Method of mineral leaching

Abstract

A method of dissolution of minerals in acid is disclosed. The method comprises providing minerals to be leached in an aqueous solution, supplying acid or an acid precursor to the aqueous solution, thereby forming a reaction mixture comprising acid; supplying energy in the form of a combination of high-voltage electric pulses and ultrasound pulses to the reaction mixture to enhance dissolution of the minerals.

Claims

1. A method of dissolution of minerals in acid comprising providing minerals to be leached in an aqueous solution, supplying an acid or an acid precursor to the aqueous solution, thereby forming a reaction mixture comprising the acid; and supplying energy in the form of a combination of electric pulses of 1000-5000V and ultrasound to the reaction mixture to enhance dissolution of the minerals.

2. The method of claim 1, wherein the minerals are silicates.

3. The method of claim 1, wherein the acid is carbonic acid.

4. The method of claim 3, wherein the method comprises supplying CO.sub.2 gas as the acid precursor to the aqueous solution, thereby forming a reaction mixture comprising carbonic acid.

5. The method of claim 4, wherein the CO.sub.2 gas originates from combustion of fossil fuel, cement production, steel production or roasting of ores.

6. The method of claim 5, wherein the purity of the supplied CO.sub.2 gas is from 80 to 100 volume %.

7. The method of claim 1, wherein the ultrasound pulses are supplied at a frequency of 10-50 kHz.

8. The method of claim 1, wherein the electric pulses are supplied at a frequency of 0.05-5 Hz.

9. The method of claim 8, wherein the electric pulses are supplied with a frequency of 0.1-0.2 Hz.

10. The method of claim 1, wherein between 400-1100 kJ ultrasound energy per kg solids per minute is supplied.

11. The method of claim 1, wherein between 200-400 kJ high voltage energy per kg solids per minute is supplied.

12. The method of claim 1, wherein in a subsequent step the dissolved minerals are extracted from the mixture.

13. The method of claim 1, wherein the mineral contains sulphide ores and the leaching process further comprises use of microbes.

14. The method of claim 1, wherein the electric pulses are applied in intervals of 5-100 seconds with interval breaks lasting 10-120 seconds.

15. The method of claim 1, wherein the method is performed at room temperature.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will be described in further detail we reference to the enclosed figures, wherein

(2) FIG. 1 schematically illustrates the leaching of olivine in carbonic acid under influence of ultra sound;

(3) FIG. 2 shows a flow chart for an embodiment of the present invention;

(4) FIG. 3 illustrates graphically the electric effect supplied by ultra sound (US) and high voltage (HV) pulses as a function of time; and

(5) FIG. 4 illustrates an embodiment of a system adapted to perform the present invention.

PRINCIPAL DESCRIPTION OF THE INVENTION

(6) The present invention provides a method which improves the efficiency of mineral leaching with a acidic leaching agent. In a preferred embodiment the leaching agent is carbonic acid.

(7) The present invention involves the use of two types of electric pulses to activate the surfaces of the grains for the leaching which again enhances the leaching rate in order to get valuable products from the leaching.

(8) The two types of electric pulse technologies that are utilized in this invention are: Ultrasound (E-Pulse US) Discharge of high voltage in milliseconds (E-Pulse HV)

(9) Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. For industrial purposes it is being used in cleaning surfaces (lenses, jewelries, surgical instruments, dental instruments etc.). It is also used for disintegration of organic cells e.g. bacteria and can be effective in cleaning sewer water for bacteria. Ultrasound can be applied continuously or in pulses, where the latter is used in this invention.

(10) FIG. 1 illustrates schematically the leaching process of olivine in carbonic acid. During the leaching an alteration zone of secondary minerals will form on the outer surface of the olivine particle. The alteration zone slows down the leaching process as the alteration results in at least a partial passivation. Supplying ultrasound pulses to the leaching mixture results in abrasion especially of the alteration zone, and reduces the precipitation of secondary minerals on the surface of the reacting minerals. Further the ultrasound pulses result in fracturing of the alteration zone and possibly the fractures reach into the non-altered mineral and increases the surface area available for reactions. Additionally the ultrasound pulses provide stirring and circulation of the leaching solution. Stirring may be necessary to be able to keep the minerals available for the leaching agent. In the present invention the ultrasound can be used both as a stirrer and for increasing the available surface area for mineral reactions.

(11) The E-Pulse HV gives high energy input onto the rock particles at pulses which results in defragmentation of the particles based their weakest bonds. The weakest bonds in a rock are commonly at the grain boundaries, but it may also be at cleavage planes, depending upon the rock particle see US2010/0025240 or WO97/10058.

(12) Some silicate minerals (olivine, pyroxcene, anorthite) have a high initial dissolution rate, however, this slows down quickly, possibly due to silica precipitation (together with clay-mineral formation) on the surface of the mineral grains. High voltage electric pulses can defragment a rock along mineral grains (US2010/0025240), resulting in electric pulse pulverization. According to the present invention lower intensive energy (ultra sound or lower intensity of the high voltage electric pulse) is able to maintain mineral surfaces free of secondary precipitation resulting in maintaining high mineral reactivity.

