Physical characteristics of nanostructured silica extracted from mineral ore precursors.

Sl. No.	Precursor	BET surface area (m2 g−1)	Particle size (nm)	Pore size (nm) and pore vol-ume (cm3 g−1)	Yield (%)	Method	Ref.
1	Silica sand	—	10 to 50	—	—	Wet process	74
						- NaOH and silica sand mass ratios of 1 : 1, 1; 5 : 1 and 2 : 1 homogeneously mixed in DI water under ultrasound waves (32 kHz, 100 W)
						- Mixed mass heated at 500 °C for 100 min
						- Resulting vitreous compounds dissolved in hot H2O to Na2O(SiO2)x·xH2O followed by H2SO4 titration
						- NPs obtained by adding
						NH4OH to form gel
						- Gel washed, dried and
						calcined at 300, 400, 500, 600
						700, 800, and 900 °C
2	Pumice rock	422	8	5.50	15–96	Leaching process	75
				0.65	Optimal: 95.83	- Milled pumice powder activated at 500 °C
						- Activated powder mixed with different molar ratio of NaOH to silica at different temperature and time to form Na2SiO3
						- Aqua gel formed by addition of H2SO4 filtered, washed and dried, purified with HCl, filtered, washed, dried and calcined at 800 °C
3	Olivine Mg2SiO4(s)	670.8	<10	5.59	86.8	Leaching process	72
				0.95		- Dissolution of ball milled olivine with NaOH and KOH solution to obtain a dense slurry
				5		- Slurry heated to fixed reaction temperatures and times: solid products obtained
						- Dissolution of solid products in DI water to extract soluble
						Species
						- Solid products separated by vacuum filtration and filtrate solution reacted with HCl to Precipitate amorphous SiO2
4	Desert sand	—	—	—	Reaction at 120 °C for 60 minutes: ∼80% Na2SiO3	^ Alkali fusion process	76
						- Sand washed, dried, ground and sieved into different fractions of 850, 600, and 425 μm
						- Each fraction reacted with NaOH at weight ratios of 1.5, 2.0 and 2.5 and temperatures of 80, 100 and 120 °C for different times of 30, 40, 50, and 60 min
						- After the reaction, mixtures leached with distilled water and the residue removed by filtration, washed with hot water, and dried
						SiO2 precipitated by H2SO4, filtered, washed, dried, and well ground
5	Quartz sand (whole rock)	A: pH 3,6,10: 194, 91, 33	Average size: 0.5 to 8 μm	At pH 3, 6, 10	86.8	Wet process	77
	Source A: menchar, B: SidiAich, C: Attaf	B: pH 3,6,10: 133, 58,24		Pore size, A: 111, 79, 129, B: 127, 85, 22, C: 107, 83, 115		- Silicate formed by heating at 1060 °C, a mixture of Na3CO3 and burdigalian or barremian quartz sand
		C: pH 3,6, 10: 178, 86, 31		Pore vol, A: 0.53, 0.23, 0.12, B: 0.46, 0.14, 0.06, C: 0.50, 0.20, 0.09		(molar ratio, n, of SiO2/Na2O equal to 1.5 or 3)
						- Dissolution of silicate in water (at 160 °C) to form hydrated Na2SiO3
						- Silica gel extracted with HCl at pH 3, 5, and 10, washed and dried
6	Pyrophyllite ore	—	<50 nm	—	—	Wet process	78
						- Ball milled 180 μm pyrophyllite refluxed with 3 M NaOH to form Na2SiO3
						- Na2SiO3 titrated with 5 M H2SO4 (5 M) to obtain SiO2 gel
						- Gel aged, filtered, washed to remove sulfate and dried followed by 1 M HCl leaching under reflux
						- SiO2 separated by centrifuge, washed, dried, and calcined at 800 °C
7	Pumice rock	422	5 to 15	Pore size: 2 to 6 avg: 5.5	—	Wet process	79
						- Washed, dried and size reduced pumice activated at 500 °C for 3 h
						- Refluxed with 3 M NaOH to form Na2SiO3 slurry, filtered and washed with boiling H2O
				Pore vol: 0.645		- Filtrate solution acidulated to neutral with 5 M H2SO4 to form SiO2 gel
						- Gel aged, filtered and washed and dried before being filtered and washed
						- SiO2 leached with 1 M HCl, filtered, washed, dried, and calcined at 800 °C
8	Perlite	Closed reaction system NaOH/SiO2 = 2.4 molar ratio at 120 °C within 60 min: ∼98	Closed reaction system NaOH/SiO2 = 2.4 molar ratio at 120 °C within 60 min: 0.3 to 1 μm	—	—	Wet process	80
						- Perlite washed, dried, and calcined at 800 °C
						- Reacted with NaOH: time 0 to 60 min; molar ratio of NaOH/SiO2, 0.6 to 2.4 and reaction temperature, 60–120 °C to form Na2SiO3
						- Na2SiO3 solution filtered and washed with boiling distilled H2O and titrated with 5 N H2SO4 to neutral to form SiO2 gel and aged
						- Gel washed to remove sulphate and dried
						- Dried gel leached with 1N HCl under reflux, washed and dried