Effect of Pre-reaction Ball Milling on Kinetics of Lanthanum Phosphate

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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Effect of Pre-reaction Ball Milling on Kinetics of Lanthanum Phosphate Roasting with Sodium Carbonate Ward A. Burgess,*,† Murphy J. Keller,‡ Jonathan W. Lekse,‡ Bret H. Howard,‡ Elliot A. Roth,† and Evan J. Granite‡ †

AECOM, 626 Cochrans Mill Road, P.O. Box 10940, Pittsburgh, Pennsylvania 15236, United States National Energy Technology Laboratory, United States Department of Energy, 626 Cochrans Mill Road, Pittsburgh, Pennsylvania 15236, United States

‡

ABSTRACT: To design economic roasting processes for the recovery of rare earth phosphates from coal-derived feedstocks, it is convenient to study model systems of rare earth phosphate plus a reactant. In this work, the kinetics of the high-temperature roasting of lanthanum phosphate with sodium carbonate were studied. It is typically necessary to heat the reaction mixture above the melting point of Na2CO3 (851 °C) to observe appreciable reaction to form La2O3. However, grinding the reactants reduces the necessary temperature to as low as 625 °C. It is believed that this increase in reactivity occurs because the ball milling process reduces the reactant particle size in addition to ensuring that the reaction mixture is thoroughly mixed. XRD hot stage results suggest a two-reaction mechanism is the route by which La2O3 is produced from the ball milled sample upon reaction. Apparent activation energy and Arrhenius prefactor for each reaction were determined from TGA experiments.

1. INTRODUCTION Rare earth (RE) elements comprise the 14 naturally occurring lanthanides plus yttrium and scandium. They are vital to industrial operations and are most widely used in the production of magnets, phosphors for video displays, metal alloys, catalysis, and glass production. The industrial world is very dependent on rare earth elements, and yet China controls the vast majority of world production. The price spike following the 2010 cutback in export quotas by the Chinese government indicated the power their decisions had over the worldwide price of RE minerals. The Chinese embargo of RE elements to Japan illustrates the importance of national self-sufficiency in RE mineral production. To this end, the United States National Energy Technology Laboratory (NETL) seeks to recover RE elements from coals and coal-derived samples originating from coal mines in the United States.1 More than 2 billion tons of coal refuse and 100 million tons of ash are produced each year in the United States. Characterization of coal samples from around the world has shown that RE elements are present at concentrations up to 5000 ppm on an ash-only basis.2 The RE minerals present in such coal ash samples are primarily monazite and xenotime at concentrations of 1000 ppm or lower. Within monazite and xenotime, the RE minerals are most likely to exist in the form of RE phosphates. Therefore, NETL has interest in the discovery of improved processes for the recovery of RE phosphates from coal-derived samples, especially coal ash. Typically, the phosphate must be removed from RE minerals before they undergo further processing. If the dephosphorization does not occur prior to metallurgical processing, the © XXXX American Chemical Society

presence of phosphates will cause the formation of phosphides, which will destroy RE complexes whose formation is necessary in metallurgical processes. Rare earth phosphates are highly stable even at temperatures to 1500 °C and do not undergo phase change.3,4 Industrial processes involve roasting in the presence of either sulfuric acid5 or sodium hydroxide6 to convert them into either RE sulfates or RE hydroxides, respectively. RE phosphates react with NaOH to form RE(OH)3 and sodium phosphate without undesired side reactions. Such processes have proven their reliability; more than 95% of the RE elements initially present in an ore can be recovered by NaOH roasting followed by a water leaching step to remove the sodium phosphate byproduct formed.7 However, new process technology for the dephosphorization of RE phosphates is an active field of research. Current technologies are suited to feedstock minerals that have a substantial concentration of RE elements (at least ∼50%), and are not economical for the coals and coal byproducts, which only contain them on the ppm level. Because the RE minerals present in the potential coal and coal-derived feedstocks are often present as RE phosphates, the great focus in this section will be new processes to recover RE phosphates. The impetus for such research stems from the desire to (1) cut the costs associated with processing high-volume feedstocks with a low RE content and (2) develop “greener” processes (i.e., Received: January 17, 2018 Revised: April 13, 2018 Accepted: April 18, 2018

A

DOI: 10.1021/acs.iecr.8b00249 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 1. Flowchart for an alternate process under investigation at NETL for RE mineral recovery from coal ash. The new proposed ball milling + solid roasting pathway is highlighted in red to distinguish from the typical NaOH solution roasting step.

