Barrick seeks an economically viable method to significantly increase silver recovery from the type of ore found at Veladero. Known processes and technologies tried to date have been unsuccessful.

	The high permeability of ore is demonstrated by the uptake of fluorescent dye.

The gold and silver at the Veladero mine were deposited in separate geological events, and are, therefore, distributed within the host quartz rock in different manners. The gold occurs primarily as the native mineral within the pores of the ore. As cyanide solution enters the pores, it dissolves the gold. Gold recovery is typically greater than 80%. Dye penetration studies on selected ore samples demonstrate that, on a micro-scale, the ore is uniformly porous and permeable.

The average silver value within the mine is 16g/t (grams/tonne), while the drill core values measured within the active mine area range from non-detectable to 100 g/t. Some crushed samples have shown localized values over 450g/t.  Overall, Veladero holds 180 million ounces of silver contained in gold reserves.

Silver process recovery averages 6.7%, but there are areas within the mine where recovery is significantly higher.

Mineralogical examination provides an explanation for the ranges in silver recovery. Studies of the Veladero ore indicate the majority of the silver exists as tiny (1 to 2 micrometer) particles of acanthite (Ag₂S), with cerargyrite (AgCl) being the second most abundant mineral. When the ore contains a high proportion of acanthite, the silver recovery is low. Conversely, high proportions of cerargyrite yield higher silver recoveries. Only very limited areas of the ore-body contain high levels of cerargyrite.

Both acanthite and cerargyrite will dissolve in a solution of cyanide; however acanthite, being a sulphide mineral, dissolves at a much slower rate than non-sulphidic forms of silver. Normally, this effect can be overcome by employing a stronger cyanide solution; however, this does not work with the Veladero ore.

Detailed studies were conducted to determine why the acanthite component did not dissolve. Micro-probe analysis of these acanthite particles detected silver, sulphur, silicon, and oxygen, suggesting the acanthite is encapsulated in silica. Encapsulation of the particles means that an impermeable layer is formed around the particle, preventing its contact with and dissolution by cyanide or other silver-dissolving solutions.

Small silver and acanthite particles are located on the surface of finely ground quartz. Note scale in micrometers.

Tests conducted on synthetic acanthite have demonstrated that this mineral dissolves more readily when a catalyst is added to the cyanide leach solution. When the test was repeated using ore ground to less than 40 microns, the positive effect of the catalyst on silver extraction was not observed. This is a further indication that silica encapsulation of the acanthite, and not the silver mineral form, is responsible for the low recovery of silver.

Laboratory tests demonstrate an inverse relationship between the level of silver extraction, and the degree of silver encapsulation.  Analysis of the ore detected no organic carbon, clay, high sulphides or other elements known to interfere with the recovery of gold and silver by the phenomenon referred to as “preg-robbing.”

Other pre-treatment and dissolution media were tested. Silver extraction did not show any marked improvement when lime-boiling, pressure oxidation, nitric acid, ferric chloride or reductive leaching systems were tested. Improved extraction was observed only when hydrofluoric acid, a known reagent for silica dissolution, was employed. Full scale use of hydrofluoric acid is impractical for a number of reasons, the most obvious being the prohibitive amount of this very hazardous chemical required to dissolve all the silica in an ore consisting of greater than 90% quartz.

Another way to improve silver dissolution is to increase the likelihood the silver sulfide will be exposed to the leaching solution. Decreasing the particle size will increase the surface area exposed. An extensive series of laboratory tests demonstrated that the reduction of the particle size of the feed from 1.7mm to 75µm improves silver recovery in the order of 20%. By further reducing the particle size to less than 10 microns, silver recovery increases to approximately 40%. An analysis of the cost of grinding versus the values of silver and gold recovered suggests that size reduction to less than 32µm would only be economical for a very minor portion of the ore body. This analysis did not include the cost added to handle fine materials in a heap leach process. Finely ground materials must be agglomerated prior to processing by heap leaching to prevent it from becoming airborne.

	Silver sulphides are the main silver bearing minerals in the ore, with acanthite being predominant. The acanthite (Ag2S) exists as 1 to 2 µm particles isolated by the quartz matrix or as sub-micronic particles embedded in the quartz matrix.

The ore has been characterized as hard and abrasive. This makes the ore difficult and expensive to grind, consuming high amounts of electricity and resulting in costly wear of equipment.

The Veladero site is located in a cold climate zone, and efforts have been made to make use of waste heat and to minimize heat loss, particularly in the cycling of leach solutions.

Preliminary attempts to upgrade the ore using flotation, gravity and magnetic separation have not proven successful. The success of these mineral processing techniques primarily relies on the liberation of the mineral to be upgraded which does not appear to occur with fine grinding. Barrick is currently exploring mineral sorting techniques.

Other methods of liberating the acanthite from the silica matrix have been examined briefly, including various types of crushing mechanisms, as well as heat-quench and microwave treatments to destabilize the quartz matrix.

None of these various methods has showed a marked improvement in silver extraction.

Barrick seeks technologies or methods that will economically increase silver recovery from this type of ore.