Quote from Runescape.com
It has been brought to our attention that some free players who used to be members have had their essence converted into pure essence which they now can’t use.
Don’t worry, we are aware of this and are working to fix it. In next week’s update we will make it so that whilst free players can’t mine any more pure essence they can however still use/trade any which was automatically converted.
If you haven’t already read it, then please see the previous news post entitled Rune Essence adjustment for more details on the Rune Essence changes.