Literature Review
2.3 Rheology modification of heavy medium suspensions
Once a particular material has been chosen for the heavy medium separation process, the rheology of the media can be modified through the following ways:
1) Addition of slimes or clays.
2) Addition of polymeric compounds or other reagents.
3) Demagnetisation of the circulating medium.
Before discussing the different ways of modifying the rheology of the medium, some properties of clays / talc's and water will be briefly discussed.
The dispersing liquid in almost all industrial applications ofDMS is water. Thus, the properties of water in relation to its solid - liquid interaction are of considerable importance. The unique properties of water include its ability to fonn hydrogen bonds, and its ionizing solvent power. It can dissolve and ionize more salts than any other solvent. Liquid water has a three dimensional structure of infinitely extending networks of anions and cations. Each cation is screened by two anions (the coordination number is 2), and each anion is tetrahedrally surrounded by four equidistant cations. In order to understand the fluidity of water one has to realize that the above picture of its structure is a description of the location of the protons over a time average. The protons can change their positions without losing screening, which means that they temporarily submerge in the electron cloud of the two 02-ions so that only over a time average do the protons assume positions which are equidistant from both 02- [Eirich (1960)].
The viscosity of water is detennined to a large extent by the mobility of the protons in its three- dimensional structure. It is also one of the anomalous properties of water. The structural picture which describes water as a three-dimensional network of ions suggests that water should be rigid.
However, the fluidity of water is abnonnally high due to the mobility of the protons. Conditions which increase the tendency of the electrons to remain within the electron clouds of 02- ions and , change water from a three-dimensional network of ions towards an aggregate of molecules lower its viscosity e.g. applying pressure to water decreases its volume and increases the electron cloud density of the 02- ions, favouring the interpenetration of the protons into the electron cloud. Thus,
CHAPTER 2 LITERATURE REVIEW
water, unlike other fluids, becomes more fluid under pressure. For charged particles in water, the field strength of the particles determines the mobility of the protons in the water molecule.
Particles with a high surface charge density (e.g. clays and talc powders) immobilize the protons, favouring the three-dimensional structure such that the viscosity of the water is increased.
The structure of all clay minerals consists essentially of mica-like layer lattices in which the ultimate building units are silica, alumina and water, frequently with appreciable quantities of iron, alkalis, and alkaline earths. In general the term clay implies a natural, earthly, fine-grained material which develops plasticity when mixed with a limited amount of water. What all Clays have in common is that they are 'infinitely extending' networks only in two dimensions. Thus, the minerals consist of stacks of sheets. On account of this structural characteristic, clay minerals occur as anisodimensional, either thin plates most less than 2 microns in diameter, or as elongated tubes and lathes [Eirich (1960)].
When minerals are comminuted or liberated from their ores, suspensions of fine (0 IJLm) or colloidal particles generally result. Such small particles are generally small enough to be unaffected by gravity and as a result; their interactions with each other and other larger particles are controlled by a range of attractive and repulsive inter-particle forces [Johnson et al. (2000)].
There are a number of forces influencing the interaction of colloidal particles. The magnitude and nature of these interactions can be controlled by manipulation of a range of experimental parameters. These include (but are not restricted to) the particle size, the solution pH, the nature and concentration of simple electrolyte ions, and the addition of other soluble and/or surface- active species including surfactants, polymers and electrolytes.
The addition of small quantities of ore slimes or clays has been found to increase significantly the viscosity and stability of DMS suspensions. In some cases the slimes represent an undesirable constituent of the ore being treated, and a proportion of the circulating medium has to be continuously removed and cleaned through magnetic separators. Laskowski et al. (1999) states that the nature of the clay contaminant has a profound effect on the viscosity increase of magnetite suspensions. They found the viscosity rise was greater with montmorillonites than with kaolinite and also that sodium montmorillonites caused a higher rise in the viscosity than did calcium variety. Aplan et al. (1964) performed experiments to find the effect of manganese ore slimes, bentonite (sodium montmorillonites), and kaolinite on the viscosity of ferrosilicon media.
They found that the addition of 10 % manganese ore slimes changed the viscosity profile of
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atomised ferrosilicon to one nearly identical to an uncontaminated, milled ferrosilicon suspension. They also showed that bentonite is about five times as effective as kaolinite in increasing the apparent viscosity of atomised ferrosilicon suspensions. .
Grobler et al. (2002) did tests to investigate the stability of ferrosilicon suspensions. They showed that particles smaller than 45 pm, and especially the ore fines, played an important role in the settling behaviour of the solids. The ore slimes they used for their tests contained mainly hematite (78%), silica (12%), and alumina (9%), and had a size distribution of70 per cent smaller than 10
).l.ffi, with 34 per cent smaller than 1 ).l.ffi. Tests were perfonned by adding 2 % and 5 % slimes.
The stability of the suspensions increased with the amount of slimes added. At 5 % slimes content, the stability of the suspensions was more than double that at 0 % slimes content. They also observed an increase in viscosity with slimes content. They attributed this to the fact that slimes or contaminants have a lower density than ferrosilicon particles; a contaminated medium will thus need to contain a higher solids concentration to achieve the same density. A higher solids concentration will result in a much higher resistance to flow. High amounts of certain types of clays, particularly bentonites, exhibit thixotropic behaviour. This means that at certain clay contamination levels, the behaviour of the suspensions could vary. This could have a significant effect on the rheological properties of the contaminated suspensions, and on the separation efficiency.