(13) FIG. 2 is a schematic overview of an embodiment of the present invention employed to leach olivine. Olivine ((Mg,Fe,Ni).sub.2SiO.sub.4) carbon dioxide and water are added to a reaction chamber 10 where the mixture is subject to pulses of ultrasound (US) and high voltage energy pulses (HV). The process is a bulk process with integrated pulverization and dissolution. After an applicable retention and/or reaction time the leaching mixture containing all reaction products, such as a mixture of carbonate, SiO.sub.2, etc. is transferred to a separation section 20. Here the different products are separated by conventional separation techniques, including mechanical, chemical and electrical techniques. In the illustrated embodiment 4 product fractions are obtained, 22 magnesite (MgCO.sub.3), 24 Silica (SiO.sub.2), 26 nickel (Ni), and 28 ferrihydrite (Fe(OH).sub.3).

(14) The energy of the single ultrasound pulse is much less than the single E-Pulse HV pulse delivered to the mineral sample, however, the US pulses are more frequent than the E-Pulse HV pulses. FIG. 3 is provided to illustrate this, however it should be noted that this figure is not to scale. The ultrasound (US) pulses are generally applied at a frequency from 10 to 100 kHz, in one embodiment from 15 to 50 kHz, or in another embodiment at 15-25 kHz and preferably at around 20 kHz. The amplitude of the US may in one embodiment be between 21-145 m and a power of 400 watt. The ultrasound pulses applicable for this purpose have a power of from 0.2 to 10 kW, in another embodiment between 0.3 and 5 kW, more preferably between 0.4 and 2.5 kW.

(15) The energy of each of the high voltage (HV) E-pulses is generally considerably higher than the effect of each US-pulse, often from 10 to 1000 times higher. The energy supplied by the high voltage pulses is between 100 J and 3000 J, preferably 2000 J range more preferably 3000 J. The pulsing regime may in one embodiment consist of pulse sequence of 20 pulses (E-Pulse HV) delivered during 5-100 seconds, preferably 10-50 seconds at a frequency of 0.1-0.2 Hz and then an interval/break of 10 to 120 seconds, preferably from 20 to 60 seconds before the pulse sequence is repeated.

(16) There are several geochemical processes going on in the process. First of all, continuously bobbling 100% CO.sub.2 gas into the leachate results in water becoming saturated in dissolved CO.sub.2. Dissolved CO.sub.2 forms carbonic acid according to the following reaction (Eq. 1):
CO.sub.2(g)+H.sub.2O.fwdarw.H.sub.2CO.sub.3(Eq. 1)

(17) The introduction of acid and carbonate in the leachate increases the solubility/reaction rate of olivine and the following reaction may take place.
Fe.sub.0.4Mg.sub.1.6SiO.sub.4+4H.sup.+.fwdarw.0.4Fe.sup.2++1.6Mg.sup.2++H.sub.4SiO.sub.4(aq)(Eq. 2)

(18) Elements substituting for iron and magnesium in the silicates will also be released as iron and magnesium are released. These elements can be removed from the solution by different processing methods (e.g. solvent-extraction electro-winning for copper and nickel). Iron occurs as ferrous iron in the silicate minerals will be oxidized to ferric iron and precipitate as iron-hydroxide according to the following reaction (Eq 3):
2Fe.sup.2++5H.sub.2O+O.sub.2.fwdarw.2Fe(OH).sub.3+4H.sup.+(Eq. 3)

(19) This iron oxidation is relatively slow with a half-life of 2-3 days depending on the oxidizing conditions. The precipitation of ferrihydrite is pH dependent and with a pH lower than approximately 3, there will be little precipitation, while at pH 5 there will be little or no iron in solution.

(20) Serpentine, as a secondary mineral, has been observed on surfaces of the olivine (in a reaction rim). This may be formed as an incongruently from olivine according to the following reaction (Eq. 4).
10Fe.sub.0.2Mg.sub.1.8SiO.sub.4+O.sub.2+12H.sup.++9H.sub.2O.fwdarw.2Fe(OH).sub.3+4Mg.sub.3Si.sub.2O.sub.5(OH).sub.4+6Mg.sup.2++2H.sub.4SiO.sub.4(aq)(Eq. 4)
Olivine+oxygen+water+acid ferrihydrite+dissolved iron+serpentine+dissolved magnesium+siliceous acid

(21) The CO.sub.2 saturation will result in a pH of approximately 4.0-4.2. Silicate-mineral dissolution can drive the pH to 9-10 depending on how effective the input of carbonic acid is and the reaction rate of the silicate minerals and the type of secondary mineral reactions. Rapid reaction rate will result in saturation of silica and precipitation of hydrous silica. This hydrous silica can also form the basis for a clean product e.g. used in the high strength cement.

