E. W. Richardson, E. D. Stobbe, and S. Bernstein Nucleur Division, Union Curbide Corp., Paducah, Ky. 42001
Ion Exchange Traps Chromates for Reuse
T
he Atomic Energy Commission’s Paducah, Ky., gaseous-diffusion plant, which is operated by Union Carbide Corp., employs a process cooling water system in which a cooling tower serves to dissipate heat. In this open recirculation system, the total dissolved solids (TDS) are concentrated in the water by evaporation at the cooling tower. Consequently, a small portion of the cooling water must be discharged (blowdown) to prevent TDS buildup. If a portion of the cooling water is not discharged, scale formation on the heat transfer surfaces or corrosion become serious problems. However, chromate ion, one of the chemicals used to inhibit corrosion in recirculating cooling water (RCW) systems, has been classified as a pollutant and, consequently, should not be discharged into receiving water in significant quantities. Several approaches leading to the resolution of this problem are being pursued: Reducing blowdown rates. Substituting nontoxic inhibitors for the chromate ion. Removing the chromate ion from the blowdown stream. Ion exchange has been used in the laboratory to recover and reuse chromates from a synthetic process cooling water system. We undertook a pilot plant demonstration project to determine if the process is feasible under plant conditions with actual process cooling water. The ion exchange pilot plant was operated for nearly 56 weeks (August 1966 to September 1967), during which time we studied the following: Extent of chromate pollution abatement. Feasibility of chromate recovery and reuse.
1006 Environmental Science and Technology
Useful resin life. Raw material and manpower requirements. Scale up criteria. Our results indicate that better than 98% of the chromate is removed, and that the chromate ion effluent concentration ranges from 0 to 2 p.p.m. In addition, we conclude that reuse of chromates in the RCW system did not significantly increase chloride and sulfate ion concentration; consequently, chromate ion can be reused. The total raw material cost is nearly 21 cents per pound of chromate, exclusive of the value of chromate recovered. To many industrial users of chromate corrosion inhibitors, the process thus offers two advantages: It reduces costs, since chromate can be reused. It reduces water pollution.
Figure 11
Ion exchange apparatus
Chromate ion was removed from the RCW blowdown stream inside two nominal 12-inch diameter, rubberlined steel downflow pressure type exchanger columns (Figure 1 ) . The internals of the exchange columns consisted of an upper distributor for influent flow during the exhaustion cycle and for backwash flow during the regeneration cycle; a center distributor for all influent flows during regeneration; an internal flat screen at the bottom for resin support; and a dished bottom to collect the effluent flow. The center distributor was added after run 26. Before that, the regenerant influent flow was added through the upper distributor. Since a two-stage regeneration with acid rinse was employed during the test program, two 30-gallon steel tanks (with portable agitators) were used for the primary and secondary re-
Columns. Chromate recovery unit consists of two 12-in. columns loaded with basic polystyrene anion exchange resin
FEATURE Pilot tests at AEC's Paducah, Ky., plant indicate 98% recovery from cooling tower blowdown; reuse of chromates would cut operating costs
generant solutions. A 55-gallon polyethylene-lined steel drum was used to store the sulfuric acid rinse solution. Corrosion resistant PVC piping and fittings were used throughout in fabrication of the pilot plant, after the point of acid addition for p H control. A 24-inch vertical pressure sand filter (Table I ) was installed (after run 3 ) to remove suspended solids in the RCW blowdown water. We learned the value of a good filter early, since we experienced excessive resin fouling when total suspended solids exceeded 2 p,p.m. in the column influent water. In addition to the above essential equipment, we installed the following instrumentation and auxiliary equipment: Acid storage tank and acid feed pump. * Integrating and indicating feed flowmeters. Regenerant solution pump and inci ic ati ng flowmeter. Indicating p H meter. * Indicating density meter. Indicating pressure gages. Nalco Chemical Co. designed all the equipment specifically for full scale industrial testing of their patented ion exchange process for removing chromate from industrial process water systems. and for subsequent reuse of the chromate as a corrosion inhibitor. Exhaustion cycle
We used strong basic polystyrene anion exchange resin (Dowex I ) throughout the pilot scale testing program. When new, this resin had a total exchange capacity of 1.38 meq. per ml. and a water retention capacity of 47.8 7% (approximately 7% divinylbenzene crosslinkage). We used resin beds 42 and 54 inches deep.
Table I. Physical data for sand filter and ion exchange columns a t chromate recovery pilot plant
Sand filter, pressure type Diameter Overall height of vessel Bed depth (sand and gravel) Pipe connection Weight of sand in bed
24 in. 68 in. 44 in. 1-in. NPS 300 Ib.
