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An evaluation of steel scrap as a reducing agent in thegalvanic stripping of iron from D2EHPA

J. Sun*

, T.J. OKeefeDepartment of Metallurgical Engineering, University of Missouri-Rolla, Materials Research Center, 101 Staumanis Hall, Rolla, MO 65401, USA

Received 30 October 2001; accepted 15 December 2001

Abstract

A new process to remove iron from metallurgical process streams using solvent extraction techniques has been developed. The

unique aspect of the process is the use of solid metal reductants directly in the organic phase, such as D2EHPA, as a means of selectively separating the undesired metal cation impurities. The process, called galvanic stripping, has been demonstrated on bothbeaker and bench scale continuous levels. Previous research focussed on using zinc or iron powders as the reducing agents. A recentstudy evaluated the use of steel scrap as a possible alternative that might enhance the process economics and product purity. A TBPmodier to the D2EHPA was also tested to determine the effect on process efficiency. The operating variables chosen for the studyincluded reductant surface area, reaction time, diluents and ferric ion concentration in the organic. Once the pertinent parameterswere identied by feasibility tests, an experimental design with statistical analysis was utilized to optimize overall process efficiencyand iron removal. 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Reduction; Solvent extraction; Hydrometallurgy; Modelling; Environmental

1. Introduction

Iron is frequently encountered in signicant quanti-ties in various leach solutions and its removal and dis-posal constitutes a major problem in manyhydrometallurgical operations. A number of processeshave been developed or proposed for the removal of Fe(III) from process streams using solvent extractiontechniques (Ritcey, 1986; Riveros et al., 1998). Thegalvanic stripping process being described was devel-oped to complement existing procedures employed insolvent extraction technology, particularly where strip-ping of a particular cation is difficult.

Galvanic stripping (OKeefe, 1993) is a spontaneouselectrochemical process, which may offer a viable alter-native technology for stripping cations from conven-tional organic solvents. Using solid metal as thereductant directly in the organic solvent is the uniqueaspect of the galvanic stripping process. The other fea-ture of the process is that relatively fast reaction ratesare possible even though the organic extractants are verypoor electrolytic conductors.

In conventional solvent extraction the metal ion dis-

tribution between the organic and aqueous phases canbe expressed in a simple equilibrium expression such as

M naq nRH org R nM org nHaq 1

where M n stands for a metal ion and RH represents anorganic extractant. Obviously, chemical stripping ispreferred if it is technically feasible and environmentallyacceptable. The conditions that favor the use of thegalvanic stripping process are those that involve situa-tions when the metal ions are difficult to remove usingonly a change in chemical driving force. The objective of galvanic stripping is to incorporate an electrochemicaldriving force to complement that provided by the usualchemical driving force. For example, while Fe 3 is dif-cult to strip from D2EHPA, even using strong acids, inreduced form as Fe 2 , stripping is relatively easy, with apH 2 being sufficient.

If an electrochemical potential difference exists be-tween two metal/ion couples in the organic solvent, areaction is theoretically possible. A more active metalwill reduce the more noble metal ion in the solution.Though each of the metal/ion potentials is unique to thespecic organic and metal/ion in question, the orderingfor values measured to date are similar to the aqueouselectromotive force series (Gu et al., 2000).

Minerals Engineering 15 (2002) 177185www.elsevier.com/locate/mine

*

Corresponding author. Tel.: +1-573-341-4392; fax: +1-573-341-2071.

E-mail address: [email protected] (J. Sun).

0892-6875/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved.PII: S0892-68 75(02 )0000 4-3

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For iron reduction with iron or steel scrap, the driv-ing force comes from the potential difference betweenmetallic iron and the two different oxidation states foriron, 2 and 3 . The primary reaction involved in ironion reduction with steel scrap in the organic phase isassumed to be

Fe solid 2R Fe3org ! 3R Fe

2org 2

As indicated by reaction (2), one mole of metallic iron orsteel will theoretically reduce two moles of ferric ion toferrous ion to give an ideal stoichiometry number of one. The actual value obtained in practice is alwayshigher because of the non-productive side reactions, inparticular hydrogen evolution. Because of this, one im-portant economic aspect of the galvanic stripping pro-cess is to identify the operating conditions that minimizethe stochiometry number or the relative amount of thesolid iron reductant used. Another important aspect isthe efficiency and amount or rate of removal of the ferricion. In this study, the reduction rate is dened as theamount of ferric ion converted to ferrous ion per minuteper square centimeter of reductant surface area.

