INTRODUCTION In chemistry, liquid-liquid extraction, or more briefly, SOLVENT EXTRACTION, is an useful method to separate a substance selectively from a mixture, or to remove unwanted impurities from a solution. Solvent extraction (SX) is based on the transfer of a solute from one liquid phase into another liquid phase. The success of this method depends upon the difference in solubility of a compound in various solvents and becomes a very useful tool if you choose a suitable extraction solvent. In the practical use, usually one phase is a water or water-based (aqueous) solution and the other an organic solvent which is immiscible with water. Solvent extraction is used in nuclear reprocessing, ore processing, the production of fine organic compounds, the processing of perfumes and other industries.
GENERAL CONCEPTS Solvent extraction (SX) is the traditional term of liquid-liquid distribution that involves the distribution of a solute between two immiscible liquid phases in contact with each other. The principle is illustrated in Figure 2.1. The vessel (a separatory funnel) contains two layers of liquids, that one is usually water (S aq) while the other, an organic solvent (Sorg). The organic phase often settles as the upper layer when it has a lower density than water, but the opposite situation also occurs. A solute A, which initially is dissolved in one of the two liquids, eventually distributes between the two phases. When this distribution reaches equilibrium, the solute has one concentration [A]aq in the aqueous phase and another concentration [A]org in the organic phase.
Figure
2.1.
A
schematic
representation
of
solvent
extraction
(liquid-liquid
distribution). A solute A is distributed between the upper layer, an organic phase (Sorg), and the lower layer, an aqueous phase (Saq).
Distribution ratio In solvent extraction, a DISTRIBUTION RATIO (D) is often quoted as a measure of how well-extracted a species is. The distribution ratio of the solute A is expressed as: D = [A]org/[A]aq where D is defined as "the total analytical concentration of the substance in the organic phase to its total analytical concentration in the aqueous phase, usually measured at equilibrium", irrespective of whether the organic phase is the lighter or heavier one. If a second solute, B, is present, the distribution ratios for the various solutes are indicated by DA , DB , and so on. If DB is different from DA , A and B can be separated from each other by (single or multistage) solvent extraction. D is also called the distribution coefficient or the extraction coefficient. Depending on the system, the distribution ratio can be a function of temperature, the concentration of chemical species in the system, and a large number of other parameters
Separation factors The SEPARATION FACTOR(SF) is one distribution ratio divided by another, it is a measure of the ability of the system to separate two solutes. For instance if the distribution ratio for nickel (DNi) is 10 and the distribution ratio for silver (DAg) is 200, then the silver/nickel separation factor (SFAg/Ni) is equal to DAg/DNi = SFAg/Ni = 20.
DESCRIPTION OF THE SOLVENT EXTRACTION PROCESS Solvent extraction is applicable in any instance where it is desirable to selectively, remove or extract a species from one solution into another. In metal recovery operations, the valuable component is normally a metal ion or a metal ion complex contained in an aqueous solution. This aqueous solution is mixed intimately with an immiscible organic phase containing the active extractant at which time the active extractant transfers the desired metal from the aqueous phase into the organic phase. The "loaded organic" phase, now containing the desired metal, is then transferred to the stripping section. Here the "loaded organic" phase is mixed intimately with an aqueous stripping solution containing a chemical that provides the driving force required to strip the loaded metal from the organic phase into the aqueous stripping phase. The "stripped organic"
is recycled back to extraction while the aqueous phase, now containing the desired metal, goes to final metal recovery. Final metal recovery usually takes place from the aqueous solution. The recovery of metal directly from the loaded organic phase is also possible but it is practiced commercially in only special cases probably because of organic contamination of the final product.
Extractable Metal Species If a metal is to be extracted from impurities or vice versa it is important to know the various metal and other species present in solution. With respect to metals, the extractable species can be divided into four categories:
Metal cations such as Cu2+, Ni2+, and Co2+ Complex metal anions. Complex metal cations. Neutral metal specials
Reagent Requirements It is obvious that if a metal species is to be transferred from an aqueous leach solution into an organic solution, there must be some chemical interaction which causes this to happen. The component in the organic phase, which chemically interacts with the metal, is properly called the "extractant" but is also commonly called the "reagent". At the present time, there are organic extractants known for virtually all metals in one form or another. However, the requirements for a successful extractant in analytical chemistry are much different than the requirements for a reagent to be successful in large scale metal recovery operations, especially as they relate to process continuity and economics. For a solvent extraction reagent to perform satisfactorily in the recovery and purification of metals present in aqueous leach solutions the reagent must meet a number of criteria. The most important of these have been summarised as follows:
1. Extraction of the desired metal selectively from the aqueous solution containing the dissolved metal. This solution is usually a specifically prepared leach solution but can also be an acid mine drainage solution or a waste stream from some metallurgical or industrial process. 2. Be able to be stripped to produce a solution from which the desired metal can be recovered in an acceptable form. The metal may be recovered in a number of forms including electrowon cathode, crystallised salts, or precipitated salts. 3. Be chemically and physically stable in the solvent extraction circuit so that it can be recycled through extraction and strip many times without experiencing undue physical loss or chemical breakdown. 4. The reagents must meet today's stringent environmental and work place regulations (must be nonflammable, nontoxic, noncarcinogenic, etc). 5. The extraction and strip kinetics must be sufficiently fast to allow these processes to take place in an industrially acceptable time frame. 6. The extractant must be soluble, both in the loaded and stripped form, in a relatively inexpensive diluent which also meets the environmental and workplace regulations. Alternatively the extractant may be capable of being used at a volume concentration of 100%, in which case the reagent forms its own diluent. 7. The made - up circuit organic must phase separate from the aqueous at a reasonable rate and the separated phases must have acceptable levels of entrainment. 8. The extractant must not transfer deleterious species back from the strip section to extraction. 9. The extractant should be tolerant of crud and should not promote crud formation or stable emulsions. 10. The extractant must have a reasonable cost, which will enable it to provide an economically attractive recovery route for the metal being treated. Meeting all or sufficient of these criteria to an acceptable level imposes great restrictions on the number of chemicals which have found commercial use as extractants. While there are many hundreds of reagents which have been developed and tested in the laboratory only very few of these have found commercial acceptance.
