A Recovery Scheme f o r . . .
Poisoned Ion Exchange Resins MAYER B. GOREN’ Kerr-McGee Oil Industries, Inc., Oklahoma City, Okla.
1 0 s exchange unit operations have played a n increasingly important role in hydrometallurgical recovery processes (6, 77): and have been extended to the recovery of uranium. This recovery scheme is susceptible to accumulation of relatively nonelutable substances in the resin beads so that operation becomes less efficient. Mechanical poisoning is due to accumulation of insoluble fouling materials within the resin matrix; chemical poisoning, to relatively irreversible adsorption of ionic material on exchange sites. Both types are characterized by premature or persistent leakage of material ordinarily sorbed (early “breakthrough“), while saturation capacity is affected less by mechanical than by chemical fouling. Mechanical fouling also interferes with normal elution by physical blocking of resin pores. Because of the value of ion exchange resins, it is not practical to discard them after short use. Among the agents contributing to the poisoning problem in uranium recovery, silica, polythionate, cobalticyanide, and molybdenum have been most prominent. Other metals (zirconium, titanium) and anions (phosphate), as well as waterdispersible organic materials, have played a lesser role (7-6, 8, 9 ) . Figure 1 compares loading characteristics of fresh IRA-400 (Rohm & Haas Co.) and after some 100 cycles when feed liquors contained minor proportions of poisoning elements. Feed used for examining loading characteristics was a synthetic liquor containing 1.0 gram of uranium oxide ( U 3 0 ~ ) per liter a t p H 1.5. A background of 30 grams of sulfate per liter was added as magnesium sulfate; retention time was 3 minutes. Thus fresh resin exhibits 1% leakage a t a uranium oxide loading of about 55 mg. per cc. of resin; poisoned resin, a t only 10 mg. per cc. Figure 2 shows elution characteristics of the same resins after loading to saturation. (Eluent, O.BAlr sodium chloride0.1-Vhydrochloric acid.) Even though poisoned resin has a lower saturation capacity, more eluent is required for Present address, 5950 hlcIntyre Rd., Rt. 1, Box 299A, Golden, Colo.
stripping it to the same degree of “barrenness” in the effluent than does fresh resin. Resin Poisoning at Shiprock
T h e solubilization scheme employed a t the Kerr-McGee ShiDrock. N. M.. mill during the first year of operation was a n “acid cure” process wherein the ore (largely carnotite-roscoelite deposits from the salt wash of the Morrison formation) is pugged with approximately 600 pounds of 60% sulfuric acid per ton of ore and allowed to cure in a pile for some 16 hours. Heat of reaction raises the temperature above 100” C., and mineral values are effectively solubilized. Water leaching then dissolves metal values, and leach liquor is separated from the solids by conventional methods. Acid cure is a very vigorous, nonselective process and solubilizes a considerable portion of undesirable minerals. T h e leach liquor, which subsequently proceeds to ion exchange, might typically have the followirig composition : us08
vzo5
AlzOj Fe
so4--
Pod---
Sios, Mo, Zr,Ti
Grams/Liter 1-1.5 4-6 8-12 3-5
50-100 0.1-0.3 Minor quantities
Pilot plant studies indicated that molybdenum might be the major poisoning agent because of a n essentially irreversible adsorption on quaternary sites. Such poisoning is ordinarily readily alleviated by regular elution with 5 to 10% sodium hydroxide or smaller amounts of causticcombinedwith cheaper reagents such as salt ( 9 ) . I n full scale plant operation poisoning was more rapid than anticipated, and zirconium and titanium, i n addition to molybdenum, were the chief contaminants. Within 40 cycles ash content of the resin was 7yo and increased to 14 to 20% for 130-cycle resin. Break-through capacity dropped to about 307, of that of unfouled resin. Effective saturation capacity decreased markedly, as columns could not be kept on stream because of excessive leakage. and “tailing-out’’
Uranium recovery by ion exchange processes may become less of a headache with the development of this process for restoring the resins. Treatment with strong caustic, then strong acid at &month intervals can effectively prolong resin life
elutions (Figure 2) became intolerably prolonged. Spectrographic analysis of ashed resins indicated that other contaminants were hafnium, silicon, iron, and phosphate; in addition organic . complex humates were found.
