CA2043717A1

CA2043717A1 - Recovery of sodium hydroxide and aluminum hydroxide from etching waste

Info

Publication number: CA2043717A1
Application number: CA002043717A
Authority: CA
Inventors: Thomas A. Davis
Original assignee: Individual
Current assignee: Marmon Industrial Water LLC
Priority date: 1990-07-06
Filing date: 1991-05-31
Publication date: 1992-01-07
Also published as: DE69103486T2; ES2057668T3; JPH05115871A; EP0465822B1; AU634661B2; ATE110123T1; EP0465822A1; DE69103486D1; AU7948091A; MX9100096A

Abstract

RECOVERY OF SODIUM HYDROXIDE AND
ALUMINUM HYDROXIDE FROM ETCHING WASTE

ABSTRACT

A method for the recovery and recycling of sodium hydroxide from the waste solution of aluminum etching operations. The method utilizes a dialysis membrane column or stack to initially remove sodium hydroxide from the waste solution and return it to the etch tank base solution sufficiently concentrated to carry on the basic etching operation. The method permits the recovery of salable quantities of aluminum hydroxide.

Description

2~37 ~ 7 ;

aLuMINu~ HYDROXIDE FROM ~TCHING WASTE

L~9k9rQLrgLo~ Thç Inv~niQn This $nvention relates to the tr atment of the s wa~te ~trea~ from aluminu~ d$~olutlon op ratlons and, or partieularly, to an improv d ~ thod ~or - r g-n~rating th- alk~ teh ~olutlon and recovering alu inum hydroxlde Treatment of alumlnum artieles of manufacture 1- earried out by ~ueh well known proee~s-s a~ etehing, el-aning or eh-mieal milling Typically these processes lnvolve the di~solution of ~luminum metal according to th- guation (1) Al + N~OH + 2H20 ~ NaAl02 + 2 5H2 Ae¢ording to r-aetion (1), there is an iner-a~- in th- eoneentration o~ the NaAl02 and a d-er a-- in th- eonc-ntration of th- alkali a~ the aluminum r tal dl~olve~ Nowever, th- aluminat- 1~ not atabl- ln water and, d-pending on xl~tlng conditlon~ o~
t-np-ratur-, eone-ntration~ and tlm-, reaet~ with the wat-r eeordlng to the following eguillbrium equation ~2) NaAl02 1 2H20 ~ NaOH + Al(OH)3 Theoretle~lly, ~urth-r addltions of NaOH ~re r guir d only to r-plaeo that whieh i~ phy~ieally tta¢h-d to th- work pl-¢e- r-nov-d ~rom th- bath How v-r, 1~ th- Al(OH)3 1~ allowed to preelpltate out in th- teh bath, lt 1~ w-ll known that th- tch ~olution v-ntually b-comes ln ~-ctlve and unu~able ~or earrying on the proce~s and must be dl~earded and replaeed Att-mpt~ h~v bsen Dade to avold th- problem6 and waste o~ ~at-rlal~ alluded to above J~

In u.S. Patent No. 4,372,805, there i8 ~hown a method for regenerating the sodium hydroxide wherein water i8 added to the ~olution containing di~olved aluminum to create a guper~aturated solution of aluminum hydroxide, crystallizing the aluminum hydroxide, removing the same from the etch wa6te ~olution by centrifugation, and then recycling the remaining liquid to the etch tank. Examining equation (2) above, it would appear that the addition of water to the etch waste solution causes a shift of the equilibrium to the right in accordance with Le Chatelier's Principle, thereby cau~ing the formation of lncreased aluminum hydroxide. However, that process is not completely satisfactory because the sodium hydroxide being recycled i8 diluted to such an extent that it i6 not ~uf~iciently concentrated for u~e in the etching bath. In thi~ regard, it i~ noted that the patent toaches the use of an evaporator in an effort to increase the concentration o~ the alkali.
In U.S. Patent No. 4,136,026, there i~ ~hown another method wherein the etch waste solution i8 trans~erred ~ir~t to a reactor vessel whexe it is apparently ag$tated to induce some precipitation of aluminum hydroxide. Some of the liquid from the reactor vessel is then tran~ferred to a separator vessel where the aluminum hydroxide is ~eparated from the solution with a vacuum drum filter. Due to the ~low precipitation rate of aluminum hydroxide, the filter medium, a8 well a8 the filter cake, collects precipitate and problems of plugging soon occurred.
There thus exists a need for a more ef~ective method of recovering and recycling the alkali from the tch wa~te ~olutions of aluminum d$ssolution operation~.

