Inventories (Tables)

6 Months Ended

Jun. 30, 2018

Inventory Disclosure [Abstract]

Schedule of inventories

	June 30, 2018		December 31, 2017
	(in thousands)
Raw materials and precious metals	$	6,374	$	6,333
Finished goods	11,100		13,594
Parts and supplies	32,028		34,170
Total inventories	$	49,502	$	54,097