Recovery of silver by crystallization of silver carbonate in a fluidized-bed reactor

War Rex. Vol. 26. No 2. pp. 235-239. Iq92 Printed in Great Brttain. All rights reserved

0043-1354,92 $5.00 + 0.00 Copyright ~ 1992Pergamonlatestpie

RECOVERY OF SILVER BY CRYSTALLIZATION OF SILVER CARBONATE IN A FLUIDIZED-BED REACTOR D. WILMS I (~ K. VERCAEMSTI and J. C. VAN DIJKJ tlnstituut voor Industri~le Scheikunde. Katholieke Universiteit Leuven. De Croylaan 46, B3001 Heverlee, Belgium and :DHV Water BV. P.O. Box 484, 3800 AL Amersfoort, The Netherlands

(First receh'ed June 1990; accepted in rerised form July 1991)

Abstract--Heavy metals can be recovered from wastewaters by growing crystals of metal carbonate in a fluidized-bed reactor. Optimal conditions for crystallizing silver carbonate have been investigated on a laboratory scale pellet reactor initially seeded with quartz sand. With an effluent-pH of 10.2, a Cr/Ag feeding ratio of 3 tool/tool, an Ag-load of less than 2 kg Ag per square meter reactor cross-section per hour and a hydraulic load of 45 m/h. the effluent silver concentration was below 8 mg/I. From chemical analysis it was found that the pellets, apart from the sand fraction, consisted (for more than 99%) of pure Ag:COj. It can be concluded that the crystallization technique is a valuable alternative for the classical methods of silver recovery from wastewater. K,,y words--silver, heavy metals, crystallization, recovery, pellet reactor, silver carbonate

INTROI)UCTION Since 1985. DIIV Water and the KU Lcuven have been developing a completely new process for recovering heavy metals from wastewater by crystallization in a pellet reactor. This process is applicable to almost all heavy metals. However, depending on the kind of metal, optimal process conditions can be quite different. Till now. experience has been gained for Zn, Cu, Co, Ni and Cr. This paper describes the experimental work for determining the optimal process conditions for the crystallization of silver carbonate. The most important process parameters are: - - t h e effluent pH - - t h e effluent Cr (Cr is the sum of the molar concentrations of CO:, hydrogen carbonates and carbonates) - - t h e hydraulic load (total flow per unit crosssection of the reactor) - - t h e metal load (mass of metal fed per unit time and unit cross-section of the reactor). The influence of other process parameters, such as the dynamic specific surface of the pellets and the total pellet mass in the reactor, has been investigated as well. The objective is the production of pure crystals of silver carbonate and of an effluent with a minimum concentration of silver. The crystallization process may offer a valuable alternative with respect to the existing techniques for recovering silver from wastewater. Basically, three methods are frequently applied at present: (I) precipitation with Na:S, (2) electroplating. (3) cementation. 235

Methods 2 and 3 produce an effluent with 0.1-20 mg/! of silver. With the first method even lower concentrations can be obtained. With the exception of electroplating, these methods offer no simple means to recover the silver in a pure form. PRINCIPLE OF TIlE CRYSTAI.I.IZATIONPROCESS IN A FI.UIDIZED-.BEDREACTOR The process of forming pellets of metal carbonate is very similar to the process of crystallizing CaCO~ in a pellet reactor, as applied in the partial softening of drinking water (Graveland, 1987; Van Dijk and Wilms, 1992). Essential in the process is to mix metal ions and carbonate ions under such conditions that the mixture becomes supersaturated with respect to the solubility of the metal carbonate, but not with respect to the solubility of metal hydroxide. Metal hydroxides are not crystalline and form a voluminous, water-rich sludge. The higher the pH, the greater the carbonate fraction of Cr is, so that the driving force for the production of metal carbonate becomes greater. The same however applies with respect to the (undesired) production of metal hydroxide. As a consequence, Cr must be high enough to allow the crystallization of metal carbonate at a not too high pH. The reactor consists of a cylindrical vessel, partially filled with suitable seed material, e.g. quartz sand. When out of operation, the fixed bed height is about half of the total reactor height. During operation the fluid up-flow velocity is so high (40-120 m/h) that the pellet bed is kept in a fluidized state, so that cementing is prevented.

