现随着外加电场E_F极性的改变,非导体的石英颗粒的摩擦荷电量的极性不变,即石英颗粒的摩擦荷电量始终为负的,而导体的磷酸盐颗粒的摩擦荷电量的极性则发生了改变。4、本文首次运用牛顿第二定律、赫兹定律和Anderson J.H的摩擦碰撞荷电的电子理论模型,推导出矿粒与复合电场的接地极圆筒表面发生碰撞时,碰撞持续时间和摩擦碰撞荷电量的计算公式。矿粒摩擦碰撞荷电量的大小取决于矿粒与圆筒的表面逸出功之差、矿粒的直径、碰撞速度。它随逸出功之差的增大而线性增大,随矿粒半径与碰撞速度的增加呈指数减小。5、在摩擦电选过程中引入了荷电量衰减的计算公式。研究了矿粒在电晕电场中的受力情况,分析了导体、半导体和非导体矿粒在电晕电场中被分选成三种产品的受力条件。为复合式摩擦电选机实验装置的研制提供了设计的依据,并且为复合式摩擦电选机数学模型的建立提供了理论基础。6、研究了复合式摩擦电选机中摩擦荷电装置的溜槽长度、倾角和材料设计方法,以及摩擦荷电装置的外加电场的设计,矿物温度对摩擦荷电量的影响;研究了电晕电极、电晕丝数和电晕电极极距的设计方法;选择一根静电极和四根百叶窗状刀片电晕电极构成复合式摩擦电选机的复合电场;研究了静电极对电晕电流的影响,及考虑相对湿度时复合电场的电场强度设计方法等。在研究了以上设计复合式摩擦电选机多方面的理论依据的基础上,设计出了复合式摩擦电选机的实验装置。7、研究了矿粒在复合式摩擦电选机摩擦荷电装置中、矿粒与圆筒碰撞摩擦以及矿粒在电晕电场三个过程中分别获得的电量,以及它们在复合电场中受到电场力作用时刻的剩余荷电量。在此基础上,推导出了复合式摩擦电选机的数学模型。在分析复合式摩擦电选机中作用在矿粒上的电场力的基础上,得出复合式摩擦电选机的分选效果和分选效率将会好于只有复合电场电选机或者是只有摩擦荷电装置电选机的。利用数字电荷测量仪与法拉第筒组成的矿粒荷电量测量系统,测量出磷酸盐颗粒的荷电量,从实验结果来看:电晕荷电量、摩擦与电晕复合荷电量的趋势线之间存在着一定的线性关系,也就说明在建立复合式摩擦电选机的数学模型时,在考虑到荷电量衰减的基础上,假设矿粒在复合式摩擦电选机摩擦荷电装置中、矿粒与圆筒碰撞摩擦以及矿粒在电晕电场中分别获得的电量,它们在复合电场中受到电场力作用时刻的剩余荷电量存在线性关系是基本成立的。从而证明了本文所建立的复合式摩擦电选机的数学模型是正确的。8、研究了矿粒摩擦荷电量与振动频率之间的关系。实验数据表明:矿粒摩擦荷电量与振动频率之间并不存在简单的线性关系。振动频率低时,磷酸盐矿粒摩擦得到的荷电量较少,但是当振动频率高于300Hz后矿粒的荷电量迅速增加,到400Hz矿粒的荷电量达到最大值39×10^-9C/g;而后当振动频率高于400Hz矿粒的荷电量反而有些减少。因此,总的来说随着振动频率的增加,矿粒与溜槽表面、矿粒与矿粒之间的摩擦速度在加快,矿粒的摩擦荷电量越来越大。但是随着摩擦速度的增加,矿粒在溜槽中滞留的时间缩短,矿粒的摩擦时间也在缩短,因而矿粒的摩擦荷电量反而会减少。同时也可以看出,摩擦荷电装置的外加电场强度E_F=0时,矿粒获得的摩擦荷电量是比较少的。摩擦荷电装置的外加电场强度对矿粒摩擦荷电量的影响是非常大的,增加E_F可以使矿粒获得较多的摩擦荷电量。9、本文利用复合式摩擦电选机的实验装置进行了粉煤灰的分选实验。粉煤灰具有重要的回收利用价值,把粉煤灰作为一种资源来加以认识和充分利用,是关系到我国电力工业和相关产业可持续发展急需的解决得重大问题;国内的粉煤灰大部分只得到低附加值的回收利用,急需研究出能工业化的制备低烧失量的脱碳灰的技术,因为低烧失量的脱碳灰有很多用途,可以作水泥混和料来生产各种粉煤灰水泥,对于提高水泥质量,降低水泥成本和大量消耗粉煤灰都是很有好处的。低烧失量的脱碳灰还可用于具有很高实用价值和经济价值的粉煤灰减水剂。10、粉煤灰一股晶体矿物为石英、磁铁矿、氧化镁,生石灰及无水石膏等,非晶体矿物为玻璃体、无定型碳和次生褐铁矿等。从美国Crystal River发电厂电除尘器收集的粉煤灰的典型的粒度分布情况为:90%以上的粉煤灰颗粒的粒度主要分布在45μm＜d＜106μm的范围内。粉煤灰中未燃尽的煤粒电阻率为10⁴～10⁵Ω·cm而属于导体;而粉煤灰中的纯净灰是以硅铝酸盐矿物为主要成分的,其电阻率为10¹¹～10¹²Ω·cm则属于非导体。因此原状粉煤灰中的煤粒和灰粒,在导电性上有着明显的差异。这一研究结果对于开展复合式摩擦电选机用于粉煤灰的分选实验是十分重要。11、首次利用复合式摩擦电选机的实验装置进行粉煤灰的分选实验,从分选的实验结果来看:①随着摩擦荷电装置外加电场强度、复合电场电压和筒转速的增加,复合式摩擦电选机的分选效果越来越好;②随着给料速率和相对湿度的降低,复合式摩擦电选机的分选效果也是越来越好。这些实验结果与复合式摩擦电选机数学模型的理论分析结果是一致的。12、利用复合式摩擦电选机的实验装置,可以从烧失量（LOI）为8.34%原状粉煤灰制备出烧失量（LOI）为1.99%和产率为56.67%为的优质脱碳灰,以及烧失量（LOI）为27.37%和产率为16.84%为的纯净煤粒。更多还原

【Abstract】 Electric separation is a very high efficiency physical separation, which uses different minerals have different electric properties in the high voltage electric field to separate. Electric separation is easy to run, economical, dose not produce waste water and friendly to environment, so electric separation is better than other physical and chemical mineral processing methods, many countries pay great attention to it, and it must be quickly developed in the world.Triboelectrostatic separation is a dry separation process based on the fact that particles can be charged differently upon contact, friction or impact. Triboelectricity can be produced by particles friction each other, and particles friction or impact some materials surface. The cause of tribocharge is electrons transfer. Triboelectricity is small, so the main problems with conventional triboelectrostatic separators are low throughput and poor efficiency, which limit their application.According to the situation of triboelectrostatic separation technology studied and used in the world, this paper presents a new concept that is particles’ compound charge for the first time, and bases on the new concept this paper presents and study a compound triboelectrostatic separator, which is made up of a tribocharge device and a compound high voltage electric field. The tribocharge device is formed by a tribotrough and an external electric field. The compound high voltage electric field is formed by an electrostatic electrode and several shutter or sheet sharp electrodes with the roller electrode connected to earth. The tribocharge and corona charge combine to make particles compound charge, this charge principle can let particles charge electricity increase. Using the charge principle we can manufacture a new type of triboelectrostatic separator, which has high throughput and high efficiency.This paper studies the charge mechanism, which combined by the tribocharge and corona charge. Setting up a mathematical model and manufacturing an experimental device of the compound triboelectrostatic separator. Using the experimental device to test particles charge electricity, and prove that the mathematical model of the compound triboelectrostatic separator is right. This paper gets good experimental results by using the experimental device to separate fly ash. We can find that the compound tnboelectrostatic separator is a new technique, which can be widely used in mineral