Precious Metals Refining By Solvent Extraction

General
If a compound (= solute) is dissolved in a liquid (water) and the solution is brought into contact with a second immiscible liquid (= solvent, maybe a hydrocarbon etc.), then a part of the solute is transferred to the second liquid phase by a force called the chemical potential. The physical-chemical process of transferring the solute between the bulk of the two immiscible liquid phases is called solvent extraction.

During intensive mixing of both liquids in the course of time the mass transfer of the solute between the two liquid phases deminishes and finally at very long contact times vanishes; an equilibrium is encountered. The time necessary to reach 90% of the equilibrium is characteristic for a given solute/solvent system and is in the range of seconds up to hours for technically relevant systems. This time is a function of the product k_t a ; (mass transfer coefficient) x (interface area/liquid volume).
In technical equipment equilibrium is never reached to a hundred percent; 90 to 99% is considered to be sufficient.

Commonly for practical process layout the equilibrium is characterized by a distribution coefficient which is defined as the ratio D = (all species of solute in organic phase)/(all species of solute in aqueous phase). D is dependent on the initial concentration of the solute and the concentration of other reaction components in question. The distribution coefficient is  independent of phase flow conditions and somewhat characteristic for a given solute/solvent/aqueous system. It is one of the key parameters in the design of a solvent extraction process.
 

The basic equipment to perform a continuous mixing and coalescence process on a technical scale is called a mixer-settler.
It is often used in laboratories as an ideal tool for basic system design of continuous solvent extraction processes because it offers reproducable phase flow and contact times. The device comprises a continuously fed and stirred mixing compartment (see sketch below) and a gravity settler compartment where the liquids are allowed to separate. At the end of the settler two individual weirs care for good separation of the liquid bulk phases. The flow capacity of a mixer-settler is reached when the emulsion phase dispersion band overflows the light phase weir or underflows the heavy phase weir.

In technical mixer-settler devices the volume of the settler compartment is often 10-fold the size of the mixer compartment to provide sufficient settling time even for systems with low coalescence rates (in case of high viscosity and/or low density and/or low interfacial tension).

For a good performance of the settler the liquid phase ratio organic/aqueous in a mixer-settler should be kept close to one.
To adjust a definite feed ratio of organic/aqueous- independent of optimum internal phase ratio - an external or internal phase recycle can be installed. In the sketch of a conventional mixer-settler below a feed ratio of  O/A = 1/10 is shown.
Hence such a process would concentrate a solute in the loaded organic by a factor of 10 if the distribution coefficient is high enough.
 

In nearly all practical cases it is necessary to have more than one mixer-settler unit. Repeated mixing and settling can provide a high concentration and purity of the solute especially if the distributuion coefficient is low or even close to one. In multistage solvent extraction mixer-settler batteries often are built as contiguous boxes which can save a lot of space.

In applications where a very high number of stages is required theoretically and in large scale industrial application of more than 100 m3/h solvent extraction is preferably run in continuous countercurrent column equipment (reciprocating plate, rotating disc, stirred column). 

In column extractors there are no longer discret compartments where mixing and settling of the liquids occurs but new formation of droplet with "fresh interface" and coalescence of droplets prevails allmost in each section of the column in the same extent.
 

The control of the hydraulic stability of solvent extraction columns is much more difficult to maintain than in  mixer-settlers.

In some cases if solvent extraction is governed by an irreversible chemical reaction of the solute with other components in the solvent the process can be performed in a stirred tank reactor as a batch mixer-settler process.

Please note that the feed for a solvent extraction process containing the solute in question is not necessarily always aqueous. There are lots of examples in industrial organic synthesis where crude organic product solutions have to be treated with aqueous solutions or water for purifying. Moreover the solute is not always the valuable target component in a solution, it might just be an impurity which has to be removed.
 

Reactive Extraction
In cases if solvent extraction is enabled by a chemical reaction (at the interphase of aqueous/organic or in one or both of the bulk phases) or if a prevailing chemical reaction is accelerating the basic physical mass tranfer, this process is called reactive extraction. The item was first used and introduced into literature in 1981 by Werner Halwachs in his publication
"Reaktivextraktion" (Habilitationsschrift, Universität Hannover, Fachbereich Chemie, 1981).
 

Metal Extraction
As far as solvent extraction of metals or precious metals is concerned an additional chemical, a so-called extractant, is necessary to form a chemical compound (= complex) with the solute and thus make it soluble in the organic phase.
Furthermore additional chemicals, so-called modifiers, are used to enhance coalescence of the emulsion and increase the solubility of metal complexes formed in the solvent to suppress formation of solids which may lead to a total collapse of the process by clogging..

Solvent extraction of metals or precious metals is used in refineries for

1. the removal of individual precious metals or group of metals from a pregnant liquor comprising other pgm, base metals and /or salts (= separation).

2. the removal/reduction of impurities from crude solutions of individual precious metals or a group of metals (= refining)

In most cases a complete industrial solvent extraction cycle for metal extraction comprises four consecutive solvent extraction steps in order to win concentrated or purified solute and recover solvent and extractant - otherwise the process would be uneconomic and environmentally hazardous; those subprocesses are:

1. extraction; transfer of the metal into the organic phase by chemical reaction with the extractant

2. scrubbing; removal of coextracted material/metals or excess acid etc. (optional)

3. stripping; transfer of the metal back into a second pure aqueous phase for winning or further processing

4. solvent make-up; treatment of the organic by a third aqueous phase for purification of solvent or extractant; removal of crud or degradation products; topping with fresh organic (optional)

In large scale industrial solvent extraction process where raffinates or barren liquids are drained to effluent treatment plants, the solubilities of solvents and extractants in the aqueous liquids are of superior importance; during solvent extraction system design in the lab phase extractant/solvent combinations must be selected comprising as low aqueous solubility as possible; otherwise those chemicals might be harmfull to the environment.
 
 
 

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