Previous studies on the effects of various operatingparameters for iron removal focussed on the use of metallic zinc or iron powders as the reductant for gal-vanic stripping (Barrera et al., 2000; Belew et al., 1993;Chia, 1994; Chang et al., 1996; Moats et al., 1995). Steelscrap was evaluated as the reductant in this study for thegalvanic stripping of iron from D2EHPA to identifypossible economical and product purity benets. Eithersimultaneous stripping or separate stripping may be

utilized with this process and both were evaluated.An experimental design method was used for plan-ning the experiments and to statistically analyze andmodel the results. The inuence of reaction time, ironconcentration in the loaded organic and the reductantsurface area on percent iron removal, the stoichiometrynumber, and the iron reduction rate were determined.Using the experimental data, a model for the quantita-tive identication of the operating parameters necessaryto optimize iron removal was constructed.

2. Background

Iron is a common impurity associated with a numberof mineral and waste materials and is very detrimentalbecause of the high concentrations encountered anddifficulty in its selective separation. One promisingmethod is the use of solvent extraction. Most of theextractant types, such as solvating, chelating, anionic orcationic, have a high affinity for Fe(III), but recoveringthe iron in an environmentally acceptable or even sale-able form has proven to be a major challenge (Pam-menter et al., 1986). Numerous studies to develop ironseparation processes have been proposed, but a truly

viable process has yet to emerge. In principal, solventextraction offers the possibility of separating and con-centrating the iron.

Ritcey (1986) reviewed the situation with regard toiron control by solvent extraction and Riveros et al.presented an extensive survey review article on the re-covery of iron from zincsulfuric acid processing solu-tions by solvent extraction or ion exchange. Highlyconcentrated strip liquor is essential for the economicalgeneration of a marketable iron product. In most of theexisting processes used for iron separation, the diluteiron strip solution formed is discarded or neutralized toform an iron-bearing sludge that must be properlystored.

One method, hydrolytic stripping, was developed toseparate iron from Versatic acid (Monhemius, 1985;Monhemius et al., 1984, 1993). In this process, iron-loaded Versatic acid is contacted with water in anautoclave to 150200 C to precipitate hematite. Thehydrolytic stripping reaction can be represented as2R 3Fe org 3H 2O Fe 2O3s 6RH org 3

It was possible to produce pigment grades of iron oxide,which could be readily separated from the organicphases. Another method to separate iron from Versaticacid, developed by the Lurgi researchers, involvesstripping with formic acid with pressure hydrolysis of the ferric formate solution to precipitate hematite.

The solvent extraction separation of Fe(III) andZn(II) with Primene 81R (a primary amine, Rohm &Hass) has been studied by Spanish researches (Juan and

Perales, 1994). Iron is precipitated from the organicphase using ammonia/ammonium sulfate. The precipi-tate is readily ltered from the aqueous phase and isthen calcined to pure iron oxide.

At low pH, iron(III) is preferentially extracted intomany organic phases relative to most metals. However,stripping iron(III) from the organic phases has been amajor problem for practically every extractant. As aresult, considerable effort has been made to identifymethods for Fe(III) stripping.

Majima et al. (1985) studied the use of an SO 2 gas-eous reductant to accelerate the stripping of Fe(III)loaded in DEHPA. The mechanism involves the strip-ping of Fe(III) rst, followed by its reduction to Fe(II)in the aqueous phase. Repeated use of the aqueous stripsolution recovered a product solution containing 50 g/liron. Hydrogen reduction of the iron in the presence of acatalyst, followed by stripping Fe(II) with H 2SO 4 isanother technique using a reductive mechanism (Riveroset al., 1998). Although using SO 2 or H 2 reductants athigh pressure can generate concentrated strip solutions,high temperature and pressure are needed.

