The group produces zinc by running EAFD (electric arc furnace dust) through its flagship plant, dubbed KRP, and is targeting monthly throughput of 17,600 tonnes.
In May the figure came in at 15,668 tonnes, which was an improvement on 14,533 tonnes in April this year.
Zinc concentrate produced in May was lifted to 3,618 tonnes from 3,006 tonnes the month before, while the target recovery rose to 91% from 90% in April.
EBITDA was US$1.010 million in May (April: US$768,000).
Chief executive Andrew Woollett said: "Generating US$1 million of EBITDA in a month is an important milestone for the company.
"The continuing optimisation of the operation in Korea is leading to greater profitability and further fine tuning of the plant should increase recovery over the next few months".
He added that the tonnage of EAFD treated in April was reduced as a result of planned maintenance in the first four days of the month.
In May, throughput was curtailed by a lack of EAFD, which has been addressed by entering into new contracts that should become effective in July 2015.
In April this year, the group said it expected to finish the ramp-up of the KRP plant to full production later this year.