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Production of sodium carbonate from soda ash via flash calcination in a drop tube furnace
Chemical Engineering and Processing 41 (2002) 215– 221 www.elsevier.com/locate/cep
Production of sodium carbonate from soda ash via flash calcination in a drop tube furnace Ayhan Demirbas * P.K. 216, 61035 Trabzon, Turkey Received 18 January 2001; received in revised form 4 April 2001; accepted 4 April 2001
Abstract This paper described the production of sodium carbonate via flash calcination of Turkish trona ore in a drop tube furnace. It has been observed that the loss of weight also increases with the increasing temperature of calcination. The production of sodium carbonate from the saturated solution of raw soda has been realized by a crystallization process in vacuum at 360 K. As a result, dense soda ash of 99.8% purity has been produced. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Sodium carbonate; Soda ash; Trona ore; Flash calcination; Drop tube furnace
1. Introduction Soda ash is the trade name for sodium carbonate (Na2CO3), a chemical refined from the mineral trona or sodium-carbonate-bearing brines (both referred to as ‘natural soda ash’) or manufactured from one of several chemical processes (referred to as ‘synthetic soda ash’) [1]. Calcining any of sodium carbonate source yields soda ash with various physical properties, crystal size, shape and bulk density [2]. Production of soda ash from trona solution was achieved in a spray dryer reactor [3]. The recovery of soda ash from trona leaching and crystallization methods was also applied by researchers [4,5]. Soda ash is used mostly in the production of glass, chemicals, soaps, and detergents, and by consumers [6,7]. In 1998, in terms of production, soda ash was 11th largest inorganic chemical of all domestic inorganic and organic chemicals, excluding petrochemical feedstocks. World production capacity of soda ash by country is presented in Table 1 [8]. Soda can be produced synthetically or from natural resource. Before 1965, almost the whole amount of soda being consumed in the world was being produced synthetically. However, after 1970, production by synthetic methods rapidly decreased, and was abandoned * Tel.: +90-462-2487429; Fax: + 90-462-2487344. E-mail address: [email protected] (A. Demirbas).
to production by natural sources. At the commercial scale, there are several methods of soda production. The first universal scheme of soda production methods was the Le Blanc process [9]. At the moment, the Solvay process is being used in the synthetic production of soda (especially in those countries that have no natural soda sources, such as Western European countries) [10]. The classical Solvay process has lost its importance enormously because of invention of natural soda. At the present, the well-known sodium carbonate minerals used commercially in the production of soda thermonatrite are natrone (Na2CO3·10H2O), (Na2CO3·H2O) and trona (Na2CO3NaHCO3·2H2O). Within these minerals, trona has the most industrial importance. There are various processes known to produce sodium carbonate from sodium sesquicarbonate. According to the ‘Sesquicarbonate process’, which is one of the most common processes, trona is crushed, ground and dissolved in water to remove the soluble organic compounds, and the solution is treated with activated carbon, filtered, and cooled so as to obtain crystals of sodium sesquicarbonate. Later on, sodium carbonate is produced by the calcination of sesquicarbonate. In another process called the ‘Monohydrate process’, trona is crushed, ground and calcined so as to transform into raw sodium carbonate. The raw soda is dissolved in water, treated with activated carbon in
0255-2701/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 5 - 2 7 0 1 ( 0 1 ) 0 0 1 3 6 - 2
A. Demirbas / Chemical Engineering and Processing 41 (2002) 215–221
216
Table 1 World production of soda ash by country (thousands of metric tons)a Country
1994
1995
1996
1997
1998
USA China Russia India Germany France United Kingdom Poland Japan Bulgaria Romania Turkey Spain Italy Netherlands Ukraine Korea, Republic of Australia Canada All others
9320 5814 1585 1500 1380 1123 1000 997 1050 451 449 385 500 500 400 660 310 300 300 966
10 100 5997 1823 1500 1400 1120 1000 1019 1049 796 504 385 500 500 400 475 310 300 300 1,035
10 200 6693 1500 1500 1400 1100 1000 909 926 800 537 400 500 500 400 375 320 300 300 1.045
10 700 7258 1700 1500 1400 1,053 1000 950 801 800 548 500 500 500 400 367 320 300 300 972
10 100 7200 1600 1500 1400 1000 1000 950 800 800 550 510 500 500 400 375 300 300 300 955
a
Source: Ref. [8].
