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Salt migration column tests.

In recent years, many mining companies have investigated alternative close-out methods for mine tailings and waste rock piles. In many cases, a single layer or composite dry cover is proposed and studied. The long-term success of single layer and composite dry covers depends on their ability to retain moisture, inhibit salt migration, and support vegetation.

A self-sustaining vegetative cover is beneficial because it stabilizes the substrate, provides a long-term source of organic carbon for bacterial activity, and reduces the net infiltration of water into the tailings.

However, historical evidence reveals that the upward migration of salts from the tailings into the overlying cover kills vegetation. Both the capillary movement of the water from the tailings and the evaporative flux from the cover layer surface may be responsible for the salt's movement and concentration at the surface.

Lakefield Research Limited's salt migration column tests determine the rate of upward migration of salts as a function of cover depth for different cover materials. This study only used single layer covers.

Case Study. Lakefield Research Limited recently conducted pilot scale projects to investigate the suitability of the following materials for covering tailings ponds:

* Peat

* Desulphurized tailings (DST)

* Desulphurized tailings with a capillary barrier (DST+CB)

* Mature compost

* Lime-stabilized sewage sludge (LSSS)

Control cells, which consisted of the tailings without a cover, were monitored for comparative purposes.

Equipment and test procedure. Eighteen columns were loaded with tailings and covers for salt migration testing. The allocation of each cover material to three separate columns allowed testing to be performed at three different cover depths. Sulphidic tailings filled the columns to a depth of 0.30 m, and the compact layers of cover materials measured 0.15 m, 0.50 m, and 1.0 m in thickness above the tailing.

The column design maintained saturation of the reactive tailing and permitted free evaporation from the surface of the cover material. A constant head tank, attached to the base of the columns, enabled the tailings to remain saturated. A drainage bed of inert nepheline syenite placed on the bottom of the cylinders and covered with a geomembrane allowed the free flow of water between the constant head tank and the reactive tailings. An air-tight polyethylene seal covered the head tank. The water level was topped up weekly to permit a direct measurement of the evaporative losses from the salt migration columns. A custom-designed building housed the equipment and ensured constant humidity and air circulation.

As the column tests progressed, the analysis of core samples determined the movement of salts. Samples collected from the surface and at 0.15 m depth intervals throughout the cover were compared to the conductivity of the covers at the start of the test and after one, two, three, and four months.

Test Results. The experiment revealed that the evaporation rate tended to decrease with increasing depth of cover and appeared to be unaffected by the capillary barrier used in this case study. This may have been due to the proximity of the capillary barrier to the water table in the underlying reactive tailings. The highest evaporation rate was found in the DST and peat, whereas the lowest evaporation rate existed in compost and LSSS.

The change in conductivity depended on the cover depth and the material type. The lowest change in conductivity values took place in the compost, followed by DST+CB, DST, LSSS, control, and peat.

Regardless of the cover depth, the control tailings column showed an increasing conductivity with time. Blue and yellow salt crystals (iron sulphate containing magnesium) on the surface of the salt migration columns accompanied the increase in conductivity.

The mature compost exhibited the lowest evaporation rates and the lowest amount of salt accumulation. This mature compost has, therefore, been found to be the best of those evaluated in terms of minimizing the effects of salt migration. The mature compost also contains high concentrations of dissolved organic carbon, which is required for the survival of sulphate-reducing bacteria.

Although desulphurized tailings showed a high evaporation rate because of favorable salt migration characteristics and low initial and final salt contents, they may prove to be a suitable cover with some surface modifications designed to reduce evaporation.

These salt migration experiments were conducted in a simulated dry environment with no rainfall. The test cells' design allows water to be added to simulate local rainfall conditions, if needed. In addition, oxygen diffusion rate testing, plant growth testing, and pore water characterization may also be conducted to provide additional information on the effects of salt migration on long-term cover performance.

RAHCO analyzes heap leaching costs

RAHCO International has supplied a variety of heap leaching systems for gold and copper mines in both North and South America, with stacking capabilities ranging from 550-9,500 st/hr. While most of these systems are based on permanent leach pads, economic analyses have established that in many cases, overall life-cycle costs using RAHCO equipment will be lower with reusable pads.

According to Richard W. Hanson, RAHCO's president, the variables considered in these analyses included production rates, labor and maintenance costs, equipment costs, pad sizes and configuration, lift heights, pumping costs, solution management, reagent consumption, metal recovery, and reclamation requirements. Particular considerations for an on-off reusable pad include tons processed, the leach cycle, lift height, and ore characteristics, Hanson said.

Although an on-off pad is normally much smaller than a permanent pad handling the same tonnage of ore, a reusable pad requires a spent-ore disposal area and stacking system. In the past, this double handling has been seen as the main drawback, but RAHCO's mobile conveyors, with their low operating costs, are changing that premise. In many cases, it is less expensive to handle the ore twice than it is to elevate the ore and pump the solution to higher lifts on a multi-lift pad. For some ores, metal recovery will be higher on a reusable pad because of better solution control and lower reagent consumptions.

RAHCO uses track-mounted, self-propelled mobile stacking and reclaiming bridge conveyors. The crawler tracks create low ground pressures (typically 6-15 lb/[in..sup.2]) that allow them to walk on either the drainage layer or the leach pile, without compacting it. A moving tripper discharges material anywhere along the conveyors length. A single operator is required for the control system, which monitors and controls the track travel, machine alignment, conveyor cross leveling, and starting and stopping of the main and tripper belts. A compact bucket wheel reclaimer removes spent ore from the pad. The bucket wheel and mobile reclaim conveyor are both controlled by Global Positioning Satellite.

The configuration and number of mobile conveyor bridge sections can be varied to allow the mobile stacking conveyor to create leach heaps in rectangular, circular, semicircular, or irregular shapes. The mobile conveyors can travel across uneven terrain and up or down slopes of up to 20%. They can build their own ramps and can stack in either an advance or retreat mode.

The only significant operating cost for a mobile conveyor system is electricity, Hanson said. Typical operating costs range from 11[cents] to as low as 1[cent] per ton.
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Publication:E&MJ - Engineering & Mining Journal
Date:Jul 1, 1996
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