阮竹恩 李翠平 钟 媛
(1.北京科技大学土木与环境工程学院,北京 100083; 2.金属矿山高效开采与安全教育部重点实验室,北京 100083)
全尾膏体制备过程中尾矿颗粒运移行为研究进展与趋势
阮竹恩1,2李翠平1,2钟 媛1,2
(1.北京科技大学土木与环境工程学院,北京 100083; 2.金属矿山高效开采与安全教育部重点实验室,北京 100083)
我国重要大宗金属矿产资源平均品位低,加之近年来我国矿产资源需求的持续增长、资源综合利用率的不断提升,导致尾矿粒度越磨越细、尾矿排放量越来越大,尾矿已成为我国金属矿山的重大危险源。目前,对于全尾矿的处置技术基本上为地表低浓度排放和井下充填,但是由于尾矿制备和输送浓度过低而存在许多问题,尾矿膏体堆放和膏体充填是尾矿最有效最具前景的处理方式,以此来实现安全生产和保护环境。超细全尾颗粒沉降速度慢,渗透性差,导致浓密困难,因此带有超细颗粒的全尾矿快速脱水浓密是膏体制备的技术关键。详细介绍了全尾膏体制备过程中全尾颗粒沉降、絮凝沉降、颗粒运移以及沉降机理的研究现状,分析了目前研究存在的问题以及其发展趋势。
全尾膏体制备 颗粒沉降 絮凝沉降 沉降机理 颗粒运移
尾矿是选矿的产物之一,随着尾矿粒度越磨越细、尾矿排放量越来越大,尾矿已成为我国金属矿山的重大危险源[1]。由于当前技术经济条件的限制,已不宜再进一步分选。尾矿可根据浓度大小分为浆体尾矿、膏体尾矿和尾矿滤饼[2]。目前,对于全尾矿的处置技术基本上为地表低浓度排放和井下充填,但是由于尾矿制备和输送浓度过低而存在许多问题,如易离析分层、强度低、脱水困难、严重磨损管道、固结时间长等[3]。地表膏体堆放和井下膏体充填则是目前很有前景的尾矿处理方式。
尾矿干堆是将选矿流程输出的尾矿浆经多级浓缩后,超细的尾矿用高频振动板除水形成膏体,再用输送泵将尾矿输送到指定的地点进行压滤干堆。浓缩尾矿地表堆积最早在1973年应用于加拿大安大略省的基德克里克(Kidd Creek )矿山[4]。直到2001年,位于坦桑尼亚的布里杨胡鲁(Bulyanhulu)金矿首个真正将膏体尾矿地表处置投入实际应用,其他如位于葡萄牙的内维斯-科伏(Neves-Corvo)铜锌矿[5]。在我国,上世纪90年代初黄金局曾大力推广尾矿干堆技术,最初在排山楼金矿、撰山子金矿[6]等地开始应用,其后在三山岛金矿[7]、大安河金矿[8]等地陆续采用尾矿干堆技术,并获得了一定的经济效益[9]。通过尾矿膏体堆存技术,提高尾矿库安全性能,延长现役尾矿库使用寿命,提高回水利用率,减少土地资源的占用,改善矿区周围的生态环境,实现矿区环境效益与企业经济效益的双赢[10]。
伴随充填技术的发展,尾矿作为主要充填料被用于采空区充填,这不仅解决尾矿处置难题、更为采空区治理提供了可行方案,可谓“一废治两害”。但传统充填工艺,基本采用分级尾砂,即去掉了-20 μm或-37 μm的细颗粒,致使存在超细尾矿仍需筑坝且难度增大、采场大量脱水污染、尾矿离析充填体强度不均等突出问题[11]。
为此,矿业界借鉴“流态化混凝土”的经验提出了全尾矿膏体充填,其突出优势在于利用了传统充填不能使用的尾矿中的细粒级部分。其细粒级尤其是-25 μm的超细粒级,在膏体输送过程中能趋于管壁形成润滑层、降低管道输送阻力、减少管道磨损,并且能阻止粗颗粒下沉离析、确保膏体形成柱塞流,同时超细颗粒使膏体具有触变性能和保水性能,从而膏体充填料不分层、不离析、不脱水,满足充填所需的稳定性、流动性和可塑性,使之成为充填技术发展的主要方向[12-17]。
最早开始膏体充填研究的是德国,1978年巴德格隆德铅锌矿(Bad Grund Mine)作为“先驱”率先开展了全尾矿膏体充填试验研究,历经6 a形成了膏体充填系统“Preussage Pumped Fill”[18],其后加拿大、澳大利亚、美国、南非等矿业发达国家相继开展试验研究并推广应用[19-20]。我国膏体充填尚属起步阶段,虽然金川公司二矿区于1987年开始膏体充填的试验研究,但因全尾矿过滤脱水等技术问题影响了工业化应用,直至2008年底其充填系统才达标达产,冬瓜山铜矿、会泽铅锌矿、伽师铜矿、金大矿业、羊拉铜矿等相继开展了工业试验与应用。
但超细全尾颗粒沉降速度慢,不及分级尾砂的1/10,渗透性差,不及分级尾砂的1/100,导致浓密困难。但按全尾矿膏体堆放和膏体充填的技术要求,尾矿脱水后的质量浓度应达到基本饱和状态(75%~80%以上),由此引出膏体堆放和膏体充填的首要技术难题——细粒级尾矿的脱水问题,即带有超细颗粒的全尾矿快速脱水浓密是膏体充填的技术关键[11,14]。
