Literature Review
2.1 Heavy Media
2.1.3 Suspensions of solids in water
The heavy medium commonly used in industrial separations is a thick pulp of some heavy mineral in water, which essentially behaves as a heavy liquid. These suspensions are prepared by adding an aggregation of solid spheres to a liquid. Heavy medium suspensions can be classified as a heterogeneous system comprised of a disperse phase, and a continuous phase. The solid particles represent the disperse phase, while the liquid represents the continuous phase [Jinescu (1974)]. The disperse phase may itself also be a heterogeneous system comprised of solid particles of different nature and form. In some industrial processes solid particles of different form and nature have been used at different concentrations to facilitate the separation of certain mineral ores. Collins et al. (1983) used a mixture ofatomised ferrosilicon and magnetite mixtures in the separation ofa tin ore in a Vorsyl and cyclone separator.
The liquid can either be a Newtonian liquid or a non-Newtonian liquid. In almost all, if not all, industrial processes the liquid used is water, which is a Newtonian fluid. Generally, if the suspension continuous phase is a Newtonian liquid (e.g. water), the behaviour ofthe suspension is Newtonian for relatively small solid volume concentrations, usually below 30% by volume [Wills (1997), Pryor (1960)]. Above this value the suspension becomes non-Newtonian due to particle- particle interactions of a hydrodynamic nature. The suspension usually acquires minimum shear stress needed before movement of the suspension can occur. Feed material comprised of small particles, or particles with densities close to the suspension density, have difficulty in overcoming the surface force offered by the suspension, requiring the use of centrifugal forces in place of gravity forces, or the reduction of the suspension viscosity through agitation. If the suspension continuous phase is a non-Newtonian liquid the suspension exhibits non-Newtonian behaviour at both high and low solid volume concentrations. At low solid volume concentrations the non- Newtonian behaviour is due the non-Newtonian nature of the continuous phase. At high solid volume concentrations the non-Newtonian behaviour is due to both particle-particle interactions and the non-Newtonian nature of the continuous phase [Jinescu (1974)].
The media particles used to prepare heavy media suspension are chosen according to the following qualities [Pryor (1960)]:
CHAPTER 2 LITERATURE REVIEW
a. Hardness: The chosen medium particles must not easily breakdown under working conditions. Particle breakdown introduces fines into the system, resulting in an increase in the apparent viscosity of the suspension.
b. Chemical stability: The medium particles must not be chemically corrosive, or liable to react with the ore minerals undergoing treatment. The products of corrosion also introduce fine material into the system. As will be shown later, ferrosilicon particles are manufactured such that the silicon content is eriough to delay the onset of particle corrOSIOn.
c. Low settling rate at moderate viscosity: The settling rate of the solid particles is one of the important rheological properties of heavy medium suspension. Suspensions are said to be stable when the settling rate of the medium particles is low. The stability of the suspension can be improved by using very fine medium particle,s. However, this increases the suspension viscosity, reducing the separation efficiency of the process. Thus, the medium particles must form a fairly stable suspension without having to be below lO !lm, on average.
d. Density: At a constant suspension density, higher particle densities reduce the viscosity of the suspension by reducing the solid volume concentration. Hence, the density of the medium particles must be high enough to give the required suspension density under reasonably non-viscous conditions.
e. Regeneration: Because of the fouling nature of heavy medium processes, the media has to be cleaned after each cycle before further use. The medium chosen must be easily recoverable from the mineral surfaces through washing and reconstitution.
f. Non-fouling: After the ore has been cleaned and washed, a certain amount of medium lodges in cracks in the lumps of the cleaned ore. The chosen medium must not be of a nature or form deleterious to subsequent treatment of the ore.
CHAPTER 2 LITERATURE REVIEW
Table 2.1 lists some the heavy media able to meet some of the above specifications. The table also lists the method of regeneration for the heavy media.
Table 2.1 Heavy media and their methods of regeneration [Pryor (1960)]
Material Method of Regeneration
Galena Froth-Flotation
Magnetite Magnetic Separation
Ferrosilicon Magnetic Separation
Pyrites No regeneration
Copper Pyrites Obsolescent
Hematite Obsolescent
Galena is a soft mineral with a specific gravity of 7.4 - 7.6. Galena is used to maintain suspensions with a specific gravity of 3.3. At densities higher than these the viscosity of the suspensions become unmanageable, making the separation of small particles or those with . densities close to the suspension density very difficult. The regeneration process of galena by froth-floatation is more expensive and more difficult than the recovery of magnetic materials.
Galena is also fairly soft and tends to slime easily, and it also has a tendency to oxidize, which has a negative effect on flotation efficiency [Wills (1997)]. Because of this, and the fact that the material itself is expensive, the use of galena as heavy media has been abandoned in almost all industrial heavy medium separations.
Magnetite is a common, relatively cheap mineral existing in many localities. Some famous localities are Binnental, Switzerland; Pfitschal, Tyrol, Austria; and the Phalaborwa mine, Limpompo, South Africa. The largest magnetite deposits are in northern Sweden. Other large deposits are in Norway, Romania, and Russia. Magnetite is the only mineral that exhibits ferromagnetism, although other minerals like ilmenite and hematite have weakly magnetic behaviour [http:// geology.about.com (22/02/04)]. Most magnetite occurs in small grains and has a specific gravity between 5.0 and 5.2. It is extensively used in coal preparation, and is used to maintain suspension densities of up to 2500 kgm-3 [Pryor (1960), Wills (1997)]. Because of its relatively low specific gravity compared to ferrosilicon, magnetite is rarely used at specific gravities above 2.5 because of the associated high viscosity of suspensions above this specific
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gravities. Aplan et al. (1960) also showed that, due to processing, magnetite media contained much higher fines content than ferro silicon media. The presence of these fines is also a contributing factor towards the non-Newtonian behaviour of magnetite suspensions at high suspension densities. Magnetite is easily recoverable from the heavy medium circuit through magnetic separation.
