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1、Minerals Engineering 70 (2015) 7791Contents lists available at ScienceDirectMinerals Engineeringjournal homepage: A review of entrainment: Mechanisms, contributing factorsand mling in otationL. Wang a, Y. Peng b, K. Runge a, D. Bradshaw aa Julius Kruttschnitt Mineral Research Centre, University of Q

2、ueensland, Australiab School of Chemical Engineering, University of Queensland, Australiaa r t i c l e i n f o a b s t r a c t Article history:Received 2 September 2013Accepted 9 September 2014Available online 29 September 2014This paper reviews the recovery by entrainment in froth otation. In otati

3、on, entrainment is a mechan ical mass transfer process by which particles suspended in the water between bubbles enter the otation froth from the top pulp region and are transferred to the concentrate. Both hydrophobic and hydrophilic mineral particles suspended in water can experience entrainment.

4、In this paper, the mechanisms of entrainment are reviewed. The essential factors affecting entrainment are also discussed. The methodsKeywords: Entrainment Froth otationfor the qucation of entrainment are identied. Entrainment ms presented in the literature arereviewed with the aim of identifying th

5、eir signicance and usefulness in industrial applications. Thereis a need to develop a more general mof entrainment since the current ms available cannotMlingbe used to predict entrainment at a changed operating or feed condition.Gangue recovery Degree of entrainment 2014 Elsevier.Contents1.2.3.Intro

6、duction78Entrainment mechanisms78Principal factors affecting entrainment80....0.Water recovery80Solids percentage in the pulp81Particle size82Impeller speed82Particle density83Gas rate83Froth height83Froth retention time84Rheology84Froth structure844.Entrainment me

7、asurement methods8.. M 5.1.Method of Trahar (1981)85Method of Warren (1985)85Method of Ross (1989)86Entrainment tracer86ling of entrainment87Direct estimation of entrainment flow8..1.3.Maachar and Dobby (1992)87Moys (1978)87Neethling and Cilliers (2002)875.2.Estimation

8、of entrainment using classification functions and water recovery88* Corresponding authors. Tel.: +61 7 3346 5983.addresses: .au (L. Wang), .au (Y. Peng).0892-6875/ 2014 Elsevier.78L. Wang et al. / Minerals Engineering 70 (2015) 779..3.Degree of entrainm

9、ent mling88Classification effect in the pulp89Mling of water recovery905.3.Summary of entrainment mling906.s90Acknowledgement90References901. Introductionentrainment and develop ms with an objective of predictingentrainment in a otation cell. The early research on the characterDue to the depletion o

10、f high grade ores on a world scale result ing from high consumption of metals, low grade ores in largeistics of entrainment was conducted by Jowett (1966) and Johnson et al. (1974) followed by a number of authors such as Bisshop and White (1976), Smith and Warren (1989), Kirjavainen (1996), Savassi

11、(1998), Zheng et al. (2006b), Yianatos et al. (2009), Yianatos and Contreras (2010), and Konopacka and Drzymala (2010) who worked on understanding the factors affecting entrainquties are being processed by otation. Low grade ores areoften difcult to be processed because they are commonly complex and

12、 valuable minerals are often associated with gangue minerals at even ne grain sizes. In order to increase the recovery of valuable minerals, some ores, such as clayey, pyritic and brous ores, have to be ground to a much smaller size. However, by doing so, gangue minerals are also ground nely. These

13、ne/ultrane gangue min eral particles not only affect sub otation processes in recoveringment, the mechanisms, the m techniques.ling as well as measurementvaluable minerals based on true otation, but also lead to high mechanical entrainment.Mechanical entrainment is a transfer process by which minera

14、l particles suspended in water enter the otation froth, move upwards, and nally leave the otation cell with the mineral par ticles recovered by true otation. Ideally, in a otation cell, hydro phobic valuable mineral particles collide with air bubbles and form2. Entrainment mechanismsEntrainment is g

