The document discusses methods for improving tailings treatment processes to generate additional profits through more complete material separation and water recycling. It presents a plant concept for treating coal tailings ponds that uses classification, flotation, filtration and drying to recover 80% of solids as a valuable product. Hyperbaric filtration is proposed as an alternative to vacuum filtration and drying to further reduce waste volumes and improve economics.

1. Ecological and economical success through advanced tailing separation
process
R. Raberger Andritz AG, Austria (rainer.raberger@andritz.com)
D. Ziaja MBE, Germany (dieter.ziaja@mbe-cmt.com)
P. Godwin Andritz Pty, Australia (peter.godwin@andritz.com)
Abstract
Worldwide high grade ores at easily accessible sites become more difficult to find and mine since there are
already exploited or located at remote areas.
Low grade ore sites receive increasing attention, which challenge beneficiation technologies to become
economical. Existing mining waste and more often newly generated mining waste - in particular tailings -
become an interesting source to produce additional concentrates, without actual mining costs at over-
average profit margins.
Furthermore environmental concerns demand actions towards the huge amounts of tailings which are
already stored in dams. They need be treated to clean-up existing environmental damage.
Moreover the liquid is used as process water, the waste volume is minimized and the hazardous impact is
reduced. The separation of the tailings into product and waste as well as in a liquid and solid fraction
represents an essential process to generate additional profits.
Examples are the treatment respectively filtration of red mud and copper tailings. The plant concept of OAO
Severstal Slurry Pond Treatment, Russia is presented, where coal tailings are recycled. Approximately 80%
of the solid content of the tailings is converted to a valuable product meeting all required specifications of
ash content and residual moisture. The process applied in this plant consists of classification, flotation and
liquid solid separation. As an alternative, hyperbaric filtration can be used to replace a dryer together with a
vacuum disc filter.
1 Introduction
Liquid solid separation represents essential unit operation in beneficiation plants for liquid solid separation
filters, belt presses, thickeners, centrifuges and dryers are used. The selection of these equipments and the
possible range of operation is an important object for process engineers and plant designers.
Big beneficiation plants have to handle up to 3000 to 5000 t/h tailings based on dry solids (DS). Such a large
amount of material poses serious challenges for dewatering equipment requiring numerous and high capacity
devices. Up to now filtration for such big amount of tailings has not been realized.
Copper tailings e.g. consist of particularly fine particles that are difficult to filter. Fine particle suspension is
filtered typically using filter presses or hyperbaric filters (HBF). Generally, a belt press (CPF) would not be
used for this application since either a large amount of flocculent agent has to be introduced or a felt is used
as a filter belt medium. For such fine tailings a large amount of flocculent agent makes a belt press
economically unattractive. On the other hand a felt becomes clogged fast by the small particles of the slurry,
resulting in poor dewatering results.
Another example is red mud. The situation in red mud filtration is similar to that of copper tailings. Red mud
consists of fine particles and has a very high pH-value due to the caustic soda content. During filtration the
temperature of red mud is generally in the range of 80°C to 90°C due to upstream process conditions. This
elevated temperature has a positive influence on the filtration because the viscosity of caustic soda (which is
a component of red mud) decreases with increasing temperature. Of course, if red mud is handled out of a
pond, the temperature would be ambient.

