This document discusses various methods used for dewatering mineral concentrates in mineral processing operations. It describes sedimentation, filtration, and thermal drying processes. Sedimentation involves settling of solids using gravity or centrifugal force. Filtration separates solids from liquids using a porous medium. Thermal drying further reduces moisture content using rotary dryers. The document provides details on classification, coagulation and flocculation to improve sedimentation, and different types of filters and factors affecting filtration rates.

1. Dewatering

2. Usefulness of Dewatering
• With few exceptions, most mineral-separation
processes involve the use of substantial quantities
of water and the final concentrate has to be
separated from a pulp in which the water-solids
ratio may be high.
• Dewatering, or solid-liquid separation, produces a
relatively dry concentrate for shipment.
• Partial dewatering is also performed at various
stages in the treatment, so as to prepare the feed
for subsequent processes.

3. Classification of Dewatering
• sedimentation;
• filtration;
• thermal drying

4. Sedimentation
• Rapid settling of solid particles in a liquid produces a
clarified liquid which can be decanted (pour out),
leaving a thickened slurry, which may require further
dewatering by filtration.
• The settling rates of particles in a fluid are governed by
Stokes' or Newton's laws, depending on the particle
size.
• Very fine particles, of only a few microns diameter,
settle extremely slowly by gravity alone, and centrifugal
sedimentation may have to be performed.
Alternatively, the particles may be agglomerated, or
flocculated, into relatively large lumps (a compact
mass), called flocs, that settle out more rapidly.

5. Coagulation and flocculation
• Coagulation causes extremely fine colloidal particles to adhere directly to each other.
• All particles exert mutual attraction forces, known as London-Van der Waals' forces, which are effective only at very close range.
• Normally, the adhesion (the joining of surfaces of different composition) due to these forces is prevented by the presence around each particle
of an electrically charged atmosphere, which generates repulsion forces between particles approaching each other.
• There is, therefore, in any given system a balance between the attractive forces and the electrical repulsion forces present at the solid-liquid
interface (Figure 15.1).
• In any given system the electrical charges on the particle surfaces will be of the same sign, aqueous suspensions of pH 4 and above generally
being negative.
• Positively charged surfaces occur mainly in strong acid solutions.
• The repulsion forces not only prevent coagulation of the particles, but also retard their settlement by keeping them in constant motion, this
effect being more pronounced the smaller the particle.
• Coagulants are electrolytes having an opposite charge to the particles, thus causing charge neutralization when dispersed in the system,
allowing the particles to come into contact and adhere as a result of molecular forces. Inorganic salts have long been used for this purpose, and
as counterions in aqueous systems are most frequently positively charged, salts containing highly charged cations, such as AI3+, Fe3+, and Ca2+,
are mainly used. Lime, or sulphuric acid, depending on the surface charge of the particles, can also be used to cause coagulation.
• Most pronounced coagulation occurs when the particles have zero charge in relation to the suspending medium, this occurring when the zeta
potential is zero.
• The nature of the zeta potential can be seen from Figure 15.2, which shows a model of the electrical double layer at the surface of a particle.
The surface shown has a negative charge, such that positive ions from solution will be attracted to it, forming a bound layer of positive ions,
known as the Stern layer and a diffuse layer of counter ions decaying in concentration with increasing distance until the solution equilibrium
concentration is attained.

7. Uses of flocculants
• the addition of flocculants to a slurry can lead to significant improvements in sedimentation characteristics, it
also affects dewatering behaviour, flocculation generally being detrimental (causing harm) to final consolidation
(combining into solid mass) of the sediment.
• It is usually beneficial, however, to filtration processes, and flocculants are widely used as filter aids.
• However, the specific requirements of a flocculant used to promote sedimentation are not necessarily the same
as for one used as a filter aid.
• The behaviour of the flocculated suspension and the performance of solid-liquid separations are determined by
the size of the flocs (a small loosely aggregated mass of flocculent material suspended in or precipitated from a
liquid) and by their structure.
• Large flocs promote settling and are desirable for clarification and thickening. Floc density is of secondary
importance in these processes.
• Conversely, dense flocs are most appropriate for consolidation of the sediment, and size is of lesser importance
in this stage.
• Therefore the optimisation of solid-liquid separation processes requires careful control of floc size and structure.
• The maximum effect of a flocculant is achieved at an optimum dosage rate and pH; excess polymer can cause
dispersion of the particles due to floc breakdown.
• Physical factors are also of great importance, growth and development of the flocs being affected by
particleparticle collisions and hydrodynamic interactions.

8. • Flocculation involves the formation of much more open
agglomerates than those resulting from coagulation and relies
upon molecules of reagent acting as bridges between separate
suspended particles.
• The reagents used to form the "bridges" are long-chain organic
polymers, which were formerly natural minerals, such as starch,
glue, gelatine, and guar gum, but which are now increasingly
synthetic materials, loosely termed polyelectrolytes.
• The majority of these are anionic in character but some of them
are non-ionic, and some cationic, but these form a minor
proportion of the commercially available products of today's
flocculant market.
• Inorganic salts are not able to perform this bridging function, but
they are sometimes used in conjunction with an organic reagent
as a cheaper means of charge neutralisation, although an ionic
polyelectrolyte can and often does perform both functions.
• The polyacrylamides, which vary widely in molecular weight and
charge density, are extensively used as flocculants (Mortimer,
1991).