15、enerally considered as a two step process. In step 1, mineral particles ascend upwards to the froth phase from the region just below the pulp/froth interface, and in step 2, entrained particles in the froth are transferred to the concentrate launder with water.Entrainment starts in the pulp phase. I

16、n conventional otation cells, the pulp phase is usually divided into two regions, that is, a turbulent zone and quiescent zone. In the turbulent zone, air is drawn into the otation cell and turned into swarms of small bub bles dispersed in all directions within the whole turbulent zone by an impelle

17、r. These bubbles adsorb frothers and collide with valu able mineral particles rapidly. Then they ascend upwards to the quiescent zone, and ultimately become metastable. When these bubbles reach the region below the pulp/froth interface, particles start the process of mass transfer to the froth.To da

18、te, three mechanisms have been proposed to characterise the mineral particles in the otation cell travelling across the pulp/ froth interface from the pulp to the froth by entrainment. They are Boundary Layer Theory, Bubble Wake Theory and Bubble Swarm Theory.In the Boundary Layer Theory, mineral pa

19、rticles are transported to the froth phase in the bubble lamella, i.e. the thin hydrodynamic layer of water surrounding the bubble (Gaudin, 1957; Moys, 1978;particle bubble aggregates moving upwardsgravity to thefroth in the pulp phase while the hydrophilic gangue mineral particles report to the tai

20、lings (Yianatos et al., 2009). However, it has been found that a number of gangue minerals, especially the ne/ ultrane liberated gangue mineral particles, are carried into the concentrate by mechanical entrainment. Thus, the quality of nal products in mineral otation is often greatly reduced due to

21、the recovery of gangue minerals.In a otation cell, entrainment occurs simultaneously along with true otation. Unlike true otation, entrainment is not chem ically selective and it occurs without a direct attachment of parti cles to bubbles. Therefore, both valuable and gangue minerals experience entr

22、ainment. Fig. 1 describes the mass transferthroughout a otation cell. The mass ows indicated in the gure are:(1) Transportation of valuable mineral particles to the froth from the pulp by true otation.(2) Transportation of valuable mineral particles to the concen trate from the froth by true otation

23、.(3) Transfer of mineral particles to the froth from the pulp by entrainment.(4) Transfer of entrained mineral particles to the concentrate from the froth by entrainment.(5) Transfer of mineral particles from the froth to the pulp due to the drainage of detached particles and entrained particles.Hem

24、s, 1981; Bascur and Herbst, 1982), while in the BubbleWake Theory, water including the mineral particles is transportedto the froth phase in the wake of an ascending bubble (Smith, 1984; Yianatos et al., 1988). Fig. 2 shows the motion of water including the suspended solids from the region below the

25、 pulp/ froth interface to the froth according to Boundary Layer Theory and Bubble Wake Theory.However, the Boundary Layer Theory and Bubble Wake Theory,alone, are not adequate tofor the total mass of mineralAlthough in industry there are many other mechanisms such as the recovery via composite parti

26、cles and slime coatings as well as entrapment, attributable to the recovery of gangue minerals, the entrainment mechanism plays a critical role especially in the pro cessing of ores with a large proportion of ne particles (Fuerstenau, 1980; Kirjavainen, 1996).Since entrainment has a detrimental effe

27、ct on the grade of the concentrate, a number of studies have been carried out to understand entrainment mechanisms, identify factors affectingparticles transported to the froth by entrainment. Bubble Swarm Theory proposed by Smith and Warren (1989) provides an addi tional supplementary mechanism for

28、 transport. Fig. 3 demon strates the transportation of water including the suspended mineral particles to the froth from the pulp phase based on Bubble Swarm Theory. Fig. 3(a) shows the bubbles travelling up through the cell. Fig. 3(b) indicates that the region below the froth/pulp interface is cong

29、ested with bubbles as the bubbles slow down and crowd together. Because of drainage through the rising bubbleL. Wang et al. / Minerals Engineering 70 (2015) 779179Fig. 1. Transportation of fully liberated mineral particles in a otation cell (after Savassi, 1998).swarm, some water including the suspe

