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Cost of sales (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Disclosure of cost of sales [abstract]        
Salaries and employee benefits $ 6,119 $ 5,277 $ 17,599 $ 16,544
Raw materials and consumables 4,754 2,560 14,034 10,105
Utilities 1,237 772 3,246 3,006
Other costs 447 343 1,948 1,931
Changes in inventories 252 (1,258) 1,116 (1,229)
Cost of sales $ 12,809 $ 7,694 $ 37,943 $ 30,357