I have refined thee, but not with silver; rather, but not as silver (Cheyne). or, but not in the manner of silver (Delitzsch); i.e. not with the severity with which silver is refined (see Psalms 12:6).
I have chosen thee; rather, I have tested thee. The furnace of affliction is here the Babylonian captivity. The object of the Captivity was to "test" and "refine," or purify God's people to a certain extent—not with extreme severity, but in such sort as to fit them to "bear his Name before the Gentiles" for another five hundred years.