heavy metal crystallization kinetics in an msmpr ... · to either waterways, industrial waste...

HEAVY METAL CRYSTALLIZATION KINETICS IN AN MSMPR CRYSTALLIZER EMPLOYING

SU IF1 DE PREClPlTATl ON -

Robert W. Peters, Young Ku, and Tsun-Kuo Chang Environmental Engineering, School of Civil Engineering, Purdue University, West Lafayette, IN 47907

c employing sulfide precipitation was studied under MSMPR conditions. Removal efficiency and particle size distribution were monitored for various initial metal concentrations, pH, and reactor detention times. Preliminary results .

indicate a phase transformation is likely as fresh zinc sulfide precipitates age to a more stable thermodynamic form.

INTRODUCTION

Heavy metals appear t o o f f e r great dangers through promiscuous release t o the environ- ment because they are toxic and re la t ive ly accessible. Elements such as Hg and Cd ex- hibit human toxic i ty a t extremely low concentrations. The elements 3, .Qr, Lu, Pb, and q, etc . exhibi t toxic properties toTumans although they are orders of magnitude higher than that required for Cd o r Hg tox ic i ty . wastewaters from such industr ies as the plating and f inishing, pulp and paper, and the chemical industr ies usually contain h i g h concentrations of heavy metals and are discharged to either waterways, industr ia l waste t r ea t - ment plants (IWTP’s) , o r publicly owned t r e a t - ment works (POTW’s). The most publicized case of industrial heavy metal pollution i s the discharge of the ca ta lys t methylated mercury chloride in to Minamata Bay, Japan, from a plastic manufacturing factory. Microorganisms converted the sedimented compound t o mono- methyl-mercury, which led t o an enrichment of this most toxic metal i n f i sh consumed by local people , causing severe chronic mercury- poisoning diseases (1). From another p o i n t of view, removals o r reductions o f to ta l heavy metal concentrations below 10 mg/l are usually desirable prior t o any wastewater treatment operation since many heavy metals can adverse- l y affect biological oxidation processes [such as trick1 ing f i 1 t e r s , activated sludge , and anaerobic dfgestion] (6, L, - 35).

A number of specialized treatment pro-

The

cesses have been devel oped f o r removing e i t h e r or bo th dissolved and suspended heavy metals from industr ia l waters and wastewaters. These uni t operations include: chemical precipi ta t ion (21, 26, 27, 3 4 ) , complexation (19, 31, 48), cementation (13 , 22) , solvent extraction (8, 28), electro-depose t i o n ( l l ) , f i l t r a t i o n (g , ion exchan e (E), ’ adsorpEon/absorption o n t o h---9- ct ivated carbon or other su i tab le medium (3 , 17, 24, 3 3 ) , and f lo t a t ion ( E , 4 6 ) . Chemical ureciortation i s by f a r the most wide$ used process t o remove heavy metals. 75% of the electroplat ing f a c i l i t i e s employ precipi ta t ion treatment t o t r e a t t h e i r waste-

.-

Reportedly (%) nearly

waters, par t icu lar ly the hydroxide prec ip iQ- t i o n techni ue (16, 18, 20-, 3). p i 1 hvdroxides have very low solu-

A t e levated.

bi 1 i t i e s and wii 1 precipi ta te out- when a1 1 ow- ed t o s e t t l e . The so lub i l i t i e s of various metal hydroxides are minimized fo r pH in the range of 8.0 t o 11.0. In th- hydroxide pre- c ip i ta t ion process, the pH of the wastewater i s adjusted usually t h r o u g h lime addition. T h i s technique has proven e f fec t ive i n indus- t r y and i s well su i ted fo r automatic control. Common l imitat ions of the process, however , i ncl ude : - Hydroxide precipi ta tes tend t o resol u -

b i l i z e i f the solution pH i s changed. - The removal of metals by hydroxide

precipi ta t ion o f mixed metal wastes may no t be e f fec t ive because of the minimum so lub i l i t i e s for d i f fe ren t metals occur a t d i f fe ren t pH values. - Hydroxide sludge quant i t ies may be

Advances in Crystallization from Solutions AlChE SYMPOSIUM SER

and are generally d i f f i c u l t ( 0 e 0

+ OH- M ( O H ) , ( "

t o dewater due t o the amorphous par t i -

have an adverse e f f ec t on metal re-

, . c l e s t ruc ture . of complexing agents may

In ?ddi t ion t o reactions (1) t o ( 4 ) , tt H S - HS - S- equi l ibr ia must a l so be taker if i to consideration. The so lub i l i t y of H S i water i s about 0.1 M a t 25°C (3. The 81s- sociat ion of H 2 S follows:

moval. - Chromium (VI) i s n o t removed by this technique. - Cyanide in te r fe res d i th heavy metal removal by hydroxide precipi ta t ion.

Sulfide precipi ta t ion has been demonstrated t o be an effective a s r n a t i v e to hydroxide precipi ta t ion (4, S , VI-, 23, 50) for removing various heavy metals from industr ia l wastewaters. The a t t r ac t ive features of the su l f ide precipi ta t ion process include: a t t a in - m e n t of a i h de removal even a t low pH ( p H m t i o n time re- uirements i n the reactor because of h i g r z -

&s of sulfide_s, the f e a s i b i l i m f selective- metal su l f ide sludge exhibi ts better thickening and dewaterabil i t y than metal hydroxide sludges, and metal sul- f ide sludge i s three times less subject t o leaching a t pH 5 as compared t o hydroxide sludge (50, 51) making f ina l disposal s a fe r and eas ie r . The metal sulf ide precipi ta tes tend t o be amorphous and colloidal i n nature, so improvement of the par t iculate properties of precipi ta tes becomes an important fac tor fo r heavy metal removal by sulf ide precipi ta- t i o n . Under various operational var iables , the analysis of PTD (pa r t i c l e s ize d i s t r i b u -

_study t h e e properties of such precipi- Sates. applications , a se r i e s of bench-scale and continuous flow experiments were performed i n this study t o determine the PSD and removal eff ic iency of heavy metal removals by su l f ide precipi ta t ion.

BACKGROUND

Metal -Sulfide Reactions

t i o n ) ' pro * convenient and d i r ec t w a y - 3 0

In order t o simulate the pract ical

Metal su l f ide precipi ta tes are formed as the metal cation reacts w i t h sulf ide ion i n aqueous solut ion. Under a lkal ine conditions, formulation of several metal -hydroxy complexes also occurs. The primary reactions ( w i t h the divalent heavy metal i o n , M++) involved i n metal sulf ide precipi ta t ion are:

-t / M++ + s= c 1% [ M++ + OH- Z M ( O H ) +

The concentration of various su l fur species i s a s t rong function of pH as shown Figure 1. predominant species i s H S T p a r t of the H2S will be l o s t i n t o t h g 6@(due t o H S H S r?ltbX-egg odor.

