Why does it say "v'Yashav Metzaref u'Metaher Kesef"?
Kidushin 71a #1: When Hashem purifies families (reveals who has proper lineage), He will begin with the Leviyim (i.e. Kohanim) - "He will sit like one who purifies silver, and purify the Leviyim like gold and silver, and they will bring Minchah offerings bi'Tzdakah (rightfully)."
Kidushin 71a #2: Money permits Mamzerim. Rashi 71a - if Pesulim intermarried with many families due to their money, Hashem will be Machshir them all, lest He disqualify those who intermarried with them.
Rashi: He turns from all His affairs to be like a smelter, who refines and purifies silver.
Radak: This is like a judge who sits to judge the nation, and separate the liable from the innocent. So that day will refine and purify silver and remove the dross (Resha'im).
Malbim: After He will refine the impurities (Resha'im), imply that when He is seen, He will dwell on a fixed basis. He will refine and purify a second time the Kesef - the good ones who remain after separating the Resha'im. This is like smelters refine silver twice - "Kesef Tzaruf ba'Alil la'Aretz Mezukak Shiv'asayim" (Tehilim 12:7).
Why does it mention "Bnei Levi"?
Refer to 3:3:1:1.
Radak: This refers to the Kohanim. Since they were discussed before, they are mentioned here in particular.
Malbim: Also Shevet Levi, who are like silver, He will purify.
What is the comparison to Zikuk of gold and silver?
Malbim: Gold is dearer than silver. It requires greater refinement. We are adamant about its impurities more than impurities in silver. So Bnei Aharon, amidst Shevet Levi, will be purified more than the rest of the Shevet.
What is the significance of "Magishei Minchah bi'Tzdakah"?
Kidushin 71a: Hashem was charitable to Yisrael, to permit disqualified families that became mixed.
Radak: This is unlike the Kohanim of this generation, who offered blemished animals.
Malbim: The Kohanim who will offer to Hashem will be those who were to Hashem in Tzedakah (i.e. good deeds Bein Adam la'Makom). It is explained in Yechezkel Perek 43 that only Kohanim of Bnei Tzadok, who stood in their virtue, will offer in the Mikdash in the future. The other Kohanim, who did not stand in their virtue, will not offer in the Mikdash in the future.
