The Recovery Of Precious Metal From Refractory Ore

THE RECOVERY OF PRECIOUS METAL FROM REFRACTORY ORE

The recovery of precious metals from refractory ore remains one of the more complex yet crucial areas in metallurgy and mining. Refractory ores, characterized by their complex mineralogy and the difficulty of breaking down their structures for extraction purposes, have posed significant challenges for traditional extraction methods. As the demand for precious metals like gold, silver, and PGM continues to rise, exploring effective strategies for recovering these valuable resources becomes increasingly important.

An In-Depth Understanding of the Ore Characteristics

An in-depth understanding of the ore characteristics in question is essential to the development of efficient and economical extraction processes. Unlike free-milling ores, which can be easily processed through standard methods such as gravity or cyanidation, refractory ores contain minerals that encapsulate precious metals. This encapsulation renders the precious metal invisible, necessitating advanced recovery techniques. Common characteristics of refractory ores include high sulfur content, oxidation states, and the presence of minerals such as arsenopyrite and pyrite. Precious metals are encapsulated within sulfide minerals such as pyrite (Fe₂S), arsenopyrite (FeAsS), and chalcopyrite (CuFeS₂). When these minerals are processed, the gold and other valuable metals will not be visible. Therefore, a series of physical and chemical beneficiation processes must be conducted to liberate the gold.

It is crucial to have a comprehensive understanding of the mineralogical composition of the ore to make informed decisions in the production process of gold and other precious metals. These factors complicate the metallurgical processes required for recovery, making it essential for mining engineers to develop targeted approaches tailored to the unique properties of each refractory ore type.

Challenges in Extracting Precious Metals from Refractory Ore

The extraction of precious metals from refractory ore presents several challenges. Firstly, the complex mineral structure of these ores often leads to lower recovery rates compared to free-milling ores. Secondly, the presence of deleterious elements, such as arsenic and antimony, poses both regulatory and operational challenges. Processing refractory ore also requires high energy inputs, making it cost-prohibitive in some cases. These challenges necessitate the evaluation of new technologies and methods capable of improving yields and reducing costs, driving ongoing research in this field.

The liberation process can be achieved through crushing and grinding in ores containing gold larger than 30-40 microns. The liberated and visible gold particles can be recovered using gravity separation methods such as shaking tables.

Flotation

Flotation is not typically employed for gold separation due to its high density, which impedes the effectiveness of this method. However, this process can be applied to sulfide minerals such as pyrite. The reasons for this are as follows:

• Sulfide minerals like pyrite possess surface characteristics that are conducive to flotation. Flotation reagents can adhere to the surface of pyrite, making it hydrophobic and allowing it to be transported into the froth phase. Consequently, gold contained within or on the surface of pyrite can also be transported and separated.
• The collector reagents used in flotation interact with the surface of sulfide minerals, imparting hydrophobic properties. This facilitates the flotation of pyrite, along with any gold associated with it, into the froth.
• Gold may be present in pyrite as microscopic or submicroscopic inclusions. When pyrite is floated, these inclusions are carried along with the pyrite, thereby enabling the separation of gold.

Flotation of pyrite can be more cost-effective and efficient compared to physical and chemical methods used for liberating gold. The flotation of pyrite, followed by gold recovery through processes such as cyanide leaching, can be economically advantageous.

Various methodologies have been developed to recover precious metals from refractory ores, with varying degrees of success.

Pirit (Fe₂S) Concentrate

Pyrite (Fe₂S) concentrate contains high amounts of iron and sulfur. From a precious metal perspective, the average silver content is 928 g/t and gold is 66 g/t. Precious metals within pyrite can be found as visible and invisible. To break the sulfide structure and liberate these metals, processes such as roasting, pressure oxidation, and bioleaching are preferred. Through these methods, the valuable metals within the ore are liberated and made ready for chemical processing.

Roasting is a process whereby ores are roasted at temperatures of approximately 600°C in air or oxygen. The reaction products that result from this process depend on the conditions of the roasting. In the case of pyrite, the main reaction products are hematite ( Fe₂O₃), magnetite (Fe₃O₄) and gaseous sulphur dioxide (SO₂). However, roasting can be environmentally damaging and energy-intensive.

Pressure oxidation is a process that takes place in autoclaves. It involves a combination of high pressure, acidified ore, high temperature (above 170°C) and oxygen, resulting in the formation of ferrous iron (Fe₂⁺), ferric iron (Fe₃⁺), sulfate (SO₄^2-) and elemental sulfur (S⁰) for pyrite. Alternatively, pressure oxidation has emerged as a promising method, employing high pressure and temperature to enhance metal recovery rates.

Additionally, bioleaching—a process that uses microorganisms to extract metals—has potential in terms of enriching recovery rates while minimizing harmful emissions. The choice of method depends heavily on the mineralogy of the ore and economic considerations.

The smelting of refractory concentrate with lead paste is a process used to extract precious metals, such as gold and silver, from refractory ores. The concentrate is first prepared through grinding and flotation or other concentration methods. The prepared refractory concentrate is mixed with a lead paste, which is typically made of lead oxide (PbO) or lead carbonate (PbCO₃) and fluxing agents. The lead paste serves as a collector for the precious metals and helps in their recovery. The mixture of refractory concentrate and lead paste is then subjected to high-temperature smelting in a furnace. The smelting process involves heating the mixture to temperatures around 1000 to 1200°C. During this stage, the lead reacts with the sulfide minerals in the concentrate. The lead paste reacts with the sulfide minerals in the smelting furnace to form a lead matte. The precious metals, such as gold and silver, are collected in this lead matte, while other impurities are removed as slag.

After smelting, the lead matte is removed from the furnace and processed further to separate the precious metals. This often involves additional refining steps, such as cupellation, where lead is removed from the matte by oxidation, leaving behind a concentrated precious metal product. The final step involves the recovery and purification of the precious metals from the lead matte. Gold and silver are typically extracted and refined to obtain pure metal products. This method is effective for extracting precious metals from refractory ores that are otherwise difficult to process using conventional methods. It leverages the ability of lead to bond with and collect precious metals during the smelting process.