WO2013021244A1

WO2013021244A1 - A methodology to determine collector adsorption on sulphide minerals using electrochemical impedance spectroscopy analysis

Info

Publication number: WO2013021244A1
Application number: PCT/IB2011/053573
Authority: WO
Inventors: Zafir EKMEKCI; Esra TEKES BAGCI; Kadir PEKMEZ; Metin Can
Original assignee: Ekmekci Zafir; Tekes Bagci Esra; Pekmez Kadir; Metin Can
Priority date: 2011-08-10
Filing date: 2011-08-10
Publication date: 2013-02-14

Abstract

The present invention relates to a methodology to measure floatability of sulphide minerals in froth flotation plants based on density of collector adsorption on different mineral phases on mineral electrodes (2) using three electrode (2) electrochemical impedance spectroscopy (EIS) analysis using different size fraction mineral electrodes (2) as working electrodes (2). The electrode assembly 10 was designed in three parts, mineral-graphite pellet electrode (2), copper wire (4) connection and Teflon body (7) by mixing mineral particles and graphite in the ratio of 2:1. A conductive binder (3) is also added to the mixture to increase the strength of the pellet and the mixture is pressed in a die under 200 kN pressure. Steps of mineralogical analysis of electrodes (2), derivation of a surface coverage 15 calibration curve for different collector concentrations, determination of equivalent electrical circuit model, EIS measurements in a measurement medium and flotation rate constant or copper recovery prediction with respect to collector adsorption, are conducted.

Description

A METHODOLOGY TO DETERMINE COLLECTOR ADSORPTION ON SULPHIDE MINERALS USING ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY ANALYSIS

Field of the Invention The present invention relates to a mineral electrode fabrication technique and a methodology to measure floatability in froth flotation plants based on density of collector adsorption on mineral electrodes using three electrode electrochemical impedance spectroscopy analysis. Background of the Invention

It is known that there are methods and models to estimate floatability in froth flotation plants and correlate this data to the flotation performance of a specific mineral. For example Gorain et. al. have developed the following model which defined flotation rate constant (k) as a function of floatability (P), bubble surface area flux (Sb) and froth recovery (Rf). k = P * 5_S * R_f Later, Savassi (1998) has elaborated the above model much further by defining recovery as a function of particle floatability (P), bubble surface area flux (Sb), residence time (τ), water recovery (Rw), degree of entrainment (ENT), and froth recovery (Rf), for a given size liberation (Vianna,2004).

(P _fS_bxR - - R_w) + ΕΝΤ^

A series of methods have been developed to measure parameters in Savassi's model such as Sb, R_f, ENT however, "P paremeter" is still predicted or back calculated further to a series of standard batch flotation tests. These batch tests are generally conducted by sampling and re-floating pulp samples from the flotation feed, concentrate and tailing of a flotation plant, in laboratory at very shallow froth depth. Kinetic flotation test is performed for this purpose. (Alexander and Morriron, 1998).

In addition to batch scale flotation tests, microflotation tests, bubble induction time and contact angle measurements are some other tests used today to determine hydrophobicity of the mineral particles. In microflotation test small amount of sample (2 gr) is used and the results are analyzed based on flotation rate and recovery. In contact angle measurement the hydrophobicity is directly related to the quantity of contact angle measured mainly in two different techniques; capillary rise and bubble contact methods. In bubble induction method, the time required for attachment of a bubble to a particle is measured with a special device. Adsorption of collector on mineral surfaces is measured by a number of surface analytical techniques such as FTIR, Raman Spectroscopy, ToF-SIMS. In addition to that adsorption of collector can also be determined indirectly by residual analysis of collector in solution. UV, HPLC techniques are used for this purpose. Currently, collector coverage is measured quantitatively by two methods, namely, "residual analysis" and "extraction from mineral surface". In residual analysis and extraction techniques, spectroscopic methods (UV visible spectroscopy, FTIR, HPLC, ToF-SIMS) are used to measure the collector concentration in solution. Batch flotation test among these is a labor intensive and expensive method. Each time the pulp chemistry or the particle size is changed, batch flotation tests must be performed and the products (feed, concentrates and tailings) should be analyzed on a size by size basis. In microflotation methods on the other hand, contact angle and bubble induction time techniques are not comparable with the industrial scale operations and can be considered as qualitative procedures. Furthermore, these methods require stable conditions and mostly restricted to laboratory use.