Normally, reagent behavior with respect to the above list is not a black and white, pass or fail situation. No one reagent is the best with respect to all of the properties in the list; rather, successful reagents possess a good balance of all of the properties in the list. It should be realized that few if any reagents are selective for only one metal under all conditions, but that many reagents are selective for only one metal under certain conditions.
Solvent Extraction Isotherms The most basic requirement for solvent extraction design is an understanding of distribution coefficients and equilibrium isotherms. These data and curves are obtained in the laboratory and to some extent by computer if enough basic data are in the computer programme. Distribution curves or EQUILIBRIUM ISOTHERMS merely illustrate the distribution of a dissolved component, between two phases. The laboratory procedure to make extractions isotherms is to place the solution to be extracted in the separatory funnels (see figure 3.1). As the organic solvent and water are not miscible with each other, you should be able to see the two layers (organic and aqueous layers) clearly. You should also have two beakers ready, one labelled "organic layer" and the other labelled "aqueous layer".
Figure 3.1. Laboratory equipment to obtain equilibrium isotherms.
To extract all inorganic substances from the aqueous layer, vigorous agitation will be necessary to increase the contact between these substances and the organic layer. It is imperative to vent the separatory funnels of any gas pressure. When the residence time has finished, the layers allowed separating. At this point the two layers can be separated into
their respective beakers. Once the extraction process is completed, drying agents can now be used and the product can be isolated from the organic solvent. After phase separation the aqueous phase is analysed with respect to an element and the element content in the organic phase is calculated. The equilibrium isotherm for extraction is obtained by plotting the concentration of the element in organic phase against the concentration of the element in the aqueous phase in a diagram.
Distribution Curves The objective of solvent extraction in the metallurgical industry is clearly to extract one metal ion and to reject the others, either to recover the metal extracted or merely to remove it as an impurity. For example, the following reactions are valid for the extraction and stripping of Cu2+ in sulphate media: EXTRACTION: 2RH(org) + Cu2+(aq) ( R2Cu(org) + 2H+(aq) STRIPPING: R2Cu(org) + 2H+(aq) ( 2RH(org) + Cu2+(aq) These reactions show that extraction is pH dependent. High pH favours extraction while low pH favours stripping. LIX reagents are used industrially for copper extraction, due to their ability to selectively extract Cu2+ from Fe3+, which is commonly encountered in leaching solutions. For instance, the pH dependence for extraction and stripping is greater for Fe 3+ than for Cu2+, which also is evident from figure 3.2. By using a pH between 1.5 and 2.0 Cu2+ can be extracted selectively.
Figure 3.2.- pH isotherms for one of the commercial LIX-reagents.
Also, figure 3.3 shows the distribution of several common metals between a sulphate medium and di 2.EHPA as a function of equilibrium pH.
Figure 3.3.- pH isotherms for D2EHPA.
These isotherms, of the types shown in figures 3.2 and 3.3, form the basis of preliminary design for any solvent extraction plant. It will not be possible for the commercial plant to perform better than predicted by the isotherms (unless conditions change, in which case a new isotherm is needed) and in all cases efficiency will dictate that it performs somewhat worse. First, the relevant extraction and strip isotherms are produced in the laboratory by the method described earlier, using stripped organic at equilibrium with the spent electrolyte. The initial choice of organic phase is largely a matter of experience but at a loading mentioned previously, i.e. about 0.5 gpl Cu per percent extractant in diluent, a concentration of about 24% should be needed in theory: actually, since no system is perfect or loads to 100 percent loading, in this case 30 percent is provisionally assumed. It is also important at this early stage' to recognise that the organic phase is never stripped down to zero copper content, in other words there is always a circulating load. The copper content of stripped organic is a function of many parameters including those in table 3.1:
Table 3.1.- PARAMETERS AFFECTING COPPER CONTENT OF STRIPPED ORGANIC 1. The organic extractant chosen 2. The organic concentration 3. Acidity of strip liquor 4. Copper content of strip liquor 5. Number of strip stages 6. Temperature
The important value to consider is the transfer capacity of the system in gpl copper, i.e. the difference in gpl copper between loaded and stripped organic per volume of reagent used.