Of
Development of Regeneration Process
Laboratory tests indicated that saturation capacity (during the not too aggravated stages of poisoning) a t long retention times was not drastically lowered, but break-through capacity a t normal flow rate had been affected considerably. This was interpreted to mean that fouling was largely mechanical (intrdresin) rather than chemical. Treatment with caustic removed onlv about half the inorganic material along with organic color bodies. On the other hand, 1 to 2 s acid (hydrochloric, nitric, or sulfuric) dissolved only a trace (largely iron) of the precipitated materials. Resins can be restored to essentially ash-free condition and original operating characteristics by prolonged treatment with (ethylenedinitri1o)tetraacetic acidcaustic (7) ; however, the procedure was too costly to be practical. S t r o n g Acid-Strong Base Regeneration. The predominantly mechanical nature of the fouling prompted consideration of the poisoned resin therefore somewhat as a n ore. Because constituents deposited within the resin matrix had been solubilized in the acid cure, similar, though necessarily less drastic, treatment might redissolve the deposited material. Exploratory experiments indicated that slow percolation of 1 to 1 sulfuric acid (about 65%) through the resin a t ambient temperature very effectively removed considerable quantities of inorganic material, among which molybdenum, titanium, zirconium, and iron were identified. However, the resin was irreversibly darkened by this treatment, and more dilute acid was tried; 48% (1 to 2.0) and 42 5% (1 to 2.5) sulfuric acid were very effective in slow percolation (about 15 hours or longer) and had no destructive influence on the resin. VOL. 51, NO. 4
APRIL 1959
539
2 : 7 c
Figure 2. Elution characteristics o f fresh and poisoned resins
5
W*( 3
I
\
-I
More eluent i s required for stripping poisoned w . resin to a given degree . of effluent barrenness
-
0 Fresh resin IO
Z“
I 30
X Fouled resin 4U
so
0
Figure 1 . Uranium oxide loading characteristics of fresh and poisoned resins
5
Poisoned resin shows 1 % leakage at a loading of only 10 mg. per cc. 0 Fresh resin X Fouled resin
540
0‘1
3r?
BED VOLUMES FEED
Examination of 40-cycle resin ( 7 7 , ash) after strong sulfuric acid treatment indicated that almost all titanium and zirconium had been removed; however, some molybdenum and silica still remained. These Lvere then largely removed by prolonged treatment with l0Yc sodium hydroxide which also displaced large (apparently) quantities of organic color bodies. Resin so treated \vas restored to a n ash level of 0.55Y0 and to operating characteristics essentially indistinguishable from those of fresh resin. The capacity of “brand ne\v” resin is actually some 120% that of “restored” resin; however, even under ideal conditions this excess 2070 capacity is lost in early use possibly as a result of conversion of quaternary to less basic sites. Thus after about 1 5 to 20 cycles. ne\v resin capacity in a nonpoisoning environment levels off a t approximately 80 to 85% of that of ”brand new” resins. This strong acid-moderately strong alkali treatment does not appear to be harmful to the resin, and in one respect (increased porosity) is actually beneficial even to new resin ( 7 ) . Heavily poisoned resin was restored by a n abbreviated strong acid-base process \vherein about 9Orc of the ash was removed with an increase in resin volume of 1.3% and in moisture content from 41.1 to 45.17,. Efficiency of ash dissolution a t moderate contact times (less than 6 hours) begins to decrease as degree of fouling increases. Thus with 100-cycle resin (ash content, 1470) some 25 column volumes of 1 to 2 sulfuric acid at 4-hour contact time and 13 column volumes of lOy0 caustic reduced ash content only to 1.51 to 2.67yG. Equal or better results are obtainable \vith smaller reagent quantities a t longer contact times. Inverse Process. During strong acid treatment of the resin, the effluent was examined periodically to check the progress of metal dissolution. After iron had been essentially completely displaced, metallic constituents in the effluent were precipitated by excess base but apparently not by excess car-
2m
2
1
4
b
5
7
8
9
80
BED V O L U M E S EFFLUENT
