Patent Application
20080069723
The invention relates generally to precious metal recovery and particularly to recovery of precious metals from refractory carbonaceous materials using thermal oxidation techniques.
Carbon is an extremely abundant natural element whose unique characteristics include the ability to physically entrap or adsorb gold particles. Where the evolution of certain gold deposits coincides with the natural presence of carbon, the carbon acts as a filter to entrap the gold, producing a class of gold mineralogy known as carbonaceous ore.
The gold-attracting quality of carbon is widely exploited in commercial gold recovery techniques where gold is produced in a solution medium to which activated carbon is intentionally introduced. The carbon becomes “loaded” with gold through adsorption, is removed from the solution environment, chemically stripped of its gold and then returned to the processing circuit to be reused in the recovery process.
When the carbon occurs naturally in the gold ore, the ore cannot be directly leached through the normal cyanide treatment (cyanidation) owing to the dominant influence of the carbon; this situation is known as “preg robbing”. Adsorption of the gold lixiviant complex by refractory carbonaceous material is very complicated, due to three major factors. First, the precise chemical and physical nature of the carbonaceous matter is difficult to define, and varies from one ore body to the next. Second, the mechanism by which the carbonaceous material adsorbs gold is still being investigated. Third, although it has been known for some time that preg-robbing carbonaceous material can be passivated, or treated so as not to adsorb gold, the mechanism by which this occurs is not fully understood.
Preg robbing is counteracted by removing or otherwise treating the organic carbon prior to cyanidation. U.S. Pat. No. 3,639,925 (Scheiner) teaches the pretreatment of the carbonaceous ore by an alkaline hypochlorite. U.S. Pat. No. 4,289,532 (Matson) combines a two stage oxygenation and chlorination pretreatment process. U.S. Pat. No. 4,188,208 (Guay) teaches the introduction of surplus organic carbon to act as a collector for gold, thus diluting the concentration of gold remaining in the carbonaceous ore. U.S. Pat. No. 4,919,715 (Smith) teaches a multi-stage roasting process for sulfidic or mixed sulfidic and carbonaceous ores utilizing an environment which effectively oxidizes the organic carbon and sulfidic sulfur, rendering the material amenable to cyanide leaching of the contained gold. U.S. Pat. No. 4,902,345 (Ball) describes an autoclave oxidation followed by thiourea leaching instead of cyanidation. U.S. Pat. No. 4,923,510 (Ball et al) further discloses a method of chlorination followed by cyanidation. U.S. Pat. No. 5,536,480 (Simmons) teaches an autoclave oxidation of mixed carbonaceous and sulfidic ores. U.S. Pat. No. 5,626,647 (Kohr) teaches a method for producing a precious metal-bearing carbonaceous concentrate but does not present the method of subsequently recovering the precious metal.
There is a need for a process to recover gold economically from refractory carbonaceous precious metal-containing materials.
These and other needs are addressed by the various embodiments and configurations of the present invention. The present invention is directed generally to recovery of precious metals from refractory carbonaceous materials. “Carbonaceous” means carbon-containing and includes any carbon-containing constituent, whether or not preg robbing.
In a first embodiment of the present invention, a method of recovering precious metals from a carbonaceous precious metal-containing material is provided that includes:
(a) passing microwave energy through the carbonaceous precious metal-containing material in a reaction chamber of a reactor vessel;
(b) during step (a), passing a molecular oxygen-containing gas through reaction chamber to oxidize carbon in the precious metal-containing material and form an oxidized precious metal-containing material; and
(c) when most, if not all, of the carbon in the precious metal-containing material is converted into gaseous carbon oxides, removing the oxidized precious metal-containing material from the reaction chamber.
The carbon-containing components of the material are heated to a temperature sufficiently high to combust the component and convert the carbon into a gaseous carbon oxide (whether carbon monoxide or dioxide). Typically, the temperature ranges from about 450° C. to about 1,000° C.
The reactor can be of any suitable design, with a fluidized bed reactor, a rotary kiln, and a plug flow reactor being more preferred.
The microwave energy source comprising one or more individual generating units has a preferred power level in the range of about 1 kw to about 150 kw per generating unit and operates at a preferred frequency ranging from about 300 MHz to about 20 GHz. The reaction chamber preferably has an unloaded Q value ranging from about 1,000 to about 25,000, and the microwave energy delivered to the material preferably ranges from about 250 to about 300,000 Joules/gm.
