Kaolin Recovery Utilizing G-Cell Flotation
The Efficient Recovery of Kaolin From
a Hydrocyclone Plant
Middlings Stream
Utilizing IMHOFLOT G-CELL Pneumatic Floatation
Michael Battersby, Maelgwyn Mineral Sevices, Cardiff, UK
Rainer Imhof,
Maelgwyn Mineral Services, Dorsten, Germany
Michael Fletcher, Maelgwyn Mineral Srevices,
Cardiff, UK
Franz Puder,
Gebr.Dorfner GmbH & Co, Hirschau,
Germany
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Prepared and Collected by Atef Helal from different Sources .
The present subject was first puplished at the SAIMM
- (South African Institute of Mining and Metallurgy)-
Conference, Flotaton Cell Technology in the 21st
Century, 20 June 2007.
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Historical note (collected by Eng. Atef
Helal) : The tehnology
originates from pneumatic flotation developments in the 1970s in Germany by Prof.S imonis (Technical
University of Berlin) and Prof. Bahr (Tehnical
University of Clausthal). Since then Dr. Rainer Imhof has been known widely for his contributions to design
at Ekof, KHD. Allmineral,
and most recently Maelgwyn Mineral Services, where he
is Technical Director (Imhof, 1998). Ingenieria de Minerals S.A. has been involved with the
technology development by Dr. Imhof for more than 10
years and is broadly experienced with flotation, from Wits University and Mintek in outh Africa (Sanchez,
1990).
Generl View of Imhoflot G-Cell (collected by A.H.) : The
term pneumatic flotation is generally associated with flotation where the
aeration of the pulp is conducted outside the flotation cell. This is the main
differentiating factor between pneumatic flotation and conventional tank
flotation. The energy required by conventional cells to keep particles in
suspension and generate bubbles is now focused solely on the production of very
fine bubbles in the Imhoflot system, and the
suspension of particles is catered for in the surplus energy of the system. The
external aeration is usually achieved either by utilizing a simple venturi system in a pipe with downcomers
or by using specialized fine bubble generation technology. This fine bubble
generation technology is a core feature of the Imhoflot
system.
The design objectives for Imhoflot pneumatic flotation are to separate and optimize
the independent process steps that make up froth flotation: aeration, bubble-particle
contact and froth separation. The aerator is self-aspirating and uses a high
shear ceramic multi-jet venturi system operating at
around 2.5 bar (250 kPa) back pressure. Bubble sizes
generated start with ultra-fine bubbles at around 5 μm–10 μm. Bubbles in the 2 mm to 3 mm size range can also be
found owing to the subsequent coalescence (growing together) of bubbles that
takes place. The high shear aerator reactor is designed to maximize the
attachment of bubbles to all hydrophobic particles. Therefore the aerator can
be seen to tend to the generation of bubbles as well as assisting the
bubble-particle contact required for successful flotation. In the original
design of the Imhoflot cell, the V-Cell, the aerated
pulp was introduced upwards into the cell by means of a ring distributor system
and nozzles.
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Abstract
The traditional processing
of kaolin is achieved by dispersion of mined ore and classification by
multistage hydrocyclon plant. Inefficiencies inherent
to cyclons produce a middlings
product that is commonly disposed of back into the quarry. The Imhoflot G-Cell is an innovative pneumatic flotation
process that can be used to recover this previously wasted middlings
stream. The technology uses centrifugal forces to assist in the separation of
the froth phase from the tailings and consequentially reduce the residence time
in the separating vessel. This paper describes the testing, design and
installation of a pneumatic flotation plant for kaolin recovery at the Dorfner kaolin plant in Germany.
Introduction
Kaolin's (china clay) major
constituent is the mineral kaolinite, which is a
hydrated aluminium silicate, Al2Si2O5(OH)4.
Kaolin is formed by the decomposition of the mineral feldspar by water and heat
and can commonly be found in two types of deposits, primary and secondary.
Primary deposits, which are most commonly found in Europe, consist of the clay
occurring in-situ in the kaolinized rock. Secondary
deposits, found in the Americas, are created when water has transported the
clay from primary deposits and laid it down in new beds. This natural washing
process leaves secondary deposits comparatively pure.
Kaolin particles are predominantly below
25μm and are flat shaped particles. This mean
that industrial separation is commonly achieved by the dispersion of the mined
ore and classification using multistage hydrocyclones.
