Parachute structure trisiloxane surfactant: synthesis and its use in reverse otation of iron ore

No single element has exerted such a deep inuence on social organization of mankind as iron. Magnetite is concentrated by froth otation and used as a raw material to produce iron. However, the conventional surfactants used in the otation process often lead to the weak collecting performance due to their analogous alkyl hydrophobic group. Here, we report a new trisiloxane surfactant N-(β-aminoethyl)-γ-aminopropyltrisiloxane (AAT) in magnetite otation, which was compared with the traditional collector dodecylamine (DA). The otation test results showed that AAT had excellent collecting ability and selectivity for quartz against magnetite. Magnetite concentrate with TFe recovery of 84.79%, TFe grade of 68.84% and SiO 2 grade of 6.15% was obtained by using 150 g/t AAT. Density functional theory calculations suggested reactive site of AAT was cationic –CH 2 N + H 3 group, and AAT showed a higher positive grouping Mulliken charge and chemical reactivity that may promote its otation performance. to enhance separation useful minerals and gangue minerals to realize ecient utilization of iron ore resources. AAT and DA have an oreophilic group and a lipophilic group. One end of the oreophilic group will be adsorbed on the mineral surface, and the other end of the lipophilic group extends into the air bubbles generated in the otation machine, so as to form stable and hydrophobic AAT with a unique parachute structure has more excellent performance, stronger chemical activity and stability than the traditional alkyl surfactant DA, which signicantly improves the reverse effect of iron ore.


Introduction
Opening Mendeleev's periodic table of elements, there is no longer an element that is as important as iron for the past, present, and future destiny of mankind. Reviewing the history of human development, the use of iron has always played a role in promoting the change of human times and the transformation to modernization, whether as a tool for peaceful labor, the foundation of culture and industry, or as a weapon in wartime [1][2][3][4][5] . As the most important element in the nature around us, nowadays, the important role of iron in social economic development and scienti c and technological progress will only increase and not decrease. Iron is the most widely used metal in the world, and its consumption accounts for about 95% of the total metal consumption. According to the latest research reports, research experts predict that the global consumption of iron will continue to grow with the development of global industrialization 6,7 . Therefore, how to improve iron production e ciently to adapt to industrial development is an urgent problem. Magnetite (Fe 3 O 4 ), the main resource of iron production, is commonly received from magnetite ore by a physical bene ciation method reverse froth otation 8,9 . Furthermore, the key of reverse otation is to selectively increase the oatability of quartz by adding collectors, and its speci c principle is that the hydrophilic groups of collectors are adsorbed on the surface of quartz, while the hydrophobic groups enter into the bubble, thus forming hydrophobic quartz-bubbles 10-12 . In recent years, various reverse otation collectors of magnetite ore have been studied and applied [13][14][15] . Dodecylamine is commonly used industrially as a collector in the reverse otation of silicate gangue from magnetite ore 16 . The mixture of sodium oleate and hexadecyl trimethyl ammonium bromide (molar ratio of 1:2) was rst introduced in reverse otation of magnetite ore at pH 5.5 15 . In addition, alkyl ether amines also have been studied as a collector for quartz in the reverse otation of magnetite ore 12,16 . Seriously, the conventional magnetite reverse otation collector mentioned above is a surfactant with an analogous and onefold alkyl hydrophobic groups, which often has problems such as low e ciency and large reagent consumption in the otation process [19][20][21] . Thus, it is of great practical signi cance to study a new collector with high otation e ciency to promote the industrial production of magnetite.
Trisiloxane surfactant (MD'M) is a new style of organosilicone surfactant. In its structure, M stands for trimethylsiloxane (CH 3 ) 3 SiO 1/2 -, D' means -O 1/2 Si(CH 3 )(R)O 1/2 -, as for R, it's the hydrophilic group connected to silicon by a propyl spacer. Because of the following characters of MD'M trisiloxane -the bond of Si-O can be attened on the surface of liquid, the low-energy methyl can be completely exposed to the air and distributed on the interface in a parachute structure, therefore, MD has the ability to abate the water surface tension to ~21 mN/m with an e cient method [22][23][24] . Compared with the traditional surfactant containing alkyl hydrophobic group, trisiloxane surfactant has more excellent properties, such as a lower surface tension, better spreading and wettability, good heat resistance and foaming performance, besides, with no toxic and side effects [25][26][27] . Therefore, the trisiloxane surfactants supported some industires such as cosmetics, lubricants, pesticide additives, coatings and daily chemicals and so on for many years 28,29 .
To the best of our knowledge, trisiloxane surfactant has not been applied as collector for magnetite reverse otation. It is worth paying attention to the otation effect and mechanism of trisiloxane surfactant on quartz and magnetite. In this work, a new "parachute" structure of trisiloxane surfactant N-(β-aminoethyl)-γ-aminopropyltrisiloxane (AAT) was prepared. The reverse otation ability of AAT on quartz and magnetite was compared with that of common collector dodecylamine (DA). The mechanism of trisiloxane surfactant AAT on quartz and magnetite was studied through the structure-adsorption relationships of collector by DFT, contact angle, zeta potential and FTIR spectroscopy. It has characteristic amphiphilic properties and an unusual "parachute" molecular structure with both organic and inorganic functions, bringing an immense potential in iron industry production. The present study showed that as a reverse otation collector, trisiloxane surfactant AAT signi cantly improved the utilization of iron ore resources, because AAT had better otation performance, chemical activity and higher positive grouping Mulliken charge of -CH 2 N + H 3 group than traditional alkyl surfactant DA (Fig. 1).

