Denicking of fatty acid pitch

Unsaturated fatty acids are usually hydrogenated with a nickel catalyst. As a side reaction, nickel dissolves in the fatty acids and forms nickel soaps. On refining the crude fatty acid feedstock by distillation, these nickel soaps are enriched in the bottom fraction (pitch) to a concentration range of 2,500–5,500 ppm. With a heat value comparable to heavy fuel oil, the pitch has been used as fuel. However, due to new environmental restrictions, the nickel concentration must be considerably reduced to limit gas emission of nickel. In this study, solvent extraction was investigated as a separation method for denickling of this valuable oleochemical by-product. At a temperature of 80°C, the nickel content in the pitch was reduced from 2,900 ppm to 4 ppm in three extraction steps by means of 1 wt% citric acid in water. Based on experimental results, a preliminary process flowsheet is presented consisting of extraction, stripping, acid washing and electrolysis. An economic analysis, performed on the extraction process and a process based on direct combustion followed by electrostatic precipitation revealed that direct combustion is economically more advantageous. However, the compatibility of the extraction alternative can be considerably improved by process optimization. The type and concentration of the stripping and acid washing agents, phase ratios, temperature, ionic strength and the equilibrium pH of the aqueous and acid phases are important process parameters that should be considered in the optimization process.