a novel recovery technology of trace precious metals from waste water by

8/6/2019 A Novel Recovery Technology of Trace Precious Metals From Waste Water By

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SciencePress Trans. Nonferrous Met. Soc.China 17(2007) 858-863TransactionsofNonferrous MetalsSociety ofChina

www.csu.edu.cdysxb/

A novel recovery technology of trace precious metals from waste water bycombining agglomeration and adsorptionZOU Hua-sheng(%%&), CHU Zhu-qiang(%%$Z), LIN Gang(# kl)

Schoo l of Chemical and Energy Engineering, South China U niversity of Technology, Guangzhou 5 10640, ChinaReceived 30 September 2006; accepted 20 April 2007

Abstract: A novel and efficient technology for separating and recovering precious metals from waste water containing traces of Pdand Ag was studied by the combination of agglomeration and adsorption. The recovery process and the impacts of operatingconditions such as pH value of waste water, adsorption time, additive quantity of the flocculant and adsorbent on the recoveryefficiency were studied experimentally. The results show that Freundlich isothermal equation is suitable for describing the behaviorof the recovery process, and the apparent first-order adsorption rate constant k at 25 "C is about 0.233 4 h-'. The optimumtechnology conditions during the recovery process are that pH value is 8-9; the volume ratio of flocculant to waste water is about1:(2 000-4 000); the mass ratio of adsorbent to waste water is 1:(30-40); and processing time is 2-4 h. Finally, the field tests weredone at the optimum technology conditions, which show that the total concentration of Pd and Ag in the waste water below 11 mg/Lcan be reduced to be less than 1 mg/L.Key words: trace precious metals; recovery; agglomeration; adsorption; waste water

1 IntroductionBecause of their special electric conductivity,precious metals are used increasingly in makingelectronic elements or equipment, and accordingly, thewaste water containing precious metals is drainedinevitably in the production of precious metal alloypowder. Recovering precious metals from the wastewater is not only beneficial to reducing cost, but also canreserve the resources of precious metals and protect theenvironment. Therefore, it's of great theoretical and

realistic significance to study the new and effectivemethod for recovering precious metals fiom the wastewater[ 11.Generally, there are several methods used for therecovery of precious metals from waste water, such asagglomeration, neutralization, electrolysis, oxidation-reduction, extraction, adsorption, ion-exchange,membrane separation, elution and electrodialysis[2-41.Each method has its own advantages and disadvantages,and may be effective in recovery of precious metals fiomthe common concentration of waste water, but it maybecome less efficient or even useless when trace preciousmetal is to be recovered from waste water[2, 51. Thus inthis study, a novel process was put forward to recover

precious metals (mainly Pd and Ag) from the waste watergenerated in the production of the precious metal alloypowder, and a kind of efficient flocculant was chosen byorthogonal tests.2 Experimental2.1 Main reagent and equipment

The waste water used was taken from PreciousMetal Alloy Powder Producing Company in GuangdongProvince, China. The waste water mainly containedprecious metals Pd and Ag with total content of 2-11mg/L and had a very small amount of impurity ions suchas sodium ions, calcium ions, and fermm ions. Precedingexperiments show that the influence of these impurityions on the recovery efficiency of the precious metalscan be ignored, and the reason for this may be that theseimpurity ions without unstable d-electrons can not reactwith the flocculant of polythiol to form coordinators, andtherefore, can not be agglomerated and adsorbed by thepretreated coco nut shell activated carbon.

The reagents used in the experiment wereanalytically pure hydrochloric acid, analytically puresodium hydroxide, the flocculant of polythiol and the

Corresponding author: ZOU Hua-sheng; Tel: +86-20-87 1 14539; E-mail: [email protected]


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ZO U Hua-sheng, et al/Trans. Nonferrous Met. S O C .China 17(2007) 859prepared activated carbon.

IB-3 timing constant temperature blender providedby Shanghai Leici New Jing Co. Ltd., ICP-AESmanufactured by LEEMAN LABS Inc. in US, wereused.2.2 Experimental fundamental

Because precious metal elements such as Pd and Aghave unpaired and unstable d-electrons, which canproduce coordination compound by combining chemicalelements or groups that have lone pair of electrons[6-11].According to the coordination theory, the flocculant withhydrosulfide radical or oxy- can provide a pair of freeelectrons that can fill in precious metals' empty atomicorbit, and then form coordination compounds or largeragglomerate particles that may readily bring aboutsedimentation. Thus, the polythiol was chosen asflocculant through orthogonal exper iments.Besides the advantages of large specific surfaceareas and a large number of homogeneous pores, theactivated carbon can also combine with other elementssuch as oxygen, hydroxyl to form functional groups ofoxygen on its surface. These hctional groups have animportant influence on the activated carbon's adsorptioncapacity and the mechanism o f adsorption. T he recoveryof precious metals from the waste water becomesunefficient when the common activated carbon is usedbecause of trace precious metals and the effect of somebase metals existing in wastewater. It's noted that largerspecific surface areas and proper granularity are alsoimportant to the adsorption of precious metals, and thus,a kind of pretreated coconut shell activated carbon wasused in the experiment.2.3 Experimental