The completion of carbon removal can be determined in many ways. One way, is to maintain the material in the reactor for a minimum residence time. Another way is to determine when a temperature of the oxidized precious metal-containing material decreases, typically by at least about 100° C., notwithstanding the continued application of microwave energy to the oxidized precious metal-containing material. When this occurs, most, if not all, of the carbon in the precious metal-containing material has been converted into gaseous carbon oxides. The temperature decrease is a result of removal of the carbon, which is more microwave absorptive than other constituents of the material.
The use of microwave energy to effect carbon removal can have substantial benefits over traditional roasters. Traditional roasters require additional fuel to be added to the carbonaceous material since the carbon content, usually of the order of a few percent of the ore mass, would not otherwise be sufficient to sustain autogenous operation (which is the basis of the roaster function). In contrast, the present invention can offer an alternative to the conventional roaster and differs fundamentally from conventional roasters in that microwave energy is used to supply the combustion energy, thus removing the need for additional fuel.
After carbon removal, the material is no longer refractory due to preg robbing carbon. The precious metal can therefore be recovered inexpensively by conventional cyanide leaching techniques.
These and other advantages will be apparent from the disclosure of the invention(s) contained herein.
As used herein, “at least one”, “one or more”, and “and/or” are open-ended expressions that are both conjunctive and disjunctive in operation. For example, each of the expressions “at least one of A, B and C”, “at least one of A, B, or C”, “one or more of A, B, and C”, “one or more of A, B, or C” and “A, B, and/or C” means A alone, B alone, C alone, A and B together, A and C together, B and C together, or A, B and C together.
It is to be noted that the term “a” or “an” entity refers to one or more of that entity. As such, the terms “a” (or “an”), “one or more” and “at least one” can be used interchangeably herein. It is also to be noted that the terms “comprising”, “including”, and “having” can be used interchangeably.
The term “carbonaceous” refers to materials containing carbon. Carbonaceous materials include a variety of compounds or minerals, including graphite, carbonates (e.g., calcite, dolomite, siderite, magnesite, aragonite, azurite, and malachite), activated carbon-type material, long-chain hydrocarbons and organic acids, such as humic acid, to name but a few.
The preceding is a simplified summary of the invention to provide an understanding of some aspects of the invention. This summary is neither an extensive nor exhaustive overview of the invention and its various embodiments. It is intended neither to identify key or critical elements of the invention nor to delineate the scope of the invention but to present selected concepts of the invention in a simplified form as an introduction to the more detailed description presented below. As will be appreciated, other embodiments of the invention are possible utilizing, alone or in combination, one or more of the features set forth above or described in detail below.
In the drawings, similar features have been given similar reference numbers.
A fluidized bed reactor, according to a first embodiment illustrated in
The microwave containment walls of the reactor 10 are preferably constructed of a microwave reflective material. The reaction vessel may optionally include an inner vessel which is constructed of essentially microwave-transparent material. Examples of suitable materials include alumina, aluminum silicate, quartz, and low-metal ceramics (which are substantially transparent to microwave energy) and stainless steel, mild steel, nickel alloys, and aluminum alloys (which are reflective of microwave energy).
A typical refractory carbonaceous precious metal-containing material includes a variety of materials. Common refractory carbonaceous materials include from about 0.1% to about 5 wt. % carbon-containing materials, from about 0.1% to about 2 wt. % sulfide sulfur, from about 0.1 to about 1 oz/ton gold, and from about 0 to about 2 oz/ton silver. The carbon can be in a variety of forms including graphite, carbonates, activated carbon-type material, long-chain hydrocarbons and organic acids, such as humic acid, to name a few. The material may be in the form of an ore, concentrate, tailings, or combinations thereof.
Because water is strongly absorptive of microwave energy, it is preferred that the material be dried before processing the reactor chamber 10. This is preferably done by heating the material at low temperature and/or exposing the material to sunlight for a prolonged period. Preferably, the water content of the material is no more than about 10 wt. % and even more preferably no more than about 1 wt. %. Excess water in the material can increase microwave energy requirements.
Carbonaceous material to be processed in the reactor chamber 10 is comminuted to a P80 size of preferably from about 10 mesh to about 50 mesh and introduced through the inlet valve 34 at a material flow rate coinciding with an average residence time in the reactor vessel of from about 30 seconds to about 5 minutes and fluidized by gas 36, which is supplied from an external source pipe 38 to control valve 40 and the gas inlet valve 26. The introduction of the gas will cause the material which has been introduced through the inlet valve to form a fluid bed 42, which is suspended through the adjustment of the gas pressure in the pressure chamber 14 and the bed fluidizer screen 12. The fluidized bed is then ready for treatment with microwave energy, which is introduced into the reaction chamber via the waveguide fittings.
