Znd. E n g . C h e m . Res. 1987,26, 1851-1854
1851
Multivalent Ion Removal from Kraft Black Liquor by Ultrafiltration S.Alvin K i r b a w y Weyerhaeuser Co., Federal W a y , Washington 98477
M a r q u i t a K. Hill* Department of Chemical Engineering, University of Maine, Orono, Maine 04469
Kraft black liquor (KBL) was ultrafiltered into permeate and retentate fractions. Whereas Na and K were concentrated in permeate solids, many higher valence ions were disproportionately found in retentate solids. Those elements concentrated included Ca, Mg, Al, and Fe. Si was less dependably retained. The retentate/permeate ratio of Ca, based on dry solids, was as little as 1.6 for a low-Ca northern softwood KBL and as high as 7.6 for a high-Ca southern hardwood KBL. Polysulfone ultrafiltration (UF) membranes of 50 000 molecular weight cutoff (MWCO) concentrated multivalent elements t o a lesser extent than did 6000 or 20000 MWCO membranes. A nylon microfiltration membrane (0.5-pm pore size) concentrated multivalents to a greater extent during a 60 "C run than it did when a run was made a t 75 "C. Kraft white liquor had only a small tendency to concentrate multivalents in the retentate, especially when care was taken to remove all carbon particulates. The ability of kraft lignin to sequester multivalent metal ions has been known for many years (West Virginia Pulp and Paper Co., 1954). More recently, Kirbawy and Weyerhaeuser (1984) found that multivalent ions in kraft black liquor (KBL), including those involved in evaporator scaling, were preferentially concentrated in the retentate fraction after ultrafiltration (UF). Consistent results were subsequently obtained by Hill (1985). Similar metal concentrating effects have previously been seen for lignocellulosic materials. For example, peat concentrates Pb, Cu, Zn, Mn, and other trace metals (McLellan and Rock, 1986). Trace metals have also been found to be differentially associated with varying molecular weight (MW) ranges of dissolved organic materials after UF of river water (Hoffman et al., 1981). In this latter case, it was determined that the metals were associated with the organics rather than being present as colloids or microparticulates. More generally, UF of a gelatin solution yielded retention of Na as well as Ca in the retentate fraction with the gelatin (Akred et al., 1980). Polymeric dyes were also noted to scavenge metallic ions into the retentate (Cooper and Booth, 1980); this was especially true a t high pH. It was suggested that UF in the presence of a chelating agent could be a generally applicable method to remove metallic impurities. Experimental Section KBLs used in this study included northern and southern softwood and hardwood liquors obtained from James River and Champion Corps. Treatment of liquors and the DDS Corp. plate-and-frame UF setup were as previously described (Hill, 1985). In addition, elemental analyses were also done on UF fractions from hollow fiber runs; these runs used the Romicon Corp. HF-Lab 5 setup. Operating conditions of the UF runs were also as previously described except, when noted, some runs were done at temperatures greater than 60 "C. Liquor volumes used in these runs were as small as 2 L or as great as 38 L. Lignin concentrations were determined by ultraviolet absorption of the diluted fractions at 280 nm and TDS by oven drying. Most elemental analyses were done by the University of Maine Plant and Soil Science Laboratory using emission spectroscopy. For a number of samples, comparative elemental analysis was done by Weyerhaeuser Co. Mass balances
were routinely done on the amount of an element in the permeate plus retentate as compared to intact liquor levels. Typically, these balanced within 1-5%. Exceptions to this statement were seen for those elements present at only a few parts per million. Other Relevant Information. White liquor is the solution of NaOH and Na,S used in the kraft process to cook wood chips; it also contains smaller amounts of other inorganic salts including trace metal salts. After wood chips are cooked in white liquor, the pulp is removed, leaving the KBL. KBL contains high concentrations of degraded wood chemicals. Degraded lignin represents up to 40% of the total solids in the KBL. Organic acid salts represent another third of the solids, inorganic salts about one-fourth, and