Preparation And Properties Of Slow Releasing Silicon-Potash Fertilizer By Using Magnesium Slag

Magnesium and its alloy are the important strategic reserve material,and China is the largest magnesium producers and exporters. Magnesium slag ?MS? is the solid waste resulting from the production of magnesium metal by the Pidgeon reduction process. One ton of primary magnesium is produced, while 6-8 tons of MS is discharged. Amounts of PMS are being dumped in slag disposal areas without any profit, which not only occupies plenty of land but also causes serious environmental pollution. In addition, the shortage of potassium resources and the lower utilization of potash fertilizer also restrict the development of agriculture in our country. In this paper, based on study of the characteristics and environmental risk of magnesium slag, a novel slow releasing potash-silicon fertilizer ?SPF? has been prepared from modified Pidgeon magnesium slag by potassium carbonate.The preparation,the crystalline properties, the nutrient release characteristics, the fertilizer efficiency and agricultural safety for SPF were systematically investigated in experiments and theory.1) The physicochemical characteristics and environmental risk of MS were comprehensively investigated by XRD?FTIR?TG-DTG?SEM-EDS, etc.. The results indicated that MS is a alkaline waste, the majority of MS were sized<147 ?m and the chemical composition of different particle size for MS was different; The apparent density, BET surface area, mean pore volume, and pore size were 1.49±0.21 g/cm3,1.543 m2/g,0.0042 cm3/g, and 14.16 nm, respectively. As surface area and pore volume of PMS are so low, it is generally not suitable for absorbing materials but for the building materials. The main elements were Ca, Si, Mg, Al, Fe and O, mainly existing as ?-Ca₂Si04??-Ca₂Si04, Periclase and f-CaO, and minor phases were Ca?OH?2,Mg?OH?2,CaCO3 and MgCO3; MS was bound within a stable crystalline phase, as occurs in natural ore deposits; the heavy metal content and the average values of IRa and Ir of MS were lower than the limit values of organic-inorganic compound fertilizer ?GB 18877-2009? and the national standard limit values ?GB6566-2010?.The leaching mass concentrations of heavy metals in MS at different leaching tests were far lower than the limit values of the relevant standards, however, the heavy metals in MS existed as stable speciation. Even at the "worst scenario", the maximum extractions of heavy metals in MS was very less. While Cr as the highest content in MS was existed as the less hazardous Cr3+. The leaching amount of Cr, Cu and Ni from MS were correlative with pH value of extractant and lower than the limit value of the national standard. Therefore, MS contains the nutritional elements to plants and soil, and its environmental risk is low, which is suitable for agricultural resource utilization?2) The process of preparing the silicon-potash fertilizer ?SPF? from modified MS by potassium carbonate were conducted by single factor test and orthogonal experiment. First, the factors and levels of orthogonal experiment were determined by single factor test, and then using the active silicon content in total silicon ?AST? and the initial release rate of K ?Kin? as index, the process conditions for preparation were optimized by orthogonal experiment. The results showed that The relative significance of the various factors on AST and Kin can be described in order as heating temperature ?T? > particle size of MS ?S?> holding time ?t?> K₂O content ?P? and S> T> t> P respectively, where T and P had significant influence on Kin and AST ??=0.05?; the optimal conditions for SPF preparation were air cooling MS, K₂O content=20%,T= 1300?, holding time= 60 min, and forced air cooling. The active silicon content of the prepared SPF was 2.5 times as much as that of magnesium slag, the yield for K is about 90%, the slow-release character of SPF met the requirement for the partly slow-release fertilizer in the national standard GB23348-2009,the character of SPF have not been affected by oxides Fe2O3?Al2O3, but MgO in MS can improve the slow-releasing for K and the availability for Si.3) The crystalline properties of SPF were analyzed by XRD, SEM and SEM-EDS. The results showed SPF existed mainly in the form of crystalline phases Ca1.197K0.166SiO₄,K₂MgSiO₄, and K₄CaSi₃O₉, and in the noncrystalline phase, and the reaction mechanisms mainly are K₂CO3 can decompose into K₂O and react with CaO, SiO2, and MgO in MS to obtain the ternary systems K₂0-Ca0-SiO2 and K₂O-MgO-SiO2; The types of MS have not influence on the crystalline and slow-release properties of SPF, but cooling method, heating temperature, particle size of MS, holding time and adding amount of K₂O have the effects on the crystalline property of SPF, and then affect dissolution of Si K; the type and composition of the crystalline phase in prepared SPF have not be affected when adding 2% Fe2O3 and 1% Al2O3 in MS, but adding 3% MgO in MS will be helpful for formation of the crystalline phase K₂MgSi04 and improving properties of SPF.4) The nutrient release characteristics of SPF were investigated by the solution culture and soil column leaching, and the K release kinetic courses were described by using the kinetics models. The results showed that the optimal sizes of SPF are 20?40 meshes, the differential dissolution rates of K are 3.79?3.95%, Kin are 23.93?24.62%; the release rate for SPF in citric acid is higher than the release rate for SPF in water, and the release rate for SPF in soil solution is lower than the release rate for SPF in aqueous solution, SPF have obvious K slow-release characteristic compared to K₂SO₄ fertilizer, which indicated K in SPF can be released effectively according to requirements for plant growth, consequently improving the utilization rate of fertilize. The best models for describing the K release kinetics in water and 2% citric acid are the Elovich model and the first-order model, the best models in soil solution by water and 2% citric acid leaching are the power function and Elovich model, respectively. The crystal structure of SPF was destroyed and transformed into the amorphous phase after leaching in 2% citric acid, whereas the crystal phases of SPF almost have no change and the relative peak intensities of the mineral phases for the potassium silicate crystal phase was weakened after leaching in water, which shows that the essence of nutrients release process in SPF can be the dissolution process of SPF, where the crystal structure of SPF was destroyed by the action of solution.5) The fertilizer efficiency and agricultural safety of SPF were studied by the maize pot experiment. The results showed that application of SPF can improve maize agronomic traits, increase the dry matter and grain yields of maize, uptakes of K and Si in maize, and the utilization ratio of K, Si in fertilizer; SPF can continuously compensate the reduction of the available potassium of soil for absorption of crop, promote the effectiveness of K and Si in the soil, can meet the nutrient needs of crop in the whole growth period; Application of SPF can make pH value of soil elevated for alkalinity of SPF, which is more obvious in acid soil than alkaline soil. The heavy metal contents of SPF conformed to the national standard for organic-inorganic compound fertilizers, and the leaching mass concentrations of heavy metals and F were far lower than the limit values of the identification standard for hazardous waste identification for extraction toxicity, and also met the class ? quality standard for ground water; the concentrations of heavy metals in the soil and maize of SPF treatment are in safe range. The evaluation of the soil quality shows the other pollution indexs are in safe range except that the nemerow indexs for ?-K and II-F threatments are in alert range.Therefore, application of SPF have not impact on the environmental safety of soil and crop.In conclusion, preparing SPF by MS and K₂CO3 is feasible, which can provide insights in terms of the preparation of slow-release fertilizers and the resource utilization of magnesium slag.