1.
International Journal For Research & Development in Technology
Volume: 2, Issue: 3, Sept-2014 ISSN (Online):- 2349-3585
5 Copyright 2014- IJRDT www.ijrdt.org
Thickener Waste Management in Mineral
Processing To Prevent Environmental Pollution
Marzieh Hosseini Nasab1
1
Department of Mining Engineering
1
University of Sistan and Baluchestan
1
Zahedan, Iran
Abstract— Water plays a vital role in mineral processing
and about 2-3 tons of water is used for the treatment of one
ton of ore. The objective of water recovery in thickener is to
increase the solids concentration at the underflow to obtain
clear water at the overflow. The particle size distribution,
that follows the Rosin- Rammler equation, is considered as
the most important factors affecting thickener choosing and
waste water treatment. If solids below 200mesh include 8%
or more of weight of feed, flocculant should be used to
increase the sedimentation rate and the water clarity.
Increasing the concentration of solids in the feed (up to 25
wt %) reduces the size and cost of the equipments required
for separating. If a high concentration of solids in the feed
is used with flocculant, thickener overflow can dilute feed
and can increase sedimentation rate and clarity. An extra
depth should be added to thickener depth due to the space
lost by the turbulence of the fluid resistance force.
Index Terms— thickener, pulp, settling velocity, Rosin-
Rammler equation, flocculant.
I.INTRODUCTION
Approximately 80-90% of the tonnages of minerals are
processed using water. It has more efficient, higher recovery
and lower costs per unit of a valuable product and no air
pollution (Antonio, et al., 2009; Jaroslaw, 2012). In most
cases, the concentration of suspended solids is high and
hindered settling occurs. Thickeners are continuous or semi-
continuous units with relatively shallow reservoirs in which
the transparent fluid is taken up and the concentrated product
is collected at the bottom. The difference between thickener
and other types of sedimentary basins such as classifiers is
that suspended solids in the feed pulp in some kinds of
floculant stick together. The first forces in the separation
process are gravity, buoyancy and friction. Some of the
factors that influence these forces are the liquid density, solid
density, particle size and shape, floculation of particles and
concentration of solids. Settling of solids and separation
velocity may be related to these factors (Muralidhara, 1986;
Roberts, 1949; Svarovsky, 2001).
Continuous thickener dimensions can be determined from
previous experiments, pilot-scale continuous tests or batch
settling tests. But often due to time constraints, stirring
restriction and lack of access to sampling materials, batch
experiments are the only method based on reality for
determining the size of the thickener.
Before 1916, concentrated operations were considered an art,
but in 1916, with the publication of Coe and Clevenger
article, concentrated operations were changed to an
engineering knowledge. The factors preventing possible
changes in feed properties, using pressure depth or increasing
thickener area to have lower pressure depth, appropriate
choice of volume for the thickener, all these are factors that
convert the concentration from an art into a real science
(Couche and Goldney, 1959; Gaudin and Fuerstenau, 1962;
Moncrieff, 1964).
The concentrated radical theory began by work of Coe and
Clevenger (1916). To determine the area of the thickener
settling, they suggested that a series of batch settling tests
should be performed at different initial concentrations. Then,
carrying capacity of the solids at each concentration and the
lowest capacity (maximum level) to determine the
dimensions of the thickener are obtained (Coe and Clevenger,
1917; Fitch, 1962).Kynch (1952) developed a mathematical
analysis by which the relationship between solids
concentration and sedimentation rate is achieved by having
just one settling curve. Fitch (1962, 1966, 1971), Fitch and
Talmage (1955) proposed some methods for determining the
dimensions of a thickener. The method of drawing the
tangent, used as a standard method for the determination of
thickener area by many companies, was compared to the
actual results of continuous concentration. The results
showed that even when the scale factor equals to 1.33, the
predicted area for low concentrations of underflow is very
large and for high concentrations is very small. Thus,
selecting an appropriate method to determine the dimensions
of the thickener is of great importance (Talmage and Fitch,
1955; Fitch, 1971; Fitch, 1966; Wilhelm and Naide, 1981).
1.1- Why dewatering is required?
Dewatering is meant to separate water from solids. Humidity
inparticles decreases benefits of separating valuable solids
from gangue minerals (especially coal). Valuable products
with excess water lead to additional costs in transportation,
heating water for evaporation and blockage of transport
carriers. For example, costs for a 1% increase in the moisture
content of coal is equivalent to the benefit of a 4.5%
reduction in coal ash.