The kaolin reports to the overflow which is then reprocessed in other smaller
cyclones. Due to inefficiencies associated with hydrocycloning,
most plants are not able to produce clean quartz feldspar and kaolinite products and after the 4th or 5th
hydrocyclone stage, the underflow cannot be separated
further, resulting in the production of a middlings
stream that either sold cheaply to the cement industry or used as back fill in
the quarry. This middlings stream can be treated via
flotation, but due to environmental impact of using cationic collectors (amines)
the flotation plant has to be operated in a closed water circuit.
The Dorfner
mine is situated just outside the city of Hirschau (Oberpflaz) in south-eastern Germany, 70 km east of
Nuremburg. The Dorfner company
has been operating for over 100 years processing and refining minerals such as
quartz, feldspar and kaolin from Hirschau-Schnaittenbach
(HS) deposit. Kaolin from the HS deposit are charachterized
by high concentrations of the rare elements, Barium (Ba),
Strontium (Sr), Lead (Pb),
Copper (Cu) and Phosphorous (P). The occurrence of these trace elements with
the kaolin originates from the decomposed potassium feldspar of the kaolinised across. The usage of flotation as a
beneficiation route at Dorfner also helps to enhance
the rejection of these rare elemts, in particular the
high lead contamination.
Development of the Imhflot G-Cell
Although all froth flotation
is pneumatic i.e. with air, the term pneumatic flotation flotation
is generally accepted to apply to froth flotation where aeration of the pulp
takes place outside of the separating vessel, and thus differentiates it from
conventional tank flotation. The external aeration is usually achieved by
either utilising a simple venture system in a pipe with
downcomers or by using a specialised
fine bubble generation technology. This fine bubble generation technology is a
core feature of the Imhoflot system.
The design objectives for Imhoflot pneumatic flotation (Figure 1 & 2) are to
separate and optimize the independent process steps that make up froth
flotation: aeration, bubble-particle contact and froth separation. The aerator
is self-aspirating and uses a high shear ceramic multi-jet venturi
system operating at around 2.5 bar (250 kPa) back
pressure. Bubble sizes generated start with ultra-fine bubbles at around 5
μm–10 μm. Bubbles in the 2 mm to 3 mm size
range can also be found owing to the subsequent coalescence (growing together)
of bubbles that takes place. The high shear aerator reactor is designed to
maximize the attachment of bubbles to all hydrophobic particles. Therefore the
aerator can be seen to tend to the generation of bubbles as well as assisting
the bubble-particle contact required for successful flotation. In the original
design of the Imhoflot cell, the V-Cell, the aerated
pulp was introduced upwards into the cell by means of a ring distributor system
and nozzles.
Residence time in the cell
was generally in the order of three to four minutes. Over the last few years
MMS has developed the concept of using centrifugal forces to speed up the
separation of concentrate and enhance the removal of the froth phase. This is
achieved by introducing the aerated feed tangentially into the separating
vessel, thus creating specific rotational speeds in the cell. The cell is not
designed as a gravity separator, and the rotational speeds are not high enough
to strip coarse particles from the froth. However, the centrifugal froth
separation has now reduced the residence time in the cell to around 30 seconds,
which results in a multi-fold increase in flotation unit capacity.
The mine uses a
sophisticated hydrocyclone plant and has extensive
knowledge of pneumatic flotation due to their exclusive use of the Bahr-Cell on
kaolin flotation. The previous float plant was limited in its throughput as
well as requiring a cleaning stage to produce final grade product. The use of
amine reagents in combination with-ultra fine kaolin results in a very stable
froth that causes significant handling and pumping problems in the cleaner
stages. With the Dorfner mine looking to expand its
production the current flotation plant could not successfully handle the
additional flowrate and Maelgwyn
Mineral Services (MMS) were approached to perform tests and trials for
upgrading the flotation plant. With the planned expansion, the middlings product would make up 5-6t/hr, with an effective
flotation plant it would have the ability to produce an additional 4t/hr of
high value kaolin.
The quality of kaolin is most commonly
measured by the mass percentage loss on ignition (LOI) at 1050oC.
The theoretical maximum LOI of pure kaolin is reported as 13.9%.
