AU633141B2 - Selective leach for oxides and therein-contained metals - Google Patents

Description

COMMONWEALTH OF AUSTRALIA r Patents Act 1952 COMPLETE SPECIFICATION (Original) FOR OFFICE USE Application Number: Lodged: Class Int. Class Complete Specification Lodged: 17th September, 1990 Accepted: Published: Priority: Related Art: TO BE COMPLETED BY APPLICANT Name of Applicant: JOHN R CLARK Address of Applicant: 7580 Deframe Colorado 80005, America.

Street, Arvada, United States of Actual Inventor: JOHN R CLARK Address for Service:- Wray Associates Primary Indust. House 239 Adelaide Te-race Perth Western Australia 6000.

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Complete Specification for the invention entitled: "Selective Leach for Oxides Therein-Contained Metals" and The following statement is a full description of this invention, including the best method of performing it known Sto me:- 1 Ii 1 SELECTIVE LEACH FOR OXIDES AND THEREIN-CONTAINED METALS 2 3 BACKGROUND OF THE INVENTION 4 i! 1. Field of the Invention This invention relates to selective leaches used in partial 6 dissolution techniques to remove oxide coatings upon soil and 7 mineral particles, and the metals contained therein, primarily 8 for use in the art of geochemical prospecting, but also for 9 chemical winning by heap-leaching.

By geochemical prospecting, it is possible to infer the 11 presence, location and magnitude of ore bodies at depth by *1.2 detecting areas of localized anomalous excess of the

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13 concentrations of ore metals in comparison with their normal concentrations in the region. Such excess may result from 15 .diffusion or capillary transport of metal ions upward through overburden, from physical transport of ore mineral, or from other 17 hydromorphic processes due to transport in ground water and 18 resulting stream flows. An account of such prospecting may be i found in "Geochemistry in Mineral Exploration" (2nd ed.) by Rose, *20 Hawkes and Webb, Academic Press, New York, 1979.

An important hydromorphic process is the transport of 22 manganese and iron ions, together with associated ore metal 23 i compounds, from the deep-seated to the surficial environment. At *24 depth, redox potential is low and manganese and iron oxides are more soluble, reprecipitating nearer the surface where the system 26 becomes oxygenated arn solubilities decrease. The fresh, 27 initially largely amorphous oxides deposit as coatings upon 28 available solid bodies, ranging from clay particles to boulders, 29 and have high scavenging capacity for heavy metals moving as solutes in the aqueous environment. Depending upon conditions, 31 metals such as Ag, Co and Cu will associate predominantly with 32 manganese oxide, Pb with iron oxide, cid Zn and Ni with either.

1 2 3 4 6 7 8 9 11 l s 4 i 15 17 18 21: 22 :'.23 26 27 28 29 Because the concentration of ore metals is higher in the oxide coatings than in the solid bodies as a whole, it is advantageous to strip off these coatings before testing for metal content; this enhances the contrast between the composition. of anomalous samples and that of the normal background in the region and permits more sensitive and extensive delineation of the anomalous area. It is further advantageous to be able to strip separately the manganese oxide and iron oxide metal-rnntaining materials, and other significant components of the sample.

For these reasons, workers in this field have developed many different selective leach solutions which, applied separately or in carefully-ordered sequences in well-known partial dissolution techniques, can approximately strip off one or more of the separate components of the coating and resolve the sample into useful fractions. A description of this art is set forth in a paper by T.T. Chao, "Use of Partial Dissolution Techniques in Geochemical Exploration", Journal of Geochemicaa Exploration, Vol. 20 (1984) pp. 101-135, Elsevier Science Publishers, Amsterdam. This paper also includes an extensive bibliography which provides a gateway to much of the art.

2. Prior Art Problem One reason for the existence of so many different leach solution formulas is that improvements in performance are still being sought, not only in specificity in separating sample components to be analyzed, such as those associated with the manganese and iron oxides, but also in finding reagents which do not later interfere with or confound the operation of the analytical instruments which must detect low concentrations of metals in the separated components.