For low pH conditions (pH < 4 ) , t

equilibrium) causing the distinc?i&'

* 60 - vl

a, 4 0 -

0 2 4 6 8 1 0 1 2 1 4 PH

Figure 1 . H2S - HS- - S= equi l ibr ia

Table 1 ( 2 , 10) l i s t s the equilibrium constants of These reactions fo r several di. valent heavy metals. Equations ( 1 ) t o (6) can be solved f o r any divalent heavy metal t using the appropriate sol u b i 1 i t y constants. The so lub i l i t i e s of various metal sulf ides and hydroxides are shown i n Figure 2 as a function of pH for comparison purposes.

I

. I' No. 240, VOl. 80

Table 1. Metal su l f ide precipi ta t ion reac$Lo_ps(g,lO)

Pb - c u - Cd - N i n React i on - M* + S= MS(s) 20.7 23.8 27.7 35.1 27.0

4.1 4.4 3.9 7.0 6.3 - + id++ + OH- .- M(OH)

5.0 6.9 3.8 3.7 4.6 3.0 2.8 1.0 3.8 3.0

?i 3.5 -0.05 2.2 --- M(OH$ + O H - ~ M ( O H ) ~ --- K~ = 1.0 x _,f

HS- =H+ + S= H2S -H + HS-

-1 3 K2 = 1.2 x 10

57

w

1 w

The Sulfide Precipitation Process

Two main processes ex i s t fo r su l f ide precipitation: soluble sulf ide precipi ta t ion (SSP) and the insoluble sulf ide precipitation (ISP). The main difference between the two processes

or sodium hydro

supply the su l f ide ions needed to precipi ta te pj the heavy metals . In the pas t , operational d i f f i c u l t i e s pre- 5

vented widespread application of the SSP process. Technological advances i n the area of ion-selective electrodes have provided a probe 2 successful fo r controlling the addition of so l - 5 uble sulf ide reagent t o match the reagent de- 5 mand from the heavy metals. Eliminating sul- v, fide reagent overdose prevents formation of the odor causing H2S. In current ly operated soluble sulf ide systems tha t do n o t match demand, the process tanks must be enclosed and vacuum evacuated t o minimize sulf ide odor problems. polyelectrolyte conditioners have been devel - oped that e f fec t ive ly f locculate the f i n e metal sulfide par t ic les t o eliminate the d i f f i cu l ty in separating the precipi ta tes from the e f f lu-

E 0 u

0 2 4 6 8 10 12 14

PH

Figure 2. So lub i l i t i e s of metal sulf ides and ent and have resul ted i n sludges which are easily dewatered (PI). roxides as a function of

58 Advances in Crystallization from Solutions

A recently patented process cal led SulfexTM has proven ef fec t ive i n separating heavy metals from plat ing waste streams. I t uses a f reshly prepared ferrous su l f ide s lu r ry prepared by reacting FeSO4 and NaHS. FeS wil l dissociate i n t o ferrous and su l f ide ions ( t o the degree predicted by i t s so lub i l i t y product) . As su l f ide ions are consumed, a d d i t i o n - a l FeS will dissociate t o maintain the equi l ib- rium concentration of su l f ide ions. FeS dis- solves t o maintain the su l f ide ion concentra- t ion a t a level of approximately 0.02 ppb. Most heavy metals have sulf ides less soluble than ferrous .sulfide enabling the heavy metals t o precipi ta te as metal su l f ides . One large advantage of this ISP process is the absence of any detectable H,S odor. ISP will a l so re- duce hexavalent chromium t o the t r i v a l e n t s t a t e eliminating need t o segregate and pre- t r e a t chromium waste streams. the ISP process include: metric sulf ide concentrations are required and h igher s ludge production than by the hydroxide o r SSP processes.

Disadvantages of larger than s toichio-

Under a lkal ine conditions i n the ISP process , chromi urn w i 11 preci p i t a t e as chromi urn hydroxi de :

H,CrO, t FeS + 4 H,O 3 C r ( O H ) 3 c + Fe(OH)3 I

+ S c +2H,O

The SSP process a lso reduces hexavalent chrom- i u m according t o the reaction:

2 H2Cr04 + 3 NaHS + 8 H20 -+ 2 C r ( O H ) , + + 3 S c + 7 H20 + 3 NaOH

In the ISP process, the ferrous ion acts as a ca ta lys t for chromium reduction allowing . l e s s than stoichiometric dosages of su l f ide t o be employed.

SSP process causes a re la t ive ly h i g h concen- t r a t ion of dissolved su l f ide to be present i n the wastewater. The high su l f ide concentra- t ion causes the rapid precipi ta t ion of metal su l f ides ( i . e . high nucleation ra tes ) often resul t ing in small par t iculate f ines and hyd- rated colloidal par t ic les t o be formed. Poor s e t t l i n g o r f i l t e r i n g f locs often r e su l t s .

As w i t h SSP, the ISP process achieves a l - most complete conversion of previously pre- c ip i ta ted metal hydroxide t o metal su l f ides . The reaction goes toward completion due t o the long residence time of the so l ids i n the t r e a t -

The d i rec t addition of su l f ide ion in the

AlChE SYMPOSIUM SERIES

i nen t system prior t o discharge.

highly concentrated heavy metal laden wastewater with the SSP process (E), The primary application of SSP has been fo r waste streams containing low concentrations of metals and complexing agents ( w h i c h interfere w i t h e f fec t ive metal removal by forming hydroxide complexes). The SSP and ISP processes can also be employed as a polishing treatment system a f t e r preliminary treatment by hydroxide precipi ta t ion. This technique not only reduces su l f ide reagent consumption , but i t a lso reduces the va r i ab i l i t y o f reagent

d . EPA has discussed several process echn ques for b o t h SSP and ISP processing

Currently, no commercial units t r e a t

@ A more recent process is one developed

by General Elec t r ic Co. t ha t uses semisoluble calcium su l f ide (Cas) t o remove copper ion in the presence o f synthet ic f a t s ( 2 3 ) . Cas precipi ta tes metal sulf ides f romytable metal complexes and, i n addi t ion, removes residual f a t s . A two-stage process fo r t rea t ing the waste was developed and demonstrated on the p i lo t and fu l l -p lan t sca le application. Poly- su l f ide s a l t s (e .g . , Na2S5) have been success- f u l l y used fo r the e f f i c i en t removal of elemental mercury from solut ion; a process in- volving sodium polysulfide has been developed t o treat the wastewater and sludges from a mercury-cell chlor-alkali plant (15) . Among the other su l f ide compounds, barium sulf ide has a l s o been suggested fo r heavy metal laden wastewater treatment (44). Par t ic le Size Distribution (PSD) The0r.y

product from a precipi ta t ion process can y ie ld valuable information regarding the kinet ics and mechanisms of the precipitation process. Several experimental works (36 , 38, - 43, 47) have demonstrated the value o f t h e s izedis t r ibut ion analysis t o precipitation processes.