In neither "residual analysis" nor "extraction from mineral surface" methods direct measurement of collector adsorption density is possible. In residual analysis method, it is assumed that all the collector ions are adsorbed on minerals and also decomposition and precipitation are not taken into account. Extraction technique is more sensitive but in this method it is assumed that collector ions are adsorbed on the valuable mineral and not on the other minerals. Besides, in order to determine collector adsorption density for each size fraction, every sample must be sized and also chemical and mineralogical analysis of each size fractions must be done. In addition, requirement for special sample preparation techniques and conducting ex-situ experiments are the disadvantages of these methods under consideration.

Other methods to measure the floatability of the minerals include measurement of the sample properties such as collector concentration, pH and electro-chemical potential of the system. Also electrochemical impedance spectroscopy (EIS), using different frequencies, is applied in relation with these other data using a calomel reference electrode, a platinum counter electrode and working electrodes generally produced from similar minerals of the pulp to be floated. These properties of the pulp are then correlated with the floatability of the mineral and necessary adjustments are performed to the slurry to improve floatability of a specific mineral. There are also some other sensitive surface analytical techniques to determine collector adsorption, but they are limited to laboratory applications and are mostly qualitative. In electrochemical experiments, mineral electrodes are usually fabricated by using mineral sections of rectangular or cylinder shape cut from high purity specimens. However, it is usually hard to find such mineral specimens from every deposit. Therefore, mineral electrodes from different deposits are used to estimate electrochemical response of an ore during flotation. This method leads usually to erroneous results due to the differences in mineralogy and crystal structure of the same mineral type from different deposits.

Previous research works have shown that floatability is proportional with degree of hydrophobicity, which is directly related to the surface coverage of collector (Vianna, 2004). However, hydrophobicity of a mineral is influenced not only by collector dosage but also many other surface and chemical factors, namely degree of oxidation, type of flotation reagents, pulp chemistry and electrochemistry. Therefore, influence of these parameters, in other words, influence of pulp chemistry on flotation performance can be monitored by measurement of degree of hydrophobicity of mineral classes. Although, there have been many works to measure hydrophobicity of mineral surfaces, many of them are again limited to laboratory scale and isolated systems. However, a flotation pulp is a complex and dynamic system, which requires in-situ measurement of degree of hydrophobicity of various minerals and particle types.

The United States patent document US5108495, an application in the state of the art, discloses a method for controlling a process by means of the electrochemical potential in which electrodes made of similar materials to be treated in flotation. Impedance measurement was conducted between a specific voltages range at one particular frequency and measured capacitance and resistance values in this voltage range.

The said method resembles the present invention in that it can perform an impedance analysis to analyze surface properties. But the present invention differs from the said method in terms of the method of impedance analysis, the electrodes used and the ability of measuring collector adsorption on the electrodes.

The United States patent document US4561970 discloses a froth flotation process in which conditions like collector concentration, electrochemical potential and pH are calculated in advance for the said mineral and collector which can form stable surface compounds.

The said method again uses standard electrodes and does not directly give information on collector adsorption.

Summary of the Invention

An objective of the present invention is to provide a methodology which measures floatability of sulphide minerals in froth flotation plants based on density of collector adsorption on different mineral phases using electrochemical impedance spectroscopy.

Detailed Description of the Invention

An apparatus realized to fulfil the objective of the present invention is illustrated in the accompanying figures, in which:

Figure 1 is the section view of copper head.

Figure 2 is the exploded perspective view of the working electrode assembly. Figure 3 is the section view of the working electrode assembly.

Figure 4 is the polished surface of a chalcopyrite electrode pellet (-20+9 μιη). Figure 5 is the equivalent circuit which was used to fitting.

