Summary of Gold Extraction Process

2019-08-22 16:50:12 XinHai Views:5716

Definition:Gold extraction refers to the process of separating gold from gold-bearing ore. The content of gold in nature is low. In order to extract gold, it is usually necessary to first crush and grind the gold ore and use the extraction method to separate gold from the ore. For different gold-bearing ores, the selection of gold extraction process is different.

The gold extraction process mainly includes the following methods:

Cyanidation Gold Extraction Process

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1. Cyanidation Gold Extraction Process Principle

Cyanidation gold extraction process is a method in which gold is extracted from gold-bearing ores using aqueous solution of cyanide as solvent, and then gold is extracted from gold-bearing leaching solution. It includes cyanide leaching process and deposition gold extraction process. First, gold in the gold-bearing ore reacts with cyanide to form a complex into the solution, and then the gold is removed from the solution to metal by a displacement reaction using reactive metal such as zinc. The reaction equation is:

4Au+8NaCN+2H2O+O2=4Na [Au (CN) 2] +4NaOH

2 Na [Au (CN) 2] +Zn=2Au+Na2 [Zn (CN) 4]

2. Cyanidation Gold Extraction Process Flow

The specific steps of cyanidation gold extraction process mainly include: ore's preparation stage, leaching stage and metal recovery stage.

(1) Ore's Preparation Stage

The preparation stage of ore is an essential process for gold extraction, which is mainly by crushing and grinding the ore to reduce the particle size of the ore. It can ensured that gold is present in the cyanide aqueous solution in the most economical form of recovery, thereby increasing the gold recovery rate.

In addition to the crushing and grinding process, gold ore containing sulphide or carbonaceous minerals requires additional treatment prior to recycling. This is because cyanide leaches out sulfide firstly rather than metal, which will cause the consumption of cyanide. To counteract this effect, these ores can first through the flotation process to limit the interaction with cyanide during the leaching process. In addition, it is also possible to change the leaching process by adding activated carbon to preferentially adsorb gold.

(2) Leaching Stage

The concentration of cyanide used in the leaching process ranges from 300 to 500 mg / l (0.03 to 0.05% in terms of NaCN), mainly depending on the mineralogy of the ore.

In the leaching process, the grinded ore is first conveyed to a series of leaching tanks, and the slurry is stirred by mechanical force or by injecting air to increase the contact between cyanide and gold and improve the leaching efficiency. The cyanide then dissolves the gold from the ore to form a stable metal-cyanide complex. At the beginning of the leaching, the pH of the slurry is raised to pH 10-11 using lime to ensure that it can’t produce toxic hydrogen cyanide gas when cyanide is added.

Activated carbon is used in the dissolved gold extraction process, either by introducing it directly into the CIL (carbon-in-leach) tanks or into separate CIP (carbon-in-pulp) tanks after leaching. The activated carbon adsorbs the dissolved metal from the leach slurry thereby concentrating it onto solids. The carbon is then separated from the slurry by screening and subjected to further treatment to recover the adsorbed metal.

(3) Metal Recovery Stage

Gold are extracted from the solution first using either cementation on zinc powder or concentrating the metal using adsorption on activated carbon, followed by elution and concluding with either cementation with zinc or electrowinning. The most cost-effective method is to make the dissolved gold adsorb to the activated carbon, making it easy to achieve the separation of the solid-solid. To achieve this, the ore particles must generally be smaller than 100 μm and the carbon particles must be larger than 500 μm. These steps can be done after CIL or CIP process.

Activated carbon in contact with a pulp containing gold and/or silver can typically recover more than 99.5% of the metals in the solution in 8 to 24 hours, depending on the reactivity of the carbon, the amount of carbon used and the mixer's efficiency. The gold adsorbed on the activated carbon is recovered from the carbon by electrolysis, and then the carbon is returned to the adsorption loop for further use. The gold obtained will be refined by calcination into gold bars containing 70-90% gold. The bullion is then further refined to either 99.99% or 99.999% fineness using chlorination, smelting and electro-refining.

