unit operations in mineral processing - aalto

Unit Operations in Mineral Processing and RecyclingProf. Rodrigo Serna and Dr. Robert HartmannSpring, 2019Aalto University

Unit operations in a Cu processing plant

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1. Comminution

From mine

A. Lossin “Copper” in Ullman’s Encyclopedia of Industrial Chemistry (2001)


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2. Screening/Classification

From mine


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3. Concentration

From mine

Concentration: FlotationPerhaps the most utilized operation in minerals processing for concentration of

valuables

• Froth flotation was developed in the 1900s

• But due to its complex nature involving liquid-solid-gad interactions, it is still not completely understood

Exploits the differences in physico-chemical surface properties between valuable minerals and unwanted gangue

• Separates hydrophobic particles by attachment to air bubbles

• Development of flotation allows good selectivity in modern operations separating complex ores

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Air

Mineralized

Froth

Agitator

Air

Bubbles

Mineral

Attached to

Air Bubbles

Pulp

Feed

Concentrate

Tailings

Air

Concentration - FlotationImportance of particle size in flotation

• Liberation of valuables

• AND

• Operating range of flotation cells and columns

The processing plant requires to operate with a feasible size of particles:

• Smaller particle sizes are not necessarily the best option… sizing operations are really important!

Reco

very

Particle Size (μm)

Particles too large

will not float:

Gravity forces are

dominant over

buoyancy

Particles too small

will not attach to

bubbles due to

turbulent currents

surrounding bubbles

Entrainment of

gangue in froth

High consumption

of collector

50 150

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Concentration - FlotationWhy do we apply flotation instead of sieving or hydrocyclones?

Particle Size (μm)

150

For particles smaller than 200 µm (hair has a diameter of 60 µm) many processes cannot be applied anymore

Complex ores make a sufficient

liberation necessary which

leads to fine powders

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Drag (mass)

forces

v.d.

Waals

forces

Chemical

bonds

1 m

10-2 m

10-4 m

10-6 m

10-8 m

10-10 m

10-12 m

Drag forces diminish with decreasing

particle sizes and other forces start

dominating

Flotation is an efficient separation technique which uses the different

surface wetting properties of solids

Concentration: Flotation

Types of flotation cells• Mechanical

- Uses an impeller (rotor-stator) to agitate the slurry and disperse the air into small bubbles

• Pneumatic- Uses air as a mean to both produce froth

and to circulate the suspension

- Not so common, as they require excessive amounts of air

• Columns- Air is introduced by a sparger at the

column base

- Use wash water to remove entrained gangue, thus useful for flotation of fine particles

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Some types of flotation cells

• How do we choose a flotation cell?

• Metallurgical performance (grade, recovery)

• Capacity (ton/h per unit volume)

• Operating costs (including power consumption, maintenance, etc.)

• Ease of operation (subjective)

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Separation stages

Conditioning tanks

• The ore requires some time to mix with reagents and to form a sufficiently constant feed

Rougher

• First stage of separation, removes fast floating valuables

Scavenger

• Treats the tailings of the first separation stages, it gives the valuables a “second chance”

Cleaners and re-cleaners

• Typically to improve the grade of the rougher products, and/or treat slow floating valuables

Regrind

• Used when it is suspected that the valuables are not sufficiently liberated

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Two cases:1. Naturally hydrophobic minerals (e.g. molybdenite) can be readily floated

2. Minerals that are not hydrophobic

• Solution: “Collectors” are chemical compounds that selectively react with the mineral of interest to make it hydrophobic

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CH3-CH2-O-CS

S-Na+

CuFeS2

+

Hydrophilic

(will not float)

CuFeS2

Hydrophobic

(floats)

O2

Non-polar Polar

Anion Cation

//

\

In today’s episode of LET’S TALK CHEMISTRY:

Adsorption!!!

• Surface phenomenon

• Molecules in fluid-phase (gas/liquid) are attached to a solid surface

• Molecules to be adsorbed (the adsorbATE) will look for active sites on the surface of the solid (the adsorbENT)

• Two kinds of adsorption:- Physisorption

• Van-der-Waals type and electrostatic interactions

• Low energy (reversible) <40 kJ/mol of adsorbed species

- Chemisorption• Strong interactions, equivalent to covalent bonds (arbitrarily >40 kJ/mol, but

typically measured at >80 kJ/mol)

• Very site-specific

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In today’s episode of: LET’S TALK CHEMISTRY

So how does collectors interact with the mineral surface?

