It gives the salt as a 100%, but that is a side benefit. You should be getting liquid metals in addition to that salt. There is a couple metals you can get from this process, but magnesium is common enough.
Also, as a side note, don't use the limestone itself, even if you smelt it into smooth limestone(which gives a full bucket instead of 900 mB); make planning chalk with it(takes two limestone and a bit of bonemeal). It gives you 2,500 mB instead of the 2,000 mB from smooth limestone proper.