The Vat Leaching Process
Vat leaching can be described as leaching in a bathtub inside a building. Treatment of ore is site-specific as determined by metallurgical tests.
Bench scale metallurgical testing of ores for their amenability to vat leaching is similar to that for heap leaching. First, bottle-roll tests of finely ground material are run to determine the ultimate recovery of gold, and for any preg-robbing characteristics of the ore. Next, leaching tests in columns are conducted for the same variables as for heap leaching. These include size of material, solution strength, agglomeration, and time of leach. For vat leaching, additional tests on flow rate, applicability of soaking cycles, and direction of flow are made to determine the optimum set of conditions.
The ore is prepared for the vat either as a finely crushed or a coarsely ground and deslimed material. It is put into a container, the vat, which has a partial filter floor. Cyanide solution can be injected through the filter section of the floor or added to the top of the ore charge. After passing through the ore, the solution is drawn off through the floor. It can either be cycled back through the vat, or taken for gold recovery.
For best results, the ore is either deslimed or agglomerated, to facilitate solution transport through the material. Vat leach time for gold dissolution approaches that of agitation leaching. Vat leaching can be considered when an ore body does not have the grade or quantity to be a candidate for normal agitation leaching, and as an alternative to heap leaching. At a location where there is excessive rainfall, such as more than 120 in./yr, or even a short and intense rainy season on the order of 50 in., the solution in heap leaching can become seriously diluted.
The result is either lower strength solution or higher reagent costs. Conversely, vat leaching is an alternative to heap leaching in a low-water-availability or desert regions. Problems of water shortages or high evaporation rates in a large exposed heap are overcome by vat leaching. Another circumstance in which vat treatment is advantageous is in excessively cold climates, as it is an indoor process. Vat leaching also contains an environmental advantage over heap leaching. The footprint of a vat plant is much smaller than that of a dedicated heap-leach operation, even when considering disposal of vat tailings. A smaller treatment area also leads to plant simplicity and reduced cost of material transport.
The main disadvantage to vat leaching is higher capital and operating costs than heap leach operations. The ore must be amenable to treatment. If desliming is used instead of agglomeration, an agitation plant, albeit of smaller size, is necessary.
The latest development in vat leaching is a continuous unit, invented by Dan Mackie of Innovat Ltd., called an Ecovat. Feed to the rectangular tank is via a slurry stream or dry ore by conveyor. If dry ore is fed, it is slurried by solution injected in the bottom of the vat. This solution fluidizes and mixes the slurry by means of periodic pulses of liquid from the bottom of the vat. These sharp injections of fluid accelerate dissolution of the gold due to the dynamic contact between the cyanide solution and solid ore particles.
In the bottom of the tank is a French drain for removal of solution for processing. Wash solution and part of the French drain liquid are pumped to a head tank set at a height approximately double the depth of the ore bed. Hydrostatic pressure generated is used to inject solution into the ore bed.
Most Ecovats would be set to “dump” solution for 10 sec every four minutes. At the end of the tank opposite the feed is a slowly revolving wheel with inlet ports and compartments, which removes material from the vat. The slow speed of the wheel, on the order of 0.2 rev/min, allows the slurry to drain during removal. The liquid drains back into the treatment section, while the solids are discharged into the next compartment at 5-15% moisture, depending on characteristics of the ore… (TO BE CONTINUED)