Sequential lixivation and precipitation of metals from refractory ores by utilising variable oxidation reduction potentials and a variable ph system

The invention relates generally to a method of separating metals from another material, and in particular, to a method for extracting platinoids, rare earths, and transition metals from disseminated and difficult matrixes by utilizing controlled parameters of a solution.

Known processes for the commercial extraction of precious metals from ores at present consist mainly of two main operations: concentration and then refining. Concentration is the separation of a metal or a metallic compound from gangue of the ore. Refining is the production of a metal into a pure or nearly pure state suitable for use. Concentration and refining processes can be mechanical, chemical, or electrical and any, combination thereof. Mechanical separation techniques include gravity separation and floatation. Chemical separation techniques include smelting, amalgamation and cyanidation. Electrical separation techniques include electrolysis.

Many groups of metals such as the rare earths, the transition metals, the neucleides, and in particular, the platinoids which consist of platinum, palladium, rhodium, osmium, ruthenium, and iridium are found in very large low grade deposits throughout the world. Because of the complexity of most of these ores and the difficulty of concentrating them, these deposits are not utilized. For example the classic procedure for separating platinum group metals (PGM) begins by concentrating the metals, typically by floatation. After floatation, the concentration is smelted in an autoclave to produce a matte that is leached of copper and nickel sulphides. The solid leach residue matte typically contains 15%-20% PGM. The solid leach residue is then treated with a series of acids and finally sodium peroxide and with the metals now in solution, they can be separated either in resin columns, by solvent extraction, or by sequential precipitation. Alternatively gravity separation and/or magnetic separation can be used prior to floatation, resulting in a concentrate containing up to 50% PGM and so making direct smelting possible and making leaching unnecessary.

The simultaneous solubilization of all of the platinum metals can be accomplished by fusing the mineral concentrate obtained from nickel and copper sulphides ores with aluminum metal, dissolving the aluminum and treating the residue with hydrochloric acid and chlorine. This dissolves the PGM, which are then subsequently separated by solvent extraction. The individual solutions are then treated by conventional techniques to recover the various metals in a pure state.

In these and other known processes for extracting PGM, the ores must be concentrated prior to extraction to be economically viable. Such concentration is energy intensive, incurs significant losses, and requires high capital investment for equipment infrastr. Also, known processes for processing concentrate require numerous complex and time consuming steps, some of which are not environmentally friendly.

It is the object of the invention to make refractory ores such as slates, mudstones, shales, serpentines and other rocks which very easily become slimes when ground, very much easier to process and filter.

It is also an object of the invention to be able to extract and solubalize metals that are chemically bound to other elements and/or found in ores as discrete elements and therefore cannot be extracted by conventional means, such as gravity or floatation.

In particular, it is the object of the invention to provide a method of separating metals from another material wherein said method requires a relatively low equipment infrastructure investment, is relatively simple to operate, and is quick and environmentally benign while having a relatively high throughput.

The invention achieves most of these objectives and relates to an improved method of varying the ph electrically and the oxidation-reduction potential of the solution as well as controlling the viscosity of the solution. The variable control of the oxidation-reduction potential makes it possible to maximize or minimize the extraction of any particular metal.

The process also allows the oxidation-reduction potential to be pushed up to new heights such that metals such as platinum, scandium, and other metals which are difficult to take into solution are now much more easily solubalized. The process utilizes a peroxide leach in conjunction with suitable anion and a variable direct (DC) current to produce a variable high oxidation-reduction potential which, by adding more energy to the solution in the form of ultra violet light, ultra sonic energy, or more reagents and electric power, can be pushed far beyond normal and can consequently livivate or leach very refractory metals in extremely difficult matrixes. The process utilizes a ground up ore slurry with a hydrogen peroxide (H2O2) leach in conjunction with a variable DC current and variable amounts of energy such as ultraviolet radiation or ultrasound until a desired oxidation-potential is reached and then the reaction is allowed to proceed until the oxidation-reduction meter indicates that the metals, after a selected period of time, have been dissolved, at which time the slurry is filtered and clarified to give a pregnant solution The pregnant solution is then treated, either electrically or chemically, to yield the metal or metal salts or both.

