Mixed Copper and Gold Ore Concentrates Processing

Processing Design and Method

Introduction

Gold and copper are two valuable metals that are often found in ores and concentrates. The extraction and recovery of these metals require different processes depending on the mineralogy and grade of the ore. In some cases, gold and copper can be associated in a complex way, making the separation and concentration of both metals more challenging and costly however, this scenario is a common occurrence (Sceresini & Breuer, 2016). This article will review some of the methods and technologies that are used for the concentration of mixed and complex ore containing gold and copper and highlight some of the current challenges and opportunities for improvement.

Flotation is a widely used technique for the concentration of gold and copper from various types of ores. Flotation relies on the differences in surface properties of minerals to selectively attach them to air bubbles and separate them from the pulp. Flotation reagents such as collectors, frothers, activators, and depressants play an important role in controlling the flotation process and enhancing the recovery and grade of the desired metals.

Copper ores are classified into two main types: sulfide ores and oxide ores. Sulfide ores are usually processed using pyrometallurgy, while oxide ores are usually processed using hydrometallurgy. Gold ores can occur alongside copper ores, as some gold deposits are associated with copper mineralization.

Pyrometallurgical processing of copper sulfide ores involves four main steps: roasting, smelting, converting, and electrorefining. The roasting step involves heating the ore in the presence of air or oxygen to convert the sulfides to oxides. The resulting calcine is then smelted in a furnace to produce a matte, a mixture of copper, iron, and sulfur. The matte is then transferred to a converter where it is oxidized, producing blister copper. Finally, the blister copper is refined electrolytically to produce pure copper.

Hydrometallurgical processing of copper oxide ores involves three main steps: leaching, solvent extraction, and electrowinning. The leaching step involves dissolving the copper oxide ore in an acidic solution, usually sulfuric acid. The resulting copper sulfate solution is then subjected to solvent extraction to separate the copper from impurities. The copper is then stripped from the organic solvent using an aqueous solution and recovered through electrowinning, a process that uses electricity to deposit copper metal from the copper sulfate solution onto cathodes.

Gold ores are usually processed using a combination of gravity separation, flotation, and cyanidation. The gravity separation step involves separating the gold from the gangue minerals using the difference in density between the gold and the other minerals. The flotation step involves adding reagents to the ore to make the gold particles hydrophobic, allowing them to be separated from the other minerals in a froth flotation cell. Finally, the gold is recovered from the resulting concentrate using cyanidation, a process that involves treating the concentrate with a dilute solution of sodium cyanide, which dissolves the gold, allowing it to be recovered by precipitation onto activated carbon or by electrorefining.

Mixed Ore Characteristics

An ore containing both gold and copper can be processed using a combination of pyrometallurgical and hydrometallurgical techniques to extract both metals. As earlier stated, pyrometallurgical processing of the ore involves the roasting and smelting of the sulfide ore to produce a matte, a mixture of copper, iron, and sulfur. During the smelting process, some of the gold contained in the ore will be captured by the copper and will be present in the resulting matte. The matte is then subjected to an oxidizing step in a converter to produce blister copper, which is then refined electrolytically to produce pure copper. During the refining step, the gold present in the copper is recovered as a byproduct. Unlike the hydrometallurgical processing which involves the ore being subjected to the leaching of the oxide ore with a dilute sulfuric acid solution to dissolve the copper. The resulting copper sulfate solution is then subjected to solvent extraction to separate the copper from impurities, including any gold that may be present. The copper is then stripped from the organic solvent using an aqueous solution and recovered through electrowinning to produce pure copper. The gold, which will not be extracted by the copper sulfate solution, can be recovered from the residue using a cyanide leaching process.

One example of an ore with gold and copper minerals is the porphyry copper-gold ore. Porphyry copper-gold ores are typically low-grade, bulk-minable deposits that contain both copper and gold minerals. These ores are typically found in large, disseminated deposits and are often associated with complex mineralogy. Another example is the skarn-type gold-copper ore. Skarn-type deposits are typically formed at the contact between a magmatic intrusion and carbonate rocks. These deposits can contain significant amounts of copper, as well as gold and other minerals. Another example is the epithermal gold-copper ore. Epithermal deposits are typically formed in volcanic environments and can contain significant amounts of gold and copper. These deposits are often characterized by high-grade, narrow veins of mineralization. In general, there are many different types of ores that can contain both gold and copper minerals. These ores can be found in a variety of geological settings, including porphyry deposits, skarns, epithermal deposits, and others.

