Heap leaching is just the first stage in the ore recovery process. Once the leachate is produced by the slow trickle of strong chemical solutions through the heaped material, it requires plenty of processing and treatment to transform into a valuable product. However, it all begins with solutions and chemicals known as lixiviants. Without the right chemicals mixed into the water leached through the heap, only minimal ore is recovered, and even low energy processes are rarely worth the effort. Heap leaching is different from conventional tank leaching because it requires far less water. The reduced volume in water is made up for by the accelerating action of the lixiviant chemicals. Yet, the same solutions that are necessary for the leaching process are also particularly hard on the pond liners. Knowing what kind of chemicals are affecting the long-term stability of the liner material is key to choosing a geomembrane with the right resistances.
What is Mining Waste or Gangue?
The primary purpose of a leaching solution, or more specifically the chemical known as a lixiviant, is to separate valuable minerals or ore from unwanted rock. Yet, the unwanted materials from run-of-mine rock can also include minerals and other metals too. No matter the material or its composition, unwanted waste from the mining process is known as gangue. The specific composition of the gangue attached to the valuable ore often determines the specific lixiviants needed to achieve high levels of recovery. Without taking into mind what chemicals work best on the limestone, bauxite, or other material attached to the ore, recovery levels will be lower than expected and extra cycles of treatment will be needed.
What Are Lixiviants and Solutions?
There are multiple solutions used in metallurgy, including those specific to heap leaching. The term solution just refers to any combination of water and other chemicals designed to assist in leaching. Lixiviants are specifically those chemicals known to dissolve the ore or desired mineral. Other chemicals may break down or weaken the gangue materials, but the lixiviants themselves will target the desired ore. The majority of leaching lixiviants do this by changing the pH of the material, causing the ore to come loose and lose its bounds to the waste rock. Other chemicals rely on the oxidation process to break down ore into a harvestable state. No matter the precise mechanism of the lixiviant, the chemical is intended to assist in what is an inevitable process once water is introduced to ore.
Cyanide as a Major Lixiviant
It’s the risks of the chemicals most commonly used as lixiviants that lead to processing ponds needing reliable liners. Cyanide is one of the riskiest and most widely used lixiviants. It’s the best chemical for extracting gold in particular, although it also has some use for silver, platinum, and uranium recovery as well. It’s toxic to plant, human, and animal life, in addition to damaging waterways. Many relatively small spills of this chemical from heap leaching pads have resulted in fish kills or even poisoned water supplies. Any facility using cyanide for the heap leaching process must use reliable liners for every stage of processing, since even a single leak or seep can cause a cascade of environmental damage to the area.
Other Chemicals Commonly Used in Mineral Processing
Cyanide may be one of the most widely used lixiviants, but it’s far from the only one that can cause human health issues or environmental damage. Highly acidic chemicals, like sulfuric acid, are most commonly used for base metals like copper and nickel. They’re also hazardous to the environment, but usually have specific methods of deactivation that work well to neutralize the byproducts after processing. Gold and platinum can be treated with certain types of chloride or ammonia that are less hazardous to the environment, but these chemicals work best on tailings previously treated with acids. The number and types of previous treatment for cycled material also affect which lixiviant chemical is the best choice. No matter which chemicals are chosen, they’re likely to affect the liner unless a highly resistant material like reinforced polyethylene is used.
Secondary Separation Methods
Lixiviants and related chemicals are the primary methods for separating ores from the gangue waste products, but they’re far from the only ones. Oxidizers are also necessary for ores like uranium, in particular, to break down stubborn bonds that keep the materials mixed. Just aerating the slurry to introduce oxygen can work, but only if the liner can handle the extra wear and tear from the movement. Sodium chlorate and hydrogen peroxide are also commonly used to oxidize uranium ores.
Storage for Solutions and Lixiviant Chemicals
These chemicals usually remain in the processed wastewater left behind after the ore is extracted. Once the runoff has reached the barren ponds, it’s ready for long-term storage and reuse. The chemicals that remain mixed into the wastewater mean that it either needs disposal in a sealed underground well, treatment to improve the water quality, or reuse through the leaching system. If the leachate wastewater isn’t being held in lined, barren ponds, the tanks holding it must be protected by lined containment basins. Don’t let a spill of barren wastewater lead to costly fines for environmental damage.
Make the most of every drop of lixiviant used in the leaching process by building secure processing ponds. Lining is just the first step in keeping lixiviant ready for reuse as needed. Without a liner, you can’t expect to control the loss of these chemicals that can cause serious environmental impact. Choose RPE liners for materials you can count on for years to come. Find all the processing pond liners you need here at BTL Liners.
