Wd. Sci. Tech. Vol. 25, No. 10pp. 5-18.1592. Rioted in Great Britain. All rights reserved.

OZ73-1223i92Sl5~ pyright Q 1992 IAWPRC

METAL RECOVERY FROM WASTEWATER USING MEMBRANES

A. G. Fane, A. R. Awang, M. Bolko, R. Macoun, R. Schofield, Y. R. Shen and F. Zha Centre for Membrane and Separation Technology, University of New South Wales, P.O. Box I , Kensington, Australia 2033

ABSTRACT

This paper outlines the requirements for metal recovery from wastewater, with particular reference to electroplating. The technical features of three alternative membrane processes arc described. Nanofiltration is shown to separate ionic species on the basis of coulombic interactions or hydrated ion size, which leads to either a ’charge’ pattcm or a ’hydration’ paaem of rejection. These rejection pattems provide ion selectivity. Ultrafiltration 0. coupled with ion-complexing polymers or ion exchange resin, also provides efficient removal of metal ions, at high flux. The effectiveness of the UF-resin process is considerably increased as the resin size is decreased. Solvent extraction in a Liquid Membrane Contactor (LMC), which is based on microporous hollow fibre modules with aqueous and organic phases circulating through the shell or fibre lumens, achieves high fluxes of metal ions. A limitation of the LMC is the need to avoid phase leakage. The factors goveming the critical displacement pressure and the effective transmembrane pressure are discussed. An LMC with a high packing density of fibres in the shell is preferred. Fmally, the paper discusses critcria for selection of ’user friendly’ technologies for the electroplating industry. The membme technologies, particularly in combined form, score highly except in t e m of simplicity. This aspect needs I

I further development.

KEYWORDS

Wastewatec plating wastes; membranes; nanofiltration; ultrafiltration; liquid membrane contactors.

INTRODUCTION

Metal-containing wastewam come from a variety of industrial processes, including the mining, mineral processing and metal-finishing industries. ’Ihis paper is concerned with the electso-plating industry, although the techniques discussed have more general application.

The electro-plating industry is characterised by urban location and a proliferation of small operators. This has lead to the discharge of metals, such as chromium, nickel, cadmium, zinc, copper etc to the sewer and

faced with increasingly stringent discharge standards and in many cases significant discharge costs. For the common heavy metals the trend is towards concentrations of 1-2 mgflitrt for sewer discharge and 0.1 to 0.5 mgflitrt for open water discharge.

I 1 consequent contamination of sewage sludge and/or heavy metals pollution of the ocean. The industry is now i

i

5

I

6 A. G . FANE et aL

The simple traditional treatment method of alkali precipitation and gravity settling has a limited future since it can only achieve a water quality of about 10 mgflitre heavy metal and produces a solid sludge that is becoming difficult to dispose of. Thus the future of the industry depends on altemative methods. Some of the p n f e d characteristics are:

(i) (5) (iii) (iv)

Membrane technology has already bccn used in the electroplating industry in the form of reverse osmosis [RO] (Crampton and Whouth , 1982). The specific attractions of RO are the high quality filtrate, suitable for discharge or xuse, the lack of chemical addition and the compact nature of the plant The disadvantages of this technology are that it can be costly, that it is sensitive to misuse or lack of careful control and that it does not provide much degree of selectivity between ionic components. It is the purpose of this paper to assess the potential benefits and limitations of 3 altemative membrane processes, Nanofilmtion 0, Ultrafiiiration 0 coupled with complexation or sorption, and the Liquid Membrane Contactor (LMC).

Plating Solutions. Wastes and Strateeies

The plating process involves a plating bath with high-strength solutions and one or more rinse tanks that may contain the plating chemicals in diluted form. Table 1 presents analyses of actual bath and rinse solutions obtained from operations in the Sydney region. Both bath and rinse solutions also contain proprietary additives that serve as brighteners, levellers and fume suppressants, and include compounds such as sodium lauryl sulphate, aromatics (nickel baths), citric acid, tartaric acid, oxalic acid and saccharides (chromium baths). Comparison of the analyses in Table 1 with the typical fresh bath plating solutions shows that contaminant species have entered the baths and rinse waters.

