Background and Description, how to perform the Experiment
‘Selective Flotation of a Lead - Zinc Ore’
Figure 1 Flotation of a Lead zinc ore. Copyright: Clausthal University of Technology
Introduction:
Flotation is one of the most complex upgrading processes of ores, determined by many parameters,
therefore we will describe only some general methods, concepts and describe in detail, exemplary,
just one upgrading process, the above mentioned flotation of a lead-Zinc ore. Flotation as a method to
sort particles is well suitable for very fine particles, the optimal range of grain size is between 20 and
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300 Micrometer (µ). Depending on the Material, a sorting of Particles down to a size of 5 µ or up to
500 µ is possible. The physical parameter which governs the separation is the surface property,
hydrophobicity versus hydrophility. In aqueous solutions hydrophobic particles or those which surface
has been turned to be hydrophobic attach to air bubbles, blown into the solution. As a three phase
froth (solid, liquid, gaseous) they swim on the surface of the solution. The froth can easily be
separated from the fluid and the solid particles are processed from it. The hydrophilic particles remain
in the solution and can, later, be drawn near the button of the cell.
Figure 2 A typical flotation cell. Copyright: The original uploader was Thermbal at Wikimedia Commons Later version(s)
were uploaded by P. B. Mann at Wikimedia Commons - Own work, Public Domain,
https://commons.wikimedia.org/w/index.php?curid=10542461
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A human, european hair has a diameter between 20 µ (elder people) up to 70 µ.
Figure 3 Pupils perform a rapidly results showing and easy flotation - deinking of paper. The first effect is visible after 20
Minutes. Copyright: Clausthal University of Technology
To cut a long story short: Flotation is a separation method due to density differences after a Hetero
coagulation.
Figure 4 Attachment of a hydrophobic 'tuned' particle to an Air Bubble. Copyright: Clausthal University of Technology
Usually an ore does not contain just one mineral but several major elements, accompanied by minor
metals and barren rock. Flotation is a since centuries well established technique in minerals
processing for sulphide ores. Each ore needs a special arrangement of chemicals, due to several, inter
mingling parameters: pH – Value and the very special combination of the suspended elements. To
illustrate the principles of a selective flotation we use a Lead-Zinc ore from the ‘Grube Hilfe Gottes’,
Bad Grund, Harz Mountains, Germany . This lead-zinc ore is especially well suited for a
demonstration experiment, because the colour of the lead-sulphite ore is silvery, while the colour of
the zinc ore is yellow. The success of the separation is visible, while the experiment is performed.
As already mentioned, the ‘fish hook’ of separation in minerals processing is the especially prepared
Hydrophilicity versus Hydrophobicity of the surface of mineral grains. The ore has in advance to be
milled to a particle size diameter between 20 – 300 micrometer. Then the surface of the interesting
mineral grains is, by addition of certain chemicals turned to become hydrophobic, while others are
still hydrophilic. Air in very fine bubbles is blown into the pulp. The particles with a hydrophobic
surface adhere to these bubbles and can be drawn from the surface of the pulp as a froth, while the
hydrophilic particles stay in the solution. Then the regime of chemicals is changed and the next
fraction of interesting mineral phases can be turned to show up with a hydrophobic surface and
recovered from the pulp by the same method. The density of the fine air bubbles, while their surface
is covered by tiny mineral grains, must be lower than the density of the same volume of the aqueous
solution. In this special sense flotation is a density driven separation technique after a hetero
coagulation process, which is initiated by a special regime of chemical reagents, had started. This
shall be described in more detail now.
The surface of particles is only in rare cases hydrophobic; just to mention some, for example
graphite, coal, Molybdenum-Sulphide ores, sulphur and many polymers. A hydrophobicity of the
surface can be reached by adding certain chemical reagents to the pulp, called collectors. They
adhere to the surface of the mineral grains and by this method a hydrophobic surface can be
‘installed’. For a successful regime of interacting reagents it is necessary to add other chemical
substances which act as depressants, activators and frother.
Collectors are surface active substances with a hydrophobic body and a hydrophilic functional group.
While they adhere to the interesting mineral phase, they turn the originally hydrophilic surface into a
hydrophobic one.
