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Preparation of high-purity sulphate lignin from spent black liquor using ultrafiltration and diafiltration processes
DESALINATION ELSEVIER
Desalination 115 (1998) l 11-120
Preparation of high-purity sulphate lignin from spent black liquor using ultrafiltration and diafiltration processes Ignacy Tanistra*, Michal Bodzek Silesian Technical University, Faculty of Environmental and Energy Engineering, Konarskiego str. 18, 44-100 Gliwice, Poland Tel. +48 (32) 371047, Fax +48 (32) 371698 email: [email protected] Received 19 March 1996 ; accepted 20 December 1997
Abstract
The paper presents investigations concerning the ultrafiltration process of black liquor using ultrafiltration polyacrylonitrile membranes. The above membranes proved useful in the process of concentration and ffactionation of these liquors. As a result, concentrates enriched with macromolecular post-sulphate lignin were obtained. Raising the transmembrane pressure, the sulphate lignin concentrates of high purity level were obtained (purity level higher than 80%). Further purity increase of the sulphate lignin is possible through raising the volumetric permeate flow yield to 90% (deep concentration), although the average permeate flux is then decreasing. Alternatively higher purity of the concentrates can be obtained by complementingthe ultrafiltration process with dialfillration, i.e., by removing residual impurities with an extra amount of water added in the process. The obtained results are reliable enough to recommend ultrafiltration as a production method for the preparation of sulphate lignin concentrates.
Keywords:Ultrafiltration; Diafiltration; Spent sulphate black liquor; Sulphate lignin
1. Introduction
In the process of wood digestion, all organic compounds, excluding cellulose ones, are dissolved in the appropriately prepared digesting liquor (about 50%). The solution obtained from the above process is called post-digesting liquor, and it is a waste by-product of cellulose *Corresponding author.
production. Organic compounds present in waste digesting liquors, especially lignin, can be treated as a very valuable chemical raw material from which many valuable substances can be achieved (cheaper than with traditional methods), as for example: vanillin, vanillic acid, active carbon, dispersing agents, synthetic tannins, ionexchangers, polymer fillers, artificial fertilizers, binding agents for the production of fibre boards, chipboards and corrugated board. So far, lignins
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were being produced from liquors obtained as a result of the sulphite method of wood digestion, which are unfit for regeneration. Recently, however, the sulphate lignin from black liquor seems to be more and more popular. Black liquors --post-digestion liquors from sulphate digestion processes--are fully disposed of and pose no problems for industry in view of environmental hazards. Their concentration and combustion allow retrieval of the chemicals and heat for the purposes of cellulose production. However, the possibility of producing auxiliary substances from sulphate lignin necessitates some changes to be assumed with regard to utilization of post-sulphate liquors. According to Pearl [1], in the United States many millions tons of lignin, which is a potential organic raw material, are used in the process of energy recovery every year. Further processing of lignin could be more profitable. The above fact should encourage the plants to change their attitude to lignin as merely a heat source and to assign at least some part of it to further processing, especially in view of recent Russian investigations [2] which prove that about 10% of lignin present in post-sulphate liquors may be retrieved in the regeneration process with no disturbance of this process. Sulphate lignin is traditionally obtained by its precipitation from black liquor using acid (most often sulphuric acid). The liquor should be initially concentrated thermally, up to about 50% of dry substance. The application ofultrafiltration (UF) in the process oflignin recovery considerably facilitates this process; the concentration of liquor takes place, whereby its further thermal concentration, if needed, limits the expenditure of energy. Furthermore, the lignin undergoes the process of fractionation. Due to the above, after UF macromolecular lignins predominate in the obtained retentate (concentrate), whereas the lowmolecular ones pass to the permeate. The modified lignin, i.e., enriched with maeromolecular fractions, is much more valuable as a raw material
for further processing than non-modified lignin. Data in the literature indicate that high-quality sulphate lignin concentrates obtained in the process of UF should contain from 25-30% of dry residue, whereof 80% is lignin. Such purity is necessary and sufficient for the production of the binding agent Karatex [3] and others. Our objective was to obtain the lignin of purity level equalling or surpassing 80%, with the application of UF and diafiltration processes on polyacrylonitrile membranes.
2. Experimental 2.1.Apparatus
The principal part of the installation used for the investigations was a large-scale plate-andframe laboratory module produced by a Danish firm, DDS. Schematically the installation is shown in Fig. 1. The circulating liquor was pumped by a high-pressure piston pump (Runnie Laboratory). In order to eliminate pressure fluctuations and to ensure stable performance of the pump, a hydraulic accumulator produced by Orsta Hydraulic (Germany) was used. The membrane module is suitable for carrying out both the UF process and reverse osmosis (RO). Modelled plates ensure laminar flows of the liquor subjected to concentration radially through narrow channels (0.3-0.5 mm), over the membranes in successive sections, at a linear speed of 0.5 m/s. The section consists of a plate with two membranes installed on both sides of the plate and two separating frames. They are installed in layers on the alignment pin. The number of layers depends on the membrane area desired. The minimum area is one section--0.036 m 2, and the maximum area is 0.36m 2. Appropriate sealing of the module is ensured by the hydraulic pump which is connected with the module by means of a pipe. The pump compresses the pile of membranes, pressing them down with two collars at very high pressure. The working pressure in the
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I. Tanistra, M. Bodzek / Desalination 115 (1998) 111-120
/• I
F-'q
r
2
1
6
7
I 11
I
\
/
8 Fig. 1. Ultrafiltration installation used in the investigations. 1 inlet of black liquor, 2 pump, 3 pressure buffer, 4 pressurereducing valve, 5 ultrafiltration module, 6 manometer, 7 outlet of permeate, 8 outlet ofretentate, 9 cut -off valve, I0 black liquor tank, 11 by-pass. module (0-10 MPa) is achieved making use of he pressure reducing valve. 2.2 Membranes and their character&tics
In our investigations we used flat asymmetric polyacrylonitrile membranes (PAN-20, PAN-21). The casting solutions for the preparation of membranes were obtained by dissolving polyacrylonitrile in dimethyloformamide. After that, the membranes were casted and gellated at room temperature. The number present at the membrane symbol stands for the percentage content of polymer in the film casting solution. Detailed characteristics of the discussed membranes and their transport and operating properties have been presented in earlier works [4-8]. Before actual investigations, each group of membranes was tested and conditioned. The process consisted of successively passing the following liquids through the membranes: distilled water, raw liquor and
aqueous solutions of dextrane preparations made by Polfa (Poland). The testing agents were passed at transmembrane pressures of 0.5, 0.75, 1.0, and 1.25 MPa for 28 h. Transmembrane pressures for this testing were selected basing on experimental data obtained in our earlier investigations [4-8]. The testing was to determine the permeability of the membranes and the retention coefficients of dextrane preparations. The value of volumetric permeate flux and water flux for particular membranes was measured by means o f a rotameter and calculated per 1 m 2 of the membrane area. Testing with raw liquor was designed to condition the groups of membranes and to make a preliminary determination of permeate flux for the proper concentration process. The characteristics of the obtained membranes are presented in Tables 1 and 2. All these tests were carried out using the batch non-concentration mode, i.e., the obtained permeate was recycled to the tank with testing liquid. As shown by the
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I. Tanistra, M. Bodzek / Desalination 115 (1998) 111-120
Table 1 Volumetric permeate flux obtained during testing of PAN membranes with distilled water and black liquor Membrane type
Volumetric permeate flux, m3/m2 d AP=0.4 MPa
AP=0.75 MPa
AP=I.0 MPa
AP=l.25 MPa
Distilled water: PAN-20 PAN-21
2.426 2.487
3.001 3.231
3.657 3.846
3.716 4.012
Black liquor: PAN-20 PAN-21
0.687 0476
0.753 0.669
0.984 0.707
1.061 0.690
Ap, transmembrane pressure.