The McCabe-Thiele diagram Properly generated extraction and stripping isotherms represent equilibrium conditions and, as such, predict the best extraction and the best stripping which can be obtained. These isotherms can be used to set the staging in a circuit. The number of steps needed in a mixer-settler to reach desired separation, can be determined by constructing a McCabe-Thiele diagram. The diagram is constructed by determining the equilibrium isotherm for extraction or stripping via experiments. The equilibrium isotherm obtained is specific for every particular system. Changes in temperature, pH, and concentrations of extractant or metal will lead to another equilibrium curve. Consider, for example, the extraction isotherm in Figure 3.4 and suppose that the stripped organic entering into the last extraction stage contains 1.80 g/l Cu and that the advance flow rates of the leach solution and the organic phase are equal.
Figure 3.4.-Equilibrium extraction isotherm and McCabe-Thiele diagram for extraction of Cu2+ with a LIX-reagent. (S.O. and L.O. stands for stripped- and loaded organic phase, respectively).
After the equilibrium isotherm has been plotted into the diagram, an operating line can be constructed by starting at the point where the copper content of stripped organic (S.O.) intersects the equilibrium isotherm and drawing the line up and to the right with a slope equal to the ratio of the aqueous / organic flow rates (one in this instance) until the operating line intersects the vertical line representing the copper content of the aqueous phase feed. Next, a horizontal line to the isotherm curve and then a vertical line to the operating line are drawn creating a step. This procedure is repeated until a satisfactory degree of extraction from the aqueous phase has been obtained. Each horizontal line in the McCabe-Thiele diagram corresponds to one step in the mixer-settler. A McCabe-Thiele diagram for stripping is constructed in a similar way, see figure 3.5.
Figure 3.5.-Equilibrium strip Isotherm and one-stage McCabe-Thiele diagram with a LIXreagent. (S.E. and P.E. is the stripped- and pregnant electrolyte, respectively)
In figure 3.4, each triangle represents a single stage of extraction, in this case, completing a two-stage McCabe-Thiele diagram. In this example, a raffinate of 0.22 g/l Cu and a loaded organic of 4.30 g/l Cu are predicted in two stages of extraction. The inverse of the slope of the operating line is the advance organic / aqueous flow rate. Even though the McCabe-Thiele diagram shown in Figure 3.4 does not represent true equilibrium, but only a first approximation, it is still quite useful. For example, if a third stage of extraction were to be added to the McCabe-Thiele diagram in Figure 3.4, (note the dotted line) a near perfect equilibrium McCabe-Thiele diagram would result. In addition, a more accurate two-stage McCabe-Thiele extraction diagram can be drawn by taking the two-stage McCabe-Thiele construction as shown in Figure 3.4 and choosing as the point from which a new operating line is to be drawn a distance about 1/2 way between the isotherm line and the raffinate line. When this is done, and then a second two stage McCabe-Thiele diagram constructed as described above, a raffinate of about 0.15 g/l Cu and a loaded organic of 4.17 g/l Cu are predicted. The construction of a true equilibrium McCabe-Thiele diagram is an iterative process. With the example under discussion, the second step of iteration is all that is needed to produce a near equilibrium McCabe- Thiele diagram. This will not always be the case. The construction of an equilibrium McCabe-Thiele diagram depicting one stage of stripping is very simple (Figure 3.5). A horizontal line is drawn from the loaded organic line (3.90 g/l Cu) to the isotherm line at the value of the pregnant electrolyte (P.E.) desired, 51 g/l Cu in this case. Then a vertical line is dropped from the point where the P.E. line intersects the isotherm line to the horizontal axis. This gives the stripped organic (S.O.) to be expected (1.77 g/l Cu). Next, the strip electrolyte (S.E.) line, 30.7 g/l Cu, is drawn horizontally from the vertical axis of the graph to the L.O. line. A line is now drawn from the point of intersection of the S.O. and S.E. lines to the intersection of the loaded organic and pregnant electrolyte lines. This is the operating line. The slope of the operating line is equal to the ratio of the advance organic flow to the advance aqueous flow needed across the strip stages to obtain the desired pregnant strip solution. A two-stage McCabe-Thiele strip diagram, constructed as described above for extraction, predicts a stripped organic of 1.12 g/l Cu when building a pregnant electrolyte of 51 g/l Cu and operating at an O/A of 7.3/1. One of the important decisions the designer or builder of an SX plant must make is to decide on the staging requirements. The capital cost of a stage must be weighed against the benefits the stage provides.