Solutions to the plating waste problem include treatment and minimisation. The treatment strategy involves removal or neutralisation of pollutants before discharge. Minimisation involves recovery and recycle of the plating chemicals and the rinse water so that the volume treated and discharged is minimised. Figure 1 depicts an idealised minimisation strategy that could require 3 separation steps, depending on the extent and nature of the contaminants.

the ability to recover metal values; the ability to produce water for reuse; the need for few additional chemicals; ease and reliability of operation.

Plating chemicals for recycle

Contaminants (discharge)

_3 Rinsewater for recycle

Contaminants (discharge)

Figure 1. Idea l id “isation strategy [1,2,3 = separatior

The separations that may be necessary for chrome and nickel plating arc summarised in Table 2. Thus a suitable scheme for chrome plating would separate the divalent anions from the monovalent anions and the multivalent cations. For nickel plating the requirement is separation of multivalent anions from the monovalent. Membrane processes or membrane-hybrid processes that provide ion selectivity could be particularly attractive for these situations.

TABLE 1. Analyses of a Selection of Actual Baths and Their First Rinses (Sydney Metropolitan Area)

SOLUTION

chrome M s e (BC) 7110 105

nickel bath 0.4 8oooO nickel rinse c 0.1 800

copper bath 0 0.2 4.5

/I copper rinse (H) I c 0.1 I < 0.1 -

cu 1370

330

-

2180

215

0.2

< 0.1

-

’ 68000 ~ 8.9 1_

820 585 60

160 56 2.1

N.D. 0.6 0.9

N.D. < 0.1 < 0.1

- N.D. 1 0.7 1 :::

’ N.D. < 0.1

Notes: Chromium typically exists as the anion: [CIO,”] Other metals, unless otherwise indicated, predominantly exist in the divalent form.

’ Estimate, due to poor reproducibility of analysis. N.D.: Not done - not analysed.

-1 3385 29880 high’

34 1.7 20 s

2 (Sl, BC, H): origin code.

I

A. G. FANE et aL 8

TABLE 2. Reauired Species and Contaminants for Chrome and Nickel Plating

I Required Species Contaminants Plating

Chrome m,", so," CT, NO;, Mw, Mw

I CT, NO; Nickel I BO3* , SO,%, N:'

N ANOFILTRATION

Nanofiltration (NF) is a pressure-driven membrane process that lies between conventional RO and UF. NF membranes demonstrate a significant variation in the rejection of inorganic ions, as illustrated in Table 3 for 8 range of commercial membranes. NF membranes typically have very small pores, 1 to 4 nm diameter, and may possess residual charge. Rejection of ions can be achieved by one or more mechanisms, namely charge (coulombic) interaction, hydrated-ion size exclusion or dielectric interaction. We have modelled these mechanisms in earlier work (Macoun et al., 1991) and typical results are shown in Figure 2. This figure shows the energy required to bring an ion from the bulk solution to the centre of the pore mouth, as a function of pore size. The ion was assumed to be K+ (in lO"M Kc1) and the membrane was assumed to have a charge of zeta potential of +130 mV and a dielectric constant of 3. As pore size decreases all of the energy requirements increase. This simple analysis suggests that the charge and hydration mechanisms tend to dominate, with dielectric effects playing a minor role. This results in two major patterns of rejection, charge and hydration.

DIELECTRIC II HYDRATION o TOTAL

- 9 - a -7 - 1 0

Log pore radius (LogO"

Fig 2. Energy rtquircd to bring R ion from bulk to pore of given radius, showing coulombic dielaCtric and hydration interactions.

m o ' UTC-20HF

UTC-60

sc-L100

NTR-7410 ' NTR-7450

CXM'

I PSA'

Note: Rejection 1 Cadone et a1 and Grieves 19

The charge pati as the membra particularly if t fact reduce the (monovalent). significant witl

The hydration hydrated size t ions increases, of watcr-struct form hydrated

Inspection of ' rejection. ha cases t ha t is 1

The exception of theNTRn The hydration the well hydra our data (Ma, plotted VQSUS

effm with a

Metal recovery from wastewater 9

3 1 UF. NF able 3 for neter, and :ly charge Ued these his figure mth, as a

; all of the iisms tend rejection,

jsumed to

TABLE 3. Reported salt reiections of various NF membranes

Membrane

NF70

xP45

m 1 UTC-2oHF *

UTC-60

sc-LIOO

NTR-7410 ' NTR-7450

CXM'

PSA '

NaQ NaSO, MgQ, MgSO4

75 70 97.5

50 83 97.5

20 85

66 99.7 99.4 99.7

85 . 99.9 85 99.8

75 97 90 98

15 55 4 9

51 92 13 32

86.6 95.6 91.1

17 95 39 92

Note: Rejection (%) = (1-(permeate concentrationlfeed concentration)) x 100 1 Cadotte et at 1988,2; Nakagawa et a1 1988,3; Ikeda et a1 1988.4; Breslau et a1 1979.5; Battacharyya and Grieves 1977.