Mechanica dewatering is divided into two major phases:
1. Sedimentation: fluid arrestes to particles
move freely.
2. Filtration: particles with a means been kept
to fluid flow from between them (Ebrahimi, 1999;
Banisi, 2007).
2- Materials and Methods
2.1 - Introduction
The gravity thickener separates suspended solids from a
mixture using a gravity settling. The purpose of thickening is

2.
International Journal For Research & Development in Technology
Paper Title:- Thickener Waste management in Mineral Processing
To prevent Enviornmental Solution (Vol.2,Issue. 3) ISSN(O):- 2349-3585
6 Copyright 2014- IJRDT www.ijrdt.org
to increase the concentration of solid particles in underflow.
Most operations are performed in cylindrical sedimentation
tanks with conical bottom. Thickener feed is continuously
enrolled, and pulp is concentrated in the floor of the thickener
and then clear liquid comes out of the overflow. Thick pulp is
driven by shoveling mechanism (inward or outward) to a
center output or radial multiple outputs. Shoveling
mechanism outwards usually is used when the vessel
diameter is large and thereby material moving toward the
outlet is difficult if the outlet is located at the center. In a
deep thickener there is no need to use the paddles because the
cone inside angle is steep enough so that the concentrated
pulp moves only with gravity to the location of the center
output (Muralidhara, 1986; Chandler, 1986; Dahlstrom,
1986; Johnston and Simic, 1996).
The first step for improving thickener performance is to find
the optimal concentration of solids in the feed based on
standard settling tests. Next, the amount of water needed to
dilute the solids concentration in the feed factory is calculated
and delivered to the desired concentration. The amount of
this water is easily obtains by using feed flow rate and the
appropriate percentage of solids it. The amount of water
required for feed dilution is provided by thickener overflow.
For continuity of operation, it is necessary to have a
transparent water tank to store the required water for diluting.
In Figure 1, the thickener and feed dilution system are shown.
There is a lateral in the figure, to regulate the added water
flow rate to the feed line. Diluted water flow rate can be
manually set by any change in the concentration of solids in
the feed using a valve located on the lateral (Banisi and
Yahyaei, 2007).
Feed (25 t/h)
10% Solids
26.5% Solids
0.1% Solids
Clear water
reservoir for
feed dilution
4% Solids
Folcculant
30 g/t
ab
c
Folcculant addition points
a: 40%
b: 40%
c: 20%
Bypass
Figure 1: A designed thickener for the recovery of coal tailing in a
coal processing plant (Banisi and Yahyaei, 2007)
2.2 How to do a batch settling test?
A series of samples with the same volume of the feed pulp
in different solids concentration is prepared. After preparing
the desired concentration of solids in the feed, floculant is
added with additive rates to see starting of floculation. Levels
of settling are recorded at different times. Once the
sedimentation rate remains stable in several experiments, the
experiment current of the sample is terminated and the
subsequent settling experiment for a new concentration of
solids in the feed is started. In general, as the amount of
concentration of solids in the feed increases, a greater amount
of floculant is need to see the level of sedimentation. During
the tests, the flocc structure, trapping small particles and
transparent in liquid phase is observed and recorded (Laros,
et al., 2002; Scott and Alderton, 1966; Scott, 1968;
Discusions, 1969; Scott, 1969).
3- Results and Discussion
3.1- The main factors affecting selection of solid –
liquid separation equipments
In metallurgy science, a pulp contains water, fine particles
and colloidal material (Bonnier, 1989). This water is either
ordinary water or water that contains dissolved chemicals.
Chemical substances in the water can affect the behavior of
settling pulp, known as electrolytes. Electrolyte has a
property which can make colloidal part of the slurry
congregation known as the floc. Floccs typically include
water, colloidal materials and fine-grained materials that are
trapped. Floccs in the aquatic environments are deposited
according to certain rules (Yalcin, 1988; Mandersloot and
Scott, 1986; Frank and Charles, 1986; Willey & Sons, 1982).
According to the results of tests conducted, the key factors
affecting the selection of solid – liquid separation equipments
are summarized below.
3.1.1- Effects of particles size distribution
For organic solids such substances found in domestic and
industrial wastes, accelerators and measuring particle size in
a logical way are impossible (Pearse, 1977). In these cases,
measuring the total suspended solids and solids separated
within 30 min after starting slow sedimentation are useful.