Table 2 Permeate flux (Jr) and retention coefficient (R) of dextrane preparations obtained during testing PAN membranes with dextrane preparations of different molecular mass (M) Mof dextrane
AP = 0.5 MPa
Ap = 0.75 MPa
AP = 1.0 MPa
AP = 1.25 MPa
J~, m3/m2 d R, %
J~, m3/m2 d R, %
Jr, m3/m2 d R, %
d~, m3/m2 d R, %
PAN-20: 3,000 20,000 80,000 110,000
1.341 1.245 1.035 1.013
31.0 85.8 95.1 97.4
1.680 1.493 1.322 1.272
33.7 79.9 93.9 95.7
2.027 1.773 1.507 1.373
27.8 78.0 92.6 95.5
2.280 1.880 1.627 1.546
24.1 75.7 94.2 96.8
PAN-21 : 3,000 20,000 80,000 110,000
1.013 0.810 0.867 0.763
24.6 80.5 93.2 96.8
1.303 1.080 1.222 0.992
23.3 76.6 89.9 94.6
1.600 1.386 1.320 1.248
23.2 89.1 89.9 92.6
1.813 1.640 1.387 1.360
12.6 67.2 87.2 92.0
(dextrane concentration: 5 kg/m3 (AP, transmembrane pressure). investigations, the m e m b r a n e s PAN-20 and PAN-21 tested with dextrane preparations retain nearly all dextrane with a molecular mass exceeding 20,000. W e can therefore assume that the "cut o f f ' o f the m e m b r a n e s PAN-20 and
PAN-21 equals approximately 20,000. While testing the m e m b r a n e s with raw liquor, it was found that the permeate flux increased with increasing pressure (for PAN-20 an increase o f 54%; for PAN-21, 45%).
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L Tanistra, M. Bodzek/Desalination 115 (1998) 111-120 2. 3. Methods
For the actual experiments we used postsulphate black liquors, not soap-skimmed, from the paper mill at ~wiecie (Poland). Samples of this liquor were taken in the paper mill from the surge chamber before its preliminary concentration. Before introducing the liquor into the UF system, it was subjected to filtration through a ceramic strainer with a mesh of about 20 mm in diameter in order to remove all mechanical impurities, particularly cellulose fibres. The process of concentrating and fractionating the liquor was carried out by means of the batchconcentration mode, in the course of which the
liquor was recycling until the required volume permeate (filtrate) flow yield was reached; for a part of the experiment it amounted to 80% (40 dm 3 of liquor yielded 32dm 3 of filtrate and 8dm 3 of retentate). A few attempts to carry out additional purification of retentates (concentrates) were accomplished using diafiltration (UF with rinsing). Diafiltration adds a specified amount of water to a concentrated solution and then subjects it to UF until the same volume of filtrate has been reached. Water passing through the membranes removes from the diafiltrated solution a specified amount of low-molecular compounds which contaminate the solution. Diafiltration was carried out using the continuous mode and batch mode (Figs. 2 and 3).
H20 (1 dm 3)
H20 (1 dm 3)
1
i
K2
Raw liquor ULTRAFILTRATION 4 dm 3
20 dm 3 I UF ULTRAFI LTRATE 16 dm 3
DIAFILTRATE I 1 dm3
H20 (4 dm3)
H=O (6 dm3)
K5
l
K4
DIAFILTRATION RETENTATE
V STAGE
DIAFILTRATE II 1 dm 3
H20 (3 dm3)
1 DIAFI LTRATION IV STAGE
DF5
DF4
DIAFILTRATE V
DIAFILTRATE IV
6 drn 3
4 dm 3
K3
1 D IAFILTRATION
III STAGE
4
DF3 DIAFILTRATE III 6 dm 3
Fig. 2. Layout of preparation of purified lignin concentrates by ultrafiltration and batch diafiltration (K, retentate; DF, diafiltrate).
I. Tanistra, M. Bodzek/ Desalination 115 (1998) 111-120
116
H20 (10 dm 3)
1
K,
ULTRa, FILTRATION
Raw liquor
I STAGE
20 dm ~
II STAGE
10 dm 3
1 ULTRAFILTRATE 10 dm ~
ULTRAFILTRATION
DIAFILTRATION
1 I
RETENTATE I, 4 dm 3 Kk
1
DIAFILTRATE 10 dm 3
ULTRAFILTRATE 6 dm ~
II
Fig. 3. Layout of preparation of purified lignin concentratesby ultrafiltrationand continuous diafiltration (K, retentate; DF, diafiltrate). The rate of UF and UF with diafiltration was determined by measuring the average volumetric permeate flux and its changes in the course of concentration. In order to control the efficiency of the process in particular fractions, i.e. in permeate, retentate as well as in raw liquor, the contents of dissolved substances and lignin compounds were subjected to analysis, and the content of organic carbon as well as density were determined. The content of lignin compounds was measured by means of a spectrometer (Specord M-40), testing the absorbance of the solution in the ultraviolet range (280nm), using the standard sample of sulphate lignin obtained from the Pulp and Paper Institute in L6di (Poland). The content of organic carbon was determined using a Tocamaster (Beckmann). The remaining determinations were carried out by means of standard methods applied in the analysis of water and sewage [9].
