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Cost of revenues and general and administrative expenses (Details) (USD $)
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Cost of revenues and general and administrative expenses [Line Items]      
Cost Of Revenue $ 40,316,252 $ 43,458,260 $ 36,132,425
Electricity [Member]
      
Cost of revenues and general and administrative expenses [Line Items]      
Cost Of Revenue 13,634,617 14,473,583 12,622,788
Depreciation [Member]
      
Cost of revenues and general and administrative expenses [Line Items]      
Cost Of Revenue 4,822,967 6,959,912 5,537,098
Fuel oil [Member]
      
Cost of revenues and general and administrative expenses [Line Items]      
Cost Of Revenue 10,106,409 10,078,724 7,195,692
Employee costs [Member]
      
Cost of revenues and general and administrative expenses [Line Items]      
Cost Of Revenue 4,422,093 4,423,899 4,276,682
Maintenance [Member]
      
Cost of revenues and general and administrative expenses [Line Items]      
Cost Of Revenue 2,332,893 2,849,453 2,140,776
Royalties [Member]
      
Cost of revenues and general and administrative expenses [Line Items]      
Cost Of Revenue 1,357,988 1,451,672 1,406,160
Insurance [Member]
      
Cost of revenues and general and administrative expenses [Line Items]      
Cost Of Revenue 1,499,201 1,625,733 1,545,771
Other [Member]
      
Cost of revenues and general and administrative expenses [Line Items]      
Cost Of Revenue $ 2,140,084 $ 1,595,284 $ 1,407,458