Ferrosilicon (FeSi) is now the most widely used media for heavy medium suspensions of ores with a specific gravity in the range of approximately 2.5 to 4.0 [Collins .et.al. (1974)].
Ferrosilicon is an alloy of iron (Fe) and silicon (Si) which usually contains not less than 82 % iron, and 14-16 % silicon. To reduce the corrodibility of the medium, the silicon content should not fall below 14 %. Also, the silica content should not exceed 16 % as this has the tendency to decrease both the magnetic susceptibility of the medium, and its specific gravity [Collins et al.
(1974), Rodis et al. (1960), and Wills (1997)].
There are two types of ferrosilicon media available in South Africa: milled ferro silicon and atomised ferrosilicon. The difference between the two types of ferrosilicon arises from the different methods of their production.
The standard method for the manufacture of milled ferrosilicon is to melt steel scrap, quartz, and a reductant in a submerged arc-furnace [Collins et al. (1974)]. The molten alloy is tapped into a sand bed, allowed to cool, after which it is broken into lumps, and finally crushed in two stages and milled to the required size range. The furnace is charged on a semi-continuous basis, and is usually tapped into a ladle every two hours. The temperature of the melt on tapping is approximately 1600 DC.
Atornised ferrosilicon is manufactured using two methods. The first method, developed by Knapsack-Grieshein AG in Germany, atomises the molten liquid after it is tapped from an induction furnace [Collins et al. (1974), Rodis et al. (1960)]. Ferrosilicon of75 percent silicon is diluted with high-grade steel scrap in an induction furnace. The furnace is tapped every three hours at a charge temperature of approximately 1550 DC. The molten alloy stream enters an atomising stream cone where it comes into contact with a cone of steam, and the melt is immediately broken into fine particles, which are quenched in water. The resultant ferrosilicon pulp is filtered, dried, and classified to remove oversize material.
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The second method of manufacturing atomised ferrosilicon was developed by ISCOR in South Africa [Collins et al. (1974)]. The atomised ferrosilicon is also produced in an induction furnace.
In this case, however, the stream of molten alloy is broken up in a series of concentric cones of high pressure water. The granulated product is dried and screened, and the oversize is milled to the required size in a ball mill in closed circuit with an air classifier. The milled powder is then atomised by allowing it to fall through a flame which has an oxidising centre and a reducing outer zone. This permits each individual particle to be melted without being oxidised. Mter atomisation the powder is quenched in waste gas to prevent oxidation.
As mentioned above, the difference in physical properties between the two types of ferro silicon arises from their different methods of production. The milled grade tends to be rough and angular in shape, whereas atomised ferrosilicon is smooth and largely spherical or rounded in shape [Williams et al. (1986)]. As will be shown in a later section, this difference in shape has marked effects on the rheological properties of aqueous suspensions of the two types of ferrosilicon. It is thus important to properly select the most suitable type of ferrosilicon for a given separation.
Selection is usually based on the viscosity and stability requirements of the medium. Collins et al.
(1974) and Ferrara et al. (1986) give the general criteria used in the selection of which type of ferro silicon medium to use:
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Operating Density: Generally, for separations above 3.0 for static baths, and 3.2 for cyclones, the use of atomised ferrosilicon is preferred because above these values the viscosity of milled ferrosilicon suspensions rises to unmanageable proportions.
Ore size: The type of separating vessel used is usually determined by the size of the ore being treated. For very large ore particle sizes (above 100 mm) static separators are used, whereas dynamic separators are used for the separation of fine ore particles. Because of the higher centrifugal forces encountered in dynamic separators, a finer medium, with a moderate viscosity, and high stability is necessary. Hence, at a particular specific gravity, atomised ferrosilicon would be chosen above the milled grade.
Sharpness of Separation: At low specific gravities either the milled or atomised grade can be used. However, depending on the accuracy of separation required, milled ferrosilicon is preferred because it is cheaper.
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Corrosion Resistance: The occurrence of corrosion is deleterious because corrosion leads to loss of ferrosilicon; The finely divided products of corrosion impair the separation efficiency of the process by increasing the viscosity of the medium; In situ corrosion often leads to the cementing of ferrosilicon particles when they stand in water, caused by the electrochemical oxidation of the ferrosilicon surface to produce non-magnetic iron oxides. Making the start-up of heavy medium processes very difficult after a long shut- down. Rodis .et.al. (1960) have shown that milled ferrosilicon is more susceptible to corrosion than the atomised grade because of its rough, angular shape. For plants operating under abnormal conditions such as high density, high temperature, and acidic or saline water, atomised ferro silicon is the preferred choice.• Costs: The cost of manufacturing atomised ferrosilicon is more than double that of milled ferrosilicon. However, when a choice between the two grades is being made, careful consideration into the overall advantage of each grade must be taken into cognizance.
These include lower consumption due to corrosion; adhesion, and magnetic 'separator losses; greater efficiency of separation; and a smaller bulk concentrate, reducing subsequent treatment costs.