Results
Synthesis and structure determination of AAT As shown in Fig. 2a, the designed trisiloxane collector (AAT) was prepared by a one-step reaction. Adding N-(β-aminoethyl)-γ-aminopropyl methyl dimethoxy silane (41.27 g, 0.2 mol), hexamethylldisiloxane (162.00 g, 1 mol), and tetramethylammonium hydroxide (1.00 g, 0.01 mol) in a reactor, stirred and heated continuously for 3 h under a nitrogen atmosphere at 95 °C. Then the quaternary ammonium hydroxide was obtained from the reaction, to make it inactivate, keep the temperature at 135 °C for 50 min, after that the excessive hexmethyldisiloxane in the mixture would be extracted by distillation. Finally, N-(βaminoethyl)-γ-aminopropyltrisiloxane (51.76 g, 0.161 mol) was obtained from fraction-puri cation operation, in a yield of 80.52%, which was a colorless liquid. interface, compared with a straight alkyl chain 30,31 . Lower surface tension is bene cial to the formation of small foams with better mineralization effect and fastness 32,33 . In addition, trisiloxane collector has better ability to prevent bubble coalesce and achieve foam stability than traditional collector 27,34 .
Fig. 2e revealed the in uence of pH on quartz and magnetite recovery with 5×10 -5 mol/L collector. When the pH value increased from 2 to 6, the quartz recoveries both with AAT and DA collectors increased rapidly, and the quartz recovery with AAT collector reached at maximum of 99.5%, while only 60% with DA collector at pH 6. However, when pH value increases from 6 to 12, quartz recoveries decreased rapidly with DA collector, whereas that with AAT collector decreased only a little. As we can see, the quartz recovery curve of AAT was consistently above that of DA throughout the pH experiment. Thus, trisiloxane surfactants AAT had better collecting ability and pH adaptability than traditional surfactant DA.

Bench-scale flotation tests
In order to compare the in uence of AAT and DA collector in magnetite ore otation, the tests were carried out with 150 g/t AAT or 300 g/t DA at optimum pH 6. As presented in Fig. 2f, the TFe grade and SiO 2 grade of magnetite concentrate with AAT collector were 68.84% and 6.15% respectively, reaching the standard of C 68 (TFe grade ≥68.0% and SiO 2 grade ≤6.5%) 35 , indicating that magnetite had been recovered greatly in magnetite concentrate. Unfortunately, DA, which received a magnetite concentrate of C 66 (TFe grade 66.0% ~ 67.0% and SiO 2 grade ≤7.0%) level with 66.35% TFe grade and 6.78% SiO 2 grade, showed a lower capability. Meanwhile, the recovery of TFe obtained by 150 g/t AAT was 11.53% higher than that of using 300 g/t DA as collector. Therefore, the results showed that AAT could achieve effective separation of magnetite and quartz to obtain high quality magnetite concentrate. Fig. 3a showed the contact angles of two minerals treated with different collector concentrations. The initial contact angles of them (collector concentration was 0) were 18.5° and 34.6°, respectively, which were similar to the reported values in related literature 36,37 . As the AAT/DA concentration increased from low, the contact angle of quartz also increased. The reason was that the hydrophilic group in the collector molecule was adsorbed on the quartz surface, and another hydrophobic group was extended in the water, which enhanced the hydrophobicity of the quartz surface and increased the contact angle. Novertheless, when the concentration was 1×10 -5 mol/L, the contact angle of quartz with AAT was 73.1°, while the contact angle with DA 65.1°. In addition, the contact angles of magnetite varied little with the change of collector concentration, indicating that AAT collectors had little effect on the hydrophobicity of magnetite. This result showed that the AAT collector could effectively enhance the hydrophobicity of the quartz surface, which was consistent with the otation experiment results.

