Chapter 2 LITERATURE SURVEY

Transcription

1 24 Chapter 2 LITERATURE SURVEY

2 PRINCIPLES OF CLASSIFICATION Classification is a method of separating mixtures of particles of different sizes, shapes, and specific gravities into two or more products based on difference in their settling velocities in a medium. The medium may be fluid, air or gas (Cho et al., 2004;Crespo.E.F.,2009; Doroodchi et al.,2002; Doroodchi et al.,2004; Doroodchi et al.,2006;egarr et al., 2009; Eisenmann et al., 2001; Elder et al.,1999 ). For industrial operations water medium is preferred when sizes treated are finer (Crespo.E.F.,2009; Doroodchi et al.,2002). 2.1 Role of classification in mineral processing Specific size ranges of particles are desired for a particular unit operation to achieve the optimum separation efficiency for which classification plays a significant role in the mineral industry. Few of these advantages are For closed circuit comminution of most lean grade ores/sulphide bearing minerals, rock phosphates, etc. For removal of the slime prior to flotation operation. For improving the separation efficiency of the gravity concentration units such as spiral concentrator, wet tables etc. in processing of chromite, manganese, iron and other heavy mineral industries. For producing high solid concentration feed to dewatering units prior to filtration Presently the processing plants are designed to treat intermediate size particles requiring fine grinding for improved liberations. Therefore fine particle classification is becoming complicated with conventional classifiers. This has necessitated to the development of an efficient classifying unit for the recovery of fine particle values. Despite significant research work in the area of hindered

3 26 settling classification, the focus towards the FDS or Teeter Bed Separators has to be amplified. The detailed conducted on of these units reveals that there exists an avenue to improve their classifying performance. 2.2 Fundamentals of classification A particle attains constant acceleration when it falls in vacuum irrespective of its size and density, (Richards, R., and Locke; C.E., 1940; Taggart 1945). This behavior is quite unlike in a medium, such as air or water, where the resistance increases with the velocity until equilibrium between gravitational and fluid resistance is achieved called terminal velocity. Technically, the terminal velocity is defined as that maximum velocity at which the drag force equals the driving force (Taggart 1945). Under viscous or laminar flow; Stokes has shown that, the terminal settling velocities ( V T ) for fine particles of size less than 100 µm can be expressed as: In other words 2 ' r g (1) 9 V T 2 2 V T r (2) The settling velocity of coarse sized particles above 1000 µm micron, as given by Newton is, V T 8Q ' g r 3 ' (3) Indicating V T r (4) In mineral processing, the intermediate regime (2000 μm to 100 μm) of particle size (where neither Stokes nor Newton s law is applicable) is of considerable importance. In this region, the predominating drag force changes from skin drag (drag due to surface roughness) to form drag (viscous pressure drag) and many equations have been developed to correlate drag coefficient and Reynolds number (Allen 1990; Wiesenberger et al., 1923). One such type is

4 27 C d = (0.63+ (4.8/ Re)) 2 (5) Many alternative methods have been proposed by different researchers for calculating the terminal settling velocities of particles. Dietrich derived a correlation where shape and angularity factors have been incorporated (Gupta et al., 2006) and expressed as: W = (V T 3 *ρ 2 )/ ((ρ ρ ) g µ) (6) Dietrich's dimensionless parameters are related by the expression: W = R 3 10 R 1 +R 2 (7) Where R 1 = Coefficient describing the effect of particle density and is given by R 1 = (log D) (log D) (log D) (log D) 4 (8) Where D can be defined as: D = ((ρ- ρ ) gd N 3 * ρ )/ µ 2 (9) Where d N = nominal diameter of the largest projected area. An irregular particle will settle in a stable orientation when the largest projected area is perpendicular to the settling direction. R 2 = Coefficient describing the effect of particle shape mathematically expressed as: CSF = Corey shape factor = d min / (d max * d mid ) 1/2 and Log ((1-(1-CSF)/0.85) (10) 3.5P CSF R tanhlog D 4. 6 (11) 2.83 P = Powers roundness factor, equal to 6 for perfect rounded particles (spheres) and 2-3 for highly angular particles. Jimenez and Madsen simplified Dietrich's approach defining the dimensionless parameters (Gupta et al., 2006) as

