Monday, 24 March 2014 16:11

Reconstituting Feldspar for Fertilizer

    Postdoctoral Associate Taisiya Skorina discovers water-based alkaline process to make potassium more readily available.
    Taisiya Skorina explains how surface roughness, a combination of pores and cavities in particles of finely ground feldspar, helps promote release potassium for fertilizer. The MIT Postdoctoral Associate has developed a process to chemically alter feldspar to produce a new compound ­– coined "hydrosyenite" – with a popcorn-like texture. Photo: Denis Paiste, Materials Processing Center

    Just as many nations depend on imports for energy, others rely on imports for essentials like potassium fertilizer. MIT researchers are developing a new process to produce a potassium fertilizer from the mineral ore feldspar, which is widely available in countries like Brazil, but which releases potassium into the soil from natural weathering at much too slow a rate to benefit farmers.

    MIT Postdoctoral Associate Taisiya Skorina has developed a process based on crushing and chemically altering feldspar to produce a new compound, hydrosyenite, which has a popcorn-like texture. The potassium is contained in water-rich layers that make it more easily separated in soil. The chemical process is based on an alkaline treatment that does not lead to waste generation, a critical criteria for sustainable processes.

    The development could be a boon for Southern Hemisphere countries, for example, Brazil, which imports 90 percent of its potassium. That potassium – from potash – is mined in the Northern Hemisphere, predominantly in three countries, Canada, Belarus and Russia.

    Skorina works in the lab of Antoine Allanore, the Thomas B. King Assistant Professor of Metallurgy at MIT, who is leading a research effort with globally important implications to produce potassium fertilizer from feldspar. "We have a rock which contains about 15 percent potassium. Now we know what we need to do to control rate of dissolving, or amount of potassium you can dissolve, in a given amount of time; we have actually been very successful," Allanore explains. (See related article.)

    Although feldspar is widespread, Skorina notes, its commonly occurs in a form called microcline that tightly "traps" potassium within aluminum, silicon and oxygen.

    Feldspar can be milled or ground into powders with particle size ranging from measurements in microns, or millionths of a meter, to millimeters, or thousandths of a meter. Skorina began with dissolution studies, hoping to increase the rate of potassium release with nothing more than physically turning the feldspar into an ultra fine powder. Mineral assays showed that just grinding is not enough and the dissolution of potassium from the material is too slow to be used as fertilizer.


    Newly created hydrosyenite powders promise to provide potassium fertilizer for places like Brazil from locally abundant natural ore deposits. The newly developed compound has a popcorn-like texture. Photo courtesy of the researchers.

    "In the industrial mill, you cannot actually break the mineral up to nanoparticles," Skorina explains. Electrostatic energy causes the particles to stick together, and with decreasing of particle size below one micron, this effect is very hard to overcome.

    About six months ago, Skorina switched to processing the powders in various water-based alkaline treatments, establishing that calcium hydroxide is the best enabler of liberating potassium from its aluminum silicate framework, using the chemical process of hydrolysis, which breaks the bonds through interaction with water.

    "For our purpose, we found that calcium hydroxide is not only the cheapest one, it's also the best one," Skorina says. The hydroxide can be produced from another common mineral, calcium carbonate (limestone). "The idea is to hydrolyze the aluminosilicate framework in alkaline medium and precipitate in new phases," Skorina explains. The hydrolysis process involves treating the crushed potassium feldspar at elevated temperature and pressure in an alkaline solution to catalyze transformation of the aluminosilicate framework to a new set of compounds with potassium preferentially trapped in an amorphous, rather than a crystalline, compound.

    Feldspars are crystal-structured compounds of an alkaline or alkaline-earth element (M) bound to aluminosilicates with the chemical formula M1+(Al,Si)4O8 or M2+(Al,Si)4O8. Potassium feldspar (K-feldspar) has a theoretical composition of 16.9 percent potassium oxide (K20) by weight but occurs in three different molecular arrangements, or polymorphs, depending upon the temperature and cooling rate at formation and subsequent heat exposure. The crystal lattice features clusters of aluminum oxide and silicon oxide in a tetrahedral, or pyramidal, shape. Potassium ions balance the extra charge created by aluminum-silicon bonds with oxygen within the lattice framework. "To take it (potassium) out, you need to break this framework," Skorina explains.