9. • forming woolly cloudlike aggregations
• Flocculation, in the field of chemistry, is a process
wherein colloids come out of suspension in the
form of floc or flake; either spontaneously or due to
the addition of a clarifying agent.

10. Gravity sedimentation or thickening
• Gravity sedimentation or thickening is the most widely applied
dewatering technique in mineral processing, and it is a relatively
cheap, high capacity process, which involves very low shear (become
deformed by forces tending to produce a shearing strain ) forces, thus
providing good conditions for flocculation of fine particles.
• The thickener is used to increase the concentration of the suspension
by sedimentation, accompanied by the formation of a clear liquid.
• In most cases the concentration of the suspension is high and
hindered settling takes place.
• Thickeners may be batch or continuous units, and consist of relatively
shallow tanks from which the clear liquid is taken off at the top, and
the thickened suspension at the bottom.
• The clarifier is similar in design, but is less robust, handling
suspensions of much lower solid content than the thickener.

11. • Construction and working principle of thickener
• The continuous thickener consists of a cylindrical tank, the diameter ranging from
about 2 to 200m in diameter, and of depth 1-7 m.
• Pulp is fed into the centre via a feed-well placed up to 1 m below the surface, in
order to cause as little disturbance as possible (Figure 15.5).
• The clarified liquid overflows a peripheral launder, while the solids which settle over
the entire bottom of the tank are withdrawn as a thickened pulp from an outlet at
the centre.
• Within the tank are one or more rotating radial arms, from each of which are
suspended a series of blades, shaped so as to rake the settled solids towards the
central outlet.
• On most modern thickeners these arms rise automatically if the torque exceeds a
certain value, thus preventing damage due to overloading.
• The blades also assist the compaction of the settled particles and produce a thicker
underflow than can be achieved by simple settling. The solids in the thickener move
contin-uously downwards, and then inwards towards the thickened underflow
outlet, while the liquid moves upwards and radially outwards. In general, there is no
region of constant composition in the thickener.

13. Filtration
• Filtration is the process of separating solids from liquid by means of a
porous medium which retains the solid but allows the liquid to pass.
Factors affecting the rate of filtration include:
• The pressure drop from the feed to the far side of the filter medium.
This is achieved in pressure filters by applying a positive pressure at
the feed end and in vacuum filters by applying a vacuum to the far
side of the medium, the feed side being at atmospheric pressure.
• The area of the filtering surface.
• The viscosity of the filtrate.
• The resistance of the filter cake.
• The resistance of the filter medium and initial layers of cake.

14. The filter medium:
• The choice of the filter medium is often the most
important consideration in assuring efficient operation
of a filter.
• Its function is generally to act as a support for the filter
cake, while the initial layers of cake provide the true
filter.
• The filter medium should be selected primarily for its
ability to retain solids without blinding. It should be
mechanically strong, corrosion resistant, and offer as
little resistance to flow of filtrate as possible.
• Relatively coarse materials are normally used and clear
filtrate is not obtained until the initial layers of cake are
formed, the initial cloudy filtrate being recycled.

15. • Filter media are manufactured from cotton, wool,
linen, jute, nylon, silk, glass fibre, porous carbon,
metals, rayon and other synthetics, and
miscellaneous materials such as porous rubber.
• Cotton fabrics are by far the most common type of
medium, primarily because of their low initial cost
and availability in a wide variety of weaves. They
can be used to filter solids as fine as 10 μm.

16. Types of filter
• Pressure filter
i. Filter presses
ii. The plate and frame press
iii. The chamber press
• Vacuum filter
i. Batch vacuum filters
ii. Horizontal leaf, or tray filters
iii. Continuous vacuum filters
iv. The rotary-drum filter

17. DRYING
Figure- Direct fired, parallel flow rotary dryer

18. • The drying of concentrates prior to shipping is the last operation performed in the mineral-
processing plant. It reduces the cost of transport and is usually aimed at reducing the moisture
content to about 5% by weight. Dust losses are often a problem if the moisture content is lower.
• Rotary thermal dryers are often used. These consist of a relatively long cylindrical shell mounted
on rollers and driven at a speed of up to 25 rev min-1. The shell is at a slight slope, so that material
moves from the feed to discharge end under gravity. Hot gases, or air, are fed in either at the feed
end to give parallel flow or at the discharge to give counter-current flow.
• The method of heating may be either direct, in which case the hot gases pass through the material in
the dryer, or indirect, where the material is in an inner shell, heated externally by hot gases. The
direct-fired is the dryer most commonly used in the minerals industry, the indirect-fired type being
used when the material must not contact the hot combustion gases. Parallel flow dryers (Figure) are
used in the majority of current operations because they are more fuel efficient and have greater
capacity than counter flow types. Since heat is applied at the feed end, build-up of wet feed is
avoided, and in general these units are designed to dry material to not less than 1% moisture. Since
counter-flow dryers apply heat at the discharge end, a completely dry product can be achieved, but
its use with heat-sensitive materials is limited because the dried material comes into direct contact
with the heating medium at its highest temperature.
DRYING