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Property, Plant and Equipment, net | 12 Months Ended
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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,
| 2020 |  | 2019
Machinery, equipment and tooling | $ | 29,002 |  |  | $ | 19,683 |
SmartSystems | 22,352 |  |  | 15,811 |
Vehicles | 2,893 |  |  | 2,419 |
Furniture and fixtures | 1,302 |  |  | 1,228 |
Plant and building | 199,867 |  |  | 170,283 |
Real estate properties | 6,458 |  |  | 4,946 |
Railroad and sidings | 27,703 |  |  | 27,701 |
Land and land improvements | 33,040 |  |  | 21,320 |
Asset retirement obligation | 19,993 |  |  | 11,480 |
Mineral properties | 7,442 |  |  | 7,442 |
Deferred mining costs | 2,123 |  |  | 2,004 |
Construction in progress | 7,489 |  |  | 9,208 |
| 359,664 |  |  | 293,525 |
Less: accumulated depreciation and depletion | 84,988 |  |  | 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.

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- Definition The entire disclosure for long-lived, physical asset used in normal conduct of business and not intended for resale. Includes, but is not limited to, work of art, historical treasure, and similar asset classified as collections. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 958 -SubTopic 360 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=120429125&loc=d3e99893-112916 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -URI http://asc.fasb.org/topic&trid=2155823 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 958 -SubTopic 360 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=120429125&loc=SL120174063-112916 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 958 -SubTopic 360 -Section 50 -Paragraph 1 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=120429125&loc=d3e99779-112916 + Details
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