2. Filtration of red mud allows to recover caustic soda and to reuse it for the Bayer process. This has already
been realized by applying vacuum drum filters which give however unsatisfying residual moisture content.
The amount of red mud tailings to be handled is at least one magnitude lower compared with copper or coal
tailings. Figure 1 gives an impression of the achievable difference when hyperbaric filters (HBF) are applied
instead of vacuum filters. [Raberger, R. and Krammer, G. (2009)]
The example of recovering coal tailings is discussed in detail in chapter 2.
Hyperbaric Filters Vacuum Filters
•
Hyperbaric Filters Vacuum Filters
• Lower Moisture (24weight-%)
• Fragments
• Higher Throughputs
• Higher Moisture (>30weight-%)
• Paste; hydraulicable moveable
• Lower Throughputs
Hyperbaric Filters Vacuum Filters
•
Hyperbaric Filters Vacuum Filters
• Lower Moisture (24weight-%)
• Fragments
• Higher Throughputs
• Higher Moisture (>30weight-%)
• Paste; hydraulicable moveable
• Lower Throughputs
Figure 1 Comparison of HBF and Vacuum Filter for Red Mud Filtration
2 Coal Tailings Pond Treatment
The company McNally Bharat Engineering Co. Limited (MBE) in Cologne/Germany (formerly KHD
Humboldt Wedag) prepared the flow sheet for the plant of OAO Severstal which is shown in Figure 2. The
objective of this plant is to recover the coal of an existing coal tailings pond which was accumulated over
years. A consortium of several different equipment supplying companies and MBE as consortium leader was
formed to supply a turn key plant for the treatment of a coal tailings pond. Liquid solid separation
technology, i.e. centrifuges and filtration equipment was supplied by Andritz.
The treatment of coal tailings to get valuable coal out of it starts at the tailings pond, where a screen (1)
prevents that too big pieces or particles enter the process. Due to inefficient beneficiation processes in the
past these ponds often contain a reasonable fraction of value material to allow economical beneficial
recovering. After the screen the material is collected in a sump (2) and cyclones (3) split the stream in a fine
and coarse fraction. The underflow (fine fraction) is treated with Humphrey Spirals (4).
After some screens the overflow of the Humphrey Spiral coarse fraction is again disposed as tailings disposal
while the underflow is combined with the fine fraction screens underflow. The overflow of the Humphrey
Spirals fine fraction screen is treated with centrifuges (5) to receive fine concentrate material.
The underflow of the Humphrey Spiral screens (fine and coarse fraction) and the centrate from the
centrifuges is floated (6) to separate the tailings from the flotation concentrate where the overflow forms the
flotation concentrate and the underflow remains as tailings. The tailings fraction is thickened (9) and
dewatered by a heavy duty belt press (10) specially designed for this application. The remaining tailings are
disposed as flotation tailings. The overflow of the thickener (9) is recycled as process water. The flotation
concentrate is filtered by vacuum disc filters (7) and dried by fluid bed dryers (8) to get out the final value
product of fine coal.

3. Module Filtration and Drying
Waste
Product
Tailings Pond
Product
Remains/Waste
1 1
1
2
22
2
2
2
2
3
4
5
6 6
7
8
9
10
Process
Water
Tank
Pre-classification
Flotation
Waste Treatment
Module Filtration and Drying
Waste
Product
Tailings Pond
Product
Remains/Waste
1 1
1
2
22
2
2
2
2
3
4
5
6 6
7
8
9
10
Process
Water
Tank
Pre-classification
Flotation
Waste Treatment
Figure 2 Simplified Flow Sheet of OAO Severstal Slurry Pond Treatment concept
2.1 Equipment
A short description with a link to the flow sheet of Figure 1 for the single pieces of equipment is given within
the following sections.
2.1.1 Pre-Classification
The pre-classification consists mainly of a couple of screens (Equipment No.1) to separate coarser particles
from the fine particles or to protect the downstream equipment from big stones or rocks. In sumps
(Equipments No.2) the material resp. suspension is collected to be pumped afterwards to next step in the
beneficiation process.
Hydro cyclones (Equipment No. 3) are used to separate and concentrate the suspension in the beneficiation
plant. High centrifugal forces separate particles with lower and higher density and small versus large
particles. In Humphrey Spirals (Equipment No. 4) the first step of separating waste from product material is
done. A Humphrey Spiral is a simple concentrator consisting of a stationary spiral trough where the material
is allowed to follow gravity. Heavier particles stay on the inside and lighter ones move to the outside (Figure
3).

