Q: What is the current status of the GEOCOAT® and GEOLEACH™ technology?
A: The amenability of a large number of gold concentrates to treatment by the GEOCOAT® process has been successfully demonstrated in laboratory amenability tests, column tests, and two major field demonstrations at mine sites in Mexico and Ghana. Column testwork on a wide variety of concentrates has been conducted at Lakefield Research Africa (Pty) Limited, in Johannesburg.

GBL believes that the results of these tests, and the results of the field demonstration tests, particularly the test carried out at the Obuasi operations of Ashanti Goldfields Company in Ghana, address design and scale-up of the GEOCOAT® process. The first commercial GEOCOAT® plant is in operation at the Agnes Mine in South Africa treating refractory sulfide gold concentrates.

The GEOLEACH™ for whole ore biooxidation is under development. The focus of the application is for primary copper ores such as those found in South America. GBL is currently working with 3 companies to develop their copper leaching technology for both whole ore and concentrates.

Q: How does the coating process work?
A: Concentrate is coated onto the sized support rock by spraying a stream of the thickened concentrate slurry onto the support rock as it discharges from the stacker on to the heap. GBL has demonstrated this aspect of the process during the demonstration tests in Mexico and Ghana, and also at trials at a mine site in Nevada, USA. Coating and stacking rates were comparable to those proposed for commercial operations and no scale-up is required. No binders or other additives are required; the naturally hydrophobic nature of the sulfide minerals results in the formation of a thin, relatively uniform coating on the support rock particles.

Q: Doesn’t the coating get washed off the support?
A: Column tests, and more particularly the field demonstration tests, have shown that the coating remains in place during biooxidation. Neither the continuous irrigation of the heap, nor heavy tropical rain, dislodged the concentrate coating from the support rock, except in the upper 2 -3 rock diameters of the heap surface. Direct impact of the spray from sprinklers and raindrops washed the concentrate from the top few layers of support rock, but the concentrate was recoated in the layers immediately below. No concentrate washed from the heap into the solution collection ponds.

Q: How is the oxidized concentrate separated from the support?
A: At the completion of biooxidation, the oxidized concentrate is separated from the support rock by washing in a trommel or on a horizontal vibrating screen. A trommel was used successfully at the Ashanti demonstration and in field trials in Nevada (the Mexico heap was rinsed in place for subsequent cyanidation by the client). No difficulty was experienced in removing the oxidized coating. Assay of the separated support rock showed gold below detection limits, confirming the complete removal of the concentrate. GBL expects similar results from a properly sized and designed wet vibrating screen. Feed rate to the trommel at the demonstration test in Ghana was comparable to that proposed for commercial operations, and no scale-up is necessary.

Q: Is the GEOCOAT® heap permeable to air and solution flows?
A: All fine material (less than approximately 6 mm) 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, such as the Ashanti Goldfields demonstration heap, solutions flow freely into the heap and no ponding is observed. Back-pressure in aeration headers has been measured to be minimal in field heaps, demonstrating that the distribution ducts and piping can be designed for internal pressure drop only, neglecting the minimal effects exerted by the heap on the aeration system.

Q: How is the temperature of the GEOCOAT® and GEOLEACH™ heap controlled?
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 even more favorable for the bacteria responsible for biooxidation. Beyond this point, however, the buildup of heat would become detrimental, necessitating the removal of the excess heat. This is accomplished in the heap by management of solution flow and aeration rates using the HotHeap™ control system.. Solution applied to the top of the heap instantly begins to percolate downward, flowing over the coated particles and reaching an equilibrium temperature with 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 is evaporated 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 with time so as to maintain a target temperature in the core of the heap, typically 38° - 42°C for mesophilic bacteria. This mode of operation has been demonstrated successfully in GeoBiotics’ field demonstration heaps in Mexico and Ghana and at our commercial operating plant in South Africa.