order to remove soluble organic compounds, filtered, and then crystallized evaporatively to obtain crystals of sodium carbonate monohydrate. Then, sodium carbonate monohydrate is dehydrated to produce anhydrous sodium carbonate [11]. To remove soluble organic impurities, two conventional methods are generally used. One of them is the calcination of trona over a temperature of 675 K. During this time, organic matters are burned and removed from trona ore. In the second method, trona is calcined below 650 K and the obtained raw soda is solvated in water. The solution is treated with activated carbon, and so organic compounds are adsorbed. This paper outlines a logical methodology for process and product slate selection. In the present work, the flash calcination behavior of Turkish-Beypazari soda ash has been investigated.
height and the heated height of the reactor were 182 and 151 cm, respectively. The outer surface of the system was insulated with ceramic wool (Fig. 1). The bad was fluidized by means of nitrogen fed through a pressure regulator and calibrated rotameter. The temperature in the reactor was measured by means of NiCr Ni thermocouples.
2. Experimental
2.1. Equipment Calcination experiments for soda ash were performed using the modified system shown in Fig. 1. A similar flash calcinator apparatus was illustrated in an early study [11]. The flash calcination was carried out of in a drop tube reactor. The reactor was made from 316 stainless steel tube of 2.3-cm internal diameter. It was placed vertically into an electrically heated furnace. The total
Fig. 1. Schematic representation of the experimental set-up. 1, N2 tank; 2, pressure regulator; 3, water trap; 4, rotameter; 5, mini-fluid bed; 6, cyclone; 7, drop tube furnace reactor; 8, cylindrical ceramic tube; 8a, resistance wire; 9, 10, isolation; 11, thermocouple insert pipe; 12a, 12b, thermocouple; 13, temperature gauge; 14, temperature controller; 15, contactor; 16, water-cooled collector probe.
A. Demirbas / Chemical Engineering and Processing 41 (2002) 215–221 Table 2 Results from the chemical analyses of the Beypazari trona sample Component
%
Component
%
Na2CO3 NaHCO3 Na2SO4 NaCl Insoluble matter
46.53 34.82 0.568 0.013 2.98
H2O Free Na2CO3 Organic matter (C) Others Trona
14.92 2.59 0.068 0.169 93.68
2.2. Preparation of samples The trona used in this study was supplied from Beypazari, a district in Central Anatolia from Turkey. The particle size of trona samples was chosen within the range −0.4259 0.300 and −0.180 9 0.150.
217
2.3. Analyses The chemical and mineralogical analyses of the soda ash samples were carried out using conventional analytical methods, X-ray diffraction (XRD) and X-ray fluorescence (XRF); the results from the analyses are presented in Table 2 and Figs. 2 and 3, respectively.
2.4. Experiments The grinding trona ore particles were fed to top of the reactor at a speed of 1.2 g/min using a fluidized bed feeding system combining water-cooled cyclone at the top of the reactor. The cyclone ensured the free falling of the particles by separating the fluidizing gas (N2). This prevents particle trona entraining nitrogen gas into the reactor. The calcined materials were collected and quenched rapidly by a water-cooled collector at the
Fig. 2. XRD of the trona ore.
Fig. 3. XRF of the trona ore.
A. Demirbas / Chemical Engineering and Processing 41 (2002) 215–221
218
Table 3 Results from the chemical analyses of product
where X is the dissolution fraction of Na2CO3, and wo and w are the initial and dissolved amount of the sample, respectively. The constant 0.946 was the purity level of Na2CO3 in the sample.
