High-efficiency mechanochemical

In this study, the conversion of Celestite to SrCO3 was studied by wet mechanochemicalsynthesis in a high-energy ball mill and treatment with Na2CO3. For thispurpose, solid strontium carbonate and soluble Na2SO4 were obtained after wet millingof Celestite powder and sodium carbonate. The solid phase was washed withwater at room temperature by filter pressing. X-Ray diffraction patterns showed that the SrCO3 nanopowder was synthesized and conversion boosted with increasing the milling time up to 8 hours Also, Rietveld refinement analysis was used to calculate the fraction of SrCO3 as well as structural properties of synthesized samples. It was found that initial Celestite could be converted to strontium carbonate with a purity
more than 98% using high-energy milling without simultaneous heating. The optimum milling time was determined as 4 hours resulting in formation of nanopowders with an average particle size around 90 nm. Field Emission Scanning Electron Microscopy (FE-SEM), clearly showed the nanoscale structure of the synthesizedpowders.

    “1 | INTRODUCTION”

    ORIGINAL ARTICLE

Strontium is the 15th abundant element in the earth’s crustwith an average concentration of 0.04%. In seawater, itis the 10th most abundant element with a concentrationof 0.0008%. Of the naturally occurring strontium compounds,only Strontianite (Strontium carbonate, SrCO3)and Celestite (Strontium sulfate, SrSO4) are of economicimportance.1 Strontium carbonate (SrCO3) is widely used as a starting material for preparing a variety of Strontium compounds.2-4 It has wide applications such as electronics and glass industries, as a constituent of magnetic ferrites,
paints, medicine, dryers and in the manufacturing of metallic strontium and other strontium compounds.5-8 SrCO3 is currently produced from Celestite concentrates using the black ash and the double decomposition (direct conversion)

processes.9-11
In black ash process, SrSO4 is reduced with coke at temperatures generally over 950°C to produce water-soluble SrS.9,12 The formed sulfide is then leached with hot water and strontium is precipitated from the solution as SrCO3 using carbonating agents such as sodium carbonate (Na2CO3),
ammonium carbonate ((NH4)2CO3), ammonium oxalate (NH4)2C2O4), or carbon dioxide (CO2).9,13-17 In the double decomposition or direct

Correspondence
Mohsen Nouri-Khezrabad, Department of Mining & Matallurgical Engineering, YazdUniversity, Yazd, Iran.
In the double decomposition or direct conversion method, which is an ion-exchange reaction of SrSO4 with any carbonate,finely powdered Celestite is reacted with aqueous sodium carbonate solution to obtain directly SrCO3 and sodium sulfate by-product via following reaction:
There are many reports discussing different aspects of celestite conversion to SrCO3. Several researches,18- focused on investigating the kinetic of the reaction. An early work by Iwai and Toguri,20 studied the conversion from both thermodynamic and kinetic points of view. They studied the effect of stirring rate, celestite particle size, temperature, and the concentration of Na2CO3 and Na2SO4 on the leaching kinetics. They found out that the conversion was dominated by the surface reaction. It explains the diffusion of carbonate ions through the pores of SrCO3 layer. Similarly, Castillejos et al21 studied the effects of several parameters including stirring speed; particle size; Na2CO3 and Na2SO4 concentrations; temperature; solution pH; and solid/liquid ratio. They found out the conversion rate increases with increasing temperature, Na2CO3 concentration, and decreasing particle size.
Hizli et al,22 used ammonium Hizli et al,22 used ammonium carbonate instead of Na2CO3 to produce Strontianite from Turkish celestite concentrate.
They found that ammonium carbonate alone was not effective in producing strontium carbonate from celestite ore. Thus, ammonia in stoichiometric amounts was needed. They also reported that Celestite ore was converted to strontianite between 3 and 6 hours depending on ammonia concentration and celestite mass. However, Bingol et al,23 synthesized SrCO3 using (NH4)2CO3 without any ammonia addition. They reported that the conversion of celestite to SrCO3 can be completed with 99% of efficiency at a milling time of 75 minutes, ball-to-grinding material mass ratio of
9.24, (NH4)2CO3: SrSO4 mole ratio of 1.86:1, and a rotational speed of mill of 400 rpm. Sezer and Arslan,24 studied