Article pubs.acs.org/IECR
Dissolution Kinetics of Magnesite Waste in HCl Solution Halit L. Hoşgün*,† and Haldun Kurama‡ †
Chemical Engineering Department and ‡Mining Engineering Department, Eskişehir Osmangazi University, 26480 Eskişehir, Turkey ABSTRACT: In this study, the dissolution of electro-filter magnesite dust samples that were collected from different places in an electrostatic bag house in HCl solution was investigated. The effects of sample composition and variations in the leaching parameters, such as reaction temperature, time, and acid concentration, were studied to elucidate the kinetics of the dust dissolution. The results obtained in this study clearly indicate that the dissolution of the magnesite dust increased with increasing acid concentration and leaching temperature. However, the tests on the effect of temperature, carried out at a constant reagent concentration of 0.5 M HCl, showed that the contribution of the increase in temperature on the dissolution rate was more powerful than that of the reagent concentration. At a temperature range between 290 and 318 K, the rate constants k were calculated as a function of temperature, and the following values were obtained: k = 2.71 × 108 exp(−7550/T) min−1 for EFDS-1, k = 1.31 × 107 exp(−6520/T) min−1 for EFDS-2 and k = 2.99 × 1010 exp(−8780/T) min−1 for EFDS-3. As a kinetic model to analyze the kinetic data, the shrinking-core model was proposed for the dissolution mechanism. The reaction was controlled by diffusion through the ash layer. The related activation energies were calculated to be 54.24 kJ mol−1 for EFDS-1, 62.77 kJ mol−1 for EFDS-2, and 73 kJ mol−1 for EFDS-3.
1. INTRODUCTION In recent years, the recycling of processing wastes or byproducts has been gaining importance for sustainable development.1 It is no doubt that waste management is a very important issue from the public, environmental, and industrial viewpoints because an everincreasing amount of materials needs to be disposed in a safe and economical way or, preferably, reused whenever possible.2 The metallurgical industry, in particular, produces vast quantities of solid wastes such as slag, ash, sludge, dross, grindings, turnings, clipping residues, and secondaries during production cycles. Because of the implementation of stricter environmental rules in production industries and the necessity for innovative and cost-effective production due to increased competition in the market, all metallurgical industries are now forced to go for ecofriendly and innovative technologies including the reuse of production wastes as secondary raw materials.3 Although the current recycling technologies are diversified and change depending on the source of waste, they can be classified into mechanical, chemical, and energetic recycling. Moreover, chemical recycling processes have become feasible means for obtaining intermediate products because of the cost reduction of the raw materials involved in production. Magnesite mineral (MgCO3) is the basic raw material for the manufacture of alkaline refractories and is used mainly in the iron− steel, cement, glass, sugar, ceramic, and paper industries. In addition, it also finds use in the paint and ink industry, in pharmacology, and in the production of many magnesium chemicals and alloys. The world magnesite ore reserve is around 3420 million tonnes, and Turkey has about 160 million tonnes of this reserve. MAŞ Magnesite Company is one of the biggest sintered magnesia producers in Turkey. The company produces 140000 tonnes of sintered magnesia (MgO) per year. The traditional method is used by the company to produce MgO, and every year, nearly 20000 t of magnesite dust is collected in the furnace electro-filters. Furthermore, the amount of waste material obtained from sintering plant has been progressively increased because of the lower recycling rate.4 © 2012 American Chemical Society