3. Discussion
3.1 Ultrafiltration of black liquors Tables 3 and 4 present the results obtained in the course of liquor concentration using the polyacrylonitrile membranes for various pressures of 0.5, 0.75, and 1.0 MPa. In the course of each test the permeate flux decreased due to a continuous increase in the
concentration of substances dissolved in the liquor being concentrated (the tables present its averaged values). The increase of working pressure results in the occurrence of two phenomena: the compression of pores in the membranes and the opening of new pores. Within the range of pressures applied, the latter effect is dominant, and therefore, the average permeate flux increases with the increase in pressure. As a result of UF concentration of black liquors, two fractions, differing in their contents of dissolved substances and lignin contents, are obtained. The obtained results were defined by means of the retention coefficient and the level of lignin purity, i.e., the percentage ratio oflignin content to the overall content of dissolved substances. With the increase of working pressure, the concentration of dissolved substances increases for both polyacrylonitrile membranes (the opening pores have small diameter, which results in the increase of retention coefficient). Also, the concentration of lignins in the obtained concentrates increases and consequently their purity level. Hence, for the PAN-20 memrane, the factor in question--most important in view of further applications-increases from 65% (pressure 0.5MPa) through 75% (0.75 MPa) to the value of 80.5% (1.0 MPa), with a simultaneous increase of the average permeate flux from 0.41m3/m2d to 0.53m3/m2d, which is by about 30%. However, for the more
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I. Tanistra, M. Bodzek / Desalination 115 (1998) 111-120 Table 3 Ultrafiltration concentration and fractionation of black liquors on a PAN-20 membrane Parameter
Raw black liquor
Permeate
Retentate
Retention coefficient, %
Pressure 0.5 MPa, average permeate flux: 0.410 m3/m2d Density, kg/m3 Dissolved substances, kg/m3 Lignins, kg/m3 Organic carbon, kg/m3 Degree of purity of lignins a, %
1081 155.0 75.0 50.0 48.4
1075 132.5 51.9 33.5
1093 221.7 144.1 120.1
-40.2 64.0
- -
65.0
- -
Pressure 0.75 MPa, average permeate flux: 0.541 m3/m2d Density, kg/m3 Dissolved substances, kg/m3 Lignins, kg/m3 Organic carbon, kg/m3 Degree of purity ofligninsa, %
1083 160.0 76.1 49.4 47.6
1070 127.5 50.2 30.2 --
1099 232.0 174.1 128.0 75.0
-45.0 71.6 76.4 --
Pressure 1.0 MPa, average permeate flux: 0.530 m3/m2d Density, kg/m3 Dissolved substances, kg/m3 Lignins, kg/m3 Organic carbon, kg/m3 Degree of purity of lignins a, %
1081 164.1 74.2 49.1 45.2
1069 119.3 43.0 26.3 --
1108 250.3 201.5 134.1 80.3
-52.3 78.7 80.3 --
aDegree of purity of lignins is the ratio of the lignin content to total dissolved substances. Note: Membrane area: 0.18 m2, permeate flow yield: 0.8, feed rate: 450 dm3/h, thickness of membrane: 0.2 mm). compact membrane PAN-21, the values are respectively: 69.1% (0.5 MPa), 79.1% (0.75 MPa) and 83.4% (1.0MPa), and the permeate flux increased from 0.348 m3/m2 d to 0.450 m3/m2d and also equaled about 30%. The most important accomplishment is the 80% purity level of lignins which has been surpassed for both membranes. As a result, the obtained retentates are not only enriched with macro-molecular lignins, but they also can be referred to as lignin concentrates of high purity. By obtaining concentrates with a lignin purity level exceeding 80%, the objectives o f this stage o f experiments have been accomplished.
3.2. Attempts aiming to increase the purity level o f lignins In order to increase the purity level o f lignins in the obtained concentrates for both polyacrylonitric membranes, two methods were applied: further concentration (until the permeate volumetric yield reached 90%) and diafiltration. The results achieved by so-called "deep concentration" are presented in Table 5. The lignin purity level increased-- on the PAN-20 membrane to 82.0% and on PAN-21 to 8 5 . 8 % - - b u t the average permeate flux on the PAN-20 membrane decreased 1.8 times, and on PAN-21 1.6 times, as
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I. Tanistra, M Bodzek / Desalination 115 (1998) 111-120
Table 4 Ultrafiltration concentration and fractionation of black liquors on a PAN-21 membrane Parameter
Raw black liquor
Permeate
Retentate
Retention coefficient, %
Pressure 0.5 MPa, average permeate flux: 0.348 m3/m2 d Density, kg/m 3 Dissolved substances, kg/m 3 Lignins, kg/m 3 Organic carbon, kg/m 3 Degree of purity oflignins a, %
1083 160.0 76.1 50.8 47.5
1071 126.1 50.0 31.3 --
1096 230.7 159.4 126.7 69.1
-45.3 68.6 75.1 --
Pressure 0.75 MPa, average permeate flux: 0.443 m3/m2d Density, kg/m 3 Dissolved substances, kg/m 3 Lignins, kg/m 3 Organic carbon, kg/m 3 Degree of purity oflignins a, %
1083 160.7 77.3 51.0 48.1
1069 120.3 48. l 28.9 --
1098 249.2 197.1 139.4 79.1
-51.7 75.6 79.3 --
Pressure 1.0 MPa, average permeate flux: 0.450 m3/m2 d Density, kg/m 3 Dissolved substances, kg/m 3 Lignins, kg/m 3 Organic carbon, kg/m 3 Degree of purity of lignins a, %
1081 164.1 74.2 48.3 45.2
1069 115.1 43.0 25.9 --
1108 260.4 217.3 146.1 83.4
-55.8 80.2 82.3 --
aDegree of purity of lignins is the ratio of the lignin content to total dissolved substances. Note: Membrane area: 0.18 m 2, permeate flow yield: 0.8, feed rate: 450 m3/h, thickness of membrane: 0.2mm). Table 5 "Deep concentration" and fractionation of black liquors Parameter
Raw black liquor
Permeate
Retentate
Retention coefficient, %
PAN-20 membrane, average permeate flux: 0.301 m3/m2 d Density, kg/m 3 Dissolved substances, kg/m 3 Lignins, kg/m 3 Organic carbon, kg/m 3 Degree of purity oflignins a, %
1085 167.0 76.1 51.0 45.6
1071 133.0 48.1 29.5 --
1215 398.0 326.0 150.2 82.0
-66.25 85.2 79.4 --
PAN-21 membrane, average permeate flux: 0.285 m3/m2d Density, kg/m 3 Dissolved substances, kg/m 3 Lignins, kg/m 3 Organic carbon, kg/m 3 Degree of purity of lignins a, %
1081 165.1 75.1 52.1 45.5
1065 128.1 45.3 26.2 --
1115 432.0 320.7 370.7 85.8
-70.3 80.8 69.5 --
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L Tanistra, M Bodzek/Desalination 115 (1998) 111-120 Table 6 Ultrafiltration and diafiltration (batch mode) of black liquors (PAN-20 membrane, pressure: 1.0 MPa) Parameter
Density, kg/m 3 TDS, kg/m3 Lignins, kg/m 3 Organic carbon, kg/m 3 Degree of purity of lignins, %
Raw black UF, K~ liquor
1080 164.1 73.9 49.9 44.9
1103 247.2 200.2 133.0 81.0
Diafiltration Stage I, Kt
Stage II, K 2 Stage III, K3 Stage IV, K4 Stage V, K 5
1091 234.1 189.9 130.0 81.1
1087 222.2 181.8 131.2 81.9
1080 198.3 163.7 120.0 82.6
1073 187.3 157.0 115.0 84.0
1061 177.2 153.8 109.0 86.9
K, retentate; TDS, total dissolved solids.