膏体的定义一直不够清晰,目前国内外许多学者对于膏体的定义进行了探索性研究。国外学者认为,当膏体中-20 μm含量为15%~20%(质量比)且膏体料浆的屈服应力大于200±25 Pa时,可以视为膏体[21-22]。国内学者通常用坍落度和分层指标来表征膏体,当塌落度在15~25 cm,分层度小于2 cm时可以视为膏体[23]。近年来,从饱和率、泌水率等角度对膏体的定义进行了新的探索,当浆体饱和率在101.5%~105.3%之间且泌水率在1.5%~5%之间即为合格的膏体[24]。同时,基于全尾砂级配提出了膏体的新定义,从而使不同矿山不同特性全尾砂所能配制的膏体得到了统一规范化和定量化[24]。总的说来,膏体的特性如下[25]:
(1)固体浓度大。按照全尾砂膏体充填的技术要求,尾砂脱水后的质量浓度应达到75%~80%以上。形成膏体时,尾矿浓度因尾矿种类不同而不同,这是因为不同尾矿的粒径分布、颗粒形态、黏土含量、矿物学性质、絮凝剂等不同。
(2)不离析。在一定的条件下,膏体内部水分子与尾砂颗粒之间、尾砂颗粒与尾砂颗粒之间受各种力的控制而出现一种相对平衡的状态。
(3)黏度大,渗透率低。由于全尾膏体中加入了絮凝剂,黏度大,全尾膏体的渗透率低,有资料显示膏体的渗透性比同等物料的渗透性低半个数量级。
2.1 颗粒沉降行为研究现状
颗粒沉降一直是流体力学的经典问题,具有广泛的工业生产背景,包括在重选、石油、环境、化工等领域[26]。不过学术界对于颗粒沉降的研究以水利工程领域的泥沙为对象的研究相对活跃,对于泥沙单颗粒的自由沉降行为研究较为成熟,基本明晰了球形颗粒所受流体外力作用的沉降速度的控制关系,并可采用计算流体动力学(CFD)的数值模拟方法来分析颗粒的沉降行为。
通过实验的方法,许多学者对流体中泥沙单颗粒所受的气动阻力、升力、阻力系数沉降速度以及旋转颗粒的马格纳斯(Magnus)力进行了深入的研究并给出了相关的计算公式[27-35]。Debasish Pal和Koeli Ghoshal[36]通过引入颗粒表观直径的概念,与沉积物-流体混合物的质量密度和无量纲颗粒直径一起构建的数学模型,能较好地计算出沉积物-流体混合物中颗粒沉降速度。同时,针对不同情况下黏性流体中多颗粒之间的相互作用问题也进行了一定的研究[37-44],包括沉降过程中颗粒特性、2个接触颗粒之间力学的相互作用、旋转运动对2个接触颗粒之间相互作用的影响、颗粒群的平衡以及高分子絮凝剂聚合颗粒的运动等。
对于尾矿颗粒沉降行为的研究很少,尤其对于全尾颗粒在整个沉降过程中液-固两相耦合作用下的运移行为未见报道,可见报道只有针对给料井中的尾矿颗粒因给料井结构不同以及絮凝剂不同而导致尾矿颗粒流动轨迹及絮凝效果的数值模拟[45-49]。
2.2 絮团压缩沉降技术现状
随着全尾膏体充填技术的不断发展,膏体制备过程中的尾矿浓密技术已从过滤分离发展到沉降分离,从离心沉降发展到絮凝沉降[50]。通过使用絮凝沉降技术,可以改善全尾颗粒的沉降性能、提高全尾颗粒的沉降速度、降低膏体制备的时间单耗,并具有良好的经济效益[51-52]。
絮团的压缩行为上,提出了絮团强度的流变宏观效应,指出压实之前需克服压应力,给出了凝胶浓度、压缩屈服应力、剪切屈服应力、干涉沉降系数等参数对絮团结构强度的宏观力学行为描述[53-55]。从连续介质力学的基本原理出发,开发絮凝悬浮液沉淀的唯象理论[56],研究在自身重力和渗透性的影响下的絮凝物的可压缩性。同时,针对圆柱状、分散状和收敛状圆锥形的5种不同截面形状沉降装置进行絮凝压缩沉降行为研究,提出一种连续沉降和浓缩数学模型,为连续浓密提供了一种数值算法,也为浓密机设计提供依据[57]。
絮团结构模型建立上,以早期的Vold的弹射凝聚模型[58]和Sutherland的多孔松散结构模型[59]为原型,基于分形理论建立了絮团的动力学生长模型,利用分形维数作为絮团结构变化的控制参数,得出分形维数大小与絮团密实程度正相关[60]。王国文[61]应用分形理论进行钛铁尾矿絮凝沉降实验,把难以用欧几里德几何表征的高浓度钛铁矿尾矿浆絮体模型基于分形理论来定量描述和分析,通过扫描电镜、光学显微镜、计算机处理等现代分析技术,首度测算出无机和有机絮凝剂两大类絮凝剂作用于常见钛铁尾矿的分形维数。同时,国外也有学者开始采用光学激光显微镜和 X 射线衍射技术,对絮团颗粒和沉积层微观结构进行三维可视化研究[62]。
影响因素的作用效果上,国外研究人员近年基于二维SEM扫描图片,宏观分析了温度、pH值、絮凝作用、耙动作用等对絮团结构的影响,定性阐述了絮团结构的变化,并提出絮团内部水及在搅动作用下排水通道的存在[63-66]。我国研究人员针对絮凝沉降的影响因素做了大量相关研究[67-72],研究了物料颗粒粒度、矿浆浓度、絮凝剂种类及添加量、泥层高度等因素对沉降速度与静止沉降极限浓度的影响。研究发现,矿浆浓度、絮凝剂浓度、絮凝剂单耗对沉降速度的影响程度为尾砂浓度>絮凝剂单耗>絮凝剂浓度[73]。同时,在静态沉降的基础上,国内学者开始研究动态浓密[74-75],采用自制尾砂动态浓密物理模型,进行不同絮凝剂单耗下尾矿动态浓密效果研究,将絮凝剂添加量由低到高划分了4个区间,并提出了不同区间下絮凝剂对尾矿浓密极限浓度的影响机理,为现场深锥运行提供技术依据。
