Extraction behaviors of rare-earths in the mixed sulfur-phosphorus acid leaching solutions of scheelite

Leaching of scheelite by adopting the sulfur-phosphorus mixed acid allowed the accompanied rare-earth elements to enter a leaching solution. The leaching solution contained ions that could be coordinated with rare-earth ions, such as phosphotungstic acid anions, sulfate anions and phosphate anions, among others. To efficiently extract rare earths from the leachate, the extraction behavior of rare-earth ions in the solution was examined. Experimental results indicated that when the solution pH was lower than 1, phosphotungstic acid existed in the form of Keggin-type (1:12) [PW12O40](3-), which exhibited a relatively poor complexing ability. Thus, secondary carbon primary-amine (N1923) could only extract phosphotungstic acid anions without extracting rare earths. As pH increased, a number of WO groups were dropped from the [PW12O40](3-) ion to form unsaturated phosphotungstic acid. Unsaturated phosphotungstic acid anions with rare-earth ions exhibited a complexing ability stronger than that with SO42-. Rare-earth ions were mainly complex with unsaturated phosphotungstic acid to form stable anions in the solution and could be extracted completely by the anionic extractant N1923. Meanwhile, the cationic extractant P204 could hardly extract rare earths. As the pH rose and exceeded 2.75, the concentration of PO43- increased, rendering the complexation between the rare-earth ions and PO43- dominant. Hence, rare-earth ions were precipitated.