CLASSIFICATION OF THE MAIN TYPES OF EXTRACTANTS There are five classes of metal extractants as characterized by structure, extraction mechanism and the metal species extracted: chelation, ion pairing, neutral or solvating, organic acids and ligand substitution. The five main classes of extractant compared are:
4.1.- CHELATING AGENTS 4.2.- ION PAIR EXTRACTANTS 4.3.- NEUTRAL OR SOLVATING EXTRACTANTS 4.4.- ORGANIC ACID EXTRACTANTS. 4.5.- LIGAND SUBSTITUTION EXTRACTANTS
Chelating agents Chelating refers to "claw", which is a graphic description of the way in which the organic extractant binds a metal ion. i.e., the extractant chemically bonds to the metal ion at two sites in a manner similar to holding an object between the ends of the thumb and the index finger. In many cases, upon bonding with the metal ion, the extractant releases a hydrogen ion into the aqueous solution from which the metal was extracted.
Table 4.1. summarise chelating agents properties
Ion pair extractants A second class of extractants is based on the principle of ion association whereby a large, positively charged organic moiety causes the extraction of an anionic metal complex into the organic phase, with concomitant expulsion of a common anion into the aqueous phase.
Table 4.2. summarise chelating agents properties
Neutral or solvating extractants#Neutral_or_solvating_extractants A third class of extractants are known as neutral or solvating type extractants. Extractants of this class are basic in nature and will coordinate to certain neutral metal complexes by replacing waters of hydration, thereby causing the resulting organo-metal complex to become organic soluble and aqueous insoluble. Solvating extractants have an atom capable of donating electron density to a metal in the formation of an adduct and are classified according to that ability: R3PO > (RO)3PO > R2CO > ROH > R2O trialkylphosphine oxides > trialkylphosphates > ketones > alcohols > ethers It takes little imagination to see that the above list is only a brief representation of the organic compounds that could function as solvating extractants. In general, extractions with solvating extractants are limited by: 1. the metal's ability to form neutral complexes with anions, 2. the co-extraction of acid at high acid concentrations, and 3. the solubility of the organometal complex in the organic carrier. An important extractant of this type is tri-noctylphosphine oxide, (C8H17)3PO, called TOPO. The extraction characteristics of TOPO with a wide variety of metals have been investigated. One important commercial application of TOPO in solvent extraction is its synergistic combination with di-2-ethylhexylphosphoric acid (DEHPA) for the extraction of uranium from wet process phosphoric acid. Other important extractants in this class are triutylphosphate (TBP), di-butyl butylphosphonate (DBBP), 2,2'-dibutoxy diethyl ether (di-butyl carbitol) and methyl isobutyl ketone (MIBK). Di-butyl carbitol and MIBK are used for the extraction of gold from acid chloride solutions.
Table 4.3. summarise chelating agents properties.
Organic acid extractants. A fourth class of extractant is the so-called acid extractants. The chemistry of this type of extractant has some characteristics which resemble chelating extractants and some which are similar to neutral or solvating extractants. Di-2-ethylhexyl phosphoric acid (DEHPA), is representative of this class of extractant.
Table 4.4. summarise chelating agents properties.
Ligand substitution extractants Ligand substitution extractants are similar to neutral or solvating extractants in that they donate an electron pair to a metal ion, but they are different in that these extractants form inner shell complexes with metals and, in doing so, will displace other ligands.
Table 4.5. summarise chelating agents properties
SOLVENT EXTRACTION SELECTIVITY The selectivity of a solvent extraction circuit for a given metal is important because it determines product purity, which is a key factor in determining the price, obtained for a given metal product. In addition impurities can affect downstream processing, for example in a typical electrowinning operation following solvent. Although the thermodynamic selectivity of the extractant for one metal over other competing metals is the single most important factor affecting the selectivity of a solvent extraction circuit it is not the only factor. The factors that have an influence on the selectivity of a solvent extraction circuit as opposed to the selectivity of an extractant are listed below:
The structure of the extractant molecule and the chemistry involved in the extraction process. This determines the thermodynamic selectivity.
The pH in extraction and stripping and the possibility of selective stripping The influence of the diluent on selectivity The aqueous chemistry of the PLS, in particular the relative concentrations of the competing metals present, the speciation of the metal complexes and the types of anion present.
Differences in the extraction kinetics of competing metals. The use of scrubbing circuits for the loaded organic The mechanical design of the equipment and the circuit layout employed. Entrainment and crud effects The strip chemistry, the possibility of selective stripping. The control of one or more of these factors will in many cases allow solvent extraction circuits using reagents having inherently low selectivity, such as ion pair extractants to achieve high selectivity.