The galvanic stripping process can be operated atnear ambient pressure and temperature and uses a solidmetal as the reductant (OKeefe, 1993). In previous

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work a continuous process using galvanic stripping totreat zinc residues (Barrera et al., 2000) produced aconcentrated iron sulfate strip solution containing90130 g/l. Zinc metal was used to reduce the Fe(III) ina DEHPA-SX12 phase in this case. Preliminary feasi-bility tests also showed that electrolytic iron and crys-tallized ferrous sulfate could be obtained as by-products.The focus of these batch tests was to evaluate the use of steel scrap as the reducing agent for removing ferric ionby galvanic stripping.

3. Experimental

3.1. Reagents

The aqueous ferric ion solution used to load the or-ganic was prepared by dissolving Fe(III) sulfate in de-ionized water. Sulfuric acid was used to adjust the pH toabout 1 for the aqueous ferric ion feed and to maintainthe stripping solution at the desired value.

Low carbon steel scrap chunks, provided by an in-dustrial collaborator, were used as the reductant for themost of the experiments. The individual pieces wereapproximately cubic and had a volume of about0.25 cm 3 .

Di-(2-ethylhexyl) phosphoric acid (D2EHPA) wasused as the extractant with tributyl phosphate (TBP) asa modier and either SX-11 or SX-12 as the diluents.D2EHPA and TBP were supplied by Rhodia and theSX11 and SX12 were obtained from Phillips 66 Mining

Chemicals. All other chemicals and reagents used wereof analytical reagent grade.

3.2. Procedure

Both galvanic stripping mode options, simultaneousand separate, were used for the experiments. For si-multaneous galvanic stripping, the loaded organicphase, the aqueous stripping solution and the desiredamount of steel reductant were mixed at the desiredratios in the same reactor. This allowed the reductionand stripping reactions to occur at the same time. Thenthe phases were separated by conventional means.

Separate galvanic stripping consisted of a reductionstage in which only the organic phase was contacted andreacted with metal. After a suitable time for reductionthe organic phase went to an aqueous stripping stagethat contacted the organic containing the reduced ironand aqueous phases.

In the reduction stage of the separate galvanic strip-ping experiments, loaded organic containing 0.8 vol% of deionized water, which helps to initiate or catalyze thereduction reaction, was contacted with the metal. Afterreduction the organic containing the ferrous ion wasthen contacted with an equal volume of sulfuric acid

solution at the desired pH. The stripped organic wasthen recycled for loading with ferric ion. All the reduc-tion and stripping reactions were conducted under aninert atmosphere. The reactor was agitated using aBurrell wrist-action shaker, usually at a medium setting,for all the experiments. A water bath controlled thetemperature at 55 C. Iron concentrations in the aque-ous and organic phases were analyzed using atomicabsorption spectroscopy (AA) and a portaspec X-rayuorescent analyzer (XRF).

4. Results and discussion

4.1. Effect of operating parameters with simultaneousstripping

Simultaneous galvanic stripping is conducted in onestep, where the electrochemical reduction and chemicalstripping occur simultaneously. Screening tests wereconducted using this mode of operation to determine theeffect of process parameters. The experiments werestudied by changing one parameter at a time whilekeeping other parameters constant.

The effect of reductant surface area on iron removalwas investigated by reacting aqueous-organic mixturescontaining 3.3 g/l Fe 3 for 10 min with 13.7 and 19.2cm 2/ml steel scrap chunks. The experimental results in-dicated that increasing the effective surface area of themetal reductant increased the iron removal percent.These results are similar to those obtained when zinc

was used as the reductant. The process rate has beenfound to be rst order with respect to the reductantsurface area (Barrera, 2000; Chia, 1994). Thus, in-creasing the surface area in the system is assumed toenhance the process reactivity. The actual amount of steel scrap dissolved was also compared to the stoi-chiometric amount of steel scrap required for iron re-moval. The iron removal percent increased as thesurface area went from 13.7 to 19.2 cm 2/ml, and thestoichiometry number decreased by about 30%. The beststoichiometry number achieved using iron was about 4compared with 2 when zinc was used as reductant(Chang, 1997). This may be due either to easier directdissolution of iron in the aqueous solution due to alower hydrogen evolution over-potential or to transportconsiderations involving iron ions in the reaction layer.