D. WILMS et al.

236

The heavy metal containing wastewater is injected at the bottom of the pellet reactor: there the influent is mixed with a recirculation stream, to which a concentrated solution of sodium carbonate is being added. The mixture of recirculation and influent streams at the bottom of the reactor is supersaturated with respect to the metal carbonate: this supersaturation acts as a driving force for crystallization of the metal carbonate onto the pellets; on the other hand, the supersaturation may not be so high that it would lead to spontaneous nucleation: therefore, the higher the metal concentration in the influent, the more effluent is to be recirculated to provide an adequate dilution. Because the crystallization reaction is very fast and because high hydraulic loads can be applied, the pellet reactor system is compact and has therefore low investment costs. The process is pH-controlled and very stable thanks to the buffering action of the carbonate system and the dampening effect of the recirculation stream; hence the operation is easy and operational costs are low. The pellets are growing and have to be removed periodically, because otherwise their specific surface would become too small. After release of the bigger pellets, which automatically concentrate at the lower part of the reactor, new seeding material is added. The metal (hydroxy-)carbonate pellets are pure and can be reused by dissolving them in a strong acid; the carbonate escapes as CO:, and a concentrated metal solution is obtained, while the seeding sand can be reused in the reactor. The pure concentrated metal solution c:m be reutilizcd in metal finishing, metal processing or the chemical industry. Depending on the type of metal and wastcwater, small quantities of suspended solids (carry-over) can form in the Pellet reactor. One part of this carryover consists of amorphous metal hydroxide and/or crystals of metal carbonate formed by spontaneous nucleation, another part consists of small fragments of pellets split off as a result of erosion in the fluidized bed. if the amount of carry-over becomes too high, a dual-media filter should be incorporated in the recirculation system.

a peristaltic pump. This recycled effluent dilutes the incoming flows at the reactor bottom and at the same time ensures that the upward flow velocity of the mixture is high enough to keep the bed in a fluidized state. The seeding material is pure quartz-sand with a diameter between 0.2 and 0.3 ram. From time to time the bigger silver carbonate pellets (0.6 mm) are withdrawn via a side-tube near the reactor bottom. Three feeding streams are pumped into the reactor by means of Peristaltic pumps with adjustable flow. Cr is introduced as a solution of Na:CO3 with a concentration of 250 or 500 mmol/l. In order to bring the effluent to a required pH-value, a solution of NaOH or HNO3 with a concentration of 100 or 250 mmol/I is fed. By means of injection needles, both streams (Cr and pH-correction) are injected into the recirculation stream, just before entering the reactor, to ensure good mixing. The silver containing wastewater is simulated by a solution of AgNO3 with a concentration of 10 mmol/I. This solution is injected directly in the reactor so that crystallization will take place upon the pellets. Since Ag:CO, is light-sensitive, the whole reactor is protected from daylight by means of aluminium foil. R I,:S U L T N

The effect on the yield and on the silver concentration of the effluent (filtered and non-filtered) has been investigated for the following factors: the effluent pH, the feeding ratio of CT/Ag, the silver load, the hydraulic load and the total pellet mass. The experiments were conducted in the indicated order: first the sensitivity of the system with regard to the factor was assessed, then at its optimum value the next factor was varied. The results together with the corresponding experimental conditions are shown in Figs I-8.