extractives, ca. 6%. The high MW component in KBL is degraded lignin with a MW ranging from 50000. KBL is concentrated to ca. 65% TDS before firing and burning. Burning produces large amounts of usable energy as well as providing for recovery of the inorganic salts. To produce a skimmed KBL, the initial ca. 15% TDS liquor is concentrated to at least 24% TDS before skimming off soaps and related materials; the resulting KBL is then further concentrated. Results a n d Discussion Figure 1 shows the concentration of four nonprocess elements in six KBLs. Values shown are those determined by the University of Maine laboratory. Weyerhaeuser values for these elements were near-identical with those obtained in Maine for Ca, Mg, and Fe. However, the Weyerhaeuser values for A1 were about 75% higher than those shown here, although relative differences among fractions were similar. When a 70% hardwood-30% softwood liquor (16% TDS) was ultrafiltered through 50000 MWCO membranes at 60 O C , Na was concentrated in the permeate solids (Figure 2). Identical behavior (not shown) was seen for K and S. Since Na and K, as their monovalent ions, are primarily associated with low molecular weight components of KBL, e.g., inorganic salts and organic acid salts, such a distribution would be expected. However, those elements present as multivalent cations, Ca, Mg, Al, and Fe, were concentrated in the retentate fraction. This retentate fraction also concentrated high MW lignin (Hill, 1985). Mg was particularly highly concentrated in the
0 1987 American Chemical Society 0888-S88S/8~/2626-18~1~01.50/0
1852 Ind. Eng. Chem. Res., Vol. 26, No. 9, 1987 PPfl
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Figure 3. Ratio of concentration of elements or of lignin in retentate relative to permeate. CA x 0.1
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Figure 4. Percentage of lignin, sodium, and three nonprocess elements retained in the retentate as a function of 1/CF. E l e w l s a s u t x (Ne1 o r p p m fCa,~g,AI,Fela l l b a s e d on d r y s o l i d s .
Figure 2. Elemental analysis of a northern 70% hardwood-30% softwood KBL and its ultrafiltration fractions.
retentate relative to the permeate (28-fold for the example shown here relative to 4.7-fold for Ca.). Concentration factor for this run was 7.1. Concentration factor (CF) = feed volume/retentate volume. Figure 3 indicates the progress of this retention effect during the course of an UF run as a function of concentration factor. The multivalent ions were progressively more concentrated in retentate solids up to a CF of 4 (representing 75% recovery of the KBL as permeate) and were thereafter concentrated only slightly further at a CF of 8 (88% recovery). Meanwhile, progressively greater amounts of the monovalent Na were found in the permeate solids. Figure 3 also shows the increasing concentration of lignin found in the retentate during the course of an UF run. At a CF of 1.33, the ratio of lignin in the retentate relative to the permeate was 1.2; this increased to 2 a t a CF of 8. Although the retentate c o n c e n t r a t i o n of the multivalents goes up as noted in Figure 3, the actual p e r c e n t a g e of total element or lignin retained goes down during the course of a run as more and more of the KBL
is recovered as permeate. Figure 4 shows this effect. Figure 5 notes the levels of Ca in five different KBLs as well as the Ca distribution between retentate and permeate after UF. The liquor having the highest Ca concentration, a southern hardwood with 1300 ppm, also had the greatest proportion (73%) of that Ca removed into the retentate. This was true although the CF for this run was 5 compared to CF = 2.5 for the first three liquors shown which retained only 56-6270 of the Ca. As with Ca, the concentration of other multivalent ions in the retentate varied with the specific KBL used. Figure 6 shows this for two different KBLs as well as for a white liquor UF run that was carried out as a control. For the two KBLs, the ratios (retentate/permeate) of lignin and TDS concentrations are also noted. Feed black liquor concentrations for the elements shown are as follows (wt 70for Na and K and ppm for other elements, all based on dry solids): Al, 50-200; Ba, 5-30; Ca, 170-1350; Fe, 15-65; Mg, 70-220; Mn, 90-105; P, 85-100; Si, 625-705; Sr, 2-13; V,
