Solvent Extraction

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> RESOURCE PROJECTS > TECHNOLOGY > INTEGRATED SERVICES
METS INSIGHTS SESSION
Solvent Extraction
Dr Denis Yan
Consulting Metallurgist

j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE
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• ACKNOWLEDGEMENTS
This document is a dynamic record of the knowledge and experience of personnel at Mineral
Engineering Technical Services. As such it has been built upon over the years and is a collaborative
effort by all those involved. We are thankful for the material supplied by and referenced from various
equipment manufacturers, vendors, industry research and project partners.

Who We Are
Mineral Engineering Technical Services
• Engineering Consultants with a focus on junior and mid-tier mining
companies
• Mineral processing since 1988
• Global greenfields and brownfields project experience
• Guiding projects through the development path from testwork to
feasibility studies through to commissioning
• Commodity experience across a wide range of minerals
• A division of Midas Engineering Group

Presentation Overview
• What is Solvent Extraction
• Origins of Solvent Extraction
• Equipment Types
• Types of Extractants
• SX operations
• Absorption isotherms
• Pilot plant

Solvent Extraction (SX)
• Solvent extraction is a Liquid-liquid
extraction
• Mass transfer operation based
upon chemical differences
• Two inputs – Feed solution
containing solute (metal species)
to be extracted and Feed solvent
which acts as solute extractor
• Two resultant streams – Extract
(solute rich solvent-organic) and
Raffinate (solute depleted residual
aqueous phase)
E-1
P-4
P-5
P-6
P-7
Feed Solution
Solvent
Extract
Raffinate
ExtractionColumn

Origins of Solvent Extraction
• Extraction of fragrances using fats predates ancient Egypt and is
widely used in Africa. However it was not until the nineteenth century
that the oils absorbed from fat were extracted using ethanol in a
process called enfleurage
• Enfleurage started in Grasse (France) with another process
(maceration) used to extract the essential oils from flowers
• Maceration used hot oil while enfleurage was a cold process

• Maceration used fat heated to 50-70oC mixed with flowers for up to 48
hours
• The fat is then mixed with alcohol to extract the fragrant oils from the
fat

• Enfleurage was specifically used to extract the essential oils from
flowers that were too delicate for the distillation method
• Flowers such as jasmine are placed on a layer of fat smeared on a
glass plate
• After 24 hours the flowers
are replaced with fresh
ones
• This is repeated for several
months until the fat had
absorbed sufficient
fragrance

• The charged fat was then mixed with alcohol to extract the fragrance
from the fat

• In 1872, Berthelot and Jungfleisch described the distribution of a metal
species between two immisible phases
• In the 1940’s the need to separate radioactive species led to the
introduction of solvent extraction on a large scale and it became
entrenched as a hydrometallurgical technique for purification of metal
species
• SX is applied to remove or extract a species from one solution to
another. Can be applied by either:
― Remove valuable component from contaminants or
― Remove contaminants from the valuable components

Solvent Extraction (SX) (cont.)
• Generalised metal recovery flowsheet incorporating leach Solvent
Extraction and electrowinning (EW)
Source: Solvent Extraction Reagents and Application, www.cognis.com

SX-EW
• Diluent specialised kerosene
• Active extractant; e.g. LIX 984 N for
copper, soluble in diluent
• Extraction-1st step to organic
• Scrubbing- optional next step
cleaning impurity from organic
• Stripping- remove Cu from organic
• Electrowinning from rich copper
stream
• SX can be used for:
– Copper
– Nickel
– Cobalt
– Uranium etc……
Source: http://www.halwachs.de/solvent-extraction.htm

Equipment Types
• Mixer Settlers
― One unit approaches one equilibrium stage
― High liquid rates and large vessel

Centrifuges Single Stage
Source: http://www.rousselet-robatel.com/images/products/Monostage-Pic-1lg.jpg
Source: //www.rousselet-
robatel.com/images/products/MonoStage-Centrif-
Extractorslg.png

Columns
• Practical for almost all Liquid-
Liquid Extraction
• Packing, trays or sprays
increase surface area for two
liquid phases to intermingle
• Commonly used – Agitated and
static

Columns
Source: http://www.alchemet.com/projects/images/solvent-extraction-
o%234EED94.jpg

Counter Current Operation
• To increase metal loading onto organic, need to contact with aqueous
solution of higher concentration
• To achieve low concentration of metal in raffinate need to contact with
organic of low concentration