5 28 and V S * * v T ' gd ' (12) d' ' gd (13) 4 ' A linear regression between (1/V * ) and (1/S * ) gave the equation: 1 V * B A (14) * S The coefficients A and B allowed a solution for the drag coefficient B Re 2 2 C D A A (15) Any particle settling in a medium, depending on its surroundings, may undergo free or hindered settling Free Settling When individual particles fall freely in a medium without getting obstructed by other particles in the near vicinity, it is termed as free settling. In free settling, the particle velocity is controlled by gravity; drag and dominated friction creates medium resistance. The equipment operating in this condition is merely a sizing device to support the comminution circuits. The exterior fluid flow patterns of medium resistance are related to the Reynolds number (Re). Particle free settling velocity is defined as (Gui et al., 2010). V T kd x ' ' y The values of constants ( k, x, y and z) vary based on Reynolds number, as tabulated in the Table 2.1. Figure 2.1 shows the particles of different sizes and specific gravities settle under free settling conditions (Subba Rao 2011). z (16)

6 29 Table Parameters of particle settling under free settling condition (Gui et al., 2010). Name k x y z Re Stoke s law Transition Zone (Initial Segment) /6 2/ Allen s Law /3 1/ Transition Zone (End Segment) /3 5/9 1/ Newton s Law > Hindered Settling The particle of mixed sizes, shapes and specific gravities, in a crowded mass, are sorted in a rising current of water and is not strictly a sizing operation alone but a concentrating operation, is called hindered settling. From the Figure 2.1, it is evident that heavier (or lighter) particles can be separated when they settle by hindered settling as the effect of specific gravity gets amplified. As the particle concentration and particle size increases the forces closely related to particle movement, such as order of different resistance magnitude, mass force, force between particles and effective gravity gradually increases. A B Figure 2.1: Schematic diagram of particle settling by (A) free (B) hindered settling conditions.

7 30 Forces acting on particles undergoing hindered settling are described (Gui et al., 2010; Li, and Zhang, 1992; Yao 1992; Luo and Zhao, 2002; Wei 2002) below i. Gravity force: The force of gravity is exerted by the gravitational field of a massive object on a body within the vicinity of its surface and is defined as F g 1 d 3 g (17) 6 ii. Buoyancy Force: It is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid given by F b 1 d 3 ' g (18) 6 iii. Compressive Stress: It is force which developed in moving particles in flow under a pressure gradient, not only develop resistance caused by fluid flow, but also pressure gradient force defined as F P 1 2 ' d (19) 3 x If acceleration of particles and fluid is more or less the same, the pressure gradient force is smaller and can be ignored. iv. Resistance: This is related to the particle motion and defined as: F d f f c 3 d v v (20) v. Added Mass Force: This is numerically equal to fluid quality with same volume of particles, along with half of the inertial force of particles acting on the same acceleration, and closely relate to particle movement

8 31 vi. Basset Force: F a d 12d 3 ' v f vs (21) dt This refers to resistance of particles forced on hard phases of acceleration (or non stable movement) in viscous fluid and is defined as vii. Magnus Force: F B 2 d 3 2 dvt dvs t ' V dt dt (22) t 1 This force is lateral force caused by particle spinning and defined as viii. Saffman Force: F M 1 3 d ' V f 8 0 V s (23) When particles move in a flow field with velocity gradient, as the velocity of upper part of particles is higher than of lower part. So, the upper pressure is less than lower pressure which forced by lift known as Saff man force. It is defined as F s 1.62d 2 dv f ' V V f Vs (24) dh It is caused by the gradient in flow field. As speed of lifting water in flow fluidised bed is low and stable, basically there is no velocity gradient, implying that the force can be ignored. ix. Forces between grains: This force cannot be ignored, when the concentration of particles in separation processes increases beyond a certain value. The requisite particle size ratio of the two minerals (having different specific gravities and shapes) to fall at equal rates is called free settling ratio. So r1 r 2 2 ' 1 ' Where (Newton) 1 > m > ½ (Stokes) m (25)

