Cyanide leaching has dominated gold hydrometallurgical processing for over 100 years. Cyanide has many techno-economic advantages: relatively low cost, high gold extraction, and selectivity for gold. However, its toxicity and recent environmental accidents as well as the limited ability to treat complex raw materials have increased the interest towards cyanide-free gold processing. Halides are one of the most promising alternatives for cyanide, and gold chlorination was industrially used already in the 1800s for gold recovery. The advantages of chloride leaching using copper(II) or iron(III) as oxidant are fast leaching kinetics and the possibility to process more complex raw materials. In this dissertation, chloride and chloride-bromide based processing of free-milling gold ore as well as refractory and double refractory gold concentrates was investigated. The work aimed at examining leaching phenomena such as preg-robbing, as well as developing mild chloride leaching processes for free-milling ores. In addition, a direct chloride leaching route for refractory gold concentrates was studied without separate pre-oxidation. Gold chloride-bromide leaching was found to have a relatively strong tendency for preg-robbing, with 0.1% carbon content decreasing gold extraction from 92% to 30%. Further, a 5% carbon content decreased gold extraction to below 10%. Gold recovery onto activated carbon in chloride-bromide leaching (CICl, carbon in chloride leach) was found to be an efficient method for preventing preg-robbing, evidenced by an increase in gold extraction from 0% to 80% when using CICl. The development of milder chloride leaching showed that only minor amounts of dissolved natural oxidants, 59 mg/L of copper and 1.5 g/L of iron, allowed 87% gold extraction at a chloride concentration of 100 g/L. The redox potential was 460 – 580 mV vs. Ag/AgCl and gold extraction was supported by simultaneous CICl. At even lower chloride concentration, ([Cl]aq,0 = 20 g/L), but higher oxidant concentration ([Cu]aq,0 = 18 g/L), 72% gold extraction could be achieved. These findings suggest that gold leaching can be enabled by lower redox potentials than those suggested by the state-of-the-art literature, and even at chloride levels similar to sea water. Another process development focused on investigating simultaneous sulfide oxidation and gold dissolution from refractory gold concentrate by aggressive chloride-bromide leaching. The results confirmed that direct chloride-bromide leaching of pyrite and arsenopyrite is possible, with high gold extraction. Gold extraction of 81% was possible with 67% sulfide oxidation from refractory concentrate, whereas 67% gold extraction was achieved with 81% sulfide oxidation from double refractory concentrate. In the future, this direct processing route may provide alternatives to the state-of-the-art pressure oxidation, roasting, and bio-oxidation processes.
|Translated title of the contribution||Helposti käsiteltävien ja refraktoristen kultamalmien ja -rikasteiden kloridipohjaisten prosessien kehitys|
|Publication status||Published - 2020|
|MoE publication type||G5 Doctoral dissertation (article)|
- gold leaching
- carbon in chloride leach