Southwest Irrigation, LLC
Southwest Irrigation is the leading supplier for Heap Leach Mining Systems and Agricultural Irrigation Systems in the southwest.


Email: swiminingorders@swirr.com                
 
Mining Solution Application

The mined ore is usually crushed into small chunks and heaped on an impermeable plastic and/or clay lined leach pad where it can be irrigated with a leach solution to dissolve the valuable metals. While sprinklers are occasionally used for irrigation, more often operations use drip irrigation to minimize evaporation, provide more uniform distribution of the leach solution, and avoid damaging the exposed mineral. The solution then percolates through the heap and leaches both the target and other minerals. This process, called the "leach cycle," generally takes from one or two months for simple oxide ores (e.g., most gold ores) to two years (for nickel laterite ores). The leach solution containing the dissolved minerals is then collected, treated in a process plant to recover the target mineral and in some cases precipitate other minerals, and then recycled to the heap after reagent levels are adjusted. Ultimate recovery of the target mineral can range from 30% of contained (run-of-mine dump leaching sulfide copper ores) to over 90% for the easiest to leach ores (some oxide gold ores).

In recent years, the addition of an agglomeration drum has improved on the heap leaching process by allowing for a more efficient leach. The rotary drum agglomerator works by taking the crushed ore fines and agglomerating them into more uniform particles. This makes it much easier for the leaching solution to percolate through the pile, making its way through the channels between particles.

The addition of an agglomeration drum also has the added benefit of being able to pre-mix the leaching solution with the ore fines, to achieve a more concentrated, homogeneous mixture, and allowing the leach to begin prior to the heap.

Precious metals

The crushed ore is irrigated with a dilute alkaline cyanide solution. The solution containing the dissolved precious metals ("pregnant solution") continues percolating through the crushed ore until it reaches the liner at the bottom of the heap where it drains into a storage (pregnant solution) pond. After separating the precious metals from the pregnant solution, the dilute cyanide solution (now called "barren solution") is normally re-used in the heap-leach-process or occasionally sent to an industrial water treatment facility where the residual cyanide is treated and residual metals are removed. In very high rainfall areas, such as the tropics, in some cases there is surplus water that is then discharged to the environment, after treatment, posing possible water pollution if treatment is not properly carried out.

The production of one gold ring through this method, can generate 20 tons of waste material.

During the extraction phase, the gold ions form complex ions with the cyanide:


Recuperation of the gold is readily achieved with a redox-reaction:

The most common methods to remove the gold from solution are either using activated carbon to selectively absorb it, or the Merrill-Crowe process where zinc powder is added to cause a precipitation of gold and zinc. The fine product can be either doré (gold-silver bars) or zinc-gold sludge that is then refined elsewhere.

Copper Ores

The method is similar to the cyanide method, above, except sulfuric acid is used to dissolve copper from its ores. The acid is recycled from the solvent extraction circuit and reused on the leach pad. A byproduct is iron sulfate, jarosite, which is produced as a byproduct of leaching pyrite, and sometimes even the same sulfuric acid that is needed for the process. Both oxide and sulfide ores can be leached, though the leach cycles are much different and sulfide leaching requires a bacterial or "bio-leach" component. The largest copper heap leach operations are in Chile, Peru, and the southwestern United States.

Although the heap leaching is a low cost-process, it normally has recovery rates of 60-70%, although there are exceptions. It is normally most profitable with low-grade ores. Higher-grade ores are usually put through more complex milling processes where higher recoveries justify the extra cost. The process chosen depends on the properties of the ore.

The final product is cathode copper.

Nickel Ores

The method is an acid heap leaching method like that of the copper method in that it utilises sulfuric acid instead of cyanide solution to dissolve the target minerals from crushed ore. The amount of sulfuric acid required is much higher than for copper ores (as high as 1,000 kg of acid per tonne of ore, but 500 kg is more common.) The method was originally patented by Australian miner BHP Billiton and is being commercialized by Cerro Matoso S.A. in Colombia, Vale in Brazil, and European Nickel PLC for the rock laterite deposits of Turkey, Talvivaara mine in Finland, Balkans, and the Philippines. There currently are no operating commercial scale nickel laterite heap leach operations, but there is a sulphide HL operating in Finland.

Nickel recovery from the leach solutions is much more complex than for copper and requires various stages of iron and magnesium removal, and the process produces both leached ore residue ("ripios") and chemical precipitates from the recovery plant (principally iron oxide residues, magnesium sulfate and calcium sulfate) in roughly equal proportions. Thus, a unique feature of nickel heap leaching is the need for a tailings disposal area.

The final product can be nickel hydroxide precipitates (NHP) or mixed metal hydroxide precipitates (MHP), which are then subject to conventional smelting to produce metallic nickel.

Uranium Ores

Similar to copper oxide heap leaching, also using dilute sulfuric acid. Rio Tinto is commercializing this technology in Namibia and Australia, the French nuclear power company Areva in Niger (two mines) and Namibia, and several other companies are studying its feasibility.

The final product is yellowcake and requires significant further processing to produce fuel-grade feed.

Apparatus

While most mining companies have shifted from a previously accepted sprinkler method to the percolation of slowly dripping choice chemicals (cyanide or sulfuric acid) closer to the actual ore bed,] heap leach pads have not changed too much throughout the years. There are still four main categories of pads: conventional, dump leach, Valley Fills, and on/off pads.] Typically, each pad only has a single, geomembrane liner for each pad, with a minimum thickness of 1.5mm (usually it is thicker).

The simplest in design, conventional pads are used for mostly flat or gentle areas and hold thinner layers of crushed ore. Dump leach pads hold more ore and can usually handle a less flat terrain. Valley Fills are pads situated at valley bottoms or levels that can hold everything falling into it. On/off pads involve the use of putting significantly larger loads on the pads, and removing it and reloading it after every cycle.

Many of these mines, which previously had digging depths of about 15 meters, are digging deeper than ever before to mine materials (approximately 50 meters, sometimes more), which means that, in order to accommodate all of the ground being displaced, pads will have to hold higher weights from more crushed ore being contained in a smaller area. With that increase in build up comes in potential for decrease in yield or ore quality, as well as potential either weak spots in the lining or areas of increased pressure buildup. This build up still has the potential to lead to punctures in the liner. As of 2004 cushion fabrics, which could reduce potential punctures and their leaking, were still being debated due to their tendency to increase risks if too much weight on too large a surface was placed on the cushioning. In addition, some liners, depending on their composition, may react with salts in the soil as well as acid from the chemical leaching to affect the successfulness of the liner. This can be amplified over time.

 

 

 

 

 

  Office 520-384-9223
  
401 E Maley
Willcox, AZ 85643
Email:
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miningorders@swirr.com
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Website: www.southwest-irrigation.com