When the bed is in a fluidized state, the solid material is heated by the dielectric and resistive effects caused by interaction between the electromagnetic field and the solid material constituents. The basic mechanism for microwave heating is a combination of dielectric and ohmic heating, whereby both electrical displacement and conduction currents are used to convert the electromagnetic energy directly into heat within the material. The efficiency of this energy conversion is dependent upon the dielectric properties of the material to be treated. Microwave receptor elements, principally carbon, are rapidly heated in a controlled manner. For most refractory carbonaceous materials, carbon has the greatest microwave-absorptive properties compared to that of the other material constituents. Thus, the carbon-containing compounds are preferentially heated.
The fluidizing gas is continuously passed through valve 26 and exhausted through port 28 during the treatment process. The exhaust stream is passed through a particulate separator 32 to clean the gas of particulate matter (principally fines blown free from the fluidized bed). The stream is passed through a heat exchanger 44 to preheat the inlet gas and conserve energy. The exhaust stream is then directed to an external bag house and ultimately discharged into the atmosphere.
Preferably, the molecular oxygen is present in an amount that is at least stoichiometric relative to the carbon content of the carbonaceous material and even more preferably in an amount that is at least about 110 to about 200% of the stoichiometric amount, and the molecular oxygen content of the fluidizing gas when introduced into the bottom of the reactor vessel is at least about 20 mole % and even more preferably ranges from about 20 to about 30 mole %.
The fluidizing velocity of the gas is preferably sufficient to suspend the material in the chamber. Preferably, the velocity of the fluidizing gas is maintained to be not less than the minimum fluidization velocity of the largest mineral particle size and not greater than the terminal velocity of the smallest mineral particle size.
The region 46 above the suspended fluidized bed 42 is generally essentially free of solid material and consists primarily of fluidizing gas and gaseous reaction products. The gas seal 20 permits the transmission of microwave energy into the reactor chamber 10 while isolating the atmosphere and contents of the chamber from the connecting waveguide via fittings 16 and 18.
The microwave energy heats the carbon-containing materials to a temperature sufficiently high to oxidize the carbon in the carbon-containing materials according to the following reaction:
C+O2→CO2.
The operating temperature of the bed is preferably at least about 450° C., more preferably ranges from about 450° C. to about 1,000° C., even more preferably ranges from about 650° C. to about 850° C., and even more preferably ranges from about 700° C. to about 750° C.
To provide a relatively high reaction rate, the inflow rate of oxygen into the reaction chamber is controlled to provide sufficient molecular oxygen to maintain the reaction and preferably convert at least most, more preferably at least about 90% and even more preferably at least about 99% of the carbon in the material into carbon dioxide. The material outputted via output 24 from the reactor chamber has a preferred maximum carbon content of about 0.2 wt. % and even more preferably of about 0.05 wt. %.
Temperature, heating rate, and/or microwave coupling can be controlled in a number of ways. As will be appreciated, dielectric loss factors of the material constituents can be temperature dependent. Accordingly, it is desirable to optimize dynamically the coupling between the magnetron or other microwave generating source and the chamber 10 and the resonant tuning of the chamber 10. The degree of coupling or matching of the cavity with the magnetron determines the efficiency with which energy is delivered to the chamber 10. Preferred coupling is as close to unity as possible. In one approach, the power of the microwaves and the time of application of the microwaves are controlled. In another approach, the power of the microwaves is adjusted according to the heating temperature. In yet another approach, an aperture size of a variable iris and/or tuner positioned in the waveguide is adjusted in response to the temperature of the bed and/or the power of the reflected microwaves (relative to the microwaves entering the reactor). Coupling is optimized as reflected energy is minimized.
Preferably, the microwave source comprising one or more individual generating units generates power levels in the range of about 1 kw to about 150 kw per generating unit. A preferred power level is from about 80 to about 125 kw. The specific energy delivered to the bed in the chamber 10 ranges from about 250 to about 300,000 Joules/gm or from about 2 to about 20 kW-h/t. The unloaded Q factor in the chamber 10 preferably ranges from about 1,000 to about 25,000, but most preferably is at least about 200,000. The frequency of the microwave source preferably ranges from about 300 MHz to about 30 GHz, with preferred microwave frequencies being within the Industrial, Scientific, and Medical (ISM) bands of about 915 MHz and about 2,450 MHz.
