Quarterly report pursuant to Section 13 or 15(d)

Inventories (Details)

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Inventories (Details) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Inventory Disclosure [Abstract]    
Supplies $ 16,912 $ 12,571
Raw materials 344 199
Work in process 2,338 3,273
Finished goods 2,520 5,259
Total inventories $ 22,114 $ 21,302