3. Cyanidation Gold Extraction Process Characteristics

(1) Advantages

The recovery rate of cyanidation gold extraction process is very high. And gold can be produced on the spot, which can avoid the various drawbacks of long-distance transportation of gold concentrate, thus conducive to improving the economic benefits of enterprises.

(2) Disadvantages
  • The gold extraction process flow lasts long, and the extraction speed is slow.
  • As a highly toxic material, cyanide poses a serious threat to the ecological environment and animal life. Therefore, the slurry waste discharge should be strictly controlled when cyanide is used for gold extraction. In order to avoid the environmental damage caused by cyanide, many mine owners begin to use new non-toxic or low toxic leaching agents for gold extraction, which can not only replace sodium cyanide without changing the original equipment and cyanidation process, but also can achieve 100% environmental protection.

4. Cyanidation Gold Extraction Process Case

There is a 270tpd gold and silver cyanidation plant in Japan, in which the metal minerals mainly include native gold, silver, and pyrite. Gangue minerals mainly composed of quartz and calcite. Gold exists mainly in free state in ore. The ore contains 5.0g/t of gold and 20g/t of silver. The plant completely adopts the all sliming cyanidation method, the final gold recovery rate is 97.6%, and the silver recovery rate is 89%. The tailings contain 0.12 g/t of gold and 2.4 g/t of silver.

Flotation Gold Extraction Process

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1. Flotation Gold Extraction Process Principle

Flotation gold extraction process is based on the difference of physical and chemical properties of the ore surface, and is processed by flotation reagent to make the useful mineral adhere to the bubble to achieve the purpose of gold extraction.

Gold is an easy-floating mineral, so flotation gold extraction process has always been one of the effective ways to process gold-bearing ores. In primary gold deposits, gold minerals often coexist with sulfide minerals such as pyrite, chalcopyrite, galena, and sphalerite. These minerals are both easy-floating minerals and form stable mineralized foams. They are the ideal carrier minerals for gold minerals. The flotation gold extraction process can maximize the enrichment of gold into the sulfide concentrate. The flotation process is mainly used to process pulse gold ore. Flotation gold extraction process can also be used to process gold alluvial, but since it can be economically recovered by the low cost gravity extraction process, flotation gold extraction process is rarely used in industrial production.

2. Flotation Gold Extraction Process Flow

(1)Preparation for the ore before the gold extraction process. The raw ore is crushed, screened, ground and classified (Sometimes need washing and desliming) to obtain the appropriate particle size, and then enters the flotation process stage.

(2)Add flotation reagent. The surface properties of minerals are changed by the action of various chemicals, the stability of the bubbles is controlled, the ion composition of the slurry is adjusted to create suitable flotation conditions, and the separation and enrichment of minerals is realized.

(3)Agitation with aeration. The air is dispersed into bubbles by mechanical agitation, and the ore particles are sufficiently suspended and promote dissolution and dispersion of the flotation reagent.

(4)Mineralization of bubbles. The extraction process can only be achieved if the selectivity of the useful mineral particles is easily adhered to the bubbles and the gangue is difficult to adhere to the bubbles.

(5)The discharge of mineralized foam. The foam product is scraped off by a scraper or automatically spilled into a concentrate.

3. Flotation Gold Extraction Process Characteristics

(1)Advantage
  • The flotation gold extraction process is commonly used to process gold-bearing sulfide ore with high floatability, because the flotation process can maximize the enrichment of gold into the sulfide mineral concentrate, and the gold extraction process cost is low.
  • The flotation gold extraction process is also used to process polymetallic gold-bearing ores such as gold-copper, gold-lead, gold-bismuth, and gold-copper-lead-zinc-sulfur ore. For this type of ore, the flotation process can effectively identify and extract various gold-bearing sulfide concentrates, which is conducive to the comprehensive recovery of mineral resources.
  • For so-called "refractory ores" that cannot be directly extracted by amalgamation or cyanidation, it is also necessary to use a combined process including flotation gold extraction process.
  • (2) Disadvantages
  • It's difficult to use flotation process for coarse-grained gold ore extraction, especially when the gold grain is larger than 0.2mm.
  • For ore hard to reach the flotation conditions, such as quartz-bearing gold ore without sulfides, it is difficult to obtain stable flotation foam after slurring. So the gold extraction flotation performance won't be excellent.
  • For gold flotation process, crushing and grinding process is necessary and a huge amount of flotation reagent must be consumed. Therefore, flotation process means higher cost than gravity separation and amalgam treatment in terms of economic indicators.
  • 4. Flotation Gold Extraction Process Case

