Extraction, advantages single-stage

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. 

As far as calix[4]pyrrole-based materials are concerned, particular emphasis should be placed on the production of chelating resins. Those obtained by the condensation reaction of calix[4]pyrrole and formaldehyde in the presence of formic acid offer potential for their use as extracting agents for fluoride removal from water. Their advantage relies on the fact that a single-step procedure is required for polymerisation. However, research in this area is in a preliminary stage. Much work needs to be done to establish the full capacity of these materials to take up fluoride from water, the kinetics of the process, and the optimum experimental conditions for fluoride extraction. Although, their ability to extract fluoride reaches a value of 88% from solutions of tetra-n-butylammonium fluoride... 

The holdups of the two phases are very small, which is advantageous in certain applications. As the phases are in the co-current flow, they can attain equilibrium. Therefore, the extractor is at best one equilibrium stage. However, multiple units can be configured to form a multi-stage countercurrent extraction unit. There is a need to evaluate the relative merits of the multiple units configuration vis vis the Podbielniak extractor, which offers 4 to 6 stages in a single unit. 

Extraction or detachment replicas provide a replica of the surface along with some thin fragments of the actual specimen. The advantage is that the surface topography is recorded and anal5 ical experiments, such as electron diffraction, may also be conducted. The method is complementary to microtomy or ultrasonication. Methods for such replication are similar to those described above for single and double stage... 

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