The s ize d i s t r i b u t i o n of a precipitation

For the continuous flow study of sulfide prec ip i ta t ion , a MSMPR (mixed suspension mixed product removal) c rys t a l l i ze r was used. A schematic diagram of the continuous reactor- c rys t a l l i ze r us2d i n t h i s study i s shown i n Figure 3 . Feed solutions of the heavy metal t o be t rea ted and the soluble su l f ide solu- t i o n are the i n l e t streams labeled 1 and 2 in ;

i the diagram. streams resu l t s i n chemical reaction t o produce metal su l f ide , which due t o i t s low sol-

? The mixing of these feed

i P

P .* NO. 240, VOI. ao

ubi1 i t y precipi ta tes o u t of solution. The solubi l i ty products f o r the metal sulf ides of nickel, z inc , cadmium, copper and lead are l i s ted i n Table 1 . The objective was t o measure the r a t e of these metal sulf ide precipita t ions as functions of the various operating parameters and chemical conditions, and simul- taniously characterize the s ize d is t r ibu t ion .

Figure 3. Schematic diagram of c rys t a l l i ze r '

The concept of the population balance f o r a MSMPR system (45) stat- e %umber of discrete p a r t i c k s must be c o n m e a i n a dis- &msd system. I f b i r t h , death, and rates are properly represented, than an accounting fo r a l l the par t ic les i s possible. Such accounting can be used t o characterize the c g s i a L i ize dis t r ibut ion fo r such systems. In recent years , c o n s i m l e progress has been achieved i n CSD analysis fo r a MSMPR system. Under MSMPR conditions , the population balance analysis and the assumption of size independent growth gives ( 4 5 ) :

n = no exp ( - L / G T ) ( 7 )

where: n = population density, number/ml-1m no = nuclei population density, number/

ml -pm L = charac te r i s t ic crystal s i z e , pm G = crys ta l growth r a t e , "min T = reactor detention time, min.

The above equation provides the funda- mental re la t ionship between the population

a density n and the size L, and thus character- izes the s i ze d is t r ibu t ion . equation, the zeroth, f i r s t , seccjnd, and t h i r d moments of the s i z e dis t r ibut ion can be determined, representing the to ta l numbers , length , surface a rea , and mass , respectively.

Using the above

59

Because of the very low suspension den- s i t ies encountered i n this research, i t is more accurate t o plot I n N versus size L , where N i s the to t a l number of pa r t i c l e s per u n i t volume i n the s i ze range L t o a. The integration of n dL y ie lds :

n o GT exp(-L/GT) A p l o t o f I n N versus L has a slope o f (-l/G T) and intercept I n ( n o & ) . (nucleation r a t e ) Bo i s computed from the product o f no and G ,

The b i r t h r a t e

BO = no G (9)

Since the nucleation ra te i s the product of no and G , b o t h the nucleation ra te and growth ra te may be obtained under ident ical condi- t i ons .

The relat ionship of the supersaturation d r i v i n g force t o nucleation and growth ra tes is of considerable importance. A t constant temperature, the nucleation and growth ra tes can be modeled (45) w i t h simple power law models which, w h e n combined, y ie ld :

where kN i s the k ine t ic ra te constant r e l a t - ing nucleation r a t e t o growth r a t e and i i s the k ine t ic order. The values of kN and i may be determined experimentally from a series o f runs conducted a t d i f fe ren t super- sa tura t ion levels . The eas ies t way t o do this i s t o vary the residence time, T. The Bo and G values obtained from a par t icu lar s e r i e s can be f i t t o Equation (10) t o yield the model constants , kN and i . OBJECTIVE

The objectives of this k ine t ic study o f metal su l f ide precipitation were several fold. Metal su l f ide and metal hydroxide pre- c ip i t a t e s are often amorphous in nature, so that improvewnt o f the par t iculate properties of such precipi ta tes becomes an impor- t an t f ac to r fo r heavy metal removal by pre- c ip i t a t ion . Whereas much data abounds i n the 1 i t e ra ture on equi 1 i bri um metal concen- t r a t ion existing i n solut ion, very l i t t l e data ex i s t s concerning the s i ze dis t r ibut ion of the precipi ta tes formed. moval eff ic iency and ease of removing the

Since the re-

i

.I


sludge are coupled and are equally important, t h i s study seeks t o eliminate much of this i n - formation void. heavy metal removal and PSD data simultaneous- l y fo r the precipi ta t ing species.

T h i s research has as i t s objectives:

T h i s research reports on

1.

2.

3 .

4.

5.

6.

7.

Measurement of the ra tes of these precipi- t a t ions as functions of the various oper- a t ing parameters (reaction time, agi ta t ion l eve l , temperature , e tc . ) and chemical conditions ( p H , sulf ide dose, heavy metal concentration , etc.) Simultaneously w i t h these precipi ta t ion r a t e measurements, characterize the pa r t i - c l e size d i s t r i b u t i o n , obtaining nuclea- t i o n and growth r a t e s , as well as the pre- c ip i ta t ion kinet ics t h r o u g h a population balance analysis. Measure the heavy metal concentrations of the influent and ef f luent streams t o determine the removal efficiency as a function of the operating conditions. Monitor the cyrstal morphology of the preci p i t a t e s . Determine the e f fec ts of multimetal systems. Determine the e f f ec t of chelants on the removal of spec i f ic heavy metals. Model the complex equi l ibr ia system using computers t o determine theoret ical residual soluble metal Concentration fo r any s peci f i ed operating condition.

T h i s paper addresses the f i r s t four ob- j e c t i ves for zinc removal by su l f ide preci p i - t a t i on , based upon our preliminary research r e su l t s obtained t o date.

EXPERIMENTS

Preliminary Batch Precipi ta t ions

Metal solution o f known concentration was t ransferred t o a reactor beaker w i t h a magnet- i c s t i r r i n g device. The agi ta t ion level was held constant a t 400 rpm. After i n i t i a l adjustment of the feed solution pH, a precalcu- la ted amount of su l f ide solution was added t o the metal feed solut ion. W i t h an automatic t i t r ime te r , the solution ptl was maintained a t desired levels by periodically adding acid or base t o the solution. Typical reaction runs las ted 40 minutes t o 2.0 hours. A t various sampling times, a por t ion of the reactor so lu- t ion was t ransferred, and f i l t e r e d immediately. The resul t ing f i l t r a t e was analyzed f o r i t s soluble metals content using a Varian AA-575 Spectrophotometer.


Continuous Precipi ta t ions

The experimental condi t ions employed for this investigation are summarized i n Table 2. Stock solutions were prepared by di ssol v i ng reagent grade ZnSOL, . 7H20 and Na,S - 9H20 in deionized water. Metal concentrations were determined using a Varian Atomic Absorpt ion Spectrophotometer, Model AA-575 , while su l - f ide concentration was cal ibrated by the t i t r i m e t r i c ( iodine) method. NaOH and HC1 solutions were prepared for pH adjustment.

Table 2 . Nominal experimental conditions for continuous precipi ta t ion o f ZnS.