Table 7 Ultrafiltration and diafiltration (continuous mode) of black liquors (PAN-20 membrane, pressure: 1.0 MPa) Parameter
Density, kg/m3 TDS, kg/m 3 Lignins, kg/m3 Organic carbon, kg/m3 Degree of purity of iignins, %
Raw liquor
1074 153.1 77.3 51.5 50.5
Ultrafiltration I
Diafiltration
Ultrafiltration II
UF I
K~
DF
Kp
UF II
Kk
1052 94.1 36.2 24.1 --
1080 211.9 120.1 70,2 56.6
1039 68.7 31.2 20.0 --
1060 146.0 100.8 64.1 69.1
1060 63.2 25.7 19.0 --
1090 260.4 218.9 94.2 84.1
UF, DF: permeate, diafiltration; K, retentate; TDS, total dissolved solids.
Table 8 Ultrafiltration and diafiltration (batch mode) of black liquors (PAN-21 membrane, pressure: 1.0 MPa) Parameter
Density, kg/m 3 TDS, kg/m 3 Lignins, kg/m 3 Organic carbon, kg/m3 Degree of purity of lignins, %
Raw black UF, Kz liquor
1081 165.2 76.1 50.4 46.1
1094 250.1 203.8 138.0 81.5
K, retentate; TDS, total dissolved solids.
Diafiltration Stage I, K~
Stage II, K2 Stage III, K3 Stage IV, K4 Stage V, K5
1085 249.2 203.9 135.5 81.9
1089 227.3 187.5 138.0 82.5
1080 204.7 168.9 129.3 83.0
1071 180.2 153.5 123.2 85.2
1061 185.3 162.6 110.2 87.8
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1. Tanistra, M. Bodzek / Desalination 115 (1998) 111-120
Table 9 Ultrafiltration and diafiltration (continuous mode) of black liquors (PAN-21 membrane, pressure: 1.0 MPa) Parameter
Density, kg/m 3 TDS, kg/m3 Lignins, kg/m3 Organic carbon, kg/m3 Degree of purity of lignins, %
Raw liquor
1078 158.5 74.8 50.2 47.2
UltrafiltrationI
Diafiltration
Ultrafiltration II
UF
K
DF
K
UF
K
1050 90.1 35.1 22.3 --
1081 216.6 125.4 75.4 57.9
1035 61.0 30.0 17.9 --
1061 154.2 111.9 66.4 72.6
1031 57.9 24.1 15.0 --
1092 275.3 237.5 98.1 86.3
UF, DF: permeate, diafiltration; K, retentate; TDS, total dissolved solids. compared to the values obtained with the volumetric permeate flow yield equaling 80%. Also, the experiments testing the application of the diafiltration method for the purification of lignin concentrates proved successful (Tables 6 9). Both kinds ofpolyacrylonitric membranes have been used in the experiment (pressure 1,0 MPa). Using this method, the highest levels of lignin purity have been obtained. For the batch diafiltration method they amounted to 86.09 and 87.1% (membranes PAN-20 and PAN-21), and for the continuous method, respectively, 84.1% and 86.3%.
4. Conclusions Based on the investigations, we found that the polyacrylonitrile membranes produced in our laboratory proved very useful in the concentration and fractionation of black liquors from the sulphate method of wood digestion. The obtained concentrates of the macromolecular post-sulphate
lignin (purity level above 80%) can be applied as attractive raw materials in many branches.
References [1] I.A.Pearl, Tappi, 65(5) (1982) 68. [2] B.D.Bogomolow et.al., Bum. Prom., 5 (1975) 14. [3] J. Wagner, Ultrafiltration and reverse osmosis in the pulp and paper industry. Unpublished data from DOW, Denmark, 1990. [4] M. Bodzek, In: Synthetic Polymeric Membranes. W.de Gruyter, Berlin, 1987, p.193. [5] M. Bodzek and I. Tanistra, Archiwum Ochrony ~rodowiska, 1/4 (1986) 75. [6] M. Bodzek and I. Tanistra, Environ. Protect. Eng., 13 (1987) 5. [7] M. Bodzek, K. Konieczny and I. Tanistra, Chemia Stosowana, 32 (1988) 311. [8] M. Bodzek J. Bohdziewicz, I. Tanistra and K. Konieczny, Environ. Protect. Eng., 15 (1988) 121. [9] W. Hermanowicz, W. Do~afiska, J. Dojlido and B. Koziorowski, Physico-chemicalAnlysis of Water and Wastewater [in Polish]. Arkady, Warsaw, 1976.
Desalination 115 (1998) l 11-120
Preparation of high-purity sulphate lignin from spent black liquor using ultrafiltration and diafiltration processes Ignacy Tanistra*, Michal Bodzek Silesian Technical University, Faculty of Environmental and Energy Engineering, Konarskiego str. 18, 44-100 Gliwice, Poland Tel. +48 (32) 371047, Fax +48 (32) 371698 email: [email protected] Received 19 March 1996 ; accepted 20 December 1997
Abstract
The paper presents investigations concerning the ultrafiltration process of black liquor using ultrafiltration polyacrylonitrile membranes. The above membranes proved useful in the process of concentration and ffactionation of these liquors. As a result, concentrates enriched with macromolecular post-sulphate lignin were obtained. Raising the transmembrane pressure, the sulphate lignin concentrates of high purity level were obtained (purity level higher than 80%). Further purity increase of the sulphate lignin is possible through raising the volumetric permeate flow yield to 90% (deep concentration), although the average permeate flux is then decreasing. Alternatively higher purity of the concentrates can be obtained by complementingthe ultrafiltration process with dialfillration, i.e., by removing residual impurities with an extra amount of water added in the process. The obtained results are reliable enough to recommend ultrafiltration as a production method for the preparation of sulphate lignin concentrates.
Keywords:Ultrafiltration; Diafiltration; Spent sulphate black liquor; Sulphate lignin
1. Introduction
In the process of wood digestion, all organic compounds, excluding cellulose ones, are dissolved in the appropriately prepared digesting liquor (about 50%). The solution obtained from the above process is called post-digesting liquor, and it is a waste by-product of cellulose *Corresponding author.
production. Organic compounds present in waste digesting liquors, especially lignin, can be treated as a very valuable chemical raw material from which many valuable substances can be achieved (cheaper than with traditional methods), as for example: vanillin, vanillic acid, active carbon, dispersing agents, synthetic tannins, ionexchangers, polymer fillers, artificial fertilizers, binding agents for the production of fibre boards, chipboards and corrugated board. So far, lignins
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1. Tanistra, M. Bodzek / Desalination 115 (1998) 111-120
were being produced from liquors obtained as a result of the sulphite method of wood digestion, which are unfit for regeneration. Recently, however, the sulphate lignin from black liquor seems to be more and more popular. Black liquors --post-digestion liquors from sulphate digestion processes--are fully disposed of and pose no problems for industry in view of environmental hazards. Their concentration and combustion allow retrieval of the chemicals and heat for the purposes of cellulose production. However, the possibility of producing auxiliary substances from sulphate lignin necessitates some changes to be assumed with regard to utilization of post-sulphate liquors. According to Pearl [1], in the United States many millions tons of lignin, which is a potential organic raw material, are used in the process of energy recovery every year. Further processing of lignin could be more profitable. The above fact should encourage the plants to change their attitude to lignin as merely a heat source and to assign at least some part of it to further processing, especially in view of recent Russian investigations [2] which prove that about 10% of lignin present in post-sulphate liquors may be retrieved in the regeneration process with no disturbance of this process. Sulphate lignin is traditionally obtained by its precipitation from black liquor using acid (most often sulphuric acid). The liquor should be initially concentrated thermally, up to about 50% of dry substance. The application ofultrafiltration (UF) in the process oflignin recovery considerably facilitates this process; the concentration of liquor takes place, whereby its further thermal concentration, if needed, limits the expenditure of energy. Furthermore, the lignin undergoes the process of fractionation. Due to the above, after UF macromolecular lignins predominate in the obtained retentate (concentrate), whereas the lowmolecular ones pass to the permeate. The modified lignin, i.e., enriched with maeromolecular fractions, is much more valuable as a raw material
for further processing than non-modified lignin. Data in the literature indicate that high-quality sulphate lignin concentrates obtained in the process of UF should contain from 25-30% of dry residue, whereof 80% is lignin. Such purity is necessary and sufficient for the production of the binding agent Karatex [3] and others. Our objective was to obtain the lignin of purity level equalling or surpassing 80%, with the application of UF and diafiltration processes on polyacrylonitrile membranes.
2. Experimental 2.1.Apparatus
The principal part of the installation used for the investigations was a large-scale plate-andframe laboratory module produced by a Danish firm, DDS. Schematically the installation is shown in Fig. 1. The circulating liquor was pumped by a high-pressure piston pump (Runnie Laboratory). In order to eliminate pressure fluctuations and to ensure stable performance of the pump, a hydraulic accumulator produced by Orsta Hydraulic (Germany) was used. The membrane module is suitable for carrying out both the UF process and reverse osmosis (RO). Modelled plates ensure laminar flows of the liquor subjected to concentration radially through narrow channels (0.3-0.5 mm), over the membranes in successive sections, at a linear speed of 0.5 m/s. The section consists of a plate with two membranes installed on both sides of the plate and two separating frames. They are installed in layers on the alignment pin. The number of layers depends on the membrane area desired. The minimum area is one section--0.036 m 2, and the maximum area is 0.36m 2. Appropriate sealing of the module is ensured by the hydraulic pump which is connected with the module by means of a pipe. The pump compresses the pile of membranes, pressing them down with two collars at very high pressure. The working pressure in the
113
I. Tanistra, M. Bodzek / Desalination 115 (1998) 111-120
/• I
F-'q
r
2
1
6
7
I 11
I
\
/
8 Fig. 1. Ultrafiltration installation used in the investigations. 1 inlet of black liquor, 2 pump, 3 pressure buffer, 4 pressurereducing valve, 5 ultrafiltration module, 6 manometer, 7 outlet of permeate, 8 outlet ofretentate, 9 cut -off valve, I0 black liquor tank, 11 by-pass. module (0-10 MPa) is achieved making use of he pressure reducing valve. 2.2 Membranes and their character&tics
In our investigations we used flat asymmetric polyacrylonitrile membranes (PAN-20, PAN-21). The casting solutions for the preparation of membranes were obtained by dissolving polyacrylonitrile in dimethyloformamide. After that, the membranes were casted and gellated at room temperature. The number present at the membrane symbol stands for the percentage content of polymer in the film casting solution. Detailed characteristics of the discussed membranes and their transport and operating properties have been presented in earlier works [4-8]. Before actual investigations, each group of membranes was tested and conditioned. The process consisted of successively passing the following liquids through the membranes: distilled water, raw liquor and
aqueous solutions of dextrane preparations made by Polfa (Poland). The testing agents were passed at transmembrane pressures of 0.5, 0.75, 1.0, and 1.25 MPa for 28 h. Transmembrane pressures for this testing were selected basing on experimental data obtained in our earlier investigations [4-8]. The testing was to determine the permeability of the membranes and the retention coefficients of dextrane preparations. The value of volumetric permeate flux and water flux for particular membranes was measured by means o f a rotameter and calculated per 1 m 2 of the membrane area. Testing with raw liquor was designed to condition the groups of membranes and to make a preliminary determination of permeate flux for the proper concentration process. The characteristics of the obtained membranes are presented in Tables 1 and 2. All these tests were carried out using the batch non-concentration mode, i.e., the obtained permeate was recycled to the tank with testing liquid. As shown by the
114
I. Tanistra, M. Bodzek / Desalination 115 (1998) 111-120
Table 1 Volumetric permeate flux obtained during testing of PAN membranes with distilled water and black liquor Membrane type
Volumetric permeate flux, m3/m2 d AP=0.4 MPa
AP=0.75 MPa
AP=I.0 MPa
AP=l.25 MPa
Distilled water: PAN-20 PAN-21
2.426 2.487
3.001 3.231
3.657 3.846
3.716 4.012
Black liquor: PAN-20 PAN-21
0.687 0476
0.753 0.669
0.984 0.707
1.061 0.690
Ap, transmembrane pressure.