    The main metal minerals in a gold-bearing quartz mine in China are galena, chalcopyrite, pyrite, native gold, and gangue minerals are mainly quart. In the flotation gold extraction process, sulfurous acid and lime are used as inhibitor, copper sulfate as activator, ethyl xanthate and butyl xanthate as collectors and pine oil as foaming agent. The dosage of every agent is 15g/t of the pine oil , 130-140g / t of xanthate, 4000g / t of lime, 350g / t of copper sulfate, and 40L/ t of sulfuric acid. The obtained gold-copper concentrate contains 23% copper and 150g/t gold, and gold-lead concentrate contains 58% lead and 40g/t gold. The total recovery rate of gold is 91%.

    Gravity Separation Gold Extraction Process

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    1. Gravity Separation Gold Extraction Principle

    Gold gravity separation refers to the gold extraction and separation process making use of different moving speed and direction of mineral with different particle sizes, traits and proportion under the combined action of gravity, centrifugal force, medium resistance and mechanical resistance.

    Gold gravity separation is the oldest gold extraction process. Gold generally occurs in a form of monomeric native gold in alluvial gold mines, and the particle size is generally greater than 16g/t. Therefore, gold gravity separation is one of the most effective and economical processing methods for alluvial gold extraction. However, in general, the single gold gravity separation process is not suitable for vein gold ore extraction and if combined with other gold extraction process, the vein gold separation effect will be better.

    2. Gravity Separation Gold Extraction Types

    (1)Jigging Gold Extraction Process

    Jigging gold extraction method is a kind of beneficiation process for gold ore with jigging machine as the main gold extraction machine. The jigging machine has some merits including simple structure, large extraction capacity of per unit area, and convenient operation and maintenance. On modern large-scale gold mining vessels, the jigging machine has become the main gold extraction equipment, which can recover monomer gold directly from alluvial gold. The main factors affecting the work of the jig are: stroke and brewing, undersize supplemental water, artificial bed stone and bed thickness, mineral water, feeding amount and screen.

    (2)Shaking Table Gold Extraction Process

    Shaking table gold extraction is a beneficiation process for gold ore with shaking table as main gold extraction equipment. The shaking table is a gravity separation equipment that combines a beveled water flow with a mechanical shaking action. The shaking table consists of three parts: the bed surface, the frame and the conveyor. The bed surface is driven by the transmission mechanism to make longitudinal reciprocating motion. After the slurry is fed to the shaking table, it is subjected to lateral water flow and longitudinal reciprocating shaking on the bed surface. The ore particles achieve mineral layer based on specific gravity and particle size, and moved in different direction of the bed surface, and finally divided into concentrate, middlings and tailings. The effective recovery particle size of the shaking table is 2-0.04mm.

    (3)Spiral Chute Gold Extraction Process

    Spiral chute gold extraction is a kind of beneficiation process with spiral chute as the main gold extraction equipment. As a wooden (or steel) narrow chute with an inclination of 3° to 4° (maximum of 14° to 16°), the spiral chute's separation principle is as follow: after the slurry is fed into the chute from the trough, under the combined action of the force of the water flow, the gravity of the ore (or centrifugal force), the friction between the ore and the bottom of the trough, the minerals of different density complete loosening layer and separation where the minerals with larger at the bottom of the tank becomes concentrate, and those with small density become tailings. The spiral chute belong to intermittent operation. When concentrate at the bottom is deposited to a certain height, feeding will be stopped and concentrate will be sorted out.