9 32 Hindered settling ratio (exhibited only when particle concentrations exceed 20% by volume of total slurry) is the ratio of apparent specific gravity against the suspension raised to a power between one half and unity. Therefore r r ' 1 ' m Where (Newton) 1 > m > ½ (Stokes) (26) The general principles of classification in a fluid medium are The relative settling velocity of particles depends on size, shape and specific gravity. If any two factors of the above are more or less uniform, the settling velocity of particle depends only on the third factor. For example if the particle having same specific gravity and shape in a given liquid the relative settling velocity is dependent on the size of the particle. Initial differential acceleration of the particle influences the classification Resistance to fall increases with density and viscosity of the liquid Velocity of a particle falling in a liquid depends on the square of the particle sizes when they are small; it depends on the square root of the particle sizes when they are large. But for intermediate particle sizes the power varies between 2 and Classifier Classifiers are the separating device which generates two or more products and are of two types based on the media used i.e. water or air. The classifier in which water is used as media is called wet classifier, widely used in mineral industry (Egarr et al.,2009). The wet classifiers broadly can be divided into two types based on their basic principle. (Egarr et al., 2009; Eisenmann 2001; Elder et al.,1999). Such as a) Gravitational classifier. b) Centrifugal classifiers.

10 33 Gravitational classifiers are simple and easy to operate. Some of the salient features of this type are as follows. These types of classifiers are best suited for coarser classification. It is having higher in capacity for coarser cut points with good separation efficiency. It can classify the particles up to 75 µm. These classifiers consume little power and are of higher capacity. In this type, capacity and cut size are not dependant variables, enabling small size equipment to be used for coarse cut size, but cut size and overflow density are related to each other, preventing fine high density overflow products. Gravitational classifiers are can be subdivided into two sub types based on the fluid movement and the direction of the particle settling. Such as 1. Sedimentation Classification: In this type, the fluid movement is horizontal and forms an angle with the particle trajectory. This types of classification is also called as pool or cross flow classification. 2. Hydraulic Classification: In this type, the fluid movement and particle movement directions are opposite to each other. This type of classification is also called as hydraulic or counter flow classifier Sedimentation Classifiers The sedimentation classifiers are subdivided into two types, based on the fluid movement and the direction of the particle settling, as sedimentation classification (where the fluid movement is horizontal like in rake and spiral classifiers) and hydraulic classification (where the fluid and particle movement directions are opposite to each other).

11 34 Figure 2.2: Classifier Tree Essentially sedimentation is a pool in which the slurry fed at one end and discharges (by overflow) at the other end with the slurry of much lower percent solids (Kelly and Spottiswood, 1982). This is of free settling nature and again grouped into two categories, depending on the mechanism of segregating the solids, as mechanical and non-mechanical classifiers Mechanical Classifiers These classifiers have an inclined settling tank (generally positioned at an angle of 18 to 20 degrees) and a raking mechanism to remove the settled particle bed from the bottom of the tank while the overflow is collected in launders (Gaudin 1971; Pryor 1965; Kelly and Spottiswood, 1982). The classifiers of this category are of rectangular-trough type such as Akin s spiral, rake or Esperanza, drag classifier, log washer, scrubbers etc. Since these classifiers primarily perform sizing, so these are also called as sizing classifiers. The feed slurry introduced to the classifier at the

12 35 rectangular cross section for quick spread along the width and heading towards the top end. The schematic diagram of the typical mechanical classifier is shown in Figure 2.3. The coarse denser particles in the slurry settle at a faster rate to the bottom initially and form a layer (as shown in region 5). Region 4 is the moving zone i.e dragging the settled particles to the under flow using the raking mechanism. Above this region 3 was marked, where hindered settling takes place and continuously vary the particle concentration arised in this region such as the upper portion contained minimum particle concentration whereas the lower portion having the maximum particles concentration. The dragging mechanism helps in breaking the agglomerated fines and helps in segregation process. The region 2 is the maximum turbulent zone, where the lighter or smaller particles are separated and discharged to the over flow launder. The surface of the top layer is the region 1 and at this zone the light particles flow over to the over flow launders. These are widely used for sizing of intermediate size range particles i.e. between µm and are mostly employed in cleaning sinter fines in iron ore mines and in closed circuit with grinding mills. Log washers and scrubbers are widely used to remove the surface adhered slimes from lumpy ores of iron, chromite, manganese etc. 1 Under flow Over flow Hindered settling Figure 2.3: Schematic diagram of the mechanical classifier

