Extraction settlers

Vancas, M. F. 2003. Solvent extraction settlers a comparison of various designs. In Proceedings copper 2003 hydrometallurgy of copper modelling, impurity control and solvent extraction, vol. 6, book 2, eds. P. A. Rivieros, D. G. Dixon, D. B. Dreisinger, and J. H. Menacho, 707- 718. Montreal Canadian Institute of Mining, Metallurgy and Petroleum. 

The earliest large-scale continuous industrial extraction equipment consisted of mixer—settlers and open-spray columns. The vertical stacking of a series of mixer—settlers was a feature of a patented column in 1935 (96) in which countercurrent flow occurred because of density difference between the phases, avoiding the necessity for interstage pumping. This was a precursor of the agitated column contactors which have been developed and commercialized since the late 1940s. There are several texts (1,2,6,97—98) and reviews (99—100) available that describe the various types of extractors. 

The General Mills mixer—settler (117), shown in Figure 13b, is a pump—mix unit designed for hydrometaHurgical extraction. It has a baffled cylindrical mixer fitted in the base and a turbine that mixes and pumps the incoming Hquids. The dispersion leaves from the top of the mixer and flows into a shallow rectangular settler designed for minimum holdup. 

In the Davy-Powergas unit (118—120), shown in Figure 13c, the Hquids mn through a draft tube and are pumped by an impeller mnning directly above the draft tube. The dispersion flows out from the top of the mixer and down through a channel into a rectangular settler. Large units of this type are used for copper extraction (7). 

The development of the novel Davy-McKee combined mixer—settler (CMS) has been described (121). It consists of a single vessel (Fig. 13d) in which three 2ones coexist under operating conditions. A detailed description of units used for uranium recovery has been reported (122), and the units have also been studied at the laboratory scale (123). AppHcation of the Davy combined mixer electrostatically assisted settler (CMAS) to copper stripping from an organic solvent extraction solution has been reported (124). 

Other Metals. Because of the large number of chemical extractants available, virtually any metal can be extracted from its aqueous solution. In many cases extraction has been developed to form part of a viable process (275). A review of more recent developments in metal extraction including those for precious metals and rare earths is also available (262). In China a complex extraction process employing a cascade of 600 mixer—settlers has been developed to treat leach Hquor containing a mixture of rare earths (131). 

However, such mixer—settler methods are continuous only by virtue of repeating a sequence of similar stages to achieve a given degree of extraction. More fully continuous methods of extraction were designed as tower systems and later as screw conveyor systems as effective methods of soHds transport became reHable. 

In addition, solvent extraction is appHed to the processing of other metals for the nuclear industry and to the reprocessing of spent fuels (see Nuclearreactors). It is commercially used for the cobalt—nickel separation prior to electrowinning in chloride electrolyte. Both extraction columns and mixer-settlers are in use. 

Modem commercial wet-acid purification processes (see Fig. 4) are based on solvents such as C to Cg alcohols, ethers, ketones, amines, and phosphate esters (10—12). Organic-phase extraction of phosphoric acid is accompHshed in one or more extraction columns or, less frequently, in a series of countercurrent mixer—settlers. Generally, 60—75% of the feed acid P2 s content is extracted into the organic phase as H PO. The residual phosphoric acid phase (raffinate), containing 25—40% of the original P2O5 value, is typically used for fertilizer manufacture such as triple superphosphate. For this reason, wet-acid purification units are almost always located within or next to fertilizer complexes. 

For solvent extraction of pentavalent vanadium as a decavanadate anion, the leach solution is acidified to ca pH 3 by addition of sulfuric acid. Vanadium is extracted in about four countercurrent mixer—settler stages by a 3—5 wt % solution of a tertiary alkyl amine in kerosene. The organic solvent is stripped by a soda-ash or ammonium hydroxide solution, and addition of ammoniacal salts to the rich vanadium strip Hquor yields ammonium metavanadate. A small part of the metavanadate is marketed in that form and some is decomposed at a carefully controlled low temperature to make air-dried or fine granular pentoxide, but most is converted to fused pentoxide by thermal decomposition at ca 450°C, melting at 900°C, then chilling and flaking. 

For solvent extraction of a tetravalent vanadium oxyvanadium cation, the leach solution is acidified to ca pH 1.6—2.0 by addition of sulfuric acid, and the redox potential is adjusted to —250 mV by heating and reaction with iron powder. Vanadium is extracted from the blue solution in ca six countercurrent mixer—settler stages by a kerosene solution of 5—6 wt % di-2-ethyIhexyl phosphoric acid (EHPA) and 3 wt % tributyl phosphate (TBP). The organic solvent is stripped by a 15 wt % sulfuric acid solution. The rich strip Hquor containing ca 50—65 g V20 /L is oxidized batchwise initially at pH 0.3 by addition of sodium chlorate then it is heated to 70°C and agitated during the addition of NH to raise the pH to 0.6. Vanadium pentoxide of 98—99% grade precipitates, is removed by filtration, and then is fused and flaked. 