Process control is normally effected using a variety of instrumentation positioned in the chamber 10. For example, temperature probes are installed at various positions within the fluidized bed and all feed and discharge lines, including the gas inlet and outlet lines. Gas pressure and product monitors are installed in all gas lines. Material flow through the reactor chamber is measured either through flow meters or by mass measurements. A microwave reflection detector is positioned in the waveguide.
Completion of carbon oxidation is normally indicated by a substantial drop in bed temperature even though microwave energy is still being passed through the bed. This is so because the microwave absorptivities of the non-carbonaceous components of the material are substantially less than that of carbon. Typically, the oxidized material is removed from the chamber when the bed temperature decreases, even more typically when the bed temperature decreases by at least about 50° C., even more typically by at least about 100° C., and even more typically by at least about 200° C. For this to occur, the residence time of the carbon-containing material in the reactor chamber 10 preferably is no more than about 15 minutes and even more preferably ranges from about 2 to about 4 minutes, though the precise residence time depends on the power of the microwave source and the nature of the other constituents of the material.
After most, if not all, of the carbon in the bed is converted into carbon oxide gas and removed in the off-gas, the oxidized material is removed from the reactor and subjected to further processing to recover gold. In one process configuration, the oxidized material is further comminuted, slurried, and subjected to cyanide leaching in a suitable vessel by known techniques. In another process configuration, the oxidized material is agglomerated onto itself or coated onto an inert substrate and subjected to cyanide heap leaching by known techniques.
In a second embodiment, a rotary kiln is used. Referring to
In a third embodiment, a plug flow vessel may be used as shown in
Interaction between the material and microwave energy is accomplished by mounting the reactor vessel 80 within a waveguide conduit apparatus 94. Microwave energy is transmitted through the waveguide and passes through the dielectric vessel material, such that substantially all of the microwave energy is absorbed within the material passing through the vessel. By means of the more microwave-absorptive properties of the carbon present compared to those of the other ore materials, the carbon is preferentially heated by the microwave energy.
A treatment process for a carbonaceous material, according to the present invention, will now be described.
The carbonaceous material is first loaded into a reaction chamber where the ore is mixed with molecular oxygen, which is generally atmospheric air. Next, microwave energy is applied. The microwave energy raises the air-material mixture to the preferred operating temperature in the range of about 700-750° C., at which temperature the reaction takes place as C+O2→CO2.
This chemical reaction is exothermic and, depending upon the quantity of carbon present, contributes heat energy to the process. The microwave energy is thus required to make up the balance of energy required for the reaction to proceed. In this respect, the use of microwave energy is advantageous since the carbon acts to preferentially convert the microwave energy into heat within the mineral.
The treatment process is operated in a continuous manner and the material flowrate through the reactor vessel is controlled such that the ore being discharged from the reactor is depleted of carbon to any desired extent, the material so discharged being continuously monitored for carbon content.
The process embodied in the present invention is further illustrated by the following examples.
A carbonaceous whole ore presented the following characteristics:
Total organic carbon 0.86% by weight
95% preg robbing
8.52 grams per ton gold
Dominant mineralization quartz, dolomite, calcite.
The material was sized to >10 mesh (Tyler) to minimize blow through of fine material in the reactor. The test material was separated into two lots and both lots were processed identically. The material was batch heated in the reactor to an indicated temperature of from about 660°-700° C. The completion of the reaction was indicated by a temperature reduction to about 350-400° C., corresponding to the depletion of carbon and the resultant lower microwave absorptive properties of the remaining calcine material.
The calcine material was ground to 100% minus 200 mesh and leached in sodium cyanide for up to 48 hours and the gold in solution was measured. The carbon content was measured to be 0.09% by weight.
The following Table 1 presents the gold recovery figures for both unprocessed and microwave processed materials.
A brecciated quartz vein mineral with included carbonaceous shale had the following characteristics:
Total organic carbon 1.07% by weight
100% preg robbing
17.01 grams per ton gold
17 grams per ton silver
Dominant mineralization quartz, albite, muscovite mica.