13 Non-Mechanical Classifiers The non-mechanical gravity type comprise of various types of settling cones with or without auto controlling discharge of underflow and these classifiers utilizes free settling conditions to effect sizing whereas specific gravity and shape factors do not interfere(galvin et al.,2009). Feed intake is from the top through a centrally placed feed-well which facilitated to prevent short-circuiting of the feed to the overflow fraction and spigot discharge is through the bottom(crespo.,2009). The schematic diagram of this type of classifier is shown in Figure 2.4. The classifiers of this category are Allen s sand cone, spitzkasten, Hukki s classifier, Linatex desliming tank, Akorel classifier etc. Generally this category of classifier is employed where there is very high particle density difference. These are used for desliming the feed material in heavy mineral industry as well as in silica sand washing purpose. Feed Over flow Under flow Figure 2.4: Schematic diagram of a typical non-mechanical classifier 2.4 Hydrocyclones Hydrocyclones are continuous operation classifying devices that utilize centrifugal force to accelerate the settling rate of particles and have gained popularity in fine particle classification particularly grinding circuits due to numerous advantages such as ease of operation, high throughput, less maintenance cost, less floor space

14 37 requirement, ease of installation etc. A typical hydrocyclone consists of a conical shaped vessel open at its apex joined to a cylindrical section, which has a tangential feed inlet as shown in Figure 2.5.The top of the cylindrical section is closed with a plate through which an axially mounted overflow pipe emerges out. This pipe is extended into the body of the cyclone by a short removable section known as vortex finder, which prevents short-circuiting of feed directly to overflow. The feed is introduced tangentially at a pressure that causes a swirling motion and generates a vortex inside the cyclone, with a low-pressure zone along the vertical axis known as Air-Core, developed along this axis is connected to the atmosphere through the apex opening. Thus the particles held within the swirling motion are subjected to two opposing forces - an outward centrifugal force and an inward acting drag force. The centrifugal force accelerates the settling rate of the particle in the radial direction by facilitating the particle to get discharge through apex. Whereas the slower-settling particles, influenced by the drag force, move towards the zone of low pressure along the axis which eventually carried upward through the vortex finder to the overflow. Figure 2.5: Schematic diagram of a typical Hydrocyclone 2.5 Hydraulic Classification Hydraulic classification is relatively an advanced technique and is consisted of a single or series of sorting columns in which a vertical current of rising water is against settling the settling of particles. In which the separation is sought based on

15 38 size, shape and density. These are also named as counter current or cross flow separators since the feed pulp flows in opposite to that of rising current of water as shown in Figure 2.6. Depending on the mode of operation and design features hydraulic classifiers are classified further into two types. The first type is of classifiers that operate without formation of fluidization. The classifiers in this category are Lewis classifier, Linatex M classifier, Linatex S classifier, Lavodune classifier etc. The other one is characterized by a controlling device to control the underflow discharge and to achieve a uniform bed density of teeter column. Overflow (particles with terminal velocities <V) Fluid velocity V Spigot product (particles with terminal velocities >V) Figure 2.6: Schematic diagram of particle classification in hydraulic classifier. Hence these classifiers are called as fluidized bed hydraulic classifiers. The classifiers under this category are stroke s hydrosizer, amberger kaolin werke TAK classifier, floatex density separator, cross flow separator, reflux classifier etc. For processing of Indian iron ore slimes using fluidized bed classifier followed by hydrocyclone and low speed spiral classifiers envisaged that a concentrate of quality with 62.5% Fe(t), 4.2% SiO 2, and 2.23% Al 2 O 3 from the feed slime assaying 50.8% Fe(t), 11.86% SiO 2 and 9.84% Al 2 O 3 at a material yield of 30% (Pan et al., 2008). In coal/mineral industries for gravity classification purpose usually the particles are fluidized in water media. The separation is a consequence of the stratification witnessed by hindered settling properties (Himsworth 1974). The feed slurry enters

16 39 these classifiers through a feed well and the teeter water is added from the bottom of the spigot. Particles form a fluidized bed or a teeter bed above the water distribution pipes and at steady state particles of density lower than the teeter bed float and are carried by the stream of water to the over flow. The remaining particles penetrate through the bed and reports to the under flow. In teeter column particles are graded in the order of decreasing terminal velocity, the slow settling particle at the top and the fast settling particles at the bottom (Drummond and Swanson 2002; Heiskanen 1993). When the rising fluid, at low velocities, is passed through a bed of solid and fluidizes it, then the pressure drop across the bed will be directly proportional to the rate of the fluid flow. According to Darcy s relation the average velocity as measured over the whole area of bed is directly proportional to the thickness of bed. p U K * (27) l In fluidized bed the particle segregation is on the basis of their specific gravity differences against the raising teeter water through a bed of heavy medium and settled high density particles are accumulated at the bottom. Since the size and density of the particles are not uniform, the particles are segregated according to their mass. In general, the coarser heavier particles form a layer at the bottom of the bed and the coarser lighter forms the top layer and all other particles are distributed throughout the bed depending on their density and size. Both the apparent density and viscosity of the bed of solid particles are higher than the liquid medium and hence the resistance to settling is more because of the drag force and the buoyant force to a moving particle. Unlike in dense media separation, where the medium density is the apparent density, both the effective suspension density and upward liquid velocity have substantial influence on the apparent density. Since the settling velocity of particles during hindered settling is quite different from free settling conditions, the terminal settling velocity of the particles need to be adjusted to obtain the actual velocity of particles, which is commonly termed as slip velocity.