(22) The leaching of olivine has been used here as an example of a mineral applicable for the process, however a person skilled in the art will appreciate that a large number of single minerals and combination of minerals can be effectively dissolved by the present method.

(23) Calcite easily dissolves is acid and a clean product can be produced by dissolving high-grade limestone and selectively precipitate calcium carbonate. Calcite dissolves also easily in carbonic acid. This dissolution process is also strongly affected by use of ultrasound. Calcite can be selectively dissolved from dolomite and graphite impurities in limestone enhanced by use of ultrasound. High-grade calcium leachate will then form the basis for calcium carbonate precipitation by raising the pH.

(24) FIG. 4 shows a possible configuration of a system for performing the present invention. A mixture of the mineral to be leached, water, CO.sub.2 and any other additives ligands or salts are added as stream 1 to a tank 6. For simplicity the different components are added as one stream through one inlet, but it is equally applicable to supply the components through separate inlets. A stirring device may be included in the tank 6 to keep the mixture homogenised. A pump 8 transports the mixture into a reaction chamber 10 which may be configured as a flow cell. An ultrasonic generator 40 is supplied with power from the power unit 30. The ultrasonic generator is connected to an ultrasonic transducer 42, which supplies US-pulses to the reaction mixture present in the reaction chamber 10. A power unit 32 supplies electric energy to a high voltage pulse generator 46. The generator 46 is electrically connected to two electrodes arranged with in the reaction chamber through the wires 47 and 48. The electrodes are supplied with high voltage electricity from the generator 46 in applicable pulse intervals. The configuration of the electrodes with in the reaction chamber is not disclosed in detail, but these are well known to a person skilled in the art and art discussed for instance in US2010/0025240 and WO97/10058.

(25) After the reaction mixture has been subject to HV and US energy pulses according to the present invention the mixture is via valve 16 and pipeline 18 introduced into a second tank 20 from where the reacted mixture may proceed through valve 21 as reacted mixture 22 to further downstream separation steps. The valve 16 and the pump 8 control the pressure within the reaction chamber. If the pressure is to lower the dissolved CO.sub.2 could be released from the liquid phase which could result in increased pH with the reaction chamber.

(26) Sulphide mineral leaching is becoming more and more important as the basis for extracting metals such as gold, silver, nickel, copper, zinc, and lead. The leaching process is taking place either in large leach dumps or in enclosed reaction vessels. The sulphide dissolution is primarily based on sulphuric acid leaching enhanced by microbes.
FeS.sub.2(s)+15/4O.sub.2+7/2H.sub.2O.fwdarw.Fe(OH).sub.3(s)+4H.sup.++2SO.sub.4.sup.2(Eq. 5)

(27) pH, microbes, iron content, temperature, available surface, oxygen availability, and other elements in the leachate control these reactions. The sulphide leaching can be combined with supplying CO.sub.2 as discussed above, thereby a synergy effect is obtained as the acidity influences the leaching and the presence of CO.sub.2 additionally has a positive influence on the microbiological processes.

EXAMPLES

(28) Several experiments have been performed with E-pulse US (Branson Sonifier ultrasonic cell disruptorANALOG UNITS Models S-450A) and E-Pulse HV (designed by Dr. Richard Bialecki). Crushed dunite with +95% olivine was used in grain sizes of less than 63 mesh, 63-250 mesh, 250-500 mesh and +500 mesh. About 20 grams testing material was used. The frequency used for E-pulse US was 20 kHz. The E-Pulse HV converted 230 volts to 3-5000 volts with discharge frequency 100-200 kHz. Duration of the E-Pulse US was approximately 20-30 minutes, while the E-Pulse HV was 20 pulses of 5 seconds with 10 seconds between the pulses. CO.sub.2 was bobbling through the reaction vessel continuously during the experiments.

(29) Analyzing the leachate from the experiments, secondary mineral precipitates, and mineralogical changes by sequential chemical extraction checked results of the leaching experiments. The pH of the reaction vessel with 1 liter de-ionized water, only with constant flow of CO.sub.2 in water was 4.0-4.2, with the 20 gram samples pH increased to 4.4-4-6, while pH increased to 5.0-5.4 when E-pulse was applied. Temperature increased with 15 C. using the ultrasound, while E-pulse HV did not effect the temperature of the leachate.

(30) Magnesium in the leachate increased from below detection limit (approximately 0.2 mg/l) to 30-40 mg/l in the most effective reaction experiments. Iron content also increased drastically from below detection limit to at the most 20 mg/l. Iron hydroxide precipitation took place in most of the experiments with E-Pulse. Secondary white mineral precipitate where observed during the experiments identified by XRPD/SEM as likely silica.

(31) Similar experiments with shorter duration using E-Pulse US have also been performed on limestone also giving an effective dissolution of calcite compared to only bobbling CO.sub.2 into the reaction vessel.