Ion exchange columns, rubber lined
2 72 in.
Number of units Column height Bed depth Column 1 Column 2 Internal cross sectional Resin volume Column 1 Column 2 Piping and valves Center distributor position
42 in. 42 and 54 in. 0.71 sq. ft.
2.5 cu ft. 2.5 and 3.2 cu ft. 1-in. PVC 3 in.
Table II. Typical analysis of cooling water Influen:, p.p.rn.
Total dissolved solids Total hardness (as CaC08) Calcium hardness (as CaCO3) Magnesium hardness (as CaCO?) Chloride (as NaCI) Sulfate (as Na,SO,) Phosphate, ortho (as PO,) Phosphate, total (as PO4) Silica (as SiO?) Total iron (as Fe) Alumina (as A1208) Chromate hexavalent (as Cr04) Copper (as C u ) Zinc (as Zn) PH" Average temperature (OF.)
1,400
620 400
220 370 1,065 1 2 31
0.4 0.8 20 0.2
4.0 4.3
Effluent, p.p.rn.
1,400 620 400 220
380 1,065 0.4
0.8 35 0.4 0.5 0 to 2
0.2 4.0
4.3
87.5
After adjusting with sulfuric acid.
Volume 2, Number 11, November 1968
1007
As the working solution, we used recirculating cooling water from an industrial cooling tower installation at the AEC gaseous diffusion plant (Table 11 shows a typical analysis of the cooling water.) The flow rates through the resin bed were limited to 5 and 10 gallons per minute (14 g.p.m. per square foot of bed area). The cooling water, containing approximately 20 p.p.m. chromate, was supplied at 87.5” F. (average) from the cooling water pump discharge line. Sulfuric acid was added to the filtered water to adjust the p H from its normal control value of 6-6.5 to within test limits. During a typical chromate adsorption cycle, the exchanger columns were operated as they are in common watersoftening-that is with a standard down-flow mixed resin bed. However, in the chromate recovery cycle, two columns in series were used to provide for more economical loading of the resin prior to regeneration. (Figure 2 ) . Actually, either column could operate as a primary column if the series column arrangement was desired. The chromate adsorption or exhaustion cycle begins after the exchange bed is regenerated and rinsed, whereupon this fresh bed is put in the secondary position. The exhaustion cycle ends when the secondary column chromate effluent concentration reaches a predetermined value. Then, the secondary column is put in the primary position to complete its loading cycle, and the exhausted primary column is regenerated. This type of multistage-cyclic-series-column operation is illustrated in Figure 3, which shows the manner in which resin is exhausted (lb. of chromate adsorbed), and the effect of regeneration on chromate effluent concentration.
Figure 3
=d
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Column =1
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Column =2
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E = V
0 2.08
Regeneration
2.10
2.12
2.14
2 16
2 18
2.20 2.22
2.24
2.26
Cumulative flow, million gallons Multistage operation-breakthrough
1008 Environmental Science and Technology
history
2 28
2.30
2.32
To regenerate the exhausted resin in the primary column, we treat with a p H adjusted 10% (weight) sodium chloride solution. The basic operations employed during regeneration of the pilot plant columns are similar to those steps normally used in all ion exchange applications-backwashing, regeneration (ion elution), rinsing. Although the basic regeneration steps are not unique, the overall utility of this method depends on a suc-
cessful regeneration procedure. Early in pilot plant testing, the regeneration procedure used was successful in chromate elution and reuse, but we had to revise the procedure when we encountered serious problems with early chromate leakage after regeneration (Figure 4) and with resin fouling from trivalent chromium and SUSpended solids. The regeneration procedure finally employed uses these nine basic steps: 1. Air sparge. 2. Backwash at 5 g.p.rn.lft.2 with normal plant process cooling water (bottom distributor). 3. Pump the primary regenerant (caustic brine) solution downflow at a 0.35 g.p.mJft.2 rate (center distributors) (Table 111). 4. Rinse with 7 gallons of sanitary water at a 0.35 g.p.m./ft.2 rate (center distributor). 5. Pump the secondary regenerant (13 5% NaC1) through the column at a 0.35 g.p.m.ift.2 rate (center distributor). 6. Rinse with 30 gallons of sanitary water at a 1.4 g.p.m./ft.2 rate (center distributor). 7.Backwash with normal process cooling water at 2.75 g.p.mJft.2 rate until clean (bottom distributor). 8. Rinse with 2 % % sulfuric acid until p H of the column effluent drops to 2 (center distributor). 9. Put the column back in service as the secondary column in the exhaustion cycle, The primary column is regenerated after each run by using a split-elution technique. This technique requires two regenerant solution tanks. The primary tank contains 14 gallons of elution effluent collected during the tail end of the previous regeneration cycle; it has been fortified with the necessary sodium hydroxide (3.6 pounds of NaOH/ft.3 resin). The secondary regenerant tank contains 14 gallons of fresh brine (6.8 pounds of NaCl/ft.3 resin). The split-elution technique, then. first employs the primary tank solution, which, after passing downflow through the column at a 0.35 g.p.m./ ft.' rate, is collected in the chromate recovery tank. The fresh brine solution from the secondary tank is after the intermediate water rinse, passed downflow through the column at the same
