1. Field of the Invention
This invention generally relates to recovering precious and/or base metals from refractory sulfide ores and/or concentrates. It is particularly concerned with recovering gold and silver from those refractory sulfide ores that have high levels of sulfide and/or have been concentrated by one or more preceding ore processing steps (e.g., flotation, gravity separation, etc.).
2. Description of the Prior Art
Both precious and base metals are often associated with various sulfide minerals. Ores containing these minerals are usually characterized as "refractory ores" when their metal values are associated with a metallic sulfide host material. Gold, for example, is often found in the form of finely disseminated sub-microscopic particles that are occluded within a refractory sulfide host of pyrite or arsenopyrite. Consequently, the gold-encapsulating sulfide host material must be at least partially oxidized in order to make the ore's gold component more amenable to subsequent recovery processes wherein the sub-microscopic gold particles are exposed to a leaching agent such as cyanide.
Various sulfide oxidizing "pre-treatments" (e.g., treatments that take place prior to leaching an ore's gold component) have been developed. The most commonly used pre-treatments for precious metal-containing refractory ores are roasting, pressure oxidation and/or bacterial oxidation processes. Unfortunately, each of these processes has certain drawbacks. For example, roasting requires that the temperature of the refractory sulfide ore be raised to levels (e.g., approximately 650.degree. C.) that will burn off its sulfide component. At such temperatures, the sulphur and arsenic components of such refractory sulfide ores react with the surrounding air's oxygen to form various noxious gases, e.g., arsenic gases and sulfur oxide gases (e.g., SO.sub.2 and SO.sub.3). In earlier times, these gases were simply vented to the atmosphere. More and more stringent governmental regulations have, however, restricted such venting practices and mandated addition of scrubbing circuits to remove these contaminants. This has greatly increased the cost of constructing and operating such roasters.
Pressure oxidation processes employ high purity oxygen, (at high temperatures and at high pressures), in order to oxidize the sulfur components of refractory ores. Aside from the hazards associated with high temperatures, high pressures and high oxygen purity requirements, these processes also have the added drawback of high capital costs. These high capital costs follow from the fact that very expensive, corrosion-resistant autoclave equipment is needed to carry out such processes. Indeed, these high costs have prohibited more extended use of pressure oxidation pretreatments, especially for those ore deposits having lower grade ores or small reserves.
Bacterial oxidation of refractory ores falls into two general categories: tank biooxidation or heap biooxidation. Each takes advantage of the fact that certain microorganisms are capable of oxidizing sulfide components of metal sulfide materials (e.g., ores, concentrates, etc.). For example, various bacteria have been used to oxidize sulfide components of iron sulfide refractory ores. The use of tank biooxidation processes is, however, generally limited to use upon those refractory ores having relatively high precious metal values. In general, such processes can not be economically justified to pre-treat those ores or concentrates where the ratio of gold, or precious metal equivalent (in g/t), divided by its sulfur content (in %) is smaller than about 0.7.
The other bacterial oxidation process used to oxidize refractory sulfide ores is open air, heap bioleaching. It begins by breeding a bacterial culture in a liquid medium. The resulting bacteria suspension is then used to inoculate an unconcentrated form of the ore that is stacked in a heap (on an appropriate pad system) in the open air and sprayed with the bacteria suspension. Under such conditions, rather long periods of time (e.g., from about 180 to about 600 days) are usually needed to oxidize a refractory ore's sulfide component. These long process time periods imply large inventory hold-ups and these hold-ups, in turn, imply greater production costs. Eventually, however, refractory ores can be pre-treated by these open air, heap biooxidation processes. After this has been accomplished these treated ores are mixed with lime in order to raise their pH, and then treated by conventional hydrometallurgical treatments such as cyanide heap leaching.
