In this work mechanical activation of realgar As4S4 was studied. The addition of sodium chloride NaCl as a solid diluent into the milling process substantially improved solid state properties of the obtained fines. However, the polymorphous transformation of �-As4S4 to �-As4S4 is reduced. Key words: Realgar, Mechanical activation, Sodium chlorid

Er Atka

Erika Dutková

Jaroslav Ková

Peter Balá

Acta Montanistica Slovaca

In this work mechanical activation of realgar As4S4 was studied. The addition of sodium chloride NaCl as a solid diluent intothe milling process substantially improved solid state properties of the obtained fines. However, the polymorphous transformationo

Peter Baláž

Alexander Šatka

Jaroslav Kováč

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 Acta  Montanistica  Slovaca     Roþník 16 (2011),   þíslo 2, 149-153              149 Particle properties and their modification in mechanically activated realgar As4S4    Peter Baláﾞž1, Erika Dutková1, Alexander ﾊŠatka2 and Jaroslav Kováþ2   In this work mechanical activation of realgar As4S4 was studied. The addition of sodium chloride NaCl as a solid diluent into the milling process substantially improved solid state properties of the obtained fines. However, the polymorphous transformation               of D-As4S4 to E-As4S4 is reduced.  Key words: Realgar, Mechanical activation, Sodium chloride   Introduction  Over the years, arsenic compounds have found application in the manufacture of cosmetics, foods, glass, insecticides, medications, pigments, pyrotechnics as well as in metallurgy. Arsenic sulphides such as realgar As4S4 and orpigment As 2S3 are of significant interest in mineral technologies because of their association with  gold  [1-2].  Recently  interesting  optical  properties  have  been  described  for  arsenic  sulphides as a consequence of their sensitivity to light exposure [3-7]. This sensitivity is pronounced in disordered solids and found applications in optoelectronic materials [8]. Mechanical  activation  by  high-energy  milling  is  a  suitable  way  to  create  disordered  solids  [9-10]. Among the different processes which are able to produce nanocrystalline powders the synthesis by means of mechanical activation is one of the most interesting from an industrial point of view. It exploits devices and processes  that  have  many  aspects  in  common  with  fine  grinding  and  comminution  of  solids  including nanoparticles [11-12], all frequent operations in minerals engineering. The aim of this work was examine the changes in solid state properties of realgar As4S4 under influence of high-energy milling.  Experimental  The investigation of dry milling was carried out with realgar mineral D-As4S4 (Getchel Mine, Nevada, USA). Small amount of quartz SiO2 (33-1161) was determined in this sample by XRD method.   High-energy milling The high-energy milling of realgar samples was performed in a planetary mill Pulverisette 6 (Fritsch, Germany).  The  following  milling  conditions were used:  dry  milling  -  loading of  the  mill  with  96  balls (10 mm diameter); material of milling chamber and balls: zirconia; ball charge 136 g; weight charge 3 g; rotational speed of the mill 400 rpm; milling time 2-20 min. Solid sodium chloride NaCl as milling diluent was added in various amounts into the milling chamber.  X-ray diffraction analysis The X-ray diffraction patterns were recorded by an X-ray diffractometer Rigaku Model D/Max-2400 with Rini 2000 wide-angle goniometer (Rigaku, Japan). The following measuring conditions were applied 2T range 10-90°, CuKD radiation (O=0.15406 nm), KE filter, voltage 30 kV, current 40 mA, scan speed 3°/min, scan step 0.020°. The crystallite size of nanopowders was calculated from the Scherrer equation [13] as follows  4O cos bKrd                 (1)  where d - the average crystallite size, K=1.2 - the shape factor [14], r - radius of goniometer, Ȝ=0.15406 nm, b-the angular line width, Ĭﾖ–the Bragg´s angle. For the calculation of the particles crystallite size the peak at d=0.540 nm of D-As4S4 has been selected.                                                                            1  prof.  RNDr.  Peter  Baláﾞž,  DrSc.,  RNDr.  Erika  Dutková,  PhD.,  Institute  of  Geotechnics  of  the  Slovak  Academy  of  Sciences, Watsonova 45, 043 53 Koﾚšice, Slovakia, Phone: +421-55-79 22 603, Fax: +421-55-79 22 604, balaz@saske.sk, dutkova@saske.sk 2  prof. Ing. Jaroslav Kováþ, CSc., prof. Ing. Alexander ﾊŠatka, CSc., Department of Microelectronics, Slovak University of Technology and International Laser Centre, 812 19 Bratislava, Slovakia, jaroslav.kovac@stuba.sk, satka@elf.stuba.sk    Peter Baláﾞž, Erika Dutková, Alexander ﾊŠatka and Jaroslav Kováþ: Particle properties and their modification in mechanically activated realgar As4S4 150 Specific surface area The  specific  surface  area  was  determined  by  the  low  temperature  nitrogen  adsorption  method. Gemini 2360 sorption apparatus (Micromeritics, USA) has been applied.   Particle size distribution The particle size distribution of the realgar was measured by a laser beam scattering in a Helos and Rodos granulometer (Sympatec GmbH, Germany). The mean particle diameter was calculated as the first moment of the volume size distribution function.  SEM The  morphology  of  the  samples  was  analyzed  using  FE-SEM  LEO 1550  scanning  microscope (Germany). The samples were not covered with any conductive material in order to avoid artifacts.   Results and discussion  X-ray diffraction patterns of the realgar particles milled for 1, 4, 15 and 20 minutes have been shown in Fig.  1.  