Gold particles emerging from sludge, surrounded by plants.

From Sludge to Gold: Unlocking Hidden Value in Mining Waste

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Rhea Morgan in Climate & Environment October 2026 • 4 min read.

"Discover how innovative hydrometallurgical processes are transforming gold-containing sludge into a valuable resource, offering a sustainable solution for the mining industry."


The gold mining industry faces a significant challenge: a growing imbalance between production and reserves, compounded by declining ore quality and the accumulation of vast amounts of technogenic waste. As easily accessible, high-grade ore deposits dwindle, the industry must find innovative ways to tap into previously overlooked resources, including placer mines containing finely dispersed gold.

One promising avenue lies in the processing of gold-containing ores, particularly refractory ores that resist conventional extraction methods. These ores often contain gold locked within sulfides or in a fine-grained form, making recovery difficult. However, various technogenic wastes, including gold mining byproducts, are emerging as competitive sources of mineral raw materials.

This article delves into the research of technogenic objects in the Far East of Russia, focusing on the silt of the settling pond "Chalgany" and the waste from the Chalganov kaolin plant. We'll explore how preliminary concentration methods and hydrometallurgical processing, including non-cyanide leaching techniques, can unlock the value hidden within these materials, offering a more sustainable and efficient approach to gold extraction.

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Hydrometallurgy’s Promise and Limits

Hydrometallurgy is used to recover gold through aqueous processing, with reported advantages including mild operation, affordability, and low gas emissions. Research on cyanide-free leaching describes green extraction as a major focus while also emphasizing that these technologies have advantages and limitations. Farmonaut identifies seven hydrometallurgical technologies shaping gold and silver recovery in 2026.

From Cyanidation to Modern Hydrometallurgy

Modern hydrometallurgy is generally traced to the end of the 19th century, when cyanidation for gold and silver extraction and the Bayer process for bauxite treatment were discovered. The cyanidation patent published by McArthur and the Forrester brothers in 1887 is described as a decisive step in recovering gold from gold-bearing ores. A later breakthrough came in the 1940s during the Manhattan Project in the United States, where hydrometallurgical methods were connected with uranium extraction.

The Science of Sludge: Unlocking Gold with Innovative Techniques

Gold particles emerging from sludge, surrounded by plants.

Traditional gravitational methods of gold recovery often leave behind significant amounts of gold, particularly in the form of fine particles. This gold, ranging in size from tens of microns to just a micron, becomes densely associated with other minerals, creating a low-grade, refractory ore. While challenging, these technogenic gold-containing objects often benefit from their proximity to existing infrastructure.

Hydrometallurgy offers a promising solution for extracting fine-grained gold from technogenic waste. However, conventional cyanide leaching, while widely used, poses environmental hazards and struggles with carbonaceous, organic, or copper-rich raw materials. Therefore, alternative leaching methods, such as thiocarbamide and ammonia-thiosulphate leaching, are gaining traction for their ability to efficiently process complex, cyanide-resistant ores.

  • Ammonia-Thiosulphate Leaching: Achieved over 90% gold recovery from model systems within 2 hours using an optimized solution (0.1 mol/l Na2S2O3; 0.2 mol/l NH3; 0.015 mol / 1 CuSO4; pH 10.4). Real samples of gold-containing silicate and aluminosilicate samples provided high efficiency of gold recovery (78-97%).
  • Thiocarbamide Leaching: Achieved laboratory gold recovery of 93.5% using determined optimal conditions: 0.65 mol/L CS(NH2)2; 0.1 mol/L H2SO4; 0.09 mol/L (NH4)2S2O8
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Processing Gold-Bearing Sludge

A study of gold-containing sludge proposes preliminary concentration followed by hydrometallurgical processing. The reported methods are presented as ways to extract gold from the investigated technogenic objects. Because the source describes proposed processing methods rather than a universal solution, their applicability depends on the characteristics of the sludge being studied.

Leaching Versus Smelting

A systematic review of gold extraction examines alternative leaching reagents to cyanide as possible routes to cleaner extraction. In contrast, smelting uses high-temperature thermal treatment to extract metals from ores, while hydrometallurgy relies on aqueous processing. Gold hydrometallurgy is also described as applying to gold recovery from ores and mineralized tailings, with 2026 demand linked to sustainable, low-emission, and water-conscious processing.

Researchers investigated the effectiveness of ammonia-thiosulphate and thiocarbamide leaching on silt from the "Chalgany" settling pond. While ammonia-thiosulphate leaching showed promise, its efficiency was limited by the presence of magnetite and titanomagnetite, which can destabilize the leaching system. Thiocarbamide leaching, however, proved more effective, achieving a gold recovery rate of 93.5% in laboratory conditions.

From Waste to Resource: A Sustainable Future for Gold Mining

The research demonstrates that technogenic wastes, such as the silts of settling ponds and kaolin plant wastes, represent a valuable source of mineral raw materials. These materials contain not only gold but also precious metals, rare earth elements, and other valuable components.

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Mapping the Research Landscape

A 2025 study presents a bibliometric and systematic literature analysis of hydrometallurgy for recovering gold, silver, and platinum group metals. It examines scientific research published from 1981 to 2025. The study combines bibliometric analysis, text mining, and content analysis to identify key themes and trends, review the literature systematically, and propose future directions.

