Lava Mining is the very pinnacle of evil mining, far more diabolical than rival techniques such as open cat mining.
A lava mine is not a practical mine, it is a mine of evil. The most vital law of lava mining is to mine only lava and nothing more, no valuable mineral, ore or rock should be excavated, just pure molten lava. This fundamental law does mean that while all of the costs of mining are incurred there is no source of income, indeed if you are offered sources of income (for example selling your igneous rock) a true evil overlord will shun the money and continue to run at a crippling loss, just to prove how evil they are.
Deny your workers tools, forcing them to work with only their bare, burning hands.
Any excavated rock should be ground up, ideally in a giant man powered grounding machine, and the resultant sand thrown into the faces of the miners.