The effect of reaction time on iron removal and sto-ichiometry number is shown in Fig. 1. Longer reactiontimes not only increased the iron removal percentage butalso lowered the stoichiometry number. A comparisonof SX11 to SX12 diluents was made to determine theeffect on stoichiometry number and iron removal andthe results are shown in Fig. 2. The SX11 gave about a30% increase in reaction rate but gave a very highstoichiometry number. Even though the SX12 gave

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somewhat lower iron removal, the stoichiometry num-ber was decreased from 8 to 4. One possible explanationmay be due to differences in composition. SX12 iscomposed of 22.2% aromatics, 41.6% naphthenes, and36.3% paraffins while SX11 contains 100% iso-paraffins.Even though the exact cause of these variations is notclearly understood, it is obvious that the diluent may bean important variable in determining the efficiency of

the galvanic stripping process and have a role in theelectrochemical reaction.

4.2. Effect of TBP modier with both simultaneous and separate stripping

Using SX11 as a diluent gave relatively fast kineticsbut a higher stoichiometry number with the simulta-neous galvanic stripping mode. The effect of adding 4%TBP as a modier to the DEHPA-SX-11 organic systemon iron removal and efficiency was examined using bothsimultaneous and separate stripping. The results areshown in Figs. 3 and 4, respectively. Adding TBP causeda slight increase in the initial ferric ion reduction rate forsimultaneous stripping. The improvement in the stoi-chiometry number was substantial, decreasing fromabout 7 to 4 for a 5-min reaction time. Excess reactiontime will only result in reductant dissolution.

For separate stripping using SX11 there was an in-crease in iron removal from 50% to nearly 80% as seenin Fig. 4. The stoichiometry number also decreasedsignicantly, going from about 3 to 2.

4.3. Design of experiments separate stripping

The design of experiments method was used to studythe separate galvanic stripping mode for iron removal.Previous studies showed that certain operating parame-ters, such as reductant type, reductant surface area,water content, diluent, organic concentration, and ironion concentration could considerably affect the iron re-

moval and process efficiency of the separate galvanic

Fig. 3. Effect of modier TBP on the iron removal and stoichiometrynumber with simultaneous stripping. Organic phase: 3.3 g/l Fe 3 loa-ded in 15% DEPHA and 4% TBP with SX11 or loaded in 15% DE-PHA with SX11; Aqueous phase: sulfuric acid solution with initial pHof 0.7; Reductant: steel scrap with a surface area of 19.2 cm 2=ml; 5-minreaction time; 55 C; A/O of 2; Agitation at medium setting.

Fig. 1. Effect of reaction time on iron removal and stoichiometrynumber with simultaneous stripping. Organic phase: 3.3 g/l Fe 3 loa-ded in 15% DEPHA with SX12; Aqueous phase: sulfuric acid solutionwith initial pH of 0.7; Reductant: steel scrap with a surface area of 19.2 cm 2/ml; 55 C; A/O of 3; Agitation at medium setting.

Fig. 2. Effect of diluents of SX11 and SX12 on the iron removal andstoichiometry number with simultaneous stripping. Organic phase:3.3 g/l Fe 3 loaded in 15% DEPHA with SX11 or SX12; Aqueous

phase: sulfuric acid solution with initial pH of 0.7; Reductant: steelscrap with a surface area of 19.2 cm 2 /ml; 10-min reaction time; 55 C;A/O of 2; Agitation at medium setting.


