Recovery of Trace Rare Earths from High-Level Fe3+ and Al3+ Waste of Oil Shale Ash (Fe-Al-OSA)

An experimental investigation was undertaken to study the high-efficient and clean enrichment of trace rare earths from high-level Fe3+ and Al3+ waste of oil shale ash (Fe-Al-OSA). The optimum leaching temperature, ratio of solid to liquid, acidity, and reaction time for Fe-Al-OSA were 30 degrees C, 1:7, 50% (v/v), and 1 h by the Taguchi method, respectively, and the leaching rate of rare earths has been reached up to 96.24%. The optimal extraction conditions for removing Fe3+ from leaching liquor of Fe-Al-OSA were as follows: the organic phase was 30% N235 + 10% isooctyl alcohol (ROH) + 60% n-heptane, acidity of aqueous phase was about 3.00 mol/L, and phase ratio (V-o:V-w) was 8:5. More than 92.09% of Fe3+ was recovered by using countercurrent extraction process with 4-5 stages. The 99.76% of high pure Fe byproduct was obtained by stripping the loaded N235 organic phase, and it can be used as chemistry pure reagent directly. After adjusting the pH of the raffinate to 6.00 with MgO and saturated Na2CO3 solution, all of the Al3+ and rare earths were transformed to hydroxide precipitation and separated from the solution, together with other small amounts of coexisting metal ions such as Ca2+ and Mg2+. Then, the precipitation was washed, collected, and dissolved by HNO3. Rare earths can be separated from the solution by solvent extraction with 30% tributyl phosphate (TBP) +70% n-heptane at the phase ratio (V-o:V-w) 3:2. The recovery rate of rare earths from Fe-Al-OSA has reached 86,30% in the whole separation process. The residual Al3+ in solution was recycled. This work shows that such a treatment route is one kind of highly efficient and clean method for separating Fe3+ and Al3+, and concentrating rare earths from Fe-Al-OSA. The solid waste (Fe-Al-OSA) from the refinery can also be utilized effectively to solve the ecological and environmental problems caused by the waste heap.