Influence of the pH Table 1 represents the results of a number of pH experiments. The first three parameters and the

2OO

LABORATORY-SCALE PELLET REACTOR

In order to determine the optimal process conditions for the crystallization of Ag.,CO3, a number of experiments have been conducted on a laboratoryscale pellet reactor. This reactor consists of a glass tube with an internal diameter of 18mm and a height of 1.50m. The reactor is filled over a height of 0.60 m (the fixed bed height) with pellets; above a narrowing of the tube at the bottom, a glass marble is installed: this marble takes care of the distribution o f the incoming flow over the cross-section of the tube, and also blocks the inlet opening when the reactor is out of operation. The greater part of the effluent is recirculated by means of

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Fig. 1. Total and dissolved silver in the effluent as a function of pH: O, total silver; O. dissolved silver, Process conditions: pH = 8.0-11.2; CT/Ag(feed) = 1.5 tool/tool; effluent

Cr = 8-10 retool/I; Ag-load = 0.95 kg/m2h; hydraulic load -4Sin/h; fixed bed height =0.55-0.70m; expanded bed height = 1.45 m.

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fifth are measured directly: the others follow from calculation of mass-balances. Total silver is the silver concentration of nonfiltered samples; the silver concentration in filtered samples is called dissolved silver. The difference between both concentrations is a measure for the concentration of amorphous silver. The supersaturation of the effluent with respect to Ag:CO3 and AgOH is defined as t a g ~'):(CO~ 2-)/K c and t a g *)(OH ~ ) / K , resp., where 0 represent analytical concentrations and Kc and K, the apparent solubility products of silver carbonate and hydroxide, dAg/dC-r is the ratio of the amount of silver that is removed to the amount of Cr removed. Removal in one pass represents the difference in silver concentration at the base and the top of the reactor. From the table it appears that although the Cr/Ag ratio was kept constant CT increases steadily at increasing pH-levels. These CT-values are smaller than the ones calculated at lower pH-values, whereas the reverse holds at higher pH. This can be explained by the fact that there is some exchange of CO 2 with the atmosphere, with which the effluent is contacted during sampling and filtration. Although the CT/Ag ratio in the feed was kept constant (at 1.50 tool/tool), this ratio is considerably higher at the bottom and at the top of the reactor (e.g. 32 and 97 at pH = 10.2). The concentration of total and dissolved silver in the effluent as a function of the effluent-pH is shown in Fig. !.

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Fig. 4. Total and dissolved silver in the effluent vs effluent CT: O. total silver: C), dissolved silver, pH = 10.2: CT/Ag (feed) = 0.75-15 mol/mol: Ag-load = 0.95 kg/m"h; hydraulic load = 45 m/h.

The solubility of Ag2CO3 is higher at lower pHvalues. The effluent contains practically no carry-over (amorphous silver). At pH-values higher than 10.2 the concentration of total and amorphous silver increases considerably (Fig. 2). This increase is due either to spontaneous nucleation of Ag:CO~ as a result of the high supersaturation in the mixing point, or to precipitation of AgOH(s)-Ag20(s), whose solubility product is exceeded above pit = 10. Apparently part of the carry-over is in colloidal form and can not be removed by filtration: this explains the increase of "dissolved" silver in the effluent at higher pH-values. If pure Ag,CO 3 crystallizes, dAg/dCr must be ~ 2. This seems indeed to be the case (Table I); deviations can be due to exchange of CO2 with the atmosphere or to the formation of some AgHCO~ (at lower pH) or AgOH (at higher pH) together with Ag:CO~. In order to measure the thermodynamic solubility constant of the solid phase that crystallizes upon the pellets, the water phase was further recireulated for 16 h after finishing each pH experiment. During this process, the system was kept isolated from 24 22

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Fig. 5. Total and dissolved silver in the effluent vs effluent O, total silver; O. dissolved silver, pH ffi9.4; Cr/Ag (fred) = 0,75-15 tool/tool; Ag-load = 0.95 kg/m:h; hydraulic load ffi 45 m/h. CT:

D. WIL~S et al.

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Fig. 6. Total and dissolved effluent silver vs silver load: O, total silver: O, dissolved silver, pH--10.2: Cr/Ag (feed) = 3 mmol/I; Ag-load =0.5-3.7 kg/m:h; hydraulic load = 45 m/h.

