16 The Patio and Oazo Process. 



there is a wooden trough through which water flows freely. 

 At the opposite end there is a trough, B, with a gate having 

 three plugged holes through which to let off the slimes. The 

 scrapings are thrown into this trough and are carried by the 

 water into the uoliche; a boy sitting on the edge of the chuza keeps 

 the material in this bowl in constant agitation with his feet. This 

 disintegrates the material. The mercury and amalgam fall into 

 and sink to the bottom of the bowl; the heavy particles other than 

 these are carried into the chuza, and the slimes run off by the 

 trough B, from which, if of value, they are collected in settling- 

 tanks, and if not, run to waste. The tails in the chuza are 

 concentrated by drawing out the plugs and letting the lighter 

 material flow away, but the work is done by hand, and yields a 

 very rich material called cabezuela, which is sold. When the 

 rich tailings have been separated, the top layer of a coarsely 

 ground ore is removed with iron scrapers and set on one side 

 for the next charge. The amalgam is scraped up and carried 

 in wooden bowls, uateas, to the washing-tanks. The gold 

 amalgam collected in the bowl is strained and retorted as the 

 silver is, but not with it. The surplus mercury is not mixed 

 with that from the straining of the amalgam from the patio. It 

 contains considerable gold and silver, and is always used over 

 again to catch the free gold in the arrastra, as amalgamated is al- 

 ways much more lively in catching free gold than pure mercury. 

 The amount of gold separated in this way varies from 30 to 50 

 per cent, of the total contents of the ore. This gives a bullion 

 that will pay to part. The rest of the gold is recovered in the 

 patio, either in the direct washing of the pulp, or in that of the 

 polvillos, or is lost in the float during the various processes of 

 washing. When the ore does not contain native silver, 10 to 12 

 per cent, of precious metals contained are taken from the arras- 

 tra. The amalgam taken at the clean-up usually contains from 

 18 to 22 per cent, of silver.* The coarser the silver is, the less 

 mercury is required. 



The loss of mercury in the arrastra is owing to the formation 

 of salts of mercury by the impurities contained in the ores, and 



Engineering and Mining Journal, Vol. 33 p. 104 to 114. 



