| In millions, except per share items | Jun-30-23 | Mar-31-23 | Dec-31-22 | Sep-30-22 | Jun-30-22 | Mar-31-22 | Dec-31-21 | Sep-30-21 |
| | 10-Q | 10-Q | 10-K | 10-Q | 10-Q | 10-Q | 10-K | 10-Q |
| Current assets: | | | | | | | | |
| Cash and cash equivalents | 2.3 | 2.4 | 3.0 | 7.0 | 8.9 | 4.3 | 2.5 | 4.5 |
| Restricted cash | 4.0 | 3.9 | 3.4 | 3.5 | 3.5 | 3.3 | 3.3 | 3.2 |
| Accounts receivable | 21.4 | 33.2 | 29.9 | 16.7 | 17.2 | 15.4 | 13.6 | 16.5 |
| Inventory | 47.9 | 49.4 | 49.8 | 13.7 | 13.8 | 8.8 | 7.7 | 11.3 |
| Parts and supplies | 33.9 | 31.2 | 28.3 | 15.0 | 12.2 | 11.5 | 10.0 | 9.8 |
| Prepaid expenses | 3.1 | 3.4 | 4.5 | 2.2 | 1.0 | 1.2 | 2.1 | 3.7 |
| Total current assets | 112.7 | 129.7 | 138.5 | 58.2 | 56.6 | 44.4 | 39.2 | 49.1 |
| |
| Property, plant and equipment: | | | | | | | | |
| Land and mineral rights | 115.5 | 115.5 | 115.6 | 115.7 | 115.8 | 115.8 | 115.8 | 115.9 |
| Buildings and equipment | 554.3 | 541.4 | 534.1 | 363.9 | 355.1 | 346.9 | 342.8 | 361.6 |
| Mine development | 149.7 | 145.2 | 140.1 | 131.8 | 123.3 | 116.2 | 112.6 | 105.9 |
| Total property, plant and equipment | 819.5 | 802.1 | 789.8 | 611.4 | 594.1 | 579.0 | 571.2 | 583.3 |
| Less - accumulated depreciation, depletion and amortization | -342.7 | -325.9 | -309.4 | -298.1 | -287.0 | -277.5 | -268.4 | -281.0 |
| Total property, plant and equipment, net | 476.8 | 476.2 | 480.5 | 313.3 | 307.1 | 301.4 | 302.8 | 302.2 |
| Investment in Sunrise Energy | 3.2 | 3.4 | 4.0 | 4.1 | 3.9 | 3.7 | 3.5 | 3.3 |
| Other assets | 7.2 | 7.3 | 7.6 | 7.8 | 8.1 | 8.3 | 8.4 | 8.4 |
| Total Assets | 599.9 | 616.6 | 630.6 | 383.4 | 375.7 | 357.9 | 354.0 | 363.1 |
| |
| Current liabilities: | | | | | | | | |
| Current portion of bank debt, net | 17.6 | 32.3 | 33.0 | 110.2 | 13.2 | 116.2 | 23.1 | 26.8 |
| Accounts payable and accrued liabilities | 70.9 | 84.5 | 83.0 | 52.4 | 48.0 | 47.7 | 41.5 | 39.3 |
| |
| Deferred revenue | | | | | | | | |
| Contract liability - power purchase agreement and capacity payment reduction | | | | | | | | |
| Total current liabilities | 177.8 | 209.3 | 239.6 | 162.6 | 61.2 | 163.9 | 64.6 | 66.0 |
| |
| Long-term liabilities: | | | | | | | | |
| Long-term bank debt, excluding current maturities, net | 54.7 | 40.1 | 49.7 | | 113.6 | | 84.7 | 83.5 |
| Convertible note payable | 10.0 | 10.0 | 10.0 | 10.0 | | | | |
| Convertible notes payable - related party | 9.0 | 9.0 | 9.0 | 9.0 | | | | |
| Deferred income taxes | 9.0 | 7.5 | 4.6 | 3.7 | 3.1 | 2.7 | 2.9 | 0.1 |
| Asset retirement obligations | 17.2 | 17.3 | 38.1 | 12.4 | 13.4 | 13.7 | 42.3 | 17.2 |
| Contract liability - power purchase agreement | 65.7 | | | | | | | |
| Other | 1.6 | 1.4 | 1.3 | 1.7 | 1.6 | 1.5 | 1.6 | 2.1 |
| Total long-term liabilities | 167.2 | 170.2 | 175.9 | 36.8 | 131.7 | 17.8 | 234.5 | 103.0 |
| Total liabilities | 345.0 | 379.4 | 415.5 | 199.4 | 192.9 | 181.7 | 167.7 | 169.0 |
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| Commitments and contingencies | | | | | | | | |
| Stockholders' equity: | | | | | | | | |
| Preferred stock, $.10 par value, 10,000 shares authorized; none issued and outstanding | | | | | | | | |
| Common stock | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 |
| Additional paid-in capital | 119.7 | 118.9 | 118.8 | 117.7 | 114.2 | 104.2 | 104.1 | 104.2 |
| Retained earnings | 134.9 | 118.0 | 95.9 | 65.9 | 64.3 | 67.7 | 77.8 | 85.5 |
| Total stockholders’ equity | 254.9 | 237.2 | 215.0 | 184.0 | 178.8 | 172.2 | 182.2 | 190.1 |
| Total liabilities and stockholders’ equity | 599.9 | 616.6 | 630.6 | 383.4 | 375.7 | 357.9 | 354.0 | 363.1 |