Table 1. Various Studies Have Attempted To Improve the Roasting Reaction by Varying Reactant, Method of Reaction, or Reaction Temperature reactant

T (°C)

time (h)

REPO4 source

REPO4:reactant ratio

ref

NaOH (s) NaOH (aq), 50 w/v % NaOH (s) CaO:NaCl:CaCl2 (s) CaO (s) Na2CO3 (s) charred coal (s) Al2O3 (s) SiO2, w(t ) > w∞,RXN1

w0,RXN2 − w(t ) w0,RXN2 − w∞,RXN2

, w0,RXN2 > w(t ) > w∞,RXN2

In eq 7, X is the conversion, t is reaction time, T is reaction temperature, and f(X) is the kinetic relationship. Rate constant k(T) is a function of the kinetic parameters of activation energy Ea and Arrhenius prefactor k0. At isothermal reaction conditions, the value of k(T) can be determined by integrating eq 7. g (X ) =

(5)

X

dX = k(T )t f (X )

(8)

The choice of a kinetic model is an important one. The kinetic model takes into consideration factors such as particle shape and diffusion limitations that can affect reaction kinetics. Several common models useful for solid-phase reactions are summarized in Table 2. Reaction 1 between LaPO4 and Na2CO3 is a solid−solid phase reaction. It has been shown that an ash diffusion-limited kinetic model describes solid-phase RE mineral decomposition reactions.8 The ash diffusion-limited model requires a plot of (3 − 2X − 3(1 − X)2/3) versus t, with the slope of this plot being equal to the apparent rate constant k(T) at that temperature.

(6)

3.3. Determination of Reaction Kinetics. Once X1 and X2 are determined as a function of reaction time, it is possible to define kinetic models that govern the respective rates of reactions 1 and 2. Reaction rate is given as dX = k(T )f (X ) dt

∫0

(7) G

DOI: 10.1021/acs.iecr.8b00249 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

the slightly lower temperature of 810 °C. After the reactants were milled in a reciprocating ball mill for 30 min prior to reaction, XRD hot stage experiments and 2 h decomposition ratio data confirm that the minimum reaction temperature for La2O3 production was reduced to as low as 625 °C, and the lanthanum oxycarbonate intermediate La2O2CO3 can be produced at temperatures of approximately 500 °C! To date, this temperature is the lowest suggested for La2O3 production by the Na2CO3 roasting of LaPO4. This increase in reactivity happens without the addition of corrosive salts to the reaction mixture. There appear to be several factors causing the increased reactivity of the BM sample. First, the ball milling process mixes the reactants much more efficiently than can be accomplished manually. Wet particle size analysis indicates that the average particle size is reduced by almost an order of magnitude. SEM results indicate that the small particles are then compacted by the ball milling process, forming aggregates. The overall effect is that the total surface area of unlike particles in contact is significantly increased relative to the HM sample. XRD hot stage experiments confirm that the Na2CO3 roasting of LaPO4·H2O occurs via a two-reaction mechanism, with La2O2CO3 and La2O3 as the rare earth products of the respective reactions. TGA experiments were used to obtain values for the reaction rate constants k1 and k2 at each reaction temperature of interest. From these values, the kinetic parameters were determined for each reaction. The apparent activation energy and Arrhenius prefactor were 25.58 kJ/mol and 0.153 min−1 for reaction 1 and 46.52 kJ/mol and 12.5 min−1 for reaction 2. We believe that the reaction kinetics would be accelerated if this reaction were conducted in a fluidized bed reactor, to purge out CO2 so that the low equilibrium constants do not limit the rate of either reaction.

Indeed, of the models in Table 2, the ash diffusion model gives the most linear fit to the experimental TGA data obtained for X1. The results for the fit of the ash diffusion model to the kinetic data are given in Figure 8a. Equations 9−11 each fail to describe the experimental data. Because of the requirement of an adjustable parameter N, the nucleation and growth model was not considered as a kinetic model for reaction 1. The loss of CO2 from La2O2CO3 in reaction 2, on the other hand, is not a solid−solid phase reaction. Rather in reaction 2, the CO2 gas is lost from a solid particle. The nucleation and growth model has previously been used successfully to model the kinetics of the reduction of hematite.22 During this process, CO2 and H2O gases are evolved as hematite is reduced in the presence of CH4. Therefore, this model was used to describe the kinetics of reaction 2. When the value of k has been calculated for a number of temperatures, the activation energy Ea can be determined from eq 14 by plotting ln k versus inverse temperature 1/T according to the Arrhenius relationship given in eq 14. As deduced from Figure 9, apparent activation energy Ea = 25.58 kJ/mol and the Arrhenius prefactor k0 = 0.153 min−1 for reaction 1. For reaction 2, apparent activation energy Ea = 46.52 kJ/mol and the Arrhenius prefactor k0 = 12.5 min−1. ln k = ln k 0 −

