Characterization of gold-bearing tailings by diagnostic leaching for reprocessing by flotation and leaching [Caracterización de relave aurífero mediante lixiviación diagnóstica para reproceso mediante flotación y lixiviación]

The purpose of the present investigation is the evaluation to reprocess the Sayapullo tailings, to extract the gold and silver that it still has, for which an initial chemical analysis, granulometric analysis and mineral flotation tests for gold and silver were made, then The percentage of gold extraction was performed by diagnosis of oxidative leaching (DLT). The DLT consists of the quantification of the percentage of gold extraction by cyanidation as the oxidation state of the mineral increases. For this, progressive chemical attacks were carried out with oxidizing agents for each stage in the following order: Sodium Carbonate (Na 2 CO 3 ), Hydrochloric Acid (HCl), Sulfuric Acid (H 2 SO 4 ) and Nitric Acid (HNO 3 ). The gold content in the solutions was determined by ICP-OES and in the solids by fire assay. It is concluded that the Sayapullo tailings have 44.68% free gold, 3.47% gold associated with sulfates, 11.64% gold associated with carbonates, 6.13% gold associated with Cu-Zn sulfides, 31.49% associated with pyrites and arsenopyrites; and 2.60% gold associated with silicates.


Introduction
Extractive mining is based on the extraction of valuable metals contained in ores, these metals can be extracted by different types of metallurgical processes, however, refractory minerals cannot be efficiently processed by any of the known metallurgical processes, In the case of conventional cyanidation of a refractory mineral, it generates a low recovery, causing an excess of cyanide consumption. In addition, it is important to consider that the particle size is a variable that influences the metallurgical processes in the extraction of valuable metals, since it has been proven that the smaller the material to be processed, the greater the extractions. (Misari, 2010).
On the other hand, in the old extractive metallurgical processes, due to ignorance or lack of adequate technology, the tailings were discharged with grades that could now be considered important for their extraction. The most widely used technique for gold extraction by hydrometallurgical is cyanidation. (Misari, 2010) One way to characterize and determine the refractoriness of gold and in which minerals gold is associated or encapsulated is diagnostic leaching (DLT), and together with scanning electron microscopy (SEM), which complements the characterization of minerals or tailings (Celep & Serbest, 2015). The complementary and very useful techniques in the characterization of minerals, is through the use of software that helps in the identification of minerals such as QUESCAM (Mohammad et al., 2017).
In this investigation, chemical analysis was carried out using ICP-OES of the tailings obtained from Sayapullo, a granulometric analysis test to determine the percentage through mesh # 200 (75 um), then with the data obtained, mineral flotation tests were carried out to determine the extraction of gold, with the concentrate obtained from the flotation tests, the characterization was carried out by diagnostic leaching (DLT), previously the concentrate was tested by grinding to obtain the granulometry of 90% through mesh #400 (38 um), with the in order to evaluate the extraction of gold and determine in which mineralogical species it is found in the highest percentage.

Materials and Methods a. Tailings sampling
The original gold tailings are from the district of Sayapullo, department of La Libertad, the sample corresponds to 200 kilos of tailings that was sampled by carrying out 04 test pits with an area of 1 m 2 and a depth of 1 m, this sample was transferred to a laboratory where it was dried at 105 ºC for 24 hours in a Schemin oven, then the entire sample was homogenized by the cone method using a shovel, followed by cracking using a Jones cracker, until representative samples were obtained for the metallurgical tests and trials.  b. Chemical analysis of tailings A 500 g sample was used for the chemical analysis of the elements silver, copper, lead, zinc, iron, arsenic and antimony, using an inductively coupled plasma optical emission spectrometer (ICP -OES), Teledyne brand, model Prody XP with dual view. For the gold analysis, a 500 g sample was used and it was carried out by means of the fire test. For the determination of the acidity of the mineral, it was carried out using the Hanna brand multiparameter pH and temperature meter, 100 g of sample was used with 200 ml of distilled water, and a magnetic stirrer where the sample was mixed with the water for 30 minutes, then the pH was measured. (Vega et al, 2021a) c. mineral flotation Mineral flotation tests were carried out, using anionic collectors due to the presence of iron sulfides, as well as MIBC foaming agent. 1 kg of ore was used with a ratio of L/S: 2/1, dosing 15 g/MT of potassium amyl xanthate anionic collector, 10 g/MT of Aero 404 dithiophosphate and 30 g/MT of Methyl Isobutyl Carbinol (MIBC). The concentrate was dried by weight and analyzed chemical for gold and silver by fire assay. Then the concentrate obtained was subjected to diagnostic leaching. (Teague, Swaminathan & Deventer, 1998).

d. Diagnostic Leaching (DLT)
Diagnostic leaching was developed in order to determine the extraction of free gold, or that is released after being oxidized the mineralogical species that may contain it, through the use of acids, bases and roasting in the case of carbonaceous minerals. (Lorenzen, 1995). The DLT stages used for the Sayapullo tailings are described below.