2.3 深锥浓密机中尾矿沉降机理研究
深锥浓密机是一种新型的固液分离设备,除了具有高效浓密机的特点之外,最大的特点是深锥高度要比其他浓密机高得多,压缩沉降带的结构也更复杂。深锥膏体浓密机多用于井下尾矿充填和地表尾矿管道输送干式堆存。
尾矿沉降机理方面的研究主要基于静态间歇沉降实验,探讨如何利用数学模型更有效地从实验数据中表达沉降过程,如何更全面分析影响沉降过程的各个因素。其理论基础是Coe-Clevenger的沉降模型,后经Kynch、Talmage、Fitch、Shannon、Tory、Landman、Usher等人发展完善,建立沉降过程的宏观分布参数方程,利用沉降曲线分析沉降速度与固体浓度、沉降通量等参数的关系,获得底流浓度与浓密机面积等工业指标[76-81]。虽然近年研究已开始涉足动态连续沉降,但其模型维度基本是一维的沉降模型,故对实际工业指标预测存在较大偏差,且很少考虑压缩沉降[82-83]。
国内因膏体充填尚处工业试验阶段,故对深锥浓密机的脱水浓密的研究主要集中在宏观浓缩效果、浓密机结构参数等方面,包括浓密机高径比、浓缩面积等结构参数与压缩程度、停留时间、泥层高度、固体通量、底流浓度等因素的影响,以及传统浓密机的系统改造等工业应用探讨等。在分析矿浆浓度影响因素的基础上,叙述了深锥浓密机槽体高度的计算过程,尤其是动态压缩层高度的计算[84]。通过推导出深锥浓密机内部料浆停留时间的理论公式,进而确定深锥浓密机内部料浆体积的计算方法[85]。基于深锥浓密机内部不同高度、不同浓度尾砂对耙子运行的影响,对耙子进行受力分析,提出复杂结构耙子扭矩的计算模型[86]。采用深锥相似模型动态沉降实验及流变参数测定方法研究深锥浓密机压耙原因[87]。针对梅山选矿厂的深锥浓密机,通过采用高效深锥浓密机进行尾矿浓缩实验,提出了采用浓密机高效化处理方案[88]。借用CFD中的Fluent对搅拌反应罐流场的各影响因素(如桨间距、罐桨径比等)进行分析和研究,提出了搅拌反应罐内部结构的改进方向和措施。
同时,国内外学者对于高径比对深锥浓密机沉降机理的影响进行了深入的研究。国外有学者认为该类浓密机高径比在1~2之间,而国内学者认为深锥浓密机高径比为2.1~2.4。 但是,随着尾矿处置规模的加大,深锥浓密机的直径也在不断增大,高径比则会随之减小。在此基础上,国内学者提出了有别于传统认识中的高径比概念,即泥层高度与浓密机直径之比,使得高径比这一概念工程意义更加明确,同时在深锥膏体浓密机底流浓度和外形结构高径比之间建立联系得出其数学模型。
3.1 存在问题
纵观国内外的研究现状,虽然针对颗粒的沉降行为、絮团的压缩沉降、尾矿的沉降机理及其生产应用的实践等方面开展了大量的研究工作,取得了一定的进展,但其成果还有一定的局限性,对于细观层面研究不够深入,如絮团与全尾颗粒的细观表征、絮团的内部细观结构在不同沉降阶段的改变、颗粒到絮团的发展演变、全尾颗粒的动态运移沉降行为等问题,仍需进一步开展研究和创新。目前仍然存在以下问题与不足:
(1)颗粒沉降行为方面的现有研究还限于固相颗粒所受液相流体单项作用下的宏观效果,尚未进行尾矿颗粒在自由沉降—干涉沉降—压缩沉降的全过程中因液-固两相耦合作用下的时空运动行为研究,更未从三维可视化的角度进行颗粒实际运移沉降过程的动态仿真。
(2)还原和表征絮团的真实结构是揭示压缩沉降机理的基础,现有研究中利用二维扫描图片对絮团的结构进行观测与识别,但是尚未洞悉絮团的三维细观结构。
(3)对于尾矿沉降机理方面的现有研究基本是宏观规律,且动态连续沉降机理仍较薄弱,尚未结合颗粒和絮团的细观结构效应开展沉降规律的研究。
3.2 研究趋势
(1)在现有的二维、宏观研究基础上,利用三维重构技术建立絮团的三维空间结构模型,开展絮团结构的细观力学行为研究,来揭示全尾砂浓密沉降的机理,实现由宏观到微观、由二维到三维的研究。
(2)在现有静态间歇沉降研究基础上,利用深锥浓密机模拟装置进行试验,以及利用流体软件进行动态连续沉降模拟,研究全尾砂在液固两相耦合条件下的时空运移行为,进行全尾砂动态连续沉降机理的研究。
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(责任编辑 石海林)
DevelopmentProgressandTrendofWhole-TailingsParticles'MigrationBehaviorduringPreparationofWhole-TailingsPaste
