2.2. Intensive Grinding with Sodium Sulfide (Na2S) As a grinding additive for increasing the lithium solubility, sodium sulfide (Na2S, SS), was used in this study. SS was prepared from sodium sulfide nonahydrate (Na2S9H2O, 96%), supplied from Junsei Chemical Co., Ltd. (Tokyo, Japan), which was calcined at 120 C for 6 h for dehydration. The