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Property, Plant and Equipment, net
 6 Months Ended
Jun. 30, 2019
Property, Plant and Equipment [Abstract]  
Property, Plant and Equipment, net
Property, Plant and Equipment, net
Net property, plant and equipment consisted of:
 
June 30, 2019
 
December 31, 2018
Machinery, equipment and tooling
$
16,080

 
$
14,858

SmartSystems
9,532

 
5,286

Vehicles
2,214

 
1,955

Furniture and fixtures
1,217

 
1,140

Plant and building
160,413

 
158,882

Real estate properties
4,855

 
4,601

Railroad and sidings
27,347

 
27,347

Land and land improvements
27,421

 
27,167

Asset retirement obligation
19,770

 
16,469

Mineral properties
10,075

 
10,075

Deferred mining costs
1,806

 
1,806

Construction in progress
25,132

 
21,619

 
305,862

 
291,205

Less: accumulated depreciation and depletion
54,884

 
42,809

Total property, plant and equipment, net
$
250,978

 
$
248,396


Depreciation expense was $6,171 and $3,994 for the three months ended June 30, 2019 and 2018, respectively, and $12,038 and $7,151 for the six months ended June 30, 2019 and 2018, respectively. Depletion expense was $13 and $18 for the three months ended June 30, 2019 and 2018, respectively, and $18 and $20 for the six months ended June 30, 2019 and 2018, respectively.
The Company capitalized no interest expense associated with the construction of new property, plant and equipment for the three and six months ended June 30, 2019 and 2018.