Figure 2 (a) details the
forces acting on the particles in the slurry inside the G-Cell. The downward
force (G) is the force exerted by gravity and can be calculated using Newton’s
second law of motion, which states:
FG = m.a
FG =
gravitational force
m = mass
a = Gravitational
Acceleration
The acceleration of an
object due to gravity is constant and equal to 9.8m/s2. For the
purpose of this example, we can ignore the mass of the object because it will
be variable for different size particles entering the separation vessel. We can
therefore say that gravitational acceleration (FG) of 9.8m/s2
in a downward direction will be exerted on all particles entering the
separating device. By using the cylindrical shape of the vessel, and injecting
the slurry tangentially into it, it is possible to create a
centrifugal force acting on the aerated pulp in the separating vessel.
The following equation is a derivative of Newton’s second law and can be used
to determine the centrifugal force experienced by a particle in the rotating
pulp:
Fc = m . v2 ∕ r
Fc = centrifugal force
V = rotational velocity
r = radius
Therefore at a predetermined
speed and radius (and ignoring the mass of the object, as before) it can be
calculated that the centrifugal acceleration experienced by the object would be
9.8m/s2. This force will be exerted in an outward direction. The
resultant force is shown in the diagram above (Figure. 2a).
FR is the
resultant force that is experienced by a particle in the pulp. To calculate the
resultant force, we use the Pythagoras theorem. For example, when the
centrifugal acceleration is 9.8m/s2, the resultant force experienced
by the slurry will be 13.86m/s2 at an angle of 45°. The resultant force
on the particle in the slurry is greater than that of the gravitational force
alone, as experienced in other flotation systems. This increased force in the
system encourages hydrophilic particles to drop out of the system faster, and thus
allows a much shorter residence time in the separating vessel. The additional
force added to the particles aids in the reduction of the entrainment of
hydrophilic particles into the froth. This results in higher selectivity’ and
hence produces better grades in the froth. The resultant force on the pulp creates
an angled pulp/froth interface. This is beneficial, as it allows the froth to
essentially ‘flow’ over the interface towards the inner channel, thus aiding in
the removal of froth from the system. This faster froth removal ensures that valuable
particles are removed from the system before they can detach from bubbles and
drop back into the pulp, to be lost to the tails. This, in combination with the
generation of fine bubbles in the high shear aerator, results in better recoveries
of valuable minerals in the finer size fractions. This increased performance in
the flotation system enables the separating device to be smaller in size and
more cost effective than standard flotation cells.
Dorfner Flotation plant Upgrade
Background
Primary kaolin is most
commonly found in Europe whilst secondary clays are more commonly found in the
Americas. Kaolin is extensively used in Europe in highly filled uncoated
mechanical papers where it contributes to gloss and smoothness. Kaolin is also
used in coating applications where good coverage of the surface is critical.
The normal industrial
separation process of kaolinite production is
achieved by the dispersion (slurry production) of the mined ore and
classification by means of multi-stage hydrocyclone
systems. Kaolinite particles are commonly found in
the size range of only a few microns and therefore report to the hydrocyclone overflow. This overflow is then further
classified in the next, smaller cyclone and so on. The hydrocyclone
plant is not able clean quarz, clean feldspar and
clean kaolinite products individually. This
inefficiency leads to the production of a middlings
stream which is either used in the cement industry or disposed of back into the
quarry.
In certain kaolin treatment
plants, the beneficiation process of kaolin is undertaken by the use of froth
flotation. Amines are used as collectors, in a low pH environment, for
effective flotation to take place. Dorfner has
exclusively been using pneumatic flotation for many years, utilizing Bahr-Cell,
the earliest development of pneumatic flotation, and were consequently
comfortable with their use.
Dorfner Flotation Plant upgrade
Two of the restrictions of
the existing flotation
Plant at Dorfner were its low throughput capacity and
the need of a cleaning stage to produce a saleable grade of concentrate. The
use of amines to float fine particles results in a very stable froth. This
causes a significant problems with froth handling and
pumping of the rougher concentrate to the cleaner flotation cells.
