CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
OPERATING ACTIVITIES:
Net income	$ 7,031	$ 5,544
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	5,341	4,655
Amortization of deferred financing costs	2	2
Deferred income taxes	750	990
Loss (gain) on disposition of property, plant and equipment	3	(13)
Stock-based compensation	547	379
Changes in operating assets and liabilities:
Trade receivables, net	(978)	(200)
Inventories, net	(199)	(146)
Prepaid expenses and other current assets	(183)	249
Other assets	16	14
Accounts payable and accrued expenses	(719)	21
Other liabilities	11	13
Net cash provided by operating activities	11,622	11,508
INVESTING ACTIVITIES:
Purchase of property, plant and equipment	(4,543)	(6,106)
Proceeds from sale of property, plant and equipment	10	16
Net cash used in investing activities	(4,533)	(6,090)
FINANCING ACTIVITIES:
Cash dividends paid	(905)	(899)
Proceeds from exercise of stock options		81
Purchase of treasury shares	(95)	(64)
Net cash used in financing activities	(1,000)	(882)
Net increase in cash and cash equivalents	6,089	4,536
Cash and cash equivalents at beginning of period	83,562	54,260
Cash and cash equivalents at end of period	$ 89,651	$ 58,796