Counter-Current
• Conserves mass transfer driving force
• More efficient use of solvent by higher loading in contact with feed
• Best recovery from raffinate in contact with fresh organic
Source: Cobre Las Cruces, S.A. (CLC)

Process Operation
• Several key parameters in operation of the SX process:
– Speciation in the feed (oxidation potential, complexation)
– Concentration
– pH
– Residence time
– Temperature
– Pressure is not usually considered. Mineral processes are at atmospheric pressure.
• The reagent driven reaction kinetics and equilibria will define the
operational parameters
• Setting operational parameters can be used to increase the selectivity of
reagents
• Multiple extraction and stripping stages are usual to improve concentration
and recovery
• Regeneration step included to reactivate the organic where there is a
gangue build-up on the extractant or reaction deactivating the extractant

Operating Limitations
• Temperature Selection
– Suitable interfacial tension and viscosity and kinetics favours higher
operating temperature
– Flash point and vapor pressure of the organic favours lower operating
temperature
• Solvent Selection
– Partially soluble with the carrier
– Easily recoverable
– Immiscible with feed components
– High selectivity towards solute
– High distribution coefficient
– Low viscosity
– Chemically stable
– Non toxic, non flammable and low cost

Brief Design Criteria
• Adequate amount of mixing is vital
– Amount of mixing depends on the physical properties between two phases
– Largest droplets controls extraction equilibrium
– Smallest droplets controls settling time
• Emulsion
– Formed due to excessive agitation or inherent nature of chemical compounds due to
contaminants
– Coagulants minimize emulsification
• Crud Layer
– Loose solid substances (foreign impurities) float at the interface
– Continuously withdrawn and filtered in continuous extraction

Extractable Metal Species
The extractable species (which have been recovered commercially in SX)
can be divided into four categories:
• Metal cations, such as Cu2+, Ni2+ and Co2+
• Complex metal anions, for example UO2(SO4)3
4- and Mo8O26
4-
• Complex metal cations, such as MoO2
2+
• Neutral metal species like UO2(NO3)2

Reagent Requirements
• For a metal extraction to be commercially successful, the extractant
(reagent) must:
― Have very low solubility in the aqueous phase and high solubility in diluent
― Extract the desired metal(s) selectively from the metal-containing aqueous
solution at a fast rate
― Be strippable at a fast rate with a solution from which eventual metal
recovery can take place
― Be stable to the circuit conditions so it can be recycled many times
― Not promote stable emulsions but have good coalescing properties when
mixed with diluent and modifier if necessary
― Have an acceptable cost
• It is desirable to be:
― nonflammable, nontoxic, noncarcinogenic

Diluent
• Selection Characteristics
― Mutually miscible with extractant (and modifier)
― High solvency of extracted metal organic species
― Immiscible (insoluble) with feed aqueous
― Low viscosity
― Low surface tension
― Low volatility and high flash point
― Density different from aqueous
― Chemically stable
― Desire non toxic, non flammable and low cost
― Aromatic content optimised for the system

Type of Extractants
• Metal extractants are classified by structure, extraction mechanism
and the metal species extracted.
• Main extractants types include:
― Chelating extractants
― Ion–Pair Extractants
― Neutral or Solvating Type Extractants
― Organic Acid Extractants
― Ligand Substitution

Chelating Extractants
> Metal extraction by LIX 84-I as
a function of pH
> Metal extraction by LIX 84-I as
a function of total NH3

Ion-Pair Extractants (cont.)

Neutral or Solvating Type
Extractants
• Are basic in nature and will coordinate to certain neutral metal complexes by
replacing waters of hydration  organic-metal complex to become organic
soluble and aqueous insoluble
• Extractions with solvating extractants are limited:
― The metal’s ability to form neutral complexes with anions
― The co-extraction of acid at high acid concentrations
― The solubility of the organo-metal complex in the organic carrier
• An important extractant of this type is : trioctyl phosphine oxide (C8H17)3PO,
called TOPO
http://upload.wikimedia.org/wikipedia/commons/thumb/c/c7/Trioctylphosphine
_oxide.png/244px-Trioctylphosphine_oxide.png

