Table 2. Reagent Classes and Molecular Formulas of Common Industrial REE Extractants Used in SX Processes^a.

reagent class	molecular formula	extractant
carboxylic acid	(R1·R2·CH3)·C·COOH	versatic acids:
		R1 + R2 = C7, Versatic 10;
		R1 + R2 = C6–C8, Versatic 911
	(R1·R2·R3·R4)·(CH2)nCOOH	naphthenic acids:
		R1–R4: varied alkyl groups
organophosphorus acids	(R1·R2)·POOH	phosphoric acids:
		R1 = R2 = C4H9CH(C2H5)CH2O–, di-2-ethylhexylphosphoric acid (D2EHPA)
		phosphonic acids:
		R1 = C4H9CH(C2H5)CH2O–, R2 = C4H9CH(C2H5)CH2–, 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (EHEHPA, HEHEHP, P507, PC88A)
		phosphinic acids:
		R1 = R2 = C4H9CH(C2H5)CH2–, di-2-ethylhexylphosphinic acid (P227)
		R1 = R2 = CH3(CH2)3CH2CH(CH3)CH2–, di-2,4,4-trimethylpentylphosphinic acid (Cyanex 272)
	(R1·R2)·PSOH	monothiophosphinic acids:
		R1 = R2 = CH3(CH2)3CH2CH(CH3)CH2–, di-2,4,4-trimethylpentyl-monothiophosphinic acid (Cyanex 302)
	(R1·R2)·PSSH	dithiophosphinic acids:
		R1 = R2 = CH3(CH2)3CH2CH(CH3)CH2–, di-2,4,4-trimethylpentyl-dithiophosphinic acid (Cyanex 301)
chelating exchangers	R1·(CO)·CH2·(CO)·R2	β-diketones:
		R1 = R–C6H5, R2 = CH3(CH2)5–, R: alkyl group (LIX 54)
solvating extractants	(R1·R2·R3)·PO	phosphorus esters:
		R1 = R2 = R3 = CH2(CH2)2CH2O–, tri-n-butyl-phosphate (TBP)
		R1 = R2 = CH2(CH2)2CH2O–, R3 = CH2(CH2)2CH2–, dibutylbutylphosphonate (DBBP)
		phosphine oxides:
		R1 = R2 = R3 = CH2(CH2)6CH2–, tri-n-octylphosphine oxide (TOPO, Cyanex 921, 923)
anion exchanger	(R1·R2·R3)·NCH3Cl	primary amines:
		R = (CH3)3C(CH)2C(CH3)2)4 (Primene JMT, N1923)
		quaternary amines: R1 = R2 = R3 = C8–C10 mixture (Aliquat 336, Adogen 464)

^aReproduced with permission from ref (2). Copyright 2015 Taylor and Francis Group.