Jamieson, Fausset and Brown on Jeremiah 15:7

7. fan--tribulation--from tribulum, a threshing instrument, which separates the chaff from the wheat (Mt 3:12).

gates of the land--that is, the extreme bounds of the land through which the entrance to and exit from it lie. Maurer translates, "I will fan," that is, cast them forth "to the gates of the land" (Na 3:13). "In the gates"; English Version draws the image from a man cleaning corn with a fan; he stands at the gate of the threshing-floor in the open air, to remove the wheat from the chaff by means of the wind; so God threatens to remove Israel out of the bounds of the land [Houbigant].