Ball Milling of Chalcopyrite: Mossbauer Xa9949653 Spectroscopy and Xrd Studies

BALL MILLING OF CHALCOPYRITE: MOSSBAUER XA9949653 SPECTROSCOPY AND XRD STUDIES H. POLLAK, M. FERNANDES, D. LEVENDIS, L. SCHONIG Mossbauer Laboratory, Departments of Physics and Chemistry, University of the Witwatersrand, Johannesburg, South Africa Abstract The aim of this project is to study the behavior of chalcopyrite under ball milling for extended periods in order to determine how it's decompose or transform. Tests were done with chalcopyrite mixed with iron and zinc with and without surfactant. The use of surfactants has various effects such as avoiding oxidation and clustering of the fine particles. In all case magnetic chalcopyrite is transformed into a paramagnetic component showing a disordered structure, thus reveailing that Cu atoms have replaced Fe atoms. In the case of ball milling in air, chalcopyrite is decomposed with the lost of iron, while in milling under surfactants, iron enters into the chalcopyrite structure. 1. Introduction Iron in chalcopyrite is present in the ferric state with high spin d5 electron configuration Fe3+ [1]. This means that iron is in tetrahedral site, Cu is Cu+, and sulphur is S". The crystal structure of chalcopyrite CuFeS2 has been extensively investigated by Hall [2] as well as the isomorphous compounds talnakhite, mooihoekite and haycockite. The principal structural aspect of the last three minerals is the presence of extra metallic atoms located at interstitial sites and can be considered as superstructure of chalcopyrite [3]. Chalcopyrite is antiferromagnetic at room temperature [4]. Chalcopyrite decomposes at 663 K in air [5]. Various studies of the effect of grinding chalcopyrite have been published, mostly by a Tchek group leads by Tkacova [6-9]. Discussing these results,1 they consider that iron goes from the high spin to the low spin form and that sulphur changes its valence from 2" to 6". This result seems somewhat curious, especially that the authors assume the formation of copper sulphates and ferric oxides. Changes in the structure of chalcopyrite brought by mechanical activation causes exothermic oxidative reactions to proceed at temperatures as 180 °C lower than in non activated sample [8]. 2. Experimental methods 2. 1. X-ray powder patterns All samples were run with the same setting; i. e. 40 kV and 20 mA, without spinning. Two types of scan differing only in the 20 range and collection time were used. The 20 ranges used were 5-140 °C in 45 minutes and 42-60° in 30 minutes for the different scans. 2. 2. Mossbauer spectroscopy The samples were those used for the XRPD measurements. The samples of about 5 mg were mixed with "cremona" to avoid any broadening due to tickness effects. All spectra were recorded with more than 750 000 counts in each of the 512 channels obtained after folding. All analysis were done using either in-house computer algorithms or MOSFUN [10]. 67 2. 3. TGA and DSC procedures The original chalcopyrite sample was run in order to see if phases' transitions or losses of material occurred and at what temperature. The TGA and DSC traces were obtained using 10 °C and 1 °C per minute scans. The typical temperature ranges used for TGA were 25-750 °C at both scan rates. The typical temperature range used for the DSC traces were 25-500 °C. Traces were obtained under air and inert atmosphere (He). 2. 4. SEM and EDS Initial and 30 hour ball milled samples were examined for particle size and an elemental analysis was done by EDS. 2. 5. Magnetic susceptibility Magnetic susceptibility measurements were done at CNRS, Laboratory of Magnetism, Bellevue, France by Dr.. J-L. Dormann. 2. 6. Ball milling Chalcopyrite (2.009 g) and irons (0.673 g) were mixed together using an agate hand mill until a fine powder was obtained. Then, the powder was placed in the milling container made of agate balls of an approximate diameter of 8 mm. The mill changes the direction of rotation every 2 minutes. The ratio of the chalcopyrite and iron corresponds roughly to a 33.3 % Fe/CuFeS2 mixture. Some other experiments have been done such as ball milling Zn and chalcopyrite as well as chalcopyrite and pyrite. The results of these experiments will not be discussed in this report, but will be used in the general discussion. 3. Experimental results: TGA and DSC The reason of this study is to get insight on the possible effect of the ball milling. Ball milling induces an increase of temperature in common in both experiments the conditions are quite different. In TGA or DSC the gradient of temperature is slow, while in milling one expects explosions due to sudden change in temperature. 