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Property, Plant and Equipment, net
9 Months Ended
Sep. 30, 2022
Property, Plant and Equipment [Abstract]  
Property, Plant and Equipment, net Property, Plant and Equipment, net
Net property, plant and equipment consisted of:
September 30, 2022December 31, 2021
Machinery, equipment and tooling$34,411 $30,813 
SmartSystems
27,654 27,343 
Vehicles3,234 3,066 
Furniture and fixtures1,345 1,325 
Plant and building200,373 199,958 
Real estate properties6,523 6,496 
Railroad and sidings33,622 27,703 
Land and land improvements40,455 35,652 
Asset retirement obligation28,818 20,536 
Mineral properties7,442 7,442 
Deferred mining costs2,455 2,455 
Construction in progress9,708 9,574 
396,040 372,363 
Less: accumulated depreciation and depletion128,830 109,898 
Total property, plant and equipment, net$267,210 $262,465 

Depreciation expense was $6,498 and $6,111 for the three months ended September 30, 2022 and 2021, respectively, and $19,309 and $18,325 for the nine months ended September 30, 2022 and 2021, respectively.