Mechanism Of Phosphorus Release Characteristicsof Mechanochemically Activated Phosphorites

The high grade phosphorite is gradually depleted in China.There are difficulties to develop the medium and low-grade phosphorites for their high enrichment cost.Mechanochemical activation technology can significantly improve the phosphorus solubility of medium and low-grade phosphorite.But previous researches mainly focused on the optimization of the processing technology of milling equipment.However,there are still unknown that the phosphorus release characteristics and their mechanism of mechanochemically activated phosphorites.Therefore,a planetary ball mill is used to mill the medium and low-grade phosphorite of magmatic type（Beipiao in Liaoning）,metamorphic type（Shimen in Hunan）and sedimentary type（Shimen in Hunan and Yichang in Hubei）with the grinding time 0-60 min.In this paper,the relationships between the physical and chemical properties and the phosphorus solubility of mechanochemically activated phosphorites are revealed.The application effects of mechanochemically activated phosphorite on oat and field corn are verified with pot and field experiments.The main results are as follows:（1）The phosphorus solubility of phosphorite can be significantly improved by mechanochemical activation.Mechanochemical activation can reduce the particle size of phosphorite significantly.The median particle size of D50 can reach 6.42-10.48 μm.Compared with non-activated phosphorite,assimilable phosphorus in citric acid increased to 12.46%-17.06% by mechanochemical activation within 60 minutes,and the phosphorus solubility in citric acid increased by 1.64-13.67 times.There were significant exponential relationships between phosphorus solubility in citric acid and milling times..Assimilable phosphorus contents in citric acid were negatively correlated with particle size of mechanochemically activated phosphorites.（2）Mechanochemical activation can result in changes in physical and chemical properties of phosphorite,which are strongly related to the phosphorus solubility.From the change in physical properties,The results of X-ray diffraction（XRD）show that the main phase apatite and associated minerals in phosphorite have been transformed into amorphous state with the extension of mechanochemical activation time.From the chemical properties,mechanochemical activation can change the phases of phosphorite.The results of IR and XRD show that the mechanochemical activation of four phosphorites carbon apatite（Ca10（PO4）6CO3）and hydroxylapatite were formed by the B type substitution of CO32-for PO43-tetrahedra and channel ions substitution of the OH-for F-,respectively.They were verified by thermogravimetric analysis（TGA）that carbon apatite and hydroxylapatite were decarburized and pyrolyzed to form Ca（PO3）2 in the process of mechanochemical activation,which increased the phosphorus solubility of mechanochemically activated phosphorites.Compared with the nonactivated phosphorites,the p H value of mechanochemically activated phosphorite are increased by 0.44-0.92 units.（3）Compared with the nonactivated phosphorites,soil available phosphorus contents could be significantly increased within 100 days when mechanochemically activated phosphorites were applied into red soil.After 25 days of incubation,the soil p H value of mechanochemically activated phosphorites were significantly higher than that of superphosphate and nonactivated phosphate rock.（4）The mechanochemically activated phosphorite could gradually release its phosphate.The total dry matter weight,total phosphorus uptake,phosphorus utilization rate(partial productivity of phosphorus nutrient P_pnp)of oat and soil available phosphorus contents under mechanochemically activated phosphorite applications were the same as that under superphosphate application with the same pure phosphorus content,especially for the continuous oat planting.（5）Compared with the treatment of 100% triple superphosphate,the combination of 20% mechanically activated phosphorite and 80% triple superphosphate could get the similar results about phosphorus uptake,phosphorus utilization rate(partial productivity of P_pnp and apparent utilization rate of Prec)and yield of corn.The net income increased by 253.8 yuan hm-2（1.53%）.More than 50% substitution of mechanically activated phosphorite will result in different degree of maize yield reduction.Compared with the treatment of 100% triple superphosphate,the combination of 10% mechanically activated phosphorite and 90% triple superphosphate could get the similar results about soil available phosphorus contents.To sum up,exploring the characteristics and mechanism of phosphorus release from mechanochemically activated phosphorite will provide technical support and theoretical guidance for reasonable utilization of medium and low-grade phosphorite and alleviating the crisis of phosphorus resources in China.