7
18 1
Cumulative water flow, million gallons Breakthrough history with SBR ion exchange resin
Table 111. Test conditions for ion exchange pilot plant ADSORPTION: Flow rate Superficial water velocity Water temperature range Water pH, RCW system Water pH, adjusted Total suspended solids i n influent Influent chromate (Cr4-) Effluent chromate (CrO,=)
5 and 10 g.p.m. 7 and 14 g.p.m./ft.2 85' to 90" F. 6.1 to 6.4 4.0 to 4.5 1t o 2 p.p.rn. 18 to 24 p.p.m. 0 to 2 p.p.m.
REGENERATION: Regenerant solution concentration Primary regenerant NaCl NaOH Secondary regenerant NaCl Regenerant solution volume Primary regenerant Intermediate water rinse Secondary regenerant Final water rinse Acid rinse (2.5% HSS04) Regenerant flow rate Recovered chromate solution volume
10 wt. % 7 wt. % 13 wt. %
5.6 gal./ftn3resin 2.7 gal./ft.a resin 5.6 gal./ft.3 resin 12.0 gal./ft.3 resin 12.0 gal./ft.3 resin 0.35 g.p.m./ft.2 20.3 gal./ft.3 resin
Volume 2, Number 11, November 1968
1009
Figure Regenerant solutions Recover e d solutions
Primary regenerant
Water rinse
To drain
Secondary regenerant
Chromate recovery tank
Water rinse Salt recovery tank
Chromate recovery tank 4
1.14
Note: The 14 gals. of recovered salt solution-
// I
1.12
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-
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contain 122 gal. I.-of NaCl (1.08 Sp. Gr.)
>
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1.06
1.04 1.02 1.00
5
0
10
15
25
20
30 35 40 Volume, gallons
45
50
55
60
65 68
Specific gravity of recovered solutions Chromate recovery ion exchange pilot plant
Figure 6 10 X
\++---
I
02
04 I 06
I
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08
I
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10
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42" bed depth
-1-
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54" bed depth ___
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18
20
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22
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24
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Cumulative flow, million gallons Resin bed loading history
1010 Environmental Science and Technology
,
28
30
32 ' 34
'
3'6
38
'
4'0 ' 4 2
rate and collected in the salt recovery (primary regenerant) tank. The excess brine and caustic values from this secondary elution are saved to be recycled in the next regeneration as the primary portion of the elution influent. The 50% excess caustic used with this split-elution technique is not all wasted, since a portion is recovered and stored in the salt recovery tank (primary regenerant tank); then it is recycled back through the resin during the next regeneration. Our testing indicates this excess (50%) caustic is necessary to get low chromate effluent concentrations at the beginning of the next run. We simplified the split-regeneration (specifically steps 3, 4, 5, and 6) considerably by measuring the specific gravity of the various recovered solutions. The results of these measurements are shown in the elution curve in Figure 5, where volume eluted, specific gravity, and various regeneration steps are correlated. These data were used in deciding which elution tank should be used to receive the various recovered solutions generated during the regeneration procedure. In addition to providing the maximum use of elution chemicals, these data also permit automation of the entire regeneration cycle. Resin reactivation
While the first million gallons of b l o ~ d o w nwater were processed, we found that resin oxidation and trivalent chromium fouling were gradually reducing operating capacity (Figure 6 ) . So. we developed three alternative methods for removing the trivalent chromium ion from the resin: Bleaching with sodium hypochlorite. Reactivating with sulfuric acid. Rinsing with sulfuric acid after regeneration. The bleaching procedure used 2.5 % bleach (NaOCI) solution to reoxidize the trivalent chromium. This bleach is recycled through the resin approximately one hour, and follows step 7 of the standard regeneration procedure. A second 2.5% bleaching step is employed for one hour, followed by a water rinse (until clean) and a 10% sodium chloride rinse. Laboratory analysis reveals that approximately 1?4 pounds of chromate nere reoxidized to the hexavalent state