the atmosphere to prevent any exchange of CO:. Thereafter a sample of the liquid was filtered and analyzed. Figure 3 shows the ionic products K, (corrected for the ionic strength of the solution) of Ag2CO~ and AgOH. The experimental pK,-values of AgOH decrease with increasing pH and are greater than the literature value 7.71 (Kragten, 1978): clearly, the solubility limit of AgOH is not attained in the equilibrium experiments. The pK,-values of Ag,CO~ remain constant as the pH varies and are close to the litcrature value 11.09. The silver concentrations in the solutions at equilibrium are evidently dominated by the solubility of silver carbonate.

Influence of the feeding ratio of Cr/Ag Since the composition of the effluent is controlled by the solubility of Ag2COj, one can expect that dosing higher amounts of Na2COj will result in lower silver concentrations of the effluent. This has been verified with two series of experiments, one at the optimal pH of 10.2 (Fig. 4), the other at a somewhat lower pH of 9.4 (Fig. 5). The same conclusions apply for both pH-values. An increase of the effluent-Cr from 3 to 20 mmol/I 20 18 18 14 AO

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results in clearly better results (higher driving force). A still further increase of CT does not lower the effluent silver any further. At CT-Values higher than 70 mmol/I important amounts of amorphous material are formed, due to the spontaneous nucleation of Ag,CO~. From an economical point of view (cost of chemicals), clearly one will select the lowest C-r that ensures equally good results, namely CT = 20 mmol/I (corresponding to a feeding ratio CT/Ag = 3 mol/mol).

Influence of the sih,er load The crystallization capacity of a pellet reactor is limited. If one still increases the silver load any further, then from some point on the effluent quality will deteriorate. From Figure 6 it appears that it is possible to apply relatively high silver loadings (up to 1.6 kg/m2h); at still higher Ioadings, effluent total silver starts to increase, due to the formation of carry-over. The silver concentration in the filtered effluent also becomes higher, probably due to the colloidal character of part of the carry-over. The crystallization of Ag2CO3 evidently takes some time: an increase of the applied silver load increases the fraction of the fluidized bed where a greater supersaturation remains; as a consequence, production of amorphous material is enhanced. On designing a crystallization reactor, one has therefore to compromise between: (I) a reactor without a post-filter, but with a relatively great diameter (to allow high recirculation ratios, which however will limit the silver load), (2) a relatively small reactor, operated at higher silver Ioadings, followed by a post-filter to remove the carry-over.

Influence of the hydraulic load On varying the recirculation flow one can change the hydraulic load of the reactor. This latter was increased from 25 to 90 m/h.

Silver recovery in a fluidized-bed reactor

239

Table I. C h a r a c t c r i s t z ~ of the effluent and the bottom liquid. Ct/A8 (feed) - 3 mol/mol, Ag-load - 0.95 k&m:h El~uent pH 8.59 8.91 9.27 9.53 9.76 9.94 10.20 10.30 10.46 10.72 gttluent total Ag (rag.l) 42.090 29.730 21.370 17.130 14.010 13.870 10.440 13.520 21.490 25.720 Et~tuent dissolved A8 (rag,1) ,10.080 28.930 19.570 15.670 13.280 12.430 9.640 11.140 12.580 14.130 Effluent amorphous A 8

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Emuent C.r Effluent supersaturation: Ag:CO3 AgOH Bottom total A8 Bottom dissolved A8 Bottom amorphous Ag Bottom CT Bottom supersaturation: Ag:CO~ AgOH dAgdC r Formation in one pass: Pellets Amorphous A8