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stripping process. Generally the effect of process pa-rameters has been studied by changing one parameter(factor) at a time while keeping other parameters con-stant. This approach, however, is not necessarily the bestway to optimize the process, and in some cases it mayeven give misleading implications, depending on the setsof parameters chosen. In particular, such testing pro-vides no information about what happens when thefactors are varied simultaneously, i.e. the interactionsbetween factors are not identied. The method also re-quires an unnecessarily high number of experiments.Experimental design provides an organized approach foridentifying the optimum conditions for iron removal andgives a better estimate of the variability and noise of thesystem, thus increasing the reliability of the results.When combined with thorough statistical analysis andmodeling, experimental results can be correlated thusyielding more useful and precise information on theprocess.

The DESIGN-EASE version 6, Stat-Ease was utilized

to set up a three factor, two level factorial test (2 3 de-sign). Two replicates were made to allow an analysis of variance or estimated error. For all combinations of thefactors, a total of 16 experiments is needed for the 2 testsets. The three factors evaluated were reaction time ( A),ferric ion concentration in the loaded organic ( B ) andthe reductant surface area ( C ). It is a 2 2 2 factorialexperiment with two observations in a completely ran-domized manner. The mathematical model is

Y l Ai B j ABij C k AC ik BC jk ABC ijk e mijk

where Y is the response value, l represents an overallmodel parameter, Ai represents the time effect in thisproblem, B j and C k the iron concentration and reductantsurface area, respectively, ABij , AC ik , BC jk , and ABC ijk the interactions, and e the random error.

A general rule is to set the levels as far apart aspossible so an effect will be more likely to occur, but theoperating boundaries must not be exceeded. The highand low values of the factor levels chosen for thisanalysis are based on prescreening tests and conditionsappropriate to possible industrial practice. The re-sponses are iron removal (%), the stoichiometry number(a ratio of actual to theoretical amount of iron re-quired), and reduction rate which was dened as theamount of iron reduced per minute by a square centi-meter of reductant surface area. The factors, with lowand high levels, are shown in Table 1. In Table 2 thevalues are given for the other parameters which werekept constant for all experiments. The experimentalfactor data and corresponding experimental results areshown in Table 3.

4.3.1. Iron removal The half-normal plot for iron removal, shownin Fig. 5,

is based on the positive half of the full normal curve andindicates that the percentage of Fe(III) stripped isstrongly dependent on the reaction time and the reduc-tant surface areas. These two variables cause signicantdeparture from the straight-line relationship in the half-normal probability plot. The analysis of the variance forthe iron removal is given in Table 4. The Model F -value

of 231.48 implies the model is signicant. There is only a0.01% chance that a Model F -value this large could

Table 1Factors levels for the experiment

Factor Factor level

Low High

A Time (min) 10 20B Fe 3 concentration (g/l) 3.2 5.6C Surface area cm 2=ml 4.8 7.7

Table 2Experimental conditions

Parameter Setting

Loaded organic Fe 3 + 16.5 vol% D2EHPA+ 83.5 vol% SX-11

Reductant Scrap steel chunks(about 0 :6 0:6 0:7 cm3)

Deionized water 0.8 vol%Temperature 55 CAgitation Medium settingInitial pH of strippingsolution

0.7

Nitrogen Sparging the system for 10 minbefore reduction and stripping

Fig. 4. Effect of modier TBP on the iron removal and stoichiometrynumber with separate stripping. Organic phase: 3.3 g/l Fe 3 loaded in15% DEPHA and 4% TBP with SX11 or loaded in 15% DEPHA withSX11; Steel scrap with a surface area of 7.68 cm 2 /ml; 0.8% deionizedwater; 10-min reduction; 55 C; Agitation at medium setting; Strippingsolution: sulfuric acid solution with initial pH of 0.7; stripping time of 10 min.


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occur due to noise. The main effects of the variables werecalculated at the 95% condence level. The model for the% iron removal in terms of coded form is

% Iron removal 52 16:25 A 11 :00 C

The results show that longer reaction time and highreductant surface area are favored for increasing thepercent of iron removal. In this design of experiments,two levels of iron concentrations of 3.2 and 5.6 g/l wereselected mainly on anticipated industrial practice andrelative iron removal or % Fe was about the same inboth cases, however the total amount of iron removed isgreater when the initial iron content was higher.

