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Property, Plant and Equipment, net
 12 Months Ended
Dec. 31, 2017
Property Plant And Equipment [Abstract]  
Property, Plant and Equipment, net

7. Property, Plant and Equipment, net

Net property, plant and equipment consists of:

 

 

 

December 31,

 

 

 

2017

 

 

2016

 

Machinery, equipment and tooling

 

$

8,242

 

 

$

4,841

 

Vehicles

 

 

1,546

 

 

 

953

 

Furniture and fixtures

 

 

720

 

 

 

305

 

Plant and building

 

 

81,561

 

 

 

64,390

 

Real estate properties

 

 

4,432

 

 

 

3,503

 

Railroad and sidings

 

 

10,254

 

 

 

7,927

 

Land and improvements

 

 

16,378

 

 

 

13,317

 

Asset retirement obligation

 

 

8,408

 

 

 

1,324

 

Mineral properties

 

 

9,878

 

 

 

9,785

 

Deferred mining costs

 

 

657

 

 

 

417

 

Construction in progress

 

 

56,493

 

 

 

16,715

 

 

 

 

198,569

 

 

 

123,477

 

Less: accumulated depreciation and depletion

 

 

26,367

 

 

 

19,381

 

Total property, plant and equipment, net

 

$

172,202

 

 

$

104,096

 

 

Depreciation expense was $7,267, $6,441 and $5,276 for the years ended December 31, 2017, 2016 and 2015, respectively. Depletion expense was $33, $3 and $13 for the years ended December 31, 2017, 2016 and 2015, respectively.

The Company capitalized $139 and $1,808 of interest expense associated with the construction of new plant and equipment for the years ended December 31, 2016 and 2015, respectively. No amounts of interest expense were capitalized in the year ended December 31, 2017.