People, Places, and Valuable Waste

Research on investigated technogenic objects reports the presence of gold, noble metals, and rare earth elements, highlighting the potential value embedded in waste materials. Another study explores two hydrometallurgical gold-recovery processes in a small-scale gold-mining case study at Ponce Enriquez, Azuay, Ecuador. A 2025 review frames circular hydrometallurgy as a response to the environmental and economic limitations of conventional pyrometallurgical and linear hydrometallurgical approaches.

Thiocarbamide leaching, followed by electrolytic precipitation, offers a promising approach for gold recovery from settling pond silts. Similarly, kaolin plant wastes can be processed to recover valuable components, with the remaining material serving as a secondary source of kaolin for producing high-purity silicon, silica, aluminum compounds, and refractory materials.

By embracing innovative hydrometallurgical processes and integrated approaches to waste processing, the gold mining industry can transform environmental liabilities into economic opportunities, paving the way for a more sustainable and resource-efficient future.

Testing leaching on Chalgany pond silt

PopulationSilt from the Chalgany settling pond and waste from the Chalganov kaolin plant[1]
SettingFar East of Russia[1]
ComparisonAmmonia-thiosulphate and thiocarbamide leaching methods[1]
Main outcomeGold recovery from Chalgany settling pond silt[1]
Effect sizeGold recovery reaches 93.5%[1]
Times cited2[1]

What the recovery result says about this sludge

The reported recovery indicates that thiocarbamide leaching can retrieve much of the gold from the particular pond silt studied. It does not establish that the same result would hold for other waste materials or processing conditions.[1]

The paper also identifies gold alongside noble metals and rare earth elements in the investigated objects, so the materials’ potential value is not limited to gold.[1]

What remains unknown about the waste materials

  • How consistently would the reported recovery be reproduced across samples from the Chalgany pond?[1]
  • How would the proposed processing perform on the Chalganov kaolin-plant waste compared with the pond silt?[1]

Sources

  1. Beneficiation and hydrometallurgical processing of gold-containing sludge (EDP Sciences, 2018)

About this Article -

Written with AI assistance from published research, and reviewed by the Mystum team. See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1051/e3sconf/20185603007, Alternate LINK

Title: Beneficiation And Hydrometallurgical Processing Of Gold-Containing Sludge

Subject: General Medicine

Journal: E3S Web of Conferences

Publisher: EDP Sciences

Authors: Sergei Ivannikov, Oleg Ageev, Svetlana Bratskaya, Michael Medkov, Evgeny Shamrai, Aleksandr Yudakov

Published: 2018-01-01

Everything You Need To Know

1

What makes mining waste a valuable resource?

Technogenic waste, such as the silt from the "Chalgany" settling pond and waste from the Chalganov kaolin plant, holds significant value because it contains gold that was not extracted by traditional methods. The presence of fine gold particles, often locked within minerals, makes this waste a potential source of gold. Turning this waste into a resource provides an economic opportunity for the gold mining industry, enabling it to tap into previously overlooked resources and address the growing imbalance between gold production and the availability of high-grade ores. This is a sustainable approach which reduces the environmental impact and enhances efficiency.

2

Why are hydrometallurgical processes important in this context?

Hydrometallurgical processes are crucial because they can efficiently extract fine-grained gold from technogenic waste that conventional methods fail to recover. These processes use chemical solutions to dissolve the gold, separating it from other materials. Specific examples include Ammonia-Thiosulphate Leaching and Thiocarbamide Leaching. These alternative leaching methods can process complex ores and mining byproducts that resist traditional techniques. Hydrometallurgy offers a sustainable and effective way to recover gold, improving the overall efficiency of gold extraction and minimizing environmental impact by using non-cyanide leaching methods.

3

How does Ammonia-Thiosulphate Leaching work, and what are its limitations?

Ammonia-Thiosulphate Leaching is a hydrometallurgical method that uses a solution containing ammonia and thiosulphate to dissolve gold. Although it achieved high gold recovery rates in certain experiments, its efficiency was limited by the presence of minerals like magnetite and titanomagnetite, which can destabilize the leaching system, making it less effective in the context of the "Chalgany" settling pond silt. Despite these limitations, Ammonia-Thiosulphate Leaching presents an important alternative, particularly in the mining of complex ores, where traditional methods like cyanide leaching cannot be used.

4

What is Thiocarbamide Leaching, and how effective is it?

Thiocarbamide Leaching is another hydrometallurgical method that uses thiocarbamide to extract gold. Laboratory tests using Thiocarbamide Leaching achieved a gold recovery rate of 93.5% on silt from the "Chalgany" settling pond. Thiocarbamide leaching works under specific conditions, and offered a more efficient method to leach gold in this case. This demonstrates its potential as a viable method for processing gold-containing waste, and also providing a sustainable path for the recovery of the precious metal.

5

What are the broader implications of using mining waste as a resource?

The implications of turning mining waste into a valuable resource are substantial. The primary implication is a shift towards a more sustainable and efficient gold mining industry. This approach reduces environmental liabilities, minimizes the need for new mining operations, and addresses the challenge of dwindling high-grade ore deposits. By recovering gold from waste materials like the silt of the "Chalgany" settling pond, the industry can increase its gold supply, reduce its environmental footprint, and promote economic growth. This transformative approach is critical to the future of gold mining, ensuring the industry's long-term viability and sustainability.

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