d.1 Cyanide Washing and Cyanidation
A mixture of two reagents in solution was prepared, adding 0.02 g/L of NaOH (extra pure) Loba Chemie brand and 0.1 g NaCN (analitical agent) Panreac brand, the prepared solution was used after each leaching with acids.
A flash leach (QLT) was done with a liquid/solid ratio of 2/1. NaOH was added to obtain a pH of 10-11, then NaCN (analitical agent) was added to obtain a concentration of 5.0 g/L, and leaching was carried out under normal conditions, that is, at room temperature (20 ºC) and a pressure atmosphere. At the end of the leaching, the pH was measured with a Hanna brand multiparameter pH and temperature meter. To measure the remaining cyanide, it was done by volumetry with AgNO3 as a titrant and 5-(4-Dimethylaminobenzylidene)-rhodanine as an indicator. The pulp was filtered, the filtered solution was analyzed for gold, then the solid was washed with distilled water and filtered. The filtered solid was dried and weighed ready for the second diagnostic leaching stage (DLT). This step is repeated throughout the DLT process between each oxidative acid leach.

d.2 Leaching with soda ash
If the ore has minerals of calcium sulfate or sodium sulfate, these compounds and some other calcium compound containing the ore are solubilized with a prepared aqueous solution of sodium carbonate, the aqueous leaching solution is prepared with a concentration of 60 g/t of sodium carbonate (analitical agent), L/S ratio of 10/1, is boiled for 2 hours, then the sample is filtered, washed and the remaining solids are mixed in a 0.1 M HCl solution at a L/S ratio of 2:1, leach at pH 1 until acid consumption is negligible.

d.3 Oxidizing Leaching with Hydrochloric Acid
Of HCl (analitical agent), it was prepared at 32% by weight with distilled water, the leaching was carried out with a L/S ratio of 3/1, at an average temperature of 60 °C, it was mixed in the magnetic stirrer for 4 hours. The sample was then washed and filtered with distilled water before leaching with sodium cyanide. The final mass of the sample (dry sample before cyanidation) was recorded, Table 1 shows the chemical analysis of the tailings from Sayapullo, where it is observed that it contains a high percentage of iron (10.5%), gold (2.55 g/t) and silver (72.45 g/t).  Table 3 shows the floatation metallurgical balance for gold and silver of the Sayapullo tailings, where a high recovery of Ag (72.83%), gold (50.47%) and with a grade of 215.32 g/t and 4.75 g/ t respectively.  Table 4 shows what is obtained by means of diagnostic leaching (DLT) carried out on the flotation concentrate, carried out in 6 stages. The average free gold extraction percentage was 44.68%, with this result it is determined that the gold in gold tailings can only be recovered up to that limit, that is, free gold, while the remaining fraction of gold in the sample is encapsulated in silicates, pyrite, copper sulfides, etc. This indicates that a pretreatment is necessary to eliminate the matrix that encapsulates the gold. Calculations were made by gold analysis of the pregnant solution obtained after each sodium cyanide leaching step. To determine the percentage of gold extraction, the metallic content of the initial ore (tailings) was used as a base of 100%. Then the accumulated gold extraction was obtained by successive stage. In Figure 2, the weight losses due to oxidative leaching are presented with the different lixiviants used for each stage, where it is observed that 85% of mass loss occurs in the oxidation with nitric acid, and corroborates with the high extraction of gold at that stage.
Figure 2. Percent weight loss by stages after DLT Figure 3 shows the results of the leaching tests carried out after each chemical attack. It is observed that, in the leaching after the attack with HCl and HNO3, including free gold, 11.64% and 31.49% extraction of gold are obtained gold respectively, with a total of 97.40% of total gold extracted, the difference remains in the silicate matrix. (Lorenzen & Tumilty, 1992).

Conclusions
The tailings from Sayapullo present metallic elements of interest (Au, Ag), to carry out reprocessing for gold and silver that are associated with pyrite minerals. The acidity of the tailings has a pH of 2.6, being very acidic, the relative density is 2.70 g/cm 3 . The granulometry of the Sayapullo tailings, presents a low percentage through the 200 mesh, (27.40%), for which it is required to do regrinding for leaching, in this case, gold and silver associated minerals were floated 72.83% and 50.47% gold, with a concentration ratio of 3.8. The concentrate obtained by having a decrease in weight by the ratio of 3.8, and an increase in gold and silver grades, is regrinded to obtain 90% through 400 mesh. Finally, with the reground concentrate, diagnostic leaching (DLT) is performed, where 44.68% gold extraction was obtained by free gold cyanidation, then the following high gold extraction was obtained in the oxidation stage with HNO3 (31.49%). The Sayapullo tailings present a good opportunity to reprocess, given the high prices of gold and silver in the international market, and also to make an adequate tailings deposit to avoid the generation of acidic waters. It is recommended to do research with the tailings from the new process, since in the flotation carried out a high presence of iron (pyrite) is observed in the concentrate, so the tailings could be used in construction materials.