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Experimental Determination of the Optimum Conditions of KVO3 Synthesis Based on KCl and V2O5 in the Presence of Steam Mieczysław Trypuc´ ,* Zbigniew Torski, and Urszula Kiełkowska Faculty of Chemistry, Nicolaus Copernicus University, 7 Gagarin Street, 87-100 Torun´ , Poland
The influence of temperature, time, and excess of KCl in a reaction mixture on the yield of KVO3 synthesis based on KCl and V2O5 in the presence of steam was determined. The reaction temperature was varied between 673 and 823K, and the time ranged from 4 to 6 h. The required optimum excess of KCl in the reaction mixture and time of processing were identified. The results are implemented in a database to determine an alternative, nonwaste method of potassium carbonate production. Introduction Potassium carbonate is commonly used in various branches of industry such as the glass-making, cosmetic, food, and pharmaceutical industries. The domestic demands in Poland alone for K2CO3 are estimated at 15 000 tons per year, with 90% provided by import suppliers. Currently, widespread application of the industrial processing of potassium carbonate is based on several methods, which are characterized by very low utilization of KCl and many engineering difficulties.1 The following large-scale industrial production methods of K2CO3 are used: the Engel-Precht method, the amine and nepheline methods, and the carbonization of aqueous solutions of KOH.2-4 The general idea of the Engel-Precht is grounded on the precipitation of the binary salt MgCO3‚KHCO3‚4 H2O from a magnesium carbonate or magnesium oxide suspension in KCl (or K2SO4) solution under CO2 pressure conditions. The solids generated are then decomposed in an autoclave within the temperature limits 388-413 K. A key disadvantage of this process is the preparation of an active form of MgCO3 to generate the binary salt. The amine process is strictly related to the application of toxic and uneconomical amines and 100% unutilized chloride ions, which are sent, after the regeneration step, to open water reservoirs. The less important nepheline method, designed in the former Soviet Union, is based on the convertion of nepheline into Al2O3, Na2CO3, K2CO3, and Portland cement. These products are of great application in industry. However, the highenergy-consuming characteristics of the process play a significant role in limiting its applicability. It is assumed, by analogy to Solvay’s method of Na2CO3 production, that K2CO3 can be generated by employing the carbonization of water + ammonia solutions of KCl. Unfortunately, a literature review revealed that such theoretical process cannot be conducted.5 The main restriction is linked with the very low yield of the carbonization process toward the potassium ions (significant solubility of KHCO3 in the mother liquor). With regard to the existing strict environmental laws (including solid waste storage and liquid waste drain* Author to whom correspondence should be addressed. E-mail:
[email protected]. Phone: (0048) 566114569. Fax: (0048)566542477.
age), research projects are being focused on the development of alternative nonwaste production methods of K2CO3 or the possible modification of conventional processes. In previous papers by the authors,6,7 experimental results on the application of V2O5 for the sodium carbonate production process based on solid NaCl were reported. This systematic study is continued with the production process of K2CO3 based on KCl and V2O5. The proposed project is grounded on the generation of intermediate potassium metavanadate, which is subsequently, through many correlated unit operations, converted into the final product of potassium carbonate. The first stage of the process is the generation of intermediate KVO3 based on solid KCl and V2O5 in the presence of steam or oxygen, expressed by eqs 1 and 2. T
2KCl + V2O5 + H2O(steam) 98 2KVO3 + 2HClv (1) T
4KCl + 2V2O5 + O2 98 4KVO3 + 2Cl2v
(2)
under suitable reaction conditions.8 Potassium metavanadate is then transformed into potassium carbonate through the following unit operations: preparation of a saturated ammoniated brine of KVO3 and brine carbonization to reach the assumed carbonization degree of the system and the maximum yield toward the ammonium ions, WNH4+. The presented model can be illustrated with eqs 3 and 4.
KVO3 + NH3 + CO2 + H2O T NH4VO3V + KHCO3 (3) 2KVO3 + 2NH3 + CO2 + H2O T 2NH4VO3V + K2CO3 (4) A sparingly soluble ammonium metavanadate is precipitated out as the final product of the reactions, whereas a mixture of KHCO3 and K2CO3 is detected in solution, limited to the solution carbonization degree. Wet NH4VO3 solids are filtered out of mother liquor and then decomposed as indicated below. T
2NH4VO3 798 2NH3 + V2O5 + H2O
10.1021/ie000588i CCC: $20.00 © 2001 American Chemical Society Published on Web 01/20/2001
(5)
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Preliminary tests revealed that NH4VO3 decomposition was initiated at 328 K and terminated at 523 K. The products of reaction 5, ammonia and vanadium(V) oxide, are returned to the appropriate stages of the K2CO3 production process. It is well recognized that some side reactions occur in the system, because of the strong oxidizing character of V2O5. Evolved gaseous NH3 is partly oxidized by vanadium(V) oxide to nitrogen according to the reactions
2NH3 + 3V2O5 f N2 + 3V2O4 + 3H2O
(6)
2NH3 + 3V2O4 f N2 + 3V2O3 + 3H2O
(7)
The reduced vanadium oxides are then oxidized to V2O5, provided that a suitable amount of oxygen is present.
