P006 ₄ PC

DRAINAGE SYSTEM FOR AERATED HEAP LEACHING AND METHOD OF ENSURING ADEQUATE DRAINAGE IN AERATED HEAP LEACHING

FIELD OF THE INVENTION

This invention relates to the drainage of solution from aerated sulphide heap leaching operations.

BACKGROUND TO THE INVENTION

The drain layer of ore heaps in sulphide heap leaching operations is typically constructed with crushed rock. Often the crushed rock is supplemented with slotted circular drainage pipe, placed in a herringbone pattern feeding a series of central collection pipes, at the base of the drain layer. Aeration pipes are typically placed midway between the base and the top of the drain layer. A slope is also provided in two or three dimensions to assist the drainage process. Typically the design basis is to contain the phreatic head below the aeration pipes in the base. However, if the permeability of the crushed rock in the drain is reduced or erroneously estimated, the phreatic head may rise into the drain layer, the air pipes may become flooded and it may become difficult or impossible to add low pressure air to the heap via the aeration pipes.

OBJECT OF THE INVENTION

It is an object of the invention to provide a drainage design for aerated heap leaching operations which at least partly overcomes the abovementioned problem.

SUMMARY OF THE INVENTION

In accordance with this invention there is provided a drainage system for an aerated heap leaching operation comprising a layer of drainage rock which includes a plurality of spaced apart substantially vertically orientated perforated tubes, preferably strip drains.

There is further provided for the drainage layer to include a plurality of aeration pipes extending substantially horizontally through the layer of drain rock, and preferably for each aeration pipe to be located proximate a perforated tube, more preferably proximate an operatively upper end of a perforated tube.

There is still further provided for the plurality of perforated tubes to equidistantly spaced apart, preferably at most about 2 meters, more preferably about 1 meter.

There is also provided for the perforated tubes to extend from the operative base of the drainage layer, preferably to a height of at least about 450 mm, more preferably to a height of at least about 900 mm.

There is further provided for the perforated tubes to be covered with a layer of geotextile fabric.

There is still further provided for the drainage layer to include a layer of synthetic material at the base thereof, preferably a layer of HDPE, and more preferably for a layer of heavy duty geonet material, preferably a layer of heavy duty tri-planar geonet, to be placed above the layer of synthetic material.

There is also provided for the drainage layer to include a top layer of drain rock above the operatively upper ends of the perforated tubes, and preferably for the top layer of drain rock to have a thickness of about 600 mm.

There is still further provided for the drainage layer to be topped by a sacrificial layer of rock which is generally finer than the drain rock, to operatively act as a trap for fine particles migrating downward.

According to a further feature of the invention there is provided a method of designing a drainage layer as defined above, which includes the steps of determining an optimum strip drain height and optimum spacing based on data of specific infiltration rates or a range of infiltration rates of a heap leaching operation.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described by way of example only and with reference to the accompanying drawings in which:

Table 1 shows the effect of herringbone pipe spacing and permeability of the drainage on the phreatic head, at an infiltration rate of 40l/m ² /hour; Figure 1 shows the performance of herringbone drains at 1 meter spacing, at various infiltration rates and drain rock hydraulic conductivities;

Figure 2 shows the performance of 900mm strip drains at 1 meter spacing, at various infiltration rates and drain rock hydraulic conductivities; Figure 3 shows the performance of 900mm strip drains at 2 meter spacing, at various infiltration rates and drain rock hydraulic conductivities; Figure 4 shows the performance of 450mm versus 900mm strip drains in a drainage system degrading upwards from the base; Figure 5 shows a preferred embodiment of the invention, described by way of example only, which is a diagrammatic representation of an aerated heap leach drain system; and Figure 6 shows a perspective view of portion of a strip drain tube used in the invention.

DETAILED DESCRIPTION OF THE INVENTION

A problem with using crushed or sized rock as the main drainage layer is that fines may migrate in to the rock. Additionally, precipitates from chemical reactions between the solution and the drainage rock may occur. Such chemical reactions and precipitates will be exacerbated where drainage rock is neither durable nor acid resistant. Fines migration from the ore, fines generated by in situ degradation of the drain rock, and chemical precipitates all will tend to reduce the hydraulic transmissivity of the drainage rock and will result in an increase in the phreatic head. As the drain layer becomes flooded, the solution will eventually reach the aeration pipes. Solution entry into the aeration pipes is highly undesirable as it (a) reduces airflow into the heap and (b) results in precipitates in the air pipes which when dried have the capacity of blocking the holes in the aeration pipes from the inside and (c) in the worst case the phreatic head may exceed the air pressure in the pipe, preventing air addition in entirety.

Such a problem can be partially solved in on-off leaching systems by replacing or repairing the drainage layer after each and every cycle. In fixed pad systems this would not be possible, without periodic installation of new drain rock, drain pipes and aeration pipes. Both propositions are not attractive economically.

The inventors have determined that there are several key components in maintaining a phreatic head below the aeration pipes and ore in the heap:

• The drain must maintain the capability to remove solution despite a potentially dramatic decline in permeability of the surrounding rock from solids migration or in situ fines generation and/or the formation of chemical precipitates;

• The drain must be capable of withstanding mechanical integrity when heavy mobile equipment is operated at the surface of the heap.

• The drain must be capable of accommodating aeration pipes in a manner that minimizes the chance of solution entry into the aeration pipes.

• The drain must be capable of handling the maximum irrigation rate applied to the heap along with any precipitation, i.e. the total infiltration rate.

• The drain design must be able to accommodate relatively small slopes as well as relatively high slopes.