    (4) Spiral Concentrator Gold Extraction Process

    The spiral concentrator gold extraction process is a gold extraction process using spiral concentrator as main equipment. This gold extraction process is as following: the ore slurry fed from above the chute flows downward along the chute in a spiral shape. During the flow, the ore particles are stratified. Large particles with small density are distributed on the outer edge of the spiral chute, and fine particles with large density are distributed on the inner edge of the spiral chute. The stratified heavy product is discharged by the discharge port of the inner chute bottom by the interceptor, and the light product is discharged by the end of the spiral chute.

    (5) Cone Concentrator Gold Extraction Process

    The cone concentrator gold extraction process is a gold extraction process using cone concentrator as main equipment. The chute of the cone concentrator is fan-shaped, the chute length is about 1 m, the feed end is 125-400 mm wide, and the chute surface is inclined.

    The ore slurry is fed from the center of the upper end and distributed through the distribute cone to enter the extraction cone. The ore particles are stratified by density during the flow on the extraction cone, and the light and heavy products are separated by the intercepting port in the end.

    3. Gravity Separation Gold Extraction Characteristics

    (1) Advantages
    • Simple process and low investment
    • No need to use chemicals, low mineral processing cost, less pollution to the environment.
    • In some cases, using gravity separation to pre-concentration, and using flotation or cyanidation to extract the middlings can save the cost.
    • Suitable for the extraction of coarse gold.
    (2) Disadvantages

    If using single gravity separation process to extract gold, the recovery rate of gold would be low. This process needs to be combined with flotation, cyanidation and other processes.

    Mercury Amalgamation Gold Extraction Process

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    1. Mercury Amalgamation Gold Extraction Process Principle

    The principle of gold extraction by using mercury (amalgamation method) is mainly that after the mercury is added to the slurry, the surface of the gold particles is wetted by mercury, and then the mercury diffuses into the interior of the gold particles, causing a chemical reaction. Au+2Hg=AuHg2. Finally, the solid of Au3 Hg will form. When the obtained solid matter is heated, the mercury will volatilize into an elemental form, and the gold will exist in the form of sponge gold.

    2. Mercury Amalgamation Gold Extraction Process Types

    Mercury amalgamation gold extraction process is generally divided into two types, mainly internal mercury amalgamation method and external mercury amalgamation method.

    (1)Internal Mercury Amalgamation Gold Extraction Process

    Internal mercury amalgamation gold extraction process refers to extracting gold from the inside of the grinding equipment. The extraction of gold is accomplished by adding mercury while the grinding operation is being carried out. The internal mercury amalgamation is mainly used to process ore with high grade, which is one of the commonly used methods for processing gravel mine. The extraction of gold becomes easier when other impurities in the gold ore are low in content, the dissemination size of gold is coarse and there is no sulfide that will pulverize mercury. At this point, the gold can be extracted by internal mercury amalgamation while grinding the ores. The commonly used equipment mainly includes the special small ball mill, mixed mercury barrel, ore pound, and roller and so on.

    (2)External Mercury Amalgamation Gold Extraction Process

    External mercury amalgamation gold extraction process refers to extracting gold outside of grinding equipment. It is mainly used to extract part of free gold. Especially when the dissemination size of gold is fine, it can use this method for gold extraction. The commonly used equipment generally includes mercury mixing board and mercury mixing machinery of various structures. The mixed mercury plate is usually assembled in ball mill, classifier or bushy machine, which is used to extract monomer native gold pellets. In addition, external mercury amalgamation method and internal mercury amalgamation method can be used in combination, which can extract some gold granules and mercury paste from internal amalgamation equipment, and improve the recovery rate of gold.