Dorfner planned to increase the flotation capacity by
building a new flotation plant. Their intial studies
indicated that an Imhoflot pneumatic flotation plant
would offer considerable capital cost savings over a conventional tank cell
plant. In addition it appeared that pneumatic flotation plant had the potential
to produce a high grade concentrate without the need for further cleaning
cells, a considerable cost advantage. If the middlings
product could be efficiently cleaned in the flotation plant it would produce
more than 4 t/h of high value kaolinite. The simplest way to measure the mass percentage loss on ignition at
1000oC. Kaolin has a theoretical maximum loss on ignition of
13.9%. This measurement is most commonly used in this paper.
Laboratory
Trials
Because of the Dorfners extensive experience with kaolin flotation,
limited laboratory flotation trials were conducted in order to determine the
correct reagent suite and operating parameters for the new flotation plant
expansion. The feed material that was tested had a head grade of 10.6% LOI
(approx. 75% kaolinite) and was 100% - 40μm. The
overall required concentrate grade was an LOI of 13.4% (96% kaolinite).
All tests were conducted in a 3 litre Denver type
laboratory flotation cell and a rougher cleaner circuit was tested as shown in
Figure 3.
The feed material was
conditioned at a high pulp density of 500 g/l for approximately four minutes
with H2SO4 to reduce the pH to 2.5 (all clays showed
alkaline pH) before flotation. Flotation reagents were also added at this point
with Amine 3305 from Clariant was used as a collector
and frother in dosages between 400 g/l and 600 g/l.
The rougher concentrate was collected during the four minute bulk float, then transferred to another flotation cell, additional water
was added and a cleaner float (re-floated) was performed for approximately
three minutes. During conditioning the impeller speed was set at 900 rpm and
increased to the generic 1300 rpm during flotation.
Effect
of pH
The standard pH of tests
conducted was set at 2.5 and established from prior flotation knowledge. Two additional
tests were conducted at higher pH's in order to
determine the effect it made on the flotatation
performance.
Table 1 shows
that there is a distinctive trend associated with the pH of the flotation pulp.
As the pulp pH is increased, the selectivity of the float decreases resulting
in a higher yield and decrease in concentrate grade. Although higher yields can
be favourable, the final product grade of the
concentrate at higher pH values was below the selection criteria of an LOI
13.4%.
Effect of conditioning time
Conditioning
the feed at high pulp density enables the mechanical energy transferred to the
particles to be increased. In addition to conditioning the material at high
pulp density, the amount of time allowed for particle/reagent contact is
essential for successful flotation. As shown in table 2, with a conditioning
time of 1 minute, the desired grade is not achieved in the final concentrate
and yield increases due to the lower selectivity of the float.
Effect of floatation pulp density
After high
density conditioning the flotation pulp is watered down significantly to a
floatable density. Previous experience dictated that a value of approximately
100g/l would ensure that the required grade was achieved. Table 3 below shows
the effect seen when the flotation density is either too thick ot too thin.
Both extremes result in lower grades in the final concentrate due to loss of
selectivity of the float.
Effect of water quality
The majoity
of laboratory tests were completed using tap water during flotation. Ue to the
nature of amine flotation, the plant's water needs to be recycled and tests
were conducted to confirm that there was no performance loss due to recycled
water.
It can be
seen that the final grade produced is not negatively affected by the water,
however there is a slight reduction in yield.
The results
from the laboratory flotation can be summarised as follows:
·
Maximum kaolin grade achievable was 13.5% LOI from a
feed grade of approximately 10.6% LOI.
·
Kaolin recoveries of 85% to 90%.
·
Kaolin yields of 65% to 75% are achievable but
dependant on the feed quality.
·
Laboratory tests show that the use of conventional
flotaion technology would require a cleaner circuit to produce the required
grade.
·
Low pulp densities, increased conditioning times and
low pH environments would be required for the flotation performance to be
satisfactoy.
Industrial Trials
Following
the results obtained in the laboratory, an industrial size G-Cell (1.7 m in
diameter) was used to rest the full middling stream. The low solids
concentration (below 100g/l) required to produce a final grade product was
treatable in this size og G-Cell without difficulty. Although pneumatic
flotation has shown in the past that it produces a higher grade product, the
target set by Dorfner was very stringent and so a froth washing system was
installed by MMS. Due to the rotational movement of the froth in the G-Cell,a
single spray bar can be used to get coverage over the entire froth area as well
as ensure deep froth penetration without destroying the produced froth. The use
of a froth washing system also resulted in a thinner froth being produced which
would be beneficial for pumping of the froth to downstream processes.