4.3.2. Stoichiometry numberThe statistical analysis results for the stoichiometry

number is given in Table 5. The Model F -value of 36.24implies the model is signicant. There is only a 0.01%chance that a Model F -value this large could occurdue to noise. The half-normal plot shows the effect of the individual variables and interactions on the stoichi-ometry number and indicated that almost all the indi-vidual variables and interactions have a signicantimpact on the stoichiometry number, particularly theiron concentration and reaction time. Experimentalobservations showed that unproductive reactions suchas hydrogen reduction are favored over ferric iron re-duction initially. With higher ferric ion concentration inthe organic phase there is a shorter induction time,

Table 3Experimental results

Std Pattern Factor Response

Time, A(min)

Fe concentration, B (g/l)

Surface area, C cm 2=ml

Fe removal, Y 1(%)

Stoichiometrynumber, Y 2

Rate, Y 3g=cm 2=min E ) 05

1 10 3.2 4.8 23 4.1 1.52 10 3.2 4.8 22 4.7 1.53 20 3.2 4.8 62 2.4 2.14 20 3.2 4.8 56 2.6 1.95 10 5.6 4.8 27 2.7 3.16 10 5.6 4.8 29 2.5 3.37 20 5.6 4.8 54 1.7 3.18 20 5.6 4.8 55 2.0 3.29 10 3.2 7.7 51 2.9 2.1

10 10 3.2 7.7 48 2.8 2.011 20 3.2 7.7 82 2.4 1.712 20 3.2 7.7 83 2.4 1.713 10 5.6 7.7 46 2.3 3.414 10 5.6 7.7 40 2.5 2.915 20 5.6 7.7 76 1.8 2.7

16 20 5.6 7.7 78 1.7 2.8

Fig. 5. Half-normal plot of effects for iron removal. The letters A andC represent reaction time and reductant surface area, respectively.

Table 4Analysis of the variance for iron removal

Source Sum of squares d.o.f. Mean square F -value Prob > F

Model 6161 2 3080.5 231.48

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therefore less iron dissolution and a smaller stoichio-metry number. Here the induction time refers to theinitial period where relatively no ferric ion has been re-duced and only hydrogen evolution has taken place. Thecube plot in Fig. 6 of the predicted response shows thatthe high levels of reaction time, iron concentration, andsurface area gave the most efficient or lowest stoichi-ometry number of 1.75. The predicted stoichiometrynumber for the coded factors is

Stoichiometry number

2:59 0:47 A 0:44 B 0:24 C 0:12 A B

0:19 A C 0:17 B C 0:17 A B C

Compared with the best stoichiometry number of 4achieved with simultaneous stripping, separate strippinghas an obvious operating advantage with respect to

control since reduction and stripping can be operatedindependently. In this mode, direct metal reductantdissolution in the aqueous phase is avoided, thus pro-ducing a better stoichiometry number.

4.3.3. Reduction rateThe analysis of variance for reduction rate is shown

in Table 6. Values of Prob> F less than 0.05 indicatemodel terms are signicant. The main effect of the ironconcentration shows the greatest impact on the ironreduction rate. In order to further understand the effectof ferric ion concentration on iron removal and stoi-chiometry number, other experiments were conductedover a larger iron concentration range. Ferric ion con-centrations of 0.44, 1.07, 3.2, and 5.6 g/l were used forthe experiments. After 10-min reaction time, about 50%iron removal was observed for organic loadings of 3.2and 5.6 g/l iron, and 26% iron removal from 1.07 g/l ironorganic solution. When the initial organic containedonly 0.44 g/l iron there was actually an increase in theiron content in the organic phase. These results indi-cated a type of equilibrium exists that must be estab-

lished before reduction occurs. One possible way toincrease the iron reduction rate from low iron concen-tration organic solutions may be to change the DEHPAconcentration in the organic phase and thus establish anew equilibrium. The interactions of AC and BC are alsosignicant factors based on the analysis of variance. Theinteraction between reaction time and reductant surfacearea indicates that two factors need to be compromised,i.e., short reaction time with more reductant or longerreaction time with less reductant lead to a faster re-duction rate. The best result was always obtained whenferric ion concentration was at its high level. The nalequation in terms of coded factors:

Rate 2:44 0:64 B 0:14 A C 0:097 B C

4.3.4. Assessment for the model For statistical purposes it is assumed that residuals are

normally distributed and independent with constantvariance. A residual is the difference between the pre-dicted value and the actual (observed) value. Two plotsof normal plot of residuals and residuals versus predictedlevel were made to check the statistical assumptions.