28~7 ~

,Summ,a~y Of The Invention The present invention provides an improved method of recovering sodium hydroxide from etch waste ~olutions that substantially eli~inates the above 5 dQscribed problems inherent in the prior art methods.
The sodium hydroxide recovered is sufficiently concentrated for recycling and use in the etching operations and is also substantially free of contamination by dissolved aluminum present in the waste ~olution being treated. The method of the invention also permits the recovery of sub6tantial amounts of aluminum hydroxide which i8 a commercially useful product.
Briefly, the invention comprises a departure from the prior art methods which add water to the etch wa~te solution in order to induce aluminum hydroxide precipltation and sodium hydroxide formation. Instead, the inventive method removes ~odium hydroxide initially from the waste solution and recycles it directly back into the etching tank. The remaining aluminum-containing Bolution i8 treated in a particle-contacting cry~tallizer where solid aluminum hydroxide is recovered.
An i~portant component of the present method i~ a diffusion dialyzer. The dialyzer includes one or more ion exchange membranes which are eubstantially permeable to ~odium hydroxide but eubstantially less permeable to aluminum salt~. The etch waste ~olution i8 fed into a dif~usion dialyzex ~tack on one side of the ion exchange membrane. Water is simultaneously fed into the ~tack on the opposite 6ide of the membrane and countercurrent to the flow of the wa~te solution.
odium hydroxide diffuses across the membrane into the receiving water stream which is returned to the etching tank. Since this addition of sodium hydroxide would 2~ ~ S~7 ~

tend to precipitate many mult~-valent cations present in tap water, it i8 beneficial to feed soft water into the diffusion dialyzer. Also, it is known that air is ~uch le~ soluble in sodium hydroxide solution than in water, ~o the diffusion of ~odium hydroxide into the water would tend to cau~e air bubbles to be released into the ~olution. Since accumulation of the air in the tops of the downward-flowing ~tream could lead to ~aldistribution of flow rate~ among the multiple parallel compartments of a diffusion dialyzer, it is beneficial to deaerate the feed water and to periodically rever~e the water flow to purge any gases that accumulate in the water compartments. The salt-containing waste solution pas~es, after cooling, to a cry~tallizer vessel for removal of precipitated aluminum hydroxide. The remalning dilute waste ~olution may be dl warded or treated furthor for the recovery of what ~mall amount~ of alkali remain therein.
80me aluminum etching operations, Qspecially chemical mllling, evolve enough heat to boil away considerable w~ter from the bath, and this water must be replaced. Since the overflow from the crystallizer in tho pre~ent method contains useful components of the bath, i.e., NaOH and other bath additives, it is a proferred ~ource of maXe-up water for the etch bath.
Moreover, return o~ the overflow to the bath eliminates the need for di~po~al or further treatment of the overflow. Howover, a high utilization of the cry~tallizer overflow a~ makeup water would eliminate a mean~ of purge or blo~down of impurities that enter with makeup water. In ~uch a case it i~ beneficial to deionize the make-up water and the feed water to the diffusion dialyzer.
The method is simple and efficient and does not require the use of many sophisticated controls.