INFLUENCE OF THE SOLVENT EXTRACTION DILUENT The diluent can influence the selectivity of a solvent extraction process through both chemical and entrainment effects. Diluents used in the solvent extraction of the major metals are usually either de-aromatized hydrocarbons containing less than 0.5% aromatics or diluents containing between 17 and 23% aromatic. Both types of diluent contain about 40% cyclo - paraffins. The selection of the diluent is usually made on the basis of flash point, viscosity and environmental parameters. However the diluent can influence the selectivity of the organic. Aromatic components in the diluent can have three main effects on the chemistry of solvent extraction. 1. To increase the solubility in the organic phase of the metal - extractant complex. 2. To act as an equilibrium modifier. 3. To influence the selectivity of the extractant. Where a contaminant is being chemically extracted then the presence of aromatics in the diluent may enhance the solubility of the metal - extractant complex in the organic phase with a consequent decrease in selectivity. The effects described above illustrate that while the choice of diluent can influence selectivity the mechanism involved can vary with the system being used and experimentation is usually required to predict the effect on selectivity of selecting a particular diluent.
INDUSTRIAL PROCESS DESIGN Typically an industrial process will use an extraction step in which solutes are transferred from the aqueous phase to the organic phase, this is often followed by a scrubbing stage in which unwanted solutes are removed from the organic phase, then a stripping stage in which the wanted solutes are removed from the organic phase. The organic phase may then be treated to make it ready for use again. After use the organic phase may be subjected to a cleaning step to remove any degradation products.
Solvent Extraction Process Solvent extraction, as applied to Hydrometallurgy, is a unit operation for the purification and concentration of a wide variety of metals. It consists of contacting an organic phase containing an extractant, with an aqueous phase containing the metal of interest. The
extractant chemically reacts with the metal to form an organic-metal complex that is soluble in the organic phase. Impurities normally do not react with the extractant and remain in the aqueous phase. The organic phase, containing the organic-metal complex, is separated from the aqueous phase. The metal is recovered and concentrated into another aqueous phase by reversing the chemical reaction. Solvent extraction was first applied to higher value metals, but now due to availability of new extractants with improved selectivity, faster kinetics and phase disengagement times, and recent developments in efficient equipment with less area and reagent inventory, the technology is now applicable to lower value metals. Extraction: The operation of transferring the metal of interest from the aqueous phase (SX Feed) to the organic phase. The extraction circuit produces a loaded organic containing the metal value and an aqueous phase depleted of the metal known as raffinate. The raffinate is sent for further treatment or effluent. Scrubbing: The selective removal of impurity metals from the loaded organic phase by treatment with either fresh scrub solution or a bleed of the strip liquor. The spent scrub solution is normally combined with the SX Feed. The scrubbed organic containing the metal of interest is advanced to stripping. Stripping: The process of removing the metal of value from the scrubbed organic phase by reversing the extraction chemical reaction. It is normally conducted under conditions in order to produce a strip liquor containing a high concentration of the metal value. The strip liquor is the product of the SX circuit. Regeneration: The treatment of the stripped organic phase for removal of metals that were not scrubbed or stripped, or for the removal of organic degradation products. The operation produces a regenerated organic phase for recycle to the extraction operation as organic feed. The spent regenerant is advanced for further processing or to effluent treatment.
Figure 7.1. A schematic representation of hydrometallurgical solvent extraction process.
Equipment While solvent extraction is often done on a small scale by synthetic lab chemists using a separating funnel it is normally done on the industrial scale using machines which bring the two liquid phases into contact with each other. Such machines include centrifugal contactors, spray columns, pulsed columns and mixer-settlers.
Mixer Settlers For the major metals recovered by solvent extraction the mixer-settler contactor design predominates. However there is a range of mixer settler designs available and in recent years there has been some attention refocused on the use of pulsed columns for plants using the kinetically fast ion- exchange extractants.
Well established with literally hundreds of operating units.
Design parameters are well established and very large units treating over 1000 cubic metres per hour of PLS can be designed from bench scale tests.
Excellent mixing characteristics with control of the optimum droplet size claimed to be possible with modern turbine designs.
Prediction of capital and operating costs is accurate.
The phases are readily accessible for sampling and examination in situ.
Several design varieties are available
Column Contactors Advantages claimed for the column contactor include:
Low area requirements
Multiple stages within one unit
Few moving parts
Low entrainment
Good vapour conservation
Column installations require piloting for each installation and the flooding conditions for the column must be determined. The long residence times in a column compared to a mixer settler can influence the selectivity of the extraction if contaminants have slow extraction kinetics.
O/A Ratio The O/A ratio of the phases in a mixer has a significant effect on entrainment as shown in Figures 7.2 and 7.3. With organic continuous dispersions, aqueous entrainment in the organic phase increases considerably at O/A ratios greater than 1.5:1. Organic entrainment in the aqueous phase is very low for organic continuous dispersions and is not dependent on the O/A ratio in the mixer. For aqueous continuous dispersions, organic entrainment in the aqueous phase increases sharply at O/A ratios of less than 1:1, and aqueous entrainment is lowest between O/A ratios of 1:1 to 2:1. Therefore, the optimum O/A for both organic and aqueous continuous dispersions is between 1:1 to 1.5:1. The O/A ratio of the phases in a mixer can be maintained between 1:1 to 1.5:1 by recycling either the organic or aqueous phase from the settler to its mixer. Another important reason for maintaining the optimum O/A ratio is to improve the mass transfer rate and stage efficiency. At the optimum O/A ratio the rate of coalescence and redispersion of the dispersed phase is enhanced.
