Hydrometallurgy For Gold Leaching

Refiners hydrometallurgy will give examples of careers and precious metals using chemistry both inorganic and organic, chemical engineering, metallurgical engineering and others. Hydrometallurgy for gold refers the processing techniques that require wet chemistry approaches to separate the precious metals from the surrounding materials vs pyrometallurgy which requires heating and smelting operations to perform the separation.

Hydrometallurgy to take many forms depending on the substrate or the material which contains the precious metals. Many of these chemicals used are very hazardous and these processes should only be done by professional refiners

Hydrometallurgy is an extractive metallurgy process using aqueous solutions to recover metals it involves technologies with solubilize, concentrate and then recover specific metals.

For example a first step maybe to chemically dissolve or leach a material to eventually extract metals by means of precipitation or electrically reducing the metals.  Leaching is the first process used to extract metals into an aqueous solution, chemical acids or alkali eyes are used to convert metals into soluble salts within the solution.

For example aqua regia is a highly corrosive acid mixture of nitric and hydrochloric acids which will dissolve most metals, including gold platinum and paladium

In the other platinum group metals however do not dissolve in aqua regia so it can be used to separate these groups, all those silver also dissolves in aqua regia it immediately precipitates to silver chloride so does not go in the solution. Nitric acid alone is used to dissolve silver.

The different gold leaching processes are descriptively named :

Gold leaching processe

1. Situ Leaching

 In situ leaching also called  solution mining. Extrating the minerals directly from ore in the mining. Leach solutions are pumped through the ore to surface.  In situ leaching is occasionally used in select mining operations to leach precious metals from surrounding rock structures.

2. Heap leaching is routinely used to remove gold from ore deposits in tailings

3.Vat leaching is most frequently used when removing gold from secondary scrap materials such as electronic scrap or even jewelry scrap

See Also : 
How To Work The Heap Leaching Mining  
Gold Leaching Process With Carbon In Pulp

4. Agitated leaching is where the material is saved in slurry with the extracting solution.

5. Pressure leaching places the material, solvent along autoclaves to pressurize heat, it is also used when gases need to be added for the leaching reaction.

The pressure oxidation process the free gold is an example of this cyanidation and chlorination are other pressure leaching processes, after leaching solution concentration and purification steps are employed to further concentrate and separate the precious metals

These refining processes can be used for precious metals extracted from primary remaining sources as well as secondary sources of industrial or recycle scrap materials

Video How To Refine Precious Metals - Hydrometallurgy

HOW TO WORK THE HEAP LEACHING MINING

Heap leach is a method developed by Henin and Lindstrom to process low-grade ore on a large scale with small production cost so that the levels below the cut-of grade can still be economical. Heap leach is the process of leaching of low grade gold rocks thrown, then the solution containing the metal particles are screened and separated by electrolysis. Heap leach done repeatedly and on a large scale.

There are several steps that are used in processing gold ores using heap leaching system

• Preparation of the foundation of leaching with an angle of 1 degree to 6 degrees. : The foundation of this leaching can be made by permanently using a hardening of the concrete floor or foundation and geo-coated membrane (High Density Polyethylene)

• Then the rock minerals that have been taken from the mine area was destroyed. Destruction of rock done to get smaller sized rocks. Size obtained usually ± ½ or up to 1 inch. machines used in the process of destruction is the engine or Stone Mill Hammer Crusher and Crusher Jaws.

• After the material to be crushed and refined, then placed into the foundation of leaching that has been provided

• Then through the spray in the spray solution of sodium sianide. And in general the use of sodium sianide NaCN. This process aims to take the gold extraction is formed in the chlorination process and transformed into compounds Dicyano Aurrat (Au (CN) 2). Because the compound that would be too absorption within the pores of activated carbon

In the cyanidation process consists of two important stages, namely the process of dissolution and separation of gold from the solution process. Solvents commonly used in the process cyanidasi is NaCN, KCN, Ca (CN) 2, or a mixture of all three. The solvent most often used is NaCN, being able to dissolve gold better than any other solvent. 

From this it will spray cyanide in the extraction process produces a chemical reaction in the form:

 Au + NaCN + O2 + H2O = NaAu (CN) 2 + NaOH (from equation Elsener and Adamson)
 Au + NaCN + O2 + H2O = NaAu (CN) 2 + H2O2 + NaOH (from equation Adamson)

Gold extraction is generally effective at pH 9.5 to 11. Optimum near 10.5. The more acidic, hydrolysis to hydrogen cyanide had cyanide (HCN), the more alkaline reaction the more it will slow down. To speed up the process of gold dissolution in cyanide solution will be added to the catalyst. Namely lead Nitrate (PbNO3), because the catalyst can also increase the recovery of gold to be 95%. Generally cyanide complex solution containing between 1 ppm to 3 ppm.

• Once the cycle is completed the leaching process of leaching in the wash with water to remove residual cyanide is used peroxide. Decision-stop solution if the gold content down to 0.005 ounces of gold per ton of solution. Taking gold in the solution can be done by using activated carbon or by direct receipts using zinc cementation process (Merrill - Crowe Process). In addition to using the carbon and cementation, other techniques can be used is elektrorefining and ion exchange resins. Elektrorefining is the process of attachment of the metal contained in the solution on the cathode rod, in this process is stailess steel rod anode and cathode in the form of steel brush.

