*Article* **An Investigation of the Adsorption of Xanthate on Bornite in Aqueous Solutions Using an Atomic Force Microscope**

**Jinhong Zhang**

Department of Mining and Geological Engineering, The University of Arizona, Tucson, AZ 85721, USA; jhzhang@email.arizona.edu

**Abstract:** An atomic force microscope (AFM) was applied to study of the adsorption of xanthate on bornite surfaces in situ in aqueous solutions. AFM images showed that xanthate, i.e., potassium ethyl xanthate (KEX) and potassium amyl xanthate (PAX), adsorbed strongly on bornite, and the adsorbate bound strongly with the mineral surface without being removed by flushing with ethanol alcohol. The AFM images also showed that the adsorption increased with the increased collector concentration and contact time. Xanthate adsorbed on bornite in a similar manner when the solution pH changed to pH 10. The AFM force measurement results showed that the probe–substrate adhesion increased due to the adsorption of xanthate on bornite. The sharp "jump-in" and "jump-off" points on force curve suggest that the adsorbate is not "soft" in nature, ruling out the existence of dixanthogen, an oily substance. Finally, the ATR-FTIR (attenuated total reflection-Fourier-transform infrared) result confirms that the adsorbate on bornite in xanthate solutions is mainly in the form of insoluble cuprous xanthate (CuX) instead of dixanthogen. This xanthate/bornite adsorption mechanism is very similar to what is obtained with the xanthate/chalcocite system, while it is different from the xanthate/chalcopyrite system, for which oily dixanthogen is the main adsorption product on the chalcopyrite surface. The present study helps clarify the flotation mechanism of bornite in industry practice using xanthate as a collector.

**Keywords:** flotation; xanthate; adsorption; bornite; cuprous xanthate; AFM; FTIR

## **1. Introduction**

Flotation has been widely studied as the most efficient separation technique in the copper extraction industry. The adsorption of the collector on the mineral surface is vital for a successful flotation process to achieve a recovery. Historically, many works have been carried out to clarify the adsorption mechanism of collectors on sulfides [1–6].

Compared to other copper minerals, such as chalcopyrite and chalcocite, the adsorption of collector on bornite has been rarely studied. Allison et al. [3] studied the reaction products of various sulfide minerals with xanthate solutions. The authors reported that the measured rest potential of bornite in 6.25 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX solution at pH 7 was +60 mV, and the reaction product of PAX on bornite was cuprous alkyl xanthate. Mielczarski and Suoninen [7,8] applied XPS and studied the adsorption of potassium ethyl xanthate on cuprous sulfide. The authors reported that there was a relatively rapid formation of a well-oriented monolayer of xanthate ions followed by a slow growth of disordered cuprous xanthate molecules on top of this layer. Buckley et al. [9] investigated the surface oxidation of bornite by linear potential sweep voltammetry and X-ray Photoelectron Spectroscopy, and proposed the adsorption products on bornite depended on the solution potential. Zachwieja et al. [10] studied the electrochemical flotation of the bornite-ethylxanthate system and reported that KEX reacted with bornite through an electrochemical oxidation reaction, forming cuprous xanthate between −0.4 v and −0.2 v (SCE, saturated calomel electrode). Hangone et al. [11] studied the flotation of a bornite-rich copper sulfide ore using thio collectors and their mixtures, and reported that the highest copper recoveries

**Citation:** Zhang, J. An Investigation of the Adsorption of Xanthate on Bornite in Aqueous Solutions Using an Atomic Force Microscope. *Minerals* **2021**, *11*, 906. https:// doi.org/10.3390/min11080906

Academic Editors: Shuai Wang, Xingjie Wang, Jia Yang and Lev Filippov

Received: 22 July 2021 Accepted: 18 August 2021 Published: 21 August 2021

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**Copyright:** © 2021 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

were obtained with the diethyl dithiophosphate (di C2-DTP). Recently, Dhar et al. [12] investigated the improvement of the copper recovery from Nussir Copper Ore Deposit in Northern Norway, using the blend of xanthate and dithiophosphate as collectors.