and recovered from column one following run number 16. The sulfuric acid reactivation also follows the standard regeneration operation. The first step includes addition of 10% H,SO, to the column until the effluent p H drops to 2 . Following this initial step, 20 gallons of 10% H,S04 are recycled through the resin for four hours at a 1.4 g.p.m./ft.z rate. The final step before putting the column back onstream provides for rinsing with 40 gallons of 0.5% H2S04 at a 1.4 g.p.m./ft.z rate. One pound of hexavalent chromate was recovered from column one, following run number 43, as a result of this acid reactivation procedure. Based on the chromate recovery data, both procedures are satisfactory; however, chemical costs plus the resin degradation factor would tend to favor using the acid reactivation procedure. Sulfuric acid rinsing after regeneration was successful following run number 40; consequently, this step is included in the current regeneration procedure (step 8 ) . The success of the acid rinse, as shown by improved column operating characteristics (minimal initial chromate effluent concentration), is probably due to better pH control when putting the column back in service after regeneration. However, use of an acid rinse strong enough to redissolve the trivalent chromium and thus to reactivate the resin, was not fully developed at the conclusion of this work. Sampling
The sampling procedures followed during the chromate adsorption cycle are designed mainly to control the process within the predetermined test limits. However, control of p H and chromate concentrations of the process cooling water were extremely good, probably because of the huge holding capacity of the cooling water system. Consequently, control sampling was limited to checking the p H six times daily, while operating data such as flows and pressures were checked and logged three times daily. Three column influent and effluent control samples were drawn once each day for analysis. Duplicate samples from the chromate and salt recovery tanks were also collected after each regeneration cycle. These samples were analyzed for: hexavalent and trivalent chromium;
chlorides; sulfates; alumina; zinc; alkalinity; hardness; and pH. Duplicate regenerant samples were sent to Nalco Chemical Co. to doublecheck our results and as an aid in obtaining regenerant material balances. In addition to the standard sampling procedures, we performed special laboratory analyses on the sand filter effluent and the ion exchange resin samples. Test results
The pilot plant tests showed that an ion exchange system could be used to recover the chromates discharged from a recirculating cooling water system and, thus, to help maintain the chromate concentration at an acceptable level for discharge into surface streams. The chromate concentration was reduced from 20 p.p.m. to an average of less than 1 p.p.m. as CrO,=, while the concentration of the other dissolved salts remained essentially unchanged. In the early phases of the testing program, one of the objectives was to determine the practical minimum effluent chromate concentration that could be maintained. With this objective in mind, most of the tests were terminated when the secondary column effluent was about 2 p.p.m., which occurred after processing about 60,000 gallons of water through the resin bed. As we gained operating experience, it was possible to process approximately the same quantity of water and maintain a maximum effluent concentration from the secondary column of about 1 p.p.m. In the early tests, the minimum chromate concentration occurred after processing 10,000 to 20,000 gallons of water (Figure 7 ) . The initial leakage in the column was the result of high p H in the secondary column after regeneration. However, the early leakage was reduced in later tests by lowering the pH of the influent water from 4.0 to 3.0 for the first 5000 gallons of water of each adsorption cycle (Figure 8 ) . We later determined that an acid rinse was more beneficial in reducing the chromate leakage than lowering the pH during the initial part of the test. Analysis of the recovered solutions showed that the chromate could be reused without detrimental effects in the recirculating water system (Table I V ) . The recovered chromated solutions consisted of the primary regenerant and all of the rinse water. Since the
Volume 2, Number 11, November 1968 1011
Figure 7 -E
Figure 8
I
E d
(data from tests 4 thru 151
I
26
7
(data from tests 33 thru 44)
1
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16