Figure 4
shows some of the results obtained during the pilot plant trials with the
single G-17 G-Cell. The feed to the cell varied from between 70 to 80 m3/hr
and the density was kept below 100 g/l. The most promising results were
obtained at densities as low as 50 g/l. This is unexpected as the lab results
dictated that such a low density would result in a loss of selectivity. It is
blieved that the advanced aeration system on the G-Cll reults in very good
bubble particle contact even at the very low densities. A single pass through
the one G-Cell was able to obtain the following results; 35% recovery of
kaolinite at 13.3% LOI with a yield of 31.5%.
With the success of the
laboratory flotation and the pilot plant proving that the IMHOFLOT G-Cell was
able to achieve the final grade without the need for cleaning, the approval was
given to invest in an IMHOFLOT flotation plant. To ensure that an economical
recovery of above 85% was achieved, the plant would require 3 G-18 G-Cell (1.8
m in diameter) operating in series. This size of plant would be comfortably
retrofitted into the old flotation plant building and the available space.
Figure 5 gives a process flow diagram of the 3 stage flotation plant. Water is
recycled in the plant from the final tailings by the use of a cyclon pack and vacuum drum filter, as well as from the
concentrate via the thickener overflow. The process water tank is then topped
up with fresh water as is required using a float valve. The conditioning time
in the cascade is approximately 4 minutes as dictated by the laboratory trials.
The cascade is designed with 3 compartments, of which the first two are open at
the bottom and allows the pulp to be conditioned with flotation reagents at a
high density. The third compartment is freed from the second via an overflow
and process water is added to dilute the pulp before flotation.
The concentrate produced is treated with a defrothing agent to aid the pumping and settling in the
lamella style thickener where the thickened concentrate is neutralized using NaOH and pumped to a storage silo for further treartment and sale. The filter cake produced from tailings
is removed via conveyor belt and stock piled.
The plant provided by MMS to
Dorfner was a full turnkey installation. Included in
the battery limits for the delivery was the electrical system, flotation cells,
pumps, automation, control protocol and integration with the existing equipment
into the control system.
Commissioning
and Installation
The
plant was commissioned successfully in June 2005 and achieved the desired
results from the very first commissioning run. The full flotation plant is
shown in Figure 6.
During
the commissioning of the plant, a sample from each of the 3 G-Cells was taken
to confirm the performance down the line of the flotation cells. The results
obtained are presented in Table 5.
When
these samples were taken, the feed grade to the flotation plant was
significantly lower than is normally expected and resulted in low concentrate
grades being produced. However, the results showed that each of the 3 G-Cells
was able to produce a high grade product with good air flow rate control on the
third cell.
The
overall performance of the plant is shown in Figure 8. As the yield of the
plant was increased, the recovery increased as expected, but the loss of grade
at the higher end was lower than expected. The yield increased from
approximately 67.2% up to 73.6% with corresponding grade decrease from an LOI
of 13.5% to 13.1%.
Conclusions
The Imhoflot G-Cell plant was delivered, installed and
commissioned within 3 months of the order being received from Dorfner. It was successfully commissioned without any major
commissioning problems and was fully operational in July 2005. With the use of
froth washing the 3 stage plant produced an acceptable concentrate without the
need for further cleaning resulting in considerable cost savings. The residence
time of the complete G-Cell installation is less than 120 seconds. This can be
compared for this application of 8 minutes of conventional roughing time and a
further 6 minutes in a required cleaning setion.
Dr. Imhof supplied his first pneumatic flotation plant in 1987.
Since then he has designed and supplied over 110 cells treating a whole range
of materials including copper sulphide and oxide
ores, gold, coal, iron ores, slags, soil remediation
and a range of industrial minerals. The development of centrifugal froth
removal providing high performance separation with significantly reduced
residence time, now offers further reductions in installation costs of
flotation plants. The first two successful installations detailed here
demonstrate the reliability of a flotation plant based on the G-Cell
development.
_____________
Note:
For more related readings Please go to the following link :
Imhoflot Flotation Technology , for more related readings.
The
schematic drawing shown below shows a modern flotation column featuring feed line
aeration (insert) and partial recycle of tailings. The main thrust of a column
in this configuration is its equivalence to a virtual flotation circuit with
the capability of generating fine bubbles for fine particle flotation.