Table 2 Permeate flux (Jr) and retention coefficient (R) of dextrane preparations obtained during testing PAN membranes with dextrane preparations of different molecular mass (M) Mof dextrane
AP = 0.5 MPa
Ap = 0.75 MPa
AP = 1.0 MPa
AP = 1.25 MPa
J~, m3/m2 d R, %
J~, m3/m2 d R, %
Jr, m3/m2 d R, %
d~, m3/m2 d R, %
PAN-20: 3,000 20,000 80,000 110,000
1.341 1.245 1.035 1.013
31.0 85.8 95.1 97.4
1.680 1.493 1.322 1.272
33.7 79.9 93.9 95.7
2.027 1.773 1.507 1.373
27.8 78.0 92.6 95.5
2.280 1.880 1.627 1.546
24.1 75.7 94.2 96.8
PAN-21 : 3,000 20,000 80,000 110,000
1.013 0.810 0.867 0.763
24.6 80.5 93.2 96.8
1.303 1.080 1.222 0.992
23.3 76.6 89.9 94.6
1.600 1.386 1.320 1.248
23.2 89.1 89.9 92.6
1.813 1.640 1.387 1.360
12.6 67.2 87.2 92.0
(dextrane concentration: 5 kg/m3 (AP, transmembrane pressure). investigations, the m e m b r a n e s PAN-20 and PAN-21 tested with dextrane preparations retain nearly all dextrane with a molecular mass exceeding 20,000. W e can therefore assume that the "cut o f f ' o f the m e m b r a n e s PAN-20 and
PAN-21 equals approximately 20,000. While testing the m e m b r a n e s with raw liquor, it was found that the permeate flux increased with increasing pressure (for PAN-20 an increase o f 54%; for PAN-21, 45%).
115
L Tanistra, M. Bodzek/Desalination 115 (1998) 111-120 2. 3. Methods
For the actual experiments we used postsulphate black liquors, not soap-skimmed, from the paper mill at ~wiecie (Poland). Samples of this liquor were taken in the paper mill from the surge chamber before its preliminary concentration. Before introducing the liquor into the UF system, it was subjected to filtration through a ceramic strainer with a mesh of about 20 mm in diameter in order to remove all mechanical impurities, particularly cellulose fibres. The process of concentrating and fractionating the liquor was carried out by means of the batchconcentration mode, in the course of which the
liquor was recycling until the required volume permeate (filtrate) flow yield was reached; for a part of the experiment it amounted to 80% (40 dm 3 of liquor yielded 32dm 3 of filtrate and 8dm 3 of retentate). A few attempts to carry out additional purification of retentates (concentrates) were accomplished using diafiltration (UF with rinsing). Diafiltration adds a specified amount of water to a concentrated solution and then subjects it to UF until the same volume of filtrate has been reached. Water passing through the membranes removes from the diafiltrated solution a specified amount of low-molecular compounds which contaminate the solution. Diafiltration was carried out using the continuous mode and batch mode (Figs. 2 and 3).
H20 (1 dm 3)
H20 (1 dm 3)
1
i
K2
Raw liquor ULTRAFILTRATION 4 dm 3
20 dm 3 I UF ULTRAFI LTRATE 16 dm 3
DIAFILTRATE I 1 dm3
H20 (4 dm3)
H=O (6 dm3)
K5
l
K4
DIAFILTRATION RETENTATE
V STAGE
DIAFILTRATE II 1 dm 3
H20 (3 dm3)
1 DIAFI LTRATION IV STAGE
DF5
DF4
DIAFILTRATE V
DIAFILTRATE IV
6 drn 3
4 dm 3
K3
1 D IAFILTRATION
III STAGE
4
DF3 DIAFILTRATE III 6 dm 3
Fig. 2. Layout of preparation of purified lignin concentrates by ultrafiltration and batch diafiltration (K, retentate; DF, diafiltrate).
I. Tanistra, M. Bodzek/ Desalination 115 (1998) 111-120
116
H20 (10 dm 3)
1
K,
ULTRa, FILTRATION
Raw liquor
I STAGE
20 dm ~
II STAGE
10 dm 3
1 ULTRAFILTRATE 10 dm ~
ULTRAFILTRATION
DIAFILTRATION
1 I
RETENTATE I, 4 dm 3 Kk
1
DIAFILTRATE 10 dm 3
ULTRAFILTRATE 6 dm ~
II
Fig. 3. Layout of preparation of purified lignin concentratesby ultrafiltrationand continuous diafiltration (K, retentate; DF, diafiltrate). The rate of UF and UF with diafiltration was determined by measuring the average volumetric permeate flux and its changes in the course of concentration. In order to control the efficiency of the process in particular fractions, i.e. in permeate, retentate as well as in raw liquor, the contents of dissolved substances and lignin compounds were subjected to analysis, and the content of organic carbon as well as density were determined. The content of lignin compounds was measured by means of a spectrometer (Specord M-40), testing the absorbance of the solution in the ultraviolet range (280nm), using the standard sample of sulphate lignin obtained from the Pulp and Paper Institute in L6di (Poland). The content of organic carbon was determined using a Tocamaster (Beckmann). The remaining determinations were carried out by means of standard methods applied in the analysis of water and sewage [9].