MgO can also be produced from raw or waste material by a hydrometallurgical method rather than by the calcination route. In that process, the raw materials are leached using suitable reagents, generally HCl, and then precipitated with bases. Alternatively, as recently practiced in the Slovak Republic, MgCl2 solution is refined by precipitation, filtration, and extraction, and the obtained MgCl2 is pyrohydrolyzed to produce MgO.5,6 The type and amount of impurities and the form of magnesium in raw/waste material can affect the purity of final product. For this reason, the leaching step and the determination of the rate-controlling step or steps is the most important stage of this treatment method. In hydrometallurgical applications, it has been recognized that the ratecontrolling step can change depending on the reaction conditions such as temperature, reagent concentration, reaction time, and reactant structure. Accordingly, rate information obtained under a set of given conditions might not be applicable under another set of conditions. Therefore, an understanding of how the individual reaction steps interact with each other is important in determining the rate-controlling step under given conditions.7 In the literature, several research studies were performed to determine the dissolution of raw magnesite and calcined or dead-burned magnesia. For example, Mejdall et al.8 studied the production of high-purity MgCl2 from magnesite in HCl solution. Kurtbaş9 also investigated the dissolution kinetics of magnesite in HCl solution and reported that the dissolution reaction was controlled by the surface reaction with an activation energy value of 48.33 kJ mol−1. Ö zbek et al.10 investigated the dissolution kinetics of magnesite mineral in water saturated by chlorine gas and proposed that the dissolution process was controlled by reaction on the solid surface in the temperature range of 12−40 °C and by film diffusion for the temperature range of 40−70 °C. The dissolution of magnesite in Received: Revised: Accepted: Published: 1087
August 23, 2011 December 29, 2011 January 1, 2012 January 2, 2012 dx.doi.org/10.1021/ie201890s | Ind. Eng.Chem. Res. 2012, 51, 1087−1092
Industrial & Engineering Chemistry Research
Article
acetic acid solution was also studied by Laçin et al.,11 who also reported that the chemical reaction step controlled the dissolution of natural magnesite. Instead of raw magnesite, the leaching of calcined magnesite with CO2 as a leaching agent instead of common acids was performed by Fernandez et al.12 They pointed out that, in the absence of magnesium carbonate precipitation, during the first 10 min of the dissolution reaction, the dissolution rate was controlled by reaction on the solid surface. It was reported by Demir et al.13 that the leaching reaction of calcined magnesite in citric acid solution was controlled by chemical kinetics with an activation energy value of 39.1 kJ mol−1. The leaching studies performed by Raschman et al.6 to examine the kinetics of the reaction between dead-burned magnesite and HCl showed that the dissolution rate was strongly affected by temperature (from 45 to 75 °C). The shrinking-particle model was used to analyze the leaching, and it was concluded that the dissolution was controlled by the chemical reaction of the MgO with H+ ions at the liquid− solid interface. As summarized above, different dissolution mechanisms have been discussed in the literature. However, there are a few studies concerning the flue dusts trapped in the dust collector during the production of the calcined or dead-burned magnesia. In this study, the dissolution of electro-filter magnesite dust sample supplied by MAŞ Magnesite Company (Eskisehir, Turkey) in HCl solution was investigated. The effects of the variation of the chemical composition of the waste and various leaching parameters such as reaction temperature, time, and acid concentration on the dissolution were studied to clarify the kinetics of dust. Furthermore, the kinetic studies served to elucidate fundamental mechanisms of reactions and to provide data for engineering applications, including improved ability to scale up process from the bench to the pilot scale. Therefore, the kinetics of the dissolution of magnesium from electro-filter waste was specially considered to increase the recyclability of the waste at the industrial scale.