20~717 Other features and advantage~ of the invention will become apparsnt from the following descript~on of preferred ~bodiment6 fro~ the clai~s and from the acco~panying drawing B~i*f Descrig~-ion Of The Drawings Figure 1 is a ~chematic representation of the ~teps and apparatus for practicing the ~ethod embodying the principles of the invention; and Figure 2 is a ~chematic representation of the invontive method and apparatus ~hown in use with a milling operation plant Dsscrigtion Of Preferred Embodiments Referring to Figure 1, there i8 shown a method of recovering and recycling sodium hydroxide and also recovering use~ul aluminum hydroxide The embodiment ~hown i~ employed in connection with a conventional aluminum etching operation wherein aluminum articlos are imm-r~-d for relatively ~hort periods in an otch tank 10 containing a bath o~ ~odium hydroxide and water Di~solution of the aluminum takes place as indicated in quation (1) above Waste ~olution is pumped from the tank 10 through linQ 12 and into a diffusion dialyzer 15 Dif~u~ion dialyzer 15 comprises a liquid flow vessel 16 divided into chambers or channels 18 and 20 on oppo~ite ~ides o2 an ion-exchange membrane 22. AB shown, the wa~te ~olution is pumped into and flows upwardly through channel 18 81multaneously, a stroam of warm water, whi¢h has been softened and degassed by boiling, iB
pumped into and f1OWB downwardly through channsl 20.
Preferably, the water and waste solution are here ~upplied to the dialyzer 15 at substantially equal rates Hembrane 22 is sub~tantially permeable to sodium hydroxide and substantially less permeable to the 2 ~ 7 dis~olved aluminum or alum$num salts. Such mem~branes are of a type commonly available and manufactured by companies like Pall/RAI under the trade designation BDM
and Tokuyama Soda under the trade de6ignation Neosepta 5 CR-2. Inside the dialyzer column 15, ~odium hydroxide ~igrate~ across the membrane 22 and into the water stream and the recovered sodium hydroxide is discharged back into the ~ch tank 10 as indicated through line 24.
The recycled sodium hydroxide is sufficiently concentrated to be useful in carrying on the basic etching operation.
The alkali-depleted wa~te stream exits from the top of channel 18 through line 26 and is cooled, preferably by a water ~acket heat exchanger or the like, and then pumped into a crystallizer vessel 28. The wa~te ~olution exiting from the dialyzer column 15 iB
believed to be supersaturated in aluminum hydroxide, which is known to be extremely d ow to precipitate from aguoou~ ~olution under normal condition~. The cry~tallizer vsosel 28 is of known construction and provides nucleation ~ites ~or enhanc~ng the formation and preclpitation of aluminum hydroxide which i8 removable from the bottom of the vessel as illustrated.
The overflow from vessel 28 is a dilute waste solution 30 low in remaining sodium hydroxide and/or aluminum hydroxide and may be disposed of as waste or in ~ome ca~es used as make-up water for the etch tank. However, if de~ired, the waste solution 30 may be ~urther treated a8 before in a second diffusion dialyzer for recovery of any remaining usable componentR.
It has been determined that optimum result~
are achieved if the water fed into the dialyzer i~
warmed to a temperature at or above that of the waste solution being fed to the dialyzer. Thus, the water 3s temperature should be preferably between 105F and 130F, and mo~t preferably about 120F. The ratio of water flow rate to waste ~olution flow rate also affect~
the results achieved. That ratio is preferably in the range of o.s and 4.0 to 1 and most preferably about 2 to 1.
Depending upon the size and nature of the particular alu~inum di~olution operation (i.e., etching, cleaning or chemical milling), the diffusion dialyzer may comprise a plurality of diffu~ion membranes properly spaced to provide a stack with waste solution and water channels on opposite ~ides of each membrane.
The nature of the operation will also determine if certain temperature and/or filtration controls of the wa~te ~olution being ~ed into the dialyzer are required.
For oxample, in a ~imple etching operation of the type already described, the temperature of the etch bath is not rai~ed ~ub~tantially above ambient. On the other hand, chemical milling operationR which di6solve larger amounts of metal produce bath temperature~ at or near the boiling point o~ water and also ~ignificant amounts o~ other motal~, ~uch a~ copper. Since waste solution temperature~ approaching 212F would be destructive o~
the membrane~ in the dialyzer, it is desirable to first cool the waate solution to temperatures near ambient.
simllarly, lt iB common practice in milling operatlon~
to add a precipitating agent like Na2S to the bath for precipitating out the di~olved copper and other metals.
The precipitated ~ul~ide~ form a ~ludge which desirably 1~ flltered from the waste ~olution before feeding lnto the dialyzer.
Referring now to Figure 2, there i~
schematically illu~trated a chemical milling operation with which the inventive method i8 used for recovering the ~odium hydroxide and aluminum hydroxide. The milling operation comprises multiple etch tanks 50, 52, 2~7~7 54, from which the waote ~olution i~ fed fir6t into settling tanks 56, 58, 60, for removal of sulfide precipitates. The upernatant solution is then pumped through filter mean~ 62, 64, to remove any remaining ~ludge. The t~mperature of the clear waste solution i8 regulated in ~uitable te~perature control mean~ 66 to approximately ~ bient, and then pumped into a diffusion dialyzer otack 75 to Slow upwardly therethrough. A
water tank 68 is provided having asso¢iated hot air or ~team means for degassing the water. The degassed water is pumped through suitable temperature control means 70 to reach a preferred temperature of around 120F and then into the top of the dialyzer 75 to flow dow~wardly therethrough. In the embodiment of Figure 2, the dialyzer 75 comprises multiple diffusion ~embranes and lnclude~ v-nt ueans 76 Sor periodically purging any air bubble~ Srom the Slow channels in the dialyzer. 8torage tank~ 78 and 80 are provided for rospectlvely rec-lvlng the oodium hydroxide and the alkall-depleted salt olutlon. 80dium hydroxlde Srom tank 78 is recycled and S-d back into the etch tanks 50, 52, 54, as desired.
The ~alt ~olutlon Srom tank 80 is fed into conventlonal crystallizing or proclpitatlng means, in this embodiment, a ~ixing tank 82, where the solution may ¢ontact previouoly precipitated Al(OH)3, and settling tank 84 Srom which precipitated aluminum hydroxide is removed. The wpernatant llquld Srom the oettling tank 84 i~, in this operation, al~o recycled back into the otch tanks, Sor recapture oS the remaining sodiu~
hydroxide and aloo to replace the water which is being ovaporated Srom the hot etch tanks.
The invention i~ illustrated further by the Sollowing examples.
Example 1 2~ ~ 3 71 ~