II I i 1 For one example, hydroxylamine hydrochloride can be a 2 selective reducing agent for rendering manganese dioxide coatings 3 soluble, and can be used along with other reagents in such dilute 4 concentrations that chemical attack upon iron oxides and substrates is very minor. But concentrations of dissolved trace 6 elements can often be so low as to require sensitive instrumental 7 techniques such as inductively-coupled plasma/mass spectrometry 8 ("ICP/MS") or flameless atomic absorption The presence 9 of chloride ions in sample solutions can produce extreme interferences for many elements when using either of these 11 techniques. Thus hydroxylamine hydrochloride, which contains chloride, is not a viable leaching agent when seeking many low- 3 level trace-element signatures.

*.14 ji For another example, ascorbic acid solutions can be used to .l5 dissolve both manganese oxide and iron oxide coatings. They can be quite effective when used in conjunction with an iodide salt, 17 producing a strong reduction couple. But these leaches are not 18 selective for a particular type of oxide; also, reagent grade ""19 iodide silts are contaminated with trace amounts of some metals.

0 Although the concentrations of these contaminants are low, in the 1 1 microgram per gram range, they are sufficient to mask anomalies 22 in the samples being analyzed. These 'two examples are typical; 23 I there are others.

it 4 There is therefore a need for an improved selective leach, for oxides and therein-contained metals, which not only is able 26 to select and separate the manganese oxide and the iron oxide 27 I components but also does not add interfering materials which 28 could interfere with or confound the operation of analytical 29 equipment used to analyze low concentrations of metals in the leachate from the sample.

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I I SUMMARY OF THE INVENTION I have discovered that a fairly weak aqueous sugar solution, to which has been added a small amount of a first enzyme capable of converting sugar to provide a low concentration of hydrogen peroxide and some organic acid, is ai: effective selective leach for dissolving manganese oxides on the particles in a sample and releasing their ore metal content, without appreciable attack on the iron oxides or the substrates of the particles. I have further discovered that the same kind of leach, to which has been added a second enzyme capable of converting hydrogen 0o peroxide to water and oxygen, or to which has been added tome ascorbic acid, has a sufficiently lowered oxidation :D 'otential to dissolve iron oxides on the particles and release their ore metal content, without attack on the substrates of the particles.

Accordingly, the invention resides in a method of selectively leaching geological materials to dissolve ooe oxides and therein-contained metals, comprising contacting said materials with a leach solution comprising water, a sugar, and a first enzyme which can act upon said sugar to produce an organic acid and a low concentration of hydrogen peroxide, the concentration of said sugar in said solution being in the range 0.0001% to 4 L L and the concentration of said first enzyme in said solution being in the range of 0.0001% to and separating said leach solution from said materials.

DESCRIPTION OF THE INVENTION While concentrated hydrogen peroxide has been widely used as an oxidant in selective leaching processes, particularly for destruction of organic portions of the sample, it is a little known and rarely used fact that hydrogen peroxide can function as a reducing agent for several metallic oxides. In an aqueous solution, it will react with manganese dioxide, consuming hydrogen ions, resulting in the manganese being reduced to the divalent state, which is soluble, thus: MnO(s) H 0 2H Mn 2 0 2 2 2 2(aq) H 2 0 In the process, all trace elements trapped in the manganese dioxide are released.

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It has been known for some time, in the rather different art of beekeeping, that natural raw honey contains a very low concentration of the heat-labile enzyme, glucose oxidase, which, acting upon the dextrose in the honey, has the interesting property of maintaining a very low, 33 parts per million, *a a ego concentration of hydrogen peroxide in the honey. This peroxide, among other factors, prevents growth of pathogenic and degradative microorganisms in the honey. In brief: Dextrose 02 H 2 0 Gluconic Acid H 2 0 2 (Glucose Oxidase) With suitable concentrations and other conditions, I find that these reactions can be exploited to accomplish selective leaching by partial dissolution of samples of geological materials by a leach solution comprising glucose and glucose oxidase.

While I do not wish to be limited by any particular theory, it appears that hydrogen peroxide is slowly generated in the glucose solution and reacts with any manganese dioxide which is present. The gluconic acid is an excellent complexer of metals and helps to hold them in solution. It also contributes hydrogen ions to the reaction between hydrogen peroxide and manganese 4* dioxide. Since the hydrogen peroxide is slowly generated, and there seems to be some sort of negative-feedback effect which S: limits its maximum concentration, it does not reach high concentrations at which it could effectively attack organic matter that could be present in the sample. Nor would there be extensive oxidation and consequent possible dissolution of reduced mineral substances. The slow rate of reaction allows the user to alter rates of sample leaching by adjusting concentrations of the reactants.

