Complex sentence: Two processing mills were built to crush the ore and separate out the galena from it, and the mine had its own smelt mill between 1828 and 1917, but after that it was more economical to sell concentrated galena to a commercial smelter.
Split into simpler sentences: Two processing mills were built to crush the ore and separate out the galena from it, and the mine had its own smelt mill between 1828 and 1917. After that it was more economical to sell concentrated galena to a commercial smelter.