The experimental procedures were as follows. ThepH value of the waste water was adjusted first; secondly,the flocculants were added into the waste water underswift stirring condition, then agglomeration took place;thirdly, the adsorbents were added to the waste waterunder slow stirring for 2 min, and then, the waste waterwas rested to make precious metals fully adsorbed andsedimented; and at last, the concentration of preciousmetals in supernates was analyzed by ICP-AES.

Precious metals display different properties and theresults of agglomeration and adsorption are differentbecause the valence of precious m etals in the waste wateris influenced by the pH value[7-91. Thus, firstly, the pHvalue of the waste water should be adjusted to 8-9 inorder to get optimum effect of agglomeration andadsorption. The ions or atoms of precious metals existedin the stable form of colloid or solution when their

concentrations were quite low, and they couldn'tsedimentate, or be separated from wa ste water. easily.Several chemical and physical methods were used tobreak down the stable colloid or solution in order toaccelerate the sedimentation of precious metalparticles[8-91. The flocculant of polythiol was added tothe solution or colloid under swift stirring condition, andthe drops of the polythiol in the solution or colloidbecame much smaller and highly dispersed, which cansignificantly enlarge mass transfer area and providemuch more chance for combining flocculant withprecious metals.

The fine particles formed from the ions or atoms ofprecious metals by agglomeration are in a highlydisperse state and their sedimentation rates are quite low.Since a kind of activated carbon with some functionalgroups on its surface is very effective in thgadsorptionof some precious metal particles, the activated carbonscan not only accelerate the enrichment process of theprecious metal particles, but also improve recoveryefficiency of precious metals from the waste water. Thesediments can be further treated by method of calcinationor super-filtration to get free precious metals or theoxides of precious metals.3 Results and an alysis

In order to obtain the recovery efficiency ofprecious metals Pd and Ag from the waste water bycombining of agglomeration and adsorption, theefficiency q of agglomeration and adsorption is definedasq= - o - c s x100% (1)

COwhere c, is original concentration of Pd and Ag in thewaste water, mg/L; c, is the concentration of Pd and Agin the supernate, mg/L.3.1 Adsorption isothermal line

The adsorption isothermal line of the system wasdetermined by experiment of statically saturatedadsorption at constant temperature of 25 "C . Theadsorption capacity o f the system q is calculated by

where c, is the equilibrium concentration of preciousmetal in supernate, mg/L; m is the mass of activatedcarbon adsorbent, g; V is the volume of wastewatersample, L.

Five test samples of the wastewater were taken w iththe initial concentration c of precious metal of 10.31,8.01, 6.02, 4.06 and 2.81 mg/L, respectively, and the


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860 ZOU Hua-sheng, et al/Trans. Nonferrous Met. SOC. hina 17(2007)volume of each sample was 500 mL, and 1 g of theadsorbent was added into the each sample under slowstirring for 10 min, then the samples were held for 4 h,and finally, the supemate of each sample was analyzedfor determining the adsorption equ ilibrium concentrationcf , from which the adsorption capacity q wasdetermined by Eq n.(2). The experimental result is show nin Fig. 1, which indicates that the relation betw eenadsorption capacity q and the concentration of preciousmetal in wastewater is linear in a d ouble logarithm ic plot,which can be expressed as Eqn.(3) with a regressioncoefficient of 0.998 6.

I_ _lg q =0.490 5 lgc +0.1289 or q = 1.352 2.04 (3)4.54.0

h 3.5Pv 3.0-I

.P2.5

Eqn.(3) shows that the behavior of the adsorptionsystem conforms to the Freundlich isothermal equationq=acb, and the value of parameter b is in the range of2-1 0, which indicates that the polyth iol flocculant react swith precious metals easily[10-131, which is inaccordance with the 4th test result in Table 1.3.2 Dynamics of adsorption

By taking 1 g adsorbent and 500 mL waste water ofprecious metal with .initial concentration of 10.31 mg/L,Table 1Results of field test

the experiments of adsorption dynamics were done atconstant temperature of 25 ' C . The supenates wereanalyzed every 2 h to determine the adsorption capacityq at different adsorption times. The experimental resultsare show n in Fig.2, which may be relate d with Eqn.(4) asfollows:R =4 = 1- xp(-kt) or -ln(l- R) = kt (4 )where k is the adsorption rate constant, h-'; R is theratio of adsorption capacity q to saturated adsorptioncapacity qe; is the ad sorption time, h.

