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Property, Plant and Equipment, net
 12 Months Ended
Dec. 31, 2021
Property, Plant and Equipment [Abstract]  
Property, Plant and Equipment, net Property, Plant and Equipment, net
Net property, plant and equipment consists of:
 December 31,
 20212020
Machinery, equipment and tooling$30,813 $29,002 
SmartSystems27,343 22,352 
Vehicles3,066 2,893 
Furniture and fixtures1,325 1,302 
Plant and building199,958 199,867 
Real estate properties6,496 6,458 
Railroad and sidings27,703 27,703 
Land and land improvements35,652 33,040 
Asset retirement obligation20,536 19,993 
Mineral properties7,442 7,442 
Deferred mining costs2,455 2,123 
Construction in progress9,574 7,489 
372,363 359,664 
Less: accumulated depreciation and depletion109,898 84,988 
Total property, plant and equipment, net$262,465 $274,676 

Depreciation expense was $24,667, $21,725 and $25,689 for the years ended December 31, 2021, 2020 and 2019, respectively. Depletion expense was $36, $18 and $49 for the years ended December 31, 2021, 2020 and 2019, respectively.
The Company capitalized no interest expense associated with the construction of new property, plant and equipment for the years ended December 31, 2021, 2020 and 2019.