## 3 Rare Earth Flotation from Primary Resources
### Flotation Reagents
#### 3.1.1 Collectors
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Collections** & **Samples** & **pH** & **Depressants** & **Recovery [\%]** & **Grade [\%]** & **References** \\ \hline  & Bestneseite & 6-8 & - & 95 & - & [32, 23] \\  & Bestneseite and calcite & 9 & Xanthian gum & 75 & - & [44, 1] \\  & Bestneseite, calcite, and barite & 5-10 & Login sulfonate & 86-90 & 11-30.4 & [9] \\  & Bestneseite, shellite, hematite, and quartz & 9 & Sodium silicate and starch & 63 & 2.3 & [49] \\  & Homene, subcellular, hematite, and quartz & 7 & – & 95 & – & [32, 33] \\  & Monazite and deolomie & 7 & – & 32 & – & [44] \\  & Monazite, hematite, and quartz & 9 & Sodium silicate and starch & 61 & 63 & [50, 51] \\ Sodium oleate & Xenonite, zircon, school, staurville, and ilmenite & 9 & Sodium silicate & 71-96 & 36-51.1 & [52] \\  & Alanine and quartz & - & Ammonium lignocellulosic & 33-52 & 82.39\({}^{+}\)6 & [53] \\  & Fergusonite, quartz, and glata & 5-7 & – & – & [54] \\  & Heavy mineral salts containing monazine, ilmenite, rutile, licenov, guaren, and pyroxene & 10 & Cyquest400 & 20-32 & 43-66 & [55] \\  & Pre-concentrates of iron oxide-alice-rich things & 9 & Sodium silicate and starch & 12 & 1.9 & [56] \\  & Kaolin by-products containing monazine & 10 & Sodium silicate & 802 & 0.54 & [42] \\  & Fine coal fruse & 9 & – & – & 0.47 & [44] \\  & Fire clay sound sound, oxide, dolomite, Illie, & 9 & – & – & 0.05-0.1 & [45] \\  & Isalime, pyrite, quartz, and rutile & 8 & Sodium silicate & 40-55 & 0.28-0.3 & [43] \\ \hline  & Sodium oleate;mixed with & & & & & \\  & serbitan monopolate & Fergusonite, zircon, quartz, and feldspar & 5 & Acid, and citric acid & 96 & – & [57] \\ \hline Fatty acid & Phosphate flotation tailing containing REMs & – & – & 64 & 0.01 & [58] \\  & & & & & & \\ \hline Fatty acid (a.e., oleic acid) & Monazite, hematite, and quartz & 7 & Sodium silicate & 66-96 & 32.9-62.8 & [59] \\  & & & & & & \\ \hline  & & & & & & \\ \hline  & & & & & & \\ \hline  & & & & & & \\ \end{tabular}
\end{table} 
Table 1: A summary of recent studies on REM flotation using carboxylate collectors. Hydroxamates are a group of chelating collectors popular in REM flotation because of their high selectivity [34; 36; 63]. The adsorption of hydroxamate-type collectors on mineral surfaces can occur via chemisorption and/or surface reactions (Figure S3). In chemisorption, hydroxamate interacts with the mineral surfaces without the movement of surface atoms [31; 32; 64; 65; 66; 67]. This type of adsorption was proposed when hydroxamate concentration was low and monolayer adsorption predominated. Meanwhile, surface reactions--combinations of chemisorption and multilayer physisorption--are dominant at high concentrations of hydroxamate and involve (i) hydrolysis of metal atoms, (ii) formation of hydroxyl-complexes, and (iii) re-adsorption to or precipitation on the mineral surface [34; 65; 68]. There are many types of hydroxamate collectors used for REM flotation (Table 2), and the most common and frequently used are octanohydroamic acid, benzohydroxamic acid, salichydroxamic acid, H205, Aero 6493, and AEROFLOAT 6494. [PERSON] et al. [65] studied the adsorption mechanism of octanohydroxamic acid on bastnaesite and proposed that, at low concentrations, monodentate adsorption was dominant while, at higher collector concentrations, the mechanism shifts to bidentate adsorption. These authors also observed that the binding enthalpies of octanohydroxamic acid interacting with La- and Ce-bastnaesite surfaces were stronger than on calcite. Moreover, the selectivity of hydroxamate was enhanced by maintaining a low collector concentration that limited hydroxamate adsorption to gangue minerals. The optimum pH for pure bastnaesite flotation using hydroxamate collectors is around pH 8-9, with 67-75% recovery [31; 32; 69; 70; 71]. Similarly, the optimum pH for pure monazite flotation using hydroxamate collectors is around 7-9, with 61-95% recovery [32; 70; 72]. It should be noted that when common gangue minerals are present, a lower grade and recovery were obtained.