9 e

tf hFig3 Relation between adsorption capacity and adsorptiontime

Eqn.(4) suggests that the relation between (1-R)and t is linear with a slope of k in a single logarithmicplot as shown in Fig.3. The slope k, adsorption rateconstant, was calculated as 0.233 4 /h by regressing theexperimental data with a relation coefficient of 0.996 2.The result indicates that the process of agglomerationand adsorption is apparent first-order reversal reaction,and mainly limited by the diffision of precious metal inliquid film[ lo] .3.3 Effect of pH value on efficiency of agglomeration

0.05mL flocculant was added into a sample of 200mL w aste water under quick stirring for 2 m in, and then

and adsorption

Test cdP d)/ c,(Ag)/ V(Flocculant)/ m(Adsorbent)/ c,(Pd)/(mg.L-') c,(Ag+)/(m g L- 'No . (mg.L-') (mg.L-') mL mg l h 2 h 3 h 4 h l h 2 h 3 h 4 h1 2.94 7.37 2.5 250 0 0 0 0 0.240 0.356 0.409 0.0522 2.94 7.37 2.5 250(recycleuse) 0 0 0 0 0.232 0.200 0.336 0

0 0 0 0 0.277 0.337 0.282 0.1322.94 7.374 2.94 7.37 0 250 2.030 2.030 1.940 1.880 4.180 4.180 3.920 3.7905 10.3 25.5 2.5 250 0 0 0 0 1.150 0.970 0.852 1.330

25 02.5 (triply cycling use)


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ZO U Hua-sheng, et aVTrans. Nonferrous Met. SO C. hina 17(2007) 861

2.718 28

7vC7 1.00000-

-

0.367 8810 2 4 6 8 10 12 14 16 18

tlhFig3 Relation between -In( 1-R) and adsorption timethe sample was rested till it became turbid. The samplewas equally divided into 10 small parts of 20 mLindividually; each part was adjusted to different pHvalues with hydrochloride or so dium hydroxide solution,and 1 g adsorbent was added to the each sm all part underslow stirring for 2 min; then each part was held for 2-4 h;and at last, the contents of Pd and Ag in the supernateswere measured. The effect of pH value on the efficiencyof agglomeration and adsorption of precious metal isshown in Fig.4. It shows that the efficiency ofagglomeration and adsorption of Ag increases with therise of the pH value when pH value is less than 6. Butwhen pH value is in the scope of 6-7, the efficiencydescends with pH value increasing. The efficiency takeson increasing trend when pH is in the range of 7-9, andreaches the top at pH value of 6 or 9. The agglomerationand adsorption efficiency of Pd increases with the rise ofpH value when pH value is below 3. But when pH valueis 6-7, the efficiency descend s with pH value increasing.The efficiency of Pd also takes on increasing trend whenpH value is in the range of 7-9, and reaches peak at thepH value of 3 or 9. The reason for this may be that theoxidation value of the precious metal and the activity ofthe functional group on the surface of the activatedcarbon are different in d ifferent ranges of pH value, andso, the precious metal Ag and Pd may react with theflocculent polythiol or functional groups on the surfaceof activated carbon to form different coordinationcompounds with d ifferent solubilities in w ater[3, 14-15].The precious metal Ag and Pd may easily react with theflocculent or the functional group, and form insolublecompounds with strong coordination link at individualoptimum pH valu e, and the activity of the polythio l andhydrolysis effect of precious metal coordinationcompound become weaker at the pH value of 7.Comprehensive results of agglomeration and adsorptionof Pd and Ag in the waste water by test show that the

