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Precious metals have been extracted since antiquity. However. the advent of modem day hydrometallurgical technique began less than a hundred years ago. Today, cyanidation extraction of precious metals supersedes all hydrometallurgical processesandits versatility is appreciated moreandmoreas thebreadth of its applications is widened through modifications such as dump and heap leaching, carbon-in-leach. carbon-in-pulp, chemical and autoclave oxidation, roast-leach, and ftotation-cyanidation. This paper reviews the advances that have taken place in the hydrometallurgical extraction of precious metals in the last century including the latest development such as bacterial oxidation and leaching with non-cyanide reagents such as thiourea and thiosulfate. Efforts have been made to cover the theoretical aspects as well as practical and economic aspects of these developments.

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The process previously used by one of us (Crozier 1960) for recovering from sediments black, magnetic spherules, of presumed meteoritic origin, was limited to sediment samples of 2 to 4 g. It was thereby very difficult, because of the very steep size distribution, to recover appreciable numbers of the spherules in sizes larger than about 30 microns diameter.The froth flotation process, used extensively for separation of ores in the mineral industry, and for other purposes (Bhappu 1961; Gaudin 1959), has been adapted to the recovery of the black, magnetic spherules, which appear essentially to be composed of magnetite. Thus far, the process has been applied to clay deposits from the San Agustin playa. The steps followed were, (1) dispersal, using a Calgon (sodium hexametaphosphate) solution, (2) flotation with air bubbles, after conditioning with H2S04 (for pH adjustment), sodium sulfonate, ferric chloride, and a commercial frother, (3) refinement of the floated fraction by elutriation and magnetic separation, and (4) mounting for microscopic study.In the smallest size classes--including spherules with diameters less than 10 microns-the numbers of spherules recovered per gram of sediment were smaller than previously found; this may be the result of losses in elutriation. For larger sizes, the numbers and size distribution agreed well with the previous results. By processing 160 g of clay, in 20-g batches, it was possible to extend the size distribution to diameters of about 60 microns. Five spherules larger than 60 microns were found, the largest having a diameter of 103 microns.