Ea RT

(14)

In eq 15, the reaction rate constant k is defined as a function of the activation energy Ea and the reaction order n, and the rate prefactor constant A.

k = A e−Ea / RT

(15)

The plot of ln k2 versus 1/T is especially noteworthy. Over the temperature range of 625−725 °C (1/T = 0.00112− 0.00100), the plot of ln k2 versus 1/T is not linear as expected. Rather, the plot of ln k2 versus 1/T in this region can be divided into two regions. In the first region, from 625 to 675 °C, the value of ln k2 grows relatively quickly. In the second region, from 675 to 725 °C, the growth of ln k2 levels off greatly. We postulate that at temperatures below 675 °C, the reaction is thermodynamically limited by the low value of the equilibrium CO2 pressure for reaction 2. This value grows from 0.0036 bar (0.36 mol % at 1 bar total pressure) at 625 °C and grows to approximately 0.006 bar (0.6 mol %) at 675 °C, while it is considerably larger at 0.025 bar (2.5 mol %) at 725 °C. We hypothesize that above 675 °C, the rate of reaction is no longer thermodynamically limited but rather kinetically limited. It is likely that the TGA purge gas is able to remove CO2 generated from the TGA chamber such that the CO2 concentration within is maintained below the thermodynamically limiting value. Therefore, in determining Ea and k0 for reaction 2, only the reaction data for the temperature range 675−725 °C were considered. Notably, it is reported that higher calcination temperatures of 800 °C are necessary to cause the loss of CO2 from La2CO5 within a time of less than 2 h.23

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (412) 386-5409. E-mail: [email protected]. ORCID

Ward A. Burgess: 0000-0001-6673-6457 Elliot A. Roth: 0000-0001-8760-6985 Evan J. Granite: 0000-0001-7668-8364 Notes

This project was funded by the Department of Energy, National Energy Technology Laboratory, an agency of the United States Government, through a support contract with AECOM. Neither the United States Government nor any agency thereof, nor any of their employees, nor AECOM, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. The authors declare no competing financial interest.

4. CONCLUSIONS Previous experiments by Kumari et al.14 suggest that it is necessary to heat an equimass mixture of LaPO4 and phosphate-rich monazite to at least the boiling point of pure Na2CO3, 851 °C, to achieve decomposition ratios of 0.9−1.0 after 2 h. In this work, comparable results were obtained for mixtures of the model monazite compound LaPO4·H2O and Na2CO3 as decomposition ratios of 0.7−0.8 were obtained at H

DOI: 10.1021/acs.iecr.8b00249 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

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(20) Chen, J. H.; Li, C. R. Thermoanalysis and Application; Science Press: Beijing, 1987; p 121. (21) Lucas, S.; Champion, E.; Bernache-Assolant, D.; Leroy, G. Rare Earth Phosphate Powders RePO4*H2O (Re = La, Ce or Y) II. Thermal behavior. J. Solid State Chem. 2004, 177, 1312−1320. (22) Monazam, E. R.; Breault, R. W.; Siriwardane, R.; Richards, G.; Carpenter, S. Kinetics of the Reduction of Hematite (Fe2O3) by Methane CH4) During Chemical Looping Combustion: A Global Mechanism. Chem. Eng. J. 2013, 232, 478−487. (23) Moothedan, M.; Sherly, K. B. Synthesis, Characterization, and Sorption Studies of Nano Lanthanum Oxide. J. Water Process Eng. 2016, 9, 29−37.

ACKNOWLEDGMENTS We thank Mary Anne Alvin from USDOE for funding support and encouragement. This technical effort was performed in support of the National Energy Technology Laboratory’s ongoing research under RES contract DE-FE0004000.

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REFERENCES

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DOI: 10.1021/acs.iecr.8b00249 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX