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Ind. Eng. Chem. Res. 1997, 36, 4768-4773
Potash Fertilizer from Biotite Chandrika Varadachari† Department of Agricultural Chemistry and Soil Science, University of Calcutta, 35 Ballygunge Circular Road, Calcutta 700 019, India
A process was developed for the utilization of biotite mica, as a source of potassium, for the production of potassium chloride. The investigation was done in two stages. In the first stage, the kinetics of the reaction between biotite and HCl was studied and various kinetic parameters were evaluated. The reaction appears to be pseudounimolecular with the rate-determining step being the replacement of K+ by H+ in the interlayer space. In the second stage of this investigation, a method for extraction and purification of KCl from biotite was developed and subsequently tested on a pilot scale. Here, KCl was recovered from the mica-acid reaction extract, using ethanol. The residue was treated for recovery of Al3+ as the ammonium alum, and Fe2+ was obtained as FeCl2. Biotite (10 kg) yielded 1.4 kg of KCl (99%). The process appears to be well suited for the commercial production of KCl (as well as ammonium alum) from biotite mica. 1. Introduction The primary sources of potassium for fertilizers are, usually, underground deposits of soluble minerals and potash-bearing brines. Such deposits, however, occur predominantly in the Northern Hemisphere; about eight countries in the North and two in the South account for the potassium produced and consumed by the rest of the world. This situation is ironic when one considers that immense deposits of insoluble potassium silicate minerals occur throughout the world, mainly in the form of micas and feldspars. Of the two major types of micas, the muscovites and biotites, the commercial utility of the latter is practically negligible. A viable means of extracting soluble potassium from biotite, which has about 8% K2O, therefore, holds great potential not only to help several potash-importing countries to attain some degree of self-sufficiency but also to augment the total reserves of usable potassium in the world. Research on the decomposition of silicates for producing potash fertilizers was initiated in the U.S.A. as a consequence of Germany’s embargo on the export of potassium salts during World War I. These early attempts have been described in a large number of patents and papers. The processes involve fusion with alkalis, hydrofluoric acid, limestone, or salts (Harley, 1953; Collins, 1962; Mellor, 1963; Sauchelli, 1967). For example, feldspars were fused with lime and salt at about 1000 °C, potash was leached out, and the residue was used as cement; similarly, Wyomingite was calcined with lime and salt, and the potassium was volatilized as KCl and recovered by cooling; greensand was fused with ferric chloride or nitrate prior to the recovery of potash salts (Jacob, 1953; Mellor, 1963). However, only a few of these processes reached the pilot plant stage and none attained commercial production on a continuing basis (Harley, 1953). Following the discovery of large deposits of potash salts in the U.S.A. after the war, the necessity of the aforesaid attempts was drastically reduced. Besides, as most of the industrially developed countries, which were advanced in research on fertilizer technology, had their own reserves of soluble potassium salts, this subject lost ground and consequently recent † Present address: Raman Centre for Applied and Interdisciplinary Sciences, 11 Gangapuri, Calcutta 700 093, India. E-mail:
[email protected]. Tel: 91-33-479 1112.
S0888-5885(97)00220-0 CCC: $14.00
ligerature pertaining to the production of soluble potash fertilizers from the insoluble silicates is by and large nonexistent. Till today, none of the earlier processes has been adopted even in potash-deficient countries because such processes are, for various reasons, uneconomical. Most raw materials are expensive and nonrecoverable in the course of operation; high temperatures (>1000 °C) are required in one or more stages of manufacture, causing large fuel consumption; the reagents, under the fusion conditions usually adopted, are extremely corrosive and consequently cause difficulties in the choice of materials for construction and fabrication. This investigation was, therefore, undertaken with a view to developing a suitable process, for the extraction of potassium from biotite, that involves inexpensive raw materials and moderate temperatures together with technically simple operating stages. It was initially developed on a laboratory scale and subsequently upgraded to a pilot scale. Preliminary investigations with biotite indicated that potassium can be readily solubilized by concentrated acid solutions. This behavior is unlike that of the other mica mineral, muscovite, which is very difficult to solubilize except by heating with phosphoric acid (Varadachari, 1992a). In fact this reaction between muscovite and phosphoric acid has been utilized for the recovery of potash salts from muscovite wastes (Varadachari, 1992b,c). Although the same reaction can also be applied to biotite, the ready solubility of biotite in HCl as well as H2SO4 provides easier alternatives. Of the various inorganic acids tested (i.e., HCl, H2SO4, and H3PO4), HCl was observed to be the most effective in terms of rate of extraction. Moreover, the volatility of HCl is an added advantage in the succeeding steps involving evaporation of excess acid. Considering these factors as well as others, such as price and availability, HCl was selected as the acid most suited for this purpose. A complete process for the extraction and purification of potash salts was developed. This has been presented here in two parts. The first part consists of fundamental studies of the kinetics and mechanism of the reaction between biotite and HCl. In the second part, methods for purification and recovery of potash salts and results of pilot-scale trials, are reported. © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4769
Figure 1. Solubilization of K+ from biotite as a function of time.
2. Kinetics of Potassium Release from Biotite by Hydrochloric Acid 2.1. Theory. Optimum concentration and amounts of reactants necessary for extraction, as well as the model of the reactant vessel and nature of contact or flow, are all determined by studies on the reaction kinetics. This is because kinetics provides information both on the influence of the amount of reactants on the rate of a reaction and also on the rate-determining step of a reaction. Thus, in the common rate equation, (dc/ dt) ) kcn (Glasstone, 1972), the higher the order of reaction n, the greater the influence of concentration on the time taken to complete the process. The rate-determining step for irreversible reaction of particles with the surrounding fluid may be one or more of the following processes (Levenspiel, 1975). (i) Film diffusion control: diffusion of liquid reactant, through the film surrounding the particle, to the surface of the particle. (ii) Ash diffusion control: penetration of liquid reactant, through the blanket of ash, to the surface of the unreacted core. (iii) Chemical control: reaction of liquid with solid at the reactant surface. One method of deducing the rate-determining step of a fluid-particle reaction is to note the process of reaction of particles measured in terms of time for complete conversion, i.e., by plotting (1 - XB) versus (t/τ) where B is the particle, here biotite, XB is the fraction of reactant B (i.e., biotite) converted to product at any time t, and τ is the time for complete conversion of reactant B (Levenspiel, 1975). By studying the nature of this curve, the rate-determining step can be evaluated. Moreover, the relation of particle radius with the time for complete reaction may also be deduced. 2.2. Materials and Methods. Large, good quality flakes of biotite (from Ajmer, Rajasthan, India) were hand-picked, washed with twice its volume of dilute HCl, made chloride free by washing with water, and airdried. The flakes were dry ground in an electric grinder and sieved to obtain the fraction of mesh size 80-150
B.S. The sample was then dialyzed till chloride-free, dried at 105 °C, and preserved (the sample contains 0.05% adsorbed water). Chemical analysis of the sample was done according to the scheme of Shapiro and Brannock (Maxwell, 1968). XRD was recorded using Nifiltered Cu KR radiation on a Philips PW 1730 instrument fitted with a Guinier camera. Thermogravimetry was done on a Gebruder-Netzsch Instrument (No. 404) at a heating rate of 10 °C/min. Reactions were carried out at room temperature (25 °C) in borosilicate glass containers. To the biotite was pipeted in different amounts HCl (11.55 N) of different dilutions (HCl:H2O, V:V, 100:0, 90:10, 80:20, and 75: 25), and the container was covered with parafilm to prevent loss by evaporation and allowed to stand (mechanical stirring was avoided since the nature and period of stirring greatly influence reaction rates and the reproducibility of the data is affected). The solutions were filtered at intervals of 48, 96, 144, 192, 240, and 312 h, washed, and diluted to volume. The K+ in solution was determined by flame photometry. The exact strength, of the HCl solutions used, was determined by titration with recrystallized borax (Vogel, 1961). 2.3. Results and Discussion. Chemical analysis of the sample gave the following results, viz., 16.67% Si, 9.67% Al, 19.24% Fe, 3.35% Ti, 0.23% Ca, 2.60% Mg, 0.26% Na, 7.75% K, 0.05% H2O- (adsorbed water), and 1.20% H2O+ (structural water). XRD showed major reflections at 10.00, 4.60, 3.32, 2.61, 2.43, 2.17, 2.02, 1.53, and 1.305 Å. TG revealed a single weight loss above 1000 °C that continued up to 1350 °C. Weight loss at this temperature is due to loss of lattice OH. Following the expulsion of OH, the biotite structure persists to about 1100 °C and subsequently is transformed to high-iron magnetic spinel, leucite, and mullite (Grim, 1968). Results on the solubilization of K+ by HCl of varying concentrations and volumes are presented in Figure 1.
4770 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 Table 2. K+ Solubilized from Biotite by HCl (8.49 N) at 100 °C treatment HCl/100 mg of biotite
Figure 2. Application of the first-order rate equation to K+ solubilization. Table 1. Kinetic Parameters of the Biotite-HCl Reaction treatment strength of HCl 11.55 N, 34.77% HCl
10.55 N, 33.41% HCl
9.37 N, 30.06% HCl
8.89 N, 28.62% HCl
vol of HCl (mL/100 mg of biotite) 10-3k (h-1) 0.375 0.500 0.625 0.750 0.375 0.500 0.625 0.750 0.375 0.500 0.625 0.750 0.375 0.500 0.625 0.750
8.73 9.38 23.26 27.60 7.64 12.15 14.93 15.63 6.51 8.85 10.94 16.15 5.76 10.42 14.06 16.67
C 8.62 1.15 0.03 -0.02 0.87 0.82 0.77 0.88 0.84 0.83 0.77 0.55 0.79 0.50 0.33 0.25
A representative plot of ln(a/a - x) versus t (a is the total K+ released at infinite time, and x is the amount released after time t) is given in Figure 2. Rate constants derived from the K+ solubilization curves are given in Table 1. From the data obtained, the following observations may be made. Rates of release of K+ from biotite increase with increasing concentrations of HCl (Table 1). In some instances, initial rates of release are greater with more dilute HCl (Figure 1). However, this effect levels off
volume, mL
mequiv
0.375 0.500 0.625
3.183 4.244 5.304
K+ solubilized with time (% ot total K+) 15 min
30 min
45 min
60 min
90 min
120 min
66.67 80.00 83.33 90.00 91.33 96.67 70.00 83.33 90.00 93.33 100.00 100.00 75.00 86.67 91.67 96.67 100.00 100.00
with increasing time of reaction, so that ultimately release with more concentrated HCl is faster than with more dilute solutions. Reaction periods may be shortened, if necessary, by heating the reactants. One such example of K+ solubilization at 100 °C is shown in Table 2. It may be seen from this table that 0.5 mL of HCl/100 mg of biotite can extract all K+ from biotite within 90 min when the reaction is carried out at 100 °C. Plots of ln(a/a - x) versus t are linear (only some representative plots have been shown here in Figure 2). The reaction is therefore of the first order (Glasstone, 1972). However, the lines will not intercept the Y-axis at zero unlike as in a first-order reaction. A likely explanation is that the initial reaction between HCl and biotite is not of the first order but of a higher order. Such initially higher orders of reaction could be due to release of exchangeable K+ from edges and surfaces. Here, H2O could also be a reacting component, in addition to HCl, and this may be the reason for observed higher initial rates with more dilute HCl, immediately as the acid comes in contact with biotite. This process of K+ release would no longer be operative when all exchangeable positions are depleted of K+. In the later stages, reaction of first order suggests a mechanism of the type K+-biotite + H+ h H+-biotite + K+. It may be deduced that the above process, unlike simple exchange, involves first a diffusion of H+ into the interlayer of biotite followed by the release of K+. This would be the slowest and hence the rate-determining step of the reaction. Subsequently, the H+ probably migrates into the octahedral and tetrahedral positions occupied by Al3+, Mg2+, Fe2+, and Fe3+, thereby solubilizing these cations. Since the first-order rate equation is not valid at lower values of time t, the straight lines in Figure 2 will not intercept the Y-axis at zero when time t ) 0. The rate equation therefore has to be represented in the form y ) kt + c, where y ) ln(a/a - x), k is the rate constant, and c is the intercept on the Y-axis. The values of k and c for the various HCl solutions used during reaction are given in Table 1. It may be mentioned here that though the reaction satisfies the first-order equation, it is actually a pseudounimolecular reaction. The rate of release has been shown to depend solely on the concentration of K+, though the reaction mechanism as depicted in the earlier equation suggests that [H+] is also a determining factor. However, since the acid is present in large excess, the amount used up in the course of reaction is negligible in comparison to the total, so that its concentration may be regarded as remaining constant throughout. Therefore, in the kinetic equation, rate ) k[K+]n[H+]m, this rate is proportional only to the concentration of K+ since [H+] is almost constant. A plot of total K+ released after 312 h versus mequiv of HCl/g of biotite (Figure 3) shows that approximately 65 mequiv of HCl is required for the release of all K+ from 1 g of biotite.
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4771
Figure 3. Amount of K+ solubilized after 312 h in terms of the milliequivalent of HCl added.
Figure 5. The chemical reaction-control equation as applied to K+ solubilization.
