"Phase Diagrams" can be used to examine a variety of diagrams. Figure 3 shows the typical analysis that can be easily performed where the tem-perature and composition are speci-fied by the user. The program deter-mines the phases present, the compo-sition of the phases and the amount of each phase.

A constant composition phase frac-tion chart for a 1 % C-99% Fe alloy is shown in Figure 4. The phases pres-ent are indicated by the labels to the left and right of the fraction chart. The arrow next to each label indicates which scale should be used to deter-mine the amount. Any composition or any temperature may be chosen for the generation of a phase fraction chart.

These programs have proven very beneficial in helping students relate textbook diagrams (cooling curves, phase diagrams and TTT diagrams) to the actual structure that is present in the material. The programs also provide students with a means to practice typical phase analysis calcula-tions.

'eo;.--r"",,-,-,- T.o, Pi"k ."'00,",,,, "', •• ,,' "'" f-l'.+-f+:+-t--1-+-t- 1 :.~~ G "l~'''~;<~ )_ t~ 1.

L'qlJl.J 'r "- -.<:, ~

"" L.LL...L--"--'-'-L-l--'-'~ F~ 1 _ ) -l '.

\""~'Jhl F~r[ .. r,l~g~ (~rbon

Figure 3. Typical phase analyses for Fe-C alloys. The alloys being analyzed are indi-cated by an X on the phase diagram.

1600

1400 c- --0 C 1':11:1

"'" "'"

, (

Fe 1 2 :: J 100 WeI'3t,t Ferc~ntMle C,t·~,)"

Figure 4. An example of a constant com-position phase fraction chart for a 1 % C-99% Fe alloy.

If you want more infotmation on this subject, please circle reader service card number 84.

References .1. J.T. Staley, J. Metals, 37 (March 1985) 71. 2. J.T. Staley, J. Metals, 37 (April 1985) 76. 3. J.T. Staley, J. Metals, 37 (May 1985) 65. 4. T.H. Kosel, J. Metals, 37 (Oct. 1985) 56.

JOURNAL OF METALS· July 1986

Reclaiming Waste Pickle Acid Carolyn Hutter Byszewski Aquatech Systems, Allied-Signal Corporation

Stainless steel surfaces are pre-pared with annealing and pickling processes where baths of HF and HN03 remove iron oxide scale from the 316SS sheet. Periodically, these baths must be neutralized, dumped and recharged. Presently, the cost for waste acid disposal at a waste dump site is between $0.20 and $0.37/gallon. With increasingly successful efforts being conducted to close the dumps for environmental reasons, disposal costs will soon become prohibitive, forcing steel plants to find an alterna-tive.

To alliviate the Situation, Allied-Signal's Aquatech Systems' business group has developed an electrodialytic process which uses bipolar membrane technology to reclaim valuable fluo-ride and nitrates from hazardous waste acids. The metals are reduced to a solid, dry metal hydroxide cake, the only waste left by the process.

Keying the technology is the bipolar membrane with a water splitting capa-bility. Under the influence of an elec-tric current, hydroxyl and hydrogen ions are generated. The bipolar membrane is flanked by conventional mono polar membranes; a cation selective mem-brane that allows only positively charged ions to pass and an anion selective membrane that allows only negatively charged ions to pass. The cell stack effectively separates acids and bases from aqueous salt streams such as KF, KN03, Na2S04, NaCl, KCI, RCOONa, etc.

With the process, the waste acid, spent with metal levels (typically 50-60 gIl), is neutralized with dilute caustic potash (nominally 1.5-2.0 normal KOH). The two streams are mixed well using conventional stirred tank methods in a ratio of 3.5:1.0 base to acid until the target pH of 10.5 is reached. The resulting solution fea-tures a suspended solids content rang-ing between 10-20%, depending on metals content originally in the waste acid.

The neutralization reaction is fast and equilibrium is normally reached within 15 minutes. The neutralization reaction is also successful at break-ing down the complexed metal fluo-ride sludge such as FeF3 to KF and Fe(OHb- It is theorized that the free nitric acid assists in the dissolution of complexed fluorides. Once the pH has stabilized, filtration can begin. Filtration

is performed using conventional recessed plate and frame filtration. Diatomaceous earth (DE) is used as a body feed which increases cake porosity, thus lowering pressures re-quired for filtration as well as increas-ing filtrate rate. Presently, approxi-mately six pounds of DE are required for every 100 gallons of waste acid processed. Experiments to minimize DE filter aid are underway.

After filtration is complete, the cake is first washed with dilute KF and then with water. Cake washings (typically 0.75 molar KF), are concentrated to 3 molar KF via electrodialysis (ED) and recycled as fresh feed. The dilute stream from the ED unit supplies the cake wash water. The cake is discard-ed as 40 wt.% solids. Some F-, N03-and K+ is lost in the cake and must be made up. Makeup requirements for K+ are approximately 5%. The cake is 60% water and 40% solids, 2% of which represent DE. The remaining solids in the filter cake are metal hydroxides: 75% iron, 12.5% chrome and 12.5% nickel. The cake byproduct is about 4 ft3 for every 450 gallons of filtrate processed. Currently, the cake is sent to disposal, however, research is investigating the possibility of pelletizing the cake for recycle in an electric furnace.

Once filtered, the water clear filtrate of 10-15 wt.% KF/KN03 is analyzed for contaminants. Filtrate quality must be less than 3 wppm total metals of Fe, Ni, and Cr. The KF/KN03 is continuously fed to the Aquatech stack at a rate of 15 gph. In turn, the stack produces -5 gph of acid product and -10 gph of base. The acid product quality is nominally 4/9 wt.% HF/HN03 (3.0 normal) while the base strength is approximately 8 wt.% (1.5 normal). The base is recycled internally in the process and used to neutralize the next batch of waste acid. The acid product is either stored or sent direct-ly to the pickling tubs. The Aquatech stack can recover over 95% of the fluoride and nitric value present in the waste acid.

Power consumption is estimated at 1275 KW hr/ton KOH produced at 68 amps/ft2 or 1525 KW hr/ton HF/HN03 acid. Aquatech is a trademark of Allied Corpora-tion. If you want more information on this subject, please circle reader service card number 82.

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