Molecular Design And Interacting Mechanisms Of Phosphonic Acid Collectors For Cassiterite

Efficient recovery of fine cassiterite has always been a difficult problem in the mineral processing industry.80%of the lost tin metals in China are lost from in slimes.Flotation is one of the most effective methods to recover fine cassiterite.It is of theoretical and practical significance to develop high-efficiency collectors for fine cassiterite flotation.In this paper,seven phosphonic cassiterite collectors are designed,and the relationships between the structure and properties of the collectors are analyzed by density functional theory(DFT)simulations and experimental detections.A new cassiterite collector CEPPA is screened out.With CEPPA as the new collector,SPA and NaOL as references,the flotation behaviours of cassiterite,quartz and fluorite are studied by single mineral flotation test.The effects of unavoidable ions Ca2+,Cu2+,Mg2+,Pb2+,Al3+ and Fe3+ on the floatability of these three minerals are investigated.The effects of sodium silicate,citric acid and sodium hexametaphosphate on the floatability of cassiterite and fluorite in the three collector systems are investigated.The separation performances of the three collectors are investigated by flotation separation tests for artificial mixed ores.Finally,the interaction mechanisms of flotation reagents on mineral surfaces are analyzed by means of adsorption measurement,contact angle measurement,zeta potential test,solution chemistry calculation,infrared spectrum analysis,XPS test and DFT simulations.The results show that the Sn,Si and Ca sites on cassiterite(110),quartz(101)and fluorite(111)surfaces are the active sites which can interact with nucleophilic reagents..The order of the surface energies of the three minerals is quartz(101)surface>cassiterite(110)surface>fluorite(111)surface.The hydration and hydroxylation strength of the three minerals are quartz(101)surface>cassiterite(110)surface>fluorite(111)surface,and correspondingly the order of the hydrophilicity of the minerals is quartz>cassiterite>fluorite.Because the hydroxylation of quartz surface is stronger than that of cassiterite surface,it is easier for quartz surface to be negatively charged.The points of zero charge of cassiterite,quartz and fluorite are 4.8,2.1 and 9.9,respectively.Based on the analysis of the structure-property relationships of the polar and hydrophobic groups of the collectors for cassiterite and according to the design principles of molecular structure of collector,four alkyl phosphonic acid collectors and three phosphinic/phosphonic acid collectors are designed,i.e.hexyl phosphonic acid(HPA),octyl phosphonic acid(OPA),decyl phosphonic acid(DPA),dodecyl phosphonic acid(DDPA)and phenyl phosphinic acid(PPIA),phenyl phosphonic acid(PPOA)and 2-carboxy ethyl phenyl phosphinic acid(CEPPA).The relationships between the structures and performances of the seven collectors are analyzed by DFT simulations and experiment tests.It is found that CEPPA has the basic characteristics of being a good collector for cassiterite.At pH 7.0,the flotation recovery of cassiterite reachs 95.5%with CEPPA as a collector,98.8%by using NaOL as the collector at pH 10.5,and 88.6%by using SPA as a collector at pH 4.0.The highest flotation recovery of fluorite using CEPPA,NaOL and SPA as the collectors are 96.8%,98.8%and 93.9%,respectively.CEPPA,NaOL and SPA have no ability to collect the unactivated quartz.The performances of these three collectors in the flotation of cassiterite,fluorite and quartz show that CEPPA has the strongest ability to resist the interference of the unavoidable metal ions,followed by SPA and NaOL.In CEPPA system,Cu2+can strongly inhibit fluorite without affecting the floatability of cassiterite or activating quartz.In CEPPA and SPA systems,sodium silicate shows good selective inhibiting effect on fluorite;citric acid shows good selective inhibiting effect on fluorite in NaOL system;in these three collector systems,sodium hexametaphosphate shows strong inhibiting effect on both cassiterite and fluorite.The results of flotation separation of artificial mixed ores show that for the cassiterite-quartz artificial mixed ore,a tin concentrate with a grade of 71%Sn and a recovery of 