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Ind. Eng. Chem. Res. 2001, 40, 826-832
PROCESS DESIGN AND CONTROL Utilization of Tannery Wastes for the Production of Sodium Chromate(VI) Zygmunt Kowalski* Institute of Chemistry and Inorganic Technology, Cracow University of Technology, Warszawska 24, 31-015 Krako´ w, Poland
Barbara Walawska Institute of Inorganic Chemistry, Sowinski 11, 44-101 Gliwice, Poland
This paper presents results of research on utilizing chromic waste, such as chromic mud from the current production and chromic tannery waste in the sodium chromate production process, in the area determined by the range of variation of selected process parameters. It was established that it was possible to substitute chromic ore used as a raw material for the production of sodium chromate with solid chromic tannery waste. The difference between the quantities of chromic mud produced and that used in the process of production has a negative value in such an alternative of the production process. Because a part of the chromic mud can be utilized, the proposed method also gives a chance for future utilization of old chromic heaps. 1. Introduction The transformation of the sodium chromate(VI) production into the wasteless process by introducing a technology with chrome waste recirculation (in-process, on-site, and off-site) makes it possible to achieve the goal of a wasteless chromium compound production model.2,5,6 This model system of chromium compound production in Poland can make it possible to utilize all solid chromic waste produced in Poland and gives a chance of utilizing old chromic waste dumps in a near, yet determinable, future.1,4-6 Chromium-containing waste is produced in Poland in many industries. One-fifth of the total quantity of the waste is created in the chemical industry. Solid chromic waste existing in Poland, which can be used in the sodium chromate production process, can be divided into three groups: (a) Waste obtained in the production of chromium compounds by their only Polish producer, the Alwernia Chemical Works. This waste has been generated as a result of the production of sodium chromate(VI) (the socalled chromic mud), sodium sulfate, and chromic acid anhydride (hydrated chromium oxides). During the last 50 years of chromium compound production, more than 3 million tons of chromic waste have been stored on heaps.6-8 (b) Waste containing hydrated chromium(III) oxides, mainly from galvanic waste treatment plants generated at hundreds of sites around Poland. During the last 20 years, between 40 000 and 50 000 tons of this waste have been accumulated on different storage heaps. * To whom correspondence should be addressed. Telephone: +48 12 633 03 00/27 16. Fax: +48 12 633 374. E-mail:
[email protected].
(c) Solid waste from tannery waste treatment plants containing hydrated chromium(III) oxides and significant quantities of organic compounds. Their quantity is estimated to be between 18 000 and 20 000 tons/ year.3,9-13 Leather waste, containing 3-7% of Cr(III), was obtained in the processes of chromic tanning, production of shoes, and fancy leather goods (leather scraps). Their quantity is estimated to be equal to about 40 000 tons/year.3,13-15 Another 1.5 million tons are stored on the heaps of the former ferrochromium producer in Siechnice. Altogether, about 5 million tons of chromic waste are stored on Polish heaps.1-4 This work presents the results of research on the utilization of chromic waste for the production of sodium chromate as a substitute for natural raw materials. Technological variants with the use of the off-site recirculation of chromic tannery waste and the use of the in-process recirculation of chromic mud have been analyzed. The determination of the optimal parameters for this method is based on the mathematical model of the process. This model, i.e., a system of equations describing the production process of sodium chromate(VI) as a function of selected process parameters, allows for evaluation of the production process indicators. These are consumption figures of the basic raw materials and technological figures, e.g., process efficiency, the content of chromium compounds in chromic waste generated in this process, and their quantity. 2. Experimental Part The main process for the production of sodium chromate is the calcination of the mixture of Na2CO3, CaO, and the chromium-containing raw material (typically chromic ore). The calcination is conducted at about 1423
10.1021/ie9905106 CCC: $20.00 © 2001 American Chemical Society Published on Web 01/12/2001
Ind. Eng. Chem. Res., Vol. 40, No. 3, 2001 827
K in an oxidizing atmosphere. The reaction17-19 runs according to the equation
2FeCr2O4 + 4Na2CO3 + 3.5O2 f 4Na2CrO4 + Fe2O3 + 4CO2 (1) At this temperature, both sodium carbonates and sodium chromate(VI) are present in a liquid phase. Sodium chromate(VI) appears already at the temperature of 873 K as a product of the reaction running in a solid phase. At the temperature of 928 K, it forms a liquid eutectic mixture Na2CrO4-Na2CO3, containing 62.5% of Na2CrO4. The presence of the liquid phase decreases the reaction rate, which, in turn, depends on the speed of the diffusion of air oxygen which runs through a liquid layer of alloys surrounding chromite grains. The use of finely ground fillers allows for an increase of the surface of the solid phase by decreasing the thickness of the alloy layer. In the classical method, dolomite was used as a filler.17-19 The chromic raw material with a high content of chromium(III) usually contains pollutants, such as Fe2O3, SiO2, and Al2O3. Calcium oxide contained in the filler fixes them into compounds barely soluble in water according to the following equations:
2CaO + SiO2 f 2CaO‚SiO2 4CaO + Al2O3 + Fe2O3 f 4CaO‚Al2O3‚Fe2O3
(2) (3)
Our previous investigations allowed us to work out a new technology using the in-process recycling of chromic mud. In this process, the dolomite filler was replaced with chromic mud with a small addition of calcium oxide. This technology was implemented in the Alwernia Chemical Works near Cracow in the summer of 1999 at a new sodium chromate unit. The implementation results showed a waste quantity of 80%, an energy consumption figure of 38%, and production cost 25% lower than that in the dolomite method. The yield of Na2CrO4 reached 88.5% (20% more than that in the classical dolomite method).6,8,16 In our further research, we wanted to substitute chromic ore with waste containing hydrated chromium oxides, coming mainly from the tannery and galvanic industries.8,12,14 Figure 1 presents the proposed flow sheet of the sodium chromate production process with the use of chromic ore or chromic tannery waste as a basic chromic raw material. In the chromic tanning process, the wastewater contains Cr(III) and organic compounds. Their quantity produced in Poland is estimated to be 1 million m3/ year.12 A standard method of chromic tannery waste treatment consists of creating suitable conditions for the precipitation of Cr(OH)3 and the subsequent precipitate filtration.17,18 In Polish tanneries Ca(OH)2, NaOH, and cement are mostly used for the neutralization of chromic waste.6,12 Solid waste from the treatment of tannery chromic waste is very diversified in terms of the content of Cr(III), organic compounds, humidity, and others pollutants. Table 1 presents some typical compositions of solid waste produced in chromic tannery waste treatment units.6 All kinds of chromic tannery wastes used in the sodium chromate production process14-24 require their initial preparation by thermal treatment. The analysis of possible solutions allowed the determination of the
Figure 1. Flow sheet of the sodium chromate production process with the use of chromic mud and chromic tannery waste as raw materials.
