Abstract
Titanium(IV) transport through a hybrid liquid membrane (HLM) and a suported liquid membrane (SLM) systems was studied. 1 mol/kg di-(2-ethylhexyl) phosphate (DEHPA) or a mixture with mono-(2-ethylhexyl) phosphate (MEHPA) in benzene was used as a carrier, impregnated into the membrane in the SLM, or as a bulk carrier solution (stirring or circulating) between membranes in the HLM. Celgard ® 2400 (25 μm thickness), 2402 (50 μm) hydrophobic microporous polypropylene films were used as the SLM support. The same hydrophobic membranes, hydrophilic Celgard ® 3400 (25 μm), cation-exchange Neosepta ® CM-1, anion exchange Neosepta ® ACH-45 membranes were used as barriers in the HLM system. 0.1 mol/kg Ti(IV) hydrochloric acid solutions, at pH=0.65 and at 7 mol/kg HCl, were studied as a feed phase. The receiver (strip) phase was a 2 mol/kg HCl solution. Titanium transport rate through the SLM from low and high acidity feed is found in the range of 3–4×10 −6 mol/m 2 s. Preloading of the carrier by 0.08 M Ti(IV) before impregnation shows two-fold increase of the transport rate from high acidity feed. The rate-controlling step of the transport is the kinetics of a back-extraction process. Presence of the MEHPA, a strong complexant of titanium, in the carrier leads to a drastic decrease in the transport rate. Elution of the carrier solution from the support and gel layer formation on the strip phase exit of the membrane is the main disadvantage of the SLM system for application in titanium separation. Titanium transport rate through the HLM with hydrophobic membranes is close to that in the SLM. Preloading by 0.08 M Ti of the initial carrier phase leads to a 6-fold increase of the titanium flux from the high acidity feed. MEHPA, as additive to the carrier, leads to fouling of the membrane. Preloading of the carrier phase by titanium may prevent the negative effect of gel formation. The rate-controlling step of the transport is the resistivity of the feed phase inside the pores (for ino-exchange membranes), competing with the back-extraction kinetics. Elimination of the carrier phase elution from the membrane pores and of the membrane fouling at the same or close mass transfer rates, are decisive advantages of the titanium separation processing by the HLM in comparison with the SLM.
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