The inventors designed a strip drain for a drainage layer which includes a plurality of spaced apart substantially vertically orientated strip drain tubes in a layer of drain rock. Detail of the strip drain used is shown in Figure 6. The strip drain comprises a flat tube which has a surface area to enclosed volume ratio which is much greater than would be the case if a right circular cylindrical perforated tube was used in the drain.

A widely-used empirical equation was used to calculate the head on the liner taking into account the drain spacing, the infiltration rate, and the hydraulic conductivity of the drain layer. This empirical method did not take into account the thickness of the drain layer or the hydraulic conductivity of the ore above the drain. Table 1 shows the effect of herringbone pipe spacing and permeability of the drainage on the phreatic head, at an infiltration rate of 40l/m ² /hour. It is apparent that whilst widely spaced herringbone drains can cope with a drain having a permeability of 10 ^'1 cm/sec, if the drain degrades to a permeability of 10 ^'4 cm/sec the drainage layer phreatic head increases substantially. Even at a drain spacing of about 1 .20 meters, the phreatic head is still about 2 meters. Whilst an infiltration rate of 40l/m ² /hour may seem high, it would not be unusual in GEOCOAT ^® applications, particularly in regions of high precipitation.

The inventors then established the phreatic head for a herringbone drain design using one meter pipe spacing, for a variety of hydraulic conductivities and infiltration rates, as illustrated in Figure 1 . With hydraulic conductivities greater than 10 ^'2 there is a minimal

head at all infiltration rates. However with hydraulic conductivities equal or less than 10 ^'3 the phreatic head starts increasing substantially depending on the infiltration rate.

The inventors then considered the use of 900mm strip drains at one meter spacing. Strip drains have a height substantially greater than width and are placed vertically in the material to be drained. The strip drains may be optionally covered in geofabric. In this instance two-dimensional finite element software SEEP/W, developed by GEO- SLOPE International, Ltd (GEO-SLOPE 2002) was used. SEEP/W is a general seepage analysis program that models both saturated and unsaturated flow using soil water characteristic curves (SWCC) and user defined boundary conditions. Steady- state seepage analyses were conducted, taking into consideration the hydraulic conductivity as well as the volumetric water content of both the ore and the drain materials. The SEEP/W model is a much more rigorous analytical method and allows consideration of many more factors than the empirical method utilized for the first round of analyses. The second round of modelling also took into consideration drain type. Figure 2 illustrates that even at hydraulic conductivities of 10 ^'4 the drain can cope with a wide range of infiltration rates, with a maximum phreatic head of under 0.3m even at an infiltration rate of 60l/m ² /hour. Such low phreatic heads suggest the air pipes could be placed at 0.5m above the heap base without any danger of flooding in a worst case scenario.

Figure 3 illustrates a similar case to Figure 2 except that the 900mm strip drains were placed at 2 meter intervals. In this instance, at the lowest hydraulic conductivity considered, the drain performance is relatively poor except at a low infiltration rate of 5l/m ² /hour. A spacing of 2m may therefore be marginally acceptable for whole ore leaching applications such as GEOLEACH™.

Figure 4 illustrates a case where a drain has degraded from the base up at 0.5m increments (bottom 0.5m K=10 ^~4 cm/sec, next 0.5m K=10 ^~3 cm/sec, next 0.5m K=10 ^{" 2} cm/sec, next 0.5m K= 10 ^'1 cm/sec). A strip drain with a height of 900 mm was compared to a strip drain with a 450 mm height. In such case the 900mm strip drains clearly outperform the 450mm strip drains as can be seen from Figure 4. Although the 450mm drains are satisfactory it would appear further degradation will reduce their performance to that shown previously.

Since the strip drains "reach up" into the drainage layer it should be possible to determine and optimum strip drain height and spacing for specific infiltration rates (or a

range of such infiltration rates) to tailor an individual operation. The aeration pipes may be placed in any location relative to the strip drains, but preferably just adjacent to the drain, where the phreatic head is lowest.

An additional layer of barren finer rock may be placed over the drain composite to act as a trap for fine particles migrating downward. Such finer rock may be reclaimed along with leached ore and replaced each cycle and act as a sacrificial layer.

In especially arduous duty tri-planar geonet may be considered above the HDPE liner.

A non-limiting example of the invention is illustrated in Figure 5, which shows a partial cross section of the drainage system. An HDPE liner (1 ) sits on a prepared surface (5) onto which strip drains, 900 mm high, (3) are placed vertically and 1000 mm apart. The space between the strip drains is filled in with drain rock (2). Aeration pipes (4) are placed adjacent to the strip drains (3) and an additional 600mm of drain rock is placed on top, bringing the total height of the drain to 1500 mm.

Figure 6 shows detail of the strip drains (3) used in the system. The strip drain (3) comprises a tube (6) with a substantially rectangular cross section with a ribbed (7) structure. Included in the tube (6) are several internal support pillars (8) which prevents the tube (6) from collapsing under external pressure.

The tube (6) is covered with a geotextile material (9) which acts as a soil filter. The tube is perforated which allows fluid to collect in the tube (6), passing through the geotextile (9) covering. Since the tube has an elongate cross section, the surface area available for draining is greater than with a conventional circular cross sectional pipe. The elongate cross section also reduces the likelihood of the pipe (6) flooding. It will be appreciated that this invention may also be applied in heaps, dumps and other systems where efficient drainage is required which are treated by aerobic and anaerobic processes. This is not only limited to heap leaching operations and also, for example, include operations in which gasses such as methane and ethanol are produced from organic materials.