•... :::>"';.
~ ----IECONDARY MAGNETIC IE'ARATOII
TO DENSE MEDIUM CIRCUIT
DILUTE IUM'
'Uti COAL
----t.~ CIRCUIT
DILUT! 'UM'
Figure 1.3 Stages in heavy media separation for dilute medium circuit (Horsfall (1993)]
CHAPTER 1 fNTRODUCTION
The main stages in heavy medium separation are divided into the sections given below:
a) Presentation of a suitably prepared feed.
b) Separation into' floats and sinks in a vessel containing the dense medium.
c) Withdrawal of products, and removal from them of adherent dense media.
d) Cleaning and reconstitution of the medium followed by a return to the separator of clean dense medium.
The feed to the cyclone is suitably prepared by screening in order to remove some of the fine material, which has a tendency of increasing the suspension viscosity. The de-slimed feed is then fed into the separating vessel (cyclone in this case) to obtain an overflow and underflow product (Figure 1.2). The floats and sinks are then sent to screens where they are washed to remove any adhered media particles. After dewatering, the products are removed, and the underflow from the screens is sent to the dilute medium circuit (Figure 1.3). Here the media is reconstituted by passing it through magnetic separators.
There are a number of solids which have been used to prepare dense medium suspensions. The most common solids used are galena, magnetite, and ferrosilicon. Minor types include pyrites, chalcopyrite, hematite, barites, and clay. The criteria used to determine which solids are suitable are based on the physical properties of the particles. The solid particles must have a reasonably high density; must be hard spheres, and must be easily recoverable from the suspension. Galena is mostly used in lead-zinc concentrators, at suspension specific gravities as high as 4.0 [Wills (1997)]. Magnetite is used predominantly in the coal industry to separate clean coal from high-ash coal. Magnetite is used to prepare suspensions with specific gravities ranging between 1.6 and 2.5. Ferrosilicon is the most widely used medium for preparation of suspensions with specific gravities greater than 2.5. Ferrosilicon is an alloy of iron and silicon. The ratio of iron and silicon is such that the corrodibility of the alloy is small, without compromising the recovery of the particles. Both magnetite and ferrosilicon are magnetically recoverable. This method of recovery is more cost effective compared to the method of recovery of galena and the minor type solids.
The latter solids are recovered through flotation, which is an expensive process [Wills (1997)).
CHAPTER I INTRODUCTION
1.3 Parameters involved in heavy medium separations
The separating vessel~ used in heavy medium suspensions can be classified under two groups:
static (gravitational) separators, and dynamic separators. For an ore particle being separated using heavy medium separation, there are three main forces acting on the particle [Napier-Munn (1990)]. The first force is a downward or penetrating force. In static baths this force is the gravitational force on the particle, and in dynamic separators (e.g. centrifugal separators) the force· is often a centrifugal force. The second force is a buoyant force, equal to the weight of the fluid displaced by the particle. This type of force is common among static separators and is one of the forces accounted for by Stoke's Law. This force always acts in the direction opposite to gravitational/centrifugal forces. The third type of force is the drag force. This type of force also acts in a direction opposite to gravitational/centrifugal forces. The drag force on a particle can either be due to viscous resistance or turbulent resistance, depending on the type of flow regime prevailing in the separating vessel. The most significant difference between the two types of drag force is that viscosity is one of the parameters in viscous resistance, while it is absent in turbulent resistance.
Particle size and density difference are important parameters common to both types of resistances. Density difference refers to the difference in density between the particles and the separating fluid. If the modulus of the density difference is high, separation of particles occurs quite easily even at small particle size ranges. However, if the modulus of density difference is small, the separation of the particles becomes difficult at small particle sizes. The efficiency of separation decreases at small particle sizes because of the slower settling-rates of the particles.
For separations to be effective on a difference in specific gravity of 0.1 or less, the particles should be larger than 3 mm in diameter. For small particle sizes (0300 J.UI1) centrifugal forces can be used to increase the settling-rate of the particles [Wills (1997)].
The two most important rheological parameters in heavy medium separation are the viscosity and stability of the suspensions [Collins et al. (1983)]. The viscosity is defined in terms of the resistance to flow of the fluid. In the context of heavy medium separations, this refers to the resistance of the fluid towards particle penetration. A high viscosity has been shown to reduce the efficiency of heavy medium separations [Collins et al. (1983) and Napier-Munn (1990)]. A high viscosity results in fine particles, and those with specific gravities close to that of the suspension being unable to penetrate the suspension. This results in the misplacement of high density
CHAPTER 1 INTRODUCTION
material to the floats product. The stability is defined in tenns of the settling rate of the suspenSIOn. Heavy medium suspensions often stratify and become non-homogenous. This is caused by the high density of the solid particles making up the suspensions. These solid particles settle out of the suspension, often leading to difficulties in achieving the correct specific gravity.
A suspension with a low settling rate is said to have a high stability i.e. stability is the reciprocal of settling rate. The stability and viscosity of heavy medium suspensions depend on the specific gravity of the suspension, the media particle size distribution, media particle shape, and presence of slimes / clays.
The viscosity and stability of heavy medium suspensions are mutually dependant on each other.
This means that altering one of them has a direct effect on the other. A high viscosity results in a higher stability, whilst a lower viscosity results in a much lower stability. This often causes problems for plant operators since most suspensions are inherently viscous because of the presence of clay and slimes in the suspensions. Thus a trade-off exists between having a suspension with a high viscosity and stability, or having a suspension with a lower viscosity and stability. Most plant operators would rather work with suspensions which are less viscous, with a much lower stability, than having to work with suspensions which have a high viscosity [Valentyik (1972)].
CHAPTER 1 INTRODUCTION
1.4 Objectives of the Investigation
The main objective of this project was to investigate the viscosity of heavy medium suspensions.
The media particles used were ferrosilicon and magnetite. The effect of a surface active agent on the viscosity of heavy medium suspensions was also to be investigated.
To achieve the above, the project was divided into the following investigations to:
• Survey the available literature on the viscosity of heavy media suspensions, and also the methods of reducing the viscosity of heavy media suspensions through the use of dispersants.
• Perform rheological measurements usmg a viscometer m order to determine the rheological profiles of the suspensions. The behaviour of the liquids was then used to infer what is likely to happen in separating vessels.
• Perform rheological measurements using a surface active agent (DPOOI). The results of the experiments can be used to predict whether or not a dispersant affects the viscosity of the suspensions.
•
•
Perform rheological measurements to determine the effect of media particle SIze distribution and shape on the viscosity of the suspensions.
Perform experiments to determine the effect of viscosity and the dispersant have on the separation efficiency in a dynamic separator. The dynamic separator chosen for the project being a dense medium cyclone.