The  patterns  show  many  overlapping  peaks  with  tendency  of  the  gradual  amorphization  as a consequence of crystal structure disordering. The identified peaks belong to various arsenic sulphide phases (as analyzed further). Peak Q belongs to the admixtured quartz SiO2 (33-1161). The whole process of realgar structure changes has been described in detail in [15]. The decrease of realgar nanosize particles from 60 to 16 nm has been registered. However, the addition of solid sodium chloride NaCl into the milling process was not applied in this paper.   Fig. 1.  XRD patterns of nanocrystalline realgar As4S4 milled at different times (milling time in patterns), Q-quartz SiO2, 2T = 10-90° [15].         Fig. 2.  SEM of realgar As4S4 milled at 20 min.  Acta  Montanistica  Slovaca     Roþník 16 (2011),   þíslo 2, 149-153              151  SEM microphotographs of milled As4S4 (Fig. 2) show glassy structures, which are characteristic for these types of compounds. The dependence of new surface area, SA on milling of realgar in a dry mode is given in Table 1. From the data one can conclude that the highest value of SA was attained at tM = 7 min. The prolonged milling is characterized by a decrease and stagnation of SA values.  Tab. 1.  Specific surface are, SA of the milled realgar As4S4. Milling time [min] SA [m2g-1] 0  0.3 2  0.9 4  1.6 7  2.0 10  1.3 15  1.1 20  1.3  There are several approaches to improve the milling behaviour of solids at higher energy input where agglomeration phenomena usually play decisive role [9-10, 16]. The application of a washable solid diluent sodium chloride NaCl directly into the milling process has been applied several times by mechanochemical synthesis of advanced nanocrystalline powders [17-21]. The use of the diluent reduces the volume fraction of nanoparticles  and  consequently  prevents  nanoparticles  being  agglomerated  and  control  the  particle  size distribution. Sodium chloride NaCl has been applied in our study as a solid diluent in realgar milling in order to modify the properties of fines. After milling, water soluble NaCl was washed out with de-ionized water using ultrasonic  bath.  In  Figure  3  the  values  of  specific  surface  area  for  selected  milling  times  and  various NaCl/As4S4 ratio are given.  Fig. 3.  Specific surface area of realgar As4S4 in dependence of NaCl/As4S4 ratio (w/w).  The addition of NaCl had positive influence on surface area of milled realgar particles which increased several times in comparison with realgar milled without diluent. However, the effect of NaCl addition is limited by the value NaCl/As4 S4=2.0. The further increase of NaCl amount has no effect. The dilution of milled realgar with sodium chloride over this critical value is so large that realgar particles loose contact and the milling energy is preferentially transferred to NaCl particles. As consequence, the particle size of realgar nanoparticles is practically constant (Fig. 3). It has been shown in our previous work that the transition of the low temperature Į-As4S4 to the high-temperature ȕ-As4S4 as well as the formation of nanosize realgar particles in the range 16-48 nm is possible [15].  However, the dilution effect of NaCl addition has detrimental influence on  E D o  transformation of realgar (Fig. 4).  Peter Baláﾞž, Erika Dutková, Alexander ﾊŠatka and Jaroslav Kováþ: Particle properties and their modification in mechanically activated realgar As4S4 152  Fig. 4.  Dependence of realgar As4S4  E D o  conversion degree, İ on dilution by NaCl, NaCl/As4S4 (milling time 7 min).  The value H=28 % has been obtained for sample milled for 7 min which is substantially lower as the value H=55 % obtained for the same sample but without addition of NaCl. The sodium chloride matrix phase effectively separated the milled powder thus allowing the nanocrystalline grain structure that was developed during milling. However, from the point of mechanochemical polymorphous transformation this effect has retarding  influence  on  realgar  E D o  c o n v e r s i o n  d e g r e e  a s  a  c o n s e q u e n c e  o f  r e d u c i n g  t h e  d i f f u s i o n  phenomena, which are predisposing for the reaction procedure.   Conclusions  1.  Increase in specific surface area of realgar As4S4 particles when using sodium chloride NaCl as a solid diluent during milling has been detected. 2.  Application of sodium chloride NaCl by milling has a detrimental effect on  E D o  conversion of As4S4.   Acknowledgements: The support through the Slovak Research  and  Developing  Agency  (project  APVV-0189-10), the Slovak Grant Agency (project VEGA 2/0009/11)  is  gratefully  acknowledged.  This  work was realized within the frame of the project ﾄ„Center of Excellence of Advanced Materials with Nano- and Submicron-  Structureﾓ“  that  is  supported  by  the Operational Program ﾓ“Research and Developmentﾔ” financed  through  European  Regional  Development Fund.   References  [1]  Lengke,  M.F.,  Tempel,  R.N.:  Natural  realgar  and  amorphous  AsS  oxidation  kinetics.  Geochim. Cosmochim. Acta 67, 2003, 859-871. [2]  Lengke, M.F., Tempel, R.N.: Geochemical modelling of arsenic sulfide oxidation kinetics in a mining environment. Geochim. Cosmochim. Acta 69, 2005, 341-356. [3]  Bonazzi, P., Menchetti, S., Pratesi, G.: The crystal structure of pararealgar As4S4. Amer. Miner. 80, 1995, 400-403. 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Particle properties and their modification in mechanically activated realgar As4S4

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