FTIR spectroscopy analysis
In order to clarify the adsorption mechanism of AAT on the mineral surface, FTIR spectroscopy of two minerals before and after treatment with AAT collector were investigated. As shown in Fig. 3b and c, the infrared spectrum of pure quartz and magnetite were basically consistent with other literature 38,39 . In Fig. 3b, after treatment with trisiloxane collector AAT, the new peaks at 2926 cm -1 and 2855 cm -1 appeared on the quartz surface, which were attributed to the stretching vibration peak of -CH 2 and -CH 3 of AAT, showing that AAT has adsorption on the quartz surface. At the same time, it was speculated that no new compounds were formed between AAT and quartz due to the absence of other peaks. In Fig. 3c, there was no change in the infrared spectrum of magnetite after AAT treatment, indicating that AAT was not adsorbed on magnetite, which was consistent with the otation experiment results.

Zeta potential measurement
Electromotive force measurement is an important means to explore the changes of mineral surface electrodynamic properties, explain the adsorption phenomenon and analyze the adsorption mechanism 40,41 . As shown in Fig. 3d and e, to further understand the adsorption mechanism of AAT on mineral surface, the changes of the zeta-potential of two minerals with pH value were measured in the presence and absence 5×10 -5 mol/L AAT/DA. In the test results, the zero electric points of quartz and magnetite were 2.63 and 5.66, respectively, which were consistent with relevant research results [42][43][44] . In Fig. 3d, the zeta potential of the quartz without collector was negative in the pH range of 4-12, indicating that the quartz surface was negatively charged. However, after interacting with the collectors, the zeta potential of the quartz with AAT and DA moved in a positive direction, suggesting that AAT + and DA + were adsorbed on the quartz surface by electrostatic force. In addition, the zeta potential of quartz with AAT was much larger than that with DA, indicating that the adsorption of AAT was much stronger than that of DA, which was consistent with the otation experiment results. For magnetite (Fig. 3e), the presence of AAT or DA had little effect on the zeta potential of magnetite, indicating that the two collectors did not adsorb on magnetite. The zeta potential measurement results indicated that AAT the main interaction between AAT and quartz was electrostatic attraction, which was veri ed with the FTIR spectroscopy analysis.

DFT calculation
The space and electronic structure of the molecule are important factors that determine whether the surfactant is effective as a collector. Recently, density functional theory (DFT) is an effective and convenient quantum mechanical method to explain the interaction mechanism between minerals and collectors 45,47 . The optimized geometric structures of the AAT + and DA + were shown in Fig. 4a. The eigenvalues of frontier orbits and the selected atomic charges were calculated and displayed in Table 1.
Molecular electrostatic potential (MEP) is usually used to reveal the electron density of the target molecule, and the MEP map is a very meaningful descriptor for predicting the nucleophilic and electrophilic active sites in the reaction 12,48 . Generally, the red area represents the most negatively charged part of the molecule, which is also the active part of electrophilic attack, while the blue area corresponds to the most positively charged part of nucleophilic attack. The MEP map of AAT and DA were shown in Fig. 4b, showing that two blue regions appear on the -N + H 3 group, but the blue degree of AAT was deeper than that of DA. Practically, the -N + H 3 groups in AAT and DA could generate electrostatic attraction with negative charge sites on the mineral surface. Here, the natural charge was calculated to discuss the net atomic charge of the collectors, as shown in Table 1. For two collectors, the positive charge was mainly concentrated on the H atoms connecting N 13 atoms. Cationic groups -CH 2 N + H 3 was the main active group of AAT and DA molecules. However, the Mulliken atomic charges of the cationic groups -CH 2 N + H 3 from AAT + and DA + were 0.355 and 0.213, respectively, showing that AAT + and DA + can generate electrostatic attraction with negative charge sites on the quartz surface, while the electrostatic attraction between AAT and quartz was stronger than that of DA, which was consistent with the zeta potential measurement (Fig. 3d), otation experiment and contact angle measurement results.
Frontier molecular orbital (FMO) theory is a famous molecular orbital theory, which is a widely used method to evaluate the properties of chemical reactions 49,50 . The highest occupied molecular orbital (HOMO) is mainly composed of Px or Pz orbitals of carbon atoms, and the lowest unoccupied molecular orbital (LUMO) is mainly composed of S-orbitals of N, C and H atoms, representing the properties of electron donor and acceptor, respectively. It can be seen from Table 1 that the HOMO values of AAT + and DA + were very low, indicating that their ability to provide electrons were very weak. This was because the valence states of these carbon atoms had been completely lled and there was no opportunity to provide p-orbital electrons to other atoms. The LUMO values of AAT + and DA + were also low, indicating that LUMO could not accept feedback electrons to form π-bonds. Thus, it was di cult for AAT + and DA + to form covalent bonds with iron on magnetite surface and silicon on quartz surface, which was consistent with the above-mentioned FTIR spectroscopy analysis (Fig. 3b). In Fig. 4c, the LUMO of the two collectors was mainly concentrated on -N + H 3 group, indicating that -N + H 3 group played a leading role in the electrostatic attraction between collectors and quartz surface. In addition, the energy gap between HOMO and LUMO could be used as an indicator of the stability of organic compounds. The smaller the energy gap, the lower the molecular stability and the higher the chemical reactivity. From Table 1, The order of energy gap ΔE LUMO-HOMO was AAT < DA, indicating that the chemical reaction activity of AAT was stronger than DA.
The above results showed the behavior of AAT and DA at the molecular level, veri ed the adsorption mechanism between AAT and mineral surface, and further proved that the active center of AAT + was mainly concentrated in the -N + H 3 group, and comparing with traditional alkyl surfactants, trisiloxane surfactant showed a higher positive grouping Mulliken charge and chemical reactivity that might improve its otation properties.