2.3.1 Surface forces acting between colloids/clays in aqueous media
The effect of the different slime contaminants is dependant on the type of inter-particle forces occurring within the medium. There are a number of forces acting between colloidal particles, and these are discussed below. Only the forces considered relevant to the project will be briefly discussed in this section. Further details on these and other forces can be found in Johnson et al.
(2000).
2.3.1.1 Van der Waals forces
Van der Waals forces occur almost everywhere within colloidal systems, and are caused by the interaction between instantaneous dipoles generated within the atoms comprising each particle.
On a microscopic scale, atoms are non-polar in nature, but dipoles are fonned through the
CHAPTER 2 LITERA TURE REVIEW
displacement of the centre of the electron cloud relative to that of the positive nucleus. Each dipole formed then induces instantaneous dipoles within atoms in neighbouring particles. Van der Waals forces are independent of conditions such as the system pH and the presence of surfactant and polymeric species. These forces have, however, been shown to be strongly dependent on the particle size, as shown by the following equation:
F
=_
aAHYAW 12H2 (a» H) (2.7)
FVDW
=
Van der Waals force, a = particle radius, H = distance between adjacent particles, and AH=
a materials property termed the Hamaker constant.2.3.1.2 Electrical double layer forces
When mineral particles are brought into contact with an aqueous environment they often acquire a substantial surface charge. This can occur through either through specific ion adsorption, isomorphous ion substitution, or differential ion dissolution and ionisation of surface sites. This process is of particular relevance to oxide minerals (e.g. Fe304), and involves the protonation / deprotonation of surface hydroxyl sites (M-OH) via the following process:
H' H'
H20 + M - 0- <;::> M - OH <;::> M - OH;
OH- ow (2.8)
where H+ and OK are referred to as the potential determining ions.
The experimental parameters which can be used to modify electrical double layer forces are changing the suspension pH, addition of charged surface -associating species to the suspension, controlling and regulating the type and quantity of electrolytes present n the suspension, and changing the particle size.
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2.3.1.3 Structural ('hydration') forces
When two particles interacting across an aqueous medium are brought into close proximity, a short range repulsive force is often observed. This interaction is believed to be related to the presence of a strongly surface~associated layer of water on each of the particle, and is enhanced when the surface posses a strong hydrogen bonding ability. The strength and range of repulsion of these forces can be manipulated through the control and type of electrolytes in solution. In addition, however, the nature of the particles (and their affinity for solution electrolyte ions) can also be controlled through both pH-induced changes to the surface charge and adsorption of other ions, polymers or surfactants from solution.
2.3.1.4 Steric and electrosteric forces
As mentioned, the rheology of mineral suspensions can be altered by the use of surface active agents to either reduce or increase the viscosity and stability of the suspension. These surface active agents are usually polymers of varying lengths. The interaction behaviour observed when two surfaces bearing layers of adsorbed polymers are brought together is often substantially different to that noted for rigid, smooth surfaces. In particular, a steric repulsion is generally measured when surfaces coated with a stable polymer approach to a separation less than twice the thickness of the polymer layer. This force is believed to result from two separate effects; a rise in osmotic pressure caused by the increased concentration of the polymer layers between the surfaces, and an elastic force due to compression of the polymer layers. Practically, Steric and electrosteric forces in mineral suspensions can most readily be manipulated by controlling the type, size and concentration of the adsorbed species used to effect dispersion.
2.3.2 Surface active agents
There are two important mineral process used for the separation of solid particles from a suspension; gravity concentration and floatation. The preparation of slurries for each of the above processes occasionally requires the use of surface active agents to improve the separation efficiency. As previously mentioned, gravity concentration uses gravity or centrifugal forces' to separate dense particles from less dense particles. The slurry used in this application, particularly in heavy medium separations, should have a low viscosity and moderate - high stability. This type
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of separation suits a dispersed suspension where the particles in the medium do not show a tendency towards agglomeration. Floatation on the other hand requires the solution to show a tendency towards sedimentation through the agglomeration of the particles in the medium.
The process by which particles in solution either become dispersed or agglomerate is believed to be due to the zeta potential of the medium particles and fine contamination particles. For example, Jones et al. (1999) showed the effect of zeta potential on the type of agglomeration in aqueous silica - iron oxide slurries. The presence of small quantities of iron oxide, particularly if the particles are small, causes co-agglomeration with silica particles into flocs. The interaction occurs because the zeta potential of iron oxide is positive and that of silica is negative at neutral pH. Thus there exists a net attractive force between their respective particles. These flocs also retain a portion of the continuous phase, which effectively increases the apparent solids volume fraction, and the viscosity of the suspension becomes very high. As the flocs are sheared they break into smaller aggregates, releasing some of the retained fluid, which decreases the apparent volume fraction. The viscosity consequently decreases until a point is reached when no more floc structure exists and the particles move freely in the slurry. The fonnation of this structure on resting gives rise to an apparent yield stress, and the breaking down of this structure leads to shear thinning behaviour, both these effects being caused by particle - particle interaction. These conditions are depicted in Figure 2.2.
•
thick slurry. large apparent volume fraction
mechanical shear dispersants
rest f10cculants