At a METS Insights Session in Perth presented by our Consulting Metallurgist, covers what solvent extraction is as well as its origins, and discusses the different equipment types and types of extractants, among others.
3. j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE
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
5. j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE
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
6. j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE
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
8. j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE
Origins of Solvent Extraction
• 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
10. j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE
• 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
12. j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE
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
19. j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE
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
20. j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE
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
21. j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE
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
23. j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE
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
24. j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE
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
28. j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE
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
29. j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE
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"
30. j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE
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)
31. j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE
(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
32. j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE
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
33. j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE
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!
34. j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE
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
35. j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE
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)
37. j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE
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
38. j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE
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
39. j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE
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
41. j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE
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
43. j ENVIRONMENTALPROCESSINGDESIGN&VERIFICATIONPRODUCTINNOVATIONPROJECTMANAGEMENTOPERATIONSTRAININGSKILLSHIRE
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