     In heap leaching using carbon adsorption techniques in coulumn Carbon (CIC) in which the solution containing the gold complex flows in tanks containing activated carbon in series and terraced. Activated carbon can absorb gold from 100 up to 400 ounces per ton of carbon, the efficiency will drop if content of the solution decreases. Precious metals in the capture of carbon by passing a hot solution of caustic soda and cyanide mixture (1% NaOH and 1% NaCN) Activated carbon can be reused by reactivation in advance using the rotary kiln at a temperature of 730 degrees Celsius for 20 minutes. In traditional mining usually carbon that has been filled with gold complexes directly in the fuel, the dust from the burning of carbon was then taken to be melted down. Contained metal can be purified directly. This is done because the aged and technological considerations.

Heap Leaching Mining

      Heap leaching mining was developed as a mineral processing low-grade metal that is more efficient and minimal cost. Compared with conventional cyanidation process of the mineral leaching process with stirred tank Heap leaching has several advantages in the form of designs that use a very simple, operating costs are also minimal, and in requiring less investment, because the heap leaching recovery is around 60% to 80 %.

 In large-scale process technology was first used and applied to the mining of gold ore at Carlin Nevada USA in 1970. This process has really made ​​a lot of mines that can easily take on geological resources that have a low grade and transforming them into the category of proven ore. Ore value as low as 0.01 oz Au can be processed by heap leaching system is economically.

       Besides gold and silver, cyanidation is also often used to recover copper content in the rock, due to the nature of this metal is very easily soluble in both the cyanide. And sometimes too much copper in the rocks encountered in the content of gold and silver cyanidation method on the rocks so that it is still economical given.

Heap leaching generally requires 60 to 90 days for processing of ore that can be leached in 24 hours in a conventional agitated leach process. Gold recovery is usually 70% compared with 90% in agitated leach plant. Even with this low performance, the heap leaching process has facilitated the processing of natural resources, because the processing cost is much reduced compared with agitated leaching.

        The process involves the leaching of rocks and placed as well as the destruction of rock minerals that contain metal ore in a heap on the building on a floor stand. The foundations of leaching (leaching pad) can be made permanently using a hardening of the concrete floor or foundation and geo-coated membrane (high density polyethylene) cyanide solution is distributed at the top of the stack and then the liquid seeps down through the pile of ore and dissolves the gold through a pile interrupted. Gold laden minerals flowing out from the bottom of the pile and is collected for gold recovery by carbon adsorption either with or zinc precipitation. Barren solution is then recycled to the next pile.

Some of the benefits or advantages from the use of heap leaching system are as follows:

1. Extraction from the heap leaching is usually carried out on the rocks -3 / 4 inches, whereas extraction in agitated leaching in stirred tank requires a reduction to -200 mesh size of 100 mesh or need. 
2. Solid liquid separation step is not necessary to heap leaching on the heap leaching system.
3. Tailings disposal costs are high enough for the modern agitated leach plant. Large dam expensive fluid retention is required. For comparison, heap leach pads can generally be left in place after reclamation.

Some Disadvantages heap leaching system, in addition to a lower recovery compared with agitated leaching heap leaching, including: 

1. Ore stacked must be porous enough to allow solution to drip or drain water solution through the slit pore. There are plenty of recovery failure due to inability to obtain the flow solution. It's a lot of experience when the ore has high clay content or in rocks containing clay very much. This problem is often solved by agglomeration prior to stacking piles. 
2. In areas of high rainfall, the solution of balance problems can arise, so it requires a lot of care and discharge processes. 
3. In areas where extremely cold, freezing the pile can result in low recovery period. Modification of operational procedures such as subsurface application solutions has been reduced, but not eliminated this concern.

System Flow Chart In a Simple Heap Leaching 
In general, processing system gold using heap leaching method can be described as the chart below

Transport of rock minerals on lands open pit using dump trucks

 

Rock mineral that is ready to be destroyed and soften

 

 

Destruction of rock done to get smaller sized rocks. Size obtained usually ± 20mm. And used machines are machines Hammer Mill, Stone Crusher or Jaws Crusher

 

 

Placement of material that has been crushed and refined into the foundation leaching 

Cyanide solution through water storage tank in the pump and the pipe through the hose and spray the spray solution of sodium cyanide on rock minerals on the leaching

 

Recharge cycles in which the metal minerals will drop to flow down through the rock broke into the flow of irrigation. The solution then percolates through the pile and dissolve both the target and other minerals. Leach solution containing dissolved minerals are then collected, processed in a factory process to recover the mineral targets. In heap leaching using carbon adsorption techniques in column Carbon (CIC) in which the solution containing the gold complex flows in tanks containing activated carbon in series and multilevel 

 

In heap leaching using carbon adsorption techniques in column Carbon (CIC) in which the solution containing the gold complex flows in tanks containing activated carbon in series and terraced. 

 

Activated carbon that has been filtered and removed from the vat of mineral-rich bath solution can absorb gold from 100 up to 400 ounces per ton of carbon, the efficiency will drop if content of the solution decreases. Precious metals in the capture of carbon by passing a hot solution of caustic soda and cyanide mixture (1% NaOH and 1% NaCN) Activated carbon can be reused by reactivation in advance using the rotary kiln at a temperature of 730 degrees Celsius for 20 minutes

 

Melting Process of activated carbon and Metal Refining Gold.

gold bullion metals