The GEOCOAT® system is designed to maximize homogeneity of the heap, and this greatly simplifies the mathematical and computer modeling of the system. Computer spreadsheets and models are used in appropriate algorithms to predict heat generation within biooxidation heaps. Examples of GEOCOAT® heap parameters used in these calculations are heap stacked height, mass coating ratio, concentrate composition, support rock particle size, thermal conductivity and heat capacity. Other factors in the model include biooxidation kinetics and site climatic data. Through the use of such calculations, optima can be predicted for heap temperatures in conjunction with irrigation and aeration rates. The performance of the Ashanti trial heap was successfully predicted using this process and as this test heap is comparable to commercial heaps in terms of coating ratio, stacked height, solution application rate, etc., no difficulties with heat management are anticipated. Similar modeling has been performed for the GEOLEACH™ system.

Q: How is the heap inoculated?
A: The mixed mesophilic bacterial culture proposed for commercial GEOCOAT® and GEOLEACH™ heaps is essentially the same as that being used in the full-scale agitated tank biooxidation plants currently in operation. Those bacteria, Thiobacillus ferrooxidans, Thiobacillus thiooxidans and Leptospirillum ferrooxidans, have been shown over many years to be effective in oxidizing sulfide minerals, and robust under plant conditions.

At start-up, it will be necessary to grow a batch of inoculum for the first batch of concentrate to be stacked on the pad. The process consists of growing an initial small batch of culture to the required cell density, then transferring the batch, with appropriate nutrients and addition of sulfide minerals, to a larger container, until the cell density has again increased to the required level. The process is repeated until the required volume of inoculum is obtained. This inoculum may then be added to the solution ponds and applied to the heap with the solution. Inoculum may also be used in making up the concentrate slurry prior to coating. This pre-inoculation process (BIOPROTM) distributes the bacteria throughout the heap, ensuring that biooxidation is rapidly initiated, reducing overall residence time on the pad.

The BIOPROTM pre-inoculation process is covered by patents held by Newmont Mining Company and made available to licensees through a cross-licensing agreement between Newmont and GBL. Newmont is presently using the BIOPROTM technology in whole-ore heap biooxidation at its Carlin, Nevada operations.

Q: Is oxidation uniform throughout the heap?
A: The biooxidation of sulfide minerals in the GEOCOAT® process 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.

Q: How is the solution composition controlled?
A: The GEOCOAT® heap is continuously irrigated with solution containing acid, ferric iron, nutrients and bacteria. The solution is collected as it drains from the heap and recycled via sprinklers or drip emitters to the top of the heap. Depending on the mineralogy of the concentrate and support rock, 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 (1.2 - 1.5), and to purge iron from the system, it will be necessary to bleed continuously a portion of the off-solution to a neutralization system. Lime and/or ground limestone are added to neutralize acidity and precipitate metals, mainly iron. The volume removed in the bleed stream is made up by the addition of fresh water to the system. The need for, and 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. During the demonstration test at Obuasi, pH was maintained in within the range desired by this method. The Obuasi concentrate was acid-consuming, and acid was therefore added to the system as needed.

Q: How is the progress of oxidation monitored?
A: Sampling of solids and solutions is used to accurately track progress of GEOCOAT® heaps. Solution samples are taken several times daily to monitor temperature, pH, solution oxidation potential or Eh, and levels of solubilized iron and other metals. Metallurgical balances are maintained by compilation of solution volumes and analyses. Solid samples are withdrawn from GEOCOAT® heap demonstrations several times during the course of biooxidation. A vacuum system is used 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. The apparatus is rinsed between sample intervals with the rinsate added to the sample. The cased hole prevents cross-contamination from one vertical interval to the next. 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 predictions of performance.