Case Examples

Benguet province in the Philippines is known for its rich mineral deposits, including gold and copper ores. One of the most significant gold-copper deposits in the province is the Kingking deposit, located in the town of Pantukan in Compostela Valley. The Kingking deposit is a porphyry copper-gold deposit that contains an estimated 5.4 billion pounds of copper and 18 million ounces of gold. The deposit is characterized by a large, disseminated zone of copper and gold mineralization that is hosted by porphyritic intrusive rocks. In addition to the Kingking deposit, there are also several other gold and copper deposits in Benguet province, including the Balatoc and Acupan gold mines, and the Lepanto and Far Southeast copper-gold mines. These deposits vary in terms of their mineralogy and geological setting, but all contain significant amounts of gold and copper.

The gold and copper ores in Benguet province in the Philippines are known to be difficult to process for a few reasons:

1. Low-grade ores: Many of the gold and copper deposits in the Benguet province are low-grade, which means they contain relatively small amounts of gold and copper per ton of ore. This makes it more difficult and expensive to extract the metals from the ore.
2. Complex mineralogy: The ores in the Benguet province often have complex mineralogy, meaning they contain a variety of different minerals that are difficult to separate. This can make the processing of the ore more complex and challenging.
3. Refractory ores: Some of the gold and copper ores in the Benguet province are considered to be refractory, which means they are resistant to conventional processing methods. Refractory ores can be difficult to treat and require specialized processing techniques such as pressure oxidation, bioleaching, or roasting.
4. Environmental regulations: The processing of gold and copper ores can have significant environmental impacts, including the release of toxic chemicals and heavy metals into the environment. The Philippine government has strict environmental regulations that must be followed by mining companies operating in the country, which can add to the complexity and cost of processing the ore.

The Design Process

• The design consists of a flotation cell that has a cylindrical tank with a conical bottom and a central air sparger. The first step is to crush and grind the ore to a fine powder, using crushers and ball mills. This liberates the copper minerals from the gangue (waste) minerals.
• The froth tank is fitted with a froth crowder device that directs the froth to a launder for collection. The tank is also equipped with a froth skimmer that removes the froth layer from the surface of the slurry.
• The ore slurry is fed into the flotation cell through a feed box and is distributed over the entire tank surface by a rotating rake mechanism. The slurry is conditioned with water and chemical reagents, such as collectors, frothers, modifiers, and pH regulators. The collectors are organic compounds that selectively adsorb to the copper minerals and make them hydrophobic. The frothers are surfactants that stabilize the air bubbles and create a persistent froth layer. The modifiers are substances that enhance or depress the flotation of certain minerals by altering their surface properties. The pH regulators are acids or bases that adjust the acidity or alkalinity of the slurry to optimize the flotation conditions.
• The air sparger injects fine air bubbles into the slurry through a porous plate or a pipe with small holes. The air bubbles attach to the hydrophobic copper minerals and carry them to the surface of the slurry, forming a mineralized froth layer. The hydrophilic gangue minerals remain in the slurry and sink to the bottom of the tank.
• The froth crowder device pushes the froth towards the center of the tank, where it overflows into a launder for collection. The froth skimmer device removes any excess froth from the surface of the slurry and returns it to the tank. The collected froth contains a high concentration of copper minerals and is transferred to another stage of flotation for further upgrading.
• The flotation cell operates continuously for 24 hours, with a retention time of 15 minutes for each pass through the cell. This allows sufficient time for the copper minerals to attach to the air bubbles and be recovered in the froth. The tailings from the flotation cell are discharged through a valve at the bottom of the tank.