4. Inner Radius
Particles of Low Density
Particles of High Density
Outer Radius
Inner Radius
Particles of Low Density
Particles of High Density
Outer Radius
Figure 3 Humphrey Spirals
Efficient dewatering of coal is one of the highest production priorities encountered by coal preparation
engineers. The objective is to meet product specifications or environmental constraints or to facilitate the
handling of coal and tailings. The reduction of moisture has become a major concern, particularly as the
amount of fine coal increases.
Screen scroll centrifuges (Equipment No.5) as shown in Figure 4 are currently the most popular machine for
dewatering fine particles in the U.S. coal industry. Although these units typically provide acceptable product
moistures, they often suffer from low recoveries of fine solids. Past attempts to improve solids recovery by
adding flocculants to the feed stream have shown little, if any, positive effect. [Schnabel, G. and Raaf, T.
(2006)]
Figure 4 Centrifuge AH 1000 (Andritz)
2.1.2 Flotation (Pneuflot Equipment No.6)
The aerator (self-aerated or “supercharged” from a blower or compressor) is installed in the vertical central
pipe. The flotation pulp is first directed to a single aerating unit arranged in the central pipe above the
flotation cell. Following aeration, the pulp flows through the central pipe to the distributor located at the
bottom of the cell where it is vertically deflected upwards.
In view of the fact that the pulp flow is first routed vertically downwards and then vertically upwards, this
model is called the “vertical” type. The air bubbles covered with hydrophobic particles ascend to the upper
cell area and form a froth layer on the surface which flows off into a froth launder surrounding the cell like a
ring. Particles not clinging to air bubbles are discharged with the pulp from the bottommost point of the cell.
The pulp level is kept constant either by a level probe which actuates a valve controlling the discharge or by
a device known as “goose neck discharge”. The flotation cell must have a minimum height above the
deflecting distributor to avoid the upward flowing pulp moving uncontrolled towards the froth layer.
[Markworth, L.; et al. (2010)]

5. Areator
Hydraulicable adjustable cone
Froth launder
Goose neck
Drain
Feed
Areator
Hydraulicable adjustable cone
Froth launder
Goose neck
Drain
Feed
Figure 5 Machine parts of Pneuflot flotation cell
2.1.3 Module Filtration and Drying
The vacuum disc filters (Equipment No. 7) are used for many different applications and the technology is
well established over the years. They are generally used in heavy duty applications such as the dewatering of
iron ore, coal, aluminium hydrate, copper concentrate, pyrite flotation concentrates and other beneficiation
processes.
The filter shown in Figure 6 consists of 10 discs, each made up from sectors which are clamped together to
form the disc. The sectors consist of stainless steel to reduce corrosion and wear. One of the main features is
that the required floor space taken up by disc filters is small and the cost per m2
of filtration area is the lowest
when compared to other filters i.e. drum filter, pan filter or filter presses. Figure 4 depicts a vacuum disc of
the type which was applied for the plant concept of OAO Severstal.
Figure 6 Vacuum Disc Filter VSF 120 (Andritz)
Fluid bed dryers are used for thermal drying after filtration. The fluid bed is generated by blowing
fluidization gas uniformly over the dryer cross-section. In the dryer the free-flowing granulate begins to
float, and it is at the same time mixed thoroughly. A fluid bed is characterized by movement of the granules,
achieved by a gas stream passing through the product layer.
2.1.4 Waste Treatment
Thickeners (Equipment No.9) are used for separating particles by gravity from the liquid. Solids that sink to
the bottom of the thickener are obtained and fluid is rejected from the surface.

