| As an important inorganic filler,nano calcium carbonate has been widely used in various industries.Non-toxic and harmless nano calcium carbonate has higher bioavailability and can be widely used in the food and pharmaceutical industries as a calcium supplement to meet need of the general calcium deficiency population for calcium.In this paper,eggshells and clamshells,which are safe,non-toxic,and widely-sourced bio-based materials,are used as green calcium sources.Nano calcium carbonate was prepared in the laboratory by high gravity carbonization.The effects of initial concentration of Ca(OH)2 suspension,gas-liquid ratio,carbonation temperature,high gravity level,CO2 concentration and calcium source on the crystal morphology and particle size of nano calcium carbonate were investigated.The effects of calcination temperature and calcination time on the activity of calcined product CaO were investigated.The bioavailability of calcium in nano calcium carbonate was also studied.X-ray fluorescence diffraction,inductively coupled plasma emission spectrometry,thermogravimetry,scanning electron microscopy,transmission electron microscopy,specific surface area,X-ray diffraction,Fourier transform infrared spectroscopy,particle size distribution,atomic absorption spectrometry characterization were carried out,the following research results have been achieved:(1)The eggshell and clamshell were analyzed by thermogravimetry.According to the thermogravimetry analysis curve,the lowest calcination temperature was determined as 800?.The calcination temperature and time would affect the activity of the calcined product CaO.Light or over calcination would reduce the activity of the product CaO.In this experiment,the best calcination condition of the eggshell was 800?,2.5 h and clamshell was 900?,2 h.The decomposition kinetics of calcium carbonate in eggshell,clamshell,limestone and nano calcium carbonate were studied by thermogravimetry.The results show that nano calcium carbonate is the most easily decomposed.(2)The activity of calcium hydroxide(Ca(OH)2)affects the crystal form and particle size of the final calcium carbonate.Firstly,the eggshell was used as the calcium source,and the effects of the initial slaking temperature,slaking time and CaO activity on the activity of Ca(OH)2 were investigated.The particle size distribution of Ca(OH)2 in the suspension was characterized by particle size analysis and scanning electron microscopy.The results show that the optimum slaking conditions were CaO obtained by calcining the eggshell at 900? for 2.5 h,slaking at 90? for 1 h.The results of carbonization show that when the initial concentration of Ca(OH)2 suspension is 6.8 wt.%,the gas-liquid ratio is 2,the temperature is 22?,the high gravity level is 490.55,and the concentration of CO2 is 100 vol.%,the average particle size of the prepared nano calcium carbonate is small,the particle size range is 50?70 nm,the distribution is narrow,and the morphology is mainly cubic.Under the optimal conditions,the cubic nano calcium carbonate with an average particle size of 69 nm and a cubic shape was prepared from clamshell.The results show that the content of heavy metals in the prepared nano calcium carbonate is very low,which is in line with the national standard of food grade calcium carbonate.(3)In this paper,the bioavailability of self-prepared calcium carbonate with an average particle size of 112nm,commercial micron grade calcium carbonate and calcium tablet(mainly composed of calcium carbonate)were studied.The results show that the dissolution rate of the prepared nano calcium carbonate is higher than that of commercially available micro calcium carbonate and commercially available calcium tablets.The smaller the particle size,the higher the dissolution of calcium in calcium carbonate,and the higher the bioavailability of calcium supplements in the human body. |
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