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12
2
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4.0
I
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2 3.0
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2.0 1.0
n 0
10
20 30 40 50 60 70 80 0 10 Cumulative water flow, thousand gallons Breakthrough history with SBR resin
chromate ion concentration was approximately equivalent to the total chloride and sulfate ion concentration, reusing the chromate in the RCW system would add only about 10 p.p.m. of chloride and sulfate ions to the existing 370 p.p.m. of sodium chloride and 1065 p.p.m. sodium sulfate in the RCW. This small addition to the total dissolved solids in the system would not affect the corrosion or scaling properties of the RCW. Resin capacity for chromates remained reasonably constant throughout the series of tests. Since we terminated most of the tests before complete exhaustion of the primary resin bed, we took an arbitrary chromate breakthrough of 4 p.p.m. for comparison of the tests. Figure 6 gives a comparison of the loading history for all tests that had a minimum breakthrough of 4 p.p.m. chromates. Except for the small amount of resin removed in sampling, the same resin and a constant bed depth of 42 inches were maintained in Column 1 throughout the tests. Column 2 bed depth was varied by the addition of 0.71 cubic feet of new resin at about the midpoint of the test
1012 Environmental Science and Technology
(run 27); after eight subsequent runs, an equal quantity of the resin was removed. Near the conclusion of the tests, the entire resin bed in Column 2 was replaced with new resin. Table V gives the average, maximum, and minimum chromate loading on the resin at a breakthrough of 4 p.p.m., o r at the time the column was taken offstream for regeneration. (At 4 p.p.m. breakthrough, the column loading was about 75% of the loading at exhaustion.) According to Helfferich (1962), high aspect ratio (ratio of bed depth to column diameter) has little effect on resin capacity utilization if the equilibrium is unfavorable. The resin capacity at bed depth of 42, 54, 84, and 96 inches indicates that the equilibrium for chromate adsorption on the resin was unfavorable (Table VI). Although the resin capacity in pounds of chromate per cubic foot of resin was better at the bed depth of 54 inches than at 42 inches, the deeper beds of 84 and 96 inches did not improve the resin capacity further. The lower loading observed on the deeper beds may not be significant, since the deeper beds were the sum of
20 30 40 50 60 Cumulative water flow, thousand gallons
70
80
the bed depths on Columns 1 and 2 ; during the latter phase of the test program, Column 2 resin gave consistently better loading capacities than Column 1. However, when resin was removed from the Column 2 bed, loading capacity decreased in the following tests. All aspect ratio tests were made at a superficial velocity of 14 g.p.mJft.2. The 96-inch bed depth (54- plus 42-inch beds) gave a 17 p.s.i. pressure drop for the system when the resin was clean. Calculations indicate that the piping, valves, and fittings in the pilot plant unit gave a pressure drop of about 4 p.s.i., and the 96-inch bed gave a pressure drop of about 13 p.s.i. at a superficial flow rate of 14 g.p.m./ft.z. During the adsorption cycle, suspended solids were trapped in the bed and the pressure drop increased to about 20 p.s.i. across the resin bed. Operation of two 48-inch beds would give good loading of the resin at a moderate preswre drop in the system. Hesler and Oberhofer found that regeneration of the resin with caustic and salt solution permits the resin to be run through repeated adsorption cycles with essentially no loss in capacity.
Table IV. Recovery solution composition average
Total dissolved solids Total hardness (CaC03) Calciu rn hard ness (CaC03) Magnesium hardness (CaC08) Chloride a s NaCl Sulfates as Na2S04 Chrornates as Cr04= Ratio: CrO,=/(CI- Sod=)
+
Recovered chromate solution, p.p.m.
Recovered salt solution, p.p.m.
88,100 15 8 6 22,900 14,000 27,700 1.18
119,900 12 11 1 112,800
Table VI. Aspect ratio data on resin columns Run number
4 27 48 50 51 52 46 47 4 50 51 52 47 48
... 1,100
...
Column number
1 1 1 2 1 2 2 2 1+2 2+1 1+2 2+1 2+1 1+2
Resin bed de t h (incLs)
42 in. 42 42 42 42 42 54 54 84 84 84 84 96 96
Chromate loading Ib.lft.8 at various' leakage 1 p.p.m. 2 p.p.m. 4 p.p.m.
2.2 2.4 1.72 2.52 1.64 2.36 3.27 2.9 2.26 2.24 2.42 2.52 2.7 2.6
3.0 3.0 2.32 3.20 2.16 3.04 3.94 3.7 3.47 2.74 3.12 3.12 3.47 3.37
... ... 2.92 3.72 2.76 3.64 4.44 4.30
... 3.46
...
3.66 3.93 3.58
Table V. Chromate loading a t various resin bed depths (1b./ft.3) 42-in. bedCOl. 1 Col. 2
Remarks
Loading at
(Avg.
4 p.p.rn. CrO,
leakage Loading a t conclusion of r u n
i;n?*
(Avg.