17 40 Therefore the particle velocities can be described by the relative velocity of each particle with respect to the velocity of water, which is called slip velocity. Particles having a slip velocity equal to the raising velocity of the water have equal chances of settling or being transported upward by water. However, if the slip velocity of a particle is greater than the raising water velocity, the particle settles downwards and reports to the under flow otherwise, it is carried away to the over flow. Galvin et al. (1999), proposed the following equation for calculating the slip velocity of any species in a homogeneous suspension (Galvin et al., 1999), Vi V T sus As per the above equation, the suspension density will have a strong effect on segregation, when particles are of different density. The dependence of the constant (ni) on the Reynold s Number is given in explicit form of equation obtained by Graside and A1-Dibouni (1977) Re ni (29) Re The Reynold s Number (Rei) of the particle at its terminal settling velocity (Uti) is obtained from the equation proposed by Zigrang and Sylvester (1981) Re ' ni1 (28) * 1.83 i g ps p p dp 3.81 (30) u Using equation (30) to calculate the Reynolds Number, terminal settling velocity of a particle (V T ) can be calculated as V i u T Re * (31) dp * p Generally these classifiers are operating based on the teeter column pressure difference which is a function of bed porosity, bed thickness, particle and fluid density along with the settling velocity. Therefore the pressure difference can be used to control the bed thickness along with the discharge of under flow fraction. The fluidized bed hydraulic classifiers are of different types based on their design such as single compartment, or multi compartment (Kohmuench et al., 2006;

18 41 Luttrell et al., 2006). The dynamic hindered-settling model was successfully developed by Kim and Klima et al. using a modified Concha and Almendra s hindered settling equation. By utilizing the model, predicting the settling velocities of the particles and the finite difference solution for mass balance while incorporating continuous mass input and output streams. Another model was proposed by Kwon et al based on discrete element Method (DEM) was used to simulate the movements of the particles in settling column (Kwon et al., 2008) Teeter Bed Separator The teeter bed separators (TBS) were developed from the concept of hydrosizer and the evolutions in the instrumentation field was made the segregation possible primarily on the basis of density; this has enabled specific gravity cut points as low as 1.35, while maintaining good separation efficiency. Consequently these units have gained the acceptance for recovery of coal from waste piles and tailing ponds as well as to treat run of mine coal. The schematic diagram of the TBS is shown in Figure 2.7. The feed slurry enters tangentially through a feed well and a fluidized or teetered bed is developed due to the upward water current. When steady state is reached, the particles which are lighter than the density of the teetered bed will float and report to the overflow stream whereas, the higher density particles will percolate and report to the under flow stream. In order to operate effectively the average relative density of the teetered bed within the tank is kept constant through a simple PID control and a capacitance type differential pressure cell receives a 4-20 ma signal from the probe. The effective density is compared to the operating set-point and the spigot valve will be actuated to discharge excessive solids, if the effective density is too high. Conversely, the control system acts to restrict the solids discharge if the effective density is too low (Drummond et al 2002; Stuart 1998).