30、nded solids drops back while some other water including the suspended solids is squeezed upwards due to the buoyancy of the bubble swarm. Fig. 3(c) dem onstrates that, as each layer of bubbles is pushed up, another layer of bubbles will form. In this way, more solids suspended in the water are pushe

31、d up to the froth. It is likely that all three mecha nisms contribute to the transfer of gangue and valuable particles per unit mass of water into the froth, providing a basis for the com mencement of the entrainment mechanism.In the froth phase, three mechanisms have been proposed to describe the t

32、ransportation of water including the suspended sol ids back to the pulp phase. When the solids suspended in the water enter the froth, some solids are entrained to the concentrate, and some other solids experience drainage back to the pulp. The drain age mechanisms range from the drainage through th

33、e Plateau bor ders, collapse of froth causing rapid transfer of water and solids downwards in the local area of froth and sedimentation induced by shear activity (Cutting, 1989).In general, drainage results in a decrease in the water and solids recovered by entrainment and thus causes a drier forth.

34、 The amount of material entrained to the concentrate is determinedFig. 2. Schematic of Boundary Layer Theory and Bubble Wake Theory (after Smith and Warren, 1989).Fig. 3. Schematic of Bubble Swarm Theory in the otation cell (after Smith and Warren, 1989).80L. Wang et al. / Minerals Engineering 70 (2

35、015) 7791by the net ow of water including suspended solids upwards. Therefore, the recovery of particles by entrainment in essence is a result of a trade off between the transfer of suspended solids upwards in the froth phase and drainage of particles downwards across the pulp/froth interface to the

36、 pulp from the froth phase.It should be noted that the Plateau borders in the froth are par ticularly important for entrainment, as they provide the drainage channels for entrained solids. The plateau borders also contain most of the liquid present within the froth. It is formed by the thin water lm

37、s (lamellae) meeting at 120 . In the Plateau borders, the unattached hydrophilic and hydrophobic particles movely in the water and undergo settling (Neethling and Cilliers, 2002). Fig. 4 shows a schematic of a Plateau border and its associated lamellae in the froth (Ross and Van Deventer, 1988).Entr

38、ainment mechanisms are related directly to the state of the suspended solids in the water or the water lm of the bubbles, the drainage in the froth phase and water recovery, and the common characteristics of entrainment are the classication effects in both the pulp and froth phases. In the pulp phas

39、e a classication func tion CFi was proposed to describe the state of solids suspension (Zheng et al., 2005), while in the froth phase the degree of entrain ment (ENTi) was proposed to describe the classication effect of the drainage of entrained particles (Engelbrecht and Woodburn, 1975; Johnson, 19

40、72; Bisshop and White, 1976; Smith and Warren, 1989; Kirjavainen, 1992; Savassi et al., 1998). Thus, the overall recovery of mineral particles by entrainment (Rent,i) can be expressed as:Fig. 5. Recovery of sized silica by entrainment as a function of water recovery (after Engelbrecht and Woodburn,

41、1975).of pulp, as it is often difcult to estimate the mass transfer of particles from the pulp to the concentrate from a practical viewpoint (Johnson, 1972; Bisshop, 1974; Savassi, 1998).3. Principal factors affecting entrainmentRent;if ðCFi; ENTi; RwÞð1Þ3.1. Water recoverywhere

42、Rent,i is the recovery of the particles in the ith size fraction by entrainment and Rw is the water recovery.In the literature, most researchers relate entrainment directly to the water recovery and the degree of entrainment without distin guishing the two classication effects for solids suspension

43、in the pulp phase and drainage in the froth phase:Entrainment is strongly dependent on the water which is con sidered as the carrying medium to transfer the mineral particles into the concentrate in otation processes. The paper by Jowett (1966) was the rst to state that the recovery of mineral parti

44、cles by entrainment was caused by water currents. After that, further research was carried out by a number of researchers to identify the relationship between the water recovery and recovery of solids by entrainment. Johnson (1972) conducted continuous pilot tests and full scale tests as well as lab

