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Property, Plant and Equipment, net
 6 Months Ended
Jun. 30, 2018
Property, Plant and Equipment [Abstract]  
Property, Plant and Equipment, net
Property, Plant and Equipment, net
Net property, plant and equipment consisted of:
 
 
June 30, 2018
 
December 31, 2017
Machinery, equipment and tooling
$
11,791

 
$
7,802

Vehicles
1,671

 
1,546

Furniture and fixtures
760

 
720

Plant and building
143,622

 
81,561

Real estate properties
4,441

 
4,432

Railroad and sidings
25,691

 
10,254

Land improvements
25,870

 
16,378

Asset retirement obligation
10,493

 
8,408

Mineral properties
9,879

 
9,878

Deferred mining costs
782

 
657

Construction in progress
22,527

 
56,493

 
257,527

 
198,129

Less: accumulated depreciation and depletion
33,534

 
26,367

Total property, plant and equipment, net
$
223,993

 
$
171,762


Depreciation expense was $3,994 and $1,688 for the three months ended June 30, 2018 and 2017, respectively, and $7,151 and $3,350 for the six months ended June 30, 2018 and 2017, respectively. Depletion expense was $18 and $5 for the three months ended June 30, 2018 and 2017, respectively, and $20 and $10 for the six months ended June 30, 2018 and 2017, 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, 2018 and 2017.