The components illustrated in the figures are individually numbered where the numbers refer to the following:

1. Mineral electrode assembly

2. Mineral pellet electrode

3. Conductive silver binder

4. Copper wire

5. Copper head 6. Screw head

7. Teflon tubular body

The methodology to determine collector adsorption on sulphide minerals using three electrode electrochemical impedance spectroscopy analysis (EIS) comprises the following steps,

- pressing of electrodes (2) to be used in the measurement using the plant mineral feed of different size fractions, graphite powders and conductive binders (3),

mineralogical analysis and surface characterisation of the said mineral electrodes (2) by optical microscopy or MLA, deriving a surface coverage calibration curve for the EIS analysis using electrode assemblies (1) comprising;

- the said mineral electrodes (2),

- copper wires (4),

- Teflon bodies (7)

as the working electrodes (2) and by measuring surface coverage for different collector concentrations,

determination of equivalent electrical circuit model for three electrode (2) electrochemical impedance spectroscopy analysis (EIS) using the aforementioned electrode (2) assemblies (1) as the working electrodes (2),

immersing the said electrodes (2) into a conditioner, flotation cell, solution or flotation pulp for which the collector adsorption will be determined (will be called "measurement medium" from now on) and then performing EIS measurements at a certain frequency range.

- predicting flotation adsorption or surface coverage using the calibration curve and equivalent electrical circuit model. Mineral electrodes (2) were employed for EIS measurements. But, in spite of pure single specimen mineral electrodes (2), mineral electrodes (2) made of mineral particles of different size fractions was used in this methodology. This electrode assembly (1) was fabricated in five parts (copper head (5), conductive silver binder (3), mineral pellet electrode (2), copper wire (4) and a teflon body (7)) and designed in order to investigate the surface of mineral pellet (2) under optical microscope or MLA for mineralogical analysis. Therefore, a new mineral electrode (2) was designed to use the feed material of a flotation plant. The electrode assembly (1) was designed in five parts, copper head (5), conductive silver binder (3), mineral-graphite pellet electrode (2), copper wire (4) connection and Teflon body (7). To fabricate mineral pellet electrode (2), mineral particles and graphite was mixed in the ratio of 2: 1. Graphite powder is used as the conductive matrix in the pellet (2), as the interaction between collectors and graphite is negligible. A conductive binder (3) is also added to the mixture to increase the strength of the pellet (2). The mixture was pressed in a die of 14 mm diameter under 200 kN pressure. A cylinder shape sample of about 15 mm height was produced and copper head (5) was pasted to the pellet (2) via conductive silver epoxy. A copper wire (4) is screwed to the copper head (5) section whenever EIS measurements were performed by a screw head (6). The pellet (2) and the copper wire (4) are mounted in a Teflon tube (7) (Figure 1 and Figure 2). The pellet section (2) can be removed before or after the measurements for mineralogical and surface characterisations.

With this design, surface of the pellet (2) can be ground and polished for mineralogical analysis to determine the mineral distribution, their surface area and liberation state under optical microscope or MLA. A chalcopyrite mineral electrode (2) of -20+9 μιη size fraction is shown as an example in Figure 3. Chalcopyrite particles mounted in graphite matrix are clearly identified and their surface area, which is required for calculation of density of collector adsorption, is measured. Chemical and mineralogical analysis of the feed sample will be performed only during the electrode (2) preparation. In order to express density of collector adsorption in terms of fractional surface coverage of collector, a calibration curve is derived for each electrode (2) by changing the collector dosage in the solution from nil to excess values. The experiments are performed in a conventional three electrodes (2) electrochemical cell in a buffer solution. pH of the buffer solution is chosen according to the pulp pH in the flotation plant. Saturated calomel electrode (2), a platinum plate electrode (2) and a mineral electrode assembly (1) which is fabricated with plant ore as mentioned above is used as reference, counter and working electrodes (2) respectively. EIS measurements are taken at minimum four collector

-3 -3

concentrations ranging from nil to 10^" M. 10^" M collector dosage can be assumed as an excess concentration for most of sulphide ore flotation applications and represent monolayer coverage of the surface. Therefore, the calibration curve is derived as a function of resistance or capacitance to estimate surface coverage of collector on fractional basis.