bonate-or at most they gave only a slight turbidity in excess carbonate. These metals were identified as zirconium and. to a lesser extent. titanium. This behavior is somewhat puzzling in that neither zirconium nor titanium is knoivn to form stable carbonate complexes (70). Nevertheless zirconyl solutions can be caused to precipitate and the precipitate “redissolved” by excess carbonate. Redissolution may \vel1 be a colloidal phenomenon. as strong heating (or addition of sodium hydroxide) produces irreversible precipitation of zirconium. Percolation of dilute (lOyc) sodium carbonate through the poisoned resin eluted considerable quantities of zirconium, molybdenum. and phosphate. along with lesser quantities of titanium. After carbonate treatment (or even after treatment with strong sodium hydroxide), residual metal oxides in the resin Lvere considerably more susceptible to leaching by acidic reagents than before alkaline treatment. so that the amount of acid required to remove metallic constituents completely from even very heavily fouled resins \vas markedly lowered. Because silicate in the resin might undergo a cerrain amount of dehydration under the influence of strong acid with resultant intractability toivard subsequent basic dissolution. it is regarded as fortuitous that alkaline treatment as a first step obviates this dehydration and promotes subsequent dissolution of titanium. zirconium, and other metals by the acid. Reagent requirements for the fundamental and inverse processes are compared in ‘Table 1: which also summarizes over-all effectiveness in terms of residual ash and loading and elution characteristics. Conversion of relatively intractable metal contaminants into acid-soluble species by alkaline treatment is interpreted as arising out of a metathetical reaction involving precipitated impurities and caustic and:or carbonate. I n the
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Shiprock resins, zirconium. titanium, and possibly other metals appear to be precipitated within the resin as phosphates, which accounts for their inertness to all but very strong acids. Metathesis Lvith carbonate or h)-droxide might convert acid-insoluble zircon)-l phosphate into acid-soluble zirconyl hydroxide: possibly according to Equaiion 1, facile dissolution of zircon)-l hydroxide proceeding according to Equation 2. 221-0? P 2 0 , 5 H 2 0 12NaOH 4Na,$PO4 ZrlOa(OHjz 15Hs0 ( 1 ) Zr?Ol(OH)4H2SOq 2Zr(S04): jH,O ( 2 )
+
+
+
-
-+
+ +
Additional Requirements. The “molybdenum blue” formed to a small extent by the in situ reduction of adsorbed molybdenum is more easil!, eluted by caustic or causlic-salt mixture if it is first reoxidized. Organic materials (humates) are also eiuted from the resin by alkaline reagents and are more effectively removed after mild oxidation. In practice, contacting the fouled resin with ammonium nitrate-sulfuric acid (about 0.75.Y-0.25.Y)effects these osidations satisfactorily. T o minimize osmotic shock it is expedient to convert the resin from a hydroxide (or carbonate) form to the sulfate or chloride form (by contact with the appropriate salt) before the resin is in contact with strong acid. This salt treatment elutes additional traces of molybdenum, organics, and generally frees some surface dirt. \Vith these refinements? the most effective sequence of operations is as follo\vs: It et ci 1tion
Column T i m e , Operation NHaNO.4H12S04
Concn., % 0.75S-0.25S
Rinse NazCOa 5-10 N a O H - N a C I 5-10 of each
i-01.
5
Hr. 2-3
5 5-10
8-10 12
5 2.5-5
16-24
Rinse NaCl &SO4
5-10 20-45
3-5
When this sequence was carried out on heavily fouled resin, ash content was
RESIN RECOVERY reduced from 14.6 to 0.1 1yc,and operating characteristics were restored to normal. Under less ideal conditions in plant practice, ash content was reduced to 0.54% (967, dissolution). In practice, the first three steps can be omitted with relatively little loss of efficiency and some saving i n reagents. This has been done a t Shiprock several times. laboratory Studies