{lynx;
42-in. bed (new 54-in. bed _ _ resin) _ Col. 2 Col. 2
3.00 3.80 1.84
3.12 3.72 1.88
4.07 4.50 3.68
5.54 6.52 4.84
3.94 5.12 3.11
4.23 4.95 3.18
4.43 5.43 3.11
6.70 7.80 5.80
During the first three tests on the pilot plant system, suspended solids in the unfiltered water fouled the resin bed and made a material balance on the resin bed impossible. On the fourth run, with adequate filtration of the influent water, a material balance showed that the resin capacity was less than half of the theoretical capacity. Efforts to increase the resin capacity by reactivation with bleach and sulfuric acid were largely unsuccessful: however, no further permanent loss of capacity occurred. Near the end of the test program, new resin was charged to Column 2 and run through four cycles of adsorption and regeneration. On the first adsorption cycle, the resin capacity was 7.8 lb. CrO,=/ft.3 at exhaustion or 6.5 lb./ft.s at breakthrough of 4 p.p.m. By the third cycle, the resin capacity had dropped to 4.8 lb. CrO,=/ft.3 at 4 p.p.m. leakage, and remained about the same for the fourth cycle. The cause of this initial rapid loss of capacity is not known, but the same phenomenon apparently happened when the tests were first started. Whether the reduced capacity of the
~
new resin could be maintained would have to be determined by further testing, but the new resin capacity was significantly better than had been observed during the previous testing and reflected the improved operating procedures for the system. Other factors that affect chromate resin loading and breakthrough are proper regeneration, resin fouling, and p H control. Figure 4 shows the results of high p H in the columns and of poor regeneration after runs 17 and 18. Prior to the start of run 17, both resin beds had been treated with a 2.5% solution of sodium hypochlorite (bleach) to remove the trivalent chromium fouling: pH in the resin bed was approximately 12. The influent water was controlled at a p H of 4.0 to 4.5, but shortly after the start of the test, chromate leakage through Column 1 was noted while the effluent water still had a pH of about 10. The chromate carried on through Column 2 before the operating pH of 4.5 was established in the columns. The pH was high in the secondary column after each regeneration until the acid rinse was made a part of the re-
generation procedure. The acid rinse served two purposes: first, it reduced leakage of chromates through the resin while pH was above 5.0; second, it removed some trivalent chromium foulant during each regeneration. The regeneration procedure was changed during the course of testing to improve the efficiency of the system. The quantity of caustic used in the primary regenerant was increased from 5 % to 7.5% early in the test program when we found that good regeneration was not obtained at the lower caustic concentration. We tried a 5% caustic solution (primary regenerant) again, following runs 17 and 18. The secondary column came on stream with a high concentration of chromates in the effluent. However, since most of the chromates were removed from the resin by regeneration, the effluent concentration of chromates decreased to less than 1 p.p.m. when the pH in the bed equalized at about 4.5. We used a water rinse between the primary and secondary regenerant 60lution to give a better split between the recovered chromate solutions and the salt solutions, and to reduce the p H in
Volume 2, Number 11, November 1968
1013
the resin at the end of the regeneration. It is questionable whether the water rinse reduced the column p H measurably, but the intermediate rinse minimized mixing of the primary and secondary regenerant solution. The specific gravity changes in the recovered solution aided in making the cut between the recovered chromate solution and salt solutions. When the secondary regenerant solution was passed through the resin bed, some of the salt was lost by mixing with the preceding and following solutions and in converting the resin to the chloride form. Initially, makeup salt had to be added to the recovered solution before it could be used as the primary regenerant in the next cycle. No additions of salt to the recovered solution were necessary after the solution strength was increased to 1 3 % , and the additional salt concentration in the secondary solution gave a better driving force for conversion of the resins to the chloride form. Fouling of the resin with trivalent chromium occurred throughout the test program, probably as a result of oxidation of the resin by the hexavalent chromium. This fouling was demonstrated by a loss of capacity of the resin and by the fact that some trivalent chromium was eluted from the resin bed with each acid rinse. Fouling with the trivalent chromium appeared to be most severe during the first few cycles of the resin, as illustrated by the rapid loss of capacity of the new resin. After the first few cycles, the rate of fouling decreased and the resin loading capacity decreased very gradually thereafter. To reactivate the resin, we removed the trivalent chromium by treatment with either a 2.5% bleach solution or a 10% sulfuric acid solution. The sulfuric acid was the preferred treatment, since the oxidizing conditions were not as severe as with the bleach. The bleach solution removed the trivalent chromium by reoxidizing it to the hexavalent state and making the chromium soluble in caustic solution. The resin could be damaged by prolonged exposure to the highly oxidizing condition of the bleach solution. The sulfuric acid removed the trivalent chromate as a soluble chromium sulfate. It is possible that if an acid rinse had been used from the beginning of the testing program, reactiva-
1014 Environmental Science and Technology
tion of the resin would not have been required. Although the influent water contained only 1-2 p.p.m. suspended solids with a particle size range of less than 1 micron, suspended solids collected in the resin bed during the adsorption cycle. We used the usual practice of backwashing the bed to remove any foreign material and resettling the bed before regeneration. Backwashing the loaded resin bed at the normal 2.5 g.p.g./fL2, for a 50% expansion of the bed, required excessive time to obtain a clean effluent. The backwash rate was increased to 5 g.p.mJft.2 after we determined that the higher rate was required to give the desired 50% expansion of the loaded resin and to remove the dirt from the bed more efficiently. When the first resin sample was taken from the column, we found that