6. In Figure 7 a heavy duty belt press (Equipment No. 10) is displayed. The material from the thickener is
dewatered between two endless belts. The filter consists of the following zones:
• Sludge feed
• Gravity dewatering zone (thickening of the suspension by gravity only)
• Wedge zone (gradual application of pressure)
• Press zone (containing S-rolls and press rolls)
In contrast to widely used sludge belt presses, a heavy-duty belt press is designed much more robust and
allows applying substantially higher dewatering pressures (see Table 1).
Gravity Dewatering
Zone
Press zone
Sludge Feed
Gravity Dewatering
Zone
Wedge zone
Press zone
Filtrate
Dewatered Pruduct
Filtrate
Gravity Dewatering
Zone
Press zone
Sludge Feed
Gravity Dewatering
Zone
Wedge zone
Press zone
Filtrate
Dewatered Pruduct
Filtrate
Figure 7 Heavy duty belt press CPF 2000 (Andritz)
Table 1 Comparison between sludge belt presses and heavy-duty belt presses
Typical conventional
sludge belt press
Heavy-duty belt press
CPF 2200 S8
Drive Motor (kW) 5.5 15
Empty Mass (t) 5 to 10 22
Max. Belt Tension (N/mm) 8 10
Linear Force (N/mm) 20 to 30 40+
+
with special design up to 200 N/mm linear force possible
2.2 Alternative and Possible Modification for Filtration and Drying
The module Filtration and Drying marked in Figure 1 consists of vacuum filtration and thermal drying. This
module can be particularly replaced by hyperbaric filtration (HBF) which depends upon the necessary
residual moisture (Figure 8). This has to be evaluated by test work on a case by case basis.
Module Filtration and Drying
7
8
=
Module Filtration and Drying
7
8
=
Figure 8 Hyperbaric Filtration instead of Vacuum Filtration and Thermal Drying

7. The HBF (Figure 9) consists of a set of filter discs mounted inside a pressure vessel. The main difference
between conventional vacuum disc filtration and the HBF is the positive pressure applied to the filtration
surface. In theory, vacuum filtration pressure is limited to 1 bar. This is never achieved, however, and
practically a differential pressure of up to 0.7 bars is utilized. Hyperbaric filtration is unlimited in theory, but
mining applications have a practical operating range of 2 to 6 bar, which results in three to eleven times the
differential pressure available compared to vacuum filtration. [Haehling, G. (2005)]
1 - Pressure Vessel
2 - Manhole
3 - Filterdisc
4 - Control head
5 - Filter drive
6 - Filter trough
7 - Agitators
8 - Discharger
1 - Pressure Vessel
2 - Manhole
3 - Filterdisc
4 - Control head
5 - Filter drive
6 - Filter trough
7 - Agitators
8 - Discharger
Figure 9 Hyperbaric Filter (HBF)
3 Conclusions
In the case OAO Severstal coal tailings treatment a viable process concept is presented that is currently in
installation. The realization is based upon advanced liquid solid separation equipment that is readily
available on the market. Further optimization is possible through replacement of the energy intensive thermal
dryer by hyperbaric filtration. Again hyperbaric filtration technology is already widely utilized especially for
large throughputs. The total tailings volume is drastically reduced by such a treatment and the potential of
ground water pollution is thereby minimized as the waste product is rather dry. Overall the solid waste
quantity reduced by a factor 5.
References
Raberger, R. and Krammer, G. (2009) How to Meet the Challenge of Tailings Filtration; Environmine 2009; Santiago
de Chile
Schnabel, G. and Raaf, T. (2006) A fine Art – A new dewatering system for coal fines, which challenges established
processes by using screen bowl centrifuges and hyperbaric filters combinations; World Coal; July pp 43 – 46
Markworth, L.; Ören, E. and Gerards, M. (2010) Cost Efficient Application for Fine Coal Beneficiation; Coal Prep,
Lexington 2010.
Haehling, G. (2005) Hyperbaric Filtration: Operating Experience with Andritz Hyperbaric Filters (HBF) for Dewatering
Fines in Coal Prep Plants; Coal Prep, Lexington 2005.