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Property, Plant and Equipment, net
6 Months Ended
Jun. 30, 2021
Property, Plant and Equipment [Abstract]  
Property, Plant and Equipment, net Property, Plant and Equipment, netNet property, plant and equipment consisted of:
June 30, 2021December 31, 2020
Machinery, equipment and tooling$30,079 $29,002 
SmartSystems
26,498 22,352 
Vehicles3,021 2,893 
Furniture and fixtures1,325 1,302 
Plant and building200,202 199,867 
Real estate properties6,478 6,458 
Railroad and sidings27,703 27,703 
Land and land improvements33,155 33,040 
Asset retirement obligation20,730 19,993 
Mineral properties7,442 7,442 
Deferred mining costs2,448 2,123 
Construction in progress6,864 7,489 
365,945 359,664 
Less: accumulated depreciation and depletion97,528 84,988 
Total property, plant and equipment, net$268,417 $274,676 
Depreciation expense was $6,037 and $5,253 for the three months ended June 30, 2021 and 2020, respectively, and $12,213 and $10,529 for the six months ended June 30, 2021 and 2020, respectively.