Component or element
%
Element
%
Na2CO3 Organic matter (C) Ca S Cl
99.8214 0.0160 0.0020 0.0010 0.0006
Mg Si Al K
0.0420 0.0100 0.0003 0.0087
bottom of the reactor. The weight loss of the materials was determined by weighing. The dissolution experiments were conducted in a jacket stirred glass reactor operating in batch mode. The temperature controlled using a thermostatic batchtemperature control circuit. The dissolved part of Na2CO3 was determined using a pH/mV-meter calibrated for different temperatures, equipped with a recorder. The stirring speed was maintained at 450 r.p.m. throughout the experiments. The dissolution experiments were conducted at 285, 290, 295, 300, 305 and 310 K using calcined materials at 575 K with particle size of −0.180 90.150 mm. The dissolution fraction of Na2CO3 was calculated by X=
w 0.946wo
(1)
3. Results and discussion The results from the chemical analyses of product are presented in Table 3. Fig. 4 shows the effect of flash calcination temperature on the weight loss. The effect of temperature on the dissolution time is shown in Fig. 5. The XRD and XRF of the product are given in Figs. 6 and 7, respectively. The flow diagram for the production of dense soda ash from trona ore is shown in Fig. 8. The dominant peaks of the sample are seen clearly in the XRD diffractogram (Fig. 2). Table 2 indicates that the trona ore contains 93.68% sesquicarbonate. The elements indicated by the XRF spectrum, Mg, Cl, Al, Si, S, K and Ca, are from the compounds forming the impurities (Fig. 3). The production of soda from trona consists of main steps of calcination–dissolution and crystallization. In general, calcination is carried out by one of the systems of fixed, fluidized or entraining flow beds. Apart from these, there are some other private calcinators developed by the some other researches. In order to calcine
Fig. 4. Effect of calcination temperature on the weight loss. Speed of feed, 1.2 g/min.
A. Demirbas / Chemical Engineering and Processing 41 (2002) 215–221
219
Fig. 5. Effect of temperature on the dissolution time. Particle size, −0.180 90.150 mm.
Fig. 6. XRD of the product.
the trona, a calcinator design has been developed that can use slow and flash calcination techniques. Compounds with Cl and S, which are water soluble, are presented in Table 2 as NaCl and Na2SO4 equivalent. The rest of the impurities (i.e. Mg, Al, Si, K and Ca), which are clay minerals, has been determined at 2.98% in total by gravimetric methods.
3.1. Calcination beha6ior of the trona The
chemical
formula
of
trona
is
given
as
Na2CO3·NaHCO3·2H2O. When it is heated from 436 to 477 K, the following reaction takes place: 2Na2CO3·NaHCO3·2H2O 3Na2CO3 + CO2 + 5H2O (2) The decomposition of trona starts at about 347 K. The effect of the temperature has been investigated in the range from 373 to 600 K for two particle sizes of − 0.1809 0.150 and − 0.42590.300 mm (Fig. 4). It is seen that the weight loss increases with increasing temperature (Fig. 4).
A. Demirbas / Chemical Engineering and Processing 41 (2002) 215–221
220
3.2. Dissolution experiments Na2CO3 can be dissolved in water approximately upped 30% weight [12]. The effect of temperature on the dissolution has been investigated at 285, 290, 295, 300, 305 and 310 K using calcined materials at 575 K with particle size of − 0.180 9 0.150 mm (Fig. 5). It is observed that the dissolution rate is strongly dependent on the temperature. From Fig. 5, a dissolution fraction of 0.93 is reached at 6.0 se for 310 K temperature. The dissolution curve shows a maximum at 310 K (Fig. 5). At this temperature, the structures with monohydrate and heptahydrate are at equilibrium. The dissolution reaction mechanism of soda ash was given as follows [7]. Na2CO3 (s)+H2O (l) ? NaHCO3 (aq) +NaOH (aq) (3) NaHCO3 (s)+ H2O (l) ? H2CO3 (aq) +NaOH (aq) (4)
Thus, the dissolution reaction can be considered to be non-catalytic fluid –solid reaction type as follows [13,14]. Fluid+solid products
(5)
3.3. Crystallization The saturated solution prepared from the raw sample was filtered, and then crystallized by evaporative crystallization process to obtain anhydrous Na2CO3. Fig. 6 shows the XRD spectrum of produced Na2CO3. It is determined that all peaks on the figure are for the Na2CO3 compound. The product was analyzed as qualitative and quantitative to determine the impurities using XRF (Fig. 7). The produced dense soda ash was 99.8% pure (Table 3). Both organic matter and other impurities in the raw material were extremely reduced. The process flow diagram for the production of dense soda ash from trona is illustrated in Fig. 8.
Fig. 7. XRF of the product.
Fig. 8. The flow diagram for the production of dense soda ash from trona.
