The role of aluminum concentration and pH in the purification of waste Li-ion battery leach solution was investigated using NaOH and LiOH as neutralization agents ([H2SO4] = 0.313 M, t = 6 h). Solution was prepared from synthetic chemicals to mimic real battery leach solution. Results demonstrate that pH (3.5–5.5) has a significant effect on the precipitation of metals (Fe, Al, Ni, Cu, Co, Mn, and Li), whereas higher temperature (T = 30 and 60 °C) decreases the precipitation pH of metals. Iron and aluminum were both found to precipitate at ca. pH 4 and the presence of aluminum in PLS clearly decreased the separation efficiency of Fe vs. active material metals (Ni, Co, Li). In the absence of dissolved aluminum, Fe precipitated already at pH 3.5 and did not result in the co-precipitation of other metals. Additionally, the Al-free slurry had a superior filtration performance. However, aluminum concentrations of 2 and 4 g/L were found to cause loss of Ni (2–10%), Co (1–2%) and Li (2–10%) to the Fe-Al hydroxide cake at pH 4. The use of LiOH (vs. NaOH) resulted in 50% lower co-precipitation of Ni, Co and Li. Overall, these results demonstrate that hydroxide precipitation can be an effective method to remove iron from battery waste leach solutions at aluminum concentrations of < 2 g/L only. Although the highest level of lithium loss in the cake was found at pH 4, the loss was shown to decrease with increasing pH.