Hydraulic and earth engineering – Soil remediation – In situ contaminant removal or stabilization
Reexamination Certificate
2000-08-01
2003-01-21
Shackelford, Heather (Department: 3673)
Hydraulic and earth engineering
Soil remediation
In situ contaminant removal or stabilization
C405S128750, C037S462000, C037S465000, C037S355000, C037S360000
Reexamination Certificate
active
06508608
ABSTRACT:
FIELD OF THE INVENTION
The present invention is generally directed to the treatment of materials such as earthen materials and, in particular, to a method and apparatus for tilling base material in a manner that forms a mixing area where additives may be applied to and mixed with the tilled base material. The method and apparatus are particularly suitable for use in treatment of a leach pad of a precious metals mining operation.
BACKGROUND OF THE INVENTION
In many in-situ material treatment processes, it is desirable to reduce compaction of the material. Uncompacted material requires less energy and less additives to process, resulting in decreased environmental impact. Reduced compaction increases the permeability and effective surface area of the material, which can enhance the effects of additives that are often applied to the material in-situ to cause a change in the material. A need exists for improved methods and devices for reducing compaction and treating materials in-situ. Contaminated soil remediation and chemical leaching operations are two processes that can benefit from reduced compaction. Because opportunities for particularly significant economic and environmental benefits exist in a leaching process of a precious metals mining operation, the present invention will be described with reference to that process.
Mining for precious metals, such as, for example, gold, platinum, silver, and copper, commonly involves a leaching process that is used to extract these metals from a low grade raw ore. In such a mining operation, the ore is typically collected on a heap leach pad built on the surface of a relatively flat land area many acres in size.
FIG. 1
is a side elevational view of a leach pad
10
being constructed in accordance with the prior art.
FIG. 2
is a diagrammatic illustration of a multi-lift leach pad in accordance with the prior art. With reference to
FIGS. 1 and 2
, the leach pad
10
is constructed on a basin
11
having a crowned surface covered by a liner. The leach pad
10
is supplied with ore
12
brought in by large dump trucks
13
to form a layer of ore called a lift
14
, typically having a depth of between 9 and 50 feet. After the lift
14
is formed, a liquid leaching agent is applied to the upper surface of the lift
14
, usually by a sprinkler system (not shown). The leaching agent percolates through the lift
14
and dissolves or otherwise binds to metals in the ore. The laden leaching solution, often called the leachate or pregnant leachate, is contained by the liner and is collected at the perimeter of the leach pad
10
for transportation to a refining facility where the metals are chemically extracted from the laden leaching solution. When the concentration of metals in the leachate decreases to a certain level, a fresh lift of ore
15
(
FIG. 2
) is then deposited over the spent ore and the process is repeated. Multiple lifts are formed so that the leaching agent continues to percolate through the lower lifts to maximize the yield of the ore.
It is important that the lift be evenly permeable so that the leaching agent can percolate completely throughout the lift. However, the permeability of the lift decreases due to the way in which it is built. In building the lift, the dump trucks
13
may deliver, for example, 38,000 loads of raw ore with each load of raw ore weighing between 75 and 300 tons. The lift
14
is compacted by the weight of the trucks
13
as they drive over it to deliver each load of ore
12
, as shown in FIG.
1
. The compaction of the lift
14
is greatest near the upper surface of the lift and decreases with depth. Substantially all truck compaction is found in the top six to nine feet of the lift. Ideally, the ore would consist entirely of clean gravel that remains highly permeable, even when compacted. However, ore commonly includes fines, silts, and clay that form a less permeable matrix with the gravel when compacted. Poor permeability inhibits the free flow of the leaching agent through the lift and lessens the yield of the ore.
A ripper is typically used to break up the ore at the upper surface of the lift. A ripper is a bulldozer that drags a shank through the upper surface of the lift.
FIG. 3
shows a prior-art ripper
16
with its shank
17
retracted.
FIG. 2
shows the path followed by the ripper
16
. Rippers have proven only partially effective in reducing compaction because they are typically unable to extend their shanks deeper than 60 inches. Additionally, known rippers have shanks that produce only a narrow path of ripped lift material, typically less than 6 inches wide. Because the shank
17
is narrow, the ripper
16
usually leaves pillars or cells of compacted ore between adjacent paths of the shank
17
, resulting in less than optimal permeability of the leach pad. The liquid leaching agent will follow the path of least resistance as it filters through the lift. The compacted cells and pillars form flow channels
18
between them that shunt the flow of the leaching agent and can prevent it from percolating through entire sections of the lift. Hydrodynamic effects of flow channels can also cause fines and silts to form dams and lenses within the lift that further hinder leachate dispersion. Lenses (subsurface ponds) and dams reduce ore yield by producing a shadowing effect that leaves dry spots in the lift. Lenses and dams have lesser effect in shallow pads (9-20 ft. deep) because there is less material to be shadowed. In a taller lift, lenses formed near the top of the lift will shadow larger amounts of ore. On the other hand, the effects of flow channeling (in the absence of dams and lenses) are typically more pronounced in shallow pads (9-20 ft. deep) due to shorter soak times and the shorter distance from the top of the lift where the leaching agent is applied, down to the relatively dead material at the bottom of the leach pad.
To improve the permeability of the leach pad, fine ores are sometimes treated with an agglomerating treatment prior to being deposited onto the leach pad so that the fine particles will agglomerate into larger clumps that are more loosely organized. Agglomerated material tends to inhibit the formation of lenses and dams within the lift because it has drainage properties that are similar to gravel. However, agglomerated material is not very resistant to compaction caused by the weight of delivery trucks driving over the agglomerated material after it is deposited on the lift. Consequently, a need exists for improved methods of agglomerating that do not subject the agglomerated materials to compaction forces.
Machines have been proposed for tilling compacted soil as a step in environmental remediation of contaminated soil. For example, U.S. Pat. No. 5,639,182 of Paris describes a method for treating soil in which a treatment material is first spread over the surface of the soil then tilled into the soil by a mixing apparatus. Paris does not disclose the use of the mixing apparatus or process in a precious metals mining operation for extracting metals from ore. The mixing apparatus includes a vertically-oriented endless cutter that is towed behind a tractor over the soil area to be treated. The endless cutter is extended into the soil to a cutting depth and activated to drive the treatment material down into the hard soil and to provide a mixing effect. The soil and treatment material is driven down and around the lower end of the cutter and back up to the soil surface at a location distal of the tractor. The mixed soil and treatment material may be moved to the side of the machine by a lateral conveyor. Because the treatment material is spread on the surface of the soil before mixing, the mixing apparatus may not always mix it evenly throughout the cutting depth. Furthermore, the teeth of the endless cutter are shaped and sized so that fines would tend to remain at the cutting depth without being drawn back to the surface for a more thorough mixing with the treatment material. It would also be ineffective for agglomeration of fines into small clumps because the
Nilson Ron
Stone Thomas C.
Shackelford Heather
Singh Sunil
Stoel Rives LLP
Tracsaw Manufacturing, Inc.
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