• …you guessed it: they adsorb on the surface of the mineral to be floated

• Depending on the type of collector it can be physical or chemical adsorption

• This depends on the type of collector

- Ionic

• Cationic (+)

• Anionic (-)

- Non-ionic

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Ionic collectors

• What makes them ionic?

- They bear a (free) electrical charge

• What are they?

- Salts of weak acids or bases …also some weak acids

- Ionic compounds in their natural state are neutralized by having both positively and negatively charged fractions

• What makes them cationic or anionic?

- The charge of the functional group determines their classification, for example:

• Amines (+) are cationic

• Xanthates (-) are anionic

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Ionic collectors (a few examples)

• Cationic

• Anionic

• Question: Why do you think it is necessary for these collectors to be ionic?

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Answer: Water-soluble when added to the aqueous media,

but hydrophobic when adsorbed


Adsorption mechanisms

Chemisorption

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Oxidation with dissolved oxygen:

MS + 2O2 ⇋ MSO4

If carbonate ions are present:

MSO4 + CO32- ⇋ MCO3 + SO4

2-

Ion exchange replacement with e.g. xanthate ions:

MSO4 + 2X- ⇋ MX2 + SO42-

MCO3 + 2X- ⇋ MX2 + CO32-

M(OH)2 + 2X- ⇋ MX2 + 2OH-

(M = Metal species, X = xanthate ion)

Chemisorption is believed to occur with reaction with e.g.,

galena, chalcocite, sphalerite

M +

M

O2

X-

X-

Chemisorbed metal-xanthate



Electrochemical oxidation

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The mineral surface works as an electron

conductive material (e.g. metal sulphides)

Two independent electrochemical reactions take

place at the surface of the mineral:

2X-⇋ X2 (adsorbed) + 2e-

1/2O2 (adsorbed) + H2O + 2e- ⇋ 2OH-

(M = Metal species, X = xanthate ion)

Electrochemical oxidation has been reported

on pyrite, arsenopyrite and pyrrhotite

M

X-

X-

2X

-2e-

Adsorbed dixanthogen on metal

+2e- + H2O

O

OH-

OH-



Electrochemical oxidation and

chemisorption are not mutually

exclusive

Both mechanisms may occur

simultaneously although one can

be dominant

In the case of chalcopyrite,

adsorption by both mechanisms

have been reported

Note that both mechanisms require

the presence of oxygen… bubbling

N2 for example, will not help!

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M

X-

X-

2X

-2e-

Adsorbed dixanthogen on metal

+2e- + H2O

O

OH-

OH-

M +

M

O2

X-

X-

Chemisorbed metal-xanthate


Adsorption of ionic collectors

• pH is very important for collector flotation

• The surface of the mineral must be negative to react with cationic collectors and positive for anionic ones

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Flotation performance of goethite

(FeO(OH)) using two type of collectors:

Anionic: Sodium dodecyl sulfate (circles)

Cationic: Dodecyl ammonium chloride (triangles)


Adsorption of ionic collectors

• pH is very important for collector flotation

• This phenomenon can be exploited for selectivity of floatable species

- The example shows minerals with active sites of similar nature:

- Pyrite (FeS2), Galena(PbS), Chalcopyrite(CuFeS2)

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Non-ionic collectors

• Typically, hydrocarbon mixtures (similar to diesel oil, i.e., paraffin mixtures)

- Relatively cheap and abundant

• Their non-ionic character makes them hydrophobic

• Question: why do you think they are not the preferred choice?

• Note: mainly used as additives in presence of ionic collectors

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Answer: Surfaces must be already

hydrophobic for them to adsorb


Flotation chemicals

• Collectors

- Used to hydrophobize the surface of the valuables

- The challenge is to find a collector that will adsorb only (or at least preferentially) with the valuables and not the gangue

- Hence, various chemicals are available to float the various types of ores

• Depressants

- Prevent the adsorption of collectors, inhibit hydrophobicity

• Activators

- Used to “clean” the surface of the ore and/or promote the interaction with collector functional groups

• Frothers

- Stabilize the foam, used to increase the lifetime of bubbles

- ALSO considered adsorption, but in this case on the air-water interface!

