FAQ

A: The optimum method for coating concentrate onto the sized support rock depends on the size of the facility and client preference. Typically, the concentrate is coated by spraying a stream of the thickened concentrate slurry onto the support rock as it discharges from the stacker onto the heap. Other methods include a coating (mixing) box either at the end of the stacker (for low tonnages) or before the stacker (high tonnages), or an agglomeration drum. Binders or other additives are not required - the naturally hydrophobic nature of the sulfide minerals results in the formation of a thin, relatively uniform coating on the support rock particles.

A: No, even heavy rain does not wash off the concentrate. The concentrate on the upper surfaces of the heap surface is washed off the support and is trapped in the heap, but the lower surfaces are still coated. Short-duration precipitation events are comparable in intensity to routine solution application operations and have no adverse effects on the system. During periods of prolonged heavy rainfall, it may be necessary to stop the addition of fresh make-up water to the system to avoid undue dilution of the solution and possible overflows from the solution pond.

Depending on climatic conditions at the site, an emergency stormwater pond may be required to handle runoff resulting from storm events. A water balance based on site-specific meteorological data is prepared to determine the surge capacity required.

The water balance issues involved with a GEOCOAT^® operation are the same as those that are routinely addressed in the design of heap operations for cyanide leaching of gold ores and acid leaching of copper ores. The principles are well understood, and based on meteorological data for the particular site, a solution management plan will ensure a safe and environmentally acceptable operation.

A: Yes. All fine material is screened from the support rock prior to coating. The concentrate coating adheres to the support, leaving open voids throughout the stacked material. These voids are up to an order of magnitude larger than those found in typical whole ore heaps, such as those used in cyanide heap leaching or secondary copper sulfide leaching. These large voids provide almost no resistance to solution and air flows through the heap. As evidenced by both laboratory columns and field heaps, solutions flow freely into the heap.

A: The oxidation of sulfide minerals produces heat, whether the reaction occurs in a roaster or in a biooxidation heap. Initially, this heat assists the biooxidation reactions in the heap by increasing the temperature into a range more favorable for the bacteria responsible for biooxidation. Beyond this point, however, the buildup of heat could become detrimental, necessitating the removal of the excess heat. GeoBiotics' HotHeap™ technology manages solution flow and aeration rates in the heap to maintain optimal temperature. Solution applied to the top of the heap instantly begins to percolate downward, flowing over the coated particles and reaching an equilibrium temperature within that portion of the heap. Upon reaching the bottom of the heap, this warm solution contacts the incoming air, and a portion of the solution evaporates into the air stream. This pre-heats and humidifies the air before the partially cooled solution exits the heap, carrying away still more heat. The partially humidified air rises through the open pores of the heap, driven by both the incoming air below and the thermal buoyancy of the warming air. The rising air stream cools the coated material still further, until the now-saturated air exits the top surface of the heap. The air injection rate is varied to maintain a target temperature in the core of the heap.

A: GEOCOAT^® and GEOLEACH™ use mixed cultures of mesophilic, moderate, and thermophilic microorganisms to promote biooxidation. Starting with a small volume of culture from the laboratory testwork, the volume required for the heap is grown in a solution of nutrient salts, with periodic addition of small quantities of sulfide concentrate and elemental sulfur. The suspension is agitated and aerated. As the bacterial density increases, the inoculum is transferred to progressively larger vessels and diluted by adding nutrient medium. This process is repeated until the required volume of inoculum is obtained. The inoculum is added to the on-solution pond, which then is used to irrigate the heap.

A: The biooxidation of sulfide minerals converts metallic sulfides to soluble metal sulfates. Depending on mineral species involved, the reaction may produce or consume acid. Additionally, elemental sulfur may be produced as an intermediate product, which is subsequently further oxidized to sulfate by the bacteria. The rate of oxidation of the minerals may vary slightly in the initial stages of the biooxidation process, with some areas outpacing others. As these areas consume the sulfide minerals, they provide heat to neighboring particles and shed bacterial cells, which assist in the colonization of the remainder of the heap. After the initial stage of colonization with the onset of biooxidation, oxidation progresses at full rate. When sulfide minerals begin to be depleted, the areas which were slightly ahead of others slow down. By the completion of the scheduled treatment period, the entire heap has achieved the target degree of biooxidation.

A: The heap is continuously irrigated with solution containing acid, ferric iron, nutrients and bacteria. The solution is collected as it drains from the heap and is recycled to the top of the heap via sprinklers or drip emitters. Depending on the mineralogy, a heap may be acid-consuming or acid-generating. For an acid-generating concentrate, the pH of the solution percolating through the heap will fall as it picks up acid from the oxidizing sulfide minerals. To maintain the pH in the optimum range for bacterial activity, and to purge iron from the system, a portion of the off-solution is bled to a neutralization system. Lime and/or ground limestone are added to neutralize acidity and precipitate metals, mainly iron. Addition of fresh water makes up the volume removed in the bleed stream. The sizing of the bleed stream and neutralization system are determined from the analyses of the concentrate and support rock, and from the results of the column tests.

A: Sampling of solids and solutions accurately track progress of biooxidation. Solution samples are taken to monitor temperature, pH, solution oxidation potential or Eh, and levels of solubilized iron and other metals. Solid samples are also taken using a vacuum system to remove samples from cased holes. The hole casing is driven downward as the solid sample is removed by the vacuum system, ensuring samples are withdrawn from undisturbed strata. Samples are analyzed for iron and other metals, sulfur compounds and gold. This sampling system provides data based on residue analyses from distinct locations within the heap, allowing comparison of actual progress of the heap to testwork predictions of performance.

A: Several types of instrumentation aid in obtaining data relating to physical and chemical conditions within the heap during operation. During heap construction, thermocouples are placed within the coated and stacked material. These thermocouple arrays provide temperature data from specific locations within the heap, and are used for monitoring conditions during biooxidation and controlling temperatures. GeoBiotics has developed a unique instrumentation and control package under the brand HotHeap™. The equipment provides for the monitoring and control of irrigation and aeration rates in response to oxygen and cooling demands of the heap.