Effect of acid treatment on the chemical composition and the structure of Egyptian diatomite

• Thermo chemical treatment improves the properties of Egyptian diatomite.
• Content of crystalline phase decreases; surface area increases after fine sieving.
• Diatomite samples show meso- and macropores in and between the particles.

In this study Egyptian diatomaceous earth from Kasr El-Sagha locality in El Fayium province, Egypt was used. A highly purified diatomite has been prepared successfully by using a two stage process. In the first stage, samples of Egyptian diatomite were subjected to several mechanical techniques (attrition scrubbing and hydrocyclone classification) to remove most of the accompanied gangue minerals like inorganic carbonate, silica sand, and clay. The fraction with a particle size of −45 μm was then submitted to a thermo-chemical treatment. This second stage started with a calcination of the diatomite samples at 1023 K for 1 h. Three modified forms of diatomite were then prepared by hot acidification of the calcined diatomite samples with aqua regia (5 M HCl + 5 M HNO3), 5 M sulphuric acid or 5 M hydrochloric acid. The refined products were characterized by X-ray diffraction, X-ray fluorescence, low temperature nitrogen adsorption, mercury intrusion and scanning electron microscopy. Silica materials with high purity (> 96 wt.% SiO2) and an accessible and interconnected mesopore structure could be obtained. Additionally, all samples displayed a large amount of macropores of about 1 μm diameter and a broad distribution of larger interstitial macropores. The treatment with hydrochloric acid led to products with a specific surface area between 120 and 150 m2 g− 1 and 63 wt.% of amorphous phase.