Figure 7.2. Effect of O/A ratio on Aqueous entrainment.
Figure 7.3. Effect of O/A ratio on Organic entrainment.
Phase Continuity A relationship exists between entrainment and phase continuity. Under organic continuous dispersions, in which the aqueous phase is dispersed as droplets in the organic phase, aqueous entrainment in the organic phase is common. Organic continuous dispersions usually produce an aqueous phase low in organic entrainment. Therefore, in order to produce a strip liquor and raffinate low in organic entrainment, it is recommended to operate the first stage of stripping and the last stage of extraction with organic continuous dispersions in the mixers. Aqueous continuous dispersions, in which the organic phase is dispersed as droplets in the aqueous phase, can produce an aqueous phase with organic entrainment and an organic phase low in aqueous entrainment. Therefore, it is recommended to operate the mixer in the last stage of stripping and the first stage of extraction under aqueous continuous conditions in order to minimize aqueous entrainment in the organic phase. This relationship between O/A ratio and phase continuity on entrainment can be used to improve SX plant operation.
De-entrainment of Organic and Aqueous Flows Entrainment of organic in aqueous and aqueous in organic can create significant problems in both SX circuits and electrowinning. The main methods used for de -entrainment of SX streams are Coalescers in the Extraction Circuit. These usually employ some form of packed bed or porous media and may not work well when there is significant crud present in the system. Examples of coalescers include:
PICKET FENCES IN THE SETTLER
Besides distributing a uniform flow in the settler, these can bank up the emulsion band on the upstream side and this band acts as a coalescer.
PACKED BED COALESCERS IN THE SETTLER
These can reduce entrainments but can also blind and cause channel flow around the edges of the packed bed or through nonblinded sections of the coalescer. For this reason, provision must be made to periodically remove the coalescers and to clean them.
COALESCING MATERIAL IN THE LOADED ORGANIC SURGE TANK
This can help reduce aqueous entrainment to the tankhouse. Provision must be made to remove and clean the coalescing material. This can be easier to carry out than cleaning of the coalescers from the settler.
COALESCERS EMPLOYING MULTI-MEDIA
Woven coalescers using an aqueous wetted fibre (stainless steel) and an organic wetted plastic fibre have given good results in pilot plants but blockage by crud can be a problem in larger installations.
PACKED BED RAFFINATE COALESCERS FOR ORGANIC RECOVERY
A coarse packed bed of 1 - 2 cm sized coke or silica particles is used in an upflow mode. Provision is made to backwash with air and water. The top of the coalescer has a "thickener froth ring" type of launder and coalesced organic remains inside this ring and is removed from time to time.
ORGANIC RECOVERY FROM AN AFTER SETTLER AHEAD OF THE RAFFINATE POND OR FROM THE RAFFINATE POND ITSELF
The after settler can be a purpose built unit such as a multiplate coalescer or simply a small raffinate pond. For small pond and for raffinate pond recovery, floating booms are used to "corral" the organic prior to recovery from the top of the aqueous.
SOLVENT EXTRACTION OF METALS The practice of hydrometallurgy contains examples of a great number of diverse solvent extraction processes. Besides the major metals there are or have been commercial solvent extraction processes operated for the recovery of metals such as tungsten, rare earths, thorium and vanadium.
Solvent extraction of metals such as copper, uranium, cobalt and nickel, besides being of great economic significance has been the spur for the development of the engineering aspects of solvent extraction. Let us briefly examine the metals for which solvent extraction has succeeded and the circumstances, which caused these metals to become candidates for recovery by solvent extraction. They have some common features: 1. They are soluble in suitable lixiviants such as sulphuric acid, ammonia and cyanide. 2. They are relatively valuable. Have a reasonable market price or have environmental properties that financially support their recovery. 3. They can be recovered from the concentrated strip solution in a suitable marketable or intermediate form by processes such as electrowinning or precipitation. 4. Suitable solvent extraction chemistry has been developed for these metals.
Uranium The first metal to be recovered in significant quantities using solvent extraction was URANIUM. Following the development of the nuclear industry during and immediately after World War II, attention was focussed on developing technologies which could be used to upgrade and purify uranium from low grade ores and in 1957 the first commercial solvent extraction plant using amines was opened in the USA. Today most of the world's uranium is recovered in hydrometallurgical circuits which involve solvent extraction and a significant proportion of this uranium is produced in circuits which use solvent extraction as the only recovery system.
