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Cost of sales (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of cost of sales [abstract]    
Salaries and employee benefits $ 22,314 $ 23,430
Raw materials and consumables 14,252 15,065
Utilities 3,902 4,113
Other costs 2,318 1,768
Changes in inventories (2,748) 1,769
Cost of sales $ 40,038 $ 46,145