EIS data of the experiments are fitted to an equivalent electrical circuit model to calculate resistance and conductance of the electrical double layer. Different electrical circuit models may be used for different minerals and collector types, depending on the surface characteristics of the mineral and adsorption mechanism of the collector.

For chalcopyrite minerals the fractional surface coverage of collector is calculated with the following equation when resistance values are used: Equation 1;

Where;

R_cti : Resistance of surface at given concentration (ohm) Rcto : Resistance of surface at zero concentration (ohm)

Rct∞ : Resistance of surface at infinite concentration (ohm)

The mineral electrode assembly (1) together with counter and reference electrodes (2) is immersed into the solution or the flotation pulp for which the measurement will be performed and it is polarized for 10 minutes in order to enhance adsorption of collector to the mineral surface. Meanwhile, the solution is mixed to ensure that the collector is dispersed homogenously. Further to conditioning, mineral electrode (2) is taken out of the pulp and placed in an electrochemical cell containing buffer solution of a certain pH. Thus, the impedance is measured at a stable environment. Impedance measurements are taken at open circuit potential, without applying any potential to the surface. EIS measurement can be repeated each time the flotation conditions are changed. In this methodology, the measured resistance and capacitance values are used as indicators of collector adsorption. Adsorption of collector increases the polarization resistance of the surface by creating an electrochemically passive surface layer. Capacitance is inversely proportional with the adsorption of the collector. Since most of the sulphide minerals are semiconductors and the mineral-collector interaction is electrochemical in nature, adsorption of collectors can be measured by electrochemical methods. Electrochemical Impedance Spectroscopy (EIS) technique measures the changes occurring in the electrical double layer of solid electrodes (2) and expresses the changes in terms of resistance and conductance. It is a practical technique and can measure surface coverage in-situ. Thiol collectors adsorb on sulphide mineral surface by different adsorption mechanisms. EIS enables the measurement of density of collector adsorption independent of collector adsorption mechanisms. It also enables the measurement of collectors adsorbed on surfaces of highly resistive or poor conductive minerals, like sphalerite. With this invention, mineral electrodes (2) are fabricated by using the ore from a flotation feed on a size by size basis. Therefore, each size fraction in the flotation feed will be simulated by the corresponding electrode (2). EIS measurements will be performed by using these electrodes (2). Therefore, density of collector adsorption is measured on a size by size basis. At this stage, it is assumed that the collector(s) is adsorbed on the floatable mineral surface. Therefore, the EIS data is correlated with the floatability of one mineral, depending on the chemistry of the flotation system. Therefore, chemical and mineralogical analysis of the feed sample will be performed only during the electrode (2) preparation, and EIS measurement can be repeated each time the flotation conditions are changed. This method has the potential for on-line measurement and control of the pulp chemistry.

Case Study-1 (Laboratory floation experiments)

The methodology was tested in laboratory batch flotation tests with pure chalcopyrite mineral electrode (2) (-150 +75 μιη fraction). Cayeli Cu-Zn ore (Rize,Turkey) ore was used in the flotation experiments. The mineral electrode (2) was placed into flotation cell and conditioned for 3 minutes starting with the addition of the collector. After the conditioning, the electrode (2) was taken out from cell and placed into the three electrodes (2) conventional electrochemical cell containing pH 9.2 buffer solution and impedance measurements were taken. Flotation of the ore, one rougher and two cleaner stages, was performed at the same time with impedance measurement. Two stage cleaning was performed to reject the particles recovered by entrainment and determine roughly the recovery by true flotation. Three different concentrations (30 g/t, 50 g/t and 100 g/t) were tested. Di iso buthyl dithiphosphinate (DTPI) was used as collector. The calibration curve was derived previously at different DTPI concentrations and surface coverage of the collector " Θ " was plotted against the resistance at different concentrations (Graph 1 and Graph 2).

O.E+00 1 . E-04 2.E-04 3. E-04 4.E-04 5.E-04 6.E-04

DTPI Concentration (M)

Graph 1: The calibration curve of chalcopyrite electrode (2) in different DTPI concentrations.

140 160 180 200 220

Z real (ohm)

Graph 2: The fractional surface coverage of chalcopyrite electrode (2).