I n i t i a l conditions: Zn = 100 Rg/l T = 25.0 C

Series pH S= dosage Residence Time,

I 8.0 1 . 0 5 ~ 5 , 10, 15

I1 10.0 1 . 0 5 ~ 5 , 10, 15

The MSMPR reactor was a 2-1 i t e r capacity glass beaker s t i r r e d w i t h a glass impeller operated a t 400 rpm t o ensure complete mixing. The flow diagram fo r the experimental equipment is shown i n Figure 4. Feed solu- t i ons o f metal and su l f ide were pumped th rough 0.20 um f i l t e r s t o remove foreign par t ic les and Passed t h r o u g h constant temper- a ture baths t o maintain the reactor temperature a t 25.0 2 0.2OC. The volumetric flow- ra te of metal and su l f ide streams were con- t ro l l ed t o maintain 2 1 of solution i n th i s MSMPR reactor a t the desired sulfide/metal dosage r a t i o . The residence times employed in t h i s research were nominally 5 , 10, and 15 minutes. The solutions were mixed and pH was monitored using a Corning pH meter Model 130. The su l f ide concentration i n the MSMPR react- or was determined using a su l f ide electrode. Samples from the reactor were withdrawn i n t o a sample vial f o r par t ic le counting a f t e r various residence times. For some experiments, the sample solution were f i l t e r e d through a 0.2 um car t r idge f i l t e r and pre-

c

No. 240, Vol. 80 61

c Rotameter

I

F i 1 t e r

--% PumD

NaOH or

HC1 Feed

l r pH electrodf I

I- Rotameter

F i 1 t e r

PumD Na2S Feed Tank ZnC12 Feed Tank

Figure 4 . Experimental equipment flow diagram.

React o r Y

- lischarge

Se t t l ing Column

served by adding few drops of n i t r i c acid f o r metal concentration analysis by AA i n order t o evaluate the performance of metal removal by sulfide precipi ta t ion.

using a Coulter Counter Model TA-I1 par t ic le - size analyzer, equipped w i t h a PCA-I1 population accessory. Periodically 1 o r 10 ml a l iquots were withdrawn from reactor , depend- ing on the number of par t ic les generated, and diluted t o 100 in1 w i t h saturated metal su l f ide solution. The suspensions were determined on the Coulter Counter us ing a 140 um aperture

achieved a f t e r u r e s i d e n c e times. Most of t h p d x a w e r e c o l l e c t e d between 8 and 16 residence times. Details of the chemical analysis performed and other experimental de t a i l s are avai lable elsewhere ( 2 5 ) . - RESULTS AND DISCUSSION

The pa r t i c l e s i ze analysis was determined

tube. Stead- Repa-tly

Due t o the low suspension densi t ies i n - volved in this research, the size d i s t r i b u t i o n is plotted in the form of I n N versus L . A p lo t o f I n N versus L has a slope of (-l/G-c) and intercept I n (n0G-c). Typical s i ze d i s - tribution plots are shown in Figure 5 fo r Series I and I1 conducted a t pH 8.0 and 10.0, respectively. The data were f i t using l i nea r

regression analysis . For the par t icu lar case shown in Figure 5 , a t pH 8.0 employing a de- ten t ion time of 5.0 minutes, the growth ra te of 0.330 pm m i n . and a nuclei density of 1.40 x l o5 no./rnl-pm were obrained from the l e a s t squares p lo t . The b i r t h ra te Bo i s computed from the product of no and G , equaling 4.61 x lo4 no./ml-min. Likewise, a t pH 10.0 with a detention time of 10.0 minutes, the growth r a t e was 0,190 um/min. , the nuclei density was 7.78 x lo4 no./ml-um, while the nuclea- t i o n r a t e was 1.62 x lo4 no./ml-min.

These two se r i e s involved the precipi- t a t ion of zinc su l f ide , conducted a t pH 8.0 and 10.0, respectively, f o r a su l f ide dose of 1 . 0 5 ~ . The growth ra tes and nucleation ra tes f o r these ser ies a re sumnarized i n Table 3 . Work i n progress investigates the precipi ta- t i o n of ZnS a t pH 6 , varying the su l f ide dosage, and conducting the precipi ta t ion i n the presence of chelating agents such as EDTA, gluconic ac id , t a r t r a t e , cyanide, e t c . The precipi ta t ion of other heavy metals ( C d , N i , C u ) a re a lso be ing investigated a t t h i s time. Results of these experiments i n progress wil l be reported in the future .

of Ser ies I were plotted together in Figure 6 . A1 t h o u g h the MSMPR cyrs ta l l i zation model

The s ize dis t r ibut ion fo r the three runs

62 Advances in Crystallization from Solutions AlChE SYMPOSIUM SENE

be insuf f ic ien t therefore t o break the flocs

morpho1 ogy (by us ing an opti cal m i croscope) . 4 L Series I Although this may provide an explanation t o

the observed behavior, the authors do not feel i t i s the l i ke ly reason. Since the system involves very low concentrations and

c le -par t ic le contact are small , particularly with the very small nature o f the sizes o f the par t ic les . As evidenced from Table 3, the dominant s i ze is less than 7.0 um. With such par t ic les , the reac tor suspension typi- ca l ly was cloudy i n appearance. W i t h th i s small a pa r t i c l e , the par t ic les tend t o follow the streamlines o f f low, t h u s tending not t o co l l ide . The authors have observed the reactor suspension under the microscope; t h e par t ic les were observed t o be s ingle par t i - c l e s . Further discussion on these microscope observations are provided i n the section on morphology. The authors thus feel t h m

robablv does not exhib i t much coagulation/ ion D r o r 6 S .

Also , note tha t the small s i z e of the pa&i-

a t ions very d i f f i c u l t i n t h i s primary nucle- a t ion system. This sugqests a c o w r coaqulant aid &i,i-b e-aexj helpful i n metal su l f ide nrpcinjta t ions . Since many industk- i a l wastewaters already Lontain such mater-

locculation i n t h i s p r p r i p i t a T

les makes sedimentation and f i l t r a t i o n oper-

i?E7tE-.s may account f o r the reason for 6 r increased s e t t l e a b i l i t y of the precipi- t a t e s .

- T h i s could be tes ted by varying the agitatio speed and observing bo th the PSD and crystal

, , , , , , , , , , , , , , * , , .

- -

Run 4 - T = 5.0 min . - suspension dens i t ies , the chances fo r parti- - -

Code t o Symbols:

. .

. .

