(excluding depletion, depreciation and amortization). We believe our cost of sales (excluding depreciation, depletion and amortization) on a per unit basis has stabilized in the short-term, although we anticipate additional efficiency opportunities as we increase REO production volumes in our milling and flotation circuit over time.
Royalty expense paid to related party. Royalty expense paid to related party increased by $0.8 million, or 76%, to $1.9 million in the nine months ended September 30, 2020, from $1.1 million in the nine months ended September 30, 2019, reflecting our increased product sales (the royalty rate is 2.5% of our gross revenue from products derived from mined ore).
General and administrative expenses. General and administrative expenses increased by $4.4 million, or 43%, to $14.6 million in the nine-months ended September 30, 2020, from $10.2 million in the nine months ended September 30, 2019, reflecting a $3.4 million increase in professional service fees, mainly accounting advisory services related to the Business Combination.
Depreciation, depletion and amortization. Depreciation, depletion and amortization increased by $1.1 million, or 29%, to $4.8 million in the nine months ended September 30, 2020, from $3.7 million in the nine months ended September 30, 2019, reflecting the impact of additional equipment purchases.
Accretion of asset retirement obligations (ARO) and environmental remediation obligations. Accretion of ARO and environmental remediation obligations remained relatively flat between periods.
One-time settlement charge. We recorded a one-time non-cash settlement charge of $66.6 million in the nine months ended September 30, 2020 in connection with the termination of the DMA. See “—Recent Developments and Comparability of Results—Our Relationship with Shenghe.”
Other income, net. Other income, net, was $0.3 million in the nine months ended September 30, 2020, reflecting interest income on restricted cash and an environmental incentive credit, compared to other income, net of $4.1 million in the nine months ended September 30, 2019, mainly reflecting a gain on the disposal of idle assets and interest income.
Interest expense. Interest expense increased by $0.9 million, or 34%, to $3.6 million in the nine months ended September 30, 2020, from $2.7 million in the nine months ended September 30, 2018, mainly reflecting an increase in the interest expense from the issuance of a non-interest-bearing debt instrument to Shenghe Resources (Singapore) in connection with the amended offtake agreement that was entered in June 2020. See “—Recent Developments and Comparability of Results—Our Relationship with Shenghe.”
Income tax expense. Income tax expense was $0.2 million in the nine months ended September 30, 2020 and negligible in the nine months ended September 30, 2019.
Net income (loss). Net loss increased by $38.1 million to $45.9 million in the nine months ended September 30, 2020, from $7.8 million in the nine months ended September 30, 2019, due to the one-time settlement charge and the other reasons discussed above.
Comparison of Year Ended December 31, 2019 to Year Ended December 31, 2018
The following table summarizes our statement of operations for the periods indicated:
| | | | | | | | | | | | | | | | |
| | Year Ended December 31, | | | 2019 vs. 2018 | |
| | 2019 | | | 2018 | | | $ Change | | | % Change | |
| | (in thousands, except percentages) | |
Product sales (including sales to related parties) | | $ | 73,411 | | | $ | 67,418 | | | $ | 5,993 | | | | 9 | % |
Operating expenses: | | | | | | | | | | | | | | | | |
Cost of sales (excluding depreciation, depletion and amortization) | | | (61,261 | ) | | | (56,252 | ) | | | (5,009 | ) | | | 9 | % |
Royalty expense paid to related party | | | (1,885 | ) | | | (1,032 | ) | | | (853 | ) | | | 83 | % |
General and administrative expenses | | | (11,104 | ) | | | (14,565 | ) | | | 3,461 | | | | -24 | % |
Depreciation, depletion and amortization | | | (4,687 | ) | | | (2,455 | ) | | | (2,232 | ) | | | 91 | % |
Accretion of asset retirement obligation and environmental remediation obligation | | | (2,094 | ) | | | (1,998 | ) | | | (96 | ) | | | 5 | % |
| | | | | | | | | | | | | | | | |
Total operating expenses | | | (81,031 | ) | | | (76,302 | ) | | | (4,729 | ) | | | 6 | % |
Operating loss | | | (7,620 | ) | | | (8,884 | ) | | | 1,264 | | | | -14 | % |
Other income, net | | | 4,278 | | | | 839 | | | | 3,439 | | | | 410 | % |
Interest expense | | | (3,412 | ) | | | (5,420 | ) | | | 2,008 | | | | -37 | % |
| | | | | | | | | | | | | | | | |
Loss before income taxes | | | (6,754 | ) | | | (13,465 | ) | | | 6,711 | | | | -50 | % |
Income tax expense | | | (1 | ) | | | (1 | ) | | | — | | | | n.m. | |
| | | | | | | | | | | | | | | | |
Net loss | | $ | (6,755 | ) | | $ | (13,466 | ) | | $ | 6,711 | | | | -50 | % |
| | | | | | | | | | | | | | | | |
15