A. Demirbas / Chemical Engineering and Processing 41 (2002) 215–221
4. Conclusion Sodium carbonate can be dissolved in water approximately upped 30% weight [12]. The dissolution curve shows a maximum at 311 K. At this temperature, the structures with monohydrate and heptahydrate are at equilibrium. When a saturated solution of sodium carbonate is cooled below 311 K, Na2CO3·10H2O is formed as this structure is more stable than monohydrate and heptahydrate structures. The saturated soda solution was prepared by solving 249.17 g trona sample (calcined at 573 K) in 500 ml water at 319 K. The solution was filtered under vacuum in order to separate insoluble impurities using filter paper. The filtration was also carried out at 319 K to prevent soda from crystallizing in a temperature-controlled apparatus. The filtrate was slightly brown. This color is probably due to soluble organic compounds. The solution was filtered at 319 K in an activated carbon bed for removing the organic compounds. The soda was crystallized from the clear solution in a vacuum oven by evaporation at 363 K. The particle size distribution of the product after calcination was − 0.30090.225 mm. These studies have been focused on the production of soda ash from natural sources because synthetic methods are expensive and have disadvantages with regard to environmental pollution. The scientific studies pertaining to production of soda from trona are generally licenced with a patent, and focus on the removal of organic matters existing in trona that could be transferred into solution after being solvated.
221
This process has produced dense soda ash of 99.8% purity. At the end of the process, 76% of the organic matter within the ore has been removed.
References [1] L. Gribovics, FY 91 Annual Inspection Report: FMC-Wyoming Corporation, Westvaco Soda Ash Refinery, Wyoming Department of Environmental Quality, Cheyenne, WY, 11 June 1991. [2] H.A. Sommers, Soda ash from trona, Chem. Eng. Prog. 56 (1960) 76 – 79. [3] M. Dogan, C ¸ . Gu¨ ldu¨ r, G. Dogu, T. Dogu, Soda ash production from trona in a spry dryer, J. Chem. Technol. Biotechnol. 68 (1997) 157 – 162. [4] J.P. Martins, F. Martins, Soda ash leaching of scheelite concentrates: the effect of high concentration of sodium carbonate, Hydrometallurgy 46 (1997) 191 – 203. [5] G. Nasu¨ n-Saygili, H. Okutan, Application of the solution mining process to the Turkish trona deposit, Hydrometallurgy 42 (1996) 103 – 113. [6] D.S. Kostick, Soda Ash, Mineral Commodity Summaries, US Department of the Interior, 1992. [7] D.S. Kostick, Soda Ash, Mineral Yearbook 1989, Vol I: Metals and Minerals, US Department of the Interior, 1990. [8] Turkish State Statistics Institute (TSSI), Ankara, 2000. [9] R.E. Kirk, D.F. Othmer, Encyclopedia of Chemical Technology, The Interscience Encyclopedia, vol. 1, Wiley, New York, 1978 (3rd ed.). [10] R.N. Shreve, Chemical Process Industries, 3rd ed., McGrawHill, New York, 1967 (Chapter 13). [11] S. C ¸ olak, A. Ekmekyapar, H. Ersahan, A. Ku¨ nku¨ l, O8 . Modoglu, Flash calcination of trona ore in a free-fall reactor and production of soda from trona, Energy Educ. Sci. Technol. 4 (2000) 48 – 59. [12] R.H. Perry, C.H. Chilton, Chemical Engineering Handbook, 5th ed., McGraw-Hill Kogakusha, Tokyo, 1973 p. 18 (Chapter 3). [13] L.K. Doraiswamy, M.M. Sharma, Heterogeneous Reactions, vol. 1, Wiley, New York, 1984, p. 452. [14] J.M. Smith, Chemical Engineering Kinetics, 3rd ed., McGrawHill, New York, 1981, p. 636.
