UPDATE 1-Japan's Mitsubishi Materials expects H2 copper output to rise 14 pct y/y


* Copper output to rise due to higher capacity at Onahama

* Conducting 21 days of maintenance at Onahama from Oct. 21

* Gold output to rise due to higher slime supply from Indonesia

(Adds comment, detail) TOKYO, Oct 2 (Reuters) - Japan's Mitsubishi Materials Corp said on Monday it plans to produce 181,830 tonnes of refined copper during October-March, up 14 percent from the same period last year as it boosts its smelting capacity. Japan's third-biggest copper smelter expects output at its Naoshima smelter and refinery in western Japan to rise 10 percent, while production at its Onahama plant, north of Tokyo, will increase by 22 percent, it said in a statement. The Onahama plant is jointly owned with Dowa Holdings Co Ltd and Furukawa Co Ltd. As part of an agreement between the partners, Mitsubishi Materials is gradually taking some of the smelting capacity previously assigned to Dowa, starting from the last financial year. "Copper demand in the domestic market has been steady," said a spokesman at Mitsubishi Materials. The company carried out 27 days of maintenance at the Naoshima plant from March 20 and it plans to begin 21 days of maintenance at the Onahama facility from Oct. 21. "The planned maintenance will have limited impact on second-half output (relative to last year) as the Onahama plant went through a 14-day maintenance period in the second half of the last financial year," the spokesman said. Mitsubishi Materials plans to increase its gold output as it expects higher supply of anode slime, a byproduct of copper concentrate processing that includes other metals such as gold and silver, to come from the Gresik smelter in Indonesia. That smelter is run by PT Smelting, which is 60.5-percent owned by Mitsubishi Materials. It stopped exporting anode slime from Jan. 11 to March 7 due to new rules introduced by the Indonesian government, the company spokesman said. At the Naoshima plant, the Japanese company extracts gold and silver from anode slime, providing it with additional revenues. "We expect to receive higher volume of slime from Gresik in the second half," the spokesman added. Mitsubishi Materials' output plan is detailed below, with comparisons against planned production in the first half of the financial 2017/18 year and actual production in the second half of 2016/17. (Copper and lead in tonnes, gold and silver in kilograms):

H2 FY17/18 H1 FY17/18 H2 FY16/17 Copper 181,830 177,186 159,582 Lead 13,380 13,728 12,744 Gold 29,748 24,798 21,420 Silver 201,000 190,998 182,286

(Reporting by Yuka Obayashi; Editing by Joseph Radford)