    3. Mercury Amalgamation Gold Extraction Process Characteristics

    (1) Advantages
    • Simple equipment, small floor space, low investment, energy saving, low production cost, uncomplicated process flow, easy operation and good economic benefits.
    • The mercury amalgamation gold extraction process has a particularly large enrichment ratio. It can obtain gold amalgam with about 30% grade from raw ore containing 2-3 g/ton of gold, and the gold amalgam can be refined into gold with more than 70% grade in the mine, which greatly reduces the transportation of semi-finished products in mines.
    (2) Disadvantages

    Mercury is a highly toxic substance, which needs to be handled carefully during use, and the sewage it produces must be strictly controlled. The pollution of mercury to the environment is also the key controlling target of environmental protection departments in various countries over the years. Mercury pollution will spread over large areas with mediums such as atmosphere and water, making it more difficult to control.

    4. Mercury Amalgamation Gold Extraction Process Case

    The metal minerals in a gold-copper-pyrite mine in China account for 10%-15%, mainly chalcopyrite, pyrite, magnetite, and gangue minerals are mainly quartz and chlorite gneiss.

    The average grade of copper raw ore is 0.15-0.20%, the average grade of iron is 4-7%, the average grade of gold is 10-20g/t, and the grade of silver is about 2.8 times that of gold. The plant uses amalgamation method to extract gold, and the total gold recovery rate of the mine is 70%.

    Gold Extraction Process Commonly Used in Actual Production

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    Gold extraction process is a combined process of various gold extraction methods (amalgamation, gravity separation, flotation, cyanidation) to extract gold from gold-bearing rocks. The selection of the gold extraction process is mainly based on the nature of the ore and the requirements for the product form. The gold extraction process is proposed according to the ore selectability test. There are many options of gold extraction process used in actual production, which are generally adopted as follows:

    1. Amalgamation-Gravity Separation Combined Process

    This kind of gold extraction process includes amalgamation before gravity separation process and gravity separation before amalgamation process.

    Gravity separation before amalgamation process is mostly used for the extraction of low grade alluvial gold or the ore with coarse disseminated grain size and contaminated surface. Amalgamation before gravity separation process is mostly used for simple quartz vein gold-bearing sulfide ore, and when gold is monomer in the ore.

    2. Gravity Separation (Amalgamation)-Cyanidation Combined Process

    This kind of gold extraction process is suitable for the gold-containing oxidized ore of quartz vein type with uneven grain size and deep oxidation. The kind of ore should be basically free of elements such as copper, arsenic and antimony. This kind of gold extraction process is suitable for small mines owned by individual.

    3. Single Flotation Process

    This gold extraction process is suitable for the extraction of gold-bearing quartz veins with fine gold particles and good floatability, gold-bearing altered rock type ores, and also for ores containing various sulfides. This gold extraction process is suitable for small to medium or large gold processing plants.

    4. Gravity Separation (Amalgamation)-Flotation Combined Process

    This gold extraction process is suitable for the extraction of gold-bearing quartz veins with fine gold particles and good floatability, gold-bearing altered rock type ores, and also for ores containing various sulfides. This gold extraction process is suitable for small to medium or large gold processing plants.

    5. Direct Cyanidation (All-Slime Cyanidation) Process

    This gold extraction process is applicable to all-ferritization pyrite quartz vein type ore or gold-bearing alteration diorite-type ore with fine grain size and deep oxidation. The ore is required to contain no elements harmful to cyanidation such as copper, arsenic or antimony.

    6. Flotation-Cyanidation Combined Process

    This gold extraction process is applicable to the gold-bearing pyrite quartz vein ore which is closely related to the symbiotic relationship between gold and sulfide. It is also suitable for the altered rock type ore of the gold-bearing granite fracture zone, especially for high sulfur ore. This gold extraction process is also one of the common processes in China's mines.

    7. Flotation-Roasting-Cyanidation Combined Process

    This gold extraction process is applicable to gold-bearing quartz vein type ores containing elements such as arsenic and antimony. The roasting process is a preparation for cyanidation in order to remove elements that are harmful to cyanidation.

    The several gold extraction processes above are typical gold extraction processes. Different mines should select the appropriate gold extraction process according to the nature of their ore and technical and economic requirements.

    
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