The anion source can be selected from the group consisting of inorganic acids, soluble inorganic salts, alkaline solutions, and in a limited way, some organic acids. In particular, the anion source can be one of, but not limited to: HCl, NaCl, HBr, HNO3H2SO4, KBr, KNO3, FeCl3 NaOH, KOH, and organic acids such as oxalic acid.

The length of the leaching period can be roughly selected based on the grade of ore and type of rock being leached or based on the metal that is to be extracted. The final length of time can be determined by the reaction in the slurry, and more precisely, by the oxidation-reduction potential meter. In order to increase the rate of lixivation, an external source of heat may be applied or the DC current may be increased to raise the temperature of the slurry. While this will increase the rate of reaction, the increased heat will decompose the H2O2 more rapidly so a balance must be struck between the increased reaction and the loss of H2O2.

The method can be directed at recovering the PGM (or any group that have similar chemical characteristics to one another such as the rare earths, the neuclides, or the transition metals), in which case the method can first be applied using hydrochloric acid (HCl) as the anion source with the oxidation-reduction potential suitable for platinum and treating the pregnant solution using techniques that are known to the art. The method can then be applied again to the remaining ore using sulfuric acid (H2SO4) as the anion source and then treating the pregnant solution to recover palladium from the solution using techniques that are known to the art. This process can, in many cases, give a much cleaner division of the individual metals.

FIG. 1 is a schematic side cut-away view of a leaching container for carrying out an electro-chemical oxidative leach process, containing a slurry comprising ground ore and a leaching solution.

According to one embodiment of the invention an electrochemical process is provided for winning certain precious metals from ground ore, such as the platinum group metals (PGM), gold, silver, and incidental metals. By extracting the metals directly from the ground ore, the costly step of concentrating the ore, with subsequent losses, is avoided. The process utilizes a peroxide leach and, as needed, additions of energy to the solution, such as ultra violet light in combination with a variable DC current to produce a controllable highly oxidative-reduction solution which gives soluble, very high valency compounds. In general, a solution of a calculated amount of hydrogen peroxide (H2O2) and a calculated amount of an anion are mixed with three litres of water in solution with approximately 150 grams of ground ore to form a slurry. At this time a direct current is run through the solution, and after a few minutes, a ph reading is taken and an oxidation-reduction potential (ORP) is also taken. From these readings, the current, amount of H2O2, and the amount of acid, it is decided what the correct balance is needed to give an ORP suitable for the desired metal to be lixivated. The solution is adjusted accordingly and a current is then run through the solution until the reaction slows down and the ORP indicates that all the desired metal has been lixivated, The electricity is stopped and the solution filtered. The filtrate is treated in a manner well known in the art and the metal or metals, is precipitated.

We theorize that during the process, hyperoxidation occurs because of the anodic oxidation at the anode where the metals are often forced into high valency combinations which stay in solution and are stable as long as the current is kept on and the amount of electrons or EMF in the solution is high. One visible sign of this is the ph of the solution which is forced downwards as the electron load is increased, sometimes as much as two or three points and possibly four, so that the solution is, in effect, very much more acidic compared to what the chemical reagents would cause. The result is that the excess electrons force the negative anions to bond to metals. When the current is withdrawn and the electrons are allowed to dissipate, the ph returns towards the alkaline. While all metals are reactive, the process can be tailored to preferentially lixivate only certain metals or certain groups of metals or a gross bulk lixivation and precipitation can be produced if so desired.

We also theorize that the chemical reactions in the process include the following reaction between platinum and hydrogen peroxide:

2Pt+2H2O2→2Pt+4(OH)→2Pt(OH)2→2Pt++2H2O2═O2−

According to our theory, hydrogen peroxide is preferentially catalyzed by platinum and to a more or lesser degree by other metals. At least some of the energy required to split the hydrogen peroxide is provided by the platinum entering into the reaction briefly to form platinum hydroxide Pt(OH)2, which is unstable. Oxygen ions (O−) in the solution that are released in the reaction combine with other oxygen ions to form gaseous oxygen (O2). The remaining water (H2O) and platinum (Pt) split and the Pt is left in a positive state (Pt+). The resulting excess of negative electrons and negative ions in the solution encourages the available anions in solution such as chloride ions (Cl−) to combine with the Pt+to form a combination that is much more stable than platinum hydroxide. The combinations formed include PtCl2, PtCl4, PtCl6 and even higher valencies. Such combinations are produced because of the negative charge in the solution and the release of energy at the site of the platinum.