3.1. TGA When chalcopyrite is analyzed in air there is a slight increase in mass of the sample in the range of 360-450 °C followed by a steady decrease in mass. This would indicate that chalcopyrite oxidizes in the ranges of 360-450 °C, most probably to copper and iron sulphate. The mass decrease could most probably be attributed to the formation of elemental sulphur and compounds such as SO2, SO3 and SO4". Run under nitrogen, the traces are similar, because most probably water in the nitrogen reacts with chalcopyrite. Run under He (Figure 1), the traces indicate a slight decrease in mass from 93 °C to 360 °C, followed by a large mass decrease after 393 °C that is due to elemental sulphur liberation. An observation during the annealing experiments was that the furnace silica tube was stained with at least three compounds that had come off the sample. Two of the compounds had a metallic appearance, one with gray color and the other with a gold/copper color. When washing the silica tube neither of these compounds could be removed with soap and water. However treatment with nitric acid removed these strains implying that they were composed of metal. The third stain was composed of a white chalky material that was not removed by acid, that suggests a sulphur component. The three compounds were most probably, copper, iron and a compound of sulphur. 68 I 1 1 1 1 I 1 1 1 1 111 1 1 1 1 1 1 0.2044 - • 95 »C 391 »C - 0.2033 •L t 360 »C 0.2032 under He 3.2028 1 1 . 1 . 1 . 1 , 1 . 1 . .1.1. 92 180 227 293 362 430 497 585 632 700 »C FIG. 1. TGA curve of chalcopyrite run under He. The curve is very flat up to 360 °C were the first weight lost occurs. This indicates a change in the structure above this temperature but not a phase transformation. (He eliminated external oxidation) 3. 2. DSC DSC under He (Figure 2) indicates that they are two temperatures at which some compound comes off 62 °C and 124 °C. The peaks are small indicating that very small quantities of materials come off. Run in the reverse direction (Figure 3), the peaks were not detected, indicating that they are not due to a phase transition. The high temperature exothermic peaks at 351° and 450 °C arise from sulphur reacting with surrounding impurity gases of the helium. 3.3. SEM and EDS If the SEM images reveal (Figures 4) that the overall "particle" size is 100 (am for the initial sample and 2 mm for the ball milled one, these are not elementary particles and represent agglomerate of smaller particles. The EDS results are given in the Table I with the calculated chemical formula. 3.4. Mossbauer spectroscopy of ball milling chalcopyrite in function of time of milling The Mossbauer spectra of the reference sample (Figure 5) present the superposition of a magnetic sextet and a quadrupole double doublet. The sextet is the signature of the antiferromagnetism of chalcopyrite while the main quadrupole doublet represents a non magnetic phase due to a randomization of the distribution of the cations (Fe-Cu) between the two possible cationic sites. The doublet with small intensity is considered as a silicate impurity and will be taken out all further discussions. The Figure 6 shows the crystal structure of chalcopyrite. Notice the planes of cations that are parallel to the ab plane. In perfect chalcopyrite all planes are equivalent and only differ by the replacement of Cu by Fe and Fe by Cu. The sextet and the main doublet are charecteristic of chalcopyrite as find from the literature. The Mossbauer parametrs (Table V) that we obtained from all the 69 1 IIII i i t i i 0.038 _ - 450 - r—~ 0.008 - \ - —' "~~35i o V -0.020 I/ 118 under He - V "37 i i i .1,1.1 , 1 , " 77 <?4 .71 ?1B - 265 312 359 40B 453 500 °C FIG. 2. DSC curve ofchalcopyrite run under He. DSC curve indicates various dips at low temperature (64 and 124 °C). They are small, so they can arise from lost of small quantities of materials, perphaps impurities. 1 1 1 1 1 1 1 1 1 1 J i J 1 1 1 1 1 0.032 - 0.014 - ) o - S -0.004 123 "\ / \y _ 62 -0.022 •- V under He 40.5 . 1 . I.I.I. .1.1. 1 , 1 . 38 SB 74 92 110 128 148 104 182 200 OC FIG. 3. DSC curve ofchalcopyrite run under He in the reverse direction (decreasing temperature). He avoid spurious oxidation. This curve is to be compared with Figure 2, taken with increasing temperature. The fact that no peaks (dips) appear at low temperature confirm the fact that the dips seen in the direct direction (increasing temperature) are not due to phase transition. 70 FIG. 4. SEMmicrograp of the initial chalcopyrite (large magnification).

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