Straight lines obtained indicate that the reaction is indeed a chemical reaction control; i.e., the rate-limiting step is the reaction between HCl and biotite at the solid surface. 3. Extraction and Purification of Potash Salts from Biotite
Figure 4. Nature of the (1 - XB) versus t/τ relation for K+ solubilization.
In order to determine the model and rate-limiting step of the reaction, plots of (1 - XB) versus (t/τ) [the symbols have been explained earlier] were drawn for two representative samples (Figure 4). From the nature of the curve, it appears that the reaction is either ash diffusion control or chemical reaction control (Levenspiel, 1975). Since the rate of dissolution of biotite is probably controlled by the rate of reaction of H+ ions with the ions in the crystal, the appropriate equation (for chemical reaction control) was chosen and the validity of this assumption tested by the following methods [Levenspiel, 1975): For particles of constant as well as shrinking size, a chemical reaction control gives a relation between fraction converted XB and time t as (t/τ) ) 1 - (1 - XB)1/3. Values of t/τ were plotted against (1 - XB)1/3 (Figure 5).
3.1. Process Outline. A flow chart for the process is depicted in Figure 6. A brief theoretical background of the various stages is as follows: When HCl is reacted with biotite, solution A is obtained, which contains all the major cations of biotite, viz., K+, Al3+, and Fe2+, in solution whereas silica is present as an insoluble residue. The silica is removed by filtration. Solution A is evaporated to remove excess HCl; solid B thus obtained contains mainly KCl, AlCl3, FeCl2, some FeCl3, and HCl as well as precipitated silica. When stirred with ethanol, the chlorides of iron and aluminum dissolve, leaving a residue of KCl and silica impurities. The KCl is purified by washing and crystallization. Ethanol is recovered by distillation. Aluminum is separated from iron by precipitation as the ammonium alum, leaving an acidic FeCl2 solution. This process was initially developed on a laboratory scale and subsequently tried out in a small pilot plant. 3.2. Production of KCl on a Pilot Scale. The laboratory studies provided valuable information on the nature of the biotite-HCl reaction that was vital for designing the pilot plant. Results of kinetic studies at room temperature revealed both the order of the reaction as well as the rate-limiting step. Such information is a prerequisite for chemical reaction engineering. Pilot scale studies were done in a rubber-lined reactor equipped with a stirrer and three silica-sheathed im-
4772 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997
Figure 6. Flow chart for production of KCl from biotite.
mersion heaters, for the biotite-HCl reaction stage. Due to the temperature limitation of the rubber lining, the reaction temperature could not exceed 80 °C. The reaction was, therefore, studied in the stirred vessel at 80 °C, whereupon it was observed that reaction was complete in 4 h. (It may be pointed out here that owing to the difference in the nature of solid-liquid contact induced by stirring, reaction periods in laboratory scale studies are quite different from those of the pilot scale.) Similarly, the purification stages were developed in the laboratory scale but the quantification of the reactants was done at the pilot level. The various stages of the reaction are shown in Figure 6. Commercial HCl (9.4 N, 40 L) was reacted with 10 kg of biotite at 80 °C for 4 h (stage 1). It was then filtered and washed in a Nutsche filter using polypropylene cloth (stage 2). The residue (stage 2a) contained silica with titanium dioxide as an impurity. The filtrate was evaporated in an all-glass distillation unit, under mild vacuum (stage 3). The residue, after evaporation (stage 3a), contained crystals of KCl, AlCl3, and FeCl2, as well as precipitated silica and a small amount of
MgCl2. This was stirred with 25 L of rectified spirit (90% ethanol) and filtered (stages 4 and 5). The residue was washed further with rectified spirit (8 L). The residue obtained on ethanol extraction (stage 5a) contained KCl together with precipitated silica and some adsorbed iron and aluminum impurities. This was dissolved in 5 L of water, and the silica impurities were filtered out (stage 6). The solution when boiled produced a precipitate of red iron oxide, which was again removed by filtration (stage 7). The remaining solution, when dried, produced crystals of KCl (stages 7b-7d). Thus, 1.4 kg of KCl of 99% purity was obtained. This amounts to recovery of about 95% of the total K+ in the biotite. The ethanolic extract (stage 5b) contained AlCl3, FeCl2, HCl, some MgCl2, and H2O. Ethanol, from this ethanolic extract, was recovered by fractional distillation (stage 8). The distillate was free of acid; recovery of ethanol was about 93%. To the residue remaining after ethanol recovery (stage 8b), 3 L of water was added, followed by 4 L of H2SO4 (36 N). The solution was stirred well to dissolve the crystals; then 2.6 L of