1001

86840 2 4 6 8 1 0 1 2 1 4

PHFig.4 Effect of pH value on agglomeration and adsorptionefficiencybest result can be obtain ed at pH value o f 8-9.3.4 Effect of additive flocculant quantity on agglome-

ration and adsorption efficiencyWhen pH value was in the range of 8-9,0.025-0.12

mL flocculant was added to 4 samples of 200 mLwastewater respectively under swift stirring condition for2 min, then a part of 50 mL was taken out from eachsample, and 2 g of adsorbent was added to the each partsrespectively under slow stim ng for 2 min, then was heldabout 2 h, and at last, the concentrations of the Pd andAg in the clear solution were measured by ICP-AES. Theeffects of additive flocculant amount on the efficiency ofagglomeration and adsorption are shown in Fig.5. It canbe seen that the efficiency of agglomeration andadsorption of precious metals in the waste wa ter does notalways increase as additive flocculant quantity rises. Theefficiency of agglomeration and adsorption of preciousmetals is improved with the increase of additiveflocculant quantity when the volume ratio of the wastewater to flo cculant is larger than 4 000: 1. The recoveryefficiency of A g and Pd approaches almost 100% whenthe additive flocculant quantity is 0.05 mL. Theefficiency of agglomeration and adsorption of Pddescends if the additive flocculant quantity exceeds 0.05mL. The reason for this may be that excessive flocculantexists in the free state and may react disadvantageouslywith the activated groups on the activated carbon surface,thus, the efliciency decreases. Moreover, excessiveflocculant cannot be dispersed sufficiently and floats onthe top surface of solution in the free state, which maypollute the environment and increase the recovery costsimultaneously. Thus, the ap propriate volume ratio of theflocculant to wastewa ter is about 1:(2 000-4 000).


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862 ZOU Hua-sheng, et al/Trans. Nonferrous Met. SO C.China 17(2007)

2 02 g 99.4% gF:-3 99.2

100.0w

-

-

a'x 99.81 /c vm r 99.6 1 I

99.0 19 - A g- d

I I0.02 0.04 0.06 0.08 0.10Flocculant quantity/mL

Fig.5 Effect of flocculant quantity on agglomeration andadsorption efficiency3.5 Effect of adsorbent quantity on agglomeration

and adsorption efficiencyWhen pH value was in the optimum range of 8-9,0.02 mL flocculant was respectively added to each partof 50 mL divided from a 200 mL sample under faststirring for 2 min, then 0.5, 1.0, 1.5 and 2.5 g adsorbentwas added to eac h part respectively by slo w shak ing for1 min, then each share was held for 2-4 h, and at last thecontents of Pd and Ag in the supernate were examined.The effect of adsorbent quantity on agglomeration andadsorption efficiency is shown in Fig.6. It indicates thatthe efficiency of agglomeration and adsorption ofprecious metals increases as the adsorbent quantityincreases when the mass ratio of the adsorbent quantityto waste water is sm aller than 1:40, but furthe r increasein adsorbent quantity has no dramatic influence on theefficiency when the mass ratio is larger than 1:30. Thus,the optimum mass ratio of adsorbent to the waste wateris 1 (30-40) from the perspectives of both econom y andadsorption efficiency.

. g- d

901 10.8 1.2 1.6 2.0 2.4Adsorbent quantitylg

Fig.6 Effect of adsorbent quantity on agglomeration andadsorption efficiency

3.6 Field testsField tests were done in the Feng Hua Precious

Metal Alloy Powder Producing Company. Two kinds ofwaste water were fetched from the effluent of thecompany, and their characte ristics are listed in Table 1. Afully mixed sample of 5 L was taken out for each run offield experiment done under the mentioned optimumconditions by the same steps as above. The results offield test are shown in Table 1. The results of field testshow that high recovery of the field test is obtainedwithin 4 h under the optimum processing conditions. Bycomparing the data of test 1 with that of tests 2 and 3 inTable 1, it can be seen that the recovery efficiency ofprecious metals keeps very good even by recycling useof the adsorbent at least three times. By comparing the4th test result with other test results in Table 1, it can befound that the flocculant plays an important role inobtaining a high recovery of precious metals from thewaste water.4 Conclusions

1) The technology of reclaiming precious metalfrom waste water that contains precious metals below 11mg/L has many advantages such as simpleness, easyoperation, less equipment and good economic effects.The efficiency of agglomeration and adsorption of Pdand Ag almost approaches 100%.2) The experimental data show that Freundlichisothermal equation is suitable for describing features ofthe system, and the isothermal equation is q = 1 . 3 5 ~ - ~ . ~ ,and the apparent first-order adsorption rate constant at25 'C is about 0.233 4 h-'.

3) T he technological parameters such as pH value,adsorption time, flocculant quantity and adsorbentquantity have great influence on the efficiency ofagglomeration and adsorption of precious metals. Theoptimum technological conditions are that pH value is8-9, the volume ratio of the flocculant quantity to thewaste water is 1:(2 000-4 000), the mass ratio of theadsorbent quantity to the waste water is 1:(30-40), andthe adsorp tion time is within 4 h.

,

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(Edited by CHE N Wei-ping)

a novel recovery technology of trace precious metals from waste water by

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