19 42 ACTUATOR PID CONTROLLER FEED INLET FEED WELL TEETER ZONE OVER FLOW High RD Particle Settling through Bed Low RD Particle Rising through Bed UCW flow Fluidizing Bed PROBE UCW Inlet Valve Spigot Valve Spigot Discharge Figure 2.7: Schematic diagram of teeter bed separator. These units have been employed since the 1960s for recovering coal from waste piles and tailing ponds and have been used to treat run of mine coal in the UK from mid 1980s. Presently over 200 such units have been installed worldwide, in mineral processing applications including silica sand for construction grade, foundry and glass making purpose, mineral sands and hematite processing (Ratlou, 2002). The main disadvantage of this unit is of the design aspect of the under flow discharge. The flat bottom and the teeter plate was designed in inclined way over it, so the underflow solid concentration was low due to the entrainment of ultrafine particles Floatex Density Separator FDS is an advanced hindered settling classifier, also referred as counter-current or autogenous Teetered Bed Separator. It uses differential particle settling rates to segregate particles according to size, shape and density, moreover it is capable of treating the material whose size in between the size of screen and hydrocyclone (Littler, 1986). The cross sectional view of FDS is shown in Figure 2.8. Floatex density separator works on the principle of hindered settling and fluidization where

20 43 the settling rate of a particle in a liquid suspension is influenced by the presence of particles and the transition from free to hindered settling occurs as the concentrate of solids in suspension increases (Galvin et al., 1999). These decrease the distance between particles and the drag force created by the settling particles that will affect the surrounding particles. However, if the concentration of solids in the suspension is too high, entrapment and misplacement of particles will dominate the particles settling; as a result the heavier particle settles against upward current. If the density of the fluid is higher the larger/heavier particle that will remain suspended in the fluid hence, it is a function of particle size, density and fluid viscosity as well as the pulp density. When pulp density increases abundantly with particles crowded, only thin film of water and surface tension keeps the mixture together in perfect suspension is called full teeter. The particles heavier than the viscous pulp can fall through and settle against the raising teeter water through a bed of artificial heavy medium and accumulated at the bottom whereas all other particles simply float above the teeter zone (Epstein, 2005). The FDS consists of an upper square tank and a lower conical section and is divided into six main zones (Honaker and Mondal, 1999) as follows Over flow collection zone (zone A) Upper intermediate zone (zone B) Feed zone (zone C) Lower intermediate zone (zone D) Thickening zone (zone E) and Under flow collection zone (zone F) Feed slurry is introduced through a central feed well that extends to one third of the length of the main tank and the teeter water is introduced over the entire crosssectional area through evenly spaced water distribution pipes at the base of the teeter chamber. The teeter water flow rate is dependent upon feed particle size distribution, density and the desired cut-point for the separation. The separator is equipped with a pressure sensor mounted in zone D above the teeter water pipes and an underflow discharge control valve. The pressure, as measured by a level sensor, is transmitted to the underflow discharge control valve using a set-point controller

21 44 resulting in maintaining a constant height of the teeter bed and a steady discharge of the underflow (Elder et at, 1999). It is effective equipment for discarding the fine impurities like silica and other gangue minerals from the ore or raw coal. Hearn (SME Publication) discussed the operational circuit at LKAB, Sweden, to beneficiate iron ore fines using combination of spiral concentrator with FDS. The primary and secondary FDS are used for cleaning and a tertiary FDS is employed for scavenging operation to achieve the performance requirement of 80% iron bearing particle recovery with 1% silica in the concentrate. The effect of hindered-bed on classification or beneficiation of minerals and throughput capacity are function of the particle size distribution of the minerals (Littler 1986). A B C Overflow Stream Feed D E Teeter Water F Underflow Stream Figure 2.8: Different zones in the floatex density separator. It was found that a commercial hindered bed unit known as the FDS was very efficient in the recovery of zircon from wet gravity tailings (Mankosa et al., 1996) and the circuit efficiency can be increased by introducing the FDS for preclassifying or pre-concentrating the spiral feed in heavy mineral recovery (Elder et al., 1999). Preconcentration of chromite plant tailings was studied by varying some important process variables of FDS (Raghu Kumar et al., 2009). Optimization studies conducted on a 75 TPH hindered-bed separator in coal preparation plant