3. Discussion
3.1 Ultrafiltration of black liquors Tables 3 and 4 present the results obtained in the course of liquor concentration using the polyacrylonitrile membranes for various pressures of 0.5, 0.75, and 1.0 MPa. In the course of each test the permeate flux decreased due to a continuous increase in the
concentration of substances dissolved in the liquor being concentrated (the tables present its averaged values). The increase of working pressure results in the occurrence of two phenomena: the compression of pores in the membranes and the opening of new pores. Within the range of pressures applied, the latter effect is dominant, and therefore, the average permeate flux increases with the increase in pressure. As a result of UF concentration of black liquors, two fractions, differing in their contents of dissolved substances and lignin contents, are obtained. The obtained results were defined by means of the retention coefficient and the level of lignin purity, i.e., the percentage ratio oflignin content to the overall content of dissolved substances. With the increase of working pressure, the concentration of dissolved substances increases for both polyacrylonitrile membranes (the opening pores have small diameter, which results in the increase of retention coefficient). Also, the concentration of lignins in the obtained concentrates increases and consequently their purity level. Hence, for the PAN-20 memrane, the factor in question--most important in view of further applications-increases from 65% (pressure 0.5MPa) through 75% (0.75 MPa) to the value of 80.5% (1.0 MPa), with a simultaneous increase of the average permeate flux from 0.41m3/m2d to 0.53m3/m2d, which is by about 30%. However, for the more
117
I. Tanistra, M. Bodzek / Desalination 115 (1998) 111-120 Table 3 Ultrafiltration concentration and fractionation of black liquors on a PAN-20 membrane Parameter
Raw black liquor
Permeate
Retentate
Retention coefficient, %
Pressure 0.5 MPa, average permeate flux: 0.410 m3/m2d Density, kg/m3 Dissolved substances, kg/m3 Lignins, kg/m3 Organic carbon, kg/m3 Degree of purity of lignins a, %
1081 155.0 75.0 50.0 48.4
1075 132.5 51.9 33.5
1093 221.7 144.1 120.1
-40.2 64.0
- -
65.0
- -
Pressure 0.75 MPa, average permeate flux: 0.541 m3/m2d Density, kg/m3 Dissolved substances, kg/m3 Lignins, kg/m3 Organic carbon, kg/m3 Degree of purity ofligninsa, %
1083 160.0 76.1 49.4 47.6
1070 127.5 50.2 30.2 --
1099 232.0 174.1 128.0 75.0
-45.0 71.6 76.4 --
Pressure 1.0 MPa, average permeate flux: 0.530 m3/m2d Density, kg/m3 Dissolved substances, kg/m3 Lignins, kg/m3 Organic carbon, kg/m3 Degree of purity of lignins a, %
1081 164.1 74.2 49.1 45.2
1069 119.3 43.0 26.3 --
1108 250.3 201.5 134.1 80.3
-52.3 78.7 80.3 --
aDegree of purity of lignins is the ratio of the lignin content to total dissolved substances. Note: Membrane area: 0.18 m2, permeate flow yield: 0.8, feed rate: 450 dm3/h, thickness of membrane: 0.2 mm). compact membrane PAN-21, the values are respectively: 69.1% (0.5 MPa), 79.1% (0.75 MPa) and 83.4% (1.0MPa), and the permeate flux increased from 0.348 m3/m2 d to 0.450 m3/m2d and also equaled about 30%. The most important accomplishment is the 80% purity level of lignins which has been surpassed for both membranes. As a result, the obtained retentates are not only enriched with macro-molecular lignins, but they also can be referred to as lignin concentrates of high purity. By obtaining concentrates with a lignin purity level exceeding 80%, the objectives o f this stage o f experiments have been accomplished.
3.2. Attempts aiming to increase the purity level o f lignins In order to increase the purity level o f lignins in the obtained concentrates for both polyacrylonitric membranes, two methods were applied: further concentration (until the permeate volumetric yield reached 90%) and diafiltration. The results achieved by so-called "deep concentration" are presented in Table 5. The lignin purity level increased-- on the PAN-20 membrane to 82.0% and on PAN-21 to 8 5 . 8 % - - b u t the average permeate flux on the PAN-20 membrane decreased 1.8 times, and on PAN-21 1.6 times, as
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I. Tanistra, M Bodzek / Desalination 115 (1998) 111-120
Table 4 Ultrafiltration concentration and fractionation of black liquors on a PAN-21 membrane Parameter
Raw black liquor
Permeate
Retentate
Retention coefficient, %
Pressure 0.5 MPa, average permeate flux: 0.348 m3/m2 d Density, kg/m 3 Dissolved substances, kg/m 3 Lignins, kg/m 3 Organic carbon, kg/m 3 Degree of purity oflignins a, %
1083 160.0 76.1 50.8 47.5
1071 126.1 50.0 31.3 --
1096 230.7 159.4 126.7 69.1
-45.3 68.6 75.1 --
Pressure 0.75 MPa, average permeate flux: 0.443 m3/m2d Density, kg/m 3 Dissolved substances, kg/m 3 Lignins, kg/m 3 Organic carbon, kg/m 3 Degree of purity oflignins a, %
1083 160.7 77.3 51.0 48.1
1069 120.3 48. l 28.9 --
1098 249.2 197.1 139.4 79.1
-51.7 75.6 79.3 --
Pressure 1.0 MPa, average permeate flux: 0.450 m3/m2 d Density, kg/m 3 Dissolved substances, kg/m 3 Lignins, kg/m 3 Organic carbon, kg/m 3 Degree of purity of lignins a, %
1081 164.1 74.2 48.3 45.2
1069 115.1 43.0 25.9 --
1108 260.4 217.3 146.1 83.4
-55.8 80.2 82.3 --
aDegree of purity of lignins is the ratio of the lignin content to total dissolved substances. Note: Membrane area: 0.18 m 2, permeate flow yield: 0.8, feed rate: 450 m3/h, thickness of membrane: 0.2mm). Table 5 "Deep concentration" and fractionation of black liquors Parameter
Raw black liquor
Permeate
Retentate
Retention coefficient, %
PAN-20 membrane, average permeate flux: 0.301 m3/m2 d Density, kg/m 3 Dissolved substances, kg/m 3 Lignins, kg/m 3 Organic carbon, kg/m 3 Degree of purity oflignins a, %
1085 167.0 76.1 51.0 45.6
1071 133.0 48.1 29.5 --
1215 398.0 326.0 150.2 82.0
-66.25 85.2 79.4 --
PAN-21 membrane, average permeate flux: 0.285 m3/m2d Density, kg/m 3 Dissolved substances, kg/m 3 Lignins, kg/m 3 Organic carbon, kg/m 3 Degree of purity of lignins a, %
1081 165.1 75.1 52.1 45.5
1065 128.1 45.3 26.2 --
1115 432.0 320.7 370.7 85.8
-70.3 80.8 69.5 --
119
L Tanistra, M Bodzek/Desalination 115 (1998) 111-120 Table 6 Ultrafiltration and diafiltration (batch mode) of black liquors (PAN-20 membrane, pressure: 1.0 MPa) Parameter
Density, kg/m 3 TDS, kg/m3 Lignins, kg/m 3 Organic carbon, kg/m 3 Degree of purity of lignins, %
Raw black UF, K~ liquor
1080 164.1 73.9 49.9 44.9
1103 247.2 200.2 133.0 81.0
Diafiltration Stage I, Kt
Stage II, K 2 Stage III, K3 Stage IV, K4 Stage V, K 5
1091 234.1 189.9 130.0 81.1
1087 222.2 181.8 131.2 81.9
1080 198.3 163.7 120.0 82.6
1073 187.3 157.0 115.0 84.0
1061 177.2 153.8 109.0 86.9
K, retentate; TDS, total dissolved solids.