2V2O3 + O2 f 2V2O4
(8)
2V2O4 + O2 f 2V2O5
(9)
The final product of K2CO3 is generated through the evaporation of the after-filtration solution and the calcinaction of KHCO3 raw precipitate, as illustrated by eq 10. evaporation+calcination
2KHCO3 98 K2CO3 + CO2 + H2O (10) Because there is a strong need for the development on nonwaste alternative routes to K2CO3 production, the authors have proposed a method based on KCl and V2O5 in the presence of steam. Special attention was paid to the elaboration of an environmentally friendly model. The total amount of chloride ions from KCl is employed for hydrogen chloride production, whereas the vanadium(V) oxide and ammonia recovered from the NH4VO3 decomposition are recycled. The only disadvantage of the method is the contamination of the K2CO3 final product with different vanadium compounds, thus eliminating its application in the food industry. Thus far, no literature data on the operating parameters of reaction 1 have been found. The main objective of the present study was the determination of the optimum conditions providing with the maximum yield of KVO3 synthesis based on KCl and V2O5. The variable parameters were temperature, time of processing, and excess of KCl in the reaction mixture. Experimental Part A schematic diagram of the experimental apparatus used in this work is shown in Figure 1. For the investigations, the temperature was varied between 673 and 823 K, a range that was chosen with regard to the operating parameters of NaVO3 synthesis based on NaCl and V2O5 in the presence of steam.6 Temperatures below 673 K (with known stoichiometric ratio of KCl to V2O5) were found to influence the reaction yield only insignificantly, i.e., generally around or below 10% of the reaction yield. It was also determined that temperature increases over 823 K resulted in melting and sintering of the reaction mixture. This fact accounts for the loss of an active surface, the formation of undesirable byproducts such as bronzes,8 and, as a consequence, a reduction in the reaction yield. Initially, the investigations were focused on the determination of the efect of temperature and time of processing on the yield of KVO3 synthesis based on KCl
Figure 1. Schematic diagram of the laboratory apparatus set. Table 1. Mass Flow Rate of Steam in the Reactor for the KVO3 Synthesis time of processing (h)
mass flow rate of steam (g/h)
1 2 3 4 5 6
4.86 2.43 1.62 1.22 0.97 0.81
and V2O5 in the presence of steam for a known stoichiometric composition of the reaction mixture. The synthesis of KVO3 was carried out within the time limits of 1-6 h. The mass flow rate was controlled to guarantee a 10-fold molar excess of steam to V2O5 detected in the reaction mixture (reaction 1). The reaction was carried out, with regard to the time parameter, employing a variable flow rate of steam in the reactor. The data obtained are collected in Table 1. The flow rate was controlled by means of an electromagnetic valve installed in the steam generator. The preliminary tests revealed that there were no significant changes in the yield of reaction 1 with increasing time and (or) steam excess. The second experimental setup was concerned with the determination of the KVO3 synthesis yield versus KCl excess relative to V2O5 in the reaction mixture for a known time of processing. Each measurement for the assumed conditions was repeated three times in order to calculate the reaction yield as an arithmetic mean. All substances used in the experiments were of analytical purity grade and supplied as follows: V2O5 (purity better than 98 mass %, Fluka) and KCl (purity better than 99.5 mass %; POCh, Poland). All experiments were carried out with stated reagent granulation. The granulometric reagent specification was determined using the method of molecular sieve analysis (FRITSCH apparatus, Germany). The grain diameter of the reaction mixture components was varied between 0.063 and 0.5 mm (Table 2). A static flow reactor, made of quartz glass, was located inside an electric heating mantle. A temperature stability of about (1 K was achieved. A weighed amount (approximately 9 g) of the reaction mixture consisting of KCl and V2O5 with a known granulometric and quantitative composition was intro-
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Table 2. Granulometric Reagent Composition in the Reaction Mixture grain diameter (nm) >0.710 0.710-0.500 0.500-0.355 0.355-0.250 0.250-0.180 0.180-0.125 0.125-0.090 0.090-0.063