45、oratory experiments. Both the lab oratory data and industrial data indicated that there was a direct correlation between the recovery of suspended solids by entrain ment and the water recovered to the concentrate. Thisfrom Johnson (1972) is in agreement with Engelbrecht and Woodburn (1975), and Lapl

46、ante et al. (1989). In general, the corre lation between the recovery by entrainment and the water recov ery shows a linear trend for ultrane particles and a parabolicRent;if ðENTi; RwÞð2ÞThe degree of entrainment is dened as the ratio of the mass of mineral of size i per unit ma

47、ss of water in the concentrate recovered by entrainment to the mass of mineral of size i per unit mass of water in the pulp (Johnson, 1972). It indicates the relative drain age of solids with respect to the water in the froth. The degree of entrainment in a specic otation system can be calculated by

48、 the amount of gangue and water in the concentrate and the pulp or via the size by size assays of both the concentrate and the pulp. It is also calculated on the basis of tailings or feed insteadFig. 4. Schematic representation of a Plateau border and its associated lamella (after Ross and Van Deven

49、ter, 1988).Fig. 6. Effect of water recovery on the concentrate grade (after Yianatos and Contreras, 2010).L. Wang et al. / Minerals Engineering 70 (2015) 779181Table 1Summary of solids suspension measurement results in industrial otation cells at different mine sites.38 lm38 lm53 lm75 lm106 lm150 lm

50、212 lm300 lmCF (Outokumpu 150 m3 tank cell at the Newcrest Cadia copper concentrator)0.990.820.4290.810.2540.670.1190.530.06310.390.03350.260.01390.100.0038CF (Outokumpu 100 m3 tank cell at the Perilya Broken Hill lead/zinc concentrator) 0.824Fig. 7. Recovery by entrainment as a function of solids p

51、ercentage below the pulp/froth interface: three size fractions ( 11 lm, 23 + 16 lm and +50 lm) are illustrated withthe degree of entrainment (ENT) of 0.75, 0.33 and 0, respectively, (after Johnson, 2005).trend for coarse particles. Fig. 5 shows the data from a continuous pilot plant system on silica

52、 gangue recovery versus water recovery reported by Engelbrecht and Woodburn (1975).An increase in the amount of water brings more suspended sol ids into the froth phase. The relationship between water recovery and entrained solids can also be indicated by the effect of water recovery on the concentr

53、ate grade, as shown in Fig. 6. It shows that an increase in the water recovery leads to a decrease in the concen trate Cu grade. This is because a higher recovery of water in a o tation cell results in a wetter froth, followed by a higher proportion of gangue minerals reporting to the concentrate. T

54、herefore, water recovery plays an important role in recovering gangue minerals by entrainment (Johnson et al., 1974; Yianatos and Contreras, 2010).It should be noted that variant denitions of water recovery are used in the literature such as a fraction of total water recov ered to the concentrate an

55、d the water ow rate of concentrate. This is because water recovery is dened in different otation sys tems for different purposes by different researchers. Hence, much attention needs to be paid to denitions to avoid the potential con fusion when the experimental results are considered and com pared

56、(Smith and Warren, 1989; Zheng et al., 2006a).settling occurs in a otation cell, the solids percentage in the pulp phase where entrainment starts undoubtedly becomes particularly important, as it determines the amount of solids entrained to the concentrate. Hence, the state of solids suspension in t

57、hat region must be taken into consideration in the non perfectly mixed case. Table 1 shows the state of solids suspension of different size frac tions at the top of the pulp region in some industrial otation cells at different mine sites (Zheng et al., 2006b). In the pulp phase, theclassication func

58、tion CFis for the state of solids suspension. Its value is known to decrease with an increase in particlesize, which indicates that the coarse particles tend to settle to the lower portion of a otation cell while the ne particles would be more likely to be uniformly dispersed in the pulp phase.Fig. 7 shows the e