• pH controllers

- pH determines floatability of minerals

- Alkaline conditions help stabilize collectors and prevent corrosion of equipment

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Frothers

Typically non-ionic surfactants

Polyglycols, alcohols

Average bubble size decreases with

increasing frother concentration

At concentrations higher than

Critical Coalescence

Concentration (CCC), bubble size

remains constant

Frothers also help stabilize the froth-

phase in flotation cells

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Bubble size (D32) as a function of frother concentration

for pentanol, methyl isobutyl carbinol (MIBC) and commercial

blends F150 and DF250

Elmahdy and Finch, Int. J. Min. Process. 123 (2013) 60

Flotation circuitsTask for next week:

• Prepare a 15 min presentation of working cases on non-sulphidic ores- Tuesday:

- Niobium: Fang Hu

- Copper oxide: Chao Peng

- Rare-earths (RE): Diederick van Roemburg and Juho Ollila

- Thursday:

- Platinum group metals (PGMs) : Marja Rinne, Felix Kornemann

- Titanium: Mehrdad Mousapour

- Gold: Pia Höner, Mika Sahlman

• What are the flotation stages? What is the logic behind this arrangements?

• Is there anything special about this flowsheet that draws your attention?

Unit Operations in Mineral Processing and RecyclingWeek 3


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Waste

From mine To Refining

Flotation types

Flotation can be classified as:

• Standard- Separates a single valuable mineral from

gangue

- The froth phase contains the valuable mineral

• Reverse- The froth phase contains the gangue

- Typically used for quality control of bulk minerals

StandardFeed

Froth phase

(Concentrate)

Pulp

(Tailings)

ReverseFeed

Froth phase

(Tailings)

Pulp

(Concentrate)

Flotation types

Flotation can be classified as:

• Bulk- More than one valuable is floated at the

same time

• Differential- Separates a bulk concentrate into streams

with two different products

BulkFeed

Froth phase

(Concentrate 1

+ Concentrate 2)

Pulp

(Tailings)

DifferentialFeed

Froth phase

(Concentrate 2)

Pulp

(Concentrate 1)

Flotation circuits

As seen so far, flotation rarely occurs in a single step

• A single step may not provide the target recovery-grade

A big part of the design of a mineral processing plant is to

define the best flotation scheme (flotation circuit)

• The complexity of such a scheme will depend on mineral parameters such as

• Mineralogy

• Liberation

• Expected grade

Flotation circuits

The typical stages in flotation are:

• Rougher

• Scavenger

• Cleaner

• These various stages are also used in other concentration operations

The ore may be processed between stages, for example by

• Regrinding

• Addition of chemicals

• Dewatering

Flotation circuit stages

1. Rougher

• Rougher flotation is the first step in concentration

• As it name suggests, its purpose is to make a “rough” separation of the valuables in the ore- Recovery at the rougher should be high, even if this compromises grade

- Tailings should contain most of the unwanted material

• The feed can be:- Product of milling circuits

- Middling return products (Scavenger concentrate or cleaner tailings)

• Rougher flotation cells are typically large, since they treat large volumes


2. Scavenger

• The purpose of scavenger flotation is to treat tailings from other stages since they may still contain valuables

• As it treats feeds with low grade, scavenger’s concentrate is typically re-circulated as rougher feed

• As tailing streams are treated, scavenger flotation must be capable of treating large volumes and process coarse particles


3. Cleaner

• A cleaner stage is used to treat concentrates when roughers cannot achieve the target grade

• Since cleaners treat smaller volumes, regrinding may be economical between cleaning stages

• Due to the characteristics of flotation columns, they are considered particularly suitable for cleaner stages

Support operations in flotation circuits

1. Conditioning

• This is a preparation step to provide the pulp with the adequate chemical characteristics before flotation

• Typically done in stirred tanks where chemical additives are mixed with the pulp

• May also be used as buffer stages to control residence time in the flotation circuit

• In some cases, high intensity stirring can be used to remove clay, precipitates or other unwanted substances from the mineral surfaces

Support operations in flotation circuits

2. Regrinding

• Additional liberation of particles to improve grade

• Mineral surface activation or re-activation

3. Thickeners

• Used to control the solids content in the pulp

Basic flotation circuit arrangements

Rougher-Scavenger

Feed 1 42 3 1 42 3 Final tailings

Final concentrate´

Rougher tailings/

Scavenger feed

Scavenger concentrate/

Recirculated rougher feed

Rougher Scavenger


Rougher-scavenger-cleaner


Final concentrate´

Rougher tailings/

Scavenger feed



Rougher Scavenger

1 42 3

Cleaner tailings

Recirculated rougher feedCleaner

Rougher concentrate/

Cleaner feed


Flotation with re-cleaners


Final concentrate´

Rougher tailings/

Scavenger feed



Rougher Scavenger

1 42 3

Cleaner tailings/


Cleaner

Rougher concentrate/

Cleaner feed

1 42 3

Regrinding

Cleaner concentrate/

Re-cleaner feed

Re-cleaner tailings/

Recirculated cleaner feedRe-cleaner

Common flotation circuits for sulphidicores

Circuit 1

• Simple circuit

• Counter-current flow of valuable and waste minerals

Pros (+) Cons (-)

