| 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 |
| Cash flows from operating activities | | | | | | | | |
| Net loss | -5.9 | -7.5 | -3.2 | -2.4 | -0.3 | -3.0 | 5.8 | 24.3 |
| |
| Adjustments to reconcile net loss to net cash (used in) from operating activities: | | | | | | | | |
| Depreciation, amortization, depletion and accretion | 2.4 | 2.1 | 1.7 | 1.7 | 1.6 | 1.5 | 1.8 | 2.1 |
| Operating lease expense | 0.7 | 0.7 | 0.8 | 0.7 | 0.6 | 0.7 | 0.6 | 0.6 |
| Stock-based compensation expense | 0.5 | 0.6 | 0.5 | 0.5 | 0.5 | 0.5 | 0.5 | 0.5 |
| Earnings from equity method investments | -0.5 | -0.6 | -0.3 | | -2.4 | -0.8 | -6.8 | -22.2 |
| Amortization of debt discount and debt issuance costs | 0.2 | | | | | | | |
| Other non-cash items, net | 0.0 | 0.0 | 0.1 | 0.0 | -0.1 | 0.6 | 0.1 | 0.0 |
| Changes in operating assets and liabilities: | | | | | | | | |
| Receivables and related party receivables | -0.2 | 3.9 | | | 0.3 | 2.1 | | |
| Prepaid expenses and other assets | -1.1 | 3.4 | 0.1 | -0.2 | -0.1 | -0.7 | -2.0 | 0.7 |
| Other long-term assets, net | -2.4 | -0.5 | -1.9 | -0.8 | -0.2 | 3.1 | -0.3 | -1.3 |
| Other current liabilities | 0.3 | -0.2 | 1.2 | 0.3 | 0.8 | -1.2 | -4.8 | -2.0 |
| Operating lease liabilities | 1.2 | -0.8 | 0.1 | -0.6 | -0.7 | 2.7 | -0.5 | 1.8 |
| Other long-term liabilities | 0.0 | 0.3 | -0.2 | -0.4 | -0.3 | 0.9 | 0.4 | -0.7 |
| Distributions from equity method investees, return on investment | | | | | 0.8 | 1.5 | 0.9 | 2.9 |
| Net cash (used in) provided by operating activities | -3.5 | -17.7 | -6.4 | -1.4 | -0.3 | 2.1 | 1.3 | 6.5 |
| |
| Cash flows from investing activities | | | | | | | | |
| Acquisition of property, plant, equipment, and intangible assets, net | -6.8 | -3.5 | -2.7 | -3.3 | -1.5 | -1.4 | -0.8 | -0.8 |
| Payment for disposal of Marshall Mine, LLC | -2.2 | | | | | | | |
| Acquisition of mine development costs | -1.2 | | | | -0.3 | | | |
| Distributions from equity method investees in excess of cumulative earnings | 0.5 | 0.6 | | | 2.3 | 1.0 | | |
| Proceeds from sale of property and equipment | | | 0.0 | 0.0 | 1.2 | | | |
| Net cash (used in) provided by investing activities | -7.6 | -2.9 | -2.6 | -3.3 | 1.7 | -0.4 | 5.4 | 18.5 |
| |
| Cash flows from financing activities | | | | | | | | |
| Net proceeds from common stock issuance | | 15.2 | | | | | | |
| Principal payments on finance lease obligations | -0.3 | -0.3 | -0.3 | -0.3 | -0.4 | -0.2 | -0.1 | -0.3 |
| Principal payments on Arq Loan | -0.2 | 0.0 | | | | | | |
| Dividends paid on common stock | | | | | 0.0 | 0.0 | 0.0 | 0.0 |
| Net cash provided by (used) in financing activities | -0.5 | 23.3 | -0.3 | -0.3 | -0.4 | -0.6 | -0.1 | -0.3 |
| (Decrease) increase in Cash and Restricted Cash | -8.8 | | | | 2.0 | | | |
| |
| Supplemental disclosure of non-cash investing and financing activities: | | | | | | | | |
| Equity issued as consideration for acquisition of business | 0.0 | 31.2 | | | | | | |
| Change in accrued purchases for property and equipment | -0.2 | 0.5 | | | 0.2 | 0.0 | | |
| Paid-in-kind dividend on Series A Preferred Stock | 0.2 | | | | | | | |
| Acquisition of property and equipment under finance lease | | | | | 1.6 | | | |