Reducing waste, increasing purity: The environmental advantages of Metso’s gold recovery technology

Originally developed in 1988, Metso’s gold recovery technology was designed to recover gold from gold mud produced in the silver refinery of the anode slime process. Anode slime process is followed by copper smelter/refinery. This batch-operated direct chloride leach process uses hydrogen peroxide as an oxidant. It is modularized unit where all the process equipment is arranged in skid mounted unit including reactors, filters, tanks, scrubber, pumps and piping. This design makes it easy to transport and assemble on-site. 

Benefits 

• Increased sustainability 
• Lower energy usage 
• Reduced chemical use 
• Ease of operation 
• High purity 
• Recycled gas outlets 

Rising demand for recovering valuable metals from various high gold (over 50%) containing  scraps or concentrates led to the development and evolution of the modularized scrap gold process. To address this, Metso introduced three key steps to the process, all of which are also modularized skid mounted units: 

1. Copper Oxide Leaching – Metso’s copper oxide leaching process extracts impurities like copper and zinc before the gold recovery stage. 

2. Gold Recovery – Gold, platinum, palladium and silver are leached using a hydrochloric acid and hydrogen peroxide method, offering a NOx-free, environmentally friendly solution 

3. Silver Recovery – Silver chloride residue from the gold recovery process is treated to produce 99.99% pure silver powder 

Metso Copper Oxide Leaching 

This process is used for leaching copper ores, concentrates, primary and secondary sulfides oxidic copper ores as well as secondary feed materials (such as recycled materials). Oxide leaching is a direct sulfuric acid leaching technology in atmospheric pressure. This step was added to scrap gold recovery to extract impurities like copper and zinc from gold, silver and platinum group metals. It operates with a retention time ranging from 4 to 9 hours, depending on particle size or if elements are in metallic form. Insoluble compounds are sent to the gold recovery process, while soluble impurities move on to either a wastewater treatment, or to copper and zinc recovery. Process gases are washed with the scrubber to ensure continuous performance and meet environmental standards. 

Gold Recovery 

In the gold recovery unit, precious metals such as gold, platinum, palladium and silver are instantly leached with hydrochloric acid using hydrogen peroxide as an oxidant. This direct chloride leach process is both efficient and environmentally friendly, as it is a NOx free method and produces gold of 99.99% purity. The leaching takes 4 hours, forming only gold, PGM chlorides, water and silver precipitates as silver chloride. Gold is reduced in only two steps using sodium metabisulphite, with fine gold being produced in the first step. Precipitated gold from the second step is recycled back to the leaching rector. Platinum and palladium are further reduced in the following step with metallic iron. The remaining solution is further reduced with zinc, confirming all precious metals are precipitated. This ensures maximum recovery of all precious metals, with dissolved impurities in the leach solution either recycled or led to wastewater facilities. 

Silver Recovery 

The silver recovery process treats silver chloride precipitate from the gold recovery with sodium hydroxide and sugar. This conversion from silver chloride to silver oxide following the reduction to silver powder is instant, and conversion is vigorous. The conversion happens in one reactor as a batch, resulting in the formed silver powder having a purity of 99.99%. Due to exothermic reactions, this highly efficient process is also environmentally sustainable, as the main residue sodium chloride can also be recycled. As an added benefit, it is nitrate and NOx-free, ensuring efficiency and sustainability at the same time. 

Key benefits of Metso’s gold recovery technology: 

• Increased sustainability: Corrosion-resistant materials are used, such as like titanium, plastics and glass, ensuring durability and long-term sustainability 

• Lower energy usage: Exothermic reactions in each module reduce energy consumption, making the technology energy-efficient. 

• Reduced chemical use: Only two acids—sulfuric acid and hydrochloric acid—are used. 

• Ease of operation: Each process step is enhanced with automation, clean operation and hygienic working conditions. Minimal manpower is required. 

• High Purity: End products are pure gold and silver powders of 99.99% purity as well as and platinum group metal (PGM) concentrates. 

• Re-using gas outlets: Off-gases, such as sulfuric acid and chloride vapors, are scrubbed, cleaned, and recycled back to related processes, further reducing the environmental footprint. 

Backed by decades of industry know-how and expertise, Metso’s gold recovery technology represents a future-focused solution, combining sustainability, efficiency and simplicity within precious metal recovery. By meeting the demands of today’s industry, it allows operations to work with more sustainable metal recovery processes, whilst also enabling quality and reliable performance. From maximizing precious natural resources to minimizing environmental impact, Metso’s aims to help customers transform the industry. That’s why, together, we are the partner for positive change.