Table 7 Ultrafiltration and diafiltration (continuous mode) of black liquors (PAN-20 membrane, pressure: 1.0 MPa) Parameter
Density, kg/m3 TDS, kg/m 3 Lignins, kg/m3 Organic carbon, kg/m3 Degree of purity of iignins, %
Raw liquor
1074 153.1 77.3 51.5 50.5
Ultrafiltration I
Diafiltration
Ultrafiltration II
UF I
K~
DF
Kp
UF II
Kk
1052 94.1 36.2 24.1 --
1080 211.9 120.1 70,2 56.6
1039 68.7 31.2 20.0 --
1060 146.0 100.8 64.1 69.1
1060 63.2 25.7 19.0 --
1090 260.4 218.9 94.2 84.1
UF, DF: permeate, diafiltration; K, retentate; TDS, total dissolved solids.
Table 8 Ultrafiltration and diafiltration (batch mode) of black liquors (PAN-21 membrane, pressure: 1.0 MPa) Parameter
Density, kg/m 3 TDS, kg/m 3 Lignins, kg/m 3 Organic carbon, kg/m3 Degree of purity of lignins, %
Raw black UF, Kz liquor
1081 165.2 76.1 50.4 46.1
1094 250.1 203.8 138.0 81.5
K, retentate; TDS, total dissolved solids.
Diafiltration Stage I, K~
Stage II, K2 Stage III, K3 Stage IV, K4 Stage V, K5
1085 249.2 203.9 135.5 81.9
1089 227.3 187.5 138.0 82.5
1080 204.7 168.9 129.3 83.0
1071 180.2 153.5 123.2 85.2
1061 185.3 162.6 110.2 87.8
120
1. Tanistra, M. Bodzek / Desalination 115 (1998) 111-120
Table 9 Ultrafiltration and diafiltration (continuous mode) of black liquors (PAN-21 membrane, pressure: 1.0 MPa) Parameter
Density, kg/m 3 TDS, kg/m3 Lignins, kg/m3 Organic carbon, kg/m3 Degree of purity of lignins, %
Raw liquor
1078 158.5 74.8 50.2 47.2
UltrafiltrationI
Diafiltration
Ultrafiltration II
UF
K
DF
K
UF
K
1050 90.1 35.1 22.3 --
1081 216.6 125.4 75.4 57.9
1035 61.0 30.0 17.9 --
1061 154.2 111.9 66.4 72.6
1031 57.9 24.1 15.0 --
1092 275.3 237.5 98.1 86.3
UF, DF: permeate, diafiltration; K, retentate; TDS, total dissolved solids. compared to the values obtained with the volumetric permeate flow yield equaling 80%. Also, the experiments testing the application of the diafiltration method for the purification of lignin concentrates proved successful (Tables 6 9). Both kinds ofpolyacrylonitric membranes have been used in the experiment (pressure 1,0 MPa). Using this method, the highest levels of lignin purity have been obtained. For the batch diafiltration method they amounted to 86.09 and 87.1% (membranes PAN-20 and PAN-21), and for the continuous method, respectively, 84.1% and 86.3%.
4. Conclusions Based on the investigations, we found that the polyacrylonitrile membranes produced in our laboratory proved very useful in the concentration and fractionation of black liquors from the sulphate method of wood digestion. The obtained concentrates of the macromolecular post-sulphate
lignin (purity level above 80%) can be applied as attractive raw materials in many branches.
References [1] I.A.Pearl, Tappi, 65(5) (1982) 68. [2] B.D.Bogomolow et.al., Bum. Prom., 5 (1975) 14. [3] J. Wagner, Ultrafiltration and reverse osmosis in the pulp and paper industry. Unpublished data from DOW, Denmark, 1990. [4] M. Bodzek, In: Synthetic Polymeric Membranes. W.de Gruyter, Berlin, 1987, p.193. [5] M. Bodzek and I. Tanistra, Archiwum Ochrony ~rodowiska, 1/4 (1986) 75. [6] M. Bodzek and I. Tanistra, Environ. Protect. Eng., 13 (1987) 5. [7] M. Bodzek, K. Konieczny and I. Tanistra, Chemia Stosowana, 32 (1988) 311. [8] M. Bodzek J. Bohdziewicz, I. Tanistra and K. Konieczny, Environ. Protect. Eng., 15 (1988) 121. [9] W. Hermanowicz, W. Do~afiska, J. Dojlido and B. Koziorowski, Physico-chemicalAnlysis of Water and Wastewater [in Polish]. Arkady, Warsaw, 1976.