g In accordance with Figure 1, etch waste solution containing about 8% sodium hydroxide was fed into a dialyzer column compri~ing a single BDM ion-exchange membrane with about 2dm2 of exposed area. The waste solution and water were fed to the dialyzer by a dual head, 6ize 13 Masterflex pump operating at 28.5 rpm to supply the ~olution~ at equal rates. The ~y~tem wa~
operated overnight and sample~ taken the following day.
The measured output flow rate~ were O.44 ml/min. for the recovered base and 1.22 ml/min. for the treated etching solution. Analysi~ of the samples by titration with HCl ~howed that the concentration of the recovered base (viz, ~ree base) was substantially higher than in the feed waste ~olution, thereby suggesting that NaAl02 was being decomposed and releasing bound ~odium hydroxide.
Titratlon o~ the treated waste ~olution indlcated that virtually all o~ the free oodium hydroxide had been romoved and that most of the dis~olved aluminum remained, although ~ome alumlnum may also have permeated the membrane and returned with the recovered sodium hydroxide.

In A ~ystem accordlng to Flgure 2, a diffusion dialy~ tack was assembled with ten ~heets of Neosepta CR-2 membrane separated by Vexar-type ~pacers about 0.75 mm thick. Each membrane sheet had about 175 cm2 of lts ~urface expo~ed to the solutlons. Alternate ~olutlon ¢ompartment~ were fed with water ~lowing downward and a ~pent aluminum chemical milling etchant ~lowing upward.
The water, which had been demineralized and boiled, was warmed to about 110F by passing it through a heating coil before it entered the stack. Analy~i~ wa~ by titration with H2SO~. In an experiment of 450 min duratlon, a 2371 ml batch of etchant was treated in the ~tack. The etchant contained 144 g/li of free NaOH, and - 2~7 ~7 476 g/li of NaAl02. a 2644 ml batch of base was recovered composed of 109 g/li of fxee NaOH, and 15 g/li of NaA102. The ~060 ml batch of base-depleted ~alt solution contained 12 g/li of free NaOH, ~nd 272 g/li of N~A102. Upon standing at room temperature, a voluminous white precipitate of Al(OH) 3 formed in the base-depleted ~alt ~olution.
The preci~e chQmistry of the method i8 not completely under~tood, but it is theorized that the salutary results obtained indicate another operation of Le Chatelier~s Principle. Referring again to equilibrium equation (2), it will be noted that removal of ~odium hydroxide causes shifting of the equilibrium to the right with the depletion of sodium aluminate and the increa~ed production of aluminum hydroxide. The treated ~olution exiting from the dialyzer apparently become~ ~uper~aturated in aluminum hydroxide which is th-n readily romovable in the nucleating crystallizer or other ~ettling ve~el. It i~ al~o theorized that the differenca in ~low rate~ and increase in concentration of free ~odium hydroxide in the recovered base was cau~ed by osmotic water removal from the water stream through the membrane.
It should be understood that the language employed herein is for de~criptive purpose6 only and is not intended to be otherwi~e limiting of the concepts of the invention. Although the illustrations and examples herein utilize flat sheet membranes, other configurations such as tubular or ~piral wound device~
could be employed. While preferred embodiment~ have been described, changes and variations may be made by those ~killed in the art without departing from the ~pirit ~nd scope of the invention as defined in the appended claims.

