distill, lutecium, surname
鎦 centers on the process of separating a substance by heating and condensing: distillation. The rare element sense (lutecium) and surname use are specific applications of this character.
鎦 combines 金 (metal) with 留, which likely contributes the sound and the idea of stopping or retaining, perhaps referring to the metal vessel used in distillation.
The left side is metal 金, and the right side 留 suggests something being held or retained. Picture a metal still that traps and condenses vapors: 鎦 is distillation.
For リュウ, imagine a metal still producing a steady stream of distilled liquid: the sound of the flowing liquid is like 'ryuu' (リュウ), a gentle, continuous stream.