The Equipment and Chemicals Needed

Some of the equipment involved in this process are flotation cells, feed boxes, launders, froth crowders, froth skimmers, air spargers, pumps, pipes, valves, etc. Some of the chemicals used are water, air, collectors (such as xanthates or dithiophosphates), frothers (such as pine oil or methyl isobutyl carbinol), modifiers (such as lime or sodium cyanide), pH regulators (such as sulfuric acid or sodium hydroxide), etc. Activators are reagents that enhance the flotation response of minerals that are otherwise difficult to float. Activators can either modify the surface properties of minerals or form complexes with collectors to increase their hydrophobicity. For example, copper sulphate can be used as an activator for gold-bearing pyrite or arsenopyrite, by forming a layer of copper xanthate on their surface. The dosage of activators depends on the mineral composition and pH of the pulp, but generally ranges from 10 to 100 g/t for copper sulphate. (Diego & Anderson, 2020). Depressants are reagents that prevent or reduce the flotation of unwanted minerals by making them hydrophilic or by interfering with their interaction with collectors. Depressants can either react with minerals or form soluble complexes with metal ions in solution. For example, lime can be used as a depressant for pyrite or other iron sulphides, by increasing the pH and forming iron hydroxide on their surface (Diego & Anderson, 2020). The dosage of depressants depends on the gangue mineralogy and collector type, but generally ranges from 500 to 2000 g/t for lime.

One possible reagent that can be considered as sustainable is biosolids, which are organic wastes derived from sewage treatment plants or animal manures. Biosolids can act as both collectors and frothers in flotation, and have been shown to improve the recovery and grade of copper ores in some studies. Biosolids are biodegradable, renewable, and low-cost sources of reagents that can reduce the environmental impact of flotation.

Copper Mining and Processing: Processing Copper Ores | Superfund (arizona.edu)

Handling complex and mixed ore containing gold and copper particularly in economically constrained-concentrates production plan, it is advisable to target the two metals at the same time using a combined approach of flotation and skimming off the concentrates using specialized collector and froth agents before the thickening process however, in such case, it is necessary to use Cyanide to dissolve the gold concentrate first the copper.

Hence, concentrating both gold and copper from a mixed ore type in the same facility requires a combination of techniques, including flotation, gravity separation, and cyanide leaching. Here is a step-by-step process for concentrating both metals from a mixed ore type:

1. Crushing and Grinding: The mixed ore type is first crushed into small particles and then ground into a powder. This process increases the surface area of the ore, allowing for better contact between the ore and the chemicals used in the subsequent steps.
2. Flotation: The powdered ore is mixed with water and chemicals, such as collectors and frothers, and then introduced into a flotation cell. Air is bubbled through the mixture, and the gold and copper minerals attach to the bubbles and rise to the surface to form a froth. The froth containing the gold and copper minerals is then thickened and filtered to remove excess water and concentrate the minerals.
3. Gravity Separation: The concentrated gold and copper minerals are then subjected to gravity separation techniques, such as shaking tables or centrifugal concentrators. This process separates the heavy minerals from the lighter minerals, allowing for further concentration of the gold and copper.
4. Cyanide Leaching: The final concentrate containing both gold and copper is then subjected to cyanide leaching. Cyanide is added to the concentrate to dissolve the gold, forming a gold-cyanide complex. The copper is not affected by the cyanide, so it remains in the concentrate. The gold-cyanide complex is then extracted from the solution using activated carbon or other methods.
5. Refining: The gold is then further refined to remove any remaining impurities and produce pure gold. The copper concentrate is smelted to remove any remaining impurities and produce pure copper.

The General Rule

The recommended dosage of collector, such as sodium xanthate, typically ranges from 10 to 200 grams per ton of ore, depending on the ore mineralogy and the specific collector used. The collector is added to the grinding circuit or the flotation cell to promote the adsorption of the copper and gold minerals to the air bubbles.

The recommended dosage of frother, such as pine oil, typically ranges from 10 to 100 grams per ton of ore, depending on the ore mineralogy and the specific frother used. The frother is added to the flotation cell to stabilize the froth and promote the separation of the copper and gold minerals from the gangue minerals. In the study of Langa et al., (2014) , Sceresini, (2005), and Dunne, (2016), The collector used for gold and copper ore is usually sodium isobutyl xanthate (SIBX) or potassium amyl xanthate (PAX), and the frother used is pine oil or cresylic acid. “The dosage of SIBX can range from 60 to 100 g/t, depending on the ore characteristics and flotation conditions” (Sceresini & Breuer, 2016). The dosage of PAX can vary from 10 to 100 g/t, depending on the ore type and grade (Diego & Anderson, 2020), more also the same authors propose the “dosage of pine oil can range from 10 to 50 g/t, depending on the froth stability and bubble size. The dosage of cresylic acid can be lower than pine oil, around 5 to 20 g/t”. However, these dosages are only indicative and may not be applicable to your specific ore.