Production of sodium carbonate from soda ash via flash calcination in a drop tube furnace Ayhan Demirbas * P.K. 216, 61035 Trabzon, Turkey Received 18 January 2001; received in revised form 4 April 2001; accepted 4 April 2001
Abstract This paper described the production of sodium carbonate via flash calcination of Turkish trona ore in a drop tube furnace. It has been observed that the loss of weight also increases with the increasing temperature of calcination. The production of sodium carbonate from the saturated solution of raw soda has been realized by a crystallization process in vacuum at 360 K. As a result, dense soda ash of 99.8% purity has been produced. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Sodium carbonate; Soda ash; Trona ore; Flash calcination; Drop tube furnace
1. Introduction Soda ash is the trade name for sodium carbonate (Na2CO3), a chemical refined from the mineral trona or sodium-carbonate-bearing brines (both referred to as ‘natural soda ash’) or manufactured from one of several chemical processes (referred to as ‘synthetic soda ash’) [1]. Calcining any of sodium carbonate source yields soda ash with various physical properties, crystal size, shape and bulk density [2]. Production of soda ash from trona solution was achieved in a spray dryer reactor [3]. The recovery of soda ash from trona leaching and crystallization methods was also applied by researchers [4,5]. Soda ash is used mostly in the production of glass, chemicals, soaps, and detergents, and by consumers [6,7]. In 1998, in terms of production, soda ash was 11th largest inorganic chemical of all domestic inorganic and organic chemicals, excluding petrochemical feedstocks. World production capacity of soda ash by country is presented in Table 1 [8]. Soda can be produced synthetically or from natural resource. Before 1965, almost the whole amount of soda being consumed in the world was being produced synthetically. However, after 1970, production by synthetic methods rapidly decreased, and was abandoned * Tel.: +90-462-2487429; Fax: + 90-462-2487344. E-mail address: [email protected] (A. Demirbas).
to production by natural sources. At the commercial scale, there are several methods of soda production. The first universal scheme of soda production methods was the Le Blanc process [9]. At the moment, the Solvay process is being used in the synthetic production of soda (especially in those countries that have no natural soda sources, such as Western European countries) [10]. The classical Solvay process has lost its importance enormously because of invention of natural soda. At the present, the well-known sodium carbonate minerals used commercially in the production of soda thermonatrite are natrone (Na2CO3·10H2O), (Na2CO3·H2O) and trona (Na2CO3NaHCO3·2H2O). Within these minerals, trona has the most industrial importance. There are various processes known to produce sodium carbonate from sodium sesquicarbonate. According to the ‘Sesquicarbonate process’, which is one of the most common processes, trona is crushed, ground and dissolved in water to remove the soluble organic compounds, and the solution is treated with activated carbon, filtered, and cooled so as to obtain crystals of sodium sesquicarbonate. Later on, sodium carbonate is produced by the calcination of sesquicarbonate. In another process called the ‘Monohydrate process’, trona is crushed, ground and calcined so as to transform into raw sodium carbonate. The raw soda is dissolved in water, treated with activated carbon in
0255-2701/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 5 - 2 7 0 1 ( 0 1 ) 0 0 1 3 6 - 2
A. Demirbas / Chemical Engineering and Processing 41 (2002) 215–221
216
Table 1 World production of soda ash by country (thousands of metric tons)a Country
1994
1995
1996
1997
1998
USA China Russia India Germany France United Kingdom Poland Japan Bulgaria Romania Turkey Spain Italy Netherlands Ukraine Korea, Republic of Australia Canada All others
9320 5814 1585 1500 1380 1123 1000 997 1050 451 449 385 500 500 400 660 310 300 300 966
10 100 5997 1823 1500 1400 1120 1000 1019 1049 796 504 385 500 500 400 475 310 300 300 1,035
10 200 6693 1500 1500 1400 1100 1000 909 926 800 537 400 500 500 400 375 320 300 300 1.045
10 700 7258 1700 1500 1400 1,053 1000 950 801 800 548 500 500 500 400 367 320 300 300 972
10 100 7200 1600 1500 1400 1000 1000 950 800 800 550 510 500 500 400 375 300 300 300 955
a
Source: Ref. [8].
order to remove soluble organic compounds, filtered, and then crystallized evaporatively to obtain crystals of sodium carbonate monohydrate. Then, sodium carbonate monohydrate is dehydrated to produce anhydrous sodium carbonate [11]. To remove soluble organic impurities, two conventional methods are generally used. One of them is the calcination of trona over a temperature of 675 K. During this time, organic matters are burned and removed from trona ore. In the second method, trona is calcined below 650 K and the obtained raw soda is solvated in water. The solution is treated with activated carbon, and so organic compounds are adsorbed. This paper outlines a logical methodology for process and product slate selection. In the present work, the flash calcination behavior of Turkish-Beypazari soda ash has been investigated.
height and the heated height of the reactor were 182 and 151 cm, respectively. The outer surface of the system was insulated with ceramic wool (Fig. 1). The bad was fluidized by means of nitrogen fed through a pressure regulator and calibrated rotameter. The temperature in the reactor was measured by means of NiCr Ni thermocouples.