beneficiation, materials purification, food process, etc.1. Conductor particle has big permittivity and high energy level, it is easy polarization and loses outer surface’s electrons; Uonconductor particle has small permittivity and low energy level, it is hard polarization and easy gets electrons. While particles are tribocharged, conductor particles get positive charges, and uonconductor particles get negative charges.2. This paper researches the corona electrification mechanism. A corona discharge is produced when a high voltage is applied between two electrodes, one of which has small radius of curvature named the sharp electrode. The electric field causes an electric discharge in a limited region near the sharp electrode, at a voltage below the spark breakdown voltage of the gap. In the region close the sharp electrode the electric field is very high, exceeding the breakdown field of gas to produce electron/ion pair. Usually the sharp electrode is negative, then in the corona electric field positive charges quickly fly to the sharp electrode, and negative charges quickly fly to the positive electrode connected to earth. In the corona electric field both conductor and uonconductor get negative charges. A compound high voltage electric field of the compound triboelectrostatic separator is combined by 4 sharp electrodes and an electrostatic electrode. After particles get corona electrification, they will be affected by electrostatic field immediately. Conductors are influenced and absorbed to electrostatic electrode after quick discharging. But uonconductors are hard discharging, which are absorbed by roller of the positive electrode connected to earth. So conductor and uonconductor particles are separated in the compound high voltage electric field. The separation effect is strengthened by combining 4 sharp electrodes and the electrostatic electrodes. There are two separation theories, one is the Leavings Charge Separation Theory, another is the Steady Charge Separation Theory, this paper finds the first one fits for calculating particles’ corona electrification, the second one fits for explaining particles charge and discharge in the corona electric field.3. This paper educes the equation for calculating conductor particles triboelectricity, in which includes tribocharge affected by an external electric field. The first time introduces the new research result of three American scientists who are Schein L.B., Laha M. and Novotny D., which about calculating uonconductor particles triboelectricity, and their theory points out that should think about in the low and high density limit. Analyzing experimental results, the triboelectricity of uonconductor quartz particles is far lesser than conductor phosphate particles’ at the same test condition, so it is right that use different equation to calculate conductor and uonconductor particles’ triboelectricity. This paper also find uonconductor quartz particles’ polarity can’t be change with the external electric field’s polarity change, quartz particles’ triboelectricity is always negative. But conductor phosphate particles’ polarity can be change with the external electric field’s polarity change for the first time.4. For calculating collision time duration and triboelectricity between particles and roller of the positive electrode connected to earth, this paper uses Newton’s second law, Hertzian theory and an electronic theory model of tribocharge developed by Anderson J.H to educe two equations for the first time. The particle specific charge is determined by the work function difference between the particle and roller, particle diameter, and impact velocity. It is clear that the particle specific charge increases linearly with the increase of the work function difference and exponentially decreases with the radius of particles and impact speed.5. This paper introduces the equation of particle charge decay, studies a charged particle affected by some forces in the corona electric field, analyses of the force condition of conductor, semiconductor and uonconductor to be separated three products, supports theory for setting up a mathematical model and manufacturing an experimental device of the compound triboelectrostatic separator.6. In order to design an experimental device of the compound triboelectrostatic separator, this paper analyses the length, obliquity and materials of the tribocharge device. Thinks triboelectricity affected by particles temperature, how to design the external electric field of the tribocharge device, how many sharp electrodes, and how long the distance