Solvent Extraction

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METS INSIGHTS SESSION

Solvent Extraction

Dr Denis Yan

Consulting Metallurgist

j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE

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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

Extr

actio

n C

olu

mn


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)34- and Mo8O26

4-

• Complex metal cations, such as MoO22+

• 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


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