Mixer-settler stage

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. 

The raffinate passes to a second extraction stage where it is contacted with a second stream of pure MIBK, leading to the transfer of more acetone. The two phases are allowed to separate in a second settler, and the raffinate from this stage is discarded. The extracts from the two mixer-settler stages are combined and fed to a distillation column. The overhead effluent is rich in acetone and is the... 

Mixer-settlers are simply tanks configured to permit mixing in one followed by decantation in a second. Each stage consists of a mixer and settler pair (i.e., two tanks). When several mixer-settler stages are configured countercurrently, they constitute a cascade as indicated in Figure 10.4. 

Organic extract is stripped of uranium with 23 liters/min of an aqueous ammonium sulfate solution in four additional countercurrent mixer-settler stages, to produce an aqueous product containing about 39 g UgOgAiter. To drive uranium into the aqueous product, pH in the first mixer stage is brought to 4.1 to 4.3 by addition of ammonia gas to react with most of the 9 g H2SO4/liter carried by the extract. 

Savannah River [P7]. At the Savannah River Purex plant, neptunium in irradiated natural uranium was recovered by the alternative method of forcing most of it into the aqueous waste stream HAW from the first extraction cycle and then recovering it from waste directly by anion exchange. Neptunium in the first extraction step was converted mostly to the inextractable pentavalent state by adding sufficient nitrite to the next-to-the-last mixer-settler stage of the HA section to make the solvent 0.007 M in HNOj. 

Fig. 2.3.4-1 Extraction with downstream separation units (schematic) (Schierbaum 1997]. Ml mixer-settler stage Kl rectification column, separation of solvent from product K2 stripper, separation of solvent from raffinate.

When a series of mixer-settler stages are connected to form a counter-current extraction plant, means must be provided to allow the two liquids to flow in opposite directions. External pumps have been used between stages, but this is rarely considered necessary. It is more usual either to utilize the pumping action of the mixing device to assist flow, or simply to rely upon the difference in densities of the two phases. 

Fig, 4.21. Airlift mixer-settler stage (a) end view (b) side view (c) types of airlift foot (U.K.A.E.A. copyright). 

Fig. 4.22. Airlift mixer-settler stage with long mixing residence time. (Jamrack,...

An amine process has been devised for the simultaneous extraction of uranium and thorium from the ore in the Bancroft area of Ontario in Canada. A sulphate leach liquor is extracted in four mixer-settler stages, with S per cent tri-iso-octyl amine and 2 per cent n-decyl alcohol in kerosene, to remove uranium. Thorium is then extracted from the raffinate by means of 5 per cent di-tridecyl amine in another four-stage mixer-settler. Uranium... 

Separation of each element by solvent extraction is carried out in multistage batteries of mixer-settlers for each rare earth element. A minimum of 50 mixer-settler stages per stream is required to obtain a product with a purity of 4 or 5 nines. For economic and technical reasons the ion-exchange method is only used for laboratory and pilot scale production to produce small toimage of rare earths. 

Boron extraction with four parts of 20% isooctanol in kerosene to one part of brine, and four mixer-settler stages. 

With -butanol the Uthium extraction was the same at pH values from 1 to 11, although pH > 8 reduced the calcium extraction. The optimum ratio of solvent to brine was 1/1, among the ratios of 1/5 to 3.2/1 that were tested. As high as a 90% lithium recovery was obtained in four mixer-settler stages from solutions containing from 30-300 g/1 Uthium. 

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