Union negotiator John Mendoza told Bloomberg that voting on demands for the three-year pact had begun and the group would solidify its demands for increased wages and better healthcare benefits by April 16.
Once in place, he told the news service, the union would have up to 60 days to settle on an agreement with Alabama-based producer Drummond.
“We’ve held the first of three assemblies,” Mendoza said Thursday.
“We’ll present the list of demands agreed to by the workers at the end of April.”
These sort of labor disputes have been common in Colombia of late.
Workers at the BHP Billiton, Xstrata and Anglo American-owned Cerrejon complex are back at work after a month-long strike ended in March over similar issues.
Drummond Coal was not without problems of its own, having restarted coal exports in March after Colombian officials lifted a three-week ban on its load-out and operations.
It is still investigating the January incident where coal was dumped into the Caribbean to keep a barge from sinking, which caused the closure.
Finance Minister Mauricio Cardenas told Bloomberg the strife had left the Andean nation with a drop in coal exports and reduced economic growth in the first quarter.
The warning led the country’s central bank to cut its benchmark rate by a half point to 3.25% on March 22, it reported.
Drummond, which has operations in Colombia and the US, holds more than 2 billion tons of reserves.