2. Experimental
2.1. Equipment Calcination experiments for soda ash were performed using the modified system shown in Fig. 1. A similar flash calcinator apparatus was illustrated in an early study [11]. The flash calcination was carried out of in a drop tube reactor. The reactor was made from 316 stainless steel tube of 2.3-cm internal diameter. It was placed vertically into an electrically heated furnace. The total
Fig. 1. Schematic representation of the experimental set-up. 1, N2 tank; 2, pressure regulator; 3, water trap; 4, rotameter; 5, mini-fluid bed; 6, cyclone; 7, drop tube furnace reactor; 8, cylindrical ceramic tube; 8a, resistance wire; 9, 10, isolation; 11, thermocouple insert pipe; 12a, 12b, thermocouple; 13, temperature gauge; 14, temperature controller; 15, contactor; 16, water-cooled collector probe.
A. Demirbas / Chemical Engineering and Processing 41 (2002) 215–221 Table 2 Results from the chemical analyses of the Beypazari trona sample Component
%
Component
%
Na2CO3 NaHCO3 Na2SO4 NaCl Insoluble matter
46.53 34.82 0.568 0.013 2.98
H2O Free Na2CO3 Organic matter (C) Others Trona
14.92 2.59 0.068 0.169 93.68
2.2. Preparation of samples The trona used in this study was supplied from Beypazari, a district in Central Anatolia from Turkey. The particle size of trona samples was chosen within the range −0.4259 0.300 and −0.180 9 0.150.
217
2.3. Analyses The chemical and mineralogical analyses of the soda ash samples were carried out using conventional analytical methods, X-ray diffraction (XRD) and X-ray fluorescence (XRF); the results from the analyses are presented in Table 2 and Figs. 2 and 3, respectively.
2.4. Experiments The grinding trona ore particles were fed to top of the reactor at a speed of 1.2 g/min using a fluidized bed feeding system combining water-cooled cyclone at the top of the reactor. The cyclone ensured the free falling of the particles by separating the fluidizing gas (N2). This prevents particle trona entraining nitrogen gas into the reactor. The calcined materials were collected and quenched rapidly by a water-cooled collector at the
Fig. 2. XRD of the trona ore.
Fig. 3. XRF of the trona ore.
A. Demirbas / Chemical Engineering and Processing 41 (2002) 215–221
218
Table 3 Results from the chemical analyses of product
where X is the dissolution fraction of Na2CO3, and wo and w are the initial and dissolved amount of the sample, respectively. The constant 0.946 was the purity level of Na2CO3 in the sample.
Component or element
%
Element
%
Na2CO3 Organic matter (C) Ca S Cl
99.8214 0.0160 0.0020 0.0010 0.0006
Mg Si Al K
0.0420 0.0100 0.0003 0.0087
bottom of the reactor. The weight loss of the materials was determined by weighing. The dissolution experiments were conducted in a jacket stirred glass reactor operating in batch mode. The temperature controlled using a thermostatic batchtemperature control circuit. The dissolved part of Na2CO3 was determined using a pH/mV-meter calibrated for different temperatures, equipped with a recorder. The stirring speed was maintained at 450 r.p.m. throughout the experiments. The dissolution experiments were conducted at 285, 290, 295, 300, 305 and 310 K using calcined materials at 575 K with particle size of −0.180 90.150 mm. The dissolution fraction of Na2CO3 was calculated by X=
w 0.946wo
(1)
3. Results and discussion The results from the chemical analyses of product are presented in Table 3. Fig. 4 shows the effect of flash calcination temperature on the weight loss. The effect of temperature on the dissolution time is shown in Fig. 5. The XRD and XRF of the product are given in Figs. 6 and 7, respectively. The flow diagram for the production of dense soda ash from trona ore is shown in Fig. 8. The dominant peaks of the sample are seen clearly in the XRD diffractogram (Fig. 2). Table 2 indicates that the trona ore contains 93.68% sesquicarbonate. The elements indicated by the XRF spectrum, Mg, Cl, Al, Si, S, K and Ca, are from the compounds forming the impurities (Fig. 3). The production of soda from trona consists of main steps of calcination–dissolution and crystallization. In general, calcination is carried out by one of the systems of fixed, fluidized or entraining flow beds. Apart from these, there are some other private calcinators developed by the some other researches. In order to calcine
Fig. 4. Effect of calcination temperature on the weight loss. Speed of feed, 1.2 g/min.