Claims

1. A method of recovering sodium hydroxide from an etch tank waste solution containing dissolved aluminum comprising:
directing a stream of the waste solution into a diffusion dialyzer containing permeable membrane means that is permeable to sodium hydroxide and substantially less permeable to dissolved aluminum on one side of the membrane means;
simultaneously directing a stream of water into the dialyzer on the opposite side of the membrane means whereby sodium hydroxide migrates through said membrane means from the waste solution stream into the water stream; and recycling the sodium hydroxide-containing stream back into the etch tank.

2. A method according to claim 1 wherein the streams of water and waste solution flow countercurrently through the dialyzer.

3. A method according to claim 1 wherein the water directed into the dialyzer is first softened.

4. A method according to claim 1 wherein the water directed into the dialyzer is first degassed.

5. A method according to claim 1 wherein the ratio of the flow rate of the water to the waste solution 18 between 0.5 and 4.0 to 1.

6. A method according to claim 1 wherein the water directed into the dialyzer is heated to a temperature between 105°F and 130°F.

7. A method according to claim 1 wherein the waste solution exiting from the dialyzer is directed into settling vessel means to precipitate aluminum hydroxide therefrom.

8. A method according to claim 7 wherein the waste solution exiting from the dialyzer is cooled to a temperature between 65°F and 115°F before being directed into the settling vessel means.

9. A method according to claim 7 wherein overflow liquid from the settling vessel means is directed back into the etch tank.

10. A method according to claim 7 comprising further directing a stream of the overflow liquid from the settling vessel means into a second dirfusion dialyzer containing permeable membrane means that is permeable to sodium hydroxide and substantially less permeable to dissolved aluminum on one side of the membrane means;
simultaneously directing a stream of deionized water into the second dialyzer on the opposite side of the membrane means; and directing the water stream into the etch tank.

11. A method according to claim 7 wherein the settling vessel means comprises a particle-contacting crystallizer adapted to provide nucleating sites for the precipitation of the aluminum hydroxide.

12. A method according to claim 1 wherein said membrane means comprises at least one ion-exchange membrane.

13. A method according to claim 12 wherein the dialyzer comprises a stack of a plurality of ion-exchange membranes providing liquid flow channels on opposite sides of each of the membranes, and periodically purging the dialyzer of gas bubbles formed in the channels by the diffusion of sodium hydroxide into the water stream.

14. Apparatus for recovering sodium hydroxide and aluminum hydroxide from an etch tank waste solution comprising:
diffusion dialysis means having channels on opposite sides of permeable membrane means that is permeable to sodium hydroxide and substantially less permeable to dissolved aluminum for receiving respectively a stream of the waste solution and a stream of water;
pumping means for directing said two streams in opposite directions through the dialysis means on opposite sides of said membrane means; and settling vessel means for receiving the waste solution exiting from the diffusion dialysis means and collecting aluminum hydroxide precipitating therein.

15. Apparatus according to claim 14 wherein said membrane means comprises an ion-exchange membrane.

16. Apparatus according to claim 15 wherein said diffusion dialysis means comprises a dialyzer having a stack of a plurality of ion-exchange membranes and means purging gas bubbles forming in the channels during flow of the streams of liquids therethrough.

17. Apparatus according to claim 14 comprising deionizing and degassing means for treating the water before pumping into the diffusion dialysis means.

18. Apparatus according to claim 14 wherein the settling vessel means comprises a particle-contacting crystallizer for providing nucleating sites for the precipitation of aluminum hydroxide.

19. A method of recovering sodium hydroxide and aluminum hydroxide from a sodium aluminate solution comprising:
directing a stream of sodium aluminate solution into a diffusion dialyzer containing permeable membrane means that is permeable to sodium hydroxide and substantially less permeable to sodium aluminate on one side of the membrane means;
simultaneously directing a stream of water into the dialyzer on the opposite side of the membrane means whereby sodium hydroxide migrates through said membrane means from the sodium aluminate solution stream into the water stream;
directing the water stream exiting from the dialyzer into a sodium hydroxide storage vessel; and directing the sodium aluminate stream exiting the dialyzer into settling vessel means to precipitate aluminum hydroxide therefrom.

20. A method according to claim 19 wherein the streams of sodium aluminate solution and water flow countercurrently through the dialyzer.

21. A method according to claim 19 wherein the water directed into the dialyzer is first softened and degassed.

22. A method according to claim 21 wherein the softened and degassed water is heated to a temperature between 105°F and 130°F.

23. A method according to claim 19 wherein the sodium aluminate solution exiting from the dialyzer is cooled to a temperature between 65°F and 115°F before being directed into the settling vessel means.