22 45 with the separator being used as a cleaner unit for the spiral product (Drummond et al., 1998), demonstrated separation densities below 1.6 with Ep values in the range of 0.10 to The separation features inside the FDS were described by treating coal fines, concluding that the performance of the FDS was influenced by both with particle density as well as size (Sarkar et al., 2008). The separation principle of the FDS was analysed with simple slip velocity model which incorporates the effects of suspension density and bed voidage; this model was successfully simulated with different coal samples (Das et al., 2009). It was also found that in single stage processing in FDS, 72% of the feed alumina in iron ore could be removed. A concentrate containing 1.66% alumina from 3.39% run-of-mine alumina was produced at a yield of 57% using FDS. Higher teeter water was found to improve alumina removal albeit with a small decrease in iron recovery. It was observed that higher bed pressure and lower pulp density are favorable for alumina rejection (Sarkar et al. 2008a). Classification performance of FDS has evaluated by using pure silica mineral and effect of particle size on particle classification was established by Raghu Kumar et al.(2011). This particular unit has been used in mineral sand, low grade iron ore, chromite and coal preparation industry (Reed et al., 1995; Dunn et al., 2000; Sarkar et al., 2008). Several studies have been carried out to characterize the teeter water flow rate on particle separation (Sarkar et al 2008a). Also the particle separation in FDS while treating different coal was characterized in terms of slip velocity models (Sarkar et al 2008; Sarkar et al 2008a; Sarkar et al., 2010). Similarly the effect of teeter water flow rate on particle separation for chromite and iron ore fines was investigated (Sarkar et al., 2008; Kapure et al., 2007). FDS works based on the difference between both the particle size and density but its detailed separation mechanism is not properly understood. A proper understanding of the process variables along with the feed material characteristics is essential, making it easy to integrate with all circuits where applicable.

23 Cross Flow Separator The Cross Flow separator is an improved version of fluidized bed classifier, which uses the feed slurry introduction to the system across the top of the separator through transition box/feed well into the teeter bed to avoid the turbulent mixing that causes a detrimental impact on performance. The schematic diagram of the cross flow separator is shown in Figure 2.9. The stilling-well smoothly passes the feed slurry horizontally across the top of the cell which ensures those variations in feed slurry characteristics (e.g., solids content) do not impact separation performance. A duct plate is also located at the discharge end of the feed introducer to prevent short-circuiting of solids directly to the overflow launder. Another design feature incorporated into Cross Flow classifier is the improved water distribution system viz. a horizontal slotted plate, located at the base of the separation chamber where water is introduced beneath the plate through a series of large diameter holes (>1.25 cm). This modification essentially eliminates problems associated with distributor pipe plugging. The combined use of the improved feed injection system and simplified water distribution system makes it possible to increase both the separation efficiency and throughput capacity while eliminating mechanical problems associated with traditional designs. Pilot-scale testing proved that the CrossFlow separator improved the separation efficiency compared to conventional hindered-bed classifiers. A full-scale retrofit of an existing hydrosizer with a CrossFlow feed system verified that at equivalent cut points, the classification efficiency is improved by more than 33% and employed at barite processing. Typical results show that over 90% of the available barite can be recovered at a silica rejection of greater than 90% (Kohmuench et al., 2002). Further it has demonstrated its supremacy in coal washing with low ash product at a high combustible recovery.

24 47 Figure 2.9: Schematic diagram of cross flow separator (Luttrell et al, 2006) Reflux Classifier Reflux classifiers, consist of a conventional fluidized bed with a set of parallel inclined plates and provides a stable response to the large fluctuations in the feed solids throughput (Galvin and Nguyentranlam 2002; Galvin et al., 2005). A schematic representation of the Reflux classifier is shown in Figure A pressure transducer senses the high density suspension, resulting in their discharge when the suspension density exceeds that of the set point. In the absence of high density particles the fluidization water maintains a suspension within the vessel and the excess suspension reports to the overflow. The effective sedimentation area of the vessel is increased by these inclined plates and fluidized suspension passes up through the inclined channels. Faster settling particles segregate onto the inclined plates and slide back down as concentrated to below the fluidized zone (Doroodchi et al., 2004a; Galvin et al., 2010).

25 Vertical fluidization section Inclined section 48 Overflow weir Over flow Feed Distributor Fluidization water Additional water Fluidization water Underflow Figure 2.10: Schematic diagram of reflux classifier (Zhao et al, 2006). The particle separation in reflux classifier is shown pictorially in Figure Extensive laboratory and pilot scale studies have been carried out at Centre for Advanced Particle Processing and Transport, University of Newcastle. The Reflux Classifier was used to separate coal and mineral matter in a number of studies, covering a broad range of vessel geometries and feed conditions. Overflow (Product) Product plates Feed Reject plates Fluidization water Overflow (Reject) Figure 2.11: schematic diagram of particle separation in reflux classifier( Galvin et al, 2004).