best one, consisting of the calcination of waste for 6 h at a temperature of 523 K or for 3 h at 873 K.4,5,15 This method allowed for removal of all water contained in the tannery waste and most of the organic contamination.4 The research on the production of sodium chromate conducted allows the formulation of equations which define the proportion of raw materials in the chromate charge.4-6,22,24
Gr + S + W + Ar ) 1000
(4)
X ) (GrCr2O3(Gr) + ArCr2O3(Ar))/1000
(5)
a ) W/CaO(stoich.)
(6)
b ) S/Na2CO3(stoich.)
(7)
CaO(stoich.) ) Gr[Cr2O3(Gr) - WGr - 1] + [Cr2O3(Ar) × Ar/Cr2O3(Gr)][Cr2O3(Gr) - WGr - 1] (8) WGr ) 1.0181Cr2O3(Gr) - 1.93791
(9)
Na2CO3(stoich.) ) 0.01395[GrCr2O3(Gr) + ArCr2O3(Ar)] (10) where Gr (chromic ore or tannery waste), Ar (chromic mud), S (soda), and W (calcium oxide) are quantities of raw materials (kg/1000 kg of charge); Cr2O3 is % Cr2O3 in Ar or Gr; and X, a, b, CaO(stoich.), and Na2CO3(stoich.) are indicators (described below). By solving this system, one can determine the quantity of raw materials in the chromate charge ratio. This system has been used in industrial practice. The following process parameters (independent variables) were selected for the examined object of research: (i) a is a stoichiometric indicator of calcium oxide, which is a ratio of the mass of calcium oxide (W) used in the process and the stoichiometric mass CaO(stoich.)
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Table 1. Some Chemical Components of Chromic Tannery Waste (na ) Not Available; nb ) Not Analyzed) content (mass % in the dry mass after drying in 378 K) supplier
H2O (%)
Cr
Ni
Zn
Cu
Fe
Cd
Pb
Na
Ca
Cl-
SO42-
Szczakowa Tannery ZACHEM Bydgoszcz PSKE ELBUD WOLMET Wolsztyn Hankelman Niepołomice Tanewski Bielsko Krako´w Tannery Radom Tannery OLEKTAN Zwolen˜ Florian Steel-works Ursus Gorzo´w Gniezno Tannery
73.1 75.2 12.0 6.8 51.5 75.8 82.3 86.2 78.6 61.5 67.0 56.0
14.03 22.33 23.84 18.58 10.57 29.18 12.69 5.96 14.63 4.17 1.39 1.22
0.007 0.16 0.021 0.361 na 0.0047 0.0010 0.003 0.0054 nb 0.30 0.0001
0.14 0.033 7.75 6.63 0.0178 0.239 0.0275 0.013 0.022 7.54 9.21 0.0038
0.001 0.042 0.0033 0.139 0.019 0.0194 0.038 0.002 0.0008 nb 1.03 0.0008
0.41 0.40 1.03 nb 0.644 0.245 0.591 0.22 0.33 21.0 9.31 ∼10
0.001 0.0027 0.0011 0.0006 0.0002 0.0035 na na na na 0.0065 na
nb nb 0.015 0.155 0.005 5.070 0.009 0.006 0.004 0.75 0.23 0.0007
7.6 1.5 na na 4.1 na 6.7 5.1 na na na na
5.7 na na na 30.1 na 5.3 7.1 na na na na
2.39 0.71 0.13 na 2.27 0.23 3.92 9.41 7.21 nb 0.31 nb
20.12 18.72 3.30 21.36 21.49 11.78 29.18 19.15 27.02 nb 0.94 nb
resulting from reactions (2) and (3). From the analysis of the compositions of the charge in the calcination process,20-22 it was accepted that the range of change of this variable would be a ∈ 〈0.2; 1.0〉. (ii) b ia a stoichiometric indicator of sodium carbonate which is a ratio of the mass of the calcium carbonate used in the process S and the stoichiometric mass Na2CO3(stoich.) resulting from reaction (1). In industrial units, this indicator in the dolomite method is equal to 0.90.95,17-19 and in the process where chromic mud is recycled, it can be even lower.20-24 This is the result of the presence of a sodium ion in the recycled waste. The accepted range of changes for this variable was b ∈ 〈0.5; 1.0〉. (iii) X is the chromium contained in the charge calculated to Cr2O3 (%). This variable bonds the mass of the chromium-containing raw materials used with their content of Cr(III+VI). The use of either excessive X values (>19.5%) or too little X values (