Depressants especially selected for each single ore deposit subdue the adherence of special film of
collector molecules on the hydrophobic surface while it would hinder the separation process of
another mineral phase.
Activators restore the previously subdued hydrophobic surface property. With this change of
electrochemical potential on the surface they are accessible for the fine hydrophobic tensed-air
bubbles.
Frother stabilize the froth on the surface of the pulp. If they would not be stable – for a while – and
would prematurely ‘explode’ the mineral grains, not supported by the air bubble, would sink back
into the water phase due to their higher density.
“The reagent scheme used in the treatment of lead–zinc ores varies considerably and depends on the
nature and mineralogy of the ore. In most cases, NaCN–ZnSO4 depressant system is used with
xanthate dithiophosphate collectors.” 2
In the field of primary ore processing, flotation is one of the most widely used methods. Compared
to other chemical upgrading processes of ores the flotation needs, to a significant amount, less (!)
reagents. Since the necessary surface property (hydrophilic or hydrophobic) is achieved by only
mono or oligo-molecular films on top of the mineral grains; some hundred Gram per Ton ore are
sufficient.
In distinct contrast to primary ore processing in the early stages of recycling strategies the engineers
were in the comfortable situation to take advantage of naturally hydrophobic versus hydrophilic
substances, for example greasy oily substances from aqueous solutions or the flotation of polymer
particles. Today widely established is the technique of deinking of paper via flotation. In a scientific
exploration phase is the flotation for the separation of synthetic mineral or metal rich phases as they
occur in waste incineration processes or in processing of metallurgical slags.
Experimental equipment:
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Flotation cell
pH Measuring strip, pH Meter
Scraper
Collector pots
Beakers for reagents
Feeding bottle
Bulatovic, Srdjan M. Flotation of Lead-Zinc Ores, Handbook of Flotation Reagents 2007, pages 323–366,
10.1016/B978-044453029-5/50023-X
 Büchner funnel
 Dropper
 Glass rod
Reagents:
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Zinc sulphate solution 1%, 45 ml (Depressant)
Potassium ethyl xanthate 1%, 3 ml (KEtX) (Collector)
Flotanol 1%, 2ml (Frother)
Copper sulphate solution 1 %, 20 ml (Activator)
Potassium amyl xanthogenat 1 %, 3 ml (KAX) (Collector)
Flotanol 1 %, 2 ml (Frother)
Lime slurry (ph-Value-Regulator)
Realisation
The lead zinc ore (300 g) has been milled down to grain size < 150 µ and shall be suspended in the
flotation cell with 300 ml water, stirred with the glass rod. The cell pot shall be placed in the
machine, the head of device put into the pulp, the cell shall be filled with water up to the marked line
and the pH-probe brought into position. The air supply shall be closed, the circulation controller shall
be in the Zero position – then the machine can be started. Then the circulation frequency shall be
regulated to 50 and the pulp shall be stirred for five minutes – this is necessary while aiming a
favourable wetting of the mineral grains. Then the zinc sulphate solution shall be added, while the
pH value shall be adjusted to 8, while adding carefully droplets of lime slurry. It is necessary to wait a
while before adding new lime slurry, because the lime slurry needs a time to solve. After five minutes
of stirring KETX shall be added and stirred for another five minutes. Then Flotanol C7 shall be added
and the solution stirred up once again. This initiates the development of froth, which can be
supported by opening the air supply, until a sustained froth crown is visible, which can carefully be
removed from the water surface with a scraper. The process reaches its end in case that no more
bubbles are formed or the bubbles get transparent, which means they bear no more ore particles.
The air supply shall be throttled, the copper sulphate solution shall be added to the pulp and the pHValue shall be adjusted to the value 10. After a stirring of five minutes KAX shall be added to the
solution and stirred once again for five minutes. The same procedure as described above starts; this
time the slightly yellow zinc ore attach to the air bubbles. The lead sulphite ore particles shine silvery.
The gained products can be vacuum filtered using the Büchner funnel and the, in this first step
reached, imperfect, purity can be watched under a light microscope.
Figure 5 copper sulphide particles in a Jameson cell. Copyright: Geomartin - Eigenes Werk, GFDL,
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