compositions. A LECO furnace analysis was also performed to identify the MgO-to-MgCO3 ratios of the samples. The calculated values based on the carbon content of the samples are given in Table 2. It was found that the values ranged from 0.185 to 0.422 with respect to the sample type. Table 2. MgO/MgCO3 Ratios of Samples
oxide (%) EFDS-2
EFDS-3
1.48 0.27 0.05 2.10 58.60 37.50
1.85 0.37 0.07 2.25 56.36 39.10
3.84 0.80 0.08 2.79 49.49 43.00
0.374 0.422 0.185
3. RESULTS AND DISCUSSION 3.1. Effect of HCl Concentration. The results of leaching tests at acid concentrations of 0.5, 1, and 2 M under a constant temperature of 290 K are given in Figure 2. As illustrated in this figure, an increase in acid concentration had a positive effect on the dissolution process. The dissolved amounts of magnesium increased with increasing concentration of HCl for all samples. Although the extraction of magnesium continued up to 8 h, higher rates of dissolution were observed within the first 2 h of reaction. At this stage, it should be noted that the initial reaction rates of dissolution were higher for high concentrations and decreased with extension of the leaching process. The reason for observing higher dissolution of magnesium at the beginning of leaching (2 h) can be explained by the higher surface area of the oxide compounds of the waste samples (MgCO3) and, hence, the easier and faster reaction of H+ with this compound and relatively smaller particle size of the dust samples as discussed in the literature. The total extracted amounts of magnesium for sample EFDS-1 were calculated as 69% for 0.5 M, 83% for 1 M, and 96% for 2 M acid concentration, and the corresponding values were determined as 72%, 84%, and 96% for sample EFDS-2 and 80%, 90%, and 83% for sample EFDS-3. 3.2. Effect of Temperature. The effect of temperature on the dissolution of magnesium from dust samples was
Table 1. Chemical Analysis Results of Magnesite Dust Samples EFDS-1
MgO/MgCO3
The identification of crystalline material phases in samples was performed with an S5000 diffractometer, with nickelfiltered Cu Kα radiation. In regard to X-ray diffraction (XRD) analysis, it was found that samples mainly consisted of original raw magnesite (MgCO3) and sintered magnesia, periclase (MgO), and small amounts of serpentine (lizardite) phases (see Figure 1). The particle size distributions of the samples were determined by wet screening (Table 3). It was revealed that samples had approximately the same size distributions. The D80 values of the samples were determined to be about 0.130 μm. 2.2. Methods. The leaching experiments were carried out in a 500 mL three-necked round-bottom glass reactor placed on a temperature-controlled magnetic stirrer. A condenser was attached to the reactor to avoid evaporating any reactor content. In the leaching tests, HCl was used as the leaching reagent, and in each test, a 5-g dust samples was added to 250 mL of acid solution that had previously been adjusted to desired temperature. The solutions were then continuously stirred at a fixed stirring speed of 600 rpm. To evaluate the effects of test parameters on magnesium dissolution, 5 mL samples were taken from the reactor at predetermined time intervals; each sample was filtered, and its magnesium concentration in the solution was analyzed by complexometric titration using ethylenediaminetetraacetic acid (EDTA).
2. EXPERIMENTAL SECTION 2.1. Materials. The magnesite dust samples used in leaching tests, designated as EFDS-1 (electro-filter dust sample), EFDS-2, and EFDS-3, were taken from different parts of the electrostatic bag house of MAŞ Magnesite Company, Eskişehir, Turkey. The samples were first characterized by chemical and physical analysis, and then they were subjected to leaching tests to determine the effects of test parameters on the dissolution process. The chemical compositions of representative waste samples determined by atomic absorption spectrometry (AAS) are given in Table 1.
SiO2 Fe2O3 Al2O3 CaO MgO LOI
sample EFDS-1 EFDS-2 EFDS-3
As can be seen from Table 1, SiO2 and CaO are the main impurities, together with very small amounts of Al2O3 and Fe2O3. The magnesium oxide content of the samples was determined as between 49% and 58% according to sample 1088
dx.doi.org/10.1021/ie201890s | Ind. Eng.Chem. Res. 2012, 51, 1087−1092
Industrial & Engineering Chemistry Research
Article
Figure 1. XRD diffractogram of raw magnesite dusts: M, magnesite; L, lizardite; P, periclase. (a) EFDS-1, (b) EFDS-2, (c) EFDS-3.
chemical reaction rate is very high. The rate of diffusion is relatively insensitive to any further increase in temperature. The total extracted amounts for samples EFDS-1, EFDS-2, and EFDS-3 were calculated as follows: 83.24%, 84.49%, and 90.22% at 290 K; 81%, 73.22%, and 95.72% at 305 K; and 85.67%, 86.83%, and 95.60% at 318 K. 3.3. Kinetic Analysis. The reaction, taking place between a fluid and a solid, can be written as
Table 3. Partical Size Distibutions of Magnesite Dust Samples size fraction (μm)
EFDS-1
EFDS-2
EFDS-3
>1000 1000−500 500−212 212−106 106−63
