Analysis Of The Namib And Kalahari Dune Sand Deposits In Namibia And Their Application In Potassium Silicate Synthesis

ABSTRACT

Silica sand is a main component in the synthesis of potassium silicates that are commonly used as binding, coating, and adhesives agents. Physical and chemical properties of the sand particles such as grain size, mineralogical and geochemical compositions, determine the viability of sand in the synthesis of potassium silicates. This study analysed sand samples from dunes in the Namib and Kalahari deserts in order to 1) evaluate their suitability in potassium silicate synthesis as well as to 2) synthesize potassium silicate and characterize microstructural and chemical properties of the potassium silicate. Sand grains were analysed and characterized using the following techniques: mechanical sieves, ICP-EOS/MS spectrometry, and X-ray diffractometry. Synthesized potassium silicate was characterized using techniques of XRD, ICP-OES, and SEM-EDX. The dunes sand grain sizes range from 45 – 2000 μm, with 98 % of the Namib dune sand are distributed in the grain size range 63 – 500 μm and only 60% of the Kalahari dune sand are distributed in the similar grain size range, while the other 20 % is confined in the 45 -63 μm grain size range and the other 20 % confined in the grain size 500 - 2000 μm. Quartz is the dominant mineral in all sand samples and feldspar minerals were observed only in the Namib dune sand samples. High SiO2 contents 93-99 % and 85-88 % were observed in the Kalahari dune sands and in the Namib dune sands, respectively. In contrast, oxides (Al2O3+Fe2O3+K2O+Na2O) contents are relatively high in the Namib dune sands (9-11%) compare to the Kalahari dune sands (0.9-2%). Similarly, trace elements and REE such as Rb, Sr, Ba, Y, Zr, La, V, Cr, Ce, and Nd are present in higher contents in the Namib dune sands compare to the Kalahari dune sands. Chemical index of alteration (CIA) values are high in Kalahari dune sands (76-82 %), compare to values in the Namib dune sands (58-59%). Finer and well-sorted grains, geochemical and mineralogical compositions of the Kalahari dune sands are attributed to long distance aeolian transportation, intense weathering and sediment recycling processes,

ii

whereas for the Namib dune sand, coarser and well-sorted grains and the presence of feldspars are ascribed to fluvial transportation by the Orange River as well as marine, and short aeolian transportation and a moderate degree of weathering.

An amorphous potassium silicate glass with a 4.24 SiO2/K2O molar ratio and a dense, uniform surface morphology based on ESEM micrographs is reported in this study. SiO2/K2O molar ratio correlates with the concentrations of Si, K, and Al recorded by EDX point analysis. The concentrations of SiO2, K2O and Al2O3 were determined as 71.67±0.21 wt.%, 26.56±1.75 wt.% and 1.76±0.01 wt. %, respectively, resulting in a silicate glass composition of K2Si2O5.

In view of specification and requirements of silica sand as raw material in the synthesis of potassium silicate such as high SiO2, low contents of Al2O3, K2O, Fe2O3 and Na2O, low contents of refractory and heavy minerals, it is recommended that Kalahari sand deposits are suitable for exploitation in the development of silicate glasses. In contrast, the Namib dune sands with high contents of Al2O3, K2O, and Na2O can be considered in the processing and manufacturing of other sand type products such as refractory sand, foundry sand, coal-washing sand and metallurgical sand.

Keywords: Kalahari and Namib Dune sands, grain size parameters, major elements, trace elements, REE, Potassium silicate, SiO2/K2O molar ratio.