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Property, Plant and Equipment, net | 6 Months Ended
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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, 2021 |  | December 31, 2020
Machinery, equipment and tooling | $ | 30,079 |  |  | $ | 29,002 |
SmartSystems | 26,498 |  |  | 22,352 |
Vehicles | 3,021 |  |  | 2,893 |
Furniture and fixtures | 1,325 |  |  | 1,302 |
Plant and building | 200,202 |  |  | 199,867 |
Real estate properties | 6,478 |  |  | 6,458 |
Railroad and sidings | 27,703 |  |  | 27,703 |
Land and land improvements | 33,155 |  |  | 33,040 |
Asset retirement obligation | 20,730 |  |  | 19,993 |
Mineral properties | 7,442 |  |  | 7,442 |
Deferred mining costs | 2,448 |  |  | 2,123 |
Construction in progress | 6,864 |  |  | 7,489 |
| 365,945 |  |  | 359,664 |
Less: accumulated depreciation and depletion | 97,528 |  |  | 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.

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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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