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Property, Plant and Equipment, net
 9 Months Ended
Sep. 30, 2020
Property, Plant and Equipment [Abstract]  
Property, Plant and Equipment, net Property, Plant and Equipment, net
Net property, plant and equipment consisted of:
September 30, 2020December 31, 2019
Machinery, equipment and tooling$52,077 $19,683 
SmartSystems
22,393 15,811 
Vehicles2,938 2,419 
Furniture and fixtures1,284 1,228 
Plant and building176,731 170,283 
Real estate properties6,458 4,946 
Railroad and sidings27,703 27,701 
Land and land improvements32,956 21,320 
Asset retirement obligation19,904 11,480 
Mineral properties7,442 7,442 
Deferred mining costs2,123 2,004 
Construction in progress8,153 9,208 
360,162 293,525 
Less: accumulated depreciation and depletion79,020 63,064 
Total property, plant and equipment, net$281,142 $230,461 

Depreciation expense was $5,328 and $6,610 for the three months ended September 30, 2020 and 2019, respectively, and $15,856 and $18,648 for the nine months ended September 30, 2020 and 2019, respectively.