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4773
ammonia solution (14 N) was added slowly with stirring (stage 9). Ammonium alum crystallized and was recovered by filtration (stage 10). About 13 kg of ammonium alum was thereby obtained with a purity of 99.9% and an Fe content of 0.005%. The solution remaining after recovery of alum (stage 10b) contained mainly FeCl2 and HCl together with some MgCl2 and FeCl3. The HCl was removed by evaporation (stage 11) to give 8 L of 9 N acid (stage 11d). The remaining soldis contained 6 kg of FeCl2 of 90% purity (stage 11a). In the various stages of the pilot plant production, no particularly difficult problems were encountered. The only stage that was somewhat problematic was the filtration (stage 2) of the silicia residue from the acidic extract. The process was quite slow and the filter cloth tended to clog; mechanical stirring of the residue with a spatula was, therefore, necessary to facilitate filtration. Apart from this stage, other processes like distillation and filtration of coarse-grained crystals are standard operations that could be quite easily carried out. Summary and Conclusion Kinetics of reaction of biotite with HCl revealed that solubilization of K+ from biotite follows first-order kinetics. It is a pseudounimolecular reaction, and the rate-determining step is the replacement of K+ by H+ in the interlayer space. Various kinetic parameters were obtained. A method for extraction and recovery of KCl from biotite was developed in the laboratory and subsequently tried out on a pilot scale. In this method, the extract from the biotite-HCl reaction was evaporated and the residue washed with ethanol; precipitated KCl was purified. Ethanol was recovered from the extract and remaining solids were treated with H2SO4 and NH4OH to precipitate Al3+ as the ammonium alum; FeCl2 was recovered in the filtrate. In this process, by utilizing low value raw materials, commercially important products and byproducts are obtained. The major raw material, biotite, is practically a waste material with little commercial value; the other raw material, hydrochloric acid, is very cheap and widely available. Apart from these, sulfuric acid and ammonia are also used but these are completely recovered as their byproducts. All the products, viz., potassium chloride, ammonium alum, ferrous chloride, and amorphous silica, have ample markets as well as commercial value. In conclusion, it appears that this process is well suited for the commercial exploitation of biotite mica
for the production of potash fertilizer. It may be particularly useful for several countries that lack soluble potash ores but contain low value biotite deposits; for others, the recovery of substantial quantities of aluminum may provide greater impetus. Acknowledgment The author thanks the Council of Scientific & Industrial Research and the Department of Science & Technology, Government of India, for financial support. She is also grateful to Professor S. K. Mukherjee and Dr. Kunal Ghosh for their advice. Literature Cited Collins, G. H. Commercial Fertilizers; Tata McGraw-Hill: Bombay, 1962. Glasstone, S. Textbook of Physical Chemistry; MacMillan: London, 1972. Grim, R. E. Clay Mineralogy; McGraw-Hill: New York, 1968. Harley, G. T. Production and Processing of Potash Minerals. In Fertilizer Technology and Resources; Jacob, K. D., Ed.; Academic: New York, 1953. Jacob, K. D. Fertilizer Technology and Resources; Academic: New York, 1953. Levenspiel, O. Chemical Reaction Engineering; Wiley Eastern: New York, 1975. Maxwell, J. A. Rock and Mineral Analysis; Interscience: New York, 1968. Mellor, J. W. A Comparative Treatise on Inorganic and Theoretical Chemistry; Longmans Green: London, 1963; Vol. II, Supplement II, Part 2. Sauchelli, V. Chemistry and Technology of Fertilizer; Reinhold: New York, 1967. Varadachari, C. An Investigation on the Reaction of Phosphoric Acid with Mica at Elevated Temperatures. Ind. Eng. Chem. Res. 1992a, 31, 357. Varadachari, C. Phosphoric Acid, Phosphates and Fertilizers for the Future. Proc. Indian Natl. Sci. Acad. 1992b, B58, 119. Varadachari, C. Phosphoric Acid Polymerization and Its Role in Fertilizer Development. In Indian Fertilizer Scene Annual; Shukla, H. C., Ed.; Commercial Publications: Bombay, 1992c. Vogel, A. I. A Textbook of Quantitative Inorganic Analysis; ELBS & Longmans Green: London, 1961.
Received for review March 18, 1997 Revised manuscript received June 25, 1997 Accepted July 1, 1997X IE970220U
X Abstract published in Advance ACS Abstracts, September 1, 1997.