Q: What instrumentation is installed to measure heap performance?
A: Several types of instrumentation have been utilized during operation of GEOCOAT® demonstration heaps in Ghana and Mexico, to obtain data relating to physical and chemical conditions within the heap. For example, during heap construction thermocouples were placed within the coated and stacked material, and the instrument locations were logged. These thermocouple arrays provided concrete temperature data from specific locations within the heap. This data was used both for monitoring conditions during biooxidation, as well as for control of temperatures through manipulation of aeration and solution application. Aeration rate and back-pressure are other examples of parameters measured during the course of the field tests. As noted previously, solution and solid samples were withdrawn and analyzed during the demonstrations. Together, these measures provided a complete picture of the biooxidation environment of the heap both by specific location and as a whole, allowing on-going measurement of heap performance.

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.

Q: What technologies does GBL own?
A: GeoBiotics, has a wide array of patents with coverage in all major mining centers. Our technology encompasses heap biooxidation and associated processes. The following figure shows the core of the GBL technologies:



GEOCOAT®
The GEOCOAT® technology offers a unique approach to the application of bacterial processing; combining the low capital and operating costs of heap leaching with the high recoveries obtained in agitated tank reactors. Both of which are well accepted processes in the minerals industry and are in commercial operation worldwide. In the GEOCOAT® process, sulfide flotation or gravity concentrate slurry is coated onto crushed and sized support rock which may be barren or may contain sulfide or oxide mineral values. The coated material is stacked on a lined pad for biooxidation. The process is applicable to the biooxidation of refractory sulfide gold concentrates and to the bioleaching of copper, nickel, cobalt, zinc, and polymetallic base metal concentrates. Mesophilic or thermophilic biological systems are used to catalyze the sulfide oxidation reactions. In the processing of chalcopyrite concentrates, the higher temperatures associated with the use of thermophile microorganisms have proven highly beneficial in increasing the rate and extent of copper leaching. In the processing of refractory gold sulfide concentrates, the GEOCOAT® process offers significant cost advantages over the established processes: roasting, pressure oxidation, and agitated tank biooxidation. In base metals operations, the GEOCOAT® process is particularly suited to the treatment of “dirty” concentrates, can reduce transport by allowing the on-site production of metal at remote deposits, and can take advantage of depletion of oxide reserves through the utilization of existing SX/EW equipment. The process is simple, robust, and ideally suited to operation in remote locations.

GEOLEACH™
The GEOLEACH™ technology is applicable for whole ore systems where the metals occur as sulfides. The driving force behind the GEOLEACH™ process is that most sulfide whole ore leaching systems have enough energy present in the sulfides to allow the heap to obtain very high temperatures but poor heat management prevents significant temperature rise. Without significant increase in temperatures beyond ambient within the heap, sulfide leaching kinetics are extremely slow and in the case of chalcopyrite extraction is limited by passivation. The GEOLEACH™ technology is designed to maximize heat conservation through careful control of aeration and irrigation rates. GEOLEACH™ has built on the best industry knowledge around the operation of bioleaching facilities and added to this the HotHeap™ control/operating philosophy. GEOLEACH™ is very similar to conventional whole ore acid heap leaching systems but with the addition of a system to maintain biological activity and maximize heat conservation. The process is simple, robust, and ideally suited to operation in remote locations.

Other Technologies
A wide variety of additional patented expertise form the framework of the GeoBiotics technology suite including high temperature bioleaching, toxin removal, HotHeap™, BioPro®, and a series of other complementary internal processes focused around pretreatment, aeration, stacking and instrumentation. HotHeap™ - a biological heap leach operating philosophy coupled with an instrumentation and control system that maximizes heat conservation and thus bioleaching kinetics.

BIOPRO® - an inoculation method for biological heap leaching systems. (licensed from Newmont)

Q: What is HotHeap™?
A: HotHeap™ is an operational and control philosophy designed to maximize the heat retention within whole ore and concentrate heaps. During the early stages of heap operation, the conservation of heat from a low level of biooxidation is critical, the heap is control based on the oxygen demand of the bacterial system. As the heap heat up and reaches its operating temperature, the control strategy shifts to maintenance of temperature through evaporative cooling. Finally as biooxidation slows down near the end of the heap life, control switches back to oxygen again to maximize the time at temperature.