Backwash extractor

The complex carbonate is soluble and consequently does not precipitate in the backwash extractor. Other alkalis have been used which form precipitates, e.g. backwashing can be carried out with a slurry of magnesium oxide, i.e. 

Design data for a Dapex plant to handle 200 tons of ore per day have been quoted. The leach liquor at 100,000 gal/day would pass, with solvent, through mixers each of 350 gal capacity and fitted with 10 h.p. drives. Settlers would be about 12 ft in diameter. In the backwash extractor, mixer capacities would be 50 gal each, with 1-5 h.p. drives, and settlers 5 ft in diameter. 

A continuous nitrate/fluoride process has been proposed to take advantage of the ready solubility of columbite ore in a mixture of nitric and hydrofluoric acids, and thus allow the feed solution to be prepared in a cheap and simple manner. The feed contains 45 g/1. of niobium, about 25 g/1. of tantalum and is 8N in hydrofluoric acid and 3N in nitric acid. It is extracted in six stages with three volumes of solvent, together with an additional stage for acid equilibration . A half volume aqueous strip solution is also 8N in hydrofluoric acid and 3N in nitric acid. About 99 per cent of the niobium is extracted into the solvent and is then backwashed in a second extractor with three volumes of 0-5N hydrofluoric acid. The solvent finally passes to a single-stage backwash extractor for removal of tantalum by sodium carbonate solution. 

The solvent which has been almost completely denuded of solute may be suitable for recycling directly back to the first extractor, for re-extraction of more solute. Alternatively, it may require treatment before re-use, e.g. an alkaline wash to remove impurities which have accumulated during its previous contact with acid aqueous phases. It is advisable for any treatment process to be continuous, as are the extraction and backwash processes, to preserve the continuity of operations. The cost of solvent can be an important item of process economics unless it is efficiently recovered. Systems are therefore chosen with low solubilities of solvent in the aqueous phases, and care should be taken to avoid entrainment of solvent, general spillages, etc. 

In this case it is advisable to carry out backwashing of the solvent extract either batchwise or in a suitable extractor, such as a pulsed column, which will handle slurries satisfactorily. 

Extraction of uranium takes place in nine forward extraction stages and five stripping stages. The object of the stripping section is to minimize the extraction of thorium, and IN nitric acid is used. The ratio of feed solvent strip flow rates is 1 1 0-2. The second extractor backwashes the uranium into an equal volume of 0 02N nitric acid, in five stages, and the solvent is recycled back to the first extractor. The uranium-to-thorium ratio is reduced from 5 per cent to less than 1 ppm and the loss of thorium in this cycle is only a fraction of one per cent. 

It is considered that the process could be modified without difficulty to a system based upon a single solvent, e.g. 30 per cent TBP/xylene, with appropriate changes in volumetric flow rates. Alternatively, it might be possible to extract both the uranium and the thorium together in the first extractor and then to backwash first the thorium in extractor 2, and then the uranium in a third extractor. This would save one extractor but it would be necessary to include a solvent stripping section in extractor 2 to prevent... 

An alternative American process uses a feed produced by oxalate precipitation of thorium and rare earths. This precipitate is calcined to the oxides and dissolved in nitric acid for extraction with undiluted TBP. After stripping with 8N nitric acid, a high proportion of cerium extracts with the thorium, but the other rare earths are eliminated. The cerium is then back-washed in a separate extractor by means of 0 1 N sodium nitrite solution, which reduces it to the solvent-insoluble cerous condition. Thorium is then backwashed in the last extractor with either water or 2 per cent sulphuric acid. In order to make this process economic it was necessary to devise an efficient system of oxalic acid recovery. This was based upon treatment of the thorium and rare earth oxalates with sodium hydroxide and recycling the resulting sodium oxalate to the precipitation stage. 

The American Battelle Memorial Institute TBP extraction process resembles the U.K. process a little more closely. In this, a feed solution made by the alkali breakdown route is extracted into a solvent mixture of TBP and solvent naphtha. Thorium is selectively backwashed with dilute nitric acid, and the uranium backwashed in a separate extractor with water. 

Although the solvent purification process is adequate for the production of say reactor-grade zirconium, it is possible to modify it so that pure hafnium may also be obtained. Distribution data are available for various solutions containing thiocyanate, sulphate and chloride from which it is possible, for example, to deduce that both hafnium and zirconium will extract into hexone provided the aqueous phase has a high thiocyanate concentration and a low chloride concentration. The zirconium may then be selectively backwashed in a second extractor using say an aqueous phase of high thiocyanate concentration and moderate sulphate concentration, where the separation factor of the system is high. The hafnium can then... 

Figure 13-24 a) Continuous flow countercurrent extractor. (Modified from Thornton, 1992.) (b) Cutaway drawing of Podbielniak extractor with mixed feeds. The feed and backwash liquids enter at the principal interface. Centrifugal force separates the heavy phase from the light phase. The direction of rotation is out of the page. 

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