• Simple to operate • Nothing is done during processing to improve

the separation characteristics of the slow-

floating particles which appear in the

scavenger concentrate and cleaner tails that

are:

• Coarse composites

• Very fine liberated valuables Fresh feed

diluted with these particles adverse effects


Circuit 2

• Extension of circuit 1, coarse composite particles returned to grinding circuit for breakage

• Counter-current flow of valuable and waste minerals

Pros (+) Cons (-)

• Further mineral liberation may occur due to

re-breakage Improved flotation

characteristics

• Fresh feed diluted with particles that are

mostly difficult to float may disturb the

behavior of the rougher bank inefficient

flotation of the fresh feed


Circuit 3

• Further extension of circuit 1

• Composite and other particles that are difficult to separate are removed from the main circuit

• Retreatment in separate circuit

• Only concentrate returned to main circuit

Pros (+) Cons (-)

• Particles that are difficult to separate can be

treated in the most suitable way for their

characteristics

• Re-treatment way is not influenced by the

processing requirements of the main stream

• Complexity

• Costs


Circuit 4

• Approach to solve the problem of “difficult” particles

Pros (+) Cons (-)

Production of “difficult”, slow floating, fine

particles is minimized (tailing from primary

flotation fed to secondary grinding)

Overgrinding may occur if particles with fast-

floating size range are further broken and the

product appears in the slow floating, fine fraction


Circuit 5

• Figure shows basic circuit, can be more complicated with additional re-grinding and flotation stages

• Used on porphyry copper ore treatment:- Coarse initial grind rougher

flotation rougher concentrate re-grinding cleaning

Pros (+) Cons (-)

Grinding costs can be minimized Valuable minerals may be lost in the rougher

tailing due to the coarse grind

Concentration - Flotation

Sizing of floatation cells

• The volume of a flotation cell depends on the volumetric flow rate and the retention time

• Vf=(Q Tr S)/(Ca)

• Vf = volume of tank, Q = volumetric flowrate, Tr = retention time

• Retention time is defined by the kinetics of flotation, which means it is determined from test work

• Some correction factors can be included: - Scale up-factor “S” (if Tr from laboratory-scale tests, 1.6-2.6)

- Aeration factor “Ca” (typically 0.85)

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…and then again to the manufacturer’s options:

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Selection Data for Metso Reactor Cell Systems

Type of

cellVolume (m3)

Maximum Bank Feed

Rate (m3/h)

Maximum cells per

section

RCS 3 3 240 4/5

RCS 5 5 320 4/5

RCS 10 10 540 4

RCS 15 15 730 4

RCS 20 20 870 4

RCS 30 30 1120 3

RCS 40 40 1360 3

RCS 50 50 1650 3

RCS 70 70 2040 2

RCS 100 100 2550 2

RCS 130 130 3050 2

RCS 160 160 3450 1

RCS 200 200 3990 1


Kinetics and probability of flotation• Flotation can be represented by a first-order kinetic equation

- m = mIexp(-kt)• mI = initial mass of particles

• m = mass of particles at time t

• k = rate constant

• Thus, recovery also follows first-order kinetics:- R = 1-exp(-kt)

• Separation as a function of probability of flotation at each stage in a flotation bank

- m = mI(1-p)n

• mI = initial mass of particles

• m = mass of particles after n stages of flotation

• p = probability of flotation at any stage

• n = number of stages

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Concentration - FlotationFlotation probability example

• A 10% galena (PbS)-containing ore

• With a liberation of 64% (probability of flotation of 50%)

• 10% probability of flotation for locked PbS

• The amount of gangue associated with locked PbS is in average 35.3% in mass

• The probability of non-associated gangue is of 0.3%

• How is the grade of galena changing at each flotation stage?

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Quote 3

Don't try to solve serious matters in the middle of the night.

-Philip K. Dick, writer

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unit operations in mineral processing - aalto

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