26 49 It also reported that coarse coal particles can also beneficiated effectively by this unit (Galvin et al., 2009). But this particular unit has been tested for coal preparation only, so their applications in different mineral industry need to be explored All Flux Separator The Allflux classifier is also a hydraulic classifier having round, central feed well with a unique combination of rising current and fluidised bed which divides the feed into three different products. The schematic diagram of a Allflux classifier is depicted in Figure Feed material is suspended in water in a separate vessel. It flows into the top of the machine and down the central core. At the bottom the material encounters an upward flow of water. The velocity of the upward water is set so that the lighter particles fluidize and are carried upwards whilst the heavy particles are able to settle downwards. During operation, the water flow rate is adjusted in order to find the optimum for the separation. The heavy particles will be discharged at the bottom of the machine. The lighter material flows up and over into the outer section of the machine where it encounters another upward flow of water. The water velocity in the outer separation section is less than that in the inner separation section. The lightest material is fluidized but the middle fraction sinks, as its settling velocity is greater than the water velocity. The light material is carried over the weir and is collected. The middle fraction meanwhile collects in the bottom of the outer section and a valve periodically opens to allow the material to flow out of the bottom of the machine. The upward water flow rate in the outer separation chamber depends on the size and density of the particles to be separated. The machine can be supplied without the outer separation section if it is only required to separate two products (heavy and light) (Breuer et al., 1994).

27 Fine classification section Fine classification section 50 Feed inlet Ultrafines collection launder Ultrafines collection launder Coarse classification Water injection Ultrafine Product Fine Product Water injection Water injection Fine Product Ultrafine Product Coarse product Figure 2.12: Sectional view of Allflux classifier (Courtesy: All minerals) This particular unit can employed in iron ore, coal, industrial minerals, chromite, slag processing industry for the coarse classification, deslime, thickened and to separate the particles based on their difference in the slip velocity. The commercial units are available for the higher throughput up to 2000 m 3 /hr. It was successfully employed in the iron ore industry to classify the feed into different products for subsequent separation (Andreas Horn., 2009).The main advantage of this unit is for high throughput, automatic three product discharge system, low power consumption and minimum maintenance. 2.6 Review on floatex density separator Littler 1986, evaluated the effect of hindered-beds on the classification of mineral particles and found that throughput capacity and teeter water requirements are a function of the particle size distributions of the mineral components being separated. Mankosa et al found that a commercial hindered-bed units known

28 51 as the FDS was very efficient in the recovery of zircon from wet gravity tailings. A classifier commercially known as the Allflux Separator has been found to provide excellent purification of silica sand, iron ore and heavy minerals (Breuer H et al., 1994; Andreas Horn 2009). The cleaning of fine coal using hindered-bed classifiers has also been the subject of several recent investigations. Mankosa et al.2007 conducted tests using a hindered-bed classifier as a pre-concentrator to spiral concentrators for the treatment of a difficult-to-clean coal. The findings suggest that the hindered-bed-spiral combination provides an efficient, low density separation that allows the production of clean coal while maintaining a relatively high recovery of combustibles. Studies comparing the separation performances achieved by spiral concentrators and two hindered-bed classifiers (i.e., Stokes and Floatex) found the classifiers to provide more efficient gravity separations for nominally 1 mm x 150µm coal. Nicol and Drummond 1997 have described the efficiency of hindered-bed density separators and several fine coal circuits for which application may prove to be beneficial. In fact, Drummond et al 2002 recently reported on the optimization studies conducted on a 75 tph hindered-bed classifier installed in an operating coal preparation plant in which the separator is being used as a cleaner unit for the spiral product. This application reportedly allows separation densities (D 50 ) below 1.6 and probable error (Ep) values in the range of 0.10 to Elder et al.1999 envisaged that the circuit efficiency can be increased by introducing the floatex density separator as a pre-classification and/or pre-concentrate of the spiral feed in up gradation of heavy mineral sand. Pan et al described that by processing iron ore slime in Fluidized Bed Classifier followed by hydro-cyclone and Slow Speed Spiral Classifier, the valuable iron ore particles with 62.5% Fe, 4.2% SiO 2, 2.23% Al 2 O 3 could be recovered from the slime assaying 50.8% Fe, 11.86% SiO 2 and 9.84% Al 2 O 3 with a yield of 30%. The separation features inside the FDS was described by Sarkar et al.2008b by treating coal. They concluded that the performance of the FDS was influenced by both with density as well as size. Das et al.2009b described the separation principle of the FDS by simple slip velocity model which incorporates the effects of suspension density and bed voidage.