For vanadium solvent extraction, Hon powder can be added to reduce pentavalent vanadium to quadrivalent and trivalent Hon to divalent at a redox potential of —150 mV. The pH is adjusted to 2 by addition of NH, and an oxyvanadium cation is extracted in four countercurrent stages of mixer—settlers by a diesel oil solution of EHPA. Vanadium is stripped from the organic solvent with a 15 wt % sulfuric acid solution in four countercurrent stages. Addition of NH, steam, and sodium chlorate to the strip Hquor results in the precipitation of vanadium oxides, which are filtered, dried, fused, and flaked (22). Vanadium can also be extracted from oxidized uranium raffinate by solvent extraction with a tertiary amine, and ammonium metavanadate is produced from the soda-ash strip Hquor. Fused and flaked pentoxide is made from the ammonium metavanadate (23). 

A similar process has been devised by the U.S. Bureau of Mines (8) for extraction of nickel and cobalt from United States laterites. The reduction temperature is lowered to 525°C and the hoi ding time for the reaction is 15 minutes. An ammoniacal leach is also employed, but oxidation is controlled, resulting in high extraction of nickel and cobalt into solution. Mixers and settlers are added to separate and concentrate the metals in solution. Organic strippers are used to selectively remove the metals from the solution. The metals are then removed from the strippers. In the case of cobalt, spent cobalt electrolyte is used to separate the metal-containing solution and the stripper. MetaUic cobalt is then recovered by electrolysis from the solution. Using this method, 92.7 wt % nickel and 91.4 wt % cobalt have been economically extracted from domestic laterites containing 0.73 wt % nickel and 0.2 wt % cobalt (8). 

For extraction, the mixing usually takes place either in a vessel which also serves as the settler (these can be baffled or unbaffled), or a separate mixing compartment (usually baffled if there is a gas-hquid interface, and usually unbaffled if it is liquid filled). 

FIG. 15-29 Kerr-McGee multistage mixer-settler, a) and (h) For uranium, (c) For vanadium extraction. 

Mixer-Settler Equipment The equipment for extraction or chemical reaction may be classified as follows ... 

Liquid-Liquid Extraction The actual configuration of mixers in multistage mixer-settlers and/or multistage columns is summarized in Section 15. A general handbook on this subject is Handbook of Solvent Extraction by Lowe, Beard, and Hanson. This handbook gives a comprehensive review of this entire operation as well. 

Equipment suitable for reactions between hquids is represented in Fig. 23-37. Almost invariably, one of the phases is aqueous with reactants distributed between phases for instance, NaOH in water at the start and an ester in the organic phase. Such reac tions can be carried out in any kind of equipment that is suitable for physical extraction, including mixer-settlers and towers of various kinds-, empty or packed, still or agitated, either phase dispersed, provided that adequate heat transfer can be incorporated. Mechanically agitated tanks are favored because the interfacial area can be made large, as much as 100 times that of spray towers, for instance. Power requirements for L/L mixing are normally about 5 hp/1,000 gal and tip speeds of turbine-type impellers are 4.6 to 6.1 i7i/s (15 to 20 ft/s). 

FIG. 23-38 Efficiency and capacity range of small-diameter extractors, 50 to 150 mm diameter. Acetone extracted from water with toluene as the disperse phase, V /V = 1.5. Code AC = agitated cell PPC = pulsed packed column PST = pulsed sieve tray RDC = rotating disk contactor PC = packed column MS = mixer-settler ST = sieve tray. (Stichlmair, Chem. Ing. Tech. 52(3), 253-255 [1980]). 

Extraction (sometimes called leaching) encompasses liquid-liquid as well as liquid-solid systems. Liquid-liquid extraction involves the transfer of solutes from one liquid phase into another liquid solvent it is normally conducted in mixer settlers, plate and agitated-tower contacting equipment, or packed or spray towers. Liquid-solid extraction, in which a liquid solvent is passed over a solid phase to remove some solute, is carried out in fixed-bed, moving-bed, or agitated-solid columns. 

In the extraction process, the LPG from the prewash tower enters the bottom of an extractor column. The extractor is a liquid/liquid contactor in which the LPG is counter-currently contacted by a caustic solution. Another option is the use of a fiber film contacting device. The mercaptans dissolve in the caustic (Equation 1-14). The treated LPG leaves the top of the extractor and goes on to a settler, where entrained caustic is separated. 

Each cell in the extraction system presented in Fig. 122 is called a mixer-settler extractor and is made up of two parts. The role of the first part, the mixer, is to emulsify the incoming aqueous and organic phases and to transfer the emulsion to the second part of the extractor-settler cell. The purpose of the settler is to stratify the phases and enable the separation of the two liquids. 

Fig. 123. Schematic structure of a mixer-settler extraction cell.

A small extraction system, consisting of a total of 18 cells, was used for laboratory evaluation of the process. The volumes of the mixer and settler were 10 ml and 30 ml, respectively. Production capacity of the system was 200-300 ml per hour. The cells were divided among the functions as follows extraction - 8 cells, washing - 3 cells, stripping - 8 cells. Several experiments were performed using the laboratory system. 

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