that... won't work. as you'll need a molten liquid inside the smeltery first, before the villagers can be smelted.
but the smeltery will only drain the liquid which was first in the smeltery, which would be the molten liquid you used to smelt the villagers.
when that's drained, the molted emerald will be left, and will be the next to be drained. but if it drains too fast, the smeltery will become empty and there won't be any molten liquid left for the smeltery to keep smelting villagers. the only way to fix this is perhaps have some block or machine which can read the liquid contents of the smeltery, and stop the redstone signal when the liquid content goes below a certain value