The particle size analysis should determine the largest range
of particle sizes (typically 98% smaller than the size), the
percentage of particles finer than 200 mesh and possibly the
percentage of those finer than 10 microns (the percentage of
colloidal particles in the largest dimension). It is always wise
to draw the distribution curve and to determine whether the
particle size distribution obeys the Rosin- Rammler formula
or not. If there is a large deviation from the formula,
significant changes should be made in the selection of
equipments.
In general, sedimentation and filtration velocities of the small
particles are low. Increasing the lateral surface of the
particles decreases the concentration of underflow and
increases the moisture of filter cake.
For example, for solids with a density of 2.7, the maximum
size of the thickener feed has about 60mesh (to protect
thickener).
The percentage of solids under 10 microns determines
whether flocculant is necessary or not. However, the
concentration of fine solids in the underflow of the thickener
and the moisture content of filter cake are not affected by this
percentage. In fact, when the solids below 200mesh include
8% or more of weight of feed, all the unit Areas required are
related to the solids below 200mesh.
It should be noted that some of the analyses of particle size
may determine particles sizes only to a certain extent (2
microns or larger). Thus, although these analyses may report
zero percents for the amounts less than the minimum
dimensions, there may be existed percentages of solids that
are significant.

3.
International Journal For Research & Development in Technology
Paper Title:- Thickener Waste management in Mineral Processing
To prevent Enviornmental Solution (Vol.2,Issue. 3) ISSN(O):- 2349-3585
7 Copyright 2014- IJRDT www.ijrdt.org
If we only use the screen analysis and the percentage of
particles below 200mesh is larger than the 8 to 10, the screen
analysis using wet screening methods should be carried out.
3.1.2-The role of particle characterization
The nature of solids, that crystal material is amorphous or
organic, affects the solid- liquid separation process. Although
crystalline or granular solids are relatively fine, they are easily
separated by all the equipments.
The shape and surface of the particles can be also important in
separation. Flat or disk-shaped lenses settle slower than others
and filtration may be considered as an obstacle to fluid flow
through the filtered cake. Usually, the optimal shape of the
particles is spherical with non-porous.
If solids are porous, due to higher moisture content in
dewatered solids, the specific surface area can be
considerably increased. So, regarding particle characteristics,
even if they are colloidal particles (bentonite clays are flat
lenses and they have low filtration rates, although they are
easily flocculated) it is almost always better to consider
particle shape and porosity. Moreover, chemical analysis of
solids should be performed if possible, especially when the
solids are gross.
As density of particles increases, sedimentation rates
increase.
3.1.3- Fluid properties
pH, the chemical composition of the fluid, and in some
cases the fluid viscosity are important for separating solid-
liquid. The sedimentation rate varies with changes in pH and
reaches to its maximum at a certain pH.
The reason seems to be related to the effect of coagulation in
game pulps. By increasing the viscosity of the fluid,
sedimentation and filtration rates decrease. In this condition,
temperature should be always maintained higher than a certain
value in the sedimentation and filtration because a higher
temperature reduces the viscosity of water. But this is
economically not viable. The fluid density also plays a similar
role in separating.
3.1.4- Effects of the concentration of solids on
the feed
In many cases, as the solids concentration increases, the
separation velocity of suspended solids in the feed increases.
In other words, increasing the concentration typically reduces
the sizes and costs of the required equipments in the solid-
liquid separation process. Thickeners are generally designed
according to the unit of area needed. Therefore, one should
always avoid from the unnecessary dilution of the feed.
Sometimes, to increase the solid separation rate, one should
dilute the feed. This is the case in flocculation process of a
thickener. When a high concentration of solids in the feed
(25wt %) is used with flocculant, we may have a different
structure of the feed that makes settling speed significantly
slow. By diluting the feed with overflow we can solve this
problem and increase settling velocity of solids. If particles
below 200mesh in the food form more than 7% of the total
weight of the pulp, the feed should be diluted.
3.1.5- Role of pulp concentration in separation
As the pulp concentration increases, the sedimentation rate
decreases. The first reason is that with substitution of solid
particles for the liquid, the liquid moves higher than of solid
particles. The relative motion between the fluid and solids
makes resistance against settling of solids. Secondly, if the
concentration increases, the pulp density increases and this
can increase buoyancy force and as a result, the driving force
for the sedimentation of solid particles decreases.
