Synthesizing LiFePO4 by phosphate & iron recovered from sludge-incinerated ash and Li extracted from concentrated brines

Abstract

Phosphorus (P) recovered from sludge-incinerated ash (SIA) could be applied to synthesize highly added-value products (FePO₄ and LiFePO₄) with in situ Fe in SIA. Indeed, LiFePO₄ is a future of rechargeable batteries, which makes lithium (Li) highly needed. Alternatively, Li could also be extracted from concentrated brines to face a potential crisis of Li depletion on lands. Based on H₃PO₄ and Fe³⁺ co-extracted from the acidic leachate of SIA by tributyl phosphate (TBP), FePO₄ (31.2 wt% Fe, 17.6 wt% P and the molar ratio of Fe/P = 0.98) was easily formed only adjusting pH of the stripping solution to 1.6. Interestingly, the organic phase from the first-stage co-extraction process of Fe³⁺ and H₃PO₄ could be utilized for Li-extraction from salt-lake brine, based on the TBP-FeCl₃-kerosene system, and a good performance (78.7%) of Li-extraction and separation factors (β) (186.0–217.4) were obtained. Furthermore, the compounds with Li-extraction are complex, possibly LiFeCl₄∙2TBP, in which Li⁺ could be stripped to form Li₂CO₃ by 4.0 M HCl (with a stripping rate up to 83%). Besides, Li₂CO₃ could also be obtained from desalinated brine by adsorption with manganese oxide ion sieve (HMO) and desorption with HCl. In the two cases, almost pure Li₂CO₃ products were obtained, up to 99.7 and 99.5 wt% Li₂CO₃ respectively, after further purification and concentration. Finally, recovered FePO₄ and extracted Li₂CO₃ were synthesized for producing LiFePO₄ that had a similar electrochemical property (69.5 and 77.8 mAh/g of the initial discharge capacity) to those synthesized from commercial raw materials.