# he will purify the descendants of Levi

"he will correct the sons of Levi and forgive them for having sinned" 

# He will refine them like gold and silver

Here persuading people not to sin any longer is spoken of as if a metalworker were making gold and silver more pure.

# they will bring offerings of righteousness to Yahweh

"they will bring acceptable offerings to Yahweh in order to worship Yahweh"