Referring to FIG. 1, experimental testing of the process was carried out in a plastic leach container 10. Plastic is preferred-as it is relatively chemically non-reactive; however, other suitable materials can be substituted. Flat plasma arc graphic electrodes 12 having dimensions of 12″ by ¾″ by ¼″ were mounted inside and on opposite sides of the container 10 and were electrically connected to a low voltage high-amperage direct current source 14. Suitable electrodes 12 can be obtained from Anachemia Ltd. Graphite or carbon electrodes are preferred because they are relatively non-reactive. However, a metallic electrode (for example, titanium, lead, and some forms of iron) can be used if the leaching solution is acidic as such solution will enable the platinum to be plated out. If the solution is alkaline, then the solution must be made acidic.

The ore is obtained from copper or nickel sulfide minerals, and is ground according to methods that are well known in the art. Copper and nickel sulfide minerals are particularly suitable as the subject ore in this process, especially sulfide ores such as nickel mineral pentalandite [(Ni, Fe)9S8] laurite (RuS2) irasite [(Ir, Ru, Rh, Pt)AsS)], osmiridium (Ir, Os), cooperite [(Pt, PD, Ni)S], and braggite. Platinum materials are usually found highly disseminated in these sulfide ores. Platinoids are also found in many other different ores but are generally too poor of a grade to be economically recovered by conventional methods.

The process involves forming a leach slurry by combining the ground ore with a solution comprising water (H2O) with a predetermined amount of hydrogen peroxide (H2O2) and an anion source. In particular, the leach slurry can be formed by placing the ground ore in the leach container 10, then adding aqueous hydrochloric acid (HCl) solution as an anion source to produce an acid solution containing chloride ions (Cl−) or, adding aqueous sodium chloride (NaCl) solution as an anion source to produce an alkali solution. As the chloride ions (Cl−) are consumed, the sodium ions (Na+) form sodium hydroxide (NaOH) with the water (H2O) and make the solution continuously more alkaline.

The amount of water and anion source added will depend on various factors, such as the type of minerals, whether the metals are occluded, etc. Generally, it is desirable to use the least amount of anion source as possible to extract the metals from the ore in the shortest amount of time and in the most economical manner. In this embodiment, 150 cc of hydrochloric acid (HCl) was used per 4000 cc solution per pound ore.

Alternatively, other anion sources can be substituted for hydrochloric acid (HCl) or sodium chloride (NaCl), such as: inorganic acids like HBr, HNO3, H2SO4, H3PO4, soluble inorganic salts such as KBr, KNO3, FeCl3, and alkaline solution compounds such as NaOH and KOH. In addition to being sources for chloride ions, these chemicals are sources of other anions such as SO4, OH—, Br— etc. It has been found that different metals respond differently to different anions and therefore the anion source selected for the slurry will depend on the metal that is targeted for recovery; for example, it has been found that although both platinum and palladium can be picked up by either chloride ions (Cl—) or sulfate ions (SO4—), that chloride ions (Cl—) are more effective to pick up platinum and sulfate ions (SO4—) are more effective to pick up palladium.

Note that NHOH should be avoided as an anion source from winning PGM because so many different ammonium compounds can be formed that it becomes difficult to make any subsequent separation of the metals.

After the ground ore and aqueous anion source solution have been combined in the container 10, the DC source is turned on and the current is directed through electrodes 12 and into the slurry. Then, the hydrogen peroxide is added to the slurry and the slurry is left to leach for a selected period of time (“oxidative leach period”). The amount of hydrogen peroxide added will also depend on various factors. Generally, it is desirable to use the least amount of peroxide as possible to extract the metals from the ore in the shortest amount of time and in the most economical manner. In this embodiment, 150 to 200 cc of peroxide was used per 4000 cc of solution per pound of ore.