0 2 4 6 8 10 12 1 4 16

Par t ic le s i ze , ilm

f igure 5. Par t ic le s i ze d is t r ibu t ions fo r Run 4 of Ser ies I and Run 6 of Series I1 a t various residence times.

does an adequate j o b t o describe the precipi- t a t ion of ZnS ( a l l correlat ion coeff ic ients i n Table 3 exceeded 0.9222), the PSD's are a l l observed t o be concave, w i t h the degree of concaveness increasing as the resi dence t i me is increased. Another in te res t ing feature i s shown i n Figure 6 . As the residence time i n - creased, more par t ic les are present. contrary t o the exoerience o f the inves t iaa t -

T h i s was . - . In our previous work (36, 37, E, &, 41, - 43, e), as the residence time was i n c r e z - fewer par t ic les were observed. Several

plausible explanations are possible t o explain these behaviors. systems are f locculat ive i n nature (causing a concave s i ze d i s t r i b u t i o n ) . cause an increase i n the number o f larger par- t i c l e s . Since the mixing speed was held con- s t a n t a t 400 rpm, the shear r a t e developed may

One poss ib i l i ty i s t ha t these

T h i s would h r t h e r

Another plausible explanation fo r the increased number of par t ic les as the residence time i s increased is shown schematically i n Figure 7. The s ize d is t r ibu t ion was measured on a Coulter Counter Model TA-11, using a 140 um aperture tube. The use of t h i s t u b e i s l ess than optimum, due t o the small nature of the par t ic les . However, use o f a smaller sized aperture tube caused much plugging of the tubes, necessi ta t ing the use of the 140 urn aperture tube. Typically the lower level of detection with t h i s tube was on the order of 3 pin. As shown i n Figure 7 , this lower l imi t may n o t be small enough t o see the large increase in the number o f par t ic les a t very f ine s i ze . Note the s imi la r i ty between Figures 6 and 7 . These researchers feel t h i s provides an adequate description o f t h i s behavior; however additional research is needed t o confirm o r deny t h i s hypothesis.

Further support f o r t h i s explanation is shown by examining the nuclei dens i t ies l is t - ed i n Table 3 . Note no increases as T in- ~ -

63 4

No. 240, Vol. 80

Table 3. Measured growth rates and nucleation ra tes for the precipitation of zinc su l f ide .

Cor re 1 a t i on Coefficient , r T G no BO LD Pun Series pH

No. m i n . pm/min no. /ml -pm no. /ml - m i n um

4 I 8.0 5.0 0.330 1.40 lo5 4.61 lo4 4.95 -0.973 5 10.0 0.165 1.51 lo5 2.49 lo4 4.95 -0.925 9 15.0 0.136 i.32 lo5 1.80 lo4 6.1 -0.922

11 I1 10.0 5.0 0.390 5.16 lo4 2.01 lo4 5.85 -0.979 1 10.0 0.208 7.78 lo4 1.56 lo4 6.25 -0.987 6 10.0 0.190 8.54 lo4 1.62 lo4 5.75 -0 9 7 0

2 15.0 0.155 9.49 lo4 1.47 lo4 7 .O -0.995

creases , contrary t o expected behavior. The %ors feel tna t i f the PSD were analyzed t o smaller sizes, then no would be la rges t a t the shortest retention time .

Comparing the two ser ies l i s t e d i n Table 3 , operating the prec iwi ta i iw 2t i%--UUL rat’her than 8.Q aplz~tnrs __tg__bu&-an&geous ., eien through both conditions give essent ia l ly the same removal of heavy metals. A larger growth r a t e and dominant s i z e is seen f o r these higher pH conditions.

Kinetic Order

To determine the kinet ic order i and k i n - e t ic constarit kN given i n Equation ( l o ) f o r each ser ies of runs, the values o f Bo and G obtained for each run were plotted on log-log paper. The data were f i t t e d w i t h a l i n e a r regression. These nucleation rate-growth r a t e relationships obtained f o r Ser ies I and I1 are shown i n Figure 8. The c rys ta l l iza t ion kinetics for these two ser ies are summarized i n Table 4. is shown by comparing Runs 1 and 6 o f Series 11, these duplicated runs gave very comparable results.

.

The consistency between experiments

very low kinet ic orders (1 - 1.0) . Because of such low kinet ic orders i n s p i t e of large supersaturations , i t suggests the precipi t a - t ion involves surface nucleation-control led growth. The source of nucleation a r i s e s from the supersaturation dependency , a1 though the ef fec t i s not very strong. Since i t i s close t o 1 .O f o r Series I , i t implies no advantage is gained from changing the residence time; LD remains essent ia l ly constant. confirmed in Table 3 . Likewise, since i < 1 i n Series 11, increasing the residence time decreases the dominant s ize . This trend i s shown i n Table 3, although somewhat obscured by experimental e r ror . These resu l t s forming two experiments w i t h s imilar suspension den- s i t i e s b u t varying supersaturation levels . I f c rys ta l l iza t ions 1 and 2 are operating so as t o produce the same suspension density, then the following relat ions (45) , obtained by population balance considerations are t rue :

This i s

The zinc su l f ide precipi ta t ion system has

64

l o 6 8 6 4

I--

E 2 L a

5 n v) 10 a 8 m 6

- C,

2 4

O 2

2 5

V

cc

L

r 8

> 'F 4 c, 4

J 2

a 6

7

5 V

l o 3 8 6 4

2

1 o 2

Advances in Crystallization from Solutions

Series I Zn = 100 mg/l

S- dose = 1.05 x

Par t ic le s i z e , m

Figure 6 . Par t ic le s i ze d is t r ibu t ions fo r Runs 4 3 , and 9 from Series I .

( i - l ) / ( i+3) (14) 'I1

Similar ly , the e f f ec t of suspension den- s i t y can be studied by operating c rys t a l l i z - a t ions 1 and 2 a t the same retention time and temperature , b u t u s i n g d i f fe ren t feed concen- t r a t ions t o create d i f fe ren t suspension den- s i t i e s . MT represents the suspension density i n units of crystal mass per u n i t volume of s lur ry and is calculated (45) - as :

(Gr) 4

(15) 0 MT = 6 k v p n

z - E L a P

vi a V

C, L ta

rc 0 L a

- .I-

n

n 5 S

a > C, 4

J

0

.I-

7

5

Fi

' I

A\ I

AlChE SYMPOSIUM SERIES '1 1. i i.

n i t o f deter the Coulter

2 'I 2

I

I

I

i

Par t ic le s i ze , L

gure 7 . Plausible s i ze dis t r ibut ions t o explain why experiments w i t h the long- est residence times ( T ~ < T~ < T ~ ) gave the highest number of counts.

.- .; . I 5 .2 .25 .3 .4 .5 .6 .E 1.1)

Growth Rate G , um/min

Figure 8. Kinetic data f o r ZnS precipitation

where k, = a shape f ac to r for volume and p =

I

f I *

No. 240, Vol. 80

Table 4 . Crystal l izat ion kinetics fo r the precipi ta t ion of ZnS.

Ser ies pH i Correlation Average LO Coefficient, r um

_- -

I 8.0 1.015 1.44 x lo5 0.991 5.3 0.978 6.2 4

I1 10.0 0.335 2.75 x 10

par t ic le density in mass per u n i t volume. Using this expression fo r MT, i t can be shown (45) - :

1 / ( i+3) (16) - G2 =(;)

G1

The suspension density can be increased by i n - creasing the feed concentration o r by means of seeding w i t h recycled sludge. suspension density causes G and LD t o increase yielding a more favorable s i z e d i s t r i b u t i o n . Systems w i t h lower kinet ic orders (such as th i s ZnS system) exhibi t greater change than those w i t h higher k ine t ic orders. Thus i n - creasing the suspension density should great- ly enhance b o t h G and LD i n out- heavy metal precipi ta t ion. gated i n the near future .

options t o accomplish given objectives. To increase the dominant s i ze , increasing T or MT are possible. Knowledge of the c rys ta l - l izat ion kinet ics is of paramount importance t o make the correct decision. For i close t o 1.0, the choice is c lear ly t o increase MT.

