Gold deposits are varied regarding their geological and geochemical settings. Hence, their processing methods must be best organized for achieving the desired recovery of precious metals. In this research, two types of hard-rock gold ores, each with specific characteristics from Kouh-zar region (south of Damghan city) were subjected to mineral processing tests at the laboratory scale. The reference (head) samples of both veins contained on average 3.5-4.5 ppm gold. According to mineralogical investigations no observable gold particles reported even in SEM analyses except for rare over-micrometer gold particles locked in the silica matrix. Therefore, the major (invisible) gold supposed to exist in sulfides" crystal structure. Flotation applied to recover the gold interlocked within sulfides. The highest grades achieved were 49.2 ppm @ R=77.61% for Baghu samples and 57.0 ppm @ R=61.00% for Darestan samples gold in the sulfide concentrates. In the CYANIDATION diagnostic tests the optimum conditions of pH, particle size, cyanide concentration and leaching time were determined for both ores. High gold recoveries (> 95%) were achievable for Baghu samples for the pH conditions between 10.5 and 11, and cyanide and hydrated lime consumptions of 0.6 and 2.5 grams per kilogram of ore respectively. As it had been expected, direct CYANIDATION of Darestan samples (containing up to 1.5% copper), couldn"t be established as an efficient procedure (due to cyanide consumption of about 20 times higher than for Baghu tests to achieve similar recoveries) and the preliminary copper removal phase was determined as a necessity for better recovery of gold.

Samples of the waste rock obtained from Marofengin South Africa ground 60, 70 and 80% passing 75µm were leached with cyanide in bottle roll tests. The results obtained showed that the percentage gold dissolution depends more on the ore size consists than on the cyanide concentration with the highest average dissolution rates of 88.75, 94.34 and 95.90% found at 600, 500 and 500 ppm, for 60, 70 and 80% passing sizes, respectively. It was also noted that the lowest average percents cyanide consumption of 62.91, 61.73 and 58.56 at 60, 70 and 80% passing 75 µm were obtained at the 500 ppm cyanide concentrations. It was further observed that a clear pattern of increasing residual lime content was only observed at the 70% grind size, with the least lime content of 150.37 ppm at 500 ppm cyanide concentration being higher than the least lime contents for the 60 and 80% grind sizes. The results obtained thus suggest the 70% grind passing 75mmore with the gold dissolution percentage very close to the conventional 80% passing size at the lowest cyanide consumption of 500 ppm, much lower daily power consumption of about 925 kWh and high residual lime content that indicated the minimization of cyanide loss as hydrogen cyanide, a good choice for the leaching of the waste rock.

By depletion of high-grade and easy-to-treat gold ores and the increase in gold price, the extraction of gold from low-grade and refractory ores, such as sulfide ores has been given attention. Biooxidation is one of the efficient, relatively low-cost and environmentally friendly processes to enhance gold extraction by releasing fine gold encapsulated in sulfide phases. So, in this research, the efficiency of concentrate biooxidation-flotation system in increasing gold recovery from sulfide-refractory low-grade ores was investigated. Biooxidation experiments with mesophilic and moderate thermophilic microorganisms were conducted on the flotation concentrate. Finally, CYANIDATION experiments were conducted on the flotation concentrate, and biooxidized flotation concentrate. Gold recovery from non-biooxidized flotation concentrate was 63.59%, while it increased to 80.21% after biooxidation with mesophilic microorganisms and to 79.84% after biooxidation with moderate thermophilic microorganisms.

The presence of copper bearing minerals in CYANIDATION of gold ores may lead to several challenges in the CIP/CIL circuits. Many solutions have been proposed to address these problems, one being the use of glycine in the CYANIDATION process. Here, the experimental as well as molecular modelling studies using Density Functional Theory (DFT) have been conducted to investigate the glycine role in CYANIDATION of copper bearing gold ores. The results show that in the presence of glycine in the solution containing copper-cyanide species and in very low or zero free cyanide content, the dissolution rate of gold is significantly improved (3.02 vs. 0.23 ppm), while no improvement is observed in copper free or cyanide enriched solutions. Molecular modeling has been performed to interpret the laboratory results as well as to identify the mechanisms. The modeling results demonstrate that in cyanide deficient solutions, cyanide complex of copper complexes (E = -319 kCal.mol-1) is replaced by glycine, and the free cyanide produced results in higher gold CYANIDATION as well as lower copper cyanide formation.

The presence of copper oxide next to gold increases the consumption of cyanide in the CYANIDATION process, reduces the absorption of gold on carbon, and causes many problems in the downstream processes of elution, electrowinning, and smelting. In the current research, increasing the recovery of gold dissolution and its selective adsorption on activated carbon using ammonia additive and the effect of different factors such as the type of ammonia additive, changing the system of contact of gold with carbon from CIL to CIC, and adding hydrogen peroxide and cyanide to the filtered solution were studied. The findings showed that adding ammonium chloride or liquid ammonia to the CYANIDATION solution, in the sample with high copper grade (1% copper), and in the sample with medium copper grade (0.5% copper), increased the dissolution recovery by 60% and 30%, respectively. Also, the recovery of gold adsorption on activated carbon in sample with a grade of 0.5% copper increased from about 25% during the normal CIL process to about 100% during the ammonia CYANIDATION process in the form of CIC. Results showed that liquid ammonia provides up to 10% higher dissolution recovery and up to 3% higher absorption recovery compared to ammonium chloride, while reduces the consumption of cyanide by half and the amount of copper absorption on carbon by a third.