A. Demirbas / Chemical Engineering and Processing 41 (2002) 215–221
219
Fig. 5. Effect of temperature on the dissolution time. Particle size, −0.180 90.150 mm.
Fig. 6. XRD of the product.
the trona, a calcinator design has been developed that can use slow and flash calcination techniques. Compounds with Cl and S, which are water soluble, are presented in Table 2 as NaCl and Na2SO4 equivalent. The rest of the impurities (i.e. Mg, Al, Si, K and Ca), which are clay minerals, has been determined at 2.98% in total by gravimetric methods.
3.1. Calcination beha6ior of the trona The
chemical
formula
of
trona
is
given
as
Na2CO3·NaHCO3·2H2O. When it is heated from 436 to 477 K, the following reaction takes place: 2Na2CO3·NaHCO3·2H2O 3Na2CO3 + CO2 + 5H2O (2) The decomposition of trona starts at about 347 K. The effect of the temperature has been investigated in the range from 373 to 600 K for two particle sizes of − 0.1809 0.150 and − 0.42590.300 mm (Fig. 4). It is seen that the weight loss increases with increasing temperature (Fig. 4).
A. Demirbas / Chemical Engineering and Processing 41 (2002) 215–221
220
3.2. Dissolution experiments Na2CO3 can be dissolved in water approximately upped 30% weight [12]. The effect of temperature on the dissolution has been investigated at 285, 290, 295, 300, 305 and 310 K using calcined materials at 575 K with particle size of − 0.180 9 0.150 mm (Fig. 5). It is observed that the dissolution rate is strongly dependent on the temperature. From Fig. 5, a dissolution fraction of 0.93 is reached at 6.0 se for 310 K temperature. The dissolution curve shows a maximum at 310 K (Fig. 5). At this temperature, the structures with monohydrate and heptahydrate are at equilibrium. The dissolution reaction mechanism of soda ash was given as follows [7]. Na2CO3 (s)+H2O (l) ? NaHCO3 (aq) +NaOH (aq) (3) NaHCO3 (s)+ H2O (l) ? H2CO3 (aq) +NaOH (aq) (4)
Thus, the dissolution reaction can be considered to be non-catalytic fluid –solid reaction type as follows [13,14]. Fluid+solid products
(5)
3.3. Crystallization The saturated solution prepared from the raw sample was filtered, and then crystallized by evaporative crystallization process to obtain anhydrous Na2CO3. Fig. 6 shows the XRD spectrum of produced Na2CO3. It is determined that all peaks on the figure are for the Na2CO3 compound. The product was analyzed as qualitative and quantitative to determine the impurities using XRF (Fig. 7). The produced dense soda ash was 99.8% pure (Table 3). Both organic matter and other impurities in the raw material were extremely reduced. The process flow diagram for the production of dense soda ash from trona is illustrated in Fig. 8.
Fig. 7. XRF of the product.
Fig. 8. The flow diagram for the production of dense soda ash from trona.
A. Demirbas / Chemical Engineering and Processing 41 (2002) 215–221
4. Conclusion Sodium carbonate can be dissolved in water approximately upped 30% weight [12]. The dissolution curve shows a maximum at 311 K. At this temperature, the structures with monohydrate and heptahydrate are at equilibrium. When a saturated solution of sodium carbonate is cooled below 311 K, Na2CO3·10H2O is formed as this structure is more stable than monohydrate and heptahydrate structures. The saturated soda solution was prepared by solving 249.17 g trona sample (calcined at 573 K) in 500 ml water at 319 K. The solution was filtered under vacuum in order to separate insoluble impurities using filter paper. The filtration was also carried out at 319 K to prevent soda from crystallizing in a temperature-controlled apparatus. The filtrate was slightly brown. This color is probably due to soluble organic compounds. The solution was filtered at 319 K in an activated carbon bed for removing the organic compounds. The soda was crystallized from the clear solution in a vacuum oven by evaporation at 363 K. The particle size distribution of the product after calcination was − 0.30090.225 mm. These studies have been focused on the production of soda ash from natural sources because synthetic methods are expensive and have disadvantages with regard to environmental pollution. The scientific studies pertaining to production of soda from trona are generally licenced with a patent, and focus on the removal of organic matters existing in trona that could be transferred into solution after being solvated.
221
This process has produced dense soda ash of 99.8% purity. At the end of the process, 76% of the organic matter within the ore has been removed.
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