Recovery of rubidium from power plant fly ash

Abstract

Although valuable rubidium (Rb) is present in large quantities in many natural sources, its concentration is low, and recovery are often uneconomical. Rubidium can concentrate on power plant fly ash when peat and wood residue is used as fuel. Accurate determination of rubidium and recovery from fly ashes originating from two Finnish combustion power plants were studied in this thesis. Three different analysis methods based on ICP-OES (inductively coupled plasma – optical emission spectrometry), GFAAS (graphite furnace atomic absorption spectrometry), and ICP-MS (inductively coupled plasma – mass spectrometry) for determination of rubidium from different fly ash derived matrices were developed and evaluated. ICPOES was found to be a useful analysis technique at ppm concentration levels when the observed ionization interference was corrected using the MLR-method. The GFAAS and ICP-MS analysis methods were found to be most suitable for the analysis of low rubidium concentrations. For the multi-elemental analysis of several elements, ICP-techniques are preferred. Properties of fly ashes originating from different fuel contents were investigated by sieving the ashes into fractions by their particle size and determining their elemental content. The highest Rb concentrations (114 ± 3 mg kg-1) were in fly ashes of 50% peat and 50% wood residue as fuel. Sieving the fly ash before the recovery of Rb did not offer any real benefit. The concentration of rubidium in different sized ash particles presumably depends on the origin of the fuel and the design of the power plant. The developed recovery technique for Rb consists of oxalic acid leaching of fly ash, preparatory treatments of the leachate (recovery of oxalate and pH-adjustment) and liquid- liquid extraction using chlorophene. The oxalic acid leaching procedure is part of a patented method for recovery of rare earth elements (REEs) and platinum group metals (PGMs) from fly ash. The capability of oxalic acid to leach Rb from fly ash was compared to the total digestion results, and pH-adjustment prior to the liquid-liquid extraction was optimized. Using 1 mol L-1 chlorophene in chloroform as an organic phase with an A:O phase ratio of 1 and four extraction steps, 94% recovery of Rb was achieved. For stripping of Rb, 0.01-1 mol L-1 HCl was applied with excellent recoveries (96-100%). After using 0.01 mol L-1 HCl as a stripping solution, the extraction cycle was repeated three times to ensure low matrix content in the final solution. In addition, the feasibility of potassium copper(II) hexacyanoferrate sorbent for the recovery of Rb was investigated. The synthesized sorbent was 3D-printed into a functional filter, which was found effective for the recovery of Rb from synthetic samples mimicking oxalic acid leached fly ash. However, the desorption of Rb from the functional filters was challenging, resulting in only a 60% recovery of Rb with 1 mol L-1 ammonium chloride solution. Desorption of Rb from the filters requires further investigations. Keywords: Rubidium, fly ash, recovery, digestion, leaching, liquid-liquid extraction, chlorophene, potassium copper hexacyanoferrate, functional filter, ICP-OES, ICP-MS, GFAAS