Arsenic removal was done in a Calciner, which was a furnace and heating chamber in which ores were roasted to drive off impurities such as sulphur and arsenic.
These were also known as Burning Houses, later patterns being of reverbaratory design.
The Brunton pattern calciner, introduced in the mid-19th century, was mechanically powered, and operated on a continuous basis, unlike earlier designs. There were a number of other calciners, most named after their designers (eg Oxland, Hocking and Loam).
Have a look at the Togus Calciner to see more about one that was operating in Cornwall,
Mines and Mining In Cornwall