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Biooxidation of sulfidic refractory gold ores

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The past years have confirmed that Biooxidation is a viable technology which is used commercially to treat certain refractory sulfide gold concentrates. There are several processes applied to achieve the oxidation of the sulfidic compounds
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   iooxidation of sulfidic refractory gold ores Rupert Kirchner, BSc Paper conducted in November 2015  Abstract The past years have confirmed that Biooxidation is a viable technology which is used commercially to treat certain refractory sulfide gold concentrates. There are several processes applied to achieve the oxidation of the sulfidic compounds encapsulating the finely distributed gold. The question, however, is to find reliable processes which are able to both gain a high recovery of gold and keep the operating and capital costs low. There exist a few biooxidation processes. This paper will cover basic information about the biooxidation technology and also describe a few processes which are commercially used to treat sulfidic refractory gold ores. Introduction For centuries the precious metal gold has been an element linked to welfare. The greed for gold was the reason for many wars and for whole nations to be exterminated. In the nineteenth century the gold rushes resulted to a mass migration of people to major gold fields. Even presently fluctuations in the gold price often end up in major social and ecological consequences, especially in the gold mining districts of Africa. As the scarcity of oxide gold ore deposits increases, the demand for more efficient ways to process refractory sulfidic gold ores increases as well. Most deposits in the USA are considered to be refractory sulfides. While, without the oxidation of the sulfidic coating, the gold recovery would be limited to around 15-30%. The recovery can be increased to 85-95% by a prior oxidation process, attacking the encapsulating matrix. Depending on the mineralogy (the composition) of the ore there are several types of oxidation procedures applied. The oxidation procedures are listed in the following table.   While in earlier times the dominant commercially used oxidation method has been roasting, recent environmental regulations of the EPA and social concerns about the emission of greenhouse and other gases lead to a gradual rethinking in the selection preference of the oxidation method applied. In these processes the encapsulating matrix covering the gold particles is oxidized and converted to either elemental sulfur or sulfate ions. The problem however is that the hydrometallurgical processes are usually more expensive. This paper will describe and detail a few hydrometallurgical processes and inventions based on the usage of bacteria and microbes in order to prepare the refractory ores for the downstream leaching process. Biooxidation systems Depending on the ore grade and dissemination of the gold there are different methods available to treat Sulfidic Refractory Gold Ores (SRGO). There are two different main categories when considering biooxidation as the ore preparation method. The percolation type, where the solids define the continuous phase of the leaching system and the agitation type, where the liquid phase defines the continuous media. Generally the percolation type is used for low grade ores and concentrates and the agitation type for higher grade concentrates. The percolation methods considered would be heap, dump and vat leaching, whereas the agitated tank leaching is the agitated type to be mentioned. In the following table the different types of leaching are illustrated. Percolation Agitated In-situ recovery Pulp Heap or Dump Pressure Vat Baking Process   As biooxidation of SRGOs is an exothermic process and the used strain of bacteria is sensitive to the environmental conditions, the pressure leaching method and the baking process are not considered to be used for this purpose. In the following lines the biooxidation methods used for the treatment of SRGOs listed above will be described briefly. Please note that only agitated tank and heap leaching are commercially used for SRGOs presently. In-situ: Also often referred to as bio-mining, is a percolate leaching method, where the ore does not have to be mined before the leaching process. The leaching agent is either sprayed on the ore or pumped into the ore containing aquifer through injection wells. Afterwards the pregnant lixiviant is either retained by recovery wells or from the bottom of the mine.   The usage is limited to areas where the ore containing aquifer is permeable and enclosed between impermeable strata. This environment can occur naturally or has to be created artificially through fracking the orebody. In permeable deposits a wellfield usually consists of 60 wells (40 injection and 20 recovery) per header house. Monitoring wells are arranged surrounding the various production fields in order to control excursions. The header houses are connected to the central processing plant (satellite system). Reverse osmosis filter usually clean the lixiviant before it is recycled back to the well field. 1% of the lixiviant has to be disposed (normally in deep disposal wells). In-situ biooxidation is not yet commercially used for gold recovery. The problems regarding the usage of bacteria in in-situ recovery operations are that the oxidation rates are limited due to the restricted solubility of oxygen in the lixiviant and the very specific ore body characteristics needed. Heap: The first step in the heap leaching process is the comminution of the ore. Depending on the dissemination of the gold in the ore and the type and mass fraction of the gangue material, the ore may be preliminarily enriched by a gravity separation circuit or sulfide flash flotation. The enriched  concentrate is agglomerated and stacked on an impervious pad. In the agglomeration process, the grinded ore (P 80  <40µm) is often coated on gangue material with a larger grain size. This should help to achieve a better permeability, faster leaching and a greater pellet stability. Also nutrients for the bacterial oxidation and acidic media are often admixed to the pellets.   Then the leaching solutions are sprayed over the ore piles. Additional air supply may be introduced to the base of the heap. During this process, the leaching solutions drain through the heap and are then captured. After the recovery the highly acidic solution (pH of 1-2) has to be neutralized with limestone and then adjusted to a pH of 9-12 for the further cyanidation treatment. This process may be applied for gold oxide tailings of a preliminary flash floatation unit, ROM ore and low grade SRGOs. The whole process is illustrated below. Dump: Differs from the heap leaching process in that few if any efforts are made to encourage the recovery. That means that there is usually no preliminary separation or comminution and the ore is simply dumped on an impermeable site and then leached like a heap. This process is directly applied to ROM ore. Vat: Ore is confined in a rectangular leaching vat and then treated with increasing concentrations of leaching solutions on a batch or continuous basis. There are normally several vats used in order to utilize the countercurrent principle with ore being the stationary phase. In the process the strong leaching agent enters the first vat with the lowest metal content and is then passed with decreasing leaching potential to vats with increasing metal contents. This drives the leaching process. There are two different types of vat leaching depending on the location the leaching solution enters the vat. If it  enters the top it is referred to as downward percolation leach, otherwise it is called upward percolation leach. The common leach circle consists of two periods. The soaking period which is followed by the drain period. After the drainage the leaching solution is passed to the next vat and the process is repeated. The solutions are noted as first advance, second advance, etc. in order to designate the position in the leach circle. The advantage of vats over agitated tank leaching is that vats provide a better control of the biooxidation environment and there is no additional agitation and air-distribution required. The problem however is that the extent and rate of oxidation are considered to be very low.  Agitated: The leaching process is accomplished by keeping the finely divided particles (P 80  <40µm) in solution. This is achieved through agitating the reactor by either a rotating impeller or gas injection. The agitation leaching can be carried out continuously or batch wise. Whereas in the batch process the reactor is charged and then agitated till the reaction is complete and finally discharged, the continuous process consists of several tanks which are connected in series where the discharge of the first leaching step is fed to the second leaching step etc. In the following table some advantages and disadvantages of using those systems are shown.  Batch Continuous Process control very high Process control can be lower Productivity can be lower Productivity can be higher Lower CAPEX Higher CAPEX Higher OPEX Lower OPEX More manpower Less manpower Less sophisticated control needed More sophisticated control needed Agitated leaching is generally used for high grade ores or concentrates. In order to achieve an optimum leaching rate a very fine grain size is needed. As metal values are hard to dissolve they require high agitation, elevated temperatures and controlled conditions.
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