What are the steps involved in cyanide heap leaching?

As mentioned in the previous article, Dasen mining focused on “From lead-zinc ore, how can silver mining process?” Furthermore, heap leaching is used to treat minerals. Next, we will discuss how cyanide heap leaching works.

1.Construction of heap leaching site.
Hoard leaching sites are typically located in gentle slope mountain areas close to mining and transportation convenience. Weeds and floating soil are removed first, and then a slope of 3 * 5% is built for the foundation, so the leaching solution can flow into the liquid storage tank by itself. A lean solution tank, an expensive solution tank, and flood control facilities should be built near the heap leaching site.

2.Ore pretreatment.
A pretreatment’s purpose is, on the one hand, to reduce the size of the ore to a specific size. Silver can be fully contacted by the cyanide leaching solution. The ore, on the other hand, has good permeability, so that the leaching solution can pass through the ore pile evenly.

Prior to pretreatment, the material is crushed into appropriate particle sizes and heap leached. After crushing, it goes through the granulation heap leaching step. As a rule, heap leaching ore can be crushed to a size of less than 30 mm, but not less than 10 mm.

3. Pile building.
The heap building process is one of the main links in heap leaching. The manner in which a heap is built is related to the permeability of the ore and the uniform distribution of coarse and fine particles, which directly affects the rate of silver leaching.
Manual or mechanical pile construction is possible.
Avoid secondary slime caused by crushing of ore, local compaction, and segregation of ore particles during stacking. In order to ensure uniform distribution of ore particles and good water and air permeability in the ore pile, so that cyanide solution and oxygen will flow through the pile evenly, and to avoid ditch flow and blockage areas during leaching.
Pile building methods include subpile building, layered pile building, and subsection layered pile building.
In general, heap leaching has a height of 2 x 5 m.

4. Pipelines are erected after the ore pile is built and are ready to spray.
5. Spray leaching.
Prior to spraying cyanide solution on the ore heap, it should be washed with water and alkali. When the pH value of the washing solution reaches 9.5 ~ 11, the cyanide solution should be sprayed.

1) Spray equipment

One of the most important factors to improve the leaching rate is uniform liquid distribution. Spray liquid distribution and drip liquid distribution are two methods of liquid distribution. No matter what liquid distribution method is used, the leaching solution should cover the entire heap of ore. Every point on the surface has a balanced flow, and evaporation and drift loss are small. Anti-corrosion materials, such as PVC plastic, should be used in liquid distribution systems.

2）leaching

① Ore washing

Before spraying the cyanide solution on the ore heap, alkali washing (or water washing before alkali washing, if necessary) can be carried out to reduce cyanide consumption. You can use CaO aqueous solution or NaOH aqueous solution. In comparison, lime aqueous solution is cheap, but takes a long time to wash the ore and is easy to form calcium, while NaOH aqueous solution is quick to wash the ore and expensive. As soon as the pH value of the solution from the ore pile reaches 9.5 * 11 (generally after 3 ~ 5 days), the mixed solution of NaCN and alkali can be used for spray leaching.

② Spray leaching
During heap leaching, appropriate concentrations of cyanide, alkali and oxygen must be maintained.
Sodium cyanide is generally controlled between 0.02 and 0.1% in the leaching solution.
NaCN concentration should be controlled in stages, generally 0.08 * 0.1 at the beginning, 0.05 * 0.06% in the middle, and 0.02 * 0.05% in the end. The cyanide concentration and pH value of leaching solutions should be measured regularly and adjusted accordingly during production. This should be done once or twice a day.

③ Spray intensity
Generally, spraying intensity is 10 ~ 20 L / m2.
Ore heaps with a large scale and poor permeability have a low value, otherwise they have a high value. It is possible to spray cyanide solution continuously or intermittently, which is conducive to oxygen supplementation and gold and silver leaching. Spray cyanide solution for 2 hours, stop and spray again after 1 hour.
Always check the nozzle pipe before spraying alkaline cyanide solution. In areas that cannot be sprayed, spray the solution to every place to ensure that silver will leach.
In order to prevent the nozzle pipe and ore heap from scaling, an appropriate amount of anti scaling agent can be added to the cyanide leaching solution.

3）Heap leaching time

The time required for heap leaching depends on factors such as ore properties and particle size, which are determined by the test. Generally, it can last between January and February, while the elderly can last until March. If the gold concentration in the effluent is less than 0.1g/m3, the spraying can be stopped.

Leached gold and silver precious liquid flows into the precious liquid pool, and gold and silver are recovered by carbon adsorption, zinc replacement, direct electrowinning, and other methods.

6. Ore heap washing:

Factors affecting heap leaching include cyanide concentration, oxygen content, pH value, spray intensity, ore properties, heap permeability, and leaching process temperature.
Following aspects are being studied to improve the leaching rate and leaching rate of gold and silver in ore.
1) When O2 is used instead of air, as for 36 days after O2, the leaching rate of gold can be increased from 82% to 91%.
2) Increase the oxygen content of the ore pile by using hydrogen peroxide or calcium peroxide.

7. Recovery of gold and silver from heap leaching solution.

In general, zinc replacement precipitation methods are applied to precious liquids with high gold and silver content and large amounts of precious liquid. Activated carbon should not be used since silver will compete with gold for adsorption, thus reducing gold’s adsorption.
For mines with a medium heap leaching scale and general gold and silver content, activated carbon or resin adsorption desorption electrowinning can be used to extract precious metals.
There is a high concentration of gold and silver in the heap leaching solution (more than 30 * 50 mg per liter). Following clarification, gold and silver are directly extracted by electrodeposition.
The activated carbon adsorption method is widely used in heap leaching because of its simplicity and low investment cost.

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