3.1. Analysis on the Selection Principle of the Main Collector and Auxiliary Collector
The aliphatic hydrocarbons and molybdenite faces have similar surface force properties and surface energy. Therefore, aliphatic hydrocarbon collectors can adsorb on the faces of molybdenite particles through a dispersion force to improve their hydrophobicity to collect molybdenite, but they cannot adsorb on the edges of molybdenite particles. As is well known, aliphatic hydrocarbons are hydrocarbons with the basic properties of aliphatic compounds. Usually, the main representative of the hydrocarbon oils used in the flotation of molybdenite is kerosene and diesel oil. Kerosene and diesel oil have good collecting ability at room temperature and are widely used in the field. However, the dispersion of diesel oil in pulp at low temperature is poor, resulting in a poor flotation effect of the molybdenite. As a result, kerosene was selected as the main collector in this study.
In this case, the key was to find a type of collector as an auxiliary collector mixed with kerosene which could adsorb on the “edge” of the molybdenite particle and improve its hydrophobicity, so as to improve the flotation effect of molybdenite. Because molybdenite edges possess a certain polarity, a kind of polar non-polar oil collector is required to selectively adsorb on the particle edges to improve their hydrophobicity. It has been shown [
18,
19,
20,
21,
22] that polar collector, syntax emulsifier, and kerosene can be mixed to improve the separation index of molybdenite, but at the same time, other non-molybdenum sulfide minerals can also be collected which increases the difficulty of flotation separation from molybdenite. Hence, the addition of polar reagents with low selectivity is only suitable for single molybdenite, but not for molybdenite with other complex minerals. In order to achieve this goal, the selection of an auxiliary collector is a strict procedure. The principle of selection is that the auxiliary collector has a certain polarity, which is not too high, and a good selectivity. What is more, it should have no collecting effect on the other non-molybdenum sulfide minerals. Aromatic hydrocarbons with benzene ring are especially suitable for this requirement.
3.2. Surface Energy Analysis of Collectors
The surface force properties and surface energy are similar between aliphatic hydrocarbon and molybdenite particles. Therefore, aliphatic hydrocarbon collectors can improve the hydrophobicity of molybdenite particles by dispersive force adsorption. Similarly, we can calculate and compare the surface energy of polycyclic aromatic hydrocarbons and the molybdenite “edge”. The surface free energy of molybdenite was calculated secondhand by Young’s equation according to the relationship between the contact angles of the three liquids on the “face” and “edge” [
26,
27]. The surface energy of the polycyclic aromatic hydrocarbons was determined by measuring the contact angle of the three liquids on the solid surface treated by the polycyclic aromatic hydrocarbons.
Young’s equation is shown as follows:
where
the surface is tension of the liquid (mJ/m
2);
θ is the contact angle in degrees (°);
is the surface free energy of the solid (mJ/m
2);
is the free energy of solid–liquid interface (mJ/m
2).
According to the viewpoint of van OSS, Fowkes, and Chaudhury [
28,
29,
30], the surface free energy of a solid can be explained by the acid base theoretical model, i.e.,
is composed of Lifshitz van der Waals
and the component electron donor acceptor
(also known as acid base interaction), which can be described as follows:
where
can be divided into the electron acceptor component
and electron donor component
. The relationship is shown in Equation (3).
By introducing Equation (3) into Equation (2), another expression of surface free energy of the solid can be obtained as follows:
Similarly, the surface free energy of the liquid can be expressed as:
The free energy of the solid–liquid interface can also be expressed as a function of the geometric average of components
and
, and can be described as follows:
By combining Equations (4)–(6) with Young’s equation, the following results are obtained:
In this study, distilled water (polar liquid), formamide (polar liquid), and di-iodomethane (nonpolar liquid) were selected. The contact angle of the common exposed surface of the molybdenite crystal under the action of three liquids is shown in
Table 2. The surface tension and component parameters of the three liquids are shown in
Table 3 [
31,
32]. The contact angle was measured by the sessile drop method [
33]. According to Equation (7), the surface energy of molybdenite “face” and “edge” can be calculated. The results are shown in
Table 4. Similarly, the surface free energy of MNap was calculated, but not for Nap. The results are shown in
Table 5.
The results in
Table 5 show that the surface energy value of kerosene is
= 44.50 mJ/m
2 close to that (
= 42.55 mJ/m
2) of the molybdenite {001} surface shown in
Table 4, which is consistent with the results of previous studies [
34,
35]. The
and
of kerosene are low, which indicates that kerosene does not adsorb on the molybdenite surface by chemical reaction, but by physical adsorption on the {001} surface of molybdenite according to the principle of similar compatibility. For fine-grained molybdenite, it is not enough to collect molybdenite particles by kerosene adsorption on the “face”. As a result, the floatability of fine-grained molybdenite in the kerosene system is lower than that of coarse-grained molybdenite. To solve this problem, the effect of auxiliary collector adsorption on molybdenite “edge” needs to be studied in a scientific way.