Optionally, the peroxide can be added to the solution before the DC source is turned on. However, if acid is added to a carbonaceous ore, then peroxide should be added first, then the current turned on, and then the acid should be added last in order to limit foaming when the acid contacts the ore.

The current is maintained throughout the oxidative leaching period. It is desirable to set the current as high as possible in order to achieve the fastest possible lixivation rate. Because of the direct current, the solution will heat up and, to a point, the reaction in a warn solution will proceed at a faster rate than a reaction in a cold solution. However, each different solution conducts current at a different rate; therefore, the voltage and current settings must be balanced such that a sufficiently high current is applied without heating the slurry to such an extent that the hydrogen peroxide is boiled away.

The oxidative leaching period depends on the amount of ore being leached. Larger quantities of ore require longer leach periods. For low grade ores used during this experiment, an oxidative leaching period of 1 to 6 hours was found to be adequate. However, for higher grade ore or refractory ore, a longer leaching period should be selected. However, even a longer oxidative leach period of six hours is substantially faster than leaching by cyanidation, which often takes 72 hours or more.

The selected oxidative leaching period also depends on which metals are to be recovered. Leaching for a shorter period causes only the most reactive metals of the anion to enter the solution. Conversely, the leaching for a longer period causes more metals to enter the solution. The metals enter the solution in order of their respective rate of reactivity with the anion and their concentration in the solution. The division between one metal and another entering into the solution is not sharp. As the more reactive metal (“metal A”) gets taken up in the slurry, leaving fewer of its atoms per volume of slurry, the more available are the next more reactive metal (“metal B”) in the slurry. As a result, both metals A and B are being taken up until eventually, nearly all of the metal A has been taken up and then metal B is the metal with the most atoms going into solution.

After the oxidative leaching period has expired, the current is turned off and the pregnant solution is filtered from the slurry by mechanical techniques well known in the art. In this experiment, a filter paper (not shown) was placed in a funnel (not shown) and the pregnant solution was allowed to filter through the filter paper. When the solution is cloudy, it can be clarified by techniques well known in the art, for example, by using a substance that will coagulate with particles of the cloudy material which can then be filtered out. When hydrochloric acid (HCl) is used as the anion source, the pregnant solution should contain mostly platinum with a smaller amount of palladium. After filtering the pregnant solution form the slurry, the remaining ore material is subject to another oxidative leach process with direct current as described above, but instead of hydrochloric acid (HCl), a Leach solution with sulfuric acid (H2SO4) is used as the anion source. With sulfate ions (SO4−) as the anion, the process extracts mostly palladium with a lesser amount of platinum.

The electrochemical oxidative leach process can be applied with different anion sources to extract different metals from the slurry, such as other PGM and copper (Cu), nickel (Ni), chromium (Cr) and other precious rare earth metals.

Tailings from Crystal Graphite Corporations mine near the city of Slocan, British Columbia were tested. One pound (454 grams or 15 assay tons) of tailings were placed in a plastic leach container. A solution comprising 3000 cc water (H2O), 150 cc hydrogen peroxide (H2O2), and 150 cc concentrated hydrochloric acid (HCl) was added to the tailings to form a slurry. Two graphite electrodes were placed spaced apart into the slurry in the container, and a twelve volt six ampere current was applied for a period of three hours. The leached solution was then filtered. The resulting acid solution was then neutralized with sodium hydroxide (NaOH) so that a dark brown precipitate formed, which was then filtered out and dried. After drying, 38 g of precipitate remained which was then fire assayed and cupelled. This produced a 3.2 mg metal bead which was found by Assayers Canada to contain $52 per ton in gold.

A 200 gram sample of siliceous rock with garnets in it was placed in a plastic reaction vessel containing an undiluted mixture of hydrochloric acid (HCl) and hydrogen peroxide (H2O2) and a six volt current was applied for six hours. At that time it was found that the garnets were untouched and the rock matrix was partially dissolved. After leaving the sample in the solution for another six hours, it was found that the remaining rock had dissolved along with some of the garnets. The solution was then diluted and the metals precipitated. On assay, it was found that the metal values were

While the present invention has been described herein by the preferred embodiments, it will be understood to those skilled in the art that various changes may be made and added to the invention. The changes and alternatives are considered within the spirit and scope of the present invention.