Aside from the long periods of time they require, heap bioleaching processes also have certain technical drawbacks. For example, these processes can not be used to treat ores that have a carbonate matrix. This is due to the low pH requirements of those sulfide digesting bacteria used in such processes. Moreover, when such processes are used on low grade whole ores, a large volume of such ores must be placed on a pad in order to recover even relatively small amounts of precious metals. This circumstance dictates that the heaps must be stacked at lift heights of as much as 20 feet. This, in turn, leads to problems when the ore contains clays and/or fine refractory sulfide materials because such fine materials tend to plug the channels of air and liquid flow in such highly stacked heaps. This results in puddling, channeling, and starvation of nutrients, carbon dioxide and/or oxygen, as well as uneven inoculum distributions. Blocked heap channels have proven to be particularly debilitating with respect to sulfide-digesting bacteria because these bacteria require especially large amounts of oxygen in order to grow and oxidize the sulfide components of such ores. Adequate air flow is also needed in such heaps in order to dissipate the heat generated by the exothermic biooxidation reactions that are carried out by sulfide digesting bacteria.
Various biodigesting processes have been the subject of a number of patents. For example, South African Patent 90/2244 teaches a tank bioleaching process for treatment of refractory sulphide ores. This process includes the steps of making a slurry from a refractory ore, subjecting the slurry to the biological oxidation action of certain Thiobacillus ferrooxidans species, separating the solid component of the slurry, and then recovering the precious metal from said solid component by, for example, cyanidation procedures.
U.S. Pat. No. 5,246,486 teaches a pre-treatment process based upon biooxidation of a sulfide component of a refractory ore. The process begins by coating refractory sulfide ore particles with an inoculate of a bacteria that is capable of attacking the sulfide component of such an ore. After various other treatments, a heap is constructed from these particles and exposed to the action of a cyanide leaching solution.
U.S. Pat. No. 5,143,543 teaches an improved method of mixing biological conversion components (e.g., nutrients and oxygen) into a biomass. To this end, a portion of a biomass is withdrawn from a reaction tank and sent to an injection zone where the conversion components are injected into a portion of biomass previously withdrawn from the reactor. The resulting mixture is then sent to a static mixer where it is combined with other streams. The resulting material is then returned to the reaction tank.
U.S. Pat. No. 5,021,088 teaches a process for pre-treating gold-bearing, carbonaceous or carbonaceous pyretic ores with one or more heterotrophic microorganisms in order to consume the ore's carbon component. The resulting ore is then colonized with at least one microorganism species whose sulfide digestion capability serves to further enhance the ore's susceptibility to subsequent cyanidation processes.
U.S. Pat. No. 4,530,763 teaches a method for removing a metal contaminant from a waste fluid by a process that begins by incubating a bacteria that is capable of attaching to a particular type of metal contaminant. A waste fluid containing the targeted metal contaminant is then introduced into the tank and porous support members with which the bacteria are associated are slowly moved through the waste fluid to allow the bacteria to attach themselves to the metal contaminant component of the waste fluid. The resulting bacteria/metal contaminant is then separated from the porous support material.
U.S. Pat. No. 5,573,575 teaches a process whereby differences in the adhering qualities of refractory ore particles of different sizes are employed to enhance the overall recovery efficiencies of an open heap leaching process. The first step in the disclosed process is to crush the refractory ore and separate it into a fine particle component and a coarse particle component. The coarse particle component is formed into a heap. The fine particle component is made into a large particle concentrate material that is then added to the coarse particle component heap. The resulting coarse particle/large particle concentrate mixture is thereafter exposed to a heap biooxidation treatment.
These prior art processes often suffer from the disadvantage of being prohibitively expensive when they are used upon low grade ores in general--and especially those that emanate from relatively small ore bodies. Indeed, there are large amounts of identified low grade refractory ores, as well as small bodies of higher grade ores and/or many stocks of mined ore, that must be set aside because they cannot be processed economically using current recovery technologies. It is therefore an object of the present invention to provide biooxidation pre-treatment processes that are particularly effective in rendering such ores amenable to lixiviation at economically acceptable costs.