the bed was still dirty after backwashing even at the higher rate. The agitation of the bed by the backwash was insufficient to break the bond between the finely divided particles and the resin bead. Air sparging prior to backwashing gave sufficient agitation to loosen the dirt and permit it to be easily washed from the resin bed. There was no evidence of excessive cracking of the beads as a result of the air sparging of the resin. Determination of the effect of the oxidizing conditions in chromate recovery on the expected resin life was one of the prime objectives of the test program. Figure 9 shows that there was a continued degradation of the resin physical properties during the year of testing. The total exchange capacity (TEC), as determined by laboratory measurements, decreased rather
Figure 9
I
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1
4
2
6
a
10
12
11
16
$13
t
m c u
x
0.7
Ll
0
.2
I
1
1 .4
.6
.8
1.0
1.2
Water processed, millions gal. /cu. ft. resin Physical properties of Column 1 SBR resin
; 1.4
i.6
rapidly during the first half of the testing period; by the end of the program, the TEC was nearly constant. The operating data did not agree with the laboratory results in that the resin capacity for chromates decreased about 50% during the first few cycles and then remained fairly constant. If the laboratory measurements are used and a 50% decrease in TEC is taken as the useful life of the resin, a resin life of several years is possible. Another measure of resin life is water retention capacity (WRC), which is indirectly proportional to the degree of styrene-divinylbenzene bead crosslinkage. The WRC data indicate that the crosslinkage decreased from about 7.5% to 5.5%. As the crosslinkage decreases, the mechanical and chemical properties of the resin change; eventually, at about 1-2% crosslinkage, the resin would become very soft and give a higher pressure drop. Oxidation by the chromate ion is probably the major cause of the decreased crosslinkage. Based on the TEC and WRC determination, the useful life of resin would appear to be two to three years. Scale-up
An additional objective of the pilot plant operation was to obtain data for the design of a plant scale unit, if the process proved feasible. We found removal of suspended solids from the influent water to be of prime importance for the successful operation of the ion exchange system. A standard pressure sand filter proved satisfactory in the pilot plant tests. The filtration system should give the ion exchange unit an influent water containing no more than 2 p.p.m. suspended solids. I n addition to suspended solids, any organics in the water should be removed prior to the ion exchange unit to prevent fouling the resin. To obtain maximum resin loading and recovery of the chromate, a minimum of two stages is required. Since approximately six hours are required to regenerate a column, three stages would be desirable if the blowdown could not be shut off during the regeneration cycle. The third stage would also permit the complete exhaustion of the primary column and maintain the effluent concentration from the system at less than 1 p.p.m. chromate. A design flow rate of 15 g.p.mJft.2 super-
ficial velocity will give satisfactory chromate recovery at a pressure drop of about 6.5 p.s.i. across a 4-foot deep bed. We observed no apparent difference in resin loading in the pilot plant tests at flow rates of either 7 or 14 g.p.m./ft.2. The influent water to the ion exchange system should be controlled at a p H of 4.3 rt 0.2. Dilute sulfuric acid was used to control the pH in the pilot plant tests since this dilute solution was the easiest to control and the least expensive. Concentrated sulfuric or hydrochloric acid could be used for p H control, provided there was adequate mixing prior to the ion exchange bed and provided proper control of the pH could be maintained. Auxiliary facilities required for regeneration of the resin consist of rubber lined primary and secondary regenerant tanks, a rubber lined acid rinse tank, and a steel tank for storing recovered chromate solution. The tank for the recovered chromate solution should be big enough to contain the solution generated from two or more regeneration cycles, since a column will occasionally require two regenerations to obtain an acceptable effluent chromate concentration. A source of softened or demineralized water should be provided for regenerant solution makeup and for water rinse. A water supply that is chromate-free should also be provided
for backwashing the filter and resin and for makeup of the acid solutions. Chemical storage facilities should be provided for sulfuric acid, sodium hydroxide, and sodium chloride. Rayongrade caustic is recommended for use in ion exchange application. Since flake caustic is deliquescent and has a tendency to cake when exposed to ambient atmospheric conditions, a concentrated solution, about 50% NaOH, could be handled most conveniently. A good grade of salt, low in iron and silicon, should also be used for regenerant solution makeup. Common practice in large ion exchange units is to use concrete wet-salt storage bins to obtain the advantages of lower cost and ease of materials handling. The construction of the acid tank could depend on the type of acid used. An ion exchange unit could be operated automatically with installation of the proper controls. In the adsorption cycle, this would require control of the inlet water flow and pH and instrumentation to indicate chromate concentration in the effluent. Automatic backwash of the filter system could also be designed into the installation. In the regeneration cycle, control of the regenerant solution flow would be required. The use of concentrated caustic and salt solutions would facilitate regeneration solution makeup, and tank level indicators could be used for control of
Table VII. Waste chromate disposal chemical cost comparison RECOVERY AND REUSE (ION EXCHANGE) Chemical usage, (lb.)/lb.