The charge pattem of rejection is characterised by the behaviour of co-ions. i.e. those with the same charge as the membrane. If rejection increases with charge of the co-ion it could indicate the charge pattem, particularly if the rejection does not increase with the charge of the counter ion. The counter ions can in fact reduce the rejection if they have siffnifcant charge (mdtivalency) and the co-ions have low charge (monovalent). Coulombic interactions are lessened as electrolyte concentration increases, and will be more significant with dilute solutions, such as rinse waters.

The hydration pattem of rejection is characterised by the rejection of large hydrated ions, the larger the hydrated size the more strongly it will be rejected. The hydration effect is reduced as the concentration of ions increases, again favouring application to dilute solutions. Hydration is also nduced by the presence of water-structure breakers, such as NO;, but increased by structure-forming ions, such as Na' and CT that form hydrated ion pairs.

Inspection of Table 3 shows that UTC-60, NTR-7410, NTR-7450 and PSA display the charge pattem of rejection. In all cases there is a higher rejection with the divalent d o n no matter what the cation. In most cases there is no increase in rejection for the divalent cation over the monovalent cation for the same anion. The exception is MgcI, for PSA which may indicate the presence of some hydration effects. In the case of the NTR membranes the influence of the divalent cation is seen as it reduces the rejection of the salt. The hydration mechanism is displayed by the UTC-2OHF and SC-L100 membranes, where the rejection of the well hydrated ions (SO,", M e ) is large whether they are paired with another well hydrated ion or not. Our data (Macoun et al. 1991) in Figure 3 show rejection of anions and cations with an NF40 membrane plotted versus hydrated ion size. This indicates the hydration pattem, although it also shows some charge effects with a larger rejection of the anions for a given size. clearly charge and hydration effects can interact.

A. G. FANE et aL

REJ.

"1

A pno size ar develo patterr

0 1 . I " 1 . 1 . '

0.30 0.32 0.34 0.28

HYDRATED RADIUS (nm)

Fig. 3. The effect of hydrated ion radius on ion rejection for NF-40 membranes (1350 Wa for 1 mM solutions. 0 = Anion radius. = Cation radius)

-20 4 I I

3000 1000 2000

R-Zn+P ---#-- R-Na+

R-CI-

I PRESSURE (@a)

Note

An ar is shc to nc con& 45 VI chw at hi€ of ~

i Metal m v a y from wastewater 11

A priori prediction of ion selectivity with NF is difficult and quires detailed knowledge of membrane pore size and surface charge as well as concentrations of the individual ions in the solution. However we have developed rules of rejection which act as a guide; these are listed in Table 4 for charge and hydration patterns.

TABLE 4. Rules of Ion Reiection for NF Membranes

Mechanism Rule I I

1 Charge c1 Rejection is dominated by the co-ion (’) c2 c 3 c4 Increased concentration reduces rejection c5

Increased charge of co-ion incrtases rejection Increased charge of counter-ion reduces rejection. or no effect

Mixed feed solutions; monovalent co-ion paired with highest valency counter-ion will pass preferentially through the membrane

II Hydration H1 Rejection increases with hydrated ion size

H2 H3 H4

Hydrated ion pairs increase rejection Structure breaking ions reduce rejection Strongly hydrated membranes increase rejection

I Note (1) Co-ion has same sign of charge as the membrane.

An application of the Filmtec XP-20 membrane to a diluted chrome plating bath solution (ea Id ppm CrO,’) is shown in Figure 4. This membrane gave reasonable rejection of the divalent 00,’ and SO,”, with 11 to negative rejection of the monavalent (3, as well as low rejection of monovalent Na’. Under some conditions, the contaminant divalent cations also leaked to the permeate. The fluxes achieved were high (ca 45 Vm’ hr at 500 P a and 160 Vm’ hr at 2000 Pa). The rejection behaviour showed characteristics of charge pattem (multivalent counter ions pass) at low fluxes, but became more typical of the hydration pattem at higher flux. This example illustrates a potentially beneficial selectivity using NF. Practical application of this membrane probably requires a slightly higher level of rejection of divalent anions.

