Por: Francisco J. Sotillo, PerUsa EnviroMet Inc.

Abstract

The use of High-Pressure Grinding Roll (HPGR) at the right conditions with the aid of chemical additives was developed to liberate silica encapsulated silver sulfides to be included into the reserves from an Andean Region deposit. These silver resources submitted to traditional processing and extractive metallurgy resulted in ~6.7% of Ag extraction using cyanidation (heap leaching technology). Thus, it was necessary to find an economically reliable method to significantly increased Ag extraction from this type of ore.

Gold occurred primarily as a native mineral within the pores of the ore. As cyanide solution entered the pore, it dissolved Au. This Au extraction was typically greater then 80%. In the case of silver, the average content was 16 g/t with areas of up to 100 g/t of Ag. Mineralogical examination showed tiny (1 or 2 microns) particles of acanthite (Ag₂S) and cerargyrite (AgCl), which should be dissolved in a cyanide solution. However, microprobe studies determined that Ag minerals were encapsulated in silica. This means that an impermeable layer was formed around particles preventing their contact with cyanide and their dissolution. Tests in bottle roll at 1.7 mm and column heap leaching tests were carried out with bottle roll tests resulting in 3% higher Ag extraction than column heap leaching. When particle size was reduced to 75 µm a modest improvement to 12%-22% of Ag extraction was observed. Here, decreasing the particle size was used to improve Ag dissolution by increasing the expose of silver sulfides to the leaching solution. However, economical analysis showed that grinding was too costly due to the hard and abrasive nature of the ore. Also, flotation and magnetic separation were not successful due to lack of liberation of silver minerals.

This proposed technology considered a combination of the following: The use of bed particles comminution technology for selective crushing of particles of encapsulated silver sulfides, and the use of chemical additives to enhance the propagation of micro-cracks produced during HPGR selective grinding (HPGR mechanism). The process could be visualized as an “m&m” type of particle in which a soft core of chocolate (silver sulfides) is covered by hard candy (silica layer). Grinding in a bed of particles that takes place in a HPGR, at the right pressure and rate will expose silver minerals (“chocolate”) covered (encapsulated) by silica (“candy”) to a leaching solution through micro-cracks produced during the crushing unit operation. Thus, in the case of encapsulated silver minerals, HPGR crushing should result in residual stresses and micro-cracks of the silica encapsulating layer. Therefore, leaching with a weak 0.05% cyanide solution will be enhanced. Moreover, the use of micro-cracks propagation chemical additives will significantly improve Ag extraction in this 40,000 tpd operation after 100 days of leaching. For this novel technology, a Physical Model-PerUsa of HPGR and four Micro-crack Propagation Additives (Reagents 1, 2, 3, and 4) were used.

The chemical analysis reported 2.45 g/t Au and 48.5 g/t Ag with the feed being prepared at minus 12.75 mm by the research laboratory of the company on a dry basis (0.03% moisture). The sample was to be submitted to a Gilson Material Testing Equipment-Compression Machine to apply the Physical Model-PerUsa to this very hard-abrasive Type I Ore in 150 g subsamples at 3% moisture (154.64 g). The material was submitted to relaxation tests; operating conditions tests at 50, 100, 150, and 200 Mpa of applied pressure at low, medium and high rates of application of force; two dosages of four Micro-crack Propagation Additives tests; screen analyses; and bottle roll cyanidation tests (carried out by the company at its Technology Center).

Relaxation tests demonstrated that the ore was suitable for comminution under compression showing an elastic-plastic behavior. Thus, these tests showed that using compression, as in a HPGR, was adequate. The next step consisted in determining the operating conditions for the Physical Model-PerUsa of HPGR. This model showed that the pressure applied for the calculation of force as a function of displacement matched data obtained. The Specific Energy Absorbed, Required or Resultant in kw-h/ton as a function of applied pressure resulted in a straight line; therefore, validating the model. It was also demonstrated that the Specific Energy Absorbed, Required or Resultant increased as the grinding rate increased. Slow Rate mode of applied pressure resulted in a more efficient Specific Energy in the absence of Micro-crack Propagation Additives, and finer Particle size distribution (PSD). Also, it was shown that the fineness of the PSD increased as the applied pressure increased.

In the presence of Micro-crack Propagation Additives of different adsorption mechanisms and chemical nature (Reagents 1, 2, 3, and 4), the results showed that the Specific Energy Absorbed decreased in the presence of these Micro-crack Propagation Additives and the PSD increased in fineness as the applied pressure increased as compared to that obtained in the absence of Micro-crack Propagation Additives. These effects of the Micro-crack Propagation Additives depended on the rheological characteristics of the reagent added, and its effect on the surface chemistry of the system, as well as, the feed PSD, the threshold of the reagent added necessary to affect the surface chemistry of the system, and the capillary effect created by the reagent solution in the pores of particles and bed of particles. Reagent 1 may involve physical adsorption; Reagent 2, a chelating and rheology modifier may specifically adsorb onto the ore’s surfaces (strong adsorption); Reagent 3, a rheology modifier that may specifically adsorb onto the ore’s surfaces (strong adsorption); and Reagent 4 may physically adsorb onto silica. Tests demonstrated that these reagents decreased the Specific Energy Absorbed, Required or Resultant and produced a finer PSD.

Selected pretreated samples using the Physical Model-PerUsa of HPGR in the absence and presence of Micro-crack Propagation Additives were submitted to cyanidation bottle roll tests at the company’s laboratory for different leaching times, 24 hours, and 432 hours (18 days). An extrapolation to 100 days was made assuming a model of a First Order Reaction Kinetics. It was demonstrated for all tests that Ag extraction kinetics was significantly increased than that obtained for the Baseline Test. By applying this novel technology Ag extraction was even higher than that reported on previous Baseline tests after 14 days of cyanidation (~6.7%), proving the concept. This was true in the absence and presence of Micro-crack Propagation Additives, but being additive depended and increasing as the applied pressure increased. The series of Ag extraction for 432 hours (18 days) of leaching for tests at 200 Mpa of applied pressure and reagent additives used was: Baseline (7.6%) <<< HPGR (15.4%) < HPGR + Reagent 1 = HPGR + Reagent 3 (17.2%) < HPGR + Reagent 2 (17.6%) < HPGR + Reagent 4 (18.6%). In general, the higher the applied pressure the higher Ag extraction.

The extrapolation to 100 days (2,400 hours) of leaching using a First Order Reaction Kinetics model rendered the following sequence for Ag extraction without reaching a complete extraction process: Baseline (10%) <<<HPGR (30%) << HPGR + Reagent 2 (37%) < HPGR + Reagent 1 = HPGR Reagent 3 (39%) << HPGR + Reagent 4 (46%). These data of the new technology applied were comparable to grinding at 35-80 µm. Also, NaCN and lime consumptions were lower than in the Baseline Test.

In the case of Au extraction, the Baseline Test reported Au extraction of 84.9%. In the presence of Micro-crack Propagation Additives at Slow Rate mode of applied pressure, this new technology resulted in higher Au extraction than those extractions obtained for the Baseline Test and the HPGR only. With HPGR + Reagent 4 Micro-Crack Propagation Additive at 200 Mpa of applied pressure a 93.1% Au extraction was obtained. Thus, this technology also increased significantly gold extraction, proving the concept.