Fig. 7 shows the normal plot of residuals for theiron removal. The residuals are normally distributed,

Fig. 6. Cube plot of the predicted response for the stoichiometrynumber. The letters of A, B , and C represent reaction time, ironconcentration, and reductant surface area, respectively.

Table 5Analysis of the variance for stoichiometry number


Model 9.35 7 1.34 36.24 < 0.0001A 3.52 1 3.52 95.34 < 0.0001B 3.15 1 3.15 85.44 < 0.0001C 0.95 1 0.95 25.78 0.0010 A B 0.23 1 0.23 6.12 0.0385 B C 0.46 1 0.46 12.36 0.0079 A C 0.60 1 0.60 16.29 0.0038 A B C 0.46 1 0.46 12.36 0.0079Pure error 0.30 8 0.037Cor total 9.65 15


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since they give a linear plot. The deviations from lin-earity are very minor, so it supports the assumption of normality.

The residuals versus predicted response plot foriron removal is shown in Fig. 8. Ideally, the verticalspread of data will be approximately the same fromleft to right. From the Fig. 8 there is no denite in-

crease in residuals with predicted level, which supportsthe underlying statistical assumption of constantvariance.

5. Conclusions

The results obtained in this study show that steelscrap can be an effective reductant for ferric ion removalfrom DEHPA using the galvanic stripping process.Either the simultaneous or separate stripping modeswere effective. Steel scrap may be benecial in someapplications not only for economic reasons, but also forproducing a purer nal iron product.

The reductant surface area is a very important pa-rameter in determining the process efficiency. Thegreater the surface area, the higher the iron strippingpercent for a given reaction time. Thus, increasing thesurface area in the system provides a major means of increasing the rate of the process.

The selection of diluent may also be an important

variable in determining the efficiency of the galvanicstripping process. The results of this study indicated thatSX11 as a diluent was favored for faster kinetics butSX12 gave the better stoichiometry numbers.

Adding 4% TBP to the DEHPA-SX11 mixture in-creases the initial ferric ion reduction rate for both si-multaneous and separate stripping and signicantlylowers the stoichiometry number for simultaneousstripping. The reason for this effect is currently un-known, further studies are needed to understand themechanism of modiers such as TBP in altering theelectrochemical reactions.

In this work it was shown that by utilizing design of experiments the iron removal percentage, stoichiometrynumber, and ferric ion reduction rate could be de-scribed by statistical models. For the separate strippingmode, the initial ferric ion concentration, reductantsurface area, and reaction time signicantly affected theprocess according to the model derived. High ironconcentration solutions always have high reductionrates, while longer reaction time and high iron con-centration gave the best stoichiometry numbers. Thebest stoichiometry number achieved in this study is1.75. Longer reaction time and high reductant surfacearea were favored for increasing the percentage of iron

Fig. 8. Residuals versus predicted iron removal.

Table 6Analysis of the variance for reduction rate


Model 6.93 3 2.31 83.68 < 0.0001B 6.49 1 6.49 234.94 < 0.0001 A C 0.29 1 0.29 10.65 0.0068 B C 0.15 1 0.15 5.44 0.038Residual 0.33 12 0.028Cor total 7.27 15

Fig. 7. Normal plot of residuals for % iron removal.


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removal. The highest iron removal attained in thisstudy was in the range of 7582% for reaction timesbetween 10 and 20 min.

In general, the process has been demonstrated tobe reasonably effective in removing dissolved ironfrom a variety of waste streams. One of the moreimportant features of galvanic stripping is that theiron can be recovered in a relatively pure and saleableform.

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