Ruan Zhuen1,2Li Cuiping1,2Zhong Yuan1,2
(1.SchoolofCivil&EnvironmentalEngineering,UniversityofScience&TechnologyBeijing,Beijing100083,China;2.MinistryofEducationKeyLaboratoryofHighEfficiencyMining&SafetyforMetalMines,Beijing100083,China)
The large bulk key metallic mineral resources in China is in a low average grade.Coupled with sustainable growth in demand of mineral resources and constant pursuit for comprehensive utilization ratio of resources in recent years,tailing particles become more and more fine and tailing emissions continue to be grown.As a result,tailing has become a major hazard source of metal mines in our country.Currently,the disposal of whole-tailings basically includes over-ground stockpiling at lower concentration and underground filling.However,there are many problems in preparation and delivery of tailings in lower tailings concentration.With the consideration of safe production and environmental protection,pasty tailings stockpiling and whole-tailing paste filling will be the most effective and promising methods.Due to the low sedimentation velocity and poor permeability of whole-tailing with ultrafine particle,dewatering and thickening become difficult.So dewatering and thickening rapidly for whole-tailings with ultrafine particles is the key technology for preparation of whole-tailings paste.The development status of whole-tailings particles' migration behavior during whole-tailings paste preparation is outlined,including whole-tailings particle sedimentation,flocculation sedimentation,particle migration as well as sedimentation mechanism.Finally,the problems in and trend of development are analyzed.
Preparation of whole tailings paste,Particle sedimentation,Flocculation sedimentation,Sedimentation mechanism,Particle migration
2014-09-11
国家自然科学基金项目(编号:51174032),教育部新世纪优秀人才支持计划项目(编号:NCET-10-0225),中央高校基本科研业务费专项资金项目(编号:FRF-TP-09-001A)。
阮竹恩(1989—),男,硕士研究生。
TD926.4
A
1001-1250(2014)-12-013-07