out one i o n for another, can:rniious u n i t s offer advantages over f i x e d bed units i n certain a p p l i c a t i o n s a n u although d e m i n e r a l i z a t i o n I S the most wctelv p r a c t i c e d use today. water and waste water treatment. m i n e r a l c o n v e r s i o n . and h y d r o m e t a l l u r g i c a l tt ea?merit of low-grade ores loom on the i.7 0' I zo r l Swapping
the chemical engineer's tool box. Originally conceived for isotope separations and the removal of trace radioactive elements from waste water, this mechanism was rapidly adapted to a wide variety of water treatment and chemical processing flowsheets. Continuous countercurrent operation applied to ion exchange offers the user the same advantages and disadvantages as any batch-type operation does in compliance with any unit operation. The major advantages are: 0 higher throughput rate per unit size 0 higher chemical utilization 0 lower dilution of process streams 0 lower process water requirements 0 more uniform product quality over the total cycle The major disadvantages are: 0 more complex mechanical operation 0 higher dependence on instrumentation
Where we are today Water treatment in the form of softening and demineralization qualifies as the present, most widely practiced application of ion exchange technology. Almost all major plants and many cities use one of these mechanisms for treating some portion of their water supply. Many of these applications are continuous ion exchange, but the majority of installations in this area employ fixed bed exchangers. The largest municipal softening plant using continuous ion exchange is one on the West Coast: it has 111 0
Environmental Science & Technology
FEATURE Irwin R. Higgins Chemical Separations Corp., Oak Ridge, Tenn. 37380
a capacity of 28 mgd. The largest industrial demineralization plant using continuous ion exchange has a capacity of just under 6 mgd. Current ion exchange technology is not limited to water treatment, however: Another major use is in the demineralization of sugar juice. This demineralization is mostly accomplished using fixed bed units, but some continuous plants have been put into operation. Some wines are also treated using ion exchange to remove undesirable contaminants. Other areas where ion exchange technology is in present practice include: 0 removal of aluminum from phosphoric acid brightdip solution 0 presoftenirig of seawater prior to purification by flash evaporation 0 treatment of wastes from fertilizer plant recovering product 0 treatment of cooling tower blowdown removing and recycling chromates 0 various mineral conversions 0 treatment of various other types of waste streams
How it works At present, the major emphasis in ion exchange seems to be in the area of waste water treatment for pollution control. Ion exchange is a natural method to consider for the removal of trace pollutants from water. In fact, no water treatment system exists that removes ions from water so fast, so completely, or at so low a pressure drop. This statement must be followed by what isn't so attractive about ion exchange. The term ion exchange is most descriptive of what happens. It is a "swap-out'' of one ion for another. For every pound of mineral salt extracted from water, at least an equivalent amount of regenerant chemical must be purchased and consumed. I f both extracted mineral and purchased mineral are dumped back to the environment, Mother Nature is twice as bad off as before. Here, electrodialysis and reverse osmosis have an edge over ion exchange. Power in the form of electromigration or pumping is substituted as the driving force rather than chemical exchange. One answer to this characteristic unfavorable feature of ion exchange is to use regenerant chemicals that form salts easily disproportionated into acid and base for reuse. For example, if HCI and NaOH are the regenerants, then NaCl makes up an equivalent portion of waste.
Instead of dumping this waste, feed this NaCl to an electrolytic cell and recycle HCI and NaOH. Nitric acid is attractive as a cation regenerant because by steam calcination of the nitrate salts, HNO3 is recovered for use. The residual alkali salts may be added to the regenerant wastes resulting from NH40H use with recycle of "3. Sulfurous acid is a strong ion exchange regenerant in the presence of acetone. With low thermal input, both SO2 and acetone may be driven off for reuse. These, and other regenerant recycle systems are being currently studied. However, they are not commercially available at this time. Another practical approach to this problem is to develop flowsheets which can, as nearly as possible, produce useful by-products from the pollutants. For example, ammonium and nitrate ions in waste water in an ammonium nitrate plant use HNO3 and NH40H as ion exchange regenerants. The regenerant waste from both cation and anion exchange is NH4N03. This material is the plant product anyway and is mixed with regular product. Another example of this type flowsheet is the treatment of cooling tower blowdown to remove hexavalent chrome. The regenerant in this case is NaOH and the product is sodium dichromate which can be recycled to the cooling tower for continued corrosion protection. Not only does this eliminate the pollutant, it sharply' reduces chemical make-up costs. Although it would be ideal, not all flowsheets can be made to fit in the latter category. For this reason there exists tremendous incentive for development of regenerant recycle systems, which has long been one of Chemical Separations' objectives.