ULTRAFILTRATION

Although UF membranes have signifcant pore sizes, typically 5 to 20 nm diameter. they can be used to retain inorganic ions if the ions are complexed with macromolecules or sorbed onto particulate resins. The application of complexation UF has been studied by many workers and reviewed by Chaufer and Deratani (1988). In their work they described the use of water-soluble polymers, such as polyacrylic acid and polyethylenimine, to recover metals such as copper b m dilute solution.

Two procedures an possible, diafilhtion or concentration. Diafiltration involves a fmed inventory of complexing agent in solution with continuous replenishment of filtrate by fresh feed. After time the complexing polymer becomes loaded and metal ions appear in the filtrate. If a large excess of complexing agent is used there should be little loss of metal ions to the filtrate. The ’concentration’ method involves a premixed volume of metal-polymer complex which is batch concentrated by a retentive membrane. For

// / I I

both diafiitration and concentration procedures the metal can be recovend by pH shift or electrolysis.

We have examined the use of a low cost complexing agent, alginic acid (molecular wt 240 k Daltons). Figure 5 shows our rejection results for the UF of 0.25 mM (16 ppm) copper solutions with and without the alginic acid present using a polysulphone UF membrane (Millipore PTGC molecular wt cut off 10 kD).

12 A. G. FANE e( aL

The copper with no alginic acid shows some rejection at low pH probably because of the residual charge on the membrane; the rejection mechanism is similar to charge pattern NF. As the pH is increased the copper is hydrolysed and forms colloidal particles that subsequently coagulate to form relatively insoluble hydrous copper oxide precipitate which is rejected by the membrane. In the presence of the macromolecular alginic acid high rejections, close to 100%. have been observed for pH values greater than 4 as copper ions are bound to the alginic acid. At low pH, rejection falls although it is still quite high. This is possible due to an alginic acid precipitation forming a gel at low pH which hinders the passage of the copper ion through the membrane even though it is not bound' to the alginic acid. The copper can be released from the alginic acid by lowering the pH to about 2.0. This leads to about 80% recovery of the copper in one batch cycle and an alginic acid solution that can be reused.

80 - - c z 60- :

40 - -J w U

20 - / o ! . 7 . I . I . 1 . I -

0 2 4 6 8 10 1 2

PH

Figure 5. Rejection of copper with and without alginic acid

A difficulty reported by Chaufer and Deratami (1988) and observed in our work is that fouling can occur due to polymer precipitation caused by solubility limits at the membrane surface.

An alternative process that has received less attention involves the use of ion exchange resin coupled with UF or MF. This approach is being used in the removal of nitrate from ground water (Exxchange 1991) and has been reported for the scavenging of metal ions (C lppm) from water (Grinstead and Paalman (1989)). In principle the method is similar to the complexation - UF method described above. It can be used in the diafiiltration or concentration mode. The extent of metal uptake will depend, in part, on the ion-resin equilibria, which means that low metal concentrations in solution will only result in partial loading of the resin. The other important factor is the kinetics of ion diffusion and uptake.

Experiments have been performed with a cation exchange resin. Amberlite IRC 71 8, and a polysulphone UF membrane (Millipore ITGC). A solution of ca 300 ppm copper was diafiltered into a vessel containing a 5 wt % slurry of resin. The effectiveness of copper nmoval was indicated by rejections of 98 to 100%. The recovery in the washing phase was >95% for a wash volume less than half the initial volume processed. For a lower copper concentration in the feed (say 30 ppm) the ratio of wash volume to feed would be about 1/20. This experiment was repeated with resins of different size, produced by grinding and sieving. Figure 6 shows details of copper rejection profiles for size ranges (178 - 231 pm). (75 - 178), (75 - 0). There was a clear advantage in the use of the smallest sized resin. This could have been due to the increased surface/volume ratio of the resin and/or the increased residence time in the slurry vessel caused by the lower flux. The combined UF-ion exchange process clearly suits significantly smaller resin beads than required in an ion-exchange column, as mognised by Grinstead and Hunter Paalman (1989). The optimum resin size will be a compromise between attainable flux and solute removal efficiency.