Table 5 shows the surface energy value (198.53 mJ/m
2) of MNap as being also extraordinarily close to that (195.18 mJ/m
2) of {100} surface of molybdenite, and
and
of MNap are relatively high and close to that of the {100} surface of molybdenite. Thus, MNap can adsorb on the {100} surface of molybdenite. In conclusion, the adsorption of polycyclic aromatic hydrocarbons on the molybdenite “edge” to enhance its hydrophobicity can be studied in a scientific way.
3.3. The Effect of Auxiliary Collector on Molybdenite Flotation
In this study, MNap and Nap were selected as auxiliary collector mixed with kerosene in the proper proportion in the flotation experiments, respectively. Micro-flotation experiments were conducted under the conditions of different mass ratios of kerosene to auxiliary collector and the results are shown in
Figure 6. As can been seen, molybdenum recovery remained about 78.85% with kerosene as collector alone. It is obvious that molybdenum recovery first increased and then decreased with increasing mass ratio of kerosene to auxiliary collector and reached a maximum of 82.21% and 82.13% when MNap and Nap were auxiliary collectors, respectively, with the mass ratio of kerosene to auxiliary collector being 95:5.
The results of micro flotation experiments illustrate that the addition of auxiliary collector is helpful to improve the flotation recovery. In order to further verify the improvement effect of auxiliary collector on actual ore flotation, the flotation experiments of molybdenite were carried out under the conditions of pulp temperature of 25 °C and 3 °C, to find the best mixing ratio of auxiliary collector and kerosene.
Figure 7 shows the effect of mass ratio of kerosene to MNap on molybdenite flotation, with the mass ratios of 100:0, 99:1, 97:3, 95:5, and 93:7, respectively. The total dosage of collector is 170 g/t, and the total dosage of frother is 90 g/t, which are distributed to rougher, scavenging I, and scavenging II in the proportion of 7:2:1. As shown in
Figure 7, under the conditions of pulp temperature of 25 °C, the Mo recovery rate of molybdenite concentrate first gradually increased and then decreased with the increase of the proportion of MNap, while the grade also increased. The optimum mass ratio of kerosene to MNap was 95:5 corresponding to 89.01% molybdenite recovery, which was increased by 4.25% compared with the use of kerosene as the collector alone. Obviously, when the content of auxiliary collector is above 5 wt%, the flotation recovery of molybdenite began to decrease. Under the condition of pulp temperature of 3 °C, the recovery curve of molybdenite concentrate is similar to that of pulp temperature of 25 °C. The optimum mass ratio of kerosene to MNap was 95:5 corresponding to 85.37% molybdenite recovery, which was increased by 3.51% compared with the use of kerosene as the collector alone.
Figure 8 shows the effect of mass ratio of kerosene to Nap on molybdenite flotation, with the mass ratios also of 100:0, 99:1, 97:3, 95:5 and 93:7, respectively. When the auxiliary collector was Nap, the change in the trend of the flotation test results was similar to that of auxiliary collector MNap. As well, the optimum mass ratio of kerosene to Nap was 95:5, and the recovery was increased by 3.47% and 3.70% under the condition of pulp temperature of 25 °C and 3 °C, respectively.
In order to more clearly express the effect of the optimal auxiliary collector dosage on the improvement of the molybdenite flotation recovery rate, the data comparison is shown in
Figure 9. We can draw the conclusion that MNap and Nap as auxiliary collector can improve the flotation recovery of molybdenite both at room temperature and low temperature.
According to the above data and the analysis of the selection basis of the auxiliary collector, the auxiliary collector may be adsorbed onto the molybdenite particles to improve their hydrophobicity. The combined use of auxiliary collector and kerosene as molybdenite collector can effectively improve the flotation index of molybdenite at the mass ratio of 95:5 kerosene to MNap or Nap. Furthermore, the degree of improvement of the molybdenite flotation index is very close. On the basis of condition tests, closed circuit experiments were carried out according to the process shown in
Figure 5. The results are shown in
Table 6.
According to the results of the closed-circuit test in
Table 6, the flotation indexes with “kerosene + MNap” and “kerosene + Nap” as collectors were better than those with kerosene as collector. Compared with kerosene as collector alone, the recoveries were increased by 3.28% and 3.21%, while “kerosene + MNap” and “kerosene + Nap” as collectors, respectively. Therefore, the closed-circuit test shows that the recovery of molybdenite can be effectively improved by the combination of two kinds of polycyclic aromatic hydrocarbons and kerosene, respectively.
3.4. Filtration Characteristics of Flotation Concentrate with Different Collectors
The flotation results demonstrated that the MNap and Nap could improve the flotation recovery under the condition of pulp temperatures of 25 °C and 3 °C. In the field application, it was found that the filtration velocities of these three collectors were different at low temperature in winter. To investigate the influence of the three collectors (kerosene, kerosene + MNap and kerosene + Nap) on the filtration velocity of flotation concentrate in winter, the filtration time of flotation concentrate with different collectors was determined. The results are shown in
Figure 10.