Low-grade ores Water and waste treatment may always be the greatest application of ion exchange. However, another great potential looms on the horizon. Ion exchange is a key unit operation in the hydrometallurgical treatment of lowgrade ores. Man started mining only the very highest grade ores. Lower grade ores are generally subjected to physical upgrading like floatation. Chemical dissolution is the most effective method of releasing values from really low-grade ore. The greatest role of ion exchange may be to extract these mineral values from a large volume of unclarified leach. The method has been termed "ResinIn-Pulp." A pilot plant using this flowsheet was operated Volume7, Number 13, December 1973
1111
Water softening. Major plants and many cities use ion exchange for treating Some portion of their water supply
in a uranium mill in Colorado. Uranium was extracted to
99.8% purity from an ore leach pulp containing 8% suspended solids in a I - m i n contact time. This technology started with uranium, but has also been considered for Ag, Au, Be, Go, Cu, Mo, Ni, W, a Mineral conversion Another future application ot ion excnange tecnnology will be in the area of mineral conversions; for example, the conversion of fetilizer KCI to more desirable formsK2S04, KN03, or K2HP04. As farming is intensified, the
detrimental effect of chloride on both soils and quality of food products is of growing concern. When it is necessary to convert one salt form to another, ion exchange is surpassed by no other method. Merely by pumping solutions at controlled flow rates, at low pressure drop, through a packed bed of ion exchange resin, one salt form is converted to another with the greatest of ease. Continuous countercurrent flow is of extreme importance iecause solutions are generally pumped at one half the esin rate without dilution or tailings. Conventional ion ex:hange cannot come close to this type of performance.
TABLE 1
Demineralizer system for high solids removal Capital a n d operating costs Basis = TDS 1800 p p m Flow = 380,000 U S . gal/day CDnYentional
CAPITAL COST Equipment cost (actual $130,000 bid) delivered uninstalled Installation and $ 26,000 start-up ~
CO"ti""0"S
$120,000 $ 12,000
___
Total investment
$156,000
$132,000
Total resin inventory Strong acid cation Weak base anion
950 fta 550 ft5
190 ft3 190 f t 3
2.92 lb/1000 ga I 38.2 lb/1000 gal $0.09 $0.67 $0.76
0.575 lb/1000gal
OPERATING COST
HNOI NHs at 36/1b HNOj at 1.75$/lb Cost of chemicals/ 1000 gal
inuous units offer cost savings
Environmental Science & Technology
$0.02 $0.39 $0.41
Resin attrition Assume equal for both cases $133 SAVINGS/DAY SAVINGS/YEAR AT 365 $48.500 DAYS/YEAR OPERATION Source: Chemical Separations Corp.
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22.1 lb/1000 gal
I
!
TABLE 2
High throughput softening system Cap tal and operating cost comparisons for sofrening 17 mgd of 250 p p m CaCO, nardness to c1 p p m CaCO, nardness CONTINUOUS UNITS FIXED BED UNITS Chemicals and waste water Chemicals and waste water unit co5t $ per m i l gal Of 85
Dosa e, Ib P&
Item
mil gal
Salt Chlorine Backwash, dilution and rinse water waste
Chemical Cost, C per Ib
3,580 16 35,300
0.585 6.75 10.74/1000 gal
Dosage. Ib per mi1 gal
ness
water
Unit Cost $ per mil
gal
ms/l hard-
Item
21.29 Salt 1.08 Chlorine 3.79 Backwash, dilution and rinse and slip water waste
Chemical cost, f per Ib
2,850
16 15,600 gal/mg
0.59
6.75 10.74/1000 gal
Labor I Chief operator $700/mo 4 Operators 8598/mo 1 laborer $447/mo
26.16
$8,400 28,684 5,364 42,448 plus overhead 15%
Total operation and maintenance cost excluding amortization
16.82 1.08 1.68
28,684 5,364 ~
PIUS
5.26
overhead 15% Waste water disposal No operating cost other than included under power and maintenance Waste discharge through waste line to Bay 1.00 1.00
33.42
Power Maintenance and repairs
0.50 1.00
26.34
Total operation and maintenance cost excluding amortization Amortization: $984,000 plant 205.800 waste line
__
$811,800 for 20 years a t 5% = $65,139
water
$8,400
42.448
5.26
Amortization: $606,000 plant 205,800 waste line Total operation, maintenance and amortization cost per million gallons
Total chemicals and waste water Labor 1 Chief operator $700/mo 4 Operators $598/mo 1Laborer $447/mo
Waste water disposal No operating cost other than included under power and maintenance Waste discharged through waste line to Bay Power Maintenance and repairs
85
19.58
~
Total chemicals and waste water
Of
mg/l hardness
$1,189,800 for 20 years a t 5% = $95,470
7.00 40.42
10.26
-
Total operation, maintenance and amortization cost per million gallons
36.60
Source Chemical Separations Corp.
