Investigation of microwave roasting for potash extraction from nepheline syenite
Introduction
The rising demand of potash for the growth of plants, human and animal life is inevitable [1]. Unfortunately, there are no potash resources such as sylvite, carnallite or polyhalite in a vast country like India. Therefore, it imports all its potash demand (about 3.5 million tons per annum) from different countries. India possesses abundant mineral sources like glauconitic sand, feldspar, muscovite, and nepheline syenite containing 3–14% K2O. However, no systematic effort has been made to recover the potash values from any of these available resources. The recovery of potassium from the silicate rocks has received little attention probably due to the slow release of potassium values, the availability of appropriate economic processes, and their know-how. Some efforts have been made to recover potassium values from feldspar and glauconitic sandstone by acid leaching, and calcium chloride roasting followed by water leaching [2], [3], [4], [5]. However, to the best of our knowledge, no commercial extraction of potassium values from nepheline syenite has been reported so far.
Nepheline syenite is a feldspathic and plutonic igneous rock. It is primarily made up of nepheline (Na,K)AlSiO4, feldspar, albite (NaAlSi3O8), and orthoclase (KAlSi3O8) [6]. It is used as a source for Al2O3, Na2O and K2O for glass and ceramics industry. In ceramic industries, nepheline syenite is used as an active agent for the formation of a glassy phase in the ceramic body and provides the physical strength to the final products. The K2O content in nepheline syenite varies from 3% to 14%. Most of the investigations on nepheline syenite are concentrated on the removal of iron values by magnetic separation and flotation techniques for its use in glass and ceramics production [6], [7].
Microwave energy based on non-ionizing electromagnetic radiation with frequencies in the range of 0.3–300 GHz is an alternative source of heating. The microwave frequency in the range of 2.45 GHz is widely used in different industries. Microwaves generate molecular motion by migration of ionic species and rotation of dipolar species. Microwave heating is material specific, which depends on the ratio of the dielectric loss and the dielectric constant termed as dissipation factor. The major advantage in microwave treatment is the non-contact energy transfer compared to the direct heat transfer in conventional heating. The rapid heating generated by microwaves can be transmitted, absorbed or reflected depending on the nature of the material [8], [9]. Some of the major mineral engineering applications of microwave energies include the treatment of ores with complex mineralogy in order to enhance the liberation, microwave assisted grinding, and carbothermic reduction of ores. The process of heating followed by sudden quenching causes the micro cracks, which decreases the lattice strength of the ore and thereby reduces the grinding cost. One major disadvantage of conventional way of heating adopted in industries is that it requires a significant amount of heat input; also the energy balance is unfavorable due to the loss of energy [10]. On the other hand, microwave heating offers advantages over the conventional heating as it selectively heats the responsive mineral in the ore and no heat loss occurs due to bulk heating. In this context, it has been reported that the ore mineralogy has a significant impact due to the selective heating of allied phases and thus high temperature heating can be avoided [11], [12], [13], [14]. The effect of pre-treatment of microwave on grindability of different ores such as coal, oolitic iron ore, and tin has been reported beneficial due to the development of micro fractures along the boundaries, and reduced bond work index [11], [12], [15], [16]. The efficiency of microwave pre-treatment is dependent on the microwave power and time of exposure.
Besides grinding, microwave heating also finds some broad application in carbothermic reduction processes of metal oxides. It offers faster heating rate, uniform heat distribution, and preferential reduction that start from the core to the surface as compared to the reverse direction in conventional heating. Therefore, microwave methods are reported as an efficient alternative for the reduction of metal oxides [17], [18], [19]. The use of microwave pre-treatment prior to leaching has also been investigated by many researchers [20], [21].
Considering the above mentioned applications of microwave energy, an attempt has been made to extract potassium recovery from nepheline syenite by using microwave assisted heating technique. The purpose of using microwave in place of the conventional roasting is because of its ability to offer faster kinetics through rapid heating. To the best of our knowledge, this methodology has not been applied to optimize the potash extraction from nepheline syenite till date. Recently, we have reported the extraction of potash values from nepheline syenite by CaCl2 roasting in a conventional furnace at ∼900 °C followed by water leaching [22]. Although it was possible to recover ∼99.6% of K2O values, it requires higher temperature and time. Hence, in the present study, an attempt has been made to use microwave energy with an objective to reduce the energy consumption in terms of time and temperature.
Section snippets
Sample pre-treatment and characterization
Nepheline syenite sample was collected from one of the mines in Odisha, India. The size reduction of the sample was carried out by using stage crushing with laboratory jaw crusher and roll crusher. The laboratory jaw crusher was supplied by Eastman Crushers Company (P) Ltd, Kolkata India, where as the roll crusher from Rajco Science and Engineering product, New Delhi, India was used in the crushing studies. The size analysis of the crushed products was carried out by standard sieves down to 100
Effect of particle size and microwave energy
The nepheline syenite sample was ground to different sizes in order to study the effect of particle size on potassium recovery. The d80 size of various fractions (75–3350 μm) was obtained by grinding the sample in different intervals of time using a laboratory ball mill. The effect of particle size on the recovery of potassium was studied with 50% CaCl2, 25% charcoal, microwave power of 900 W, and 8 min of microwave treatment followed by 15 min of water leaching. The results shown in Fig. 1 suggest
Conclusions
Nepheline syenite rock containing around 5.4% K2O was considered as an alternative source for potash values. The characterization studies revealed the presence of orthoclase, nepheline and biotite as the major potassium bearing mineral phases. The use of microwave assisted roasting followed by water leaching was found successful in recovering up to 85–88% potassium values from this unconventional silicate rock. CaCl2 was found to be a potential K-extracting agent, whereas charcoal was primarily
Acknowledgements
The authors are thankful to the Director, CSIR-IMMT, Bhubaneswar for his kind consent to publish this paper. We are also thankful to the Council of Scientific and Industrial Research, India for their financial support to carry out the research work.
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