Extraction and Stripping Chemistry The vast majority of uranium is recovered from sulphuric acid leach solutions using C8-C10 tertiary amines. When uranium is dissolved in sulphuric acid two anionic sulphate complexes are formed: UO22+ + 2SO42- ( UO2(SO4)22UO2(SO4)22- + SO42- ( UO2(SO4)34Uranium leaching normally involves an aggressive, oxidising agitation leach at elevated temperatures of around 40 - 80ยบC and such a leach is not selective for uranium. As a result the SX feed will contain a number of anion species which can cause problems in the solvent extraction of uranium.
Extractant and diluent Type Tri-octyl and tri-decyl amines are used almost exclusively for uranium extraction however trilaurylamines are used when molybdenum is present as the amine molybdenate complex formed with the C8 and C10 amines may not be sufficiently organic soluble. In practice the theoretical maximum loading is not attained due to the presence of competing anions in the leach liquor. Ion exchange extractants are non-selective and, although the uranyl sulphate anion is very strongly extracted by tertiary amines, other anions will also be extracted. The uranium - amine complex formed from the C8-C10 tertiary amines has limited organic solubility in the 1-20% aromatic diluents typically used and third phase inhibitors are added to the circuit organic to improve this solubility. Isodecanol at a concentration of about 50% of the amine concentration is the most commonly used modifier although high aromatic diluents can also be used. The use of isodecanol can increase phase separation times and increase crud formation. Isodecanol is also a nutrient for bacteria and can lead to bacterial activity in the circuit. The solubility of isodecanol in water is about 150-170 ppm compared to 5ppm for tertiary amine and isodecanol losses to the raffinate will be similar to the amine losses.
Uranium Circuit Layout The circuit configuration of any SX plant is dictated by the extraction and stripping chemistry. A typical acid leach - SX uranium circuit is shown in Figure 8.1. Let us consider some of the important features of this circuit. . Clarification Uranium circuits normally use an agitation leach. Clarification to remove suspended solids may involve special equipment such as hopper clarifiers. . Soluble silica removal. The leach is sufficiently aggressive to leach significant quantities of silica and this can cause crud formation and phase separation problems if allowed to go through to SX. High molecular weight polyethylene oxides or similar chemicals with significant hydrogen bonding capability are used, often in conjunction with floc bed clarifiers to precipitate soluble silica.
. Extraction Uranium circuits usually aim at recoveries in excess of 95% and four stages of extraction is considered the minimum. The function of E4 may be more amine protonation than uranium recovery. Competing anions may load in E3 and E4 but are crowded off in E1. . Scrubbing Ion exchange is not a selective process and scrubbing to remove impurities such as iron, silica, and zirconates is required. . Stripping Most circuits employ a deprotonation scrub using NH3 plus ADU precipitation spent liquor as the strip aqueous. A carefully controlled pH profile is required in strip if ADU precipitation is to be avoided. The pH decreases from about 5.0 in S4 to 3.0 in S1. . ADU precipitation Ammonia is used to precipitate ADU. The temperature must be maintained about 30oC if the sulphate content and the particle size of the precipitate are to be optimum.
Copper Today around 25% of the world's copper is recovered using solvent extraction and solvent extraction is considered to be the lowest cost production route for the production of quality cathode. The scope of solvent extraction for copper is only limited by the availability of acid leachable ore and it is not surprising that considerable attention is being directed towards development of suitable leaching techniques for chalcopyrite, the most ubiquitous of all copper minerals. The use of solvent extraction for copper ores dates from the Ranchers Bluebird mine which started operation in 1968. Acceptance of solvent extraction technology for copper using Henkel's (previously General Mills Chemicals Inc) oxime based extractants took a giant step forward in 1974 when ZCCM commissioned their 80,000 tonne per annum SX-EW plant at Nchanga.
Extraction Chemistry Copper extractants for acid leach solutions are exclusively oximes. For extraction from ammoniacal solutions beta diketones may be used. The chemistry of oxime extraction of copper is relatively simple:
2RH(org) + Cu2+ + SO42- ( R2Cu(org) + 2H+ + SO42There are other reactions which take place in copper extraction which can influence extraction and stripping such as dissociation of H2SO4 or dimerisation of the oxime.
Extractant Types Oxime based extractants for copper are largely based on salicyaldoximes which have been modified with one of three modifier types. Examples of the three main extractant types currently in use are: 1. A mixture of oxime and aldoxime in a high flash diluent. The acetophenone oxime modifies the aldoxime and also performs as an extractant in its own right.An example of this type of extractant is LIX(r) 984N 2. An aldoxime modified with an ester in a high flash diluent. An example is Acorga(r) M5640. 3. An aldoxime modified with tridecyl alcohol in a high flash diluent. The LIX(r) 622N is an example of this extractants. Each of the extractants marketed by the major chemical suppliers has been designed for a specific type of PLS with regard to pH and copper tenor. Used under the conditions for which they were designed they all deliver very similar copper net transfer values. The vol% concentration of the commercially available extractants is limited by organic viscosity constraints to about 30-33% and this means that the maximum net transfer of copper will be about 10g/l. For leach solutions containing significantly higher copper tenors than this the throughput O/A ratio will have to be increased above 1.0. Typical copper and acid concentrations for an SX plant treating a dump leach solution of 3.0g/l and pH1.8 are shown in the following figure:
Copper solvent extraction plant configurations A variety of circuit configurations are used:
2E X 1S(The usual circuit for heap leach plants)
2E X 2S(Used where the copper tenor is above about 7-10 g/l or the PLS pH is less than about 1.2. The decision to use a second strip stage is influenced by the life of the project and the sensitivity of the circuit to the copper tenor of the raffinate. Agitation leach plants for example are sensitive to the loss of copper in a raffinate bleed or in the wash liquor used in the solid liquid separation stages.
