What was the "free Coynage" referred to in Dudley North's (1691) Discourses Upon Trade? Dudley North (1691):


  I call to witness the vast Sums that have been coyned in England, since the free Coynage was set up; What is become of it all? no body believes it to be in the Nation, and it cannot well be all transported, the Penalties for so doing being so great. The case is plain, it being exported, as I verily believe little of it is, the Melting-Pot devours all.


What was the "free Coynage" referred to here? When was it "set up"? (Was it for example some specific law?)
It seems to refer to the free coinage act of 1666 introduced by British Parliament under the rule of Charles II.

The name of the act was:


  An Act for encourageing of Coynage.


But it is often referred to as free coinage act. The full text of the act can be read in this source.

According to the act:


  Gold and Silver to be coined gratis. For every Pound Troy of Gold or Silver brought, there is to be delivered a Pound Troy of Current Coin.; For every Pound Troy of Gold and Silver finer than Standard so much more than a Pound Troy delivered; and for baser Gold or Silver so much less.


So the act basically abolished seigneurage and allowed people to bring their silver/gold to the royal mint and having it minted into coins. Hence here the free literary means people did not had to pay for coinage of the coins.