Increasing the

This aspect will be invest i -

Designers and operators have several

65

Due t o the highly concentrated nature of most plating wastewaters, t h i s explains wny the s izes are generally much la rger and eas i e r t o s e t t l e out.

Morphology of the Precipitates-

the dried prec ip i ta te ( a f t e r f i l t e r i n g through a Buchner funnel) were observed under an optical microscope. par t ic les were typical ly egg-shaped, w i t h no regular crystal s t ruc ture observable. As the solution evaporated, the par t ic les became a rod-1 ike s t ruc ture , t h u s transforming from an amorphous k ine t ica l ly favored phase t o a more c rys ta l l ine thermodynamically favored phase. The ident i f ica t ion of these par t ic les has n o t been completed t o date; such s tudies are under invest igat ion. For the f i l t e r e d dried sludge, again no regular crystal s t ructure was observed, a1 htough the material appeared t o be flaky o r plate- l ike. Several c rys ta l s were t ransparent , a1 though most were opaque. No photomicrographs are avai lable a t t h i s time, although the sample ident i f ica t ion and morphology arc! currently under investigation. The major conclusion drawn from our microscope observations is tha t the par t ic les are primari ly amorphous i n nature.

Samples of the reactor suspension and

In solut ion, the

Removal of Heavy Metals

metal su l f ide precipi ta t ion system including the formation of various hydroxide complexes. These equations can be solved fo r any divalent

Equations ( 1 ) t h r o u g h (6 ) describe the

Advances in Crystallization from Solutions AlChE SYMPOSIUM SERIES;

heavy metal using the appropriate so l ub i 1 i ty constants, l i s t e d i n Table 1. l t should be noted, however, t h a t Ksp values of f resh pre- c i p i t a t e s may be h igher than l i t e r a t u r e values o f aged prec ip i ta tes . Such behavior has a l so been observed i n heavy metal p r e c i p i t a t i o n w i t h hydroxide (34) and i s i n d i c a t i v e o f a phase t ransformat ion from an amorphous k ine t - i c a l l y favored p r e c i p i t a t e t o a more ordered c r y s t a l l i n e form. the i n i t i a l p r e c i p i t a t e transforms t o the more s tab le thermodynamic form o f lower s o l u b i l i t y , thus lower ing the res idua l heavy metal concen- t r a t i o n remaining i n so lu t i on . This same type o f behavior has a lso been observed i n the pre- c i p i t a t i o n o f ca l c i um carbonate (36,37,38,39 , - 40) i n which the k i n e t i c a l l y favored aragoni te c r y s t a l l i n e form transforms t o the thermo- dynamical ly favored c a l c i t e c r y s t a l l i n e form under h igh pH and/or h igh suspension dens i ty condi t ions.

As the p r e c i p i t a t e ages,

about 0.125. about 70% of EDTA formed 1 :1 chelates w i t h z inc fo r EDTA concentrat ions less than 500 mg/l and EDTA/Zn r a t i o s less than 1.0; tha t i s , about 12.5 mg/l of so lub le z inc concen- t r a t i o n present w i t h 100 mg/l o f EDTA. This assumption agreed w i t h the f a c t t h a t almost no p r e c i p i t a t i o n occurred when the i n i t i a l z inc concentrat ion equaled 10 mg/l w i t h an EDTA concentrat ion equal t o 100 mg/l ( 3 . 4 x l o - . M ) .

The assumption can be made t h a t ,

I i ~~~~~~~ r tt’ Zn - S - EDTA

Dosage = 1 . 0 5 ~

60 /’ 0 500 mg/l Zn A 100 mg/l Zn El 10 mg/1 Zn

_Solid l i n e s : Experimental Figure 2 showed the s o l u b i l i t i e s o f I-- . Dashed 1 ines : Calcu lated

50

several metal s u l f i d e s i n d i s t i l l e d water a t 2 various pH values. For comparison purposes , the s o l u b i l i t i e s o f t he metal hydroxide pre- 6 l o _ _ c i p i t a t e s were a lso ca lcu la ted and shown i n ‘r the f i g u r e . The s o l u b i l i t i e s o f metal hydro- ? xides are considerably h igher than those o f t h e 2 metal su l f i des except a t very h igh pH. Very $ l i t t l e metal hydroxide forms f o r pH < 6.0. Note 5 3L -- t h a t t he p r e c i p i t a t i o n o f metal hydroxides occurs on ly w i t h i n a narrow pH range.

The res idua l metal concentrat ions were measured f o r the ser ies o f runs repor ted here- i n as w e l l as f o r p re l im inary batch p r e c i p i - t a t i o n runs (42 ) . l i s t e d below for the batch system conducted a t various pH condi t ions, both i n the absence o f and presence o f che la t ing agents.

Pre l iminary Batch P r e c i p i t a t i o n Results

equ i l ib r ium, the res idua l metal concentrat ion increases w i t h an increase i n the amount o f che la t ing agent present i n the i n i t i a l feed so lu t i on . This i s v e r i f i e d t o be t r u e f o r t he

= 1.6 x 10 ,

Prel iminary r e s u l t s are

According t o Le Cha te l i e r ’ s p r i n c i p l e o f 0 0 100 200 300 400 500 6

FDTA, mg/l

zinc-EDTA system. Experiments were conducted a t pH 8.0 i n the presence o f various amounts o f EDTA f o r the zinc-EDTA system, the r e s u l t s o f

Figure 9 . E f f e c t of EDTA concentrat ion or ZnS p r e c i p i t a t i o n ( 2 5 ) .

which are shown i n Figure 9. concentrat ion increased w i t h increas ing amounts o f EDTA present i n the so lu t ion . High concen- t r a t i o n o f EDTA caused severe in te r fe rence on the p r e c i p i t a t i o n o f z inc s u l f i d e . For exam- p l e , 62 mg/l o f z inc res idua l d i d no t prec”pi- t a t e w i t h s u l f i d e i n the presence o f 500 mg/l o f EDT#. The slope o f t h e ’ l i n e i n Figure 9 i s

Residual z inc

No. 240, Vol. 80 67

The residual metal concentration a t f ix - ed chelating agent concentration should n o t be affected by the i n i t i a l concentration of zinc a t s toichiometric dosage of su l f ide and fixed pH. T h i s phenomenon was ver i f ied t o be t rue for the zinc-EDTA system where 10, 100 and 500 mg/l of zinc solution were t rea ted by su l f ide . The r e su l t s are shown i n Figure 9 by d i f fe ren t symbols fo r the various zinc concentrations.