In princip makes the are most :

L1

Liquid m chemistry (organic) by Babco (ii) low in

A signific capillary : Critical di

where y i

the n o m incgular 1

where r, I

Membran practice r, n"!d about 40( POlyprOP)

An dtcm; This invo

13 Metal recovery from wastewater

uge the ible

In principle these hybrid UF processes offer the advantages of selectivity and high flux. The selectivity makes the processes applicable to removal of contaminants from recycle streams or discharge streams. They are most suited to batch operation.

ular

due

inic icle

ons i

ugh

cur

qith md 3)). the :sin the

UF g a h e ed. out ure V a S

sed xer red .ize

100

99 E P + 98 s 3

z

K

97

SIZE ( P m)

0 75-178

200 300 400 500 E

PERMEATE VOLUME ( ml)

Fig. 6. Profiles of copper rejection for Amberlite. ion exchange resin and UF membrane. UF fluxes shown ( ) in Vm’ hr.

LIQUID MEMBRANE CONTACTOR

Liquid membranes provide an altemative to solvent extraction, and can be based on similar extractant chemistry. Supported liquid membranes (SLM’s) make use of a microporous substrate to immobilise a thin (organic) liquid layer within the membrane wall. This technique has been evaluated for metal-ion transport by Babcock et al (1986). The intrinsic advantages of the SLM include (i) low capital and operating costs, (ii) low inventory of extractants, (iii) the possibility of separating and concentrating metals in a single device.

A significant limitation of the SLM is the membrane lifetime and stability. The membrane is held by capillary forces in the pores of the substrate, and can be displaced once the pressure differential exceeds the critical displacement pressure, P,. For straight, cylindrical pores the Laplace-Young equation applies, i.e.

P, = 2 y Cos 8 I r (1)

where -y is interfacial tension, 8 is contact angle and r is the capillary radius. For typical substrates with irregular tortuous pores, charackrised by a structure angle (K, the maximum angle of the pore relative to the normal direction) we have shown (Zha 1991) that the displacement pressure is given by:

P, = 2 y Cos (e + a,,,) / r,, (2)

where r, is the hydraulic radius and allows for nonckular pores.

Membranes with larger values of P, will be more stable, and this is favoured by small r, and small %. In practice r,, is in the preferred range 0.02 to 0.5 pm, to avoid hindered diffusion. The values of 4. we have measured vary from about 20 to > 60“. For the system kerosene-water the measured P, values range from about 400 kPa for a polypropylene Celgard membrane (pores 0.075 x 0.25 pm) to about 180 kPa for a polypropylene Accurel membrane (nominal pore diameter 0.2 pm).

An altemative and potentially more robust approach is the Liquid Membrane Contactor (LMC) (Kim, 1984). This involves, for example, a hollow-fibre module with organic flowing in the shell and aqueous phase in

14 A. G. FANE et aL

the lumen (or vice versa). Two modules can be used, one for extraction, the other for stripping, with organic phase cycling between the modules. The application of LMCs to metal ion extraction has been analysed and modelIed by de Haan et al (1989).

We have tested a small LMC for the extraction of copper from water. The LMC had 280 polypropylene hollow fibres (i.d. = 0.3mm, 0.d = 0.67mm, length 250") in a shell 15.4" diameter, giving a packing density of about 53%. A 1 litre solution of 0.01 M CuSO, was circulated through the lumens and an extractant (10% LIX 984 in kerosene) was circulated through the shell in cocurrent flow. Approximately 40% of the copper was removed in 60 minutes of batch extraction, which is equivalent to a copper flux of about 2 glmz hr.

