> can you render a fat that have been stored dried and got rancid and make the
> rancid taste disappear?

The short answer is, maybe to some extent.

There are two kinds of rancidity. Oxidative rancidity affects
unsaturated fats and definitely cannot be reversed by heating or
rendering. Polyunsaturated vegetable oils are highly sensitive to heat,
light, and oxygen, and should be stored in a cool, dark place. Because
animal fats are mostly saturated, they are much less sensitive to
oxidation, but any unsaturated content can become oxidized.

Hydrolytic rancidity involves the splitting off of free fatty acids from
the glycerol backbone of a triglyceride fat molecule by reaction with
water. This is what makes butter go rancid. I suppose that prolonged
heating might partially reverse this reaction by recombining the fatty
acid and glycerol and driving off the resulting free water, but that
doesn't strike me as a promising solution. Well-rendered fat that is
kept dry and sealed from humidity should be safe from this type of
degradation indefinitely.

I did find one reference to "cleaning" rancid fat...on a soap-making
website. They advised boiling the fat with 4 times its volume of water
to remove impurities. This could not remove all impurities, such as the
stubs of broken fatty acids that are still part of a triglyceride
molecule, but could remove some of the broken-off pieces and other
oxidation products of low molecular weight. If you did that, you'd have
to render the fat again to get rid of the water. I'm not sure I'd want
to eat the result, but in a time of famine it might be a way to improve
the taste of rancid fat to some degree.

Bill Dooley