The amount of aqua-regia required for a given batch varies and depends on the proportion that is acid soluble and the quality of the metal present. This quantity requires considerable agitation and stirring to keep the reaction going and is better done in a jar under a fully ventilated fume hood. One to two kilos of scrap material in a 6 liter Erlenmeyer flask is typical though up to as much as 35 kilos have been treated in these flasks. Teflon plastic is also suitable for the strong oxidizing conditions of the aqua-regia and Teflon will tolerate heating if done with care.įor many batches 6 liter Erlenmeyer flasks are used successfully. The scrap gold is placed in the digesting vessel. The aqua-regia can be used immediately, days or weeks and probably months after preparation. A closed aqua-regia vessel can develop enough chlorine pressure to burst. No heat is evolved when mixing but the aqua-regia at once starts to emit chlorine gas, which evolves slowly for several days. These acids mix quietly.īoth acids, and especially hydrochloric, emit acrid fumes. Avoid splashes, protect eyes and work in the open or under a fume hood. The precautions for mixing the acids are simple. If muriatic acid is used (it is usually less costly) the proportions are calculated to be: 1 nitric acid to 4.5 muriatic acid by volume. This and many of the operations described here should be carried out under an efficient fume extraction hood.Īqua regia is a combination of nitric acid and hydrochloric acid (the industrial grade of hydrochloric is sometimes called muriatic acid), it is made by mixing 1 volume of concentrated nitric acid (69% concentration) with 4 volumes of concentrated hydrochloric acid (38% concentration). Any such large pieces should be shotted first to increase surface area. Large pieces of metal take a very long time to digest in aqua-regia. If the scrap contains shellac, rubber wheel particles, rouge or similar material it is best simmer it in lye (sodium hydroxide and water (a saturated solution) in a ratio of 10 volumes of lye/water to 1 volume of bench sweeps. Such golds or any high silver alloy must be melted with several times their weight of copper or brass and shotted to permit dissolution. So-called "green gold" and some low carat white golds contain considerable silver and are very difficult or impossible to dissolve in aqua-regia due to the formation of an insoluble silver chloride film which prevents further action by the aqua-regia. Note: The dust from floor sweepings or from polishing wheel vacuum collectors and similar low-grade scrap requires extensive preliminary treatment which is not described here.
Such material has been found to contain from 20% to 70% gold. The processes described here assume the use of gold scrap comprised of old jewelry and the waste material from the making of gold jewelry-filings, clippings, scrap jewelry pieces, grinding wheel dust, casting spills, sprues, etc.
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