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[SCG53-12] Synthesis of siliceous potassic mineral fertilizer from drifted pumice
Keywords:Drifted pumice, Mineral fertilizer, Kaliophilite, Akermanite
In 2021 a large amount of pumice stones produced by the explosive volcanic eruption of Fukutoku-Oka-no-Ba drifted widely along the coast from Okinawa to Kyushu region in Japan, which caused serious damage to shipping and fishing industry as well as tourist business. Although a part of these pumice stones removed from beaches and bays was used for concrete materials and soil improvement, most of them still remain undisposed in temporary storages. In this study, we investigated the possibility of utilizing the drifted pumice stones for silicious potassic mineral fertilizers, which have been produced usually from fly ash, a by-product of coal-fired power generation and used for rice paddy for more than 40 years. In addition to potassium, silica is also important to supply to the soil for rice plants, which accumulate it in the form of opal in their tissue. The silicious potassic fertilizer synthesized by the calcination of a mixture of fly-ash, KOH and other minor additives consists of kaliophilite (KAlSiO4) and akermanite (Ca2MgSi2O7), from which the fertilizer components are slowly leached out to the soil. The felsic pumice stones from Fukutoku-Oka-no-Ba are expected to be a potential raw material for this mineral fertilizer, since the chemical composition is similar to that of fly ash.
We performed a series of sintering experiments using the pumice stones collected from Uppama beach in Okinawa, Japan in an electric furnace. The pumice stones were crushed into powder and mixed with KOH solution with/without Al(OH)3, Mg(OH)2 and Ca(OH)2 chemical reagent in ceramic crucibles. The samples were heated to 800 ~ 1100℃, kept for a certain duration and cooled down to room temperature at 1 atm. The recovered samples were examined by XRD and SEM-EDS.
XRD patterns showed that after heating at 800°C for 2 hours, broad diffraction peaks of kaliophilite were detected and they became more intense and shaper with increasing temperature. The formation of akermanite and more Ca-rich merwinite (Ca3Mg(SiO4)2) were detected in the samples heated above 900°C. The sample heated at 1000°C consists of kaliophilite, akermanite and small amount of merwinite, which is almost comparable to the commercial product synthesized from fly ash. When heated to 1100°C, the formation of kalsilite, a polymorph of kaliohilie, became dominant, although the peak intensity of akermanite also became stronger. We also conducted experiments at 1100°C for different heating duration. In 5 minutes heating, kalsilite formation was limited and kaliophilite is dominant, but as the heating duration increased, kalsilite became more dominant, eventually becoming a single phase after 12 hours.
SEM observation on the cross sections of the recovered samples showed that the samples heated at 800 and 900°C composed of loosely aggregated irregular grains that retained the grain shape of the starting glass (pumice), while those heated at 1000°C and 1100°C show rather massive aggregate where the shape of the starting grains cannot be recognized and they were adhered to each other. However, no evidence of melting was observed, indicating that the formation of crystalline phases occurred by solid-solid reactions. In the sample heated at 900°C despite that K penetrated deep into glass particles, Al diffused only to the periphery, indicating a significant difference in the diffusion rate of the two elements. This is likely associated to the crystal structure of kaliophilite, in which Al is coordinated by oxygen atoms to form tetrahedron that build up the vertex-sharing framework together with SiO4 tetrahedron, while K is coordinated in the gaps between the framework. Therefore, the diffusion rate of Al, which requires tetrahedral recombination in the glass, is considered to be slower than that of K.
In order to confirm the effectiveness of the present products as a fertilizer, a dissolution test in ammonium citrate solution adjusted to pH 4.5 was conducted and the concentrations of ions dissolved in the solution were analyzed by ICP-OES. The products synthesized at 1000°C and 1100°C were found to show high K and Si elution rates equivalent to those of the commercial products produced from fly ash. Our study lead to the effective utilization of pumice stone drifted ashore, which is regarded as a nuisance and difficult to dispose of.
We performed a series of sintering experiments using the pumice stones collected from Uppama beach in Okinawa, Japan in an electric furnace. The pumice stones were crushed into powder and mixed with KOH solution with/without Al(OH)3, Mg(OH)2 and Ca(OH)2 chemical reagent in ceramic crucibles. The samples were heated to 800 ~ 1100℃, kept for a certain duration and cooled down to room temperature at 1 atm. The recovered samples were examined by XRD and SEM-EDS.
XRD patterns showed that after heating at 800°C for 2 hours, broad diffraction peaks of kaliophilite were detected and they became more intense and shaper with increasing temperature. The formation of akermanite and more Ca-rich merwinite (Ca3Mg(SiO4)2) were detected in the samples heated above 900°C. The sample heated at 1000°C consists of kaliophilite, akermanite and small amount of merwinite, which is almost comparable to the commercial product synthesized from fly ash. When heated to 1100°C, the formation of kalsilite, a polymorph of kaliohilie, became dominant, although the peak intensity of akermanite also became stronger. We also conducted experiments at 1100°C for different heating duration. In 5 minutes heating, kalsilite formation was limited and kaliophilite is dominant, but as the heating duration increased, kalsilite became more dominant, eventually becoming a single phase after 12 hours.
SEM observation on the cross sections of the recovered samples showed that the samples heated at 800 and 900°C composed of loosely aggregated irregular grains that retained the grain shape of the starting glass (pumice), while those heated at 1000°C and 1100°C show rather massive aggregate where the shape of the starting grains cannot be recognized and they were adhered to each other. However, no evidence of melting was observed, indicating that the formation of crystalline phases occurred by solid-solid reactions. In the sample heated at 900°C despite that K penetrated deep into glass particles, Al diffused only to the periphery, indicating a significant difference in the diffusion rate of the two elements. This is likely associated to the crystal structure of kaliophilite, in which Al is coordinated by oxygen atoms to form tetrahedron that build up the vertex-sharing framework together with SiO4 tetrahedron, while K is coordinated in the gaps between the framework. Therefore, the diffusion rate of Al, which requires tetrahedral recombination in the glass, is considered to be slower than that of K.
In order to confirm the effectiveness of the present products as a fertilizer, a dissolution test in ammonium citrate solution adjusted to pH 4.5 was conducted and the concentrations of ions dissolved in the solution were analyzed by ICP-OES. The products synthesized at 1000°C and 1100°C were found to show high K and Si elution rates equivalent to those of the commercial products produced from fly ash. Our study lead to the effective utilization of pumice stone drifted ashore, which is regarded as a nuisance and difficult to dispose of.