3E X 2S(These can be justified when the PLS has a very high copper tenor, say above 20-25g/l and a high 93% plus copper recovery is desired. It is possible to use a 2EX2S circuit under these conditions but the throughput O/A ratios will be high, in excess of 2.5:1 and the capital cost of the 2EX2S plant may be higher than that of the 3EX2S plant. The 3EX2S plant will also be much more flexible in operation).
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2E X 1W X 1S(The wash stage is used to remove entrained impurities, usually chloride, and entrained and chemically loaded iron).
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2E X 1P X 1S(Series parallel circuits are used to treat high volumes of low tenor leach solutions. The parallel stage is often retrofitted to maintain copper production when the copper tenor of the PLS falls below project design and there is also the possibility to increase the volume of the PLS flow. Extractant concentrations are higher and copper recoveries are lower in series parallel circuits than in series circuits).
Nickel In comparison to copper and uranium the percentage of the world's nickel which is recovered using SX is relatively small, however recent developments may well change this situation. Unlike uranium and copper extraction where one type of extractant and circuit predominates there are a number of potential nickel extractants and circuit configurations. Sulphide nickel is usually treated using pyrometallurgical routes but in recent years there has been intensive activity in the development of hydrometallurgical routes for both sulphide concentrates and laterites. Nickel deposits can contain valuable quantities of cobalt and copper and these must also be recovered by SX if they are present in sufficient quantity. 1. The base metals (Ni, Cu, Co,Zn) may be precipitated as hydroxides from sulphate leach solutions , redissolved in ammonia, the cobalt may be oxidised to Co(3) and the copper and nickel co-extracted with ketoxime (LIX 84-I ). This is the process most commonly used for treating Ni Laterite.
Figure 8.2. Nickel Laterite Treatment.
2. Direct solvent extraction of copper, cobalt and nickel from acid leach solutions using oximes, phosphinic acids and versatic acids to extract copper, cobalt and nickel in sequence. While this circuit may function on leach solutions derived from sulphide concentrates, laterite leach solutions contain significant manganese and magnesium, both of which are extracted by phosphinic acid extractants. In addition the aqueous solubility of versatic acid at the pH used for extraction necessitates the inclusion of a versatic acid recovery stage.
Figure 8.3. Nickel Recovery from Acid PLS.
3. Matte leach chloride solutions may be purified by iron extraction with TBP followed by cobalt and copper co extraction as chloride complexes with tertiary amine. Nickel does not form chloride complexes and remains in the raffinate. It may be recovered by crystallization and hydrogen reduction.
Figure 8.4. Nickel-Cobalt Separation using Chloride Leach Solutions and tertiary A
Precious Metals Although the quantity of precious metals currently recovered using circuits that involve solvent extraction is small the value of these metals is significant. A gold refining process has been recently developed based on solvent extraction and there exists a potential for gold recovery by solvent extraction. In the refining of platinum group elements solvent extraction plays an important role. A variety of solvent extraction processes are used in the refining of the platinum group elements. Phosphinic acids have been used to remove cobalt from nickel in sulphate leach solutions. The PGE's form a wide range of chloride complexes and this makes possible the separation of these elements using ion-exchange extractants. The majority of these separation systems are not widely discussed in the literature. Gold as the aurocyanide complex can be extracted at pH values below 9.0 by tertiary amines and over a wide pH band by quaternary amines. The relatively low pH required for tertiary amine separation makes this an unattractive route for most circuits while the quaternary amines can only be stripped in a two stage process using zinc tetracyanide stripping followed by sulphuric acid regeneration of the organic and HCN recovery.
AUTHORS / CONTRIBUTORS Maria Frades Francisco Sรกnchez
Hydrometallurgy & Electrochemistry c/ Sierrra Nevada, 16 - P. Ind. San Fernando II 28830 San Fernando de Henares (Madrid)
ACKNOWLEDGEMENT The authors gratefully acknowledge the EU commission for funding this work within the Sixth Framework Programme. Project: NMP2-CT-2005-500329 Acronym: BioMinE Project Title: Biotechnology for Metal bearing materials in Europe Instrument: Integrated Project Thematic Priority: Priority 3 - NMP Nanotechnologies and nanosciences, knowledge-based multifunctional materials, and new production processes and devices