    The reconstituted compounds are layered with many water molecules between the alumina silicate sheets. "It increases volume drastically after this synthesis, and it becomes just loose, so it's not like a hard rock before, which is impossible to break. Now it's like popcorn. It absorbs a lot of water during the aqueous treatment, and this water helps to loosen the structure, in which potassium now is much more available. About 20 percent of potassium releases in the first 24 hours in aqueous solution of pH5, which is the pH of acidic soils, Skorina says. Skorina and Allanore have applied for a patent for the process of transforming the potassium feldspar to a new material called hydrosyenite. Transmission electron microscope images show the amorphous structure, which is enriched with potassium and aluminum, along with newly formed crystalline structures that are different from the initial crystalline material, and appear to be mostly calcium silicate hydrate, she says. "From these basic reactions, we know what happens, but we need to optimize the process, the ratio between these two newly formed phases."

    Taya research Structures Web
    Schematic representation of crystalline structure in original feldspar  microcline (a), top left, and schematic representation of amorphous component of the reconstituted material showing a semi-crystalline aluminosilicate matrix (b), top right – water molecules and OH groups are omitted for clarity; scanning electron microscope (SEM) image (c), bottom left, and transmission electron microscope (TEM) image (d), bottom right, show co-existence of crystalline and amorphous components. Images courtesy of Taisiya Skorina

    "We convert the rock to popcorn, but we are still interested in this surface specific area, again, because we are going to extract potassium from it," Skorina explains. In the lab, Skorina uses a Micromiretics gas sorption analyzer to measure the surface area and porous structure of the powders, two important parameters for establishing control of potassium release from the compound. In the analyzer, a gas such as krypton or nitrogen condenses on the sample material (sorption) and then the process is reversed (desorption). The resulting data, measured in sorption isotherms, can be translated to a measure of the specific surface area of the sample, expressed in square meters per gram. "We've worked with powders with specific surface area ranging from 2 meters-squared per gram to 20 meters-squared per gram that's our range," Skorina says.

    "We assume that this interaction is just physical interaction; there is not chemical bonding between our gas and the material," Skorina explains. Defects on the surface of the powders – referred to as roughness when they are wider than they are deep – provide contact points for chemical interactions to release potassium from the powder.

    A second measurement from the analyzer, an isotherm, sometimes in the form of a hysteresis curve, gives information about the pore size distribution in the material. "We try to control porosity also, but it's not so easy. You can see that it's light. It's very light, and it's very easy to break, which is good for fertilizer," Skorina says while holding a sample in the lab.

    The ideal outcome from the research would be to develop a processing method which transforms the feldspar to a form that releases potassium at the rate plants need it to grow.

    Another interesting finding from Skorina's research is that the transformation of potassium feldspar to hydrosyenite is not always complete. Along with the new amorphous material, some crystalline potassium aluminosilicate remains in a finely ground form. This provides the possibility that the new material could have a long-lasting effect both by releasing potassium immediately, helping farmers with their current crop, and enriching the soil as the remaining initial micro feldspar releases potassium over time.

    In a related study, MIT Postdoctoral Associates Davide Ciceri is using microfluidic techniques to study how potassium leaches from feldspar under exposure to acidic solutions. (See related article.)

    Before joining the Allanore Lab, Skorina was a Visiting Scientist at Arizona State University, Department of Chemistry and Biochemistry,  where she worked on synthesis and characterization of novel alkaline-activated aluminosilicate-based nano porous materials. She earned her master's and doctorate in Materials Science from Mendeleev University of Chemical Technology of Russia.

    Written by Denis Paiste, Materials Processing Center

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    Last modified on Saturday, 29 March 2014 09:34