It will be a mistake to use Aqua Regia on sweeps. Sulphur is not the
problem here. Start by loading sweeps on iron sheet and roast it,
from underneath, until there are no smoke at all. Then sweeps should
be smelted with silver in proportion 3 parts of silver and 1 part of
sweeps. Resulting ingot is rolled thinly and treated with
concentrated nitric acid, until acid is completely exhausted. Fresh
acid should be added until reaction completely stops. Used acid
should be preserved to extract silver and what is left on the bottom
after acid is poured off, is treated with Aqua Regia.
This very general description of refining process, which should not
be attempted by anybody who is not trained in handling dangerous
chemicals. It is also strongly recommended to acquaint oneself with
rather large volume of material on refinement of precious metals.
There are numerous details on intermediate steps of treatment to
handle variety of contaminants, usually present in sweeps, which
cannot be addressed in this form of communication.