Conclusions
Herein, a novel trisiloxane collector N-(β-aminoethyl)-γ-aminopropyltrisiloxane (AAT) was synthesized, characterized and rst applied as a superior collector for the reverse otation of magnetite ore. The otation performance of AAT and the conventional collector dodecylamine (DA) for pure quartz, magnetite and magnetite ore were studied by both micro-otation tests and bench-scale otation tests.
Flotation results show that at the optimal pH 6, 5×10 −5 mol/L AAT could obtain the highest quartz recovery of 99%, amount of which was 1/3 of the optimal dosage of DA leading to a higher quality magnetite concentrate (C 68 level) by 150 g/t AAT. The adsorption mechanism shows that AAT could be strongly adsorbed on quartz surface to greatly improve the hydrophobicity of quartz surface, and had little effect on the hydrophobicity of magnetite. In addition, AAT was electrostatic adsorbed on the quartz surface, rather than on magnetite surface since cationic groups -CH 2 N + H 3 from AAT + is more positive and thus leads to a stronger electrostatic attraction with negative charge sites on the quartz surface. Economic estimation results further indicated that the use of AAT as a collector in a medium-sized iron ore plant could add signi cant economic pro ts. Consequently, the molecular design of "parachute" structure of trisiloxane surfactant AAT is a new and excellent collector to satisfy the imperious demands of improving property and reducing chemical depletion in practical industrial application, which has great theoretical and practical signi cance for the sustainable production of iron ores.
The contact angle of water droplets on the surfaces of magnetite and quartz was measured with Kruss DSA100 contact angle analyzer (Bruker, Germany) by the xed drop method. The large samples of magnetite and quartz are polished to be smooth and cleaned with ethanol for contact angle measurement. For each measurement, the magnetite/quartz sample was immersed in 30 mL of the AAT/DA solution of the designed concentration. After being soaked for 15-20 min, the samples were fetched out and dried naturally. The nal result was the average of three repeated tests.

FTIR spectroscopy analysis
The infrared spectrum of magnetite and quartz with and without AAT collector were recorded by a Nicolet iS50 spectrometer on the KBr disk. At pH 7 and 25 °C, 2.0 g of magnetite/quartz samples (below 5 μm) was mixed with 30 mL of aqueous solution with or without 5.0×10 -5 mol/L AAT. After stirred for 2 h, the mineral samples were ltered and dried for 2 days. The infrared absorption spectrum recorded on the KBr disk was from 500 cm -1 to 4,000 cm -1 .
Zeta potential measurement polymetallic ore. Appl. Surf. Sci. 551, 149420 (2021). Schematic illustration showing a reverse otation process of using a "parachute" structure trisiloxane surfactant AAT to enhance the effective separation of useful minerals and gangue minerals to realize e cient utilization of iron ore resources. AAT and DA have an oreophilic group and a lipophilic group. One end of the oreophilic group will be adsorbed on the mineral surface, and the other end of the lipophilic group extends into the air bubbles generated in the otation machine, so as to form stable and hydrophobic mineral-froths. The trisiloxane surfactant AAT with a unique parachute structure has more excellent performance, stronger chemical activity and stability than the traditional alkyl surfactant DA, which signi cantly improves the reverse otation effect of iron ore. The synthesis route, molecular structure and otation behavior of AAT. a, Synthetic route of AAT. b, The chemical molecular structures of AAT and DA. c and d, Effect of collector dosage on otation recovery of quartz and magnetite at pH 7. e, Effect of pH on otation recovery of quartz using 5×10 -5 mol/L collector. f, TFe and SiO 2 grade and recovery in magnetite concentrate obtained by AAT and DA.