Suitable diluents can be water-immiscible aliphatic, aro› matic solvents or blends of such solvents. Most preferred are  solvents that are classified nonflammable (flash point> 140°  F.), nonhalogenated, low-odor aliphatic, aromatic, or a blend  of aliphatic and aromatic solvents. The aliphatic diluent(s)  can be linear but are preferably branched. The aromatic  diluent(s) can be unsubstituted aromatic liquids but are  F.), nonhalogenated, low-odor, aliphatic, either linear or  branched, with a carbon number of 8-16, most preferably  9-13, or mixtures thereof. Specific examples are given in  Table 3.  1s 2-50% in the feed solution and the S2M mixture 1s  preferably maintained at -pH 1-2 or less. Sl mixer pH is  preferably maintained at pH 2-4, or less. Sulfate ions are  subsequently stripped by basic reagents that do not cause  decomposition of the extractant preferably selected from  alkali and alkali metal hydroxides, and/or alkali carbonates.  Base strength for sulfate ion stripping is preferably -pH  10-11 or greater. Specific examples of suitable of reagents  for stripping and regenerating, and pH adjustment of the  extractant are listed elsewhere herein.  preferably aliphatically-substituted aromatic liquid com› pounds. Extractant mixtures suitable for the invention con› tain at least 25% diluent (v/v), preferably 60% (v/v), and  most preferably 85% (v/v). Specific examples of suitable  diluents are shown in Table 3.  For a further embodiment of the invention suitable  reagents for stripping of metal ions, regeneration, solute  concentration, and/or pH adjustment include mineral acids  that do not cause decomposition of the extractant. Suitable  strip acids are preferably selected from hydrochloric, sulfu› ric, phosphoric, blends of these and the like. Acid strength  Referring to Table 3, effective "Type extractant phase"  extract compounds of the invention. All of the components  of the E-phase are oil soluble with a total carbon number of  at least eight (8), can be charged or neutral, and can have  additional functional groups such a halogens, ether linkages,  ester linkages, alkyl phenolic, and the like, so long as the  extraction chemistry and the oil solubility of the reagent is  not adversely effected relative to the descriptions herein.  Effective extractant compounds of the invention are all oil  soluble with a total carbon number of at least eight (8), but  preferably about 16, and most preferably about 25 or more,  TABLE 3  Typical Compounds Useful for the Extractant Phase  Chemical  Class (used  alone or in  Extractant  combination  Formulation  with any other  Component  extractant)  Extractant  Modifier  Diluent  Quaternary  Amines  Mono  guanadinium  Quaternary  phosphonium  b-isodecanol  (or decyl  alcohol, or  Exxal fi 10)  Isotridecanol  (orb- Tridecyl  alcohol  Nonyl phenol  Dodecyl  phenol  Aromaticfi 150  Aromatic fi 200  Calumetfi  400-500  Conocofi 170  Isoparfi M  Specific Compounds  N-methyl tri-(n› octyl)ammonium ion  N-methyl tri-(n› decyl)anunonium ion  N-methyl tri-(n› dodecyl)ammonium ion  Aliquatfi 134  Aliquat fi 336  Tri-octyl methylarnmoniwn ions  Mixture of tridecyl- and trioctyl› methylarnmoniwn ions  Level of  Preference  Most preferred  Most preferred  Most preferred  Most preferred  Most preferred  Most preferred  Most preferred  HOE S 2706  Most preferred  Adogen fi 464  Most preferred  Tri(C$_{8}$-C$_{10}$) methylammonium ions  Most preferred  R$^{1}$R$^{2}$R$^{3}$N+CH$_{3 }$ R$^{1}$=R$^{2}$=R$^{3}$=CHs(CH$_{2}$ ) 9 - Most preferred  R$^{1}$=R$^{2}$=R$^{3}$=CHs(CH$_{2}$ h- Most preferred  and  Most preferred  R$^{1}$=R$^{2}$=R$^{3}$=CHs(CH$_{2}$ ) 9 - Blends of the above quaternary  Most preferred  arnmoniwn ions in any proportion  e.g. LIX-79  fi  N-methyl tri(n-octyl) phosphonium  aromatic  aromatic  Most preferred  Functional  Most preferred  Most preferred  Functional  Functional  Functional  Functional  Most Preferred  Preferred  Preferred  US 10,494,270 B2  42  41 