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Property, Plant and Equipment, net
 12 Months Ended
Dec. 31, 2020
Property, Plant and Equipment [Abstract]  
Property, Plant and Equipment, net Property, Plant and Equipment, net
Net property, plant and equipment consists of:
 December 31,
 20202019
Machinery, equipment and tooling$29,002 $19,683 
SmartSystems22,352 15,811 
Vehicles2,893 2,419 
Furniture and fixtures1,302 1,228 
Plant and building199,867 170,283 
Real estate properties6,458 4,946 
Railroad and sidings27,703 27,701 
Land and land improvements33,040 21,320 
Asset retirement obligation19,993 11,480 
Mineral properties7,442 7,442 
Deferred mining costs2,123 2,004 
Construction in progress7,489 9,208 
359,664 293,525 
Less: accumulated depreciation and depletion84,988 63,064 
Total property, plant and equipment, net$274,676 $230,461 

Depreciation expense was $21,725, $25,689 and $17,117 for the years ended December 31, 2020, 2019 and 2018, respectively. Depletion expense was $18, $49 and $47 for the years ended December 31, 2020, 2019 and 2018, respectively.
The Company capitalized no interest expense associated with the construction of new property, plant and equipment for the years ended December 31, 2020, 2019 and 2018.