Theoretical zinc s o l u b i l i t i e s fo r cor- responding experimental conditions were plotted in Figure 9 for comparison purposes. Ex- perimental data always show higher residual zinc concentration than values calculated w i t h the l i t e r a t u r e Ks values fo r zinc su l f ide . This phenomenon cgn be explained by the fac t t h a t zinc su l f ide ex i s t s i n several polymorphic forms (32). ature are spha ler i te (cubic close packing) and wurtzite (hexagonal close packing) which has different so lub i l i t y products i n water. The value? of K varies from lO-Z4 (sphaler i te ) t o 10 2 2 ( w i k z i t e ) . The experimental data always f a l l between the s o l u b i l i t i e s calculated from these two K s p values indicating tha t zinc sulfide may ex i s t i n both forms when precipi- t a t i o n occurs. Calculated residual zinc concentration f o r d i f fe ren t Ksp values are compared and shown by dotted l ines i n Figure 9 .

action time on zinc sulf ide precipi ta t ion, experiments were carr ied o u t fo r various zinc- chelating agent systems. withdrawn a t various time periods and f i l t e r e d for analysis. The resu l t s are shown i n Figure 10. Zinc su l f ide precipi ta t ion was very f a s t

and reached equilibrium in a very short time. No s igni f icant change i n residual zinc concentration was observed a f t e r a reaction time of 5 minutes.

The s tab le forms a t room temper-

In order t o determine the e f f ec t of re-

Sample solutions were

A s e r i e s of experiments were conducted t o study the e f f ec t of chelating agents on copper sulfide precipi ta t ion. i n Table 5 which indicates t ha t the removal of copper w i t h su l f ide precipi ta t ion was s a t i s - factory even i n the presence of EDTA. For ex- ample, a t pH 8.0 and 100 mg/l EDTA, residual copper concentration was only about 0.6 mg/l.

The r e su l t s are shown

Reaction times fo r most Cu-chelating agent systems were short; Figure 11 shows tha t no change i n copper concentration was observed after a reaction time of 5 minutes. In the presence of high concentration (grea te r than 300 mg/l) of c i t r a t e , the residual copper concentration increased dramatically when reaction time was greater than 20 minutes.

Table 5. Effect of chelating agents on CuS precipi ta t ion(25) .

Cu Concentration = 100 mg/l Na2S Dosage = 1 . 0 5 ~

Chelating Agent Concentration = 100 mg/l Reaction Time = 30.0 m i n

Chelating Residual Metal Concentration ,mg/l Agent (0.025 pin f i l t e r )

pH = 4.0 pH = 8.0

EDTA 0.85 Ci t ra te 0.65 Gluconic Acid 0.25 Tartrate 0.15 No Chelating o.08

Agent

0.7 0.4

0.1

0.05

---

Evaluation of the e f f ec t of pH was made w i t h EDTA of 0 mg/l and 100 mg/l a t 1 . 0 5 ~ stoichiometric su l f ide dosage. Figure 12 shows tha t copper removals were complete a t pH values between 4 and 8. B u t i t should be noted tha t evenat very low pH (pH -1 .5) , the precipi ta t ion of copper su l f ide a t low pH i s fur ther complicated because of the formation of Cu2S. i n g reactions :

Cu,S formation involves the follow-

2cu++ + s= + + + 2 c u -I. s (19)

2CU-I. -I. s= : c u 2 s (20)

The theoret ical so lub i l i t y of Cu2S i s less than CuS indicating tha t a t very low pH, the d i s s o l u t i o n of Cu2S must be taken i n t o account i n addition t o CuS. Some experiments were conducted f o r the Cu-citrate system. s ign i f icant difference of residual copper concentrations were observed a t d i f fe ren t pH values, even a t low pH 4 o r h i g h pH 10.

No

Continuous MSMPR Precipitation Results

The zinc removals from each r u n involved i n the MSMPR study are summarized i n Table 6 . A reactor detention time o f 5.0 - 15.0 min- u tes , and a su l f ide dose of 1 . 0 5 ~ were used i n these zinc su l f ide precipi ta t ion experiments. The i n i t i a l z inc concentration was 100.0 mg/l. of zinc exceeded 99.7% fo r b o t h pH levels used, showing excel len t metal removals.

As shown in Table 6 , the removal

68

Phas .-


14

12

System: Znt+ - S - Chelating Agent Zn = 100 mg/l Sulfide Dose = 1 . 0 5 ~

A : EDTA El : Cit ra te @ : Gluconic Acid

AlChE SYMPOSIUM SERlEIl

1 Chelating Agent = 100 mg/l pH = 8.0 t Code t o symbols :

0 0 20 40 60

Reaction Time. min

Figure 10 . Effect of reaction time on ZnS precipitation i n the presence o f various chelating agents (25).

Transformation Indications

The residual zinc concentration f o r the constant pH batch precipi ta t ion systems (EDTA versus no E D T A ) a re shown i n Figure 13. Experiments in progress invest igate the effects of varying i n i t i a l metal concentra- t ion (20 - 100 mg/l heavy metal) , type of metal to be removed ( Z n , C u , o r Cd) pH < l o ) , reaction time (5 - 120 minutes), and presence of EDTA ( 0 - 300 mg/l) ¶

residual metal concentration and resul t j n g Zn (0 - 300 mg/l), upon the residual metal con- cenbration and resul t ing par t ic le size dis- t r ibu t ion . Preliminary resu l t s from constant

pH ( 3 <

pH batch experiments are l i s t e d i Tab1 7 f o r zinc sulf ide precipitation conduct€ b o t i n the absence and in the presence of a che- la t ing agent ( E D T A ) . The su l f idemeta l dosage was 1.05:l f o r a l l experiments l i s t e d i n the table . For the pH range under investi- gat ion¶ pH does not have a s ign i f icant effec on the PSD of the metal sulf ide precipi ta te . Although the system i s chemically a t equili- b r i u m within 5-10 minutes of reaction time, the system i s i n a dynamic s t a t e ; the PSD changed with time due t o smaller par t ic les dissolving and recrystal l iz ing on larger par t ic les . After 40 minutes of reaction t i s the PSD remained constant indicating true

No. 240, Vol. 80 69 z

I I I

I Sy s tern : Cu++ -S-Che 1 a t i ng Agent

Sulfide Dosage = 1 . 0 5 ~ Chelating Agent = 100 mg/l pH = 8.0 o : EDTA A : Cit ra te

c u = 100 mg/l

: No Chelating agent

% a c t , 3 n time, minutes Figure 1 1 . E f f e c t o f reaction time on CuS

precipi ta t ion in the presence of chelating agents (25).

equi 1 ib r i um conditions were present.

Figure 14 compares the experimental values of the residual Zn concentration with reported theoret ical s o l u b i l i t i e s ; the experimental data were always higher than the theoretical values. The values f o r so lubi l i ty products l i s t e d i n the l i t e r a t u r e are based on aged preci p i t a t e s . In these s tudies , however, the zinc su l f ide precipi ta te formed were fresh precipitates of varying morphology. cipitates were usually amorphous i n nature. The KSp value of fresh precipi ta tes may be 1 t o 3 l o g cycles higher than those fo r aged precipitates. comparison of ZnS experimental data t o calculated values fo r aged precipi ta tes ( K S p = 1.6 x and corrected K ( K s = 1 . 6 x respectively. Such beha8for I! indicative of a phase transformation from an amorphous k i n - et ically favored precipi ta te t o a more ordered crystall ine form. the i n i t i a l precipi ta te transforms t o the more stable thermodynamic form. Such behavior was a l s o observed visual ly under the microscrope.