between two electrodes. The compound high voltage electric field of the compound triboelectrostatic separator is combined by 4 shutter or sheet sharp electrodes and an electrostatic electrode.7. Studying three charge courses which are tribocharge, impact charge between particles and roller, and corona electrification in the compound triboelectrostatic separator. While particles are affected by Coulomb force, using particles leavings charges of three charge courses to set up a mathematical model of the compound triboelectrostatic separator. So we can find particles charges in the compound triboelectrostatic separator are more than conventional triboelectrostatic separators’, the compound triboelectrostatic separator’s separation effect will be improved. Using a particles charge measurement system including a Digital Charge Measurement and a Faraday Cage to measure particles charge. Experimental results show that the line of tribocharge, trendlines of corona electrification and tribocharge + corona electrification have linear relationship, which can prove the hypothesis of setting up the compound triboelectrostatic separator mathematical model is right.8. The experimental result illuminates that between the triboelectricity and frequency of the tribocharge device vibration does not have linear relationship. When the frequency is low, the triboelectricity is small. With the frequency increases, the triboelectricity also increases. The triboelectricity reaches maximum that is 39×10^-9C/g when the frequency at 400Hz. But the triboelectricity decreases to 34×10^-9C/g when the frequency at 450Hz. This result can explain with the frequency increases, particles tribocharge time in the tribocharge device is decreased, so let triboelectricity reduces. At the same time can be found that the triboelectricity is small when E_F = 0 of the external electric field. Big triboelectricity will be get by using big intensity of the external electric field.9. Using the experimental device of the compound triboelectrostatic separator to separate fly ash. Fly ash is an important mineral resource. Its handling, processing, and ultimate use or disposal are important to the utility industry and our environment. In the last decade, commercial interest in removing unburned carbon from fly ash to get two products which are unburned carbon and fly ash of low LOI. Fly ash of low LOI can be used to produce cement and water reduce agent, which has great value and very important for the construction industry. For separating fly ash and other minerals has stimulated research and development of triboelectrostatic separation technology. The triboelectrostatic separation, a dry processing technology, is one of the best technologies for processing fly ash due to its fine particle size and dry state.10. Fly ash is a crystal mineral containing quartz, magnetite, magnesia, calcium oxide, anhydrite, etc.; the non-crystal mineral contains vitreous body, unformed carbon, hypo-limonite, etc. The fly ash comes from American Crystal River power station, which more than 90% particles size is between 45μm to 106μm. unburned carbon particles’ resistance rate is about 10⁴～10⁵Ω·cm, which are conductors; fly ash particles’ resistance rate is about 10¹¹～10¹²Ω·cm , which are nonconductors. So unburned carbon particles and fly ash particles have great different electric properties, which is important for separating fly ash by the compound triboelectrostatic separator.11. The result using the experimental device of the compound triboelectrostatic separator to separate fly ash for the first time, which shows that with the electric intensity of the tribocharge device external electric field and the voltage of the compound electric field increases, separation effects are better and better. With feed rate and relative humidity decrease, the separation effects are better and better. Such results accord with the forecast of the compound triboelectrostatic separator mathematical model. The result of testing the relationship between separation effect and roller rotating speed, which shows the separation effect when the roller rotating speed at 35 rpm is better than that when the roller rotating speed at 40 rpm, it is just to prove that the particle specific charge exponentially decreases with the radius of particles and impact speed.12. From original state fly ash, using the experimental device of the compound triboelectrostatic separator we can get fly ash particles of low LOI with the LOI at 8.34% and the yield at 56.67%, unburned carbon particles with the LOI at 27.37% and the yield at 16.84%.更多还原