It is evident from the results in
Figure 10, that the filtration time of the same quality concentrate obtained respectively by the three collectors shows little difference to room temperature of 25 °C. This shows that the addition of the two auxiliary collectors has little negative effect on the filtration velocity of the concentrate at room temperature. It is to be noted that in
Figure 10, the filtration time of the concentrate with “kerosene + Nap” as the collector was much longer than that of the other two collectors at different filtration pressures at the low temperature of 3 °C.
By comparison of the filtration times at 25 °C and 3 °C, under the same filtration pressure, for the concentrates with “kerosene” and “kerosene + MNap” as collectors, the filtration time at room temperature was similar to that at low temperature. However, when “kerosene + Nap” is used as collector, the filtration time of concentrate at low temperature was longer than that at room temperature. This shows that the auxiliary collector “Nap” may be only suitable for use at relatively high temperature, but not in winter. As such, it can be considered that MNap added into kerosene as auxiliary collector of molybdenite has little effect on the filtration speed of flotation concentrate at low temperature in winter. MNap not only improves the recovery, but also has little negative influence on the subsequent treatment of concentrate.
3.5. Crystallization Characteristics of Different Collectors
Due to the low temperature in winter in the northern part of our country, the ambient temperature may have a negative impact on the collectors, thus affecting collecting performance. Consequently, in order to study the existing state of collectors (Kerosene, Kerosene + MNap, Kerosene + Nap) at different temperatures, three collectors were observed and pictures taken with a camera. The results are shown in
Figure 7.
It is clear that in
Figure 11, the three collectors all existed in the form of liquid at 25 °C. When the temperature was reduced to 3 °C, there was no change in “kerosene” and “kerosene + MNap”, but a small amount of white crystals began to precipitate in “kerosene + Nap”. When the temperature dropped to −10 °C, there was still no change in “kerosene” and “kerosene + MNap”, while the white crystals precipitated from “kerosene + Nap” gradually increased with more at −15 °C. These results demonstrated that Nap crystallized readily in liquid at low temperature. Although the indoor temperature is above 0 °C in winter in northern China, the outdoor temperature is generally below 0 °C, which is not conducive to the transportation, storage, and use under the conditions of low temperature in northern winter.
Therefore, it can be concluded that although “kerosene + Nap” can improve the flotation index of molybdenite, it is not suitable for molybdenite flotation in the northern cold winter area. However, the existing state of MNap at room temperature and low temperature shows no change. As a result, MNap can be used as auxiliary collector and mixed with kerosene to improve molybdenite flotation.
3.6. Crystallization Characteristics of the Collectors on the Flotation Concentrate Surface at Different Temperatures
Based on the above experimental results, Nap can precipitate from kerosene at low temperature. In the field application tests, it was also found that the filtration time of flotation concentrate was longer when “kerosene + Nap” was used as collector. Therefore, to find out the reason affecting the filtration time, the molybdenite single mineral flotation experiments with different collectors were carried out at low temperature (3 °C) and normal temperature (25 °C), and then flotation concentrates were observed and compared under the ordinary optical microscope respectively. The benefit was to more intuitively watch the changes on the surface of the molybdenite with the different collectors, affecting the filtration velocity at low and room temperature.
Figure 12 presents the results.
It is obvious in
Figure 12 that at room temperature of 25 °C, there was no crystal precipitation on the surface of the flotation concentrate with the three collectors after filtration respectively, indicating that the addition of the two auxiliary collectors has no effect on the filtration of the concentrate. At low temperature of 3 °C, crystal precipitation cannot be observed on the surface of the concentrate after filtration with “kerosene” and “kerosene + MNap” as collectors, but there were white crystals on the surface of the concentrate obtained by “kerosene + Nap” as collector after filtration, leading to the slow filtration speed. This is mainly due to the addition of auxiliary collector “Nap”. Nap is a solid lamellar crystal that can be dissolved in kerosene at room temperature. It can be seen from
Figure 12 that it was precipitated from kerosene at low temperature. The precipitation of Nap at low temperature was the fundamental reason that affected the filtration velocity of concentrate, which led to the longer filtration time in winter. Furthermore, this effect will be more obvious when nap is applied in industrial experiments.
Based on these findings, it can be inferred that using “kerosene + MNap” as the collector for molybdenite separation, white crystals were not precipitated from the surface of the flotation concentrate at room and low temperature, thus MNap did not have adverse impact on the filtration velocity of concentrate and had strong adaptability to the environmental temperature. Furthermore, when MNap was added as an auxiliary collector of molybdenite, we noted that the recovery rate of molybdenite can be improved at room temperature and at low temperature. Based on these considerations, we considered MNap to be a good auxiliary collector to improve the flotation index of molybdenite.