SUMMARY

The pre-

Figures 14 and 15 show the

As the prec ip i ta te ages,

IC--

The precipi ta t ion of zinc su l f ide has been studied under MSMPR conditions fo r

1 .o

0.8

0.6

I n i t i a l C u concentration = 100 I I N J ~ I Sulfide/Metal dosage = 1 . 0 5 ~

0 : no EDTA A : 100 m d l EDTA

I I I I

2 4 6 8 10 12

PH

Figure 1 2 . Effect o f pH on the precipi ta t lon of cus.

Table 6. Zinc removal from the MSMPR study.

Initial Zinc Concentration = 100.0 mg/l Sulfide Dose = 1.05 x

Series Run pH Residence Time, Residual Zn* X Removal

I 4 8.0 5 .o 0.25 99.15 5 8.0 10.0 0.30 99.70

No. mi n . Concentration ,mg/ i

9 8.0 15.0 0.15 99.85

I1 3 10.0 5 .O 0.20 w.eo 1 10.0 10.0 0.20 99.80 6 ;;.< 15.3 2 . 2 5 9 ; . i 5 2 13.; 15.3 1.15 99 .as

1 1 iG 0 15.0 0.10 99.90


I n i t i a l Zn concentration = 100 mg/l Sulfide/Metal dosage = 1 . 0 5 ~

@ : no EDTA Q : 100 mg/l EDTA

E

S 0

c, a L

aJ V E 0 0

N

16 *r

1: 12

E 8 - iu 2 = 4 *I- VI aJ m

0

2 4 6 8 10 12 PH

various pH values. The growth r a t e , nuclea- t ion r a t e , nuclei densi ty , removal e f f ic iency , and k ine t ic order were measured for various pH val ues. A1 t h o u g h very high supersatura- t i ons were achieved, the kinet ic order f o r this system i s low (1.015 fo r Ser ies I ; 0.335 for Series 111, suggesting a small dependency on supersaturation fo r the nucleation r a t e . Due t o such low k ine t ic orders, changing the residence time has l i t t l e e f f ec t on the pa r t i - c l e s i ze dis t r ibut ion and dominant s i z e . I t i s expected tha t increasing the suspension dens-i ty wi 11 enhance the growth ra te Tnd

dominant s i ze . The removal of zinc exceeded 99.7% fo r b o t h pH levels used. The pa r t i c l e s i ze d is t r ibu t ion was very narrow. Very few par t ic les greater than 20 pm were observed; the dominant s i ze was generally on the order of 5-7 v m , causing a cloudy appearance fo r tbe reactor suspension. Such f ine precipi- t a t e s make sedimentation and f i l t r a t i o n extremely d i f f i c u l t . suspension densi t ies and/or coagulants should

* be employed. The par t ic les were amorphous in nature.

To overcome t h i s , higher

The f ac t t ha t greater number of partic1,es fand hence larger nuclei dens i t ies ) were obtained fo r the longer residence time runs


was a t t r ibu ted t o the inab i l i t y analytically t o detect s izes smaller t h a n 3 pm on the Coulter Counter Model TA-I1 us ing the 140 pin aperture tube. were employed, the tubes would plug very quickly .

When tubes of smaller size

Due t o microscopic observations and corrections on the so lub i l i t y product of zinc su l f ide (by 2 log cycles) when comparing the experimental resu l t s t o values calculated u s i n g constants found i n the l i t e r a t u r e , a phase transformation from an amorphous kin- e t i c a l l y favored precipi ta te t o a more ordered c rys ta l l ine form i s indicated. fresh precipi ta te ages, the i n i t i a l precipi- t a t e transforms t o the more s t ab le form. Similar behavior has been observed i n heavy metal precipitation with hydroxides.

As the

ACKNOWLEDGMENTS

The authors wish t o acknowlege the co- operation of the School of Animal Science a t Purdue University fo r t he i r loan of the Coulter Counter Model TA-I1 enabling the pa r t i c l e s ize dis t r ibut ions t o be obtained essent i a1 1 y instantaneously,

.I 9'

No. 240, VOl. 80

50

=I 40 F 5 .- 30 .)

c, L c, E

E 0 u E N

m

20

r 10 m 7 0 v) aJ .r

= o 0 10 20 30 40 50

Calculated Zn concentration, mg/l

Figure 14 . Comparison of calculated and experimental results f o r ZnS-pre- c ip i t a t ion ( K 1 i t e ra ture vaf8e) .

= 1.6 x 10 z4,

71

0 10 20 30 40 50

Calculated Zn Concentration, m g i l

Figure 15. Comparison o f calculated dnd experimental resu l t s f o r ZnS pre- c ip i ta t ion ( K = 1.6 x corrected valh@).

Table 7 . Residual metal concentrations from constant pH batch su l f ide precipi ta t ions(25) .

I n i t i a l Zinc Concentration = 100 mg/l SulfidejMetal Dosage = 1.05 x

Metal PH No EDTA 100 mg/l EDTA Residual Metal % Removal. Residual Metal % Removal

Concentration, mg/l Concentration, mg/l 3 ---- ---- Zn 3.0 12.0 99 .o

4.0 0.3 99.7 16.5 83.5 i 6.0 --- - -- 15.0 85 .O

8.0 0.2 i 1 10.0 0.15

99.8 99.85

12.8 12.0

87.2 88.0

72

NOTATION

Advances in Crystallization from Solutions A

AIC~IE SYMPOSJUM SERIES''

Bo pa r t i c l e nucleation r a t e , number/ml-min

C, residual metal concentration, moles/l

CSD crys ta l s i ze d is t r ibu t ion

G pa r t i c l e growth r a t e , pm/min

i kit.etic exponent re la t ing nucleation r a t e t o growth r a t e

ISP insoluble su l f ide precipitation

kN k ine t i c r a t e constant r e l a t ing nucleation r a t e t o growth r a t e

k, shape f ac to r f o r volume

K,

K2

f i r s t ionization constant for H S

second ionization constant f o r H2S

solubi 1 i t y product

( a s )

(4 KS P L pa r t i c l e s i z e , pm

LD dominant pa r t i c l e s i z e , um

MSMPR

MT suspension density , mg/l

n

no

N

pH -log [H']

PSD pa r t i c l e size d is t r ibu t ion

mi xed suspension m i xed product remova

population density a t s i ze L , number/ml-vm

nuclei density , number/ml -pm

cumulative number of c rys ta l s per m l

r correlat ion coef f ic ien t

ST

SSP soluble su l f ide precipitation

equilibrium t o t a l soluble su l f ide concentrat ion, mg/l

Greek

P par t i c l e density, gm/cm3

-

'I residence time, m i n

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heavy metal crystallization kinetics in an msmpr ... · to either waterways, industrial waste...

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