Deportment of gold in refractory gold ores by diagnostic leaching technique to estimate their suitability for biooxidation pre-treatment

Mineralogy and ore properties on process selection and optimization steps in mineral dressing industry as both prefeasibility studies and quality control measures in on-going plant processes, make mineralogical investigation more critical and important aspect. This aspect is getting more important especially in the case of gold, not only due to its forms, carriers, and low occurence in ores, but also includes the gangue minerals and other species that may affect the processing. While the various techniques, as optical microscopy, scanning electron microscopy, electron microprope analysis, proton X-ray emission, dynamic secondary ion mass spectrometry etc., have been using for the ore characterization, Diagnostic Leaching Technique offers a simple and cheap alternative. Diagnostic leaching starts with the detailed mineralogical examination that has to be the first step in the selection of processing steps of a gold deposit. The term of refractory gold ore stands for the gold particles that are not amenable to recovery by conventional cyanidation process by the low extraction (less than 80%) even after fine grinding. Thus, refractoriness could be occured within or by various gang minerals, for example gold in sulfide-type minerals or carbonaceous matter. Some gold ores would be termed as double refractory when they occur in both organic carbonaceous matter and sulfide minerals. Diagnostic leaching was developed by the Anglo American Research Laboratories (AARL) to find answers upon the mineralogical assays of the gold containing ores. Diagnostic leaching is a series of various acid leaches, which are more aggressive than the previous leaching steps, that combined with inter-cyanadation steps. Different solutions are used to leach different mineral matrixes, it has to be notated that, Diagnostic Leaching is not a single-route procedure that could be applied to all samples. In this study, diagnostic leaching procedure was used to determine the level of the refractoriness of two different gold ore samples from the two different regions of Turkey: Kışladağ and İliç gold mines. The samples are carefully collected and investigated according to their sequential solubility by diagnostic leaching in order to check their suitability of biooxidative pre-treatment in the further experiments. Another aim of this study is to understand whether the collected gold bearing ore samples are suitable for biooxidative pre-treatment for the further experiments. Two gold bearing ore samples were supplied and collected from Kışladağ and İliç gold mines and the regions of Turkey, respectively. Samples were reduced to -106 µm in particle size. First, they were crushed using a primary jaw crusher followed by a roll crusher and the samples were homogenized throughly and a representative sample of 2 kg was drawn and kept sealed in separate polyethylene bags for grinding. The representative ore sample (125 gr) was then subjected to ring mill and was ground for 1 minute to get the entire product passing through 106 µm. Diagnostic leaching followed by cyanide leaching tests were performed into three stages using a propeller-mixer at 1200 rpm. In our experiments, Stage I is the NaCN leaching. Stage II is the leaching step that contains simple hydrochloric acid leach followed by cyanide leaching to extract gold. Stage III is the HNO3 leaching step together with cyanide leaching . In these experiments, 20 % v/v of HCl and 30 % v/v of HNO3 were used as extractants. At the end of all leaching steps, the samples were filtered, washed with water, dried and weighted to obtain the weight loss, and this operation has been applied for all leaching steps. The cakes were dried and analyzed by ALS Laboratory for fire assay of gold and silver. Iron has been traced for acidic leaching steps to obtain the specific mineral dissolutions via Atomic Adsorption Spectroscopy. Cyanide concentration was determined by the silver nitrate titration method using rhodanine as an indicator. Gold recovery of İliç and Kışladağ samples were obtained 73% and 74% in the first cyanide leaching steps (48 hours). Guay (1981) classified refractoriness with a gold recovery less than 80 %, thus İliç and Kışladağ gold samples represent low refractoriness ores. However, gold recovery rate of HCl and HNO3 pre-leached samples were increased upto 84% and 98% for Iliç, and to 85% and 96% for Kışladağ sample, respectively. Additionally, low Fe and S dissolution were obtained until HNO3 leaching step and presented that these elements are related to the sulfide type minerals such as pyrite, chalcopyrite etc. Kışladağ gold sample, which is known as industrially scale heap leaching gold extraction process by Tüprag Gold, would be suitable for a whole-ore heap biooxidative pre-treatment due to its low grade and suitable iron and sulfur content (1.3 g Au/t, 3.81 % Fe, and 3.47 % S). Such a bio-heap leaching technology, called BIOPROTM, has been commercialized by Newmont for low grade (1-3 g Au/t) sulfidic refractory ores with a range of 1-2.5 % sulfide content. Further investigations have been needed for the application of a biooxidative heap-leaching technology to the Kışladağ gold sample in order to increase the gold recovery. İliç gold sample has been used before by Ciftci et al., (2013) with a rate of <90% gold recovery. In these experiments, gold recovery from direct cyanide leaching of İliç ore was 42% for 24 hours leaching, while 73% in our study for 48 hours leaching. Although the chemical analyses of the samples nearly equal or close to each other, the amounts of gold, iron and sulfur, calcium oxide and quartz show huge varieties. These differences could be discussed as followings: 1- Sulfidic gold-bearing ores have been classified as 5 different classes at İliç gold mine (Diorite, Manganese- Diorite, Meta-sediment, Gossan, Massive Pyrite, Master Composite) (Alacer Gold, 2013), thus we could collected our samples from different sulfidic ore beds. 2- Those non-feed sulfide-type minerals have been stocking randomly, since the mine was commercialized. Additionally, Iliç minerals contain high amounts of arsenic (As) in form as realgar and arsenopyrite. Arsenic does not make stable complexes with cyanide, and stabile in the leach solution. As a result of arsenic oxides formation, a passivation layer forms on gold surface and lower the gold extraction efficiency. Therefore, alternatively pressure oxidation or Archaeal Biooxidation would be a better choice rather than mesophillic biooxidation for the selection of suitable process for the formation of scorodite/bio-scorodite structures. Biooxidation tests were carried out on three different type of ores (Ilıc, Kısladag and Carlin). In these experiments, mixed cultures preparated from pure mesophillic bacteria cultures (Acidithiobacillus ferrooxidans (DSM 583), Acidithiobacillus thiooxidans(DSM 11478) ve Leptospirillum ferrooxidans (DSM 2705)) were used to oxidise the ores. In cyanide leaching tests gold recovery and cyanide consumption were determined. All experiments have been studied with 10% pulp density. Dissolution of iron and pH changes were monitored for biooxidation experiments. Iron dissolution with 35% for Ilıc, %37 for Kısladag and 19% for Carlin were achieved while pH decreases up to 1.2 for Ilıc, 1.4 for Kısladag and 1.62 for Carlin from 2.0. Therefore, gold leaching rates increased up to 80% only in 15 days of biooxidation. Thus, most significant differences have been obtained in the meaning of cyanide consumptions.