The material was sized to between about 10 and 50 mesh (Tyler). The test material was separated into two lots and both lots were processed identically. The material was batch heated in the reactor to an indicated temperature of approximately 700° C. The completion of the reaction was indicated by a temperature reduction to about 500° C., corresponding to the depletion of carbon and the resultant lower microwave absorptive properties of the remaining calcine material.
The calcine was ground to 100% minus 200 mesh and leached in sodium cyanide for up to 48 hours and the gold and silver in solution were measured. The carbon content was measured to be 0.14% by weight.
The following Tables 2 and 3 present the gold and silver recovery figures for both unprocessed and microwave processed materials.
A sulfidic carbonaceous whole ore had the following characteristics:
Total carbon 1.42% by weight
Organic carbon 0.26% by weight
7.24 ppm gold
80% preg robbing
A total of approximately 4000 lbs of test material was used throughout 8 test sequences. The material was finely ground as conventional roaster feedstock. Each test was operated as a continuous operation in which the reactor charge of approximately 55 lbs of test material was initially heated to reaction temperature. Upon reaching operating temperature, fresh feed was introduced while calcined product was recovered from the calcine discharge and cyclone underflow. All of the samples used for gold recovery were obtained while the reactor was operating in continuous steady state condition. Operating temperatures throughout the test series ranged from 470-660° C.
Cyanide soluble gold recoveries from processed materials over all tests ranged from 85.8-96.0%. For the best test, conducted at an operating temperature of between 480-550° C., the average cyanide soluble gold recovery was 92.9% for the calcine discharge material and 88.3% for the cyclone underflow material.
By observation of these experimental data, notwithstanding the fact that the metal extraction process has not been optimized with respect to cyanide consumption or other performance measures, it is evident that the microwave process is effective in greatly reducing the carbon content which is the known preg robbing agent. Metal recoveries, both for gold and silver, also indicate a great improvement over the unprocessed situation.
It is also evident that the microwave process is effective at indicated temperatures, which are substantially lower than the known reaction temperature of the oxidation of carbon, being above approximately 600° C. This is indicative of the selective absorption and heating properties of carbon whereby the total bulk (ore) temperature is not required to reach reaction temperatures in order for the localized carbon temperatures to reach the necessary levels. This feature leads to potential economic savings through more efficient energy utilization.
It is also evident that the microwave process is effective without the addition of combustible fuels to sustain operating temperatures and that, in fact, the microwave process continues to be effective at all values of carbon content until the organic carbon is totally depleted.
While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations as fall within the spirit and broad scope of the invention.
A number of variations and modifications of the invention can be used. It would be possible to provide for some features of the invention without providing others.
For example in one alternative embodiment, the process is used to recover other metals the recovery of which is complicated by the presence of carbon in the metal-containing material.
The present invention, in various embodiments, includes components, methods, processes, systems and/or apparatus substantially as depicted and described herein, including various embodiments, subcombinations, and subsets thereof. Those of skill in the art will understand how to make and use the present invention after understanding the present disclosure. The present invention, in various embodiments, includes providing devices and processes in the absence of items not depicted and/or described herein or in various embodiments hereof, including in the absence of such items as may have been used in previous devices or processes, e.g., for improving performance, achieving ease and\or reducing cost of implementation.
The foregoing discussion of the invention has been presented for purposes of illustration and description. The foregoing is not intended to limit the invention to the form or forms disclosed herein. In the foregoing Detailed Description for example, various features of the invention are grouped together in one or more embodiments for the purpose of streamlining the disclosure. The features of the embodiments of the invention may be combined in alternate embodiments other than those discussed above. This method of disclosure is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the following claims are hereby incorporated into this Detailed Description, with each claim standing on its own as a separate preferred embodiment of the invention.
Moreover, though the description of the invention has included description of one or more embodiments and certain variations and modifications, other variations, combinations, and modifications are within the scope of the invention, e.g., as may be within the skill and knowledge of those in the art, after understanding the present disclosure. It is intended to obtain rights which include alternative embodiments to the extent permitted, including alternate, interchangeable and/or equivalent structures, functions, ranges or steps to those claimed, whether or not such alternate, interchangeable and/or equivalent structures, functions, ranges or steps are disclosed herein, and without intending to publicly dedicate any patentable subject matter.
The present application claims the benefits of U.S. Provisional Application Ser. No. 60/826,346, filed Sep. 20, 2006, of the same title, which is incorporated herein by this reference.
| Number | Date | Country | |
|---|---|---|---|
| 60826346 | Sep 2006 | US |