Difficult chemical separations More limited, perhaps, in application, but certainly a potential for ion exchange is the area of separations. Ion exchange is one of the most powerful tools for separating ions of similar chemical properties-even isotopes. Examples are: rare earths, K-Na, Ca-Mg. CszRb, Zr-Hf. Isotope systems that have been considered are: Li6-Li7, B'O-B", N"-N15, and U235-Uz38.The outstanding reason for consideration of ion exchange is that typical Btu values are the lowest of any mass transfer system, this meaning that equipment is the smallest for doing a unit amount of work. Continuous ion exchange holds the record for this type of separations. Lithium was enriched by a factor of IO' from sodium with a separation factor of three in a 3-ft operating height. This area is where chemical separations started. Company outlook CHEM-SEPS technnlnov is "snin-nff" frnm Oak Rirlna
ticuiar
continuous device,
comminly
known as ' t h e
Volume 7, Number 13, December 1973
1113
"Higgins" contactor, was first conceived a L ,,, 1951. ORNL carried out intensive studies in diverse fields of application that laid a solid foundation for CHEM-SEPS potential development. The first serious application was for the extremely tough separation of isotopes. Although lithium isotope separation was not practical with ion exchange, it did prove to have a superior capability in the area of tough separations. The potential of ion exchange in the pollution area was demonstrated by work with long-lived fission products SrS0 and C S ' ~in~ waste streams. Chemical Separations Gorp objective was to apply ion exi cal processing where it mighr coniriuuie aavariiage. I I was recognized as a new technology that would take a certain amount of time. Company emphasis has always been on process development and pilot plant demonstration. Water softening was chosen for first practical scaleup because it was simple and best understood of all ion ~. ....... . earl" .. , unit was installed tD exchange operations. Another remove aluminum from a phosphoric acid bright-dip solution. This unit has operat'ed for over eight years. The question is repea tedly asked, "If continuous countercurrent ion exchange is better than conventional fixed bed ion exchange, why aren't all new installations of the continuous t w e ? " Altht,ugh continuous operation offers distinct operaiional advimtages, its more complex nature results in fairly. hiah - cap ita1 cost. For certain waters, fixed bed ion exchange rema ins the best overall process. High solids removal and/or h igh throughput rates favor continuous equipment. Capital and operating costs of a )r high solids removal (Table 1) demineralizer system fi and a high throughput softening system (Table 2) are compared for both coritinuous and fixed bed systems. . . . ~ . Other cases could be cnosen TO Snow equal cosls lor both systems. From the above discussion it can be seen that continuous ion exchanoe can offer tremendous advantaae for 1~ cer tain applications. I t does not offer a complete solution for all problems, however. Each application must be vie!Ned and the alternatives realistically compared before the optimun treatment process can be chosen. Realizing the need for process evaluation and development of new Drocess technology, Chemical Separations maintains an act ive pilot program for this purpose. 1.here is presently a wide-open future for ion exchange ~~~~~
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Circle No I on
Readen' Service card
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Lan Energy and Development: A Case Study compiled and edited by William W. Seifert, MohammedA. Bskr, and M. A l i Ketfani $11.00 paperbound
North Sea Science: Papers Presented at the NATO Science Committee Conference, Awemore, Scotland, 15-20 November, 1971 edited by Edward D. Goldbers $18.95 hardcover
$15.00 hardcover; $3.95 paperbound
The MIT Press Massachusetts Institute ofTechnology Cambridge, Massachuretts 02142 Circle No. 8 on R e i d m ' Sewice Card
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Environmental Science
Technology
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new applications are explored, this technology will continue to grow. Many existing applications are still not fully explored or developed. The opportunities which still exist in this area are one of the reasons that a major engineering construction corporation-Foster Wheeler Corp. (Livto make Chemical Separations ingston, N.J.) -decided Corp. its subsidiary. Foster Wheeler has also expressed particular interest in hydrometallurgical applications. The greatest contribution may be the application of continuous ion exchange to low-grade ore treatment. Application to general waste treatment will also be encouraged; even air pollution as it is converted to water pollution by way of water scrubber.
Irwin R. Higgins is vice-president and technical director, Chemical Separations Corp. He founded the corporation to market the continuous counter~~~.~ current exchanger lor w h i m he holds U. S. uafent rronfs. Listed m "Men in Science;' M i Higgins has supervised a n d developed new processes which lead themselves to the ion exchange apparatus tor private industry as well as the U. S. Government. ~~
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