3.1.6- Effects of differences between the solid –
liquid densities
Differences between the solid – liquid densities are
important factors in classification, centrifugation and
sedimentation. However, to determine the concentration of
solids in the feed and content of dewatering product moisture,
apart from the weight percentage, the volume percentage
should also be considered.
3.1.7- Effects of the amount of compressive
force on the compression zone (the depth of the
compression zone)
As the compressive force of the upper layers to the lower
layers increases (or as the height of the compression zone is
greater), the sedimentation rate is greater. However, increases
in the pressure force make the concentration of underflow in
the continuous thickeners increase.
3.1.8- Effects of the amount and types of
flocculants
By increasing the flocculant content up to a certain value,
because of higher connection of particles together, the
settling velocity increases. After this value, increases in the
flocculant amount no longer change the sedimentation rate.
This is related to breaking chains due to increasing its length.
On the other hand, increases in the amount of flocculant, can
confine considerable amount of the liquid within the flocs, it
can increase floc porosity. This reduces the pulp viscosity. It
is worth noting that the amount of the pulp mixed after
adding flocculant affects the sedimentation rate. As the
degree of mixing or stirring the pulp is higher, the deposition
rate of separation decreases due to breaking the chains.
Flocculant types depending on the chain length and other
characteristics affect the sedimentation rate.
3.1.9- Effects of sedimentation vessel diameter
If the sedimentation container diameter is more than 2
inches, the container diameter can not affect the
sedimentation rate. Otherwise, the sedimentation rate in a pot
with a diameter less than 2 inches is higher until it reachs the
critical point, and before the critical point, the sedimentation
rate is lower, compared to the sedimentation rate in container
with a diameter more than 2 inches.
3.1.10- Role of operational settings
If the real situation of the work is not sufficiently known, it
may not be possible to achieve the desired goals. However,
before finding a final solution to the problem of solid- liquid
separation, one should clear the logical and optimal
conditions. Sometimes it is an exaggeration to say that there
is no suspended solid material in a liquid flow. Some of the
solids in the refined liquid may be allowed and is not
harmful. In addition, the complete removal of solids increases

4.
International Journal For Research & Development in Technology
Paper Title:- Thickener Waste management in Mineral Processing
To prevent Enviornmental Solution (Vol.2,Issue. 3) ISSN(O):- 2349-3585
8 Copyright 2014- IJRDT www.ijrdt.org
the required filtration area. Due to high costs of thermal
drying, dewatering is often more favorable than thermal
drying. Making a good tank and installing a "door" in the
thickener wall, can reduce the retention time, because the
loader, bulldozer or other machines can easily be imported
the thickener. Moreover, the maximum residence time
required and easier access to the equipments should be also
considered.
As a result, the solid - liquid separation process is costly in
many cases, and it is need to consider different points of view
so that the operational and capital costs are minimized.
Conclusion
The results of this study show that in the thickener choice
and wastewater refinery, particles size distribution should
follow the Rosin- Rammler formula.If solids below 200 Mesh
include 8% or more of the feed weight, we should use
flocculant to increase the sedimentation rate and the water
clarity.Increases in the concentration of solids in the feed (up
to 25 Wt %) reduce the size and cost of the equipments
required for the separation. However, when a high
concentration of solids in the feed is used with flocculant, we
can use thickener overflow to dilute feed and increase
sedimentation rate and water clarity. As the degree of mixing
or stirring the pulp is higher, the deposition rate decreases
due to breaking the chains.Regarding depth of the container,
the following results are obtained:There should be enough
depth for feed to avoid surface turbulence.There should be
enough depth for the clear fluid avoiding the feed amount
chsnges and the pulp properties of continues thickeners
variation.There should be enough depth to have maximum
capacity to avoid high cost and frequent over-discharge of
continues thickeners.
The required depth of thickener is obtained by calculating the
capacity of concentrated zone, so that the stored solids is
equal to the total capacity of the reservoir during the
necessary time for pulp concentration that reaches the proper
density in the underflow. We should add extra depth to the
thickner due to the space lost by the turbulence of the fluid
resistance force in the thickener. Furthermore, a depth
between ft
2
1
1 and ft
2
1
2 should be considered for the feed
and another depth for material storage capacity when the
underflow is close.
Acknowledgements
The author would like to thank University of Sistan and
Baluchestan for supporting this work. She is grateful to the
reviewers for their useful comments.
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To prevent Enviornmental Solution (Vol.2,Issue. 3) ISSN(O):- 2349-3585
9 Copyright 2014- IJRDT www.ijrdt.org
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