It is important to note that the dosages of collectors and frothers may need to be optimized through laboratory and pilot plant tests to achieve the desired results. Factors such as ore mineralogy, particle size distribution, and pH can all affect the efficiency of the collector and frother. Therefore, it is recommended to consult with a qualified mineral processing engineer or metallurgist to determine the appropriate dosages for your specific case. The flotation performance of gold-copper ores can be affected by various factors, such as mineralogy, particle size distribution, reagent dosage, pulp density, pH, Eh, etc. These factors can influence the recovery and grade of both gold and copper, as well as their selectivity against each other or against other minerals. Therefore, it is important to optimize the flotation conditions for each ore type.

Bioleaching As An Emerging Biotechnology for Ore Processing

Bioleaching is a process that uses microorganisms to extract metals from ores, and it has become an important method for processing low-grade and refractory gold ores. Here are five common bioleaching agents used for gold ore processing:

1. Acidithiobacillus ferrooxidans: This is a type of bacterium that can oxidize sulfide minerals in the ore, releasing gold particles that can be recovered using conventional methods. Acidithiobacillus ferrooxidans is commonly used in bioleaching operations for gold ores.
2. Acidithiobacillus thiooxidans: Another type of bacterium used in bioleaching, Acidithiobacillus thiooxidans can oxidize sulfur compounds in the ore, releasing gold particles that can be recovered.
3. Leptospirillum ferriphilum: This is a type of bacterium that can oxidize ferrous iron in the ore, releasing gold particles that can be recovered.
4. Sulfolobus metallicus: This is a type of archaea that can thrive in extremely acidic environments, making it useful for bioleaching of acidic gold ores.
5. Bacillus subtilis: This is a type of bacterium that can produce organic acids, which can dissolve gold from ores. Bacillus subtilis is used in some bioleaching operations for gold ores.

These bioleaching agents are often used in combination with one another to maximize gold recovery from the ore. The microorganisms are added to the ore along with a nutrient-rich solution that provides the microorganisms with the energy and nutrients they need to grow and metabolize the sulfide minerals in the ore. As the microorganisms oxidize the sulfide minerals, they release gold particles that can be recovered using conventional methods. Bioleaching is a relatively low-cost and environmentally friendly way to extract gold from ores, making it an attractive alternative to traditional processing methods.

References

1. B. Sceresini, Gold-copper ores, Editor(s): Mike D. Adams, B.A. Wills,
   Developments in Mineral Processing, Elsevier, Volume 15, 2005, Pages 789–824, ISSN 0167–4528, ISBN 9780444517302, https://doi.org/10.1016/S0167-4528(05)15032-7.
2. B. Sceresini, P. Breuer, Chapter 43 — Gold-Copper Ores, Editor(s): Mike D. Adams, Gold Ore Processing (Second Edition), Elsevier, 2016,
   Pages 771–801, ISBN 9780444636584, https://doi.org/10.1016/B978-0-444-63658-4.00043-8.
3. Langa, N.T.N., Adeleke, A.A., Mendonidis, P. et al. Evaluation of sodium isobutyl xanthate as a collector in the froth flotation of a carbonatitic copper ore. Int J Ind Chem 5, 107–110 (2014). https://doi.org/10.1007/s40090-014-0025-5
4. R. Dunne, Chapter 20 — Flotation of Gold and Gold-Bearing Ores,
   Editor(s): Mike D. Adams, Gold Ore Processing (Second Edition),
   Elsevier, 2016, Pages 315–338, ISBN 9780444636584, https://doi.org/10.1016/B978-0-444-63658-4.00020-7. (https://www.sciencedirect.com/science/article/pii/B9780444636584000207)