I have observed that sugar, as in a 1% glucose solution, tends to suppress the desorption of metal ions adsorbed on the surfaces of mineral phases that are not attacked by hydrogen peroxide, such as clays. This can enhance the detectibility of trace-element anomalies found in the metal oxide coatings in the sample. Again, I do -not wish to be limited by any particular thory, but it appears since sugars are non-polar but are watersoluble, there are polar sites along the molecule, and hydrogen bonding can occur at those sites. Sugars can then effectively lower the ionic strength of the leach solution because a portion of the polar water molecules would be bound by hydrogen bonding, while the molecule that produced the hydrogen bonding was itself nonpolar.

In discusEions to follow, reactant concentrations are given in perceht, weight/volume, gm/ml. For example 1% represents a concentration of 0.01 grams of reactant per milliliter of solution. Further, amounts of enzyme reactants are expressed in terms of weight of the commercial prepared powders in which they are obtained, which may typically contain only about 0.05% actual enzyme content. Hereinafter, enzyme concentration numbers "represent the concentrations, in the solution, of the powders in which they are available.

I find that this selective leach solution must have a glucose concentration of at least 0.0001%, in order to be effective in a leaching sample having a solution-to-sample ratio of 50 to 1 and proportionately more for smaller ratios, and no more than 20%, in order to avoid sticky solutions. I recommend 0" 20 the range 0.1% to with the further note that, above about o carbi n can deposit on the torch of an ICP/MS instrument, if that is used to analyze the leached metals. I prefer to use a 1% glucose concentration.

I find that the glucose oxidase concentration should be at least 0.0001%, in order to avoid too slow action, and no more 0 than in order to avoid excessive cost. I recommend the range 0.001% to 0.10%. I prefer to use a 0.03% glucose oxidase concentration.

For reasonably rapid reaction, the ratio of the amount of glucose oxidase to the amount of glucose should be at least 0.00001. To balance the amounts of glucose and glucose oxidase, I recommend that the ratio of glucose oxidase to glucose should be in the range 0.001 to 1.0. I prefer to use a ratio of 0.03.

1 2 3 4 6 7 8 9 11 6 20 22 23 *24 27 28 29 31 32 33 26 28 29 31 32 33 Commercial preparations of glucose oxidase frequently contain large amounts of another enzyme, catalase, which decomposes hydrogen peroxide into water and oxygen, lowering the oxidative potential of the solution. Too much catalase in the selective leach solution can lower that potential sufficiently that higher iron oxides can be reduced to the ferrous state and thus rendered soluble, along with the manganese oxides. This is undesirable when the intent is to analyze the two oxide suites separately.

There are purified grades of glucose oxidase powder which contain only moderate amounts of catalase, of the order of one-tenth of their glucose oxidase content, and these can be used for leaches which are selective for manganese oxides alone.

But when the objective is to dissolve the iron oxides, either in a second and separate treament of the sample after the manganese oxide components have been stripped or in a combined single treatment to strip both the manganese and iron oxides, a selective leach comprising catalase, as well as glucose and glucose oxidase, can be effective. For this purpose, the concentration of catalase can be about equal to that of the glucose oxidase. I recommend that the concentration of catalase in the solution be in the range 0.001%to 0.10%. I prefer to use a 0.03% catalase concentration.

Catalase is obtained commercially from microbial and fungal sources and also from mammalian livers. That from livers should be avoided, since it is frequently contaminated with a wide spectrum of heavy metals. And it must be pointed out that various commercial enzyme powders may contain other unwanted components, such as sodium chloride, which can complicate thN task of analyzing the resulting leachate; these preparations must Le chosen and used judiciously.

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1 Alternatively, one can lower the oxidative potential of a 2 glucose-glucose oxidase selective leach solution by adding a 3 reducing organic acid such as ascorbic acid. I recommend that the 4 i concentration of ascorbic acid in the solution be in the range I 0.01% to I prefer to use a 0.1% concentration; it performs 6 i well and does not deposit excessive carbon in analytical 7 l I instrumentation.

