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23 hours agoI’m no expert, but what about waste sulfur in smelting (also mentioned in the article)?
https://www.sciencedirect.com/science/article/abs/pii/S0013935125017244
https://link.springer.com/chapter/10.1007/978-3-032-00102-3_4
https://www.sciencedirect.com/science/article/abs/pii/S0013935125017244

Can you know for certain?