Por: Julio Guizado, Carlos Briceño y Miguel Huanca, Compañía Minera Poderosa.

Abstract

Gold is rarely found in nature forming compounds, unlike other metals such as copper or zinc. Its usual presentations are in native state or forming alloys, generally with silver (electrum). It occurs in nature disseminated in small quantities and is commonly found in veins that are genetically related to silicic igneous rocks. 

The main source of gold is the so-called hydrothermal quartz-gold veins, where along with pyrite and other sulfides, the gold was deposited by ascending mineral solutions containing it, i.e. it is simply mechanically mixed with the sulfides and not in the form of any chemical combination. At and near the earth's surface, the sulfides containing gold are usually oxidized, leaving it free and thus making its extraction very easy.

The leaching process using cyanide in an alkaline medium as a leaching agent has been the universal practice for the extraction of precious metals since J.S. MacArthur and brothers R. and W. Forrest patented the technique in 1889. Cyanidation is a hydrometallurgical process based on the use of alkaline cyanide solutions as a chemical means to leach gold and silver contained in auriferous/argentiferous ores. The most important cyanides used in this process are potassium and sodium cyanide.

There are several alternative reagents to cyanide for Au and Ag leaching, but considering the mineralogical variability of each mining operation we will have to perform multiple metallurgical tests varying the operating conditions such as pH, particle size, leachant consumption and leaching time. 

This document details the leaching tests with an alternative reagent to sodium cyanide considering the different mineralogical characteristics of the ore processed in the plant. Compañía Minera Poderosa has two ore processing plants which have a corporate processing capacity of 1,800 metric tons/day, the Marañon plant processes an average of 800 metric tons/day and Santa Maria 1,000 metric tons/day. 

As part of the metallurgical control, the ore entering the milling circuit is sampled on a daily basis, these samples are accumulated monthly and leaching tests have been performed with these composites. The Au leaching values in the metallurgical tests with the ore from the Marañon plant reached 86.5% with sodium cyanide, while the leaching with the alternative reagent reached 87.63%. Considering the Au leaching values in the metallurgical tests with the ore from the Santa Maria plant, these reach 86.44% with sodium cyanide, while the leaching with the alternative reagent reaches 88.05%. 

The Ag leaching values in the metallurgical tests with the MN Plant ore reached 50.21% with sodium cyanide, while the leaching with the alternate reagent reached 55.98%. Considering the Ag leaching values in the metallurgical tests with the SM Plant ore, these reach 48.31% with sodium cyanide, while the leaching with the alternate reagent reaches 51.5%.