It was observed that under some conditions liquid would transfer across the membrane; a similar problem was encountered by de Haan et al(1989). The reason for this is that the capillary pressure equations (1) and (2) stiU apply to the LMC. If the critical displacement pressure is exceeded organic will flow into the aqueous phase, or vice versa. Thus it is important to avoid high transmembrane pressures and/or develop membranes with a high critical displacement pressure 6.e. h e surface pores, coatings, etc). The transmembrane pressure will be determined by the phase flow rates, the number of fibres (which determines the packing density in the shell) and the length of the module. Figure 7 shows experimental and calculated LIP, versus phase flow rates for various packing densities in co- and counter-cumnt flow. The calculated data are based on a module with similar shell and fibre dimensions to that used in our work, applying the laminar flow equation:

AP = K p 1 u/a2 (3)

For the lumen of the fibres K = 32, d, = d, (i.e. the Hagen-Poiseuille equation applies), and for the shell d, = the hydraulic diameter of the available flow area and K varies with packing density (Shah and London, 1978). The experimental data for our module (fibres giving 53% packing density) are close to the predicted values. Calculations for a 20% packing density (106 fibres) suggest a much higher Om, due to the much larger lumen pressure drop. For a 75% packing density (396 fibres) the bp, should drop substantially. However, for counter-current flow the 75% packing density would give a higher dpnn than the 53% density in co-current flow.

1; 20

lob

0 0 5

Phase Flowrata ( m l h )

Fig. 7. Transmembrane pressure drop versus phase flow rates for LMC (Copper-LIX-Kerosene system). Stable and unstable conditions shown for 53% packing density module.

Figure i became the mea One rea. reason c tension (Takeuc!

To sum particuk require module and all0

7

The nicl i

plating t b i 1 of conta

separatic b chrome J

to prod1 recyclab

Such co technolo NF is us permeatc

I

1. SepiWiltiC

Rom a prolifcral to estabb

(1) v (2) s (3) c

Metal recovay from wastewater 15

Figure 7 shows that at a flow rate of about 5 to 6 mVs. equivalent to a AP,,,, of 25 to 30 Pa, operation became ’unstable’. i.e. lumen phase (aqueous) started to leak into the shell. This N,,,, is much lower than the measured displacement pressure P, of 120 kPa for kerosene-water and a similar polypropylene membrane. One reason for this could be a greater proportion of larger pores (0.8 to 1.0 pm) in the LMC fibres. Another reason could be that the copper - LIX complex, which was present in the LMC tests. lowers both interfacial tension and contact angle (y and 8 in equations (1) or (2)). which would lower P,. It has been reported (Takeuchi and Nakano, 1989) that metal complexes have more surface-active properties.

th organic tlysed and

propylene a packing ns and an .oximately Kr flux of

,r problem iations (1) w into the )r develop tc). The fetennines calculated calculated ‘plying the

(3)

lr the shell !d London, : predicted

the much ostantially. 5% density

,mer-)

y module.

To summarise, the LMC is a potential means for removal of metals from plating rinse and waste waters, particularly as it introduces the selectivity of liquid extractants. However, contactor design and operation require opthisation in terms of membrane type (to maximise P, without compromising mass transfer). module packing density (preferably high to maximise shell-side mass transfer, but minimise lumen-side AP) and allowable phase flow rates.

f THE POTENTIAL FOR MEMBRANE APPLICATIONS

The niche for membrane separations is likely to be in the recycling of plating wastes. This involves separation of dilute rinse waters to produce a reusable rinse water and plating chemicals for return to the plating bath. Figure 1 indicates that this could involve up to 3 separation functions, depending on the degree of contamination encountered. The requirements could probably be achieved by combining 2 stages of separation of the rinse waters, Le. (i) selective concentration of the plating chemicals (i.e. CrO,”, SO,” for chrome plating) and the decontamination of the permeate to produce quality rinse water. or (E) deionisation to produce a reusable rinse water and then decontamination of the mixed ion concentrate to produce recyclable plating chemicals.

Such combinations may include only membrane processes or membrane processes combined with other technologies, such as ion exchange or precipitation. Figure 8 illustrates one option for chrome plating, where NF is used to recover the divalent cations, CrO,” and SO,” and a second process (S) is used to clean up the permeate. S could be complexation-UF, ion exchange-UF, LMC, ion exchange, electrodialysis etc.

MAKE-UP WATER RECYCLE

:-w

CHROME > BLEED

PLATING m -

.. 00;- so;-

CHEMICAL RECYCLE X- M”’

Figure 8. Water and Chemical Recycle for Chrome Plating Using Combined Membrane F’rocesses (S = UF + Or, LMC, IX etc)

From a practical viewpoint a major challenge is the development of “user friendly” systems for the proliferation of small operators with relatively unskilled personneL We have surveyed a range of operators to establish user ’criteria’, and these are given below, ranked in order of importance.

