recovering metal values from dilute aqueous solutions can be used with advantage typically such an operation features countercurrent flow of the organic and aqueous phases employing a battery of extraction units a cyclic process for carbonylating propylene in 80 aqueous hf is schematically shown in fig2 this schematic further serves to illustrate the recovery of the aqueous hf for recycling in accordance with this invention in effecting the carbonylation reaction referred to therein 42 g of propylene 1 0 mole were slowly pumped against about 190 atmospheres of carbon monoxide to a charge containing 75 g water 4 16 moles and 300 g hf 15 moles at 87 u00b0 92 u00b0 c while continuously repressuring with carbon monoxide to maintain 185 190 atmospheres a selectivity to iba of 97 was attained in 43 minutes the composition of the resultant reaction mixture is as shown in the schematic this product is thereupon distilled to provide 250 g anhydrous hf for direct recycle to the carbonylation reactor as shown in the schematic a distillation residue is provided having the composition noted therein since 18 g of water were consumed in the reaction a make up of this amount of water is eventually needed in order to provide 80 aqueous hf in the carbonylation reactor in the absence of this make up water the distillation residue contains about 47 aqueous hf in view of the fact that the extraction efficiency is somewhat higher at a lower hf concentration it is preferred to add the requisite water to the residue prior to extraction as.