After Z_reai values were determined, Θ can be calculated for each DTPI concentration value (Graph 2). Cu recovery of the flotation tests was also calculated. Graph 3 shows that Θ is directly proportional to Cu recovery and DTPI concentration. This work showed that EIS measurements can be used to determine Θ of chalcopyrite particles and flotation recovery of copper minerals predicted usin the relationship between Θ and Cu Recovery.

0 0.2 0.4 0.6 0.8 1

Fractional Surface Coverage

Graph 3: The corelation between fractional surface coverage of Cp electrode (2) and Cu recovery of Cayeli ores.

Case Study 2 (Plant measurements) Further to batch flotation experiments, impedance method and mineral electrode (2) were tested in Cayeli Cu-Zn Flotation Plant (Rize, Turkey). Two types of measurements were performed;

EC Cell: The mineral electrode (2) was conditioned in the conditioner tank for 5 minutes and the EIS measurement was performed in a three electrode (2) electrochemical cell at pH 9.2 buffer solutions immediately after the conditioning.

In-Situ: Both conditioning and EIS measurement were performed in the conditioner.

A compact three electrode system was designed and manufactured for plan scale measurements using the mineral electrode assembly (1) as the working electrode (2). A graphite rode was used as the counter electrode (2) and a silver foil coated with AgCl was used as the reference electrode (2). The reference electrode (2) was mounted in a Teflon tube (7) and protected from the particles by a glass frit. The impedance measurement was fitted to the equivalent circuit model shown in Figure 4 and the resistance value was calculated. Pure chalcopyrite mineral electrode (2) and plant ore electrode (2) were used as three different size fractions (+36, -36+20, -20+9μ). DTPI is used as a collector in Cayeli Copper Plant. As it was mentioned above, calibration curves were derived in the laboratory for each electrode (2) used in the plant measurements. The calibration curve of chalcopyrite mineral electrode (2) for +36 μιη fraction and the values measured in the plant with two methods are plotted in Graph 4.

O.OE+00 5.0E-04 1.GE-G3 1.5E-03

DTPI {M)

Graph 4: Calibration curve for chalcopyrite electrode (2) of +36 μιη fraction and the plant measurements. The resistance in the adsorbed layer increased as a function of collector dosage, indicating higher amount of collector adsorption. The collector dosage in the plant was calculated as 3.41xl0^"5 M and the measured Ret values were plotted for this concentration in the same graph. The plant data is in correlation with the calibration curve. In the next step, fractional surface coverage of collector (Θ) was calculated using the equation (1) for a given collector concentration. The relationship between Ret and fractional surface coverage of DTPI is illustrated in Graph 5. The results showed that the fractional surface coverage for chalcopyrite particles coarser than 36 μηι was found to be 0.26 with EC Cell method and 0.40 with in-situ method. The same experiments were also performed with the other electrodes (2) (-38+14, -14+8). The calculated fractional surface coverage of DTPI on cayeli electrodes (2) of different size fractions is illustrated in Graph 6.

1.2

Cp (+36 μιη)

1 w 0.8

0.6

> Calibration

O

u

w 0.4 0.40 EC Cell

(tf 0.26

¾ IrvSitu

0.2

0

1000 1500 2000 2500 3000

Ret {ohm}

Graph 5: Fractional surface coverage as a function of charge transfer resistance at adsorption layer for chalcopyrite +36 μιη electrode (2).

Graph 6: Calculated fractional surface coverage of DTPI on Cayeli electrodes (2) of different size fractions. In parallel to EIS measurements in the plant, batch flotation tests were performed using Cu Rougher Feed from the plant to determine floatability of copper minerals (Alexander and Morrison 1998). Flotation rate constant of copper minerals was calculated by using the first order rate equation. The relationship between surface coverage of collector and flotation rate constant is shown in Graph 7.

K4 (I S

Surface coverage o/D TPI

Graph 7: The relationship between surface coverage of collector and flotation rate constant. The results showed clearly that floatability of copper minerals can be measured rapidly and precisly using the methodology developed in this work.

Within the scope of these basic concepts, it is possible to develop a wide variety of embodiments of the inventive "A methodology to determine collector adsorption on sulphide minerals". The invention cannot be limited to the examples described herein; it is essentially according to the claims.