All materials were ordinary C.P. reagents. Ion exchange resins were samples of -4mberlite IR.4-400 (Rohm & Haas C o . ) , taken a t various times from cells of the Navajo Uranium Division mill, Shiprock, K.M. Capacity tests were carried out in small laboratory columns, 17-mni. inside diameter and 110 cm. long. Columns had the usual feed, backwash, and effluent lines, the resin being supported on a 1inch bed of fine gravel resting on a Witt plate. Influent liquors \rere gravity fed by a constant-head device to obviate changes in flow rate. Capacity determinations were carried out xvith synthetic pregnant liquors prepared by dissolving relatively pure ammonium diuranate in about 407, sulfuric acid. diluting, and adding sufficient magnesium sulfate to provide 30 grams of sulfate per liter a t ultimate dilution. Final adjustments i n volume afforded 1.0 gram of uranium oxide ( U 3 0 ~ )per liter. and p H was adjusted to 1.5 with either sulfuric acid or concentrated ammonium hydroxide as needed. Flow rate in loading synthetic pregnant liquor was maintained a t 10 ml. per minute through a 75-ml. bed (about 3.3-minute retention time). Samples of
effluent \vere collected periodically and assayed by standard fluorometric procedures to determine extent of uranium leakage. T o compare resin samples, break-through capacity was arbitrarily defined as the loading a t which lYO leakage occurred (10 mg. of uranium oxide per liter i n the effluent). After break-through capacity had been determined, a n equal volume of synthetic pregnant liquor was passed through the column to load the resin to saturation. Before elution, the resin was rinsed and back-washed as in standard practice. Elution characteristics of resin samples were determined on resins loaded to saturation. Eluent was a solution 0 . 9 S in sodium chloride and 0.1-j- in hydrochloric acid and was fed a t 3 ml. per minute (about 10-minute retention time). Effluent was sampled periodically to determine the degree of eluate barrenness a t a given level. Table I summarizes results of the various procedures. Regeneration Processes
EDTA-Sodium Hydroxide Process ( 7 ) . A sample of freshly eluted resin (68 cycles, 9.17, ash) was transferred to a column and treated as follows : (Ethvlenedinitri1o)tetraacetic acid (EDTA). 100 grams, and 58 grams of sodium chloride were suspended in about 900 ml. of water; 40 g r a m s of sodium hydroxide as a SOYc solution was added to dissolve EDTA. T h e pH of the solution after dilution to 1 liter was 6 0. T\senty-seven column volumes of this solution fed a t 30-minute retention time Lvere required to elute all material precipitable by base. T h e resin \vas then well \sashed.
Table I.
Four column volumes of 1070 sodium chloride solution were fed at 10-minute retention t i n e to elute EDTA. Next, 10 column volumes of 1070 sodium hydroxide removed considerable organic material (dark red-broLvn) during a n %hour contact. Molybdenum and silica \\'ere likewise dissolved. Finally, three column volumes of 67, sodium chloricle solution [rere followed by rinse. T h e washed resin ( 0 . 6 0 j , ash) exhibited break-through and saturation capacities for uranium oxide of 51 and 61 mg. per cc. of \vet settled resin, respectively. Eluate a t 13 bed volumes assayed 7.5 mg. of uranium oxide per liter. By comparison, used unpoisoned resin has break-through and saturation capacities for uranium oxide of 57 to 60 and 65 to 70 mg. per cc. of resin. \Vhen 40-cycle resin (77; ash) was treated in the same manner, ash content was reduced to 0.337;: break-through and saturation capacities were 55 and 69 mg. per cc. of resin, respectively. Strong Sulfuric Acid-Sodium Hydroxide Process. Resin (40 cycles; 7Yo ash) was treated IS follo~vs: Fifteen bed volumes of 1 to 2 sulfuric acid were percolated through the resin for 12 hours. Effluent \vas turbid \vith dissolved and reprecipitated solids. ?rletals identified by qualitative tests included iron, molybdmum. zirconium, titanium. and uranium. To\vard the end of this treatment qualitative tests for titanium and zirconium \\-ere faint. T h e resin \vas well rinsed and treated with 10 column volumes of 107; sodium hydroxide for 8 hours. This treatment removed essentially the remainder of the molybdenum? silica, and organic color bodies. T h e resin \\-as then converted
Resin Regeneration Processes
In plant practice regeneration with the condensed inverse process was as effective as the complete version and required much less time
Process EDTA
Complete inverse process
lteageiit Sequence 10% EDTA, 1.V NaCl, 1.1- NaOH 10% NaCl 10Tc NaOH 6% NaCl 48% H i s 0 1 10% NaOH lOc/, NaCl 0.8-1-NH~NOB-O.Z~.V
Fresh resin capacity
27 4 10 3
33 2
15 10 4-5
12 8 3
8
10% NaOH 10% NaCl 427, H2S01
8 4 20
10% NaOH-10% NaCl lOTc NaCl 207, H2S01
6 6 7
15 6 24
...
Ash, yo Filial
Initial
Cagarity. Mg. L O ? 'c'c. ILesiii_____ Break-through Saturatiori
Effluent *issay, Mg.G OI / Liter"
...
7 9.1
0.33 0.6
55 51
69 61
< 10
7
0.55
55
67