94.5%is obtained with CEPPA as the collector at pH 7.0.When CEPPA is used as the collector and CuCl2 as a modifier,a tin cassiterite with a grade of 71.2%Sn and a recovery of 91.9%is achieved for the cassiterite-fluorite artificial mixed ore at pH 4.0,and a tin concentrate assaying 70.7%Sn and recovery 90.5%can be obtained for the cassiterite-quartz-fluorite artificial mixed ore at pH 4.4.At pH 7.0 and using CEPPA as the collector and sodium silicate as the inhibitor,a tin concentrate grade of 71.2%Sn with 92.3%recovery is achieved for the cassiterite-fluorite artificial mixed ore,and a tin concentrate assaying 68.5%Sn and recovery 85.5%is yielded for the cassiterite-quartz-fluorite artificial mixed ore.The zeta potential detection and chemical calculation analysis show that SPA monoanion is the effective component for collecting cassiterite.DFT simulations show that SPA monoanion can replace the adsorbed OH-on cassiterite(110)surface to form a single point adsorption,and then H2 protons dissociate,and the O2 atom bonds with Sn2 atoms to form a further cyclization.Finally,SPA forms a stable "Sn-O-P-O-Sn"five-membered ring adsorption morphology on cassiterite(110)surface.The measurements of adsorption density and contact angle show that the range of pH from 7.1 to 8.0 is the most favorable for the adsorption of CEPPA on cassiterite surface.CEPPA enhances the hydrophobicity of cassiterite surface.Zeta potential,infrared spectroscopy and XPS analysis show that the adsorption of CEPPA on cassiterite surface involves chemical adsorption and hydrogen bonding.A complex similar to Sn-CEPPA precipitation is formed by combining four O atoms in the CEPPA molecule with Sn atoms on cassiterite surface.DFT simulations show that CEPPA molecule forms two hydrogen bonds(-OH…O)on cassiterite surface via its two-OH groups with two O atoms on cassiterite(110)surface,and the adsorption of CEPPA monoanion forms a "Sn-O-P-O-Sn" five membered ring by bonding its O1 and O2 atoms with two adjacent Sn atoms on cassiterite(110)surface.After dissociation to form dianions,the O1 and O2 atoms in the phosphinic acid group of CEPPA have the same chemical properties,while the O3 and O4 atoms in the carboxyl group have the same chemical properties.CEPPA dianions can form a "Sn-O-P-O-Sn" pentacyclic ring by bonding O1 and O2 atoms with two Sn atoms on cassiterite(110)surface,or form a"Sn-O-C-O-Sn" pentacyclic ring by bonding O4 and O3 atoms with two Sn atoms on cassiterite(110)surface.They can also form a chelating adsorption ring by the bonding with two Sn atoms on cassiterite(110)surface via one O atom in the phosphinic acid group and one O atom in the carboxyl group.During the adsorption process,electron gain and loss occur between Sn atoms on cassiterite(110)surface and O1,O2,O3,O4 atoms in CEPPA,forming a series of Sn-O bonds.By analyzing the population of the Sn-O bonds,it can be found that these Sn-O bonds have certain covalent bond properties,and the adsorption of CEPPA dianions on cassiterite(110)surface through O1 and O2 atoms is stronger than that by O3 and O4 atoms.At pH 4.0,the main component in CuCl2 solution is Cu2+,and the dominating species in CEPPA solution is its monoanion.XPS analysis shows that Cu2+ in solution can act on cassiterite surface by forming Cu-O bonds,DFT simulations demonstrate that both adsorption and substitution of Cu2+ on cassiterite(110)surface can occur.The monoanion of CEPPA can be adsorbed on the Cu containing cassiterite surface by the O atom which lost the H proton,so that the adsorption of CEPPA on cassiterite surface is not affected.However,the Cu2+ in the solution cannot iact on fluorite surface,and the CEPA monoanion in the solution will preferentially combine with Cu2+.Therefore,the presence of Cu2+ can reduce the adsorption of CEPPA on fluorite surface,thereby reducing or even eliminating the improvement of the hydrophobicity of fluorite surface by the adsorption of CEPPA,and ultimately the flotation of fluorite in CEPPA system is selectively inhibited by Cu2+.The research results in this paper enrich the collectors for cassiterite,develop the structure-activity relationships of the collectors for cassiterite,and have a certain guiding significance for the flotation of fine cassiterite.