Heap Leach Solution Irrigation

Heap Leach Solution Irrigation

The application of drip irrigation in Heap Leach,  has been proven to be a significant improvement over conventional spray irrigation techniques. While still under evaluation, it is evident that the use of drip irrigation will increase the operating time of a heap leach during winter and may significantly reduce operating costs through reduction in reagent costs and water pumping costs. This advantage alone will increase the profitability of operations having short summers and long winters. The additional advantages include:

  1. Reduction in maintenance of the Heap Leach irrigation system.
  2. Improved environmental controls. The system will not overspray or wash out sides of the heap.
  3. Reduction in evaporation and heat loss due to the low surface area of solution exposed to the atmosphere.
  4. Reduction in sodium cyanide consumption due to reduced evaporation and ultra violet light exposure.
  5. Increased solution application rate. The solution droplets do not fall eight feet to the surface destroying the agglomerates and washing the fines into low spots on the heap and sealing the surface.

The disadvantages of the drip irrigation compared to spray irrigation include:

  1. Inability to reuse buried drip lines. Drip lines on the surface during summer months have been reused with no detrimental effects.
  2. The formation of scale will plug the emitters. This can be a problem with spray irrigation. But the drip irrigation appears to be more sensitive than spray irrigation. The scaling problem is under control and we have not experienced any scaling problems.
  3. The in-line emitters are supposed to be not affected by particles less than 150 mesh. If the irrigation lines are not properly flushed, any large particles will plug either drip or spray units.
  4. The surface of the heap is not totally contacted with solution using drip irrigation as compared to spray irrigation. This is not a significant problem where successive layers of ore are leached. Also, during summer months the tubing may be easily moved over to the middle of the 3 ft spacing to supplement the surface contact.

Drip Tubing Installation

The first heap test panel was placed under drip irrigation using the in-line emitter system selected in the screening. Buried to a depth of approximately three and one-half feet, the initial tubing spacing was twelve feet with the emitters on twelve-inch centers as recommended by the manufacturer for the desired flowrate. The one-half-inch diameter tubing was direct-buried behind a D-9 bulldozer. The tubing was fed through a pipe attachment welded to the center rip shank of the bulldozer. As the dozer advanced across the pad, the tubing was secured in place, below the heap surface, by the heap material collapsing behind the center shank.heap leach perpendicular solution profile

Both ends of the tubing were left exposed above the surface to enable flow verification of breakage detection. The feed end of the drip line was connected directly to the Yelomine pipe header above ground. The other end, after verification of flow, was sealed and then buried for freeze protection. The flowrate to the drip system was then set to yield a percolation rate of 0.004 gpm/sq ft; As had been anticipated, the solution flow was easily established well away from the influence of the low temperatures and to quote an old saying “Light appeared at the end of the tunnel.

Operational Benefits of Drip Irrigation 

In addition to extending the heap leaching season to year-round, there have been several other side benefits associated with utilizing drip irrigation for leach solution application. While the exact contribution of these side benefits are still under evaluation, preliminary results are positive enough to have prompted full commitment to drip irrigation. At the present, over 80 percent of the heap panels, or over one million square feet, is under drip. The remaining 20 percent is under conventional spray in an effort to evaporate the abnormally high amount of meteorological water collected this spring. These sprays will be removed and replaced with drip tubing as soon as the water level in the storage area is decreased to normal operating levels.

Some of the side benefits under evaluation include:

  • Decreased sodium cyanide consumption
  • Increased solution application rates
  • Decreased maintenance
  • Decreased evaporation
  • Decreased heat loss

The effect of the use of drip irrigation on sodium cyanide consumption at this point is more perceived than measurable particularly as less than one year’s operational data is available. However, it might be assumed that as the solution losses through evaporation are negligible and the exposure to sunlight is minimal, even with above ground installation, the loss of cyanide through volatilization and ultra violet light degradation should therefore also be minimal. In any event, the present cyanide consumption is less than half that indicated in the feasibility test program.

heap leach solution application at coeur-rochester

Even though the crushed ore sent to the leach pad contains less than 5 percent minus 200 mesh, solution application rates in excess of 0.006 gpm/sq ft are not possible without excessive puddling with the conventional sprinkler system. The impact of the solution droplet from the sprinkler dislodges the finer particles from the agglomerates and washes them to a low spot on the heap surfaces. The low spot fills with the fine particles which then seals the low spot preventing effective percolation below that point. The continued spraying increases the sealed area further reducing effective percolation until a sizeable area of the spray panel is sealed. A wide range of application rates, up to 0.01 gpm/ sq ft, have been achieved with the drip emitters, with only localized puddling if at all. The availability of a variety of flow ranges increases the flexibility of leach pad operation particularly when juggling fresh panels, not-so-fresh panels and worn-out panels of ore.

The use of drip irrigation tubing has virtually eliminated the labor intensive, aggravating, hygienically unsuitable chore of continual sprinkler maintenance. A leach pad the size would require over 1900 sprinklers. One man was dedicated entirely to unsticking, adjusting, or changing sprinklers on day shift and countless plant operation manhours were expended on the off-shifts attending to stuck sprinklers, which as we all know only aggravates the pending problem mentioned above. The pad maintenance has dropped to virtually nothing with the installation of the drip tubing.

Under other circumstances, the decrease in evaporation losses as a result of the drip tubing would be most beneficial and certainly cost effective. Conventional sprinklers are being used on about 20 percent of the pad area in an effort to evaporate this excess meteorological water in order to restore the solution storage area to a normal operating level. As soon as this level is attained, the sprinklers will be replaced with drip tubing as possible to maintain the plant water balance.

Drip Irrigation Disadvantages

The most significant drawback of the drip irrigation tubing noted to date is its re-usability. Obviously, depending on the burial depth, retrieval and reuse after winter operation might not be possible whereas the conventional sprinkling system can be retrieved, when the ice melts, and reused. However, compared to the possible reduced revenue impact from a rapid spring thaw of an iced pad and the reduced manpower requirements more than offsets the tubing costs. The current plan is to buy new tubing in the spring, use on the surface during the spring, summer, and fall and then bury it for winter operation, replacing it again in the spring.

Two other major problems that influence the operation of drip irrigation are:

  1. Scale build-up
  2. Particles in the leach solution.

Scale formation is a problem common to all mining operations. The operation has been using a descalent from the start. Scale formation in the drip system has been negligible. The scale can form at the final two rings in the emitter due to the large pressure drop and reduced flowrate. The problems of particles in the solution has not been a factor. This is because of the two-stage filtration common to all Merrill-Crowe plants. The leach pipe array is always flushed to remove sand and gravel. The drip system manufacture has stated that the emitter will pass particles less than 150 mesh. A minor disadvantage is the loss of solution contact on the surface. The amount of ore not contacted by the solution using the approximate 36-inch square spacing is less than 3 percent of the total volume. This would be significant for operations where the heap is leached for short periods then removed from the leach pad. The operation will have additional layers added on top. Any ore that was not leached the first pass will be leached on the second pass.