The Concept of Liquid-Liquid
Extraction
• Liquid-liquid extraction is based on the transfer of a solute substance
from one liquid phase into another liquid phase according to the
solubility
• Extraction becomes a very useful tool if you choose a suitable
extractant
• You can use extraction to separate a substance selectively from a
mixture, or to remove unwanted impurities from a solution.
• One phase is a polar (aqueous) solution and the other an organic
(non-polar) solvent
• The difference in solubility is key to success of this processing method
• The ratio of solubility in the two solvents is termed "distribution
coefficient"

At a certain temperature, the ratio of concentrations of a solute in each
solvent is constant (over a certain concentration range). This ratio is called the
distribution coefficient, K.
when solvent1 and solvent2 are immiscible liquids
For example, suppose the
compound has a distribution
coefficient K = 2 between solvent1
and solvent2
By convention the organic solvent
is (2) and water is (1)

(1) 30 particles
of compound distributed between
equal volumes of solvent1
and solvent2.
(2) 300 particles of compound , the
same distribution ratio is observed
in solvents 1 and 2
(3) Double the volume of solvent2
(i.e., 200 mL of solvent2 and 100
mL of solvent1), the 300 particles
of compound distribute as shown
If you use a larger amount of extraction solvent, more solute is extracted

What happens if you extract twice with 100 mL of solvent2 ?
In this case, the amount of extraction solvent is the same volume as was used in
Figure 3, but the total volume is divided into two portions and you extract with each.
The first extraction is as in figure 2
You still have 100 mL of solvent1,
containing 100 particles. Now you add a
second 100 mL volume of fresh
solvent2. According to the distribution
coefficient K=2, you can extract 67
more particles from the remaining
solution
100
67
33

An additional 67 particles are extracted
with the second portion of extraction
solvent (solvent2).The total number of
particles extracted from the first (200
particles) and second (67 particles)
volumes of extraction solvent is
267.This is a greater number of particles
than the single extraction (previous at
240 particles) using one 200 mL portion
of solvent2!
It is more efficient to carry out two
extractions with 1/2 volume of
extraction solvent than one large
volume!

Organic:Aqueous (O:A) Ratio
• Volumetric ratio between the organic and the aqueous phase
• Variation in O:A ratio will change recovery performance
• It is preferable where possible to have a series of countercurrent
contacts (for both loading and stripping). This results in an increase in
concentration for the desired species
• This will decrease the amount of solution required for downstream
processing and improve the recovery of the final product
• Testwork will determine the optimal advancing O:A ratio to achieve
high recovery of the desired species with reasonable number of stages
and extractant flow

Extractant
• The extractant is the key organic phase component used for the metal
recovery
– A wide range of extractants are available to suit a wide range of applications
– Some reagents can be more selective than others, some offer a better overall
recovery
• Extractant properties (price, flash point, viscosity, polarity) must be
considered prior to selection
• Testwork will determine what works for a specific process
• Extractant must be suitable to the system, not degraded by the
leaching and stripping liquors nor detrimental to the downstream
treatment
• Extractant is usually the most expensive component and minimising
losses of this component is key to minimising cost
(eg. Cyanex 272 - ~$50,000/t)

Diluent
• Diluent is used to solvate and mobilise the organic extractant, loaded
and unloaded. Extractant alone can be quite viscous
• Immiscible in the aqueous phase
• High flash point, low vapour pressure
• Diluent should be stable in the process. Eg oxidation to carboxyl can
results in non-selective extraction
• Effects transfer kinetics and phase disengagement

Modifiers
• Several modifier types available – selected dependant on application
• The extraction takes place at the interface. The extractant can be
assisted at this interface by a modifier component which changes
interface parameters. The effect can be:
– Improved kinetics
– Improved settling
• Loaded extractant can have low solubility in the diluent. A modifier can
improve loaded organic solubility
• Interaction with the extractant can improve extraction or stripping
extent or rate
• Modifiers which increase transfer rate should be called accelerators or
catalysts

Eh / pH
• Speciation in the aqueous phase depends on oxidation potential, pH
and complexing reagents
Eg iron could be in the species Fe2+, Fe3+, FeCl+, FeCl2+ depending on conditions
• Selective extraction depends on this speciation
• Therefore control of these factors is vital to good operation
• Transfer can involve exchange of proton which then changes pH and
equilibrium. The pH may have to be controlled to drive loading and
selectivity