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0.800

1.800

1.460

0.730

1.440

0.800

2.380

8.910

7.870

7.840

8.389

7.921

8.502

8.605

8.640

8.448

9.214 10.985

(- ) 1.303 1.378 1.455 1.492 1.702 1.956 1.556 2.256 3.440 5.493 (- ) 0.042 0.063 0.103 0.167 0.248 0.355 0.428 0.703 1.066 2.049 (re&l) 63.709 51.039 43.223 39.460 36.422 36.292 30.017 35.968 42.911 47.400 (m8,1) 61.748 50.258 41.466 38.035 35.710 34.886 29.233 33.643 34.198 36.071 (rag/l) 1.961 0.781 1.757 1.425 0.713 1.406 0.784 2.325 8.713 11.329 (mmoLl) 7.990 7.954 8.492 8.044 8.612 8.716 8.731 8.562 9.31M 11.036 (- ) 3.137 (- ) 0.065 (moLmol) 1.673 (mg/l) (mgl)

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8.910 12.443 15.573 14.427 20.793 25.598 35.893 0.404 0.667 0.997 1.297 2.123 2.896 5.229 1.684 1.887 1.876 1.988 1.831 2.235 3.966

21.619 21.309 21.853 22.330 22.412 22.422 19.577 22.448 21.421 21.680 0.049 0.019 0.043 0.035 0.017 0.034 0.016 0.055 0.197 0.261

A higher recireulation ratio results in a lower silver concentration and a lower supersaturation at the base of the reactor, so that homogenous nucleation is prevented: this however will promote erosion of the pellets. From Fig. 7, however, it appears that the hydraulic load has only a negligible effect on the effluent quality. This is not always the case: e.g. the pellets o f NiCO~ have been found to bc very sensitive to erosion (Wilms et al., 1988).

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pelh't mass in the reactor

The rate of hcterogenous crystallization is proportional to the specific pellet surface. For pellets o f uniform size, tile total surface is proportional to the fixed bed height in the reactor. One may expect that the allowable supersaturation increases with the available total pellet surface. In order to assess the influence of the pellet mass, a series of experiments was conducted in which each time a fraction of the pellets was removed. In the first experiment the fixed bed height was 668 ram, in the last run 115 ram. From Fig. 8 it appears that the same effluent quality is obtained as long as the fixed bed height exceeds ~m. F r o m the bed porosity and from the mean pellet diameter under the experimental conditions, one can calculate that the crystallization rate of Ag.,CO~ corresponds to 0.3 g Ag/m" pellet surface, h.

Characteristics o f the pellets After drying and dissolving the pellets in a strong acid, the composition of the pellets has been determined. The pellets consist of more than 99% o f Ag:CO3, and contain traces of Na, CI and Si. The

density o f the pellets was 6020 kg/m ~. The color is pH-dependent: on increasing pH, the color shifts from yellow to green. CONCLUSIONS The aim of this work was to optimize the different process parameters for the crystallization of Ag:CO3. This resulted in following conditions: optimal pH ~ 10.2 optimal effluent C r ~ 20 mmol/I silver load ~< 2 kg Ag/m: reactor cross-section, h. The hydraulic load was of minor importance. To ensure a reliable operation of the reactor, it is also required to provide a sufficient pellet surface, so that the silver loading does not exceed 0.3 g Ag/m" pellet surface, h. Under these conditions one may expect an ellluent silver concentration of 8-10 mg/I (not filtered), or o f 7-8 mg/I (filtered). A lower effluent concentration could only be reached on applying excessive doses of Na,CO~, which is not admissible from an economical or environmental point of view. REFERENCES

Graveland A. (1987) Amsterdam: van hard naar zacht water. H,O 20, 290-293. Kragten J. (19781 Atlas of MetaI-Ligand Equilibria in Aqueous Solution. Wiley, New York. Van Dijk J. C. and Wilms D. 0992) Drinking water softening with pellet reactors: fundamentals and state-ofthe-art. Aqua. In press. Wilms D., Buldeo Rai P., Van Dijk J. C. and Sch611er M. 0988) Recovery of nickel by crystallization of nickel carbonate in a fluidized-bed reactor. In Water Pollution Control in Asia (Edited by Panswad T., Polprasert C. and Yamamoto K.), pp. 449-456. Pergamon Press, Oxford.