*4.6. Adsorption of the Mixture of Xanthate and Dialkyl Dithiophosphate on Chalcocite Surface and Its Implication for Chalcocite Flotation*

In froth flotation, specific chemicals, i.e., collectors or promoters, are added to pulp to increase the surface hydrophobicity of a target mineral. This will result in the increase of both attractive hydrophobic force and adhesion force between mineral particles and bubbles. The former can facilitate a particle/bubble attachment and the latter can retard a particle/bubble detachment, which are both beneficial for froth flotation.

AFM force measurements [17,21,22] have shown that, compared to insoluble metal xanthate, an oily substance such as dixanthogen adsorbs on the mineral surface resulting in a large adhesion force both in magnitude and range between an AFM tip and the mineral surface, suggesting a strong adhesion between a bubble and a mineral surface, which is beneficial for better flotation. That is, on the mineral surface, an oily adsorbate is better than insoluble precipitates when the surface coverage of adsorbate is the same.

The AFM images as obtained in the present work show that xanthate adsorbs on chalcocite in the form of insoluble cuprous xanthate, while dialkyl dithiophosphate adsorbs on chalcocite mainly in the form of oily Cu(DTP)2. Figure 24 shows the AFM images of chalcocite in the mixture of 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M PAX and 300 ppm Aerofloat 238 solution at pH 6. One can clearly see that there are two very different types of adsorption morphology. Firstly, as indicated by the "Red" arrow, the morphology is of oily patches, i.e., Cu(DTP)2, due to adsorption of dialkyl dithiophosphate; Secondly, as indicated by the "Blue" arrow, the morphology is of precipitates, i.e., cuprous xanthate, due to the adsorption of xanthate. Figure 24 shows that xanthate and dialkyl dithiophosphate can co-adsorb with both insoluble cuprous xanthate and oily Cu(DTP)<sup>2</sup> on the chalcocite surface. The former increases the surface coverage of the collector on the mineral surface, while the latter increases the adhesion between a bubble and chalcocite. Clearly, this mixed collectors scheme is beneficial for an improved flotation of chalcocite. The conclusions obtained from the present AFM images analysis explain a recent flotation practice of chalcocite, which claims that using a mixture of xanthate and dithiophosphate collector can largely improve both grade and recovery of copper flotation concentrate [15].

#### **5. Conclusions**

AFM image analysis has been applied to study the adsorption morphology of KEX, PAX and Aerofloat 238 on chalcocite in situ in aqueous solutions changing with solutions' chemistry such as collector type and dosage, and pH. AFM images show that: (1) All these collectors, i.e., xanthate and dialkyl dithiophosphate, adsorb strongly on chalcocite but in different manners. (2) Xanthate adsorbs mainly in the form of insoluble cuprous xanthate (CuX), which binds strongly with the mineral surface without being removed by flushing with ethanol alcohol. This xanthate/chalcocite adsorption mechanism is very similar to the one obtained with the xanthate/bornite system; while it is different from the one of the xanthate/chalcopyrite system. (3) On the other hand, dibutyl dithiophosphate adsorbs on chalcocite in the form of hydrophobic patches, which can be removed by rinsing with ethanol alcohol. (4) AFM images show that the adsorption of collectors increases

with increasing adsorption time and collectors' concentration. (5) Using the mixture of xanthate and dithiophosphate as a collector can help with the adsorption of collectors on chalcocite surface, as shown by the co-adsorption images of xanthate and dithiophosphate on chalcocite, and therefore an improved flotation. The present AFM image analysis work clarifies the flotation mechanism of chalcocite in industry practice using xanthate and dialkyl dithiophosphate as collectors.

**Author Contributions:** Conceptualization, J.Z.; methodology, J.Z. and W.Z.; formal analysis, J.Z.; investigation, J.Z. and W.Z.; resources, J.Z.; data curation, J.Z. and W.Z.; writing—original draft preparation, J.Z.; writing—review and editing, J.Z.; supervision, J.Z.; project administration, J.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available due to ethical.

**Conflicts of Interest:** The authors declare no conflict of interest.