(1) (2) (3)

Water Quality - refers to the ability of the water to comply with discharge or reuse standards. Simplicity - includes ease of operation, training required, maintenance and operating stability. Compactness - smaller units score higher because of space limitations in many plating shops.

16

(4) (5) (6) (7) (8)

The economic factor is also important, and the cost of technology will have to be offset by savings in discharge costs, water costs and chemical costs. In Table 5 we have ranked the membrane processes discussed in this paper against the criteria. This table is of necessity subjective, but gives a useful focus for discussion. The membrane processes score favourably, particularly as a coupled system, except for the criterion of simplicity. This criterion is clearly important and should be emphasised in future research and development of membranes for plating waste treatment.

A. G. FANE et aL

Versatility - the adaptability of the technology to a variety of rinse waters. Products - production of by-pdicts or sludges for disposal score poorly. Feed - tolerance of the applied technology to changes in the feed concentrations. Energy - relative energy rtquirCments. Selectivity - specificity of the technology to the recovery of a particular species.

TABLE 5. Rankine. of Technoloeles aminst User Criteria

Criteria (order of importance) NF UF LMC Combined”’ Precipitationo’

Score (ma. 10) for various technolo&s

Water quality 7 7 6 9 2 Simplicity 5 5 4 4 8 Compactness 8 8 8 6 3 Versatility 8 7 6 9 7 Products 8 8 7 8 2 Feed 7 8 7 8 7 Energy 5 5 8 7 8

9 2 Selectivity 6 8 9

(1) (2)

Combined = combined membrane systems Precipitation = (traditional) chemical precipitation

CONCLUSIONS

The membrane technologies, Nanofdtmtion, Ulaafltration coupled with complexing polymers or ion exchange resins, and Liquid Membrane Contactors, provide practical means for the treatment of plating wastes to rtcovc~ metals and produce rinse watm for recycle.

Nanofilters separate ions on the basis of charge or hydration mechanisms, and with careful selection they could provide the means to concenmtc and fractionate in a single step. Ultrafiltration, coupled with complexation or sorption, provides good selectivity and high flux, but requires a batch process. Liquid Membrane Contactors have high selectivity, but need careful optimisation to avoid instability in the form of phase leakage.

The three membrane processes score favourably in terms of plating industry criteria, except for the aspect of simplicity, which needs further development.

ACKNOWLEDGEMENT

We gratefully acknowledge the support of the Urban Water Research Association of Australia, the Sydney Water Board, the Waste Management Authority (New South Wales) and the Australian Research Council. We also acknowledge the assistance of Ms Wendy Hoong Ang and helpful discussions with MI Michael Costello.

r

NO

d , = 4 = K = 1 - A P = AP,.,., =

-

P, = - r -

rh - U -

- -

RE

Babcock, coupled-tr.

Bhatsachar Sepurutior

Breslau, B technolog!

Cadotte, J the use of

Chaufer. I soluble m

Crampton - 80 (3), 21

de Haan. of the ma

Exxchang denitrific2

Grinsttad exchanger

Bada, K. osmosis r

B.h JMembrc

Mmun, WaratiOl

Nakagaw C h t a - alw

Metal recovery from wastewater 17

NOMENCLATURE

avings in processes focus for it for the x c h and

rs or ion af plating

:tion they pled with is. Liquid the form

h e aspect

ie Sydney I Council. r Michael

hydraulic diameter (flow channel) a, = porestructureangle inside diameter 0 = contact angle coefficient (eqn (3)) 7 = interfacialtension length of flow channel p = viscosity pressure loss down channel . transmembrane pressure drop critical displacement pressure

pore hydraulic radius velocity

pore radius

REFERENCES

Babcock, W.C., Brooke, J.W., Burback, V.A., Friesen. D.T. and Lorenz, D.A. (1986). Fundamentals of coupled-transport membranes. Final Report to U.S., DOE (DE-AC06-79ERO1047).

Bhattacharyya, D. and Grieves, R.B. (1977). Charged membrane ultrafiltration. Recent Developments in Separation Science, Vol III (Part B), Ed N.N. Li, CRC Press, 261-284.

Breslau, B.R., Testa, A.J., Milnes, B.A. and Medjonis, G. (1980). Advances in hollow fibre ultrafiltration technology. Ultr@ltration Membranes and Applications, Ed A.R. Cooper, Plenum Press, 109-127.

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