These previous studies have revealed a significant amount of information, such as the possible reaction product and reaction mechanism of the adsorption of collectors on the bornite surface. Technically, it is also of great interest to directly obtain the image of the collector on the bornite surface changing with the pulp chemistry, such as the type and dosage of chemicals, solution pH and adsorption time. An autoradiography technique was first applied by Polkin et al. [13] and Plaksin et al. [14] to obtain the images of xanthate radioactive isotopes absorbed on sulfide minerals. The authors reported that there was a mosaic distribution of xanthate collectors on the sulfide mineral surface. Later, scanning tunneling microscopy (STM) was applied by Kim et al. [15] and Smart et al. [16] to collect surface images for the study of the reaction, i.e., oxidation, reaction and absorption, of the galena surface under flotation-related conditions. The reported STM images showed that the pulp chemistry, such as the pH and chemical dosage, impacted the reaction and its products on the galena surface. Recently, the AFM imaging technique has also been successfully applied for the in situ study of the adsorption of chemicals on various mineral surfaces [17–22]. The novel analysis method has greatly expanded the understanding of the impact of solution chemistry on the collectors' adsorption on mineral surface and the flotation mechanism.

In the present investigation, an AFM was applied to obtain the surface morphology of bornite in KEX and PAX solutions. By comparing the AFM images obtained under different conditions, such as the collector's type, dosage and contacting time, we studied the impact of water chemistry on the adsorption of collectors on bornite. The results will help answer important questions, such as (1) What is the morphology of the adsorbate on mineral surface? (2) What is the impact of the collector's dosage, the contacting time and the solution pH on the adsorption of the collector on the mineral surface? This information will help to clarify the reaction and adsorption mechanisms of xanthate on bornite changing with aqueous solutions, and therefore its impact on bornite flotation.

#### **2. Materials and Methods**

#### *2.1. Materials*

Research-grade bornite (Cu5FeS4) and chalcopyrite (CuFeS2) were obtained from Wards Natural Science Establishment Inc. Mineral samples were finely polished by consecutively using #800, #1200 and #2400 sandpaper, and then diamond-polishing paste of 10, 5, 2.5 and 1 microns. (MTI Inc., Richmond, CA, USA) Mineral samples were further cleaned by rinsing thoroughly with ethanol and water. A 1.2 cm × 1.2 cm sample was used for the surface characterization, i.e., AFM and ATR-FTIR analysis. The DI (deionized) water used in the present work had a conductivity of 18.2 MΩ·cm−<sup>1</sup> at 22 ◦C and a surface tension of 72.8 mN/m at 22 ◦C. Potassium amyl xanthate (PAX, >98%), potassium ethyl xanthate (KEX, >98%) and NaOH (>99%) were obtained from Alfa Aesar and used without further purification. Xanthate solutions were freshly prepared at various concentrations and pH levels as needed each time right before an experiment was carried out.

## *2.2. AFM Surface Image and Force Measurements*

AFM surface image measurements were carried out with a Digital Instrument Nanoscope IIID (Veeco, San Jose, CA, USA) AFM using the contact mode at room temperature (22 ± 1 ◦C). SNL cantilevers were obtained from Veeco (San Jose, CA, USA). Triangular Si3N<sup>4</sup> cantilevers with a nominal spring constant of 0.12~0.58 N/m were used for both AFM imaging and force measurements. For the force measurements, the separation distance (H) between the probe and the substrate (bornite plate) was measured by monitoring the deflection of the cantilever.

To study the mineral surface in water, surface image measurements were carried out after 5 mL DI water was gently injected into an AFM fluid cell. Extreme care was taken to avoid the entrapment of air in the cell. After force data and surface images were collected

in water, a 10 mL solution of a specific chemical's concentration was flushed through the liquid cell, and the cell was left undisturbed for the adsorption of chemicals on mineral surface. AFM image analysis and force measurement were commenced after the exposure of the mineral plate to the chemical's solution for a specific time. The AFM images as reported in this study, which were processed by no image modification other than being flattened, include both height and deflection images obtained in the contact mode. The same silicon nitride probe used for the force measurement was also applied to obtain the AFM image of the mineral plate in the solutions at different conditions.