8 I recommend that, after a sample has been leached and the 9 supernatant leachate drawn off, enough ultrapure nitric acid be Ii added to make the leachate 1% HNO 3 to stabilize it against 11 reprecipitation of some elements. Also, some formaldehyde should .2 iH be added to prevent spoilage by microorganisms.

1.3: Many variations of the foregoing scheme can be devised. For S.I4. i one example, addition of a small amount of halide salt to the 5 glucose-glucose oxidase leach solution would produce an aqueous solution of the halogen, which could attack gold, platinum group 17 I metals and some sulfide minerals. Addition of glucose-glucose 18 oxidase leach solution to a leach containing a pseudo-halide such as cyanide or thiocyanate, by raising the oxidation potential, would increase the rate of leaching of certain metals or sulfide II minerals.

22 i For another example, while this invention has been described 23 i, in the context of the use of glucose as the sugar acted upon by 24 the oxidase enzyme, other sugars may be employed, converted by enzymes such as invertase when necessary. And while glucose 26 oxidase is readily available in sufficient purity, other oxidase 27 enzymes could be employed, providing they act upon the sugar to 28 yield hydrogen peroxide and an organic acid.

29 31 32 33 8 L 3 I 1 For yet another example, while it is certainly more precise 2 ;i and convenient to use carefully-made and commercially-available 3 1 enzyme powders in these selective leaches, it is possible to 4 I produce the necessary enzyme content in the sugar solution by inoculating it with suitable microorganisms together with added 6 i nutrients as required. This scheme could be economically 7 attractive if the selective leach is to be used for large-scale 8 hI heap leaching rather than for dissolution of gram-size samples.

9 The various features and advantages of the invention are thought to be clear from the foregoing description. Various other 11 features and advantages not specifically enumerated will 2 undoubtedly occur to those versed in the art, as likewise many variations and modifications of the embodiments described, all of 9 I, 1t4 which may be achieved without departing from the spirit and scope "i of the invention as defined by the following claims: 17 18 .9 22 Ii 23 1I 24 27 28 29 1| Ij 31 ii 32 33

Claims (9)

1. A method of selectively leaching geological materials to dissolve oxides and therein-contained metals, comprisi-g contacting said materials with a leach solution comprising water, a sugar, and a first enzyme which can act upon said sugar to produce an organic acid and a low concentration of hydrogen peroxide, the concentration of said sugar in said solution being in the range 0.0001% to 20%, and the concentration pf said first enzyme in said solution being in the range of 0.0001% to and separating said leach solution from said materials. e •i S. 2. A method according to claim 1 wherein said selective leach solution hr a sugar concentration of 1%.

3. A method according to claim 1 or claim 2 wherein said eeeoe: selective leach solution has a first enzyme concentration o o of 0.03%.

4. A method according to any one of claims 1 to 3 wherein said selective leach solution additionally comprises a second enzyme which can act upon said hydrogen peroxide to produce water and oxygen, the concentration of said second enzyme in said solution being in the range of 0.001% to 0.10%. 10 W L. A method according to claim 4 wherein said second enzyme comprises catalase.

6. A method according to any one of claims 1 to 5 wherein said sugar comprises glucose.

7. A method according to any one of claims 1 to 6 wherein said first enzyme comprises glucose oxidase.

8. A method according to any one of claims 1 to 7 wherein said selective leach solution additionally comprises a reducing organic acid, the concentration of said reducing acid in said solution being in the range 0.01% to

9. A method according to any one of claims 1 to 7 wherein said selective leach solution additionally comprises a small amount of a halide salt. A method according to any one of claims 1 to 7 wherein said selective leach solution contains a pseudo-halide.

11. A method according to any one of claims 1 to 6 wherein said selective leach solution includes a first enzyme provided by action of microorganisms in said solution. 11

12. A method according to claim 1 substantially as herein described in relation to the described examples. DATED this EIGHTEENTH day of NOVEMBER 1992 JOHN R CLARK Applicant. WRAY ASSOCIATES, Perth, Western Australia, Patent Attorneys for the Applicant. o o 12