Claims

A methodology to determine collector adsorption on sulphide minerals in a flotation plant using three electrode (2) electrochemical impedance spectroscopy analysis (EIS) characterized by the steps of;

fabrication of electrodes (2) to be used in the measurement using the plant mineral feed of different size fractions, graphite powders and conductive binders (3),

mineralogical analysis and surface characterisation of the said mineral electrodes (2) by optical microscopy or MLA,

deriving a surface coverage calibration curve for the EIS analysis using electrode (2) assemblies (1) comprising;

- the said mineral electrodes (2),

- copper wires (4) and

- Teflon bodies (7)

immersing the said electrodes (2) into a conditioner, flotation cell, solution or flotation pulp for which the collector adsorption will be determined (will be called "measurement medium" from now on) and then performing EIS measurements on a certain frequency range,,

predicting flotation rate constant or copper recovery with respect to collector adsorption or surface coverage using the calibration curve and equivalent electrical circuit model.

2. A methodology to determine collector adsorption on sulphide minerals in a flotation plant using three electrode (2) electrochemical impedance spectroscopy analysis according to Claim 1, characterized in that mineral particles and graphite is mixed in the ratio of between 1 : 1 and 3: 1 to form mineral electrodes (2) .

3. A methodology to determine collector adsorption on sulphide minerals in a flotation plant using three electrode (2) electrochemical impedance spectroscopy analysis according to any of the preceding claims, characterized in that mineral feed, graphite powders and conductive binders (3) are pressed in a cylindrical die of between 13 and 14 mm diameter and 14 to 16 mm height under between 150 to 250 kN pressure.

4. A methodology to determine collector adsorption on sulphide minerals in a flotation plant using three electrode (2) electrochemical impedance spectroscopy analysis according to any of the preceding claims, characterized in that the mineral pellet electrode (2) of the electrode assembly (1) is detached from the copper wire (4), which was pasted via conductive silver epoxy, before or after the measurements for mineralogical and surface characterisations to determine the mineral distribution, their surface area and liberation state.

5. A methodology to determine collector adsorption on sulphide minerals in a flotation plant using three electrode (2) electrochemical impedance spectroscopy analysis according to any of the preceding claims, characterized in that the copper wire (4) is screwed to the mineral electrode (2) section by a screw head (6) whenever EIS measurements are performed.

6. A methodology to determine collector adsorption on sulphide minerals in a flotation plant using three electrode (2) electrochemical impedance spectroscopy analysis according to any of the preceding claims, characterized in that the calibration curve is plotted for each electrode (2) as a function of collector concentration versus resistance or capacitance using at least four collector dosage in the solution from zero to 10^" M in a buffer solution pH of which is chosen according to the pulp pH of the flotation plant.

7. A methodology to determine collector adsorption on sulphide minerals in a flotation plant using three electrode (2) electrochemical impedance spectroscopy analysis according to any of the preceding claims, characterized in that the fractional surface coverage (Θ) of collector is plotted and a linear relation with measured impedance is obtained using the equation;

A methodology to determine collector adsorption on sulphide minerals in a flotation plant using three electrode (2) electrochemical impedance spectroscopy analysis according to any of the preceding claims, characterized in that the electrode assembly (1) is placed into the measurement medium and conditioned together with the medium starting with the addition of the collector.

A methodology to determine collector adsorption on sulphide minerals in a flotation plant using three electrode (2) electrochemical impedance spectroscopy analysis according to any of the preceding claims, characterized in that the said electrode assembly (1) is taken out from the measurement medium and EIS measurements are done after placing the assembly (1) immediately in a buffer solution.

10. A methodology to determine collector adsorption on sulphide minerals in a flotation plant using three electrode (2) electrochemical impedance spectroscopy analysis according to claims 1 to 8, characterized in that EIS measurements are performed in the measurement medium during conditioning.

11. A methodology to determine collector adsorption on sulphide minerals in a floatation plant using three electrode (2) electrochemical impedance spectroscopy analysis according to any of the preceding claims, characterized in that flotation rate constant of minerals is calculated by using surface coverage plots.