Stripping
• The loaded organic is stripped using an aqueous liquor. This is a
stronger lixiviant than the feed liquor or more dilute in the solute
• In some cases several components can load during the extraction
stage, not just the desired metal
– Manipulation of the Eh/pH/complexation can effect this, however to achieve a high
level of recovery other species may be loaded
• Methods to deal with the impurities include selective stripping stage-
wise by differing pH, stripping agent or stripping agent strength
• This enables a higher purity product stream, can allow for multiple
product recovery or a return as part of the leach reagent

Solvent Losses
• Solvent losses can be a large cost factor for a solvent extraction
process
• Losses can occur through crud formation, phase entrainment and
misting
• Crud formation
– Crud formation is driven by the contact of aqueous, organic and a solid phase /
particle
– Crud can cause impurities to carry over into other phases resulting in poor process
performance

Solvent Losses - Crud
• Control of crud
– As there is no relationship between reagent additions and crud, other prevention
methods must be used
– Primary cause is solids or incipient solids within the feed to the SX circuit
– Particles are sub-micron – large settling tanks and or filters prior to SX feed to remove
solids
– Flocculants / agglomerating agents can be added to increase settling speed and
reducing solids in SX circuit
– Fast variation in pH can also increase crud formation – It can be difficult to manage
following a leaching step
• Removal of crud
– Pumping out of circuit at the weir
– Intermittent flooding of organic phase to remove and treat
– requires reduced production or shutdown
• Treatment of crud
– Centrifugal treatment or filtration of the crud containing organic
– Treated organic returned to the SX circuit

3rd Phase Formation
• Third phase formation occurs when the organic phase splits into two
distinct organic phases
• The organic will typically split into the diluent and the metal rich
extractant phase
10% Cyanex 272, 5% TBP 20% DEHPA, 5% TBP

Third Phase
• The formation of a third phase is not always an issue, many processes
will not have this issue arise
• Serious issues for recovery and processing can arise with third phase
formation – Phase inversion can result in the loaded organic flowing
out with the aqueous phase
• Studies have shown that ensuring operation below the LOC (limiting
organic concentration) can prevent the separation of the diluent and
the loaded extractant
• Use of a modifier can maintain solubility and prevent third phase

Equilibrium
• Previously spoke of distribution coefficient K or D. This value is not
constant over the entire concentration range. Equilibrium compositions
are determined experimentally.
http://www.chem1.com/acad/webtext/chemeq/eq-images/sepfunnelransir.jpg
Source:
http://firstyear.chem.usyd.edu.au/prelab/images/E28extractionimage2.gif

McCabe Thiele
Source: http://firstyear.chem.usyd.edu.au/prelab/images/E28extractionimage2.gif

Stripping
• Similar must be performed on stripping isotherm plot, operating line
and stages
• The two ends of the operating lines are joined by the loaded organic
concentration and the stripped organic concentration
• Usual to plot strip liquor concentration against organic concentration.
In that case operating line slope is O/A

Pilot Plant

SX Fire Hazard
• Olympic Dam, 2001 - $170 million damage, effected production for 2 years
Source: “Factors Influencing the Effectiveness of Firewall Designs for Metalliferous Solvent
Extraction Plants”, A. MacHunter, ALTA Uranium-REE Conference, May 2014

SX Fire Hazard
• Morenci Metclaf, Phelps Dodge, 2003

SX Fire Hazard
Source: “Factors Influencing the Effectiveness of Firewall Designs for Metalliferous Solvent Extraction Plants”, A. MacHunter, ALTA Uranium-REE Conference, May
2014

References
• Solvent Extraction Reagents and Application, www.cognis.com
• http://www.halwachs.de/solvent-extraction.htm
• http://www.rousselet-robatel.com/images/products/Monostage-Pic-1lg.jpg
• http://www.outotec.com/Global/Products%20and%20services/Outocompact3_300.jpg
• http://www.rousselet-robatel.com/images/products/MonoStage-Centrif-Extractorslg.png
• http://www.alchemet.com/projects/images/solvent-extraction-o%234EED94.jpg
• Cobre Las Cruces, S.A. (CLC)
• http://www.chem1.com/acad/webtext/chemeq/eq-images/sepfunnelransir.jpg
• http://firstyear.chem.usyd.edu.au/prelab/images/E28extractionimage2.gif
• “Factors Influencing the Effectiveness of Firewall Designs for Metalliferous Solvent Extraction Plants”, A. MacHunter, ALTA
Uranium-REE Conference, May 2014

THANK YOUwww.metsengineering.com

Solvent Extraction

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