Effect of raw material particles sizes and hydrothermal synthesis time on properties of Hydrated Magnesium Borate (Admontite) minerals

IntroductionMagnesium borate is a very important kind of material in many application areas due to its excellent mechanical properties, attractive thermal properties, thermo-luminescent properties, its high resistance to... [ view full abstract ]

Introduction

Magnesium borate is a very important kind of material in many application areas due to its excellent mechanical properties, attractive thermal properties, thermo-luminescent properties, its high resistance to corrosion and good antiwear behavior which increase with increasing boron trioxide content. Admontite is an important magnesium borate mineral with a high boron trioxide content which is functional in application areas. In this study, the effect of raw material particles sizes and hydrothermal synthesis time on properties of admontite mineral were investigated.

Methods

Magnesite (MgCO3) was used as source of magnesium and boric acid as boron source in admontite synthesis. A stoichiometric mixture (Mg/B=1/6) of MgO, H3BO3 and distilled water (w/s=2) was ground and mixed 30 min at 250 rpm in the agate ball mill as the precursor of synthesis of admontite. The precursor was treated from 1 to 5 hours at 110 C in PTFE inserts within 5 different steel autoclaves. After reaction in autoclave under autogenous steam pressure, products were washed and filtered with hot distilled water. Synthesized minerals were characterized by XRD, particle size analyzer, FTIR, SEM and EDS.

Results

Finer grain sized admontite minerals were synthesized after grinding raw materials to finer sizes. When 19,3 µm sized (d₅₀) MgO was used, 6,1 µm sized (d₅₀) admontite minerals were synthesized. The XRD scores of the synthesized admontite minerals were higher than 90 independently from the particle sizes of MgO used (Fig.1). FTIR analyses also proved admontite synthesis (Fig.2). It appears that higher reaction durations such as 5 hours is not necessary for admontite minerals. At 1 hour reaction durations, admontite minerals with high XRD scores and nano-flake morphologies were successfully synthesized (Fig.3).

Discussion

After this study, it is concluded that the particle sizes of raw materials determine the final sizes of synthesized admontite minerals. Probably, raw materials were also grinded to finer sizes during precursor preparation using ball mill. This explains 6,1 µm sized admontite minerals synthesized by using 19,3 µm sized MgO particles. Higher reaction durations are unnecessary, reaction at 1 hour was sufficient for pure nano-flake shaped admontite minerals.