Chernica cost
Itern
CrOa
Sulfuric acid Sodium hydroxide Sodium chloride Ion exchange resin replacement
1.2 0.9 1.7
$0.013 0.037 0.036 0.122
Total"
(b/lb: CrOa)
$0.208
DESTRUCTION BY REDUCTION & PRECIPITATION 0.65 $0.007 Sulfuric acid 10.4 0.234 Ferrous sulfate 3.1 0.023 Calcium hydroxide Total
$0.264
Fees on the patented process are subjected t o negotiation with the Nalco Chemical Co. and are not included in t h e costs.
Volume 2, Number 11, November 1968 1015
solution makeup. Partitioning the salt solution (secondary regenerant) from the recovered chromate solution is necessary to minimize chemical usage and chloride concentration in the recovered chromate. The change in the specific gravity of the effluent regenerant solutions could he used to control the partitioning of the recovered solutions. costs
We calculated material costs for the recovery and reuse of the chromates from pilot plant data. From published data, we obtained the chemical usage for another popular chromate pollution abatement scheme-the reductionneutralization-precipitation (destruction) process. Table VI1 compares the two. The sulfuric acid requirements are based on reducing the influent water from p H 6.0 to 4.0. All chemical costs are based on the market price in August of last yearplus delivery charges. The same cost criteria were used in calculating the chemical cost for destruction of the chromates by the reduction and nrecipitation process. Thf qiirements (FeSO, and CaOH) are based on another plant's experience. Normal operating labor costs for the two processes could he assumed to be ahout the same, depending on the de-
**. mIcoaro6"" 1s process engmrrr at Union Carbide Corp., Paducah, K y . H e received his B.S.from Georgia Institute of Technology (1951), and did graduate work a f Southern Illinois University. Richardson worked in the design of uranium solvent extraction systems and ion exchange systems f o r technetium and neptunium recovery, and in the production of uranium hexafluoride f o r use in gaseous diffusion. Coinventor o f a process f o r making porous sodium fluoride pellets used in separation of uranium hexafluoride f r o m other volatile impurities, he is a member o f the American Chemical Society and the Western Kenfucky Chemical Engineers Club. D.
1016 Environmental Science and Technology
gree of automation designed into the respective systems. The resin replacement cost is based on an assumed resin life of three years and a price of $60.25 per cubic foot. The chemical costs for the recovery and reuse (ion exchange process) are significantly better than for the destruction process. However, if the effective resin life were only two years, the chemical costs for the two processes would he ahout the same. The economic analysis we have discussed here is not complete, since it obviously does not include all costs associated with the entire water and waste treatment operations. For example, reuse of the chromates would reduce the chromate feed requirements in the treated water; hut this reduced treatment cost could he partially offset by the license fee for using a patented process, which is subject to negotiation. In addition, if proprietary water treatment chemicals were being used, makeup chemical costs may not he in direct proportion to the reduced requirements. Also, the effluent stream DH would he around 4.5. which may require neutralization or 'mixing with other waste streams of higher pH. On the other hand, destruction of the chromate by reduction and precipitation generates large quantities of
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chemical process engineering at Union Carbide Corp., Paducah, Ky., a position he has held since 1965. Previously (1960-1965), he was process engineer with. thir eiouo. and ~.~Tame msineerinp ~ v " worked as engineer and assistant area supervisor in Union Carbide's cascade operations department during the construction and sfartup of the Paducah n - - ~. 5 "(I~JpLIusu..n r~~ a->.~. s r o nr L U n l ,, I, A7 J
